Electrocatalytic water oxidation by copper(ii) complexes with a pentadentate amine-pyridine ligand

Abstract

Two five-coordinate mononuclear copper complexes, [Cu(mpydien)](ClO₄)₂ (1, mpydien = 5-methyl-1,9-bis(2-pyridyl)-2,5,8-triazanonane) and [Cu(pydien)](ClO₄)₂ (2, pydien = 1,9-bis(2-pyridyl)-2,5,8-triazanonane), are synthesized and found to be molecular electrochemical water oxidation catalysts (WOCs) under neutral conditions. Complex 1 with one –NCH₃ group and two –NH groups displays an onset overpotential of 640 mV and low turnover frequency (TOF) of 0.04 s⁻¹, while complex 2 with three –NH groups catalyzes water oxidation and shows a much lower onset overpotential of 530 mV and higher TOF of 0.44 s⁻¹. The possible water oxidation catalytic mechanism for the two complexes is proposed based on electrochemical test results. Comparative results reveal that the –NH group of the pentadentate amine-pyridine ligand has a detrimental effect on the catalytic activity of the two Cu complexes. It also suggests that only a minor alteration of the coordinating nitrogen atom has a detrimental effect on the catalytic activity of homogeneous water oxidation electrocatalysts based on transition metal complexes.