A Squaramide-Crown\nEther-Based Receptor and Polymer\nfor Enhanced Lithium Chloride Extraction

A non-mulitmacrocyclic ion pair receptor with the ability\nto cooperatively\nbind lithium cations and halide anions is presented. These receptors\nfeature a squaramide function for anion binding and are directly linked\nto a benzo-4-crown-12 unit to achieve selectivity toward lithium salts.\nIon pair receptors <b>1</b> and <b>3</b> demonstrated\nstronger anion binding in the presence of cobound cations, while anion\nreceptor <b>2</b> lacks this property. X-ray measurements supported\nthe simultaneous binding capability of ion pair receptors for both\ncations and anions. Under solid–liquid extraction (SLE) conditions\nin acetonitrile, ion pair receptors <b>1</b> and <b>3</b> selectively extract lithium salts. The more lipophilic receptor <b>1</b> can extract LiCl in liquid–liquid extraction (LLE)\nand SLE, with the latter process notably more selective and effective.\nImproved LLE extraction of lithium chloride was achieved using copolymer <b>5</b> containing receptor units in the structure. The inability\nof receptor <b>2</b> or benzo-12-crown-4 ether to extract salts\nis attributed to the absence of a heteroditopic binding domain in\ntheir structures. This underscores the significant influence of both\nanion and cation binding domains in the receptor structures on the\neffective and selective extraction of lithium halide salts. The selectivity\nof these receptors and the polymeric material toward lithium salts\nsuggests significant potential for extracting lithium from brine solutions\nwith elevated halide content.