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

The formation rates of Ce(DOTA)-, Eu(DOTA)-, and Yb(DOTA)- have been studied at 25-degrees-C and I = 1.0 M (NaC1) by spectrophotometry (Ce3+, Eu3+) and an indicator method (Yb3+). (H4DOTA = 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid). In the formation reaction, a diprotonated intermediate, Ln(H2DOTA)+, is formed very rapidly and slowly rearranges to the product. The intermediates Ce(H2DOTA)+ and Eu(H2DOTA)+ have been detected by spectrophotometry. The stability constants of the intermediates were determined by pH-metry and spectrophotometry. H-1-NMR studies indicate that, in the diprotonated intermediates, only the carboxylate groups are coordinated to the metal ions. The rearrangements of the intermediates occuring by the loss of two protons are OH- ion catalyzed processes. The rate constants, k(OH), obtained for Ce3+, 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 monoprotonated complex. The rate constant for the proton-assisted dissociation 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 formation of mono- and diprotonated intermediates. The protonation constants 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.