Palladium complexes with picolyl functionalized N-heterocyclic carbene ligands and their application in the Mizoroki-Heck reaction

Article abstract of DOI:10.1007/s11243-013-9700-1

A series of Pd complexes of picoline-functionalized N-heterocyclic carbenes with different substituents were synthesized. The molecular structures of selected complexes were determined by X-ray diffraction studies, showing a pseudo-square-planar structure with a Pd center surrounded by carbene, pyridine, and two chloride ligands. The influence of the different substituents on the structure and reactivity of the complexes has been studied. The catalytic properties of the complexes were investigated in the Mizoroki-Heck reaction for cross-coupling of bromobenzene with a variety of aryl halides. Their performances varied in these reactions but showed good product regioselectivities.

Full text of DOI:10.1007/s11243-013-9700-1

Transition Met Chem (2013) 38:351–358
DOI 10.1007/s11243-013-9700-1
Palladium complexes with picolyl functionalized N-heterocyclic
carbene ligands and their application in the Mizoroki–Heck
reaction
•
Changsheng Cao Rui Sun Qian Chen
•
•
•
Lanlan Lv Yanhui Shi Guangsheng Pang
•
Received: 10 December 2012 / Accepted: 15 January 2013 / Published online: 26 January 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A series of Pd complexes of picoline-func-
tionalized N-heterocyclic carbenes with different substitu-
ents were synthesized. The molecular structures of selected
complexes were determined by X-ray diffraction studies,
showing a pseudo-square-planar structure with a Pd center
surrounded by carbene, pyridine, and two chloride ligands.
The influence of the different substituents on the structure
and reactivity of the complexes has been studied. The
catalytic properties of the complexes were investigated in
the Mizoroki–Heck reaction for cross-coupling of bromo-
benzene with a variety of aryl halides. Their performances
varied in these reactions but showed good product
regioselectivities.
centers, which makes their complexes are of considerable
interest as potential catalysts. The chelate effect is a well-
known means of increasing the stability of metal com-
plexes. Furthermore, a ligand bearing a stronger and weaker
donor can exhibit hemilabile behavior, which is often
important for the generation of active species in homoge-
neous catalysis. Various complexes of donor-functionalized
NHCs and their applications in catalysis have been explored
[10–14]. Among these complexes, the use of pyridine- and
picoline-functionalized chelating NHCs as ligands for var-
ious transition metals is well established [15–23]. However,
to the best of our knowledge, no systematic study of the
relationship between catalysis and structure concerning
the substitution at the N of NHCs has been reported. Herein,
we present the synthesis and structural characterization of
several picolyl functionalized NHC ligands with different
N-substituents. Screening of these palladium complexes in
catalytic application for the Mizoroki–Heck reaction is also
described.
Introduction
Over the past few decades, N-heterocyclic carbenes (NHCs)
have been widely used in coordination chemistry and
catalysis [1–9]. NHCs can both stabilize and activate metal
Experimental
C. Cao Á R. Sun Á Q. Chen Á L. Lv Á Y. Shi (&)
School of Chemistry and Chemical Engineering, Jiangsu Key
Laboratory of Green Synthetic Chemistry for Functional
Materials, Jiangsu Normal University, Xuzhou 221116, Jiangsu,
People’s Republic of China
Materials and Methods
General procedures
e-mail: yhshi@jsnu.edu.cn
All reagents were commercially sourced and were used
1
Y. Shi
without further purification. H and 13C NMR spectra were
State Key Laboratory of Coordination Chemistry,
Nanjing University, Nanjing 210093, Jiangsu,
People’s Republic of China
recorded on a Bruker DPX 400 MHz spectrometer at room
temperature and referenced to the residual signals of the sol-
vent. Elemental analyses were performed on a EuroVektor
Euro EA-300 elemental analyzer. GC–MS was performed on
an Agilent 6,890–5,973 N system with electron ionization
(EI) mass spectrometry. X-ray crystallography was conducted
G. Pang
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University, Changchun 130012, Jilin,
People’s Republic of China
123

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Transition Met Chem (2013) 38:351–358
on a Rigaku Mercury CCD device with graphite-monochro-
˚
2-chloromethylpyridine (0.702 g, 5.5 mmol) at 100 °C
for 6 h. A grey-white powder was isolated. Yield: 1.3 g
(80 %). 1H NMR (CDCl3, 400 MHz): 10.51 (s, 1H),
8.55 (d, J = 4.0 Hz, 1H), 8.13 (s, 1H), 8.04 (d, J =
8.0 Hz, 1H), 7.81–7.77 (m, 1H), 7.49 (t, J = 7.6 Hz,
1H), 7.33–7.28 (m, 3H), 7.14 (s, 1H), 6.23 (s, 2H),
2.43–2.25 (m, 4H), 1.15 (t, J = 7.2 Hz, 6H). 13C NMR
(CDCl3, 100 MHz): 152.9, 149.5, 140.5, 138.8, 137.7,
132.0, 131.4, 127.2, 124.3, 123.8, 123.6, 122.9, 53.9,
24.0, 14.8.
mated Mo Ka radiation (k = 0.71073 A). The crystal data
collection and refinement parameters are summarized in
Table 1. Absorption correction was performed using the
SADABS program. The structure was solved by directed
methods using the SHELXS-97 program and refined by full-
matrix least-squares techniques on F2 [29].
1-Mesityl-3-(pyridin-2-ylmethyl)-1H-imidazol-3-ium
chloride (1a)
1-Mesityl-1H-imidazole (0.745 g, 4 mmol) and 2-chloro-
methyl-pyridine (0.561 g, 4.4 mmol) were placed in a pressure
tube, and the mixture was stirred for 6 h at 130 °C. The product
was recrystallized from CH2Cl2/ether to give a grey-white
1-(2,6-Diisopropylphenyl)-3-(pyridin-2-ylmethyl)-1H-
imidazol-3-ium (1c)
1
powder. Yield: 1.1 g (85 %). H NMR (CDCl3, 400 MHz):
The synthesis of 1c was similar to 1a, reacting of 1-(2,6-
diisopropylphenyl)-1H-imidazole (0.913 g, 4 mmol) and
2-chloromethyl-pyridine (0.530 g, 4.2 mmol) at 120 °C for
6 h, and a grey-white powder was isolated. Yield: 1.2 g
10.57 (s, 1H), 8.52 (d, J = 4.8 Hz, 1H), 8.02–7.98 (m, 2H),
7.75 (t, J = 8.0 Hz, 1H), 7.29–7.26 (m, 1H), 7.09 (s, 1H), 7.01
(s, 2H), 6.14 (s, 2H), 2.32 (s, 3H), 2.03 (s, 6H). 13C NMR
(CDCl3, 100 MHz): 152.7, 149.5, 141.0, 138.3, 137.5, 134.1,
130.6, 129.6, 123.9, 123.7, 123.6, 122.6, 53.7, 20.9, 17.3.
1
(81 %). H NMR (CDCl3, 400 MHz): 10.33 (s, 1H), 8.50
(d, J = 4.4 Hz, 1H), 8.27 (s, 1H), 8.03 (s, 1H), 7.77
(t, J = 7.6 Hz, 1H), 7.52 (t, J = 8.0 Hz, 1H), 7.31–7.28
(m, 3H), 7.11 (s, 1H), 6.24 (s, 2H), 2.33–2.23 (m, 2H), 1.19
(d, J = 6.8 Hz, 6H), 1.12 (d, J = 6.8 Hz, 6H). 13C NMR
(CDCl3, 100 MHz): 152.6, 149.2, 145.2, 138.7, 137.6,
131.6, 130.1, 124.4, 124.0, 123.8, 123.4, 53.6, 28.4, 24.2,
23.9.
1-(2,6-Diethylphenyl)-3-(pyridin-2-ylmethyl)-1H-
imidazol-3-ium (1b)
The synthesis of 1b was similar to 1a, using of 1-(2,6-
diethylphenyl)-1H-imidazole (1.001 g, 5 mmol) and
Table 1 Crystal data and
Complex
3a
3b
3e
structure refinement details for
3a, 3b and 3e
Formula
C18H19Cl2N3PdÁCH3CN
495.72
C19H21 Cl2N3 Pd
468.69
C28H33Cl2N3Pd
588.87
Formula weight
Temperature of measurement (K)
Crystal system
293(2)
293(2)
293(2)
Monoclinic
P2(1)/n
Monoclinic
P2(1)/n
Orthorhombic
P2(1)2(1)2(1)
Space group
Unit cell dimensions
˚
a (A)
11.077(2)
16.282(3)
12.629(2)
90.00
8.3668(17)
15.411(3)
15.046(3)
90.00
9.6838(19)
15.439(3)
18.050(4)
90.00
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
106.852(16)
90.00
102.36(3)
90.00
90.00
90.00
3
˚
Volume (A )
2,179.9(7)
4
1,895.1(7)
4
2,698.6(9)
4
Z
F(000)
1,000
944
1,208
Theta range for data collection (°)
Goodness-of-fit on F2
Final R indices [I [ 2r (I)]
2.16–25.00
0.982
3.07–25.20
1.072
3.09–27.46
1.061
R1 = 0.0279
wR2 = 0.0763
R1 = 0.0354
wR2 = 0.0807
R1 = 0.0176
wR2 = 0.0419
R1 = 0.0187
wR2 = 0.0424
R1 = 0.0202
wR2 = 0.0469
R1 = 0.0215
wR2 = 0.0474
R indices (all data)
123

Transition Met Chem (2013) 38:351–358
353
1-(tert-Butyl)-3-(pyridin-2-ylmethyl)-1H-imidazol-3-ium
chloride (1d)
Ag–NHC complex 2b
The synthesis of 2b was similar to that of 2a, involving
the reaction of 1b (0.656 g, 2 mmol) and Ag2O (0.255 g,
1.1 mmol). A grey-white power was isolated. Yield: 0.57 g
(65 %). 1H NMR (CDCl3, 400 MHz): 8.60 (d, J = 4.4 Hz,
1H), 7.73 (t, J = 7.6 Hz, 1H), 7.40–7.36 (m, 2H), 7.32–
7.27 (m, 2H), 7.18 (d, J = 8.0 Hz, 2H), 7.00 (s, 1H), 5.50
(s, 2H), 2.33–2.21 (m, 4H), 1.10 (t, J = 7.6 Hz, 6H). 13C
NMR (CDCl3, 100 MHz): 155.1, 150.0, 140.9, 137.5,
136.5, 130.2, 127.0, 123.8, 123.6, 122.4, 121.6, 77.2, 57.3,
24.2, 15.2. Anal. Calc. for C19H22AgClN3 (435.7 g/mol): C
52.0 (calc. 52.4); H 4.9 (5.1); N 9.5 (9.7) %.
Compound 1d was synthesized by the reaction of 1-(tert-
butyl)-1H-imidazole (0.497 g, 4 mmol) with 2-chloro-
methyl-pyridine (0.530 g, 4.2 mmol) at 80 °C for 6 h,
giving a grey-white powder. Yield: 0.8 g (81 %). 1H NMR
(CDCl3, 400 MHz): 11.06 (s, 1H), 8.52 (d, J = 4.8 Hz,
1H), 7.96 (d, J = 7.6 Hz, 1H), 7.74 (t, J = 7.6 Hz, 1H),
7.65 (s, 1H), 7.28–7.25 (m, 2H), 5.84 (s, 2H), 1.69 (s, 9H).
13C NMR (CDCl3, 100 MHz): 152.8, 149.5, 137.6, 136.1,
124.3, 123.8, 122.7, 119.1, 60.1, 53.7, 29.9. Anal. Calc. for
C13H18ClN3 (251.8 g/mol): C 61.8 (calc. 62.0); H 7.1 (7.2);
N 16.8 (16.7) %.
Ag–NHC complex 2c
4-Benzyl-1-(2,6-diisopropylphenyl)-3-(pyridin-2-ylmethyl)-
4,5-dihydro-1H-imidazol-3-ium chloride (1e)
The synthesis of 2c was similar to that of 2a, involving
the reaction of 1c (0.712 g, 2 mmol) and Ag2O (0.255 g,
1.1 mmol). A grey-white power was isolated. Yield: 0.62 g
(67 %). 1H NMR (CDCl3, 400 MHz): 8.62 (d, J = 4.4 Hz,
1H), 7.75 (dt, J = 7.6 Hz, J = 0.8 Hz, 1H), 7.46 (t,
J = 7.6 Hz, 1H), 7.38 (s, 1H), 7.34–7.29 (m, 2H), 7.24 (d,
J = 6.8 Hz, 2H), 7.00 (s, 1H), 5.52 (s, 2H), 2.39–2.33 (m,
2H), 1.21 (d, J = 6.8 Hz, 6H), 1.11 (d, J = 6.8 Hz, 6H).
13C NMR (CDCl3, 100 MHz): 152.9, 149.5, 141.2, 138.6,
137.7, 134.2, 130.7, 129.8, 124.4, 123.9, 123.5, 122.4,
54.0, 21.0, 17.5. Anal. Calc. for C21H26AgClN3 (463.8 g/
mol): C 54.1 (calc. 54.4); H 5.4 (5.6); N 9.4 (9.1) %.
Compound 1e was synthesized by the reaction of (S)-4-
benzyl-1-(2,6-diisopropylphenyl)-4,5-dihydro-1H-imidaz-
ole (1.602 g, 5 mmol) with 2-chloromethyl-pyridine
(0.957 g, 7.5 mmol) at 90 °C for 10 h, giving a grey-white
powder. Yield: 2.1 g (93 %). 1H NMR (CDCl3, 400 MHz):
9.94 (s, 1H), 8.56 (d, J = 4.8 Hz, 1H), 7.77–7.70 (m, 2H),
7.39–7.27 (m, 5H), 7.22–7.15 (m, 4H), 6.29 (d, J =
15.6 Hz, 1H), 4.97–4.90 (m, 2H), 4.01 (t, J = 11.6 Hz,
1H), 3.85–3.80 (m, 1H), 3.35 (dd, J = 16.0 Hz, J =
4.4 Hz, 1H), 3.16–3.10 (m, 1H), 2.95–2.88 (m, 1H),
2.46–2.39 (m, 1H), 1.26–1.19 (m, 9H), 1.14 (d, J =
6.8 Hz, 3H). 13C NMR (CDCl3, 100 MHz): 160.9, 153.3,
148.9, 146.4, 138.0, 133.8, 131.0, 129.7, 129.6,
129.2, 127.7, 124.7, 124.0, 123.6, 61.4, 58.1, 57.0, 50.0,
37.1, 25.0, 24.9, 24.1, 24.0, 18.3. Anal. Calc. for
C28H34ClN3 (448.0 g/mol): C 74.7 (calc. 75.1); H 7.7 (7.6);
N 9.5 (9.4) %.
Palladium complex 3a
1,5-Cyclooctadienpalladium dichloride (0.285 g, 1 mmol)
and 2a (0.421 g, 1 mmol) were stirred in dichloromethane
(10 mL) for 12 h at room temperature under exclusion of
light. AgCl was filtered off, and diethyl ether was added to
the concentrated filtrate to give a yellow solid. The solid
was collected and was recrystallized from DMSO/ether.
Yield: 0.30 g (67 %). 1H NMR (DMSO-d6, 400 MHz):
8.98 (s, 1H), 8.10 (s, 1H), 7.79–7.73 (m, 2H), 7.57 (s, 1H),
7.33 (s, 1H), 6.96 (s, 2H), 5.80–5.70 (m, 2H), 2.26 (s, 3H),
2.04 (s, 3H), 1.85 (s, 3H). Anal. Calc. for C18H19Cl2N3Pd
(454.7 g/mol): C 47.2 (calc. 47.5); H 4.0 (4.2); N 9.5
(9.2) %.
Ag-NHC complex 2a
Imidazolium salt 1a (0.627 g, 2 mmol) and Ag2O
(0.255 g, 1.1 mmol) were stirred in dichloromethane
(10 mL) for 12 h at room temperature under exclusion of
light. Excess Ag2O was then removed, and diethyl ether
(15 mL) was added to the concentrated filtrate. Compound
2a was collected as a grey-white powder. Yield: 0.58 g
(69 %). 1H NMR (CDCl3, 400 MHz): 8.60 (d, J = 4.4 Hz,
1H), 7.73 (t, J = 7.6 Hz, 1H), 7.37–7.27 (m, 3H), 6.95 (s,
3H), 5.50 (s, 2H), 2.33 (s, 3H), 1.97 (s, 6H). 13C NMR
(CDCl3, 100 MHz): 157.4, 156.8, 155.1, 150.0, 139.6,
137.5, 134.6, 129.5, 123.5, 123.0, 122.3, 121.8, 57.3, 21.0,
17.7.
Palladium complex 3b
Complex 3b was synthesized by a procedure analogous to
that for 3a, from 2b (0.436 g, 1 mmol), and 1,5-cyclooc-
tadienpalladium dichloride (0.285 g, 1 mmol). A light
yellow powder was isolated. Yield: 0.32 g (68 %). 1H
NMR (DMSO-d6, 400 MHz): 8.99 (s, 1H), 8.16–8.13 (m,
1H), 7.83–7.78 (m, 2H), 7.63–7.60 (m, 1H), 7.44–7.38 (m,
123

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Transition Met Chem (2013) 38:351–358
2H), 7.23 (s, 2H), 5.85–5.74 (m, 2H), 2.33–2.17 (m, 4H),
1.14–1.06 (m, 6H). Anal. Calc. for C19H21Cl2N3Pd (468.7 g/
mol): C 48.4 (calc. 48.7); H 4.3 (4.5); N 9.2 (9.0) %.
3.22–3.07 (m, 2H), 2.56–2.37 (m, 3H), 1.68–1.64 (m, 3H),
1.31–1.17 (m, 3H), 1.05–0.89 (m, 6H). 13C NMR (DMSO-
d6, 100 MHz): d 165.4, 164.4, 148.4, 146.7, 146.5, 139.9,
136.8, 135.8, 128.8, 128.3, 126.5, 124.3, 123.6, 121.6,
61.1, 53.7, 48.3, 36.2, 28.0, 27.6, 25.3, 24.8, 23.7, 23.2.
Anal. Calc. for C28H33Cl2N3Pd (588.9 g/mol): C 56.8
(calc. 57.1); H 5.5 (5.6); N 7.4 (7.1) %.
Palladium complex 3c
Complex 3c was synthesized by a similar procedure to 3a,
from 2c (0.464 g, 1 mmol) and 1,5-cyclooctadienpalladi-
um dichloride (0.285 g, 1 mmol). A light yellow power
Procedure for Heck coupling reactions
1
was isolated. Yield: 0.34 g (69 %). H NMR (DMSO-d6,
400 MHz): 8.99–8.98 (m, 1H), 8.16–8.12 (m, 1H),
7.83–7.78 (m, 2H), 7.62–7.59 (m, 1H), 7.47–7.43 (m, 2H),
7.30–7.25 (m, 2H), 5.86–5.74 (m, 2H), 2.08 (s, 2H), 1.60
(s, 3H), 1.27 (s, 3H), 1.25 (s, 3H), 0.81 (s, 3H). Anal. Calc.
for C21H25Cl2N3Pd (496.8 g/mol): C 51.0 (calc. 50.8); H
4.9 (5.0); N 8.6 (8.5) %.
In a typical run, a 5 mL vial equipped with a magnetic
stirrer bar was charged with a mixture of aryl bromide
(1 mmol), styrene (1.5 mmol), Pd catalyst (0.005 mmol),
K3PO4 (2 mmol), and 2 mL of dimethylacetamide in
argon. The mixture was heated at 110 °C for 18 h, then
cooled to room temperature, and brine was added into it.
The resulting mixture was extracted with ethyl acetate 3
times, and the crude product was obtained by removing
the volatiles. The product was purified by flash column
chromatography on silica gel.
Palladium complex 3d
Imidazolium chloride (1d) (0.252 g, 1 mmol) and Ag2O
(0.116 g, 0.5 mmol) were stirred in dichloromethane
(10 mL) for 12 h at room temperature under exclusion of
light, and then Pd(COD)Cl2 (0.285 g, 1 mmol) was added.
The mixture was stirred at room temperature for another
12 h, then filtered through celite. The filtrate was concen-
trated to 2 mL, and diethyl ether was added to precipitate
Results and discussion
Synthesis of the imidazoli(ni)um chloride salts
1
the product as a yellow solid. Yield: 0.24 g (61 %). H
The imidazolium salts 1a–1d were prepared by the reac-
tion of the corresponding readily available alkyl or aryl
imidazoles with 2-chloromethyl-pyridine (1.1 eq.) under
solvent-free conditions at 130 °C in good yields. As all of
the N-substituted imidazoles used were melted at elevated
temperature, these reactions actually took place homoge-
nously, and the completion of the reactions was easily
monitored by the formation of solid compounds in the
reaction mixture. The chiral imidazolinium chloride 1e
was synthesized by the reaction of 2-chloromethyl-pyri-
dine with the chiral imidazoline in acetonitrile in 93 %
yield (Scheme 1). The chiral imidazoline used here was
synthesized effectively from the relatively inexpensive
amino acid L-phenylalanine in five steps, according to our
previous report [24]. The imidazolium chlorides 1a–1c
have been reported previously [25–27]; however, imi-
dazolium chlorides 1d–1e are new compounds and were
characterized by their spectroscopic data and elemental
analyses.
NMR (DMSO-d6, 400 MHz): 8.85 (d, J = 5.6 Hz, 1H),
8.08–8.05 (m, 1H), 7.75 (d, J = 7.6 Hz, 1H), 7.63
(d, J = 2.0 Hz, 1H), 7.55 (s, 2H), 5.95–5.91 (m, 1H), 5.63
(d, J = 15.2 Hz, 1H), 1.85 (s, 9H). Anal. Calc. for
C13H17Cl2N3Pd (392.6 g/mol): C 39.6 (calc. 39.8); H 4.3
(4.4); N 10.8 (10.7) %.
Palladium complex 3e
The same procedure for the synthesis of compound 3d
was used for 3e with imidazolium chloride (1e) (0.448 g,
1 mmol), Ag2O (0.116 g, 0.5 mmol) and Pd(COD)Cl2
(0.285 g, 1 mmol). The product was obtained as a pale
yellow solid. Yield: 0.41 g (70 %). [a]2D5 = 79.3° (c 1.0,
DCM). 1H NMR (DMSO-d6, 400 MHz): 8.93 (s, 1H),
8.18–8.14 (m, 1H), 7.92–7.80 (m, 1H), 7.62 (s, 1H), 7.38–
7.23 (m, 6H), 7.14–7.05 (m, 2H), 5.40 (d, J = 16.0 Hz,
1H), 5.27–5.14 (m, 1H), 4.74–4.55 (m, 1H), 3.63 (s, 1H),
Scheme 1 Synthesis of 1e from
L-phenylalanine
N
Cl
Ph
Ph
Ph
H2N
O
5 steps
N
N
N
N
OH
CH3CN, 93%
overall yield: 26%
N
Br
1e
123

Transition Met Chem (2013) 38:351–358
355
Synthesis and characterization of the NHC-silver
and NHC-palladium complexes
an ethyl substituent, and further to 10.33 ppm for 1c with
an isopropyl substituent. In addition, the proton in the
imidazolinium chloride 1e was very basic with a chemical
shift of 9.94 ppm, indicating stronger electron-donating
properties of this carbene ligand.
The reaction of imidazolium salts 1a–1e with Ag2O in
DCM yields NHC–Ag–Cl complexes, which can possess
different structures, depending on N-substituent of the
NHC [28]. For example, 2a was determined to exist in
[Ag(NHC)l-Cl]2 dimer form by its crystal structure, as
reported previously [25]. Although all of the other Ag–
NHC–Cl complexes are hitherto unknown, the bromide
analoges of 2c and 2d were reported to exist in
[(NHC)2Ag][AgBr2] form [28]. Our effort to determine the
structures of these NHC–Ag–Cl complexes was unsuc-
cessful due to the difficulties of obtaining the single crys-
tals and pure compounds. Complexes 2a–2c were readily
isolated in moderate yield and high purity as a grey-white
powders (Scheme 2), whereas the pure NHC–Ag com-
plexes of 1d and 1e could not be isolated. Therefore, NHC–
Pd complexes (3a–3c) were prepared by the transmetala-
tion reaction of Pd(COD)Cl2 with well-defined NHC–Ag
compounds (2a–2c), while 3d and 3e were prepared in a
one-pot process by treating PdCl2(COD) with NHC–Ag
formed in situ. The Pd–NHC complexes were isolated in
moderate yields and were characterized by NMR spec-
troscopy and elemental analyses. Due to their poor solu-
bility in common solvents, no 13C NMR could be obtained
for 3a–3d. These complexes are air- and moisture-stable
and can be under air in the solid state for more than
6 months without any noticeable decomposition.
Molecular structures of complexes 3a, 3b, and 3e
The molecular structures of 3a, 3b, and 3e were determined
by X-ray diffraction studies. Crystals of 3a and 3b were
obtained by slow diffusion of diethyl ether into acetonitrile
solutions, while crystals of 3e were obtained by slow
evaporation of an acetonitrile solution. The molecular
structures are shown in Figs. 1, 2 and 3. Crystal data col-
lection and refinement parameters are summarized in
Table 1. The structures of all three complexes consist of a
square planar molecule with a palladium center surrounded
by carbene, pyridine, and two chloride ligands in a cis
configuration. The plane of the heterocylic ring is twisted
out of the distorted metal square plane with a torsion angle
of 36°–43°. The Pd–C1 distances in these complexes
˚
˚
˚
(1.955(3) A for 3a, 1.9475(13) A for 3b and 1.9610(19) A
for 3e) are similar to that shown by other palladium(II)-
related species [20]. The Pd–N (pyridine) distance is
˚
almost identical in these three compounds (2.063(2) A for
˚
˚
3a, 2.0744(11) A for 3b and 2.0596(17) A for 3e). The
different Pd–Cl distances in these complexes indicate dif-
ferent trans influences of the NHC and pyridine ligands.
˚
For example, the distances are 2.3620(8) A for Pd1–Cl1
˚
trans to NHC, compared to 2.2923(8) A for Pd1–Cl2 trans
It is interesting that the proton signal of NCHN (H
on pre-carbene carbon) of N-t-butylimidazolium chloride
(1d, 11.06 ppm) is shifted downfield relative to those of
N-arylimizolium chlorides, which means the proton of
N-alkylimidazolium chloride is more acidic than that of N-
arylimidazolium chloride. In addition, the acidity of this
proton in N-arylimidazolium chlorides was decreased with
increasing steric hindrance of ortho-substituents on the
phenyl group. It shifted from 10.57 ppm for 1a, which
carries methyl substituent, to 10.51 ppm for 1b, which has
to pyridine in 3a. The difference between two Pd–Cl dis-
tances is increased by increasing steric effect of the N-
˚
substituents of the NHC ligand, being 0.0697 A in complex
3a, 0.0830 in 3b, and 0.1059 in 3e. All other distances and
angles lie in the expected ranges, and representative bond
distance and angles are given in the corresponding figure
captions. Complex 3e has one stereogenic center on the
backbone of the heterocyclic ring with a specific rotation of
79.3° in 1.0 M dichloromethane solution.
Scheme 2 Synthesis of
palladium (II) complexes 3a–3e
R1
R1
N
N
R2
Ag2O, DCM, 12h
Pd(COD)Cl2
DCM, rt, 12h
N
N
R2
Ag-NHC-Cl
N
N
Pd Cl
Cl
Cl
1
2
3
a: R1 = H, R2 = 2,4,6-Me3C6H2
b: R1 = H, R2 = 2,6-Et2C6H3
c: R1 = H, R2 = 2,6-i-Pr2C6H3
d: R1 = H, R2 = t-Bu
e: R1 = Bn, R2 = 2,6-i-Pr2C6H3
123

356
Transition Met Chem (2013) 38:351–358
Fig. 1 Molecular structure of
compound 3a. Selected bond
˚
distances [A] and angles [°]:
Pd1–C1 1.955(3), Pd1–N3
2.063(2), Pd1–Cl1 2.3620(8),
Pd1–Cl2 2.2923(8), C1–Pd1–
N3 86.03(10), C1–Pd1–Cl2
91.16(8), N3–Pd1–Cl2
176.22(7), C1–Pd1–Cl1
176.70(8), N3–Pd1–Cl1
91.68(7), Cl2–Pd1–Cl1 91.23(3)
Fig. 2 Molecular structure of
compound 3b. Selected bond
˚
distances [A] and angles [°]:
Pd1–C1 1.9475(13), Pd1–N1
2.0744(11), Pd1–Cl1 2.2923(5),
Pd1–Cl2 2.3753(5), C1–Pd1–
N1 86.15(5), C1–Pd1–Cl1
89.71(4), N1–Pd1–Cl1
173.18(3), C1–Pd1–Cl2
175.27(4), N1–Pd1–Cl2
92.30(3), Cl1–Pd1–Cl2
92.239(19)
Fig. 3 Molecular structure of
compound 3e. Selected bond
˚
distances [A] and angles [°]:
Pd1–C1 1.9610(19), Pd1–N3
2.0596(17), Pd1–Cl1 2.2726(6),
Pd1–Cl2 2.3785(6), C1–Pd1–
N3 87.54(7), C1–Pd1–Cl1
91.54(6), N3–Pd1–Cl1
174.32(5), C1–Pd1–Cl2
176.25(6), N3–Pd1–Cl2
91.32(5), Cl1–Pd1–Cl2 89.94(3)
123

Transition Met Chem (2013) 38:351–358
357
Table 2 Mizoroki–Heck cross-coupling of bromobenzene with aryl halides catalyzed by complexes 3
R
X
0.5 mol% Pd, K 3PO4
DMAC, 100 oC, 18 h
+
+
R
R
A
B
Entry
R (X = Br)
3a
3b
3c
3d
3e
Yield
A:B
Yield
A:B
Yield
A:B
Yield
A:B
Yield
A:B
1
2
H
69
64
56
61
73
56
70
35
82
36
20:1
26:1
18:1
20:1
99:1
28:1
31:1
24:1
21:1
99:1
83
88
79
66
86
79
68
40
90
27
28:1
22:1
12:1
13:1
99:1
32:1
31:1
20:1
17:1
99:1
93
89
81
75
74
81
78
43
94
55
24:1
20:1
15:1
14:1
99:1
23:1
35:1
22:1
18:1
99:1
85
60
64
49
60
64
50
25
88
61
38:1
22:1
45:1
9:1
97
92
92
80
92
92
85
46
97
87
25:1
26:1
17:1
7:1
p-Me
3
p-OMe
p-NMe2
p-NH2
p-Cl
4
5
99:1
20:1
40:1
25:1
14:1
99:1
99:1
23:1
36:1
25:1
20:1
99:1
6
7
o-Me
8
o-OMe
o-OMe (X = I)
p-COMe
9
10
Palladium-catalyzed Heck reaction
activities, in the trend of 3c [ 3b [ 3a. In addition, the
bulky complex 3e based on imidazoline showed the best
activity for all chosen substrates. The probable reason for
the high performance of 3e is that the NHC ligand in 3e is
more electronic rich, which facilitates the oxidative addi-
tion step; while, its steric hinderance also facilitates the
reductive elimination step.
The palladium-catalyzed arylation of olefins has found
wide application in organic synthesis. The activities of
complexes 3a–3e for the catalytic arylation of styrene were
tested in order to elucidate the influence of the ligands.
The reactions were conducted in a vial in the presence of
catalytic amounts of palladium catalysts 3a–3e and K3PO4
in a mixture of aryl halides and styrene. A variety of aryl
halides with different electronic and steric properties was
chosen for the reactions, which were heated at 110 °C for
18 h. Details of the experimental setup are given in
experimental section. The results are given in Table 2.
From the results, complexes 3a–3e showed different
performances for the arylation of olefins with different
alkyl halides. Generally all the complexes showed good
regioselectivities for the products; in that, only small
amounts of the corresponding cis products could be
detected after the reaction. Furthermore, the trans isomers
were the only products for the substrates of 4-bromoben-
zenamine and 1-(4-bromophenyl)ethanone. The steric
effect of the aryl halide plays an important role; for
example, the reactions of aryl halides with para groups
gave better results than those with ortho groups (entries
2–3 versus entries 7–8). Better yields were generally
obtained with N-aryl substituted Pd complexes (3a–3c, 3e)
than with the N-alkyl substituted Pd complex (3d). Typi-
cally, the Pd complexes with bulky substituents at the 2,6
positions of the phenyl group showed better catalytic
Conclusion
A series of Pd(II) complexes of picoline-functionalized
N-heterocyclic carbenes with different substituents were
synthesized and characterized. The molecular structures
of 3a, 3b, and 3e were determined by X-ray diffraction
studies. The complexes were tested in the Mizoroki–Heck
reaction with only 0.5 mol % of the catalyst, and the best
conversion and good regioselectivity were reached using
complex 3e in the reactions of styrene with aryl halides.
Supplementary material
CCDC 914054 (3a), 914055 (3b), and 914056 (3e) contain
the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
123

358
Transition Met Chem (2013) 38:351–358
Acknowledgments We are grateful to the foundation of NSFC
(21071121, 21172188 and 21104064), Graduate Foundation
(CXLX11_0909 and CXLX11_0910), SRF for ROCS, SEM, PAPD,
and the State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry at Jilin University (2013-03) for financial support.
15. Warsink S, van Aubel CMS, Weigand JJ, Liu S-T, Elsevier CJ
(2010) Eur J Inorg Chem 35:5556
16. Wang C-Y, Liu Y-H, Peng S-M, Chen J-T, Liu S-T (2007) J
Organomet Chem 692:3976
17. Zeng F, Yu Z (2006) J Org Chem 71:5274
18. Nielsen DJ, Cavell KJ, Skelton BW, White AH (2006) Inorg
Chim Acta 359:1855
19. Canovese L, Visentin F, Levi C, Santo C, Bertolasi V (2012)
Inorg Chim Acta 390:105
References
20. Tulloch AAD, Winston S, Danopoulos AA, Eastham G, Hurst-
house MB (2003) Dalton Trans 4:699
21. Tulloch AAD, Danopoulos AA, Cafferkey SM, Kleinhenz S,
Hursthouse MB, Tooze RP (2000) Chem Commun 14:1247
22. Winston S, Stylianides N, Tulloch AAD, Wright JA, Danopoulos
AA (2004) Polyhedron 23:2813
23. Khlebnikov V, Meduri A, Mueller-Bunz H, Montini T, Fornasi-
ero P, Zangrando E, Milani B, Albrecht M (2012) Oganometallics
31:976
24. Yang L, Sun R, Zhang L, Li Y, Cao C, Pang G, Shi Y (2011) J
Chem Res 35:608
25. Cisnetti F, Lemoine P, El-Ghozzi M, Avignant D, Gautier A
(2010) Tetrahedron Lett 51:5226
26. Kuhl S, Schneider R, Fort Y (2003) Organometallics 22:4184
27. Tulloch AAD, Winston S, Danopoulos AA, Eastham G, Hurst-
house MB (2003) Dalton Trans 4:699
1. Moore JL, Rovis T (2010) Top Curr Chem 291:77
2. Diez-Gonzalez S, Marion N, Nolan SP (2009) Chem Rev 109:
3612
3. Bugaut X, Glorius F (2012) Chem Soc Rev 41:3511
4. Arnold PL, Casely IJ (2009) Chem Rev 109:3599
´ ´
5. Marion N, Dıez-Gonzalez S, Nolan SP (2007) Angew Chem Int
Ed 46:2988
6. Diez-Gonzalez S, Nolan SP (2007) Coord Chem Rev 251:874
7. Droge T, Glorius F (2010) Angew Chem Int Ed 49:6940
8. Mata JA, Poyatos M, Peris E (2007) Coord Chem Rev 251:841
9. Peris E, Crabtree RH (2004) Coord Chem Rev 248:2239
10. Normand AT, Cavell KJ (2008) Eur J Inorg Chem 18:2781
11. Pugh D, Danopoulos AA (2007) Coord Chem Rev 251:610
12. Boronat M, Corma A, Gonzalez-Arellano C, Iglesias M, Sanchez
F (2010) Organometallics 29:134
28. Tulloch AAD, Danopoulos AA, Winston S, Kleinhenz S, Eas-
tham G (2000) Dalton Trans 24:4499
29. Sheldrick G (2008) Acta Crystallogr Sect A 64:112
13. Danopoulos AA, Tulloch AAD, Winston S, Eastham G, Hurst-
house MB (2003) Dalton Trans 5:1009
14. Fiddy SG, Evans J, Neisius T, Newton MA, Tsoureas N, Tulloch
AAD, Danopoulos AA (2007) Chem Eur J 13:3652
123