Structural, magnetic, and electronic properties of a pentacoordinated intermediate-spin (S=3/2) iron(III) complex with a macrocyclic [N-4](2-) ligand

The complex (6,13-bis(ethoxycarbonyl)-5,14-dimethyl-1,4,8,11-tetraazacyclot etradeca-4,6,12,14-tetraenato(2-))- iron(III) iodide, 3 (chemical formula C 18H26O4N4FeI . 0.5 C7H8), has been characterized by X-ray crystallography ( a = 16.063(3) Angstrom, b = 15.895(3) Angstrom, c = 19.703(4) Angstrom, bet a = 107.11(3)degrees; monocline; I2/a; Z = 8), EPR and Mossbauer spectrosco py, and temperature-dependent magnetic susceptibility as well as by MO calc ulations. The pentacoordinated iron atom is 0.34 Angstrom outside of the [N -4] plane. The equatorial Fe-N distances (1.92 Angstrom for the N's in neig hborhood of the methylated C-atoms and 1.88 Angstrom for the others) are no t significantly different from those found in octahedral low-spin iron(III) complexes with the same macrocyclic [N-4(2-)] ligand. The magnetic moment (mu(eff) = 4.19 mu(B) up to 20 K; Theta = -7.37 K), the EPR measurements (g (y)(eff) = 5.38, g(x)(eff) = 2.70, and g(z)(eff) = 1.74), and the Mossbauer data (delta = 0.18 mm/s, Delta E-Q = 3.56 mm/s, T = 120 K) indicate a pure intermediate-spin state of S = 3/2. The MO calculations yield a complete p icture of bonding modes, intramolecular interactions, and the electronic st ructure around the iron center, as indicated by the correct determination o f the ground state and by the agreement between the measured and calculated quadrupole splitting.