Synthesis of binuclear palladium complexes with a rigid phenylene bridge linker

Article abstract of DOI:10.1007/s11243-014-9851-8

The complexes [PdX₂Py]₂(di-NHC) (X = Br or Cl) in which di-NHC represents a di-N-heterocyclic carbene, featuring a rigid phenylene spacer between the carbene units, have been prepared from reactions of the corresponding diimidazolium

Full text of DOI:10.1007/s11243-014-9851-8

Transition Met Chem (2014) 39:691–698
DOI 10.1007/s11243-014-9851-8
Synthesis of binuclear palladium complexes with a rigid phenylene
bridge linker
•
Guangying Wang Gang Liu Yufeng Du
•
•
•
Wenshu Li Shuhui Yin Shuzhan Wang
•
•
•
Yanhui Shi Changsheng Cao
Received: 6 May 2014 / Accepted: 13 June 2014 / Published online: 15 July 2014
Ó Springer International Publishing Switzerland 2014
Abstract The complexes [PdX2Py]2(di-NHC) (X = Br
or Cl) in which di-NHC represents a di-N-heterocyclic
carbene, featuring a rigid phenylene spacer between the
carbene units, have been prepared from reactions of the
corresponding diimidazolium halide salts with PdCl2 in
pyridine. The molecular structures of three of the com-
plexes were determined by X-ray diffraction studies. The
influences of different substitutions and of the halide ligand
(Br or Cl) on the structure and reactivity of the complexes
have been studied. The catalytic activity of the binuclear
palladium complexes was tested in the Mizoroki–Heck
reaction of styrene with bromobenzene.
of bidentate bis-NHC ligands and their complexes have
been studied. In these ligands, a pair of NHC moieties are
connected mostly by flexible alkyl [9], ether [10], benzyl
[11], or picolyl [12] linkers, whereas only a few reports
have featured rigid liners [13–16]. An interesting feature of
metal complexes with rigid linkers, especially having p
conjugating systems, is their potential for metal–metal
interactions, which could also have an impact on the cat-
alytic activity of the metal centers.
Palladium is one of the most versatile metals in cata-
lyzing reactions involving C–C bond formation [17]. For
example, the Heck coupling reaction, involving the palla-
dium-catalyzed arylation of olefins, has found a wide
application in organic synthesis. In continuation of our
previous research into di-NHC dipalladium complexes with
flexible alkyl bridges [18–21], in this paper we describe the
synthesis of a series of dipalladium di-NHC complexes
bridged with a rigid phenylene spacer (Scheme 1), and
their use as catalysts for the Heck reaction. The effects of
different substituents and choice of halide (X = Br or Cl)
on the catalytic activity were studied.
Introduction
The chemistry of N-heterocyclic carbenes (NHCs) has been
intensively studied in recent years, and they have been
successfully employed in a variety of applications such as
ligands in catalytically active metal complexes [1–5], or-
ganocatalysis [6], biologically active compounds [7], and
the construction of molecular devices [8]. A large number
Experimental
Guangying Wang and Gang Liu have contributed equally to this
work.
Materials and methods
All reactions were performed under an argon atmosphere
using standard Schlenk or glovebox techniques. 1,4-Bis(1-
imidazolyl)benzene was synthesized according to a litera-
ture procedure [22]. Pyridine was distilled from calcium
hydride under argon atmosphere, and potassium carbonate
was ground to a fine powder prior to use. All other
chemicals were obtained from common suppliers and used
without further purification. 1H and 13C spectra were
G. Wang ꢀ G. Liu ꢀ Y. Du ꢀ W. Li ꢀ S. Yin ꢀ S. Wang ꢀ
Y. Shi (&) ꢀ C. Cao (&)
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
e-mail: yhshi@jsnu.edu.cn
C. Cao
e-mail: chshcao@jsnu.edu.cn
123

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Transition Met Chem (2014) 39:691–698
(s, 4H, Ar), 8.12 (t, J = 2.0 Hz, 2H, NCH), 7.54 (m, 4H,
1
Ar), 7.44 (m, 6H, Ar), 5.55 (s, 4H, CH2). H NMR (D2O,
N
N
N
N
R
R
400 MHz): 8.00 (s, 2H), 7.89 (s, 4H), 7.76 (s, 2H), 7.54 (s,
10H), 5.56 (s, 4H) (two H of 2,20-CH are missing due to
H/D exchange in D2O). 13C NMR (100 MHz, d6-DMSO):
137.3, 136.5, 134.7, 131.2, 131.1,130.5, 125.8, 125.0,
123.7, 123.6, 55.1. Anal. Calc. for C26H24Cl2N4 (463.4 g/
mol): C, 67.4; H, 5.2; N, 12.1. Found: C, 67.2; H, 5.2; N,
12.2 %.
X Pd
N
X
X Pd
N
X
Scheme 1 Stucture of the dipalladium di-NHC complexes with a
phenylene linkers. X = Cl or Br; R = Bn, n-Bu or i-Pr
recorded on a Bruker AV 400 MHz spectrometer at room
temperature and referenced to the residual signals of the
solvent. GC–MS was performed on an Agilent
6,890–5,973N system with electron ionization (EI) mass
spectrometry. Elemental analyses were performed on a
EuroVektor Euro EA-300 elemental analyzer. X-ray crys-
tallography was conducted with a Rigaku Mercury CCD
device using graphite-monochromated Mo Ka radiation
1,10-Di-n-butyl-3,30-(1,4-phenylene)bisimidazolium
dibromide
1
Yield: 0.42 g (87 %). H NMR (D2O, 400 MHz): 9.35 (s,
2H, NCHN), 7.92 (s, 2H, NCH), 7.85 (s, 4H, Ar), 7.69 (s,
2H, NCH), 4.28 (t, J = 7.6 Hz, 4H, CH2), 1.89 (m, 4H,
CH2), 1.33(m, 4H, CH2), 0.89 (t, J = 7.2 Hz, 6H,CH3).
13C NMR (100 MHz, D2O-DMSO): 137.3, 136.3, 125.7,
125.0, 123.3, 123.2, 51.6, 32.8, 20.4, 14.3. Anal. Calc. for
C20H28Br2N4 (484.3 g/mol): C, 49.6; H, 5.8; N, 11.6.
Found: C, 49.3; H, 5.3; N, 12.0 %.
˚
(k = 0.71073 A). Absorption correction was performed by
the SADABS program. The structures were solved by
direct methods using the SHELXS-97 program and refined
by full-matrix least-square techniques on F2.
1,10-Di-n-butyl-3,30-(1,4-phenylene)bisimidazolium
dichloride
Synthesis of bisimidazolium dihalides
1
Yield: (62 %). H NMR (400 MHz, D2O): 9.39 (s, 2H,
In a typical run, a 10 mL thick wall pressure tube was
charged with 1,4-di(1H-imidazol-1-yl)benzene (0.210 g,
1 mmol), the corresponding alkyl halide (2.5 mmol) and
acetonitrile (2 mL). The reaction mixture was heated at
100–120 °C for 6–12 h. After the completion of the reac-
tion, the solid was filtered off and rinsed with CH2Cl2
(5 mL) to give the product as a white powder.
NCHN), 7.95 (s, 2H, NCH), 7.88 (s, 4H, Ar), 7.72 (s, 2H,
NCH), 4.31 (t, J = 7.0 Hz, 4H, CH2), 1.92 (m, 4H, CH2),
1.37 (m, 4H, CH2), 0.92 (t, J = 7.3 Hz, 6H, CH3). 13C
NMR (100 MHz, D2O-DMSO): 137.2, 136.3, 125.6, 124.9,
123.2, 51.5, 32.7, 20.4, 14.3. Anal. Calc. for C20H28Cl2N4
(395.4 g/mol): C, 60.8; H, 7.1; N, 14.2. Found: C, 60.9; H,
7.2; N, 14.0 %.
1,10-Dibenzyl-3,30-(1,4-phenylene)bisimidazolium
dibromide
1,10-Diisopropyl-3,30-(1,4-phenylene)bisimidazolium
dibromide
Yield: 0.52 g (95 %). 1H NMR (d6-DMSO, 400 MHz):
10.27 (s, 2H, NCHN), 8.48 (t, J = 1.6 Hz, 2H, NCH), 8.16
(s, 4H, Ar), 8.12 (t, J = 2.0 Hz, 2H, NCH), 7.56 (m, 4H,
Yield: 78 %. 1HNMR (D2O, 400 MHz): 9.41 (s, 2H,
NCHN), 7.95 (d, J = 2.0 Hz, 2H, NCH), 7.89 (s, 4H, Ar),
7.80 (d, J = 2.0 Hz, 2H, NCH), 4.76 (octet, J = 6.8 Hz,
2H, CH), 1.62 (d, J = 6.4 Hz, 12H,CH3). 13C NMR
(100 MHz, D2O-DMSO): 137.5, 135.2, 126.1, 125.8,
123.4, 123.2, 55.6, 23.6. Anal. Calc. for C18H24Br2N4
(456.22 g/mol): C, 47.4; H, 5.3; N, 12.3. Found: C, 47.9;
H, 5.0; N, 12.6 %.
1
Ar), 7.44 (m, 6H, Ar), 5.56 (s, 4H, CH2). H NMR (D2O,
400 MHz): 7.93 (s, 2H), 7.82 (s, 4H), 7.68 (s, 2H), 7.46 (s,
10H), 5.48 (s, 4H) (two H of 2,20-CH are not observed due
to H/D exchange in D2O). 13C NMR (100 MHz, d6-
DMSO): 137.3, 136.5, 134.7, 131.2, 131.1, 130.5, 125.8,
125.0, 123.7, 123.6, 55.1. Anal. Calc. for C26H24Br2N4
(552.3 g/mol): C, 56.5; H, 4.4; N, 10.1. Found: C, 56.1; H,
4.0; N, 9.8 %.
Synthesis of palladium bromide complexes (1, 3, 5)
1,10-Dibenzyl-3,30-(1,4-phenylene)bisimidazolium
dichloride
To a mixture of the required bisimidazolium dibromide
(1 mmol), PdCl2 (0.351 g, 1.98 mmol), NaBr (1.029 g,
10 mmol), and K2CO3 (1.382 g, 10 mmol) was added
pyridine (10 mL). The reaction mixture was stirred and
heated at 80–95 °C for 8–10 h, then filtered through Celite
Yield: 0.41 g (90 %). 1H NMR (d6-DMSO, 400 MHz):
10.37 (s, 2H, NCHN), 8.48 (t, J = 1.6 Hz, 2H, NCH), 8.16
123

Transition Met Chem (2014) 39:691–698
693
and washed with CH2Cl2. The solvent was removed under
vacuum, and the crude product was washed with diethyl
ether (3 9 5 mL). The pure compounds were obtained as
yellow solids by recrystallization from CH2Cl2/ether.
C30H36Br4N6Pd2 (1,013.10 g/mol): C, 35.6; H, 3.6; N, 8.3.
Found: C, 35.8; H, 3.2; N, 8.6 %.
Complex 4
1
Yield: 0.72 g (87 %). H NMR (400 MHz, CDCl3): 8.82
Synthesis of palladium chloride complexes (2, 4)
(d, J = 4.8 Hz, 4H, Py), 8.24 (s, 4H, Ar), 7.58 (t,
J = 7.6 Hz, 2H, Py), 7.28 (d, J = 2.0 Hz, 2H, NCHC),
7.24 (t, J = 6.4 Hz, 4H, Py), 7.13 (d, J = 2.0 Hz, 2H,
NCHC), 4.67 (m, 4H, CH2), 2.17 (sextet, J = 7.2 Hz, 4H,
CH2), 1.58 (septet, J = 6.8 Hz, 4H, CH2), 1.08 (t,
J = 7.2 Hz, 6H, CH3). 13C NMR (100 MHz, CDCl3):
152.0, 151.4, 129.0, 128.2, 127.3, 127.0, 124.4, 122.2,
51.2, 32.4, 20.0, 13.8. Anal. Calc. for C30H36Cl4N6Pd2
(835.3 g/mol): C, 43.1; H, 4.3; N, 10.1. Found: C, 43.7; H,
4.1; N, 9.6 %.
To a mixture of the required bisimidazolium dichloride
(1 mmol), PdCl2 (0.351 g, 1.98 mmol), and K2CO3
(1.382 g, 10 mmol) was added pyridine (10 mL). The
reaction mixture was stirred and heated at 80–95 °C for
8–10 h, then filtered through Celite and washed with
CH2Cl2. The solvent was removed under vacuum, and the
crude product was washed with diethyl ether (3 9 5 mL).
The pure compounds were obtained as yellow solids by
recrystallization from CH2Cl2/ether.
Complex 5
Complex 1
1
Yield: 0.61 g (76 %). H NMR (400 MHz, CDCl3): 8.84
Yield: 0.86 g (80 %). 1H NMR (400 MHz, d6-DMSO):
8.71 (d, J = 4.8 Hz, 4H, Py), 8.46 (s, 4H, Ar), 7.94 (d,
J = 2.0 Hz, 2H, NCHC), 7.81 (t, J = 7.6 Hz, 2H, Ar),
7.69 (m, 4H, Py), 7.52 (d, J = 2.0 Hz, 2H, NCH),
7.45–7.36 (m, 10H, Ar, Py), 5.89 (s, 4H, CH2). 13C NMR
(100 MHz, CDCl3): 154.3, 152.6, 138.4, 137.5, 134.8,
129.4, 129.0, 128.7, 127.4, 125.0, 124.8, 124.5, 53.4. Anal.
Calc. for C36H32Br4N6Pd2 (1,081.1 g/mol): C, 40.0; H, 3.0;
N, 7.8. Found: C, 40.3; H, 3.2; N, 7.6 %.
(d, J = 4.8 Hz, 4H, Py), 8.24 (s, 4H, Ar), 7.61 (t,
J = 7.6 Hz, 2H, Py), 7.33 (d, J = 2.0 Hz, 2H, NCHC),
7.27–7.16 (m, 6H, Py, NCHC) 5.96 (m, 2H, CH), 1.67 (m,
12H, CH3). 13C NMR (100 MHz, d6-DMSO): 151.6, 143.1,
138.5, 130.6, 125.0, 124.9, 124.2, 119.8, 50.0, 22.2. Anal.
Calc. for C28H32Br4N6Pd2 (985.0 g/mol): C, 34.1; H, 3.3;
N, 8.5. Found: C, 34.4; H, 2.9; N, 8.9 %.
Procedure for the Mono–Heck coupling reaction
Complex 2
In a typical run, a 5 mL vial equipped with a magnetic
stirrer was charged with a mixture of phenyl bromide
(0.5 mmol), styrene (62.5 mg, 0.6 mmol), Pd catalyst
(0.005 mmol), K3PO4 (207 mg, 1.5 mmol), and DMAC
(1 mL) under argon. The mixture was heated at 100 °C for
5 h and then cooled to room temperature. Brine was added,
and the resulting mixture was extracted with ethyl acetate
(3 9 5 mL). The GC–MS samples were prepared by
diluting 10 lL of ethyl acetate solution to 1 mL.
1
Yield: 0.74 g (82 %). H NMR (400 MHz, CDCl3): 8.85
(d, J = 4.8 Hz, 4H, Py), 8.29 (s, 4H, Ar), 7.68 (t,
J = 7.6 Hz, 2H, Ar), 7.60 (m, 4H, Py), 7.45–7.37 (m, 6H,
Ar, Py), 7.26 (m, 6H, Ar, NCH), 6.91 (d, J = 2.0 Hz, 2H,
NCH), 6.00 (s, 4H, CH2). 13C NMR (100 MHz, CDCl3):
151.7, 151.4, 139.6, 137.9, 134.8, 129.2, 129.0, 128.7,
127.0, 124.5, 123.4, 121.8, 55.1. Anal. Calc. for C36H32-
Cl4N6Pd2 (903.3 g/mol): C, 47.9; H, 3.6; N, 9.3. Found: C,
47.6; H, 3.8; N, 9.6 %.
Results and discussion
Complex 3
Synthesis
Yield: 0.84 g (83 %). 1H NMR (400 MHz, d6-DMSO):
8.68 (d, J = 4.8 Hz, 4H, Py), 8.42 (s, 4H, Ar), 7.91 (d,
J = 2.0 Hz, 2H, NCHC), 7.81 (t, J = 7.6 Hz, 2H, Py),
7.75 (d, J = 2.0 Hz, 2H, NCHC), 7.32 (t, J = 6.4 Hz, 4H,
Py), 4.58 (t, J = 7.6 Hz, 4H, CH2), 2.14 (sextet,
J = 7.2 Hz, 4H, CH2), 1.47 (septet, J = 7.6 Hz, 4H, CH2),
1.03 (t, J = 7.2 Hz, 6H, CH3). 13C NMR (100 MHz, d6-
DMSO): 151.8, 148.1, 138.9, 138.3, 125.8, 124.6, 123.6,
123.3, 50.4, 31.3, 19.2, 13.5. Anal. Calc. for
1,4-Bis(1-imidazolyl)benzene was synthesized by the CuI-
catalyzed C–N coupling reaction of 1,4-dibromobenzene
with 1H-imidazole, using cheap hexamethylenetetramine
(HMTA) as additive and dimethylacetamide (DMAC) as
solvent [22]. The bisimidazolium dihalides were prepared
by the reaction of bisimidazolylbenzene and the corre-
sponding dihaloalkanes at elevated temperature. n-Butyl
[14] and benzyl [23] substituted bisimidazolium bromides
123

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Transition Met Chem (2014) 39:691–698
N
N
N
N
R
R
PdCl2
N
N
N
N
R
R
X Pd X
N
X Pd X
N
Py, K2CO3
2X-
if X = Br, + NaBr
Scheme 2 Synthesis of complexes 1–5. X = Br, Cl, R = CH2Ph (10); X = Br, R = CH2Ph (1); X = Cl, R = CH2Ph (2); X = Br, R = n-Bu
(3); X = Cl, R = n-Bu (4); X = Br, R = i-Pr (5)
have been reported previously, whereas the other imi-
dazolium halides are unknown. Unfortunately, an attempt
to prepare i-propyl substituted imidazolium chloride failed,
most likely due to the steric hindrance of i-propyl chloride.
The identities of the new compounds were confirmed by 1H
NMR, 13C NMR, and elemental analysis.
given below the figures. The molecular structures of these
complexes confirm the bridging coordination mode of the
ditopic carbene ligand and the halide exchange under the
reaction conditions.
All the complexes show slightly distorted square-planar
geometries around both palladium centers, which are
coordinated by the imidazolylidene, two halide ligands in a
trans configuration, and a pyridine ligand. The solid-state
structures of these complexes are largely dependent on the
different substituents and halide ligands. For complex 10
with benzyl substitution and bromo/chloro ligands, two
pseudo-square-planar subunits are in a trans configuration
with torsion angle of 180° involving the backbone atoms
N3–C13–C13A–N3A, and the angle between the planes of
the imidazole and phenylene rings is 34.30°. For complex 3
with n-butyl substitution and bromo ligands, the two sub-
units are in an X configuration with torsion angle of
-72.68° involving the backbone atoms N1–C7–C7A–
N1A, and the angle between the planes of the imidazole
and phenylene rings is 51.44°. For complex 4 with n-butyl
substitution and chloride ligands, the two subunits are in a
trans configuration with torsion angle of 180° involving the
backbone atoms N1–C7–C7A–N1A, and the angle between
the planes is 41.72°. Hence, the two NHC–Pd–Py subunits
in 3 are mutually staggered, whereas the subunits in 10 and
4 are parallel. It is worth noting that the observed geom-
etries of these complexes are the result of symmetry.
Although the spacer between two NHCs is the same in
these complexes, the Pd to Pd distance is very different,
due to the diff0erent geometries. The Pd–Pd distances are
The bimetallic palladium complexes 1–5 were prepared
by the procedure shown in Scheme 2. Reaction of the
corresponding imidazolium chloride with one equivalent of
PdCl2 in pyridine in the presence of K2CO3 as a base
afforded palladium complexes with chloro co-ligands (2
and 4) in good yield. The synthesis of di-NHC di-Pd
complexes bearing bromo ligands (1, 3, and 5) was similar
to that of 2 and 4, except that NaBr was also added to the
reaction (Scheme 2). The exchange of chloride to bromide
was tested by adding different amounts of NaBr into the
reaction. It was found that the exchange of chloride to
bromide was incomplete with only 3 equivalents of NaBr,
and complex 10 with mixed halo ligands was observed. The
complete exchange of chloride to bromide was detected
with 10 equivalents of NaBr. These complexes were fully
characterized by NMR spectroscopy and gave satisfactory
elemental analyses. The formation of the metal complexes
1–5 was indicated by the expected 1:1 pyridine/imidazol-
1
ylidene ratio in the H NMR spectra. The proton signal of
NCHN from the imidazolium salts (9.35–10.37 ppm) was
1
absent from the H NMR spectra of the palladium com-
plexes, confirming the carbene generation. The complexes
are air and moisture stable and can be stored at air in the
solid state for many months without any noticeable
decomposition.
˚
˚
˚
8.630 A for 1 , 6.493 A for 3, and 8.556 A for 4. In
addition, from the point of view of structure, the two Pd
centers most likely have weak or no communication
because the NHC fragment and benzene ring are nonco-
planar, resulting in a lack of significant conjugation
between them. The Pd–C bond lengths are comparable
within the margin of error. The same applies to the Pd–N,
Pd–Cl, and Pd–Br bonds. The Pd–Ccarbene distance is
Description of the structures
Suitable crystals of complexes 10, 3, and 4 for X-ray dif-
fraction analysis were obtained by slow evaporation of a
dichloromethane or chloroform-saturated solution at room
temperature. The selected crystallographic data and
refinement parameters are summarized in Table 1. The
molecular diagrams of 10, 3, and 4 are shown in Figs. 1, 2,
3, respectively, and selected bond lengths and angles are
0
˚
1.975(3) A for 1 , 1.953(4) A for 3, and 1.943(7) A for 4,
˚
˚
similar to the values for other palladium-related species
0
˚
[18]. The Pd–Npyridine distances in complex 1 [2.086(2) A],
123

Transition Met Chem (2014) 39:691–698
695
Table 1 Selected
crystallographic data for
complexes 10, 3, and 4
Compound
10
3
4
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
C38H36Br2.53Cl5.47N6Pd2
1,184.28
C30H36Br4N6Pd2
1,013.09
293(2)
C34H40Cl16N6Pd2
1,312.76
298(2)
298(2)
Monoclinic
P 21/c
Monoclinic
C2/c
Triclinic
P-1
Crystal size/mm
0.30 9 0.20 9 0.10
10.3247(2)
8.26410(10)
25.5408(4)
90
0.3 9 0.25 9 0.2
26.4139(11)
8.8683(5)
15.2547(6)
90
0.30 9 0.20 9 0.10
9.2177(18)
12.368(3)
13.079(3)
75.06(3)
˚
a/A
˚
b/A
˚
c/A
a/°
b/°
c/°
93.339(3)
90
93.049(3)
90
69.91(3)
70.68(3)
3
˚
V/A
2175.55(6)
2
3,568.3(3)
4
1,303.9(4)
1
Z
D
calcd./mg cm-3
1.808
1.886
1.672
Absorption coefficient/mm-1
3.496
5.516
1.542
F(000)
1,158
1,960
650
h range/°
3.16 to 25.02°
15,882/3,834
1.055
2.42 to 25.01°
9,806/3,150
0.972
3.33 to 25.01°
10,285/4,572
1.081
Reflections collected/unique
Goodness-of-fit on F2
Final R indices [I [ 2r (I)]
R1 = 0.0264
wR2 = 0.0622
R1 = 0.0293
wR2 = 0.0608
R1 = 0.0335
wR2 = 0.0638
R1 = 0.0491
wR2 = 0.069
R1 = 0.0714
wR2 = 0.2244
R1 = 0.0753
wR2 = 0.2297
R indices (all data)
˚
˚
3 [2.113(3) A], and 4 [2.109(6) A] are comparable to those
of related palladium carbene analogues [18]. All other
distances and angles lie in the expected ranges.
In order to elucidate the influence of the reaction con-
ditions, we studied the arylation with catalyst
4
(Scheme 4), with the results shown in Table 3. Of the
various bases used, K3PO4 gave the best yield and good
regioselectivity, while almost no product was obtained with
pyridine (Table 3, entries 1–4). The choice of solvents also
has a great effect on the reaction. When the reaction was
conducted in DMSO, only trace amount of product was
detected by GC. With DMAC as solvent, the yield and
regioselectivity were both good. Although all the reactions
were tested with 1 mol% catalyst loadings, comparable
yields can be obtained with 0.5 % mol catalyst loading and
longer reaction times (Table 3, entry 9). The arylation of
styrene with different substituted bromobenzenes catalyzed
by 4 was also tested (Table 3, entries 10–12). The results
show that the reactions with para-methoxybromobenzene
(electron donating substituent) and para-bromoflouroben-
zene (electron withdrawing substituent) gave high yields
and good selectivity, whereas the reaction with the steri-
cally hindered ortho-methoxybromobenzene gave moder-
ate yield.
Catalytic studies
The palladium-catalyzed arylation of olefins has wide
application in organic synthesis. The activity of complexes
1–5 in the catalytic Heck reaction was tested in order to
elucidate the influence of substituents and halide ligands
(Scheme 3). Table 2 shows their relative reactivities for a
model system (styrene with bromobenzene) at 0.5 M sub-
strate concentration in DMAC solvent at 100 °C for a
reaction period of 5 h. Details of the experimental setup are
given in the Experimental section. Complex 4 gave the
highest yield with good selectivity, while complex 1
showed lowest activity, albeit still with good selectivity
under identical conditions (Table 2). The different sub-
stituents and halide ligands do indeed affect the catalytic
activities of these palladium complexes. Thus, the activity
increased as benzyl \ i-Pr \ n-Bu and Br \ Cl.
123

696
Transition Met Chem (2014) 39:691–698
Fig. 1 ORTEP structure of complex 10 with the probability ellipsoids
drawn at the 50 % level. Hydrogen atoms and solvents have been
Pd1–Cl2 2.29(6), N1–C13–N2 104.6(2), N1–C13–Pd1 128.1(2), N2–
C13–Pd1 127.28(19), C13–N1–C12 125.5(2), C13–N2–C16 126.2(2),
C5–N3–C1 118.2(3), C5–N3–Pd1 121.77(19), C1–N3–Pd1 119.9(2),
C13–Pd1–N3 176.96(10), C13–Pd1–Br2 90.1(4), N3–Pd1–Br2
88.4(4), C13–Pd1–Br1 90.2(2), N3–Pd1–Br1 91.3(2), Br2–Pd1–Br1
179.3(4)
˚
omitted for clarity. Selected bond distances (A) and angles (°): C1–N3
1.348(4), C5–N3 1.343(4), C12–N1 1.471(3), C13–N1 1.349(3),
C13–N2 1.361(4), C13–Pd1 1.975(3), C16–N2 1.435(3), N3–Pd1
2.086(2), Br1–Pd1 2.439(9), Br2–Pd1 2.423(15), Pd1–Cl1 2.30(4),
Fig. 2 ORTEP structure of complex 3 with the probability ellipsoids
drawn at the 50 % level. Hydrogen atoms have been omitted for
89.31(11), N1–Pd1–Br1 92.88(9), C7–Pd1–Br2 86.10(11), N1–Pd1–
Br2 92.11(9), Br1–Pd1–Br2 173.034(19), C15–N1–C11 118.3(4),
C15–N1–Pd1 123.8(3), C11–N1–Pd1 117.8(3), C7–N2–C4 125.3(3),
C7–N3–C9 124.5(3), N2–C7–N3 105.7(3), N2–C7–Pd1 128.7(3),
N3–C7–Pd1 125.2(3)
˚
clarity. Selected bond distances (A) and angles (°): Pd1–C7 1.953(4),
Pd1–N1 2.113(3), Pd1–Br1 2.4392(5), Pd1–Br2 2.4435(5), N1–C15
1.333(5), N1–C11 1.336(5), N2–C7 1.349(5), N2–C4 1.456(5), N3–
C7 1.350(5), N3–C9 1.435(5), C7–Pd1–N1 175.07(15), C7–Pd1–Br1
123

Transition Met Chem (2014) 39:691–698
697
Fig. 3 ORTEP structure of
complex 4 with the probability
ellipsoids drawn at the 50 %
level. Hydrogen atoms and
solvents have been omitted for
clarity. Selected bond distances
˚
(A) and angles (°): Pd1–C7
1.943(7), Pd1–N1 2.109(6),
Pd1–Cl2 2.3348(17), Pd1–Cl1
2.3950(16), N1–C11 1.339(10),
N1–C15 1.340(10), N2–C7
1.365(9), N2–C8 1.437(9), N3–
C7 1.363(9), N3–C4 1.462(10),
C7–Pd1–N1 177.9(3), C7–Pd1–
Cl2 86.5(2), N1–Pd1–Cl2
91.68(18), C7–Pd1–Cl1 88.7(2),
N1–Pd1–Cl1 93.15(18), Cl2–
Pd1–Cl1 175.15(5), C11–N1–
C15 117.6(7), C11–N1–Pd1
121.2(5), C15–N1–Pd1
121.2(5), C7–N2–C8 125.3(6),
C7–N3–C4 125.5(7), N3–C7–
N2 104.4(6), N3–C7–Pd1
127.9(5), N2–C7–Pd1 127.7(5)
1 mol% Pd,
Br
+
1.25 eq. K3PO4
DMAC, 100 °C, 5 h
+
1 eq.
1.2 eq.
trans
cis
Scheme 3 Arylation of styrene catalyzed by complexes 1–5
Table 2 Arylation of styrene catalyzed by Pd complexes 1–5
Conclusion
Entrya
Catalyst
GC yieldb (%)
Trans:cis ratio
We have synthesized five dicarbene palladium complexes
with a rigid phenylene linker. X-ray studies showed that the
molecular geometry of the solid Pd–NHC complexes is
influenced significantly by the substituents and halide
ligands. The Pd to Pd distances of these complexes having
the same spacer vary considerably, and the two Pd centers
most likely have weak or no communication. These new
binuclear complexes were successfully tested as catalysts
for the Heck reaction of styrene with bromobenzene, and
complex 4 gave the best yield of the product with good
1
2
3
4
5
a
1
2
3
4
5
63
67
82
92
79
53:1
49:1
78:1
85:1
72:1
Reaction conditions: bromobenzene (0.5 mmol), styrene
(0.6 mmol), catalyst (0.005 mmol), K3PO4 (1.25 mmol), DMAC
(1 mL), 100 °C, 5 h
b
Yield was determined by GC with dodecane as an internal standard
Scheme 4 Arylation of styrene
catalyzed by 4
complex 4,
base
R
Br
R
+
+
R
solvent
trans
cis
1 eq.
1.2 eq.
123

698
Transition Met Chem (2014) 39:691–698
Entrya
R
Cat.
(mol%)
Solvent
Base
Temp
(°C)
Time
(h)
GC
yieldb (%)
A:B
ratio
Table 3 Arylation of styrene
catalyzed by 4 under different
reaction conditions
1
H
1
Toluene
Toluene
Toluene
Toluene
DMAC
Dioxane
DMSO
Toluene
DMAC
DMAC
DMAC
DMAC
Et3N
110
110
110
110
100
100
100
100
100
100
100
100
12
12
12
12
8
62
77:1
39:1
–
2
H
1
K2CO3
Pyridine
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
20
3
H
1
Trace
99
4
H
1
76:1
84:1
77:1
–
5
H
1
98
a
6
H
1
8
75
Reaction conditions:
bromobenzene (0.5 mmol),
styrene (0.6 mmol), complex 4
(0.0025–0.005 mmol), base
(1.25 mmol), solvent,
7
H
1
8
Trace
73
8
H
1
8
62:1
81:1
72:1
65:1
80:1
9
H
0.5
1
8
89
100–110 °C, 8–12 h
10
11
12
p-OMe
o-OMe
p-F
8
87
b
Yield was determined by GC
with dodecane as an internal
standard
1
8
65
1
8
99
11. Iqbal MA, Haque RA, Budagumpi S, Khadeer Ahamed MB,
Abdul Majid AMS (2013) Inorg Chem Commun 28:64
12. Hernandez-Juarez M, Vaquero M, Alvarez E, Salazar V, Suarez
A (2013) Dalton Trans 42:351
selectivity. The catalytic activity of these complexes
increased in the order of benzyl \ iso-propyl \ n-butyl
and Br \ Cl.
13. Maity R, Rit A, Schulte to Brinke C, Koesters J, Hahn FE (2013)
Organometallics 32:6174
14. Tronnier A, Strassner T (2013) Dalton Trans 42:9847
15. Mercs L, Neels A, Stoeckli-Evans H, Albrecht M (2011) Inorg
Chem 50:8188
Acknowledgments We are grateful to the National and Provincial
Innovative Entrepreneurial Training Programs for Undergraduate
Project (HXKYZD201203, 201310320047, and 201310320047Z) and
PAPD for financial support.
16. Maity R, Koppetz H, Hepp A, Hahn FE (2013) J Am Chem Soc
135:4966
´ ´
17. Dıez-Gonzalez S, Marion N, Nolan SP (2009) Chem Rev
109:3612
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