Isostructurality, polymorphism and mechanical properties of some hexahalogenated benzenes: the nature of halogen···halogen interactions

Abstract

The nature of intermolecular interactions between halogen atoms, X···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 (C₆Cl_6-nB_rn, C₆Cl_6-nI_n, C₆Br_6-nI_n) crystallise in two main packing modes, which take the monoclinic space group P2₁/n and the triclinic space group P. 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 ··· Cl contacts and also pp 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 ··· Cl interactions compared with the stronger pp 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 ··· X interactions that result in threefold-symmetrical X₃ 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 … X interactions in hexahalogenated benzenes. The X ··· X interactions in the monoclinic group are nonspecific, whereas in the triclinic group some X ··· X interactions are anisotropic, chemically specific and crystal-structure directing.