Homogeneous electrocatalytic water oxidation catalyzed by a mononuclear nickel complex

Highlights: •Three (Me₄N)₂[NiLⁱ] complexes were first used as water oxidation electrocatalysts. •The mechanism of water oxidation reaction catalyzed by (Me₄N)₂[NiL¹] was proposed. •A stability relationship between the molecular catalyst and ligand structure was obtained. -- Abstract: Three nickel (II) complexes of o-phenylenebis(N′-methyloxamidate) (L¹), o-phenylene(N′-methyloxamidate)oxamate (L²) and o-phenylenebis(oxamate) (L³) with tetradentate ligands are synthesized and characterized. These four-coordinate complexes, (Me₄N)₂[NiLⁱ] (i = 1–3 for complex 1–3), are investigated for electrocatalytic water oxidation in basic phosphate buffer solution. Experimental results show that (Me₄N)₂[NiL¹] (1) is a homogeneous catalyst for electrochemical water oxidation. However, under the same condition, 2 and 3 decompose into NiO_x nanoparticles, which acts as precatalyst for the electrocatalytic water oxidation. The catalytic mechanism for electrocatalytic water oxidation by 1 is proposed using cyclic voltammetry experiments, kinetic isotope effect and Pourbaix diagram. By constructing the relationship between the molecular structure and stability of these catalysts, nitrogen atom is found to be more beneficial than carboxyl group for the stability of nickel based homogeneous electrochemical water oxidation catalysts.