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

Co-diglyoxime complexes catalyze H-2 evolution from protic solutions at modest overpotentials. Upon reduction to Col, 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 x 10(-8) - 2.6 x 10(-5) M-1 s(-1);). = 3.9 (+/-0.3) eV: Co-II-Co-I self-exchange k = 1.2 (+/-0.5) x 10(5) M-1 s(-1); lambda = 1.4 (+/-0.05) eV). Examination of both the barriers and driving forces associated with the two pathways indicates that the homolytic reaction ((CoH)-H-III + (CoH)-H-III --> 2 Co-II + H-2) is favored over the route that goes through a Co-III intermediate ((CoH)-H-III + H+ --> Co-III + H-2).