High-spin (meso-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 C_meso-C_α bonds. The activation free energies for rotation have been determined as 8.0 (-72 °G), 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.