research communications
of 3,3′-biisoxazole-5,5′-bis(methylene) dinitrate (BIDN)
aUS Army Research Laboratory, RDRL-WML-B, Aberdeen Proving Ground, MD 21005, USA, and bUS Army Research Laboratory, RDRL-WML-C, Aberdeen Proving Ground, MD 21005, USA
*Correspondence e-mail: rosario.c.sausa.civ@mail.mil
The molecular structure of the title energetic compound, C8H6N4O8, is composed of two planar isoxazole rings and two near planar alkyl-nitrate groups (r.m.s deviation = 0.006 Å). In the crystal, the molecule sits on an inversion center, thus Z′ = 0.5. The dihedral angle between the isoxazole ring and the nitrate group is 69.58 (8)°. van der Waals contacts dominate the intermolecular interactions. Inversion-related rings are in close slip-stacked proximity, with an interplanar separation of 3.101 (3) Å [centroid–centroid distance = 3.701 (3) Å]. The measured and calculated densities are in good agreement (1.585 versus 1.610 Mg m−3).
Keywords: crystal structure; 3,3′-bis-isoxazole-5,5′-bis-methylene dinitrate; energetic material; density; FTIR; Raman; and ultraviolet absorption peaks.
CCDC reference: 1540757
1. Chemical context
Isoxazole compounds have attracted much interest in recent years because of their potential usefulness in medicine, agriculture, and in the field of energetic materials (Galenko et al., 2015; Wingard et al., 2017). The title compound is an isoxazole-based energetic material that has been synthesized recently in our laboratory. It has potential use as a trinitrotoluene replacement in melt-castable and Composition B formulations, and as an energetic plasticizing ingredient in nitrocellulose-based propellant formulations. The compound is composed of two heterocyclic isoxazole rings, each bonded to an alkyl nitric ester group. The heterocyclic base has non-bonded electron lone pairs which can exhibit Lewis-base behavior towards electrophilic materials such as nitrocellulose, whereas the alkyl nitric provide miscibility and compatibility with commonly used energetic plasticizers.
2. Structural commentary
The molecule (see Fig. 1) consists of two isoxazole rings bonded to two alkyl nitric ester groups. There are no unusual bond lengths or angles. The rings are planar (r.m.s. deviation = 0.0003 Å), and adopt a co-planar trans geometry, perhaps to minimize lone-pair interactions of the nitrogen atoms, similar to 3,3′-bisoxazole and 5,5′-diphenyl-3,3′-bisoxazole (Cannas & Marongiu, 1968; van der Peet et al., 2013). Atom C4 is co-planar with the ring [deviation = 0.062 (3) Å]. Similarly, atoms C4/O2/N2/O3/O4 adopt a near planar conformation (r.m.s deviation = 0.006 Å). The dihedral angle between the isoxazole ring and the nitrate group is 69.58 (9)°.
3. Supramolecular features
Figs. 2 and 3 show the packing of the title compound along the a and b axes, respectively. Bifurcated contacts between the N1 and H atoms of adjacent molecules [N1⋯H4Ai = 2.704 (4) Å and N1⋯H2ii = 2.656 (4) Å); symmetry codes: (i) 1 − x, 1 − y, 1 − z; (ii) x, y − 1, z] dominate the intermolecular interactions. Inversion-related (1 − x, 1 − y, 1 − z) isoxazole rings are in close slip-stacked proximity, with an interplanar separation of 3.101 (3) Å [ring centroid–centroid distance = 3.701 (3) Å].
4. Database survey
An open literature search, as well as a search of the Cambridge Structural Database (Groom et al., 2016) and the Crystallography Open Database (Gražulis et al., 2009) yielded many hits for bis-isoxazole-containing compounds and several on 3,3′ and 5,5′ bis-isoxazole-based compounds, the most pertinent studies relating to the title compound being the crystal structures of 3,3′-bisoxazole (Cannas & Marongiu, 1968; CCDC 1111317, BIOXZL) and 5,5′-diphenyl-3,3′-bisoxazole (van der Peet et al., 2013; CCDC 935274). In these compounds, the rings also adopt planar trans conformations, similar to that observed in the title compound.
5. Synthesis and crystallization
The synthesis of the title compound has been reported recently (Wingard et al., 2017). Briefly, a solution of sodium bicarbonate was added to a mixture of dichloroglyoxime (0.191 mol), propargyl alcohol (0.956 mol), and 1.9 L of methanol to produce the intermediate compound 5,5′-dihydroxymethyl-3,3′-bis-isoxazole (75% yield). Then, this compound (0.120 mol) was added portionwise over ten minutes to 90% nitric acid (150 ml) placed in a 250 ml round-bottom flask equipped with a stir bar, and cooled in an ice–water bath. No exotherm was observed during the addition. The reaction mixture was stirred for four hours while the water–ice bath was warmed to room temperature. The reaction mixture was poured onto ice, resulting in the formation of a white precipitate, which was collected by Büchner filtration and dried, giving the title compound (92% yield). Slow solvent evaporation of a solution in acetonitrile yielded suitable single crystals for the X-ray diffraction experiments at room temperature. Based on the cell dimensions and molecular weight, the calculated crystal density of 1.609 Mg m−3 at 297 K is in excellent agreement with the value of 1.585 Mg m−3 measured using a pycnometer at room temperature.
Spectroscopic data: FTIR (Nicolet iS50, attenuated total reflectance, cm−1): 3144 (w), 3032 (w), 2923 (w), 1643 (m), 1605 (m), 1421 (m), 1359 (m), 1351(m), 1278 (s), 1259 (m), 1209 (m), 1075 (m), 1021 (w), 955 (m), 926 (s), 912 (s), 845 (s), 824 (s), 753 (s), 649 (m), 582 (m). Raman (Nicolet iS50, 1064 nm; cm−1): 3143 (m), 3027 (w), 2977 (m), 2855.59 (w), 1621 (w), 1552 (s), 1476 (m), 1422 (w), 1354 (w), 1299 (w) 1279 (w), 1146 (w), 1020 (w) 960 (m), 922 (w), 847 (m), 728 (w), 667 (w), 645 (w), 585 (m), 489 (m), 449 (w), 381 (w), 373 (w), 249 (w), 218 (w), 161.70 (w). UV (acetonitrile solvent, nm): 220 nm (max).
6. Refinement
Crystal data, data collection and structure . The hydrogen atoms were refined using a riding model with C—H = 0.93 or 0.97 Å and Uiso(H) = 1.2Ueq(C).
details are summarized in Table 1Supporting information
CCDC reference: 1540757
Data collection: CrysAlis PRO (Rigaku OD, 2015); cell
CrysAlis PRO (Rigaku OD, 2015); data reduction: CrysAlis PRO (Rigaku OD, 2015); program(s) used to solve structure: ShelXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009) and Mercury (Macrae et al., 2008); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).C8H6N4O8 | F(000) = 292 |
Mr = 286.17 | Dx = 1.609 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
a = 6.1917 (5) Å | Cell parameters from 1878 reflections |
b = 5.5299 (5) Å | θ = 2.4–25.2° |
c = 17.4769 (12) Å | µ = 0.15 mm−1 |
β = 99.233 (7)° | T = 297 K |
V = 590.65 (8) Å3 | Irregular, colourless |
Z = 2 | 0.4 × 0.2 × 0.1 mm |
SuperNova, Dualflex, EosS2 diffractometer | 1079 independent reflections |
Radiation source: fine-focus sealed X-ray tube, Enhance (Mo) X-ray Source | 903 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.027 |
Detector resolution: 8.0945 pixels mm-1 | θmax = 25.3°, θmin = 2.4° |
ω scans | h = −7→7 |
Absorption correction: multi-scan (SCALE3 ABSPACK in CrysAlisPro; Rigaku OD, 2015; Bourhis et al., 2015) | k = −6→5 |
Tmin = 0.678, Tmax = 1.000 | l = −21→21 |
4487 measured reflections |
Refinement on F2 | Hydrogen site location: inferred from neighbouring sites |
Least-squares matrix: full | H-atom parameters constrained |
R[F2 > 2σ(F2)] = 0.041 | w = 1/[σ2(Fo2) + (0.045P)2 + 0.168P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.105 | (Δ/σ)max < 0.001 |
S = 1.06 | Δρmax = 0.19 e Å−3 |
1079 reflections | Δρmin = −0.18 e Å−3 |
92 parameters | Extinction correction: SHELXL-2016/4 (Sheldrick 2015b), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
0 restraints | Extinction coefficient: 0.077 (8) |
Primary atom site location: dual |
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. |
x | y | z | Uiso*/Ueq | ||
C4 | 0.5869 (4) | 0.8027 (4) | 0.63740 (11) | 0.0621 (6) | |
H4A | 0.714559 | 0.780169 | 0.612468 | 0.075* | |
H4B | 0.553186 | 0.974123 | 0.636451 | 0.075* | |
C3 | 0.1082 (3) | 0.5201 (3) | 0.52299 (9) | 0.0446 (5) | |
C1 | 0.3989 (3) | 0.6693 (3) | 0.59298 (10) | 0.0513 (5) | |
C2 | 0.1987 (3) | 0.7337 (3) | 0.55899 (10) | 0.0512 (5) | |
H2 | 0.133409 | 0.885170 | 0.559012 | 0.061* | |
N1 | 0.2447 (3) | 0.3392 (3) | 0.53455 (9) | 0.0558 (5) | |
N2 | 0.5082 (3) | 0.8284 (4) | 0.76635 (11) | 0.0660 (5) | |
O2 | 0.6372 (2) | 0.7230 (3) | 0.71689 (8) | 0.0610 (5) | |
O1 | 0.4340 (2) | 0.4322 (2) | 0.57971 (8) | 0.0603 (4) | |
O4 | 0.3714 (3) | 0.9669 (4) | 0.73937 (12) | 0.0920 (6) | |
O3 | 0.5589 (3) | 0.7599 (4) | 0.83151 (10) | 0.1017 (7) |
U11 | U22 | U33 | U12 | U13 | U23 | |
C4 | 0.0688 (13) | 0.0653 (15) | 0.0515 (11) | −0.0108 (11) | 0.0070 (10) | −0.0053 (9) |
C3 | 0.0621 (11) | 0.0348 (9) | 0.0377 (9) | −0.0046 (8) | 0.0103 (7) | 0.0013 (7) |
C1 | 0.0674 (13) | 0.0421 (11) | 0.0444 (10) | −0.0060 (9) | 0.0088 (9) | −0.0012 (8) |
C2 | 0.0678 (13) | 0.0361 (10) | 0.0478 (10) | −0.0027 (9) | 0.0037 (9) | −0.0010 (8) |
N1 | 0.0665 (11) | 0.0421 (10) | 0.0571 (9) | −0.0043 (8) | 0.0046 (8) | −0.0043 (7) |
N2 | 0.0596 (11) | 0.0725 (13) | 0.0640 (12) | −0.0003 (10) | 0.0042 (9) | −0.0189 (9) |
O2 | 0.0597 (8) | 0.0670 (10) | 0.0536 (8) | 0.0124 (7) | 0.0009 (6) | −0.0115 (7) |
O1 | 0.0649 (9) | 0.0490 (9) | 0.0637 (8) | 0.0005 (7) | 0.0002 (7) | −0.0044 (6) |
O4 | 0.0747 (11) | 0.0866 (13) | 0.1133 (14) | 0.0257 (10) | 0.0113 (10) | −0.0237 (11) |
O3 | 0.1055 (14) | 0.144 (2) | 0.0538 (10) | −0.0022 (13) | 0.0082 (9) | −0.0135 (11) |
C4—H4A | 0.9700 | C1—C2 | 1.334 (3) |
C4—H4B | 0.9700 | C1—O1 | 1.355 (2) |
C4—C1 | 1.487 (3) | C2—H2 | 0.9300 |
C4—O2 | 1.443 (2) | N1—O1 | 1.402 (2) |
C3—C3i | 1.465 (4) | N2—O2 | 1.395 (2) |
C3—C2 | 1.411 (2) | N2—O4 | 1.182 (2) |
C3—N1 | 1.304 (2) | N2—O3 | 1.193 (2) |
H4A—C4—H4B | 107.9 | O1—C1—C4 | 115.83 (18) |
C1—C4—H4A | 109.1 | C3—C2—H2 | 127.8 |
C1—C4—H4B | 109.1 | C1—C2—C3 | 104.42 (17) |
O2—C4—H4A | 109.1 | C1—C2—H2 | 127.8 |
O2—C4—H4B | 109.1 | C3—N1—O1 | 105.57 (15) |
O2—C4—C1 | 112.39 (17) | O4—N2—O2 | 117.91 (19) |
C2—C3—C3i | 129.4 (2) | O4—N2—O3 | 130.4 (2) |
N1—C3—C3i | 118.8 (2) | O3—N2—O2 | 111.70 (19) |
N1—C3—C2 | 111.83 (16) | N2—O2—C4 | 114.35 (16) |
C2—C1—C4 | 133.91 (19) | C1—O1—N1 | 107.99 (14) |
C2—C1—O1 | 110.19 (16) | ||
C4—C1—C2—C3 | 176.7 (2) | C2—C1—O1—N1 | −0.1 (2) |
C4—C1—O1—N1 | −177.37 (15) | N1—C3—C2—C1 | 0.0 (2) |
C3i—C3—C2—C1 | 179.9 (2) | O2—C4—C1—C2 | 115.6 (2) |
C3i—C3—N1—O1 | −179.97 (18) | O2—C4—C1—O1 | −67.9 (2) |
C3—N1—O1—C1 | 0.08 (19) | O1—C1—C2—C3 | 0.0 (2) |
C1—C4—O2—N2 | −82.9 (2) | O4—N2—O2—C4 | 1.0 (3) |
C2—C3—N1—O1 | −0.1 (2) | O3—N2—O2—C4 | −178.72 (19) |
Symmetry code: (i) −x, −y+1, −z+1. |
Acknowledgements
We thank Dr Eric Reinheimer of Rigaku for his help and useful discussions regarding the structure solution and
of the title compound and Dr D. Taylor of the US Army Research Laboratory for review of this manuscript.References
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