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-2-bpy)Ru-IV(mu-O)Ru-IV(trpy)(O)(H2O)](4+) (X=H, 1-dn(4+); X=F, 2-dn(4+); 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(4+) and 2-dn(4+) 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(4+) and 2-dn(4+) as well as the previously proposed [Ru-VI(trpy)(O)(2)(H2O)](2+) complex (3(2+)) are formed. Complex 3(2+) is a pivotal intermediate that provides access to the formation of dinuclear species. Single-crystal X-ray diffraction analysis of the isolated complex [Ru-IV(O)(trpy)(5,5'-F-2-bpy)](2+) 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(2+).