Bis(imino)pyridine palladium(II) complexes: Synthesis, structure and catalytic activity

Article abstract of DOI:10.1016/j.jorganchem.2009.03.021

Bis(imino)pyridine palladium(II) complexes 3-6 were synthesized by two different methods. The structure of complexes 3 and 4 has been confirmed by X-ray structure analysis. The catalytic studies show that bis(imino)pyridine palladium(II) complexes are highly efficient catalysts in the Suzuki-Miyaura reaction and the complex 4 was used to catalyze the synthesis of fluorinated liquid crystalline compounds via Suzuki coupling reaction. Crown Copyright

Full text of DOI:10.1016/j.jorganchem.2009.03.021

Journal of Organometallic Chemistry 694 (2009) 2290–2294
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Bis(imino)pyridine palladium(II) complexes: Synthesis, structure
and catalytic activity
*
Ping Liu, Li Zhou, Xiaogang Li, Ren He
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Bis(imino)pyridine palladium(II) complexes 3–6 were synthesized by two different methods. The struc-
ture of complexes 3 and 4 has been confirmed by X-ray structure analysis. The catalytic studies show that
bis(imino)pyridine palladium(II) complexes are highly efficient catalysts in the Suzuki–Miyaura reaction
and the complex 4 was used to catalyze the synthesis of fluorinated liquid crystalline compounds via
Suzuki coupling reaction.
Received 11 November 2008
Received in revised form 5 March 2009
Accepted 6 March 2009
Available online 21 March 2009
Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
Keywords:
Bis(imino)pyridine
Palladium(II)
Suzuki
Fluorinated liquid crystalline compounds
1. Introduction
CH2Cl2 at room temperature for 24 h (Scheme 1) and the anion is
one PF6ꢀ, not one [PdCl3]ꢀ. Which is identified by elemental anal-
2,6-Diiminopyridines are versatile ligands that have found wide
applications in coordination chemistry and catalysis. Despite being
known for more than 30 years, these 6-electron donors have re-
ceived considerable attention over the past decade, mainly due
to the discovery of the high catalytic activity of their Fe and Co
complexes in olefin polymerization catalysis [1–4]. Moreover,
these ligands have found use in many other catalytic processes,
such as olefin epoxidation [5], hydrogenation and hydrosilation
[6], and aerobic oxidation reactions [7–9]. To the best of our
knowledge, Few investigation has been carried out on the synthe-
ses and catalytic properties of bis(imino)pyridine palladium(II)
complexes [10]. Herein we report their syntheses, crystal struc-
tures, and catalytic activity and application.
ysis, 1H NMR and IR spectroscopies. Complexes 3 and 4 are poorly
soluble in acetonitrile and chloroform, slightly soluble in dichloro-
methane and ethanol and soluble in dimethylsulfoxide. Compared
with them, the complexes 5 and 6 are soluble in above any solvent.
The proposed coordination pattern is different from those com-
plexes containing 2, 6-bis(imino)pyridine as bidentate chelate li-
gands have been reported, [11–14] Which was testified by their
1H NMR spectra (Fig. 1). For example, complexes 4 and 6 exhibited
downfield shifts of the proton signals relative to those for ligand 2,
change of the pyridine proton chemical shift values is obvious, For
complexes 4, 6 and ligand 2, the para proton A of pyridine appear
as a triplet at d 9.65, 8.96 and 7.92 ppm, respectively, and the meta
protons B of pyridine protons appear as a doublet at d 9.11, 8.61
and 8.48 ppm. IR spectra of complex 4 and 6 show
m (C@N) around
1624 and 1635 cmꢀ1, ligand 2 show (C@N) around 1644 cmꢀ1
m
.
2. Result and discussion
Which indicate the three nitrogen atoms of bis(imino)pyridine li-
gand coordinate with Pd(G). To confirm the solid-state structure
of the complexes 3 and 4, X-ray crystallography of 3 and 4 was
studied. Suitable crystal for X-ray analysis was obtained by slow
diffusion of hexane into saturated dichloromethane solution of
the complex. The crystal structure plot is shown in Fig. 2. Crystal
data and other details of the structure analysis are presented in Ta-
ble 1. Important bond lengths and angles are summarized in Table
2. The coordination geometry of 3 and 4 is a distorted square pla-
nar, The Pd–NPy bond lengths (Pd(1)–N(1): 1.925(3) Å) of 4 are
slightly longer than that of 3 (1.921(4) Å), but the Pd–Nimine and
PdꢀCl bond lengths (Pd(1)ꢀN(3): 2.043(3) Å, Pd(1)ꢀN(2):
2.050(3) Å, Pd(1)ꢀCl(1): 2.2782(10) Å) of 4 are slightly shorter
2.1. Synthesis of complexes
Bis(imino)pyridine palladium(II) complexes 3 and 4 were pre-
pared by reaction of PdCl2(CH3CN)2 with 1.0 equiv. of ligands (1
and 2) in acetonitrile at room temperature for 2 h (Scheme 1). Their
structures contained one [PdLCl]+ and one [PdCl3]ꢀ were confirmed
by X-ray crystallography. Noteworthy is that the complexes 5 and
6 could be obtained by reaction of the mixture of PdCl2(CH3CN)2
and 1.5 equiv. of NH4PF6 with 1.0 equiv. of ligands (1 and 2) in
* Corresponding author. Tel.: +86 0411 39893861; fax: +86 0411 39893687.
E-mail address: liuping1979112@yahoo.com.cn (R. He).
0022-328X/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.03.021

P. Liu et al. / Journal of Organometallic Chemistry 694 (2009) 2290–2294
2291
Scheme 1. Synthesis of bis(imino)pyridine palladium(II) complexes.
2.2. Catalytic activity
The catalytic activity of complexes 3–6 in the Suzuki–Miyaura
coupling reaction of 4-bromoanisole with phenylboronic acid
was investigated in water at 80 °C (Scheme 2) and representative
results are summarized in Table 3, it was found the employ of com-
plexes 3 and 4 as a precatalyst provided good yields of biaryls (93%
and 94%, respectively, Table 3, entries 1, 2), the use of complexes 5
and 6 also gave good yield (90% and 92%, respectively, Table 3, en-
tries 3, 4). The [PdCl3]ꢀ had no effect obviously for the catalytic
activity of these complexes.
The cross-coupling reactions of various aryl bromides and aryl-
boronic acids have been investigated. The electron-poor aryl bro-
mide, such as 4-bromoacetophenone, reacted with phenylboronic
acids to give high yield (97%, Table 4, entries 2, 3). The electron-
rich aryl bromide, such as 4-bromoanisole, also gave good yield
at the same condition (90%, Table 4, entry 1). When the 3-bromo-
pyridine was treated with phenylboronic acid, 4-methylphenylbo-
ronic acid, and 4-fluorophenylboronic acid, respectively, the
moderate yields were obtained (67–90%, Table 4, entries 4, 6, 7),
but the reaction of 2-bromopyridine with phenylboronic acid re-
sulted in a low yield of the product, which may be due to the coor-
dination of the pyridine nitrogen with palladium metal (Table 4,
entry 5). The cross-coupling reactions of various aryl chlorides
and arylboronic acids have been also investigated in DMA at
100 °C. The coupling reaction of aryl chlorides bearing an elec-
tron-withdrawing group, such as 4-CN, 4-COCH3, and 4-NO2, with
phenylboronic acid gave biaryls in good yields ranging from 82% to
96% for 10 h (Table 4, entries 8ꢀ11). The electron-rich aryl chlo-
rides reaction with aryl boronic acid, such as 4-Me, also gave mod-
erate yields at the same condition (71%, Table 4, entry 12).
In addition, the liquid crystalline materials containing monoflu-
oro-, difluoro- or trifluoro-substituted phenyls [15–20] are the
most prominent for application in thin-film transistor liquid crys-
Fig. 1. 1H NMR spectra (aromatic region) of ligand 2 (in CDCl3), complexes 4 (in
CD2Cl2) and 6 (in CDCl3).
than that of 3 (Pd(1)ꢀN(3): 2.059(4) Å, Pd(1)ꢀN(2): 2.065(4) Å,
Pd(1)ꢀCl(1):
2.2817(13) Å),
The
and
average
angles
of
and
N(1)ꢀPd(1)ꢀN(3)
(79.36(16)°
80.15(12)°)
N(1)ꢀPd(1)ꢀN(2) (79.72(15)° and 79.76(12)°) in 3 and 4 are nearly
80° and the angles of N(3)ꢀPd(1)ꢀCl(1) (100.27(12)° and
99.15(9)°) and N(2)ꢀPd(1)ꢀCl(1) (100.70(12)° and 100.94(9)°) in
3 and 4 are 100° approximately.
Fig. 2. Structure of complexes 3 and 4 at 30% probability level. Hydrogen atoms are omitted for clarity.

2292
P. Liu et al. / Journal of Organometallic Chemistry 694 (2009) 2290–2294
Table 1
pling reactions very important in synthesis. We can use this new
protocol to synthesize liquid crystal compounds, but consider sol-
ubility of arylbromides, we select ethanol as solvent. The products
8m–8q can be synthesized by this reaction to give excellent yield
(ꢁ92%) for 3 h (Table 5, entries 1–4). However, 8q is obtained with
only 75% yield because the aryl bromide is not soluble in ethanol at
80 °C. Thus, this method provides an efficient way to prepare
biphenyl derivatives used as liquid crystal compounds.
Crystal data and structure refinement for complexes 3 and 4.
Complex 3
Complex 4
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C4H3.62Cl0.76N0.57Pd0.38 C66H86Cl8N6Pd4
127.24
1672.61
273(2)
0.71073
Monoclinic
P2(1)/n
8.8274(9)
26.711(3)
15.2801(16)
90.00
27 273(2)
0.71073
Monoclinic
P21/c
10.9479(4)
16.3875(5)
16.3875(5)
90.00
b (Å)
c (Å)
3. Conclusion
a
(°)
b (°)
97.223
90.00
2283.17(13)
4
1.943
103.847
90.00
3498.10(6)
2
1.588
1.360
15632/7952
[0.0327]
1688
0.39 ꢃ 0.38 ꢃ 0.17
2.44–25.87
7952/0/379
0.0679
In conclusion, we have synthesized air- and moisture-stable
bis(imino)pyridine palladium(II) complexes 3–6 and have investi-
gated their catalytic activity in the Suzuki cross-coupling reaction.
The complex 4 are found to be excellent catalysts for Suzuki cross-
coupling reactions of arylboronic acids and aryl bromides or chlo-
rides and the synthesis of various fluorinated biphenyl derivatives
was readily achieved via complex 4 catalyzed Suzuki cross-cou-
pling reaction.
c
(°)
V (Å3)
Z
DC (mg mꢀ3
)
l
(mmꢀ1
)
1.101
13283/4009 [0.0672]
Reflections collected/unique
[R(int)
]
F(000)
1304
Crystal size (mm3)
0.60 ꢃ 0.41 ꢃ 0.25
2.25–25.00
4009/0/271
0.0578
0.1904
1.101
h range for data collection (°)
Data/restraints/parameters
R1 [I > 2
4. Experimental
r
(I)]
(I)]
wR2 [I > 2
r
0.1026
1.018
Goodness-of-fit on F2
4.1. General
Infrared spectra were obtained as KBr pellets on a Perkin–Elmer
FT–IR 430 spectrometer. 1H NMR spectral data were recorded on a
Bruker DPX-400 spectrometers using TMS as internal standard and
CDCl3, CD2Cl2 as solvent. EI-Mass spectra were measured on a LC/
Q-TOF MS (Micromass, England). Acetonitrile was dried over CaH2,
distilled and stored under nitrogen. Methanol were dried and dis-
tilled from Mg. All other reagents were of analytical grade quality
purchased commercially and used as received unless noted
otherwise.
Table 2
Selected bond lengths (Å) and angles (°) for complexes 3 and 4.
Complex 3
Complex 4
Bond lengths
Pd(1)ꢀN(1)
Pd(1)ꢀN(3)
Pd(1)ꢀN(2)
Pd(1)ꢀCl(1)
1.921(4)
2.059(4)
2.065(4)
2.2817(13)
1.925(3)
2.043(3)
2.050(3)
2.2782(10)
Bond angles
4.2. Synthesis of ligands 1 and 2
N(1)ꢀPd(1)ꢀN(3)
N(1)ꢀPd(1)ꢀN(2)
N(3)ꢀPd(1)ꢀCl(1)
N(2)ꢀPd(1)ꢀCl(1)
79.36(16)
79.72(15)
100.27(12)
100.70(12)
80.15(12)
79.76(12)
99.15(9)
Bis(imino)pyridine ligands 1 and 2 were synthesized according
to literature methods [21].
100.94(9)
1: Yellow solid, yield 85%. 1H NMR (400 MHz, CDC13): d 8.36 (d,
J = 7.60 Hz, 2H, Py-Hm), 7.89 (t, J = 7.60 Hz, 1H, Py-Hp), 7.41–6.84
(m,10H, Ar-H), 2.42 (s, 6H, N@CMe). IR (KBr, cmꢀ1): 1637 (C@N).
2: Yellow solid, yield 84%. 1H NMR (400 MHz, CDC13): d 8.49 (d,
J = 7.60 Hz, 2H, Py-Ho), 7.92 (t, J = 7.60 Hz, 1H, Py-Hp), 7.19–7.13
(m, 6H, Ar-H), 2.78 (t, J = 8.40 Hz, 4H, CH), 2.74 (s, 6H, N@CMe),
i
1.17 (d, J = 8.40 Hz, 24H, Pr-H). IR (KBr, cmꢀ1): 1644 (C@N).
4.3. Synthesis of complexes 3 and 4
Scheme 2.
0.20 mmol of PdCl2(CH3CN)2 and 0.20 mmol of ligand were dis-
solved in 10 mL of acetonitrile. The solution was stirred at room
temperature for 2 h. Ether (100 mL) was added to the reaction mix-
ture, and this mixture was placed for an appropriate period of time,
after which it was filtered and the solid was obtained.
3: Yellow solid, yield 84%. Elemental Anal. Calc. for
C21H19ClN3Pd ꢂ PdCl3 ꢂ 1.5CH3CN: C, 39.51; H, 3.25; N, 8.64.
Found: C, 41.00; H, 3.44; N, 8.52%. 1H NMR (400 MHz, CDC13): d
8.69 (t, J = 8.0 Hz, 1H, Py-Hp), 8.50 (d, J = 8.0 Hz, 2H, Py-Hm),
7.48–7.19 (m, 10H, Ar-H), 2.46 (s, 6H, N@CMe). IR (KBr, cmꢀ1):
1636 (C@N); HRMS (EI), m/z: [MꢀPdCl3]+, calculated for:
454.0302; found, 454.0222.
Table 3
Effect of complexes 3–6 on Suzuki cross-coupling reaction.a
Entry
Complexes
Yield (%)b
1
2
3
4
3
4
5
6
93
94
90
92
a
Reaction conditions: 4-bromoanisole 0.50 mmol, phenylboronic acid 0.75
m!mol, K3PO4 ꢂ 3H2O 1.2 mmol, 3–6 0.50 mmol%, H2O 2.0 mL, 80 °C, reaction time
3 h.
b
Isolated yields.
4: Yellow solid, yield 80%. Elemental Anal. Calc. for
C33H43Cl2N3Pd ꢂ PdCl3: C, 47.39; H, 5.18; N, 5.02. Found: C,
47.60; H, 5.50; N, 5.05%. 1H NMR (400 MHz, CD2C12): d 8.94 (t,
J = 8.0 Hz, 1H, Py-Hp), 9.45 (d, J = 8.0 Hz, 2H, Py-Ho), 7.36–7.19
(m, 6H, Ar-H), 3.18 (t, J = 6.8 Hz, 4H), 2.51 (s, 6H, N@CMe), 1.31
tal displays (TFT-LCDS). The long, lath-like molecular structure of
most fluorinated liquid crystalline compounds demanded by
thin-film transistor liquid crystal displays makes Suzuki cross-cou-

P. Liu et al. / Journal of Organometallic Chemistry 694 (2009) 2290–2294
2293
Table 4
Suzuki cross-coupling reaction catalyzed by complex 4.a,b
Entry
1
ArB(OH)2
Ar-X
Product
Time (h)
3
Yield (%)c
90
8b
B(OH)2 7a
Me
Br
Me
2
7a
7a
7a
7a
3
90
97
90
trace
81
67
90
84
96
82
71
8c
F
Br
F
3
3
8d
MeOC
Br
MeOC
8e
8f
Br
Br
Br
Br
4
5
N
N
N
N
5
5
N
F
8g
6
5
7b
F
B(OH)2
N
8h
Me
7
5
Me
B(OH)2
7b
N
N
8
7a
10
10
10
10
10
8i
NC
Cl
NC
9
7a
7a
7b
7b
8d
MeOC
O2N
Cl
Cl
MeOC
10
11
8j
Cl
O2N
Me
COMe
8k
MeOC
Me
12
Me
Me
8l
Cl
a
Reaction conditions: aryl bromide 0.50 mmol, arylboronic acid 0.75 mmol, K3PO4 ꢂ 3H2O 1.20 mmol, complex 4 (0.50 mmol%), H2O 2.0 mL, 80 °C.
Reaction conditions: aryl chloride 0.25 mmol, arylboronic acid 0.40 mmol, K3PO4 ꢂ 3H2O 0.75 mmol, complex 4 (0.25 mmol%), DMA 2.0 mL, 100 °C.
Isolated yields.
b
c
i
i
(d, J = 6.8 Hz, 12H, Pr-H1), 1.19 (d, J = 6.8 Hz, 12H, Pr-H2). IR (KBr,
cmꢀ1): 1624 (C@N). HRMS (EI), m/z: [MꢀPdCl3]+, calculated for:
622.2175; found, 622.2180.
4.5. X-ray crystallography
Crystal data and other details of the structure analysis are pre-
sented in Table 1. Suitable crystal for X-ray diffraction was
mounted on a glass fiber. Data collection was performed on a Bru-
ker Smart APEX CCD diffractometer using graphite monochromat-
4.4. Synthesis of complexes 5 and 6
ed Mo Ka radiation (k = 0.71073 Å) at 273 K. The diffraction frames
0.20 mmol of PdCl2(CH3CN)2 and 0.30 mmol of NH4PF6 were
dissolved in 10 mL of CH2Cl2. The solution was stirred at room tem-
perature for 12 h. The ligand was added to the reaction mixture,
and continue to be stirred for 24 h, after which it was filtered, ether
(100 mL) was added to the filtrate and this mixture was placed for
an appropriate period of time, after which it was filtered and the
solid was obtained.
5: Yellow solid, yield 70%. Elemental Anal. Calc. for
C21H19ClF6N3PPd: C, 42.02; H, 3.19; N, 7.00. Found: C, 41.35; H,
3.32; N, 6.96%. 1H NMR (400 MHz, CDCl3): d 8.55 (t, J = 8.0 Hz,
1H, Py-Hp), 8.18 (d, J = 8.0 Hz, 2H, Py-Hm), 7.46–7.13 (m, 10H,
Ar-H), 2.50 (s, 6H, N@CMe). IR (KBr, cmꢀ1): 1632 (C@N).
were integrated using the SAINT package. The structure was solved
by direct methods using the program SHELXS97. Structure refine-
ment by the full-matrix least-squares on F2 was carried out with
the program SHELXL97. All non-hydrogen atoms of the complex were
assigned anisotropic displacement parameters. The hydrogen
atoms were constrained to idealized geometries and assigned iso-
tropic displacement parameters equal to 1.2 times the Uiso values
of their respective parent atoms.
4.6. General procedure for Suzuki cross-coupling reaction
6: Yellow solid, yield 72%. Elemental Anal. Calc. for
C33H43ClF6N3PPd: C, 51.57; H, 5.64; N, 5.47. Found: C, 50.46; H,
5.46; N, 5.54%. 1H NMR (400 MHz, CDC13): d 8.96 (t, J = 8.40 Hz,
1H, Py-Hp), 8.61 (d, J = 8.40 Hz, 2H, Py-Ho), 7.31 (t, J = 8.00 Hz,
2H, Ar-H), 7.18 (d, J = 8.00 Hz, 2H, Ar-H), 3.07 (m, 4H), 2.46 (s,
A 5 mL flask was charged with 3 or 4 (0.25 mmol%),
K3PO4 ꢂ 3H2O (1.20 mmol), and arylboronic acid (0.75 mmol), tolu-
ene, water or ethanol (2 mL) and aryl bromide (0.5 mmol) were
added. The reaction was stirred at 80 °C for the certain time. After
the reaction was allowed to cool to room temperature, the result-
ing mixture was extracted with ether (5 ꢃ 2 mL). The combined
ether extracts were dried (MgSO4) and the solvent was removed
i
6H, N@CMe), 1.33 (d, J = 6.40 Hz, 12H, Pr-H1), 1.20 (d, J = 6.40 Hz,
i
12H, Pr-H2). IR (KBr, cmꢀ1): 1635 (C@N).

2294
P. Liu et al. / Journal of Organometallic Chemistry 694 (2009) 2290–2294
Table 5
Suzuki cross-coupling reaction catalyzed by complex 4 for synthesis of TFT-LCDS.a
Entry
ArBr
Ar0B(OH)2
Product
Yield (%)b
95
1
C3H7
F
Br
8m
F
C3H7
B(OH)2 7b
C5H11
F
Br
2
3
93
94
8n
C5H11
F
B(OH)2
7b
C5H11
F
Br
Br
F
8o
8p
C5H11
F
F
F
B(OH)2
B(OH)2
7c
7d
C5H11
F
F
F
4
92
75
C5H11
F
F
C5H11
Br
C5H11
F
5
8q
F
B(OH)2
7b
a
Reaction conditions: aryl bromide 0.50 mmol, arylboronic acid 0.75 mmol, K3PO4 ꢂ 3H2O 1.20 mmol, 4 (0.50 mmol%), EtOH 2.0 mL, 80 °C, reaction time 3 h.
b
Isolated yields.
[5] B. Cetinkaya, E. Cetinkaya, M. Brookhart, P.S. White, J. Mol. Catal. A: Chem. 142
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under reduced pressure. The crude material was flash chromato-
graphed on a short silica gel column.
Acknowledgement
We thank Dr. Cheng He for X-ray crystallography analyses and
helpful discussions.
[10] K.J. Miller, T.T. Kitagawa, M.M. Abu-Omar, Organometallics 20 (2001) 4403–
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Appendix A. Supplementary material
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Organomet. Chem. 631 (2001) 125–134.
CCDC 686605 and 635773 contain the supplementary crystallo-
graphic data for 3 and 4. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2009.03.021.
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