The coordination chemistry of pyridyl oximeststamatatos.brockubeta.ca/publications/7.pdfThe...

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Polyhedron 25 (2006) 134–194

The coordination chemistry of pyridyl oximes

Constantinos J. Milios, Theocharis C. Stamatatos, Spyros P. Perlepes *

Department of Chemistry, University of Patras, GR-265 04 Patras, Greece

Received 14 July 2005Available online 24 August 2005

Abstract

The coordination chemistry of pyridyl oximes is reviewed. Simple pyridyl oximes have the general formula (py)C(R)NOH, wherepy is a pyridyl group (2-, 3- or 4-) attached to the oxime carbon atom and R can be a donor or a non-donor group. There are alsoligands containing more pyridyl and/or oxime groups. The coordination chemistry of twenty-three such ligands is described, includ-ing 2-acetylpyridine N-oxide oxime (which strictly speaking is not a pyridyl oxime) and of four polydentate ligands containing pyr-idyl groups that are not directly attached to the oxime carbon. References are given to methods for the synthesis of the ligands thatare not available in the market. The coordination chemistry of each ligand with all metals is detailed, with emphasis being placed onstructural features and physical properties (mainly magnetic) of the resulting metal complexes. This report shows that the anions ofpyridyl oximes are versatile ligands for a variety of objectives/advantages, including l2 and l3 behavior, preparation of polynuclearcomplexes (clusters) and coordination polymers, mixed-metal chemistry and interesting magnetic characteristics. The activation of2-pyridyl oximes by 3d-metal centers towards further reactions seems to be an emergent area of synthetic chemistry.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Clusters; Coordination chemistry; Coordination polymers; Molecular magnetism; Oximate group; Oxime group; Pyridyl oximes

1. Introduction and information for the organization of

this report

Simple pyridyl oximes have the general structures de-picted in Fig. 1 and consist of a pyridyl group (2-, 3- or4-) attached to the oxime carbon atom. R can be a do-nor or a non-donor group. There are also pyridyl oximescontaining more pyridyl and/or oxime groups.

The anionic forms of these molecules are versatile li-gands for a variety of objectives, including l2 and l3behavior, formation of polynuclear complexes (clusters),isolation of coordination polymers, mixed-metal chemistryand significant magnetic characteristics. The activationof 2-pyridyl oximes by 3d-metal centers towards furtherreactions is also becoming a fruitful area of research.The majority of the metal complexes of these ligands

0277-5387/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2005.07.022

* Corresponding author. Tel.: +30 2610 997146; fax: +30 2610997118.

E-mail address: [email protected] (S.P. Perlepes).

have been prepared only in the last 15 years and muchof their chemistry remains to be explored in more detail.

This report presents a review of the chemistry of pyr-idyl oxime ligands. It aims not to be comprehensive interms of a discussion of every known complex contain-ing a pyridyl oxime ligand (such a task would create amonograph for the complexes of di-2-pyridyl ketoneoxime alone!); rather, it aims to provide the reader someidea of the range of chemistry that has been carried out(and indeed remains still to do) with these ligands. Thisreview will also deal with the coordination chemistry of2-acetylpyridine N-oxide oxime (which is not a pyridyloxime) and of some polydentate oxime ligands contain-ing pyridyl groups that are not directly attached to theoxime carbon (Sections 5.6 and 7.3, respectively).

The report contains 10 sections. The first four areintroductory. Section 2 briefly describes the already pub-lished reviews on metal oxime and oximato complexes.In Sections 3 and 4, the reader can find informationon the organic, supramolecular and coordination chem-

mailto:[email protected]

N

C

R

NOH

N

C

R

NOH

N

C

R

NOH

Fig. 1. General structures of simple pyridyl oximes.

C.J. Milios et al. / Polyhedron 25 (2006) 134–194 135

istry of the simple oxime group. Sections 5–9 describethe chemistry of metal complexes that have neutraland anionic pyridyl oximes as ligands. The ligands� clas-sification is based both on the nature of R and the num-ber of pyridyl or oxime groups per molecule. Section 10gives some conclusions and areas for further investiga-tion. Most sections are divided into parts. Each part, de-voted to the coordination chemistry of a particularpyridyl oxime, gives its structural formula, systematicname and abbreviation, details the synthesis of the freeligand, and discusses some of its important metal com-plexes, with particular emphasis being placed on struc-tural aspects of these.

As in the case of many organic ligands, most pyridyloxime ligands are known by an abbreviation, usually ofthree or four letters, nominally derived from the fullname of the ligand. For example, the systematicallynamed ligand pyridine-2-carbaldehyde oxime (Fig. 1,R = H; Section 5.2) is better known as paOH, the abbre-viation being derived from the non-systematic namespyridine-2-aldoxime or 2-pyridinaldoxime; other abbre-viations used for this ligand are HPOX and PyAH. Theabbreviations of other pyridyl oximes are somewhathaphazard and confusing. Obviously, this is an unsatis-factory situation and ideally a systematic abbreviationsystem should be developed. In what follows, we adopta common abbreviation system based on the use of theconstituent py for a 2-pyridyl group, 3-py for a 3-pyridyland 4-py for a 4-pyridyl group; the oxime group(s) andthe nature of R (Fig. 1) will complete the abbreviation.For example, the abbreviation of pyridine-2-carbalde-hyde oxime (Fig. 1, R = H; Section 5.2) will be (py)CH-NOH, while the anionic ligand will be abbreviated as(py)CHNO. The abbreviation of 1-pyridine-2-yl-etha-none oxime (Fig. 1, R = CH3; Section 5.4) will be(py)C(Me)NOH, etc. We hope that this abbreviationsystem is more convenient for the reader than abbrevia-tions with letters derived from the name of the ligand.

2. Background

A review article on the coordination chemistry of pyr-idyl oximes has never appeared. However, metal com-plexes of pyridyl oximes have been incorporated intomore general reviews on the chemistry of metal oxime/oximate complexes. This chemistry has been actively

investigated since 1890, when Tschugaeff [1] first intro-duced dimethylglyoxime as a reagent for the gravimetricdetermination of Ni(II). Oximes as ligands have playeda significant role in the development of transition metalchemistry. This development has been documented in anumber of review articles and we refer the readers tosome of these excellent treatises [2–9]. An early treatiseby Chakravorty [2] is a comprehensive review on thestructural chemistry of simple oximes, vic-dioximes, qui-nonemonoximes, and carbonyl-, imine-, pyridine-, azo-,hydroxy- and amidoximes. A review by Bertrand andEller [3] covers oxime-bridged complexes of transitionmetals, while a concise review by Mehrotra [4] deals withthe syntheses, structures and reactivity of complexescontaining ‘‘simple’’ and vic-dioximes. In more recentyears five excellent reviews have been published [5–9].The survey by Tasker and co-workers [5] describes therich coordination chemistry of phenolic oxime ligands.The strategy of using ‘‘metal oximate’’ building blocksas ligands to synthesize various homo- and heterometal-lic paramagnetic complexes has been reviewed by Chau-dhuri [6]. This review is an important contribution to thefield of Molecular Magnetism; the oximato groups(�C@N–O�) can mediate exchange interactions ofvarying range, from moderate ferromagnetic to strongantiferromagnetic. Metal-ion mediated reactions of oxi-mes, and reactivity of oxime-containing and oximatemetal complexes have been described and classified byKukushkin and Pombeiro [7–9]; the three reviews illus-trate the fact that the chemistry of oxime/oximato metalcomplexes is rich since these species display an amazingvariety of reactivity modes.

3. Brief information on the organic and supramolecular

chemistry of the oxime group

3.1. Isomerism

The oxime group (�C@N–OH) is a well-exploredgroup in organic chemistry. The type of isomerismabout a C@C double bond [10] is also possible withthe C@N bond, though in this case only three groupsare connected to the double-bond atoms. The method,which can be applied, is based on the Cahn–Ingold–Pre-log system [10]. The two groups at the carbon atom areranked by the sequence rules. Then that isomer with the

C N

OH

R2

R1

C N

OH

R1

R2

Z or syn E or anti

Fig. 2. The Z–E isomerism of the oxime group assuming that R1 takesprecedence over R2 according to the Cahn–Ingold–Prelog system.

136 C.J. Milios et al. / Polyhedron 25 (2006) 134–194

higher ranking group and the –OH group on the sameside of the double bond is called Z (for the Germanword zusammen meaning together); the other is calledE (for entgegen meaning opposite). In the case of oximes,the Z isomer may be called syn and the E isomer anti

[11]. The isomerism of the oxime group is illustrated inFig. 2 for the case in which the substituent R1 is a higherranking group than R2. If there is more than one oximegroup in a molecule, the number of isomers can be in-creased, e.g., Z, Z, Z, E and E, E.

3.2. Formation of the oxime group

Some synthetic schemes that lead to the oxime groupmay be useful for coordination chemists. These arebriefly mentioned below [10].

3.2.1. Nitrosation at a carbon bearing an active hydrogenCarbon atoms adjacent to a Z group (Z may be CO-

OR 0, CHO, COR 0, CONR02, COO�, CN, NO2, SOR 0,

SO2R0, SO2OR 0, SO2NR0

2 or similar groups) can benitrosated with nitrous acid or alkyl nitrates. The initialproduct is the C-nitroso compound, but these are stableonly when there is no tautomerizable hydrogen. Whenthere is, the product is the most stable oxime (Eq. (1)).

RCH2-Z + HONO2 R C

N

Z

OHð1Þ

3.2.2. Addition of NOCl to olefins

The initial product is always the b-halo nitroso com-pound, but these are stable only if the carbon bearingthe nitrogen has no hydrogen (Eq. (2a)). If it has, the ni-troso compound tautomerizes to the oxime (Eq. (2b)).

C C + NOCl C C

Cl N Oð2aÞ

C C

N

H

Cl O

C C

NCl OH

ð2bÞ

3.2.3. Addition of hydroxylamine to aldehydes or ketones

This is the commonly used method for the synthesisof new oxime ligands by coordination chemists (Eq. 3).It has been shown that the rate of formation of oximesis maximum at a pH which depends on the substrate

but is usually 4, and that the rate decreases as the pHis either raised or lowered from this point.

C

O

+ NH2OH C

N OHð3Þ

3.2.4. Addition of Grignard reagents to the conjugate

bases of nitro compounds

The conjugate bases of nitro compounds (formed bytreatment of the nitro compound with BuLi) react withGrignard reagents in the presence of ClCH@NMe2

þCl�

to give oximes (Eq. (4)).

RCH@NðOÞOLiþR0MgX ! RR0C@NOH ð4Þ

3.2.5. Oxidation of primary aliphatic amines

Primary aliphatic amines can be oxidized to nitrosocompounds by Caro�s acid (H2SO5) or with H2O2 in Me-CO2H. Hydroxylamines, which are probably intermedi-ates in most cases, can sometimes be isolated, but underthe reaction conditions are generally oxidized to the ni-troso compounds. The nitroso compound is stable onlyif there is no a hydrogen; if there is an a hydrogen, thecompound tautomerizes to the oxime.

3.2.6. Reduction of aliphatic nitro compounds

Nitro compounds that contain an a hydrogen can bereduced to oximes with Zn dust in acetic acid (Eq. (5)) orwith other reagents, among them Co–Cu(II) salts inalkanediamines, CS2–Et3N and CrCl2.

RCH2NO2 !ZnHOAc

RCH@N–OH ð5Þ

3.3. The oxime group in supramolecular chemistry

In supramolecular chemistry a major goal is to con-trol the aggregation of molecules via intermolecularinteractions [12,13]. This is most readily achieved whensuch interactions are strong and directional. For thisreason hydrogen bonds are often employed. More spe-cifically, molecular building blocks can be designed tocarry particular functional groups that are capable ofrecognition of other groups or self-recognition throughthe formation of one or more hydrogen bonds. By sucha synthetic approach even quite complex molecularaggregates (supermolecules) can be prepared in a de-signed manner [14–16]. Where infinite assemblies areformed, the opportunity arises to construct crystallinesolids in which 1D, 2D or 3D networks are propagatedby hydrogen bonds [17,18].

Despite earlier studies establishing its capability toform hydrogen bonds [19–21], the oxime group has re-ceived far less attention in supramolecular chemistryand crystal engineering [22] than have other groups ascarboxyl [23], amide [24] and alcohol [25]. Oximes are

N

R'

R

O H A(a)

H

D

(b)

H

D

(c)

Fig. 3. Formation of hydrogen bonds by oxime groups [22].A, hydrogen bond acceptor; D, hydrogen bond donor.


able to form three types of hydrogen bond (Fig. 3). For-mation of only an O–H� � �A hydrogen bond is typical ofthe situation in which another strong hydrogen bond

Fig. 4. Common hydrogen bonding arrangements [22] for oxime (I, II)and carboxyl (III–V) groups.

Fig. 5. Less common hydrogen bonding arrangements [22] for oxime groupaldoxime R2

2ð8Þ C–H� � �O dimer (VIII).

acceptor group is present, as is illustrated by the struc-tures of pyridyl oximes [26]. The situation in which bothO–H� � �A and D–H� � �N hydrogen bonds form is typifiedby the absence of other hydrogen bonding functionalgroups, or at least ones strong enough to compete withthe oxime. Thus, O–H� � �N hydrogen bonds form be-tween oxime groups, most often as either an R2

2ð6Þ[27] dimeric arrangement (I) or a C(3) catemer (II)[22], resembling the R2

2ð8Þ (III) and C(4) arrangements(IV,V) that are well established [13] for carboxyl groups(Fig. 4).

Less common, crystallographically established hydro-gen bond patterns in oximes are presented in Fig. 5. Onepotential advantage [22] to the use of oximes is the pos-sibility of greater tunability by facile variation of thesubstituent R 0 (Fig. 3), which is not present in carboxylicacids or primary amides. Careful choice of this substitu-ent also permits the solubility of the oxime ligand to bemodified, facilitating supramolecular synthesis in awider range of solvent systems.

4. The importance of oxime and oximate groups in

coordination chemistry

4.1. General information

There is currently a renewed interest in the coordi-nation chemistry of oximes [6,9]. The research effortsare driven by a number of considerations. These in-clude the solution of pure chemical problems [28–34],the desire to provide useful bioinorganic models (oxi-mes may be considered to be reasonable models forthe biologically significant imidazole donor group ofthe amino acid histidine) [35], the design of Ca2+-and Ba2+-selective receptors based on site-selectivetransmetalation of multinuclear polyoxime–zinc(II)complexes [36], the development of new oxygen activa-tion catalysis based on nickel(II)–polyoximate com-plexes [37], the application of metal ion/oximesystems as simple and efficient catalysts for the hydrolysis

s: R44ð12Þ oxime tetramer (VI), related R3

3ð9Þ oxime trimer (VII) and


of organonitriles [38] (metal ions can behave as extre-mely strong activators of RCN toward nucleophilic at-tack by OH�/H2O), the mechanistic study of corrosioninhibition by Acorga P5000 (a modern corrosion inhib-itor comprising 5-nonylsalicylaldoxime as a mixture ofcarbon-chain isomers) on iron surfaces [39] and theemployment of oximate ligands in the synthesis ofhomometallic [6,32,40,42] and heterometallic [6,29,41]clusters [6,29,32,40–42] and coordination polymers[43] with interesting magnetic properties including sin-gle-molecule magnetism [42] and single-chain magne-tism [43] behaviors.

For example, the pure chemical interest in the coor-dination chemistry of oximes arises from the ability ofthe oximate(�1) group to stabilize higher oxidationstates of metals, e.g., Ni(III) or Ni(IV) [33,34] andthe fact that the activation of oximes by transitionmetal centers towards further reactions seems to bean emergent area of modern synthetic chemistry (Sec-tion 4.3). Another example of the importance of metaloximate complexes are the excellent studies on surfacecoordination chemistry by Collison, Garner, Taskerand co-workers; they have postulated (based on syn-thetic models) that the corrosion inhibition by P5000on an iron surface is due to the generation of a tetra-nuclear iron(III) cluster complex [39]. Iron ions mustbe available for the formation of such species, andthe efficiency of the inhibitor is therefore enhancedby treatment of an oxidized iron surface with a mild

CN

OH

M

1.01

CN

O

M

1.01

CN

O

1.10

CN

O

3.21

M

M

M

Fig. 6. The crystallographically established coordination modes of oxime amodes. Note that the upper right mode combines one formally neutral oxim

acid. The polyfunctional P5000 oxime ligand tendsto produce compact polymetallic assemblies, and whenformed at the surface of a metal, such species canform a protective barrier which inhibits further corro-sion [39].

4.2. Coordination modes of oxime and oximato groups

Oxime and oximato groups can bind a metal ion indifferent modes [9]; these coordination modes areshown in Fig. 6. The numbers below each bondingmode refer to the Harris notation [44]. Harris notationdescribes the binding mode as XÆY1Y2Y3� � �Yn, whereX is the overall number of metals bound by the wholeligand, and each value of Y refers to the number ofmetal atoms (ions) attached to the different donoratoms. The ordering of Y is listed by Cahn–Ingold–Prelog priority rules, hence (for most of the ligandsincluded in this report) O before N. In the case ofchelating/bridging ligands, to distinguish between sev-eral alternatives, a subscript number is included toshow to which metal ion the donor is attached. Inthe following, the binding mode of the ligands willbe often described using Harris notation. Since thereader will always have recourse to diagrams, we shallavoid using subscript numbers. Occasionally we shalluse the currently approved notation based on Greekletters l and g. We do believe that Harris notation

CN

O

M

1.0011

HO

NC

M

CN

O

2.11

M

M

CN

OM

1.11

nd oximato groups, and the Harris notation [44] that describes thesee group and one formally anionic oximato group.


is, at least for pyridyl oximes, more convenient thanthe notation based on Greek letters.

4.3. Reactivity of coordinated oximes

4.3.1. A brief introductionAs said in Section 2, oxime and oximato metal spe-

cies exhibit versatile reactivity. Their reactions can beclassified according to the extent of involvement of the{C@NO} moiety and to the bond at which the reac-tion is centered (see below). The reviews by Pombeiroand Kukushkin [7–9] are excellent sources on thistopic.

The general modes of reactivity concerning nucleo-philic or electrophilic additions to the polarized C@Nbond are illustrated in Scheme 1. Nucleophilic reagentscan add to the carbon atom of the azomethine linkage

C N

O(H)

Nuc

[M]

E

Scheme 1. General reactivity modes of the coordinated oxime group[9]. Nuc = nucleophile; E = electrophile.

[M] N

[M]

N

[M](2) (3)

- 'OH' (2)

HON / [M]

(1)

HO-[M]-N

(1) (3)

(11)

[M] NO

(13)

ne-/mH(7)

(8) - H2O(-R'OH)

[M] N

(10)

C

N C

(11)

-[M](9)

H2O(10)

Fig. 7. Reactions of coordinated oximes that le

(a reaction that is promoted by coordination of theoxime, in particular via the N-atom), whereas electro-philic reagents can attack the O- or the N-sites.

4.3.2. Reactions with preservation of the {CNO} fragment

[9]The reactions can either be centered at any of the

atoms of the {CNO} moiety, leading to oxime (or oxi-mato), imine or other types of complexes, or occur atanother part of the oxime molecule.

On account of the nucleophilic character of the oximeO-atom, the oximes can add, via this atom, to unsatu-rated species such as organonitriles, anhydrides, ke-tones, isocyanates, aldehydes, olefins and the olefinicgroup of an a,b-unsaturated oxime. Few examples ofoxime coupling via the N-atom acting as the nucleo-phile, are known. These include reactions of oximes withallene-PtII complexes to produce metallacycles and reac-tions of Cu(II) or Ni(II) complexes of o-quinone monox-ime with electrophilic acetylenes to give N-containingheterocycles. The electrophilicity of the C-atom of theNCO group of an oxime is expected to be promotedby oxidation and formal two-electron oxidations pro-mote not only H+ loss from the NOH group, but alsoaddition of a nucleophile to that C-atom to yield nitro-soalkyl species.

4.3.3. Reactions with rupture of the {CNO} fragment

Several N–O bond rupture reactions are known(Fig. 7); these reactions usually involve the formation

HNH2O-NH3

(5)O(5)

(6)

[M] NH

(4)

-NH3(6)

NH

N

(7)+

NH2 CH (8)

NH4+ (9)

NH2C

O

(12)

(4) -[M]

ad to N–O and N@C bond cleavage [9].


of an N–metal bond. The reactions include [9] oxida-tive addition of oximes to an electron-rich metal cen-ter (Reaction (1)), dehydroxygenation of oximes by ahydride metal center (Reaction (2)), deoxygenationof oximes (Reactions (3)–(7)), dehydration or alcoholelimination (Reactions (8) and (9)) and the Beckmannrearrangement of aldoximes into amides (Reactions (8)and (10)). Reactions with complete N@C bond cleav-age are also known [9], for example, Reactions (3)–(5),(7) and (11) in Fig. 7.

5. Ligands containing one oxime group, one pyridyl group

and no other donor atoms

5.1. Coordination modes

Ligands containing one oxime group, one pyridylgroup and no other donor atoms are popular. Mostof these ligands contain a 2-pyridyl group. The crys-

NC

R

NOHM

R= H, Me, Ph

1.011

NC

R

NOM

R= H, Me, Ph

1.011

NC

R

NOM

R= H, Ph

3.211

M

M

N

R= M

1.10

M

N

R=2.

Fig. 8. The crystallographically established coordination modes of the neutra[44] that describes these modes.

tallographically established coordination modes ofthese 2-pyridyl oximes are shown in Fig. 8. Their an-ions can bridge two or three metal ions.

5.2. Pyridine-2-carbaldehyde oxime, (py)CHNOH

NC

N

OH

H

(py)CHNOH

The free ligand is commercially available. Its crystalstructure has been determined [45]. There are two uniquemolecules in the asymmetric unit; the molecules relatedby a 21 screw axis, form infinite 1D chains held togetherby hydrogen bonds. The predominant hydrogen bond

NC

R

N OM

R= H

2.111

H

M

NC

R

NOM

R= H, Me, Ph

M(M')

2.111

CR

N

O

e

0

CR

N O

Me

MM

101

NC

R

N

O

R= Ph

1.100

M

l and anionic forms of simple 2-pyridyl oximes, and the Harris notation

Fig. 10. A drawing of the trinuclear molecule [Cu3(OH)(SO4)-{(py)CHNO}3] down its threefold axis; the SO4

2� group has beenomitted for clarity [6].


interaction assembling the chains is a head-to-tailO–H� � �N hydrogen bond involving the oxime O–Hand the 2-pyridyl nitrogen atom. There is also aC–H� � �N hydrogen bond involving the oxime C–H andthe oxime nitrogen atom (C� � �N 3.274, 3.283 A). Thechains are arranged in an anti-parallel fashion and packin a typical herringbone motif.

Potentiometric titrations at 25 �C in aqueous 0.1 MNaCl solution have provided the values of the logarithmof the protonation constant, logb, of the 2-pyridyl nitro-gen and the acid dissociation constant, pKa, of theoxime group [46]; these values are 3.59 and 10.01,respectively.

The first studies on the coordination chemistry of(py)CHNOH were reported in the late 50s and early60s [47–54]. No single-crystal X-ray structures wereavailable at that time. Investigations based on physicaland spectroscopic data showed [49,51,53] that squareplanar cationic complexes of divalent transition metalswere capable of intramolecular hydrogen bonding inthe type of structure shown in Fig. 9. However, thisstructural type has never been proven by crystallogra-phy. The groups of Busch [48] and Liu [50,54] werethe first to suggest that the deprotonated oxygen atomof coordinated (py)CHNO� can act as donor givinghomo- and heteropolynuclear complexes.

The first structurally characterized metal complex ofpyridine-2-carbaldehyde oxime was [Cu3(OH)(SO4)-{(py)CHNO}3] [55]. The CuII atoms fall at the cornersof an exact equilateral triangle of side 3.22 A due tothe presence of a threefold crystallographic axis. Themetal ions are held together by three distinct bridgingsystems: (i) the l3-hydroxo group, (ii) the sulfato group,lying on the threefold axis but below the plane contain-ing the metal ions, and acting as a tripod bridge bondingto all three CuII atoms through three of its oxygen atoms(g1:g1:g1:l3), and (iii) the three symmetry-related(py)CHNO� ligands each of which functions as a biden-tate chelate to one of the CuII atoms, through its twonitrogen atoms, and as a CuII� � �CuII bridging groupthrough the nitrogen and oxygen atoms of the oximatomoiety (Fig. 10). Thus, the (py)CHNO� ligands adopt

N

CH

N O

M

N

CH

N O

H

Fig. 9. The square planar structural type proposed [49,51,53] for thecomplex cations [M{(py)CHNO}{(py)CHNOH}]+ (M = Ni, Cu, Pd).

the coordination mode 2.111, see Fig. 8. Two of thethree electrons of the Cu3

II core are completely pairedand only the doublet spin state (ST = 1/2) is populatedat room temperature (leff = 1.0 BM per CuII in the80–300 K range) [6], which is evidence for strong antifer-romagnetic coupling.

Addition of I� to a solution of Cu2+ normally leadsto reduction to CuI, but in the presence of nitrogen do-nors reduction is inhibited. Chaudhuri and co-workers[56] have synthesized the complex [LCuII{(py)CHNO}2-CuIII] (ClO4) [56], where L is the capping tridentateligand 1,4,7-trimethyl-1,4,7-triazacyclononane. Themolecular structure consists of dinuclear cations(Fig. 11). The CuII atoms are bridged by 2.111 oximatoligands. Both metal ions have a distorted square pyrami-dal (spy) geometry with a CuII� � �CuII separation of 3.45A; the CuII–I bond length is 2.74 A. The chloro and ace-tato analogues of the iodo complex have also been pre-pared and structurally characterized [6]. The chlorocomplex has a very similar structure to that of the iodocompound. On the other hand, the acetate ion bridgesthe copper centers and, thus, one metal ion is five-coor-dinate and the other is six-coordinate. The magnitude ofthe exchange parameter J (2J being the singlet-tripletsplitting) depends on the nature of the axial ligand:MeCO2

� (J = �358 cm�1), Cl� (J = �390 cm�1) andI� (J = �460 cm�1). Interestingly, the strength of thespin interaction is not related to the N–O and C@Nbond distances, suggesting that a p exchange mechanismvia the ring system is not the major pathway.

The coordination chemistry of (py)CHNOH with Feand Co is practically unknown. Early aqueous solutionstudies by Hanania and Irvine [47,52] have shown thatwhen FeII forms a complex with (py)CHNOH the acidstrength of the oxime group increases considerably.The localized mixed-valence cation [LFeIII{(py)CH-NO}3Fe

II]2+ (L = 1,4,7-trimethyl-1,4,7-triazacyclonon-ane) contains a low-spin FeII atom [6], the complexbehaves magnetically as a mononuclear high-spinFe(III) species, with a leff value of 5.83 ± 0.02 BM at10–290 K.

Fig. 11. X-ray molecular structure of the [LCuII{(py)CHNO}2CuIII]+ cation [56].


A structurally impressive dodecanuclear Fe(III) clus-ter was recently reported by Christou�s group [57]. Thecomplex [Fe12(l3-O)8(l-OMe)2(O2CPh)12{(py)CHNO}6]was obtained from the reaction of [Fe3O(O2CPh)6-(H2O)3](O2CPh), (py)CHNOH and NaOMe in MeCN.The complicated core of the complex contains a centralFe6 unit that can be described in various ways, one ofwhich is as four edge-sharing {Fe3(l3-O)}7+ triangularunits and two additional {Fe3(l3-O)}7+ units attachedto the flanks. Variable-temperature solid-state suscepti-bility studies on the cluster in the temperature range5.0–300K reveal that this possesses anS = 0ground state;this behavior is not suprising given the dominance of anti-ferromagnetic interactions in high-spin Fe(III) chemistry.

Blackmore and Magee [58a] reported a slow reactionbetween Co(II) and (py)CHNOH; their explanation wasthat the low rate is due to the slowness of the interconver-sion of the syn and anti forms of the ligand. A later photo-metric and pHmetric solution study by Beck�s group [58b]showed that this reaction is complicated. The complexformation reaction itself is very fast, in contrast to thedata by Blackmore and Magee [58a], and is followed bya slow redox process where the metal ion is oxidised bythe ligand to yield an inert cobalt(III) complex. Recently,our group investigated the solid-state coordination chem-istry of (py)CHNOH with Co [59]. The refluxing reac-tion mixtures Co(O2CMe)2 Æ 4H2O/(py)CHNOH/NaClO4

(1:2:1) in MeOH, CoCl2 Æ 6H2O/(py)CHNOH/LiOH/NaClO4 (1:2:2:1) in H2O or Co(ClO4)2 Æ 6H2O/(py)CH-

NOH/Me4NOH (1:2:2) in MeOH led (under aerobicconditions) to the clean preparation of dark red complex½Co2

IIICoIIfðpyÞCHNOg6�ðClO4Þ2 (Fig. 12). The centralCoII atom, Co(2), which sits on a threefold axis of sym-metry, is octahedrally coordinated by six oxygen atomsbelonging to six crystallographically equivalent 2.111(py)CHNO� ligands. The six sites on each of the dis-torted octahedral, terminal CoIII atoms, Co(1), whichsit on a threefold axis of symmetry, are occupied bythe nitrogen atoms that belong to the ‘‘chelating’’ partof three (py)CHNO� ligands, with the three oximatoN atoms in the fac (or cis) configuration.

The fact that the two mononuclear neutral fac-CoIII

{(py)CHNO}3 units of the above mentioned mixed-valence, trinuclear cluster can be considered as actingas tridentate chelating ‘‘ligands’’ to the central CoII cen-ter, Co(2), led us to suspect that the mononuclear 1:3Co(III) complex would be capable of existence. Our sus-picion was both correct and incorrect. It proved correctbecause the desired product has been, indeed, preparedand, simultaneously, it proved incorrect because the dis-crete mononuclear complex, [CoIII{(py)CHNO}3], thatwe managed to isolate and structurally characterize isthe ‘‘wrong’’, i.e., the mer (or trans) isomer. The CoIII

atom is coordinated by three N,N 0-bidentate chelating(or 1.011 [44], Fig. 8) ligands.

The mononuclear distorted octahedral Co(III)complexes [Co{(py)CHNO}2(L–L)]Cl (L–L = bpy,phen) and [Co(acac)2{(py)CHNO}] have been recently

Fig. 12. The molecular structure of the mixed-valence (III/II/III) cation [Co3{(py)CHNO}6]2+; identical atoms are used for atoms generated by

symmetry [59].


prepared by our group; single-crystal X-ray crystallog-raphy revealed that (py)CHNO� behaves as an 1.011ligand [60].

Few homometallic Ni(II) complexes containing theneutral and/or anionic ligand have been published[61–63]. All are mononuclear: [Ni{(py)CHNO}2{(py)-CHNOH}] [61], [Ni{(py)CHNO}2L2] (L = pyridine,4-picoline, 4-ethylpyridine) [62] and [Ni{(py)CH-NO}2(L–L)] (L–L = bpy, phen) [63]. Representativedrawings are shown in Figs. 13 and 14. The deproto-nated ligands are N,N 0-bidentate (or 1.011). All com-pounds containing monodentate aromatic N-ligandshave the same structural motif with trans coordinationfashion for the identical donor groups [62], while thosecontaining bpy or phen are racemic [63]. PolynuclearNi(II) complexes comprise [64] several salts of theenneanuclear cation [Ni9(OH)6{(py)CHNO}10(H2O)6]

2+

and [Ni3(acac){(py)CHNO}2{(py)CHNOH}3](ClO4)3.

Fig. 13. A drawing of [Ni{(py)CHNO}2{(py)CHNOH}].

The latter (Fig. 15) contains two 2.111 neutral ligands,one 1.011 neutral ligand and two 3.211 deprotonatedligands (Fig. 8).

Although complex formation equilibria involving(py)CHNOH and ZnII were studied [65], only one struc-turally characterized Zn(II)/(py)CHNO� complex hasbeen reported [66]; this is [Zn4(OH)2Cl2{(py)CHNO}4](Fig. 16). The molecule of this complex features aninverse 12-metallacrown-4 motif [67] with the oximatoligand adopting the 2.111 coordination mode.

Fig. 14. The molecular structure of [Ni{(py)CHNO}2(bpy)] [63].

Fig. 15. The structure of the trinuclear cation [Ni3(acac){(py)CHNO}2{(py)CHNOH}3]3+ [64].


A dinuclear mixed-valence d3/d4 complex [LCrIII-{(py)CHNO}3Cr

II](ClO4)2 has been prepared [68], inwhich the CrIII and CrII centers are antiferromagneti-cally coupled (J = �7.9 cm�1) [68]; L = 1,4,7-tri-methyl-1,4,7-triazacyclononane. No structural data areavailable for this compound.

The Mn/(py)CHNOH chemistry is better developed.A modular approach (vide infra) using tris(pyridine-2-aldoximato)manganate(II), [MnII{(py)CHNO}3]

�, and[MnIIIL]-units (L = 1,4,7-trimethyl-1,4,7-triazacyclo-nonane) yielded the localized mixed-valence complex[LMnIII{(py)CHNO}3MnII]2+ [6]. The two manganesecenters are bridged by the –NO oximato linkages ofthe three 2.111 (py)CHNO� ligands. The interaction

Fig. 16. X-ray structure of [Zn4(OH)2Cl2{(py)CHNO}4]; atoms O(3)are the hydroxo oxygen atoms [66].

between the two metal ions was found to be ferromag-netic (J = +1.8 cm�1, ST = 9/2).

The structure of the 1D polymer [Mn(SO4){(py)CH-NOH}(H2O)]n (Fig. 17) consists of double chains, inwhich the MnII ions are bridged by g1:g1:g1:l3 sulfatoligands [66]; the neutral oxime ligand behaves as N,N 0-bidentate chelate (1.011, Fig. 8).

The preparation and crystal structures of four Mn(II)carboxylate complexes containing neutral (py)CHNOHwere recently reported [69]. The 1:1 reaction betweenMn(O2CPh)2 Æ 2H2O and the ligand inMeCN led to isola-tion of [Mn4(O2CPh)6{(py)CO2}2{(py)CHNOH}2](Fig. 18). The most interesting synthetic feature of thisreaction is the in situ formation of the picolinate(�1) li-gand, ðpyÞCO2

�. The centrosymmetric tetranuclear clus-ter consists of an exactly planar zig-zag array of MnII

atoms and is held together by four syn, syn g1:g2:l2 andtwog1:g2 :l3 PhCO2

� groups, twog1:g2 :l2ðpyÞCO2� li-

gands and two N,N 0-bidentate chelating (py)CHNOHmolecules.

The 1:4:7 ½Mn3II;III;IIIOðO2CPhÞ6ðpyÞ2ðH2OÞ�=

Me3SiCl=ðpyÞCHNOH reaction mixture in MeCN(py = pyridine) yielded the 1D coordination polymer[Mn(O2CPh){(py)CO2}{(py)CHNOH}]n, in which thepartial ðpyÞCHNOH ! ðpyÞCO2

� transformation hasagain occurred. Its structure is shown in Fig. 19. The1:3 reaction between Mn(O2CMe)2 Æ 4H2O and (py)CH-NOH in EtOH led to the isolation of the dinuclearcomplex [Mn2(O2CMe)2{(py)CO2}2{(py)CHNO}2];crystallography again revealed the partial ðpyÞCHNOH ! ðpyÞCO2

� transformation. A simplifiedscheme for this transformation was proposed (Fig. 20).Reaction of Mn(hfac)2 Æ 3H2O (hfacH = hexafluoroacet-

Fig. 17. A small portion of the 1D double chain present in complex [Mn(SO4){(py)CHNOH}(H2O)]n [66].

Fig. 18. The molecular structure [69] of the complex [Mn4(O2CPh)6{(py)CO2}2{(py)CHNOH}2]. Only the ipso carbon atoms of the phenyl groups ofthe benzoate ligands are shown.


ylacetone)with one equivalent of (py)CHNOH inCH2Cl2yields complex [Mn(O2CCF3)2{(py)CHNOH}2]; theCF3CO2

� ligand is one of the decomposition productsof the hfac� ligand. The MnII ion is coordinated bytwo CF3CO2

� groups and two 1.011 neutral oximeligands.

[Pt{(py)CHNO}2] Æ 2H2O is the only structurallycharacterized (py)CHNOH- or (py)CHNO�-based com-plex with 4d or 5d metals [70]. The coordination aroundthe PtII center is roughly trans square planar, the ligat-ing atoms being the two nitrogen atoms from each oftwo deprotonated ligands. The planar units form a chain

parallel to the crystallographic c-axis (Pt� � �Pt = 3.245A). A powdered sample of the complex shows enhancedelectrical conductivity which is in the same range as thatobserved for Magnus�s Green salt, [Pt(NH3)4][PtCl4].

The main-group metal chemistry of (py)CHNOH isvirtually non-existent. The only exception is the organo-metallic complex [In2Me4{(py)CHNO}2] (Fig. 21), inwhich the InIII atoms are five-coordinate adopting a dis-torted trigonal bipyramidal geometry [71].

Up to now, we have discussed homometal complexes of(py)CHNOH/(py)CHNO�.We now continue our discus-sion with the description of heterometal complexes that

Fig. 19. Views of the complex [Mn(O2CPh){(py)CO2}{(py)CHNOH}]n along b-axis (up) and c-axis (down) [69].

Fig. 20. A simplified view for the transformation of an account of (py)CHNOH to picolinate(�1) during the preparation of someMn(II) carboxylatecomplexes.


are (exclusively or partially) based on bridging(py)CHNO� ligands. All these complexes have aestheti-cally beautiful structures, while some of them presentinteresting magnetic properties. Magnetic investigationsof heteronuclear complexes are more informative than

those of homonuclear complexes as new exchange path-ways can be expected for two different spin carriers withina molecular unit, because unusual sets of magnetic orbi-tals are brought in close proximity [6]. Thus, oximato-bridged heterometal complexes are central players in the

Fig. 21. A drawing of the molecule of [In2Me4{(py)CHNO}2].


field of molecular magnetism [72]. Most of these speciesare prepared by the so-named ‘‘modular’’ synthesis [6];other names of this approach are ‘‘complexes as ligands’’or ‘‘complexes as ligands and complexes as metals’’ strat-egies [73].Different ‘‘modules’’, i.e., complexes containingone or more metal centers, are able to react further withother modules (different metal centers or metal com-plexes) through available appropriate donor atoms.

In an excellent paper [68], Chaudhuri, Wieghardt andco-workers reported that tris(pyridine-2-aldoxi-mato)metalates(II), [MII{(py)CHNO}3]

�, are capable ofacting as ‘‘ligands’’ reacting with [LCrIII(MeOH)3]

3+

(‘‘metals’’) to give various asymmetric dinuclear com-plexes of the general type [LCrIII{(py)CHNO}3M

II]2+

(MII = MnII, FeII, NiII, CuII, ZnII; L is the ‘‘end-cap’’1,4,7-trimethyl-1,4,7-triazacyclononane), see Scheme 2.In the case of cobalt(II), oxidation occurs and the result-ing complex is [LCrIII{(py)CHNO}3Co

III](ClO4)3. Thesecompounds contain three 2.111 (py)CHNO� ions asbridging ligands. The complexes are isostructural in thesense that they all contain a terminal CrIII atom in a dis-torted octahedral CrIIIN3O3 environment and a secondsix-coordinate metal ionM in a mostly trigonal prismaticMN6 geometry. Analysis of the variable-temperaturemagnetic susceptibility data indicates the presence ofweak ferro- or antiferromagnetic exchange interactionsbetween the paramagnetic centers. A qualitative rationaleon the basis of Goodenough–Kanamori rules [74,75] wasprovided to explain the differences in magnetic behavior.

Using the ‘‘complexes as ligands and complexes asmet-als’’ strategy Clerac, Miyasaka and co-workers [62] per-

Scheme 2. General structural typeof the [LCrIII{(py)CHNO}3M]2+ or 3+

cations (L = 1,4,7-trimethyl-1,4,7-triazacyclononane); the metal ion M isCrII, MnII, FeII, CoIII, NiII, CuII, ZnII [68].

formed the reaction between ½Mn2III;IIIðsaltmenÞ2-

ðH2OÞ2�ðClO4Þ2 (the ‘‘metal’’) and [Ni{(py)CHNO}2(py)2](the ‘‘bridging ligand’’), where saltmen2� isN,N 0-(1,1,2,2-tetramethylethylene)bis(salicylideneiminate), seeScheme3.The product [43] consists of two fragments, the out-of-plane dimer [Mn2(saltmen)2]

2+ as a coordination acceptorbuilding block and the neutral mononuclear unit [Ni-{(py)CHNO}2(py)2] as a coordination donor buildingblock, forming an alternating chain having the [–MnIII–(O)2–MnIII–(ON)–Ni–(NO)–] repeating unit. The chainsare well isolated and there are no interchain p–p overlapsbetween organic ligands; these features ensure a goodmagnetic isolation of the chains. The NiII� � �MnIII ex-change is antiferromagnetic (J = �21K) andmuch stron-ger than the ferromagnetic intrachain MnIII� � �MnIII

interaction (J 0 = +0.67 K). Hysteresis loops are observedbelow 3.5 K, indicating a magnet-type behavior. Com-bined ac (Fig. 22) anddcmeasurements showa slow relax-ation of the magnetization. The material constitutes anelegant design of a heterometallic chain with ST = 3mag-netic units showing a ‘‘single-chain magnet’’ behaviorpredicted in 1963 by Glauber [76] for an Ising 1D systemand first experimentally documented by Gatteschi�sgroup [77,78]. Complexes f½Mn2

III;IIIðsaltmenÞ2NiIIfðpyÞ-CHNOg2ðLÞ2�ðAÞ2gn (L = N-methylimidazole, A =ClO4; L = py, A = PF6; L = py, A = ReO4) were alsostructurally characterized, and found to have similarstructures and properties [79] with those of the L = py,A = ClO4 archetype described above.

Employing [Ni{(py)CHNO}(bpy)2]+ as ‘‘terminal li-

gand’’, the groups of Miyasaka and Clerac characterized[80] the heterometallic linear tetramers [Mn(5-R-salt-men)Ni{(py)CHNO}(bpy)2]2(ClO4)2 (R = H, Cl, Br,MeO; Scheme 4). These tetramers can be seen as oligo-meric units (components) of the aforementioned ‘‘sin-gle-chain magnets’’. Magnetic studies on the former [80]confirm the nature of the magnetic interactions reportedfor the latter [43,79]; a strong antiferromagneticMnIII� � �NiII coupling via the oximato bridge (JMn� � �Ni

ranges from�23.7 to�26.1 K) and a weak ferromagneticMnIII� � �MnIII coupling through thebis(phenolato) bridge(JMn� � � Mn ranges from +0.4 to +0.9 K). These magneticinteractions lead to tetramers with an S = 2 ground state.

5.3. 6-Methylpyridine-2-carbaldehyde oxime,

(6-Mepy)CHNOH

NC

N

HO

(6-Mepy)CHNOH

HH3C

Fig. 22. Temperature and frequency dependence of (a) the real (v 0)and (b) the imaginary (v00) parts of the ac susceptibility; the solid linesare guides for the eye [43].

Scheme 3. Synthesis of the ‘‘single-chain magnet’’ f½Mn2III;IIIðsaltmenÞ2NiIIfðpyÞCHNOg2ðpyÞ2�ðClO4Þ2gn; paO is another abbreviation for the anion

of pyridine-2-carbaldehyde oxime (see Section 2).


The free ligand is synthesized [46] by the reaction of6-methylpyridine-2-carbaldehyde, (6-Mepy)CHO, withan equimolar amount of H2NOH in MeOH underreflux. The values of the logarithm of the protonationconstant of the 2-pyridyl nitrogen and pKa of the oximegroup are [46] 4.26 and 9.94, respectively. The coordina-tion chemistry of this ligand is practically unknown.Complex formation equilibria involving (6-Mepy)CH-NOH and Cu(II) [46], Zn(II) [65] and Cd(II) [65] have

Scheme 4. A drawing of the cations ½MnIIIð5-R-saltmenÞNiIIfðpyÞCHNOgðbpyÞ2�24þ which are the tetrameric components ofthe ‘‘single-chain magnets’’ depicted in Scheme 3.


been studied by Saarinen�s group. Only one complex hasbeen structurally characterized [81]. This is the enneanu-clear Ni(II) cluster [Ni9(l3-OH)2(l2-OH)4{l3-(6-Mepy)CHNO}4{l2-(6-Mepy)CHNO}6(H2O)6](ClO4)2, preparedby the reaction between NiCl2 Æ 6H2O and the ligand inH2O at pH 8 (Eq. (6)). The chromophores in the struc-ture are NiO6, NiN2O4 and NiN4O2. The nine NiII

atoms are held together via l3-OH�, l2-OH�, 2.111and 3.211 (6-Mepy)CHNO� ligands.

9NiCl2 � 6H2Oþ 10ð6-MepyÞCHNOHþ 16NaOH

þ 2NaClO4

!H2O

pH 8½Ni9ðOHÞ6fð6-MepyÞCHNOg10ðH2OÞ6�ðClO4Þ2

þ 18NaClþ 58H2O ð6Þ

5.4. 1-Pyridin-2-yl-ethanone oxime, (py)C(Me)NOH

NC

N

OH

CH3

(py)C(Me)NOH

The free ligand can be synthesized [46] by the reac-tion of equimolar quantities of 1-pyridin-2-yl-ethanone(2-acetylpyridine), (py)C(Me)O, H2NOH Æ HCl andNaOMe in EtOH. The values of logb of the 2-pyridylnitrogen (b is the protonation constant) and pKa ofthe oxime group are [46] 3.97 and 10.87, respectively.The acidity of the oxime group of (py)C(Me)NOH islower than that of (py)CHNOH (pKa = 10.01) due tothe adjacent methyl group in the former. Complex for-mation equilibria involving the neutral and/or thedeprotonated ligand and Cu(II) [46], Zn(II) [65] andCd(II) [65] have been studied in aqueous solution bypotentiometic methods.

The published coordination chemistry of (py)C(Me)-NOH is limited compared with (py)CHNOH. Thestructurally characterizedmetal complexes of (py)C(Me)-NOHand/or (py)C(Me)NO� [82–91] are listed inTable 1,along with the coordination modes of the ligands and fewstructural details. Of particular note are the coordinationmodes 1.100 and 2.101 observed in organometallic com-pounds of Sb(V) and Sn(IV) [89–91], which are uniquefor the (py)C(R)NO� ligands (R = H, Me, Ph).

Molecular structures of representative complexes areshown in Figs. 23–27. In general terms, the comparisonof the coordination chemistry of (py)C(Me)NOH withthat of (py)C(H)NOH is not possible because of the differ-ent nature of the reaction systems studied.However, threemetal/(py)C(Me)NO� complexes, i.e., [Co{(py)C(Me)-

NO}3] [59], [Zn4(OH)2Cl2{(py)C(Me)NO}4] [84] and[Pt{(py)C(Me)NO}2] [88], have rather similar molecularstructures to their (py)CHNO� counterparts, i.e.,[Co{(py)CHNO}3] [59], [Zn4(OH)2Cl2{(py)CHNO}4][66] and [Pt{(py)CHNO}2] [70]. It should be mentionedthat the Co(III)/(py)C(Me)NO� complex is the fac iso-mer, whereas the Co(III)/(py)C(H)NO� complex is themer isomer. The nature of R affects the structural identityof the organometallic complexes [R8Sn4O2{(py)C-(Me)NO}4] [90,91], see Table 1.

The crystal structure of the free ligand consists ofchains of molecules arising from intermolecular hydro-gen bonding with the ring nitrogen atom as acceptor[90].

5.5. Phenyl-pyridin-2-yl-methanone oxime,

(py)C(ph)NOH

NC

N

OH

(py)C(ph)NOH

The free ligand can be synthesized [92] by thereaction of 2-benzoylpyridine, (py)C(ph)O, with anexcess of NH2OH (NH2OH Æ HCl + NaOH) inEtOH/H2O. The first structurally characterizedmetal complexes of (py)C(ph)NOH were the carbonylcompounds [93] [Os3(CO)8{(py)C(ph)NO}2], [Os3-(CO)8{(py)C(ph)NO}{(py)C(ph)HNH}], [Os3H(CO)9-{(py)C(ph)NO}] and [Os3H(CO)11{(py)C(ph)NO}],which exhibit interesting structural features. Thestructurally characterized, non-organometallic metalcomplexes of the neutral or anionic ligand are listedin Table 2.

The comproportionation reaction between Mn(O2-CPh)2 Æ 2H2O and nBu4MnO4 (3:1) in the presence of(py)C(ph)NOH in MeCN/EtOH/CH2Cl2 leads to theisolation of the mixed-valent cluster ½Mn4

IIMn4IIIO2-

ðOHÞ2ðO2CPhÞ10fðpyÞCðphÞNOg4�. The centrosymmet-ric octanuclear molecule (Fig. 28) contains four MnII

and four MnIII ions held together by two l4-O2� ligands

and two l3-OH� ions to give the {Mn8(l4-O)2-(l3-OH)2}

14+ core (Fig. 29), with peripheral ligationprovided by 10 PhCO2

� ligands that exhibit three differ-ent coordination modes and four 2.111 (py)C(ph)NO�

ions [94,95]. The molecular structure of this complex isvery similar with that of ½Mn4

IIMn4IIIO2ðOHÞ2-

ðO2CPhÞ10fðpyÞCðMeÞNOg4� [82].

Table 1Structurally characterized metal complexes containing (py)C(Me)NOH and/or (py)C(Me)NO� ligands

Complexa Coordination mode of theoxime/oximate ligand

Coordination spheres; coordination geometries Reference

[Mn(O2CPh)2{(py)C(Me)NOH}2] 1.011 cis,cis,trans-MnIIO2(Npy)2(Nox)2; oct [82][Mn3O(O2CMe)3{(py)C(Me)NO}3](ClO4) 2.111 MnIII(l3-O)(Ocarb)2(Oox)N2; oct [82]½Mn4

IIMn4IIIO2ðOHÞ2ðO2CPhÞ10fðpyÞCðMeÞNOg4� 2.111 MnII(l4-O)(l3-OH)(Ocarb)4, MnII(l3-OH)(Ocarb)3N2,

MnIII(l4-O)2(Ocarb)3(Oox), MnIII(l4-O)(l3-OH)(Ocarb)(Oox)N2;oct, oct, oct, oct

[82]

[Co{(py)C(Me)NO}3] 1.011 fac-CoIIIN6; oct [59][NiBr2{(py)C(Me)NOH}2] 1.011 cis,cis,cis-NiIIBr2(Npy)2(Nox)2; oct [83][Ni{(py)C(Me)NO}{(py)C(Me)NOH}(H2O)2](NO3) 1.011, 1.011 trans,cis,cis-NiIIO2(Npy)2(Nox)2; oct [83][Ni(SO4){(py)C(Me)NOH}(H2O)3] 1.011 NiII(Osulf)(Oaqua)3N2; oct [84][Ni(SO4){(py)C(Me)NOH}2(H2O)] 1.011 NiII(Osulf)(Oaqua)N4; oct [84][Ni{(py)C(Me)NO}}{(py)C(Me)NOH}(H2O)2](ClO4) 1.011, 1.011 trans, cis, cis-NiIIO2(Npy)2(Nox)2; oct [85][ZnCl2{(py)C(Me)NOH}2] 1.011 cis,cis,trans-ZnIICl2(Npy)2(Nox)2; oct [84][Zn(NO3)2{(py)C(Me)NOH}2] 1.011 all trans-ZnIIO2(Npy)2(Nox)2; oct [86][Zn(SO4){(py)C(Me)NOH}(H2O)3] 1.011 ZnII(Osulf)(Oaqua)3N2; oct [84][Zn4(OH)2Cl2{(py)C(Me)NO}4] 2.111 ZnII(l3-OH)(Oox)2Cl, Zn

II(l3-OH)2N4; tet, oct [84]{[Cd(SO4){(py)C(Me)NOH}(H2O)] Æ [Cd(SO4){(py)C(Me)NOH}(H2O)2]}n

b 1.011 CdII(Osulf)3(Oaqua)N2, CdII(Osulf)2(Oaqua)2N2; oct, oct [84]

[RhCl2{(py)C(Me)NO}{(py)C(Me)NOH}] 1.011, 1.011 trans,cis,cis-RhIIICl2(Npy)2(Nox)2; oct [87][Pt{(py)C(Me)NO}2] 1.011 trans-PtIIN4; sp [88][Ph3Sb{(py)C(Me)NO}2] 1.100 trans-SbVC3(Oox)2; tbp [89][nBu8Sn4O2{(py)C(Me)NO}4] 1.100, 2.101 SnIVC2(l3-O)(Oox)(Nox), Sn

IVC2(l3-O)2(Oox); spy, tbp [90][Et8Sn4O2{(py)C(Me)NO}4] 1.100, 2.101 SnIVC2(l3-O)(Oox)2, Sn

IVC2(l3-O)2(Nox); tbp, spy [90][Me8Sn4O2{(py)C(Me)NO}4] 1.100, 2.111 SnIVC2(l3-O)(Oox)2, Sn

IVC2(l3-O)2(Npy)(Nox); tbp, oct [91]

Abbreviations: Nox, oxime or oximato nitrogen; Npy, 2-pyridyl nitrogen; Ocarb, carboxylate oxygen; Oox, oximate oxygen; Osulf, sulfate oxygen; oct, octahedral; sp, square planar; spy, squarepyramidal; tbp, trigonal bipyramidal; tet, tetrahedral.a Solvate and other lattice molecules have been omitted.b The crystal structure of this coordination polymer consists of single and double chains.

150C.J.Milio

set

al./Polyhedron25(2006)134–194

Fig. 23. X-ray structure of the [Mn3O(O2CMe)3{(py)C(Me)NO}3]+

cation [82].


The reaction of Mn(O2CPh)2 Æ 2H2O with the sodiumsalt of (py)C(ph)CNOH and NaN3 in MeOH gives a tet-ranuclear cage (Fig. 30) with a fMn3

IIMnIVðl4-OÞ-

Fig. 24. Molecular structure of complex [Co{(py)C(Me)NO}3];

ðg1:l2-N3Þg7þ core; the four oximate anions behave as2.111 ligands [96]. Magnetic and EPR (Fig. 31) studiesshow the cage has an S = 6 ground state.

Complexes ½Co2III;IIICoIIfðpyÞCðphÞNOg6�ðPF6Þ2 and

[Co{(py)C(ph)NO}3] are structurally similar to their(py)CHNO� partners. The use of ðpyÞCðphÞNO�=MeCO2

� and ðpyÞCðphÞNO�=SO42� ‘‘blends’’ in Ni(II)

chemistry leads to a variety of structurally interestingclusters [84,97]. Of particular note are the complexes[Ni3{(py)C(ph)NO}6] and [Ni6(OH)(SO4)4{(py)C(ph)-NO}3{(py)C(ph)NOH}3(MeOH)3]; both complexes havebeen isolated from the NiSO4 Æ 6H2O/(py)C(ph)NOH/NaOMe reactionmixtures. In the former [84], the oximateligands adopt four different coordination modes, includ-ing the unique 1.110 mode (Fig. 8) which gives rise to asix-membered chelating ring. The molecular structure ofthe latter consists of two parallel triangles (Fig. 32). Themetal ions in the ‘‘small’’ triangle (defined by Ni(2),Ni(5) and Ni(6)) are held together by the l3-OH�, theg1:g1 :g1:l3-SO4

2� and the deprotonated oximate groupsof the three 3.211 (py)C(ph)NO� ligands; this triangle canbe viewed as an inverse 9-MC-3-subunit [67]. The metalions in the ‘‘large’’ triangle (defined by Ni(1), Ni(3),Ni(4)) are held together by the three g1:g2 :l3-SO4

2�

groups. Monoatomic oxygen bridges from the

the CoIII atom sits on a threefold axis of symmetry [59].

Fig. 25. A small portion of the double chain [Cd(SO4){(py)C(Me)NOH}(H2O)]n present in the complex {[Cd(SO4){(py)C(Me)NOH}(H2O)] Æ[Cd(SO4){(py)C(Me)NOH}(H2O)2]}n [84].


g1:g2 :l3-SO42� and the 3.211 (py)C(ph)NO� ligands link

up the two triangles.Despite the four complexes listed in Table 2, the

Cu(II), Zn(II) and Cd(II) chemistry of (py)C(ph)NOHis virtually non-existent. In the dinuclear complex[Cu2(hfac)2{(py)C(ph)NO}2] (Fig. 33) [97], the two

Fig. 26. X-ray structure of [nBu8S

bridges are the oximato groups of the two 2.111(py)C(ph)NO� ligands, whereas complexes [M2(SO4)2-{(py)C(ph)NOH}4] (M = Zn, Cd) contain neutral1.011 oxime ligands and g1:g1:l2 sulfato groups(Fig. 34) [84]. Complex [Zn4(OH)2(N3)2{(py)C(ph)-NO}4] features [84] an inverse 12-metallacrown-4 motif,

n4O2{(py)C(Me)NO}4] [90].

Fig. 27. X-ray structure of [Me8Sn4O2{(py)C(Me)NO}4] Æ 2(py)C(Me)CNOH [91].


like its (py)CHNO� [66] and (py)C(Me)NO� [84] chloroanalogs.

5.6. Metal complexes of 2-acetylpyridine N-oxide oxime,

(Opy)C(Me)NOH

NC

NOH

(Opy)C(Me)NOH

CH3

O

Strictly speaking this ligand is not a pyridyl oxime.The (Opy)C(Me)NOH ligand was first mentioned inthe literature in 1977 in a paper describing Co(II) andNi(II) complexes of 2-substituted pyridine N-oxides,although the synthesis of (Opy)C(Me)NOH was not de-tailed [98]. The detailed synthesis was reported in 1982and involves the reaction of NH2OH Æ HCl with 2-acet-ylpyridine N-oxide in warm H2O in the presence ofNaO2CMe Æ 3H2O [99].

The only metal complexes of (Opy)C(Me)NOH thathave been structurally characterized are [CoBr2{(Opy)-C(Me)NOH}2] [100] and [Co2Cl4{(Opy)C(Me)NOH}2-(MeOH)2] [101]. The CoII ion in the mononuclearcomplex is coordinated by two bromo ions and twoON-oxide, Noxime-bidentate chelating (Opy)C(Me)NOHligands (1.101, Fig. 35) in a cis–cis–trans fashion (the transdonor atoms are the Noxime atoms) [100]. The ligandadopts the 2.201 coordination mode (Fig. 35) in the cen-trosymmetric dinuclear complex [101]. Each CoII ion hasa six-coordinate O3NCl2 environment, produced byO,N-coordination from one ligand, bridgingN-oxide bondingfrom the second ligand, two terminal chlorides and onecoordinated MeOH.

5.7. Pyridine-3-carbaldehyde oxime, (3-py)CHNOH, and

1-pyridin-3-yl-ethanone oxime, (3-py)C(Me)NOH

N

CN OH

(3-py)CHNOH (R= H)

(3-py)C(Me)NOH (R= Me)

R

Compounds (3-py)CHNOH and (3-py)C(Me)NOHare the 3-pyridyl analogs (isomers) of (py)CHNOH (Sec-tion 5.2) and (py)C(Me)NOH (Section 5.4), respectively.The free ligand (3-py)CHNOH is commercially available.Its crystal structure has been determined [45]. Infinite 1Dchains are assembled through a head-to-tail O–H� � �Nhydrogen bond involving the oximeO–Hand the pyridinenitrogen atom. Adjacent chains are related by a glideplane and the two chains are linked through a C–H� � �Ohydrogen bond to form an 1D ribbon; the ribbons are ar-ranged in a herringbone motif and are hydrogen bondedto neighboring ribbons via C–H� � �Noxime interactions toproduce an overall 3D hydrogen bonded structure. Thefree ligand (3-py)C(Me)NOH can be synthesized by thereactionof3-acetylpyridine, (3-py)C(Me)O,withNH2OH ÆHCl in EtOH/H2O under reflux in the presence of excessNa2CO3 [45]. In the crystal structure of this compound,the molecules form infinite 1D chains assembledthrough a head-to-tail O–H� � �Npyridine hydrogen bond;additional C–H� � �Noxime and C–H� � �O hydrogen bondscross-link the 1D chains to produce a 3D hydrogenbonded infinite architecture [45]. In contrast to their2-pyridyl analogs, the published coordination chemistryof (3-py)CHNOH and (3-py)C(Me)NOH is very limited.

Aakeroy and co-workers [102,103] have employed(3-py)CHNOH and (3-py)C(Me)NOH as versatile tools

Table 2Structurally characterized metal complexes containing (py)C(ph)NOH and/or (py)C(ph)NO�ligands

Complexa Coordination mode ofthe oxime/oximate ligand

Coordination spheres; coordination geometries Reference

[Mn(O2CPh)2{(py)C(ph)NOH}2] 1.011 cis,cis,trans-MnIIO2(Npy)2(Nox)2; oct [82]½Mn4

IIMn4IIIO2ðOHÞ2ðO2CPhÞ10fðpyÞCðphÞNOg4� 2.111 MnII(l4-O)(l3-OH)(Ocarb)4, MnII(l3-OH)(Ocarb)3N2,

MnIII(l4-O)2(Ocarb)3(Oox), MnIII(l4-O)(l3-OH)(Ocarb)(Oox)N2;oct, oct, oct, oct

[94,95]

½Mn3IIMnIVOðN3ÞðO2CPhÞ3fðpyÞCðphÞNOg4� 2.111 MnII(l4-O)(Ocarb)2(Oox)(Npy)(Nox), MnII(l4-O)(Ocarb)2(Nazido)N2,

MnIV(l4-O)(Oox)3(Npy)(Nox); oct, oct, oct [96]½Co2IIICoIIfðpyÞCðphÞNOg6�ðPF6Þ2 2.111 fac-CoIIIN6, Co

IIO6; oct [59][Co{(py)C(ph)NO}3] 1.011 mer-CoIIIN6; oct [59][Ni(O2CPh)2{(py)C(ph)NOH}2] 1.011 cis,cis,trans-NiIIO2(Npy)2(Nox)2; oct [97][Ni4(O2CMe)2(NCS)2{(py)C(ph)NO}4(PrOH)(H2O)] 3.211 NiII(Ocarb)(Oox)2(Nisothiocyanato)(Npy)(Nox),

NiII(Ocarb)(Oaqua/PrOH)(Oox)2(Npy)(Nox); oct[97]

[Ni4(O2CMe)4{(py)C(ph)NO}4(MeOH)2] 3.211 NiII(Ocarb)2(Oox)2N2, NiII(Ocarb)(Oox)2(OMeOH)N2; oct, oct [97][Ni3{(py)C(ph)NO}6] 1.110, 1.011, 2.111, 3.211 NiIIO2N4; oct [84][Ni{(py)C(ph)NOH}3](SO4) 1.011 fac-NiIIN6; oct [84][Ni2(SO4)2{(py)C(ph)NOH}4] 1.011 NiII(Osulf)2N4; oct [84][Ni6(OH)(SO4)4{(py)C(ph)NO}3{(py)C(ph)NOH}3(MeOH)3] 3.211, 1.011 NiII(l3-OH)(Osulf)2(Oox)N2, NiII(Osulf)2(OMeOH)(Oox)N2; oct, oct [84][Cu2(hfac)2{(py)C(ph)NO}2] 2.111 CuII(Ohfac)2(Oox)N2; spy [97][Zn4(OH)2(N3)2{(py)C(ph)NO}4] 2.111 ZnII(l3-OH)(Oox)2(Nazido), Zn

II(l3-OH)2(Npy)2(Nox)2; tet, oct [84][Zn2(SO4)2{(py)C(ph)NOH}4] 1.011 ZnII(Osulf)2N4; oct [84][Cd2(SO4)2{(py)C(ph)NOH}4] 1.011 CdII(Osulf)2N4; oct [84]

Abbreviations: Nox, oxime or oximate nitrogen; Npy, 2-pyridyl nitrogen; Ocarb, carboxylate oxygen; Oox, oximate oxygen; Osulf, sulfate oxygen; oct, octahedral; spy, square pyramidal; tet, tetrahedral.a Solvate molecules have been omitted.

154C.J.Milio

set


Fig. 28. X-ray structure of complex [Mn8O2(OH)2(O2CPh)10{(py)C(ph)NO}4]; only the ipso carbon atoms of the phenyl groups of the benzoato andoximato ligands are shown.

Fig. 29. ORTEP representation of the {Mn8(l4-O)2(l3-OH)2}14+ core. Mn(2), Mn(2 0), Mn(3) and Mn(30) are MnII atoms.


for supramolecular assembly of silver(I)- and copper(I)-containing hydrogen-bonded architectures. The crystalstructure of [Ag{(3-py)CHNOH}2](PF6) [102] containscations (Fig. 36) comprised of two ligands coordinatedthrough the 3-pyridyl nitrogen atoms to a AgI ion (coor-dination mode 1.010, Fig. 37). The oxime moieties arecis with respect to each other, and cations are linkedby complementary O–H� � �N hydrogen bonds betweenoxime moieties on neighboring ligands (I in Fig. 4), gen-erating infinite 1D chains. Adjacent chains are linked byC–H� � �O hydrogen bonds, resulting in 2D cationicsheets, Fig. 36. The PF6

� counterions occupy the result-ing ‘‘holes’’ within the cationic sheet, and are held in po-sition by several C–H� � �F hydrogen bonds. The result isan anisotropic, lamellar structure. The crystal structureof [Ag{(3-py)CHNOH}2](ClO4) [102] is very similar tothat of the PF6

� salt, even though the size of the anion

has changed significantly from the PF6� to the ClO4

�

salt (molecular volumes of 72 and 55 A3, respectively).The crystal structure of [Ag{(3-py)C(Me)-

NOH}2](PF6) [102] contains cations comprised of two1.010 ligands. The oxime moieties are arranged trans

with respect to each other and neighboring cations arelinked by oxime O–H� � �N hydrogen bonds, R2

2ð6Þ, into1D chains. The chains are arranged within well-defined3D regions, connected by intermolecular hydrogenbonds. The anions, positioned between layers, act as‘‘bridges’’, via C–H� � �F hydrogen bonds (Fig. 38). Thecrystal structure of [Ag{(3-py)C(Me)NOH}2](ClO4)[102] is very similar to that of the PF6

� salt.The persistence of the intermolecular R2

2ð6Þ motif inthe presence of different counterions and ligand substit-uents in the crystal structures of the above describedAg(I) complexes is testimony to the utility of the oxime

0 5000 10000 15000 20000

experimental

magnetic field /G

simulation

Fig. 31. The measured and simulated Q-band EPR spectra for thecomplex [Mn4O(N3)(O2CPh)3{(py)C(ph)NO}4].

Fig. 30. The molecular structure of the complex½Mn3

IIMnIVOðN3ÞðO2CPhÞ3fðpyÞCðphÞNOg4� emphasizing its core.


moiety as a versatile intermolecular connector which canallow coordination complexes to be directed into or-dered networks.

Fig. 32. X-ray structure of [Ni6(OH)(SO4)4{(py)C(ph)NO}3{(py)C(ph)NOHnitrogen atoms of (py)C(ph)NOH and (py)C(ph)NO� are shown.

In the crystal structure of [CuI{(3-py)CHNOH}]n[103] each tetrahedral CuI atom is coordinated to threel3-I

� ligands to generate an infinite 1D motif consist-ing of ‘‘staircases’’ of CuI; the (3-py)CHNOH ligand is

}3(MeOH)3]. Only the two carbon atoms that intervene between the

Fig. 33. X-ray structure of [Cu2(hfac)2{(py)C(ph)NO}2].


attached to each metal ion through the 3-pyridyl nitro-gen atom, to complete the coordination sphere. Adja-cent oxime moieties are connected via O–H� � �N

Fig. 34. Molecular structure of the complex [Zn2(SO4)2{(py)-C(ph)NOH}4].

hydrogen bonds, in a catemer-like fashion, to propa-gate the 1D polymeric chains into an infinite 2D sheet(Fig. 39).

NC

Me

N

OHO

CoCo

2.201

NC

Me

N

OHO

Co

1.101

Fig. 35. The coordination modes of (Opy)C(Me)NOH in its structur-ally characterized Co(II) complexes.

Fig. 36. Hydrogen-bonded cationic sheet in [Ag{(3-py)CHNOH}2](PF6) [102].

N

CN OH

R

M

1.010; R= H, Me

Fig. 37. The crystallographically established coordination mode of theligands (3-py)CHNOH and (3-py)C(Me)NOH in their Ag(I) and Cu(I)complexes.


5.8. Pyridine-4-carbaldehyde oxime, (4-py)CHNOH

N

C NH OH

(4-py)CHNOH

Fig. 38. Edge-on view of the packing in [Ag{(3-py)C(Me)NOH}2](PF

The free ligand is commercially available. In a generalproject that is aimed at assembling metal complexesthrough hydrogen bonds to form porous molecularmaterials, Aakeroy and co-workers [104] synthesizedthe complex [Ni{(4-py)CHNOH}4(H2O)2]Br2 Æ 6H2O2-(4-py)CHNOH. The ligand adopts the 1.010 coordina-tion mode (see Fig. 40).

The oxime hydroxy groups link through complemen-tary O–H� � �O hydrogen bonds to form sheets withlarge, hourglass-shaped holes. The sheets are cross-linked by hydrogen bonds between the axially coordi-nated H2O molecules and the bromide counterions,forming a 3D network, where the large holes are alignedinto channels. Twofold interpenetration of the 3D net-work blocks the center of the large hole, leaving twosmaller channels at each end. A host–guest complex isformed, and the guest molecules, (4-py)CHNOH, arecontained inside the channels, held in the lattice byhydrogen bonds to the bromide ion and the coordinatedoxime ligands. The structure of this complex demon-strates that an octahedral system can generate a 3D net-work with holes large enough to hold relatively smallorganic molecules.

6), with PF6� anions positioned between cationic sheets [102].

N

Ni

C NH OH

1.010

Fig. 40. The coordination mode of (4-py)CHNOH in the structurallycharacterized complex [Ni{(4-py)CHNOH}4(H2O)2]Br2 Æ 2(4-py)-CHNOH.

Fig. 41. A structural drawing of [NiIV{(py){C(Me)NO}2}2].

NCMe

NM

CMe

NOHHO

NCMe

NM

CMe

NOHO

M

NCMe

NM

CMe

NOO

1.00111 2.10111

1.00111

Fig. 42. The crystallographically established coordination modes of(py){C(Me)NOH}2 and its mono- and dianionic forms.

Fig. 39. Infinite 2D sheets of [CuI{(3-py)CHNOH}]n [103].


6. Ligands containing two oxime groups and one or two

pyridyl groups, with no other donor atoms

6.1. 1-[6-(1-Hydroxyimino-ethyl)pyridin-2-yl]-ethanone

oxime, (py){C(Me)NOH}2

NCH3C

N

CCH3

NOHHO

(py){C(Me)NOH}2

The free ligand can be synthesized [105,106] by thereaction of 2,6-diacetylpyridine, (py){C(Me)O}2, with2 equiv. of NH2OH Æ HCl and 2 equiv. of NaOH inMeOH/H2O under heating. The two oxime groups havestrongly overlapping titration curves; the pKa1 and pKa2

values are �10.1 and �10.8, respectively [107]. Theligand was first investigated by Hartkamp [108], who re-ported that aqueous Ni(II) solutions of (py){C(Me)-

NOH}2 are oxidized by air. The ligand was also investi-gated by Irvine and co-workers [107], who studied Fe(II)complexes in solution. It was found that pKa for the li-gand dropped markedly upon coordination, an effectthat was attributed to resonance stabilization of the an-ionic conjugate base.

Early solid-state coordination chemistry with this li-gand involved nickel. Following Hartkamp�s report[108], Baucom and Drago [105] isolated several com-plexes, including [Ni{(py){C(Me)NOH}2}2]

2+, thedeprotonated species [Ni{(py){C(Me)NO}2}2]

2� andthe formally Ni(IV) complex [Ni{(py){C(Me)NO}2}2].Subsequently, Sproul and Stucky [109] reported thecrystal structure of [Ni{(py){C(Me)NO}2}2] (Fig. 41),and showed that the ligand is planar with coordinationthrough nitrogen (1.00111, Fig. 42) and that consider-able strain is introduced into the ðpyÞfCðMeÞNOg22�moiety when it coordinates to NiIV. A comparison ofthe nickel-nitrogen bond distances with those found inanalogous Ni(II) complexes suggests a shortening of0.17 A in the NiIV–N bond.

X-ray structures of cationic octahedral complexes ofthe general formula [M{(py){C(Me)NOH}2}2]X2, whereM = Mn, X = ClO4 [106], M = Fe, X = Cl [110] andM = Cu, X = ClO4 (Fig. 43) [106] have been reported.X-ray diffraction studies of the 1:1 five-coordinate

Fig. 43. X-ray structure of [Cu{(py){C(Me)NOH}2}2](ClO4)2 [106].

Fig. 45. View of small portions (three monomer units) of two chains incomplex [Mn{(py){C(Me)NOH}2}Cl2]n [113].


complexes [M{(py){C(Me)NOH}2}Cl2], where M = CuandZn (Fig. 44), were also performed [111]. For theCu(II)complex, the coordination environment about the metalcenter resembles a distorted square pyramid with achloro ligand at the apex. For the Zn(II) complex, theenvironment about the metal ion can be viewed as a dis-torted trigonal bipyramid, with the equatorial positionsoccupied by the two chloro ligands and the pyridinenitrogen. A second form (monoclinic) of the Cu(II) com-plex was structurally characterized in 1994 [112]. Thestoichiometrically similar Mn(II) complex [113,114],[Mn{(py){C(Me)NOH}2}Cl2]n, has an interesting poly-meric structure. The metal ion coordinates to form pen-tagonal bipyramids MnII(N3Cl2)Cl2 in which eachchloro ligand occupies axial and equatorial sites on

Fig. 44. A structural drawing of [Zn{(py){C(Me)NOH}2}Cl2].

adjacent monomer units in the helical chains (Fig. 45).Variable-temperature magnetic susceptibility studies[112] indicate weak ferromagnetic coupling.

The (2,6-diacetylpyridine dioxime)copper(II) unit isalso present in the trinuclear cluster [Cu3{(py){C(Me)-NOH}2}2Cl6] [115]. The complex is a ‘‘sandwich’’ madeof two Cu{(py){C(Me)NOH}2}Cl

+ cationic units (the‘‘bread’’) and a CuCl4

2� anionic unit (the ‘‘filling’’);the latter can be viewed as a bis(monodentate) ‘‘bridg-ing’’ ligand, see Fig. 46. The three CuII centers are ferro-magnetically coupled.

A series of monomeric In(III) complexes containingneutral 1.00111 (py){C(Me)NOH}2 ligands are alsoknown. The ligand reacts with InCl3 in MeOH to givethe seven-coordinate complex [InCl3{(py){C(Me)-NOH}2}(MeOH)] (Fig. 47) [116]. The MeOH ligandcan be replaced by Cl� or H2O to give the complexanion [InCl4{(py){C(Me)NOH}2}]

� and [InCl3{(py)-{C(Me)NOH}2}(H2O)], respectively. The pentagonalbipyramidal coordination environment of the metal ispreserved during the substitution reactions [116]. TheMeOH-containing complex can react with bidentate

Fig. 46. The ‘‘sandwich’’ structure of [Cu3{(py){C(Me)NOH}2}2Cl6][115].

Fig. 47. X-ray structure of [InCl3{(py){C(Me)NOH}2}(MeOH)] [116].

CuN

CH3

N

OH

CH3

N O

Cu

O N

CH3

N

CH3

N

HO

Fig. 49. A structural drawing of the dinuclear dication [Cu2{(py)-{C(Me)NOH}{C(Me)NO}}2(H2O)2]

2+; the weakly bound aqua ligandshave been omitted for clarity.


ligands by ligand exchange [117]. A seven-coordinatecomplex of the composition [InCl(ox){(py){C(Me)-NOH}2}(H2O)] is formed with potassium oxalate(K2ox); the oxalato(�2) ligand occupies the equatorialplane of a pentagonal bipyramid together with the tri-dentate chelating oxime. With sodium 1,2-dicyanoeth-ene-1,2-dithiolate (Na2mnt) the analogous reactionproduces the six-coordinate, mixed-ligand complex [In-Cl(mnt){(py){C(Me)NOH}2}], which has a distortedoctahedral coordination sphere (Fig. 48). Ligands whichform four-membered chelate rings, like dialkyldithiocar-bamates or pyridine-2-thiolate, are able to replace all li-gands of [InCl3{(py){C(Me)NOH}2}(MeOH)] to formneutral tris chelates [117].

Fig. 48. X-ray structure of [InCl(mn

We have up to now discussed in this part metal com-plexes containing the neutral, (py){C(Me)NOH}2, or thedianionic, ðpyÞfCðMeÞNOg22�, ligand. Two complexescontaining the monoanionic ligand (py){C(Me)NOH}-{C(Me)NO}� have been also reported. Complex [Cu-{(py){C(Me)NOH}2}2Cl2] [111] is remarkably acidic[118], the value of pKa being 2.8 at 25 �C. Deprotona-tion of this complex in alcoholic solution leads to thedinuclear dication [Cu2{(py){C(Me)NOH}{C(Me)-NO}}2(H2O)2]

2+ which has been isolated [118] as thechloride or tetrafluoroborate salt. The crystal structureof the latter has been determined [119] to establish thecoordination mode of the monoanionic ligand, seeFig. 49. The structural analysis revealed that the nearlyplanar (py){C(Me)NOH}{C(Me)NO}� ion behaves as abridging tetradentate ligand adopting the 2.10111 coor-dination mode, see Fig. 42. The same coordinationmode is adopted by the ligand in the mixed-valence tri-nuclear cluster ½FeIIfðpyÞfCðMeÞNOHgfCðMeÞNOgg2-Fe2

IIIðl-OÞCl4� [110].

t){(py){C(Me)NOH}2}] [117].

Scheme 5. Synthesis of (py)(CNOH)2(py) and 3,4-di(2-pyridyl)-1,2,5-oxadiazole.


The kinetics and mechanism of ester hydrolysis bymetal complexes of (py){C(Me)NOH}2 and (py)-{C(Me)NOH} {C(Me)NO}� were studied by Yatsimir-sky�s group a few years ago [120,121]. The rate constantsof the cleavage of 4-nitrophenyl acetate by the ligandover the pH interval 6–8 increase 103–104 times in thepresence of Pb(II), Mn(II) and Cd(II), and 2–100 timesin the presence of Ni(II), Hg(II), Pr(III) and Zn(II).The reactive species are monomeric complexes of thetype M{(py){C(Me)NOH}{C(Me)NO}}+.

6.2. Di-2-pyridylglyoxal dioxime, (py)(CNOH)2(py)

The free ligand was first reported in 2002 as an inter-mediate in the synthesis of 3,4-di(2-pyridyl)-1,2,5-oxadiazole [122], see Scheme 5. The dioxime wasprepared, in 52% yield, by reacting the commerciallyavailable diketone di-2-pyridylglyoxal (2,2 0-pyridil) withan excess of aqueous NH2OH. The compound was char-acterized by melting point, 1H and 13C NMR spectros-copy, positive-ion EI mass spectrometry and elementalanalysis. Subsequent heating of the dioxime at 185 �Cfor 18 h in a sealed tube effected cyclodehydration togive the substituted oxadiazole [122].

N

C

N

N

OTl

Tl''

Tl'

3.2110

Fig. 50. The basic unit in compound [Tl{(py)C(CN)NO}]n.

7. Ligands containing one pyridyl group, one oxime group

and a third donor group

7.1. Hydroxyimino-pyridin-2-yl-acetonitrile,

(py)C(CN)NOH, and the 2-quinolyl analogue

N CCN

N

OH

NC

N OH

CN

(py)C(CN)NOH

(qu)C(CN)NOH

These compounds belong to a relatively new class ofligands which have the general name cyanoximes. Thecoordination chemistry of cyanoximes first received de-tailed attention about two decades ago [123,124]. These

ligands have the general formula RC(CN)NOH, whereR is usually an electron withdrawing group such as anamide, ester or keto group [125]. The presence of the cy-ano group close to the oxime fragment makes the acidityof cyanoximes about 103–105 times greater than that ofcommon oximes or dioximes. Thus, all currently knowncyanoximes readily form yellow-colored conjugated an-ions in water or alcoholic solutions. The deprotonatedcyanoximes form numerous complexes with differentmetal ions [126–131]. These anions demonstrate ambid-entate properties participating in complex formationthrough different donor atoms in complexes with differ-ent metal ions. In addition, some cyanoximes and theirmetal complexes have demonstrated biological activitiessuch as growth-regulating [132], antimicrobial [133],detoxifying agricultural pesticide [134] and antiprolifer-ating [131] properties.

The free ligands (py)C(CN)NOH and (qu)C(CN)-NOH are synthesized by the reaction of equimolarquantities of 2-pyridylacetonitrile and 2-quilonylaceto-nitrile, respectively, and KNO2 in glacial CH3COOHat �50 �C [135]. The structures of the two free ligandshave been determined by single-crystal X-ray crystallog-raphy [135]. There are two planar fragments in themolecular structure of (py)C(CN)NOH, the 2-pyridylgroup and the cyanoxime NCCNO group. The dihedralangle between these planes is 10.6(1)�. This compoundexists in a cis–anti configuration with respect to the ori-entation of the CNO group and the nitrogen atom of the2-pyridyl ring. The crystal structure of the monohydrateof (qu)C(CN)NOH, (qu)C(CN)NOH Æ H2O, reveals thatthe molecule adopts a trans–anti configuration (see thestructural formulae of the free ligands in the beginningof this part). Compound (qu)C(CN)NOH exhibits a ni-troso-oxime equilibrium in polar solvents [136].

The crystal structure of [Tl{(py)C(CN)NO}]n revealspolymer formation (Fig. 50) [137]. The anionic ligand isplanar, exists in its nitroso form and adopts the cis–anticonformation; it exhibits a simultaneously chelating andbridging behavior. Compound [Cs{(py)C(CN)NO}]n isalso polymeric. The anionic ligand is planar, exists inits nitroso form and adopts the trans–anti configuration[138]. Complexes of the general composition

Fig. 51. Molecular structure of (py)C(NH2)NOH [142].

Fig. 52. Molecular structure of the cation present in complex[Cu{(py)C(NH2)NOH}2(H2O)]Cl2 [142].

NC

NH2

N OHM

1.0110

Fig. 53. The coordination mode of (py)C(NH2)NOH in its structurallycharacterized transition metal complexes.


[M{(py)C(CN)NO}2L2], where M = FeII, NiII, CuII andL = pyridine, 3-picoline, have been prepared [139].The authors used spectroscopic methods to propose thestructures of the complexes. Based on such data thecomplexes appear to have monomeric, trans-octahedralstructures in the solid state. The anionic ligand seemsto adopt its nitroso form exhibiting the N(2-pyridyl),N(nitroso)-chelating mode. The anions (py)C(CN)NO�

and (qu)C(CN)NO� have an important analytical appli-cation as reagents for the photometric determination ofFe2+, because of the great stability and large molarabsorptivities of the low-spin, 1:3 monoanionic Fe(II)complexes [135]. It has been established that the pres-ence of other metal ions, such as Co2+, Mn2+ andNi2+, does not affect the quantitative determination ofFe2+.

7.2. N-Hydroxy-pyridine-2-carboxamidine,

(py)C(NH2)NOH

NC

NOH

NH2

(py)C(NH2)NOH

Amidoximes and their metal complexes find a widerange of applications in technology, medicinal chemistryand agriculture [140]. As a bidentate ligand,(py)C(NH2)NOH incorporates the structural featuresof pyridine-2-carbaldehyde oxime (Section 5.2) and2-pyridylamine, (py)CH2NH2, in a single molecule. Theexperimental procedure for its synthesis [141] consistsof liberating the hydroxylamine from its hydrochlorideby means of sodium carbonate in water, adding anequivalent amount of 2-cyanopyridine and enough etha-nol to obtain a clear solution, and finally keeping themixture at 85 �C for 2 h; the yield of the crude productcan reach 98%. The crystal structure of the free ligand[142] reveals that the molecule exists as the syn isomer(Fig. 51), with the N atom of the 2-pyridyl ring onthe same side of the exocyclic C–C bond as the NH2

group.(py)C(NH2)NOH had been known to form stable

complexes with various metal ions (characterized byspectroscopic methods) [143], some of which wereexploited in analytical chemistry [144]. Few transitionmetal complexes containing the neutral ligand havebeen structurally characterized [142,145–147]. TheX-ray structure of [Cu{(py)C(NH2)NOH}2(H2O)]Cl2(Fig. 52) was reported independently by two groups in1989 [142,145]. The structure of the complex shows thefive-coordinate nature of the metal ion which is bound

through the heterocyclic and oxime nitrogen atoms(Fig. 53) of two trans-oriented bidentate ligands plus aH2O molecule to give a square-based pyramidal chelate.When an ethanolic solution of Ni(NO3)2 Æ 6H2O is trea-ted with (py)C(NH2)NOH, a dark blue solution resultsif the ratio of ligand to metal does not exceed 2:1. Fromthis solution the blue complex [Ni(NO3)2{(py)C(NH2)-


NOH}2] crystallizes [146]. If three equivalents of ligandare added to one equivalent of Ni(NO3)2 Æ 6H2O in aque-ous solution the initial color is blue, which changes towine-red and the 1:3 complex [Ni{(py)C(NH2)NOH}3]-(NO3)2 is obtained as red-brown crystals. In both com-plexes the neutral ligand adopts the 1.0110 chelatingmode (Fig. 53). The octahedral 1:2 complex (Fig. 54)has two coordinated nitrato groups in the equatorialplane which are cis to each other [146]. As (py)C(NH2)-NOH is added, the two cis equatorial monodentate nit-rato groups are replaced by the third ligand, resultingin the 1:3 cationic complex which has the structure[146] shown in Fig. 55. In the neutral 1:2 complex, thetwo heterocyclic nitrogens are in a cis arrangement.The three organic ligands in the cationic 1:3 complexhave their heterocyclic nitrogens co-planar with one

Fig. 54. Block diagram structure of [Ni(NO3)2{(py)C(NH2)NOH}2].

Fig. 55. X-ray structure of the octahedral cation presen

oxime nitrogen and the metal ion; this results in thetwo remaining oxime nitrogens being trans to each other.

In an attempt to deprotonate (py)C(NH2)NOH,Jones and co-workers [147] performed the 1:2 reactionbetween Ni(O2CMe)2 Æ 4H2O and the ligand in EtOH.The ligand is not deprotonated during complex forma-tion, as was confirmed by the X-ray structure of theresulting mononuclear octahedral complex [Ni(O2C-Me)2{(py)C(NH2)NOH}2] (Fig. 56). It is possible [147]that the NH2 group stabilizes the oxime group by delo-calization of the lone pair at the N atom of the aminogroup; the ensuing reduced electron density is the prob-able reason for its low affinity for the metal ion, i.e.,the non-participation of the NH2 group in coordination,in all the structurally characterized complexes of this

t in complex [Ni{(py)C(NH2)NOH}3](NO3)2 [146].

Fig. 56. Block diagram structure of [Ni(O2CMe)2{(py)C(NH2)N-OH}2].


ligand. Two mutually cis positions in the nickel(II) dis-torted octahedron are occupied by the O atoms of thetwo monodentate acetato ligands and the two otherpairs by two 1.0110 (Fig. 53) ligands. The oxime nitro-gens are mutually trans and the 2-pyridyl nitrogensmutually cis.

The coordination chemistry of the anionic ligand(py)C(NH2)NO� remains to be studied.

Although the 3- and 4-pyridyl isomers of(py)C(NH2)NOH have been synthesized [141,148], theircoordination chemistry has not been investigated.

7.3. 3-(2-Pyridin-2-yl-methylimino)-butan-2-one oxime

(pmiboH), 3-(2-Pyridin-2-yl-ethylimino)-butan-2-oneoxime (peiboH) and their reduced amino analogs

(pmaboH, peaboH)

The above shown ligands contain 2-pyridyl groupsthat are not directly attached to the oxime carbon atom.The abbreviations of the ligands in this part derive [149]from the non-systematic names 2-[2-(a-pyridyl)methyl]imino-3-butanone oxime (pmiboH), 2-[2-(a-pyr-idyl)ethyl]imino-3-butanone oxime (peiboH), 2-[2-(a-pyridyl)methyl]amino-3-butanone oxime (pmaboH)and 2-[2-(a-pyridyl)ethyl]amino-3-butanone oxime (pea-boH); H denotes the oximic hydrogen.

Detailed syntheses of the four free ligands were re-ported by Randacci and co-workers [150]. The com-pounds pmiboH and peiboH were synthesized startingfrom diacetyl monoxime and the appropriate aminopyri-dine following the usual procedure for the preparation ofSchiff bases; the solvent used was diisopropyl ether. Thecompounds pmaboH and peaboH were prepared bytreating pmiboH and peiboH, respectively, with NaBH4

in methanol; the reactions involves hydrogenation ofthe �C@N–CH2– imino groups to �CHNH–CH2– aminogroups. The crystal structure of pmiboH has been deter-mined [151].

The crystallographically established coordinationmodes of the neutral and monoanionic ligands areshown in Fig. 57.

The first structurally characterized complex of theseligands was [Cu2(peibo)2(MeCN)2](ClO4)2 [152]. Thecation (Fig. 58) contains a six-membered ring formedby two CuII atoms and two oximate groups; the ring is

distinctly non-planar with a twisted-boat conformation.Each peibo� ligand adopts the 2.1111 coordinationmode (Fig. 57). Perchlorate and nitrate salts containingthe structurally similar cations [Cu2(peibo)2]

2+, [Cu2-(peibo)2(H2O)2]

2+ and [Cu2(pmibo)2]2+ have been pre-

pared [149]. The �2J values (H = �2JS1S2) are in the510–835 cm�1 range, indicative of strong antiferromag-netic coupling. The dinuclear complexes showed rela-tively narrow 1H NMR signals in the �0.5–30 ppmrange (Dm1/2 = 60–1500 Hz), indicating that the antifer-romagnetic interaction is maintained in DMSO-d6[149]. The �2J values roughly correlate with the 1HNMR parameters; the larger the �2J values, the smallerthe chemical shifts and linewidths. The cation [Cu2(pmi-bo)2]

2+ was found to undergo an autoreduction reactionin DMSO, DMF and DMA. The triply-bridged dinu-clear copper(II) complexes [Cu2(peibo)2(pz)](ClO4) and[Cu2(peibo)2(phta)](ClO4)2 (Fig. 59), where pz� is thepyrazolate anion and phta is phthalazine, have been pre-pared [153]; a very strong antiferromagnetic interaction(2J = �760 cm�1) between the metal ions was observedfor the latter. In the dinuclear complexes [Cu2(OMe)-(ClO4)(peibo)(bpy)](ClO4) and [Cu2(O2CMe)2(peibo)-(bpy)](ClO4) the two CuII atoms are bridged by one2.1111 peibo�, one methoxo and one perchlorato li-gands, and one 2.1111 peibo� and two acetato ligands,respectively [154]. Matsumoto and co-workers [155]prepared and structurally characterized the dinuclear,end-on azido-bridged complexes [Cu2(N3)(peibo)(bpy)]-(ClO4)2 and [Cu2(N3)(peibo)(pmdt)](ClO4)2, where pmdtis the tridentate chelating ligand N,N,N 0,N00,N00-pentam-ethyldiethylenetriamine. The peibo� ligand is in the2.1111 (Fig. 57) coordination mode. For both com-plexes, the two CuII atoms are antiferromagneticallycoupled with a singlet-triplet separation of 2J = �520and �296 cm�1 for the bpy and pmdt complexes, respec-tively. The complexes are EPR-silent in the solid state atroom temperature. Two other peibo�-containing Cu(II)complexeshavebeen reported; these are [Cu3(N3)2(peibo)2-(NO3)2(H2O)2] [156] and [Cu2(peibo)2(OClO3)2]n [157],in which the ligand is in the 2.1111 mode. In the perchl-orato coordination polymer, the dinuclear units arebridged by one inorganic anion which adopts ang1:g1:g1:l3 mode [157].

The Ni(II) coordination chemistry of pmiboH andpeiboH is practically unknown. The crystal structureof the complex [Ni(peiboH)2](NO3)2 has been reported[158]. In the all-trans mononuclear octahedral cation,the neutral ligand adopts the 1.0111 mode (Fig. 57).

Contrary to Ni chemistry, the Co coordination chem-istry of pmiboH and peiboH is well developed; most ofthe known complexes are organometallic. The first refer-ence to a Co complex of these ligands in the literaturecame in 1983 in the X-ray structure of [CoIII(peibo)2]-(ClO4) (Fig. 60) [159]. The CoIII ion is coordinated tosix N atoms from two tridentate chelating anionic

M

N N

MeHMe

HHO (CH2)n

N

M

N N

MeHMe

HO (CH2)n

N

H

M

N N

C C

MeMe

O (CH2)n

N

M

N N

C C

MeMe

O (CH2)n

N

MM

N N

C C

MeMe

O (CH2)n

N

M

N N

C C

MeMe

HO (CH2)n

N

M

N N

C C MeHMe

HO (CH2)2

N

CH2

1.0111 1.0011

1.0111 2.1111

1.0111

M

N N

MeHMe

HHO (CH2)n

N

1.0011

1.0111

M

N N

MeHMe

HO (CH2)n

N

1.0011

Fig. 57. The crystallographically established coordination modes of pmiboH, peiboH, pmaboH, peaboH and their monoanions, and the Harrisnotation [44] that describes these modes. The dashed lines indicate hydrogen bonds.


ligands (1.0111, Fig. 57) in a slightly distorted octahe-dral arrangement. Each peibo� ligand forms one five-and one six-membered chelating ring. The ligands arealmost planar in a mer configuration around the metal.The molecular structure of the cation [Co(pmibo)2]

+ issimilar (Fig. 60) [150]. The decrease in the CoIII–Npy dis-tances and Npy–Co–Nim angles from [Co(peibo)2]

+ to[Co(pmibo)2]

+ has been ascribed to the steric constraintimposed by the closure of the five-membered ring, con-taining the Npy atom in the latter [150]. The aminooxi-mes pmaboH and peaboH react with CoCl2 Æ 6H2O inMeOH in the presence of ClO4

� under atmospheric con-

ditions to give the complexes [Co(pmabo)(pmaboH)]-(ClO4)2 and [Co(peabo)(peaboH)](ClO4)2 [150]. In bothstructures (Fig. 61), one protonated 1.0111 and onedeprotonated 1.0111 ligand coordinate the CoIII ionthrough their N donors in a fac configuration, in sucha way that the two 2-pyridyl rings are trans to eachother. The two O atoms make a strong intramolecularhydrogen bond. The formation of the hydrogen bondserves further to stabilize the fac arrangement of theligands.

The reduction of [CoIII(peibo)2](ClO4) with NaBH4

in alkaline media produces a nucleophilic Co(I) species

Fig. 58. Block diagram structure of the cation [Cu2-(peibo)2(MeCN)2]

2+.Fig. 60. Block diagram structure of the cations [Co(pmibo)2]

+ and[Co(peibo)2]

+.

Fig. 61. Block diagram structure of the cations [Co(pmabo)(pma-boH)]2+ (n = 1) and [Co(peabo)(peaboH)]2+ (n = 2).


which, upon reaction with alkyl halides, gives stableorganocobalt dinuclear complexes (Scheme 6). Thisreactivity pattern parallels that observed for cobaloxi-mes and other vitamin B12 models [160]. The molecularstructure of the methyl complex [161] is shown inFig. 62. The R = CH2CF3, Cy analogues have similarstructures [162]. The peibo� ligand adopts the 2.1111mode. On the contrary, the reduction of [CoIII(pmi-bo)2](ClO4) involves hydrogenation of the ligand fromimino- to amino-oxime, with the formation of a stableCo(II) species; the latter can be oxidized to afford [Co-(pmabo)(pmaboH)](ClO4)2, see Scheme 6. The differentreactivity was attributed [150] to the more strained coor-dination in [CoIII(pmibo)2](ClO4) with respect to that in[CoIII(peibo)2](ClO4).

Addition of NaBH4 to an alkaline solution of [CoIII-(peibo)2](ClO4) under a nitrogen atmosphere, followedby addition of benzyl chloride gave the mononuclearcomplex [CoIII(C6H5CH2)(peibo)L](ClO4), where L is

Fig. 59. X-ray structure of the cation present in

2-[(2-pyridylethyl)amino]-3-aminobutane [163]. Thepeibo� tridentate ligand in a mer configuration coordi-nates CoIII through its N-donors (1.0111). The complex

complex [Cu2(peibo)2(phta)](ClO4)2 [153].

2 mer-[CoIII(peibo)2]+ NaBH4 2 [CoI]RX, OH-

[{CoIIIR(peibo)}2(µ-OH)]+ + 2 (peibo)- + 2 X-

mer-[CoIII(pmibo)2]+ NaBH4 [CoII]O2 fac-[CoIII(pmabo)(pmaboH)]2+

Co2+ + pmaboH Co2+ + pmaboH

N2 O2

Scheme 6. Redox reactivity pattern of [CoIII(peibo)2]+ and [CoIII(pmibo)2]

+.

Fig. 62. The molecular structure of the dinuclear cation present incomplex [{CoIII(Me)(peibo)}2(l-OH)](ClO4) [161].


undergoes Co–C homolytic cleavage under acidic condi-tions, giving dibenzyl under a nitrogen atmosphere andbenzaldehyde in the presence of air [163].

The oxidative addition of alkyl halides to the CoI

species generated by the reduction of [CoIII(peabo)-

Fig. 63. X-ray structure of the cation present in the c

(peaboH)](ClO4)2 (Fig. 61) [150], led to the formationof a new class of organocobalt complexes of general for-mula [CoIIIR(peabo)(peaboH)](ClO4) [164], where R =Me, Et, CH2CF3,

nBu and CH2Cl. The X-ray structuresof the R = Me, Et (Fig. 63) and CH2CF3 compoundsprovide conclusive evidence for a distorted octahedralstructure, where peaboH and peabo� act as 1.0011(Fig. 57) and 1.0111 (Fig. 57) ligands, respectively. In fact,the non-organometallic ligand system about CoIII can beconsidered as (peabo� � �H� � �peabo)�; adopting thisformulation, the hydrogen bridged anion behaves as apentadentate chelating ligand. The axial geometry in theR = Me compound is closer to that found in methylco-balamin than that reported for other models, suggestingsteric and electronic cis influences of the equatorialligands close to those of the corrin nucleus [164].

Treatment of the complexes [CoIIIR(peabo)(pea-boH)](ClO4) [164], where R = CH2X (X = halogen),with diluted NaOH afforded [165] the complex [CoIII-(peabo)L](ClO4), where L is the monoanionic ligandwhose coordination mode is shown on the bottom ofFig. 57. The three-membered ring is formed by a path-way involving intramolecular nucleophilic addition ofan equatorial nitrogen donor to the axial carbon.

omplex [CoIII(Et)(peabo)(peaboH)](ClO4) [164].

Fig. 64. X-ray structure of the cation present in complex [CoIII(peabo)-L](ClO4) [165].


X-ray analysis reveals a highly distorted structure(Fig. 64). The C–Co–N angle is acute (42.8�). Thepeabo� ion behaves as 1.0111 ligand.

Complex mer-[CoIII(pmibo)2](ClO4) (Fig. 60) [150]gives cobalt alkyl derivatives after reduction withNaBH4/Pd

2+ to CoI and alkylation [166]. The formationof the Co–C bond is accompanied by the reduction ofthe amino form of one or both imino ligands (dependingon the experimental conditions) initially present in thestarting material. In one series of experiments, com-plexes of the type fac-[CoIIIR(pmibo)(pmaboH)](ClO4)(R = Me, i-Pr, CH2Cl, CH2Br, CH2CF3, Bz) were ob-tained, in which only one of the two ligands was reduced

Fig. 65. Molecular structures [166] of the cations present in comp(i-Pr)(pmibo)(pmaboH)](ClO4) (right).

to the amino form (pmaboH). The molecular structuresof two representative cations are shown in Fig. 65. Theanion of the unmodified imino ligand acts as a bidentateNimino, Noximate-ligand (1.0011, Fig. 57), whereas theneutral amino-oxime molecule (pmaboH) acts as a tri-dentate ligand (1.0111, Fig. 57). The saturation of oneazomethine group causes the product to assume a fac-configuration and induces the formation of one asym-metric carbon and one asymmetric nitrogen center inthe chelating system. When an excess of reducing agentwas used, both azomethine groups were saturated, caus-ing the introduction of one pair of chiral carbons andone pair of chiral nitrogens. Two isomers of the methylderivative [CoIII(Me)(pmabo)(pmaboH)]+ were isolated(Fig. 66) [166]. The pmabo� and pmaboH ligands adoptthe 1.0011 and 1.0111 modes, respectively. One isomerdiffers from the other in the opposite configuration ofthe C and N centers located on the bidentate ligand.One isomer closely resembles the peabo�/peaboH ana-log [164]. Similarities and differences in the reactivityexhibited by [CoIII(pmibo)2]

+ and [CoIII(peibo)2]+ were

discussed [166].

7.4. Other ligands

Ligands featuring a 6-alkylaminomethyl-2-pyr-idinealdoxime moiety (alkyl = CH3, n-C12H25) havebeen synthesized as outlined in Scheme 7 [167]. Thereactivity of their Ni(II), see Fig. 67, and Zn(II) com-plexes in the cleavage of p-nitrophenylacetate andp-nitrophenylhexanoate has been investigated in theabsence (R = CH3) or in the presence (R = n-C12H25)

lexes fac-[CoIII(Me)(pmibo)(pmaboH)](ClO4) (left) and fac-[CoIII-

Fig. 66. The two [CoIII(Me)(pmabo)(pmaboH)]+ isomers [166].

Scheme 7. Synthesis of ligands featuring a 6-alkylaminomethyl-2-pyridinealdoxime moiety. Conditions: (i) di-tert-butyl dicarbonate,triethylamine, dioxane, 20 �C; (ii) selenium dioxide, dioxane, reflux;(iii) hydroxylamine hydrochloride, Na2CO3, EtOH, 60 �C; (iv) triflu-oroacetic acid, 20 �C.

Fig. 67. Proposed structure of the 1:2 nickel(II)/6-methylaminom-ethyl-2-pyridinealdoximate complex.


of hexadecyltrimethylammonium bromide micelles. Themicellar complexes are effective in promoting the cleav-age of the substrate with accelerations strongly depen-dent on pH, being larger in moderately acidic than inneutral solutions. The coordination chemistry of these2-pyridyl oximes remains completely unexplored.

8. The rich coordination chemistry of di-pyridin-2-yl-

methanone oxime (di-2-pyridyl ketone oxime),

(py)2CNOH

8.1. Introduction

NC

(py)2CNOH

N

N

HO

Di-pyridin-2-yl-methanone oxime (di-2-pyridyl ke-tone oxime), (py)2CNOH, occupies a special positionamongst the 2-pyridyl oximes. One area to which theanionic ligand (py)2CNO� is relevant is the chemistryof metallamacrocycles. Another attractive aspect of(py)2CNO� is its great coordinative flexibility and ver-satility, characteristics that have led to polynuclear3d-metal complexes with impressive structures andinteresting magnetic properties. A third interesting fea-ture is the activation of (py)2CNOH by 3d-metal cen-ters, which appears to be a fruitful area of syntheticinorganic chemistry; examples of this activation willbe described below.

The published coordination chemistry of (py)2-CNOH is rich [168–188]. The free ligand is commerciallyavailable. Since several complexes of (py)2CNO� canbe considered as metallamacrocycles, and especiallyas metallacrowns, we feel obliged to give brief informa-tion about these compounds. Metallamacrocycles havegained increasing attention over the past decade due totheir potentially unique properties. These moleculeshave already been used in applications as diverse ascatalysts [189], sensors [190] or as chiral building


blocks for 2D and 3D solids [191,192]. Metallamacro-cycles include complexes such as metallacrowns[67,193], metallacrowns containing carbon in the mac-rocycle [194,195], metallacrown ethers [196], azametal-lacrowns [197,198], anticrowns [199], metallahelicates[200], metallacalixarenes [201], metallacryptates [202],molecular squares and boxes [203], and the aestheti-cally pleasant polynuclear fluoro, alkoxo or oxo metalcomplexes [204–206]. Metallacrowns (MCs) [67], theinorganic structural and functional analogs of crownethers [207], are usually formed when a transition metalion and a nitrogen replace the methylene carbonatoms. MCs exhibit selective recognition of cationsand anions, and can display intramolecular magneticexchange interactions. The isolation of metallacrowns

NC

N

N

M OH

NC

NM

NC

NN

M OHH

NC

NM

NC

NN

M OM

NC

NM

NC

NN

M O M

M

1.0110 2.0

1.0111.011

2.0111 2.111

3.2111

Fig. 68. The crystallographically established coordination modes of (py)2CNmodes.

requires the employment of tri- and tetradentate li-gands containing hydroxamate or oximate functional-ities to provide a scaffolding within which the desiredmetal-containing core can be realized. One exampleof such a ligand is (py)2CNO�. This approach yieldsclusters with M–N–O–M networks. Metallacrownnomenclature has been given in refs. [67,181] and[193]. There are nine metals in four oxidation states(II–IV) that have been incorporated into the MC ring,while more than 20 metal ions, i.e., lanthanide, acti-nide, alkali, alkaline earth and transition metal ionshave been captured in the central cavity of MCs. For12-MC-4 complexes, two structural motifs have beenreported: classical or regular [67,181,193,208] and in-verse [66,67,172,179,184,185]. In the regular motif,

N

OH

NC

N

N

M OM

H

M

N

O

NC

N

N

MO

M

NC

NN

M OM

M

N

O M

NC

NN

M O M

M

1112.1110

2.11100

1

3.1111

3.2110

OH and (py)2CNO�, and the Harris notation [44] that describes these

Fig. 69. Block diagram structure of the square pyramidal complex[AuCl{(py)2CNO}2].


there is an N–O–M–N–O–M linkage, i.e., an [M–N–O]n repeat unit, with the oxygen atoms oriented to-wards the center of the cavity and capable of bindingcations. In the inverse motif, which has been realizedonly for Zn and Co, the ring metal ions are orientedtowards the center of the cavity which is now capableof encapsulating anions, whereas the connectivity istransposed to N–O–M–O–N–M. It should be men-tioned at this point that some researchers prefer toconsider MCs simply as one sub-area of metal wheel(ring) chemistry, avoiding the use of their specializednomenclature.

The crystallographically established coordinationmodes of (py)2CNOH and (py)2CNO� are shown inFig. 68.

Fig. 70. A small portion of one of the zig-zag chains present in thecomplex [Cu(NCS){(py)2CNOH}]n [171].

8.2. Metal complexes containing terminal (py)2CNOH

and/or (py)2CNO� ligands

These complexes are listed in Table 3. Molecularstructures of representative complexes are shown inFigs. 69–71. Six out of the eight complexes listed in Ta-ble 3 are mononuclear and present no special structuralinterest. In the trinuclear complex [Ni3(shi)2{(py)2-CNOH}2(py)2], the NiII ions are bridged by the shi3�

ligands [169]; two metal ions have a square planar geom-etry while the third one is in an octahedral environment.The open array of the three metal ions is angular, withan Ni� � �Ni� � �Ni dihedral angle of 46.5�. The structureof [Cu(NCS){(py)2CNOH}]n features tetrahedral geom-etry around CuI atoms (Fig. 70) with a N,S-bridgingthiocyanate group creating zig-zag chains along the c-axis of the unit cell [171].

Complex ½Cu2IIðl-ClÞ2Cl2fðpyÞðpyHÞCNOHg2-

ðH2OÞ2�Cl2 (Fig. 71), not listed in Table 3, is unique, be-cause it contains the monocation of di-2-pyridyl ketoneoxime as a ligand [186b]. The cation (py)(pyH)CNOHbehaves as a terminal 1.011 ligand, see Fig. 68.

Table 3Structurally characterized metal complexes containing exclusively terminal (py)2CNOH and/or (py)2CNO� ligands

Complexa Coordination mode Coordination sphere; coordination geometry Reference

[Mn(O2CPh)2{(py)2CNOH}2] 1.0110 cis,cis,trans-MnIIO2(Npy)2(Nox)2; oct [168][Ni3(shi)2{(py)2CNOH}2(py)2] 1.0110 NiIIO3N, NiO4N2; sp, oct [169][Co(NO2){(py)2C(OH)O}{(py)2CNO}]b 1.0110 CoIIION5; oct [170][Ni(O2CPh)2{(py)2CNOH}2] 1.0110 cis,cis,trans-NiIIO2(Npy)2(Nox)2; oct [97][CuCl{(py)2CNO}{(py)2CNOH}]c 1.0110, 1.0110 CuIIN4Cl; spy [97][Cu(NCS){(py)2CNOH}]n 1.0110 CuIN3S; tet [171][ZnCl2{(py)2CNOH}2] 1.0110 ZnIIN2Cl2; tet [172][AuCl{(py)2CNO}2] 1.0110 AuIIIN4Cl; spy [173]

Abbreviations: Nox, oxime or oximate nitrogen; Npy, 2-pyridyl nitrogen; oct, octahedral; (py)2C(OH)O�, the mononanion of the gem-diol derivativeof di-2-pyridyl ketone; shi3�, the fully deprotonated form of salicylhydroxamic acid; sp, square planar; spy, square pyramidal; tet, tetrahedral.a Solvate molecules have been omitted.b The NO2

� ligand is in its nitro form.c In fact, the ligand system about CuII can be considered as {(py)2CNO� � �H� � �ONC(py)2}

�; adopting this formulation, the hydrogen-bridgedanion behaves as a tetradentate chelating ligand.

Fig. 71. X-ray structure of the dinuclear cation present in the complex [Cu2Cl4{(py)(pyH)CNOH}2(H2O)2]Cl2, which contains the monocation of(py)2CNOH as a ligand [186b].


8.3. Metal complexes containing bridging (py)2CNOH

and/or (py)2CNO� ligands

These complexes are listed in Table 4. Molecularstructures of representative complexes are shown inFigs. 73–95. The majority of the listed complexes aredinuclear and polynuclear (clusters).

The Mn/(py)2CNO� chemistry is interesting[168,174–178]. Using a variety of synthetic routes thecomplexes ½Mn2

IIMn2IIIðO2CRÞ2fðpyÞ2CO2g2fðpyÞ2-

CNOg2X2�, where R =Me, Ph and X = Cl, Br, NO3,etc. have been isolated in good yields [168,176]. Remark-able features of the reactions are the in situ trans-formation of an amount of (py)2CNOH to yield thecoordinated dianion, ðpyÞ2CO2

2�, of the gem-diol deriv-ative of di-2-pyridyl ketone, (py)2CO (Fig. 72) and thecoordination of nitrate ligands in the X� ¼ NO3

�

cluster (Fig. 73) although the starting materials werenitrate-free (Scheme 8). It is noteworthy that a simi-lar transformation of (py)2CNOH to the monoanion(and not to the dianion as observed in the Mn2

IIMn2III

clusters) of the gem-diol derivative of di-2-pyridyl ke-tone, (py)2C(OH)O� (Fig. 72), was also reported by Jen-sen and co-workers [170]. The 1:2 reaction between[CoIII-(CO3)(NH3)4](NO3) and (py)2CNOH in H2Oyielded the complex [CoIII(NO2){(py)2C(OH)O}{(py)2-CNO}], see Table 3. The authors avoided mechanisticdiscussions and it is not clear whether the nitro ligandpresent in the product resulted from oxidation of theoxime or from reduction of the NO3

� counterion ofthe starting material. The tetranuclear clusters havecompletely analogous molecular structures [168,176].The centrosymmetric tetranuclear molecule containstwo MnII and two MnIII six-coordinate ions (the MnII

ions are seven-coordinate in the nitrato cluster becauseof the chelating behavior of the NO3

� ions) held to-gether by four l-oxygen atoms from the two 3.2211ðpyÞ2CO2

2� ligands to give the {MnII(l-OR00)MnIII-(l-OR00)2MnIII(l-OR00)MnII}6+ core consisting of aplanar zig-zag array of the four metal ions (R00 = (py)2-C(OH)–). Peripheral ligation is provided by two 2.1110(py)2CNO�, two 2.11 RCO2

� and two terminal X� li-

gands. Variable-temperature magnetic susceptibilitystudies in the 2–300 K range reveal weak antiferromag-netic exchange interactions, leading to non-magneticS = 0 ground states.

Reaction of Mn(hfac)2 Æ 3H2O (hfac� = hexafluoro-acetylacetonate) with one equivalent of (py)2CNOH inCH2Cl2 gives the dinuclear complex [Mn2(O2CCF3)2-(hfac)2{(py)2CNOH}2] in 70% yield. The CF3CO2

� li-gand is one of the decomposition products of the hfac�

ligand [168]. The twoMnII ions are bridged by twoneutral(py)2CNOH ligands which adopt the 2.0111 coordinationmode.

The trinuclear complexes ½Mn2IIMnIVðOMeÞ2-

X2fðpyÞ2CNOg4� are rare examples of complexes simul-taneously containing MnII and MnIV ions (X� = Cl�,NCO�, NCS�) [174,175]. The molecular structure ofthe X� = NCS� cluster is shown in Fig. 74. X-ray crys-tallography and XANES spectroscopy clearly distin-guish the Mn2

IIMnIV valence isomer from the morecommonly observed Mn2

IIIMnII formulation. There isa central six-coordinate MnIV ion in an MnO6 coordina-tion environment and two terminal six-coordinate MnII

ions having an MnOWN4 chromophore. Fits to vari-able-temperature magnetic susceptibility data (Fig. 75)indicate that the MnII and MnIV ions are ferromagneti-cally coupled and that the compounds have an S = 13/2ground state.

Complex ½Mn3IIMnIVOð3; 4-DÞ4fðpyÞ2CNOg4�,

where 3,4-D� is the anion of 3,4-dichlorophenoxyaceticacid, has the fMn3

IIMnIVðl4-OÞðg1 : l2-O2CRÞg7þ core[177]; its molecular structure (Fig. 76) is very similarto that (Fig. 30) of the complex ½Mn3

IIMnIVO-ðN3ÞðO2CPhÞ3fðpyÞCðphÞNOg4� [96], the only essentialdifference being the presence of one g1:l2 carboxylategroup instead of one g1:l2 azido group. Magnetizationmeasurements (Fig. 77) support an S = 6 ground state.

Reaction of Mn(ClO4)2 Æ 6H2O with (py)2CNOH inMeOH in the presence of NaOH gives the mixed-valentcluster ½Mn4

IIMn6IIIMn2

IVðl3-OHÞ4ðl3-OÞ4ðl4-OÞ2-ðl-OMeÞ2fðpyÞ2CNOg12�ðOHÞðClO4Þ3. The clustercontains a 24-MC-8 ring which wraps a 16-membered,star-shaped ring containing four metal ions [178]. This

Table 4Structurally characterized metal complexes containing bridging (py)2CNOH and/or (py)2CNO� ligands

Complexa Coordination mode Coordination sphere; coordination geometry Reference

[Mn2(O2CCF3)2(hfac)2{(py)2CNOH}2] 2.0111 MnII(Ohfac�)2(OCF3CO2�)(Npy)2(Nox); oct [168]½Mn2

IIMnIVðOMeÞ2W2fðpyÞ2CNOg4� 2.1110 MnII(Omethoxo)(Npy)2(Nox)2W, MnIV(Omethoxo)2(Oox)4; oct, oct [174,175]½Mn2

IIMn2IIIðO2CRÞ2fðpyÞ2CO2g2fðpyÞ2CNOg2X2� 2.1110 MnIIO2N3X, MnIIIO5N; oct, oct [168,176]

½Mn2IIMn2

IIIðO2CMeÞ2fðpyÞ2CO2g2fðpyÞ2CNOg2ðNO3Þ2� 2.1110 MnIIO4N3, MnIIIO5N; pbp, oct [168]½Mn3

IIMnIVOð3; 4-DÞ4fðpyÞ2CNOg4� 2.1110 MnII(l4-O)(Ocarb)3(Npy)(Nox), MnII(l4-O)(Ocarb)2(Oox)(Npy)(Nox),MnIV(l4-O)(Oox)3(Npy)(Nox); oct, oct, oct

[177]

[Mn12(OH)4O6(OMe)2{(py)2CNO}12](ClO4)3(OH) 2.1110 MnII(l3-OH)(l3-O)(Nox)4, MnII(l3-OH)(l3-O)2(l4-O)2(Oox), MnIII

(l3-OH)(l3-O)(l4-O)(Oox)(Nox)2, MnIII(l3-OH)(l3-O)(Oox)2(Nox)2,MnIII(l3-OH)(l4-O)(Omethoxo)(Nox)2, MnIV(l3-OH)(l3-O)(Oox)2(Nox)2; oct, oct, oct, tbp, oct

[178]

½Co2IICo2IIIðOR0Þ2ðO2CRÞ2fðpyÞ2CNOg4S2�ðClO4Þ2 2.1110, 2.1111 CoII(OR�O�)(Ocarb)(Oox)2(OS)(Npy), CoIII(OR�O�)(Ocarb)(Npy)2(Nox)2;

oct, oct[179]

[Ni4{(py)2CNO}6(MeOH)2](ClO4)(OH) 2.1110, 2.1111 fac-NiII(Npy)3(Nox)3, NiII(Oox)4(OMeOH)(Npy); oct, oct [180][Ni4(NCS)2(Hshi)2{(py)2CNO}2(DMF)(H2O)] 2.1111 NiIIO3N3, NiIIN3O; oct, sp [181][Ni4Na2(acac)4{(py)2CNO}4](ClO4)2 2.1111b NiII(Oacac�)2(Npy)2(Nox)2, NiII(Oacac�)2(Npy)2(Oox)2; oct, oct [97][Ni4(O2CMe)2{(py)2CNO}4](SCN)(OH) 3.2111 NiII(Ocarb)(Oox)2(Npy)2(Nox); oct [97][Ni4{(py)2CNO}4{(py)2CNOH}2(H2O)2](ClO4)4 2.1110, 3.2111, 2.1110 fac-NiII(Npy)3(Nox)3, NiII(Oaqua)(Oox)4(Npy); oct, oct [97][Ni5(O2CMe)2(shi)2{(py)2CNO}2] 3.2111 NiIIO4N2, NiIIO6, NiIIO2N2; oct, oct, sp [181][Ni5(acac)2{(py)2CNO}6(H2O)(MeOH)](ClO4)2 2.1110, 3.1111, 3.2111 NiII(Osolvent)(Oox)2(Npy)2(Nox), NiII(Oox)2(Npy)2(Nox)2,

NiII(Oacac�)2(Oox)(Npy)2(Nox)[97]

[Ni5(O2CMe)7{(py)2CNO}3(H2O)] 2.1111, 3.2110, 3.2111 NiII(Oaqua)(Ocarb)2(Oox)(Npy)(Nox), NiII(Ocarb)3(Oox)2(Npy),NiII(Oaqua)(Ocarb)3(Npy)(Nox), NiII(Ocarb)4(Npy)(Nox),NiII(Ocarb)2(Oox)3(Npy); oct, oct, oct, oct, oct

[97]

[Ni5{(py)2CNO}5(H2O)7](NO3)5 2.1110, 3.2111 NiII(Oaqua)2(Oox)2(Npy)2, NiII(Oaqua)(Oox)5,NiII(Oaqua)2(Oox)(Npy)2(Nox), NiII(Oaqua)2(Npy)2(Nox)2,NiII(Oox)(Npy)3(Nox)2; oct, oct, oct, oct, oct

[97]

[Ni7(O2CMe)6(N3)2{(py)2CNO}6(H2O)2]c 1.0110, 3.2111 NiII(Ocarb)2(Oox)2(Npy)2, NiII(Oaqua)(Ocarb)2(Oox)2(Npy),

NiII(Ocarb)(Nazido)(Npy)2(Nox)2, NiII(Ocarb)2(Oox)(Nazido)(Npy)(Nox);oct, oct, oct, oct

[97]

[Ni10(MCPA)2(shi)5{(py)2CNO}3(MeOH)3(H2O)] 3.2111 Various chromophores; 8 NiII ions oct, two NiII ions sp [181]

174C.J.Milio

set


[Mn2Ni2(O2CMe)2(shi)2{(py)2CNO}2(DMF)5] 2.1110 MnIIIO5N, NiIIO4N2; oct, oct [181][Cu2Cl2{(py)2CNOH}2] 2.0111 CuICl(Npy)2(Nox) [171][Cu2{(py)2CNO}4]

c 1.0110, 2.1110 CuII(Oox)(Npy)2(Nox)2; tbp [182][Cu2(hfac)2{(py)2CNO}2] 2.1110 CuII(Ohfac�)2(Oox)(Npy)2(Nox); tbp [97][Cu3(OH)(O2CR)2{(py)2CNO}3] 2.1110 CuII(l3-OH)(Ocarb)(Oox)(Npy)(Nox); spy [183][Zn4(OH)2X

02{(py)2CNO}4] 2.1110 ZnII(l3-OH)(Oox)2X�, ZnII(l3-OH)2(Npy)2(Nox)2; tet, oct [172,184]

[Zn4(OH)2Z2{(py)2CNO}4] 2.1110 ZnII(l3-OH)(OZ)2(Oox)2, ZnII(l3-OH)2(Npy)2(Nox)2; tbp, oct [184,185]

[Zn5Cl2{(py)2CNO}6][ZnCl(NCS)3] 2.1110, 3.1111, 3.2111 cis,cis,trans-ZnII(Oox)2(Npy)2(Nox)2, ZnII(Oox)2(Npy)2(Nox),

ZnII(Oox)(Npy)2(Nox)Cl; oct, tbp, spy[184]

[Zn5(NCS)2{(py)2CNO}6(MeOH)][Zn(NCS)4] 2.1110, 3.1111, 3.2111 cis,cis,trans-ZnII(Oox)2(Npy)2(Nox)2, ZnII(Oox)2(Npy)2(Nox),

ZnII(Oox)(Nisothiocyanato)(Npy)2(Nox),ZnII(OMeOH)(Oox)(Nisothiocyanato)(Npy)2(Nox); oct, tbp, tbp, oct

[184]

[Zn8(shi)4{(py)2CNO}4(MeOH)2] 2.1110, 3.1111 ZnIIO6, ZnIIO3N3, Zn

IIO3N2, ZnIIO4; oct, oct, tbp, tet [172]

[Ru3{(py)2CNO}2(CO)8]d 2.1110 RuC2(Oox)(Npy)(Nox)Ru, RuC4Ru2 [186a]

[Ru2{(py)2CNO}2(CO)4]d 2.1110 RuC2(Oox)(Npy)(Nox)Ru [186a]

[Os3(l-H){(py)2CNO}(CO)9]d 2.1110 OsC4Os2, OsHC2(Npy)(Nox)Os2, OsHC3(Oox)Os2 [186a]

[Ag2(NO3)2{(py)2CNOH}2] 2.0111 AgI(Onitrato)(Oox)(Npy)2(Nox); tet [186b][Cd2(O2CR)4{(py)2CNOH}2] 2.0111 CdII(Ocarb)3(Npy)2(Nox); oct [187][Cd(O2CMe)(SCN){(py)2CNOH}]n 2.0111 CdII(Ocarb)(NSCN�)(Npy)2(Nox)S; oct [187][Hg2(O2CMe)4{(py)2CNOH}2] 2.0111 HgII(Ocarb)3(Npy)2(Nox); oct [97][Hg2(O2CPh)3{(py)2CNO}{(py)2CNOH}2(MeOH)] 2.0111, 2.0111 HgII(Ocarb)3(Npy)2(Nox), HgII(OMeOH)(Ocarb)2(Npy)2(Nox); oct, oct [97][HgCl(O2CPh){(py)2CNOH}]n 2.0111 HgIICl2(Ocarb)(Npy)2(Nox); oct [97][Tb3Zn3{(py)2CNO}6(NO3)9] 3.2111 TbIII(Onitrato)6(Oox)2(Npy)2, Zn

II(Npy)3(Nox)3, ZnII(Oox)6; sph, oct [187]

[Ln2Ni(hfac)6{(py)2CNO}2(py)2] 2.1111 No details available [188][LnNi2(hfac)2{(py)2CNO}4(MeOH)][Ln(hfac)4(MeOH)] 2.1110, 3.2111 No details available [188]

Abbreviations: acac�, acetylacetonate; 3,4-D�, 3,4-dichlorophenoxyacetate(�1); hfac�, hexafluoroacetylacetonate; Hshi2�, the dianion of salicylhydroxamic acid; M = Ru, Os; MCPA�, 2-methyl-4-chlorophenoxyacetate(�1); Nox, oxime or oximate nitrogen; Npy, 2-pyridyl nitrogen; Ocarb, carboxylate oxygen; Oox, oximate oxygen; oct, octahedral; pbp, pentagonal bipyramidal; ðpyÞ2CO2

2�, thedianion of the gem-diol derivative of di-2-pyridyl ketone; R =Me, Ph; R 0 = H, Me; S, solvate molecule; shi3�, the trianion of salicylhydroxamic acid; sp, square planar; sph, sphenocorona; spy,square pyramidal; tbp, trigonal bipyramidal; tet, tetrahedral; X� = Cl�, Br�, NO3

�, X0� = Cl�, N3�, OCN�, SCN�, Z� = acac�, MeCO2

�.a Solvate molecules have been omitted.b Considering only the NiII; the O atom of (py)2CNO� interacts with Na+ ions.c These complexes contain simultaneously terminal and bridging (py)2CNO� ligands.d These molecules have metal–metal bonds.

C.J.Milio

set


175

N C N

O

N C N

O-

OH

N C N

O--O

(py)2CO (py)2C(OH)O- (py)2CO22-

Fig. 72. The formulae of the (py)2CO-based ligands discussed in the text; note that (py)2C(OH)O� and ðpyÞ2CO22� do not exist as free species but

exist only in their respective metal complexes.

Fig. 73. X-ray structure of ½Mn2IIMn2

IIIðO2CMeÞ2fðpyÞ2CO2g2fðpyÞ2CNOg2ðNO3Þ2�. Mn(1) and Mn(10) are assigned as the MnIII ions [168].


complex is the first metallacrown with ring metal ions inthree different oxidation states.

Aerobic reactions of Co(O2CR)2 Æ 4H2O with (py)2-CNOH, in the presence of counterions ðClO4

�;PF6�Þ,

give complexes [179] containing the tetranuclear,mixed-valence cobalt(II/III) cations ½Co2

IICo2IIIðOR0Þ2-

ðO2CRÞ2fðpyÞ2CNOg4S2�2þ (R =Me, Ph; R 0 = H, Me;S = MeOH, EtOH) depending on the solvent mixture[179]. These complexes are the first Co members in thefamily of metallacrowns adopting the extremely rare in-verse 12-MC-4 motif (Fig. 78). The (py)2CNO� ligandscomprise two pairs arranged along the edges and thesides of the Co4 rectangle. Edge (py)2CNO� ions func-tion as 2.1110 ligands, whereas long side (py)2CNO�

ions adopt the 2.1111 coordination mode.All known polynuclear Ni(II)/(py)2CNO� complexes

come from Pecoraro�s, Kessissoglou�s and our groups.Complex [Ni4{(py)2CNO}6(MeOH)2](ClO4)(OH) [180],containing both 2.1110 and 2.1111 (py)2CNO� ligands,is a rare example of an {Ni4(OR)2}

6+ core based on a

chair or butterfly ‘‘out-of-face’’ topology (Fig. 79). Thecomplex is characterized by the presence of both ferro-magnetic and antiferromagnetic exchange interactions.

The ‘‘monomeric’’, vacant mixed-ligand metalla-crown [Ni4(NCS)2(Hshi)2{(py)2CNO}2(DMF)(H2O)]shows [181] the connectivity pattern [–O–Ni–O–N–Ni–N–]2. Two NiII ions are bound only to nitrogenatoms along the metallacrown core and are in asquare planar arrangement. The other two NiII ionsare coordinated only to oxygen atoms along themetallacrown ring and are in an octahedral environ-ment. This complex can be described as the acid formof metallacrown, i.e., H212MC4 [181]. The refinementof [Ni4Na2(acac)4{(py)2CNO}4](ClO4)2 has not beencompleted yet [97]; the MC ring encapsulates twoNa+ ions.

Complex [Ni5(O2CMe)2(shi)2{(py)2CNO}2] has aninteresting structure [181]. The two shi3� and two(py)2CNO� (in the 3.2111 mode) ligands are arrangedin a trans configuration to construct a 12-MC-4 core

C N OH2 + 2H2O C O + 2H2N-OH

2H2N-OH + 3O2 2HNO3 + 2H2O

2(+)

C N OH2 + 3O2 C O2 + 2HNO3

C O2 + 2O2-C

O-

O-

(+)

C N OH2 + 3O2 + 2O2-

2

C

O-

O-

2 + 2HNO3

2HNO3 + O2- H2O + 2NO3-

(+)

C N OH2 + 3O2 + 3O2-C

O-

O-

2 H2O + 2NO3-+

(I)

(II)

(III)

(IV)

(V)

(VI)

(VII)

Scheme 8. A proposed simplified reaction scheme for the metal-mediated, partial transformation of di-2-pyridyl ketone oxime, (py)2CNOH, towardsthe dianion of the gem-diol form of di-2-pyridyl ketone, ðpyÞ2CO2

2�, involving NO3� generation. The O2� species can be derived from H2O and/or

the oxidation of MnII by atmospheric dioxygen. The 2-pyridyl rings have been omitted for clarity.

Fig. 74. X-ray structure of ½Mn2IIMnIVðOMeÞ2ðNCSÞ2fðpyÞ2CNOg4� [174].


with a fifth NiII encapsulated ion (Fig. 80). The twoMeCO2

� ions bridge the encapsulated metal ion totwo ring metal ions giving an overall neutral charge to

the molecule. The (py)2CNO� ligand is nonplanar dueto steric hindrance between the two pyridyl rings; thisconfers a nonplanar conformation to the metallacrown

Fig. 75. Variable-temperature magnetic susceptibility for ½Mn2IIMnIVðOMeÞ2ðNCSÞ2fðpyÞ2CNOg4� [174].

Fig. 76. X-ray structure of ½Mn3IIMnIVOð3; 4-DÞ4fðpyÞ2CNOg4� [177].

Fig. 77. Magnetization measurements for ½Mn3IIMnIVOð3; 4-DÞ4-

fðpyÞ2CNOg4� in the field range 0–6.5 T at 2.5 K (*) and 4.5 K (�).The solid lines represent the simulations according to the Brillouinfunction of a system with an S = 6 ground state and D = 0.025 cm�1

[177].


ring or a ‘‘saddle’’ shape. Two NiII ions in the ring areoctahedral and two are square planar; the encapsulatedmetal ion is also in an octahedral oxygen environmentwith four oxygens coming from the MC cavity andtwo from the bridging syn, anti MeCO2

� ligands. Therelationship between the bridging acetates and the MCring requires that stereoisomers are present. Magneti-cally the compound is characterized by weak antiferro-magnetic exchange interactions. Our group have alsoisolated a series of pentanuclear Ni(II) clusters [97]based on (py)2CNO�, see Table 4 and Figs. 81–83. Theirmagnetic properties are being studied.

The molecular structure of the azido-bridged hepta-nuclear cluster [Ni7(O2CMe)6(N3)2{(py)2CNO}6(H2O)2]

is based on the fusion (at a central metal ion) of two tet-ranuclear fragments [97].

The X-ray structure of [Ni10(MCPA)2(shi)5{(py)2-CNO}3(MeOH)(H2O)] (Fig. 84), where MCPA� is theanion of 2-methyl-4-chlorophenoxyacetic acid, consistsof two 12-MC-4 units with charges of +1 and �1[181]. Each tetranuclear unit has one additional encap-sulated NiII ion. The cationic unit is bound to the anio-nic unit via O bridges. The ground state of this cluster isS = 0, with S = 1, 2 low-lying excited states [193]; thisleads to a non-Brillouin behavior of the magnetization.

Complex [Mn2Ni2(O2CMe)2(shi)2{(py)2CNO}2-(DMF)5] consists of a mixed metal/mixed ligand ‘‘col-lapsed’’ 12-MC-4 motif (Fig. 85) [181]. The oxophilic

Fig. 78. X-ray structure of the cation of ½Co2IICo2

IIIðOHÞ2-ðO2CMeÞ2fðpyÞ2CNOg4ðMeOHÞ2�ðClO4Þ2. O(21) and O(21 0) are thehydroxo oxygen atoms, while OM(1) and OM(1 0) are the oxygenatoms of the methanol ligands. The metallacrown ring is outlined inbold [179].

Fig. 79. X-ray structure of [Ni4{(py)2CNO}6(MeOH)2]2+ [180].

Fig. 80. X-ray structure of [Ni5(O2CMe)2(shi)2{(py)2CNO}2]; themolecule shown here is the K isomer as defined by the screw axisoriented along the C2-axis [181].


MnIII ions are bound to the O, N chelating part of theshi3� ligand and then bind across the core to the oximeoxygens instead of the pyridyl nitrogens of the 2.1110

(py)2CNO� ligands. Paramagnetic 1H NMR studiesdemonstrate that the mixed metallacrown retains itsstructure in solution [181].

The only copper(I) complex containing bridging(py)2CNOH or (py)2CNO� ligands is [171] the dimer[Cu2Cl2{(py)2CNO}2], in which each CuI ion is coordi-nated by one chloride and three nitrogen atoms in a dis-torted tetrahedral environment.

Copper(II) complexes possessing bridging (py)2-CNOH or (py)2CNO� ligands have not been studiedextensively. The crystal structure of [Cu2{(py)2-CNO}4] Æ 2H2O [182,183] consists of dinuclear moleculescontaining both 1.0110 and 2.1110 (py)2CNO� ligands(Fig. 86); the CuII coordination geometry is slightlydistorted trigonal bipyramidal. Employment of carb-oxylates in the reaction mixtures gives trinuclearcomplexes of the general formula [Cu3(OH)(O2CR)2-{(py)2CNO}3]; the molecular structure of the acetatecomplex is shown in Fig. 87. The trinuclear cluster,which has an inverse 9-MC-3 motif, is held togetherby one l3-OH group, one g1 : g1 : l2-MeCO2

2� ligandand three 2.1110 (py)2CNO� ions [183]; a monodentateacetate completes five coordination at one CuII center.Two trinuclear molecules are ‘‘dimerized’’ in the crystallattice through weak interactions between two CuII

ions and ‘‘free’’ 2-pyridyl nitrogen atoms creating ahexamer. The benzoate analogue consists of well-iso-lated trinuclear molecules [183]. In the dinuclear com-plex [Cu2(hfac)2{(py)2CNO}2], the two five-coordinateCuII ions are bridged by two 2.1110 (py)2CNO�

ligands.

Fig. 81. X-ray structure of the cation present in the complex [Ni5(acac)2{(py)2CNO}6(H2O)(MeOH)](ClO4)2; many carbon atoms have been omittedfor clarity [97].

Fig. 82. X-ray structure of [Ni5(O2CMe)7{(py)2CNO}3(H2O)]; many carbon atoms of the (py)2CNO� ligands have been omitted [97].


The use of (py)2CNO�/X� ‘‘blends’’ ðX� ¼ MeCO2�;

PhCO2�;Cl�;N3

�;NCO�; acac�;NCS�;Cl�=NCS�Þ inZnII chemistry yields neutral tetranuclear and cationicpentanuclear clusters [172,184,185], see Scheme 9.Various synthetic procedures have led to the synthesisof compounds [Zn4(OH)2X

02{(py)2CNO}4] ðX0� ¼

Cl�;N3�; NCO�Þ, [Zn4(OH)2Z2{(py)2CNO}4] ðZ� ¼

MeCO2�; acac�Þ, [Zn5Cl2{(py)2CNO}6][ZnCl(NCS)3]

and [Zn5(NCS)2{(py)2CNO}6(MeOH)][Zn(NCS)4]; rep-resentative structures are shown in Figs. 88 and 89. Thetetranuclear molecules have an inverse 12-MC-4 topol-ogy. The triply bridging hydroxides are accommodatedin the center of the metallacrown ring. The (py)2CNO� li-gands form a propeller configuration that imposes abso-lute stereoisomerism with K and D chirality. Two metalions are in distorted O2N4 octahedral environments,

Fig. 83. X-ray structure of the cation present in complex [Ni5{(py)2-CNO}5(H2O)7](NO3)5 [97].

Fig. 84. A view of [Ni10(MCPA)2(shi)5{(py)2CNO}3(MeOH)(H2O)];many atoms have been omitted for clarity [181].

Fig. 85. X-ray of the mixed metal/mixed ligand ‘‘collapsed’’ 12-MC-


whereas the rest are in severely distorted tetrahe-dral ðX0� ¼ Cl�;N3

�;NCO�;PhCO2�Þ or tbp ðZ� ¼

MeCO2�; acac�Þ environments. The five ZnII ions of the

pentanuclear cations are held together by six (py)2CNO�

ligands which adopt three different coordination modes(2.1110, 3.1111, 3.2111); the chloro and isothiocyanato li-gands in these cluster cations are terminal. The five ZnII

ions define two nearly equilateral triangles sharing a com-mon apex, and the novel Zn5 topology can be described astwo ‘‘collapsed’’ 9-MC-3 structures sharing a commonmetal apex.

Employment of shi3� in ZnCl2/(py)2CNO� chemistryyields [172] the octanuclear cluster [Zn8(shi)4{(py)2-CNO}4(MeOH)2]. The molecule contains a 12-MC-4core constructed by four metal ions, i.e., Zn(3), Zn(5),Zn(7) and Zn(8) in Fig. 90, and four shi3� ligands.The MC core accommodates a dinuclear Zn2{(py)2-CNO}4 component (Zn(1), Zn(4)), while the ring metalions Zn(3) and Zn(5) create dinuclear units with Zn(2)and Zn(6), respectively, through oxygen bridges.

There also exist (py)2CNO�-based complexes of 4d-and 5d-metals, including organometallic compounds.Treatment of [Ru3(CO)12] with (py)2CNOH in refluxingTHF leads [186a] to a separable mixture of [Ru3{(py)2-CNO}2(CO)8] (Fig. 91) and [Ru2{(py)2CNO}2(CO)4].Compounds [M3(CO)10(MeCN)2] (M = Ru, Os) reactwith (py)2CNOH in THF at room temperature to give[M3(l-H){(py)2CNO}(CO)9] [186a]. The thermal reac-tion of [Os3(l-H){(py)2CNO}(CO)9] with (py)2CNOHgives [186a] [Os3{(py)2CNO}2(CO)8], which is isostruc-tural with the Ru analogue. These complexes displaylow activity as DNA cleavage agents. The crystals of[Ag2(NO3)2{(py)2CNOH}2] were found [186b] to con-tains neutral dinuclear molecules (Fig. 92). The (py)2-CNOH ligands bridge the two AgI ions in a 2.0111fashion, see Fig. 68. The nitrato ligands are monodentate

4 complex [Mn2Ni2(O2CMe)2(shi)2{(py)2CNO}2(DMF)5] [181].

Fig. 86. X-ray structure of [Cu2{(py)2CNO}4] [183].

Fig. 87. X-ray structure of [Cu3(OH)(O2CR)2{(py)2CNO}3]; the ‘‘dimerization’’ of the trinuclear molecules is not shown [183].


Scheme 9. The ZnII/(py)2CNOH/X� reaction scheme that leads toneutral tetranuclear and cationic pentanuclear clusters.

Fig. 89. X-ray structure of the pentanuclear cation present in thecomplex [Zn5Cl2{(py)2CNO}6][ZnCl(NCS)3].


and the geometry around the metal centers can bedescribed as distorted tetrahedral. The 2.0111 ligationmode is also adopted by the neutral ligand in the dinu-clear complexes [Cd2(O2CR)4{(py)2CNOH}2] (R =Me, Ph) and the coordination polymer [Cd(O2CMe)(SCN){(py)2CNOH}]n [187]. The structurally character-ized mercury(II) complexes are [97] [Hg2(O2CMe)4-{(py)2CNOH}2], [Hg2(O2CPh)3{(py)2CNO}{(py)2-CNOH}(MeOH)] (Fig. 93) and [HgCl(O2CPh){(py)2-CNOH}]n; details are given in Table 4. The threecomplexes contain 2.0111 ligands.

Working with (py)2CNOH, we have been able to ap-ply the ‘‘metal complexes as ligands’’ strategy [73,209] toisolate mixed-metal 3d/4f complexes [187]. Complexes

Fig. 88. X-ray structure of [Zn4(OH

[M{(py)2CNOH}2(H2O)2](NO3)2 [187], which have yetto be structurally characterized, most probably contain1.0110 neutral oxime ligands. Since these species containtwo potentially free (the oxime oxygen, the second2-pyridyl nitrogen) coordination sites, they can be re-garded as ‘‘ligands’’. Reactions between equimolarquantities of [M{(py)2CNOH}2(H2O)2](NO3)2, whereM = Mn, Ni, Cu, Zn, and Ln(NO3)3 Æ xH2O (Ln = lan-thanide) in various solvents lead to hexanuclear clustersof the general formula [M3Ln3{(py)2CNO}6(NO3)9](Eq. (7)).

)2(acac)2{(py)2CNO}4] [184].

Fig. 91. X-ray structure of [Ru3{(py)2CNO}2(CO)8] [186a].

Fig. 92. X-ray structure of [Ag2(NO3)2{(py)2CNOH}2] [186b].

Fig. 90. The connectivity pattern of the complex [Zn8(shi)4{(py)2-CNO}4(MeOH)2] [172].


3½MfðpyÞ2CNOHg2ðH2OÞ2�ðNO3Þ2 þ 3LnðNO3Þ3� xH2Oþ 6LiOH

! ½M3Ln3fðpyÞ2CNOg6ðNO3Þ9� þ 6LiNO3

þ 3ð4þ xÞH2O

The molecular structure of the M = Zn, Ln = Tb com-plex is shown in Fig. 94. The (py)2CNO� ligands adoptthe 3.2111 coordination mode, each bridging two ZnII

ions and one TbIII center; three chelating nitrates arebound to each TbIII ion. The two donor atoms, thatwere free in the mononuclear ‘‘ligand’’ 3d-metal com-plex, are indeed coordinated to terbium(III), as antici-pated; however, the deprotonated oximate oxygen isbound to a second ZnII ions and this does not permita full synthetic control of the reaction. Five, out of thesix, metal ions define a trigonal bipyramid (Fig. 95).The TbIII ions occupy the equatorial positions, whilethe third ZnII ion lies in the middle of the equatorialplane. Trinuclear NiII/LnIII clusters based on(py)2CNO� and containing chelating hfac� as ancillaryligand have also been communicated at a conference[188].

8.4. Metal-ion assisted transformations of (py)2CNOH

We have already mentioned the in situ transforma-tions of (py)2CNOH to give the coordination dianionand monoanion of the gem-diol derivative of di-2-pyr-idyl ketone (Fig. 72) during the preparation of ½Mn2

II-Mn2

IIIðO2CRÞ2fðpyÞ2CO2g2fðpyÞ2CNOg2X2� (Fig. 73,Scheme 8) [168,176] and [Co(NO2){(py)2C(OH)O}-{(py)2CNO}] (Table 3) [170], respectively. In these com-plexes, an amount of the oximate ligand still remainscoordinated in the product. We shall briefly discuss herethe few cases in which the initially (py)2CNOH ligandemployed does not appear in the products.

The synthetic utility of the metal-mediated organictransformations and the reactions of coordinated li-gands is an important subject [210]. It is based onthe enhancement in reactivity of organic ligands as aconsequence of metal coordination. For example, themetal can act as a �super acid� and cause enhancednucleophilic attack on coordinated carbonyl and imineligands. The metal ion can also enable the ligand itselfto act as a nucleophile, sometimes by direct activation,sometimes by protecting other parts of the ligand andsometimes by a combination of both. The ligandswhich undergo reaction can be bound to the metalion in the transition state or in relatively stable, isola-ble complexes.

The reaction between [VCl3(THF)3] and (py)2CNOHis solvent dependent (Scheme 10) [211]. In THF, theproduct is [VOCl2{(py)2CNH}(THF)], where (py)2CNHis di-2-pyridylimine. The molecular structure of this

Fig. 94. X-ray structure of [Zn3Tb3{(py)2CNO}6(NO3)9] [187].

Fig. 93. X-ray structure of [Hg2(O2CPh)3{(py)2CNO}{(py)2CNOH}(MeOH)]; many carbon atoms of the oxime and oximate ligands have beenomitted for clarity [97].


compound is shown in Fig. 96. The structure reveals twoimportant features: (a) the oxidation of the initially VIII

ion to the oxovanadium(IV) ion, VIVO2+ and (b) thetransformation of the oxime group to an imino group.In EtOH the product is [VOCl2{(py)2C(OEt)(NH2)}],

where (py)2C(OEt)(NH2) is amino-di-2-pyridyl-methylethyl ether. The new ligand exhibits an (Npy)2(Namino)chelating behavior (Fig. 97). The reaction in MeOHgives a mixture of two-coordination isomers of[VOCl2{(py)2C(OMe)(NH2)}], where (py)2C(OMe)-

Fig. 95. The topological arrangement of the six metal ions in the 3d/4fcluster [Zn3Tb3{(py)2CNO}6(NO3)9] [187].

V(1)

N(2)N(1) C(6)

C(5)C(4)

C(3)

C(2)

C(1)

N(3)H2 C(7)

C(8)C(9)

C(10)C(11)

O(1)Cl(2) Cl(1)

O(3)

C(12)

C(13)

Fig. 97. X-ray structure of [VOCl2{(py)2C(OEt)(NH2)}].

V(1)

N(2)

N(1)

C(6)

C(5)

C(4)

C(3)

C(2)C(1)

N(3)

C(7)

C(8)

C(9)

C(10)

C(11)

O(1)

Cl(2)

Cl(1)

O(2)

C(12)

C(13)

C(14) C(15)H(N2)

Fig. 96. X-ray structure of [VOCl2{(py)2CNH}(THF)].


(NH2) is amino-di-2-pyridyl-methyl methyl ether.One isomer bears striking structural resemblance to[VOCl2{(py)2C(OEt)(NH2)}] (Fig. 97), while in theother isomer an (Npy)2(Oether) chelating behavior ofthe ligand is realized (Fig. 98). One pyridyl nitro-gen atom exhibits no interaction with VIV in [VOCl2-{(py)2CNH}(THF)] due to the sp2 character of thecentral carbon atom (C(6)), whereas the coordinationof both pyridyl nitrogen atoms has been observed inthe rest of the complexes as a result of flexibility ofC(6) due to sp3 hybridization [211]. It was concludedthat the vanadyl oxygen comes from N–O bondcleavage.

NC

NN

OH

(py)2CNOH

+ [VCl3(THF)3]

Scheme 10. The [VCl3(THF)3]/(py)2

Recent results [97] reveal that the Co(O2CMe)2 Æ4H2O/(py)2CNOH/NaN3 reaction mixture in MeCNgives the complex [Co4(N3)2(O2CMe)2{(py)2C(OH)-O}4]. The remarkable feature of the reaction is thein situ transformation of (py)2CNOH to yield the triply-

THF

ROHR= Me, Et

NC

NN

H

(py)2CNH

NC

NOR

(py)2C(OR)(NH2)

NH2

CNOH reaction system [211].

Fig. 98. X-ray structure of the second coordination isomer of[VOCl2{(py)2C(OMe)(NH2)}].


bridging monoanion, (py)2C(OH)O�, of the gem-diolderivative of di-2-pyridyl ketone, (py)2CO, see Fig. 72.The X-ray diffraction analysis shows (Fig. 99) a defectivedouble-cubane, tetranuclear entity in which the CoII ionsare linked by end-on (2.100) azido ligands and two kindsof O-bridges from two 3.3011 and two 2.2011 (py)2C-(OH)O� ligands. Magnetic susceptibility studies of thiscompound in the 2–300 K range indicate bulk ferromag-netic coupling. Efforts are in progress to elucidate themechanism of this transformation.

Fig. 99. X-ray structure of [Co4(N3)2(O2CMe)2{(py)2C(OH)O}4]; this (py)2C(py)2CNOH and NaN3 in MeCN.

9. The coordination chemistry of tris(2-aldoximo-6-

pyridyl)phosphine, P{(py-H)CHNOH}3, and of the

related clathro-chelates

P

NN

N

N

OH

N

HON

OH

P{(py-H)CHNOH}3

The ligand was synthesized by Holm�s group some 35years ago [212,213] during his successful project for thedesign of complexes containing encapsulated metal ionswith trigonal prismatic coordination. As shown inScheme 11, P{(py-H)CHNOH}3 can be obtained in�20% overall yield from 2,6-dibromopyridine in a six-step process.

Reaction of FeII, CoII, NiII, CuII and ZnII salt withP{(py-H)CHNOH}3 in MeCN yields the cations[M(P{(py-H)CHNOH}2{(py-H)CHNO})]+, which canbe isolated as analytically pure perchlorate salts; the

NO�-free complex is prepared by the reaction of Co(O2CMe)2 Æ 4H2O,

Scheme 11. Synthesis of P{(py-H)CHNOH}3.


formation of these cations and their proposed structures(based on physical and spectroscopic data) [213] areshown in Scheme 12.

The chlathro-chelate fluoroborotris(2-aldoximo-6-pyridyl)phosphinemetal(II) cations, [M(P{(py-H)CH-NO}3BF)]

+, containing Fe(II), Co(II), Ni(II) andZn(II), were prepared by closure reactions of [M(P-{(py-H)CHNOH}2{(py-H)CHNO})]+ with boron triflu-oride etherate or tetrafluoroborate ion and isolatedas BF4

� salts (Scheme 12). X-ray results have shown[214] that the desired trigonal prismatic coordinationhas been very closely approached in [Ni(P{(py-H)-CHNO}3BF)](BF4), and that the Co(II) and Zn(II) saltsare isomorphous with the NiII compound. The generalsynthetic procedure shown in Scheme 12 failed whenapplied to the synthesis of the CuII clathro-chelate.

Scheme 12. Synthesis of [M(P{(py-H)CHNO}3BF)]+ complexes from P{(py-

as ring-closure reagents. No specific stereochemistry of the intermediate six-coring structure is shown in each species (M = Fe, Co, Ni, Zn) [213].

In a synthetically smart and magnetically elegant pa-per [68], Chaudhuri and co-workers used the metal-con-taining fragment {CrIIIL}3+, instead of the B-cappingunit, for encapsulation of the [Ni(P{(py-H)CHNO}3)]

�

unit to yield the bicyclic chlathro-chelate (L = 1,4,7-trimethyl-1,4,7-triazacyclononane). The molecularstructure of the resulting cation [LCrIII(P{(py-H)-CHNO}3)NiII]2+ is shown in Fig. 100. The CrIII andNiII coordination geometries are distorted octahedraland trigonal prismatic, respectively. The effective mag-netic moment, leff, for this complex exhibits an essen-tially temperature-independent behavior in the range290–30 K (Fig. 101). Below 30 K the leff decreasesreaching a value of 3.82 BM at 2 K. The solid line inFig. 4 represents the best fit with the parameters J = 0,gCr = 1.98, gNi = 2.16, h = �1.42 K. The isoelectronic

H)CHNOH}3 using boron trifluoride etherate or tetrafluoroborate ionordinate complexes is implied. For purposes of clarity only one chelate

Fig. 100. X-ray structure of the cation [LCrIII(P{(py-H)CHNO}3)NiII]2+ in its perchlorate salt [68].

Fig. 101. Plots of leff vs. T for solid [LCrIII{(py-H)CHNO}3NiII]-(ClO4)2 (a) and [LCrIII(P{(py-H)CHNO}3)NiII](ClO4)2 (b). The solidlines represent the best fit of the data to the Heisenberg-Dirac-vanVleck model.


analogue [LCrIII{(py-H)CHNO}3NiII](ClO4)2 exhibits(Fig. 101) a weak antiferromagnetic exchange interac-tion between the CrIII and NiII ions (J = �9.2 cm�1,gCr = 2.0, gNi = 2.19) and has an St = 1/2 ground state[68]. These two complexes are rare examples of weakantiferromagnetic or no coupling at all between CrIII

and NiII. Interestingly, most of the known CrIII–NiII

interactions in the literature are in accord with the pre-dictions made nearly 35 years ago [215], namely ferro-magnetic in nature.

10. Conclusions and perspectives

It is obvious from the preceding pages that, justover a half century since the first preparations of metalcomplexes of pyridyl oximes, the coordination chemis-try of such ligands is an expanding field of great cur-rent interest. This chemistry is an area that hassomething for everyone: from smart synthetic inorganicchemistry to complexes (both polynuclear and poly-meric) with aesthetically pleasant structures, and fromhigh-spin molecules to single-chain magnets. For exam-ple, in the area of homo- and heterometallic polynu-clear transition metal complexes we hope that thisreport illustrates what is possible through very simplecoordination chemistry. The monoanions of simple2-pyridyl oximes have fulfilled their promise as asource of polynuclear 3d-metal complexes with interest-ing structures and properties. The immense structuraldiversity of the complexes described here stems fromthe ability of the (py)C(R)NO� ligands to exhibit manydistinct coordination modes (Figs. 8 and 68). Presum-ably, the presence of dissimilar donor atoms withinthese anionic ligands leads to this coordinative flexibil-ity; however, their versatility was unexpected. Employ-ment of carboxylates, b-diketonates and sulfates in the(py)C(R)NO� metal chemistry gives an extraordinarystructural flexibility in the resulting mixed-ligandsystems (‘‘blends’’ [216]). The remarkable diversityof structures has prevented any guiding structuralprinciples from being proposed. It is clear that the

Scheme 13. Synthesis of 1,10-phenanthroline-2,9-dicarbaldehyde diox-ime (L) [221].

Fig. 102. Schematic representation of a model ternary metal ion/monoanion of L/phosphodiester system [221].


pyridyloximato clusters do not correspond in a straight-forward manner to fragments of common minerals or topolyhedral archetypes, but rather, display a richnessof topology and nuclearity that is unpredictable butintriguing. In the area of molecular magnetism, the oxi-mato group of the anionic pyridyl oximes can mediateexchange interactions of varying range, from weak andmoderate ferromagnetic to strong antiferromagnetic.Although almost all of the complexes discussed in thisreport have been structurally characterized, our currentknowledge of magnetostructural correlations is stillpoor. As also emphasized by Chaudhuri [6], the ‘‘irreg-ular spin-state structure’’ approach, resulting from aparticular spin topology, is more effective in obtaininghigh-spin molecules than the more rational approachof obtaining ferromagnetically coupled systemsthrough the approach of the strict orthogonality ofthe magnetical orbitals of the interacting metal centers.An additional important chemical lesson of this reportis that the activation of the oxime group of 2-pyridyloximes by 3d-metal ions towards further reactionsseems to be an emergent area of synthetic inorganicchemistry.

This area of research will undoubtedly continue to ex-pand, given the relatively recent nature of the majorityof references in this paper, and the numerous syntheticroutes now documented for the isolation of pyridyloxi-mato metal complexes. Obvious areas for further inves-tigation include:

1. The chemistry of other 3d-metals, and second andthird row transition metals with such ligands. Thereactions of V, Cr and Fe sources with pyridyl oximesshould be studied in detail, considering how interest-ing the magnetic properties of the products could be.For example, it is surprising that iron(II) and iro-n(III) pyridyloximato complexes have little beeninvestigated; high-spin FeIII (S = 5/2) complexes arepromising candidates to obtain large S values in theground state.

2. Studies of the chemistry of pyridyl oximes with thegenerally oxophilic lanthanide ions. Such studies arecompletely lacking.

3. Further studies of the chemistry of heterometallicpyridyloximato complexes. For example, 3d/4f clus-ters are extremely rare, and in the context of therecent discovery that such complexes can be single-molecule magnets [217–220], could be veryinteresting.

4. The use of pyridyl oximes in supramolecular systems;the published studies are interesting but, simulta-neously, limited in number. Several pyridyl oximescan be proven versatile tools for supramolecularassembly of metal-containing supramolecular archi-tectures and interesting building blocks for crystalengineering.

5. Studies of the reactivity of some known pyridyloxi-mato complexes; such studies are lacking. The para-magnetism of many known compounds makesNMR a method of limited applicability, especiallyin case where some ligands are weakly bound, creat-ing additional problems of fluxionality. Therefore,solution studies have been limited. The growing useof electrospray mass spectrometry suggests moremay be done. According to our experience, thereare several systems in which several clusters can becrystallized from very similar reaction mixtures, andan examination of which of these clusters is presentin solution would be a step toward understandinghow, and when, these complexes form.

6. The use of new pyridyl oximes in metal chemistry.Synthesizing new ligands will be challenging andmay lead to novel properties. A characteristic exampleis provided by the polydentate ligand 1,10-phenan-throline-2,9-dicarbaldehyde dioxime (L), synthesized[221] as illustrated in Scheme 13. The attachment oftwo oxime groups to a metal-chelating, pyridyl-basedligand is an attractive way of developing the design ofsmall metal complexes which are potentially able tohydrolyze the phosphodiester backbone of nucleicacids (chemical nucleases). The oximate group hasbeen chosen because it can effectively act as a nucleo-phile endowed with nucleolytic activity [221]. Thecomplexes of the monoanion of L are designed tointeract with the initial negatively charged phosphate,and to promote hydrolysis of the phosphodiester P–Obond through nucleophilic attack on the P atom byone oximate group, the other oxime group stabilizingthe leaving oxygen atom (Fig. 102).


The results of the above proposed future investiga-tions will be probably described in another polyhedronreport.

Acknowledgments

The described work from our group is in the mainbased on the Ph.D. work of two of us (C.J.M., Th.C.S.)and eight talented scientists: Dr. Eugenia Katsoulakou,Dr. Eleanna Diamantopoulou, Dr. Elena Kefalloniti,Dr. Athanassios Boudalis, Constantina Papatriantafyl-lopoulou-Efthymiou, Gina Vlahopoulou, KonstantinaPriggouri and Constantinos Stoumpos. We alsoacknowledge our longstanding collaboration withDr. Aris Terzis, Dr. Catherine P. Raptopoulou andDr. Vassilis Psycharis (NCSR ‘‘Demokritos’’, Athens)for X-ray crystallography, Dr. Vassilis Tangoulis andDr. Nikolia Lalioti (University of Patras, Greece),Dr. Yiannis Sanakis (NCSR ‘‘Demokritos’’, Athens),Professors Albert Escuer and Ramon Vicente (Univer-sity of Barcelona, Spain) for performing magnetic andEPR studies, Lecturer Panagiotis Kyritsis (Universityof Athens, Greece) for electrochemistry and ProfessorEvangelos G. Bakalbassis (University of Thessaloniki,Greece) for quantum-chemical calculations. We thankProfessors George Christou (University of Florida,USA), Richard E.P. Winpenny (University of Man-chester, UK), Dimitris Kessissoglou (University ofThessaloniki, Greece), and Lecturers Euan Brechin(University of Edinburgh, UK), Sarah L. Heath (Uni-versity of Manchester, UK), Anastasios Tasiopoulos(University of Cyprus, Cyprus) for helpful discussions.Th.C.S. and S.P.P. are grateful to the European SocialFund (ESF), Operational Program for Educational andVocational Training II (EPEAEK II), and particularlythe Program PYTHAGORAS (Grant b.365.037), forfunding our research efforts in the area of the pyridy-loximato clusters. S.P.P. also thanks the ResearchCommittee of the University of Patras (K. Caratheod-ory Program No. 03016) for support of our work onthe coordination polymers of pyridyl oximes.

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