Halogen-Bonded Supramolecular Capsules in the Solid State, in Solution, and in the Gas Phase

Area of Science:

• Supramolecular Chemistry
• Crystal Engineering
• Physical Organic Chemistry

Background:

• Supramolecular capsules are self-assembled structures with potential applications in molecular recognition and encapsulation.
• Halogen bonding (XB) is a non-covalent interaction increasingly utilized in constructing complex supramolecular architectures.
• Understanding factors influencing the stability and guest-binding properties of these capsules is crucial for their design and application.

Purpose of the Study:

• To assemble and characterize supramolecular capsules formed via neutral halogen bonding.
• To investigate the influence of donor and acceptor components on capsular association strength.
• To explore the stability and guest-binding capabilities of these capsules in different phases (solid, solution, gas).

Main Methods:

• X-ray crystallography to determine the solid-state structure of the capsules.
• Solution-state studies to quantify association constants (Ka) and binding free energies (ΔG).
• Electrospray ionization mass spectrometry (ESI-MS) to confirm gas-phase stability.

Main Results:

• Successfully assembled supramolecular capsules featuring two halogen bonding (XB) hemispheres, rigidified by hydrogen bonding to methanol and encapsulating benzene guests.
• Demonstrated that tuning the XB donor is more effective than tuning the XB acceptor for enhancing capsular association strength, likely due to desolvation penalties.
• Quantified high association constants (Ka = 2.11×10^5 M^-1) for a specific donor-acceptor pair in solution.
• Confirmed the stability of the XB capsules in the gas phase using ESI-MS.
• Discovered a novel guest binding site within an elongated iodoethynyl capsule.

Conclusions:

• Neutral halogen bonding is a viable strategy for constructing robust supramolecular capsules.
• Optimizing the halogen bonding donor is key to maximizing capsule association strength in solution.
• The studied capsules exhibit stability across different phases and possess potential for new guest binding applications.