Neutral Nickel(II) Complexes Bearing Aryloxide Imidazolin-2-imine Ligands for Efficient Copolymerization of Norbornene and Polar Monomers

Article abstract of DOI:10.1021/acs.organomet.8b00752

A series of novel aryloxide imidazolin-2-imine bidentate neutral Ni(II) complexes with different substituents on the imidazolin-2-imine ligand were synthesized and characterized. The complex Ni2 bearing a 2,6-diisopropylphenyl substituent adopted an almost square planar geometry, while the bulkier 2,6-bis(diphenylmethyl)-4-methylphenyl substituent ligated complex Ni3 was obtained in an allyl adduct base-free η³ coordination mode. In the presence of B(C₆F₅)₃, these Ni(II) complexes exhibited remarkably high activity (up to 2.7 × 10⁷ g of PNB (mol of Ni)^-1 h^-1) and particularly good thermal stability toward the addition polymerization of norbornene. Most importantly, these catalysts were able to promote the direct copolymerization of norbornene with various polar monomers with high activity (～10⁵ g (mol of Ni)^-1 h^-1), reasonable comonomer incorporation (0.14-3.08%), and high copolymer molecular weight (M_n up to 2.0 × 10⁵). The strategy of installing a strongly electron donating imidazolin-2-imine ligand on the nickel complex demonstrates a great advantage for the copolymerization of an olefin with polar monomers.

Full text of DOI:10.1021/acs.organomet.8b00752

Article
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Neutral Nickel(II) Complexes Bearing Aryloxide Imidazolin-2-imine
Ligands for Efficient Copolymerization of Norbornene and Polar
Monomers
Mingyuan Li,† Zhengguo Cai,*,† and Moris S. Eisen*,‡
†State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering,
Donghua University, Shanghai 201620, People’s Republic of China
‡Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
S
* Supporting Information
ABSTRACT: A series of novel aryloxide imidazolin-2-imine
bidentate neutral Ni(II) complexes with different substituents
on the imidazolin-2-imine ligand were synthesized and
characterized. The complex Ni2 bearing a 2,6-diisopropylphenyl
substituent adopted an almost square planar geometry, while the
bulkier 2,6-bis(diphenylmethyl)-4-methylphenyl substituent
ligated complex Ni3 was obtained in an allyl adduct base-free
η3 coordination mode. In the presence of B(C6F5)3, these Ni(II)
complexes exhibited remarkably high activity (up to 2.7 × 107 g
of PNB (mol of Ni)−1 h−1) and particularly good thermal
stability toward the addition polymerization of norbornene.
Most importantly, these catalysts were able to promote the
direct copolymerization of norbornene with various polar
monomers with high activity (∼105 g (mol of Ni)−1 h−1),
reasonable comonomer incorporation (0.14−3.08%), and high copolymer molecular weight (Mn up to 2.0 × 105). The strategy
of installing a strongly electron donating imidazolin-2-imine ligand on the nickel complex demonstrates a great advantage for
the copolymerization of an olefin with polar monomers.
late-transition-metal catalysts provide promising potentials to
promote the concept for the synthesis of polar-functionalized
polyethylene and polypropylene.7 A milestone discovery in this
regard was made in the 1990s by Brookhart et al., who
reported the copolymerization of olefins with acrylate
monomers using α-diimine palladium catalysts.7a−c Later in
the 2000s, neutral salicylaldimine nickel catalysts were reported
by Grubbs et al., which are capable for the copolymerization of
ethylene with polar-substituted norbornene and a few other
special polar monomers.7d,e However, the activity and
molecular weight are low and the scope of polar functional
groups was limited in these catalyst systems. In addition,
examples of catalysts for the copolymerization of norbornene
with polar monomers are still limited, since late-transition-
metal complexes commonly show poor performance for the
copolymerization of norbornene and higher α-olefins.8
INTRODUCTION
■
Cyclic olefin copolymers (COCs) obtained via coordination
polymerization are an attractive class of polyolefin with unique
properties, such as high glass transition temperature, good
chemical resistance, excellent transparency, and low water
adsorption.1 Among them, the most common and versatile
COC is the copolymer of norbornene and ethylene as a kind of
commercial engineering thermoplastic.2 In addition, the
properties of copolymers can be modified by changing the
comonomer contents and sequence distributions as well as the
comonomer structures employed.2,3 Numerous single-site
catalysts have been developed in both academia and industry
for the copolymerizations of norbornene with ethylene,
propylene, and higher α-olefins to give various COCs.4
Despite these developments in the synthesis and the wide
range of applications of COCs, their nonpolar nature remains a
significant limitation. Incorporation of even a small amount of
polar functional groups on the polyolefin backbone can
significantly improve the performance of the polyolefins,
such as adhesion, dye retention, printability, and compati-
bility.5 The direct copolymerization of an olefin with polar
monomers is a desirable and economical synthetic strategy.5c−f
However, early-transition-metal catalysts are easily poisoned by
the polar functional groups.6 In contrast, the less electrophilic
Recent developments of late-transition-metal catalysts have
achieved excellent performances in the copolymerization of
ethylene or propylene with polar monomers,9 strategies for
which can be classified into two categories. One is to block the
metal center by the installation of sterically bulky substituents
Received: October 15, 2018
© XXXX American Chemical Society
A
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on the ligand,9a−g,i−m and the other is to reduce the
electrophilicity of the metal center by the incorporation of
electron-donating ligands.9a,b,h−l Earlier observation of the
former has been demonstrated extensively.9a−g,i−m The latter
was successfully applied by Nozaki et al. using aryloxycarbene-
based Ni/IzQO and Pd/IzQO catalysts supported by N-
heterocyclic carbenes (NHCs) as strong σ donors (Chart 1,
I).9i,j
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Ligands and
Complexes. The aryloxide imidazolin-2-imine ligands L1−L3
were prepared by the reaction of o-aminophenol (4) with
substituted dichloroimidazolium salts (3) in the presence of
trimethylamine.13 These dichloroimidazolium salts (3) can be
easily obtained by the deprotonation of NHC precursors (1)
with 1.2 equiv of sodium hydride using potassium tert-butoxide
as a catalyst in THF,14 followed by the addition of 1.1 equiv of
hexachloroethane at −35 °C.15
Chart 1. Selected Examples of Previously Reported
Catalysts for Olefin Polymerization
In order to prepare nickel complexes, the ligands were
deprotonated with 1.1 equiv of potassium hydride in THF,
respectively. The obtained potassium salt of each ligand was
then treated with an appropriate nickel precursor (trans-
[NiMeCl(PMe3)2]), which led to the formation of the
expected complexes Ni1 and Ni2 in good yields (80.9−
85.7%). To our surprise, the bulkier 2,6-bis(diphenylmethyl)-
4-methylphenyl-substituted ligand L3 did not react by the
same procedure, which may be due to a larger steric hindrance
effect. Therefore, the potassium salt of L3 was treated with 0.5
equiv of [Ni(Br)allyl]2 dimer, and the allyl adduct base-free
complex Ni3 was successfully prepared in a good yield of
81.6%. All of these novel ligands and complexes were
characterized by NMR spectroscopy, HRMS, and elemental
analysis. In addition, the molecular structures of the ligands
L1−L3 (Figure 1) and the complexes Ni2 and Ni3 (Figures 2
and 3) were determined by single-crystal X-ray diffraction.
In Figure 1, all of the ligands showed a similar [N,O]
bidentate chelating structure, in which the N1−C1 distance
(∼1.29 Å) and C2−C3 distance (∼1.33 Å) are similar to those
of typical NC double bonds (∼1.28 Å) and CC double
bonds (∼1.34 Å), respectively.10 The structure in Figure 2
Stimulated by these strategies,9 we speculate that an
imidazolin-2-iminato(imine) ligand would serve as a promising
ligand, owing to its strong 2σ,4π-electron N donor with
mesomeric structures (Chart 2).10 In addition, the steric
Chart 2. Mesomeric Forms of Imidazolin-2-iminato(imine)
Ligands
hindrance at the axial position on the metal can also be tailored
by the substituent of the corresponding R groups.10 Excellent
performances of their early-transition-metal complexes with
the monoanionic forms were observed in the field of olefin
polymerization (Chart 1, II−IV).11 However, the study of late-
transition-metal complexes combined with these ligands for
olefin polymerization is still very limited, although they have
already been used in organometallic and coordination
chemistry.10−12
Our recent work gave rare examples of neutral Ni(II) and
Pd(II) complexes bearing aryloxide imidazolidin-2-imine
ligands toward the vinyl-type polymerization of norbornene
(Chart 1, V).13 However, the addition of polar monomers
quenched the polymerization. Here, we report the synthesis of
neutral Ni(II) complexes bearing strongly electron donating
imidazolin-2-imine fragments and their excellent performances
in the copolymerization of norbornene with polar monomers.
To the best of our knowledge, this is the first example of
neutral Ni(II) complexes bearing aryloxide imidazolin-2-imine
ligands with different substituents for olefin copolymerization,
affording functionalized COCs with various polar functional
groups and suitable comonomer contents.
Figure 1. ORTEP representation of the ligands L1−L3 (50% thermal
ellipsoids). Hydrogen atoms have been omitted for clarity. Selected
bond lengths (Å) and angles (deg): L1, C1−N1 1.288(3), C1−N2
1.382(3), C1−N3 1.384(3), C2−C3 1.326(4), N2−C1−N3
105.1(2); L2, C1−N1 1.303(3), C1−N2 1.384(3), C1−N3
1.376(3), C2−C3 1.333(4), N2−C1−N3 105.1(2); L3, C1−N1
1.286(4), C1−N2 1.393(4), C1−N3 1.385(4), C2−C3 1.329(4),
N2−C1−N3 103.9(3).
B
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Table 1. Polymerization of Norbornene with Ni(II)
a
Complexes
b
c
c
entry
cat.
T (°C)
yield (g)
A
105Mn
PDI
d
1
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni2
Ni3
Ni3
Ni3
Ni3
Ni3
Ni1
Ni1
Ni1
Ni1
Ni1
25
25
25
25
40
60
80
100
25
40
60
80
100
25
40
60
80
100
0.147
0.157
0.226
0.195
0.352
0.343
0.246
0.093
0.245
0.267
0.302
0.186
0.085
0.409
0.448
0.362
0.239
0.077
8.82
9.42
5.37
5.45
5.02
4.76
4.85
4.78
4.22
3.06
3.53
4.10
4.65
4.73
3.35
6.06
6.19
5.24
4.03
3.17
1.71
1.70
1.74
1.79
1.89
1.79
1.82
1.99
1.79
1.88
1.82
1.85
2.03
1.64
1.69
1.74
1.83
2.11
e
2
3
4
13.56
11.70
21.12
20.58
14.76
5.58
14.70
16.02
18.12
11.16
5.10
24.54
26.88
21.72
14.34
4.62
f
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Figure 2. ORTEP representation of complex Ni2 (50% thermal
ellipsoids). Hydrogen atoms are removed for clarity. Selected bond
lengths (Å) and angles (deg): Ni1−P1 2.1037(8), Ni1−O1
1.8864(16), Ni1−N1 1.9590(18), Ni1−C34 1.921(2), N1−C1
1.328(3), C2−C3 1.328(3); O1−Ni1−P1 90.14(6), O1−Ni1−N1
85.19(7), O1−Ni1−C34 168.75(10), N1−Ni1−P1 171.11(6).
a
Polymerization conditions unless specified otherwise: complex, 0.5
μmol; cocat., B(C6F5)3, 10 μmol, 20 equiv; norbornene, 10 mmol;
b
V
total(toluene) = 10 mL; t = 2 min. Acitivity in units of 106 g of PNB
c
(mol of Ni)−1 h−1. GPC data in 1,2,4-trichlorobenzene at 165 °C
with polystyrene standards. B(C6F5)3, 5.0 equiv. B(C6F5)3, 10
d
e
f
equiv. B(C6F5)3, 50 equiv.
106g of PNB (mol of Ni)−1 h−1 with an increase in the B/Ni
ratio from 5 to 20 (entries 1−3, Table 1). Further increase in
the B/Ni ratio to 50 slightly decreased the activity (entry 4,
Table 1). Therefore, the B/Ni ratio was fixed at 20. The
polymerization temperature significantly affected the activity,
where the highest activities of Ni1 (26.88 × 106 g of PNB (mol
of Ni)−1 h−1), Ni2 (21.12 × 106 g of PNB (mol of Ni)−1 h−1),
and Ni3 (18.12 × 106 g of PNB (mol of Ni)−1 h−1) were
obtained at 40 and 60 °C, respectively, indicating the better
thermal stability of Ni3 by the bulkier substituent. However, all
of the nickel complexes still maintained high activity (107 g of
PNB (mol of Ni)−1 h−1) even at 80 °C, suggesting the great
thermal stability of the aryloxide imidazolin-2-imine nickel
catalysts (entries 7, 12, and 17, Table 1). Ni1 bearing a 2,4,6-
trimethylphenyl substituent showed higher activity in compar-
ison to complexes Ni2 and Ni3 at the same polymerization
temperature range from 25 to 80 °C (entries 3, 5, 6, 9−12, and
14−17, Table 1), indicating that the bulky substituent seems to
hinder the coordination site to prohibit the insertion of the
norbornene monomer. A similar steric effect of ligands, in the
norbornene polymerization, has also been observed in many
previous reports.8a,13 Interestingly, a different phenomenon
was observed that the activity and molecular weight of Ni2 and
Ni3 are slightly higher than those of Ni1 at 100 °C (entries 8,
13, and 18, Table 1). This observation indicate higher thermal
stability of Ni2 and Ni3, which is possibly ascribable to the
greater steric hindrance effects of Ni2 and Ni3 at high
temperature. The similar microstructures of PNBs obtained
Figure 3. ORTEP representation of complex Ni3 (50% thermal
ellipsoids). Hydrogen atoms are removed for clarity. Selected bond
lengths (Å) and angles (deg): Ni1−O1 1.827(3), Ni1−N1 1.972(4),
Ni1−C76 2.000(6), Ni1−C77 1.940(6), Ni1−C78 1.969(5), N1−C1
1.339(5), C76−C77 1.399(9), C77−C78 1.384(9), C2−C3
1.332(5); O1−Ni1−N1 87.28(15), O1−Ni1−C76 92.2(2), N1−
Ni1−C78 106.61(19), C76−Ni1−C78 73.1(2).
revealed that complex Ni2 adopted an almost square planar
geometry, in which the distance from the metal center to the
coordination plane is 0.029 Å. The phosphine in complex Ni2
is trans to the nitrogen atom, which is consistent with
previously reported aryloxide imidazolidin-2-imine Ni(II)
complexes.13 The Ni−C(allyl) distances in complex Ni3
(2.000(6) Å for Ni1−C76; 1.940(6) Å for Ni1−C77;
1.969(5) Å for Ni1−C78) are similar to those in previously
reported phosphine sulfonate allyl Ni(II) complexes, suggest-
ing a typical η3 coordination mode in Ni3.8b,16 Although
different nickel precursors were installed in complexes Ni2 and
Ni3, not much difference was observed in the corresponding
bond lengths and angles.
Norbornene Polymerization. The catalytic behaviors of
Ni1−Ni3 toward norbornene homopolymerization were
studied in the presence of B(C6F5)3 as a phosphine scavenger
(Table 1). We selected Ni2 to investigate the effects of the
polymerization conditions in detail, at room temperature (25
°C). The activity of Ni2 increased from 8.82 × 106 to 13.56 ×
1
were characterized by H NMR and 13C NMR spectra in
1
CDCl3, at 25 °C (see the Supporting Information). The H
NMR spectra of PNBs indicates that the PNBs were vinyl
addition type products.
On the other hand, these complexes showed poor behaviors
toward ethylene polymerization. The reason is not yet clear;
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a
Table 2. Copolymerization of Norbornene with Polar Monomers
a
Polymerization conditions unless specified otherwise: complex, 5 μmol; cocat., B(C6F5)3, 10.0 equiv, 50 μmol; norbornene, 10 mmol, 0.9415 g;
b
c
V
total(toluene) = 10 mL; time = 30 min. Acitivity in units of g (mol of Ni)−1 h−1. Determined by 1H NMR spectra in CDCl3 at room temperature.
d
e
GPC data in 1,2,4-trichlorobenzene at 165 °C with polystyrene standards. time = 60 min.
one plausible reason can be attributed to the addition of the
electron density to the Ni center by the installation of the
strongly electron donating imidazolin-2-imine ligand, which
reduces the electrophilicity of the metal center to prohibit
ethylene coordination.
Norbornene Copolymerization. On the basis of the
norbornene homopolymerization results, the Ni(II) complexes
were employed in the copolymerization of norbornene with
various polar monomers (Table 2). In this system, the addition
of some commercial polar monomers, such as methyl acrylate,
ethyl acrylate, n-butyl acrylate, and allyl chloride, completely
shuts down polymerization. Although the copolymerization of
norbornene and vinyl acetate was performed with moderate
polymerization activity, no comonomer incorporation was
observed. When a spacer was put between the double bond
and the polar functional groups, high activity, high copolymer
molecular weight ,and reasonable comonomer incorporation
were achieved during the copolymerization of norbornene with
allyl acetate, allyl ethyl ether, allyl benzene, and some OH/
COOMe/Br/Cl-containing long-chain monomers.
The complexes Ni1 and Ni2 bearing Mes and Dipp
substituents conducted efficient copolymerization of norbor-
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Figure 4. Plots of copolymerization activity and comonomer incorporation against polar monomer concentration in the feed: (a) copolymerization
of norbornene with 10-undecen-ol; (b) copolymerization of norbornene with methyl 10-undecenoate (Tables S1 and S2).
Figure 5. GPC curves of copolymers obtained with complexes Ni2 and Ni3: (a) norbornene and 10-undecen-1-ol copolymers; (b) norbornene and
methyl 10-undecenoate copolymers (Tables S1 and S2).
nene and polar monomers with considerable activities and
copolymer molecular weights. In contrast to the homopolyme-
rization of norbornene, complex Ni3 bearing the bulkiest 2,6-
bis(diphenylmethyl)-4-methylphenyl substituent showed the
highest activity toward the copolymerization of norbornene
with polar monomers, indicating that the effective blockage of
the axial position on the metal is efficient for enhancing the
tolerance of the nickel complex toward polar functional groups.
This could also be proved by our previous study,13 in which
the polymerization of norbornene was quenched when polar
monomers were added to the system by using similar aryloxide
imidazolidin-2-imine Ni(II) complexes bearing a smaller
methyl substituent.
As shown in Table 2, the polar comonomer incorporation in
the copolymers can be improved by increasing the
concentration of the feeding polar monomers (from 1.67%
to 2.40% for allyl acetate (entries 1 and 2, Table 2); or from
0.70% to 1.06% for 11-chloro-1-undecene (entries 13 and 14,
Table 2). Therefore, the effects of the concentration of the
feeding polar monomers on the copolymerization activity,
molecular weight, and polar comonomer incorporation in the
copolymers were studied in detail with complexes Ni2 and Ni3
using 10-undecen-1-ol and methyl 10-undecenoate as the polar
monomers. The results are shown in Figures 4 and 5 (see also
Tables S1 and S2).
When the polar monomer concentration was increased from
0.1 to 0.4 mol/L, the polar monomer incorporation in the
copolymer was effectively increased regardless of the complex
and monomer used, accompanied by a decrease in the
activities (Figure 4). However, it should be noted that both
catalytic systems still maintained considerable activities (∼104
g (mol of Ni)−1 h−1) to give high-molecular-weight copolymers
(4 × 104 g mol−1), indicating the good tolerance of these
Ni(II) complexes toward polar functional groups. The higher
activity, polar monomer incorporation, and molecular weight
of Ni3 (Figures 4 and 5) even at high polar monomer
concentration also indicated the improved tolerance of Ni3
toward polar functional groups by the large steric hindrance
effect around the nickel center. Furthermore, GPC curves of all
the copolymers were unimodal with a narrow molecular weight
distribution (∼2.0), indicating the single-site catalytic behavior
of these catalysts (Figure 5).
The glass transition temperatures (Tg) of PNB and
copolymers were determined by DSC (Figure 6). The Tg
value of PNB was not observed below 350 °C, while each
copolymer with the highest comonomer incorporation
exhibited a corresponding Tg value (328.2 °C for norbor-
nene−10-undecen-1-ol copolymer (entry 8, Table S1); 327.4
°C for norbornene−methyl 10-undecenoate copolymer (entry
8, Table S2); 324.8 °C for norbornene−6-bromo-1-hexene
copolymer (entry 17, Table 2); 326.3 °C for norbornene−allyl
E
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imidazolidin-2-imine ligand and their behaviors toward olefin
(co)polymerization are in progress.
EXPERIMENTAL SECTION
■
All manipulations of air-sensitive materials were performed with the
rigorous exclusion of oxygen and moisture in flamed Schlenk-type
glassware or J. Young Teflon valve sealed NMR tubes on a dual-
manifold Schlenk line interfaced to a high-vacuum (10−5 Torr) line or
in a nitrogen-filled Innovative Technologies glovebox with a medium-
capacity recirculator (1−2 ppm of O2).
Materials. Argon and nitrogen were purified by passage through
an MnO oxygen-removal column and a Davison 4 Å molecular sieve
column. Dichloromethane, tetrahydrofuran, diethyl ether, toluene,
and n-hexane were purified by a PS-MD-5 (Innovative Technology)
solvent purification system. Benzene-d6 (Cambridge Isotopes) was
distilled under vacuum from Na/K alloy. Norbornene was dissolved in
dry toluene and stirred with CaH2 for 24 h, and then the solution was
distilled under reduced pressure. Comonomers (polar α-olefins) were
dried with CaH2 overnight and then freshly distilled before use. 2-
Aminophenol, hexachloroethane, trimethylphosphine (1.0 M in
toluene), anhydrous nickel(II) chloride, Ni(COD)2, potassium
hydride, and tris(pentafluorophenyl)boron were purchased from
Sigma-Aldrich and used without any purification.
Figure 6. DSC curves of selected copolymers: (a) norbornene and 10-
undecen-1-ol copolymer (entry 8, Table S1); (b) norbornene and
methyl 10-undecenoate copolymer (entry 8, Table S2); (c)
norbornene and 6-bromo-1-hexene copolymer (entry 17, Table 2);
(d) norbornene and allyl acetate copolymer (entry 3, Table 2).
Norbornene (Co)polymerization. Norbornene (co)-
polymerization was performed in a 50 mL flask equipped with a
magnetic stirrer and carried out by the following methods. At first, the
flask was charged with a certain amount of norbornene, tris-
(pentafluorophenyl)boron, polar α-olefins (for copolymerization),
and toluene under nitrogen. After the mixture was kept at the desired
temperature for 10 min, (co)polymerization was initiated by
introduction of the Ni(II) complexes in toluene into the flask via
syringe, and the reaction was started. The desired time later, the
(co)polymerization was terminated by addition of 10% HCl in
ethanol. The precipitated polymer was washed with ethanol and water
and dried at 60 °C under vacuum to a constant weight. For all of the
(co)polymerization procedures, the reaction volume in total was
constant, which can be achieved by variation of the added toluene
when necessary.
Measurements. The molecular weights and distributions of
polymers were determined by a Polymer Laboratory PL GPC-220
instrument equipped with a triple-detection array consisting of a
differential refractive index (DRI) detector, a two-angle (45, 90°)
light scattering (LS) detector at a laser wavelength of 658 nm, and a
four-bridge capillary viscosity detector. This system included one
guard column (PL# 1110-1120) and three 30 cm columns (PLgel 10
μm MIXED-B 7.5 × 300 mm). Polymer characterization was carried
out at 165 °C using 1,2,4-trichlorobenzene as eluent and calibrated by
polystyrene standards. The NMR spectra were measured on Bruker
Avance 200, Bruker Avance 300, Bruker Avance III 400, and Bruker
Avance III 500 spectrometers. Elemental analysis was performed using
an Element Vario EL III elemental analyzer. HRMS experiments were
performed at 200 °C (source temperature) on a Maxis Impact
(Bruker) mass spectrometer with an APCI solid probe method. The
glass-transition temperatures (Tg) of the copolymers were measured
with a TA Q2000 Differential Scanning Calorimeter at a heating rate
of 10 °C/min from 40 to 350 °C. The TGA measurements were
performed on a NETZSCH TG 209F3 instrument from 40 to 550 °C
at a rate of 10 °C/min under a nitrogen atmosphere.
acetate copolymer (entry 3, Table 2)). The lower Tg value of
these copolymers in comparison to PNB also indicated that
various functionalized COCs were obtained by direct
copolymerization of norbornene with polar monomers. The
thermal stabilities of selected copolymers were studied by TGA
analysis. According to the TGA data (see the Supporting
Information), all of the copolymers exhibited similar thermal
stabilities and decomposition temperatures (Td) up to 421.5
°C, suggesting the good thermal stability of the copolymers
prepared in this catalytic system.
In general, the copolymerization of ethylene with polar
monomers catalyzed by Ni(II) catalysts exhibit significantly
decreased activity (∼104 g (mol of Ni)−1 h−1) and copolymer
molecular weight (∼104 g mol−1) because of the poisoning
effect of polar groups on the metal center.8a The polymer-
ization of norbornene was commonly quenched when polar
monomers were involved.7,8 In this paper we present a
promising catalytic system in which high activity (∼105 g (mol
of Ni)−1 h−1) and high copolymer molecular weight (Mn up to
197600) were achieved during the copolymerization of
norbornene with allyl acetate, allyl ethyl ether, allyl benzene,
and some OH/COOMe/Br/Cl-containing long-chain mono-
mers. The high performances of these nickel catalysts may
originate from the lower electrophilicity of the nickel center
due to the installation of the electron-donating ligand.
CONCLUSION
■
In summary, the synthesis of novel neutral Ni(II) complexes
bearing strongly electron donating imidazolin-2-imine biden-
tate ligands and their catalytic behavior toward norbornene
copolymerization with various polar monomers is reported.
Upon activation with B(C6F5)3, these Ni(II) complexes
showed good thermal stabilities toward norbornene polymer-
ization. Most importantly, excellent performance of the nickel
catalysts including high activity, reasonable comonomer
incorporation, high copolymer molecular weight, and good
solubility of the copolymers were observed in the copoly-
merization of norbornene with various polar monomers
without the use of large amounts of a cocatalyst or any
protecting reagents, affording functionalized COCs. Further
studies on other late-transition-metal complexes bearing this
Crystal Structure Determinations. A single crystal was
immersed in perfluoropolyalkylether oil and quickly mounted on a
Kappa CCD diffractometer underflow of liquid nitrogen. Data
collection was performed using monochromated Mo Kα radiation
using φ and ω scans to cover the Ewald sphere. Accurate cell
parameters were obtained with the amount of indicated reflections.
The structure was solved by SHELXS-97 direct methods and refined
by the SHELXL-97 program package. The atoms were refined
anisotropically. Hydrogen atoms were included using the riding
model.
F
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Article
Synthesis of Aryloxide Imidazolin-2-imine ligands. The N-
heterocyclic carbene precursors (compound 1, Scheme 1) with
bis(2,6-diisopropylphenyl)-2-dichloroimidazolium chloride (20
mmol) in MeCN (60 mL) at 0 °C with strong agitation. After the
whole mixture was refluxed overnight, volatiles were removed by
evaporation under vacuum. The mixture was then added to Et2O (200
mL), stirred for 0.5 h, and filtered. Then the light yellow Et2O
solution was collected and evaporated under vacuum. The crude
product was dissolved in the minimum amount of MeCN (∼50 mL)
and stored at −20 °C overnight to recrystallize. The ligands were used
as white crystals or a microcrystalline powder. Yield: 45.8%. 1H NMR
(400 MHz, CDCl3): δ 7.35−7.28 (m, 2H, Ar-CH), 7.20−7.10 (m,
4H, Ar-CH), 6.56−6.48 (m, 1H, Ar-CH), 6.42 (s, 2H, NCHCHN),
6.40−6.32 (m, 1H, Ar-CH), 6.04−5.97 (m, 1H, Ar-CH), 5.95−5.88
(m, 1H, Ar-CH), 3.11−2.98 (m, 4H, CH(CH3)2), 1.24 (dd, J = 12.9,
6.9 Hz, 24H, CH(CH3)2). 13C NMR (100 MHz, CDCl3): δ 149.1,
146.6, 145.1, 133.9, 129.5, 124.0, 120.0, 119.8, 117.8, 115.9, 111.6,
28.9, 24.9, 22.9. Anal. Calcd for C33H41N3O: C, 79.96; H, 8.34; N,
8.48. Found: C, 79.84; H, 8.42; N, 8.37. HRMS (APCI, m/z): calcd
for C33H42N3O [M + H]+ 496.3322, found 496.3376.
Scheme 1. Synthesis of Aryloxide Imidazolin-2-imine
a
Ligands and Complexes
Ligand L3. The 2-aminophenol (5 mmol) and triethylamine (10
mmol) in MeCN (40 mL) were added to a solution of N,N’-1,3-
bis(2,6-bis(diphenylmethyl)-4-methylphenyl)-2-dichloroimidazolium
chloride (5 mmol) in MeCN (80 mL) at 0 °C with strong agitation.
After the whole mixture was refluxed overnight, a large amount of
white precipitate appeared in the hot MeCN solution. The pure
ligand was obtained by filtration and washed with a small amount of
MeCN. Yield: 82.5%. 1H NMR (400 MHz, CDCl3): δ 7.19−7.09 (m,
24H, Ar-CH), 7.01−6.93 (m, 8H, Ar-CH), 6.91−6.85 (m, 8H, Ar-
CH), 6.75 (s, 4H, Ar-CH), 6.74−6.72 (m, 1H, Ar-CH), 6.50−6.39
(m, 1H, Ar-CH), 6.05−5.93 (m, 1H, Ar-CH), 5.90−5.76 (m, 1H, Ar-
CH), 5.60 (s, 4H, CH(Ph)2), 4.93 (s, 2H, NCHCHN), 2.16 (s, 6H,
CH3). 13C NMR (100 MHz, CDCl3): δ 149.6, 144.2, 143.8, 143.5,
142.0, 138.6, 133.4, 133.2, 130.1, 129.9, 129.5, 128.4, 128.3, 126.5,
126.4, 119.7, 119.3, 117.9, 115.2, 111.9, 51.9, 21.8. Anal. Calcd for
C75H61N3O: C, 88.29; H, 6.03; N, 4.12. Found: C, 88.17; H, 5.98; N,
4.23. HRMS (APCI, m/z): calcd for C75H62N3O [M + H]+
1020.4815, found 1020.4887.
a
Abbreviations: Mes, 2,4,6-trimethylphenyl; Dipp, 2,6-diisopropyl-
phenyl; IPr*, 2,6-bis(diphenylmethyl)-4-methylphenyl.
different substituents (2,4,6-trimethylphenyl (Mes), 2,6-diisopropyl-
phenyl (Dipp), and 2,6-bis(diphenylmethyl)-4-methylphenyl (Ipr*))
were prepared according to literature procedures.14a The correspond-
ing N-heterocyclic carbenes (compound 2, Scheme 1) were obtained
by the deprotonation of these imidazolium salts, which has been
reported before.14a N,N′-1,3-Bis(2,4,6-trimethylphenyl)-2-dichloroi-
midazolium chloride (compound 3, Mes, Scheme 1)15a and N,N′-1,3-
bis(2,6-diisopropylphenyl)-2-dichloroimidazolium chloride (com-
pound 3, Dipp, Scheme 1)15b were prepared according to published
procedures. N,N′-1,3-Bis(2,6-bis(diphenylmethyl)-4-methylphenyl)-
2-dichloroimidazolium chloride (compound 3, Ipr*, Scheme 1) was
prepared by the same method as for N,N′-1,3-Bis(2,6-diisopropyl-
phenyl)-2-dichloroimidazolium chloride (compound 3, Dipp, Scheme
1).15b
Ligand L1. The 2-aminophenol (20 mmol) and triethylamine (40
mmol) in MeCN (40 mL) were added to a solution of N,N’-1,3-
bis(2,4,6-trimethylphenyl)-2-dichloroimidazolium chloride (20
mmol) in MeCN (60 mL) at 0 °C with strong agitation. After the
whole mixture was refluxed overnight, sodium hydroxide (40 mmol)
in 10 mL of H2O was added. Volatiles were removed by evaporation
under vacuum. Then, 50 wt % potassium hydroxide in H2O (10 mL)
was added. The products were extracted with MeCN, and the organic
phase was collected and dried with anhydrous MgSO4. Then the dry
MeCN solution was evaporated under vacuum. The crude product
was dissolved in the minimum amount of MeCN (∼35 mL) and
stored at −20 °C overnight to recrystallize. The ligands were used as
colorless crystals or a white microcrystalline powder. Yield: 33.2%. 1H
NMR (400 MHz, CDCl3): δ 6.83 (s, 4H, Ar-CH), 6.56−6.49 (m, 1H,
Ar−CH), 6.40−6.34 (m, 1H, Ar-CH), 6.33 (s, 2H, NCHCHN),
6.11−6.02 (m, 2H, Ar-CH), 2.24 (s, 6H, CH3), 2.20 (s, 12H, CH3).
13C NMR (100 MHz, CDCl3): δ 148.8, 144.4, 138.3, 135.7, 134.7,
133.3, 129.1, 120.0, 119.6, 117.9, 114.53, 111.6, 21.0, 18.0. Anal.
Calcd for C27H29N3O: C, 78.80; H, 7.10; N, 10.21. Found: C, 78.67;
H, 7.02; N, 10.33. HRMS (APCI, m/z): calcd for C27H30N3O [M +
H]+ 412.2383, found 412.2390.
Synthesis of Complexes. The nickel(II) precursors used in this
work are trans-NiMeCl(PMe3)2 and [Ni(Br)allyl]2 dimer, which were
prepared according to the work reported previously.17,18
Complex Ni1. To a solution of L1 (0.5 mmol) in dry THF (20
mL) was added KH (0.55 mmol, 1.1 equiv). After it was stirred at 25
°C for 6 h, the resulting mixture was filtered, and the filtrate was
evaporated. The light yellow-green residue was washed with n-hexane
(5 mL × 3) and dried in vacuo. This salt was used for the next
synthesis immediately without further purification. A solution of this
salt in dry THF (10 mL) was added dropwise at 25 °C into a flask
containing trans-NiMeCl(PMe3)2 (0.5 mmol) in dry THF (10 mL). A
rapid color change from yellow to red-orange was observed. After it
was stirred overnight at 25 °C, the reaction mixture was filtered. The
volatiles were then partially removed from the filtrate in vacuo, and to
the residue was added excess n-hexane. A yellow-orange powder of
1
complex Ni1 was obtained by filtration. Yield: 80.9%. H NMR (300
MHz, C6D6): δ 6.96−6.86 (m, 1H, Ar-CH), 6.86−6.72 (m, 1H, Ar-
CH), 6.65 (s, 4H, Ar-CH), 6.62−6.54 (m, 1H, Ar-CH), 6.37−6.11
(m, 1H, Ar-CH), 5.96 (s, 2H, NCHCHN), 2.48 (s, 12H, CH3), 1.99
(s, 6H, CH3), 0.91 (d, J = 10.0 Hz, 9H, P(CH3)3), −1.25 (d, J = 6.7
Hz, 3H, Ni-CH3). 13C NMR (75 MHz, C6D6): δ 165.1, 165.0, 141.9,
137.4, 134.4, 133.4, 129.7, 120.5, 116.9, 115.6, 114.8, 109.5, 20.4,
20.1, 12.7 (d, J = 27.2 Hz, P(CH3)3), −18.0 (d, J = 43.7 Hz, Ni-CH3).
31P NMR (240 MHz, C6D6): δ −6.02. Anal. Calcd for
C31H40N3NiOP: C, 66.45; H, 7.20; N, 7.50. Found: C, 66.24; H,
7.37; N, 7.38. HRMS (ESI, m/z): calcd for C31H41N3NiOP [M + H]+
560.2341, found 560.2340.
Complex Ni2. To a solution of L2 (0.5 mmol) in dry THF (20
mL) was added KH (0.55 mmol, 1.1 equiv). After it was stirred at 25
°C for 6 h, the resulting mixture was filtered, and the filtrate was
evaporated. The yellow residue was washed with n-hexane (5 mL × 3)
and dried in vacuo. This salt was used for the next synthesis
immediately without further purification. Then a solution of this salt
in dry THF (10 mL) was added dropwise at 25 °C into a flask
Ligand L2. The 2-aminophenol (20 mmol) and triethylamine (40
mmol) in MeCN (40 mL) were added to a solution of N,N’-1,3-
G
DOI: 10.1021/acs.organomet.8b00752
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
containing trans-NiMeCl(PMe3)2 (0.5 mmol) in dry THF (10 mL). A
rapid color change from yellow to dark red was observed. After it was
stirred overnight at 25 °C, the reaction mixture was filtered. The
volatiles were then partially removed from the filtrate in vacuo, and to
the residue was added excess n-hexane. A red-orange powder of
complex Ni2 was obtained by filtration. Yield: 85.7%. The single
crystal for XRD was obtained in n-hexane/THF (3/1 v/v). 1H NMR
(600 MHz, C6D6): δ 7.13−7.06 (m, 6H, Ar-CH), 6.88−6.83 (m, 1H,
Ar-CH), 6.77−6.70 (m, 1H, Ar-CH), 6.48 (s, 2H, NCHCHN), 6.47−
6.46 (m, 1H, Ar-CH), 6.26−6.19 (m, 1H, Ar-CH), 3.57−3.49 (m,
4H, CH(CH3)2), 1.52 (d, J = 6.6 Hz, 12H, CH(CH3)2), 1.08 (d, J =
6.6 Hz, 12H, CH(CH3)2), 0.88 (d, J = 10.0 Hz, 9H, P(CH3)3), −1.39
(d, J = 6.7 Hz, 3H, Ni−CH3). 13C NMR (75 MHz, C6D6): δ 165.2,
165.1, 153.6, 145.1, 141.4, 133.8, 129.2, 124.4, 120.7, 117.8, 115.5,
114.1, 110.0, 28.8, 25.9, 22.7, 12.6 (d, J = 27.0 Hz, P(CH3)3), −19.1
(d, J = 42.6 Hz, Ni-CH3). 31P NMR (240 MHz, C6D6): δ −6.52. Anal.
Calcd for C37H52N3NiOP: C, 68.95; H, 8.13; N, 6.52. Found: C,
68.81; H, 8.17; N, 6.38. HRMS (ESI, m/z): calcd for C37H53N3NiOP
[M + H]+ 644.3280, found 644.3256.
ORCID
Zhengguo Cai: 0000-0001-5784-3920
Moris S. Eisen: 0000-0001-8915-0256
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21774018), Program for New Century
Excellent Talents in University, the Program for Professor of
Special Appointment (Eastern Scholar) at Shanghai Institu-
tions of Higher Learning, and “Shu Guan” project supported
by Shanghai Municipal Education Commission and Shanghai
Education Development Foundation and the Fundamental
Research Funds for the Central Universities. We thank Dr.
Heng Liu (Technion-Israel Institute of Technology) for
helpful discussions. This research was supported by the Israel
Science Foundation underContract No. 184/18.
Complex Ni3. To a solution of L3 (0.2 mmol) in dry THF (20
mL) was added KH (0.2 mmol, 1.0 equiv). After the mixture was
stirred at 25 °C overnight, a large amount of yellow precipitate
appeared and the resulting mixture was filtered. The yellow powder
was washed with n-hexane (5 mL × 3) and dried in vacuo. This salt
was used for the next synthesis immediately without further
purification. A suspension of this salt in dry THF (10 mL) was
added dropwise at 25 °C into a flask containing [Ni(Br)allyl]2 (0.1
mmol) in dry THF (10 mL). A rapid color change from yellow to
dark red was observed. After it was stirred overnight at 25 °C, the
reaction mixture was filtered. The volatiles were then partialy removed
from the filtrate in vacuo and to the residue was added excess n-
hexane. An orange powder of complex Ni3 was obtained by filtration.
Yield: 81.6%. Single crystals for XRD were obtained in n-hexane/
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00752.
Polymerization tables, NMR spectra, and HRMS report
of the ligands and complexes and NMR spectra, GPC
data, and DSC data of the (co)polymers (PDF)
Accession Codes
CCDC 1872890−1872894 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for Z.C.: caizg@dhu.edu.cn.
*E-mail for M.S.E.: chmoris@tx.technion.ac.il.
■
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