Phenyl-perfluorophenyl synthon mediated cocrystallization of carboxylic acids and amides

Abstract

The significance of face-to-face π-π stacking of phenyl and perfluorophenyl rings (Ph-Ph^F), a robust supramolecular synthon in aromatic and perfluoroaromatic crystal structures, is studied in the cocrystallization of simple aromatic carboxylic acids and amides. X-ray crystal structures of C₆H₅COOHπC₆F₅COOH 1, C₆H₅CONH₂πC₆F₅CONH₂ 2, and C₆H₅CONH₂πC₆F⁵COOH 3 are analyzed to understand the role of Ph-Ph^F synthon in directing self-assembly and hydrogen bonding in these cocrystals. The strong hydrogen bond donor acidity of C₆F₅COOH and C₆F₅CONH₂ together with mixed stacks of phenyl and perfluorophenyl rings steer acid···acid and amide···amide hydrogen bonding in cocrystals 1 and 2. Acid···amide hydrogen bonding is sufficiently strengthened by donor acidity and acceptor basicity in 3 that the role of the Ph-Ph^F synthon is weaker because the aromatic rings stack with lateral offset. The complex C₆H₅COOH·C₆F₅CONH₂ 4 could not be obtained under similar crystallization conditions. The crystal structure of C₆F₅CONH₂ is determined to compare molecular conformation and hydrogen bonding with motifs in the cocrystals. This study shows the viability of the Ph-Ph^F synthon in the presence of strong hydrogen bonding COOH and CONH₂ groups for crystal engineering.