Expanding the Solid Form Landscape of Bipyridines

Two bipyridine isomers (2,2′- and 4,4′-), used as coformers and ligands in coordination chemistry, were subjected to solid form screening and crystal structure prediction. One anhydrate and a formic acid disolvate were crystallized for 2,2′-bipyridine, whereas multiple solid-state forms, anhydrate, dihydrate, and eight solvates with carboxylic acids, including a polymorphic acetic acid disolvate, were found for the 4,4′-isomer. Seven of the solvates are reported for the first time, and structural information is provided for six of the new solvates. All twelve solid-state forms were investigated comprehensively using experimental [thermal analysis, isothermal calorimetry, X-ray diffraction, gravimetric moisture (de)sorption, and IR spectroscopy] and computational approaches. Lattice and interaction energy calculations confirmed the thermodynamic driving force for disolvate formation, mediated by the absence of H-bond donor groups of the host molecules. The exposed location of the N atoms in 4,4′-bipyridine facilitates the accommodation of bigger carboxylic acids and leads to higher conformational flexibility compared to 2,2′-bipyridine. For the 4,4′-bipyridine anhydrate ↔ hydrate interconversion hardly any hysteresis and a fast transformation kinetics are observed, with the critical relative humidity being at 35% at room temperature. The computed anhydrate crystal energy landscapes have the 2,2′-bipyridine as the lowest energy structure and the 4,4′-bipyridine among the low-energy structures and suggest a different crystallization behavior of the two compounds.
The solid form landscapes of 2,2′- and 4,4′-bipyridine were explored experimentally and computationally. The position of the nitrogen atoms defines not only the conformational flexibility of the molecule but also the crystallization behavior. 4.4′-Bipyridine was found to be more prone to solvate (including hydrate) formation than its 2,2′-isomer.