Urea vs. thiourea in anion recognition.

Area of Science:

• Supramolecular Chemistry
• Anion Recognition
• Hydrogen Bonding

Background:

• Neutral receptors (LH) form stable 1:1 hydrogen-bonded complexes with various anions (X-).
• Anion complexation is crucial in biological and chemical systems.
• Understanding anion binding and subsequent reactions is key to designing new receptors.

Purpose of the Study:

• To investigate the deprotonation of neutral anion receptors upon complexation with anions.
• To explore the factors influencing the release of HX fragments and formation of [HX2]- species.
• To correlate receptor acidity and [HX2]- stability with deprotonation tendency.

Main Methods:

• Synthesis and characterization of neutral anion receptors (urea and thiourea derivatives).
• Spectroscopic studies (e.g., NMR) to monitor complex formation and deprotonation.
• Thermodynamic and kinetic analysis of anion binding and release.

Main Results:

• Neutral receptors form stable 1:1 hydrogen-bonded complexes with carboxylates, halides, and phosphates.
• Excess anions can induce deprotonation of the receptor (L-) and formation of [HX2]- self-complexes.
• Receptor acidity and [HX2]- stability dictate the extent of deprotonation; more acidic receptors and stable [HX2]- favor deprotonation.

Conclusions:

• The deprotonation of neutral anion receptors is influenced by both receptor properties and the nature of the bound anion.
• Thiourea receptors show broader deprotonation capabilities than urea receptors due to higher acidity.
• The formation of stable [HX2]- anions, like [HF2]-, plays a critical role in driving receptor deprotonation.