Electron Paramagnetic Resonance Analysis of a Transient Species Formed During Water Oxidation Catalyzed by the Complex Ion [(bpy)2Ru(OH2)]2O4+

The ruthenium “blue dimer” [(bpy)2Ru(OH2)]2O4+—the first well-defined molecular complex able to catalyze water oxidation at low overpotentials—has been the subject of numerous experimental and computational studies. However, elements of the reaction mechanism remain controversial. Of particular interest is the nature of the O–O bond-forming step. Herein, we report the first advanced electron paramagnetic resonance (EPR) spectroscopic studies of a high-valent intermediate that appears under conditions in which the catalyst is actively turning over. Results from previous studies have suggested that this intermediate is derived from [(bpy)2RuV(O)]2O4+, denoted {5,5}. Under photooxidizing conditions, the corresponding EPR signal disappears at a rate comparable to the turnover rate of the catalyst once the illumination source is removed. In the present work, the electronic and geometric structures of this species were explored using a variety of EPR techniques. Continuous wave (CW) EPR spectroscopy was used to...