Proton Electroreduction Catalyzed by Cobaloximes:  Functional Models for Hydrogenases

Cobaloximes have been examined as electrocatalysts for proton reduction in nonaqueous solvent in the presence of triethylammonium chloride. Co(III)(dmgH)2pyCl, working at moderate potentials (-0.90 V/(Ag/AgCl/3 mol x L(-1) NaCl) and in neutral conditions, is a promising catalyst as compared to other first-row transition metal complexes which generally function at more negative potentials and/or at lower pH. More than 100 turnovers can be achieved during controlled-potential electrolysis without detectable degradation of the catalyst. Cyclic voltammograms simulation is consistent with a heterolytic catalytic mechanism and allowed us to extract related kinetic parameters. Introduction of an electron-donating (electron-withdrawing) substituent in the axial pyridine ligand significantly increases (decreases) the rate constant of the catalytic cycle determining step. This effect linearly correlates with the Hammet coefficients of the introduced substituents. The influence of the equatorial glyoxime ligand was also investigated and the capability of the stabilized BF2-bridged species Co(dmgBF2)2(OH2)2 for electrocatalyzed hydrogen evolution confirmed.