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

H-1 NMR, C-13 NMR, and EPR studies of a series of low-spin (meso-tetraalkyl porphyrinato)iron(III) complexes, [Fe(TRP)(L)(2)]X where R = Pr-n,Pr-c, and Pr-i and L represents axial ligands such as imidazoles, pyridines, and cya nide, have revealed that the ground-state electron configuration of [Fe((TP PrP)-P-n)(L)(2)]X and [Fe((TPrP)-Pr-c)(L)(2)]X is presented either as the c ommon (d(xy))(2)(d(xz),d(yz))(3) or as the less common (d(xz),d(yz))(4)(d(x y))(1) depending on the axial ligands. The ground-state electron configurat ion of the isopropyl complexes [Fe(Tt-PrP)(L) (2)]X is, however, presented as (d(xz),d(yz))(4)(d(xy))(1) regardless of the kind of axial ligands. In e very case, the contribution of the (d(xz),d(yz))(4)(dxy)(1) state to the el ectronic ground state increases in the following order: HIm < 4-Me2NPy < 2- MeIm < CN- < 3-MePy < Py < 4-CNPy. Combined analysis of the C-13 and (HNMR) -H-1 isotropic shifts together with the EPR S values have yielded the spin densities at the porphyrin carbon and nitrogen atoms. Estimated spin densit ies in [Fe((TPrP)-Pr-i)(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 a s follows: meso-carbon, +0.045; alpha-pyrrole carbon, +0.0088; beta-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 resu lt 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 beta-pyrrole car bon 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, delta +917.5 ppm at -50 degrees C in [Fe((TPrP)- Pr-i)(4-CNPy)(2)](+), is ascribed to the large spin densities at these carb on atoms. In contrast, the large upfield shift of the alpha-pyrrole carbon signal, delta -293.5 ppm at the same temperature, is caused by the spin pol arization from the adjacent meso-carbon and pyrrole nitrogen atoms.