trans-1,2-Bis(3-chloro­phenyl)­diazene oxide

The title mol­ecule, C₁₂H₈Cl₂N₂O, lacks a centre of symmetry. However, as a result of the positional disorder of the O atom, mol­ecules of the title compound lie across a crystallographic inversion centre in the centrosymmetric space group P2₁/n. The trans orientation of the aromatic rings around the N=N bond is confirmed.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.005 Å
• R factor = 0.048
• wR factor = 0.140
• Data-to-parameter ratio = 12.5

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

Diazine oxide derivatives, commonly known as azoxybenzenes, are useful in the preparation of liquid crystals (Tsuji et al., 2000). They are also used as ligands in coordination chemistry (Bassi & Scordamaglia, 1975). They are photochemically active (Rhee & Jaffe, 1973), like the related azo (Tamai & Miyasika, 2000) and nitrone derivatives (Hamer & Macaluso, 1964). Preparation of non-centrosymmetric azoxybenzene derivatives will be of interest to study their non-linear optical properties (Long, 1995) because of electron delocalization within the molecule. The structures of para-substituted azoxybenzenes are known, and most of them assume trans configurations of the benzene ring around the N=N bond in the solid state (Ejsmont et al., 2000). During our investigations of the use of tin(II) chloride for the reductive coupling of nitro compounds with aldehydes and ketones, the title compound, (I), was isolated. The structure determination was undertaken to assign its configuration and the space-group symmetry.

Molecules of the title compound lie across crystallographic inversion centres and the asymmetric unit therefore contains one-half of the molecule. Although the molecule lacks a centre of symmetry, it lies across the inversion centre in the centrosymmetric space group P2₁/n, due to the positional disorder of O atom. Further, the molecule adopts a thermodynamically stable configuration in which the two cholorophenyl groups are oriented trans to the N=N bond.

The C—C bond lengths in the phenyl ring vary from 1.368 (6) to 1.384 (5) Å. The N=N [1.267 (5) Å] bond distance is comparable to the corressponding bond distance of 1.27 Å observed in trans azoxybenzene through electron diffraction studies. The computed values for the above bond in the same molecule is 1.232 Å at the RHF/6–31G** level of calculation (Tsuji et al., 2000). The N=N bond distance in 5,5'-dichloro-2-hydroxy-2'-(phenylsulfonyl)azoxybenzene is 1.31 (1) Å (Cameron et al., 1976), which is different from that in the title compound. As a consequence of disorder in the title molecule, the N—O and C—N distances are elongated compared to the corresponding distances of 1.27 (1) Å and 1.43 (1) Å in p-azoxyanisole (Krigbaum et al., 1970).

In the crystal structure, the inversion-related molecules are linked by C6—H6···O1(2 − x, 2 − y, 1 − z) hydrogen bonds (Table 2) involving the disordered O atom, to form molecular chains along [1 1 0].

S2. Experimental

The compound was formed by the reduction of 1-chloro-3-nitrobenzene (2.0 g) by tin(II) chloride (2.5 g) in tetrahydrofuran. The product was isolated by the addition of dilute hydrochloric acid followed by extraction with diethyl ether. Yield was 45.5%. Diffraction quality crystals were obtained by recrystallization of the crude product in hexane.

S3. Refinement

The O atom is disordered over two centrosymmetric positions, O1 and O1ⁱ [symmetry code: (i) 1 − x, 1 − y, 1 − z]. H atoms were placed in geometrical positions and refined using a riding model, with U(H) = 1.2U_eq(C).

Computing details

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: MolEN (Fair, 1990); program(s) used to solve structure: SIR92 (Atomare et al., 1993); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 and PARST (Nardelli, 1983).

Figures

	Fig. 1. The molecular structure of the title compound, showing 30% probability displacement ellipsoids for non-hydrogen atoms. The O atom is disordered over two inversion related positions, O1 and O1^i [symmetry code: (i) 1 − x, 1 − y, 1 − z.
	Fig. 2. Packing of the molecules, viewed down the a axis, showing the chain formation.

trans-Bis(3-chlorophenyl)diazine oxide

Crystal data

C12H8Cl2N2O	F(000) = 272
Mr = 267.10	Dx = 1.521 Mg m^−3
Monoclinic, P21/n	Melting point: 367-368 K K
Hall symbol: -P 2yn	Mo Kα radiation, λ = 0.71069 Å
a = 3.911 (5) Å	Cell parameters from 1179 reflections
b = 5.893 (5) Å	θ = 3.2–24.6°
c = 25.367 (5) Å	µ = 0.54 mm^−1
β = 94.155 (5)°	T = 293 K
V = 583.1 (9) Å^3	Needle, pale yellow
Z = 2	0.3 × 0.2 × 0.2 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	807 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.015
Graphite monochromator	θmax = 25.0°, θmin = 3.2°
ω–2θ scans	h = 0→4
Absorption correction: ψ scan (North et al., 1968)	k = 0→7
Tmin = 0.813, Tmax = 0.898	l = −30→30
1179 measured reflections	3 standard reflections every 100 reflections
1021 independent reflections	intensity decay: neglible

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)] = 0.048	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.140	H-atom parameters constrained
S = 1.11	w = 1/[σ^2(Fo^2) + (0.0644P)^2 + 0.337P] where P = (Fo^2 + 2Fc^2)/3
1021 reflections	(Δ/σ)max < 0.001
82 parameters	Δρmax = 0.21 e Å^−3
0 restraints	Δρmin = −0.23 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. The occupancy of O1 is fixed as 0.5 during refinement which resulted in an R value of 0.0477. Restricting the occupancy factor for oxygen to unity led to a higher R value of 0.077.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Cl1	0.3752 (2)	0.3764 (2)	0.29551 (3)	0.0852 (4)
O1	0.7582 (14)	0.7545 (8)	0.52152 (16)	0.0767 (14)	0.50
N1	0.5810 (7)	0.5880 (4)	0.49375 (9)	0.0594 (7)
C1	0.6025 (7)	0.6347 (5)	0.43800 (11)	0.0525 (7)
C2	0.4869 (7)	0.4899 (5)	0.39766 (9)	0.0462 (6)
H2	0.3863	0.3516	0.4050	0.055*
C3	0.5245 (7)	0.5552 (5)	0.34645 (10)	0.0500 (7)
C4	0.6758 (8)	0.7591 (7)	0.33474 (15)	0.0689 (10)
H4	0.7005	0.8004	0.2998	0.083*
C5	0.7886 (9)	0.8994 (6)	0.3753 (2)	0.0815 (12)
H5	0.8904	1.0372	0.3678	0.098*
C6	0.7538 (9)	0.8399 (6)	0.42696 (16)	0.0702 (10)
H6	0.8310	0.9366	0.4543	0.084*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.0925 (7)	0.1179 (9)	0.0432 (5)	0.0156 (6)	−0.0083 (4)	−0.0249 (5)
O1	0.116 (4)	0.069 (3)	0.045 (2)	−0.043 (3)	0.003 (2)	−0.008 (2)
N1	0.0684 (17)	0.0669 (17)	0.0417 (13)	0.0107 (13)	−0.0051 (11)	−0.0121 (13)
C1	0.0526 (16)	0.0594 (18)	0.0445 (15)	0.0138 (15)	−0.0033 (12)	−0.0097 (13)
C2	0.0518 (15)	0.0471 (15)	0.0400 (14)	0.0014 (13)	0.0048 (11)	0.0004 (12)
C3	0.0478 (15)	0.0627 (18)	0.0394 (14)	0.0098 (13)	0.0018 (11)	0.0011 (13)
C4	0.0593 (18)	0.077 (2)	0.072 (2)	0.0133 (17)	0.0186 (16)	0.0311 (19)
C5	0.066 (2)	0.052 (2)	0.126 (4)	−0.0037 (17)	0.009 (2)	0.021 (2)
C6	0.0617 (19)	0.055 (2)	0.091 (3)	0.0027 (16)	−0.0122 (18)	−0.0190 (18)

Geometric parameters (Å, º)

Cl1—C3	1.735 (3)	C2—H2	0.93
O1—N1	1.367 (5)	C3—C4	1.381 (5)
N1—N1^i	1.267 (5)	C4—C5	1.368 (6)
N1—C1	1.449 (4)	C4—H4	0.93
C1—C2	1.383 (4)	C5—C6	1.372 (6)
C1—C6	1.384 (5)	C5—H5	0.93
C2—C3	1.373 (4)	C6—H6	0.93
N1^i—N1—O1	134.6 (3)	C4—C3—Cl1	119.6 (2)
N1^i—N1—C1	117.7 (3)	C5—C4—C3	118.9 (3)
O1—N1—C1	107.7 (3)	C5—C4—H4	120.5
C2—C1—C6	120.7 (3)	C3—C4—H4	120.5
C2—C1—N1	124.5 (3)	C4—C5—C6	121.0 (3)
C6—C1—N1	114.8 (3)	C4—C5—H5	119.5
C3—C2—C1	118.4 (3)	C6—C5—H5	119.5
C3—C2—H2	120.8	C5—C6—C1	119.3 (3)
C1—C2—H2	120.8	C5—C6—H6	120.3
C2—C3—C4	121.6 (3)	C1—C6—H6	120.3
C2—C3—Cl1	118.8 (2)
N1^i—N1—C1—C2	−6.4 (5)	C1—C2—C3—Cl1	−179.2 (2)
O1—N1—C1—C2	172.5 (3)	C2—C3—C4—C5	−0.4 (5)
N1^i—N1—C1—C6	174.1 (3)	Cl1—C3—C4—C5	179.3 (3)
O1—N1—C1—C6	−7.0 (4)	C3—C4—C5—C6	0.1 (5)
C6—C1—C2—C3	−0.3 (4)	C4—C5—C6—C1	0.1 (5)
N1—C1—C2—C3	−179.8 (2)	C2—C1—C6—C5	0.0 (5)
C1—C2—C3—C4	0.5 (4)	N1—C1—C6—C5	179.5 (3)

Symmetry code: (i) −x+1, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2···O1^i	0.93	2.08	2.735 (6)	126
C6—H6···O1	0.93	2.05	2.450 (7)	104
C6—H6···O1^ii	0.93	2.48	3.271 (7)	144

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