Short N···N and CH···N Contacts in the Ambient and High-Pressure Polymorphs of a High-Nitrogen-Content Compound

The compression of high-nitrogen-content compound 6-azido-1,2,3,4-tetrazolo[1,5-b]pyridazine, (C4H2N8), was studied in situ in a diamond-anvil cell by single-crystal X-ray diffraction. The compression of ambient-pressure phase α is monotonic to 2.5 GPa at least, and high-pressure isochoric recrystallizations yield phase α, too. A new polymorph β could also be recrystallized from the low-concentration acetone solution, at high pressure below 0.5 GPa and temperature lower than 320 K. However, above 0.5 GPa again only the ambient-pressure form α was obtained. Both polymorphs are built of the azide-tetrazole tautomer, and intermolecular CH···N and N···N interactions are similar, but the molecules aggregate into planar sheets in phase α and into a three-dimensional network in phase β. Polymorph β can be stored for a few years in an open vial in ambient conditions with no signs of changes.