An allosteric heteroditopic receptor for neutral guests and contact ion pairs with a remarkable selectivity for ammonium fluoride salts

Two novel calix[6]cryptamides bearing a tren-based cap have been synthesized and their host-guest properties have been investigated by ¹H and ¹⁹F NMR spectroscopy. One of them behaves as a remarkable heteroditopic receptor toward either polar neutral guests, anions or contact ion pairs. It has been shown that only F^- can be encapsulated into the tris-amido cap of this host. Moreover, the fluoride anion acts as an allosteric activator by favoring the inclusion of ammonium ions into the calixarene cavity. The ammonium fluoride salts are bound as contact ion pairs and, remarkably, the calix[6]cryptamide host is reluctant to other ammonium salts. To our knowledge, such an highly cooperative and selective process toward contact ammonium fluoride salts is unique in the literature. Allosteric regulation of all the host-guest systems can also be achieved through protonation of the aza-cap. Indeed, guest release can be triggered by addition of various acids. In comparison to related calixarene-based receptors, all these unique properties are due to the smallness and the higher preorganization of the binding site provided by the convergent hydrogen bond donor groups of the tris-amido cap.