Substituent effects in trans-p,p'-di­substituted azo­benzenes: X-ray structures at 100 K and DFT-calculated structures

The crystal and mol­ecular structures of two para-substituted azo­benzenes with [pi]-electron-donating -NEt₂ and [pi]-electron-withdrawing -COOEt groups are reported, along with the effects of the substituents on the aromaticity of the benzene ring. The deformation of the aromatic ring around the -NEt₂ group in N,N,N',N'-tetra­ethyl-4,4'-(diazene­diyl)dianiline, C₂₀H₂₈N₄, (I), may be caused by steric hindrance and the [pi]-electron-donating effects of the amine group. In this structure, one of the amine N atoms demonstrates clear sp²-hybridization and the other is slightly shifted from the plane of the surrounding atoms. The mol­ecule of the second azo­benzene, diethyl 4,4'-(diazene­diyl)dibenzoate, C₁₈H₁₈N₂O₄, (II), lies on a crystallographic inversion centre. Its geometry is normal and comparable with homologous compounds. Density functional theory (DFT) calculations were performed to analyse the changes in the geometry of the studied compounds in the crystalline state and for the isolated mol­ecules. The most significant changes are observed in the values of the N=N-C-C torsion angles, which for the isolated mol­ecules are close to 0.0°. The HOMA (harmonic oscillator model of aromaticity) index, calculated for the benzene ring, demonstrates a slight decrease of the aromaticity in (I) and no substantial changes in (II).

S1. Introduction

Azo­benzenes and related hydrazo and az­oxy compounds have attracted significant attention owing to their electronic structure. Inter­est in the development of azo­benzenes is connected with their potential applications in photoresponsive materials, as well as in linear and nonlinear optics or optical data-storage devices (Zeitouny et al., 2009). The inter­esting structural properties of the compounds are connected with their molecular dynamics (cis–trans isomerization) and their ability to transform reversibly from the thermodynamically stable trans form to the cis form by irradiation with UV or visible light (Takamiya et al., 1986; Tsuchiya, 1999; Harada & Ogawa, 2004; Zeitouny et al., 2009). The need to understand the stability of the different isomers in particular motivates analysis of azo­benzene-type compounds (Allmann, 1997). From the point of view of structural chemistry, the symmetrically para-substituted azo­benzenes are especially important. Different π-electron-withdrawing and/or π-electron-donating groups may change the aromaticity of the benzene rings and strongly influence the geometry and electronic structure of the system. The aim of this work is to analyse how the substituent effects of amino (di­ethyl­ene), carb­oxy­lic acid and ester groups, located in para positions to the azo bridge, work in azo­benzenes. The crystal and molecular structures of 4,4'-azinodi­benzoic acid (Yu & Liu, 2009) and its ethyl ester (Niu et al., 2011) have been determined previously at room temperature.

We present herein the crystal and molecular structures at 100 K of two para-substituted azo­benzenes, namely N,N,N',N'-tetra­ethyl-4,4'-(diazenediyl)dianiline, (I), and di­ethyl 4,4'-(diazenediyl)dibenzoate, (II) (Fig. 1), with a π-electron-withdrawing group [–NEt₂] in (I) and a π-electron-donating group (–COOEt) in (II), along with a description of the effects of the substituents on the aromaticity of the benzene ring and the geometry of the azo bridge. To extend the structural studies, the molecular geometries of the studied compounds were optimized using quantum-mechanical density functional theory (DFT) calculations.

S2. Experimental

S2.1. Synthesis and crystallization

Compounds (I) and (I) were prepared according to literature procedures, viz. Kalyanaraman & George (1973) for (I) and Onto et al. (1998) for (II). Crystals of (I) and (II) suitable for X-ray crystal structure analysis were grown from solutions in a benzene–n-heptane mixture (1:1 v/v).

Based on the solid-state geometry, the molecular structures of (I) and (II) were optimized using the B3LYP hybrid functional (Becke, 1988, 1993; Lee et al., 1988) with the 6-311++G(d,p) level of theory. All species correspond to the minima at the B3LYP/6-311++G(d,p) level with no imaginary frequencies. All calculations were performed using the GAUSSIAN09 program package (Frisch et al., 2010).

S2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were generated in idealized positions, with C—H = 0.93 Å and U_iso(H) = 1.2U_eq(C) for aromatic H atoms, C—H = 0.97 Å and U_iso(H) = 1.2U_eq(C) for methyl­ene H atoms, and C—H = 0.96 Å and U_iso(H) = 1.5U_eq(C) for methyl H atoms. Their parameters were not refined.

S3. Comment

The experimental and calculated geometric parameters of the azo bridge of (I) and (II) reported in Table 2 are supplemented with those for two additional azo­benzene structures, viz. di­methyl 4,4'-(diazenediyl)dibenzoate (Harada & Ogawa, 2004; Harada et al., 1997; Gajda et al., 2013) and 4,4'-azinodi­benzoic acid (Yu & Liu, 2009). The geometry-based index of aromaticity, the harmonic oscillator model of aromaticity (HOMA), may serve as a convenient, reliable and easily accessible qu­anti­tative measure of π-electron delocalization (aromaticity) in chemical compounds. The HOMA index stipulates that bond lengths in aromatic systems lie between values typical for single and double bonds (Kruszewski & Krygowski, 1973; Krygowski, 1993; Krygowski & Cyrański, 1996).

The molecular structures of (I) and (II) and the atom-numbering schemes are presented in Fig. 1. The molecules of (I) and (II) are composed of two para-substituted aromatic rings and an –N═N– bridge. In the asymmetric part of the unit cell of (I), there is one crystallographically independent molecule, while in (II) the asymmetric unit contains only half a molecule with the inversion centre at the azo bridge. The aromatic rings in (II) are coplanar as the aromatic ring and ester group are coplanar, whereas in (I) they are slighty twisted. The angle between the planes calculated from the mid-positions of all six C atoms (C1–C6 and C1'–C6') is 15.27 (6)°. Taking into account the torsion angles surrounding the azo group, the C1–C6 aromatic ring is twisted by 8.59 (15)° and the C1'–C6' ring by 5.52 (16)°. This suggests a notable deformation of the benzene rings. The largest deviation from the mean plane of the aromatic rings is 0.0142 (8) Å. The deformation is also revealed in the bond lengths and angles. The bond lengths which deviate most from the average C_ar—C_ar value are those connected with the –NEt₂ group [Δ_max = 0.021 (1) and 0.024 (1) Å, respectively, for the C and C' rings]. The valence angles which deviate most from the ideal value of 120° are also associated with the –NEt₂ group, with values of 117.11 (10) and 116.92 (10)°. Also, the C2—C1—C6 and C2'—C1'—C6' angles at the –N═N– bridge are reduced [From what value?] by 1.59 (10) and 1.96 (10)°, respectively.

The deformation around the –NEt₂ group of (I) may be caused by steric hindrance of the group. Such effects are not observed for (II). This may also suggest that the π-electron-withdrawing effects of amine have a stronger influence on aromatic ring deformation than the ethyl ester group of (II). The geometry of the –N═N– bridge in both structures is almost the same. The length of the N═N bond in (I) [1.2655 (13) Å] corresponds well with the value in (II) [1.258 (2) Å]. They are slightly longer than the corresponding distances found for 4,4'-(COOH)₂. In (I), the length of the one of the NN—Ph bonds is slightly longer than other [N1—C1 = 1.4230 (14) Å and N1'—C1' = 1.4196 (13) Å] and these are shorter than the corresponding bonds in the remaining structures (Table 2). The average value of the C—N—N angle is 114.17 (10)°, close to the corresponding valence angle in (II) [113.89 (15)°]. These values are in good agreement with 4,4'-(COOH)₂, 4,4'-(COOMe)₂ and azo­benzene.

Amine atom N3 of (I) lies -0.0970 (12) Å out of the plane defined by the three neighbouring C atoms (C4, C7 and C9). On the other hand, atom N4 is exactly in the plane defined by atoms C4', C11 and C13. The sum of the valence angles around the N atom of 359.99 (9)° demonstrates clear sp² hybridization. The geometry of the ester group of (II) is typical and corresponds well with the geometry of 4,4'-(COOMe)₂ (differences in bond lengths, and in valence and dihedral angles, do not exceed 3σ). There are no significant differences between the values of the bond lengths and angles of (I) and (II) in the solid state and those found for the calculated structures. Nevertheless, some deviations are observed for the torsion angles. The main discrepancy involves the –N═N– bridge. In the calculated structures of azo­benzene derivatives, the N—N—C—C torsion angle is close to 0.00°.

The substituents of azo­benzene presented here differ in their electron-withdrawing properties: for the phenyl­azo group σ_p = 0.39; for N(Et)₂ σ_p = -0.72; for COOEt, COOMe and COOH σ_p = 0.45; and for H σ_p = 0.00 (Hansch et al., 1991). Accordingly, the influence of the above-mentioned groups on the π-electron delocalization of aromatic rings has been investigated by means of HOMA. The values of HOMA are presented in Table 2. The data suggest that the –NEt₂ group has the greatest influence on aromaticity. The values of the HOMA index in (I) are 0.924 and 0.908 for aromatic rings connected to atoms N3 and N4, respectively. The above-mentioned differences in the geometry of the amino groups may influence this effect.

In (I), the HOMA values are slightly different for the rings. The value for the ring connected to atom N4 (sp²-hydridized) corresponds well with the optimized structure, where both amine N atoms are strictly planar. Simultaneously, the HOMA value for the aromatic ring connected to atom N3 (partial sp³-hybridization) is decreased, indicating greater aromaticity. The lone pairs of the planar N atoms, which are shifted partially onto the N—C_ar bond may influence formation of a quinoid-like structure (Fig. 2) which contribute to the resonance hybrid of the molecule.

The HOMA index for (II) is 0.990, comparable with values found for 4,4'-(COOH)₂ (Δ = 0.001) and azo­benzene (0.985). This may suggest that these substituents do not influence the π-electron delocalization of the aromatic ring. In the case of 4,4'-(COOMe)₂, the HOMA index is clearly reduced (0.960) and is close to the value obtained for calculated structures of isolated molecules [0.966 for 4,4'-(COOH)₂, and 0.967 for 4,4'-(COOMe)₂ and (II)]. That effect may be caused by hydrogen bonding to surrounding molecules of (II) or 4,4'-(COOH)₂. Lone-electron pairs of O atoms do not take part in the electron-transmission effects as they are involved in hydrogen bonds [C5—H5···O1ⁱ for (II) (Table 4) and O1—H1A···O2ⁱⁱ for 4,4'-azinodi­benzoic acid (Yu & Liu, 2009); symmetry codes: (i) x, -y + 7/2, z - 1/2; (ii) -x + 1, -y + 1, -z + 1], contrary to the situation in 4,4'-(COOMe)₂ where no hydrogen bonding is observed (Gajda et al., 2013).

The molecular arrangement of (I) in the crystalline state is presented in Fig. 3(a). The molecules form columns with a perpendicular arrangement. Molecules belonging to neighbouring columns form a dihedral angle of 69.2 (3)°. No classical hydrogen bonds are found for (I), but there are at least three C—H···π contacts (Table 3) between columns of molecules, which stabilize the crystal structure. These contacts are formed between ethyl groups and the aromatic rings of the perpendicular molecules. The molecular arrangement of (II) in the crystalline state is presented in Fig. 3(b). The structure of (II) is stabilized by one C—H···O hydrogen bond between the ester group and the aromatic ring of a neighbouring molecule (Table 4). The dihedral angle between connected molecules [not clear] is 86.1 (3)°, and as a result molecules form chains in the [001] direction. Perpendicular molecules are arranged in stacks forming two columns. This is achieved by π–π inter­actions between the –N═N– bridge and the aromatic ring of a parallel molecule, resulting in a staircase-like formation. The perpendicular separation of the mean planes through the rings of adjacent molecules is 3.288 (1) Å.

Related literature

For related literature, see: Harada et al. (1997); Allmann (1997); Becke (1988, 1993); Frisch et al. (2010); Gajda et al. (2013); Hansch et al. (1991); Harada & Ogawa (2004); Kalyanaraman & George (1973); Kruszewski & Krygowski (1973); Krygowski (1993); Krygowski & Cyrański (1996); Niu et al. (2011); Takamiya et al. (1986); Tsuchiya (1999); Yu & Liu (2009); Zeitouny et al. (2009).

Computing details

For both compounds, data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis RED (Oxford Diffraction, 2008); data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

Fig. 1. The molecular structures in the solid state of (a) (I) and (b) (II), showing the atom-numbering schemes. Displacement ellipsoids are drawn at the 50% probability level. The molecule of (II) is located on an inversion centre.

Fig. 2. Canonical forms of (I), with charge separation showing the resonance effect of the π-donating amino group and azobenzene.

Fig. 3. Packing diagrams for (a) (I) and (b) (II), showing the C5—H5···O2ⁱ hydrogen bonds as dashed lines. [Symmetry code: (i) x, -y + 7/2, z - 1/2].

(I) N,N,N',N'-Tetraethyl-4,4'-(diazenediyl)dianiline

Crystal data

C20H28N4	F(000) = 704
Mr = 324.46	Dx = 1.166 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 3245 reflections
a = 14.0805 (3) Å	θ = 2.9–25.0°
b = 7.9682 (2) Å	µ = 0.07 mm^−1
c = 21.1890 (5) Å	T = 100 K
β = 128.989 (2)°	Plate, red
V = 1847.82 (9) Å^3	0.30 × 0.20 × 0.05 mm
Z = 4

Data collection

Oxford Xcalibur diffractometer	3433 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.019
Graphite monochromator	θmax = 29.0°, θmin = 2.9°
Detector resolution: 1024 x 1024 with blocks 2 x 2 pixels mm^-1	h = −17→19
ω scans	k = −10→10
14494 measured reflections	l = −28→21
4685 independent reflections

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)] = 0.044	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.126	H-atom parameters not refined
S = 1.08	w = 1/[σ^2(Fo^2) + (0.0734P)^2 + 0.0149P] where P = (Fo^2 + 2Fc^2)/3
4685 reflections	(Δ/σ)max < 0.001
217 parameters	Δρmax = 0.39 e Å^−3
0 restraints	Δρmin = −0.28 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
N1	0.17967 (9)	0.50576 (12)	0.00057 (6)	0.0237 (2)
N1'	0.23450 (9)	0.42196 (12)	−0.01860 (6)	0.0230 (2)
C1	0.23961 (11)	0.50331 (14)	0.08522 (7)	0.0218 (2)
C2	0.17922 (11)	0.57673 (14)	0.11037 (7)	0.0240 (3)
H2	0.1020	0.6224	0.0718	0.029*
C3	0.23111 (11)	0.58374 (15)	0.19146 (7)	0.0235 (3)
H3	0.1887	0.6348	0.2066	0.028*
C4	0.34813 (10)	0.51408 (14)	0.25185 (6)	0.0197 (2)
C5	0.40912 (11)	0.44079 (15)	0.22546 (7)	0.0235 (3)
H5	0.4864	0.3949	0.2636	0.028*
C6	0.35628 (11)	0.43603 (15)	0.14432 (7)	0.0240 (3)
H6	0.3986	0.3876	0.1286	0.029*
C1'	0.18136 (10)	0.43145 (14)	−0.10183 (6)	0.0205 (2)
C2'	0.23709 (10)	0.33452 (14)	−0.12477 (6)	0.0207 (2)
H2'	0.3014	0.2645	−0.0865	0.025*
C3'	0.20008 (10)	0.33874 (14)	−0.20250 (6)	0.0197 (2)
H3'	0.2392	0.2714	−0.2158	0.024*
C4'	0.10336 (10)	0.44435 (14)	−0.26205 (6)	0.0181 (2)
C5'	0.04457 (10)	0.53957 (15)	−0.23872 (7)	0.0221 (2)
H5'	−0.0213	0.6076	−0.2770	0.027*
C6'	0.08265 (10)	0.53358 (15)	−0.16096 (7)	0.0226 (2)
H6'	0.0426	0.5978	−0.1474	0.027*
N3	0.40020 (9)	0.51423 (12)	0.33281 (5)	0.0222 (2)
N4	0.06828 (8)	0.45639 (12)	−0.33873 (5)	0.0213 (2)
C7	0.34264 (11)	0.59969 (16)	0.36161 (7)	0.0253 (3)
H7A	0.3702	0.5486	0.4122	0.030*
H7B	0.2548	0.5846	0.3222	0.030*
C8	0.37085 (13)	0.78535 (17)	0.37524 (8)	0.0368 (3)
H8A	0.3305	0.8355	0.3939	0.055*
H8B	0.3425	0.8372	0.3252	0.055*
H8C	0.4575	0.8013	0.4153	0.055*
C9	0.52685 (10)	0.46242 (15)	0.39539 (7)	0.0234 (3)
H9A	0.5607	0.5252	0.4447	0.028*
H9B	0.5738	0.4904	0.3777	0.028*
C10	0.54047 (12)	0.27654 (17)	0.41435 (8)	0.0322 (3)
H10A	0.6253	0.2502	0.4557	0.048*
H10B	0.5089	0.2135	0.3662	0.048*
H10C	0.4958	0.2482	0.4332	0.048*
C11	0.12613 (11)	0.35856 (15)	−0.36443 (7)	0.0246 (3)
H11A	0.0669	0.3378	−0.4223	0.029*
H11B	0.1511	0.2509	−0.3370	0.029*
C12	0.23654 (13)	0.44583 (17)	−0.34632 (9)	0.0363 (3)
H12A	0.2713	0.3766	−0.3642	0.054*
H12B	0.2962	0.4647	−0.2889	0.054*
H12C	0.2120	0.5514	−0.3743	0.054*
C13	−0.02870 (10)	0.56978 (15)	−0.39938 (7)	0.0225 (2)
H13A	−0.0143	0.6028	−0.4368	0.027*
H13B	−0.0262	0.6703	−0.3725	0.027*
C14	−0.15447 (11)	0.49120 (17)	−0.44692 (7)	0.0305 (3)
H14A	−0.2150	0.5707	−0.4857	0.046*
H14B	−0.1698	0.4597	−0.4103	0.046*
H14C	−0.1582	0.3934	−0.4749	0.046*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0261 (5)	0.0232 (5)	0.0224 (5)	−0.0024 (4)	0.0155 (5)	0.0001 (4)
N1'	0.0270 (5)	0.0216 (5)	0.0233 (5)	−0.0055 (4)	0.0172 (4)	−0.0033 (4)
C1	0.0239 (6)	0.0190 (6)	0.0196 (6)	−0.0037 (4)	0.0123 (5)	0.0008 (4)
C2	0.0248 (6)	0.0216 (6)	0.0213 (6)	0.0016 (4)	0.0125 (5)	0.0033 (4)
C3	0.0262 (6)	0.0219 (6)	0.0226 (6)	0.0017 (4)	0.0154 (5)	0.0012 (4)
C4	0.0228 (6)	0.0193 (6)	0.0171 (5)	−0.0029 (4)	0.0126 (5)	−0.0004 (4)
C5	0.0213 (6)	0.0274 (6)	0.0211 (6)	−0.0019 (5)	0.0130 (5)	−0.0006 (5)
C6	0.0255 (6)	0.0262 (6)	0.0265 (6)	−0.0032 (5)	0.0194 (5)	−0.0025 (5)
C1'	0.0215 (5)	0.0216 (6)	0.0179 (5)	−0.0042 (4)	0.0121 (5)	−0.0012 (4)
C2'	0.0188 (5)	0.0202 (6)	0.0193 (5)	−0.0009 (4)	0.0101 (5)	0.0021 (4)
C3'	0.0197 (5)	0.0191 (6)	0.0224 (6)	0.0007 (4)	0.0142 (5)	−0.0003 (4)
C4'	0.0168 (5)	0.0195 (5)	0.0167 (5)	−0.0020 (4)	0.0100 (4)	−0.0019 (4)
C5'	0.0189 (5)	0.0253 (6)	0.0195 (6)	0.0036 (4)	0.0109 (5)	0.0004 (4)
C6'	0.0231 (6)	0.0246 (6)	0.0226 (6)	−0.0002 (4)	0.0156 (5)	−0.0041 (4)
N3	0.0224 (5)	0.0269 (5)	0.0168 (5)	0.0041 (4)	0.0120 (4)	0.0008 (4)
N4	0.0216 (5)	0.0255 (5)	0.0179 (5)	0.0058 (4)	0.0129 (4)	0.0018 (4)
C7	0.0246 (6)	0.0344 (7)	0.0198 (6)	0.0066 (5)	0.0154 (5)	0.0051 (5)
C8	0.0501 (9)	0.0312 (7)	0.0325 (7)	0.0130 (6)	0.0276 (7)	0.0039 (6)
C9	0.0194 (6)	0.0317 (7)	0.0171 (5)	0.0016 (5)	0.0106 (5)	−0.0014 (5)
C10	0.0256 (6)	0.0345 (7)	0.0336 (7)	0.0091 (5)	0.0173 (6)	0.0065 (5)
C11	0.0334 (6)	0.0230 (6)	0.0218 (6)	0.0048 (5)	0.0195 (5)	−0.0004 (5)
C12	0.0469 (8)	0.0279 (7)	0.0570 (9)	0.0080 (6)	0.0437 (8)	0.0060 (6)
C13	0.0232 (6)	0.0246 (6)	0.0184 (5)	0.0033 (4)	0.0125 (5)	0.0023 (4)
C14	0.0239 (6)	0.0362 (7)	0.0236 (6)	0.0003 (5)	0.0112 (6)	−0.0042 (5)

Geometric parameters (Å, º)

N1—N1'	1.2655 (13)	N4—C13	1.4575 (14)
N1—C1	1.4230 (14)	N4—C11	1.4580 (13)
N1'—C1'	1.4196 (13)	C7—C8	1.5117 (18)
C1—C2	1.3855 (16)	C7—H7A	0.9700
C1—C6	1.4020 (16)	C7—H7B	0.9700
C2—C3	1.3818 (15)	C8—H8A	0.9600
C2—H2	0.9300	C8—H8B	0.9600
C3—C4	1.4168 (16)	C8—H8C	0.9600
C3—H3	0.9300	C9—C10	1.5149 (18)
C4—N3	1.3776 (13)	C9—H9A	0.9700
C4—C5	1.4126 (15)	C9—H9B	0.9700
C5—C6	1.3789 (15)	C10—H10A	0.9600
C5—H5	0.9300	C10—H10B	0.9600
C6—H6	0.9300	C10—H10C	0.9600
C1'—C2'	1.3888 (15)	C11—C12	1.5156 (18)
C1'—C6'	1.4044 (16)	C11—H11A	0.9700
C2'—C3'	1.3804 (15)	C11—H11B	0.9700
C2'—H2'	0.9300	C12—H12A	0.9600
C3'—C4'	1.4116 (15)	C12—H12B	0.9600
C3'—H3'	0.9300	C12—H12C	0.9600
C4'—N4	1.3722 (13)	C13—C14	1.5160 (17)
C4'—C5'	1.4214 (15)	C13—H13A	0.9700
C5'—C6'	1.3759 (15)	C13—H13B	0.9700
C5'—H5'	0.9300	C14—H14A	0.9600
C6'—H6'	0.9300	C14—H14B	0.9600
N3—C7	1.4544 (14)	C14—H14C	0.9600
N3—C9	1.4606 (14)
N1'—N1—C1	113.55 (10)	C8—C7—H7A	109.0
N1—N1'—C1'	114.80 (10)	N3—C7—H7B	109.0
C2—C1—C6	118.41 (10)	C8—C7—H7B	109.0
C2—C1—N1	117.27 (10)	H7A—C7—H7B	107.8
C6—C1—N1	124.30 (10)	C7—C8—H8A	109.5
C3—C2—C1	121.53 (11)	C7—C8—H8B	109.5
C3—C2—H2	119.2	H8A—C8—H8B	109.5
C1—C2—H2	119.2	C7—C8—H8C	109.5
C2—C3—C4	120.78 (10)	H8A—C8—H8C	109.5
C2—C3—H3	119.6	H8B—C8—H8C	109.5
C4—C3—H3	119.6	N3—C9—C10	113.31 (10)
N3—C4—C5	120.90 (10)	N3—C9—H9A	108.9
N3—C4—C3	121.98 (10)	C10—C9—H9A	108.9
C5—C4—C3	117.11 (10)	N3—C9—H9B	108.9
C6—C5—C4	121.30 (11)	C10—C9—H9B	108.9
C6—C5—H5	119.4	H9A—C9—H9B	107.7
C4—C5—H5	119.3	C9—C10—H10A	109.5
C5—C6—C1	120.86 (11)	C9—C10—H10B	109.5
C5—C6—H6	119.6	H10A—C10—H10B	109.5
C1—C6—H6	119.6	C9—C10—H10C	109.5
C2'—C1'—C6'	118.04 (10)	H10A—C10—H10C	109.5
C2'—C1'—N1'	115.65 (10)	H10B—C10—H10C	109.5
C6'—C1'—N1'	126.25 (10)	N4—C11—C12	112.66 (10)
C3'—C2'—C1'	122.13 (10)	N4—C11—H11A	109.1
C3'—C2'—H2'	118.9	C12—C11—H11A	109.1
C1'—C2'—H2'	118.9	N4—C11—H11B	109.1
C2'—C3'—C4'	120.58 (10)	C12—C11—H11B	109.1
C2'—C3'—H3'	119.7	H11A—C11—H11B	107.8
C4'—C3'—H3'	119.7	C11—C12—H12A	109.5
N4—C4'—C3'	121.75 (10)	C11—C12—H12B	109.5
N4—C4'—C5'	121.33 (10)	H12A—C12—H12B	109.5
C3'—C4'—C5'	116.92 (10)	C11—C12—H12C	109.5
C6'—C5'—C4'	121.65 (10)	H12A—C12—H12C	109.5
C6'—C5'—H5'	119.2	H12B—C12—H12C	109.5
C4'—C5'—H5'	119.2	N4—C13—C14	112.62 (10)
C5'—C6'—C1'	120.64 (10)	N4—C13—H13A	109.1
C5'—C6'—H6'	119.7	C14—C13—H13A	109.1
C1'—C6'—H6'	119.7	N4—C13—H13B	109.1
C4—N3—C7	121.49 (9)	C14—C13—H13B	109.1
C4—N3—C9	121.30 (9)	H13A—C13—H13B	107.8
C7—N3—C9	115.84 (9)	C13—C14—H14A	109.5
C4'—N4—C13	121.79 (9)	C13—C14—H14B	109.5
C4'—N4—C11	122.36 (9)	H14A—C14—H14B	109.5
C13—N4—C11	115.84 (9)	C13—C14—H14C	109.5
N3—C7—C8	112.93 (10)	H14A—C14—H14C	109.5
N3—C7—H7A	109.0	H14B—C14—H14C	109.5
C1—N1—N1'—C1'	−175.90 (9)	C3'—C4'—C5'—C6'	2.12 (16)
N1'—N1—C1—C2	−173.26 (10)	C4'—C5'—C6'—C1'	−0.34 (17)
N1'—N1—C1—C6	8.59 (15)	C2'—C1'—C6'—C5'	−1.42 (16)
C6—C1—C2—C3	−0.35 (17)	N1'—C1'—C6'—C5'	175.70 (11)
N1—C1—C2—C3	−178.61 (10)	C5—C4—N3—C7	175.50 (10)
C1—C2—C3—C4	−0.61 (18)	C3—C4—N3—C7	−5.73 (16)
C2—C3—C4—N3	−177.74 (11)	C5—C4—N3—C9	9.43 (16)
C2—C3—C4—C5	1.07 (16)	C3—C4—N3—C9	−171.81 (11)
N3—C4—C5—C6	178.22 (11)	C3'—C4'—N4—C13	−177.80 (10)
C3—C4—C5—C6	−0.61 (16)	C5'—C4'—N4—C13	1.46 (16)
C4—C5—C6—C1	−0.34 (17)	C3'—C4'—N4—C11	1.50 (16)
C2—C1—C6—C5	0.82 (17)	C5'—C4'—N4—C11	−179.24 (10)
N1—C1—C6—C5	178.95 (10)	C4—N3—C7—C8	−83.07 (13)
N1—N1'—C1'—C2'	−177.30 (10)	C9—N3—C7—C8	83.72 (12)
N1—N1'—C1'—C6'	5.52 (16)	C4—N3—C9—C10	−90.23 (13)
C6'—C1'—C2'—C3'	1.37 (16)	C7—N3—C9—C10	102.95 (12)
N1'—C1'—C2'—C3'	−176.05 (10)	C4'—N4—C11—C12	−88.41 (13)
C1'—C2'—C3'—C4'	0.46 (17)	C13—N4—C11—C12	90.94 (12)
C2'—C3'—C4'—N4	177.12 (10)	C4'—N4—C13—C14	−86.72 (13)
C2'—C3'—C4'—C5'	−2.17 (16)	C11—N4—C13—C14	93.93 (12)
N4—C4'—C5'—C6'	−177.18 (10)

Hydrogen-bond geometry (Å, º)

Cg1 and Cg2 are the centroids of the C1'–C6' and C1–C6 rings, respectively.

D—H···A	D—H	H···A	D···A	D—H···A
C11—H11B···C3^i	0.97	2.81	3.7155 (17)	156
C13—H13B···C1′^ii	0.97	2.81	3.6049 (16)	140
C12—H12A···C6^i	0.96	2.72	3.5441 (17)	144
C8—H8A···Cg1^iii	0.96	2.75	3.446 (2)	130
C5′—H5′···Cg1^ii	0.93	2.94	3.767 (2)	148
C11—H11B···Cg2^i	0.97	2.84	3.549 (2)	131

Symmetry codes: (i) x, −y+1/2, z−1/2; (ii) −x, y+1/2, −z−1/2; (iii) x, −y+3/2, z+1/2.

(II) Diethyl 4,4'-(diazenediyl)dibenzoate

Crystal data

C18H18N2O4	F(000) = 344
Mr = 326.34	Dx = 1.419 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 1345 reflections
a = 14.7061 (11) Å	θ = 2.8–25.0°
b = 4.5005 (4) Å	µ = 0.10 mm^−1
c = 11.5889 (8) Å	T = 100 K
β = 95.033 (6)°	Plate, red
V = 764.05 (10) Å^3	0.35 × 0.25 × 0.10 mm
Z = 2

Data collection

Oxford Xcalibur diffractometer	1282 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.031
Graphite monochromator	θmax = 29.0°, θmin = 2.8°
Detector resolution: 1024 x 1024 with blocks 2 x 2 pixels mm^-1	h = −20→19
ω scans	k = −4→6
5744 measured reflections	l = −15→15
1938 independent reflections

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)] = 0.042	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.117	H-atom parameters not refined
S = 0.95	w = 1/[σ^2(Fo^2) + (0.071P)^2] where P = (Fo^2 + 2Fc^2)/3
1938 reflections	(Δ/σ)max < 0.001
109 parameters	Δρmax = 0.39 e Å^−3
0 restraints	Δρmin = −0.22 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
O1	0.28116 (7)	2.0280 (2)	0.76163 (8)	0.0165 (3)
O2	0.34556 (7)	1.9518 (3)	0.59436 (9)	0.0212 (3)
N1	0.00148 (8)	1.0611 (3)	0.54895 (10)	0.0137 (3)
C1	0.07332 (10)	1.2740 (3)	0.56888 (12)	0.0130 (3)
C2	0.07752 (10)	1.4146 (3)	0.67623 (12)	0.0140 (3)
H2	0.0347	1.3696	0.7280	0.017*
C3	0.14540 (9)	1.6214 (3)	0.70618 (12)	0.0143 (3)
H3	0.1479	1.7152	0.7779	0.017*
C4	0.20979 (9)	1.6885 (3)	0.62903 (12)	0.0133 (3)
C5	0.20441 (10)	1.5497 (3)	0.52072 (12)	0.0152 (3)
H5	0.2468	1.5964	0.4686	0.018*
C6	0.13674 (10)	1.3437 (3)	0.49025 (12)	0.0153 (3)
H6	0.1335	1.2525	0.4180	0.018*
C7	0.28612 (9)	1.9012 (3)	0.65726 (12)	0.0143 (3)
C8	0.35446 (10)	2.2329 (4)	0.80038 (13)	0.0190 (4)
H8A	0.3317	2.3792	0.8521	0.023*
H8B	0.3750	2.3369	0.7341	0.023*
C9	0.43345 (11)	2.0682 (4)	0.86218 (14)	0.0268 (4)
H9A	0.4809	2.2065	0.8869	0.040*
H9B	0.4565	1.9252	0.8106	0.040*
H9C	0.4133	1.9679	0.9285	0.040*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0153 (5)	0.0223 (6)	0.0118 (5)	−0.0038 (4)	0.0007 (4)	−0.0032 (4)
O2	0.0181 (5)	0.0314 (7)	0.0149 (5)	−0.0043 (5)	0.0049 (4)	−0.0010 (5)
N1	0.0158 (6)	0.0156 (6)	0.0095 (6)	0.0007 (5)	−0.0003 (5)	0.0002 (5)
C1	0.0132 (7)	0.0144 (8)	0.0111 (7)	0.0017 (6)	−0.0003 (5)	0.0014 (6)
C2	0.0151 (7)	0.0186 (8)	0.0086 (6)	0.0013 (6)	0.0032 (5)	0.0025 (6)
C3	0.0174 (7)	0.0173 (8)	0.0079 (7)	0.0018 (6)	0.0008 (5)	−0.0003 (6)
C4	0.0138 (7)	0.0154 (8)	0.0107 (7)	0.0016 (6)	0.0004 (5)	0.0019 (6)
C5	0.0150 (7)	0.0204 (8)	0.0104 (7)	0.0009 (6)	0.0032 (5)	0.0019 (6)
C6	0.0188 (7)	0.0195 (8)	0.0076 (6)	0.0011 (6)	0.0011 (5)	−0.0007 (6)
C7	0.0149 (7)	0.0179 (8)	0.0097 (7)	0.0021 (6)	−0.0003 (5)	0.0008 (6)
C8	0.0181 (8)	0.0229 (9)	0.0157 (7)	−0.0058 (6)	−0.0001 (6)	−0.0019 (6)
C9	0.0199 (8)	0.0387 (11)	0.0208 (8)	−0.0052 (8)	−0.0033 (6)	0.0048 (8)

Geometric parameters (Å, º)

O1—C7	1.3451 (17)	C4—C5	1.398 (2)
O1—C8	1.4591 (17)	C4—C7	1.490 (2)
O2—C7	1.2080 (17)	C5—C6	1.383 (2)
N1—N1^i	1.258 (2)	C5—H5	0.9300
N1—C1	1.4301 (19)	C6—H6	0.9300
C1—C2	1.3922 (19)	C8—C9	1.505 (2)
C1—C6	1.396 (2)	C8—H8A	0.9700
C2—C3	1.386 (2)	C8—H8B	0.9700
C2—H2	0.9300	C9—H9A	0.9600
C3—C4	1.3913 (19)	C9—H9B	0.9600
C3—H3	0.9300	C9—H9C	0.9600
C7—O1—C8	116.61 (11)	C5—C6—H6	120.3
N1^i—N1—C1	113.89 (15)	C1—C6—H6	120.3
C2—C1—C6	120.10 (14)	O2—C7—O1	124.18 (14)
C2—C1—N1	115.03 (12)	O2—C7—C4	124.05 (13)
C6—C1—N1	124.87 (13)	O1—C7—C4	111.77 (12)
C3—C2—C1	120.21 (13)	O1—C8—C9	110.72 (13)
C3—C2—H2	119.9	O1—C8—H8A	109.5
C1—C2—H2	119.9	C9—C8—H8A	109.5
C2—C3—C4	120.04 (13)	O1—C8—H8B	109.5
C2—C3—H3	120.0	C9—C8—H8B	109.5
C4—C3—H3	120.0	H8A—C8—H8B	108.1
C3—C4—C5	119.46 (13)	C8—C9—H9A	109.5
C3—C4—C7	122.73 (13)	C8—C9—H9B	109.5
C5—C4—C7	117.81 (13)	H9A—C9—H9B	109.5
C6—C5—C4	120.76 (13)	C8—C9—H9C	109.5
C6—C5—H5	119.6	H9A—C9—H9C	109.5
C4—C5—H5	119.6	H9B—C9—H9C	109.5
C5—C6—C1	119.42 (13)
N1^i—N1—C1—C2	179.40 (15)	C2—C1—C6—C5	1.0 (2)
N1^i—N1—C1—C6	−1.0 (2)	N1—C1—C6—C5	−178.62 (13)
C6—C1—C2—C3	−0.9 (2)	C8—O1—C7—O2	−1.6 (2)
N1—C1—C2—C3	178.79 (13)	C8—O1—C7—C4	177.84 (11)
C1—C2—C3—C4	−0.2 (2)	C3—C4—C7—O2	176.01 (14)
C2—C3—C4—C5	1.1 (2)	C5—C4—C7—O2	−3.1 (2)
C2—C3—C4—C7	−178.02 (13)	C3—C4—C7—O1	−3.48 (19)
C3—C4—C5—C6	−0.9 (2)	C5—C4—C7—O1	177.39 (12)
C7—C4—C5—C6	178.21 (13)	C7—O1—C8—C9	−87.27 (15)
C4—C5—C6—C1	−0.1 (2)

Symmetry code: (i) −x, −y+2, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C5—H5···O1^ii	0.93	2.56	3.3175 (17)	140

Symmetry code: (ii) x, −y+7/2, z−1/2.