Synthesis and Characterisation of Novel High Energy Density Metal-Based Poly(nitro) Azolates and Azines

This work presents the experimental results of a research project in the search for novel energetic materials to replace the conventional primary and secondary explosives such as lead azide and RDX. These ‘legacy’ materials are currently being phased out due to issues with toxicity and environmental pollution. Work in this field has focused on developing non-toxic, green, but powerful energetic materials with sensitivities in the appropriate range for the application. A typical approach to the problem utilises poly(nitro)azoles and azines which are promising candidates for replacement. The beneficial properties of these compounds have directed our efforts in synthesis of novel materials.
Six novel transition metal coordination compounds based on the energetic ligand, 5-(3,5-dinitro-1H-pyrazol-4-yl)-1H-tetrazole (H2DNPT) have been synthesised. The compounds were characterised by a combination of IR spectroscopy, microanalysis, X-ray crystallography and thermal analysis. Their energetic performance was estimated using conventional methods based on the approach devised by Kamlet and Jacobs. The crystal structures of the copper(II), zinc(II), nickel(II) and cobalt(II) compounds were determined, whereas the copper(I) and silver(I) compounds could not be crystallised. The crystalline materials form infinitely extending coordination polymers leading to high densities. The compounds display decomposition temperatures over 300 ºC, with the silver(I) compound being exceptionally thermally stable (Tdec > 400 ºC). Intriguingly, the estimated detonation parameters were found to be high and in the range of secondary explosives, with the copper(II) coordination compound comparable to RDX. Given that metal-based energetics usually give rise to the behaviour of primary explosives, these results warrant further investigation into their initiation and sensitivity characteristics.
Based on the energetic ligand, 5-(5-nitro-1H-triazol-3-yl)-1H-tetrazole (H2NTT), another seven novel transition metal coordination compounds were synthesised. These were characterised by IR spectroscopy, microanalysis, and crystallographic methods. The copper(I), copper(II), nickel(II), cobalt(II) and silver(I) coordination compounds were obtained as amorphous powders, whereas the zinc(II) and a mixed-valence copper(I)/copper(II) compounds formed crystalline phases allowing X-ray crystallographic structure characterisation. The density of the mixed-valence copper compound was found to be comparable to that of DBX-1 and copper(II) DNPT, which renders it a candidate if the energetic performance of which matches these materials. Further investigation into its thermochemistry and detonation properties required to confirm this prediction.
The synthesis of a novel energetic explosophoric ligand, (5-(4,5-dinitro-1H-imidazol-2-yl)-1H-tetrazole (H2DNIT) was attempted. The ligand shows similarity with H2DNPT and H2NTT and helps facilitate the investigation into trends in structure-property relationship at the centre of research into the coordination chemistry of energetic coordination compounds. While 5-(5-iodo-4-nitro-1H-imidazol-2-yl)-1H-tetrazole was identified and characterised as a key intermediate in the attempts to synthesise H2DNIT, the latter could not be obtained. This finding indicates that that nitration is a viable route to H2DNIT. Two additional novel compounds were isolated and characterised in these attempts, which are 4,5-dinitro-imidazol-2-carboxaldehyde (DNICA) and diimidazopyrazine-5,10-diimine (DIPDI). These were characterised spectroscopically, and their crystal structures determined. Even though these compounds cannot constitute energetic material, functionalisation of DNICA using the formyl group and nitration of DIPDI to the tetranitro compound may lead to further novel energetic materials.
The synthesis and structure of novel clathrates based on the energetic 2-bromo-4,5-dinitroimidazole (BrDNI) are reported. In a range of solvents, BrDNI crystallises forming hydrogen and halogen-bonded networks akin to the organic acid clathrates reported earlier. Cages formed by BrDNI, and water enclose pockets containing the solvent of crystallisation. Isostructural clathrates were found to form from acetonitrile, acetone, and nitromethane. As expected, thermal analysis showed these clathrate structures to give rise to little heat of decomposition and are of low performance compared to classical energetic materials. However, the findings suggest that the BrDNI-H-H2O system may be suitable for hosting a range of energetic small molecules, such as tetranitromethane or hexanitroethane, which will significantly affect sensitivity and performance.
Over the last 20 years, 2,4,6-trinitro-1,3,5-triazine (TNTA) has been one of the most sought-after energetic materials. Three novel routes to synthesise TNTA were explored. While neither reaction with NO2, nor oxidation of exocyclic N-containing functional groups produced TNTA, the nucleophilic substitution involving cyanuric iodide and sodium nitrite under anhydrous conditions shows promise as TNTA can be tentatively identified in 13C NMR of the reaction solution.
N,N’-dinitrourea (DNU) has been neglected as an energetic material due to its tendency to decompose in air and its low decomposition temperature. In this work. attempts to stabilise DNU were made, consisting of coordination of DNU to copper(I), and the synthesis of N,N’-dinitroparabanic acid (DNPA) as a DNU derivative retaining much of the energetic character. A novel copper(I) coordination compound was formed that displayed an increased decomposition temperature; however, the obtained material decomposed during crystallisation precluding structural characterisation. Two synthetic routes to DNPA were attempted, via cyclisation of DNU with oxalyl chloride or nitration of parabanic acid. Neither route formed the target compound. Ab initio calculations show that DNPA is a stable molecule making it a promising synthetic target.