Among all crystalline complexes of calix-type calix[n]arenes those with organic molecules which are biologically relevant are especially of interest due to their potential medical and pharmaceutical applications. The co-crystallization of drugs with water-soluble calixarenes offers the opportunity to modify chemical and physical properties of APIs (active pharmaceutical ingredients) and to control drug conformation. Such co-crystallization can improve solubility, bioavailability and stability of pharmaceutically active molecules and/or eliminate polymorphism. In this article the solid-state interactions between calixarene hosts and biologically relevant guest molecules are summarized for the currently available structures solved by single-crystal X-ray crystallography.

Easily prepared, cone-like, extended cavity calix[6]pyrrole is shown to form strong complexes with iodine and other halide ions as well as with trihaloalkanes and electron deficient aromatic systems.