The protonation constants (KiH) 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 KiH 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 KCeL = 24.6 and log KYbL = 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, kOH, characterizing the formation of [Ce(dota)]- and [Yb(dota)]- are 2.7 × 105 and 9.3 × 107 M-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 kOH = kLnHLH/KLnHLHKw, where KLnHLH is the protonation constant of the monoprotonated intermediates [Ce(Hdota)] and [Yb(Hdota)], KCeHLH = 4.4 × 108, KYbHLH = 2.5 × 108 and kLnHL is the rate constant, characterizing the H2O-assisted deprotonation of the intermediates, kCeHL = 18.5 and kYbHL = 245 s-1. At pH > 8 the OH-8-assisted deprotonation of the monoprotonated intermediate needs to be considered in the formation rates of [Ce(dota)]-; the rate constant for this pathway is kCeHLOH = 1.9 × 107 M-1 s-1.
|Number of pages||6|
|Journal||Journal of the Chemical Society - Dalton Transactions|
|Publication status||Published - Jan 21 1998|
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