A mechanistic study of proton reduction catalyzed by a pentapyridine cobalt complex: evidence for involvement of an anation-based pathway

The pentapyridine cobalt complex Co(PY5Me2)2+ and its congeners have been shown to catalyze proton reduction to hydrogen in aqueous solution over a wide pH range using electrical or solar energy input. Here, we employ electrochemical and spectroscopic studies to examine the mechanisms of proton reduction by this parent complex under soluble, diffusion-limited conditions in acetonitrile with acetic acid as the proton donor. Two pathways for proton reduction are identified via cyclic voltammetry: one pathway occurring from an acetonitrile-bound CoII/I couple and the other pathway operating from an acetate-bound CoII/I couple. Kinetics studies support protonation of a CoI species as the rate-determining step for both processes, and additional electrochemical measurements further suggest that the onset of catalysis from the acetonitrile-bound CoII/I couple is highly affected by catalyst electronics. Taken together, this work not only establishes the CoPY5Me2 unit as a unique molecular platform that catalyzes the reduction of protons under soluble, diffusion-limited conditions in both aqueous and organic media, but also highlights the participation of anation processes that are likely relevant for a wide range of hydrogen-producing and related catalytic systems.