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

© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim The irreversible conversion of single-site water-oxidation catalysts (WOC) into more rugged catalysts structurally related to [(trpy)(5,5′-X2-bpy)RuIV(μ-O)RuIV(trpy)(O)(H2O)]4+(X=H, 1-dn4+; X=F, 2-dn4+; 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-dn4+and 2-dn4+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-dn4+and 2-dn4+as well as the previously proposed [RuVI(trpy)(O)2(H2O)]2+complex (32+) are formed. Complex 32+is a pivotal intermediate that provides access to the formation of dinuclear species. Single-crystal X-ray diffraction analysis of the isolated complex [RuIV(O)(trpy)(5,5′-F2-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 32+.