MULTINUCLEAR MAGNETIC-RESONANCE STUDY ON THE STRUCTURE AND DYNAMICS OF LANTHANIDE(III) COMPLEXES OF CYCLIC DTPA DERIVATIVES IN AQUEOUS-SOLUTION

The lanthanide coordination of the macrocyclic ligands cy(DTPA-EN) and cy(DTPA-EN-DTPA-EN) was studied in aqueous solution. O-17 NMR measure ments on the Dy-III complexes showed that, in both complexes, the firs t coordination sphere of the Ln(III) ion contains one water molecule, leaving eight coordination sites for the ligand molecule. Y-89 and La- 139 NMR analysis confirmed that the coordination mode of cy(DTPA-EN-DT PA-EN) is similar to that of the acyclic DTPA-bis(amide) derivatives. However, as a result of the cyclic nature of the ligands considered, t he number of isomers in solution is lower than for the acyclic compoun ds. From variable-temperature H-1 and C-13 NMR data, we conclude that, in the respective Ln(III) complexes in solution, the cy(DTPA-EN) liga nd is present in two rapidly interconverting isomers, whereas the cy(D TPA-EN-DTPA-EN) ligand exists in four isomeric forms. Two types of exc hange processes are observed for the cy(DTPA-EN-DTPA-EN) complexes; on e is fast on the NMR time scale and does not require decoordination of the ligand, and the second is relatively slow and decoordination is n ecessary to realize the interconversion. The complexes of cy(DTPA-EN) and the lighter Ln(III) ions (Ln = La --> Eu) precipitated, probably d ue to the formation of binuclear complexes. Comparisons are made with the previously studied acyclic DTPA-bis(amides).