Spin Distribution in Low-Spin (<i>m</i><i>eso</i>-Tetraalkylporphyrinato)iron(III) Complexes with (d<i>_xz</i>,d<i>_y</i><i>_z</i>)⁴(d<i>_x</i><i>_y</i>)¹ Configuration. Studies by ¹H NMR, ¹³C NMR, and EPR Spectroscopies

Abstract

1H NMR, 13C NMR, and EPR studies of a series of low-spin (meso-tetraalkylporphyrinato)iron(III) complexes, [Fe(TRP)(L)2]X where R = nPr, cPr, and iPr and L represents axial ligands such as imidazoles, pyridines, and cyanide, have revealed that the ground-state electron configuration of [Fe(TnPrP)(L)2]X and [Fe(TcPrP)(L)2]X is presented either as the common (dxy)2(dxz,dyz)3 or as the less common (dxz,dyz)4(dxy)1 depending on the axial ligands. The ground-state electron configuration of the isopropyl complexes [Fe(TiPrP)(L) 2]X is, however, presented as (dxz,dyz)4(dxy)1 regardless of the kind of axial ligands. In every case, the contribution of the (dxz,dyz)4(dxy)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(TiPrP)(4-CNPy)2]+, which has the purest (dxz,dyz)4(dxy)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 (dxy)2(dxz,dyz)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(TiPrP)(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.