Electronic structures of five-coordinate iron(III) porphyrin complexes with highly ruffled porphyrin ring

The spin states of the iron(III) complexes with a highly ruffled porphyrin ring, [Fe(TEtPrP)X] where X = F^-, Cl^-, Br^-, l^-, and ClO₄^-, have been examined by ¹H NMR, ¹³C NMR, EPR, and Mössbauer spectroscopy. While the F^-, Cl^-, and Br^- complexes adopt a high-spin (S = 5/2) state, the l^- complex exhibits an admixed intermediate-spin (S = 5/2, 3/2) state in CD₂Cl₂ solution. The l^- complex shows, however, a quite pure high-spin state in toluene solution as well as in the solid. The results contrast those of highly saddled [Fe(OETPP)X] where the l^- complex exhibits an essentially pure intermediate-spin state both in solution and in the solid. In contrast to the halide-ligated complexes, the ClO₄^- complex shows a quite pure intermediate-spin state. The ¹³C NMR spectra of [Fe(TEtPrP)-ClO₄] are characterized by the downfield and upfield shifts of the meso and pyrrole-α carbon signals, respectively: δ(meso) = +342 and δ(α-py) = -287 ppm at 298 K. The data indicate that the meso carbon atoms of [Fe-(TEtPrP)ClO₄] have considerable amounts of positive spin, which in turn indicate that the iron has an unpaired electron in the d_xy orbital; the unpaired electron in the d_xy orbital is delocalized to the meso positions due to the iron (d_xy)-porphyrin(a_2u) interaction. Similar results have been obtained in analogous [Fe(TⁱPrP)X] though the intermediate-spin character of [Fe(TⁱPrP)X] is much larger than that of the corresponding [Fe(TEtPrP)X]. On the basis of these results, we have concluded that the highly ruffled intermediate-spin complexes such as [Fe(TEtPrP)-ClO ₄] and [Fe(TⁱPrP)ClO₄] adopt a novel (d _xz, d_yz)³(d_xy)1(d_z ²)¹ electron configuration; the electron configuration of the intermediate-spin complexes reported previously is believed to be (d _xy)²(d_xz, d_yz)²(d _z²)¹.