Complexing mechanism of the lanthanide cations Eu3+, Gd3+, and Tb3+ with 1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane (dota)-characterization of three successive complexing phases: study of the thermodynamic and structural properties of the complexes by potentiometry, luminescence spectroscopy, and EXAFS |
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Authors: | Moreau Juliette Guillon Emmanuel Pierrard Jean-Claude Rimbault Jean Port Marc Aplincourt Michel |
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Institution: | GRECI, Université de Reims Champagne-Ardenne, BP1039, 51687 Reims Cedex 2, France. |
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Abstract: | Complexation of the lanthanides Eu3+, Gd3+, and Tb3+ 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 LnHn(dota)](n-1)+** and intermediate LnHn(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 LnHn(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 LnHn(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. |
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