2-Quinolinecarboxaldehyde: Polymorphic behavior of a small rigid molecule

► A new example of a small rigid molecule with a complex polymorphic behavior. ► Two crystalline structures, forms I and III, solved at room temperature. ► In forms I and III the hydrogen formyl does not participate in the hydrogen bonds. ► XRPD diffractograms point to preferential orientation effects. ► XRPD point to the existence of structures with only slight differences amongst them. This work reports an investigation on the polymorphism of 2-quinolinecarboxaldehyde, a quinoline derivative, frequently used as a ligand in the synthesis of metal complexes. 2-Quinolinecarboxaldehyde lacks both molecular flexibility and the ability to form strong hydrogen bonds, two characteristics often seen as driving forces for the occurrence of polymorphism. Nevertheless, a rich polymorphic behavior was found for this substance. Polymorphic forms were generated by crystallization from solutions, and by melt cooling. Four polymorphic forms could be clearly identified by thermal analysis investigation and the crystalline structures of forms I and III were solved by single-crystal X-ray diffraction, at room temperature. In polymorph I, molecules are joined by π–π and weak C−H⋯O interactions while in polymorph III helicoidal chiral chains are formed and very weak C−H⋯O intermolecular interactions can be identified. Neither of these intermolecular interactions involves the formyl hydrogen atom. Concomitant polymorph crystallization from the melt was often observed. XRPD diffractograms which showed similarities to that of polymorph I but presented striking differences were obtained in some experiments. In certain cases the discrepancies may be ascribed to effects of preferential orientation. However, the existence of multiple but slightly different structures with small differences seems to be a better explanation for these experimental observations.