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

A kinetic study of the oxidation of substituted phenols with either vanadium(V) polyoxotungstate, [a-SiVVW11O40]5 (viz. SiW11V), or manganese(III) polyoxotungstate, [a-SiMnIIIW11(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 1C in buffered (pH = 4) water solution, has been found to correlate with the s+ parameter of the substituents, yielding an Okamoto–Brown r 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 (kH/kD = 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 (l) 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.