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 proton from the NH⁺ group. Another reaction pathway is the OH⁻-assisted deprotonation of the monoprotonated intermediate, followed by the rate-controlling rearrangement of the deprotonated intermediate. The rate constants obtained for the formation of Ce(DCTA)⁻ and Eu(DCTA)⁻ are (6.6 ± 0.1) × 10⁸ M⁻¹ s⁻¹ and (2.2 ± 0.12) × 10⁹ M⁻¹ s⁻¹, respectively. The activation parameters for the reactions of Ce³⁺ and Eu³⁺, involving the transformation of the monoprotonated intermediates, proved to be practically equal, indicating the formation of similar transition states. The rate constants characterizing the formation of the complexes exhibit an increasing trend with increase in the atomic number of the lanthanide.