Coenzyme B₁₂ model studies: Electronic and steric <i>trans </i>effects of cobaloximes

Abstract

The equilibrium, kinetics and thermodynamic parameters of axial ligand substitution of cobaloximes by -CONH₂ and -CSNH₂ containing molecules in three different cobaloximes viz ., <i>trans</i> RCo(DH)₂ H₂O where DH = mono anion of dimethyl glyoxime and R = CH₃, CH₂ I and CH₂Br have been studied as a function of <i>p</i>H, incoming ligand concentration and temperature. It has been found that the incoming ligand substitutes the coordinated water molecule <i>trans </i>to the alkyl group with equilibrium constants that follow the order of stability: CH₂Br > CH2I >CH₃. Rate and activation parameters enable the formulation of the reaction mechanism that can account for the substitution reaction of the investigated cobaloximes. The stability of the alkyl (aquo) cobaloximes has been explained on the basis of <i>pK</i>a,<i> </i>basicity and steric influence of the incoming ligand. Alkyl cobaloximes are interesting compounds not only due to their analogies with organo cobalamins but also for their extensive coordination chemistry ^1-6. For these complexes the kinetics of substitution or the axial ligand L has been extensively studied. It has been shown that the nature of the alkyl group (R) significantly affects the substitution rate of the labile <i>trans </i>group. i.e., as the electron donating ability of the R group increases the rate constant also increases.