Highly Selective Coextraction of H⁺/SO₄^2– Using a Strapped Calix[4]pyrrole

We report on the synthesis of a cage-type calix[4]pyrrole (1) bearing an additional basic pyridinebisthiazolamine group on the strap. The receptor in its protonated form shows strong affinity and selectivity for sulfate over a wide range of inorganic anions. With receptor 1 as a liquid-liquid extractant, H+/SO42- in the form of H2SO4 is almost quantitatively extracted from an aqueous solution containing HNO3 at a high concentration to CH2Cl2 in a recyclable manner.