Rotation and orientation of axially coordinated imidazoles in low-spin ferric porphyrin complexes

The variable temperature ¹H NMR spectra of a series of low-spin bis(imidazole)tetrakis(2, 4, 6-trialkylphenyl)porphinatoiron(III) chlorides, (R-TPP)Fe(L)₂Cl, showed that the rotation of axially coordinated 2-alkyl- and benzimidazoles slowed down on the NMR time scale at low temperature to give four pyrrole signals. The ¹³C NMR spectra of the ¹³C enriched (Me-TPP)Fe(2- methylimidazole)₂Cl at the meso positions gave two meso signals with equal intensity at the temperature range below -25 °C. These results indicate that each of the axial ligands is perpendicularly aligned over a diagonal C_mesoFeC_meso axis. The shift range of the pyrrole protons reached as much as 12 ppm at -56 °C. The apparent unfavorable orientation of the ligands was explained in terms of the S₄ deformed structure of the porphinatoiron core in solution. The relatively small slopes in Curie plots of the pyrrole-H and the meso ¹³C signals were also ascribed to the deformed structure of the core. In the mixed ligand complexes having a hindered 2-isopropylimidazole and an unhindered 1-methylimidazole ligand, it was suggested from the ¹H and ¹³C NMR splitting patterns that the rotation of only one of the ligands, 2-isopropylimidazole, was frozen. In these complexes the spread of the pyrrole signals increased to nearly 20 ppm. The activation free energies for rotation were determined by the dynamic NMR technique. They changed in the range of 11.3 to 13.6 kcal mol^-1 depending upon the bulkiness of the axial ligands and o-alkyl substituents. Close examination of the dynamic process using the saturation transfer technique revealed that the dissociation of the axial ligands occurred concomitantly during the rotation process in the case of the bis(2-isopropylimidazole) complexes. In contrast, pure rotation process was observed in the complexes with 2-methyl-, 2-ethyl-, 1, 2-dimethyl- and benzimidazole. X band ESR spectra of these complexes were taken at 4.2 K in CH₂Cl₂ glass. Although complexes with perpendicularly aligned planar ligands tend to exhibit so called 'strong g_max' type signals, the complexes studied here showed signals with smaller g_z values.