Syntheses and properties of energetic cyclo-pentazolate cocrystals

Abstract

As a new type of polynitrogen species that is stable at room temperature, the pentazolate anion (cyclo-N₅⁻) has attracted much attention in the field of high-energy density materials, but its energy and stability are unbalanced. Cocrystallisation can balance their properties to some extent by forming new chemical compositions from existing cyclo-N₅⁻ compounds through non-covalent interactions. This article reviews the research progress of cyclo-N₅⁻ cocrystals in recent years, including synthetic methods, cocrystals of metal-N₅⁻ compounds, and cocrystals of nonmetallic pentazolate salts. The cocrystals of metal-N₅⁻ compounds mainly include metal-N₅⁻ solvates, cocrystals composed of metal-N₅⁻ compounds and amines/MSM, and metal-containing composite salts. The cocrystals of nonmetallic pentazolate salts include cocrystals composed of cyclo-N₅⁻ salts and solvents, cocrystals composed of cyclo-N₅⁻ salts and N-heterocyclic molecules, and non-metallic composite salts. The fascinating crystal structures (in some cases topological structures), stable forms, and physicochemical properties of representative cocrystals were highlighted. In addition, the future directions that need to be focused on in this field were pointed out, including the development of more preparation methods, especially those suitable for scaling up; higher precision calculation or testing of enthalpy of formation; improvement of their thermal stabilities; creation of cocrystals of cyclo-N₅⁻ salts and high-density, high-oxygen balance, high-energy oxidizers; and exploration of the formation mechanism.

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Fanle Meng

Fanle Meng received his bachelor's degree from the Jiangsu University of Science and Technology in 2020. He is currently a Ph.D. candidate at the School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology, China. His current research focuses on the design and synthesis of high-nitrogen energetic compounds.

Ming Lu

Ming Lu obtained his BSc in 1984, MSc in 1989 and PhD in 1999 from Nanjing University of Science and Technology. He then became a professor in 2001. He has published more than 200 papers, owns over 10 patents and has authored 5 books. His current research interest is focused on the synthesis and crystal engineering of energetic materials, pharmaceutical intermediates and green chemistry.

Yuangang Xu

Yuangang Xu is a professor and doctoral supervisor at the School of Chemistry and Chemical Engineering, Nanjing University of Science and Technology (NJUST). He received his PhD degree in Chemical Engineering and Technology (2020) from NJUST. His research interests include energetic cocrystals, polynitrogen materials, and hydrogen bonding organic frameworks.

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1 Introduction

High nitrogen energetic materials, as an important class of high-energy density materials (HEDMs), have great potential for military and civilian applications. Among them, polynitrogen species have attracted much attention due to their ultra-high energy and environmental friendliness.¹ The pentazolate anion (cyclo-N₅⁻) is a pentagon composed entirely of aromatic N–N bonds. Its thermal stability (a thermal decomposition temperature of about 100 °C, with an activation energy of about 100 kJ mol⁻¹)² is much better than that of the V-shaped N₅⁺ cation, and its enthalpy of formation is as high as 3.66 kJ g⁻¹ (256 kJ mol⁻¹).³ The stability and energy of cyclo-N₅⁻ have been accurately predicted by quantum chemistry studies.⁴ Over the past half century, extensive theoretical studies and experimental explorations have been carried out on stable HN₅ and cyclo-N₅⁻; however, the isolation of room-temperature stable cyclo-N₅⁻ compounds has been a great challenge for a long time. It was not until 2017 that Zhang et al.⁵ successfully synthesized a cyclo-N₅⁻ composite salt (N₅)₆(H₃O)₃(NH₄)₄Cl, whose structure was controversial but clearly confirmed the existence of cyclo-N₅⁻ in solid crystals. Subsequently, Xu et al.² reported a series of metal cyclo-N₅⁻ hydrates. Since then, synthetic studies of cyclo-N₅⁻ derivatives have entered a new phase. At present, the highest detonation velocity of the synthesized cyclo-N₅⁻-containing compounds exceeds 10 000 m s⁻¹, but their density (<1.7 g cm⁻³) and decomposition temperature (<125 °C) are generally not high, and they also have varying degrees of hygroscopicity.⁶ These defects seriously limit the development of cyclo-N₅⁻ based energetic materials, and currently, the commonly used modification methods mainly include coating and doping. However, these methods do not change the internal composition and crystal structure of energetic materials, and the addition of non-energetic components reduces their energetic performance. Therefore, to further improve the modification effect, it is necessary to start from the internal composition and structure of the target molecules and combine the cyclo-N₅⁻ compounds and suitable coformers microscopically in the same crystal lattice through intermolecular non-covalent interactions to form cyclo-N₅⁻-containing cocrystals with enhanced stability and performance.

In recent years, cocrystallization has attracted much attention and has become an effective way to balance the energy and safety of energetic materials.⁷ By purposefully introducing a second molecule called a coformer, the intermolecular interactions that determine the stacking arrangement of the target energetic molecule can be disrupted and replaced, resulting in a novel crystal structure called an energetic cocrystal, which incorporates both molecules in a well-defined stoichiometry. Energetic cocrystals have been reported to have improved thermal stability, enhanced detonation properties, and reduced sensitivity to external stimuli.⁸ However, while cocrystallization has now been widely studied, the advancement of cocrystallization of energetic ion salts has been slow, with typical examples being 2,4,6,8,10,12-hexanitrate-2,4,6,8,10,12-hexaazaisowoodzane (CL-20) : 1-amino-3-methyl-1,2,3-triazolium nitrate,⁹ 2 ammonium dinitramide (ADN) : pyrazine-1,4-dioxide,¹⁰ 2 ammonium nitrate (AN) : 5,5′-dinitro-2H,2H′-3,3′-bi-1,2,4-triazole (DNBT), and 2 ADN : DNBT cocrystals.¹¹ The cyclo-N₅⁻ ion carries a negative charge and is not only a good hydrogen bond acceptor but also a planar structure that easily experiences π–π stacking, exhibiting the structural characteristics of cocrystallization. Therefore, cyclo-N₅⁻-containing energetic cocrystals can be formed by suitable coformers to change the molecular composition and crystal structure of cyclo-N₅⁻ salts, increase their decomposition temperatures and densities, reduce their sensitivities, improve their hygroscopicity, and prepare new high-performance and high-stability cyclo-N₅⁻ based energetic materials.

This frontier article reviews the methods for synthesizing cyclo-N₅⁻ containing energetic cocrystals in recent years, their crystal structures, classification, thermal stabilities, energy properties (density, detonation velocity, and detonation pressure), and sensitivities. In order to encourage innovative research on cyclo-N₅⁻ cocrystals, the challenges and application prospects related to them are outlined.

2 Synthetic methods for accessing cyclo-N₅⁻ cocrystals

Cyclo-N₅⁻ has D_5h symmetry, with all of its five nitrogen atoms being sp² hybridized.¹² As shown in Fig. 1, it has a dual aromatic skeleton with six π-electrons delocalized in a conjugated manner above and below the cyclo-N₅⁻ ring plane, conferring cyclo-N₅⁻ π-aromaticity; meanwhile, in the equatorial plane of the cyclo-N₅⁻ ring, the lone pair of electrons from the five N atoms leads to flower-like delocalization of the electrons, indicating the σ-aromaticity of cyclo-N₅⁻. Thus, its unique structure implies good adaptability to form cocrystals with other molecules.

Fig. 1 The dual aromaticity of the cyclo-N₅⁻ ring.

Cocrystal preparation methods include solvent evaporation, solvent/nonsolvent, cooling crystallization, grinding methods, melting/condensation crystallization, resonant acoustic methods, slurry methods, solvent-suspension methods, and self-assembly methods, all of which have been widely reported thus far (Fig. 2).¹³ Currently, there are two main methods for the syntheses of cyclo-N₅⁻ cocrystals: solvent evaporation and self-assembly methods. However, neither method can proceed without a metathesis reaction. Since 2017, our group has reported on metathesis reactions of [Na(H₂O)(N₅)]·2H₂O with chlorides or nitrates.^2,3 Similar to our work, metathesis reactions of [Mg(H₂O)₆(N₅)₂]·4H₂O with some sulfates were subsequently developed.¹⁴ However, unless the target product can precipitate, both routes require repeated recrystallization in other cases and are cumbersome and inefficient since NaCl and MgSO₄ are difficult to remove completely from commonly used solvents. In 2019, we found that protonation and metathesis reactions can be completed in one pot,¹⁵ but only two cyclo-N₅⁻ hydrates were synthesized. Then metathesis reactions driven by the precipitation of AgCl and BaSO₄ were invented in the same year.^16,17 In particular, the method driven by AgCl has the advantages of simple, rapid, high yields, and high universality and has become the most widely used method for synthesizing cyclo-N₅⁻ derivatives. These methods provide a variety of coformers for creating cyclo-N₅⁻ cocrystals.

Fig. 2 Synthetic methods of cocrystals.

3 Cocrystals of metal-N₅⁻ compounds

Cyclo-N₅⁻ cocrystals can be divided into cocrystals of metal-N₅⁻ compounds and non-metallic pentazolate salts. Among them, cocrystals of metal-N₅⁻ compounds can be further divided into three categories: metal-N₅⁻ solvates, cocrystals composed of metal-N₅⁻ compounds and amines/MSM, and metal-containing composite salts. Similarly, cocrystals of non-metallic pentazolate salts can also be divided into three categories: cocrystals composed of cyclo-N₅⁻ salts and solvents, cocrystals composed of cyclo-N₅⁻ salts and N-heterocyclic molecules, and non-metallic composite salts.

3.1 Metal-N₅⁻ solvates

Our early research suggested that the coordination of cyclo-N₅⁻ to metals reduces its decomposition barrier in the gas phase, making it kinetically unstable (e.g. the thermal and kinetic stabilities have the order of Fe(N₅)₂ < Ni(N₅)₂ < NaN₅ < cyclo-N₅⁻).^18,19 Subsequent experiments proved that these metal-N₅⁻ compounds formed hydrate cocrystals with water molecules through hydrogen bonds and sometimes coordination bonds, improving their stability and safety. To date, 12 metal-N₅⁻ hydrates (1–12) have been synthesized,^2,20–26 of which 4 is also a solvate (Fig. 3). [Na(H₂O)(N₅)]·2H₂O (2) was first reported by Xu et al.² and is considered the most important precursor for the synthesis of metal hydrates. Two types of hydrogen bonds, O–H⋯O and O–H⋯N, constitute the main stabilization mechanism for stabilizing crystal 2 and its cyclo-N₅⁻, constructing an intralayer 3D hydrogen bond network in 2.²⁷ The O–H⋯O bond is stronger and can stabilize the crystal, while O–H⋯N is connected with N in cyclo-N₅⁻, so it mainly stabilizes cyclo-N₅⁻. For [M(H₂O)₄(N₅)₂]·4H₂O (M = Mn, Fe, Co, and Zn; 7, 8, 10, 12), two types of water (i.e., coordinated water (c-H₂O) and hydrogen-bonded water (h-H₂O)) significantly contribute to stability but via different modes (Fig. 4). The role of c-H₂O is to bind with M to reduce the interactions between M and cyclo-N₅⁻, leading to a less active cyclo-N₅⁻ and higher kinetic barriers (E_as) and reaction energies (ΔEs) for its decomposition. The E_as and ΔEs increased by an average of 7.6 and 15.1 kcal mol⁻¹, respectively. Compared with the reactant, h-H₂O enforces fewer electrostatic interactions on cyclo-N₅⁻ in the transition state to suppress decomposition, similar to c-H₂O.²⁸

Fig. 3 Metal-N₅⁻ solvates 1–12.

Fig. 4 The two types of first-shell water surrounding and stabilizing an M(N₅)₂.

Some representative cocrystals with different but fascinating structures can be obtained by adjusting the proportion of the coformer (H₂O). The 1D coordination chain in the single-crystal structure of 2 is demonstrated in Fig. 5a, where Xu et al.²¹ synthesized a new 3D open-framework (3/MPF-1) by changing the stoichiometric ratio of NaN₅ to H₂O from 1 : 3 to 8 : 3. Compound 3/MPF-1 exhibits an aesthetic zeolitic MEP topology featuring two types of nanocages, Na₂₀N₆₀ and Na₂₄N₆₀, in which the strong coordination bonds between cyclo-N₅⁻ and Na⁺ play vital roles in stabilizing the cyclo-N₅⁻ anions (Fig. 5b–f).

Fig. 5 (a) Single-crystal X-ray structure of 2 (a 1D chain). (b) Coordination environments of Na⁺ and cyclo-N₅⁻ in 3. (c) Na₂₀N₆₀ nanocage and simplified 5¹² topology. (d) Na₂₄N₆₀ nanocage and simplified 6²·5¹² topology. (e) Connection mode between the 5¹² and 6²·5¹² nanocages. (f) 3D framework of 3 and its zeolitic MEP topology.

The different types of coformers can also lead to different cocrystal structures. Xu et al.²³ synthesized a UNJ-type zeolite topology framework, [Na₄(N₅)₄(H₂O)₂]·H₂O·2MeOH (4), by introducing MeOH molecules into 2. The framework features multiple 1D tubular channels filled with MeOH and H₂O molecules through coordination and hydrogen bonding, whose walls are constructed from Na⁺ and cyclo-N₅⁻ (Fig. 6). Each channel is enclosed by six identical channels, indicating that the adjacent nanotubes are fused together by edge sharing. The network has helical channels along the c-axis direction. Each enclosed helical channel is assembled from two right-handed helical chains by sharing the sides of multiple pentagons (Fig. 6d).

Fig. 6 Single-crystal X-ray structure of 4. (a) Coordination environments of cyclo-N₅⁻, Na⁺, and water molecules in 4. (b) 3D framework of 4, the single channel-like structure, and the simplified topology. (c) The 3D zeolite-like UNJ topological network viewed along the c axis. (d) A right-handed double helical chain composed of cyclo-N₅⁻ and Na⁺. The hydrogen and coordination bonding interactions in the channels are also shown.

Due to the low density of the coformer, the crystal densities of cocrystals 1–12 are below 1.7 g cm⁻³, even at low temperatures.^2,20–26 These cocrystals are stable at room temperature, and most of them have a decomposition temperature of approximately 100 °C (Table 1). Notably, cocrystal 6 has the highest thermal stability among all metal-N₅⁻ solvates, and studies have shown that hydrogen bonding, van der Waals, and π–π stacking interactions play significant roles in the stabilization mechanism.²⁴ Cocrystals 1 and 3 also exhibit remarkable thermal decomposition temperatures, reaching 139 °C and 129 °C, respectively. In contrast, the decomposition temperature of 10 is only 59 °C, which may be due to the stronger interaction between Co²⁺ and cyclo-N₅⁻ (Co–N: 2.122 Å) compared to other complexes. Once c-H₂O is lost, cyclo-N₅⁻ undergoes N–N bond cleavage and decomposition.

Table 1 Properties of 1–12

Comp.	d (g cm^−3)	T d (°C)
a Density from single-crystal X-ray diffraction. b Decomposition temperature (DSC).
1	1.37 (298 K)	139
2	1.47 (170 K)	104
3	1.30 (100 K)	129
4	1.61 (100 K)	110
5	1.44 (205 K)	104
6	1.38 (150 K)	141
7	1.61 (205 K)	104
8	1.60 (205 K)	115
9	1.43 (193 K)	109
10	1.70 (170 K)	59
11	1.65 (296 K)	102
12	1.67 (205 K)	108

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3.2 Cocrystals composed of metal-N₅⁻ compounds and amines/MSM

The structural formula of the cocrystals composed of metal-N₅⁻ compounds and amines/dimethyl sulfone (MSM) is shown in Fig. 7, where three are NaN₅ cocrystals (13–15)^29–31 and the other two are AgN₅ cocrystals (16 and 17).^32,33 We recently discovered that 13²⁹ also has a UNJ-type zeolite topology framework (Fig. 8) similar to that of 4. The framework features multiple 1D tubular channels filled with 4-amino-1,2,4-triazole molecules through coordination and hydrogen bonds. Each channel is also surrounded by six identical channels, indicating that adjacent nanotubes are fused together through edge sharing. Similar to 4, the network also has helical channels along the c-axis. But each enclosed helical channel is assembled by two left-handed helical chains sharing multiple pentagonal sides (Fig. 8d).

Fig. 7 Cocrystals (13–17) composed of metal-N₅⁻ compounds and amines/MSM.

Fig. 8 Single-crystal X-ray structure of 13. (a) Coordination environments of cyclo-N₅⁻, Na⁺, and 4-amino-1,2,4-triazole molecules in 13. (b) 3D framework of 13, the single channel-like structure, and the simplified topology. (c) The 3D zeolite-like UNJ topological network viewed along the c axis. (d) A left-handed double helical chain composed of cyclo-N₅⁻ and Na⁺. The hydrogen and coordination bonding interactions in the channels are also shown.

Cao et al. obtained two cocrystals, 14 (MPF-2) and 15 (MPF-4), by adding NaN₅ hydrate and MSM at a specific ratio to a DMSO/amine aqueous solution using MSM as a coformer.^30,31 Each cyclo-N₅⁻ ring in 14 bridges Na⁺ through η³, η⁴, and η⁵ coordination modes to form a “chiral bowl-shaped” Na₁₆N₅₀ molecular container (Fig. 9). These containers are sealed and assembled by parallel arranged trapezoidal 1D helical chains through a 2D chiral layer “molecular plane” formed by sharing cyclo-N₅⁻. Each container has an ovoid cavity with a volume of approximately 13.4 × 9.1 × 8.8 ų occupied by one DMSO guest molecule fixed by hydrogen bonds. It is worth mentioning that the Na₁₆N₅₀ bowl in 14 is closely related to the Na₂₀N₆₀ (simplified 5¹²) nanocage in 3, which is formed by removing 4 Na⁺ vertices from the 5¹² nanocage (Fig. 9c). Both 4-amino-1,2,4-triazole and MSM can be regarded as bidentate ligands, but the stoichiometric ratios of NaN₅ to them in their cocrystals (13, 15) are different (2 : 1 vs. 3 : 1). Even more surprising, both of these cocrystals have a homochiral framework with two left-handed helices interpenetrating each other to form a UNJ topology (Fig. 10). Since the structure of 15 is extremely similar to that of 13, we do not discuss it in detail here.

Fig. 9 Single-crystal X-ray structure of 14. (a) Coordination environments of Na⁺ in 14. (b) Coordination modes of cyclo-N₅⁻ in 14. (c) The 3D framework of 14 viewed along the a axis, and the schematic drawing of the assembly of the bowl and plane.

Fig. 10 Single-crystal X-ray structure of 15. (a) Coordination environments of cyclo-N₅⁻, Na⁺, and MSM molecules in 15. (b) 3D framework of 15 viewed along the a axis. (c) Tubular channels composed of cyclo-N₅⁻ and Na⁺ running along the a axis. (d) The zeolite-like UNJ topological network and a schematic drawing of the channel structures. (e) A left-handed double helical chain, the topological representation and a schematic drawing of the double helical chain.

AgN₅ ³⁴ is one of the most important intermediates for the syntheses of various cyclo-N₅⁻ energetic materials. In order to improve the thermal stability and safety performance of AgN₅, cocrystals 16 and 17 were synthesized (Fig. 11).^32,33 Their coformers are all amines, and their stoichiometric ratio to AgN₅ is 1 : 2. Similar to AgN₅, there are also two types of coordinated Ag⁺ (different connected configurations) in 16. However, only one type of Ag⁺ was observed in 17. Besides, only one type of cyclo-N₅⁻ ring is present in AgN₅ and 17, while in 16, there are two types of cyclo-N₅⁻ rings that are coordinated to three and four Ag⁺ ions. Moreover, the π–π interactions between cyclo-N₅⁻ anions in AgN₅ are much stronger than those in 16 and 17. Therefore, the 3D frameworks of 16 and 17 are different from that of AgN₅ (PtS topology). The negatively charged 3D supramolecular framework Ag₃(N₅)₄ in 16 is regularly filled with Ag(NH₃)₂⁺ counter ions by hydrogen bonds, while the 2D AgN₅ networks in 17 are connected layer-by-layer by ethylenediamine molecules through Ag–N coordination bonds, thereby forming a regular 3D supramolecular network.

Fig. 11 Single-crystal X-ray structures of 16, 17, and AgN₅. (a) Coordination environments of Ag⁺ and cyclo-N₅⁻ in 16. (b) 3D framework of 16. (c) 3D framework of Ag₃(N₅)₄ in 16. (d) Coordination environments of Ag⁺, cyclo-N₅⁻, and EDA in 17. (e) 3D framework of 17. (f) 2D structure in17. (g) Coordination environments of Ag⁺ and cyclo-N₅⁻ in AgN₅. (h) 3D framework of AgN₅. (i) The 1D structure in AgN₅. Yellow-dashed lines indicate the hydrogen bonds.

The crystal densities of two AgN₅ cocrystals (16 and 17) are higher than those of three NaN₅ cocrystals (13–15), with the highest density of 16 reaching 3.21 g cm⁻³ (Table 2). However, the thermal stabilities of 16 and 17 (≤105 °C) are worse than those of 13–15 (119–127 °C). The detonation velocities (D: 6427 and 6272 m s⁻¹) and detonation pressures (P: 29.00 and 23.51 GPa) of 16 and 17 are slightly lower than those of AgN₅. Compared with those of extremely sensitive AgN₅, the mechanical sensitivities of the two cocrystals are significantly reduced (Table 2).

Table 2 Properties of 13–17 and AgN₅

Comp.	d (g cm^−3)	T d (°C)	D (m s^−1)	P (GPa)	IS^e (J)	FS^f (N)
a Density from single-crystal X-ray diffraction. b Decomposition temperature (DSC). c Detonation velocity. d Detonation pressure. e Impact sensitivity. f Friction sensitivity.
13	1.73 (173 K)	127	7863	26.4
14	1.65 (296 K)	119
15	1.66 (295 K)	119
16	3.21 (220 K)	90	6427	29.00	7.5	60
17	2.73 (170 K)	105	6272	23.51	15	120
AgN5	3.02 (173 K)	98	7782	34.67	0.5	1

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3.3 Metal-containing composite salts

There are 14 metal-containing composite salts in cyclo-N₅⁻ cocrystals, including two (18²⁹ and 19³⁵) with two cations, nine (20–23,^25,29,36,3726,³⁴27,³⁴ and 29–31³⁸) with two anions, two (24²⁵ and 25²⁵) with three anions, and one (28³⁴) with two cations and two anions (Fig. 12). Cocrystal 18, the earliest synthesized metal-containing composite salt, has a zeolite-like 3D framework with a SOD topology. 19 has a 3D heterometallic framework with left- and right-handed helical chains. Although 20 is a byproduct obtained during the synthesis of pentazolate N-oxide, we discovered the miraculous effect of coformers containing acetate ions on the structural regulation of metal-N₅⁻ compounds through the structure of 20 and subsequently developed cocrystals 21 and 22 on the basis of KN₅. The 3D topological network of KN₅ is shown in Fig. 13a, which consists of a number of parallelograms neatly arranged in the framework.¹⁶ Unlike KN₅, 21 has a zeolite-like UNJ topological network through hexagons (Fig. 13b), and cocrystal 22 has the same zeolite-like UNJ topological network through inverted triangles consisting of three small triangles and one hexagonal shape (Fig. 13c).

Fig. 12 Metal-containing composite salts 18–31.

Fig. 13 The 3D topological network of KN₅ (a), 21 (b), and 22 (c).

Cocrystals 23–25 have the same hexaaminocobalt(III) cation [Co (NH₃)₆]³⁺, but with different types and numbers of anions, which crystallize in the monoclinic C1₂/m1 (23) and C2/m (24 and 25) space groups, respectively. However, 30 and 31 have the same [Pb₄(OH)₄]⁴⁺ cubic cation and number of anions, except for the types of anions and the number of H₂O. They crystallize in triangular R3 (30) and orthorhombic Pca2₁ (31) space groups, respectively. It is worth mentioning that 26 is the only cyclo-N₅⁻ cocrystal composed of only two elements (Cu and N), but once it separates from the mother liquor, it can easily cause an explosion.

The densities of cocrystals 18–31 vary from 1.21 g cm⁻³ to 4.42 g cm⁻³ (Table 3), which is attributed mainly to the types of metal ions, coformed ions, and crystal water content, as well as the resulting differences in the crystal stacking patterns. Among these 14 cocrystals, 25 has the highest thermal decomposition temperature (163 °C), followed by 20 (139 °C) and 27 (130 °C). In order to evaluate the energetic performance of 23, detonation tests conducted in the literature have shown that the cocrystal has high priming ability and is expected to become a potential green primary explosive.²⁵

Table 3 Properties of 18–31

Comp.	d (g cm^−3)	T d (°C)
a Density from single-crystal X-ray diffraction. b Decomposition temperature (DSC).
18	1.21 (170 K)	118
19	1.55 (173 K)	106
20	1.71 (100 K)	139
21	1.75 (298 K)	100
22	2.10 (298 K)	100
23	1.72 (170 K)	98
24	1.63 (296 K)	104
25	1.60 (296 K)	163
26	2.62 (173 K)	90
27	2.59 (100 K)	130
28	2.34 (100 K)	121
29	2.85 (296 K)	89
30	3.91 (296 K)	97
31	4.42 (296 K)	110

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4 Cocrystals of non-metallic pentazolate salts

4.1 Cocrystals composed of cyclo-N₅⁻ salts and solvents

Hydrates (or solvates) of non-metallic pentazolate salts are very common. On the one hand, cations in non-metallic pentazolate salts are good donors of hydrogen bonds (rich in N–H), cyclo-N₅⁻ anions are good acceptors of hydrogen bonds, and water molecules are both excellent donors and acceptors of hydrogen bonds. During the crystallization process, owing to weak interactions or crystal stacking, smaller water molecules are needed to fill the crystal cell to form a cocrystal. On the other hand, water, as a green solvent with a strong ability to dissolve energetic salts, is inevitable in the synthesis of cyclo-N₅⁻ and its conversion to non-metallic salts. Fig. 14 shows the chemical formula of 10 hydrates (or solvates) of non-metallic pentazolate salts (32–41)^{15–17,39–43} and the stoichiometric ratio of cyclo-N₅⁻ salts to water (or solvent molecules) (1 : 0.5/1/2/3). Among them, 34–36 have –NH₂ rich triazolium cations; 37 and 38 have cage-like cations; and 39 and 40 have the same 5/5 fused imidazolium pentazolate. These cocrystals have a density below 1.6 g cm⁻³ and a thermal decomposition temperature of ≤115 °C (Table 4).

Fig. 14 Cocrystals (32–41) composed of cyclo-N₅⁻ salts and solvents.

Table 4 Properties of 32–41

Comp.	d (g cm^−3)	T d (°C)
a Density from single-crystal X-ray diffraction. b Decomposition temperature (DSC). Except for 83 and 100, other data are from the corresponding anhydrous salts.
32	1.49 (173 K)	106
33	1.46 (100 K)	98
34	1.49 (173 K)	96
35	1.58 (170 K)	83
36	1.52 (173 K)	95
37	1.23 (296 K)	79
38	1.48 (293 K)	92
39	1.66 (193 K)	115
40	1.61 (193 K)	115
41	1.49 (295 K)	100

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One of the strategies for reducing the mechanical sensitivity of highly sensitive energetic materials (such as metal-N₅⁻ compounds) is to form cocrystals of energetics and solvent molecules.⁴⁴ The sensitivities of non-metallic pentazolate salts are moderate, and there is no need to passivate them through hydrates or solvates. Therefore, researchers do not want to obtain hydrates or solvates of non-metallic pentazolate salts, but rather pure salts, to obtain more accurate structures, interactions, and physicochemical properties. However, H₂O₂ is both a solvent and a green oxidizer, with low environmental impact and insensitivity in its solvent form. If H₂O₂ can be embedded into cyclo-N₅⁻ salts with a negative oxygen balance through a solvation strategy, it is expected that the oxygen balance and energy of the developed cyclo-N₅⁻ cocrystal will be correspondingly improved. In 2020, Luo et al. successfully produced 41 by utilizing the effective strategy of a CL-20 : H₂O₂ (2 : 1) cocrystal⁴⁵ proposed by Matzger et al.

In cocrystal 41, H₂O₂ molecules are incorporated into the crystal lattices of NH₄N₅ to construct a novel 3D hydrogen-bonding network, where cyclo-N₅⁻ rings are layer-by-layer stacked and NH₄⁺ cations are embedded in the layers and connected by hydrogen bonding (Fig. 15). This cocrystal has a high oxygen balance and high D and P values (8938 m s⁻¹ and 26.37 GPa), which are much higher than those of NH₄N₅. In addition, 41 exhibits an astonishing specific impulse (I_sp: 260 s), which is approximately 15% higher than that of NH₄N₅ (225 s). Furthermore, the combustion products of 41 are mostly composed of clean N₂, H₂ and H₂O, indicating its great potential as a component of high-energy propellants.

Fig. 15 (a) Single-crystal X-ray structure of 41 (with Hirshfeld surfaces). (b and c) Stacking diagram of 41 viewed along the a and c axis.

4.2 Cocrystals composed of cyclo-N₅⁻ salts and N-heterocyclic molecules

Cocrystals composed of cyclo-N₅⁻ salts and N-heterocyclic molecules are relatively rare, with only four cocrystals (42–45) being reported (Fig. 16). Therefore, obtaining novel cocrystals of cyclo-N₅⁻ salts with a balance between high energy and good stability is a considerable challenge. The combination of a nonmetallic cyclo-N₅⁻ salt and a neutral molecule in a certain proportion to form a cocrystal compound has a positive effect on the detonation performance and hygroscopic properties of the cyclo-N₅⁻ salt. 42⁴⁶ is a cocrystal composed of 3,3-dinitroazetidine (DNAZ) and HDNAZ⁺N₅⁻ at a 1 : 1 stoichiometry, which is an unexpected product targeting a DNAZ⁺N₅⁻ salt. However, by studying its crystal structure, we found that the crystal is disordered with the H3A proton of the DNAZ cation fixed at 50% occupancy, indicating that the cation is completely occupied and semi-protonated. Therefore, it could be a (2 DNAZH)⁺N₅⁻ salt.

Fig. 16 Cocrystals (42–45) composed of cyclo-N₅⁻ salts and N-heterocyclic molecules.

In order to solve the hygroscopicity problem of N₂H₅N₅, pyrazine-1,4-dioxide (PDO) molecules were introduced to synthesize cocrystal 43, which reduced the moisture absorption of N₂H₅N₅ from 45% to 15%.⁴⁷ The PDO and cyclo-N₅⁻ rings are parallel, and the distance between them is 3.44 Å, indicating the presence of extensive π–π interactions. There are numerous hydrogen bonds in 43, which play an important role in the construction of 3D-cube layered stacking (Fig. 17). In addition, after the formation of the cocrystal, the density increased compared with that of N₂H₅N₅, but the sensitivity decreased.

Fig. 17 (a) Single-crystal X-ray structure of 43. (b and c) Stacking diagram of 43. (d) Moisture content curves of N₂H₅N₅ and 43 under a 75% relative humidity at 25 °C.

Cocrystals 44³⁹ and 45⁴⁸ differ only in their coformer, with one being 3,6,7-triamine-7H-[1,2,4]triazole[4,3-b][1,2,4]triazole (TATOT) and the other being melamine. Both of them have a layered stacking structure, and each cyclo-N₅⁻ in the 2D layer is fixed by seven hydrogen bonds (Fig. 18). Owing to the combined effects of various non-covalent interactions, 45 forms a hydrogen-bonded organic framework (HOF) with 1D pores, and cyclo-N₅⁻ rings occupy these pores and are stabilized by hydrogen bonds, π–π interactions between the cyclo-N₅⁻ rings, and attraction between the cyclo-N₅⁻ anions and the cationic HOF. These factors increase its thermal stability to 153 °C (Table 5), exceeding that of 44 (122 °C) and TATOTN₅ (121 °C).¹⁶ Overall, the D and P values of the four cocrystals composed of cyclo-N₅⁻ salts and N-heterocyclic molecules are 8029–8735 m s⁻¹ and 24.6–29 GPa, respectively (Table 5), which are slightly lower than those of RDX. But they are significantly less sensitive than RDX.

Fig. 18 Single-crystal X-ray structures of 44 and 45. (a) The position of cyclo-N₅⁻ in the 3 + 2 membered ring and the hydrogen bonds (dotted lines) accepted. (b) The position of cyclo-N₅⁻ in the 3 + 3 membered ring and the hydrogen bonds (dotted lines) accepted. (c) Layer-by-layer stacking of 44. (d) Layer-by-layer stacking of 45.

Table 5 Properties of 42–45

Comp.	d (g cm^−3)	T d (°C)	D (m s^−1)	P (GPa)	IS^e (J)	FS^f (N)
a Density from single-crystal X-ray diffraction. b Decomposition temperature (DSC). c Detonation velocity. d Detonation pressure. e Impact sensitivity. f Friction sensitivity.
42	1.67 (193 K)	116	8441	29.0	11	252
43	1.61 (296 K)	101	8735	26.6	35	300
44	1.66 (193 K)	122	8605	26.0	>40	>360
45	1.69 (100 K)	153	8029	24.6	>40	>360

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4.3 Non-metallic composite salts

The number of non-metallic composite salts of cyclo-N₅⁻ is much less than that of its metal-containing composite salts of only four (46–49).^5,15,49,50 Their common feature is that they contain both cyclo-N₅⁻ and Cl⁻ ions (Fig. 19). Among them, both 46 and 47 crystallize in the cubic Fd3̄m space group, with almost identical unit cell parameters (Fig. 20). Although the unusual structure of 46 was resolved by single-crystal X-ray diffraction analysis, the presence of hydronium ions (H₃O⁺) has not been confirmed by other experiments. Thus, a series of debates^51–54 have ensued, such as the correctness of O2 atom assignment, the possible existence of a HN₅ species in this salt, and the pathways and influencing factors for interconversion among HN₅ and cyclo-N₅⁻.

Fig. 19 Non-metallic composite salts 46–49.

Fig. 20 Single-crystal X-ray structures of 46 and 47. (a) Single-crystal X-ray structure of 46. (b) Hydrogen bonds (dotted lines) in the crystal of 46. (c) Unit cell stacking diagram of 46. (d) Single-crystal X-ray structure of 47. (e) Hydrogen bonds (dotted lines) in the crystal of 47. (f) Unit cell stacking diagram of 47.

A comparison of the crystal structures of these two cocrystals revealed the following points. First, the ordered H₃O⁺ (O1) in 46 formed only three hydrogen bonds with three cyclo-N₅⁻ rings, but a pair of lone-pair electrons on O1 did not have a hydrogen-bonding interaction with other atoms. The ordered NH₄⁺ (N6) in 47, at the same position as H₃O⁺ (O1) in 46, can form good hydrogen bonds with all the surrounding cyclo-N₅⁻ rings. Second, the isotropic temperature factors of the atomic thermal vibration for the two oxygen atoms, O1 and O2, in 46 are 0.082(3) and 0.071(3), respectively, which are 1.5–2.0 times greater than those of the other atoms (N1–N4 and Cl1). While the thermal vibration temperature factors of all the atoms in 47 are at the same level. Finally, 47 has lower R and wR indices than 46, and the goodness-of-fit on F² of 47 (1.075) is closer to 1 than that of 46 (1.208). In addition, the correctness of structure 47 was also supported by the experimental results of DSC-TG-MS, SEM-EDX, IR spectroscopy and Raman spectroscopy.

The properties of the four non-metallic composite salts are shown in Table 6. Compared with their corresponding cyclo-N₅⁻ salts, cocrystals 47 and 48 have lower friction and impact sensitivities. Moreover, 47 exhibited better thermal stability (10 °C higher than that of NH₄N₅) and good detonation performance (D: 8300 m s⁻¹, P: 21.4 GPa) than did FOX-12.⁴⁹

Table 6 Properties of 46–49

Comp.	d (g cm^−3)	T d (°C)	D (m s^−1)	P (GPa)	IS^e (J)	FS^f (N)
a Density from single-crystal X-ray diffraction. b Decomposition temperature (DSC). c Detonation velocity. d Detonation pressure. e Impact sensitivity. f Friction sensitivity.
46	1.34 (123 K)	117
47	1.34 (150 K)	109	8300	21.4	31	300
48	1.59 (100 K)	95	8260	23.8	>40	>360
49	1.62 (173 K)	99	7615	23.6

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5 Summary and outlook

The pentazolate anion and its derivatives represent one of the most important research advances in the field of energetic materials. In the past seven years, many energetic cyclo-N₅⁻ cocrystals have been developed through metathesis reactions combined with solvent evaporation or self-assembly methods. Cyclo-N₅⁻ cocrystals can be divided into cocrystals of metal-N₅⁻ compounds and non-metallic pentazolate salts according to their composition. Each major category can be further divided into three subcategories: solvates (mainly hydrates), cocrystals composed of cyclo-N₅⁻ compounds and neutral molecules, and composite salts. Among them, metal composite salts represent the largest number (14), followed by solvates of metal (12) and non-metallic salts (10). Overall, the number of water-containing cocrystals (32) accounts for nearly two-thirds of that of cyclo-N₅⁻ cocrystals, and synthesizing anhydrous cyclo-N₅⁻ cocrystals remains a challenge.

Cyclo-N₅⁻ cocrystals, especially those containing metals, have fascinating crystal structures, including pentasil-zeolite topological networks, metal pentazolate frameworks, and hydrogen-bonded organic frameworks. The thermal stabilities of some cocrystals exceed those of their pentazolate precursors, providing ideas for the development of more promising cyclo-N₅⁻ derivatives.

Suggestions for future key research directions for cyclo-N₅⁻ cocrystals are as follows:

(1) On the basis of the existing synthetic methods for cyclo-N₅⁻ cocrystals, more preparation methods should be developed, especially methods suitable for scale up.

(2) All reported enthalpies of formation of cyclo-N₅⁻ cocrystals were theoretically determined. In order to accurately evaluate their energy, more attention must be paid to using high-precision calculation methods and experimental measurements.

(3) The activation energy barrier for the decomposition of cyclo-N₅⁻ is only about 100 kJ mol⁻¹, so the decomposition temperatures of cyclo-N₅⁻ cocrystals are relatively lower than those of traditional energetic materials such as RDX and HMX. In the direction of application, it is necessary to further increase the thermal stability of cyclo-N₅⁻ cocrystals.

(4) The energetic properties of cyclo-N₅⁻ cocrystals are often between those of the precursors, which means that the energy of a cyclo-N₅⁻ cocrystal is slightly lower than that of a cyclo-N₅⁻ precursor. Therefore, further research is expected to increase the energy through the cocrystallization of cyclo-N₅⁻ compounds and high-density, high-oxygen balance, high-energy oxidizer molecules.

(5) By using advanced methods such as artificial intelligence, machine learning, and high-throughput experiments to study the formation mechanism of cyclo-N₅⁻ cocrystals, we can grasp the key influencing factors of cocrystal formation, which has guiding significance for the future creation of energetic cocrystals and even polynitrogen compounds.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 22105102 and 22135003), the Young Elite Scientist Sponsorship Program by CAST (no. YESS20210074), and the Fundamental Research Funds for the Central Universities (no. 30921011204).

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