Spin Distribution in Low-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes with (dxz,dyz)4(dxy)1 Configuration. Studies by 1H NMR, 13C NMR, and EPR Spectroscopies

1 H NMR, 13 C 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-Me 2 NPy < 2-MeIm < CN - < 3-MePy < Py < 4-CNPy. Combined analysis of the 13 C and 1 H 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.