Pentazole anion cyclo-N5- is an attractive five-membered ring comprised of only nitrogen atoms. After being recently first synthesized, it is found that the cyclo-N5- anion shows good adaptability to take part in ionic, coordination, and hydrogen bonding interactions and therefor is a possibility to form different types of derivatives.
Ming Lu and co-workers, from Nanjing University of Science and Technology, developed two new energetic coordination polymers based on multidentate pentazole cyclo-N5-, recently published in Science China Materials, 2018, doi: 10.1007/s40843-018-9268-0.
Lu's group was devoted to the development for high energy density materials including polynitrogen and nitrogen-rich compounds for a long time. He said "The cyclo-N5-, as a polynitrogen structure, is high-energy specie that, if properly assembled with other ions, is likely to develop into a new generation of energetic materials, breaking through the energy limits of current energetic materials. "
At present, their group has realized the synthesis of metal-N5? complexes, but the introduction of organic cations or molecules and the formation of organic salts with the assembly of N5- ions have not yet been achieved.
"Although metal-N5- compounds contain energy and can explode in certain condition, the overall energy level of them is always very low. It is necessary to introduce energetic organic ions to increase the energetic performance", he says, "At the same time, the design ability and different physicochemical properties of organic cations provides a possibility to develop other new substances such as coordination polymers."
Coordination to form polymer is emerging as a new technology for modifying or enhancing the properties of the existed energetic substances in energetic materials area. "However, compared with metal ions, organic ions often have weaker coordination ability." he says, "It is still challenging to introduce organic ions to coordinate with N5- by ion exchange."
To achieve the purpose of organic ions instead of metal ions, the strategy for using metal ions with weak coordination effect is a good choose. Lu emphasized "It is essential to the crystal growth and ion/ligand exchange processes of coordination polymer."
Prof. Lu, as the leader of the research group, tells us "Previous experience motivates us to employ sodium salt as precursor because we have prepared more than 5 kinds of metal salts from it." By self-assembling, anhydrous coordination polymers(NaN5)5[(CH6N3)N5](N5)3- (CP 1) and (NaN5)2(C2H4N4) (CP 2) have been synthesized.
Lu says, "Experimentally, we used methanol solution containing sodium pentazole salt and guanidine or amino-triazole. Colorless crystals can be obtained by maintaining the solutions in air at room temperature naturally for several days, with more than 80% yield."
He adds "The evaporation rate of the solvent is more critical because the coordination capacity is relatively weak. If solvent evaporates too quickly, it is very likely that the coordination polymer will not be formed. Only mixed salts can be obtained."
The DSC curves for both CPs show that their decomposing temperatures are at 118.4 and 126.5°C, respectively. He highlights "These values are 7.4 and 15.5°C higher than precursor sodium pentazole salt."
"This observation indicates that coordination and hydrogen bonding interactions are beneficial for stabilizing the N5 ring." Zhang adds.Detonation heat, detonation velocity and detonation pressure are key parameters of energetic materials. The calculated nitrogen content (>66%) and heat of formation (>800 kJ mol-1) of the two CPs are significantly higher than those of traditional energetic materials (TNT, RDX and HMX).
The detonation heat of CP 2 (1.65 kcal g-1) is higher than that of TNT, RDX, HMX and CL-20 (about 1.5 kcal g-1). Detonation and detonation pressures (7,863 m s-1, 26.44 GPa) are higher than TNT.
"If we can improve the density of organic salts, it is very promising to reach level of HMX and CL-20." Lu said. "The detonation performance of CP 1 is poor, which is attributed to the low density caused by its porous structure" he added, "From another point of view; this porous structure can load other small molecules to enhance the energy performance."
"Two breakthroughs, removing coordinated water and combining with organic ligands, are achieved here. It makes pentazole derivative a step closer to energetic materials." Lu concluded.
This research was funded by the National Natural Science Foundation of China (11702141, 21771108 and U1530101).
See the article: Peng-Cheng Wang, Yuan-Gang Xu, Qian Wang, Yan-Li Shao, Qiu-Han Lin and Ming Lu. "Self-assembled energetic coordination polymers based on multidentate pentazole cyclo-N5-" Sci. China Mater. 2018, doi: 10.1007/s40843-018-9268-0.