Cooperative host-guest recognition in crystalline clathrates: Steric guest ordering by molecular gears

Supramolecular host frameworks consisting of guanidinium (G) and 4,4'-biphenyldisulfonate (BPDS) ions include a diverse variety of aromatic guests to afford clathrates with the composition (G)(2)BPDS.(guest). Single-crystal X-ray diffraction reveals that these materials have a bilayer architecture in which the BPDS ions serve as pillars that connect opposing two-dimensional hydrogen-bonded networks of guanidinium and sulfonate ions, resulting in the formation of galleries with one-dimensional pores occupied by the guests. The solid-state structures of clathrates derived from various monosubstituted benzene guests (C6H5X; X = Cl, CH3, Br, I) and disubstituted isomers (o,m,p-YC6H5X; X, Y = Cl, CH3, Br) reveal cooperative host-guest and guest-guest interactions that affect the observed pore structure and molecular ordering in the two-dimensional galleries. The pore heights and corresponding pore sizes decrease with the introduction of halogenated guests relative to their hydrocarbon analogues, suggesting attractive host-guest interactions between halogen atoms and the guanidinium sulfonate sheets. The two-dimensional ordering of o- and m-disubstituted benzene guests in the porous galleries can be explained by cooperative steric interactions between the guests and the conformationally flexible BPDS pillars, the latter behaving as synchronous molecular gears that relay the instructions for guest ordering from one pore to another. In some cases these steric effects produce guest ordering that appears to be less than optimal with respect to guest-guest dipolar interactions. Steric effects are particularly pronounced in the 1,4-dibromobenzene clathrate, which crystallizes in an alternative "brick" architecture with larger one-dimensional pores. These materials provide insight into the molecular recognition sequences that govern the ordering of guest molecules in clathrates, which is crucial to the design of new materials based on these systems.