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

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 aqueo us solution (I = 0.16 M NaCl, 25degreesC) by potentiometric pH, spectrophot ometric, and H-1 NMR titrations. In total, eight pK(a) values could be dete rmined by these diverse methods. From the H-1 NMR titrations, all the depro tonation steps could be determined and assigned (in groups). Strong hydroge n bonding among two nitrogen atoms and one oxygen atom on the pyrone ring w as observed, and the corresponding pK(a) values were estimated to be 13.2 a nd 14.0. By comparison with similar systems, an equilibrium constant for th e formation of the hydrogen bond was estimated, K-I = 10(4.4). Eight stabil ity constants in total were determined for various protonation states of th e La3+ and Gd3+ complexes, but for Lu3+ only thelast four constants were ob tained. The species distribution diagram showed that [M(EDDA-KA)(2)-4H](2-) predominated in the range 5 less than or equal to pH less than or equal to 9 and [M(EDDA-KA)(2)-6H](4-) predominated in the region 9 less than or equ al to pH 11. In the former complex, two strong hydrogen bonds anchored the macrocycle and Ln(3+) 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, andDT PA-BMA.