Kinetics of Electron Transfer Reactions of H_2-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).