Structures of substituted di-aryl-1,3,4-oxadiazole derivatives: 2,5-bis(pyridyl)- and 2,5-bis(aminophenyl)-substitution

Article abstract of DOI:10.1016/j.molstruc.2006.03.076

Crystal structures of four different di-aryl-1,3,4-oxadiazole compounds (aryl = 2-pyridyl-, 3-pyridyl-, 2-aminophenyl-, 3-aminophenyl-) are determined. Crystallization of di(2-pyridyl)-1,3,4-oxadiazole yielded monoclinic and triclinic polymorphs. The stru

Full text of DOI:10.1016/j.molstruc.2006.03.076

Journal of Molecular Structure 800 (2006) 74–84
www.elsevier.com/locate/molstruc
Structures of substituted di-aryl-1,3,4-oxadiazole derivatives:
2,5-bis(pyridyl)- and 2,5-bis(aminophenyl)-substitution
a,
b
a
a
Franziska Emmerling *, Ingo Orgzall , Gunter Reck , Burkhard W. Schulz ,
¨
c
d
Sabine Stockhause , Burkhard Schulz
a
Bundesanstalt fu¨r Materialforschung und – pru¨fung, Richard-Willsta¨tter-Strasse 11, D-12489 Berlin, Germany
b
Institut fu¨r Du¨nnschichttechnologie und Mikrosensorik e.V., Kantstrasse 55, D-14513 Teltow, Germany
Anorganisch-chemisches Institut, Universita¨t Zu¨rich, Winterthurer Strasse 190, CH-8057 Zu¨rich, Switzerland
c
d
Institut fu¨r Physik, Universita¨t Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany
Received 20 February 2006; received in revised form 22 March 2006; accepted 25 March 2006
Available online 24 May 2006
Abstract
Crystal structures of four different di-aryl-1,3,4-oxadiazole compounds (aryl = 2-pyridyl-, 3-pyridyl-, 2-aminophenyl-, 3-aminophe-
nyl-) are determined. Crystallization of di(2-pyridyl)-1,3,4-oxadiazole yielded monoclinic and triclinic polymorphs. The structures are
characterized by the occurrence of p–p interactions. Additionally, in case of the aminophenyl compounds intra- as well as intermolecular
hydrogen bonds are found that influence the packing motif as well. Since these molecules are often used as ligands in metal–organic com-
plexes similarities and differences of the molecular conformation between the molecules in the pure crystals and that of the ligands in the
complexes are discussed.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Crystal structure; 1,3,4-Oxadiazole; Molecular conformation; Hydrogen bonds
1. Introduction
[1]). This kind of hybrid materials shows sophisticated
combinations of properties and complex network struc-
tures with unusual topologies.
The rapid technological development requires the direct-
ed search for new functional materials with well-defined
structures. In particular, organic materials offer a broad
structural variability and flexibility that allows to obtain
specific application properties and functions. Their combi-
nation with inorganic elements leads to new functional
materials, so-called metal–organic materials (MOMs).
The formation of complexes consisting of organic mole-
cules and transition metal ions recently led to the develop-
ment of strategies for the construction of specific structures
offering interesting possibilities for the formation of supra-
molecular assemblies (metal–organic frameworks – MOFs,
Oxadiazole compounds including their diphenyl and
dipyridyl derivatives have been investigated for a long
time due to their expected pharmacological properties
and biological activities. So, for instance, several com-
pounds are known as antimicrobial agents [2–4]. However,
specifically substituted diphenyl-1,3,4-oxadiazoles may
also show corrosion inhibitory effects [5,6]. More recently,
various oxadiazole-containing polymers have been used as
electron injection layers in electroluminescent devices.
Low molecular compounds containing the oxadiazole
moiety are also used in polymer blends as emitting layers
[7]. An improvement of the emitting properties and an
effective tuning of the emitted wavelength can be achieved
by complexation of these organic molecules with different
metal ions [8–13]. Further improvements should be
realized.
*
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), sabines@aci.unizh.ch (S. Stock-
hause), buschu@rz.uni-potsdam.de (B. Schulz).
0022-2860/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.03.076

F. Emmerling et al. / Journal of Molecular Structure 800 (2006) 74–84
75
Recently, a concept was developed for MOFs based on
diaryl-1,3,4-oxadiazole molecules. In particular, 2,5-bis(4-
pyridyl)-1,3,4-oxadiazole (p-PYO) ([14–20] and references
cited therein), and 2,5-bis(3-pyridyl)-1,3,4-oxadiazole mol-
ecules (m-PYO) (cf. [14,16,17,20–28]), or compounds
containing amino-substituted phenyl rings, such as
2,5-bis(3-aminophenyl)-1,3,4-oxadiazole (m-APO) [27,29]
and 2,5-bis(2-aminophenyl)-1,3,4-oxadiazole (o-APO) [30]
(see Scheme 1), were used as ligands. Due to their bent
structure these molecules give rise to a variety of different
bonding patterns. Additionally, also the variability of the
molecular conformation contributes to the versatile appli-
cation of these ligands, enabling them to meet the specific
coordination requirements. In these complexes the ortho
and meta molecules may adopt three different possible con-
formations [31]. For instance, for m-PYO this means that
in the cisoid I conformation both pyridyl-N-atoms point
in the direction of the oxadiazole oxygen, leading to a
closed structure in the supramolecular arrangement while
in the cisoid II form both N-atoms point in the direction
of the nitrogen atoms of the oxadiazole ring. This produces
open structures. In the transoid conformation, both pyridyl
nitrogen atoms of the molecule point in different directions
(cf. Scheme 2).
complexes, it is obvious that almost all ligands show con-
siderable structural variability in different supramolecular
arrangements and surroundings. Hence the following ques-
tions arise: how are the pure ligand molecules arranged in
the solid state and which conformation will they adopt?
The answers will not only contribute to a better under-
standing of oxadiazole complex formation itself, but also
to a general systematics of organic crystal structures that
will possibly allow better forecasts for yet unknown struc-
tures built from specifically designed molecules. At the
same time, they will also provide better insight into the pos-
sible variations of the molecular conformation and the pro-
cesses during crystallizations.
Accordingly, in the following crystal structures of the
ortho and meta compounds 2,5-bis(2-pyridyl)-1,3,4-oxadia-
zole (o-PYO) and m-PYO will be discussed in detail with
respect to their molecular conformation and crystalline
arrangement. The corresponding bis(aminophenyl)-1,3,4-
oxadiazole analogs, o-APO, and m-APO, are included here.
It can be expected that these compounds containing the
amino group behave in a like manner as tectones in the
complex formation as the pyridyl-containing oxadiazoles.
Similarities and differences of the molecular conformation
between the pure ligand crystals and the various metal
complexes will be discussed.
However, although a number of structural studies have
been devoted to such metal complexes and coordination
polymers, only little is known about the crystal structure
of the ligands themselves. From the study of the various
2. Experimental setup
2.1. Sample synthesis
Both pyridyl-containing compounds, m-PYO and
o-PYO, were synthesized by direct condensation of nicotin-
ic acid and picolinic acid, respectively, with hydrazine
hydrate. Both reactants were stirred in polyphosphoric acid
at 180 ꢁC (m-PYO) and 150 ꢁC (o-PYO) for 4 h. After cool-
ing the solutions were poured into water, filtered and the
products were dried in vacuum. The products were several
times recrystallized from ethanol.
o-PYO: Fp: 153–154 ꢁC (Lit. 153–154 ꢁC [32,33]), IR
(ATR, cmꢀ1): 3096, 3085, 3058, 3020, 2992, 1699, 1588,
1571, 1556, 1547, 1451, 1436, 1418, 1307, 1288, 1279,
1249, 1158, 1144, 1095, 1048, 995, 990, 971, 897, 798,
741, 727, 714, 620.
N
N
O
O
N
N
N
N
NH2
H2N
o-PYO
o-APO
N
N
O
O
NH2
H2N
N
N
N
N
m-PYO
m-APO
H2N
NH2
N
N
O
O
m-PYO: Fp: 189 ꢁC (Lit. 188–189 ꢁC [32,33]), IR (ATR,
cmꢀ1): 3061, 3042, 1599, 1576, 1547, 1536, 1479, 1410,
1367, 1339, 1279, 1241, 1192, 1126, 1097, 1081, 1019,
999, 976, 963, 819, 730, 704, 622.
N
N
N N
p-PYO
p-APO
Scheme 1. Molecules.
N
N
N
O
O
O
N
N
N
N
N
N
N
N N
cisoid I
cisoid II
transoid
Scheme 2. Possible conformations of the molecules.

76
F. Emmerling et al. / Journal of Molecular Structure 800 (2006) 74–84
The amino derivatives m-APO and o-APO are obtained
atoms were derived from the parent atoms. Further exper-
imental details are given in Table 1.
by reduction of the respective nitrophenyl-1,3,4-oxadiazole
compounds. The final products are again several times
recrystallized from ethanol.
Crystallographic data for the structure reported in this
paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication Nos.
CCDC 298165–298169. Tables S1–S10 compiling the
atomic positions and details on bond distances and angles
for the five different structures may be found in Supplemen-
tary data.
o-APO: Fp: 225–228 ꢁC (Lit. 228–230 ꢁC [30]), IR
(ATR, cmꢀ1): 3438, 3373, 3335, 3299, 3201, 1633, 1605,
1576, 1534, 1493, 1443, 1334, 1316, 1254, 1155, 1081,
1054, 1037, 1003, 962, 942, 854, 770, 735, 704, 682.
m-APO: Fp: 247–249 ꢁC (Lit. 247 ꢁC [34], 250–252 ꢁC
[35]), IR (ATR, cmꢀ1): 3421, 3323, 3218, 1635, 1621,
1605, 1593, 1542, 1477, 1467, 1458, 1366, 1316, 1288,
1275, 1161, 1110, 1078, 1044, 993, 972, 886, 874, 861,
778, 728, 692, 679.
3. Results and discussion
3.1. Structural analysis of pyridyl compounds o-PYO and
m-PYO
2.2. X-ray structure determination
The compound o-PYO crystallizes in two modifications,
both containing two crystallographically independent
o-PYO molecules in the asymmetric unit (see Fig. 1a,b for
the corresponding ORTEP plots of the molecules). o-PYO
I crystallizes in the monoclinic space group P21. Both
o-PYO I molecules adopt the cisoid I conformation.
The dihedral angles between the oxadiazole ring and both
pyridyl rings amount to 3.8(2)ꢁ/3.6(3)ꢁ (molecule A) and
9.5(2)ꢁ/10.5(2)ꢁ (molecule B). In the crystal structure, stacks
of crystallographically equivalent molecules are formed
along the b axis (Fig. 2). Within the stacks, p–p electron
interactions between the neighbouring oxadiazole rings
and pyridyl rings are possible, as described by the distances
of the ring centroids (DC), the perpendicular distances of
the centroids of one ring from the plane of the other (DP)
Single-crystal X-ray diffraction data of the investigated
compounds were collected with the use of graphite mono-
chromatized MoKa radiation (k = 0.71073 A) at 293 K
˚
on a Bruker SMART diffractometer, equipped with a
CCD area detector. The structures were solved by direct
methods using SHELXS–97 [36] and refined with SHEL-
XL–97 [37]. An empirical absorption correction (W-scan)
was applied. All non-hydrogen atoms were refined aniso-
tropically. The positions of carbon-bound hydrogen atoms
were calculated corresponding to their geometrical condi-
tions and refined using the riding model. Starting positions
for amino-hydrogen atoms were located in difference Fou-
rier syntheses. The refinement was completed using the rid-
ing model. Isotropic displacement parameters of hydrogen
Table 1
Crystal data and structure refinements for the investigated compounds
o-PYO I
o-PYO II
m-PYO
o-APO
m-APO
Formula
Formula weight
Temperature (K)
C12H8N4O
224.22
293 (2)
C12H8N4O
224.22
293 (2)
C12H8N4O
224.22
293 (2)
C14H12N4O
252.28
293 (2)
C14H12N4O
252.28
293 (2)
˚
Wavelengths (A)
0.71073
0.71073
0.71073
0.71073
0.71073
Crystal system
Space group
Unit cell dimensions (A, ꢁ)
Monoclinic
P21 (No. 4)
a = 15.863(11)
b = 3.885(3)
c = 17.575(12)
b = 100.77(2)
Triclinic
Monoclinic
P2/c (No. 13)
a = 17.547(6)
b = 6.169(2)
c = 10.369(4)
b = 106.790(6)
Monoclinic
C2/c (No. 15)
a = 27.897(5)
b = 5.0834(6)
c = 18.092(4)
b = 107.74(2)
Monoclinic
P21/c (No. 14)
a = 12.25050(10)
b = 4.8537(2)
c = 20.4518(7)
b = 98.181(2)
P-1 (No. 2)
a = 7.9355(14)
b = 11.076(2)
c = 12.393(2)
a = 93.666(4)
b = 91.392(3)
c = 96.241
1080.0(3)
˚
3
˚
Volume (A )
1063.9(13)
4, 1.400
1074.5(6)
4, 1.386
2443.6(7)
4, 1.371
1.9–28.2
ꢀ36 6 h 6 36
ꢀ6 6 k 6 6
ꢀ23 6 l 6 23
2128
1203.69(7)
4, 1.392
Z, calculated density (g cmꢀ3
Range for data collection (ꢁ)
Limiting indices
)
4, 1.379
1.18–25.00
ꢀ20 < h < 20
ꢀ5 < k < 5
ꢀ25 < l < 18
5346
1.85–24.00
ꢀ10 < h < 10
ꢀ15 < k < 14
ꢀ17 < l < 7
4983
1.21–25.00
ꢀ22 < h < 24
ꢀ3 < k < 7
ꢀ12 < l < 14
4055
1.68–23.28
ꢀ5 < h < 13
ꢀ5 < k < 5
ꢀ21 < l < 22
3771
Reflection unique
Refinement method
Full-matrix
least-squares on F2
0.965
Full-matrix
least-squares on F2
0.841
Full-matrix
least-squares on F2
1.193
Full-matrix
least-squares on F2
0.989
Full-matrix
least-squares on F2
1.295
Goodness-on-fit
Final R indices
R1 = 0.0896
R2 = 0.1646
R1 = 0.0394
R2 = 0.0894
R1 = 0.0949
R2 = 0.2576
R1 = 0.0364
R2 = 0.0613
R1 = 0.1119
R2 = 0.2493

F. Emmerling et al. / Journal of Molecular Structure 800 (2006) 74–84
77
Fig. 1. The molecular structures of the investigated compounds, showing 30% probability displacement ellipsoids.
˚
˚
and the interplanar angle (a). The following values can be
given: for ring Cg1 [O1–C2–N3–N4–C5] and Cg1 at
DC = 3.884 A, DP = 3.260 A, a = 0.0ꢁ, ring Cg2 [C6–C7–
˚
C8–C9–C10–N11] and Cg2 at (x,1 + y,z), DC = 3.884 A,
0
˚
˚
˚
(x,1 + y,z), DC = 3.884 A, DP = 3.395 A, a = 0.0ꢁ, ring
Cg10 [O10–C20–N30–N40–C50] and Cg10 at (x,1 + y,z),
DP = 3.421 A, a = 0.0ꢁ, ring Cg2 [C60–C70–C80–C90–
C100–N110] and Cg20 at (x,1 + y,z), DC = 3.884 A,
˚

78
F. Emmerling et al. / Journal of Molecular Structure 800 (2006) 74–84
a
c
o
Cg3
Cg3’
Cg2’
Cg2’
Cg2
Cg1’
Cg1’
Cg1’
Cg1
Cg1
Cg1
Cg3’
Cg3
b
Cg2
Cg3’
Cg3
Cg2’
a
o
Cg2
c
Fig. 2. Molecular packing in the crystal structure of o-PYO I.
˚
DP = 3.545 A, a = 0.0ꢁ, ring Cg3 [C12–C13–C14–C15–
12.28(14)/4.16(13)ꢁ (molecule A) and 10.62(13)/2.40(14)ꢁ
(molecule B). The differences between the angles within
one molecule result from the different participation in the
intermolecular interactions.
˚
C16–C17] and Cg3 at (x,1 + y,z), DC = 3.884 A,
0
0
0
0
˚
DP = 3.485 A, a0 = 0.0ꢁ, and ring Cg3 [C12 –C13 –C14 –
0
0
0
˚
C15 –C16 – C17 ] and Cg3 at (x,1 + y,z), DC = 3.884 A,
˚
DP = 3.506 A, a = 0.0ꢁ. Along the [101] direction alternat-
Fig. 3 illustrates the formation of molecular stacks along
the a axis inclined by 33.28ꢁ to this axis. Among the adja-
cent layers p–p electron interactions between the crystallo-
graphically independent oxadiazole molecules occur. Pairs
of molecules are formed in which two molecules are related
by a center of inversion providing the possibility for p–p
electron interactions between the oxadiazole and the pyr-
idyl ring. In case of molecule A, the second pyridyl ring
is not involved in p contacts with oxadiazole, but interacts
with a symmetry equivalent pyridyl ring instead. The slight
ing layers of head to tail arranged molecules connected via
the 21 screw axis are formed.
The second modification of 2,5-bis(2-pyridyl)-1,3,4-oxa-
diazole (o-PYO II) crystallizes in the triclinic space group
P-1 with two crystallographically independent molecules
in the asymmetric unit. As in p-PYO I, the molecules adopt
cisoid I conformation. Both molecules show a slight devia-
tion from planarity. The dihedral angle between the central
oxadiazole ring and both pyridyl rings amounts to

F. Emmerling et al. / Journal of Molecular Structure 800 (2006) 74–84
79
c
o
Cg1’
Cg2’
Cg1
b
Cg2’
Cg3
Cg3’
Cg3
Cg1’
a
Cg1
Cg1’
Cg2’
Cg1’
Cg3’
Cg2’
Fig. 3. Three-dimensional molecular network formed by p–p electron interactions in the crystal structure of o-PYO II.
torsion of the rings gives rise to better p–p electron interac-
tions. Further p–p electron interactions between the mole-
cules leads to the formation of a three-dimensional
network as illustrated in Fig. 3.
c-axis arranged in a herring bone structure motif forming
a dihedral angle of 73.0(9)ꢁ. Within a stack the oxadiazole
ring is located between two pyridyl rings of adjacent mole-
cules, leading to p–p interactions between the rings (cf.
Fig. 4). The corresponding parameters are as follows: for
ring Cg2 [C4A–C5A–N6A–C7A–C8A–C9A] and Cg1
[O1A–C2A–N3A–N3A0–C2A0] at (ꢀx,1 ꢀ y,1 ꢀ z) as well
The numerical values of the parameters characterizing
the p–p electron interactions are as follows: for ring Cg1
[O1–C2–N3–N4–C5] and Cg3 [C12–C13–C14–C15–C16–
˚
˚
N17] at (1 ꢀ x,1 ꢀ y,ꢀz), DC = 3.8042 A, DP = 3.254
as Cg2 and Cg1 at (x,ꢀy,1/2 + z), DC = 3.935 A,
˚
˚
and 3.369 A, a = 4.16ꢁ; for ring Cg2 [C6–C7–C8–C9–
DP = 3.215 and 3.439 A, a = 6.65ꢁ; and for ring Cg4
˚
C10–N11] and Cg2 at (2 ꢀ x,ꢀy,2 ꢀ z), DC = 3.669 A,
[C4B–C5B–N6B–C7B–C8B–C9B] and Cg3 [O1B–C2B–
0
0
0
0
0
DP = 3.634 A, a = 0ꢁ, and for Cg1 [O1 –C2 –N3 –N4 –
N3B–N3B0–C2B0] at (1 ꢀ x,1 ꢀ y,1 ꢀ z) as well as Cg4
˚
C50]
and
Cg20
[C60–C70–C80–C90–C100–N110],
˚
and Cg3 at (x,1 ꢀ y,1/2 + z), DC = 3.893 A, DP = 3.231
˚
˚
˚
DC = 3.742 A, DP = 3.377 and 3.318 A, a = 2.39ꢁ, for
and 3.415 A, a = 5.58ꢁ. Along the b axis ribbons of symme-
Cg3 and Cg30 at (1 ꢀ x,1 ꢀ y,1 ꢀ z) [C120–C130–C140–
try equivalent molecules are formed, generated by hydro-
C150–C160–N170],
˚
DC = 3.845 A,
DP = 3.735
and
gen bonds C5–H5ꢁ ꢁ ꢁN3 (with d(Dꢁ ꢁ ꢁA) = 3.583 A,
˚
˚
3.403 A, a = 14.03ꢁ, and for Cg20 and Cg20 at
d(Hꢁ ꢁ ꢁA) = 2.58 A, and \(D–Hꢁ ꢁ ꢁA) = 146ꢁ) and
˚
˚
˚
˚
(1 ꢀ x,ꢀy,1 ꢀ z), DC = 4.081 A, DP = 3.460 A, a = 0.0ꢁ.
Fig. 1c shows an ORTEP plot of m-PYO. The asymmet-
ric unit is built by two crystallographically independent
molecular fragments, each of them leading to a complete
molecule generated by a twofold rotation axis through
the central oxygen atom and the diazole basis. In m-PYO
(with cisoid I conformation) the angles between the oxadia-
zole ring and both pyridyl groups amount to 6.6(3)ꢁ for
molecule A and 5.6(3)ꢁ for molecule B with the correspond-
ing torsion angles O1A–C2A–C4A–C5A = 5.7(8)ꢁ and
O1B–C2B–C4B–C5B = 4.5(7)ꢁ. In the crystal structure
(space group P2/c), the crystallographically independent
molecules of m-PYO form alternating stacks along the
C9–H9ꢁ ꢁ ꢁN6 (with d(Dꢁ ꢁ ꢁA) = 3.469 A, d(Hꢁ ꢁ ꢁA)
=
˚
2.47 A, and \(D–Hꢁ ꢁ ꢁA) = 156ꢁ) between adjacent mole-
cules. Within the stacks adjacent ribbons are aligned anti-
parallel connected via the center of inversion at Æ000æ.
The structure of the para compound p-PYO ([38], see
also [20]) is known from previous research. The structural
properties are recounted here only as far as is necessary
for comparison with those of the ortho and meta com-
pounds. The main features of the crystal structure of
p-PYO and its packing motif are described by stacks along
the a-direction formed by nearly planar and parallel mole-
cules. The molecular plane is inclined to this axis by 40ꢁ, all
molecules sharing the same orientation. Within a stack, the

80
F. Emmerling et al. / Journal of Molecular Structure 800 (2006) 74–84
Fig. 4. p–p interactions and non-classical hydrogen bonds in the crystal structure of m-PYO.
oxadiazole ring is embedded between two pyridyl rings of
adjacent molecules giving rise to p–p interactions between
the rings. Therefore, these interactions are oriented in the
a,c-plane. Van der Waals interactions link the stacks. The
molecules are nearly planar, the dihedral angle between
the oxadiazole ring and both pyridyl rings amounting to
3.3ꢁ. Both rings are rotated in opposite directions with
respect to the oxadiazole ring plane [38].
dihedral angle between both pyridyl rings is mainly below
10ꢁ, exceptions are found for a specific silver complex
(25.7ꢁ) [18], a Cu(II) complex (20ꢁ) [19] or the proton trans-
fer salt with pyromellitic acid (19.5ꢁ) [16]. The twist
between the individual pyridyl rings and the oxadiazole
ring is also rather small, mainly in the range around
4ꢁ–5ꢁ, but in special cases it may also be larger as in the
case of the mentioned Ag complex for one of the two mol-
ecules forming the complex (6.1ꢁ/21.4ꢁ) [18]. Larger torsion
angles are also observed for cocrystals of p-PYO with
trimesic and pyromellitic acids [16].
In most metal complexes, the p-PYO molecule is also
almost flat, with complete coplanarity of all rings observed
in some Zn(II), Co(II) and Cu(II) complexes [14,19]. The

F. Emmerling et al. / Journal of Molecular Structure 800 (2006) 74–84
81
The molecular conformations of o-PYO and m-PYO as
obtained for the pure ligand may now be compared to
those derived from structure determinations of the different
metal complexes. Among these, structural variability is
most pronounced for the meta-pyridyl compound
m-PYO. All three theoretically possible different molecular
conformations according to Scheme 2 were observed in
dependence on the metal ion forming the complex. These
may be arranged in complex coordination networks or
polymers. In Zn-, Cu-, Hg- or Ag-complexes, the m-PYO
molecule adopts the cisoid I conformation leading to a
closed structure in the supramolecular arrangement
[22,25–27] (cf. Table S11 in the Supplementary data) while
the cisoid II form produces open structures and is mainly
observed for some copper complexes [23,25] or in cocrys-
tallization experiments with phthalic, isophthalic or tere-
phthalic acid [17]. The rare transoid conformation of the
molecule has only been found in an Fe and Mn complex
or in cocrystallization experiments of m-PYO with trimesic
acid [16].
copper complexes [23,25]. In most complexes, however,
m-PYO is characterized by a mean dihedral angle around
10ꢁ. It is interesting to note that in the aforementioned
cocrystallization experiments, the molecule adopts a flatter
form with dihedral angles mainly below 5ꢁ despite some
exceptions [16,17]. The dihedral angles between the central
oxadiazole ring and both adjacent pyridyl rings are in most
cases below or around 10ꢁ. Larger values of approximately
18ꢁ and 20ꢁ are only found for specific copper complexes
[23,25].
The complexation behavior of the ortho-pyridyl com-
pound o-PYO was less intensively studied up to now. Only
some Cu and Ni complexes have been prepared [39–41]
where the dihedral angle between both pyridyl rings varies
in larger regions between approximately 1ꢁ for a mononu-
clear Cu complex [39] and 24ꢁ for a Ni complex [41].
3.2. Structural analysis of aminophenyl compounds o-APO
and m-APO
Apart from the general shape of the molecule as deter-
mined by the orientation of the pyridyl rings also the dihe-
dral angle between the different rings is variable thus
providing the ability to adapt to different coordination
requirements. Additionally, this ligand may bind metal
ions simultaneously by the nitrogen atoms of the pyridyl
rings as well as those of the oxadiazole ring giving rise to
different supramolecular topologies.
The crystal structure of o-APO is plotted in Fig. 5. The
molecules of o-APO (see Fig. 1d) adopt the cisoid II confor-
mation and have almost planar structure, showing only a
slight torsion of the outer aminophenyl rings. The rotation
around the bond between the oxadiazole ring and the phen-
yl rings amounts to +1.3ꢁ and ꢀ2.2ꢁ, respectively, the two
phenyl rings in this molecule being tilted in opposite direc-
tions to the oxadiazole ring. The planar structure of the
molecule is further stabilized by two intramolecular hydro-
gen bonds between amino N–H and the oxadiazol nitrogen
The dihedral angle between both pyridyl rings varies
between 0ꢁ for complete coplanarity as determined for
Ag complexes [26,27] up to approximately 18ꢁ in different
˚
atoms, namely N18–H181ꢁ ꢁ ꢁN4 with d(Dꢁ ꢁ ꢁA) = 2.829 A,
Cg2
Cg3
Cg1
N18
N4
N3
N19
N19
N4
Fig. 5. Hydrogen bonding and p–p interactions in the crystal structure of o-APO.

82
F. Emmerling et al. / Journal of Molecular Structure 800 (2006) 74–84
˚
d(Hꢁ ꢁ ꢁA) = 2.07 A, and \(D–Hꢁ ꢁ ꢁA) = 134ꢁ and N19–
small deviations from planarity, the dihedral angles
between the central oxadiazole ring and the adjacent ami-
nophenyl rings amount to 0.1(9)ꢁ and 0.8(9)ꢁ. The molecule
adopts a cisoid II configuration.
˚
˚
H192ꢁ ꢁ ꢁN3 with d(Dꢁ ꢁ ꢁA) = 2.801 A, d(Hꢁ ꢁ ꢁA) = 2.04 A,
and \(D–Hꢁ ꢁ ꢁA) = 136ꢁ.
In the three-dimensional structure the molecules form
stacks along the b-axis, inclined by 41.47ꢁ against the stack
axis. The resulting herring bone-type of structure is illus-
trated in Fig. 5. In the stacks, the planes of the molecules
are aligned parallel to each other and oriented in the same
direction regarding the oxygen atom of the oxadiazole ring.
Within the stacks electron interactions between an oxadia-
zole ring and the two neighbouring phenyl rings situated
above and below it can be observed, namely between ring
Cg1 [O1–C2–N3–N42–C5] and ring Cg2 [C6–C7–C8–C9–
C10–C11] at (x,ꢀ1 + y,z) as well as Cg2 and Cg1 at
The arrangement of the m-APO molecules in the crystal
structure is influenced by hydrogen bonds and p–p electron
interactions (Fig. 6). The p–p electron interactions give rise
to the formation of stacks in b-direction. Within these
stacks, each oxadiazole ring is located between two amino-
phenyl moieties of adjacent molecules, leading to the
following p–p interactions: ring Cg1 [O1–C2–N3–N4–C5]
and Cg2 [C6–C7–C8–C9–C10–C11] at (x,1 + y,z) as well
˚
as Cg2 and Cg1 at (x,ꢀ1 + y,z), DC = 3.565 A, DP =
˚
3.309 and 3.288 A, a = 1.16ꢁ, and ring Cg1 and Cg3
˚
˚
(x,1 + y,z), with DC = 3.634 A, DP = 3.402 A and
3.344 A, a = 2.75ꢁ and between ring Cg1 and Cg3 [C9–
C10–C11–C12–C13–C14] at (x,1 + y,z) as well as between
[C12–C13–C14–C15–C16–C17] at (x,ꢀ1 + y,z) as well as
˚
˚
˚
Cg3 and Cg1 at (x,1 + y,z), DC = 3.572 A, DP = 3.349 A
˚
and 3.338 A, a = 0.82ꢁ. In the b and c plane the stacks
˚
ring Cg3 and Cg1 at (x,ꢀ1 + y,z), with DC = 3.663 A,
are arranged in a herring bone pattern connected via
hydrogen bonds formed between the amino groups of
adjacent molecules. There are two symmetry independent
˚
˚
DP = 3.366 A and 3.296 A, a = 3.98ꢁ.
Along the a-axis the molecules are arranged in corrugat-
ed double layers, consisting of two ribbons of oxadiazole
molecules with the amino groups oriented towards each
other. This orientation gives rise to intermolecular hydro-
gen bonds, connecting each o-APO to three neighbouring
molecules, leading to the formation of two-dimensional
layers. The numerical values are: N18–N18ꢁ ꢁ ꢁN4 at
hydrogen bonds, namely: N18–H18Aꢁ ꢁ ꢁN19 (x,ꢀ1/2 ꢀ y,
˚
1/2 + z)
with
d(Dꢁ ꢁ ꢁA) = 3.374 A,
d(Hꢁ ꢁ ꢁA) =
˚
2.54 A, and \(D–Hꢁ ꢁ ꢁA) = 149ꢁ and N19–H19Aꢁ ꢁ ꢁN3
˚
(x,1/2 ꢀ y,ꢀ1/2 + z)
with
d(Dꢁ ꢁ ꢁA) = 3.142(10) A,
˚
d(Hꢁ ꢁ ꢁA) = 2.28 A, and \(D–Hꢁ ꢁ ꢁA) = 147ꢁ. Two of these
layers are connected via hydrogen bonds built between the
oxadiazole nitrogen atoms and the amino groups, namely
˚
(1/2 ꢀ x,ꢀ1/2 ꢀ y,ꢀz), d(Dꢁ ꢁ ꢁA) = 3.338 A, d(Hꢁ ꢁ ꢁA) =
˚
˚
2.61 A, and \(D–Hꢁ ꢁ ꢁA) = 130ꢁ and N18–H18ꢁ ꢁ ꢁN18
N19–H19Bꢁ ꢁ ꢁN3 (ꢀx,1 ꢀ y,ꢀz) with d(Dꢁ ꢁ ꢁA) = 3.207 A,
˚
˚
(1/2 ꢀ x,1/2 + y,1/2 ꢀ z), d(Dꢁ ꢁ ꢁA) = 3.323 A, d(Hꢁ ꢁ ꢁA)
d(Hꢁ ꢁ ꢁA) = 2.46 A, and \(D–Hꢁ ꢁ ꢁA) = 136ꢁ and N18–
˚
= 2.37 A, and \(D–Hꢁ ꢁ ꢁA) = 167ꢁ.
H18Bꢁ ꢁ ꢁN4 (ꢀx,1/2 + y,1/2 ꢀ z) with d(Dꢁ ꢁ ꢁA) =
˚
˚
Fig. 1e shows an ORTEP plot of a molecule of m-APO.
3.136(8) A, d(Hꢁ ꢁ ꢁA) = 2.22(7) A, and \(D–Hꢁ ꢁ ꢁA) =
154ꢁ to form double layers stacked along the a axis.
Again, for comparison, the crystal structure of p-APO
as determined previously [42] shall be discussed shortly.
Both aminophenyl groups and the central oxadiazole ring
are nearly planar with rms deviations of 0.006 A,
0.006 A, and 0.007 A, respectively. The molecule shows
˚
˚
˚
Fig. 6. Molecular packing in the crystal structure of m-APO.

F. Emmerling et al. / Journal of Molecular Structure 800 (2006) 74–84
83
The molecular packing in the crystal structure of p-APO is
4. Conclusions
governed by hydrogen bonds, p–p electron interactions and
van der Waals contacts. Two symmetry independent
hydrogen bonds and the symmetry related bonds give rise
to the formation of a three-dimensional hydrogen bond
network. Main components of these bonds are preferential-
ly located in the a,c-plane. Interactions of the p electrons of
the different rings develop between the oxadiazole and phe-
nylene rings of symmetry related p-APO molecules mainly
in b-direction. The molecules are slightly twisted, the tor-
sion angle between both phenylene rings and the central
oxadiazole ring amounts to 15.4ꢁ and 14.2ꢁ while that
between both phenylene rings is approximately 28.1ꢁ [42].
It is worth mentioning that this compound also shows
pseudopolymorphism. A second structure was determined
for p-APO where water molecules are incorporated into
the crystal lattice in channels along the z-direction. Under
heating the water is released and the polymorph transforms
to the structure of the water-free polymorph [42].
Metal complexes of this molecule have been obtained
with silver(I) cations [29]. Here, the torsion angles of the
rings against each other are changed slightly when com-
pared to the pure ligand crystal. The two independent
ligands show angles of 19.6ꢁ/4.8ꢁ and 7.9ꢁ/7.4ꢁ between
phenylene and oxadiazole rings and 18.2ꢁ and 13.4ꢁ,
respectively, for both phenylene rings.
A copper(II) complex of o-APO was investigated in [30]
where a cisoid II configuration was found for the ortho
amino groups on both phenylene rings. m-APO is able to
form complexes with silver(I) ions [27,29], where these mol-
ecules also adopt the cisoid II conformation although the
amino groups are now located in the meta positions of
the phenylene rings.
All investigated structures show substantial similarities,
summarized in Table 2, even though the chemical structure
of the molecules is different. The molecules are almost flat
and the torsion angles between the central oxadiazole ring
and the adjacent aryl rings in the range around 5–10ꢁ for
the pyridyl compounds and between 1ꢁ and 2ꢁ for the ami-
nophenyl compounds are nearly comparable to those
found for the different complexes. It should be noted that
the aminophenyl molecules are flatter compared to the pyr-
idyl compounds due to the formation of intramolecular
(o-APO) and intermolecular hydrogen bonds (o- and m-
APO). The interring bonds are well in the range of
sp2–sp2 r-bonds. For both amino compounds, the forma-
tion of hydrogen bonds leads to a preferred cisoid II con-
formation, whereas the pyridyl compounds adopt the
cisoid I form. The rare transoid conformation is not found
in the pure crystals of both compounds although it cannot
be ruled out that further polymorphs may exist as have
been found for o-PYO. The crystal structures are mainly
characterized by the formation of molecular stacks that
lead to p–p interactions. In some cases, the herring bone
motif is found.
As the investigation of various metal–organic complexes
using the studied molecules reveals, these molecules show a
larger structural variability with regard to their conforma-
tion (Table S11 in the Supplementary information compiles
some characteristic examples). However, the summarizing
Table 2 suggests a different view. The crystal structure anal-
ysis of the pure ligand crystals leads to the conclusion that
the conformation of the molecules is already determined by
the ligand molecule itself, leading to characteristic syn-
thons. The complexing metal atoms have only minor influ-
ence. However, the above results and the analysis of the
existing data on complex structures clearly suggest that fur-
ther investigations focused on metal ligand interactions are
required to elucidate the principles of structure formation
in systems containing the discussed molecules.
It should be noted that in many of the investigated com-
plexes or cocrystals independent of the specific pyridyl- or
aminophenyl-oxadiazole compound also face-to-face p–p
interactions occur between complexed or uncomplexed
ligands due to the stack arrangement of the different aro-
matic rings.
Table 2
Conformation and intermolecular interactions in the crystal structures of o-, m-, p-PYO and o-, m-, p-APO
Compound reference
Dihedral angle
R1-Oxa/Oxa-R2 (ꢁ)
Interring distance
R1-Oxa/Oxa-R2 (A)
Conformation
Intramolecular
hydrogen bonds
Intermolecular
interactions
˚
p–p
Hydrogen bonds
o-PYO I Mol A
o-PYO I Mol B
o-PYO II Mol A
o-PYO II Mol B
m-PYO Mol A
m-PYO Mol B
p-PYO [38]
o-APO
m-APO
p-APO I [42]
p-APO II [42]
3.8/3.6
9.5/10.5
12.3/4.2
10.6/2.4
6.6/6.6
5.6/5.6
3.7/3.7
2.2/1.3
0.8/0.1
8.8/8.8
15.4/14.3
1.466/1.451
1.463/1.482
1.459/1.450
1.460/1.454
1.458/1.458
1.468/1.468
1.464/1.464
1.452/1.465
1.478/1.469
1.454/1.454
1.461/1.463
cisoid I
cisoid I
cisoid I
cisoid I
cisoid I
cisoid I
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
+
ꢀ
ꢀ
ꢀ
+
+
+
+
+
+
+
+
+
+
+
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
+
+
+
+
cisoid II
cisoid II
ꢀ
ꢀ

84
F. Emmerling et al. / Journal of Molecular Structure 800 (2006) 74–84
[17] M. Du, Z.-H. Zhang, X.-J. Zhao, Cryst. Growth Des. 5 (2005)
Appendix A. Supplementary data
1199.
[18] M. Du, X.-J. Zhao, J. Mol. Struct. 694 (2004) 235.
[19] Y.-B. Dong, J.-P. Ma, R.-Q. Huang, F.-Z. Liang, M.D. Smith,
Dalton Trans. (2003) 1472.
[20] J.-Y. Cheng, Y.-B. Dong, J.-P. Ma, R.-Q. Huang, M.D. Smith, Inorg.
Chem. Commun. 8 (2005) 6.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.
2006.03.076.
[21] M. Du, X.-J. Jiang, X.-J. Zhao, Acta. Crystallogr., E. 61 (2005) m485.
[22] Z. Huang, M. Du, H.-B. Song, X.-H. Bu, Cryst. Growth Des. 4 (2004)
71.
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