Nitro-tetrazole based high performing explosives: Recent overview of synthesis and energetic properties

Saira Manzoor, Qamar-un-nisa Tariq, Xin Yin, Jian-Guo Zhang

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing,100081, China

Keywords:Tetrazole Energetic materials Functional groups Heterocyclic High-performing explosives

ABSTRACT Heterocyclic skeleton (Azoles) and different energetic groups containing high performing explosives are highly emerged in recent years to meet the challenging requirements of energetic materials in both military and civilian applications with improved performance. For this purpose tetrazole (Azole) is identified as an attractive heterocyclic backbone with energetic functional groups nitro (-NO2), nitrato(-ONO2),nitrimino(-NNO2),and nitramino(-NH-NO2)to replace the traditionally used high performing explosives. The tetrazole based compounds having these energetic functional groups demonstrated advanced energetic performance (detonation velocity and pressure), densities, and heat of formation(HOF)and became a potential replacement of traditional energetic compounds such as RDX.This review presents a summary of the recently reported nitro-tetrazole energetic compounds containing poly-nitro,di/mono-nitro, nitrato/nitramino/nitrimino, bridged/bis/di tetrazole and nitro functional groups,describing their preparation methods, advance energetic properties, and further applications as highperforming explosives, especially those reported in the last decade. This review aims to provide a fresh concept for designing nitro-tetrazole based high performing explosives together with major challenges and perspectives.

1. Introduction

In modern times, life without high performing explosives is considered impossible due to their extensive use during peace time and armed conflicts. Therefore, to fulfill the demand, many researchers all over the world are working to develop modern energetic materials based high performing explosives with enhanced insensitivity[1,2].The design and synthesis of new high performing explosive as energetic materials remain an interesting area of research in material sciences to replace the traditional high performing explosives [3-5]. The fundamental properties of high performing explosives include the ratio of C/H/N,density,the heat of formation (HOF), sensitivity towards impact as well as friction,thermal stability, detonation performance (velocity and pressure),and acceptability to the environment [6,7]. The traditional energetic compounds containing oxygen, hydrogen, and nitrogen includes 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazine(RDX) and 1,3,5,7-tetranitro-1,3,5,7tetrazoctane (HMX), Pentaerythritol tetranitrate (PETN), 2,4,6-triamino-1,3,5-trinitrobenzene(TATB), octanitrocubane (ONC) and Hexanitrohexaazaisowurtzi tane (CL-20) (Fig.1) [8-13]. Unfortunately, the release of most of these nitro functional groups containing high-performance explosives often contaminated the surface, groundwater, soils, and sediments,which makes them highly non-desirable[14,15].Therefore,the synthesis of high nitrogen-containing energetic materials is promoted to fulfill the requirements of future defense and space sectors.The energy of these nitrogen-rich compounds comes from high HOF unlike the intramolecular oxidation of a carbon backbone in traditional high-performance explosives [16].

Fig.1. Traditional high-performing energetic materials based on different backbones.

In recent years heterocyclic skeleton(Azoles)based compounds with improved performance, have emerged to meet the challenging requirements of energetic materials [17-20]. In addition,combining various energetic groups with different heterocyclic backbones is one of the most significant and modern techniques to develop novel high-performing explosives [21-24]. In this regard,tetrazole is found to be an attractive heterocyclic backbone for the synthesis of modern high performing explosives due to the practical significance and variety of properties[25-27].The structure of tetrazole contains 79.8% nitrogen content and a large number of N-N and C-N due to that it conserves more energy than the other azole derivatives.The tetrazole possessed a high HOF of 320 kJ/mol due to the presence of a large number of nitrogen-nitrogen bonds,high density, and ring strain which are important properties required for the development of high-performing explosives[28,29]. Furthermore, the physical and energetic properties of tetrazole based compounds can be easily improved by the introduction or replacement of energetic functional groups as substituents on the ring. The interesting energetic properties and unique chemical structure combinations have an attraction for researchers to this novel class of compounds. The combination of tetrazole moiety and energetic functional groups produces many new energetic materials to meet the challenging requirements of high performing explosive. Therefore, for this purpose, the tetrazole combined framework with the nitro (-NO2), nitrato (-ONO2),nitrimino(-NNO2),and nitramino(-NH-NO2)energetic functional groups could result in favorable energetic performance [30,31].However, the achievement of species containing these energetic functional groups is still challenging owing to the high sensitivity and low thermal stability. The properties of most of them either predicted theoretically or exist only under specific conditions.

In this review,the current developments in the field of tetrazole based energetic compounds containing energetic functional groups(-NO2, -ONO2,-NNO2, and -NH-NO2) were designed and synthesized are discussed since 2010. Most of the energetic compounds highlighted here exhibit appropriate properties to be classified as high performing explosives. The properties of these compounds such as density, thermal decomposition temperature,HOF, detonation performance (detonation pressure and velocity),impact sensitivity, and friction sensitivity are reviewed in detail.TNT, TATB, RDX, HMX, and CL-20 are used as conventional highperformance explosives for comparison as listed in Table 1 [32]. It is expected that this review will present a summary of the major methodologies used for the design and synthesis of tetrazole holding energetic compounds with -NO2, -ONO2,-NNO2,and-NH-NO2energetic functional groups. While the comparison of these synthesized compounds with standard conventional highperformance explosives will stimulate more research to proposed

Table 1 Physicochemical and energetic properties of typically used energetic compounds[18,33].

similar energetic compounds with advanced properties.

2. Energetic compounds based on tetrazole framework in a combination of nitro groups (-NO2, -ONO2, -NNO2, and-NHNO2)

In the development of high energy explosives,the main focus is on the basic skeleton of compound (such as pentazole, tetrazole,triazole, pyrazole, etc.), and energetic substituents [34,35]. Tetrazole is considered as one of the ideal organic frameworks for the synthesis of high energy explosive owing to the high nitrogen content and stability [36]. In addition, the compounds containing energetic groups such as nitro (-NO2), nitrato (-ONO2), nitrimino(-NNO2), and nitriamino (-NH-NO2) are of special interest [37].These groups are important to attain desirable energetic properties owing to their high density,HOF,and good oxygen balance[38-41].Currently,the combination of a tetrazole along with these energetic moieties(-NO2,-ONO2,-NNO2,and-NH-NO2)has widely used to get satisfactory properties for high energy explosives [42,43].However,the insensitivity and high energy are found contradictory which makes the development of these new high energy explosives an interesting and challenging work[44,45].

2.1. Poly-nitro groups containing tetrazole based compounds

The incorporation of the poly-nitro groups to the heterocyclic compounds is interesting to prepare high-performing explosives with advance energetic properties[46,47].In continuing effort the polynitro group containing tetrazole based compounds 1 and 2 were synthesized according to the procedure given in Scheme 1[48]. The synthesis was originated from commercially accessible ethyl 2-cyanoacetate. The synthesis of 1a (ammonium 5-(fluorodinitromethyl)-2 H-tetrazolate) was conducted by using decarboxylative nitration of ethyl 2-(1H-tetrazol-5-yl)acetate [49].Further, ammonium 5-(fluorodinitromethyl)-2 H-tetrazolate (2a)was attained from potassium dinitroacetonitrile, and the addition of NaN3, modified from the reported method to maintain stability.Differing from the literature the fluorination of potassium dinitroacetonitrile was performed with Selectfluor in anhydrous acetonitrile at ambient temperature in a closed vessel to attain the compound 1a. The alkylation reactions of compounds 1a and 2a were performed with 2-nitro-2-azapropyl chloride in anhydrous acetone,furnishingthe2-(2-nitro-2-azapropyl)-5-(polynitromethyl)-2H-tetrazoles 1 and 2 as colorless crystalline solids in good yields of 78% each. The physiochemical and energetic properties of these prepared compounds 1 and 2 are listed in Table 2.The obtained densities were 1.75-1.83 g/cm3and detonation velocities were 8704 and 8773 m/s which were higher than to TNT and comparable RDX in the case of compound 2. While the detonation pressure of compounds 1 and 2 was 33.4 and 34.2 GPa respectively,which are comparable to RDX.These compounds were obsessed with high positive HOFs. Whereas, these compounds were found thermally less stable (134 and 184°C) owing to their high sensitivity towards impact and friction. Hence, due to high density and detonation performance (velocity and pressure),compound 1, and 2 can be used as high performance explosive.

The synthesis of compound 3 was attained by using 5 (Trinitromethyl)-2H-tetrazole which has a density [49] of 1.92 g/cm3as described in Scheme 1 [50,51]. At first, the 5 (Trinitromethyl)-2Htetrazole was allowed to react with Bromo acetone to produce an intermediate compound 1-(5-(trinitromethyl)-1H-tetrazol-1-yl)propan-2-one, which further undergoes nitration with a nitrating mixture (HNO3+ H2SO4) to achieve the targeted compound 3.Furthermore, the potassium salt of 3 was synthesized to achieve another compound 2,5-bis(dinitromethyl)-2H-tetrazole (4) after the acidification with sulfuric acid.The prepared compounds 3 and 4 were thermally less stable (73°C and 88°C respectively) due to the presence of poly nitro groups. The densities of compounds 3 and 4 were 1.88 and 1.92 g/cm3.Moreover,the detonation velocity and pressure of compound 3 was 8511 m/s and 31.2 GPa, and compound 4 was 9123 m/s and 37.9 GPa, which are higher than RDX and comparable to HMX. The calculated HOF for these compounds was high and oxygen balance was positive. Hence, poly nitro groups along with tetrazole play an important role to enhance the density and detonation performance of the targeted compounds.

Scheme 1. Synthetic pathway of poly-nitro groups containing tetrazole based compounds (1-4).

Table 2 Physiochemical and energetic properties of poly-nitro groups containing tetrazole based energetic compounds 1-23.

A family of dense energetic compounds 5-13, N-trinitroethylsubstituted mono-, bis-, and tri-5-aminotetrazoles attained via reaction of a primary amine with cyanogen azide, tailed by the trinitroethyl functionalization (Scheme 2) [52]. The trinitroethyl functionality was introduced through condensation of a hydroxymethyl intermediate which was synthesized after the reaction of formaldehyde with trinitromethane. The synthesized energetic compounds 5-13 exhibited lower decomposition temperature as compared to standard high-performance explosives in the range of 141-183°C as illustrated in Table 2. The measured densities were ranged from 1.60 to 1.85 g/cm3, greater than RDX in the case of compounds 5 and 12 (1.84 and 1.84 g/cm3). Whereas, most of the prepared compounds have lower densities due to the manifestation of the bulky space-oriented trinitroethyl group. The HOFs were positive and higher than standard energetic materials in the range of 96.2-682.2 kJ/mol. The calculated detonation velocity and pressure for these trinitroethyl-substituted compounds were fell in the range of 7859-8771 m/s and 24.9-35.7 GPa respectively,advance than TNT, and approaching RDX in the case of 5 and 12(8771 &8734 m/s and 35.7 & 35.7 GPa respectively). All the prepared compounds were sensitive towards impact(except 7 and 13)and friction. Although, these compounds particularly 5 and 12 retain good detonation performance, density, and HOF. However,their thermal stability and sensitivities towards impact and friction were not appropriate to use as high-performance explosives.

Poly nitro tetrazole energetic compounds 14-19 and their derivatives connected with pyrazole by N,N-ethylene/propylene bridges were synthesized as demonstrated in Scheme 2[53].The 4-amino-3,5-dinitropyrazolate was brought into contact with 2-bromoethylamine hydrobromide in the presence of phase transfer catalyst TEAB (tetraethylammonium bromide) to give 14, which was further used to achieve compound 15,an N,N-ethylene bridge compound bonded by 4-amino-3,5-dinitropyrazole with aminotetrazole. The compound 16 was also synthesized by sodium trinitropyrazolate with 2-bromoethylamine hydrobromide and then reacted with ammonium 4-amino-3,5-dinitropyrazolate in the DMF and TEAB. The oxidation of 15 was ended with sulfuric acid and hydrogen peroxide to carry out the synthesis of 16. Whereas,the analogous propylene bridge compounds 17-19 were also prepared in the same way as 14-16 initiated through 3-bromopropylamine hydrobromide. The asymmetric N,N-ethylene/propylene bridges were decomposed in the range of 120°C-267°C as expressed in Table 2. The thermal stabilities of diamino compounds 15 and 18 were found higher(257°C and 265°C)than RDX and lower than TNT.The measured densities were greater than TNT expect compound 17 (1.61 g/cm3) and less than other traditional high-performance explosives.In addition,due to the huge content of nitrogen, the HOF was found high positive as compared to RDX and TNT falling from 241.7 to 463.0 kJ/mol.The compounds 14,15,and 17-19 were insensitive towards impact and friction.However,the detonation performance was the only advance than TNT in the range of 21.8-27.1 GPa (pressure) and 7709-8172 m/s. Therefore,owing to the presence of fewer nitro groups and a large number of carbon and hydrogen atoms the prepared energetic compounds were not better than standard high-performance explosives except TNT. Nevertheless, this technique can be used to attain better energetic compounds with enhanced sensitivities.

The starting reagent 4-amino-3-hydrazino-1,2,4-triazolelium chloride experiences cyclization with sodium azide and triethyl orthoformate to yield compound (1H-tetrazol-1-yl)-4H-1,2,4-triazole-3,4-diamine. The nitro groups containing compound 20 was synthesized by the reaction of (1H-tetrazol-1-yl)-4H-1,2,4-triazole-3,4-diamine with trinitroethanol as represented in Scheme 3 [54]. The thermal stability and density of compound 20 were 182°C and 1.77 g/cm3respectively as listed in Table 1,which is not much reasonable as compared to standard high-performance explosives. The calculated density was only advanced than TNT and HOF was 1091.8 kJ/mol for multi-nitro groups containing compound 20. The detonation performance (velocity; 9004 and pressure;34.9) was found comparable to RDX. Therefore, the high detonation performance and HOF of these compounds making them suitable to be used as high-performance explosives.

The perception of synthesis of small molecules that keep tetrazole with a high content of N and O- attained substantial attention due to their better performance and high thermal stability[55,56]. Therefore, the methoxy group of 1-(4-methoxy-3,5-dinitrophenyl)-1H-tetrazole was consumed to convert NH2, NO2,and NH-NO2to boost the energetic abilities as shown in Scheme 3[57].The methoxy group transformed directly to NH2and NH-NO2using aqueous ammonia and fuming nitric acid to conquer 21 and 22 respectively. Whereas, the oxidation 21 by sodium tungstate/H2SO4/H2O2directed to the trinitro substituted aryl-tetrazole 23.The crystal density of compounds 22 and 23(1.78 and 1.79 g/cm3)was closer to RDX (Table 1). The detonation velocities for synthesized compounds 21-23 were 7899, 8208, and 8298 m/s respectively, which were closer to RDX. The decomposition temperature of compound 21 was 223°C,which was comparable to RDX among others.The impact sensitivities of synthesized compounds 21 to 23 were >40 J. Thus, a large number of nitro groups enhanced the energetic abilities of the synthesized compounds.Furthermore,the synthesized compounds possess high positive HOFs due to the enrichment of nitrogen in the range of 293.7-882.5 kJ/mol essential factor for high-performance explosives. Hence, 23 was found captivating energetic material due to retaining better detonation performance, density, and insensitivity towards impact.

Scheme 2. Synthetic pathway of poly-nitro groups containing tetrazole based compounds (5-19).

2.2. Di/mono-nitro groups containing tetrazole based compounds

N-(3,5-dinitro-1H-pyrazol-4-yl)-1H-tetrazol-5-amine (24) energetic compound was achieved through the synthesis of 3,4,5-trinitro-1H-pyrazole (TNP) (Scheme 2) [12,58]. For the synthesis of TNP, the 4-chloro-1H-pyrazole as starting material was used,which subsequently undergoes nitration,amination,and oxidation[59]. The nitration was proceeding using concentrated HNO3and H2SO4to afford 3,5-dinitro-4-chloropyrazole.The amination of 3,5-dinitro-4-chloropyrazole was achieved at 170°C in a sealed tube with 25%aqueous ammonia.The obtained product was 3,5-dinitro-4-aminopyrazole (LLM-116) having an excellent yield of 85%.Following amination, the oxidation was accomplished in concentrated H2SO4and 30% hydrogen peroxide to get TNP. Finally, the TNP was reacted with commercially available 5- amino-1Htetrazole (ATZ) in 4 equivalent solution of KOH maintaining temperature in the range of 140-160°C to produce the targeted compound.Furthermore,the product was acidified with 20%H2SO4and recrystallized from ethanol-water to give the desired deep yellow solid compound with 63%yield. This compound was found thermally more stable than RDX as required for high-performance explosives (Table 3). The density and HOF were 1.86 g/cm3and 856.4 kJ/mol, which were greater than RDX. In addition, the detonation velocity and pressure of the compound were 9289 m/s and 38.6 GPa respectively. The detonation pressure was higher than RDX and lower than HMX. While the detonation velocity was higher than RDX and comparable to HMX.Thus,compound 24 with a distinctive assembly of polynitropyrazole and nitrogen-rich tetrazole moiety bridged by nitrogen atoms have high thermal stability, detonation performance, and acceptable impact and friction sensitivities can be used as high performance explosive.

The energetic compound 5-(5-amino-2H-1,2,3-triazol-4yl)-1Htetrazole and its nitro derivative 24 was prepared as shown in Scheme 4 [60]. The main compound 5-(5-amino-2H-1,2,3-triazol-4yl)-1H-tetrazole was prepared in five steps using benzyl azide and cyanoacetamide with sodium in ethanol to get 5-amino-1benzyl-1,2,3-triazole-4-carboxamide. In the next step benzyl protective group was remain on the triazole and treated with trifluoroacetic acid, which was further reacted with sodium azide to carried out the synthesis of 5-(5-amino-1-benzyl-1,2,3-triazol-4-yl)tetrazole.In the final step, the debenzylation of 1-benzyl-4-(1H-tetrazol-5-yl)-1H-1,2,3-triazol-5-amine was conducted with sodium in liquid ammonia to achieve the targeted compound 5-(5-amino-2H-1,2,3-triazol-4yl)-1H-tetrazole. Furthermore, the nitro group was introduced through potassium superoxide in tetrahydrofuran to yield compound 25. The compound 25 has a thermal decomposition temperature (188°C) lower than standard HEDMs (Table 3). The detonation velocity was 8251 m/s and the detonation pressure was found 26.3 GPa,which was superior to TNT.Moreover,the HOF was 516 kJ/mol and it was sensitive towards impact and friction.Therefore,the prepared compound 25 was only nearby to TNT in a sense of energetic performance.

Scheme 3. Synthetic pathway of Poly-nitro groups containing tetrazole based compounds energetic compounds (20-23).

Table 3 Physiochemical and energetic properties of energetic compounds based on the tetrazole framework in a combination of nitro groups 24-34.

To enhance density, HOF, and thermal stability compounds 26 and 27 were intended to synthesize by adopting a unique method of combining the nitropyrazole unit and tetrazole ring. The synthesis of these compounds is elaborated in Scheme 4 [61]. Moreover,in these energetic compounds,a fluorine atom is incorporated in C(NO2)2F and C(NO2)2NF2form to increase further energetic properties like density and detonation performance. The 1-(1Hpyrazol-3 (5)-yl)-1H-tetrazole was used as a precursor to incorporate C(NO2)2F and C(NO2)2NF2groups to prepare energetic compounds with improved properties. The precursor 1-(1H-pyrazol-3 (5)-yl)-1H-tetrazole was nitrated to obtain 1, 4-nitropyrazole, which was further reacted with bromoacetone to generate acetonyl pyrazole and dinitromethyl intermediate which was consequently used to synthesized potassium 1 (dinitromethyl)-pyrazole salt after the treatment with KOAc. The salt was fluorinated by Selectfluor to attain efficiently fluorodinitromethyl product 26 in(82%yield).Further,the salt was converted to difluoroamino)dinitromethyl (27) with the help of F2NOSO2F.Associating densities of these compounds exposed that 27 has the advanced value(1.91 g/cm3),greater than RDX,and comparable to HMX (Table 3). While the other compound 26 has a density near RDX.The HOFs values were positive in the range of 386 and 589 kJ/mol progressive than RDX due to high nitrogen contents. The detonation pressure and velocity were 31 & 36 GPa, and 8470 &8990 m/s respectively. Hence, compound 27 has superior detonation properties to RDX. These compounds decomposed at 111°C and 138°C and their impact sensitivity were lies in the range of 1.5 J-1.6 J, tremendously inferior to standard HEDMs. Therefore,the only compound which exhibited good density, HOF, nitrogen content, detonation performance, and ability to serve as a highperformance explosive was 27.

Scheme 4. Synthetic pathway of energetic compounds based on the tetrazole framework in a combination with nitro groups.

A large number of tetrazole-N-aryl/heteroaryl derivatives (28a-28q)were produced and further injected with energetic functional groups (-NO2and -NHNO2) [57]. The synthetic route of tetrazole-N-aryl/heteroaryl derivatives and the further introduction of energetic functional groups are briefly described in Scheme 5. The conversion of amine moiety into tetrazole employing condensation and cycloaddition in the company of NaN3and triethyl orthoformate lead to the formation of tetrazole-N-aryl/heteroaryl derivatives (28a-28q). Further, the nitration of selected derivatives(from 28a-28r) with mixed acids (98% HNO3+H2SO4) yielded a wide range of nitrated tetrazole-N-aryl/heteroaryl derivatives.Whereas, the synthesized compounds 28a-1 to 28i-1/2 behave as high performing explosive owing to their encouraging energetic properties.

The measured density of synthesized compounds was revealed 1.63-1.70 g/cm3noticeably imitating the increase from mononitro to dinitro substituted N-aryl-tetrazoles (Table 4). The detonation velocities and pressure were ranged from 7142 m/s to 7812 m/s and 18.9 GPa-21.9 GPa respectively,equivalent to TNT while lower than other references high-performance explosives. The prepared compounds were decomposed between 152 and 194°C,presenting the thermal stability which was tempting for compound 28d-1(194°C). Furthermore, the synthesized compounds possess high positive HOFs due to the enrichment of nitrogen in the range of 287.4-654.3 kJ/mol essential factor for high-performance explosives.Hence,28i-2 was found captivating energetic material due to retaining better detonation performance and density as compared to TNT.

7-nitro-3-(1H-tetrazol-5-yl)-[1,2,4]triazolo[5,1-c][1,2,4]triazin-4-amine was prepared using 3-nitro-1H-1,2,4-triazol-5-amine as precursor. The synthesis method of energetic compound 29 is represented in Scheme 6[62].The synthesis was easily achieved by the preparation of diazonium salts with malononitrile in the weak base solution followed by the introduction of a tetrazole ring using sodium azide in hydrochloric acid.The precursor 3-nitro-1H-1,2,4-triazol-5-amine was commercially available and retain a -NO2group. Therefore, the tetrazole moiety was introduced to enhance the energetic properties of compound 29. The impact and friction sensitivities are 35 J and 300 N respectively presenting its insensitive nature (Table 3). The detonation pressure was 32.8 GPa,detonation velocity was 9010 m/s, and density was 1.87 g/cm3higher than RDX. Furthermore, the calculated HOF was found 721.6 kJ/mol and thermal decomposition temperature of 181°C.Thus,compound 29 exhibited less thermal stability,better density,HOF, and detonation performance than RDX.

Scheme 5. Synthetic method of different nitro-tetrazole derivatives.

Table 4 Physiochemical and energetic properties of different Nitrato/Nitramino/Nitriminotetrazole energetic compounds (35-45).

The design of energetic compound 30 was the result of inspiration from FOX-7, which was prepared by utilizing 4,6-dichloro-5 nitropyrimidine as starting material in various steps as articulated in Scheme 6 [63,64]. The starting material was prepared through the reaction of 6-chloro-5-nitropyrimidin-4-amine and liquid ammonia. Which was further reacted with sodium azide to attain 8-nitrotetrazolo [1,5-c]pyrimidin-7-amine followed the reaction with methanol HCl gas mixture to reach planned compound 30.The calculated density was found 1.83 g/cm3comparable to RDX and higher than TNT (Table 3). The compound 30 has good detonation velocity (8499 m/s) and detonation pressure (26.7 GPa) as compared to RDX and TNT, owing to the high density (1.83 g/cm3)and positive HOF (181.5 kJ/mol). The impact and friction sensitivities(30 J and>360 N)were found acceptable and thermally stable up to the 275°C.Therefore,Fox-7 like compound 30 can be used as a secondary explosive.

Keeping in view the importance of nitrogen-rich heterocycle such as tetrazole for the development of high performing explosive,compound 31 was designed and synthesized according to the method shown in Scheme 6[65].The sodium salt of precursor 3,5-diamino-4-nitropyrazole reacted with cyanogen azide to achieve the designed compound 31.The thermal stability up to 272°C of the compound was superior to TNT and RDX as shown in Table 3. The calculated density was 1.76 g/cm3HOF was 407.7 kJ/mol.Owing to the reasonable density and HOF,the detonation velocity(8257 m/s)and pressure(25.9)GPa were also acceptable.Moreover,the impact and friction sensitivities were 30 J and >360 N respectively.Therefore,this compound with tuning properties of higher thermal stability, high detonation velocity, and low sensitivities was found interesting to use as high performing explosive. Furthermore, 1-((1H-tetrazol-5-yl)methyl)-1H-tetrazol-5-amine (Scheme 6) [66]was easily obtained after the reaction of 2-(5-amino-1H-tetrazol-1-yl)acetonitrile with sodium azide.

Scheme 6. Synthetic method of different nitro-tetrazole derivatives.

Another effort was done to attain the tetrazolo [1,5-b]pyridazine-compounds i.e.[6-azido-8-nitrotetrazolo[1,5b]pyridazine-7-amine (32) and 8-nitrotetrazolo [1,5-b]pyridazine-6,7-diamine(33) as shown in Scheme 6 [67]. The synthesized compounds 32 and 33 have thermal decomposition temperature 163°C and 290°C and densities 1.82 g/cm3and 1.80 g/cm3(Table 3). These results indicated that densities were comparable to RDX, while thermal stability was greater than RDX in the case of 33. The thermal stability of 32 was very low due to the presence of the azido group.Because these compounds have an excess of N-N or C-N bonds,therefore the HOFs were positive (811.2 kJ/mol and 446.2 kJ/mol).Whereas, the detonation pressure (31.5 GPa and 27.2 GPa) and velocity (8746 m/s and 8446 m/s) were relatively lower than reference high-performance explosive except for TNT. However,these compounds synthesized by a simple technique, but due to high detonation performance, compound 32 predicated as a primary explosive, while good thermal stability, sensitivity, and detonation performance of compound 33 making it suitable as a secondary explosive. The starting material 3,6-dichloropyridazin-4-amine was nitrated with mixed acids (100% HNO3+H2SO4) to obtain 3,6-dichloropyridazin-4-amine. The intermediate 3,6-dichloropyridazin-4-amine was further treated with sodium azide to yield 32 interestingly in azido-tetrazole form instead of a mixture of three tautomers (diazide, azide-tetrazole, and ditetrazole). For the synthesis of compound 33, in the first step hydrazine monohydrate was reacted with 3,6-dichloropyridazin-4-amine to obtain 3-chloro-6-hydrazinyl-5-nitropyridazin-4-amine.Which was further undergo two-step reactions,with sodium nitrite in dilute HCl and aqueous ammonia resultantly the formation of 33.The purpose of the synthesis of 4-nitro-3-(5-tetrazole)furoxan(HTNF, 34) was with furazan ring-substituted tetrazole to attain high thermal stability, density, and HOF [68]. The synthesis was started from 4-amino-3-cyanofuroxan as shown in Scheme 6. The 4-amino-3-cyanofuroxan was reacted with sodium azide in the presence of zinc bromide to develop the tetrazole ring[69],further the reaction continue for nitration and produced HTNF (34). The decomposition temperature of HNTF (34) was 169°C assimilated into Table 3.Moreover,the calculated density was 1.81 g/cm3which was comparable to RDX. The high HOF (499.5 kJ/mol) detonation pressure and velocity (34.3 GPa and 8681 m/s) made this compound to be used as high performance explosive.

2.3. Nitrato/nitramino/nitrimino -tetrazole compounds

The heterocyclic compounds comprise alkyl and alkoxy principally nitrato/nitramino/nitrimino-tetrazole have gained enormous attention among chemical researchers for the production of high performing explosive. Therefore, nitrimino containing energetic compounds 35, and 36 were designed and synthesized to achieve the appropriate energetic properties as referred to Scheme 7 [70].The raw material 5-amino-1H-1,2,4-triazole-3-carbonitrile was used to yield the 5-(1,2,4-triazol-C-yl)tetrazoles(35) and 5-(1,2,4-triazol-C-yl)tetrazol-1-oles (36) [71,72]. 3-isocyano-1H-1,2,4-triazol-5-amine was undergone cycloaddition in the presence of sodium azide and ZnCl2, to yield 3-(1H-tetrazol-5-yl)-1H-1,2,4-triazol-5-amine. Subsequently, the reaction continues to nitration to harvest compound 35 containing nitrimino energetic groups.However,while preparing compound 35 the energetic compounds introduced first and then undergo cyclization in numerous step reactions.The thermal decomposition temperature was 215°C and 116°C respectively for these prepared compounds (35 and 36) as represented in Table 4. The calculated HOFs were superior to RDX which was 576 kJ/mol and 515 kJ/mol respectively. Moreover, the calculated detonation pressure of compounds 35, and 36 was 26.2 GPa and 34.2 GPa and detonation velocities were 8062 m/s and 8776 m/s respectively. The impact (<1, and <1 J) and friction (18,and 60 N) sensitivities were not very reasonable. Therefore, from theses designed compounds the 36 demonstrated superlative energetic performance in terms of detonation performance,HOF,and density in the range of RDX.

Scheme 7. Synthesis of Nitrimino-tetrazole energetic compounds 35 and 36.

Thus, energetic compounds 37, 38, and 39 were also planned according to this approach (Scheme 8) [73].The starting materials(methyl-2-(5-amino-1H-tetrazol-1-yl) acetate and ethyl-2-(5-amino-2H-tetrazol-2-yl)acetate respectively) were different for the synthesis of these compounds.The raw materials were reacted with hydrazine monohydrate, N-methyl-N-nitroso-N′-nitroguanidine, and finally refluxing in sodium hydroxide solution by acidifying with concentrated hydrochloric acid to produce 37 and 39. Furthermore, the nitration of compound 37 by concentrated HNO3leads to the formation of energetic compound 38. The densities of prepared compounds(37,38,and 39)were found 1.71,1.81,and 1.72 g/cm3respectively as given away in Table 4.The obtained density and HOF of compound 38 was high(1.81 g/cm3)among all and comparable to RDX as well. The disintegration temperature was 204,159, and 218°C, which is not appropriate in the case of compound 37 and 39.The impact(15,10,and 18 J)and friction(240,159, and 360 N) sensitivities of all the compounds were found acceptable. Furthermore, the detonation velocities were 8187,8750, and 8213 m/s and detonation pressures were 24.9, 31.8, and 25.2 GPa respectively for compounds 37,38,and 39.The detonation velocity of compounds 38 was 8750 m/s which is comparable to RDX. Whereas, the detonation pressure was not higher than RDX for these compounds. The high HOF, density, and detonation performance made compound 38 as promising high performance explosive.

The synthesis of nitramino tetrazole compound 40 was accomplished over dimethyl carbonate, which was promoted to react with hydrazine hydrate to yield methyl carbazate as articulated in Scheme 8 [74]. Further, the methyl carbazate was treated with cyanogen azide to provide N-methoxycarbonyl 1,5-diaminotetrazole.TheproductN-methoxycarbonyl1,5-diaminotetrazole was additionally nitrated (N2O5in methyl cyanide) and decomposed in the KOH solution. Finally, the potassium salt undergoes acidification by aqueous hydrochloric to yield compound 40. The compound 40 possessed a thermal decomposition temperature of 240°C, calculated detonation velocity of 10,011 m/s, and pressure of 52.2 GPa respectively, as shown in Table 4. However, compounds 40 was found highly sensitive towards impact and friction due to which it can be used as a primary explosive. The 1-methoxy-5-aminotetrazole (41) was obtained after the reaction of cyanogen azide and methoxy amine and subsequently neutralization with NaOH, and nitration via 100% HNO3as represented in Scheme 9[21].The synthesized compound 41 was highly sensitive as indicated by the physiochemical and energetic properties given in Table 4. This compound has less thermal stability(116°C)and having low density(1.66 g/cm3)as compared to standard high performing explosive. However, the detonation velocity was 8660 m/s and the detonation pressure was 31.5 GPa which was closer to RDX,making it a considerable high performing explosive.

Researchers found that the nitrogen-rich heterocyclic skeletons were an effective and adequate way to enhance the abilities of new high-performance explosives [75]. For this purpose, the C-N hetero-aromatic compounds like triazole and tetrazole are unique classes due to their brilliant energetic strengths with great HOF and high thermal stability resulting from their aromaticity[76,77].The synthesis of long catenated nitrogen atom chains having heterocyclic compounds might be an interesting approach to expand the energetic properties [78]. In this regard, the research group of Guangbin Cheng synthesized the long catenated nitrogen atoms inclosing tetrazole compound(1H-tetrazol-1-yl)-4H-1,2,4-triazole-3,4-diamine and it’s nitro group derivatives as represented in Scheme 9 [54]. The starting reagent 4-amino-3-hydrazino-1,2,4-triazolelium chloride experiences cyclization with sodium azide and triethyl orthoformate to yield compound (1H-tetrazol-1-yl)-4H-1,2,4-triazole-3,4-diamine.Further,the compounds(nitramino)42 and(nitro)43 were obtained by the nitration of(1H-tetrazol-1-yl)-4H-1,2,4-triazole-3,4-diamine with fuming HNO3and a mixture of nitration acids (100% HNO3+ 98% H2SO4) respectively in good yield. The compounds 42, and 43 exhibited thermal stability of 177°C and 186°C and densities of 1.78 g/cm3and 1.72 g/cm3respectively as listed in Table 4.The calculated densities were only advanced than TNT.The HOF was positive for both compounds(42 and 43) which was 923.3 and 950.6 kJ/mol respectively. The calculated detonation performance (velocity; 9141, and 8885 m/s and pressure; 33.6, and 32.1 GPa) was found closer or higher than RDX. The synthesized compounds were found sensitive to impact and friction. Additionally, these compounds have gained their importance to be used as high-performance explosives due to their high HOFs and detonation performances.

Scheme 8. Synthesis of Nitrimino/Nitramino -tetrazole energetic compounds 37-40.

Nitramino and tetrazole bearing energetic compound 44 was synthesized by Shreeve’s group as demonstrated in Scheme 8[53].The starting reaction was conducted between 2-chloroethylamine and pyrazole, in the presence of NaOH to yield 2-(1H-pyrrol-1-yl)ethan-1-amine,which was further reacted with cyanogen azide to carry out compound 1-(2-(1H-pyrrol-1-yl)ethyl)-1H-tetrazol-5-amine, N,N-ethylene bridge un-substituted pyrazole, and aminotetrazole. The reaction further continues and the former product was reacted with 100% nitric acid to attain the compound 44. Unfortunately,the energetic properties of this synthesized compound are only better than TNT.

The compound 5-nitro-2-nitratomethyl-1,2,3,4-tetrazole (45)was prepared by nitration of 5-nitro-2-hydroxymethyl-tetrazole by nitric acid and acetic anhydride as described in Scheme 9[79].The 5-nitro-2-hydroxymethyl-tetrazole compound was attained after the reaction of 5-nitrotetrazole sodium salt with formaldehyde in the presence of a strong acid like H2SO4. The compound 45 exhibited high density, detonation performance, and HOF, which were 1.80 g/cm3, 9260 m/s (detonation velocity) 37.92 GPa(detonation pressure) and 228.07 kJ/mol (Table 4). Therefore, the energetic performance was better than TNT, RDX, and comparable to HMX. However, the compound has low sensitivity towards impact (4.1 J) and less thermal decomposition temperature(170.81°C)as compared to reference energetic materials.Thus,the importance of this energetic compound is due to its high density,HOF, and detonation performance which make it an appropriate high performance explosive.

2.4. Bridged/bis/di tetrazole and nitro groups containing energetic compounds

Bridged/Bis/Di tetrazole connecter linkage is very important to obtain high-performance explosives[80].3,6-dinitropyrazolo[4,3-c]pyrazole (DNPP) was prepared through a series of reaction steps followed by the literature method, which was further used to synthesized 3,6-dinitro-1,4-di (1H-tetrazol-5-yl)-pyrazolo [4,3-c]pyrazole (DNTPP (46)) as shown in Scheme 10 [81,82]. Thepotassium salt of DNPP was prepared through the acid-base neutralization method in the presence of KOH. In this potassium salt of the DNPP, the tetrazole moiety was introduced via in-situ produced cyanogen azide(a mixture of sodium azide and cyanogen bromide)to achieved the targeted compound 46.The compound 46 possesses good thermal stability of 281°C(Table 5).The detonation velocity was 8721 m/s and detonation pressure was 30.9 GPa,which is not higher than RDX. However, the compound was insensitive towards impact and friction, exhibited high HOF(1111.5 kJ/mol),density(1.79 g/cm3),and thermal stability(281°C).

The nitramino functional group was introduced to offer efficient energetic properties by the reaction of 1-((1H-tetrazol-5-yl)methyl)-1H-tetrazol-5-amine with conc. HNO3to finalized compound 47 [83]. The physiochemical and energetic properties of compound 47 are represented in Table 5. The nitrogen-rich framework (tetrazole) and the nitro group-containing compounds 47 have a decomposition temperature of 178°C and density was 1.76 g/cm3.The calculated HOF was found 615.9 kJ/mol higher than TNT and RDX. The compound 47 has adequate density and HOF,therefore the detonation performance (velocity and pressure) was promising (8652 m/s and 29.6 GPa). Moreover, the impact (>12)and friction(160 J)sensitivities were found better in comparison to RDX.Thus,the energetic compound 47 can be used as a secondary explosive.

Scheme 10. Synthetic routes to Bridged/Bis/Di tetrazole and nitro groups containing energetic compounds 46-50.

Table 5 Physiochemical and energetic properties of different Bridged/Bis/Di tetrazole and nitro groups containing energetic compounds (46-50).

The oxy nitroiminotetrazoles,derivatives of nitroiminotetrazole(48 and 49) was synthesized to reach good physical and energetic properties as presented in Scheme 10 [84,85]. The reaction of cyanogen azide and methoxy amine and subsequently neutralization with NaOH, and nitration via 100% HNO3used to obtain the energetic compounds 48 and 49.The produced compounds(48 and 49)were found thermally less stable(157,and 134°C respectively)in comparison to standard high-performance explosives. The calculated densities of these compounds were 1.90,and 1.81 g/cm3.The density of compound 48 was higher (1.90 g/cm3), which was reachable to RDX,HMX,and TATB.These compounds were sensitive in comparison to the high performance explosive as the sensitivity range was 1-1.5 J. Moreover, the detonation pressure and velocity were 46.7, and 38.4 GPa and 9867, and 9200 m/s respectively,reachable to the standard high performance explosive. Therefore,compound 48 exhibited high detonation performance (velocity 9867 m/s and pressure 46.7 GPa)which is greater than HMX.Thus,although these compounds(48 and 49)were exhibited less thermal stability and high impact sensitivities, they possess excellent detonation performance and can be used as high-performance explosives. In Scheme 10 the synthesis system of unsymmetrical tetrazole-based N,N′-ethylene-bridged energetic compound 50 was also particularized [86]. The reaction of bromoethylamine hydrobromide with cyanogen azide in the presence of sodium hydroxide yielded initiating compound 1-(2-Bromoethyl)-5-aminotetrazole.The reaction of starting material 1-(2-Bromoethyl)-5-aminotetrazole in the presence of TEAB in DMF with the sodium salt of 5-nitrotetrazole yielded the targeted compound 50. The prepared compound 50 was expected to rich in energetic properties such as high thermal stability, density, HOF, and detonation performance due to nitrogen augment heterocyclic moieties [87].Therefore, the energetic and physicochemical properties are given in Table 5, which shows that the thermal decomposition temperature was 234°C. The density (1.70 g/cm3), detonation velocity(8279 m/s), detonation pressure (25.6 GPa) were higher than TNT.However,insensitivity towards impact and friction of the prepared compound and high thermal stability makes it useful as high performing explosive.

3. Conclusion and future trends

Recently, the design and synthesis of modern high-performing explosives especially heterocyclic compounds combined with energetic substituents possessing high energy, density, and detonation properties attained prevalent attention as the potential replacement for the traditional energetic materials. Tetrazole a heterocyclic moiety is considered an important precursor for the development of a wide range of energetic compounds due to its unique properties and practical significance. The ideal properties for the high performing explosives are high density, thermal stability, detonation performance (velocity and pressure), positive heat of formation(HOF),insensitivity,and environmentally friendly nature. Therefore, to achieve these properties, tetrazole moiety combined with different energetic functional groups such as nitro(-NO2), nitrato (-ONO2), nitrimino (-NNO2), and nitramino(-NH-NO2).

In this review,the current developments in the field of tetrazole based energetic compounds containing energetic functional groups(-NO2, -ONO2,-NNO2, and -NH-NO2) were designed and synthesized are discussed since 2010, which can be considered as a potential replacement for the traditionally used compounds such as RDX or HMX. Ideally, the detonation velocities of new compounds should be greater than 8500 m/s, thermal stability up to 200°C,impact sensitivity, and friction sensitivities should be greater than 7 J and 120 N respectively. Moreover, the products of decomposition should be environmentally friendly, the yield of reaction should be high and associated with low-cost synthesis. Among all of the nitro-tetrazole energetic compounds discussed here many exhibit physical and detonation properties competitive with TNT or TATB, and numerous compounds (1, 2, 5,12, 20, 27, 29, 30, 34, 36,38,41,42,43,and 49)possess similar or even improved detonation properties as compared to RDX. The thermal stabilities of the compounds 15,18,23,24,30,31,33,35,and 50 are advanced than RDX.While looking at densities of compounds 2,3,5,12,23,24,29,30, 32, 33, 38, 41, 45, and 49 found promising to replace RDX.Whereas,the detonation properties(4,24,45,and 48)and densities of(4,27,and 48)some compounds are even better than HMX.The compound 40 demonstrated promising detonation performance and density than CL-20 but it exhibits high sensitivities towards impact and friction therefore it can be used as a primary explosive.However, this is worth noting that the poly-nitro tetrazole energetic compounds such as 1-13 enjoyed greater detonation performance and density as compared to the fewer number of these energetic groups containing compounds.Although these properties of high detonation performance and densities are achieved on the contrary to low thermal stability and high sensitivity towards impact and friction.Hence,the poly-nitro tetrazole compounds are evidenced for enhancement of densities and detonation performance whereas, the less number favor high disintegration temperature and insensitivities towards impact and friction.Therefore,this strategy to attain the molecular design of new high performing explosives with balanced physiochemical and energetic properties in the future becomes more challenging and demanding.

Furthermore,there are some areas demanding improvement for the further synthesis of tetrazole based energetic compounds containing energetic functional groups(-NO2,-ONO2,-NNO2,and-NH-NO2). First, traditional nitration is generally used in the synthesis of these energetic compounds which does not meet the requirements of modern green chemistry.It is necessary to find out the appropriate green nitrating agents and catalysts in the future synthesis process. Second, the synthesis of many nitro-tetrazole energetic compounds requires several steps, leading to a low yield and high cost. Therefore, it is vital to search for an efficient route when preparing new high performing explosives.

Declaration of competing interest

There are no conflicts to declare.

Acknowledgments

We are thankful to the NSAF (U1830134), NSFC (21905023 and 21911530096) for their generous financial support.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dt.2021.02.002.