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 b via spacers of different lengths and with varying alkyl substituents on the amidic nitrogen of DGA (L-I-L-IV), 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-III (NMe) vs L-II (NH)] at 1 M HNO3. 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-Eu value [L-I (C3) vs L-II (C2)]. Slope analyses showed that Eu' was extracted as a bis-solvated species with all four ligands. The compositions of the Eu3+/L complexes were further confirmed by spectroscopic measurements, its formation constants following the order: L-III > L-IV > L-I > L-II . Luminescence spectroscopy and electrospray ionization mass spectrometry revealed that all four ligands form [Eu(L)(2)(NO3)(3)] complexes. Density functional theory and thermodynamic parameters corroborated the existence of [Eu(L)(2)(NO3)(3)] complexes.