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

Abstract

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]SbC...