Preference of High-Nitrogen-Content Compounds to Form Hydrates and the Tandem Contacts of Azide Groups

Under ambient conditions, the high-nitrogen-content compound, 6-azido-7-methyl[1,2,4]triazolo[4,3-b]pyridazine (C6H5N7), favorably crystallizes in the form of two concomitant monohydrate polymorphs: α-C6H5N7·H2O and β-C6H5N7·H2O; also anhydrate C6H7N7 is obtained from the acetone solution. High-pressure isochoric recrystallizations lead to new forms of these compounds. The α-hydrate can be reversibly isothermally compressed to 0.6 GPa at the least and the β-hydrate can be compressed to and from 1 GPa. However, the isochoric recrystallizations above 0.3 GPa favor the α-hydrate. This hydrate occurs as metastable phase, in accordance with the Ostwald rule of stages. Compound C7H7N7 could be recrystallized only from aqueous solution in the form of sesquihydrate C7H7N7·1.5H2O. The molecular packing in all hydrates is governed by hydrogen bonds OH···N and tandem contacts between azide groups.