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

Abstract

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 1H NMR spectroscopy established that these calix[6]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 tert-butyl groups of anisole units and the lower hemisphere is closed by a mesitylene cap and three anisole methoxy groups. The 1H NMR spectrum was scarcely changed at wide temperature range (30 ∼ 130°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 RNH 3 +. 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.