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 gmax 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 (dxz, dyz)4-(dxy)1 in contrast to the other low-spin complexes where ferric ions have the (dxy)2(dxz, dyz)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 (dxz, dyz)4(dxy)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 (dxz, dyz)4(dxy)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 (dxy)2(dxz, dyz)3 configuration.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry