NMR Quantification of Halogen-Bonding Ability To Evaluate Catalyst Activity.

Area of Science:

• Organic Chemistry
• Supramolecular Chemistry
• Spectroscopy

Background:

• Halogen bonding (XB) is a crucial non-covalent interaction.
• Quantifying XB donor strength is essential for designing XB catalysts and materials.
• Existing methods for XB quantification can be time-consuming or require specialized equipment.

Purpose of the Study:

• To develop a rapid and reliable method for quantifying the halogen-bonding abilities of organic molecules.
• To investigate the relationship between halogen-bond donor strength and catalytic performance in organic reactions.
• To assess the utility of phosphorus-31 NMR spectroscopy for evaluating halogen-bond donors.

Main Methods:

• Synthesis of benzimidazolium-, imidazolium-, and bis(imidazolium)-based halogen-bond donors.
• Quantification of halogen-bonding using phosphorus-31 NMR spectroscopy (³¹P NMR).
• Calculation of activation free energy (ΔG^‡) for a model reaction.
• Evaluation of catalytic activity in a Friedel-Crafts indole addition reaction.

Main Results:

• ³¹P NMR chemical shift changes (Δδ(³¹P)) effectively quantified halogen-bonding abilities.
• Measured Δδ(³¹P) values showed a strong correlation with calculated activation free energy (ΔG^‡).
• The NMR method accurately predicted the catalytic activity of the halogen-bond donors in a Friedel-Crafts reaction.
• The method demonstrated sensitivity to trace amounts of Brønsted acid impurities.

Conclusions:

• Phosphorus-31 NMR spectroscopy provides a rapid and effective means to quantify halogen-bonding strengths.
• The developed NMR method serves as a valuable tool for screening and optimizing halogen-bond donors for catalysis.
• This technique offers a sensitive detection method for acidic impurities in organic samples.