Crystal Engineering - Non-covalent Control of Structure

Non-covalent Control of Structure

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 more recent extension to inorganic systems, the coordination bond has also emerged as a powerful tool. Other intermolecular forces such as π…π, halogen…halogen, and Au…Au interactions have all been exploited in crystal engineering studies, and ionic interactions can also be important. However, the two most commonly used strategies in crystal engineering exploit 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 300 000 crystal structures, and this forms the basis for heuristic or synthon-based or "experimental" crystal engineering.