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 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 x 10(-8) - 2.6 x 10(-5) M(-1) s(-1); lambda = 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 (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)).