Taming Dinitramide Anions Within an Energetic Metal–Organic Framework

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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 − fi fi ion [N(NO2)2 ], have attracted signi cant interest in the eld 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.

■ INTRODUCTION Scheme 1. Various Strategies for the Stabilization of the − − [N(NO2)2 ] Anion 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 (T )is∼25 °C; [NH N(NO ) ] (ADN), T = ∼ 130 °C}, d 4 2 2 d − 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 hydrogen- − bonding 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 14−24 nitrogen-rich cations; besides, concomitantly the detonation standing the chemical and biological mechanism. The − properties of these salts sometimes decrease.8 13 containers function as protective, nanometer-sized cavities to Recently, the utilization of molecular containers for stabilizing labile species has attracted much attention, because Received: December 20, 2015 of the fact that the guest species as stable forms will not only Revised: February 5, 2016 permit spectroscopic observation but also facilitate under-

− ⊂ 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. entrap guest molecules and prevent their decomposition or containers for the capture, encapsulation, and stabilization of reaction with external reagents. More importantly, the labile energetic anions through simple anion exchange to encapsulation of guest molecules inside the containers usually develop new high energy materials (Scheme 1). Moreover, their engenders new features and/or improves the intrinsic proper- energetic properties could also be tuned by changing the − ties of the guests by host−guest interaction.25 27 Chemists have encapsulated quantity of guest energetic anions. It seems that devoted much effort to create a number of host containers such we could achieve many things at one stroke by applying an as metal coordination polymers, organic (covalent) cages, “energetic cationic MOF encapsulating labile energetic anions” discrete metal coordination complexes, and noncovalent strategy. − organic frameworks;28 35 however, a majority of host contain- Using this strategy, we herein reported the encapsulation of − ers are constructed from aliphatic or aryl subunits and thus have N(NO2)2 within a three-dimensional (3D) energetic cationic low energy, which could make them unsuited as hosts for the MOF through one-step anion-exchange reaction at room capture, encapsulation, and stabilization of labile energetic temperature (Figure 1). The resultant inclusion complex not anions. only possesses remarkable stabilities, but also exhibits good On the other hand, the modulation of properties of energetic energetic properties, which can be conveniently tuned by materials has also attracted growing attention not only for simply changing the mole ratio of the starting materials gaining insight into the correlation between structure and ammonium dinitramide (ADN) and [Cu(atrz)3(NO3)2]n property but also for meeting various applications including (named MOF(Cu); atrz = 4,4′-azo-1,2,4-triazole) without − explosives, propellants, pyrotechnics,36 43 carbon nitride complicated chemical modifications. Interestingly, the exchange precursors,44,45 and gas generating agents.46,47 For example, process underwent a single-crystal to single-crystal (SC−SC) gas generating agents should ideally produce more gases and transformation. Moreover, by adding the competing guest less heat when used for air bags, and primary explosives should anion, the encapsulated anion could also be released in a be sensitive enough to be initiated, while secondary explosives controlled fashion without disrupting the framework, concom- should possess considerably higher detonation heats and lower itantly forming another new energetic MOF that also possesses sensitivities. Over the past decade, a variety of protocols for a high thermal stability and tunable properties. tuning the properties of energetic materials have devel- − oped.1,2,36 51 Among them, introduction of different energetic ■ RESULTS AND DISCUSSION 36−38 39,40 42 43 groups (e.g., nitro, nitroamine, azido, and amino ) Synthesis and Structure. According to the literature as substituents on an energetic backbone is perhaps the most procedure,55 MOF(Cu) was prepared from a hydrothermal ′ commonly utilized method (Figure S1). For example, the reaction of 4,4 -azo-1,2,4-triazole (atrz) with Cu(NO3)2 (Figure introduction of a nitro group improves the oxygen balance and S2); this material can be synthesized with high yield and purity densities of energetic materials and thus the detonation and is chemically stable in pH 1−10 aqueous solutions (Figure properties, while the introduction of an amino group enhances S3). Given that MOF(Cu) has a positive porous energetic − the stability and lowers the sensitivities. However, this method framework, a number of charge-balancing NO3 anions occupy usually suffers from complicated synthetic steps and harsh the framework channels and are uncoordinated to the copper reaction conditions (e.g., relatively high reaction temperature centers, and given that enough large channels are available for and use of highly concentrated HNO3 and/or concentrated anion access (Figure 1), an anion-exchange experiment was 36−43 H2SO4). performed. Immersion of as-synthesized MOF(Cu) crystals in a Energetic cationic metal−organic frameworks (MOFs) are an 3-fold molar excess of ADN aqueous solution at room emerging class of energetic materials and possess highly regular temperature produced the highly crystalline phase solids 52−57 · − ⊂ channels, high densities, and high heats of detonation, ({Cu(atrz)3[N(NO2)2]2 0.46H2O}n, namely, N(NO2)2 which have exhibited promising applications in pyrotechnics58 MOF(Cu), Figures S4 and S5). The whole exchange process and energetic composites.59 Their positive energetic frame- was followed visually, and no crystal dissolution was observed. − ⊂ works can be constructed by using energetic nitrogen-rich The elemental analysis and TOF-MS of N(NO2)2 ligands and metal ions. The extra-framework energetic anions MOF(Cu) samples revealed that NO − was almost fully − − − 3 such as NO3 and ClO4 usually occupy the framework substituted by N(NO2)2 (Figure S6). Its IR spectrum and channels and are sometimes just weakly coordinated or even powder X-ray diffraction (PXRD) pattern were identical to uncoordinated to metal centers. We envisaged that these those of MOF(Cu), suggesting that the framework remained features could make energetic cationic MOFs as ideal host intact throughout the exchange process (Figure 2 and Figure

B DOI: 10.1021/acs.chemmater.5b04891 Chem. Mater. XXXX, XXX, XXX−XXX Chemistry of Materials Article

Figure 4. View of coordination environments of CuII atoms and atrz − ⊂ ligands in N(NO2)2 MOF(Cu). Hydrogen atoms, guest water − Figure 2. IR spectra of anion-exchange products with highlighting molecules, and more N(NO2)2 anions have been omitted for clarity. band positions of the corresponding anions: (a) MOF(Cu), (b) − ⊂ − ⊂ − ﬁ N(NO2)2 MOF(Cu), (c) N3 MOF(Cu). encapsulated N(NO2)2 anions are ordered and well-de ned. Interestingly, the N8−N9−N10 angle (108°) of the encapsu- − S10). It is worth noting that the resultant complex remained lated N(NO2)2 anion is slightly lower than those of many − ∼ ° single crystalline and was heterogeneous throughout the energetic materials containing free N(NO2)2 ( 116 , Table exchange at the bottom of the aqueous solution and could be S7).9,11,12 It is possible that the channels restrain the ﬁ − gathered through simple ltration, facilitating the observation of conformation of the N(NO2)2 anions, resulting in the molecular interaction and structure by X-ray crystallography decrease of this bond angle. − ⊂ (Figure 3). In contrast, many reported solution-state molecular 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

− ⊂ Figure 5. Unit cell packing diagram of N(NO2)2 MOF(Cu) viewed along the a axis. The green dashed lines indicate strong intermolecular Figure 3. Photographs show the color of crystals before and after hydrogen bonding. The red dashed lines indicate the interaction − trapping releasing process. between the oxygen atom of the nitro group and the π electrons of the ∼ ··· π triazole ring (contact distance: 3.209(2) Å [O Cg( ring)]). Symmetry code: (i) −0.5 + x, 1.5 − y, 0.5 + z; (ii) 1.5 − x, −0.5 + containers need complicated crystallization after encapsulation y, 1.5 − z; (iii) 1 − x,1− y,2− z; (iv) 1 − x,1− y,2− z; (v) 1.5 − x, labile guest molecules; in addition, guest molecules usually − − − − − − − 20−24 0.5 + y, 1.5 z; (vi) 2 x,1 y,3 z; (vii) 0.5 + x, 1.5 y, 0.5 + escape the cage chambers during crystallization. z. To further confirm the framework stability and encapsulation process, a crystal obtained after anion exchange was subjected ff − ⊂ to single-crystal X-ray di raction. N(NO2)2 MOF(Cu) is from 2.148 to 2.333 Å, which is considerably shorter than the isostructural to MOF(Cu)55 and crystallizes in a monoclinic previously reported C−H···O distances (2.657−4.000 Å),60,61 system with space group P21/n. The asymmetry unit is made up indicating strong hydrogen-bonding interactions. Additionally, II − of one Cu atom, three energetic atrz ligands, and two one of the oxygen atoms of the N(NO2)2 anion is also − II π N(NO2)2 anions. The Cu atom is also six-coordinated by six involved in an interaction with the electrons of the triazole atrz nitrogen atom in a regular octahedron (Figure 4 and ring. Thus, the whole positive framework is still intact after − − − Figures S11 S13), while each atrz molecule acts as a bidentate anion exchange; the N(NO2)2 anion has replaced the NO3 bridge connecting two adjacent CuII centers. The overall result anion and fills the channels in the framework to balance the ultimately constructs a 3D cationic energetic framework. The charge. These observations indicate that the exchange of − − framework possesses a one-dimensional (1D) triangular N(NO2)2 took place through an interesting SC SC process. channel with the size of 11.825 × 8.658/2 Å2, which is almost To our knowledge, the SC−SC transformation of encapsulation identical to that of MOF(Cu) (11.823 × 8.644/2 Å2). The labile energetic anions within MOFs has not been explored yet, fi − − channels are lled with N(NO2)2 anions; in addition, the although a few examples of solid solid exchange have been

C DOI: 10.1021/acs.chemmater.5b04891 Chem. Mater. XXXX, XXX, XXX−XXX Chemistry of Materials Article 62−69 − ⊂ fi ° 11 reported. Moreover, N(NO2)2 MOF(Cu) is the rst 1,2,4-triazolium dinitramide (Td = 200 C), the most stable − reported example of a porous compound able to take up compounds containing N(NO2)2 ever known. The high − − ⊂ N(NO2)2 counteranions. stability of N(NO2)2 MOF(Cu) is presumably caused by Stability and Detonation Properties. Surprisingly, N- the strong structural reinforcement of the 3D framework. − − fi (NO2)2 encapsulated in the framework was found to be quite Additionally, N(NO2)2 is encapsulated inside the con ned stable in air and common solvents and was to be almost space of the tight framework, which could restrain the nonhygroscopic in air for one month (Figure 6 and Table S1). 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 Figure 6. Comparison of the hygroscopic property of ADN and rigid ligands and metal ions generates a stable and insensitive − ⊂ N(NO2)2 MOF(Cu). structural framework. Additionally, the sensitive anion N- − (NO2)2 is encapsulated inside the host framework, resulting in The PXRD patterns and IR analysis indicated that the resultant lower sensitivity. inclusion complex was still intact even after being heated at 150 The resultant host−guest complex possesses good energetic °C for 24 h (Figures S10 and S15). In contrast, ADN, one of − ⊂ properties. The density of N(NO2)2 MOF(Cu) is 1.78 g the most promising eco-friendly energetic oxidizers containing −3 −3 − cm , which is comparable to ADN (1.81 g cm ), but is higher 70,71 ff − free N(NO2)2 , su ers from severe hygroscopicity and low than that of the parent complex [MOF(Cu), 1.64 g cm 3]. The stability; it decomposes even at temperatures below its melting constant-volume combustion energy (Δ U) for N(NO ) − ⊂ point (90 °C),72,73 which affects its normal usage. The thermal c 2 2 fi MOF(Cu) was measured by an oxygen bomb calorimeter, stability of ADN does not signi cantly improve even when along with MOF(Cu) and RDX as reference compounds. The physically mixed with MOF(Cu) (Figure 7). However, Δ ° Δ enthalpy of combustion ( cH ) was calculated from cU, and a correction for change in gas volume during combustion was included (Scheme 2, eq 1). The standard enthalpies of Δ ° − ⊂ formation ( fH ) 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 77 Δ ° − ⊂ Information). The calculated fH value of N(NO2)2 −1 Δ ° MOF(Cu) is 3667 kJ mol , while the fH 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 Figure 7. DSC curves of various samples measured with a heating rate the prediction of heat of detonation of energetic compounds − − of 5 °C min 1: (a) MOF(Cu); (b) the mixture of MOF(Cu) and containing CHON elements.78 80 However, an effective ADN through physical mixing (the mole ratio of MOF(Cu) to ADN is − ⊂ − ⊂ method for the accurate prediction of heats of detonation of 1:1); (c) ADN; (d) N(NO2)2 MOF(Cu); (e) N3 MOF(Cu). energetic MOFs is still scarce. In a recent study, we developed a simple method to calculate the heats of detonation of some − ⊂ thermogravimetric analysis of N(NO2)2 MOF(Cu) samples metal-containing explosives on the basis of the empirical showed a main loss weight of 70% in the temperature regime Kamlet formula.81 Here, using the experimental determined − ° ff −Δ 220 400 C. Moreover, the thermogravimetric/di erential (back-calculated from cU) enthalpy of formation, we scanning calorimetry (TG/DSC) analysis unambiguously adopted our developed method to predict the heats of − ⊂ demonstrated that its onset decomposition temperature reaches detonation of N(NO2)2 MOF(Cu) and MOF(Cu) (see up to 221 °C(Figures S16 and S17), which is even higher than the Supporting Information). According to the largest ° those of N-guanylurea dinitramide (FOX-12, Td = 201 C, at a exothermic principle proposed by Kamlet and our developed ° −1 ′ ′ ′ − ⊂ heating rate of 5 C min ) and 4,4 ,5,5 -tetraamino-3,3 -bi- method, during detonation of N(NO2)2 MOF(Cu) and

D DOI: 10.1021/acs.chemmater.5b04891 Chem. Mater. XXXX, XXX, XXX−XXX Chemistry of Materials Article

Table 1. Physicochemical Properties of Various Energetic Compounds a ρb d e Ω f g h i −Δ j −Δ ok Δ °l m − n compd Td N% O+N% CO IS FS ESD cU cH fH Vo Q −⊂ − s s N(NO2)2 221 1.78 54.81 71.4 22.92 9 73 1.9 10299 10261 3667 642 7176 MOF(Cu) (1.80c) (723t) (6824t) MOF(Cu) 223 1.64 53.53 67.7 −28.24 16 112 24.75 8275 8244 1651 626s 4562s (1.68c) (698t) (4388t) RDXo 210 1.81 37.84 81.06 0 7.5 120 0.2 2103.0 2092 53.8 756s 6166s (791t) (5668t) ADNp 130 1.81 52.00 96.8 26 3−5 64 >0.16 −150.0 986t 2789t RDXq 210 1.81 37.84 81.06 0 7.5 120 0.2 70.3 785t 5845t Cl-20r 221 2.04 38.36 82.18 10.95 4 48 0.13 365 715t 6168t aThe 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 −1 l −Δ −1 m combustion (kJ mol ). Experiment determined (back-calculated from cU) enthalpy of formation (kJ mol ). Volume 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 −Δ p − experimental determined (back-calculated from cU) enthalpy of formation. Properties of ADN are taken from refs 74, 75, and 83 85. qProperties 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.

− Scheme 2. Combustion Reactions of Energetic MOFs and Information). The calculated heat of detonation of N(NO2)2 − RDX, and Hess’ Law for These Combustion Reactions ⊂ 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 −1 − ⊂ kg ). 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 Scheme 3. Detonation Reactions of Energetic MOFs and molecules, but they had to be subsequently removed or broken RDX, and the Empirical Kamlet Formula for Their Heats of to make use of guest molecules. In contrast, not only can the Detonation 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 MOF(Cu), all N atoms are converted to N2; O atoms form fi these complexes maintained the parent structures (Figures S19 H2O with H atoms rst and then form CO2 with C atoms. The remaining C atoms are retained in the solid state; if there are O and S20). As the mole ratio of ADN/MOF(Cu) in the reaction mixtures gradually increased from 0 to 3, the resultant complex atoms left, they will form O2. In addition, the copper atoms should be converted to their reduction state (Cu) during also showed a gradual increase in the encapsulated quantity of − −3 N(NO2)2 anions, density from 1.64 to 1.78 g cm , volume of detonation since the heat of formation of copper oxide − Δ ° − −1 1 [ fH (CuO, s) = 156.06 kJ mol ] is higher than that of gases after detonation from 626 to 642 L kg , and heat of Δ ° − −1 77 −1 water [ fH (H2O, g) = 241.83 kJ mol ]. On the basis of detonation from 4562 to 7176 kJ kg , while the impact the above theories, the detonation reactions of our as- sensitivity showed a gradual decrease from 16 to 9 J (Figure 8, synthesized energetic MOFs were proposed in Scheme 3, and Figures S21 and S22). Therefore, in comparison with the previous methods for tuning the properties of energetic their heats of detonation were evaluated by the empirical − Kamlet formula (Scheme 3,eq1).79,81 To confirm the materials by using complicated chemical modifications,36 47 prediction accuracy of this method, we also employed the this approach appears simpler and more convenient. EXPLO5 computer code in its new version 6.01 to calculate Anion Release. In addition to stabilization of labile anions their heats of detonation (Table 1,seetheSupporting in host containers and modulation of their properties, the facile

E DOI: 10.1021/acs.chemmater.5b04891 Chem. Mater. XXXX, XXX, XXX−XXX Chemistry of Materials Article

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/ − ⊂ release of encapsulated guest anions from host containers is N(NO2)2 MOF(Cu) in the reaction mixtures. also important for the subsequent delivery or utilization of the − anions. Although the N(NO2)2 anion appeared to be fi 1.5 J, indicating that the solid product can be tuned from a inde nitely stable within the host framework, releasing the potential secondary explosive to a high sensitivity primary anion from the framework through the addition of a competing explosive. energetic guest anion proved to be straightforward (Figure 9). ■ 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. Further- more, the resultant host−guest complex possesses a rather high − nitrogen and oxygen content (71.4%), and good energetic Figure 9. Schematic representation of release process for N(NO2)2 . properties, which can be conveniently tuned by changing the mole ratio of the starting materials ADN and MOF(Cu) at By simply immersing the crystals of N(NO ) − ⊂ MOF(Cu) in room temperature without complicated chemical modifications. 2 2 − 10 mL of 2-fold molar NaN3 aqueous solution for 30 min, the Despite its stabilization, the encapsulated N(NO2)2 can also crystals underwent a rapid naked-eye detectable change in color be released in a controlled fashion without disrupting the − (from blue to deep brown), as illustrated in Figure 3. According framework by adding the competing guest anion N3 . − − to HPLC MS analysis, the release of N(NO2)2 from the Therefore, MOF(Cu) could facilitate the handing and use of − − framework into the aqueous solution was evidenced by the N(NO2)2 , while the resultant host guest complex is also a appearance of a mass peak at 105.9 m/z (M+, Figures S23−25 new potential high energy density material. This present study and S34). In addition, FT-IR spectra of the concomitant solid may not only shed new insight into the stabilization, storage, − ⊂ product [N3 MOF(Cu)] showed a strong band associated and release of labile energetic anions under ambient conditions, − −1 with the encapsulated anion N3 (2050 cm ), along with the but also provide a convenient approach for the preparation of − −1 disappearance of band of N(NO2)2 (1390 cm , Figure 2). new energetic MOFs as well as the modulation of their Other bands in the spectra remained almost unchanged, while energetic properties. its PXRD pattern appeared to be considerably similar to that of − ⊂ ■ EXPERIMENTAL SECTION N(NO2)2 MOF(Cu) (Figure S28), suggesting that the framework remained intact throughout the release process and Safety Precautions. Although none of the energetic MOFs − that the N(NO2)2 anions in the framework had been almost described herein have exploded or detonated in the course of this completely released. The concomitant solid product also research, these materials should be handled with extreme care using exhibits a remarkable thermal stability (T = 216 °C, Figure the best safety practices. d General Methods. 7 and Figure S30), which further confirms that the host MOF(Cu) and NH4N(NO2)2 (ADN) were prepared according to the previous literature studies.55,82 All other framework can serve as a polymeric container for safekeeping materials were commercially available and used without further labile energetic anions. Interestingly, the encapsulated N- fi ff − puri cation. Powder X-ray di raction (PXRD) patterns of the samples (NO2)2 anion can be controllably released, and the properties were analyzed with monochromatized Cu Kα (λ = 1.54178 Å) incident of the concomitant solid product can also be tuned by changing radiation by Bruker D8 Advance X-ray diffractometer operating at 40 − ⊂ ° the mole ratio of NaN3 to N(NO2)2 MOF(Cu) in the kV voltage and 50 mA current. PXRD patterns were recorded from 5

F DOI: 10.1021/acs.chemmater.5b04891 Chem. Mater. XXXX, XXX, XXX−XXX Chemistry of Materials Article to 80° (2θ) at 298 K. IR spectrum was recorded on a Bruker Tensor ■ ASSOCIATED CONTENT 27 spectrophotometer with HTS-XT (KBr pellets). Elemental analysis *S Supporting Information was performed on an Elementar Vario EL (Germany). To determine The Supporting Information is available free of charge on the the thermal stabilities of as-synthesized energetic MOFs and ADN, a ACS Publications website at DOI: 10.1021/acs.chemma- TA-DSC Q2000 differential scanning calorimeter (heating rate, 5 °C − − ter.5b04891. 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 Experimental methods, additional figures and tables by using Automatic Density Analyzer, ULTRAPYC 1200e. The MS described herein (PDF) spectra of as-synthesized MOFs were measured by MALDI-TOF mass spectrometry (Bruker Corporation). The concentration of the aqueous ■ AUTHOR INFORMATION solution and its ESI-MS were analyzed by HPLC−MS (HPLC, Agilent Corresponding Authors 6100 series; ESI-MS, an Agilent Technologies 6120 mass analyzer; * fl −1 E-mail: [email protected]. eluent, 10% CH3OH in water; ow rate, 1 mL min ). Before the * measurement of density, constant-volume combustion energy, E-mail: [email protected]. − ⊂ sensitivities, and hygroscopicity, N(NO2)2 MOF(Cu) crystals Notes − ⊂ fi have been desolvated. The desolvated procedure follows: N(NO2)2 The authors declare no competing nancial interest. MOF(Cu) crystals were immersed in anhydrous methanol for 3 days, during which the exchanged solvent was decanted and freshly ■ ACKNOWLEDGMENTS ° replenished three times, then dried in vacuum at 50 C for 24 h in The authors acknowledge financial support from the National order to remove the guest water locating in the channel. − ⊂ Natural Science Foundation of China (21442003, 21576026, X-ray Crystallography. The crystal structure of N(NO2)2 ff and U153062) and the opening project of State Key Laboratory MOF(Cu) was determined by a Rigaku RAXIS IP di ractometer and of Science and Technology (Beijing Institute of Technology, SHELXTL crystallographic software package of molecular structure. ZDKT12-03). The single crystals were mounted on a Rigaku RAXIS RAPID IP ff α di ractometer equipped with a graphite-monochromatized Mo K ■ REFERENCES radiation (λ= 0.71073 Å). Data were collected by the ω scan (1) Gao, H.; Shreeve, J. M. Azole-Based Energetic Salts. Chem. Rev. technique. The structure was solved by direct methods with SHELXS- − 97 and expanded by using the Fourier technique. The non-hydrogen 2011, 111, 7377 7436. (2) Zhang, Q.; Shreeve, J. M. Energetic Ionic Liquids as Explosives atoms were refined anisotropically. The hydrogen atom was fi and Propellant Fuels: a New Journey of Ionic Liquid Chemistry. 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G DOI: 10.1021/acs.chemmater.5b04891 Chem. Mater. XXXX, XXX, XXX−XXX Chemistry of Materials Article

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I DOI: 10.1021/acs.chemmater.5b04891 Chem. Mater. XXXX, XXX, XXX−XXX