¹H NMR Studies of Paramagnetic Metalloporphyrin Complexes:  Rotation of Axial Ligands, Proton Relaxation, and NOE Detectability.

Abstract:

For three low-spin (S=1/2) iron(III) six-coordinated porphyrin complexes, the temperature dependences of relaxation times and chemical shifts have been studied.  At low temperatures, similar trends have been observed in all three complexes.  At higher temperatures, two of the complexes behave differently from the third, indicating the presence of different chemical exchange processes.  In the first two complexes, chemical exchange is with the high-spin (S=5/2) forms of those complexes and is induced by the dissociation of the axial ligands.   In the third complex, the chemical exchange is a four-site cyclic exchange between inequivalent pyrrole protons and is induced by the synchronous rotation of the axial ligands.  The rate and activation parameters of the rotation have been measured by two different NMR methods.  The observed temperature dependences have been used to develop a method to estimate the detectability of NOE between structurally rigid protons in these complexes.