Palladium complexes bearing an N-heterocyclic carbene–sulfonamide ligand for cooligomerization of ethylene and polar monomers

Article abstract of DOI:10.1002/pola.29270

Herein, we report the synthesis of palladium complexes bearing an N-heterocyclic carbene (NHC)-sulfonamide bidentate ligand and their application in ethylene oligomerization and ethylene/polar monomer cooligomerization. These catalysts could smoothly catalyze ethylene oligomerization and ethylene/methyl acrylate cooligomerization albeit the performance was lower compared to that of a NHC–phenoxide catalyst.

Full text of DOI:10.1002/pola.29270

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Palladium Complexes Bearing an N-Heterocyclic Carbene–Sulfonamide
Ligand for Cooligomerization of Ethylene and Polar Monomers
This manuscript is dedicated to Professor Mitsuo Sawamoto’s outstanding achievements in polymer chemistry and recognizes his
recent retirement from 40 years of exceptional service to Kyoto University.
Wenjie Tao , Xiaoming Wang, Shingo Ito , Kyoko Nozaki
Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan
Correspondence to: K. Nozaki (E-mail: nozaki@chembio.t.u-tokyo.ac.jp)
Received 12 September 2018; accepted 17 October 2018; published online 4 December 2018
DOI: 10.1002/pola.29270
ABSTRACT: Herein, we report the synthesis of palladium complexes
bearing an N-heterocyclic carbene (NHC)-sulfonamide bidentate
ligand and their application in ethylene oligomerization and ethyl-
ene/polar monomer cooligomerization. These catalysts could
smoothly catalyze ethylene oligomerization and ethylene/methyl
acrylate cooligomerization albeit the performance was lower
compared to that of a NHC–phenoxide catalyst. © 2018 Wiley Peri-
odicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 474–477
KEYWORDS: catalysis; cooligomerization; ethylene; functionaliza-
tion of polymers; N-heterocyclic carbene; polar monomer;
polyolefins; sulfonamide
catalysts exhibited activity for olefin/polar monomer copoly-
merization, presumably due to the short lifetime of the catalysts.
In this context, our group developed palladium/imidazo[1,5-a]
quinolin-9-olate-1-ylidene (IzQO) complexes (2), which promote
the copolymerization of ethylene and propylene with polar
monomers.11 The key to the success of these catalysts is the
rigid backbone structure of the IzQO ligands to make the NHC
plane and the palladium coordination plane almost coplanar,
thus increasing the catalyst stability.
INTRODUCTION The incorporation of polar functional groups
into polyolefins, which are normally hydrophobic, is a power-
ful method of improving their properties. The functionalized
polyethylenes are endowed with hydrophilic properties such
as adhesiveness, dyeability, printability, and compatibility.1
Transition-metal-catalyzed coordination–insertion copolymeri-
zation of ethylene and polar monomers has emerged as a
direct and efficient synthetic method2 since the seminal dis-
covery that palladium complexes bearing an α-diimine3 or
phosphine–sulfonate (1)4 ligand can incorporate polar mono-
mers into polyethylene. In particular, the latter catalysts, that
is, those bearing an unsymmetrical bidentate ligand, play cen-
tral roles in this field, owing to their ability to incorporate a
wider variety of polar monomers into polyolefins. Experimen-
tal and theoretical investigations have revealed that the com-
bination of a strong σ-donor and a weak σ-donor is the key to
achieving this unique catalytic performance.5 Therefore, sub-
stantial efforts have been devoted to synthesizing many types
of unsymmetrical bidentate ligands for the development of
more active catalysts.2
Based on the aforementioned insights, we have investigated
NHC-based rigid bidentate ligands11,12 and hypothesized that
further screening of weak σ-donors would lead to more active
catalysts. In the present study, we focus on sulfonamide as a
weak σ-donor counterpart,13,14 although palladium/phos-
phine–sulfonamide complexes (3) have been reported to
exhibit no or low catalytic activity in ethylene polymeriza-
tion.14 Herein, we report the synthesis of palladium complexes
4 bearing an NHC–sulfonamide bidentate ligand and their
application in ethylene oligomerization and ethylene/polar
monomer cooligomerization (Fig. 1).
One strategy for developing active catalysts is to replace the
phosphine moiety by other stronger σ-donors such as
N-heterocyclic carbene (NHC).6 A variety of bidentate NHC
ligands have thus far been investigated in metal-catalyzed olefin
polymerization reactions, including those bearing arenolate,7
enolate,8 sulfonate,9 and pyridine10 as a counterpart, but few
RESULTS AND DISCUSSION
Ligand precursors L1 and L2 were synthesized as shown in
Figure 2. The synthesis of L1 started from the oxidation of the
methyl group of 2-methyl-8-nitroquinoline. The resulting alde-
hyde was converted to an imidazolium salt and then the nitro
Additional supporting information may be found in the online version of this article.
© 2018 Wiley Periodicals, Inc.
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FIGURE 3 Synthesis of palladium/NHC–sulfonamide complexes
4a and 4b.
silver triflate to form the silver carbene intermediates. The sil-
ver intermediates were reacted with a (methyl)(chlorido)(2,-
6-lutidine)palladium dimer to afford neutral palladium
complexes 4a and 4b.
The solid-state structures of complexes 4a and 4b, which
were determined by single crystal X-ray diffraction analysis,
revealed that 4a and 4b have similar structures (X-ray CCDC
1866966 (4a), 1866967 (4b), and 1871362 (L1) contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/structures).
In both complexes, the methyl group is located trans to the
NHC moiety. This type of trans isomer, in which the alkyl
group is located trans to the strong σ-donor, is thermodynami-
cally less stable than the corresponding cis isomer, which is
reflected by the results that few trans palladium complexes
have been isolated.9b,11,15
FIGURE 1 Representative palladium catalysts for ethylene/polar
monomer copolymerization.
group was reduced into an amino group. The final step was a
sulfonation reaction with 2,4,6-trimethylbenzenesulfonyl chlo-
ride to afford L1. Notably, this reaction route was not applica-
ble for the synthesis of L2 because the last step, that is,
methanesulfonylation of the amino group, afforded a complex
mixture of products. Thus, the synthesis of L2 started from
the palladium-catalyzed hydrogenation of the nitro group of
2-methyl-8-nitroquinoline. After methanesulfonylation of the
amino group, the resulting substrate was oxidized with sele-
nium oxide to afford the corresponding aldehyde. Finally, the
aldehyde was reacted with 2,6-diisopropylaniline and chloro-
methyl ethyl ether to afford imidazolium salt L2.
We investigated ethylene polymerization and ethylene/polar
monomer copolymerization using palladium cꢀomplexes
4 (Table 1ꢀ). Ethylene polymerization with 4a at 60 C (Entry
1) and 80 C (Entry 2) afforded oligoethylenes with polymeri-
zation activities of 16.2 and 98.0 kg mol−1 −1, respectively.
h
The observed activities are lower than those of typical
Next, we synthesized palladium complexes 4a and 4b using a
procedure similar to that used for Pd/IzQO complexes
(Fig. 3).11 Ligand precursors L1 and L2 were deprotonated
with potassium bis(trimethylsilyl)amide and reacted with
palladium catalysts such as palladium/phosphine–sulfonate
(120–5330 kg mol−1 h−1 5b,16
)
and palladium/IzQO (33–
690 kg mol−1 h−1) catalysts,11a but higher than those of
FIGURE 2 Synthesis of ligand precursors L1 and L2.
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TABLE 1 Ethylene Oligomerization and Ethylene/Polar Monomer Cooligomerization by Palladium Complexes 4a and 4ba
Polar
Temp
(ꢀC)
Time
(h)
Yield
(g)
Activity
Mw/
b
Incorporation
Ratio (%)c
Entry Catalyst Monomer
(kg mol−1 h−1
)
Mnb/1000 Mn
1
2
3
4
4a
4a
4a
4a
4b
4a
–
60
80
80
80
80
80
1.0
1.0
2.0
4.0
2.0
12.0
0.043
0.294
0.762
0.885
0.387
0.342
16.2
98.0
127.0
73.7
64.5
2.8
0.70
0.42
0.32
0.36
0.40
0.35
1.7
1.6
1.8
1.7
1.9
1.5
–
–
–
–
–
–
–
5
–
–
6d
MA
2.8
a
d
A mixture of catalyst (4.0 μmol) in toluene (10.0 mL) was stir-
red under ethylene atmosphere (4.0 MPa).
Molecular weights determined by size exclusion chromatography.
Comonomer incorporation ratio determined by 1H NMR
analysis.
A mixture of catalyst 4a (10.0 μmol), MA (2.0 mL), and
2,6-di-tert-butyl-4-methylphenol (100 mg) in toluene (8.0 mL)
b
c
was stirred under ethylene atmosphere (4.0 MPa).
FIGURE 4 X-ray structures of 4a and 4b with thermal ellipsoids set to 50%. Hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and angles (ꢀ) for 4a: Pd1–C1 = 2.070(5), Pd1–C2 = 2.105(5), Pd1–N3 = 2.074(4), Pd1–N4 = 2.062(4), C2–Pd1–N3 = 82.15(17).
Selected bond lengths (Å) and angles (ꢀ) for 4b: Pd1–C1 = 2.065(2), Pd1–C2 = 2.084(2), Pd1–N3 = 2.0539(19), Pd1–N4 = 2.0694(19), C2–
Pd1–N3 = 82.85(8).
palladium/phosphine–sulfonamide catalysts.14 Thus, we
CONCLUSION
decided to perform the oligomerization at 80 ꢀC for the fol-
In conclusion, palladium complexes bearing an NHC–
lowing entries. The time course of the reaction indicated that
sulfonamide bidentate ligand were synthesized and character-
the highest activity was observed when the polymerization
ized. These catalysts could smoothly catalyze ethylene
was stopped after 2.0 h (Entry 3) rather than 1.0 h (Entry 2)
oligomerization and ethylene/MA cooligomerization.
or 4.0 h (Entry 4). These results suggest the presence of an
induction period to initiate the oligomerization, followed by
the slow deactivation of the catalyst during the oligomeriza-
tion process. In all the entries, the obtained products were oli-
EXPERIMENTAL
goethylenes with molecular weights of Mn = about 400. When
catalyst 4b, which has a methanesulfonyl group, was used, the
catalytic activity (64.5 kg mol−1 h−1; Entry 5) was much lower
than that of 4a (127 kg mol−1 h−1; Entry 3). Finally, catalyst
4a was applied to the copolymerization of ethylene and
methyl acrylate (MA). The reaction afforded an ethylene/MA
cooligomer with an MA incorporation ratio of 2.8%. Nuclear
magnetic resonance (NMR) analyses revealed that MA was
incorporated into the chain ends rather than into the chain
(see Supporting Information).
A 50-mL stainless steel autoclave was dried in an oven at
120 ꢀC, and then allowed to cool to room temperature
inside a drybox. Into the autoclave was added catalyst
4 (4.0 μmol) and toluene (10 mL). The autoclave was
charged with ethylene (4.0 MPa), sealed, and stirred in an
isothermal heating block under the indicated conditions.
After cooling to room temperature, releasing the redundant
ethylene gas, and removing the volatile matter in vacuo, the
residue (oil) was dried under vacuum for 4.0 h at 80 ꢀC
and analyzed.
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ACKNOWLEDGMENTS
X.-X. Gou, Y.-F. Han, Chem. Asian J. 2018, 13, 2257.
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erences cited therein.
This work was supported by JST CREST Grant Number
JPMJCR1323, Japan. A part of this work was conducted at the
Research Hub for Advanced Nano Characterization, The Uni-
versity of Tokyo, supported by MEXT, Japan.
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