Racemic and Enantiopure Camphene and Pinene Studied by the Crystalline Sponge Method

Area of Science:

• Crystallography
• Supramolecular Chemistry
• Organic Chemistry

Background:

• Chiral compounds present challenges in structure determination.
• The crystalline sponge method offers a potential solution for resolving chiral guest molecules.
• Achiral frameworks can host chiral guests, but ordering is not always guaranteed.

Purpose of the Study:

• To demonstrate the use of an achiral metal-organic framework for the structure determination of chiral compounds, specifically camphene and pinene.
• To investigate the conditions under which enantiomeric pairs of guest molecules order within the crystalline sponge host.
• To evaluate powder X-ray diffraction as a method for monitoring guest molecule ordering and determining optimal soaking times.

Main Methods:

• Utilizing an achiral metal-organic framework as a crystalline sponge host.
• Soaking the host framework with enantiopure and impure samples of camphene and pinene.
• Employing powder X-ray diffraction to analyze the incorporation and ordering of guest molecules.
• Observing crystal color changes as an indicator of guest uptake.

Main Results:

• Enantiopure β-pinene was successfully resolved using the crystalline sponge method.
• α-pinene and (+)-camphene did not show ordering with enantiopure material alone.
• Enantiomeric pairs of camphene and α-pinene, as well as mixtures of α-pinene and β-pinene, exhibited ordering within the host framework.
• Powder X-ray diffraction directly visualized the incorporation of ordered guest molecules, allowing for optimization of soaking times.
• Crystal color change indicated guest uptake but not necessarily ordering, and could be caused by host degradation.

Conclusions:

• An achiral crystalline sponge can resolve chiral compounds like camphene and pinene by facilitating the ordering of enantiomeric pairs.
• Powder X-ray diffraction is a reliable method for confirming guest ordering and optimizing experimental conditions, unlike crystal color change.
• The presence of an inversion center in the host framework influences the ordered incorporation of guest molecules.