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(C(8-10))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).