Taming Dinitramide Anions Within an Energetic Metal–Organic Framework

Article pubs.acs.org/cm 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- © XXXX American Chemical Society A DOI: 10.1021/acs.chemmater.5b04891 Chem. Mater. XXXX, XXX, XXX−XXX Chemistry of Materials Article − ⊂ 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

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