Studies of lanthanide(III) pyridine-2,6-dicarboxylate complexes in aqueous solution. Part 1. Structures and 1H nuclear magnetic resonance spectra

Abstract

Conformations of lanthanide(III) pyridine-2,6-dicarboxylate (dipicolinate) complexes have been studied in solution by analysis of their 1H n.m.r. spectra. The structure of the tris complexes(Ln = any lanthanide) could be described as a tricapped-trigonal prism for cations in the second half of the series. The bis complexes are shown to exist in states of fast exchange between different conformations. The ligands are mobile on the surface of the complexes and the carboxylate groups move into different positions within the complexes. A decrease in the rate of exchange occurs with decreasing radius of the central ion, reaching a shallow minimum at the thulium(III) complex. Using the paramagnetic perturbation of the n.m.r. signals of different protons in these bis complexes, a structure for the dispositions of the dipicolinate anions similar to that expected for a tricapped-trigonal prism is obtained for all the different lanthanide(III) complexes. Investigation of the n.m.r. spectra of the mono complexes has indicated that he mobility of the ligand is higher than in the bis complexes, but that the disposition of the dipicolinate ligands is similar.