EQUILIBRIUM AND KINETIC-STUDIES ON COMPLEXES OF L]-1,4,7,10-TETRAAZACYCLODODECANE-1,4,7-TRIACETATE

Complexation properties of the ligand ]-1,4,7,10-tetraazacyclododecane -1,4,7-triacetatic acid (D03A-B) were studied and compared with those of yl)-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (HP-D03A) a nd ,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA). Tne protonation constants of D03A-B (K-i(H)) and the stability constants (K) of the complexes formed with Ca2+, Sr2+, Ba2+, Zn2+, Cu2+, Fe3+, C e3+, Nd3+, Eu3+, Gd3+, Dy3+, Tm3+ and Lu3+ were determined in differen t media (I=0.1 M; 25 degrees C). The first protonation constants (log K-1(H)) in Me(4)NCl, KCl and NaCl were found to be 11.75, 11.27 and 9. 46, respectively, indicating the formation of Na+ and weaker K+ comple xes. The complexes of lanthanides, alkaline earths and Zn2+ form slowl y and the complexation equilibria could be studied by means of an out- of-cell technique. The stabilities of the complexes Ln(D03A-B) increas e from La to Eu while the log K values are practically constant for th e heavier lanthanides. The stability constants of the D03A-B complexes of Ce, Gd and Lu are 1-2 orders of magnitude lower than those of the HP-D03A complexes. The coordinated alcoholic hydroxy groups dissociate at relatively low pH. The dissociation constant (K-d) obtained pH-met rically for Gd(D03A-B) (pK(d) = 9.48) is about 100 times higher than t hat for Gd(HP-D03A) (pK(d) = 11.36), indicating the higher basicity of the alcoholic oxygen in HP-D03A, which may contribute to the larger s tability constants of the complexes formed with HP-D03A. The kinetic s tability of the complexes Gd(D03A-B) and Gd(HP-D03A) were studied by s pectrophotometry in the pH range 3.2-5.3 by following the exchange rea ctions between the complexes and EU(3+). The rates of the exchange rea ctions proved to be linearly proportional to the H+ concentration. Thi s was interpreted in terms of the rate-determining role of the rearran gement and dissociation of the monoprotonated complexes. The rate cons tants obtained for the proton-assisted dissociation of Gd(D03A-B) and Gd(HP-D03A) were (2.8 +/- 0.1) x 10(-5) and (2.6 +/- 0.1) x 10(-4) M(- 1) s(-1), respectively.