THERMODYNAMICS AND NMR-STUDIES OF DTPA-BIS(METHOXYETHYLAMIDE) AND ITSDERIVATIVES - PROTONATION AND COMPLEXATION WITH LN(III)

The thermodynamic parameters (Delta G, Delta H, Delta S) of protonatio n and of complexation of trivalent La, Nd, Gd, Dy and Lu with DTPA-bis (methoxyethylamide) (MEA), DTPA-bis(methylmethoxyethylamide) (MEA) and DTPA-bis(hydroxyethylmethoxyethylamide) (HEMEA) have been measured by potentiometric and calorimetric titration at I=0.1 M and 25 degrees C . Especially in the first two protonation steps, there are significant differences in the pK(a) and the protonation enthalpy among the three DTPA-bis( amides) and DTPA. The fractional proton population determin ed by NMR shifts for each basic site of MEA and MMEA is compared with the thermodynamic parameters of those compounds and discussed in terms of the role of NH and CO of the amide groups in the protonation. The complexation parameters are compared with other Ln-polyaminopolycarbox ylate complexes to understand the role of the carboxylate, amino and m ethoxyethylamide groups of the DTPA-bis( amides). The luminescence lif etime of the Eu(III) complexes of the bisamide ligands indicated the c oordination of a single water molecule to the metal in the aqueous com plexes which is evidence that the DTPA-bis(amide) ligands have octaden tate coordination with the Ln(III) ions.