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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.

A series of organometallic complexes derived by organocobaloximes and organorhodoximes in which either one or both the hydrogen bridges have been replaced by BPh 2 groups, RM(DH)(DBPh 2 ) N -MeIm and RM((DBPh 2 ) 2 N -MeIm, respectively, have been synthesised and characterised, both in solution and in solid state. 1 H NMR spectra show that they assume different interconverting conformations in solution. With increasing steric bulk of R, the axial phenyls of the BPh 2 group tend to face N -MeIm, forcing the latter in an orientation which is quite unusual in organocobaloximes and causing a lengthening of the Co-N bond. Some possible implications on the strength of the trans Co-C bond are discussed.

The reaction between methylrhodoxine, CH 3 Rh(DH 2 ) 2 L (L = N-Melm or Py ), and diphenylborinic anhydride affords derivatives containing either one or two diphenylboryl bridges, depending on the ratio complex:diphenylborinic anhydride. Kinetic studies show that the insertion of the diphenylboryl bridges is a two-step process involving the formation of a relatively stable intermediate. The X-ray structure of CH 3 Rh(DH)(DBPh 2 )NMelm shows that the axial phenyl faces the NMeIm, whereas the main conformation in solution, as inferred from 1 H NMR spectra, has the axial phenyl facing the methyl. The bis(diphenylborylated) derivatives are less stable than those of the corresponding cobaloximes, and adopt a conformation with trans axial phenyls.

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)).

The analysis of the electronic spectra of adenosylcobalamin (AdoCbl) and its derivative in which the trans axial base was replaced by a water molecule (AdoCbi–H2O) has been performed by means of time-dependent density functional theory (TDDFT). The latter corresponds to the situation encountered in strongly acidic conditions. The TDDFT electronic transitions and oscillator strengths were calculated at the optimized B3LYP and BP86 ground state equilibrium geometries. A comparison of the orbital energy diagrams obtained with the B3LYP and BP86 functionals reveals a different orbital order and composition of the highest occupied and lowest unoccupied molecular orbitals. In B3LYP the lowest-energy transitions are of π/d→π*, π/d→σ*, and π/d→d characters while in the case of BP86 they are mainly d/π→π* and d→π*. The long range charge transfer transitions involving excitations from adenine π orbitals to antibonding corrin π* orbital can be observed at low energies, especially in BP86 results. Calculated electronic excitations were used to simulate the absorption spectra for a direct comparison with the absorption spectra recorded for AdoCbl at different pH values. As previously found for methylcobalamin [ see Andruniów et al., J. Chem. Phys. 129, 085101 (2008) ] also for AdoCbl the two-parameter scaling technique is required to obtain a satisfactory agreement between theoretical and experimental spectra. Both functionals correctly predict the shifting of the lowest intense transition toward blue by approximately 13 nm upon changing pH from 7 to 1.

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.

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.

Time-dependent density functional theory (TDDFT) has been applied to the analysis of the electronic spectra of methylcobalamin (MeCbl) and its derivative in which the trans axial base was replaced by a water molecule (MeCbi[Single Bond]H(2)O). The latter corresponds to the situation encountered in strongly acidic solutions. The study primarily focuses on the accuracy of two functionals, the hybrid B3LYP and the gradient corrected BP86, in dealing with the electronic excitations. The high resolution crystal structure of MeCbl was the source of the initial coordinates. To generate the initial structures, the full MeCbl was simplified by replacing the corrin side chains by H atoms. The vertical excitation energies, together with the corresponding oscillator strengths, were calculated at the optimized BP86 and B3LYP structures of the ground electronic state of the complexes. The NBO analysis shows that the B3LYP functional gives a bonding description of the ground state as a more polarized covalent bond compared to that given by BP86. The latter functional has more covalent bonding and is thus more appropriate for modeling the axial bonding properties. To validate the accuracy of the present TDDFT analysis, the computed excitations were directly compared to the absorption spectra of MeCbl. In order to obtain a reliable agreement between experiment and theory, the two-parameter scaling technique was introduced, which compensates differently the low-energy and high-energy excitations. Electronic excitations strongly depend on the choice of the functional. Transitions involving corrin pi-->pi(*) excitations are better described by the B3LYP functional while transitions associated with metal-to-ligand (dpi-->pi(*)d) excitations are better described by BP86. These differences can be associated with the different bonding descriptions obtained by B3LYP and BP86.

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)

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.

The syntheses of the organometallic complexes RCo(DBPh2)2(MeCN), where R = Me (1), n-Pr (2), trans-β-styryl (β-Sty, 3), and MeCo (DBPh2)2TCNE (4) are described. The complexes were characterized by X-ray crystallography. Compound 1 is monoclinic, space group P2 1 /n, a = 9.081(2), b = 14.491(3), c = 14.178(3) A , β = 95.35(2)°, Z = 2, R = 0.053 compound 2 is monoclinic, space group P2 1 /a, a = 18.973(6), b = 9.369(2), c = 21.059(6) A , β = 106.65(2)°, Z = 4, R = 0.079 ; compound 3 is monoclinic, space group P2 1 /n, a = 15.130(2), b = 16.055(3), c = 15.591(3) A , β = 91.70(1)°, Z = 0.071 ; compound 4 is triclinic, space group P I , a = 15.390(6), b = 15.529(7), c = 18.038(7) A , α = 107.44(2)°, β = 103.54(2)°, γ = 101.19(2)°, Z = 4, R = 0.058 . Compound 1 is found to have an up-down conformation of the equatorial ligand, with one phenyl group facing the Me axial ligand and the other facing the axial MeCN ligand, whereas compounds 2–4 in the solid state show the down-down conformation, with the two axial phenyls of the equatorial ligand facing the neutral one. The present results sshow that the conformation adopted by these complexes is determined primarily by the difference in bulk between the axial ligands. When the bulk of R is similar to that of L. the π = π interactions become the conformational driving force.

The crystal structure of pyRh(DH)2Cl (I) and pyCo(DH)2Cl (II) are reported together with the synthesis and the crystal structure of pyRh(DH)2I (III).I: RhClO4N5C13H19; a=7.5090(8), b=8.588(1), c=14.479(2) A; α=96.45(8), β=87.07(8), γ=76.12(9)°; triclinic P 1 ; Z=2; R=0.022. II: CoClO4N5C13H19; a=14.738(2), b=8.606(1), c=13.369(2) A; β=96.30(1)°; monoclinic Cc; Z=4; R=0.022. III: IRhO4N5C13H19; a=8.875(1), b=14.741(2), c=14.163(3) A; β=97.47(8)°; monoclinic P21/c; Z=4; R=0.031. The Rh-py distance of 2.046(1) A in I, compared to that of 2.079(3) A in III, indicates that I is a slightly better trans-influencing ligand than Cl. The difference in the M-py distances when X=Cl and i-pr is 0.18 A for M=Rh and 0.14 A for M=Co. This result suggests that the interaction between steric and electronic factors is reduced in rhodoximes, owing to the larger size of RhIII.

Axial ligand exchange reactions of the methylbis(dimethylglyoximato) rhodium(III) complex were studied in dichloromethane. When exchanging ligands have similar dissociation rate constants, reactions do not go to completion in the presence of an excess of added leaving ligand. Kinetic results for the examined reactions are fully consistent with a D mechanism, where the five-coordinate intermediate reacts at approximately the same rate with all the incoming ligands.

The reaction of methylaquobis(dimethylglyoximato)cobalt(III) with 3-pyridylboronic acid in CH2Cl2/CH3OH affords a dinuclear complex containing two methylcobaloxime units held together by two B(OCH3)(3-Py) residues, which is very stable in solution. The dimer is arranged around a crystallographic symmetry center so that the pendant pyridine residue of one moiety coordinates to the Co atom of the other in one axial position, the two pyridine rings being nearly coplanar.

New dinuclear complexes (μ-OH)[RCoIII(LE-py)]2ClO4 (R = Me, Et; LE-py = 2-[2-(2-pyridyl)ethyl]imino-3-butanone oximato) were obtained. The synthesis involves the reduction of [CoIII(LE-py)2]ClO4 and the alkylation of the resulting Co(I) nucleophile.The compound with R = Me has been studied as a single crystal by X-ray crystallography. Each of the two cobalt atoms has a distorted octahedral geometry. The equatorial positions are occupied by the three LE-py N donors and the bridging OH group. The axial positions are occupied by Me and oxime O atoms of the LE-py ligand coordinated to the other Co atom. The mean Co−C value of 1.970(7) A is close to the value of 1.990(5) A reported for the Co−C bond in MeCo(Hdmg)2OH2.

The tridentate iminooximes HL2 and HL1 have been prepared by condensation of biacetyl monoxime with 2-(2-aminoethyl)pyridine and 2-(aminomethyl)pyridine, respectively. They reacted with cobalt(II) ion in the presence of oxygen to give the complexes [CoIIIL2]ClO4(L = L1 or L2). The iminooxime may be hydrogenated in the presence of NaBH4 to yield the corresponding aminooximes HL3 and HL4, which, on reaction with CoII, formed the complexes [CoIIIL(HL)][ClO4]2(L = L3 or L4). All these complexes have been structurally characterized by single-crystal X-ray diffraction. The cobalt has an octahedral environment with the iminooxime ligands in mer configuration and the aminooxime ones in fac configuration. The reduction of [CoIIIL22]ClO4 with NaBH4 produces a nucleophilic cobalt(I) species which, on reaction with MeI, gives a stable organocobalt dinuclear complex, previously reported. On the contrary, the reduction of [CoIIIL12]ClO4 involves hydrogenation of the ligand from imino- to amino-oxime, with the formation of a stable cobalt(II) species. Under these conditions, no reduction to CoI was observed. The different reactivity is attributed to the more strained co-ordination in [CoIIIL12]ClO4 with respect to that in [CoIIIL22]ClO4.

Axial alkyl or aryl groups, o -bonded to the metal atom in some tetradentate Co III complexes, are readily removed by iodine or iodine monochloride; electrophilic and radical attack on the carbon atom both can occur, depending upon the nature of the axial organic group and the chelating agent.

The kinetics of axial ligation by ammonia of some organometallic complexes RCo(DH)2H2O (R = CH3, CH2Cl, C2)H(in5) were studied in aqueous solution as a function of the hydroxide ion concentration. The kinetic data indicate that a conjugate base mechanism is operative. The behaviour of these complexes in basic medium is interpreted as the result of two counterbalancing effects: a decrease in reactivity due to the formation of the inert hydroxocomplex, and an enhancement in the reactivity due to the formation of the conjugate base, identified as the species deprotonated at the equatorial ligand system.

Heterogeneous electron transfer rate constants were determined as a function of electrode potential for one-electron oxidation in acetonitrile (AN) at O °C of a series of organocobaloximes [R-Co(DH)2L] bearing widely different organic groups. Reaction entropies were determined by voltammetric half-wave potential (Er 1 2 ) measurements in a non-isothermal cell. The electron transfer coefficients and reorganization parameters were calculated following the Marcus theory. The reaction free energies relative to a reference couple ΔG∘ are linearly correlated with the polar Taft constant of the organic substituent R. The steric effects on ΔG∘ are shown by the correlation of Er sol1 2 with the CoC bond distance. Assuming constancy of double layer effects along the series in the given solution composition, the trends of the apparent rate constants kapp were considered in order to evaluate the effects of the nature of the organic ligand on the activation energy ΔG‡ of the electron transfer. The steric effects on ΔG‡ are pointed out i.a. by consideration of the relationship between ΔG‡ and ΔG∘.

Intermediates are revealed by kinetic studies on the transalkylation by mercury(II) of alkyl- and aryl-aquo-1,3-bis(diacetylmonoximeimino)propanatocobalt(III) mono cations and of alkylaquo- N , N ′-ethylenebis(salicylideneiminato)- cobalt(III).

The mechanism of the reaction between the organocobaloximes RCo(DH)2H2O (DH = dimethylglyoximate) and iodine in methanol has been studied and a comparison made with the corresponding reaction in chloroform. The effect of the solvent on the reaction mechanism and the role of nucleophilic assistance are discussed.

The activation parameters for the water substitution reactions of some organocobaloximes with thiourea in aqueous solution at I = 1 M (NaNO3) are reported and discussed. Scatter in an isokinetic plot indicates that more than one interaction mechanism is at play. The ΔH* values are shown to be influenced by the electron withdrawing power and by the steric bulk of alkyl group. The influence of these factors on ΔS* cannot be rationalized.

The water substitution reactions of the monocation Co(III)(H 2 O) 2 (salen) + [salen = N,N′-ethylene-bis(salicylideneiminate)] were studied in aqueous solution. The nucleophiles used were pyridine, thiourea, morpholine, aniline, imidazole, NCO − , HSO 3 − , S 2 O 3 2− . The kinetics of mono-substitution reactions were found to be very rapid, with second order rate constants nearly independent of the nature of the incoming ligand. A comparison is made with the corresponding reactions of Co(III)hematoporphyrin and some other macrocyclic complexes.

Organocobalt complexes containing axial haloalkyl groups afford metallacycles of different size by N or O alkylation of the macrocyclic equatorial ligands. The reaction mechanism involves the intra-molecular nucleophilic attack of a negatively charged atom of the equatorial ligand on the axial XCH2 haloalkyl group with simultaneous detachment of a halide ion, X−. In imino/oxime and amino/oxime derivatives, the generation of the negatively charged nitrogen requires the abstraction of a proton and the reaction occurs only in alkaline medium. In bis(dimethylglioximato) and Schiff base complexes, a negatively charged oxygen is present in the equatorial ligand and the reaction occurs even in neutral medium. Three-, six- and seven- membered metallacycles are obtained, with the common feature that the Co–C bond is shorter and more resistant toward homolysis than in parent complexes or in closely related derivatives. Keywords

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.

The reaction between methylcobaloxime [CH 3 Co(DH) 2 L] and diphenylborinic anhydride affords derivatives containing either one or two diphenylboron bridges, depending on the ratio of complex: diphenylborinic anhydride. The crystal structures of [CH 3 Co(DH)(DBPh 2 )( N -MeIm)] ( Ib ) and [CH 3 Co(DBPh 2 ) 2 CH 3 OH] ( IId ) show that in Ib the axial phenyl of the BPh 2 moiety faces the axial Me, while in IId the axial phenyls are “trans”. The preferred conformations of these complexes in solution, as inferred from NMR spectra, are analogous to those found in the solid state.

A series of binuclear complexes of the type [(μ-OH)(RCoIIIL)2]+ were L is the tridentate 2-(2-pyridylethyl)imino-3-butanone oximato ligand and R = Me, Et, CH2CF3, CH2Cl or Cy have been prepared and characterised. Some of their physico-chemical properties, namely the Co–R bond lengths, the νCo–Me Raman frequencies and the E1/2 half-wave potentials of the one-electron reduction process have been determined. Comparison of these properties with those of the metalloorganic derivatives containing tetradentate ligands, such as cobaloximes, Costa model derivatives and cobalamins, suggests that in the present compounds the Co–C bond strength and the electrochemical behaviour are more influenced by electronic than by steric factors.

Kinetic studies on the substitution of alkyl-aquo-1,3-bis(biacetyl monoximeimino)propanatocobalt ions reveal evidence for intermediate complexes when the incoming ligand contains an aromatic group.