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

Abstract

Resonance Raman (RR) and optical spectroelectrochemical titrations of the cis,cis-[(bpy)2Ru(OH2)]2O4+ 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 O2 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. 18O 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 Ce4+ by an 18O 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 Ce4+ to form additional ligand bridges.

Experimental demonstration of radicaloid character in a RuV=O intermediate in catalytic water oxidation

“…S2). The 816-to 818-cm −1 peak arises from the Ru = O stretch (35,38,39) and was observed in samples prepared at 100 ms-7s (Fig. S2), which is in agreement with earlier EPR characterization [ figure 8 in the work by Moonshiram et al (37)].…”

“…The BD catalyst intermediates BD [4,5] and BD [5,5], which are reactive to water, were characterized in previous studies (29,(35)(36)(37) and are characterized in more detail in this work. BD [4,5] can be prepared in 0.1 M HNO 3 by oxidation of the BD [3,4] with excess of cerium ammonium nitrate [Ce(IV)] and fast (less than 10-15 s) freezing (37).…”

Structure and Electronic Configurations of the Intermediates of Water Oxidation in Blue Ruthenium Dimer Catalysis

“…Such UV−vis absorption kinetics are known to be complex and their interpretation has led to controversial results in the past. 32,33 Here we attempted to improve the information content of UV−vis absorbance measurements by parallel EPR analysis of reaction mixtures. Figure 4A shows the EPR spectra of the initial blue dimer [3,4] and reaction mixtures frozen within 30 s after addition of 1, 2, and 3 equiv of Ce(IV).…”

Functional models for the oxygen-evolving complex of photosystem II

“…In addition to the four mechanisms shown in Figure 6, a new and somewhat unorthodox mechanism has recently been put forth that involves the oxidation of one of the bipyridine ligands to a diol and subsequent reduction of this diol to release oxygen and regenerate the bipyridine ligand [59]. This fifth mechanism is supported by EPR measurements that suggest that the proposed (V,V) oxidation state may in fact be a (IV,V) species with a radical on one of the bipyridine ligands [68,69] and by observations of monomeric Ru(bpy) 3 complexes trapped in zeolite cages [70].…”