Tubular hydrogen-bonded networks sustained by water molecules

The design concept of functional solids relies on controlling the topology of crystal packing through exploitation of weak intermolecular forces. In the context of cyclic aggregates, the ability to anticipate the consequences of ring constituents and their stereochemistries on ring conformation is vitally important since even an apparently slight structural change effected on molecules can dramatically alter the crystal structure. We have found that solid-state structures formed by hydroxy acids with a general structure (+/-)-1 depend on steric interactions. Thus, with the exception of molecules 1b and 1e, compounds (+/-)-1a-(+/-)-1m, which possess bulky and conformationally rigid substituents, aggregate by forming tapes and sheets by alternating (+) and (-) subunits held together through carboxylic acid-to-alcohol hydrogen bonds. Homologue (+/-)-1n, with conformationally flexible substituents which allow conformational deformation, gives, by incorporation of molecules of water, an efficient hexagonal assembly which extends to the third dimension to form tubular H-bonding networks. Each puckered channel can be described as interconnected closely packed hexagons in chairlike conformations. The ethyl groups presented in (+/-)-1n gave the volume required to lock the inner hexagonal wall into a rigid structure. Attempts to obtain cyclic aggregates using small substituents, compounds (+/-)-1o-(+/-)-1q, failed. The observed supramolecular assemblies of the anhydrous compounds can be classified into one-dimensional strands and two-dimensional sheets, while three-dimensional networks are present only in the hydrated molecules (1b, 1e, and 1n). The crystal structure of the anhydrous (+/-)-1n compound confirms the important role played by water molecules in the formation of tubular structures.