Heteroaryl Backbone Strategy in Bisphosphine Monoxide Palladium-Catalyzed Ethylene Polymerization and Copolymerization with Polar Monomers

Article abstract of DOI:10.1021/acs.organomet.9b00340

The backbone structure of ligand is of critical importance to modulate the selectivity and the reactivity of catalyst. In this contribution, heteroaryl backbone (benzo)thiophene was for the first time installed on bisphosphine-monoxide (PO) ligands. Corre

Full text of DOI:10.1021/acs.organomet.9b00340

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Heteroaryl Backbone Strategy in Bisphosphine Monoxide
Palladium-Catalyzed Ethylene Polymerization and Copolymerization
with Polar Monomers
Junhao Ye, Hongliang Mu,* Zhen Wang,*,†,‡,§ and Zhongbao Jian*,†,‡
†
,‡
,†
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Renmin Street 5625, Changchun 130022, China
‡
University of Science and Technology of China, Hefei 230026, China
§
Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Zhongguan West Road 1219, Ningbo
3
15201, China
*
S Supporting Information
ABSTRACT: The backbone structure of ligand is of critical
importance to modulate the selectivity and the reactivity of
catalyst. In this contribution, heteroaryl backbone (benzo)-
thiophene was for the first time installed on bisphosphine-
monoxide (PO) ligands. Corresponding palladium complexes
2
{
κ -2-P(2-OMe-Ph) -3-P(O)(Ph) -4−Br-C HS}PdMeCl (2a)
2
2
4
2
1
2
1
2
and {κ -2-PR R -3-P(O)(Ph) -C H S}PdMeCl {2b: R = R =
2
8
4
1
2
1
2
2
(
-OMe-Ph; 2c: R = R = Cy; 2d: R = Ph, R = 2-(2′,6′-
OMe) C H )-C H } and the compared phenylene-linked
2
6
3
6
4
2
palladium complex {κ -1-P(2-OMe-Ph) -2-P(O)(Ph) -C H }-
PdMeCl (2e) were synthesized and fully characterized by H,
C, P, and 2D-NMR spectroscopy, elemental analysis, and
2
2
6
4
1
1
3
31
single-crystal X-ray diffraction (2a, 2b, and 2d). In the presence
of Na B[3,5-(CF ) C H ] (NaBAr ), these complexes showed high activities (up to 10 000 kg mol h−1) for ethylene
+
−
F
−1
3
2
6
3 4
4
polymerization that are comparable to the prototype phenylene-linked BPMO-Pd catalysts, producing moderate to high
molecular weight (M up to 90 kDa) linear polyethylenes. These catalysts also copolymerized ethylene with various
n
commercially available polar monomers, such as acrylates, acrylic acid, and vinyl butyl ether, to give linear functionalized
polyethylenes with reasonable catalytic activities and moderate copolymer molecular weights and comonomer incorporations.
This work provides new ligand backbones that span donor fragments, and points to new opportunities in catalyst designing for
olefin (co)polymerization.
INTRODUCTION
and co-workers synthesized functionalized polyethylenes with
■
6
7
8
various complex chain architectures. Guan and Chen further
developed this system by introducing bulky substituents or side
arm heteroatoms. Recently, phosphino-phenolate nickel
The application of inherently nonpolar polyolefins in certain
fields is restricted due to their limited printability, adhesion,
and compatibility with other materials, which can be promoted
significantly by incorporating small amounts (even 0.5 mol %)
9
10
complexes were reported by Shimizu and Li to be efficient
catalysts for the copolymerization of ethylene with acrylates
and acrylamides, affording high molecular weight linear
1
of polar functional groups. Thus, controlled synthesis of
functional polyolefins has attracted much attention from both
academic and industrial communities over the past two
decades. Late transition metal catalyzed insertion copolymer-
ization of commercially available polar monomers with olefins
represents a rather promising method to this end. With
significant effort, benchmark late transition metal catalysts
ligated by α-diimine (Pd) and (P O) ligands (Ni or Pd) have
been proven to be the most successful catalytic systems. α-
Diimine palladium catalysts reported by Brookhart and co-
∧
functionalized polyethylenes. Among these (P O) bidentate
catalysts, phosphine-sulfonate palladium catalysts represented
the most thoroughly studied system due to their excellent
catalytic properties for a much broader scope of polar
2
2
a,11
comonomers.
Mechanism studies indicate that the
∧
combination of strong/weak σ-donors is the crucial structural
12
feature for phosphine-sulfonate palladium catalysts. Follow-
ing this principle, Nozaki and co-workers designed bi-
sphosphine monoxide (BPMO) palladium catalysts (A) that
3
3
workers copolymerized olefins with acrylates, vinyl ketones,
4
5
silyl vinyl ether, and vinylalkoxysilanes, giving copolymers
with functional groups located at the ends of the branches. By
taking advantage of the characteristic chain walking process, Ye
Received: May 21, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00340
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
could copolymerize ethylene with a wide range of polar
monomers (Chart 1). After this initial advance, much efforts
Scheme 1. Synthesis of Heteroaryl- and Aryl-Bridged
BPMO Ligands and Palladium Complexes
13
Chart 1. Bisphosphine Monoxide Palladium Catalysts
Bearing Various Backbones
1
4
15
16
by Jordan (B), Chen (C) and Carrow (D) has been
devoted to the optimization of −P or −PO substituents,
leading to several efficient new catalysts.
Steric and electronic effects of one catalyst are decisive for
reaction including olefin polymerization. While these effects
could be readily tuned by changing the type of coordination
sites and their corresponding substituents, the backbone
structure of the bidentate ligand is also of critical importance.
It is believed that backbone structures are responsible for
modulating the bite angle, ligand flexibility, distance between
coordination sites, and electron-donating ability of the ligand,
H and 4-Br sites lithiated by n-BuLi, the yield of pure ligand 1a
formed by lithiation of the active 2-H site was low. This
prompted us to choose an alternative synthetic pathway.
Introduction of an additional phenyl ring into 4,5-positions of
thiophene would eliminate the aforementioned side reactions
and potentially improve the catalyst stability as well. Therefore,
ligands 1b−d were further designed (Scheme 1). Additionally,
phenylene linked BPMO ligand 1e was also designed and
synthesized for comparison. All ligands were fully characterized
17
thus influencing selectivity and reactivity of the catalysts.
Several reports on late transition metal catalysts bearing α-
1
8,19
20
diimine
and phosphine-sulfonate chelate ligands also
highlighted the pivotal role of backbone structures. As a result,
Nozaki et al. designed new BPMO-Pd catalysts bearing a
methylene (E) linker for efficient copolymerization of ethylene
with various polar monomers, even challenging 1,1-disub-
1
13
31
by H, C, and P NMR spectroscopy. The reaction of the
resultant BPMO ligands with PdMeCl(COD) in dichloro-
methane at room temperature gave desired palladium
complexes 2a−e as white powders. The structures and purities
of all these palladium complexes were characterized
21
stituted methyl methacrylate. Chen et al. also disclosed the
diphosphazane monoxide palladium catalysts (F) with a N-
linker for copolymerization of ethylene and polar monomers
and computationally compared this system with phosphine-
1
13
31
unambiguously by comprehensive H, C, P, and 2D-
NMR spectroscopy and elemental analysis. Upon coordination
2
2
sulfonate catalysts. These ligand frameworks also provided
23
3
1
efficient nickel copolymerization catalyst. In addition, other
BPMO-Pd catalysts containing longer flexible linkers have also
to palladium center, the P NMR chemical shifts of phosphine
units moved downfield relative to the free ligands (for more
24
1
been tried, but only oligomerization occurred.
details, see the Supporting Information). In the H NMR
In recent years, we have been concentrating on the design of
late transition metal catalysts and polar monomers for the
improvement on the copolymerization of olefins and polar
spectra, the doublets of newly formed Pd-Me protons arising
from the coupling with P atom at 0.62, 0.66, 0.70, 0.33, and
0.41 ppm for 2a−e respectively clearly indicated the successful
2
5
∧
monomers. In this contribution, we now report the synthesis
of a new class of BPMO-Pd catalysts supported by heteroaryl
backbones that have found applications in bisphosphine
coordination of PdMeCl moieties to the (P O) bidentate
3
ligands. The JPH values (2.7−3.5 Hz) indicate a cis
arrangement between methyl group and phosphine moiety.
26
ligands for metal-catalyzed coupling reactions. The hetero-
aryl-based BPMO palladium catalysts in this work were all
highly active for ethylene polymerization and produced linear
copolymer of ethylene with various commercially available
polar monomers.
An obvious difference between complex 2b and its phenylene
3
1
31
analogue 2e was observed in P NMR spectra. P NMR
spectrum of 2b contains two doublets at 30.58 and 9.4 ppm,
while these signals for complex 2e are at 38.3 and 25.4 ppm,
respectively, indicating possible influence of electron-donating
nature of bezothiophene backbone on both −P and −PO
donors.
RESULTS AND DISCUSSION
■
Thienylene or benzothienylene groups were designed as the
BPMO ligand backbone because of their rigidity and tunable
electronics at 2,3-positions. Thienylene- and benzothienylene-
bridged BPMO ligands 1a−d were synthesized according to
Scheme 1. Due to the competitive side reaction of the active 5-
The solid-state structures of 2a, 2b, and 2d were further
identified by X-ray single crystal diffraction analysis (Figure 1).
In each case, a near-square-planar Pd(II) center is chelated by
phosphorus and oxygen donor atoms. The bulky 2-[2′,6′-
(OMe) C H )]C H biaryl group in complex 2d effectively
2
6
3
6
4
B
DOI: 10.1021/acs.organomet.9b00340
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 1. Molecular structures of Pd(II) complexes 2a, 2b, and 2d drawn with 30% probability ellipsoids. Selected bond lengths (Å): 2a: Pd1−
C31: 2.052(4), Pd1−O1: 2.194(3), Pd1−P2: 2.2071(10), Pd1−Cl1: 2.3644(10). 2b: Pd1−C35: 2.047(3), Pd1−O1: 2.193(2), Pd1−P1:
2
.2108(8), Pd1−Cl1: 2.3670(8). 2d: Pd1−C41: 2.048(3), Pd1−O1: 2.186(2), Pd1−P1: 2.2068(8), Pd1−Cl1: 2.3695(8). Selected bond angles
(
deg): 2a: C31−Pd1−O1: 175.17(14), C31−Pd1−P2: 92.75(12), O1−Pd1−P2: 84.31(7), C31−Pd1−Cl1: 90.58(12), O1−Pd1−Cl1: 92.49(7),
P2−Pd1−Cl1: 176.22(4). 2b: C35−Pd1−O1: 174.93(11), C35−Pd1−P1: 92.19(9), O1−Pd1−P1: 85.23(6), C35−Pd1−Cl1: 90.83(9), O1−
Pd1−Cl1: 91.90(6), P1−Pd1−Cl1: 176.44(3). 2d: C41−Pd1−O1: 179.08(11), C41−Pd1−P1: 92.67(9), O1−Pd1−P1: 87.06(6), C41−Pd1−
Cl1: 90.34(9), O1−Pd1−Cl1: 89.92(6), P1−Pd1−Cl1: 176.90 (3).
shields the metal center from the axial position. The bite angles
of complexes 2a (84.31 (7)°), 2b (85.23 (6)°), and 2d (87.06
evidenced by the similar percent buried volume value. In
contrast, complex 2d provided a significantly more crowded
environment around the palladium center, consistent with the
percent buried volume of 52.3%. This steric protection is
important for late transition metal catalysts in suppressing
(
(
6)°) are all smaller than those in phenylene-linked complex D
1
2
16
R = iPr, R = 2-OMe-C H , 89.02 (3)°) but slightly larger
6
4
1
2
13,16
than those of A (R = 2-OMe-C H , R = tBu, 83.18 (4)°).
6
4
The Pd−P bond lengths for 2a (2.2071 (10) Å), 2b (2.2108
chain transfer reactions.
After the abstraction of chloride unit with NaBAr , all
F
4
(
8) Å), and 2d (2.2068 (8) Å) are slightly shorter than those
1
2
palladium precursors 2a-d were used for ethylene polymer-
in A (R = 2-OMe-C H ) (2.2239 (15) Å) and D (R = 2-
6
4
ization. All these complexes displayed very high activities at a
OMe-C H ) (2.2181 (4) Å). In addition, the Pd−O1 bond
6
4
6
7
−1 −1
level of 10 −10 g mol h . Among these palladium catalysts,
lengths for 2a (2.194 (3) Å), 2b (2.193 (2) Å), and 2d (2.186
2) Å) are longer than that in A (2.1784 (12) Å). It is also
interesting that the Pd−CH distances for 2a (2.052 (4) Å), 2b
2
c with flexible cyclohexyl substituents showed the lowest
(
6
−1 −1
catalytic activities (1.9−3.3 × 10 g mol h ), which were
3
still among the most active BPMO palladium cata-
(
2.047 (3) Å), and 2d (2.048 (3) Å) are obviously longer
those reported for arylene-linked complexes A (2.019(2)Å), B
2.0147(7)Å), and even D (2.0345(17)Å) bearing the same
1
3−16,21,23,27
lysts.
The molecular weights of the polymer
products given by 2c were very low (M = ca. 1000, Table
n
(
1
, entries 7−9). Catalyst 2d bearing the more bulky biaryl
phosphine moieties, presumably reflecting enhanced trans
effect of −PO donor originated from heteroaryl skeleton.
These solid-state structures were also analyzed by SambVca
7
−1
group showed the highest activity of up to 1.1 × 10 g mol
h
−
1
(Table 1, entry 12) under optimized conditions (20 bar, 80
°
C), and produced polymers with the highest molecular
2
.0 program to visualize and quantify the steric hindrance
weights (M = 25 100) in these heteroaryl-based catalysts,
n
around the palladium center (Figure 2). As expected,
complexes 2a and 2b, differing in the ligand backbone that
remotes to the coordination center, showed almost identical
filled space, mostly in the eastern hemisphere, which was also
suggesting a crucial role of the bulky biaryl group in
suppressing chain transfer. Moreover, complexes 2a and 2b,
bearing the same BPMO substituents but different backbones,
showed similar catalytic activities under identical conditions.
The activities and the molecular weights of the polymers
produced by 2b bearing an additional phenyl group in the
backbone were slightly higher (Table 1, entries 1−3 vs 4−6).
As a comparison with 2a, 2b, and 2d, N-linked catalysts F
bearing the same BPMO substituents were much less active
1
2
3
under similar conditions (80 °C, 8 bar, R = R = Ph, R = R =
6
−1 −1
1
2
3
2
-OMe-C H : 2.5 × 10 g mol h ; R = R = R = Ph, R = 2-
6 4
5
−1 −1 23
(
2′,6′-OMe -C H )-C H : 4.0 × 10 g mol h ), but the
2
6
3
6
4
M of the obtained polymers was similar (ca. 10 000). As a
n
1
direct comparison, phenylene-based complex 2e (A, R = o-
OMePh, R = Ph) with the same BPMO substituents as 2a and
2
2
b was also very active for ethylene polymerization and
produced polymers with even higher molecular weights (Table
, entries 15 and 16). This seems to be completely different
1
from the results reported by Nozaki using the original BPMO
1
2
Figure 2. Topographic steric maps of palladium catalysts (2a, 2b, and
system where the catalyst with Ph PO donor (A, R = Ph, R
2
2d) based on heteroaryl BPMO ligands.
= Ph) showed low activity and produced very low molecular
C
DOI: 10.1021/acs.organomet.9b00340
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
13
Table 1. Ethylene Polymerization
weight polymers. We think that this discrepancy could be
attributed to the important role of phosphine donors in this
yield
(g)
act.
M
T
16
n
m
d
6
b
4
c
c
n
e
type of catalysts. As was also reported by Carrow and
the (P O) nickel and palladium catalysts with
simple phenyl-substituted phosphine Ph P led to inferior
entry cat. p (bar)
(10 )
(10 ) M /M
(°C)
Brs
w
10a,14
∧
others,
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
2a
2a
2a
2b
2b
2b
2c
2c
2c
2d
2d
2d
2d
2d
2e
2e
5
10
20
5
10
20
5
10
20
5
10
20
10
20
10
20
3.6
10.7
12.8
5.5
11.3
13.1
2.8
4.7
4.9
9.5
15.0
16.7
1.9
2.1
11.3
2.4
7.1
8.5
3.7
7.5
8.7
1.9
3.2
3.3
0.77
0.82
0.95
1.10
1.26
1.18
0.09
0.10
0.11
2.03
2.51
2.14
9.00
8.26
3.20
2.60
2.2
2.3
2.6
2.2
2.5
2.3
2.2
2.2
1.6
2.3
2.1
2.2
1.8
1.9
2.1
2.1
130
130
130
130
131
132
117
118
120
130
135
131
135
136
129
129
1
1
1
1
2
2
2
2
2
1
<1
1
<1
<1
4
2
catalytic performance. In short, we assume that reasonable
backbone, suitably substituted R2 O, and suitably sub-
P
stituted R P are necessary for an excellent BPMO-type catalyst.
2
The catalytic activities remarkably increased when ethylene
pressure increased from 5 to 10 bar, but this trend was less
prominent when ethylene pressure elevating from 10 to 20 bar,
suggesting that the active sites were saturated and that the rate-
∧
determining step was the insertion reaction of (P O)Pd
0
1
2
3
4
5
6
6.3
R(ethylene) species at high ethylene concentration. Notably,
the polymer molecular weights did not increase proportionally
with ethylene concentration and in some cases even decreased
under higher ethylene pressure (Table 1, entry 5 vs 6 and entry
1 vs 12), indicating serious chain transfer to monomer. This
was also observed in the phosphine−phosphonic diamide
palladium catalyst systems disclosed by Carrow. Overall, the
polymer molecular weights produced by heteroaryl-bridged
BPMO-Pd catalysts were comparable to those phenylene-,
methylene-, and N-bridged analogues with the same P-
substituents such as the anisyl group reported by Nozaki and
In addition, preferred catalyst 2d was also
employed at lower temperatures, exhibiting high activity of
10.0
11.2
1.3
1.4
7.5
f
f
1
15.3
10.2
4
16
aReaction conditions: catalyst (3 μmol), NaBArF4 (3.6 μmol),
toluene/CH Cl2 (96 mL/4 mL), polymerization time (0.5 h),
polymerization temperature (80 °C), 750 rpm, at least two runs,
unless noted otherwise. Brs: branching density/1000C. Activity is in
Determined by GPC in 1,2,4-
trichlorobenzene at 150 °C vs linear polystyrene standards, and
2
b
unit of 106 g mol
−1
h
−1
.
c
1
6,21,23
Chen.
corrected by universal calibration vs linear polyethylene via Mark−
d
6
−1 −1
Houwink−Sakurada equation. Determined by DSC (second
up to 1.4 × 10 g mol
molecular weight greatly improved (Table 1, entry 13, M =
0 000) due to slower chain transfer reaction. Again, lower
h
at 60 °C. Remarkably, the polymer
e
1
heating). Number of branches per 1000C, as determined by H
f
n
NMR in C D Cl at 110 °C. Polymerization temperature (60 °C).
2
2
4
9
molecular weight was observed under higher ethylene pressure
a
Table 2. Copolymerization of Ethylene and Polar Vinyl Comonomers
comono. (mol L−1)
yield (g)
activity
b
XM (mol %)
c
FG distribution
c
M (10 )
3
d
M /Mn
w
d
Tm (°C)
e
Brs
c
entry
cat.
n
1
2
3
4
5
6
7
8
9
2b
2b
2e
2d
2d
2d
2b
2d
2b
2d
2b
2b
2e
2d
2d
2b
2d
2b
2d
2b
2d
2b
2d
2d
MA (0.3)
MA (2.0)
MA (2.0)
MA (0.3)
MA (0.6)
MA (2.0)
tBuA (2.0)
tBuA (2.0)
nBuA (2.0)
nBuA (2.0)
AA (0.3)
AA (2.0)
AA (2.0)
AA (0.3)
AA (2.0)
VTMoS (2.0)
VTMoS (2.0)
nBuVE (0.3)
nBuVE(0.3)
VAc (0.3)
VAc (0.3)
AN (0.3)
0.75
0.30
0.15
4.28
2.13
0.50
0.08
0.18
0.18
0.35
0.83
0.10
0.08
2.22
0.36
0
7.5
3.0
1.5
42.8
21.3
5.0
0.8
1.8
1.8
3.5
8.3
1.0
0.8
22.2
3.6
1.4
7.1
5.4
0.1
0.2
0.9
5.1
1.0
5.5
0.9
0.9
3.9
4.7
0.2
0.8
96:4
97:3
96:4
99:1
99:1
92:8
95:5
99:1
95:5
99:1
78:22
87:13
95:5
78:24
68:32
8.6
6.6
5.7
32.7
20.7
3.4
3.2
5.6
3.8
7.2
2.5
0.8
0.8
9.1
1.8
2.1
1.9
1.8
1.9
1.8
2.0
1.8
2.1
2.2
1.8
2.1
1.5
3.0
1.9
2.3
118
95
1
1
2
1
1
1
1
<1
2
2
2
2
<1
1
1
101
131
129
125
101
125
101
125
123
107
107
130
126
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
0.15
0
1.5
1.0
f
5.0
2.2
123
1
0.14
0.01
0.34
0.09
1.30
0.09
1.4
0.1
3.4
0.9
13.0
0.9
0.2
0
0
0
0
64:36
1.9
g
12.2
3.3
25.9
1.5
2.5
g
2.2
2.1
1.8
1.8
124
g
136
123
133
121
3
g
2
3
1
3
AN (0.3)
AN (2.0)
0
aReaction conditions: catalyst (10 μmol), NaBArF4 (12 μmol), BHT (30 mg), toluene/CH Cl (46 mL/4 mL), polymerization time (1.0 h), 80
2
2
°
C, ethylene (10 bar), 750 rpm, at least two runs, unless noted otherwise. X : Incorporation ratio of polar monomer, FG distribution: The ratio of
M
b c
4
−1 −1
1
main chain unit to chain end unit, Brs: Branching density/1000C. Activity is in units of 10 g mol h . Determined by H NMR in C D Cl at
2
2
4
d
1
10 °C. Determined by GPC in 1,2,4-trichlorobenzene at 150 °C vs linear polystyrene standards and corrected by universal calibration vs linear
e f g
polyethylene via Mark−Houwink−Sakurada equation. Determined by DSC (second heating). Not distinguished. Not determined.
D
DOI: 10.1021/acs.organomet.9b00340
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Organometallics
Table 1, entry 13 vs 14). The high T values (up to 136 °C)
Article
(
Subsequently, vinyl trimethoxysilane (VTMoS) and elec-
tron-rich polar vinyl monomer n-butyl vinyl ether (nBuVE)
were also tested for ethylene copolymerization, in which
catalysts 2b and 2d showed remarkably different functional
group tolerance. For VTMoS, catalyst 2b generated no
polymer at all, while 2d gave 0.15 g of copolymer after 1.0
h, with a comonomer incorporation of 1.0 mol % (Table 2,
entry 16 vs 17). Catalyst 2b was not active in the presence of
.3 M nBuVE, while 2d produced 0.14 g of copolymer with a
.2 mol % comonomer incorporation (Table 2, entry 18 vs
9). These results again exemplified the important role of the
biaryl group in shielding the palladium center in 2d. We also
explored the copolymerization of ethylene with vinyl acetate
VAc) and acrylonitrile (AN) using catalysts 2b and 2d (Table
, entries 20−24). The latter catalyst was more active in the
presence of different concentrations of these two comonomers.
However, no observable incorporation of functional group was
observed in the H NMR spectra of the polymer products. All
copolymers with a variety of polar groups were fully identified
by NMR spectroscopy, GPC, and DSC. In-depth NMR
analysis reveals that all copolymers obtained are highly linear
m
of the polymers produced by 2a−d were consistent with a
highly linear structure (<2/1000C) fashion that is also
1
confirmed by H NMR spectroscopy.
Preferred catalysts 2b, 2d, and 2e were chosen to
copolymerize ethylene with various challenging polar vinyl
monomers (Table 2). Copolymerizations of ethylene (10 bar)
with methyl acrylate (MA) (0.3 M) were performed at 80 °C.
Catalyst 2d showed a significantly higher activity (ca. 6 times)
0
0
1
with a polymer molecular weight (M up to 32 700) higher
n
4
−1 −1
than that of 2b (7.5 vs 42.8 × 10 g mol h ), while the latter
incorporated much more comonomers (1.4 mol % vs 0.1 mol
%
) into the polymer chain under otherwise identical
(
2
conditions (Table 2, entry 1 vs 4). As expected, increasing
comonomer concentration from 0.3 M through 0.6 to 2.0 M
led to higher incorporations (up to 7.1 mol %) along with
decreased catalytic activity and molecular weight. It should be
noted that MA incorporation under a high comonomer
concentration (2.0 M) using 2d increased 9-fold (0.9 vs 0.1
mol %, Table 2, entry 6 vs 4), but was still lower than that
using 2b at 0.3 M (0.9 vs 1.4 mol %, Table 2, entry 6 vs 1).
This may originate from the blockage of bulky comonomer
MA from the metal center due to the presence of the large
sterically demanding biaryl group in 2d (Figure 2), which was
in line with that reported for bulky phosphine-sulfonate
1
(
methyl branching <3/1000C) with the polar functional group
mostly incorporated into the polyethylene main chain (for
details, see the Supporting Information).
CONCLUSIONS
2
8
■
palladium catalysts. Furthermore, catalyst 2b displayed
higher catalytic activity for ethylene/MA copolymerization
than phenylene-linked analogues 2e (Table 2, entry 2 vs 3),
and slightly more comonomers were incorporated (7.1 vs 5.4
To conclude, with the aim of developing high-performance late
transition metal catalysts for the copolymerization of olefins
with fundamental polar monomers, we have developed a series
of BPMO palladium catalysts based on heteroaryl backbones.
These catalyst are among the most active late transition metal
catalysts for ethylene polymerization. The polymer molecular
weights are comparable with that produced by the pioneering
phenylene-, methylene-, and N-bridged analogues. Preferred
catalysts 2b and 2d could copolymerize ethylene with various
challenging polar vinyl comonomers including MA, nBuA,
tBuA, AA, VTMoS, and nBuVE with high activities (up to 4.3
10 g mol h ) and moderate copolymer molecular
weights (up to 32 700) and comonomer incorporations (up to
.1 mol %), giving highly linear (<3/1000C) copolymers. This
work reveals that heteroaryl framework is a promising
candidate as the ligand backbone for enhanced transition
metal olefin (co)polymerization catalysts, since it provides
more opportunity for tuning the metal center. Thus, we are
further developing the heteroaryl backbone strategy into olefin
polymerization catalysts for addressing issues on insertion
1
mol %). H NMR measurement showed that most of the
incorporated MA units (≥92%) were built into the main chain
rather than chain end of the copolymer.
Acrylates bearing more bulky tert-butyl (tBuA) and n-butyl
(nBuA) substituents were also utilized as comonomers. The
catalytic activities, incorporations, and copolymer molecular
weights decreased slightly relative to ethylene/MA copoly-
merizations (Table 2, entries 2, 7, and 9) under the same
conditions.
Acrylic acid (AA) as a large-scale industrial product was
subsequently employed as the comonomer (Table 2, entries
5
−1
−1
×
7
1
0−13). AA insertion polymerizations would suffer from dual
challenges brought about by acidic proton (COOH) and
coordinating groups (CO), and thus required higher
1
1e,16,29
tolerance for the catalysts used.
Under similar
incorporation, the molecular weights of ethylene/AA copoly-
mers were lower than those of ethylene/MA copolymers
(
co)polymerization of polar monomers and are trying to give a
comprehensive picture on the function of heteroaryl frame-
work.
(Table 2, entries 1 and 2 vs 11 and 12), indicating probably
accelerated chain transfer reactions after the AA insertion. This
1
was consistent with the observation by H NMR that the ratio
of end-capped comonomer units was higher than those in
ethylene/MA copolymerization (entry 2 vs 12 and entry 6 vs
EXPERIMENTAL SECTION
■
General Procedures and Materials. All manipulations involving
air- and moisture sensitive compounds were carried out using
standard Schlenk techniques or in a glovebox under a nitrogen
atmosphere. Hexane, THF, CH Cl , and Et O were purified by the
1
5). However, the catalytic activities were comparable to those
in ethylene/MA copolymerization, reflecting the superior
tolerance toward acidic and nucleophilic reagents. Phenylene-
based catalyst 2e showed comparable catalytic activity and
copolymer molecular weight with 2b for ethylene/AA
copolymerization (Table 2, entry 12 vs 13), and the
incorporation was slightly higher (4.7 vs 3.9 mol %).
Moreover, a higher ratio of main-chain to chain-end enchain-
ment was observed for phenylene-based catalyst 2e than that
for 2b (entry 12 vs 13) bearing heteroaryl backbone, signifying
the influence of backbone structures on catalytic behaviors.
2
2
2
MBraun SPS system. NMR spectra for the ligands, complexes, and
1
polymers were recorded on either a Bruker AV400 spectrometer ( H:
1
3
31
4
00 MHz, C: 100 MHz, P: 162 MHz) or a Bruker AV500
1
13
31
1
spectrometer ( H: 500 MHz, C: 125 MHz, P: 202 MHz). H and
13
1
13
C chemical shifts are internally referenced to residual H and
C
31
solvent resonances. P chemical shifts are externally referenced to
1
1
8
5% H PO (δ 0.0). NMR assignments were confirmed by H− H
3
4
1 13 1 13
COSY, H− C HSQC, and H− C HMBC experiments when
necessary. The molecular weights and molecular weight distributions
E
DOI: 10.1021/acs.organomet.9b00340
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
M /M ) of polyethylenes and copolymers were measured by means
4H), 7.38 (ddd, J = 8.2, 7.1, 1.2 Hz, 1H), 7.31 (ddd, J = 8.3, 7.1, 1.2
w
n
3
1
of gel permeation chromatography (GPC) on a PL-GPC 220-type
Hz, 1H). P NMR (202 MHz, CDCl ) δ 22.56.
3
high-temperature chromatograph equipped with three PL-gel 10 μm
Preparation of Ligand 1b.
Mixed-B LS type columns at 150 °C. Melting points (T ) of
m
polyethylenes and copolymers were measured through DSC analyses,
which were carried out on a Q 100 DSC from TA Instruments under
a nitrogen atmosphere at heating and cooling rates of 10 °C/min
(
temperature range: 20−160 °C). Elemental analysis were performed
at the National Analytical Research Centre of Changchun Institute of
Applied Chemistry.
X-ray Diffraction. Data collections were performed at −88.5 °C
on a Bruker SMART APEX diffractometer with a CCD area detector,
using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å).
The determination of crystal class and unit cell parameters was carried
n-BuLi (0.93 mL, 1.6 M in hexane, 1.5 mmol, 1.0 equiv) was slowly
added to a solution of 3-(diphenylphosphinyl)benzo[b]thiophene
30
out by the SMART program package. The raw frame data were
(0.50 g, 1.5 mmol, 1.0 equiv) in THF (50 mL) at −78 °C. The
processed using SAINT and SADABS to yield the reflection data
3
1
mixture was stirred for 4 h at −78 °C, and chlorobis(2-
methoxyphenyl) phosphine (0.42 g, 1.5 mmol, 1.0 equiv) was
added. The reaction mixture was allowed to warm slowly to room
temperature and stirred overnight. The salts were removed by
extraction with CH Cl (30 mL) and distilled water (30 mL). The
file. All structures were solved by direct methods and refined by full-
2
32
matrix least-squares procedures on F using SHELXTL 2014.
2
Refinement was performed on F anisotropically for all non-hydrogen
atoms by the full-matrix least-squares method. The hydrogen atoms
were placed at the calculated positions and were included in the
structure calculation without further refinement of the parameters.
The program SQUEEZ, a part of the PLATON package of
crystallographic software, was used to calculate the disordered solvent
contribution and remove it from the overall intensity data.
2
2
organic layer was dried with Na SO and concentrated under reduced
2
4
33
pressure. The target ligand was obtained by recrystallization from a
1
mixture of CH Cl and n-hexane at −35 °C (0.56 g, 57%). H NMR
2
2
(
(
8
500 MHz, CDCl ): δ 8.85−8.78 (m, 1H), 7.81 (br, 4H), 7.72−7.64
3
m, 1H), 7.41−7.22 (m, 10H), 6.81 (t, J = 7.4 Hz, 2H), 6.76 (dd, J =
General Procedure for the Homopolymerization of Ethyl-
ene. In a typical experiment, a 300 mL stainless pressure reactor
connected with a high-pressure gas line was first dried at 90 °C under
vacuum for at least 1 h. The reactor was then adjusted to the desired
polymerization temperature. Next, 96 mL of toluene was added to the
1
3
1
.2, 5.1 Hz, 2H), 6.69−6.66 (m, 2H), 3.66 (s, 6H, H ). C{ H}
19
NMR (125 MHz, CDCl ): δ 161.11 (d, J = 16.9 Hz, C ), 150.93 (dd,
3
14
1
2
1
2
JPC = 42.0, J = 16.6 Hz, C ), 143.73 (dd, J = 13.6, J = 4.4 Hz,
PC
8
2
PC
PC
1
1
C
), 143.2 (dd, JPC = 12.7, JPC = 2.0 Hz, C
), 133.9, 133.3 ( JPC
), 132.7 (d, JPC = 3.7
Hz), 132.7 (d, JPC = 3.7 Hz), 131.7 (d, JPC = 2.8 Hz), 130.9, 128.0 (d,
PC = 12.7 Hz), 126.4, 124.9, 124.8, 121.5, 121.0, 109.9 (d, JPC = 1.8
=
6
5
F
2
1
reactor under N atmosphere, and the desired amount of NaBAr in
102.9, JPC= 24.9, C ), 133.4 ( JPC= 105.5, C
7
9
2
4
2
mL of CH Cl and the desired amount of Pd catalyst in 2 mL of
2 2
CH Cl were injected into the polymerization system via syringe.
J
2
2
3
1
1
With rapid stirring, the reactor was pressurized and maintained at 20
bar of ethylene. After 0.5 h, the pressure reactor was vented, and the
polymer was precipitated in ethanol, filtered, and dried at 50 °C for at
least 24 h under vacuum.
Hz), 55.5 (s, C19). P{ H} NMR (202 MHz,CDCl
3
): δ 26.14 (d, JPP
= 3.6 Hz, P = O), −41.00 (d, JPP = 3.3 Hz, P-anisyl). Elemental
−1
analysis: Calcd for C34
H O P S (578.60 g mol ): C, 70.58; H, 4.88.
28 3 2
Found: C, 70.87; H, 4.97.
Preparation of the Complexes. Preparation of 2b.
General Procedure for the Copolymerization of Polar
Monomer with Ethylene. In a typical experiment, a 300 mL
stainless pressure reactor connected with a high-pressure gas line was
first dried at 90 °C under vacuum for at least 1 h. The reactor was
then adjusted to the desired polymerization temperature. Next, 46 mL
of toluene with 30 mg of BHT was added to the reactor under N2
atmosphere; then, the desired polar monomer, the desired amount of
F
NaBAr 4 in 2 mL of CH Cl , and the desired amount of Pd catalyst in
2
2
2
mL of CH Cl were subsequently injected into the polymerization
2 2
system via syringe. With rapid stirring, the reactor was pressurized and
maintained at the desired pressure of ethylene. After 1 h, the pressure
reactor was vented, and the copolymer was precipitated in ethanol,
filtered, and dried at 50 °C for at least 24 h under vacuum.
A mixture of ligand 1b (579 mg, 1.0 mmol) and (COD)PdMeCl (265
mg, 1.0 mmol) in 15 mL of CH Cl was stirred at room temperature
2
2
for 12 h. Following filtration, target complex 2b was obtained as a
P r e p a r a t i o n o f L i g a n d s . P r e p a r a t i o n o f 3 -
1
white powder (610 mg, 83%) by precipitation from hexane. H NMR
(
Diphenylphosphinyl)benzo[b]thiophene. 3-Bromo-benzo[b]-
(
500 MHz, 298 K, CDCl , 7.26 ppm): 7.76−7.73 (m, 5H, H and
thiophene (3.0 g, 14.1 mmol, 1.0 equiv) was stirred in diethyl ether
3
10
3
H ), 7.59−7.56 (m, 2H, H ), 7.49 (t, J = 7.5 Hz, 2H, H ), 7.45
(
(
50 mL) and the mixture was cooled to −78 °C. n-BuLi (8.8 mL, 1.6
1
12
HH
16
3
4
3
td, J = 7.5 Hz, J = 3.2 Hz, 4H, H ), 7.34 (t, J = 7.6 Hz,
M in hexane, 14.1 mmol, 1.0 equiv) was slowly added and the mixture
HH PH 11 HH
1
6
3
H, H ), 7.17−7.11 (m, 1H, H ), 7.08−7.05 (m, 3H, H and H ),
was stirred for 4 h at −78 °C. PPh Cl (2.5 mL, 14.1 mmol, 1.0 equiv)
2
3
4
18
2
3
.95−6.89 (m, 4H, H and H ), 3.67 (s, 6H, H ), 0.66 (d, J
=
PH
was then added dropwise and the mixture stirred for 2 h at −78 °C.
The reaction mixture was allowed to warm slowly to room
temperature and stirred overnight. After extraction with diethyl
ether (30 mL), the organic layer was concentrated under reduced
pressure. Then, the residue was dissolved in THF (20 mL), to which
H O (2 mL, 30%) was added. After 3 h, THF was removed under
15
17
19
1
3
1
.4 Hz, 3H, H ). C{ H} NMR (125 MHz, 298 K, CDCl , 77.16
20
3
2
1
2
ppm): δ 161.1 (d, J = 5.9 Hz, C ), 149.5 (dd, J = 31.1, J =
PC
14
PC
PC
2
3
1
0.8 Hz, C ), 142.9 (dd, J = 13.0, J = 2.4 Hz, C ), 140.3 (dd,
8 PC PC 6
2
2
4
JPC = 15.5, J = 8.5 Hz, C ), 134.3 (br, C ), 133.4 (d, J = 1.8
PC
5
18
PC
4
1
2
Hz, C16), 133.1 (d, JPC = 2.9 Hz, C12), 132.8 (dd, JPC = 99.5 Hz, JPC
= 12.3, C
C
(s, C
52.1 Hz, C13), 111.5 (d, JPC = 4.9 Hz, C15), 55.8 (s, C19), −0.78 (s,
C ). P{ H} NMR (202 MHz, CDCl3): δ 30.58 (d, J = 7.5 Hz),
20 PP
2
2
2
1
vacuum, and the residue was extracted with CH Cl . The organic
7
), 132.8 (d, JPC = 11.1 Hz, C10), 130.3 (d, JPC = 110.6 Hz,
2
2
3
layer was dried over Na SO and taken to dryness to give a crude
9
), 128.9 (d, JPC = 12.8 Hz, C11), 125.8 (s, C
3
), 125.3 (s, C
), 124.5
2
2
4
3
1
product as white powder, which can be used directly for the next step
1
), 122.3 (s, C
4
), 120.9 (d, JPC = 9.3 Hz, C17), 117.2 (d, JPC =
3
or purified by column chromatography on silica gel (2:1 ethyl acetate/
1
31
1
hexane) (2 g, 6.0 mmol, 43%). H NMR (500 MHz, CDCl ) δ 7.96−
3
7
.93 (d, J = 7.7 Hz, 1H), 7.90 (d, J = 7.2 Hz, 1H), 7.78−7.70 (m,
H), 7.60−7.55 (m, 2H), 7.53 (d, J = 8.9 Hz, 1H), 7.51−7.45 (m,
9.40 (d, JPP = 7.6 Hz). Elemental analysis: calcd for C H ClO P PdS
35 31 3 2
−1
4
(764.34 g mol ): C, 57.15; H, 4.25. Found: C, 56.85; H, 4.34.
F
DOI: 10.1021/acs.organomet.9b00340
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
4) Luo, S. J.; Jordan, R. F. Copolymerization of Silyl Vinyl Ethers
ASSOCIATED CONTENT
■
with Olefins by (α-diimine)PdR+. J. Am. Chem. Soc. 2006, 128,
2072−12073.
5) Chen, Z.; Liu, W.; Daugulis, O.; Brookhart, M. Mechanistic
*
S
Supporting Information
1
(
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00340.
Studies of Pd(II)-Catalyzed Copolymerization of Ethylene and
Vinylalkoxysilanes: Evidence for a β-Silyl Elimination Chain Transfer
Mechanism. J. Am. Chem. Soc. 2016, 138, 16120−16129.
Experimental details and data on catalysts and polymers
(
6) (a) Wang, J. L.; Zhang, K. J.; Ye, Z. B. One-Pot Synthesis of
(PDF)
Hyperbranched Polyethylenes Tethered with Polymerizable Meth-
acryloyl Groups via Selective Ethylene Copolymerization with
Heterobifunctional Comonomers by Chain Walking Pd-Diimine
Catalysis. Macromolecules 2008, 41, 2290−2293. (b) Xia, X. W.; Ye,
Z. B.; Morgan, S.; Lu, J. M. ″Core-First″ Synthesis of Multiarm Star
Polyethylenes with a Hyperbranched Core and Linear Arms via
Ethylene Multifunctional ″Living″ Polymerization with Hyper-
branched Polyethylenes Encapsulating Multinuclear Covalently
Tethered Pd-Diimine Catalysts. Macromolecules 2010, 43, 4889−
4901. (c) Dong, Z. M.; Ye, Z. B. Hyperbranched Polyethylenes by
Chain Walking Polymerization: Synthesis, Properties, Functionaliza-
tion, and Applications. Polym. Chem. 2012, 3, 286−301. (d) Ye, Z. B.;
Xu, L. X.; Dong, Z. M.; Xiang, P. Designing Polyethylenes of Complex
Chain Architectures via Pd-Diimine-Catalyzed ″Living″ Ethylene
Polymerization. Chem. Commun. 2013, 49, 6235−6255.
Accession Codes
CCDC 1911046, 1911053, and 1911098 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
1
223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
*
*
*
E-mail: muhongliang@ciac.ac.cn (H.M.).
E-mail: wz@nimte.ac.cn (Z.W.).
E-mail: zbjian@ciac.ac.cn (Z.J.).
(7) (a) Popeney, C. S.; Camacho, D. H.; Guan, Z. Efficient
Incorporation of Polar Comonomers in Copolymerizations with
Ethylene Using a Cyclophane-Based Pd(II) α-Diimine Catalyst. J. Am.
Chem. Soc. 2007, 129, 10062−10063. (b) Leung, D. H.; Ziller, J. W.;
Guan, Z. Axial Donating Ligands: A New Strategy for Late Transition
Metal Olefin Polymerization Catalysis. J. Am. Chem. Soc. 2008, 130,
ORCID
Hongliang Mu: 0000-0002-7086-0992
Zhongbao Jian: 0000-0002-2627-454X
Notes
7538−7539. (c) Popeney, C. S.; Rheingold, A. L.; Guan, Z. Nickel(II)
and Palladium(II) Polymerization Catalysts Bearing a Fluorinated
Cyclophane Ligand: Stabilization of the Reactive Intermediate.
Organometallics 2009, 28, 4452−4463.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(8) (a) Dai, S.; Sui, X.; Chen, C. Highly Robust Palladium(II) α-
Diimine Catalysts for Slow-Chain-Walking Polymerization of Ethyl-
ene and Copolymerization with Methyl Acrylate. Angew. Chem., Int.
Ed. 2015, 54, 9948−9953. (b) Dai, S.; Chen, C. Direct Synthesis of
Functionalized High-Molecular-Weight Polyethylene by Copolymer-
ization of Ethylene with Polar Monomers. Angew. Chem., Int. Ed.
We are thankful for financial support from the National
Natural Science Foundation of China (No. 21871250), the
Jilin Provincial Science and Technology Department Program
(
No. 20190201009JC), and the Technology Innovation Fund
of Chinese Academy of Sciences (CXJJ-17-M159). We are
grateful to the reviewers for improving this publication.
2
016, 55, 13281−13285. (c) Li, M.; Wang, X.; Luo, Y.; Chen, C. A
Second-Coordination-Sphere Strategy to Modulate Nickel- and
Palladium-Catalyzed Olefin Polymerization and Copolymerization.
Angew. Chem., Int. Ed. 2017, 56, 11604−11609.
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DOI: 10.1021/acs.organomet.9b00340
Organometallics XXXX, XXX, XXX−XXX