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

Abstract

Cage-type calix­[4]­pyrroles <b>2</b> and <b>3</b> bearing two additional pyrrole groups on the strap have been synthesized. Compared with the parent calix[4]­pyrrole (<b>1</b>), 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 <b>2</b> and <b>3</b> 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 <b>1</b>). These two receptors also solubilize into chloroform the otherwise insoluble sulfate salt, (TMA)₂SO₄ (tetramethylammonium sulfate)