An Ion Pair Receptor for Selective Solid-Liquid Extraction of LiCl

We have synthesized a new ion pair receptor (2) consisting of a cone-calix[4]arene bearing a combination of ethyl esters and ethers as a cation binding site and a calix[4]pyrrole subunit acting as an anion binding site. Ion pair receptor 2 complexes LiCl, NaCl, and CsCl among alkali metal chloride salts with distinct complexation modes and affinities in 10 % methanol-d₄ in chloroform-d. For instance, relatively small Li⁺ and Na⁺ cations are bound to the oxygen atoms of the functionalized calix[4]arene lower rim with Cl^- being hydrogen bonded to the calix[4]pyrrole NHs while the larger Cs⁺ cation is bound to the cone-shaped calix[4]pyrrole cavity via a π-cation interaction. By contrast, in the co-presence of all MCl salts (M=Li, Na, K, Rb, and Cs), receptor 2 binds LiCl with complete selectivity. Besides, receptor 2 is capable of extracting LiCl selectively into a chloroform or a dichloromethane organic phase from its solid source containing all other MCl salts.