Electrocatalytic water oxidation using a chair-like tetranuclear copper(II) complex in a neutral aqueous solution

The development of viable molecular water oxidation catalysts is an important issue in the conversion of electricity or solar fuel into chemical fuels via water splitting, and copper complexes have become promising candidates for catalyzing water oxidation because of their low cost, well-defined redox properties and relatively high reactivity. Herein, we describe the first tetranuclear Cu(II)-based water oxidation catalyst: [Cu-4(bpy)(4)(mu(2)-OH)(2)(mu(3)-OH)(2)(H2O)(2)](2+). The complex comprises a chair-like Cu4O4 core with aqua and bridging mu-hydroxo ligands, and the multinuclear core is expected to be advantageous for promoting multi-electron transfer. In pH 7.0 phosphate buffer, the complex shows as being hydrolytically stable and a relatively low overpotential of ca. 730 mV is obtained according to a cyclic voltammetry experiment. Bulk electrolysis measurements at 1.80 V vs. a normal hydrogen electrode provided a stable current density of 0.78 mA cm(-2) and the current persists for at least 10 h. A Faradaic efficiency of nearly 98% is achieved. Importantly, the electrochemical data support that this tetranuclear complex works as a robust homogeneous water oxidation catalyst.