Spin distribution in low-spin (meso-tetraalkylporphyrinato)iron(III) complexes with (d(xz),d(yz))⁴(d(xy))¹ configuration. Studies by ¹H NMR, ¹³C NMR, and EPR spectroscopies

H NMR, ¹³C NMR, and EPR studies of a series of low-spin (meso- tetraalkylporphyrinato)iron(III) complexes, [Fe(TRP)(L)₂]X where R = (n)Pr, (c)Pr, and (i)Pr and L represents axial ligands such as imidazoles, pyridines, and cyanide, have revealed that the ground-state electron configuration of [Fe(T(n)PrP)(L)₂]X and [Fe(T(c)PrP)(L)₂]X is presented either as the common (d(xy))²(d(xz),d(yz))³ or as the less common (d(xz),d(yz))⁴(d(xy))¹ depending on the axial ligands. The ground-state electron configuration of the isopropyl complexes [Fe(T(i)PrP)(L) ₂]X is, however, presented as (d(xz),d(yz))⁴(d(xy))¹ regardless of the kind of axial ligands. In every case, the contribution of the (d(xz),d(yz))⁴(d(xy))¹ state to the electronic ground state increases in the following order: HIm < 4-Me₂NPy < 2-MeIm < CN^- < 3-MePy < Py < 4-CNPy. Combined analysis of the ¹³C and ¹H NMR isotropic shifts together with the EPR g values have yielded the spin densities at the porphyrin carbon and nitrogen atoms. Estimated spin densities in [Fe(T(i)PrP)(4-CNPy)₂]⁺, which has the purest (d(xz),d(yz))⁴(d(xy))¹ ground state among the complexes examined in this study, are as follows: meso-carbon, +0.045; α-pyrrole carbon, +0.0088; β-pyrrole carbon, -0.00026; and pyrrole nitrogen, +0.057. Thus, the relatively large spin densities are on the pyrrole nitrogen and meso-carbon atoms. The result is in sharp contrast to the spin distribution in the (d(xy))²(d(xz),d(yz))³ type complexes; the largest spin density is at the β-pyrrole carbon atoms in bis(1-methylimidazole)(meso- tetraphenylporphyrinato)iron(III), [Fe(TPP)(1-MeIm)₂]⁺, as determined by Goff. The large downfield shift of the meso-carbon signal, δ +917.5 ppm at - 50 °C in [Fe(T(i)PrP)(4-CNPy)₂]⁺, is ascribed to the large spin densities at these carbon atoms. In contrast, the large upfield shift of the α-pyrrole carbon signal, δ -293.5 ppm at the same temperature, is caused by the spin polarization from the adjacent meso-carbon and pyrrole nitrogen atoms.