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 CoIIIH and CoIIH intermediates of electrocatalytic H2 production by [CoII(PtBu 2NPh 2)(CH3CN)3]2+ and CoII(dmgBF2)2(CH3CN)2. In both cases, reduction of a transient catalytic intermediate occurs at a potential that coincides with the CoII/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 H2. A bridge-protonated CoI species was ruled out as a catalytic intermediate for CoII(dmgBF2)2(CH3CN)2 from voltammograms recorded at 1000 psi of H2. Density functional theory was used to calculate CoIII-H and CoII-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 CoIIIH and CoIIH intermediates.
ASJC Scopus subject areas
- Colloid and Surface Chemistry