S. Pramanik, I. Popovs, A. Ivanov, J. Einkauf, M. Lamb, D-E Jiang, S. Jansone-Popova
Oak Ridge National Laboratory,
United States
Keywords: ligands, preorganization, extraction, crystallization, REE, anions
Summary:
Rare earth elements (REEs) play a critical role in the United States’ economy, energy infrastructure, and national security due to their indispensable applications in advanced technologies. However, their separation and purification remain significant challenges, driven by high costs, environmental concerns, and the chemical similarity across the lanthanide series. Our recent results present the first example of a radioactive promethium complex formed using water-soluble diglycolamide (DGA) ligands, marking a major step toward safer and more efficient lanthanide chemistry. Building on this foundation, our current research focuses on the design and synthesis of multidentate preorganized ligands for the selective separation of lanthanides. This work highlights the profound effect of donor group configuration within N,O-based tetradentate ligands and the influence of solvation environments on the separation process. The bis-lactam-1,10-phenanthroline (BLPhen) ligand, with its structural rigidity, enforces size-based selectivity, showing exceptional affinity for larger lanthanides such as La³⁺. In contrast, modifying the ligand by removing one preorganization element (bipyridine) yields the BLBPy variant, which initially forms complexes rapidly with light lanthanides but gradually shifts selectivity toward middle lanthanides (e.g., Sm³⁺) over time—demonstrating time-resolved separation behavior. At low nitric acid concentrations, these neutral tetradentate ligands coordinate directly with lanthanide ions. However, at higher acid concentrations, a shift in extraction mechanism occurs, favoring the formation of anionic heavy lanthanide ensembles, [Ln(NO₃)₅]²⁻, that self-assemble with protonated ligands into intricate supramolecular architectures. We also have designed preorganized multidentate pincer ligands that enable selective separation of specific REEs chlorides through crystallization. Notably, by fine-tuning separation parameters (e.g., solvent, anions, additives), selectivity can be shifted across the REE series, offering an additional layer of control over the separation process. More recently, we have expanded this research to include the synthesis of redox-active DGA ligands, offering a potential pathway toward environmentally sustainable and energy-efficient separation of trivalent lanthanides. This presentation will discuss our latest findings in ligand synthesis, coordination chemistry, and selective separation strategies, providing new insights into advancing rare earth element purification through rational molecular design.