Alternative crystal forms produced by a dialcohol inclusion host Resolutions and polymorphs. Part 3

Abstract

The racemic C₂-symmetric dialcohol endo-4,endo-8-dimethylbicyclo[3.3.1]nonane-endo-2,endo-6-diol 2 yields isostructural inclusion compounds in cubic space group Ia from nearly all recrystallisation experiments, but it has a different apohost structure. The X-ray structures of the alternative inclusion crystal forms, (2)₆·(p-xylene) and (2)₂·(water), are described here. Both are formed in space groupP2₁/c but their structures are entirely dissimilar. The p-xylene inclusion compound is constructed from a repeat of four different layers that are cross-linked by means of hydrogen bonding, whereas the hemihydrate co-crystal has a network structure involving two different hydrogen bonded layers that are cross-linked through the water molecules. The structures of these alternative crystal forms are discussed and contrasted in the context of crystal design. These help define the limits of solvents that can be incorporated into the baseline Ia structure in terms of size, awkwardness and hydrogen bonding capability.