No abstract

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.

Any organic crystal structure can be simplified to a network wherein the molecules are the nodes and the supramolecular synthons are the node connections. This approach to crystal engineering is illustrated in this paper with reference to organic structures based on the diamond network. By introducing N‚ ‚ ‚Br synthons into this network, a 2-fold-catenated structure is obtained for the 1:1 complex between hexamethylenetetramine (HMT) and CBr 4 . The use of C-H‚ ‚ ‚N mediated synthons in the same network results in the 1:2 complex of 1,3,5,7-tetrabromoadamantane (AdBr 4 ) with HMT. Further structural flexibility is achieved by the interchange of molecular and supramolecular synthons. Accordingly, the diamond-based crystal structures of tetrakis-(4-bromophenyl)methane and the 1:1 molecular complex of tetraphenylmethane and CBr 4 are very similar. This near-identity arises because of the structural equivalence of the CBr 4 molecular synthon and the Br 4 supramolecular synthon and the ability of the CBr 4 molecule to participate in Br‚ ‚ ‚phenyl interactions. In general, there is much topological correspondence between organic and inorganic crystal structures, and this can be utilized in the description of organic crystal structures as networks. Such a depiction is of much practical utility and is different from Kitaigorodskii's model which distinguishes fundamentally between molecular and crystal structure. In the network model, molecular and supramolecular synthons are interchangeable within the same network structure.