Concerted proton-coupled electron transfer from a metal-hydride complex

Metal hydrides are key intermediates in the catalytic reduction of protons and CO2 as well as in the oxidation of H-2. In these reactions, electrons and protons are transferred to or from separate acceptors or donors in bidirectional proton-coupled electron transfer (PCET) steps. The mechanistic interpretation of PCET reactions of metal hydrides has focused on the stepwise transfer of electrons and protons. A concerted transfer may, however, occur with a lower reaction barrier and therefore proceed at higher catalytic rates. Here we investigate the feasibility of such a reaction by studying the oxidation-deprotonation reactions of a tungsten hydride complex. The rate dependence on the driving force for both electron transfer and proton transfer-employing different combinations of oxidants and bases-was used to establish experimentally the concerted, bidirectional PCET of a metal-hydride species. Consideration of the findings presented here in future catalyst designs may lead to more-efficient catalysts.