SYNTHESIS AND METAL-COMPLEX SELECTIVITY OF MACROCYCLIC DTPA AND EDTA BIS(AMIDE) LIGANDS

Several macrocyclic bis(amide) derivatives of DTPA and EDTA were prepared. Protonation constants as well as Gd3+ and Zn2+ complex stability constants of these ligands were determined by potentiometric titrations. The stability constants of the Gd3+ complexes increase with increasing ring size of the DTPA bis(amide) macrocycle series, reflecting the enhanced participation of the amide carbonyl oxygens in metal ion coordination. On the other hand, the stability constants of the Zn2+ complexes do not exhibit any particular trend throughout the series. The logarithmic selectivity constant of the 15-membered EDTA-DAM ligand for Gd3+ over Zn2+ (the difference between logarithmic stability constants of Gd(EDTA-DAM) and Zn(EDTA-DAM)) is 6.12. This indicates that the ligand EDTA-DAM exerts by far the highest selectivity for Gd3+ over Zn2+ among known polyamino polycarboxylates, presumably through a more favorable structural discrimination by this ligand. In contrast, the structural isomer DTPA-EAM shows a less favorable selectivity for Gd3+ over Zn2+.