Efficient Suppression of Chain Transfer and Branching via Cs-Type Shielding in a Neutral Nickel(II) Catalyst

Article abstract of DOI:10.1002/anie.202013069

An effective shielding of both apical positions of a neutral Ni^II active site is achieved by dibenzosuberyl groups, both attached via the same donors’ N-aryl group in a C_s-type arrangement. The key aniline building block is accessibl

Full text of DOI:10.1002/anie.202013069

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Accepted Article
Title: Efficient Suppression of Chain Transfer and Branching via Cs-
Type Shielding in a Neutral Ni(II) Catalyst
Authors: Chaoqun Wang, Xiaohui Kang, Shengyu Dai, Fengchao Cui,
Yunqi Li, Hongliang Mu, Stefan Mecking, and Zhongbao Jian
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10.1002/anie.202013069
Angewandte Chemie International Edition
COMMUNICATION
Efficient Suppression of Chain Transfer and Branching via Cs-
Type Shielding in a Neutral Ni(II) Catalyst
Chaoqun Wang,#[a,b] Xiaohui Kang,#[d] Shengyu Dai,#[e] Fengchao Cui,[f] Yunqi Li,[a] Hongliang Mu,[a]
Stefan Mecking,*[c] and Zhongbao Jian*[a,b]
Abstract: An effective shielding of both apical positions of a neutral
Ni(II) active site is achieved by dibenzosuberyl groups, both attached
via the same donors’ N-aryl group in a Cs-type arrangement. The
key aniline building block is accessible in a single step from
commercially available dibenzosuberol. This shielding approach
suppresses chain transfer and branch formation to such an extent
that ultrahigh molecular weight polyethylenes (5 × 106 g mol-1) are
pathways that compete with chain growth is essential. Accessing
ultrahigh molecular weight polyethylenes[7,8] (UHMWPEs)
requires a 105-fold favoring of chain growth events over chain
transfers.
In olefin polymerization with late transition metal Ni(II) and Pd(II)
catalysts, high polymer molecular weights are favored by steric
shielding of apical positions which hinders chain transfer by
associative displacement.[9-15] The most efficient way of doing so
is arrangement of aryl groups parallel to the square-planar
coordination plane, above and below the active site. Several
examples for 2-N,N diimine complexes are based on the
principle of a C2-type arrangement of the shielding groups, by
attaching each shielding group via the substituent framework of
one donor.
accessible, with
a
strictly linear microstructure (< 0.1
branches/1000C). Key features of this highly active (4.3 × 105
turnovers h-1) catalyst are an exceptionally facile preparation,
o
thermal robustness (up to 90 C polymerization temperature), ability
for living polymerization and compatibility with THF as a polar
reaction medium.
Compared to traditional olefin polymerization catalysts based on
d0-metal sites,[1,2] late transition metal catalysts offer benefits
such as a tolerance towards polar substrates and reaction
media.[3-6] Late transition metal complexes are generally prone to
-hydride elimination (BHE), however. This results in chain
transfer and branch formation pathways. To achieve substantial
polymer molecular weights, an effective suppression of these
[a]
C. Wang, Dr. H. Mu, Prof. Dr. Y. Li, Prof. Dr. Z. Jian
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Renmin Street 5625
Changchun 130022 (China)
E-mail: zbjian@ciac.ac.cn
[b]
[c]
C. Wang, Prof. Dr. Z. Jian
University of Science and Technology of China
Hefei 230026 (China)
Prof. Dr. S. Mecking
Chair of Chemical Materials Science
Department of Chemistry
Chart 1. C2-type arrangement of aryl substituents parallel to the coordination
plane (top, left) and Cs-approach pursued in this work (top, right). C2-type
shielded neutral enolatoimine Ni(II) complex (bottom, left). C1-type N-(8-
phenylnaphty-1-yl) salicylaldiminato Ni(II) complex (bottom, right).
University of Konstanz
Konstanz 78457 (Germany)
E-mail: stefan.mecking@uni-konstanz.de
Prof. Dr. X. Kang
College of Pharmacy
Dalian Medical University
To transfer this approach to neutral complexes with their
unsymmetric 2-X,Y-chelating environment, both different
donors' substitution pattern needs to be suited for attachment of
a shielding 'lid'. Brookhart and Daugulis solved this elegantly by
using enolatoimine complexes most recently (Chart 1; bottom,
left).[16] This approach is not generally applicable to other
popular neutral catalyst motifs, however, in which the hard
oxygen donor is surrounded by a ligand framework less suitable
to achieve C2-type shielding. Thus, for the popular and versatile
salicylaldiminato motif[17-20] so far only a parallel shielding on one
side has been achieved, by means of N-(8-phenylnaphty-1-yl)
groups (Chart 1; bottom, right).[21,22]
[d]
[e]
Dalian 116044 (China)
Prof. Dr. S. Dai
Institutes of Physical Science and Information Technology
Key Laboratory of Structure and Functional Regulation of Hybrid
Materials of Ministry of Education
Anhui University
Hefei, Anhui, 230601 (China)
Prof. Dr. F. Cui
Key laboratory of Polyoxometalate Science of the Ministry of
Education
Faculty of Chemistry
[f]
We now show how an effective shielding on both apical
positions can be achieved by a Cs-type arrangement. This is
achieved via readily accessible dibenzosuberyl substituents, and
affords strictly linear UHMWPE devoid of any branches.
Northeast Normal University
Changchun 130024 (China)
These authors contributed equally.
[#]
Supporting information for this article is given via a link at the end of
the document
Catalyst Design. As
a
reference point,
a
set of
dibenzhydryl[22,23] (Ar2CH-) substituted salicylaldimines covering
1
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10.1002/anie.202013069
Angewandte Chemie International Edition
COMMUNICATION
a range of steric and electronic properties were generated, and
converted to corresponding Ni(II) complexes (Ni2 to Ni10, Chart
2. For full synthesis and characterization of metal complexes
and ligands cf. the Supporting Information, SI). Additionally, the
similar – even slightly lower – Vbur% value than Ni2 (Ph) (45%
and 46% vs. 48%).
prototypical reference Ni1 was prepared by
procedure (Chart 2).[17]
a reported
Chart 2. Reference Ni(II) catalysts studied.
Without addition of any activating cocatalysts, these neutral
single-component Ni(II) catalysts were evaluated for ethylene
polymerization under 8 bar at various temperatures. Despite
their considerably different steric and electronic substitution
patterns, Ni1-Ni10 did not differ dramatically in their catalytic
properties (Table 1 and Figure 1; cf. SI for full polymerization
data).
To rationalize these findings, steric maps based on single crystal
structures were optimized by a DFT method (BP86-D3 method,
cf. SI for computational details) and the buried volume (Vbur) as a
measure of the steric environment around the active Ni(II) site
was calculated[25] (Figure 2). Notably, despite their sterically
demanding substituents Ni8 (p-Ph-Ph) and Ni9 (Np) feature a
Figure 1. Comparison of polymer molecular weights produced by Ni1-Ni12 at
30 oC and 90 oC, respectively (8 bar ethylene)
Table 1. Ethylene polymerization with reference Ni(II) catalysts[a]
[c]
entry
1
2
3
4
5
6
7
8
cat.
Ni1
Ni1
Ni2
Ni2
Ni3
Ni3
Ni4
Ni4
Ni5
Ni5
Ni6
Ni6
Ni7
Ni7
Ni8
Ni8
T (°C)
30
90
30
90
30
90
30
90
30
90
30
90
30
90
30
90
30
90
30
90
30
50
70
90
30
50
70
90
50
yield (g)
0.72
0.56
1.94
1.33
0.61
3.31
0.88
2.18
0.63
2.53
0.84
2.32
1.60
2.29
0.83
2.53
0.94
1.22
2.31
0.80
2.93
3.94
1.87
1.65
2.00
1.75
0.95
0.78
4.61
act. (106)[b]
0.58
0.45
1.55
1.06
0.49
2.65
0.70
1.74
0.50
2.02
0.67
1.86
1.28
1.83
0.66
2.02
0.75
0.98
1.85
0.64
2.34
3.15
1.50
1.32
1.60
1.40
0.76
0.62
3.69
Mn (104)[c]
11.8
0.6
27.3
0.6
8.5
0.4
14.6
0.7
15.0
0.9
17.8
0.6
25.6
0.7
14.3
0.7
14.0
0.3
22.5
1.1
57.4
39.9
32.4
9.5
63.7
44.2
18.5
10.0
36.1
Mw (104)[c]
16.5
0.8
31.9
1.1
12.6
0.4
19.0
1.2
18.2
1.2
22.0
0.9
30.4
1.3
17.7
1.4
18.7
0.5
31.4
1.8
67.2
70.4
53.1
16.9
73.9
71.9
32.8
17.6
56.0
Mw/Mn
1.40
1.44
1.17
1.77
1.47
1.15
1.30
1.55
1.21
1.35
1.24
1.46
1.19
1.95
1.24
1.92
1.34
1.48
1.40
1.70
1.17
1.77
1.64
1.78
1.16
1.63
1.77
1.76
1.55
brs[d]
3
63
3
61
3
43
3
65
2
63
3
60
2
62
10
53
6
84
11
55
3
14
23
46
4
17
42
74
14
Tm[e] (°C)
129
/
134
81
135
101
134
80
135
79
136
85
133
78
133
85
129
/
118
/
132
123
106
84
128
110
83
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29[f]
Ni9
Ni9
Ni10
Ni10
Ni11
Ni11
Ni11
Ni11
Ni12
Ni12
Ni12
Ni12
Ni11
/
118
[a] Reaction conditions: Ni catalyst (5 μmol), toluene (100 mL), ethylene (8 bar), polymerization time (15 min), all entries are based on at least two runs, unless
noted otherwise. [b] Activity is in unit of 106 g mol-1 h-1. [c] Determined by GPC in 1,2,4-trichlorobenzene at 150 °C using a light scattering detector. [d] brs =
Number of branches per 1000C, as determined by 1H NMR spectroscopy. [e] Determined by DSC (second heating). [f] THF (100 mL) as solvent.
2
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COMMUNICATION
Figure 2. Comparison on steric maps of Ni2, Ni3, Ni8, Ni9, and Ni11 optimized by a DFT computational method (BP86-D3).
Viewing the substituents in single crystal maps, we find that due
to their free rotation, these bulky substituents such as Ph, p-Ph-
Ph, and Np consistently orient away from the Ni(II) active center
(cf. SI. Notably, solution NMR data of Ni2 confirms free rotation
of the phenyl groups around the CH-Ph bond, cf. SI). As a
consequence, shielding of the axial sites to retard -H
elimination is limited. Most notably, the axial sites above and
below the square planar coordinated Ni(II) center are not both
shielded (Figure 2a-d).
To appropriately restrict conformational freedom and force an
aryl ring into each apical position, we sought to annulate a given
dibenzhydryl moieties’ aryl groups to a suitable mutual ring
structure. In view of its considerable tension, a seven-membered
ring in the form of a dibenzosuberyl motif[26] was targeted.
Remarkably, the key 2,6-disubstituted aniline is accessible in a
single step from commercially available compounds in good
yield (70 %, Scheme 1). This contrasts the multi-step synthesis
of the aniline component for state-of-the-art N-terphenyl- and N-
8-arylnaphthyl- salicylaldiminato Ni(II) catalysts[21,22,27,28] (cf.
Chart 1).
details), the buried volume of Ni11 was determined by the
aforementioned DFT methods. This yields a significantly higher
value compared to the reference cases (Vbur% value 52 %). Most
significantly, two dibenzosuberyl substituents very effectively
shield the axial sites above and below the square planar Ni(II)
center (Figure 2e). This effective shielding by the suberyl aryl
1
groups is also reflected by an upfield H NMR resonance of the
Ni-Me moiety (Ni11: -0.93 ppm) relative to the reference catalyst
precursors (Ni2: -0.29 ppm). Most notably, in pressure reactor
ethylene polymerization experiments with Ni11 and Ni12 yield
much higher molecular weights vs. all reference systems (Table
1 and Figure 1).
Condensation with salicylaldehydes, and reaction with
[(tmeda)NiMe2] and pyridine yielded dibenzosuberyl-based 9-
anthracenyl- and meta-terphenyl-substituted salicylaldiminato
Ni(II) complexes Ni11 and Ni12, respectively (Scheme 1. For full
characterization data cf. the SI). Ni11 and Ni12 were
undoubtedly corroborated by NMR spectroscopy and elemental
analysis. Based on single crystal structure data (cf. SI for
Scheme 1. Synthesis of Cs-shielded neutral salicylaldiminato Ni(II) catalysts.
Table 2. Effect of time and pressure on ethylene polymerization with Ni11[a]
[c]
entry
1[f]
T (°C)
30
30
30
30
30
30
30
90
p (bar)
8
8
8
8
30
40
40
30
40
40
t (min)
5
yield (g)
0.31
0.60
0.91
1.09
2.61
2.26
2.95
2.94
3.70
2.31
act. (106)[b]
2.19
2.12
2.14
1.92
6.96
12.05
3.93
3.92
4.93
Mn (104)[c]
22.3
39.5
57.8
70.9
208.7
248.9
304.7
21.1
22.4
141.3
Mw (104)[c]
25.5
45.4
69.5
86.5
242.3
372.2
505.1
31.8
34.1
202.0
Mw/Mn
1.15
1.15
1.20
1.22
1.16
1.50
1.66
1.51
1.52
1.43
brs[d]
3.5
2.9
Tm[e] (°C)
131 / 135
133 / 136
133 / 138
132 / 138
136 / 144
134 / 142
135 / 144
99 / 106
2[f]
10
15
20
15
15
60
15
15
60
3[f]
2.7
2.9
4[f]
5[g]
6[h]
7[h]
8[i]
0.2[k]
0.1[k]
< 0.1[k]
29
9[i]
90
30
28
101 / 106
135 / 143
10[h,j]
3.08
0.3[k]
[a] Reaction conditions: toluene (100 mL), all entries are based on at least two runs, unless noted otherwise. [b] Activity is in unit of 106 g mol-1 h-1. [c] Determined
by GPC in 1,2,4-trichlorobenzene at 150 °C using a light scattering detector. [d] brs = Number of branches per 1000C, as determined by 1H NMR spectroscopy.
[e] Determined by DSC at 10 K min-1 (second heating / first heating). [f] Ni11 (1.7 μmol). [g] Ni11 (1.5 μmol). [h] Ni11 (0.75 μmol). [i] Ni11 (3 μmol). [j] THF (100
mL). [k] brs = Number of branches per 1000C, as determined by 13C NMR spectroscopy.
o
Polymerization properties. Ethylene polymerization with Ni11
and Ni12 as single-component catalyst precursors was studied
in more detail (Table 1, entries 21-28 and Table 2). Notably,
Ni11 and Ni12 are thermally robust to give a high molecular
weight (Mw = 16.9  104 g mol-1) even at a high polymerization
temperature of 90 C, more than an order of magnitude higher
than those produced by Ni1-Ni10 (Figure 1), indicating the
effectiveness of our Cs-type shielding strategy.
Compared to these polymerizations in toluene, catalytic activity
is even higher in tetrahydrofuran (THF) as a polar reaction
3
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10.1002/anie.202013069
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COMMUNICATION
medium, although molecular weight decreases slightly (Table 1,
entries 22 vs. 29). Remarkably, polymerization even occurs in
this challenging reaction medium – which is not compatible with
early transition metal and also many late metal polymerization
catalysts[3-6,20] – to high molecular weight polymer (Table 2).
The polymerization to high molecular weight polyethylenes
o
proceeds in a living fashion at 30 C and 8 bar (Table 2, entries
1-4). Both yield and molecular weight increase linearly with
polymerization time (Figure 3). Within 20 min Mw = 86.5  104 g
mol-1 is reached. Notably, polymer precipitation during the
reaction possibly has a small effect on molecular weight
distributions.
Figure 4. 13C NMR spectra (100 MHz, C2D2Cl4, 74.0 ppm) of strictly linear
branched UHMWPEs (conc.: 100 g/L): (a) Table 2, entry 7; (b) Table 2, entry 5.
In summary, the approach reported enables an effective
shielding of both apical positions of a square-planar coordinated
active metal site, through a Cs-type arrangement of both
shielding groups by attachment via the same N-donor atoms’
framework. This overcomes the general propensity of late
transition metal catalysts for -H elimination, and hinders chain
transfer and branching pathways very effectively, as evidenced
by a living nature of the polymerization and the formation of
strictly linear ultrahigh molecular weight polyethylene. Such a
microstructure is desirable for this important material, but difficult
to access, particularly with late transition metal catalysts.
Notably, these novel catalysts are also thermally robust and
tolerant to polar reaction media. At the same time, this novel
type of shielding is easily accessible. The key aniline building
block is formed from commercially available compounds in a
single step, with good yield. Thus, this chemistry provides
broader perspectives for polymerization catalysis and beyond.
Figure 3. Living nature of ethylene polymerization with Ni11 at 30 °C (Table 2,
entries 1-4).
Polymerization at higher 40 bar ethylene pressure results in
remarkably higher catalyst activities (TOF: 4.3  105 h-1) and
molecular weights well in the ultrahigh molecular weight regime
(Mw: 3.7  106 g mol-1), while branching is further reduced to only
0.1/1000C (Table 2, entry 6). This compares favorably with the
state-of-the-art neutral Ni(II) catalysts[16,21,28] and other
catalysts.[29-32]
Due to the triple characteristics of high activity, low degree of
branching, and ultrahigh molecular weight, rapid precipitation of
the product polymer occurred during polymerization, which may
impact the reaction. Thus, an even lower catalyst loading of 7.5
 10-7 mol was explored. Notably, despite the absence of any
alkyl aluminum cocatalysts or scavenger, polymerization
performance is retained (Table 2, entries 6 and 7). At a
prolonged reaction time of 1 h UHWMPE with Mw of 5.1  106 g
mol-1 (Mn: 3.1  106 g mol-1) is formed. Most importantly, the
degree of branching is below the detection limit (< 0.1/1000C)
(Figure 4, Table 2, entry 7). This is also reflected in a very high
melting point (Tm) of up to 144 °C. As anticipated, higher
ethylene pressure of 30 bar and 40 bar at challenging 90 oC
resulted in elevated activities and enhanced molecular weights
(Mw: 3.4  105 g mol-1) (Table 2, entries 8 and 9). Most notably,
in THF reaction medium Ni11 even enabled the formation of
linear UHMWPE (Mw: 2.0  106 g mol-1; 0.3 branches/1000C) at
40 bar (Table 2, entry 10).
Acknowledgements
Z. J. and X. K. are thankful for financial support from the
National Natural Science Foundation of China (Nos. 21871250
and 21704011), the Jilin Provincial Science and Technology
Department Program (No. 20200801009GH). Open access
funding enabled and organized by Projekt DEAL.
Author Contributions: C.W. performed all experimental works,
X.K., F.C. and Y.L. optimized the crystal structures and afforded
steric maps, S.D. gave key discussions, H.M. advised key
syntheses. Z.J. conceived the study and supervised the
research, S.M. and Z.J. discussed the data and wrote the
manuscript. All authors have given approval to the final version
of the manuscript.
Ni11 was also capable of promoting copolymerization of
ethylene and ester-functionalized undecylenic acid methyl ester
o
(cf. SI, Table S2). Studies at various temperatures from 30 C to
90 oC gave copolymers with high molecular weights (3.9 to 26.4
× 104 g mol-1), tunable degrees of branching (4 to 28/1000C),
and reasonable incorporations of comonomer (0.2 to 1.5 mol %).
Conflict of interest
The authors declare no conflict of interest.
4
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10.1002/anie.202013069
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COMMUNICATION
Keywords: coordination polymerization, Cs shielding,
homogeneous catalysis, linear UHMWPE, nickel catalyst
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