Halogen Bonding in Crystal Engineering

The structural features of a molecule are determined by the covalent bonds within the molecule. Modification of the structure requires the breaking and creation of covalent bonds. Similarly, the intrinsic reactivity of a molecule arises from the covalently bonded functional groups and active sites on a molecule. Again, changing the properties requires modifications of the covalent bonds. Even if the ability of a molecule to react is dependent on the covalent molecular structure, the reaction itself is typically launched by non-covalent intermolecular contacts. The reacting molecules need to “see each other” and they need to be brought close enough to each other for the reaction to take place. In other words, the noncovalent interactions provide the glue that brings and holds molecules together, thus making the intermolecular interplay possible. Supramolecular chemistry and processes such as self-assembly and crystallization are strongly guided by non-covalent interactions. These interactions cover a range of different types of intermolecular forces including Columbic interactions, hydrogen bonds, π-interactions, metallophilic interactions, agostic interactions, and halogen bonds. Building predesigned supramolecular entities and molecular assemblies imposes some requirements on intermolecular contacts. They must be sufficiently strong and they must have directional preferences. If these conditions are met it is possible to design molecular building blocks with suitable acceptor/and donor sites for different types of contacts and also to build molecular assemblies in a controlled way.