Rigid Calix[4]arene as a Building Block for the Synthesis of New Quaternary Ammonium Cation Receptors

Abstract

The synthesis of a new series of neutral hosts derived from the head-to-head linkage of two calix[4]arene-bis(crown-3) units fixed in the rigid cone conformation with bridges of different nature and length is described. The binding abilities of the new receptors towards tetraalkylammonium and N-methylpyridinium cations of different size and shape in CDCl3 solution have been extensively investigated by 1H NMR spectroscopy. In this solvent, ion-pairing is found to have a strong effect on the binding efficiency. In the more polar CDCl3/CD3CN solvent mixture, a substantial decrease in the Kass values is observed, which can be attributed, in part, to CD3CN competing for the binding sites of the host. In all cases, the double calixarenes 2, 7, 8, and 9 have been found to exhibit efficiencies much higher than that of the corresponding rigid calix[4]arene-bis(crown-3) 1. The bridge present in these double calix[4]arenes dictates the orientation and distance between the two rigid caps and thus represents a new control element in determining the efficiency and selectivity of binding. The orientation and span imposed by the different bridges to the two calixarene subunits is responsible for the different binding modes observed with N-methylpyridinium and isomeric N-methylpicolinium cations.