A Review on the Reactivity of 1-Amino-2-Nitroguanidine (ANQ)

1-Amino-2-nitroguanidine (ANQ) is a high-energy nitrogen-rich compound with good detonation properties and low sensitivities. ANQ has only a central carbon atom with three small groups around it, including an amino, a hydrazine and a nitroxyl group. Though the molecular structure of ANQ is very simple, its reactivity is surprisingly abundant. ANQ can undergo various reactions, including reduction reaction, acylation reaction, salification reaction, coordination reaction, aldimine condensation reaction, cyclization reaction and azide reaction. Many new energetic compounds were purposely obtained through these reactions. These reactions were systematically summarized in this review, and detonation properties of some energetic compounds were compared. In the field of energetic materials, ANQ and some derivatives exhibit good application prospects.


Introduction
1-Amino-2-nitroguanidine (ANQ, 1) was first synthesized by Phillips in 1928 [1] and has played an important role in the synthesis of valuable biologically active compounds, such as pesticides and medicines [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. In recent years, Klapötke has done a lot of research on the application of ANQ in high-energy nitrogen-rich materials, which began to attract significant interest in the field of energetic materials [17,18]. The molecular structure of ANQ is very simple, containing only an amino, a hydrazino and a nitro group around the central carbon atom and its non-hydrogen atoms are almost on the same plane. Moreover, the bond angles of three main groups in the molecule are about 120 • with a relatively uniform distribution, indicating that the structure is comparatively regular and conforms to the structural requirements of the energetic materials. One kind of intramolecular and five kinds of intermolecular hydrogen bonds exist in ANQ, which extend it into a two-dimensional planar structure. Meanwhile, a stable spatial structure of ANQ is formed by electrostatic attraction and van der Waals force between layers, as seen in Figure 1. The crystals of ANQ are transparent yellow and crystallize in the monoclinic crystal system with the space group P2 1 /c [19]. ANQ can dissolve in water to the extent of 0.34 % (mass fraction) at 20 • C and 3.0% at 70 • C, and is easily soluble in sodium hydroxide solution, but cannot be readily dissolved in common organic solvents [1]. The decomposition behavior of ANQ presents two intense exothermal processes, and peak temperatures at a heating rate of 5 • C min −1 are 192. 48 and 196.20 • C, respectively, as presented in Figure 1 [20]. Furthermore, the self-accelerating decomposition temperature (T SADT ) and thermal explosion critical temperature (T b ) for ANQ are 184. 5 and 192.7 • C [20]. ANQ has good features, such as high nitrogen content (58.8%), big density (1.767 g cm −3 ), decent detonation velocity (8.25 km s −1 ) and detonation pressure (30.7 GPa) [21,22], suggesting that ANQ is a potential high-energy material.

Reduction Reaction
The reduction of ANQ by hydrogen can obtain diaminoguanidine (8) [1], as seen in Scheme 5, but no relevant papers confirmed the feasibility of the synthesis method. However, compound 8 can  The molecular structure of ANQ can be regarded as a resonance hybrid between the mesomer 1 and 1-a, 1-b, 1-c, as seen in Scheme 4 [41]. It is noted that the structure is similar to that of NQ. We know that NQ has long been considered to exist in two isomers. However, in terms of reactivity and spectroscopic data, the structure of NQ is not a nitramine (B), but a nitrimine (A), as seen in Figure 3. This fact suggests that ANQ is a nitrimine structure as well [23]. Due to the misunderstanding of the structure of ANQ, it has long been considered as an amphoteric substance [19,42,43]. However, according to the nitrimine structure of ANQ, it is an alkaline substance and is easily protonated. Of course, owing to its electron-withdrawing nitro group and electron-donating hydrazino group, ANQ possesses a high reactivity, and the reactivity can be described as the following four aspects: (a) due to the protonation of the terminal amino group, ANQ can generate salification reaction; (b) due to the high electron density of amino and nitro groups, it can be used as a good ligand in many coordination reactions; (c) the nucleophilic substitution of the hydrazino group; (d) other reactions of the nitro group and the amino group.

Reduction Reaction
The reduction of ANQ by hydrogen can obtain diaminoguanidine (8) [1], as seen in Scheme 5, but no relevant papers confirmed the feasibility of the synthesis method. However, compound 8 can

Reduction Reaction
The reduction of ANQ by hydrogen can obtain diaminoguanidine (8) [1], as seen in Scheme 5, but no relevant papers confirmed the feasibility of the synthesis method. However, compound 8 can be obtained in the synthesis process of ANQ, as seen in Scheme 1, if the reaction temperature and time are not properly controlled [18].

Salification Reaction
Since the hydrazine group is electron-donating, ANQ is easily protonated to form cations under strong acidic conditions, as seen in Scheme 9, which leads to the formation of the inorganic salts (22)(23)(24)(25)(26)(27) and organic salts (28-42) of ANQ. For example, ANQ can react with nitric acid to form 1-amino-2-nitroguanidinium nitrate (ANGN, 22), which crystallizes in the triclinic crystal system with space group Pī [47]. In addition, the halides as well as the sulfate salt also can be prepared by dissolving ANQ in dilute aqueous solutions of the respective mineral acids HCl, HBr, HI and H2SO4. However, it cannot react with HF, since the acidic strength of HF in aqueous solution is the weakest among all the hydrogenides, and HF is not able to protonate ANQ in aqueous solution [48]. Especially, the

Salification Reaction
Since the hydrazine group is electron-donating, ANQ is easily protonated to form cations under strong acidic conditions, as seen in Scheme 9, which leads to the formation of the inorganic salts (22)(23)(24)(25)(26)(27) and organic salts (28-42) of ANQ. For example, ANQ can react with nitric acid to form 1-amino-2-nitroguanidinium nitrate (ANGN, 22), which crystallizes in the triclinic crystal system with space group Pī [47]. In addition, the halides as well as the sulfate salt also can be prepared by dissolving ANQ in dilute aqueous solutions of the respective mineral acids HCl, HBr, HI and H2SO4. However, it cannot react with HF, since the acidic strength of HF in aqueous solution is the weakest among all the hydrogenides, and HF is not able to protonate ANQ in aqueous solution [48]. Especially, the detonation pressure (42.7 GPa) and detonation velocity (9.551 km s -1 ) of ANGN are both superior to

Salification Reaction
Since the hydrazine group is electron-donating, ANQ is easily protonated to form cations under strong acidic conditions, as seen in Scheme 9, which leads to the formation of the inorganic salts (22)(23)(24)(25)(26)(27) and organic salts (28-42) of ANQ. For example, ANQ can react with nitric acid to form 1-amino-2-nitroguanidinium nitrate (ANGN, 22), which crystallizes in the triclinic crystal system with space group Pī [47]. In addition, the halides as well as the sulfate salt also can be prepared by dissolving ANQ in dilute aqueous solutions of the respective mineral acids HCl, HBr, HI and H 2 SO 4 . However, it cannot react with HF, since the acidic strength of HF in aqueous solution is the weakest among all the hydrogenides, and HF is not able to protonate ANQ in aqueous solution [48]. Especially, the detonation pressure (42.7 GPa) and detonation velocity (9.551 km s −1 ) of ANGN are both superior to those of RDX, which can be considered as a potential candidate for high-energy-density compounds [18].
Since the hydrazine group is electron-donating, ANQ is easily protonated to form cations under strong acidic conditions, as seen in Scheme 9, which leads to the formation of the inorganic salts (22)(23)(24)(25)(26)(27) and organic salts (28-42) of ANQ. For example, ANQ can react with nitric acid to form 1-amino-2-nitroguanidinium nitrate (ANGN, 22), which crystallizes in the triclinic crystal system with space group Pī [47]. In addition, the halides as well as the sulfate salt also can be prepared by dissolving ANQ in dilute aqueous solutions of the respective mineral acids HCl, HBr, HI and H2SO4. However, it cannot react with HF, since the acidic strength of HF in aqueous solution is the weakest among all the hydrogenides, and HF is not able to protonate ANQ in aqueous solution [48]. Especially, the detonation pressure (42.7 GPa) and detonation velocity (9.551 km s -1 ) of ANGN are both superior to those of RDX, which can be considered as a potential candidate for high-energy-density compounds [18]. Scheme 9. ANQ and its protonated cation.

Coordination Reaction
In 1928, Phillips firstly reported that ANQ reacted with nickel sulfate to form a nickel complex, which owned a high decomposition temperature of 220 • C [1]. ANQ can form energetic metal complexes [M 2+ (ANQ) 2 (X − ) 2 (H 2 O) n (n = 0, 2), in the case of M = Co, Ni, Cu, Zn (43)(44)(45)(46), and M + (ANQ) 2 (X − )(H 2 O) y in the case of M = Ag (47)] with perchlorate or nitrate solution of transition metal [60]. Herein, nitrate, perchlorate and chloride are the anions of the complex, and ANQ is the ligand for the synthesis of high energy transition metal complexes, as seen in Scheme 11. Complexes M 2+ (ANQ) 2 (X − ) 2 (H 2 O) 4 (48,49) with X=N(NO 2 ) 2 can be synthesized in the case of M=Co and Ni, in a stoichiometric ratio of perchlorate complexes of cobalt (nickel): Ammonium dinitramide (ADN) = 1:2, as seen in Scheme 12 [60]. All those complexes containing perchlorate, nitrate and chloride crystallize as dihydrates, except silver complexes (47) are water free. Dinitramide crystallizes as tetrahydrate (48,49), and most of the complexes containing crystal water can be dehydrated without decomposition. In these complexes, the nickel complex is the most stable, having the highest decomposition temperature of 250 • C, while the copper and silver complexes decompose at a low temperature of about 77 • C. Compared with nitrates and ammonium dinitramide (ADN), the decomposition temperatures of chlorides and perchlorates tend to increase. Moreover, all the complexes present high impact and friction sensitivities, especially the solvate water free silver complex (47) and the copper nitrate complex [60]. ANQ can also have coordination reaction with cobalt (II), nickel (II), copper (II) and 2-hydroxybenzaldehyde or 4-(dimethylamino)benzaldehyde [61]. In 1928, Phillips firstly reported that ANQ reacted with nickel sulfate to form a nickel complex, which owned a high decomposition temperature of 220 °C [1]. ANQ can form energetic metal complexes [M 2+ (ANQ)2(X -)2(H2O)n (n = 0, 2), in the case of M = Co, Ni, Cu, Zn (43)(44)(45)(46), and M + (ANQ)2(X -)(H2O)y in the case of M = Ag (47)] with perchlorate or nitrate solution of transition metal [60]. Herein, nitrate, perchlorate and chloride are the anions of the complex, and ANQ is the ligand for the synthesis of high energy transition metal complexes, as seen in Scheme 11. Complexes M 2+ (ANQ)2(X -)2(H2O)4 (48, 49) with X=N(NO2)2 can be synthesized in the case of M=Co and Ni, in a stoichiometric ratio of perchlorate complexes of cobalt (nickel): Ammonium dinitramide (ADN) = 1:2, as seen in Scheme 12 [60]. All those complexes containing perchlorate, nitrate and chloride crystallize as dihydrates, except silver complexes (47) are water free. Dinitramide crystallizes as tetrahydrate (48,49), and most of the complexes containing crystal water can be dehydrated without decomposition. In these complexes, the nickel complex is the most stable, having the highest decomposition temperature of 250 °C, while the copper and silver complexes decompose at a low temperature of about 77 °C. Compared with nitrates and ammonium dinitramide (ADN), the decomposition temperatures of chlorides and perchlorates tend to increase. Moreover, all the complexes present high impact and friction sensitivities, especially the solvate water free silver complex (47) and the copper nitrate complex [60]. ANQ can also have coordination reaction with cobalt (II), nickel (II), copper (II) and 2-hydroxybenzaldehyde or 4-(dimethylamino)benzaldehyde [61].

Aldimine Condensation Reaction
ANQ can undergo condensation reactions with aldehydes and ketones to form hydrazine or Schiff base compounds. Phillips found that ANQ can react with aldehydes and ketones by using an acid or a base as catalyst, as shown in Scheme 13 [1]. When an acid is used as catalyst, its hydrogen ion can be combined with a carbonyl group to form an oxonium salt, thereby increasing the electrophilicity of the carbonyl group [62]. The nucleophilic addition-elimination reaction mechanisms under acid-base conditions are shown in Scheme 14,15. In 1928, Phillips firstly reported that ANQ reacted with nickel sulfate to form a nickel complex, which owned a high decomposition temperature of 220 °C [1]. ANQ can form energetic metal complexes [M 2+ (ANQ)2(X -)2(H2O)n (n = 0, 2), in the case of M = Co, Ni, Cu, Zn (43)(44)(45)(46), and M + (ANQ)2(X -)(H2O)y in the case of M = Ag (47)] with perchlorate or nitrate solution of transition metal [60]. Herein, nitrate, perchlorate and chloride are the anions of the complex, and ANQ is the ligand for the synthesis of high energy transition metal complexes, as seen in Scheme 11. Complexes M 2+ (ANQ)2(X -)2(H2O)4 (48, 49) with X=N(NO2)2 can be synthesized in the case of M=Co and Ni, in a stoichiometric ratio of perchlorate complexes of cobalt (nickel): Ammonium dinitramide (ADN) = 1:2, as seen in Scheme 12 [60]. All those complexes containing perchlorate, nitrate and chloride crystallize as dihydrates, except silver complexes (47) are water free. Dinitramide crystallizes as tetrahydrate (48,49), and most of the complexes containing crystal water can be dehydrated without decomposition. In these complexes, the nickel complex is the most stable, having the highest decomposition temperature of 250 °C, while the copper and silver complexes decompose at a low temperature of about 77 °C. Compared with nitrates and ammonium dinitramide (ADN), the decomposition temperatures of chlorides and perchlorates tend to increase. Moreover, all the complexes present high impact and friction sensitivities, especially the solvate water free silver complex (47) and the copper nitrate complex [60]. ANQ can also have coordination reaction with cobalt (II), nickel (II), copper (II) and 2-hydroxybenzaldehyde or 4-(dimethylamino)benzaldehyde [61].

Aldimine Condensation Reaction
ANQ can undergo condensation reactions with aldehydes and ketones to form hydrazine or Schiff base compounds. Phillips found that ANQ can react with aldehydes and ketones by using an acid or a base as catalyst, as shown in Scheme 13 [1]. When an acid is used as catalyst, its hydrogen ion can be combined with a carbonyl group to form an oxonium salt, thereby increasing the electrophilicity of the carbonyl group [62]. The nucleophilic addition-elimination reaction mechanisms under acid-base conditions are shown in Scheme 14,15.

Aldimine Condensation Reaction
ANQ can undergo condensation reactions with aldehydes and ketones to form hydrazine or Schiff base compounds. Phillips found that ANQ can react with aldehydes and ketones by using an acid or a base as catalyst, as shown in Scheme 13 [1]. When an acid is used as catalyst, its hydrogen ion can be combined with a carbonyl group to form an oxonium salt, thereby increasing the electrophilicity of the carbonyl group [62]. The nucleophilic addition-elimination reaction mechanisms under acid-base conditions are shown in Schemes 14 and 15.  (51), as seen in Scheme 16 [63]. Only the hydrazino group of ANQ could react with aldehyde group, and no condensation reaction of amino group with the aldehyde group was found. This may be because the strong electron-withdrawing effect of the nitro group reduced the reactivity of the amino group. Additionally, the treatment of ANQ with formaldehyde resulted in the formation of 1hydroxymethylamino-2-nitroguanidine (52). The terminal hydroxyl function of compound had high activity and could further react with nitroform in methanol, ethanol or water to obtain 2-nitro-1-(2,2,2-trinitroethylamino)guanidine (TNEANG) (53) [64,65]. Compound 53 was a typical trinitromethyl derivative, which could be reduced by iodide to form a potassium salt of 1-(2,2dinitroethylamino)-2-nitroguanidine (54) [65]. It was also able to react with denitration agents such as K2CO3 or KOH, followed by acidification with hydrochloric acid to obtain 1-(2,2dinitroethylamino)-2-nitroguanidine (DNEANG) (55) containing a dinitromethyl structure. The separation phase of potassium salt 54 was successfully eliminated, increasing the yield of 55 from 44%  (51), as seen in Scheme 16 [63]. Only the hydrazino group of ANQ could react with aldehyde group, and no condensation reaction of amino group with the aldehyde group was found. This may be because the strong electron-withdrawing effect of the nitro group reduced the reactivity of the amino group. Additionally, the treatment of ANQ with formaldehyde resulted in the formation of 1hydroxymethylamino-2-nitroguanidine (52). The terminal hydroxyl function of compound had high activity and could further react with nitroform in methanol, ethanol or water to obtain 2-nitro-1-(2,2,2-trinitroethylamino)guanidine (TNEANG) (53) [64,65]. Compound 53 was a typical trinitromethyl derivative, which could be reduced by iodide to form a potassium salt of 1-(2,2dinitroethylamino)-2-nitroguanidine (54) [65]. It was also able to react with denitration agents such as K2CO3 or KOH, followed by acidification with hydrochloric acid to obtain 1-(2,2dinitroethylamino)-2-nitroguanidine (DNEANG) (55) containing a dinitromethyl structure. The separation phase of potassium salt 54 was successfully eliminated, increasing the yield of 55 from 44% Scheme 14. Reaction mechanism of ANQ with aldehydes and ketones under acidic conditions.  (51), as seen in Scheme 16 [63]. Only the hydrazino group of ANQ could react with aldehyde group, and no condensation reaction of amino group with the aldehyde group was found. This may be because the strong electron-withdrawing effect of the nitro group reduced the reactivity of the amino group. Additionally, the treatment of ANQ with formaldehyde resulted in the formation of 1hydroxymethylamino-2-nitroguanidine (52). The terminal hydroxyl function of compound had high activity and could further react with nitroform in methanol, ethanol or water to obtain 2-nitro-1-(2,2,2-trinitroethylamino)guanidine (TNEANG) (53) [64,65]. Compound 53 was a typical trinitromethyl derivative, which could be reduced by iodide to form a potassium salt of 1-(2,2dinitroethylamino)-2-nitroguanidine (54) [65]. It was also able to react with denitration agents such as K2CO3 or KOH, followed by acidification with hydrochloric acid to obtain 1-(2,2dinitroethylamino)-2-nitroguanidine (DNEANG) (55) containing a dinitromethyl structure. The separation phase of potassium salt 54 was successfully eliminated, increasing the yield of 55 from 44% Scheme 15. Reaction mechanism of ANQ with aldehydes and ketones under alkaline conditions. ANQ reacted with 3-methyl-4-furoxancarbaldehyde and 2,4,6-trinitrobenzaldehyde leading to the formation of 3-methyl-4-((2-(N -nitrocarbamimidoyl)hydrazono)methyl)-1,2,5-oxadiazole-2-oxide (50) and N -nitro-2-(2,4,6-trinitrobenzylidene)hydrazinecarboximidamide (51), as seen in Scheme 16 [63]. Only hydrazino group of ANQ could react with aldehyde group, and no condensation reaction of amino group with aldehyde group was found. This may be because the strong electron-withdrawing effect of nitro group reduced the reactivity of amino group.  (51), as seen in Scheme 16 [63]. Only the hydrazino group of ANQ could react with aldehyde group, and no condensation reaction of amino group with the aldehyde group was found. This may be because the strong electron-withdrawing effect of the nitro group reduced the reactivity of the amino group. Additionally, the treatment of ANQ with formaldehyde resulted in the formation of 1hydroxymethylamino-2-nitroguanidine (52). The terminal hydroxyl function of compound had high activity and could further react with nitroform in methanol, ethanol or water to obtain 2-nitro-1-(2,2,2-trinitroethylamino)guanidine (TNEANG) (53) [64,65]. Compound 53 was a typical trinitromethyl derivative, which could be reduced by iodide to form a potassium salt of 1-(2,2dinitroethylamino)-2-nitroguanidine (54) [65]. It was also able to react with denitration agents such as K2CO3 or KOH, followed by acidification with hydrochloric acid to obtain 1-(2,2dinitroethylamino)-2-nitroguanidine (DNEANG) (55) containing a dinitromethyl structure. The separation phase of potassium salt 54 was successfully eliminated, increasing the yield of 55 from 44% Scheme 16. Aldehyde ketone reaction of ANQ.

Scheme 17. Synthesis of 1-hydroxymethylamino-2-nitroguanidine and its derivatives.
Based on the reaction of nitroguanidine and formaldehyde to form methylene dinitroguanidine, our research group hoped that designing a route of linking two molecules of ANQ with methylene to synthesize a symmetric energetic compound 56 or 57, but failed even though various methods were attempted. However, methyleneaminonitroguanidine (MANG) (58), as shown in Scheme 18, was unexpectedly synthesized with a high yield of 86% [68]. The reason should be that the amino of the hydrazino group is more nucleophilic. The nucleophilic addition reaction firstly occurs since the positively charged carbon atom on the carbonyl group is attacked, and the resulting intermediate is further dehydrated to form MANG. This process is a classical aldimine condensation reaction. MANG crystallizes in the orthorhombic crystal system with space group Pnn2 containing four molecules per unit cell. The crystal density is 1.63 g cm −3 . Its impact sensitivity, detonation velocity and detonation pressure are >7.9 J, 7.1 km s −1 and 20.9 GPa, respectively. In addition, we hoped to obtain a more stable five-membered nitrogen heterocyclic compound 59 through the cyclization reaction, but also failed. It illustrates that aldehyde ammonia condensation is more advantageous in this system, and it is difficult to undergo chain reaction and cyclization reaction [68]. Based on the reaction of nitroguanidine and formaldehyde to form methylene dinitroguanidine, our research group hoped that designing a route of linking two molecules of ANQ with methylene to synthesize a symmetric energetic compound 56 or 57, but failed even though various methods were attempted. However, methyleneaminonitroguanidine (MANG) (58), as shown in Scheme 18, was unexpectedly synthesized with a high yield of 86% [68]. The reason should be that the amino of the hydrazino group is more nucleophilic. The nucleophilic addition reaction firstly occurs since the positively charged carbon atom on the carbonyl group is attacked, and the resulting intermediate is further dehydrated to form MANG. This process is a classical aldimine condensation reaction. MANG crystallizes in the orthorhombic crystal system with space group P nn 2 containing four molecules per unit cell. The crystal density is 1.63 g cm −3 . Its impact sensitivity, detonation velocity and detonation pressure are >7.9 J, 7.1 km s −1 and 20.9 GPa, respectively. In addition, we hoped to obtain a more stable five-membered nitrogen heterocyclic compound 59 through the cyclization reaction, but also failed. It illustrates that aldehyde ammonia condensation is more advantageous in this system, and it is difficult to undergo chain reaction and cyclization reaction [68]. to 78%, as seen in Scheme 17 [53,[64][65][66][67]. The detonation properties of TNEANG (53) and DNEANG (55) have not been reported, but their melting points are as low as 95-96 °C and 93-94 °C respectively, which are promising as melting phase for melt casting explosives [64].

Scheme 17. Synthesis of 1-hydroxymethylamino-2-nitroguanidine and its derivatives.
Based on the reaction of nitroguanidine and formaldehyde to form methylene dinitroguanidine, our research group hoped that designing a route of linking two molecules of ANQ with methylene to synthesize a symmetric energetic compound 56 or 57, but failed even though various methods were attempted. However, methyleneaminonitroguanidine (MANG) (58), as shown in Scheme 18, was unexpectedly synthesized with a high yield of 86% [68]. The reason should be that the amino of the hydrazino group is more nucleophilic. The nucleophilic addition reaction firstly occurs since the positively charged carbon atom on the carbonyl group is attacked, and the resulting intermediate is further dehydrated to form MANG. This process is a classical aldimine condensation reaction. MANG crystallizes in the orthorhombic crystal system with space group Pnn2 containing four molecules per unit cell. The crystal density is 1.63 g cm −3 . Its impact sensitivity, detonation velocity and detonation pressure are >7.9 J, 7.1 km s −1 and 20.9 GPa, respectively. In addition, we hoped to obtain a more stable five-membered nitrogen heterocyclic compound 59 through the cyclization reaction, but also failed. It illustrates that aldehyde ammonia condensation is more advantageous in this system, and it is difficult to undergo chain reaction and cyclization reaction [68]. Treatment of ANQ with glyoxal in sodium hydroxide solution led to a N -nitro-2-(2-oxoethylidene)hydrazinecarboximidamide (60) as a mixture of syn and anti-isomers (ratio of 1:1), as seen in Figure 4. When ANQ was 2-fold excess, N -nitro-2-[(5-nitroamino-2H-1,2,4-triazol-3-yl) methyl]hydrazinecarboximidamide (61) and 3-nitroamino-4,5-dihydro-1,2,4-triazin-5-ol (62) were formed. However, under acidic conditions, ANQ reacted with glyoxal to form glyoxal dihydrazone (63), which could also be synthesized by hydrazone 60 and ANQ in boiling glacial acetic acid for a long time [69,70], as seen in Scheme 19. Under the catalysis of acid, a linear monohydrazone (64) was prepared by an equimolar ratio of ANQ and butane-2,3-dione. A 2-fold excess of ANQ was used to generate a 5:8 ratio of a mixture of monohydrazone (64) and dihydrazone (65) [71], as seen in Scheme 20. Similarly, ANQ reacted with acetylacetone to generate acetylacetonenitroguanylosazone [72].   (62) were formed. However, under acidic conditions, ANQ reacted with glyoxal to form glyoxal dihydrazone (63), which could also be synthesized by hydrazone 60 and ANQ in boiling glacial acetic acid for a long time [69,70], as seen in Scheme 19.  Under the catalysis of acid, a linear monohydrazone (64) was prepared by an equimolar ratio of ANQ and butane-2,3-dione. A 2-fold excess of ANQ was used to generate a 5:8 ratio of a mixture of monohydrazone (64) and dihydrazone (65) [71], as seen in Scheme 20. Similarly, ANQ reacted with acetylacetone to generate acetylacetonenitroguanylosazone [72].
We suspected that trichlorotriazine did not work in the reaction. So we designed the reaction of ANQ only with acetone in the absence of trichlorotriazine and obtained N -nitro-2-(propan-2-ylidene)hydrazine-1-carboximidamide (NPYHC, 68) successfully, which was a classic reaction of addition condensation reaction of ammonia and ketone. Therefore, trichlorotriazine should play a catalytic role in the reaction, as seen in Scheme 21.

Cyclization Reaction
ANQ can undergo many cyclization reactions to synthesize corresponding azoles and azines under different conditions. Compared with traditional carbon-based high-energy compounds such as TNT and TATB, high-energy materials with high nitrogen heterocycles have more advantages, including higher molecular density, fine environmental compatibility and it is easier to achieve good oxygen balance. High nitrogen heterocyclic materials have higher energy and stability due to their high-energy N-N, C-N, N=N bonds and molecular ring strain, so they are more widely used in energetic materials [73]. ANQ was reacted in KNO2/AcOH solution firstly, followed by acidifying it with hydrochloric acid to get 5-nitroaminotetrazole (69) which melts at 195 °C and explodes at a slightly higher temperature [74,75]. 5-Nitroaminotetrazole is a superior precursor to energetic salts, and the derivatives of 69 have attracted much attention for their positive formation enthalpies and high nitrogen contents [76,77]. Compound 69 can be reduced with benzaldehyde in the presence of Zn to obtain a benzene derivative, or can be reacted with others to obtain corresponding tetrazole ammonium salt [78], diammonium salt [77], methylammonium salt [79], metal salts [80][81][82] (such as lithium salt, sodium salt, rubidium salt, cesium salt) and 1,2,4-triazolium salts [76]. Five different 1,2,4-triazolium salts were synthesized and shown in Scheme 22: 4-amino-1,2,4-triazolium (70), 5amino-tetrazolium (71), 1,2,4-triazolium (72), 1-propyl-1,2,4-triazolium (73) and 3-azido-1,2,4triazolium (74) [76]. Although compound 73 contains energetic anions, its low thermal stability (Tm = 69 °C) and density (1.48 g cm −3 ) restrict its practical applications. Compound 74 exhibits the highest heat of formation (694.2 kJ mol) and good detonation properties, but the triazolium derivative (72) has the highest density in this group compounds [76]. Compared with ordinary tetrazole ions, 5nitroaminotetrazole is easier to form a corresponding complex with transition metal ions (such as Cu, Ni, Pb, Hg and Ag) [83,84]. These derivatives have potential applications in energetic materials, for instance, using as initiators of explosive materials. Scheme 21. Reaction of ANQ with acetone.

Azide Reaction
ANQ reacted in a strong acid solution of HCl/KNO2 to obtain azide nitroguanidine (84) at 60 °C with the yield of 77% [75]. It was found that ANQ could react with nitrous acid in weak or strong acid solution to obtain azide nitroguanidine, and the yield in strong acid solution was higher. Compound 84 has a high nitrogen content (64.62%) and presents a potential application value. It can be reduced to nitroguanidine by hydrogen sulfide, as a characteristic reaction of azide. Compound 84 was cyclized with inorganic base or organic base to obtain the corresponding alkyl or aryl ammonium salts of 5-nitroaminotetrazole (85)(86)(87)(88)(89)(90)(91), which could also be obtained directly by alkalizing 5-nitroaminotetrazole, as seen in Scheme 28 [74].

Azide Reaction
ANQ reacted in a strong acid solution of HCl/KNO 2 to obtain azide nitroguanidine (84) at 60 • C with the yield of 77% [75]. It was found that ANQ could react with nitrous acid in weak or strong acid solution to obtain azide nitroguanidine, and the yield in strong acid solution was higher. Compound 84 has a high nitrogen content (64.62%) and presents a potential application value. It can be reduced to nitroguanidine by hydrogen sulfide, as a characteristic reaction of azide. Compound 84 was cyclized with inorganic base or organic base to obtain the corresponding alkyl or aryl ammonium salts of 5-nitroaminotetrazole (85)(86)(87)(88)(89)(90)(91), which could also be obtained directly by alkalizing 5-nitroaminotetrazole, as seen in Scheme 28 [74].

Detonation Properties
A series of derivatives of ANQ were summarized systemically and some compounds, as potential energetic materials, display good energetic properties, as shown in Table 2. We use the classification in "UN Recommendations on the Transport of Dangerous Goods" to evaluate their safety [89].

Detonation Properties
A series of derivatives of ANQ were summarized systemically and some compounds, as potential energetic materials, display good energetic properties, as shown in Table 2. We use the classification in "UN Recommendations on the Transport of Dangerous Goods" to evaluate their safety [89]. , supporting the view that high density contributes markedly to the detonation performances. In summary, the compounds, 22, 33, 38-42 and 69 exhibit good detonation properties, and can be regarded as potential candidates in the application of high-energy-density materials.

Conclusions
Since the first report, ANQ has been regarded as an important raw material in the fields of pesticides, medicines and energetic materials. As a high-energy insensitive material, ANQ is likely to replace traditional RDX in application of solid propellants.
Though the molecular structure is very simple, its reactivity is abundant. Seven kinds of reactions about ANQ are systematically summarized, including reduction reaction, acylation reaction, salification reaction, coordination reaction, aldimine condensation reaction, cyclization reaction and azide reaction. Many excellent derivatives have been synthesized by these reactions. Some high-energy nitrogen-rich derivatives, such as ANGN, 1-amino-2-nitroguanidinium 5,5 -bis(tetrazole-2-oxide) salt and 5-nitroaminotetrazole, exhibit a good application prospect in the field of energetic materials.
According to the summary of the reactivity for ANQ, we can see that the adjacent amino and hydrazino group is a high activity group and the key factor for synthesizing these derivatives, especially to the heterocyclic derivatives. Therefore, in the design of nitrogen-rich or heterocyclic compounds, the introduction of the adjacent amino and hydrazino group is an effective way. We believe that many new nitrogen-rich materials can be synthesized according this method. The research of the reactivity of ANQ contributes significantly to expanding the understanding of the chemistry of guanidine compounds.