Homogeneous Electrochemical Water Oxidation at Neutral pH by Water-Soluble NiII Complexes Bearing Redox Non-innocent Tetraamido Macrocyclic Ligands

Water oxidation is the bottleneck reaction in artificial photosynthesis. Exploring highly active and stable molecular water oxidation catalysts (WOCs) is still a great challenge. In this study, a water-soluble Ni-II complex bearing a redox non-innocent tetraamido macrocyclic ligand (TAML) is found to be an efficient electrocatalyst for water oxidation in neutral potassium phosphate buffer. Controlled-potential electrolysis experiments show that it can sustain at a steady current of approximately 0.2 mA cm(-2) for >7 h at 1.75 V versus normal hydrogen electrode (NHE) without the formation of NiOx. Electrochemical and spectroelectrochemical tests show that the redox-active ligand, as well as HPO42- in the buffer, participate in the catalytic cycle. More importantly, catalytically active intermediate [Ni-III(TAML(2-))-O-.] is formed via several proton-coupled electron transfer processes and reacts with H2O with the assistance of base to release molecular oxygen. Thus, the employment of redox non-innocent ligands is a useful strategy for designing effective molecular WOCs.