Proton NMR Relaxation in Six-Coordinate Low-Spin Iron(III) Tetraphenylporphyrinates: Temperature Dependence of Proton Relaxation Rates and Interpretation of NOESY Experiments

Abstract

The temperature dependence of longitudinal and transverse relaxation times (T1 and T2) has been studied for the pyrrole protons of [(p-Cl)3(p-NEt2)TPPFe(III)(N-MeIm)2]Cl (1), [(p-Cl)(p-NEt2)3TPPFe(III)(N-MeIm)2]Cl (2), and [TMPFe(III)(2-MeImH)2]Cl (3), where TMP = tetramesitylporphyrin and TPP = tetraphenylporphyrin, in the temperature range 190−310 K. All three complexes are paramagnetic and have electron spin S = 1/2. Up to 273 K, all complexes exhibit four distinct pyrrole proton signals, with the asymmetry caused by unsymmetrical substitution in complexes 1 and 2 and by axial ligands fixed in a definite orientation in complex 3. Above 273 K the four-peak pattern in complex 3 collapses into a single peak due to fast synchronous rotation of axial ligands. At low temperatures, T1s and T2s in all complexes increase as temperature increases. At higher temperatures, T1s continue to increase and equalize in complex 3, but decrease in complexes 1 and 2. T2s in complexes 1 and 2 mimic the T1s at all temperatur...