PROTON NMR RELAXATION IN 6-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 t imes (T-1 and T-2) has been studied for the pyrrole protons of [(p-Cl) (3)(p-NEt2)TPPFe(III)(N-Melm)(2)]Cl (1), [(p-Cl)(p-NEt2)(3)TPPFe(III)( N-MeIm)(2)]- (2), and [TMPFe(III)(2-MeImH)(2)]Cl (3), where TMP = tetr amesitylporphyrin and TPP = tetraphenylporphyrin, in the temperature r ange 190-310 K. All three complexes are paramagnetic and have electron spin S = 1/2. UP to 273 K, all complexes exhibit four distinct pyrrol e proton signals, with the asymmetry caused by unsymmetrical substitut ion in complexes 1 and 2 and by axial ligands fixed in a definite orie ntation in complex 3. Above 273 K the four-peak pattern in complex 3 c ollapses into a single peak due to fast synchronous rotation of axial ligands. At low temperatures, T(1)s and T(2)s in all complexes increas e as temperature increases. At higher temperatures, T(1)s continue to increase and equalize in complex 3, but decrease in complexes 1 and 2. T(2)s in complexes 1 and 2 mimic the T(1)s at all temperatures. In co mplex 3, T(2)s decrease as the four-peak pyrrole proton pattern collap ses and increase again when the collapse is complete. This behavior ha s been attributed to chemical exchange induced by the rotation of 2-me thylimidazole ligands. In complexes 1 and 2, the decrease in both T(1) s and T(2)s 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, T(2)s are considerably shorter than T(1)s. Relaxation times in the TMP complex are generally larger than the corresponding values for the TPP complexes. The tempe rature dependence of the chemical shift follows the Curie law in compl ex 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 theoreti cal prediction based on the Stokes-estimated rotational correlation ti me and interproton distance measured from the MM2-minimized structure. A method has been proposed to predict the detectability of the NOE be tween a pair of structurally rigid protons in similar complexes, as we ll as to predict optimum detection conditions. Contrary to previous st udies, 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.