Isostructurality, polymorphism and mechanical properties of some hexahalogenated benzenes:: The nature of halogen•••halogen interactions

The nature of intermolecular interactions between halogen atoms, X center dot center dot center dot X (X=Cl, Br, I), continues to be of topical interest because these interactions may be used as design elements in crystal engineering. Hexahalogenated benzenes (C6Cl6-nBrn, C6Cl6-nIn, C6Br6-nIn) crystallise in two main packing modes, which take the monoclinic space group P2(1)/n and the triclinic space group P (1) over bar. The former, which is isostructural to C6Cl6, is more common. For molecules that lack inversion symmetry, adoption of this monoclinic structure would necessarily lead to crystallographic disorder. In C6Cl6, the planar molecules form Cl center dot center dot center dot Cl contacts and also pi center dot center dot center dot pi stacking interactions. When crystals Of C6Cl6 are compressed mechanically along their needle length, that is, [010], a bending deformation takes place, because of the stronger interactions in the stacking direction. Further compression propagates consecutively in a snakelike motion through the crystal, similar to what has been suggested for the motion of dislocations. The bending of C6Cl6 crystals is related to the weakness of the Cl center dot center dot center dot Cl interactions compared with the stronger pi center dot center dot center dot pi stacking interactions. The triclinic packing is less common and is restricted to molecules that have a symmetrical (1,3,5- and 2,4,6-) halogen substitution pattern. This packing type is characterised by specific, polarisation-induced X center dot center dot center dot X interactions that result in threefold-symmetrical X-3 synthons, especially when X=I; this leads to a layered pseudohexagonal structure in which successive planar layers are inversion related and stacked so that bumps in one layer fit into the hollows of the next in a space-filling manner. The triclinic crystals shear on application of a mechanical stress only along the plane of deformation. This shearing arises from the sliding of layers against one another. Nonspecificity of the weak interlayer interactions here is demonstrated by the structure of twinned crystals of these compounds. One of the compounds studied (1,3,5-tribromo-2,4,6-triiodobenzene) is dimorphic, adopting both the monoclinic and triclinic structures, and the reasons for polymorphism are suggested. To summarise, both chemical and geometrical models need to be considered for X center dot center dot center dot X interactions in hexahalogenated benzenes. The X center dot center dot center dot X interactions in the monoclinic group are nonspecific, whereas in the triclinic group some X center dot center dot center dot X interactions are anisotropic, chemically specific and crystal-structure directing.