Abstract
Complexation of the lanthanides Eu^3+^, Gd^3+^, and Tb^3+^ with 1,4,7,10‐tetrakis(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane (dota) has been studied in solution by using potentiometry, luminescence spectrometry, and EXAFS. Three series of successive complexes were characterized by at least two of these methods: the immediate [LnH~n~(dota)]^(n−1)+^** and intermediate [LnH~n~(dota)]^(n−1)+^* complexes with 0≤n≤2, and the final [Ln(dota)]^−^ complexes. The formation constants of the intermediate and final complexes were determined by using potentiometry. From the results, a complexation mechanism involving three steps has been proposed. In the [LnH~n~(dota)]^(n−1)+^** complexes that are instantaneously formed, the lanthanide is bound to four oxygen atoms of the carboxylate groups and to five water molecules. These species evolve rapidly: the lanthanide moves into the macrocycle cavity, two new bonds are formed with two nitrogen atoms diametrically opposed in the tetraaza cycle and only three water molecules remain bound to the lanthanide in the [LnH~n~(dota)]^(n−1)+^* (0≤n≤2) complexes, which appear after a two‐day wait. These compounds are stable for about four days. After 4–8 weeks, a concerted rearrangement occurs which leads to the formation of thermodynamically stable [Ln(dota)]^−^ complexes in which the lanthanide is bound to four nitrogen atoms, four carboxylate oxygen atoms, and one water molecule.