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.