X-ray Structure and Alkyl Ammonium Recognition of Quadruply Bridged Calix[6]arene:A Selective Inclusion of Primary Alkyl Ammonium with Calix[6]arene

Abstract

Numerous attempts have been made to design new host systems which can selectively interact the target guest and perform intriguing molecular recognition processes. The selective recognition of many important biogenic amines is one of the fundamental research interests in the field of the biomimetic chemistry. Particularly, selective binding of organic ammonium guests attracts an important research interest due to the application of biological systems, which results in developing of many sophisticated host systems. Calixarenes present attractive possibilities in the hostguest chemistry as molecular frameworks for the preparation of conformationally preorganized polyfunctional derivatives, whose convergent ligating groups can act cooperatively to selectively bind ions or neutral molecules. Especially, the architecture of calixarenes is such that they also possess hydrophobic cavities generated by the aromatic walls of phenol residues, which are potentially useful for the inclusion of alkyl ammonium ions. Whereas the calix[4]arene cavity in the cone conformation is too narrow to accommodate even small amine salts, that of calix[6]arene is large enough to include small to medium-sized ammonium ions. The parent calix[6]arenes, however, are conformationally mobile. The most effective approach to make them immobile is to build a bridge at the lower or upper rim. Ever since Gutsche created the lower rim-bridged calix[6]arene in 1993, several lower rim-multibridged calix[6]arenes have been reported. Also the calix[6](aza)cryptand which has a tripodal poly-aza cap on the lower rim as a efficient host for small ammonium was reported by U. Darbost et al. The quadruply bridged calix[6]arene derivatives, which had been reported by our group, are also known for having a well-defined cavity capable of incorporating various cations. Here we report the X-ray structure of quadruply bridged calix[6]arene (3) and the binding properties for various alkylammonium picrates by the H NMR analysis. Binding properties also were analyzed from the X-ray structure and the NOE study.