Synthesis, X-ray Crystal Structure, and Redox and Electronic Properties of lron(lll)-Polyimidazole Complexes Relevant to the Metal Sites of Iron Proteins

A new tripod N3 ligand (L), containing three imidazole rings, was synthesized in good yield. At variance with usual aromatic ligands with N2 or N3 donor sets such as pyridine or pyrazole derivatives, L stabilizes the Felll oxidation state. The corresponding iron(lll) complexes [Fe(L)Cl3] (1) and[Fe(L)2](ClO4)3 (2) were prepared and characterized by X-ray structural analysis and spectroscopic methods. The coordination environment around all the Felll centers has a distorted octahedral geometry. [Fe(L)Cl3] (1) belongs to the monoclinic system, space group R21/n, a = 9.7406(5) A, b = 17.207(2) A, c = 14.615(2) A, beta = 104.448(9)°, Z= 4, V = 2372.1(4) A^3; R = 0.044, Rw = 0.055. [Fe(L)2](CIO4)3 (2) belongs to the monoclinic system, space group P2i/c, a = 16.1057(15) A, b = 11.1079(12) A, c = 26.283(2) A, beta = 102.062(10)°, Z-4, V= 4598.2(8) A^3; R = 0.0465, Rw = 0.0902. The Fe-N(iPrim) bond lengths are systematically longer than the Fe-N(Melm) ones. Compound 2 is a highly anisotropic low-spin Felll complex displaying a rather unusual EPR spectrum with a sharp signal at g= 3.5 and a broad one at g ~1.6. The fitting of this EPR spectrum is discussed.