Abstract
The polyaminocarboxylate anions have been identified as compounds of interest in the operations of actinide separation, in actinide migration in the environment and in human radiotoxicology. The structural characterization of complexes formed between actinides and polyaminocarboxylates ligands is essential for a better understanding of actinide-ligands interactions. Among the polyaminocarboxylate anions, the DOTA ligand (1,4,7,10-tetraazacyclododecane tetraacetic acid) is described as a very strong complexing agent of the lanthanides(III), but has been little studied with actinides. The objective of this thesis is to describe the complexes formed between the actinides (III) and (IV) and the DOTA ligand, and compare them with the lanthanide complexes. For this, an approach has been introduced to characterize the complex by complementary analytical techniques (spectrophotometry, electrospray ionization mass spectrometry, NMR, EXAFS, electrochemistry), but also by calculations of theoretical chemistry to help the interpretation of the experimental data. The formation of a 1:1 complex is observed with the actinides(III) (plutonium and americium) as in the case of lanthanides(III): rapid formation of intermediate species which evolves slowly towards the formation of a limit complex. Within this complex, the cation is located inside the cavity formed by the ligand. Four nitrogen atoms and oxygen atoms from the carboxylate functions are involved in the coordination sphere of the cation. However, differences were observed in the bond lengths formed between the cation and the nitrogen atoms (the bonds are somewhat shorter in the case of actinide complexes) as well as the complexation kinetics, which is slightly faster for the actinides(III) than for lanthanide(III) ions of equivalent radius. The same behavior was observed in solution upon complexation of actinides(IV) (uranium, plutonium and neptunium): slow formation of a 1:1 complex (actinide(IV):ligand) in wherein the cation is located inside the cavity formed by the macrocycle DOTA, although the kinetics of actinide(IV) complexation is slower than for the actinides(III) complexation. Nevertheless, the study of thorium(IV)-DOTA complexes shows differences since 1:1 and 1:2 complexes in solution are detected, and where only the carboxylate functions are involved in the coordination sphere of the cation. Finally, an initial estimate of the stability constant of the plutonium(IV)-DOTA complexes by electrochemical measurements indicates that complexes of actinide(IV) are approximately 10 orders of magnitude more stable than the complex of actinides(III), as previously observed with other polyaminocarboxylate anions.