Coordination Chemistry of U(Ⅵ) and N-tris(2-aminoethyl)amino-N',N',N'',N'',N''',N'''-hexaacetic Acid in Aqueous Solution

Uranium, as one of the actinide metal elements with relatively high abundance in nature, its natural existence forms are of great significance for research on environmental and chemical behaviors. Typically, it exists in the +6 oxidation state as \mathrmUO_2^2+ , especially in aqueous solutions under aerobic conditions. N-tris(2-aminoethyl)amine-N',N',N'',N'',N''',N'''-hexaacetic acid (TTAHA, H₆L), as a typical amino polycarboxylic acid ligand, exhibits rich coordination ability and diversity due to its multiple coordinating groups. It can form stable coordination complexes with different metal ions through various binding modes, depending on their charge, radius, and other characteristics. However, based on a review of current literature, although the TTAHA ligand has been widely studied with various metal ions, detailed chemical research on its coordination mechanism with uranium has not been systematically reported, and this field remains in the exploratory stage. In this paper, potentiometry was used to determine the seven protonation constants of TTAHA at I_\mathrmN\mathrma\mathrmC\mathrml\mathrmO_4 =1.0 mol/L. Subsequently, a combination of potentiometry and spectrophotometry identified five U(Ⅵ)-TTAHA complexes, namely (UO₂)₂LH₂, (UO₂)₂LH⁻, UO₂LH³⁻, UO₂L⁴⁻, and UO₂L(OH)⁵⁻. The stability constants and molar absorption spectra of these complexes were calculated. The structures of the U(Ⅵ)-TTAHA complexes were inferred using Raman spectroscopy. The (UO₂)₂LH⁻ complex adopts O-N-O tridentate coordination, involving two oxygen atoms from two carboxylic groups and one nitrogen atom from the amine group. Furthermore, the UO₂LH³⁻ complex exhibits O-N-N-O tetradentate coordination. In summary, the systematic data obtained in this study, including protonation constants, the precise chemical composition of each uranyl complex and their corresponding stability constants, as well as the molar absorption spectra of key representative complex species. The obtained data provide basic parameters for the study of the coordination interaction between TTAHA and uranium ions. The determination of the complex composition, stability constants, and coordination patterns provides experimental and theoretical basis for understanding the interaction mechanism between amino polycarboxylic acid ligands and uranium.