Three Polymorphic Forms of the Co-Crystal 4,4 '-Bipyridine/Pimelic Acid and their Structural, Thermal, and Spectroscopic Characterization

Three crystal forms of the co-crystal 4,4'-bipy/pimelic acid (bipy: bipyridine), [NH4C5-C5H4N]center dot[HOOC-(CH2)(5)COOH], have been prepared and their relationship investigated by single-crystal X-ray diffraction, variable-temperature X-ray powder diffraction, differential scanning calorimetry and solid-state NMR spectroscopy. Both X-ray and NMR spectroscopic results indicate that no proton transfer takes place, that is, the three crystal forms are true co-crystals of neutral molecules. Forms I and II both convert into Form III at high temperature, Forms II and III being the thermodynamically stable forms at room and high temperature, respectively.