Structural, Magnetic, and Electronic Properties of a Pentacoordinated Intermediate-Spin (<i>S</i> = ³/₂) Iron(III) Complex with a Macrocyclic [N₄]^2- Ligand

The complex (6,13-bis(ethoxycarbonyl)-5,14-dimethyl-1,4,8,11-tetraazacyclotetradeca-4,6,12,14-tetraenato(2−))iron(III) iodide, 3 (chemical formula C18H26O4N4FeI·0.5 C7H8), has been characterized by X-ray crystallography (a = 16.063(3) Å, b = 15.895(3) Å, c = 19.703(4) Å, β = 107.11(3)°; monocline; I2/a; Z = 8), EPR and Mössbauer spectroscopy, and temperature-dependent magnetic susceptibility as well as by MO calculations. The pentacoordinated iron atom is 0.34 Å outside of the [N4] plane. The equatorial Fe−N distances (1.92 Å for the N's in neighborhood of the methylated C-atoms and 1.88 Å for the others) are not significantly different from those found in octahedral low-spin iron(III) complexes with the same macrocyclic [N42-] ligand. The magnetic moment (μeff = 4.19 μB up to 20 K; ϑ = −7.37 K), the EPR measurements (gyeff = 5.38, gxeff = 2.70, and gzeff = 1.74), and the Mössbauer data (δ = 0.18 mm/s, ΔEQ = 3.56 mm/s, T = 120 K) indicate a pure intermediate-spin state of S = 3/2. The MO calculations yield a complete picture of bonding modes, intramolecular interactions, and the electronic structure around the iron center, as indicated by the correct determination of the ground state and by the agreement between the measured and calculated quadrupole splitting.