Structural and Solution Properties of Rhodoximes: the Rh Analogues of Cobaloximes, a Vitamin B12 Model

Cobaloximes, LCo(DH)2 where L = neutral ligand, DH = monoanion of dimethylglyoxime and X = monoanionic ligand, have been widely studied in solution and in the solid state. Since then, they have been proposed as a model of the vitamin B12 system. These studies have furnished a foundation for understanding the mechanism of the Co-C bond cleavage in the vitamin B12 coenzyme. However, problems relating to the role played by the electronic and steric properties of the ligands around Co require further investigation. Therefore, the study has been extended to the pyRh(DH)2R complexes (rhodoximes). In fact, the ionic radius of Rh, larger than that of Co, should diminuish the steric interaction between the ligands. The NMR and kinetic results indicate that the transmission of the electronic effects from R to the trans neutral ligand in rhodoximes is similar to that observed in cobaloximes: the frans-influence and the irons-effect have the same trend as already found in the Co analogues. Structural data agree about a less hindered coordination around the metal centre than in the corresponding cobaloximes. Variation of the M-C distances with an increase in the bulk of axial ligand is significantly smoother in the Rh than in the Co complexes. Correspondingly, the irans-influence and -effect appear to be more enhanced in rhodoximes. Comparison between the behaviour of cobaloximes and rhodoximes is discussed on the basis of electronic and steric influences

Intramolecular Cyclization Reactions in Haloalkyl-Cobalt Complexes with Macrocyclic Equatorial Ligands

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

Steric Influences in Octahedral Cohaloximes. The Crystal and Molecular Structure of trans-methylhis(dimethylglyoximato) (phosphite)cobalt(III) Complexs with Phosphite=P(OMe)2Ph and P(OMe)Ph2

The crystal structures of the title compounds, (P(OMe)2Ph)Co( DH)2Me (I) and (P(0Me)Ph2)Co(DH)2Me (II), are reported. Compound I crystallizes in the orthorhombic system, space group Pbca, with cell parameters a = 14.652(7), b = 26.45(1) and c = 11.330(6) A and z = 8; compound II crystallizes in the monoclinic system, space group P21 with cell parameters a = 9.423(7), b = 15.184(8), c = 8.948 (7) A and ~ = 100.50(9)0 , Z = 2. Both structures were solved by conventional Patterson and Fourier methods and refined by the block diagonal least-squares method to final R values of 0.033(1) and 0.034(II), using 2108 for I and 2063 for II independent reflections with I > 30\u27(1). In both compounds the cobalt atom has a distorted octahedral geometry, the two DH units occupying the equatorial positions and Me and the P-ligand the axial ones. The Co-C and Co- P bond lengths are 2.013(5) and 2.287(1) A in I and 2.019(6) and 2.352(1) A in II, respectively. Linear correlation between Co-P bond lengths and Tolman\u27s cone angles in the series RaPCo(DH)2Me is found, with the exception of the P(OMe)Ph2 derivative. Furthermore, the linear relationship between the above Co-P distances and those reported for analogous chloro-derivatives, already found for phosphines, is valid for I but not for II

Steric Influences in Octahedral Cohaloximes. The Crystal and Molecular Structure of trans-methylhis(dimethylglyoximato) (phosphite)cobalt(III) Complexs with Phosphite=P(OMe)2Ph and P(OMe)Ph2

The crystal structures of the title compounds, (P(OMe)2Ph)Co( DH)2Me (I) and (P(0Me)Ph2)Co(DH)2Me (II), are reported. Compound I crystallizes in the orthorhombic system, space group Pbca, with cell parameters a = 14.652(7), b = 26.45(1) and c = 11.330(6) A and z = 8; compound II crystallizes in the monoclinic system, space group P21 with cell parameters a = 9.423(7), b = 15.184(8), c = 8.948 (7) A and ~ = 100.50(9)0 , Z = 2. Both structures were solved by conventional Patterson and Fourier methods and refined by the block diagonal least-squares method to final R values of 0.033(1) and 0.034(II), using 2108 for I and 2063 for II independent reflections with I > 30\u27(1). In both compounds the cobalt atom has a distorted octahedral geometry, the two DH units occupying the equatorial positions and Me and the P-ligand the axial ones. The Co-C and Co- P bond lengths are 2.013(5) and 2.287(1) A in I and 2.019(6) and 2.352(1) A in II, respectively. Linear correlation between Co-P bond lengths and Tolman\u27s cone angles in the series RaPCo(DH)2Me is found, with the exception of the P(OMe)Ph2 derivative. Furthermore, the linear relationship between the above Co-P distances and those reported for analogous chloro-derivatives, already found for phosphines, is valid for I but not for II

The Weak Metal-Water Bond in Diphenylborylated Organoaquacobaloximes and Rhodoximes Leads to the Formation of an Unusual Dinuclear Rh(III) Complex

The synthesis and characterization of a series of RM(DBPh2)2(H2O) complexes (M = Co and Rh) and the X-ray structure of the dinuclear [n-PrRh(DH)(DBPh2)]2 (1) obtained by recrystallization of n-PrRh(DBPh2)2(H2O) in non coordinating solvent are reported. The crystals of (1) are built up by neutral centrosymmetric dinuclear [n-PrRh(DH)(DBPh2)]2 units and CH2Cl2 crystallization molecules in the ratio 1:1. The Rh ion of one n-PrRh(DH)(DBPh2) unit achieves hexacoordination by coordination of one of the oxime O at-oms of the other unit. The formation of the dimer by recrystallization of n-PrRh(DBPh2)2(H2O) in non coordinating solvent may be ascribed to the concomitant effect of the weak metal-water bond in the monomeric aqua complex, the strong tendency of Rh to achieving hexacoordination in such organo derivatives, and the ease of losing a BPh2 bridge of the diphenylborylated organorhodoximes

The Weak Metal-Water Bond in Diphenylborylated Organoaquacobaloximes and Rhodoximes Leads to the Formation of an Unusual Dinuclear Rh(III) Complex

The synthesis and characterization of a series of RM(DBPh2)2(H2O) complexes (M = Co and Rh) and the X-ray structure of the dinuclear [n-PrRh(DH)(DBPh2)]2 (1) obtained by recrystallization of n-PrRh(DBPh2)2(H2O) in non coordinating solvent are reported. The crystals of (1) are built up by neutral centrosymmetric dinuclear [n-PrRh(DH)(DBPh2)]2 units and CH2Cl2 crystallization molecules in the ratio 1:1. The Rh ion of one n-PrRh(DH)(DBPh2) unit achieves hexacoordination by coordination of one of the oxime O at-oms of the other unit. The formation of the dimer by recrystallization of n-PrRh(DBPh2)2(H2O) in non coordinating solvent may be ascribed to the concomitant effect of the weak metal-water bond in the monomeric aqua complex, the strong tendency of Rh to achieving hexacoordination in such organo derivatives, and the ease of losing a BPh2 bridge of the diphenylborylated organorhodoximes

Intramolecular Cyclization Reactions in Haloalkyl-Cobalt Complexes with Macrocyclic Equatorial Ligands

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

Solid State and Solution Properties of Vitamin B12 Models

Octahedral organometallic derivatives of cobalt(III) dimethylglyoximates, LCO(DH)2R, where L = neutral ligand, R = alkyl group, DH = monoanion of dimethylglyoxime, the so called alkylcobaloximes, and other organocobalt (III) complexes with macrocyclic equatorial ligands, have been extensively studied for their ability to mimic the behaviour of the more complex system of vitamin B12, the well known cobalamins. Hence, their study has produced an incredible amount of data both in solution and in solid state, which have allowed us to establish empirical relationships between structure and solution properties of these models. Up to date, results relative to the model and comparison, when possible, with cobalamins support the idea that the homo- lytic cleavage of the Co-C bond in the B12 coenzyme is due to a »trigger« mechanism, originated by steric interactions. However, the weakening of this bond may be »helped« by electronic influences due to the lengthening of the ligand trans to the carbon atom bonded to Co. Analysis of the chemical properties of a large number of derivatives with different alkyl groups shows approximately linear relationships between bond lengths, kinetic, and spectroscopic NMR data. For R = CH2Y group the above properties can be rationalized on the basis of the 01 and OR constants used in organic chemistry for each Y residue. An alternative interpretation is more general, since it can be applied to any alkyl group. It is based on the finding that, when the alkyl group is changed, both Co-L axial distances and kinetics properties vary linearly with an electronic parameter (EP) derived from NMR data for each R group