Halogen Bond Asymmetry in Solution.

Area of Science:

• Supramolecular Chemistry
• Chemical Bonding
• Computational Chemistry

Background:

• Halogen bonding involves halogen atoms acting as electron acceptors in noncovalent interactions.
• Unlike rapidly equilibrating asymmetric hydrogen bonds ([D···H···D]⁺), analogous halogen bonds ([D···X···D]⁺) typically exhibit static and symmetric geometries.
• The potential for asymmetry and dynamic behavior in halogen bonds remains largely unexplored.

Purpose of the Study:

• To investigate the induction of asymmetry in three-center halogen bonds ([D-X···D]⁺) by modulating electronic or steric factors.
• To explore the conversion of static three-center halogen bonds into rapidly exchanging asymmetric isomers, mirroring hydrogen bond behavior.
• To understand the factors governing the geometry and dynamics of halogen-bonded complexes.

Main Methods:

• Utilized ¹⁵N Nuclear Magnetic Resonance (NMR) spectroscopy.
• Employed Intermolecular Potential Energy (IPE) NMR.
• Performed Density Functional Theory (DFT) calculations.

Main Results:

• Demonstrated that desymmetrization of electron density leads to a static, asymmetric halogen bond geometry ([D-X···D]⁺).
• Computationally showed that increasing donor-donor distance can enable dynamic exchange between asymmetric isomers ([D···X-D]⁺ ⇄ [D-X···D]⁺).
• Observed a strong preference for forming dimers with two static, symmetric three-center halogen bonds over dynamic, asymmetric forms due to high energetic gain.

Conclusions:

• Halogen bonding exhibits fundamentally different secondary bonding preferences compared to hydrogen bonding.
• Static asymmetry in halogen bonds is achievable through electronic manipulation.
• Understanding electronic and steric influences is crucial for designing improved halonium transfer agents and advancing chemical bonding knowledge.