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 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 FeIII oxidation
state. The corresponding iron(III) 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 FeIII centers
has a distorted octahedral geometry. [Fe(L)Cl3] (1) belongs to the monoclinic system, space group P21/n, a =
9.7406(5) Å, b = 17.207(2) Å, c = 14.615(2) Å, β = 104.448(9)°, Z = 4, V = 2372.1(4) Å3; R = 0.044, Rw =
0.055. [Fe(L)2](ClO4)3 (2) belongs to the monoclinic system, space group P21/c, a = 16.1057(15) Å, b = 11.1079(12) Å, c = 26.283(2) Å, β = 102.062(10)°, Z = 4, V = 4598.2(8) Å3; R = 0.0465, Rw = 0.0902. The Fe−N(iPrIm) bond lengths are systematically longer than the Fe−N(MeIm) ones. Compound 2 is a highly anisotropic
low-spin FeIII 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.