Abstract
We have prepared and characterized 1,4-dibromo-2,5-bis(2-phenylethoxy)benzene (1) and 1,4-dibromo-2,5-bis(3-phenylpropoxy)benzene (2). Their single-crystal structures confirm that, at the molecular level, they are similar with the phenylalkoxy chains in extended conformations. However, there are significant differences in packing interactions. The packing in 1 is dominated by C–Br...π(arene) interactions, with each Br located over one C–C bond of the central arene ring of an adjacent molecule. In contrast, the packing of molecules of 2 involves a combination of C–H...Br hydrogen bonds, Br...Br interactions, and arene–arene π-stacking. The single-crystal structures of both orthorhombic and triclinic polymorphs of 1 have been determined and the packing interactions are shown to be essentially identical.
Highlights
In 2013, the IUPAC provided the following definition of a halogen bond [1]: “A halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity”
The nature of the halogen bond and the scope of close contacts that fall under this umbrella remain a subject of debate, and the IUPAC
Of particular note are discussions which demonstrate the distinction between a halogen bond and a halogen...halogen interaction [11,12]
Summary
In 2013, the IUPAC provided the following definition of a halogen bond [1]: “A halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity”. In addition to halogen bonds, solid-state structures may exhibit close (≤the sum of the van der Waals radii) contacts between halogen atoms. Such interactions typically involve Cl, Br and I atoms and may be classified as type I or type II interactions as depicted in Scheme 1 [13]. Analyses of scatter plots of angles θ1 against θ2 (defined in Scheme 1) by Desiraju and coworkers have illustrated that there is a clear distinction between type I and II interactions for I...I contacts.
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