Phenyl-perfluorophenyl synthon mediated cocrystallization of carboxylic acids and amides

The significance of face-to-face pi-pi 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 C6H5COOH.C6F5COOH 1, C6H5CONH2.C6F5CONH2 2, and C6H5CONH2.C6F5COOH 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.