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

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 N 4 -coordinated Ni complexes, [Ni(dmabpy)](ClO 4 ) 2 ( 1 , dmabpy = 6,6′-bis(dimethylamino methyl)-2,2′-bipyridine) and [Ni(mabpy)](ClO 4 ) 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. Two novel nickel complexes are reported for the first time. Both complexes can catalyze water oxidation under neutral conditions with low onset overpotential. Combined experiments confirm that they are genuine molecular water oxidation catalysts.