Text of Novel metal–organic frameworks assembled from the
Cite this: Dalton Trans., 2021, 50,12906
Received 23rd June 2021,Accepted 16th August 2021
Novel metal–organic frameworks assembled fromthe combination of polynitro-pyrazole and5-nitroamine-1,2,4-oxadiazole: synthesis,structure and thermal properties†
Feng Yang, Yuangang Xu, Pengcheng Wang, Qiuhan Lin andMing Lu *
Energetic metal organic frameworks (EMOFs) is a hot topic in the field of energetic materials research.
This paper reports two kinds of EMOFs based on methylene-linked polynitropyrazole and nitroamine
1,2,4-oxadiazole. Their structures were fully characterized by crystallography and their detonation per-
formance and stability performance were explored. The results showed that the crystals of compounds 4
and 5 exhibited a 3D stacking phenomenon due to the action of a large number of hydrogen bonds and
coordination bonds inside the crystal. In terms of stability, both 4 and 5 showed good thermal stability
(TSADT (4) = 204.4 °C and TSADT (5) = 216.2 °C), but due to the difference in the number of energetic
groups (–NO2), the sensitivity of 4 (IS = 6.0 J and FS = 100 N) to mechanical stimuli is significantly lower
than that of compound 5 (IS = 1.2 J and FS = 40 N). In terms of energy performance, it is this great advan-
tage in the number of energetic groups that makes compound 5’s (Dv = 8.059 km s−1 and P = 30.9 GPa)
detonation performance superior to that of 4 (Dv = 7.704 km s−1 and P = 26.9 GPa). This research broad-
ens the horizon for the development of EMOFs based on polynitropyrazole derivatives.
High-energy-density materials (HEDMs) are special com-pounds or complexes that have been spawned to meet thedevelopmental needs of military, aerospace and civilianfields.1–3 For traditional organic energetic compounds (OECs),it is difficult to meet the needs of energetic materials indifferent fields. In most OECs, it is difficult to avoid the for-mation of active protons during the synthesis process, andthese active protons are an important factor leading to theinstability of energetic compounds.4,5
Compared with traditional organic energetic compounds,energetic metal–organic frameworks (EMOFs) have promisingcontrollability in terms of loading density, energy performanceand stability, and it is these advanced characteristics thatmake the EMOFs widely used in detonating devices, pyrotech-nics, heat-resistant explosives, and as catalysts inpropellants.6–15 Therefore, reacting high-energy-densityorganic compounds containing active protons with metal salts
to prepare high-energy metal–organic compounds is currentlythe simplest method for preparing energetic materials thatmeet different needs.
Nowadays, the ligands used as high-energy metal–organicframeworks are mostly nitrogen heterocyclic energetic com-pounds that contain nitroamine groups, nitroformyl groups orcarboxyl groups.16–18 This not only provides coordination sitesfor the formation of the metal organic framework but alsoensures the proper oxygen content of the MOFs. Among thesenitrogen-rich heterocycles, pyrazole and 1,2,4-oxadiazole arethe more commonly used frameworks for preparing high-energy-density materials.19–25 Compounds 3,4-dinitro-1H-pyrazol-5-amine (A) and 5-nitroamine-3,4-dinitropyrazole (B)are pyrazole-based high-energy-density materials (Fig. 1 A andB). 3,4-Dinitro-1H-pyrazol-5-amine not only shows a detonationperformance that is superior to 3,5-dinitro-1H-pyrazol-4-amine
Fig. 1 Commonly used high-energy moieties of pyrazole and 1,2,4-oxadiazole.
†Electronic supplementary information (ESI) available. CCDC 2089156, 2045198and 2005558. For ESI and crystallographic data in CIF or other electronic formatsee DOI: 10.1039/d1dt02093e
School of Chemical Engineering, Nanjing University of Science and Technology,
(LLM-116) but also possesses good stability.26,27 It is worthnoting that 5-nitroamine-3,4-dinitropyrazole not only exhibitsthe best detonation performance among the carbon-substi-tuted monocyclic pyrazole energetic compounds but also exhi-bits better detonation performance than 3,4,5-trinitro-1H-pyrazol-1-amine (ATNP) and 1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane (HMX). Unfortunately, the low stability of thenitroamine group and the active protons on the pyrazole ringlead to the low decomposition temperature and high acidity of5-nitroamine-3,4-dinitropyrazole, which prevents its directapplication in energetic equipment. As for 1,2,4-oxadiazole, itis mostly in the form of 5-nitroamine-1,2,4-oxadiazole (C) inhigh energy density materials. Due to the production of nitro-amine, there must be highly active protons in the overall struc-ture, which is also disadvantageous as an energetic material.In order to remove these instabilities and maintain the energyperformance of the compound, we noticed that potassium isvery popular in the formation of energetic MOFs as it can notonly significantly improve the overall thermal stability but it isalso more “green” and environmentally friendly than otherheavy metals such as lead and barium.
Continuing research on high-energy-density materials, wefocus on combining two energetic structures with differentcharacteristics to prepare energetic materials with advancedproperties. After taking into account the excellent character-istics of 3,4-dinitro-1H-pyrazol-5-amine (A), 5-nitroamine-3,4-dinitropyrazole (B) and 5-nitroamine-1,2,4-oxadiazole (C), newneutral energetic compound 2 and two compound 2 basedEMOFs 4 and 5 with high energy levels were synthesized bycombining 5-nitroamino-3,4-dinitropyrazole with 5-nitrosa-mine-1,2,4-oxadiazole and azobis-1,2,4-oxadiazole,respectively.
Results and discussionSynthesis
The synthetic routes of the target compounds 4 and 5 areshown in Scheme 1. The original compound 1 and the substi-tuted 5-amino-3,4-dinitropyrazole compound 2 were obtainedby the method we reported earlier . After dissolving com-pound 2 in methanol, slowly adding potassium hydroxide tothe above solution and stirring at room temperature for6 hours, the potassium salt 4 of compound 2 can be obtainedas a precipitate. Nitration of compound 2 with fuming nitricacid at 0 °C can provide viscous solid product 3 in higheryields. After dissolving this viscous solid in methanol, slowlyadding potassium hydroxide to it and stirring at room temp-erature for 6 hours, the dipotassium salt 5 of 3 was obtainedas a precipitate. The method of acidifying compound 5 withdilute hydrochloric acid to obtain compound 3 has also beentried, but the obtained product is still viscous.
Suitable crystals of 2, 4 and 5 were obtained by volatizing thecorresponding saturated solvents of these compounds.
Compound 2 crystallizes in space group P21/c with a calcu-lated density of 1.749 g cm−3 at the temperature of 296 K. Eachunit cell unit contains 4 energetic molecules (Z = 4).Obviously, in addition to the energetic molecules, the crystalalso contains water molecules. Similar to the ammonium saltof compound 2 (we reported in ref. 4), these water moleculesare coplanar to the structure of methylene nitroamine oxadia-zole (MNAO) (Fig. 2A). It is interesting that the angle (79.3°)between the 5-amino-3,4-dinitropyrazole (ADNP) plane and thenitroamine oxadiazole (MNAO) plane of compound 2 and thecrystal packing of compound 2 show a similar situation to thatof the ammonium salt of 2. Regarding the layout of hydrogenbonds, each compound 2 molecule is directly related to thesurrounding eight compound 2 molecules and one water mole-cule by hydrogen bonds (Fig. 2C and D).
From the crystallographic data obtained, it can be knownthat 4 crystallizes in the triclinic space group P1̄ with a calcu-lated density of 1.857 g cm−3 at the temperature of 296 K. Inthe crystal, each energetic molecule interacts with four sur-rounding potassium atoms and one water molecule throughcoordination bonds (Fig. 3A). Obviously, except for the potass-
Scheme 1 The synthetic route of compounds 2 to 5.
Fig. 2 (A) The molecular structure of compound 2 (the green dottedline represents the hydrogen bond). (B) The crystal packing diagram ofcompound 2 (the green dotted line represents the hydrogen bond). (C)The distribution of hydrogen bonds between the ADNP part and the sur-rounding energetic molecules (the green dotted line represents thehydrogen bond). (D) The distribution of hydrogen bonds between theMNAO part and the surrounding energetic molecules (the green dottedline represents the hydrogen bond).
ium atoms and the water molecule, the rest of the groups arealmost on the plane of the two heterocyclic rings and theangle between the two heterocyclic planes is 105.3°, which islarger than that of the previously mentioned neutral com-pound 2. In addition, a large number of intramolecular hydro-gen bonds and intermolecular hydrogen bonds exist inside thecrystal (C(4)–H(4B)⋯O(3), C(4)–H(4A)⋯O(5), O(10)–H(10A)⋯O(1) and O(10)–H(10A)⋯O(2)), and the connection of thesehydrogen bonds makes the whole crystal present a regular 3Dconfiguration (Fig. 3C and D) and the insertion of potassiumatoms promotes the more compact packing of crystals(Fig. 3B).
As for EMOF 5, it crystallizes in the monoclinic space groupP21/c with a calculated density of 1.934 g cm−3 at 296 K. Theresult of crystal analysis showed that each unit cell containedtwo energetic molecules inside the crystal (Z = 2), and eachenergetic ligand molecule coordinated with the surroundingseven potassium atoms and one water molecule (Fig. 4A). Forthe energetic ligand molecule, except for the nitroamine groupon the pyrazole, the other nitro groups all penetrate the planeof the aza mother ring, and the angle between the two azamother rings is 102.8° (see ESI†). Notably, different from MOF4, there are two coordination situations of potassium in 5,namely, potassium 1 (K1) and potassium 2 (K2). K1 is co-ordinated with four surrounding energetic ligand molecules,while K2 is coordinated with surrounding three energeticmolecules and two water molecules (Fig. 4C and D). Inaddition to the coordination bonds, there are also abundantintramolecular and intermolecular hydrogen bonds inside thecrystal (C(1)–H(1B)⋯O(6), N(9)–H(9A)⋯N(1), N(9)–H(9B)⋯O(7),N(9)–H(9B)⋯O(8), O(8)–H(8A)⋯O(3), O(8)–H(8B)⋯O(2) andO(8)–H(8B)⋯O(3)), (Fig. 4E) and the connection of thesehydrogen bonds promotes the crystal to assume a 3D stackingconfiguration (Fig. 4B).
Thermal performance is one of the important parameters forevaluating the stability of energetic compounds. Usually, thethermal stability of energetic compounds can be quickly andeffectively judged by the thermal decomposition temperature(Tdec), thermal explosion temperature (Tbp), self-acceleratingdecomposition temperature (TSADT) and activation energy (E)of thermal decomposition. In order to explore the thermalstabilities of compounds 2, 4 and 5, their thermal decompo-sition kinetics were investigated by testing their decompositiontemperatures at four different heating rates (5, 10, 15 and20 °C min). And based on the temperature of decompositionat four different heating rates, the kinetic parameters includ-ing apparent activation energy (E) and pre-exponential con-stant (A) can be obtained by using the widely recognizedKissinger28 (eqn (1)) and Ozawa29 (eqn (2)) methods.
� �¼ ln
log β þ 0:4567ERTp
¼ C ð2Þ
where β is the linear heating rate (°C min−1), TP is the peaktemperature (K) of decomposition, A is the pre-exponentialconstant, R is the gas constant (8.314 J (mol °C)−1), E is theapparent activation energy (kJ mol−1) and C is a constant.
The DSC curves of the newly synthesised compounds at theheating rates of 5, 10, 15 and 20 °C min−1 are shown in theFig. 5, 6 and 7, respectively. Based on the peak temperature ofdecomposition at different heating rates, the non-isothermalkinetic parameters of these three compounds can be obtained
Fig. 3 (A) The coordination of energetic molecule 2 with surroundingpotassium atoms. (B) The crystal stacking diagram of MOF 4. (C) The dis-tribution of hydrogen bonds inside the crystal. (D) The crystal packingdiagram without considering the coordination of potassium atoms (thegreen dotted line represents the hydrogen bond).
Fig. 4 (A) The coordination of energetic molecule 3 with surroundingpotassium atoms. (B) The crystal stacking diagram of MOF 5. (C) Thecoordination of potassium atom 1 and energetic molecule 3. (D) Thecoordination of potassium atom 2 and energetic molecule 3. (E) Thecrystal packing diagram without considering the coordination of potass-ium atoms (the green dotted line represents the hydrogen bond).
(Table 1). It is known from the calculation results that the acti-vation energy data obtained by Kissinger’s method (Ek) andOzawa’s method (Eo) show a high degree of consistency.Among these three compounds, EMOF 4 exhibits extremelyhigh apparent activation energy (318.8 kJ mol−1), which isreflected in its insensitivity to temperature changes.Compound 2 has the lowest E among these three compounds.This indicates that the EMOFs obtained by compound 2exhibit thermal insensitivity compared to 2. This phenomenoncan be seen intuitively from the change in the decompositionpeak temperature (ΔT (4) = 9.3 °C and ΔT (2) = 8.6 °C) with thechange in heating rate (Fig. 5 and 6). As a compound withmore nitro groups than EMOF 4, EMOF 5 also shows a higheractivation energy value (215.4 kJ mol−1), although its precursor
(3) cannot exist as a dispersed solid at room temperature.Through the obtained apparent activation energy E and ln A,the Arrhenius equation of the thermal decomposition processof the above compound can be expressed as: ln k = 22.5 −132.4 × 103/RT, ln k = 67.1 − 318.3 × 103/RT and ln k = 39.8 −214.8 × 103/RT.
Furthermore, through the decomposition temperature dataat four different heating rates, the values of TSADT and Tbp canbe obtained by applying eqn (3)–(5).30
T ðe or pÞi ¼ T ðe0 or p0Þi þ aβi þ bβi2 þ cβi3; i ¼ 1; 2; 3; 4 ð3Þ
TSADT ¼ Te0 ð4Þ
Tb ¼ Ek �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiEk2 � 4EkRTp0
where, a, b and c are coefficients; Ek is the apparent activationenergy obtained by Kissinger’s method, and R is the universalgas constant 8.314 J (mol °C)−1. The calculated TP0 and Tpb areshown in Table 1. Obviously, the TSADT and Tbp values ofEMOFs 4 (204.4 and 211.9 °C, respectively) and 5 (216.2 and227.8 °C, respectively) are higher than the current require-ments for the thermal stability of energetic materials (200 °C).In particular, for EMOF 5, its onset decomposition tempera-ture (Td = 230.8 °C) is higher than most energetic potassiumsalts (see Table 2). This implies that EMOF 5 can be used asenergetic materials in related equipment.
Physical and chemical properties
For a compound with high energy density characteristics, itsdetonation properties and stability performance are essentialindicators for preliminary judgment of its application field.Among them, detonation velocity (Dv) and pressure (P) are themost commonly used detonation performance parameters,while decomposition temperature (Td), impact sensitivity (IS)
Fig. 5 The DSC curves of 2 at the heating rates of 5, 10, 15, and 20 °Cmin−1.
Fig. 6 The DSC curves of 4 at the heating rates of 5, 10, 15, and 20 °Cmin−1.
Fig. 7 The DSC curves of 5 at the heating rates of 5, 10, 15, and 20 °Cmin−1.
and friction sensitivity (FS) are the most important stabilityparameters.
In this study, the detonation performance of the neutralcompound 2 is calculated by explo5 31 according to the esti-mated heat of formation and the measured density at roomtemperature, while the EMOFs (4 and 5) are characterized onthe basis of the highly recognized Kamlet–Jacobs eqn (6)–(8)using the method proposed by Pang.32 In terms of stability,since thermal stability has been explored in depth, this sectiononly discusses the sensitivity of the abovementioned com-pounds to mechanical stimuli (IS and FS). The mechanicalsensitivity of energetic compounds is usually directly tested bya standard BAM fall hammer (for IS) and BAM friction tester(for FS).
where Dv and P represent the detonation velocity (km s−1) andpressure (GPa), N expresses the number of moles of detonationgases per gram of explosive (mol g−1), M̄ is the gaseousproduct of the average molecular weight (g mol−1), ρ rep-resents the measured density (g cm−3) at 25 °C, and Q is theheat of detonation (kcal g−1). Taking into account the highdensity (greater than 1.4 g cm−3) and condensed character-istics of the above compounds, in order to be able to highlighttheir energy performance in the most realistic way, theexplosion equation of the largest exothermic principle (H2O–CO2) (ESI†) is applied to calculate their detonation velocity (Dv)
and detonation pressure (P). For the estimation of energy per-formance, to obtain reliable calculation results, the constantpressure reaction heat (ΔCU) values of the above compoundswere obtained by using an oxygen bomb calorimeter (ESI†). Toexpress the energy performance of the above complexes intui-tively, their detonation velocity (Dv), detonation pressure (P)and heat of detonation (Q) are listed in Table 2.
Obviously, the detonation performance (Dv(2) = 8.402 kms−1 and P(2) = 32.0 GPa) of neutral compound 2 is not satisfac-tory due to its low density (ρ = 1.80 g cm−3) and undesirableoxygen balance (OB = −7.6%). Such unfavorable conditionsfurther lead to the low detonation performance (Dv(4) =7.704 km s−1 and P(4) = 26.9 GPa) of its potassium salt (4).However, due to its unique layered stacked crystal structureand abundant hydrogen bonds, 2 exhibits low sensitivity toexternal mechanical stimuli (IS(2) = 12 J and FS(2) = 200 N),which also makes it possible for its potassium salt (4) tohave a lower (compared with most energetic potassium salts)sensitivity to external mechanical stimuli (IS(4) = 6 J and FS(4) = 100 N). The detonation performance (Dv(5) = 8.059 kms−1 and P(5) = 30.9 GPa) of EMOF 5 obtained after thefurther nitration of 2 has reached a satisfactory level (com-pared with other energetic potassium salts), which is due toits sufficient energetic groups (two nitro groups and twonitroamine) and high density (ρ = 2.04 g cm−3). However,these diverse energetic functional groups (nitro and nitro-amine) also bring the characteristics of being extremely sen-sitive to external mechanical stimuli (IS(5) = 1.2 J and FS(5)= 40 N). Considering its high decomposition temperature,good energy performance and appropriate sensitivity to exter-nal mechanical stimuli, complex 5 can be used as a poten-tial green primary explosive.
Table 1 The Ek, Eo, Tpo, Tbp and TSADT values of compounds 2, 4 and 5
Compound Ek (kJ mol−1) ln[A (s−1)] Rk Eo (kJ mol−1) Ro Tp0 (°C) Tbp (°C) TSADT (°C)
a Thermal decomposition temperature (onset) under nitrogen gas (DSC, 5 °C min−1). bDensity measured by gas pycnometer at 25 °C. cNitrogencontent. d The oxygen balance based on CO, for CaHbOcNd energetic compounds, the calculation formula of OB is OB = ((c − a − 0.5b) × 16/M) ×100%, where M is the molecular weight of the explosive. eHeat of detonation. fCalculated detonation velocity. g Calculated detonation pressure.h Impact sensitivity. i Friction sensitivity.
In summary, 5-amino-3,4-dinitropyrazolemethylene bis-3-nitro-amine-1,2,4-oxadiazole (2), the potassium salt of 5-amino-3,4-dinitropyrazolemethylene bis-3-nitroamine-1,2,4-oxadiazole (4)and the dipotassium salt of 5-nitroamine-3,4-dinitropyrazole-methylene bis-3-nitroamine-1,2,4-oxadiazole (5) were syn-thesized for the first time, and their structures were character-ized by crystallography. From the crystal stacking, 4 and 5show a 3D stacking phenomenon due to the abundant coordi-nation bonds and hydrogen bonds inside the crystal. As poten-tial energetic compounds, the energy performance and stabi-lity of 2, 4 and 5 have been fully explored. From the results ofnon-isothermal kinetic studies, it can be seen that they allhave good thermal stability (TSADT (2) = 191.2 °C, TSADT (4) =204.4 °C and TSADT (5) = 216.2 °C), especially EMOF 5,although its precursor is a non-dispersed solid at room temp-erature. In terms of sensitivity to external mechanical stimuli,EMOF 5 exhibits extremely high sensitivity (IS(5) = 1.2 J and FS(5) = 40 N) due to its rich variety of energetic functional groups(–NO2 and –NHNO2). The structural feature of EMOF 5, whichis rich in various energetic functional groups, is also a reasonfor its excellent energy performance (Dv(5) = 8.059 km s−1 andP(5) = 30.9 GPa). This provides the possibility of using it as aprimary explosive.
Caution! The reported compounds are all potentially explosivecompounds, especially energetic metal salts, which are highlysensitive to external mechanical stimuli. Therefore, handling asmall amount and wearing protective equipment during theexperiment are strongly encouraged.
Synthesis of compound 2
Compound 1 (0.690 g, 2.0 mmol) was dissolved in 20 ml ofmethanol and 1.0 ml of ammonia (25%) was added dropwiseto the methanol solution under temperature control by anice bath. After the addition, the reaction mixture was slowlywarmed to room temperature and reacted at this temperaturefor 6 hours (during this time a large amount of solid is pre-cipitated out, which is the ammonium salt of compound 2).After this, the solvent is removed by rotary evaporation, theremainder is placed in 10.0 ml water, acidified with 10%hydrochloric acid to pH = 3, and filtered. The filter cake waswashed with 10 × 3 ml of ice water and dried to obtain alight yellow solid product 2; yield 0.525 g, 83.3%. IR (KBrpellet): 3420, 3196, 1640, 1550, 1520, 1370, 1280, 1250, 1085,1000, 918, 861, 830, 806, 743, 710, 659 cm−1. 1H NMR(500 MHz, DMSO) δ 8.17 (s, 1H), 7.10 (s, 2H), 5.32 (s, 2H).13C NMR (126 MHz, DMSO) δ 175.73, 164.96, 148.23, 147.86,108.49, 44.84. Elemental analysis calcd (%) for C6H5N9O7
(315.03): C (22.87), H (1.60), N (40.00); found: C (23.01), H(1.52), N (40.04).
Synthesis of EMOF 4
Compound 2 (0.315 g, 1.0 mmol) was dissolved in 15 ml ofmethanol and potassium hydroxide (0.056 g, 1.0 mmol) wasadded to the methanol solution in batches under ice cooling.After half an hour, the ice bath was removed and the mixturewas warmed to room temperature and reacted at this tempera-ture for 12 hours. The mixture was filtered and the filter cakewas washed with 10 ml of methanol. After drying, compound 4was obtained as a yellow solid; yield 0.327 g, 92.5%. IR (KBrpellet): 3416, 3262, 2967, 1645, 1556, 1524, 1481, 1430, 1381,1316, 1251, 1113, 1087, 918, 831, 777, 745, 710, 660, 617 cm−1.Elemental analysis calcd (%) for C6H4N9O7K (352.99): C(20.40), H (1.14), N (35.69); found: C (20.31), H (1.20), N(35.75).
Synthesis of EMOF 5
In a salt bath, compound 2 (0.630 g, 2.0 mmol) was carefullyadded to 6 ml of fuming nitric acid in batches. The tempera-ture of the system was controlled to not exceed 0 °C. After theaddition, the mixture was reacted at 0 °C for 1 hour and thenthe reaction mixture was poured into 60 grams of crushed iceand stirred. After extracting with 3 × 10 ml of ethyl acetate, theorganic phase was deacidified with saturated brine and driedover anhydrous magnesium sulfate. After ethyl acetate wasremoved by rotary evaporation, a yellow viscous product wasobtained (compound 3). It was dissolved in 10 ml of methanoland then a methanol saturated solution of potassium hydrox-ide was added to it in batches until the pH value of the mixedsystem is 11. After the addition, the reaction was carried out atroom temperature for 12 hours, and then the mixture was fil-tered, the filter cake was washed with 10 ml of methanol and5 ml of cold water, and dried to obtain 0.567 g (64.7%) ofyellow solid 5. IR (KBr pellet): 3393, 1567, 1547, 1520, 1433,1410, 1380, 1347, 1304, 1279, 1240, 1160, 1085, 1018, 998, 911,851, 807, 776, 738, 702, 621 cm−1. Elemental analysis calcd (%)for C6H2N10O9K2 (435.93): C (16.52), H (0.46), N (32.10); found:C (16.63), H (0.61), N (32.18).
Conflicts of interest
There are no conflicts to declare.
This work was supported by the National Natural ScienceFoundation of China (no. 11972195).
Notes and references
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