High-Spin (<i>meso</i>-Tetraalkylporphyrinato)iron(III) Complexes As Studied by X-ray Crystallography, EPR, and Dynamic NMR Spectroscopies

1H NMR spectra of a series of high-spin (meso-tetraalkylporphyrinato)iron(III) chlorides, [Fe(TRP)Cl] where R = Me, Et, Pr, or iPr, have been measured at various temperatures in CD2Cl2 solution. In the case of the Et, Pr, and iPr complexes, either the methyl or the methylene signal split into two signals with equal integral intensities at low temperature. In contrast, the Me complex did not show any splitting even at −100 °C. The results have been ascribed to the hindered rotation of the meso-alkyl groups about Cmeso−Cα bonds. The activation free energies for rotation have been determined as 8.0 (−72 °C), 8.5 (−60 °C), and 8.9 (−62 °C) kcal·mol-1 for the Et, Pr, and iPr complexes, respectively, at coalescence temperatures given in parentheses. The small activation free energy for rotation of the isopropyl groups observed in the present system is explained in terms of the nonplanarity of the porphyrin ring, which has been verified both by the X-ray crystallographic analysis and by the EPR spectrum taken in a frozen CH2Cl2−toluene solution. The success in observing the hindered rotation of less bulky primary alkyl groups such as ethyl and propyl groups at an easily accessible temperature range is attributed to the large difference in chemical shifts of the mutually exchanging protons, ca. 3500 Hz in the case of the Et complex, caused by the paramagnetism of the five-coordinated ferric porphyrin complexes.