Iron-assisted, base-catalyzed biomimetic activation of dioxygen by dioximatoiron(II) complexes: Kinetics and mechanism of model catecholase activity

Abstract

The dioximatoiron complexes [Fe(Hdmed)]+, [Fe(Hdmpd)]+ and [Fe(H2dmdt)]2+ catalyze the oxidation of 3,5-di-tert-butylcatechol (H2dtbc) by O2 to the corresponding o-benzoquinone (dtbq) at room temperature in MeOH solution. The reaction was followed by measuring the rate of dioxygen absorption as a function of catalyst, substrate and dioxygen concentration. Kinetic measurements reveal first-order dependence on the catalyst and O2 concentration and saturation type behavior with respect to the substrate. The proposed reaction mechanism involves prior binding of the substrate H2dtbc and O2 to the iron complex, forming a ternary active intermediate, decomposing in the rate-limiting step to a semiquinonato anion radical (dbsq−), detected by ESR spectroscopy. It is then rapidly oxidized to the dtbq product. Added triethylamine accelerates the reaction to an extent much greater than that expected from the parallel base-catalyzed oxidation route. The kinetic behavior is similar to the TEA-free systems except for a saturation type dependence on TEA. This feature is due to a novel iron-enhanced oxidation path in which Hdtbc− binds O2 to form the hydroperoxide HdtbcO2−, coordinating to the iron(II) complexes as a hydroperoxo ligand. Subsequently, the hydroperoxo complex eliminates dbsq−, which is directly oxidized by O2 to dtbq. According to Mössbauer spectroscopy, the catalyst species are predominantly low-spin iron(II) complexes.