Imidazole and Imidazolate Iron Complexes: On the Way for Tuning 3D-Structural Characteristics and Reactivity. Redox Interconversions Controlled by Protonation State Franc ¸ ois Lambert,* ,† Clotilde Policar,* ,† Stephanie Durot, † Miche `le Cesario, ‡ Lu Yuwei, § Hafsa Korri-Youssoufi, † Bineta Keita, § and Louis Nadjo § Laboratoire de Chimie Bio-organique et Bio-inorganique, UMR8124, Institut de Chimie Mole ´ culaire d’Orsay, Ba ˆ timent 420, UniVersite ´ Paris XI, F-91405 Orsay Cedex, France, Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-YVette, France, and Laboratoire d’Electrochimie et de Photoe ´ lectrochimie, Ba ˆ timent 420, UniVersite ´ Paris-Sud, 91405 Orsay, France Received February 2, 2004 X-ray structures for six Fe(II) and Fe(III) complexes from two closely heptadentate N-tripodal ligands, L1H 3 ) tris[(imidazol-4-yl)-3-aza-3-butenyl]amine and L2H 3 ) tris[(imidazol-2-yl)-3-aza-3-butenyl]amine, are described: three complexes in the L1 series (namely, [Fe(II)(L1H 3 )] 2+ and [Fe(III)(L1H 3 )] 3+ at low pH and [Fe(III)(L1)] 0 at high pH) and three complexes in the L2 series (namely, [Fe(II)(L2H 3 )] 2+ at low pH and [Fe(II)(L2H)] 0 and [Fe(III)(L2)] 0 at high pH). Most of these complexes are stable in both Fe(II) and Fe(III) redox states and with the ligand in various protonation states. In the solid state, hydrogen bonds networks were obtained. Structural differences induced by 2- or 4-imidazole substitution are described and discussed. In solution, interconversions between different forms, with regard to oxidation and protonation states, were investigated by UV-visible spectroscopy, cyclic voltammetry, and potentiometry. The deprotonation pattern of these polyimidazole iron(II) and iron(III) complexes is described in detail. pK a s of the imidazolate/imidazole moieties in MeOH/H 2 O are reported. Two new species, namely, [Fe(II)(L1)] - and [Fe(II)(L2)] - , were shown to be obtained in DMSO upon strong base addition and characterized by UV-vis spectroscopy and cyclic voltammetry. Half-wave potentials of Fe(III)/Fe(II) complexes with ligand moieties in several protonation states are reported, both in DMSO and in MeOH/H 2 O. Because of the presence of free imidazole groups coordinated to the iron, the potential of the iron(III)/iron(II) couples can be tuned by pH. A shift of ∆E ) E deprot - E prot ranging from -270 to -320 mV per exchanged proton in DMSO was measured. This study shows moreover that interconversions (with regard to both redox and protonation states) can be reversed several times. As the complexes have been isolated in order to be tested as superoxide dismutase mimics, preliminary reactions with dioxygen and with superoxide, considered as oxidant and reducer of biological importance, are reported. In these two series, O 2 - behaves either as a base or as a reducer and no adducts have been observed. Introduction Histidyl imidazole ligand is probably one of the most common ligands encountered at metalloenzyme active sites, and its acidobasic behavior is certainly of first importance in the control of catalytic activity of these enzymes. A better understanding of such a fine-tuning can be provided by the study of model complexes. Iron porphyrins bearing axial imidazole and/or imidazolate have been studied in order to understand the effect of hydrogen bonding 1,2 or the depro- tonation 3 of an axial imidazole on the redox potential of the iron(III)/iron(II) couple. Recently this topic has been ex- tended to the study of non-porphyrinic complexes. 3-7 Depro- tonation of a coordinated imidazole has also been shown, by us and others, to be an easy way for spontaneous oligomerization of metal complexes (Fe(II) and -(III), * Authors to whom correspondence should be addressed. E-mail: flambert@icmo.u-psud.fr (F.L.); cpolicar@icmo.u-psud.fr (C.P.). Fax (international): 0033169157231. Tel (international): 0033169154725. ² Universite ´ Paris XI. ‡ CNRS. § Universite ´ Paris-Sud. (1) Quinn, R.; Mercer-Smith, J.; Burstyn, J. N.; Valentine, J. S. J. Am. Chem. Soc. 1984, 106, 4136-4144. (2) O’Brien, P.; Sweigart, D. A. Inorg. Chem. 1985, 24, 1405-1409. (3) Quinn, R.; Strouse, C. E.; Valentine, J. S. Inorg. Chem. 1983, 22, 3934-3940. Inorg. Chem. 2004, 43, 4178-4188 4178 Inorganic Chemistry, Vol. 43, No. 14, 2004 10.1021/ic0498687 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/17/2004