New insights into the mechanisms of the thermal Fenton reactions occurring using different iron(II)-complexes

Although the Fenton reagent (a mixture of hydrogen peroxide and an iron(II) salt) has been known for more than a century, the manifold mechanisms occurring during the thermal Fenton reaction are still under discussion. Indeed, this discussion served as a powerful driving force for the steadily increasing insight into the field of inorganic radical and electron transfer chemistry. In this work, an experimental approach towards the elucidation of the first steps taking place in the reaction between several iron(II)-complexes and hydrogen peroxide (H₂O₂) in water at pH = 3.0 is presented. 2,4-xylidine (2,4-dimethylaniline) reacts differently with reactive intermediates via the addition or hydrogen abstraction by the hydroxyl radical (HO·) or electron transfer reactions to higher valent iron-species, such as a hydrated ferryl-complex (Fe(IV)). The chemical reactivity of the employed iron(II)-complexes with H₂O₂ differed strongly depending on their ground-state one-electron oxidation potentials. The results are interpreted in accordance with the paradigm originally developed by Goldstein et al. which is based on the evidence obtained from the Marcus theory that outer-sphere electron transfer reactions between metal complexes are not likely to occur because they are too slow. Therefore, most of the “Fenton-reagents” form transient metal complexes, which can be described as [L_nFe-H₂O₂]^m+. They form, depending on the reaction conditions, either the hydroxyl radical or higher-valent iron complex species.