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

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 temperatures. In complex 3, T2s decrease as the four-peak pyrrole proton pattern collapses and increase again when the collapse is complete. This behavior has been attributed to chemical exchange induced by the rotation of 2-methylimidazole ligands. In complexes 1 and 2, the decrease in both T1s and T2s at high temperatures is attributed to equilibrium between low-spin and high-spin complexes induced by dissociation of imidazole ligands in the TPP complexes. In all complexes, T2s are considerably shorter than T1s. Relaxation times in the TMP complex are generally larger than the corresponding values for the TPP complexes. The temperature dependence of the chemical shift follows the Curie law in complex 3 and is close to Curie behavior in complexes 1 and 2, with slight deviations at high temperatures in the two latter complexes attributed to the low spin-high spin equilibrium. The NOE buildup curve for the pair of NOE-exhibiting pyrrole protons of complex 3 has been measured; the rate of NOE buildup has been found to be consistent with theoretical prediction based on the Stokes-estimated rotational correlation time and interproton distance measured from the MM2-minimized structure. A method has been proposed to predict the detectability of the NOE between a pair of structurally rigid protons in similar complexes, as well as to predict optimum detection conditions. Contrary to previous studies, no NOE is detected between pyrrole protons of 1 and 2, and this fact is justified and discussed in light of our findings for complex 3