Mechanistic Insight into Peroxydisulfate Reactivity: Oxidation of the <i>cis</i>,<i>cis</i>-[Ru(bpy)₂(OH₂)]₂O⁴⁺ “Blue Dimer”

One-electron oxidation of the μ-oxo dimer (<i>cis</i>,<i>cis</i>-[Ru^III(bpy)₂(OH₂)]₂O⁴⁺, <b>{3,3}</b>) to <b>{3,4}</b> by S₂O₈^2– can be described by three concurrent reaction pathways corresponding to the three protic forms of <b>{3,3}</b>. Free energy correlations of the rate constants, transient species dynamics determined by pulse radiolysis, and medium and temperature dependencies of the alkaline pathway all suggest that the rate-determining step in these reactions is a strongly nonadiabatic dissociative electron transfer within a precursor ion pair leading to the <b>{3,4}</b>|SO₄^2–|SO₄^•– ion triple. As deduced from the SO₄^•– scavenging experiments with 2-propanol, the SO₄^•– radical then either oxidizes <b>{3,4}</b> to <b>{4,4}</b> within the ion triple, effecting a net two-electron oxidation of <b>{3,3}</b>, or escapes in solution with ∼25% probability to react with additional <b>{3,3}</b> and <b>{3,4}</b>, that is, effecting sequential one-electron oxidations. The reaction model presented also invokes rapid <b>{3,3}</b> + <b>{4,4}</b> → 2<b>{3,4}</b> comproportionation, for which <i>k</i>_com ∼5 × 10⁷ M^–1 s^–1 was independently measured. The model provides an explanation for the observation that, despite favorable energetics, no oxidation beyond the <b>{3,4}</b> state was detected. The indiscriminate nature of oxidation by SO₄^•– indicates that its fate must be quantitatively determined when using S₂O₈^2– as an oxidant