Molecular mechanisms of cobalt-catalyzed hydrogen evolution

Abstract

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 Co^I produces a transient Co^(IIII)-H complex that was characterized by nuclear magnetic resonance spectroscopy. The Co^(IIII)-H intermediate decays by second-order kinetics with an inverse dependence on acid concentration. Analysis of the kinetics suggests that Co^(IIII)-H produces hydrogen by two competing pathways: a slower homolytic route involving two Co^(IIII)-H species and a dominant heterolytic channel in which a highly reactive Co^(II)-H transient is generated by Co^I reduction of Co^(IIII)-H.