Spectroscopic Analysis of Catalytic Water Oxidation by [Ru^II(bpy)(tpy)H₂O]²⁺ Suggests That Ru^VO Is Not a Rate-Limiting Intermediate

Modern chemistry’s grand challenge is to significantly improve catalysts for water splitting. Further progress requires detailed spectroscopic and computational characterization of catalytic mechanisms. We analyzed one of the most studied homogeneous single-site Ru catalysts, [Ru^II(bpy)­(tpy)­H₂O]²⁺ (where bpy = 2,2′-bipyridine, tpy = 2,2′;6′,2″-terpyridine). Our results reveal that the [Ru^V(bpy)­(tpy)O]³⁺ intermediate, reportedly detected in catalytic mixtures as a rate-limiting intermediate in water activation, is not present as such. Using a combination of electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy, we demonstrate that 95% of the Ru complex in the catalytic steady state is of the form [Ru^IV(bpy)­(tpy)O]²⁺. [Ru^V(bpy)­(tpy)O]³⁺ was not observed, and according to density functional theory (DFT) analysis, it might be thermodynamically inaccessible at our experimental conditions. A reaction product with unique EPR spectrum was detected in reaction mixtures at about 5% and assigned to Ru^III-peroxo species with (−OOH or −OO– ligands). We also analyzed the [Ru^II(bpy)­(tpy)­Cl]⁺ catalyst precursor and confirmed that this molecule is not a catalyst and its oxidation past Ru^III state is impeded by a lack of proton-coupled electron transfer. Ru–Cl exchange with water is required to form active catalysts with the Ru–H₂O fragment. [Ru^II(bpy)­(tpy)­H₂O]²⁺ is the simplest representative of a larger class of water oxidation catalysts with neutral, nitrogen containing heterocycles. We expect this class of catalysts to work mechanistically in a similar fashion via [Ru^IV(bpy)­(tpy)O]²⁺ intermediate unless more electronegative (oxygen containing) ligands are introduced in the Ru coordination sphere, allowing the formation of more oxidized Ru^V intermediate