Conformational polymorphism of (E,E)-N,N′-bis-(4-nitrobenzylidene) benzene-1,4-diamine

Article abstract of DOI:10.1107/S0108270111010109

Two polymorphs of (E,E)-N,N′-bis-(4-nitro-benzyl-idene)benzene-1,4- diamine, C₂₀H₁₄N₄O₄, (I), have been identified. In each case, the mol-ecule lies across a crystallographic inversion centre. The supra-molecula

Full text of DOI:10.1107/S0108270111010109

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
crystallographic inversion centre. Its central ring (B) is twisted
out of the plane defined by the peripheral nitro-substituted
aromatic ring (A) by 56.99 (7)ꢁ (Fig. 1); this is further illu-
strated by the torsion angles presented in Table 1. These
twisted conformations are well known within this class of
compounds and originate from electronic effects rather than
ISSN 0108-2701
Conformational polymorphism of
(E,E)-N,N0-bis(4-nitrobenzylidene)-
benzene-1,4-diamine
Alain Collas,a Matthias Zellerb and Frank Blockhuysa*
aDepartment of Chemistry, University of Antwerp, Universiteitsplein 1, B-2610
Antwerp, Belgium, and bDepartment of Chemistry, Youngstown State University,
One University Plaza, Youngstown, Ohio 44555-3663, USA
from steric hindrance (Collas et al., 2011). The supramolecular
structure of (I-1) is based on extended chains of molecules,
generated by a mutual weak hydrogen bond involving the
relatively acidic aromatic atom H5 and an O atom from the
nitro group in the 4-position of a neighbouring molecule (Fig. 2
and Table 2, entry 1). ꢀ–ꢀ stacks of nitro-substituted rings
extend along the direction of the a axis (Fig. 2 and Table 2,
entries 2 and 3). Within these stacks, an additional weak
hydrogen bond involving atom H10 of the central ring [C10—
H10ꢀ ꢀ ꢀO1(ꢂx, ꢂy + 1, ꢂz + 1)] operates in the same direction
Correspondence e-mail: frank.blockhuys@ua.ac.be
Received 26 January 2011
Accepted 17 March 2011
Online 14 April 2011
Two polymorphs of (E,E)-N,N0-bis(4-nitrobenzylidene)ben-
zene-1,4-diamine, C20H14N4O4, (I), have been identified. In
each case, the molecule lies across a crystallographic inversion
centre. The supramolecular structure of the first polymorph,
(I-1), features stacking based on ꢀ–ꢀ interactions assisted by
weak hydrogen bonds involving the nitro groups. The second
polymorph, (I-2), displays a perpendicular arrangement of
molecules linked via the nitro groups, combined with weak
C—Hꢀ ꢀ ꢀO hydrogen bonds. Both crystal structures are com-
pared with that of the carbon analogue (E,E)-1,4-bis[2-(4-
nitrophenyl)ethenyl]benzene, (II).
Comment
Oligomers of poly(p-phenylenevinylene) (PPV) and its
isoelectronic counterpart poly(1,4-phenylenemethylidyne-
nitrilo-1,4-phenylenenitrilomethylidyne) (PPI) constitute an
interesting class of organic semiconductors, mainly because
these materials display useful opto-electronic properties.
Using oligomers instead of polymers has proved to be a
worthwhile endeavour, as the former are far more straight-
forward to produce, characterize and process than the latter
Figure 1
The molecular structures of the two polymorphs of (I), showing the atom-
numbering schemes [note that, in (II), C8 replaces N2]. Displacement
ellipsoids are drawn at the 50% probability level. H atoms are
represented by spheres of arbitrary radii and they bear the same number
as the C atom to which they are attached.
¨
(Mullen & Wegner, 1998). Also, such low-molecular-weight
compounds can be subtly tailored to enhance the molecular
properties desired for a specific application, by substituting
them with functional groups such as electron donors and/or
acceptors, thereby making optimal use of their potential as
new materials for a variety of applications. However, since
these materials are often used in the solid state, it is of equal
importance to gain insight into their solid-state structures to
correlate their structural characteristics with the experimen-
tally determined optoelectronic properties of interest. In this
paper, we present two conformational polymorphs of (E,E)-
N,N0-bis(4-nitrobenzylidene)benzene-1,4-diamine, (I), a nitro-
substituted PPI oligomer.
Figure 2
The first polymorph, (I-1), crystallizes in the triclinic space
group P1, with one molecule per unit cell lying across a
View of the ꢀ–ꢀ stacking and hydrogen bonding in polymorph (I-1).
[Symmetry codes: (i) ꢂx + 1, ꢂy, ꢂz + 1; (iii) ꢂx, ꢂy + 1, ꢂz + 1.]
Acta Cryst. (2011). C67, o171–o174
doi:10.1107/S0108270111010109
# 2011 International Union of Crystallography o171

organic compounds
Figure 3
View of the C—Hꢀ ꢀ ꢀꢀ contacts in polymorph (I-1). [Symmetry code: (iv)
x, ꢂ1 + y, z.]
Figure 6
View of the relevant interactions in MUBTEJ, (II). [Symmetry codes: (ii)
1
2
1
2
1
2
x, ꢂy + 12, z ꢂ ; (iii) x + 1, ꢂy + , z + .]
Figure 4
Interactions involving the nitro group generating a herringbone pattern
crystallographic inversion centres (Z0 = 0.5). In its structure
one can also easily see that the central ring is twisted out of the
plane of the peripheral ring (Fig. 1), but to a lesser extent than
for (I-1); the angle between the two planes through rings A
and B is 36.29 (8)ꢁ. As can be seen from Table 1, the torsion
angle ꢁ (C11—C9—N2—C7) is the largest contributor to the
twists in both structures. The crystal packing of (I-2) is quite
different from that of (I-1), as contacts between the nitro
groups dominate the supramolecular structure. A T-shaped
contact between the partially positively charged N atom and
the partially negatively charged O atom of the nitro groups
results in a herringbone pattern (Fig. 4 and Table 3, entry 1).
This type of stacking is reinforced by mutual N—Oꢀ ꢀ ꢀꢀ
interactions (Fig. 4 and Table 3, entries 2 and 3). Further
stacking of the molecules is achieved by two additional weak
C—Hꢀ ꢀ ꢀO hydrogen bonds involving atoms H6 and H7 (Fig. 5
and Table 3, entries 4 and 5).
in polymorph (I-2). [Symmetry codes: (i) ꢂx ꢂ 1, y + 12, ꢂz + 21; (ii) x, y + 1,
z.]
(Fig. 2 and Table 2, entry 4). Finally, a mutual weak hydrogen
bond, linking atom H6 to the ꢀ-system of the central ring of
another molecule, holds the stacks together (Fig. 3 and Table 2,
entry 5). It is interesting to note that the most acidic H atom,
H7, is not involved in the crystal packing.
The second polymorph, (I-2), crystallizes in the monoclinic
space group P21/c, with two molecules per unit cell lying across
The distyrylbenzene (DSB) analogue of the title com-
pounds, namely (E,E)-1,4-bis[2-(4-nitrophenyl)ethenyl]ben-
zene, (II), originally reported by Pham (2009) [Cambridge
Structural Database (CSD; Allen, 2002) refcode MUBTEJ], is
quasi-planar, with a dihedral angle of 11.86 (7)ꢁ between the
planes through the peripheral and central rings. The dihedral
angle is only 2.29 (10)ꢁ in the previously reported dimethyl-
formamide (DMF) solvate (Bartholomew et al., 2000; CSD
refcode REGBEK), but as the solvent molecules interfere in
the inter-oligomer contacts we will only consider the crystal
packing of MUBTEJ (Fig. 6). Here, ꢀ–ꢀ stacking is present
but, unlike in the structure of (I-1), it occurs between the two
different rings A and B (Table 4, entry 1), which leads to a
photoreactive structure that may undergo a topochemical
Figure 5
View of the weak hydrogen bonds in polymorph (I-2). [Symmetry codes:
1
(iii) ꢂx, y ꢂ , ꢂz + 12; (iv) x + 1, ꢂ1 + y, z.]
2
ꢃ
o172 Collas et al. Two polymorphs of C20H14N4O4
Acta Cryst. (2011). C67, o171–o174

organic compounds
Table 1
Dihedral angles (ꢁ) in the compounds under discussion.
polymerization. Indeed, the distance between the vinyl
˚
spacers is 3.95 A, which is smaller than the experimentally
˚
determined limit of 4.2 A (Schmidt, 1971). Perpendicular to
(I-1)
12.3 (2)
(I-2)
ꢂ0.5 (3)
MUBTEJ, (II) REGBEK
these photoreactive stacks, molecules are connected through
three weak hydrogen bonds (Table 4, entries 2, 3 and 4), one in
which aromatic atom H5 contacts the ꢀ-system of an adjacent
ring A and two in which one of the O atoms of the nitro group,
O2, acts as an acceptor for aromatic atoms H6 and H7 from
another molecule.
C2—C1—C7—N2/C8
9.5 (2)
179.9 (1)
ꢂ177.6 (1)
ꢂ5.1 (4)
ꢂ178.4 (2)
ꢂ175.8 (2)
C1—C7—N2/C8—C9 ꢂ174.49 (13) ꢂ172.48 (15)
C11—C9—N2/C8—C7
43.7 (2)
35.2 (2)
Table 2
Short contacts in the crystal packing of polymorph (I-1) (A, ).
ꢁ
˚
Thus, the three structures show completely different
packing schemes. While in the structure of polymorph (I-1)
the central ring B acts as an acceptor for aromatic atom H6
and as a hydrogen-bond donor to a nitro group [C10—
H10ꢀ ꢀ ꢀO1(ꢂx, ꢂy + 1, ꢂz + 1), Table 2, entry 4], this ring does
not take part in the crystal packing of (I-2). Besides the fact
that (I-2) and (II) share the same space group, they also share
two active sites on their carbon backbone; in both structures,
atoms H6 and H7 are engaged in weak hydrogen bonds with
the O atoms of the nitro groups of neighbouring molecules.
However, in the latter only one O atom is available, while in
the former both O atoms are used as acceptor sites. The
presence of the N atom in (I), which results in a twist of the
central ring, precludes the possibility of closer stacking and, as
a result, a photosensitive supramolecular structure based on
ꢀ–ꢀ interactions, as seen in (II), cannot be formed.
The angle related to a pair of centroids is defined as the angle between the
CgIꢀ ꢀ ꢀCgJ vector and the normal to plane I.
Entry
D
X
A
Xꢀ ꢀ ꢀA
D—Xꢀ ꢀ ꢀA
1
2
3
4
5
C5
H5
O1i
2.55
153
CgA
CgA
C10
C6
CgAii
CgAiii
O1iii
3.5212 (9)
3.5981 (9)
2.52
15.64
24.53
155
H10
H6
CgBiv
2.83
129
Symmetry codes: (i) ꢂx + 1, ꢂy, ꢂz + 1; (ii) ꢂx + 1, ꢂy + 1, ꢂz + 1; (iii) ꢂx, ꢂy + 1, ꢂz + 1;
(iv) x, y ꢂ 1, z.
Table 3
Short contacts in the crystal packing of polymorph (I-2) (A, ).
ꢁ
˚
Entry
D
X
A
Xꢀ ꢀ ꢀA
D—Xꢀ ꢀ ꢀA
1
2
3
4
5
N1
N1
N1
C6
C7
O1
O1
O2
H6
H7
N1i
2.982 (2)
3.5301 (17)
3.9359 (18)
2.66
121.20 (11)
86.14 (10)
67.91 (9)
135
CgAii
CgAii
O1iii
O2iv
Experimental
2.54
152
1
2
Benzene-1,2-diamine (2.6 g, 25 mmol) and 4-nitrobenzaldehyde
(3.8 g, 50 mmol) were dissolved in ethanol (200 ml) and the resulting
solution was boiled under reflux for 2 h. The resulting precipitate was
filtered off, yielding a yellow–orange powder, part of which was
recrystallized from acetonitrile to produce needles of (I-1) with a
golden lustre. Slow evaporation of a CHCl3 solution yielded poly-
morph (I-2) as orange plates [m.p. (uncorrected) 512 and 502 K for
(I-1) and (I-2), respectively]. UV–visible (CH2Cl2): ꢂmax = 391 nm
(log " = 4.45); 1H NMR (CDCl3, 400 MHz, TMS): ꢃ 7.36 (s, 4H, H10
and H11), 8.10 (d, 4H, 3J = 8.8 Hz, H2 and H6), 8.35 (d, 3J = 8.8 Hz,
H3 and H5), 8.62 (s, 2H, H7); 13C NMR (CDCl3, 100 MHz, TMS):
ꢃ 122.20 (C3 and C5), 124.09 (C10 and C11), 129.46 (C2 and C6),
141.53 (C1), 149.76 (C9), 149.76 (C4), 157.03 (C7).
Symmetry codes: (i) ꢂx ꢂ 1, y + 12, ꢂz + 21; (ii) x, y + 1, z; (iii) ꢂx, y ꢂ , ꢂz + 12; (iv) x + 1,
y ꢂ 1, z.
Refinement
R[F2 > 2ꢈ(F2)] = 0.042
wR(F2) = 0.106
S = 1.08
127 parameters
H-atom parameteꢂrs3 constrained
˚
Áꢉmax = 0.20 e A
ꢂ3
˚
1731 reflections
Áꢉmin = ꢂ0.22 e A
Polymorph (I-2)
Crystal data
3
˚
V = 860.4 (2) A
Z = 2
C20H14N4O4
Mr = 374.35
Polymorph (I-1)
Monoclinic, P21=c
Mo Kꢄ radiation
ꢇ = 0.10 mmꢂ1
T = 293 K
0.55 ꢄ 0.37 ꢄ 0.07 mm
˚
a = 6.567 (1) A
Crystal data
˚
b = 5.0227 (7) A
˚
C20H14N4O4
Mr = 374.35
Triclinic, P1
ꢆ = 88.071 (1)ꢁ
3
c = 26.723 (5) A
ꢅ = 102.558 (4)ꢁ
˚
V = 427.20 (7) A
Z = 1
˚
˚
a = 6.9357 (7) A
Mo Kꢄ radiation
ꢇ = 0.11 mmꢂ1
T = 173 K
0.29 ꢄ 0.22 ꢄ 0.1 mm
Data collection
b = 7.3036 (7) A
Bruker SMART APEX CCD
diffractometer
Absorption correction: multi-scan
(APEX2; Bruker, 2008)
Tmin = 0.955, Tmax = 0.993
4451 measured reflections
1753 independent reflections
1181 reflections with I > 2ꢈ(I)
Rint = 0.026
˚
c = 8.8768 (8) A
ꢄ = 73.295 (1)ꢁ
ꢅ = 82.707 (1)ꢁ
Data collection
Refinement
Bruker SMART APEX CCD
diffractometer
Absorption correction: multi-scan
(APEX2; Bruker, 2008)
Tmin = 0.973, Tmax = 0.991
3748 measured reflections
1731 independent reflections
1496 reflections with I > 2ꢈ(I)
Rint = 0.018
R[F2 > 2ꢈ(F2)] = 0.040
wR(F2) = 0.106
S = 1.02
127 parameters
H-atom parameteꢂrs3 constrained
˚
Áꢉmax = 0.12 e A
Áꢉmin = ꢂ0.18 e A
ꢂ3
˚
1753 reflections
ꢃ
Acta Cryst. (2011). C67, o171–o174
Collas et al.
Two polymorphs of C20H14N4O4 o173

organic compounds
Table 4
Short contacts in the crystal packing of (II) (A, ).
a predoctoral grant. The X-ray diffractometer was funded by
NSF grant No. 0087210, Ohio Board of Regents grant No.
CAP-491 and Youngstown State University. Financial support
by the University of Antwerp under grant No. GOA-2404 is
gratefully acknowledged.
ꢁ
˚
The angle related to a pair of centroids is defined as the angle between the
CgIꢀ ꢀ ꢀCgJ vector and the normal to plane I.
Entry
D
X
A
Xꢀ ꢀ ꢀA
D—Xꢀ ꢀ ꢀA
1
2
3
4
CgA
C5
C6
C7
CgBi
CgAii
O1iii
O1iii
3.8933 (11)
2.764 (15)
2.659 (15)
2.419 (15)
29.52
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: BM3102). Services for accessing these data are
described at the back of the journal.
H5
H6
H7
160.2 (13)
146.0 (11)
161.8 (12)
1
2
Symmetry codes: (i) x ꢂ 1, y, z ꢂ 1; (ii) x, ꢂy + 12, z ꢂ 21; (iii) x + 1, ꢂy + 21, z +
.
References
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˚
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ꢃ
o174 Collas et al. Two polymorphs of C20H14N4O4
Acta Cryst. (2011). C67, o171–o174