1,2,3,4,5,7-Hexa­nitro­cubane

The hexanitro­cubane structure, C₈H₂N₆O₁₂, reported herein, is one of the last in the series of nitro­cubanes, ranging from mono- to octa­nitro­cubane. In this mol­ecule, the H atoms participate in short hydrogen-bonding contacts to the nitro O atoms in adjoining mol­ecules [2.50 (1) Å], thus linking mol­ecules into a two-dimensional sheet in the bc plane. In addition, the O...O contacts in hexanitro­cubane are shorter than van der Waals contact distances. The shortest of these [2.766 (3) Å] involve the O atoms of one of the attached nitro groups. These contacts involve mol­ecules in an extended two-dimensional sheet parallel to the ab plane.

Key indicators

• Single-crystal X-ray study
• T = 295 K
• Mean [sigma](C-C) = 0.002 Å
• R factor = 0.046
• wR factor = 0.136
• Data-to-parameter ratio = 10.5

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level C:



PLAT_320  Alert C Check Hybridisation of  C3     in main residue          ?

PLAT_320  Alert C Check Hybridisation of  C4     in main residue          ?

PLAT_320  Alert C Check Hybridisation of  C5     in main residue          ?

PLAT_320  Alert C Check Hybridisation of  C6     in main residue          ?

PLAT_320  Alert C Check Hybridisation of  C7     in main residue          ?

PLAT_320  Alert C Check Hybridisation of  C8     in main residue          ?


 1 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 6 Alert Level C = Please check

---

S1. Comment

Nitrocubanes have long been sought after as powerful, shock-insensitive, high-density explosives (Eaton, 1992). In these compounds are combined the energy associated with the highly strained cubane skeleton (Eaton, 1992) with that associated with polynitrated compounds. One of the more interesting stories in organic synthesis in recent years has been the long (ca 20 years) development of methods (Eaton at al., 2000) to make octanitrocubane (Zhang et al., 2000), the `holy grail' of the energetic materials community. Theoretical predictions (Alster et al., 1981) had indicated that this compound would have one of the highest densities observed in compounds containing only C, H, N and O. In this search, the structures of 1,4-dinitro (Eaton et al., 1984), 1,3,5-trinitro- (Eaton et al., 1993), 1,3,5,7-tetranitro- (Eaton et al., 1993), 1,2,3,5,7-pentanitro- (Lukin et al., 1997), 1,2,3,4,5,6,8-heptanitro- (Zhang et al., 2000), and 1,2,3,4,5,6,7,8-octanitrocubane (Zhang et al., 2000) have been reported. However, to date the structure of the simplest member of this series, nitrocubane, is only now being reported (Butcher & Gilardi, 2002).

The structure of 1,2,3,4,5,7-hexanitrocubane, (I), is reported herein; previously the structure of its acetonitrile solvate was reported (Lukin et al., 1997). Pure hexanitrocubane crystallizes in the orthorhombic space group Pnma with four molecules in the unit cell, thus the molecule has crystallographically imposed mirror symmetry, which passes through atoms C3, N3, O3A, O3B, C5, N5, O5A, O5B, C6, H6, C8, and H8. Positions C4A and C7A, generated by the mirror plane, correspond to positions C2 and C1 in the conventional cubane numbering scheme. In hexanitrocubane, the C—C bond lengths average 1.565 (6) Å, which is somewhat longer than the average found in all cubane structures, but well within the normal range (Butcher et al., 1995). The nitro O atomd attached to N7 are disordered over two positions, with occupancies of 0.600 (4) and 0.400 (4), which is not unusual for such moieties. As the number of nitro substituents increase, H atoms attached to the cubane skeleton become increasingly acidic (Lukin et al., 1997), and thus participate in stronger hydrogen-bonding interactions than is commonly observed for hydrocarbon H atoms. In this instance, both H6 and H8 participate in short hydrogen-bonding contacts to O7AB in adjoining molecules (2.501 Å), thus linking molecules into a two-dimensional sheet in the bc plane, as shown in Fig. 1. In addition, just as is the case in pentanitrocubane (Lukin et al., 1997), heptanitrocubane (Zhang et al., 2000), and octanitrocubane (Zhang et al., 2000), several O···O contacts in hexanitrocubane are shorter than the van der Waals contact distance of 3.04 Å (Rowland & Taylor, 1996). The shortest of these, 2.766 Å, involve the O atoms of the nitro group attached to C4. In hexanitrocubane, the nitro groups attached to C4 (and C4A which is related to C4 by the crystallographic mirror plane) are the only nitro groups that are ortho to three other nitro groups. These contacts are between the molecules that lie in an extended two-dimensional sheet parallel to the ab plane, as shown in Fig. 2.

Density is of critical importance to the performance of an explosive. The overall packing in hexanitrocubane described above leads to a density of 1.931 Mg m⁻³ at 294 K. While this is less than that observed for pentanitrocubane (1.959 Mg m⁻³; Lukin et al., 1997), heptanitrocubane (2.028 Mg m⁻³; Zhang et al., 2000), and octanitrocubane (1.979 Mg m⁻³; Zhang et al., 2000), this is still an impressive value for a compound containing only C, H, N, and O, and is much higher than the value previously observed for its acetonitrile solvate (1.676 Mg m⁻³; Lukin et al., 1997). The metrical parameters found for this unsolvated structure of hexanitrocubane are very similar to those found for the acetonitrile solvate (Lukin et al., 1997).

S2. Experimental

The preparation of hexanitrobenzene has been reported previously (Lukin et al., 1997). Solvent-free crytals were grown by slow evaporation from a mixed HNO₃/H₂SO₄ solution at 313 K.

S3. Refinement

All H atoms were initially located in a difference Fourier map and refined isotropically. The nitro O atoms attached to N7 are disordered over two positions, with occupancies of 0.600 (4) and 0.400 (4), as is commonly found for such moieties.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SMART; data reduction: SAINT (Bruker, 2001); program(s) used to solve structure: SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. Displacement ellipsoid plot (50% probability level) of the title molecule, showing the atomic labeling scheme.
	Fig. 2. The molecules of (I) linking into a two-dimensional sheet in the bc plane through short hydrogen-bonding contacts to C7 in adjoining molecules.
	Fig. 3. Short O···O contacts from the C4 nitro group linking up molecules in an extended two-dimensional sheet parallel to the ab plane.

(I)

Crystal data

C8H2N6O12	Dx = 1.931 Mg m^−3
Mr = 374.16	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Pnma	Cell parameters from 4545 reflections
a = 13.936 (2) Å	θ = 2.8–28.3°
b = 10.8870 (19) Å	µ = 0.19 mm^−1
c = 8.4833 (14) Å	T = 295 K
V = 1287.1 (4) Å^3	Plate, orange
Z = 4	0.74 × 0.29 × 0.11 mm
F(000) = 752

Data collection

Bruker P4/CCD diffractometer	1655 independent reflections
Radiation source: fine-focus sealed tube	1202 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.026
ω scans	θmax = 28.3°, θmin = 2.8°
Absorption correction: multi-scan (Bruker, 2001)	h = −18→18
Tmin = 0.819, Tmax = 0.962	k = −14→14
9511 measured reflections	l = −11→9

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.046	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.136	All H-atom parameters refined
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0747P)^2 + 0.3718P] where P = (Fo^2 + 2Fc^2)/3
1655 reflections	(Δ/σ)max < 0.001
157 parameters	Δρmax = 0.35 e Å^−3
0 restraints	Δρmin = −0.24 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
C3	0.33820 (19)	0.2500	1.0939 (3)	0.0356 (5)
N3	0.3347 (2)	0.2500	1.2689 (3)	0.0518 (7)
O3A	0.4094 (2)	0.2500	1.3397 (3)	0.0794 (8)
O3B	0.2542 (2)	0.2500	1.3251 (3)	0.0773 (8)
C4	0.29000 (12)	0.14906 (14)	0.98930 (18)	0.0323 (4)
N4	0.24031 (12)	0.03958 (13)	1.05379 (18)	0.0427 (4)
O4A	0.27462 (14)	−0.00370 (15)	1.17313 (19)	0.0716 (5)
O4B	0.17065 (13)	0.00225 (15)	0.9865 (2)	0.0725 (5)
C5	0.24530 (16)	0.2500	0.8802 (3)	0.0315 (5)
N5	0.14369 (16)	0.2500	0.8305 (3)	0.0452 (6)
O5A	0.12710 (16)	0.2500	0.6895 (3)	0.0625 (6)
O5B	0.08567 (15)	0.2500	0.9385 (3)	0.0760 (8)
C6	0.33648 (18)	0.2500	0.7732 (3)	0.0325 (5)
H6	0.336 (2)	0.2500	0.658 (3)	0.045 (7)*
C7	0.38139 (12)	0.14914 (14)	0.88278 (18)	0.0329 (4)
N7	0.43102 (11)	0.03600 (15)	0.8312 (2)	0.0438 (4)
O7AA	0.4809 (6)	0.0462 (8)	0.7150 (7)	0.0646 (16)	0.60
O7AB	0.4202 (5)	−0.0520 (6)	0.9158 (7)	0.0768 (17)	0.60
O7BA	0.5027 (10)	0.0394 (12)	0.7631 (17)	0.121 (5)	0.40
O7BB	0.3923 (6)	−0.0640 (8)	0.8610 (9)	0.0605 (18)	0.40
C8	0.42940 (19)	0.2500	0.9894 (3)	0.0363 (5)
H8	0.4926 (19)	0.2500	1.021 (3)	0.042 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C3	0.0498 (14)	0.0238 (11)	0.0332 (11)	0.000	−0.0025 (10)	0.000
N3	0.092 (2)	0.0281 (11)	0.0351 (11)	0.000	−0.0040 (14)	0.000
O3A	0.117 (2)	0.0733 (17)	0.0480 (11)	0.000	−0.0317 (14)	0.000
O3B	0.111 (2)	0.0656 (16)	0.0549 (13)	0.000	0.0323 (14)	0.000
C4	0.0395 (9)	0.0229 (8)	0.0345 (7)	0.0000 (6)	0.0022 (7)	−0.0006 (6)
N4	0.0555 (10)	0.0282 (7)	0.0446 (8)	−0.0059 (7)	0.0095 (7)	−0.0016 (6)
O4A	0.1071 (14)	0.0452 (9)	0.0623 (9)	−0.0093 (9)	−0.0083 (9)	0.0233 (8)
O4B	0.0769 (12)	0.0621 (10)	0.0784 (11)	−0.0336 (9)	−0.0040 (9)	0.0073 (9)
C5	0.0329 (12)	0.0247 (11)	0.0369 (11)	0.000	−0.0004 (10)	0.000
N5	0.0376 (12)	0.0329 (11)	0.0650 (14)	0.000	−0.0057 (11)	0.000
O5A	0.0580 (13)	0.0601 (14)	0.0694 (14)	0.000	−0.0271 (11)	0.000
O5B	0.0426 (12)	0.0911 (19)	0.0943 (18)	0.000	0.0200 (13)	0.000
C6	0.0373 (13)	0.0277 (11)	0.0326 (10)	0.000	0.0003 (10)	0.000
C7	0.0333 (8)	0.0266 (8)	0.0388 (8)	0.0023 (6)	−0.0005 (7)	−0.0020 (6)
N7	0.0370 (9)	0.0352 (8)	0.0593 (10)	0.0051 (7)	−0.0041 (7)	−0.0106 (7)
O7AA	0.061 (4)	0.061 (3)	0.0717 (18)	0.012 (3)	0.029 (2)	−0.0052 (17)
O7AB	0.110 (5)	0.038 (2)	0.083 (3)	0.030 (3)	0.005 (2)	0.011 (2)
O7BA	0.053 (5)	0.060 (6)	0.251 (15)	−0.012 (4)	0.060 (8)	−0.062 (8)
O7BB	0.069 (4)	0.030 (2)	0.082 (5)	0.003 (3)	0.021 (3)	0.001 (3)
C8	0.0380 (14)	0.0276 (11)	0.0433 (12)	0.000	−0.0080 (11)	0.000

Geometric parameters (Å, º)

C3—N3	1.486 (3)	N5—O5A	1.218 (3)
C3—C8	1.550 (4)	N5—O5B	1.222 (3)
C3—C4	1.564 (2)	C6—C7^i	1.569 (2)
C3—C4^i	1.564 (2)	C6—C7	1.569 (2)
N3—O3A	1.202 (4)	C6—H6	0.98 (3)
N3—O3B	1.218 (4)	C7—N7	1.479 (2)
C4—N4	1.483 (2)	C7—C8	1.572 (2)
C4—C7	1.562 (2)	N7—O7BA	1.155 (14)
C4—C5	1.566 (2)	N7—O7AB	1.206 (7)
N4—O4B	1.197 (2)	N7—O7AA	1.211 (7)
N4—O4A	1.215 (2)	N7—O7BB	1.241 (9)
C5—N5	1.478 (3)	C8—C7^i	1.572 (2)
C5—C6	1.562 (3)	C8—H8	0.92 (3)
C5—C4^i	1.566 (2)
N3—C3—C8	126.8 (2)	C5—C6—C7	88.85 (13)
N3—C3—C4	123.56 (14)	C7^i—C6—C7	88.82 (16)
C8—C3—C4	91.57 (14)	C5—C6—H6	125.3 (17)
N3—C3—C4^i	123.56 (14)	C7^i—C6—H6	126.5 (9)
C8—C3—C4^i	91.57 (14)	C7—C6—H6	126.5 (9)
C4—C3—C4^i	89.27 (17)	N7—C7—C4	123.51 (14)
O3A—N3—O3B	127.0 (3)	N7—C7—C6	126.46 (14)
O3A—N3—C3	118.1 (3)	C4—C7—C6	91.03 (13)
O3B—N3—C3	114.9 (3)	N7—C7—C8	123.57 (14)
N4—C4—C7	126.49 (13)	C4—C7—C8	90.84 (14)
N4—C4—C3	123.77 (14)	C6—C7—C8	91.25 (12)
C7—C4—C3	88.72 (14)	O7BA—N7—O7AB	115.6 (8)
N4—C4—C5	126.60 (15)	O7BA—N7—O7AA	25.0 (9)
C7—C4—C5	88.97 (13)	O7AB—N7—O7AA	129.0 (5)
C3—C4—C5	90.75 (12)	O7BA—N7—O7BB	120.4 (8)
O4B—N4—O4A	125.80 (17)	O7AB—N7—O7BB	29.3 (4)
O4B—N4—C4	118.39 (16)	O7AA—N7—O7BB	119.7 (6)
O4A—N4—C4	115.81 (16)	O7BA—N7—C7	121.7 (7)
N5—C5—C6	127.9 (2)	O7AB—N7—C7	115.3 (4)
N5—C5—C4^i	123.35 (13)	O7AA—N7—C7	115.6 (4)
C6—C5—C4^i	91.14 (13)	O7BB—N7—C7	117.9 (5)
N5—C5—C4	123.35 (13)	C3—C8—C7	88.86 (15)
C6—C5—C4	91.14 (13)	C3—C8—C7^i	88.86 (15)
C4^i—C5—C4	89.14 (16)	C7—C8—C7^i	88.63 (17)
O5A—N5—O5B	127.6 (3)	C3—C8—H8	127.9 (17)
O5A—N5—C5	117.5 (2)	C7—C8—H8	125.2 (9)
O5B—N5—C5	114.9 (2)	C7^i—C8—H8	125.2 (9)
C5—C6—C7^i	88.85 (13)
C8—C3—N3—O3A	0.000 (1)	C4—C5—C6—C7	−0.16 (11)
C4—C3—N3—O3A	122.53 (19)	N4—C4—C7—N7	0.2 (3)
C4^i—C3—N3—O3A	−122.53 (19)	C3—C4—C7—N7	132.63 (15)
C8—C3—N3—O3B	180.0	C5—C4—C7—N7	−136.60 (15)
C4—C3—N3—O3B	−57.47 (19)	N4—C4—C7—C6	136.61 (16)
C4^i—C3—N3—O3B	57.47 (19)	C3—C4—C7—C6	−90.93 (12)
N3—C3—C4—N4	−3.4 (3)	C5—C4—C7—C6	−0.16 (11)
C8—C3—C4—N4	134.15 (16)	N4—C4—C7—C8	−132.13 (16)
C4^i—C3—C4—N4	−134.30 (13)	C3—C4—C7—C8	0.33 (12)
N3—C3—C4—C7	−137.9 (2)	C5—C4—C7—C8	91.10 (12)
C8—C3—C4—C7	−0.33 (12)	C5—C6—C7—N7	134.57 (16)
C4^i—C3—C4—C7	91.22 (14)	C7^i—C6—C7—N7	−136.56 (13)
N3—C3—C4—C5	133.2 (2)	C5—C6—C7—C4	0.16 (12)
C8—C3—C4—C5	−89.29 (14)	C7^i—C6—C7—C4	89.04 (13)
C4^i—C3—C4—C5	2.26 (17)	C5—C6—C7—C8	−90.70 (14)
C7—C4—N4—O4B	−98.4 (2)	C7^i—C6—C7—C8	−1.83 (17)
C3—C4—N4—O4B	144.17 (19)	C4—C7—N7—O7BA	−173.7 (8)
C5—C4—N4—O4B	23.1 (3)	C6—C7—N7—O7BA	65.2 (9)
C7—C4—N4—O4A	82.3 (2)	C8—C7—N7—O7BA	−56.3 (9)
C3—C4—N4—O4A	−35.2 (2)	C4—C7—N7—O7AB	−25.1 (4)
C5—C4—N4—O4A	−156.29 (17)	C6—C7—N7—O7AB	−146.2 (3)
N4—C4—C5—N5	1.9 (3)	C8—C7—N7—O7AB	92.3 (4)
C7—C4—C5—N5	138.61 (18)	C4—C7—N7—O7AA	158.6 (4)
C3—C4—C5—N5	−132.68 (19)	C6—C7—N7—O7AA	37.6 (5)
N4—C4—C5—C6	−136.53 (16)	C8—C7—N7—O7AA	−84.0 (5)
C7—C4—C5—C6	0.16 (12)	C4—C7—N7—O7BB	7.6 (4)
C3—C4—C5—C6	88.87 (14)	C6—C7—N7—O7BB	−113.5 (4)
N4—C4—C5—C4^i	132.34 (12)	C8—C7—N7—O7BB	125.0 (4)
C7—C4—C5—C4^i	−90.97 (13)	N3—C3—C8—C7	135.67 (8)
C3—C4—C5—C4^i	−2.26 (17)	C4—C3—C8—C7	0.33 (12)
C6—C5—N5—O5A	0.0	C4^i—C3—C8—C7	−88.98 (12)
C4^i—C5—N5—O5A	122.84 (16)	N3—C3—C8—C7^i	−135.67 (8)
C4—C5—N5—O5A	−122.84 (16)	C4—C3—C8—C7^i	88.98 (12)
C6—C5—N5—O5B	180.0	C4^i—C3—C8—C7^i	−0.33 (12)
C4^i—C5—N5—O5B	−57.16 (16)	N7—C7—C8—C3	−132.58 (16)
C4—C5—N5—O5B	57.16 (16)	C4—C7—C8—C3	−0.33 (12)
N5—C5—C6—C7^i	135.58 (8)	C6—C7—C8—C3	90.72 (14)
C4^i—C5—C6—C7^i	0.16 (11)	N7—C7—C8—C7^i	138.53 (13)
C4—C5—C6—C7^i	−89.00 (11)	C4—C7—C8—C7^i	−89.22 (14)
N5—C5—C6—C7	−135.58 (8)	C6—C7—C8—C7^i	1.83 (17)
C4^i—C5—C6—C7	89.00 (11)

Symmetry code: (i) x, −y+1/2, z.