Cocrystal

The definition of a cocrystal has been debated in the crystallography field. This definition can encompass many types of compounds, including hydrates, solvates and clathrates, which represent the basic principle of host-guest chemistry. Hundreds of examples of cocrystallization are reported annually.

Although the exact definition of a cocrystal remains topic of debate, most solid-state chemists agree nowadays that they can be defined as “solids that are crystalline single phase materials composed of two or more different molecular and/or ionic compounds generally in a stoichiometric ratio which are neither solvates nor simple salts.” Several subclassifications exist.

The first reported cocrystal, quinhydrone, was studied by Friedrich Wöhler in 1844. Quinhydrone is a cocrystal of quinone and hydroquinone (known archaically as quinol). He found that this material was made up of a 1:1 molar combination of the components. Quinhydrone was analyzed by numerous groups over the next decade and several related cocrystals were made from halogenated quinones.

Many cocrystals discovered in the late 1800s and early 1900s were reported in Organische Molekulverbindungen, published by Paul Pfeiffer in 1922. This book separated the cocrystals into two categories; those made of inorganic:organic components, and those made only of organic components. The inorganic:organic cocrystals include organic molecules cocrystallized with alkali and alkaline earth salts, mineral acids, and halogens as in the case of the halogenated quinones. A majority of the organic:organic cocrystals contained aromatic compounds, with a significant fraction containing di- or trinitro aromatic compounds. The existence of several cocrystals containing eucalyptol, a compound which has no aromatic groups, was an important finding which taught scientists that pi stacking is not necessary for the formation of cocrystals.

Cocrystals continued to be discovered throughout the 1900s. Some were discovered by chance and others by techniques. Knowledge of the intermolecular interactions and their effects on crystal packing allowed for the engineering of cocrystals with desired physical and chemical properties. In the last decade there has been an enhanced interest in cocrystal research, primarily due to applications in the pharmaceutical industry.

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