Accelerating or Slowing: Fine Tuning the De-Threading Kinetics of a T-Shaped Benzimidazolium Pumping Cassette

Area of Science:

• Supramolecular Chemistry
• Materials Science
• Nanotechnology

Background:

• Artificial molecular machines (AMMs) require precise control over molecular motion.
• Mechanically interlocked molecules, such as pseudorotaxanes, are key architectures for AMMs.
• Understanding structure-property relationships in pseudorotaxanes is crucial for designing advanced AMMs.

Purpose of the Study:

• To systematically investigate how structural parameters influence the stability and de-threading kinetics of [2]pseudorotaxanes.
• To elucidate the role of stopper size and macrocycle rigidity in governing pseudorotaxane dynamics.
• To provide design principles for engineering pseudorotaxane-based modules for AMMs.

Main Methods:

• Synthesis of a library of 10 neutral [2]pseudorotaxanes using ring-closing metathesis and hydrogenation.
• Kinetic studies in DMSO-d₆ at 338 K to determine de-threading half-lives (t₁/₂) and activation free energies (ΔG‡).
• Gas-phase modeling and NMR analyses to probe host-guest interactions and energy barriers.

Main Results:

• De-threading half-lives varied over five orders of magnitude, from 0.237 hours to 436 hours.
• Increasing stopper size and macrocycle rigidity (e.g., using B24C6) significantly reduced de-threading rates.
• A specific pseudorotaxane (4d⁻) with a benzylic phenyl stopper and B24C6 exhibited a t₁/₂ of 168 hours and ΔG‡ of 29.04 kcal/mol.

Conclusions:

• Structural parameters, particularly stopper geometry and macrocycle rigidity, are critical for controlling pseudorotaxane stability and kinetics.
• Design principles derived from this study can guide the development of more robust and programmable molecular machines.
• This research paves the way for the creation of advanced AMMs, including molecular pumps.