Equilibrium and kinetic studies on the formation of the lanthanide(III) complexes, [Ce(dota)]– and [Yb(dota)]– (H4dota = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)

Abstract

The protonation constants (K _i^H) of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (H₄dota) were redetermined at 25 °C in 0.1 M NMe₄Cl. The log K_i ^H values (i = 1–5) are 12.6, 9.70, 4.50, 4.14 and 2.32, respectively. The stability constants of [Ce(dota)]^– and [Yb(dota)]^– were determined by pH-potentiometry as log K_CeL = 24.6 and log K_YbL = 26.4. The formation rates of [Ce(dota)]^– and [Yb(dota)]^– were studied by the stopped-flow method at much higher pH values than before. In the range pH 3.5–9.3 the spectra were interpreted in terms of the formation of diprotonated, [Ce(H₂dota)]⁺, and (at around pH > 8) monoprotonated, [Ce(Hdota)], intermediates. These two species are characterized by the same spectra (and structure). At pH < 7.5 the formation rates of the complexes were directly proportional to the OH^– concentration. The rate constants, k_OH, characterizing the formation of [Ce(dota)]^– and [Yb(dota)]^– are 2.7 × 10⁵ and 9.3 × 10⁷M^–1 s^–1, respectively. However, at pH > 7.5 the order of reaction in [OH^–] is higher than one. The results were interpreted in terms of rate-controlling deprotonation of the monoprotonated intermediate [Ln(Hdota)] followed by rearrangement of the deprotonated intermediate into the product. The deprotonation is a general base-catalysed process, which occurs with the assistance of a H₂O molecule at pH < 7.5. At higher pH values the OH^–-assisted deprotonation of the intermediate, as another pathway, increases the formation rate of the complexes. By assuming these reaction pathways a general rate expression was deduced and it was shown that k_OH = k^H_LnHL/K ^H_LnHLK_w, where K ^H_LnHL is the protonation constant of the monoprotonated intermediates [Ce(Hdota)] and [Yb(Hdota)], K ^H_CeHL = 4.4 × 10⁸, K ^H_YbHL = 2.5 × 10⁸ and k_LnHL is the rate constant, characterizing the H₂O-assisted deprotonation of the intermediates, k_CeHL = 18.5 and k_YbHL = 245 s^–1. At pH > 8 the OH^–-assisted deprotonation of the monoprotonated intermediate needs to be considered in the formation rates of [Ce(dota)]^–; the rate constant for this pathway is k^OH_CeHL = 1.9 × 10⁷M^–1 s^–1.