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 t he context of cyclic aggregates, the ability to anticipate the consequences of ring constituents and their stereochemistries on ring conformation is v itally important since even an apparently slight structural change effected on molecules can dramatically alter the crystal structure. We have found t hat solid-state structures formed by hydroxy acids with a general structure (+/-)-1 depend on steric interactions. Thus, with the exception of molecul es 1b and 1e, compounds (+/-)-1a-(+/-)-1m, which possess bulky and conforma tionally rigid substituents, aggregate by forming tapes and sheets by alter nating (+) and (-) subunits held together through carboxylic acid-to-alcoho l hydrogen bonds. Homologue (+/-)-1n, with conformationally flexible substi tuents which allow conformational deformation, gives, by incorporation of m olecules of water, an efficient hexagonal assembly which extends to the thi rd dimension to form tubular H-bonding networks. Each puckered channel can be described as interconnected closely packed hexagons in chairlike conform ations. The ethyl groups presented in (+/-)-1n gave the volume required to lock the inner hexagonal wall into a rigid structure. Attempts to obtain cy clic aggregates using small substituents, compounds (+/-)-1o-(+/-)-1q, fail ed. The observed supramolecular assemblies of the anhydrous compounds can b e 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 conf irms the important role played by water molecules in the formation of tubul ar structures.