Preferential axial protonation in a zwitterionic calix[4]arene

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Crystal Engineering

Background:

• Calixarenes are macrocyclic compounds with a unique three-dimensional structure.
• Substituent effects significantly influence calixarene properties and behavior.
• Understanding molecular conformation and charge distribution is crucial in supramolecular chemistry.

Purpose of the Study:

• To investigate the solid-state and solution-state structure of a novel substituted calixarene.
• To determine the influence of dimethylamino substituents on calixarene conformation and charge.
• To elucidate the formation of zwitterionic species in calixarene derivatives.

Main Methods:

• Single-crystal X-ray diffraction analysis to determine solid-state structure.
• Spectroscopic methods (e.g., NMR, IR) to study behavior in polar solvents.
• Computational modeling to support experimental findings (if applicable).

Main Results:

• Calixarene 3, substituted with dimethylamino groups at two methylene bridges, was synthesized.
• X-ray crystallography revealed a zwitterionic structure in the crystal state.
• Solution-state studies in polar solvents confirmed the persistence of the zwitterionic form, with protonation of an axial dimethylamino group.

Conclusions:

• The presence of dimethylamino groups induces a zwitterionic character in Calixarene 3.
• This zwitterionic nature is stable in both the crystalline state and polar solvents.
• The findings highlight the role of specific substituents in controlling the electronic and structural properties of calixarenes.