Evaluation of weak interactions in [2]pseudorotaxanes

Area of Science:

• Supramolecular Chemistry
• Host-Guest Chemistry
• Computational Chemistry

Background:

• Viologens and bis-p-phenylene crown ethers form [2]pseudorotaxanes.
• Steric hindrance in 3,3'-dimethylviologens significantly weakens complex formation.

Purpose of the Study:

• To investigate the binding affinity of sterically hindered viologens in [2]pseudorotaxanes.
• To quantify weak interactions in host-guest complexes using novel methods.

Main Methods:

• Density Functional Theory (DFT) calculations to determine energy costs.
• X-ray crystallography for structural analysis.
• Dilution method and host-guest Nuclear Overhauser Effect (NOE) correlations for thermodynamic measurements.

Main Results:

• DFT calculations revealed a substantial energy cost of 55 kJ/mol for 'flattening' hindered viologens.
• X-ray crystallography and NMR confirmed DFT findings.
• NOE correlations quantified a 8.5 kJ/mol weaker binding in hindered analogues compared to regular ones.

Conclusions:

• The energy cost of conformational changes prevents stable complex formation in sterically hindered systems.
• Host-guest NOE is a valuable technique for quantifying weak interactions in [2]pseudorotaxanes and other host-guest complexes.