First Report on the Complexation of Actinides and Lanthanides Using 2,2′,2′′-(((1,4,7-Triazonane-1,4,7-triyl)tris(2-oxoethane-2,1-diyl)) tris(oxy)) tris(N, N-dioctylacetamide): Synthesis, Extraction, Luminescence, EXAFS, and DFT Studies

A novel tripodal diglycolamide ligand containing a triazamacrocycle center (2,2′,2′′-(((1,4,7-triazonane-1,4,7-triyl)tris(2-oxoethane-2,1-diyl)) tris(oxy)) tris(N,N-dioctylacetamide), abbreviated as T9C3ODGA) was synthesized and characterized by conventional techniques. The ligand resulted in efficient extraction of actinide/lanthanide ions yielding the trend: Eu³⁺ > Pu⁴⁺ > Am³⁺ > NpO₂ ²⁺ > UO₂ ²⁺ > Sr²⁺ > Cs⁺. Similar to most of the other diglycolamide (DGA) ligands, Eu³⁺ was preferentially extracted as compared to Am³⁺ the separation factor (D_Eu/D_Am) value at 3 M HNO₃ was ca. 4.2. In contrast, separation from UO₂ ²⁺ ion was less effective as compared to that of other tripodal DGA ligands studied earlier. Solvent extraction studies indicated extraction of species of the ML₂ (where L is T9C3ODGA) stoichiometry. The formation of an inclusion complex with no inner-sphere water molecule was confirmed from luminescence spectral studies. DFT computations predicted the presence of an inner-sphere nitrate ion in the most preferred complex, which was also supplemented by EXAFS and luminescence studies. The selectivity of T9C3ODGA could be explained on the basis of its more favorable interactions with Eu³⁺ as compared to those with Am³⁺ both in the gas and the solution phases.