Dinuclear NHC–palladium(II) complexes: synthesis,characterization and application to Suzuki–Miyaura cross-coupling reactions

Article abstract of DOI:10.1007/s11243-018-0224-6

Abstract: Four dinuclear N-heterocyclic carbene–palladium(II) complexes 1–4 were prepared and characterized by elemental analysis and spectroscopic methods. The X-ray crystal structure of complex 2 showed a dinuclear framework in which N-heterocyclic ligands bridge between two square planar palladium(II) units. Each palladium center is surrounded by an imidazolylidene, a nitrogen atom from the central linking ligand, and two trans-chloride ligands. These dinuclear NHC–palladium(II) complexes exhibited efficient catalytic activities for the Suzuki–Miyaura coupling of aryl and benzyl chlorides with arylboronic acids.

Full text of DOI:10.1007/s11243-018-0224-6

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0224-6
Dinuclear NHC–palladium(II) complexes: synthesis,characterization
and application to Suzuki–Miyaura cross‑coupling reactions
Tao Wang1,2 · Kai Xu1,2 · Wanli Wang1,2 · An’an Zhang1,2 · Lantao Liu1,2
Received: 28 December 2017 / Accepted: 7 March 2018
©
Springer International Publishing AG, part of Springer Nature 2018
Abstract
Four dinuclear N-heterocyclic carbene–palladium(II) complexes 1–4 were prepared and characterized by elemental analysis
and spectroscopic methods. The X-ray crystal structure of complex 2 showed a dinuclear framework in which N-heterocyclic
ligands bridge between two square planar palladium(II) units. Each palladium center is surrounded by an imidazolylidene, a
nitrogen atom from the central linking ligand, and two trans-chloride ligands. These dinuclear NHC–palladium(II) complexes
exhibited eꢀcient catalytic activities for the Suzuki–Miyaura coupling of aryl and benzyl chlorides with arylboronic acids.
Introduction
the PEPPSI NHC–palladium complexes [21–24] show high
levels of activity in the Suzuki–Miyaura coupling of aryl
chlorides with arylboronic acids. Shao and Lu have prepared
a well-deꢂned N-heterocyclic carbene–Pd(II)-1-methylim-
idazole [NHC–Pd(II)–Im] [25–28] complex, which acted
as a highly eꢁective pre-catalyst for the Suzuki–Miyaura
coupling of aryl as well as benzyl chlorides with arylboronic
acids under mild conditions. Meanwhile, the NHC–Pd(II)-
2-phenylimidazole complexes [29] have been found to cat-
alyze the Suzuki cross-coupling of aryl as well as benzyl
chlorides with a series of boronic acids in excellent yields,
using a very low catalyst loading of 0.05 mol%. The use
of imine–Pd–NHC complexes as active catalysts for the
more challenging Suzuki–Miyaura cross-couplings of aryl
chlorides have also been reported [30]. Recyclable sup-
ported N-heterocyclic carbene–palladium complexes have
attracted recent interest for Suzuki–Miyaura coupling reac-
tions [31]. Despite the impressive progress made so far in
the use of N-heterocyclic carbene–palladium complexes as
catalysts, very little research has been reported on dinuclear
or multi-nuclear carbene–palladium complexes bearing
bidentate or multidentate bridging ligands [32–35]. We have
recently reported a simple route to the synthesis of trinu-
clear NHC–palladium complexes with bridging tridentate
N-heterocyclic ligands [36]. These trinuclear palladium(II)
complexes proved to be eꢁective catalyst precursors for the
Suzuki–Miyaura coupling of both aryl and benzyl chlorides
with arylboronic acids. Moreover, the trinuclear NHC–Pd(II)
complexes were found to exhibit a much higher activity
than the analogous mononuclear NHC–Pd(II) complexes
under the same reaction conditions. Encouraged by these
During recent decades, transition-metal-catalyzed coupling
reactions have contributed greatly to the straightforward
construction of carbon–carbon and carbon–heteroatom
bonds. Among such reactions, the palladium-catalyzed
Suzuki–Miyaura coupling of aryl halides and organoboron
reagents have attracted much attention in organic synthe-
sis [1–11]. Consequently, considerable eꢁorts have been
made toward the discovery of more active catalysts for the
Suzuki–Miyaura coupling reaction. Recently, N-heterocy-
clic carbene (NHC)–palladium complexes [12–20], which
often show advantages such as higher air, moisture, and
thermal stability, have been found to be good catalysts for
the Suzuki–Miyaura coupling of aryl halides. For example,
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-0224-6) contains
supplementary material, which is available to authorized users.
*
Tao Wang
wt67751726@126.com
*
Lantao Liu
liult05@iccas.ac.cn
1
2
Henan Engineering Laboratory of Green Synthesis
for Pharmaceuticals, School of Chemistry and Chemical
Engineering, Shangqiu Normal University,
Shangqiu 476000, Henan, People’s Republic of China
Henan Key Laboratory of Biomolecular Recognition
and Sensing, School of Chemistry and Chemical
Engineering, Shangqiu Normal University, Shangqiu 476000,
Henan, People’s Republic of China
Vol.:(0
1
123456789)
3

Transition Metal Chemistry
results, and also in continuation of our interest [37–39] in
the construction of functionalized complexes that can enter
an array of di- and multi-nuclear systems, herein we have
extended the methodology to the introduction of bidentate
bridging N-heterocycles into NHC–palladium complexes. In
this paper, we report the facile synthesis and characteriza-
tion of four dinuclear NHC–Pd(II) complexes (Scheme 1).
Furthermore, the catalytic applications of these dinuclear
NHC–Pd(II) complexes in the Suzuki–Miyaura coupling of
aryl and benzyl chlorides with a range of arylboronic acids
have been explored.
General procedure for synthesis of the complexes
Under an N atmosphere, a mixture of the imidazolium salt
2
(1,3-bis(2,6-diisopropylphenyl)-1H-imidazol-3-ium chlo-
ride, 2.2 mmol), the required N-heterocycle (1.0 mmol),
PdCl (2.2 mmol) and K CO (2.2 mmol) was stirred in
2
2
3
anhydrous THF (10 mL) under reꢃux for 12 h. After cool-
ing, ꢂltration and evaporation, the residue was puriꢂed by
preparative TLC on silica gel plates, eluting with CH Cl
2
2
to aꢁord the corresponding dinuclear N-heterocyclic car-
bene–palladium(II) complexes 1–4.
Complex (1)
Experimental
1
Yield: 39%, pale yellow solid. mp: 229–231 °C. H NMR
General
(
400 MHz, CDCl ): δ 7.45 (t, J = 7.7 Hz, 2H, ArH),
3
7
.30–7.26 (m, 4H, ArH), 7.03 (s, 2H, NCH=), 5.29 (s, 1H,
Both the procedures preparations of the dinuclear N-heter-
ocyclic carbene–palladium(II) complexes and all the cata-
lytic reactions were carried out under a nitrogen atmosphere.
NH), 2.99–2.93 (m, 4H, CH(CH ) ), 2.64–2.59 (m, 2H,
3
2
CH ), 2.37–2.35 (m, 2H, CH ), 1.36 (d, J = 6.6 Hz, 12H,
2
2
1
3
CH CHCH ), 1.04 (d, J = 6.8 Hz, 12H, CH CHCH ).
C
3
3
3
3
1
,4-bis(pyrid-4-yl)benzene was prepared according to the
NMR (100 MHz, CDCl ): δ 157.1, 146.6, 135.0, 130.1,
3
literature method [40]. Solvents were dried by standard
methods and freshly distilled prior to use. All other chemi-
cals were used as purchased. Melting points were measured
on a XT4A melting point apparatus and are uncorrected. IR
spectra were collected on a Bruker VECTOR22 spectropho-
−1
1
2
9
24.7, 123.8, 47.0, 28.6, 26.2, 23.1. IR(KBr, cm ): 2966(s),
867(s), 1467(s), 1412(s), 1383(s), 1079(m), 1012(s),
44(s), 887(m), 801(s), 755(m), 707(s). Anal. Calcd. for
C H Cl N Pd : C, 57.20; H, 6.79; N, 6.90. Found: C,
5
8
82
4
6
2
5
7.38; H, 6.98; N, 6.70%.
1
13
tometer using KBr pellets. H and C NMR spectra were
recorded on a Bruker DPX 400 instrument using TMS as an
internal standard. Elemental analyses were measured on a
Thermo Flash EA 1112 elemental analyzer.
Complex (2)
1
Yield: 64%, pale yellow solid. mp: 198–201 °C. H NMR
400 MHz, CDCl ): δ 8.63 (d, J= 6.5 Hz, 2H, ArH), 7.48
(
3
Scheme 1 Synthesis of the dinuclear N-heterocyclic carbene–palladium(II) complexes 1–4.
1 linker=piperazine (39% yield); 2
linker=4,4ʹ-bipyridine (64%, yield); 3 linker=1,2-bis(4-pyridyl)ethane (58%, yield); 4 linker=1,4-bis(pyrid-4-yl)benzene (41% yield)
1
3

Transition Metal Chemistry
(
t, J=7.7 Hz, 2H, ArH), 7.33 (d, J=7.7 Hz, 4H, ArH), 7.15
d, J = 6.6 Hz, 2H, ArH), 7.13 (s, 2H, NCH=), 3.17–3.11
m, 4H, CH(CH ) ), 1.46 (d, J=6.6 Hz, 12H, CH CHCH ),
Crystal structure determination and data collection
(
(
Crystals of complex 2 (CCDC 1570267) were obtained by
recrystallization from CH Cl /n-hexane at ambient tem-
3
2
3
3
13
1
.11 (d, J=6.8 Hz, 12H, CH CHCH ). C NMR (100 MHz,
3 3
2
2
CDCl ): δ 153.4, 148.0, 136.3, 131.6, 126.4, 125.3, 123.0,
perature. Data were collected on an Oxford Diꢁraction
Gemini E diꢁractometer with graphite-monochromated
Mo Kα radiation (λ = 0.7107 Å). The structure was solved
by direct methods using the SHELXS-97 program, and
all non-hydrogen atoms were reꢂned anisotropically on
3
−
1
3
1
1
7
0.0, 27.6, 24.5. IR(KBr, cm ): υ 2964(s), 2866(s), 1609(s),
530(m), 1488(s), 1456(m), 1411(s), 1382(s), 1280(s),
223(m), 1132(s), 934(s), 863(m), 801(s), 755(m), 746(s),
03(s). Anal. Calcd. for C H Cl N Pd : C, 59.68; H, 6.26;
6
4
80
4
6
2
2
N, 6.52. Found: C, 59.78; H, 6.48; N, 6.30%.
F by the full-matrix least-squares technique, using the
SHELXL-97 crystallographic software package [41, 42].
The hydrogen atoms were included but not reꢂned. Details
of the crystal structure determination are summarized in
Table 4. The crystallographic data can be obtained free of
charge from The Cambridge Crystallographic Data Centre
via http://www.ccdc.cam.ac.uk/data_request/cif.
Complex (3)
1
Yield: 58%, pale yellow solid. mp: 237–240 °C. H NMR
(
(
400 MHz, CDCl ): δ 8.42 (d, J= 6.3 Hz, 2H, ArH), 7.48
3
t, J=7.7 Hz, 2H, ArH), 7.34 (d, J=7.7 Hz, 4H, ArH), 7.11
s, 2H, NCH=), 6.86 (d, J = 6.2 Hz, 2H, ArH), 3.20–3.13
m, 4H, CH(CH ) ), 2.59 (s, 2H, CH ), 1.47 (d, J=6.6 Hz,
(
(
3
2
2
1
2H, CH CHCH ), 1.11 (d, J=6.8 Hz, 12H, CH CHCH ).
3 3 3 3
1
3
C NMR (100 MHz, CDCl ): δ 155.0, 151.3, 151.2,
3
1
2
1
1
46.6, 135.1, 130.2, 125.0, 124.0, 123.8, 35.2, 28.7,
−
1
6.3, 23.3. IR(KBr, cm ): υ 2965(s), 2868(s), 1593(s),
467(m), 1412(s), 1383(s), 1331(m), 1208(s), 1120(s),
012(m), 886(s), 801(m), 756(m), 707(s). Anal. Calcd.
for C H Cl N Pd : C, 60.23; H, 6.43; N, 6.39. Found: C,
6
6
84
4
6
2
6
0.33; H, 6.60; N, 6.20%.
Complex (4)
Yield: 41%, pale yellow solid. mp: 219–222 °C. H NMR
1
(
400 MHz, CDCl ): δ 8.60 (d, J = 6.6 Hz, 2H, ArH),
3
7
7
.51–7.47 (m, 4H, ArH), 7.35 (d, J = 7.7 Hz, 4H, ArH),
.29–7.26 (m, 2H, ArH), 7.13 (s, 2H, NCH=), 3.21–3.15
(
m, 4H, CH(CH ) ), 1.49 (d, J=6.6 Hz, 12H, CH CHCH ),
3
2
3
3
13
1
.12 (d, J=6.8 Hz, 12H, CH CHCH ). C NMR (100 MHz,
3 3
CDCl ): δ 154.9, 151.7, 148.7, 146.7, 138.2, 135.1, 130.3,
3
−
1
1
27.9, 125.1, 124.0, 121.8, 28.8, 26.3, 23.3. IR (KBr, cm ):
υ 2966(s), 2867(s), 1612(s), 1466(m), 1410(s), 1383(s),
1
8
6
346(m), 1280(s), 1223(m), 1120(s), 1059(m), 944(s),
15(s), 757(m), 705(s). Anal. Calcd. for C H Cl N Pd : C,
70
84
4
6
2
1.63; H, 6.21; N, 6.16. Found: C, 61.68; H, 6.25; N, 6.10%.
General procedure for the Suzuki–Miyaura reaction
Fig. 1 Molecular structure of the dinuclear N-heterocyclic car-
bene–palladium(II) complex 2·2CH Cl . Hydrogen atoms are
2
2
omitted for clarity. Selected bond lengths (Å) and angles (deg) in
A Schlenk ꢃask was charged with the required aryl chlo-
ride (0.50 mmol), arylboronic acid (0.75 mmol), dinuclear
complex 2·2CH Cl : Pd1–C11 1.970(5), Pd1–N2 2.110(5), Pd1–
2
2
Cl1 2.2950(17), Pd1–Cl2 2.2883(17); C11–Pd1–Cl1 88.80(16),
N2–Pd1–Cl1 92.18(15), C11–Pd1–Cl2 90.72(16), N2–Pd1–Cl2
t
NHC–palladium(II) complex (1.0 mol%), KO Bu (2.0
i
8
1
8.46(15), C11–Pd1–N2 176.6(2), Cl1–Pd1–Cl2 177.17(9); Pd2–C38
.972(5), Pd2–N1 2.107(5), Pd2–Cl3 2.2940(19), Pd2–Cl4 2.294(2);
equiv), PrOH (0.4 mL) and H O (0.8 mL). The mixture was
2
stirred at 80 °C for 4 h under N . After cooling, the reaction
2
C38–Pd2–Cl3 89.02(18), N1–Pd2–Cl3 91.70(16), C38–Pd2–Cl4
mixture was evaporated, and the product was isolated by
9
0.59(18), N1–Pd2–Cl4 88.94(17), C38–Pd2–N1 176.8(2), Cl3–Pd2–
preparative TLC on silica gel plates.
Cl4 175.24(7)
1
3

Transition Metal Chemistry
Results and discussion
with the N-heterocyclic ligand bridging between two
square planar palladium(II) units. Each palladium center
is coordinated by a carbene, a nitrogen atom and two chlo-
ride ligands in a slightly distorted square planar geometry.
The values of bond lengths and angles compare well with
those of related dinuclear NHC–palladium(II) complexes
[32–35]. The Pd(1)–Ccarbene bond length of 1.970(5) Å in
complex 2 is shorter than that of Pd(1)–N(2) (2.110(5)
Å. The Ccarbene–Pd(1)–N(2) and Cl(1)–Pd(1)–Cl(2)
bond angles are close to 180° (see caption to Fig. 1),
while the Ccarbene–Pd(1)–Cl(1), Ccarbene–Pd(1)–Cl(2),
N(2)–Pd(1)–Cl(1) and N(2)–Pd(1)–Cl(2) angles are all
close to 90°. The coordination environment around Pd(2)
is very similar to that of Pd(1).
Synthesis and characterization of the complexes
Following the synthetic route reported previously [38], the
required dinuclear N-heterocyclic carbene–palladium(II)
complexes 1–4 were easily synthesized in a one-step
reaction from the commercially available imidazolium
salts, palladium chloride, and the bridging N-heterocycle
ligands, as shown in Scheme 1. Complexes 1–4 were iso-
lated in fair yields (39–64%) after puriꢂcation and char-
1
13
acterized by elemental analysis, H NMR, C NMR, and
IR spectroscopy. Additionally, the X-ray crystal structure
of complex 2 was obtained. All four complexes isolated as
pale yellow and air-stable solids.
The X-ray crystal structure of complex 2 is shown in
Fig. 1. Two dichloromethane molecules from the recrystal-
lization solvent were found in the lattice unit of complex
Catalytic studies
In order to investigate the catalytic activities of the pre-
sent dinuclear NHC–Pd(II) complexes, initial experiments
were carried out using 1-chloro-4-methoxybenzene and
2
. The crystal structure of 2 reveals a dinuclear framework
Table 1 Optimization of reaction conditions for Suzuki–Miyaura coupling of 1-chloro-4-methoxybenzene with phenylboronic acid catalyzed by
the dinuclear NHC–Pd(II) complexes
Entry
Cat.
Base
Solvent
Yield (%)a
t
1
2
3
4
5
1
KO Bu
2
EtOH/H O (1/3)
85
60
65
75
21
89
95
>99
>99
99
79
97
91
93
2
1
Cs CO
EtOH/H O (1/3)
3
2
1
Na CO
EtOH/H O (1/3)
2
3
2
t
1
NaO Bu
K PO
EtOH/H O (1/3)
2
1
EtOH/H O (1/3)
3
4
2
b
t
6
7
8
9
1
KO Bu
EtOH/H O (1/3)
2
b
t
i
1
KO Bu
PrOH/H O (1/3)
2
c
t
i
1
KO Bu
PrOH/H O (1/2)
2
c,d
t
i
1
KO Bu
PrOH/H O (1/2)
2
c,e
t
i
1
1
1
1
1
1
0
1
2
3
4
5
1
KO Bu
PrOH/H O (1/2)
2
c,f
c,e
c,e
c,e
c,e
t
i
1
KO Bu
PrOH/H O (1/2)
2
t
i
2
KO Bu
PrOH/H O (1/2)
2
t
i
3
KO Bu
PrOH/H O (1/2)
2
t
i
4
KO Bu
PrOH/H O (1/2)
2
t
i
PdCl2
KO Bu
PrOH/H O (1/2)
15
2
All reactions were carried out using 5a (0.50 mmol), 6a (0.75 mmol), base (2.0 equiv), Catalyst. (1.0 mol%) in solvent (2.0 mL) at 80 °C for 12 h
a
Isolated yields
Solvent (1.0 mL)
Solvent (1.2 mL)
Reaction time was 8 h
b
c
d
e
Reaction time was 4 h
f
Reaction time was 2 h
1
3

Transition Metal Chemistry
Table 2 Catalytic Suzuki–Miyaura reactions of aryl chlorides using complex 1 as the catalyst
Entry
5 (Ar1)
6 (Ar2)
Product
Yield (%)a
1
2
3
4
5
6
7
8
9
5a (4-MeO–C H )
6a (Ph)
7a
7b
7c
7d
7e
7f
99
98
90
>99
>99
99
99
94
>99
72
>99
99
99
74
98
99
63
71
6
4
5b (3-MeO–C H )
6a (Ph)
6
4
5c (2-MeO–C H )
6a (Ph)
6
4
5d (4-Me–C H )
6a (Ph)
6
4
5e (3-Me–C H )
6a (Ph)
6
4
5f (2-Me–C H )
6a (Ph)
6
4
5 g (4-CH CO–C H )
6a (Ph)
7 g
7 h
7i
3
6
4
5 h (4-NO –C H )
6a (Ph)
2
6
4
5i (2-Pyridyl)
6a (Ph)
10
11
12
13
14
15
16
17
18
5j (3-Pyridyl)
6a (Ph)
7j
5a (4-OMe–C H )
6b (4-Me–C H )
7 k
7 l
7 m
7n
7o
7p
7q
7r
6
4
6
4
5a (4-OMe–C H )
6c (3-Me–C H )
6
4
6 4
5a (4-OMe–C H )
6d (2-Me–C H )
6
4
6
4
5a (4-OMe–C H )
6e (2,6-Me –C H )
6
4
2
6
3
5a (4-OMe–C H )
6f (4-F–C H )
6
4
6
4
5a (4-OMe–C H )
6 g (4-CF –C H )
6
4
3
6
4
5a (4-OMe–C H )
6 h (1-Naphthyl)
6i (2-Naphthyl)
6
4
5a (4-OMe–C H )
6
4
t
i
All reactions were carried out using 5 (0.50 mmol), 6 (0.75 mmol), KO Bu (2.0 equiv), 1 (1.0 mol%) in PrOH/H O (1.2 mL, V:V=1:2) at 80 °C
2
for 4 h
a
Isolated yields
phenylboronic acid as the reactants in order to evaluate
the catalytic properties of complex 1 in a solvent mix-
ture consisting of ethanol (0.5 mL) and water (1.5 mL)
excellent yields were still achieved (91–97%, entries
12–14). Meanwhile, in a control experiment with PdCl
2
as catalyst, only 15% yield was obtained.
(
Table 1). When 1.0 mol% of complex 1 was used as
With the optimized conditions in hand, the generality
and limitations of the reaction for a variety of aryl chlo-
rides and aryl boronic acids were evaluated. As shown in
Table 2, most of the coupling reactions proceeded rapidly
to provide the corresponding biaryl products in good to
almost quantitative yields. Both electron-donating and
electron-withdrawing substituents in the phenyl ring of the
aryl chlorides were tolerated, giving good results (entries
1–8). In general, ortho-, para-, and meta-substituted aryl
chlorides exhibited similar reactivities in this catalytic sys-
tem. To our pleasure, when heteroaromatic chlorides such
as 2-chloropyridine and 3-chloropyridine were used as the
substrates, the corresponding products 7i–j were obtained
in good to high yields (entries 9–10). Next, the scope of the
arylboronic acid component was examined (entries 11–18).
A wide range of arylboronic acids containing various
t
t
catalyst, and either KO Bu, Cs CO , Na CO or NaO Bu
2
3
2
3
as the base, the corresponding biaryl product 7a was
isolated in good yields (up to 85%) after 12 h (entries
1
–4). K PO proved to be inferior as a base, affording the
3 4
product in only 21% yield (entry 5). Thus, the choice of
base is crucial to the yield of the reaction [43]. The yield
did not increase significantly upon varying the amount
of solvent (entry 6). However, when a solvent mixture of
isopropanol and water was tested, the yield of the product
improved to 95% (entry 7). When the ratio of isopropanol
to water was changed to 1: 2, an almost quantitative yield
was obtained (entry 8). Even when the reaction time was
reduced to 4 h, the product 7a was still obtained in 99%
yield (entry 10). Further reduction of the reaction time to
2
h, afforded the product in 79% yield (entry 11). When
the other three complexes 2–4 were used as catalysts,
1
3

Transition Metal Chemistry
Table 3 Catalytic Suzuki–Miyaura reactions of benzyl chlorides using complex 1 as the catalyst
Entry
8 (Ar3)
6 (Ar2)
Product
Yield (%)a
1
2
3
4
5
6
7
8
9
8a (Ph)
8a (Ph)
8a (Ph)
8a (Ph)
8a (Ph)
8a (Ph)
8a (Ph)
6b (4-Me–C H )
9a
9b
9c
9d
9e
9f
9 g
9 h
9i
>99
99
95
75
99
99
99
99
99
99
99
99
6
4
6c (3-Me–C H )
6
4
6d (2-Me–C H )
6
4
6e (2,6-Me –C H )
2
6
3
6f (4-F–C H )
6
4
6 g (4-CF –C H )
3
6
4
6 h (1-Naphthyl)
6a (Ph)
t
8b (4- Bu–C H )
6
4
8c (4-Me–C H )
6a (Ph)
6
4
1
1
1
0
1
2
8d (3-Me–C H )
6a (Ph)
9j
9 k
9 l
6
4
8e (2-Me–C H )
6a (Ph)
6
4
8f (4-F–C H )
6a (Ph)
6
4
t
i
All reactions were carried out using 8 (0.50 mmol), 6 (0.75 mmol), KO Bu (2.0 equiv), 1 (1.0 mol%) in PrOH/H O (1.2 mL, V:V=1:2) at 80 °C
2
for 4 h
a
Isolated yields
functional groups were well tolerated, giving the desired
products 7k–7r in good to almost quantitative yields. Nev-
ertheless, the use of sterically hindered boronic acids such
as 2,6-dimethylphenylboronic acid resulted in a signiꢂcant
decrease in yield (entry 14). Furthermore, the reactions of
heteroaromatic boronic acids, such as 3-pyridinylboronic
acid or 4-pyridinylboronic acid, did not occur (data not
shown in Table 2). A possible reason for this observation
would be their susceptibility to hydrolysis under the reac-
tion conditions.
reaction. However, a slightly lower yield (75%) of 9d was
obtained when the sterically hindered 2,6-dimethylphenylbo-
ronic acid was used (entry 4). Overall, these results indicate
that these dinuclear NHC–Pd(II) complexes are highly eꢀ-
cient catalysts for the Suzuki–Miyaura coupling of both aryl
and benzyl chlorides with arylboronic acids.
Conclusion
In order to further evaluate the catalytic activities of these
complexes, the reactions of benzyl chlorides and arylboronic
acids were investigated under the same reaction conditions.
All of these reactions also proceeded smoothly to aꢁord the
corresponding diarylmethanes in good to almost quantita-
tive yields (Table 3, entries 1–12). In general, the electronic
eꢁect of the substituent has no signiꢂcant eꢁect on the
In conclusion, a new set of dinuclear N-heterocyclic car-
bene–palladium(II) complexes have been prepared and used
as catalysts for the Suzuki–Miyaura coupling of both aryl
and benzyl chlorides with arylboronic acids. Studies on fur-
ther modiꢂcations of such dinuclear NHC–Pd(II) complexes
1
3

Transition Metal Chemistry
Table 4 Crystal data and structure reꢂnement parameters for complex
3. Valente C, Calimsiz S, Hoi K-H, Mallik D, Sayah M, Organ MG
(2012) Angew Chem Int Ed 51:3314
2
4
. Jiang L, Li Z, Zhao D (2010) Chin. J. Org. Chem. 30:200
2
·2CH Cl
2 2
5. Molander GA, Canturk B (2009) Angew Chem Int Ed 48:9240
6
. Fu GC (2008) Acc Chem Res 41:1555
Empirical formula
Mr
Temperature (K)
Wavelength (Å)
Crystal system
Crystal size (mm)
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C H Cl N Pd
66 84 8 6 2
1457.79
291.15
0.71073
Triclinic
0.20×0.20×0.20
12.6745(9)
18.1032(10)
18.3027(13)
73.785(6)
75.626(6)
75.932(6)
3837.5(4)
2
7. Alonso F, Beletskaya IP, Yus M (2008) Tetrahedron 64:3047
8. Marion N, Nolan SP (2008) Acc Chem Res 41:1440
9. Corbet J-P, Mignani G (2006) Chem Rev 106:2651
1
0. Suzuki A (1999) J Organomet Chem 576:147
1
1
1. Miyaura N, Suzuki A (1995) Chem Rev 95:2457
2. Menon RS, Biju AT, Nair V (2015) Chem Soc Rev 44:5040
13. Tang Y, Yang F, Nie S, Wang L, Luo Z, Lu H (2015) Chin J Org
Chem 35:705
1
4. Budagumpi S, Haque RA, Salman AW (2012) Coord Chem Rev
2
56:1787
1
5. Yuan D, Huynh HV (2012) Molecules 17:2491
16. Fortman GC, Nolan SP (2011) Chem Soc Rev 40:5151
17. Droge T, Glorius F (2010) Angew Chem Int Ed 49:6940
1
8. Diez-Gonzalez S, Marion N, Nolan Steven P (2009) Chem Rev
γ (°)
V (Å )
1
09:3612
3
1
9. Nair V, Vellalath S, Babu BP (2008) Chem Soc Rev 37:2691
Z
20. Hahn FE, Jahnke MC (2008) Angew Chem Int Ed 47:3122
21. Dunsford JJ, Cavell KJ (2014) Organometallics 33:2902
22. Rajabi F, Thiel WR (1873) Adv Synth Catal 2014:356
Space group
P−1
1.262
0.786
Dcalcd (g cm−3)
2
3. Teci M, Brenner E, Matt D, Toupet L (2013) Eur J Inorg Chem
−
1
μ (mm
)
1
6:2841
θ range (°)
F(000)
3.16–26.37
1500.0
24. Tu T, Sun Z, Fang W, Xu M, Zhou Y (2012) Org Lett 14:4250
25. Zhang Y, Yin S-C, Lu J-M (2015) Tetrahedron 71:544
2
2
6. Gu Z-S, Chen W-X, Shao L-X (2014) J Org Chem 79:5806
7. Shen X-B, Zhang Y, Chen W-X, Xiao Z-K, Hu T-T, Shao L-X
No. of data collected
No. of unique data
R(int)
24,087
15,053
0.0702
R1 =0.0762
(
1984) Org Lett 2014:16
28. Xiao Z-K, Yin H-Y, Lu J-M (2014) Inorg Chim Acta 423:106
29. Micksch M, Tenne M, Strassner T (2014) Organometallics
Final R indices (I>2σ(I))
3
3:3966
wR =0.1960
R1 =0.1169
2
3
0. Shen A, Ni C, Cao Y-C, Zhou H, Song G-H, Ye X-F (2014) Tet-
rahedron Lett 55:3278
R indices (all data)
wR =0.2196
3.64 and −1.16
2
31. Yuan D, Zhang Q, Liao S, Xiong W, Yuan L, Cai Q, Yang M, Li
X, Jiang Y, Liu Y, Li P, Xu Z, Sun P, Geng H (2015) Chin J Org
Chem 35:961
Largest diꢁ peak and hole (e·Å−3
)
3
3
3
3
3
2. Yang J, Li P, Zhang Y, Wang L (2014) J Organomet Chem 766:73
3. Yang J, Li P, Zhang Y, Wang L (2014) Dalton Trans 43:7166
4. Yang J, Wang L (2012) Dalton Trans 41:12031
5. Han Y, Huynh HV, Tan GK (2007) Organometallics 26:6447
6. Wang T, Liu L, Xu K, Xie H, Shen H, Zhao W-X (2016) RSC Adv
6:100690
and their catalytic applications in other reactions are in pro-
gress in our laboratory (Table 4).
Acknowledgements We are grateful to the National Natural Sciences
Foundation of China (nos. U1404205, 21572126 and 21202095), the
Program for Science & Technology Innovation Talents in Universities
of Henan Province (14HASTIT016), Innovation Scientists and Tech-
nicians Troop Construction Projects of Henan Province, and Key Sci-
entiꢂc and Technological Project of Henan Province (152102410056)
for ꢂnancial support.
37. Wang T, Xu K, Meng T, Zhang A, Wang H, Shen S, Liu L (2017)
Chin J Org Chem 37:1794
38. Wang T, Xie H, Liu L, Zhao W-X (2016) J Organomet Chem
804:73
39. Wang T, Xu K, Liu L, Xie H, Li Y, Zhao W-X (2016) Transit Met
Chem 41:525
4
0. Barnes JC, Jurícek M, Strutt NL, Frasconi M, Sampath S,
Giesener MA, McGrier PL, Bruns CJ, Stern CL, Sarjeant AA,
Stoddart JF (2013) J Am Chem Soc 135:183
4
4
4
1. Sheldrick GM (1997) SHELXS-97, program for crystal structure
solution. University of Göttingen, Göttingen
References
2. Sheldrick GM (1997) SHELXL-97, program for crystal structure
reꢂnement. University of Göttingen, Göttingen
1
2
.
.
Liu C, Liu G, Zhao H (1048) Chin J Chem 2016:34
Zhang G, Zhang W, Luan Y, Han X, Ding C (2015) Chin J Chem
3. Ouyang K, Xi Z (2013) Acta Chim Sin 71:13
3
3:705
1
3