Hindered rotation of axially coordinated 2-methylimidazoles in tetramesitylporphyrinato-iron(III) complexes

Hindered rotation of coordinated imidazoles in the title complex has been observed by the dynamic NMR method. At low temperature where the rotation of the imidazoles was slow on the NMR time scale, only the conformer with C₂ symmetry was observed. The dispersion of the pyrrole signals, 7.3 ppm at -48°C, has been ascribed to the deviations from planarity of the porphyrin ring rather than an orientation effect of the axial ligands, since they must be perpendicular to each other. The activation enthalpy and entropy for ligand rotation were determined to be 12.9 ± 0.4 Kcal mol and 3.7±1.6 e. u., respectively.