Intermolecular Interactions Drive the Unusual Co-Crystallization of Different Calix[4]arene Conformations

Crystallization of 5,17-dibromo-11,27,23,25-tetraone-26,28-dipropoxycalix[4]arene results in the rare observation of two different calix[4]arene conformations (partial cone and 1,3-alternate) co-crystallized within the same single crystal X-ray structure. Analysis using H-1 and C-13 NMR spectroscopy revealed that only a single conformation (the cone) was present in solution, and in contrast to the structures of other reported calix[4]arenes and calix[4]quinones, both conformations of the compound present in this crystal structure have a "pinched" shape, drastically reducing Br-Br separation and associated cavity sizes.