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

Abstract

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 pK a on the substitution rate of H 2 O varies with the pK a 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.

26 Mechanisms of reactions in solution

“…117 Cobaloximes remain popular models for vitamin B 12 and there has been a study of the kinetics of reactions of ethyl-and methylester(aqua)cobaloximes with aromatic and aliphatic N-donor ligands. 118 There has also been a detailed study of the kinetics of substitution reactions of the cobaloxime trans-[Co(Hdmg) 2 (R)L] (R = CH 3, PhCH 2 , L = H 2 O, MeOH) with a very wide range of entering ligands (imidazole, pyrazole, 1,2,4-triazole, N-acetylimidazole, 5-chloro-1-methylimidazole, NO 2 2 , PPh 3 , Ph 3 As, Ph 3 Sb). 119 Activation parameters, including DV { , support a mechanistic picture described as dissociatively activated substitution, in which the dissociative nature of the process varies from a dissociative interchange to a limiting dissociative mechanism.…”

Solution Equilibria of Ni(II), Cu(II), and Zn(II) Complexes Involving Pyridoxine and Imidazole Containing Ligands: pH Metric, Spectral, Electrochemical, and Biological Studies

“…The imidazole group plays an important role in numerous bioactive compounds, and the imidazole ring was highly useful in pharmacology. It acts as a proton donor/acceptor and charge transfer agent and is ligated to the metal ions in the B12 coenzyme …”

“…It acts as a proton donor/acceptor and charge transfer agent and is ligated to the metal ions in the B12 coenzyme. 6 Pyridoxine displays different coordination sites with metal ions with different charges and hard/soft character. Chelation through phenolate oxygen and adjacent hydroxymethyl groups is common for pyridoxine metal complexes.…”

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

“…5,6 In recent years, there were several attempts to replicate them in experimental model systems with varying success. [7][8][9][10] Among the vitamin B 12 models, organocobaloximes are noteworthy because of their ability to accommodate a wide variety of metal-bound alkyl groups containing a large number of different types of substituents. 11 There has been continual interest to examine the steric cis and trans effects in these cobaloximes to study the factors influencing the Co-C bond cleavage.…”