The paramagnetic dinuclear Ru2+Ru3+ complex, {RuCl[P(OMe)3]2}2(μ-S2)(μ-Cl)(μ-N2H4) (1), reduces dioxygen by one-electron at room temperature in CH3CN to form 1 +O2 -, in which O2 - is coordinated to the μ-S2 ligand. The dioxygen reduction is preceded by protonation to yield HO2 + by the bridging hydrazine ligand. Complex 1 is electrochemically more negative than HO2 +, and a redox reaction generates 1 + and HO2 (→ O2 - + H+). Spectroscopic studies (NMR and UV−vis) show that 1 and 1 +O2 - are under equilibrium, and the succeeding disproportionation reaction of 1 +O2 - produces two intermediates, 1 + and the dithionite complex, 1 +(O2 -)2, as an intermediate. The dithionite intermediate undergoes Cl- dissociation and further oxidation to yield the dinuclear Ru complex {Ru2+Cl[P(OMe)3]2}2(μ-S2O5)(μ-N2H4) (3). The liberated Cl- attacks the dithionite complex, and induces SO4 2- dissociation to prepare the dinuclear complex {Ru3+Cl2[P(OMe)3]2}2(μ-S)(μ-N2H4) (5). On the other hand, intermediate 1 + forms an ion pair with HSO4 -, which stems from the liberated SO4 2-, to yield [{Ru3+Cl[P(OMe)3]2}2(μ-S2)(μ-Cl)(μ-N2H4)]+HSO4 - (4 = 1 +HSO4 -). A trace of {Ru3+Cl[P(OMe)3]2}2(μ-S2)(μ-Cl)2 (2) is also detected in the supernatant. The UV−vis spectroscopic study revealed that complexes 2−5 are the exclusive products and the oxidation reaction can be balanced as follows: 14 × 1 + 4.5 O2 = 2 + 5 × 3 + 4 × 4 + 4 × 5. The 18O2/16O2 mixed-labeled oxygen studies proved that the external oxygen was the source of oxygen for the formation of the S2O5 and HSO4 - groups in 3 and 4, respectively. The structures of 3, 4, and 5 are characterized by X-ray crystallographic structure determinations. The dinuclear framework of complex 4 remains identical with that of 1 except for the significantly shorter Ru−S distance. The hydrazine hydrogens of complex 3 show intramolecular hydrogen bonding, which is reflected on a significantly low field shift of the hydrazine 1H NMR resonance. Complex 3 possesses the μ-S2O5 2- ligand, in which the two μ-SO2-O,S ‘ bridges are linked at the sulfur atoms through the μ-oxo bonds. Comparison of the site symmetries around the Ru atoms of 1, 3, and 5 strongly supports the occurrence of coordination rearrangements.
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