Taming Dinitramide Anions within an Energetic MetalâOrganic

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Taming Dinitramide Anions within an Energetic Metal−Organic Framework: A New Strategy for Synthesis and Tunable Properties of High Energy Materials Jichuan Zhang,† Yao Du,† Kai Dong,† Hui Su,† Shaowen Zhang,‡ Shenghua Li,*,† and Siping Pang*,† †

School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China School of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China

‡

S Supporting Information *

ABSTRACT: Energetic polynitro anions, such as dinitramide ion [N(NO2)2−], have attracted significant interest in the field of energetic materials due to their high densities and rich oxygen contents; however, most of them usually suffer from low stability. Conveniently stabilizing energetic polynitro anions to develop new high energy materials as well as tuning their energetic properties still represent significant challenges. To address these challenges, we herein propose a novel strategy that energetic polynitro anions are encapsulated within energetic cationic metal−organic frameworks (MOFs). We present N(NO2)2− encapsulated within a three-dimensional (3D) energetic cationic MOF through simple anion exchange. The resultant inclusion complex exhibits a remarkable thermal stability with the onset decomposition temperature of 221 °C, which is, to our knowledge, the highest value known for all dinitramide-based compounds. In addition, it possesses good energetic properties, which can be conveniently tuned by changing the mole ratio of the starting materials. The encapsulated anion can also be released in a controlled fashion without disrupting the framework. This work may shed new insights into the stabilization, storage, and release of labile energetic anions under ambient conditions, while providing a simple and convenient approach for the preparation of new energetic MOFs and the modulation of their energetic properties.

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INTRODUCTION Dinitramide ion [N(NO2)2−], an exclusive oxy anion of nitrogen, plays a significant role as an energetic anion in the development of environmentally friendly oxidizers and energetic materials, as its salts possess impressively high densities and rich oxygen contents.1,2 Moreover, this anion has an intriguing molecular structure and has also attracted great interest in structural chemistry.3,4 However, most of its salts tend to be unstable and easily decomposed by heat {e.g., for [Me3S][N(NO2)2], its onset decomposition temperature (Td) is ∼25 °C; [NH4N(NO2)2] (ADN), Td = ∼ 130 °C }, which has limited their practical applications.5−7 Alternatively, an efficient strategy has been developed through the introduction of polyamino-based nitrogen-rich cations into the energetic salts and the formation of multiple hydrogenbonding interactions with N(NO2)2− anions (Scheme 1), thus improving their stabilities, but this method requires tedious synthetic steps for the preparation of polyamino-based nitrogen-rich cations; besides, concomitantly the detonation properties of these salts sometimes decrease.8−13 Recently, the utilization of molecular containers for stabilizing labile species has attracted much attention, because of the fact that the guest species as stable forms will not only permit spectroscopic observation but also facilitate under© 2016 American Chemical Society

Scheme 1. Various Strategies for the Stabilization of the [N(NO2)2−] Anion

standing the chemical and biological mechanism.14−24 The containers function as protective, nanometer-sized cavities to Received: December 20, 2015 Revised: February 5, 2016 Published: February 5, 2016 1472

DOI: 10.1021/acs.chemmater.5b04891 Chem. Mater. 2016, 28, 1472−1480

Article

Chemistry of Materials

Figure 1. (Left) Crystal packing of MOF(Cu) viewed along the crystallographic a axis. (Right) Crystal packing of N(NO2)2− ⊂ MOF(Cu) viewed along the crystallographic a axis. The scheme is shown for the exchange process of trapping N(NO2)2− anions and loss of NO3− anions. Hydrogen atoms and guest water molecules have been omitted for clarity.

containers for the capture, encapsulation, and stabilization of labile energetic anions through simple anion exchange to develop new high energy materials (Scheme 1). Moreover, their energetic properties could also be tuned by changing the encapsulated quantity of guest energetic anions. It seems that we could achieve many things at one stroke by applying an “energetic cationic MOF encapsulating labile energetic anions” strategy. Using this strategy, we herein reported the encapsulation of N(NO2)2− within a three-dimensional (3D) energetic cationic MOF through one-step anion-exchange reaction at room temperature (Figure 1). The resultant inclusion complex not only possesses remarkable stabilities, but also exhibits good energetic properties, which can be conveniently tuned by simply changing the mole ratio of the starting materials ammonium dinitramide (ADN) and [Cu(atrz)3(NO3) 2]n (named MOF(Cu); atrz = 4,4′-azo-1,2,4-triazole) without complicated chemical modifications. Interestingly, the exchange process underwent a single-crystal to single-crystal (SC−SC) transformation. Moreover, by adding the competing guest anion, the encapsulated anion could also be released in a controlled fashion without disrupting the framework, concomitantly forming another new energetic MOF that also possesses a high thermal stability and tunable properties.

entrap guest molecules and prevent their decomposition or reaction with external reagents. More importantly, the encapsulation of guest molecules inside the containers usually engenders new features and/or improves the intrinsic properties of the guests by host−guest interaction.25−27 Chemists have devoted much effort to create a number of host containers such as metal coordination polymers, organic (covalent) cages, discrete metal coordination complexes, and noncovalent organic frameworks;28−35 however, a majority of host containers are constructed from aliphatic or aryl subunits and thus have low energy, which could make them unsuited as hosts for the capture, encapsulation, and stabilization of labile energetic anions. On the other hand, the modulation of properties of energetic materials has also attracted growing attention not only for gaining insight into the correlation between structure and property but also for meeting various applications including explosives, propellants, pyrotechnics,36−43 carbon nitride precursors,44,45 and gas generating agents.46,47 For example, gas generating agents should ideally produce more gases and less heat when used for air bags, and primary explosives should be sensitive enough to be initiated, while secondary explosives should possess considerably higher detonation heats and lower sensitivities. Over the past decade, a variety of protocols for tuning the properties of energetic materials have developed.1,2,36−51 Among them, introduction of different energetic groups (e.g., nitro,36−38 nitroamine,39,40 azido,42 and amino43) as substituents on an energetic backbone is perhaps the most commonly utilized method (Figure S1). For example, the introduction of a nitro group improves the oxygen balance and densities of energetic materials and thus the detonation properties, while the introduction of an amino group enhances the stability and lowers the sensitivities. However, this method usually suffers from complicated synthetic steps and harsh reaction conditions (e.g., relatively high reaction temperature and use of highly concentrated HNO3 and/or concentrated H2SO4).36−43 Energetic cationic metal−organic frameworks (MOFs) are an emerging class of energetic materials and possess highly regular channels, high densities, and high heats of detonation,52−57 which have exhibited promising applications in pyrotechnics58 and energetic composites.59 Their positive energetic frameworks can be constructed by using energetic nitrogen-rich ligands and metal ions. The extra-framework energetic anions such as NO3− and ClO4− usually occupy the framework channels and are sometimes just weakly coordinated or even uncoordinated to metal centers. We envisaged that these features could make energetic cationic MOFs as ideal host

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RESULTS AND DISCUSSION Synthesis and Structure. According to the literature procedure,55 MOF(Cu) was prepared from a hydrothermal reaction of 4,4′-azo-1,2,4-triazole (atrz) with Cu(NO3)2 (Figure S2); this material can be synthesized with high yield and purity and is chemically stable in pH 1−10 aqueous solutions (Figure S3). Given that MOF(Cu) has a positive porous energetic framework, a number of charge-balancing NO3− anions occupy the framework channels and are uncoordinated to the copper centers, and given that enough large channels are available for anion access (Figure 1), an anion-exchange experiment was performed. Immersion of as-synthesized MOF(Cu) crystals in a 3-fold molar excess of ADN aqueous solution at room temperature produced the highly crystalline phase solids ({Cu(atrz)3[N(NO2)2]2·0.46H2O}n, namely, N(NO2)2− ⊂ MOF(Cu), Figures S4 and S5). The whole exchange process was followed visually, and no crystal dissolution was observed. The elemental analysis and TOF-MS of N(NO2)2− ⊂ MOF(Cu) samples revealed that NO3− was almost fully substituted by N(NO2)2− (Figure S6). Its IR spectrum and powder X-ray diffraction (PXRD) pattern were identical to those of MOF(Cu), suggesting that the framework remained intact throughout the exchange process (Figure 2 and Figure 1473


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Figure 4. View of coordination environments of CuII atoms and atrz ligands in N(NO2)2− ⊂ MOF(Cu). Hydrogen atoms, guest water molecules, and more N(NO2)2− anions have been omitted for clarity.

Figure 2. IR spectra of anion-exchange products with highlighting band positions of the corresponding anions: (a) MOF(Cu), (b) N(NO2)2− ⊂ MOF(Cu), (c) N3− ⊂ MOF(Cu).

encapsulated N(NO2)2− anions are ordered and well-defined. Interestingly, the N8−N9−N10 angle (108°) of the encapsulated N(NO2)2− anion is slightly lower than those of many energetic materials containing free N(NO2)2− (∼116°, Table S7).9,11,12 It is possible that the channels restrain the conformation of the N(NO2)2− anions, resulting in the decrease of this bond angle. In the structure of N(NO2)2− ⊂ MOF(Cu), the NO2 groups of the N(NO2)2− anion and the C−H groups of the triazole ring participate in the formation of C−H···O hydrogen bonding (Figure 5). The C−H···O hydrogen-bonding distances vary

S10). It is worth noting that the resultant complex remained single crystalline and was heterogeneous throughout the exchange at the bottom of the aqueous solution and could be gathered through simple filtration, facilitating the observation of molecular interaction and structure by X-ray crystallography (Figure 3). In contrast, many reported solution-state molecular

Figure 5. Unit cell packing diagram of N(NO2)2− ⊂ MOF(Cu) viewed along the a axis. The green dashed lines indicate strong intermolecular hydrogen bonding. The red dashed lines indicate the interaction between the oxygen atom of the nitro group and the π electrons of the triazole ring (contact distance: ∼3.209(2) Å [O···Cg(πring)]). Symmetry code: (i) −0.5 + x, 1.5 − y, 0.5 + z; (ii) 1.5 − x, −0.5 + y, 1.5 − z; (iii) 1 − x, 1 − y, 2 − z; (iv) 1 − x, 1 − y, 2 − z; (v) 1.5 − x, 0.5 + y, 1.5 − z; (vi) 2 − x, 1 − y, 3 − z; (vii) −0.5 + x, 1.5 − y, −0.5 + z.

Figure 3. Photographs show the color of crystals before and after trapping−releasing process.

containers need complicated crystallization after encapsulation labile guest molecules; in addition, guest molecules usually escape the cage chambers during crystallization.20−24 To further confirm the framework stability and encapsulation process, a crystal obtained after anion exchange was subjected to single-crystal X-ray diffraction. N(NO2)2− ⊂ MOF(Cu) is isostructural to MOF(Cu)55 and crystallizes in a monoclinic system with space group P21/n. The asymmetry unit is made up of one CuII atom, three energetic atrz ligands, and two N(NO2)2− anions. The CuII atom is also six-coordinated by six atrz nitrogen atom in a regular octahedron (Figure 4 and Figures S11−S13), while each atrz molecule acts as a bidentate bridge connecting two adjacent CuII centers. The overall result ultimately constructs a 3D cationic energetic framework. The framework possesses a one-dimensional (1D) triangular channel with the size of 11.825 × 8.658/2 Å2, which is almost identical to that of MOF(Cu) (11.823 × 8.644/2 Å2). The channels are filled with N(NO2)2− anions; in addition, the

from 2.148 to 2.333 Å, which is considerably shorter than the previously reported C−H···O distances (2.657−4.000 Å),60,61 indicating strong hydrogen-bonding interactions. Additionally, one of the oxygen atoms of the N(NO2)2− anion is also involved in an interaction with the π electrons of the triazole ring. Thus, the whole positive framework is still intact after anion exchange; the N(NO2)2− anion has replaced the NO3− anion and fills the channels in the framework to balance the charge. These observations indicate that the exchange of N(NO2)2− took place through an interesting SC−SC process. To our knowledge, the SC−SC transformation of encapsulation labile energetic anions within MOFs has not been explored yet, although a few examples of solid−solid exchange have been 1474


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Chemistry of Materials reported.62−69 Moreover, N(NO2)2− ⊂ MOF(Cu) is the first reported example of a porous compound able to take up N(NO2)2− counteranions. Stability and Detonation Properties. Surprisingly, N(NO2)2− encapsulated in the framework was found to be quite stable in air and common solvents and was to be almost nonhygroscopic in air for one month (Figure 6 and Table S1).

1,2,4-triazolium dinitramide (Td = 200 °C),11 the most stable compounds containing N(NO2)2− ever known. The high stability of N(NO2)2− ⊂ MOF(Cu) is presumably caused by the strong structural reinforcement of the 3D framework. Additionally, N(NO2)2− is encapsulated inside the confined space of the tight framework, which could restrain the conformation or configuration change of the anion and prevent the interactions or reactions between the anion and outside reactants; besides, multiple intermolecular interactions such as hydrogen-bonding and electrostatic interactions could also contribute to enhancing its stability. Besides its high thermal stability, N(NO2)2− ⊂ MOF(Cu) also exhibits relatively low sensitivities toward impact, friction, and static electricity (Table 1). Its impact sensitivity (IS) and friction sensitivity (FS) are 9 J and 73 N, respectively, which are lower than those of ADN (IS = 3−5 J, FS = 64 N).74,75 Furthermore, the human body can generate up to 0.025 J of static electricity, which can easily set off the most sensitive metal complexes such as lead azide or silver fulminate. However, N(NO2)2− ⊂ MOF(Cu) has an electrostatic sensitivity of 1.9 J, which is far higher than the human body can generate, allowing a comparable margin of safety when handling.76 It is probable that the tight integration between the rigid ligands and metal ions generates a stable and insensitive structural framework. Additionally, the sensitive anion N(NO2)2− is encapsulated inside the host framework, resulting in lower sensitivity. The resultant host−guest complex possesses good energetic properties. The density of N(NO2)2− ⊂ MOF(Cu) is 1.78 g cm−3, which is comparable to ADN (1.81 g cm−3), but is higher than that of the parent complex [MOF(Cu), 1.64 g cm−3]. The constant-volume combustion energy (ΔcU) for N(NO2)2− ⊂ MOF(Cu) was measured by an oxygen bomb calorimeter, along with MOF(Cu) and RDX as reference compounds. The enthalpy of combustion (ΔcH°) was calculated from ΔcU, and a correction for change in gas volume during combustion was included (Scheme 2, eq 1). The standard enthalpies of formation (ΔfH°) of N(NO2)2− ⊂ MOF(Cu), MOF(Cu), and RDX were back calculated from the heats of combustion on the basis of combustion equations (Scheme 2, eqs 2−4), Hess’ Law as applied in thermochemical equations (Scheme 2, eqs 5−7), and known standard heats of formation for copper oxide, water, and carbon dioxide (see the Supporting Information).77 The calculated ΔfH° value of N(NO2)2− ⊂ MOF(Cu) is 3667 kJ mol−1, while the ΔfH° value of RDX is 53.83 kJ mol−1. The energy given off by an energetic material during detonation, the heat of detonation (Q), is a critical performance metric. The empirical Kamlet formula (Scheme 3, eq 1) and a commercial program EXPLO5 are two common methods for the prediction of heat of detonation of energetic compounds containing CHON elements.78−80 However, an effective method for the accurate prediction of heats of detonation of energetic MOFs is still scarce. In a recent study, we developed a simple method to calculate the heats of detonation of some metal-containing explosives on the basis of the empirical Kamlet formula.81 Here, using the experimental determined (back-calculated from −ΔcU) enthalpy of formation, we adopted our developed method to predict the heats of detonation of N(NO2)2− ⊂ MOF(Cu) and MOF(Cu) (see the Supporting Information). According to the largest exothermic principle proposed by Kamlet and our developed method, during detonation of N(NO2)2− ⊂ MOF(Cu) and

Figure 6. Comparison of the hygroscopic property of ADN and N(NO2)2− ⊂ MOF(Cu).

The PXRD patterns and IR analysis indicated that the resultant inclusion complex was still intact even after being heated at 150 °C for 24 h (Figures S10 and S15). In contrast, ADN, one of the most promising eco-friendly energetic oxidizers containing free N(NO2)2−,70,71 suffers from severe hygroscopicity and low stability; it decomposes even at temperatures below its melting point (90 °C),72,73 which affects its normal usage. The thermal stability of ADN does not significantly improve even when physically mixed with MOF(Cu) (Figure 7). However,

Figure 7. DSC curves of various samples measured with a heating rate of 5 °C min−1: (a) MOF(Cu); (b) the mixture of MOF(Cu) and ADN through physical mixing (the mole ratio of MOF(Cu) to ADN is 1:1); (c) ADN; (d) N(NO2)2− ⊂ MOF(Cu); (e) N3− ⊂ MOF(Cu).

thermogravimetric analysis of N(NO2)2− ⊂ MOF(Cu) samples showed a main loss weight of 70% in the temperature regime 220−400 °C. Moreover, the thermogravimetric/differential scanning calorimetry (TG/DSC) analysis unambiguously demonstrated that its onset decomposition temperature reaches up to 221 °C (Figures S16 and S17), which is even higher than those of N-guanylurea dinitramide (FOX-12, Td = 201 °C, at a heating rate of 5 °C min−1) and 4,4′,5,5′-tetraamino-3,3′-bi1475


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Chemistry of Materials Table 1. Physicochemical Properties of Various Energetic Compounds compd N(NO2)2−⊂

ρb

Tda 221

ΩCOf

54.81

71.4

−22.92

9

73

1.9

10299

10261

3667

ISg

FSh

ESDi

−ΔcUj

−ΔcHok

O + N%e

N%d

ΔfH°l

53.53

67.7

−28.24

16

112

24.75

8275

8244

1651

210

1.78 (1.80c) 1.64 (1.68c) 1.81

37.84

81.06

0

7.5

120

0.2

2103.0

2092

53.8

130 210 221

1.81 1.81 2.04

52.00 37.84 38.36

96.8 81.06 82.18

26 0 10.95

3−5 7.5 4

64 120 48

>0.16 0.2 0.13

MOF(Cu) MOF(Cu)

223

RDXo ADNp RDXq Cl-20r

Vom s

−150.0 70.3 365

642 (723t) 626s (698t) 756s (791t) 986t 785t 715t

−Qn 7176s (6824t) 4562s (4388t) 6166s (5668t) 2789t 5845t 6168t

a The onset decomposition temperature (DSC, °C). bDensity measured from gas pycnometer (g cm−3). cDensity from X-ray diffration analysis (g cm−3). dNitrogen content. eOxygen and nitrogen content. fOxygen balance. gImpact sensitivity (J). hFriction sensitivity (N). iElectrostatic sensitivity (J). jExperimental determined (oxygen bomb calorimetry) contant volume energy of combustion (kJ mol−1). kExperimental molar enthalpy of combustion (kJ mol−1). lExperiment determined (back-calculated from −ΔcU) enthalpy of formation (kJ mol−1). mVolume of gases after detonation (L kg−1). nHeat of detonation (kJ kg−1). oThe detonation properties of RDX (as a reference compound) were obtained on the basis of its experimental determined (back-calculated from −ΔcU) enthalpy of formation. pProperties of ADN are taken from refs 74, 75, and 83−85. q Properties of RDX are taken from ref 79. rProperties of CL-20 are taken from ref 80. sThe detonation properties were calculated by our developed method. tThe detonation properties were calculated by EXPLO5 v6.01.

Information). The calculated heat of detonation of N(NO2)2− ⊂ MOF(Cu) is 7176 kJ kg−1, while its value obtained from EXPLO5 v6.01 is 6824 kJ kg−1, which confirms that this method possesses acceptable accuracy. The heat of detonation of N(NO2)2− ⊂ MOF(Cu) is superior to MOF(Cu) (4562 kJ kg−1). It is possible that N(NO2)2− ⊂ MOF(Cu) contains oxygen-rich N(NO2)2− anions and nitrogen-rich atrz ligands; thus, it possesses higher density, higher nitrogen content, and better oxygen balance in comparison with MOF(Cu), which could contribute to releasing more energy during detonation. Remarkably, its heat of detonation is even higher than that of CL-20 (6168 kJ kg−1), the most powerful organic explosive. Moreover, its nitrogen and oxygen content is 71.4%, which is possibly the highest value for all recently reported energetic MOFs (Table S4). In many previous studies,14,21,23 protective molecular containers were used to stabilize labile guest molecules, but they had to be subsequently removed or broken to make use of guest molecules. In contrast, not only can the host framework act as a protective container, but the resultant host−guest complex is a new potential high energy density material. Tunable Properties. The energetic properties of the resultant host−guest complex can be conveniently tuned under ambient condition. The solid samples of MOF(Cu) were immersed in ADN solutions with different concentrations at room temperature; a series of desolvated complexes with general formula {Cu(atrz)3(NO3)x[N(NO2)2]2‑x}n (0 ≤ x ≤ 2) were obtained (Figure S18 and Table S2, see the Supporting Information). The PXRD patterns and IR spectra indicated that these complexes maintained the parent structures (Figures S19 and S20). As the mole ratio of ADN/MOF(Cu) in the reaction mixtures gradually increased from 0 to 3, the resultant complex also showed a gradual increase in the encapsulated quantity of N(NO2)2− anions, density from 1.64 to 1.78 g cm−3, volume of gases after detonation from 626 to 642 L kg−1, and heat of detonation from 4562 to 7176 kJ kg−1, while the impact sensitivity showed a gradual decrease from 16 to 9 J (Figure 8, Figures S21 and S22). Therefore, in comparison with the previous methods for tuning the properties of energetic materials by using complicated chemical modifications,36−47 this approach appears simpler and more convenient. Anion Release. In addition to stabilization of labile anions in host containers and modulation of their properties, the facile

Scheme 2. Combustion Reactions of Energetic MOFs and RDX, and Hess’ Law for These Combustion Reactions

Scheme 3. Detonation Reactions of Energetic MOFs and RDX, and the Empirical Kamlet Formula for Their Heats of Detonation

MOF(Cu), all N atoms are converted to N2; O atoms form H2O with H atoms first and then form CO2 with C atoms. The remaining C atoms are retained in the solid state; if there are O atoms left, they will form O2. In addition, the copper atoms should be converted to their reduction state (Cu) during detonation since the heat of formation of copper oxide [ΔfH°(CuO, s) = −156.06 kJ mol−1] is higher than that of water [ΔfH°(H2O, g) = −241.83 kJ mol−1].77 On the basis of the above theories, the detonation reactions of our assynthesized energetic MOFs were proposed in Scheme 3, and their heats of detonation were evaluated by the empirical Kamlet formula (Scheme 3, eq 1).79,81 To confirm the prediction accuracy of this method, we also employed the EXPLO5 computer code in its new version 6.01 to calculate their heats of detonation (Table 1, see the Supporting 1476


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reaction mixtures (Figure 10, Figures S31 and S32, and Table S3). Particularly, the impact sensitivity can be tuned from 9 to

Figure 8. Tunable impact sensitivities and heats of detonation of the resultant complexes through changing the mole ratio of ADN/ MOF(Cu) in the reaction mixtures.

Figure 10. Tunable nitrogen contents and impact sensitivities of the resultant complexes through changing the mole ratio of NaN3/ N(NO2)2− ⊂ MOF(Cu) in the reaction mixtures.

release of encapsulated guest anions from host containers is also important for the subsequent delivery or utilization of the anions. Although the N(NO2)2− anion appeared to be indefinitely stable within the host framework, releasing the anion from the framework through the addition of a competing energetic guest anion proved to be straightforward (Figure 9).

1.5 J, indicating that the solid product can be tuned from a potential secondary explosive to a high sensitivity primary explosive.

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CONCLUSION In summary, we present a strategy to develop new energetic materials by encapsulating labile energetic anions within energetic cationic MOFs. By applying this strategy, we successfully achieved the encapsulation of the dinitramide anion within an energetic cationic MOF via anion exchange at room temperature. The encapsulated anion in the framework was significantly stabilized with an onset decomposition temperature of 221 °C, which is, to our knowledge, the highest value known for all dinitramide-based compounds. Furthermore, the resultant host−guest complex possesses a rather high nitrogen and oxygen content (71.4%), and good energetic properties, which can be conveniently tuned by changing the mole ratio of the starting materials ADN and MOF(Cu) at room temperature without complicated chemical modifications. Despite its stabilization, the encapsulated N(NO2)2− can also be released in a controlled fashion without disrupting the framework by adding the competing guest anion N3−. Therefore, MOF(Cu) could facilitate the handing and use of N(NO2)2−, while the resultant host−guest complex is also a new potential high energy density material. This present study may not only shed new insight into the stabilization, storage, and release of labile energetic anions under ambient conditions, but also provide a convenient approach for the preparation of new energetic MOFs as well as the modulation of their energetic properties.

Figure 9. Schematic representation of release process for N(NO2)2−.

By simply immersing the crystals of N(NO2)2− ⊂ MOF(Cu) in 10 mL of 2-fold molar NaN3 aqueous solution for 30 min, the crystals underwent a rapid naked-eye detectable change in color (from blue to deep brown), as illustrated in Figure 3. According to HPLC−MS analysis, the release of N(NO2)2− from the framework into the aqueous solution was evidenced by the appearance of a mass peak at 105.9 m/z (M+, Figures S23−25 and S34). In addition, FT-IR spectra of the concomitant solid product [N3− ⊂ MOF(Cu)] showed a strong band associated with the encapsulated anion N3− (2050 cm−1), along with the disappearance of band of N(NO2)2− (1390 cm−1, Figure 2). Other bands in the spectra remained almost unchanged, while its PXRD pattern appeared to be considerably similar to that of N(NO2)2− ⊂ MOF(Cu) (Figure S28), suggesting that the framework remained intact throughout the release process and that the N(NO2)2− anions in the framework had been almost completely released. The concomitant solid product also exhibits a remarkable thermal stability (Td = 216 °C, Figure 7 and Figure S30), which further confirms that the host framework can serve as a polymeric container for safekeeping labile energetic anions. Interestingly, the encapsulated N(NO2)2− anion can be controllably released, and the properties of the concomitant solid product can also be tuned by changing the mole ratio of NaN3 to N(NO2)2− ⊂MOF(Cu) in the

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EXPERIMENTAL SECTION

Safety Precautions. Although none of the energetic MOFs described herein have exploded or detonated in the course of this research, these materials should be handled with extreme care using the best safety practices. General Methods. MOF(Cu) and NH4N(NO2)2 (ADN) were prepared according to the previous literature studies.55,82 All other materials were commercially available and used without further purification. Powder X-ray diffraction (PXRD) patterns of the samples were analyzed with monochromatized Cu Kα (λ = 1.54178 Å) incident radiation by Bruker D8 Advance X-ray diffractometer operating at 40 kV voltage and 50 mA current. PXRD patterns were recorded from 5° 1477



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to 80° (2θ) at 298 K. IR spectrum was recorded on a Bruker Tensor 27 spectrophotometer with HTS-XT (KBr pellets). Elemental analysis was performed on an Elementar Vario EL (Germany). To determine the thermal stabilities of as-synthesized energetic MOFs and ADN, a TA-DSC Q2000 differential scanning calorimeter (heating rate, 5 °C min−1; the flow rate of nitrogen gas, 60 mL min−1; the sample size, about 2.0 mg) was used. Densities of energetic MOFs were measured by using Automatic Density Analyzer, ULTRAPYC 1200e. The MS spectra of as-synthesized MOFs were measured by MALDI-TOF mass spectrometry (Bruker Corporation). The concentration of the aqueous solution and its ESI-MS were analyzed by HPLC−MS (HPLC, Agilent 6100 series; ESI-MS, an Agilent Technologies 6120 mass analyzer; eluent, 10% CH3OH in water; flow rate, 1 mL min−1). Before the measurement of density, constant-volume combustion energy, sensitivities, and hygroscopicity, N(NO2)2− ⊂ MOF(Cu) crystals have been desolvated. The desolvated procedure follows: N(NO2)2− ⊂ MOF(Cu) crystals were immersed in anhydrous methanol for 3 days, during which the exchanged solvent was decanted and freshly replenished three times, then dried in vacuum at 50 °C for 24 h in order to remove the guest water locating in the channel. X-ray Crystallography. The crystal structure of N(NO2)2− ⊂ MOF(Cu) was determined by a Rigaku RAXIS IP diffractometer and SHELXTL crystallographic software package of molecular structure. The single crystals were mounted on a Rigaku RAXIS RAPID IP diffractometer equipped with a graphite-monochromatized Mo Kα radiation (λ= 0.71073 Å). Data were collected by the ω scan technique. The structure was solved by direct methods with SHELXS97 and expanded by using the Fourier technique. The non-hydrogen atoms were refined anisotropically. The hydrogen atom was determined with theoretical calculations and refined with an isotropic vibration factor, CCDC 1057051. Preparation of {Cu(atrz)3[N(NO2)2]2·0.46H2O}n [N(NO2)2− ⊂ MOF(Cu)]. The crystals of as-prepared MOF(Cu) (0.2 mmol, 0.14 g) were immersed in a 10 mL aqueous solution of ADN (0.06 mol/L) for 7 days under static ambient conditions (room temperature). After decanting the solution, the resultant light blue crystals were washed thoroughly with deionized water, and dried in vacuum at 50 °C for 24 h. Yield: 70% based on the Cu. Elemental analysis (%) calculated for C12H12.92CuN30O8.46 (M = 776): C, 18.56; H, 1.66; N, 54.12; O, 17.44. Found: C, 18.57; H, 1.67; N, 54.02; O 17.21. IR (KBr pellets, λ, cm−1): 3552 (m), 3454 (m), 3117 (m), 1508(s), 1392 (m), 1188 (s), 1040 (s), 884 (s), 695 (s), 630 (s), 556 (s), 425 (s). Raman (1064 nm, 200 mW, 25 °C): 1535, 1482, 1345, 1207, 1173, 1049, 946, 905, 829. The elemental analysis of desolvated sample (%) calculated for C12H12CuN30O8 (M = 768): C, 18.75; H, 1.56; N, 54.69; O 16.67. Found: C, 18.69; H, 1.57; N, 54.81; O 16.72. IR (KBr pellets, λ, cm‑1): 3454 (w), 3117 (m), 1502(s), 1385 (m), 1185 (s), 1036 (s), 887 (m), 700 (m), 621 (s), 556 (m). N(NO2)2− Release from N(NO2)2− ⊂ MOF(Cu) and Preparation of {Cu(atrz)3[N3]2}n [N3− ⊂ MOF(Cu)]. The crystals of N(NO2)2− ⊂ MOF(Cu) (0.2 mmol, 0.16 g) were immersed in a 10 mL aqueous solution of NaN3 (0.04 mol/L) for 30 min under static ambient conditions (room temperature). After decanting the solution, the resultant deep brown crystals were washed thoroughly with deionized water, but the X-ray diffraction image of the crystal did not show clear spots owing to the loss of its single crystallinity. The crystals were then immersed in anhydrous methanol for 3 days, during which the exchanged solvent was decanted and freshly replenished three times, and then dried in vacuum at 50 °C for 24 h. Yield: 79% based on Cu. Elemental analysis (%) calculated for C12H12CuN30 (M = 640): C, 22.50; H, 1.86; N, 65.63. Found: C, 22.33; H, 1.85; N, 64.82. IR (KBr pellets, λ, cm−1): 3086 (m), 2050 (s), 1507 (s), 1180 (s), 1041 (s), 882 (s), 694 (s), 617 (s), 554 (s), 443 (s). The released N(NO2)2− from N(NO2)2− ⊂ MOF(Cu) into the aqueous solution was determined by HPLC−MS (See the Supporting Information).

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04891. Experimental methods, additional figures and tables described herein (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (21442003, 21576026, and U153062) and the opening project of State Key Laboratory of Science and Technology (Beijing Institute of Technology, ZDKT12-03).

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REFERENCES

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