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

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 center dot center dot center dot Br interactions in the crystal structure of alpha,alpha,alpha,alpha',alpha'-tetrabromo-o-xylene. A new polymorph of the compound alpha,alpha,alpha',alpha'-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 alpha,alpha,alpha'-tribromo-o-xylene. Lines of evidence for the noncovalent intermolecular Br center dot center dot center dot 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 center dot center dot center dot Br interaction energy in terms of electrostatic and exchange-correlation components. Despite the presence of s holes as characterized by electrostatic potential analysis, the Br center dot center dot center dot 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 center dot center dot center dot 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.

Automated summary

This study explores the variations in bromine substitution for halogen synthons and identifies a novel cyclic supramolecular Br-8 synthon formed exclusively through hypervalent Br⋯Br interactions. The study also demonstrates the potential of non-electrostatic weak halogen⋯halogen contacts to generate diverse supramolecular patterns. The quantitative analysis of noncovalent interactions provides insight into the stabilizing factors of Br-8 and Br-4 synthons.