Capturing the Elusive [RuV═O]+ Intermediate in Water Oxidation

Although several Ruⁿ⁺ (n = 2 or 3) complexes have been reported to be excellent biomimetics for the water oxidation process of photosystem II, investigation and spectroscopic characterization of the reactive intermediates such as [Ru^IV/V═O]ⁿ⁺ involved in the catalytic process are not only scarce but also a daunting task. Here, we report a catalyst [Ru^III(L)(PPh₃)(H₂O)]⁺ (2) found to show electrochemical water oxidation efficiency with a considerably low overpotential of 195 mV compared to other Ruⁿ⁺ water oxidation catalysts reported in the nonaqueous media. Besides, the Schiff base ligand (L) employed in this study facilitates the stabilization of a [LRu^V═O]⁺ species. By the use of multispectroscopic techniques (spectroelectrochemistry, electron paramagnetic resonance, and resonance Raman), we have shed light on the electronic structure of the elusive [LRu^V═O]⁺ species. Based on the experimental results, a plausible intermolecular radical coupling (I2M) mechanism is proposed, which is corroborated by theoretical calculations.