The ferryl generation by fenton reaction driven by catechol

Abstract

The Fenton and Fenton-like reactions are based on the decomposition of hydrogen peroxide catalyzed by Fe(II), primarily producing highly oxidizing hydroxyl radicals (HO∙). While HO∙ is the main oxidizing species in these reactions, Fe(IV) (FeO²⁺) generation has been reported as one of the primary oxidants. FeO²⁺ has a longer lifetime than HO∙ and can remove two electrons from a substrate, making it a critical oxidant that may be more efficient than HO∙. It is widely accepted that the preferential generation of HO∙ or FeO²⁺ in the Fenton reaction depends on factors such as pH and Fe: H₂O₂ ratio. Reaction mechanisms have been proposed to generate FeO²⁺, which mainly depend on the radicals generated in the coordination sphere and the HO∙ radicals that diffuse out of the coordination sphere and react with Fe(III). As a result, some mechanisms are dependent on prior HO∙ radical production. Catechol-type ligands can induce and amplify the Fenton reaction by increasing the generation of oxidizing species. Previous studies have focused on the generation of HO∙ radicals in these systems, whereas this study investigates the generation of FeO²⁺ (using xylidine as a selective substrate). The findings revealed that FeO²⁺ production is increased compared to the classical Fenton reaction and that FeO²⁺ generation is mainly due to the reactivity of Fe(III) with HO∙ from outside the coordination sphere. It is proposed that the inhibition of FeO²⁺ generation via HO∙ generated from inside the coordination sphere is caused by the preferential reaction of HO∙ with semiquinone in the coordination sphere, favoring the formation of quinone and Fe(III) and inhibiting the generation of FeO²⁺ through this pathway.

Keywords: Fenton reaction driven by catechol; Ferryl; Semiquinone.