Electron Configuration and Spin Distribution in Low-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes Carrying One or Two Orientationally Fixed Imidazole Ligands

To understand the orientation effect of coordinated imidazole ligands, a series of low-spin (tetraalkylporphyrinato)-iron(III) complexes, [Fe(TRP)(L)₂J⁺ and [Fe(TRP)(L)(CN)], carrying at least one orientationally fixed imidazole (L) have been prepared. The ¹H NMR pyrrole signals of a series of [Fe(TRP)(2-MeIm)₂]⁺ have shown considerable downfield shifts as the meso substituent becomes bulkier, from -30.4 (R = H) to +5.6 ppm (R = ⁱPr) at -71 °C. These complexes have exhibited four pyrrole signals at lower temperature due to the hindered ligand rotation. The spread of the pyrrole signals decreases from 9.4 (Me) to 8.2 (Et) and then to 5.7 (ⁱPr) ppm. The downfield pyrrole signals together with the small spread in [Fe(TⁱPrP)(2-MeIm)₂]⁺ are in sharp contrast to the other low-spin complexes with orientationally fixed imidazole ligands; the chemical shifts and spreads of the pyrrole signals in [tetrakis(2,4.6-trialkylphenyl)porphyrinato]iron(III) complexes [Fe(R-TPP)(2-MeIm)₂]⁺ (R = Me. Et, ⁱPr) are ca. -20 and ca. 9 ppm, respectively, at -71 °C. The EPR spectra of a series of [Fe(TRP)(2-MeIm)₂]⁺ were then taken at 4.2 K. While the R = H, Me, and Et complexes have shown so-called "large g_max type" spectra as in the case of [Fe(R-TPP)(2-MeIm)₂]⁺, the ⁱPr complex has exhibited an "axial type" spectrum. The result indicates that the electron configuration of the ferric ion of [Fe(TⁱPrP)(2-MeIm)₂]⁺ is presented by the unusual (d_xz, d_yz)⁴-(d_xy)¹ in contrast to the other low-spin complexes where ferric ions have the (d_xy)²(d_xz, d_yz)³ configuration. When one of the 2-MeIm ligands in [Fe(TRP)(2-MeIm)₂]⁺ is replaced by CN^-, not only the ⁱPr but also the Me and Et complexes have shown the (d_xz, d_yz)⁴(d_xy)¹ configuration as revealed from the EPR spectra. The pyrrole signals of the ⁱPr complex [Fe(TⁱPrP)(2-MeIm)(CN)] have been observed at 12.2, 14.1, 14.8, and 16.2 ppm at -71 °C. Thus, the spread is only 4.0 ppm. The value is quite different from that of the corresponding [Fe(Me-TPP)(2-ⁱPrIm)(CN)] where the spread reaches as much as 11.4 ppm. On the basis of these results, it is concluded that the spin distribution on the pyrrole β-carbons in the complexes with (d_xz, d_yz)⁴(d_xy)¹ is rather homogeneous even if the coordinated imidazole is orientationally fixed. On the contrary, the fixation induces a larger asymmetric spin distribution on these carbons in the complexes with (d_xy)²(d_xz, d_yz)³ configuration.