Homogenous electrochemical water oxidation by a nickel(II) complex based on a macrocyclic N-heterocyclic carbene/pyridine hybrid ligand.

Water-soluble homogeneous nickel catalysts are not much investigated in the context of catalytic water oxidation compared to its heterogeneous catalysts. Herein, we report homogenous electrochemical water oxidation by a nickel(II) complex based on a macrocyclic N-heterocyclic carbene/pyridine hybrid ligand, ([NiL](PF6)2 (L = bis(2‒pyridyl-methylimidazolylidene)methane) in neutral and alkaline conditions. The catalyst displayed a stable catalytic current of oxygen evolution in long‒term bulk electrolysis of 0.65 mA/cm2 at 0.8 V overpotential at pH 9.0. The results of CV, UV‒Vis, ESI‒MS, SEM, and EDX demonstrate that the catalyst is impressively stable even after long‒term controlled potential electrolysis (CPE) (11 h), and homogeneous in nature. The catalyst synthesis is straightforward and the complex is air and moisture stable. To the best of our knowledge, this is the first report of Ni‒NHC complex investigated in the context of water oxidation under aqueous conditions (acetate/phosphate). According to literature, the role of phosphate ion in homogenous nickel-catalysed water oxidation was found to vary from catalyst poising to activation. Interestingly, the catalytic activity of our catalyst in phosphate buffer was much higher than in acetate ion at the same pH value, which might be an indication of a key role of the phosphate ion to act as a proton acceptor and boost the catalyst activity via enhanced PCET during catalysis.