Polymorphic Crystals Formed by an Achiral Diol under Ambient Conditions

The crystallization behavior of 3,7-bis­(2-pyridyl)­bicyclo[3.3.0]­octane-endo-3,endo-7-diol (4) is very different from that of its known 3,7-bis­(phenyl) analogue 3. It yields two solvent-free (apohost) polymorphs, a clathrate structure containing benzene, and two monohydrate polymorphs, which are produced simply by employing different recrystallization solvents at room temperature and pressure. The structures of the apohost compounds share little commonality and use quite different methods of crystal assembly. One of these polymorphs has, however, a very close relationship with the benzene clathrate crystal form. The monohydrate polymorph structures share gross structural features, but nonetheless differ considerably in their detailed construction. The X-ray structures of these five disparate crystal forms are analyzed in crystal engineering terms and illustrate how even a simple achiral molecule can yield polymorphs by using alternative interaction arrangements.