1 Introduction
Energetic material is a kind of special energy material, which has a special and important position in the field of national defense and civil use. Energetic metal complexes and their polymers are a new type of energetic materials, which have high application value and broad application prospects in various fields of energetic materials, such as primary explosives, high explosives, solid propellants, burning rate catalysts and pyrotechnic agents
[1⇓⇓⇓⇓⇓⇓~8]. Unlike traditional Organic Energetic materials, energetic Metal complexes are composed of organic functional energetic ligands and central metal atoms, and can further form one-dimensional, two-dimensional and three-dimensional polymer energetic metal-organic Frameworks (Energetic Metal Organic Framework s, EMOFs) through coordination and topology with metal atoms as nodes under appropriate synthesis conditions such as temperature, solvent and pH
[9,10]. The type of metal atom has a certain influence on the properties of energetic metal complexes, but the energy level is mainly determined by the organic energetic ligands, so the selection of appropriate functional ligands before design and synthesis is the key to obtain high-quality energetic metal complexes. In recent years, azoles have become popular energetic ligands for the development of new energetic metal complexes and their polymers due to their high heat of formation, high density, high energy and good thermal stability. Nitrogen-rich azoles mainly include imidazole, pyrazole, triazole, tetrazole and pentazole. The nitrogen atom in the azole skeleton has a lone pair of electrons, which is conducive to the formation of monodentate or multidentate coordination, and the hydrogen in the skeleton has a high chemical activity, which is easy to be replaced by energetic groups (such as —NO
2, —NH
2 and —NHNO
2) or other nitrogen-rich heterocycles such as azole heterocycles.When the performances of the ligand are further improved, the coordination sites of the ligand can be increased and the coordination modes can be enriched, so that the structure and the performance of the finally obtained energetic metal complex can be flexibly regulated and controlled, thereby further relieving the contradiction between the energy and the sensitivity of an energetic material and realizing high energy and low sensitivity
[11⇓⇓⇓⇓~16][17,18][19⇓⇓⇓~23]. In this paper, the recent progress in the theoretical design, experimental synthesis, performance evaluation and application fields of energetic metal complexes of imidazole, pyrazole, triazole, tetrazole and pentazole is reviewed.The advanced strategies for structural control, synthesis optimization and performance improvement are reviewed, and the existing problems are discussed.Finally, the future development directions of theoretical and experimental research are prospected.
2 Synthesis and Properties of Imidazole Metal Complex
The introduction of nitro group into imidazole can improve the density and oxygen balance of imidazole ligands, thus enhancing the energy of imidazole ligands
[24,25]. And because the chemical bond C—NO
2 is more stable than the N—NO
2, the nitro group is introduced on the C atom in the imidazole skeleton through nitration reaction, which is beneficial to avoiding high sensitivity. For example, it has been reported that 4,5-dinitro-1,3-imidazole (DNI) is an energetic ligand with excellent thermal stability, which is still very stable under impact and friction
[26]. Klap Klapötke et al used DNI as the ligand and Li, Na, K, Ca, Sr and Ba as the central metal atoms.Li(DNI)(H
2O)
3(imi-1), Na(DNI)(H
2O)(imi-2), K(DNI)(H
2O)(imi-3),Ca(DNI)(H
2O)
3(imi-4), Sr(DNI)(H
2O)
5(imi-5) and Ba(DNI)(H
2O)
4(imi-6),Its synthetic route is very simple (Fig. 1), which is divided into two steps: 1) preparation of DNI
[27]; 2) synthesize a metal complex at room temperature. As the DNI prepared by the reaction 1) is very stable at room temperature, the reaction 2) can be directly prepared under the condition that acetone is used as a solvent, so that the synthesis difficulty of the metal complex is greatly reduced. The density (ρ), decomposition temperature (T
d), impact sensitivity (IS), friction sensitivity (FS), detonation velocity (D) and detonation pressure (P) of the compounds are listed in Table 1. It can be seen from Table 1 that compound imi-6 has the largest density, which IS a 2.220 g·cm
-3, while compound imi-1 has the smallest density (which is a 1.634 g·cm
-3); The decomposition temperatures of all the complexes were in the range of 231 ~ 268 ℃, showing good thermal stability. The results of impact sensitivity (IS) and friction sensitivity (FS) show that the impact sensitivity of the compound IS higher when the coordination metal IS Na (IS = 25 J) and Ca (IS = 3 J), and the impact sensitivity of the compound IS lower in other cases. The electrostatic sensitivity of the complexes is also low, ranging from 0.10 to 0.60 J. Therefore, the metal complexes based on nitroimidazole ligands have good thermal stability and safety. However, its energy level is ordinary, and there is a certain distance from the energy requirements of high-energy and high-density materials.
The number of nitrogen atoms in the imidazole skeleton is small, which leads to a relatively single coordination mode. If other nitrogen heterocycles are further introduced, not only the nitrogen content can be increased to improve the energy, but also the coordination sites and coordination modes can be increased to enrich the structure of imidazole metal complexes
[28]. In 2017, Xu et al. synthesized an imidazole derivative 4,5-bis (1H-5-tetrazolyl) -1H-imidazole (H
3dtim) by introducing tetrazole groups to two adjacent carbon atoms in the imidazole skeleton
[29]. H
3dtim is a kind of aromatic ligand, which is beneficial to the formation of π-π stacking interaction and improves the stability of metal complexes. In addition, the H
3dtim has 10 nitrogen atoms, and the nitrogen content is as high as 68.63%, which is beneficial to improving the energy and increasing the coordination sites. The ligand is combined with metal atoms Zn (II) and Mn (II) to prepare metal complexes [Zn(Hdtim)(H
2O)
2]
4(imi-7] and [Mn(Hdtim)(H
2O)
2]
4(imi-8], and the two complexes are prepared by an in-situ hydrothermal method in one step. Metal complexes imi-7 and imi-8 have excellent properties with densities of 2.108 g·cm
-3 and 1.968 g·cm
-3, respectively (Table 1); The decomposition temperatures of T
d are 229 ℃ and 234 ℃ respectively, which are better than that of 1,3,5-trinitro-1,3,5-triazane (RDX,T
d=210℃). The two complexes are very insensitive, with IS > 40 J and FS > 360 N; The D of complex imi-7 is a 9.61 km·s
-1 with P = 44.6 GPa, and the D of complex imi-8 is a D=9.06 km·s
-1 with P = 38.2 GPa, which is significantly higher than that of most reported energetic metal complexes. The above properties show that compounds imi-7 and imi-8 have both the requirements of high energy and low sensitivity, and are expected to be developed into high energy density materials. Yang et al
[30].
A novel 3D solvent-free complex [Pb(Hdtim)]n(imi-9, fig. 2] was prepared by hydrothermal reaction of H3dtim with PbNO3 under acidic conditions, in which Pb was monodentate or multidentate coordinated to the deprotonated Hdtim2-. They believe that in the synthesis of EMOF, solvent molecules, such as water molecules, can easily coordinate with metal atoms or remain in the pores and unit cells of EMOF due to the multiphase and multicomponent reaction system.Solvent-free EMOFs are an effective strategy to construct EMOFs with excellent performance, because the thermal stability of EMOFs decreases, and even the density of EMOF decreases, which leads to the performance of EMOF decreases. Mimi-9 is a solvent-free 3D EMOF. It has excellent thermal stability with a thermal decomposition temperature of 325 ℃, an accelerated decomposition temperature of TSADT=288.0℃ and a thermal explosion critical temperature of Tb=298.9℃, which are superior to those of some general energetic compounds and energetic complexes, such as FOX-7(TSADT=206.0℃,Tb=207.1℃) and TNPDU(TSADT=206.3℃,Tb=219.5℃). The density of mi-9 is as high as 3.186 g·cm-3, and its detonation heat, detonation velocity and detonation pressure are 1.158 kcal·g-1, 7.84 km·s-1 and 35. 9 GPa, respectively, which are obviously superior to Pb(N3)2 and TNT and close to RDX. The IS and FS of MG-9 are > 40 J and > 360 N, respectively, and its safety performance IS better than that of RDX (IS = 7.5 J, FS = 120 N). In addition, imi-9 can also effectively promote the thermal decomposition of ammonium perchlorate (AP), a rocket propellant.
In 2012, Liu et al. Also prepared energetic catalysts [Co(en)(H
2dtim)
2]
2·enH
3dtim as ligands combined with transition metals Co and Cu, and studied their catalytic effect on the thermal decomposition of AP
[31]. They found that the thermal decomposition temperatures of [Co(en)(H
2dtim)
2]
2·en and [Cu
2(en)
2(Hdtim)
2]
2,AP added to AP were 333.7 ℃ and 336.1 ℃, respectively, which decreased to a certain extent. After adding them, the thermal decomposition process of AP changed from the typical two-step decomposition to one-step decomposition, and the latent heat released in the decomposition process increased from 319 J·g
-1 to 2583 J·g
-1 and 2526 J·g
-1, respectively. Compared with other energetic catalysts, [Co(en)(H
2dtim)
2]
2·en and [Cu
2(en)
2(H
2dtim)]
2 not only have excellent catalytic performance, but also can improve the combustion heat of AP and improve the performance of AP.Therefore, [Co(en)(H
2dtim)
2]
2·en and [Cu
2(en)
2(H
2dtim)]
2 have potential applications in the catalytic decomposition of AP-based propellants.
In addition to the introduction of azole substituents on the imidazole skeleton, alkyl groups such as methyl and ethyl can also be introduced on the imidazole skeleton, and such imidazole derivatives have good self-ignitability under the action of oxidants. In recent years, it has been used to construct Zeolitic Imidazolate Frameworks (ZIFs), which can be used as hypergolic fuel in rocket propellants and is a new hypergolic propellant with broad development prospects, and is expected to replace the traditional hydrazine fuel
[32]. Titi et al. Introduced vinyl and ethynyl to the imidazole skeleton to obtain 2-vinyl-1H-imidazole (HAlm) and 2-ethynyl-1H-imidazole (HVlm), respectively, and combined with Zn
2+, Co
2+ and Cd
2+.It has constructed Zn(Alm)
2, Co(Alm)
2, Cd(Alm)
2,Six kinds of ZIFs, Zn(Vlm)
2, Co(Vlm)
2 and Cd(Vlm)
2,They found that,The formation of coordination bonds in ZIFs can promote the spontaneous combustion reaction between HAlm and HVlm, and the topology of geometric structure in ZIFs can improve the stability of ligands when they are heated and impacted
[33]. Laser ignition tests show that the ignition delay of the six ZIFs is short (2 ms). The mass energy density (E
g) and volumetric energy density (E
v) of the six ZIFs are comparable to those of trinitrotoluene (E
g=14.9 kJ·g
-1) and hydrazine fuel (E
g=19.5 kJ·g
-1,E
v=19.3 kJ·cm
-3), so they believe that the six ZIFs may be candidates for the development of safe, environmentally friendly and efficient rocket propellants in the future, compared with the harmful hydrazine fuel which is widely used in rocket propellants.
Nitroimidazole metal complexes have good thermal stability and safety, but their energy level is low, and they are limited by the number of nitrogen atoms and the coordination mode, and their structure is single, so they often can not get high energy nitroimidazole metal complexes, so they are often used as pyrotechnics with low energy requirements. The recently developed heterocyclic substituted imidazoles are the mainstream ligands for the development of new imidazole metal complexes due to their rich types, high nitrogen content, high density and high heat of formation.It has broad prospects and potential application value in the development of new high-energy and high-density materials and rocket propellants, especially ZIFs materials, which are expected to become a development direction of new imidazole metal complexes in the future.
3 Synthesis and Properties of Metal Complexes with Pyrazole
Nitropyrazole has excellent thermal stability and stability, and has a π-electron system in its structure, which is easy to undergo electrophilic substitution reactions such as nitration, sulfonation and halogenation
[34,35][36]. 4-Amino-3,5-dinitro-1H-pyrazole (LLM-116) is one of the excellent nitropyrazole-based energetic organic ligands. The hydrogen on the pyrazole ring has high reactivity and can be easily substituted
[37]. In 2019, Shreeve et al. synthesized 3,5-dinitro-4-hydroxypyrazole (HDNP), and prepared energetic complexes pyr-1 and pyr-2 (Fig. 3) using a two-step method. pyr-3 and pyr-4 with LLM-116 as the ligand were also synthesized
[38]. In previous studies, they found that the thermal stability and sensitivity of pyrazole metal complexes mainly depend on the number and strength of coordination bonds. The more the number and strength of coordination bonds, the better the thermal stability and the lower the sensitivity of pyrazole metal complexes. Compared with LLM-116, the greatest advantage of HDNP is that the oxygen on the hydroxyl group can form shorter coordination bonds when coordinating with metals, for example, when coordinating with alkali metals such as K and Na, compared with the coordination bonds formed by the N atom on LLM-116, the coordination bonds are shorter and have higher strength. The DSC test results show that the thermal stability of complexes pyr-1 and pyr-2 is better than that of complexes pyr-3 and pyr-4, and the decomposition temperatures of the former two are 395 ℃ and 376 ℃, which are higher than those of the latter two at 317 ℃ and 315 ℃ (Table 2).The density, impact sensitivity and friction sensitivity of the complexes pyr-1 and pyr-2 with HNIP as ligand are better than those of the complexes pyr-3 and pyr-4 with LLM-116 as ligand, which indicates that the density, thermal stability and sensitivity of metal complexes can be effectively improved by increasing the number of ligand coordination sites and the strength and number of coordination bonds. pyr-1 and pyr-2 have excellent thermal stability, density and sensitivity, but their detonation velocity and detonation pressure are very common, so they can only be used as super-heat-resistant energetic materials in the fields of aviation exploration and deep well drilling. Yu et al. Synthesized the furoxan-pyrazole fused ring compound 6-nitro-4H-pyrazolo [3,4-c] [1,2,5] -oxadiazole-5-oxide by intramolecular condensation reaction using malononitrile as raw material for the first time. After reacting with KI and NaOH respectively, they successfully prepared potassium 6-nitro-5-oxo-pyrazolo [3,4-c] [1,2,5] -oxadiazole-4-oxide (pyr-5) and 6-nitro-5-oxo-pyrazole-5-oxide (pyr-5
[39]. pyr-6 has excellent safety performance, but its energy level IS average, and its energy level IS 16 J, which IS better than that of TNT (15 J), HMX (7. 4 J) and RDX (7. 4 J).Pyr-6 can be used as a candidate of environmentally friendly energetic materials.
Similar to nitroimidazole, nitropyrazole also has the problems of simple coordination mode and common energy level. However, due to the high chemical activity of the carbon atoms on the pyrazole, the substitution reaction is easy to occur, so the bridged pyrazole can be prepared by using coupling bonds or coupling groups such as — C — and — N N —, which can not only increase the nitrogen content to improve the energy, but also increase the coordination sites and enrich the coordination mode
[40,41].
In 2016, Zhang et al., using LLM-116 as the starting material, reacted in an alkaline solvent containing KMnO
4 and KOH at 80 ℃ for 2 H to obtain a metal complex K
2NPA(pyr-7), in which NPA is trans-1,2-bis (3,5-dinitro-1H-4-pyrazolyl) ethylazene
[42]. pyr-7 is synthesized by a one-step method, and the synthesis route is very simple (Figure 5). According to the synthesis route, two molecules of dinitropyrazole are connected by an azo bond, so that the content of N + O reaches 64.1%, which not only improves the energy performance, but also provides more coordination sites, which is conducive to the formation of a 3D skeleton structure. DSC test showed that pyr-7 had excellent heat resistance and its decomposition temperature was 315 ℃ (Table 2); The detonation properties of pyr-7 were predicted by EXPLO5, and the calculated D and P were 8. 28 km/s and 31. 1 GPa, respectively, which were significantly better than the traditional Pb(N
3)
2 of primary explosive. And metal K is a kind of environmentally friendly metal. Taken together, pyr-7 is a potential candidate for high quality primary explosive. In addition, the hydrogen on the carbon atom in the bridged pyrazole is also easy to be replaced by halogen, and the number of hydrogen on the carbon atom is large, and the halogen content of the substitute is high, based on this, in recent years, the bridged pyrazole is very suitable for developing unconventional energetic materials and iodine-rich energetic anti-biological warfare agents
[43,44]. Chinnam et al. synthesized 5- ( (3,4,5-triiodo-1H-pyrazol-1-yl) methyl) -1H-tetrazole bridged with rich iodine from 3,4,5-triiodo-pyrazole, and successfully prepared metal coordination polymer anti-biological warfare agent ECP3 (pyr-8) by reacting it with ZnI
2[45]. DSC test showed that the thermal decomposition temperature was 275 ℃. The results of sensitivity test show that the impact sensitivity is 80 J, the friction sensitivity is more than 353 N, the electrostatic sensitivity is more than 250 mJ, and the safety performance is outstanding.
In addition, the density of pyr-8 is outstanding, up to a 3.189 g·cm-3,D and P of 3.10 km·s-1 and 5.1 GPa, respectively. Therefore, pyr-8 has good comprehensive performance, and has a very broad application prospect and potential application value in the future development of efficient anti-biological warfare agents.
Recently, heterocyclic substituted pyrazole derivatives have attracted much attention due to their high nitrogen content, high density, high heat of formation, good thermal stability, and flexible coordination modes
[46,47]. For example, Fu et al. synthesized the energetic ligand 3,4-dinitro-1- (1H-tetrazol-5-yl) -1H-pyrazole-5-amine (HANTP) by replacing the H at position 1 of 5-amino-3,4-dinitro-1H-pyrazole with tetrazolyl, and HANTP reacted with KCl and NaCl in one step to form energetic complexes [K(ANTP)(H
2O)
2](pyr-9] and [Na(ANTP)(H
2O)
2](pyr-10], respectively, with the synthetic routes shown in Figure 6
[48]. pyr-9 and pyr-10 have 2D and 1D skeletons, and their densities are 1.980 and 1.930 g·cm
-3, respectively, which are higher than those of RDX and HMX. pyr-9 and pyr-10 have thermal decomposition temperature of 253 ℃ and 269 ℃, respectively, which meet the standard of heat-resistant energetic materials (T
d>250℃). The D=8.43 km·s
-1 of the complex pyr-10 is P = 32.0 GPa, which is better than the pyr-9(D=8.38 km·s
-1,P=31.4 GPa) of the complex. The sensitivities of complex pyr-9 (IS = 4 J, FS = 144 N) and complex pyr-10 (IS = 7 J, FS = 160 N) are significantly lower than those of the Pb(NO
3)
2. In conclusion, the complexes pyr-9 and pyr-10 have excellent thermal stability, appropriate sensitivity and good detonation properties, which are expected to be used as new green primary explosives. Myers et al prepared tetrazine-pyrazole ligands 3-amino-6- (3,5-dimethylpyrazole) -tetrazine and 3- (3,3 '-dinitroazetidine) -6- (3,5-dimethylpyrazole) -tetrazine, and reacted with metal salts of transition metal elements Fe, Cu, Co and Ni to prepare a variety of tetrazine-pyrazole-derived metal complexes with different properties from most energetic metal complexes for the first time.They all show excellent electrical and optical properties, and their electrical and optical properties can be independently regulated, so that while the sensitivity of the complex is reduced by other means,O that the complex can still keep the original good electrical and optical properties, and the tetrazine-pyrazole type metal complex can be initiated by laser, thereby providing a new perspective for exploring novel laser-initiated energetic pyrazole metal complex
[49].
In a word, nitropyrazole metal complexes have excellent thermal stability, appropriate sensitivity and certain energy, and are expected to be candidates for green and environmentally friendly primary explosives. And that heterocyclic substitute pyrazole-type metal complexes construct by more abundant coordination modes have more excellent comprehensive properties and wider application.
4 Synthesis and Properties of Triazole Metal Complex
4.1 Nitro/aminotriazole metal complex
the triazole ring has higher heat of formation and more coordination modes than the imidazole ring and the pyrazole ring,and the introduction of amino and nitro groups on the triazole skeleton can effectively improve the performance of ligands and regulate the energy and sensitivity of expected metal complexes while increasing the coordination sites and further enriching the coordination modes
[50⇓⇓~53]。 In recent years,many metal complexes of nitro/amino triazoles have been reported.For example,in 2016,Xue et al selected 4-amino-1,2,4-triazole(ATZ)and azide ligand(N
3)to synthesize an energetic complex[Zn(ATZ)
2(N
3)
2]
3(H
2O)
2(tri-1]in zinc acetate aqueous solution by constant temperature water bath method.ATZ and Zn in the molecule have two modes of monodentate and multidentate coordination
[54⇓⇓⇓~58][59]。 The main properties of tri-1 are listed in Table 3.It has a density of 1.819 g·cm
-3,a decomposition temperature of 250°C and a thermal initiation critical temperature of 262°C 。
In 2017,Su et al.Also used ATZ as an energetic ligand to synthesize an energetic metal polymer[Cu(ATZ)(NO
3)(OH)]
n(tri-2,fig.7]by a hydrothermal method in an aqueous solution of Cu(NO
3)
2.The hydroxyl groups contained in the polymer can not only improve the oxygen balance of the polymer,but also form hydrogen bonds,effectively increasing the density of the polymer
[60]。 It has a high density of 2.410 g·cm
-3,higher than the ATZ energetic metal polymer without hydroxyl groups[Cu(ATZ)
2(NO
3)
2]
n(2.072 g·cm
-3];Its decomposition temperature IS 288℃,and its IS,FS and ESD are 11 J,180 N and 12 J,respectively,which are also better than RDX,indicating that it has good safety performance.However,the energy level is moderate,with D and P being 7.46 km·s
-1and 29.7 GPa,respectively 。
5-Amino-3-nitro-1H-1,2,4-triazole(ANTA)ligand is a kind of high energy and high density ligand with excellent thermal stability,which is the main component of most secondary explosives.It has good coordination functionality and selectivity,and has been widely reported and applied
[61,62]。 In 2017,Seth et al.First combined ANTA with two different transition metals,Co and Zn,to prepare energetic metal polymers with the same 2D skeleton structure[Co(C
2H
2N
5O
2)(OH)]
n(tri-3]and[Zn(C
2H
2N
5O
2)(OH)]
n(tri-4,Fig.8 ]
[63]。 The results show that the types of metal have different effects on the thermal stability,density and sensitivity of energetic metal polymers.Firstly,the decomposition temperature of complexes tri-3 and tri-4 is between 310 and 330℃(Table 3),which is higher than the decomposition temperature of ANTA at 225℃,indicating that the thermal stability is greatly improved after the formation of coordination polymers,and the decomposition of tri-4 is higher than that of tri-3,indicating that Zn is more conducive to improving thermal stability than Co.The densities of tri-3 and tri-4 are 2.297 g·cm
-3and 2.336 g·cm
-3,respectively,which are higher than those of RDX,HMX and 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane(CL-20,2.035 g·cm
-3).Finally,both of them have low sensitivity,and the impact sensitivity of tri-4 is 217 cm(the highest standard of laboratory test),which is better than that of tri-3(94 cm),indicating that metal Zn is more helpful for Co sensitivity reduction 。
In 2018,Gu et al.First reported the synthesis method of 4,5-bis(gem-dinitromethyl)-1,2,3-triazole,and reacted with KI to prepare energetic metal complex K
2BDNMTZ(tri-5 by in-situ synthesis method.Fig.9),it is found that due to the introduction of gem-dinitromethyl,there are many coordination modes in the complex,thus forming a cage-like 3D skeleton structure,and forming intermolecular hydrogen bonds to improve the density of the metal complex
[64]。 Its decomposition temperature IS 131℃and its IS and FS are 1 J and 60 N,respectively.tri-5 has a density of 2.040 g·cm
-3and a higher positive oxygen balance(+15.9%),which makes it burn more fully.This gives it a higher energy.Its D is 8.72 km·s
-1,which is higher than the DDNP(7.65 km·s
-1)and Pb(N
3)
2(5.88 km·s
-1)of the widely used primary explosives,and is comparable to RDX.The above performance shows that tri-5 has application value in the field of lead-free primary explosive 。
In 2019,Wang et al.Prepared an energetic metal polymer that can be detonated by laser with a nitrogen-rich ligand 3-amino-1H-1,2,4-triazole-5-carbohydrazide(ATCA)[Ag(ATCA)ClO
4]
n(tri-6,Fig.10].There are many coordination sites on the ligand ATCA.The polymer tri-6 can form two different chiral helical structures,more hydrogen bonds can be formed between the ATCA and the Cl
,and the formed hydrogen bonds can improve the density and enhance the stability of the polymer structure
[65]。 Its density is 2.534 g·cm
-3(Table 3),which is greatly improved compared with simply blending ATCA with AgClO
4(2.207 g·cm
-3),because the chain structure and intermolecular hydrogen bonds formed after coordination make the structure more dense and compact.The decomposition temperature of the complex is 230 deg C,and the thermal stability of the complex is more excellent than that of a triazole metal complex[(TriTzPyr)
3Fe][ClO
4]
2(T
d=180℃)]which is synthesized and has the capability of laser initiation.At the same time,the decomposition temperature of tri-6 is much higher than that of ATCA(174℃),which indicates that the thermal stability of polymers constructed by energetic ligands can be effectively improved.The impact sensitivity(5 J)and friction sensitivity(72 N)of tri-6 are significantly lower than those of lead Stephen(LS)and lead azide(LA),which are currently used as primary explosives,and its safety is reliable.The detonation velocity of tri-6 is 6.80 km·s
-1,which is also superior to these two primary explosives.Therefore,tri-6 has excellent comprehensive properties and can be used as an alternative candidate to lead primary explosives 。
4.2 Bridged triazole metal complex
the bridged triazole has the excellent characteristics of high density,high heat generation,good thermal stability and the like,and the prepared energetic metal complex has considerable energy performance due to the high nitrogen content of the bridged triazole
[12,41]。 At the same time,the aromaticity and theπ-πstacking effect between molecules are favorable for improving the stability and the mechanical property of the prepared metal complex to a certain extent
[66]。 Bridged triazole metal complexes have been reported recently,of which 4,4'-azo-1,2,4-triazol-5-one(ZTO)is the most abundant ligand.There are a large number of lone pair electrons on the oxygen atom of the carbonyl group and the nitrogen atom of the azole ring of ZTO,which are very easy to coordinate with metal atoms and have a variety of coordination modes.At the same time,the introduction of azo bonds can effectively improve the heat of formation,density and molecular stability
[67]。 Zhu et al.compared ZTO with alkali metal ion Li
+(tri-7),Na
+(tri-8),K
+(tri-9),The Cs
+(tri-10)reacts with hydroxides of alkaline earth metal ions such as Mg
2+(tri-11),Ca
2+(tri-12),Sr
2+(tri-13)and Ba
2+(tri-14),ZTO-based alkali metal energetic complexes were prepared(fig.11 )
[68]。 The metal atom is coordinated with the oxygen atom of ZTO and the nitrogen atom of the azole ring,and the compounds tri-7 to tri-14 exhibit excellent thermal stability.They all have high density,good detonation performance and superior impact sensitivity(IS>60 cm),and the impact sensitivity IS obviously lower than that of RDX,HMX and CL-20.Among them,compounds tri-10,tri-13 and tri-14 are outstanding.Their densities are 2.261 g·cm
-3,2.077 g·cm
-3and 2.378 g·cm
-3,respectively,and their thermal decomposition temperatures are 249℃,287℃and 234℃,respectively.Besides high density and high thermal stability,they also have low impact and friction sensitivity,which can be used as potential high density insensitive materials.In order to further study the outstanding thermal stability of ZTO-based alkali metal energetic complexes,Zhu et al first applied adiabatic accelerating rate calorimetry(ARC)to the thermal stability study of ZTO and its alkali metal complex KZTO·H
2O.The experimental data of ARC showed that both ZTO and KZTO·H
2O had quite high initial thermal decomposition temperatures under adiabatic conditions.230.6℃and 235.6℃,respectively,which indicates that ZTO and KZTO·H
2O have quite high thermal stability.Zhu et al.Used ARC/DSC to characterize the thermal properties of a new ZTO-based metal complex Ca(H
2O)
6·(HZTO·2H
2O)
2,and studied its kinetics
[69][67]。 At the heating rate of 5℃/min,the thermal decomposition temperature was 290.5℃,which was higher than that of HMX,showing excellent thermal stability.the report of Zhu et al.Shows that ZTO-based alkali metal complexes have high density,excellent safety and outstanding thermal stability,but the detonation properties of ZTO-based alkali metal energetic complexes are not involved in the study,which needs further study.in addition,whether ZTO can coordinate with transition metals is also worthy of further discussion and research in order to enrich the types of ZTO-based metal complexes。
Due to the multiple coordination sites and multiple coordination modes of bridged triazoles,metal complexes of bridged triazoles can often be developed into EMOFs
[70⇓~72]。 In recent years,Xu et al.Synthesized solvent-free energetic metal coordination polymers[M(N
3)
2(atrz)]
n(M=Co tri-15,Cd tri-16]using 4,4'-azo-1,2,4-triazole(atrz)and azide(N
3)as ligands,combined with transition metals Co and Cd by one-step hydrothermal method
[70]。 Figure 12).tri-15 and tri-16 have the same 2D skeleton structure,with densities of 2.094 g·cm
-3and 2.282 g·cm
-3,respectively(Table 3);In addition,the sensitivities of tri-15(IS=1.2 J,FS=5 N,ESD=4.08 mJ)and tri-16(IS=1.6 J,FS=12 N,ESD=8.9 mJ)are lower than those of commercial primary explosives LS and LA.In order to further study the relationship between structure and mechanical sensitivity,they used non-covalent interaction analysis(NCI)to study the intermolecular interaction of compounds tri-15 and tri-16.Theπ-πinteraction between two parallel triazole rings was found,which was proved to be able to effectively buffer the mechanical interaction and reduce the mechanical sensitivity of the system in previous studies.In addition,the results of NCI analysis show that there is a mutual repulsion effect between two parallel azide ligand atoms,and the increase of the mutual repulsion effect is unfavorable to the sensitivity.In tri-15 and tri-16,the average interaction distance of the mutual repulsive effect is 3.0202Åand 3.2929Å,respectively,which proves that the mutual repulsive effect of tri-15 is greater than that of tri-16.Therefore,the sensitivity of tri-15 is higher than that of tri-16,which is consistent with the experimental results.Their detonation heatΔH
detare 4.407 kJ·g
-1and 4.248 kJ·g
-1,respectively,which are more than twice as high as those of several primary explosives currently in commercial use,so tri-15 and tri-16 can be used as candidates for high-quality primary explosives 。
In addition,Zhang et al.reported that the structure of atrz-based EMOFs significantly affects their performance.They successfully synthesized{Zn(atrz)
2[C(NO
2)
3]
2(H
2O)
2·atrz·2H
2O}
nand{Cu(atrz)
2[C(NO
2)
3]
2(H
2O)
2·atrz·2H
2O}
nwith layered structures by using anion-ligand tandem exchange strategy,using 3D EMOF,[Zn(atrz)
3(ClO
4)
2·2H
2O]
nand[Cu(atrz)
3(NO
3)
2·2H
2O]
nwith columnar layered structures as precursors,high-density oxygen-rich anion[C(NO
2)
3]
-as external anions,and water as a small single ligand
[71]。 Compared with the original 3D skeleton,the 2D skeleton is more densely packed,so its density,explosion heat,nitrogen content and oxygen balance are improved to a certain extent.The anion-ligand tandem exchange strategy used in their research is an improvement of the previous ion exchange strategy.The early research has confirmed that the use of atrz group EMOF to stabilize polynitro anion N(NO
2)
2-has a good effect,which can effectively improve the density,heat of explosion,sensitivity and other properties
[72]。 their research provides a new synthesis method and development direction for the development of new EMOF materials.However,the detonation properties of atrz-based EMOFs were not involved in their studies.Therefore,the detonation properties of atrz-based EMOFs deserve further study in order to accurately evaluate the energy performance of this class of materials,further improve them,and broaden their application fields。
In 2016,Lu et al.Successfully prepared a novel Cu(I)-Na(I)heterocyclic energetic organic framework[Cu
3Na(ZTO)
2]
n·nCH
3CN(tri-17,Fig.13]by solvothermal reaction of CuCl with NaN
3and ZTO,and desolvated tri-17 by heating to generate[Cu
3Na(ZTO)
2]
n(tri-18 ]
[73]。 Both tri-17 and tri-18 are relatively insensitive(tri-17:is=40 J,FS>360 N;tri-18:IS=31 J,FS>360 N)due to their 3D backbone structure,which allows the compounds to better absorb and buffer energy when stimulated.DSC tests showed that the solvent-free compound tri-18 had higher thermal stability than tri-17,with a decomposition temperature of 260℃,but their energy levels were similar.in addition,they also found that ZTO can coordinate with transition metals,which IS rare In previous studies,which also expands the previous research on ZTO base metal complexes by Zhu et al.,and its novel bimetallic design provides a new way and possibility for the subsequent development of new solvent-free 3D EMOF
[67⇓~69]。
4.3 Nitrogen-heterocyclic substituted triazole metal complex
Nitrogen-heterocyclic substituted triazole metal complexes have been reported in a large number of triazole metal complexes,because the triazole derivative ligands of triazole derivative-based metal complexes are diverse
[74⇓⇓⇓⇓~79]。 the biazole type triazole derivative containing the azole substituent is an organic energetic ligand which is reported to be widely adopted in triazole derivative-based metal complexes at present,the biazole type triazole derivative is easy to synthesize,the introduction of the azole group,on the one hand,the density,heat of formation,chemical stability and other properties are greatly improved,and On the other hand,the number of coordination sites is increased,which is conducive to the formation of stable polycyclic metal complexes and polymers
[80][81⇓⇓~84]。 In recent years,Du et al.Found that biazole triazole derivatives based metal complexes have higher density,heat of formation and chemical stability than bridged triazole metal complexes
[85]。 In 2016,Chen et al prepared a low-sensitivity metal complex[Cd(HTRTR)
2(H
2O)
4](HTNR)
2(tri-19,fig.14]in aqueous solution with 3-(1,2,4-triazol-4-yl)-1H-1,2,4-triazole(HTRTR)as the ligand,Co as the central metal atom,and the anion of styphnate(HTNR
-)as the ligand of the complex.The yield was 80%,which has potential applications for industrial mass production
[82]。 The Cd(Ⅱ)atom is coordinated by four water molecules and two HTRTR molecules,and the crystal density is 1.940 g·cm
-3.The complex dehydrates at 151.8℃and decomposes at 196℃.The critical temperature of thermal initiation is 440.03 K.The heat of explosion at constant pressure is-10356.1 kJ·kg
-1,which is higher than that of RDX(-9600 kJ·kg
-1)and HMX(-9880 kJ·kg
-1).The IS(>40 J)and FS(>360 N)of the complex are far superior to those of the industrial primary explosive containing lead,and the complex has excellent safety 。
Li et al.prepared a heat-resistant energetic material KCPT(tri-20,Fig.15)with 3D skeleton structure by a simple one-step reaction of 5-amino-3-nitro-1H-1,2,4-triazole and 3,4,5-trinitropyrazole in KOH aqueous solution.CPT is 4-(5-amino-3-nitro-1H-1,2,4-triazol-1-yl)-3,5-dinitropyrazole
[83]。 The 3D skeleton structure of KCPT brings outstanding performance.First,KCPT has a high density(1.980 g·cm
-3).At the same time,it has a high thermal decomposition temperature(323℃),which is close to the thermal decomposition temperature of the widely used heat-resistant insensitive energetic material TATB(T
d=330℃).KCPT also has a low sensitivity,with IS and FS of 7.5 J and 240 N,respectively.Finally,KCPT has higher energy with D and P of 8.46 km·s
-1and 32.5 GPa,respectively,which is superior to most heat-resistant energetic materials,so KCPT is a kind of heat-resistant energetic material with development potential 。
Seth et al.mixed 5,5'-dinitro-2H,2H'-3,3'-bi-1,2,4-triazole(DNBT)with nitrates of Ni and Cu,respectively,and prepared energetic metal polymers Ni-DNBT(tri-21,Figure 16)and Cu-DNBT(tri-22,Figure 17)by one-step reaction in DMF aqueous solution,and studied the relationship between the structure and properties of the polymers
[84]。 The Single crystal XRD results show that Ni(Ⅱ)is six-coordinated to two DNBT molecules and two water molecules in an octahedral configuration,in which DNBT is bidentate to Ni,forming two new five-membered rings.However,there are two coordination modes in tri-22,namely,Cu(Ⅰ)is four-coordinated with two DNBT molecules and Cu(Ⅱ)is six-coordinated with two DNBT molecules and two water molecules,which makes Cu-DNBT have a 1D chain skeleton structure.Therefore,Cu-DNBT(1.960 g·cm
-3at room temperature)has a higher density than Ni-DNBT(1.694 g·cm
-3at room temperature),which is superior to the original ligand DNBT(1.903 g·cm
-3),tri-21,and tri-22 has excellent thermal stability,especially tri-22,whose thermal decomposition temperature is 300℃,and its thermal stability is better than that of previously reported DNBT energetic salts and DNBT eutectic energetic compounds.The explosion heats of tri-21 and tri-22 are about 1950 J·g
-1and 1980 J·g
-1,respectively.The impact sensitivities of tri-21 and tri-22 are both more than 145 cm,which are lower than those ofε-CL-20(29 cm)andβ-HMX(55 cm)under the same test conditions,and the safety performance is better.tri-21 and tri-22 can be thermally detonated,and the thermal detonation test results show that the thermal explosion temperature of tri-21 is 260℃,and that of tri-22 is above 340℃.In conclusion,the properties of tri-22 are better than those of tri-21,and tri-22 is expected to be a new type of high density and heat resistant energetic material due to its high density and excellent thermal stability.It is also pointed out that the coordination mode of energetic metal polymers has a great influence on their properties,and exploring the appropriate coordination mode is conducive to the synthesis of energetic metal polymers with excellent properties 。
Chen et al.Used 4,5-bis(1H-5-tetrazolyl)-2H-1,2,3-triazole(H
3BTT)with high nitrogen content and excellent thermal stability as the ligand.[Sr
2(HBTT)
2(H
2O)
9]
n(tri-23],[Ba(HBTT)(H
2O)
4]
n(tri-24],[Na(H
2BTT)(H
2O)
2]
n(tri-25),[Cs
2(H
2BTT)
2(H
2O)
2]
n(tri-26),[Ca(HBTT)(H
2O)]
n(tri-27)and[Ba
3(BTT)
2(H
2O)
9]
n(tri-28 )
[86]。 All the six energetic metal polymers show excellent thermal stability,sensitivity and green environmental protection.All the six complexes form 1D,2D or 3D skeletons.Except for complex tri-25,the densities of the other five complexes are greater than the 2.0 g·cm
-3(Table 3).The thermal decomposition temperatures of tri-27 and tri-28 are 362℃and 397℃,respectively,which are higher than those of most reported energetic metal polymers.The impact sensitivity(IS>40 J)and friction sensitivity(FS>360 N)of the six complexes are lower than those of HMX,and are better than those of TZTR-3(IS=32 J,FS>360 N)and ATRZ-1(IS=22.5 J,FS=353 N),which have been reported recently.The detonation velocity and detonation pressure of complex tri-27 are 9.12 km·s
-1and 34.5 GPa,respectively.The energy performance and safety performance of the complex tri-27 are superior to those of HMX,and the complex tri-27 is a potential energetic metal polymer which can replace HMX.Recently,Zhang et al prepared a new C-N bonded ligand 3,5-ditetrazolyl-1,2,4-triazole(DTT)from 3,5-diamino-1,2,4-triazole by a one-step method,and successfully synthesized two DDT-based metal complexes NaDTT(tri-29)and KDTT(tri-30)by reacting DTT with alkali metal hydroxide,as shown in Figure 18
[87]。 Both tri-29 and tri-30 have 1D skeleton structure,and there are a lot of hydrogen bonds in the molecule.The safety performance of tri-29 and tri-30 IS particularly outstanding(IS>40 J,FS>360 N),and they also have certain thermal stability.Their decomposition temperatures are 208℃and 185℃,respectively.At the same time,both of them have high heat of formation(above 600 kJ·mol
-1)and certain detonation performance(tri-29:D=7.96 km·s
-1,P=23.2 GPa;tri-30:D=7.55 km·s
-1,P=20.7 GPa),which are superior to TNT(D=6.88 km·s
-1,P=19.5 GPa).Therefore,tri-29 and tri-30 can be used as candidates for environmentally insensitive energetic materials.Based on the research of C-N bonded heterocyclic compounds and their corresponding metal complexes,they provide a new perspective for the design and synthesis of new energetic materials 。
In a word,the partial nitro/amino triazole metal complexes have good safety performance and have potential In the development of new lead-free primary explosives.Therefore,most nitro/amino metal complexes do not have rich and varied topological structures,which is not conducive to performance regulation,thus hindering the further improvement of the performance of nitro/amino triazole metal complexes.In addition,the energy of nitro and amino triazole ligands mainly comes from monocyclic triazole,and the energy of ligands is limited,so the detonation properties of most reported nitro/amino triazole metal complexes are general.Bridged triazole and other nitrogen heterocyclic substituted triazole metal complexes not only retain the properties of nitro/amino metal complexes,but also make up for their shortcomings to a certain extent,especially their detonation properties,some of which are better than HMX,even better than CL-20.Moreover,they exhibit a variety of coordination modes and skeleton structures,and some of them can be further developed into novel EMOFs。
5 Synthesis and Properties of Metal Complexes with Tetrazole
5.1 Aminotetrazole metal complex
the introduction of nitro or amino groups into the tetrazole skeleton can improve the performance,but because of the environmental pollution problems in the synthesis process of nitrotetrazole,it is not in line with the current concept of"green chemistry",so there are few reports in the past five years
[22,23,88,89][90]。 At present,aminotetrazole is one of the more popular tetrazole energetic ligands.For example,in 2016,Qu et al successfully prepared a complex-type energetic organic framework with ultra-high density(ρ=3.400 g·cm
-3)using 5-amino-tetrazole(5-ATZ),NaN
3and AgNO
3as raw materials[Ag
2(5-ATZ)(N
3)](tetra-1,Fig.19].The thermal stability of tetra-1 is outstanding,and the thermal decomposition temperature is 300℃(Table 4 )
[91]。 The impact sensitivity of tetra-1 is 40 J and the friction sensitivity is more than 360 N,which is related to the stable and dense skeleton structure of tetra-1.tetra-1 has a certain energy with D and P of 7.00 km·s
-1and 29.5 GPa,respectively 。
Szimhardt et al.Selected Mn(II),Fe(II),Cu(II)and Zn(Ⅱ)as the coordinating metals,Cl
- 、N
、Cl
、Cl
、HTNR
-and H
2TNPG
-(trinitrophloroglucinol ion)as external anions,A series of 1-AT-based and 2-AT-based transition metal complexes were prepared by using 1-amino-5H-tetrazole(1-AT)and 2-amino-5H-tetrazole(2-AT)as ligands
[92]。 the sensitivity of iron and copper complexes is higher than that of manganese and zinc complexes.the stronger the oxidation of the external anion is,the higher the sensitivity of the complex is,and the sensitivity of 1-AT-based metal complex is higher than that of 2-AT-based metal complex.In addition to being initiated by physical stimuli,the two series of metal complexes can also be initiated by infrared light,which greatly broadens the application of the two series of metal complexes In the future.In addition,the 1-AT and 2-AT metal complexes have good initiation ability,and have the potential to be used as lead-free primary explosive candidates。
1,5-diaminotetrazole(DAT)is also a good ligand.In recent years,Li et al.Prepared energetic metal complexes with HDNBA(3,5-dinitrobenzoic acid)and DAT as ligands Cu(DAT)
2(DNBA)
2(tetra-2,Fig.20 )
[93⇓~95][96]。 In the tetra-2 molecule,the oxygen atom of DNBA and the N atom of DAT are coordinated to Cu(Ⅱ)in a planar quadrilateral configuration.The initial decomposition temperature of tetra-2 is 177℃and the activation energy of the initial decomposition process is 182.5 kJ·mol
-1.The sensitivity of tetra-2(IS=12.5 J,FS>360 N)IS superior to that of RDX.In addition,it was also found that the thermal decomposition temperature of AP decreased by 68℃after adding tetra-2,indicating that tetra-2 had a catalytic effect on the thermal decomposition of AP.All of the above results confirm the potential application of tetra-2 as an additive in solid rocket propellants 。
In 2019,Zhang et al synthesized a metal complex[Cu(DAT)
3]NO
3(tetra-3,Figure 21]by solvothermal method,which is intended to be used as a new environmentally friendly primary explosive
[97]。 tetra-3 has a planar structure and a weakπ-πstacking interaction between molecules,which IS beneficial to reduce the sensitivity.The IS and FS of tetra-3 are 1.5 J and 84 N,respectively,which are better than those of BNCP and LA.Its density is 1.834 g·cm
-3(Table 4)and detonation velocity is 7.60 km·s
-1,which is also higher than the BNCP(5.70 km·s
-1)and LA(4.55 km·s
-1)of traditional primary explosives,so tetra-3 is expected to be used as a substitute for BNCP and LA 。
5.2 Bridged tetrazole metal complex
The bridged tetrazole connects two monocyclic tetrazoles with bridging bonds such as—CH
2—,—NH—,and—NN—.Compared with monocyclic tetrazoles,the bridged tetrazole has more C—N,N—N and NN bonds,so it has higher energy
[98,99][78,100,101][102]。 In addition,the bridged tetrazole has more coordination sites and rich coordination diversity,and can construct metal complexes with different spatial structures.In China,Liu Wei et al.Used 5,5'-azotetrazole sodium salt as a precursor and ferrous sulfate as a metal ion to prepare a 1D metal organic framework complex with the[Fe(ATZ)(H
2O)
4·2H
2O]
n(tetra-4),ATZ of 5,5'-azotetrazole,which can be used as an excellent high-energy primary explosive
[103]。 tetra-4 has a one-dimensional infinite chain-like spatial structure,its D and P are 8.83 km·s
-1and 34.5 GPa,respectively,and its impact sensitivity is 39.0 cm.In addition,raw materials such as 5-aminotetrazole,ferrous nitrate hexahydrate and the like required for synthesizing the tetra-4 are cheap and easy to obtain,toxic metals,nitric acid,sulfuric acid and other polluting reagents are not contained in the preparation process,and the synthesis process is green and environment-friendly 。
Liu et al.Successfully synthesized nitrogen-rich energetic complex-type energetic organic framework Pb(bta)·2H
2O(tetra-5)by one-step hydrothermal method using N,N-bis(1H-tetrazol-5-yl)-amine(H
2bta)as ligand
[104]。 tetra-5 has a high heat of explosion(16.1 kJ·cm
-3),a high density(3.250 g·cm
-3),and excellent thermal stability(T
d=614.9 K).The detonation velocity and pressure are 8.96 km·s
-1and 43.5 GPa,respectively.tetra-5 also has good safety(IS>40 J).tetra-5 has high energy,excellent safety performance and thermal stability,and is expected to be a candidate for high energy density materials 。
Yang et al.Also successfully prepared and characterized three new energetic bimetallic complexes[Pb(bta)(H
2O)]
n(tetra-6],[PbCu(bta)
2(H
2O)
5]·2H
2O(tetra-7]and PbCu(bta)
2(tetra-8)with H
2bta as the ligand.It is worth mentioning that tetra-8 was prepared by dehydrating tetra-7 at 190℃
[105]。 tetra-7 exhibits a 3D supramolecular structure.The thermal decomposition temperatures of tetra-6 and tetra-7 are 314℃and 231℃,respectively.The D and P of tetra-6 are 8.92 km·s
-1and 47.7 GPa,and the D and P of tetra-7 are 8.75 km·s
-1and 39.6 GPa,respectively.In addition,DSC analysis showed that tetra-6,tetra-7 and tetra-8 could effectively accelerate the thermal decomposition process of RDX,and both tetra-7 and tetra-8 showed better acceleration effect than tetra-6.They believed that energetic complexes with suitable thermal decomposition temperature,high explosion heat and bimetallic composition were conducive to accelerating the thermal decomposition of RDX,so the three compounds had good application prospects in combustion accelerators 。
Freis et al.Synthesized bis(1H-5-tetrazolyl)methane(5-DTM),and used it as a ligand to react with the corresponding metal salts of Cu,Co,Ni,Mn and Zn,and prepared a series of metal complexes(tetra-9~tetra-15,Figure 22)through a two-step method.DTM coordinated with the coordination metal bidentate to form a polycyclic structure,which has good stability and safety
[106]。 For example,tetra-9~tetra-11 showed good safety(is>40 J,FS>360 N)and low electrostatic sensitivity(0.20~1.50 J)under impact and friction.However,when the strong oxidizing anion N
IS used to adjust The oxygen balance in the complex structure,the impact sensitivity will increase(such as tetra-13 and tetra-14).the thermal decomposition temperatures of complexes tetra-9,tetra-10,tetra-12 and tetra-15 are all above 200℃,showing good thermal stability.the introduction of strong oxidizing anions also has a negative effect on the thermal decomposition temperature of complexes。
Zhang et al.Reacted 5-DTM with MnCO
3to prepare chelated energetic metal complex{[Mn(5-DTM)(H
2O)
3]
2·H
2O}
n(tetra-16 )}
[107]。 Mn(Ⅱ)is six-coordinated with two 5-DTM molecules and three water molecules,and the 5-DTM is tridentate.The density IS 1.909 g·cm
-3,the initial thermal decomposition temperature is more than 600 K,and the impact sensitivity is more than 30 J.It has potential applications in heat-resistant energetic materials.Recently,Xu et al.Successfully synthesized four energetic metal polymers[perylene bis(tetrazol-1-yl)ethane],[Cd
2(N
3)
3Cl(btze)
2]
n(tetra-19),[Cd
2(N
3)
2Br
2(btze)
2]
n(tetra-20]as primary explosive candidates by hydrothermal method according to the synthesis route shown in Fig.23
[108]。 All four compounds exhibit high heat of explosion(two times higher than LA),good thermal stability(>200℃)and relatively low sensitivity(IS=1.5~2.0 J,FS=48~360 N).Although there are different halogen ions in the structure of tetra-19 and tetra-20,they have no effect on the sensitivity。
5.3 Nitrogen-heterocyclic substituted tetrazole metal complex
In recent years,in addition to the introduction of nitrogen heterocycles,a variety of novel tetrazole metal complex
[109~131]have been synthesized by introducing oxygen-containing heterocycles to replace the hydrogen of tetrazole or introducing N—O bonds to the ring to regulate the structural properties of ligands.For example,Chen et al prepared three new energetic polymers[CuBT(H
2O)]
n(tetra-21],[ZnBT(H
2O)
2]
n(tetra-22]and[CdBT(H
2O)
2]
n(tetra-23]by hydrothermal method using 5,5'-bitetrazole(H
2BT)as ligand and combining Cu(Ⅱ),Zn(Ⅱ)and Cd(Ⅱ)respectively
[109]。 It was found that tetra-21 had a 3D skeleton structure,while tetra-22 and tetra-23 had a 1D skeleton structure.They all have very outstanding thermal stability,and their thermal decomposition temperatures are 349℃,335℃and 394℃,respectively.In order to further improve the thermal stability of H
2BT based metal complexes,Wang et al added hydrazine carbonate(CHZ)during the hydrothermal synthesis process to synthesize[Ba(CHZ)(BT)(H
2O)
2]
n(tetra-24,fig.24 )
[110]。 The results show that tetra-24 will lose crystal water at 187℃and decompose at 432℃,and its thermal stability is indeed better than the above three.tetra-24 also has high density(2.356 g·cm
-3),high explosion heat(1962.6 kJ·mol
-1,298K)and low sensitivity,which is expected to be a candidate for insensitive materials.Fischer et al.Synthesized a metal complex K
2DNABT(tetra-25,fig.25)through a series of complex reactions using dimethyl carbonic acid as a raw material,in which DNABT is 1,1'-dinitramino-5,5'-bitetrazole
[111]。 The water solubility of the K
2DNABT is poor,so that the K
2DNABT can be well separated and purified with high yield.The K
2DNABT has a six-coordinate structure,a higher density(2.172 g·cm
-3),and its energy(detonation velocity is 8.33 km·s
-1,and detonation pressure is 31.7 GPa)is higher than the Pb(N
3)
2of the primary explosive.Because its structure contains N—NO
2which is easy to break,it has high sensitivity and is easy to detonate 。
In order to avoid the introduction of N—NO
2in the bitetrazole ligand,and taking into account the oxygen balance of the system,the researchers introduced N—O bond
[112~114]in the ring,which can not only improve the oxygen balance,increase the density and detonation properties,improve the sensitivity,but also enrich the coordination.For example,in 2016,Wang Xiaojun et al.Prepared 5,5'-bitetrazole-1,1'-dioxide lithium salt tetrahydrate[Li
2(BTO)(H
2O)
4]
n(tetra-26,BTO:5,5'-bitetrazole-1,1'-dioxide ]
[115]。 The decomposition temperature of tetra-23 is 349℃,the impact sensitivity is 71.8 cm,and the friction sensitivity explosion percentage is 0.tetra-23 has good thermal stability and safety performance.In 2012,Guo et al designed and synthesized[Cu(DOBT)(H
2O)
2]
n(tetra-27]with 1D crystal structure and{[Cu
2(DOBT)
2](tma)}
n(tetra-28,tma=tetramethylamine}with 3D crystal structure based on 1H,1'H-[5,5'-bitetrazole]-1,1'-diol(DHBT)and Cu(Ⅱ).The density,sensitivity,thermal decomposition behavior and detonation properties were studied,and the relationship between the structure and properties of energetic metal polymers constructed by the same ligand and metal was revealed for the first time,which is conducive to the design and synthesis of energetic metal polymers
[116]。 The results show that the density of the polymer tetra-27 and tetra-28 is 2.432 g·cm
-3and 1.943 g·cm
-3,respectively,indicating that the density of tetra-27 is very high.The impact and friction sensitivities of tetra-27 and tetra-28 were 11 J and 40 N and 15 J and 120 N,respectively,indicating that the 3D skeleton was more resistant to mechanical stimuli,especially to friction stimuli,than the 1D skeleton.The decomposition temperatures of tetra-27 and tetra-28 are 252℃and 265℃,respectively,which further reflects the better stability of the 3D structure.Both of them have certain energy,and the detonation performance(D=7.43 km·s
-1,P=37.3 GPa)of tetra-27 is better than that of tetra-28(D=6.99 km·s
-1,P=21.7 GPa).The above results show that when the same ligand and metal are used to construct energetic metal polymers,the 3D skeleton structure has lower mechanical sensitivity and higher thermal stability than the 1D skeleton structure,but the energetic metal polymers with 1D skeleton structure have higher density,so their detonation properties are better than those of the 3D skeleton polymer 。
In 1997,Zhang et al.Also chose DHBT as organic ligand to coordinate with Mn(Ⅱ),Co(Ⅱ),Ni(Ⅱ)and Zn(II)to synthesize five new green energetic polymer primary explosives.the deprotonated DHBT has two N—O bonds,In addition,DHBT has high density and excellent stability,and is easy to prepare from its dihydroxy ammonium salt TKX-50.Compared with the traditional primary explosive,the preparation process of DHBT is simple,efficient and environment-friendly
[117]。 The densities of all five compounds are greater than the 2.100 g·cm
-3.All of the five energetic polymers will lose crystal water at about 200℃,and then all of them have two obvious endothermic peaks,and their thermal decomposition temperatures are 313/350℃,282/330℃,274/365℃,276/339℃and 318/344℃respectively.Compared with the decomposition temperature 221℃of the dihydroxy ammonium salt TKX-50 of BTO
2-,the thermal stability of the five energetic polymers is obviously improved,and the thermal stability of Mn-based and Zn-based metal complexes is the best.The impact sensitivities of the five compounds are in the range of 12.5~60 J,and the friction sensitivities are in the range of 180~260 N,respectively.The safety performance of the five compounds IS obviously better than that of industrial primary explosive LA(IS=2.5~4 J,FS=0.1~1 N).The five compounds also have certain energy,among which the Cu-based complex has the best performance(D=7.97 km·s
-1,P=32.4 GPa).In a word,by introducing N—O bond into the azole skeleton,the O atom on the N—O bond is easy to coordinate with the oxygen-philic metal,forming a strong coordination,so that the metal complexes have good sensitivity properties.The introduction of N—O bond can improve the oxygen balance of the system to a certain extent and enhance the detonation properties of the complexes,so it is an effective design and synthesis strategy to construct metal complexes with tetrazole ligands containing N—O bond 。
In addition to the introduction of N—O bonds to improve the oxygen balance of the system,the introduction of oxygen-containing heterocycles such as furazan can also play the same role,and can also improve other properties.Shreeve et al.reported a series of energetic salts based on 4-amino-3-(5-tetrazolyl)-furazan(HAFT)and found that they all had fairly high densities,suitable impact and friction sensitivities,and good detonation velocities and pressures
[118]。 In 2017,Wu et al.Successfully prepared a high-nitrogen energetic metal complex polymer with 2D skeleton[Cd(H
2O)
2(AFT)
2]
n(tetra-29),tetra-29,which has high density(2.252 g·cm
-3),good thermal stability(initial thermal decomposition temperature>250℃)and low sensitivity
[119]。 It is worth mentioning that AFT is a multidentate ligand with many coordination sites,so it is very possible to design and synthesize AFT-based EMOFs with 3D skeleton structure by introducing bidentate or multidentate ligands,such as hydrazine,ethylenediamine and azide ligands,into the original structure.In the same year,Wu et al.Further prepared Cd(Ⅱ)metal polymers with 1D skeleton structure[Cd(NH
2NH
2)(AFT)
2·0.7H
2O]
n(tetra-30]and[Cd(ODH)
1.5(AFT)
2·5H
2O](tetra-31,ODH=NH
2NH COCONHNH
2],both of which have good impact sensitivity(IS>25 J )
[120]。 At the same time,tetra-30 and tetra-31 have their own characteristics.tetra-30 is one of the few energetic materials whose thermal decomposition temperature is higher than 300℃,and tetra-31 has a unique seven-coordination mode.It is worth mentioning that the tetrazole ring of the ligand AFT shows a single coordination mode,and the ODH molecule shows a typical three and four coordination mode,which is conducive to the formation of a 1D chain skeleton structure。
In recent years,Ding et al.Reacted HAFT with Cu(NO
3)
2·3H
2O in the environment of ammonia and propylenediamine solution,and prepared the complex Cu(NH
3)(AFT)
2(tetra-32)and Cu(C
3H
6N
2H
4)
2(AFT)
2(tetra-33)respectively
[121]。 The impact sensitivity of tetra-32(IS>12 J)and tetra-33(IS>9 J)IS better than that of HMX and RDX(7.4 J),which may have the potential to be used as safe energetic materials.In 2019,Ma et al.Successfully prepared Co(HCONHNH
2)
2(AFT)
2·H
2O(tetra-34)and[Pb(HCONHNH
2)
2(AFT)
2]
n(tetra-35],in which the HCONHNH
2was formylhydrazine
[122]。 tetra-34 has 1D skeleton structure,and tetra-35 has 2D skeleton structure.Their densities are up to 2.107 g·cm
-3and 3.119 g·cm
-3,respectively.Both of them have good thermal stability and low sensitivity 。
In addition to the use of the popular bicyclic ligands to construct tetrazole metal complexes,some other tetrazole derivatives have also been used to synthesize energetic metal complex
[123~131].For example,Feng et al.First used 5-methyltetrazole(Mtta)for the synthesis of 3D tetrazole complex-type metal-organic frameworks
[123]。 In 2018,Liu et al.Synthesized the energetic thin film Cu(ntz)]
n(ntz=3-on the copper substrate for the first time through electrochemical assistance.Compared with the synthesized energetic thin film[Cu(ntz)]
n(ntz=3-nitro-1H-1,2,4-triazole],the initial thermal decomposition temperature of[[Cd
5(Mtta)
9]
n]is 389℃,which is higher than that of[Cu(ntz)]
n(302℃],indicating that[[Cd
5(Mtta)
9]
n]has better thermal stability
[124][125]。 In addition,the[Cd
5(Mtta)
9]
ncan be initiated by laser and can be used as a candidate for the ignition material.It is worth noting that the in-situ synthesis method used in the preparation of energetic thin film[Cd
5(Mtta)
9]
nis very simple and has promotional value in the development of new energetic thin films.Recently,Gao et al.Used Mtta as the energetic ligand,combined with the metal atom Cu(Ⅰ)with strong coordination ability,and prepared the energetic metal polymer[Cu(Mtta)]
n·0.17n(H
2O]through a one-step solvothermal reaction,which showed a 3D metal topological structure and a higher density(2.130 g·cm
-3),superior to the previously reported energetic complex[Ni(NH
2NH
2)
5(ClO
4)
2]
n(1.980 g·cm
-3 ]
[126]。 Its thermal decomposition temperature is 213℃,and an exothermic peak appears at 400℃,which is caused by the decomposition of the skeleton structure.In this process,a large amount of energy is released,so[Cu(Mtta)]
n·0.17n(H
2O]has high density and excellent thermal stability,and also has potential high-energy properties.In addition,metal complexes composed of tetrazole derivative ligands such as 5-oxotetrazole,5-mercapto-1-methyl-tetrazole,3-pyridyltetrazole,1-vinyl-5H-tetrazol,1-azidovinyl-5H-tetrazol and ethyl formate tetrazolium have also been reported.They show a variety of spatial structures,good density,heat of formation,energy performance or appropriate sensitivity,and have good development prospects.It is expected that more and more special tetrazole derivatives will be applied to energetic metal complexes
[127][128][129][130][130][131]。
in a word,various types of tetrazole metal complexes obtained by improving the performance of ligands through different strategies have different advantages,and have high application value In various fields of energetic materials such as primary explosives,combustion accelerators and energetic films.Among them,bridged tetrazoles,heterocyclic substituted tetrazoles and tetrazoles containing N—O bonds have more abundant coordination modes,higher heat of formation,density and oxygen balance,and some of the metal complexes synthesized by them have both high energy and low sensitivity.If we can further study the relationship between their structures and properties,it will help to develop more excellent new metal complexes,and further promote the development of energetic metal-organic frameworks with rich and changeable structures and outstanding comprehensive properties。
6 Synthesis and Properties of Metal Complexes with Pentazol
Pentazoles (N
5) and their anions (
) have attracted great attention due to their extremely high nitrogen content and outstanding energy properties. However, N
5 and
are unstable at room temperature, which limits their further research and application
[132~134]. However, in 2017, a breakthrough was made in the report of pentazolium salt which can exist stably at room temperature. Since then, related research has been very popular and many
[135~145] have been achieved. In 2017, Xu et al. Synthesized five kinds of pentazole metal hydrated complexes [Na(H
2O)N
5]·2H
2O, [M(H
2O)
4(N
5)
2]·4H
2O(M=Mn,Fe,Co] and Fe(Gly)
2. Based on the synthesis of arylpentazoles, they used m-chloroperoxybenzoic acid (m-CPBA) and ferrous glycinate [Fe(Gly)
2] as cutting reagents and stabilizing additives respectively to form NaN
5, which were then prepared by ion exchange reaction with metal salts
[136]. Except for the compound [Mg(H
2O)
6(N
5)
2]·4H
2O, the thermal decomposition temperatures of the other complexes are over 100 ℃, showing good thermal stability. [Na(H
2O)(N
5)]·2H
2O and Zn(NO
3)
2·6H
2O (Fig. 26)
[137]. The two N atoms of the pentazole anion are axially coordinated to Zn (Ⅱ), and the O atoms of the four water molecules are coordinated to Zn (Ⅱ) to form a planar quadrilateral, so the [Zn(H
2O)
4(N
5)
2]·4H
2O has a six-coordinated octahedral configuration with a density of 1.669 g·cm
-3; Water plays an important role in stabilizing the pentazole anion. Dehydration does not lead to the immediate decomposition of the pentazole anion. However, when the temperature is raised to 107.9 ℃, the pentazolium anion will still decompose into
and N
2. In addition, the pentazole anion in the [Zn(H
2O)
4(N
5)
2]·4H
2O of the complex has C
2v symmetry, which is different from the previously reported D
5h symmetry, and is a novel stable pentazole metal complex. A large number of intermolecular hydrogen bonds are conducive to the formation of a stable 3D framework structure. In the same year, Xu et al. successfully constructed a zeolite-like 3D framework structure (Figure 27) based on the Na-N5
- framework with 1D structure, which showed a high nitrogen content of 70.19% and a high thermal decomposition temperature (129 ℃). However, its detonation performance (D=5.01 km·s
-1;P=8.8 GPa) was lower than that of most energetic materials due to its low density (1.260 g·cm
-3). Therefore, increasing the density of the pentazole metal complex could effectively improve its detonation performance
[138].
The synthesis yield and efficiency of the metal pentazole complex are low due to the strong water solubility of the Na
+ and the Mg
2+ after the ion exchange reaction between the intermediate NaN
5 and the MgN
5 used in the synthesis process of the metal pentazole complex and a metal salt. In order to solve this problem, Xu et al. Changed the synthesis process in 2012, and used AgN
5 as an intermediate to perform ion exchange reaction with KCl, and AgCl was precipitated (Figure 29). The yield of the prepared metal complex KN
5 was more than 90%, and it had the advantages of high efficiency and simple process
[142]. The density of KN
5 is 1.959 g·cm
-3 and the heat of formation is 136.8 kJ·mol
-1, which is superior to HMX(104.8 kJ·mol
-1);KN
5 and has certain energy.Its D is the 6.98 km·s
-1 and P is the acceptable sensitivity of the 20.9 GPa;KN
5, its impact sensitivity is 5 J and its friction sensitivity is 40 N. In the same year, Wang et al. Co-crystallized the
of guanidine cationic CH
3 (GU) and amino-1,2,4-triazole C
2H
4N
4(ATZ) with NaN
5.Two energetic coordination polymers, (NaN
5)
5[(CH
3H
6)N
5](N
5)
3 and (NaN
5)
2(C
2H
4N
4), were prepared by the self-assembly process, respectively
[143]. The crystal structure shows that the chelated pentazole anion coexists with other organic components in the solid state. The polymer (NaN
5)
5[(CH
3H
6)N
5](N
5)
3,Na
+ and the
form a cage shape, and the guanidine cation [C(NH
2)
3]
+ is enclosed inside; The polymer (NaN
5)
2(C
2H
4N
4) is a mixed ligand system, in which the
and ATZ form two independent but interwoven networks with the Na
+, respectively. All these show that it is feasible to change the specific properties of energetic materials by crystallizing coordination polymers. Benefiting from the coordination and weak interactions, the thermal decomposition temperatures of the two polymers were increased from 111 ℃ (one-dimensional structure) to 118. 4 and 126. 5 ℃, respectively. In addition, they successfully removed the crystal water in the product and obtained the anhydrous salt of pentazoles (>800 kJ·mol
-1) with high heat of formation. The heat of formation of polymer (NaN
5)
5[(CH
3H
6)N
5](N
5)
3 and (NaN
5)
2(C
2H
4N
4) is much higher than that of traditional energy materials, which indicates that
has a good prospect as a precursor of high energy density materials (HEDMs).
In a word, pentazoles have the advantages of ultra-high nitrogen content and very rich coordination, and the stability, yield and performance of pentazoles metal complexes have been significantly improved by various advanced means and methods recently, which lays a solid foundation for the synthesis of pentazole energetic compounds with outstanding comprehensive performance. However, compared with high energy density materials, there is still a certain gap in the thermal stability, sensitivity and energy level of the existing pentazole energetic metal complexes, and there is still a lot of room for people to explore.More research is needed to bring the advantages of pentazole into full play and obtain high-quality pentazole energetic compounds with ultra-high energy.
7 Theoretical Studies on Metal Complexes of Azoles
Theoretical research can well assist experimental synthesis. Before experimental synthesis, it is an efficient way to design and screen new energetic compounds with better performance as target candidates for experimental synthesis through various advanced strategies
[144]. Density functional theory (DFT) and other theoretical methods have been widely used to simulate the structure and predict the properties of energetic compounds. In the studies of Sharma et al and Wu et al, Politzer method is used to predict the density of metal complexes. This method is based on the electrostatic potential analysis (ESP). The relevant parameters are calculated by using the self-compiled program WFA-SAS of Politzer et al, and then substituted into the density
[145][146,147][148,149]. The empirical formula of Posp Pospíšil et al. Is used to measure the impact sensitivity of the safety performance of metal complexes
[150]; Yan et al. Used this method to calculate the density and sensitivity of the synthesized metal complex Cu(1-MeAtNO
2)
2(NH
3)
2, and the calculated value was ρ=1.87 g·cm
-3,h
50=14 J, which was very close to the experimental value (ρ=1.85 g·cm
-3,h
50=15 J)
[151]. The heat of formation of metal complexes in gas phase was calculated by the method of atomization energy, and then the heat of formation of metal complexes in solid phase was calculated by Hess law
[152]. Wang et al improved the Kamlet-Jacobs method to take the metal elements into account, and the relative error between the calculated results and the predicted values by EXPLO5 is less than 5%. This method is used to predict the energy of metal complexes
[153][154,155].
Sharma et al. And Wu et al. Selected four kinds of high-nitrogen bitetrazoles with different bridging groups ( "-", "-NH-", "-NN-" and "-NH-NH-") as the main energetic ligands, and designed a series of new nitrogen-rich Cu/Ni energetic metal complexes by combining the NH
3 and NH
2NO
2 of regulatory small ligands (Fig.(1 ~ 3). These compounds have high heat of formation and density, so they have high energy level. Their detonation velocity (7.20~8.80 km·s
-1) and detonation pressure (25.9 ~ 38.6 GPa) are excellent, and the energy of some compounds is higher than RDX and close to HMX
[145][146]. The impact sensitivity is also acceptable (17 ~ 48 cm), and the impact sensitivity of some substances is slightly lower than that of RDX and HMX. However, due to the low oxygen content, the energy of these new complexes does not fully meet the requirements of high energy density compounds. Therefore, Wu et al. Designed two series of metal complexes with similar structures and metals using the nitrogen-rich and oxygen-rich triazole compound 3,3 '-dinitro-5,5' -bi-1,2,4-triazole-1,1 '-diol (DNBTBO) as a bidentate ligand to replace the nitrogen-rich tetrazole ligand (fig.(4) It is found that the energy of these two series of metal complexes has been significantly improved (D=7.79~9.67 km·s
-1,P=30.5~46.9 GPa) compared with the complexes designed by nitrogen-rich tetrazole ligands, and the energy of some new metal complexes not only meets the requirements of high energy density materials, but also exceeds that of CL-20
[147]. They also have the impact sensitivity (22 ~ 27 cm) close to RDX, and have good comprehensive properties. In addition, it is found that both the metal and the ligand have significant effects on the structure, heat of formation and stability of the complexes, and these effects are often coupled with each other, which is conducive to the further development of metal complexes with high energy. Subsequently, in 2019, Yan et al adopted the complementary mixed ligand strategy, combining two ligands of 1,1 '-dinitroamino-5,5' -bitetrazole (DNABT) with high energy but moderate sensitivity and 4-amino-4H-1,2,4-triazole-3,5-dione (ATDO) with low sensitivity but moderate energy, and modified the structure with energetic groups —NO
2 and —NH
2, and designed a variety of new M (DNABT) (ATDO) (M = Cu, Ni) energetic metal complexes (
[151]. The designed complexes have high heat of formation (673~868 kJ·mol
-1), high density (2.06~2.14 g·cm
-3) and near zero oxygen balance (-19.7% ~ -6.7%), which make their detonation properties (D=8.76~9.84 km·s
-1,P=37.4~46.6 GPa) higher than those of RDX, HMX or CL-20. The complementary strategy makes the sensitivity of some compounds better than these three substances at the same time. At the same time, it has outstanding energy and moderate sensitivity, and some substances of the system are expected to become potential candidates for high energy density materials for experimental synthesis.
In addition, theoretical studies are also helpful to understand the structure and properties of complexes in different environments, especially in the newly developed pentazoles. Arhangelskis et al. Used the periodic density functional theory to study the role of pentazole in the construction of its skeleton structure. For the first time, they found that pentazole has something in common with other azole skeletons (diazole, triazole, tetrazole) in the formation of skeleton structure. The probability of skeleton formation is not only the same as other azole skeletons, but also affected by the coordination environment around pentazole
[156]. Zhao et al studied the high-pressure behavior of [Co(H
2O)
4(N
5)
2]·4H
2O (0 ~ 20 GPa) by using the periodic density functional theory with dispersion force correction. It was found that the covalent interactions of Co-N and Co-O were obvious at 11.5, 13.0 and 15.8 GPa, which indicated that the internal structure of [Co(H
2O)
4(N
5)
2]·4H
2O might become ordered after a certain pressure was applied, which was consistent with the results of density of States analysis
[157]. They also observed that the applied pressure also led to the uneven distribution of electron density and different degrees of covalent interaction between pentazole, coordinated water and Co. They believe that the above studies are of great significance for explaining the intramolecular/intermolecular interactions and structural stability of [Co(H
2O)
4(N
5)
2]·4H
2O, and also provide theoretical guidance for the design of pentazole metal complexes. Luo et al studied the effect of water molecules on the stability of metal pentazoles [M(N
5)
2(H
2O)
4]·4H
2O(M=Mn,Fe,Co,Zn] by molecular dynamics method
[158]. The results show that the stability of [M(N
5)
2(H
2O)
4]·4H
2O(M=Mn,Fe,Co,Zn] comes from the water molecules involved in coordination (c-H
2O) and hydrogen bond formation (h-H
2O) in the molecule, and their mechanisms are different, but both of them can inhibit the decomposition of N
5. They point out that in the design of future pentazole metal complexes, a variety of molecules, such as NH
3, H
2S and PH
3, can be used to construct pentazole complexes. They can combine closely with metal atoms, reduce the interaction between metal atoms and N
5, and form permanent electrostatic interaction with N
5 to inhibit the decomposition of N
5, so as to achieve the same effect as water molecules.
In addition, theoretical studies have also been used to design potential pentazole metal complexes. Recently, Xia et al. Used machine learning to accelerate the crystal structure search method and first principles to design alkaline earth metal pentazolium salts, MN
10(M=Be,Mg)
[159]. The MN
10 exhibits a metal-centered octahedral configuration, which makes the MN
10 more energetic than a mixture of pure nitrogen and M
3N
2 or MN
4.It also makes it very likely that MN
10 will be synthesized at lower pressures (12 GPa for MgN
10). It is worth noting that the decomposition of MN
10 can release a large amount of energy and has good detonation performance, in which the detonation velocity and pressure of BeN
10 are about 2 to 4 times that of TNT, respectively. Subsequently, Xia et al. Designed a series of metastable trivalent metal pentazolium salt MN
15(M=Al,Ga,Sc,Y) by using the crystal structure search method based on the first principles
[160]. The MN
15 exhibits high energy density and extremely high nitrogen content, which is the highest nitrogen content compound among the nitrides studied so far. They found that the molecular structure of MN
15 remained stable at both high temperatures (about 1000 K) and room temperature. More importantly, the detonation pressure of MN
15 (about 80 GPa) is excellent, far exceeding that of TNT and HMX, which makes MN
15 a candidate for high-energy green energetic materials.
In a word, the theoretical design of energetic metal complexes can optimize the target products for their synthesis, save experimental costs, avoid the waste of experimental resources, reduce the risk in the design process, and also provide new design strategies and ideas for experimental synthesis. It can also provide guidance for understanding the structure and performance changes in different environments, and jointly accelerate the development of different types of energetic metal complexes and polymers. However, at present, the relevant theoretical research is relatively small, and more energy can be devoted to the relevant research in the later period. In addition, with the rapid development of computer technology, in the later period, in order to continuously improve the accuracy and accuracy of calculation methods, theoretical research can also be combined with artificial intelligence theory to carry out structural design, performance calculation and mechanism interpretation more efficiently through machine learning.
8 Conclusion and prospect
Rich azoles energetic metal complexes and their polymers have attracted wide attention due to their excellent properties, and many series and types of new rich azoles energetic metal complexes with different advantages and disadvantages and different application fields have been reported successively. Among them, the development of pentazole metal complexes is particularly rapid. In recent years, great progress has been made in the synthesis technology and structural stability, and the research value is great. Further research is expected to develop new pentazole metal complexes with higher energy level and better safety performance, giving full play to their advantages of ultra-high nitrogen content, green decomposition products and rich coordination modes, and their application prospects are very broad.
In addition, the current research fully reflects the application prospects of the energetic metal complexes of azoles, but there are also some problems that need to be further explored and solved: (1) Most of the current energetic metal complexes of azoles have the problem that the energy level is not high enough, and few of them have detonation properties exceeding CL-20. One of the main reasons is that the ligands used have common energy or single coordination mode, so it is necessary to create high-quality ligands with more prominent energy and more abundant coordination mode. Another reason may be that the current energetic metal complexes are mainly constructed by one ligand, which can not give full play to their strengths and make up for their weaknesses. Therefore, more advanced strategies are needed to design and synthesize substances with more prominent detonation properties, and to give full play to the huge energy contained in their structures, especially the metal pentazoles. (2) Fully consider and draw lessons from the characteristics of the reported azol-rich energetic metal complexes, understand the relationship between their structure and performance more deeply, and improve their performance according to the needs of specific application fields, so as to achieve a better balance between combustion rate, thermal stability, sensitivity and energy level. (3) For the synthesized energetic metal complexes of azoles with good application prospects, on the one hand, we should vigorously improve their synthesis methods, on the other hand, we should vigorously carry out the study of their compatibility with other energetic components, so as to lay a solid foundation for their subsequent production and application. (4) In order to further expand its application scope and value, it is necessary to strengthen the development of civil energetic metal complexes such as fireworks and primary explosives, and to strengthen the research of special military complexes such as multi-iodine energetic anti-biological warfare agents. (5) Deeply carry out theoretical research on structural simulation, design and performance prediction of azol-rich energetic metal complexes, which not only provides more high-quality candidates for experimental synthesis, but also helps to understand the relationship between structure and performance, and improves the accuracy and accuracy of calculation methods. (6) Vigorously promote the application of artificial intelligence in energetic metal complexes, combine with the existing abundant experimental data, use machine learning to develop new azol-rich energetic metal complexes purposefully and efficiently, reduce experimental costs and risks, and reduce environmental pollution.
the National Natural Science Foundation of China(51802156)
Natural Science Foundation of Jiangsu(BK20170761)
Natural Science Foundation of Jiangsu(BK20181302)
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