Enantiomeric crystallization of (±)-trans-1,2-diaminocyclohexane under pressure

Abstract

The isothermal and isochoric crystallizations of (±)-trans-1,2-diaminocyclohexane, C₆H₁₀(NH₂)₂, (±)-DACH, yield enantiomeric conglomerates of orthorhombic symmetry, space groupP2₁2₁2, the same as obtained by isobaric crystallization. Despite the compression to 73% of ambient pressure volume, no racemic compound of DACH was formed. This contradicts for this compound the possibility of reverse causality between density and preferred enantiomorphic/racemic crystallization in Wallach's rule. The crystal structures have been determined at 0.36, 0.52, 0.65, 1.19 and 2.04 GPa at 296 K by single-crystal X-ray diffraction. It has been established that the enantiomeric-conglomerate crystallization of DACH at ambient pressure is mainly due to the molecular arrangement, governed by the close-packing rule in the crystal. High pressure enhances the close packing, and it does not change the course of crystallization of (±)-DACH. The shortest intermolecular contacts, NH⋯N and CH⋯N, become most compressed between 0.36 and 2.04 GPa, indicating that such weak hydrogen bonds are enhanced by pressure. The phase diagram of (±)-DACH has been outlined to 2.04 GPa.