Insight into the mechanism of thermal stability of α-diimine nickel complex in catalyzing ethylene polymerization

Article abstract of DOI:10.1021/acs.organomet.7b00066

The union of experimental and computational methods can accelerate the development of polymerization catalysts for industrial applications. Herein, we report complementary experimental and computational studies of the thermal stability of α-diimine nickel complexes by using thermally stable Cat. 1 and a typical Brookhart catalyst (B-Cat) as models. Experimentally, we found that many more nickel atoms could be activated for Cat. 1 at elevated temperature during the ethylene polymerization process compared to those for B-Cat. Computationally, first-principle calculations showed that the decomposition energies of Cat. 1 were found to be higher than those of B-Cat, contributing to the activation observed for Cat. 1. We found that the presence of ethydene evidently affected the conformation of C1-N1-Ni-N2-C2 five-membered ring (where the nickel center is located) of Cat. 1, turning the envelope conformation (B-Cat) into a half-chair conformation (Cat. 1). According to calculations, the decomposition energy of the latter was 17.4 kJ/mol higher than that of the former. These results provide information to elucidate the mechanism of thermal stability of α-diimine nickel catalyst and significantly advance the development of thermally stable α-diimine nickel catalysts used in industry.

Full text of DOI:10.1021/acs.organomet.7b00066

Article
pubs.acs.org/Organometallics
Insight into the Mechanism of Thermal Stability of α‑Diimine Nickel
Complex in Catalyzing Ethylene Polymerization
Liang Zhu,†,‡ Dandan Zang,† Yi Wang,‡ Yintian Guo,† Baiyu Jiang,† Feng He,† Zhisheng Fu,*,†
Zhiqiang Fan,† Michael A. Hickner,‡ Zi-Kui Liu,‡ and Long-Qing Chen‡
†MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, 38
Zheda Road, Hangzhou 310027, China
‡Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United
States
S
* Supporting Information
ABSTRACT: The union of experimental and computational methods can
accelerate the development of polymerization catalysts for industrial
applications. Herein, we report complementary experimental and
computational studies of the thermal stability of α-diimine nickel
complexes by using thermally stable Cat. 1 and a typical Brookhart
catalyst (B-Cat) as models. Experimentally, we found that many more
nickel atoms could be activated for Cat. 1 at elevated temperature during
the ethylene polymerization process compared to those for B-Cat.
Computationally, first-principle calculations showed that the decom-
position energies of Cat. 1 were found to be higher than those of B-Cat,
contributing to the activation observed for Cat. 1. We found that the
presence of ethydene evidently affected the conformation of C1−N1−Ni−
N2−C2 five-membered ring (where the nickel center is located) of Cat. 1,
turning the envelope conformation (B-Cat) into a half-chair conformation (Cat. 1). According to calculations, the
decomposition energy of the latter was 17.4 kJ/mol higher than that of the former. These results provide information to elucidate
the mechanism of thermal stability of α-diimine nickel catalyst and significantly advance the development of thermally stable α-
diimine nickel catalysts used in industry.
diazadiene ligands derived from glyoxal produced low
molecular weight and linear PE. In contrast, catalysts with
backbones of methyl and acenaphthyl substituents provided
higher molecular weight and branched PE.3a,11 Based on a
strategy to improve thermal stability of α-diimine nickel
catalysts by modifying the α-diimine backbone, Wu et al.2b
prepared a series of α-diimine nickel complexes with bulky
camphyl-substituted backbones (VI in Chart 1). These
researchers attributed the thermal stability improvement to
bulky substituents on the ortho-position of aniline moiety or on
the backbone that exerted steric effects on nickel centers by
blocking the axial site for the metal center and suppressing
potential rotation of the CAr−N bond. Although Brookhart
pointed out that decomposition of α-diimine palladium
complex occurred by C−H activation of the ortho alkyl,4d he
thought C−H activation was not a major route for deactivation
of α-diimine nickel catalysts bearing ortho alkyl substituents.3a
Because they found that thermal stability of α-diimine nickel
catalysts containing 2-C6F5 and 6-CH3 (I in Chart 1) was
higher than that only containing 2-C6F5.3a
INTRODUCTION
■
α-Diimine catalyst1,2 developed by Brookhart and co-workers in
1995 can produce multiple polyethylene (PE) topologies
ranging from linear to hyperbranched by varying polymer-
ization temperature,3a ethylene pressure,2e,4 and ligand
structure.2a,5 These PE products have shown clear promise
for industrial applications. However, in general, at 60 °C and
above, lifetimes of α-diimine catalyst are sufficiently shortened
so that the total turnovers over 10−30 min polymerization time
are significantly reduced. While a decrease in ethylene solubility
at elevated temperatures may play a role, it is clear that the
major effect on total turnover number should be attributed to
catalyst decay.3 Thus, the commercialization and overall
industrial application of α-diimine catalyst has been significantly
retarded, although α-diimine catalyst has been developed for
more than 20 years.6
Since 2000, a growing number of researchers have examined
the effects that bulky substituents have on the thermal stability
of α-diimine catalysts. Most of them modified those
substituents on the ortho-position of aniline moiety of the
ligand to obtain thermally stable α-diimine catalysts (I−V in
Chart 1).7−10 However, changes to the α-diimine backbone
have also shown a remarkable effect on polymer microstructure
and catalyst activity.1,3a For instance, catalysts bearing 1,4-
Received: January 26, 2017
© XXXX American Chemical Society
A
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Chart 1. α-Diimine Nickel Complexes
Angle strain, torsional strain, and steric strain contribute to
the overall energy of a cyclic molecule.12 The higher the energy,
the poorer the stability of the cyclic molecule. In an α-diimine
nickel complex, two carbon atoms, two nitrogen atoms, and a
nickel atom form a five-membered α-diimine nickel ring (as
shown in Figure 1).13 According to the single crystal structure
same plane, while the nickel atom lies about 7.49° above the
plane of C1−N1−N2−C2. It indicates that the five-membered
α-diimine nickel ring in B-Cat exists in envelope conformation.
In B-Cat, the 2,6-diisopropyl phenyls connecting directly to
the two N atoms in the five-membered α-diimine nickel ring
significantly affect the conformation of the ring. However, it
should also be noted that the naphthyl connecting to C1 and
C2 would also affect the conformation of the ring. According to
the single crystal structure, it is found that the naphthyl in B-
Cat deforms seriously. As shown in Figure S3a, the dihedral
angle between the two benzene rings is not 180° but 176.70°.
This deformation will influence the conformation of the five-
membered ring formed by the naphthyl and C1−C2 and finally
will change the conformation of the C1−N1−Ni−N2−C2 ring.
We thus envisioned introducing an ethydene bridge to the
naphthyl backbone could make it more rigid and conjugated.
Thus, the deformation of naphthyl could be eased, and the
C1−N1−Ni−N2−C2 ring could exist in a state of relatively
lower energy. Herein, we synthesized a new α-diimine nickel
complex with an acenaphthene-derived backbone (Cat. 1 in
Scheme 1) in three steps with commercially available starting
materials.
Figure 1. Conformation of C1−N1−Ni−N2−C2 five-membered rings
in B-Cat.13
of a typical Brookhart catalyst (B-Cat),13 the dihedral angle
between plane α (C1−C2−N2) and plane β (C1−N1−N2) is
179.56°. The dihedral angle between plane β and plane γ (N1−
Ni−N2) is 172.90°. The dihedral angle between plane α and
plane γ is 172.51°. C1, N1, N2, and C2 are in approximately the
Scheme 1. Synthetic Routes for α-Diimine Nickel Complex with Acenaphthene-Derived Backbone
B
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obtained by global refinement of the positions of all collected
reflections. Intensities were corrected for Lorentz and polarization
effects and empirical absorption. The structures were solved by direct
methods and refined by full-matrix least-squares on F2. All hydrogen
atoms were placed in calculated positions. Structure solution and
refinement were performed by using the SHELXL−97 package.15
Details of the X-ray structure determinations and refinements are
provided in Tables S1 and S2.
Most of the researchers owed the thermal stability improve-
ment of α-diimine nickel complex to bulky substituents on the
ortho-position of aniline moiety or on the backbone that
exerted steric effects on nickel centers by blocking the axial site
for the metal center and suppressing potential rotation of the
CAr−N bond. However, the introduction of an ethydene bridge
to the naphthyl backbone of a typical α-diimine nickel complex
significantly enhanced its thermal stability, although the
position of ethydene was too far to suppress the rotation of
the CAr−N bond. We found that the presence of ethydene
evidently affected the conformation of C1−N1−Ni−N2−C2
five-membered ring (where nickel center located) of Cat. 1,
turning the envelope conformation (B-Cat) into half-chair
conformation (Cat. 1). According to calculations, the
decomposition energy of the latter was 17.4 kJ/mol higher
than that of the former. This result demonstrates a new strategy
to develop and design thermally stable α-diimine nickel catalyst.
Polymerization Procedure and Quenching Reaction. Ethyl-
ene polymerizations were carried out in a 100 mL Schlenk flask
equipped with a magnetic stirrer. The Schlenk flask was heated by a
heat-gun under vacuum for several minutes, during which period the
Schlenk flask was swept with dry N2 at least three times. The Schlenk
flask was first filled with 25 mL of n-heptane at room temperature and
then saturated with ethylene of 0.1 MPa. After the Schlenk flask was
filled with solvent and ethylene, MAO/MMAO and the catalyst
solution (dissolved in 25 mL of toluene) were introduced in turn with
syringes. The polymerization was quenched with excess ethanol/HCl
mixture. The obtained solution was poured into 200 mL of ethanol to
precipitate the polymer. The product was filtered and further washed
with ethanol several times. A white polymer was collected and dried at
60 °C under vacuum overnight. Finally, the polymer was weighed, and
the catalytic activity was calculated in units of g(PE)/mol(Ni)·h.
Method of Counting Active Centers in Olefin Polymer-
ization. Ethylene polymerizations were performed in a 100 mL
Schlenk flask containing about 50 mL of toluene at 60 °C and 0.1 MPa
ethylene. The reagents were added in the order of solvent, MMAO,
and Cat. 1 with [Ni] = 100 μmol/L and Al/Ni = 200 (mol/mol). Five
minutes later, a TPCC toluene solution (in TPCC/Al = 3) was
injected into the flask to quench the polymerization and stirred at 60
°C for 5 min. Subsequently, an ethanol/HCl mixture (95/5) was
added to decompose the catalyst, and the polymer was precipitated
with excess of ethanol. The resultant polymer was treated by refluxing
the quenched polymer in excess of ethanol/HCl mixture for 60 min.
Then, the polymer was isolated, washed, dried, and weighed. Each
polymer sample was then thoroughly purified by one dissolution−
precipitation operation, extracted with fresh ethanol in a Soxhlet
extractor for 12 h, and then dried in vacuum at 60 °C. The sulfur
content of the quenched polymer was measured in a GLC-200
microcoulometry sulfur analyzer with a lower detection limit of 0.05
ppm (Jiangyan Yinhe Instrument Co., Jiangyan, China). The polymer
sample for analysis was solid powder (2−4 mg, weighed to 0.01 mg),
and the average value of three parallel measurements was taken as the
sulfur content. Propagation rate constant (kp) of polymerization was
calculated according to eq 1:
EXPERIMENTAL SECTION
■
Materials. All manipulations of air- and/or moisture-sensitive
compounds were performed under a nitrogen atmosphere using
standard Schlenk techniques. Toluene, diethyl ether, and n-heptane
were distilled under nitrogen from benzophenoneketyl. Methylene
chloride was distilled under nitrogen from CaH2. Acenaphthene,
carbon disulfide, oxalyl bromide, aluminum bromide, carbon disulfide,
hydrochloric acid, activated carbon, Celite, sodium bisulfite,
acetonitrile, acetic acid, 2,6-diisopropylaniline, (DME)NiBr2, methyl-
aluminoxane (MAO), modified methylaluminoxane (MMAO), and 2-
thiophenecarbonyl chloride (TPCC) were purchased from Acros and
used as received.
Synthesis of C1. 1,2-Diketopyracene (C1) was prepared according
to the literature.14
Synthesis of L1. 1,2-Diketopyracene (C1) (0.305 g, 1.47 mmol) in
acetonitrile (30 mL) and acetic acid (7 mL) was heated (90 °C) under
reflux for 5 h. To this solution, 0.68 mL (3.23 mmol) of 2,6-
diisopropylaniline was added directly, and the solution was heated
under reflux for a further 24.5 h. The solution was evaporated under
reduced pressure. The residual solids were further purified by silica
column chromatography (petroleum ether/methylene chloride = 1,
0.75% triethylamine) to afford 0.301 g of L1 (chrome yellow, 65%
1
3
yield). H NMR (300 MHz, CDCl33, δ in ppm): 7.27−7.28 (d, JHH
=
3.0 Hz, 2H, Ar−H), 7.24−7.25 (d, JHH = 3.6 Hz, 4H, Ar−H), 7.12−
7.13 (d, 3JHH = 5.1 Hz, 2H, Py−H), 6.49−6.51 (d, 3JHH = 5.4 Hz, 2H,
Py−H), 3.44 (s, 4H, CH2), 2.98−3.14 (sept, 4H, CH(CH3)2), 1.22−
1.24 (d, 3JHH = 5.1 Hz, 12H, CH(CH3)2), 0.98−0.96 (d, 3JHH = 5.1 Hz,
12H, CH(CH3)2). 13C NMR (75 MHz, CDCl3), δ (ppm): 162.14
(CN), 147.88 (CPh−N), 148.31 (CNa−CH2), 139.68, 137.71
(CNaH−CNa−CNaH), 135.97 (CPh−CH), 125.60 (CNa−CN),
125.12, 121.07 (CNa−H), 124.18, 123.36 (CPh−H), 32.20 (CH2),
28.57 (CH(CH3)2), 23.51, 23.18 (CH3). Elem. Anal. Calcd For
C38H42N2: C, 86.65%; H, 8.04%; N, 5.32%. Found: C, 86.09%; H,
8.04%; N, 5.31%. ESI-MS: m/z 527.10([M + H]+).
Rp = kp[C*][M]
(1)
where Rp is the rate of polymerization calculated by dividing the
polymer yield with polymerization time, [C*] is the concentration of
active center, and [M] is ethylene concentration.16
1
Characterization. H nuclear magnetic resonance (NMR) spectra
were recorded at 300 MHz on a Varian Mercury-Plus300 MHz
spectrometer, and chemical shifts were listed in parts per million
(ppm) downfield from tertramethylsilane (TMS). 13C nuclear
magnetic resonance (NMR) spectra were recorded at 500 MHz
Bruker on an Avance DMX500 spectrometer, and chemical shifts were
listed in parts per million (ppm) downfield from CDCl3. IR spectra
were recorded on a Thermo Nicolet 6700 spectrometer. Elemental
analysis was carried out using a Thermo Electron SPA EA1112
microanalyzer. Molecular weight and molecular weight distribution
(Mw/Mn) of the obtained polyethylene was determined by gel
permeation chromatograph (GPC) on a PL GPC220 equipped with
two PL gel 5 m MIXED-C columns using polystyrene standards and
THF as the eluent at a flow of 1.0 mL min−1 at 40 °C.
Synthesis of Cat. 1. Cat. 1 was synthesized by the reaction of
(DME)NiBr2 with L1 in methylene chloride. A typical procedure of
synthesizing Cat. 1 is described as follows. L1 (0.41 g, 0.78 mmol) was
dissolved in 10 mL of methylene chloride in an Schlenk tube under
nitrogen atmosphere. This solution was added via cannula to a
suspension of (DME)NiBr2 (0.25 g, 0.79 mmol) in methylene chloride
(10 mL). The resulting red mixture was stirred at room temperature
for 24 h. The solvent was evaporated under reduced pressure to afford
a red solid. The product was washed (3 × 20 mL) with diethyl ether
and dried in vacuum. Yield: 0.53 g, 86.9%. IR (KBr; cm−1) CN:
1610 (s), 1645 (s). Elem. Anal. Calcd For C38H42Br2N2Ni: C, 61.24%;
H, 5.68%; N, 3.76%. Found: C, 59.78%; H, 5.53%; N, 3.65%.
RESULTS AND DISCUSSION
■
X-ray Crystallographic Studies. Single crystal of Cat. 1 suitable
for X-ray diffraction analysis was obtained by laying hexane on the
dichloromethane solution at −10 °C. With graphite monochromated
Mo Kα radiation (λ = 0.71073 Å) at 173(2) K, cell parameters were
Synthesis and Characterization of Ligands and
Complexes. In this paper, an α-diimine nickel complex with
acenaphthene-derived backbone (Cat. 1 in Scheme 1) was
C
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synthesized in three steps from commercially available starting
materials. Acenaphthene was acylated using 1 equiv of oxalyl
bromide in the presence of stoichiometric aluminum bromide
to yield diketopyracene,14 which was subsequently condensed
with 2,6-diisopropylaniline to form ligand L1 (Scheme 1).1 L1
was metalated in methylene chloride at room temperature using
the nickel(II) dibromide dimethoxyethane adduct to form Cat.
and plane γ is 171.76°. Namely, the five-membered α-diimine
nickel ring in Cat. 1 exists in a half-chair conformation which is
significantly different from B-Cat.
Ethylene Polymerization. Ethylene polymerization with
B-Cat and Cat. 1 were respectively evaluated using
methylaluminoxane (MAO) or modified methylaluminoxane
(MMAO) as an activator. As shown in Table 1, at 60 °C using
1
1. L1 was characterized by H NMR (Figure S1), 13C NMR
a
(Figure S2), IR spectra, and elemental analysis. Cat. 1 was
characterized by IR spectra and elemental analysis.
X-ray Crystallographic Studies. X-ray quality crystal of
Cat. 1 was grown by layering a saturated methylene chloride
solution with pentane (Figure 2). The observed bond lengths
Table 1. Ethylene Polymerization with B-Cat and Cat. 1
b
c
d
entry
cat.
cocat.
act.
Mw
Mw/Mn
B
1
2
3
4
B-Cat
B-Cat
Cat. 1
Cat. 1
MAO
3.2
1.2
6.1
3.9
15.8
8.4
1.9
2.1
1.8
2.2
128
126
123
129
MMAO
MAO
16.9
11.3
MMAO
a
Polymerization conditions: [Ni] = 100 μmol/L, Al/Ni = 200, 25 mL
b
of toluene, 25 mL of n-heptane, 0.1 MPa, 60 °C, 30 min. 105 g(PE)/
c
d
mol(Ni)·h. 104 g/mol. Branching density per 1000 carbons
1
determined by H NMR.
MAO as activator, the polymerization activity of Cat. 1 was
twice that of B-Cat. Using MMAO as activator, the
polymerization activity of Cat. 1 was thrice that of B-Cat.
Regardless of the type of activator, the molecular weight of
polyethylene produced by Cat. 1 was higher than that of
polyethylene produced by B-Cat. However, the molecular
weight distribution and the branching density of polyethylene
produced by Cat. 1 were similar to those of polyethylene
produced by B-Cat.
So far, knowledge of the active centers in Brookhart catalysts
and micro kinetics of the α-diimine nickel catalysis process is
still limited. The activation of α-diimine nickel catalyst,
coordination and insertion of ethylene, and formation of
branches in ethylene homopolymerization were elucidated by
Brookhart et al.1 However, it is still unclear how many nickel
atoms could be activated by MAO or MMAO during the
polymerization process. Recently, we developed a method of
counting active centers in olefin polymerization with supported
Ziegler−Natta catalysts, metallocene catalysts, or α-diimine
nickel catalysts using 2-thiophenecarbonyl chloride (TPCC) as
the quenching agent to label the propagation chains.17 The
possible quenching reactions of the active centers by TPCC in
the initial stage of ethylene polymerization catalyzed by α-
diimine nickel catalyst is shown in Scheme 2. The sulfur
content of the quenched polymer ([S]) was measured with a
microcoulometry sulfur analyzer. Since one sulfur atom comes
from one propagation chain that is produced by one active
center, according to [S] the concentration of active centers
([C*]) in the polymerization system can be obtained.
Subsequently, the chain propagation rate constant (kp) can
be calculated based on Rp = kp[C*][M], where Rp is the
averaged polymerization rate of the whole reaction period (eq
1).
Figure 2. ORTEP diagram of Cat. 1. Hydrogen atoms have been
omitted for clarity.
are typical for α-diimine nickel complexes with N1−Ni = 2.018
(4) Å and N2−Ni = 2.031 (3) Å. The Ni−Br bond lengths
were 2.337 (8) and 2.330 (8) Å for Ni−Br1 and Ni−Br2,
respectively. These data are similar to those of B-Cat.
Moreover, the size of the ethydene on the naphthyl in Cat. 1
is small, and the position of it is too far to suppress the rotation
of the CAr−N bond. However, as shown in Figure S3b, the
conformation of the naphthyl on the backbone of Cat. 1 is
much different from that of B-Cat. The dihedral angle between
those two benzene rings of the former is 179.39° which is more
coplanar than the two benzene rings of the latter. Meanwhile,
the naphthyl on the backbone of Cat. 1 is more symmetrical
than that of B-Cat. Moreover, the conformation of C1−N1−
Ni−N2−C2 five-membered ring in Cat. 1 was also quite
different from that in B-Cat. As shown in Figure 3, the dihedral
angle between plane α (C1−C2−N2) and plane β (C1−N1−
N2) is 178.37°. The dihedral angle between plane β and plane γ
(N1−Ni−N2) is 173.23°. The dihedral angle between plane α
It is reasonable to consider the reaction between TPCC and
Al-terminated polymer chains formed by chain transfer to
trimethylaluminum in MMAO might overestimate the number
of active centers in the polymerization reaction.17a To rule out
the possibility of overestimation of the active centers, we
plotted the changes of the number of polymer chains (Npol/
NNi) and active centers (Nsites/NNi) as a function of
polymerization time. As shown in Figure 4, the increase of
N
sites was proportional to that of Npol/NNi for the first 10 min. If
Figure 3. Conformation of C1−N1−Ni−N2−C2 five-membered rings
in Cat. 1.
the reaction between TPCC and Al-terminated polymer chains
D
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Scheme 2. Possible Quenching Reactions of the Active Centers by TPCC in Ethylene Polymerization Catalyzed by α-Diimine
Nickel Catalyst
([C*]/[Ni]) was considerably lower than 100%. Actually, the
value of [C*]/[Ni] is lower than 16% even at 30 °C. As the
polymerization temperature was increased from 30 to 80 °C,
the value of [C*]/[Ni] dropped significantly. In the case of B-
Cat, the polymerization activity was too low at 80 °C to
accurately measure the [C*]/[Ni] value. In the case of Cat. 1,
the value of [C*]/[Ni] at 30 °C was just a little higher than
that of B-Cat. However, at 60 °C the value of [C*]/[Ni] of
Cat. 1 was 50% higher than that of B-Cat. Moreover, at 80 °C,
the [C*]/[Ni] value of Cat. 1 (5.1%) was significantly higher
than that of B-Cat, as [C*]/[Ni] of B-Cat was too low to be
detected. The values of kp of B-Cat were similar to that of Cat.
1, whether at 30 or 60 °C. The thermal stabilities of Cat. 1 and
B-Cat were examined by thermogravimetric analysis (TGA)
under a nitrogen atmosphere at a heating rate of 10 °C min−1
up to 800 °C. As shown in Figure S4, both Cat. 1 and B-Cat
Figure 4. Changes of Npol/NNi and Nsites/NNi with polymerization
displayed two stages of weight loss. The first stage of weight
time. Conditions: nNi = 5 μmol; cocatalyst: MAO; T = 30 °C; AlMAO
Ni = 260; toluene 50 mL; TPCC/AlMAO = 3; tquenching = 5 min; Pethylene
= 1 atm.
/
loss below 340 °C can be attributed to the decomposition via
detaching of NiBr2 from (α-diimine)NiBr2. The decomposition
temperature of Cat. 1 (337 °C) is much higher than that of B-
Cat. (306 °C), demonstrating the higher thermal stability of
Cat. 1.
occurred, then the values of Nsites were supposed to keep
increasing along with rising Npol, since the number of Al-
terminated polymer chains increased with polymerization time.
In contrast, a significant decrease of Nsites was observed when
the polymerization time was prolonged to 15 min, indicating
that the possible reaction between TPCC and Al-terminated
polymer chains is negligible.
Calculated Decomposition Energies of α-Diimine
Nickel Complexes. All these experimental results indicate
that Cat. 1 has much higher thermal stability and reactivity than
does B-Cat. We hypothesized that the conformation of C1−
N1−Ni−N2−C2 five-membered rings should be a highly
influential factor in tuning the thermal stability of α-diimine
nickel(II) complexes. As mentioned above, the exact
deactivation pathways and catalytic mechanisms of α-diimine
nickel catalysts in the polymerization process has been unclear
and argued since the first report by Brookhart et al.4e Despite
these, the formation of nickel dialkyl species and their
subsequent reductive elimination to Ni(0) make up one
possible deactivation mechanisim.18 To further prove the
difference in thermal stability between Cat. 1 and B-Cat, we
calculate their decomposition energies of the two α-diimine
nickel complexes using the first-principles calculations based on
the density functional theory. We have employed the projector-
augmented wave (PAW) method19,20 as implemented in the
Vienna ab initio simulation package (VASP, version 5.2). The
exchange-correlation part of the density functional was treated
within the generalized gradient approximations (GGA) of
Perdew−Burke−Ernzerhof (PBE) with the interpolation
formula of Voskoet al.21 The ferromagnetisms of Ni and its
As shown in Table 2, the molar fraction of nickel atoms that
were turned to active centers after being activated by MMAO
Table 2. Ethylene Polymerization with B-Cat and Cat. 1
a
Activated by MMAO
b
c
d
e
entry
cat.
T
Act.
[C*]/[Ni]
kp
1
2
3
4
5
6
30
60
80
30
60
80
1.73
0.58
0.08
1.95
1.05
0.42
14.4
5.0
1.19
1.66
B-Cat
15.9
7.9
1.22
1.90
1.37
Cat. 1
5.1
a
Polymerization conditions: [Ni] = 100 μmol/L, Al/Ni = 200, 50 mL
toluene, 0.1 MPa, Tp = 5 min, quenching conditions: 60 °C, Tq = 5
b
c
min, TPCC/Al = 3. Polymerization temperature, °C. 106 g(PE)/
d
e
mol(Ni)·h. mol %. 103 L/mol·s.
E
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compounds were accounted for under the spin-polarized
framework.
Although the calculations show that both α-diimine nickel
complexes are ferromagnetic with almost the same net
difference of 2 electrons between the total spin-up and -down
electrons, the space distributions of the spin density for the two
complexes show an observable difference, as shown in Figure 5.
It is seen that in the case of B-Cat the C−C bond next-nearest-
neighbor to Ni lost more spin-down electrons to Ni, resulting
in increased spin-up electrons around Ni. Compared to B-Cat,
C−C bond of Cat. 1 next-nearest-neighbor to Ni lost fewer
spin-down electrons to Ni, which results in a more electrophilic
nickel center, increasing its catalytic activity. This result is
consistent with the substituent effects reported by Brookhart.3a
We have considered decomposition via detaching of NiBr2
from (α-diimine)NiBr2 (reaction R1) which resulted in the
opening of C1−N1−Ni−N2−C2 five-membered ring. Two
steps of decompositions, in which the first step is detaching of
Br2 from (α-diimine)NiBr2 (reaction R2) and the second step is
detaching of Ni metal from (α-diimine)Ni (reaction R3), hasso
been considered. Although it is very difficult for (α-diimine)-
NiBr2 to decompose directly into α-diimine, Ni, and Br2
(reaction R4) this reaction as well as the other decomposition
reactions are shown as:
(α‐diimine)NiBr2 → α‐diimine + NiBr2(solid)
(R1)
CONCLUSIONS
■
In summary, we report a synthetically simple α-diimine nickel
catalyst which demonstrates substantially higher thermal
stability than that of B-Cat at elevated temperature. A
comparison of the crystal structures between B-Cat and Cat.
1 revealed that the deformation of naphthyl was eased because
of the introduction of an ethylene bridge, resulting in a more
thermally stable catalyst. The determination of active center
quenched by 2-thiophenecarbonyl chloride showed that many
more active centers remained for Cat. 1 at elevated temperature
during ethylene polymerization process. To elucidate the
mechanism of thermal stability of α-diimine nickel catalyst,
the decomposition energies of the two α-diimine nickel
complexes were calculated via using the first-principles
calculations based on the density functional theory. The
decomposition energies of Cat. 1 were found to be much
higher than those of B-Cat, contributing to the high thermal
stability of Cat. 1. Compared to B-Cat, the C−C bond of Cat.
1 next-nearest-neighbor to Ni lost fewer spin-down electrons to
Ni, which is the reason for higher catalytic activity of Cat. 1.
The enhanced thermal stability of Cat. 1, as well as
experimental and calculation methods to prove its thermal
stability, provide a significant advance toward developing α-
diimine nickel catalyst for industrial application.
(α‐diimine)NiBr2 → (α‐diimine)Ni + Br2(molecular)
(R2)
(α‐diimine)Ni → α‐diimine + Ni(metal)
(R3)
(α‐diimine)NiBr → α‐diimine + Ni(metal)
2
+ Br2(molecular)
As summarized in Table 3, it can be seen that Cat. 1 is more
stable than B-Cat. For example, for the decomposition into (α-
(R4)
Table 3. Decomposition Energies of B-Cat and Cat. 1 (kJ/
mol)
decomposition
reaction
reaction
R1
reaction
R2
reaction
R3
reaction
R4
B-Cat
Cat. 1
19.3
36.7
339.6
368.6
−185.3
−195.9
155.3
172.7
diimine) + NiBr2, the decomposition energy of Cat. 1 is 36.7
kJ/mol, which is 17.4 kJ/mol higher than that of B-Cat. For the
decomposition into (α-diimine)Ni + Br2, the decomposition
energy of Cat. 1 is 29 kJ/mol higher than the decomposition
energy of B-Cat. For the decomposition into (α-diimine) + Ni
+ Br2, the decomposition energy of Cat. 1 is 17.4 kJ/mol higher
than the decomposition energy of B-Cat. The higher thermal
stability of Cat. 1 than B-Cat can thus be attributed to higher
decomposition energy of the former. This extra stability of Cat.
1 might be caused by its more conjugated backbone.
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.7b00066.
Figure 5. Space distribution of the spin density difference between the spin-up and spin-down electrons for B-Cat (a) and Cat. 1 (b). The plots are
rendered by isosurfaces aided by the density of the stipples. The net spin-up density at 0.008 electrons/Å3 surrounding the Ni is depicted by red
color and the net spin-down density at −0.008 electrons/Å3 around the C−C bonding region of α-diimine is depicted by blue color.
F
DOI: 10.1021/acs.organomet.7b00066
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Synthesis and characterization of Cat. 1 (PDF)
Article
(II)-α-Diimine-Catalyzed Olefin Polymerizations1. J. Am. Chem. Soc.
2000, 122, 6686−6700. (e) Mecking, S. Cationic Nickel and Palladium
Complexes with Bidentate Ligands for the C-C Linkage of Olefins.
Coord. Chem. Rev. 2000, 203, 325−351. (f) Guan, Z.; Popeney, C. S.
Recent Progress in Late Transition Metal α-Diimine Catalysts for
Olefin Polymerization. Top.Organometal.Chem. 2009, 26, 179.
Crystallographic information file for Cat. 1 (CIF)
checkCIF (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: fuzs@zju.edu.cn.
■
(5) Schmid, M.; Eberhardt, R.; Klinga, M.; Leskela, M.; Rieger, B.
̈
New C2v-and Chiral C2-Symmetric Olefin Polymerization Catalysts
Based on Nickel (II) and Palladium (II) Diimine Complexes Bearing
2, 6-Diphenyl Aniline Moieties: Synthesis, Structural Characterization,
and First Insight into Polymerization Properties. Organometallics 2001,
20, 2321−2330.
ORCID
Liang Zhu: 0000-0003-4551-0963
Author Contributions
(6) Xie, T. Y.; McAuley, K. B.; Hsu, J. C. C.; Bacon, D. W. Gas Phase
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Am. Chem. Soc. 2007, 129, 9182−9191.
(8) Ionkin, A. S.; Marshall, W. ortho-5-Methylfuran-and Benzofuran-
Substituted η3-Allyl (α-diimine) nickel (II) Complexes: Syntheses,
Structural Characterization, and the First Polymerization Results.
Organometallics 2004, 23, 3276−3283.
(9) Liu, H.; Zhao, W. Z.; Yu, J. G.; Yang, W. H.; Hao, X.; Redshaw,
C.; Chen, L. Q.; Sun, W. H. Synthesis, Characterization and Ethylene
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α-Diimine Catalyst for High Temperature Ethylene Polymerization. J.
Am. Chem. Soc. 2013, 135, 16316−16319. (b) Rhinehart, J. L.;
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L.Z. and D.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support by the National Natural Science Foundation of China
(Grant No. 21544004), the Fundamental Research Funds for
the Central Universities (Grant 2016QNA4032), and the Open
Research Project of the State Key Laboratory of Industrial
Control Technology, Zhejiang University (No. ICT1600234) is
gratefully acknowledged. First-principles calculations were
carried out partially on the LION clusters at the Pennsylvania
State University (Wang). This work was in part supported by
the Hamer Professorship in Materials Science and Engineering
(Chen) and by National Science Foundation (NSF) through
Grant CHE-1230924 (Wang and Liu).
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