1 Introduction

Energetic materials (EMs) refer to substances whose chemical reactions are accompanied by energy liberation. According to the energy amount contained in EMs and the rate of release, EMs are classified as pyrotechnics, propellants, “tertiary” explosives, secondary explosives and primers. Preferable EMs should satisfy the following requirements: tailored and high performance, stability, insensitivity, high thermal stability, environmental safety, vulnerability, hydrolytic stability and small solubility in water, compatibility and longevity [1,2,3]. Traditional energetic materials such as 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) [4], trinitrotoluene (TNT) [5], hexanitrohexaazaisowurtzitane (CL-20) [6] and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) [7] suffer from several deficiencies such as being environmentally unfriendly, expensive, showing polymorphism, involving harsh synthetic methods and so on. The key challenge to developing new suitable EMs is the compatibility of large energy content with mechanical and chemical stability, even though the two properties are generally contradictory [8,9,10,11].

Nitrogen plays an important role among the elements. The strong and short nitrogen–nitrogen triple bond involved is a powerful energetic driving force for the generation of nitrogen molecules. The more contiguous nitrogen atoms in the doubly and singly bonded system result in larger formation heats and thus better performances [12, 13]. In recent years, development of novel nitrogen-rich energetic materials has become an interesting research field because of their outstanding features like high energy density and content, good thermal stability, suitable solubility in ordinary solvents and lower sensitivity towards mechanical stimuli [14,15,16,17]. Besides, the decomposition of these high-nitrogen energetic materials release a large amount of environmental-friendly gas N2; thus, they are improved and “green” candidates for applications which require environmental protection [18].

In the past decade, high-nitrogen heterocycles have attracted increasing attention in the field of high energy density materials (HEDMs) meanwhile obtaining a balance of environmental concerns, sensitivity and high detonation performance [19, 20]. Heterocyclic five-membered cycles like pyrazole, oxazole, tetrazole, triazole and six-membered cycles like tetrazine and triazine are usually utilized in nitrogen-rich energetic materials to improve nitrogen and energy contents, densities, oxygen balance, planarity and detonation performance [21,22,23,24]. The formation enthalpies of these heterocycles increase with the catenated N atoms number, while pentazole is not isolated in macroscopic amounts and alkyl substituted pentazoles are often thermally instable and extremely sensitive to mechanical stimuli [25]. Among the stable azoles, tetrazole has the largest nitrogen content and imparts the highest energy content [26]. Introducing diverse explosophoric groups to the tetrazole scaffolds and their aromatic nature would be beneficial toward realizing the contradictory balance between detonation performance and safety, making the tetrazole motifs the most prospective backbones. Introduction of electron-donating groups to tetrazole rings (methyl, amine, etc.) enhanced the stability of tetrazole-based Ems [27, 28]. Additionally, linkages like methylene not only lowered the mechanical sensitivity but also increased the density and thermal stability [29]. Tetrazole-based EMs with –NH- bridges have superior inter- and intramolecular H-bonding, effectively reducing the sensitivity [30, 31]. Combination of tetrazoles with other heterocyclic rings could improve the performance [32, 33]. Compared to monotetrazole-based EMs, bistetrazole- or polytetrazole-based EMs have additional superiorities resulting from the larger number of N − N bonds and higher formation heats [34, 35].

Although some reviews on high-nitrogen energetic materials have been published recently, just two or three examples of tetrazole scaffolds have been reported [36,37,38]. Because of the well-established synthetic method, distinctive performance and potential application, a systematic review on tetrazole-based nitrogen-rich EMs is needed. In this review, we present the recently developed synthetic approaches, interesting performance and potential use of nitrogen-rich tetrazole EMs. High-nitrogen EMs of both tetrazole and trazolate are covered in detail. According to the number of tetrazole rings involved, the EMs based on tetrazole and trazolate in this review were mainly divided into the following categories: (1) nitrogen-rich EMs based on mono-tetrazole skeletons, (2) nitrogen-rich EMs based on bis-tetrazole skeletons, (3) nitrogen-rich EMs based on tri- and tetra-tetrazole skeletons and (4) other nitrogen-rich EMs based on tetrazole skeletons. References in this review on nitrogen-rich energetic materials of tetrazole were collected up to May 2023. We apologize to the authors whose contributions were not presented here because of the limitations of the search profiles and tools employed.

2 Nitrogen-Rich EMs Based on Mono-Tetrazole Skeleton

Numerous nitrogen-rich EMs based on tetrazole have been developed in recent years. Among them, amino-, nitro-, aryl-, and bridged tetrazole derivatives are important categories of high-nitrogen EMs with some advantages like high nitrogen and energy content, excellent performance, suitable stability and being environmentally friendly.

2.1 Nitrogen-Rich Molecules Based on Mono-Tetrazole

Early in 2011, Zhang investigated the tautomers/rotamers of diazido-tetrazole (I − IV, CN10) bearing two azide groups by theoretical computations (Fig. 1). The geometries were wholly measured at the level of B3LYP/6–311 +  + G(d). The harmonic vibrational frequencies, thermodynamic properties and electronic structures were studied at the same level for the optimal structure. The predicted stability order at MP2/6–311 +  + G(d) was II < I < V < IV < III. Additionally, the detonation property was investigated. The results showed that CN10 has high detonation energy and heat formation and should be a novel potential HEDM candidate if synthesized successfully [39]. This investigation could give significant information toward developing novel HEDMs with high performance and good stability.

Fig. 1
figure 1

Optimized geometries and numbering of CN10 isomers. (Images reproduced from Ref. 39 with permission of Elsevier) [39]

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1,5-Diamino-1H-1,2,3,4-tetrazole (DAT), a common building framework for a broad range of HEDMs, has attracted increasing attention because of its large nitrogen content and high thermal stability. Wang studied the pressure structural (Fig. 2) and vibrational nature of DAT in 2021 [40]. First, the author investigated the static compression property of DAT at 0 − 20 GPa pressure range through theoretical calculations. The calculated geometry, vibrational modes and crystal parameters were matched with those obtained by experimental results at ambient pressure. Additionally, the compressibility of the lattice parameters demonstrated an isotropic pattern. Subtle abrupt changes were obtained in the lattice angle, vibrational modes, dihedral angles and bond lengths at 17 and 7 GPa, indicating the transitions of isostructural phase took place. The results of vibrational modes and Hirschfeld surface analysis showed that the molecular interaction strengthening enhanced crystal packing. This report not only revealed the influence of hydrostatic pressure on the structure and crystal of DAT but also showed the intermolecular interactions and molecular packing mode in response to pressure, which were very important for exploring the high-pressure structural stability of DAT.

Fig. 2
figure 2

Crystal structure of DAT. (Images reproduced from Ref. 40 with permission of Elsevier) [40]

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To enhance the density of 5-aminotetrazole, 4-amino-3,5-dinitropyrazole was introduced to 5-aminotetrazole as a skeleton. The target compounds N,N-methylene linked pyrazole and 5-aminotetrazole DMPT-1 and DMPT-2 were synthesized successfully through treatment of chloroiodomethane with ammonium 4-amino-3,5-dinitropyrazolate followed by treatment with 5-aminotetrazole. These two compounds were systematically determined via NMR and IR, differential scanning calorimetry, elemental and x-ray diffraction analysis. The experimental results and theoretical calculation showed that nitropyrazole functionalization improved mechanical sensitivity and energetic performance. In comparison with N,N’-ethylene-linked molecules, the two methylene-linked molecules DMPT-1 and DMPT-2 exhibited higher detonation performance and density. DMPT-2 showed a large crystal density (1.806 g·cm−3), high detonation pressure (P = 30.2 GPa), good pressure velocity (vD = 8610 m·s−1) and low impact sensitivity (30 J) (Table 1) [41]. This research showed that involvement of 4-amino-3,5-dinitropyrazole in the 5-aminotetrazole skeleton could improve the energetic performance, which was very useful for designing efficient nitrogen-rich EMs based on 5-aminotetrazole.

Table 1 Synthesis and physicochemical properties of DMPT [41]
figure 1
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One-step synthesis of tetrazole compounds bearing N5 group as well as fused compounds 2–9 was developed by Cheng in 2020 (Table 2) [42]. The structure of 2–9 was well characterized through NMR, elemental analysis, thermal analysis and IR spectroscopy. Compounds 2, 4, 6, 8 and 9 were characterized by x-ray diffraction analysis as well. These compounds exhibited high formation heats (3308 − 6180 kJ·kg−1) and nitrogen content (50.9% − 76.4%). Compounds 2–9 showed high energy performances and low sensitivities because of their distinctive structure particularly for compound 4 (D = 9004 m·s−1, IS = 22 J, IsP = 289 s). Additionally, all of these compounds displayed low sensitivities of friction (204 to > 360 N) and impact sensitivities (18 to > 40 J). The hydrogen bonding and π-π interaction in 2 − 9 were proved through fingerprint plots and NCI plots according to Hirschfeld surfaces. Moreover, the structure-property relationship was well established through theoretical calculations, crystal structure analysis and detonation performance. These above results demonstrated the potential applications of these tetrazole derivatives as novel nitrogen-rich HEDMs. This study not only offered guidelines for development of energetic materials but also opened a prospect in this field.

Table 2 Synthesis and energetic performance of tetrazole compounds 2 − 9 [42]
figure 2
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Incorporation of oxygen-rich scaffolds, C(NO2)3, C(NO2)2NF2 or C(NO2)2F, onto an endothermic skeleton, which integrated nitropyrazole and tetrazole, was used to develop new oxygen- and nitrogen-rich energetic compounds [45]. The desired compounds 18 − 20, 24 were obtained by the synthetic routes demonstrated in Table 3. The structures of 18 − 20, 24 were characterized by x-ray analysis. According to a combination of the ΔOED criterion and molecular density (dmol), the authors presented a useful and convenient method for crystal packing analysis, which was the determination of molecular packing tightness upon formation of crystal. Target compounds 18 − 20 and 24 decomposed at the range of 110 to 170 °C, indicating 18 − 20, 24 were more stable thermally than the analogous one with no pyrazole bridge. Difluoroamine 20 exhibited the largest onset decomposition point at 138 °C. Notably, compound 19 bearing C(NO2)2F module melted along with decomposition while other compounds decomposed without melting. For safety testing, friction (FS) and impact (IS) sensitivities of 18 − 20, 24 was measured through a standard BAM techniques. Interestingly, the group (F, NO2, NF2) at the dinitromethyl framework showed a few effects on the sensitivity, and a narrow range (1.5 J to 1.6 J) of impact sensitivity was observed (Table 3). Difluoroamino compound 20, showing good nitrogen and oxygen contents, high density, acceptable sensitivities and high enthalpy of formation, could serve as a promising candidates for nitrogen-rich EMs. This investigation, introducing oxygen- and fluorine-rich groups to tetrazole derivatives, is of considerable interest for the development of fluorine-containing EMs.

Table 3 Synthesis and energetic performance of compounds 10 − 16, 19 [45]
figure 3
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5-Nitro-2-nitratomethyl-1,2,3,4-tetrazole 23 was prepared successfully in 38% yield by nitration of the alcohol 22 with acetic anhydride and dilute nitric acid. Intermediate 22 could be generated from 5-nitrotetrazole sodium salt (21), which was obtained from 5-aminotetrazole by a one-pot reaction [47]. The structure of 23 was characterized via NMR, FT-IR, MS techniques and x-ray diffraction. Target 23 was the orthorhombic system with space group Pna2(1). The results of theoretical and experimental study showed that 23 had high energetic performances with large formation heat (228.07 kJ·mol−1), high pressure of detonation (37.92 GPa), high velocity of detonation (9260 m·s−1) and good oxygen balance (Scheme 1) [48]. This report introduced oxygen atoms into the tetrazole motif, achieving good balance of oxygen and high performances.

Scheme 1
scheme 1

Preparation of 5-nitro-2-nitratomethyl-1,2,3,4-tetrazole [48]

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The preparation and energetic nature of a new N-oxide high-nitrogen energetic molecule, 6-amino-tetrazolo[1,5-b]-1,2,4,5-tetrazine-7-N-oxide (4), were reported [49]. Compared with known nitrogen-rich molecules, like 2,4,6-tri(azido)-1,3,5-triazine (TAT) [50], 4,4’,6,6’-tetra(azido)azo-1,3,5-triazine (TAAT) [51] and 3,6-diazido-1,2,4,5-tetrazine (DiAT) [52], target 26 displayed excellent properties of detonation, acceptable sensitivities of friction and impact, and high density. The high performance of 26 might result from the fused ring system and N-oxide (Table 4). This study suggested that fused tetrazole compound 26 had potential application as a nitrogen-rich EM.

Table 4 Synthesis and physical properties of 25 and 26 [49]
figure 4
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1,1-Diamino-2,2-dinitroethene (FOX-7) is a skeleton having good energetic property, high stability and low sensitivity [53]. In 2021, Shreeve and Tang modified the structure of FOX-7 by introducing a tetrazole block and hydrazino group, developing (Z)-1-amino-1-hydrazinyl-2-nitro-2-(1H-tetrazol-5-yl)ethene (HTz-FOX) as a promosing nitrogen-rich compound [54]. The structure of HTz-FOX was measured via elemental analysis, NMR and x-ray diffraction. The authors discovered that the target HTz-FOX, which displayed parallel molecular layers, was a planar molecule. For this coplanar structure, HTz-FOX showed a high temperature of decomposition (TD = 237 °C) and good performance of detonation (P = 28.3 GPa, Dv = 8883 m·s−1), which was superior to FOX-7 (P = 31.6 GPa, Dv = 8613 m·s−1, TD = 220 °C) and 1-amino-1-hydrazino-2,2-dinitroethene (H-FOX) [55] (P = 33.8 GPa, Dv = 8803 m·s−1, TD = 125 °C). HTz-FOX displayed an outstanding combustion nature with shorter delay combustion time (0.09 m·s−1) and larger Pmax (20.13 GPa) (Table 5). This work offered potential guidance to develop advanced nitrogen-rich EMs through structural modification, constructing planar configuration for allowing parallel molecular layers to balance the thermostabilities, sensitivities and detonation performances.

Table 5 Synthesis, energetic and detonation property of HTz-FOX [54]
figure 5
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2.2 Nitrogen-Rich Salts Based on Mono-Trazolate

Several high-nitrogen energetic salts 28–39 of tetrazole azasydnone were prepared and characterized, leading to the development of novel secondary and primary explosives. The structures of 28–39 were measured by IR, NMR and x-ray analysis. The authors also studied their energetic performances and sensitivity. All salts had larger densities than azidotetrazole analogs [59, 63]. Density of 30 exceeded RDX (1.820 g·cm−3) and the densities of 31, 32 and 37 − 39 exceeded their analogous nitrotetrazole [59, 64]. The silver 34 salt and free acid 30 showed higher sensitivity than RDX. Compared to RDX, 30 possessed the best performance of detonation (327 kbar, 8906 m·s−1), and 30 also exhibited the highest heat of formation although its nitrogen content was lower. The silver salt 34 displayed a suitable sensitivity regarding primary explosive power. Among these compounds, 30 and 34 were the best replacements for the primary explosives (Scheme 2) [64]. Compared to the lead-based Ems [65], this silver salt is obviouly less toxic.

Scheme 2
scheme 2

Structure of nitrogen-rich energetic salts 28–39 [64]

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A novel high-nitrogen methylene and azo linked mixed azole 41 was prepared by simple reactions as well as the methylene bridged azole molecule 40, which was not just easily converted to its salts 42 − 44 but also could be applied as a starting material to synthesize a nitroimino involved azole 45 and its energetic salts 46 − 48 (Scheme 3). Most of the molecules exhibited high thermostabilities and low sensitivities. These energetic molecules also showed high heats of formation for compound 45 with P, 32.87 GPa; Dv, 8759 m·s−1. The salts 48 (P, 31.92 GPa; Dv, 8870 m·s−1), 47 (P, 30.53 GPa; Dv, 8933 m·s−1) and 44 (P, 28.04 GPa; Dv, 8590 m·s−1) displayed higher detonation performances than ANTA and TNT [66]. This research provided possible candidates which could be applied to the field of insensitive EMs.

Scheme 3
scheme 3

Synthesis of nitrogen-rich energetic compounds and salts 42–48 [66]

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Two kinds of fused high-nitrogen EMs 49 − 50 and their salts 51 − 56 were prepared. The structures were characterized by NMR and IR, elemental and thermal analysis. Because of the ring-strain energy and large nitrogen contents (59.6%–76.8%), these high-nitrogen compounds showed a large range of formation heats (2.35 to 4.23 kJ·g−1), which were much larger than those of HMX (0.25 kJ·g−1) and RDX (0.32 kJ·g−1). The larger noncovalent interactions and enthalpies (hydrogen bonds) provided them with significant detonation performance. Their energetic properties were tested with EXPLO5. 49, 52 and 53 displayed large densities, good mechanical stabilities and high detonation performances, thus accentuating their potential uses as new nitrogen-rich HEDMs (Table 6) [67]. This work developed a series of high-nitrogen compounds with triazolo-triazine and tetrazolotriazine as the skeletons, bearing amino and tetrazole substituents. They not only realized good balance between detonation performance and molecular stability but also offered a new strategy to create novel nitrogen-rich HEDMs with excellent performance and suitable stability.

Table 6 Physiochemical properties of compounds 49 − 56 [67]
figure 6
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Several salts of C-N bridged 1-(2H-tetrazol-5-yl)-5-nitraminotetrazole were successfully prepared from easily available 5-aminotetrazole (Scheme 4). These novel high-nitrogen energetic molecules were totally measured by NMR, IR, elemental analysis, and x-ray diffraction. Among them, 58 (3.25 kJ·g−1) was found to show interesting energetic properties with excellent calculated data (vD = 9822 m·s−1, IS = 8 J and FS = 192 N) [69]. This good performance of detonation and suitable sensitivities made compound 7 an improved candidate to use as high-performance EMs.

Scheme 4
scheme 4

Structure of C-N bridged 1-(2H-tetrazol-5-yl)-5-nitraminotetrazole and its salts [69]

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Dharavath and the co-authors synthesized high-nitrogen energetic salts 60 − 62 with good performance and thermal stability from readily obtainable starting material (Scheme 5). These molecules were fully measured by NMR, IR, TGA–DSC measurements, ESI–MS and EA techniques. All the salts showed high density, low sensitivity towards friction and impact, large heat of formation and good detonation velocity. Because of the large nitrogen content, they could be potentially employed as pyrotechnic applications and gas generators [70]. The large thermal decomposition temperature (> 220 °C) enabled them to be good replacements of heat-resistant explosives and thermally stable secondary EMs.

Scheme 5
scheme 5

Synthesis of high-nitrogen energetic tetrazole salts 60 − 62 [70]

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Aminotetrazole (AT) showed an acidic property and could be applied to synthesize high-nitrogen energetic ionic liquids and salts 63 − 70 (Table 7). These prepared AT salts were fully determined by NMR, IR, phase behavior, density, thermal stability and elemental analysis. The nitrogen content of salt 63 was 82%, while 68 possessed the largest nitrogen content (68%) among the reported room temperature ionic liquid. These salts also displayed other promising properties of energy, including high content and densities of nitrogen, and excellent thermal and hydrolytic stabilities. The results of theoretical calculations and experiments demonstrated that these salts showed large densities of energy (> 3.0 kJ·g−1) and good heats of formation. Additionally, all of them were impact insensitive EMs with excellent calculated detonation performances [71]. These reported AT ionic liquids and salts would be applied as potential safe EMs. Importantly, this work provided some advice for development of insensitive EMs.

Table 7 Physiochemical properties of compounds 63 − 70 [71]
figure 7
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(Trinitromethyl)-2H-tetrazole (HTNTz) 77 could be synthesized conveniently through nitration of acid 76 and basic hydrolysis. HTNTz could be transformed to the salts 78–83. Moreover, the ammonia adducts, 82a and 83a, could be obtained by addition of NH3. Tetrazolate 84 and 85 were also produced via treatment of 77 with hydroxylamine and hydrazine. Acid treatment of 84 and 85 led to 5-(dinitromethylene)-4,5-dihydro-1H-tetrazole 86, which was transformed into 87 by reaction with K2CO3 (Scheme 6). The authors also implemented thermal stability testing (DTA) and initial safety measurements (friction, electrostatic and impact sensitivity) and found that introduction of ammonia or water to the crystal lattice of the salts led to lowered impact sensitivity. The authors also demonstrated that the target salts 80 − 82 and 84 possessed a positive oxygen balance and were overoxidized, while 79, 83a and 85 − 87 had a negative oxygen balance and ammonium salt 78 was oxygen-balanced. The salts 84, 85 and 87 showed higher decomposition temperatures than the related free acid 86 [74]. Notably, this report provided meaningful guidance to design EMs with good oxygen balance and energetic performance.

Scheme 6
scheme 6

Synthesis of high-nitrogen energetic tetrazole salts 78–87 [74]

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3 Nitrogen-Rich EMs Based on Di-Tetrazole Skeleton

3.1 Nitrogen-Rich Molecules Based on Di-Tetrazole

In 2012, Sinditskii and the co-author synthesized 3,6-bis(1H-1,2,3,4-tetrazol-5-ylimino)-1,2,4,5-tetrazine (BTATz) as well as 3,6-dihydrazino-1,2,4,5-tetrazine (DHT) and examined their thermal stability through nonisothermal and isothermal means (Scheme 7). The authors considered that the first period for the decomposition of DHT was a unique redox process, during which tetrazine was reduced by hydrazine functional group to generate nitrogen molecule as well as diaminodihydrotetrazine. For BTATz, decomposition started with tetrazole units. The existence of preliminary isomerization in tetrazole decomposition caused the high observable activating energy for decomposition (57.5 kJ/mol or 240.6 kcal/mol) in the 250 − 334 °C interval. At higher temperatures in the wave of combustion, the activating energy for the decomposition of BTATz was relatively lower (128.4 kJ·mol−1 or 30.7 kcal·mol−1), close to theoretical data [75]. This report provided value to obtain thermostable EMs.

Scheme 7
scheme 7

Synthesis of BTATz and DHT [75]

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Among the azo-stabilized high-nitrogen molecules, 1,19-azobis(tetrazole) (N10), a possible candidate eco-friendly compound, possessed the largest content of nitrogen. Zhang and the co-authors investigated the potential energy surface of N10 in details by theoretical calculations and also calculated the major decomposition manners through theory modeling of canonical transition state [76]. The authors predicted that the cycle break of the N10 and the release of N2 to produce the linear Im8° were a main decomposition pathway. The generated Im8° was determined through a fournitrogen atom linkage, which was stabilized via two HNC substituents. Its complete decomposition was dramatically exothermic having an energy barrier of 67.5 kcal·mol−1. The overall liberated heat of N10 complete decomposition was 167.12 kcal·mol−1, with the formation of final products including 2HNC and 4N2. According to the mechanism of N10 thermal decomposition, the new species N14 and N12 were demonstrated (Scheme 8). The energy barriers for beginning N2 elimination from the tetrazoles of N14 and N12 were 5.1 kcal mol−1 less than that of N10, which implied that the new compounds N14 and N12 have longer nitrogen linkages stabilized through two tetrazole cycles, which should be obtained predictably in the near future.

Scheme 8
scheme 8

Structure of N10, N12 and N14 [76]

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1,4-Bis-[1-methyltetrazol-5-yl]-1,4-dimethyl-2-tetrazene 92 is facilely obtained by a one-pot process by applying 89 as a reactant (Scheme 9) [77]. First, 89 was transformed to the N-nitrosamino-1H-tetrazole 90 and then reduced by CH3COOH and Zn system to form the desired 1,1-substituted hydrazine derivative 91. In the presence of Br2, the in situ oxidation of 91 occurred to produce the target 92. The structure of 92 was determined by x-ray, NBO and MO analysis. The physical natures of 92 were demonstrated by MS and IR pyrolysis experiments and drop hammer. The results showed that 92 could be described as a novel stable HEDM with the high performances required for a potential gas generator [78]. Notably, the author successfully stabilized an acyclic nitrogen with four N atoms (2-tetrazene) by two tetrazolyl groups in this report.

Scheme 9
scheme 9

Synthesis of 92 [77]

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In 2022, Liu and Tang developed a lithium-promoted cycloaddition of salt 93 and diazoacetonitrile, which resulted in the generation of two cyanotetrazoles (94a, 94b). Additionally, the cyano in cyanotetrazoles (94b and 94a) was transformed to tetrazole N-oxide or tetrazole for producing heterocyclic compounds (95a, 95b, 99a and 99b) (Scheme 10). The energetic performances of these compounds were investigated. 1,5-Disubstituted tetrazoles (94a, 95a and 99a) displayed lower sensitivities and higher decomposition temperatures than 2,5-disubstituted tetrazoles (94b, 95b and 99b). Compared to RDX, compound 95a (D = 9052 m·s−1, TD = 220 °C, FS = 200 N, IS = 13 J) exhibited higher stability, lower sensitivities and better detonation performances, which was expected for use as a potential Ems [79]. This work provided important references to design heterocyclic nitrogen-rich energetic EMs with good stability and high performance.

Scheme 10
scheme 10

Synthesis of cyanotetrazoles and tricyclic heterocyclic compounds [79]

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3.2 Nitrogen-Rich Salts Based on Di-Trazolate

In 2015, Klapötke and co-authors prepared a series of salts based on H2BTF through metathesis of the barium salt with the sulfate salts obtained in advance by treatment of Ag2SO4 with chloride or iodide salts. The authors used the following nitrogen-containing counter anions to produce these energetic salts: hydrazinium (Hy), ammonium (A), aminoguanidinium (AG), guanidinium (G), triaminoguanidinium (TAG), diaminoguanidinium (DAG), 1-methyl-3,4,5-triamino-1,2,4-triazolium (H1,2,4TAMTr), N-carbamoylguanidinium (CG) and 1-amino-3-methyl-1,2,3-triazolium (H1,2,3AMTr) (Scheme 11). Compared to the corresponding neutral molecule, most of these salts exhibited higher decomposition temperatures. TAG2BTF displayed the lowest temperature of decomposition (TD = 2208 °C), while the diaminoguanidinium (TD = 2908 °C) and barium (TD = 2978 °C) salts possessed the best thermal stability. The densities were located at the range of 1.56 to 1.85 g·cm−3. Additionally, these salts were endothermic molecules (formation enthalpies were 471.6 to 1762.0 kJ·mol−1). Moreover, Hy2BTF salt displayed the best detonation performance (pC-J = 32.0 GPa, VDet = 8915 m·s−1) [80]. Notably, the authors realized high energetic performance, suitable thermal stability and good oxygen balance of EMs through cooperation of tetrazolate with oxygen involved furoxan.

Scheme 11
scheme 11

Synthesis of dianionic salts of H2BTF [80]

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Triaminoguanidinium salt (TAG2AzTF) was prepared successfully by treatment of ammonia with (E)-1,2-bis(4-(1H-tetrazol-5-yl)-1,2,5-oxadiazol-3-yl)diazene (AzTF) (Scheme 12). Benefiting from the high-nitrogen content, this TAG salt possessed a ∆fH of 1481 kJ·mol−1. Compared to compound AzTF, the salt TAG2AzTF exhibited relatively large impact sensitivity and low thermal stability [81]. AzTF showed the most promise for application as an explosive because of the high thermal stability as well as good performance, while the corresponding salt (TAG2AzTF) was favored as a high-nitrogen gas generator regarding its large nitrogen content and suitable overall sensitivity.

Scheme 12
scheme 12

Synthesis of triaminoguanidinium salt (TAG2AzTF) [81]

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The high-nitrogen energetic ionic salt (2CH7N4+-C4H2N142−-2H2O, TAG2AzTF) of 3,6-bis[(1H-1,2,3,4-tetrazol-5-yl)-amino]-1,2,4,5-tetrazine (BTATz) could be prepared and determined through Fourier transform IR spectrometry, x-ray analysis, NMR and elemental analysis (Scheme 13). The authors also studied thermal decomposition of TAG2AzTF via TGA and DSC. The temperature of the exothermic peak was 509.72 K, which demonstrated that hydrated salt showed excellent thermostability. The authors also found that TAG2AzTF displayed good thermal safety compared with other salts of BTATz [82].

Scheme 13
scheme 13

Synthesis of triaminoguanidinium salt (TAG2AzTF) [82]

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The high-nitrogen ionic compound, uranyl(VI), was produced successfully from sodium salt (Scheme 14). The structure of UO2(ZT)-5H2O was measured by x-ray analysis and vibrational spectroscopy. UO2(ZT)-5H2O displayed an uncommonly distorted angle of 172.4° for O = U = O. The authors showed that this molecule was one of the highest nitrogen uranium molecules (26.72% N per weight). In the presence of neutron bombardment, this compound did not decompose, indicating its exceptional stability [83]. Notably, this work provided significant access to obtaining high-nitrogen uranium compounds with radiation stability.

Scheme 14
scheme 14

Synthesis of UO2(ZT)-5H2O [83]

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A series of novel high-nitrogen energetic tetrazole salts, 102a-102g and 103a-103d, were obtained successfully according to the reported procedure (Scheme 15) [84,85,86,87,88,89]. These energetic tetrazoles salts showed large nitrogen content, pressure density, good thermostabilities, excellent properties of detonation, acceptable sensitivities of friction and impact. Particularly, salts 103c and 103b possessed large formation heats, good velocities of detonation (8839 m·s−1 and 9050 m·s−1) and pressures (28.8 GPa and 28.7 GPa) and attractive sensitivities (20 J and 35 J) [90]. These novel molecules showed potential to take place of current nitrogen-rich EMs like TATB, TNT and RDX.

Scheme 15
scheme 15

Synthesis of tetrazole salts 102a − 102 g, 103a − 103d [90]

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4 Nitrogen-Rich EMs Based on Tri- and Tetra-Tetrazole Skeleton

Nitrogen-rich salts (106a − f) were prepared from intermediate 106, and product 109 was produced from cyanuric chloride through an efficient, simple two-step synthetic procedure (Scheme 16). All of these salts (106a − f) displayed positive formation heats (205 − 1889 kJ·mol−1), high densities (1.65 − 1.83 g·cm−3), high pressures of detonation (20.73 − 27.34 GPa) and velocities (7876 − 8832 m·s−1) and acceptable thermal stabilities (TD = 165 − 269 °C). Compound 109 showed better detonation properties (DP = 30 GPa, VOD = 8660 m·s−1) and the highest density (1.85 g·cm−3) [91]. Based on their facile synthesis, good detonation properties, high content of nitrogen and suitable thermal stabilities, these molecules had potential applications as nitrogen-rich EMs.

Scheme 16
scheme 16

Synthesis of 106a-106f, 108–109 [91]

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In 2022, Shreeve and co-authors described the synthesis of nitrogen-rich azoles, which were derived from the combination of triazole and tetrazoles. Treatment of 110 [92] with chloroacetone in MeCN under reflux formed the product 111 in 87% yield. Using ZnCl2 as a catalyst, nitriles of 111 reacted with sodium azide to produce bis-tetrazole 112 in good yield. In the presence of mixed acids, nitration of 112 occurred to yield 113. Treatment of bases like hydroxylamine, hydrazine and ammonia with 113 generated the target energetic salts 114a − c in good yield. Potassium salt 110 reacted with chloroacetonitrile to produce acetonitrile 115, which was then transformed into tri-tetrazole 116 by treatment with NaN3 in the presence of NH4Cl. Neutralization of the three tetrazole rings in 116 obtained the target tri-ionic salts 117a − c (Scheme 17). Interestingly, the salts 114b and 114c showed good detonation velocities (Dv = 9376 m·s−1 and Dv = 9418 m s−1), while the detonation velocities of 117c-0.5H2O (Dv = 9058 m·s−1) and 117b-H2O (Dv = 8998 m·s−1) were better than that of RDX (Dv = 8795 m·s−1). From the theoretical analyses, the authors deduced that almost all these compounds were insensitive to stimuli for the present hydrogen bond interactions [93]. Importantly, this work was the first case of insensitive tri-cationic nitrogen-rich salts with good performance and suitable stability to our knowledge.

Scheme 17
scheme 17

Synthesis of salts 114, 117 [93]

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Deprotonation of a novel high-nitrogen compound, 2,3,5,6-tetra(1H-tetrazol-5-yl)pyrazine (H4TTP), with various bases resulting in a series of salts: aminoguanidinium (130), guanidinium (129), hydroxylammonium (128), hydrazinium (127), ammonium (126), caesium (125), rubidium (124), potassium (123) and sodium (122) (Table 8). Among these salts, the guanidinium (297 °C) and caesium (300 °C) salts showed the highest decomposition temperature, while hydroxylammonium salt displayed the lowest temperature of decomposition (207 °C). Hydroxylammonium (HA)4TTP (128; 23.7 GPa, -2450 kJ·kg−1, 183.5 s, 8364 m·s−1) and hydrazinium (Hy)4TTP (127; 24.9 GPa, − 3166 kJ·kg−1, 192.5 s, 8360 m·s−1) salts possessed remarkable datas of performance characteristics [94].

Table 8 Physiochemical properties of compounds 122–130 [94]
figure 8
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1,1,3,3-Tetra(1H-tetrazol-5-yl)propane-involved high-nitrogen salts 131–142 could be prepared through a straight and convenient method. The target tetraanionic salts 131 − 142 were obtained from TTP (Scheme 18). The enthalpy of formation of 133, 137 and 139 were large (> 1900 kJ mol−1). The authors also found that the introduction of a nitro to the imidazole cycle increased the HOF. The salts 141 (D = 7.51 km·s−1, P = 24.13 GPa) and 137 (D = 7.53 km·s−1, P = 24.41 GPa) showed good performance of detonation [9]. This investigation gave some important information to understand the effect of nitro, amino and heterocycles in the manufacture of EMs with improved performance.

Scheme 18
scheme 18

Synthesis of 1,1,3,3-tetra(1H-tetrazol-5-yl)propane-involved energetic salts [9]

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5 Other Nitrogen-Rich EMs Based on Tetrazole Skeleton

The nitro-azolate DAT salts 143–145 could be prepared and determined via NMR, IR, thermal stability, elemental analysis, density and phase behavior (Scheme 19). The calculated detonation pressure (P) data of 143–145 were located between 28.05 to 29.88 GPa, and the velocities (D) were 8343 to 8655 m·s−1, making them promising EMs. These compounds possessed a balance of oxygen near zero (-23.8 to -33.5%). All these salts showed suitable thermal stabilities (Td from 176 to 187 °C) and large energy densities (1.65 to 1.74 g·cm−3). The impact sensitivities of 143–145 were < 1 J, 28 J and > 60 J, respectively. Thus, salt 143 could be applied as a potential detonator or initiator, and 145 had a potential application as an impact insensitive EM [95].

Scheme 19
scheme 19

Synthesis of DAT nitro-substituted azolate salts [95]

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High-nitrogen energetic compounds 1,4,5-triaminotetrazolium tosylate 146 and salts 147 − 149 were synthesized through amination of diamino-1,2,3,4-tetrazole and metathesis reactions (Scheme 20). For the dramatically large content of nitrogen, the formation heats, thermal and explosive sensitivities of these compounds were very high. Tosylate salt 146 showed the lowest sensitivity with 240 J and 10 N of impact and friction sensitivities. For all compounds 147 − 149, friction sensitivities were < 10 N and impacts were ≤ 2 J. The velocities of detonation (Vdet) of 148 and 149 were 8779 m·s−1 and 8872 m·s−1. Regarding a potential application as propellant ingredients, 148 and 149 exhibited large specific impulses (260 s for 148 and 259 s for 149). All salts 147 − 149 decomposed at 78 to 102 °C, indicating the limited practical use as Ems [96]. This study did offer the insight that limited thermal stability appeared when the nitrogen content and formation heats were enhanced.

Scheme 20
scheme 20

Synthesis of 1,4,5-triaminotetrazolium salts [96]

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The energetic performances of high-nitrogen salts 150a − c, 151a − c were also inverstigated (Scheme 21). The calculated detonation velocity for 150a was very high (9127 m s−1). The author found that the decomposition temperature of the methylated nitrate and perchlorate salts 151a − b was significantly higher than that of the corresponding protonated species 150a and 150b, while the explosive performances decreased because of the methylation. Compared to 150a and 150b with the corresponding methylated salts 151a and 151b, the impact sensitivity offered the same data (IS (150b, 151b) = 2 J, IS (150a, 151a) = 3 J) [97]. This study did offer the insight that limited thermal stability appeared when the nitrogen content and formation heats were enhanced.

Scheme 21
scheme 21

Structure of high-nitrogen salts 150a-b, 151a-c[97]

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Treatment of cyanogen azide with aqueous hydroxylamine afforded 152. Using 152 as a precursor, various energetic salts (156–160) were obtained successfully (Scheme 22). Compound 152 and its salt (156) showed high detonation velocities (8609 m·s−1 and 9056 m·s−1), with friction sensitivities of 108 and 360 N and an impact sensitivity of 10 J. The monoclinic configuration of compound 153 showed a higher velocity of detonation (9312 m s−1) for the higher density. Salt 157 displayed remarkably high performance (338 kbar and 9032 m·s−1). The authors also suggested that salts 157 − 159 could be applied as propellant or explosive ingredients [98].

Scheme 22
scheme 22

Synthesis of energetic salts 156–159 [98]

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A new high-nitrogen molecule (NABTI) bearing bis(tetrazole)imidazole and salt (161) was prepared successfully (Scheme 23). The authors also demonstrated that both of these two compounds had layer-by-layer packing models and regular planar configurations, and the sensitivity to mechanical stimuli could be weakened by the interactions of π–π and 3D hydrogen-bonding network inside the crystals. NABTI also displayed positive formation heat (916.8 kJ·mol−1), large nitrogen content (63.6%), high density (1.80 g·cm−3) and good detonation performance (P = 31.4 GPa and D = 8329 m·s−1) [99]. These two energetic compounds based on bis(tetrazole)imidazole were good candiates for EMs.

Scheme 23
scheme 23

Synthesis of NABTI and its salt 161 [99]

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The coordination compounds based on tetrazole were also developed as high-nitrogen energetic complexes. For example, the high-nitrogen complex [Cd(DAT)6](NO3)2 was obtained via treatment of DAT with Cd(NO3)2-6H2O. The structure of this complex was determined by FT-IR, elemental and x-ray analysis. The Cd2+ cation chelated with six N atoms in the ligand of DAT to give a hexacoordinating distorted octahedral molecule. The [Cd(DAT)6](NO3)2 molecules were connected by two kinds of hydrogen bonds, giving a stable structure with a three-dimensional net. The authors also found that the exothermic activation energy of this complex was 121.7 kJ·mol–1[100]. Other complexes like copper(II) dicyanamide compounds based on N-substituted tetrazole were also synthesized. These complexes displayed good energetic performance as well as moderate sensitivities (FS > 80N, IS > 6) [101].

Two environmental-friendly high-nitrogen energetic metal–organic frameworks (MOFs), [(AG)3(Co(btm)3)] (Fig. 3) and {[(AG)2(Cu(btm)2)]}n, in which H2btm was bis(tetrazole)methane and AG was aminoguanidinium (Fig. 4), were manufactured and determined through x-ray analysis. The authors also demonstrated that the two energetic MOFs exhibited abundant hydrogen bonding with very large content of nitrogen, suitable insensitivities, favorable thermal stabilities and good detonation performances [102]. Compared to other known high-energy MOFs [103, 104], these two anionic MOFs also showed some advantages like good detonation performances and environment friendly.

Fig. 3
figure 3

a Coordination environment of Co(III) ions in [(AG)3(Co(btm)3)], b coordination sites of btm, c 3D supramolecular network, d 3D supramolecular network. (Images reproduced from Ref. 103 with permission of Royal Society of Chemistry) [102]

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Fig. 4
figure 4

a Coordination environment of Cu(II) ions in {[(AG)2(Cu(btm)2)]}n, b coordination sites of btm, c 3D supramolecular network, d 3D supramolecular network. (Images reproduced from Ref. 103 with permission of Royal Society of Chemistry) [102]

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Combining the advantages of furazan and tetrazole, a 2D MOF complex [Cd(H2O)2(AFT)2]n (HAFT: 4-amino-3-(5-tetrazolate)-furazan) was prepared and measured by FTIR, elemental and x-ray analysis. In the structure of HAFT, Cd2+ was hexacoordinated by two water molecules and four AFT groups. Tetrazole showed a classic model of bidentate coordination, in which furazan did not participate. The authors also demonstrated that the decomposition temperature of HAFT was > 250 °C, which indicated the thermal stability of HAFT [105]. This 2D coordination complex could be a foundation for the development of the 3D high-nitrogen energetic MOFs.

The high-nitrogen energetic coordination polymers (ECPs) were manufactured using high-nitrogen ligands 5,5’-bistetrazole-1,1’-diolate dehydrate (BTO) and azotetrazole (AT) with promising thermostability but large photosensitivity. By intercalating 5 wt% graphene oxide (GO), the activation energy of decomposition (Ea) for Cu-AT was enhanced between 135.7 and 151.9 kJ·mol−1; meanwhile, the temperature of the exothermic peak (Tp) rose by 12.6 °C. In sharp contrast to this, the Ea of decomposition for Cu-BTO reduced because of the influence of the same amount GO with little influence on Tp. This demonstrated that GO had a stabilizing influence on the crystal of Cu-AT. Moreover, when 3% GO was added, the resulting GO0.03-Cu-AT showed good thermostability (TP = 293.7 °C) and a larger density (2.88 g·cm−3). This ECP could be ignited at a wavelength of 976 nm with an energy of < 1 mJ (Fig. 5). This small energy laser initiation was recognized as safer [106].

Fig. 5
figure 5

High-nitrogen energetic coordination polymers GO-Cu-AT and GO-Cu-BTO. (Images reproduced from Ref. 107 with permission of ACS publications) [106]

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6 Summary and Outlook

In summary, the nitrogen-rich tetrazole-based EMs have attracted increasing popularity because of their outstanding advantages such as high content of nitrogen and energy density, good thermostability, suitable sensitivity, high energetic performance, environmental-friendly, etc. The nitrogen-rich energetic EMs based on mono-, di-, tri- and tetra-tetrazole skeletons were prepared and characterized successfully; meanwhile, their energetic properties were also investigated systematically. In design and manufacture of practically useful high-nitrogen tetrazole-based EMs, the big issue of achieving the contradictory balance of excellent performance of detonation and good insensitivity still existed.

Although various nitrogen-rich tetrazole-based EMs with good performances have been manufactured, development of other kinds of nitrogen-rich EMs based on tetrazole skeleton is still urgently required. The following challenges should focused on: (1) Since ionic liquid usually shows remarkable stability, design and preparation of ionic liquid nitrogen-rich EMs based on tetrazole should be enhanced in future research; (2) nitrogen-rich EMs with 2D coordination MOF structure were prepared; thus, 3D high-nitrogen energetic EMs require more attention.