Acta Crystallographica Section B

Structural Science

Volume 57, Part 4 (August 2001)

research papers

Acta Cryst. (2001). B57, 560-566 [ doi:10.1107/S0108768101008412 ]

Crystal engineering in the gem-alkynol family: the key role of water in the structure of 2,3,5,6-tetrabromo-trans-1,4-diethynyl-cyclohexa-2,5-diene-1,4-diol dihydrate determined by X-ray and neutron diffraction at 150 K

C. Bilton, J. A. K. Howard, N. N. L. Madhavi, G. R. Desiraju, F. H. Allen and C. C. Wilson

Abstract: The structure of the title compound has been determined using low-temperature (150 K) single-crystal X-ray and neutron diffraction data. Crystals adopt the uncommon space group P4₂/ncm and display a complex set of intermolecular interactions in which the water molecules play the crucial role: the water O-atom [O2(w)] accepts two hydrogen bonds and both water H atoms act as bifurcated donors. A set of O-HO hydrogen bonds is formed around the 4₂ axis comprising (a) a cyclic tetrameric synthon involving four donor-H from two water molecules and two O(hydroxy) acceptors from two parent molecules, and (b) short discrete O(hydroxy)-HO2(w) hydrogen bonds which link these tetramers along the c axis. Four BrBr interactions [3.708 (1) Å] form cyclic Br₄ tetramers around the $\bar 4$ axis and are linked to the O-HO system via O2(w)-HBr bonds with HBr = 2.995 (2) Å. Finally, the O-HO system is further linked to the parent molecules via C [equivalent to] CHO2(w) bonds of 2.354 (3) Å. The supramolecular structure of the title hydrate is compared with that of the non-hydrated parent molecule, which also forms cyclic O-HO bonded tetrameric synthons, and with its (non-hydrated) tetrachloro analogue, which forms cyclic tetrameric Cl₄ synthons [Madhavi, Desiraju et al. (2000b). Acta Cryst. B56, 1063-1070].

Keywords: crystal engineering; supramolecular synthons.