New Shapes For Selective Molecular Recognition From Calixarenes

Abstract

The functionalization of calixarenes at the “lower rim” reduces their conformational mobility creating new shapes which have been exploited to organize binding groups in space, thus producing efficient and selective Molecular Receptors. Calix[4]arene- ester and amide ligands in the fixed cone conformation show high efficiency and selectivity in the complexation of Na+ and Ca2+; the amide is able to encapsulate lanthanide ions (Eu3+and Tb3+) in its polar cavity, giving water soluble luminescent complexes. Calix[4]arene-based siderophores, which strongly complex iron (ΙII) in ethanol/water, have been obtained by the introduction of three hydroxamate chelating chains on the bottom rim of the calix fixed in the cone conformation. The selective 1,3,5-methylation of p-tert-butylcalix[6]- arene has been achieved and the subsequent introduction of ester and amide binding groups fixes the calix in a “flattened cone” conformation. The resulting ligands have a C3 symmetry and are able to transport guanidinium cation through a Supported Liquid Membrane. Water soluble calix[4] arenes in the fixed cone conformation, which selectively bind organic cations and neutral molecules in aqueous solution, have been synthesized, together with lipophilic cage molecules derived from calix[4]arenes which are potentially host for the encapsulation of organic molecules in apolar media.