Bipyrrole-Strapped Calix[4]pyrroles: Strong Anion Receptors That Extract the Sulfate Anion

Cage-type
calix­[4]­pyrroles 2 and 3 bearing
two additional pyrrole groups on the strap have been synthesized.
Compared with the parent calix[4]­pyrrole (1), they were
found to exhibit remarkably enhanced affinities for anions, including
the sulfate anion (TBA+ salts), in organic media (CD2Cl2). This increase is ascribed to participation
of the bipyrrole units in anion binding. Receptors 2 and 3 extract the hydrophilic sulfate anion (as the methyltrialkyl­(C8–10)­ammonium (A336+) salt) from aqueous
media into a chloroform phase with significantly improved efficiency
(>10-fold relative to calix[4]­pyrrole 1). These two
receptors
also solubilize into chloroform the otherwise insoluble sulfate salt,
(TMA)2SO4 (tetramethylammonium sulfate).