Metastable Macrocyclic Bis-Meisenheimer Adduct

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Chemical Crystallography

Background:

• Calixarenes are macrocyclic compounds with diverse applications.
• Meisenheimer complexes are anionic intermediates in nucleophilic aromatic substitution.
• Metastability in chemical systems is rare and scientifically significant.

Purpose of the Study:

• To investigate the reversible cyanide capture by an electron-deficient tetranitroazacalixarene.
• To characterize the resulting dianionic macrocycle and its unique properties.
• To explore the dynamic behavior and stabilization mechanisms of the adduct.

Main Methods:

• Single-crystal X-ray diffraction for structural determination.
• Nuclear Magnetic Resonance (NMR) spectroscopy for characterization.
• Electronic absorption spectroscopy to study electronic properties.
• Computational analyses to understand stabilization.

Main Results:

• Formation of a novel class of metastable dianionic macrocycles containing two Meisenheimer units.
• Demonstration of reversible cyanide capture and release, acting as a chemical fuel.
• Identification of intramolecular hydrogen bonding as a key stabilization factor.
• Tunable dynamic behavior influenced by concentration, counterion, solvent, and temperature.

Conclusions:

• Tetranitroazacalixarene enables reversible cyanide capture, forming a stable Meisenheimer complex adduct.
• Macrocyclic preorganization and non-covalent interactions are crucial for this reversible reactivity.
• The system exhibits dynamic behavior akin to out-of-equilibrium assemblies, offering new possibilities in molecular design.