Polymorphism in cocrystals of urea:4,4′-bipyridine and salicylic acid:4,4′-bipyridine

Abstract

Polymorphic cocrystals of urea:4,4′-bipyridine and salicylic acid:4,4′-bipyridine were obtained by crystallization from different solvents. The urea tape is a rare phenomenon in cocrystals but it is consistent in urea:4,4′-bipyridine polymorphic cocrystals. The polymorph obtained from MeCN has symmetrical N–H⋯N hydrogen bond distances on either side of the urea tape. However, the other form obtained from MeOH has unsymmetrical N–H⋯N hydrogen bond lengths. In the polymorphic cocrystals of salicylic acid:4,4′-bipyridine, the basic supramolecular synthon acid–pyridine is the same but the 3D packing is different. Both the polymorphic pairs of cocrystals come under the category of packing polymorphs. All polymorphs were characterized by single-crystal X-ray diffraction (SCXRD), PXRD, DSC, FT-IR and HSM. N–H⋯N and the acid–pyridine supramolecular synthons were insulated by FT-IR vibrational spectroscopy.