Electronically and sterically tuned trans labilization controls the substitution behaviour of cobaloximes

摘要

The kinetics of axial water substitution by cysteine in six different cobaloximes, viz.trans-RCo(Hdmg)_2H_2O, where Hdmg = dimethylglyoximate, R =cyclo-C_5H_9(c-P), CH_3CH_2(Et), CH_3(Me), C_6H_5CH_2(Bz), C_6H_5(Ph) and CF_3CH_2, were studied as a function of cysteine concentration, temperature and pressure. It was found that cysteine substitutes the coordinated H_2O molecule trans to the alkyl group with second order rate constants that follow the order of reactivity: c-P > Et > Bz > Me > Ph > CF_3CH_2. Rate and activation parameters (H, S and V) enable the formulation of a reaction mechanism that can account for the substitution behaviour of the investigated alkylcobaloximes. In particular, a gradual mechanistic changeover from Id to I is observed along the series of R groups from c-P to CF_3CH_2.