Crystal Engineering

Crystal engineering is the design and synthesis of molecular solid state structures with desired properties, based on an understanding and use of intermolecular interactions. The two main strategies currently in use for crystal engineering are based on hydrogen bonding and coordination bonding. These may be understood with key concepts such as the supramolecular synthon and the secondary building unit.

The term ‘crystal engineering’ was first used in 1971 by Gerhard Schmidt in connection with photodimerization reactions in crystalline cinnamic acids. Since this initial use, the meaning of the term has broadened considerably to include many aspects of solid state supramolecular chemistry. A useful modern definition is that provided by Gautam Desiraju, who in 1988 defined crystal engineering as "the understanding of intermolecular interactions in the context of crystal packing and the utilization of such understanding in the design of new solids with desired physical and chemical properties." Since many of the bulk properties of molecular materials are dictated by the manner in which the molecules are ordered in the solid state, it is clear that an ability to control this ordering would afford control over these properties.

Crystal engineering relies on noncovalent bonding to achieve the organization of molecules and ions in the solid state. Much of the initial work on purely organic systems focused on the use of hydrogen bonds, though with the recent extension to inorganic systems, the coordination bond has also emerged as a powerful tool. In addition to this, especially through studies during the last decade, the use of halogen bonds has proved beneficial in providing additional control in crystal design. Other intermolecular forces such as π…π and Au…Au interactions have all been exploited in crystal engineering studies, and ionic interactions can also be important. However, the two most common strategies in crystal engineering still employ only hydrogen bonds and coordination bonds.

Molecular self-assembly is at the heart of crystal engineering, and it typically involves an interaction between complementary hydrogen bonding faces or a metal and a ligand. By analogy with the retrosynthetic approach to organic synthesis, Desiraju coined the term "supramolecular synthon" to describe building blocks that are common to many structures and hence can be used to order specific groups in the solid state. The carboxylic acid dimer represents a simple supramolecular synthon, though in practice this is only observed in approximately 30% of crystal structures in which it is theoretically possible. The Cambridge Structural Database (CSD) provides an excellent tool for assessing the efficiency of particular synthons. The supramolecular synthon approach has been successfully applied in the synthesis of one dimensional tapes, two dimensional sheets and three dimensional structures. The CSD today contains atomic positional parameters for nearly 800 000 crystal structures, and this forms the basis for heuristic or synthon based or "experimental" crystal engineering.

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