Halogen bonds in crystal engineering: like hydrogen bonds yet different.

Area of Science:

• Supramolecular Chemistry
• Crystal Engineering
• Chemical Physics

Background:

• Halogen bonds are attractive interactions where an electrophilic halogen atom interacts with a nucleophilic species.
• These interactions have been observed in crystals for approximately 50 years, existing in symmetrical (Type I) and bent (Type II) forms.
• The research group has utilized halogen bonding in crystal engineering for nearly three decades, focusing on halogen-halogen (X···X) and halogen-heteroatom (X···B) interactions.

Purpose of the Study:

• To illustrate crystal engineering applications using the modern definition of halogen bonds.
• To compare and contrast halogen bonds with hydrogen bonds, highlighting similarities and differences.
• To provide insights into the geometric and chemical factors governing halogen bond interactions.

Main Methods:

• Analysis of X···X interactions using the Cambridge Structural Database (CSD).
• Experimental studies on 3,4-dichlorophenol and related compounds to investigate halogen···halogen interactions.
• Variable temperature studies to differentiate between Type I and Type II contacts.

Main Results:

• A clear geometric and chemical distinction exists between Type I (non-halogen bond) and Type II (halogen bond) contacts.
• Cl/Br isostructurality can be explained through a geometric model, and experimental data reveal chemical differences between Cl and Br.
• Halogen bonds offer tunable strength, size, and gradation, facilitating the design of ternary cocrystals and structural modularity.

Conclusions:

• Halogen bonds are versatile tools in crystal engineering due to their directionality, strength, and tunable nature.
• They enable the rational design of molecular crystals with specific properties, including mechanical characteristics and the organization of larger structural units (LSAMs).
• Understanding the distinctions between halogen and hydrogen bonds allows for more sophisticated applications in crystal engineering.