Homogeneous electrochemical water oxidation catalyzed by cobalt complexes with an amine–pyridine ligand

Abstract

Efficient water oxidation catalysts are highly desired because of their great significance for overall water splitting, which is a promising strategy to produce hydrogen on a large scale. Herein, the water oxidation catalytic ability of two five-coordinate cobalt complexes has been investigated under alkaline conditions. Stabilized by similar organic scaffolds, their catalytic behaviors are distinct in phosphate-buffered saline at pH 11.0. Complex [Co(N2Py3)](ClO₄)₂ (1, N2Py3 = 2,6-bis[(methyl(2-pyridylmethyl)amino)methyl]pyridine) with pentadentate pyridine-amine containing three pyridine rings homogeneously catalyzes the oxidation of water to oxygen efficiently, exhibiting satisfactory stability and high faradaic efficiency. Replacing one of the pyridine rings of the ligand with an azaalkyl structure, the corresponding complex [Co(N3Py2)(OH₂)](ClO₄)₂ (2, N3Py2 = 2,5,8-trimethyl-1,9-bis(2-pyridyl)-2,5,8-triazanonane) shows much lower catalytic activity. Electrochemical measurements reveal that the non-innocent N2Py3 undergoes a ligand oxidation process and participates in water oxidation catalysis, further assisting the generation of Co(IV) species via intramolecular electron transfer, resulting in the higher catalytic activity of complex 1. However, consecutive oxidation of the Co(II) center to Co(V) species is required for complex 2 to achieve water oxidation. The reactivity of Co(IV) intermediates of complex 2 with substrate water molecules is much lower than that of complex 1. Therefore, the tripyridine coordination environment of N2Py3 benefits the catalytic activity of complex 1via the ligand oxidation process. This work indicates that the ligand-assisted catalytic cycle is a favorable method for the accumulation of intermediate species that account for electrochemical water oxidation.