Synthesis, Structures, and Norbornene Polymerization Behavior of Imidazo[1,5- a]pyridine-sulfonate-Ligated Palladacycles

Article abstract of DOI:10.1021/acs.organomet.9b00495

Two imidazo[1,5-a]pyridine-sulfonate proligands, L1 and L2, were synthesized in five-step reactions. Treatment of the proligands with palladacycles {[Pd(OAc)(8-Me-quin-H)]₂, [Pd(dmba)(μ-Cl)]₂, and [Pd(o-acetanilido)(μ-trifluoroacetato)]₂} yielded the desired five-membered C(sp³),N-chelated (Pd1, Pd2), C(sp²),N-chelated (Pd3, Pd4), and six-membered C(sp²),O-chelated (Pd5, Pd6) palladacycles, respectively. All these complexes were fully characterized by ¹H and ¹³C NMR, IR, high-resolution mass spectrometry, and elemental analysis. The molecular structures of complexes Pd1, Pd2, Pd4, and Pd5 were determined by single-crystal X-ray diffraction analysis. In the presence of MAO or Et₂AlCl, Pd1-Pd6 exhibited activities toward the addition polymerization of norbornene which decreased in the order Pd6 > Pd5 > Pd4 > Pd2 > Pd3 > Pd1. The Pd1-Pd6/MAO catalytic system showed high thermal stability and reached the highest activity at 100 °C (6.0 × 10⁷ g of PNB (mol of Pd)^-1 h^-1 with 99.9% conversion). In the presence of Et₂AlCl with low loading (100 equiv), Pd5 and Pd6 exhibited high activities (up to 2.9 × 10⁷ g of PNB (mol of Pd)^-1 h^-1 with 96.5% conversion). It was demonstrated that the structures of palladacycles and the substituents on the ligands significantly affected the activities of these complexes.

Full text of DOI:10.1021/acs.organomet.9b00495

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis, Structures, and Norbornene Polymerization Behavior of
Imidazo[1,5‑a]pyridine-sulfonate-Ligated Palladacycles
†
†
,†,‡
Jie Dong, Minliang Li, and Baiquan Wang*
†
State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, PR China
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
‡
Shanghai 200032, PR China
*
S Supporting Information
ABSTRACT: Two imidazo[1,5-a]pyridine-sulfonate proli-
gands, L1 and L2, were synthesized in five-step reactions.
Treatment of the proligands with palladacycles {[Pd(OAc)(8-
Me-quin-H)] , [Pd(dmba)(μ-Cl)] , and [Pd(o-acetanilido)-
2
2
(
μ-trifluoroacetato)] } yielded the desired five-membered
2
3
2
C(sp ),N-chelated (Pd1, Pd2), C(sp ),N-chelated (Pd3,
Pd4), and six-membered C(sp ),O-chelated (Pd5, Pd6)
2
palladacycles, respectively. All these complexes were fully
1
13
characterized by H and C NMR, IR, high-resolution mass spectrometry, and elemental analysis. The molecular structures of
complexes Pd1, Pd2, Pd4, and Pd5 were determined by single-crystal X-ray diffraction analysis. In the presence of MAO or
Et AlCl, Pd1−Pd6 exhibited activities toward the addition polymerization of norbornene which decreased in the order Pd6 >
2
Pd5 > Pd4 > Pd2 > Pd3 > Pd1. The Pd1−Pd6/MAO catalytic system showed high thermal stability and reached the highest
7
−1 −1
activity at 100 °C (6.0 × 10 g of PNB (mol of Pd)
h
with 99.9% conversion). In the presence of Et AlCl with low loading
2
7
−1 −1
(
100 equiv), Pd5 and Pd6 exhibited high activities (up to 2.9 × 10 g of PNB (mol of Pd)
demonstrated that the structures of palladacycles and the substituents on the ligands significantly affected the activities of these
h
with 96.5% conversion). It was
complexes.
3
INTRODUCTION
olefin polymerization. {PO}Pd(R)(L) species (L = labile
ligand) can polymerize ethylene to linear polyethylene and
copolymerize ethylene with several unprecedented types of
polar monomers including acrylates, vinyl acetate, acrylonitrile,
■
The vinyl addition polymer of norbornene (PNB) is an unique
material, owing to its high chemical resistance, good UV
resistance, low dielectric constant, high glass transition
temperature, excellent transparency, large refractive index,
3
,4
and carbon monoxide to functionalized linear polymers. The
outstanding behavior of {PO}Pd species results from their
electronic asymmetry that may inhibit β-H and β-X
eliminations, thus enabling the generation of linear polymers.
The cis arrangement of the phosphine ligand, a strong donor,
and the sulfonate ligand, a very weak donor, plays an important
1
and low birefringence. Therefore, PNB has been applied to
the microelectronic devices and LCD protective coating and
has been used as a high-temperature-resistant protective
material, which has attractive business prospects.
Palladium(II) complexes bearing phosphine-sulfonate li-
gands ({PO}, Chart 1, I) exhibit unique characteristics in
2
4
role here.
Since first reported by Arduengo and co-workers in 1991, N-
heterocyclic carbenes (NHCs) have been widely employed in
Chart 1. Phosphine-sulfonate and NHC-sulfonate
Complexes
5,6
catalysis due to their distinctive capabilities.
However,
successful applications of NHC-derived transition metal
complexes for olefin polymerizations are rather rare so
7
,8c−h
far.
As donors stronger than typical phosphines, NHCs
can be used as substitutes for the phosphines. The chelating
NHC-sulfonate ligands may possess enhanced electronic
asymmetry and lower metal electrophilicity and thus are
potential candidates as ancillary ligands for late transition-
metal olefin polymerization catalysts. In 2009, Nozaki et al.
reported two methyl palladium complexes in which the NHC
and sulfonate units were linked by a methylene spacer (Chart
Received: July 23, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00495
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
8
a
1
, II), but their catalytic properties for olefin polymerization
Scheme 1. Synthesis of Imidazo[1,5-a]pyridine-sulfonate
has not been investigated. Similar seven-membered chelating
palladium(II) complexes containing saturated or unsaturated
ortho-NHC-arenesulfonate (NHC-sulfonate) ligands were
Proligands L1 and L2
8b
8c,d
synthesized by Jordan’s group and our group, respectively
Chart 1, III). Neither saturated nor unsaturated NHC-
(
sulfonate Pd(II) complexes showed activity for ethylene
polymerization. Nevertheless, the unsaturated NHC-sulfonate
Pd(II) complexes had been demonstrated active in norbornene
polymerization in the presence of methylaluminoxane
8
c,d
8e−h
(
MAO).
Recently, Nozaki et al.
reported a series of
Ni(II) and Pd(II) complexes bearing six-membered chelating
imidazo[1,5-a]quinolin-9-olate-1-ylidene (IzQO) ligands
(Chart 1, IV), which showed high activities for ethylene
polymerization and copolymerization with some polar
monomers. As one of the few successful demonstrations of
NHC-based ligands for olefin polymerization, the Pd/IzQO
system highlighted the significance of the rigid skeleton.
3
Synthesis of Palladacycles. C(sp ),N-chelated pallada-
2
cycles Pd1 and Pd2, C(sp ),N-chelated palladacycles Pd3 and
Pd4, and C(sp ),O-chelated palladacycles Pd5 and Pd6 were
synthesized by the reactions of proligands L1 and L2 with
Pd(OAc)(8-Me-quin-H)]2 (8-Me-quin-H = 8-CH -quino-
line), [Pd(dmba)(μ-Cl)]2 (dmba = Me NCH C H ), and
Pd(o-acetanilido)(μ-trifluoroacetato)]2 (acetanilido =
C H NHCOCH ), respectively (Scheme 2). All of these
2
8
Stimulated by these strategies, we speculated that an
imidazo[1,5-a]pyridine-sulfonate ligand would serve as a
promising ligand, owing to its electronic asymmetry and six-
membered chelating rigid backbone.
[
2
2
2
6
4
[
Imidazo[1,5-a]pyridines have attracted extensive attention
9
due to their bicyclic nature. The electronic and steric effects
6
4
3
can be well tuned by modifiable substituents on the nitrogen
atom. Thus far, the transition-metal complexes containing the
imidazo[1,5-a]pyridine ligand have been widely studied and
Scheme 2. Synthesis of Imidazo[1,5-a]pyridine-sulfonate
Palladacycles Pd1−Pd6
10
applied to various catalytic reactions, but few examples for
catalytic olefin polymerization. To date, the imidazo[1,5-
a]pyridine-sulfonate ligands and their transition metal
complexes have not been reported.
11
Since the first synthesis by Cope and Siekman in 1965,
palladacycles have become an important class of catalysts on
account of their versatile frameworks, extraordinary catalytic
activity, synthetically easy accessibility, extra stability toward
12
air and moisture, and relatively low toxicity. Despite a wide
variety of heteroatom-derived palladacycles being synthesized
and utilized in catalytic hydrogenation reactions, coupling
1
3
reactions, the Aldol reaction, and other organic reactions,1
4
reports on palladacycles with NHCs are rather limited.
Recently, we reported that aryloxide-NHC-ligated pallada-
cycles exhibited high catalytic activities in the vinyl polymer-
8c,15f−h
ization of norbornene.
Inspired by the above research, we believe the imidazo[1,5-
a]pyridine-sulfonate-ligated palladacycles, combining the elec-
tronic effects of the ligands and the stability of the
palladacycles, should be regarded as polymerization catalysts.
Herein, we report the synthesis and structures of a series of
C(sp ),N-chelated, C(sp ),N-chelated, and C(sp ),O-chelated
imidazo[1,5-a]pyridine-sulfonate palladacycles and their per-
formances in the vinyl polymerization of norbornene upon
palladacycles are stable in air and moisture. They have been
1
13
fully characterized by H NMR, C NMR, IR, HRMS, and
1
elemental analysis. In their H NMR spectra, the signals of the
3
2
2
imidazole proton at the C-2 position for the proligands
completely disappeared. The signals of the carbene carbons in
1
3
C NMR spectra supported their structures (158.0 ppm for
activation with either diethylaluminum chloride (Et AlCl) or
2
Pd1; 157.4 ppm for Pd2; 161.1 ppm for Pd3; 158.4 ppm for
Pd4; 154.8 ppm for Pd5; 154.9 ppm for Pd6).
methylaluminoxane (MAO).
The molecular structures of Pd1, Pd2, Pd4, and Pd5 were
determined by single-crystal X-ray diffraction analysis. As
shown in Figures 1−4, they all possess spiro structures with
palladium as the spiroatom. Complexes Pd1, Pd2, and Pd4
each contain a six-membered {C−O}Pd chelate ring and a
five-membered C,N-chelated palladacycle, while complex Pd5
consists of two six-membered chelate rings. The Pd−N bond
RESULTS AND DISCUSSION
Synthesis of Proligands. Proligands L1 and L2 were
synthesized in five-step reactions from commercially available
■
2
,6-dibromopyridine (Scheme 1). Both the proligands were
1
13
characterized by H NMR, C NMR, IR, and high-resolution
1
mass spectrometry (HRMS). Their H NMR spectra exhibited
3
singlets at 9−10 ppm characteristic for the imidazole,
lengths in C(sp ),N-chelated palladacycles (2.0655(14) Å for
consistent with those reported in the literature for similar
Pd1, 2.058(2) Å for Pd2) are shorter than the sum of the
1
5
16
imidazole salts.
covalent radii of the Pd and N atoms (2.10 Å). However, the
B
DOI: 10.1021/acs.organomet.9b00495
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 3. ORTEP diagram of Pd4. Thermal ellipsoids are shown at
the 60% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angle (deg): Pd(1)−C(18)
Figure 1. ORTEP diagram of Pd1. Thermal ellipsoids are shown at
the 60% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angle (deg): Pd(1)−C(17)
1
.9754(10), Pd(1)−C(1) 2.0067(9), Pd(1)−N(3) 2.1149(9),
Pd(1)−O(1) 2.1788(8), C(18)−Pd(1)−C(1) 97.91(4), C(18)−
Pd(1)−N(3) 81.71(4), C(1)−Pd(1)−N(3) 168.91(3), C(18)−
Pd(1)−O(1) 171.83(3), C(1)−Pd(1)−O(1) 88.84(3), N(3)−
Pd(1)−O(1) 92.54(3).
2
.0109(16), Pd(1)−C(1) 2.0238(17), Pd(1)−N(1) 2.0655(14),
Pd(1)−O(1) 2.1773(13), C(17)−Pd(1)−C(1) 97.10(6), C(17)−
Pd(1)−N(1) 178.97(6), C(1)−Pd(1)−N(1) 83.02(6), C(17)−
Pd(1)−O(1) 88.35(5), C(1)−Pd(1)−O(1) 174.44(5), N(1)−
Pd(1)−O(1) 91.51(5).
Figure 4. ORTEP diagram of Pd5. Thermal ellipsoids are shown at
the 30% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angle (deg): Pd(1)−O(1) 2.173(7),
Pd(1)−O(4) 2.063(8), Pd(1)−C(7) 1.974(10), Pd(1)−C(17)
1.974(10), O(4)−Pd(1)−O(1) 90.0(3), C(7)−Pd(1)−O(1)
89.0(3), C(7)−Pd(1)−O(4) 167.7(4), C(17)−Pd(1)−O(1)
170.3(4), C(17)−Pd(1)−O(4) 89.5(4), C(17)−Pd(1)−C(7)
93.5(4).
Figure 2. ORTEP diagram of Pd2. Thermal ellipsoids are shown at
the 60% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angle (deg): Pd(1)−O(1) 2.1976(16),
Pd(1)−N(3) 2.058(2), Pd(1)−C(7) 2.005(2), Pd(1)−C(18)
2
8
9
8
.023(2), N(3)−Pd(1)−O(1) 92.05(7), C(7)−Pd(1)−O(1)
9.01(8), C(7)−Pd(1)−N(3) 178.11(8), C(7)−Pd(1)−C(18)
5.77(9), C(18)−Pd(1)−O(1) 173.89(8), C(18)−Pd(1)−N(3)
3.07(9).
bonds (1.9754(10) Å for Pd4, 1.974(10) Å for Pd5). It should
be noted that the Pd−O(SO ) bond lengths (in the range of
3
2
.173(7)−2.1976(16) Å) are analogous to that of reported
NHC-sulfonate Pd(II) complexes (in the range of
2.1442(12)−2.182(2) Å, Chart 1, III)
complexes (2.159 Å, Chart 1, I). The Pd−C(NHC) bond
lengths are similar in all complexes (2.0109(16) Å for Pd1,
2.005(2) Å for Pd2, 2.0067(9) Å for Pd4, and 1.974(10) Å for
Pd5), approaching that of reported NHC-sulfonate Pd(II)
2
8b,c
Pd−N bond length (2.1149(9) Å) in C(sp ),N-chelated
and {PO}Pd(II)
3j
palladacycle Pd4 and the Pd−O(palladacycle) bond length
2
(
2.063(8) Å) in C(sp ),O-chelated palladacycle Pd5 are
slightly longer than the sum of the covalent radii of the Pd
and N atoms and of the Pd and O atoms (2.05 Å),
1
6
8c
respectively. The different bond lengths indicated that the
coordination of Pd and heteroatom on the palladacycles in
Pd1 and Pd2 were stronger than those in Pd4 and Pd5. In
addition, the Pd−C(sp ) bonds (2.0238(17) Å for Pd1,
2
complexes (∼1.98 Å) but definitely shorter than Pd−P bond
length (2.199(2)−2.2841(12) Å) in {PO}Pd(II) complex-
3j,m
es.
These results revealed that the coordination abilities of
3
the imidazo[1,5-a]pyridine-sulfonate ligands are close to that
of the NHC-sulfonate ligands and are slightly stronger than
2
.023(2) Å for Pd2) are evidently longer than the Pd−C(sp )
C
DOI: 10.1021/acs.organomet.9b00495
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
that of the {PO} ligands, because the tendency of the metal−
ligand interaction is usually represented by the length of the
metal−ligand bonds. In Pd1 and Pd2, the quinoline rings are
perpendicular to the aromatic rings on the ligands, while Pd4
and Pd5 exhibit π−π stacked arrangements of their phenyl
groups on the palladacycles and the aryl backbone on the
ligands. From the decomposition temperatures of these
complexes (Pd1, 250 °C; Pd2, 240 °C; Pd5, 236 °C; Pd4,
activity to decrease significantly (entry 7, Table 1). If we
reduce the Al/Pd ratio to 1000, then only trace polymer could
be observed. With the optimal conditions in hand, we
continued testing the catalytic properties of the other five
palladacycles. Similarly, complexes Pd5 and Pd4 also exhibited
6
excellent polymerization activities ((54.8−55.6) × 10 g of
−
1
−1
PNB (mol of Pd) h ) and conversions (91.3−92.6%) for
the polymerization of norbornene (entries 9 and 10, Table 1).
6
2
03 °C), it seems that Pd1 and Pd2 with vertical structures are
Pd2 showed moderate activity (42.8 × 10 g of PNB (mol of
−
1
−1
more thermally stable than Pd4 and Pd5 with π−π stacking
structures. These structural differences are hypothesized to
influence the catalytic properties of these palladium catalysts in
Pd) h ), with 71.3% conversion (entry 13, Table 1), while
complexes Pd3 and Pd1 displayed obvious lower activities, as
6
−1
the highest activity was only 4.8 × 10 g of PNB (mol of Pd)
17
−1
norbornene polymerization.
h
(entries 11, 12, 14, and 15, Table 1). The conversions did
Norbornene Polymerization. Recently, some nickel and
palladium complexes containing NHC ligands have proved
highly active catalysts for addition polymerization of
not significantly improve even if the reaction time was
extended to 3 min (20.0% for Pd3, entry 11, and 9.6% for
Pd1, entry 14; Table 1). The results showed that the activities
of these six complexes decreased in the order of Pd6 > Pd5 >
Pd4 > Pd2 > Pd3 > Pd1.
7
,8c,d,15
norbornene.
In order to explore the potential
application of these imidazo[1,5-a]pyridine-sulfonate pallada-
cycles, the catalytic behaviors of complexes Pd1−Pd6 toward
norbornene polymerization were studied. First, no polymer
was obtained in the absence of cocatalyst. For the purposes of
determining the most suitable cocatalyst, both MAO and
For different palladacycles containing the same ligand,
2
C(sp ),O-chelated six-membered palladacycles (Pd5 and Pd6)
2
were more active than the C(sp ),N-chelated five-membered
palladacycles (Pd3 and Pd4), and the latter were more active
3
Et AlCl were examined for norbornene polymerization.
than the C(sp ),N-chelated five-membered palladacycles (Pd1
2
MAO has excellent performance in many olefin polymer-
and Pd2). That is to say, for Pd2, Pd4, and Pd6, all containing
1
,15
6
ization systems.
Therefore, we first choose MAO as a
a tert-butyl substituent, the order of activity is Pd6 (60.0 × 10
−
1
−1
6
cocatalyst for the experiment. Complex Pd6 was employed as a
precatalyst for the study of the polymerization in detail (Table
g of PNB (mol of Pd) h ) > Pd4 (54.8 × 10 g of PNB
1 6
−
−1
−1
(mol of Pd) h ) > Pd2 (42.8 × 10 g of PNB (mol of Pd)
1
−
1
). On activation with 2000 equiv of MAO, the activity of Pd6
h ); For Pd1, Pd3, and Pd5, all containing a mesityl
6
substituent, the order of activity is Pd5 (55.6 × 10 g of PNB
−
1
−1
6
−1
Table 1. Vinyl Polymerization of Norbornene with Pd1−
(mol of Pd) h ) > Pd3 (4.8 × 10 g of PNB (mol of Pd)
6
a
−1
−1 −1
Pd6 Activated by MAO
h ) > Pd1 (1.9 × 10 g of PNB (mol of Pd) h ). This
could be due to the six-membered ring having the looser
chelation than that of the five-membered ring, coupled with
the coordination of oxygen atom with palladium being weaker
than that of nitrogen atom, which were conducive to the rapid
coordination and insertion of norbornene monomer. As can be
seen from the X-ray structure, π−π stacking between the
phenyl of the palladacycles and the aryl backbone of the
ligands of Pd4 and Pd5 may efficiently remove the steric
influence of the substituents in ligands, leaving the palladium
b
entry
cat.
T (°C)
Al/Pd
t (min)
conv. (%)
activity
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
Pd6
Pd6
Pd6
Pd6
Pd6
Pd6
Pd6
Pd6
Pd5
Pd4
Pd3
Pd3
Pd2
Pd1
Pd1
40
60
80
2000
2000
2000
2000
2000
2000
1500
1000
2000
2000
2000
2000
2000
2000
2000
1
1
1
1
1
1
1
1
1
1
1
3
1
1
3
20.0
73.0
82.7
99.9
83.7
83.2
57.2
trace
92.6
91.3
8.0
12.0
43.8
49.6
60.0
50.2
49.9
34.3
100
120
140
100
100
100
100
100
100
100
100
100
17
center accessible, which leads to the rapid chain initiation.
55.6
54.8
4.8
4.0
42.8
Chain initiation is considered to be the rate-determining step
in this system. After activation, the π−π stacked arrangement
disappears and does not affect the polymerization. Further-
more, as for the same kind of palladacycles, the substituents on
nitrogen atom have great influence on the catalytic activity.
The Pd(II) complexes substituted by 4-tert-butylphenyl
showed higher activities than those substituted by mesityl
0
1
2
3
4
5
20.0
71.3
trace
9.6
1.9
a
Conditions: Pd complexes, 1 μmol; solvent, 1,2-dichlorobenzene;
(
Pd2 > Pd1, Pd4 > Pd3, Pd6 > Pd5). The presence of a
b
V
1
, 10 mL; norbornene, 1.0 g; MAO, 1.5 M in toluene. In units of
total
methyl group at the ortho position of mesityl may hinder the
coordination and insertion of norbornene, while the presence
6
−1 −1
0 g of PNB (mol of Pd) h .
18
of a 4-tert-butyl group improves the solubility of the
palladacycles to some extent. It is hypothesized that the
combination of these effects leads to the order of activities of
the catalysts in the current system.
However, its high price and the strict demands for
production, transportation, and storage limit the application
6
−1 −1
was relatively low (12.0 × 10 g of PNB (mol of Pd) h )
when the temperature was below 60 °C, but it increased to
high levels in a wide range of temperatures from 60 to 140 °C,
6
with the best activity at 100 °C (60.0 × 10 g of PNB (mol of
−
1
−1
Pd)
h ) (entries 1−6, Table 1). Pd6 can catalyze
norbornene polymerization quantitatively in 1 min at an Al/
Pd ratio of 2000 at 100 °C. It proved that Pd6 has good
thermal stability and still has high activities above 100 °C in
the presence of MAO. The high temperature tolerance makes
it possible for these complexes to be used in industry.
Reducing the Al/Pd ratio from 2000 to 1500 caused the
of MAO. Et AlCl, in contrast, is a common cocatalyst, and the
2
price is much cheaper. Thus, we chose it for comparison.
When Et AlCl was used as the cocatalyst, complex Pd6 showed
2
6
−1 −1
the highest activity of 29.0 × 10 g of PNB (mol of Pd)
h
at an Al/Pd ratio of 500 at 40 °C (entry 2, Table 2). Different
from the Pd/MAO system, the activities decreased obviously
D
DOI: 10.1021/acs.organomet.9b00495
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 2. Vinyl Polymerization of Norbornene with Pd1−
The difference in catalytic behaviors between MAO and
a
Pd6 Activated by Et AlCl
Et AlCl comes from their different structures and Lewis
2
2
acidities. MAO is a linear or cyclic oligomer. MAO has a higher
b
entry
cat.
T (°C)
Al/Pd
t (min)
conv. (%)
activity
Lewis acidity than Et AlCl and leads to faster formation of
2
1
2
3
4
5
Pd6
Pd6
Pd6
Pd6
Pd6
Pd6
Pd5
Pd5
Pd4
Pd4
Pd4
Pd3
Pd2
Pd1
20
40
60
80
40
40
40
40
40
60
60
60
60
60
500
500
500
500
100
100
100
100
100
1500
1500
1500
1500
1500
2
2
2
2
2
2
2
3
3
3
5
5
5
5
24.5
96.5
70.9
29.2
94.5
38.5
81.7
99.8
trace
66.4
81.8
12.5
71.0
7.2
7.4
29.0
21.3
8.8
28.4
14.4
24.5
20.0
active metal cation species with empty coordination site to
21
initiate polymerization. In contrast, the polymeric MAO can
act as a large weak-coordinated counteranion to make the
active metal cation species more free, active, and thermally
22
stable. The Pd/MAO catalytic system showed the highest
activity at higher temperature, and its activity is higher than the
c
6
7
8
9
1
1
1
1
1
Pd/Et AlCl system. Also due to the polymeric structure, some
2
aluminum centers in MAO may be shielded. As a result, a
23
larger Al/Pd ratio is needed for the Pd/MAO system.
0
1
2
3
4
13.3
9.8
1.5
8.5
0.9
The PNBs obtained by the Pd/MAO system are insoluble in
7,8c,d,15
most organic solvents as in many previous reports,
while
those obtained by Pd/Et AlCl systems are slightly soluble in
2
1
,2-dichlorobenzene. Therefore, we only characterized the
microstructures of the polymers obtained by Pd/Et AlCl
2
a
1
Conditions: Pd complexes, 1 μmol; solvent, 1,2-dichlorobenzene;
systems by H NMR. The signals appearing in the 0.9−3.0
b
Vtotal, 10 mL; norbornene, 1.0 g; Et AlCl, 2.0 M in hexane. Acitivity
in units of 10 g of PNB (mol of Pd) h . Pd complexes, 0.8 μmol.
1
2
ppm range and the absence of resonance at 5.0−6.0 ppm in H
6
−1 −1
c
NMR spectra (Figure S33) indicated that the polymers were
vinyl-addition-type products. The missing absorption of a
−
1
double bond at 1600−1700 cm in the IR spectra of the
polymers (Figures S34−45) further revealed that the polymer-
ization was not a ROMP type. According to the TGA study
when the temperature was above 60 °C, which would cover the
different temperature tolerance of active species produced by
different cocatalysts when they interact with metal catalysts.
Even if the Al/Pd ratio dropped to 100, the conversion was still
over 90% (entry 5, Table 2). Reducing the amount of catalyst
to 0.8 μmol led to a lower conversion (38.5%) and activity
(
Figure S52), the polymers are thermally stable up to 400 °C.
2
The polymers obtained with C(sp ),N-chelated palladacycles
Pd3 and Pd4 displayed glass transition temperatures close to
310 °C as analyzed by means of differential scanning
calorimetry (DSC) techniques, while others could not be
determined as the glass transition temperature of vinyl
polynorbornene is located close to the temperature range
6
−1 −1
(
14.4 × 10 g of PNB (mol of Pd) h , entry 6, Table 2).
Pd5 was then explored under the optimal conditions, with the
activity of 24.5× 10 g of PNB (mol of Pd)
6
−1 −1
h
and 81.7%
conversion. By prolonging the reaction time to 3 min, the
conversion was increased to 99.8% accompanied by a slight
24
where decomposition tends to begin.
6
−1 −1
decrease in activity (20.0× 10 g of PNB (mol of Pd) h ,
entry 8, Table 2). The C,N-chelated palladacycles showed
activities much lower than those of the C,O-chelated
palladacycles. Pd4 hardly polymerized norbornene under the
same conditions. By adjusting the Al/Pd ratio and the reaction
temperature (see the Supporting Information), the yield of
PNB was increased to 66.4%. When the reaction time was
extended to 5 min, the conversion reached 81.1% (entry 11,
Table 2). For the purposes of comparison, the activities of the
remaining three palladacycles were investigated under the same
conditions (entries 12−14, Table 2). The results showed that
the order of activities of the six palladacycles was same as that
in Pd/MAO system. As the reaction time was further extended
to 10 min, the conversion grew continuously (99.6% for Pd4,
CONCLUSION
■
In summary, a series of novel imidazo[1,5-a]pyridine-
sulfonate-ligated palladacycles were synthesized, and their
catalytic behaviors toward norbornene polymerization were
studied with MAO or Et AlCl as cocatalyst. The order of
2
activities toward the addition polymerization of norbornene
was Pd6 > Pd5 > Pd4 > Pd2 > Pd3 > Pd1. The structure of
palladacycles, the type of coordination atom, and the
substituent group on the ligands all showed significant
influences on the polymerization activities. The Pd1−Pd6/
MAO catalytic system showed high thermal stability and
7
reached the highest activity at 100 °C (6.0 × 10 g of PNB
−1 −1
(
mol of Pd)
h
with 99.9% conversion). With very low
4
9.5% for Pd3, 97.9% for Pd2, 29.0% for Pd1), proving that
Et AlCl loadings (100 equiv), Pd5 and Pd6 exhibited high
activities (up to 2.9 × 10 g of PNB (mol of Pd)
6.5% conversion). Further studies on late-transition-metal
complexes based on modified imidazo[1,5-a]pyridine-sulfonate
ligands and their behaviors toward other α-olefin (co)-
polymerization processes are in progress.
2
these palladacycles have excellent stability (entry 8−11, Table
S1). Ethylene polymerization with these complexes was also
tested. Unfortunately, no polymer was obtained. Considering
the many reported palladium− or nickel−NHC complexes
suffering from poor thermal stability and low molecular weight
of the obtained polyethylene, the stereo and electronic effects
of metal−NHC complexes may require more modification to
obtain polyethylene.
chelate ring also plays important roles in determining the
properties of transition metal catalysts.
7
−1 −1
h
with
9
EXPERIMENTAL SECTION
7
,8a−d,19
■
Moreover, the size of the metal−
General Considerations. All experiments were carried out under
an atmosphere of dry argon using standard Schlenk techniques. All
solvents were distilled from appropriate drying agents under argon
before use. Both MAO (1.5 M in toluene, purchased from Beijing
20
It is difficult to obtain detailed evidence for the mechanism
of vinyl polymerization of norbornene catalyzed by palladium
InnoChem Co., Ltd.) and Et AlCl (2.0 M in hexane, purchased from
2
catalysts with MAO or Et AlCl as cocatalyst. It is generally
2
J&K Scientific, Ltd.) were used as received. Norbornene (purchased
from Alfa Aesar China (Beijing) Chemical Co., Ltd.) and 1,2-
dichlorobenzene were purified by distillation over calcium hydride. 6-
accepted that the aluminum cocatalysts capture the ligand to
8c
create an empty site for coordination and insertion of olefin.
E
DOI: 10.1021/acs.organomet.9b00495
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2
5
Bromopyridine-2-carbaldehyde (1), 2-bromo-6-(dimethoxymethyl)
119.4, 116.0, 115.8, 20.6, 16.8. HRMS (ESI, m/z): calcd for
25
26
27
+
pyridine (2), [Pd(OAc)(8-Me-quin-H)]2, [Pd(dmba)(μ-Cl)] ,
and [Pd(o-acetanilido)(μ-trifluoroacetato)]2 were synthesized
according to the literature. Other commercially available reagents
were purchased and used without further purification. H and
NMR spectra were recorded on a Bruker AV400 spectrometer.
HRMS analyses were carried out on Agilent 6520 Q-TOF mass
spectrometers. Elemental analyses were performed on a PerkinElmer
40C analyzer. IR spectra were recorded as KBr disks on a Nicolet
80 FT-IR spectrometer. TGA and DSC data were obtained via TA
C H N O S [M + H] 317.0960. Found 317.0958.
2
16 16
2
3
2
8
Ligand L2. The same procedure as that described for the synthesis
of L1 was used. Reaction of 4 (1.2 g, 5 mmol), 4-tert-butyl
benzenamine (746 mg, 5 mmol), and p-toluenesulfonic acid (86 mg,
1
13
C
1
0.5 mmol) gave L2 (972 mg, 64%) as a white solid. Mp: >300 °C. H
NMR (400 MHz, DMSO-d ): δ 9.80 (s, 1H, NCHN), 8.85 (s, 1H,
6
im-H), 8.00 (d, J = 9.2 Hz, 1H, py-H), 7.83 (d, J = 8.6 Hz, 2H, Ar−
H), 7.71 (d, J = 8.6 Hz, 2H, Ar−H), 7.55 (d, J = 6.9 Hz, 1H, py-H),
2
3
1
3
7.42 (dd, J = 9.2, 7.0 Hz, 1H, py-H), 1.35 (s, 9H, Ar−C(CH ) ).
C
3
3
Instrument SDT-2960 and SC-2910 thermal analyzers, respectively.
NMR (100 MHz, DMSO-d ): 153.4, 138.2, 132.5, 130.4, 126.9,
6
Isobutyl 6-(Dimethoxymethyl)pyridine-2-sulfonate (3).
125.0, 124.2, 123.1, 119.2, 115.8, 113.9, 34.6, 30.8. HRMS (ESI, m/
n
n
+
BuLi (6.88 mL, 2.5 M in hexane, 17.2 mmol) and Bu Mg (17.2
z): calcd for C H N O S [M + H] 331.1116. Found 331.1117.
2
17 18
2
3
mL, 1.0 M in hexane, 17.2 mmol) were diluted in THF (60 mL). To
this stirring mixture, a solution of 2 (10.2 g, 43 mmol) in THF (30
mL) was added dropwise while maintaining the temperature at −5
Complex Pd1. A mixture of L1 (316 mg, 1.0 mmol),
[Pd(OAc)(8-Me-quin-H)] (0.308 g, 0.5 mmol), and 6 equiv of
2
potassium carbonate (829 mg, 6 mmol) was placed in a round-
bottomed flask and dissolved in 20 mL of acetonitrile. The reaction
mixture was stirred under reflux for 24 h and then cooled to room
temperature. After removal of solvent under reduced pressure, the
°
C. The reaction mixture was stirred at −5 °C for 2 h and added
slowly at −78 °C to a THF solution of sulfonyl chloride (5.2 mL, 64.5
mmol). The resulting yellow solution was stirred at −78 °C for 30
min and then allowed to warm to room temperature. Stirring
continued overnight, and the solution was cooled to 0 °C. Pyridine
residue was dissolved in CH
chromatography (silica gel, CH Cl
a white solid (293 mg, 52%). Mp: 250 °C (dec.). Anal. Calcd for
PdS: C, 55.37; H, 4.11; N, 7.45. Found: C, 55.51; H,
Cl
/EtOAc = 10:1) to give Pd1 as
2
and purified by column
2
2
2
(
10.4 mL, 129 mmol), 4-dimethylaminopyridine (525 mg, 4.3 mmol),
and isobutanol (5.9 mL, 64.5 mmol) were added slowly in order.
When the addition was over, the reaction was warmed to room
temperature and stirred for 48 h. The volatiles were removed under
reduced pressure, and the residue was taken up in ethyl acetate. The
mixture was washed with aqueous citric acid solution (15%) and
saturated NaCl solution and dried over Na SO . The organic solution
C H N O
26 23 3 3
1
4.30; N, 7.27. H NMR (400 MHz, CDCl ): δ 9.08 (d, J = 4.5 Hz,
3
1H, py-H), 8.22 (d, J = 8.2 Hz, 1H, Ar−H), 7.61 (d, J = 6.6 Hz, 1H,
Ar−H), 7.52−7.44 (m, 3H, Ar−H), 7.34 (t, J = 7.6 Hz, 1H, Ar−H),
7.26 (s, 1H, im-H), 7.12 (d, J = 7.0 Hz, 1H, py-H), 7.04 (s, 2H, Ar−
H), 6.96 (dd, J = 9.0, 6.9 Hz, 1H, py-H), 2.57 (s, 2H, Ar−CH −Pd),
2
4
2
1
3
was evaporated under reduced pressure, and the residue was purified
by column chromatography (silica gel, PE/EtOAc = 7:1) to give 3 as
2.40 (s, 3H, Ar−CH ), 2.16 (s, 6H, Ar−CH ). C NMR (100 MHz,
CDCl ): 158.0, 150.7, 149.2, 148.3, 140.2, 139.7, 137.6, 136.7, 134.9,
3
3
3
1
a white solid. (4.4 g, 15 mmol, 35%). Mp: 55−57 °C. H NMR (400
132.5, 129.1, 128.5, 128.1, 127.6, 123.1, 122.5, 121.4, 119.7, 115.9,
MHz, CDCl ): δ 8.01−7.98 (m, 2H, py-H), 7.82 (dd, J = 6.7, 2.3 Hz,
114.4, 21.1, 18.2, 16.9. HRMS (ESI, m/z): calcd for C H N O PdS
3
26 23
3
3
+
1
H, py-H), 5.37 (s, 1H, CH(OCH ) ), 4.17 (d, J = 6.5 Hz, 2H,
[M + H] 564.0573. Found 564.0570.
3
2
CH CH(CH ) ), 3.42 (s, 6H, CH(OCH ) ), 2.03 (dt, J = 13.4, 6.7
Complex Pd2. The same procedure as that described for the
synthesis of Pd1 was used. L2 (330 mg, 1.0 mmol), [Pd(OAc)(8-Me-
2
3
2
3 2
Hz, 1H, CH CH(CH ) ), 0.91 (d, J = 6.7 Hz, 6H, CH CH(CH ) ).
2
3
2
2
3 2
13
C NMR (100 MHz, CDCl ): δ 158.8, 153.8, 138.7, 124.9, 122.7,
quin-H)] (0.308 g, 0.5 mmol), and potassium carbonate (829 mg, 6
3
2
1
03.8, 78.9, 54.3, 28.2, 18.5. HRMS (ESI, m/z): calcd for
mmol) were used to give Pd2 (353 mg, 61%) as a white solid. Mp:
+
C H NO S [M + H] 290.1062. Found 290.1059.
240 °C (dec.). Anal. Calcd for C27
7.27. Found: C, 56.18; H, 4.33; N, 7.35. H NMR (400 MHz,
CDCl ): δ 9.08 (dd, J = 1.6, 4.8 Hz, 1H, py-H), 8.23 (dd, J = 1.6, 8.4
H N O PdS: C, 56.11; H, 4.36; N,
25 3 3
12
19
5
1
Isobutyl 6-Formylpyridine-2-sulfonate (4). A flask was
charged with 3 (4.4 g, 15 mmol) and p-toluenesulfonic acid (336.0
mg, cat.). Acetone (60 mL) and water (9 mL) were added via syringe.
The reaction was refluxed at 90 °C for 6 h. The mixture was cooled to
room temperature, and most of the solvent was removed. The mixture
was alkalized by the addition of saturated NaHCO aqueous solution
and extracted three times with dichloromethane (3 × 100 mL). The
extracts were combined, dried over MgSO , and concentrated under
3
Hz, 1H, Ar−H), 7.65−7.63 (m, 2H, Ar−H), 7.61 (dd, J = 1.2, 6.8 Hz,
1H, Ar−H), 7.55 (d, J = 3.6 Hz, 2H, Ar−H), 7.53 (t, J = 2.0 Hz, 1H,
py-H), 7.50−7.47 (m, 2H, Ar−H), 7.44 (dd, J = 1.2, 9.2 Hz, 1H, Ar−
H), 7.31 (t, J = 7.2, 1H, Ar−H), 7.01 (dd, J = 0.8, 6.8 Hz, 1H, py-H),
3
6.97 (dd, J = 6.8, 9.2 Hz, 1H, im-H), 2.57 (s, 2H, Ar−CH
−Pd), 1.39
(s, 9H, Ar−C(CH ) ). C NMR (100 MHz, CDCl ): 157.4, 153.0,
3 3 3
2
1
3
4
vacuum. The crude sulfonate was purified using column chromatog-
raphy (silica gel, PE/EtOAc = 5:1), yielding 2.95 g (12 mmol, 81%)
150.7, 149.4, 148.4, 140.3, 138.1, 137.6, 132.5, 128.6, 127.7, 127.5,
126.6, 126.0, 123.1, 122.8, 121.6, 119.5, 116.0, 114.6, 34.9, 31.3, 19.8.
1
+
of sulfonate product 4 as white solid. Mp: 30−32 °C. H NMR (400
HRMS (ESI, m/z): calcd for C27
H
25
N
3
O
3
PdS [M + H] 578.0730.
MHz, CDCl ): δ 10.10 (s, 1H, CHO), 8.26−8.16 (m, 3H, py-H), 4.21
Found 578.0728.
3
(
d, J = 6.5 Hz, 2H, CH CH(CH ) ), 2.04 (dt, J = 13.3, 6.6 Hz, 1H,
Complex Pd3. The same procedure as that described for the
synthesis of Pd1 was used. L1 (316 mg, 1.0 mmol), [Pd(dmba)(μ-
2
3 2
1
3
CH CH(CH ) ), 0.94 (d, J = 6.7 Hz, 6H, CH CH(CH ) ). C NMR
2
3
2
2
3 2
(
2
100 MHz, CDCl ): δ 191.3, 155.3, 152.7, 139.7, 126.8, 124.6, 79.2,
8.3, 18.5. HRMS (ESI, m/z): calcd for C H NO S [M + H]
Cl)]
mmol) were used to give Pd3 (476 mg, 0.86 mmol, 86%) as a white
solid. Mp: 200 °C (dec.). Anal. Calcd for C25 PdS: C, 54.01;
H, 4.89; N, 7.56. Found: C, 53.97; H, 5.00; N, 7.47. H NMR (400
MHz, CDCl ): δ 7.55 (d, J = 9.2 Hz, 1H, py-H), 7.45 (d, J = 9.2 Hz,
2
(307.5 mg, 0.5 mmol), and potassium carbonate (829 mg, 6
3
+
10 13 4
2
44.0644. Found 244.0633.
Ligand L1. A solution of 4 (1.2 g, 5 mmol), 2,4,6-trimethylaniline
676 mg, 5 mmol), and p-toluenesulfonic acid (86 mg, 0.5 mmol) in
H N O
27 3 3
1
(
3
toluene (20 mL) was stirred at 120 °C for 8 h. The suspension was
cooled to room temperature before it was added to a solution of
paraformaldehyde (187.6 mg, 6.25 mmol) in toluene (20 mL). HCl
1H, py-H), 7.32 (s, 1H, im-H), 6.96 (dd, J = 9.0, 6.9 Hz, 1H, Ar−H),
6.63−6.62 (m, 4H, Ar−H), 6.50 (d, J = 7.4 Hz, 1H, Ar−H), 6.43−
6.39 (m, 1H, py-H), 3.72 (s, 2H, Ar−CH -N), 2.63 (s, 6H,
2
1
3
(
1.25 mL, 4 M in dioxane) was added. The resulting brown solution
N(CH ) ), 2.36 (s, 6H, Ar−CH ), 2.05 (s, 3H, Ar−CH ).
NMR (100 MHz, CDCl ): 161.1, 146.0, 144.8, 140.1, 138.4, 137.7,
C
3
2
3
3
was stirred at 70 °C for 12 h. Upon cooling to room temperature, the
mixture was separated. After removal of solvents, the residue was
purified by column chromatography (silica gel, CH Cl /MeOH =
3
136.3, 133.7, 131.4, 128.9, 123.7, 122.8, 122.5, 121.3, 119.6, 116.1,
114.9, 71.4, 49.1, 20.6, 19.8. HRMS (ESI, m/z): calcd for
2
2
+
4
0:1) to give L1 as a white solid. (1.33 g, 4.2 mmol, 84%). Mp: >300
C H N O PdS [M + H] 556.0886. Found 556.0886.
2
5
27
3
3
1
°
C. H NMR (400 MHz, CDCl ):δ 9.72 (s, 1H, NCHN), 7.85 (d, J =
.8 Hz, 1H, py-H), 7.71 (d, J = 9.2 Hz, 1H, py-H), 7.60 (s, 1H, im-
Complex Pd4. The same procedure as that described for the
synthesis of Pd1 was used. L2 (330 mg, 1.0 mmol), [Pd(dmba)(μ-
3
6
H)), 7.43 (dd, J = 6.8, 8.8 Hz, 1H, py-H), 7.08 (s, 2H, Ar−H)), 2.40
Cl)] (307.5 mg, 0.5 mmol), and potassium carbonate (829 mg, 6
2
1
3
(
s, 3H, Ar−CH ), 2.04 (s, 6H, Ar−CH ). C NMR (100 MHz,
mmol) were used to give Pd4 (568 mg, 1.0 mmol, >99%) as a white
solid. Mp: 185 °C. Anal. Calcd for C H N O PdS: C, 54.78; H,
3
3
DMSO-d ): 140.3, 138.3, 133.9, 131.6, 130.4, 129.2, 127.0, 125.1,
6
26 29
3
3
F
DOI: 10.1021/acs.organomet.9b00495
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
5
.13; N, 7.37. Found: C, 55.03; H, 4.89; N, 6.99. H NMR (400
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00495.
■
MHz, CDCl ): δ 7.73 (d, J = 8.4 Hz, 2H, Ar−H), 7.65 (s, 1H, im-H),
.57 (d, J = 6.8 Hz, 1H, py-H), 7.45 (d, J = 9.6 Hz, 1H, py-H), 7.13
3
*
S
7
(
(
7
d, J = 8.0 Hz, 2H, Ar−H), 6.99 (dd, J = 7.6, 8.8 Hz, 1H, Ar−H), 6.62
d, J = 7.6 Hz, 1H, Ar−H), 6.53 (t, J = 7.2 Hz, 1H, py-H), 6.23 (t, J =
.6 Hz, 1H, Ar−H), 6.13 (d, J = 7.6 Hz, 1H, Ar−H), 2.71 (br, 6H,
N(CH ) ), 2.51 (s, 1H, Ar−CH -N), 1.40 (s, 1H, Ar−CH -N), 1.13
3
2
2
2
Polymerization tables, NMR spectra for 1−4, NMR and
IR spectra for L1−L2 and Pd1−Pd6; NMR and IR
spectra, DSC, and TGA spectra of the PNB obtained
(PDF)
1
3
(
1
1
s, 9H, Ar−C(CH ) ). C NMR (100 MHz, CDCl ): 158.4, 151.8,
3
3
3
45.5, 145.3, 140.3, 137.7, 137.2, 132.4, 125.8, 124.9, 124.5, 122.9,
22.8, 121.0, 119.5, 116.1, 113.4, 71.3, 34.4, 31.0. HRMS (ESI, m/z):
calcd for C H N O PdS [M + H] 570.1043. Found 570.1034.
+
26
29
3
3
Complex Pd5. The same procedure as that described for the
synthesis of Pd1 was used. L1 (316 mg, 1.0 mmol), [Pd(o-
Accession Codes
CCDC 1899681, 1899685, 1899686, and 1907389 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
acetanilido)(μ-trifluoroacetato)] (354 mg, 0.5 mmol), and potassium
2
carbonate (829 mg, 6 mmol) were used to give Pd5 (253 mg, 43%) as
a pale yellow solid. Mp: 236 °C (dec.). Anal. Calcd for
C H N O PdS: C, 51.85; H, 4.17; N, 7.56. Found: C, 51.68; H,
2
4
23
3
4
1
3
.96; N, 7.41. H NMR (400 MHz, DMSO-d ) δ 11.00 (s, 1H, N−
H), 8.07 (s, 1H, im-H), 7.72 (d, J = 8.8 Hz, 1H, py-H), 7.35 (d, J =
6
6
7
.8 Hz, 1H, py-H), 7.11 (dd, J = 6.8, 9.2 Hz, 1H, Ar−H), 6.72 (t, J =
.2 Hz, 1H, Ar−H), 6.65 (d, J = 7.2 Hz, 1H, Ar−H), 6.60 (s, 2H, Ar−
H), 6.54 (d, J = 7.2 Hz, 1H, Ar−H), 6.47 (t, J = 7.2 Hz, 1H, py-H),
AUTHOR INFORMATION
Corresponding Author
E-mail: bqwang@nankai.edu.cn. Tel./fax: +86-22-23504781.
■
*
2
.22 (s, 3H, CO−CH ), 2.18 (s, 6H, Ar−CH ), 2.05 (s, 3H, Ar−
3
3
13
CH ). C NMR (100 MHz, DMSO-d ) δ 169.37, 154.85, 140.87,
3
6
1
1
1
5
39.92, 137.87, 136.35, 133.63, 133.46, 131.42, 128.90, 127.47,
24.35, 123.09, 122.55, 121.25, 117.35, 116.11, 115.58, 21.87, 20.77,
ORCID
+
9.74. HRMS (ESI, m/z): calcd for C H N O PdS [M + H]
2
4
23
3
4
Baiquan Wang: 0000-0003-4605-1607
Notes
The authors declare no competing financial interest.
56.0522. Found 556.0518.
Complex Pd6. The same procedure as that described for the
synthesis of Pd1 was used. L2 (330 mg, 1.0 mmol), [Pd(o-
acetanilido)(μ-trifluoroacetato)] (354 mg, 0.5 mmol), and potassium
2
ACKNOWLEDGMENTS
This work was financially supported by the National Natural
Science Foundation of China (Grants 21672108).
carbonate (829 mg, 6 mmol) were used to give Pd6 (148 mg, 26%) as
a pale yellow solid. Mp: 238 °C (dec.). Anal. Calcd for
C H N O PdS: C, 52.68; H, 4.42; N, 7.37. Found: C, 52.43; H,
■
2
5
25
3
4
1
4
7
2
1
.65; N, 7.09. H NMR (400 MHz, CDCl ) δ 9.70 (s, 1H, N−H),
3
.65 (s, 1H, im-H), 7.53−7.49 (m, 4H, Ar−H), 7.08 (d, J = 8.4 Hz,
H, Ar−H), 6.96 (dd, J = 7.2, 8.8 Hz, 1H, py-H), 6.59 (d, J = 8.0 Hz,
H, Ar−H), 6.39 (t, J = 7.2 Hz, 1H, Ar−H), 6.25 (d, J = 7.6 Hz, 1H,
REFERENCES
■
(
1) (a) Janiak, C.; Lassahn, P. G. Metal catalysts for the vinyl
polymerization of norbornene. J. Mol. Catal. A: Chem. 2001, 166,
93−209. (b) Buchmeiser, M. R. Homogeneous metathesis polymer-
py-H), 6.12 (t, J = 7.2 Hz, 1H, py-H), 2.05 (s, 3H, CO−CH ), 1.12
3
1
3
1
(
s, 9H, Ar−C(CH ) ). C NMR (100 MHz, CDCl ) δ 168.94,
3
3
3
ization by well-defined group VI and group VIII transition-metal
alkylidenes: fundamentals and applications in the preparation of
advanced materials. Chem. Rev. 2000, 100, 1565−1604. (c) Coates, G.
W. Precise control of polyolefin stereochemistry using single-site
metal catalysts. Chem. Rev. 2000, 100, 1223−1252. (d) Blank, F.;
Janiak, C. Metal catalysts for the vinyl/addition polymerization of
norbornene. Coord. Chem. Rev. 2009, 253, 827−861. (e) Ma, R.; Hou,
Y.; Gao, J.; Bao, F. Recent progress in the vinylic polymerization and
copolymerization of norbornene catalyzed by transition metal
catalysts. Polym. Rev. 2009, 49, 249−287. (f) Chen, Z.; Brookhart,
M. Exploring ethylene/polar vinyl monomer copolymerizations using
Ni and Pd α-diimine catalysts. Acc. Chem. Res. 2018, 51, 1831−1839.
(g) Tan, C.; Chen, C. Emerging palladium and nickel catalysts for
copolymerization of olefins with polar monomers. Angew. Chem., Int.
Ed. 2019, 58, 7192−7200. (h) Chen, C. Designing catalysts for olefin
polymerization and copolymerization: beyond electronic and steric
tuning. Nat. Rev. Chem. 2018, 2, 6−14. (i) Wang, F.; Chen, C. A
continuing legend: the Brookhart-type α-diimine nickel and palladium
catalysts. Polym. Chem. 2019, 10, 2354−2369.
1
1
1
54.86, 151.59, 140.17, 139.97, 137.47, 132.38, 132.33, 126.51,
25.89, 125.15, 124.34, 123.49, 122.54, 119.99, 116.57, 116.05,
14.21, 34.47, 30.99, 22.00. HRMS (ESI, m/z): calcd for
+
C H N O PdS [M + H] 570.0679. Found 570.0674.
25
25
3
4
Crystallographic Studies. Single crystals of Pd1, Pd2, Pd4, and
Pd5 suitable for X-ray diffraction were obtained from CH Cl /hexane.
2
2
Data collections were carried out on a Rigaku Saturn 724 CCD
diffractometer equipped with a rotating anode system at 113(2) K by
using graphite-monochromated Mo Kα radiation (ω−2θ scans, λ =
0
.71073 Å). Semi-empirical absorption corrections were applied for all
complexes. The structures were solved by direct methods and refined
using the full-matrix least-squares method. Calculations were
performed by using the SHELXL-97 program. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were assigned
idealized positions and were included in structure factor calculations.
The molecular structures of complexes Pd1, Pd2, and Pd4 contain
one CH Cl molecule of solvation, respectively.
2
2
Norbornene Polymerization. In a typical procedure, 1.00 g of
norbornene in 5.0 mL of 1,2-dichlorobenzene and an exact amount of
MAO or Et AlCl solution were added into a flask (100 mL) with
(2) Ahmed, S.; Bidstrup, S. A.; Kohl, P. A.; Ludovice, P. J.
Development of a new force field for polynorbornene. J. Phys. Chem. B
1998, 102, 9783−9790.
2
stirring under an Ar atmosphere. After the mixture was kept at the
desired temperature for 3 min, the right amount of the palladium
complex dissolved in dichlorobenzene was injected into the flask with
a syringe, and the reaction was started. After the desired time, the
polymerization was terminated by the addition of 10% HCl in
ethanol. The precipitated polymer was washed with ethanol and water
and dried at 60 °C in vacuo to a constant weight. For all the
polymerization procedures, the total reaction volume was 10 mL,
which can be achieved by the variation of the amount of added
dichlorobenzene when necessary.
(3) (a) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R.
I. Palladium catalysed copolymerisation of ethene with alkylacrylates:
polar comonomer built into the linear polymer chain. Chem. Commun.
2002, 744−745. (b) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort,
B.; Pugh, R. I. The first example of palladium catalysed non-perfectly
alternating copolymerisation of ethene and carbon monoxide. Chem.
Commun. 2002, 964−965. (c) Chen, M.; Zou, W.; Cai, Z.; Chen, C.
Norbornene homopolymerization and copolymerization with ethylene
G
DOI: 10.1021/acs.organomet.9b00495
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
by phosphine-sulfonate nickel catalysts. Polym. Chem. 2015, 6, 2669−
Chem. Soc. Rev. 2007, 36, 592−607. (h) Arnold, P. L.; Pearson, S.
Abnormal N-heterocyclic carbenes. Coord. Chem. Rev. 2007, 251,
596−609. (i) Pugh, D.; Danopoulos, A. A. Metal complexes with
‘pincer’-type ligands incorporating N-heterocyclic carbene function-
alities. Coord. Chem. Rev. 2007, 251, 610−641. (j) Lin, I. J. B.; Vasam,
C. S. Preparation and application of N-heterocyclic carbene
complexes of Ag(I). Coord. Chem. Rev. 2007, 251, 642−670.
(k) Dragutan, V.; Dragutan, I.; Delaude, L.; Demonceau, A. NHC−
Ru complexesFriendly catalytic tools for manifold chemical
transformations. Coord. Chem. Rev. 2007, 251, 765−794. (l) Mata,
J. A.; Poyatos, M.; Peris, E. Structural and catalytic properties of
chelating bis- and tris-N-heterocyclic carbenes. Coord. Chem. Rev.
2007, 251, 841−859. (m) Hahn, F. E.; Jahnke, M. C. Heterocyclic
carbenes: synthesis and coordination chemistry. Angew. Chem., Int. Ed.
2008, 47, 3122−3172. (n) Lin, J. C.; Huang, R. T.; Lee, C. S.;
Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. Coinage metal−N-
heterocyclic carbene complexes. Chem. Rev. 2009, 109, 3561−3598.
(7) (a) McGuinness, D. Alkene oligomerisation and polymerisation
with metal-NHC based catalysts. Dalton Trans. 2009, 0, 6915−6923.
(b) Khlebnikov, V.; Meduri, A.; Mueller-Bunz, H.; Montini, T.;
Fornasiero, P.; Zangrando, E.; Milani, B.; Albrecht, M. Palladium
carbene complexes for selective alkene di- and oligomerization.
Organometallics 2012, 31, 976−986. (c) Khlebnikov, V.; Meduri, A.;
Mueller-Bunz, H.; Milani, B.; Albrecht, M. Synthesis of a sterically
modulated pyridine−NHC palladium complex and its reactivity
towards ethylene. New J. Chem. 2012, 36, 1552−1555. (d) Wang, X.;
Liu, S.; Jin, G. Preparation, structure, and olefin polymerization
behavior of functionalized nickel(II) N-heterocyclic carbene com-
plexes. Organometallics 2004, 23, 6002−6007. (e) Sujith, S.; Noh, E.
K.; Lee, B. Y.; Han, J. W. Synthesis, characterization, and norbornene
2
676. (d) Wada, S.; Jordan, R. F. Olefin insertion into a Pd−F bond:
catalyst reactivation following β-F elimination in ethylene/vinyl
fluoride copolymerization. Angew. Chem., Int. Ed. 2017, 56, 1820−
1
824. (e) Black, R. E.; Jordan, R. F. Synthesis and reactivity of
palladium(II) alkyl complexes that contain phosphine-cyclopentane-
sulfonate ligands. Organometallics 2017, 36, 3415−3428. (f) Wimmer,
F. P.; Caporaso, L.; Cavallo, L.; Mecking, S.; Falivene, L. Mechanism
of insertion polymerization of allyl ethers. Macromolecules 2018, 51,
4
525−4531. (g) Wucher, P.; Goldbach, V.; Mecking, S. Electronic
influences in phosphinesulfonato palladium(II) polymerization
catalysts. Organometallics 2013, 32, 4516−4522. (h) Jian, Z. B.;
Falivene, L.; Boffa, G.; Sanchez, S. O.; Caporaso, L.; Grassi, A.;
́
Mecking, S. Direct synthesis of telechelic polyethylene by selective
insertion polymerization. Angew. Chem., Int. Ed. 2016, 55, 14378−
1
4383. (i) Parisi, L. R.; Scheibel, D. M.; Lin, S.; Bennett, E. M.;
Lodge, J. M.; Miri, M. J. Eugenol as renewable comonomer compared
to 4-penten-1-ol in ethylene copolymerization using a palladium aryl
sulfonate catalyst. Polymer 2017, 114, 319−328. (j) Noda, S.;
Nakamura, A.; Kochi, T.; Chung, L. W.; Morokuma, K.; Nozaki, K.
Mechanistic studies on the formation of linear polyethylene chain
catalyzed by palladium phosphine-sulfonate complexes: experiment
and theoretical studies. J. Am. Chem. Soc. 2009, 131, 14088−14100.
(
k) Ota, Y.; Ito, S.; Kobayashi, M.; Kitade, S.; Sakata, K.; Tayano, T.;
Nozaki, K. Crystalline isotactic polar polypropylene from the
palladium-catalyzed copolymerization of propylene and polar
monomers. Angew. Chem., Int. Ed. 2016, 55, 7505−7509.
(
l) Lanzinger, D.; Giuman, M. M.; Anselment, T. M. J.; Rieger, B.
Copolymerization of ethylene and 3,3,3-trifluoropropene using
phosphine-sulfonate)Pd(Me)(DMSO) as catalyst. ACS Macro Lett.
(
3
2
014, 3, 931−934. (m) Zhang, D.; Chen, C. Influence of polyethylene
polymerization of η -benzylnickel(II) complexes of N-heterocyclic
glycol unit on palladium- and nickel- catalyzed ethylene polymer-
carbenes. J. Organomet. Chem. 2008, 693, 2171−2176. (f) Sung, W.
Y.; Park, M. H.; Park, J. H.; Eo, M.; Yu, M.; Do, Y.; Lee, M. H.
Triarylborane-functionalized polynorbornenes: Direct polymerization
and signal amplification in fluoride sensing. Polymer 2012, 53, 1857−
1863. (g) Deng, J.; Gao, H.; Zhu, F.; Wu, Q. Synthesis and structure
of imine−N-heterocyclic carbene palladium complexes and their
catalytic behavior in norbornene polymerization. Organometallics
2013, 32, 4507−4515.
ization and copolymerization. Angew. Chem., Int. Ed. 2017, 56,
1
4672−14676. (n) Zhang, W.; Waddell, P. M.; Tiedemann, M. A.;
Padilla, C. E.; Mei, J.; Chen, L.; Carrow, B. P. Electron-rich metal
cations enable synthesis of high molecular weight, linear functional
polyethylenes. J. Am. Chem. Soc. 2018, 140, 8841−8850. (o) Rezabal,
E.; Ugalde, J. M.; Frenking, G. The trans effect in palladium
phosphine sulfonate complexes. J. Phys. Chem. A 2017, 121, 7709−
7
(
716.
4) (a) Nakamura, A.; Ito, S.; Nozaki, K. Coordination insertion
copolymerization of fundamental polar monomers. Chem. Rev. 2009,
09, 5215−5244. (b) Nakamura, A.; Anselment, T. M. J.; Claverie, J.;
(8) (a) Nagai, Y.; Kochi, T.; Nozaki, K. Synthesis of N-heterocyclic
carbene-sulfonate palladium complexes. Organometallics 2009, 28,
6131−6134. (b) Zhou, X.; Jordan, R. F. Synthesis, cis/trans
isomerization, and reactivity of palladium alkyl complexes that
contain a chelating N-heterocyclic-carbene sulfonate ligand. Organo-
metallics 2011, 30, 4632−4642. (c) Li, M.; Song, H.; Wang, B.
Synthesis, structures, and norbornene polymerization behavior of N-
heterocyclic carbene-sulfonate-ligated palladacycles. Organometallics
2015, 34, 1969−1977. (d) Li, M.; Song, H.; Wang, B. Synthesis,
structures, and norbornene polymerization behavior of palladium
methyl complexes bearing N-heterocyclic carbene-sulfonate ligands. J.
Organomet. Chem. 2016, 804, 118−122. (e) Tao, W. J.; Nakano, R.;
Ito, S.; Nozaki, K. Copolymerization of ethylene and polar monomers
by using Ni/IzQO catalysts. Angew. Chem., Int. Ed. 2016, 55, 2835−
2839. (f) Nakano, R.; Nozaki, K. Copolymerization of propylene and
polar monomers using Pd/IzQO catalysts. J. Am. Chem. Soc. 2015,
137, 10934−10937. (g) Tao, W.; Wang, X.; Ito, S.; Nozaki, K.
Palladium complexes bearing an N-heterocyclic carbine-sulfonamide
ligand for cooligomerization of ethylene and polar monomers. J.
Polym. Sci., Part A: Polym. Chem. 2019, 57, 474−477. (h) Yasuda, H.;
Nakano, R.; Ito, S.; Nozaki, K. Palladium/IzQO-catalyzed coordina-
tion−insertion copolymerization of ethylene and 1, 1-disubstituted
ethylenes bearing a polar functional group. J. Am. Chem. Soc. 2018,
140, 1876−1883. (i) Tao, W.; Akita, S.; Nakano, R.; Ito, S.;
Hoshimoto, Y.; Ogoshi, S.; Nozaki, K. Copolymerisation of ethylene
with polar monomers by using palladium catalysts bearing an N-
heterocyclic carbene−phosphine oxide bidentate ligand. Chem.
Commun. 2017, 53, 2630.
1
Goodall, B.; Jordan, R. F.; Mecking, S.; Rieger, B.; Sen, A.; Van
Leeuwen, P. W. N. M.; Nozaki, K. Ortho-phosphinobenzenesulfonate:
a superb ligand for palladium-catalyzed coordination−insertion
copolymerization of polar vinyl monomers. Acc. Chem. Res. 2013,
4
6, 1438−1449. (c) Ito, S.; Nozaki, K. Coordination−insertion
copolymerization of polar vinyl monomers by palladium catalysts.
Chem. Rec. 2010, 10, 315−325.
(
5) (a) Arduengo, A. J.; Harlow, R. L.; Kline, M. A stable crystalline
carbene. J. Am. Chem. Soc. 1991, 113, 361−363. (b) Arduengo, A. J.
Looking for stable carbenes: the difficulty in starting anew. Acc. Chem.
Res. 1999, 32, 913−921.
(
6) For recent reviews, see (a) Bourissou, D.; Guerret, O.; Gabbai,
F.; Bertrand, G. Stable carbenes. Chem. Rev. 2000, 100, 39−92.
b) Herrmann, W. A. N-heterocyclic carbenes: a new concept in
organometallic catalysis. Angew. Chem., Int. Ed. 2002, 41, 1290−1309.
c) Peris, E.; Crabtree, R. H. Recent homogeneous catalytic
(
(
applications of chelate and pincer N-heterocyclic carbenes. Coord.
Chem. Rev. 2004, 248, 2239−2246. (d) Crudden, C. M.; Allen, D. P.
Stability and reactivity of N-heterocyclic carbene complexes. Coord.
Chem. Rev. 2004, 248, 2247−2273. (e) Garrison, J. C.; Youngs, W. J.
Ag(I) N-heterocyclic carbene complexes: synthesis, structure, and
application. Chem. Rev. 2005, 105, 3978−4008. (f) Kantchev, E. A. B.;
O’Brien, C. J.; Organ, M. G. Palladium complexes of N-heterocyclic
carbenes as catalysts for cross-coupling reactionsA synthetic
chemist’s perspective. Angew. Chem., Int. Ed. 2007, 46, 2768−2813.
(9) (a) Alcarazo, M.; Roseblade, S. J.; Cowley, A. R.; Fernandez, R.;
́
Brown, J. M.; Lassaletta, J. M. Imidazo[1,5-a]pyridine: a versatile
(
g) Ku
̈
hl, O. The chemistry of functionalised N-heterocyclic carbenes.
H
DOI: 10.1021/acs.organomet.9b00495
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
architecture for stable N-heterocyclic carbenes. J. Am. Chem. Soc.
reactions involving palladacycle-directed aryl coupling steps. Acc.
Chem. Res. 2008, 41, 1512−1522.
2
005, 127, 3290−3291. (b) Burstein, C.; Lehmann, C. W.; Glorius, F.
Imidazo[1,5-a]pyridine-3-ylidenes-pyridine derived N-heterocyclic
(13) (a) Molitor, S.; Schwarz, C.; Gessner, V. H. Mono- and bis-
cyclometalated palladium complexes: synthesis, characterization, and
catalytic activity. Organometallics 2016, 35, 159−167. (b) O’Duill, M.
L.; Matsuura, R.; Wang, Y.; Turnbull, J. L.; Gurak, J. A.; Gao, D.-W.;
Lu, G.; Liu, P.; Engle, K. M. Tridentate directing groups stabilize 6-
membered palladacycles in catalytic alkene hydrofunctionalization. J.
Am. Chem. Soc. 2017, 139, 15576−15579. (c) Liu, M.; Yang, P.;
Karunananda, M. K.; Wang, Y.; Liu, P.; Engle, K. M. C(alkenyl)−H
activation via six-membered palladacycles: catalytic 1, 3-diene
synthesis. J. Am. Chem. Soc. 2018, 140, 5805−5813. (d) Jin, L.;
Wei, W.; Sun, N.; Hu, B.; Shen, Z.; Hu, X. Unsymmetrical CNN-
palladacycles with geometry-constrained iminopyridyl ligands: an
efficient precatalyst in Suzuki coupling for accessing 1, 1-diarylalkanes
from secondary benzylic bromides. Org. Chem. Front. 2018, 5, 2484−
2491. (e) Oliva-Madrid, M. J.; Garcia-Lopez, J. A.; Saura-Llamas, I.;
Bautista, D.; Vicente, J. Insertion reactions on carbopalladated
benzyne: from eight- to nine- and ten-membered palladacycles.
Applications to the synthesis of N-Heterocycles. Organometallics
2014, 33, 6420−6430. (f) Koller, S. G.; Martín-Romo, R.; Melero, J.
carbene ligands. Tetrahedron 2005, 61, 6207−6217.
(
10) (a) Furstner, A.; Alcarazo, M.; Krause, H.; Lehmann, C. W.
̈
Effective modulation of the donor properties of N-heterocyclic
carbene ligands by “through-space” communication within a planar
chiral scaffold. J. Am. Chem. Soc. 2007, 129, 12676−12677. (b) Chien,
C.-H.; Fujita, S.; Yamoto, S.; Hara, T.; Yamagata, T.; Watanabe, M.;
Mashima, K. Stepwise and one-pot syntheses of Ir(III) complexes
with imidazolium-based carbene ligands. Dalton Trans. 2008, 7, 916−
9
23. (c) Kriechbaum, M.; Winterleitner, G.; Gerisch, A.; List, M.;
Monkowius, U. Synthesis, characterization and luminescence of gold
complexes bearing an NHC ligand based on the imidazo[1,5-
a]quinolinol scaffold. Eur. J. Inorg. Chem. 2013, 2013, 5567−5575.
(
d) Tsui, E. Y.; Agapie, T. Carbon dioxide cleavage by a Ni complex
2
supported by a binucleating bis(N-heterocyclic carbene) framework.
Polyhedron 2014, 84, 103−110. (e) Nonnenmacher, M.; Kunz, D.;
Rominger, F.; Oeser, T. Palladium(II) complexes bearing methylene
and ethylene bridged pyrido-annelated N-heterocyclic carbene ligands
as active catalysts for Heck and Suzuki−Miyaura cross-coupling
reactions. J. Organomet. Chem. 2007, 692, 2554−2563. (f) Roseblade,
S.; Colquhoun, V. P.; Schildbach, D.; Strohmann, C.; Villafane, F.
̃
́
S. J.; Ros, A.; Monge, D.; Alcarazo, M.; Alvarez, E.; Lassaletta, J. M.;
Fernandez, R. Imidazo[1,5-a]pyridin-3-ylidene/thioether mixed C/S
ligands and complexes thereof. Organometallics 2007, 26, 2570−2578.
g) Samanta, T.; Kumar Rana, B.; Roymahapatra, G.; Giri, S.; Mitra,
Structural consequences of an extreme difference between the trans
influence of the donor atoms in a palladacycle. Organometallics 2014,
33, 7329−7332. (g) Gaikwad, V. V.; Bhanage, B. M. Oxime
palladacycle in PEG as a highly efficient and recyclable catalytic
system for phenoxycarbonylation of aryl iodides with phenols. Appl.
Organomet. Chem. 2019, 33, No. e4741.
́
(
P.; Pallepogu, R.; Kumar Chattaraj, P.; Dinda, J. Synthesis, structure
and theoretical studies of Hg(II)−NH carbene complex of annulated
ligand pyridinyl[1,2-a]{2-pyridylimidazol}-3-ylidene hexaflurophos-
phate. Inorg. Chim. Acta 2011, 375, 271−279. (h) Grohmann, C.;
̈
Hashimoto, T.; Frohlich, R.; Ohki, Y.; Tatsumi, K.; Glorius, F. An
iron(II) complex of a diamine-bridged bis-N-heterocyclic carbene.
(14) Selected reviews and articles: (a) Marion, N.; Nolan, S. P. Well-
defined N-heterocyclic carbenes−palladium(II) precatalysts for cross-
coupling reactions. Acc. Chem. Res. 2008, 41, 1440−1449. (b) Navarro,
O.; Kelly, R. A.; Nolan, S. P. A general method for the Suzuki−
Miyaura cross-coupling of sterically hindered aryl chlorides: synthesis
of di- and tri-ortho-substituted biaryls in 2-propanol at room
temperature. J. Am. Chem. Soc. 2003, 125, 16194−16195. (c) Peh,
G.; Kantchev, E. A. B.; Er, J.; Ying, J. Y. Rational exploration of N-
heterocyclic carbene (NHC) palladacycle diversity: a highly active
and versatile precatalyst for Suzuki−Miyaura coupling reactions of
deactivated aryl and alkyl substrates. Chem. - Eur. J. 2010, 16, 4010−
4017. (d) Chiang, M.; Li, Y.; Krishnan, D.; Sumod, P.; Ng, K. H.;
Leung, P. Synthesis and characterisation of a novel chiral bidentate
pyridine-N-heterocyclic carbene-based palladacycle. Eur. J. Inorg.
Chem. 2010, 2010, 1413−1418. (e) Kim, Y.; Lee, J.; Kim, T.; Ham,
J.; Zheng, Z. N.; Lee, S. W. C,N-Palladacycles containing N-
heterocyclic carbene and azido ligands−effective catalysts for Suzuki−
Miyaura cross-coupling reactions. Eur. J. Inorg. Chem. 2012, 2012,
Organometallics 2012, 31, 8047−8050. (i) Schmidt, A.; Grover, N.;
Zimmermann, T. K.; Graser, L.; Cokoja, M.; Pothig, A.; Kuhn, F. E.
̈
̈
Synthesis and characterization of novel cyclopentadienyl molybdenum
imidazo[1,5-a]pyridine-3-ylidene complexes and their application in
olefin epoxidation catalysis. J. Catal. 2014, 319, 119−126.
∧
(
j) Tronnier, A.; Schleicher, D.; Strassner, T. (C C*)-cyclometalated
platinum(II) imidazo[1,5-a]pyridine NHC complexes-synthesis and
characterization. J. Organomet. Chem. 2015, 775, 155−163.
(
11) Cope, A. C.; Siekman, R. W. Formation of covalent bonds from
platinum or palladium to carbon by direct substitution. J. Am. Chem.
Soc. 1965, 87, 3272−3273.
(
12) Selected reviews for palladacycles: (a) Bedford, R. B.; Cazin, C.
S. J.; Holder, D. The development of palladium catalysts for C−C and
C−heteroatom bond forming reactions of aryl chloride substrates.
Coord. Chem. Rev. 2004, 248, 2283−2321. (b) Dupont, J.; Consorti,
C. S.; Spencer, J. The potential of palladacycles: more than just
precatalysts. Chem. Rev. 2005, 105, 2527−2571. (c) Phan, N. T. S.;
Van Der Sluys, M.; Jones, C. W. On the nature of the active species in
palladium catalyzed Mizoroki−Heck and Suzuki−Miyaura couplings−
homogeneous or heterogeneous catalysis, a critical review. Adv. Synth.
Catal. 2006, 348, 609−679. (d) Nomura, H.; Richards, C. J. Allylic
imidate rearrangements catalyzed by planar chiral palladacycles. Chem.
6011−6017. (f) Nagele, P.; Herrlich, U.; Rominger, F.; Hofmann, P.
̈
Transition metal complexes of bulky, electron-rich N-phosphanyl-
substituted N-heterocyclic carbenes (NHCP ligands). Small bite angle
four-membered (κ-C, κ-P) chelate structures. Organometallics 2013,
32, 181−191. (g) Sabater, S.; Mata, J. A.; Peris, E. Chiral palladacycles
with N-heterocyclic carbene ligands as catalysts for asymmetric
hydrophosphination. Organometallics 2013, 32, 1112−1120.
(h) Baier, H.; Metzner, P.; Korzdorfer, T.; Kelling, A.; Holdt, H.
̈ ̈
-
Asian J. 2010, 5, 1726−1740. (e) Alonso, D. A.; Naj
́
era, C. Oxime-
Efficient palladium(II) precatalysts bearing 4,5-dicyanoimidazol-2-
ylidene for the Mizoroki−Heck reaction. Eur. J. Inorg. Chem. 2014,
2014, 2952−2960. (i) Xu, C.; Li, H.; Wang, Z.; Fu, W. Synthesis,
crystal structures and catalytic activity of three cyclopalladated 6-
bromo-2-ferrocenylquinoline complexes with N-heterocyclic carbenes
(NHCs) and triphenylphosphine. Inorg. Chim. Acta 2014, 423, 11−
derived palladacycles as source of palladium nanoparticles. Chem. Soc.
Rev. 2010, 39, 2891−2902. (f) Wu, Y.; Yang, F.; Zhang, J.; Cui, X.;
Gong, J.; Song, M.; Li, T. Cyclopalladated ferrocenylimines as
efficient catalysts for homogeneous catalysis: a brief introduction to
our preliminary achievements. Chin. Sci. Bull. 2010, 55, 2784−2793.
(
g) Mo, D.; Zhang, T.; Ge, G.; Huang, X.; Ding, C.; Dai, L.; Hou, X.
15. (j) Naya, L.; Vaz
M.; Marcos, I.; Lenis, O. A.; Pereira, M. T.; Vila, J. M.; Fernan
́
quez-García, D.; Fernan
́
dez, A.; Lop
́
ez-Torres,
dez, J. J.
The applications of palladacycles as transition-metal catalysts in
organic synthesis. Synlett 2014, 25, 2686−2702. (h) Kapdi, A. R.;
Fairlamb, I. J. S. Anti-cancer palladium complexes: a focus on PdX L ,
́
Novel palladacycle N-heterocyclic carbene complexes with bidentate
[C,N] and terdentate [C,N,N] and [C,N,O] Schiff bases. Synthesis,
characterization and crystal structure analysis. J. Organomet. Chem.
2014, 772, 192−201. (k) Xu, C.; Wang, Z.; Yuan, X.; Han, X.; Xiao,
Z.; Fu, W.; Ji, B.; Hao, X.; Song, M. Solid state fluorescent
palladacycles with N-heterocyclic carbene-based ancillary ligands at
room temperature. J. Organomet. Chem. 2015, 777, 1−5.
2
2
palladacycles and related complexes. Chem. Soc. Rev. 2014, 43, 4751−
4
777. (i) Bruneau, A.; Roche, M.; Alami, M.; Messaoudi, S. 2-
Aminobiphenyl palladacycles: the “most powerful” precatalysts in C−
C and C−heteroatom cross-couplings. ACS Catal. 2015, 5, 1386−
1
396. (j) Catellani, M.; Motti, E.; Della Ca’, N. Catalytic sequential
I
DOI: 10.1021/acs.organomet.9b00495
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
15) (a) Ren, H.; Yao, P.; Xu, S.; Song, H.; Wang, B. A new class of
(20) (a) Ito, S. Effect of the backbone structure of bidentate ligands
in palladium-and nickel-catalyzed polar monomer copolymerization.
Sci. China: Chem. 2018, 61, 1349−1350. (b) Mitsushige, Y.; Yasuda,
H.; Carrow, B. P.; Ito, S.; Kobayashi, M.; Tayano, T.; Watanabe, Y.;
Okuno, Y.; Hayashi, S.; Kuroda, J.; Okumura, Y.; Nozaki, K.
Methylene-bridged bisphosphine monoxide ligands for palladium-
catalyzed copolymerization of ethylene and polar monomers. ACS
Macro Lett. 2018, 7, 305−311. (c) Chen, M.; Chen, C. A versatile
ligand platform for palladium- and nickel- catalyzed ethylene
copolymerization with polar monomers. Angew. Chem., Int. Ed.
2018, 57, 3094−3098. (d) Zou, C.; Pang, W.; Chen, C. Influence of
chelate ring size on the properties of phosphine-sulfonate palladium
catalysts. Sci. China: Chem. 2018, 61, 1175−1178.
o-hydroxyaryl-substituted N-heterocyclic carbine ligands and their
complexes with palladium. J. Organomet. Chem. 2007, 692, 2092−
2
098. (b) Kong, Y.; Ren, H.; Xu, S.; Song, H.; Liu, B.; Wang, B.
Synthesis, structures, and norbornene polymerization behavior of
bis(aryloxide-N-heterocyclic carbene) palladium complexes. Organo-
metallics 2009, 28, 5934−5940. (c) Kong, Y.; Cheng, M.; Ren, H.; Xu,
S.; Song, H.; Yang, M.; Liu, B.; Wang, B. Synthesis, structures, and
norbornene polymerization behavior of bis(aryloxide-N-heterocyclic
carbene) nickel complexes. Organometallics 2011, 30, 1677−1681.
(
d) Kong, Y.; Xu, S.; Song, H.; Wang, B. Synthesis, structures, and
norbornene ROMP behavior of o-aryloxide-N-heterocyclic carbene p-
cymene ruthenium complexes. Organometallics 2012, 31, 5527−5532.
e) Kong, Y.; Tang, Y.; Wang, Z.; Xu, S.; Song, H.; Wang, B.
Ruthenium ring-opening metathesis polymerization catalysts bearing
o-aryloxide-N-heterocyclic carbenes. Macromol. Chem. Phys. 2013,
(
(21) Li, M.; Zhang, H.; Cai, Z.; Eisen, M. S. Norbornene
polymerization and copolymerization with 1-alkenes by neutral
palladium complexes bearing aryloxide imidazolin-2-imine ligand.
Polym. Chem. 2019, 10, 2741−2748.
2
14, 492−498. (f) Kong, Y.; Wen, L.; Song, H.; Xu, S.; Yang, M.; Liu,
(
22) (a) Zijlstra, H. S.; Harder, S. Methylalumoxane−history,
B.; Wang, B. Synthesis, structures, and norbornene polymerization
behavior of aryloxide-N-heterocyclic carbene ligated palladacycles.
Organometallics 2011, 30, 153−159. (g) Dang, L.; Song, H.; Wang, B.
Synthesis, structures, and norbornene polymerization behavior of o-
aryloxide-substituted NHC-ligated σ, π-cycloalkenyl palladium
complexes. Organometallics 2014, 33, 6812−6818. (h) Dang, L.;
Guo, J.; Song, H.; Liu, B.; Wang, B. Synthesis, structures, and
production, properties, and applications. Eur. J. Inorg. Chem. 2015,
2
015, 19−43. (b) Tanaka, R.; Kawahara, T.; Shinto, Y.; Nakayama, Y.;
Shiono, T. An alternative method for the preparation of
trialkylaluminum-depleted modified methylaluminoxane (dMMAO).
Macromolecules 2017, 50, 5989−5993.
(
23) Liu, H.; Zhao, W.; Hao, X.; Redshaw, C.; Huang, W.; Sun, W.
3
H. 2, 6-Dibenzhydryl-N-(2-phenyliminoacenaphthylenylidene)-4-
methylbenzenamine nickel dibromides: synthesis, characterization,
and ethylene polymerization. Organometallics 2011, 30, 2418−2424.
norbornene polymerization behavior of C(sp ), N-chelated pallada-
cycles bearing o-aryloxide-N-heterocyclic carbene ligands. Dalton
Trans. 2014, 43, 17177−17183. (i) Li, M.; Song, H.; Wang, B.
Synthesis and structures of N-heterocyclic carbene−sulfonate
ruthenium complexes and their applications in the ring-opening
metathesis polymerization of norbornene. Eur. J. Inorg. Chem. 2015,
(
b) Zhang, R.; Wang, Z.; Flisak, Z.; Hao, X.; Liu, Q.; Sun, W. H.
Achieving branched polyethylene waxes by aryliminocycloocta[b]-
pyridylnickel precatalysts: synthesis, characterization, and ethylene
polymerization. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2601−
2
015, 4055−4061. (j) Yang, D.; Tang, Y.; Song, H.; Wang, B. o-
2
610.
Aryloxide-N-heterocyclic carbenes: efficient synthesis of the proli-
(
24) Haselwander, T. F. A.; Heitz, W.; Krugel, S. A.; Wendorff, J. H.
̈
gands and their p-cymene ruthenium complexes. Organometallics
Polynorbornene: synthesis, properties and simulations. Macromol.
Chem. Phys. 1996, 197, 3435−3453.
(
2
015, 34, 2012−2017. (k) Yang, D.; Dong, J.; Wang, B. Homo- and
copolymerization of norbornene with tridentate nickel complexes
25) Hicks, R. G.; Koivisto, B. D.; Lemaire, M. T. Synthesis of
bearing o-aryloxide-N-heterocyclic carbene ligands. Dalton Trans.
multitopic verdazyl radical ligands. Paramagnetic supramolecular
2018, 47, 180−189. (l) Yang, D.; Tang, Y.; Song, H.; Wang, B.
synthons. Org. Lett. 2004, 6, 1887−1890.
Synthesis, structures, and norbornene polymerization behavior of
palladium complexes bearing tridentate o-aryloxide-N-heterocyclic
carbene ligands. Organometallics 2016, 35, 1392−1398.
(26) Deeming, A. J.; Rothwell, I. P. Palladation behaviour of 8-
methyl-, 8-ethyl-, and 8-isopropyl-quinolines and some of their 2-
substituted derivatives. J. Organomet. Chem. 1981, 205, 117−131.
(
16) Cordero, B.; Gom
Echeverría, J.; Cremades, E.; Barragan
revisited. Dalton Trans. 2008, 0, 2832−2838.
17) (a) Song, G.; Pang, W.; Li, W.; Chen, M.; Chen, C. Phosphine-
́
ez, V.; Platero-Prats, A. E.; Reves
́
, M.;
(
27) Cope, A. C.; Friedrich, E. C. Electrophilic aromatttic
́
, F.; Alvarez, S. Covalent radii
substitution reactions by platinum(II) and palladium(II) chlorides
on N,N-dimethylbenzylamines. J. Am. Chem. Soc. 1968, 90, 909−913.
(
(
28) Tischler, O.; Bokanyi, Z.; Novak, Z. Activation of C−H
́ ́
sulfonate-based nickel catalysts: ethylene polymerization and
copolymerization with polAr−functionalized norbornenes. Polym.
Chem. 2017, 8, 7400−7405. (b) Yang, B.; Xiong, S.; Chen, C.
Manipulation of polymer branching density in phosphine-sulfonate
palladium and nickel catalyzed ethylene polymerization. Polym. Chem.
activation: the beneficial effect of catalytic amount of triaryl boranes
on palladium-catalyzed C−H activation. Organometallics 2016, 35,
741−746.
2
(
017, 8, 6272−6276.
18) Li, M.; Shu, X.; Cai, Z.; Eisen, M. S. Synthesis, structures, and
norbornene polymerization behavior of neutral nickel(II) and
palladium(II) complexes bearing aryloxide imidazolidin-2-imine
ligands. Organometallics 2018, 37, 1172−1180.
(
19) (a) Ketz, B. E.; Ottenwaelder, X. G.; Waymouth, R. M.
Synthesis, structure, and olefin polymerization with nickel(II) N-
heterocyclic carbene enolates. Chem. Commun. 2005, 45, 5693−5695.
(
b) Waltman, A. W.; Grubbs, R. H. A new class of chelating N-
heterocyclic carbene ligands and their complexes with palladium.
Organometallics 2004, 23, 3105−3107. (c) Benson, S.; Payne, B.;
Waymouth, R. M. Synthesis and reactivity of allyl nickel(II) N-
heterocyclic carbene enolate complexes. J. Polym. Sci., Part A: Polym.
Chem. 2007, 45, 3637−3647. (d) Waltman, A. W.; Ritter, T.; Grubbs,
R. H. Rearrangement of N-heterocylic carbenes involving heterocycle
cleavage. Organometallics 2006, 25, 4238−4239. (e) Hameury, S.; de
Fremont, P.; Breuil, P. A. R.; Olivier-Bourbigou, H.; Braunstein, P.
Bis(ether-functionalized NHC) nickel(II) complexes, trans to cis
isomerization triggered by water coordination, and catalytic ethylene
oligomerization. Organometallics 2015, 34, 2183−2201.
J
DOI: 10.1021/acs.organomet.9b00495
Organometallics XXXX, XXX, XXX−XXX