Hemispherand-Strapped Calix[4]pyrrole: An Ion-pair Receptor for the Recognition and Extraction of Lithium Nitrite

The hemispherand-strapped calix[4]­pyrrole (1) acts as an ion pair receptor that exhibits selectivity for lithium salts. In organic media (CD₂Cl₂ and CD₃OD, v/v, 9:1), receptor 1 binds LiCl with high preference relative to NaCl, KCl, and RbCl. DFT calculations provided support for the observed selectivity. Single crystal structures of five different lithium ion-pair complexes of 1 were obtained. In the case of LiCl, a single bridging water molecule between the lithium cation and chloride anion was observed, while tight contact ion pairs were observed in the case of the LiBr, LiI, LiNO₃, and LiNO₂ salts. Receptor 1 proved effective as an extractant for LiNO₂ under both model solid–liquid and liquid–liquid extraction conditions.