High-spin (meso-Tetraalkylporphyrinato)iron(III) complexes as studied by X-ray crystallography, EPR, and dynamic NMR spectroscopies

H-1 NMR spectra of a series of high-spin (meso-tetraalkylpsrphyrinato)iron( III) chlorides, [Fe(TRP)Cl] where R - Me, Et, Pr, or Pr-i, have been measur ed at various temperatures in CD2Cl2 solution. In the case of the Et,Pr, an d Pr-i complexes, either the methyl or the methylene signal split into two signals with equal integral intensities at low temperature. In contrast, th e Me complex did not show any splitting even at -100 degrees C. The results have been ascribed to the hindered relation of the meso-alkyl groups about C-meso-C-alpha bonds. The activation free energies for rotation have been determined as 8.0 (-72 degrees C), 8.5 (-60 degrees C), and 8.9 (-62 degree s C) kcal . mol(-1) for the Et, Pr, and Pr-i complexes, respectively, at co alescence temperatures given in parentheses. The small activation free ener gy for rotation of the isopropyl groups observed in the present system is e xplained in terms of the nonplanarity of the porphyrin ring, which has been verified both by the X-ray crystallographic analysis and by the EPR spectr um taken in a frozen CH2Cl2-toluene solution. The success in observing the hindered rotation of less bulky primary alkyl groups such as ethyl and prop yl groups at an easily accessible temperature range is attributed to the la rge difference in chemical shifts of the mutually exchanging protons, ca. 3 500 Hz in the case of the Et complex, caused by the paramagnetism of the fi ve-coordinated ferric porphyrin complexes.