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Synthon polymorphism and pseudopolymorphism in co-crystals. The 4,4'-bipyridine-4-hydroxybenzoic acid structural
Arijit Mukherjee1, Gautam R Desiraju
1Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, India.
This study reveals synthon polymorphism in 4,4'-bipyridine and 4-hydroxybenzoic acid co-crystals, with a 1:2 ratio showing greater stability. A 2:1 co-crystal exhibits pseudopolymorphism due to distinct roles of bipyridine nitrogen atoms.
Area of Science:
- Crystallography
- Supramolecular Chemistry
- Materials Science
Background:
- Co-crystals are formed by combining two or more neutral molecules in the same crystalline lattice.
- Polymorphism and pseudopolymorphism are critical phenomena influencing the physical and chemical properties of crystalline materials.
- Understanding the structural behavior of co-crystals is essential for designing materials with desired characteristics.
Purpose of the Study:
- To investigate the synthon polymorphism and pseudopolymorphism in co-crystals of 4,4 -bipyridine and 4-hydroxybenzoic acid.
- To elucidate the distinct structural roles of the nitrogen atoms in 4,4 -bipyridine within the co-crystal lattice.
- To explore the potential for mimicry by related compounds, such as 4-phenylpyridine.
Main Methods:
- Single-crystal X-ray diffraction was employed to determine the crystal structures.
- Analysis of hydrogen bonding and intermolecular interactions was performed.
- Comparison of crystal structures to identify polymorphism and pseudomorphism.
Main Results:
- A 1:2 co-crystal of 4,4 -bipyridine and 4-hydroxybenzoic acid exhibits synthon polymorphism, with one form being more stable.
- A 2:1 co-crystal demonstrates pseudopolymorphism within the same structural framework.
- The two nitrogen atoms of 4,4 -bipyridine play distinct roles in the co-crystal structure, as confirmed by mimicry using 4-phenylpyridine.
Conclusions:
- The co-crystal system of 4,4 -bipyridine and 4-hydroxybenzoic acid displays complex polymorphic and pseudopolymorphic behavior.
- The distinct roles of the bipyridine nitrogen atoms are crucial in defining the observed structural variations.
- 4-phenylpyridine can effectively mimic the structural behavior of 4,4 -bipyridine in similar co-crystal systems, offering insights into structure-property relationships.
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