Kinetics of Electron Transfer Reactions of H2-Evolving Cobalt Diglyoxime Catalysts

Abstract

Co−diglyoxime complexes catalyze H 2 evolution from protic solutions at modest overpotentials. Upon reduction to Co I, a Co III -hydride is formed by reaction with a proton donor. Two pathways for H 2 production are analyzed: one is a heterolytic route involving protonation of the hydride to release H 2 and generate Co III; the other is a homoytic pathway requiring association of two Co III -hydrides. Rate constants and reorganization parameters were estimated from analyses of laser flash−quench kinetics experiments (Co III −Co II self-exchange k = 9.5 × 10 −8 − 2.6 × 10 −5 M −1 s −1; λ = 3.9 (±0.3) eV: Co II −Co I self-exchange k = 1.2 (±0.5) × 10 5 M −1 s −1; λ = 1.4 (±0.05) eV). Examination of both the barriers and driving forces associated with the two pathways indicates that the homolytic reaction (Co III H + Co III H → 2 Co II + H 2 ) is favored over the route that goes through a Co III intermediate (Co III H + H + → Co III + H 2 ).