Bidirectional and Unidirectional PCET in a Molecular Model of a Cobalt-Based Oxygen-Evolving Catalyst

The oxidation of water to molecular oxygen is a kinetically demanding reaction that requires efficient coupling of proton and electron transfer. The key proton-coupled electron transfer (PCET) event in water oxidation mediated by a cobalt-phosphate-based heterogeneous catalyst is the one-electron, one-proton conversion of Co-III-OH to Co-IV-O. We now isolate the kinetics of this PCET step in a molecular Co4O4 cubane model compound. Detailed electrochemical, stopped-flow, and NMR studies of the CoIII-OH to Co-IV-O reaction reveal distinct mechanisms for the unidirectional PCET self-exchange reaction and the corresponding bidirectional PCET. A stepwise mechanism, with rate-limiting electron transfer is observed for the bidirectional PCET at an electrode surface and in solution, whereas a concerted proton electron transfer displaying a moderate KIE (4.3 +/- 0.2), is observed for the unidirectional self-exchange reaction.