Mechanistic Insight into Peroxydisulfate Reactivity: Oxidation of the cis,cis-[Ru(bpy)(2)(OH2)](2)O4+ 'Blue Dimer'

摘要

One-electron oxidation of the pt-oxo dimer (cis,cis-[Ru-III(bpy)(2)(OH2)](2)O4+, {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 rate-determining step in these reactions is a strongly nonadiabatic dissociative electron transfer within a precursor ion pair leading to the {3,4}vertical bar SO42-vertical bar SO4 center dot- ion triple. As deduced from the SO4 center dot- scavenging experiments with 2-propanol, the SO4 center dot- 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, similar to 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 k(com) similar to 5 x 10(7) 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 center dot- indicates that its fate must be quantitatively determined when using S2O82- as an oxidant.