Ion-pair recognition by a neutral [2]rotaxane based on a bis-calix[4]pyrrole cyclic component

Area of Science:

• Supramolecular Chemistry
• Organic Synthesis

Background:

• Rotaxanes are mechanically interlocked molecules with potential applications in molecular machines.
• Bis(calix[4]pyrrole) macrocycles offer unique binding properties for guest molecules.
• Amidepyridyl-N-oxide derivatives serve as versatile components in supramolecular assemblies.

Purpose of the Study:

• To synthesize and characterize a novel [2]rotaxane incorporating a bis(calix[4]pyrrole) unit and a specific axle.
• To investigate the self-assembly of the rotaxane precursor into pseudorotaxane structures.
• To explore the binding interactions of the synthesized [2]rotaxane with tetraalkylammonium salts.

Main Methods:

• Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) for rotaxane synthesis.
• Optimization of an "in situ" capping strategy for high-yield isolation.
• 1H NMR titrations and Isothermal Titration Calorimetry (ITC) for binding studies.

Main Results:

• A [2]rotaxane was synthesized with a 50% yield using an optimized CuAAC reaction.
• The rotaxane precursor formed a [2]pseudorotaxane with tetrabutylammonium salts.
• The [2]rotaxane demonstrated efficient heteroditopic receptor behavior for tetraalkylammonium salts, forming stable 1:1 complexes.
• Higher order aggregates (2:1 complexes) formed upon addition of excess salt.

Conclusions:

• The developed synthetic strategy enables efficient access to novel bis(calix[4]pyrrole)-based [2]rotaxanes.
• The synthesized [2]rotaxane acts as a potent host for tetraalkylammonium salts in solution.
• The study highlights the potential of such rotaxanes in selective ion recognition and supramolecular assembly.