Crystal structure and thermal behaviors of the tetrapotassium salt of octahydroimidazo-[4,5-d]imidazol-1,3,4,6-tetrasulfonic acid (TACOS-K)

The tetrahydrate of the tetrapotassium salt of octahydroimidazo-[4,5-d]imidazol-1,3,4,6-tetrasulfonic acid (TACOS-K), an new energetic material, was investigated in terms of its crystal structure, heat of combustion, thermal stability and decomposition kinetics. Its heat of combustion is −5487 ± 96 J g⁻¹ from which a heat of formation of −3150 kJ mol⁻¹ was estimated. It has been found that this compound has a hexagonal crystal space group with a density of 2.026 g mol⁻¹ at 150 K. The organic anionic skeleton of the TACOS-K molecule is distinctly deformed. Six tetraanions interconnected by coordination to the potassium cations and hydrogen bridges to water molecules form hydrophilic as well as hydrophobic cavities. As an intermediate to synthesize cis-1,3,4,6-tetranitro-octahydroimidazo-[4,5-d]imidazole, it decomposes at around 31 °C with a first peak temperature range of 46.6–94.3 °C due to loss of water, depending on the heating rates. Hydrolysis of the N–S bonds might play an important role here. Crystalline water evaporation competes with this hydrolysis. TACOS-K has a residual mass of about 46 % at 2 °C min⁻¹, which increases with the heating rate. Peak of the exothermic process occurs at 235.6 °C at the same heating rate with an enthalpy change of 164 J g⁻¹. Dehydration occurs with an energy barrier of 36.7 kJ mol⁻¹ followed by a shoulder mass loss process with a much higher activation energy 110.6 kJ mol⁻¹, while the activation energy for the main exothermic reaction is about 136.2 kJ mol⁻¹.