Weak hydrogen bonding as a basis for concentration-dependent guest selectivity by a cyclophane host

Area of Science:

• Supramolecular Chemistry
• Crystal Engineering
• Host-Guest Chemistry

Background:

• Cyclophane hosts form inclusion complexes with guest molecules.
• Understanding host-guest selectivity is crucial for molecular recognition and separation.
• Weak hydrogen bonds play a role in the stability of crystalline inclusion compounds.

Purpose of the Study:

• To investigate the formation and properties of crystalline inclusion complexes between cyclophane 1 and picoline/lutidine isomers.
• To analyze the selectivity of the cyclophane host towards different isomers using competition experiments.
• To correlate observed selectivities with structural and energetic factors.

Main Methods:

• Synthesis and crystallization of inclusion complexes.
• X-ray diffraction analysis for structural determination.
• Thermal gravimetry (TG) and differential scanning calorimetry (DSC) for property analysis.
• Guest competition experiments to assess host selectivity.

Main Results:

• Cyclophane 1 discriminates between picoline and lutidine isomers, forming crystalline inclusion complexes.
• Selectivity is concentration-dependent in some cases.
• Inclusion of 4-picoline is favored at higher concentrations.
• 2,4-lutidine is preferred, indicating a preference for the methyl group at the 4-position.
• Observed selectivities align with adduct class assignments and calculated lattice energies.

Conclusions:

• Cyclophane 1 exhibits significant selectivity for specific picoline and lutidine isomers.
• The host's preference is influenced by isomer structure and concentration.
• Lattice energies can help explain the observed selectivity in picoline isomer complexes.
• Caution is advised when inferring selectivity from guest competition experiments alone.