KINETICS OF FORMATION AND DISSOCIATION OF LANTHANIDE(III)-DOTA COMPLEXES

The formation rates of Ce(DOTA)-, Eu(DOTA)-, and Yb(DOTA)- have been s tudied at 25-degrees-C and I = 1.0 M (NaC1) by spectrophotometry (Ce3, Eu3+) and an indicator method (Yb3+). (H4DOTA = 10-tetraazacyclodode cane-N,N',N'',N'''-tetraacetic acid). In the formation reaction, a dip rotonated intermediate, Ln(H2DOTA)+, is formed very rapidly and slowly rearranges to the product. The intermediates Ce(H2DOTA)+ and Eu(H2DOT A)+ have been detected by spectrophotometry. The stability constants o f the intermediates were determined by pH-metry and spectrophotometry. H-1-NMR studies indicate that, in the diprotonated intermediates, onl y the carboxylate groups are coordinated to the metal ions. The rearra ngements of the intermediates occuring by the loss of two protons are OH- ion catalyzed processes. The rate constants, k(OH), obtained for C e3+, Eu3+, and Yb3+ are (3.5 +/- 0.06) x 10(6), (1.1 +/- 0.07) x 10(7) , and (4.1 +/- 0.4) x 10(7) M-1 s-1, respectively. The dissociation of Gd(DOTA)- and Eu(DOTA)- is catalyzed by H+ ions. In the pH range of 3 .2-5.0 the dissociation of Gd(DOTA)- occurs via the formation of a mon oprotonated complex. The rate constant for the proton-assisted dissoci ation is k1' = (2.0 +/- 0.1) x 10(-5) M-1 s-1. In the interval 0.018 M < [H+] < 2.5 M, the dissociation of Eu(DOTA)- proceeds through the fo rmation of mono- and diprotonated intermediates. The protonation const ants of Gd(DOTA)- and HGd(DOTA) are K1 = 14 +/- 1 and K2 = 0.12 +/- 0. 03, while the rate constants characteristic for their dissociation are (1.0 +/- 0.5) x 10(-6) and (6.2 +/- 0.5) x 10(-4) s-1, respectively.