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

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Abstract
Co−diglyoxime complexes catalyze H2 evolution from protic solutions at modest overpotentials. Upon reduction to CoI, a CoIII-hydride is formed by reaction with a proton donor. Two pathways for H2 production are analyzed: one is a heterolytic route involving protonation of the hydride to release H2 and generate CoIII; the other is a homoytic pathway requiring association of two CoIII-hydrides. Rate constants and reorganization parameters were estimated from analyses of laser flash−quench kinetics experiments (CoIII−CoII self-exchange k = 9.5 × 10−8 − 2.6 × 10−5 M−1 s−1; λ = 3.9 (±0.3) eV: CoII−CoI self-exchange k = 1.2 (±0.5) × 105 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 (CoIIIH + CoIIIH → 2 CoII + H2) is favored over the route that goes through a CoIII intermediate (CoIIIH + H+ → CoIII + H2).