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 (T<INF>1</INF> and T<INF>2</INF>) has been studied for the pyrrole protons of [(p-Cl)<INF>3</INF>(p-NEt<INF>2</INF>)TPPFe(III)(N-MeIm)<INF>2</INF>]Cl (<BO>1</BO>), [(p-Cl)(p-NEt<INF>2</INF>)<INF>3</INF>TPPFe(III)(N-MeIm)<INF>2</INF>]Cl (<BO>2</BO>), and [TMPFe(III)(2-MeImH)<INF>2</INF>]Cl (<BO>3</BO>), where TMP = tetramesitylporphyrin and TPP = tetraphenylporphyrin, in the temperature range 190−310 K. All three complexes are paramagnetic and have electron spin S = ¹/<INF>2</INF>. Up to 273 K, all complexes exhibit four distinct pyrrole proton signals, with the asymmetry caused by unsymmetrical substitution in complexes <BO>1</BO> and <BO>2</BO> and by axial ligands fixed in a definite orientation in complex <BO>3</BO>. Above 273 K the four-peak pattern in complex <BO>3</BO> collapses into a single peak due to fast synchronous rotation of axial ligands. At low temperatures, T<INF>1</INF>s and T<INF>2</INF>s in all complexes increase as temperature increases. At higher temperatures, T<INF>1</INF>s continue to increase and equalize in complex <BO>3</BO>, but decrease in complexes <BO>1</BO> and <BO>2</BO>. T<INF>2</INF>s in complexes <BO>1</BO> and <BO>2</BO> mimic the T<INF>1</INF>s at all temperatures. In complex <BO>3</BO>, T<INF>2</INF>s 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 <BO>1</BO> and <BO>2</BO>, the decrease in both T<INF>1</INF>s and T<INF>2</INF>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<INF>2</INF>s are considerably shorter than T<INF>1</INF>s. 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 <BO>3</BO> and is close to Curie behavior in complexes <BO>1</BO> and <BO>2</BO>, 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 <BO>3</BO> 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 <BO>1</BO> and <BO>2</BO>, and this fact is justified and discussed in light of our findings for complex <BO>3</BO>.