Electronically and sterically tuned trans labilization controls the substitution behaviour of cobaloximes
Abstract
The kinetics of axial water substitution by cysteine in six different cobaloximes, viz.trans-RCo(Hdmg)2H2O, where Hdmg = dimethylglyoximate, R = cyclo-C5H9 (c-P), CH3CH2 (Et), CH3 (Me), C6H5CH2 (Bz), C6H5 (Ph) and CF3CH2, were studied as a function of cysteine concentration, temperature and pressure. It was found that cysteine substitutes the coordinated H2O molecule trans to the alkyl group with second order rate constants that follow the order of reactivity: c-P > Et > Bz > Me > Ph > CF3CH2. Rate and activation parameters (ΔH≠, ΔS≠ and ΔV≠) 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 CF3CH2.
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