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: alpha-C6H5N7.H2O and beta-C6H5N7.H2O; also anhydrate C6H5N7 is obtained from the acetone solution. High-pressure isochoric recrystallizations lead to new forms of these compounds. The alpha-hydrate can be reversibly isothermally compressed to 0.6 GPa at the least and the beta-hydrate can be compressed to and from 1 GPa. However, the isochoric recrystallizations above 0.3 GPa favor the alpha-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 & BULL;1.5H2O. The molecular packing in all hydrates is governed by hydrogen bonds OH & BULL;& BULL;& BULL;N and tandem contacts between azide groups.