One-electron oxidation of the μ-oxo dimer
(cis,cis-[Ruᴵᴵᴵ(bpy)₂(OH₂)]₂O⁴⁺, {3,3}) to {3,4} by S₂O₈²⁻ can
be described by three concurrent reaction pathways corresponding
to the three protic forms of {3,3}. 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 ratedetermining
step in these reactions is a strongly nonadiabatic
dissociative electron transfer within a precursor ion pair
leading to the {3,4}|SO₄²⁻|SO₄•− ion triple. As deduced from the SO₄•− scavenging experiments with 2-propanol, the SO₄•−
radical then either oxidizes {3,4} to {4,4} within the ion triple, effecting a net two-electron oxidation of {3,3}, or escapes in
solution with ∼25% probability to react with additional {3,3} and {3,4}, that is, effecting sequential one-electron oxidations. The
reaction model presented also invokes rapid {3,3} + {4,4} → 2{3,4} comproportionation, for which kcₒₘ ∼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 {3,4} state was detected. The indiscriminate nature of oxidation by SO₄•− indicates that its fate must be
quantitatively determined when using S₂O₈²⁻ as an oxidant