Selective binding of oxalate by a tris-ureido calix[6]tube in a protic environment

Due to their significant role in industry and biological systems, the interest in selectively recognizing and detecting small dicarboxylates has grown in recent years. In this study, we report on the binding properties of a family of tubular-shaped heterotritopic receptors based on bis-calix[6]arenes, which contain three (thio)urea bridges (C3U and C3TU) or six urea bridges (C6U), toward dicarboxylates. While poor binding properties were observed by NMR for the newly synthesized C6U, receptors C3U and C3TU exhibited a unique ability to cooperatively complex a dicarboxylate anion sandwiched between two ammonium ions. The three ions are complexed in contact and aligned within the tubular shape of the receptor, forming cascade complexes that are stable even in a competitive environment. The different binding properties between the receptors were rationalized in terms of size, flexibility, H-bond donor ability, and intramolecular H-bonding within the anion binding pocket between the calixarene cavities. With C3U, a rare selectivity for oxalate over other small dicarboxylates and various bicharged anions was observed. Molecular modeling of the cascade complex indicated that the oxalate anion is stabilized by an array of hydrogen bonds with the urea bridges of the receptor and both propylammonium cations nested within the calixarene cavities.