Synthesis, Structure and Thermal Properties of BFFO Bull. Korean Chem. Soc. 2012, Vol. 33, No. 10 3317 http://dx.doi.org/10.5012/bkcs.2012.33.10.3317
Synthesis, Structure and Thermal Properties of
Bifurazano[3,4-b:3',4'-f ]furoxano[3'',4''-d]oxacyclohetpatriene (BFFO)
Yanshui Zhou, Kangzhen Xu,†,* Bozhou Wang,* Hang Zhang,† Qianqian Qiu,† and Fengqi Zhao
Xi’an Modern Chemistry Research Institute, Xi’an 710065, China. *E-mail: wbzboo@163.com
†Department of Chemical Engineering, Northwest University, Xi’an 710069 China. *E-mail: xukz@nwu.edu.cn Received June 26, 2012, Accepted July 15, 2012
A novel energetic compound, bifurazano[3,4-b:3',4'-f ]furoxano[3'',4''-d] oxacyclohetpatriene (BFFO), was synthesized through special etherification and its structure was determined by single crystal X-ray diffraction.
The crystal of BFFO·H2O is monoclinic, space group P2(1)/c with crystal parameters of a = 9.324(4) Å, b = 9.727(4) Å, c = 10.391(4) Å, β = 106.305(6)°, V = 904.5(6) Å3, Z = 4, μ = 0.17 mm−1, F(000) = 512 and Dc= 1.866 g cm−3. Spectroscopic properties and thermal behaviors of BFFO were studied. BFFO presents good detonation properties.
Key Words : Bifurazano[3,4-b:3',4'-f ]furoxano[3'',4''-d]oxacyclohetpatriene (BFFO), Furazan, Crystal struc- ture, Thermal properties
Introduction
As an energetic nitrogen heterocyclic group, furazan ring features high heat of formation, fine thermal stability, high density and active oxygen, so that it is a highly efficient structural unit in the development of high energy density materials (HEDMs).1-6 Furoxans have the same properties.
Moreover, substituting a furazan ring or a furoxan ring for a nitro, the density and detonating velocity can increase about 0.06-0.08 g cm−3 and 300 ms−1,5,6 which leads many researchers to synthesize multi-furazan ring compounds and multi-furazan ring oxides, such as difurazans, chained furazans, macrocyclic fururanzans, ring-fused furazans and other furazan derivatives.7-12 Many derivatives, for example 3,3'-dinitro-4,4'-azoxyfurazan (DNOAF) and 3,4-dinitro- furanzanfuroxan (DNTF), all have shown good physico- chemical properties and applied prospect.6 Furazanether compounds are also main research objectives. After ether bond was drawn into molecule, the flexibility of compound rises, but the melting point reduces.13 Those properties are valuable for the usage in solid propellant. In this paper, we will report a novel macrocyclic ether-furoxan, bifurazano [3,4-b:3',4'-f ]furoxano[3'',4''-d] oxacyclohetpatriene (BFFO), using 3,4-dinitrofurzanfuroxan (DNTF) as raw material through special etherification (Scheme 1).
Experimental
Synthesis. In room temperature, 3,4-dinitrofurzanfuroxan (DNTF) (10.0 g, 0.0321 mol) and anhydrous sodium carbo- nate (4.6 g, 0.0434 mol) were put into 25 mL acetonitrile.
After reacting at 80 °C for 3.5 h, the resulting solution was transferred into 80 mL water, and then extracted with 60 mL chloroform for three times. The obtained organic phase was dried by anhydrous magnesium sulfate and then filtered.
After the above filtrate was evaporated, white solid (BFFO) was obtained, yield 3.8 g (50.1%). mp 97 ºC. 13C NMR (DMSO-d6, 500 MHz) δ 160.50, 159.98, 144.39, 137.78, 135.52, 105.03. 15N NMR (DMSO-d6, 500 MHz) δ 422.7, 421.4, 386.0, 383.5, 376.1, 358.3. IR (KBr, cm−1): 1655, 1623, 1562, 1543, 1470, 1384, 1151, 997. Anal. calcd for C6N6O5 (%): C 30.52, N 35.59, found C 30.87, N 35.99.
Determination of the Single Crystal Structure. Single crystals suitable for X-ray measurement were obtained by slow evaporation of an acetonitrile-water solution of BFFO.
A crystal of bifurazano[3,4-b:3',4'-f]furoxano[3'',4''-d]oxa- cyclohetpatriene monohydrate (BFFO·H2O) with dimensions of 0.23 × 0.18 × 0.15 mm3 was chosen for X-ray determina- tion. The data were collected on a Bruker SMART APEX II CCD X-ray diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.071073 nm). The structure was solved by the direct methods (SHELXTL-97) and refined by the full-matrix-block least-squares method on F2 with aniso- tropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms were added according to the theoretical models. Crystal data, experimental details and refinement results were summarized in Table 1.
Thermal Analysis Condition. The DSC experiment for BFFO·H2O was performed using a Q2000 apparatus (TA, USA) under a nitrogen atmosphere at a flow rate of 100 mL min−1. The heating rate used was 5.0 min−1 from ambient Scheme 1. The designed synthetic route of BFFO.
3318 Bull. Korean Chem. Soc. 2012, Vol. 33, No. 10 Yanshui Zhou et al.
temperature to 400 °C.
The TG-DTG experiment was performed using a SDT- Q600 apparatus (TA, USA) under a nitrogen atmosphere at a flow rate of 100 mL min−1. The heating rate used was 5.0 ºC min−1 from ambient temperature to 400.0 ºC.
Results and Discussion
Structural Description. Selected bond lengths and bond angles of BFFO·H2O were summarized in Table 2. Mole- cular structure, 2D monolayer structure and 3D network of BFFO·H2O were illustrated in Figures 1-3.
The analytical results indicate that the molecule of BFFO·H2O is made up of two furazan rings, an furoxan ring, a seven-membered ether ring based on the above three furazan rings and a water molecule (Fig. 1). No hydrogen atom exists in BFFO, so there is on direct hydrogen bond interaction between BFFO molecules. Water molecule plays an important part in the crystal packing. Each BFFO mole- cule connects with three surrounding water molecules through four hydrogen bond interactions (three O−H…N and one O−H…O), which were listed detailedly in Table 3.
And so, each water molecule connects with three surround- ing BFFO molecules through the same hydrogen bond inter- actions. All form a 2D waved monolayer infinite network, as shown in Figure 2 and Figure 3(a). There is no hydrogen bond between 2D layers (Fig. 3 (a)). It is the Van der Waals force that leads to the formation of a 3D framework, in
which various channels are formed and sustained by cova- lent bonds and hydrogen bonds. Many methods have been tried to obtain the single crystal of BFFO without other solvent molecules, but all failed. Moreover, two furazan rings and one furoxan ring possess huge energy. The crystal density of BFFO·H2O is 1.866 g cm−1, so we can forecast the density of pure BFFO can reach 1.9 g cm−1. So, BFFO is a good high energy density material (HEDM).
Form Table 2, we can see that the seven-membered ether ring based on two furazan rings and one furoxan ring is a distorted seven-membered ring, owing to the effect of N→O in the furoxan ring. However, the whole molecule of BFFO Table 1. Crystal data and structure refinement details
Chemical formula C6H2N6O6
Formula weight/(g·mol−1) 254.14
Temperature/K 296(2)
Crystal system Monoclinic
Space group P2(1)/c
a/Å 9.324(4)
b/Å 9.727(4)
c/Å 10.391(4)
α/(º) 90.00
β/(º) 106.305(6)
γ/(º) 90.00
Volume /Å3 904.5(6)
Z 4
Dcalc/g·cm−3 1.866
Absorption coefficient/mm−1 0.169
F(000) 512
θ range/(º) 2.93-28.09
Index ranges -12≤ h ≤ 5, -12 ≤ k ≤ 12, -13≤ l ≤ 13
Reflections collected 2146
Reflections unique 1086 [R(int) = 0.0255]
Refinement method Full-matrix least-squares on F2 Goodness-of-fit on F2 1.035
Final R indices [l > 2σ(l)] R1=0.0363, wR2=0.0910 R indices (all data) R1=0.0442, wR2=0.0969 Largest diff. peak and hole/(e·Å−3) 0.262 and -0.180
Table 2. Selected bond lengths ( Å )and bond angles (º) Bond lengths
C1-N1 1.290(2) C4-N4 1.3135(18) N2-O1 1.3611(19) C1-O5 1.3459(18) C4-C5 1.442(2) N3-O2 1.3598(18) C1-C2 1.414(2) C5-N5 1.295(2) N4-O3 1.2186(17) C2-N2 1.292(2) C5-C6 1.422(2) N4-O2 1.4444(18) C2-C3 1.446(2) C6-N6 1.2867(19) N5-O4 1.3716(18) C3-N3 1.3035(19) C6-O5 1.3449(17) N6-O4 1.3773(18) C3-C4 1.395(2) N1-O1 1.3744(19)
Bond angles
N1-C1-O5 116.47(13) N6-C6-O5 116.83(13) N1-C1-C2 109.63(14) N6-C6-C5 109.95(13) O5-C1-C2 133.88(13) O5-C6-C5 133.21(13) N2-C2-C1 108.37(13) C1-N1-O1 104.97(13) N2-C2-C3 122.83(13) C2-N2-O1 106.04(13) C1-C2-C3 128.80(13) C3-N3-O2 106.07(13) N3-C3-C4 112.18(13) O3-N4-C4 136.06(14) N3-C3-C2 122.97(14) O3-N4-O2 117.75(12) C4-C3-C2 124.82(12) C4-N4-O2 106.19(12) N4-C4-C3 107.14(12) C5-N5-O4 105.73(12) N4-C4-C5 124.74(13) C6-N6-O4 104.91(12) C3-C4-C5 128.12(13) N2-O1-N1 110.99(12) N5-C5-C6 108.29(13) N3-O2-N4 108.41(10) N5-C5-C4 123.98(13) N5-O4-N6 111.13(11) C6-C5-C4 127.73(13) C6-O5-C1 123.18(11)
Figure 1. Crystal structure of BFFO·H2O.
Synthesis, Structure and Thermal Properties of BFFO Bull. Korean Chem. Soc. 2012, Vol. 33, No. 10 3319
is almost a planar configuration, and the biggest deviation of torsion angle (C2-C1-O5-C6) is only 6.4°. Water molecule was affected by these hydrogen bond interactions markedly.
The bond lengths of two O-H bonds are 0.779 and 0.900 Å, which should be equivalent to each other, although hydrogen atoms were appended in structure-solving process. The bond length of N4-O3 is 1.219 Å, and equivalent to the relevant bond length of (C)NO2. The N4 atom provides a pair of lone pair electrons and presents certain positive charges, but the O5 atom provides unoccupied molecular orbital and has some negative charges.
Spectroscopic Properties. The IR spectrum of BFFO was performed as KBr pellets in the range of 4000-400 cm−1. From the determined result, we can see that the peaks at 1151 and 1072 cm−1 are the characteristic absorption peaks of C-O-C. The absorption peaks at 1655, 1624, 1563and 1543 cm−1 indicate that there exists in C=N structure in BFFO molecule. The peaks at 997 and 975 cm−1 are the characteristic absorption peaks of N-O-N. 1219 cm−1 is the characteristic absorption of N→O group. No absorption peak
at ~3000 cm−1 indicates that there is no C-H or N-H bond in BFFO.
Ultraviolet-absorption spectrum of BFFO indicates that the maximum absorption peak occurred at 255.4 nm, which came from the π→π* transition of C=N group. The other obvious absorption peak at 291.0 nm was caused by the n→π* transition of N-O-N group. Owing to the conjugative effect of furazan ring, the absorption peak was red-shifted.
Thermal Behaviors. From DSC and TG-DTG curves (Figs. 4 and 5), we can see that the thermal behaviors of BFFO·H2O present multiple stages. The first stag is a small endothermic decomposition process, which occurs at 81-94 ºC with a mass loss of about 5.6%. The weight loss is rough- ly consistent with the theoretical content of a water molecule in BFFO·H2O as 7.1%. The peak temperature of the de- hydration process is 84.6 °C at the heating rate of 5 °C min−1. The second stage is a typical melting process, and the extra- polated onset temperature, peak temperature and melting enthalpy at the heating rate of 5 °C min−1 are 95.1 ºC, 96.8 ºC and 34.2 Jg−1, respectively. When heating temperature is greater than 180 ºC, the melting samples begin to slowly evaporate, so the thermal behavior presents slight endo- thermic trend, and corresponding TG curve begins to fall. At about 250 ºC, the thermal behavior presents an obvious exothermic trend, so we believe that BFFO begins to decom- pose at the temperature. Soon after, the thermal behavior Figure 2. 2D monolayer structure of BFFO·H2O.
Figure 3. 3D framework of BFFO·H2O.
Table 3. Hydrogen bonds of BFFO·H2O
D−H…A d(D−H)/Å d(H…A)/Å d(D…A)/Å ∠DHA/(º)
O6−H…N1#i 0.779 2.523 3.068 128.45
O6−H…N3#ii 0.779 2.572 3.234 144.02
O6−H…N5#iii 0.900 2.396 3.201 149.01
O6−H…O3#iii 0.900 2.464 3.092 127.15
Symmetry codes: #i x, -y+2/3, z+1/2; #ii -x+1, -y+1, -z+1; #iii -x, -y+1, -z+1
Figure 4. DSC curve of BFFO·H2O at a heating rate of 5 °C min−1.
Figure 5. TG-DTG curves of BFFO·H2O at a heating rate of 5 °C min−1.
3320 Bull. Korean Chem. Soc. 2012, Vol. 33, No. 10 Yanshui Zhou et al.
presents an obvious endothermic process again, because decomposition heat is less than evaporation heat of melting BFFO. Thus, it can be seen that thermal decomposition temperature of BFFO is high.
Estimating of Detonation Properties. The empirical Kamlet- Jacobs (K-J) equations are widely applied to estimate the values of detonation velocity (D) and detonation pressure (P) for the explosives only containing C, H, O and N.13-16 D and P of energetic materials can also be predicted with the nitrogen equivalent equations,17 which don’t need the value of heat of formation (HOF).
The heat of formation (HOF) of BFFO was calculated as 275.2 kJ mol−1, according to the semiempirical theoretical calculation. So, the calculated values of D and P for BFFO were listed in Table 4. Characteristic height H50 is an impor- tant parameter for estimating explosive sensitivity. Charac- teristic height H50 of BFFO was determined as 57.5 cm. From the comparison with other typical energetic materials,14,15 we can see that BFFO presents good detonation properties, and is less sensitive.
Conclusion
Bifurazano[3,4-b:3',4'-f ]furoxano[3'',4''-d]oxacyclohetpa- triene (BFFO) was synthesized and structurally determined.
BFFO has planar geometry, and presents lower melting point and higher exothermic decomposition temperature. BFFO has good detonation properties.
Supplementary Material. Crystallographic data for the structural analysis have been deposited in the Cambridge Data Center (CCDC), CCDC number: 851878 for C6 H2 N6 O6.
Acknowledgments. This investigation received financial assistance from the National Natural Science Foundation of China (Grant No. 20803058) and Basal Science Foundation of National Defense (Grant No. B0920110005).
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Table 4. Detonation properties of BFFO and comparison with other energetic materials
Compounds D (km s−1) P (GPa) Characteristic height H50 (cm)
BFFO 8.3 #1 33.2 #1
8.6 #2 34.6 #2 57.5
FOX-7 9.0 36.1 72
RDX 8.7 33.8 24
TNT 6.8 19.3 160
NTO 8.7 34.9 290
HMX 9.1 39.0 26
TATB 7.9 29.7 320
(#1) obtained by K-J equations; (#2) obtained by nitrogen equivalent equations. FOX-7--1,1-Diamino-2,2-dinitroethylene. RDX--1,3,5-Trinitro- perhydro-1,3,5-triazine. TNT--2,4,6-Trinitrotoluene. NTO--3-Nitro-1,2,4- triazol-5-one. HMX--1,3,5,7-Tetranitro-1,3,5,7-tetrazocane. TATB--2,4,6- Triamino-1,3,5-trinitrobenzene
