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Conformational polymorphism in a Schiff-base macrocyclic organic ligand: an experimental and theoretical study
Leonardo Lo Presti1, Raffaella Soave, Mariangela Longhi
1Dipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, Via Golgi 19, 20133 Milano, Italy. leonardo.lopresti@unimi.it
Acta Crystallographica. Section B, Structural Science
|September 16, 2010
Summary
This study explores polymorphism in the organic Schiff-base macrocycle DIEN, revealing new solid forms and conformations. The endo conformation is most stable in solid and gas phases, with a facile interconversion pathway.
Area of Science:
- Supramolecular Chemistry
- Crystallography
- Computational Chemistry
Background:
- Polymorphism is crucial for understanding the solid-state properties of organic molecules.
- The Schiff-base macrocycle ligand DIEN (C(24)H(30)N(6)) is known only in solvated forms.
- Investigating new solid forms of DIEN is essential for its material applications.
Purpose of the Study:
- To characterize new polymorphic and solvated forms of DIEN.
- To elucidate the conformational preferences and stability of DIEN in different phases.
- To explore the relationship between crystal packing, molecular conformation, and thermodynamic stability.
Main Methods:
- Single-crystal X-ray diffraction was used to determine the crystal structures.
- Solid-state and gas-phase density functional theory (DFT) calculations were performed.
- Hirshfeld surface analysis was employed to study intermolecular interactions.
Main Results:
- Two new unsolvated polymorphs and one new solvated form of DIEN were discovered.
- Two distinct molecular conformations, 'endo' and 'exo', were identified.
- The 'endo' conformation is prevalent in the solid state and most stable in the gas phase.
- Phase (II°) contains both conformers and is the most stable, aligning with Ostwald's rule.
- DFT calculations revealed a two-step interconversion pathway between 'endo' and 'exo' conformers with low activation energies.
Conclusions:
- The study provides comprehensive insights into DIEN polymorphism and conformational behavior.
- The findings highlight the importance of crystallization conditions in controlling solid-state forms.
- The identified interconversion mechanism offers a potential route for controlling DIEN's solid-state properties.
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