Synthesis, X-ray crystal structure, and redox and electronic properties of iron(III)-polyimidazole complexes relevant to the metal sites of iron proteins

A new tripod N-3 ligand (L), containing three imidazole rings, was synthesized in good yield. At variance with usual aromatic ligands with N-2 or N-3 donor sets such as pyridine or pyrazole derivatives, L stabilizes the Fe-III oxidation state. The corresponding iron(III) complexes [Fe(L)Cl-3] (1) and [Fe(L)(2)](CIO4)(3) (2) were prepared and characterized by X-ray structural analysis and spectroscopic methods. The coordination environment around all the Fe-III centers has a distorted octahedral geometry. [Fe(L)Cl-3] (1) belongs to the monoclinic system, space group P2(1)/n, a = 9.7406(5) Angstrom, b = 17.207(2) Angstrom, c = 14.615(2) Angstrom, = 104.448(9)degrees, Z = 4, V = 2372.1(4) Angstrom(3); R = 0.044, R-w = 0.055. [Fe(L)(2)](CIO4)(3) (2) belongs to the monoclinic system, space group P2(1)/c, a = 16.1057(15) Angstrom, b = 11.1079(12) Angstrom, c = 26.283(2) Angstrom, beta = 102.062(10)degrees, Z = 4, V = 4598.2(8) Angstrom(3); R = 0.0465, R-w = 0.0902. The Fe-N((i)-Prlm) bond lengths are systematically longer than the Fe-N(Melm) ones. Compound 2 is a highly anisotropic low-spin Fe-III complex displaying a rather unusual EPR spectrum with a sharp signal at g = 3.5 and a broad one at g similar to 1.6. The fitting of this EPR spectrum is discussed.