trans-Diphenyl­diazene oxide

The crystal structure of the title compound (common name: trans-azoxybenzene, t-AXB), C₁₂H₁₀N₂O, was determined at 299 ± 0.5 K, i.e. 6 K below its melting point. The az­oxy group has the expected geometry, and the O atom is affected by libration normal to the N=N—O plane. The O atom is disordered over two positions; the site-occupancy ratio is ∼3:2. Phenyl rings are out of the NNO plane, with dihedral angles ranging from 20 (1) to 34 (1)°. The dihedral angle between the two phenyl rings is 46.19 (11)°. This geometry contrasts strongly with the gas-phase conformation of t-AXB, previously determined by electron diffraction combined with ab initio calculations, which showed that the stable conformation is planar. This discrepancy is, however, consistent with low rotational barriers for phenyl rings. These features could be of inter­est in relation to the actual reaction mechanism of the Wallach rearrangement (acid-catalyzed isomerization of t-AXB into 4-hydroxy­azobenzene), which remains a controversial matter.

Key indicators

• Single-crystal X-ray study
• T = 299 K
• Mean [sigma](C-C) = 0.004 Å
• Disorder in main residue
• R factor = 0.055
• wR factor = 0.156
• Data-to-parameter ratio = 12.0

checkCIF/PLATON results

No syntax errors found



Alert level C
RINTA01_ALERT_3_C  The value of Rint is greater than 0.10
            Rint given   0.115
PLAT020_ALERT_3_C The value of Rint is greater than 0.10 .........       0.12
PLAT301_ALERT_3_C Main Residue  Disorder .........................       6.00 Perc.
PLAT331_ALERT_2_C Small Average Phenyl C-C Dist.   C1     -C6            1.36 Ang.

   0 ALERT level A = In general: serious problem
   0 ALERT level B = Potentially serious problem
   4 ALERT level C = Check and explain
   0 ALERT level G = General alerts; check

   0 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   1 ALERT type 2 Indicator that the structure model may be wrong or deficient
   3 ALERT type 3 Indicator that the structure quality may be low
   0 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

---

S1. Comment

The title molecule, (I), has been structurally characterized using a variety of techniques, e.g. IR and Raman spectroscopy (Gruger & Le Calvé, 1972), or gas electron diffraction combined with ab initio calculations (Tsuji et al., 2000). However, the X-ray structure has not been determined so far, probably because of the low melting point of this compound, 305–309 K. A preliminary study about space group determination, P2₁/a was done by Herrington & Bernard (1966). This reference is however not quoted in the CSD (Version 5.28, updated May 2007).

Two complexes including (I) as ligand are reported, with W^VI (Bassi & Scordamaglia, 1975) and Ru^I (Cambie et al., 1996). In both complexes, azoxybenzene is found in an almost planar conformation, the dihedral angle between phenyl rings being 13.2 and 7.4°, respectively. The same conformation has been observed in derivatives of (I) including a para-substituted phenyl ring (Ejsmont et al., 2000, 2002, 2004, 2005 Domański et al., 2001). Non-planar conformations have been observed e.g. in the case of trans-4-aminoazoxybenzene: the α isomer is stabilized in a planar (2.6°) conformation at 293 K (Domański et al., 2001) but is twisted [43.7 (1)°] at 90 K (Ejsmont et al., 2002). A similar flexible behavior is described for the β isomer.

Unexpectedly, the title molecule is found to belong to non-planar azoxybenzene family. The dihedral angle between phenyl rings is 46.19 (11)° (Fig. 1), while electron diffraction studies showed that the molecule is stabilized in a planar conformation in the gas phase. This difference confirms that rotational barriers for phenyl rings in (I) are low (Tsuji et al., 2000). Such a flexibility may be of interest regarding the actual mechanism of the Wallach rearrangement [acid-catalyzed isomerization of (I) into 4-hydroxyazobenzene], which still remains controversial, despite of a number of experimental evidences.

Otherwise, the geometry of (I) is as expected, for example for the central azoxy functionality, with bond lengths and angles similar to those found in above cited derivatives. A feature deserves however to be commented, regarding the O atom, which is disordered over two sites. This disorder seems to be a consequence of a libration motion normal to the N═N—O plane for this atom, which, in turn, may be related with the low melting point of this molecule.

Related literature

The vibrational spectra of the title molecule have been analyzed (Gruger & Le Calvé, 1972). Preliminary X-ray data were reported by Herrington & Bernard (1966) and the molecular structure in the gas-phase was established on the basis of gas electron diffraction studies combined with ab initio calculations (Tsuji et al., 2000). Two coordination complexes including the title molecule have been structurally characterized (Bassi & Scordamaglia, 1975; Cambie et al., 1996). For structures of para-substituted azoxybenzenes, see: Ejsmont et al. (2000, 2002, 2004, 2005); Domański et al. (2001).

S2. Experimental

A sample of (I) was donated by the Chemistry Stores at Universidad Autónoma de Nuevo León, and recrystallized from ethanol. Batch used for X-ray crystallography was checked by I.R. spectroscopy (solid sample, cm^-1): ν(N═N) = 1436, ν (N—O) = 1328 (assignation follows studies published by Gruger & Le Calvé, 1972).

S3. Refinement

Single crystals systematically decomposed on contact with all available glues we attempted. A single-crystal was then transferred in a capillary tube, and data collected without special precautions regarding the atmosphere around the sample. Data collection was carried out avoiding continuous irradiation, by closing the shutter during dead times. Azoxy O atom is disordered over two sites, O1A and O1B, with fixed site occupancies 0.6 and 0.4, respectively. All H atoms were placed in idealized positions, and refined using a riding model approximation, with C—H bond lengths constrained to 0.97 Å and isotropic displacement parameters set to U_iso(H) = 1.2 U_eq(carrier C).

Computing details

Data collection: XSCANS (Siemens, 1999); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXTL-Plus (Sheldrick, 1998); program(s) used to refine structure: SHELXTL-Plus; molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXTL-Plus.

Figures

Fig. 1. The structure of (I), with displacement ellipsoids at the 50% probability level. Atom O1B, disordered with O1A, has been omitted for clarity.

trans-Diphenyldiazene oxide

Crystal data

C12H10N2O	F(000) = 416
Mr = 198.22	Dx = 1.318 Mg m^−3
Monoclinic, P21/n	Melting point = 305–309 K
Hall symbol: -P 2yn	Mo Kα radiation, λ = 0.71073 Å
a = 5.844 (3) Å	Cell parameters from 68 reflections
b = 15.864 (7) Å	θ = 4.5–12.0°
c = 11.073 (5) Å	µ = 0.09 mm^−1
β = 103.30 (4)°	T = 299 K
V = 998.9 (8) Å^3	Needle, yellow
Z = 4	0.60 × 0.20 × 0.16 mm

Data collection

Siemens P4 diffractometer	Rint = 0.115
Radiation source: fine-focus sealed tube, FN4	θmax = 25.1°, θmin = 2.3°
Graphite monochromator	h = −6→6
ω scans	k = −17→18
4750 measured reflections	l = −13→13
1746 independent reflections	2 standard reflections every 48 reflections
1034 reflections with I > 2σ(I)	intensity decay: 1%

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)] = 0.055	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.156	H-atom parameters constrained
S = 1.05	w = 1/[σ^2(Fo^2) + (0.043P)^2 + 0.2419P] where P = (Fo^2 + 2Fc^2)/3
1746 reflections	(Δ/σ)max < 0.001
145 parameters	Δρmax = 0.16 e Å^−3
0 restraints	Δρmin = −0.15 e Å^−3

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
O1A	−0.0378 (16)	0.4162 (9)	0.5903 (12)	0.080 (3)	0.60
O1B	−0.015 (3)	0.3914 (15)	0.6080 (18)	0.109 (7)	0.40
N1	0.1816 (4)	0.42455 (16)	0.6174 (2)	0.0637 (7)
N2	0.3345 (4)	0.39334 (15)	0.5698 (2)	0.0634 (7)
C1	0.2717 (5)	0.48817 (17)	0.7134 (3)	0.0566 (7)
C2	0.1399 (6)	0.50318 (19)	0.7979 (3)	0.0713 (9)
H2	−0.0068	0.4734	0.7936	0.086*
C3	0.2203 (6)	0.5616 (2)	0.8893 (3)	0.0805 (10)
H3	0.1325	0.5713	0.9525	0.097*
C4	0.4186 (6)	0.6056 (2)	0.8930 (3)	0.0819 (10)
H4	0.4712	0.6475	0.9573	0.098*
C5	0.5464 (6)	0.5911 (2)	0.8052 (3)	0.0792 (9)
H5	0.6882	0.6232	0.8067	0.095*
C6	0.4729 (5)	0.53096 (18)	0.7152 (3)	0.0663 (8)
H6	0.5638	0.5194	0.6539	0.080*
C7	0.2709 (5)	0.32809 (17)	0.4804 (2)	0.0551 (7)
C8	0.4621 (5)	0.28069 (19)	0.4705 (3)	0.0621 (8)
H8	0.6153	0.2926	0.5236	0.075*
C9	0.4369 (5)	0.2169 (2)	0.3863 (3)	0.0658 (8)
H9	0.5719	0.1826	0.3815	0.079*
C10	0.2243 (5)	0.20073 (18)	0.3086 (3)	0.0627 (8)
H10	0.2077	0.1550	0.2490	0.075*
C11	0.0350 (5)	0.24936 (18)	0.3153 (3)	0.0645 (8)
H11	−0.1155	0.2389	0.2586	0.077*
C12	0.0550 (5)	0.31344 (18)	0.4019 (3)	0.0615 (7)
H12	−0.0804	0.3474	0.4071	0.074*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1A	0.041 (3)	0.096 (7)	0.101 (4)	−0.014 (3)	0.011 (3)	−0.035 (4)
O1B	0.114 (11)	0.106 (13)	0.140 (11)	−0.055 (9)	0.094 (10)	−0.062 (9)
N1	0.0650 (16)	0.0607 (15)	0.0694 (16)	−0.0037 (13)	0.0234 (13)	0.0043 (13)
N2	0.0612 (14)	0.0642 (15)	0.0664 (14)	0.0049 (13)	0.0179 (12)	0.0031 (13)
C1	0.0650 (16)	0.0479 (15)	0.0564 (16)	−0.0003 (14)	0.0125 (14)	0.0005 (14)
C2	0.0797 (19)	0.0635 (19)	0.077 (2)	−0.0014 (17)	0.0313 (17)	−0.0041 (17)
C3	0.096 (2)	0.076 (2)	0.076 (2)	0.004 (2)	0.033 (2)	−0.0108 (19)
C4	0.094 (2)	0.068 (2)	0.081 (2)	0.002 (2)	0.014 (2)	−0.0191 (18)
C5	0.079 (2)	0.069 (2)	0.090 (2)	−0.0103 (17)	0.0196 (19)	−0.0081 (19)
C6	0.0767 (19)	0.0585 (18)	0.0676 (18)	−0.0028 (15)	0.0247 (16)	−0.0053 (16)
C7	0.0698 (17)	0.0515 (16)	0.0449 (14)	−0.0079 (14)	0.0150 (13)	−0.0028 (13)
C8	0.0591 (16)	0.0658 (19)	0.0599 (16)	−0.0018 (15)	0.0105 (14)	−0.0029 (16)
C9	0.0684 (18)	0.0688 (19)	0.0629 (17)	0.0038 (16)	0.0205 (16)	−0.0032 (16)
C10	0.0743 (18)	0.0572 (18)	0.0568 (16)	−0.0059 (16)	0.0157 (15)	−0.0073 (14)
C11	0.0696 (18)	0.0638 (18)	0.0570 (16)	−0.0062 (15)	0.0083 (14)	−0.0020 (15)
C12	0.0659 (17)	0.0625 (18)	0.0568 (16)	0.0046 (15)	0.0156 (14)	0.0051 (15)

Geometric parameters (Å, º)

O1A—N1	1.255 (10)	C5—H5	0.9700
O1B—N1	1.243 (18)	C6—H6	0.9700
N1—N2	1.240 (3)	C7—C8	1.372 (4)
N1—C1	1.473 (4)	C7—C12	1.378 (4)
N2—C7	1.421 (3)	C8—C9	1.361 (4)
C1—C6	1.354 (4)	C8—H8	0.9700
C1—C2	1.363 (4)	C9—C10	1.363 (4)
C2—C3	1.373 (5)	C9—H9	0.9700
C2—H2	0.9700	C10—C11	1.364 (4)
C3—C4	1.346 (5)	C10—H10	0.9700
C3—H3	0.9700	C11—C12	1.384 (4)
C4—C5	1.375 (5)	C11—H11	0.9700
C4—H4	0.9700	C12—H12	0.9700
C5—C6	1.374 (4)
N2—N1—O1B	122.4 (10)	C1—C6—C5	118.9 (3)
N2—N1—O1A	130.7 (6)	C1—C6—H6	120.5
N2—N1—C1	114.0 (2)	C5—C6—H6	120.5
O1B—N1—C1	121.3 (9)	C8—C7—C12	120.1 (3)
O1A—N1—C1	114.9 (6)	C8—C7—N2	111.6 (3)
N1—N2—C7	119.2 (2)	C12—C7—N2	128.1 (3)
C6—C1—C2	121.9 (3)	C9—C8—C7	120.0 (3)
C6—C1—N1	121.3 (3)	C9—C8—H8	120.0
C2—C1—N1	116.8 (3)	C7—C8—H8	120.0
C1—C2—C3	118.2 (3)	C8—C9—C10	120.8 (3)
C1—C2—H2	120.9	C8—C9—H9	119.6
C3—C2—H2	120.9	C10—C9—H9	119.6
C4—C3—C2	121.3 (3)	C9—C10—C11	119.5 (3)
C4—C3—H3	119.4	C9—C10—H10	120.3
C2—C3—H3	119.4	C11—C10—H10	120.3
C3—C4—C5	119.7 (3)	C10—C11—C12	120.8 (3)
C3—C4—H4	120.2	C10—C11—H11	119.6
C5—C4—H4	120.2	C12—C11—H11	119.6
C6—C5—C4	120.0 (3)	C7—C12—C11	118.7 (3)
C6—C5—H5	120.0	C7—C12—H12	120.6
C4—C5—H5	120.0	C11—C12—H12	120.6
O1B—N1—N2—C7	13.5 (13)	C2—C1—C6—C5	0.2 (5)
O1A—N1—N2—C7	−10.0 (10)	N1—C1—C6—C5	178.5 (3)
C1—N1—N2—C7	176.8 (2)	C4—C5—C6—C1	1.4 (5)
N2—N1—C1—C6	23.9 (4)	N1—N2—C7—C8	−158.1 (3)
O1B—N1—C1—C6	−172.6 (13)	N1—N2—C7—C12	27.2 (4)
O1A—N1—C1—C6	−150.4 (8)	C12—C7—C8—C9	−2.5 (4)
N2—N1—C1—C2	−157.7 (3)	N2—C7—C8—C9	−177.7 (2)
O1B—N1—C1—C2	5.8 (13)	C7—C8—C9—C10	1.8 (4)
O1A—N1—C1—C2	28.0 (8)	C8—C9—C10—C11	0.3 (4)
C6—C1—C2—C3	−2.2 (5)	C9—C10—C11—C12	−1.6 (4)
N1—C1—C2—C3	179.4 (3)	C8—C7—C12—C11	1.2 (4)
C1—C2—C3—C4	2.7 (5)	N2—C7—C12—C11	175.5 (2)
C2—C3—C4—C5	−1.2 (5)	C10—C11—C12—C7	0.9 (4)
C3—C4—C5—C6	−0.9 (5)