Communication pubs.acs.org/IC
Metal Effects on the Sensitivity of Isostructural Metal−Organic Frameworks Based on 5‑Amino-3-nitro‑1H‑1,2,4-triazole Saona Seth,† Kyle A. McDonald,† and Adam J. Matzger*,†,‡ †
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States Department of Macromolecular Science and Engineering, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
‡
S Supporting Information *
position products, and low impact sensitivities, in addition to having multiple potential donor sites for coordination bonding. The predicted detonation parameters of energetic CPs derived from functionalized triazoles and tetrazoles are exceptional.11,12,14 5-Amino-3-nitro-1H-1,2,4-triazole (ANTA) is a secondary explosive with well-characterized energetic parameters that has the appropriate coordinating functionality to produce a CP.15 ANTA has been employed as a precursor to synthesize several high-energy organic compounds with high power and thermal stabilities,16,17 but its coordination polymerization is unexplored. We aim to leverage the well-established energetic properties of common explosives, here ANTA, to realize new materials through coordination polymerization that have modified properties.15 ANTA is an ideal system to study the effects of different metal ions on the energetic properties of isostructural frameworks. The chemical nature of metal ions has remarkable effects on the properties of isostructural MOFs, in areas such as gas adsorption and catalysis, based on their variable ionic radii, Lewis acidic nature, and redox properties.18−21 Surprisingly, the current literature rarely highlights modulation of sensitivity/stability to impact or thermal stimuli with isostructural energetic CPs containing different metal nodes.10 Understanding the variances in the energetic properties with isostructural CPs assists in the rational design of new energetic materials with desirable attributes (e.g., high sensitivity to impact for primary explosives and impact insensitivity for secondary explosives). Therefore, coordination polymerization of ANTA was investigated in the presence of different transition-metal ions to access energetic CPs and study the effect of metal ions on the energetic properties. The reaction of ANTA with cobalt nitrate leads to a 2D MOF, Co-ANTA, that exhibits high density, oxygen balance, and superior thermal stability to ANTA. Furthermore, CoANTA has a much higher impact sensitivity than ANTA. The presence of unpaired electrons on the metal ion presumably renders the compound more impact-sensitive. To assess the effect of the metal ion on the sensitivity of the energetic MOF, Zn-ANTA, a MOF isostructural to Co-ANTA was synthesized through the reaction of ANTA with Zn2+. Zn-ANTA exhibits lower sensitivities to heat as well as impact compared to CoANTA, which may stem from its d10 electronic configuration. Co-ANTA and Zn-ANTA were synthesized by solvothermal reaction of ANTA with cobalt nitrate and zinc nitrate respectively
ABSTRACT: Two energetic metal−organic frameworks (MOFs), Co-ANTA and Zn-ANTA, are synthesized from 5-amino-3-nitro-1H-1,2,4-triazole (ANTA) and exhibit superior oxygen balance, density, and thermal stability compared to ANTA. The superior oxygen balance is achieved through a combination of hydroxide ligands and deprotonated linkers. Although the materials are isostructural and have similar density, oxygen balance, and sensitivity to heat, their impact sensitivities are significantly different. Similar to ANTA, Zn-ANTA is fairly insensitive to impact. By contrast, the impact sensitivity of ANTA is increased significantly after coordination polymerization with cobalt. The disparate impact sensitivities of the compounds might be attributed to the different electronic configurations of the metal ions constituting the frameworks.
E
nergetic materials, compounds typically containing both oxidizer and fuel, undergo self-sustained combustion when initiated, resulting in energy release. These materials are in constant demand in different military as well as civilian applications from munitions to mining. An energetic compound should have high density and heat of detonation in order to have formidable explosive power (measured by the detonation velocity and detonation pressure). Additionally, stability and sensitivity to external stimuli are two important parameters that are critical for the practical use of energetic materials. It is important, however, to have a balance of all of these parameters in order to obtain energetic materials with optimized properties for their particular use. The functional properties of an energetic material can be tuned by modifying the molecular-level interactions. A considerable amount of research in the field of energetic materials has focused on the fine-tuning of energetic properties by modulating the crystal packing through the synthesis of polymorphs,1−3 salts,4,5 cocrystals,6,7 and coordination polymers.8,9 Energetic coordination polymers (CPs), a relatively new strategy in this area, have shown promise to exhibit tunable properties such as density, thermal stability, sensitivity, and heat of detonation.10−12 Many of these compounds meet the criteria of two- or three-dimensional (2D or 3D) crystalline frameworks to be considered metal−organic frameworks (MOFs).13 Nitrogen-rich polytopic linkers, commonly azoles, are primarily employed to construct energetic CPs.8 Many azoles have high heats of formation, environmentally benign decom© 2017 American Chemical Society
Received: July 21, 2017 Published: August 24, 2017 10151
DOI: 10.1021/acs.inorgchem.7b01865 Inorg. Chem. 2017, 56, 10151−10154
Communication
Inorganic Chemistry (see experimental section). Single-crystal X-ray diffraction indicates that the compounds constitute 2D CPs that are isostructural (Figure 1b,c). In each structure, the metal ion
Figure 2. Oxygen balance of the energetic MOFs of ANTA in comparison with selected energetics.
results in lower hydrogen contents of the MOFs compared to ANTA, which leads to higher oxygen balances than the latter (Figure 1a). In fact, the hydroxo bridge offers an oxygen balance advantage similar to the incorporation of peroxide2 without concerns about metal-catalyzed decomposition. It should be noted that the oxygen balances of Co-ANTA and Zn-ANTA are higher than those of a number of high-energy CPs with good predicted detonation properties (ATRZ-1, −58.8%; [Cu(Htztr)]n, −56.1%) and comparable to those of conventional high explosives, such as HMX and RDX (−21.6%).11,14,22 The energetic properties of Co-ANTA and Zn-ANTA were investigated to determine the thermal stability and sensitivity to impact. The thermal decomposition temperatures of these compounds were determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) by heating the samples under nitrogen at a ramp rate of 10 °C min−1. Whereas ANTA exhibits a thermal stability of up to 225 °C, both MOFs are stable beyond 300 °C, with the Zn CP having slightly higher thermal stability. Co-ANTA and Zn-ANTA decompose above 310 and 330 °C, respectively (Figures 3 and 4). Therefore, thermal desensitization of ANTA is realized through coordination polymerization with cobalt and zinc ions.
Figure 1. Zn-ANTA structure: (a) zinc ion tetrahedrally coordinated by two deprotonated ANTA and two hydroxide ions; (b) 2D corrugated network (note that the nitro and amino groups of the neighboring ANTA anions are projected alternately out of the 2D sheet); (c) side view of the 2D sheet; (d) packing of 2D sheets through hydrogen bonding between the hydroxo and nitro groups of the neighboring 2D layers.
exhibits tetrahedral disposition and is coordinated by two deprotonated ANTAs and two hydroxide ions (Figure 1a). The hydroxides bridge the metal ions and form one-dimensional (1D) [M(OH)]+n chains that are linked by the anions of ANTA, leading to 2D networks. The anions of ANTA are oriented perpendicularly to the corrugated 2D layer such that the nitro and amino groups of the neighboring ANTA anions are projected alternately out of the layer. The crystal structures rely on the hydrogen bonding between the hydroxo and nitro groups from the neighboring 2D layers (Figure 1d). The bulk purity of the compounds were determined by comparing the powder X-ray diffraction (PXRD) patterns of the powdered samples with the PXRD profiles simulated from the respective single-crystal structures (Supporting Information, Figures S1 and S2). Energetic properties, such as the detonation velocity and detonation pressure, are functions of density and, therefore, high density is crucial for an energetic material. The crystallographic density of Co-ANTA is 2.326 g cm−3. Similarly, ZnANTA has a crystallographic density of 2.405 g cm−3 at 100 K. Both compounds exhibit high densities at room temperature as well (2.297 g cm−3 for Co-ANTA and 2.336 g cm−3 for ZnANTA). The densities of Co-ANTA and Zn-ANTA are higher than many organic energetics, such as TNT (1.64 g cm−3), RDX (1.81 g cm−3), HMX (1.91 g cm−3), and ε-CL-20 (2.035 g cm−3), and energetic CPs including CHP (1.948 g cm−3), NHP (1.983 g cm−3), and ATRZ-1 (1.68 g cm−3).8 The oxygen balance of an energetic compound indicates the amount of available oxygen (excess or deficit) in the compound for its self-combustion and is related to the exothermicity of its explosive decomposition. The oxygen balance of an energetic compound should ideally be zero, meaning that the compound can undergo complete self-sustained combustion, leading to maximum heat release. In general, most energetic compounds suffer from low oxygen balances. The calculated oxygen balances for Co-ANTA and Zn-ANTA are −27.6% and −26.6%, respectively, values that are significantly higher than that of the energetic linker ANTA (−43.8%; Figure 2). The presence of deprotonated ANTA as well as hydroxide ions in the structure
Figure 3. TGA of ANTA, Co-ANTA, and Zn-ANTA.
Sensitivity to external stimuli (impact, friction, heat, etc.) is a critical parameter for energetic compounds that influences both the controlled initiation of detonation and safety. Impact sensitivities for both of the MOFs were determined experimentally based on small-scale drop-testing experiments and are represented by Dh50, the drop height for which the probability of detonation of the energetic compound is 50%. Typically, 2.0 mg (±10%) of the energetic material is weighed into a nonhermetic aluminum DSC pan, and a 5 lb. drop weight is allowed to free fall on the sample. ANTA and Zn-ANTA are insensitive to impact from a height of 217 cm (the maximum limit of the experimental setup in our laboratory). By contrast, Co-ANTA is much more sensitive to impact. The Dh50 value of Co-ANTA was determined to be 94 cm.23 Thus, Co-ANTA and Zn-ANTA have very different sensitivities to impact, in spite of their isostructural 10152
DOI: 10.1021/acs.inorgchem.7b01865 Inorg. Chem. 2017, 56, 10151−10154
Communication
Inorganic Chemistry
(method 1). The resulting red crystals were dried in vacuo overnight to yield 1.86 mg (23.5%, based on ANTA) of material. CHN anal. Calcd for C2H3N5O3Co: C, 11.77; H, 1.47; N, 34.33. Found: C, 11.92; H, 1.59; N, 33.77. IR (solid, cm−1): ν̃ 3620, 3400, 3280, 3126, 1643, 1571, 1543, 1492, 1448, 1372, 1298, 1125, 1109, 1090, 938, 887, 846. Although the compound obtained by this procedure is pure, this method has an issue of reproducibility in the case of CoANTA. Therefore, the compound was scaled up following a different procedure (method 2). ANTA (42.0 mg, 0.326 mmol), Co(NO3)2·6H2O (116.0 mg, 0.398 mmol), and NaN3 (26.0 mg, 0.400 mmol) were dissolved in 4 mL of water in a digestion bomb. The reaction was heated at 160 °C for 72 h. Co-ANTA synthesized by this method contains a solid impurity of smaller particle size. This impurity could be removed by first sonicating the crude product in acetone and then using a sieve and thoroughly washing; the smaller impurities were removed from Co-ANTA. The crystals were dried in vacuo overnight to yield 8.19 mg (24.6%, based on ANTA) of material. CHN anal. Calcd for C2H3N5O3Co: C, 11.77; H, 1.47; N, 34.33. Found: C, 11.64; H, 1.68; N, 34.47.
Figure 4. DSC of ANTA, Zn-ANTA, and Co-ANTA.
nature, comparable densities, similar oxygen balances, and thermal stabilities, which demonstrates that the metal ions have a significant role in the decomposition processes of the energetic CPs. The presence of unpaired electrons in the metal ion presumably renders Co-ANTA more sensitive to impact than Zn-ANTA, which contains a metal ion of the d10 configuration. Coordination polymerization of the organic explosive ANTA was accomplished to obtain high-density energetic materials with tunable properties. The energetic MOFs Co-ANTA and ZnANTA, synthesized by this strategy, are isostructural and exhibit excellent densities and high thermal stabilities, with decomposition occurring above 300 °C. Thus, the thermal stability of ANTA can be improved remarkably through coordination polymerization. The higher oxygen balances of these materials compared to ANTA are due, in part, to the presence of hydroxo groups in the networks, species that contribute to more positive oxygen balances. Both of the MOFs are secondary explosives based on their impact sensitivities, and the sensitivities of the isostructural energetic MOFs are significantly different; CoANTA exhibits higher sensitivity to impact compared to both ANTA and Zn-ANTA. Thus, coordination provides a method to improve the thermal stability of an organic energetic and modulate the sensitivity of the energetic MOF to impact based on the nature of the metal ion. Caution! No unplanned explosions were encountered during our experiments. However, all of these compounds are highly explosive and, hence, should be handled following proper safety protocols to avoid accidents. Synthesis of Energetic MOFs. Zn-ANTA ([Zn(C2H2N5O2)(OH)]n). ANTA (5.05 mg, 0.0388 mmol) and Zn(NO3)2·6H2O (53.8 mg, 0.181 mmol) were dissolved in 0.5 mL of dimethylformamide (DMF) in a 4 mL vial, and 1 mL of water was subsequently added to the resulting solution. The homogeneous reaction mixture was heated at 100 °C for 48 h. Yellow plates of the crystals were formed as clusters. The mother liquor was filtered off, and the crystals were washed thoroughly with a solution containing a 2:1 water/DMF mixture to remove unreacted starting materials. The crystals were dried in vacuo overnight to yield 4.17 mg (51.2%, based on ANTA) of material. CHN anal. Calcd for C2H3N5O3Zn: C, 11.41; H, 1.43; N, 33.29. Found: C, 11.49; H, 1.37; N, 32.42. IR (solid, cm−1): ν̃ 3632, 3395, 3290, 3123, 1647, 1577, 1549, 1505, 1451, 1378, 1298, 1131, 1118, 1099, 940, 897, 846. Co-ANTA ([Co(C2H2N5O2)(OH)]n). The compound can be synthesized following the synthetic procedure for Zn-ANTA by using Co(NO3)2·6H2O (50.2 mg, 0.173 mmol) instead of Zn(NO3)2·6H2O and 10.0 mg (0.0775 mmol) of ANTA
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01865. Experimental details, unit cell parameters, IR spectroscopy, TGA, DSC, PXRD data, and Fourier transform IR spectra of the energetic MOFs (PDF) Accession Codes
CCDC 1565874−1565875 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (A.J.M.). ORCID
Adam J. Matzger: 0000-0002-4926-2752 Author Contributions
The manuscript was written by all of the authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Defense Threat Reduction Agency (Grant HDTRA1-15-1-0001). We thank Dr. Philip Pagoria of Lawrence Livermore National Laboratory for providing ANTA. We acknowledge Dr. Jeff Kampf for singlecrystal X-ray analysis. We thank Rosalyn Kent for assistance with drop-height determination. K.A.M. gratefully acknowledges support from the National Science Foundation Graduate Research Fellowship Program and the Rackham Merit Fellowship. 10153
DOI: 10.1021/acs.inorgchem.7b01865 Inorg. Chem. 2017, 56, 10151−10154
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Inorganic Chemistry
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(21) Johnson, J. A.; Luo, J.; Zhang, X.; Chen, Y.-S.; Morton, M. D.; Echeverría, E.; Torres, F. E.; Zhang, J. Porphyrin-Metalation-Mediated Tuning of Photoredox Catalytic Properties in Metal−Organic Frameworks. ACS Catal. 2015, 5, 5283−5291. (22) Gao, W.; Liu, X.; Su, Z.; Zhang, S.; Yang, Q.; Wei, Q.; Chen, S.; Xie, G.; Yang, X.; Gao, S. High-energy-density materials with remarkable thermostability and insensitivity: syntheses, structures and physicochemical properties of Pb(ii) compounds with 3-(tetrazol-5-yl) triazole. J. Mater. Chem. A 2014, 2, 11958−11965. (23) For details about the drop-height apparatus and Bruceton analysis, see: Landenberger, K. B.; Bolton, O.; Matzger, A. J. Energetic− Energetic Cocrystals of Diacetone Diperoxide (DADP): Dramatic and Divergent Sensitivity Modifications via Cocrystallization. J. Am. Chem. Soc. 2015, 137, 5074−5079.
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DOI: 10.1021/acs.inorgchem.7b01865 Inorg. Chem. 2017, 56, 10151−10154
