Reduction of oxygen by ruthenium(II) ammines

Article abstract of DOI:10.1021/ic50204a048

The reduction of O₂ to H₂O₂ by a series of ruthenium(II) ammines has been studied in aqueous acidic solution at 25.0 °0C and 0.1 M ionic strength in noncomplexing media. The rate law is -d[Ru(II)]/dt = 2k₁[Ru(II)][O₂], with k₁ = 1.08 × 10^-1, 1.38 × 10^-1, 3.03 × 10^-2, and 7.73 × 10^-3 M^-1 s^-1 for [Ru(NH₃)₅isn]²⁺ (isn = isonicotinamide), cis-[Ru(NH₃)₄isn(H₂O)]²⁺, trans-[Ru(NH₃)₄isn(H₂O)²⁺, and [Ru(NH₃)₄phen]²⁺, respectively. The reaction of [Ru(NH₃)₅isn]²⁺ is inhibited by [Ru(NH₃)₅isn]³⁺, and the inhibition increases with decreasing acidity. These results are accommodated by a mechanism involving outer-sphere formation of O₂^-; the Ru(III)/pH effect arises from a competition between the reaction of O₂^- with Ru(III) and the protonation of O₂^- followed by its reaction with Ru(II). The rate constants are correlated by a linear free-energy relation, (LFER), and they are consistent with the Marcus cross relation. Its application yields a self-exchange rate for the O₂/O₂^- couple of about 1 × 10³ M^-1 s^-1. In the presence of Cl^-, the reaction of trans-[Ru(NH₃)₄isn(H₂O)]²⁺ has two additional terms in the rate law: -d In [Ru(II)]/dt = 2(k₁ + k₅L_Cl[Cl^-] + k₆K_Cl[Cl^-][H⁺])[O₂], with k₅ = 7.84 × 10^-1 M^-1 s^-1, k₆ = 1.40 × 10² M^-2 s^-1, and K_Cl = 0.39 M^-1. The k₅ path fits the LFER when it is treated as the autoxidation of trans-[Ru(NH₃)₄isnCl]⁺, and the k₆ path probably involves direct formation of HO₂ by the reaction of O₂ with [Ru(NH₃)₄(isn)Cl]⁺.