Electronic structure of a series of five-coordinate Fe(OArTAzP)X (OAr = octaaryltetraazaporphyrin, X = Cl-, Br, I-; Ar = 4-tert-butylphenyl) have been examined on the basis of 1H NMR, 13C NMR, and EPR spectroscopy as well as SQUID magnetometry. These complexes adopt the intermediate-spin state as in fhe case of analogous complexes reported by Fitzgerald et al. (Inorg. Chem. 1992; 31: 2006-2013) and Stuzhin et al. (Inorg. Chim. Acta 1995; 236: 131-139). The 13C NMR studies using 13C-enriched complexes at the pyrrole α positions have revealed that fhe pyrrole-Cα signals appear at extraordinary upfield positions, i.e. -130 to -250 ppm at 273 K, due to the dz2-a2u and dπ-3eg interactions. The Curie plots of the pyrrole-Cα signals have further revealed that the iodide complex adopts a much purer intermediate-spin state than the bromide and chloride complexes. In contrast to fhe case of Fe(OArTAzP)X, six-coordinate [Fe(OArTAzP)(CN)2]- showed the pyrrole-Cα signal at 47 ppm at 273 K, which indicates that the complex adopts the low-spin state with the (dxy)2(d xz, dyz)3 electron configuration. Thus, the 13C NMR chemical shift of the pyrrole-Cα signal turns out to be quite a good probe to elucidate the spin state and electron configuration of iron(III) tetraazaporphyrins, where fhe 1H NMR spectroscopy is less useful because of the absence of the hydrogen atoms as well as the alkyl or aryl groups directly attached to the meso positions.
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