2,3,4,5,6-Penta­nitro­aniline 1,2-dichloro­ethane disolvate: `push–pull' deformation of aromatic rings by intra­molecular charge transfer

The title compound, C₆H₂N₆O₁₀·2C₂H₄Cl₂, forms layered stacks of penta­nitro­aniline mol­ecules, which possess twofold symmetry. The voids between these stacks are occupied by dichloro­ethane mol­ecules, which reside near a 2/m symmetry element and display pseudo-inversion symmetry. The C atoms in one of the two solvent mol­ecules are threefold disordered. In the penta­nitro­aniline mol­ecule, considerable distortion of the benzenoid ring, coupled with the short C—N(H₂) bond and out-of-plane NO₂ twistings, point to significant intra­molecular `push–pull' charge transfer at the amino- and nitro-substituted (ortho and para) positions, as theoretically quantified by natural bond orbital analysis of the π-electron density.

S1. Comment

Organic molecules with pronounced intramolecular charge transfer have recently attracted significant attention owing to their potential solid-state applications (Lee et al., 2001; Techert & Zachariasse, 2004; Perepichka et al., 2001, 2002). Such `push–pull' systems usually consist of bridged but relatively independent donor and acceptor constituents; and structural/spectroscopic studies have focused on the effects of charge distribution between these parts, which vary between limiting neutral and ionic states (Yang et al., 2002; Thallapally et al., 2002).

In this communication, we turn to the structural effects of the intramolecular charge transfer in pentanitroaniline, which contains intimately bonded donor and acceptor parts. This molecule was prepared according to the literature procedure (Nielsen et al., 1980) and crystallized as the dichloroethane disolvate, (I). As shown in Fig. 1, the pentanitroaniline molecule exhibits C₂ symmetry, with the C—N(amine) and C—N(para-nitro) bonds lying along the twofold rotation axis. Two crystallographyically independent solvent molecules reside near a 2/m symmetry element and display pseudo-inversion symmetry. One of the two solvent moleules has ordered Cl atoms but threefold disorder amongst the C atoms, with C6 in a general position and C7 residing on a mirror plane. The packing diagram (Fig. 2) shows layers in which pentanitroaniline molecules are arranged in a rectangular fashion, with centroid-to-centroid distances of c/2 and b/2. Two adjacent layers, separated by 3.799 (3) Å, are shifted by 1/2 along the c axis, which leads to the formation of stacks in which aromatic rings from every other layer are positioned directly below each other. Solvent molecules occupy the voids between these stacks in the ac plane.

The pentanitroaniline molecule contains an essentially planar but distorted aromatic ring (Fig. 1), in which the C1—C2 bond is significantly longer than the C2—C3 and C3—C4 bonds (Table 1), and the former is comparable to the average carbon–carbon bond length of 1.442 Å in 1,3,5-triamino-2,4,6-trinitrobenzene (Cady & Larson, 1965). It is notable that the structure of hexaaminobenzene (Dixon et al., 1989) and hexanitrobenzene (Akopyan et al., 1966) are both characterized by C—C bond lengths close to the standard aromatic value 1.39 Å. As such, the lengthening of the C—C bonds adjusted to the amino group in (I) is apparently related to the presence of both strong donor and strong acceptor substituents. Another feature of pentanitroaniline is the very short C1—N1 bond, compared with 1.432 Å in 1,3,5-triamino-2,4,6-trinitrobenzene, which is consistent with electron release from the amino group. On the other hand, the acceptor part of (I) shows that (i) the C3—N3 bond is longer than the C2—N2 and C4—N4 bonds (Table 1); (ii) the twistings of the NO₂-groups relative to the benzene core are more pronounced in meta-positions (torsion angle of 65.6°) than that in ortho- and para-positions (angles are 36.3 and 39.5°, respectively). The earlier studies of nitrobenzene derivatives showed that (partial) reduction of a nitro group results in its planarization and the elongation of the C—N bond (Lü et al., 2005; Akopyan et al., 1966; Cady & Larson, 1965). Therefore, the features are consistent with preferential charge transfer to the ortho and (slightly less) para-positions of (I). (Note that N—H···O—N hydrogen bonding may also affect the twisting of the NO₂ group in the o-position.)

To quantify this conclusion, we carried out the natural bond orbital (NBO) analysis of the electron distribution within (I). Thus, DFT (B3LYP/6–11 G^*) computations with GAUSSIAN98 (Frisch et al., 1998) produced a geometry in good agreement with the experimentally obtained structure. The NBO analysis indicates that the negative charge in pentanitroaniline is concentrated mostly on the O atoms, which leads (together with some positive charge on N atoms) to overall negative charges on the ortho-, meta- and para- nitro groups of −0.20, −0.11 and −0.15, respectively. All the C₆-core atoms bear positive charges, and the electrostatic interaction leads to a very short intermolecular N—O···C distance between the negatively charged O2 atom and the electron deficient C3 atom. As such, all the data indicate significant contributions of the resonance structures in Fig. 3 that result from charge transfer via the amino- to the nitro-containing fragments and lead to the molecular geometry of (I).

Thus, the analysis of the structure of pentanitroaniline along with the other polynitroaminobenzenes indicates that (i) the deformation of the aromatic ring results mainly from favorable resonance structures related to the asymmetrical presence of both donor and acceptor substituents as opposed to any symmetrical release or withdrawal of electron density, and (ii) the twistings and bond lengths to NO₂ groups are determined by the charges on the entire C—NO₂ fragment, not merely on the constituent C—NH₂ or NO₂ parts.

S2. Experimental

Pentanitroaniline was prepared by nitration of 3,5-dinitroaniline with nitric acid in 100% sulfuric acid according to the literature procedure (Nielsen et al., 1980). The dissolution of the yellow solid in dichloroethane followed by slow cooling to 243 K resulted in the precipitation of clear yellow plates suitable for X-ray analysis.

S3. Refinement

The disordered 1,2-dichloroethane molecule has two crystallographically independent C atoms, C6 and C7. They were located in a difference Fourier map and their site occupancy factors, 0.36 (1) and 0.28 (2), respectively, were obtained by SHELXTL (Bruker, 2003). These two C atoms were refined isotropically. The H atoms of the solvent molecules were placed in calculated positions and were treated as riding atoms, with C—H distances of 0.99 Å (CH₂). Hydrogen atoms in the NH₂ group, which are involved in hydrogen bonds, were located in the difference Fourier map and subsequently allowed to refine as riding atoms. All H atoms were refined isotropically with U_iso(H) = 1.2U_eq(N,C).

Computing details

Data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT and SADABS (Bruker, 2003); program(s) used to solve structure: SHELXTL (Bruker, 2003); program(s) used to refine structure: SHELXTL; molecular graphics: XP (Bruker, 1999); software used to prepare material for publication: SHELXTL and XCIF (Bruker, 1999).

Figures

	Fig. 1. The molecular components of pentanitroaniline disolvate. Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. A view of the molecular arrangement of (I) as the dichloroethane solvate, along the a axis. H atoms have been omitted for clarity. For the disordered solvent molecule, only one set of its three possible conformations is shown.
	Fig. 3. The principal resonance structures in (I), resulting in significant charge transfer.

2,3,4,5,6-Pentanitroaniline 1,2-dichloroethane disolvate

Crystal data

C6H2N6O10·C2H4Cl2	F(000) = 840
Mr = 417.09	Dx = 1.838 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 38 reflections
a = 11.327 (4) Å	θ = 6.7–20.3°
b = 21.621 (7) Å	µ = 0.50 mm^−1
c = 7.911 (3) Å	T = 173 K
β = 128.941 (6)°	Plate, yellow
V = 1506.9 (9) Å^3	0.5 × 0.5 × 0.01 mm
Z = 4

Data collection

Bruker SMART diffractometer	2294 independent reflections
Radiation source: fine-focus sealed tube	1574 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.044
ω scans	θmax = 30.6°, θmin = 1.9°
Absorption correction: multi-scan (SADABS; Bruker, 2003)	h = −15→15
Tmin = 0.800, Tmax = 0.995	k = −30→30
10284 measured reflections	l = −11→11

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)] = 0.053	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.148	H-atom parameters constrained
S = 1.09	w = 1/[σ^2(Fo^2) + (0.0793P)^2 + 0.3853P] where P = (Fo^2 + 2Fc^2)/3
2294 reflections	(Δ/σ)max < 0.001
127 parameters	Δρmax = 0.43 e Å^−3
0 restraints	Δρmin = −0.39 e Å^−3

Special details

Experimental. The charge distribution was evaluated using the POP=NBO option in single point DFT(B3LYP/6–11 G^*) computations that were optimized with the same DFT methodology as employed with the geometry of pentanitroaniline (Frisch, et al., 1998).

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)
C1	0.5000	0.18527 (15)	1.0000	0.0255 (6)
C2	0.3638 (2)	0.22105 (10)	0.8695 (3)	0.0229 (4)
C3	0.3658 (2)	0.28477 (10)	0.8679 (3)	0.0214 (4)
C4	0.5000	0.31758 (14)	1.0000	0.0220 (6)
N1	0.5000	0.12398 (14)	1.0000	0.0377 (7)
H1	0.4203	0.1064	0.9089	0.045*
N2	0.2166 (2)	0.18952 (9)	0.7465 (3)	0.0271 (4)
N3	0.2214 (2)	0.31955 (9)	0.7267 (3)	0.0250 (4)
N4	0.5000	0.38574 (13)	1.0000	0.0277 (6)
O1	0.2038 (2)	0.13964 (8)	0.6627 (3)	0.0412 (5)
O2	0.11600 (19)	0.21496 (8)	0.7378 (3)	0.0321 (4)
O3	0.15092 (19)	0.31401 (8)	0.5304 (2)	0.0341 (4)
O4	0.18633 (19)	0.35124 (8)	0.8162 (3)	0.0331 (4)
O5	0.4114 (2)	0.41110 (8)	0.8253 (3)	0.0398 (5)
Cl1	0.18816 (10)	0.0000	0.84463 (13)	0.0404 (3)
C5	0.0004 (5)	0.0000	0.5908 (5)	0.0625 (14)
H5	−0.0546	−0.0370	0.5824	0.075*
Cl2	0.40609 (12)	0.0000	0.66638 (16)	0.0422 (3)
C6	0.4374 (8)	0.0202 (3)	0.4728 (13)	0.0426 (18)*	0.36
H6A	0.3436	0.0130	0.3224	0.051*	0.36
H6B	0.4651	0.0644	0.4880	0.051*	0.36
C7	0.543 (2)	0.0000	0.6196 (10)	0.069 (5)*	0.28
H7	0.6083	0.0372	0.6831	0.083*	0.28

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0343 (17)	0.0243 (15)	0.0208 (13)	0.000	0.0187 (13)	0.000
C2	0.0251 (10)	0.0281 (11)	0.0167 (9)	−0.0047 (9)	0.0137 (8)	−0.0024 (8)
C3	0.0249 (10)	0.0260 (11)	0.0148 (8)	0.0024 (8)	0.0132 (8)	0.0028 (8)
C4	0.0253 (15)	0.0254 (15)	0.0163 (12)	0.000	0.0136 (12)	0.000
N1	0.0394 (17)	0.0265 (15)	0.0387 (16)	0.000	0.0205 (14)	0.000
N2	0.0303 (10)	0.0320 (10)	0.0196 (8)	−0.0060 (8)	0.0160 (8)	−0.0011 (8)
N3	0.0240 (9)	0.0291 (10)	0.0200 (8)	−0.0004 (8)	0.0130 (8)	0.0036 (7)
N4	0.0257 (13)	0.0249 (14)	0.0301 (13)	0.000	0.0163 (12)	0.000
O1	0.0449 (11)	0.0309 (10)	0.0445 (11)	−0.0126 (8)	0.0265 (9)	−0.0118 (8)
O2	0.0268 (8)	0.0441 (10)	0.0243 (8)	−0.0035 (7)	0.0155 (7)	−0.0011 (7)
O3	0.0312 (9)	0.0474 (11)	0.0162 (7)	0.0017 (8)	0.0113 (7)	0.0059 (7)
O4	0.0323 (9)	0.0375 (10)	0.0309 (9)	0.0048 (7)	0.0205 (8)	−0.0015 (7)
O5	0.0392 (10)	0.0302 (10)	0.0369 (10)	0.0023 (8)	0.0176 (9)	0.0119 (8)
Cl1	0.0336 (5)	0.0545 (6)	0.0220 (4)	0.000	0.0121 (4)	0.000
C5	0.0274 (19)	0.128 (5)	0.0216 (17)	0.000	0.0103 (16)	0.000
Cl2	0.0486 (6)	0.0457 (6)	0.0406 (5)	0.000	0.0321 (5)	0.000

Geometric parameters (Å, º)

C1—N1	1.325 (4)	N4—O5	1.214 (2)
C1—C2^i	1.429 (3)	Cl1—C5	1.785 (4)
C1—C2	1.429 (3)	C5—C5^ii	1.431 (6)
C2—C3	1.378 (3)	C5—H5	0.9900
C2—N2	1.468 (3)	Cl2—C7	1.80 (2)
C3—C4	1.382 (3)	Cl2—C6^iii	1.829 (7)
C3—N3	1.480 (3)	Cl2—C6	1.829 (7)
C4—C3^i	1.382 (3)	C6—C6^iii	0.872 (14)
C4—N4	1.474 (4)	C6—C6^iv	1.195 (14)
N1—H1	0.8131	C6—C6^v	1.479 (14)
N2—O1	1.226 (3)	C6—H6A	0.9900
N2—O2	1.227 (3)	C6—H6B	0.9900
N3—O4	1.217 (2)	C7—C7^v	1.488 (10)
N3—O3	1.227 (2)	C7—H7	0.9900
N4—O5^i	1.214 (2)
N1—C1—C2^i	122.78 (14)	O5^i—N4—C4	116.84 (15)
N1—C1—C2	122.78 (14)	O5—N4—C4	116.84 (15)
C2^i—C1—C2	114.4 (3)	C5^ii—C5—Cl1	112.4 (4)
C3—C2—C1	122.1 (2)	C5^ii—C5—H5	109.0
C3—C2—N2	118.4 (2)	Cl1—C5—H5	109.3
C1—C2—N2	119.2 (2)	C6^iii—C6—Cl2	76.2 (2)
C2—C3—C4	121.5 (2)	C6^iv—C6—Cl2	120.6 (9)
C2—C3—N3	119.89 (19)	C6^v—C6—Cl2	105.7 (7)
C4—C3—N3	118.6 (2)	C6^iii—C6—H6A	81.0
C3—C4—C3^i	118.2 (3)	C6^iv—C6—H6A	124.5
C3—C4—N4	120.90 (14)	C6^v—C6—H6A	111.4
C3^i—C4—N4	120.90 (14)	Cl2—C6—H6A	110.1
C1—N1—H1	117.9	C6^iii—C6—H6B	165.1
O1—N2—O2	125.05 (19)	C6^iv—C6—H6B	75.1
O1—N2—C2	117.73 (19)	C6^v—C6—H6B	111.2
O2—N2—C2	117.21 (19)	Cl2—C6—H6B	110.0
O4—N3—O3	126.9 (2)	H6A—C6—H6B	108.4
O4—N3—C3	117.17 (18)	C7^v—C7—Cl2	107.7 (19)
O3—N3—C3	115.91 (18)	C7^v—C7—H7	109.1
O5^i—N4—O5	126.3 (3)	Cl2—C7—H7	111.3

Symmetry codes: (i) −x+1, y, −z+2; (ii) −x, −y, −z+1; (iii) x, −y, z; (iv) −x+1, y, −z+1; (v) −x+1, −y, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O1	0.81	2.08	2.690 (2)	132