MULTINUCLEAR MAGNETIC-RESONANCE STUDY OF THE STRUCTURE AND DYNAMICS OF LANTHANIDE(III) COMPLEXES OF THE BIS(PROPYLAMIDE) OF DIETHYLENETRIAMINEPENTAACETIC ACID IN AQUEOUS-SOLUTION

The structure and dynamics of the lanthanide(III) complexes of the bis(propylamide) of diethylenetriaminepentaacetate (DTPA-PA2) in aqueous solution have been investigated by C-13 and O-17 NMR. With the use of the longitudinal C-13 relaxation times of the Nd(III) complex it is shown that the organic ligand is bound to the lanthanide(III) ion in an octadentate fashion via the three nitrogens of the diethylenetriamine backbone, the three carboxylate groups, and the two amide oxygens. O-17 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 C-13 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-coordination. 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 Ln(III) diethylenetriaminepentaacetate (DTPA) complexes.