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 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.

Properties and Stabilities of Cyclic and Open Chains of Halogen Bonds

“…There does not appear to be any particular limit to the number of I atoms that can participate in an I···I ring structure, with such geometries optimized for as high as n = 8. The latter large size has in fact been identified within the confines of a crystal of substituted o -xylene . On the other hand, when each XB involves the substituent lone pair instead of I itself, n = 6 appears to be the maximal size.…”

“…The latter large size has in fact been identified within the confines of a crystal of substituted o-xylene. 57 On the other hand, when each XB involves the substituent lone pair instead of I itself, n = 6 appears to be the maximal size. The angular aspects of each I•••I XB lead the two terminal molecules of a chain toward one another, particularly as the chain grows longer.…”

“…For example, a zigzag open chain involving XBs between I atoms occurs in the crystal structure of MeOPyO·I 2 and ClQnO·I 2 . , The cyclic X 3 synthon has also been noted on several occasions. ,− Four-membered square Br rings occur, , e.g., within the crystal structure of anti-α-bromoacetophenone. And there are also instances observed of X 6 and X 8 The latter workers had suggested that the X 8 system might take on a sort of a modified cubic geometry, with all X atoms located at the vertices of a parallelipiped.…”

“…53 There are also examples in the literature of the X 4 synthon 54−56 as in Figure 1b, as well as X 6 and X 8 . 57 The linear configuration is also prevalent in crystals. 58 In fact, these motifs are not limited to halogen atoms but are common to chalcogen atoms as well.…”

“…Indeed, the cyclic trimer analogous to Figure b is not an uncommon motif in crystal structures, so much so that it has been designated as the X 3 synthon, ,− which appears in crystals and two-dimensional self-assembled nanoarchitectures, as, for example, in a set of trihalomesitylenes . There are also examples in the literature of the X 4 synthon − as in Figure b, as well as X 6 and X 8 . The linear configuration is also prevalent in crystals .…”