Complexation and Crystal Structures of Trivalent Lanthanides with Cyclic Diglycolamide Exhibiting High Extraction Efficiency at Low Ligand Inventory

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.