A Nickel(II) Complex as a Homogeneous Electrocatalyst for Water Oxidation at Neutral pH: Dual Role of HPO42- in Catalysis

A new mononuclear nickel complex, featuring two cis-relative labile sites, is reported as a homogeneous electrocatalyst for water oxidation in a neutral phosphate buffer. The water oxidation catalysis can be initiated at a moderate overpotential of approximately 0.48V. Mechanistic studies reveal that the oxidation of water by a two-electron oxidized species of the nickel catalyst is feasible, with formation of a peroxide intermediate through intramolecular O-O coupling. Interestingly, it has been found that the buffer base (HPO42-) plays a dual role in the catalytic system: it acts as a proton acceptor to expedite proton-coupled electron transfer (PCET) during catalysis, but it also suppresses the electrocatalysis through an anation process.