A NEW TRIPODAL TETRADENTATE LIGAND AND ITS IRON(III) COMPLEX, AS A MODEL FOR MONONUCLEAR NONHEME IRON ACTIVE-SITES - REACTIVITY STUDIES TOWARD DIOXYGEN AND SUPEROXIDE

The new biomimetic ligand (bis((1-methylimidazol-2-yl)methyl)amino) ac etic acid (LH) has been synthesized, which increases the growing famil y 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) Angstr om, 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 w ith F-o > 4 sigma(F) and 452 variable parameters). The iron(III) cente r is located in a distorted octahedral environment consisting of four donor atoms from the ligand L, two imidazole nitrogens, an amine nitro gen, and one oxygen of the carboxylate group, with the 6-fold coordina tion being completed by two chloride anions. UV-vis and EPR spectra ar e solvent dependent due to the two labile chloride positions. Cyclic v oltammograms run under an argon atmosphere exhibit a one-electron quas i-reversible redox process (E(o) = -0.015 V vs SCE in DMSO). The react ion of the complex with dioxygen and superoxide has been investigated through UV-vis spectroscopic and electrochemical studies. Direct UV-vi s spectroscopic observations upon reaction with potassium superoxide, KO2, dissolved in DMSO indicate that a new species is formed, symbolyz ed [Fe(III)-O-2(-)]. Comparison of the cyclic voltammograms obtained u nder 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 experi ments of the associated voltammograms in the presence of dioxygen. Thi s new species, symbolyzed [Fe(II)-O-2], has been shown to be stable ev en in the absence of dioxygen. Its UV-vis spectrum is superimposable w ith the spectrum of [Fe(III)-O-2(-)], indicating that the same oxygena ted entity has been formed. Reaction of superoxide with the iron(III) complex has also been monitored through cyclic voltammetry, which conf irms that superoxide is reactive toward the complex.