Nickel(II) complexes with three-dimensional geometry α-diimine ligands: Synthesis and catalytic activity toward copolymerization of norbornene

Article abstract of DOI:10.1021/om300883h

A series of three-dimensional geometry 9,10-dihydro-9,10-ethanoanthracene- 11,12-diimines (L1-L4) and their nickel(II) dibromide complexes (C1-C4) were synthesized and characterized. The nickel complexes C1-C4, with three-dimensional geometry, exhibited very high activities for norbornene (NB) homopolymerization with only B(C₆F₅)₃ as cocatalyst,: for C2 even up to 5.53 × 10⁷ g of polymer/((mol of Ni) h). To investigate the activation of polar monomer, complexes C2 and C3 were selected for copolymerization of NB and 5-norbornene-2-yl acetate (NB-OCOMe) in relatively high activities (1.6-5.8 × 10⁵g of polymer/((mol of Ni) h)) and high molecular weights ((0.2-2.8) × 10 ⁵ g/mol) as well as narrow molecular weight distributions (MWD < 2 for all polymers) depending on the variation of feed ratios. The reactivity ratios of the NB and NB-OCOMe monomers for C2/B(C₆F₅) ₃ system by the Kelen-Tuedoes method were determined to be r_NB-OCOMe = 0.05 and r_NB = 6.72, respectively. Moreover, the mechanism of polymerization catalyzed by the novel three-dimensional geometry nickel(II) complexes was presented and supported by an end group analysis of the polymer and density functional theory (DFT) calculations of the reaction. The substituent effect of the catalysts and the interaction between Ni²⁺ and NB were discussed, and the results showed that α-diimine nickel complexes with greater steric hindrance and smaller HOMO-LUMO gaps could achieve higher reactivity.

Full text of DOI:10.1021/om300883h

Article
pubs.acs.org/Organometallics
Nickel(II) Complexes with Three-Dimensional Geometry α‑Diimine
Ligands: Synthesis and Catalytic Activity toward Copolymerization of
Norbornene
Ping Huo,†,‡ Wanyun Liu,†,‡ Xiaohui He,*,† Hongming Wang,† and Yiwang Chen*,†
†Department of Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, People’s Republic of
China
‡Key Laboratory of Jiangxi University for Applied Chemistry and Chemical Biology, Yichun University, Yichun 336000, People’s
Republic of China
S
* Supporting Information
ABSTRACT: A series of three-dimensional geometry 9,10-
dihydro-9,10-ethanoanthracene-11,12-diimines (L1−L4) and
their nickel(II) dibromide complexes (C1−C4) were synthe-
sized and characterized. The nickel complexes C1−C4, with
three-dimensional geometry, exhibited very high activities for
norbornene (NB) homopolymerization with only B(C6F5)3 as
cocatalyst,: for C2 even up to 5.53 × 107 g of polymer/((mol
of Ni) h). To investigate the activation of polar monomer,
complexes C2 and C3 were selected for copolymerization of
NB and 5-norbornene-2-yl acetate (NB-OCOMe) in relatively
high activities (1.6−5.8 × 105g of polymer/((mol of Ni) h))
and high molecular weights ((0.2−2.8) × 105 g/mol) as well as
narrow molecular weight distributions (MWD < 2 for all
polymers) depending on the variation of feed ratios. The reactivity ratios of the NB and NB-OCOMe monomers for C2/
B(C F ) system by the Kelen−Tudos method were determined to be rNB‑OCOMe = 0.05 and rNB = 6.72, respectively. Moreover,
̈
̈
6
5 3
the mechanism of polymerization catalyzed by the novel three-dimensional geometry nickel(II) complexes was presented and
supported by an end group analysis of the polymer and density functional theory (DFT) calculations of the reaction. The
substituent effect of the catalysts and the interaction between Ni2+ and NB were discussed, and the results showed that α-diimine
nickel complexes with greater steric hindrance and smaller HOMO−LUMO gaps could achieve higher reactivity.
toward deactivation by polar functionalities, the design of late-
transition-metal catalysts has received renewed interest.33−44
The common late-transition-metal catalysts are Ni(II) and
Pd(II) complexes. Extensive investigations have favored Ni(II)
complexes, because of their better catalytic performance and
their illustrated reaction mechanisms.45
Bulky α-diimine nickel complexes which show high catalytic
activity for α-olefin polymerization have been successfully
developed by Brookhart and his collaborators.46−48 The
utilization of α-diimine Ni(II) complexes for norbornene
polymerization has attached much interest in both the academic
and industrial fields over the past dozen years. The catalytic
features rely on the electronic or steric influence of the ligand
used; therefore, substantial work has been conducted on
substituent variation of the α-diimine ligands.49,50 Much of this
work has focused on changing the N-bound aryl group51−53 or
modifying the backbone,54 including the use of axial donating
diimine ligands.55 Considering that a substituted bulky
INTRODUCTION
■
Bicyclo[2.2.1]hept-2-ene (norbornene, NB) and its derivatives
have three polymerization methods: ring-opening metathesis
polymerization (ROMP),1−8 cationic or radical polymer-
ization,9 and vinyl-type polymerization.10−20 Due to the vinyl-
type polynorbornene (PNB) high glass transition temperature,
high optical transparency, low dielectric constant, and low
birefringence, it has been the subject of many studies.
Meanwhile, incorporation of polar functionalities into the
polynorbornene backbone can change their physical proper-
ties,21−25 such as toughness, adhesion, surface properties
(paintability, wettability, and printability), solvent resistance,
blend compatibility with other functional polymers, and
rheological properties.
Different catalysts can lead to different polymerization
methods of norbornene; thus, the design and synthesis of
catalysts are very important. It is well-known that the vinyl
addition polymerization of norbornene can be efficiently carried
out using conventional Ziegler−Natta catalysts,26,27 metal-
locene catalysts,28−30 and late-transition-metal catalysts.31,32
Recently, because of the lower oxophilicity and resistance
Received: September 15, 2012
© XXXX American Chemical Society
A
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Scheme 1. Synthesis Route of α-Diimine Nickel Complexes
backbone may enhance the stability of nickel α-diimine
complexes by hindering the rotation of aniline moieties and
protecting the metal center effectively, three-dimensional
geometry diimine ligands with great hindrance have been
selected. Especially, to the best of our knowledge, the research
into three-dimensional geometry diimine ligands of nickel
catalysts is still rare.
In this study, a series of three-dimensional geometry 9,10-
dihydro-9,10-ethanoanthracene-11,12-diimine ligands (L1−L4)
and their nickel(II) dibromide complexes (C1−C4) were
designed and synthesized, and the catalytic activities of
complexes C1−C4 with only B(C6F5)3 as cocatalyst in NB
polymerization were also investigated. All complexes appeared
to be highly active catalysts of vinyl NB polymerization;
complexes C2 and C3 displayed high activities toward
copolymerization of NB and NB-OCOMe. In particular, we
had aimed at investigating the reaction mechanisms and the
different reactivities of different catalysts by the 19F NMR
analysis of the polymer end group and density functional theory
(DFT) calculations of the reaction.
Figure 1. ORTEP plot of C1 showing the atom-labeling scheme.
Hydrogen atoms and some carbon atoms are omitted for clarity.
RESULTS AND DISCUSSION
■
Synthesis of α-Diimine Ligands and of Nickel(II)
Complexes. The stoichiometric condensation reaction of
9,10-dihydro-9,10-ethanoanthracene-11,12-dione and various
anilines produced α-diimine ligands L1−L4. All α-diimine
ligands L1−L4 formed α-diimine nickel(II) dibromide
complexes C1−C4 in good yields by reaction with dimethoxy-
ethane nickel dibromide in dichloromethane (Scheme 1). The
α-diimine ligands and nickel complexes were characterized by
elemental analysis, single-crystal X-ray diffraction analysis, and
NMR spectroscopy.
X-ray Crystallographic Studies. Dark red crystals were
obtained by slow diffusion of hexane into a dichloromethane
solution of complexes C1−C4, whereas yellow single crystals of
ligands L1−L4 suitable for X-ray diffraction analysis were
obtained by slow diffusion of petroleum ether into their ethyl
acetate solutions. The ORTEP plots of all complexes and
ligands are shown in Figures 1−4 and Figures S1−S4 (in the
Supporting Information). The crystallographic data are
Figure 2. ORTEP plot of C2 showing the atom-labeling scheme.
Hydrogen atoms and some carbon atoms are omitted for clarity.
summarized in Tables S1 and S2 (Supporting Information).
Tables S3 and S4 (Supporting Information) give the selected
B
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10000/1 and B/Ni ratio at 5/1 is the most effective. All of the
complexes demonstrated high activities on the order of (0.01−
5.53) × 107 g of polymer/((mol of Ni) h) under optimal
conditions. Among the complexes, complex C2 had the highest
activity, 5.53 × 107 g of polymer/((mol of Ni) h). Especially, it
was far higher than the activities of conventional Ni diimines
(105 g of polymer/((mol of Ni) h)).56,57 Large steric
substituents on the aryl rings can effectively shield the axial
plane of the central metal and suppress the side reaction of
chain growth. Furthermore, with an increasing number of
electron-donating substituents on the aryl rings, C3 demon-
strated higher activity in comparison to C1, while complex C4
with an electron-withdrawing para substituent on the aryl ring
exhibited the lowest catalytic activity. The obtained PNBs were
completely soluble in chlorobenzene and dichlorobenzene at
room temperature, which indicated a low stereoregularity.
Copolymerization of NB and NB-OCOMe. Since the
Ni(II)/B(C6F5)3 system had high activity (5.53 × 107 g of
polymer/((mol of Ni) h) toward NB homopolymerization, C2
and C3 were selected as catalysts to investigate whether they
also had high activities toward copolymerization of NB with
NB-OCOMe. A series of copolymerizations of NB with NB-
OCOMe were investigated with the catalyst system Ni(II)/
B(C6F5)3 in chlorobenzene at 60 °C for 1 h under a nitrogen
atmosphere (Scheme 3).
Figure 3. ORTEP plot of C3 showing the atom-labeling scheme.
Hydrogen atoms and some carbon atoms are omitted for clarity.
From Table 2, we could also find that the catalytic activity of
C2/B(C6F5)3 was higher than that of C3/B(C6F5)3. However,
NB-OCOMe could not be homopolymerized under the same
conditions. This was common for functionalized NB
derivatives, in which the oxygen atom competes with the
double bond for coordination. A high concentration of oxygen
atoms would impede double-bond coordination.58−60 The
copolymers of NB and NB-OCOMe were soluble in common
organic solvents (such as CHCl3, CH2Cl2, cyclohexane, THF)
as well as chlorobenzene, exhibiting better solubility than PNB.
To obtain the reactivity ratios of NB and NB-OCOMe
monomers, we selected C2/B(C6F5)3 as the catalytic system
and changed the polymerization time and the feed content of
catalysts to control the conversion rate to be under 10%. The
results are shown in Table S5 (Supporting Information). The
Figure 4. ORTEP plot of C4 showing the atom-labeling scheme.
Hydrogen atoms and some carbon atoms are omitted for clarity.
bond lengths and angles of complexes and ligands. In these
complexes, the coordination geometries of the complexes C1,
C3, and C4 were demonstrated to be very similar in the solid
state. They were both mononuclear and four-coordinate.
However, the crystal structure of C2 showed a binuclear
complex, where each nickel atom is five-coordinate. The two
nickel atoms together with two bromine atoms form a planar
four-membered ring.
Homopolymerization of NB. First of all, NB polymer-
ization activities of the synthesized complexes were investigated
(Scheme 2). In all polymerizations, B(C6F5)3 was selected as
cocatalyst; it could be seen that C6F5 transferred from B(C6F5)3
to nickel in norbornene polymerization.16,20
reactivity ratios were determined by the Kelen−Tudos method
̈
̈
(Figure 5),61 which is shown in eqs 1−4. For C2/B(C6F5)3, the
reactivity ratio of NB-OCOMe was 0.05 and the reactivity ratio
of NB was 6.72.
Scheme 2. Homopolymerization of NB Catalyzed by α-
[NB‐OCOCH3]
d[NB‐OCOCH3]
Diimine Nickel Complexes
x =
y =
[NB]
d[NB]
(1)
(4)
⎛
rNB
α
⎞
rNB
α
⎜
⎟
⎠
η = rNB‐OCOCH
+
ξ −
3
⎝
x2
y
x(y − 1)
F =
η =
G =
α = Fmax × Fmin
y
(2)
(3)
Four Ni(II) complexes were tested as catalysts in the same
system with B(C6F5)3 as cocatalyst and chlorobenzene as the
reaction solvent (Table 1). Polymer yields and catalytic
activities depended on the reaction conditions. In order to
determine the most suitable homopolymerization conditions,
initial investigations of NB homopolymerization catalyzed by
C2/B(C6F5)3 were conducted under various conditions, such as
different temperatures, NB/Ni ratios, and B/Ni ratios. It was
found that the conditions at 90 °C with the NB/Ni ratio at
G
α + F
F
ξ =
α + F
Characterizations of Copolymers. The obtained copoly-
mers were characterized by GPC, and the corresponding data
are shown in Table 2. The molecular weights of the obtained
C
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a
Table 1. Norbornene Polymerization on α-Diimine Nickel Complexes
b
entry
complex
NB/Ni (mol/mol)
B/Ni (mol/mol)
T (°C)
reaction time (min)
PNB yield (%)
activity
1
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C1
C3
C4
5000
10000
15000
20000
10000
10000
10000
10000
10000
10000
10000
10000
10000
10000
10000
10000
10000
10000
10000
20
20
20
20
0
60
60
60
60
60
60
60
60
60
60
60
0
10
10
10
10
c
48.13
73.64
22.65
6.69
1.36
4.16
1.92
0.75
c
2
3
4
5
c
6
2.5
5
10
2
41.60
96.48
82.03
73.64
47.97
16.82
4.78
2.35
27.25
23.17
4.16
2.71
0.95
0.09
0.74
27.25
55.30
45.35
16.45
18.98
8.64
7
8
10
20
40
60
5
2
9
10
10
10
30
30
2
10
11
12
13
14
15
16
17
18
19
5
30
60
90
120
90
90
90
39.30
96.48
97.93
80.30
29.12
33.60
5
5
1
5
1
5
1
5
1
5
1
15.29
a
b
c
Reaction conditions: chlorobenzene, 22 mL; Ni complex, 5 × 10−6 mol. In units of 106 g of polymer/((mol of Ni) h). Not determined.
Scheme 3. Copolymerization of NB and NB-OCOMe
Catalyzed by α-Diimine Nickel Complexes
Figure 5. Kelen-Tudos method plot of η versus ξ for NB/NB-
̈
̈
OCOMe copolymerization (based on NMR analysis of the copolymers
catalyzed by C2/B(C6F5)3 systems).
copolymers were up to 105, and the molecular weight
distributions (MWDs) of the obtained polymers were all
relatively narrow (MWD < 2). The molecular weights of
copolymers in the C2/B(C6F5)3 system were somewhat higher
than those of polymers obtained from C3/B(C6F5)3.
These obtained copolymers were characterized by spectra
(FTIR, 1H NMR) and thermal analysis (DSC, TGA). In Figure
S5 (Supporting Information), the signal of the double bond at
1710 cm−1, assigned to the CC bond of the ROMP structure
of PNB, was not observed. The signals at about 941 cm−1 could
be attributed to the bicyclo[2.2.1]heptane ring. The observa-
tion of a signal at 1740 cm−1 in the spectra confirmed that the
NB-OCOMe monomer had been inserted into the chains of
the copolymers in vinyl-type addition rather than physical
a
Table 2. Copolymerization of NB and NB-OCOMe Catalyzed by Nickel Complex C2 or C3 Combined with B(C6F5)3
b
c
c
d
entry
cat.
NB/NB-OCOMe (mol/mol)
reaction time (min)
yield (%)
activity
Mw
Mw/Mn
NB-OCOMe (mol %)
1
2
3
4
5
6
7
8
9
C2
C2
C2
C2
C2
C3
C3
C3
C3
100/0
80/20
60/40
50/50
0/100
100/0
80/20
60/40
50/50
1
60
60
60
60
2
82.03
55.23
34.2
32.6
trace
90.42
46
2.3 × 107
5.8 × 105
4.0 × 105
1.6 × 105
e
1.6 × 106
4.8 × 105
2.9 × 105
e
e
e
e
2.8 × 105
1.5
1.4
1.4
e
7.28
8.23
10.23
e
1.9 × 104
1.5 × 104
e
e
e
e
60
60
60
1.0 × 105
1.5 × 104
e
1.5
1.5
e
6.10
7.18
e
25
trace
a
Conditions: c[cat.] = 5.0 × 10−6 mol/L; cocatalyst B(C6F5)3, B/complex/monomer (n/n/n) 10/1/10000, polymerization temperature 60 °C,
b
c
solvent chlorobenzene, total volume 22 mL. In units of g of polymer/((mol of Ni) h). Determined by GPC vs polystyrene standards in THF.
d
e
1
Determined by H NMR spectroscopy in CDCl3. Not determined.
D
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blends. Figure S6 (Supporting Information) proved that the
polymers were vinyl-addition type by the absence of a
resonance of the proton hydrogen connected to the double
bond at 5.3−5.9 ppm. The peaks at 0.9−1.3 ppm were assigned
to the hydrogen corresponding to 5′/6′/6, the peaks at 1.5−1.7
ppm were assigned to the hydrogen corresponding to 7′/7, the
peaks at 1.9−2.2 ppm were assigned to the hydrogen
corresponding to 1′/2′/3′/4′/1/2/3/4/8, and the peaks at
5.0 ppm qwew assigned to the hydrogen corresponding to 5.
and the polymers were thus not suitable for NMR
determination of end groups. The unambiguous character-
ization of the end groups for copolymers (NB-OCOMe
content 7.28%) in Table 2 by 19F NMR spectra enabled us
to construct a mechanism of chain initiation and termination.
The 19F NMR spectrum (Figure 6) of copolymers
demonstrated that the resonances observed came from a
1
All the H NMR spectra of the obtained copolymer had two
blocks of broadened signals (0.9−2.2 and 4.9−5.0 ppm). Each
block corresponded to 1′/2′/3′/4′/5′/6′/7′/1/2/3/4/6/8 and
5. If we assume the NB-OCOMe content as x, each block was
calculated to contain 10 + x and x protons. The ratio (10 + x)/
x corresponds to the integral ratio of these two blocks (A/B).
From this equation, x could be calculated as 10B/A − B.
Figure 6. 19F NMR spectrum of NB/NB-OCOMe copolymers (NB-
OCOMe content 7.28%) obtained with C2/B(C6F5)3.
C6F5 group appended to the polymer, presumably at the end of
the polynorbornene chain. The p-fluorine was assigned to the
peak at −158.1 ppm (1F). The two m-fluorines, which
theoretically should also be nonequivalent, were evidently
accidentally degenerate and resonate at −162.9 ppm (2F) and
apparently were not affected by the stereochemistry of the last
polymer chain units.64 The two o-fluorines should be
nonequivalent; one of them was syn to the norbornene
bridging methylene, and the other was anti. If the
pentafluorophenyl end group experienced hindered rotation
around the phenyl ipso carbon−norbornene carbon single
bond, separate resonances would be expected for the o-
fluorines. Indeed, two groups of signals were observed at
−133.1 (1F) and −140.1 ppm (1F) and were assigned to the
two o-fluorines. The chemical shifts and the number of the
absorption peaks were consistent with those reported.16 It
could be concluded that the polymerization initiation
mechanism via insertion of the norbornene monomer into
the Ni−C bond of the C6F5 ligand was supported by the 19F
NMR spectrum of a copolymer containing the resonances of
C6F5.
In Figure 7, in the first step, the NB molecule coordinates
perpendicularly to the complex R1 to form the π complex IN1.
For IN1, the two NB carbon atoms are located 2.113 and 2.205
Å from the nickel atom. The coordination of NB to complex R1
leads to a decrease in energy to 27.61 kcal mol−1, which
indicates that this process is exothermic. The formation of a
new carbon−metal bond (complex IN1) is complete at this
stage. The rate-determining step of the chain propagation is NB
insertion from complex NB to PR1 through transition state 1
(TS1) (Figure S10, Supporting Information). The NB
molecule in complex IN1, which is also perpendicular to the
plane of the metal coordination sphere, rotates into the
equatorial plane and forms two new bonds from the two carbon
atoms originally bonded to the metal. Simultaneously, the alkyl
group on the catalyst migrates from the metal to one of the C
The content of NB-OCOMe in copolymers was calculated
from 6.1 to 10.2% by varying the comonomer feed ratios (NB-
OCOMe/NB) from 10 to 50%.62
The TGA curves of the copolymers with different NB-
OCOMe contents prepared by C2/B(C6F5)3 and C3/B(C6F5)3
are shown in Figure S7 (Supporting Information). The
decomposition temperatures (Tds) of the polymers were as
high as 321.5−408.1 °C and decreased with an increase of the
NB-OCOMe content in the copolymers. The DSC curves and
glass transition temperatures (Tgs) of the copolymers with
different NB-OCOMe ratios prepared by Ni(II)/B(C6F5)3 are
shown in Figure S8 (Supporting Information). The Tgs of the
polymers were reduced to 265.9−304.2 °C when the content of
NB-OCOMe was increased in the copolymers.
All of the copolymers, irrespective of the polymerization
conditions employed, showed identical powder WXRD
patterns. Figure S9 (Supporting Information) shows the
WXRD curves of the copolymers with different NB-OCOMe
contents prepared by C2/B(C6F5)3 and C3/B (C6F5)3. There
was no trace of Bragg reflections in the characteristic crystalline
regions; thus, the polymers are noncrystalline. The occurrence
of two halos at 2θ values of 9.82−11.14 and 17.86−19.03° are
characteristic for PNB. The packing density became smaller
with an increase of NB-OCOMe content in the copolymers,
and the packing densities of the copolymers obtained by C2/
B(C6F5)3 were smaller than those obtained by C3/B(C6F5)3.
Mechanism of NB Polymerization. In order to establish
the NB polymerization mechanisms catalyzed by Ni catalysts
used in the experiments, we analyzed the end group of the
polymer and carried out a DFT calculation for the reaction.63
Toward the polymerization initiation step, the most direct
proof of an initiation mechanism would come from elucidation
of the end group structure of the polymers.16,20 The solubilities
of the polymers produced in CDCl3 in Table 1 were too poor
E
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Figure 7. Proposed mechanism and energy profile of NB polymerization catalyzed by Ni catalyst.
atoms of NB through TS1 to produce the product PR1. This
complexation assists the insertion of NB into the metal−carbon
bond via a four-membered cyclic transition state. For this
process, the energy barrier is about 19.93 kcal mol−1. At the
TS1 point, the distance between C1 and C2 lengthens from
1.391 Å in IN1 to 1.492 Å in TS1. After TS1, the complex PR1
formed, which possesses an energy of 3.22 kcal mol−1 above
IN1. The product of insertion (PR1) resembles the structure of
IN1 in that a new vacant coordination site is available for the
next NB to coordinate and propagate the chain. The
coordination of NB to PR1 results in a decrease in energy of
about 14.37 kcal mol−1 to produce the complex R2.
between LUMONi and HOMONB and the charges are
transferred from NB to Ni catalyst in the reaction process.
All these results indicate that a smaller HOMO−LUMO gap
was more reactive. From Figure 8, the HOMO−LUMO gaps
In order to understand the effect of the catalyst structure
variation on the Ni catalysis activity, the substituent effect of the
catalyst and the interaction between Ni2+ and NB will be
discussed in detail. The orbital interaction diagram is given in
Figure S11 (Supporting Information). According to these
orbital models, the interactions between the ligand (NB) and
Ni2+ in IN1 comprise mainly two components. Therefore, the
two NB−metal bonding orbitals are occupied in IN1. First, the
interactions of the occupied π orbital of the ligand (NB) with
Figure 8. Relevant FMOs of NB and different Ni catalysts in this
reaction.
predicted for the reactions of NB with four different Ni
catalysts are 0.205, 0.210, 0.211, and 0.216 au for C2, C3, C1,
and C4, respectively. Thus, the complex C2 has a better
catalyzing activity than the other three kinds of Ni catalysts,
which is consistent with the experimental results.
the dx −y orbital of Ni2+ result in σ and σ* orbitals. In fact, the
2
2
HOMO-1 orbital represents the orbital interaction of the p
2
2
orbital of the ligand (NB) with the dx −y orbital of the Ni atom.
At the same time, the π* orbital of NB and dxy orbital of Ni2+
also overlap to produce the π and π* orbitals between NB and
Ni2+ (Figure S11). The HOMO orbital represents the
interaction of the π* orbital of NB and the dxy orbitals of
Ni2+. In fact, the interaction of the π* orbital of NB and dxy
orbitals of Ni2+ result in a lengthening of the CC bond of
NB, which leads to activation of the carbon−carbon double
bond (Figure S12, Supporting Information). Thus, the HOMO
is very important for the activation of NB in that there are
strong π interactions between NB and Ni2+.
CONCLUSIONS
■
A series of new three-dimensional geometry α-diimine nickel
complexes were synthesized and characterized by single-crystal
X-ray techniques. Despite the fact that there have been many
reported α-diimine systems in the literature, the three-
dimensional geometry α-diimine ligands show superior steric
hindrance on the nickel center and herein represent a new
polymerization mechanism for α-diimine catalytic systems.
Toward NB polymerization, these new three-dimensional
geometry α-diimine nickel complexes showed high activities,
for C2 even up to 5.53 × 107 g of polymer/((mol of Ni) h),
which was the highest activity for NB polymerization over α-
diimine nickel catalysts in the current report. α-Diimine nickel
complexes with greater steric hindrance and a smaller HOMO−
LUMO gap could achieve even higher reactivity. The
polymerization initiation mechanism via insertion of the
norbornene monomer into the Ni−C bond of the C6F5 ligand
was supported by the 19F NMR analysis of the polymer end
The relevant frontier molecular orbitals (FMOs) of Ni
catalyst and NB in this reaction are presented in Figure S13
(Supporting Information). It is easy to find that the energy
difference between HOMONi (HOMO of Ni catalyst) and
LUMONB (LUMO of NB) is greater than that of HOMONB
(HOMO of NB) and LUMONi (LUMO of Ni catalyst);
therefore, the main HOMO−LUMO interaction occurs
F
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ppm): 17.78, 20.90, 51.05, 125.20, 125.43, 127.60, 128.42, 132.59,
138.22, 145.40, 159.89.
group and DFT calculations of the reaction. These findings
identify a promising strategy to explore high-reactivity catalysts
by designing three-dimensional geometry α-diimine catalytic
systems with large steric hindrance.
Ligand L4 was obtained as yellow crystals in 75.6% yield. Mp: 194−
195 °C. Anal. Found: C, 64.21; H, 4.41; N, 4.65. Calcd: C, 64.23; H,
1
4.38; N, 4.68. H NMR (CDCl3, δ, ppm): 1.84 (s, 12 H), 4.85 (s, 2
H), 7.20−7.25 (m, 8H), 7.28 (s, 4H). 13C NMR (CDCl3, δ, ppm):
17.68, 51.09, 116.33, 125.28, 127.88, 128.02, 130.51, 137.64, 146.82,
160.32.
EXPERIMENTAL SECTION
■
General Considerations. All the reactions were performed under
an atmosphere of dry and oxygen-free argon using standard vacuum
and Schlenk techniques or under a nitrogen atmosphere in a glovebox
(MBraun). Chlorobenzene was refluxed and distilled from sodium
under dry nitrogen; dichloromethane was dried over CaH2. NB was
purchased from Alfa Aesar, purified through drying by sodium and
distilled under dry nitrogen, and then dissolved in chlorobenzene. 5-
Norbornen-2-yl acetate was purchased from Aldrich and purified by
anhydrous magnesium sulfate. Other commercially available reagents
were purchased and used without purification. The intensity data of
the single crystals were collected on a CCD-Bruker Smart APEX II
system. Elemental analysis was performed on an Elementar-Vario EL
cube elemental analyzer. The nuclear magnetic resonance (NMR)
spectra of the polymers were recorded on a Bruker ARX 400 NMR
spectrometer at ambient temperature, with CDCl3 as the solvent.
Chemical shifts for the 1H and 13C NMR spectra are given in parts per
million from the peak for internal trimethylsilane. The 19F NMR
spectra are referenced to external CFCl3. An XT4A melting point
instrument (BeiJing KeYi electro-optic instrument factory, China) was
used to test the melting point temperature. Gel permeation
chromatography (GPC) was conducted with a Breeze Waters system
using polystyrene as the standard and tetrahydrofuran as the eluent at
a flow rate of 1.0 mL/min. The FTIR spectra were recorded with a
Shimadzu IR Prestige-21 FTIR spectrophotometer. Wide-angle X-ray
diffraction (WXRD) curves were recorded on a Bruker D8 Focus X-
ray diffractometer, operating at 40 kV and 40 mA with a copper target
(λ = 1.54 Å) and a scanning rate of 2°/min. The thermal gravimetric
analysis (TGA) measurements were performed on a Perkin-Elmer
TGA 7 instrument from room temperature to 700 °C at a rate of 10
°C/min under a nitrogen atmosphere. Differential scanning calorim-
etry (DSC) measurements were obtained on a Shimadzu DSC-60
instruemtn with a heating/cooling rate of 10 °C/min under a nitrogen
atmosphere.
Synthesis of α-Diimine Nickel Dibromide Complexes C1−
C4. The α-diimine nickel dibromide complexes C1−C4 were
synthesized by the reaction of dimethoxyethane nickel dibromide
with the corresponding aniline in dichloromethane. A typical synthetic
procedure for C1 is as follows: 0.2 mmol of ligand L1 and 0.2 mmol of
dimethoxymethane nickel dibromide were added in a Schlenk tube
together with 10 mL of dried dichloromethane. The reaction mixture
was then stirred for 8 h at room temperature, and 10 mL of absolute
hexane was added. The dark red complex crystallized from a mixture of
dichloromethane and hexane in 85% yield. Complex C1 was
characterized by elemental analysis, single-crystal X-ray diffraction
1
analysis, and H NMR spectroscopy. Mp: 343−344 °C. Anal. Found:
C, 58.34; H, 4.26; N, 4.24. Calcd: C, 58.32; H, 4.28; N, 4.25. 1H NMR
(CDCl3, δ, ppm): 1.92 (s, 12 H), 5.05 (s, 2 H), 7.03−7.52 (m, 14H).
Complex C2 was obtained as dark red crystals in 89% yield. Mp:
320−321 °C. Anal. Found: C, 62.26; H, 5.76; N, 3.62. Calcd: C, 62.29;
1
H, 5.75; N, 3.63. H NMR (CDCl3, δ, ppm): 1.03 (d, 12 H, J = 6.8
Hz), 1.16 (d, 12 H, J = 6.8 Hz), 2.50 (m, 4 H), 5.02 (s, 2 H), 7.11−
7.27 (m, 14H).
Complex C3 was obtained as dark red crystals in 80% yield. Mp:
351−352 °C. Anal. Found: C, 59.45; H, 4.65; N, 4.09. Calcd: C, 59.43;
H, 4.69; N, 4.08. 1H NMR (CDCl3, δ, ppm): 1.86 (s, 12 H), 2.39 (s, 6
H), 5.06 (s, 2 H), 7.24 (s, 4H), 7.33−7.37 (m, 8H).
Complex C4 was obtained as dark red crystals in 88% yield. Mp:
362−364 °C. Anal. Found: C, 47.03; H, 3.25; N, 3.45. Calcd: C, 47.05;
H, 3.21; N, 3.43. 1H NMR (CDCl3, δ, ppm): 1.85 (s, 12 H), 4.85 (s, 2
H), 7.27−7.52 (m, 8H), 7.96 (s, 4H).
Procedure for Homopolymerization of NB. In a typical
procedure, the appropriate B(C6F5)3 solid was introduced into a
round-bottom glass flask, a certain amount of a chlorobenzene solution
of NB (0.5 g/mL) and quantitative nickel complex solution were
syringed into the well-stirred solution in order, and the reaction
mixture was continuously stirred for an designated period at the
polymerization temperature. Polymerizations were terminated by
addition of 50 mL of acidified ethanol (10/1 v/v ethanol/HCl).
The precipitate of polynorbornene was filtered off, washed with
ethanol (3 × 10 mL), and dried under vacuum at 40 °C to a constant
weight.
Procedure for Copolymerization of NB and NB-OCOMe. In a
typical procedure, the appropriate B(C6F5)3 solid was introduced into
a round-bottom glass flask, a certain amount of a chlorobenzene
solution of NB (0.5 g/mL), the corresponding amount of NB-
OCOMe, and quantitative nickel complex solution were syringed into
the well-stirred solution in order, and the reaction mixture was
continuously stirred for an designated period at 60 °C. Polymer-
izations were terminated by addition of 50 mL of the acidified ethanol
(10/1 v/v ethanol/HCl). The precipitate of polynorbornene was
filtered off, washed with ethanol (3 × 10 mL), and dried under vacuum
at 40 °C to a constant weight.
Calculation Details. The geometries were optimized by DFT,
specifically with the Becke three-parameter hybrid exchange functional
and the Lee−Yang−Parr correlation function (B3LYP).66,67 All
theoretical calculations were performed using the Gaussian 09 suite
of programs.63 All structures were optimized by employing the hybrid
density functional B3LYP method66,67 and the 6-31G(d,p) basis set.68
A vibrational frequency calculation was then performed at the
optimized geometry belonging to each reactant, product, transition
state, and intermediate. We confirmed that all reactants and
intermediates had no imaginary frequencies and each transition state
had one, and only one, imaginary frequency. The zero-point energies
(ZPE) were calculated using the vibrational frequencies. Intrinsic
reaction coordinate (IRC) calculations, at the same level of theory,
Synthesis of α-Diimine Ligands L1−L4. The α-diimine ligands
L1−L4 were synthesized by the reaction of 9,10-dihydro-9,10-
ethanoanthracene-11,12-dione65 with the corresponding aniline in
toluene. A typical synthetic procedure for L1 is as follows: under a
nitrogen atmosphere, to a solution of 30 mmol of 2,6-dimethylaniline
in 150 mL of toluene were added 10 mmol of 9,10-dihydro-9,10-
ethanoanthracene-11,12-dione and a catalytic amount of p-toluene-
sulfonic acid. The mixture was heated under reflux. The resulting water
was removed as an azeotropic mixture using a Dean−Stark apparatus.
After 24 h, the reaction mixture was cooled to room temperature and
evaporated at reduced pressure. The residual solids were further
purified by silica column chromatography (20/1 v/v petroleum ether/
ethyl acetate) and were crystallized from a mixture of petroleum ether
and ethyl acetate to give the ligand L1 as yellow crystals in 96.1% yield.
Ligand L1 was characterized by elemental analysis, single-crystal X-ray
diffraction analysis, and 1H and 13C NMR spectroscopy. Mp: 213-214
°C. Anal. Found: C, 87.31; H, 6.40; N, 6.29. Calcd: C, 87.24; H, 6.41;
1
N, 6.36. H NMR (CDCl3, δ, ppm): 1.88 (s, 12H), 4.86 (s, 2H),
7.04−7.25 (m, 14H). 13C NMR (CDCl3, δ, ppm): 17.81, 51.13,
123.51, 125.61, 126.08, 127.71, 128.39, 138.05, 147.90, 159.64.
Ligand L2 was obtained as yellow crystals in 95.2% yield. Mp: 196−
197 °C. Anal. Found: C, 86.94; H, 7.94; N, 5.12. Calcd: C, 86.91; H,
8.02; N, 5.07. 1H NMR (CDCl3, δ, ppm): 1.02 (d, 12 H, J = 6.8 Hz),
1.15 (d, 12 H, J = 6.8 Hz), 2.49 (m, 4 H), 4.978 (s, 2 H), 7.05−7.25
(m, 14H). 13C NMR (CDCl3, δ, ppm): 22.49, 23.29, 51.10, 122.79,
124.12, 125.40, 127.27, 136.38, 138.57, 145.56, 158.44.
Ligand L3 was obtained as yellow crystals in 87.2% yield. Mp: 227−
228 °C. Anal. Found: C, 87.23; H, 6.91; N, 5.86. Calcd: C, 87.14; H,
1
6.88; N, 5.98. H NMR (CDCl3, δ, ppm): 1.84 (s, 12 H), 2.36 (s, 6
H), 4.89 (s, 2 H), 7.23 (s, 4H), 7.21 (m, 8H). 13C NMR (CDCl3, δ,
G
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were performed to ensure that the transition states led to the expected
reactants and products.
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ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files, tables, and figures giving crystallographic data and
additional characterization data for α-diimine ligands L1−L4)
and α-diimine nickel(II) dibromide complexes C1−C4 and
details of the calculations. This material is available free of
charge via the Internet at http://pubs.acs.org.
(28) Natta, G.; Pino, P.; Mazzanti, G.; Giannini, U.; Mantica, E.;
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AUTHOR INFORMATION
■
Corresponding Author
*Tel.: +86 791 83969562. Fax: +86 791 83969561. E-mail:
ywchen@ncu.edu.cn (Y.C.); hexiaohui@ncu.edu.cn (X.H.).
(30) Golding, R. M.; Pascual, R. O.; Suvanprakorn, C.; Dance, I. G.
Bull. Korean Chem. Soc. 2006, 27, 1752−1756.
Notes
(31) Raspolli Galletti, A. M.; Hayatifar, M. J. Polym. Sci., Part A:
Polym. Chem. 2012, 50, 4459−4464.
The authors declare no competing financial interest.
(32) Zhang, J. S.; Gao, W.; Lang, X. M.; Wu, Q. L.; Zhang, L.; Mu, Y.
Dalton Trans. 2012, 41, 9639−9645.
ACKNOWLEDGMENTS
■
(33) Lassahn, P. G.; Janiak, C.; Oh, J. S. Macromol. Rapid Commun.
2002, 23, 16−20.
This work was supported by the National Natural Science
Foundation of China (No. 21164006).
(34) Piche, L.; Daigle, J.-C.; Rehse, G.; Claverie, J. P. Chem. Eur. J.
2012, 18, 3277−3285.
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