Recent progress in taming FOX-7 (1,1-diamino-2,2-dinitroethene)

Abstract

2,2-Dinitroethene-1,1-diamine (FOX-7) is an insensitive, energetic material with a simple structure but very unusual properties. With interest among chemists in expanding the chemistry of FOX-7, continuous efforts are underway to improve the performance of the compound by modifying and optimizing its structure and creating various derivatives. In this Review, the chemical reactivity of 1,1-diamino-2,2-dinitroethene (FOX-7) towards nucleophilic (transamination), and electrophilic substitution, and its acid–base properties are explored as well as an account of the molecular structure, physical and detonation properties of derivatives, salts and metal complexes of FOX-7.

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Considerable effort has been focused on the design, and synthesis of new and improved high-energy-density materials (HEDMs). New HEDMs with high densities, high heats of formation (HOF), and positive oxygen balances (OB) are highly desirable because they exhibit good detonation properties, which makes them suitable in roles as propellants, pyrotechnics or explosives.^1,2 In general, the search for new HEDMs is driven by the desire to achieve detonation performance better or comparable to that of the current benchmark HEDMs such as cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX).^1,2 In the search for these materials, researchers must improve upon the energy capacity of compounds, which requires new classes of compounds with a variety of functional groups, advanced synthesis strategies, and accurate theoretical prediction techniques. Desirable characteristics for new HEDMs include high density; positive HOF and OB; good thermal stability; simple synthesis; high detonation performance; low sensitivity toward external stimuli, and environmental friendliness.^1–3 However, achieving a fine balance between high detonation performance and low sensitivity is an interesting but challenging task because enhancement in detonation performance most often comes at the expense of molecular stability.

1,1-Diamino-2,2-dinitroethylene (FOX-7) is a novel, insensitive HEDM that exhibits many of the excellent energetic properties required for great performance with few negative points.^4,5 It was first prepared via the nitration of 2-methyl-imidazole (Scheme 1, Method 1) by N. Latypov, et al. in 1998.⁶ Other routes to synthesize FOX-7 were developed later (Scheme 1, Method 2–4).^6–8

Scheme 1 Typical methods used to synthesize FOX-7.

The structure of FOX-7 is a highly polarized carbon–carbon double bond with two electron-withdrawing nitro groups on one carbon and two electron-donating amino groups on the other carbon^4,5 FOX-7 has a crystalline molecular packing similar to that observed in other relatively insensitive HEDMs such as TATB and NTO.^4,5 The wave-shaped layers and intramolecular hydrogen bonds stabilize this molecule, resulting in insensitivity toward external stimuli—a very desirable safety characteristic in HEDMs.

Research and development work involving FOX-7 is currently being pursued with great interest among academic and industrial chemists. The existence of FOX-7 as a variety of tautomers and resonance structures¹⁵ (Scheme 2) suggests the possibility of several main type reactions for FOX-7, e.g., neutralization, nucleophilic and electrophilic substitution, addition to double-bonds, etc.

Scheme 2 Tautomers and resonance structures of FOX-7.

This variety of reactions enables the optimization of the structure of FOX-7 to enhance its detonation properties in conjunction with its insensitivity. From the very first synthesis of FOX-7, a broad range of reactions has been performed. The main derivatives of FOX-7, including salts, metal complexes, and modification with energetic functional groups (Scheme 3), have been discussed in extensive reviews, e.g., by Bellamy⁵ and Simkova, et al.⁴ Perhaps the most challenging characteristic of FOX-7 is that it is chemically unpredictable and exhibits poor solubility in most organic solvents which makes monitoring its reaction behavior extremely difficult. Its derivatives are limited, and most of its reactions are simple and can be achieved in a straightforward manner in a single step reaction (Scheme 3).

Scheme 3 Typical reactions of FOX-7.

With interest among chemists in expanding the chemistry of FOX-7, continuous efforts are underway to improve the performance of the compound by modifying and optimizing its structure and creating different derivatives. However, no relevant summary has been published describing the latest chemistry of FOX-7 and its derivatives. This review gives an overview of the work published from 2010 to 2015 that focuses on the synthesis of the derivatives of FOX-7, as well as earlier references that were omitted in other reviews.^4,5 These earlier references include reports of condensation, halogenation, and nitration reactions, and preparation of salts, metal complexes and derivatives. Also, covered are the experimentally determined and theoretically predicted detonation properties of these new compounds where available.

1. Derivatives of FOX-7 from condensation reactions

Condensation of FOX-7 with glyoxal ((CHO)₂) and formaldehyde (HCHO) in aqueous solution at pH 7–8 leads to 2-(dinitromethylene)imidazolidine-4,5-diol (16) and 1,1-diamino-N,N′-bis(hydroxymethyl)-2,2-dinitroethylene (17) (Scheme 4). An attempt to carry out the Ritter reaction of acetonitrile (MeCN) with 17 gave rise to 4-(dinitromethylene)-1,3,5-oxadiazinane (18). Under similar reaction conditions, the diol 16 reacted with MeCN to precipitate the sulfate of the diacetamide derivative, 4,5-diacetamido-2-(dinitromethylene)imidazolidine sulfate (19).¹⁶ The free base 4, 5-diacetamido-2-(dinitromethylene)imidazolidine (20) was achieved by treating 19 with ice water (H₂O) or ethanol (EtOH). Addition of a small amount of H₂O to the suspension of 19 in MeCN at 0–5 °C gave 2-(dinitromethylene)-5-methyl-1,2,3,3a,4,6a-hexahydroimidazo-[4,5-d]imidazole (21).¹⁶ Similarly, condensation of 16 with urethane or benzonitrile resulted in 2-(dinitromethylene)-4,5-bis-(ethoxycarbonylamino)-imidazolidine (22) or 4,5-dibenzamido-2-(dinitromethylene)-imidazolidine (23), respectively.¹⁶ Upon treating the diol 16 in a mixture of acetic acid (AcOH) and sulfuric acid (H₂SO₄), the corresponding hydantoin 2-(dinitromethylene)-2,3-dihydro-1H-imidazol-4-ol (24) is formed. However, 16 easily undergoes etherification with MeOH or EtOH or acetylation with AcOH, catalyzed with H₂SO₄, yielding the products 4,5-diethoxy-2-(dinitromethylene)imidazolidine (25), 4,5-dimethoxy-2-(dinitromethylene)imidazolidine (26) or 4,5-diacetoxy-2-(dinitromethylene)imidazolidine (27), respectively.¹⁶ Further condensation of the acetoxy derivative 27 with diaminofurazan resulted in the cyclic compound 6-(dinitromethylene)-4a,5,6,7,7a,8-hexahydroimidazo[4,5-b]furazano[3,4-e]pyrazine (28).¹⁶

Scheme 4 Condensation products of FOX-7 with ((CHO)₂) and HCHO and their derivatives.

Obviously, the condensation products of FOX-7 could be used as precursors for the synthesis of various cyclic and acyclic derivatives of FOX-7. Unfortunately the physical and detonation properties of these condensation products of FOX-7 were not reported.¹⁶

2. Halogenated derivatives of FOX-7

Reactivity of FOX-7 toward strong electrophiles was implied by the work of Hervé, et al., who reacted N-chlorosuccinimide (NCS) or N-bromosuccinimide (NBS) with FOX-7 to give 1-chloro-1,1-dinitro-2-(N-chloroamidino)-ethane (4) or 1-bromo-1,1-dinitro-2-(N-bromoamidino)-ethane (5), respectively (Scheme 3).^9,15

In 2012, trichloroisocyanuric acid (TCICA) was used in converting the amine group in FOX-7 to a new azo-bridged product, (E)-1,2-bis{(Z)-2-chloro-1-(chloroimino)-2,2-dinitroethyl}diazene (33,¹⁰ in addition to the previously known dichloro substrate 4 (Scheme 5)).¹⁵ Later, energized halogenated derivatives of FOX-7 were reported by our group.⁹ Treating FOX-7 with a slight excess of TCICA resulted in 4 (major) and azo compound 33 (minor). However, using a larger excess (five equivalents) of TCICA with FOX-7 or 4 led to the formation of 33 as the major product.⁹ These results support the likelihood that 4 is the intermediate which reacted subsequently with additional TCICA to generate 33 (Scheme 5).

Scheme 5 Halogenated derivatives of FOX-7.

Commercial bleach (NaClO) was used as an oxidizing reagent with FOX-7 to give a higher yield of 33 (42.5% vs. 11% with TCICA as oxidizer) and simultaneously obtained a novel isomer azo-bridged product, (E)-1,2-bis{(Z)-2-chloro-1-(chloroimino)-2,2-dinitroethyl}diazene (34) (yield 21.0%) (Scheme 5).¹⁰ Compound 34 has physical properties similar to those of 33 but has a slightly higher HOF. Further treatment of 33 with aqueous ammonia or Me₂S led to N₁,N₂-dichloro-1,2-diazenedicarboximidamide (35) or (E,E)-N,N′-1,2-ethanediylidenebis(2,2-dinitro-2-chloroethanamine) (36) (Scheme 5). Compared with their parent molecule 33, compounds 35 and 36 exhibit superior stability with less favorable detonation properties (Table 1).¹⁰

Table 1 Physical properties of derivatives of FOX-7 compared with those of TNT, TATB, RDX, HMX and PETN

d	Tm^a [°C]	Td^b [°C]	Ω^c [%]	D^d [g cm^−3]	ΔfH^e [kJ mol^−1]	D^f [m s^−1]	P^g [GPa]	IS^h (J)	FS^i (N)	Ref.
a Melting point.b Decomposition temperature (onset).c OB for CaHbOcNd, 1600(c − a − b/2)/Mw; molecular weight.d Density (measured/crystal density).e Calculated heat of formation.f Detonation velocity.g Detonation pressure.h Impact sensitivity (IS).i Friction sensitivity (FS).j Ref. 14.
1	—	274.0	0	1.89	−53.1	8930	34.0	60		9
4	91.0	127.0	7.4	1.86 [1.84]	274.1	8389	32.5	10		9
5	70.0	128.0	5.2	2.47 [2.48]	872.8	7159	36.7	7.5		9
9	—	257.0 (261–262)^j	−27.6	1.69	−5.7	8394	28.6	>40	>360	10
12	—	124.5	−4.9	1.78	71.6	8803	33.8	6	—	11
18	—	209.2	−16.8	1.79	−74.2	8280	28.9	>40	>360	10
29	—	136.0	12.3	2.08 [2.10]	1315.3	7102	25.4	<2		9
30	—	133.0	11.4	2.39 [2.33]	707.7	7403	34.3	<2
33	—	139.0	14.9	1.97 [1.99]	1010.3	8348	32.4	3.5	40	9 and 10
34	—	137.7	14.9	1.95 [2.00]	1060.1	7851	30.1	3.5	40	10
35	—	159.6	−35.0	1.71 [1.77]	348.3	6460	16.4	>40	>360	10
36	—	120.7	8.8	1.95	−94.3	8250	30.8	5		10
37	129.0	147.0	21.1	1.92 [1.93]	−42.2	8406	30.3	≤9		9
40	131.0	136.0	−12.2	1.70 [1.74]	−64.4	7382	22.9	>40		9
41	125.0	141.0	−6.9	1.79 [1.77]	297.4	8013	27.5	32.0		9
42	91.0	137.0	30	1.87 [1.84]	—	—	—	19		12
44	—	—	—	2.09	—	7208	25			12
45	—	129.8	12.1	1.75	256.2	8504	31.9	3.5	40	10
48	—	180.7	−75.0	1.46	125.4	6903	15.2	>40	>360	10
49	—	141.2	19.7	1.87	−60.7	9112	38.1	6	80	10
50	—	140.4	17.1	1.83	−148.6	8807	34.9	3.5	60	10
62	—	270.0	−10.7	1.88	104.8	7252	22.1	4	240	10
64	—	123.5	−15.8	1.73	304.2	8492	29.4	8	360	10
65	—	125.6	−9.2	1.85	348.2	8862	35.2	7.5	160	10
69	—	181.6		1.92	441.6	8973	37.3	10		11
74	—	138.6		1.67	721.9	9482	37.8	11		11
76	—	157.9		1.56	553.5	8424	27.2	25		11
77	—	74.0	−10.7	1.94 [1.94]	−1230.1	6784	19.3			13
78	—	76.0	12.9	1.94 [1.94]	−86.1/−0.35	8485	32.6			13
79	—	118.0	−12.8	1.98 [1.98]	−1086.6	7219	21.9			13
80	—	120.0	9.1	—/1.84	67.8	8482	32.0			13
81	—	60.0	−20.0	—/1.64	61.1	7657	23.7			13
TNT	81.0	295.0	−18.6	1.65	−67.0	6881	19.5	15	—
TATB		324.0	−18.6	1.93	−139.3	8144	31.2	50	360
RDX	206.0	230.0	0	1.82	80.0	8748	35.2	7.4	120
HMX	282	287	0	1.94	116.1	9221	41.5	7.0
PETN	143.0	160	15.2	1.78	−502.8	8564	31.4	2.9

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The analogous reactions of the dibromo substrate 5 with TCICA showed that the products obtained were a function of the quantity of TCICA used (Scheme 5). The azo complex (E)-1,2-bis[(E)-1-(bromoimino)-2-chloro-2,2-dinitroethyl}-diazene (29) was found as the major product when a slight excess of TCICA was reacted with 5. When five equivalents of 5 were reacted with one equivalent of TCICA, the three azo compounds (E)-1-[(E)-2-bromo-1-(bromoimino)-2,2-dinitroethyl-2-[(E)-2-bromo-1-(chloroimino)-2,2-dinitroethyl}diazene (30),⁹ (E)-1,2-bis[(E)-2-bromo-1-(bromoimino)-2,2-dinitroethyl}diazene (31),⁹ and 1-bromo-1,1-dinitro-2-(N-chloroamidino)-ethane (32)⁹ were formed. However, only 30 could be isolated. As in the preparation of 4 and 29, the quantity of products formed was a function of the amount of TCICA used (Scheme 5) and 31 was not observed.⁹ Attempts to incorporate additional nitro groups into azo-bridged compound (37b) via treatment of 29 or 37 with 100% HNO₃/acetic anhydride (Ac₂O) failed (Scheme 5).⁹

It was expected that the introduction of a methyl group would enhance the stability of the azo derivatives of FOX-7. However, when substrate 38 (ref. 9) was reacted with TCICA, the azo compound 39 was not detected; rather, only the chlorinated compounds, 2-chloro-N-methyl-2,2-dinitroacetimidamide (40) and (E)-N,2-dichloro-N-methyl-2,2-dinitroacetimidamide (41), resulted (Scheme 5).⁹

The physical properties of the halogenated FOX-7 derivatives (4, 5, 29, 30, 33–37, 40, 41) are summarized in Table 1.^9,10 The calculated detonation pressures lie between 23 GPa for 40 and 37 GPa for 5, and the detonation velocity range from 7102 m s⁻¹ for 29 to 8406 m s⁻¹ for 37, exceeding those of PETN (except 40 and 41). Only the performance of 5 and 30 exceeds that of RDX. The impact sensitivities (IS) of the halogenated FOX-7 derivatives range from 2 to >40 J, with most of the azo compounds (29, 30, 33, 34, 36, 39) being very sensitive. The remaining compounds, 4, 5, 36, 37 are less sensitive than RDX.^9,10

Hypergolic fuels are special propellants that ignite spontaneously upon contact with an oxidizer.² An interesting feature is that the halo FOX-7 derivatives behave as hypergolic oxidizers. Hypergolic testing of the pure halogenated FOX-7 derivatives with hydrazine hydrate and monomethylhydrazine indicate that these materials could be used as hypergolic oxidizers with common rocket fuels. This is a new role for compounds 33 and 34 and allows us to gain a better understanding of the chemistry of FOX-7 itself.^9,10

3. Polynitro functional derivatives of FOX-7

The nitro group is an important constituent of HEDMs; its presence in molecules contributes greatly to the overall energetic performance. Additionally, the nitro group enhances the density and oxygen balance of energetic materials, which improves their detonation performance.¹⁷ Introducing additional nitro groups is a practical strategy to generate energetic FOX-7 derivatives.

The first nitration reaction of FOX-7 was achieved using a mixture of nitric acid, trifluoroacetic anhydride, and trifluoroacetic acid, resulting in compound 6 (Scheme 6).¹⁵ However, 6 is very unstable at RT. During the reaction of FOX-7 with different acids, the simple single-step reaction of FOX-7 with an excess of fuming HNO₃ gave the interesting compound tetranitroacetimidic acid (TNAA, 42) in high yield (≥93%). It is stable at RT for long periods and is non-hygroscopic.¹² TNAA has an interesting molecular structure and was observed to be more stable toward impact than ADN and AP, with enhanced or comparable properties. It suggests that TNAA has a high potential for replacing AP as an oxidizer for applications in solid rocket propellants and missiles.

Scheme 6 Polynitro derivatives of FOX-7.

The reaction chemistry of TNAA (42) leads to some interesting insights regarding its synthesis and reactivity. When it was reacted with pyridine/HF, the long-known pyridinium trinitromethanide (43) was isolated. The reactivity of 42 with simple bases (NaOH, Na₂CO₃, NaHCO₃, etc.) resulted in the corresponding trinitromethanide salts. These results reflect the highly reactive nature of the amide moiety toward bases. The formation of 1-amino-2,2,2-trinitroethaniminium triflate–triflate hydrate (44) represents a very promising energetic precursor for energetic salts because of its high nitrogen and oxygen content (Scheme 6).¹²

The cyclic FOX-7 derivatives of 2-(dinitroethylene)-imidazolidine (9), 2-(dinitromethylene)hexahydropyrimidine (10) and 2-(dinitromethylene)-1,3-diazepan (11) were prepared by Baum, et al. through the reaction of the same diamines with 1,1-diiodo-2,2-dinitroethene.¹⁸ Currently, these derivatives can be easily prepared via the cyclic products of the reactions of FOX-7 with these diamines. The nitration of 9, 10 and 11 resulted in 3-nitro-2-(trinitromethyl)-1,3-diazacyclopent-1-ene (45), 3-nitro-2-(trinitromethyl)-1,3-diazacyclohex-1-ene (46), and 3-nitro-2-(trinitromethy1)-1,3-diazacyclohept-1-ene (47), respectively.^10,18 These products were formed by attack of the nitro group on the dinitro carbon and amidine nitrogen (Scheme 6). This behavior was also observed by Baum, et al.¹⁸

More recently,¹⁰ hexahydro-3-tert-butyl-2,2-dinitromethylene-1,3,5-triazine (48) was prepared from the Mannich condensation of FOX-7, HCHO, and tert-butylamine (Scheme 6). Compound 48 was nitrated using a mixture of HNO₃ and Ac₂O to give 1,2,3,4-tetrahydro-1,3-dinitro-6-trinitromethyl-1,3,5-triazine (49). The nitration of another cyclic amine derived from FOX-7, 18, gave a similar trinitromethyl derivative, 4-(trinitromethyl)-3-nitro-dihydro-2H-1,3,5-oxadiazin (50) (Scheme 6).¹⁰ Their relatively high hydrolytic and thermal stabilities, coupled with their good calculated detonation properties, suggest that these oxygen-rich polynitro derivatives of FOX-7 may be attractive candidates for energetic applications (Table 1). Unfortunately, those polynitro derivatives 45, 49 and 50 are quite sensitive to impact and friction based on their reported IS and FS values (Table 1).¹⁰

4. 1-Amino-1-hydrazo-2,2-dinitroethene (H-FOX) (12) and its derivatives

The synthesis of 1-amino-1-hydrazo-2,2-dinitroethene (12 or H-FOX¹³) through the reaction of FOX-7 with hydrazine is well known (Scheme 1).¹⁹ However, based on the literature and first-hand experience in our laboratories, anhydrous 12 is prone to spontaneous energetic decomposition, which makes it extremely dangerous relative to FOX-7 which is both insensitive and chemically stable.^4,5 In view of the unexpected instability of 12, a warning has been issued regarding the potential hazards associated with the preparation and handling of this material.²⁰ Likely for this reason, except for the hydrazinium salt (3), only a limited number of derivatives of 12 were synthesized prior to 2011. For example, in 2009, the reaction of 12 with NaNO₂ in AcOH and H₂O provided 1-tetrazolium-5-dinitromethylide (51) (Scheme 7).²¹

Scheme 7 Synthesis of H-FOX (12) and its derivatives.

The presence of the hydrazine moiety in 12 enables this compound to react readily with aldehydes and ketones. Recrystallization in acetone gives rise to crystals of hydrazone 52 [2,2-dinitro-1-(2-(propan-2-ylidene)hydrazinyl)ethanamine].²¹ In 2011, Astrat'ev et al. carried out a series of reactions of 12 to introduce a dinitromethylene group into the molecule.²¹ The reactions of 12 with aromatic aldehydes and ketones (Scheme 7) proceed more smoothly than those with aliphatic aldehydes. In particular, 12 does not react with β-diketones to form pyrazoles, but instead produces the corresponding monoazomethine derivatives, such as 2-amino-1,1-dinitro-7-oxo-3,4-diazaocta-2,4-diene (54)²¹ and 2-amino-1,1-dinitro-7-carboethoxy-3,4-diazaocta-2,4-diene (55).²¹

Interaction of 12 with acetonylacetone results in the formation of bis-azomethine (53). Furthermore, the observation of enhanced reactivity of the N2 atom of the hydrazine moiety is supported by the results of acylation. Thus, the acylation of 12 with Ac₂O yields the monoacetyl derivatives acetic 2-(1-imino-2,2-dinitroethyl)hydrazide (56) and propanoic 2-(1-imino-2,2-dinitroethyl)hydrazide (57). Compound 56 failed to give hydrazones with aldehydes because of the acylation of the N2 atom. The reaction between phenyl glyoxal and 12 at the aldehyde group results in 2-[(1-amino-2,2-dinitroethylidene)-hydrazino]-1-phenyl-ethanone (58); however, cyclic condensation products are not formed.²¹

The product obtained when 12 is reacted with (CHO)₂ is pH dependent, e.g., at pH 2–3, bis-hydrazone 2,9-diamino-1,1,10,10-tetranitro-3,4,7,8-tetraazadeca-2,4,6,8-tetraene (59) is obtained. Meanwhile, at pH 8–9, condensation gives the heterocyclic 3-(dinitromethyl)-1,2,4-triazine potassium salt (62). Acidification of a solution of 62 yields the hydration product 3-dinitromethyl-4,5-dihydro-l,2,4-triazin-5-ol (63).

l,2,4-Triazine (62) in aqueous media in the presence of a mineral acid was condensed with 1-nitro-3-amino-guanidine or 12 to prepare the bicyclic derivative of 7-(dinitromethylene)-octahydro-N-nitro-1,2,4-triazino[1,2,4]triazin-3-amine (64) and 3,7-bis(dinitromethylene)-octahydro-[1,2,4]-triazino-[6,5-e][1,2,4]triazine (65), respectively. Theoretical calculations indicate that 65 exhibits better detonation properties than 64 and compares favorably with RDX (Table 1); compound 65 may thus may have potential as an HEDM.

3-Dinitromethyl-l,2,4-triazole (66) was prepared by the reaction of 12 with triethylorthoformate in AcOH. Compound 12 with triethylorthoformate in the presence of sodium acetate gives only 2-amino-l,1-dinitro-5-ethoxy-3,4-diazaocta-2,4-diene (67). Furthermore, the high reactivity of 12 makes it a valuable precursor for the syntheses of nitrogen heterocycles, including the dinitromethylene moiety.

5. Salts and complexes of FOX-7 and its derivatives

FOX-7 is a representative “push–pull” nitro-enamine compound that contains a highly polarized carbon–carbon double bond. For this reason, FOX-7 and H-FOX have been shown to react with strong alkalis to generate the corresponding anionic substrates (1b and 12b) (Scheme 8).¹³

Scheme 8 Acid–base equilibria of FOX-7 and H-FOX under various pH conditions.

Guanidinium (68) and potassium (69) salts of FOX-7 (Scheme 9) were first synthesized by Anniyappan et al.²² Ag(FOX-7) (70) was first prepared by metathesis of 68 or 69 with AgNO₃ in an aqueous solution (Scheme 9). Compound 70 is light-sensitive and somewhat sensitive to impact.²³ Compound 71, Cu(FOX-7)₂(H₂O)₂, was prepared via the metathetical reaction of 69 in an aqueous solution of Cu(NO₃)₂ to give essentially the quantitative precipitation of 71. It is less stable thermally than FOX-7 or 68.²⁴ In 2010, Xu et al. prepared Rb(FOX-7)·H₂O (72) and Cs(FOX-7)·H₂O (73) and studied their thermal behaviors.²⁵ However, the lithium salt and sodium salt could not be prepared because of the weak acidity of FOX-7.²¹

Scheme 9 Salts of FOX-7 and H-FOX.

All alkali-metal salts of FOX-7 contain a H₂O molecule of coordination – confirmed by the crystal structure and thermal decomposition of 69·H₂O.²⁵ Anion–cation interactions with coordinated H₂O are strong, as indicated by analysis of the crystal structure; thus, the coordinated H₂O is quite difficult to volatilize during the thermal decomposition processes. However, 68 does not contain any coordinated H₂O. The T_b values (critical temperature of thermal explosion) for this series of compounds decrease in the order 73 (223.73 °C) > 72 (216.22 °C) > 69·H₂O (212.02 °C) > FOX-7 (207.08 °C) > 68 (201.72 °C).⁹

The hydrazinium (74) and potassium (75) salts of 12 were synthesized through the reaction of FOX-7 (1) or 69 with hydrazine hydrate. The guanidinium salt of H-FOX (76) was prepared by treating 12 with guanidine carbonate.²¹ On the basis of their relatively high detonation properties, these anionic salts of 12 have potential applications as energetic materials.

Although salts with anions of FOX-7 and 12 have been prepared, there are few reports that describe the reaction of FOX-7 and H-FOX with acids to obtain the corresponding cationic substrates. In 2015, Vo et al. synthesized a series of salts containing the first cationic species of both FOX-7 and 12 (ref. 13) by reacting each with a variety of strong acids. The cationic FOX-7 and 12 salts, viz., 1-amino-2,2-dinitroethaniminium triflate (77), 1-amino-2,2-dinitroethaniminium perchlorate (78), 1-amino-1-hydrazino-2,2-dinitroethaniminium triflate (79), 1-amino-1-hydrazino-2,2-dinitroethaniminium perchlorate (80) and 1-amino-1-hydrazino-2,2-dinitroethaniminium chloride (81) were obtained. All of the salts exhibit energetic properties superior to TNT, and some exhibit properties comparable to those of FOX-7 and RDX, which demonstrates their strong potential.

In 2011, following our research regarding the preparation of salts of FOX-7, the syntheses of a series of FOX-7-based metal complexes to investigate their reaction behavior was initiated.²³ Ag–NH₃–FOX (82) was the first reported complex of FOX-7; it was readily obtained from the reaction of 70 with aqueous ammonia or the reaction of 68 or 69 with AgNO₃ in aqueous ammonia (Scheme 10).²³ Since the first report of 82, the syntheses of a series of FOX-7 complexes (82–98) with different ligands {amines – (ammonia,^23,24 methylamine,²³ propylamine,²³ dimethylamine,²³ ethylenediamine,²⁴ and 1,3-propanediamine,^24,26 etc.), (bipyridyl (bipy) or 1,10-phenanthroline (phen), etc.)} and metals – (Ag,²³ Zn,²³ Cu,^24,26 Ni,^24,26 etc.) have been reported.

Scheme 10 Metal complexes of FOX-7.

Silver complexes of FOX-7, 82–84, exhibit low IS (>40 J) and good thermal stability (T_d > 184 °C) and are hydrolytically stable.²³ The copper complexes of FOX-7 perform less well; however, 85–87 are less sensitive (12–15 J) than compound 88 (5 J). With the exception of 86 (T_d = 128.6 °C), all the other compounds (85, 87, and 88) are thermally (T_d = 166.8 °C (85) 163.2 °C (87), 169.9 °C (88)) and hydrolytically stable. Compound 89, Zn(NH₃)₂(FOX-7)₂, (89) decomposes at 183.2 °C. The thermal stability of 89 is higher than that of the analogous 88.²³ Different from the monoamino ligand in FOX-7 copper complexes, 85–87, the bidentate amines were used as ligands in the copper and nickel complexes 90–98. These materials exhibit thermal stabilities between 156 and 225 °C; thus, they are slightly less stable than FOX-7 (261 °C) and K-FOX (226 °C).²³

These metal complexes of FOX-7 are either inner-sphere or outer-sphere depending on the amine used to stabilize the complex. The discovery of these new complexes contributes significantly to expanding the understanding of the chemistry of FOX-7, and demonstrates that much remains to be learned.

6. Conclusion

The main advances in the reactions of FOX-7 over the last five years, including condensation, halogenation, nitration, acid–base reactions and coordination reactions are covered. FOX-7 behaves as an excellent precursor to acyclic or cyclic compounds. Therefore, synthetic chemists have shown increased interest in both the syntheses and applications of its new derivatives. The continuing creative development of FOX-7 reactions will enable the synthesis of a range of nitro-containing compounds from readily available starting materials, which can be transformed into a range of other functionalities. FOX-7 reactions hold great promise to become a foundation precursor in EMs synthesis that will be of value to the wider synthetic chemistry community. Such applications of FOX-7 continue to increase rapidly in number.

Acknowledgements

The authors are grateful for the support of the Office of Naval Research (N00014-16-1-2089), the Defense Threat Reduction Agency (HDTRA 1-15-1-0028) and the National Natural Science Foundation of China (21377163).

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