Cubanecarboxylic acids.: Crystal engineering considerations and the role of C-H•••O hydrogen bonds in determining O-H•••O networks

A family of 4-substituted-1-cubanecarboxylic acids have been synthesized and their X-ray crystal structures analyzed. The rare syn-anti O-H ... O catemer 6 is a recurring pattern in this series of compounds. Catemer 6 is observed in the crystal structures of 4-chloro-1-cubanecarboxylic acid (10), 4-bromo-1-cubanecarboxylic acid (11), 4-iodo-1-cubanecarboxylic acid (12), and 4-(methoxycarbonyl)-1-cubanecarboxylic acid (13). The ready occurrence of catemer 6 in this family is ascribed to its stabilization by auxiliary C-H ... O hydrogen bonds formed by the relatively acidic cubyl C-H groups. The frequency of occurrence of 6 also facilitates its definition as a useful supramolecular synthon. As is true in many catemers, the formation of 6 is sensitive to steric factors. Therefore, the robustness of this synthon may be assessed by analyzing the crystal structures of molecules wherein the 4-substituent is too small (R = H, 14), too large (R = Ph, 15), or has a specific hydrogen bonding preference of its own (R = CONH2, 16). In these structures, either dimer 3 (in 14 and 15) or heterodimer 22 (in 16) is observed. Powder diffraction shows that the previously noted structure of 1,4-cubanedicarboxylic acid (7) that contains catemer 6 is characteristic of the bulk material. In summary, the syn-anti catemer is the dominant supramolecular synthon in this family of cubanecarboxylic acids.