One-electron oxidation of iron(II)–imidazole and iron(II)–bis[imidazol-2-yl]methane complexes: a pulse radiolysis study

Abstract

The radical anion, Br₂^˙–, a strong one-electron oxidant, has been used to oxidise iron(II)–imidazole, Fe^II–ImH, and iron(II)–bis(imidazol-2-yl)methane, Fe^II–2-BIM, complexes in aqueous solution, the latter being regarded as good models of the iron(II) site in non-haem iron-containing enyzmes such as lipoxygenase. The rates of oxidation of Fe^II–lmH, Fe^II(ImH)₂, Fe–2-BIM and Fe^II(2-BIM)₂ were measured as 1.0 × 10⁷, 2.0 × 10⁷, 1.8 × 10⁸ and 3.6 × 10⁸ dm³ mol^–1 s^–1. From measurements of the rates of oxidation of the ligand, it is clear that Br₂^˙– oxidises the ligand in the metal complexes in the first instance. The same studies also show that the 2-BIM ligand is easier to oxidise than the closely related imidazole ligand by a factor of 10. Measurements of the rate of oxidation of 2-methylimidazole indicate that the difference is attributable to the inductive effect of the —CH₂— group.

The spectra of the transient initial products of the iron(II)–imidazole oxidation are very similar to the imidazole free radical spectra suggesting either very weak metal–ligand charge transfer, MLCT, character in the metal–free radical complex or that the complex dissociates rapidly (> 10⁶ s^–1) to yield an imidazole free radical. In contrast, the initial iron(II)–2-BIM products exhibit spectra which are three to six times more intense than the 2-BIM free radical spectrum. For the ML product, this is attributed to MLCT transitions of the metal–2-BIM free radical species, whereas for ML₂, it is proposed that the spectrum is assigned to an Fe^III–2-BIM complex, formed following fast intramolecular electron transfer (>10⁶ s^–1) within the Fe^II–2-BIM free radical complex.

The data are in contrast to similar data obtained for iron(II)–histidine complexes in an earlier study (Parsons M. Al-Hakim, G. O. Phillips and A. J. Swallow, J. Chem. Soc., Faraday Trans. 1, 1986, 82, 1575) where the oxidation process was not found to be controlled by initial oxidation of the histidine ligand. It is suggested that these differences are attributable to a greater degree of covalent character in the metal–ligand bonding in the iron(II)–histidine complex compared with the weaker, largely electrostatic bonds, in iron(II)–imidazole complexes.