STRUCTURAL INVESTIGATIONS OF THE CATALYTIC MECHANISMS OF WATER OXIDATION BY THE [(BPY)(2)RU(OH2)]O-2(4+) ION

Electron paramagnetic resonance and Raman spectroscopies have been used to identify component species in acidic solutions containing the water oxidation catalyst (mu-oxo)bis[cis-aquabis(2,2'-bipyridine) ([3,3]) and Ce4+ ions. One-electron oxidation with Ce4+ 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 Ce4+ 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 similar to 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 pi-cation in a complex whose metal oxidation state is [5,5].