Impact of coordinated nitrogen atoms on the electrocatalytic water oxidation properties of copper complexes with pentadentate ligands

The electrocatalytic water oxidation by a mononuclear Cu complex with an aliphatic pentaamine ligand was conducted and is reported herein. In the phosphate buffer solution at pH 8.0, the [Cu(tetren)](ClO4)2 (tetren = tetraethylenepentamine) complex functioned as an efficient homogeneous water oxidation catalyst, operating via a single-site catalysis mechanism. The catalytic properties of [Cu(tetren)](ClO4)2, featuring an all-aliphatic amine nitrogen coordination environment, were investigated and compared with those of another mononuclear Cu complex, [Cu(pydien)](ClO4)2 (pydien = 1,9-bis(2-pyridyl)-2,5,8-triazanonane), with similar distorted square pyramidal geometry formed by azamine nitrogen atoms and pyridine rings. Experimental results indicated that [Cu(tetren)]2+ accelerated water oxidation to oxygen through a consecutive proton-coupled electron transfer process, resulting in a lower catalytic overpotential for water oxidation and higher catalytic current density than [Cu(pydien)](ClO4)2. Related measurement results revealed that the chain aliphatic amine pentadentate ligand, with its greater flexibility, was more beneficial for achieving higher water oxidation catalytic activity in the Cu complex than the chain pyridine-amine ligand, providing valuable insights for designing and screening Cu complexes as efficient electrochemical water oxidation catalysts.