Solvatomorphism and first-time observation of acid-acid catemer in 4-phenylamino-benzoic acids

To investigate the polymorphism in 4-phenylamino-benzoic acids (4-PABAs) in general, and the effect on the polymorphism of these compounds exerted by substitution in particular, a series of 4-PABAs (1-8) varying in the substitution position and pattern were synthesized, and their polymorphic behavior was investigated for the first time. A relatively comprehensive polymorph screening led to the discovery of two forms, one solvent-free and the other solvate, for compounds 1, 3 and 8, and one form for the other compounds. The crystal structures were determined by single-crystal XRD. All the 4-PABAs in the crystal structures are highly twisted, and all the solvent-free crystals are based on the conventional acid-acid dimer motif, except for 2, which has a rarely observed acid-acid catemer motif. Two of the solvates (1-S and 8-S) have pyridine in the lattice while the other (3-S) has dichloromethane. The observation indicates that neither conformational flexibility or substitution alone nor the combination of both leads to polymorphism in these compounds, which is in dramatic contrast to the polymorphism of fenamic acids. The thermal properties of each system were investigated by differential scanning calorimetry and desolvation of the solvates was studied by thermogravimetric analysis. Hirshfeld surface analysis and molecular dynamics simulation were performed to study the mechanism of polymorphism and the intermolecular interactions contributing to the formation and stability of each crystal form.