Kinetics and mechanisms of formation of the lanthanide(III)-trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetate complexes†
Abstract
The rates of formation of the DCTA complexes of Ce3+, Eu3+ and Yb3+ are significantly lower than expected on the basis of the water-exchange rates of Ln3+(aq). The kinetics of formation of these complexes were studied by means of a stopped-flow method. The reactions were found to be first order in the reactants, and the formation of monoprotonated and diprotonated intermediates was detected. The reactions occur through the deprotonation and rearrangement of the monoprotonated intermediates, which are in equilibrium with the diprotonated ones. The rate constants obtained for this pathway are 15 ± 0.2 s−1, 28 ± 1.3 s−1 and 144 ± 3 s−1 for the formation of Ce(DCTA)−, Eu(DCTA)− and Yb(DCTA)−, respectively. General base catalysis is not valid for the reactions, indicating a fast deprotonation of the intermediate. The rate-determining step is presumably the rearrangement of the monoprotonated intermediate, when Ln3+ enters into the coordination cage and displaces the