Calix[6]arene tris-carboxylic acid derivatives: X-ray and NMR characterization of their remarkable host-guest properties toward ammonium ions

The ability of calix[6]arene tris-carboxylic acid derivatives to include ammonium guests has been investigated both in solution and in the solid state. NMR studies and crystallographic data showed that the highly flexible calix[6]arene structures can be shaped in a well defined cone conformation thanks to the formation of an ion-paired cap between the carboxylate groups of the calixarene and their ammonium counter-ions. The resulting supramolecular edifices exhibit remarkable host-guest properties toward ammonium ions even in polar and protic solvents. The recognition process has been rationalized in the solid state by the combination of hydrogen bonding, electrostatic and CH-pi interactions, and a remarkable C-3-complementarity between the well-organized binding carboxylates of the host and the ammonium guest. Very interestingly, the endo-cavity complexation of large polycyclic ammonium ions as well as bioactive ammonium ions has been clearly demonstrated through NMR spectroscopy. Finally, the tuning of the nature of the ammonium ions involved in the supramolecular cap led to highly responsive molecular receptors.