A new tripodal tetradentate ligand and its iron(III) complex, as a model for mononuclear non-heme iron active sites. Reactivity studies toward dioxygen and superoxide

The new biomimetic ligand (bis((1-methylimidazol-2-yl)methyl)amino) acetic acid (LH) has been synthesized, which increases the growing family of tripodal tetradentate ligands. Reaction of LH with FeCl3 . 6H(2)O allowed us to obtain the corresponding iron(III) complex [Fe(L)Cl-2]. The crystal structure reveals mononuclear neutral molecules with four chiral molecules per asymmetric unit (crystal data: monoclinic, P2(1), a = 12.564(5) Angstrom, b = 25.960(10) Angstrom, c = 9.875(4) Angstrom, alpha = 90.000, beta = 60.494(4)degrees, gamma = 90.00 degrees, V = 3221(2) Angstrom(3), Z = 8, R = 0.0601 for 2433 unique reflections with F-o > 4 sigma(F) and 452 variable parameters). The iron(III) center is located in a distorted octahedral environment consisting of four donor atoms from the ligand L, two imidazole nitrogens, an amine nitrogen, and one oxygen of the carboxylate group, with the 6-fold coordination being completed by two chloride anions. UV-vis and EPR spectra are solvent dependent due to the two labile chloride positions. Cyclic voltammograms run under an argon atmosphere exhibit a one-electron quasi-reversible redox process (E(o) = -0.015 V vs SCE in DMSO). The reaction of the complex with dioxygen and superoxide has been investigated through UV-vis spectroscopic and electrochemical studies. Direct UV-vis spectroscopic observations upon reaction with potassium superoxide, KO2, dissolved in DMSO indicate that a new species is formed, symbolyzed [Fe(III)-O-2(-)]. Comparison of the cyclic voltammograms obtained under argon or in the presence of dioxygen unambiguously indicates that the reduced form of the complex reacts with dioxygen. The buildup of a new species is confirmed by controlled potential electrolysis experiments of the associated voltammograms in the presence of dioxygen. This new species, symbolyzed [Fe(II)-O-2], has been shown to be stable even in the absence of dioxygen. Its UV-vis spectrum is superimposable with the spectrum of [Fe(III)-O-2(-)], indicating that the same oxygenated entity has been formed. Reaction of superoxide with the iron(III) complex has also been monitored through cyclic voltammetry, which confirms that superoxide is reactive toward the complex.