Synthesis and properties of iridium complexes based 1,3,4-oxadiazoles derivatives

Article abstract of DOI:10.1016/j.tet.2007.11.099

A series of iridium complexes with 2,5-diaryl-[1,3,4]-oxadiazole ligands were synthesized and their electrochemical, photophysical, and electroluminescent (EL) properties studied. It was found that electron-withdrawing or donating substituents on the phenyl ring affected the emission maxima. Complex 3, iridium(III) bis(2,5-bis-(2-hydroxyphenyl)-[1,3,4]oxadiazolato-C²′,N³) (acetyl acetonate), was characterized by single-crystal X-ray structural determination. Three organic light emitting diodes devices were fabricated, which showed stable green-yellow luminescence.

Full text of DOI:10.1016/j.tet.2007.11.099

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 1860e1867
www.elsevier.com/locate/tet
Synthesis and properties of iridium complexes based
1,3,4-oxadiazoles derivatives
Zhaowu Xu a, Yang Li a, Xuemei Ma a, Xindong Gao b, He Tian a,
*
a Key Laboratory for Advance Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai, 200237, China
b State Key Laboratory on Surface Physics, Fudan University, Shanghai 200433, China
Received 29 September 2007; received in revised form 26 November 2007; accepted 28 November 2007
Available online 4 December 2007
Abstract
A series of iridium complexes with 2,5-diaryl-[1,3,4]-oxadiazole ligands were synthesized and their electrochemical, photophysical, and elec-
troluminescent (EL) properties studied. It was found that electron-withdrawing or donating substituents on the phenyl ring affected the emission
2
0
3
maxima. Complex 3, iridium(III) bis(2,5-bis-(2-hydroxyphenyl)-[1,3,4]oxadiazolato-C ,N ) (acetyl acetonate), was characterized by single-
crystal X-ray structural determination. Three organic light emitting diodes devices were fabricated, which showed stable green-yellow lumine-
scence.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
thermal, and chemical stabilities. It has been shown previously
that 2-(4-biphenylyl)-5-(4-tertbutylphenyl)-[1,3,4]-oxadiazole
(PBD) functions very well as an excellent electron-transport
material (ETM) in multilayer OLEDs.7
Organic light emitting diodes (OLEDs) have attracted a great
deal of interest due to their potential application in flat displays.1
Particularly, phosphorescent materials like iridium(III) com-
plexes have received considerable attention because of their
high phosphorescence quantum efficiency, which is due to mix-
ing the singlet and the triplet excited states via spin-orbit coup-
ling and enhancing the triplet-state subsequently.2e4 The
research on iridium complexes as well as their application in
OLEDs are mainly focused on designing new and facile ligands.
Based on (iso) quinoline, pyrazine, pyrimidine, pyridine, and
pyrazole derivatives, several OLEDs with efficient blue, green,
and red emission iridium(III) phosphorescencewere fabricated.5
Recently, some heteroleptic iridium(III) complexes based
on fluorinated 1,3,4-oxadiazole were reported, which could
be tuned to blue by dithiolate ancillary ligands.61,3,4-Oxadi-
azole derivatives are one of the most widely studied classes
of electron-injection/hole-blocking materials due to their elec-
tron deficiency, high photoluminescence quantum yield, good
In this paper, we designed and synthesized a series of iridium-
(III)eoxadiazole complexes as emitting materials. The ligands
of iridium with different substituents show different electronic
properties, which can be expected to synthesize color tunable
emissive iridium complexes. Subsequently, we expected to in-
crease the understanding of the structureeproperty relationship
of these iridium complexes better. Finally, the usefulness of
three complexes as triplet emitters in electroluminescent de-
vices was examined by applying them as guests in CBP hosts
in two different multilayered device architectures. It is expected
that incorporation of an electron-deficient oxadiazoles can in-
crease the electron affinity and transporting properties of these
complexes, which would lead to the reduction of driving volt-
age and the increase in energy conversion efficiency.
2. Results and discussion
Synthetic routes for the ligands and their iridium complexes
are outlined in Scheme 1. 2-Aryl-5-alkyl-[1,3,4]-oxadiazoles
* Corresponding author. Fax: þ86 21 64252288.
E-mail address: tianhe@ecust.edu.cn (H. Tian).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.11.099

Z. Xu et al. / Tetrahedron 64 (2008) 1860e1867
1861
N
N
Ar' COOH
H2O
NH2NH2
Ar CONHNH2
Ar
Ar'
a
b
Ar COOC2H5
POCl3,reflux,5h
O
C2H5OH,reflux,17h
N
N
(i)OHCH2CH2OCH3:H2O=3:1,reflux,24h
Iridium complexes
+
IrCl3·3H2O
Ar'
Ar
(ii)OHCH2CH2OCH3,Na2CO3,acetyl acetone
O
reflux,24h
H3CO
N
N
N
N
O
O
OCH3
O
Ar=Ar'=
Ar=Ar'=
O
O
O
Ir
Ir
2
F
1
OCH3
4
F
F
N
N
Ar=
O
O
N
N
F
F
Ar=Ar'=
Ir
O
O
F
O
Ir
O
OCH3
Ar'=
2
F
2
2
OCH3
5
OH
N
N
O
O
Ar=Ar'=
Ir
Ar=Ar'=
O
HO
N
N
HO
O
O
2
Ir
O
3
2
6
N
N
N
O
O
O
Ar=Ar'=
N
Ir
2
N
7
Scheme 1. Synthesis of IreOXD complexes.
were so unstable under the cyclometalated reaction condition
used that did not yield the desired iridium complexes. The syn-
theticmethodusedtopreparethesecomplexesinvolvestwosteps.
In the first step, IrCl3$3H2O was allowed to react with an excess
of the cyclometalated ligands (2.5ꢀ) to give a chloride-bridged
dinuclear complexes, i.e., (xOXD)2Ir2(m-Cl)2(xOXD)2. The chlo-
ride-bridged dinuclear complexes can be readily converted to
emissive, mononuclear complexes (xOXD)2Ir(acac) by replacing
the two bridging chlorides with bidentate acetyl acetonate. These
reactions result in (xOXD)2Ir(acac) with a yield of 70e85%.
To have an insight into the highest occupied molecular
orbital (HOMO) energy levels of the complexes, cyclic
voltammetry experiments were carried out for all complexes.
However, no reduction processes were observed in dichloro-
methane in the range of ꢁ2 to 2 V.8 The oxidation potentials
of all complexes were collected in Table 1. Figure 1 depicts
the cyclic voltammetry of complex 1, 2, 4, and 5 in CH2Cl2
solution at a 100 mV/s scan rate under argon atmosphere.
The decreasing order of oxidation potential of the complexes
has been observed in the following: 2 (4,40-difluoro)>5 (2,6-
difluoro-40-methoxyl)>1 (unsubstituted)>4 (4, 40-dime-
thoxyl). This observation shows that different substituent
groups can tune the electronic nature of these complexes.
Complex 2 displays a significantly anodic shift (210 mV),

1862
Z. Xu et al. / Tetrahedron 64 (2008) 1860e1867
Table 1
Photophysical and electrochemical data for (OXD)2Ir(acac)
b
lem (nm)
c
Eox (V)
Complex
Absorption l (log 3)a (nm)
DE (eV)
Fem
1, (OXD)2Ir(acac)
267 (4.9), 289 (4.8), 415 (3.9)
290 (4.8), 397 (3.8)
524
508
516
512
519
569
536
2.4
2.5
2.4
2.5
2.5
2.2
2.4
0.41
0.33
0.12
0.10
0.19
0.28
0.65
1.24
1.45
1.24
1.22
1.38
1.07
1.44d
2, (dfOXD)2Ir(acac)
3, (dhOXD)2Ir(acac)
4, (dmOXD)2Ir(acac)
5, (dfmOXD)2Ir(acac)
6, (nOXD)2Ir(acac)
7, (dcOXD)2Ir(acac)
266 (4.9), 293 (4.8), 410 (4.0)
269 (4.8), 300 (4.9), 407 (3.9)
290 (4.8), 415 (3.8)
261 (4.9), 315 (4.6), 477 (3.2)
290 (4.9), 344 (4.5), 445 (3.7)
a
Measured in THF solution.
b
Measured in THF at 298 K excited at 340 nm. (ppy)2Ir(acac) was used as the reference for the quantum yield.
Measured in CH2Cl2 at a concentration of 10ꢁ3 M and scan rate was 100 mV/s.
Complex 7 has an reversible oxidation potential.
c
d
0.6
1
2
1
2
4
5
0.5
4
5
0.8
0.4
0.3
0.4
0.2
0.1
0.0
0.0
300
400
500
Wavelength/nm
600
700
-0.1
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.8 1.6 2.0 2.2
Potential (V), Vs Ag/AgCl
Figure 2. Absorption spectra of complexes 1, 2, 4, and 5 in THF (1ꢀ10ꢁ5 M).
Figure 1. Cyclic voltammograms of complexes in 0.1 M n-Bu4NClO4 at a scan
rate of 100 mV/s.
phosphorescence quantum yields (Fp¼0.1e0.65). Addition-
ally, large Stokes shift between the lowest energy absorption
and emission bands and resolved vibronic structure of phos-
phorescent emission spectra suggested that these iridiume
oxadiazole complexes mainly emit from pep* transitions.
As shown in Figure 3, complex 6 showed a nearly 44 nm
bathochromic emission shift as compared to that of complex
compared to the unsubstituted complex 1. The fact indicated
that electron-withdrawing groups can lower the HOMO energy
levels effectively. However, the electron-donating substituent
group, eOCH3, does not affect the HOMO level effectively,
as evidenced by the fact that the oxidation potential of com-
plex 4 exhibits a slight lower (DE¼ꢁ20 mV) than that of
complex 1. Furthermore, it should be noted that the oxidation
potential of complex 5, which contains eF and eOCH3 to-
gether is in the middle of that of complex 2 and complex 4.
The photophysical and electrochemical data of (OXD)2Ir-
(acac) complexes are collected in Table 1. Figure 2 shows
the absorption spectra of complex 1, 2, 4, and 5 as representa-
tive examples of this series of complexes. It is suggested that
the bands observed in the ultraviolet part of the spectrum
(250e350 nm) can be assigned to the allowed ligand-centered
(pep*) transitions. Somewhat weaker, broad, and featureless
absorption bands located in the lower part of energy
(lmax>400 nm) suggest that these bands are attributed to the
coexistent spin-allowed metal-to-ligand charge transfer
1MLCT and spin-forbidden 3MLCT/3pep* transitions as
result of the spin-orbit coupling of iridium(III) according to
the bands position, size, and the extinction coefficients.3a
All of these complexes show strong luminescence in the
THF solutions. Most of the complexes exhibit high solution
3
1
1.0
2
4
6
0.8
7
0.6
0.4
0.2
0.0
450
500
550
Wavelength/nm
600
650
Figure 3. Normalized photoluminescence spectra of the complexes in THF
(1ꢀ10ꢁ5 M).

Z. Xu et al. / Tetrahedron 64 (2008) 1860e1867
1863
1 owing to the p-system increasing effect. Apparently, the
shift is smaller than that of ppy-based system of Ir(ppy)2(acac)
and Ir(NaPy)2(acac) [NaPy¼2-(1-naphthalenyl)pyridine] but
similar with the pip-based system of Ir(pip)2(acac) and Ir(Na-
pip)2(acac) [Napip¼2-(1-naphthalenyl) imidazo[1,2-a]pyri-
dine].3 However, unlike the pipiridium system, the HOMO
of complex 6 has remarkable change, which could be sup-
ported by the electrochemical data. This difference can be
explained that the HOMO of these complexes are somewhat
extended to the phenyl-p and Ir-d orbitals like revealed by
the MO calculations of ppy-based complexes.
We still investigated the substituents attached to the phenyl
rings, including electron-withdrawing and donating substitu-
ents. The emission maximum of complex 2 with fluorine
atoms on the phenyl rings revealed a 16 nm blue shift with re-
spect to that of the complex 1, which is due to the increase of
HOMOeLUMO gap. A similar blue shift was observed in the
end-absorption of complex 1 and 2. This observation is consis-
tent with the research of Ireppy complexes. Introducing fluo-
rine atoms to the phenyl rings would lower and stabilize the
HOMO levels. Similarly, a blue shift of 12 nm of the emission
maximum was observed by using electron-donating groups,
eOCH3, instead of fluorine atoms. This observation concern-
ing the substitution effect is like the results in the iridiume
phenylbenzthiazole system. Chen et al. revealed that the iri-
diumephenylbenzthiazole complexes with various substituents
(eF, eOCH3, eCH3) exhibit shorter emission maxima than that
of unsubstituted complex.9 In addition, the emission maxima of
complex 5 exhibits longer by 9 nm than that of complex 2 and
shorter by 5 nm than that of complex 1, which is consistent
with the observation of electrochemical properties. It can be in-
ferred that the photophysical properties of oxadiazoles based
iridium complexes are mainly dependent on the properties of
the phenyl ring iridium cyclometalated. Based on the discussion
above, it is suggested that controlling the substituents on the
phenyl may realize color emission tunable.3b,3d
It should be noted that converting the substituents with 4,40-
dicarbazole, the maximum emission peak of complex 7 shows
a large bathochromic shift compared to the unsubstituted com-
plex owing to electron-donating effect and increase of ligand
p-system. Additionally, complex 7 exhibits the highest phos-
phorescent quantum efficiency in this series of complexes,
up to 0.65. There may be mainly two reasons: one is the
high efficiency of carbazole group, the other is sterically hin-
dered effect.10,11 We hope this fact would provide useful infor-
mation on fabricating OLEDs devices.
Figure 4. ORTEP figure of (2-OHOXD)2Ir(acac) with 50% probability
ellipsoids.
and these observations reflect the large trans influence of the
phenyl groups. All other bond lengths and bond angles within
the chelate ligands are analogous to the similar type of
complexes.3
We have also used complex 1, 2, and 5 as dopants in the
emitting layer to fabricate electroluminescent (EL) device
(Fig. 5), the maximum brightness is 5430 cd/m2 at 13 V for
device 1, 4969 cd/m2 at 15 V for device 2, and 2091 cd/m2
at 15 V for device 3. It was found that the power efficiency
decreased with increasing current density for the devices as
indicated in Figures 6 and 7. The luminescence efficiency of
device 1 shows a maximum of 9.6 cd/A at 74.8 mA/cm2.
Then the efficiency starts to decrease with increasing current
density, which can be attributed to the increasing tripletetrip-
let annihilation of the phosphor-bound excitons. However,
even at a current density of 537 mA/cm2, the luminescence ef-
ficiency still remains as high as 7.99 cd/A, corresponding to
a 17% loss from the maximum value. We also used complexes
1 and 5 as the dopants and changed the hole blocking material,
from BCP to TPBI, to fabricate devices 2 and 3. Though the
power efficiency, luminescence efficiency, and brightness of
devices 2 and 3 are lower than that of device 1, it is also ob-
served that the decrease of the efficiency is small with increas-
ing current density. This performance is significantly better
than that of most other iridium complexes (Table 2). The
results of luminescence efficiency of these devices are consis-
tent with the study of photophysical properties.
As shown in Figure 8, the device 1 having fluoro-
substituted dopant exhibits the expected blue shift as com-
pared to that of device 2, which was consistent with the PL
investigation. The EL emission spectra matched the emission
spectra of the complex in solution, which indicated that the
EL emission was originated from the triplet excited states of
the phosphors.
Complex 3 has octahedral coordination geometry around Ir
and has crystallographically imposed twofold symmetry, with
the Ir atom lying on a twofold axis (shown in Fig. 4). The
˚
length of IreC bonds of the complex ((IreCav) 1.996(5) A)
˚
are shorter than that of IreN bonds ((IreNav) 2.041(5) A).
The IreC bond length is similar to those in the analogous
complexes reported. Furthermore, the IreN bond lengths
also fall within the range of values for those of the reported
complexes. The IreO bond lengths of 2.131(4) and
3. Conclusions
A series of new iridium complexes using 2,5-diaryl-[1,3,4]-
oxadiazoles with different substituents as the ligands have
been prepared and investigated their electrochemical,
˚
2.138(4) A are longer than the mean IreO bond length of
˚
2.088 A reported in the Cambridge Crystallographic Database

1864
Z. Xu et al. / Tetrahedron 64 (2008) 1860e1867
Al
N
N
LiF
BCP
Alq3
BCP(device 1)/TPBI(device2, 3)
N
N
CBP+7% Ir complex
NBP
ITO
CBP
F
N
N
O
O
O
Ir
N
N
F
Complex 2 for device 1
NPB
N
N
O
O
O
Ir
Complex 1 for device 2
N
N
F
N
N
N
N
F
O
O
O
Ir
N
N
OCH3
TPBI
Complex 5 for device 3
Figure 5. Schematic diagram of EL device configuration and the structures of the compounds.
photophysical, and EL properties. It is demonstrated that the
emission color can be tuned depending upon different ligands.
The electron-withdrawing and donating substituents on the
phenyl of the OXD ligands effectively lower the HOMO levels
to afford emissions with shorter wavelengths. In particular,
replacement of the phenyl group by naphthyl group in 2,5-di-
phenyl-[1,3,4]-oxadiazole results in a red shift of the emission
maxima to 569 nm. Additionally, electroluminescent devices
fabricated using complexes 1, 2, and 5 as the phosphorescent
dopants yielded efficient green-yellow emission. Especially,
device 1 exhibits the best performance within the three de-
vices, having a maximum luminescence efficiency of 9.6 cd/
A at 8.5 V (J¼74.8 mA/cm2) and a maximum brightness of
5430 cd/cm2 at 13 V. It is suggested that these complexes
could be good candidates of new phosphorescent materials
for OLEDs.
4. Experimental section
4.1. General
Absorption spectra were measured on a Varian Cary 500
UVevis spectrophotometer. Fluorescence spectra were mea-
sured on a Varian Cary Eclipse Fluorescence Spectrophoto-
1
meter. H NMR spectra were recorded on a Bruker AM500
spectrometer with tetramethyl silane as internal reference.
Mass spectra were obtained on a HP5989 mass spectrometer.
Luminescence lifetime measurements were performed on an
Edinburgh FL 900 Fluorescence Spectrometer equipped with
a blue laser (l¼372 nm). The quantum efficiency (QE) mea-
surements were carried out at room temperature in degassed
THF solutions. Cyclic voltammetry was performed using an
Potentiostat/Galvanostat Model K0264 (Princeton Applied

Z. Xu et al. / Tetrahedron 64 (2008) 1860e1867
1865
6000
5000
4000
3000
2000
1000
0
device 1
device 2
device 3
1.0
0.8
0.6
0.4
0.2
0.0
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
device 1
device 2
device 3
480 500 520 540 560 580 600
300
400
500 600
Wavelength/nm
700
800
0
2
4
6
8
Voltage/V
10
12
14
16
Figure 8. EL spectra of OLED devices 1, 2, and 3.
Figure 6. Luminance versus voltage characteristics of devices 1, 2, and 3.
Table 3
Selected bond distances (A) and angles ( ) for (2-OHOXD)2Ir(acac)
ꢂ
˚
10
8
10
8
˚
Bond
Distances (A)
2.041(5)
2.041(5)
1.996(5)
1.996(5)
2.116(4)
2.116(4)
device 1
device 2
device 3
Ir(1)eN(1)
Ir(1)eN(1)*
Ir(1)eC(10)
Ir(1)eC(10)*
Ir(1)eO(1)
Ir(1)eO(1)*
6
6
Angles (ꢂ)
171.0(2)
79.4(2)
4
4
N(1)eIr(1)eN(1)*
N(1)eIr(1)eC(10)
N(1)*eIr(1)eC(10)*
N(1)eIr(1)eO(1)
C(10)*eIr(1)eO(1)
O(1)eIr(1)eO(1)*
79.4(2)
2
2
89.75(16)
176.03(15)
89.1(2)
0
0
0
300
600
900
1200
Current density (mA/cm2)
1500
1800
the data using the SADABS program. The structure was solved
using the SHELXL-97 program. The crystallographic refine-
ment parameters of complex 3 are summarized in Table 2, while
the selected bond distances and angles are given in Table 3.
The OLED device is fabricated on indium tin oxide (ITO)
coated glass plate, which is cleaned by routine procedure
including ultrasonication in detergent and de-ionized water se-
quentially and a final UVozone treatment. In a vacuum chamber
at a pressure of <8ꢀ10ꢁ5 Pa, 50 nm of NPB as the hole trans-
porting layer, 20 nm of the complex doped (7%) CBP as the
emitting layer, 10 nm of 2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline (BCP) or 1,3,5-tris(2-N-phenylbenzimidazolyl)-
benzene (TPBI) as a hole and exciton blocking layer (HBL),
40 nm of Alq3 as the electron transporting layer, and a LiF
Figure 7. Current and power efficiencies of the devices 1, 2, and 3.
Research). Anhydrous CH2Cl2 was used as the solvent under
inert atmosphere, and 0.1 M tetra(n-butyl)ammonium pero-
chlorate was used as the supporting electrolyte. A platinum
disk electrode was used as the working electrode, a platinum
wire was used as the counter electrode, and a saturated Ag/
AgCl reference electrode.
A single crystal of complex 3 with the approximate dimen-
sions 0.51ꢀ0.36ꢀ0.33 mm3 was mounted on a Rigaku RAXIS
RAPID IP imaging plate system with Mo Ka radiation
˚
(l¼0.71073 A) at 293(2) K. An empirical absorption was
based on the symmetry-equivalent reflections and applied to
Table 2
Electroluminescent performance of the devices
Device1
Device 2
Device 3
EL peak wavelength
CIE-x
CIE-y
504 nm
0.312
520 nm
0.333
511 nm
0.283
0.429
0.493
0.435
Luminance
720 cd/m2 (at 74.8 mA/cm2)
9.62 cd/A (at 74.8 mA/cm2)
5.24 lm/W (5 V)
4 V
387 cd/m2 (at 74.8 mA/cm2)
5.14 cd/A (at 74.8 mA/cm2)
2.29 lm/W (6.5 V)
3 V
238 cd/m2 (at 74.8 mA/cm2)
3.11 cd/A (at 74.8 mA/cm2)
1.15 lm/W (8 V)
4.5 V
Luminescence efficiency
Power efficiency
Turn-on voltage

1866
Z. Xu et al. / Tetrahedron 64 (2008) 1860e1867
(0.7 nm)/Al (100 nm) bilayer as cathode were sequentially
deposited onto the substrate to give the device structure. The
currentevoltage (IeV) profiles and light intensity characteris-
tics for the above fabricated devices were measured in
atmosphere at ambient temperature. The electroluminescence
(EL) spectra were measured with a Spectra Scan PR705. The
currentevoltage (IeV) and luminanceevoltage (LeV) charac-
teristics were measured with a Keithley 236 source unit and
Keithley 2000-20 multimeter with ST-86LA luminance meter.
4.3.2. Iridium(III) bis(2,5-bis-(4-fluorophenyl)-
0
[1,3,4]oxadiazolato-C2 ,N3) (acetyl acetonate)
(dfOXD)2Ir(acac)
1
Yield: 62%, H NMR (CDCl3) d: 8.26 (m, 2H), 7.61 (dd,
2H, J¼12.5, 3.4 Hz), 7.38 (m, 2H), 7.32 (m, 2H), 6.86
(dꢀd, 2H, J¼13.7, 1.7 Hz), 6.62 (m, 2H), 6.45(d, 2H,
J¼7.6 Hz), 5.31 (s, 1H), 1.93 (s, 6H). MS (m/e, ESI): 829.1
(100, MþNaþ) C33H21F4IrN4O4 (806.11) calcd C 49.19, H
2.63, N 6.95; found C 48.98, H 2.75, N 6.81.
4.2. Synthesis of oxadiazoles ligands
4.3.3. Iridium(III) bis(2,5-bis-(2-hydroxyphenyl)-
0
[1,3,4]oxadiazolato-C2 ,N3) (acetyl acetonate)
(dhOXD)2Ir(acac)
The oxadiazole derivatives were synthesized via a general
procedure where we used the synthesis of 2,5-diphenyl-[1,3,4]-
oxadiazole as an example.7h Benzoate (15 g, 100 mmol) was
added dropwise to the ethanol solution of hydrazine monohy-
drate (26 g, 518 mmol) and refluxed for 17 h. Ethanol was
removed under reduced pressure and the residue was poured
into water. The solid was collected by filtration and then recrys-
tallized from ethanol to yield the pure crystal product, benzohy-
drazide (10.6 g, 70%). Benzohydrazide (7.6 g, 50 mmol) and
benzoic acid (7.5 g, 50 mmol) were dissolved in 50 mL
POCl3. The mixture was stirred at room temperature for 2 h
and then refluxed 5 h. After cooled to room temperature, the
mixture was poured into ice water carefully. The solid was col-
lected by filtration and then recrystallized from ethanol to give
the product, 2,5-diphenyl-[1,3,4]-oxadiazole, in a yield of 45%
(5 g, 23 mmol).
1
Yield: 16%, H NMR (CDCl3) d: 8.27 (t, 2H, J¼7.4 Hz),
7.61 (m, 2H), 7.36 (m, 4H), 6.86 (dꢀd, 2H, J¼8.7, 5.8 Hz),
6.62 (t, 2H, J¼9.0 Hz), 6.45 (d, 2H, J¼7.62 Hz), 5.31 (s,
1H), 1.95 (s, 6H). MS (m/e, ESI): 821.1 (100, MþNaþ)
C33H25IrN4O8 (797.79) calcd C 49.68, H 3.16, N 7.02; found
C 48.77, H 3.08, N 7.14.
4.3.4. Iridium(III) bis(2,5-bis-(4-methoxyphenyl)-
0
[1,3,4]oxadiazolato-C2 ,N3) (acetyl acetonate)
(dmOXD)2Ir(acac)
1
Yield: 72%, H NMR (CDCl3) d: 8.13 (dd, 4H, J¼7.7,
1.9 Hz), 7.51 (d, 2H, J¼8.4 Hz), 7.05 (d, 4H, J¼8.9 Hz),
6.48 (dd, 2H, J¼8.5, 2.4 Hz), 6.19(d, 2H), 5.28 (s, 1H),
3.91(s, 6H, J¼2.4 Hz), 3.62 (s, 6H), 1.91 (s, 6H). MS (m/e,
ESI): 877.2 (100, MþNaþ) C37H33IrN4O8 (853.9) calcd C
52.04, H 3.90, N 6.56; found C 54.01, H 3.76, N 6.63.
4.3. Synthesis of iridiumeoxadiazoles complexes
4.3.5. Iridium(III) bis(2-(2,6-difluorophenyl)-5-
0
4.3.1. Synthesis of (OXD)2Ir(acac)
(4-methoxyphenyl)-[1,3,4]oxadiazolato-C2 ,N3)
(acetyl acetonate) (dfmOXD)2Ir(acac)
All cyclometalated Ir(III) complexes were synthesized by
the same method reported. The synthesis 0 of iridium(III)
bis(2,5-diphenyl)-[1,3,4]oxadiazolato-N3,C2 ) (acetyl aceto-
nate) ((OXD)2Ir(acac)) will be described in detail. Other com-
plexes were synthesized following in a similar procedure.
IrCl3$3H2O (50 mg, 0.14 mmol) and 2,5-diphenyl-[1,3,4]-
oxadiazole (96 mg, 0.43 mmol) were added in a 10 mL mix-
ture of 2-ethoxyethanol and water (v/v¼3:1). The mixture
was refluxed for 24 h and cooled to room temperature. The
solid was collected by filtration and pumped dry to give crude
chloro-bridged dimer complex. Without further purification,
the dimer was added in a mixture of Na2CO3 (88.8 mg,
0.83 mmol), acetyl acetone (5 mL), and 2-ethoxyethanol
(10 mL). After refluxed for 24 h, the solution was cooled to
room temperature and poured into water. The brown precipi-
tate was filtered off and washed with water, hexane, and ether.
The crude product was purified by chromatography on silica
gel using CH2Cl2ehexane¼1:1. A yellow powder was ob-
tained with a total yield of 40%.
1
Yield: 55%, H NMR (CDCl3) d: 8.13 (dd, 4H, J¼7.7,
1.9 Hz), 7.51 (d, 2H, J¼8.4 Hz), 7.05 (d, 4H, J¼8.9 Hz),
6.48 (dd, 2H, J¼8.5, 2.4 Hz), 6.19 (d, 2H), 5.28 (s, 1H),
3.91 (s, 6H, J¼2.4 Hz), 3.62 (s, 6H), 1.91 (s, 6H). ESI:
889.1 (100, MþNaþ) C33H21F4IrN4O3 calcd C 48.55, H
2.91, N 6.47; found C 47.26, H 2.85, N 6.79.
4.3.6. Iridium(III) bis(2,5-bis-(2-naphthalenyl)-
[1,3,4]oxadiazolato-C2 ,N3) (acetyl acetonate)
0
(nOXD)2Ir(acac)
Yield: 68%, 1H NMR (CDCl3) d: 8.88 (s, 2H), 8.39 (dd, 2H,
J¼8.5, 1.5 Hz), 8.21 (s, 2H), 8.08 (d, 4H, J¼5.4 Hz), 7.98 (d,
2H, J¼7.6 Hz), 7.69 (m, 6H), 7.38 (d, 2H, J¼7.9 Hz), 7.19 (m,
4H), 7.11 (s, 2H), 5.10 (s, 1H), 1.98 (s, 6H). MS (m/e, ESI):
957.2 (100, MþNaþ) C49H33IrN4O4 (934.03) calcd C 63.01,
H 3.56, N 6.00; found C 65.04, H 3.42, N 6.02.
4.3.7. Iridium(III) bis(2,5-bis-(4-(9H-carbazol-9-yl)phenyl)-
0
1
(OXD)2Ir(acac). Yield: 60%, H NMR (CDCl3) d: 8.23(dd,
[1,3,4]oxadiazolato-C2 ,N3) (acetyl acetonate)
(dcOXD)2Ir(acac)
4H, J¼4.6, 1.6 Hz), 7.59 (m, 8H), 6.98 (t, 2H, J¼6.3 Hz), 6.83
(t, 2H, J¼7.5 Hz), 6.72 (d, 2H, J¼7.58 Hz), 5.30 (s, 1H), 1.92
(s, 6H). MS (m/e, ESI): 757.1 (100, MþNaþ) C33H25IrN4O4
(734.15) calcd C 54.01, H 3.43, N 7.64; found C 53.13, H
3.44, N 7.67.
1
Yield: 69%, H NMR (CDCl3) d: 8.35 (dd, 4H, J¼6.7,
1.8 Hz), 8.16 (d, 4H, J¼7.8 Hz), 8.04 (d, 4H, J¼7.8 Hz),
7.95 (d, 2H, J¼8.1 Hz), 7.75 (dꢀd, 4H, J¼6.7, 1.9 Hz), 7.46
(m, 8H), 7.35 (m, 12H), 7.30 (d, 2H, J¼1.9 Hz), 5.44 (s,

Z. Xu et al. / Tetrahedron 64 (2008) 1860e1867
1867
1H), 2.05 (s, 6H). MS (m/e, ESI): 1417.5 (100, MþNaþ)
C33H25IrN4O4 (1394.56) calcd C 69.76, H 3.83, N 8.04; found
C 68.53, H 3.71, N 8.21.
Yamashita, Y. Chem. Lett. 2005, 34, 1378; (f) Yang, C.-H.; Fang, K.-H.;
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Acknowledgements
Authors acknowledge the support by NSFC/China
(90401026, 60508011), National Basic Research 973 Program
(2006CB806200) and Scientific Committee of Shanghai.
Authors thank Prof. Xiao-Yuan Hou (Dept. of Physics, Fudan
University) for his help in measurements of OLEDs.
5. (a) Jiang, C. Y.; Yang, W.; Peng, J. B.; Xiao, S.; Cao, Y. Adv. Mater. 2004,
16, 537; (b) Yang, C. H.; Tai, C. C.; Sun, I. W. J. Mater. Chem. 2004, 14,
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Supplementary data
Crystallographic information files for complex 3 and CV
traces of all complexes were provided. Supplementary data
associated with this article can be found in the online version,
at doi:10.1016/j.tet.2007.11.099.
6. Chen, L. Q.; You, H.; Yang, C. L.; Ma, D. G.; Qin, J. G. Chem. Commun.
2007, 1352.
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