Electronic structure of five- and six-coordinate iron(III) tetraazaporphyrin complexes: Pyrrole-C, chemical shift as a useful probe

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-C_α signals appear at extraordinary upfield positions, i.e. -130 to -250 ppm at 273 K, due to the d_z2-a_2u and d_π-3e_g interactions. The Curie plots of the pyrrole-C_α 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-C_α signal at 47 ppm at 273 K, which indicates that the complex adopts the low-spin state with the (d_xy)²(d _xz, d_yz)³ electron configuration. Thus, the ¹³C NMR chemical shift of the pyrrole-C_α 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.