Proton- and lanthanide-coordinating properties of the tetraaza macrocycle (EDDA-KA)2 in aqueous solution

Abstract

The new potentially multidentate ligand (EDDA-KA)2 has been prepared and its acid-base properties determined. Its coordination properties with three lanthanide ions (La3+, Gd3+, and Lu3+) and Fe3+ have been studied in aqueous solution (I = 0.16 M NaCl, 25°C) by potentiometric pH, spectrophotometric, and 1H NMR titrations. In total, eight pKa values could be determined by these diverse methods. From the 1H NMR titrations, all the deprotonation steps could be determined and assigned (in groups). Strong hydrogen bonding among two nitrogen atoms and one oxygen atom on the pyrone ring was observed, and the corresponding pKa values were estimated to be 13.2 and 14.0. By comparison with similar systems, an equilibrium constant for the formation of the hydrogen bond was estimated, KI = 104.4. Eight stability constants in total were determined for various protonation states of the La3+ and Gd3+ complexes, but for Lu3+ only the last four constants were obtained. The species distribution diagram showed that [M(EDDA-KA)2-4H]2- predominated in the range 5 [Formula: see text] pH [Formula: see text] 9 and [M(EDDA-KA)2-6H]4- predominated in the region 9 [Formula: see text] pH [Formula: see text] 11. In the former complex, two strong hydrogen bonds anchored the macrocycle and Ln3+ was coordinated with the four carboxylate groups; in the latter, the two hydrogen bonding protons were absent and the metal ion might also bind to the nitrogens on the macrocycle. The stability of Gd3+-(EDDA-KA)2 is lower than the analogous Gd3+ complexes of DOTA, DTPA, and DTPA-BMA.Key words: acidity constants, stability constants, La3+, Gd3+, Lu3+, Fe3+, amino carboxylate chelator.