Coordination Polymers with High Energy Density: An Emerging Class

Nov 9, 2015 - CPs have shown potential to improve energetic performance relative to conventional organic energetic materials with regard to density, o...
3 downloads 15 Views 7MB Size
Download PDF
Review pubs.acs.org/crystal

Coordination Polymers with High Energy Density: An Emerging Class of Explosives Kyle A. McDonald,† Saona Seth,† and Adam J. Matzger*,†,‡ †

Department of Chemistry and ‡Macromolecular Science and Engineering Program, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109-1055, United States ABSTRACT: Coordination polymers (CPs) have recently emerged as a promising class of high energy materials useful for the synthesis of tailored energetic materials. CPs have shown potential to improve energetic performance relative to conventional organic energetic materials with regard to density, oxygen balance, sensitivity, and heat of detonation. Thus far, a variety of energetic linkers have been applied, and success has been achieved across a number of structure classes including nitrogen-rich heterocycles and azides. Here, the current progress in the field of energetic CPs, both from the standpoint of structure and properties, is reviewed, and a perspective on current challenges and promising future directions for the field are delineated. Inasmuch as structure/function relationships have been elucidated for CPs with particular applications in mind, these are discussed as well as shortcomings in experimental work required to firmly establish predictive principles within the realm of energetic materials.

1. INTRODUCTION Among the numerous applications proposed for coordination polymers (CPs and in particular the subset referred to as metal−organic frameworks or MOFs), including separations,1,2 gas storage,3−5 catalysis,6−9 and sensing,10,11 a surprising potential application has recently emerged to use CPs as energetic materials (explosives, propellants, and pyrotechnics).12−40 In the case of explosives, this class of high energydensity materials typically derives from the combination of fuel (carbon, hydrogen) and oxidizer covalently bonded with the inclusion of atoms such as nitrogen, which can provide additional heat upon decomposition. At first there appears to be a tension between the need for energetic materials to have high density, a requirement to achieve superior detonation velocity/pressure, and the tendency of CPs to be among the most porous (least dense) crystalline materials.41 However, in addition to having high-energy density, the ideal energetic material should also display good thermal stability, controllable impact sensitivity, and produce environmentally benign reaction products upon detonation.42,43 These requirements point to the potential advantages of CP tunability in producing tailored energetics. Explosives can be divided, according to their sensitivity, into primary and secondary explosives. Primary explosives can be initiated via a small stimulus; this class of energetic materials has characteristically fast deflagration to detonation transitions upon initiation.44 Sensitive functionality in the form of azides, for example, often leads to the high sensitivity of primary explosives. Traditional primary explosives, including mercury(II) fulminate and lead azide, itself a CP, have been widely used in commercial applications since the late 1800s.45,46 However, due to health and environmental concerns, much attention has been given to replacing these primary explosives with new © XXXX American Chemical Society

materials having decreased toxicity and environmental impact.42,44 As discussed below CPs are of potential interest for this application. With regard to the use of CPs as secondary (low sensitivity, high power) explosives, the challenges are considerable as there are many high energy-density organic molecules manufactured safely and on a large scale (TNT, RDX, HMX, CL-20, etc.) for applications from munitions to mining.43 However, there is a potential advantage with CPs over the dominant class of organic molecular materials; by being connected in one or more dimensions, the weak (distant) intermolecular interactions of common (molecular) explosives might be replaced with shorter coordination bonds leading to higher density. Incorporation of energetic linkers into CPs is the most straightforward route to produce energetic CPs. Unfortunately the ubiquitous metal-carboxylate coordination chemistry is undesirable for energetic materials since the nearly fully oxidized carbons detract from performance. Fortunately, metal−nitrogen as well as other coordination chemistries that have been developed are much more attractive for this application.47−50 CPs based on energetic linkers, such as hydrazines,22,25−27,51 triazoles,28,30,37−39 and tetrazoles,24,29,33,36 can form one-, two-, and three-dimensional structures and have emerged as promising candidates for primary or secondary explosives wherein energetic properties such as sensitivity and stability can be modulated. MOFs derived from energetic linkers have been present in the literature for well over a decade,52−54 although the energetic properties of materials derived from such building blocks have only been explored in detail over the last 10 years.12−23,40 Recently, progress in Received: October 7, 2015

A

DOI: 10.1021/acs.cgd.5b01436 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Review

Figure 1. Nitrogen-rich heterocyclic linkers used in the synthesis of energetic CPs: (a) azide, (b) hydrazine, (c) hydrazinecarboxylate, (d) 5methyltetrazole (Mtta), (e) 3-amino-1H-1,2,4-triazole (atrz), (f) 3-nitro-1H-1,2,4-triazole (Hntz), (g) 3-(tetrazole-5-yl)triazole (H2tztr), (h) 5aminotetrazole (HATZ), (i) 1,5-diaminotetrazole (DAT), (j) 4,4′-azo-1,2,4-triazole (ATRZ), (k) N,N-bis(1H-tetrazole-5-yl)-amine (H2bta).

energetic metal complexes and CPs has been cataloged by Gao and co-workers.55 The present short review focuses on the synthesis, structure, and properties of selected energetic CPs for their use as explosives and examines current challenges as well as future directions for the field.

2. DESIGN OF ENERGETIC COORDINATION POLYMERS The opportunity to control coordination polymer properties through selection of the linker(s) employed in their synthesis offers a vast parameter space for exploration that is further augmented when considering the array of metals available to connect these groups.56 Construction of CPs with nitrogen-rich molecular building blocks is an attractive approach because many such compounds are themselves energetic materials.42,57 Moreover, the polytopicity of nitrogen-rich heterocyclic linkers makes them natural targets for producing CPs with higher dimensionality. Employing linkers which are obtained by derivatization of different commonly used energetic materials allows introducing new metal coordination modes and can modulate the properties of the energetic material. Below some of the prominent ligand classes explored thus far for energetic CPs are discussed in the context of the dimensionality of coordination networks observed (Figure 1). 2.1. Low-Dimensional Coordination Polymers (1D/ 2D). Lead azide is the conceptual precursor for many of the energetic coordination polymers made since.44,45 Azide is a 100% nitrogen-containing linker, which can exhibit μ1,1 and/or μ1,3 coordination modes; azide is known to be energetic and can considerably increase the heat of formation of CPs.50,58 Azide has been employed with different metal ions in the presence of colinkers to synthesize energetic CPs. This enables fine-tuning of their structures and properties. For example, Yang and co-workers synthesized a hydrazine and azide based nitrogen-rich (65.5% nitrogen) energetic CP [Zn(N2H4)2(N3)2]n from the reaction of zinc acetate, hydrazine, and sodium azide.25 The compound is a 1D CP formed through the bridging of octahedral metal ions by hydrazine in an anti conformation, while the azide groups are terminal to the metal centers (Figure 2a). This CP has a high density of 2.083 g cm−3, which is notably higher than common organic energetic materials (HMX, RDX, TNT; see below). Zhang and coworkers synthesized the 2D CP [Cd(DAT)2(N3)2]n containing bridging azide linkers and 1,5-diaminotetrazole (DAT) as terminal ligands (Figure 2g). The DAT ligands are coordinated to the octahedral Cd(II) and project out of the 2D square

Figure 2. 1D and 2D coordination polymers: (a) [Zn(N2H4)2(N3)2]n, (b) NHN, (c) NHP, (d) [Cu(bta)(NH3)2]n, (e) CHHP, (f) ZnHHP, (g) [Cd(DAT)2(N3)2]n, and (h) [Cu(Htztr)]n. Hydrogens and counterion guests are omitted for clarity (Zn: orange, Ni: green, Cu: dark turquoise, Co: maroon, Cd: light yellow, C: gray, N: blue, O: red).

lattice toward the cavities of the neighboring layers. The crystallographic density of this CP is 2.144 g cm−3.23 There are other examples of 2D CPs containing azides;[Cu(en)2(N3)4]n (en = ethylenediamine) and [Cu(pn)(N3)2]n (pn = 1,2diaminopropane), in which en and pn act as chelating ligands, have both bridging and terminal azides.15,18 The effects of anions on the structures and energetic performance of hydrazine-based CPs was studied by HopeWeeks and co-workers.26 Two cobalt- and nickel-based CPs were synthesized, namely, cobalt hydrazine perchlorate (CHP) and nickel hydrazine perchlorate (NHP). Their objective was to improve the energetic properties of the previously known material nickel hydrazine nitrate (NHN) by changing the nitrate anion to perchlorate. NHN consists of 1D metalhydrazine chains, wherein hydrazine acts as a sole innersphere bridging linker and connects the adjacent metal centers in a staggered conformation (Figure 2b). Nitrate ions are uncoordinated and occupy the interstitial space of the CP. CHP and NHP are isostructural 1D coordination polymers. Unlike NHN, the octahedral metal centers in CHP and NHP are monocoordinated by four hydrazines and bridged by two hydrazine linkers coordinated in an anti-fashion (Figure 2c). The perchlorate ions are present in between the 1D chains. The CPs have densities of 1.948 g cm−3 and 1.983 g cm−3 for CHP and NHP, respectively. B

DOI: 10.1021/acs.cgd.5b01436 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Review

Hydrazine has a limited number of coordination modes that are available for the formation of CPs; therefore, the derivitization of hydrazine allows for the formation of more diverse CPs. Absorption of atmospheric CO2 into a reaction mixture containing metal perchlorate salts and hydrazine resulted in the partial conversion of hydrazine to hydrazinecarboxylate, which acts as a three-connecting colinker, leading to the formation of 2D sheets.27 The CPs obtained with zinc and cobalt perchlorates are referred to as cobalt hydrazine hydrazinecarboxylate perchlorate (CHHP) and zinc hydrazine hydrazinecarboxylate perchlorate (ZnHHP). CHHP consists of one crystallographically independent octahedral Co(II) ion that is chelated by one hydrazinecarboxylate coordinated through the terminal nitrogen and one oxygen atom (Figure 2d). The metal ion is further coordinated by one oxygen atom of another hydrazinecarboxylate unit, two bridging hydrazines, and one terminal hydrazine. ZnHHP consists of three crystallographically independent metal ions, all of which are octahedral but with different coordination environments leading to a different 2D sheet-like network (Figure 2e). In both cases, the perchlorate ions occupy the interstitial spaces between the layers. The density of CHHP and ZnHHP are 2.000 g cm−3 and 2.117 g cm−3, respectively. The reported values for the density are marginally higher than the 1D CHP and NHP analogues, although this does not translate into improved detonation performance (see section 3.4). A series of triazole- and tetrazole-functionalized linkers have been employed to access CPs to investigate different properties, such as magnetism59,60 and photoluminescence,59,61 in addition to applications such as CO2 capture.62,63 However, energetic properties of the CPs based on these heterocycle derivatives have only recently been explored in the literature. The early work by Friedrich et al. employed the semirigid nitrogen rich heterocyclic linker N,N-bis(1H-tetrazole-5-yl)-amine (H2bta) with Cu(II) salts leading to the CP [Cu(bta)(NH3)2]n.40 The metal ion behaves as a distorted square pyramidal node and the linker serves as a chelating as well as bridging unit leading to a 1D CP with a density of 1.988 g cm−3 (Figure 2d). Chen and co-workers investigated the mixed-heterocyclic linker 3(tetrazole-5-yl)-1,2,4-triazole (H2tztr) which has seven potential N-coordination sites in its completely deprotonated state and exhibits variable coordination modes.37 In a series of 2D energetic CPs with Pb(II) and Cu(I) ions, the H2tztr linker serves as a chelating group for the metal as well as a bridging unit.30 In the crystal lattice of [Pb(Htztr)2(H2O)]n, the Pb(II)ions act as an 8 coordinated node (Figure 3a); each Pb(II)-ion is chelated by three Htztr linkers and further coordinated by one Htztr linker and one water molecule. The Htztr linker exhibits variable coordination modes acting as both a threeconnecting and four-connecting unit (Figure 3b). The CP [Pb(H2tztr)(O)]n contains one crystallographically independent metal ion with distorted pyramidal disposition, which is coordinated by one chelating linker, one monocoordinated linker, and three bridging oxo groups (Figure 3c). H2tztr acts as a three-connecting linker in this case. [Pb(Htztr)2(H2O)]n and [Pb(H2tztr)(O)]n have densities of 2.519 g cm−3 and 3.511 g cm−3 respectively. The large difference represents the wide range of densities that can be achieved in CPs using the same metal and a similar ligand sphere. The H2tztr linkers in [Pb(H2tztr)(O)]n are oriented such that the metal centers are brought closer decreasing the porosity, which in turn makes this structure more dense compared to [Pb(Htztr)2(H2O)]n. These densities are notably high when compared to other CPs and

Figure 3. (a) Coordination geometry of the metal ion and (b) 2D polymeric layer of the CP [Pb(Htztr)2(H2O)]n. (c) Coordination environment of the metal ion and (d) 2D coordination network of the CP [Pb(H2tztr)(O)]n. Notice more densely packed 2D network in [Pb(H2tztr)(O)]n as compared to [Pb(Htztr)2(H2O)]n. Hydrogens are omitted for clarity (Pb: lime green, C: gray, N: blue, O: red).

commonly used organic energetic materials. However, the density of these two Pb(II)-CPs are much lower than lead azide (4.71 g cm−3), which is a 3D coordination polymer.64 The lower density of the azole-based CPs can be attributed to the lower weight percentage of lead in addition to the increase in crystallographic void space relative to lead azide. With toxicity in mind H2tztr was used to synthesize environmentally benign high-density energetic CPs with copper salts. The CP [Cu(Htztr)]n was obtained with CuCN and resulted in a structure in which both the metal ion and the linker act as 4connecting units (Figure 2h).37 The CP has a density of 2.44 g cm−3. Recently, substituent effects on linkers were investigated as a means to modify energetic properties of CPs. Qu et al. employed the linkers 3-amino-1H-1,2,4-triazole (Hatrz) and 3nitro-1H-1,2,4-triazole (Hntz) and synthesized two Ag(I)-CPs: [Ag(atrz)]n (Ag-atrz) and [Ag(ntz)]n (Ag-ntz).38 The CPs are isostructural and possess 2D network structures in which the metal ions, as well as the linkers, serve as 3-connecting units (Figures 4a,c). The shortest distance between the metal ions of the neighboring layers, in the case of Ag-atrz, is 3.46 Å, which is very close to the sum of van der Waals radii of two Ag(I) ions (3.44 Å) (Figure 4b). In the case of Ag-ntz, the Ag(I)−Ag(I) distance is substantially longer (3.60 Å). The 2D layers in Agntz are more corrugated than those of Ag-atrz presumably due to the bulkier nitro-groups, which are projected out of the 2D layers, in the former (Figure 4b,d). The crystallographic densities are 2.996 g cm−3 and 3.121 g cm−3 for Ag-atrz and Ag-ntz respectively. 2.2. Three-Dimensional Coordination Polymers. Because of the robust nature of many 3D CPs, it might be expected that energetic CPs with 3D structures will lead to formation of more stable energetic materials relative to 1D and 2D coordination networks. Chen and co-workers proposed 3,5dinitrobenzoic acid (3,5-DNBA) as a colinker to improve the stability and insensitivity of azide-based coordination networks.31 They obtained an energetic CP, [Cu(3,5-DNBA)(N3)]n, in which 3,5-DNBA is involved in the construction of C

DOI: 10.1021/acs.cgd.5b01436 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Review

produces the 3D CP ATRZ-1 (Figure 6a), a doubly interpenetrated network structure. The density of ATRZ-1 is only 1.68 g cm−3, which is lower than that of the other 1D and 2D CPs mentioned in the previous section. The linker exhibits variable coordination modes to serve as both 2- and 3connecting units in the presence of Ag(I) (acts as tetrahedral unit) and leads to the 3D CP ATRZ-2, with a density of 2.16 g cm−3 (Figure 6b). In both cases, nitrate ions occupy the interstitial space of the framework. Another study by Gao and co-workers reported a 3D energetic CP, [Co5(atrz)7(N3)3]n, containing a triazole based linker (atrz = 3-amino-1H-1,2,4triazole to be distinguished from ATRZ above), with azide bridging functionality.39 The atrz linker acts as a 3-connecting unit (Figure 7d). The CP contains three crystallographically independent Co(II)-ions (Figure 7a−c). Co1 and Co2 ions are octahedral and are coordinated by four atrz linkers and two azides (Figure 7a,b). In contrast, Co3 is tetrahedral and is coordinated by four atrz linkers. The Co1 and Co2 ions form 1D chains which propagate in perpendicular directions; these chains are bridged by [Co(atrz)2] units (containing the tetrahedral Co3) resulting in the 3D framework structure (Figure 7d). Gao and co-workers synthesized a 3D CP {[Cu(tztr)]H2O}n (Figure 8a), based on the mixed-heterocyclic linker H2tztr.37 The CP consists of distorted square pyramidal metal ions which are coordinated by one chelating linker and three other linkers in monodentate fashion (Figure 8b). In this case, the linker is 5connecting and acts as both a chelating and a bridging unit (Figure 8c). The density of the 3D framework is 2.36 g cm−3, which is lower than the 2D CPs derived from the same linker. Tetrazole-based linkers have the potential to form 3D CPs through a number of readily accessible coordination modes. A series of linkers based solely on tetrazolate heterocycles, namely, 5-aminotetrazole (HATZ), 5-methyl tetrazole (Mtta), N,N-bis(1H-tetrazole-5-yl)-amine (H2bta), has been explored for energetic CP synthesis. Lin et al. employed HATZ as the linker to synthesize energetic CPs in the presence of tetraalkylammonium cation templates.29 They obtained a Cd(II)-CP, {(Et4N)[Cd6Br5(ATZ)8]·H2O}n, by using tetraethylammonium bromide as templating agent. The CP is an anionic 3D framework structure wherein the Et4N+ guests occupy the channels of the porous network. This CP has a density of 2.530 g cm−3, which is comparable to other CPs containing mixed tetrazole/triazole linker systems. The 3D heterometallic CP {[Cu4(Mtta)5][Na(CH3CN)]}n exemplifies the use of tertazole-based linkers for energetic CP synthesis in mixed metal systems.36 The Mtta linker serves as a 4-connecting (μ4-η 1:η 1:η 1:η 1) linker, half of which are coordinated to only Cu(I) ions and the other half are coordinated to both Cu(I) ions and Na(I) ions (Figure 9a,b).

Figure 4. 2D coordination networks of (a) Ag-atrz and (c) Ag-ntz. Packing of the 2D layers in the crystal structures of (b) Ag-atrz and (d) Ag-ntz. Hydrogens in the triazole rings are omitted for clarity (Ag: baby blue, C: gray, N: blue, O: red).

the 1D chains along with azide bridging groups (Figure 5a). Each metal ion is bridged by two azide ions in μ1,1 fashion and the two carboxylate groups from two 3,5-DNBA molecules. The authors note that the 1D chains are held together by weak coordination interactions between the oxygen of the nitrate functional groups and the Cu(II) ions (Cu−O distance = 2.609 Å) resulting in a 3D coordination network (Figure 5b,c). This CP has a density of 2.030 g cm−3. Pang and co-workers employed the 1,2,4-triazole-functionalized hydrazine derivative 4,4′-azo-1,2,4-triazole (ATRZ) as the linker to form 3D energetic CPs, [Cu(ATRZ)3(NO3)2]n (ATRZ-1) and [Ag(ATRZ)1.5(NO3)]n (ATRZ-2).28 The linker ATRZ contains six potentially coordinating sites, which can lead to 3D CPs when reacted with metal salts. The linker acts as a 2-connecting module with Cu(II), which serves as an octahedral node, and

Figure 5. (a) 1D chain of Cu(II) ions bridged by 3,5-DNBA and azide, (b) coordination mode of the 3,5-DNBA, and (c) the 3D coordination network of [Cu(3,5-DNBA)(N3)]n. Hydrogens are omitted for clarity (Cu: dark turquoise, C: gray, N: blue, O: red).

Figure 6. 3D coordination networks of (a) ATRZ-1, and (b) ATRZ-2. Hydrogens and perchlorate ions are omitted for clarity (Cu: dark turquoise, Ag: baby blue, C: gray, N: blue). D

DOI: 10.1021/acs.cgd.5b01436 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Review

Figure 7. Coordination environments of the metal ions (a) Co1, (b) Co2, and (c) Co3 in the CP [Co5(atrz)7(N3)3]n. (d) Bridging of the 1D metal ion chains by [Co(atrz)2] units to construct the 3D network of the CP. Hydrogens are omitted for clarity (Co1/Co2: maroon, Co3: gold, C: gray, N: blue).

Figure 9. Different coordination environments of the Mtta linker in the CP {[Cu4(Mtta)5][Na(CH3CN)]}n: (a) coordinated to Cu(I) and Na(I) ions and (b) coordinated to only Cu(I) ions. Coordination modes of the metal ions (c) Na(I) and (d) Cu(I). (e) 3D coordination network of {[Cu4(Mtta)5][Na(CH3CN)]}n. Hydrogens in the network structure are omitted for clarity (Cu: dark turquoise, Na: purple, C: gray, N: blue).

Figure 8. (a) Crystal packing and coordination environments of the (b) metal ion and (c) linker in the 3D CP {[Cu(tztr)]H2O}n. Hydrogens and guest water molecules are omitted for clarity (Cu: dark turquoise, C: gray, N: blue, O: red).

Figure 10. Different coordination modes of the H2bta linker in the CP {[Co9(bta)10(Hbta)2(H2O)10]·22H2O}n: (a) μ2-1,1′:4′, (b) μ31,1′:3:4′, (c) μ3-1,1′:4:4′, and (d) μ3-1,1′:3:3′. (e) 3D crystal structure of the CP {[Co9(bta)10(Hbta)2(H2O)10]·22H2O}n. Hydrogens and guest water molecules are omitted for clarity (Co: maroon, C: gray, N: blue).

The Cu(I) ions are tetrahedral and are coordinated by four nitrogen atoms from four different linkers (Figure 9d). The Na(I) ions in the framework exhibit a distorted trigonal bipyramidal coordination mode with four nitrogen atoms from the linkers and one from CH3CN (Figure 9c); this leads to a 3D framework with a density of 1.975 g cm−3 (Figure 9e). The same group later reported a 3D porous mixed-valence Co(II)− Co(III) energetic CP, {[Co9(bta)10(Hbta)2(H2O)10]·22H2O}n, from a tetrazole-derived linker H2bta.33 The linker exhibits four different types of coordination modes in the CP; these are μ21,1′:4′, μ3-1,1′:3:3′, μ3-1,1′:3:4′ and μ3-1,1′:4:4′ (Figure 10a− d). The CP contains an alternating array of discrete Co(II) and Co(III) ions, coordinated by the linker and water molecules, in the 3D framework structure (Figure 10e). The dehydrated material has a nitrogen content of 59.85% and exhibits different energetic properties than the CP in the presence of guest molecules. These compounds demonstrate guest-dependent energetic properties of CPs (see Section 3.2). One trend that can be seen from the previous sections is that the density of energetic CPs generally increases as one moves from 1D to 2D structures based on the replacement of weak intermolecular interactions in the crystal lattice with shorter coordination interactions, whereas density often decreases

moving from 2D to 3D structures, where the 3D structures are based on nitrogen rich heterocycles. The latter can be attributed to the creation of void spaces in the 3D structure. It is important to note that the density reported for many CP structures includes materials with low energy guests, such as [Pb(Htztr)2(H2O)]n, and solvates, such as {[Cu4(Mtta)5][Na(CH3CN)]}n, which gives an overall increased crystallographic density that is not expected to translate into improved performance. Moreover, the guest dependent density of energetic CPs makes it difficult to compare the density of energetic CPs with traditionally used organic energetic materials.

3. PROPERTIES OF ENERGETIC COORDINATION POLYMERS The key properties that characterize energetic materials (thermal stability, sensitivity, and heat of detonation) have the potential to be fine-tuned by utilization of coordination polymers. By employing the inherent properties of CPs, framework rigidity and structural/chemical modularity, novel E

DOI: 10.1021/acs.cgd.5b01436 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Review

CHHP CPs. In this case, the low decomposition temperature can be attributed primarily to the terminal azide groups, which sensitize this CP toward thermal decomposition. Energetic CPs containing bridging azides tend to display increased thermal decomposition temperatures relative to the 1D [Zn(N2H4)2(N3)2]n CP containing terminal azides. The nature of other ligands that coordinate to the metal center can affect the thermal decomposition temperature of the CP. For example, [Cu(en)2(N3)4]n and [Cu(pn)(N3)2]n, containing both bridging and terminal azides, have thermal decomposition temperatures of 202 (TGA) and 216 °C (DSC), respectively.15,18 Additionally, [Cd(DAT)2(N3)2]n, containing only bridging azides and terminal DAT ligands, has an increased thermal decomposition temperature of 232 °C (TGA/DSC), relative to the CPs containing terminal azide functionality.23 The energetic CPs containing bridging azide functionality and bridging colinkers, such as 3,5-DNBA in [Cu(3,5-DNBA)(N3)]n (Figure 5) and atrz in [Co(atrz)7(N3)3]n (Figure 7), have increased thermal decomposition temperatures relative to [Zn(N2H4)2(N3)2]n, [Cu(en)2(N3)4]n, [Cu(pn)(N3)2]n, and [Cd(DAT)2(N3)2]n.31,39 These energetic CPs have thermal decomposition temperatures of 268 °C (TGA/DSC) and 285 °C (TGA/DSC) for [Cu(3,5-DNBA)(N3)]n and [Co5(atrz)7(N3)3]n, respectively.31,39 Among azide-containing energetic CPs, it generally is observed that CPs containing bridging azides have increased thermal decomposition temperatures relative to CPs containing terminal azides.15,18,20,22,23,25,31,39 CPs derived from heterocyclic linkers, such as HATZ, Hntz, H2tztr, and Mtta (shown in Figure 1) have been demonstrated to be stable beyond 300 °C. The 2D CPs Ag-atrz and Ag-ntz (Figure 4) have thermal decomposition temperatures of 348 and 302 °C respectively (TGA/DSC).38 The H2tztr linker produces 2D CPs, [Pb(Htztr)2(H2O)]n (Figure 3b), [Pb(H2tztr)(O)]n (Figure 3d), and [Cu(Htztr)]n (Figure 2h), which show impressive thermal decomposition temperatures of 335 °C (TGA), 314 °C (TGA), and 355 °C (TGA), respectively.30,37 The authors claim that the formation of a 3D structure from the 2D layer structure by virtue of hydrogen bonding contributes to the elevated thermal decomposition temperature in the case of [Cu(Htztr)]n. The thermostability of the 2D [Cu(Htztr)]n CP may also increase relative to the Pb(II) based CPs due to the rigid coordination environment that makes up the 2D sheets. The 3D energetic CP {[Cu4(Mtta)5][Na(CH3CN)]}n (Figure 9) obtained with the Mtta linker has a thermal decomposition temperature above 335 °C (TGA/DSC).36 Throughout the literature, there seems to be a correlation between rigid linker systems and high thermostability. However, there is no nonrigid (flexible) version of the nitrogen rich heterocyclic linkers used in energetic CP synthesis to support the theory that thermostability results from linker rigidity and the tendency of rigid ligands to yield more open structures with lower energy density cannot be neglected. As mentioned previously, coordination with metal nitrates sensitizes the ATRZ linker toward thermal decomposition; however, this is the only example we are aware of in the energetic CP literature where the thermostability of the pure linker has been compared to the resulting CP. This is particularly important for new and more complex energetic linkers where energetic performance and stability has not been previously reported in the literature. 3.2. Sensitivity. Sensitivity to impact, spark, and friction are critical issues in the design of safe energetic materials. Energetic CPs show potential to act both as primary or secondary

energetic materials can be synthesized with optimized properties. Furthermore, the energetic properties of the CP can be fine-tuned by functionalization of the linker(s). In this section the properties of energetic CPs examined in the literature are discussed in the context of thermal decomposition, sensitivity to impact, spark, and friction, oxygen balance, and heat of detonation. Moreover, we draw some parallels between structure and properties of energetic CPs and ultimately offer guidance for the synthesis of new energetic CPs with improved performance. 3.1. Thermal Decomposition. Thermal decomposition is one of the most fundamental physiochemical properties of energetic materials. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) are commonly used to examine the thermal decomposition of energetic materials as well as, for DSC, the energy released upon decomposition. For example the energetic CP lead azide decomposes above 190 °C.64 CPs offer the opportunity to control their properties via modification of the building blocks. Therefore, it is possible to improve the thermal stabilities of energetic CPs by modification of the metal ions or linkers employed in synthesis. The 1D CP CHP (Figure 2c) shows a single exotherm between 187 and 193 °C by DSC.26 The inclusion of hydrazinecarboxylate as a colinker with hydrazine to form 2D CPs with perchlorate salts resulted in more robust, thermally stable 2D energetic CPs: ZnHHP and CHHP (Figure 2e,f).27 The decomposition temperature for CHHP (231 °C, detonation) shows an increase in the thermal stability of the CP relative to CHP in spite of being composed of the same metal and sharing some of the same ligand sphere. The authors claim that the 2D scaffold of CHHP results in an increase in structural rigidity of the frameworks relative to CHP leading to a more thermally stable energetic material. Similarly, ZnHHP, a 2D CP based on the same linkers as CHHP, but with a different network structure shows a relatively high decomposition temperature of 293 °C (deflagration). However, the influence of structural rigidity is difficult to deconvolute from the overall lower energetic content of the CPs caused by introduction of the carboxylate functionality (see Section 3.4). The utilization of 4,4′-azo-1,2,4-triazole increases the available coordination modes, relative to hydrazine-based linkers, leading to the formation of 3D energetic CPs ATRZ1 and ATRZ-2 (Figure 6a,b).28 These 3D CPs based on nitrogen-rich heterocycles have thermal stabilities of 243 °C for ATRZ-1 and 257 °C for ATRZ-2 (DSC). It is important to note that the thermostability of the ATRZ linker (313 °C) is higher than the corresponding CPs. In other words coordinating to the metal nitrate sensitizes the linker toward thermal decomposition. Azides are used for the synthesis of energetic CPs based on their high energy content and available coordination modes. However, energetic CPs based on azides tend to display relatively low thermal decomposition temperatures. One example of such a CP is the 1D coordination polymer [Zn(N2H4)2(N3)2]n (Figure 2a); this CP has a thermal decomposition temperature of 150 °C (TGA), which is much lower than the above-mentioned CPs, specifically CHP, CHHP, ZnHHP, ATRZ-1, and ATRZ-2.25 DSC analysis reveals one endothermic event (centering at 232 °C) followed by two exothermic events between (268−375 °C), which occur after the decomposition of the framework. The [Zn(N2H4)2(N3)2]n CP also contains bridging hydrazine functionality, which is reminiscent of the bridging in the ZnHHP and F

DOI: 10.1021/acs.cgd.5b01436 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Review

Figure 11. Illustration of the relative impact sensitivities for selected energetic CPs discussed in section 3.2.

based on nitrogen rich heterocyclic linkers in the literature that have low sensitivity, such as [Pb(Htztr)2(H2O)]n, [Pb(H2tztr)(O)]n, [Cu(Htztr)]n, Ag-atrz, Ag-ntz (impact sensitivity >40 J, except for [Cu(Htztr)]n: impact sensitivity = 32 J), which are comparable to 3D energetic CPs.30,36−38 Energetic CPs derived from heterocyclic linker systems, such as H2tztr, Mtta, HATZ, and Hntz tend to result in frameworks which have low impact sensitivity. Clearly through coordination chemistry, it is possible to produce energetic materials with decreased impact sensitivities relative to commonly used energetics today. Gao and co-workers reported the guest dependent sensitivity of a 3D porous mixed-valence Co(II)−Co(III) energetic CP, {[Co(bta)10(Hbta)2(H2O)10]·22H2O}n (Figure 10e).33 This energetic CP has a low impact sensitivity (impact sensitivity > 40 J); however, after dehydration, a more sensitive energetic CP was isolated (impact sensitivity = 27 J). The authors suggest that this guest dependent strategy represents a new approach in the storage of sensitive energetic materials by activating an insensitive form.66 It is difficult to ascertain many structure− function relationships concerning sensitivity, because many groups have not reported impact, friction, and spark sensitivity. However, the results reported thus far point to a measure of control in sensitivity difficult to achieve with purely organic systems. 3.3. Oxygen Balance. In the field of energetic materials oxygen balance is of crucial importance when considering the decomposition products of an energetic. The optimal oxygen balance for an energetic material in most applications is 0%; this means that the fuel and oxidant are internally balanced for efficient reaction.42 The role of oxygen balance in the design of new energetic CPs is not well explored. High nitrogen content in the linker has been a target for the synthesis of energetic CPs, and this can increase the oxygen balance relative to conventional carbon-rich CP linkers. The oxygen balance of lead azide, a prototypical primary explosive, is −5.5%.64 The reported oxygen balance values for NHN, NHP, and CHP are −5.7%, −11.5%, and −11.5%, respectively; these values are improved when compared to HMX/RDX (−21.6%) and TNT (−74.0%).26,30 The inclusion of oxidizing perchlorate anions in NHP and CHP (e.g., ammonium perchlorate oxygen balance = +27.2%) plays a key role in this difference.27 Nitrogen-rich heterocyclic linkers lead to CPs with high nitrogen content, thereby resulting in CPs with increased oxygen balance. However, the reported oxygen balances for these CPs are only fair, typically ranging from −28.6%, for [Pb(Htztr)(O)]n, to −58.8%, for ATRZ-1.28,30 On the other hand, the oxygen balance values are still comparable to HMX/

explosives based on the ability to design CPs with specific coordination environments, which result in either more or less sensitive structures (shown in Figure 11 for selected CP structures). Consider the cases of the 1D CPs NHP and CHP, compounds that have extremely high impact sensitivities.26 NHP detonates even without external stimuli and therefore quantification of the impact sensitivity by the drop height method is not feasible. On the other hand, CHP has an impact sensitivity of 0.5 J or 20 cm (using a 2.5 kg weight) which is comparable to pentaerythritoltetranitrate (PETN), a commonly used secondary explosive (impact sensitivity ≈12 cm ±5).26,65 The sensitivities for NHP and CHP are significantly different despite being isostructural and illustrate the role of metal in controlling sensitivity. This offers a rare glimpse into the effect of metals on the properties of energetic CPs, an area which is in need of further development. For the 2D energetic CP ZnHHP the impact sensitivity is 50 cm (using a 5 kg weight) and for CHHP the impact sensitivity is 30 cm (using a 2.5 kg weight). The sensitivity to impact for CHHP is slightly improved relative to CHP albeit in a material with a decreased heat of detonation.27 Moreover, CHP and CHHP detonate when subjected to spark and flame sensitivity tests, whereas ZnHHP is insensitive to spark and deflagrates in flame. The 1D energetic CP [Zn(N2H4)2(N3)2]n, containing both bridging hydrazine and terminal azide functionality, has an impact sensitivity of 28.5 cm (10 kg weight).25 The use of 3,5-DNBA as a bridging colinker for the formation of the energetic CP [Cu(3,5-DNBA)(N3)]n containing bridging azide functionality results in an energetic 1D CP with an impact sensitivity of 23.5 J or 120 cm (using a 2 kg weight), which is much lower than commonly used secondary explosives which tend to have impact sensitivities greater than 4 J, such as RDX (7.5 J), HMX (7.4 J), and TNT (15 J).30,31 Many 3D energetic CPs, such as ATRZ-1 and ATRZ-2, have relatively low impact sensitivities and are resistant to spark, shock, and friction.28 The authors claim that the improved sensitivity in these compounds when compared to NHP, CHP, CHHP, and ZnHHP, stems from the robust 3D structure of the materials. The ATRZ linker used to synthesize ATRZ-1 and ATRZ-2 has an impact sensitivity of 14 J, whereas the CP structures have impact sensitivities of 22.5 and 30 J, respectively.57 This means that, relative to the pure linker, the impact sensitivity is substantially decreased in the CP framework. Other 3D CP structures, such as {[Cu(tztr)]H2O}n (Figure 8a, impact sensitivity >40 J) and {[Cu4(Mtta)5][Na(CH3CN)]}n (Figure 9e, impact sensitivity = 36 J) also have improved impact sensitivities relative to most 1D and 2D energetic CPs.36,37 There are examples of 2D energetic CPs G

DOI: 10.1021/acs.cgd.5b01436 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Review

respectively.26 These values are similar to HMX (5.52 kJ g−1) and RDX (5.79 kJ g−1).30 The utilization of the hydrazinecarboxylate colinker for the synthesis of the 2D ZnHHP and CHHP energetic CPs resulted in a decrease in the heat of detonation (2.93 kJ g−1 and 3.14 kJ g−1 by DFT, respectively), relative to the NHP and CHP energetic CPs.27 The incorporation of the fully oxidized carboxylate functionality in the ZnHHP and CHHP structures is likely the cause of the low heat of detonation. Furthermore, detonation velocity for ZnHHP (7.02 km s−1) and CHHP (6.21 km s−1) was predicted to be low relative to NHP (9.18 km s−1) and CHP (8.23 km s−1) 1D energetic CPs.26,27 The detonation velocities for NHP and CHP are higher than the detonation velocity of TNT (7.18 km s−1).30 Energetic CPs containing nitrogen-rich heterocyclic linkers have reported heats of detonation which range from low, as in the case of [Pb(H2tztr)(O)]n (1.07 kJ g−1) to extremely high, as in the case of ATRZ-1 (15.12 kJ g−1) and [Cu(Htztr)]n (16.55 kJ g−1).28,30,37 ATRZ-2 (5.77 kJ g−1) has a significantly lower heat of detonation than ATRZ-1. In addition, the detonation velocity of ATRZ-1 and ATRZ-2 was predicted to be 9.16 km s−1 and 7.77 km s−1, respectively, by using the empirical Kamlet formula.28,70 The detonation velocity for ATRZ-1 is notably higher than the pure ATRZ linker (7.52 km s−1). The 2D energetic CP [Cu(Htztr)]n has among the highest heat of detonation values (16.55 kJ g−1) relative to other CP structures and commonly used energetic materials. It is difficult to conclude any structure−function relationships from the reported energetic CPs in the literature based on a lack of systematic examination of structure effects on the detonation performance. The reported heats of detonation for energetic CPs in the literature have shown potential to compete not only with commonly used energetics like HMX (5.52 kJ g−1) and RDX (5.79 kJ g−1), but also thermites, such as Fe2O3/Al (reaction enthalpy of 3.95 kJ g−1).72 There are a few cases in the energetic CP literature where the DSC exothermic event was explicitly integrated to give the heat released. Exothermic peaks in DSC are proportional to the enthalpy change of the reaction and can therefore be integrated to give the heat released for explosives upon decomposition.73,74 In spite of the fact that this is not technically a detonation, it does give some experimental insight into the heat release and avoids ambiguities associated with assuming particular detonation products. For example, the reported heat evolution by DSC for [Zn(N 2 H 4 ) 2 (N 3 ) 2 ] n and [Co5(atrz)7(N3)3]n was determined to be 1.8 kJ g−1 and 7.81 kJ g−1, respectively.25,39 The heat evolution measured by DSC for CHP is 4.33 kJ g−1, which, in this case, can be compared to the calculated heat of detonation (DFT) of 5.26 kJ g−1.26 Experimentally determining the heat released during decomposition is the most straightforward method to examine the effectiveness of an energetic CP, although underexplored in the energetic CP literature.

RDX and higher than TNT. Methods for achieving optimal oxygen balance in energetic CPs are underdeveloped in the literature. As a further complication, it must be noted that the commonly used calculation for oxygen balance assumes that the detonation products for metal-containing energetic materials are metal oxides. Because there are multiple oxidation states for many metals this leads to ambiguity when computing oxygen balance. In addition, there are other potential detonation products, such as metal salts or elemental metal, for example, which can affect oxygen balance. Therefore, without proper characterization of the reaction products, which is uncommon in the energetic CP literature, it is not possible to determine the oxygen balance, and the reported values should be treated with caution. In the Summary and Outlook section below, we address this challenge and other challenges related to the design of energetic CPs with optimal oxygen balance and other energetic properties. The transfer of oxygen is not the only method to transfer energy and increase oxygen balance. Recently, Coles and coworkers have introduced a series of MOFs composed of fuel and oxidizer in the form of an integrated scaffold to optimize the mixing of ingredients in a pyrotechnic material35 For this study, group 1 and 2 metal nitrates were chosen with phthalic, isophthalic, and terephthalic acid linkers and their fluorinated analogues. Group 1 and 2 metal nitrates were chosen for their burn color change within the group. MOFs derived from the fluorinated analogues of benzenedicarboxylate linkers exhibit pyrotechnic properties based on the oxidizing nature of fluorine. This study highlights the potential for modification of existing linker types with fluoride to tune pyrotechnic properties. No doubt this strategy is one that can be adapted to the formation of energetic CP structures to tune the properties of the energetic and, in particular, the oxygen balance. 3.4. Heat of Detonation and Detonation Velocity. The energy given off by an energetic material during detonation, the heat of detonation, is a critical performance metric. The heat of detonation is commonly measured by calorimetric methods, although for energetic CPs there are no reported detonation calorimetry results perhaps due to the relatively large sample requirements of the technique. Instead density functional theory (DFT) has been applied to estimate the energy of detonation (ΔEdet), from which heat of detonation (ΔHdet) is calculated by using a linear correlation equation.26,67 Experimental determination of detonation velocity has also not been carried out for CPs. Instead estimations using the CHEETAH thermochemical code, the Kamlet−Jacobs equation, or the empirical Kamlet formula have been employed.68−70 The Kamlet−Jacobs equation is traditionally used for organic energetic materials; however it has been applied to energetic materials containing metals.33 Recently, Pang and co-workers, inspired by the further development of metal-containing energetic compounds, extended the Kamlet−Jacobs equation to cover these types of materials assuming that their reaction products include metal oxides.71 One key factor for the calculation of heats of detonation is that the composition of the detonation products directly affects the detonation energy. What this means is that identity of the products needs to be known in order to ensure proper calculation of the heats of detonation. There are some energetic CPs for which calculated heat of detonation and detonation velocity values have been reported, these are discussed below. NHP and CHP heats of detonation were evaluated using the DFT method and found to be 5.73 kJ g−1 and 5.23 kJ g−1,

4. SUMMARY AND OUTLOOK The potential for coordination polymers to act as next generation energetic materials is considerable. However, there are many fundamental issues concerning structure−function relationships that are under-investigated and need to be established for more rational design of energetic CPs. Energetic CPs reported in the literature, thus far, tend to have higher densities than organic energetic materials. It is important to note, however, that crystallographic density of CPs are often H

DOI: 10.1021/acs.cgd.5b01436 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

■

increased due to the high density of the metal ions; this high crystallographic density does not necessarily relate to the CP energy density per unit mass of the materials in the same fashion as for typical organic energetics. Considering that most of the energy for energetic CPs comes from the linker, increased density based on the use of heavier metal ions is not a practical means for improving energetic properties, although it may be useful for hard target penetration. Moreover, the heat of detonation values for most energetic CPs are determined computationally and the detonation products are, in many cases, assumed rather than experimentally determined. Bearing in mind the issues associated with the calculation of heat of detonation and detonation performance with energetic CPs, calorimetric methods can give valuable information about the performance of the energetic CP relative to known energetic materials. There is an overall lack of systematic investigations connecting structure and property relationships of CPs in the literature, which limits the ability to design energetic CPs specifically for applications as primary or secondary explosives. For example, it is often not discussed how the energetic properties of the linker are altered in the resulting CPs. The aforementioned issues are not unexpected given the short time scale over which progress has occurred in this area, and further studies are expected to address these knowledge gaps. Throughout the energetic CP literature porosity has been seen as a detriment in the generation of high-energy-density CPs. Additionally, effects of guest molecules on the properties of CPs have been underexplored in the literature. The usage of porous CPs for the adsorption of energetic guest molecules presents a promising route for the production of energetic CPs with improved properties. This concept has even led to a patent, which utilizes the inherently porous nature of many CPs (particularly MOFs) for the adsorption of energetic or oxidant species.75 Adsorption of oxidants and/or energetics, such as nitroalkanes and nitroarenes, has the potential to lead to increased overall density, improved detonation performance, and tunable oxygen balances. This strategy also makes it possible to incorporate sensitive energetic molecules into CPs, by adsorption or direct mixing rendering the CP energetic. Additionally, adsorption of sensitive energetic guests has the potential for safer transportation of these types of explosives. This strategy has not been widely explored as a means to develop energetic materials. The use of CPs for the synthesis of energetic materials is no doubt promising and represents a growing field that will result in a new generation of high-energy materials specifically designed with particular applications in mind.

■

Review

REFERENCES

(1) Li, J.-R.; Kuppler, R. J.; Zhou, H.-C. Chem. Soc. Rev. 2009, 38, 1477. (2) Li, J.-R.; Sculley, J.; Zhou, H.-C. Chem. Rev. 2012, 112, 869. (3) Getman, R. B.; Bae, Y.-S.; Wilmer, C. E.; Snurr, R. Q. Chem. Rev. 2012, 112, 703. (4) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W. Chem. Rev. 2012, 112, 782. (5) Sumida, K.; Rogow, D. L.; Mason, J. A.; McDonald, T. M.; Bloch, E. D.; Herm, Z. R.; Bae, T.-H.; Long, J. R. Chem. Rev. 2012, 112, 724. (6) Lee, J.; Farha, O. K.; Roberts, J.; Scheidt, K. A.; Nguyen, S. T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450. (7) Ma, L.; Abney, C.; Lin, W. Chem. Soc. Rev. 2009, 38, 1248. (8) Corma, A.; García, H.; Llabrés i Xamena, F. X. Chem. Rev. 2010, 110, 4606. (9) Yoon, M.; Srirambalaji, R.; Kim, K. Chem. Rev. 2012, 112, 1196. (10) Kreno, L. E.; Leong, K.; Farha, O. K.; Allendorf, M.; Van Duyne, R. P.; Hupp, J. T. Chem. Rev. 2012, 112, 1105. (11) Liu, D.; Lu, K.; Poon, C.; Lin, W. Inorg. Chem. 2014, 53, 1916. (12) Xia, Z.; Chen, S.; Wei, Q.; Qiao, C. J. Solid State Chem. 2011, 184, 1777. (13) Wang, S.-W.; Yang, L.; Feng, J.-L.; Wu, B.-D.; Zhang, J.-G.; Zhang, T.-L.; Zhou, Z.-N. Z. Anorg. Allg. Chem. 2011, 637, 2215. (14) Xu, C.-X.; Zhang, J.-G.; Yin, X.; Jin, X.; Li, T.; Zhang, T.-L.; Zhou, Z.-N. J. Solid State Chem. 2015, 226, 59. (15) Wu, B.-D.; Bi, Y.-G.; Li, F.-G.; Yang, L.; Zhou, Z.-N.; Zhang, J.G.; Zhang, T.-L. Z. Anorg. Allg. Chem. 2014, 640, 224. (16) Wang, S.-H.; Zheng, F.-K.; Wu, M.-F.; Liu, Z.-F.; Chen, J.; Guo, G.-C.; Wu, A. Q. CrystEngComm 2013, 15, 2616. (17) Wu, B.-D.; Zhang, T.-L.; Li, Y.-L.; Tong, W.-C.; Zhou, Z.-N.; Zhang, J.-G.; Yang, L. Z. Anorg. Allg. Chem. 2013, 639, 2209. (18) Wu, B.-D.; Zhou, Z.-N.; Li, F.-G.; Yang, L.; Zhang, T.-L.; Zhang, J.-G. New J. Chem. 2013, 37, 646. (19) Tao, G.-H.; Parrish, D. A.; Shreeve, J. n. M. Inorg. Chem. 2012, 51, 5305. (20) Yang, L.; Wu, B.; Zhang, T.; Liu, Z.; Zhang, J. Propellants, Explos., Pyrotech. 2010, 35, 521. (21) Cudziło, S.; Nita, M. J. Hazard. Mater. 2010, 177, 146. (22) Liu, Z.; Zhang, T.; Zhang, J.; Wang, S. J. Hazard. Mater. 2008, 154, 832. (23) Tang, Z.; Zhang, J.-G.; Liu, Z.-H.; Zhang, T.-L.; Yang, L.; Qiao, X.-J. J. Mol. Struct. 2011, 1004, 8. (24) Zhang, K.-L.; Pan, Z.-C.; Chang, Y.; Liu, W.-L.; Ng, S. W. Mater. Lett. 2009, 63, 2136. (25) Wu, B.-D.; Yang, L.; Wang, S.-W.; Zhang, T.-L.; Zhang, J.-G.; Zhou, Z.-N.; Yu, K.-B. Z. Anorg. Allg. Chem. 2011, 637, 450. (26) Bushuyev, O. S.; Brown, P.; Maiti, A.; Gee, R. H.; Peterson, G. R.; Weeks, B. L.; Hope-Weeks, L. J. J. Am. Chem. Soc. 2012, 134, 1422. (27) Bushuyev, O. S.; Peterson, G. R.; Brown, P.; Maiti, A.; Gee, R. H.; Weeks, B. L.; Hope-Weeks, L. J. Chem. - Eur. J. 2013, 19, 1706. (28) Li, S.; Wang, Y.; Qi, C.; Zhao, X.; Zhang, J.; Zhang, S.; Pang, S. Angew. Chem., Int. Ed. 2013, 52, 14031. (29) Lin, J.-D.; Wang, S.-H.; Cai, L.-Z.; Zheng, F.-K.; Guo, G.-C.; Huang, J.-S. CrystEngComm 2013, 15, 903. (30) Gao, W.; Liu, X.; Su, Z.; Zhang, S.; Yang, Q.; Wei, Q.; Chen, S.; Xie, G.; Yang, X.; Gao, S. J. Mater. Chem. A 2014, 2, 11958. (31) Liu, X.; Yang, Q.; Su, Z.; Chen, S.; Xie, G.; Wei, Q.; Gao, S. RSC Adv. 2014, 4, 16087. (32) Zhang, Q.; Shreeve, J. M. Angew. Chem., Int. Ed. 2014, 53, 2540. (33) Zhang, S.; Liu, X.; Yang, Q.; Su, Z.; Gao, W.; Wei, Q.; Xie, G.; Chen, S.; Gao, S. Chem. - Eur. J. 2014, 20, 7906. (34) Bennion, J. C.; McBain, A.; Son, S. F.; Matzger, A. J. Cryst. Growth Des. 2015, 15, 2545. (35) Blair, L. H.; Colakel, A.; Vrcelj, R. M.; Sinclair, I.; Coles, S. J. Chem. Commun. 2015, 51, 12185. (36) Feng, Y.; Liu, X.; Duan, L.; Yang, Q.; Wei, Q.; Xie, G.; Chen, S.; Yang, X.; Gao, S. Dalton Trans. 2015, 44, 2333. (37) Liu, X.; Gao, W.; Sun, P.; Su, Z.; Chen, S.; Wei, Q.; Xie, G.; Gao, S. Green Chem. 2015, 17, 831.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This work was supported by the Defense Threat Reduction Agency (DTRA) (HDTRA1-15-1-0001). K.A.M. acknowledges the National Science Foundation Graduate Research Fellowship Program (NSF GRFP) for funding. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Jonathan C. Bennion for helpful discussions related to the design and properties of energetic materials. I

DOI: 10.1021/acs.cgd.5b01436 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Review

(71) Wang, Y.; Zhang, J.; Su, H.; Li, S.; Zhang, S.; Pang, S. J. Phys. Chem. A 2014, 118, 4575. (72) De Luca, L. T.; Galfetti, L.; Maggi, F.; Colombo, G.; Paravan, C.; Reina, A.; Dossi, S.; Fassina, M.; Sossi, A. In Metal Nanopowders; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2014; p 301. (73) Akhavan, J. In The Chemistry of Explosives, 3rd ed.; Royal Society of Chemistry: London, 2011; pp 116−130. (74) Agrawal, J. P. High Energy Materials; Wiley-VCH VerlagGmbH & Co. KGaA, Weinheim, 2010; pp 183−188. (75) Nelson, A. P.; Trivedi, N. J. Host-guest complexes of solid oxidizers and metal-organic frameworks. U.S. Patent 8,197,619 B1, June 12, 2012.

(38) Qu, X.; Zhang, S.; Yang, Q.; Su, Z.; Wei, Q.; Xie, G.; Chen, S. New J. Chem. 2015, 39, 7849. (39) Chen, S.; Shu, S.; Gao, S. Inorg. Chim. Acta 2009, 362, 3043. (40) Friedrich, M.; Gálvez-Ruiz, J. C.; Klapötke, T. M.; Mayer, P.; Weber, B.; Weigand, J. J. Inorg. Chem. 2005, 44, 8044. (41) The advantages of dense MOFs in certain applications is increasingly being recognized. See for example: Tominaka, S.; Hamoudi, H.; Suga, T.; Bennett, T. D.; Cairns, A. B.; Cheetham, A. K. Chem. Sci. 2015, 6, 1465. (42) Steinhauser, G.; Klapötke, T. M. Angew. Chem., Int. Ed. 2008, 47, 3330. (43) Sikder, A. K.; Sikder, N. J. Hazard. Mater. 2004, 112, 1. (44) Oyler, K. D. In Green Energetic Materials; John Wiley & Sons, Ltd: New York, 2014; p 103. (45) Glen, G. L. J. Am. Chem. Soc. 1963, 85, 3892. (46) Beck, W.; Evers, J.; Göbel, M.; Oehlinger, G.; Klapötke, T. M. Z. Anorg. Allg. Chem. 2007, 633, 1417. (47) Xue, H.; Gao, Y.; Twamley, B.; Shreeve, J. M. Chem. Mater. 2005, 17, 191. (48) Wang, K.; Zeng, D.; Zhang, J.-G.; Cui, Y.; Zhang, T.-L.; Li, Z.M.; Jin, X. Dalton Trans. 2015, 44, 12497. (49) Yang, Q.; Wei, Q.; Chen, S.-P.; Zhang, G.-C.; Zhou, C.-S.; Gao, S.-L. J. Anal. Appl. Pyrolysis 2013, 99, 66. (50) Talawar, M. B.; Sivabalan, R.; Mukundan, T.; Muthurajan, H.; Sikder, A. K.; Gandhe, B. R.; Rao, A. S. J. Hazard. Mater. 2009, 161, 589. (51) Shunguan, Z.; Youchen, W.; Wenyi, Z.; Jingyan, M. Propellants, Explos., Pyrotech. 1997, 22, 317. (52) Jiang, C.; Yu, Z.; Wang, S.; Jiao, C.; Li, J.; Wang, Z.; Cui, Y. Eur. J. Inorg. Chem. 2004, 2004, 3662. (53) Su, C.-Y.; Goforth, A. M.; Smith, M. D.; Pellechia, P. J.; zur Loye, H.-C. J. Am. Chem. Soc. 2004, 126, 3576. (54) Dincǎ, M.; Dailly, A.; Liu, Y.; Brown, C. M.; Neumann, D. A.; Long, J. R. J. Am. Chem. Soc. 2006, 128, 16876. (55) Zhang, S.; Yang, Q.; Liu, X.; Qu, X.; Wei, Q.; Xie, G.; Chen, S.; Gao, S. Coord. Chem. Rev. DOI:201510.1016/j.ccr.2015.08.006. (56) The number of environmentally acceptable metals is much smaller; however, at the start of a field this consideration can be overlooked to allow the fundamental science to develop unconstrained by such practicalities. (57) Qi, C.; Li, S.-H.; Li, Y.-C.; Wang, Y.; Zhao, X.-X.; Pang, S.-P. Chem. - Eur. J. 2012, 18, 16562. (58) Petrie, M. A.; Sheehy, J. A.; Boatz, J. A.; Rasul, G.; Surya Prakash, G. K.; Olah, G. A.; Christe, K. O. J. Am. Chem. Soc. 1997, 119, 8802. (59) Habib, H. A.; Sanchiz, J.; Janiak, C. Inorg. Chim. Acta 2009, 362, 2452. (60) Habib, H. A.; Sanchiz, J.; Janiak, C. Dalton Trans. 2008, 1734. (61) Huang, Y.-Q.; Ding, B.; Song, H.-B.; Zhao, B.; Ren, P.; Cheng, P.; Wang, H.-G.; Liao, D.-Z.; Yan, S.-P. Chem. Commun. 2006, 4906. (62) Yan, Y.; Suyetin, M.; Bichoutskaia, E.; Blake, A. J.; Allan, D. R.; Barnett, S. A.; Schroder, M. Chem. Sci. 2013, 4, 1731. (63) Sumida, K.; Horike, S.; Kaye, S. S.; Herm, Z. R.; Queen, W. L.; Brown, C. M.; Grandjean, F.; Long, G. J.; Dailly, A.; Long, J. R. Chem. Sci. 2010, 1, 184. (64) Hariharanath, B.; Chandrabhanu, K. S.; Rajendran, A. G.; Ravindran, M.; Kartha, C. B. Def. Sci. J. 2006, 56, 383. (65) Manner, V. W.; Tappan, B. C.; Scott, B. L.; Preston, D. N.; Brown, G. W. Cryst. Growth Des. 2014, 14, 6154. (66) For an example of a related strategy to produce a smart energetic cocrystal see: Bolton, O.; Matzger, A. J. Angew. Chem., Int. Ed. 2011, 50, 8960. (67) Delley, B. J. Chem. Phys. 1990, 92, 508. (68) Bastea, S.; Fried, L. E.; Glaesemann, K. R.; Howard, W. M.; Kuo, I.-F. W.; Sourers, P. C.; Vitello, P. A. Cheetah 6.0, Users Manual; Lawrence Livermore National Laboratory: Livermore, CA, 2010. (69) Kamlet, M. J.; Jacobs, S. J. J. Chem. Phys. 1968, 48, 23. (70) Bingham, R. C.; Dewar, M. J. S.; Lo, D. H. J. Am. Chem. Soc. 1975, 97, 1285. J

DOI: 10.1021/acs.cgd.5b01436 Cryst. Growth Des. XXXX, XXX, XXX−XXX