A biscalix[4]arene-based ditopic hard/soft receptor for K+/Ag+ complexation

A biscalixarene bearing tert-butylated and de-tert-butylated calix[4]arene unit connected by four ethylenoxy bridges has been prepared. This compound adopts the pinched cone conformation (C2v symmetry) and hence, is suitably preorganised for Ag+ complexation. X-ray crystallography showed that the silver cation (soft) is sandwiched by the two distal coplanar phenyl rings of the de-tert-butylated part of the molecule possessing a η1 binding mode. The complexation of the K+ cation (hard) into the central cavity created by the bridging moieties leads to a change of conformation (C4v symmetry) and hence, to the complete loss of complexation ability towards the Ag+ cation.