Giant Supramolecular Synthons via Cyclic Halogen···Halogen Contacts in Substituted o-Xylenes

Abstract

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 bromine-substituted o-xylenes. We report an unprecedented giant cyclic supramolecular Br8 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 Br4 tetrahedral synthon was isolated and characterized by single-crystal X-ray diffraction. Additionally, a rectangular Br4 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 Br8 and the Br4 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 Br8 and Br4 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.