Water oxidation by [(tpy)(H2O)2RuIIIORuIII(H2O)2(tpy)]4+

The complex [(tpy)(C2O4)(RuORuIII)-O-III(C2O4)(tpy)]. 8H(2)O (1.8H(2)O) (tpy is 2,2':6',2 ",terpyridine) has been prepared and characterized by X-ray crystallography and FTIR, resonance Raman, and H-1 NMR spectroscopies. From the results of the X-ray analysis, angle RuORu is 148.5 degrees with a torsional angle (O-22-Ru-2-O-1-Ru-1-O-12) Of 22.7 degrees and there is a short Ru-O bridge distance of 1.843 Angstrom. 1 undergoes a chemically reversible one-electron, pH-independent oxidation at 0.73 V vs NHE (0.49 V vs SCE) from pH = 4-8 and a pH-dependent, two-electron, chemically irreversible reduction at -0.35 V below pH = 4.0. Addition of 1.8H(2)O to strong acid generates [(tpy)(H2O)(2)-(RuORuIII)-O-III(H2O)2(tpy)](4+) (2), which has been characterized by UV-visible, resonance Raman, and H-1 NMR measurements. In pH-dependent cyclic voltammograms there is evidence for a series of redox couples interrelating oxidation states from (RuORuII)-O-II to (RuORuV)-O-V. In contrast to the "blue dimer", cis,cis- [(bpy)(2)(H2O)(RuORuIII)-O-III-(OH2)(bpy)(2)](4+), oxidation state (RuORuIV)-O-IV ((RuORuIII)-O-V?) does appear as a stable oxidation state. Oxidation of (RuORuIV)-O-IV by Ce-IV in 0.1 M HClO4 is followed by rapid O-2 production and appearance of an anated form of (RuORuIV)-O-IV. O-2 formation is in competition with oxidative cleavage of (RuORuV)-O-V by Ce-IV to give [Ru-VI(tpy)-(O)(2)(OH2)](2+). Anation and oxidative cleavage prevent this complex from being a true catalyst for water oxidation.