Receptor that can capture a discrete monohydrated fluoride anion

Area of Science:

• Supramolecular Chemistry
• Anion Recognition
• Crystal Engineering

Background:

• Fluoride anion (F-) recognition and stabilization in solution remains challenging due to its small size and high charge density.
• Calixpyrrole derivatives have emerged as promising hosts for anion binding, but stabilizing specific hydration states, like monohydrated fluoride, is difficult.

Purpose of the Study:

• To synthesize and characterize a 'picket calix[4]pyrrole' host capable of selectively binding and stabilizing a monohydrated fluoride anion.
• To investigate the structural and spectroscopic properties of the fluoride-host complex.
• To explore the solid-state assembly of the resulting complex.

Main Methods:

• Synthesis of a 'picket calix[4]pyrrole' receptor.
• Anion binding studies using cesium fluoride (CsF) in aqueous acetonitrile.
• Structural characterization via single crystal X-ray diffraction.
• Spectroscopic analysis using low-temperature proton ((1)H) and fluorine ((19)F) Nuclear Magnetic Resonance (NMR) spectroscopy.

Main Results:

• A well-defined binding domain in the calixpyrrole host successfully stabilized a monohydrated fluoride anion.
• The monohydrated fluoride was exclusively observed when using CsF as the fluoride source, not tetrabutylammonium fluoride (TBAF).
• Single crystal X-ray diffraction and low-temperature NMR confirmed the structure of the receptor-bound, monohydrated fluoride.
• The complex self-assembled into a three-dimensional, salt-mediated organic framework in the solid state.

Conclusions:

• The 'picket calix[4]pyrrole' host effectively stabilizes a monohydrated fluoride anion, demonstrating precise control over anion hydration state.
• The choice of counterion (Cs+ vs. TBA+) is critical for achieving the desired monohydrated fluoride complex.
• The study highlights the potential of tailored host molecules for specific anion recognition and the formation of novel solid-state architectures.