Fundamental Aspects of Salts and Co-crystals
The chapter provides background information about fundamental aspects of salts and co-crystals and describes some of the approaches that have been taken to prepare strategic co-crystal or salts. Definitions of co-crystals and salts are considered and a flexible approach is encouraged, without application of arbitrary borderlines. The supramolecular synthon is presented as the basis for rational co-crystal or salt design strategies and the essential supporting role of the Cambridge Structural Database is noted. It is stressed that the synthon approach is probabilistic in its nature and therefore that co-crystal design strategies must also be probabilistic. However, it is possible to exert a significant degree of control over specific chemical systems by considering a synthon hierarchy. Examples of successful targeted realisation of co-crystals and salts are given, including binary and ternary examples. Finally, the expectation of obtaining a salt rather than a co-crystal is discussed in the specific context of proton transfer from an acid to a base and it is shown that the ionisation states of the molecules in a multi-component crystal can be difficult to predict.