A novel cis-chelated Pd(II)-NHC complex for catalyzing Suzuki and Heck-type cross-coupling reactions

Article abstract of DOI:10.1016/j.tet.2005.09.010

A novel Pd(II)-NHC complex, which has a 'normal' cis-chelating, bidentate structure is fairly effective in Suzuki and Heck-type cross-coupling reaction to give the products in good to excellent yields in most cases.

Full text of DOI:10.1016/j.tet.2005.09.010

Tetrahedron 61 (2005) 11225–11229
A novel cis-chelated Pd(II)–NHC complex for catalyzing Suzuki
and Heck-type cross-coupling reactions
Qin Xu,a Wei-Liang Duan,b Zhi-Yu Lei,a Zhi-Bin Zhua and Min Shia,
*
aEast China University of Science and Technology, 130 MeiLong Lu, Shanghai 200237, China
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Lu, Shanghai 200032, China
Received 22 July 2005; revised 1 September 2005; accepted 5 September 2005
Available online 23 September 2005
Abstract—A novel Pd(II)–NHC complex, which has a ‘normal’ cis-chelating, bidentate structure is fairly effective in Suzuki and Heck-type
cross-coupling reaction to give the products in good to excellent yields in most cases.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
¨
In 1968, Ofele and Wanzlick concurrently prepared the first
described metal complexes of N-heterocyclic carbenes
(NHC).1 However, these reports received little attention
until Arduengo III synthesized stable free carbenes.2
Herrmann’s group further expanded this field by preparing
numerous NHCs and their metal complexes, and applying
these complexes in homogeneous catalysis.3 Recently,
numerous papers concerning this topic have appeared, and
complexes of NHCs have been applied as catalysts in a
broad range of reactions. Significantly, a number of Pd–
NHC complexes have emerged as effective catalysts for a
variety of coupling reaction.4,5
Scheme 1. Preparation of a novel Pd(II)–NHC complex.
either under reflux in THF or by treatment with a base such
as potassium tert-butoxide in THF affords the desired
complex 2 in 77 and 70% yields, respectively. Its crystal
structure was determined by X-ray diffraction (Fig. 1).7,8
Previously, we reported the synthesis of a novel axiall0y
chiral Rh(III)–NHC complex derived from binaphthyl-2,2 -
diamine (BINAM) and its application in the enantio-
selective hydrosilylation of methyl ketones.5 In this paper,
we wish to report the synthesis of a novel cis-chelating,
Pd(II)–NHC complex and the application in the Suzuki and
Heck-type cross-coupling reaction.6
Structural features. The single crystals of this complex
suitable for X-ray crystal structure analysis were grown
from dichloromethane/petroleum ether 1:2 solution. This is
a NHC cis-chelating, bidentate Pd(II) complex. The bite
angle of C–Pd–C is slightly more than 908 (94.48). The bond
length of C–Pd is 1.981(10)–1.982(9) A, which is slightly
contracted in comparison to those in the Herrmann’s
diiodide complexes.3e,f The bond length of the Pd–I bond
˚
2. Results and discussion
˚
trans to the carbene is 2.6617(10)–2.6658(10) A and the bite
angle of Pd–I–Pd is slightly more than 908 [94.87(4)8],
which is resulted from the different steric effects of the
terminal iodine atom and the NHC ligand.
The synthesis of the novel cis-chelated, Pd(II)–NHC
complex 2 is shown in Scheme 1. Reaction of dibenzi-
midazolium iodide (NHC precursor) 15 with Pd(OAc)2
The use of 2 as a catalyst for Suzuki cross-coupling
reactions was examined where both solvent and base effects
were carefully examined in the reaction of phenylboronic
acid with bromobenzene under ambient atmosphere. The
Keywords: Pd(II)–NHC complex; Suzuki–Miyaura cross-coupling
reaction; Heck reaction.
*
Corresponding author. Fax: C86 21 64166128;
e-mail: mshi@pub.sioc.ac.cn
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.09.010

11226
Q. Xu et al. / Tetrahedron 61 (2005) 11225–11229
with Cs2CO3 gave 3a in 98% yield after 24 h (Table 2, entry
1). Thus, DMA is the solvent of choice and Cs2CO3 is the
preferred base for this reaction. Using these optimized
reaction conditions, the Suzuki cross-coupling reactions of a
variety of aryl halides, including aryl bromides, aryl
chlorides and iodobenzene, with phenylboronic acid were
examined. The results are summarized in Table 3. As can be
seen, aryl bromides and iodobenzene afforded coupling
products 3 in 75–99% yield under ambient atmosphere
(Table 3, entries 1–6 and 9). Aryl chlorides afforded
moderate yields of the coupling products 3 (Table 3, entries
7 and 8). Under argon atmosphere, the isolated yields of 3a
and 3d from chlorobenzene and 4-chloroacetophenone with
phenylboronic acid could reach 54 and 80%, respectively
(Table 3, entries 7 and 8). This is because aryl chlorides
involved in a slower oxidative addition with Pd(0)–carbene
complex might be more reactive under argon atmosphere,
which increases the probability to have a Pd(0) complex
rather than an unreactive Pd(II) complex.
Table 3. Pd(II)–NHC complex 2 catalyzed Suzuki coupling reaction
between phenylbronic acid (1.2 mmol) and arylhalides (1.0 mmol) under
optimized conditions
Figure 1. The ORTEP draw of Pd(II)–NHC complex 2.
results are summarized in Tables 1 and 2, respectively. We
found that using Cs2CO3 as the base in N,N-dimethyl-
acetamide (DMA) or THF at 80 8C gave the coupled product
3a in 66 and 70% yield, respectively (Table 1, entries 6 and
8). Increasing the reaction temperature to 100 8C in DMA
Entry
X
R
Time (h)
Yield (%) 3a
1
2
3
4
5
6
7
8
9
Br
Br
Br
Br
Br
Br
Cl
Cl
I
o-Me
p-Me
p-COMe
p-OMe
o-Cl
3,5-Me,Me
H
p-COMe
H
24
24
24
24
24
24
24
24
24
3b, 95
3c, 75
3d, 90
3e, 86
3f, 95
Table 1. Pd(II)–NHC complex 2 catalyzed Suzuki coupling reaction
between phenylboronic acid (1.2 mmol) with bromobenzene (1.0 mmol) in
various solvents
3g, 89
3a, 40 (54)b
3d, 45 (80)b
3a, 99
a Isolated yields.
b Isolated yield under argon atmosphere.
Entry
Solvent
Time (h)
Yield (%) 3aa
Heck coupling reactions were also examined in DMA by the
reaction of bromobenzene with butyl acrylate in the
presence of various bases (Table 4, entries 1–7). We
found that KF afforded the best results for this reaction and
offered the coupling product 4a in 53% under ambient
1
2
3
4
5
6
7
8
1,4-Dioxane
DMSO
DMF
CH2ClCH2Cl
DME
DMA
24
24
24
24
24
24
24
24
40
14
36
22
65
66
40
70
CH3CN
THF
Table 4. Pd(II)–NHC complex 2 catalyzed Heck coupling of bromoben-
zene with n-butyl acrylate under ambient or argon atmosphere
a Isolated yields.
Table 2. Pd(II)–NHC complex 2 catalyzed Suzuki coupling reaction
between phenylboronic acid (1.2 mmol) with bromobenzene (1.0 mmol) in
the presence of various bases
Entry
Base
Time (h)
Yield (%) 4aa
1
2
3
4
5
6
7
Na2CO3
K2CO3
KF
30
30
30
30
30
30
30
38
40
Entry
Base
Time (h)
Yield (%) 3aa
53 (98)b
31
1
2
3
4
5
6
CS2CO3
Na2CO3
K2CO3
KF
K3PO4$3H2O
KOBut
24
24
24
24
24
24
98
56
67
85
77
6
K3PO4$3H2O
NaOAc
KOBut
9
Trace
48
NaOAcc
a Isolated yields.
b Under argon atmosphere.
a Isolated yields.
c Bu4NBr (0.2 mmol) was added to the reaction solution.

Q. Xu et al. / Tetrahedron 61 (2005) 11225–11229
11227
Table 5. Pd(II)–NHC complex 2 catalyzed Heck coupling of aryl halides with butyl acrylate under argon atmosphere
R0
Entry
X
R
Time (h)
Yield (%) 4a
1
2
3
4
5
6
7
8
9
10
Br
Br
Br
Br
Br
Br
Br
Cl
Br
Br
o-Me
p-Me
p-COMe
p-OMe
o-Cl
3,5-Me,Me
H
p-COMe
H
p-CHO
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
Me
30
30
30
30
30
30
30
30
30
30
4b, 86
4c, 84
4d, 99
4e, 28
4f, 90
4g, 70
4a, 98
4d, 50
4h, 76
4i, 83
Me
a Isolated yields.
atmosphere (Table 4, entry 3). Using an argon atmosphere,
the yield of 4a was improved to 98% (Table 4, entry 3).
purification. All reactions were monitored by TLC with
Huanghai GF254 silica gel coated plates. Flash column
chromatography was carried out using 300–400 mesh silica
gel at increased pressure.
Using these reaction conditions, we next examined the Heck
coupling of a variety of aryl halides with butyl and methyl
acrylate. The results are summarized in Table 5. We found
that the Heck reaction products 4 were obtained in good to
high yields in most cases when an argon atmosphere was
used (Table 5, entries 1–3 and 5–10). For electron rich
4-bromoanisole, the reaction was sluggish and product 4e
was obtained in only 28% yield under the same conditions
(Table 5, entry 4). Product structures were determined by 1H
and 13C NMR spectroscopy and HRMS or microanalyses
3.1.1. 1,10-(1,10-Binaphthyl)-3,30-dimethyldibenzimida-
zolium diiodide 1. This is a known compound.5 The
synthesis of 1 has been summarized in Supporting
information.
3.1.2. Preparation of Pd(II)–NHC complex. The com-
pound 1 (154 mg, 0.20 mmol) and Pd(OAc)2 (44 mg,
0.20 mmol) was refluxed in THF (10 mL) for 16 h. The
solvent was removed under reduced pressure and the residue
was separated by silica gel chromatography to give Pd(II)–
NHC complex 2 (134 mg, 77%) as a yellow solid [eluent:
CH2Cl2/ethyl acetate 0:1–1:1]. The single crystal for X-ray
diffraction was obtained by recrystallization from CH2Cl2/
petroleum ether. MpO300 8C; IR (KBr): n 3538, 1583,
1509, 1382, 747 cmK1; 1H NMR (300 MHz, CDCl3, TMS):
d 3.82 (6H, s, CH3), 6.66 (2H, d, JZ8.1 Hz, ArH), 6.73–
6.93 (10H, m, ArH), 7.19–7.24 (2H, m, ArH), 7.70 (2H, d,
JZ8.7 Hz, ArH), 8.03–8.10 (4H, m, ArH); MS (ESI) m/e
747.0 (MCKI). Anal. Calcd for C36H26I2N4Pd$H2O
requires: C, 48.43; H, 3.16; N, 6.27%. Found: C, 48.27;
H, 3.24; N, 6.15%.
1
(for H NMR charts see Supporting information).
We believe that the cis-chelated configuration of 2 is
responsible for allowing it to be an effective catalyst in these
coupling reactions since the 1,1-reductive elimination from
a cis-oriented coordination site is favored.9 Moreover,
carbene ligands are less likely to dissociate than the
corresponding phosphine10 or bis-pyridine11 systems, so
the catalysis via less saturated intermediates is disfavored.12
In conclusion, we disclosed novel cis-chelating, Pd(II)–
NHC complex 2, which has a ‘normal’ bidentate structure
and is an effective catalyst for Suzuki and Heck cross-
coupling reactions. Efforts are underway to elucidate the
mechanistic details of these C–C bond forming reactions
and the use of 2 to catalyze other C–C bond forming
transformations.
3.2. General procedure for the Suzuki reactions of aryl
bromides with boronic acids
A typical procedure is given below on the reaction
expressed in entry 3 of Table 3. Complex 2 (8.7 mg,
0.01 mmol), cesium carbonate (646 mg, 2.0 mmol),
4-bromoacetophenyl (198 mg, 1.0 mmol), phenylboronic
acid (148 mg, 1.2 mmol), and DMA (2.0 mL) were
introduced to an Schlenk tube under ambient atmosphere.
The mixture was stirred at 100 8C for 24 h. The reaction
mixture was diluted with H2O (15 mL) and Et2O (15 mL),
followed by extraction twice with Et2O. The combined
organic layers were dried over MgSO4, filtered, and
evaporated under reduced pressure to give the crude
product. The pure product was isolated by column
chromatography (eluent: hexane/ethyl acetate 15:1) on
silica gel to give 176 mg (90%) of 4-acetylbiphenyl as a
3. Experimental
3.1. General remarks
1H NMR spectra were recorded on a Bruker AM-300
spectrometer for solution in CDCl3 with tetramethylsilane
(TMS) as an internal standard; J values are in Hz. Mass
spectra were recorded with a HP-5989 instrument. THF and
toluene were distilled from Na under Ar atmosphere. All of
the solid compounds reported in this paper gave satisfactory
CHN microanalyses with a Carlo-Erba 1106 analyzer.
Commercially obtained reagents were used without further

11228
Q. Xu et al. / Tetrahedron 61 (2005) 11225–11229
1
colorless solid, which was analyzed by H NMR and IR
spectroscopy.
t, JZ7.2 Hz, CH3), 1.37–1.50 (2H, m, CH2), 1.64–1.74 (2H,
m, CH2), 4.21 (2H, t, JZ7.2 Hz, OCH2), 6.44 (1H, d, JZ
15.9 Hz, ]CH), 7.36–7.38 (3H, m, ArH), 7.50–7.53 (2H,
m, ArH), 7.68 (1H, d, JZ15.9 Hz, ]CH).
3.2.1. Compound 3a. A white solid, mp 70.5–72.0 8C; IR
(KBr): n 3037, 1569, 1480, 1429, 729, 697 cmK1; 1H NMR
(CDCl3, 300 MHz, TMS): d 7.35–7.37 (2H, m, ArH), 7.42–
7.47 (4H, m, ArH), 7.59–7.62 (4H, m, ArH).
3.3.2. Compound 4b. A yellow liquid; IR (KBr): n 3070,
3020, 2959, 2867, 1713, 1634, 1600, 1520, 1461, 1380,
1
1313, 1275, 1173, 981, 763, 731 cmK1; H NMR (CDCl3,
3.2.2. Compound 3b. A colorless oil; IR (KBr): n 3059,
3020, 2933, 1590, 1479, 1439, 1372, 1010, 774, 748, 726,
701 cmK1; 1H NMR (CDCl3, 300 MHz, TMS): d 2.27 (3H,
s, CH3), 7.23–7.61 (9H, m, ArH).
300 MHz, TMS): d 0.97 (3H, t, JZ7.2 Hz, CH3), 1.38–1.50
(2H, m, CH2), 1.64–1.75 (2H, m, CH2), 2.44 (3H, s, CH3),
4.22 (2H, t, JZ7.2 Hz, OCH2), 6.36 (1H, d, JZ15.9 Hz,
]CH), 7.19–7.30 (3H, m, ArH), 7.54–7.57 (1H, m, ArH),
7.98 (1H, d, JZ15.9 Hz, ]CH).
3.2.3. Compound 3c. A white solid, mp 45.0–50.0 8C; IR
1
(KBr): n 3029, 1486, 1398, 823, 755, 735, 690 cmK1; H
NMR (CDCl3, 300 MHz, TMS): d 2.40 (3H, s, CH3), 7.24–
7.60 (9H, m, ArH).
3.3.3. Compound 4c. A yellow liquid; IR (KBr): n 3020,
2956, 2859, 1713, 1638, 1605, 1520, 1464, 1380, 1310,
1256, 1204, 1168, 983, 813 cmK1 1H NMR (CDCl3,
;
300 MHz, TMS): d 0.88 (3H, t, JZ7.2 Hz, CH3), 1.29–1.42
(2H, m, CH2), 1.57–1.66 (2H, m, CH2), 2.28 (3H, s, CH3),
4.21 (2H, t, JZ7.2 Hz, OCH2), 6.31 (1H, d, JZ15.6 Hz,
]CH), 7.09 (2H, d, JZ7.2 Hz, ArH), 7.34 (2H, d, JZ
7.2 Hz, ArH), 7.58 (1H, d, JZ15.6 Hz, ]CH).
3.2.4. Compound 3d. A white solid, mp 120.4–121.3 8C; IR
(KBr): n 3050, 1680, 1600, 1570, 1450, 1394, 1353, 1261,
958, 835, 765, 721, 690 cmK1; 1H NMR (CDCl3, 300 MHz,
TMS): d 2.65 (3H, s, CH3), 7.40–7.48 (3H, m, ArH), 7.62–
7.71 (4H, m, ArH), 8.04 (2H, d, JZ7.2 Hz, ArH).
3.3.4. Compound 4d. A yellow liquid; IR (KBr): n 3020,
2941, 2852, 1715, 1685, 1638, 1601, 1560, 1460, 1380,
3.2.5. Compound 3e. A white solid, mp 91.1–92.3 8C; IR
(KBr): n 3050, 2956, 2837, 1605, 1520, 1486, 1446, 1283,
1312, 1265, 1174, 982, 827 cmK1 1H NMR (CDCl3,
;
1
1269, 1180, 1035, 834, 760, 688 cmK1; H NMR (CDCl3,
300 MHz, TMS): d 0.97 (3H, t, JZ7.2 Hz, CH3), 1.40–1.48
(2H, m, CH2), 1.68–1.73 (2H, m, CH2), 2.62 (3H, s, CH3),
4.23 (2H, t, JZ7.2 Hz, OCH2), 6.53 (1H, d, JZ15.9 Hz,
]CH), 7.61 (2H, d, JZ8.4 Hz, ArH), 7.69 (1H, d, JZ
15.9 Hz, ]CH), 7.97 (2H, d, JZ8.4 Hz, ArH).
300 MHz, TMS): d 3.87 (3H, s, OCH3), 6.97–7.00 (2H, m,
ArH), 7.26–7.30 (1H, m, ArH), 7.39–7.44 (2H, m, ArH),
7.52–7.57 (4H, m, ArH).
3.2.6. Compound 3f. A yellow solid, mp 32.5–33.5 8C; IR
(KBr): n 3059, 3031, 2924, 1380, 1498, 1467, 1425, 1128,
3.3.5. Compound 4e. A yellow liquid; IR (KBr): n 3070,
3020, 2959, 2852, 1710, 1635, 1604, 1564, 1513, 1464,
1387, 1251, 1170, 1031, 983, 828 cmK1; 1H NMR (CDCl3,
300 MHz, TMS): d 0.89 (3H, t, JZ7.2 Hz, CH3), 1.32–1.40
(2H, m, CH2), 1.56–1.63 (2H, m, CH2), 3.76 (3H, s, OCH3),
4.12 (2H, t, JZ7.2 Hz, OCH2), 6.24 (1H, d, JZ15.6 Hz,
]CH), 6.81–6.84 (2H, m, ArH), 7.39–7.42 (2H, m, ArH),
7.57 (1H, d, JZ15.6 Hz, ]CH).
1
1075, 1036, 1009, 770, 748, 699 cmK1; H NMR (CDCl3,
300 MHz, TMS): d 7.24–7.61 (9H, m, ArH).
3.2.7. Compound 3g. A white solid, mp 22.2–23.1 8C; IR
(KBr): n 3059, 3031, 2919, 1603, 1577, 1482, 1380, 850,
761, 737, 698 cmK1; 1H NMR (CDCl3, 300 MHz, TMS): d
2.37 (6H, s, CH3), 6.99–7.61 (8H, m, ArH).
3.3.6. Compound 4f. A yellow liquid; IR (KBr): n 3070,
3020, 2960, 2934, 2873, 1717, 1637, 1591, 1520, 1471,
1443, 1380, 1315, 1269, 1202, 1176, 1053, 980, 760,
738 cmK1; 1H NMR (CDCl3, 300 MHz, TMS): d 0.97 (3H,
t, JZ7.2 Hz, CH3), 1.42–1.48 (2H, m, CH2), 1.55–1.74 (2H,
m, CH2), 4.22 (2H, t, JZ7.2 Hz, OCH2), 6.43 (1H, d, JZ
15.6 Hz, ]CH), 7.25–7.31 (2H, m, ArH), 7.38–7.41 (1H,
m, ArH), 7.59–7.62 (1H, m, ArH), 8.08 (1H, d, JZ15.6 Hz,
]CH).
3.3. Typical reaction procedure for Heck reaction
In a Schlenk tube fitted with a septum and a reflux condenser
were placed aryl halide (1.0 mmol), butyl acrylate
(1.5 mmol), potassium fluoride (116 mg, 2.0 mmol), tetra-
butylammonium bromide (64.4 mg, 0.20 mmol), and N,N-
dimethylacetamide (DMAC, 2.0 mL). After repeated
degassing by oil pump vacuum and flushing with argon,
complex 2 (8.7 mg, 0.01 mmol) was added under an argon
atmosphere. The mixture was stirred at 140 8C for 30 h. The
reaction mixture was diluted with H2O (15 mL) and Et2O
(15 mL), followed by extraction twice with Et2O. The
combined organic layers were dried over anhydrous
MgSO4, filtered, and evaporated under reduced pressure to
give crude product. A pure product was isolated by column
chromatography (eluent: hexane/ethyl acetate 15:1) on
3.3.7. Compound 4g. A yellow liquid; IR (KBr): n 3020,
2959, 2867, 1714, 1637, 1594, 1442, 1380, 1285, 1254,
1
1164, 982, 843, 678 cmK1; H NMR (CDCl3, 300 MHz,
TMS): d 0.97 (3H, t, JZ7.2 Hz, CH3), 1.40–1.48 (2H, m,
CH2), 1.66–1.71 (2H, m, CH2), 2.33 (6H, s, CH3), 4.20 (2H,
t, JZ7.2 Hz, OCH2), 6.42 (1H, d, JZ15.6 Hz, ]CH), 7.02
(1H, s, ArH), 7.15 (2H, s, ArH), 7.62 (1H, d, JZ15.6 Hz,
]CH).
1
silica gel. The purified product was analyzed by H NMR
and IR spectroscopy.
3.3.1. Compound 4a. A yellow liquid; IR (KBr): n 3020,
2960, 2874, 1714, 1638, 1571, 1500, 1450, 1380, 979, 768,
711 cmK1; 1H NMR (CDCl3, 300 MHz, TMS): d 0.96 (3H,
3.3.8. Compound 4h. A white solid, mp 35.8–36.5 8C; IR
(KBr): n 3050, 3020, 2944, 1717, 1638, 1576, 1496, 1450,
1
1380, 1314, 1275, 1200, 1170, 980, 768, 712 cmK1; H

Q. Xu et al. / Tetrahedron 61 (2005) 11225–11229
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NMR (CDCl3, 300 MHz, TMS): d 3.81 (3H, s, OCH3), 6.45
(1H, d, JZ15.6 Hz, ]CH), 7.38–7.41 (3H, m, ArH), 7.51–
7.55 (2H, m, ArH), 7.70 (1H, d, JZ15.6 Hz, ]CH).
3.3.9. Compound 4i. A white solid, mp 82.1–83.0 8C; IR
(KBr): n 3020, 2958, 2840, 2740, 1712, 1691, 1637, 1602,
1540, 1425, 1392, 1315, 1203, 1163, 984, 820, 796 cmK1
;
1H NMR (CDCl3, 300 MHz, TMS): d 3.84 (3H, s, OCH3),
6.57 (1H, d, JZ15.9 Hz, ]CH), 7.69 (2H, d, JZ8.0 Hz,
ArH), 7.73 (1H, d, JZ15.9 Hz, ]CH), 7.91 (2H, d, JZ
8.0 Hz, ArH), 10.04 (1H, s, HC]O).
Acknowledgements
We thank the State Key Project of Basic Research (project
973) (no. G2000048007), Shanghai Municipal Committee
of Science and Technology, and the National Natural
Science Foundation of China (203900502, 20472096 and
20272069) for financial support.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tet.2005.09.
010. The spectroscopic charts of co0upling products 3 and 04
as well as the synthesis of 1,1 -(1,10-binaphthyl)-3,3 -
dimethyldibenzimidazolium diiodide 1 and the X-ray
crystal data of NHC–Pd(II) complex 2.
5. This is a known compound. (a) Duan, W.-L.; Shi, M.; Rong,
G.-B. Chem. Commun. 2003, 2916–2917. (b) Shi, M.; Duan,
W. L. Appl. Organomet. Chem. 2005, 19, 40–44.
6. Nolan and co-workers recently disclosed that ‘unusual’ Pd(II)–
NHC complex synthesized from imidazolium salts is a more
suitable precursor for the Suzuki and Heck coupling reactions.
Lebel, H.; Janes, M. K.; Charette, A. B.; Nolan, S. P. J. Am.
Chem. Soc. 2004, 126, 5046–5047.
7. The crystal data of 2 has been deposited in CCDC with number
209242. Empirical formula: C36H28N4OI2Pd; formula weight:
892.82; crystal color, habit: colorless, prismatic; crystal
dimensions: 0.548!0.097!0.041 mm; crystal system: mono-
clinic; lattice type: primitive; lattice parameters: aZ
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