Stoichiometric Lanthanide Compounds with Diglycolamides: A Synthetic Approach toward Understanding Rare-Earth...

A fundamental understanding of coordination chemistry across the lanthanide series is essential for explaining the chemical behavior of rare-earth metals in complex liquid-liquid extraction processes. Probing the exact bonding between the extractant and the metal is sometimes done through the synthesis of solid-state compounds that can serve as models for metal speciation in solution. In the case of diglycolamide (DGA), a commonly used neutral diamide extractant, extensive studies identify the stepwise formation of 1:1 [Ln(DGA)(H₂O)₆]³⁺, 1:2 [Ln(DGA)₂(H₂O)₃]³⁺, and 1:3 [Ln(DGA)₃]³⁺ complexes in solution. The crystallographic reports, however, exclusively show a 1:3 [Ln(DGA)₃]³⁺ moiety in the solid state, while the structures of 1:1 and 1:2 complexes are yet to be isolated and comprehensively studied. In this work, we report the synthesis and characterization of three new families of stoichiometric N,N,N',N'-tetramethyldiglycolamide (TMDGA) compounds: [Ln(TMDGA)(H₂O)₅Cl]Cl₂·2H₂O, [Ln(TMDGA)₂(H₂O)₃]Cl₃·3H₂O, and [Ln(TMDGA)₃]Cl₃·7H₂O, where Ln = Nd, Eu, Gd, with Ln:TMDGA ratios of 1:1, 1:2, and 1:3, respectively. The compounds have been analyzed using vibrational spectroscopy (both Raman and FT-IR), as well as variable temperature fluorescence spectroscopy. A spectrophotometric titration of Eu(III) was performed to confirm the presence of the [Eu(TMDGA)]³⁺ (n = 1, 2, and 3) species in solution and to compare the individual solid-state emission spectra to their respective analogues in solution.