Nitrogen-rich alkali metal5,5′-hydrazinebistetrazolate salts: environmentally friendly compounds in pyrotechnic mixtures

Abstract

Straightforward preparations of energetic compounds based on the nitrogen-rich 5,5′-hydrazinebistetrazolate anion ([C₂H₂N₁₀]²⁻) and alkali metals (Li⁺, 2, Na⁺, 3, K⁺, 4, Rb⁺, 5 and Cs⁺, 6) are given by reaction of the free acid 5,5′-hydrazinebistetrazole and a suitable alkali hydroxide or carbonate salts. The new compounds were synthesized in high yields and purities and characterized using analytical (mass spectrometry and elemental analysis) and spectroscopic (IR, Raman and multinuclear NMR) methods. In addition, the solid state structure of the [C₂H₂N₁₀]^2– anion in compounds 2, 5 and 6 was solved by means of low temperature X-ray crystallography, showing non-coplanar tetrazole rings. Also, the thermal stabilities of the compounds were assessed by differential scanning calorimetry revealing high thermal: stabilities above 215 °C. Initial safety testing gave insight into the low sensitivity of the salts towards impact (≥30 J) and friction (>360 N). Due to the energetic nature of compounds 2–6 the (constant volume) energies of combustion were determined using oxygen bomb calorimetry and were used to calculate the corresponding heats of formation. The response of the compounds in the “flame test” is a burning, deflagration or explosion reaction, depending on the heating mode and the rate of oxidation to the 5,5′-azobistetrazolate ([C₂N₁₀]²⁻) derivative. The ready oxidation of the [C₂H₂N₁₀]²⁻ anion to form the [C₂N₁₀]²⁻ salt provides a safer method for the synthesis of the latter compounds. Lastly, the materials reported here might be of interest as a more environmentally friendly alternative to commonly used compounds in pyrotechnic mixtures.