A Failed Cocrystallization Attempt Resulted in Novel First Polymorphic Structure: Crystal Structure and Hirshfeld Surface Analysis of the Polymorph of 4-Oxo-4-(pyridin-2-ylamino)butanoic Acid

Abstract

An attempt to cocrystallize 4-oxo-4-(pyridin-2-ylamino)butanoic acid (APS) with adipic acid in the ratio 1:1 was made. Solvent assisted grinding followed by solvent evaporation technique using ethanol as solvent yielded single crystals. Single crystal X-ray diffraction (SCXRD) studies of the obtained crystals showed that an attempt to obtain cocrystals has failed and instead has yielded a novel and very first polymorphic structure of APS (1). Compound 1 crystallizes with one molecule in the asymmetric unit in monoclinic P21/n system, whereas, the earlier reported polymorph, 2, crystallizes in triclinic P-1 system with two molecules in the asymmetric unit. The crystal structure of 1 features aminopyridine···carboxylic O-H···N and N-H···O=C interactions between molecules resulting in R2 2(8) supramolecular hetero-synthon, similar to that observed in polymorph 2. The R2 2(8) dimer propagates into a 2D sheet along the body diagonal plane (intersecting a and c axis) via a pair of C-H···O intermolecular interactions having R2 2(14) motif. Polymorph 2, on the other hand, features several C-H···O intermolecular interactions that extends the R2 2(8) supramolecular architecture into complex 1D columns. The Hirshfeld surface analyses including dnorm plots and two dimensional fingerprint analyses were conducted to confirm the presence of various hydrogen bonds/intermolecular interactions existing in the crystal structure of 1. H···H contacts (dispersion interactions) contributes most to the Hirshfeld surface with a contribution of 40.8%, followed by O···H/H···O (28.4%), C···H/H···C (12.4%), N···H/H···N (9.3%) and others (9.1%). Further, the molecular structure, crystal structure and Hirshfeld surfaces including dnorm and fingerprint plots of 1, 2 and a positional isomer 4-oxo-4-(pyridin-3- ylamino)butanoic acid (3) were compared to observe the similarities and differences in the three compounds.