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Mechanisms of Hydrogen Evolution


Molecular mechanisms of cobalt-catalyzedhydrogen evolution

Smaranda C. Marinescu, Jay R. Winkler, and Harry B. Gray;  PNAS 109, 12127 September 18, 2012

Several cobalt complexes catalyze the evolution of hydrogen from acidic solutions, both homogeneously and at electrodes. The detailed molecular mechanisms of these transformations remain unresolved, largely owing to the fact that key reactive intermediates have eluded detection. One method of stabilizing reactive intermediates involves minimizing the overall reaction free-energy change. Here, we report a new cobalt(I) complex that reacts with tosylic acid to evolve hydrogen with a driving force of just 30 meV/Co. Protonation of CoI produces a transient CoIII-H complex that was characterized by nuclear magnetic resonance spectroscopy. The CoIII-H intermediate decays by second-order kinetics with an inverse dependence on acid concentration. Analysis of the kinetics suggests that CoIII-H produces hydrogen by two competing pathways: a slower homolytic route involving two CoIII-H species and a dominant heterolytic channel in which a highly reactive CoII-H transient is generated by CoI reduction of CoIII-H.

Schematic energy landscape for H2 evolution from CoI and TsOH•H2O (HA). The vertical dimension in the plot corresponds to free energy; the lateral dimensions represent different routes through configuration space. The lowest energy pathway involves nearly isoenergetic protonation of {CoI}+, producing the common intermediate {CoIII-H}2+. Endergonic reduction by {CoI}generates a highly reactive hydride {CoII-H}+, that produces H2 upon reaction with a second equivalent of HA. Overprotonation of {CoI}traps the metal complex as {CoIII-H}2+, from which only a high-energy bimolecular pathway is available for the production of {CoII}2+ and H2. Protonation of {CoIII-H}2+ is an unfavorable pathway, owing to formation of a high-energy {CoIII}3+ intermediate.