[1,2,5]Oxadiazolo[3,4-d]pyridazine 1,5,6-trioxides: efficient synthesis via the reaction of 3,4-bis(hydroxyimino)methyl)-1,2,5-oxadiazole 2-oxides with a mixture of concentrated nitric and trifluoroacetic acids and structural characterization

An efficient synthesis of [1,2,5]oxadiazolo[3,4-d]pyridazine 1,5,6-trioxides (1) from 3,4-bis(hydroxyimino)methyl)-1,2,5-oxadiazole 2-oxides using a mixture of concentrated nitric and trifluoroacetic acids has been developed. The scope of the unconventional reaction was established. The 4,7-dinitro[1,2,5]oxadiazolo[3,4-d]pyridazine 1,5,6-trioxide 1f represents a new high energy compound, unfortunately with low thermal stability. The parent [1,2,5]oxadiazolo[3,4-d]pyridazine 1,5,6-trioxide 1c was studied by single-crystal X-ray diffraction analysis which revealed a planar molecule with an unusually long intracyclic N–N bond of 1.668(5) Å and unexpected exo-cyclic bond angles at the nitroxyl nitrogen atoms. In the crystal, the molecules of 1c are bound to each other by strong π-π stacking and C–H⋯O hydrogen bonding interactions into a three-dimensional framework that results in a high crystal density of 1.833 gcm−3. © 2018