Identification of homogeneous [Co4(H2O)4(HPMIDA)2(PMIDA)2]6− as an effective molecular-light-driven water oxidation catalyst

Abstract

Transition-metal complexes, especially cobalt complexes, are used as precatalysts in homogeneous water oxidation catalysis. However, a practical method of identifying the effective active species in cobalt-complex homogeneous catalysts under highly oxidizing conditions have not yet been established. In this work, (H3O)6·[Co4(H2O)4(HPMIDA)2(PMIDA)2]·2H2O (1) was introduced, and its photocatalytic performance was evaluated. The compound exhibited effective water oxidation catalysis, with a turnover number of 661.5 at pH 9.0 using [Ru(bpy)3](ClO4)2 as photosensitizers and sodium persulfate as sacrificial electron acceptor. Accordingly, 1 was used as target catalysts, and the active species was thoroughly investigated under catalytic conditions. The stability of 1 was further tested and confirmed by a series of experiments (cyclic voltammetry (CV), ultraviolet–visible (UV–vis) spectrometry, dynamic light-scattering (DLS), aging experiments, and extraction techniques). Results showed that 1 maintained its structural integrity under the given photocatalytic conditions. Furthermore, cathodic adsorptive stripping voltammetry and inductively coupled plasma-mass spectrometry were used to quantify the amount of Co2+ ions released from 1 into borate buffer after oxidation. The pH dependence of the oxygen-evolution performance of 1 and other cobalt species were then compared. Data collectively revealed that 1 was stable during water oxidation; it did not release free Co2+ ions and was not hydrolyzed to cobalt oxide or hydroxide, thereby confirming that 1 was the effective active species in water-oxidation catalysis.