Symmetric halogen bonding is preferred in solution.

Area of Science:

• Chemistry
• Molecular Interactions
• Supramolecular Chemistry

Background:

• Halogen bonding is a key non-covalent interaction with growing importance in drug design and materials science.
• While hydrogen bond symmetry is well-studied, the symmetry of halogen bonds, particularly three-center systems, remains less understood.
• Understanding halogen bond symmetry is crucial for harnessing their full potential in molecular design.

Purpose of the Study:

• To investigate the symmetry of three-center halogen bonds ([N-I-N]+ and [N-Br-N]+) in solution.
• To compare the symmetry of halogen bonds with that of hydrogen bonds.
• To explore the energetic favorability of symmetric halogen bonding arrangements.

Main Methods:

• Utilized the isotopic perturbation of equilibrium (IPE) technique.
• Employed carbon-13 Nuclear Magnetic Resonance ((13)C NMR) spectroscopy for detection.
• Investigated regioselectively deuterated pyridine complexes in solution.
• Performed Density Functional Theory (DFT) level computations for confirmation.

Main Results:

• Observed a clear preference for symmetric arrangements in both [N-I-N]+ and [N-Br-N]+ halogen bonds in solution.
• Confirmed symmetric preference in both flexible and conformationally restricted model systems.
• Demonstrated that a nearby counterion does not disrupt the symmetric halogen bond arrangement.
• Computational results corroborated the experimental findings, predicting a significant energetic gain for symmetric structures.

Conclusions:

• Symmetric, three-center, four-electron halogen bonds ([N-X-N]+) are energetically favored in solution.
• Unlike hydrogen bonds, which are typically asymmetric in solution, analogous bromine and iodine halogen bonds prefer symmetric configurations.
• These findings highlight halogen bonding as a distinct and valuable interaction for rational molecular design.