Synthesis, structure, and photophysical properties of tributyl phosphine bisbenzothienyl iridium(III) complex and its application on transfer hydrogenation of acetophenone

Article abstract of DOI:10.1002/zaac.201400329

A phosphine bisbenzothienyl iridium(III) complex was synthesized and characterized by IR and¹H NMR spectroscopy as well as X-ray diffraction methods. The TG data suggests that the complex has an excellent thermal stability than that with an amidate ancillary ligand. Photophysical properties showed that the complex emits typical green emission.

Full text of DOI:10.1002/zaac.201400329

Journal of Inorganic and General Chemistry
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Zeitschrift für anorganische und allgemeine Chemie
DOI: 10.1002/zaac.201400329
Synthesis, Structure, and Photophysical Properties of Tributyl Phosphine
Bisbenzothienyl Iridium(III) Complex and its Application on Transfer
Hydrogenation of Acetophenone
[a]
[a]
[a]
[a]
Dawei Wang* Keyan Zhao, Shuyan Yang, and Yuqiang Ding*
Keywords: Bisbenzothienyl; Phosphine; Iridium; Photophysical property; Catalysis
Abstract. A phosphine bisbenzothienyl iridium(III) complex was syn- has an excellent thermal stability than that with an amidate ancillary
1
thesized and characterized by IR and H NMR spectroscopy as well
ligand. Photophysical properties showed that the complex emits typical
as X-ray diffraction methods. The TG data suggests that the complex green emission.
thesis of phosphine iridium(III) complex,[7] herein we report
the synthesis, characterization, structure, and photophysical
properties of the tributylphosphine bisbenzothienyl (bt)
iridium(III) complex and its catalytic activity attemption on
transfer hydrogenation of acetophenone.
1
Introduction
Great efforts have been made over the past decades toward
developments in the design and synthesis of cyclometalated
iridium(III) complexes, owing to their potential application in
the field of new material, such as luminescent biological la-
bels, photoelectrochemical solar cells, electrogenerated chemi-
luminescence and organic light-emitting diodes (OLEDs).[
Generally, there are two kinds of cyclometalated iridium com-
[1]
2]
2
Results and Discussion
2.1 Synthesis and Characterization
plexes: neutral complexes such as homoleptic Ir(CN) and het-
3
eroleptic (CN) Ir(LX) complexes, and cationic complexes such
Chloro-bridged dimer [{(bt) Ir( u˛ -Cl)} ] (1) and tributyl-
2
2
2
+
as [Ir(CN) (NN)] . Both neutral and ionic Ir-based complexes phosphine were placed together in a Schlenk tube containing
2
[3]
[8]
have the photophysical properties. Neutral Ir-based com- CH Cl as solvent under nitrogen atmosphere. The mixture
2
2
plexes have been used in OLED structures, given the good was stirred at room temperature for 6 h and thus resulted in
chemical and photochemical stability they provide. Cationic Ir the desired complex 2 with 76% yield (Scheme 1).
complexes have been used in light-emitting electrochemical
cells (LEC), which typically consist of a single active layer,
which is responsible for both carrier transport and light emis-
sion,[ but ionic complexes may provide better catalytic reac-
tivity.[
4]
5]
Our research in developing amides as ancillary ligand to
adjust transition-metal reactivity and photophysical properties
has led to the recent discovery of four-member cyclometalated
III
[6]
Ir complexes. To date, a series of amide iridium complexes
and enhanced photophysical properties has been reported.[
However, the emission spectrum is only around the green re-
gion and the thermal stability of these amide auxiliary ligands
four-member cyclometalated compounds is not good, and they
also have no catalytic activity.[ As a continuing work to syn-
7]
6a]
*
Dr. D. Wang
Fax: +86-510-85917763
E-Mail: wangdw@jiangnan.edu.cn
Dr. Y. Ding
Scheme 1. The synthesis of complex 2.
Crystal Structure of 2
*
E-Mail: yding@jiangnan.edu.cn
[
a] The Key Laboratory of Food Colloids and Biotechnology
Ministry of Education
School of Chemical & Material Engineering
Jiangnan University
The crystal structure of complex 2 was determined through
X-ray crystallography. Single-crystal analysis has revealed that
1800 Lihu Road, Wuxi, Jiangsu Province, P. R. China
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compound 2 belongs to the P2 /c space group. In the molecular weight loss of 60% at 390 °C. Such data suggests that the IrIII
1
structure of compound 2, the Ir(1) atom is coordinated to one complex has much better thermal stability than that with an
tributylphosphine, one chlorine atom, and two bt ligands all in amidate ancillary ligand.[
bidentate-chelating fashions (Figure 1). The C(1)/N(2) from
6]
the benzothiazole are coordinated to Ir(1) forming a nearly
planar Ir(1)–C(1)–C(6)–C(7)-N(2) five-membered ring and the
angle 79.2(3)° of C(1)–Ir(1)–N(2). The bond length of Ir(1)–
P(1) [2.426(2) Å] is much shorter than that of Ir(1)–Cl(1)
[2.479(2) Å] (Table 1).
Figure 2. Thermogravimetric curves of complex 2.
2.3 Photophysical Characterization
Additionly, the photophysical characterizations of the new-
type bisbenzothienyl iridium(III) complex were tested. The re-
sults are shown in Figure 3. The absorption spectrum displays
bands in the UV at 320 nm, being dramatically different from
III
those of Ir complexes with amides as auxiliary ligands,
which might be caused by different cloud density of phosphine
substituent group with amides. The coordination ability of
phosphine ligand usually is much stronger than that of
amides.[ One of our research is focused on the development
of color tuning function through changing ligands. Here, when
phosphine groups were introduced into Ir complexes, to our
delight, the wavelength of solution photoluminescence (PL) of
the complex was observed around 555 nm, thus falling into the
green light region. This provides us a possible method to turn
our attention to the synthesis of new iridium(III) complex with
phosphine ligands instead of amidate ligands.
Figure 1. ORTEP diagram of complex 2 with thermal ellipsoids shown
at the 30% probability level.
6b]
Table 1. Selected bond lengths /Å and bond angles /° of complex 2.
Bond length
Bond angle
Ir(1)–C(26)
Ir(1)–C(1)
Ir(1)–N(1)
Ir(1)–N(2)
Ir(1)–P(1)
Ir(1)–Cl(1)
2.009(8)
2.064(8)
2.070(6)
2.075(7)
2.426(2)
2.479(2)
C(26)-Ir(1)-(C1)
C(26)–Ir(1)–N(1)
C(1)–Ir(1)–N(1)
C(26)–Ir(1)–N(2)
C(1)–Ir(1)–N(2)
N(1)–Ir(1)–N(2)
C(26)–Ir(1)–P(1)
C(1)–Ir(1)–P(1)
N(1)–Ir(1)–P(1)
N(2)–Ir(1)–P(1)
C(26)–Ir(1)–Cl(1)
C(1)–Ir(1)–Cl(1)
N(1)–Ir(1)–Cl(1)
N(2)–Ir(1)–Cl(1)
P(1)–Ir(1)–Cl(1)
C(2)–C(1)–Ir(1)
C(6)–C(1)–Ir(1)
87.1(3)
79.7(3)
89.2(3)
91.9(3)
79.2(3)
166.1(3)
90.9(2)
177.2(2)
88.61(19)
102.77(19)
177.0(2)
90.7(2)
102.2(2)
85.7(2)
91.40(7)
130.4(7)
113.6(6)
2.2 Thermal Property
Furthermore, a TG experiment was investigated and the
thermal curve was as shown in Figure 2. The experiment
shows that this phosphine-assisted iridium(III) complex in-
curred a weight loss of approximately 10% at 350 °C, but a Ir complex 2.
Figure 3. UV/Vis absorption and normalized PL emission spectra of
III
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2
.4 Catalytic Activity
Table 3. Transfer hydrogenation of ketones with 2-propanol by catalyst
a)
2
.
Next, we sought to expand its catalytic activity in the trans-
fer hydrogenation reaction, so the classical acetophenone with
-propanol was chosen as the model substrates for reaction
2
examination. The effects of base, solvent, and temperature
were all checked for the purpose of a better yield, the results
of which are shown in Table 2. We found that the best reaction
conditions could be summarized as follows: 2 mol-% of 2 cata- Entry
lyst, 1.5 equivalent of tBuOK, toluene as the reaction solvent,
at 110 °C (Table 2, entry 5), and in this conditions, the product
Alcohol
Product
Yield /%
(
4a) was checked with 65% conversion. The comparison to
1
4a (65)
[6]
catalytic activity of reported amide iridium complexes was
also conducted. The results showed that these amide iridium
complexes had nearly no catalytic reactivity (Table 2, entries
8
–12 and Scheme 2).
2
4b (61)
4c (47)
Table 2. Transfer hydrogenation of acetophenone with 2-propanol by
bisbenzothienyl iridium(III) complex 2
a)
.
3
4
Entry
Base
none
Solvent
Temp. /°C
Conv. /%
1
1
2
3
4
5
6
7
8
9
Toluene
Toluene
DMF
110
110
110
110
110
110
90
24
37
24
25
49
65
38
57
Ͻ5
Ͻ5
Ͻ5
Ͻ5
Ͻ5
K
2
K
2
K
2
CO
CO
CO
3
3
3
4d (54)
DMSO
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
KOH
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
130
110
110
110
110
110
b)
c)
5
6
4e (63)
4f (68)
d)
10
e)
1
1
f)
12
a) Reaction conditions: Cat. 2 (0.02 mmol), acetophenone (1 mmol),
-propanol (20 mmol), base (1.5 mmol), solvent (2 mL), 12 h, N at-
2
2
mosphere, the conversion was determined by GC. b) Catalyst 5a was
used. c) Catalyst 5b was used. d) Catalyst 6a was used. e) Catalyst 6b
was used. f) Catalyst 6c was used.
a) Reaction conditions: Cat. 2 (0.02 mmol), ketone 3 (1 mmol), 2-pro-
panol (20 mmol), tBuOK (1.5 mmol), toluene (2 mL), 12 h, N atmo-
2
sphere, 110 °C, the yields were determined by GC.
were obtained with moderate yields. The best yield was
achieved in 68% yield (entry 6, Table 3). Although the yield is
not good, the introducing phosphine ligand gives us a possible
method to enhance catalytic activity about iridium-catalyzed
reactions.
3
Conclusions
Scheme 2. Several pyridyl- and quinolyl-substituted iridium(III) com-
plexes.
A benzothienyl skeleton tributyl phosphine iridium(III)
complex (2) was synthesized and characterized through X-ray
Under the best reaction conditions, the substrate scope was crystallography. The TG data suggests that the complex
also explored (Table 3). In general, all the desired products showed a weight loss of approximately 10% at 350 °C, and
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has much better thermal stability than that with an amidate Acknowledgements
ancillary ligand. Photophysical properties showed that com-
plex 2 gave the typical green emission. Moreover, the iridium We gratefully acknowledge financial support of this work by the
National Natural Science Foundation of China (21401080, 21371080),
complex could be applied on the transfer hydrogenation reac-
the Natural Science Foundation of Jiangsu Province (BK20130125),
tion of acetophenone and 2-propanol with moderate yield.
333 Talent Project of Jiangsu Province (BRA2012165) and China Post-
Compared to amide iridium complexes, the enhanced catalytic
activity of phosphine iridium complex provides us a possible
method toward iridium-catalyzed reactions.
doctoral Science Foundation (2014M550262).
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Zeitschrift für anorganische und allgemeine Chemie
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