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.