Ethylene polymerization by new chromium catalysts based on carborane [SSO] ligands

Article abstract of DOI:10.1039/c3dt00040k

Tridentate carborane [S, S, O] ligands 2a-2b [(HOC₆H ₂R₂-4,6)(CH₂)SC(B₁₀H ₁₀)C(Ph)₂PS, R = ^tBu (2a), R = Me (2b)] were synthesized and characterized. Reaction of CrCl<

Full text of DOI:10.1039/c3dt00040k

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Ethylene polymerization by new chromium catalysts
based on carborane [SSO] ligands†
Cite this: Dalton Trans., 2013, 42, 9089
Ping Hu,a Ya-Lin Qiao,a Zhen-Hua Li,a Jian-Qiang Wangb and Guo-Xin Jin*a
t
Tridentate carborane [S, S, O] ligands 2a–2b [(HOC6H2R2-4,6)(CH2)SC(B10H10)C(Ph)2PvS, R = Bu (2a), R =
Me (2b)] were synthesized and characterized. Reaction of CrCl3(THF)3 with the sodium salts of ligands 2a
and 2b afford six-coordinated chromium complexes 3a and 3b. EXAFS spectroscopy performed on
complex 3a to describe the coordination chemistry of ligand 2a around chromium center. DFT calcu-
lations were also performed on complex 3a to analyze the structure. The preliminary screening results
revealed that six-coordinated chromium complexes 3a–3b displayed good catalytic activities towards
ethylene polymerization in the presence of modified methylaluminoxane. The effect of polymerization
parameters such as cocatalyst, reaction temperature, ethylene pressure, and reaction time on polymeri-
zation behavior were investigated in detail. The polymer obtained from this homogeneous catalytic reac-
tion has a fibroid morphology.
Received 4th January 2013,
Accepted 13th February 2013
DOI: 10.1039/c3dt00040k
www.rsc.org/dalton
incorporated ligands exhibited high activities in ethylene
polymerization or ethylene copolymerization with other
Introduction
The search for new “non-metallocene” olefin polymerization α-olefins.6–8 Li’s group developed new chromium catalysts
catalysts based on transition metal complexes is a field of based on aminopyrrolide ligands which were efficient catalysts
major interest involving many academic and industrial for ethylene polymerization.9 Furthermore, they found that the
research groups.1 The relatively ill-defined nature of these CrCl3(thf)3–ligand system containing soft donor (sulfur or
paramagnetic olefin polymerization catalysts with respect to phosphorous atoms) ligands displayed higher activity than the
coordination environment and oxidation state, however, have hard donor ones (oxygen or nitrogen atoms), in line with the
prompted considerable efforts to synthesize well-defined results of the titanium-based catalyst system reported by
homogeneous single-site Cr-based catalysts.1b,2 Anionic non- Gibson and co-workers.10
Cp ligands have produced effective polymerization catalysts,
A three-dimensional aromatic, soluble and high thermally
with the ligand structure displaying significant effect on cata- stable bulky carborane cage, has been used as a ligand to con-
lyst activity and polymer properties. Chromium(III) complexes struct various metal complexes since the first report on a
with β-diimine ligands were reported as efficient catalysts both metalladicarbollide complex by Hawthorne and co-
for ethylene homopolymerization and copolymerization with workers.11,12 We are interested in using carborane ligands to
α-olefins.3 Chromium(III) complexes based on monoanionic synthesize effective olefin polymerization catalysts.13 These
phenoxyamide ligands were reported as efficient catalysts for carborane ligands contain a highly electron-deficient carbo-
ethylene polymerization.4 The iminopyrrolide chromium com- rane moiety, which should enhance the Lewis acidity of the
plexes showed moderate ethylene polymerization activity in central metal ion and, thus, increase the activity of the result-
the presence of diethylaluminum chloride.5 Recent research ing transition metal complexes. On the other hand, the good
suggested that chromium catalysts based on phosphorous- solubility of transition metal complexes with carborane
ligands is of benefit for high catalytic activity. As an extension
of our investigation of transition metal complexes with carbo-
rane ligands, tridentate [S, S, O] carborane ligands 2a–2b were
aState Key Laboratory of Molecular Engineering of Polymers, Department of
Chemistry, Fudan University, Shanghai 200433, P. R. China.
E-mail: gxjin@fudan.edu.cn; Fax: (+86)-21-65643776
prepared and characterized. The soft pendant donors in this
ligand are sulfur atoms, while the hard donor is an oxygen
atom. In this work, we report the chromium complexes based
on this type of [S, S, O] carborane ligands, which show high
activities for ethylene polymerization to produce high mole-
cular weight PE when activated with MMAO.
bShanghai Synchrotron Radiation Facility, Chinese Academy of Sciences,
Shanghai 201204, P. R. China
†CCDC 916907 (2a). For crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3dt00040k
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sodium hydride at 0 °C in THF, ligands 2a and 2b could react
with CrCl3(THF)3 to give the six-coordinated complexes 3a and
3b. The severe line broadening in the 1H NMR spectra for com-
Results and discussion
Synthesis and characterization
The synthesis of ligands 2a–2b are outlined in Scheme 1. Com- plexes 3a and 3b in CDCl3 indicates that these chromium com-
pound 1 [1-(2′-(S)PPh2)-o-carborane] was prepared by the plexes are paramagnetic species. However, their IR spectra and
monophosphino o-carborane reacted with elemental sulfur in elemental analysis are consistent with the described structure
the presence of Et3N.13d The lithium salt of compound 1 was (see Experimental section). The FTIR spectra of 2a and 3a were
treated with elemental sulfur then reacted with 2,4-di-tert- studied and the results indicated the disappearance of a broad
butyl-6-chloromethylphenol or 2,4-dimethyl-6-chloromethyl- peak representing stretching frequencies of OH at about
phenol affording tridentate carborane [S, S, O] ligands 2a 3000 cm−1. Instead, stretching frequencies of tetrahydrofuran
t
[(HOC6H2 Bu2-4,6)(CH2)SC(B10H10)C(Ph)2PvS]
and
2b at about 3405, 3060, and 2970 cm−1 were observed. In
[(HOC6H2Me2-4,6)(CH2)SC(B10H10)C(Ph)2PvS] in good yields. addition, the peak representing stretching frequencies of PvS
These ligands are stable in air and moisture, and were resolva- bond at around 680 cm−1 has no obvious shift. Like the com-
ble in common organic solvents such as THF, CH2Cl2, toluene, parison of 2a and 3a, a similar difference in FTIR spectra was
and ethyl ether.
observed for 2b and 3b.
NMR spectroscopic data of ligands 2a–2b are consistent
However, the structures of complexes 3a–3b are not the
with the described structures. The crystal of compound 2a, expected structures. The chromium atom in complex 3a is
which was grown by the slow evaporation from toluene solu- coordinated with oxygen atoms of solvent (THF) instead of the
tion, was determined by X-ray diffraction (Fig. 1).
terminal PvS type sulfur atom of the ligand.
Chromium complexes 3a–3b were obtained by easy syn-
The local atomic environment and charge state of metal
thetic routes in good yields (Scheme 2). After deprotonation by center in complex 3a were further confirmed by X-ray absorp-
tion spectroscopy, including extended X-ray absorption fine
structure (EXAFS) and X-ray absorption near-edge structure
(XANES) (Fig. 2). The EXAFS spectra allows us to get infor-
mation on the environment around the metal center, which
has already been used to characterize transition metal com-
plexes for olefin polymerization.14 The Cr K-edge absorption
spectrum, plotted in Fig. 2, shows a typical line peak at
6013 eV for Cr compounds. The fitting of the EXAFS spectra
(Table 1) of 3a was performed with a three-shells model, in
which the first coordination shell at 1.98 Å consists of the
Scheme 1 Synthetic route to ligands 2a–2b.
three oxygen atoms from the functional carborane ligand and
THF solvent. The second shell is the chlorine backscatterer at
about 2.31 Å. The third shell at 2.93 Å consists of the sulfur
atom from the functional carborane ligand.
The efforts to separate the two kinds of coordinating
oxygen failed due to the close bond lengths of Cr–O(ligand)
Fig. 1 Molecular structure of ligand 2a with thermal ellipsoids drawn at the
30% level.
Fig. 2 Cr K-edge XANES spectra of sample for 3a; Fourier transform of experi-
Scheme 2 Synthesis of chromium complexes 3a–3b.
9090 | Dalton Trans., 2013, 42, 9089–9095
mental data and fit for 3a (inset).
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Ethylene polymerization. Upon activation with modified
methylaluminoxane (MMAO), chromium complexes 3a–3b
were found to be active for ethylene polymerization. The
polymerization results are summarized in Table 2.
The polymerization was performed in a 100 mL Schlenk
flask. The flask was repeatedly evacuated and refilled with
argon, and finally filled with ethylene gas. MMAO or other
cocatalysts were added via a syringe. The catalyst, dissolved in
toluene under dry argon, was transferred into the Schlenk
flask to initiate the polymerization. The polymerization was
initiated by the addition of a heptane solution of MMAO and a
toluene solution of the chromium catalyst.
Table 1 Fit parameters of Cr EXAFS spectra for complexes 3a
Shell
Na
Rb
σ
2 c (10−3 Å2)
ΔE0d (eV)
3a
Cr–O
Cr–Cl
Cr–S
3.3 0.6
1.8 0.4
0.8 0.2
1.98 0.01
2.31 0.01
2.93 0.02
5.3 0.6
4.8 0.8
2.2 1.6
−1.0 0.9
−0.3 0.2
19.4 1.1
a Coordination number. b Distance between absorber and backscatterer
atoms. c Debye–Waller factor. d Inner potential correction.
and Cr–O(THF). With the help of EXAFS analysis, the solid-
state structure of 3a was confirmed to be the six coordinated
model as [O, O, O, Cl, Cl, S].
The reaction conditions in ethylene polymerization have
been optimized for precatalyst 3a. We began our studies by
searching for the optimal cocatalyst to chromium catalyst
system (entries 1–4, Table 2). Good catalytic performances are
observed in the presence of MAO or MMAO as the cocatalyst,
of which MMAO gives better results than MAO. For example,
for the molar ratio of Al/Cr = 2000, under 1 atm ethylene
pressure, the catalytic activity is 170 kg PE mol−1(Cr) h−1 using
MAO, while the system with MMAO gives 225 kg PE mol−1(Cr)
h−1. In addition to MAO and MMAO, triethylaluminum (TEA)
and triisobutylaluminum (TIBA) were also used as cocatalysts.
However, only a trace amount of polymer was obtained in
those cases, as shown in Table 2 (entries 3–4). Therefore
detailed investigations have been carried out employing
MMAO through varying the ratio of Al/Cr, reaction tempera-
ture, and time, and the results are tabulated in Table 2.
The amount of cocatalyst MMAO was found to have a dra-
matic effect on catalyst activity and molecular weights of the
polymer. According to the data at different Al/Cr ratios ranging
from 1000 to 3000 (entries 4–7), the maximum activity is
shown at an Al/Cr molar ratio of 2000 (entry 4). The reaction
temperature also considerably affects the catalytic activities
and molecular weights. As can be seen from Table 2 (entries 4,
8–10), with increasing temperature from 0 to 80 °C, the cata-
lytic activities of complex 3a first increase and reach a
maximum at room temperature and then decrease. The mole-
cular weights of polyethylene decrease with elevating tempera-
ture due to a faster chain transfer and termination at higher
temperature.
DFT calculations were performed on complex 3a to analyze
the structure.15 Possible structures were considered and opti-
mized. The geometries of six possible structures are shown in
Fig. 3. Relative energies (in kcal mol−1) of these structures are
also shown in Fig. 3.
The results indicate that structure S1, in which the chro-
mium atom is bonded to only one sulfur atom and two tetra-
hydrofuran, is the lowest energy structure. The thioether-type
sulfur has a better bonding ability to chromium than the ter-
minal PvS type sulfur (Fig. 3, S1, S2 vs. S3, S4). Complexes with
more or less than two tetrahydrofuran ligands are less stable
(Fig. 3, S1, S2 vs. S3, S4, S5, S6). Moreover, the structures with
two chloride atoms in the trans-position have lower relative
energies (Fig. 3, S1 vs. S2, S3 vs. S4). These DFT calculation
results had proved that the structure S1 which was described
as complex 3a in Scheme 2 was reasonable.
To determine the effect of reaction time on the activity and
molecular weight (Mv) of the resulting polymers, ethylene
polymerization using the complex 3a/MAO catalytic system was
conducted over different time periods, namely, 10, 30, 60 and
90 min (entries 4, 11–13 in Table 2), the highest activity
(310 kg of PE (mol of Cr)−1 h−1) was obtained in a period of
10 min.
With the ethylene pressure increased, the catalytic activity
generally notably improved. Increasing the ethylene pressure
from 1 to 10 atm, the polymerization activity further increased
to 630 kg PE mol−1(Cr) h−1 in current catalytic system (entry
14, Table 2). Moreover, no significant enhancement in molecu-
lar weight was seen even at 10 atm ethylene pressure (entries 4
and 14, Table 2), which was similar to the literature results.9
The apparent nondependence of the molecular weight on
Fig. 3 Geometries of the six optimized structures and their relative energies (in
kcal mol−1). Green, yellow, red, blue, orange, pink, and gray balls are chlorine,
sulfur, oxygen, chromium, phosphorus, boron and carbon atoms, respectively.
The hydrogen atoms are omitted for clarity.
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Table 2 Ethylene polymerization results with chromium complexes 3a–3ba
Activityb
Mv
c
Entry
Cat.
Cocatalyst
Al/Cr ratio
Temp (°C)
Time (min)
d
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
TEA
TIBA
MAO
2000
2000
2000
2000
3000
1500
1000
2000
2000
2000
2000
2000
2000
2000
2000
25
25
25
25
25
25
25
0
50
80
25
25
25
25
25
30
30
30
30
30
30
30
30
30
30
10
60
90
30
30
Trace
Trace
170
225
90
180
97
102
120
32
310
110
54
—
—
d
64
63
57
66
68
69
59
42
50
71
83
64
69
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
MMAO
9
10
11
12
13
14e
15
630
110
a Conditions: [Cr] 2 μmol, toluene (total volume 50 mL), 1 atm of ethylene. b Activity in kg PE mol−1(Cr) h−1
.
c Mv measured by the Ubbelohde
calibrated viscosimeter technique in decahydronaphthalene at 135 °C, units of 104 g mol−1 d Undetermined. e 10 atm of ethylene.
.
possibilities for copolymerization of ethylene with other
olefins using these types of catalysts are underway.
Experimental
General considerations
All manipulations of air and/or water-sensitive compounds
were carried out under dry argon using standard Schlenk tech-
niques. Tetrahydrofuran (THF), hexane, and toluene were dis-
tilled from sodium benzophenone. Commercial reagents,
namely, NaH, CrCl3(THF)3, n-BuLi (1.6 M in hexane) were pur-
chased from Aldrich. Methylaluminoxane (MAO, 1.46 M in
toluene), modified methylaluminoxane (MMAO, 1.88 M in
heptane solution), triethylaluminum (TEA) and triisobutyl-
aluminum (TIBA) were purchased from Akzo Nobel Chemical
Inc. Commercial ethylene was directly used for polymerization
without further purification. The other reagents and solvents
were commercially available.
Fig. 4 SEM image of polyethylene obtained with complex 3a/MMAO.
ethylene pressure is a strong indication for chain transfer by
β-hydride transfer to the monomer rather than to the metal.
The R group on the ligand has a significant effect on catalytic
t
activity. Complex 3a with a Bu group on the ortho-position,
show higher activities than complex 3b, indicating that proper
steric hindrance on the ligand may avoid active species deacti-
vating to some extent.
The fibroid morphology of polyethylene obtained from the
reaction catalyzed by the homogeneous catalyst 3a/MMAO (see
Fig. 4) is different from the sponge-like morphology catalyzed
when other homogeneous catalysts are used.16
1H NMR and 13C NMR measurements were obtained on a
Bruker AC 400 spectrometer in CDCl3 solution. 31P NMR
(162 MHz) spectra were measured with a VAVCE DMX-400 spec-
trometer. IR spectra were measured on a Nicolet Avatar-360
spectrophotometer. Elemental analyses for C and H were
carried out on an Elementar III Vario EI analyzer.
The investigation of the structure of polyethylene was per-
formed by scanning electron microscopy (SEM) using a Philips
XL30 FEG FE-SEM instrument at 10 kV.
Conclusion
In conclusion, novel chromium complexes based on carborane
Preparation of ligands 2a and 2b. Compound 1 was syn-
[SSO] ligands were synthesized and employed as catalysts for thesized according to our previously reported methods.13d To a
ethylene polymerization to produce high molecular weight PE solution of compound 1 (180 mg, 0.5 mmol) in dry Et2O
with good activities under mild conditions in the presence of (10 mL) at −78 °C was dropwise added 1.6 M of n-BuLi in
MMAO as a cocatalyst. The reaction parameters such as co- hexane (0.32 mL, 0.5 mmol) and the reaction mixture stirred
catalyst concentration, polymerization temperature, ethylene for 1 h at −78 °C. Sulfur (16 mg, 0.5 mmol) was added after
pressure, and reaction time significantly affect the catalytic the reaction mixture was warmed to room temperature. Stir-
activity. Further investigations exploring higher activity chro- ring was maintained for 1 h. After a solution of 2,4-di-tert-
mium catalysts based on other carborane ligands and butyl-6-chloromethylphenol (127 mg, 0.5 mmol) or 2,4-di-
9092 | Dalton Trans., 2013, 42, 9089–9095
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methyl-6-chloromethylphenol (85 mg, 0.5 mmol) in Et2O
(10 mL) was added, a white precipitate was generated. Stirring
was maintained for 2 h at room temperature. The mixture was
filtered and the solvent of the filtrate was removed under
vacuum. Recrystallization of the product from hexane afforded
ligands 2a and 2b as white needles in yields of 70%(2a) and
68%(2b).
Table 3 Summary of crystallographic data for 2a
2a
29H43B10OPS2
Formula
fw
T (K)
Cryst syst
Space group
a (Å)
b (Å)
c (Å)
α (°)
C
610.82
296(2)
Monoclinic
P2(1)/n
12.437(3)
15.925(4)
17.870(4)
90
2a: IR (KBr, cm−1), 2995 m(OH), 2575 s(B-H), 1650 w,
1550 m, 1450 s(Ph), 1188 m(P-Ph), 680 m(PvS). 1H NMR
(CDCl3, 400 MHz): δ 8.37–8.32 (m, 5H, Ph-H), 7.59–7.53 (m,
5H, Ph-H), 6.97 (s, 1H, Ph-H), 6.49 (s, 1H, Ph-H), 4.38 (S, 2H,
β (°)
101.657(4)
90
3466.6(13)
4
1.170
0.223
γ (°)
t
CH2), 3.39–1.80 (br, 10H, B-H), 1.45 (s, 9H, Bu), 1.26 (s, 9H,
Volume (Å3)
Z
tBu). 13C NMR (CDCl3, 100 MHz): δ 159.13 (Ph-C), 151.72
(Ph-C), 143.07 (Ph-C), 138.22 (Ph-C), 125.23 (Ph-C), 128 (PPh2),
121.01 (Ph-C), 89.78 (carborane-C), 85.36 (carborane-C), 39.49
(SCH2), 35.40 (tBu), 33.94 (tBu), 31.51 (tBu), 28.44 (tBu) ppm.
31P NMR (162 MHz, CDCl3, H3PO4, 25 °C): δ 41.49 (s, −P(S)
Ph2) ppm. Anal. Calcd for C29H43B10OPS2: C, 57.02; H, 7.10.
Found: C, 57.11; H, 7.16.
Dcalc (Mg m−3
)
Absorp coeff (mm−1
F(000)
)
1288
Cryst size (mm)
2θ range (°)
0.22 × 0.09 × 0.07
1.83–27.63
25 401/7994 [R(int) = 0.0829]
7994/1/408
No. of reflns collected/unique
No. of data/restraints/params
Goodness of fit on F2
0.980
Final R indices [I > 2σ(I)]a
R1 = 0.0541, wR2 = 0.1305
0.338 and −0.297
2b: IR (KBr, cm−1), 2990 m(OH), 2575 s(B-H), 1656 w,
1562 m, 1450 s(Ph), 1184 m(P-Ph), 676 m(PvS). 1H NMR
(CDCl3, 400 MHz): δ 8.36–8.31 (m, 5H, Ph-H), 7.61–7.53 (m,
5H, Ph-H), 6.95 (s, 1H, Ph-H), 6.47 (s, 1H, Ph-H), 4.38 (S, 2H,
CH2), 3.37–1.76 (br, 10H, B-H), 1.32 (s, 3H, Me), 1.21 (s, 3H,
Me). 13C NMR (CDCl3, 100 MHz): δ 158.88, (Ph-C), 154.66 (Ph-
C), 147.02 (Ph-C), 140.13 (Ph-C), 127.03 (Ph-C), 125.34 (PPh2),
121.28 (Ph-C), 89.55 (carborane-C), 85.37 (carborane-C), 39.45
(SCH2), 31.40 (Me), 29.94 (Me) ppm. 31P NMR (162 MHz,
CDCl3, H3PO4, 25 °C): δ 41.52 (s, –P(S)Ph2) ppm. Anal. Calcd
for C23H31B10OPS2: C, 52.45; H, 5.93. Found: C, 57.42; H, 5.95.
Preparation of complexes 3a–3b. To a stirred suspension of
NaH (48 mg, 2.0 mmol) in 10 mL of THF was added ligand 2a
(76.2 mg, 0.125 mmol) or 2b (65.8 mg, 0.125 mmol) dropwise
at 0 °C. Stirring was maintained for 2 h at room temperature.
The mixture was filtered. Then the filtrate was added dropwise
to CrCl3(THF)3 (46.3 mg, 0.125 mmol) in dried THF (20 mL)
over a 30 min period at −78 °C. The mixture was allowed to
warm to room temperature and stirred overnight. A green solu-
tion was obtained, and the solvent was removed under
vacuum. After the residual solid was dissolved in CH2Cl2 and
filtered to remove inorganic salts, the filtrate was concentrated
to about 2 mL and mixed with hexane (20 mL). Cooling to
−30 °C gave green needles in yields of 65% (3a) and 69% (3b).
3a: IR (KBr, cm−1), 3405, 3060, 2970 (furan), 2962 m(Bu),
2571 s(B-H), 1637 w, 1555 m, 1478 s(Ph), 1188 m(P-Ph), 687
m(PvS). Anal. Calcd for C37H58B10Cl2CrO3PS2: C, 50.67; H,
6.67. Found: C, 50.64, H, 6.69.
Lgst diff peak and hole (e Å−3
)
a R1 = ||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(|Fo2| − |Fc2|)2/∑w|Fo2|2]1/2
.
room temperature. The intensity data of the single crystal were
collected on a CCD-Bruker Smart APEX system. All determi-
nations of the unit cell and intensity data were performed with
graphite-monochromated MoKα radiation (λ 0.71073 Å). All
data were collected at room temperature using the ω-scan tech-
nique. These structures were solved by direct methods, using
Fourier techniques, and refined on F2 by a full-matrix least-
squares method. All the calculations were carried out with the
SHELXTL program.17 All the non-hydrogen atoms were refined
anisotropically, and all the hydrogen atoms were included but
not refined. Crystallographic data are summarized in Table 3.
XAFS data collection
The X-ray absorption data at the Cr K-edge of the sample were
measured in the fluorescent mode with a Lytle fluorescence
detector on beam line BL14W1 of the Shanghai Synchrotron
Radiation Facility (SSRF), China. The station was operated with
a double crystal monochromator, Si(111), detuned to 40%
intensity to minimize the presence of higher harmonics. All
measurements were done in transmission or fluorescent mode
using ion chambers filled with a mixture of He and N2 to have
an X-ray absorbance of 20% in the first and 80% in the second
chamber. Data processing and analysis data processing was
performed using the program ATHENA.18 All fits to the EXAFS
data were performed using the program ARTEMIS.18
3b: IR (KBr, cm−1), 3410, 3057, 2969 (furan), 2960, 2870 m
(Me), 2575 s(B-H), 3411, 3058, 2965 (CH, furan), 1620 w,
1560 m, 1455 s(Ph), 1181 m(P-Ph), 685 m(PvS). Anal. Calcd
for C31H46B10Cl2CrO3PS2: C, 46.96; H, 5.85. Found: C, 46.93;
H, 5.89.
DFT calculations
All total energy density functional theory (DFT) calculations
were carried out using the SIESTA package with numerical
atomic orbital basis sets and Troullier–Martins norm-conser-
Single-crystal X-ray structure determination of ligand 2a
For compound 2a, no samples showed signals of decompo- ving pseudopotentials.19,20 The DFT functional utilized is the
sition during X-ray data collection, which was carried out at PBE functional,21 a generalized gradient approximation DFT
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method. A double-ζ plus polarization (DZP) basis set was
employed. The orbital-confining cutoff radii were determined
from an energy shift of 0.01 eV. The energy cutoff for the real
space grid used to represent the density was set as 150 Ry. The
“External” function of the Gaussian 03 program22 was used to
call SIESTA to return the energy and Cartesian forces on the
atoms to the geometry optimizer of the Gaussian 03 program.
The default geometry convergence criteria of the Gaussian 03
program was used where the forces and displacements of the
relaxed coordinates were all less than 0.00045 Hartree Bohr−1
and 0.0018 Å, respectively. The molecule was placed in the
center of a cubic supercell with a length of 50 Å for each edge.
Only Γ-point was used to sample the Brillouin zone in our
calculations due to the large lattice parameter of the supercell.
(c) E. Groppo, C. Lamberti, S. Bordiga, G. Spoto and
A. Zecchina, Chem. Rev., 2005, 105, 115–184.
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3 (a) W.-K. Kim, M. J. Fevola, L. M. Liable-Sands,
A. L. Rheingold and K. H. Theopold, Organometallics, 1998,
17, 4541–4543; (b) V. C. Gibson, C. Newton, C. Redshaw,
G. A. Solan, A. J. P. White and D. J. Williams, Eur. J. Inorg.
Chem., 2001, 1895–1903; (c) L. A. McAdams, W.-K. Kim,
L. M. Liable-Sands, I. A. Guzei, A. L. Rheingold and
K. H. Theopold, Organometallics, 2002, 21, 952–960.
4 (a) D. J. Jones, V. C. Gibson, S. M. Green and P. J. Maddox,
Chem. Commun., 2002, 1038–1039; (b) D. J. Jones,
V. C. Gibson, S. M. Green, P. J. Maddox, A. J. P. White and
D. J. Williams, J. Am. Chem. Soc., 2005, 127, 11037–11046.
5 V. C. Gibson, P. J. Maddox, C. Newton, C. Redshaw,
G. A. Solan, A. J. P. White and D. J. Williams, Chem.
Commun., 1998, 1651–1652.
Procedures for ethylene polymerization under 1 atm of
ethylene
6 H. Hanaoka, Y. Imamoto, T. Hino, T. Kohno, K. Yanagi and
Y. Oda, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 3668–
3676.
7 D. S. McGuinness, P. Wasserscheid, D. H. Morgan and
J. T. Dixon, Organometallics, 2005, 24, 552–556.
8 (a) D. D. Wet-Roos, A. D. Toit and D. J. Joubert, J. Polym.
Sci., Part A: Polym. Chem., 2006, 44, 6847–6856;
(b) A. D. Toit, D. D. Wet-Roos, D. J. Joubert and
A. J. VanReenen, J. Polym. Sci., Part A: Polym. Chem., 2008,
46, 1488–1501.
9 L.-P. He, J.-Y. Liu, P. Li, J.-Q. Wu, B.-C. Xu and Y.-S. Li,
J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 713–721.
10 D. C. H. Oakes, B. S. Kimberley, V. C. Gibson, D. J. Jones,
A. J. P. White and D. J. Williams, Chem. Commun., 2004,
2174–2175.
A 100 mL flask was equipped with an ethylene inlet, a mag-
netic stirrer, and a vacuum line. The flask was filled with
30 mL of freshly distilled toluene, MMAO (1.88 M in heptane)
was added, and the flask was placed in a bath at the desired
polymerization temperature for 30 min. The polymerization
reaction was started by adding a toluene solution of the cata-
lyst precursor (0.002 mmol) with a syringe. Then the solvent
toluene was added to make the total volume of the solution
50 mL. The polymerization was carried out for the desired
time and then quenched with 3% HCl in ethanol (250 mL).
The precipitated polymer was filtered and then dried overnight
in a vacuum oven at 80 °C.
General procedure for ethylene polymerization at high
pressure
11 M. F. Hawthorne, D. C. Young and P. A. Wegner, J. Am.
Chem. Soc., 1965, 87, 1818–1819.
12 A. M. Spokoyny, C. W. Machan, D. J. Clingerman,
M. S. Rosen, M. J. Wiester, R. D. Kennedy, C. L. Stern,
A. A. Sarjeant and C. A. Mirkin, Nat. Chem., 2011, 3, 590–
596.
13 (a) X. Wang and G.-X. Jin, Organometallics, 2004, 23, 6319–
6322; (b) X. Wang, L. H. Weng and G.-X. Jin, Chem.–Eur. J.,
2005, 11, 5758–5764; (c) P. Hu, J.-Q. Wang, F. S. Wang and
G.-X. Jin, Chem.–Eur. J., 2011, 17, 8576–8583; (d) P. Hu,
Z.-J. Yao, J.-Q. Wang and G.-X. Jin, Organometallics, 2011,
30, 4935–4940.
A 100 mL autoclave was charged with 50 mL of toluene under
argon. MMAO (1.88 M in heptane) was added. A solution of
the precatalyst in toluene was added. After three times of ethy-
lene gas exchange, the ethylene pressure was raised to 10 atm
and maintained for a certain time. The polymerization was ter-
minated by addition of ethanol and dilute HCl (3%). The solid
polyethylene was filtered, washed with ethanol, and dried at
80 °C under vacuum.
Acknowledgements
14 S. O. Brien, J. Tudor, T. Maschmeyer and D. O. Hare, Chem.
Commun., 1997, 1905–1906.
We are grateful to the National Science Foundation of China
(91122017), the Program for Changjiang Scholars and Innova-
tive Research Team in University (IRT1117), the National Basic
Research Program of China (2011CB808505) and the Shanghai
Science and Technology Committee (12DZ2275100).
15 For the S1–S4 structures, the values in the parentheses are
the relative energies (in kcal mol−1) of these structures. For
the S5 and S6 structures, the values in the parentheses are
the estimated free energy change of the following two reac-
tions at 298.15 K: S1 → S5 + tetrahydrofuran, and S1 + tetra-
hydrofuran → S6, respectively.
16 (a) K. S. Lee, C. G. Oh, J. H. Yim and S. K. Ihm, J. Mol.
Catal. A: Chem., 2000, 159, 301–308; (b) C. Guo, G.-X. Jin
and F. Wang, J. Polym. Sci., Part A: Polym. Chem., 2004, 42,
4830–4837.
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