Synthesis And Properties Of 1,2,4-oxadiazole-derived Energetic Ionic Salts With Nitrogen-linkages

Owing to the low volatility,tailored performance and environmental friendliness,energetic ionic salts have become one of the most active research topics of energetic materials.As a isomer of furazan,1,2,4-oxadiazole has received increasing attentions due to its excellent thermal stability and insensitivity and become one of the most promising heterocycles for developing insensitive and high-performance energetic materials.A series of 1,2,4-oxadiazole-derived energetic ionic salts with nitrogen-linkages were synthesized,and their properties have been investigated in this paper.Initially,various oxidants were selected to react with 3,5-diamino-1,2,4-oxadiazole（DAO）in our preliminary experiments and it turned out that H₂O₂/Na₂WO₄/H₂SO₄ was relatively effective.The optimum conditions have been established by screening reactant ratio and amount and nature of acids.And 5 g scale of DAO was converted into 3-nitro-5-amino-1,2,4-oxadiazole（NOA）in 10%yield with H₂O₂（50%,25 m L）,Na₂WO₄?2H₂O（4.95g,15 mmol）,and conc.H₂SO₄（3 m L）.Further oxidation of NOA with sodium dichloroisocyanurate（SDCI）gave yellow solids sodium bis（3-nitro-1,2,4-oxadiazol-5-yl）amide（3-1）and sodium 1,3-bis（3-nitro-1,2,4-oxadiazol-5-yl）triaz-2-en-1-ide（3-2）in 48%and 44%yield,respectively.Different nitration conditions were also explored with 3-2.Subsequently,a new family of energetic ionic salts（3-4～3-9）containing 1,2,4-oxadiazole ring and amino or triazene linkage were synthesized by a cation metathesis.Their structures were fully characterized by ¹H and ¹³C NMR and IR spectra,while three ionic salts3-4,3-5 and 3-7 along with two sodium salt precursors 3-1 and 3-2 were confirmed by single crystal X-ray diffraction.Amino-bridged salts（3-4～3-6）demonstrate more advantages over triazene-linked ones（3-7～3-9）in terms of density and detonation performance.Nitrogen-bridged salts（3-4～3-9）possess a moderate onset decomposition temperatures ranging from166°C to 249°C and a low impact sensitivities,most of which show an excellent impact sensitivities of>40 J.Guanidinium bis（3-nitro-1,2,4-oxadiazol-5-yl）amide（3-4）exhibits a relatively good performance with acceptable detonation properties(D:8351 m s^-1,P:29.0GPa),good stability（T_d:241 ^oC,onset）and low impact sensitivity（IS>40 J）.