The structure and dynamics of the lanthanide(II1) complexes of the bis(propy1amide) of diethylenetriaminepentaacetate (DTPA-PA3 in aqueous solution have been investigated by I3C and I7O NMR. With the use of the longitudinal 13C relaxation times of the NdlI1 complex it is shown that the organic ligand is bound to the lanthanide(II1) ion in an octadentate fashion via the three nitrogens of the diethylenetriamine backbone, the three carboxylate groups, and the two amide oxygens. I7O NMR measurements show that the coordination sphere is completed by one water ligand. The inversion of the lanthanide-bound nitrogens is precluded, and therefore four diastereomeric pairs of isomers are possible for these complexes. Low-temperature I3C NMR spectra show that all these isomers occur in solution. Three dynamic processes have been identified by means of variable-temperature NMR measurements: (i) rapid rotations of the carboxylate groups, (ii) racemization at the middle N atom via interconversions between the two possible conformations of the ethylene bridges, and (iii) racemization at the terminal N atoms of the diethylenetriamine backbone via decoordination-inversion-cmrdination. The barriers of the interconversions between the enantiomers have been determined by line-shape analysis and from coalescence temperatures. A comparison is made with the LnlI1 diethylenetriaminepentaacetate (DTPA) complexes.

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The lanthanide coordination of the macrocyclic ligands cy(DTPA-EN) and cy(DTPA-EN-DTPA-EN) was studied in aqueous solution. 17O NMR measurements on the DyIII complexes showed that, in both complexes, the first coordination sphere of the LnIII ion contains one water molecule, leaving eight coordination sites for the ligand molecule. 89Y and 139La 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 1H and 13C NMR data, we conclude that, in the respective LnIII 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 LnIII ions (Ln = La → Eu) precipitated, probably due to the formation of binuclear complexes. Comparisons are made with the previously studied acyclic DTPA-bis(amides).

NMR shift and relaxation measurements show that the lanthanide(iii) complexes of the bis(propy1amide) of diethylenetriaminepentaacetic acid (H3L) occur in at least eight isomeric forms, in which the metal ion is coordinated