GUEST INCLUSION PROPERTIES OF CALIX[6]ARENE-BASED UNIMOLECULAR CAGE COMPOUNDS - ON THEIR HIGH CS-RATES( AND AG+ SELECTIVITY AND VERY SLOW METAL EXCHANGE)

The reaction of 1,3,5-tri-O-alkylated calix[6]arenes with 1,3,5-tris(b romomethyl)benzene yielded capped calix[6]arenes (2 with tert-butyl gr oups on the upper rim and 3 without tert-butyl groups) in unexpectedly high yields (80 - 91%). Combined studies of 2 and 3 by MM3 computatio n, X-ray analysis, and H-1 NMR spectroscopy established that these cal ix[b]arenes feature a unique structure consisting of alternately-arran ged 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 an isole units and the lower hemisphere is closed by a mesitylene cap and three anisole methoxy groups. The H-1 NMR spectrum was scarcely chang ed at wide temperature range (30 similar to 130 degrees C), indicating that the structure is extremely rigidified Both solvent extraction an d spectroscopic studies established that this cavity shows the high se lectivity toward Cs+ among alkali metal cations, the high affinity wit h Ag+, and the moderate affinity with RNH3+. Very surprisingly, the as sociation-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 bas ed on the desolvation. (C) 1997 Elsevier Science Ltd. All rights reser ved.