Abstract
On the basis of 1,2-bis(5-tetrazolo)ethane (BTE) the corresponding twofold vinyl and allyl N-substituted derivatives were synthesized using 1,2-dibromoethane and allyl bromide, respectively. The compounds were obtained as two different constitutional isomers. Both species were analyzed using NMR and IR spectroscopy, elemental analysis, as well as mass spectrometry. In the case of the diallyl bistetrazoles, the two isomers were characterized using 2D NMR spectroscopy. The synthesis of the divinyl compounds gave crystals of the 2,2′-N-substituted isomer, which were analyzed by single-crystal X-ray diffraction. The thermal stability of the compounds was determined using differential scanning calorimetry (DSC) and gave decomposition temperatures around 190°C and 230°C. For the investigation of the inherent energetic potential, sensitivities toward physical stimuli and detonation parameters were determined. The compounds turned out to be insensitive toward friction and impact and possess moderate energetic properties.
1 Introduction
In the broad field of heterocyclic chemistry, heterocycles containing nitrogen make fascinating research objects. Among these nitrogen rich compounds, especially tetrazoles stand out due to their broad field of applications. They find use in pharmaceutical [1], [2], [3], [4] and biomedical applications [5], [6], [7], as well as in energetic materials or gas-generating agents [8], [9] for both, civil and military applications. In fact, tetrazoles are applied in every subfield of energetic materials, primary explosives [10], [11], [12], secondary explosives [13], propellants [14], [15], [16], [17], [18], [19], [20], [21], [22] as well as pyrotechnics [23], [24], [25], [26]. Besides this, on the basis of tetrazoles, polymers were developed for the use as binders in energetic compositions [27], [28], [29]. Here, tetrazoles are well suited as monomeric units for these kinds of polymers, due to their high nitrogen content (up to >80%), good thermal stability, along with their energetic character [29]. Examples of tetrazole-based polymers are poly(vinyltetrazole) [30], poly(1-vinyl-1H-hydroxytetrazole) [30] or a polycondensate with N1-[1-(4-aminophenyl)-1H-tetrazol-5-yl]benzene-1,4-diamine [31]. To date, most of the tetrazole-containing polymers are synthesized via radical polymerization. Herein, we report the syntheses of bistetrazolo derivatives with twofold double bond functionalities, which offer diverse possibilities for further (polymeric) processing.
2 Results and discussion
2.1 Syntheses
On the basis of 1,2-bis(tetrazole-5-yl)ethane (BTE, 1), which was synthesized in close accordance to a procedure described in the literature [32], the corresponding divinyl and diallyl derivatives were synthesized (Scheme 1).
The vinyl-substituted compounds 2a, b were prepared in an substitution/elimination reaction using BTE and 1,2-dibromoethane in analogy to described procedures for related divinyl bistetrazoles (Scheme 1a) [33]. The reaction gave a mixture of 2,2′- and 1,2′-N substituted compounds, 1,2-bis(2-vinyl-2H-tetrazol-5-yl)ethane (2a, 2,2-DvBTE) and 1-vinyl-5-(2-(2-vinyl-2H-tetrazol-5-yl)ethyl)-1H-tetrazole (2b, 1,2-DvBTE). After purification via column chromatography 2a and b were obtained as colorless solids in low yields (31% and 18%). Recrystallization of 2a from an n-hexane-ethyl acetate mixture gave crystals suitable for X-ray diffraction. The diallyl derivatives of BTE were obtained in a substitution reaction using allyl bromide (Scheme 1b). After the purification via column chromatography compounds 3a (1,2-bis(2-allyl-2H-tetrazol-5-yl)ethane, 2,2-BaBTE) and 3b (1-allyl-5-(2-(2-allyl-2H-tetrazol-5-yl)ethyl)-1H-tetrazole, 1,2-BaBTE) were obtained as colorless liquids in low yields with 28% and 15%, respectively.
2.2 Spectroscopic analyses
The synthesized compounds were characterized using elemental analysis, mass spectrometry, as well as 1H, 13C NMR and IR spectroscopy. The crystal structure of 2a was determined using single-crystal X-ray diffraction data.
[D6]DMSO was used as solvent for the NMR measurements. BTE (1) gave consistent results with the literature values [32]. For a clear assignment of the exact positions of the carbon and hydrogen atoms in 3a and 3b 2D NMR measurements were carried out. The spectra of the HMQC and HMBC 2D NMR measurements are depicted in Figs. S1–S4 of the Supporting Information.
Both isomers of 3 can be distinguished by their signal patterns in the 1D 1H and 13C NMR spectra (Figs. 1 and 2). In the 1H NMR spectrum of 3a five different, partly overlapping signals can be observed at 6.04 ppm (CH of the allyl group), 5.29 ppm (CHcis of the terminal CH2 of the allyl group), 5.28 ppm (aliphatic CH2 of the allyl group), 5.20 ppm (CHtrans of the terminal CH2 of the allyl group) and 3.33 ppm (CH2) in a 2:6:2:4 intensity ratio. The signals show different coupling patterns due to their different interactions with the surrounding hydrogen atoms. The CH of the allyl group shows a doublet of doublets of triplets assigned to its 3JHH couplings to the CHtrans and CHcis of the terminal CH2 group and to the aliphatic CH2 group. The geminal hydrogen atoms of the terminal CH2 group also show a doublet of doublets of triplets splitting, representing the respective 3JHH (to the vinyl CH), 4JHH (to the aliphatic CH2) and the 2JHH couplings. Both geminal hydrogen atoms can be distinguished because of their differing 3JHH coupling constants (17.2 Hz (Htrans) and 10.1 Hz (Hcis)). The aliphatic CH2 group of the allyl group shows a doublet of doublets of doublets representing the respective couplings to the CH and the terminal CH2 hydrogen atoms.
The 1H NMR spectrum of 3b shows 10 different overlapping signals (Fig. 1) with an intensity ratio of 2:5:3:4. The signal around 3.39 ppm represents the two different CH2 groups attached to the respective Cq of the tetrazole rings and shows an A2B2 spin system of higher order. The signal is also overlapped by the signal of residual water, which explains the higher integral value.
A comparison of the 13C NMR spectra of 3a and b (Fig. 2) also proves the existence of two isomers. Whereas the spectrum of 3a shows only five signals (as an indicator for symmetrically substituted tetrazole rings), 3b must be the asymmetric 1,2′-N-substituted isomer, because of its 10 signals. To determine, which kind of symmetrically N-substituted isomer was formed as 3a (1,1′- or 2,2′-), along with the general assignment of the proton and carbon positions in 3a and 3b, HMQC and HMBC 2D NMR measurements were the methods of choice (see Supporting Information, Figs. S1–S4). Here, the 1- or the 2-N substituted position can be distinguished by the occurring 3JCH long-range heteronuclear carbon-proton coupling between the quaternary carbon in the tetrazole ring and the protons of the aliphatic CH2 group attached to the N1 atom of the tetrazole ring (Fig. 3).
The HMBC experiments (Figs. S3 and S4) verified the 2,2′-N-substitution pattern for 3a, as no 3JCH coupling between the H4 hydrogens and the C1 carbon atoms are visible in the 2D NMR spectrum. However, in the 2D HMBC spectrum of 3b, the signal of the 3JCH coupling between C2 and H8 is visible, which confirms the asymmetric 1,2′-N-substituted structure.
For the assignment of the remaining carbon and hydrogen positions in 3a and 3b the HMQC measurements were beneficial (Figs. S1 and S2). The resulting spectra enabled the allocation of the carbon signals C3 and C4 of 3b to the overlapping signals of their corresponding protons H3 and H4. A precise distinction between the two different C9 carbon atoms and their corresponding H9 protons of 3b was not possible.
The 1H spectrum of 2a shows four different signals with a 2:2:2:4 intensity ratio (Fig. 4). At 7.79 ppm the CH of the vinyl group is visible as a doublet of doublets, representing the interactions with the geminal hydrogen atoms of the terminal =CH2. These two geminal hydrogen atoms of the vinyl group also show doublets of doublets as coupling patterns. The signal at 6.06 ppm can be assigned to the Htrans of the terminal vinyl-CH2, because of its larger 3JHH coupling value (15.5 Hz) compared to the 3JHH coupling value (8.7 Hz) of the Hcis at 5.47 ppm. The protons of the aliphatic CH2 group occur at 3.41 ppm. Similar to the results of the measurement of 3a, the 1H NMR spectrum of 3b shows eight different, partly overlapping signals with an intensity ratio of 1:1:1:1:2:4, representing the asymmetric 1,2′-N-substituted divinyl compound. The new signals of the CH and the CHtrans protons of the 1-N-substituted vinyl group can clearly be assigned at 7.47 ppm and 5.97 ppm, whereas the signals of the two CHcis protons are overlapping and cannot be allocated properly. The signal around 3.47 ppm represents the two different CH2 groups attached to the respective Cq of the tetrazole rings and shows an A2B2 spin system of higher order.
The results of the 2D NMR measurements of 3a and 3b can be applied to assignments in the 1D NMR spectra of 2a and 2b. Here again, the comparison of the 13C NMR spectra of 2a and b (Fig. 5) proves the existence of two different isomers. Whereas the spectrum of 2a shows only four different signals (as indicators for symmetrically substituted tetrazole rings), 2b must be the asymmetric 1,2′-N-substituted isomer because of its eight different signals. In analogy to the diallyl compounds 3a and b, 2a and b show signals at 164.9 ppm (Cq), 130.1 ppm (CH vinyl), 109.0 (CH2 vinyl) and around 22.9 ppm (aliphatic CH2), which represent the 2-N-substituted fragment. The additional signals in the 13C NMR spectrum of 3b at 153.3, 126.5, 109.9 and 20.5 ppm can be assigned in an analogous manner to the 1-N-substituted isomer.
2.3 Structure determination by X-ray diffraction
Single crystals suitable for X-ray diffraction were obtained by recrystallization from an n-hexane-ethyl acetate mixture. A summary of crystallographic refinement parameters and structure data for 2a is given in Table 1.
Chemical formula | C8H10N8 |
Molecular weight, g mol−1 | 218.22 |
Color, habit | colorless block |
Size, mm3 | 0.07×0.31×0.51 |
Crystal system | monoclinic |
Space group | P21/c |
a, Å | 9.137(4) |
b, Å | 8.166(4) |
c, Å | 6.941(4) |
β, deg | 98.164(5) |
V, Å3 | 512.61(4) |
Z | 2 |
ρcalcd, g cm−3 | 1.414 |
μ, mm−1 | 0.10 |
F(000), e | 228 |
θ range, deg | 4.27–26.35 |
T, K | 173 |
Dataset h | −7≤h≤11 |
Dataset k | −10≤k≤10 |
Dataset l | −8≤l≤8 |
Reflecions coll. | 3803 |
Independent refl. | 1049 |
Observed refl. | 930 |
Rint | 0.022 |
Ref. parameters | 94 |
R1a/wR2b (2 σ data) | 0.0302/0.0698 |
R1a/wR2b (all data) | 0.0351/0.0739 |
Weighting scheme A/Bb | 0.0292/0.1084 |
GoFc | 1.085 |
Residual density max/min, e Å−3 | −0.148/0.191 |
aR1=Σ||Fo|–|Fc||/Σ|Fo|; bwR2=[Σw(Fo2–Fc2)2/Σw(Fo2)2]1/2, w=[σ2(Fo2)+(AP)2+BP]−1, where P=(Max(Fo2, 0)+2Fc2)/3; cGoF=S=[Σw(Fo2–Fc2)2/(nobs–nparam)]1/2.
Compound 2a crystallizes in the monoclinic space group P21/c with two formula units per unit cell. Calculated density for T=173 K is 1.414 g cm−3. The bond lengths and angles within the crystal structure of 2a are consistent with comparable values in the literature [34], [35]. The formula unit of 2,2-DvBTE (2a) is shown in Fig. 6 along with selected bond lengths, angles and torsion angles. The molecular structure itself shows a slightly twisted assembly with a torsion angle of 110.8° for the C1i–C1–C2–N2 fragment. The vinyl group is nearly in one plane with the tetrazole ring with a torsion angle of the vinyl group towards the tetrazole ring of 4.0°.
Due to the lack of suitable donors no hydrogen bonds are observed in the crystal system to stabilize the supramolecular structure. As shown in Fig. 7, the crystal structure of 2a consists of stacks of alternately oriented molecules which form infinite zig-zag rows along the crystallographic a axis. The layers are stacked above each other.
2.4 Thermal stability
The behavior at high temperatures of compounds 2–3 was determined via differential scanning calorimetry with a heating rate of 5°C min−1. The obtained plots are depicted in Fig. 8.
The compounds containing the same functional groups show similar melting and decomposition temperatures. The vinyl-substitued compounds 2a and 2b show melting points around 90°C and decomposition temperatures around 190°C, whereas the liquid allyl analogues 3a and 3b are stable up to higher temperatures with Tdec around 230°C.
2.5 Energetic data
To determine their inherent energetic potentials, sensitivities and energetic properties of compounds 2–3 were investigated. The impact and friction sensitivities of 2–3 were explored by BAM methods [16], [17], [18], [19], [20], [21], [22]. All compounds were found as insensitive towards impact (>40 J) and friction (>360 N).
For calculating the energetic properties of compounds 2–3 quantum chemical calculations were carried out. Initial structure optimizations for 2b and 3a,b were performed at the B3LYP/cc-pVDZ level of theory using Gaussian 09 (revision A.02) [36]. The enthalpies (H) and free energies (G) were calculated by the atomization method using electronic energies (CBS-4M method) [37], [38].
All calculations concerning the detonation parameters were carried out using the program package Explo5 (version 6.02) [39], [40] and were based on the calculated solid and liquid state heats of formation and attributed to the corresponding densities. For a discussion of the methods that were used for calculations, see the Supporting Information. The calculated detonation values of compounds 2–3 are given in Table 2.
2a | 2b | 3a | 3b | |
---|---|---|---|---|
Formula | C8H10N8 | C8H10N8 | C10H14N8 | C10H14N8 |
FW, g mol−1 | 218.22 | 218.22 | 246.27 | 246.27 |
IS, Ja | >40 | >40 | >40 | >40 |
FS, Nb | >360 | >360 | >360 | >360 |
N, %c | 51.35 | 51.35 | 45.50 | 45.50 |
TDec, °Cd | 190 | 186 | 225 | 230 |
ρ, g cm−3e | 1.4 | 1.4 | 1.2 | 1.2 |
ΔfHm°, kJ mol−1f | 635 | 643 | 605 | 622 |
ΔfU°, kJ kg−1g | 2907 | 2970 | 2452 | 2504 |
Explo5 V6.02 values | ||||
−ΔEU°, kJ kg−1h | 3681 | 3743 | 3398 | 3448 |
T, Ki | 2317 | 2341 | 2131 | 2150 |
pCJ, kbarj | 148 | 149 | 99 | 99 |
VDet, m s−1k | 7109 | 7132 | 6148 | 6167 |
Gas vol., L kg−1l | 702 | 703 | 737 | 737 |
Is, sm | 197 | 198 | 188 | 189 |
aBAM drop hammer (1 of 6); bBAM friction tester (1 of 6); cnitrogen content; dtemperature of decomposition by DSC (onset values); ederived from pycnometer measurements; fmolar enthalpy of formation; genergy of formation; henergy of explosion; iexplosion temperature; jdetonation pressure; kdetonation velocity; lassuming only gaseous products; mspecific impulse (isobaric combustion, chamber pressure 70 bar, equilibrium expansion).
The obtained detonation values show moderate energetic properties for 2–3. Due to their higher ΔfHm° value and density, as well as lower carbon content, vinyl based 2a and 2b show an about 1000 m s−1 increased detonation velocity (Vdet) and an about 50 kbar higher detonation pressure (pCJ), when compared to the respective allyl based isomers 3a and 3b. A comparison of the corresponding isomers, in relation to each other, revealed slightly increased detonation values for the unsymmetrically substituted compounds, due to their higher enthalpy of formation.
3 Conclusion
In the course of our work aimed at generating suitable precursors for further (polymeric) processing, divinyl and diallyl derivatives of 1,2-bis(tetrazol-5-yl)ethane were synthesized and characterized. Two different constitutional isomers of each compound were isolated. The thermal behavior, as well as the sensitivities and energetic properties of the compounds were investigated. The compounds were found to be stable up to 190°C and 230°C, respectively, and insensitive towards impact and friction. Due to the dual terminal double bonds present in the compounds, they offer possibilities for diverse processing steps.
4 Experimental section
All chemical reagents and solvents of analytical grade were obtained from Sigma-Aldrich, Acros Organics or ABCR and used without further purification. BTE was synthesized in close accordance to a procedure described in the literature [32].
Purification by column chromatography was performed using Merck silica gel 60 (Ø 35–70 μm). The eluent is given in the respective compound section. 1H, 13C and 2D NMR spectra were recorded with a JEOL 400 or a Bruker 400 (TR) instrument. The spectra were measured at 25°C. The chemical shifts are given relative to tetramethylsilane as external standard. Infrared spectra were measured with a Perkin-Elmer Spectrum BX-FTIR spectrometer equipped with a Smiths DuraSamplIR II ATR device. All spectra were recorded at ambient temperature; all samples were neat liquids or solids. Elemental analysis (C/H/N) was performed with an Elementar Vario EL analyzer.
Decomposition temperatures were determined by differential scanning calorimetry (DSC) with a Linseis DSC PT10 calibrated by standard pure indium and zinc, using a heating rate of 5°C min−1 in covered aluminum pans and a nitrogen flow of 20 mL min−1. Pycnometric measurements were carried out with a Quantachrome Ultrapyc 1200e pycnometer. High resolution measurements were recorded on a Finnigan MAT 95 instrument. Impact and friction sensitivity tests were carried out according to STANAG 4489 and STANAG 4487 modified instructions using a BAM (Bundesanstalt für Materialforschung) drop hammer and friction tester [16], [17], [18], [19], [20], [21], [22]. The classification of the tested compounds results from the “UN Recommendations on the Transport of Dangerous Goods”.
4.1 1,2-Bis(2-vinyl-2H-tetrazol-5-yl)ethane (2,2-DvBTE, 2a) and 1-(2-vinyl-2H-tetrazol-5-yl)-2-(1-vinyl-1H-tetrazol-5-yl)ethane (1,2-DvBTE, 2b)
1,2-Dibromoethane (8.3 mL, 96 mmol) was dissolved in acetonitrile (30 mL) and heated to 80°C. BTE (12, 4.00 g, 24.08 mmol) was dissolved in acetonitrile (20 mL) and triethylamine (8.3 mL, 96 mmol). This mixture was added into the reaction flask over 5 h using a dropping funnel. Stirring at 80°C was continued for 2 days. After cooling to ambient temperature, brine (50 mL) was added and the aqueous phase was extracted with ethyl acetate (3×100 mL). The combined organic phases were dried over MgSO4 and the volatiles were removed in vacuo. The crude product was purified using column chromatography on silica gel (eluent: n-hexane-DCM-EtOAc=5:3:2).
4.2 1,2-Bis(2-vinyl-2H-tetrazol-5-yl)ethane (2,2-DvBTE, 2a)
Compound 2a was obtained as a colorless solid in 31% yield (1.63 g, 7.46 mmol, Rf =0.6) and was recrystallized from a n-hexane-ethyl acetate mixture.
M. p. 92°C. – dec. 190°C. – IR (ATR, cm−1):
4.3 1-(2-Vinyl-2H-tetrazol-5-yl)-2-(1-vinyl-1H-tetrazol-5-yl)ethane (1,2-DvBTE, 2b)
Compound 2b was obtained as a colorless solid in 18% yield (0.95 g, 4.33 mmol, Rf=0.2).
M. p. 88°C. – dec. 186°C. – IR (ATR, cm−1):
4.4 1,2-Bis(2-allyl-2H-tetrazol-5-yl)ethane (2,2-BaBTE, 3a) and 1-(2-allyl-2H-tetrazol-5-yl)-2-(1-vinyl-1H-tetrazol-5-yl)ethane (1,2-BaBTE, 3b)
Allyl bromide (4.8 mL, 55 mmol) was dissolved in acetonitrile (20 mL) and heated to 55°C. BTE (1, 4.00 g, 24.08 mmol) was dissolved in acetonitrile (20 mL) and triethylamine (7.7 mL, 55 mmol). This mixture was added into the reaction flask over 5 h using a dropping funnel. Stirring at 65°C was continued for 2 days. Brine (50 mL) was added and the aqueous phase was extracted with ethyl acetate (3×100 mL). The combined organic phases were dried over MgSO4 and the volatiles were removed in vacuo. The crude product was purified using column chromatography on silica gel (eluent: n-hexane-DCM-EtOAc=5:3:2).
4.5 1,2-Bis(2-allyl-2H-tetrazol-5-yl)ethane (2,2-BaBTE, 3a)
Compound 3a was obtained as yellowish liquid in 28% yield (1.71 mg, 6.94 mmol, Rf=0.4).
Dec. 225°C. – IR (ATR, cm−1):
4.6 1-(2-Allyl-2H-tetrazol-5-yl)-2-(1-vinyl-1H-tetrazol-5-yl)ethane (1,2-BaBTE, 3b)
Compound 3b was obtained as a yellowish liquid in 15% yield (0.89 g, 3.71 mmol, Rf=0.2).
Dec. 230°C. – IR (ATR, cm−1):
4.7 Crystal structure determination
The crystallographic data were collected using an Oxford XCalibur3 diffractometer equipped with a Spellman generator (voltage 50 kV, current 40 mA) and a Kappa CCD area detector was employed for data collection using MoKα radiation (λ= 0.71073 Å). An absorption correction was applied based on multi-scans. The structure was solved by Direct Methods using Sir-97 [41], refined with Shelxl-97 [42], finally checked using the Platon software [43] integrated in the WinGX software suite [44]. All H atoms were located from a Fourier map and refined isotropically, with Uiso (H)=1.2Ueq (N, O). All non-hydrogen atoms were refined anisotropically. Diamond [45] plots are showing displacement ellipsoids at the 50% probability level for the non-hydrogen atoms. Table 1 contains the crystal data and numbers pertinent to data collection and structure refinement.
CCDC 1484921 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.
5 Supporting information
The spectra of the HMQC and HMBC 2D NMR measurements (Figs. S1–S4), together with detailed discussions of the methods used for calculations are given as Supporting Information available online (DOI: 10.1515/znb-2016-0146).
Dedicated to: Professor Dr. Jürgen Evers on the occasion of his 75th birthday.
Acknowledgments
Financial support of this work by the Ludwig-Maximilian University of Munich (LMU), the Office of Naval Research (ONR) under grant no. ONR.N00014-16-1-2062, and the Bundeswehr – Wehrtechnische Dienststelle für Waffen und Munition (WTD 91) under grant no. E/E91S/FC015/CF049 is gratefully acknowledged. The authors acknowledge collaborations with Dr. Mila Krupka (OZM Research, Czech Republic) in the development of new testing and evaluation methods for energetic materials and with Dr. Muhamed Suceska (Brodarski Institute, Croatia) in the development of new computational codes to predict the detonation and propulsion parameters of novel explosives. We are indebted to and thank Drs. Betsy M. Rice, Jesse Sabatini and Brad Forch (ARL, Aberdeen, Proving Ground, MD, USA) for many inspiring discussions. Stefan Huber is thanked for assistance with sensitivity measurements. Christina Hettstedt and Carolin Pflüger are thanked for help with X-ray diffraction measurements.
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