Binuclear palladium complexes bridged by dicarbene with different lengths and their application in Mizoroki-Heck reaction

Article abstract of DOI:10.1016/j.ica.2010.07.054

A series of di-NHC ligands with alkyl bridges of different chain lengths (n = 2-4) and their bi-palladium complexes (4, 5 and 6) have been prepared. The molecular structure of 4 and 6 were determined by X-ray diffraction studies. The structures of complex 4 consist of two pseudo-square-planar subunits in a trans configuration, however 6 consist of two subunits in a cis configuration with π-π stacking between two pyridines. The influence of the different bridges on the structure and reactivity of the complexes has been studied. The catalytic activity of the new binuclear palladium complexes was successfully tested in the Mizoroki-Heck reaction of styrene with bromobenzene. The complex with 2 and 4 carbon linker gave higher yield, while the one with 3 carbon linker gave better regioselectivity.

Full text of DOI:10.1016/j.ica.2010.07.054

Inorganica Chimica Acta 363 (2010) 3914–3918
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Binuclear palladium complexes bridged by dicarbene with different lengths
and their application in Mizoroki–Heck reaction
Changsheng Cao a, Yongzhong Zhuang a, Jianfeng Zhao a, Yu Peng b, Xiuling Li a, Zhan Shi b,
Guangsheng Pang b, Yanhui Shi a,
*
a College of Chemistry and Chemical Engineering and Key Laboratory of Biotechnology for Medical Plant, Xuzhou Normal University, Xuzhou, Jiangsu 221116, PR China
b State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, Jilin 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 May 2010
Received in revised form 16 July 2010
Accepted 16 July 2010
Available online 24 July 2010
A series of di-NHC ligands with alkyl bridges of different chain lengths (n = 2–4) and their bi-palladium
complexes (4, 5 and 6) have been prepared. The molecular structure of 4 and 6 were determined by X-ray
diffraction studies. The structures of complex 4 consist of two pseudo-square-planar subunits in a trans
configuration, however 6 consist of two subunits in a cis configuration with p–p stacking between two
pyridines. The influence of the different bridges on the structure and reactivity of the complexes has been
studied. The catalytic activity of the new binuclear palladium complexes was successfully tested in the
Mizoroki–Heck reaction of styrene with bromobenzene. The complex with 2 and 4 carbon linker gave
higher yield, while the one with 3 carbon linker gave better regioselectivity.
Keywords:
Binuclear
N-Heterocyclic carbene
Palladium
Ó 2010 Elsevier B.V. All rights reserved.
Heck reaction
1. Introduction
2. Results and discussion
Since the first isolation of a stable, free N-heterocyclic carbine
by Arduengo in 1991 [1], NHC has attracted considerable atten-
tion as a new class of ligands in organometallic coordination
chemistry due to its successful application in homogeneous catal-
ysis [2–4]. The majority of NHC ligands used are monodentate
[4], and much fewer complexes are known that involve bidentate
di-N-heterocyclic carbene (di-NHC) ligands, in which pairs of
NHC groups are joined by an appropriate linker group [5–7].
Most of the complexes containing di-NHC’s employ these groups
as chelating ligands at a single metal. Investigations of binuclear
di-NHC complexes have been limited so far, although some have
been reported in which the di-NHC group bridges pairs of metals
[8]. On the basis of an extensive chemistry of binuclear, late
metal complexes with diphosphine ligand [9], the design and
synthesis of binuclear complexes with di-NHC are of considerable
interest.
2.1. Synthesis of the bisimidazolium chloride salts
1-Mesitylimidazole was synthesized according to the method
reported by Liu et al. (Scheme 2) with a little modification [10].
The product was purified by sublimation instead of column chro-
matography, giving 65% yield. The synthesis of bisimidazolium
salts from imidazoles is accomplished by a substitution reaction
of alkyl dihalide with 2 equiv. of imidazoles [11,12]. Usually, the
reactions worked well with bromides and iodides, and it generally
required high temperature and longer reaction time with chlorides.
In addition, hazard organic solvent, such as DME [13], PEG [14] or
THF [15] was usually used. We developed a green and easy method
to synthesize bisimidazolium chloride salts. As the 1-mesitylimi-
dazole was melted at about 110 °C, the bisimidazolium dichlorides
could be prepared in neat condition. As the high concentration of
the reaction mixture due to the lack of solvent, the reaction is com-
pleted in a shorter time and lower temperature in high yield. The
completion of the reaction is easy to monitor by initial colorless
solution turning to final white solid. This method is also suitable
for synthesis of other bisimidazolium salts [16].
In this paper we describe the synthesis of a series of bi-palla-
dium di-NHC complexes with different length, diagrammed in
Scheme 1, and tested them in the C–C coupling reaction of Heck
reaction.
2.2. Synthesis and characterization of the bi-palladium di-NHC
complexes
The synthesis of bi-palladium di-NHC complexes 4, 5 and 6 are
achieved by refluxing of the corresponding imidazolium salt, with
* Corresponding author. Fax: +86 516 83500349.
E-mail address: yhshi_2000@126.com (Y. Shi).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.07.054

C. Cao et al. / Inorganica Chimica Acta 363 (2010) 3914–3918
3915
the 1H NMR of 5 having one carbon shorter linkage between two
palladium compared to 6 showed two distinctive signals for the
four CH3 (2.29 ppm and 2.25 ppm), and the 1H NMR of 4 with
two carbon shorter linkage showed multiple peaks from 2.30 to
2.25 ppm.
N
N
N
N
n
Cl Pd Cl
N
Cl Pd Cl
N
The molecular structures of 4 and 6 were determined by means
of X-ray diffraction studies. The molecular diagrams of 4 and 6 are
shown in Figs. 1 and 2 and selected crystallographic data are
shown in Table 1. The structures of complex 4 consist of two pseu-
do-square-planar subunits in a trans configuration bridged with
ethylene. The structures of complex 6 consist of two pseudo-
square-planar subunits in a cis configuration bridged with butyl-
n = 2-4
Scheme 1. Bi-palladium di-NHC chloride complexes with different alkyl bridge
lengths.
ene, and there is probably a face-to-face
p–p stacking between
palladium chloride in presence of K2CO3 in pyridine, as shown in
Scheme 3. These complexes were fully characterized by NMR spec-
troscopy and gave satisfactory elemental analyses. The complexes
are air and moisture stable and can be stored at air atmosphere in
solid state for more than 6 months without any noticeable
decomposition.
two pyridine rings. The distance of the centers of the two pyridines
is 3.913 Å and the shortest atom distance on the two pyridines is
3.525 Å. Two complexes show slightly distorted square-planar
geometries around two palladium centers, which are surrounded
by imidazolylidene, two chloro ligands in a trans configuration,
and
a pyridine. The Pd–Ccarbene distance is 1.951(18) Å and
The formation of the metal complexes was evident from the dis-
tinctive metal-bound carbene peak, which appeared at 152.5 and
151.3 ppm for complex 4, 152.5 and 151.2 ppm for 5, and 153.3
and 150.1 ppm for 6. It is noteworthy that the 1H NMR patterns
of four methyl groups from 2, 6-dimethyl on mesityl group of the
imidazolylidene ligand in complexes 4, 5 and 6 are very different.
The 1H NMR of 6 which has longer linker between two Pd centers
showed only one signal (2.25 ppm) for the four methyl groups on
the phenyl group meaning the molecule has a binary symmetry
and methyl groups and phenyl rings could rotate rapidly within
NMR time scale in the solution at room temperature, so there is al-
most no steric hindrance around Pd center in complex 6. However,
1.988(17) Å for 4 and 1.965(5) Å for 6 (Table 2), similar to that
shown by other palladium-related species [17,18]. The Pd–Npyridine
distance in complex 6 [2.094(4) Å], 4 [2.070(9) Å, 2.075(14) Å] was
comparable to that of its related saturated mono-palladium car-
bene analogues [17]. All other distances and angles lie in the ex-
pected range (Table 2).
3. Heck reaction
The palladium-catalyzed arylation of olefins has found wide
application in organic synthesis. The activity of complexes 4–6 in
MeOH, rt
18 h
NH2
N
H3PO4, HCHO
O
O
O
+
N
N
NH4Cl, reflux
Scheme 2. Synthesis of 1-mesitylimidazole.
N
N
N
N
N
N
N
N
Cl Pd Cl
N
Cl Pd Cl
N
l
2C
(4)
N
N
N
N
N
N
N
N
K2CO3
l
2C
Cl Pd Cl
N
Cl Pd Cl
N
PdCl2
N
, 85 °C
(5)
(6)
N
N
N
N
N
N
N
N
2Cl
Cl Pd Cl
N
Cl Pd Cl
N
Scheme 3. Synthesis of the bi-palladium di-NHC complexes 4, 5 and 6.

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C. Cao et al. / Inorganica Chimica Acta 363 (2010) 3914–3918
Fig. 1. ORTEP structure of complex 4 with the probability ellipsoids drawn at the 30% level. Hydrogen atoms have been omitted for clarity.
Fig. 2. ORTEP structure of complex 6 with the probability ellipsoids drawn at the 30% level. Hydrogen atoms have been omitted for clarity.
the catalytic arylation of styrene was tested in order to elucidate
the influence of the ligands (Scheme 4). The reactions were con-
ducted in a vial in the presence of catalytic amounts of palladium
catalysts 4–6 and K2CO3 in a mixture of bromobenzene and sty-
rene. The reaction is heated at 110 °C for 18 h. Details of the exper-
imental setup are given in Section 5. The results are given in Table
3. From the results we can see the complexes of 4–6 have been
shown good catalysts for the arylation of olefins. The complex 4
and 6 gave higher yield, while 5 gave better regioselectivity. We
are currently optimizing the conditions to further improve the cat-
alytic results
plexes were successfully tested in the catalytic Heck reaction of
styrene with bromobenzene. The complexes of 4–6 have been
shown good catalysts for the arylation of olefins.
5. Experimental
5.1. General procedures
Pyridine was distilled from calcium hydride under argon atmo-
sphere. 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 re-
corded on a Bruker AV 400 MHz spectrometer at room temperature
and referencedto theresidualsignals of thesolvent. GC–MSwas per-
formed on an Agilent 6890-5973N system with electron ionization
4. Conclusion
We have synthesized a serious of bi-palladium dicarbene com-
plexes with different length of linker. The new binuclear com-

C. Cao et al. / Inorganica Chimica Acta 363 (2010) 3914–3918
3917
Table 3
Table 1
Results of the Mizoroki–Heck reaction.
Selected crystallographic data for complexes 4 and 6.
Entry
Catalyst
Catalyst
(mmol%)
Conversion
(%)
Isolated
yield (%)
A:B ratio
Complex
4
6
Formula
Formula weight
T (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
C
36H40Cl4N6Pd2
C38H44Cl4N6Pd2
939.39
293(2)
monoclinic
C2/c
1
2
3
4
5
6
0.5
0.5
0.5
98
92
95
77
66
75
8.6:1
10:1
5.2:1
911.4
293(2)
monoclinic
P2(1)/c
of Ace pressure tube. After the completion of the reaction, the solid
was dissolved in DCM (2.5 mL) and precipitated out with diethyl
ether (5 mL) to give product as a white powder. Yield: 74%
(0.5961 g). 1H NMR (CDCl3, 400 MHz): d 10.29 (s, 2H, NCHN),
9.14 (s, 2H, NCH), 7.11 (s, 2H, NCH), 7.02 (s, 4H, Ar-H), 5.57 (s,
4H, NCH2), 2.36 (s, 6H,CH3), 2.05 (s, 12H, CH3).
8.7156(17)
23.869(5)
24.434(5)
90.00
101.43(3)
90.00
21.134(4)
22.016(4)
14.758(3)
90.00
133.31(3)
90.00
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
4982.3(17)
4
4996.2(27)
4
Z
Crystal size
F(0 0 0)
h range for data collection (°)
Goodness-of-fit on F2
0.30 ꢀ 0.22 ꢀ 0.2
0.55 ꢀ 0.05 ꢀ 0.4
5.3.2. Synthesis of [H2L2]Cl2
1796
1896
1,10-Dimesityl-3,30-(1,3-trimethylene)bisimidazolium
dichloride
1.2–25
1.001
3.08–25.35
1.024
(2). 1-Mesitylimidazole (27 mmol, 5.04 g) and 13.5 mmol of 1,3-
dichloropropane (1.53 g) were heated to 110 °C for 5 h in a 10 mL
of Ace pressure tube. The crude was dissolved in DCM (10 mL)
and precipitated out with ether (50 mL) to give product as a white
powder. Yield: 81% (5.3 g). 1H NMR (CDCl3, 400 MHz): d 10.15 (s,
2H, NCHN), 8.67 (s, 2H, NCH), 7.08 (s, 2H, NCH), 7.00 (s, 4H, Ar-
H), 4.96 (t, 4H, CH2), 3.18 (m, 2H, CH2), 2.35 (s, 6H,CH3), 1.92 (s,
12H, CH3).
Final R indices [I > 2
r
(I)]
R1 = 0.1242
wR2 = 0.3651
R1 = 0.2102
wR2 = 0.4225
R1 = 0.0494
wR2 = 0.1300
R1 = 0.0601
wR2 = 0.1383
R indices (all data)
Table 2
Selected bond lengths and angles for complexes 4 and 6.
Distance
(Å)
4
6
Angle (°)
4
6
5.3.3. Synthesis of [H2L3]Cl2
1,10-Dimesityl-3,30-(1,4-tetramethylene)bisimidazolium dichlo-
ride (3). 1-Mesitylimidazole (4 mmol, 0.745 g) and 2 mmol of 1,4-
dichlorobutane (0.254 g) were heated to 110 °C for 4 h in a 10 mL
of Ace pressure tube. The crude solid was dissolved in DCM
(2.5 mL) and precipitated out with ether (5 mL) to give product
C1–Pd1
N1–Pd1
Pd1–Cl1
Pd1–Cl2
C4–Pd2
N2–Pd2
Pd2–Cl3
Pd2–Cl4
1.951(18)
2.070(9)
2.410(3)
2.384(4)
1.988(17)
2.075(14)
2.400(3)
2.371(4)
1.965(5)
2.094(4)
2.2915(14)
2.3078(14)
C1–Pd1–N1
C1–Pd1–Cl1
C1–Pd1–Cl2
N1–Pd1–Cl1
N1–Pd1–Cl2
C4–Pd2-N2
C4–Pd2–Cl3
C4–Pd2–Cl4
N2–Pd2–Cl3
N2–Pd2–Cl4
177.6(6)
91.5(5)
89.1(5)
90.9(3)
88.5(3)
177.1(6)
91.7(5)
89.6(5)
90.8(4)
87.9(4)
179.61(17)
89.32(13)
89.89(13)
90.96(12)
89.84(12)
as
a
white powder. Yield: 80% (0.7992 g). 1H NMR (CDCl3,
400 MHz): d 10.14 (s, 2H, NCHN), 8.62 (s, 2H, NCH), 7.08 (s, 2H,
NCH), 6.99 (s, 4H, Ar-H), 4.96 (m, 4H, NCH2), 2.79 (m, 4H,CH2),
2.34 (s, 6H, CH3), 2.06 (s, 12H, CH3).
(EI) mass spectrometry. Elemental analyses were performed on a
EuroVektor Euro EA-300 elemental analyzer.
5.4. Synthesis of the palladium complexes
5.4.1. Synthesis of Pd2L1Cl4(C5H5N)2 (4)
To a mixture of 1,10-dimesityl-3,30-(1,2-dimethylene)bisimi-
dazolium dichloride (1) (1.414 g, 3 mmol), PdCl2 (1.069 g,
6.0 mmol), and K2CO3 (8.293 g, 60 mmol) in a 50 mL round-bot-
tomed flask with a stir bar was added 35.5 mL of pyridine. The
reaction mixture was heated at 85 °C for 18 h, after which time
the mixture was filtered through Celite and washed with DCM.
The solvent was removed under vacuum, and the crude was
washed by diethyl ether (15 mL). The pure compound 4 was ob-
tained as yellow solid by recrystallization with DCM/ether in 83%
yield (2.27 g).
5.2. Solid state structure determination of 4 and 6
The crystals for the solid state structures of the complexes 4 and
6 could be obtained by slowly evaporating a saturated solution of
DCM. Preliminary examination and data collection were carried
out on a Rigaku Mercury CCD device at the window of a sealed
X-ray tube with graphite-monochromated Mo
Ka radiation.
Absorption correction was performed by SADABS program. All the
structures were solved by directed methods using the SHELXS-97
program and refined by full-matrix least squares techniques on F2.
1H NMR (CDCl3, 400 MHz): d 8.81 (m, 4H, Py-H), 7.70 (m, 2H,
NCH), 7.57 (m, 2H, Py-H), 7.28 (m, 4H, Py-H), 7.03 (s, 4H, Ar-H),
6.72 (m, 2H, NCH), 5.80 (m, 4H, NCH2), 2.37 (s, 6H, CH3), 2.30–
2.25 (m, 12H, CH3). 13C NMR (CDCl3, 100 MHz): d 152.5 (NCN–
Pd), 152.0, 151.3 (NCN–Pd), 139.2, 137.9, 137.8, 137.7, 136.1,
136.0, 135.9, 135.8, 134.8, 129.3, 124.3, 123.7, 123.4, 123.1, 51.3,
5.3. Synthesis of imidazolium salts under neat condition
5.3.1. Synthesis of [H2L1]Cl2
1,10-Dimesityl-3,30-(1,2-dimethylene)bisimidazolium
dichloride
(1). 1-Mesitylimidazole (4 mmol, 0.745 g) and 2 mmol of 1,2-
dichloroethane (0.199 g) were heated to 110 °C for 6 h in a 10 mL
50.8, 31.6, 22.6, 21.1, 14.1. Anal. Calc. for
C36H40Cl4N6Pd2
Br
cat. [Pd], K2CO3
+
+
110 °C, 18 h, tol.
(A)
(B)
Scheme 4. Condition of the Mizoroki–Heck reaction.

3918
C. Cao et al. / Inorganica Chimica Acta 363 (2010) 3914–3918
(911.40 g/mol): C, 47.44; H, 4.42; N, 9.22. Found: C, 47.82; H, 4.13;
N, 9.55%.
(103.7 mg, 0.75 mmol) in the glove box and sealed with a cap con-
taining PTFE septum. Toluene (1 mL) and styrene (70 L,
a
l
0.60 mmol) were injected sequentially. The mixture was stirred
at 110 °C for 18 h. The crude was purified by column chromatogra-
phy on silica gel with petroleum ether.
5.4.2. Synthesis of Pd2L2Cl4(C5H5N)2 (5)
To a mixture of 1,10-dimesityl-3,30-(1,3-trimethylene) bisimi-
dazolium dichloride (2) (1.457 g, 3.0 mmol), PdCl2 (1.069 g,
6.0 mmol), and K2CO3 (8.293 g, 60 mmol) in a 50 mL round-bot-
tomed flask was added 10.5 mL of pyridine. The reaction mixture
was heated at 85 °C for 18 h, after which time the mixture was fil-
tered through Celite and washed with DCM. The solvent was re-
moved under vacuum, and the crude was washed by diethyl
ether (15 mL). The pure compound 5 was obtained as yellow solid
by recrystallization with DCM/ether in 77% yield (2.142 g).
1H NMR (CDCl3, 400 MHz): d 8.81 (m, 4H, Py-H), 7.70 (m, 2H,
NCH), 7.54 (m, 2H, Py-H), 7.32–7.23 (m, 4H, Py-H), 7.02 (s, 4H,
Ar-H), 6.81 (m, 2H, NCH), 4.95 (m, 4H, NCH2), 3.40 (m, 2H, CH2),
2.36 (s, 6H, CH3), 2.29 (s, 6H, CH3), 2.25 (s, 6H, CH3). 13C NMR
(CDCl3, 100 MHz): d 152.5 (Pd–NCN), 151.9, 151.2 (Pd–NCN),
139.1, 137.8, 137.7, 137.6, 136.3, 136.1, 136.0, 134.7, 129.2,
124.5, 124.2, 124.0, 123.0, 122.9, 49.7, 49.5, 49.3, 30.5, 30.4, 30.3,
Acknowledgements
We are grateful to the foundation of Xuzhou Normal University
(KY2008103, KY2008104, 09XLY02 and 2010YLB030), Qing Lan
Project (08QLT001 and 08QLD006) of Jiangsu Education Commit-
tee, State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry (2009-06) at Jilin University and NSFC 20801047 for
financial support.
Appendix A. Supplementary material
CCDC 772515 and 784315 contain the supplementary crystallo-
graphic data for this paper. 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 associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2010.07.054.
21.08, 19.9, 19.6, 19.1, 18.9. Anal. Calc. for
(925.42 g/mol): C, 48.02; H, 4.57; N, 9.08. Found: C, 48.62; H,
4.25; N, 9.31%.
C37H42Cl4N6Pd2
5.4.3. Synthesis of Pd2L3Cl4(C5H5N)2 (6)
References
To a mixture of 1,10-dimesityl-3,30-(1,4-tetramethylene)bisimi-
dazolium dichloride (3) (1.499 g, 3 mmol), PdCl2 (1.069 g,
6.0 mmol), and K2CO3 (8.293 g, 60 mmol) in a 50 mL round-bot-
tomed flask was added 35.5 mL of pyridine. The reaction mixture
was heated at 85 °C for 18 h, after which time the mixture was fil-
tered through Celite and washed with DCM. The solvent was re-
moved under vacuum, and the crude was washed by diethyl
ether (15 mL). The pure compound 6 was obtained as yellow solid
by recrystallization with DCM/ether in 64% yield (1.807 g).
1H NMR (CDCl3, 400 MHz): d 8.80 (s, 4H, Py-H), 7.66 (m, 2H, Py-
H), 7.28 (d, J = 2 Hz, 2H, NCH), 7.24 (m, 4H, Py-H), 7.02 (s, 4H, Ar-H),
6.85 (d, J = 2 Hz, 2H, NCH), 4.92 (m, 4H, NCH2), 2.47 (m, 4H, CH2),
2.36 (s, 6H, CH3), 2.25 (s, 12H, CH3). 13C NMR (CDCl3, 100 MHz):
d 153.3 (Pd–NCN), 151.4, 150.1 (Pd–NCN), 139.1, 137.7, 136.3,
134.9, 129.2, 124.2, 124.1, 122.1, 53.4, 50.7, 27.3, 21.2, 19.0. Anal.
Calc. for C38H44Cl4N6Pd2 (939.45 g/mol): C, 48.58; H, 4.72; N,
8.95. Found: C, 48.93; H, 4.36; N, 8.47%.
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5.5. Heck reaction of styrene
An oven-dried 4 mL vials containing a stirrer bar was charged
with bromobenzene (0.5 mmol), catalyst (0.5 mol%) and K2CO3