Oxo-Bridge Scenario behind Single-Site Water-Oxidation Catalysts

High-oxidation-state decay of mononuclear complexes [RuTB­(H₂O)]²⁺ (<b>X</b>²⁺, where B = 2,2′-bpy or bpy for X = 1; B = 5,5′-F₂-bpy for X = 2; B = 6,6′-F₂-bpy for X = 3; T = 2,2′:6′,2″-terpyridine) oxidized with a large excess of Ce^IV generates a manifold of polynuclear oxo-bridged complexes. These include the following complexes: (a) dinuclear [TB-Ru^IV-O-Ru^IV-(T)­(O)­OH₂]²⁺ (<b>1-dn</b>⁴⁺), [TB-Ru^III-O-Ru^III-T­(MeCN)₂]⁴⁺ (<b>1-dn-N</b>⁴⁺), and {[Ru^III(trpy)­(bpy)]₂(μ-O)}⁴⁺ (<b>1-dm</b>⁴⁺); (b) trinuclear {[Ru^III(trpy)­(bpy)­(μ-O)]₂Ru^IV(trpy)­(H₂O)}­(ClO₄)₅⁶⁺ (<b>1-tr</b>⁶⁺) and {[Ru^III(trpy)­(bpy)­(μ-O)]₂Ru^IV(pic)₂}­(ClO₄)₄ (<b>1-tr-P</b>⁴⁺, where P is the 2-pyr­i­dine­car­box­yl­ate anion); and (c) tetranuclear [TB-Ru^III-O-TRu^IV(H₂O)-O-TRu^IV(H₂O)-O-Ru^III-TB]⁸⁺ (<b>1-tn</b>⁸⁺), [TB-Ru^III-O-TRu^IV(AcO)-O-TRu^IV(AcO)-O-Ru^III-TB]⁶⁺ (<b>1-tn-Ac</b>⁶⁺), and [TB-Ru^II-O-TRu^IV(MeCN)-O-TRu^IV(MeCN)-O-Ru^II-TB]⁶⁺ (<b>1-tn-N</b>⁶⁺). These complexes have been characterized structurally by single-crystal X-ray diffraction analysis, and their structural properties were correlated with their electronic structures. Dinuclear complex <b>1-dm</b>⁴⁺ has been further characterized by spectroscopic and electrochemical techniques. Addition of excess Ce^IV to <b>1-dm</b>⁴⁺ generates dioxygen in a catalytic manner. However, resonance Raman spectroscopy points to the in situ formation of <b>1-dn</b>⁴⁺ as the active species