Oxidation of phenols employing polyoxometalates as biomimetic models of the activity of phenoloxidase enzymes

Abstract

A kinetic study of the oxidation of substituted phenols with either vanadium(V) polyoxotungstate, [α-SiV^VW₁₁O₄₀]⁵⁻ (viz. SiW₁₁V), or manganese(III) polyoxotungstate, [α-SiMn^IIIW₁₁(H₂O)O₃₉]⁵⁻ (viz. SiW₁₁Mn), has been carried out. Because the redox potentials of the polyoxometalate SiW₁₁V and SiW₁₁Mn compounds, viz. POMs, are 0.67 and 0.76 V/NHE, respectively, and those of the phenolic substrates are in the range of 0.4–0.9 V/NHE in water, the resulting oxidation reactions are exoergonic or slightly endoergonic. The reactivity of oxidation of substituted phenols by SiW₁₁V, at 50 °C in buffered (pH = 4) water solution, has been found to correlate with the σ⁺ parameter of the substituents, yielding an Okamoto–Brown ρ value (i.e., −3.1) consonant with the electronic requirements of a rate-determining electron-transfer route. The negligible value of the solvent kinetic isotope effect (k_H/k_D = 1.06) obtained for the oxidation of p-MeO-phenol with SiW₁₁V in H₂O vs. D₂O solution was also in favour of a rate-determining one-electron abstraction from the substrate, followed by fast deprotonation of the intervening radical cation. Additional support to an outer-sphere oxidation with SiW₁₁V was provided by a satisfactory correlation of the reactivity vs. redox potential of the phenols, and by a Marcus analysis of the experimental oxidation data. A sizeable value for the reorganisation energy (λ) was accordingly obtained. The slightly stronger oxidant SiW₁₁Mn resulted to be more reactive than SiW₁₁V by ca. two powers of magnitude. Our scrutiny of the reactive behaviour of the two POMs towards phenols may provide a model of interpretation for the phenoloxidase activity of the laccase enzymes, a class of multicopper oxidases endowed with redox potentials well comparable with those of the two POMs.