2-Quinolinecarboxaldehyde: Polymorphic behavior of a small rigid molecule

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. olinecarboxaldehyde 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. iffractograms 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.