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 [N4 2-] ligand. The magnetic moment (μ eff = 4.19 μB up to 20 K; ϑ = −7.37 K), the EPR measurements (g y eff = 5.38, g x eff = 2.70, and g z eff = 1.74), and the Mössbauer data (δ = 0.18 mm/s, Δ 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 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.

The molecular structures and the spin ground-state of the iron(II/III) complexes Fe1, Fe2 and Fe3, which contain dianionic macrocyclic [N 4 ] ligands derived from substituted acetylacetaldehydes and 1,2-diamines, have been investigated by X-ray analysis and temperature-dependent susceptibility measurements, respectively. The iron (II) . The iron centre in Fe II 2 is nearly square-planar and shows a strong decrease of the magnetic moment below T Ϸ 250 K. This is probably due to an S = 0 to S = 1 spin-crossover or, more likely, to intermolecular

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