The First Single-Step Immobilization of a Calix-[4]-arene onto the Surface of Silica

A novel synthetic method for immobilizing calixarene macrocycles onto the surface of silica is reported that does not require calixarene synthetic derivatization with a flexible tether. In part because of the rigidity of the linkage between the calixarene and the silica resulting from this method, immobilized site densities in excess of 0.2 mmol/g are obtained, which are the highest values reported to date on a per gram of material basis for anchored calixarenes on silica. The immobilized calixarenes contain lipophilic cavities that can be accessed with gas physisorption experiments at cryogenic temperatures, as well as with neutral organic molecular guests at room temperature. The toluene adsorption isotherm from aqueous solution clearly demonstrates a saturation coverage that is commensurate with formation of a 1:1 stoichiometric complex between the immobilized host and guest. The reversibility of adsorption within this class of materials is demonstrated with phenol, which is shown to bind reversibly via thermal desorption spectroscopy.