Highly Efficient N-Pivot Tripodal Diglycolamide Ligands for Trivalent f-Cations: Synthesis, Extraction, Spectroscopy, and Density Functional Theory Studies.

A series of four N-pivot tripodal diglycolamide (DGA) ligands, where three DGA moieties are attached to the central N atom via spacers of different lengths and with varying alkyl substituents on the amidic nitrogen of DGA (L-L), were studied for their extraction and complexation ability toward trivalent lanthanide/actinide ions, including solvent extraction, complexation using spectrophotometric titrations, and luminescence spectroscopic studies. Introduction of a methyl group on the amidic nitrogen atom gives rise to a 400 fold increase of the Eu distribution ( D) value [L (NMe) vs L (NH)] at 1 M HNO. Enlargement of the spacer length between the pivotal N atom and the DGA moieties with one carbon atom results in a 14 times higher D value [L (C3) vs L (C2)]. Slope analyses showed that Eu was extracted as a bis-solvated species with all four ligands. The compositions of the Eu/L complexes were further confirmed by spectroscopic measurements, its formation constants following the order: L > L > L > L. Luminescence spectroscopy and electrospray ionization mass spectrometry revealed that all four ligands form [Eu(L)(NO)] complexes. Density functional theory and thermodynamic parameters corroborated the existence of [Eu(L)(NO)] complexes.