Efficient electrochemical water oxidation catalyzed by N4-coordinated nickel complexes under neutral conditions

Abstract

Fine modulation of the ligand structure of metal-complex-based homogeneous water oxidation catalysts is a feasible method to realize efficient electrochemical oxygen evolution. Herein, the synthesis and electrocatalytic water oxidation activity of two novel mononuclear N₄-coordinated Ni complexes, [Ni(dmabpy)](ClO₄)₂ (1, dmabpy = 6,6′-bis(dimethylamino methyl)-2,2′-bipyridine) and [Ni(mabpy)](ClO₄)₂ (2, mabpy = 6,6′-bis(methylamino methyl)-2,2′-bipyridine), are reported for the first time. Both the Ni complexes are capable of accelerating water oxidation to oxygen with low onset overpotential under neutral conditions. Combined experiment and measurement prove that the two complexes are genuine homogeneous water oxidation catalysts. Besides, they maintain their molecular structure during long-term catalytic cycles. Catalytic activity tests indicate that, though they are stabilized by similar bipyridine-based ligands, complex 1 shows lower overpotential and higher catalytic current for water oxidation than complex 2, revealing that methyl groups on the coordinated N atoms of the organic ligand obviously benefit the water oxidation catalytic activity. Detailed electrochemical measurements reveal that the water oxidation process was accomplished with the assistance of Ni^IV species generated by two successive e⁻/H⁺ proton coupled electron transfer (PCET) processes of the Ni^II center for both the Ni complexes. The facilitation effect of methyl on the catalytic activity of the Ni complex is realized by modulating the redox potential of the metal center via an electronic effect.