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 measurements on the Dy-III complexes showed that, in both complexes, the first 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-DTPA-EN) is similar to that of the acyclic DTPA-bis(amide) derivatives. However, as a result of the cyclic nature of the ligands considered, the number of isomers in solution is lower than for the acyclic compounds. 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) ligand is present in two rapidly interconverting isomers, whereas the cy(DTPA-EN-DTPA-EN) ligand exists in four isomeric forms. Two types of exchange processes are observed for the cy(DTPA-EN-DTPA-EN) complexes; one is fast on the NMR time scale and does not require decoordination of the ligand, and the second is relatively slow and decoordination is necessary to realize the interconversion. The complexes of cy(DTPA-EN) and the lighter Ln(III) ions (Ln = La --> Eu) precipitated, probably due to the formation of binuclear complexes. Comparisons are made with the previously studied acyclic DTPA-bis(amides).