Structural Investigations of the Catalytic Mechanisms of Water Oxidation by the [(bpy)_2Ru(OH_2)]_2O^(4+) Ion

Electron paramagnetic resonance and Raman spectroscopies have been used to identify component species in acidic solutions containing the water oxidation catalyst (µ-oxo)bis[cis-aquabis(2,2'-bipyridine)ruthenium(III)] ([3,3]) and Ce^(4+) ions. One-electron oxidation with Ce^(4+) gave rise to an anisotropic pH-dependent EPR signal at g = 1.77-1.90, assigned to the S = 1/2 ground state of a spin-delocalized Ru-O-Ru orbital. Appearance of the EPR signal correlated with formation of a resonance-enhanced symmetric band at 405-410 cm^(-1) in the Raman spectrum, attributable to the Ru-O-Ru symmetric (v_s) stretching mode. Upon further addition of Ce^(4+) in 0.1 M acid, a second anisotropic EPR signal was detected at g = 1.87, which correlated with a v_s(Ru-O-RU) band appearing at 398 cm^(-1). By analogy with the one-electron oxidation product ([3,4]), this species was assigned as the S = 1/2 three-electron oxidized ion ([4,5]). In 1 M acid, however, the EPR spectrum of highly oxidized samples also exhibited an unusual isotropic signal centered at g = 1.95, whose relative intensity correlated with resonance Raman bands at ~357 cm^(-1) (v_s(Ru-O-Ru)) and 817 cm^(-1) (v(Ru=O)). By analogy with very similar signals reported for reduced Ru(bpy)_3^+ ions and related species which contain bipyridine radical anions as ligands, the g = 1.95 signal is tentatively assigned to a coordinated bipyridine radical π-cation in a complex whose metal oxidation state is [5,5].