Electrocatalytic water oxidation by Cu(ii) complexes: influence of ternarification and chelate ring size

The electrocatalytic water oxidation activity of two mononuclear ternary copper(ii) complexes, 1 [Cu^II(L₁H)(L₂)](ClO₄)₂ [L₁H = N ¹-(2-aminoethyl)ethane-1,2-diamine and L₂ = pyridin-2-ylmethanamine] and 2 [Cu^II(L₁H)(L₃)](ClO₄)₂ [L₃ = 2-(pyridin-2-yl)ethan-1-amine], has been explored. In a neutral phosphate buffer, complexes 1 and 2 catalyse water oxidation at overpotentials of 423 and 483 mV with turnover frequencies (TOFs) of 157 and 704 s^-1, respectively. Electrochemical studies reveal that ligands capable of forming six-membered chelate rings enhance the catalytic activity of the copper complexes relative to ligands capable of forming five-membered chelate rings. Furthermore, the incorporation of a redox-active tridentate ligand together with a bidentate ligand capable of forming six-membered chelate rings in ternary copper complexes synergistically enhances the rate of water oxidation catalysis. These findings underscore an effective strategy for improving water oxidation catalysis through the incorporation of cooperative structural elements into synthetic ternary metal complexes.