Electron Configuration and Spin Distribution in Low-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes Carrying One or Two Orientationally Fixed Imidazole Ligands1

Abstract

To understand the orientation effect of coordinated imidazole ligands, a series of low-spin (tetraalkylporphyrinato)iron(III) complexes, [Fe(TRP)(L)2]+ 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 c...