Calix[4]pyrrole-Based Molecular Capsule: Dihydrogen Phosphate-Promoted 1:2 Fluoride Anion Complexation.

Area of Science:

• Supramolecular Chemistry
• Host-Guest Chemistry
• Chemical Sensing

Background:

• Anion recognition is crucial in biological and chemical processes.
• Designing receptors with tunable selectivity remains a challenge.
• Polytopic receptors offer enhanced binding capabilities through multiple binding sites.

Purpose of the Study:

• To synthesize and characterize a novel molecular capsule anion receptor (1).
• To investigate the binding modes, stoichiometries, and selectivity of receptor 1 towards various anions.
• To explore the influence of counterions and concentrations on anion binding preferences.

Main Methods:

• Synthesis of a molecular capsule comprising two calix[4]pyrrole units linked by ethylene diamide.
• ¹H NMR spectroscopy in CD₂Cl₂ to study anion binding.
• Gas-phase computational studies to rationalize binding behavior.

Main Results:

• Receptor 1 binds anions with varying stoichiometries (1:1 or 1:2) depending on the anion and counterion (tetrabutylammonium vs. tetraethylammonium).
• Selective binding of fluoride over dihydrogen phosphate is observed, tunable by relative anion concentrations.
• A preformed dihydrogen phosphate complex can be converted to a fluoride complex, rationalized by reduced reorganization energy.

Conclusions:

• Receptor 1 demonstrates flexible anion recognition capabilities.
• Anion selectivity can be fine-tuned by controlling anion concentrations and exploiting counterion effects.
• This work presents a new strategy for designing selective polytopic anion receptors.