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-, I-, 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 I- complex exhibits an admixed intermediate-spin (S = 5/2, 3/2) state in CD2Cl2 solution. The I- 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 I- 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(a2u) 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)ClO4] 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 z 2 )1.