Solvent-Dependent Linear Free-Energy Relationship in a Flexible Host-Guest System

Area of Science:

• Supramolecular Chemistry
• Host-Guest Chemistry
• Physical Organic Chemistry

Background:

• Supramolecular chemistry utilizes synthetic hosts for molecular recognition.
• Understanding how environments influence host-guest binding is crucial but less explored.
• Flexible receptors present unique challenges in binding dynamics.

Purpose of the Study:

• To investigate the effect of solvent polarity on host-guest binding.
• To quantify binding affinities in various solvents using a bis(arylethynyl phenylurea) host and chloride guest.
• To establish a relationship between solvent properties and binding free energy.

Main Methods:

• Host-guest binding experiments were conducted using a bis(arylethynyl phenylurea) host and a chloride guest.
• Binding was studied in eight different solvents with varying polarities (E-T(30) values from 40.7 to 47.4 kcal mol-1).
• Free energy of binding was measured and correlated with solvent polarity parameters.

Main Results:

• Binding affinity decreased significantly in polar solvents compared to nonpolar solvents.
• A linear free-energy relationship was observed between the E-T(30) solvent polarity parameter and the binding free energy.
• Solvent effects, particularly in polar media, can magnify contributions to the free energy of binding.

Conclusions:

• Solvent environment plays a critical role in governing host-guest binding, especially for flexible receptors.
• The study emphasizes the importance of host reorganization energy in the binding process.
• Preorganization of the host may be less critical for weak binding than previously assumed, particularly when solvent effects are dominant.