Mechanistic Complications Caused by Redox Equilibration: Ascorbate Reduction of a Ruthenium(III) Complex under Low Driving Force Conditions

Abstract

A detailed kinetic study of the reduction of cis‐dichloridobispicolinatoruthenate(III) by l‐ascorbic acid that leads to the formation of the corresponding RuII complex, was carried out spectrophotometrically using the stopped‐flow technique. The reaction was studied as a function of [AscH2]T, [RuIII], and pH. The observed kinetic traces could only be fitted to a three‐exponential function, characteristic for three parallel reaction paths. However, an earlier study clearly showed that under the selected experimental conditions only a single complex species is present in solution. The observed complication is suggested to arise from the low driving force of the reaction during which the re‐oxidation of RuII by the semi‐oxidized l‐ascorbic acid, i.e. ascorbyl radical Asc·–, accounts for the apparent three‐exponential behavior of the reaction. A detailed account of different modeling efforts and a comparison with related systems is presented.

Redox Equilibration Observed for the Reduction of a Ruthenium(III) Complex by Ascorbate under Low‐Driving‐Force Conditions

“…Detailed kinetic studies of the reduction of the complex by ascorbic acid, as a function of pH and ascorbic acid concentration, were performed. The results are in excellent agreement with our earlier findings, and they demonstrate, in general, the importance of back reactions during the reduction of such Ru III complexes under low‐driving‐force conditions.…”

“…the fully protonated acid (AscH 2 ) is a much weaker reductant than its conjugate base (AscH – ), which, in turn, is much less reactive than the fully deprotonated Asc 2– . The corresponding redox potentials are reported in our recent paper . The clear dependence of the second‐order rate constants ( k b and k c K ) for all three reaction phases on the pH of the solution (Table ) shows that the ascorbate dianion (Asc 2– ), though present in a very low concentration (0.003 to ca.…”

“…Our intention was to synthesize the bis(dipicolinato)Ru III complex as a modification of the [RuCl 2 (picolinato) 2 ] – complex ion studied before,, with the goal of changing the redox potential of the complex. Under the selected synthetic conditions, we were not able to synthesize this complex, and we could only isolate a bis(dipicolinato) complex of which one carboxylate group of each dipicolinate ligand was esterified by ethanol.…”

“…In a recent paper from our group, we reported mechanistic findings for the reduction of cis ‐[Ru III Cl 2 (pic) 2 ] – , where pic = picolinate anion, by ascorbic acid and found kinetic traces that were typical for three parallel reactions, independent of the selected pH. Furthermore, at lower pH and ascorbic acid concentrations, the kinetic data and overall conversion to Ru II clearly showed evidence for a reversible reaction for all three reactions paths.…”

“…In an earlier study, we demonstrated that the Ru III compounds used in the latter study were absolutely pure and no evidence for any other species in solution was found that could account for the apparent three‐parallel‐reaction behavior. It was concluded, from a detailed investigation, that the observed behavior was due to redox equilibration under low‐driving‐force conditions that involved a back reaction of the ascorbyl radical anion with Ru II to re‐form the Ru III complex …”