Structural studies on trifluoromethyl substituted 2,5-diphenyl-1,3,4-oxadiazoles

Article abstract of DOI:10.1016/j.molstruc.2006.08.013

Three new compounds have been synthesized based on the molecular motif 2-[2,6-bis(trifluoromethyl)phenyl]-5-phenyl-1,3,4-oxadiazole, with subsequent CF₃-substitution in the ortho-positions of the phenylene ring. The crystal structures of the co

Full text of DOI:10.1016/j.molstruc.2006.08.013

Journal of Molecular Structure 832 (2007) 124–131
www.elsevier.com/locate/molstruc
Structural studies on triXuoromethyl substituted
2,5-diphenyl-1,3,4-oxadiazoles
Franziska Emmerling a,¤, Ingo Orgzall b, Birgit Dietzel b, Burkhard W. Schulz a,
Günter Reck a, Burkhard Schulz c
a Bundesanstalt für Materialforschung und-prüfung, Richard-Willstätter-Strasse 11, D-12489 Berlin, Germany
b Institut für Dünnschichttechnologie und Mikrosensorik e.V., Kantstrasse 55, D-14513 Teltow, Germany
c Universität Potsdam, Institut für Physik, Am Neuen Palais 10, D-14469 Potsdam, Germany
Received 9 June 2006; received in revised form 11 August 2006; accepted 15 August 2006
Available online 2 October 2006
Abstract
Three new compounds have been synthesized based on the molecular motif 2-[2,6-bis(triXuoromethyl)phenyl]-5-phenyl-1,3,4-oxadia-
zole, with subsequent CF3-substitution in the ortho-positions of the phenylene ring. The crystal structures of the compounds have been
determined by single crystal X-ray diVraction. All compounds have a monoclinic structure. The solid state structure of the compounds is
inXuenced by the electronic properties of the Xuorine atoms, leading to the occurrence of C–H,F, and C–F,ꢀ interactions, partly
replacing ꢀ–ꢀ interactions usually observed in the crystal structures of 2,5-diphenyl-1,3,4-oxadiazole derivatives. Other signiWcant interac-
tions than those involving Xuorine appear only in rare cases. The strong impact of the Xuorine atoms on the intra- and intermolecular
interactions, and the molecular conformation lead to novel inputs for the understanding of molecular recognition, supramolecular assem-
bly, and crystal packing of Xuorine containing compounds.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Crystal structure; 1,3,4-oxadiazole; Molecular conformation; Weak interactions
1. Introduction
zole containing materials are also interesting candidates for
the use in thin-Wlm-transistors.
Due to their structural variability organic materials oVer
many possibilities to tune their properties to actual needs
and to become versatile high tech materials with a broad
application spectrum. Aromatically substituted 1,3,4-oxa-
diazoles are widely used as scintillators, Xuorescence and
photographic materials [1,2] or for non-linear optical appli-
cations, but they are also known as biologically active
agents [3]. With the development of organic light emitting
diodes these materials obtained a speciWc interest as elec-
tron transport and injection or, more important, as hole
blocking layers [4–7] as well as emitting layers [8]. Oxadia-
The design of compounds with novel and improved
chemical and physical properties as advanced functional
materials with a speciWc application spectrum requires the
knowledge about possible supramolecular packing motifs
and their experimental control. This is one of the main top-
ics of crystal engineering. Besides the structure of the indi-
vidual molecule also non-covalent interactions play a
signiWcant role for the molecular conformation, the forma-
tion of a certain three-dimensional supramolecular archi-
tecture of a crystal as well as for molecular recognition
processes, bioactivity etc. Functional groups may contrib-
ute to the formation of a speciWc packing motif due to their
deWnite interactions. In their entirety the strength and direc-
tionality of these interactions create the characteristic
supramolecular synthons which can be used for the design
of supramolecular arrangements by the development of
*
Corresponding author. Tel.: +49 30 8104 1133; fax: +49 30 8104 5827.
E-mail addresses: franziska.emmerling@bam.de (F. Emmerling), orgzall
@rz.uni-potsdam.de (I. Orgzall), buschu@rz.uni-potsdam.de (B. Schulz).
0022-2860/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.08.013

F. Emmerling et al. / Journal of Molecular Structure 832 (2007) 124–131
125
appropriate strategies for the precise control of the topol-
ogy. Such non-covalent interactions are the characteristic
ꢀ-stacking or hydrogen bonds that both have been exten-
sively studied. If the molecules contain halogen atoms also
halogen,halogen, C–H,halogen, or halogen,ꢀ interac-
tions contribute to the formation of speciWc motifs. Such
interactions have been studied recently for a variety of
model compounds experimentally as well as by theoretical
considerations (see for instance [9–15]). But they are still a
matter of interest and discussion since halogen,ꢀ interac-
tions are not commonly observed and their role in molecular
recognition is not understood in detail yet [9]. A speciWc
problem seems to be the presence of organic Xuorine, whose
behavior is diVerent from that of the other halogen atoms
[16,17]. The introduction of a Xuorine atom leads to a mod-
iWcation of the packing motif compared to that of the non-
Xuorinated counterpart [12]. Nevertheless, although the
interactions between Xuorine atoms and C–H groups or ꢀ
systems are weak they also contribute to the stability of the
crystal structure, especially in the absence of other, strong
interactions like hydrogen bonding, ꢀ–ꢀ or C–H,ꢀ inter-
actions [12]. However, the use of such synthons to predict
possible supramolecular arrangements is less developed.
In the following solid state structures of some newly syn-
thesized heteroaromatic compounds containing the triXuoro-
methyl group will be presented as a contribution to broaden
the knowledge about the Xuorine,ꢀ interactions, the associ-
ated synthons and resulting characteristic packing motifs. The
basic molecular structure is the 2,5-diphenyl-1,3,4-oxadiazole
core. This study continues a series of previous investigations
[18–21] where crystal structures for several diVerently substi-
tuted diphenyl-1,3,4-oxadiazole compounds have been inves-
tigated to derive common motifs and characteristic diVerences
of the crystal packing with varying molecular structure.
Familiar structural features for compounds with substitution
in para position are planar or nearly planar molecules. This
picture changes if the substituent is located in ortho position
as could be shown in [20] for Xuorine substitution. Here, the
torsion angle between the central oxadiazole ring and both
phenylene rings increases remarkably. The most obvious com-
mon packing motif is the occurrence of molecular stacks in
most of the analyzed crystal structures. Within these stacks
intense ꢀ–ꢀ interactions dominate between adjacent molecules
between oxadiazole ring (acceptor) and phenylene rings
(donor) although also deviations from this basic scheme may
be found. The intermolecular interactions between the stacks
are preferentially van der Waals forces or dipole–dipole inter-
actions.
determine the packing motifs accompanied by C–H,F
interactions. While both, para and meta compounds, show
ꢀ–ꢀ interactions between phenylene and oxadiazole rings of
adjacent molecules, stacking interactions are restricted to the
oxadiazole rings in the ortho compound. Molecular stacks
are found for meta and ortho substitution, the para substitu-
tion is characterized by the occurrence of molecular layers
with chains of molecules connected by C–H,F contacts.
Interestingly, no ꢀ interactions are observed for the com-
pletely Xuorinated compound. Here, the crystal structure is
stabilized by close Xuorine–Xuorine contacts. Additionally,
also Xuorine,ꢀ interactions occur between Xuorine atoms
and the central oxadiazole ring.
This substitution also inXuences the planarity of the
molecule. The meta compound is considerably Xat with
dihedral angles of approximately 6° and 4° between both
phenylene rings and the oxadiazole ring. The para and
the perXuorinated compounds show increased values with
13°/16° and 14°/14°, respectively, while the ortho substitu-
tion leads to the largest deviation from planarity (29°/29°).
All of these compounds only contain Xuorine atoms as
substitutents. In the following, a more bulky group, the tri-
Xuoromethyl group, is introduced into the basic diphenyl-
1,3,4-oxadiazole molecule as substituent. All three subse-
quently investigated derivatives contain the triXuoromethyl
groups in the ortho positions of the phenylene rings. Com-
pound OXA1 (2-[2,6-bis(triXuoromethyl)phenyl]-5-phenyl-
1,3,4-oxadiazole) contains two CF3-substituents only on one
phenylene ring, while compound OXA2 (2-[2,6-bis(triXuoro-
methyl)phenyl]-5-[2-(triXuoromethyl)phenyl]-1,3,4-oxadia-
zole) and compound OXA3 (2,5-bis[2,6-bis(triXuoromethyl)
phenyl]-1,3,4-oxadiazole) contain CF3 groups on both rings,
the latter in all four possible ortho positions. The aim is to give
some new contributions for studies of the relations between
molecular structure and crystalline packing motif. Molecular
interactions between an aromatic ꢀ-electron system and Xuo-
rine atoms are an important topic in supramolecular chemis-
try. Therefore, the variations of the molecular conformation
with the substitution scheme and the implications for the solid
state structure will be investigated.
2. Experimental setup
2.1. Synthesis
The investigated triXuoromethyl group containing 2,5-di
(phenyl)-1,3,4-oxadiazole compounds OXA1–OXA3 were
synthesized using the classical procedure from tetrazole and
the corresponding acid chlorides [22,23]. The products were
dried in vacuum and repeatedly recrystallized from petrole-
ther.
This synthesis procedure is outlined in more detail for
OXA3, as prepared here for the Wrst time. To obtain the tetra-
zole a mixture of 1g (4.2mmol) 2,6-bis(triXuoromethyl)phe-
nylnitrile, 0.98g (15mmol) sodiumazide, and 2.05g (15mmol)
triethylamine hydrochloride in 50ml toluene is kept at 100°C
under stirring for 10h. After cooling, the product is extracted
A series of Xuorine containing, symmetrically substituted
diphenyl-1,3,4-oxadiazoles has been studied recently [20].
Both phenyl rings were substituted by one Xuorine atom in
ortho, meta, and para positions, respectively. The resulting
crystal structures were compared to that of the completely
Xuorinated compound. All structures diVer in relation to the
unsubstituted diphenyl-1,3,4-oxadiazole [19], although also
some common motifs may be found. In the bis(monoXuor-
ophenyl) compounds mainly intermolecular ꢀ–ꢀ interactions

126
F. Emmerling et al. / Journal of Molecular Structure 832 (2007) 124–131
with water (4£50ml). Concentrated hydrochloric acid is
dropwise added to the aqueous phase and the resulting 5-[2,6-
bis(triXuoromethyl)phenyl]tetrazole is salted out. After Wltra-
tion, the white solid is washed with water and dried under
vacuum (yield: 90%; Fp: 210°C).
The corresponding oxadiazole compound OXA3 is syn-
thesized by solving 1.04g (37mmol) of the tetrazole and 1g
(36 mmol) 2,6-bis(triXuoromethyl)benzoyl chloride in 15ml
dry pyridine. This solution is heated under reXux for 4 h,
cooled and transferred into water. The precipitate is
Wltrated, dried and recrystallized from petrolether (yield:
90%; 99% purity by HPLC.)
2-[2,6-bis(triXuoromethylphenyl)]-5-phenyl-1,3,4-oxa-
diazole (OXA1): Fp: 102°C, IR (ATR, cm¡1): 3087, 3069,
3038, 1607, 1590, 1575, 1551, 1483, 1462, 1451, 1343, 1298,
1261, 1217, 1180, 1138, 1124, 1070, 1048, 1023, 1005, 961,
923, 839, 823, 781, 764, 745, 705, 687, 676. 1H NMR (CDCl3,
400MHz): ꢀ [ppm] 8.08 (d, 2H, C7, C11), 8.06 (d, 2H, C14,
C16), 7.88 (dd, 1H, C15), 7.48–7.58 (m, 3H, C8, C9, C10).
2-[2,6-bis(triXuoromethylphenyl)]-5-(2-triXuoromethylphe-
nyl)-1,3,4-oxadiazole (OXA2): Fp: 96°C, IR (ATR, cm¡1):
1593, 1570, 1539, 1468, 1460, 1452, 1343, 1316, 1294, 1273,
1255, 1210, 1186, 1176, 1145, 1133, 1116, 1069, 1037, 969, 961,
881, 838, 826, 773, 767, 759, 737, 711, 691, 677, 646. 1H NMR
(CDCl3, 400MHz): ꢀ [ppm] 8.06 (d, 3H, C14, C16, C10), 7.89
(t, 2H, C15, C7), 7.85 (d, 1H, C10), 7.71 (t, 2H, C8, C9).
2,5-bis[2,6-bis(triXuoromethylphenyl)]-1,3,4-oxadiazole
(OXA3): Fp: 177°C, IR (ATR, cm¡1): 3104, 3071, 3037,
1593, 1578, 1567, 1456, 1345, 1294, 1251, 1214, 1187, 1151,
1131, 1068, 1035, 1007, 987, 842, 827, 759, 732, 710, 696, 677.
1H NMR (CDCl3, 400 MHz): ꢀ [ppm] 8.06 (d, 2H, C14,
C16), 8.06 (d, 1H, C10), 8.06 (d, 1H, C8), 7.89 (t, 1H, C15),
7.89 (t, 1H, C9).
2.2. X-ray structure determination
The crystal structures of OXA1–OXA3 have been deter-
mined by single crystal X-ray structure analysis. X-ray
measurements of crystals of compound OXA1 and OXA2
were performed at 293(2) K on a Bruker AXS SMART/
CCD diVractometer with Mo Kꢁ radiation (ꢁ D0.71073 Å).
The reXection intensities of compound OXA3 were
recorded at 293(2) K on an Enraf Nonius CAD4, using Mo-
radiation. The structures were solved by direct methods
using SHELXS-97 [24] and reWned by full matrix least
squares method using SHELXL-97 [25]. An empirical
absorption correction (ꢂ-scan) was applied. All non-hydro-
gen atoms were reWned anisotropically. In case of com-
pound OXA3 the positions of carbon-bound hydrogen
atoms were calculated corresponding to their geometrical
conditions and reWned using the riding model. Isotropic dis-
placement parameters of hydrogen atoms were derived
from the parent atoms. Further experimental details are
given in Table 1.
Crystallographic details on the structure analyses
of the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication Nos. CCDC
609038–609040.
X
F3C
2
3
O
1
N
N
Y
F3C
Fig. 1. Schematic representation of the substitution pattern in the investi-
gated compounds. X,Y D H for compound OXA1; X D CF3, Y D H for
compound OXA2; X,Y D CF3 for compound OXA3.
Table 1
Crystal data and structure reWnements for the investigated compounds
OXA1
OXA2
OXA3
Empirical formula
Formula weight
C16H8F6N2O
358.24
C17H7F9N2O
426.25
C18H6F12N2O
494.25
Temperature (K)
293(2)
293(2)
293(2)
Wavelength (Å)
0.71073
0.71073
0.71073
Crystal system, space group
Unit cell dimensions [Å, °]
Monoclinic, C2/c (No. 15)
a D 26.828(4)
Monoclinic, P21/c (No. 14)
a D 12.084(2)
Monoclinic, P21/c (No. 14)
a D 9.313(2)
b D 10.713(2)
b D 16.732(2)
b D 15.7248(14)
c D 14.778(4)
c D 11.755(2)
c D 8.320(1)
ꢂ D 115.834(3)
ꢂ D 103.447(7)
ꢂ D 122.606(8)
Volume (Å3)
3040.9(8)
1636.2(4)
1823.1(6)
Z, Calculated density (Mg/m3)
8, 1.565
0.150
1440
4, 1.730
0.180
848
4, 1.801
0.198
976
Absorption coeYcient (mm¡1
F(0 0 0)
)
Theta range for data collection (°)
Limiting indices
2.08–25.00
2.12–25.00
2.09–28.37
¡31 6 h 6 31, ¡12 6 k 6 11,
¡13 6 l 6 12
7312/2671 [R(int) D 0.0350]
Full-matrix least-squares on F2
2671/0/259
¡12 6 h 6 14, ¡19 6 k 6 19,
0 6 h 6 12, ¡20 6 k 6 20,
¡19 6 l 6 16
9725/4502 [R(int) D 0.1107]
Full-matrix least-squares on F2
4502/0/299
¡9 6 l 6 9
ReXections collected/unique
ReWnement method
7888/2877 [R(int) D 0.0518]
Full-matrix least-squares on F2
2877/0/291
Data/restraints/parameters
Goodness-of-Wt on F2
Final R indices [I > 2ꢃ(I)]
R indices (all data)
0.969
0.967
1.144
R1 D 0.0446, wR2 D 0.1376
R1 D 0.0655, wR2 D 0.1480
0.290 and ¡0.299
R1 D 0.0444, wR2 D 0.1343
R1 D 0.0444, wR2 D 0.1343
0.234 and ¡0.230
R1 D 0.0677, wR2 D 0.2174
R1 D 0.1228, wR2 D 0.2571
0.469 and ¡0.411
Largest diV. peak and hole (e.Å¡3
)

F. Emmerling et al. / Journal of Molecular Structure 832 (2007) 124–131
127
Fig. 2. The molecular structures of the investigated compounds OXA1 (a), OXA2 (b), and OXA3 (c) showing 30% probability displacement ellipsoids.

128
F. Emmerling et al. / Journal of Molecular Structure 832 (2007) 124–131
Table 4
3. Results and discussion
3.1. Molecular conformation
Intramolecular C–F,ꢀ electron interactions
Compound
OXA1
F,C
D(F–C) (Å)
DP (Å)
Shift (Å)
F9,C5
F10,C5
2.687(2)
2.882(2)
2.652
2.760
0.432
0.830
Fig. 1 provides the schematic diagrams of the molecules
referred to in the discussion, whereas Fig. 2 shows the
ORTEP [26] plots of the molecules OXA1–OXA3. Accord-
ing to the X-ray diVraction data, each oxadiazole and phe-
nylene ring in these molecules is perfectly planar in the
crystalline state. The rms deviations from the mean planes
are 0.0012 Å (ring 1), 0.0024 Å (ring 2), and 0.0057 Å (ring 3)
for molecule OXA1, 0.0033 Å (ring 1), 0.0022 Å (ring 2), and
0.0112Å (ring 3) for molecule OXA2, and 0.0025 (ring 1),
0.0082 Å (ring 2), and 0.0036 Å (ring 3) for molecule OXA3,
respectively.
The conformation of the molecules can be described by
the dihedral angles between the central oxadiazole and the
adjacent phenylene rings. The angles between the planes as
depicted in Fig. 1 are given in Table 2. Interestingly, the
dihedral angle between the oxadiazole and phenylene ring 2
increases with the introduction of triXuoromethyl groups
from nearly planar 6.7(1)° observed for OXA1 to an almost
perpendicular orientation 81.8(2)° in OXA3. In all com-
pounds studied the dihedral angles between the ring planes
1 and 3 remain nearly unaVected by the ortho substitutions
in ring 2. The angle between the planes of the two pheny-
lene moieties changes from nearly perpendicular 84.7(1)° in
compound OXA1 to 50.8(1)° in OXA2 and 50.0(2) in
OXA3.
OXA2
OXA3
F9,C5
F10,C5
2.733(3)
2.670(2)
2.704
2.617
0.396
0.530
F3,C2
F5,C2
F9,C5
F10,C5
2.741(2)
2.861(2)
2.703(2)
2.763(2)
2.680
2.691
2.666
2.687
0.575
0.972
0.446
0.644
sp2–sp2 ꢃ-bonds, indicating at least a very weak conjuga-
tion of the whole molecule. The mean C–F bonds in all
compounds amount to 1.320(5) Å (OXA1), 1.332(7)Å
(OXA2), and 1.327(8) Å (OXA3).
Intramolecular C–H,F interactions between the tri-
Xuoromethyl group and neighboring CH groups in the phe-
nylene ring are observed for all three compounds. In case of
compound OXA1 and OXA2 additional intermolecular
hydrogen bonds including the oxadiazole oxygen (C11–
H11,O1 with d(D,A)D 2.823(3)Å, d(H,A) D2.50(2)Å,
and \(D–H,A)D 100.3(18)°) and nitrogen (C7–H7,N3
O1 with d(D,A)D2.877(3)Å, d(H,A)D2.53(3)Å, and
\(D–H,A)D100.8(16)°), respectively, are formed.
The rotation of the CF3 substituted phenylene rings with
respect to the central oxadiazole ring gives rise to intramo-
lecular C–F,ꢀ interactions listed in Table 4. As a measure
of C–F,ꢀ interaction the distances between the Xuorine
atoms and the C atoms of the oxadiazole ring are consid-
ered. The shift values given in Table 3 indicate the deviation
from a perpendicular arrangement of both atoms. As may
be seen for the example of OXA2 and depicted in Fig. 2 the
The mean N–C distances of 1.271(3)Å of the discussed
compounds conWrm the double bond character between
these atoms in the oxadiazole ring. The C–C distances for
the bond between the oxadiazole and the phenylene rings
range from 1.461(3) to 1.487(3)Å in good agreement with
Table 2
Conformation and intermolecular interactions in the crystal structures of OXA1–OXA3
Compound Dihedral angles (°)
Dihedral angles (°)
Interring contact (Å)
Intramolecular interactions
Intermolecular interactions
Phenyl(2)-
oxadiazole(1)/phenyl(3)-
oxadiazole(1)
Phenyl(2)–phenyl(3) phenyl(2)-oxadiazole(1)/ C–H,F
C–F,ꢀ
ꢀ,ꢀ
C–F,ꢀ
F–F
phenyl(3)-oxadiazole(1)
OXA1
OXA2
OXA3
6.7(1)/79.7(1)
29.1(1)/69.8(1)
81.8(2)/78.0(2)
84.7(1)
50.8(1)
50.0(2)
1.461(3)/1.479(3)
1.471(3)/1.487(3)
1.486(2)/1.473(6)
+
+
+
+
+
+
+
+
¡
+
+
+
¡
+
+
Table 3
Intermolecular C–F,Cg interactions in the crystal structures of OXA1–OXA3
Compound
OXA1
C–F,Cg
F,Cg (Å)
F,Perp (Å)
C,Cg (Å)
ꢄ\(C–F, Cg) (°)
ꢅ\(C–F, ꢀ) (°)
C20–F7,Cg2i
3.302(2)
3.123(2)
3.301
3.109
4.214(3)
4.079(2)
125.6(1)
127.5(2)
34.05
35.46
C20–F9,Cg3ii
OXA2
OXA3
C21–F10,Cg3i
C21–F12,Cg3i
3.670(2)
3.642(2)
3.947
3.947
3.947(2)
3.947(2)
92.3(1)
93.1(2)
29.16
21.64
C21–F11,Cg3i
3.376(5)
3.243(3)
3.276
3.195
4.592(6)
3.974(5)
154.4(3)
114.7(3)
61.74
34.26
C21–F12,Cg2ii
(OXA1) i D 1/2 ¡ x, 1/2 + y, 1/2 ¡ z; ii D 1/2 ¡ x, ¡1/2 + y, 1/2 ¡ z. (OXA2) i D 1 ¡ x, ¡y, 1 ¡ z. (OXA3) i D ¡x, 1 ¡ y, 1 ¡ z; ii D ¡1 + x, 1/2 ¡ y,¡1/2 + z.
The Cg numbers refer to the ring centre-of-gravity numbers given in Fig. 1.

F. Emmerling et al. / Journal of Molecular Structure 832 (2007) 124–131
129
Fig. 3. Intermolecular interactions in the crystal structure of OXA1. ꢀ–ꢀ interactions are indicated via dotted lines, C–F,ꢀ interactions via solid lines.
Intramolecular interactions are excluded for clarity.
intramolecular C–F,ꢀ interactions between the CF3 group
and the oxadiazole ring are formed only in the case of dou-
ble ortho substitution. The energy gain obtained by the
rotation of the mono-CF3 substituted phenylene ring
against the plane of the oxadiazole ring is not suYcient to
induce a stronger conformation change.
axis. The intermolecular interactions lead to the formation
of layers in the (¡101) plane stacked along the c axis, as
illustrated in Fig. 3. These layers are interconnected via
weak ꢀ–ꢀ electron interactions between the CF3-substituted
phenylene rings (Cg3,Cg3). The values for the ꢀ–ꢀ elec-
tron interactions are given in Table 5. Short F–F contacts
and intermolecular C–H,F interactions were not
observed.
3.2. Intermolecular interactions
The molecular packing of OXA2 is shown in Fig. 4. This
compound crystallizes in the monoclinic space group P21/c.
Along the c axis stacks of head to tail oriented OXA2 mole-
cules, generated by the c glide plane parallel to the a–c
plane are formed, providing the basis for ꢀ–ꢀ electron inter-
actions between neighboring 2,6-bis(triXuoromethyl)
phenylene rings (Cg3,Cg3). The separation of the molecu-
lar planes, 3.856(2)Å (Fig. 4), indicates weak ꢀ–ꢀ interac-
tions. The values for the interactions are given in Table 5.
Weak stacking interactions between adjacent (triXuoro-
methyl)phenylene rings complete the intermolecular
interactions in the crystal structure. The associated values
are given in Table 5. The close intermolecular F–F contacts,
2.748(2)Å (assuming a van der Waals radius of 1.47Å for the
Xuorine atom [28]), resulting from the packing arrangement,
In the following the crystal structures of OXA1–OXA3
will be discussed separately focused on the diVerent kinds
of intermolecular interactions and resulting packing
motifs.
The compound OXA1 crystallizes in the space group
C2/c. The basic building block of its crystal structure
includes a dimer of two oxadiazole molecules which are
connected via ꢀ–ꢀ electron interactions between two
unsubstituted phenyl rings of adjacent molecules
(Cg2,Cg2), symmetrically related by the inversion centre
in ͗000͘. The dimer is also linked to adjacent molecules via
C–F,ꢀ interactions (C20–F7,Cg2, C20–F9,Cg3; see
Table 3 for details). The involved molecules are symmetri-
cally related by a 21-rotation axis running parallel to the b
Table 5
ꢀ–ꢀ electron interactions in the crystal structures of OXA1 and OXA2
Compound
OXA1
CgI,CgJ
Cg–Cg (Å)
CgI Perp (Å)
CgJ Perp (Å)
ꢄ (°)
Cg2,Cg2i
3.890(2)
4.090(2)
3.461
3.712
3.461
3.712
0.0
0.0
Cg3,Cg3ii
OXA2
Cg2,Cg2i
Cg2,Cg2ii
Cg3,Cg3iii
4.169(2)
4.169(2)
3.856(2)
3.680
3.696
3.701
3.696
3.680
3.701
3.32
3.32
0.03
(OXA1) i D 1 ¡ x, 1 ¡ y, 1 ¡ z; ii D 1/2 ¡ x, 3/2 ¡ y, ¡z. (OXA2) i D x, 1/2 ¡ y, 1/2 + z; ii D x, 1/2 ¡ y, ¡1/2 + z; iii D 1 ¡ x, ¡y, 2 ¡ z.
The Cg numbers refer to the ring centre-of-gravity numbers given in Fig. 1.

130
F. Emmerling et al. / Journal of Molecular Structure 832 (2007) 124–131
Fig. 4. Molecular packing in the crystal structure of OXA2 viewed along the b axis. ꢀ–ꢀ interactions are indicated via dotted lines, C–F,ꢀ interactions via
solid lines. Intramolecular interactions are excluded for clarity.
are tolerated by the structure. Intermolecular C–H,F inter-
actions are not observed.
As a consequence of the conformational features short F–F
contacts of 2.866(4)Å are detected between adjacent mole-
cules. Similarly, as observed for OXA1 and OXA2 no inter-
molecular C–H,F interactions occur.
Compound OXA3 (Fig. 5) crystallizes in the space group
P21/c. The crystal structure is composed of a dimeric unit
consisting of two OXA3 molecules, symmetrically dependent
via inversion centres. Fig. 5 illustrates the occurrence of C–
F,ꢀ interactions (C21–F11,Cg3) between the OXA3 mol-
ecules. In the b–c plane neighboring dimers are connected via
additional C–F,ꢀ interactions (C21–F12,Cg2), as
depicted in Fig. 5. The numerical values are given in Table 3.
4. Conclusion
The organo halogene compounds are known to generate
structural motifs via intermolecular interactions like
C–H,X, X,X, C–X,ꢀ (XD Cl, Br, I). Detailed CSD [29]
and PDB [15] investigations concerning the role of C–X,ꢀ
interactions (X D F, Cl, Br, I) showed, that the tendency of
the formation of C–X,ꢀ interactions is higher in case of
Xuorine than in those of the other halogens. In this work,
we investigated three diVerent diphenyl-oxadiazole mole-
cules containing triXuoromethyl ortho substitutents. First
of all, this substitution considerably inXuences the molecu-
lar conformation. A triXuoromethyl group in ortho position
of the phenylene ring leads to the rotation of this ring with
respect to the central oxadiazole ring. However, the rota-
tion of the second, doubly substituted ring is unaVected of
this, keeping a nearly constant torsion angle to the oxadia-
zole ring. Thus, only weakly or not conjugated molecules
result indicated by the interring bonds between the diVerent
aromatic rings. And, additionally, this molecular twist leads
to the formation of intramolecular CF,ꢀ interactions with
the oxadiazole ring. Such intramolecular C–F,ꢀ interac-
tions can be observed in all the compounds, but, interest-
ingly, only in the case of a doubly substituted phenylene
ring. A single CF3 group is not involved in such interactions.
Additionally, also intramolecular C–H,F interactions
between neighbored atoms in a molecule are found for
every investigated compound.
Fig. 5. Intermolecular interactions in the crystal structure of OXA3.
Molecular layers in the crystal structure of OXA3 parallel to the (¡201)
plane. C–F,ꢀ interactions are represented via solid lines.
Usually, the crystal structures of diphenyl-oxadiazoles
are dominated by strong ꢀ–ꢀ stacking interactions. Such

F. Emmerling et al. / Journal of Molecular Structure 832 (2007) 124–131
131
[2] J. Hill, 1,3,4-Oxadiazoles, in: A.R. Katritzky, C.W. Rees (Eds.), Com-
prehensive Heterocyclic Chemistry, Pergamon Press, Oxford, 6 (1984)
426.
interactions are only found in the case of OXA1 and OXA2
although being relatively weak. In the crystal structure of the
fully ortho substituted compound OXA3 the ꢀ–ꢀ stacking
interactions are replaced by C–F,ꢀ interactions. Generally,
the tendency to form ꢀ–ꢀ interactions is reduced with
increasing ortho substitution that complicates a stack-like
arrangement. It is noteworthy, that the C–F,ꢀ interactions
are also observed in compound OXA1. Here, and in OXA3
they provide the stability of the packing motif. Contrary to
possible assumptions based on common synthons, that
strongly twisted molecules with aromatic rings could form
T-like arrangements with corresponding interactions, such
structures are not found for the investigated compounds.
With increasing substitution also the number of close F–F
contacts increases. Such interactions are not observed in
OXA1 but in OXA2 and OXA3. No intermolecular C–F,H
hydrogen bonds were observed in the crystal structures of the
three compounds. Since Xuorine can only act as a hydrogen-
bond acceptor, the lack of strong hydrogen donors within
the molecules might be a reason. Therefore, the observed
C–F,ꢀ interactions and close F–F contacts as main inter-
molecular interactions in the studied compounds obviously
provide the stability to form molecular assemblies in absence
of any other strong intermolecular forces as i.e. hydrogen
bonds like in the case of OXA3. The strength and the num-
ber of these intermolecular interactions, the packing motifs,
and thus the crystal structures clearly depend on the number
of triXuormethyl substituents.
[3] A. Hetzheim, in: Houbon-Weyl, Methoden der organische Chemie,
vol. E 8c, Georg Thieme Verlag, Stuttgart, 1994, p. 526.
[4] B. Schulz, M. Bruma, L. Brehmer, Advanced Materials 9 (1997) 601.
[5] Z.H. Peng, Z.N. Bao, M.E. Galvin, Advanced Materials 10 (1998) 680.
[6] Z. Bao, J.A. Rogers, A. Dodabalapur, A.J. Lovinger, H.E. Katz, V.R.
Raju, Z. Peng, M.E. Galvin, Optical Materials 12 (1999) 177.
[7] J.F. Wang, G.E. Jabbour, E.A. Mash, J. Anderson, Y. Zhang, P.A. Lee,
N.R. Armstrong, N. Peyghambarian, B. Kippelen, Advanced Materi-
als 11 (1999) 1266.
[8] M.A. Karasz, G.E. Wnek, Electrochimica Acta 43 (1998) 1623.
[9] H. Addams, S. Cockroft, C. Guardigli, C.A. Hunter, K.R. Lawson, J.
Perkins, S.E. Spey, C.J. Urch, R. Ford, Chembiochem 5 (2004) 657.
[10] I.Y. Bagryanskaya, Y.V. Gatilova, A.M. Maksimov, V.E. Platonov,
A.V. Zibarev, Journal of Fluorine Chemistry 126 (2005) 1281.
[11] A.R. Choudhury, K. Nagarajan, T.N.G. Row, Crystal Engineering 6
(2003) 43.
[12] A.R. Choudhury, T.N.G. Row, Crystal Growth & Design 4 (2004) 47.
[13] A.R. Choudhury, U.K. Urs, P.S. Smith, R. Goddard, J.A.K. Howard,
T.N.G. Row, Journal of Molecular Structure 641 (2002) 225.
[14] S. Kawahara, S. Tsuzuki, T. Uchimaru, Journal of Physical Chemistry
A 108 (2004) 6744.
[15] I. Saraogi, V.G. Vijay, S. Das, K. Sekar, T.N.G. Row, Crystal Engi-
neering 6 (2003) 69.
[16] G.R. Desiraju, R. Parthasarathy, Journal of the American Chemical
Society 111 (1989) 8725.
[17] L. Shimoni, J.P. Glusker, Structural Chemistry 5 (1994) 383.
[18] O. Franco, I. Orgzall, G. Reck, S. Stockhause, B. Schulz, Journal of
Physics and Chemistry of Solids 66 (2005) 994.
[19] O. Franco, G. Reck, I. Orgzall, B.W. Schulz, B. Schulz, Journal of
Molecular Structure 649 (2003) 219.
[20] I. Orgzall, O. Franco, G. Reck, B. Schulz, Journal of Molecular Struc-
ture 749 (2005) 144.
[21] S. Stockhause, M.S. Wickleder, G. Meyer, I. Orgzall, B. Schulz, Jour-
nal of Molecular Structure 561 (2001) 175.
Acknowledgement
[22] J. Sauer, R. Huisgen, H.J. Sturm, Tetrahedron 11 (1960) 241.
[23] R. Huisgen, Angewandte Chemie-International Edition 72 (1960) 359.
[24] G.M. Sheldrick, SHELXS-97, Program for the Solution of Crystal
Structures, Universität Göttingen, 1997.
[25] G.M. Sheldrick, SHELXL-97, Program for the Crystal Structure
ReWnement, Universität Göttingen, 1997.
We thank D. Pfeifer for NMR measurements and
U. Resch-Genger for fruitful discussions and support.
References
[1] A. Hetzheim, K. Möckel, in: A.R. Katritzky, A.J. Boullten (Eds.),
Advances in Heterocyclic Chemistry, vol. 7, Academic Press, New
York, 1966, p. 185.
[26] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
[28] A. Bondi, J. Phys. Chem. 68 (1964) 441.
[29] M.D. Prasanna, T.N. Guru Row, Crystallogr. Eng. 3 (2000) 135.