Complexation and Separation of Palladium with Tridentate 2,6-Bis-triazolyl-pyridine Ligands: Synthesis, Solvent Extraction, Spectroscopy, Crystallography, and DFT Calculations

Selective extraction of palladiumfrom high-level liquidwaste(HLLW) is desirable for the sustainable development of nuclear energyand resource recovery. In this work, three tridentate 2,6-bis-triazolyl-pyridineligands (L-I, L-II, and L-III) bearing different alkyl sidechains were synthesized and systematically studied for the complexationand extraction of palladium. Altering the alkyl side chains of theligands led to pronounced differences in extraction performance. Amongthe three ligands, L-II decorated with two n-octyl groups exhibited the highest Pd-(II) extraction efficiencyat acidity levels of 1-5 M HNO3 and outstandingselectivity over 13 coexisting competing metal ions. Results fromUV-vis titration experiments and theoretical calculations suggestedthat the differentiated extraction abilities of the ligands couldbe because of their different hydrophilicity rather than electron-donatingeffects. Slope analyses and electrospray ionization-high resolutionmass spectrometry (ESI-HRMS) experiments revealed the formation ofboth L/Pd 1:1 and 2:1 species during the extraction process.These stoichiometries were further confirmed by job plots and NMRtitration experiments. The ligands were found to aggregate slightly,especially at higher concentrations, which could result from multipleintermolecular hydrogen bonds as illustrated by X-ray crystallography.The configurations of PdL and PdL (2) were further elucidated by analysis of single crystal structureand density-functional theory (DFT) calculations, respectively, wherethe first coordination sphere of Pd-(II) was surrounded by four nitrogenor oxygen atoms in a quadrangular manner. This study provides an alternativemethod to separate palladium from HLLW and brings a new understandingof the coordination and complexation behaviors of Pd-(II) with tridentatenitrogen ligands. Tridentatenitrogen-rich 2,6-bis-triazolyl-pyridine ligandswere investigated for highly efficient complexation and separationof Pd-(II) from high-level liquid waste.