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.