Octahydroxypyridine[4]arene self-assembles spontaneously to form hexameric capsules and dimeric aggregates

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Physical Chemistry

Background:

• Resorcin[4]arenes and pyrogallol[4]arenes are known to form hydrogen-bonded hexameric capsules in nonpolar solvents.
• Understanding the self-assembly of related macrocycles is crucial for designing novel supramolecular structures.

Purpose of the Study:

• To investigate the self-assembly and aggregation modes of octahydroxypyridine[4]arene (1 b) in chloroform using NMR spectroscopy.
• To compare the aggregation behavior of octahydroxypyridine[4]arene with known resorcin[4]arene and pyrogallol[4]arene systems.
• To elucidate the role of solvent encapsulation and the effect of acid on aggregate stability.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy, with a specific focus on diffusion NMR.
• Spectroscopic studies utilizing a C-undecylresorcin[4]arene (2 b) hexameric capsule as a reference compound.
• Analysis of (1)H NMR and diffusion NMR data to identify aggregate species, solvent encapsulation, and monomerization.

Main Results:

• Octahydroxypyridine[4]arene (1 b) spontaneously self-assembles into both hexameric and dimeric aggregates in chloroform solutions.
• Diffusion NMR confirmed the presence of encapsulated chloroform molecules within the hexameric structures.
• Addition of trifluoroacetic acid induced disassembly of the hexamer and dimer of 1 b into monomers, while leaving the reference hexamer (2 b) unaffected.

Conclusions:

• Octahydroxypyridine[4]arene (1 b) forms stable hexameric capsules in nonpolar solvents, similar to resorcin[4]arenes and pyrogallol[4]arenes.
• The self-assembly process is spontaneous and influenced by solvent and the presence of acids.
• Diffusion NMR is a powerful tool for characterizing aggregate formation, solvent encapsulation, and disassembly dynamics in supramolecular systems.