Multiple Molecules in the Asymmetric Unit (Z′ > 1) and the Formation of False Conglomerate Crystal Structures

Handed molecules that pack awkwardly with each other in the solid state are prone to forming crystals containing multiple independent molecules in the asymmetric unit (Z′ > 1). Three racemic examples with (Z′ = 2, 2, and 5) each crystallize as a 1:1 mixture of enantiomorphous crystals by self-resolution of each independent molecule present. Their X-ray structures are analyzed to demonstrate how these enantiomorphs are generated in the solid state. These false conglomerate materials are rather similar to conglomerates and can be readily confused with them. They differ, however, in their solution properties. On dissolution, a conglomerate crystal yields an optically active solution, but a false conglomerate crystal will give an optically inactive solution (since independent molecules are not a solution-state property). The frequency of false conglomerate formation is estimated to be about 1% for organic compounds.