Electrochemical Detection of Transient Cobalt Hydride Intermediates of Electrocatalytic Hydrogen Production

A large variety of molecular cobalt complexes are used as electrocatalysts for H₂ production, but the key cobalt hydride intermediates are frequently difficult to detect and characterize due to their high reactivity. We report that a combination of variable scan rate cyclic voltammetry and foot-of-the-wave analysis (FOWA) can be used to detect transient Co^IIIH and Co^IIH intermediates of electrocatalytic H₂ production by [Co^II(P^tBu ₂N^Ph ₂)(CH₃CN)₃]²⁺ and Co^II(dmgBF₂)₂(CH₃CN)₂. In both cases, reduction of a transient catalytic intermediate occurs at a potential that coincides with the Co^II/I couple. Each reduction displays quasireversible electron-transfer kinetics, consistent with reduction of a Co^IIIH intermediate to Co^IIH, which is then protonated by acid to generate H₂. A bridge-protonated Co^I species was ruled out as a catalytic intermediate for Co^II(dmgBF₂)₂(CH₃CN)₂ from voltammograms recorded at 1000 psi of H₂. Density functional theory was used to calculate Co^III-H and Co^II-H bond strengths for both catalysts. Despite having very different ligands, the cobalt hydrides of both catalysts possess nearly identical heterolytic and homolytic Co-H bond strengths for the Co^IIIH and Co^IIH intermediates.