Resonance Raman, Optical Spectroscopic, and EPR Characterization of the Higher Oxidation States of the Water Oxidation Catalyst, cis,cis-[(bpy)₂Ru(OH₂)]₂O⁴⁺

Resonance Raman (RR) and optical spectroelectrochemical titrations of the cis, cis -[(bpy) 2 Ru(OH 2 )] 2 O 4+ ion (denoted [3,3] to indicate the formal oxidation state of the Ru−O−Ru unit) were made over the range 0.8−2.0 V vs Ag/AgCl in 0.5 M trifluoromethanesulfonic acid; the results revealed sequential accumulation of three higher oxidation states. Two of these states were identified by redox titration with Os(bpy) 3 2+ as one-electron ([3,4]) and four-electron oxidized species ([5,5]); spectroscopic analysis of reaction products formed upon mixing the [3,3] and [5,5] ions indicated that the third oxidation state is a two-electron oxidized species ([4,4]). The [5,5] ion underwent first-order decay to the [4,4] ion with a rate constant, k ≃ 9.5 × 10 - 3 s - 1, that was nearly identical with the catalytic turnover rate for O 2 evolution, k cat ≃ 1.3 × 10 - 2 s - 1 measured under comparable conditions. The [4,4] ion underwent degradation more slowly to the [3,4] ion, which was stable on these time scales. An analogue bearing 4,4‘-dimethyl-2,2‘-bipyridine ligands exhibited very similar behavior, except that the oxidation steps were shifted by ∼50 mV to lower potentials. 18 O isotope labeling experiments on the underivatized complex established that there was no oxygen exchange at the bridging μ-oxo position during catalytic turnover. Frozen solutions of the [5,5] ion displayed unusual low-temperature spectroscopic features, including the following: (i) a narrow g = 2.02 axial EPR signal exhibiting an apparent six-line hyperfine interaction from a minor component; (ii) a concentration-dependent broad rhombic EPR signal in mixtures also containing the [4,4] ion; and (iii) a concentration-dependent replacement of its characteristic ruthenyl Ru O stretching mode at 818 cm - 1 in the RR spectrum when chemically oxidized with Ce 4+ by an 18 O isotope sensitive set of three bands in the 650 cm - 1 region. The RR spectrum of this new species is consistent with further coordination of the terminal oxo ligands by Ce 4+ to form additional ligand bridges.