Synthesis and host–guest properties of a calix[6]arene based receptor closed by an internal ion-paired cap

The straightforward synthesis of a Cs symmetrical calix[6]arene possessing carboxylic acid groups as well as an ammonium arm is described. This calixarene can encapsulate ammonium ions through a highly selective recognition process thanks to the presence of an internal ion-paired cap that preorganizes the cavity and constitutes an efficient binding site.