Architecting unique supramolecular structures requires robust and reproducible supramolecular synthons. Noncovalent halogen bonding offers rich crystal packing possibilities through diverse synthons and thus constitutes a useful structural domain in crystal groups. The variations in halogen synthon patterns with bromine substitution was explored by means of crystal packing analyses of a series of brominesubstituted o-xylenes. We report an unprecedented giant cyclic supramolecular Br 8 synthon with a fused triangular prism-like geometry formed exclusively through hypervalent Br•••Br interactions in the crystal structure of α,α,α′,α′-tetrabromo-o-xylene. A new polymorph of the compound α,α,α′,α′-tetrabromo-o-xylene with a Br 4 tetrahedral synthon was isolated and characterized by single-crystal X-ray diffraction. Additionally, a rectangular Br 4 synthon was found to direct the crystal packing in α,α,α′-tribromo-o-xylene. Lines of evidence for the noncovalent intermolecular Br•••Br interactions rendering the Br 8 and the Br 4 synthons were quantitatively obtained by Bader's quantum theory of atoms in molecules (QTAIM). Noncovalent interaction index isosurfaces exposed the intramolecular interactions, which were not evident from QTAIM. Penda's interacting quantum atom energy partitioning provided insight into the Br•••Br interaction energy in terms of electrostatic and exchange−correlation components. Despite the presence of σ holes as characterized by electrostatic potential analysis, the Br•••Br interactions constituting the Br 8 and Br 4 synthons are stabilized through nonelectrostatic components. The current study intends to reinforce the potential of nonelectrostatic weak halogen•••halogen contacts to give rise to a wide variety of synthon patterns and develop solid supramolecular assemblies that find applications in numerous fields of research.
Non‐covalent halogen bonding interactions are quintessential in crystal engineering for the construction of distinctive supramolecular synthons. Here, we report the first crystalline evidences of unique boat and chair shaped cyclic hexahalogen synthons in the crystal structures of α,α,α′,α′,4‐pentabromo‐o‐xylene (PBX) and α,α,α′,α′,4,5‐hexabromo‐o‐xylene (HBX) respectively. Nature and stability of constituent interactions in the supramolecular synthons are scrutinized with the help of quantum‐chemical calculations. Pendás’ interacting quantum atoms approach confirmed the stability of Br⋅⋅⋅Br interactions leading to boat and chair shaped synthons with major contribution from exchange‐correlation. Although both the molecules are achiral in nature, the packing forces guide PBX to crystallize in the chiral space group P21 with a helix‐like orientation while HBX packs in a centrosymmetric P21/n space group. The extended furcations in the pentabromo derivative construct a molecular framework consisting of macrocycles realized through halogen bonding.
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