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)C l]SbCl6 is estimated to be 14.8 kcal mol(-1) (72 degrees 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 ob served 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 acros s the porphyrin ring; the latter process is verified because the, activatio n free energies obtained from the line shape analysis of the meta and methy l signals are quite close, 18.1 (113 degrees C) and 17.9 (113 degrees C) kc al mol(-1), respectively. Thus, the inversion barrier in [Fe(OETPP)Cl]SbCl6 is supposed to be much greater than 18 kcal mol(-1). Since the inversion b arrier in the corresponding neutral porphyrin complex [Fe(OETPP)Cl] is repo rted to be 15.8 kcal mol(-1), the barrier has increased by much greater tha n 2 kcal mol(-1). These results are interpreted in terms of the enhanced no nplanarity of the radical cations as compared with the corresponding neutra l complexes. The interaction between porphyrin a(2u) and iron d(x2-y2) orbi tals in the radical cations, which is expected to be strengthened in the de formed structure, could be one of the reasons for the conformational change .