Crystalline Organic Cavitands as Microcavity Materials

Abstract

There has been much interest in inefficiently packed molecular materials and their applications in gas storage, separations, catalysis, etc. Such known materials include metal-organic frameworks (MOFs), polymers of intrinsic microporosity (PIMs), container molecule materials, etc. One way to design inefficiently packed materials is to construct them from compounds that are incapable of close-packing, that is rigid scaffolds with enforced cavities that cannot be filled by self-packing.

Cavitand molecules, tetrameric macrocycles derived from calix[4]resorcinarene derivatives, are well known for their propensity to form crystalline inclusion compounds with small molecules; for example, of the 169 examples of calix[4]resorcinarene scaffolds found in the Cambridge Structural Database (CSD), no guest-free forms exist. The guest-free forms of various cavitands, synthesized by literature methods, have been obtained as single crystals by sublimation. Gas inclusion compounds of these cavitands have also been isolated and studied by single crystal x-ray diffraction, thermogravimetric analysis, and 1H NMR. Furthermore, some cavitand derivatives have shown promise as media for industrial separations (Kr vs. Xe, MeCl vs. DME, Propene vs. Propane).