Improved methods for achieving the selective extraction of lithium salts from lithium sources, including rocky ores, salt-lake brines, and end-of-life lithium-ion batteries, could help address projected increases in the demand for lithium. Here, we report an ion pair receptor (2) capable of extracting LiCl and LiBr into an organic receiving phase both from the solid state and from aqueous solutions. Ion pair receptor 2 consists of a calix[4]pyrrole framework, which acts as an anion binding site, linked to a phenanthroline cation binding motif <i>via</i> ether linkages. Receptor 2 binds MgBr₂ and CaCl₂ with high selectivity over the corresponding lithium salts in a nonpolar aprotic solvent. The preference for Mg²⁺ and Ca²⁺ salts is reversed in polar protic media, allowing receptor 2 to complex LiCl and LiBr with high selectivity and affinity in organic media containing methanol or water. The effectiveness of receptor 2 as an extractant for LiCl and LiBr under liquid-liquid extraction (LLE) conditions was found to be enhanced by the presence of other potentially competitive salts in the aqueous source phase.

A molecular capsule (<b>1</b>) consisting of two calix[4]pyrroles connected via ethylene diamide linkers has been prepared as an anion receptor. ¹H NMR spectroscopic studies carried out in CD₂Cl₂ revealed that receptor <b>1</b> recognizes a variety of anions with different binding modes and stoichiometries. For instance, receptor <b>1</b> binds fluoride and acetate with 1:2 receptor/anion stoichiometry and other test anions with 1:1 stoichiometry in solution when their respective tetrabutylammonium (TBA⁺) salts were used. In contrast, with tetraethylammnium (TEA⁺) salts, receptor <b>1</b> forms 1:2 complexes with chloride and bromide in addition to fluoride, overcoming expected Columbic repulsions between the anions co-bound in close proximity. Receptor <b>1</b> is also able to bind oxoanions, such as oxalate (C₂O₄^2-), dihydrogen phosphate (H₂PO₄^-), sulfate (SO₄^2-), and hydrogen pyrophosphate (HP₂O₇^3-), in the form of 1:1 complexes as the result of presumed cooperation between the two calix[4]pyrrole subunits. The selectivity of receptor <b>1</b> for fluoride versus dihydrogen phosphate varies depending on their relative concentrations. For instance, in the presence of less than 1.0 equiv of an equimolar mixture of fluoride and dihydrogen phosphate, receptor <b>1</b> shows high selectivity for dihydrogen phosphate. In contrast, in the presence of ≥2.0 anion equiv, receptor <b>1</b> binds fluoride preferentially, forming a 1:2 complex. Moreover, when treated with F^-, the preformed 1:1 H₂PO₄^- complex of receptor <b>1</b> is converted to the corresponding 1:2 receptor/fluoride complex with the release of the prebound dihydrogen phosphate anion. As inferred from gas-phase computations, this seemingly counterintuitive behavior is rationalized in terms of the precomplexed dihydrogen phosphate serving to reduce the reorganization energy required to bind two fluoride anions. The presence of a water molecule in addition to the bound fluoride anions may also favor the formation of the 1:2 F^- complex. The present study provides a new approach for fine-tuning the binding selectivity of polytopic anion receptors.