Kinetics and mechanism of the activation of molecular hydrogen by bis(dimethylglyoximato)cobalt(II) derivatives

Abstract

The reaction of H₂ with bis(dimethylglyoximato)cobalt(II), [Co(Hdmg)₂], and its 1 : 1 pyridine (py) adduct yields 3-aminobutan-2-one oxime via hydrogenation of the co-ordinated Hdmg^–. Excess of Hdmg^– is hydrogenated catalytically. The initial rate of H₂ uptake is first order with respect to H₂ pressure and second order with respect to overall cobalt concentration. Axial co-ordination of py, NEt₂H, and PPh₃ enhances the rate, the five-co-ordinate adducts being more reactive than the parent cobaloxime(II). The spectrophotometric stability constants for the 1 : 1 and 1 : 2 py adducts are 186 and 0.75 dm³ mol^–1, respectively, at 20 °C. The rate-determining step of H₂ uptake is a bimolecular reaction of two hydridocobaloxime(III) species. In the absence of py, the overall rate coefficient is 3.1 and 9.4 dm⁶ mol^–2 s^–1 in MeOH and 50% v/v methanol–water, respectively, at 20 °C. For the reactions involving the 1 : 1 adduct with py, the corresponding values are 34.7 and 323 dm⁶ mol^–2 s^–1, respectively. Hydrogenation of the intermediate, containing one CH–NHOH moiety, is interpreted in terms of N–O bond cleavage by cobaloxime(II) followed by reduction of the resulting cobaloxime(III) by hydridocobaloxime(III).