Synthetic Approaches to Halogen Bonded Ternary Cocrystals

Area of Science:

• Solid-state chemistry
• Supramolecular chemistry
• Crystallography

Background:

• Cocrystal synthesis relies on understanding supramolecular synthons.
• Ternary cocrystals, involving three components, present unique synthetic challenges.
• Halogen bonding is a key non-covalent interaction in crystal engineering.

Purpose of the Study:

• To report three synthetic approaches for creating ternary halogen-bonded cocrystals.
• To illustrate the specificity and generality of these synthetic methods.
• To provide insights into the design principles for ternary cocrystal formation.

Main Methods:

• Investigated three distinct synthetic strategies for ternary cocrystal assembly.
• Analyzed the role of electrophilicity/nucleophilicity differences in halogen bond site selection.
• Evaluated the relative strengths of halogen bonds within ternary structures (A⋅⋅⋅B vs. B⋅⋅⋅C).
• Utilized interaction mimicry of hydrogen bonds by halogen bonds, exemplified by a pyrene-based system.

Main Results:

• Demonstrated three viable synthetic routes towards ternary halogen-bonded cocrystals.
• Highlighted the necessity of differing electrophilicity/nucleophilicity at halogen bonding sites.
• Confirmed that the two halogen bonds in a ternary system (A⋅⋅⋅B and B⋅⋅⋅C) must possess different strengths.
• Showcased the successful application of halogen bond mimicry of hydrogen bonds using a pyrene structure.

Conclusions:

• Successful ternary cocrystal synthesis is achievable through a knowledge of supramolecular synthons.
• Specific conditions, including differential halogen bond strengths and site reactivity, are crucial for ternary cocrystal formation.
• Halogen bonds can effectively mimic hydrogen bonds, offering a powerful strategy for designing complex cocrystal architectures.
• Crystal engineers can anticipate and design for halogen bonds stronger than hydrogen bonds.