On the inclusion of solvent molecules in the crystal structures of organic compounds

The Cambridge Structural Database has been searched for all crystal structures including organic solvent molecules (solvates) and solvent water molecules (hydrates). Well above 300 different solvent molecules were identified and the frequencies with which they occur in crystal structures, as a function of the year of publication, were established. The crystal structures are classified as `organic' and `metalloorganic'; it is shown that the relative prevalences of various cocrystallized solvents are different in the two groups. Several frequently used organic solvents are also common ligands for metal ions. Special interest has been focused on the existence of heterosolvates, i.e. crystal structures which include more than one type of solvent molecule. Up to five different types of solvent molecules were found in a single crystal structure. It is suggested that the use of solvent mixtures during crystallizations may prove to be a more useful and versatile approach for obtaining crystals of high-molecular-weight organic compounds than has hitherto been recognized.