Crystal structures of two substituted 2,5-diaryl-1,3,4-oxadiazoles

Article abstract of DOI:10.1016/S0022-2860(00)00921-2

The crystal structures of 2,5-di(4-pyridyl)-1,3,4-oxadiazole, I, and 2-(4-cyanophenyl)-5-(4-dimethylaminophenyl)-1,3,4-oxadiazole, II, were determined from single crystal data (I: monoclinic, C2/c (no. 15), a = 5.3129(9) ?, b = 12.142(3) ?, c = 16.771(3) ?, β = 93.41(2)°, R_all = 0.0774; II: triclinic, P1? (no. 2), a = 7.3908(12) ?, b = 8.7992(15) ?, c = 12.036(2) ?, α = 77.04(2)°, β = 89.29(2)°, γ = 87.58(2)°, R_all = 0.0552). In both structures the formation of stacks of the almost planar molecules is observed as known for other oxadiazoles. These compounds exhibit differences to other substituted 2,5-diaryl-1,3,4-oxadiazoles in the crystalline structure.

Full text of DOI:10.1016/S0022-2860(00)00921-2

Journal of Molecular Structure 561 (2001) 175±183
www.elsevier.nl/locate/molstruc
Crystal structures of two substituted 2,5-diaryl-1,3,4-oxadiazoles
a,
S. Stockhause , M.S. Wickleder , G. Meyer , I. Orgzall , B. Schulz
b
b
c
c
*
aInstitut Charles Sadron (CNRS-ULP), 6, rue Boussingault, 67083 Strasbourg, France
b
È
Institute of Inorganic Chemistry, University at Cologne, Greinstr. 6, 50939 Koln, Germany
cResearch Center Thin Organic and Biochemical Films, University of Potsdam, Am Neuen Palais 10, 14669 Potsdam, Germany
Received 1 September 2000; revised 10 November 2000; accepted 10 November 2000
Abstract
The crystal structures of 2,5-di(4-pyridyl)-1,3,4-oxadiazole, I, and 2-(4-cyanophenyl)-5-(4-dimethylaminophenyl)-1,3,4-
ꢀ
ꢀ
oxadiazole, II, were determined from single crystal data (I: monoclinic, C2/c (no. 15), a ˆ 5:3129ꢀ9† A; b ˆ 12:142ꢀ3† A; c ˆ
ꢀ
ꢀ
ꢀ
ꢀ
16:771ꢀ3† A; b ˆ 93:41ꢀ2†8; Rall ˆ 0:0774; II: triclinic, P1 (no. 2), a ˆ 7:3908ꢀ12† A; b ˆ 8:7992ꢀ15† A; c ˆ 12:036ꢀ2† A; a ˆ
77:04ꢀ2†8; b ˆ 89:29ꢀ2†8; g ˆ 87:58ꢀ2†8; Rall ˆ 0:0552†. In both structures the formation of stacks of the almost planar
molecules is observed as known for other oxadiazoles. These compounds exhibit differences to other substituted 2,5-diaryl-
1,3,4-oxadiazoles in the crystalline structure. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: 1,3,4-oxadiazoles; Crystal structure; X-ray diffraction
1. Introduction
These physical and physicochemical properties can
be tailored by the electronic and three-dimensional
structure of the molecules as well as, in the solid
state, by the crystalline structure. Systematic study
of the crystal structures of differently substituted
2,5-diaryl-1,3,4-oxadiazoles should make it possible
to understand the formation of certain structure
elements desired for speci®c applications. For
example, the design of oxadiazoles with a non-centro-
symmetric structure is of great interest for optical
applications. Besides, the knowledge of the bulk
structure will be helpful to understand the formation
of highly ordered thin layers, which are needed for
microelectronic and micro-optoelectronic devices.
Nevertheless, little is known about the crystalline
structures of aromatic 1,3,4-oxadiazoles. The few
known structures [3±7] show mainly monoclinic or
orthorhombic symmetry and consist of stacks of mole-
cules with one or two orientations of these molecules
within the stacks, but with a different arrangement of
the stacks relative to each other.
Aromatic 1,3,4-oxadiazoles have attracted much
interest during the past 30 years because of their use
as laser dyes, photographic materials or scintillators
[1]. Besides, they show a combination of interesting
properties, which makes them suitable for the devel-
opment of new electrical and electro-optical devices:
The electronic spectrum of the 1,3,4-oxadiazole
system is equivalent to that of benzene and the
maxima are only slightly hypsochromically shifted.
Substituted 2,5-diaryl-1,3,4-oxadiazoles show strong
¯uorescence in solution on stimulation by UV or b-
irradiation, and some of them are electroluminescent
with radiation of blue light [2]. Additionally, they
have extremely high thermal and oxidative stability.
* Corresponding author. Institut fur Physik, Universitat Potsdam,
Am Neuen Palais 10, 14469 Potsdam, Germany. Tel.: 149-331-
977-1046; fax: 149-331-977-1083.
E-mail address: stockhau@rz.uni-potsdam.de (S. Stockhause).
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(00)00921-2

176
S. Stockhause et al. / Journal of Molecular Structure 561 (2001) 175±183
Table 1
Data collection parameters and crystal data for 2,5-di(4-pyridyl)-1,3,4-oxadiazole (I) and 2-(4-cyano-phenyl)-5(4-dimethylamino-phenyl)-
1,3,4-oxadiazole (II)
Compound
I
II
Ê
Lattice parameters (A, 8)
a ˆ 5:3129ꢀ9†
b ˆ 12:142ꢀ3†
c ˆ 16:771ꢀ3†
a ˆ 7:3908ꢀ12†
b ˆ 8:7992ꢀ15†
c ˆ 12:036ꢀ2†
a ˆ 77:04ꢀ2†
b ˆ 89:29ꢀ2†
g ˆ 87:58ꢀ2†
762.1(2)
b ˆ 93:41ꢀ2†
Molar volume (cm3/mol)
Number of formula units, Z
Crystal system
Space group
Diffractometer
Radiation
1080.0(4)
4
4
Triclinic
Monoclinic
C2/c (no. 15)
Stoe IPDS
Mo±Ka (graphite
monochrom., l ˆ
71:07 pm†
293
3.8 , 2u , 56.3
26 # h # 6
216 # k # 16
222 # l # 22
0 , w , 250; Dw ˆ 28
125
P1 (no. 2)
Stoe IPDS
Mo±Ka (graphite
monochrom., l ˆ
71:07 pm†
293
3.8 , 2u , 56.3
29 # h # 9
210 # k # 10
215 # l # 15
0 , w , 250; Dw ˆ 28
125
Temperature (K)
Data range (8)
Index range
Rotation angle (8); w-increment
Number of images
Exposure time (min)
Detector distance (mm)
Data corrections
6
60
4
60
Polarization/Lorentz
None
69.0
Polarization/Lorentz
None
126.5
Absorption corrections
m (cm21
)
Number of collected re¯ections
Number of unique re¯ections
Number of re¯ections with
I0 . 2s (I)
4761
899
545
6121
2251
1565
Rint
0.1346
0.0908
Structure determination
Scattering factors
Goodness of ®t
shelxs86 and shelxs93 [9]
Intern. tables, vol. C
1.062
shelxs86 and shelxs93 [9]
Intern. tables, vol. C
1.068
R1; wR2 (I0 . 2s(I))
R1; wR2 (all data)
0.0431; 0.0912
0.0774; 0.1039
0.0375; 0.0939
0.0552; 0.1015
In this paper, we present the structures of two
substituted 2,5-diaryl-1,3,4-oxadiazoles, compare
their structural features to those of similarly substi-
tuted oxadiazoles, and discuss the existing similarities
and differences.
phoric acid. After precipitation in water the product
was recrystallized from water (melting point: 1188C).
2-(4-cyanophenyl)-5-(4-dimethylaminophenyl)-
1,3,4-oxadiazole (II) was synthesized via precursor
route as described in [8]. The product was recrystal-
lized from toluene (melting point: 2388C).
2. Experimental
2.2. Structure analysis
From both compounds single crystals were trans-
ferred into glass capillaries (d ˆ 0.2±0.3 mm). The
scattering intensities of a suitable crystal were
collected with an imaging plate diffractometer. Struc-
ture solution and re®nement were carried out using the
2.1. Preparation
2,5-di(4-pyridyl)-1,3,4-oxadiazole (I) was prepared
by one-step condensation reaction of 4-pyridine-
carboxylic acid and hydrazine hydrate in polyphos-

S. Stockhause et al. / Journal of Molecular Structure 561 (2001) 175±183
177
Table 2
Positional
parameters
and
equivalent
isotropic
displacement
parameters
for
2,5-di(4-pyridyl)-1,3,4-oxadiazole
(I)
(Ueq ˆ 1=3‰U22 1 1=sin2 bꢀU11 1 U33 1 2U13 cos b†Š [10].)
Atom
Site
x/a
y/b
z/c
Ueq £ 1021/pm2
O1
N1
N2
C1
C2
C3
H3
C4
H4
C5
H5
C6
H6
4e
8f
8f
8f
8f
8f
8f
8f
8f
8f
8f
8f
8f
0.0000
0.55218(14)
0.37965(15)
0.6019(2)
0.4818(2)
0.5255(2)
0.4524(2)
0.3729(15)
0.4947(2)
0.4444(20)
0.6704(2)
0.7533(21)
0.6362(2)
0.6912(18)
0.2500
45.3(6)
54.2(6)
63.7(6)
43.7(6)
44.2(6)
50.9(6)
44(5)a
20.1068(4)
20.7615(4)
20.1610(4)
20.3664(4)
20.5365(4)
20.5147(39)
20.7287(5)
20.8481(53)
20.5974(6)
20.6187(43)
20.3991(5)
20.2772(47)
0.22300(10)
0.05390(11)
0.20903(12)
0.15617(11)
0.11845(13)
0.1275(12)
0.06821(14)
0.0430(15)
0.0903(2)
57.9(7)
72(8)a
65.0(8)
68(7)a
0.0786(14)
0.14236(14)
0.1664(15)
57.0(7)
64(7)a
a
Isotropic displacement parameters.
programs shelxs86 and shelxl93 [9]. The data
collection parameters and determined crystallo-
graphic data are summarized in Tables 1±5. The H-
atoms were located in successive difference Fourier
syntheses.
3. Results and discussion
3.1. 2,5-Di(4-pyridyl)-1,3,4-oxadiazole
In the crystal, the molecules of the 2,5-di(4-
pyridyl)-1,3,4-oxadiazole, I, Fig. 1, have an almost
planar structure. All three rings of the molecule are
planar but they show a slight torsion relative to each
other. The deviation of the planes of the two pyridyl
rings relative to the plane of the oxadiazole ring is
13.38 for one ring and 23.38 for the other. Due to
the absence of signi®cant steric hindrance the torsion
angle between two neighboring rings is small, hence
partial conjugation is present.
Table 3
Selected distances (A) and angles (8) for 2,5-di(4-pyridyl)-1,3,4-
oxadiazole (I)
Ê
Bond
Distance
Bond
Angles
O1±C1
N1±N1
N1±C1
C1±C2
C2±C3
C3±C4
C4±N2
N2±C5
C5±C6
C6±C2
C3±H3
C4±H4
C5±H5
C6±H6
1.365(2)
1.409(3)
1.292(3)
1.464(3)
1.391(3)
1.383(3)
1.334(3)
1.327(3)
1.391(3)
1.373(3)
0.98(2)
C1±O1±C1
C1±N1±N1
N1±C1±O1
N1±C1±C2
O1±C1±C2
C6±C2±C3
C6±C2±C1
C1±C2±C3
C2±C3±C4
C4±C3±H3
C2±C3±H3
C3±C4±N2
N2±C4±H4
C3±C4±H4
C4±N2±C5
N2±C5±C6
N2±C5±H5
C6±C5±H5
C5±C6±C2
C2±C6±H6
C5±C6±H6
102.5(2)
106.2(1)
112.5(2)
127.5(2)
120.0(2)
118.4(2)
122.6(2)
119.0(2)
118.5(2)
122.3(2)
119.2(2)
123.7(2)
117.7(2)
118.6(2)
117.0(2)
123.7(2)
117.6(1)
118.8(1)
118.8(2)
121.0(1)
120.2(1)
In the three-dimensional structure the molecules of
I form stacks, Fig. 3(a). The shortest contact between
Ê
the stacks in c-direction is 3.75 A so that no intermo-
lecular interactions such as hydrogen bridges/bonds
can exist, even though they could be expected for
this compound. The axis of the stacks is parallel to
the a-axis of the unit cell and the molecules are
inclined to this axis by 508. The planes of the mole-
cules are all parallel as illustrated in Fig. 2(a), and the
molecules are all oriented in the same direction within
one stack regarding the oxygen atom of the oxadiazole
ring. The shortest interplanar distance within the
0.96(3)
1.03(2)
1.00(2)
Ê
stacks is 3.80 A, and the oxadiazole ring of each mole-
cule is situated between two pyridyl rings of neigh-
boring molecules so that p±p interactions exist which

178
S. Stockhause et al. / Journal of Molecular Structure 561 (2001) 175±183
Table 4
Positional parameters and equivalent isotropic displacement parameters for 2-(4-cyano-phenyl)-5(4-dimethylamino-phenyl)-1,3,4-oxadiazole
(II) (Ueq ˆ 1=3‰U11ꢀaap†2 1 U22ꢀbbp†2 1 U33ꢀccp†2 1 2U12abapbp cos g 1 2U13acapcp cos b 1 2U23bcbpcp cos aŠ [10].)
Atom
Site
x/a
y/b
z/c
Ueq £ 1021/pm2
O1
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
2i
20.05248(15)
0.1358(2)
0.32412(13)
0.45194(8)
0.31602(12)
0.39257(12)
0.1024(2)
70.5(3)
88.4(5)
88.5(5)
119.1(7)
103.0(6)
69.7(5)
69.8(5)
67.9(4)
75.3(5)
95(5)a
N1
0.4318(2)
0.3116(2)
N2
0.2310(2)
N3
N4
20.7083(3)
0.2572(3)
0.9386(3)
20.2527(2)
0.2519(2)
0.84454(14)
0.47015(13)
0.35348(13)
0.30378(13)
0.21019(14)
0.1814(15)
0.15873(15)
0.0942(15)
0.20060(14)
0.2947(2)
C1
0.1155(2)
C2
C3
20.0285(2)
20.1774(2)
20.1409(2)
20.0110(25)
20.2764(3)
20.2522(23)
20.4529(2)
20.4902(3)
20.6144(27)
20.3536(2)
20.3821(25)
0.1471(2)
0.4358(2)
0.5408(2)
C4
0.6627(2)
H4
0.6746(21)
0.7637(2)
C5
H5
78.8(5)
94(6)a
0.8517(22)
0.7467(2)
C6
75.3(5)
84.4(5)
109(6)a
79.2(5)
103(6)a
68.4(4)
75.7(5)
95(5)a
C7
0.6271(2)
H7
C8
0.6161(23)
0.5250(2)
0.3217(16)
0.3453(2)
H8
0.4358(25)
0.1223(2)
0.0562(2)
0.4094(16)
0.56758(13)
0.58742(14)
0.5394(15)
0.67738(15)
0.6853(14)
0.75359(14)
0.7331(2)
C9
C10
H10
C11
H11
C12
C13
H13
C14
H14
C15
C16
H161
H162
H163
C17
H171
H172
H173
0.3202(3)
0.4203(25)
0.3581(3)
0.0996(22)
20.0679(2)
20.1076(21)
20.1304(2)
20.0639(2)
20.1016(23)
0.0599(2)
81.0(5)
85(6)a
0.4770(24)
0.2208(3)
80.3(5)
82.4(5)
101(6)a
77.1(5)
86(5)a
0.0459(3)
20.0533(27)
0.0081(3)
0.7847(16)
0.64169(15)
0.6264(14)
0.1473(2)
20.1194(25)
20.5950(3)
0.4409(5)
0.1083(21)
0.8524(3)
91.8(6)
121.8(9)
164(12)a
164(11)a
197(16)a
158(2)
158(10)a
180(13)a
19(4)a
20.3098(4)
20.2354(38)
20.3985(37)
20.3514(42)
20.3069(9)
20.3967(36)
20.2222(38)
20.3162(33)
0.8718(3)
0.5179(39)
0.4356(37)
0.5062(46)
0.0993(9)
0.8967(23)
0.9328(25)
0.8141(30)
0.9255(5)
0.1995(38)
0.0582(45)
0.1530(25)
0.9828(24)
0.9709(26)
0.9237(22)
a
Isotropic displacement parameters.
ꢀ
ꢀ
are perpendicular to the planes of the molecules. In
this view every second molecule belongs to the same
stack, the molecules seen directly next to each other
are situated at a different height regarding the b-axis
and therefore they belong to different stacks.
The structure of a closely related oxadiazole is
already known: In the 2,5-diphenyl-1,3,4-oxadiazole,
DPO, the terminal nitrogen atoms of the pyridyl
substituents are replaced by CH-groups. This
compound crystallizes in the monoclinic space
group P21/c with a ˆ 5:203ꢀ2† A; b ˆ 18:143ꢀ6† A;
ꢀ
c ˆ 12:184ꢀ4† A: The two structures are compared in
Figs. 2 and 3.
The view along the a-axis (Fig. 3) shows the
arrangement of the stacks in the bc-plane. Within
the stacks the molecules are oriented in the same
direction for both structures. But for compound I the
stacks are related by translation along the b-axis. In
contrast to this highly periodic arrangement in the
DPO the stacks are shifted relative to each other so

S. Stockhause et al. / Journal of Molecular Structure 561 (2001) 175±183
179
Table 5
Ê
Selected distances (A) and angles (8) for 2-(4-cyanophenyl)-5-(4-
dimethylaminophenyl)-1,3,4-oxadiazole (II)
Bond distance
Angles
Bond distance
Angles
O1±C1
1.368(2)
1.374(2)
1.292(2)
1.407(2)
1.294(2)
1.451(2)
1.403(2)
1.367(3)
1.395(2)
1.396(3)
1.375(3)
1.390(2)
1.02(2)
C1±O1±C2
102.82(12)
106.97(13)
106.13(14)
112.36(14)
127.4(2)
120.20(13)
111.7(2)
127.35(15)
120.75(13)
119.1(2)
122.6(2)
118.28(14)
120.9(2)
120.6(1)
118.5(1)
119.7(2)
121.8(1)
118.5(1)
119.9(2)
119.8(2)
120.2(2)
178.8(2)
120.1(2)
121.3(2)
118.6(1)
120.4(1)
119.9(2)
119.7(2)
118.4(2)
119.13(14)
122.4(2)
121.8(2)
117.8(2)
120.3(2)
120.5(2)
117.4(2)
122.1(2)
121.3(2)
121.1(2)
117.6(2)
121.7(2)
117.7(2)
120.6(2)
120.0(2)
121.8(2)
118.2(2)
120.8(2)
116.8(3)
121.7(3)
O1±C2
C2±N1±N2
C1±N2±N1
N2±C1±O1
N2±C1±C9
N1±C2
N1±N2
N2±C1
C2±C3
O1±C1±C9
C3±C4
C4±C5
N1±C2±O1
N1±C2±C3
Fig. 1. Molecular structure of 2,5-di(4-pyridyl)-1,3,4-oxadiazole
(I).
C5±C6
O1±C2±C3
C6±C7
C8±C3±C4
C7±C8
C8±C3
C8±C3±C2
C4±C3±C2
that the oxadiazole rings are not aligned along the
vertical axis. Along the horizontal axis pairs of mole-
cules related by a center of inversion are formed in
both structures.
C4±H4
C5±C4±C3
C5±H5
0.99(2)
C5±C4±H4
C7±H7
C8±H8
0.97(2)
1.00(2)
C3±C4±H4
C4±C5±C6
This slight shift of the stacks can also be seen in
Fig. 2 where for the DPO the layers of stacks overlap
slightly which is not the case for I. The most obvious
difference between these two structures is the orienta-
tion of neighboring stacks relative to each other: For
compound I neighboring stacks are oriented in the
same direction whereas in the DPO neighboring
stacks show two different orientations (^508 relative
to the axis of the stacks). However, the single orienta-
tion of the molecules is preferable for optical applica-
tions as it might lead to polarized emission.
In both structures common motifs of packing are
found such as the parallel arrangement of the mole-
cules in the stacks, which allows p±p interactions
between the oxadiazole ring of one molecule and
the phenyl groups of the neighboring molecules.
Despite the small molecular differences the two
compounds show signi®cant differences in their
three-dimensional structure and in the orientation of
their molecules in the unit cell.
One possible explanation for this difference could
involve the formation of hydrogen bonds/bridges in
the pyridyl structure with the terminal nitrogen but
this is not observed in this structure even though
there should not be any steric hindrance. Conse-
quently, the only molecular interaction in these
solid state structures is the p±p interaction for
which the same motif is observed so that this is not
explaining the differences between the two crystal
structures.
C6±C15
C15±N3
C1±C9
1.431(3)
1.158(3)
1.454(2)
1.384(3)
1.376(3)
1.404(3)
1.397(3)
1.386(3)
1.399(2)
0.97(2)
C4±C5±H5
C6±C5±H5
C5±C6±C7
C9±C10
C10±C11
C11±C12
C12±C13
C13±C14
C14±C9
C10±H10
C11±H11
C13±H13
C14±H14
C12±N4
N4±C16
N4±C17
C16±H161
C16±H162
C16±H163
C17±H171
C17±H172
C17±H173
C5±C6±C15
C7±C6±C15
N3±C15±C6
C8±C7±C6
C8±C7±H7
C6±C7±H7
C7±C8±C3
C7±C8±H8
0.93(2)
0.98(2)
C3±C8±H8
1.02(2)
C10±C9±C14
C10±C9±C1
C14±C9±C1
C11±C10±C9
C11±C10±H10
C9±C10±H10
C10±C11±C12
C10±C11±H11
C12±C11±H11
N4±C12±C13
N4±C12±C11
C13±C12±C11
C14±C13±C12
C14±C13±H13
C12±C13±H13
C13±C14±C9
C13±C14±H14
C9±C14±H14
C12±N4±C16
C12±N4±C17
C16±N4±C17
1.373(3)
1.443(4)
1.532(6)
0.98(3)
0.95(3)
0.97(4)
1.17(3)
1.05(3)
0.40(2)

180
S. Stockhause et al. / Journal of Molecular Structure 561 (2001) 175±183
Fig. 2. Perspective view of the crystal structures along the axis of the stacks of: (a) 2,5-di(4-pyridyl)-1,3,4-oxadiazole (I); (b) 2,5-diphenyl-
1,3,4-oxadiazole (DPO)
Fig. 3. Crystal structures of: (a) 2,5-di(4-pyridyl)-1,3,4-oxadiazole (I); (b) 2,5-diphenyl-1,3,4-oxadiazole (DPO) illustrating the arrangement of
the stacks relative to each other.

S. Stockhause et al. / Journal of Molecular Structure 561 (2001) 175±183
181
Fig. 4. Molecular structure of 2-(4-cyano-phenyl)-5(4-dimethylamino-phenyl)-1,3,4-oxadiazole (II).
3.2. 2-(4-Cyanophenyl)-5-(4-dimethylaminophenyl)-
1,3,4-oxadiazole
by a benzyl group so that the molecules have an angle
of 1178.
In the crystal, the molecules of 2-(4-cyanophenyl)-
5-(4-dimethylaminophenyl)-1,3,4-oxadiazole, II, Fig.
4, have an almost planar structure. The planarity of the
three single rings is nearly perfect but again the rings
show a slight torsion relative to each other. The rota-
tion of the inter-ring bond between the oxadiazole ring
and the benzonitrile is 16.58 and between the oxadia-
zole ring and the dimethylaniline is 14.28. In this
molecule the two substituents are tilted in the same
direction relative to the oxadiazole ring whereas in I
the rings are tilted in opposite directions.
This compound is the ®rst 2,5-diphenyl-1,3,4-
oxadiazole that crystallizes in the triclinic crystal
system. The only other triclinic oxadiazole known
so far is the 2-(4-methoxybenzyl)-5-phenyl-1,3,4-
oxadiazole [11] where the conjugation is interrupted
In the three-dimensional structure, Fig. 5, the mole-
cules of II form stacks, too. The axis of these stacks is
the b-axis and the molecules in the stacks are inclined
by 508 to this axis. Neighboring stacks are oriented in
the same direction like already observed for
compound I. The shortest distance between the stacks
Ê
is around 3.5 A which is too long for any hydrogen
bonds/bridges between these layers. Within the stacks
pairs of molecules are formed in which the two mole-
cules are related by a center of inversion so that the
molecules show alternately opposite orientations
regarding the oxygen atoms of their oxadiazole
rings. The planes of the oxadiazole rings of the mole-
Ê
cules are separated by 3.60 A. These pairs of mole-
Ê
cules also show a distance of 3.54 A to the next
oxadiazole molecule so that within the stacks the
molecules are almost equidistant. Between the pairs
the shortest contact exists between the oxadiazole ring
of one molecule and the dimethylaniline ring of the
other so that the p±p interaction here is not exactly
perpendicular to the planes of the rings. The slight
torsion of the rings allows a better p±p interaction.
Accordingly, the interaction of the molecules in the
Fig. 5. Unit cell of 2-(4-cyanophenyl)-5-(4-dimethylaminophenyl)-
1,3,4-oxadiazole (II).

182
S. Stockhause et al. / Journal of Molecular Structure 561 (2001) 175±183
Fig. 6. Unit cell of 2-(4-amino-phenyl)-5(4-dimethylamino-phenyl)-1,3,4-oxadiazole (III).
stacks is different for II compared to that observed in I
or DPO.
This complex structure does not show the motifs of
packing (like the formation of stacks) that are
observed for the solid state structures of oxadiazoles
so far. The small change in the molecular structure did
not introduce any additional interactions between the
molecules but, nevertheless, leads to a complete
change in the crystalline structure.
If the CN-moiety of II is replaced by a NH2-group
we get to the 2-(4-amino-phenyl)-5(4-dimethylamino-
phenyl)-1,3,4-oxadiazole, III, for which the structure
is also known. We observe that just a very small
change in the molecular substitution leads to a
completely different structure with other motifs of
packing. Compound III crystallizes in the monoclinic
ꢀ
space group P21/n with a ˆ 11:4585 A; b ˆ
4. Conclusions
ꢀ
ꢀ
16:9604 A; c ˆ 16:0758 A and b ˆ 109:498 [12].
The structure is much more complex, Fig. 6 consists
of oxadiazole molecules with four different orienta-
tions. The formation of stacks of molecules like seen
in the other structures is not observed. The complex
packing of the molecules in this structure increases the
The 2,5-diaryl-1,3,4-oxadiazole structures deter-
mined so far consist all of almost planar molecules
in which the planar rings show only slight torsion
relative to each other. In nearly all structures these
molecules form stacks. Within these stacks the planes
of the molecules are parallel to each other and sepa-
Ê
distance between the aromatic rings to about 4 A so
Ê
rated by approximately 3.5 A, and two possible
arrangements occur: Regarding the oxygen atom of
the oxadiazole ring the molecules can be oriented all
that the p±p interactions are rather weak. The
shortest distance between molecules in this structure
Ê
is 2.5 A between the amino groups.

S. Stockhause et al. / Journal of Molecular Structure 561 (2001) 175±183
183
in the same direction (I) or alternating in opposite
directions (II). An arrangement observed for different
oxadiazoles is the shift of the molecules relative to
each other in the way that the oxadiazole ring of
one molecule is sandwiched between two phenyl
rings of neighboring molecules to allow good p±p
interaction perpendicular to the planes of the mole-
cules. However, in II a different arrangement of the
molecules within the stacks is found where two
oxadiazole rings are situated next to each other.
Generally, the planes of the molecules are inclined
to the axis of the stacks by around 508. Between
neighboring stacks two different relations are
observed: Neighboring stacks are oriented in the
same direction (I, II) or in opposite directions
(DPO) so that the neighboring stack is inclined to
the axis of the stacks by 2508.
in¯uences must be responsible for the formation of a
certain structure and a de®nite type of packing.
Acknowledgements
We thank the European Commission under contract
number FMRX-CT97-0106 (TMR-EUROLED) for
®nancial support and for introducing the collabora-
tion.
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