Supramolecular Synthons in Crystal Engineering. 4. Structure Simplification and Synthon Interchangeability in Some Organic Diamondoid Solids1

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 CBr4. The use of C−H· · ·N mediated synthons in the same network results in the 1:2 complex of 1,3,5,7-tetrabromoadamantane (AdBr4) 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 CBr4 are very similar. This near-identity arises because of the structural equivalence of the CBr4 molecular synthon and the Br4 supramolecular synthon and the ability of the CBr4 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.