Investigating the Role of Weak Interactions to Explore the Polymorphic Diversity in Difluorinated Isomeric N-Phenylcinnamamides

A total of nine difluoro derivatives of N-phenylcinnamamides have been synthesized from fluoro-substituted cinnamic acids and anilines in order to investigate the formation of polymorphs arising due to the conformational flexibility around the amide and vinyl group. Among them, four compounds have been found to exist in multiple polymorphic forms, which includes concomitant polymorphism, solvatomorphism, and packing polymorphism, while the remaining five compounds display monomorphic behavior. Crystal structure analyses of all the forms belonging to these four compounds reveal that, although the molecules are primarily held by strong N-H center dot center dot center dot O hydrogen bonds, the relative interplay of weak C-H center dot center dot center dot F, C-H center dot center dot center dot O, C-H center dot center dot center dot pi, and pi center dot center dot center dot pi interactions allows the flexible molecules to adopt different orientations and exhibit polymorphism. These forms interestingly also display different thermal stabilities, and they have been quantified by intermolecular interaction topological analyses. The occurrence of different primary packing motifs in these crystal structures has been further investigated by the crystal structure prediction (CSP) computational method, wherein an energy landscape of an unsubstituted N-phenylcinnamamide was generated and a number of hypothetical structures were accessed with experimentally obtained crystal structures of its difluoro-substituted derivatives.