posted on 2015-06-03, 00:00authored byYating Zhou, Fang Guo, Colan E. Hughes, Duncan L. Browne, Thomas R. Peskett, Kenneth D. M. Harris
Members of a homologous family of
1:2 co-crystals comprising even-chain
α,ω-dihydroxyalkanes and urea have been reported previously
to fall into three well-defined structure types, although surprisingly
polymorphism was not observed for any member of this series. Here
we report the discovery of the first examples of polymorphism within
this family of materials, specifically for 1,6-dihydroxyhexane-(urea)2 and 1,8-dihydroxyoctane-(urea)2. The new polymorphs
have been prepared by mechanochemical milling, and the crystal structures
have been determined directly from powder X-ray diffraction data.
On the basis of periodic density functional theory calculations, the
new polymorphs are assigned as metastable with respect to the polymorphs
reported previously. Under ambient conditions, the new polymorph of
1,6-dihydroxyhexane-(urea)2 transforms to the previously
known polymorph over a matter of days. However, the new polymorph
of 1,8-dihydroxyoctane-(urea)2 has significantly greater
kinetic stability, which can be rationalized on the basis of the specific
structural reorganization required to transform to the previously
known polymorph.