Catalytic Four-Electron Oxidation of Water by Intramolecular Coupling of the Oxo Ligands of a Bis(ruthenium-bipyridine) Complex

A bis(rutheniumbipyridine) complex bridged by 1,8-bis(2,2':6',2''-terpyrid-4'-yl)anthracene (btpyan), [Ru2(mu-Cl)(bpy)2(btpyan)](BF4)3 ([1](BF4)3; bpy=2,2'-bipyridine), was prepared. The cyclic voltammogram of [1](BF4)3 in water at pH 1.0 displayed two reversible [RuII,RuII]3+/[RuII,RuIII]4+ and [RuII,RuIII]4+/[RuIII,RuIII]5+ redox couples at E1/2(1)=+0.61 and E1/2(2)=+0.80 V (vs. Ag/AgCl), respectively, and an irreversible anodic peak at around E=+1.2 V followed by a strong anodic currents as a result of the oxidation of water. The controlled potential electrolysis of [1]3+ ions at E=+1.60 V in water at pH 2.6 (buffered with H3PO4/NaH2PO4) catalytically evolved dioxygen. Immediately after the electrolysis of the [1]3+ ion in H216O at E=+1.40 V, the resultant solution displayed two resonance Raman bands at $\tilde \nu $=442 and 824 cm-1. These bands shifted to $\tilde \nu $=426 and 780 cm-1, respectively, when the same electrolysis was conducted in H218O. The chemical oxidation of the [1]3+ ion by using a CeIV species in H216O and H218O also exhibited the same resonance Raman spectra. The observed isotope frequency shifts (?$\tilde \nu $=16 and 44 cm-1) fully fit the calculated ones based on the Ru?O and O?O stretching modes, respectively. The first successful identification of the metal?O?O?metal stretching band in the oxidation of water indicates that the oxygenoxygen bond at the stage prior to the evolution of O2 is formed through the intramolecular coupling of two Ruoxo groups derived from the [1]3+ ion.