Accelerating electrocatalytic water oxidation mediated by Cu complexes via modulation of geometry configuration

The development of robust and efficient catalysts for water oxidation is challenging. Herein, we demonstrate the significant regulation of water oxidation catalytic activity in mononuclear Cu complexes by disrupting their square pyramidal geometry through modulation of the alkyl chain length in pyridine-amine ligands. The electrocatalytic properties of two Cu complexes with homologous pyridine-amine ligands toward water oxidation under neutral conditions were investigated. Characterizations indicate that a slight increase in the length of the azaalkyl backbone of the ligand modulates the geometry of the metal center, as evidenced by increasing geometric parameters. This suggests a subtle but predictable trend in the geometry change from near-perfect square pyramidal to trigonal bipyramidal. Electrochemical measurements reveal that the pentacoordinated complex with a geometry closer to trigonal bipyramidal exhibits significantly higher catalytic activity and much lower overpotential for electrochemical water oxidation. Collective experiments and theoretical calculations elucidate that the alkyl chain structure significantly influences the intrinsic redox properties and electrocatalytic water oxidation activity of Cu complexes. This is achieved by favoring water nucleophilic attack on the Cu(II) center and facilitating the subsequent proton-coupled electron transfer process through the creation of a non-standard square pyramidal geometry.