Remarkable Enhancement in Extraction of Trivalent -Block Elements Using a Macrocyclic Ligand with Four Diglycolamide Arms: Synthesis, Extraction, and Spectroscopic and Density Functional Theory Studies.

A multiple diglycolamide (DGA)-containing ligand having four DGA arms tethered to a tetraaza-12-crown-4 ring, viz. 2,2',2'',2'''-(((1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetrakis(2-oxoethane-2,1-diyl)) tetrakis (oxy)) tetrakis(-dioctylacetamide) (T12C4ODGA), was synthesized and evaluated for the extraction of different actinide and lanthanide ions, viz. Am, Eu, Pu, Np, and UO. The extraction efficiency of the present ligand was found to be the highest reported so far, more specifically for the trivalent metal ions Am and Eu, when one considers the very low ligand concentration used in the present study, compared to that of the various previously reported multiple DGA-based ligands. The nature of the complexes formed during the extraction of Eu was investigated using time-resolved fluorescence (TRFS) and extended X-ray absorption fine structure (EXAFS) spectroscopy. Both the solvent extraction and TRFS studies indicated the presence of 1:1 and 1:2 complexes during the extraction of Am and Eu having three inner-sphere water molecules in the 1:1 complex. Density functional theoretical (DFT) studies were performed on the Am and Eu complexes of both T12C4ODGA and an analogous compound having methyl groups in place of the -octyl groups, and the DFT results of the T12C4ODGA nicely explain the extraction behavior of Am and Eu.