Experimental quantification of anion-π interactions in solution using neutral host-guest model systems

Area of Science:

• Supramolecular Chemistry
• Physical Chemistry
• Organic Chemistry

Background:

• Cation-π interactions are well-established, involving attraction between cations and electron-rich aromatic systems.
• Anion-π interactions are theorized as the electrostatic opposite, requiring electron-deficient aromatic systems to attract anions.
• While observed in gas and solid states, attractive anion-π interactions in solution are less common and weaker.

Purpose of the Study:

• To explore and provide examples of neutral molecular receptors that bind anions in solution via anion-π interactions.
• To investigate the role of anion-π interactions, both solely and in conjunction with hydrogen bonding, in molecular recognition.
• To experimentally determine the binding energies of anion-π interactions in solution and assess their significance.

Main Methods:

• Computational studies to model anion-π interactions and estimate binding energies.
• Synthesis of neutral molecular receptors designed to interact with anions.
• Experimental techniques to study anion binding in solution, focusing on halide recognition.
• Analysis of binding data, considering the influence of hydrogen bonding and using appropriate reference systems.

Main Results:

• Confirmation of attractive anion-π interactions in solution, though typically weaker than in other phases.
• Binding energies for anion-π interactions in solution are estimated to be less than 1 kcal/mol per substituted phenyl group.
• Anion-π interactions can be indirectly detected through modulation of stronger hydrogen bonding interactions.
• Quantification is complex due to the need for model systems and the use of salt precursors.

Conclusions:

• Anion-π interactions exist and are experimentally verifiable in solution.
• These interactions are generally weak in solution, limiting their role in selective anion binding.
• Potential applications exist in catalysis and transport within synthetic and biological systems.
• Further research is needed to refine quantification methods and explore applications.