Geometry constrained N-(5,6,7-trihydroquinolin-8-ylidene)arylaminopalladium dichloride complexes: Catalytic behavior toward methyl acrylate (MA), methyl acrylate-co-norbornene (MA-co-NB) polymerization and heck coupling

Article abstract of DOI:10.3390/molecules21121686

A new pair of plladium complexes (Pd4 and Pd5) ligated with constrained N-(5,6,7-trihydroquinolin-8-ylidene)arylamine ligands have been prepared and well characterized by ¹H-, ¹³C-NMR and FTIR spectroscopies as well as elemental analysis. The molecular structure of Pd4 and Pd5 in solid state have also been determined by X-ray diffraction, showing slightly distorted square planar geometry around the palladium metal center. All complexes Pd1-Pd5 are revealed highly efficient catalyst in methyl acrylate (MA) polymerization as well as methyl acrylate/norbornene (MA/NB) copolymerization. In the case of MA polymerization, as high as 98.4% conversion with high molecular weight up to 6282 kg·mol^-1 was achieved. Likewise, Pd3 complex has good capability to incorporate about 18% NB content into MA polymer chains. Furthermore, low catalyst loadings (0.002 mol %) of Pd4 or Pd5 are able to efficiently mediate the coupling of haloarenes with styrene affording up to 98% conversion.

Full text of DOI:10.3390/molecules21121686

molecules
Article
Geometry Constrained N-(5,6,7-Trihydroquinolin-8-
ylidene)arylaminopalladium Dichloride Complexes:
Catalytic Behavior toward Methyl Acrylate (MA),
Methyl Acrylate-co-Norbornene (MA-co-NB)
Polymerization and Heck Coupling
Yanning Zeng 1,2,*, Qaiser Mahmood 1,3, Tongling Liang 1 and Wen-Hua Sun 1,2,3,
*
1
Key Laboratory of Engineering Plastics, Beijing National Laboratory for Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China;
mqaiser@iccas.ac.cn (Q.M.); ltl@iccas.ac.cn (T.L.)
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, China
2
3
International School, University of Chinese Academy of Sciences, Beijing 100049, China
Correspondence: ynzeng@iccas.ac.cn (Y.Z.); whsun@iccas.ac.cn (W.-H.S.); Tel.: +86-10-6255-7955 (W.-H.S.)
*
Academic Editors: Diego A. Alonso and Isidro M. Pastor
Received: 11 November 2016; Accepted: 3 December 2016; Published: 7 December 2016
Abstract: A new pair of plladium complexes (Pd4 and Pd5) ligated with constrained N-(5,6,7-
1
trihydroquinolin-8-ylidene)arylamine ligands have been prepared and well characterized by H-,
13
C-NMR and FTIR spectroscopies as well as elemental analysis. The molecular structure of Pd4 and
Pd5 in solid state have also been determined by X-ray diffraction, showing slightly distorted square
planar geometry around the palladium metal center. All complexes Pd1 Pd5 are revealed highly
efficient catalyst in methyl acrylate (MA) polymerization as well as methyl acrylate/norbornene
–
(
MA/NB) copolymerization. In the case of MA polymerization, as high as 98.4% conversion with high
−
1
molecular weight up to 6282 kg
·
mol was achieved. Likewise, Pd3 complex has good capability
to incorporate about 18% NB content into MA polymer chains. Furthermore, low catalyst loadings
0.002 mol %) of Pd4 or Pd5 are able to efficiently mediate the coupling of haloarenes with styrene
(
affording up to 98% conversion.
Keywords: constrained ligand framework; palladium complexes; homopolymerization of MA;
MA/NB copolymerization; Heck coupling
1
. Introduction
The abundant chemistry of palladium-based complex catalysts has attracted considerable
attention, particularly in the field of C–C bond formation, C–H functionalization, hydrocarbon
oxidation, as well as radical insertion and addition reaction in polymerization, indicating that
palladium is one of the most catalytically versatile transition metals [1–3].
The Mizoroki–Heck reaction [4–11] using palladium catalyst has been extensively utilized in
the synthesis of natural products [12], pharmaceuticals, bioactive compounds [13], fine chemical
substances [14], as well as polymeric materials [15]. Besides mild reaction conditions, Mizoroki–Heck
reactions also tolerate various functional groups including amino [16], hydroxyl [17], aldehyde [18],
ketone [19], carboxylic acid [20], ester [21], cyano [22], and nitro groups [23] within both the aryl or
vinyl halides and alkenes. The extensive use have driven the price of palladium higher and higher,
making it is necessary to enhance the catalytic efficiency by designing simpler and general palladium
catalyst systems which can work under accessible reaction conditions.
Molecules 2016, 21, 1686; doi:10.3390/molecules21121686
www.mdpi.com/journal/molecules

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On the other hand, commercial copolymerization processes plays a key role in the production
of new materials, which is indispensable in all fields of human life [24 30]. For instance,
–
the copolymerization of polar monomers, particularly methyl acrylate (MA), with olefins confers
novel properties on the resulting polymers: improved thermal stability, superior etch resistance, good
adhesion due to the ester groups and a hydrophobic character derived from the C–C backbone [31–33].
Although recent progress in polymerization based on palladium catalysts allows the efficient
copolymerization of polar and nonpolar monomers, the catalyst activity and second monomer insertion
ratio remain quite low [34–37]. Subsequently, the design of suitable catalyst systems for efficient
copolymerization of polar and non-polar monomers viz. acrylate and norbornene (NB) is still a great
challenge [38–43].
In view of the aforementioned perspectives and targeting higher efficiency catalysts, many
newfangled ligand frameworks have been reported to provide subtle control over the palladium
driven catalysis of either coupling reactions or polymerization. For instance; Nozaki et al. developed
a highly efficient organic palladium catalyst framework for copolymerization of polar and nonpolar
monomers [44]. Similarly, Chen et al. reported highly robust palladium(II)
for copolymerization of -olefins with MA [45]. Besides, our group successfully established
highly efficient nickel complexes comprising constrained cycloalkyl-fused pyridine ligands, viz
N-(5,6,7-trihydroquinolin-8-ylidene)arylamine ligands [46] (Figure 1, ) and 9-arylimino-5,6,7,8-
tetrahydro-cyclohepta[b]pyridine [47] (Figure 1, ). Meanwhile, recently highly efficient palladium
α-diimine catalysts
α
A
B
complexes ligated with the same N-(5,6,7-trihydroquinolin-8-ylidene)arylamine ligands were reported
for Heck and Suzuki coupling reactions [48]. Owing to these encouraging result, we were interested in
extending the scope of N-(5,6,7-trihydroquinolin-8-ylidene)arylamine ligand-containing palladium
complexes (Figure 1,
C). In this research, we have prepared a pair of new palladium complexes
Pd4 and Pd5 along with recently reported Pd1
–
Pd3 complexes. Owing to the simplicity and ease of
preparation of the title palladium complexes, we have employed them in MA polymerization and
copolymerization of MA with NB under mild conditions. The resultant PMA has high molecular
−
1
weight, up to 6282 kg·mol , and gave 98.4% conversion. The incorporation of NB was improved
up to 18%. Additionally Pd4 and Pd5 were also used in the coupling of haloarenes with styrene
substrates. Full characterization of new complexes and structural features of PMA and PM-co-NB
polymer are reported.
Figure 1. Geometry-constrained iminopyridylmetal halide complexes.
2
. Results and Discussion
2
.1. Synthesis and Characterization of Palladium Complexes
In order to explore the catalytic potential of palladium complexes, a series of ligands L1
and their corresponding palladium complexes Pd1–Pd5 have been prepared. Complexes Pd1–
–
L5
Pd3
have already been reported as highly active catalysts for Heck and Suzuki coupling reactions [48].
On the basis of these encouraging results, we extended the scope of the reported Pd1 Pd3 complexes
–
and additionally synthesized the new compounds Pd4 and Pd5 for homopolymerization of MA and
copolymerization of MA and NB. Additionally Pd4 and Pd5 were also employed in Heck coupling
reactions. Ligands L4 and L5 and their complexes Pd4 (51%) and Pd5 (89%) have been prepared
according to the same procedure reported in our previous work [46] as shown in Scheme 1. The new
1
13
synthesized complexes were fully characterized by H-, C-NMR and FTIR spectroscopies as well as

Molecules 2016, 21, 1686
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elemental analysis. All the distinctive peaks for proton and carbon atoms in the H- and 13C-NMR are
1
consistent with the proposed structures of the palladium complexes. Moreover, the V stretching
C=N
−
1
vibration absorption underwent a blue shift at around 1577 cm in the FT-IR spectra of the palladium
complexes, which is consistent with the reported data [49 50]. The palladium complexes are highly
,
stable in the solid state as well as for several months in their solutions in haloalkanes (dichloromethane
and chloroform), dimethylacetamide (DMA), dimethylformamide (DMF), toluene and acetonitrile.
In addition, the molecular structures of both newly synthesized palladium complexes Pd4 and Pd5
have been determined by single crystal X-ray diffraction studies.
Scheme 1. Synthesis of palladium complexes Pd1–Pd5.
2
.2. Single Crystal X-ray Diffraction Study
The crystals of complexes Pd4 and Pd5 were grown by layering diethyl ether on their
dichloromethane solutions. Both complexes Pd4 and Pd5 displayed a similar molecular structure that
adopts a distorted square planar geometry around the palladium metal center and the square plane
is comprised of two nitrogen atoms from the ligand as forming a chelate ring with Pd metal center
and two chloride atoms. The structural features correspond to classical N,N-bidentate palladium
complexes [51–56]. The molecular structures of Pd4 and Pd5 are shown in Figure 2 and the selected
bond lengths and bond angles are listed in Table S1 (ESI). The N2–C8 bond length shows double
bonding features characteristic of imino groups: 1.292(3) Å in Pd4 and 1.287(3) Å in Pd5, whilst the
C8–C7 bond length characteristically revealed single bonding features: 1.494(3) Å (Pd4) and 1.497(3) Å
(
Pd5) contrary to the ligand which shows single as well as double bonding features [46]. The bite angle
◦
N(2)–Pd(1)–N(1): 80.31(7) for Pd4 and 80.37(7) for Pd5 was observed to be smaller than 90 which
is accompanied by opening of the other three angles attributed to the non-aromatic six membered
framework. In addition, the C2 atoms in the non-aromatic six membered framework fused with the
pyridine ring are constrained from planarity, possibly due to minimization of the angle ring strain.
The Nimine aryl ring is inclined nearly perpendicular with respect to the plane of the chelate ring
◦
N1–C9–C8–N2–Pd1 in Pd4 (87.98 ), while, slightly less twisting of the N
aryl ring is observed
imine
◦
in case of Pd5 (80.30 ). Both bond lengths and bond angles are in good agreement with previously
reported analogy palladium complexes [57].
(
a)
(b)
Figure 2. ORTEP diagrams of Pd4
(a
) and Pd5
(b
). Thermal ellipsoids set at the 30% probability level.
Hydrogen atoms are omitted for clarity.

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2
.3. Polymerization of MA and Copolymerization of MA/NB
To establish the optimal conditions for polymerization using a catalyst system composed of Pd3
and (C F ) BC H NH(CH ) , reaction temperature and run time were systematically investigated;
6
5 4
6
5
3 2
the results are summarized in Table 1. Generally, N PdCl type palladium complexes are sensitive
2
2
to the temperature and easily dissociate at elevated temperature, subsequently affecting the catalytic
activity of polymerization, hence homopolymerization of MA was conducted at different temperatures
to explore the best catalyst activity. It was observed that when the temperature was increased from
◦
◦
◦
8
0 C to 120 C, the highest conversion (95%) was obtained at 100 C. In contrast, in polymerization
◦
either at lower or higher temperatures than 100 C, Pd3 exhibited lower activities, illustrating that
partial deactivation started at elevated temperature. From the perspective of polymer microstructure,
the raising temperature also causes in increase of the molecular weight of the polymers, consistent
with the literature [58] and even higher than the corresponding polymerization data catalyzed by
other late transition metals viz. Fe(II) and Co(II)pyridyl bis(imine)/MAO α-diimine Ni(II)/MAO
systems [59]. Besides, the obtained PMAs show a broad bimodal molecular weight distribution with
◦
a high molecular weight tail at 100 C as shown in the GPC traces in Figure 3.
Table 1. The catalytic properties of Pd complexes for MA homopolymerization a.
◦
Conversion (%) b
Mw c (kg·mol−1
Mw/Mn c
Run
Pre-Catalyst
T ( C)
Time (h)
Yield (g)
)
1
2
3
4
5
6
7
8
9
Pd3
Pd3
Pd3
Pd3
Pd3
Pd3
Pd1
Pd2
Pd4
Pd5
Pd3
80
4
4
4
1
2
3
4
4
4
4
4
7.30
9.01
6.01
2.21
5.22
7.44
7.70
8.76
9.15
9.28
-
77.0
95.0
63.4
24.2
56.1
79.4
82.0
93.4
97.2
98.4
-
1498
2252
1091
1534
1734
1830
1936
1163
1596
6282
-
2.33
11.5
1.74
3.54
2.39
3.46
3.21
3.79
3.98
3.73
-
100
120
100
100
100
100
100
100
100
100
10
d
1
1
a
3 2
NH(CH )
; 10 mL MA. b Calculated
Conditions: 10.0
µ
mol of Pd complexes activated with 1.2 eq (C
6
F
5
)
4
BC
6
H
5
by H-NMR. Determined by GPC. d Polymerization in absence of light.
1
c
(
a)
(b)
Figure 3. GPC traces of PMA produced by complex Pd3 at different polymerization temperature (
a)
and by different complexes Pd1–Pd5 (b).
To check the efficiency of the catalyst regarding the reaction time, the homopolymerization of
MA has been performed for different time durations such as 1, 2, 3 and 4 h: conversion was increased
from 24.2 to 95.0% as the reaction duration was prolonged from 1 to 4 h (Runs 2, 4, 5 and 6, Table 1).

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Interestingly, the molecular weight of the polymers increased regularly with the prolonged reaction
−
1
time; a maximum molecular weight of 2252 kg
Table 1).
·
mol was observed after 4 h reaction run time (Run 2,
Under the optimized polymerization conditions, the catalytic behavior of additional complexes
Pd2 Pd4 and Pd5) was also examined. The order of catalytic efficiency observed was:
(Pd1
,
,
Pd5 > Pd4 > Pd3 > Pd2 > Pd1. By examining the data in Table 1, complexes Pd5 and Pd4 showed
improved conversion (Runs 9 and 10, Table 1) when compared with their analogous complexes Pd2
and Pd1, respectively (Runs 7 and 8 Table 1). Likewise Pd5 showed slightly higher conversion of MA
as compared to Pd4 and a similar correlation was observed by comparing the complexes Pd3 with Pd2
or Pd1. Contrary to Wu et al.’s observations [35,36], sterically hindered Pd5 gave improved conversion,
so it seemed a ligand effect was involved during the initial insertion of the MA monomer. Considering
the molecular weight of the obtained polymers (Figure 3), Pd5 gave the highest molecular weight, up to
−
1
6
282 kg
·
mol . However, Pd1, Pd2, Pd4 and Pd5 showed narrow polydispersity (M /M = 3.21–3.98)
w n
while Pd3 exhibited a broad polydispersity of 11.5.
In order to verify either a free radical or coordination mechanism, MA polymerization was
performed in the absence of ambient light under optimized reaction conditions (Run 11, Table 1).
Polymerization was totally hindered in the absence of ambient light, confirming the free radical
mechanism of our catalytic system. According to the mechanistic studies of Albéniz et al., free radicals
were produced by light induced-homolytic cleavage of the Pd–C bond after the incorporation of
one MA unit into the Pd–C F bond of 1 [60]. A similar mechanistic pathway was also proposed by
6
5
Ye et al. [35]. Although at this stage less experimental evidence is available, we reason that radicals
were similarly produced from the light induced-decomposition of the palladium complex intermediate
after initial incorporation of MA monomer into the palladium active species. Herein the palladium
active specie could be formed only after the addition of (C F ) BC H NH(CH ) initiator followed
6
5 4
6
5
3 2
by insertion of one MA monomer. In fact no polymerization was observed in the absence of initiator.
Furthermore, 13C-NMR measurements of representative samples (Run 2, Table 1) indicated that the
obtained PMAs are atactic. Similar results were also observed by Sen et al. and established a radical
mechanism for such type of PMAs [61,62].
Polyolefins functionalized with polar and non-polar monomers remain an area of great interest
for providing hybrid polymeric properties which afford useful polymeric materials in industry
as well as academia. However, it remains a great challenge to design suitable catalyst systems
for efficient copolymerization of MA with NB [34
,39–43,59]. In this essence, we employed the
Pd3/(C F ) BC H NH(CH ) catalytic system for copolymerization of MA with NB under the
6
5 4
6
5
3 2
aforementioned optimum conditions for homopolymerization of MA (Run 2, Table 1). Various feed
ratios of MA/NB (9:1–5:5) were used and results are tabulated in Table 2.
Table 2. Copolymerization of MA and NB by complexes Pd3 a.
Run a
MA/NBE Yield (g)
Conversion of MA (%) b
NB/MA b
Mw /kg·mol
c
−1
Mw/Mn c
1
2
3
4
5
1:0
9:1
7:3
5:5
0:1
9.01
7.38
5.21
3.49
-
95
68
49
30
-
-
8
16
18
-
2252
1699
202
152
-
1.74
2.69
5.74
4.59
-
a
1
at 100 C. b Calculated by
◦
General conditions: 10.0
µ
mol of Pd3 activated with 1.2 eq (C F ) BC H NH(CH )
6 5 4 6 5 3 2
H-NMR. c Determined by GPC.
It was observed that the incorporation of NB monomer gradually increased with an increase of
feed ratio and a maximum 18% conversion was observed at a 5:5 feed ratio. The homopolymerization
of NB did not give polymer using same Pd3/(C F ) BC H NH(CH ) catalytic system under similar
6
5 4
6
5
3 2
reaction conditions. Conversely, 18% NB was incorporated during copolymerization of MA with NB,
which assured the copolymerization was happened instead of simple physical mixing of both polymers.

Molecules 2016, 21, 1686
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Further confirmation of copolymer formation was obtained from the by H- and 13C-NMR spectrum
vide infra, Figures 4 and 5) [37 38]. As mentioned in Table 2, increasing the MA/NB feed ratio caused
a substantial decrease in molecular weight of the obtained MA/NB copolymer from 2252 kg
1
(
,
−
mol
1
·
−
1
to 152 kg
·
mol . The molecular structure of PMA and MA-co-NB polymer was characterized by
H- and C-NMR measurements. According to the H-NMR spectrum the methoxy peak appeared
1
13
1
at 3.67 ppm and the methine and methylene protons of acrylate units as well as norbornene units
appeared as broad resonances between 0.9–2.5 ppm illustrating the copolymerization was successful.
The methylene protons for the acrylate units appeared around 1.98 ppm and overlapped with the
methine protons from the norbornene as well as MA units. Furthermore, an intense methoxy group
peak showed the MA enriched polymer was formed. All the assigned peaks are in agreement with the
−
,63,64]. Moreover, as expected, no vibration for the C=C bond appeared at 1680–1620 cm
1
literature [61
in FT IR spectrum indicating the absence of ring opening polymerization of NB [31].
Figure 4. 1H-NMR spectra of polymer: PMA (
feed ratios of MA: NB = 9:1 (b), MA:NB = 7:3 (c), MA:NB = 5:5 (d).
a), MA-co-NB originated from copolymerization with
Figure 5. 13C-NMR spectra of polymer: PMA (
feed ratios of MA:NB = 9:1 (b), MA:NB = 7:3 (c), MA:NB = 5:5 (d).
a), MA-co-NB originated from copolymerization with
Similarly, for comparison 13C-NMR spectra of PMA and MA-co-NB copolymer with different
feed ratios (9/1, 7/3 and 5/5) are compiled together in Figure 5 and interpreted according to the
literature [65]. The peaks for the carbonyl (–C(O)OMe), methoxy (–OCH ), methine (–CH) and
3
methylene (–CH –) carbons of PMA appear at 175.0 ppm, 51.8 ppm, 41.8 ppm and 35.0 ppm,
2
respectively. On the other hand, in spectra (b) and (c), the methine and methylene carbon peaks
became slightly less intense as well as broader. A clearer difference was observed in spectrum (c),
where even the methylene carbon peaks of both monomer units emerge in the range of 34.0–44.1 ppm.
Furthermore, the methoxy (–OCH ), methine (–CH) and methylene (–CH –) signals in spectrum (b),
3
2
(c) and (d) were slightly shifted toward higher field as compared to spectrum (a), verifying that the

Molecules 2016, 21, 1686
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copolymerization had happened. A similar observation was reported by Wu and his co-workers [35,36]:
NB are highly non-polar units which act as inert diluent that would be responsible for slightly shifting
the chemical shift of MA segments to higher field. Such a shifting of peaks would not possible in
a physic mixture of both polymers because no specific interaction between MA and NB monomers
is possible. Moreover, some additional peaks was observed at 25.1 ppm, 30.2 ppm and 48.3 ppm
which was assigned to the MA and NB sequence and the appearance of the peak intensity indicated
a random sequence of co-monomers; such a sequence of MA and NB in polymer chains mostly
occurs by a radical polymerization mechanism in agreement with the literature [34,39–43,59]. Though,
a precise mechanism is unknown, like MA homopolymerization, MA free radicals are produced from
the palladium intermediates, followed by propagation of MA-co-NB polymer chain by reacting with
MA as well as NB monomers.
2
.4. Heck Coupling Reaction
Generally the catalytic efficiency for Heck coupling reaction is highly influenced by the reaction
conditions namely inorganic base, organic solvent, temperature and reaction time. In this context,
firstly, the reaction conditions were optimized. The optimized reaction conditions were investigated
by employing 0.002 mol% loading of Pd5 complex for Heck coupling of classical substrates;
p-bromo-toluene with 1.2 equivalents of styrene and explored the effect of base, temperature, solvent
and reaction duration. The data are compiled in Table 3.
Table 3. Optimization of reaction conditions using Pd1 as catalyst a.
◦
Conversion (%) b
ToF c
Run
Catalyst
Base
K CO3
T ( C)
Solvent
Time (h)
1
2
3
4
5
6
7
8
9
Pd5
Pd5
Pd5
Pd5
Pd5
Pd5
Pd5
Pd5
Pd5
Pd5
Pd5
Pd5
Pd5
150
150
150
150
150
150
100
60
DMA
DMA
DMA
DMA
DMA
DMF
8
8
8
8
8
8
8
8
8
8
2
4
12
08
08
08
93
72
74
82
70
92
-
-
-
-
86
88
98
93
22
41
5813
4500
4625
5125
4375
5750
-
-
-
-
21500
11000
4083
5813
28
2
Na CO3
2
NaHCO3
NaOAc
NaOH
K CO3
2
K CO3
Toluene
2
K CO3
CH CN
2
3
K CO3
100
60
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
2
1
1
1
1
1
0
1
2
3
4
K CO3
2
K CO3
150
150
150
150
150
150
2
K CO3
2
K CO3
2
Pd4
PdCl2
Pd(OAc)2
K CO3
2
d
d
1
1
5
6
K2CO3
K2CO3
50
a
−5
Conditions: 4
×
10 mmol Pd5 as catalyst, 2.0 mmol p-bromotoluene, 2.4 mmol styrene, 2.2 mmol base,
.0 mL solvent. GC yield. c ToF = product/(catalyst
b
×
time), d Conditions: 2
×
10 mmol Pd, 2.2 mmol
−3
4
Na2CO3 as a base [56].
On careful inspection of the data, the best conversion was found when the coupling was performed
◦
in dimethylacetamide at 150 C in the presence of K CO for 12 h, which resulted in 98% conversion
2
3
(
Run 13, Table 3). Other bases including Na CO , NaHCO , NaOAc and NaOH were also tested but
2 3 3
showed lower efficiency (Runs 2–5, Table 3). In considering the crucial rule of the solvent in Heck
coupling reactions, various polar and non-polar solvents were employed: the polar aprotic solvent
N,N-dimethylformamide (DMF) exhibited similar activity as DMA and the coupling product could be
obtained in 92% yield after 8 h reaction run time (Run 6, Table 3). An the other hand, due to lower

Molecules 2016, 21, 1686
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◦
◦
boiling points of the solvents toluene and acetonitrile, the reaction was conducted at 100 C and 60 C
respectively, but no conversion was observed (Runs 6–8, Table 3). In order to distinguish the effect
◦
of solvent or/and temperature, the coupling reaction was performed at lower temperature (60 C
◦
and 100 C) using the optimal solvent (DMA), and still no conversion was obtained, illustrating the
dynamic role of temperature in the coupling reactions (Runs 9 and 10, Table 3). On the inspection of
the reaction run time, surprisingly, slightly less conversion (93%) was observed after 8 h reaction run
time (Run 1, Table 3). Less reaction time, namely 2 and 4 h, gave less conversion (Runs 12 and 13,
Table 3), while 12 h reaction gave slightly more conversion, but considering good the ToF values we
used eight hour reaction time to explore the scope for further reaction substrates.
Using the optimum reaction conditions as established for Pd5, Pd4, PdCl and Pd(OAc) were
2 2
additionally employed to evaluate the catalytic efficiency for the coupling of p-bromotoluene and
styrene. According to the results as mentioned in Table 3, Pd4 like Pd5 exhibited excellent conversion
(
93%) and high ToF value (Run 14, Table 3). In contrast, PdCl and Pd(OAc) showed only 22% and
2 2
41% conversion, respectively, albeit at higher catalyst loading, illustrating the role of the ligand in the
catalytic efficiency (Runs 15 and 16, Table 3). Complexes Pd4 and Pd5 have additional methyl groups
on the para-position, which showed a detrimental effect that led to a slightly lower conversion rate as
compared to analogous complexes [48].
Furthermore, keeping the optimal conditions in mind, the scope of the catalytic system Pd5 was
further investigated in the coupling of styrene with a variety of haloarene substrates as listed in Table 4.
High catalytic efficiency was achieved in the case of bromobenzene (Run 1, Table 4), but the efficiency
abruptly dropped in the case of 2,4,6-trimethylbromobenzene (Run 2, Table 4), being attributed to the
steric crowding of both ortho-methyl substituents.
Table 4. Heck reaction of various aryl bromides and styrene using complex Pd5 a.
Yield (%) b
Conversion (%) c
ToF d
Run
Ar–X
1
Br
74
81
3905
2
Br
-
3.6
225
3
4
5
Br
Br
31 (15)
64
43 (21)
77
1938 (938)
4000
H
N
Br
2
MeO
Br
65
76
3847
Br
OH
Br
6
37
53
2312
OMe
7
8
-
-
6
8
375
500
I
N
a
−5
10 mmol Pd complex as the catalyst, 2.0 mmol p-bromotoluene, 2.4 mmol styrene, 2.2 mmol
Conditions: 4
×
base, 4.0 mL DMA, 150 C for 8 h. Isolated yield. c GC yield. d ToF = product/(catalyst × time).
◦
b

Molecules 2016, 21, 1686
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However, 1,4-dibromobenzene gave 43% conversion in the first coupling stage and but second
coupling of styrene seemed more difficult as it gave only 21% conversion caused of the high steric
hindrance (Run 3, Table 4). Meanwhile, introducing electron donating groups, namely –NH and –OMe
2
at the para-position of bromobenzene gave slightly less conversion as compared to bromobenzene: 77%
and 76%, respectively (Runs 4 and 5, Table 4). In comparison with the self-encapsulated Pd-diimine
catalyst reported by Ye et al., Pd5 is an efficient catalyst showing high values of ToF (4000 and
−
1
3
847 h ) [66]. In the case of 3-methoxybromobenzene, the catalytic efficiency was further decreased
up to 53% (Run 6, Table 4) consistent with the literature [67]. On comparison with our earlier
reported work [48], the current catalytic system exhibited similar efficiency viz electron withdrawing
groups positively enhanced the catalytic efficiency and electron donating groups showed contrasting
results. At the same time, the coupling reaction of highly inert 2-bromopyridine substrates were also
investigated with styrene (Runs 7 and 8, Table 4) but lower activity was exhibited, possibly due to
coordination of Npyridine with Pd metal center leading to lower efficiency. This indicated that it is
worthy to explore more efficient catalyst systems for the coupling of less reactive substrates, albeit
a plethora of highly efficient catalysts are available. The structures of the isolated products were
1
13
elucidated by H- and C-NMR spectroscopy and interpreted according to the literature [68–70].
3
. Experimental Section
3
.1. General Information
All the manipulations of air and/or moisture sensitive compounds were conducted under
a nitrogen atmosphere by standard Schlenk techniques. All the reagents were purchased from Aldrich
Beijing, China). NMR spectra were recorded on a DMX 400 MHz instrument (Bruker, Karlsruhe,
(
Germany) at ambient temperature with TMS as an internal standard. FTIR spectra were determined on
a System 2000 FTIR spectrometer (Perkin-Elmer, Shanghai, China). Elemental analysis was performed
by an HPMOD 1106 microanalyzer (Thermo Electron SPA, Beijing, China).
3
.2. Synthesis of Ligands L1–L5
Ligands L1 L5 have been prepared according to the procedure as reported in our previous
study [47].
–
3
.3. Synthesis of Palladium Complexes
A similar procedure was used as described for the synthesis of Pd1–Pd3 in our previous study [48].
A solution of (CH CN) PdCl (2 mmol) in CH Cl2 (20 mL) was added with the corresponding ligand
3
2
2
2
(
2 mmol) at room temperature. After overnight stirring, the color of the reaction mixture turned from
light yellow to dark yellow, then the complex was precipitated by adding an excess of diethyl ether,
filtered and washed with diethyl ether (3 5 mL). Finally, the precipitate was dried under vacuum
×
afforded a yellow microcrystalline powder in high yield.
N-(5,6,7-Trihydroquinolin-8-ylidene)-2,4,6-trimethylphenyliminopalladium(II) dichloride (Pd4): Yellow
microcrystalline solid, 51% yield. 1H-NMR (CDCl ): 9.33 (d, J = 8.0 Hz, 1H); 7.92 (d, J = 8.0 Hz
,
3
1
2
3
H); 7.66 (t, J = 6.0 Hz, 1H); 6.91 (s, 2H), 3.03 (t, J = 8.0 Hz, 2 H); 2.45 (t, J = 4.0 Hz, 2H); 1.99 (t, J = 6.0 Hz,
H); 1.67 (s, 9H). 13C-NMR (CDCl3):
1.7, 28.8, 28.1, 23.6, 21.8. FTIR (KBr, disk, cm ): 3074, 2965, 2906, 2876, 1604, 1577, 1453, 1376, 1288,
190, 1128, 1035, 956, 851, 789, 660. Anal. calcd for C H Cl N Pd (442): C, 48.95; H, 4.56; N, 6.34.
δ 178.8, 152.3, 149.5, 142.7, 140.4, 140.2, 140.1, 129.0, 128.5, 123.7,
−
1
1
18
20
2
2
Found: C, 49.35; H, 4.68; N, 6.36.
N-(5,6,7-Trihydroquinolin-8-ylidene)-2,6-diethyl-4-methylphenyliminopalladium(II) dichloride (Pd5): Yellow
1
microcrystalline solid, 89% yield: H-NMR (CDCl ): 9.35 (d, J = 4.0 Hz, 1H); 7.95 (d, J = 8.0 Hz, 1H);
3
7
.68 (t, J = 6.0 Hz, 1H); 6.69 (s, 1H), 3.03 (t, J = 6.0 Hz, 2 H); 2.84-2.74 (m, 2H); 2.52–2.43 (m, 4H); 1.79 (t,
J = 6.0 Hz, 2H), 1.29 (t, J = 6.0 Hz, 6H). 13C-NMR (CDCl3):
δ
178.4, 152.4, 150.0, 142.3, 140.4, 139.5, 137.9,

Molecules 2016, 21, 1686
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34.7, 129.1, 126.8, 31.3, 28.4, 24.6, 22.1, 21.6, 13.7. FTIR (KBr, disk, cm 1): 3073, 2967, 2928, 2871, 1604,
−
1
1
579, 1456, 1375, 1286, 1188, 1128, 934, 854, 791, 668. Anal. calcd for C H Cl N Pd (442): C, 51.14;
18
20
2
2
H, 5.15; N, 5.96. Found: C, 50.97; H, 5.11; N, 5.93.
3
.4. Homopolymerization of MA
Typical procedure (Run 2, Table 1): A 50 mL oven-dried Schlenk flask was charged with Pd3
complex (9.2 mg, 20 mol) and (C F ) BC H NH(CH ) (0.017 mg, 24 mol, 1.2 equiv.) under nitrogen,
µ
µ
6
5 4
6
5
3 2
then MA (8.6 g, 100 mmol) was injected into the reaction mixture by syringe. The reaction was stirred
◦
for 4 h at 80 C. After the required time of polymerization, methanol was added to quench the
◦
polymerization. After filtering the polymer was dried under vacuum in an oven for 12 h at 45 C.
3
.5. Copolymerization of MA and NB
Typical procedure: A 50 mL oven-dried Schlenk flask was charged with Pd3 complex (9.2 mg,
2
0 µmol) and (C F ) BC H NH(CH ) (0.017 mg, 24 µmol, 1.2 equiv.) under nitrogen. Then the
6 5 4 6 5 3 2
required molar ratio of MA and NB were added into the reaction mixture by syringe. After 4 h reaction
◦
time at 100 C, methanol was added to quench the polymerization. The polymer was obtained by
◦
filtration and dried under vacuum in an oven for 12 h at 45 C.
3
.6. Heck Reaction
In a typical procedure for Run 1 in Table 3, a 50 mL oven-dried Schlenk flask was charged
with p-bromotoluene (342 mg, 2.0 mmol), styrene (254 mg, 2.4 mmol), anhydrous K CO (304 mg,
2
3
2.2 mmol) and dimethylacetamide (DMA, 4.0 mL) under nitrogen. A 4 µmol Pd5 solution in DMA
solvent was added into reaction mixture using a syringe, and then the reaction tube was kept
◦
stirring at 150 C for 8 h. After cooling to room temperature, the reaction mixture was diluted
with water and EtOAc. The combined organic extract was washed with brine, dried over Na SO
2
4
and concentrated. The crude product was purified by flash chromatography on silica gel to afford the
desired product 1-methyl-4-styrylbenzene.
3
.7. X-ray Crystallographic Study
Single crystals of Pd4 and Pd5 suitable for X-ray diffraction analysis were obtained from their
dichloromethane solution using diethyl ether at room temperature. X-ray diffraction study were
conducted on a Saturn 724 + CCD diffractometer (Rigaku, Tokyo, Japan) with graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å) at 173(2) K. Cell parameters were obtained by global refinement of
the positions of all collected reflections. Intensities were corrected for Lorentz and polarization effects
and empirical absorption. The structures were solved by direct methods and refined by full-matrix
2
least squares on F . All hydrogen atoms were placed in calculated positions. Structure solution and
refinement were performed by using the SHELXL–97 package [71]. Details of the X-ray structure
determinations and refinements are provided in Table S2.
4
. Conclusions
Along with reported complexes Pd1
–
Pd3, a pair of new complexes Pd4 and Pd5 have been
1
13
prepared and fully characterized by H-NMR, C-NMR and FT-IR spectroscopies as well as elemental
analysis. The molecular structures of Pd4 and Pd5 in the solid state have also been determined by X-ray
diffraction, showing slightly distorted square planar geometry around the palladium metal center.
All complexes Pd1
–Pd5 displayed excellent catalytic activity, with as high as 98% conversion, for MA
−
1
polymerization. From the perspective of PMA microstructure, a high molecular weight (6282 kg
·
mol
)
was achieved. Moreover, Pd3 has good capability in copolymerization of MA with NB and exhibted
up to 18% incorporation. Additionally Pd4 and Pd5 was employed for the coupling of halorenes with
styrene and gave almost quantitative conversions. In summary, the reported complexes are easy to

Molecules 2016, 21, 1686
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prepare and are highly efficient catalysts for different applications. Further research on modification of
these palladium complexes by changing the spectator ligands is undergoing.
Supplementary Materials: CCDC 1507785 and 1507784 contain the supplementary crystallographic data for
Pd4 and Pd5. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Table S1: the selected bond lengths and bond angles of Pd4 and Pd5
Table S2: Crystal data and structure refinement for complexes Pd4 and Pd5. Supplementary materials can be
accessed at: http://www.mdpi.com/1420-3049/21/12/1686/s1.
.
Acknowledgments: This work was supported by National Natural Science Foundation of China (No. 51273202
and U1362204). QM’s doctoral studies were supported by a CAS-TWAS President’s Fellowship administered by
University of Chinese Academy of Sciences (UCAS) and the World Academy of Sciences (TWAS).
Author Contributions: W.-H.S. and Y.Z. were the principle investigators of the project and designed the
experiments. Y.Z. performed the experiments. T.L. interpreted the data obtained from the single crystal X-ray
diffraction. Q.M. and Y.Z. interpreted the results and wrote the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1
.
Sivaramakrishna, A.; Clayton, H.S.; Mogorosi, M.M.; Moss, J.R. Hydrocarbon (
palladium and platinum: Synthesis, reactivity and applications. Coord. Chem. Rev. 2010, 254, 2904–2932.
CrossRef]
π- and δ-) complexes of nickel,
[
2
.
Budagumpi, S.; Haque, R.A.; Salman, A.W. Stereochemical and structural characteristics of single- and
double-site Pd(II)–N-heterocyclic carbene complexes: Promising catalysts in organic syntheses ranging from
C–C coupling to olefin polymerizations. Coord. Chem. Rev. 2012, 256, 1787–1830. [CrossRef]
3
.
.
Mirica, L.M.; Khusnutdinova, J.R. Structure and electronic properties of Pd(III) complexes. Coord. Chem. Rev.
2
013, 257, 299–314. [CrossRef]
4
Alonso, F.; Beletskaya, I.P.; Yus, M. Non-conventional methodologies for transition-metal catalysed
carbon–carbon coupling: A critical overview. Part 1: The Heck reaction. Tetrahedron 2005, 61, 11771–11835.
[CrossRef]
5
6
7
8
9
1
1
1
1
.
.
.
.
.
Whitcombe, N.J.; Hii, K.K.; Gibson, S.E. Advances in the hecking chemistry of aryl bromides and chlorides.
Tetrahedron 2001, 57, 7449–7476. [CrossRef]
Beletskaya, I.P.; Cheprakov, A.V. The Heck Reaction as a Sharpening Stone of Palladium Catalysis. Chem. Rev.
2
000, 100, 3009–3066. [CrossRef] [PubMed]
Crisp, G.T. Variations on a theme-recent developments on the mechanism of the Heck reaction and their
implications for synthesis. Chem. Soc. Rev. 1998, 27, 427–436. [CrossRef]
Cabri, W.; Candiani, I. Recent developments and new Perspectives in the Heck Reaction. Acc. Chem. Res.
1
995, 28, 2–7. [CrossRef]
Mizoroki, T.; Mori, K.; Ozaki, A. Arylation of olefin with aryl iodide catalyzed by palladium. Bull. Chem.
Soc. Jpn. 1971, 44, 581. [CrossRef]
0. Dieck, H.A.; Heck, R.F. Organophosphinepalladium complexes as catalysts for vinylic hydrogen substitution
reactions. J. Am. Chem. Soc. 1974, 96, 1133–1136. [CrossRef]
1. Dieck, H.A.; Heck, R.F. A palladium-catalyzed conjugated diene synthesis from vinylic halides and olefinic
compounds. J. Org. Chem. 1975, 40, 1083–1090. [CrossRef]
2. Minto, R.E.; Blacklock, B.J. Biosynthesis and function of polyacetylenes and allied natural products.
Prog. Lipid Res. 2008, 47, 233–306. [CrossRef] [PubMed]
3. Goa, K.L.; Wagstaff, A. A review of its pharmacology, clinical efficacy and tolerability in the management of
hypertension. J. Drugs 1996, 51, 820–845. [CrossRef]
1
1
4. Vries, J.G. The heck reaction in the production of fine chemicals. Can. J. Chem. 2001, 79, 1086–1092. [CrossRef]
5. Sun, L.; Zou, Y.; Liang, Z.; Yu, J.; Xu, R. A one-pot synthetic strategy via tandem Suzuki–Heck reactions for
the construction of luminescent microporous organic polymers. Polym. Chem. 2014, 5, 471–478. [CrossRef]
6. Rafiee, E.; Ataei, A.; Nadri, S.; Joshaghani, M.; Eavani, S. Combination of palladium and oleic acid
coated-magnetite particles: Characterization and using in Heck coupling reaction with magnetic recyclability.
Inorg. Chim. Acta 2014, 409, 302–307. [CrossRef]
1

Molecules 2016, 21, 1686
12 of 14
1
1
7. Chaudhari, T.Y.; Hossian, A.; Manna, M.K.; Jana, R. Chemo-, regio-, and stereoselective Heck–Matsuda
arylation of allylic alcohols under mild conditions. Org. Biomol. Chem. 2015, 13, 4841–4845. [CrossRef]
PubMed]
8. Sabouncheia, S.J.; Ahmadia, M.; Panahimehra, M.; Bagherjeria, F.A.; Nasri, Z. Phosphine mono- and bis-ylide
palladacycles as homogeneous molecular precatalysts: Simple and efficient protocol greatly facilitate Suzuki
and Heck coupling reactions. J. Mol. Catal. A Chem. 2014, 383–384, 249–259. [CrossRef]
[
1
2
2
2
9. Hajipour, A.R.; Azizi, G. Immobilized Pd nanoparticles on Tris-modified SiO : Synthesis, characterization,
2
and catalytic activity in Heck cross-coupling reactions. Chin. J. Catal. 2014, 35, 1547–1554. [CrossRef]
0. Bachmann, D.G.; Wittwer, C.C.; Gillingham, D.G. Stereoselective Heck Reactions with Vinyl Sulfoxides,
Sulfides and Sulfones. Adv. Synth. Catal. 2013, 355, 3703–3707. [CrossRef]
1. Potier, J.; Menuel, S.; Rousseau, J.; Tumkevicius, S.; Hapiot, F.; Monflier, E. Multifunctional cyclodextrin-based
N,N-bidentate ligands for aqueous Heck arylation. Appl. Catal. A 2014, 479, 1–8. [CrossRef]
2. Quinteros, G.J.; Uberman, P.M.; Martín, S.E. Bulky monodentate biphenylarsine ligands: Synthesis and
evaluation of their structure effects in the palladium-catalyzed heck reaction. Eur. J. Org. Chem. 2015, 2015,
2
698–2705. [CrossRef]
2
3. Chou, C.; Yang, C.; Syu, H.; Kuo, T. Monomeric Pd(II) complexes with trans-chelated pyrazole ligands as
effective pre-catalysts for Heck cross-coupling reaction under mild aerobic conditions. J. Organomet. Chem.
2
013, 745–746, 387–392. [CrossRef]
2
4. Ye, J.; Ye, Z.; Zhu, S. Synthesis and characterization of hyperbranched polyethylenes containing cross-linking
structures by chain walking copolymerization of ethylene with diacrylate comonomer. Polymer 2008, 49,
3
382–3392. [CrossRef]
2
2
2
5. Dong, Z.; Ye, Z. Hyperbranched polyethylenes by chain walking polymerization: Synthesis, properties,
functionalization, and applications. Polym. Chem. 2012, 3, 286–301. [CrossRef]
6. Ye, Z.; Xu, L.; Dong, Z.; Xiang, P. Designing polyethylenes of complex chain architectures via Pd–diimine-
catalyzed “living” ethylene polymerization. Chem. Commun. 2013, 49, 6235–6255. [CrossRef] [PubMed]
7. Wang, J.; Zhang, K.; Ye, Z. One-Pot Synthesis of Hyperbranched Polyethylenes Tethered with Polymerizable
Methacryloyl Groups via Selective Ethylene Copolymerization with Heterobifunctional Comonomers by
Chain Walking Pd-Diimine Catalysis. Macromolecules 2008, 41, 2290–2293. [CrossRef]
2
2
3
8. Wang, J.; Ye, Z.; Joly, H. Synthesis and characterization of hyperbranched polyethylenes tethered with
polyhedral oligomeric sSilsesquioxane (POSS) nanoparticles by chain walking ethylene copolymerization
with acryloisobutyl-POSS. Macromolecules 2007, 40, 6150–6163. [CrossRef]
9. Morgan, S.; Ye, Z.; Zhang, K.; Subramanian, R. One-pot synthesis of hyperbranched polyethylenes tethered
with pendant acryloyl functionalities by chain walking copolymerizations. Macromol. Chem. Phys. 2008, 209,
2
232–2240. [CrossRef]
0. Zhang, K.; Wang, J.; Subramanian, R.; Ye, Z.; Lu, J.; Yu, Q. Chain walking ethylene copolymerization with
an ATRP inimer for one-Pot synthesis of hyperbranched polyethylenes tethered with ATRP initiating sites.
Macromol. Rapid Commun. 2007, 28, 2185–2191. [CrossRef]
3
3
3
3
1. Boffa, L.S.; Novak, B.M. Copolymerization of polar monomers with olefins using transition-metal complexes.
Chem. Rev. 2000, 100, 1479–1494. [CrossRef] [PubMed]
2. Gibson, V.C.; Sptzmesser, S.K. Advances in non-metallocene olefin polymerization catalysis. Chem. Rev.
2
003, 103, 283–316. [CrossRef] [PubMed]
3. Berkefeld, A.; Mecking, S. Coordination Copolymerization of Polar Vinyl Monomers H C=CH .
2
X
Angew. Chem. Int. Ed. 2008, 47, 2538–2542. [CrossRef] [PubMed]
4. Huang, C.F.; Wang, S.K.; Kuo, S.W.; Huang, W.J.; Chang, F.C. Homopolymerization and copolymerization of
tert-butyl methacrylate and norbornene with nickel-based methylaluminoxane catalysts. J. Appl. Polym. Sci.
2
004, 92, 1824–1833. [CrossRef]
3
3
3
5. Liu, S.; Elyashiv, S.; Sen, A. Copper-mediated controlled copolymerization of methyl acrylate with 1-alkenes
under mild conditions. J. Am. Chem. Soc. 2001, 123, 12738–12739. [CrossRef] [PubMed]
6. Wang, L.Y.; Li, Y.F.; Zhu, F.M.; Wu, Q. Copolymerization of norbornene and methyl acrylate by β-ketoiminato
Palladium complexes/MAO. Polym. Bull. 2006, 57, 73–81. [CrossRef]
7. Gao, H.; Liu, X.; Pei, L.; Wu, Q. Bis(b-ketoamino)copper(II)/methylaluminoxane systems for homo- and
copolymerizations of methyl acrylate and 1-Hexene. J. Polym. Sci. Part A Polym. Chem. 2010, 48, 1113–1121.
[CrossRef]

Molecules 2016, 21, 1686
13 of 14
3
3
8. Chen, B.; Fang, J.; Cong, Y.; Shao, D.; Hu, M.; Wang, J.; Yang, J.; Gao, H. Copolymerization of norborene and
methyl acrylate catalyzed by Nd(naph) -Al(i-Bu) . Polym. Bull. 2011, 67, 975–983. [CrossRef]
9. Gu, B.; Liu, S.; Leber, J.D.; Sen, A. Nitroxide-mediated copolymerization of methyl acrylate with 1-alkenes
and norbornenes. Macromoleculars 2004, 37, 5142–5144. [CrossRef]
3
3
4
4
4
0. Morris, J.R. Substituted Norbornene Compounds. U.S. Patent 3,536,681, 27 October 1970.
1. Goodall, B.L.; McIntosh, H.L., III. Polycyclic Copolymer Compositions. U.S. Patent 6,303,724, 16 October 2001.
2. Hennis, A.D.; Sen, A. Palladium(II) Catalyzed Copolymerization of Norbornene with Polar Vinyl Monomers.
Am. Chem. Soc. Polym. Prepr. 2000, 41, 1383.
4
4
4
3. Hennis, A.D.; Hilt, D.C.; Kacker, S.; Sen, A. Palladium(II) Catalyzed Copolymerization of Norbornene and
Methyl Acrylate. Am. Chem. Soc. Polym. Prepr. 1998, 39, 412.
4. Nakano, R.; Nozaki, K. Copolymerization of propylene and polar monomers using Pd/IzQO catalysts. J. Am.
Chem. Soc. 2015, 137, 10934–10937. [CrossRef] [PubMed]
5. Dai, S.; Sui, X.; Chen, C. Highly robust palladium(II) a-diimine catalysts for slow-chain walking
polymerization of ethylene and copolymerization with methyl acrylate. Angew. Chem. Int. Ed. 2015
4, 9948–9953. [CrossRef] [PubMed]
,
5
4
4
4
6. Yu, J.; Zeng, Y.; Huang, W.; Hao, X.; Sun, W.-H. N-(5,6,7-trihydroquinolin-8-ylidene)arylaminonickel
dichlorides as highly active single-site pro-catalysts in ethylene polymerization. Dalton Trans. 2011, 40,
8
436–8443. [CrossRef] [PubMed]
7. Huang, F.; Sun, Z.; Du, S.; Yue, E.; Ba, J.; Hu, X.; Liang, T.; Galland, G.B.; Sun, W.-H. Ring-tension
adjusted ethylene polymerization by aryliminocycloheptapyridyl nickel complexes. Dalton Trans. 2015, 44,
1
4281–14292. [CrossRef] [PubMed]
8. Tang, Y.; Zeng, Y.; Hu, Q.; Huang, F.; Jin, L.; Mo, W.; Sun, N.; Hu, B.; Shen, Z.; Hu, X.; et al. Efficient catalyst
for both suzuki and heck cross-coupling reactions: Synthesis and catalytic behaviour of ceometry constrained
iminopyridylpalladium chlorides. Adv. Synth. Catal. 2016, 358, 2642–2651. [CrossRef]
4
9. Lavery, A.; Nelson, S.M. Dinuclear intermediates in the oxidation of pendant olefinic groups of
palladium(II)—co-ordinated schiff-base ligands. Dalton Trans. 1984, 4, 615–620. [CrossRef]
5
0. Chen, R.; Bacsa, J.; Mapolie, S.F. {N-alkyl-N-[pyridin-2-ylmethylene] amine}dichloro palladium(II) complexes:
Synthesis, crystal structures and evaluation of their catalytic activities for ethylene polymerization. Polyhedron
2
003, 22, 2855–2861. [CrossRef]
5
5
5
1. Mahdavi, H.; Zirakzadeh, A.; Amani, J. Modified cross-linked polyacrylamide supported palladium salts as
a new heterogeneous catalyst for Heck reaction. React. Funct. Polym. 2007, 67, 716–722. [CrossRef]
2. Chen, W.; Xi, C.; Yang, K. 2-Iminopyridylpalladium dichloride as highly active catalyst for the Heck reaction.
Appl. Organomet. Chem. 2007, 21, 641–644. [CrossRef]
3. Ban, Q.; Zhang, J.; Liang, T.; Redshaw, C.; Sun, W.-H. 2,6-Dibenzhydryl-N-(2-aryliminoacenaphthylenylidene)-
4-chlorobenzenaminopalladium dichlorides: Synthesis, characterization, and use as catalysts in the
Heck-reaction. J. Organomet. Chem. 2012, 713, 151–156. [CrossRef]
5
5
5
4. Ye, B.; Wang, L.; Hu, X.; Redshaw, C.; Sun, W.-H. 2-(1-Aryliminoethyl)cycloheptapyridylpalladium
complexes: Synthesis, characterization and the use in the Heck-reaction. Inorg. Chim. Acta 2013, 407,
2
81–288. [CrossRef]
5. Kong, S.; Liu, Q.; Sun, W.-H.; Redshaw, C. Dichloropalladium complexes ligated by 4,5-bis(arylimino)
pyrenylidenes: Synthesis, characterization, and catalytic behavior towards Heck-reaction. J. Organomet.
Chem. 2014, 751, 453–457.
6. Zhao, T.; Xing, Q.; Song, K.; Ban, Q.; Liang, T.; Liu, Q.; Sun, W.-H. Enhancing catalytic activity towards
Heck-reaction by using 4,5,9,10-tetra(arylimino)-pyrenylidenyldipalladium tetrachlorides. RSC Adv. 2015, 5,
1
4228–14234. [CrossRef]
5
5
5
7. Laine, T.V.; Klinga, M.; Leskelä, M. Synthesis and X-ray Structures of New mononuclear and dinuclear
diimine complexes of late transition metals. Eur. J. Inorg. Chem. 1999, 6, 959–964. [CrossRef]
8. Nagel, M.; Paxton, W.F.; Sen, A.; Zakharov, L.; Rheingold, A.L. Metal-mediated polymerization of acrylates:
Relevance of radical traps? Macromolecules 2004, 37, 9305–9307. [CrossRef]
9. Elyashiv, S.; Greinert, N.; Sen, A. Copolymerization of methyl acrylate with Nnorbornene derivatives by
atom transfer radical polymerization. Macromolecules 2002, 35, 7521–7526. [CrossRef]

Molecules 2016, 21, 1686
14 of 14
6
0. Lopez-Fernandez, R.; Carrera, N.; Albeniz, A.C.; Espinet, P. Dual behavior of cationic palladium
pentafluorophenyl complexes as catalysts for the homopolymerization of acrylates and of nonpolar olefins.
Organometallics 2009, 28, 4996–5001. [CrossRef]
6
6
6
6
1. Xiang, P.; Ye, Z. Homo- and Co-polymerization of norbornene and methyl acrylate with Pd-diimine catalysts.
J. Organomet. Chem. 2015, 798, 429–436. [CrossRef]
2. Tian, G.; Novak, B.M. Neutral palladium complexes as catalysts for olefin–methyl acrylate copolymerization:
A cautionary tale. Macromolecules 2001, 34, 7656–7663. [CrossRef]
3. Matsuzaki, K.; Uryu, T.; Ishida, A. Stereoregularity of poly(methy1 acrylate) stereoregularity of poly(methy1
Acrylate). J. Polym. Sci. Part A Polym. Chem. 1967, 5, 2167–2177. [CrossRef]
4. Uryu, T.; Shiroki, H.; Okada, M.; Hosonuma, K.; Matsuzaki, K. Stereoregularity of radically polymerized
poly(alkyl acrylates) and poly(trimethylsilyl acrylates) and the preparation of syndiotactic poly(methyl
acrylate). J. Polym. Sci. Part A Polym. Chem. 1971, 9, 2335–2342. [CrossRef]
5. Kawamura, T.; Toshima, N.; Matsuzaki, K. Assignment of finely resolved l3C-NMR spectra of polyacrylates.
Macromol. Chem. Phys. 1995, 196, 3415–3424. [CrossRef]
6
6
6. Dong, Z.; Ye, Z. Heterogeneous palladium catalyst constructed with cross-linked hyperbranched
poly(phenylacetylene) as polymer support: A reusable highly active ppm-level catalyst for multiple
cross-coupling reactions. Appl. Catal. A Gen. 2015, 489, 61–71. [CrossRef]
6
7. Mahmood, Q.; Yue, E.; Zhang, W.; Solan, G.A.; Liang, T.; Sun, W.-H. Bisimino-functionalized
dibenzo[a,c]acridines as highly conjugated pincer frameworks for palladium(II): Synthesis, characterization
and catalytic performance in Heck coupling. Org. Chem. Front. 2016, 3, 1668–1679. [CrossRef]
8. Wang, Z.; Ding, Q.; He, X.; Wu, J. Palladium-catalyzed decarboxylative cross-coupling reaction of cinnamic
acid with aryl iodide. Org. Biomol. Chem. 2009, 7, 863–865. [CrossRef] [PubMed]
9. Sawant, D.; Wagh, Y.; Bhatte, K.; Panda, A.; Bhanage, B. Palladium polyether diphosphinite complex
anchored in polyethylene glycol as an efficient homogeneous recyclable catalyst for the Heck reactions.
Tetrahedron Lett. 2011, 52, 2390–2393. [CrossRef]
6
6
7
0. Kantam, M.L.; Reddy, P.V.; Srinivas, P.; Bhargava, S. Ligand and base-free Heck reaction with heteroaryl
halides. Tetrahedron Lett. 2011, 52, 4490–4493. [CrossRef]
7
1. Sheldrick, G.M. SHELXTL-97, Program for the Refinement of Crystal Structures; University of Göttingen:
Göttingen, Germany, 1997.
Sample Availability: Samples of the Pd1–Pd5 complexes and polymers are available from the authors.
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(