ONE-ELECTRON OXIDATION OF IRON(II) IMIDAZOLE AND IRON(II) BIS(IMIDAZOL-2-YL)METHANE COMPLEXES - A PULSE-RADIOLYSIS STUDY

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-co ntaining enzymes such as lipoxygenase. The rates of oxidation of Fe(II )-ImH, Fe(II)(ImH)2, Fe-2-BIM and Fe(II)(2-BIM)2 were measured as 1.0 x 10(7), 2.0 x 10(7), 1.8 x 10(8) and 3.6 x 10(8) 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 instanc e. The same studies also show that the 2-BIM ligand is easier to oxidi se than the closely related imidazole ligand by a factor of 10. Measur ements 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)-imidazo le oxidation are very similar to the imidazole free radical spectra su ggesting either very weak metal-ligand charge transfer, MLCT, characte r in the metal-free radical complex or that the complex dissociates ra pidly (> 10(6) s-1) to yield an imidazole free radical. In contrast, t he 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 M L product, this is attributed to MLCT transitions of the metal-2-BIM f ree radical species, whereas for ML2, it is proposed that the spectrum is assigned to an Fe(III)-2-BIM complex, formed following fast intram olecular electron transfer (> 10(6) 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, 1 986, 82, 1575) where the oxidation process was not found to be control led by initial oxidation of the histidine ligand. It is suggested that these differences are attributable to a greater degree of covalent ch aracter in the metal-ligand bonding in the iron(II)-histidine complex compared with the weaker, largely electrostatic bonds, in iron(II)-imi dazole complexes.