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.任何有机晶体结构均可简化为以分子为节点、超分子合成子 (supramolecular synthons) 为节点连接关系的网络。本研究以基于金刚石型网络的有机晶体结构为例，阐释了这一晶体工程研究路径。通过向该网络中引入N···Br合成子，可得到六亚甲基四胺 (hexamethylenetetramine，缩写HMT) 与四溴化碳 (CBr4) 按1:1配比形成的配合物的二重互穿结构。在同一网络中使用C−H···N介导的合成子，则可得到1,3,5,7-四溴金刚烷 (1,3,5,7-tetrabromoadamantane，缩写AdBr4) 与HMT按1:2配比形成的配合物。通过分子合成子 (molecular synthons) 与超分子合成子的互换，可进一步实现结构的柔性调控。据此，四(4-溴苯基)甲烷的金刚石型晶体结构，以及四苯基甲烷与CBr4按1:1配比形成的分子配合物的晶体结构极为相似。这种近乎完全一致的结构源于：CBr4分子合成子与Br4超分子合成子具有结构等效性，且CBr4分子能够参与Br···苯基相互作用。总体而言，有机与无机晶体结构之间存在广泛的拓扑对应性，这一特性可用于将有机晶体结构描述为网络形式。该表征方式具有较高的实际应用价值，且与基泰戈罗茨基模型 (Kitaigorodskii's model) 截然不同——后者从根本上区分了分子结构与晶体结构。在网络模型中，分子合成子与超分子合成子在同一网络结构内可实现互换。