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-n)Br(n), C6Cl(6-n)I(n), C6Br(6-n)I(n)) crystallise in two main packing modes, which take the monoclinic space group P2(1)/n and the triclinic space group P1. 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 pi...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...Cl interactions compared with the stronger pi...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...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.

X-ray crystal structures of halogen containing nucleobase derivatives in unsolvated and DMSO solvated forms

“…Different conventional O-HÁÁÁO and N-HÁÁÁO hydrogen bonds as well as a weaker C-HÁÁÁO contact are formed that link the ribofuranosyl unit to solvent molecules, to the guanine moiety and to the ribofuranosyl residue of a neighboring molecule (Table 3). However, a significant contact involving the bromine atom, such as an interaction of BrÁÁÁBr type [50] or between Br and another heteroatom [36,37], is not detectable in the structure. Structure of 2-i-butyramido-8-bromo-9-(2 0 ,3 0 ,5 0 -tri-…”

On the Importance of Halogen–Halogen Interactions in the Solid State of Fullerene Halides: A Combined Theoretical and Crystallographic Study

“…Finally, we have performed a search in the CSD (Cambridge Structural Database) in order to find experimental evidence of the impact of halogen-halogen interactions on the solid state architectures of halogenated C 60 s. exponentially, due to the recognition of its importance in biological systems and the design of new materials, leading to a plethora of theoretical and experimental studies devoted to this fruitful line of research [22][23][24][25]. Apart from the classical halogen bond, there is growing interest amongst the scientific community to properly understand weaker halogen-halogen interactions [26], which have shown a promising future as stabilizing agents of metal complexes [27,28], by governing the formation of novel solid state architectures [29][30][31] and even tuning the properties of biological and conjugated materials [32][33][34][35]. In this regard, the presence of both an electron-rich "belt" and an electro-positive σ-hole (Cl, Br and I) are key to describe the two types of X···X interactions, which are usually classified as either type I and II [36].…”

Probing halogen–halogen interactions in solution

“…[15][16][17][18][19][20] For instance, the introduction of halogen atoms at key positions allows to organize molecules in the solid state to favour topochemical polymerization, 21 or inclusion complex formation. 22 Moreover, the weak but nonetheless predictable XX interactions have been used to design crystalline solids with unusual plastic 23 or elastic 24,25 mechanical properties. Theoretical studies have allowed to rationalize the influence of the molecular structure and the electronic density distribution, on the strength of XX interactions (such as the nature of the halogen atoms, the hybridization of the carbon atoms, or substitution effects).…”