S. Satyanarayana

Coenzyme B12 model studies: An HSAB approach to the equilibria and kinetics of axial ligation of alkyl(aquo)-cobaloximes by imidazole and cyanide

Kinetics and equilibria of the axial ligation of alkyl(aquo)cobaloximes by imidazole and cyanide have been measured spectrophotometrically in aqueous solutions of ionic strength 1.0 M at 25°C as a function of pH. Comparison of KIMD and KCN- of CH3, C2H5 and BrCH2cobaloximes indicates that their stability is in the order BrCH2>CH3>C2H2. As the electron-withdrawing capacity of the alkyl grouptrans to water increases, the electron density of the cobalt(III) decreases and thus it becomes a stronger Lewis acid and binds more strongly to imidazole and cyanide. The association and dissociation rate constants are better correlated to the relative softness of the ligand showing that cyanide binds 30 times faster than imidazole. These complexes are isolated and are characterized by IR and1H NMR spectra.

Equilibria and kinetics forpH-dependent axial ligation of alkyl(aquo) cobaloximes with aromatic and aliphatic N-donor ligands

Equilibria and kinetics of the reaction of bromomethyl(aquo) cobaloxime with histamine, histidine, glycine and ethyl glycine ester and iodomethyl(aquo) cobaloxime with cyanide, imidazole and substituted imidazoles were studied as a function ofpH at 25°C, 10 M ionic strength (KCl) by spectrophotometry technique. The rate of substitution of H2O varies with thepKa of the incoming ligand, thus establishing the existence of nucleophilic participation of the ligand in the transition state. Dissociation kinetic reactions were also studied as a function ofpH. Binding and kinetic data were interpreted based on the basicity, steric crowd of the entering ligand and HSAB principle. To compare the rate constants of the entering ligandspH independent second-order rate constants were calculated.

Coenzyme B12 model studies: Equilibrium constants for the pH-dependent axial ligation of benzyl(aquo)cobaloxime by various N- and S-donor ligands

Equilibria of the axial ligation of benzyl(aquo)cobaloximes by imidazole, 1-methyl imidazole, histidine, histamine, glycine, ethyl glycine ester, thiourea and urea have been spectrophotometrically measured in aqueous solutions of ionic strength 1.0M (KCl) at 25°C as a function of pH. The equilibrium constants are in the order CN−> 1-methyl imidazole > imidazole > histidine > histamine>glycine>ethyl glycine ester > thiourea > urea. The order of stability of benzyl(ligand)cobaloxime is explained based on the basicity of the ligand, Co(III) →>L dπ- pπback bonding and soft-soft and soft-hard interaction. Imidazole, substituted imidazoles, histidine and histamine form more stable complexes than glycine, ethyl glycine ester in contrast to the basicity of the ligands. Benzyl(ligand)cobaloximes were isolated and characterized by elemental analysis, IR and1H NMR spectra.

Equilibria and kinetics for pH-dependent axial ligation of ethylester and methylester(aquo)cobaloximes with aromatic and aliphatic N-donor ligands and a molecular mechanistic study of the Co-C bond

Equilibrium constants are determined for the reaction of ethylester and methyl ester (aquo) cobaloximes with histamine, histidine, glycine and ethyl glycine ester as a function of pH at 25°C, using spectrophotometric technique. The functional dependence of pKa on the substitution rate of H2O varies with the pKa of the incoming ligand, establishing the existence of nucleophilic participation of the ligand in the transition state. This data is interpreted with the help of kinetic data where dissociation kinetic reactions were also studied as a function of pH. Binding and kinetic data were correlated based on the basicity, steric hindrance of the entering ligand and HSAB principle. To compare the rate constants of the entering ligands pH-independent second-order rate constants were calculated. The effect of incoming ligand on Co-C bond is studied using molecular mechanics

Equilibrium and kinetic studies on ligand substitution reactions of chloromethyl(aquo)cobaloxime with aromatic and aliphatic N-donor ligands

Equilibria and kinetics of the reactions of chloromethyl(aquo)cobaloxime with histamine, histidine, glycine and ethyl glycine ester were studied as a function of pH at 25°C, 10 M ionic strength (KCl) by spectrophotometric techniques. Comparison of equilibrium constants and rate constants tells that the order isKHisdn >KHiamn >KGly >KEtGlyest. The rate of substitution of H2O varies with the pKa of the incoming ligand and nucleophilic participation of the ligand in the transition state. The rate constants and equilibrium constants are correlated to the hardness and softness of the ligands and the Co(III) of cobaloxime.

Atrans influence study in propyl (aquo)cobaloxime by imidazoles and amino acids

Substitution reactions of propyl cobaloxime with imidazole, substituted imidazoles, histidine, histamine, glycine and ethyl glycine ester are carried out as a function of pH. Trends in the formation constants are explained based on the steric hindrance, extent of π-bonding and Σ-donor capacity of the incoming ligand. Molecular mechanics is used to theoretically determine the bond length and bond strain values by MM2 parametrization and these are correlated with the experimental data.

Equilibria and kinetics for pH-dependent axial ligation of bromomethyl(aquo)cobaloxime by aliphatic amine ligands

Kinetics and equilibria of axial ligation of bromomethyl(aquo) cobaloxime by a series of straight chain primary amines (methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine), cycloamines (cyclopentylamine, cyclohexylamine, cycloheptylamine) and secondary amines (N,N-dimethylamine, N,N-diethylamine) have been measured as functions of pH by spectrophotometric technique in aqueous solution, ionic strength 1 M (KCl) at 25°C. The rate of substitution of H2O varies with the pKa of incoming ligand, thus establishing nucleophilic participation of the ligand in the transition state. Binding and kinetic data are interpreted based on the basicity and steric influence of the entering ligand. To compare the rate constants of the entering ligands, pH independent second-order rate constants (kon) are calculated.

Synthesis,13C and15N NMR and infrared studies of cyano(ligand) cobaloximes

Mixed ligand complexes of [CNCo(D2H2)L] (whereL=imidazoles, benzimidazoles, thiourea or hydroxylamine) were prepared by treating the [CNCo(D2H2)SCN] with ‘L’ in alcohol. Imidazoles and benzimidazoles bind to Co(III) through N3, thiourea throughS and hydroxylamine throughN of NH2. The analogous series of complexes enriched in13C and15N of the cyanide ligand have been used to locate the cyanide’s13C and15N NMR resonances and the cyanide stretching frequencies in the infrared spectra. As thetrans axial ligand is varied, an inverse dependence of the15N chemical shift on the13C chemical shift is observed. This data together with trends inδ15N,δ13C andν CN with the basicity of thetrans axial ligand are interpreted to indicate that the cobalt to cyanideπ bonding is important in these cyano (ligand) complexes.