Halogen bonds in some dihalogenated phenols: applications to crystal engineering

Area of Science:

• Solid-state chemistry and crystallography
• Supramolecular chemistry
• Materials science

Background:

• Halogen bonding is a crucial non-covalent interaction in crystal engineering.
• Distinguishing between different types of halogen bonds (e.g., type I and type II) is essential for understanding their influence on crystal packing and properties.
• Previous studies have explored halogen bonding, but a detailed comparison of type I and type II interactions, particularly involving chlorine and bromine, is needed.

Purpose of the Study:

• To elucidate the distinct structural and energetic characteristics of type I and type II halogen bonds.
• To investigate the role of bromine and chlorine in directing crystal structures and intermolecular interactions.
• To explore the potential for designing materials with specific mechanical properties based on halogen bonding.

Main Methods:

• Single-crystal X-ray diffraction was used to determine the crystal structures of several halogenated phenols.
• Crystal structure prediction protocols and computational methods (COMPASS26 force field) were employed to analyze energetically favorable packing arrangements.
• Variable-temperature crystallography was performed to study the temperature-dependent behavior of halogen bonds.

Main Results:

• 3,4-Dichlorophenol exhibits both type I and type II Cl⋯Cl interactions, while 4-bromo-3-chlorophenol shows a type II Br⋯Cl interaction.
• 3-Bromo-4-chlorophenol adopts a different crystal structure, with bromine forming a Br⋯O contact, indicating a preference for type II interactions over type I.
• Type II halogen bonds show greater length variations with temperature and are stronger than type I interactions, leading to elastic deformation in bromine-containing compounds.

Conclusions:

• Type I and type II halogen bonds are chemically distinct and exhibit different structural and mechanical properties.
• Bromine atoms show a preference for type II interactions, influencing crystal packing and enabling elastic deformation.
• The findings provide a foundation for designing novel materials with tailored properties through strategic control of halogen bonding.