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Chapter-1 Introduction
1
The chemistry of metal alkoxide dates back to 1840s when Liebig first time observed
the reaction of alcohol with metal like sodium and potassium In the same year the
term alkoxides was used by Kuhlmann1
for the alkaline derivatives of alcohols
Thereafter the chemistry of alkoxides has been the center of attraction for the
researchers due to their tremendous applicability especially in the fields like catalysis
sol-gel and MOCVD methods (metal-organic chemical vapor-phase deposition) for
synthesis of pure metal oxides precursors for nano materials ceramics etc
Alkoxides M(OR)n are compounds with one or more metal atoms or semi-metal atoms
(M) with alkoxy groups (OR) as ligands An alkoxy ligand is produced when
hydrogen atom is stripped from an alcohol (ROH) or by dividing an ether (ROR)
molecule in such way that oxygen is retained along with R group (ie OR) In other
words alkoxides may be regarded similar to hydroxides (MOH) in which the H has
been substituted by an alkyl or aryl group(R) The alkoxides are known for most of
the elements in the periodic table
Most of the metallic elements are reactive towards oxygen because an oxide exists in
stable single or mixed phases Molecular precursors derived from alkoxide complexes
can generate ceramic materials in a single step (thus-called single-source precursors ndash
SSPs)2
Metal alkoxides are very good single source molecular precursor (SSP) for
oxide synthesis This is mainly due to their molecular structure and high reactivity
which depends on the electronegativity of the metal ion ability to increase their
coordination numbers and the steric hindrance in the alkoxy groups This helps in the
synthesis of corresponding colloidal metal oxides with high homogeneity
There has been a growing interest in the development of the chemistry of mixed-metal
bi and polynuclear alkoxo and alkoxo-organometallic complexes for the last two
decades Motivation force for such interest derives from their fascinating structural
chemistry interesting catalytic properties and high potential for industrial
applications34
The important fact that most of the heterometallic alkoxo species can
generate bimetallic or multimetallic oxides has resulted in high research activity in the
field Their attractiveness lies in the fact that they are easily accessible and are
inexpensive Furthermore alkoxide ligands are easily removable via thermal
treatments Finally these compounds already have established metal-oxygen bonds It
is worth mentioning that their thermal deposition or decomposition processes can be
performed at relatively low temperatures compared to conventional methods
Chapter-1 Introduction
2
involving other inorganic salts These features make the metal oxides derived from
metal alkoxides highly pure with specific properties like high hardness chemical and
mechanical resistance and high temperature stability The softndashchemical approach is
a flexible simple and novel synthesis for industrial production Different types of
precursor can lead to change in unique features of the structural framework that
affects the physical and chemical properties in the final product by retaining the
stoichiometric ratio under control This will help to predict favorable morphology for
its futuristic applications with high reliability accuracy and reproducible results5-14
From above mention feature it is evident that metal alkoxides play the key role for
preparing materials of excellent functions and shapes
A range of catalytic applications of Group (IV) (ieTi Zrand Hf) is mainly due to the
Lewis acidic nature of M(IV) complexes Group (IV) elements are important
components of electro ceramics such as lead zirconium titanate (PZT) and barium
titanate Sol-gel processing of these and related materials is based upon hydrolysis of
alkoxide solutions It is found that the species produced when acetylacetone (acacH)
is added to Ti(OR)4 is 11 reaction product [Ti(OR)3(acac)] which is dimeric readily
under-go ligand redistribution in solution to give Ti(OR)4 and [Ti(OR)2(acac)2] 17
O
NMR spectroscopy has been used to investigate the hydrolysis of the resulting
mixture for R = Pri and the results were interpreted in terms of the hydrolysis behavior
of the individual mononuclear components Titanium complexes of alkoxide and
aryloxide ligands exhibit a rich coordination chemistry and reactivity and find
applications in various fields15
The interest in the development of non-metallocene
complexes for the polymerization of α-olefins16
has created a new range of chelating
dialkoxo17-24
ligands for group IV transition metals Complex formation may be
effected by the steric bulk on the ligands Furthermore the olefin polymerization
reactivity by a possible catalyst may strongly depend on the crowding around the
metal as substantial bulk on one hand may hinder the approach of an incoming
olefin and on the other hand decelerate termination processes25-28
Several types of
amine bis (phenolate) titanium complexes having different steric crowding around
the metal have been synthesized The [ONNO]-type ligands bind to the metal in a
tetradentate fashion leads to octahedral bis-(isopropoxide) complexes regardless of
the steric bulk of the aromatic ring substituents Various complexes involving
ethylenediamine and metal alkoxides with particular reference to complexes involving
Chapter-1 Introduction
3
titanium isopropoxide have been reported The alkoxides Titanium(IV) are
diamagnetic tetrahedral molecules and are being used in many organic synthesis
and in material science The structures of titanium alkoxides are often complex
Alkoxides derived from bulkier alcohols such as isopropanol aggregate less Titanium
isopropoxide is mainly a monomer in non- polar solvents Titanium isopropoxide is a
component of the Sharpless epoxidation a method used for the synthesis of chiral
epoxides Titanium isopropoxide is also used as a catalyst for the preparation of
certain cyclopropanes in the Kulinkovich29
reaction Prochiral thioethers are oxidized
enantio-selectively using a catalyst derived from Ti(OPri)4
The aluminium alkoxides chemistry has progressed significantly in the last fifty years
due to advances in their synthetic methodology and in the understanding of the role
ligands and co-ligands play in stabilizing the compounds and ensuring solubility
During column chromatography Aluminium oxide is well known as the stationary
phase It has also been used as solid support for various reactions including dehydro-
genation30
reactions with formic and acetic acid31
selective oxidation of alcohols to
carbonyl compounds using iodobenzene diacetate32
and aziridination and
cyclopropanation reactions using copper nanoparticles33
Aluminium alkoxides are thermally stable and even the insoluble [Al(OMe)3]n may be
sublimed at 240oC under reduced pressure The higher alkoxides are all soluble
Distillable products and the melting points of the solids increase with increasing
complexity of the alkyl chain Degree of association of aluminium alkoxides have
been reported by number of investigators rather widely based on molecular weight
determinations RCMehrotra34
revealed that freshly distilled aluminium isopropoxide
was trimeric in boiling benzene and it changed into a tetrameric form on ageing With
increasing branching of alkoxo groups the complexity diminished and it reduced to
the dimeric state in the tert-butoxide
Tris(acetylacetonato) aluminium(III)35
was the first complex to be obtained using a β-
diketone ligand Tris(β-diketonato) aluminium(III) complexes have then been
synthesized via a variety of routes as shown in Figure 1 Most frequently applied
Method is I which involves dissolution of the β-diketone in aqueous ammonia (if the
ligand is water-soluble) or a mixture of aqueous ammonia and methanol36
Aqueous
ammonia forms the ammonium salt by removing the methane proton from the β
diketone This is due to the acidity of the methine protons of β-diketones The
Chapter-1 Introduction
4
ammonium salt is then reacted with aluminium sulphate The reaction completes with
precipitation of product The synthesis of tris(ferrocenyl-13-butanedionate)
aluminium (III) by Zanello et al37
also take place via Method I This complex
[(FcCOCHCOCH3)3 Al] is the only known ferrocene containing β-diketonato
aluminium (III) complex
Method I Al2(SO4)3 + 3O
R1
O
R2
NH4
e-3(NH
4)2SO
4H2OCH
3OH
Al
O
O
R1
R2
3
3O
R1
O
R2
Method II AlCl3 +
3O
R1
O
R2
Method III Al(OH)3 +
(freshly prepared)
-3 HCl(g)
-3 H 2O(g)
EtOH
Figure 1 Methods commonly used for synthesis tris(β-diketonato aluminium(III)
complexes
Method II involves refluxing the β-diketone with aluminium chloride in benzene and
the reaction completes by the removal of gaseous HCl38
The method III shows that
the tris(β-diketonato) aluminium(III) complex can be synthesized from freshly
precipitated aluminium hydroxide34
This reaction thus illustrates that β-diketones is
acidic enough to attack freshly precipitated aluminium hydroxide
Due to existence of many hydrolysis species39
the aqueous chemistry of aluminium is
complex At low pH (25 - 35) 40
Tomany and co-workers performed an investigation
on the kinetics and mechanism of the reactions of aluminium (III) with acetylacetone
trifluoro actylacetone and hetpane-35-dione Aluminium alkoxide and the β-diketone
(Figure 2)3441
directly synthesized mixed alkoxy β-diketonato aluminium (III)
complexes of the type Al(R1COCHCO R
2)n(OR)3-n Driving force behind the reaction
is the azeotropic removal of the alcohol with benzene It is also interesting to note that
the alkoxide groups are significantly more reactive than the β-diketonate ligands
Another reason why the β-diketonate ligands displace the alkoxide is that bidentate
Chapter-1 Introduction
5
ligands (β-diketonates) form stronger bonds with the coordinating metal than
monodentate (alkoxide) ligands
AlH
O
O
R1
R2
n
RO Al
OR
OR
++ nROHRO
3-n
nO
R1
O
R2
R = CH2CH3 OPri R1= CH3 R2 = Ph
Figure 2 The synthesis of mixed alkoxy β-diketonato aluminium(III) complexes
from aluminium alkoxides41
Structures of Alkoxides
Alkoxide ions are the conjugate bases of alcohols and metal alkoxides are the
coordination compounds formed between metal ions and alkoxide ligands (fig 3i-iii)
As alcohols are only weak acids alkoxide ions are strong bases and metal alkoxides
tend to hydrolyze under condensation and elimination of alcohol when exposed to
water
1)
HO
2)
-O HOR -OR= =
3)
Mx(O Mx(OR)z=)z
Figure 3 i) Alcohol = hydrocarbon chain with a hydroxyl group ii) Alkoxide
Ion = deprotonated alcohol and iii) Metal alkoxide = coordination
compound with alkoxide ligands
Some possible hydrolysis and condensation reactions for metal with alkoxide ligands
are shown
M OR + H2O M OH + HOR
M OR + MHO M O + HORM
M OH + MHO M O +M H2O
Chapter-1 Introduction
6
According to Bradleyrsquos concept42
alkoxides have a strong tendency for
polymerization creating coordination polymers [M(OR)x]y (where y represents the
degree of polymerization) Degree of polymerization increases with the metal atomic
ratio Alkoxides take the smallest structural unit for the highest possible coordination
number of the metal Metal alkoxides [M(OR)x]y are well soluble in common organic
solvents and creates small oligomers with y = 2 3 or 442
Alkoxo RO- anion
possesses donor oxygen atom with three unpaired electrons which form covalent
bond with metal These anions might be coordinated to metal sites in terminal or
bridging way Alkoxides have physical properties that vary according to the nature of
the metal and alkoxy group They range from non-volatile insoluble solids to volatile
soluble solids This great variation in physical properties is due to the differing
molecular complexities observed in alkoxide chemistry those forming large
polymeric frameworks are insoluble and non-volatile while those forming small
oligomers are generally volatile and soluble
Alkoxides have a tendency to form oligomeric compounds [M(OR)x]y where RO-
groups are connected to two or even more metal sites This phenomenon affects the
reactivity and properties of these compounds The formation of oligomers or larger
polymeric frameworks is due to alkoxide groups bridging two or more metal centers
(ie acting as micro2 micro3 micro4 ligands) and the tendency of metals to increase their
coordination number The extent of oligomerization is affected by
(a) The Alkoxy Group For a given metal the more bulky the alkoxy group the lower
the degree of association
(b) The Metal The oxidation state of the metals determines the number of alkoxy
groups present per metal which in turn affects the bridging Low oxidation state
requires more bridges to achieve a given coordination number as compared to higher
oxidation state The size of the metal also play an important role to affect the degree
of bridging as larger the size of the metal atom more easily it can accommodate bulky
alkoxy groups and therefore more easily it can increase its coordination number
The versatile coordinating abilities of an alkoxo ligands leads to the formation of
structural pattern which range from simple bimetallic compounds to very complex
aggregates
Chapter-1 Introduction
7
R
O
M
R
O
M
R
O
M
O
R
MM
O
R
M M
O
R
M M
O
R
M M
O
R
MM
O
R
M MM
Figure 4 Coordination modes of an alkoxo ligand
Steric and electron demand of alkoxo groups have an influence on metal alkoxides
they form Electrophilic nature of metal cations allows attaching neutral ligands (eg
tetrahydrofuran pyridine etc) to the metal spheres Due to the saturating of metal
sites it is possible to obtain monomeric alkoxides [M(OR)xLy] (where L = neutral
ligand)
Alkoxides are highly versatile precursors for sol-gel synthesis4344
Alkoxides when
condensed form volatile alcohols andor ethers allowing for the formation of pure
products without impurities due to the precursor ligands In metal-organic chemical
vapour deposition (MOCVD) alkoxides are sometimes used as precursors4546
Examples of some of metal alkoxides having different structural features are depicted
below
TiOiPr
OiPr
PrOi
PrOi
= Ti(OiPr)4
Ti EtOEtO
EtO
EtO
EtO
OEt Ti
Ti
EtO OEt
OEt
EtO OEt
EtO Ti
OEt OEt
OEt
OEt
=Ti(OEt)16
Monomer47
Oligomer4849
Chapter-1 Introduction
8
Ce
PriO OiPr
PriO OiPr
PriO OPri
Ce
PriO OPri
OPriPriO
H
H
Alkoxide ndash alcohol adduct 50 51
Various so-called alkoxides are in fact oxo- or hydroxo-alkoxides which are the
condensation andor hydrolysis products of true alkoxides In oxo-alkoxides one or
more centrally placed bridging oxo ligands help to increase the coordination number
of the metal atoms The reactivity of oxo-alkoxides decreases with the ratio of
(bridging) oxo to alkoxo ligands Alkoxide derivatives may also contain other
ligands such as chloride ions or organic nonalkoxide ligands Chloro-alkoxides often
acquire structures similar to alkoxide structures but are normally avoided in sol-gel
synthesis as the chloride ions tend remain in the gel after hydrolysis as impurities in
the final materials
Ti
PriOPri
O
PriO
PriOO
OH
Ti
OPri
OPri
OPri
Ti
OPriPriO
=Ti3O(OH)(OiPr)9
Ti
PrOi PrOi
PrOi
PriO
PrOi
OiPr
Ti
OiPr
OiPr
OiPr
Y
ClCl
=YTi2(OiPr)9Cl2
oxo-hydroxo-alkoxide 52
chloro-alkoxide53
Apart from being versatile and important precursors in materials synthesis alkoxides
are also interesting from a structural point of view For example the choice of alkyl
group provides a means of systematic variation for the investigation of coordination
chemistry around metal and oxygen atoms The other parameters which can be
systematically varied are the number of oxo bridges and nuclearity
In the literature there are many examples of metal alkoxides it would be very
difficult to formulate a precise rule that could fully predict the final geometry of
forming alkoxide complex
Chapter-1 Introduction
9
Different structural pattern of metal alkoxides 54-69
Complex Structural pattern
[(C5H4CH3)4Y(micro-OCH=CH2)]2 Y2(micro-O)2 core
[Y3(micro3-OtBu)( micro3-Cl)( micro-O
tBu)3(O
tBu)4(thf)2 Y3(micro3-O)( micro-O)3O4 core
[Ti2(micro-OR)2(OR)4(acac)2]a (R=MeEt
iPr) [Ti2(micro-O)2O4 core
[Me4Zn4(micro3-OtBu)4 Zn4(micro3-O)4 core
[W2(OCMe2CMe2O)3] O3W= WO3 core
[Ga2(micro-OtBu)2
tBu4] Ga2(micro-O)2
core
[Mg2V2(thffo)6Cl4]b Mg2V2(micro3-O)2(micro-O)4 core
[(thf)(OtBu)Y(micro-O
tBu)(micro-CH3)AlMe23] YAl3(micro-O)3O core
[Zr2Co2(micro3-OnPr)2(micro-O
nPr)4(O
nPr)4(acac)2]
a Zr2Co2(micro3-O)2(micro-O)4O4 core
[Al(OEt)2GaMe23] AlGa3(micro-O)6 core
[Nb2(micro-OMe)2(OMe)2(HOMe)2Cl4] Nb-Nb(micro-O)2O4 core
[Mo2(OiPr)4(HO
iPr)4 O4Mo=MoO4 core
[Pr3(micro3-tftb)2(micro-tftb)3(tftb)2]c Pr3(micro3-O)2(micro-O)3O4 core
[YNa8(micro9-Cl)( micro4-OtBu)( micro3-O
tBu)8(O
tBu)] YNa8(micro4-O)( micro3-O)8O core
[Na4Zr6(micro5-O)2( micro3-OEt)4(micro-OEt)14(OEt)6 Na4Zr6(micro5-O)2( micro3-O)4(micro-O)14O
core
[Ti7(micro4-O)( micro3-O)2(micro-OEt)8(OEt)12 Ti7(micro4-O)( micro3-O)2(micro-O)8O12 core aacac = acetylacetonato
bthffo=tetrahydrofuryloxo
ctftb= OCMe2(CF3)
In fact even minor changes in a ligand structure or reaction conditions can lead to the
geometry of the whole compound to be fundamentally different
Classification based on Alkoxide Complexes Structure
Metal alkoxide complexes can have very complex structures due to formation of
oligomeric and sometimes even polymeric aggregates Formation of alkoxy bridges
M-O(R)-M help the complexes to obtain maximal and preferred coordination even
though the number of bonded ligands per metal atom are too few Complexes are
categorized based on number of metal atoms in the complex Optimal coordination is
obtained by chelating ligand or by a shared (bridging) ligand atom
Mononuclear complexes ndash Mononuclear complexes are highly charged metal ions
where the coordination requirements are satisfied by the number of OR-ligands The
ligands are often large and branched with chelating abilities
Binuclear complexes - In binuclear complexes an oxygen atom in the ligand connect
the two metal atoms Usually at least two alkoxy bridges are connecting the metal
atoms and thus stabilizes the complex
Chapter-1 Introduction
10
Mo and W complexes can have metal-metal bonds to stabilize the complex (without
any bridging ligand) the multiplicity depends on the number and nature of ligands
Trinuclear complexes ndash These complexes are often triangular structures linear
chains or non-linear chains with the same type of connections as in the binuclear
complexes
Tetranuclear complexes ndash These complexes have several different types of
configura- tion The tetrahedral configuration has a core of μ4-O and four metal atoms
connected by the oxo-ligand but this is not a common configuration
Ti4(OR)
16 type is common and is built up by a M
4-rhomb with 2μ
3-O and 4μ-O The
R-groups are most often-primary alkyl groups for the 3d-metals
The cubane-like structure contains metal atoms in four opposite corners of a cube and
oxygen atoms in the other corners (4μ3-O)
Al4(μ4-O)(μ-OPr
i)5
complex70
and the [Eu4(OPri)10(HOPr
i)3]middot2HOPr
i
complex71
are
some of the examples without a metal-metal bond The Al4(OPr
i)12
type (the propeller
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
2
involving other inorganic salts These features make the metal oxides derived from
metal alkoxides highly pure with specific properties like high hardness chemical and
mechanical resistance and high temperature stability The softndashchemical approach is
a flexible simple and novel synthesis for industrial production Different types of
precursor can lead to change in unique features of the structural framework that
affects the physical and chemical properties in the final product by retaining the
stoichiometric ratio under control This will help to predict favorable morphology for
its futuristic applications with high reliability accuracy and reproducible results5-14
From above mention feature it is evident that metal alkoxides play the key role for
preparing materials of excellent functions and shapes
A range of catalytic applications of Group (IV) (ieTi Zrand Hf) is mainly due to the
Lewis acidic nature of M(IV) complexes Group (IV) elements are important
components of electro ceramics such as lead zirconium titanate (PZT) and barium
titanate Sol-gel processing of these and related materials is based upon hydrolysis of
alkoxide solutions It is found that the species produced when acetylacetone (acacH)
is added to Ti(OR)4 is 11 reaction product [Ti(OR)3(acac)] which is dimeric readily
under-go ligand redistribution in solution to give Ti(OR)4 and [Ti(OR)2(acac)2] 17
O
NMR spectroscopy has been used to investigate the hydrolysis of the resulting
mixture for R = Pri and the results were interpreted in terms of the hydrolysis behavior
of the individual mononuclear components Titanium complexes of alkoxide and
aryloxide ligands exhibit a rich coordination chemistry and reactivity and find
applications in various fields15
The interest in the development of non-metallocene
complexes for the polymerization of α-olefins16
has created a new range of chelating
dialkoxo17-24
ligands for group IV transition metals Complex formation may be
effected by the steric bulk on the ligands Furthermore the olefin polymerization
reactivity by a possible catalyst may strongly depend on the crowding around the
metal as substantial bulk on one hand may hinder the approach of an incoming
olefin and on the other hand decelerate termination processes25-28
Several types of
amine bis (phenolate) titanium complexes having different steric crowding around
the metal have been synthesized The [ONNO]-type ligands bind to the metal in a
tetradentate fashion leads to octahedral bis-(isopropoxide) complexes regardless of
the steric bulk of the aromatic ring substituents Various complexes involving
ethylenediamine and metal alkoxides with particular reference to complexes involving
Chapter-1 Introduction
3
titanium isopropoxide have been reported The alkoxides Titanium(IV) are
diamagnetic tetrahedral molecules and are being used in many organic synthesis
and in material science The structures of titanium alkoxides are often complex
Alkoxides derived from bulkier alcohols such as isopropanol aggregate less Titanium
isopropoxide is mainly a monomer in non- polar solvents Titanium isopropoxide is a
component of the Sharpless epoxidation a method used for the synthesis of chiral
epoxides Titanium isopropoxide is also used as a catalyst for the preparation of
certain cyclopropanes in the Kulinkovich29
reaction Prochiral thioethers are oxidized
enantio-selectively using a catalyst derived from Ti(OPri)4
The aluminium alkoxides chemistry has progressed significantly in the last fifty years
due to advances in their synthetic methodology and in the understanding of the role
ligands and co-ligands play in stabilizing the compounds and ensuring solubility
During column chromatography Aluminium oxide is well known as the stationary
phase It has also been used as solid support for various reactions including dehydro-
genation30
reactions with formic and acetic acid31
selective oxidation of alcohols to
carbonyl compounds using iodobenzene diacetate32
and aziridination and
cyclopropanation reactions using copper nanoparticles33
Aluminium alkoxides are thermally stable and even the insoluble [Al(OMe)3]n may be
sublimed at 240oC under reduced pressure The higher alkoxides are all soluble
Distillable products and the melting points of the solids increase with increasing
complexity of the alkyl chain Degree of association of aluminium alkoxides have
been reported by number of investigators rather widely based on molecular weight
determinations RCMehrotra34
revealed that freshly distilled aluminium isopropoxide
was trimeric in boiling benzene and it changed into a tetrameric form on ageing With
increasing branching of alkoxo groups the complexity diminished and it reduced to
the dimeric state in the tert-butoxide
Tris(acetylacetonato) aluminium(III)35
was the first complex to be obtained using a β-
diketone ligand Tris(β-diketonato) aluminium(III) complexes have then been
synthesized via a variety of routes as shown in Figure 1 Most frequently applied
Method is I which involves dissolution of the β-diketone in aqueous ammonia (if the
ligand is water-soluble) or a mixture of aqueous ammonia and methanol36
Aqueous
ammonia forms the ammonium salt by removing the methane proton from the β
diketone This is due to the acidity of the methine protons of β-diketones The
Chapter-1 Introduction
4
ammonium salt is then reacted with aluminium sulphate The reaction completes with
precipitation of product The synthesis of tris(ferrocenyl-13-butanedionate)
aluminium (III) by Zanello et al37
also take place via Method I This complex
[(FcCOCHCOCH3)3 Al] is the only known ferrocene containing β-diketonato
aluminium (III) complex
Method I Al2(SO4)3 + 3O
R1
O
R2
NH4
e-3(NH
4)2SO
4H2OCH
3OH
Al
O
O
R1
R2
3
3O
R1
O
R2
Method II AlCl3 +
3O
R1
O
R2
Method III Al(OH)3 +
(freshly prepared)
-3 HCl(g)
-3 H 2O(g)
EtOH
Figure 1 Methods commonly used for synthesis tris(β-diketonato aluminium(III)
complexes
Method II involves refluxing the β-diketone with aluminium chloride in benzene and
the reaction completes by the removal of gaseous HCl38
The method III shows that
the tris(β-diketonato) aluminium(III) complex can be synthesized from freshly
precipitated aluminium hydroxide34
This reaction thus illustrates that β-diketones is
acidic enough to attack freshly precipitated aluminium hydroxide
Due to existence of many hydrolysis species39
the aqueous chemistry of aluminium is
complex At low pH (25 - 35) 40
Tomany and co-workers performed an investigation
on the kinetics and mechanism of the reactions of aluminium (III) with acetylacetone
trifluoro actylacetone and hetpane-35-dione Aluminium alkoxide and the β-diketone
(Figure 2)3441
directly synthesized mixed alkoxy β-diketonato aluminium (III)
complexes of the type Al(R1COCHCO R
2)n(OR)3-n Driving force behind the reaction
is the azeotropic removal of the alcohol with benzene It is also interesting to note that
the alkoxide groups are significantly more reactive than the β-diketonate ligands
Another reason why the β-diketonate ligands displace the alkoxide is that bidentate
Chapter-1 Introduction
5
ligands (β-diketonates) form stronger bonds with the coordinating metal than
monodentate (alkoxide) ligands
AlH
O
O
R1
R2
n
RO Al
OR
OR
++ nROHRO
3-n
nO
R1
O
R2
R = CH2CH3 OPri R1= CH3 R2 = Ph
Figure 2 The synthesis of mixed alkoxy β-diketonato aluminium(III) complexes
from aluminium alkoxides41
Structures of Alkoxides
Alkoxide ions are the conjugate bases of alcohols and metal alkoxides are the
coordination compounds formed between metal ions and alkoxide ligands (fig 3i-iii)
As alcohols are only weak acids alkoxide ions are strong bases and metal alkoxides
tend to hydrolyze under condensation and elimination of alcohol when exposed to
water
1)
HO
2)
-O HOR -OR= =
3)
Mx(O Mx(OR)z=)z
Figure 3 i) Alcohol = hydrocarbon chain with a hydroxyl group ii) Alkoxide
Ion = deprotonated alcohol and iii) Metal alkoxide = coordination
compound with alkoxide ligands
Some possible hydrolysis and condensation reactions for metal with alkoxide ligands
are shown
M OR + H2O M OH + HOR
M OR + MHO M O + HORM
M OH + MHO M O +M H2O
Chapter-1 Introduction
6
According to Bradleyrsquos concept42
alkoxides have a strong tendency for
polymerization creating coordination polymers [M(OR)x]y (where y represents the
degree of polymerization) Degree of polymerization increases with the metal atomic
ratio Alkoxides take the smallest structural unit for the highest possible coordination
number of the metal Metal alkoxides [M(OR)x]y are well soluble in common organic
solvents and creates small oligomers with y = 2 3 or 442
Alkoxo RO- anion
possesses donor oxygen atom with three unpaired electrons which form covalent
bond with metal These anions might be coordinated to metal sites in terminal or
bridging way Alkoxides have physical properties that vary according to the nature of
the metal and alkoxy group They range from non-volatile insoluble solids to volatile
soluble solids This great variation in physical properties is due to the differing
molecular complexities observed in alkoxide chemistry those forming large
polymeric frameworks are insoluble and non-volatile while those forming small
oligomers are generally volatile and soluble
Alkoxides have a tendency to form oligomeric compounds [M(OR)x]y where RO-
groups are connected to two or even more metal sites This phenomenon affects the
reactivity and properties of these compounds The formation of oligomers or larger
polymeric frameworks is due to alkoxide groups bridging two or more metal centers
(ie acting as micro2 micro3 micro4 ligands) and the tendency of metals to increase their
coordination number The extent of oligomerization is affected by
(a) The Alkoxy Group For a given metal the more bulky the alkoxy group the lower
the degree of association
(b) The Metal The oxidation state of the metals determines the number of alkoxy
groups present per metal which in turn affects the bridging Low oxidation state
requires more bridges to achieve a given coordination number as compared to higher
oxidation state The size of the metal also play an important role to affect the degree
of bridging as larger the size of the metal atom more easily it can accommodate bulky
alkoxy groups and therefore more easily it can increase its coordination number
The versatile coordinating abilities of an alkoxo ligands leads to the formation of
structural pattern which range from simple bimetallic compounds to very complex
aggregates
Chapter-1 Introduction
7
R
O
M
R
O
M
R
O
M
O
R
MM
O
R
M M
O
R
M M
O
R
M M
O
R
MM
O
R
M MM
Figure 4 Coordination modes of an alkoxo ligand
Steric and electron demand of alkoxo groups have an influence on metal alkoxides
they form Electrophilic nature of metal cations allows attaching neutral ligands (eg
tetrahydrofuran pyridine etc) to the metal spheres Due to the saturating of metal
sites it is possible to obtain monomeric alkoxides [M(OR)xLy] (where L = neutral
ligand)
Alkoxides are highly versatile precursors for sol-gel synthesis4344
Alkoxides when
condensed form volatile alcohols andor ethers allowing for the formation of pure
products without impurities due to the precursor ligands In metal-organic chemical
vapour deposition (MOCVD) alkoxides are sometimes used as precursors4546
Examples of some of metal alkoxides having different structural features are depicted
below
TiOiPr
OiPr
PrOi
PrOi
= Ti(OiPr)4
Ti EtOEtO
EtO
EtO
EtO
OEt Ti
Ti
EtO OEt
OEt
EtO OEt
EtO Ti
OEt OEt
OEt
OEt
=Ti(OEt)16
Monomer47
Oligomer4849
Chapter-1 Introduction
8
Ce
PriO OiPr
PriO OiPr
PriO OPri
Ce
PriO OPri
OPriPriO
H
H
Alkoxide ndash alcohol adduct 50 51
Various so-called alkoxides are in fact oxo- or hydroxo-alkoxides which are the
condensation andor hydrolysis products of true alkoxides In oxo-alkoxides one or
more centrally placed bridging oxo ligands help to increase the coordination number
of the metal atoms The reactivity of oxo-alkoxides decreases with the ratio of
(bridging) oxo to alkoxo ligands Alkoxide derivatives may also contain other
ligands such as chloride ions or organic nonalkoxide ligands Chloro-alkoxides often
acquire structures similar to alkoxide structures but are normally avoided in sol-gel
synthesis as the chloride ions tend remain in the gel after hydrolysis as impurities in
the final materials
Ti
PriOPri
O
PriO
PriOO
OH
Ti
OPri
OPri
OPri
Ti
OPriPriO
=Ti3O(OH)(OiPr)9
Ti
PrOi PrOi
PrOi
PriO
PrOi
OiPr
Ti
OiPr
OiPr
OiPr
Y
ClCl
=YTi2(OiPr)9Cl2
oxo-hydroxo-alkoxide 52
chloro-alkoxide53
Apart from being versatile and important precursors in materials synthesis alkoxides
are also interesting from a structural point of view For example the choice of alkyl
group provides a means of systematic variation for the investigation of coordination
chemistry around metal and oxygen atoms The other parameters which can be
systematically varied are the number of oxo bridges and nuclearity
In the literature there are many examples of metal alkoxides it would be very
difficult to formulate a precise rule that could fully predict the final geometry of
forming alkoxide complex
Chapter-1 Introduction
9
Different structural pattern of metal alkoxides 54-69
Complex Structural pattern
[(C5H4CH3)4Y(micro-OCH=CH2)]2 Y2(micro-O)2 core
[Y3(micro3-OtBu)( micro3-Cl)( micro-O
tBu)3(O
tBu)4(thf)2 Y3(micro3-O)( micro-O)3O4 core
[Ti2(micro-OR)2(OR)4(acac)2]a (R=MeEt
iPr) [Ti2(micro-O)2O4 core
[Me4Zn4(micro3-OtBu)4 Zn4(micro3-O)4 core
[W2(OCMe2CMe2O)3] O3W= WO3 core
[Ga2(micro-OtBu)2
tBu4] Ga2(micro-O)2
core
[Mg2V2(thffo)6Cl4]b Mg2V2(micro3-O)2(micro-O)4 core
[(thf)(OtBu)Y(micro-O
tBu)(micro-CH3)AlMe23] YAl3(micro-O)3O core
[Zr2Co2(micro3-OnPr)2(micro-O
nPr)4(O
nPr)4(acac)2]
a Zr2Co2(micro3-O)2(micro-O)4O4 core
[Al(OEt)2GaMe23] AlGa3(micro-O)6 core
[Nb2(micro-OMe)2(OMe)2(HOMe)2Cl4] Nb-Nb(micro-O)2O4 core
[Mo2(OiPr)4(HO
iPr)4 O4Mo=MoO4 core
[Pr3(micro3-tftb)2(micro-tftb)3(tftb)2]c Pr3(micro3-O)2(micro-O)3O4 core
[YNa8(micro9-Cl)( micro4-OtBu)( micro3-O
tBu)8(O
tBu)] YNa8(micro4-O)( micro3-O)8O core
[Na4Zr6(micro5-O)2( micro3-OEt)4(micro-OEt)14(OEt)6 Na4Zr6(micro5-O)2( micro3-O)4(micro-O)14O
core
[Ti7(micro4-O)( micro3-O)2(micro-OEt)8(OEt)12 Ti7(micro4-O)( micro3-O)2(micro-O)8O12 core aacac = acetylacetonato
bthffo=tetrahydrofuryloxo
ctftb= OCMe2(CF3)
In fact even minor changes in a ligand structure or reaction conditions can lead to the
geometry of the whole compound to be fundamentally different
Classification based on Alkoxide Complexes Structure
Metal alkoxide complexes can have very complex structures due to formation of
oligomeric and sometimes even polymeric aggregates Formation of alkoxy bridges
M-O(R)-M help the complexes to obtain maximal and preferred coordination even
though the number of bonded ligands per metal atom are too few Complexes are
categorized based on number of metal atoms in the complex Optimal coordination is
obtained by chelating ligand or by a shared (bridging) ligand atom
Mononuclear complexes ndash Mononuclear complexes are highly charged metal ions
where the coordination requirements are satisfied by the number of OR-ligands The
ligands are often large and branched with chelating abilities
Binuclear complexes - In binuclear complexes an oxygen atom in the ligand connect
the two metal atoms Usually at least two alkoxy bridges are connecting the metal
atoms and thus stabilizes the complex
Chapter-1 Introduction
10
Mo and W complexes can have metal-metal bonds to stabilize the complex (without
any bridging ligand) the multiplicity depends on the number and nature of ligands
Trinuclear complexes ndash These complexes are often triangular structures linear
chains or non-linear chains with the same type of connections as in the binuclear
complexes
Tetranuclear complexes ndash These complexes have several different types of
configura- tion The tetrahedral configuration has a core of μ4-O and four metal atoms
connected by the oxo-ligand but this is not a common configuration
Ti4(OR)
16 type is common and is built up by a M
4-rhomb with 2μ
3-O and 4μ-O The
R-groups are most often-primary alkyl groups for the 3d-metals
The cubane-like structure contains metal atoms in four opposite corners of a cube and
oxygen atoms in the other corners (4μ3-O)
Al4(μ4-O)(μ-OPr
i)5
complex70
and the [Eu4(OPri)10(HOPr
i)3]middot2HOPr
i
complex71
are
some of the examples without a metal-metal bond The Al4(OPr
i)12
type (the propeller
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
3
titanium isopropoxide have been reported The alkoxides Titanium(IV) are
diamagnetic tetrahedral molecules and are being used in many organic synthesis
and in material science The structures of titanium alkoxides are often complex
Alkoxides derived from bulkier alcohols such as isopropanol aggregate less Titanium
isopropoxide is mainly a monomer in non- polar solvents Titanium isopropoxide is a
component of the Sharpless epoxidation a method used for the synthesis of chiral
epoxides Titanium isopropoxide is also used as a catalyst for the preparation of
certain cyclopropanes in the Kulinkovich29
reaction Prochiral thioethers are oxidized
enantio-selectively using a catalyst derived from Ti(OPri)4
The aluminium alkoxides chemistry has progressed significantly in the last fifty years
due to advances in their synthetic methodology and in the understanding of the role
ligands and co-ligands play in stabilizing the compounds and ensuring solubility
During column chromatography Aluminium oxide is well known as the stationary
phase It has also been used as solid support for various reactions including dehydro-
genation30
reactions with formic and acetic acid31
selective oxidation of alcohols to
carbonyl compounds using iodobenzene diacetate32
and aziridination and
cyclopropanation reactions using copper nanoparticles33
Aluminium alkoxides are thermally stable and even the insoluble [Al(OMe)3]n may be
sublimed at 240oC under reduced pressure The higher alkoxides are all soluble
Distillable products and the melting points of the solids increase with increasing
complexity of the alkyl chain Degree of association of aluminium alkoxides have
been reported by number of investigators rather widely based on molecular weight
determinations RCMehrotra34
revealed that freshly distilled aluminium isopropoxide
was trimeric in boiling benzene and it changed into a tetrameric form on ageing With
increasing branching of alkoxo groups the complexity diminished and it reduced to
the dimeric state in the tert-butoxide
Tris(acetylacetonato) aluminium(III)35
was the first complex to be obtained using a β-
diketone ligand Tris(β-diketonato) aluminium(III) complexes have then been
synthesized via a variety of routes as shown in Figure 1 Most frequently applied
Method is I which involves dissolution of the β-diketone in aqueous ammonia (if the
ligand is water-soluble) or a mixture of aqueous ammonia and methanol36
Aqueous
ammonia forms the ammonium salt by removing the methane proton from the β
diketone This is due to the acidity of the methine protons of β-diketones The
Chapter-1 Introduction
4
ammonium salt is then reacted with aluminium sulphate The reaction completes with
precipitation of product The synthesis of tris(ferrocenyl-13-butanedionate)
aluminium (III) by Zanello et al37
also take place via Method I This complex
[(FcCOCHCOCH3)3 Al] is the only known ferrocene containing β-diketonato
aluminium (III) complex
Method I Al2(SO4)3 + 3O
R1
O
R2
NH4
e-3(NH
4)2SO
4H2OCH
3OH
Al
O
O
R1
R2
3
3O
R1
O
R2
Method II AlCl3 +
3O
R1
O
R2
Method III Al(OH)3 +
(freshly prepared)
-3 HCl(g)
-3 H 2O(g)
EtOH
Figure 1 Methods commonly used for synthesis tris(β-diketonato aluminium(III)
complexes
Method II involves refluxing the β-diketone with aluminium chloride in benzene and
the reaction completes by the removal of gaseous HCl38
The method III shows that
the tris(β-diketonato) aluminium(III) complex can be synthesized from freshly
precipitated aluminium hydroxide34
This reaction thus illustrates that β-diketones is
acidic enough to attack freshly precipitated aluminium hydroxide
Due to existence of many hydrolysis species39
the aqueous chemistry of aluminium is
complex At low pH (25 - 35) 40
Tomany and co-workers performed an investigation
on the kinetics and mechanism of the reactions of aluminium (III) with acetylacetone
trifluoro actylacetone and hetpane-35-dione Aluminium alkoxide and the β-diketone
(Figure 2)3441
directly synthesized mixed alkoxy β-diketonato aluminium (III)
complexes of the type Al(R1COCHCO R
2)n(OR)3-n Driving force behind the reaction
is the azeotropic removal of the alcohol with benzene It is also interesting to note that
the alkoxide groups are significantly more reactive than the β-diketonate ligands
Another reason why the β-diketonate ligands displace the alkoxide is that bidentate
Chapter-1 Introduction
5
ligands (β-diketonates) form stronger bonds with the coordinating metal than
monodentate (alkoxide) ligands
AlH
O
O
R1
R2
n
RO Al
OR
OR
++ nROHRO
3-n
nO
R1
O
R2
R = CH2CH3 OPri R1= CH3 R2 = Ph
Figure 2 The synthesis of mixed alkoxy β-diketonato aluminium(III) complexes
from aluminium alkoxides41
Structures of Alkoxides
Alkoxide ions are the conjugate bases of alcohols and metal alkoxides are the
coordination compounds formed between metal ions and alkoxide ligands (fig 3i-iii)
As alcohols are only weak acids alkoxide ions are strong bases and metal alkoxides
tend to hydrolyze under condensation and elimination of alcohol when exposed to
water
1)
HO
2)
-O HOR -OR= =
3)
Mx(O Mx(OR)z=)z
Figure 3 i) Alcohol = hydrocarbon chain with a hydroxyl group ii) Alkoxide
Ion = deprotonated alcohol and iii) Metal alkoxide = coordination
compound with alkoxide ligands
Some possible hydrolysis and condensation reactions for metal with alkoxide ligands
are shown
M OR + H2O M OH + HOR
M OR + MHO M O + HORM
M OH + MHO M O +M H2O
Chapter-1 Introduction
6
According to Bradleyrsquos concept42
alkoxides have a strong tendency for
polymerization creating coordination polymers [M(OR)x]y (where y represents the
degree of polymerization) Degree of polymerization increases with the metal atomic
ratio Alkoxides take the smallest structural unit for the highest possible coordination
number of the metal Metal alkoxides [M(OR)x]y are well soluble in common organic
solvents and creates small oligomers with y = 2 3 or 442
Alkoxo RO- anion
possesses donor oxygen atom with three unpaired electrons which form covalent
bond with metal These anions might be coordinated to metal sites in terminal or
bridging way Alkoxides have physical properties that vary according to the nature of
the metal and alkoxy group They range from non-volatile insoluble solids to volatile
soluble solids This great variation in physical properties is due to the differing
molecular complexities observed in alkoxide chemistry those forming large
polymeric frameworks are insoluble and non-volatile while those forming small
oligomers are generally volatile and soluble
Alkoxides have a tendency to form oligomeric compounds [M(OR)x]y where RO-
groups are connected to two or even more metal sites This phenomenon affects the
reactivity and properties of these compounds The formation of oligomers or larger
polymeric frameworks is due to alkoxide groups bridging two or more metal centers
(ie acting as micro2 micro3 micro4 ligands) and the tendency of metals to increase their
coordination number The extent of oligomerization is affected by
(a) The Alkoxy Group For a given metal the more bulky the alkoxy group the lower
the degree of association
(b) The Metal The oxidation state of the metals determines the number of alkoxy
groups present per metal which in turn affects the bridging Low oxidation state
requires more bridges to achieve a given coordination number as compared to higher
oxidation state The size of the metal also play an important role to affect the degree
of bridging as larger the size of the metal atom more easily it can accommodate bulky
alkoxy groups and therefore more easily it can increase its coordination number
The versatile coordinating abilities of an alkoxo ligands leads to the formation of
structural pattern which range from simple bimetallic compounds to very complex
aggregates
Chapter-1 Introduction
7
R
O
M
R
O
M
R
O
M
O
R
MM
O
R
M M
O
R
M M
O
R
M M
O
R
MM
O
R
M MM
Figure 4 Coordination modes of an alkoxo ligand
Steric and electron demand of alkoxo groups have an influence on metal alkoxides
they form Electrophilic nature of metal cations allows attaching neutral ligands (eg
tetrahydrofuran pyridine etc) to the metal spheres Due to the saturating of metal
sites it is possible to obtain monomeric alkoxides [M(OR)xLy] (where L = neutral
ligand)
Alkoxides are highly versatile precursors for sol-gel synthesis4344
Alkoxides when
condensed form volatile alcohols andor ethers allowing for the formation of pure
products without impurities due to the precursor ligands In metal-organic chemical
vapour deposition (MOCVD) alkoxides are sometimes used as precursors4546
Examples of some of metal alkoxides having different structural features are depicted
below
TiOiPr
OiPr
PrOi
PrOi
= Ti(OiPr)4
Ti EtOEtO
EtO
EtO
EtO
OEt Ti
Ti
EtO OEt
OEt
EtO OEt
EtO Ti
OEt OEt
OEt
OEt
=Ti(OEt)16
Monomer47
Oligomer4849
Chapter-1 Introduction
8
Ce
PriO OiPr
PriO OiPr
PriO OPri
Ce
PriO OPri
OPriPriO
H
H
Alkoxide ndash alcohol adduct 50 51
Various so-called alkoxides are in fact oxo- or hydroxo-alkoxides which are the
condensation andor hydrolysis products of true alkoxides In oxo-alkoxides one or
more centrally placed bridging oxo ligands help to increase the coordination number
of the metal atoms The reactivity of oxo-alkoxides decreases with the ratio of
(bridging) oxo to alkoxo ligands Alkoxide derivatives may also contain other
ligands such as chloride ions or organic nonalkoxide ligands Chloro-alkoxides often
acquire structures similar to alkoxide structures but are normally avoided in sol-gel
synthesis as the chloride ions tend remain in the gel after hydrolysis as impurities in
the final materials
Ti
PriOPri
O
PriO
PriOO
OH
Ti
OPri
OPri
OPri
Ti
OPriPriO
=Ti3O(OH)(OiPr)9
Ti
PrOi PrOi
PrOi
PriO
PrOi
OiPr
Ti
OiPr
OiPr
OiPr
Y
ClCl
=YTi2(OiPr)9Cl2
oxo-hydroxo-alkoxide 52
chloro-alkoxide53
Apart from being versatile and important precursors in materials synthesis alkoxides
are also interesting from a structural point of view For example the choice of alkyl
group provides a means of systematic variation for the investigation of coordination
chemistry around metal and oxygen atoms The other parameters which can be
systematically varied are the number of oxo bridges and nuclearity
In the literature there are many examples of metal alkoxides it would be very
difficult to formulate a precise rule that could fully predict the final geometry of
forming alkoxide complex
Chapter-1 Introduction
9
Different structural pattern of metal alkoxides 54-69
Complex Structural pattern
[(C5H4CH3)4Y(micro-OCH=CH2)]2 Y2(micro-O)2 core
[Y3(micro3-OtBu)( micro3-Cl)( micro-O
tBu)3(O
tBu)4(thf)2 Y3(micro3-O)( micro-O)3O4 core
[Ti2(micro-OR)2(OR)4(acac)2]a (R=MeEt
iPr) [Ti2(micro-O)2O4 core
[Me4Zn4(micro3-OtBu)4 Zn4(micro3-O)4 core
[W2(OCMe2CMe2O)3] O3W= WO3 core
[Ga2(micro-OtBu)2
tBu4] Ga2(micro-O)2
core
[Mg2V2(thffo)6Cl4]b Mg2V2(micro3-O)2(micro-O)4 core
[(thf)(OtBu)Y(micro-O
tBu)(micro-CH3)AlMe23] YAl3(micro-O)3O core
[Zr2Co2(micro3-OnPr)2(micro-O
nPr)4(O
nPr)4(acac)2]
a Zr2Co2(micro3-O)2(micro-O)4O4 core
[Al(OEt)2GaMe23] AlGa3(micro-O)6 core
[Nb2(micro-OMe)2(OMe)2(HOMe)2Cl4] Nb-Nb(micro-O)2O4 core
[Mo2(OiPr)4(HO
iPr)4 O4Mo=MoO4 core
[Pr3(micro3-tftb)2(micro-tftb)3(tftb)2]c Pr3(micro3-O)2(micro-O)3O4 core
[YNa8(micro9-Cl)( micro4-OtBu)( micro3-O
tBu)8(O
tBu)] YNa8(micro4-O)( micro3-O)8O core
[Na4Zr6(micro5-O)2( micro3-OEt)4(micro-OEt)14(OEt)6 Na4Zr6(micro5-O)2( micro3-O)4(micro-O)14O
core
[Ti7(micro4-O)( micro3-O)2(micro-OEt)8(OEt)12 Ti7(micro4-O)( micro3-O)2(micro-O)8O12 core aacac = acetylacetonato
bthffo=tetrahydrofuryloxo
ctftb= OCMe2(CF3)
In fact even minor changes in a ligand structure or reaction conditions can lead to the
geometry of the whole compound to be fundamentally different
Classification based on Alkoxide Complexes Structure
Metal alkoxide complexes can have very complex structures due to formation of
oligomeric and sometimes even polymeric aggregates Formation of alkoxy bridges
M-O(R)-M help the complexes to obtain maximal and preferred coordination even
though the number of bonded ligands per metal atom are too few Complexes are
categorized based on number of metal atoms in the complex Optimal coordination is
obtained by chelating ligand or by a shared (bridging) ligand atom
Mononuclear complexes ndash Mononuclear complexes are highly charged metal ions
where the coordination requirements are satisfied by the number of OR-ligands The
ligands are often large and branched with chelating abilities
Binuclear complexes - In binuclear complexes an oxygen atom in the ligand connect
the two metal atoms Usually at least two alkoxy bridges are connecting the metal
atoms and thus stabilizes the complex
Chapter-1 Introduction
10
Mo and W complexes can have metal-metal bonds to stabilize the complex (without
any bridging ligand) the multiplicity depends on the number and nature of ligands
Trinuclear complexes ndash These complexes are often triangular structures linear
chains or non-linear chains with the same type of connections as in the binuclear
complexes
Tetranuclear complexes ndash These complexes have several different types of
configura- tion The tetrahedral configuration has a core of μ4-O and four metal atoms
connected by the oxo-ligand but this is not a common configuration
Ti4(OR)
16 type is common and is built up by a M
4-rhomb with 2μ
3-O and 4μ-O The
R-groups are most often-primary alkyl groups for the 3d-metals
The cubane-like structure contains metal atoms in four opposite corners of a cube and
oxygen atoms in the other corners (4μ3-O)
Al4(μ4-O)(μ-OPr
i)5
complex70
and the [Eu4(OPri)10(HOPr
i)3]middot2HOPr
i
complex71
are
some of the examples without a metal-metal bond The Al4(OPr
i)12
type (the propeller
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
4
ammonium salt is then reacted with aluminium sulphate The reaction completes with
precipitation of product The synthesis of tris(ferrocenyl-13-butanedionate)
aluminium (III) by Zanello et al37
also take place via Method I This complex
[(FcCOCHCOCH3)3 Al] is the only known ferrocene containing β-diketonato
aluminium (III) complex
Method I Al2(SO4)3 + 3O
R1
O
R2
NH4
e-3(NH
4)2SO
4H2OCH
3OH
Al
O
O
R1
R2
3
3O
R1
O
R2
Method II AlCl3 +
3O
R1
O
R2
Method III Al(OH)3 +
(freshly prepared)
-3 HCl(g)
-3 H 2O(g)
EtOH
Figure 1 Methods commonly used for synthesis tris(β-diketonato aluminium(III)
complexes
Method II involves refluxing the β-diketone with aluminium chloride in benzene and
the reaction completes by the removal of gaseous HCl38
The method III shows that
the tris(β-diketonato) aluminium(III) complex can be synthesized from freshly
precipitated aluminium hydroxide34
This reaction thus illustrates that β-diketones is
acidic enough to attack freshly precipitated aluminium hydroxide
Due to existence of many hydrolysis species39
the aqueous chemistry of aluminium is
complex At low pH (25 - 35) 40
Tomany and co-workers performed an investigation
on the kinetics and mechanism of the reactions of aluminium (III) with acetylacetone
trifluoro actylacetone and hetpane-35-dione Aluminium alkoxide and the β-diketone
(Figure 2)3441
directly synthesized mixed alkoxy β-diketonato aluminium (III)
complexes of the type Al(R1COCHCO R
2)n(OR)3-n Driving force behind the reaction
is the azeotropic removal of the alcohol with benzene It is also interesting to note that
the alkoxide groups are significantly more reactive than the β-diketonate ligands
Another reason why the β-diketonate ligands displace the alkoxide is that bidentate
Chapter-1 Introduction
5
ligands (β-diketonates) form stronger bonds with the coordinating metal than
monodentate (alkoxide) ligands
AlH
O
O
R1
R2
n
RO Al
OR
OR
++ nROHRO
3-n
nO
R1
O
R2
R = CH2CH3 OPri R1= CH3 R2 = Ph
Figure 2 The synthesis of mixed alkoxy β-diketonato aluminium(III) complexes
from aluminium alkoxides41
Structures of Alkoxides
Alkoxide ions are the conjugate bases of alcohols and metal alkoxides are the
coordination compounds formed between metal ions and alkoxide ligands (fig 3i-iii)
As alcohols are only weak acids alkoxide ions are strong bases and metal alkoxides
tend to hydrolyze under condensation and elimination of alcohol when exposed to
water
1)
HO
2)
-O HOR -OR= =
3)
Mx(O Mx(OR)z=)z
Figure 3 i) Alcohol = hydrocarbon chain with a hydroxyl group ii) Alkoxide
Ion = deprotonated alcohol and iii) Metal alkoxide = coordination
compound with alkoxide ligands
Some possible hydrolysis and condensation reactions for metal with alkoxide ligands
are shown
M OR + H2O M OH + HOR
M OR + MHO M O + HORM
M OH + MHO M O +M H2O
Chapter-1 Introduction
6
According to Bradleyrsquos concept42
alkoxides have a strong tendency for
polymerization creating coordination polymers [M(OR)x]y (where y represents the
degree of polymerization) Degree of polymerization increases with the metal atomic
ratio Alkoxides take the smallest structural unit for the highest possible coordination
number of the metal Metal alkoxides [M(OR)x]y are well soluble in common organic
solvents and creates small oligomers with y = 2 3 or 442
Alkoxo RO- anion
possesses donor oxygen atom with three unpaired electrons which form covalent
bond with metal These anions might be coordinated to metal sites in terminal or
bridging way Alkoxides have physical properties that vary according to the nature of
the metal and alkoxy group They range from non-volatile insoluble solids to volatile
soluble solids This great variation in physical properties is due to the differing
molecular complexities observed in alkoxide chemistry those forming large
polymeric frameworks are insoluble and non-volatile while those forming small
oligomers are generally volatile and soluble
Alkoxides have a tendency to form oligomeric compounds [M(OR)x]y where RO-
groups are connected to two or even more metal sites This phenomenon affects the
reactivity and properties of these compounds The formation of oligomers or larger
polymeric frameworks is due to alkoxide groups bridging two or more metal centers
(ie acting as micro2 micro3 micro4 ligands) and the tendency of metals to increase their
coordination number The extent of oligomerization is affected by
(a) The Alkoxy Group For a given metal the more bulky the alkoxy group the lower
the degree of association
(b) The Metal The oxidation state of the metals determines the number of alkoxy
groups present per metal which in turn affects the bridging Low oxidation state
requires more bridges to achieve a given coordination number as compared to higher
oxidation state The size of the metal also play an important role to affect the degree
of bridging as larger the size of the metal atom more easily it can accommodate bulky
alkoxy groups and therefore more easily it can increase its coordination number
The versatile coordinating abilities of an alkoxo ligands leads to the formation of
structural pattern which range from simple bimetallic compounds to very complex
aggregates
Chapter-1 Introduction
7
R
O
M
R
O
M
R
O
M
O
R
MM
O
R
M M
O
R
M M
O
R
M M
O
R
MM
O
R
M MM
Figure 4 Coordination modes of an alkoxo ligand
Steric and electron demand of alkoxo groups have an influence on metal alkoxides
they form Electrophilic nature of metal cations allows attaching neutral ligands (eg
tetrahydrofuran pyridine etc) to the metal spheres Due to the saturating of metal
sites it is possible to obtain monomeric alkoxides [M(OR)xLy] (where L = neutral
ligand)
Alkoxides are highly versatile precursors for sol-gel synthesis4344
Alkoxides when
condensed form volatile alcohols andor ethers allowing for the formation of pure
products without impurities due to the precursor ligands In metal-organic chemical
vapour deposition (MOCVD) alkoxides are sometimes used as precursors4546
Examples of some of metal alkoxides having different structural features are depicted
below
TiOiPr
OiPr
PrOi
PrOi
= Ti(OiPr)4
Ti EtOEtO
EtO
EtO
EtO
OEt Ti
Ti
EtO OEt
OEt
EtO OEt
EtO Ti
OEt OEt
OEt
OEt
=Ti(OEt)16
Monomer47
Oligomer4849
Chapter-1 Introduction
8
Ce
PriO OiPr
PriO OiPr
PriO OPri
Ce
PriO OPri
OPriPriO
H
H
Alkoxide ndash alcohol adduct 50 51
Various so-called alkoxides are in fact oxo- or hydroxo-alkoxides which are the
condensation andor hydrolysis products of true alkoxides In oxo-alkoxides one or
more centrally placed bridging oxo ligands help to increase the coordination number
of the metal atoms The reactivity of oxo-alkoxides decreases with the ratio of
(bridging) oxo to alkoxo ligands Alkoxide derivatives may also contain other
ligands such as chloride ions or organic nonalkoxide ligands Chloro-alkoxides often
acquire structures similar to alkoxide structures but are normally avoided in sol-gel
synthesis as the chloride ions tend remain in the gel after hydrolysis as impurities in
the final materials
Ti
PriOPri
O
PriO
PriOO
OH
Ti
OPri
OPri
OPri
Ti
OPriPriO
=Ti3O(OH)(OiPr)9
Ti
PrOi PrOi
PrOi
PriO
PrOi
OiPr
Ti
OiPr
OiPr
OiPr
Y
ClCl
=YTi2(OiPr)9Cl2
oxo-hydroxo-alkoxide 52
chloro-alkoxide53
Apart from being versatile and important precursors in materials synthesis alkoxides
are also interesting from a structural point of view For example the choice of alkyl
group provides a means of systematic variation for the investigation of coordination
chemistry around metal and oxygen atoms The other parameters which can be
systematically varied are the number of oxo bridges and nuclearity
In the literature there are many examples of metal alkoxides it would be very
difficult to formulate a precise rule that could fully predict the final geometry of
forming alkoxide complex
Chapter-1 Introduction
9
Different structural pattern of metal alkoxides 54-69
Complex Structural pattern
[(C5H4CH3)4Y(micro-OCH=CH2)]2 Y2(micro-O)2 core
[Y3(micro3-OtBu)( micro3-Cl)( micro-O
tBu)3(O
tBu)4(thf)2 Y3(micro3-O)( micro-O)3O4 core
[Ti2(micro-OR)2(OR)4(acac)2]a (R=MeEt
iPr) [Ti2(micro-O)2O4 core
[Me4Zn4(micro3-OtBu)4 Zn4(micro3-O)4 core
[W2(OCMe2CMe2O)3] O3W= WO3 core
[Ga2(micro-OtBu)2
tBu4] Ga2(micro-O)2
core
[Mg2V2(thffo)6Cl4]b Mg2V2(micro3-O)2(micro-O)4 core
[(thf)(OtBu)Y(micro-O
tBu)(micro-CH3)AlMe23] YAl3(micro-O)3O core
[Zr2Co2(micro3-OnPr)2(micro-O
nPr)4(O
nPr)4(acac)2]
a Zr2Co2(micro3-O)2(micro-O)4O4 core
[Al(OEt)2GaMe23] AlGa3(micro-O)6 core
[Nb2(micro-OMe)2(OMe)2(HOMe)2Cl4] Nb-Nb(micro-O)2O4 core
[Mo2(OiPr)4(HO
iPr)4 O4Mo=MoO4 core
[Pr3(micro3-tftb)2(micro-tftb)3(tftb)2]c Pr3(micro3-O)2(micro-O)3O4 core
[YNa8(micro9-Cl)( micro4-OtBu)( micro3-O
tBu)8(O
tBu)] YNa8(micro4-O)( micro3-O)8O core
[Na4Zr6(micro5-O)2( micro3-OEt)4(micro-OEt)14(OEt)6 Na4Zr6(micro5-O)2( micro3-O)4(micro-O)14O
core
[Ti7(micro4-O)( micro3-O)2(micro-OEt)8(OEt)12 Ti7(micro4-O)( micro3-O)2(micro-O)8O12 core aacac = acetylacetonato
bthffo=tetrahydrofuryloxo
ctftb= OCMe2(CF3)
In fact even minor changes in a ligand structure or reaction conditions can lead to the
geometry of the whole compound to be fundamentally different
Classification based on Alkoxide Complexes Structure
Metal alkoxide complexes can have very complex structures due to formation of
oligomeric and sometimes even polymeric aggregates Formation of alkoxy bridges
M-O(R)-M help the complexes to obtain maximal and preferred coordination even
though the number of bonded ligands per metal atom are too few Complexes are
categorized based on number of metal atoms in the complex Optimal coordination is
obtained by chelating ligand or by a shared (bridging) ligand atom
Mononuclear complexes ndash Mononuclear complexes are highly charged metal ions
where the coordination requirements are satisfied by the number of OR-ligands The
ligands are often large and branched with chelating abilities
Binuclear complexes - In binuclear complexes an oxygen atom in the ligand connect
the two metal atoms Usually at least two alkoxy bridges are connecting the metal
atoms and thus stabilizes the complex
Chapter-1 Introduction
10
Mo and W complexes can have metal-metal bonds to stabilize the complex (without
any bridging ligand) the multiplicity depends on the number and nature of ligands
Trinuclear complexes ndash These complexes are often triangular structures linear
chains or non-linear chains with the same type of connections as in the binuclear
complexes
Tetranuclear complexes ndash These complexes have several different types of
configura- tion The tetrahedral configuration has a core of μ4-O and four metal atoms
connected by the oxo-ligand but this is not a common configuration
Ti4(OR)
16 type is common and is built up by a M
4-rhomb with 2μ
3-O and 4μ-O The
R-groups are most often-primary alkyl groups for the 3d-metals
The cubane-like structure contains metal atoms in four opposite corners of a cube and
oxygen atoms in the other corners (4μ3-O)
Al4(μ4-O)(μ-OPr
i)5
complex70
and the [Eu4(OPri)10(HOPr
i)3]middot2HOPr
i
complex71
are
some of the examples without a metal-metal bond The Al4(OPr
i)12
type (the propeller
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
5
ligands (β-diketonates) form stronger bonds with the coordinating metal than
monodentate (alkoxide) ligands
AlH
O
O
R1
R2
n
RO Al
OR
OR
++ nROHRO
3-n
nO
R1
O
R2
R = CH2CH3 OPri R1= CH3 R2 = Ph
Figure 2 The synthesis of mixed alkoxy β-diketonato aluminium(III) complexes
from aluminium alkoxides41
Structures of Alkoxides
Alkoxide ions are the conjugate bases of alcohols and metal alkoxides are the
coordination compounds formed between metal ions and alkoxide ligands (fig 3i-iii)
As alcohols are only weak acids alkoxide ions are strong bases and metal alkoxides
tend to hydrolyze under condensation and elimination of alcohol when exposed to
water
1)
HO
2)
-O HOR -OR= =
3)
Mx(O Mx(OR)z=)z
Figure 3 i) Alcohol = hydrocarbon chain with a hydroxyl group ii) Alkoxide
Ion = deprotonated alcohol and iii) Metal alkoxide = coordination
compound with alkoxide ligands
Some possible hydrolysis and condensation reactions for metal with alkoxide ligands
are shown
M OR + H2O M OH + HOR
M OR + MHO M O + HORM
M OH + MHO M O +M H2O
Chapter-1 Introduction
6
According to Bradleyrsquos concept42
alkoxides have a strong tendency for
polymerization creating coordination polymers [M(OR)x]y (where y represents the
degree of polymerization) Degree of polymerization increases with the metal atomic
ratio Alkoxides take the smallest structural unit for the highest possible coordination
number of the metal Metal alkoxides [M(OR)x]y are well soluble in common organic
solvents and creates small oligomers with y = 2 3 or 442
Alkoxo RO- anion
possesses donor oxygen atom with three unpaired electrons which form covalent
bond with metal These anions might be coordinated to metal sites in terminal or
bridging way Alkoxides have physical properties that vary according to the nature of
the metal and alkoxy group They range from non-volatile insoluble solids to volatile
soluble solids This great variation in physical properties is due to the differing
molecular complexities observed in alkoxide chemistry those forming large
polymeric frameworks are insoluble and non-volatile while those forming small
oligomers are generally volatile and soluble
Alkoxides have a tendency to form oligomeric compounds [M(OR)x]y where RO-
groups are connected to two or even more metal sites This phenomenon affects the
reactivity and properties of these compounds The formation of oligomers or larger
polymeric frameworks is due to alkoxide groups bridging two or more metal centers
(ie acting as micro2 micro3 micro4 ligands) and the tendency of metals to increase their
coordination number The extent of oligomerization is affected by
(a) The Alkoxy Group For a given metal the more bulky the alkoxy group the lower
the degree of association
(b) The Metal The oxidation state of the metals determines the number of alkoxy
groups present per metal which in turn affects the bridging Low oxidation state
requires more bridges to achieve a given coordination number as compared to higher
oxidation state The size of the metal also play an important role to affect the degree
of bridging as larger the size of the metal atom more easily it can accommodate bulky
alkoxy groups and therefore more easily it can increase its coordination number
The versatile coordinating abilities of an alkoxo ligands leads to the formation of
structural pattern which range from simple bimetallic compounds to very complex
aggregates
Chapter-1 Introduction
7
R
O
M
R
O
M
R
O
M
O
R
MM
O
R
M M
O
R
M M
O
R
M M
O
R
MM
O
R
M MM
Figure 4 Coordination modes of an alkoxo ligand
Steric and electron demand of alkoxo groups have an influence on metal alkoxides
they form Electrophilic nature of metal cations allows attaching neutral ligands (eg
tetrahydrofuran pyridine etc) to the metal spheres Due to the saturating of metal
sites it is possible to obtain monomeric alkoxides [M(OR)xLy] (where L = neutral
ligand)
Alkoxides are highly versatile precursors for sol-gel synthesis4344
Alkoxides when
condensed form volatile alcohols andor ethers allowing for the formation of pure
products without impurities due to the precursor ligands In metal-organic chemical
vapour deposition (MOCVD) alkoxides are sometimes used as precursors4546
Examples of some of metal alkoxides having different structural features are depicted
below
TiOiPr
OiPr
PrOi
PrOi
= Ti(OiPr)4
Ti EtOEtO
EtO
EtO
EtO
OEt Ti
Ti
EtO OEt
OEt
EtO OEt
EtO Ti
OEt OEt
OEt
OEt
=Ti(OEt)16
Monomer47
Oligomer4849
Chapter-1 Introduction
8
Ce
PriO OiPr
PriO OiPr
PriO OPri
Ce
PriO OPri
OPriPriO
H
H
Alkoxide ndash alcohol adduct 50 51
Various so-called alkoxides are in fact oxo- or hydroxo-alkoxides which are the
condensation andor hydrolysis products of true alkoxides In oxo-alkoxides one or
more centrally placed bridging oxo ligands help to increase the coordination number
of the metal atoms The reactivity of oxo-alkoxides decreases with the ratio of
(bridging) oxo to alkoxo ligands Alkoxide derivatives may also contain other
ligands such as chloride ions or organic nonalkoxide ligands Chloro-alkoxides often
acquire structures similar to alkoxide structures but are normally avoided in sol-gel
synthesis as the chloride ions tend remain in the gel after hydrolysis as impurities in
the final materials
Ti
PriOPri
O
PriO
PriOO
OH
Ti
OPri
OPri
OPri
Ti
OPriPriO
=Ti3O(OH)(OiPr)9
Ti
PrOi PrOi
PrOi
PriO
PrOi
OiPr
Ti
OiPr
OiPr
OiPr
Y
ClCl
=YTi2(OiPr)9Cl2
oxo-hydroxo-alkoxide 52
chloro-alkoxide53
Apart from being versatile and important precursors in materials synthesis alkoxides
are also interesting from a structural point of view For example the choice of alkyl
group provides a means of systematic variation for the investigation of coordination
chemistry around metal and oxygen atoms The other parameters which can be
systematically varied are the number of oxo bridges and nuclearity
In the literature there are many examples of metal alkoxides it would be very
difficult to formulate a precise rule that could fully predict the final geometry of
forming alkoxide complex
Chapter-1 Introduction
9
Different structural pattern of metal alkoxides 54-69
Complex Structural pattern
[(C5H4CH3)4Y(micro-OCH=CH2)]2 Y2(micro-O)2 core
[Y3(micro3-OtBu)( micro3-Cl)( micro-O
tBu)3(O
tBu)4(thf)2 Y3(micro3-O)( micro-O)3O4 core
[Ti2(micro-OR)2(OR)4(acac)2]a (R=MeEt
iPr) [Ti2(micro-O)2O4 core
[Me4Zn4(micro3-OtBu)4 Zn4(micro3-O)4 core
[W2(OCMe2CMe2O)3] O3W= WO3 core
[Ga2(micro-OtBu)2
tBu4] Ga2(micro-O)2
core
[Mg2V2(thffo)6Cl4]b Mg2V2(micro3-O)2(micro-O)4 core
[(thf)(OtBu)Y(micro-O
tBu)(micro-CH3)AlMe23] YAl3(micro-O)3O core
[Zr2Co2(micro3-OnPr)2(micro-O
nPr)4(O
nPr)4(acac)2]
a Zr2Co2(micro3-O)2(micro-O)4O4 core
[Al(OEt)2GaMe23] AlGa3(micro-O)6 core
[Nb2(micro-OMe)2(OMe)2(HOMe)2Cl4] Nb-Nb(micro-O)2O4 core
[Mo2(OiPr)4(HO
iPr)4 O4Mo=MoO4 core
[Pr3(micro3-tftb)2(micro-tftb)3(tftb)2]c Pr3(micro3-O)2(micro-O)3O4 core
[YNa8(micro9-Cl)( micro4-OtBu)( micro3-O
tBu)8(O
tBu)] YNa8(micro4-O)( micro3-O)8O core
[Na4Zr6(micro5-O)2( micro3-OEt)4(micro-OEt)14(OEt)6 Na4Zr6(micro5-O)2( micro3-O)4(micro-O)14O
core
[Ti7(micro4-O)( micro3-O)2(micro-OEt)8(OEt)12 Ti7(micro4-O)( micro3-O)2(micro-O)8O12 core aacac = acetylacetonato
bthffo=tetrahydrofuryloxo
ctftb= OCMe2(CF3)
In fact even minor changes in a ligand structure or reaction conditions can lead to the
geometry of the whole compound to be fundamentally different
Classification based on Alkoxide Complexes Structure
Metal alkoxide complexes can have very complex structures due to formation of
oligomeric and sometimes even polymeric aggregates Formation of alkoxy bridges
M-O(R)-M help the complexes to obtain maximal and preferred coordination even
though the number of bonded ligands per metal atom are too few Complexes are
categorized based on number of metal atoms in the complex Optimal coordination is
obtained by chelating ligand or by a shared (bridging) ligand atom
Mononuclear complexes ndash Mononuclear complexes are highly charged metal ions
where the coordination requirements are satisfied by the number of OR-ligands The
ligands are often large and branched with chelating abilities
Binuclear complexes - In binuclear complexes an oxygen atom in the ligand connect
the two metal atoms Usually at least two alkoxy bridges are connecting the metal
atoms and thus stabilizes the complex
Chapter-1 Introduction
10
Mo and W complexes can have metal-metal bonds to stabilize the complex (without
any bridging ligand) the multiplicity depends on the number and nature of ligands
Trinuclear complexes ndash These complexes are often triangular structures linear
chains or non-linear chains with the same type of connections as in the binuclear
complexes
Tetranuclear complexes ndash These complexes have several different types of
configura- tion The tetrahedral configuration has a core of μ4-O and four metal atoms
connected by the oxo-ligand but this is not a common configuration
Ti4(OR)
16 type is common and is built up by a M
4-rhomb with 2μ
3-O and 4μ-O The
R-groups are most often-primary alkyl groups for the 3d-metals
The cubane-like structure contains metal atoms in four opposite corners of a cube and
oxygen atoms in the other corners (4μ3-O)
Al4(μ4-O)(μ-OPr
i)5
complex70
and the [Eu4(OPri)10(HOPr
i)3]middot2HOPr
i
complex71
are
some of the examples without a metal-metal bond The Al4(OPr
i)12
type (the propeller
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
6
According to Bradleyrsquos concept42
alkoxides have a strong tendency for
polymerization creating coordination polymers [M(OR)x]y (where y represents the
degree of polymerization) Degree of polymerization increases with the metal atomic
ratio Alkoxides take the smallest structural unit for the highest possible coordination
number of the metal Metal alkoxides [M(OR)x]y are well soluble in common organic
solvents and creates small oligomers with y = 2 3 or 442
Alkoxo RO- anion
possesses donor oxygen atom with three unpaired electrons which form covalent
bond with metal These anions might be coordinated to metal sites in terminal or
bridging way Alkoxides have physical properties that vary according to the nature of
the metal and alkoxy group They range from non-volatile insoluble solids to volatile
soluble solids This great variation in physical properties is due to the differing
molecular complexities observed in alkoxide chemistry those forming large
polymeric frameworks are insoluble and non-volatile while those forming small
oligomers are generally volatile and soluble
Alkoxides have a tendency to form oligomeric compounds [M(OR)x]y where RO-
groups are connected to two or even more metal sites This phenomenon affects the
reactivity and properties of these compounds The formation of oligomers or larger
polymeric frameworks is due to alkoxide groups bridging two or more metal centers
(ie acting as micro2 micro3 micro4 ligands) and the tendency of metals to increase their
coordination number The extent of oligomerization is affected by
(a) The Alkoxy Group For a given metal the more bulky the alkoxy group the lower
the degree of association
(b) The Metal The oxidation state of the metals determines the number of alkoxy
groups present per metal which in turn affects the bridging Low oxidation state
requires more bridges to achieve a given coordination number as compared to higher
oxidation state The size of the metal also play an important role to affect the degree
of bridging as larger the size of the metal atom more easily it can accommodate bulky
alkoxy groups and therefore more easily it can increase its coordination number
The versatile coordinating abilities of an alkoxo ligands leads to the formation of
structural pattern which range from simple bimetallic compounds to very complex
aggregates
Chapter-1 Introduction
7
R
O
M
R
O
M
R
O
M
O
R
MM
O
R
M M
O
R
M M
O
R
M M
O
R
MM
O
R
M MM
Figure 4 Coordination modes of an alkoxo ligand
Steric and electron demand of alkoxo groups have an influence on metal alkoxides
they form Electrophilic nature of metal cations allows attaching neutral ligands (eg
tetrahydrofuran pyridine etc) to the metal spheres Due to the saturating of metal
sites it is possible to obtain monomeric alkoxides [M(OR)xLy] (where L = neutral
ligand)
Alkoxides are highly versatile precursors for sol-gel synthesis4344
Alkoxides when
condensed form volatile alcohols andor ethers allowing for the formation of pure
products without impurities due to the precursor ligands In metal-organic chemical
vapour deposition (MOCVD) alkoxides are sometimes used as precursors4546
Examples of some of metal alkoxides having different structural features are depicted
below
TiOiPr
OiPr
PrOi
PrOi
= Ti(OiPr)4
Ti EtOEtO
EtO
EtO
EtO
OEt Ti
Ti
EtO OEt
OEt
EtO OEt
EtO Ti
OEt OEt
OEt
OEt
=Ti(OEt)16
Monomer47
Oligomer4849
Chapter-1 Introduction
8
Ce
PriO OiPr
PriO OiPr
PriO OPri
Ce
PriO OPri
OPriPriO
H
H
Alkoxide ndash alcohol adduct 50 51
Various so-called alkoxides are in fact oxo- or hydroxo-alkoxides which are the
condensation andor hydrolysis products of true alkoxides In oxo-alkoxides one or
more centrally placed bridging oxo ligands help to increase the coordination number
of the metal atoms The reactivity of oxo-alkoxides decreases with the ratio of
(bridging) oxo to alkoxo ligands Alkoxide derivatives may also contain other
ligands such as chloride ions or organic nonalkoxide ligands Chloro-alkoxides often
acquire structures similar to alkoxide structures but are normally avoided in sol-gel
synthesis as the chloride ions tend remain in the gel after hydrolysis as impurities in
the final materials
Ti
PriOPri
O
PriO
PriOO
OH
Ti
OPri
OPri
OPri
Ti
OPriPriO
=Ti3O(OH)(OiPr)9
Ti
PrOi PrOi
PrOi
PriO
PrOi
OiPr
Ti
OiPr
OiPr
OiPr
Y
ClCl
=YTi2(OiPr)9Cl2
oxo-hydroxo-alkoxide 52
chloro-alkoxide53
Apart from being versatile and important precursors in materials synthesis alkoxides
are also interesting from a structural point of view For example the choice of alkyl
group provides a means of systematic variation for the investigation of coordination
chemistry around metal and oxygen atoms The other parameters which can be
systematically varied are the number of oxo bridges and nuclearity
In the literature there are many examples of metal alkoxides it would be very
difficult to formulate a precise rule that could fully predict the final geometry of
forming alkoxide complex
Chapter-1 Introduction
9
Different structural pattern of metal alkoxides 54-69
Complex Structural pattern
[(C5H4CH3)4Y(micro-OCH=CH2)]2 Y2(micro-O)2 core
[Y3(micro3-OtBu)( micro3-Cl)( micro-O
tBu)3(O
tBu)4(thf)2 Y3(micro3-O)( micro-O)3O4 core
[Ti2(micro-OR)2(OR)4(acac)2]a (R=MeEt
iPr) [Ti2(micro-O)2O4 core
[Me4Zn4(micro3-OtBu)4 Zn4(micro3-O)4 core
[W2(OCMe2CMe2O)3] O3W= WO3 core
[Ga2(micro-OtBu)2
tBu4] Ga2(micro-O)2
core
[Mg2V2(thffo)6Cl4]b Mg2V2(micro3-O)2(micro-O)4 core
[(thf)(OtBu)Y(micro-O
tBu)(micro-CH3)AlMe23] YAl3(micro-O)3O core
[Zr2Co2(micro3-OnPr)2(micro-O
nPr)4(O
nPr)4(acac)2]
a Zr2Co2(micro3-O)2(micro-O)4O4 core
[Al(OEt)2GaMe23] AlGa3(micro-O)6 core
[Nb2(micro-OMe)2(OMe)2(HOMe)2Cl4] Nb-Nb(micro-O)2O4 core
[Mo2(OiPr)4(HO
iPr)4 O4Mo=MoO4 core
[Pr3(micro3-tftb)2(micro-tftb)3(tftb)2]c Pr3(micro3-O)2(micro-O)3O4 core
[YNa8(micro9-Cl)( micro4-OtBu)( micro3-O
tBu)8(O
tBu)] YNa8(micro4-O)( micro3-O)8O core
[Na4Zr6(micro5-O)2( micro3-OEt)4(micro-OEt)14(OEt)6 Na4Zr6(micro5-O)2( micro3-O)4(micro-O)14O
core
[Ti7(micro4-O)( micro3-O)2(micro-OEt)8(OEt)12 Ti7(micro4-O)( micro3-O)2(micro-O)8O12 core aacac = acetylacetonato
bthffo=tetrahydrofuryloxo
ctftb= OCMe2(CF3)
In fact even minor changes in a ligand structure or reaction conditions can lead to the
geometry of the whole compound to be fundamentally different
Classification based on Alkoxide Complexes Structure
Metal alkoxide complexes can have very complex structures due to formation of
oligomeric and sometimes even polymeric aggregates Formation of alkoxy bridges
M-O(R)-M help the complexes to obtain maximal and preferred coordination even
though the number of bonded ligands per metal atom are too few Complexes are
categorized based on number of metal atoms in the complex Optimal coordination is
obtained by chelating ligand or by a shared (bridging) ligand atom
Mononuclear complexes ndash Mononuclear complexes are highly charged metal ions
where the coordination requirements are satisfied by the number of OR-ligands The
ligands are often large and branched with chelating abilities
Binuclear complexes - In binuclear complexes an oxygen atom in the ligand connect
the two metal atoms Usually at least two alkoxy bridges are connecting the metal
atoms and thus stabilizes the complex
Chapter-1 Introduction
10
Mo and W complexes can have metal-metal bonds to stabilize the complex (without
any bridging ligand) the multiplicity depends on the number and nature of ligands
Trinuclear complexes ndash These complexes are often triangular structures linear
chains or non-linear chains with the same type of connections as in the binuclear
complexes
Tetranuclear complexes ndash These complexes have several different types of
configura- tion The tetrahedral configuration has a core of μ4-O and four metal atoms
connected by the oxo-ligand but this is not a common configuration
Ti4(OR)
16 type is common and is built up by a M
4-rhomb with 2μ
3-O and 4μ-O The
R-groups are most often-primary alkyl groups for the 3d-metals
The cubane-like structure contains metal atoms in four opposite corners of a cube and
oxygen atoms in the other corners (4μ3-O)
Al4(μ4-O)(μ-OPr
i)5
complex70
and the [Eu4(OPri)10(HOPr
i)3]middot2HOPr
i
complex71
are
some of the examples without a metal-metal bond The Al4(OPr
i)12
type (the propeller
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
7
R
O
M
R
O
M
R
O
M
O
R
MM
O
R
M M
O
R
M M
O
R
M M
O
R
MM
O
R
M MM
Figure 4 Coordination modes of an alkoxo ligand
Steric and electron demand of alkoxo groups have an influence on metal alkoxides
they form Electrophilic nature of metal cations allows attaching neutral ligands (eg
tetrahydrofuran pyridine etc) to the metal spheres Due to the saturating of metal
sites it is possible to obtain monomeric alkoxides [M(OR)xLy] (where L = neutral
ligand)
Alkoxides are highly versatile precursors for sol-gel synthesis4344
Alkoxides when
condensed form volatile alcohols andor ethers allowing for the formation of pure
products without impurities due to the precursor ligands In metal-organic chemical
vapour deposition (MOCVD) alkoxides are sometimes used as precursors4546
Examples of some of metal alkoxides having different structural features are depicted
below
TiOiPr
OiPr
PrOi
PrOi
= Ti(OiPr)4
Ti EtOEtO
EtO
EtO
EtO
OEt Ti
Ti
EtO OEt
OEt
EtO OEt
EtO Ti
OEt OEt
OEt
OEt
=Ti(OEt)16
Monomer47
Oligomer4849
Chapter-1 Introduction
8
Ce
PriO OiPr
PriO OiPr
PriO OPri
Ce
PriO OPri
OPriPriO
H
H
Alkoxide ndash alcohol adduct 50 51
Various so-called alkoxides are in fact oxo- or hydroxo-alkoxides which are the
condensation andor hydrolysis products of true alkoxides In oxo-alkoxides one or
more centrally placed bridging oxo ligands help to increase the coordination number
of the metal atoms The reactivity of oxo-alkoxides decreases with the ratio of
(bridging) oxo to alkoxo ligands Alkoxide derivatives may also contain other
ligands such as chloride ions or organic nonalkoxide ligands Chloro-alkoxides often
acquire structures similar to alkoxide structures but are normally avoided in sol-gel
synthesis as the chloride ions tend remain in the gel after hydrolysis as impurities in
the final materials
Ti
PriOPri
O
PriO
PriOO
OH
Ti
OPri
OPri
OPri
Ti
OPriPriO
=Ti3O(OH)(OiPr)9
Ti
PrOi PrOi
PrOi
PriO
PrOi
OiPr
Ti
OiPr
OiPr
OiPr
Y
ClCl
=YTi2(OiPr)9Cl2
oxo-hydroxo-alkoxide 52
chloro-alkoxide53
Apart from being versatile and important precursors in materials synthesis alkoxides
are also interesting from a structural point of view For example the choice of alkyl
group provides a means of systematic variation for the investigation of coordination
chemistry around metal and oxygen atoms The other parameters which can be
systematically varied are the number of oxo bridges and nuclearity
In the literature there are many examples of metal alkoxides it would be very
difficult to formulate a precise rule that could fully predict the final geometry of
forming alkoxide complex
Chapter-1 Introduction
9
Different structural pattern of metal alkoxides 54-69
Complex Structural pattern
[(C5H4CH3)4Y(micro-OCH=CH2)]2 Y2(micro-O)2 core
[Y3(micro3-OtBu)( micro3-Cl)( micro-O
tBu)3(O
tBu)4(thf)2 Y3(micro3-O)( micro-O)3O4 core
[Ti2(micro-OR)2(OR)4(acac)2]a (R=MeEt
iPr) [Ti2(micro-O)2O4 core
[Me4Zn4(micro3-OtBu)4 Zn4(micro3-O)4 core
[W2(OCMe2CMe2O)3] O3W= WO3 core
[Ga2(micro-OtBu)2
tBu4] Ga2(micro-O)2
core
[Mg2V2(thffo)6Cl4]b Mg2V2(micro3-O)2(micro-O)4 core
[(thf)(OtBu)Y(micro-O
tBu)(micro-CH3)AlMe23] YAl3(micro-O)3O core
[Zr2Co2(micro3-OnPr)2(micro-O
nPr)4(O
nPr)4(acac)2]
a Zr2Co2(micro3-O)2(micro-O)4O4 core
[Al(OEt)2GaMe23] AlGa3(micro-O)6 core
[Nb2(micro-OMe)2(OMe)2(HOMe)2Cl4] Nb-Nb(micro-O)2O4 core
[Mo2(OiPr)4(HO
iPr)4 O4Mo=MoO4 core
[Pr3(micro3-tftb)2(micro-tftb)3(tftb)2]c Pr3(micro3-O)2(micro-O)3O4 core
[YNa8(micro9-Cl)( micro4-OtBu)( micro3-O
tBu)8(O
tBu)] YNa8(micro4-O)( micro3-O)8O core
[Na4Zr6(micro5-O)2( micro3-OEt)4(micro-OEt)14(OEt)6 Na4Zr6(micro5-O)2( micro3-O)4(micro-O)14O
core
[Ti7(micro4-O)( micro3-O)2(micro-OEt)8(OEt)12 Ti7(micro4-O)( micro3-O)2(micro-O)8O12 core aacac = acetylacetonato
bthffo=tetrahydrofuryloxo
ctftb= OCMe2(CF3)
In fact even minor changes in a ligand structure or reaction conditions can lead to the
geometry of the whole compound to be fundamentally different
Classification based on Alkoxide Complexes Structure
Metal alkoxide complexes can have very complex structures due to formation of
oligomeric and sometimes even polymeric aggregates Formation of alkoxy bridges
M-O(R)-M help the complexes to obtain maximal and preferred coordination even
though the number of bonded ligands per metal atom are too few Complexes are
categorized based on number of metal atoms in the complex Optimal coordination is
obtained by chelating ligand or by a shared (bridging) ligand atom
Mononuclear complexes ndash Mononuclear complexes are highly charged metal ions
where the coordination requirements are satisfied by the number of OR-ligands The
ligands are often large and branched with chelating abilities
Binuclear complexes - In binuclear complexes an oxygen atom in the ligand connect
the two metal atoms Usually at least two alkoxy bridges are connecting the metal
atoms and thus stabilizes the complex
Chapter-1 Introduction
10
Mo and W complexes can have metal-metal bonds to stabilize the complex (without
any bridging ligand) the multiplicity depends on the number and nature of ligands
Trinuclear complexes ndash These complexes are often triangular structures linear
chains or non-linear chains with the same type of connections as in the binuclear
complexes
Tetranuclear complexes ndash These complexes have several different types of
configura- tion The tetrahedral configuration has a core of μ4-O and four metal atoms
connected by the oxo-ligand but this is not a common configuration
Ti4(OR)
16 type is common and is built up by a M
4-rhomb with 2μ
3-O and 4μ-O The
R-groups are most often-primary alkyl groups for the 3d-metals
The cubane-like structure contains metal atoms in four opposite corners of a cube and
oxygen atoms in the other corners (4μ3-O)
Al4(μ4-O)(μ-OPr
i)5
complex70
and the [Eu4(OPri)10(HOPr
i)3]middot2HOPr
i
complex71
are
some of the examples without a metal-metal bond The Al4(OPr
i)12
type (the propeller
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
8
Ce
PriO OiPr
PriO OiPr
PriO OPri
Ce
PriO OPri
OPriPriO
H
H
Alkoxide ndash alcohol adduct 50 51
Various so-called alkoxides are in fact oxo- or hydroxo-alkoxides which are the
condensation andor hydrolysis products of true alkoxides In oxo-alkoxides one or
more centrally placed bridging oxo ligands help to increase the coordination number
of the metal atoms The reactivity of oxo-alkoxides decreases with the ratio of
(bridging) oxo to alkoxo ligands Alkoxide derivatives may also contain other
ligands such as chloride ions or organic nonalkoxide ligands Chloro-alkoxides often
acquire structures similar to alkoxide structures but are normally avoided in sol-gel
synthesis as the chloride ions tend remain in the gel after hydrolysis as impurities in
the final materials
Ti
PriOPri
O
PriO
PriOO
OH
Ti
OPri
OPri
OPri
Ti
OPriPriO
=Ti3O(OH)(OiPr)9
Ti
PrOi PrOi
PrOi
PriO
PrOi
OiPr
Ti
OiPr
OiPr
OiPr
Y
ClCl
=YTi2(OiPr)9Cl2
oxo-hydroxo-alkoxide 52
chloro-alkoxide53
Apart from being versatile and important precursors in materials synthesis alkoxides
are also interesting from a structural point of view For example the choice of alkyl
group provides a means of systematic variation for the investigation of coordination
chemistry around metal and oxygen atoms The other parameters which can be
systematically varied are the number of oxo bridges and nuclearity
In the literature there are many examples of metal alkoxides it would be very
difficult to formulate a precise rule that could fully predict the final geometry of
forming alkoxide complex
Chapter-1 Introduction
9
Different structural pattern of metal alkoxides 54-69
Complex Structural pattern
[(C5H4CH3)4Y(micro-OCH=CH2)]2 Y2(micro-O)2 core
[Y3(micro3-OtBu)( micro3-Cl)( micro-O
tBu)3(O
tBu)4(thf)2 Y3(micro3-O)( micro-O)3O4 core
[Ti2(micro-OR)2(OR)4(acac)2]a (R=MeEt
iPr) [Ti2(micro-O)2O4 core
[Me4Zn4(micro3-OtBu)4 Zn4(micro3-O)4 core
[W2(OCMe2CMe2O)3] O3W= WO3 core
[Ga2(micro-OtBu)2
tBu4] Ga2(micro-O)2
core
[Mg2V2(thffo)6Cl4]b Mg2V2(micro3-O)2(micro-O)4 core
[(thf)(OtBu)Y(micro-O
tBu)(micro-CH3)AlMe23] YAl3(micro-O)3O core
[Zr2Co2(micro3-OnPr)2(micro-O
nPr)4(O
nPr)4(acac)2]
a Zr2Co2(micro3-O)2(micro-O)4O4 core
[Al(OEt)2GaMe23] AlGa3(micro-O)6 core
[Nb2(micro-OMe)2(OMe)2(HOMe)2Cl4] Nb-Nb(micro-O)2O4 core
[Mo2(OiPr)4(HO
iPr)4 O4Mo=MoO4 core
[Pr3(micro3-tftb)2(micro-tftb)3(tftb)2]c Pr3(micro3-O)2(micro-O)3O4 core
[YNa8(micro9-Cl)( micro4-OtBu)( micro3-O
tBu)8(O
tBu)] YNa8(micro4-O)( micro3-O)8O core
[Na4Zr6(micro5-O)2( micro3-OEt)4(micro-OEt)14(OEt)6 Na4Zr6(micro5-O)2( micro3-O)4(micro-O)14O
core
[Ti7(micro4-O)( micro3-O)2(micro-OEt)8(OEt)12 Ti7(micro4-O)( micro3-O)2(micro-O)8O12 core aacac = acetylacetonato
bthffo=tetrahydrofuryloxo
ctftb= OCMe2(CF3)
In fact even minor changes in a ligand structure or reaction conditions can lead to the
geometry of the whole compound to be fundamentally different
Classification based on Alkoxide Complexes Structure
Metal alkoxide complexes can have very complex structures due to formation of
oligomeric and sometimes even polymeric aggregates Formation of alkoxy bridges
M-O(R)-M help the complexes to obtain maximal and preferred coordination even
though the number of bonded ligands per metal atom are too few Complexes are
categorized based on number of metal atoms in the complex Optimal coordination is
obtained by chelating ligand or by a shared (bridging) ligand atom
Mononuclear complexes ndash Mononuclear complexes are highly charged metal ions
where the coordination requirements are satisfied by the number of OR-ligands The
ligands are often large and branched with chelating abilities
Binuclear complexes - In binuclear complexes an oxygen atom in the ligand connect
the two metal atoms Usually at least two alkoxy bridges are connecting the metal
atoms and thus stabilizes the complex
Chapter-1 Introduction
10
Mo and W complexes can have metal-metal bonds to stabilize the complex (without
any bridging ligand) the multiplicity depends on the number and nature of ligands
Trinuclear complexes ndash These complexes are often triangular structures linear
chains or non-linear chains with the same type of connections as in the binuclear
complexes
Tetranuclear complexes ndash These complexes have several different types of
configura- tion The tetrahedral configuration has a core of μ4-O and four metal atoms
connected by the oxo-ligand but this is not a common configuration
Ti4(OR)
16 type is common and is built up by a M
4-rhomb with 2μ
3-O and 4μ-O The
R-groups are most often-primary alkyl groups for the 3d-metals
The cubane-like structure contains metal atoms in four opposite corners of a cube and
oxygen atoms in the other corners (4μ3-O)
Al4(μ4-O)(μ-OPr
i)5
complex70
and the [Eu4(OPri)10(HOPr
i)3]middot2HOPr
i
complex71
are
some of the examples without a metal-metal bond The Al4(OPr
i)12
type (the propeller
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
9
Different structural pattern of metal alkoxides 54-69
Complex Structural pattern
[(C5H4CH3)4Y(micro-OCH=CH2)]2 Y2(micro-O)2 core
[Y3(micro3-OtBu)( micro3-Cl)( micro-O
tBu)3(O
tBu)4(thf)2 Y3(micro3-O)( micro-O)3O4 core
[Ti2(micro-OR)2(OR)4(acac)2]a (R=MeEt
iPr) [Ti2(micro-O)2O4 core
[Me4Zn4(micro3-OtBu)4 Zn4(micro3-O)4 core
[W2(OCMe2CMe2O)3] O3W= WO3 core
[Ga2(micro-OtBu)2
tBu4] Ga2(micro-O)2
core
[Mg2V2(thffo)6Cl4]b Mg2V2(micro3-O)2(micro-O)4 core
[(thf)(OtBu)Y(micro-O
tBu)(micro-CH3)AlMe23] YAl3(micro-O)3O core
[Zr2Co2(micro3-OnPr)2(micro-O
nPr)4(O
nPr)4(acac)2]
a Zr2Co2(micro3-O)2(micro-O)4O4 core
[Al(OEt)2GaMe23] AlGa3(micro-O)6 core
[Nb2(micro-OMe)2(OMe)2(HOMe)2Cl4] Nb-Nb(micro-O)2O4 core
[Mo2(OiPr)4(HO
iPr)4 O4Mo=MoO4 core
[Pr3(micro3-tftb)2(micro-tftb)3(tftb)2]c Pr3(micro3-O)2(micro-O)3O4 core
[YNa8(micro9-Cl)( micro4-OtBu)( micro3-O
tBu)8(O
tBu)] YNa8(micro4-O)( micro3-O)8O core
[Na4Zr6(micro5-O)2( micro3-OEt)4(micro-OEt)14(OEt)6 Na4Zr6(micro5-O)2( micro3-O)4(micro-O)14O
core
[Ti7(micro4-O)( micro3-O)2(micro-OEt)8(OEt)12 Ti7(micro4-O)( micro3-O)2(micro-O)8O12 core aacac = acetylacetonato
bthffo=tetrahydrofuryloxo
ctftb= OCMe2(CF3)
In fact even minor changes in a ligand structure or reaction conditions can lead to the
geometry of the whole compound to be fundamentally different
Classification based on Alkoxide Complexes Structure
Metal alkoxide complexes can have very complex structures due to formation of
oligomeric and sometimes even polymeric aggregates Formation of alkoxy bridges
M-O(R)-M help the complexes to obtain maximal and preferred coordination even
though the number of bonded ligands per metal atom are too few Complexes are
categorized based on number of metal atoms in the complex Optimal coordination is
obtained by chelating ligand or by a shared (bridging) ligand atom
Mononuclear complexes ndash Mononuclear complexes are highly charged metal ions
where the coordination requirements are satisfied by the number of OR-ligands The
ligands are often large and branched with chelating abilities
Binuclear complexes - In binuclear complexes an oxygen atom in the ligand connect
the two metal atoms Usually at least two alkoxy bridges are connecting the metal
atoms and thus stabilizes the complex
Chapter-1 Introduction
10
Mo and W complexes can have metal-metal bonds to stabilize the complex (without
any bridging ligand) the multiplicity depends on the number and nature of ligands
Trinuclear complexes ndash These complexes are often triangular structures linear
chains or non-linear chains with the same type of connections as in the binuclear
complexes
Tetranuclear complexes ndash These complexes have several different types of
configura- tion The tetrahedral configuration has a core of μ4-O and four metal atoms
connected by the oxo-ligand but this is not a common configuration
Ti4(OR)
16 type is common and is built up by a M
4-rhomb with 2μ
3-O and 4μ-O The
R-groups are most often-primary alkyl groups for the 3d-metals
The cubane-like structure contains metal atoms in four opposite corners of a cube and
oxygen atoms in the other corners (4μ3-O)
Al4(μ4-O)(μ-OPr
i)5
complex70
and the [Eu4(OPri)10(HOPr
i)3]middot2HOPr
i
complex71
are
some of the examples without a metal-metal bond The Al4(OPr
i)12
type (the propeller
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
10
Mo and W complexes can have metal-metal bonds to stabilize the complex (without
any bridging ligand) the multiplicity depends on the number and nature of ligands
Trinuclear complexes ndash These complexes are often triangular structures linear
chains or non-linear chains with the same type of connections as in the binuclear
complexes
Tetranuclear complexes ndash These complexes have several different types of
configura- tion The tetrahedral configuration has a core of μ4-O and four metal atoms
connected by the oxo-ligand but this is not a common configuration
Ti4(OR)
16 type is common and is built up by a M
4-rhomb with 2μ
3-O and 4μ-O The
R-groups are most often-primary alkyl groups for the 3d-metals
The cubane-like structure contains metal atoms in four opposite corners of a cube and
oxygen atoms in the other corners (4μ3-O)
Al4(μ4-O)(μ-OPr
i)5
complex70
and the [Eu4(OPri)10(HOPr
i)3]middot2HOPr
i
complex71
are
some of the examples without a metal-metal bond The Al4(OPr
i)12
type (the propeller
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
11
type) has an octahedron (with Al in the center) in the center and three tetrahedra of Al
connected by pairs of OR-bridges
A heterometallic example is the Nd[Al(OPr
i)4]
3 with the Nd atom in the center of the
complex72
Pentanuclear complexes ndash These complexes belong most often to either of two
different coordination
First the trigonal bipyramidand the square pyramid both with a μ
5-O in the center of
the M5O-cluster Second the structure with two triangles sharing a vertex Many
lanthanide oxo-isopropoxide complexes belong to the square pyramidal coordination
eg the [Eu4III
EuIIO(OPr
i)12
(HOPri)] HOPr
i complex
73 and Ln5O(OPr
i)13 Ln = Nd
Gd or Er74
Hexanuclear complexes - Most common ones in hexanuclear complex are octahedral
M6-arrangement with a μ6-O in the center
or a structure with two M
3-triangles connected by the ligands the double propeller
type
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
12
Sol-Gel routes to metallic oxides
The goal is not only to obtain heterometallic alkoxides for fundamental studies but
should preferably be suitable for sol-gel processing and implementation in different
matrices The ligands plays important roles in the complexes changing the ligands
greatly affect their chemical behaviour and the way they act in sol-gel preparations
Sol-gel process is an efficient way of producing highly homogeneous pure
heterometallicoxides7576
with a well-controlled specific composition In Sol-gel
processes an alkoxide is first dissolved in a water-free organic solvent The precursor
solution can then be used to manufacture a great variety of different products such as
fine powders thin films fibers and ceramics depending on different manufacturing
steps as shown in Figure 5
Figure 5 Different steps in Sol-Gel process leading to different product
77-79
In the sol-gel synthesis two fundamental types of routes are possible (i) the metal-
organic (or organic) route and (ii) the inorganic route
The metal-organic route gives a better control over the process and is particularly
good when preparing high quality heterometallic oxides The inorganic route related
to ACG (Aqueous Chemical Growth) is much cheaper and easier to handle and is
often efficient for preparing highly crystalline oxides of specific shapes and size at
low temperatures but is not so useful for heterometallic oxides 75
The inorganic route
In the inorganic route metal salts such as acetates chlorides nitrates or sulphates are
dissolved in an aqueous solution and sol or precipitate is formed at a change of pH
temperature or concentration
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
13
Depending on the charge of the metal ion and the pH for the solution different
complexes such as aqua hydroxo and oxo complexes are formed
M (OH2)z+ M OH(z-1)+ + H+ M O(z-2)+ + 2H+ (Equ 1)
Figure 6 Metal ion charge (Z) vs pH 80
Figure 6 shows pH versus the metal ion charge (Z) areas typical of aqua hydroxo and
oxo ions The figure clearly shows that in acidic conditions M-OH2 complexes
observed for low-valence metal cations and in basic conditions M-O complexes are
observed for high-valence metal cations The area of the M-OH complexes is between
these areas Formation of a sol or precipitate occurs in the M-OH area
From the sol different condensation reactions can occur The condensation reactions
can be divided in two sub-categories olation where hydroxyl bridges are formed and
oxolation where oxo-bridges are formed 75
2M OH M (Equ 2)
(Equ 3)2M OH M + H2O
Olation
Oxolation
2(OH)
O M
M
Olation occurs for large metal ions with low charge Oxolation occurs very fast if the
metal ion is coordinatively unsaturated 75
The aqua-ligands are good leaving groups and poor nucleophiles while the oxo-
ligand has the opposite properties ie they have poor leaving groups and good
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
14
nucleophiles This means that no condensation can occur and no stable colloidal
solution can be obtained
The inorganic synthesis route is difficult to control for systems consisting of more
than one metal-ion due to the different properties of the metal-ions leading to different
pH ranges for precipitation Therefore there will be a preferential precipitation of one
metal-ion before the other in a multi-ion system On the other hand the inorganic sol-
gel route is a good choice for the monometallic systems when a desired shape and
phase can be produced at a low temperature and with cheap chemicals and
equipment7576
The metal-organic route
In metal-organic route metal-organic precursors mostly alkoxides are dissolved in
water-free organic solvents to form a homogeneous solution An alkoxide is a
derivative of an alcohol and consists of a metal or a semi-metal (M) an oxygen (O)
attached to an alkyl group (R) M-OR The starting chemicals for the synthesis and the
solvents must be carefully dried467576
as most of the alkoxides are extremely sensitive
to moisture and sometime also to oxygen
The first step in the metal-organic route is hydrolysis step where the alkoxo group is
changed for a hydroxo group while an alcohol molecule is expelled
M OR + H2O M OH + ROH (Equ 4)
In the next step the hydroxyl complexes M-OH react with another alkoxide or
hydrolyzed alkoxide molecule in one of two different ways olation or oxolation Both
these reactions are condensation reactions because metal-oxygen bridges are formed
while a small molecule is expelled The condensation phase can proceed as long as
sufficient water is available to form either a gel or a precipitate75
Olation
M OH M OHR M OH M+ + ROH
M OH M OH2 M OH M+ + H2O
(Equ5)
(Equ6)
Oxolation
M OH M OR M O M+ + ROH
M OH M OH M O M+ + H2O
(Equ7)
(Equ8)
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
15
A gel with specific desired structure and properties can be obtained by control of the
hydrolysis and condensation steps
Two fundamental types of gels can be formed particulate gels and polymeric gels
Particulate gels consist of spherical shape particles with dense or highly branched
polymers in size around a few nanometres to micrometres Polymeric gels on the
other hand have a low degree of branching of the polymer strands81
If the hydrolysis
and condensation reactions occur sequentially a polymeric gel is formed The
particular gel is formed when the hydrolysis is slow and the condensation reaction is
rapid Rapid hydrolysis and condensation reactions give colloidal gels or gelatinous
precipitates and low reaction rates result in a particle sols being formed75
General Synthetic Routes to Different Alkoxides
In alkoxides the metal is highly charged because of the low degree of electron
donation from the alkoxo oxygen to the metal75
The alkoxides are normally
polynuclear through sharing of alkoxo groups or oxo-oxygens and can be classified in
two groups homometallic alkoxides and heterometallic alkoxides All alkoxides are
with few exceptions (small p-block Si As P B S) very reactive to water Alkoxides
are very useful for producing hetero-metal oxides with exact composition and
ordering of the metals which is difficult to achieve with most techniques such as
CVD PVD and electrochemistry Homometallic alkoxides can be prepared in many
ways which to a great degree are dependent on the oxidation number of the metal ion
Hetero bi- or hetero polymetallic alkoxo complexes constitute an enormous family of
compounds with a very broad structural diversity Heterometallic alkoxides are
alkoxides containing two or more different kinds of metal-ions connected through
oxygenrsquos of the alkoxo-ligands
Synthetic Routes to Homometallic Alkoxides
The methods for synthesizing metal alkoxides are well established482
and the method
required for the synthesis of alkoxy derivatives of an element generally depends upon
its electronegativity Alkoxides can be prepared by several different synthetic
routes483-85
Some of the synthetic methods to produce desired metal Alkoxide are
described below
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
16
bull Reaction between metal and alcohol
M + n R-OH rarr M(OR)n + n2 H2 (g)
This method is limited to the most reactive metals such as alkaline metals alkaline
earth metals rare earth metals and aluminium Hydroxyl hydrogen gets replaced by
suitable metal cation with evolution of H2
bull Anodic oxidation of metal in alcohol
In this method due to the oxidation of metal at the anode cation and electron are
formed The electron and alcohol create hydrogen radical H and alkoxide anion
Molecular hydrogen exudes at the cathode
LiCl + R-CH2 -OH rarr Li-O-CH2 -R + frac12 H2 (g) + Cl
2 Cl + R-CH2 -OH rarr 2 HCl + R-CHO
The metal alkoxide produced by anodic oxidation is insoluble in the solvent and
therefore precipitates This method works for less reactive metals such as Zr Ta Nb
Co Fe and Ni An electro conductive additive (a halide) must often be added
The lithium chloride can react with the solvent alcohol and produces a lithium
alkoxide complex along with hydrogen and chlorine radical This radical reacts
further with the alcohol and produces hydrogen chloride and an aldehyde 86
bull Metal oxide or hydroxide reaction with alcohol
Metal hydroxides and oxides react with alcohols forming alkoxides and water
M-O + 2R-OH M-(OR)2 + H2O
M-OH + R-OH M-OR + H2O
Due to the reversible nature of these reactions it is necessary to remove water from
the reaction system Alkoxides of Mg Ca or Al are often used for obtaining water-
free alcohol since their reactions with water are irreversible
bull Reaction of Metal Halides with alcohol
MXn + n R-OH rarr M(OR)n + n HX
(X = H alkyl CequivC equivN NH2 NR2 SH N(SiR3)2 hellip)
Here the reaction between alcohol and metal halide leads to the substitution of halide
anion into RO- group forming appropriate metal alkoxide The hydrogen in the
alcohol interacts with the produced anion (from eg the metal hydride) and HX is
produced along with the metal alkoxide
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
17
bull Metathesis between two different metal complexes
MXn + n MOR rarr M(OR)n + n MX X=halide
This is the most common method for synthesis of metal alkoxides The solvent is
usually an alcohol mixed with another organic solvent used to decrease the solubility
of MX One disadvantage of this method is the formation of bimetallic complexes
However this can be avoided if ammonia is used instead of alkaline alkoxides
bull Alcohol exchange or transesterfication
One of the characteristic properties of metal alkoxides is their activity in the
substitution reactions of alkoxo groups
M(OR)n
+ nR-OH rarr M(OR)n + nR-OH
M(OR)n + nRCOOR rarr M(OR)n + nRCOOR
The alcohol produced in this reaction can normally be distilled off or the new metal
alkoxide can be precipitated to enhance the yield A drawback is that it can be
difficult to exchange all of the alkoxy groups in the complex leaving a mixed ligand
complex
Synthetic Routes to Heterometallic Alkoxides
Heterometallic complexes are of interest not only because of their attractive structural
chemistry catalytic properties and potential for industrial applications but also
because they constitute a group of molecular precursors for various metal oxide
materials In heterometallic alkoxide- or aryloxide- based complexes two or more
different metals might be held together by alkoxo or aryloxo bridging ligands
Coordinated alkoxo or aryloxo groups and alcohol or phenol molecules both attach to
the metal center resulting in excellent anchors for organometallic compounds
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues The methods described
above are the present ways to produce homometallic alkoxides Producing
heterometallic alkoxides ie alkoxides containing two different metal atoms requires
different approaches
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
18
Reaction between two alkoxide complexes
M(OR)n + qM(OR)m rarr MMq (OR)n+qm
This route is effective in the cases where one of complex is an alkaline metal or
alkaline earth metal alkoxide and the other is an alkoxide of a transition metal
preferably multivalent87
Reaction between a metal halide and an alkaline metal alkoxide
MXn + nMM(OR)m rarr MMn (OR)nm
+ nMX (s)
This route is used when one metal alkoxide is difficult to access whereas the halide
complex can be easily produced83
Synthetic Routes to Heterobimetallic Complexes
The formation of heterobimetallic complexes can occur due to one of the following
reactions
bull Alkoxide Routes
Mixed-metal species MMprime(OR)x+y generation depends on the difference in the
electronegativity between different metals ieM and Mprime insaturation stereolability of
alkoxides or oxoalkoxides of metal alkoxides M(OR)x Such reactions can be sensitive
to solvent presence of impurities such as water oxygen parent alcohol and method of
purification of alkoxides etc88
yM(OR)n + M(OR)n MMY(OR)n (OR)ny
Mostly studied heterometallic alkoxides are of the type MMprime(OR)6 where M = Li Na
K and Mprime = Nb Ta
The nature of the OR ligand can modify the stoichiometry between the metals as
shown in the following Ba-Zr system89
2Ba(OPri)2+ Zr4Ba2(OPri)20 4PriOH+2Zr2(OPri)(PriOH)2
Zr(OBut)2 Ba(OBut)2+ 12[ZrBa(OBut)6]2
Triphenylbismuth reacts with salicylic acid and the metal alkoxides
Ti(OCH(CH3)2)4 and M(OCH2CH3)5 (M = Nb Ta) to produce the heterobimetallic
complexes Bi2M2(sal)4(Hsal)4(OR)4
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
19
Figure 7 Bi2M2(sal)4(Hsal)4(OR)4
By the Reaction of Alkoxides with Metal β-diketonates
An interesting way to the preparation of heterometallic complexes is the reaction
between a metal alkoxides and β-diketonate complex of another metal atom The Ba-
Ti and Sr-Ti examples demonstrate that a convenient set of ligands can stabilize
mixed-metal β-diketonato alkoxides and even tune their MMprime stoichiometry Some
examples of these are as BaTi2(thd)4(OEt)8(EtOH)2 where thd =(ButCOCHOCO
But)90
formed by reacting titanium ethoxide and barium tetra methyl heptanedionate
in 11 stoichiometry Reaction of titanium isopropoxide with strontium tetra methyl
heptanedionate gave Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4(OPr
i)2 having rhombus
structure (Figure 8)91
Figure 8 Molecular structure of Sr2Ti2(η2-thd)4(μ3-OPri)2(μ-OPr
i)4 (OPr
i)2
bull By the Reaction of Alkoxides with Metal Carboxylates
The solubility of metal acetates in organic solvents is very low but can be improved
in the presence of metal alkoxides by the formation of heterobimetallic species For
example anhydrous metal acetates M(OAc)2 (M = Mg Pb Cd) are solubilized in
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
20
hydrocarbons in the presence of niobium alkoxides at room temperature giving
trimetallic species Nb2M(μ-OAc)2(OR)10
M(OAc)2 + [Nb(OR)5]2
HexaneNb2M(OAc)2(OR)10
room temp
Where M = Mg Pb Cd and R = Pr Et
bull Salt Elimination Reactions
Heterobimetallic complexes are also synthesized by substitution of all halide ligands
in a metal halide by anionic alkoxo- metallates
MCln + uMMy(OR)2 M[My(OR)2]n + nMCl
ZnCl2 + Ti2Sn(OEt)6 ZnSn(OEt)6 + 2TiCl4
Metal halides (MCln) are of three categories ie (i) divalent and trivalent transition
metals such as Cr Mn Fe Co Ni Cu etc (ii) lanthanides and actinides (iii) Pb(II)
Sb(III)
bull Condensation Reactions
In Condensation reactions the elimination of small molecules such as ether alcohol
water carboxylic acid or ester as volatile by-product takes place For example
heterobimetallic oxoalkoxide bridges can be obtained according to the following
chemical reactions
M(OR)n + M(OL)n (RO)n-1M-O-M(OL)n-1 + ROL
Where L = CH3COO- group and RʹOL is a volatile by-product
Sometimes heating could be required for the dissolution of some metal acetates and
condensation takes place with the elimination of ester92
The condensation of metal
(II) acetate with alkoxides leads to the product of type (RO)nM-O-M(II)-O-M(OR)n
where M = Al(II) Ti(IV) M(II) = Mg Cr Mn Fe Co Zn Mo Pb
For the last two decades evolutionary studies have been carried out for the synthesis
and characterization of polymetallic clusters and cages93-98
as these compounds have
proved importance in developments of several fields of bioinorganic chemistry99-104
magnetochemistry105-107
solid-state physics108-114
and material science
Almost all transition metals throughout the periodic table form metal compounds
utilizing different kinds of bridging organic and inorganic ligands93-96115116
The
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
21
involvement of carboxylato oxo and alkoxo bridges provide interesting exchange
coupling in various cases In this sense alkoxo-aliphatic ligands or simply the
aminoalcohol ligands can be expected to improve the coupling between two or more
metal centers forming homo or heteronuclear complexes374546117118
Homoleptic Alkoxides
A great number of homoleptic Cu- and Zn-alkoxides with simple aliphatic or aromatic
alkoxide ligands (eg OMe OEt OiPr O
tBu OCEt3 OCH2CH2NMe2 OCH2CH2O
Me and OAr) are known119-127
Homoleptic heterometallic alkoxides suitable as precursors for materials such as
M[Al(OR)4]2 [ClMndashZr2(OPri)9]2 or M[Zr2(OPr
i)9]2 (M=Cu Zn) are available via salt
metathesis eg by KCl elimination or reaction of anionic nucleophilic Al or Zr-
alkoxide complexes with MCl2 89128
Synthetic Routes to Heteroleptic Alkoxides
Alkoxide complexes with two or more different ligands known as heteroleptic
alkoxides and can be prepared by chemical modifications A metal alkoxide reacts
with an acidic organic ligand or by reaction of the metal alkoxide with a β-diketonate
(eg acetylacetone (Hacac H3C-C(O)-CH2-C(OH)-CH3) or a carboxylate metal
complex
M(OR)n + mHZ rarr M(OR)n-m Zm
+ mROH
Z=acidic organic ligand
Complexes with a β-diketonate or a carboxylate ligand are less reactive to hydrolysis
as compared to ordinary alkoxide complex due to larger negative charge on the
carboxylate or β-diketonate ligand and a chelating effect
The reaction with a β-diketonate complex is another way to prepare heterometallic
alkoxides but it may also result in heterometallic heteroleptic alkoxides For
heterometallic complexes the solvent should be purely hydrocarbon based such as
toluene or hexane129-131
If alcohol is added it behaves as a Lewis base and formation
of heterometallic complex would be interrupted and a ligand exchange reaction could
occur instead132
Properties and Reactivity of Metal Alkoxides
In metal alkoxides M-OR the organic moiety R attached to oxygen may be alkyl
substituted alkyl chelating alkyl or alkenyl and has a substantial influence on the
structure and properties of the metal alkoxides The steric effect of the R group has a
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
22
controlling influence on the volatility of the metal alkoxides82
Thus the alkoxides
with less bulky alkyl groups eg methyl and ethyl proved to be oligomers (eg
dimers trimers and tetramers) due to the bridging property of the alkyl group which
may be bonded through its oxygen to two or three metals through μ2 or μ3 fashion
respectively by means of conventional two-electron covalent bonds 133134
Bond lengths vary in the order M-OR terminal lt M-μ2-OR lt M-μ3-OR These
structures are retained in non-polar organic media Polynuclear species can also be
obtained via an oxo ligand and the elements with large metallic radii having small
valency such as divalent (Ba Sr) and trivalent (In Ln Fe Al) and this favour the
stability of oxo-derivatives rather than alkoxides oligomers and their alcohol solvated
analogs135136
The oxo ligand is an versatile ligand which can be linked to more
metals around 2minus6 than an OR ligand and thus increase the metal coordination
number in the absence of a neutral ligand L137138
Metal alkoxides M(OR)n are very reactive towards wide variety of molecules having
acidic protons which helps in chemical modifications of organic hydroxyl derivatives
such as alcohols silanols R3SiOH glycols OH(CH2)nOH carboxylic and hydroxyl
carboxylic acids hydroxyl surfactants etc to achieve tuneable properties
1m[M(OR)n]m + aXH 1m[M(OR)n-aXa]m + aROH
X= RCO2 β-dik
Hydrolysis
Metal alkoxides are rapidly hydrolyzed leading to the formation of hydroxides or
hydrated oxides
2Al(OR)3 + 6H2O Al2O33H2O + 6ROH
or 2Al(OH)3
This means that during handling such materials great care must be taken to exclude
moisture However if a restricted amount of water is used then this may lead to
formation of oxyalkoxides
2Ti(OBut)4 + H2O (OBut)3Ti-O-Ti(OBut)3 + 2ButOH
When a restricted amount of water is added partial hydrolysis occurs sometimes-
yielding products of definite composition known as oxide alkoxides
2Al(OR)3 + 2H2O Al2O(OR)4 + 2ROH
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
23
2Al(OR)3 + 2H2O Al2O2(OR)2 + 4ROH or Al2(OH)2(OR)4
Reaction with Alcohols
Functionalized alcohols at room temperature easily interchange alcoholic groups in
the metal alkoxides while heating is required for complete exchange by classical
alcohols These are known as alcoholysis reactions which increase the solubility of
metal alkoxides The reaction can be represented by the following general equation
M(OR)m + xROH M(OR)m-n(OR)n + xROH
These reactions appear to proceed through the SN2 type mechanism involving a four-
centered cyclic transition state
O
H
RM
RO
ROOR
ORO
R
HM
RO
ROOR
OR
(+ROH)
M
RO
ROOR
OR
(+ROH)
and so on
Figure 9 Mechanism of the reaction
Functionalized alkoxide ligands such as O(CH2)nX [X = OR (alkoxyalcohols) NR2
(aminoalcohols)] with intermolecular O or N donor sites can be bridging or chelating
(Figure 10) Chelation generally requires formation of a cycle which takes place by
bonding the alkoxides oxygen and the donor site X to the metal The size of the ring
depends upon the value of lsquonrsquo in the (CH2)n eg the value n = 2 is for 2-
methoxyethanol and it forms five membered rings in complexes
Group replacement by functional alcohols has also been found to solubilize some
insoluble alkoxides as polymeric metal alkoxides of some metals such as Ni Cu Sn
etc It usually depends upon their ability to act as a chelating ligand rather than a
bridging one and in this respect aminoalcohols are often more efficient than
alkoxyalcohols This behaviour is shown by polymeric Cu(II) alkoxides [Cu(OR)2]infin
(R = Me Pri Bu
t) in which alcohol exchange reactions afford insoluble copper(II) 2-
methoxyethoxide [Cu(OC2H4OMe)2]2 whereas Cu(OC2H4NMe2)2 is a monomer
volatile and soluble139
Similar is the case with the soluble Ba(teaH2)2 2EtOH and
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
24
[Cu(teaH2)]43teaH3 species which are obtained by alcoholysis of insoluble methoxide
by triethanolamine N(C2H4OH)3(teaH3) and volatility can be enhanced by steric
effects such as substitution in the α-position a strategy used for forced chelation140
The different modes of coordination of functionalized alcohols in monoanionic
alkoxides (x = OR NR2 and M is atom of same or different elements) are as follows
O
M
X
O
M M
X
Terminal or pendant ƞ1
Bridging micro2-ƞ1
X
MO
X
MO
M
Chelating ƞ2
Bridging-chelating micro2-ƞ2
Figure10 Different modes of coordination of functionalized alcohols
Reaction with β-diketones
Metal alkoxides reacts readily with chelating β-diketones because of the availability
of number of M-OR bonds for hydrolysis Titanium isopropoxide [Ti(OPri)4] is highly
reactive towards air and moisture due to unsaturated four coordinate Ti(IV) The
moisture sensitivity of the Ti based precursors can be reduced by the insertion of
chelating β-diketone groups to increase the coordinative saturation of the Ti(IV)
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
25
center to make Ti(OPri)2(acac)2
141 Similarly [Zr(OPr
i)3(thd)]2 the symmetric dimer is
the most stable complex which has significant advantages over Zr(OPri)4 and Zr(thd)4
due to its high volatility and stability142
Thermal stability of the Ta(OEt)4(dbm)
complex is due to delocalization of the negative charge into an extended conjugated
electron system involving the orbitals of the phenyl groups in the dibenzoylmethanate
ligand143
Reaction with Carboxylic Acid
The reactivity of metal alkoxides with carboxylic acids is rather complex as compared
to β-diketones as the competitive reactions can occur The three different situations
are as follows
Substitution
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Generation of oxo ligands by either non-hydrolytic condensation or elimination of an
ester from an unstable carboxylatoalkoxide
M(OR)n-x(RCO2)x 12[(OR)n-x-1M-O-M(RCO2)x-1] + RCO2R
Hydrolysis which leads to esterification
ROH + RCO2H RCO2R + H2O
This depends on the experimental conditions as stoichiometry acidM(OR)n tempera-
ture nature of the acid solvent and duration The increase in temperature causes an
increase in the number of oxo ligands Polynuclear complexes of titanium alkoxides
such as Ti6O4(μ-OBu)4(OBu)4(μ-OAc)8144
is obtained at room temperature while
heating drives the reaction towards more oxo species Ti6O6(OEt)6(μ-O2CR)6145146
Reactions wih Hydrogen halides Halogens and Acyl halides
Metal halides are used as the starting materials for the synthesis of metal alkoxides
However the alkoxides can be converted to metal halides or mixed alkoxy-halides by
reaction with halogen hydrogen halide or acyl halide
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
26
i) xHX + M(OR)n M(OR)n-x(X)x + xROH
ii) X2 + M(OCH2R)nMX2(OCH2R)n-2 + 2RCH2O
RCH2OH + RCHO
iii) xRCOX M(OR)n M(OR)n-xXx + RCOOR+
Reactions with Organic Esters and Silyl Esters
Metal alkoxides react with organic esters to form new alkoxy derivatives
i) M(OR)n + xCH3COOR M(OR)n-x(OR)x + xCH3COOR
ii) M(OR)n + xR3SiOH M(OSiR3)n-x(OR)x + xROH
Reactions with Glycols
Glycols are di-hydroxy alcohols and react readily with metal alkoxides to form
glycolates(chelated or bridged) or mixed alkoxide glycolates Due to presence of a
large organic chain glycolates tend to form highly polymeric derivatives compared to
the analogous alkoxide derivatives and are more resistant to hydrolysis Mixed
glycolates can be obtained by reactions of monoalkoxide monoglycolates with
different glycols in equimolar ratios
M(OR)x + n HO
HO
(RO)x-2n M
O
O
+ 2nROH
Reactions with Schiff Bases and β-ketoamines
General mode of reaction of Metal alkoxides with Schiff bases and β-ketoamines is as
shown below
i) M(OR)n + x(HO)RC=NR (RO)n-xM(O(R)C=NR)x + xROH
ii) M(OR)n + x NR
HOR
HOR
(RO)n-x (M
OR
OR
NR)x
+ 2nROH
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
27
Reactions with Oximes and Hydroxylamines
The reaction of metal alkoxides with oximes and Hydroxylamines provides many
different routes for synthesis of variety of derivatives of Boron aluminium tin
titanium silicon etc
i) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
ii) M(OR)n + xR2C=NOH M(OR)n-x(ON=CR2)x + xROH
Meerwein-Ponndorf-Verley Reaction
Metal alkoxides catalyzed the reduction of ketones by alcohols of which aluminium
alkoxides are the best The reaction completes by the removal of the volatile ketone
formed
Me2HC-OH + R2C=O Me2C=O + RHC-OH
Thermal Decomposition of Alkoxides
Metal alkoxides decompose on heating to the metal oxides hydroxides or to the metal
itself with the evolution of organic species The mode of decomposition depends not
only on the alkoxide but on the conditions of the pyrolysis
Uses of Alkoxides
Alkoxides are moisture-sensitive and require special handling techniques but this
property does not restrict their uses in various fields They have many industrial
applications A brief summary of some of these is given below
Catalysts
The alkoxides are used as catalysts in the Meerwein-Ponndorf-Verley reaction and in
ring opening polymerizations However there are other systems catalysed by
alkoxides Ziegler-Natta polymerisations147
trans esterifications148-151
and polyester
formation152-154
Polymer Cross-Linking Agents
Many alkoxides have the ability of to promote cross-linking which makes them useful
in a variety of ways For example titanium and zirconium alkoxides may be used in
films where rapid drying is required155
while aluminium titanium and zirconium
alkoxides may be used in conjunction with silicones in the waterproofing of leather
where it is believed that the alkoxides promote the curing of the silicone156
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
28
Precursors to Metal Oxides (Glasses and Ceramics)
Metal alkoxides are very susceptibility to hydrolysis This property has led to a major
use of alkoxides the formation of high purity metal oxides by the pyrolysis of the
hydroxides formed on the controlled hydrolysis of alkoxides157
The metal alkoxides
are readily purified by distillation under reduced pressure or by recrystallisation so the
oxides produced are free from impurity
Heterometallic Oxo-alkoxides
Preparation by ester elimination reactions
This method has general applicability for synthesis of heterobimetallicalkoxidesof a
number of metals158
and even of organometallic moieties159
as illustrated by the
following equations
M(OAc)2 + Al(OPri)3
Xylene
RefluxM(OAc)OAl(OPri)2 + 2PriOAc
The solvent like pyridine play the role of a coordination leading to ligand exchange
rather than ester elimination reactions between Sn(OBut)4 and Sn(OAc)4 Me3Si(OAc)
In hydrocarbon solvents (eg toluene) contrary to the ester elimination reaction
occurring generally in such systems158-162
formation of an addition product Nb2Cd(micro-
OAc)2(micro OPri)4(micro OPr
i)6has been reported
163 from Nb(OPr
i)5 and Cd(OAc)2
Cd(OAc)2 + 2Nb(OPri)5 CdNb2(OAc)2(OPri)10
Condensation Reactions between oxo- and normal metal alkoxides
The commonly utilized route for bimetallic alkoxides synthesis heterometallic oxo-
alkoxides is synthesis by the condensation of component alkoxides and oxo-alkoxide
In view of the importance of Y-Ba precursors for 123 superconductors a novel
barium yttrium oxo-alkoxide [Y4Ba2(micro6-O)( micro3-OEt)(dpm)6] has been synthesized164
by the following reaction
[Y4Ba2(micro6-O)( micro3-OEt)(dpm)6]Y5O(OPri)13 + Ba + PriOH + EtOH
(dpm= ButC(O)CHC(O)Bu
t)
Reactions between Metal Halides and Alkali Alkoxo- metallates
This type of reaction which has been utilised extensively for synthesis of
heterometallic normal alkoxides has been reported for heterometaloxo-alkoxides The
reaction between SmI and NaTi(OPrl) yields165
[Sm4Ti(micro5-O)(micro3-OPri)2(micro-
OPri)6(OPr
i)6] which could also be isolated by the reaction between Sm5O(OPr
i)3 and
Ti(OPri)4
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
29
Similarly the reaction at room temperature between ZnI2 and KTa(OPri)6 (12
stoichiometry) yields ZnTa202(OPri)8
166 Another interesting micro-oxo-centered iron
heterometal methoxide derivative Na2Fe6O(OMe)186MeOH has been synthesized by
the reaction between iron(III) chloride and sodium methoxide
Na2Fe6O(OMe) 186MeOH+ 6MeOH + Me2O6FeCl3 + 20 NaOMe
Other Methods for Synthesis of Heterometallic oxoalkoxide Derivatives
In addition to the condensation of a metal oxo-alkoxide with the alkoxide of another
metal the interaction of normal alkoxides of two metals also under some conditions
yields a heterometaloxo-alkoxide eg
Fe(acac)3 + 3Zr(OPrn)4Zr3Fe(O)(OPrn)10(acac)3
Decomposition method
At high temperatures volatile thermolysis products of alkoxides can contain alcohols
ethers aldehydes saturated and unsaturated hydrocarbons etc The structures of
crystalline products provide evidence for condensation attendant on this process
Copper oxosilane oxide [Cu18O2(OSiMe3)14] was prepared by vacuum distillation of
CuOSiMe3 Evidently the process is accompanied by destruction followed by
condensation of the resulting fragments Decomposition of W4(OPri)10 to
[WIII
4O2(OPri)8]2 occurs with elimination of propane
167 Thermolysis of bimetallic
isopropoxides Sb(OPri)4 afforded crystalline [K2Sb2O(OPr
i)6]2 and Pr
i2O Refluxing
of toluene solutions of KMIII
(OBut)4 (M
III=Sb Bi) over a long period of time resulted
in elimination of isobutylene and afforded the oxo complexes [K4MIII
2O(OBut)8] It
should be noted that Na-containing compounds with the same composition are
generated already in the step of the reaction of NaOBut with M
III(OBu
t)3
168
If decomposition of alkoxides occurs at rather low temperatures the reaction gives
ethers as the major products For instance thermolysis of methoxides Al(OMe)3
Pb(OMe)2 and NaAl(OMe)4 (at ~120 oC) afforded dimethyl ether as the only gaseous
product169- 171
Heating of an alcoholic solution of Ti(OEt)4 in an autoclave at 100 oC
led to crystallisation of Ti16O16(OEt)32 and elimination of Et2O172 173
Condensation with elimination of ethers proved to be one of the main pathways of
spontaneous decomposition of alkoxides
M-OR + RO-M M-O-M + R2O
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
30
The formation of oxo bridges is undoubtedly thermodynamically favourable and in
the case under consideration is analogous to ageing of oxide hydrates accompanied
by condensation of two hydroxy groups with elimination of a water molecule This
type of reaction174
was used for the preparation of oxo compounds by
transesterification of niobium ethoxide with tert-butyl alcohol
Nb(OEt)5 + ButOH Nb(OBut)3 + Nb2O(OBut)8 + But2O + EtOH
The proposed mechanism involves the heterolytic cleavage of the O-R bond followed
by the attack of the resulting carbocation on the M-O bond of another OR group
Ethers (like other volatile decomposition products for example unsaturated
hydrocarbons) are not always detectable against the background of alcohols175
Decomposition of bimetallic alkoxides to oxoalkoxometallates MmMrsquonOp(OR)q
containing heterometallic M-O-Mrsquo bridges is of most importance among the reactions
under consideration
Sn(OR)4 + Cd(OAc)2 Cd4Sn4O2(OR)10(OAc)10 + AcOR
R=CH2But
Since such complexes readily eliminate ester they were proposed as precursors in the
synthesis of complex oxides from the gaseous phase (CVD method)
Applications of Mixed-Metal oxides
Heterometallic oxides have a wide range of applications in electronics optics
magnetism catalysis biomedical and environmental issues Some important
examples are mentioned here
Lead titanate (PbTiO3) has pyroelectric and piezoelectric properties due to its
ferroelectric nature This is used in pyro-detectors and acoustic transducers
In capacitors and sensors Barium titanate (BaTiO3) is used as it is dielectric
material
(LiNbO3Ti) has electro-optic properties and is used in second harmonic
generation wave-guide devices and optical modulators
[K (TaNb)O3] is also a pyroelectric electro-optic material and has applications in
pyrodetectors wave guide devices and frequency doublers130
In semiconductor devices Magnesia aluminate (MgAl2O4) used as coating on
silicon
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
31
Yttrium-barium-copper oxide (YBa2Cu3O7) a high temperature super conductor
has some commercial applications176
Lead zirconate titanate [Pb(ZrTi)O3 PZT] and lead lanthanum zirconate
titanate [(PbLa)(ZrTi)O3 PLZT] have many applications They are used in
pyrodetectors non-volatile memory surface acoustic wave devices wave-guide
devices optical memory display due to their dielectric pyroelectric piezoelectric
and electro optic properties130
Ba2Cu3O5+x and CuO have been employed as catalysts for CO oxidation one of
the most important reactions in air pollution control processes177178
Nickel-cobalt catalyst is useful for hydrogen or synthesis gas production through
the partial oxidation of methane179
Cu and ZnO-based catalysts are used for large-scale industrial synthesis of
methanol from COCO2H2180
New high-temperature superconductors eg REBa2Cu3O7 (where RE = rare
earth) play a key role in various technological applications181
Among the common oxide precursors such as metal β -diketonates M(β-dik)n
carboxylates and alkoxides the latter are the most versatile for customizing properties
at a molecular level and conversion into extended arrays31581
Rational design of
precursors and optimization of the ligand requires a knowledge of the relationships
between the properties of the materials and of their precursors130149
which should thus
be structurally well defined
Metal β-diketonates
β-diketonate chelating system with six membered metal containing ring is the most
commonly used ligand in the coordination chemistry182183
(A B C = CR where R = H Alk Ar Het n = oxidation state of metal)184
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
32
β-diketonates have coordination capabilities along with the formation of chelates
(intra complex compounds) The possible modes of O- and O O
- coordination in
mono di and polynuclear β-diketonates shown in following structures (Figure 11)
β-Diketonates have been used as chelating ligands for almost 120 years184
Metal β-
diketonates [M (RCOCHCORprime)n]m are mostly used in material science due to their
high volatility They are mostly monomeric due to chelating behaviour of the ligand
but association take place for divalent and large elements such as alkaline earth
metals185
In Cancer treatment especially β-diketonate complexes of titanium antitumor agents
are a promising replacement for the platinum heavy metal complex cisplatin186187
β-
Diketonate supported metal-alkoxide aryloxide and halogenate complexes are easily
synthesized from available commercial metal precursors utilizing reliable and
reproducible syntheses which are important considerations from an industrial view
point
Here (R1 R2 R3) = H alkyl aryl (M M
1 M
2) = different metal atoms and m =
oxidation state of metal
Figure 11 Some O- and O O
- coordination modes of β-diketonates
The β-diketones or 1 3-diketones bear two carbonyl groups that are separated by one
carbon atom This carbon atom is the α-carbon In most β-diketones the substituents
on the α- carbon are hydrogen atoms The substituent on the carbonyl function can be
an alkyl group a fluorinated alkyl group an aromatic or a heteroaromatic group The
parent and most common 1 3-diketone is acetylacetone (Hacac) which is prepared by
the reaction of acetone and acetic anhydride with the addition of BF3 catalyst (Figure
12) were the substituents on both carbonyl groups are methyl groups
Various different β-diketones can be considered as derived from acetylacetone by
substitution of the CH3 groups by other groups and therefore they are well-known
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
33
chelating ligands mostly available commercially at relatively low cost Examples of
other common β-diketones are benzoylacetone (Hbzac) benzoyltrifluoroacetone
(Hbtfac) dibenzoylmethane (Hdbm) hexafluoroacetylacetone (Hhfac) 2-thenoyl
trifluoroacetone (Htta) 2266-tetramethyl-35-heptanedione (Hthd) and 6677888-
heptafluoro-22-dimethyl-35-octanedione (Hfod)
O
+O
O O OH O
Figure12 Preparation of acetylacetone
Exchange of ligand is a common method to coordinate β-diketonate ligands to the
metal center resulting in the formation of complexes with many transition metals
where both oxygen atoms bind to the metal
β-diketonates undergo keto-enol tautomerism 188
(Figure 13) These tautomers are in
equilibrium with each other and structurally they show a cis configuration (enol) and
a syn (cisoid) conformation (keto)
O O
R R
O OH
RR
O OH
RR
Keto form enol forms
Figure 13 keto-enol tautomerism
The amount of keto and enol form can be determined by integration of the keto and
the enol resonance peaks in the 1H NMR spectrum The position of the ketondashenol
equilibrium depends on a various factors such as the substituents on the β-dicarbonyl
system the solvent the temperature and the presence of other species in solution that
are capable of forming hydrogen bonds The presence of an alkyl substituent on the α-
carbon decreases the amount of enol form Bulky alkyl groups such as the isopropyl
group or the sec-butyl group reduces the amount of enol form to almost 0The
presence of a methyl group in the α-position depresses the amount of enol form in
other β-diketones than acetylacetone For example presence of a methyl group in the
α-position of benzoylacetone reduces the amount of enol form from 98 in pure
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
34
benzoylacetone to 4 in the methyl-substituted benzoylacetone During
deprotonation of the β-diketone the proton is removed from the α-carbon (if the β-
diketone is in the keto form) or from the alcohol group (if the β-diketone is in the enol
form) β-diketone acidity depends on the substituents Electron-withdrawing groups
increase the acidity whereas electron-donating groups decrease it Because of the
presence of the two carbonyl groups the proton on the α-carbon is quite acidic and
relatively weak bases can remove it Ammonia sodium hydroxide piperidine and
pyridine are some examples of bases that are used for deprotonation of β-diketones
are A much stronger base is required for removing second proton
The enolic hydrogen atom of the β-diketonate can be replaced by a metal cation to
give a six-membered chelate ring shifting the keto-enol equilibrium towards the
enolate form (Figure 14)189
O O
RR
M
Figure 14 Six-membered chelate ring
β-Diketonate chelates are synthesized by the reaction of ligand with metal salts in
water organic solvents or in solvent mixture β-Diketonate ligand replaces the
ligands of metal salts For example
TiCl4 + 4K(ligand) Ti(ligand)4 + 4KCl
The direct syntheses of metal β-diketonates may be carried out from a number of
starting reagents ie M MOx M(CO3)x MHx Metal alkoxides will undergo
exchange reactions in a simple stoichiometric ratio This synthetic route has
advantages over direct methods in the sense of isolation of very pure materials if
enough care is taken in the preparation of the starting metal oxides (ie the use of
anhydrous oxygen free solvents and rigorous handling techniques) 190
Ti(OPri)4 + n(-dik)Hexane
[Ti(OPri)4-n(-dik)n]x + nHOPri
Metal β-diketonate complexes are attractive and extensively used precursors in oxide
MOCVD due to their high volatility The volatility of β-diketonate complexes be
increased by increasing the steric bulk of the R group
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
35
Structure of Titanium β-Diketonates
Over the fifty years reaction between a tetraalkoxy titanium and β-diketones has been
known The initial studies191192
failed to isolate pure compounds or to provide
convincing analytical data Yamamoto and Kambara 193
in 1957 on basis of IR
spectroscopy and cryoscopy first isolated and predicted structures of titanium β-
diketonate complexes for the ethoxide and n-propoxide derivatives (Figure 15) They
described the octahedral coordination around the titanium metal centers
O
O
H3C
R
Ti(OR)3
O
O
H3C
R
Ti
OR
OR
O
O
R
CH3
R = CH3OC2H5
R = C2H5 n-C3H7-C4H8
Figure 15 Structures (proposed) by Yamamoto and Kambara (11 and 12 ratio)
Mehrotra and co-workers153-155
later prepared the chloro and a wider range of alkoxy
derivatives However it remained unclear whether the complexes had cis-substituted
or trans-substituted structures with respect to the metal center In separate studies
Bradley194
and Fay195196
rejected the possibility of the trans configuration in favour of
cis based on variable temperature 1H NMR and IR spectroscopy studies They
observed a splitting of the acetyl acetonate (acac) methyl proton resonance into a
doublet at low temperatures for several homologous titanium compounds
Ti(acac)2(OR)2 which they explained as having a cis configuration where the two
methyls have magnetically inequivalent positions (eg Figure 16 where R = Rrsquo = Me)
In 1993 Keppler and co-workers197
proposed that solution NMR data and crystal
structures of known bis(BDK) titanium(IV) complexes (BDK = β-diketonate)
indicates that an equilibrium mixture of three cis isomers in solution is obtained as
shown below
M
O
O
XO
XO
R
R
R
R
cis-cis-cis(C1) cis-cis-trans(C2) cis-trans-cis(C2)
M
O
O
XO
XO
R
R
R
R
M
O
O
XO
XO
R
R
R
R
Figure 16 Isomers in solution for cis-[Ti(BDK)2X2]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
36
Thus it is believed that the cis configurations are more strained as compared to trans
But still cis preferred by electronic effects due to the significance of π-bonding (pπ
oxygen rarr dπ metal) 197198
as all three d orbitals of titanium would participate in the
cis complex whereas only two d orbitals would be involved in the trans complex
Furthermore β-diketonates are bonded more efficiently to the metal center than the X
groups (usually oxo alkoxo aryloxo or halogenato ligands) and therefore they are
the trans-directing group
In monomeric structures of titanium β-diketonate complexes significant distortion
from the ideal octahedral geometry indicates that the distances between titanium
metal and the oxygen atoms in β-diketonate chelates of titanium (IV) are usually not
symmetrical For example the cis-[Ti(BDK)2(OR)2] complexes show relatively short
Ti-OR bonds (18 Aring) and longer TiO(BDK) bonds with Ti-O distances trans to OR
distinctly longer than the bonds cis to OR (206 vs 200 Aring) 199
In the reaction of titanium alkoxides with β-diketonates due to a preferred
coordination number of six for titanium188
the third or fourth alkoxy groups are not
replaced and bis- β-diketonate derivatives were always obtained even if excess of
these chelating ligands was used
The first crystal structure of a mixed acetylacetonearyloxide complex of titanium
(Figure17) was synthesised by Bird and co-workers200
who observed that the
phenoxide ligands were in a cis position same was observed for mixed acetyl-
acetonealkoxide complexes
Figure17 Molecular structure of C34H48O6Tin-bis-(24-pentanedionato)
bis(26diisopropylphenoxo)titanium(IV)200
Brown et al201
in 2005 published two more mixed β-diketonatearyloxide complexes
of titanium using BINOL(11-Bi-2-naphthol) as the aryloxide ligand and
dibenzoylmethane(DBM) and (DMHD) Dimethyl-heptandionate They studied the
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
37
electronic dissymmetry of these compounds by DFT calculations and showed that a
chiral electronic structure can exist even in a symmetrical fragment such as
bis(diketonate)titanium(IV)
Serpone et al202
in 1972 first resolved monosubstituted compounds [Ti(BDK)(Hal)3]
The compound was surprisingly a μ2-Cl bridged dimer as shown in Figure 18
Figure18 Structure of [Ti(acac)Cl3]2
Schiff bases
Schiff base was first reported by Hugo Schiff in 1864203
Schiff base metal complexes
have been studied extensively because of their attractive chemical and physical
properties and their wide range of applications in numerous scientific areas Ligand a
metal surrounded by a cluster of ions or molecule is used for the preparation of the
complex compounds named as Schiff base which are condensation product of
primary amine and aldehyde or ketone The speciality of Schiff base is that many
kinds of amine can be chosen to react with aldehyde or ketone to get the ligand with
different structures as well as some variable properties
The findings of structural studies are interesting in that the Schiff base ligands can
control the stereochemistry of the complex and provide us with numerous examples of
unusual geometries about the central metal ion Therefore they can serve to illustrate
the coordination flexibility of these ions
Schiff bases play an important role as ligands in metal coordination chemistry even
after almost a century since their discovery Modern chemists still prepare Schiff
bases and nowadays active and well-designed Schiff base ligands are considered as
ldquoprivileged ligandsrdquo Schiff bases are important class of ligands due to their synthetic
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
38
flexibility their selectivity and sensitivity towards the central metal atom structural
similarities with natural biological substances and also due to the presence of the
imine group (N=Clt) which imparts in elucidating the mechanism of transformation
and rasemination reaction in biological system
Schiff bases can be prepared by condensing carbonyl compounds and amines in
different conditions and in different solvents with the elimination of water molecules
A Schiff base is a nitrogen analog of an aldehyde or ketone in which the C=O group is
replaced by C=N-R group It is formed by condensation of an aldehyde or ketone with
a primary amine according to the following scheme
R NH2
Primary amine
R C R
O
Aldehyde or ketone
+ C + H2O
R
R
N R
Schiff base
The common structural feature of these compounds is the azomethine group with a
general formula RHC=N-R where R may be alkyl aryl cyclo alkyl or heterocyclic
groups which may be variously substituted
Schiff bases that contain aryl substituents are substantially more stable and more
readily synthesized as compared to those which contain alkyl substituents Schiff
bases of aliphatic aldehydes are relatively unstable and readily polymerizable while
those of aromatic aldehydes having effective conjugation are more stable
The formation of a Schiff base from an aldehydes or ketones is a reversible reaction
and generally takes place under acid or base catalysis or upon heating
R C R
O
+ R NH2
R C
OH
NHR
R
Aldehydeor ketone
Primaryamine
Carbinolamine
R C R
NR
+ H2O
N-substituted imine
Water
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
39
The formation generally moves to the completion by separation of the product or
removal of water or both By aqueous acid or base many Schiff bases can be
hydrolyzed back to their aldehydes or ketones and amines
The presence of a dehydrating agent normally favours the formation of Schiff bases
Though the Schiff bases are stable solids care should be taken in the purification
steps as it undergoes degradation Excellent chelating ability and considerable
chemical importance of Schiff bases is due to presence of a lone pair of electrons in
sp2 hybridised orbital of nitrogen atom of the azomethine group Examples of a few
compounds are given in Figure 19 This chelating ability of the Schiff bases combined
with the ease of preparation and flexibility in varying the chemical environment about
the C=N group makes it an interesting ligand in coordination chemistry
NN
HH
NH HN
NH2N
H
HN
NN-bis(pyrrole-2-carboxalidene)-12-diaminobenzene
N-pyrrole-2-carboxalidene-12-diaminobenzene
NHS
H
HO
NH2N
H
HO
N-salicylidene-2-aminothiophenol N-salicylidene-12-diaminobenzene
NH2
NN NHO
H
HO
N-salicylidene-2-aminophenolN-pyridine-2-carboxalidene-11-binaphthyl-22-diamine
Figure 19 Some examples of Schiff bases
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
40
Treating metal salts with Schiff base ligands under suitable experimental conditions
generally prepare metal complexes of the Schiff bases However for some catalytic
application the Schiff base metal complexes are prepared in situ in the reaction
system Cozzi204
in his review has outlined five synthetic routes that are commonly
used for the preparation of Schiff base metal complexes and are depicted as shown
below
R1
R1
OYN
N
R2
R2 OY
R1
R1
1) Y=H M(OR)n
2) Y=H M(NR2)n
3) Y=H MRn
4) Y=H M(OAc)n
5) Y=NaK MXn
R=AlkylAryl
X=ClBr
R1
R1
ON
N
R2
R2 O
R1
R1
M Xn-2
Figure 20 Preparation of Schiff base complexes
The use of metal alkoxides (M(OR)n) is shown in method 1 Alkoxides of early
transition metals (M = Ti Zr) are commercially available and easy to handle In the
case of highly moisture-sensitive derivatives of lanthanides the use of other alkoxide
derivatives is not easy Metal amides M(NMe2)4 (M = Ti Zr) are also employed as the
precursors in the preparation of Schiff base metal complexes (method 2) The reaction
occurs via the elimination of the acidic phenolic proton of the Schiff bases through the
formation of volatile NHMe2
Other synthetic routes include reaction of metal alkyl complexes with Schiff bases
(method 3) or reaction of the Schiff base with the corresponding metal acetate under
reflux conditions (method 4) The synthetic scheme presented in method 5 consists of
a two-step reaction involving the deprotonation of the Schiff bases followed by
reaction with metal halides
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Chapter-1 Introduction
41
SCOPE OF THE PRESENT INVESTIGATIONS
Literature review has revealed that there has been ever growing interest in the field of
metal alkoxides and their derivatives with different types of ligands and their
application in various fields Hence it was considered worthwhile to synthesize some
new heterometallic micro-oxo compounds and carry out their reactions with different
ligands such as β-diketones Salicylates Cycloalcohols and Schiff bases in order to
get an insight its structural features The compounds and there derivatives have been
synthesized and characterized on the basis of elemental analysis infrared 1H NMR
13C NMR and Mass spectral studies
The work in the thesis has been broadly classified into the following sections
1 Synthesis of Heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-iso propoxide micro-oxo
n-propoxide [SnO2TiAl(OPri)2(OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Schiff bases derivatives of heterotrimetallic [Al(III)Sn(II)Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
Cycloalcoholic derivatives of heterotrimetallic [Al(III)Sn(II) Ti(IV)]-micro-oxo-
isopropoxide micro-oxo n-propoxide [SnO2TiAl(OPri)2(OPr
n)3]
2 Synthesis of Heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-oxo-isopropoxide
micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2 (OPr
n)3] and its derivatives
β-diketone derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV)Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-prop- oxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Schiff base derivatives of heterotrimetallic dibutyl [Al(III) Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]
Salicylate derivatives of heterotrimetallic dibutyl [Al(III)Sn(IV) Ti(IV)]-micro-
oxo-isopropoxide micro-oxo n-propoxide [Bu2SnO2TiAl(OPri)2(OPr
n)3]