DYNAMIC NMR CONFORMATIONAL STUDIES OF RUTHENIUM(II) TETRAAZAPORPHYRINOGEN COMPLEXES

A series of Ru(II) tetraaazaporphyrinogen complexes with heteroaromatic axial ligands have been prepared in order to study their dynamic conformational behavior by variable-temperature NMR spectroscopy. Axial ligand rotation and macrocyclic interconversion proceed slowly at temperatures which depend mainly on the steric interactions occurring between the macrocycle and the axial ligands, both processes being concomitant. The free energies of activation increase in the following order: alpha-unsubstituted five-membered heterocycles < alpha-unsubstituted six-membered heterocycles < alpha-substituted five-membered heterocycles. The structure of the complexes when the dynamic processes are restricted on the NMR scale has been shown to be such that the macrocycle has a double saddle-shaped conformation. The axial heterocyclic ligands are perpendicular to the macrocycle and perpendicular to each other.