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Three polymorphic forms of the molybdenum- (VI) complex [MoO2(L)(EtOH)] (1α, 1β, and 1γ) (L2− = 4- methoxy-2-oxybenzaldehyde isonicotinylhydrazonate) were synthesized by the reaction of H2L with the dioxobis- (acetylacetonato)molybdenum(VI) complex, [MoO2(acac)2], in ethanol. Removal of the coordinated ethanol molecule upon grinding or heating led to the solid-state transformation of the polymorphs 1α, 1β, or 1γ into the coordination polymer [MoO2(L)]n (2a). The square inclusion complex [MoO2(L)]4⊃CH2Cl2·4CH2Cl2 (2b⊃CH2Cl2·4CH2Cl2) was obtained by a self-assembly reaction in dichloromethane. Standard Gibbs energies of binding for molybdenum(VI) compounds [MoO2(L)(D)] with the sixth coordination site occupied by a nitrogen or an oxygen donor D were estimated using quantum chemical calculations. Crystal and molecular structures of the molybdenum(VI) compounds [MoO2(L)(EtOH)] (1α, 1β, and 1γ), [MoO2(L)]4⊃CH2Cl2·4CH2Cl2 (2b⊃CH2Cl2·4CH2Cl2), [MoO2(L)(γ-pic)]·γ-pic (3·γ-pic), [MoO2(L)(py)] (4), and [MoO2(L)(DMSO)] (5) were determined by the single crystal X-ray diffraction method. The compounds were further characterized by chemical analysis, thermogravimetric, and differential scanning calorimetry measurements, IR, UV−vis, one- and two-dimensional NMR spectroscopies, and the powder X-ray diffraction method.

The reaction of [CoIII(4,4′dmsalen)(CH2Cl)(L)], where 4,4′dmsalen = 4,4′-dimethylsalen and L = solvent, with pyridine led to the isolation of [CoIII(4,4′dmsalen)(CH2py)(Cl)], in a two-step process involving a metallacyclized intermediate, cis β [CoIII(4,4′dmsalenCH2)(py)(L)]+, whereas a parallel direct conversion through the displacement of a Cl atom from the axial CH2Cl group of [CoIII(4,4′dmsalen)(CH2Cl)(L)] may be ruled out. Natural bond orbital analysis of the structures show that the ylide carbon atom has lower orbital occupancies in the cyclized complex, thus making it more susceptible to a nucleophilic attack of pyridine. The calculated potential energy surfaces indicate that the most energy feasible reaction path would be for the complex [CoIII(4,4′dmsalenCH2)(py)ax(H2O)]+, with an entering pyridine molecule and a leaving equatorial water molecule. Experimental kinetic studies of the formation of the ylide complex starting from [CoIII(4,4′dmsalenCH2)(py)eq(H2O)]+ strongly suggest that it is possible to obtain the ylide complex through an alternative, less energetically favourable path.

A new multicomponent material with nanoporous structure has been synthesized by co-crystallization of a mixture of cationic meso-tetrakis(4-N-methylpyridyl)porphyrin (H2T4) and meso-tri(4-N-methylpyridyl)porphyrin (H2T3py) with polyanionic 5,11,17,23-tetrasulfonato-25,26,27,28-tetrakis(hydroxylcarbonylmethoxy)calix[4]arene (C4TsTc) in the presence of Ni2+ ions. The structural analysis indicates that the overall architecture is assembled by interpenetrated two-dimensional (2D) meshes where the nodes are built up by a central tetracationic H2T4 porphyrin with arms hosted in sulphonated rims of four cavitands. The approximately 2D square network is formed by Ni2+ ions bridging the calixarene carboxylate rims in a tail-to-tail fashion. The central H2T4 stacks with two external H2T3py molecules having the neutral pyridine arm N-coordinated to Ni2+ ions. These metal centers interconnect the orthogonal 2D meshes by further coordination of calixarene–carboxylate groups. Self-organization of the new multicomponent material, featuring large channels (60% of volume accessible to solvent molecules) and potential readily accessible metal active sites, has been driven by both supramolecular host–guest recognition and coordinative assembly. The thermal behavior of native and nickel-containing crystals was studied by hot stage microscopy and differential scanning calorimetry. The decomposition temperatures of the multicomponent materials, 465–470 °C, are about 100 °C higher than those of the single building blocks.

Substitution of the acetylacetonate ligands in [MO2(acac)2] (M = Mo or W) by 2-oxy-1-naphthaldehyde izonicotinoyl hydrazonate (NIH2−) or 3-methoxy-2-oxybenzaldehyde izonicotinoyl hydrazonate (VIH2−) gives rise to either zigzag chain polymers, [MO2(NIH)]n (1 and 2), square complexes [MO2(VIH)]4 (3 and 4), mononuclear complexes [MO2(VIH)(C2H5OH)] (5 and 6), or to polyoxomolybdate hybrid compounds [{; ; ; MoO2(HNIH)}; ; ; 2Mo6O19] (7) and [MoO2(HVIH)(H2O)]2Mo6O19 (8), depending on the reaction conditions. In all of these compounds the ligand is coordinated to the cis-MO22+ core via phenolic-oxygen, azomethine-nitrogen and enolic-oxygen atoms, while the remaining sixth coordination site is occupied either by the nitrogen atom of the izonicotinyl part, or by an oxygen atom from the solvent molecule, or by an oxygen atom from the hexanuclear anion. Crystal and molecular structures of the molybdenum(VI) compounds [MoO2(NIH)]n (1), [MoO2(VIH)]4 (3), [{; ; ; MoO2(HNIH)}; ; ; 2Mo6O19] (7) and [MoO2(HVIH)(H2O)]2Mo6O19 (8) were determined by the single crystal X-ray diffraction method. Dioxotungsten(VI) compounds [WO2(NIH)]n (2) and [WO2(VIH)]4 (4) were structurally characterized by the X-ray powder diffraction method. All of the investigated compounds were further characterized by elemental analysis, thermogravimetric analyses, cyclic voltammetry, FT-IR, UV-Vis and NMR spectroscopy.

New trinuclear mixed-valent CoIII-CoII-CoIII complexes with salen-type ligands have been synthesized and characterized both in the solid state and in solution. These complexescomprise a central CoCl2 unit connecting two [RCo(4,4′,7,7′tmsalen)] fragments with chlorides acting as bridging ligands between the metals, while the central CoII ion completes its octahedral coordination through the salen oxygen donors. Complex 1 (R = CH2Cl), which has cis Cl ligands at the central CoII ion, has approximate C2 symmetry, whereas the centrosymmetric complex 2 (R = CF3CH2) contains trans Cl ligands (C2h symmetry). Complexes 1 and 2 are paramagnetic with magnetic moments of 4.7 and 4.4 μB, respectively, which indicate the presence of a high-spin CoII center in the molecule. The 1H NMR spectra, which spread over a range of nearly 190 ppm, are consistent with the presence of both cis and trans isomers in solution (with cis being the prevailing species), and indicate that exchange is slow on the NMR time-scale. Isotropically shifted 1H NMR signals have been fully assigned by comparison with those of appropriately substituted complexes and by T1 measurements and 2D exchange experiments (EXSY). The K constant for the trans [rlarr2] cis equilibrium, as evaluated from the 1H NMR signal intensity ratio at various temperatures, has allowed us to estimate, at least qualitatively, the thermodynamic parameters ΔH° and ΔS°.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

We prepared two new analogues of ([CH 3 CO((DO)(DOH)pn)L] + ) [(DO)(DOH)pn = N 2 ,N 2 '-propane-1,3-diylbis(2,3-butanedione-2-imine-3-oxime)] B 12 models but with an O-BF 2 -O unit replacing the O-H···O unit as follows: [CH 3 Co((DO)(DOBF 2 )pn)L]PF 6 with L = pyridine (py) and 1,5,6-trimethylbenzimidazole (Me 3 Bzm). Our goal was to compare the properties of these new O-BF 2 -O complexes with the well-established O-H···O analogues. The Co-CH 3 1 H NMR shifts indicate that the BF 2 group makes the Co(III) less electron rich. The X-ray crystal structures determined for the new compounds were compared to the one known structure with L = imidazole (Im). With increasing size of L, in the series Im < py < Me 3 Bzm, the plane of L orients so as to avoid the bulky BF 2 group. This orientation effect becomes apparent in the L 1 H NMR shifts, which are not sensitive to Co(III) electronic properties. Thus, in the O-BF 2 -O versus the O-H···O analogue, the Me 3 Bzm H4 signal shifts 0.41 ppm upfield from the anisotropic effect of the equatorial ligand double bonds. We advance the concept (applicable to a broad series of complexes) that steric interactions between L and the equatorial ligands are alleviated by a combination of Co-N ax bond elongation and opening of the N eq -Co-N ax angles.

This short review describes the use of organocobaloximes in the self-assembly of supramolecular systems by means of functionalized aromatic boronic or diboronic acids. With the former, the cyclization is achieved by the coordination of the nitrogen donor of the boronic acid to one of the Co axial positions, with simultaneous insertion of the boron in the hydrogen bond of the other cobaloxime unit. Cyclic supramolecular species of different nuclearity, depending either on the kind of boronic acid or on the side substituents of the cobaloxime, can be obtained. The use of diboronic acids, which link two organonaquocobaloxime units, allows the formation of cyclic oligomers only in the presence of suitable guests.

trans-[Co(CH2Cl)(tmsalen)(S)] undergoes metallacyclization in water/methanol, affording the cis β organometallic derivative [Co(tmsalenCH2)(S)2]Cl. The presence of N-donor ligands, L, increases the...

Structural and 1 H NMR data have been obtained for cobaloximes with the bulkiest substituted pyridines reported so far. We have isolated in noncoordinating solvents the complexes CH 3 Co(DH) 2 L (methylcobaloxime, where DH = the monoanion of dimethylglyoxime) with L = sterically hindered N-donor ligands: quinoline, 4-CH 3 quinoline, 2,4-(CH 3 ) 2 Pyridine, and 2-R-pyridine (R = CH 3 , OCH 3 , CH 2 CH 3 , CH=CH 2 ). We have found that the Co-N ax bond is very long in the structurally characterized complexes. In particular, CH3Co(DH)2(4-CH3quinoline) has a longer Co-N ax bond (2.193(3) A) than any reported for methylcobaloximes. The main cause of the long bonds is unambiguously identified as the steric bulk of L by the fairly linear relationship found for Co-N ax distance vs CCA (calculated cone angle, CCA, a computed measure of bulk) over an extensive series of methylcobaloximes. The linear relationship improves if L basicity (quantified by pK a ) is taken into account. In anhydrous CDCl 3 at 25 °C, all complexes except the 2-aminopyridine adduct exhibit 1 H NMR spectra consistent with partial dissociation of L to form the methylcobaloxime dimer. 1 H NMR experiments at -20 °C allowed us to assess qualitatively the relative binding ability of L as follows: 2,4-(CH 3 ) 2 pyridine > 4-CH 3 quinoline ≈ quinoline ≈ 2-CH 3 pyridine > 2-CH 3 Opyridine > 2-CH 3 CH 2 pyridine > 2-CH 2 =CHpyridine. The broadness of the 1 H NMR signals at 25 °C suggests a similar order for the ligand exchange rate. The lack of dissociation by 2-aminopyridine is attributed to an intramolecular hydrogen bond between the NH 2 group and an oxime O atom. The weaker than expected binding of 2-vinylpyridine relative to the Co-N ax bond length is attributed to rotation of the 2-vinyl group required for this bulky ligand to bind to the metal center, a conclusion supported by pronounced changes in 2-vinylpyridine signals upon coordination.

Bis(methylphenylglyoximate)cobalt(III) complexes exist both as cis and trans isomers due to the asymmetry of the equatorial ligand, and, when the axial ligands are different, the trans isomer is chiral. The reaction of racemic trans-[CH3Co(mpgH)2py] (1) with either 3- or 4-pyridylboronic acid affords dimeric units arranged on a crystallographic symmetry center such that the pyridyl nitrogen of one moiety coordinates to the Co atom of the symmetry-related unit. In principle, three structurally different dimeric species (two homodimers and one heterodimer) can be obtained. Time-resolved 1H NMR spectra of a 1:1 mixture of racemic 1 and either 3- or 4-pyridylboronic acid in CDCl3/CD3OD show that the reaction does not converge toward a unique species in solution. Nevertheless, X-ray structures show that the heterochiral dimers are the only products that crystallize from the reaction mixture. The nature of the dioximate side groups does not affect the geometry of the dimeric arrangements assembled by 4-pyridylboronic acid (“molecular box”). On the contrary, the geometry of the species assembled by 3-pyridylboronic acid varies from the “molecular parallelogram” obtained from the bis(dimethylglyoximates) to the highly squeezed “molecular box” obtained from bis(methylphenylglyoximates). Cyclic voltammetry studies show that the metal centers in the dimeric species do not interact with each other and undergo a simultaneous redox process. However, depending on the geometry of the systems, the redox process involves a single four-electron reduction for 3 and 5 or two consecutive two-electron reduction steps for 4 and 6. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Cobalt(III) complexes of the type [Co(HL)(L)] were synthesized under solvothermal conditions staring from [Co(C5H7O2)3] and the corresponding ligand H2L (salicylaldehyde 4-hydroxybenzhydrazone, 3-methoxysalicylaldehyde 4-hydroxybenzhydrazone, 4-methoxysalicylaldehyde 4-hydroxybenzhydrazone, salicylaldehyde benzhydrazone, 3-methoxysalicylaldehyde benzhydrazone, 4-methoxysalicylaldehyde benzhydrazone). The presence of differently protonated forms of the same ligand in the complexes was supported by IR and NMR spectroscopy as well as by the single crystal X-ray diffraction method. The effects of weak interactions on the supramolecular architecture and their role on the ligand form stabilization have been analysed. Molecular interactions within the unit cells were investigated and quantified by extensive quantum chemical analysis on models built from crystal structures using density functional theory and empirical dispersion. Ligands used in this study were prepared under environmentally friendly conditions by mechanochemical synthesis. Their thermal behaviour and phase transitions were investigated using TG and DSC analysis and the powder X-ray diffraction method.

The guest-induced synthesis of a molecular box from methylaquacobaloxime and 1,4-phenylenebisboronic acid, with pyrazine (pz) as guest, is described. The resulting supramolecular species was characterized by X-ray structural analysis, 1H and 13C NMR spectroscopy, and low-resolution electrospray ionization (ES) mass spectrometry. The assembly was monitored by a time dependent 1H NMR experiment, which showed that the guest thermodynamically drives the assembly of the host. The effectiveness of pyrazine in this role arises from its having both the correct geometry and a sufficiently low pKa value. Several other ditopic ligands were tested, but none led to formation of an analogous product. However, a second supramolecular species was formed in the case of ethylenediamine (en). X-ray structural analysis and 1H and 13C spectroscopy revealed that this is closely related to the first, with a phenyl side removed and the pz bridge substituted by the en bridge.

The reactions of methylaquacobaloxime and ethylaquacobaloxime with 3-aminophenylboronic acid in water produce supramolecular triangles 3a and 3b respectively, through the reaction of boron with the oxime oxygens of the alkylcobaloxime and the coordination of pyridine to cobalt. The reaction is reversible and strongly pH dependent. The complexes 3a and 3b precipitate immediately from the solution at pH 7, but dissolve quickly at both acidic (below 4) and basic (above 9) pH values. The aggregates are re-formed by readjusting the pH to neutrality and the precipitation-dissolution cycle may be repeated several times without an apparent decrease in yield. The complexes were fully characterised by 1H and 13C NMR spectroscopy. The X-ray structure shows that the neutral chiral supramolecule 3a is a triangle of cobalt atoms with an approximate C3 symmetry. The side of the triangle is about 9.5 A. The ring closure is ensured by the torsion angles around the B−C and C−NH2 bonds, which vary in the range 25−30° and 97−109°, respectively. The reaction of methylaquarhodoxime with 3-aminophenylboronic acid in water at pH 7 results in the displacement of H2O by 3-aminophenylboronic acid only. The polymeric form 4 precipitates from CD3OD in the NMR tube with time. The X-ray structure shows that 4 is a polymeric species with a zig-zag chain arrangement, with Rh···Rh distances of 9.2 A. The chain arrangement is characterized by torsion angles around the B−C and C-NH2 bonds of 61° and 92°, respectively. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Treatment of R = -CH(2)X (X = halogen) derivatives of the type [RCo(III)(LNH-py)(HLNH-py)](+), where HLNH-py = 2-(2-pyridyl-ethyl)amino-3-butanone oxime and LNH-py its conjugated base with diluted NaOH, afforded a new complex containing a three-membered ring by a pathway involving the intramolecular nucleophilic addition of an equatorial nitrogen donor to the axial carbon. The X-ray analysis reveals a highly distorted structure. The C-Co-N angle is acute (42.8 degrees ) with the distortion of the coordination sphere concentrated in the Co-C axial and Co-N bonds, which move away from the pseudo-octahedral positions in the CH(2)X parent complex to form the C-N bond of the metallocycle. Kinetic studies of the formation of this novel complex starting from [(XCH(2)Co(III)(LNH-py)(HLNH-py)](+) (X = Cl,Br,I) showed that the metallocycle formation rates increase in the order ClBrI. Kinetic data are consistent with a mechanism involving an intermediate species resulting from the deprotonation of an amine equatorial nitrogen in a rapid preequilibrium, followed by the slow step of the ring closure.

The oxidative addition of alkyl halides to the CoI species generated by the reduction of [CoIII(LNHpy)(HLNHpy)](ClO4)2 (1), where HLNHpy is the tridentate 2-(2-pyridylethyl)amino-3-butanone oxime ligand and LNHpy is its conjugate base, led to the formation of a new class of organocobalt complexes of general formula [RCoIII(LNHpy)(HLNHpy)](ClO4) [R = Me (2a), Et (2b), CH2CF3 (2c), nBu (2d), and CH2Cl (2e)]. All the complexes were characterised by 1H and 13C NMR spectroscopy. The X-ray structures of 2a, 2b and 2c provide evidence for a pseudo-octahedral configuration, where HLNHpy and LNHpy act as bi- and tridentate ligands, respectively. The axial geometry in 2a is closer to that found in methylcobalamin than that reported for other models, suggesting steric and electronic cis influences of the equatorial ligands close to those of the corrin nucleus. The solution properties and the reactivity show strong analogies with those of the previously known Vitamin B12 models.

The reaction of [CoIII(4,4′dmsalen)(CH2Cl)(S)], where 4,4′dmsalen = 4,4′-dimethylsalen and S = solvent, with pyridine led to the formation of [CoIII(4,4′dmsalen)(CH2py)(Cl)], containing a nonstabilized pyridinium ylide as axial ligand. The complex has been unambiguously characterized by single-crystal X-ray diffraction analysis. Time-resolved 1H NMR spectra showed that the formation of [CoIII(4,4′dmsalen)(CH2py)(Cl)] occurs in a two-step process involving a metallacyclized intermediate, cis-β-[CoIII(4,4′dmsalenCH2)(py)(S)]+. A similar experiment carried out in the presence of different nitrogen bases having higher pKa values (4-Me-py or 4-t-Bu-py) allowed a better separation of the two consecutive reactions. The almost complete conversion of [CoIII(4,4′dmsalen)(CH2Cl)(S)] in the cyclized intermediate before the formation of the ylide indicates that the ylide complex forms exclusively through the nucleophilic attack of the nitrogen base at the −CH2O– carbon of the cyclized species, whereas a parallel direct conversion through the displacement of Cl– from the axial CH2Cl group of [CoIII(4,4′dmsalen)(CH2Cl)(S)] may be ruled out.

A series of electronically tuned trans- [CoIII(chel)(CH2Cl)]2 complexes, where chel is a salen derivative (salen = 2, 2′- ethylenebis(nitrilomethylidene)diphenol) containing either two or four methyl substituents in different positions, has been synthesized and characterized, both in solution and in the solid state. These complexes undergo an intramolecular cyclization reaction in methanolic solution to form the corresponding cis β organometallic derivative containing a seven-membered metallacycle, by replacement of the Cl atom of the axial CH2Cl by the salen phenolate oxygen. The cyclization rate increases on going from two to four methyl substituents in the chelate, in agreement with the electrochemical data that evidence a general shift toward more negative values with an increase in the number of methyl substituents. The cyclization rate is also affected by the substituent position, and both electrochemical and kinetic data evidence a remarkable influence of the methyls on the −C═N– groups of the chelate. The X-ray structures of cyclized complexes, [CoIII(chelCH2)(py)(H2O)]+, show a dependence of the conformation of the seven-membered metallacycle on the different positions of substituents in the chelate. In fact, in the complex having methyls on the −C═N– groups, the conformation is characterized by having the methylene carbon atom significantly displaced (ca. 1.26 Å) from the aromatic ring plane, whereas in the complex lacking methyl groups in those positions, the atoms of the Ph–O–CH2 fragment are coplanar. The standard Gibbs energies obtained by quantum chemical calculation reveal that the different conformations observed in the solid state are mainly the result of the energetically unfavorable setup of the methyls on the −C═N– groups and of the energetically favorable displacement of the CH2 group out of plane of the aromatic ring. 1H NMR data suggest that the different conformations of the metallacycle are, at least partially, retained in solution.

CF2HCbl, CF3Cbl , and CF3CH2Cbl have been synthesized and characterized in solution by (1)H NMR and UV-vis spectroscopy, and their X-ray crystal structures have been determined using synchrotron radiation. The structure of CF3CH2Cbl is reported for the first time, whereas those of CF2HCbl and CF3Cbl are re-examined to obtain more precise structural data. Comparison of the structural data obtained with the alkylcobalamin analogues, MeCbl and EtCbl, indicates that the Co-C and Co-NB3 bond lengths are shorter in the fluoroalkylcobalamins. The structural data of the fluoroalkylcobalamins previously reported in the literature had been conflicting in this regard. Thus, a much less dramatic shortening of the two axial bonds was found for CF3Cbl, whereas in the case of CF2HCbl, the Co-NB3 bond length is shorter than in MeCbl. Direct comparison of the structures of CF3CH2Cbl and EtCbl indicates a large distortion of the axial fragment in the former case that can be attributed to steric effects. A number of previously reported correlations of the effect of the β-ligand on the structure and properties of cobalamins are re-examined in light of the present results. Particular emphasis is placed on the axial fragment. This analysis substantially confirms and, with the new data reported here, adjusts and expands the data set for correlations between trans and cis influences of the β-ligand of cobalamins and their structure (Co-X and Co-NB3 distances and corrin fold angle) and properties (UV-vis spectra, NMR spectra, and pK(base-off)).

Cobaloximes are bis(dioximate) cobalt complexes with the metal ion having different oxidation and coordination numbers. The most common cobaloximes are octahedral complexes of formula trans-LCo(dioxH)2X, where dioxH is the dioxime monoanion, L a neutral ligand and X a monoanionic ligand. Due to the possibility to obtain and characterize CoIII compounds with a large variety of dioxH, X and L ligands, they constitute a unique system of octahedral cobalt complexes. The availability of a large number of complexes has allowed to rationalize their structural properties in solution and in the solid state and their reactivity in term of cis and trans influences and effects. These studies were also prompted by the widely accepted proposal that cobaloximes are model systems of cobalamins, the cofactors of B12 enzymes. Cobaloximes behave as functional models of cobalamin also towards the reductive dechlorination of chloroethylenes. Alkylcobaloximes have catalytic applications in organic chemistry and in polymerization reactions. Recently, inorganic cobaloximes have been studied as catalysts for the hydrogen evolution in the presence of a proton source. All these aspects of cobaloximes and of some related compounds, such as dioximate chlatrochelates, are presented in this chapter. Keywords: Cobaloximes; Structure; Properties; Reactivity; Cis Influence/effect; Trans influence/effect; B12 model systems; Polymerization catalysts; Hydrogen evolution

A new series of inorganic and organometallic cobalt Schiff base complexes containing the tetradentate ligand {[Z-Tyr(3-Ac)-OMe] 2 en} derived from the condensation of two equivalents of protected 3-acetyl- l -tyrosine with one equivalent of ethylenediamine has been synthesized. As the amino acids have l configuration, the resulting ligand is chiral. The complexes have been characterized by 1 H and 13 C NMR, ESI-MS spectrometry and UV–Vis spectroscopy. The UV–Vis spectra recorded in CH 2 Cl 2 show an absorption band at about 650 nm, characteristic of penta-coordinated alkylcobalt(III)(salen), but an evident deviation from the Lambert Beer law in the concentration range 0.20–2.0 × 10 −3 M suggests some kind of association in this solvent. In coordinating solvent the complexes are hexa-coordinated. The equilibrium constants for pyridine ligation in CH 3 OH decrease with increasing electron-donor power of the axial alkyl group. Cyclic voltammetry studies indicate that after the reduction RCo(III)/RCo(II), the complexes decompose giving a Co(I) species that is stable on the CV time scale and can be reoxidized to Co(II) and Co(III) species. Both the E pa and the E pc values show that the peak potentials are shifted to more negative values with increasing inductive effect of the axial alkyl group.

The reaction of [MoO2(acac)2] (where acac = acetylacetonate ligand) with the salicylaldehyde isonicotinyl hydrazonate ligand (SIH2−) yielded a zigzag chain polymer [MoO2(SIH)]n (1), an interwoven hexagon [MoO2(SIH)]6 (2), or the mononuclear complexes [MoO2(SIH)(C2H5OH)] (3EtOH) and [MoO2(SIH)(C3H7OH)] (3PrOH). Diversity in the formation of dioxomolybdenum(VI) compounds illustrates their sensitivity to the reaction conditions. Crystal and molecular structures of all of the investigated molybdenum(VI) compounds were determined by the single crystal X-ray diffraction method. Solid-state reactions lead to the transformation of the supramolecular hexagon or the mononuclear complexes into the chain coordination polymer. These thermally induced conversions were characterized by the X-ray powder diffraction method. All of the investigated compounds were further characterized by elemental analysis, thermogravimetric analyses, Fourier transform infrared (FT-IR), and NMR spectroscopy.

Cobaloximes are bis(dioximate) cobalt complexes with the metal ion having different oxidation and coordination numbers. The most common cobaloximes are octahedral complexes of formula trans-LCo(dioxH)2X, where dioxH is the dioxime monoanion, L a neutral ligand and X a monoanionic ligand. Due to the possibility to obtain and characterize CoIII compounds with a large variety of dioxH, X and L ligands, they constitute a unique system of octahedral cobalt complexes. The availability of a large number of complexes has allowed to rationalize their structural properties in solution and in the solid state and their reactivity in term of cis and trans influences and effects. These studies were also prompted by the widely accepted proposal that cobaloximes are model systems of cobalamins, the cofactors of B12 enzymes. Cobaloximes behave as functional models of cobalamin also towards the reductive dechlorination of chloroethylenes. Alkylcobaloximes have catalytic applications in organic chemistry and in polymerization reactions. Recently, inorganic cobaloximes have been studied as catalysts for the hydrogen evolution in the presence of a proton source. All these aspects of cobaloximes and of some related compounds, such as dioximate chlatrochelates, are presented in this chapter. Keywords: Cobaloximes; Structure; Properties; Reactivity; Cis Influence/effect; Trans influence/effect; B12 model systems; Polymerization catalysts; Hydrogen evolution

We described the guest-induced synthesis of a rectangular host derived from methylaquacobaloxime CH3Co(dmgH)2H2O and 4,4′-biphenyldiboronic acid containing 4,4′-bipyridine (bipy) molecule as guest. The assembly, monitored by a time dependent 1H NMR experiment, showed that the guest thermodynamically drives the organization of the host. The structural characterization of the complex evidenced a centrosymmetric rectangular box having two biphenyldiboronate units and a central 4,4′-bipyridine spacer connecting the cobaloxime units (1). The spacers have coplanar rings, being negligible the tilt angle around the central C–C bond. The molecular structure of 1 and of the dinuclear complex [CH3Co(dmgH)2]2(bipy) indicates close comparable intermetallic distance for the rigid bipy bridge and a coplanar arrangement of the cobaloxime moieties that facilitates the formation of the molecular box. The other tested ditopic ligands L (α,α′-diamino-p-xylene and 1,6-diaminohexane) used in this study allowed only a partial assembly. These derivatives and the related dinuclear complexes [CH3Co(dmgH)2]2(L), which were synthesized with the aim to obtain further insights about the formation of the molecular boxes, were also characterized by X-ray structural analysis. The structures of [CH3Co(dmgH)2]2(L), beside the expected flexibility of L, indicate not facing cobaloxime moieties. CV measurements suggest the formation of dinuclear non organometallic Co(II) and Co(I) species after the first reduction for all the dinuclear compounds that are stable in solution. For 1 no electronic coupling is shown, in spite of coplanar conformation of the 4,4′-bipyridine rings.

This short review describes the use of organocobaloximes in the self-assembly of supramolecular systems by means of functionalized aromatic boronic or diboronic acids. With the former, the cyclization is achieved by the coordination of the nitrogen donor of the boronic acid to one of the Co axial positions, with simultaneous insertion of the boron in the hydrogen bond of the other cobaloxime unit. Cyclic supramolecular species of different nuclearity, depending either on the kind of boronic acid or on the side substituents of the cobaloxime, can be obtained. The use of diboronic acids, which link two organonaquocobaloxime units, allows the formation of cyclic oligomers only in the presence of suitable guests.

Two probes consisting of vitamin B(12) (CNCbl) conjugated to Gd chelates by esterification of the ribose 5'-OH moiety, Gd-DTPA-CNCbl (1; DTPA = diethylenetriamine-N,N,N',N'',N''-pentaacetic acid) and Gd-TTHA-CNCbl (2; TTHA = triethylenetetramine-N,N,N',N'',N''',N'''-hexaacetic acid), have been synthesised and characterised. The crystal structure of a dimeric form of 1, obtained by crystallisation with an excess of GdCl(3), has been determined. The kinetics of binding to and dissociation from transcobalamin II show that 1 and 2 maintain high-affinity binding to the vitamin B(12) transport protein. Complex 2 is very stable with respect to Gd(3+) release owing to the saturated co-ordination of the Gd(3+) ion by four amino and five carboxylate groups. Hydrolysis of the ester functionality occurs on the time scale of several hours. The lack of saturation and the possible involvement of the ester functionality in co-ordination result in lower stability of 1 towards hydrolysis and in a considerable release of Gd(3+) in vitro. Gd(3+) ions released from 1 are avidly taken up by the K562 tumour cells to an extent corresponding to approximately 10(10) Gd(3+) per cell. The internalisation of toxic Gd(3+) ions causes a marked decrease in cell viability as assessed by Trypan blue and WST-1 tests. On the contrary, the experiments with the more stable 2 did not show any significant cell internalisation of Gd(3+) ions and any influence on cell viability. The results point to new avenues of in situ generation of cytotoxic pathways based on the release of toxic Gd(3+) ions by vitamin B(12) bioconjugates.

The reaction of a cis beta folded organocobalt derivative with a salen-type ligand, 1, isolated as racemic compound of Delta and Lambda enantiomers, with enantiomerically pure alpha-l amino acids is reported. The reaction between racemic 1 and l-tyrosine afforded a mixture of the two diastereoisomers Delta-2 and Lambda-2, which could be separated by fractional crystallization owing to the lower solubility of Delta-2. The absolute configuration of the two diastereomers was unequivocally assigned from the X-ray structure, using the known absolute configuration of the asymmetric carbon of the amino acid as internal reference. The reaction of racemic 1 with trans-4-hydroxy-l-proline afforded only the diastereoisomer with a Delta configuration of the tetradentate ligand, as proved by X-ray diffractometric analysis. For both l-tyrosine and trans-4-hydroxy-l-proline, the amino acid initially coordinates both to the Delta and to the Lambda enantiomers of 1, leading to an about equimolar mixture of diastereoisomers. In the case of l-tyrosine the diastereoisomers have about the same energy, so that the successive isomerization is negligible. In the case of trans-4-hydroxy-l-proline, Delta-3 is much more stable than Lambda-3, and the isomerization reaction Lambda-3 --Delta-3 goes practically to completion.

The synthesis and characterization of the products of the reactions between the bis(diphenylglyoximato)cobalt(III) complexes [Co(CH3)(dpgH)2L] [L = py or H2O; dpgH2 = bis(diphenylglyoxime)] and 4-pyridinylboronic acid (1) and 3-aminophenylboronic acid (2) are described. The crystals of both 1 and 2 are built up by dimeric units arranged on a crystallographic symmetry center, so that the 4-pyridinyl (in 1) or the 3-aminophenyl residue (in 2) of one moiety coordinates to the Co atom of the symmetry-related unit. 1H NMR spectra show that both 1 and 2 are stable in solution in aprotic solvents: they do not dissociate and are not in equilibrium with species having different nuclearity. The geometry of 1 is very similar to that of the species assembled from the corresponding bis(dimethylglyoximato)cobalt(III) complex and 4-pyridinylboronic acid. On the contrary, the replacement of the methyls by phenyls prevents the trimeric arrangement previously found in the supramolecular species assembled from methylcobaloxime and 3-aminophenylboronic acid and leads to the dimeric species 2. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The reduction of [Co(III)(tmsalen)py(2)](+)ClO(4)(-), where tmsalen = 4,4',7,7'-tetramethylsalen, with NaBH(4)/PdCl(2) in alkaline methanolic solution, followed by the oxidative addition of CH(2)ClI, leads to the expected trans organometallic dimeric species 1, [CH(2)ClCo(tmsalen)](2), provided that the product is recovered from the reaction mixture immediately after the completion of the reaction. If 1 is left for longer time in contact with the reaction mixture, the intramolecular reaction of the axial chloromethyl group with the equatorial chelate leads to the formation of the monocationic complex 2, containing a seven-membered ring. In this complex the novel tetradentate ligand coordinates Co in a cis fashion, the other two positions being occupied by one py and one water molecule. The resulting complex is chiral, even if the reaction product is a racemic compound. The unidentate ligands of 2 have been exchanged quantitatively for N-MeIm, and the resulting complex 3 still maintains the beta cis geometry. Therefore, 2 may be considered the precursor of a new class of organocobalt derivatives with a folded tetradentate ligand and two adjacent exchangeable sites. On the basis of the geometry of the tetradentate Schiff bases in complexes, where they adopt a planar geometry, it was suggested that there is a significant electron density delocalization involving the metal center over the two chemically equivalent moieties of Co(chel). Comparison of the geometry of the planar salicylaldiminate (sal) moiety with that of the cyclized methoxy-iminate one (imi) in 2 and 3 strongly supports that the delocalization, still present in sal, is essentially either lost or strongly reduced in imi.

A new series of inorganic and organometallic cobalt Schiff base complexes containing the tetradentate ligand tmsalen (4, 4', 7, 7', tetramethylsalen) have been synthesized. They have been characterized by H-1 NMR and UV-Vis spectroscopy and by ESI MS spectrometry, both in coordinating and non-coordinating solvents. Two inorganic complexes [Co(tmsalen)(L)(2)]ClO4, where L = py (2) and N-Melm (3), two dimeric organometallic derivatives [RCo(tmsalen)](2) (R = CH3 (4) and CH2CF3 (7)) and one organometallic penta-coordinated species (R = i-Pr (6)) have been structurally characterized. It is noteworthy that the formation of organometallic dimeric and penta-coordinated species occurs even if the synthesis is carried out in presence of py. Solution data confirm the strong tendency to the penta-coordination of the complexes containing the most electron-donating alkyl groups, which maintain penta-coordination even in methanol. (C) 2003 Elsevier Science B.V. All rights reserved.

The reaction of CH(3)Co(DH)(2)H(2)O with 4-pyridinyl boronic acid in methanol or water affords the dinuclear complexes [MeCo(DH)(DB(OR)(4-Py))](2), with R = Me (2) or H (3), respectively, through reaction of boron with the oxime oxygens of the alkylcobaloxime and coordination of the pyridinyl N to cobalt. The reaction is strongly pH dependent, and the formation of the complexes requires a neutral medium. The complexes have been fully characterized by (1)H and (13)C NMR spectroscopy, ESI-MS spectrometry, and elemental analysis. The X-ray structure shows that in 2, the pyridinyl groups are facing each other and nearly perpendicular both to the plane of the Co B Co1 B1 atoms and to the mean equatorial plane, so that the complex may be considered a molecular box. A dimeric arrangement has already been found in the related [MeCo(DH)(DB(OMe)(3-Py))](2) (1) complex, which forms a distorted molecular rectangle [Dreos, R.; Nardin, G.; Randaccio, L.; Tauzher, G.; Vuano, S. Inorg. Chem. 1997, 36, 2463]. The dimerization is possible in both cases, as the conformational freedom of the B bridge compensates for the different position (3- or 4-) of the pyridinyl N donor.

Some new derivatives of organocobaloximes containing para -substituted diphenylboryl groups, RCo(DH) 2− n (DB( p- XPh) 2 ) n L (R=alkyl or aryl group, L= N -MeIm, Py or H 2 O, X=OCH 3 , CH 3 or Cl, n =1 or 2) have been synthesized. The X-ray structures and the 1 H-NMR spectra are compared with those of the corresponding RCo(DH) 2− n (DBPh 2 ) n L and RCo(DBF 2 ) 2 L complexes. The insertion of X groups in the phenyl rings does not significantly affect the equatorial CoN distances, whereas the CoPy distances increase slightly in the order (DB( p- OCH 3 Ph) 2 ) 2 p- ClPh) 2 ) 2 2 ) 2 . 1 H-NMR spectra suggest that the conformational distribution in solution is similar to that observed in the corresponding BPh 2 derivatives. Electrochemical studies on the corresponding MeCo(DB( p- XPh) 2 ) 2 H 2 O compounds show a mono-electron Co(III)/Co(II) transfer reaction followed by two parallel reactions: (a) mono-electron Co(II)/Co(I) transfer; (b) homolytic dissociation of the CoC bond with the formation of Co(I) species, the relative rates of the two processes being dependent on X. As the electron-withdrawing power of the equatorial ligand increases, the reduction potentials associated with both Co(III)/Co(II) and Co(II)/Co(I) processes shift towards more positive values, indicating a decrease of electron density on the Co atom. The effects are comparable with those observed by changing the axial ligands.