Two trans-4-aminoazoxybenzenes

Article abstract of DOI:10.1107/S0108270101000518

Two isomeric trans-4-aminoazoxybenzenes, trans-1-(4-aminophenyl)-2-phenyldiazene 2-oxide (α, C₁₂H₁₁N₃O) and trans-2-(4-aminophenyl)-1-phenyldiazene 2-oxide (β, C₁₂H₁₁N₃O), have been charact

Full text of DOI:10.1107/S0108270101000518

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
properties of the azoxy group are applied to both rings, the
proposal of Duffey & Hendley (1968) would seem to be
reasonable. If not, the assumption of a symmetrical inter-
mediate seems to be necessary (Buncel & Lawton, 1965).
Another reason that trans-4-aminoazoxybenzenes are inter-
esting subjects for structural investigation is their applicability
to the syntheses of various derivatives of trans-azoxybenzene.
Transformation or substitution of the amino group with
another substituent can provide some para-substituted
azoxybenzenes belonging to the same (ONN or NNO) series
as the parent compound (Risaliti & Monti, 1961; Risaliti,
1963). This provides an opportunity to prepare isomers which
are not accessible by the direct oxidation of azoxybenzenes,
provided that the position of the amino group is unequivocally
established. In our experience, polar substituents bound to the
aromatic ring facilitate separation of the isomers. Conse-
quently, we have oxidized 4-aminoazobenzene (protected with
an N-acetyl group) to the corresponding title azoxybenzenes,
separated the isomers to give compounds (I) (ꢀ isomer) and
(II) (ꢁ isomer), and examined their structures.
ISSN 0108-2701
Two trans-4-aminoazoxybenzenes
Â
Andrzej Domanski, Krzysztof Ejsmont, Janusz B. Kyzioø
and Jacek Zaleski*
Institute of Chemistry, University of Opole, Oleska 48, 45-052 Opole, Poland
Correspondence e-mail: zaleski@uni.opole.pl
Received 31 July 2000
Accepted 4 January 2001
Two isomeric trans-4-aminoazoxybenzenes, trans-1-(4-amino-
phenyl)-2-phenyldiazene 2-oxide (ꢀ, C12H11N3O) and trans-2-
(4-aminophenyl)-1-phenyldiazene 2-oxide (ꢁ, C12H11N3O),
have been characterized by X-ray diffraction. The ꢀ isomer
is almost planar, having torsion angles along the CarylÐN
bonds of only 4.9 (2) and 8.0 (2)ꢀ. The relatively short CarylÐN
Ê
bond to the non-oxidized site of the azoxy group [1.401 (2) A],
together with the signi®cant quinoid deformation of the
respective phenyl ring, is evidence of conjugation between the
aromatic sextet and the ꢂ-electron system of the azoxy group.
The geometry of the ꢁ isomer is different. The non-substituted
phenyl ring is twisted with respect to the NNO plane by ca 50ꢀ,
whereas the substituted ring is almost coplanar with the NNO
plane. The non-oxidized N atom in the ꢁ isomer has increased
sp3 character, which leads to a decrease in the NÐNÐC bond
angle to 116.8 (2)ꢀ, in contrast with 120.9 (1)ꢀ for the ꢀ isomer.
The deformation of the CÐCÐC angles (1±2ꢀ) in the phenyl
rings at the substitution positions is evidence of the different
character of the oxidized and non-oxidized N atoms of the
azoxy group. In the crystal structures, molecules of both
isomers are arranged in chains connected by weak NÐHÁ Á ÁO
(ꢀ and ꢁ) and NÐHÁ Á ÁN (ꢁ) hydrogen bonds.
We have initially assumed that the azoxy group is an elec-
tron-withdrawing substituent, due to the formal positive
charge on the oxidized N atom, analogous to the nitro group.
The facts that the 4-nitroaniline molecule, the amino group
and the aromatic ring lie in the same plane, and that the sum of
the valence angles around the amino nitrogen is 360ꢀ indicate
trigonal hybridization of the amino N atoms (Tonogaki et al.,
1993). On the other hand, in the 2-methyl-5-nitroaniline
molecule, the amino group diverges from the ring plane and
the sum of the angles about the amino nitrogen is less than
360ꢀ (Ellena et al., 1999). Interaction of the complementary
substituents across the ring depends on their relative positions.
The aim of the present work is to answer the question of
whether the azoxy group interacts with both aromatic rings in
the same or a different way.
Comment
We have recently (Ejsmont et al., 2000) compared the struc-
ture of 4-hydroxy-ONN-azoxybenzene with some other para-
substituted azoxybenzenes. We could not explain their mole-
cular structures due to the diversity of substituents. Because of
a lack of relevant structural data for the isomeric unsymme-
trically substituted azoxybenzenes in the literature, we were
prompted to prepare two isomeric trans-4-aminoazoxy-
benzenes and compare their geometries. We have selected
amino derivatives since the amino group may be considered as
a probe indicative of an intramolecular interaction of the
azoxy group with the aromatic rings. Its in¯uence on the
amino group bonded to the ring, and on the oxidized and
unoxidized sides of the azoxy bridge, should provide some
information on the electronic properties of the azoxy group.
The question is essential in investigations of the mechanism of
the Wallach rearrangement. If the electron-withdrawing
In the ꢁ isomer, (II) (Fig. 1), the sum of the valence angles
around the amino nitrogen (ca 341ꢀ) and the distance of atom
Ê
N3 from the C4/H32/H31 plane (0.25 A) suggest a tetrahedral
ꢁ
Acta Cryst. (2001). C57, 467±470
# 2001 International Union of Crystallography
Printed in Great Britain ± all rights reserved 467

organic compounds
con®guration around the amino nitrogen. The internal coor-
dinate ꢃN = (H32ÐN3ÐC4ÐC3)
The character of the substituent on the benzene rings may
be inferred by the relative change in the CÐCÐC bond angle
at the ipso position. A decrease in this angle to below 120ꢀ is
characteristic of an electron-releasing substituent (ca 2ꢀ in the
case of aniline), whereas in the case of an electron acceptor
bound to the ring, the angle is increased (ca 3ꢀ in the case of
¯uorobenzene; Domenicano, 1992). This effect is con®rmed in
the values of the C3ÐC4(ÐNH2)ÐC5 angles in both (I) and
(II) (ca 118ꢀ). It should be noted that the C20ÐC10ÐC60 angle
in (I) is enlarged, as expected, to 121.0 (2)ꢀ. The non-oxidized
N atom bound to the other ring leads, however, to a decrease
of the C2ÐC1ÐC6 bond angle to 117.7 (2)ꢀ. This in¯uence is
almost the same as in the case of the NH2 group. It seems to
af®rm that the character of the N atoms (non-oxidized and
oxidized) is different. In the case of (II), the C6ÐC1ÐC2
angle remains almost the same as the respective angle in (I),
whereas the C60ÐC10ÐC20 angle is only slightly smaller than
120ꢀ. The deformation is most probably much smaller due to
the twist of this phenyl ring with respect to the NNO plane,
which prevents conjugation.
In the trans-azoxybenzenes, the benzene ring bound to the
oxidized site of the azoxy bridge is almost always coplanar
with it, irrespective of the substituent present (Ejsmont et al.,
2000, and references therein). While the reason for this is not
clear, it may result from the interaction of the O atom of the
NNO group with the ring bound to the oxidized N atom. On
the other hand, the other phenyl ring shows a much larger
tendency to become twisted with respect to the NNO plane.
The geometry of the azoxy group in (I) and (II) is consistent
with such a picture. In both isomers, the NÐN, CarylÐNO and
NÐO bonds have typical lengths (Ejsmont et al., 2000),
although the latter are slightly shorter. The differences
between the CarylÐNO and CarylÐN bond lengths are
observed in both isomers, with the CarylÐN bond always
shorter. The most signi®cant difference between (I) and (II) is
the conformation of the rings with respect to the NNO plane.
In (II), the unsubstituted ring is signi®cantly twisted along the
CarylÐN bond [torsion angles 54.6 (3) and 134.9 (2)ꢀ measured
to both ortho-C atoms]. The twist of the benzene ring with
respect to the NNO plane is associated with an increase of the
(H32ÐN3ÐC4ÐC5) +
ꢂ(mod2ꢂ), which is a measure of the degree of nitrogen
pyramidalization, is 46ꢀ. This is less than the extreme (60ꢀ)
value for regular sp3 hybridization, but is much more than
zero, which would indicate planar (sp2) geometry. The corre-
sponding numbers for the ꢀ isomer, (I), are as follows: the sum
of the valence angles is ca 354ꢀ, the dꢀistance of N3 from the
Ê
plane is 0.14 A and the factor ꢃN is 26 . It is inferred that the
amino group situated on the unoxidized side of the azoxy
bridge is more planar than in (II), but the deviation from
regular sp3 hybridization is the same as in 2-methyl-5-nitro-
aniline (Ellena et al., 1999). The lengths of the CarylÐNH2
Ê
Ê
bonds [1.369 (2) A in (I) and 1.385 (3) A in (II)] con®rm that
the interaction with the azoxy bridge is stronger in (I). It
should be mentioned that a mesomeric interaction with the
nitro group in a conjugated position shortens this bond to
Ê
1.355 A (Tonogaki et al., 1993).
It can be expected that an interaction of the substituents
across the ring will induce a quinonoid deformation of an
aromatic hexagon. The effect is small, even in the case of
4-nitroaniline (Tonogaki et al., 1993). In the present 4-amino
azoxybenzenes, the average CÐC bond length in the rings is
Ê
Ê
1.379 A and the deviations do not exceed 0.01 A, with one
exception. In the substituted ring of (I), the bonds parallel to
Ê
the N±Caryl±N axis are shorter (average 1.370 A) than the
Ê
remaining ones (average 1.394 A). The effect is minute, but it
is limited to one ring only, hence we can conclude that the
amino group interacts with the azoxy substituent when the
substituted ring is bound to the non-oxidized N atom. The
conclusion seems to con®rm the results obtained from the
geometries of the amino groups. The interaction of the azoxy
group with aromatic rings is of inductive character on one side
and mesomeric on the other.
Ê
CarylÐN bond length [1.420 (3) A in (II) compared with
3
Ê
1.401 (2) A in (I)]. It also increases the sp character of the
non-oxidized N atom [ꢃN factor of 9.6ꢀ in (II) compared with a
ꢃN value of 0.4ꢀ in (I)]. This is con®rmed by the relative shift of
atom H6 in (II), which interacts with the lone pair on N2. This
leads to an increase of the C5ÐC6ÐH6 angle to 125 (1)ꢀ and
a decrease of the C1ÐC6ÐH6 angle to 116 (1)ꢀ. The
respective angles in (I) are 120 (1) and 118 (1)ꢀ. The twist does
not result from steric hindrance around the O atom. The
Ê
distances to H atoms in the ortho positions are 2.36 A in the
Ê
coplanar ring and 2.63 A in the twisted ring.
The geometry of the ꢀ isomer, (I), is different; the molecule
is essentially planar, so the torsion angles along the CarylÐN
bonds are only 4.8 (2) and 8.0 (2)ꢀ. The conjugation between
the aromatic sextet and the ꢂ-electron system of the azoxy
group bound to the non-oxidized N atom cannot be excluded,
especially if one takes into acccount the shortening of the
Figure 1
The molecular structures of (a) the ꢀ isomer (I) and (b) the ꢁ isomer (II).
Displacement ellipsoids are shown at the 50% probability level and H
atoms are drawn as small spheres of arbitrary radii.
ꢁ
Â
468 Andrzej Domanski et al.
Two isomers of C12H11N3O
Acta Cryst. (2001). C57, 467±470

organic compounds
C2ÐC3 and C5ÐC6 bonds with respect to the others of the
same phenyl ring. The increase in the NÐNÐO angle to
126.4 (1)ꢀ and the N2ÐC1ÐC2 angle to 129.7 (1)ꢀ brings the
Experimental
To prepare 4-aminoazoxybenzene, 4-(N-acetylamino)azobenzene
(9.57 g, 0.040 mol) was dissolved in a mixture of acetic acid (100 ml)
and acetic anhydride (30 ml). To the stirred solution, hydrogen
peroxide (30%, 40 ml) was added and the mixture was stirred for 4 h
at 323 K. It was then poured onto ice, and a yellow precipitate was
collected by ®ltration and dried. The crude product was dissolved in
ethanol (300 ml), sodium hydroxide (4.80 g, 0.12 mol) was added as a
concentrated aqueous solution and the mixture was stirred under
re¯ux for 2 h. It was then concentrated and poured onto ice. A yellow
precipitate (7.41 g, 87%) was collected by ®ltration and dried. It was a
mixture of equal amounts of both isomers, (I) and (II) (m.p. 375±
383 K), contaminated with small amounts of the substrate, as indi-
cated by high-performance liquid chromatography (HPLC) analyses.
To prepare the ꢁ isomer, (II), the aforementioned product (1.00 g)
was dissolved in benzene (20 ml) and chromatographed on a column
with silica gel, using a benzene±n-heptane mixture (1:1, v/v) as the
eluent. The substrate was eluted ®rst and the next fraction (0.34 g)
contained mainly (73%, HPLC) compound (II). This was crystallized
twice from the same solvent yielding (II) (0.15 g) as yellow needles
melting at 410.6±411.5 K (differential scanning calorimetry, onset
406 K, ꢅH = 33 kJ mol 1). Spectroscopic analysis: MS, m/z (intensity):
213 (M+, 65), 185 (17), 121 (28), 106 (77), 92 (100), 77 (21), 65 (62);
13C NMR (DMSO-d6, ꢅ, p.p.m.): 152.8 (C4), 144.2 (C10), 136.2 (C1),
128.3 (C40), 112.5 (C3), 128.7, 124.5 and 123.7 (remaining C atoms);
IR (KBr, cm 1): 3441, 3333 (stretching vibrations of the NH2 group),
0
Ê
H2Á Á ÁO1 distance to 2.18 A, compared with an H2 Á Á ÁO1
Ê
distance of 2.39 A. The H2Á Á ÁO1 distance is rather short and
must result in a signi®cant repulsion between the atoms in
question. In spite of that, the monosubstituted benzene ring
remains almost coplanar with the azoxy group. We may
therefore conclude that the inductive/mesomeric interactions
of the non-oxidized N atom of the NNO group and the N atom
of the amino group are energetically more favourable than the
repulsion interactions between the O atom of the NNO group
and the phenyl ring.
Intermolecular interactions contribute signi®cantly to the
crystal structures of the compounds under discussion. In
particular, NÐHÁ Á ÁO or NÐHÁ Á ÁN hydrogen bonds are
expected. The FT±IR spectra of the ꢀ and ꢁ isomers are very
similar; in the region of ꢄ > 3000 cm 1, two peaks (see
Experimental) characteristic of the primary amino group are
observed. In the spectra registered in chloroform solutions,
the absorption bands at 3503 and 3410 cm 1 for (I), and 3504
and 3411 cm 1 for (II), obey the rule ꢄasym = 0.876ꢄsym + 345.5,
indicating the presence of free amino groups. In the spectra
registered in KBr pellets, these maxima are shifted toward
lower wave numbers by ca 60 cm 1, i.e. by only about 2%.
According to the Desiraju & Steiner (1999) classi®cation of
hydrogen bonds, such spectral characteristics of this interac-
tion indicate that it is weak (< 4 kcal mol 1; 1 cal = 4.184 J).
1629 (amino group deformations), 1460, 1324 (NÐO and N
N
stretching vibration within the azoxy group). To prepare the ꢀ isomer,
(I), the last fraction from the column was collected. This provided
orange crystals (0.52 g) of (I) (88% purity according to HPLC). After
repeated crystallization, pure (I) (0.32 g) was obtained (m.p. 411±
412.3 K; differential scanning calorimetry, onset 404 K, ꢅH =
28 kJ mol 1). Spectroscopic analysis: MS, m/z (intensity): 213 (M+,
80), 121 (2), 106 (100), 92 (10), 77 (33), 65 (10); 13C NMR (DMSO-d6,
ꢅ, p.p.m.): 151.4 (C4), 147.7 (C10), 133.3 (C1), 130.6 (C40), 112.9 (C3),
129.0, 128.3 and 121.3 (remaining aromatic C atoms); IR (KBr, cm 1):
3447, 3351 (primary amino group), 1632 (deformations of the amino
group), 1472, 1317 (asymmetric and symmetric stretch of the azoxy
group).
Ê
When HÁ Á ÁO distances are greater than 2.0 A they are of
electrostatic character (Desiraju & Steiner, 1999). The present
X-ray diffraction studies con®rm the spectroscopic data. The
molecular packing of the ꢀ and ꢁ isomers is shown in Fig. 2.
The molecules are arranged in chains connected to each other
by weak NÐHÁ Á ÁO [in (I) and (II)] and NÐHÁ Á ÁN [in (II)]
hydrogen bonds.
Compound (I)
Crystal data
C12H11N3O
Mr = 213.24
Monoclinic, P21/n
Ê
a = 5.733 (1) A
Mo Kꢀ radiation
Cell parameters from 28
re¯ections
ꢆ = 8±13ꢀ
1
Ê
b = 11.602 (2) A
ꢇ = 0.089 mm
T = 293 (2) K
Ê
c = 16.038 (3) A
ꢁ = 93.16 (3)ꢀ
V = 1065.1 (3) A
Prism, orange
0.7 Â 0.6 Â 0.2 mm
3
Ê
Z = 4
Dx = 1.330 Mg m
3
Data collection
Kuma KM-4 diffractometer
!/ꢆ scans
h = 6 ! 6
k = 13 ! 0
4164 measured re¯ections
1888 independent re¯ections
1480 re¯ections with I > 2ꢈ(I)
Rint = 0.024
l = 19 ! 19
2 standard re¯ections
every 50 re¯ections
intensity decay: 0.67%
Figure 2
The packing diagram of (a) the ꢀ isomer (I) and (b) the ꢁ isomer (II),
showing the hydrogen-bonding (dashed lines).
ꢆ
max = 25.05ꢀ
ꢁ
Â
Acta Cryst. (2001). C57, 467±470
Andrzej Domanski et al.
Two isomers of C12H11N3O 469

organic compounds
Table 3
Selected geometric parameters (A, ) for (II).
Re®nement
ꢀ
Ê
Re®nement on F2
R[F2 > 2ꢈ(F2)] = 0.037
wR(F2) = 0.101
S = 1.033
1888 re¯ections
190 parameters
All H-atom parameters re®ned
w = 1/[ꢈ2(Fo2) + (0.0736P)2
+ 0.1764P]
N2ÐC10
N3ÐC4
1.421 (3)
1.385 (3)
where P = (Fo2 + 2Fc2)/3
(Á/ꢈ)max < 0.001
O1ÐN1
N1ÐN2
N1ÐC1
1.257 (3)
1.266 (3)
1.450 (3)
3
Ê
Áꢉmax = 0.22 e A
3
Ê
0.25 e A
Áꢉmin
=
O1ÐN1ÐN2
O1ÐN1ÐC1
N2ÐN1ÐC1
N1ÐN2ÐC10
125.6 (2)
116.9 (2)
117.5 (2)
116.8 (2)
C2ÐC1ÐN1
C6ÐC1ÐN1
C20ÐC10ÐN2
C60ÐC10ÐN2
118.5 (2)
120.5 (2)
122.4 (2)
117.1 (2)
Table 1
Selected geometric parameters (A, ) for (I).
ꢀ
Ê
O1ÐN1ÐN2ÐC10
O1ÐN1ÐC1ÐC2
N2ÐN1ÐC1ÐC2
7.7 (3)
4.7 (3)
175.9 (2)
N2ÐN1ÐC1ÐC6
N1ÐN2ÐC10ÐC20
N1ÐN2ÐC10ÐC60
2.1 (3)
54.6 (3)
134.9 (2)
O1ÐN1
N1ÐN2
N1ÐC10
1.268 (2)
1.261 (2)
1.456 (2)
N2ÐC1
N3ÐC4
1.401 (2)
1.369 (2)
N2ÐN1ÐO1
N2ÐN1ÐC10
O1ÐN1ÐC10
N1ÐN2ÐC1
126.4 (1)
116.3 (1)
117.2 (1)
120.9 (1)
C6ÐC1ÐN2
C2ÐC1ÐN2
C60ÐC10ÐN1
C20ÐC10ÐN1
112.7 (1)
129.7 (1)
120.7 (1)
118.2 (1)
Table 4
Hydrogen-bonding geometry (A, ) for (II).
ꢀ
Ê
DÐHÁ Á ÁA
DÐH
HÁ Á ÁA
DÁ Á ÁA
DÐHÁ Á ÁA
N1ÐN2ÐC1ÐC6
N1ÐN2ÐC1ÐC2
171.8 (1)
7.8 (2)
N2ÐN1ÐC10ÐC60
N2ÐN1ÐC10ÐC20
5.2 (2)
174.5 (1)
N3ÐH32Á Á ÁO1ii
0.96 (3)
0.79 (3)
2.09 (3)
2.68 (3)
2.918 (3)
3.432 (2)
143 (3)
161 (3)
N3ÐH31Á Á ÁN3iii
1
2
Symmetry codes: (ii) 1 ‡ x; y; z; (iii) 1 x; y
; z.
Table 2
Hydrogen-bonding geometry (A, ) for (I).
ꢀ
Ê
Ê
Re®ned CÐH distances were in the range 0.93 (2)±0.99 (2) A for
Ê
the ꢀ isomer (I) and in the range 0.87 (2)± 0.98 (3) A for the ꢁ isomer
(II).
DÐHÁ Á ÁA
DÐH
HÁ Á ÁA
DÁ Á ÁA
DÐHÁ Á ÁA
N3ÐH31Á Á ÁO1i
Symmetry code: (i)
0.88 (2)
2.13 (2)
2.999 (2)
171 (2)
For both compounds, data collection: Kuma Diffraction Software
(Kuma, 1998); cell re®nement: Kuma Diffraction Software; data
reduction: Kuma Diffraction Software; program(s) used to solve
structure: SHELXS97 (Sheldrick, 1997); program(s) used to re®ne
structure: SHELXL97 (Sheldrick, 1997); molecular graphics:
SHELXTL (Sheldrick, 1990); software used to prepare material for
publication: SHELXL97.
x; 12 ‡ y; 12 z.
1
2
Compound (II)
Crystal data
3
C12H11N3O
Mr = 213.24
Monoclinic, P21
Dx = 1.333 Mg m
Mo Kꢀ radiation
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: DA1152). Services for accessing these data are
described at the back of the journal.
Cell parameters from 25
re¯ections
Ê
a = 8.764 (2) A
b = 6.103 (1) A
ꢆ = 8±12ꢀ
Ê
1
Ê
c = 10.791 (2) A
ꢇ = 0.089 mm
T = 293 (2) K
ꢁ = 112.97 (3)ꢀ
Ê
V = 531.4 (2) A
Z = 2
References
3
Prism, yellow
0.6 Â 0.4 Â 0.4 mm
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È
Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of Gott-
ingen, Germany.
Data collection
Kuma KM-4 diffractometer
!/ꢆ scans
h = 11 ! 11
k = 7 ! 7
2506 measured re¯ections
1432 independent re¯ections
973 re¯ections with I > 2ꢈ(I)
Rint = 0.018
l = 11 ! 15
2 standard re¯ections
every 50 re¯ections
intensity decay: 0.03%
ꢆmax = 30.05ꢀ
Re®nement
Re®nement on F2
R[F2 > 2ꢈ(F2)] = 0.031
wR(F2) = 0.096
S = 1.044
1432 re¯ections
w = 1/[ꢈ2(Fo2) + (0.0472P)2
+ 0.0451P]
where P = (Fo2 + 2Fc2)/3
(Á/ꢈ)max = 0.081
3
Ê
Áꢉmax = 0.15 e A
3
Ê
0.13 e A
189 parameters
All H-atom parameters re®ned
Áꢉmin
=
Tonogaki, M., Kawata, T., Ohba, S., Iwata, Y. & Shibuya, I. (1993). Acta Cryst.
B49, 1031±1039.
ꢁ
Â
470 Andrzej Domanski et al.
Two isomers of C12H11N3O
Acta Cryst. (2001). C57, 467±470