Electrochemical and Resonance Raman Spectroscopic Studies of Water-Oxidizing Ruthenium Terpyridyl-Bipyridyl Complexes.

The irreversible conversion of single-site water-oxidation catalysts (WOC) into more rugged catalysts structurally related to [(trpy)(5,5'-X -bpy)Ru (μ-O)Ru (trpy)(O)(H O)] (X=H, 1-dn ; X=F, 2-dn ; bpy=2,2'-bipyridine; trpy=2,2':6',2"-terpyridine) represents a critical issue in the development of active and durable WOCs. In this work, the electrochemical and acid-base properties of 1-dn and 2-dn were evaluated. In situ resonance Raman spectroscopy was employed to characterize the species formed upon the stoichiometric oxidation of the single-site catalysts and demonstrated the formation of high-oxidation-state mononuclear Ru=O and RuO-O complexes. Under turnover conditions, the dinuclear intermediates, 1-dn and 2-dn as well as the previously proposed [Ru (trpy)(O) (H O)] complex (3 ) are formed. Complex 3 is a pivotal intermediate that provides access to the formation of dinuclear species. Single-crystal X-ray diffraction analysis of the isolated complex [Ru (O)(trpy)(5,5'-F -bpy)] reveals a clear elongation of the Ru-N bond trans to the oxido ligand that documents the weakness of this bond, which promotes the release of the bpy ligand and the subsequent formation of 3 .