Electrochemical behaviour of mononuclear Fe(iii) complexes as models for oxygenases: reactivity of Fe(ii) species electrochemically formed in situ toward dioxygen

Abstract

In this paper, we report the electrochemical study of a family of mononuclear Fe^III complexes [Fe(BMPA)Cl₃] 1, [Fe(MPBMPA)Cl₃] 2, [Fe(PBMPA)Cl₂] 3 and [Fe(PABMPA)Cl₂](ClO₄) 4, where the ligand BMPA is bis-(2-pyridylmethyl)amine, and MPBMPA, PBMPA and PABMPA are the N-methylpropanoate, N-propanoate and N-propanamide BMPA-derivatives, respectively. It was possible to verify the influence of the different ligands on the redox properties of the complexes and from this to classify the complexes according to their Lewis acidity through the Fe^III/Fe^II redox process, resulting in the following decreasing order in CH₃CN solution: 4 > 2 > 1 > 3. The effect of the solvents CH₃CN and DMSO on their electrochemical properties was also determined. Furthermore, we investigated the reactivity of the electrochemically-generated Fe^II complexes toward dioxygen and of the Fe^III complexes toward superoxide through cyclic voltammetry. All the complexes reacted with dioxygen and superoxide in DMSO solution. Redox processes attributed to oxygenated species were observed in a more cathodic potential than those of the original compounds. According to the data, the new species Fe^II–O₂ converts itself to Fe^III–O₂⁻, which presents a new redox wave attributed to the process Fe^III–O₂⁻ + e⁻ → Fe^II–O₂⁻. The same species Fe^III–O₂⁻ is formed from the reaction of the Fe^III form of the complexes and KO₂.