Synthesis and characterization of a new bis-NHC palladium complex and its catalytic activity in the Mizoroki–Heck reaction

Article abstract of DOI:10.1177/1747519820917883

A new bis-N-heterocyclic carbene palladium complex, (C₁₃H₉N₂F₂)₂PdCl₂, is synthesized by a three-step reaction and characterized by ¹H NMR and ¹³C NMR spectroscopy as well as by X-ray crystallography. This new bis-N-heterocyclic carbene palladium complex has excellent stability and is capable of efficiently catalyzing the Mizoroki–Heck coupling reaction of aryl halides with acrylates.

Full text of DOI:10.1177/1747519820917883

9
research-arti
1cle2020 7883CHL0010.1177/1747519820917883Journal of Chemical ResearchFeng et al.
Research Paper
Journal of Chemical Research
–6
1
Synthesis and characterization of a new
bis-NHC palladium complex and its catalytic
activity in the Mizoroki–Heck reaction
© The Author(s) 2020
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journals.sagepub.com/home/chl
Can Feng1 , Cheng-xin Liu , Yu-fang Wang , Jin Cui
and Ming-jie Zhang
1
2
3
1
Abstract
A new bis-N-heterocyclic carbene palladium complex, (C H N F ) PdCl , is synthesized by a three-step reaction and
1
3
9
2
2 2
2
1
13
characterized by H NMR and C NMR spectroscopy as well as by X-ray crystallography. This new bis-N-heterocyclic
carbene palladium complex has excellent stability and is capable of efficiently catalyzing the Mizoroki–Heck coupling
reaction of aryl halides with acrylates.
Keywords
Mizoroki–Heck reaction, N-heterocyclic carbene, palladium complex
Date received: 9 January 2020; accepted: 14 March 2020
1
A new bis-NHC palladium complex, (C H N F ) PdCl , is synthesized by a three-step reaction and characterized by H
1
3
9
2
2 2
2
1
3
NMR and C NMR spectroscopy as well as by X-ray crystallography. This new bis-NHC palladium complex has excellent
stability and is capable of efficiently catalyzing the Mizoroki–Heck coupling reaction of aryl halides with acrylates.
Introduction
1Department of Chemistry, School of Sciences, Tianjin University,
In recent years, the catalytic cross-coupling reactions of
N-heterocyclic carbene (NHC) metal complexes have become
a popular research topic. Compared with traditional phos-
phine and nitrogen ligands, NHC ligands have the follow-
ing advantages: they are electron rich and strong electron
donors, they strongly coordinate to metals, they are relatively
Tianjin, P.R. China
2
Scientific Research Department, Shijiazhuang University of Applied
Technology, Shijiazhuang, P.R. China
National Foodstuff Inspection Center, Tianjin Product Quality
Inspection Technology Research Institute, Tianjin, P.R. China
1,2
3,4
3
Corresponding authors:
5–20
stable in synthesized metal complexes and easy to modify, Jin Cui, National Foodstuff Inspection Center, Tianjin Product Quality
21,22
and so on.
They are typically represented by an NHC-Pd Inspection Technology Research Institute, Tianjin 300384, P.R. China.
Email: tjzjchem@163.com
complex which exhibits excellent reactivity in catalytic reac-
tions, mainly due to the strong σ electron-donating properties
of the NHC, which makes the central Pd atom more suscepti-
ble to oxidative addition at the inactive bond.
Ming-jie Zhang, Department of Chemistry, School of Sciences, Tianjin
University, Tianjin 300350, P.R. China.
Email: mjzhangtju@163.com

2
Journal of Chemical Research 00(0)
There are many reports on the catalytic cross-coupling
2
3–27
F
F
reactions of NHC-Pd complexes.
In 1991, Arduengo
3
CH OH
2
1
+
et al. first synthesized a stable NHC complex and deter-
mined its molecular structure. Seven years later, Herrmann
synthesized a bidentate NHC-Pd catalyst that exhibited
NH
2
N
N
N
CHO
F
F
1
yield 90%
2
8
higher activity for brominated aromatic hydrocarbons.
His project was then developed to give a NHC-Pd catalyst
containing a bis-NHC that achieved the coupling of a chlo-
rinated aromatic hydrocarbon at room temperature in high
yield. In 2005, Singh et al. synthesized a monodentate
NHC-Pd complex by a simple method. This NHC-Pd com-
plex could effectively catalyze the Suzuki–Miyaura cross-
coupling reaction of highly hindered chlorinated aromatic
F
N
N
(
HCHO)n
EtOAc
F
F
PdCl
2
3
F
Cl
N
N
Pd
TMS-Cl
K CO
2
F
Cl
-
THF
Cl
N
N
2
yield 85%
F
2
9
hydrocarbons, benzyl chloride, and allyl chlorides.
3
3
0
yield 67%
Recently, Lin et al. synthesized a series of NHC-Pd com-
plexes that catalyze the Mizoroki–Heck reaction, which
was achieved with very high yields using 1mol% of the Scheme 1. Synthesis of complex 3.
3
1
catalyst. Zhong et al. synthesized a new NHC-Pd com-
plex that catalyzed the Suzuki–Miyaura reaction in an
aqueous solvent at room temperature, a good yield being
obtained by using 1mol% of their catalyst.
Among the NHC metal complexes that have been
reported so far, most structures are monoligands, while bis-
3
2,33
ligands are relatively rare.
A new NHC ligand contain-
ing a F atom, which is helpful in forming hydrogen bonds,
was designed and synthesized in this work. This new bis-
ligand complex, whose stability was greatly enhanced by
the presence of the hydrogen bond and the bis-ligand struc-
ture, was obtained after complexing with palladium. A sin-
gle crystal of the complex was left in the air at room
temperature and standard atmospheric pressure for 1month
without any resulting changing in its properties. The crystal
structure of the complex and its catalytic activity in Heck
coupling reactions were then studied.
Figure 1. X-ray single-crystal structure of complex 3.
Results and discussion
the angle of the Pd atom was at a right angle to the
N-heterocyclic carbenes.
Synthesis and crystal structure
The synthesis of the new (C H N F ) PdCl complex is
Complex 3 is further stabilized by hydrogen bonds. The
1
3
9
2
2 2
2
depicted in Scheme 1. Reacting 2 with PdCl and potassium F(3) atom formed hydrogen bonds with C(10) of another
2
carbonate in anhydrous tetrahydrofuran (THF) at 80°C molecular complex with a D. . .A distance of 2.6020 (25)
gave the desired palladium complex 3 as a yellow solid in (Å). The dimeric structure is shown in Figure 2. The F(4)
1
6
7% yield. Complex 3 was characterized by H NMR and atom of the NHC ligand formed a hydrogen bond with the
1
3
C NMR spectroscopy as well as by X-ray crystallography. C(15) atom in another molecular complex. The one-dimen-
Complex 3 is monoclinic, P-21/c space group. The crystal sional (1D) chain structure is shown in Figure 3 where the
structure of complex 3 is depicted in Figure 1. X-ray single- D. . .A distance is 2.7421 (25) (Å). Apart from these inter-
crystal and structural refinement data are shown in Table 1. actions, the F(1) atom forms a hydrogen bond with C(10) in
Selected bond lengths and bond angles are listed in Table 2. the same ligand and the F(2) atom forms a hydrogen bond
The crystal structure of complex 3 indicates the Pd(II) is with a molecule of CH Cl
2
2.
bound to the C atom in the NHC ligand and the Cl anion is
located on an isosceles trapezoid center. The Pd(1)-Cl(1)
distance of 2.3621(10) (Å) and the Pd(1)-Cl(2) distance of
Catalytic activity
2
.3644(10) (Å) are almost equal. The Pd(1)-C(7) distance The catalytic activity of the palladium complex 3 in the
of 1.974(3) (Å) and the Pd(1)-C(20) distance of 1.984(3) Mizoroki–Heck coupling reaction of aryl halides with
(
(
(
Å) are almost the same. The angle of C(7)-Pd(1)-Cl(1) acrylates was investigated. To optimize the reaction condi-
176.24 (9)°) is similar to that of C(20)-Pd(1)-Cl(2) (176.53 tions, different bases and different solvents were investi-
9)°) which is approximately 180°. The angle of C(7)- gated using 4-bromotoluene (1.3mmol) and methyl acrylate
Pd(1)-C(20) (89.42 (13)°) is close to C(7)-Pd(1)-Cl(2) (1.3mmol) as the substrates with 0.1mol% of complex 3 at
88.82 (3)°) which is approximately 90°. This shows that 140°C for 20h under a N atmosphere. The results are
(
2

Feng et al.
3
Table 1. Crystal data and structure refinement for complex 3.
Formula
C H Cl F N Pd
26 18 2 4 4
FW
T (K)
639.77
113.15
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P-21/c
13.905(3)
12.426(3)
16.428(3)
90
α (°)
β (°)
100.61(3)
90
2790.2(10)
4
1.720
1.101
1432.0
0.2×0.18×0.12
2.98–55.75
γ (°)
3
V (Å )
Z
−
3
ρ
(gcm )
calc
−
1
μ (mm )
F (000)
3
Crystal size (mm )
Θ range for data collection (°)
2
Figure 2. View of the hydrogen bonding of F(3) in two
molecules of complex 3.
Index ranges
–18⩽h⩽18,
–
16⩽k⩽16, –21⩽l⩽21
Reflections collected
32,887
Independent reflections
Data/restraints/parameters
Goodness of fit on F2
6638 [R =0.0560]
6638/12/362
1.083
int
Final R indexes (I⩾2σ(I))
R =0.0467, wR =0.1144
1
2
Final R indexes (all data)
Largest diff. peak/hole (eÅ )
R =0.0562, wR =0.1209
1.08/−1.23
1
2
−
3
Table 2. Selected bond lengths (Å) and bond angles (°).
Pd(1)-Cl(1)
Pd(1)-Cl(7)
2.3621(10) Pd(1)-Cl(2)
1.974(3) Pd(1)-Cl(20)
2.3644(10)
1.984(3)
Cl(1)-Pd(1)-Cl(2)
C(7)-Pd(1)-Cl(2)
C(20)-Pd(1)-Cl(1)
93.16(3) C(7)-Pd(1)-Cl(1) 176.24(9)
88.82(9) C(7)-Pd(1)-C(20) 89.42(13)
88.76(9) C(20)-Pd(1)-Cl(2) 176.53(9)
Figure 3. View of the hydrogen bond of F(4) in the chain
structure of complex 3.
Symmetry transformations used to generate equivalent atoms: (a)
x+2, −y, −z; (b) −x, −y+1, −z.
−
entries 4 and 5). Even after lowering the loading, the cata-
lyst was still quite effective for the reaction of the 4-meth-
yliodobenzene (Table 4, entry 6). For 4-nitrochlorobenzene,
much lower reactivity was observed (Table 4, entry 7) even
with a higher loading of catalyst 3.
presented in Table 3. When dimethyl sulfoxide (DMSO)
was used as the solvent, a low yield (10%) was obtained in
the presence of Et N (Table 3, entry 2). Other solvents such
3
We found that ethyl acrylate was a better substrate for
the reaction than methyl acrylate. Again the results showed
that the reaction was sensitive to the electronic properties
of the phenyl substituents. Much higher yields were
obtained when electron-withdrawing groups such as nitro
and formyl were present on the phenyl rings. In contrast,
lower yields were obtained when electron-donating groups
such as methyl and methoxyl were present on the phenyl
rings (see Table 5). All the products in Tables 3–5 had been
as dimethylacetamide (DMA) and dimethylformamide
(
(
DMF) gave better yields (25% and 35%, respectively)
Table 3, entries 1 and 3). Screening of the base showed
that Na CO was the best (Table 3, entries 4–7). Overall,
2
3
the highest yield (78%) was obtained using DMA as the
solvent and Na CO as the base (Table 3, entry 7).
2
3
The scope of this reaction was then investigated under
the optimal conditions and the results are summarized in
Tables 4 and 5. Using methyl acrylate as a fixed substrate,
various aryl halides were subjected to the coupling reac-
tion. When the substituent on the aryl group was electron-
withdrawing, such as in 1-bromo-4-nitrobenzene and
3
4–36
reported.
Conclusion
4-bromobenzaldehyde, high yields (93%, 92%) were
obtained (Table 4, entries 1 and 2). Electron-donating sub- A new bis-NHC palladium complex, (C H N F ) PdCl,
1
3
9
2 2 2
stituents, such as those in 4-methylbromobenzene and has been synthesized by a three-step reaction and character-
1
13
4
-methoxybromobenzene (Table 4, entries 4 and 5), led to ized by H NMR and C NMR spectroscopy as well as by
moderate yields (78%, 36%) being obtained (Table 3, X-ray crystallography. The bis-NHC palladium complex

4
Journal of Chemical Research 00(0)
Table 3. Optimization of the reaction conditions for the
coupling of 4-bromotoluene with phenylboronic catalyzed by
complex 3.a
Table 5. Heck reactions of aryl halides and ethyl acrylatea
catalyzed by complex 3.
3
mmol)
(
0.0013
COOEt
ArX
+
COOEt
Ar
o
1
40 C
3
mmol)
0.0013
(
+
4
6
COOMe
Br
1
40 oC, solvent
base
COOMe
Br
Time (h) Product Yield (%)b
Entry ArX
1
10
10
10
10
10
2
6a
6b
6c
6d
6e
6f
99
99
91
83
42
99
25
Entry
Base
Solvent
Time (h)
Yield (%)b
Br
NO2
1
2
3
4
5
6
7
Et N
DMF
20
20
20
20
10
20
10
35
10
25
40
45
65
78
3
2
Et N
DMSO
DMA
DMF
DMA
DMF
3
Br
CHO
Et N
3
K PO
3
4
3
Br
Br
Br
K PO
3
4
Na CO
2
3
4
Na CO
DMA
2
3
DMSO: dimethyl sulfoxide; DMA: dimethylacetamide; DMF:
dimethylformamide.
5
O
a
Reactions were carried out using 3 (1mg, 0.0013mmol),
4
-bromotoluene (222mg, 1.3mmol, 1.0equiv.), methyl acrylate (134mg,
6c
7d
1
.3mmol, 1.0equiv.), base (2.6mmol, 2.0equiv.), solvent (1mL), N2
I
atmosphere, 140°C.
b
Isolated yields.
10
6g
Cl
NO2
Table 4. Heck reactions of aryl halides and methyl acrylatea
catalyzed by complex 3.
DMA: dimethylacetamide.
a
3
mmol)
Reactions were carried out using 3 (0.0013mmol), aryl halide
(
0.0013
COOMe
+
ArX
COOMe
Ar
(1.3mmol, 1.0equiv.), ethyl acrylate (1.3mmol, 1.0equiv.), Na CO3
2
o
1
40 C
(2.6mmol, 2.0equiv.), DMA (1mL), N atmosphere, 140°C, 10h.
2
4
5
b
c
Isolated yields.
Reaction was carried out using 3 (0.00013mmol).
Reaction was carried out using 3 (0.0026mmol).
Entry ArX
Time (h) Product Yield (%)b
d
1
10
10
10
10
10
2
5a
5b
5c
5d
5e
5f
93
92
85
78
36
99
19
Br
NO2
with acrylates. According to the catalytic study, the com-
plex showed good catalytic activity in the Heck reaction.
2
Br
CHO
Experimental
1H NMR and 13C NMR spectra were collected on a Bruker
3
Br
Br
Br
Avance III 400 MHz spectrometer with CDCl or DMSO as
4
3
the solvent and tetramethylsilane (TMS) as the internal ref-
erence. Chemicals and reagents were obtained from com-
mercial sources and used directly.
5
O
6c
7d
Synthesis of compound 1 [(E)-N-(2,6-
difluorophenyl)-1-(pyridin-2-yl)methanimine]
I
10
5g
NO2
To a 100-mL three-neck round-bottom flask, pyridine-
Cl
2
-carbaldehyde (2.00g, 18.69mmol) and methanol (6mL)
3
7,38
were added under an N atmosphere.
Next, a mixed
2
DMA: dimethylacetamide.
solution of 2,6-difluoroaniline (2.40g, 18.60mmol) in
methanol (6mL) was added dropwise. The mixture was
stirred for 6h at room temperature. On cooling in an ice
bath, a solid was precipitated, and the solvent was removed
under reduced pressure to give product 1 as a bright yel-
a
Reactions were carried out using 3 (0.0013mmol), aryl halide
(1.3mmol, 1.0equiv.), methyl acrylate (1.3mmol, 1.0equiv.), Na CO3
2
(
2.6mmol, 2.0equiv.), DMA (1mL), N atmosphere, 140°C, 10h.
Isolated yields.
2
b
c
Reactions were carried out using 3 (0.00013mmol).
d
Reaction was carried out using 3 (0.0026mmol).
1
low solid, yield 90% (3.60g, 16.51mmol). H NMR
(
400MHz, CDCl ): δ=8.76 (s, 1H), 8.73 (d, J=4.6Hz,
3
has excellent stability and enriches the diversity of NHC 1H), 8.28 (d, J=8.0Hz, 1H), 7.84 (t, J=7.8Hz, 1H), 7.41
ligands and will aid further development of new NHC-Pd (t, J=6.2Hz, 1H), 7.08–7.12 (m, 1H), 6.99 (t, J=8.0Hz,
1
3
complexes. The catalytic activity of complex 3 was exam- 2H).
C NMR (101MHz, CDCl ): δ=167.29 (t,
3
ined in Mizoroki–Heck reactions of a variety of aryl halides JCF =3.4Hz), 155.14 (dd, JCF =250.4, 5.1Hz) 154.23,

Feng et al.
5
1
1
49.74, 136.70, 128.26, 125.95 (t, JCF =9.8Hz), 125.70, mini (600 W, SHINE, CCD, 75mm, 0.1 electrons/pixel/s)
21.89, 111.92 (dd, JCF =17.5, 6.9Hz).
X-ray single-crystal diffractometer. The structure was
solved and refined by direct methods using the SHELXS 97
3
9,40
program.
The non-hydrogen atoms were refined using
Synthesis of compound 2
2-(2,6-difluorophenyl)imidazo[1,5-a]
pyridin-2-ium chloride)
anisotropic thermal parameters. All the hydrogen atoms
were located at geometrically calculated positions.
(
Product 1 (1.4g, 6.42mmol), paraformaldehyde (183.01mg, Declaration of conflicting interests
6
.11mmol), and ethyl acetate (100mL) were added to a 250-
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
mL three-neck round-bottom flask under an N atmosphere.
2
After heating to reflux, trimethylchlorosilane (1mL) was article.
added to the flask slowly, and the healing was continued for
6
h. After cooling to room temperature and filtering, the Funding
solid was washed with ethyl acetate several times. Product 2
The author(s) disclosed receipt of the following financial support
for the research, authorship, and/or publication of this article: This
work was supported by the Natural Science Foundation of Tianjin
was obtained as a yellow solid after drying, yield 85%
1
(
5.01g, 18.79mmol). H NMR (400MHz, CDCl ): δ=11.78
3
(s, 1H), 9.75 (d, J=7.1Hz, 1H), 7.95 (s, 1H), 7.80 (d, (17JCQNJC05500), P. R. China.
J=9.4Hz, 1H), 7.63–7.67 (m, 1H), 7.28 (dt, J=26.8, 8.3Hz,
13
3
H), 7.13 (t, J=7.0Hz, 1H). C NMR (101MHz, CDCl3): ORCID iD
δ=158.76–153.30 (m), 134.11–132.59 (m), 130.33–130.07
m), 129.87 (d, JCF =2.0Hz), 126.25, 126.08, 118.15, 117.41,
13.51, 113.26 (dd, JCF =19.0, 3.8Hz).
Can Feng
https://orcid.org/0000-0002-9602-3068
(
1
Supplemental material
Supplemental material for this article is available online.
Synthesis of complex 3
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