Resolved structures of two picolinamide polymorphs. Investigation of the dimorphic system behaviour under conditions relevant to co-crystal synthesis

The polymorphism of picolinamide, one of the three isomeric pyridinecarboxamides, a group of co-formers with relevance for co-crystal research, has been investigated. Particular attention has been focused on phase transitions brought about in DSC scanning experiments or during ball mill grinding, a common strategy in co-crystal synthesis. Two polymorphs, which the Burger and Ramberger empirical rules predict to be enantiotropically related, were identified. The crystal structure of the room temperature stable polymorph II, Tfus,II = 102.0 °C, was redetermined, while that of the ambient temperature metastable polymorph I, Tfus,I = 106.4 °C, was determined for the first time. This was produced as single crystals by sublimation in an oven at 90 °C. In the crystalline structure of this polymorphic form, hydrogen bonds link the molecules in tetramers, which are then packed in piles along the a axis in an arrangement that has not been found in any of the previously solved crystalline structures of isomeric pyridinecarboxamides. Hirshfeld surface analysis was performed in order to facilitate comparison of the intermolecular contacts in both polymorphs. Ball mill grinding of commercial polymorph II gives rise to different outcomes, depending on the experimental conditions: neat grinding for 120 minutes results in conversion to polymorph I, while the addition of 10 μL of toluene, ethyl acetate, dimethylsulfoxide, methanol, ethanol or isopropyl alcohol and liquid assisted grinding stabilizes polymorph II.