Electrochemical Detection of Transient Cobalt Hydride Intermediates of Electrocatalytic Hydrogen Production

A large variety of molecular cobalt complexes are used as electrocatalysts for H-2 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 (CoH)-H-III and (CoH)-H-II intermediates of electrocatalytic H-2 production by [Co-II(PtBu2NPH2)(CH3CN)(3)](2+) and CoII(dmgBF(2))(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 CoIIIH intermediate to CoIIH, which is then protonated by acid to generate H-2. A bridge-protonated Co-I species was ruled out as a catalytic intermediate for Co-II(dmgBF(2))(2)(CH3CN)(2) from voltammograms recorded at 1000 psi of H-2. 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 (CoH)-H-III and (CoH)-H-II intermediates.