Chapter 2

Polymer Supported Metal Complexes and Mixed Ligand Complexes: An Overview

T he Chem~stn, and technology of polymer supports and polymer supported

react~ons has been a subject of intense investigation during the last three

decades. The polymer supported strategy is widely used in synthetic organic and

Inorganic chem~stry to overcome some of the serious limitations of the reactions in

the homogeneous phase. The concept of the use of insoluble crosslinked polymers

as supports for reactlve functional species has reduced the efforts of the usual

reactlon work-up cons~derably. For this and many other reasons, the polymer-

supported strategy has been extended to innumerable multistep synthesis in the

field of chem~stry and other related areas.

The molecular structural features of the polymeric support material have

been proved to contribute significantly to the reactivity of the attached functions.

The nature of the Interaction between the polymeric backbone and the reactive

specles IS e~ther phys~cal or chemical and therefore the support itself creates a

specific microenv~ronment for the anchored group and control the reactivity. It is

then no longer possible to consider the support as a rigid and inert material. Thus

the support materlal can play an active role in the chemistry of transition metal

complexes also. Mlxed ligand complexes deserve special attention because the

rlgld polymeric support materlal can offer stability to these systems in a dramatic

way

2.1 Functionalised polymers and their metal complexes: Synthetic and

characterisational aspects

In 1963, Merrifield introduced the concept of solid phase peptide synthesis,

after which solid polymer supports were used extensively in other areas of

chemistry ' The advantages of solid phase reactions are the operational simplicity,

possibility of us~ng one of the reagents in excess, and the ease of purification.

Polymer supports have gained wide application not only in solid phase peptide'

synthesis but also in different areas like immobilisation of enzymes, biomolecules,

catalysts, reagents and in metal ion ~ e ~ a r a t i o n . ~ . ~

The use of polymers as supports for chelates and catalysts has increased

tremendously nowadays. The polymer support should be functionalised before

exploiting them for chemical processes like peptide synthesis, catalysis, chelation

or metal ion separation. Functionalisation involves the incorporation of a functional

group to the polymer support.

Chloromethyl polystyrene crosslinked with 1-2% divinyl benzene is the

most commonly used support. Functionalisation of styrene polymers primarily

~nvolves electrophilic substitution on the aromatic ring. Chloromethylation has

been the most widely used r ea~ t ion .~ Chloromethylation of styrene polymers is

carried out uslng a Lewls acid catalyst and chloromethylmethylether as the solvent.

In addition to their direct use, chloromethyl groups can be readily modified into

other functional g r ~ u ~ s . ~ . ~ Different functional groups may be directly introduced

into the polystyrene support by well known reaction

Several ligand functions were anchored to polymer support by polymer

analogous reactions to get polymer supported ligands. These polymer supported

complexing agents find wide application in metal ion ~e~ara : ion , '~ . ' ~

preconcentration and recovery of trace metal catalysis,16~" organic

nuclear chemistry,19 water and waste water treatrnent,20,z1 pollution

22 control and ~ndustr~al processes, hydrometallurgy,23.24 polymer drug and

var~ous m~scellaneous appl~cat~ons in analytical chem~stry

A polymer-metal complex IS composed of synthet~c polymer and metal ~ o n s

Many synthet~c polymer metal complexes have been found to e x h ~ b ~ t h ~ g h catalyt~c

eficlency I h Metallo-enzyme 1s a k ~ n d of polymer metal complex present in nature

where metal Ions are surrounded by g~an t proteln molecule l 6 A wwlde var~ety of

~nkestlgatlons have been carr~ed out on polymer metal complexes, these mclude

stud~es of thermal conduct~k~ty, thermodynam~c stab~l~ty, redox reacttons, collect~on

of metal Ions, b~omed~cal effects, etc

Even though polymer~c ligands and their metal complexes were used

extcns~vely tbr var~ous applicat~ons, the co-ordination structure around the metal

Ion was not studled properly A detailed knowledge of the co-ordination geometry

around the metal ion 1s of great importance for preparing metal ion specific

on-exchange resins and for following the mechanism of catalysis.

2.11 Polymeric ligands

Polymers w ~ t h nitrogen, oxygen, sulphur and phosphorous as hetero atoms

are commonly found as monodentate or bidentate ligands. The specificity, or

selectivity of a polymer supported ligand is usually correlated to that of the

monomeric compound with the same functionality. The preparation,

characterisat~on and use of polymer supported ligands are reviewed briefly in the

following sections.

(u) Prepurution of polymer supported ligands

Polymer matrix

Fundamentally two types of polymeric matrices can be considered, i.e.,

Inorganic or organic. Silica 1s the most extensively used inorganic support.'6

Among the naturally occurring polymers used as supports, cellulose has been used

most extens~vely. The chem~stry of cellulose as a polymeric support, its ion

exchange properties and the use of chelating cellulose for preconcentration of trace

elements, etc . have been rev~ewed by Knapp and ~ ieser* ' Among the synthetic

organlc rnatrlces two types of polymers can be considered i.e. condensation and

add~tion polymers wh~ch are extensively used as supports or matrices for chelating

ton exchangers. The synthesis and properties of these polymers have been

described earher 2X.2" A polymer support with desired mechanical stability,

swellability, hydrophilic-hydrophobic equilibrium, etc. can be constructed by

properly selecting the monomer systems, and by adjusting the monomer-crosslinker

ratio.

incorporation of ligand,

Two approaches exist for the preparation of functional polymers, namely the

polymerisation or copolymerisation of monomers which carry the desired

functionalityJu and the chemical modification of the preformed polymer.31 The

tormer is the more d~rect approach and many functional linear polymers can be

prepared without difficulty by free radical, anionic, cationic, co-ordination or group

transfer polymer~sation However for most purposes crosslinked polymers are

more attractive than linear polymers. The preparation of crosslinked polymers in a

good physical form is most readily achieved by suspension polymerisation.5

The alternat~ve to direct copolymerisation for the preparation of

functionalised polymers is the chemical modification of preformed polymers.31

'This method 1s preferred to others in view of the fact that the degree of

functionalisat~on can be controlled by varying the amount of crosslinking agents

and the extent of modification in preformed matrices.

The method lnvolvlng copolymer~sat~on of monomers contrunlng the des~red

funct~onal~ty 1s wtdely used to produce Ion exchange and complex~ng sorbents

Vlnyl monomers are usually used and the synthes~s lnvolves the polymens&t~on of

vlnyl compounds contalnlng chelating groups like pyridine, 8-hydroxyquinoline,

~m~dazole, carboxylic acid etc. with divinyl ~ o m ~ o u n d s . ' ~ . ~ ~ Yeh el a/. reported the

preparztlon of polystyrene based acetyl acetone by direct emulsion polymerisation

of a vlnyl benzyl acetyl acetone with styrene uslng the conventional emulsion

system or by bulk polymerisat~on using azobisisobutyronitrile as solvent and by

irradiating the monomer by UV radiatiom3' This method of synthesis makes it

poss~ble to obtaln sorbents of high capaclty and a uniform structure of the polymer

Modlficat~on of the preformed polymer is the easiest method for the

synthes~s ot' a w ~ d e varlety of macromolecular ligand systems. This method is

generally used for the functionalisation of a polymer matrix. The required ligand

function IS Introduced on to the polymer matrix by simple polymer analogous

organlc reactions Im~dazole supported on styrene divinyl benzene copolymers can

be prepared from chloromethylated styrene DVB copolymers and the sodium or

lithlum salts of ~m~dazo le using dimethyl formamide or tetrahydrofuran as

solvent ""'"e b~dentate ligand 2,2'-bipyridine has successfully incorporated into

polystyrene by Card and ~ e c k e r s ' ~ according to Scheme 2.1.

Scheme 2.1

Shambu rr 01.'' syntheslsed several chelatiny resins contiuning polyamine

funct~onal groups by the reaction between polyethylene polyamines and

chloromethylated DVB-crosslinked polystyrene resins using pyridine as readtion

medium. Drago el 01.'' described the strategy for covalently attaching multidentate

chelating Ilgands to polystyrene matrix. Polymeric substrates containing

polydentate amlnes can be obtained by reacting chloro or iodomethylated

polystyrene w~th bis (cyanoethylamine) followed by BF,-THF reduction.

Preparation of polymer attached bis(3-aminopropyl) m i n e is depicted in

Scheme 2.2.

Scheme 2.2

Sugi~ etal. prepared two macroreticular polystyrene resins containing

8-hvdroxy qurnolinic The resin with an oxine group attached to

polystyrene vra a throureido linkage was prepared by treating arninopolystyrene

with 8-hydroxyqu~nolyl isothiocyanate in a mixed solution of triethyl amine and

dioxane. Reactlon of chloromethyl polystyrene resin and zinc oxine chelate in

nitrobenzene produces a resrn in which 8-hydroxyquinoline is directly attached to

methylene group of the polymer matrix. 8-Hydroxy quinoline immobilised on

s~llca gel was prepared by Sturgeon el and was used for the preconcentration

of metals.

Bhadun e l ul. ~ncorporzted acetyl acetone into a styrene divinyl benzene

(8%) copolymer by chloromethylation and treatment with acetyl acetone in the

presence of a catalytic amount of sod~um ethoxide in a swell~ng solvent such as

tetra hydrofuran "

Bhadurl and ~ h w a j a ' ~ synthesised some polymer supported dithiocarbamate

ligand from chloromethylated styrene divinyl benzene copolymers (8%) using the

sequence of reactions given below.

Scheme 2.3

Polycondensatlon 1s another important method for synthesising polymers

bearing chelating llgands This method involves the copolymerisation of certain

ligand molecule w ~ t h phenols and aldehydes. For example, anthranilic acid,43

;mthranillc acid dlacetic ac~d,'" m-phenylene diamine," diglycine, m-phenylene

t i i t i~~i~ne tetra acettc acid etc" can be copolymerised with formaldehyde and

rnono or polvphenols to synthesise condensation chelating polymers. Unicellex

llR-50 1s a chelating Ion exchange resin prepared by the copolymerisation of

N-(0-hydroxybennl) lmldodiacetlc acid with phenol and forma~dehyde.'~

Recently Patel rt a1 prepared a polymer by condensation of 2-hydroxy 4-methoxy

acetophenone and 1.4-butane d ~ o l "'

(b) Choructerkation of polymeric lignnds

Chemical methods can be used for the determination of functional groups in

polymers. Chemlcal reactlon based analytical methods are generally slow when

compared with the physical methods that m~ght be used to determine functionalitles

in polymers

The determ~nat~on of functional groups by chemical methods includes

elemental analysis and reactions of functional groups. If the polymers conhn

functional groups of some specific element the polymers can be analysed for the

particular element and degree of functionalisation can be determined from the

percentage of the element.

Methods based on the reactions of functional groups are widely accepted for

the characterisat~on of polymeric ligating systems with sufficient accuracy. When

the functionalised polymer contain acid or basic groups acid-base titration method

are suitable for functional group dete~mination.~~ The free amino acid in a

polymer4' was determined by titration with acid. Histidine incorporation in a resin

pept~de can be determined by potentiometric titration with perchloric acid."

Acylation methods are used for the determination of amino and hydroxyl groups in

polymers.5'

IJh.y.sical and physrc~chrrnrcal methods

With su~table modifications many physical methods can be adopted for the

analysis of functional groups in solid insoluble polymers IR spectra prov~des an

excellent tool for characteris~ng funct~onallsed polymers 52 IR spectra of functional

groups anchored on polymers do not dlffer appreciably from those in small

molecules NMR spectroscopy has also been used to study polymeric compounds i 13 I9 1 . C and F NMR spectroscopy were used In monitonny solid phase

5 3 I I reactions C NMR spectroscopy 1s used nowadays as a powerful spectroscop~c

method for the study of crossl~nked polymers j4 The cross polar~sation (CP) and

magc angle splnning (MAS) techniques are appl~c-d for the rapid srannlng of "C

spectra T h ~ s glves a clearer plcture of the h~ghly complex carbon skeleton of the

polymer network

2.1.2 Polymer metal complexes

The synthes~s ot a polymer metal complex represents an attempt to glve an

organic polymer with lnorganlc funct~ons The properties of the lnorganlc molety

are greatly controlled by the polymer support and the polymer propert~es are

modlfied by the lnorganlc funct~ons The following sectlon deds w ~ t h the synthes~s,

character~sat~on and appl~cat~on of polymer metal complexes

(a) Synthesis of polymer metal compleres

When a polymer~c l~gand is mixed with a metal salt or metal complex under

suitable reaction condit~ons, a polymer metal complex is formed. A polymer metal

complex 1s also obtruned bv polymerising a low molecular weight metal complex.

Cbmplexatron ~Spolynterrc lrgund wrth metal ion

This type of complex formation is achieved by mixing a polymer containing

ligand funct~ons like amine, ketone, dithiocarbamate, carboxyljc acid, thiol, schiff

base etc w~th metal Ion or metal complex solution. The reaction usually resulted in

various types of co-ordinat~on structures like pendant, inter and intramolecular

bridged complexes

I Pendant complexes

When a metal on or metal colnplex has only one labile ligand which is

easilv subst~tuted bv a p o l p e n c ligand and when other co-ordination sites are

subst~tut~on inactive. a monodentate pendant complex is formed.

L L L L L L L L L L

This type of polymer metal complex is very often soluble in water or in

organlc solvents slnce it contains only very few bridged structures. Kaneko and

~such ida '~ tried the reaction of poly(4-vinyl pyridine), PVP, with various metal

chelates and succeeded in obtaining polymer metal complexes containing only

simple structures of monodentate type. The polymer complex cis

[Co(en)zPVP.CI]C12 was prepared by mixing an ethanolic solution of PVP with an

aqueous solution of Co(1ll) chelates and heating at 80°C for 2-6 h. The solution

was filtered and the filtrate was dialysed in cold water. After the water was

evaporated thin films of reddish violet PVP complex was obtained.

A polymer~c ligand having polydentate structure will form polydentate

pendant complexes.

High stability IS expected for the metal complex in this case. The best known

example is the crossl~nked polystyrene containing iminodiacetic acid type pendant

groups. It has been espec~ally used as a metal binding resin, showing excellent

ability to adsorb metal ions selectively.s6 Polyamine ligands such as ethylene

diamine, diethylene triamine or triethylene tetramine are excellent chelating agents.

They were bound to chloromethyl polystyrene and the resulting polyamine resins

adsorb CU' effectively " Polymers containing dithiocarbamate groups have been

used as metal chelatrng agents. These ligands showed high ability to adsorb

vanous metal ions 'R

2 Inter and Intra molecular bridging

The reaction of a polymer llgand with metal ions very often results in inter

andlor ~ntrarnolecular br~dglng.

L L L L L L L L L L

L L L L L .

It is usually difficult to distinguish between inter and intramolecular

br~dging. Polyvinyl alcohol (PVA) ~ ~ ( 1 1 ) ~ ~ complex i s a typical example of this

type of polymer complex. Molecular weight dependencies of the inter and intra

molecular complex formations of PVA with cu2 ' were studied spectrophoto-

metncally." Polymers with a degree of polymerisation larger ?ha! 260 form a

stable complex with the co-ordination of four hydroxyl groups of the same PVA

molecule while polymers wrth a degree of polymerisation smaller than 160 forms

an inter molecular complex resulting in precipitatior. by crosslinking.

l'olymensatron of monotnenc n~etal complexes

If a rnonomerlc metal complex containing a vinyl group is polymerised

without s ~ d e reactions, a polymer metal complex having uniform structure is

obtained.

L ____, L L L L L

Typical example of the polymerisation of a vinyl monomer containing

transltlon metal Ion IS the radlcal polymerisation of vinyl ferrocene." Vinyl

t'errocene and its derlvatlves are polyrnerised by a radical or cationic initiator to

t o m a polvmer of hlgh molecular weight. Methacrylate monomer co-ordinated to

(:o(llI) complex e g . methacrylate pentamlne Co(l11) perchlorate was radically

p o l p e r ~ s e d to get the polymer metal complex.62

( 'omplexatron of b~firnctronal ligand with metal ion

When blfunct~onal l~gands form a complex wrth metal Ions hawng mote

than two lab~le llgands whlch are easy to be subst~tuted a polymer complex IS

formed through metal Ion br~dglng Thls type of polymer metal complex has been

used as semlconduct~ng organic materials, heat resistant orgalcc polylners or

polymer catalysts ''

(b) Cluuoclerkation oJpolymer metal complexes

The chemlcal methods of analysis applied to low molecular weight species

have been found to be satisfactory with linear polymers. But with crosslinked

polymers such methods which require solubilisation of the samples cannot be

applied. The detect~on and estimation of elements present in the polymer

complexes are canied out by elemental analysis. The complexed metal ioniare

estimated by volumetric, spectrophotometric or gravimetric methods. The co-

ordination of a polymeric ligand to metal ion and the structure of the resulting

polymer metal complex are studied spectroscopically and by measuring the

magnetic properties. Infrared (IR), UV-visible, electron spin resonance (ESR),

nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), X-ray

optical rotatory dispers~on (ORD) and circular dichroism (CD) can be made use of

for the structure elucidation of polymer metal complexes.i6 Thermal studies have

also been used to explore composition, structure and thermal stabilities of

polymeric ligands and their metal complexes.64

The format~on of polymer metal complexes can be followed from their

characteristic absorpt~on bands in infra red and far infra red spectra and comparing

them with the corresponding low molecular weight complexes. The IR absorptions

by a ligand are usually shifted by complex formation with metal ions. The

absorption band at 1600 c~n- ' of y(C=C) or y(C=N) of poly(4-vinyl pyndine), (PVP)

shifts to a higher wave number by about 20 cm.' in cis [CO(~~)~PVP.CI ]CI~ and

cis[~o(trien)~~~~l]~l~.~' The y(C=C) and S(CH) of PVP also shift to higher wave

number in the Co(I1I) complex. When two kinds of ligands capable of co-

ordination are present in the polymer, the IR spectra can be used to find out the

group which partic~pates in co-ordination. In the complex of Cu(l1) with vinyl 66 amine vinyl alcohol copolymer the IR spectrum shows that the Cu(I1) ion is co-

ordinated exclusively w~th the nitrogen atom of the copolymer.

In the 1R spectrum of poly(im~no ethylene) dithio carbarnate copolymer, the

band at 1480 cm-' and a strong band at 965 cm'l have been assigned to

y(NCS2) + 6(CH2) and y(CNC) + y(CS2) vibrations respectively.67 In the spectra of

the metal complexes the band at 1450 cm-' moved to higher frequencies and the

lower band at 965 cm-' broadened and showed a splitting of less than 20 cm-I

~ndlcat~ng b~dentate mode of co-ordination of CS2 group. The bands observed in

the range 352-384 cm-' have been assigned to y(M-S) vibrations.

The 1R spectra of the metal complexes of the copolymer glycidyl

methacrylate ethylene d~methacrylate with attached ethyl mines differs from that

of the copolymer.bx The absorption band at 1640 (6NH2, def.) is shifted towards

lower wave numbers (1630 cm-'). The weak absorption band at 1560 cm" in the

onginal polymers (FNH) disappears and the band at 1390 cm-' (yNH) is overlapped

with a strong absorption band due to nitrate ion at 1380 cm" in the metal

complexes

Polystyrene supported bipyridine and 1,lO-phenanthroline were used for

preparing rhodium complexes which were used as catalysts. The IR spectra of the

complexes show peaks due to the ligands. Peak at 330 cm.' is ascribed to Rh-CI

vibration.69

NMR has not been used for structural analysis of polymer complex due to

its low solubility In general the complex formation with metal ions leads to shifts

splitt~ng or broadening of the peaks due to ligand molecule.

ESR spectral studies were used to determine the co-ordination structure of

the polymer metal complexes. The ESR spectrum of Cu(11) complex of the

copolymer of methacrylate ethylene dimethacrylate, which contains chemically

bonded ethylene d ~ a m ~ n e molecule is a typical spectrum for a square planar

ordering of Cu(I1) and is identical with that of the tetra~nitle'~ Cu(I1) complex.

Consequently. the copolymer-Cu(1I) complex can be described as planar or almost

planar w~th nitrogen atoms of the two ethylene dtamlne groups on the x and y axis

and with nltrate lons or water molecules weakly co-ord~nated along z axis

The ESR parameters give an lndlcation about the nature of bond between

metal Ion and ltgand Covalent character of a bond becomes more pronounced

when the parameters y and y decrease and when A. and A1 Increase l6 The ESR

spectra of the Cu(I1) complexes of polymer supported Sch~ff bases were slmllar to

the frozen glass spectra of the parent complexes establ~shlng the ex~stence of the

complexes on polymer "

The ESR data of the Cu(l1) complex of the chelating ion exchange resin

spheron oxine 1000 show that in some resins two kinds of copper species are

present. The spectral data of the first copper speciesT2 (g; = 2.23-2.24, A,, = 181-

195 G ) corresponds to squareplanar Cu(8HQ)2 complexes and the spectral data of

the other copper specles (g, = 2.29-2.32, A: = 148-161 G) agree very well with the

spectral data of reference compounds in water containing host lattices, having

octahedral geometry.

The electronic transition energy values can be used in conjunction with ESR

parameters for determining the co-ordination structure of polymer metal

complexes. The diffuse reflectance spectra of the metal complexes of spheron-

oxlne 1000 agree well with the data expected.72 Since polymer metal complexes

are very often amorphous, there are only very few reports concerning X-ray

analysis.73 However it was demonstrated in recent years that extended X-ray

absorption fine structure (EXAFS) can be used to determine interatomic distances

and co-ordination of polymer bound Wilkinson's catalyst."

2.1.3 Applications of polymeric ligands and polymer metal complexes

The Important appl~cat~ons of polymer supported llgands are as chelatlng

ion exchange resins for the selective separation and preconcentrat~on of metal lons

and in the lmmob~l~sat~on of catalysts In add~t~on to their use as catalysts polymer

metal complexes find wide application in various areas like conductivity,

adsorption and in the model investigation of metal complexes of bioinorganic

(a) Chelating sorbenb

Chelat~ng sorbents are assumed to form chelate rings in the wrbent phase

during the sorption of metals. So the criteria for a polymeric ligand to act as a

chelating resin is the ability of the ligand function to form chelate rings with the

corresponding metal ions. At present a large number of chelating ion exchange

resins incorporat~ng ligands such as amino carboxylic acids, hydroxamic acids,

oximes, pyridine carboxylic acids, dithiocarbamates, thiols, crown ethers etc. have

been prepared wtth ligands incorporated into a variety of both organic and

Inorganic m a t n ~ e s . ~ ~

Majority of commercial ion exchange resins that find wide application

posses styrene-DVB copolymer. The groups taking part in the formation of chelate

ring usually include nitrogen, oxygen and sulphur atoms. ?he selectivity of the

ligand for various metal ions determines the selectivity of the respective chelating

resin. P-diketone resins prepared from styrene-DVB copolymers and diketone was

applicable to the separation of ~ e ~ ' and coZ' by a column operation7'

An Ion exchanger in which 8-hydroxyqu~noline is attached to polystyrene

matnx was found to absorb copper, nlckel and cobalt strongly m the pH range

2-3 7m 8-Hydroxyquinol~ne ~rnmob~llsed on sll~ca gel has been used for the

preconcentratlon of several transltlon metals from sea water 79

Chelattng polymers derived from amino acids are expected to be more

selective than the corresponding axnino carboxylic acid resins. Phenyl alanine resin

shows high selectivity for Hg(1I) and Cu(l1) in the pH range 2-3.80

Donald E Leydon and G.H. L,utterel immobi!ised dithio cabamate groups

on silica gel and used them as preconcentration matrix for X-ray analysi~.~' The

advantage of polymer supported dithiocarbamate is their ability to quantitatively

concentrate a large number of metal ions simultaneously while not complexing

alkal~ and alkaline earth metal Ions

A deta~led survey of available chelating ion exchange resins was made by

Sahnl and Reedijk "" 4 large number of chelating resins were used commercially

under d~fferent trade names The analytical properties and uses of such commercial

reslns and many other Ion exchange resins were ~ u r n m a r i s e d . ~ ~ Some of the

polymer supported l~gands used for metal uptake or separation is given in

Table 2 1

Table 2.1. Polymer supports employed for metal uptakelseparation

No

1

2

3

4

5

6

7

8

9

Metal lons studied

el*, co2-

-

Polymer matrix (hgand funct~on)

Styrene-DVB copolymer (pyndyl ~rmdazole)

Styrene-DVB copolymer (2-pyridyl methyl amine)

Styrene-DVB-copolymer (polyethylene amine)

Styrene-DVB-copolymer (phenyl alanine)

Cellulose (EDTA)

Styrene DVB-copolymer (8-hydroxy quinoline)

Styrene-DVB copolymer (Imino diacetic acid)

DVB-crosslinked polyacrylamide (amine)

Polyactylonitrile copolymer (Cystein)

Nature of study

Separation

Ref

83

CU", PJi2', co2' , 2nZ'

H ~ ~ ' , cu2', cd2'

Hg2', cuZ '

~ g ~ ' , cu2+

Hg2', cu2*, 2n2+, ~ i ~ ' , co2 '

~ g " , H ~ ~ ' , AU~ ' , pt2'

Separation

Metal ion uptake

Selectivity

Selective concentration of micro elements

Selectivity

Group concentration of micro elements

Separation

Selectivity

84

85

80

86

39

87

88

89

of acrylonitrile Ti4-, vS, Cr", Concentration of 90 with DVB (oxine) Mn2-, c o 2 , ~ i ? . , elements

cuZ, z n L , ~ d " ,

- ~ g ? - , pb2', Fe",

of styrene with PJb5', Mo6', snJ , Separation and 91 DVB (N-phenyl hydroxamic U6., sb3-, zn2*, w6' selectivity

I ac~d)

1- 11 1 Cellulose (pyrogallol) / u", cu2+. zn2-. 1 Selectivity ! Fe". sb3' 1 92 1 r3 I 14 I

j ~ 1 ~ ~ 1 se4., cu2., pb2' concentration

15

Cellulose (Salicylic acid)

Styrene-DVB copolymer (tluourea)

(b) Polymer bound catalysts

Styrene-DVB copolymer (thioglycolate)

17

18

-

Even though homogeneous catalysts often have higher substrate selectivity

and better reproducibility in the catalysed reactions their recovery by separation

tiom low molecular weight reactants and products is difficult and results in a

considerable loss of catalysts and the products. This problem has been

circumvented by preparing a heterogeneous catalyst in which a homogeneous

catalyst is bound to inorganic carrier materials or organic polymers.

--

A ~ " , s4 ' , co2+, ~ i ~ ' , Cu2-, U6'. y4-,

zn2', ~e~ , ~ h ' .

Aui*, pt2*, pd2'.

The first example of catalysis by a polymer metal complex was presented by

I,autsch er olY9 To a poly(pheny1 alanine) chain metalloporphyrin was linked by a

peptide bond The catalyt~c activity of polymer-Fe(II1) porphyrin complex was

twice as large as that for the Fe(II1) porphyrin analogue.

pb2-, ~ i ~ * , ~ g ? - , sn2', As3*, sb2*, zn2', cd2', phi'

Styrene-DVB copolymer (guanidine)

Polyaminophenolic condensate (thiol)

m2+ 1

Selectivity and separation

Selectivity

93

94

Separation

,

pd2'. pt2.

Agi*, AU", pdZ*, R4*. sb3*

95

Separation

Concentration of elements

97

98

The metal complexes of poly(styry1) bipyridine have manly been used as

hydrosenation catalysts '' Polymer supported rhod~um spec~es has been shown to

be ef?icient catalyst for the hydrogenation of a variety of substrates '00 A number of

studies in t h ~ s regard hake been carried out with phosphine l~nked complexes 101 anchored to polystyrene

To okercome some of the inherent dtsadvantages of phosphine ligands like

rhelr sensitibitv towards mr and moisture, bidentate ligands like anthran~lic acid,

7.2'-bipyridln and I . l 0-phenanthrollne were used for prepartng rhodium complexes

m d were supported on poly(styrene-co-dtvinyl benzene) (2%) 69

Pecht etul reported some detatled studies on the catalysts of the Cu(II)

complex of poly(histid~ne) I o 2 Hirai eta1 studied the hydrogenation of olefines

catalysed by poly(acryl~c ac~d)-Rh(I1) complexes tn homogeneous solutions lo3 The

catalytic act~vlty of the polymer-Rh complex was about 10' times that of the

acetato-Rh complex The hydrolysis of sodium pyrophosphate was effected by

uslng some metal complexes of poly(methacry1 acetone) as catalysts I"

Quaterntsed PVP Cu(I1) complex catalyse the oxidation of ascorbic acid.'05

f i e polymer complex was 200-1500 times more effective than cupric ions. The

acetyl acetonato Rh complex of biphenyl phosphinated poly(chloromethylated

styrene) ( I ) catalysed the hydrogenation of 1-hexene, the hydorformylation of

I -hexene. and cyclodimenzation of butadiene.'"

Such a solid phase catalyst can easily be separated from the products and used

repeatedly. Some of the polymer supports used in catalysis are listed in Table 2.2.

1 I 1 3 . Cyclodimerisation of butadiene I I

Ref.

107

108

102

109

42

110

38

103

111

112

I Styrene LIVB copolymer (d~phenyl phosphine)

catalysis.

Reaction

1 . Oxidation of thioderivatives by molecular oxygen.

2. Oxidative polymerisation of phenols.

Oxidation of phenylene diamine.

Hydrolysis of oligo phosphates

1 . Conversen of (CH,)*SO to (CH3)2SOz

2. Cyclohexane to cyclohexane oxide by Bu02H

Hydrogetlation of cyclohexene

Oxidation of 2,6-dimethyl phenol

Hydrogenation of olefines

Hydrogenation of methyl acetoacetates

Oxidation of thiols to disulphides

Table 2.2. Polymer supports

Polymer support (hnctional group)

-- - Poly(4-vinyl pyridine)

! t ~- j Poly(pheny1 alanine)

I Poly(L-lysine) - - - - -

Styrene DVB copolymer (-N-CS;)

-

Styrene DVB copolymer I COCH 3

-CH \ COCH

I Stvrene DVB copolymer (Schifl base)

Poly(acrylic acid) - Poly(L-glutamic acid)

Polystyrene (amine)

used in

Metaumetal complex involved

Cu(1I)

Fe(II1) as iron

porphyrins

Cu(I1)

Mo(V)

.%(I) as Rh(CO12

(acac)

Co(1I)

Rh(I1)

Ru(I1)

Co(II1) as cobalt

porphyrin

Rh(l) as Rh(CO' (acac)

/ Styrene-DVB copolymer i

( antiuanilic acid) (bipyridinej

(phenanthrolini

I Hydrogenation of I-hexene

2 Hydroformylation of 1-hexene

i '06

Rh(1) Hydrogenation of mono olefines I and dienes. 69

(c) . S e m i c o ~ c t i v ~

The semtconducting properties of polymer metal complexes such as

phthalocyanine, polyferrocene. polyacetic acid metal complex, polyamino quinone

metal complex etc. have been well known and well studied.63 These are widely

employed in the research of semiconductors.

2.2 Yaed ligand complexes-synthetic, structural and stability a s w

A metal complex in which the metal ion is co-ordinated by more than one

type of ligand system IS designated as a mixed ligand complex. These mixed ligand

complexes occur tn the transition states of metal ion catalysed reactions and also

the metal tons present in many biological systems are in the mixed ligand fom.

' f ie heme part of haemoglobin contain Fe porphyrin complex, in which the fifth

co-ordination site of iron is co-ordinated by an imidazole group from the protein

chain. The synthesis and structural characterisation of mixed ligand complexes is

~mportaat for a clear understanding of the mechanism of catalysis and many

biologcal processes. The synthesis, characterisation and stability aspects of mixed

l~gands are bnefly reviewed in the following sections.

22. f Qnthesis of mixed ligiind complexes

General method used for the preparation of mixed ligand complexes

involves the reaction between the metal salt and the ligands in I : 1:I ratio.

Synthesis of rn~xed ligand complex of copper(I1) and cobalt(I1) with Schiff bases

and heterocycltc organlc compounds were reported.lt3 The complexes were

prepared by refluxtng ethanolic solution of metal salt, Schiff base and the

heterocyclic base tn 1 : 1 : I mole ratio for 2-5 h. The wloured complexes

precipitated were filtered washed and dried. For the preparation of Co(II), Cu(I1)

and Ni(LI) ternary wmplexes of amino acids and imidazoleStt4 a mixture of metal

salt, amino acid and imidazole were heated in ethanol under magnetic stirring and

then refluxed on a water bath for 2 h. The solid ternary complexes separated

dunng refluxlng as powders

Madhu Gupta and Sr~vastava prepared mixed complexes of Cu(II), Ni(II),

CTo(ll) and Zn(I1) by mlxtng CuC12 and ligands alanine and uracil in 1 : 1 : 1 ratio."'

Another method used for the preparation of mixed complexes involves the reaction

between the stable metal complexes. 116,117

2.2.2 Formation constants of mixed complexes

When two klnds of metal complexes having bidentate ligands are reacted to

tbrm the mlxed complex, the kind of equilibrium is expressed by the following

general formulat~on

Reciprocal of Km 1s the dismutation constant introduced by Waners el al:"*

If the bond energles of M-A and M-B are not altered in the two sides of this

equilibrium and there IS no interaction between MAB, MA? and MB2 three

complex specles would be distributed statistically and in this condition Krn

will be 4 Watters er al. succeeded in attaining the formation constants of mixed

complexes such as (Cu en P207) and (Cu en ox) using isosbestic

~ i d a " ~ modlfied Waners' isosbestic method and obtained the values of log Km

and absorption spectra of three mixed ligand complexes (Cu en ox), (Cu en aca)'

and (Cu dgH aca) (where, en - ethylene diarnine, ox - oxalate, aca - acetyl

acetonate, dgH - dimethyl glyoximate). Log Km values are 1 .1 , 0.42 and -1.96,

respectively. Log Km values vary considerably from the statistically expected

values of 0.6 This observation supported the suggestion of Watters and De Witt

that the varlatlon of Km can be ascribed to the difference of the mutual ionic

repulsion of ligands. They explained that in the case of (Cu en ox) electrical

repulsion between ethylene diamine and oxalate ion is smaller than that between

two oxalate Ions. Therefore Krn is greater than the statistical value. On the other

hand, in the case of (Cu en aca)+ and its parent complexes the contribution of

electrical repulsion of the ligands to Km is negligibly small so the value of Km of

this complex is expected to be near the statistical value. But the found value is

smaller than the expected value. ICidaI2O defined log & as follows.

where log K , is the statistically expected value i.e., 0.6. Accordingly if log k t ,

has a positive value the mixed complex is easily formed from its parent complexes.

Kida verified by calculation that the electrostatic effect exerts an influence so as to

promote the formation of a mixed complex. The effect of a-covalency of the co-

ordinate bond can be considered analogously to the electrostatic one and is

expected to have a s~mllar Influence upon mixed complex formation.

Isobe el al. 116,117 reported an ESR study of the mixed ligand complexes

using as bidentate ligands ethylene diamine, its analogues and amino acids. For the

systems (1) [cu(enhlz* and [Cu(a alanine)z] and (2) [ ~ u ( e n k ] ~ ' and [~u(deen)j]"

[where, en - ethylene diamine, deen - N,N-diethyl ethylene diamine] the ESR

spectra are due to a single complex species only and so mixed ligand complexes are

formed quantitatively in these systems. But for the mixed solutions of (1)

o en)]'' and [Cu($ alaninek], and (2) [cu(enh12' and [Cu(dimethyl glycineh],

the ESR spectra are made up of the superposition of three different spectra

Indicating the coexistence of three different complex species. The molar ratio of

the complex specles were determined from thejr ESR spectra and log K values

were ~alculated."~ Log K values were also determined by spectrophotometric

methods. Results show that for systems having K values greater than 20 only

mixed complex IS detected in solution. For the systems with comparatively smaller

K values the reactants and products coexist in solution. Thermodynamic constants

were derived and the variation in enthalpy term is consistent with fida's proposal

that the promotive formation of a mixed ligand complex depends upon the

; i 1 < ', , , . A ,,

electrostatic effect and also upon the effect &%.o-covalency of the co-ordinate , , :-. bond slnce enthalpy term ts related to the bond energies.: - . - -

2.2.3 Determination of K values

(a) By spectrophotometrie method

Watters er al. developed an isosbestic method for obtaining the formation

constants of mtxed complexes such as (Cu en P207) and (Cu en ox) by

spectrophotometrtc methods."%~ida'~~ modified Watters method and obtained the

balues of Km for the three m~xed complexes, (Cu en ox), (Cu, en aca)+ and (Cu

d@ aca)

When MA2 and MB2 are m~xed to form MAB the equilibrium is expressed

by the formula

(2.1)

4%' I(\$ -

( C , xXC? x )

where CI and C2 are the concentration of MA2 and MB2 respectively and El , t 2 and

E? are the molar extinction coefficients of MA2 MB2 and MAB respectively. D is

the measured opttcal dens~ty, log (Iofl), of the solution in equtlibrium and 2x is the

concentration of the m~xed complex. From equation (2.4)

where E ? mt I

the value of m 1s chosen for the convenience of experiment and the wavelength

where ~3 IS equal to m&l 1s found out.

(b) L)etermination of K bq. ESR

lsobe el a1'" developed a method to determine the K values for the systems

( 1 ) cu(en)z2 and Cu(P-alan~ne)~ and (2) [cu(en)212- and Cu(dimethy1 glycine)~

from rhe ESR spectra. The ESR spectra of the systems at various volume ratios

were recorded The figures ~ndicate that the ESR spectra are made up of the

superposition of three different spectra. The three complex species correspond to

the two parent complexes and one mixed ligand complex. The molar ratio of these

three complex specles were estimated from the ESR spectra by Gaussian analysis

and the K values were calculated using equation (2.2).

(c) pH metric method

pH metrlc methods developed by Irving and ~ o s s o t t l ~ ~ were used

extens~vely for the determ~nation of stability constants of mixed complexes. The

method 1s relatively s~mple and can be conducted under ordinary laboratory

conditions. This simplifies the task of an inorganic chemist considerably.

2.2.4 Steric effects in the formation of mixed complexes

Bulky ligands like terpyridinelZ2 and tetramethyl ethylene diaminelZ3 (tmen)

form stable mlxed ligand complexes with metal ions. The secondary ligands used

were common over with no bulky groups around the donor atoms. Sone and

~o-worke r s '~~ studled mixed Cu(I1) chelates containing 2.2'-bipyridine or

1.10-phenanthrol~ne and another bidentate ligand A-A (en, ply, ox, acac) and

concluded that the strong inter ligand steric hindrance in [ ~ u ( b i ~ y r ) 2 ] ~ ' plays an

Important role in the equilibrium.

The equil~bnum of this type is forced strongly to the nght hand slde by the

fact that the repulsion between two bulky ligands in [~u(b l~yr )2]~ ' disappears in the

course of mixed chelate formation

It is seen that in an equimolar mixture of cu2-, tmen and (A-A) an extreme

case of thls equlllbrium take place where one of the components on the left hand

slde of equat~on ( 1 ) I e , [cu(tmen)212+ is so unstable stencally that it cannot ex~st at

all The facts that the mixed tmen chelates can be obta~ned easily from such

solutlons and they seem to be quite stable in aqueous solutlons are in confomlty

wlth this view

2.2.5 Spectral analysis of mixed ligand complexes

(a) Electronic spectra

~ i d a " ' venfied that the frequency of maximum absorption of a mixed

complex coincides with the mean values of the parent complexes In many types of

CuiII) complexes Kida's rule holds well which can be expressed as

Y W = %(yu + Y H ~ ) . where ~ A I , is the y,, value expected for a mixed chelate and 9 ~ 2

and YB2 are the y,, values of the parent chelates

in the case of tmen complexesi23 Kida's rule does not hold and y h of a

mixed chelate is remarkably larger than its y ~ . Natarajan et synthesised and

spectrally studied mixed ligand complexes of Ni(II), Zn(I1) and Cu(I1) with acetyl

acetone as one ligand and substituted cyclohexanones as the second ligand. They

observed that the d-d bands occurred at relatively higher energies in the mixed

ligand complex than in the bis chelates. The greater splitting of d orbitals suggests

increased thermodynamic stability

(b) I R spectra

The formation of a mixed ligand complex can be verified from its IR

spectrum. The IR spectrum of a mixed complex differs considerably from that of

the parent complexes or the ligand molecules. The groups which take part in

co-ordinatlon and the mode of co-ordination were also obtained from the IR 113.114 spectra The mixed complexes of Cu(II), Ni(II), Co(I1) and Zn(II), where the

ltgands are uracll and alanine were studied by IR methods.Il5 vNH~' band of

alan~ne d~sappears on coordination and vNH frequency appears in 3400-3200 cm-I

In metal complex. v, COO and v,COO frequencies appear around 1610 and

14 10 cm" and vC=O of uracil are in general little affected in the metal complex.

(c) ESR spectra

In the ESR spectral analysis of mixed ligand Cu(I1) complexes 116,117 by

Isobe er al. considering the line shapes of the spectra it was concluded that the

llgand field posses axial or near axial symmetry. g,, values of the mixed ligand

complexes were almost equal to the average gll value of the parent complexes in

that case. The mixed complexes of Cu(I1) with cinnamate and phenanthroline

llgands were analysed by ESR and the parameters, gl > g1 > 2.04, suggests a

d ~ * - ~ * ground state characteristic of an essentially square planar stereo chemistry.lZ6

Magnetic moment 123.125.127 and electro chemical 126-128 studies were also used for the

structural analysts of mixed complexes.

2.2.6 Mixed complexes on polymer supports

Some of the pendant type polymer metal complexes can be considered as

mlxed ligand complexes. For their preparation a polymer ligand is made to co-

ordinate to a vacant site of a previously prepared stable, low molecular weight 65.129 metal complex. Tsuchida eta/. prepared a series bf pendant type polymer

metal complexes hav~ng a unlform structure by the substitution reaction between a

polymer llgand and a Co(Il1) or Cr(lI1) chelate, the chelate belng Inert in llyand

substltut~on reactions

where u is the degree of co-ordination. If all the ligands in the polymer are

co-ord~nated to M the value of x become unity. A polymer Co(III) complex65

C'ls[Co(en)2 PVPCI1C12 [en - ethylene diamine; PVP - poly(4-vinylpyridin)] was

prepared by mlxlng PVP and [Co(en)2CI2]C1.

In the foregoing sections, an attempt was made to give a brlef overview of

the prevlous literature regarding the synthesis, characterisation and applications of

polymer supports, polymer-supported metal complexes and mixed ligand

complexes. Extensive studies were conducted and wealthy information is available

In the areas of polymer-supported strategy and synthetic and characterisational

aspects and catalytic appllcatlons of polymer-supported metal complexes. The

chemistry of polymer-supported mixed ligand complexes is a growing area o f

complex chemistry which needs further development.

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