Complexation and Crystal Structures of Trivalent Lanthanides with Cyclic Diglycolamide Exhibiting High Extraction...

Tetracyclohexyldiglycolamide (TCHDGA) was synthesized using a two-step method. It demonstrated extraordinarily high extraction capability for trivalent lanthanides with a very low ligand inventory. For the first time, the ML3 complexes of Eu-TCHDGA and Nd-TCHDGA have been synthesized, and their crystal structure parameters are reported. The slope ratio method indicated the predominant formation of [Eu(TCHDGA)3][NO3]3 in the organic phase. UV-visible investigations revealed the formation of ML, ML2, and ML3 as the predominant species, with complexation constants of Logβ1 = 3.79, Logβ2 = 7.60, and Logβ3 = 11.2, respectively. In contrast, photoluminescence investigations indicated the formation of similar species with complexation constants higher than those of Nd3+, attributed to lanthanide contraction. The asymmetry factor of the ML3 species was 1.3, with no inner-sphere water molecules having J.O. parameters: Ω2 = 2.14 × 10-20 and Ω4 = 1.26 × 10-20, respectively. The extraction efficiency of TCHDGA for trivalent lanthanides was found to be atleast two to three order magnitude higher than several diglycolamides reported in the literature (the highest D values for trivalent lanthanides reported so far). The radiolytic stability of the present solvent system was also found to be superior compared to the most prominent tetra-n-octyldiglycolamide (TODGA) ligand.