Barriers to Inversion of Nonplanar Ferric Porphyrin Radical Cations:  Comparison with the Corresponding Neutral Complexes

Barriers to inversion of the highly deformed porphyrin rings in two ferric porphyrin radical cations, [Fe(OMTPP)Cl]SbCl₆ and [Fe(OETPP)Cl]SbCl₆ where OMTPP and OETPP are dianions of 2,3,7,8,12,13,17,18-octamethyl- and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, respectively, have been determined by the dynamic NMR method. Inversion barrier of [Fe(OMTPP)Cl]SbCl₆ is estimated to be 14.8 kcal mol^-1 (72 °C) which is higher than that of the corresponding neutral porphyrin complex [Fe(OMTPP)Cl] by more than 4 kcal mol^-1. Increase in barrier to inversion has also been observed in [Fe(OETPP)Cl]SbCl₆, though the precise determination is hampered in this case due to the occurrence of site exchange of the ferric ion across the porphyrin ring; the latter process is verified because the activation free energies obtained from the line shape analysis of the meta and methyl signals are quite close, 18.1 (113 °C) and 17.9 (113 °C) kcal mol^-1, respectively. Thus, the inversion barrier in [Fe(OETPP)Cl]SbCl₆ is supposed to be ≫18 kcal mol^-1. Since the inversion barrier in the corresponding neutral porphyrin complex [Fe(OETPP)Cl] is reported to be 15.8 kcal mol^-1, the barrier has increased by ≫2 kcal mol^-1. These results are interpreted in terms of the enhanced nonplanarity of the radical cations as compared with the corresponding neutral complexes. The interaction between porphyrin a_2u and iron d_x²-y² orbitals in the radical cations, which is expected to be strengthened in the deformed structure, could be one of the reasons for the conformational change.