Molecular Capsules in Modular Frameworks

In a rare example of crystal engineering through empirical design, the metric and symmetry complementarity between the four sulfonates appended to the upper rim of tetrasulfonated calix[4]arenes and the sulfonate nodes of the guanidinium-sulfonate hydrogen-bonded sheet permits the formation of single crystals comprising antiparallel interdigitated arrays of calixarene capsules embedded in discrete bilayers wherein each calixarene is suspended from a single guanidinium-sulfonate sheet through four sulfonate attachments. A hydrate of the parent compound tetraguanidinium calix[4]arenetetrasulfonate, in which water occupies the calixarene as well as positions in the hydrogen-bonded sheet, undergoes a single crystal-single crystal phase transition with loss of all water molecules, but with remarkable retention of crystallinity. These compounds suggest a path to a new class of crystalline materials with molecular capsules that can be tailored for selective guest inclusion for separations and regulation of solid-state properties through the encapsulation of functional guests.