Rational Design of High-Performance Phosphine Sulfonate Nickel Catalysts for Ethylene Polymerization and Copolymerization with Polar Monomers

Article abstract of DOI:10.1021/acscatal.6b03394

Use of palladium catalysts in olefin polymerization and copolymerization has evolved rapidly. In contrast, earth-abundant and low-cost nickel catalysts generally suffer from drawbacks that include low thermal stability and generation of low-molecular-weight polymers in the presence of polar monomers. By taking advantage of several design strategies, high-performance phosphine-sulfonate-based nickel catalysts were developed. These nickel catalysts demonstrated high activities and thermal stabilities to afford high-molecular-weight polyethylene. Most importantly, high-molecular-weight copolymers could be generated through the copolymerization of ethylene with a variety of polar monomers.

Full text of DOI:10.1021/acscatal.6b03394

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Letter
Rational Design of High-Performance Phosphine Sulfonate Nickel Catalysts
for Ethylene Polymerization and Copolymerization with Polar Monomers
Min Chen, and Changle Chen
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03394 • Publication Date (Web): 12 Jan 2017
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Rational Design of High-Performance Phosphine Sulfonate Nickel
Catalysts for Ethylene Polymerization and Copolymerization with
Polar Monomers
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Min Chen, Changle Chen*
CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy
Materials), Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei,
230026, China
ABSTRACT: Use of palladium catalysts in olefin polymerization and copolymerization has evolved rapidly. In contrast,
Earth-abundant and low-cost nickel catalysts generally suffer from drawbacks that include low thermal stability and
generation of low-molecular-weight polymers in the presence of polar monomers. By taking advantage of several
design strategies, high-performance phosphine sulfonate-based nickel catalysts were developed. These nickel
catalysts demonstrated high activities and thermal stabilities to afford high-molecular-weight polyethylene. Most
importantly, high-molecular-weight copolymers could be generated through the copolymerization of ethylene with a
variety of polar monomers.
Keywords: nickel catalysts, ligand design, olefin polymerization, polar monomer, copolymerization
Polyolefins are the most common synthetic polymers,
representing almost half of the 300 million tons of plastics
produced worldwide.1 Despite this huge annual production
and the wide range of applications of polyolefins, their
nonpolar nature remains
a
significant limitation.
Incorporation of polar functional groups can greatly
improve the properties of these polymers. The most direct
and economical synthetic strategy involving coordination
copolymerization of olefins with polar monomers is highly
challenging.2 As major players in the polyolefin industry,
early transition metal catalysts are prone to poisoning from
polar groups, making them incompetent for the required
task. In contrast, because of their low oxophilicity, late
transition metal catalysts provide great potential for
addressing this issue. A milestone discovery was made in the
1990s by Brookhart et al., who reported the
copolymerization of olefins with acrylate monomers using α-
diimine palladium catalysts.3 Subsequently, great effort has
been directed toward the preparation of a variety of polar
functionalized copolymers.4
Chart 1. Strategies to improve the performance of
phosphine-sulfonate nickel catalysts.
limitation for industrial applications.5 As a result, relatively
few high-performance Ni(II) catalyst systems and narrow
polar monomer scope have been reported.
Some of the most notable examples include the neutral
salicylaldimine Ni(II) catalysts reported by Grubbs et al.,
which are capable of copolymerizing ethylene with polar-
substituted norbornene and a few other special polar
monomers.6 The groups of Agapi and Takeuchi reported the
copolymerization of ethylene with polar monomers using
dinuclear salicylaldimine Ni(II) catalysts.7a, 7b Coates et al.
showed that the α-diimine Ni(II)/MAO system can
copolymerize ethylene with methyl 10-undecenoate.7c,7d
Nozaki et al. described the copolymerization of ethylene
with allyl acetate and allyl ether, using carbene-based nickel
catalysts at low activities (~1 kg/mol·h).7e SHOP (Shell
Higher Olefin Process)-type nickel catalysts and a few other
nickel-based catalysts have been demonstrated to mediate
ethylene copolymerization with polar monomers.8
Significant progress has been made in palladium-
catalyzed olefin/polar monomer copolymerization chemistry
since then. However, utilization of nickel metal has proven
to be very difficult because of its intrinsically high
oxophilicity. This translates into a much higher suppression
of polymerization in the presence of polar monomers than
observed for the analogous palladium chemistry. For
example, Mecking et al. showed that the challenging nature
of nickel-catalyzed copolymerization arises from the
retarded reactivity of the functionalized olefin insertion
products, as well as some deactivation reactions.4r In
addition, many nickel catalysts show greatly reduced
activities at temperatures above 80 oC, which is a serious
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R1
PH+
R1
PH+
Ar
Ar
Recently, some palladium complexes based on phosphine
SO3H
nBuLi
2 nBuLi
sulfonate ligands and their derivatives have emerged as
superior catalysts for the copolymerization of ethylene with
a variety of polar monomers.9 In contrast, the corresponding
nickel catalysts usually afford polyethylene of low molecular
weight (Mn around 1000).10 Nozaki et al. demonstrated the
phosphine sulfonate nickel catalyzed copolymerization of
ethylene with allyl acetate and allyl silyl ether, with low
activity (~1 kg/mol·h), low comonomer incorporation (<
0.24%), and low copolymer molecular weight (Mn < 1000).11
Several strategies have been reported for improving the
properties of late transition metal catalysts in literature. (1)
It is well known that steric bulk at the axial positions in α-
C6F6
or Me3SiCl
ArR1PCl
O-
O-
S
S
R2
O
O
O
O
R1 = Ph;
R2 = C6F5
t
L1
L2
L3
L4 L5
), SiMe3 ( )
R1 = Ph ( ), Cy ( ), Bu (
)
(
R1
Ar
Me
R1
1) Ni(COD)2
2,6-lutidine
Ar
1
P
: R1=Ph, R2=H;
2: R1=Cy, R2=H;
4: R1=Ph, R2=C6F5
H+
P
Ni
2) allyl chloride
3) AlMe3
Lut
S
O
R2
O-
S
R2
O
O
O
O
O
R1
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Ar
Ar=
P
3: R1=tBu, R2=H;
: R1=Ph, R2=SiMe3
Ph
1) NaH
Ni
5
OMe
2) (PPh3)2NiPhCl
PPh3
MeO
S
O
R2
O
diimine Pd(II) and Ni(II) catalysts can decrease the rate of Scheme 1. Synthesis of the phosphine sulfonate ligands
chain transfer to monomer, leading to increased polymer and the corresponding nickel complexes.
molecular weight.4 This strategy was successfully applied by
Mecking et al. and Scott et al. to phosphine sulfonate Pd(II)
and Ni(II) catalysts through the incorporation of biaryl
substituents (Chart 1, I).12 (2) Electronic perturbation is
important for modulating the properties of metal catalysts,
and electron-donating substituents may lower the
oxophilicity of a nickel center. Recently, Mecking et al.
showed that electron-donating substituents (R) on
phosphine sulfonate Pd(II) catalysts (Chart 1, II) can
increase the polyethylene molecular weight.13 (3) In
salicylaldimine Ni(II) and phosphine sulfonate Pd(II)
systems, the steric bulk of the ortho R' substituent (Chart 1,
II and III) is crucial to obtaining high-molecular-weight
polymers.6,9m (4) The presence of strongly coordinating
Figure 1. Molecular structures of catalysts 2 and 5. Hydrogen
bases (such as PPh3 in catalyst III) may require a scavenger
atoms and the Ph3P phenyl groups in 5 were omitted for
clarity. Selected bond lengths (Å) and angles (deg) for 2:
Ni2-P2 = 2.1397(7), Ni2-C41 = 1.939(3), Ni2-N2 = 1.979(2),
Ni2-O6 = 1.9775(18), C41-Ni2-N2 = 88.86(11), O6-Ni2-P2 =
94.11(6); for 5: Ni2-P3 = 2.2314(11), Ni2-C83 = 1.894(4), Ni2-P4
= 2.2615(11), Ni2-O6 = 1.984(3); C83-Ni2-P4 = 88.03(11), O6-
Ni2-P3 = 90.65(9).
to activate the catalyst precursor. Moreover, the
coordinating base may competitively bind to the metal
14
center as opposed to the polar monomer.12a,
In this
contribution, we demonstrate that the abovementioned
drawbacks for nickel catalysts can be addressed through
rational catalyst design.
The phosphine sulfonate ligands were prepared by the
reaction of benzenesulfonic acid with two equiv. of nBuLi,
followed by the addition of one equiv. of R1ArPCl (Scheme 1;
Ar = 2-[2,6-(MeO)2C6H3]C6H4; R1 = Ph (L1), Cy (L2), tBu (L3)).
group is positioned cis to the phosphine. The biaryl
substituent does not lie on the axial position of the nickel
metal, however, during polymerization, and after the
dissociation of the coordinating base, chelate ring inversion
and/or pseudoaxial aryl group rotation could bring the
biaryl substituent to the axial positions.15
n
The reaction of ligand L1 with two equiv. of BuLi, followed
by the addition of C6F6 or Me3SiCl, afforded ligands L4 (R1 =
Ph, R2 = C6F5) and L5 (R1 = Ph, R2 = SiMe3), respectively, in
57-64% yields. Our initial target was the nickel 2,6-lutidine
complex, because the labile nature of the 2,6-lutidine base
may likely lead to the formation of a single component
nickel catalyst. Complexes 1, 2 and 4 were generated in 46-
62% yields using the strategy developed by Nozaki et al.11
Similar procedures utilizing ligands L3 and L5 generated
complex mixtures from which the desired products could
not be isolated. Therefore, the PPh3 adducts (3 and 5) were
prepare instead. These metal complexes were characterized
by 1H, 13C, and 31P NMR spectroscopy, and elemental
First of all, these nickel complexes (1-5) exhibit good
o
thermal stability, maintaining high activities at 80 or 100 C
(Table 1). With activities well above 106 g·mol-1·h-1, they are
among the most active neutral nickel catalysts for ethylene
polymerization.6a,10e,14,16 Complexes 2 and 3, bearing electron-
donating R1 groups, are less active than complex 1, affording
polyethylene of lower molecular weight. This is the most
important reason that we decided to prepare ligands L4 and
L5, bearing phenyl groups at the R1 position. With the extra
ortho substituent, catalyst 4 demonstrated much higher
activities, as well as higher polymer molecular weights.
Interestingly, the polymer molecular weights produced from
the various catalysts followed the order: 4 > 1 > 5. This is
analysis. The molecular structures of
2 and 5 were
determined by x-ray diffraction (Figure 1). The geometry at
the nickel center is square planar, and the R (Me or Ph)
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Table 1. Ethylene homopolymerization at 80 oC and 100 Table 2. Ethylene-polar monomer copolymerization at
oC.a
80 oC.a
c
Ent Cat. T(oC) Yieldb Act.(106)b Mn
PDIc m (oC)d
T
d
Mn PDId
Comonom [M] Yield Act. XM
Tm
(oC)e
Ent. Cat.
1
2
1
2
3
4
5
1
80
80
3.7
2.3
3.0
4.5
3.2
3.2
2.0
2.2
4.0
2.9
3.7
2.3
3.0
4.5
3.2
3.2
2.0
2.2
4.0
2.9
3200 3.02 122.6
2900 3.17 121.8
1700 2.74 120.4
4700 2.70 126.1
2900 2.64 121.9
2300 3.56 121.1
2000 3.33 121.0
1500 3.12 120.0
3800 3.49 124.6
2100 3.28 120.8
er
mol/L (g)b (104)b (%)c
1
2
1
1
2
1
1.2
1.2
6.0 3.4 12500 1.49 124.1
6.0 2.1 12200 2.13 119.5
1.0 1.6 19600 1.78 128.8
1.0 0.7 4300 2.52 116.3
0.5 0.8 2000 1.58 122.6
5.0 3.1 10400 2.61 126.4
11.5 1.6 10400 2.50 125.9
11.5 2.4 3700 2.14 114.7
4.5 1.6 7900 2.76 119.1
5.5 1.8 4800 2.50 122.5
3
80
4
5
80
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3
1
1
0.2
0.2
0.1
1.0
2.3
2.3
0.9
1.1
80
6
7
100
100
100
100
100
4
1
1
5f
1
1
2
3
4
5
8
9
10
6
1
2
2
2
2
1
7
1
aPolymerization conditions: catalyst = 2 μmol; toluene = 48
mL, CH2Cl2 = 2 mL, ethylene = 8 atm, time = 0.5 h. bThe
yields and activities are average of at least two runs. Activity
is in unit of 106 g·mol-1·h-1. cDetermined by GPC in
trichlorobenzene at 150 oC. dDetermined by differential
scanning calorimetry (DSC).
8
1
9
1
10
11
1
1
1
1.5
7.5
4.0 7.6 3000 2.60 94.5
15.5 2800 2.87 118.0
2
4500 2.76 118.7
12
13
14
15
16
17
18
19
20
21
22
1
3
1
0.8
3.1
consistent with comparisons made between catalysts 2/3
and 1. Based on time-dependence studies at 80 oC (Figure
S1), catalyst 1 became deactivated within 20 min, while
catalyst 4 maintained high activity for up to 1 h.
2
3
4
5
2
3
4
5
4
4
4
3
1
2.7
2.1
13.5 1.9 1600 3.53 120.0
10.5 4.4 9800 3.54 118.7
1
Secondly, this is one of the very few nickel catalyst
systems that can copolymerize ethylene with a variety of
polar monomers (Table 2). Without the use of large
amounts of cocatalyst or protecting reagents, very few
single-component nickel catalysts are capable of initiating
the efficient copolymerization of ethylene with polar
monomers. In this system, methyl acrylate completely shuts
down polymerization.4r Moderate polymerization activity
was observed with butyl vinyl ether, and no comonomer
incorporation was observed. High activity, good comonomer
1
1.5
7.5
3
4000 3.52 118.9
2
2
2
2
1
1.2
6.0 4.4 4700 2.38 117.4
7.5 2.5 3600 2.33 120.4
1.5
1.2
6.0
6
12000 2.68 115.5
0.8
0.2
1.6
4.9
4.0 3.4 4500 2.53 118.0
1.0
1.2 5400 2.06 118.5
CH3COOEt
Et2O
1
8.0
-
10700 3.70 126.9
incorporation (0.7-7.6%), and high copolymer molecular 23
weights (Mn up to 19600) were achieved during 1-catalyzed
ethylene copolymerizations with silicon containing
vinyl/allyl comonomers, allyl ether, allyl cyanide, polar
functionalized norbornenes, and some OH/COOMe/Br/Cl
containing, long chain, comonomers. Among these nickel
catalysts, 4 gave the highest comonomer incorporation, and
the highest copolymer molecular weight. Interestingly,
catalysts 2 and 3, bearing electron donating substituents,
showed higher activities than 1. This may originate from the
lower electrophilicity of the nickel center in catalysts 2 and
3, compared to catalyst 1.
1
24.5
-
48000 1.80 134.9
aPolymerization conditions: total volume of solvent and
polar monomer = 20 mL, catalyst = 20 μmol, ethylene = 8
atm, 80 C, time = 1 h. bThe yields and activities are average
o
of at least two runs. Activity is in unit of 104 g·mol-1·h-1.
cDetermined by H NMR in C2D2Cl4 at 120 oC. dDetermined
1
o
e
by GPC in trichlorobenzene at 150 C. Determined by DSC.
f100 μmol B(C6F5)3 was added.
incorporate these types of comonomer.8 Interestingly,
acetonitrile was shown to greatly inhibit polymerization in
the phosphine sulfonate palladium system, even at very low
acetonitrile loadings (0.05 M).14a In our system, this
inhibition effect is less dramatic; 1.2 g of polyethylene was
generated by catalyst 4 in the presence of 1 M of acetonitrile
under the conditions shown in Table 2.
Nozaki et al. previously showed that allyl chloride
completely deactivated phosphine sulfonate nickel
catalysts.11 In our system, small amounts of product were
generated during 1-catalyzed ethylene copolymerization
with allyl chloride. We previously showed that the addition
of B(C6F5)3 to phosphine sulfonate nickel catalysts greatly
Thirdly, very high molecular weights are able to be
increases their ethylene polymerization activity.10c Here,
1/B(C6F5)3 was shown to initiate efficient ethylene/allyl
chloride copolymerization, with moderate activity,
comonomer incorporation, and molecular weight. The
ability of this catalyst system to incorporate the allyl cyanide
comonomer (Table 2, entries 4 and 21) represents a big
advantage, since SHOP-type catalysts are unable to
achieved
in
both
ethylene
polymerization
and
copolymerization reactions (Table 3). At 25 oC,
semicrystalline polyethylenes, with high molecular weights
(
Mn up to 446,000), high melting temperatures (Tm up to
o
138.5 C), and low branching densities (5/1000 carbon atoms
for 3 and 2/1000 carbon atoms for 4) were generated. Similar
to observations made under high temperatures, catalyst 4
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Table 3. Ethylene (co)polymerization at 25 oC.a
ethylene homopolymerization and copolymerization
reactions (Table 3, entries 3, 5 vs 12, 13, entries 9, 11 vs 14, 15,
similar behavior was observed at 80 oC, see Table S1).
Probably, the strong trans influence of the phosphine
moiety increases the lability of the PPh3 coordination, such
that it no longer affects ethylene coordination or insertion
under polymerization conditions. This is an important
advantage of this system, since the PPh3 complex is more
stable and much easier to synthesize, compared to the 2,6-
lutidine complex.
Comonom Yield Act. XM
Mn
Tm
Ent. Cat.
PDId
er
(g) b (104)b (%)c (104)d
(oC)e
1
2
1
2
3
4
5
1
-
2.7 13.5
trace --
-
-
-
-
-
36.2
--
2.21 137.2
-- --
2.42 134.5
-
-
-
-
3
0.7
3.5
6.2
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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60
4
2.6 13.0
4.3 21.5
44.6 1.96 138.5
26.4 2.42 137.5
5
Copolymerization always resulted in lower molecular
weights because insertion at a tertiary carbon (resulting
from the 2,1 insertion of the monomer into the metal-carbon
bond) is always slower than insertion into a secondary
carbon (product of ethylene insertion). In addition, chain
transfer is more likely to occur when the insertion process is
slower; the polymerization and copolymerization studies at
25 oC agree with this trend. It is quite surprising and
unexpected that the opposite trend was observed for studies
6
0.4 2.0 0.35
8.9
2.14 131.3
1.85 130.0
1.85 126.4
2.02 132.9
2.12 128.7
2.39 130.6
1.96 136.2
7
1
1.3
1.0
1.2
6.5 0.35
15.0
8
1
5.0 0.46 14.6
9
3
4
5
3
5
3
5
6.0 0.30
5.9
17.9
12.4
7.5
10
2.0 10.0 0.6
3.3 16.5 0.7
11
o
12f
13f
14f
15f
-
-
0.9 4.5
4.4 22.0
-
-
at 80 C (Table 1 vs. Table 2). The activities of these nickel
o
catalysts are so high at 80 C that polymerization becomes
highly exothermic. Although the temperature of the 80 oC
oil bath remained unchanged, it is highly possible that the
temperature of the polymerization solution itself may have
increased.17 The presence of polar monomers greatly
reduced these catalytic activities by one to two orders of
magnitude. This temperature difference may lead to
different polymer molecular weights, considering the high
sensitivity of molecular weight toward temperature observed
in our system. The presence of 1 M of CH3COOEt or Et2O
could lower the catalytic activities and increase the polymer
molecular weights (Table 2, entries 22 and 23).
29.0 1.90 136.5
1.1
5.5
0.5
5.5
2.32 133.1
2.27 131.2
3.8 19.0 0.9
13.7
aPolymerization conditions: total volume of solvent and
polar monomer = 20 mL, polar monomer = 1 M, ethylene = 8
o
atm, 25 C, catalyst = 20 μmol, time = 1 h (10 μmol and 2 h
for ethylene homopolymerization). bThe yields and activities
are average of at least two runs. Activity is in unit of 104
c
1
g·mol-1·h-1. Determined by H NMR in C2D2Cl4 at 120 oC.
dDetermined by GPC in trichlorobenzene at 150 oC.
eDetermined by DSC. f2 eq. Ni(COD)2 was added.
To conclude, high-performance phosphine sulfonate
nickel catalysts were designed and prepared by the
installation of sterically bulky substituents at axial positions
on the metal, ortho positions on the ligands as well as the
installation of substituents with different electronic
properties and labile coordinating bases. These nickel
catalysts can initiate ethylene homopolymerization in the
25-100 oC temperature range, without the need for any
cocatalyst, and with activities of up to 4.5 × 106 g·mol-1·h-1,
providing Mn values of up to 446,000. A large number of
polar monomers were copolymerized with ethylene with
high activities (up to 1.5 × 105 g·mol-1·h-1), good comonomer
incorporations (up to 7.6%) and high molecular weights (Mn
up to 179,000). This is a very rare example of a nickel-based
catalyst system that possesses the combined properties of
high activity and high thermal stability to produce polymers
of high molecular weight, while displaying wide scope for
polar monomer substrates. This also represents an excellent
demonstration of the capacity of rational ligand design for
the improvement of the performance of olefin
polymerization catalysts.
afforded the highest-molecular-weight polymers. This can
be explained by the electron-withdrawing feature of the C6F5
substituent and is consistent with the fact that catalysts 2
and 3, bearing electron-donating groups, gave lower-
molecular-weight polyethylene than catalyst 1. In addition,
the control over the orientation of the -SO2- moiety in
relation to the ortho substituent may also contribute to this
phenomenon.9d,9m At low temperature, the comonomer
incorporation ratio was reduced by factors of 4-7. Probably,
the ethylene concentration is significantly higher at lower
temperatures, resulting in a lower fraction of comonomer in
solution and thus copolymer. However, the copolymer
molecular weights were improved by up to 37 times. In all
cases, semicrystalline copolymers, with high molecular
weights (Mn up to 179,000) and high melting temperatures
(
T
m up to 132.9 oC), were generated.
In some nickel catalysts containing coordinating
phosphine bases (such as triphenylphosphine in catalyst III),
phosphine scavenger, such as Ni(COD)2, is usually
a
required because of the difficulties in activating the catalyst
precursor.6 In this system, the presence or absence of
Ni(COD)2 had minimal effect on the catalytic activities of 3
or 5, or the polymer molecular weights obtained, in both
AUTHOR INFORMATION
Corresponding Author
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L. A.; Long, B. K. J. Am. Chem. Soc. 2013, 135, 16316-16319. (e)
Rhinehart, J. L.; Mitchell, N. E.; Long, B. K. ACS Catal. 2014, 4,
2501-2504.
changle@ustc.edu.cn
Notes
The authors declare no competing financial interests.
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A: Polym. Chem. 2002, 40, 2842-2859. (c) Wang, C. M.; Friedrich,
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Göttker-Schnetmann, I.; Mecking, S. Organometallics 2013, 32,
5239-5242. (h) Takeuchi, D.; Chiba, Y.; Takano, S.; Osakada, K.
Angew. Chem. Int. Ed. 2013, 52, 12536-12540. (i) Stephenson, J.;
McInnis, J. P.; Chen, C. L.; Weberski, M. P.; Motta, A.; Delferro,
M.; Marks, T. J. ACS Catal. 2014, 4, 999-1003. (j) Hu, X. H.; Dai,
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J. Am. Chem. Soc. 2013, 135, 3784-3787. (b) Takeuchi, D.; Chiba,
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12540. (c) Long, B. K.; Eagan, J. M.; Mulzer, M.; Coates, G. W.
Angew. Chem. Int. Ed. 2016, 55, 7222-7226. (d) It was
demonstrated that ethylene-methyl acrylate copolymerization
could be realized using α-diimine Ni(II) catalyst under very
harsh conditions (1000 psi ethylene pressure and 120 oC) with
the addition of large amount of tris(pentafluorophenyl)borane.
Johnson, L. K.; Wang, L.; McLain, S.; Bennett, A.; Dobbs, K.;
Hauptman, E.; Ionkin, A.; Ittel, S. D.; Kutnisky, K.; Marshall, W.;
McCord, E.; Radzewich, C.; Rinehart, A.; Sweetman, K. J.; Wang,
Y.; Yin, Z.; Brookhart, M. ACS Symp. Ser. 2003, 857, 131-142. (e)
Tao, W.; Nakano, R.; Ito, S.; Nozaki, K. Angew. Chem. Int.
Ed. 2016, 55, 2885-2889.
ASSOCIATED CONTENT
Supporting Information
Synthesis and characterization of the ligands and metal
complexes, X-ray diffraction information, polymerization
data and polymer characterization. This material is
available free of charge via the Internet at
http://pubs.acs.org.
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ACKNOWLEDGMENT
This work was supported by National Natural
Science Foundation of China (NSFC, 21374108,
51522306 and 21960071), the Fundamental Research
Funds for the Central Universities (WK3450000001),
and the Recruitment Program of Global Experts.
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