Synergistic system of bipyridine derivatives and ionic liquid for highly selective lithium ion extraction from complex salt lake brine

A 6-Bipi/[NTf 2 ] − synergistic system for Li extraction from brine was developed. • Superior Li + selectivity over Na + , Mg 2+ , and Ca 2+ was achieved. • SC-XRD confirmed the tetrahedral coordination structure of [Li(6-Bipi) 2 ][NTf 2 ]. • DFT and IRI analyses revealed methyl steric hindrance as a driver of selectivity. Efficient Li + extraction from brine rich in Na + , Mg 2+ , and Ca 2+ is crucial for meeting the ever-growing demand for Li. This paper proposes a novel Li + extraction system based on the N-heterocyclic chelating ligand 6,6′-dimethyl-2,2′-bipyridine (6-Bipi) and the ionic liquid bis (trifluoromethanesulfonyl)imide ([NTf 2 ] − ), enabling highly selective Li + extraction from complex brine. A systematic investigation of how methyl substitution positions on the bipyridine rings regulate on the extraction performance revealed that the precise steric hindrance introduced by ortho-methyl substitution substantially enhances the separation of Li + from Mg 2+ , Na + , and Ca 2+ . Single-crystal X-ray diffraction confirmed the formation of a four-coordinate distorted tetrahedral complex, [Li(6-Bipi) 2 ][NTf 2 ], which serves as the extractant. Density functional theory calculations helped elucidate the microscopic mechanism underlying the extraction selectivity through thermodynamic, electronic structure, and energy analyses, confirming that the steric hindrance of the ortho-methyl group is the primary driver of the high selectivity. Under optimal conditions, the single-stage Li + extraction efficiency of this system reached 92.2%, while the extraction efficiencies of Na + , Mg 2+ , and Ca 2+ remained below 5%. Even in brine with a Li + concentration as low as 0.02 mol/L, the system maintained excellent Li + extraction performance (73%). This study presents a novel Li + extraction system characterized by high selectivity and strong adaptability to complex brine and provides a valuable theoretical basis and experimental support for the molecular design of N-heterocyclic Li extractants. Download: Download high-res image (201KB) Download: Download full-size image