Synthesis and characterization of dinuclear NHC-palladium complexes and their applications in the Hiyama reactions of aryltrialkyoxysilanes with aryl chlorides

Article abstract of DOI:10.1039/c2dt31174g

Four linear dinuclear N-heterocyclic carbene (NHC)-palladium complexes {[PdCl₂(NHC)]₂(μ-L)·xCH₂Cl ₂} (1-4, L = pyrazine, DABCO) were synthesized through one-pot reactions of imidazolium salts, PdCl₂ and various bidentate N-heterocycles under mild conditions. The compounds were fully characterized by NMR, FT-IR and elemental analysis. Among them, complexes [PdCl₂L ^Mes]₂(μ-pyrazine)·CH₂Cl₂ (1), [PdCl₂L^iPr]₂(μ-pyrazine) (2), and [PdCl₂L^Mes]₂(μ-DABCO) (3), were elucidated by single-crystal X-ray crystallography. Moreover, the catalytic activity of the NHC-palladium complexes was examined in the Hiyama reactions and the results showed that the dinuclear palladium complexes were the effective catalyst precursors for the reactions of aryltrialkyoxysilanes with aryl chlorides.

Full text of DOI:10.1039/c2dt31174g

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PAPER
Synthesis and characterization of dinuclear NHC–palladium complexes and
their applications in the Hiyama reactions of aryltrialkyoxysilanes with aryl
chlorides†
Jin Yanga and Lei Wang*a,b
Received 31st May 2012, Accepted 26th July 2012
DOI: 10.1039/c2dt31174g
Four linear dinuclear N-heterocyclic carbene (NHC)–palladium complexes {[PdCl2(NHC)]2(μ-L)·
xCH2Cl2} (1–4, L = pyrazine, DABCO) were synthesized through one-pot reactions of imidazolium salts,
PdCl2 and various bidentate N-heterocycles under mild conditions. The compounds were fully
characterized by NMR, FT-IR and elemental analysis. Among them, complexes
[PdCl2LMes]2(μ-pyrazine)·CH2Cl2 (1), [PdCl2LiPr]2(μ-pyrazine) (2), and [PdCl2LMes]2(μ-DABCO) (3),
were elucidated by single-crystal X-ray crystallography. Moreover, the catalytic activity of the NHC–
palladium complexes was examined in the Hiyama reactions and the results showed that the dinuclear
palladium complexes were the effective catalyst precursors for the reactions of aryltrialkyoxysilanes with
aryl chlorides.
comparable to the conventional phosphine ligands capable of
Introduction
coordination with a wide range of transition metals.5 Recently, it
Transition-metal-catalyzed coupling reactions have contributed
greatly to the straightforward construction of carbon–carbon
bonds in recent years. Significant progress in this area has been
achieved with a variety of palladium catalysts.1 Among them,
the palladium-catalyzed Hiyama cross-coupling reactions of aryl-
trialkyoxysilanes with aryl halides have attracted much attention
in organic synthesis.2 Over the last few decades, considerable
efforts have been made to develop more active catalysts for the
Hiyama coupling reaction.3 Generally, aryl iodides and bromides
have been employed as substrate partners in the presence of
palladium precursors and air-sensitive phosphine ligands.
Despite the lower reactivity of aryl chlorides compared with the
corresponding aryl iodides and bromides, aryl chlorides are the
most desirable substrates because of their lower cost and ready
availability. Therefore, the development of high-performance
catalysts for Hiyama cross-couplings with aryl chlorides is still
relatively unexplored.4
has been shown that N-heterocyclic carbene–palladium com-
plexes offer distinctive advantages as possible alternatives to
Pd–phosphine systems in C–C cross-coupling reactions.6 Some
highly active palladium systems with carbene ligands for the
activation of aryl chlorides have been developed.7 Recent studies
have illustrated that the provision of functional ligands to comp-
lement the strongly binding NHCs could promote a reversible
coordination and dissociation mechanism which is highly desir-
able for catalytic applications.8 Among the known NHC–Pd
complexes, most are tetracoordinate containing an ancillary
ligand, such as pyridine, pyrazole, or imidazole, and adopt a
square-planar geometry for stability.9 Despite the great interest in
N-heterocyclic carbene–palladium complexes as catalysts, very
little research has been reported on dinuclear or multinuclear
carbene complexes bearing bidentate or multidentate bridging
ligands.10 Recently, the design of regularly shaped molecules
has aroused more and more interest as such a process can yield
self-assembled architectures not only with regular, well-defined
inner structures but also with tailored functionalities.11 An
emerging trend in coordination chemistry is to construct functio-
nalized complexes that can enter into an array of di- and multi-
nuclear systems which are versatile and generally constructed
from simple mononuclear sources with bridging ligands. Both
Stang and co-workers,12 and Fujita and Ogura13 have elegantly
demonstrated the structural beauty arising from the application
of diverse functional building blocks. Known as common brid-
ging ligands, N-heterocycles such as 4,4′-bipyridine, pyrazine,
and DABCO have been the most widely used linkers in this
field. Herein, we reported the synthesis and structural character-
ization of four linear dinuclear N-heterocyclic carbene
Since the successful isolation of the first stable carbene by
Arguengo in 1991, N-heterocyclic carbenes (NHCs) have been
extensively studied in the field of organometallics as ligands
aDepartment of Chemistry, Huaibei Normal University, Huaibei,
Anhui 235000, P R China. E-mail: leiwang@chnu.edu.cn;
Fax: +86-561-309-0518; Tel: +86-561-380-2069
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032,
P R China
†Electronic supplementary information (ESI) available: 1H and 13C
NMR spectra of the complexes 1–4 and all products. CCDC
885564–885566. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt31174g
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Scheme 1 Overview of the synthesis of the dinuclear N-heterocyclic
carbene (NHC)–palladium complexes (1–4): imidazolium salts
(1.0 mmol), PdCl2 (1.0 mmol), K2CO3 (1.1 mmol) and bidentate
N-heterocycles (0.6 mmol), THF, reflux, 6 h.
(NHC)–palladium complexes. The catalytic behavior of the com-
plexes in Hiyama reactions of a range of aryl chlorides was also
studied.
Results and discussion
Synthesis of NHC–palladium complexes
The synthesis of the NHC–palladium complexes 1–4 was
achieved through the procedure shown in Scheme 1. The reac-
tions of imidazolium salts, PdCl2 and bidentate N-heterocycles
(DABCO, and pyrazine) in the presence of K2CO3 afforded the
corresponding NHC–palladium complexes in moderate to good
yields. The compounds were fully characterized by NMR, FT-IR
and elemental analysis. The elemental analysis results are con-
sistent with the formation of dinuclear species of the general
formula [PdCl2(NHC)]2(μ-L) (L = DABCO or pyrazine), and
there is no evidence of the occurrence of other stoichiometries.
The 1H-NMR spectrum of compounds 1–4 in CDCl3 shows
complete disappearance of the proton signal of NCHN from the
imidazolium salts which appears at 10.73 ppm for LMes·HCl and
9.97 ppm for LiPr·HCl. In addition, the formation of the dinuc-
lear complexes was evident from the distinctive stoichiometric
proton signal resonances of the bidentate N-heterocycles
(Fig. 1). These spectra strongly support the formation of the
dinuclear N-heterocyclic carbene–palladium complexes. Further-
more, the 13C-NMR spectra revealed the appearance of a diag-
nostic Pd–C(carbene) peak at 149.9 ppm for 1, 152.4 ppm for 2,
150.6 ppm for 3 and 153.1 ppm for 4, respectively. These values
are significantly shifted downfield relative to that of the imidazo-
linium NCHN peak of the starting ligand precursor (141.0 ppm
for LMes·HCl, 145.0 ppm for LiPr·HCl).
Fig. 1 1H NMR spectra of LMes·HCl, LiPr·HCl and compounds 1–4 in
CDCl3 at room temperature.
PdCNCl2 coordination plane of the compound 1 is oriented
approximately perpendicularly to the carbene ring plane with
dihedral angle of 76.97°, which is typical for NHC complexes to
relieve steric congestion, and is twisted from the pyrazine ring
plane by 26.43°. These values are significantly different from
the mononuclear N-heterocyclic carbene–palladium complexes
Pd-PEPPSI (66.70 and 40.48° for dichloro-(3-chloropyridine)-
(1,3-dimesitylimidazol-2-ylidene)-palladium hexane solvate).9a
However, the Pd–C(carbene) bond (1.968(3) Å), Pd–N bond
(2.151(2) Å) and Pd–Cl bonds (2.2863(9) and 2.3035(7) Å) in
compound 1 are comparable to those found in the mononuclear
N-heterocyclic carbene–palladium complexes Pd-PEPPSI
(Pd–C: 1.962(3) Å, Pd–N: 2.117(3) Å, Pd–Cl: 2.290(1) and
2.298(1) Å for dichloro-(3-chloropyridine)-(1,3-dimesitylimid-
azol-2-ylidene)-palladium hexane solvate). The major difference
in the structure of compound 1 from the mononuclear Pd-
PEPPSI complex (dichloro-(3-chloropyridine)-(1,3-dimesitylimi-
dazol-2-ylidene)-palladium hexane solvate) is that the dihedral
angle between the carbene ring plane and the pyrazine ring plane
(76.69°) is significantly bigger than that found in Pd-PEPPSI
complex (27.57°).9a
The structures of compounds 1–3 were further investigated by
single-crystal X-ray diffraction, and a summary of the crystallo-
graphic data is provided in Table 1. The crystal structures of 1–3
showed a dinuclear framework with bidentate N-heterocycle
ligands bridging across two square planar Pd(II) units. Com-
pounds 1 and 2 have similar compositions and structures, in
which each palladium center is surrounded by an imidazolyli-
dene, a pyrazine, and two chloro ligands in an almost square-
planar fashion. Compound 1 crystallizes in the trigonal space
The iPr-substituted analogue 2 crystallizes in the monoclinic
space group P2(1)/c and the Pd–C(carbene) and Pd–N bonds are
1.959(7) and 2.116(6) Å, respectively, which are similar to those
observed for compound 1 and other related mononuclear
carbene–palladium Pd-PEPPSI analogues.9a As shown in Fig. 3,
the dihedral angles between the carbene ring plane and the
PdCNCl2 coordination plane, and between the pyrazine ring
ˉ
group R3 with a trans-configuration. As shown in Fig. 2, the
12032 | Dalton Trans., 2012, 41, 12031–12037
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Table 1 Crystallographic data for compounds 1–3
1
2
3
Formula
Fw
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
C46H52Cl4N6Pd2
1043.54
C58H76Cl4N6Pd2
1211.85
Monoclinic
P21/c
11.785(5)
19.420(8)
14.692(6)
90.00
C48H60Cl4N6Pd2
1075.62
Tetragonal
P42/n
19.2748(5)
19.2748(5)
13.3673(6)
90.00
Trigonal
ˉ
R3
21.2224(4)
21.2224(4)
28.0170(7)
90.00
β/°
90.00
107.196(4)
90.00
3212(2)
2
90.00
90.00
4966.2(3)
4
1.439
γ /°
120.00
10 928.0(4)
9
V/Å3
Z
D
calc/g cm−3
1.427
1.253
F(000)
4770
0.997
1252
0.764
2200
0.603
μ/mm−1
GOF
1.086
1.035
19 415
5631 (0.1032)
3051
324
0.0588
0.1858
0.979
9723
4364 (0.0390)
2915
272
0.0504
0.1110
Reflections collected
Independent reflections (Rint
Observed reflections [I > 2σ(I)]
Refined parameters
R1 [I > 2σ(I)]
23 441
4284 (0.0307)
3674
268
0.0286
0.0795
)
wR2 (all data)
imidazol-2-ylidene)-dichloro-(3-chloropyridine)-palladium dichloro-
menthane solvate).9a In addition, compared to the mononuclear
N-heterocyclic carbene–palladium complex (1,3-bis(2,6-diiso-
propylphenyl)imidazol-2-ylidene)-dichloro-(3-chloropyridine)-
palladium dichloromenthane solvate) a dramatically decreased
dihedral angle between the carbene ring plane and the pyrazine
ring plane (from 69.16 to 41.15°) was observed.
The crystal structure of compound 3 confirmed a dinuclear
complex bridged by DABCO (Fig. 4). Compound 3 crystallizes
in the tetragonal space group P42/n and also show a square-
planar geometries around palladium center (largest deviation
0.080 Å), which are surrounded by an imidazolylidene, a
DABCO, and two chloro anions in a trans-configuration. The
Pd–Ccarbene and Pd–N distance are 1.970(5) and 2.184(3) Å,
respectively, similar to that observed in compounds 1 and 2 and
other related carbene–palladium analogues.9a The dihedral angle
between the PdCNCl2 coordination plane and the carbene ring is
76.97°, which is in the range of the compounds 1 and 2.
Catalytic studies
The palladium-catalyzed Hiyama coupling reaction is a powerful
method for the preparation of biaryl derivatives. Many efforts
have been directed toward the development of efficient catalytic
systems for the Hiyama reaction. Herein, compounds 1–4 were
used as catalysts for Hiyama coupling reactions of aryltrialkyoxy-
silanes with aryl chlorides. The Pd-PEPPSI (1,3-bis(2,6-diiso-
propylphenyl)imidazol-2-ylidene)-dichloro-(3-chloropyridine)-
palladium)9b and palladium–NHC complexes, generated in situ
from the corresponding imidazolinium salts and palladium
acetate14 were also examined as a comparison. The coupling
reaction of phenyltrimethoxysilane with 4-chloroanisole was
tested as the model reaction in toluene at 120 °C and the results
are shown in Table 2. Upon treatment with TBAF, which is a
well-known promoter of the Hiyama reaction, all the dinuclear
Fig. 2 ORTEP diagram of 1 with thermal displacement parameters
drawn at 30% probability. Hydrogen atoms have been omitted for clarity.
The suffix A in 1 denotes symmetry operation 1.66667 − x, 1.33333 − y,
0.33333 − z. Selected bond lengths (Å) and angles (°) in 1: Pd1–C1
1.968(3), Pd1–N3 2.151(2), Pd1–Cl1 2.2863(9), Pd1–Cl2 2.3035(7);
C1–Pd1–Cl1 89.19(8), N3–Pd1–Cl1 90.02(6), C1–Pd1–Cl2 90.31(8),
N3–Pd1–Cl2 90.55(6).
plane and the PdCNCl2 coordination plane are 70.99 and 30.63°,
respectively. These values are somewhat shorter than that in the
mononuclear N-heterocyclic carbene–palladium complexes Pd-
PEPPSI (79.07 and 32.61° for (1,3-bis(2,6-diisopropylphenyl)-
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Fig. 4 ORTEP diagram of 3 with thermal displacement parameters
drawn at 30% probability. Hydrogen atoms have been omitted for clarity.
The suffix A in 3 denotes symmetry operation 1 − x, 1 − y, −z. Selected
bond lengths (Å) and angles (°) in 3: Pd1–C1 1.970(5), Pd1–N3
2.184(3), Pd1–Cl1 2.2842(17), Pd1–Cl2 2.2954(19); C1–Pd1–Cl1
89.14(16), N3–Pd1–Cl1 92.88(12), C1–Pd1–Cl2 86.90(16), N3–Pd1–
Cl2 91.05(12).
Table 2 Hiyama reactions of phenyltrimethoxysilane with 4-
chloroanisole in the presence of the NHC–palladium catalystsa
Fig. 3 ORTEP diagram of 2 with thermal displacement parameters
drawn at 30% probability. Hydrogen atoms have been omitted for clarity.
The suffix A in 2 denotes symmetry operation 1 − x, −y, 1 − z. Selected
bond lengths (Å) and angles (°) in 2: Pd1–C1 1.959(7), Pd1–N3
2.116(6), Pd1–Cl2 2.286(2), Pd1–Cl1 2.292(2); C1–Pd1–Cl2 88.5(2),
N3–Pd1–Cl2 90.63(17), C1–Pd1–Cl1 90.6(2), N3–Pd1–Cl1 90.29(17).
Entry
Catalyst
Yieldb (%)
1
2
3
4
5
6
7
1
2
3
4
80
88
78
84
86
43
51
Pd-PEPPSI
Pd(OAc)2 + LMes·HCl
Pd(OAc)2 + LiPr·HCl
N-heterocyclic carbene (NHC)–palladium complexes showed
high catalytic activities for the Hiyama reaction. The results
show that the sterically bulkier catalysts 2 and 4 exhibited higher
activity. This is mainly due to the different steric topography
imparted by the NHC ligand around the palladium centre, which
has an impact on the rate of transmetalation and/or reductive
elimination. The sterically bulky ligands facilitate reductive elim-
ination (or transmetalation) of the catalytic process, which results
in the sterically bulkier catalysts displaying a higher conversion
to product.9a–c,15 Some multinuclear complexes exhibit coopera-
tive reactivity between multiple metal centers and are far more
catalytically active than their mononuclear counterparts.16 Unfor-
tunately, in our system, the dinuclear palladium complexes only
demonstrated a similar level of catalytic activity compared with
Pd-PEPPSI catalyst, but showed higher activities in comparison
with the palladium–NHC complexes generated in situ. The
Hiyama reactions of a wide range of aryl chlorides with
a Reaction conditions: phenyltrimethoxysilane (0.75 mmol), 4-
chloroanisole (0.50 mmol), palladium catalyst (containing Pd
0.0025 mmol), TBAF (1.0 mmol) in toluene (2.0 mL) at 120 °C for 5 h.
b Isolated yield.
aryltrialkyoxysilanes were further studied in the presence of
compound 2 and the results are summarized in Table 3.
The results in Table 3 show that compound 2 is an effective
catalyst for the Hiyama coupling of aryl chlorides with organo-
silanes. When 0.5 mol% of 2 was employed in the Hiyama reac-
tion, both electron-rich and electron-deficient aryl chlorides
generated moderate to good yields of the corresponding biphenyl
products (Table 3, entries 1–11). It should be noted that the
reaction could tolerate ortho-substituted groups, and a small
12034 | Dalton Trans., 2012, 41, 12031–12037
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Table 3 Hiyama reactions of aryltrialkyoxysilanes with aryl chloridesa
carbene–palladium complex 1,3-bis(2,6-diisopropylphenyl)imi-
dazol-2-ylidene)-dichloro-(3-chloropyridine)-palladium dichloro-
menthane solvate was prepared according to the literature.9a
Elemental analyses were performed on an Elementar VarioEL
instrument. IR spectra were recorded on a Bruker IFS 120HR
spectrometer as KBr disks. NMR spectra were recorded at
ambient temperature on a Bruker Avance-400 MHz spectrometer
for solution in CDCl3 with tetramethylsilane (TMS) as an
internal standard. Commercially obtained reagents were used
without further purification. Flash column chromatography was
carried out using 300–400 mesh silica gel.
Entry
Aryltrialkyoxysilane
Aryl chloride
Yieldb (%)
1
2
3
4
5
6
7
8
R = CH3
R = CH3
R = CH3
R = CH3
R = CH3
R = CH3
R = CH3
R = CH3
R = CH3
R = CH3
R = CH3
R = CH3
R = CH3
R = CH2CH3
R = CH2CH3
R = CH2CH3
R1 = 4-OCH3
R1 = 3-OCH3
R1 = 2-OCH3
R1 = 4-NO2
R1 = 3-OCH3
R1 = 4-CN
88
80
70
92
89
90
88
82
85
61c
54c
80
62
86
89
87
R1 = 4-COCH3
R1 = 4-COOC2H5
R1 = 4-F
9
Synthesis
10
11
12
13
14
15
16
R1 = 4-H
R1 = 4-CH3
[PdCl2LMes]2(μ-pyrazine)·CH2Cl2 (1). A mixture of 1,3-bis-
(2,4,6-trimethylphenyl)imidazolium chloride (1.0 mmol), PdCl2
(1.0 mmol), K2CO3 (1.1 mmol), and pyrazine (0.60 mmol) was
stirred in anhydrous THF (5.0 mL) under reflux for 6 h. The
solvent was removed under reduced pressure, and the residue
was purified by a flash chromatography on silica gel (hexane/
CH2Cl2 = 1 : 1) to give compound 1 as a yellow solid (0.88 g,
84% yield). The single crystal for X-ray diffraction was obtained
by recrystallization from CH2Cl2 and diethyl ether. 1H-NMR
(400 MHz, CDCl3): δ = 8.56 (s, 4H, pyrazine-H), 7.06 (s, 4H,
Im-H), 7.02 (s, 8H, m-ArH), 2.37 (s, 12H, p-CH3), 2.29 (s, 24H,
o-CH3). 13C-NMR (100 MHz, CDCl3): δ = 149.9, 146.1, 139.3,
136.2, 134.8, 129.3, 124.4, 21.2, 19.0. IR (KBr, cm−1): 3179,
3124, 2949, 2916, 1606, 1483, 1420, 1409, 1380, 1328, 1280,
1227, 1162, 1133, 1060, 929, 861, 849, 914, 738. Anal. Calc.
for [PdCl2LMes]2(μ-pyrazine) (C46H54Cl4N6Pd2): C, 52.84;
H, 5.21; N, 8.04%. Found: C, 52.51; H, 5.30; N, 8.18%.
2-chloropyridine
3-chloropyridine
R1 = 4-OCH3
R1 = 4-CN
R1 = 4-NO2
a Reaction conditions: aryltrialkyoxysilane (0.75 mmol), aryl chloride
(0.50 mmol), NHC–palladium catalyst (2, 0.0025 mmol), TBAF
(1.0 mmol) in toluene (2.0 mL) at 120 °C for 5 h. b Isolated yields. c At
120 °C for 10 h.
ortho-position effect was observed (entry 3). The coupling reac-
tion of activated 4-chloronitrobenzene and 4-chlorobenzonitrile
proceeded smoothly to afford 92 and 90% yields, respectively
(entries 4 and 6). For the less active chlorobenzene and
4-methylchlorobenzene, the yields of the products dropped to 61
and 54% even with prolonged reaction times (entries 10 and 11).
In addition, the catalytic protocols were also extended to the
cross-coupling of heteroaryl chlorides with phenyltrimethoxy-
silane. 2-Chloropyridine and 3-chloropyridine were able to
undergo the coupling reactions and generated the corresponding
products in moderate yields (entries 12 and 13). Meanwhile,
phenyltriethoxysilane, as the organosilane substrate was emplo-
yed to react with aryl chlorides under the identical reaction con-
ditions, and good yields of the corresponding products were
obtained (entries 14–16).
[PdCl2LiPr]2(μ-pyrazine) (2). Compound 2 was synthesized
by a similar method for the preparation of 1 except that 1,3-bis-
(2,6-diisopropylphenyl)imidazolium chloride was used instead
of 1,3-bis-(2,4,6-trimethylphenyl)imidazolium chloride. Yield:
77%. The single crystal for X-ray diffraction was obtained by
recrystallization from CH2Cl2 and ethyl acetate. 1H-NMR
(400 MHz, CDCl3): δ = 8.56 (s, 4H, pyrazine-H), 7.47 (t, 4H,
J = 8.0 Hz, p-ArH), 7.30 (d, 8H, J = 8.0 Hz, m-ArH), 7.12 (s,
4H, Im-H), 3.06 (heptet, 4H, J = 6.4 Hz, CH(CH3)2), 1.39 (d,
24H, J = 6.8 Hz, CH(CH3)2), 1.09 (d, 24H, J = 6.8 Hz,
CH(CH3)2). 13C-NMR (100 MHz, CDCl3): δ = 152.4, 146.6,
146.1, 134.8, 130.3, 125.2, 124.0, 28.7, 26.3, 23.2. IR (KBr,
cm−1): 3129, 2964, 2868, 1465, 1416, 1382, 1350, 1333, 1287,
1211, 1160, 1125, 1069, 946, 819, 802, 763, 757, 707. Anal.
Calc. for [PdCl2LiPr]2(μ-pyrazine) (C58H78Cl4N6Pd2): C, 57.39;
H, 6.48; N, 6.92%. Found: C, 57.41; H, 6.27; N, 6.98%.
Conclusions
In summary, we have successfully prepared four linear dinuclear
N-heterocyclic carbene (NHC)–palladium complexes 1–4
through the reaction of imidazolium salts, PdCl2 and bidentate
N-heterocycles. The catalytic behaviour of the N-heterocyclic
carbene–palladium complexes 1–4 in Hiyama coupling reactions
was investigated. Among 1–4, NHC–Pd 2 exhibited the best
activity in the Hiyama reaction of aryl chlorides with aryltrialky-
oxysilanes. The reactions generated the corresponding products
in moderate to good yields.
[PdCl2LMes]2(μ-DABCO) (3). Compound 3 was synthesized
by a similar method for the synthesis of 1 except that DABCO
was used instead of pyrazine. Yield: 82%. The single crystal for
X-ray diffraction was obtained by recrystallization from CH2Cl2
and diethyl ether. 1H-NMR (400 MHz, CDCl3): δ = 7.01 (s, 8H,
m-ArH), 6.99 (s, 4H, Im-H), 2.76 (s, 12H, DABCO-H), 2.36 (s,
12H, p-CH3), 2.27 (s, 24H, o-CH3). 13C-NMR (100 MHz,
CDCl3): δ = 150.6, 139.0, 136.2, 135.1, 129.1, 124.1, 48.8,
21.1, 19.2. IR (KBr, cm−1): 3146, 3108, 3092, 2959, 2918,
1608, 1484, 1468, 1411, 1379, 1331, 1298, 1228, 1185, 1054,
Experimental
General
The ligands LMes·HCl and LiPr·HCl were prepared according
to the literature.17 The mononuclear N-heterocyclic
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1035, 849, 812, 706. Anal. Calc. for [PdCl2LMes]2(μ-DABCO)
(C48H62Cl4N6Pd2): C, 53.49; H, 5.80; N, 7.80%. Found: C,
53.51; H, 5.38; N, 7.67%.
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1
was used instead of pyrazine. Yield: 54%. H-NMR (400 MHz,
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CDCl3): δ = 7.47 (t, 4H, J = 7.6 Hz, p-ArH), 7.30 (d, 8H,
J = 7.6 Hz, m-ArH), 7.05 (s, 4H, Im-H), 3.04 (heptet, 4H, J =
6.8 Hz, CH(CH3)2), 2.74 (s, 12H, DABCO-H), 1.38 (d, 24H, J =
6.4 Hz, CH(CH3)2), 1.06 (d, 24H, J = 6.8 Hz, CH(CH3)2).
13C-NMR (100 MHz, CDCl3): δ = 153.1, 146.7, 135.1, 130.0,
124.9, 123.8, 49.0, 28.9, 26.2, 23.2. IR (KBr, cm−1): 3131,
3069, 2965, 2869, 1696, 1591, 1465, 1446, 1416, 1381, 1364,
1349, 1330, 1286, 1268, 1212, 1161, 1124, 1068, 1059, 946,
820, 801, 757, 746, 705. Anal. Calc. for [PdCl2LiPr]2(μ-DABCO)
(C60H86Cl4N6Pd2): C, 57.84; H, 6.96; N, 6.74%. Found: C,
57.98; H, 6.64; N, 7.02%.
General procedure for the NHC–Pd (2) catalyzed Hiyama
reactions
A sealable reaction tube equipped with a magnetic stir bar was
charged with aryl chloride (0.50 mmol), phenyltrialkyoxysilane
(0.75 mmol), TBAF (1.0 mmol), NHC–Pd complex (2, 3.0 mg,
0.5 mmol%), and anhydrous toulene (2.0 mL). The mixture was
heated in an oil bath at 120 °C and stirred for 5 h. The organic
phase was evaporated under reduced pressure and the product
was purified by column chromatography using silica gel.
X-Ray crystallography
Data collection was performed on a Bruker-AXS SMART CCD
area detector diffractometer at 293 K using ω rotation scans with
a scan width of 0.3° and Mo Kα radiation (λ = 0.71073 Å).
Multi-scan corrections were applied using SADABS.18 Structure
solutions and refinements were performed with the SHELX-97
package.19 All non-hydrogen atoms were refined anisotropically
by full-matrix least-squares on F2. The hydrogen atoms were
included in idealized geometric positions with thermal par-
ameters equivalent to 1.2 times those of carbon and nitrogen
atoms. In compound 1, the unit cell includes a disordered
solvent CH2Cl2 molecule (it can be also observed from the
1H-NMR), which could not be modeled as discrete atomic sites.
We employed PLATON/SQUEEZE to calculate the diffraction
contribution of the solvent CH2Cl2 molecule and, thereby, to
produce a set of solvent-free diffraction intensities. Crystallo-
graphic data for the compounds are summarized in Table 1.
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Financial supports from the National Natural Science Foundation
of China (Nos. 21172092, 20972057, and 21002039), the
Natural Science Foundation of Anhui (No. 090416223), and the
Key Project of Science and Technology of the Department of
Education, Anhui Province (No. ZD2010-9) are gratefully
acknowledged.
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