Electrochemical water oxidation catalyzed by a mononuclear cobalt complex of a pentadentate ligand: the critical effect of the borate anion

Electrochemical water oxidation catalyzed by a cobalt complex, [Co(N3Py2)(H2O)] (ClO4)(2) (1) (N3Py2 = 2,5,8-trimethyl-1,9-bis(2-pyridyl)-2,5,8-triazanonane), is reported. The release of dioxygen (O-2) occurs with a moderate onset overpotential of 540 mV and turnover frequency (k(cat)) of 0.79 s(-1). Collective experimental results confirm that 1 homogeneously catalyzes water oxidation without undergoing decomposition into cobalt oxides or cobalt hydroxides. Detailed electrochemical investigation clarified the mechanism of water oxidation, indicating that the implementation of a proton-coupled electron transfer (PCET) process and the formation of an O-O bond in the catalytic cycle are assisted by the ion of the buffer solution. Compared to hydrogen phosphate, borate, which is more receptive to protons, Leads to a more remarkable catalytic activity. This work suggests that the buffer solution may have a critical effect on the catalytic performance of metal complexes. The disclosed information is expected to inspire new ideas for the development of efficient homogeneous complex catalysts for water oxidation.