Exploring the Competition between Halogen Bonding and CH Hydrogen Bonding in Bromoarenes Using an Aryldiyne Template

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Computational Chemistry

Background:

• Halogen bonding (often involving bromines) is a well-established non-covalent interaction.
• CH hydrogen bonding, particularly with electron-deficient hydrogens, is increasingly recognized as a significant interaction.
• Understanding the interplay between these competing interactions is crucial for designing molecular systems and materials.

Purpose of the Study:

• To experimentally and computationally investigate the competitive nature of CH hydrogen bonding versus halogen bonding in functionalized bromoarenes.
• To explore the influence of electron-withdrawing groups on the strength and preference of these interactions.
• To assess the feasibility of selective crystal engineering based on these non-covalent forces.

Main Methods:

• Synthesis of a templated system featuring an aryldiyne bridge to control the spatial arrangement of interacting groups.
• Experimental investigation of CH hydrogen bonding and halogen bonding interactions within the templated system.
• Computational modeling using M06-2x/6-311+G-(2d,p) and B3LYP/6-311++G** levels of theory to calculate interaction energies and map molecular electrostatic potentials.

Main Results:

• Experimental evidence and computational support indicate that CH hydrogen bonds can be competitive with, and sometimes preferred over, halogen bonds to bromine.
• The preference for CH hydrogen bonding is attributed to a greater positive electrostatic potential on the hydrogen atom compared to the bromine atom in electron-deficient systems.
• Computational studies showed a preference for the CH hydrogen bond conformer irrespective of the type or position of electron-withdrawing substituents (-F or -CF3).

Conclusions:

• CH hydrogen bonding in functionalized arenes presents a viable alternative to halogen bonding for molecular recognition.
• The energy differences between CH hydrogen bonding and halogen bonding conformers are often small (≤5 kJ/mol).
• Achieving selective crystal design based solely on CH hydrogen bonding or halogen bonding interactions presents a significant challenge due to the comparable interaction strengths.