Boat and Chair Shaped Hexahalogen Synthons

Area of Science:

• Crystal Engineering
• Supramolecular Chemistry
• Organic Chemistry

Background:

• Non-covalent halogen bonding is crucial for designing supramolecular structures.
• Cyclic hexahalogen synthons are key building blocks in crystal engineering.

Purpose of the Study:

• To report the first crystalline evidence of boat and chair-shaped cyclic hexahalogen synthons.
• To investigate the nature and stability of interactions within these synthons.
• To analyze the crystal packing of pentabromo-o-xylene (PBX) and hexabromo-o-xylene (HBX).

Main Methods:

• X-ray crystallography to determine crystal structures.
• Quantum-chemical calculations, specifically Pendás' interacting quantum atoms approach.
• Analysis of intermolecular interactions, focusing on halogen bonding (Br⋅⋅⋅Br).

Main Results:

• First crystalline evidence of unique boat and chair-shaped cyclic hexahalogen synthons in PBX and HBX, respectively.
• Quantum-chemical calculations confirm the stability of Br⋅⋅⋅Br interactions, driven by exchange-correlation.
• PBX crystallizes in a chiral space group (P2₁) with helix-like packing, while HBX packs in a centrosymmetric space group (P2₁/n).
• Extended furcations in PBX lead to a molecular framework with macrocycles formed via halogen bonding.

Conclusions:

• Novel boat and chair-shaped cyclic hexahalogen synthons have been successfully synthesized and characterized crystallographically.
• Halogen bonding interactions are essential for the formation and stability of these unique supramolecular structures.
• Crystal packing forces dictate the specific chiral or centrosymmetric arrangements of these molecules, leading to distinct supramolecular architectures.