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

Abstract

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 of pH at 25°C, 1⋅0 M ionic strength (KCl) by spectrophotometry technique. The rate of substitution of H 2 O varies with the pKa 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 of pH. 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 ligands pH independent second-order rate constants were calculated.

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

“…A soft or class b character has been assigned to Co(III) in cobaloximes 19 and is consistent with the observed greater ligand affinity of cyanide, imidazole, [20][21][22][23] histidine or histamine than that of the hard glycine or ethyl glycine ester. Furthermore, softness appears to be related to the ability of a cobalt complex to stabilize a Co-C bond.…”

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

“…A soft or class b character has been assigned to cobaloximes (III) 33 and is consistent with its observed greater ligand affinity of cyanide, imidazole [34][35][36][37] histidine or histamine than for hard glycine or ethyl glycine ester. The order of binding of various ligands to methyl and ethyl estercobaloximes is of the order, histidine > histamine > glycine > ethylesterglycine.…”

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

“…[15][16][17] Recently, the equilibria and kinetics of axial ligation of alkyl(aquo)cobal-*For correspondence oximes with imidazole and substituted imidazoles were studied. [18][19][20][21] Compared to both cobalamins and other model systems, cobaloximes have stronger Co-C bonds 22 and shorter Co-L (L = pyridine, substituted pyridines) bonds. 13 Many model complexes [23][24][25][26][27][28] have shown that the ligand in the trans position to the Co-C bond can affect the kinetics and thermodynamic stability of the bond.…”