Guest inclusion properties of calix[6]arene-based unimolecular cage compounds. On their high Cs+ and Ag+ selectivity and very slow metal exchange rates [Review]

摘要

The reaction of 1,3,5-tri-O-alkylated calix[6]arenes with 1,3,5-tris(bromomethyl)benzene yielded capped calix[6]arenes (2 with tert-butyl groups on the upper rim and 3 without tert-butyl groups) in unexpectedly high yields (80 - 91%). Combined studies of 2 and 3 by MM3 computation, X-ray analysis, and H-1 NMR spectroscopy established that these calix[b]arenes feature a unique structure consisting of alternately-arranged three flattened mesitylene-linked phenyl units and three stand-up anisole units. Particularly, compound 2 possesses a closed ionophoric cavity: the upper hemisphere is closed by three ten-butyl groups of anisole units and the lower hemisphere is closed by a mesitylene cap and three anisole methoxy groups. The H-1 NMR spectrum was scarcely changed at wide temperature range (30 similar to 130 degrees C), indicating that the structure is extremely rigidified Both solvent extraction and spectroscopic studies established that this cavity shows the high selectivity toward Cs+ among alkali metal cations, the high affinity with Ag+, and the moderate affinity with RNH3+. Very surprisingly, the association-dissociation processes for 2 and cesium picrate was so slow that the rate could be followed by a conventional spectroscopic method The thermodynamic parameters determined by kinetic studies disclosed that the major driving-force for Cs+ inclusion is the entropy term based on the desolvation. (C) 1997 Elsevier Science Ltd. All rights reserved. [References: 97]