Cucurbituril slippage: cations as supramolecular lubricants

Area of Science:

• Supramolecular Chemistry
• Host-Guest Chemistry
• Molecular Dynamics

Background:

• Cucurbit[7]uril (CB[7]) is a macrocyclic host known for its ability to encapsulate guest molecules.
• The dethreading process of guest molecules from CB[7] is a key aspect of host-guest complex dynamics.
• Polyamidated axles with crown ether stoppers represent a class of molecules studied for their interactions with macrocyclic hosts.

Purpose of the Study:

• To investigate the effect of aqueous metallic and organic cations on the dethreading rate of a specific polyaminated axle from CB[7].
• To elucidate the mechanism by which cations influence the dethreading kinetics.
• To explore the potential for external stimuli to modulate molecular motion within supramolecular cages.

Main Methods:

• Synthesis of a polyaminated axle functionalized with benzo-15-crown-5 stoppers.
• Host-guest complexation studies with Cucurbit[7]uril (CB[7]).
• Kinetic measurements of the dethreading process in the presence of various aqueous cations using spectroscopic techniques.

Main Results:

• The dethreading rate of the polyaminated axle from CB[7] was enhanced by up to 500-fold in the presence of aqueous metallic and organic cations.
• The observed rate enhancement suggests a significant influence of cations on the dethreading kinetics.
• The interaction of cations with both the crown ether stoppers and the CB[7] cavity appears to be crucial for stabilizing the transition state.

Conclusions:

• Aqueous cations can dramatically accelerate the dethreading of specific guest molecules from Cucurbit[7]uril.
• Cation-π interactions and coordination effects likely play a role in stabilizing the high-energy transition state of dethreading.
• This finding opens avenues for controlling molecular release and motion in supramolecular systems using external ionic stimuli.