A Squaramide-Crown Ether-Based Receptor and Polymer for Enhanced Lithium Chloride Extraction

A non-mulitmacrocyclic ion pair receptor with the ability to cooperatively bind lithium cations and halide anions is presented. These receptors feature a squaramide function for anion binding and are directly linked to a benzo-4-crown-12 unit to achieve selectivity toward lithium salts. Ion pair receptors 1 and 3 demonstrated stronger anion binding in the presence of cobound cations, while anion receptor 2 lacks this property. X-ray measurements supported the simultaneous binding capability of ion pair receptors for both cations and anions. Under solid–liquid extraction (SLE) conditions in acetonitrile, ion pair receptors 1 and 3 selectively extract lithium salts. The more lipophilic receptor 1 can extract LiCl in liquid–liquid extraction (LLE) and SLE, with the latter process notably more selective and effective. Improved LLE extraction of lithium chloride was achieved using copolymer 5 containing receptor units in the structure. The inability of receptor 2 or benzo-12-crown-4 ether to extract salts is attributed to the absence of a heteroditopic binding domain in their structures. This underscores the significant influence of both anion and cation binding domains in the receptor structures on the effective and selective extraction of lithium halide salts. The selectivity of these receptors and the polymeric material toward lithium salts suggests significant potential for extracting lithium from brine solutions with elevated halide content.