Electrochemical water oxidation using a copper complex

Abstract

Herein, we report the application of the mononuclear copper complex 1, [Cu^II(L)]²⁺, in electrochemical water oxidation catalysis (L = 1,3-bis(((1-methyl-1H-imidazol-2-yl)methyl)amino)propan-2-ol). The complex exhibits a N₄ donor set consisting of two amine and two imidazole units and a dangling OH unit in close proximity to the copper ion. 1 exhibits a moderate apparent rate constant k_cat of 0.12 s⁻¹ in catalysis and operates at an overpotential of 0.83 V. Detailed investigations allowed us to derive a mechanism for water oxidation. The catalysis proceeds only under basic conditions, where [Cu^II(L)(OH)]⁺, 1H₋₁, is the main solution species, which indicates that a negatively charged ligand is necessary to drive the catalysis. Initial oxidation of 1H₋₁ is coupled to proton loss forming a copper(III) species and further oxidation initiates oxygen evolution. Initial oxidation of 1 under neutral, i.e. non-catalytic, conditions is pH independent, highlighting the importance of PCET steps during catalysis. We collected reasonable evidence that catalysis proceeds via a water nucleophilic attack mechanism. The electrolyte presumably acts as a proton acceptor in catalysis as the onset potential depends on the buffer employed.