Crystal structure of 3,3′-biisoxazole-5,5′-bis­(methyl­ene) dinitrate (BIDN)

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).

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 esters 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)°.

Figure 1 Mol­ecular conformation and atom-numbering scheme. Non-labeled atoms are generated by inversion (−x, 1 − x, 1 − z). Non-hydrogen atoms are shown as 50% probability displacement ellipsoids.

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].

Figure 2 Crystal packing viewed along the a axis. Dashed lines represent contacts between atoms N1⋯H2, N11⋯H4A, and C11⋯O4 (blue) and O41⋯H4B (red).

Figure 3 Crystal packing viewed along the b axis. Dashed lines represent contacts between atoms N1⋯H4A and C11⋯O4 (blue), and O4⋯H4B (red).

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 refinement details are summarized in Table 1. The hydrogen atoms were refined using a riding model with C—H = 0.93 or 0.97 Å and U_iso(H) = 1.2U_eq(C).

Table 1
Experimental details

Crystal data
Chemical formula	C8H6N4O8
Mr	286.17
Crystal system, space group	Monoclinic, P21/n
Temperature (K)	297
a, b, c (Å)	6.1917 (5), 5.5299 (5), 17.4769 (12)
β (°)	99.233 (7)
V (Å^3)	590.65 (8)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	0.15
Crystal size (mm)	0.4 × 0.2 × 0.1
Data collection
Diffractometer	Agilent SuperNova, Dualflex, EosS2
Absorption correction	Multi-scan (SCALE3 ABSPACK in CrysAlis PRO; Rigaku OD, 2015; Bourhis et al., 2015)
Tmin, Tmax	0.678, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	4487, 1079, 903
Rint	0.027
(sin θ/λ)max (Å^−1)	0.602
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.041, 0.105, 1.06
No. of reflections	1079
No. of parameters	92
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å^−3)	0.19, −0.18
Computer programs: CrysAlis PRO (Rigaku OD, 2015), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b), OLEX2 (Dolomanov et al., 2009) and Mercury (Macrae et al., 2008).

Supporting information

Computing details

Data collection: CrysAlis PRO (Rigaku OD, 2015); cell refinement: 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).

3,3'-Biisoxazole-5,5'-bis(methylene) dinitrate

Crystal data

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) Å^3	Irregular, colourless
Z = 2	0.4 × 0.2 × 0.1 mm

Data collection

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

Refinement on F^2	Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: full	H-atom parameters constrained
R[F^2 > 2σ(F^2)] = 0.041	w = 1/[σ^2(Fo^2) + (0.045P)^2 + 0.168P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 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+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.077 (8)
Primary atom site location: dual

Special details

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	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)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
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)

Geometric parameters (Å, º)

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—C3^i	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—C3^i	129.4 (2)	O4—N2—O3	130.4 (2)
N1—C3—C3^i	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)
C3^i—C3—C2—C1	179.9 (2)	O2—C4—C1—C2	115.6 (2)
C3^i—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.