2-Benzimidazolyl-N-phenylquinoline-8-carboxamide chromium(III) trichlorides: Synthesis and application for ethylene oligomerization and polymerization

Article abstract of DOI:10.1021/om2001124

A series of chromium(III) complexes, LCrCl₃ (C1-C6: L = 2-(1H-benzo- [d]imidazol-2-yl)-N-(2,6-R¹-4-R²-phenyl) quinoline-8-carboxamide; C1: R¹ = ⁱPr, R² = H; C2: R¹ = Et, R²= H; C3: R¹ = Me, R² = H; C4: R1 = Me, R2 = Me; C5: R¹ = H, R² = H; C6: R ¹ = F, R² = H), was synthesized and characterized by IR spectroscopy, elemental analysis, mass spectra, and magnetic moments. The X-ray crystallographic analysis of complexes C2 and C3 reveals distorted octahedral geometries around the chromium atoms. Upon activation with MAO, these complexes exhibit high activities for ethylene oligomerization (up to 1.28 × 107 g.mol^-1(Cr).h^-1) and ethylene polymerization (up to 6.30×10⁶ g.mol^-1(Cr).h^-1). The oligomers were produced with high selectivity for R-olefins (>99%), and the distribution closely resembled the Schultz-Flory rule. Various reaction parameters were investigated in detail, and the results revealed that both the steric and electronic effects of the ligands effect the catalytic activities of these chromium complexes as well as the distribution of the products formed.

Full text of DOI:10.1021/om2001124

ARTICLE
pubs.acs.org/Organometallics
2-Benzimidazolyl-N-phenylquinoline-8-carboxamide
Chromium(III) Trichlorides: Synthesis and Application for
Ethylene Oligomerization and Polymerization
Deligeer Wang,† Shaofeng Liu,†,‡ Yanning Zeng,‡ Wen-Hua Sun,*,‡ and Carl Redshaw*,§
†Department of Chemistry, Inner Mongolia Normal University, Hohhot 010022, China
‡Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
§School of Chemistry, University of East Anglia, Norwich, NR4 7TJ, U.K.
S Supporting Information
b
ABSTRACT: A series of chromium(III) complexes, LCrCl3 (C1ꢀC6: L = 2-(1H-benzo-
[d]imidazol-2-yl)-N-(2,6-R1-4-R2-phenyl)quinoline-8-carboxamide; C1: R1 = iPr, R2 = H; C2:
R1 = Et, R2 = H; C3: R1 = Me, R2 = H; C4: R1 = Me, R2 = Me; C5: R1 = H, R2 = H; C6: R1 = F,
R2 = H), was synthesized and characterized by IR spectroscopy, elemental analysis, mass spectra,
and magnetic moments. The X-ray crystallographic analysis of complexes C2 and C3 reveals
distorted octahedral geometries around the chromium atoms. Upon activation with MAO, these
complexes exhibit high activities for ethylene oligomerization (up to 1.28 ꢁ 107 g molꢀ1(Cr) hꢀ1) and ethylene polymerization
3
3
(up to 6.30 ꢁ 106 g molꢀ1(Cr) hꢀ1). The oligomers were produced with high selectivity for R-olefins (>99%), and the distribution
3
3
closely resembled the SchultzꢀFlory rule. Various reaction parameters were investigated in detail, and the results revealed that both
the steric and electronic effects of the ligands effect the catalytic activities of these chromium complexes as well as the distribution of
the products formed.
10-phenanthrolines (A, Scheme 1),20 2,6-bis(benzimidazolyl)-
1. INTRODUCTION
pyridines (B, Scheme 1),21 2-quinoxalinyl-6-iminopyridines
Polyolefins are by far the most important and the most
(C, Scheme 1),22 2-benzimidazolyl-6-aryliminopyridines
produced synthetic polymers today.1 Along with the fast devel-
(D, Scheme 1),23 2-benzazole-1,10-phenanthrolines (X = N,
opment of coordination and organometallic chemistry, polyole-
O, or S) (E, Scheme 1),24 and 2-benzoxazolyl-6-arylimino-
fins remain highly technology-driven and are indeed the fastest
pyridines (F, Scheme 1).25 During the course of this research,
growing segment of the polymer industry.2 The ever-growing
demand for high-performance polyolefins has inspired extensive
it became clear that careful tuning of the environments of
the ligands, namely, incorporation of different substituents or
heteroatoms in the framework of the ligands, can tremendously
affect the observed catalytic activities and resulting properties of
the products. Therefore, we are continuing our program of
designing new catalyst models. In our earlier work, we studied
industrial and academic research into developing new catalysts
for polyolefin production.3 While the Cr-based Phillips catalyst
accounts for more than one-third of the polyethylene sold
worldwide,4 recently Cp-free chromium complexes have at-
tracted attention as efficient homogeneous catalysts.3d,f,5 In
the 2-benzimidazolyl-N-phenylquinoline-8-carboxamide titanium
particular, interest has focused on the selective trimerization5d,6
complexes, which exhibited high activity for ethylene polymeriza-
or tetramerization7 of ethylene by chromium catalysts, producing
tion and ethylene/higher R-olefin copolymerization.26 In extend-
1-hexene or 1-octene as comonomers in the production of linear
ing this research, the chromium complexes bearing the same
low-density polyethylene (LLDPE). On the other hand, homo-
ligands have now been prepared herein and applied as procatalysts
geneous chromium catalysts provide the opportunity of under-
for ethylene oligomerization and polymerization.
standing both the nature of the active species and the mechanism
of the polymerization/oligomerization process, which in turn
might well be helpful in designing new advanced catalysts.6h,n,8
2. RESULTS AND DISCUSSIONS
Recently, considerable attention has been paid to designing
homogeneous catalyst systems based on chromium complexes
2.1. Synthesis and Characterization of 2-(Benzimidazol-2-
through developing and utilizing multidentate ligand sets, such as
N∧N,6g,7a,9 N∧O,9b,10 P∧P or P∧N∧P6a,b,n,q,11 P∧P∧P,12
N∧P∧N,9f,13 S∧N∧S,6i,p,14 N∧S∧N,15 C∧N∧C,16 N∧N∧O,6e,17
N∧N∧N,6c,17d,18 and N∧N∧N∧N19 ligands.
Over the past few years, our group has studied chromium
complexes bearing tridentate ligands such as 2-imino-1,
yl)-N-phenylquinoline-8-carboxamide Chromium Trichlor-
ides. A series of 2-(benzimidazol-2-yl)-N-phenylquinoline-8-
carboxamide derivatives (L1ꢀL6) was prepared following
Received: February 6, 2011
Published: May 17, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om2001124 Organometallics 2011, 30, 3001–3009
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Organometallics
ARTICLE
Scheme 1. Representative Procatalysts of Chromium Complexes
Scheme 2. Synthesis of Complexes C1ꢀC6
Figure 2. ORTEP view of the molecular structure of C3 DMF
3
(ellipsoids enclose 30% electronic density; H atoms are omitted for
clarity).
showed the mononuclear feature with the main fragments of
[M ꢀ 2Cl ꢀ H þ CH3OH]þ ions in their methanol solutions.
Their magnetic moments were typically between 3.73 and
4.22 μB, which are measured in methanol-d4 by the Evans
method,27 which indicate the three unpaired electrons and are
consistent with observations of the paramagnetic mono-
chromium(III) complexes.9f,i,28
Figure 1. ORTEP view of the molecular structure of C2 CH3OH
(ellipsoids enclose 30% electronic density; H atoms are omitted for
clarity).
Single crystals of complexes C2 CH3OH and C3 DMF
3
3
3
suitable for X-ray diffraction analysis were grown by slow layering
of diethyl ether into a methanol solution of C2 and a DMF
solution of C3, respectively. The molecular structures are illu-
strated in Figures 1 and 2, with selected bond lengths and angles
previously reported procedures.26 The chromium complexes
C1ꢀC6 (Scheme 2) were synthesized as green solids by the
stoichiometric reactions of [CrCl3(THF)3] and the correspond-
ing ligands in THF in high yields (91ꢀ97%). All complexes are
air-stable powders, and elemental analysis confirmed that the
complexes are of the LCrCl3 composition. According to their IR
spectra, the CdO stretching frequencies of C1ꢀC6 are shifted
to lower values (1618ꢀ1625 cmꢀ1) compared to those of the
free ligands (1639ꢀ1665 cmꢀ1). In addition to the mononuclear
structures of C2 CH3OH and C3 DMF confirmed by the
collected in Table 1. Complex C2 CH3OH exhibits a distorted
3
octahedral coordination geometry at the Cr center. One chloride
atom trans to the quinoline-N is replaced by one methanol
molecule and acts as a counterion, which is also found for the Ni
complexes.29 The tridentate ligand is situated around the chro-
mium center in a meridional manner (N∧N∧O) and adopts a
puckered chelating style with the dihedral angle 6.21° defined by
the two chelating rings (Cr1ꢀN1ꢀC7ꢀC8ꢀN3, Cr1ꢀN3ꢀ
Cl6ꢀCl5ꢀC17ꢀO1), which is much smaller than that of an
analogous titanium complex.26 The phenyl ring linked to the
3
3
single-crystal X-ray diffraction, the mass spectra of complexes
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Organometallics
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Table 1. Selected Bond Lengths and Angles for C2 CH3OH
Table 2. Effect of Co-catalyst on Ethylene Reactivity with C3a
3
and C3 DMF
3
oligomersc
C2 CH3OH
C3 DMF
3
3
entry cocatalyst Al/Cr activityb distribution
K
waxes activityb
Bond Lengths (Å)
1.956(3)
1
2
3
4
MAO
1500
1500
200
3.05
1.37
0.105
C4ꢀC28
C4ꢀC28
C4
0.73
0.64
1.39
Cr1ꢀO1
Cr1ꢀO2
Cr1ꢀN1
Cr1ꢀN3
Cr1ꢀCl1
Cr1ꢀCl2
O1ꢀC17
N4ꢀC17
1.945(2)
1.993(2)
2.015(3)
2.055(2)
2.3179(1)
2.3177(1)
1.255(4)
1.320(4)
MMAO
Et2AlCl
Et3Al
0.650
1.993(3)
2.017(4)
200
2.061(4)
a Conditions: 2 μmol of Cr; 1 atm of ethylene; 20 °C, 30 min, toluene
(total volume 50 mL). b In units of 106 g molꢀ1 (Cr) hꢀ1
.
2.321(1)
3
3
3
c Determined by GC.
2.306(1)
1.254(5)
1.341(5)
some interesting differences can be observed between C2
CH3OH and C3 DMF. The dihedral angle defined by the
3
Bond Angles (deg)
178.98(5)
89.55(1)
3
Cl1ꢀCr1ꢀCl2
Cl1ꢀCr1ꢀO2
O1ꢀCr1ꢀO2
N1ꢀCr1ꢀN3
O1ꢀCr1ꢀN3
N1ꢀCr1ꢀO2
O1ꢀCr1ꢀN1
O2ꢀCr1ꢀN3
175.79(4)
93.40(8)
92.57(9)
81.08(1)
90.68(1)
96.26(1)
171.69(1)
176.53(1)
two chelating rings (Cr1ꢀN1ꢀC7ꢀC8ꢀN3, Cr1ꢀN3ꢀCl6ꢀ
Cl5ꢀC17ꢀO1) is 2.0°, which is smaller than that in C2
3
CH3OH. In contrast, the dihedral angle between the phenyl ring
92.73(1)
and the coordination plane is 88.6°, which is much larger than the
corresponding one found in C2 CH3OH. In turn, these differ-
ences also influence their catalytic behavior during olefin polym-
erization, which will be discussed in the next section.
80.49(1)
90.72(1)
3
95.68(1)
170.36(1)
175.38(1)
2.2. Ethylene Oligomerization and Polymerization. The
effects of various cocatalysts, such as methylaluminoxane
(MAO), modified methylaluminoxane (MMAO), Et2AlCl, and
Et3Al, on the activity toward ethylene reactivity were first studied
in detail with C3. The results are collected in Table 2 and indicate
that the ethylene reactivity is strongly influenced by the chosen
cocatalyst. The catalytic system with Et2AlCl (entry 3, Table 2)
produced only butenes with low oligomerization activities, and it
is inactive when Et3Al was employed as the cocatalyst. However,
high activity could be achieved by the catalytic system of complex
C3 when combined with MAO or MMAO (entries 1 and 2,
Table 2). Further investigations were carried out with MAO as
cocatalyst, and all the complexes were found to exhibit high
catalytic activities for ethylene oligomerization and polymeriza-
tion with high R-olefin selectivity (>99%). The distributions of
obtained oligomers closely resemble the SchulzꢀFlory rules,
which is characteristic of the constant K, where K represents the
probability of chain propagation (K = rate of propagation/((rate
of propagation) þ (rate of chain transfer)) = (moles of
Cnþ2)/(moles of Cn)).31 The K values in this context are
determined by the molar ratio of C12 and C14.
2.2.1. Effect of Reaction Parameters. The catalytic system of
C3/MAO was typically investigated with varying reaction con-
ditions, such as the reaction temperature, the molar ratio of
Al/Cr, and the reaction time. Elevating the reaction temperature
from 0 to 20 °C speeds the combined catalytic reactions of
oligomerization and polymerization (entries 1 and 2, Table 3).
However, further increasing the reaction temperature to 40 °C
leads to a decrease of catalytic activity (entry 4, Table 3), which
could be attributed to the decomposition of the active species
and lower ethylene solubility in the reaction solution at higher
temperature.32 This is consistent with the chromium complexes
ligated by 2-benzimidazolyl-6-aryliminopyridine, which also
showed the best activity at 20 °C.23a Although the activities of
the C3/MAO system were affected by various reaction tempera-
tures, the reaction temperatures did not significantly change the
distributions of oligomers, for which the K values fall in the range
0.70 to 0.74 (entry 1ꢀ4, Table 3). Fewer polyethylene waxes
were produced at elevated temperatures (entries 1 vs 4, Table 3)
amide group orients nearly perpendicularly to the coordination
plane with a dihedral angle of 70.8°. Similar phenomena have
been observed in reported chromium complexes ligated by
bis(imino)pyridine,17d,18h,22 2-carbethoxy-6-iminopyridines,17b
2-imino-1,10-phenanthrolines,20 2-quinoxalinyl-6-iminopyrid-
ine,22 and 2-benzimidazolyl-6-aryliminopyridine.23 Compared
to the half-titanocene analogue, this angle is slightly smaller
due to more congestion.26 The chromium atom and the mutually
trans-disposed chlorines are almost in one line [Cl1ꢀCr1ꢀCl2 =
178.98(5)°] with N1, N3, O1, and O2 atoms located in an
equatorial plane. Most of the bond angles in the equatorial plane
are close to a right angle [O1ꢀCr1ꢀN3 = 90.72(1)°;
O1ꢀCr1ꢀO2 = 92.73(1)°; N1ꢀCr1ꢀN3 = 80.49(1)°;
N1ꢀCr1ꢀO2 = 95.68(1)°]. The coordination sphere contains
six bonds of three types, for which the bond lengths of the same
type are very similar [Cr1ꢀO1 = 1.956(3) Å; Cr1ꢀO2 =
1.993(3) Å; Cr1ꢀN1 = 2.017(4) Å; Cr1ꢀN3 = 2.061(4) Å;
Cr1ꢀCl1 = 2.321(1) Å; Cr1ꢀCl2 = 2.306(1) Å]. The O1ꢀC17
bond length [1.254(5) Å] is slightly longer than the characteristic
CdO bond (1.19ꢀ1.23 Å), indicating partial electron donation
by oxygen to the central metal, but this is shorter than the
corresponding one found in 2-benzimidazolyl-N-phenylquino-
line-8-carboxamide half-titanocenes [1.330(4) Å], which exhibit
a conjugated feature within the carboximidate group.26 Similar
phenomena are also observed in our other reported systems
containing the carboximidate group as a ligand.30
Complex C3 DMF (Figure 2) possesses similar coordination
features to C2 CH3OH; selected structural parameters for both
complexes are presented in Table 1. Given the lack of structural
deviation between C2 CH3OH and C3 DMF, we believe that
the other complexes will possess similar coordination character-
istics. In C3 DMF, one DMF molecule coordinates to the Cr
metal center, while the chloride atom trans to the quinoline-N is
dissociated as a counterion. The CꢀO bond length (O1ꢀC17 =
1.255(4) Å) of the amidate group is almost equal to that in
3
3
3
3
3
C2 CH3OH and displays partial conjugated features. However,
3
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Table 3. Oligomerization and Polymerization of Ethylene
with C3/MAOa
Table 5. Oligomerization and Polymerization of 10 atm of
Ethylene with C1ꢀC6/MAOa
T
oligomersb
Al/Cr t (min) activityc (wt %)
waxes
T
oligomersb
(wt %)
waxes
entry (°C)
K
(wt %)
entry complex (°C) Al/Cr activityc
K
(wt %)
1
0
20
30
40
20
20
20
20
20
20
20
1500
1500
1500
1500
500
30
30
30
30
30
30
30
30
10
20
60
2.78
4.44
2.62
1.97
0.256
2.61
3.68
2.15
4.88
4.76
3.11
68.0
68.7
74.0
74.5
0.73
0.73
0.70
0.74
0.76
0.74
0.74
0.78
0.78
0.76
0.73
32.0
31.3
26.0
25.5
trace
26.4
26.9
27.4
31.1
31.9
32.8
1
C1
C2
C3
C4
C5
C6
C3
C3
C3
C3
C3
C3
20
20
20
20
20
20
30
40
20
20
20
20
1500
1500
1500
1500
1500
1500
1500
1500
1000
2000
1500
1500
15.0
15.9
18.8
19.1
61.5
63.5
67.0
67.0
65.6
66.7
72.0
87.6
65.6
74.5
66.7
66.9
0.74
0.73
0.73
0.73
0.72
0.74
0.74
0.72
0.76
0.74
0.74
0.73
38.5
36.5
33.0
33.0
34.4
33.3
38.0
12.4
34.4
25.5
33.6
32.5
2
2
3
3
4
4
5
100
5
4.80
6
1000
2000
2500
1500
1500
1500
73.6
73.1
72.6
68.9
68.1
67.2
6
1.89
8.14
4.81
7
7
8
8
9
9
16.0
10
10
11d
12e
11.2
18.9
18.1
11
a Conditions: 2 μmol of Cr; 1 atm of ethylene; toluene (total volume
50 mL). b Determined by GC. c In units of 106 g molꢀ1 (Cr) hꢀ1
.
a Conditions: 2 μmol of Cr; 30 min, toluene (total volume 100 mL).
3
3
3
b Determined by GC. In units of 106 g molꢀ1 (Cr) hꢀ1
.
d Toluene
c
3
3
3
(total volume 200 mL). e 1 μmol of Cr.
Table 4. Oligomerization and Polymerization of 1 atm of
Ethylene with C1ꢀC6/MAOa
entry complex activityc oligomersb (wt %)
K
waxes (wt %)
at 20 °C under 1 atm of ethylene. The data listed in Table 4 show
that the structures of the ligands have a considerable effect on the
catalytic behavior of the complexes. It is evident that an electron-
donating effect positively influences the activities of the chro-
mium complexes. Complexes C1ꢀC4, containing electron-do-
1
2
3
4
5
6
C1
C2
C3
C4
C5
C6
2.86
3.38
4.44
4.66
1.48
1.05
81.8
74.0
68.7
68.9
69.6
71.0
0.71
0.71
0.73
0.75
0.73
0.70
18.2
26.0
31.3
31.1
30.4
29.0
i
nating groups, namely, Pr, Et, and Me on the anilines, exhibit
much higher activity comparing to complex C6, which bears
electron-withdrawing groups. On the other hand, increasing the
steric hindrance at the ortho positions of the anilines renders a
less active catalyst (C3 (R1 = Me) > C2 (R1 = Et) > C1 (R1 =
iPr)). This could be explained in terms of the reaction rate, which
depends on the insertion of ethylene at the active species, and so
a bulkier group would cause a slower insertion reaction and
consequently lower activity. Meanwhile, with increased bulk, the
content of longer chain oligomers and polyethylene is decreased,
which might be attributed on the basis that the catalysts with
more steric bulkiness facilitate the β-hydrogen elimination reac-
tion and not the reaction of propagation.
2.2.3. Ethylene Oligomerization and Polymerization of 10
atm of Ethylene. Kinetic studies have shown that the rate of
oligomerization increases with increasing ethylene concent-
ration.7e,34 Therefore, ethylene oligomerization and polymeriza-
tion with complexes C1ꢀC6 were also carried out at 10 atm of
ethylene with MAO as cocatalyst, and the results are listed in
Table 5. The results show that the ethylene pressure significantly
affects the catalytic behavior of the complexes. This observation
of increased activity at elevated ethylene pressure is in accor-
dance with previously reported results for homogeneous chro-
mium catalysts.14b,21,23a Among the catalytic systems herein, the
C4/MAO system shows the highest activity of up to 1.91 ꢁ 107
g molꢀ1 (Cr) hꢀ1 (entry 4, Table 5). At 10 atm of ethylene, the
a Conditions: 2 μmol of Cr; 20 °C, 30 min, Al/Cr = 1500, toluene
(total volume 50 mL). b Determined by GC. c In units of
106 g molꢀ1 (Cr) hꢀ1
.
3
3
3
because of faster β-hydrogen elimination than chain propagation,
which is consistent with the observation for [2,6-bis(2-benzimi-
dazolyl)pyridyl]chromium trichlorides.21,33 It is notable that
increasing the temperature does not lead to a decrease in the
selectivity for R-olefins.
The amount of cocatalyst was found to have significant
influence on the catalytic behavior of the C3/MMAO system.
Increasing the Al/Cr molar ratio from 500 to 1500 leads to an
enhancement of the ethylene oligomerization and polymeriza-
tion activities (see entries 2, 5, 6, Table 3). When the ratio of
Al/Cr is 1500, the catalytic system displays the highest activity
of 4.44 ꢁ 106 g molꢀ1 (Cr) hꢀ1 (entry 2, Table 3). A further
3
3
3
increase in the Al/Cr molar ratio to 2000 or 2500 leads to
decreased catalytic activity (entries 7 and 8, Table 3). Similar
phenomena were observed in previously reported chromium
complexes, and the deactivation might be caused by the impu-
rities present in commercial MAO such as alkyl aluminum, which
can react with the active species.20,24,25
With regard to the lifetime of the C3/MAO system, the ethylene
polymerization was conducted over different time periods, namely,
10, 20, 30, and 60 min (entries 2, 9ꢀ11, Table 3). The activities were
almost maintained over the initial 30 min, but decreased somewhat on
extending the runs to 60 min, indicating that the active species slowly
deactivated over prolonged reaction times.
2.2.2. Effect of the Ligand Environment. On the basis of the
results observed with the C3/MAO system, the title complexes
C1ꢀC6 were investigated with an MAO/Cr molar ratio of 1500
3
3
3
effect of the ligand environments and the reaction parameters
(the reaction temperature and the molar ratio of Al/Cr) on the
activity is similar to the results obtained at 1 atm of ethylene. In
comparison with the results with 1 atm of ethylene, the percen-
tages of polyethylene waxes increased remarkably, which can be
explained by the rate of propagation being faster than that of
hydrogen elimination at higher pressure. The distributions of the
oligomers achieved by the C1ꢀC3/MAO systems (entries 1ꢀ3,
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Organometallics
ARTICLE
Figure 3. Plot of the molar amount of oligomers formed by C1ꢀC3/MAO (entries 1ꢀ3 in Table 5) versus carbon number (C6ꢀC28) and the
calculated distribution for a K value of 0.73.
Figure 4. 13C NMR spectrum of the waxes obtained by the C3/MAO system in entry 3 in Table 5.
Table 5) are shown in Figure 3, indicating an approximate
SchulzꢀFlory distribution. It is worthy to mention that dilute
catalyst concentration through either more solvent (entry 11,
Table 5) or loading less catalyst (entry 12, Table 5) resulted in a
slight decrease of the catalytic activity.
the polymers. The resonances at about δ 138.7 (CHd) and
113.8 (CH2d) ppm are typical for a vinyl-unsaturated chain
end;35 the other ethylene waxes showed very similar NMR
spectra.
Along with the high selectivity for R-olefins, polyethylene
waxes were also obtained. As characterized by IR spectra (KBr
disks), the polyethylene waxes were confirmed to be composed
of highly linear R-olefins by the characteristic vibration absorp-
tion bands of the CꢀH and CdC bonds. A 13C NMR spectrum
(Figure 4) of the polyethylene waxes obtained by the C3/MAO
system (entry 3, Table 5) was recorded at 110 °C in
o-dichlorobenzene-d4 using TMS as the internal standard and
demonstrates that linear R-olefins of the waxes predominate in
3. CONCLUSION
The chromium(III) complexes (LCrCl3, C1ꢀC6) have been
synthesized and characterized, including by single-crystal X-ray
diffraction for C2 and C3. All procatalysts show high activity, up
to 1.91 ꢁ 107 g molꢀ1 (Cr) hꢀ1, toward ethylene oligomeriza-
3
3
3
tion and polymerization in the presence of MAO. The procata-
lysts bearing ligands with electron-donating groups give higher
activity, while the activity is favored for less bulky substituents at
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Table 6. Crystallographic Data and Refinement Details for C2 CH3OH and C3 DMF
3
3
C2 CH3OH
C3 DMF
3
3
formula
fw
C
27H23Cl3CrN5O CH3OH
C25H21Cl3CrN4O DMF
3
3
623.90
610.89
T (K)
173(2)
0.71073
monoclinic
P2(1)/c
8.7764(1)
20.349(4)
21.050(4)
90.00
173(2)
0.71073
triclinic
P1
wavelength (Å)
cryst syst
space group
a (Å)
9.4487(1)
10.408(2)
16.144(3)
84.10(3)
73.53(3)
79.76(3)
1495.9(5)
2
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
Z
92.29(3)
90.00
3756.3(1)
4
Dcalcd (g cmꢀ3
)
1.103
1.356
μ (mmꢀ1
)
0.545
0.681
F(000)
1284
630
θ range (deg)
1.39ꢀ27.48
1.99ꢀ27.44
limiting indices
ꢀ11 e h e 11, ꢀ24 e k e 26, ꢀ27 e l e 23
ꢀ12 e h e 12, ꢀ13 e k e 12, ꢀ20 e l e 20
no. of rflns collected
no. of unique reflns
completeness to θ (%)
abs corr
30 509
20 234
8582
99.6 (θ = 27.48o)
6818
99.7 (θ = 27.44o)
numerical
numerical
no. of params
352
347
goodness of fit on F2
final R indices (I > 2σ(I))
R indices (all data)
largest diff peak, hole (e Åꢀ3
1.068
1.123
R1 = 0.0666, wR2 = 0.1711
R1 = 0.0870, wR2 = 0.1825
0.537 and ꢀ0.589
R1 = 0.0823, wR2 = 0.1930
R1 = 0.1085, wR2 = 0.2071
0.589 and ꢀ0.500
)
the ortho positions of the amidate aryl rings. The oligomers were
produced with high selectivity for R-olefins (>99%), and the
distributions closely resemble the SchultzꢀFlory rule. The
polyethylene waxes were verified to be long linear olefins with
vinyl end groups by their characteristic IR and 1H and 13C NMR
spectra.
with a LC-MS 2010 (constant infusion 6 mL hꢀ1, needle voltage 4.6 kV,
capillary voltage 50 V). GC analyses were performed with a Varian CP-
3800 gas chromatograph equipped with a flame ionization detector and a
30 m (0.25 mm i.d., 0.25 μm film thickness) CP-Sil 5 CB column. The
yield of oligomers was calculated by referencing with the mass of the
solvent on the basis of the prerequisite that the mass of each fraction was
approximately proportional to its integrated areas in the GC trace.
Selectivity for the linear R-olefin was defined as (amount of linear
R-olefin of all fractions)/(total amount of oligomer products) in
4. EXPERIMENTAL SECTION
1
percent. H NMR and 13C NMR spectra of the polyethylene waxes
were recorded on a Bruker DMX-300 MHz instrument at 110 °C in
deuterated 1,2-dichlorobenzene with TMS as an internal standard.
4.2. General Procedure. Solid [CrCl3(THF)3] was added to a
solution of the ligand in THF (20 mL) to slowly give a green suspension,
which was stirred for 6 h. The solvent was removed under vacuum,
affording a green powder, which was washed with diethyl ether (2 ꢁ
20 mL) and dried in vacuo overnight. All of the complexes were prepared
in high yield following this same procedure.
2-(1H-Benzo[d]imidazol-2-yl)-N-(2,6-diisopropylphenyl)quinoline-
8-carboxamide CrCl3 (C1). Obtained as a green powder in 95.0% yield.
FT-IR (KBr, cmꢀ1): 3436m, 3210w, 3065m, 2966m, 2868w, 1623s,
1604s, 1570s, 1540m, 1442m, 1363w, 1339m, 1232w, 1146m, 1013w,
997m, 856w, 765m. m/z 566.1, [M ꢀ 2Cl ꢀ H þ CH3OH]þ (ESI-MS).
μeff. = 3.82 μB. Anal. Calcd for C29H28Cl3CrN4O: C, 57.39; H, 4.65; N,
9.23. Found: C, 57.13; H, 4.55; N, 9.11.
4.1. General Considerations. All manipulations of air- and/or
moisture-sensitive compounds were performed under a nitrogen atmo-
sphere in a glovebox or using standard Schlenk techniques. 2-
(Benzimidazol-2-yl)-N-phenylquinoline-8-carboxamide derivatives were
prepared as previously reported.26 Tetrahydrofuran (THF) and toluene
were refluxed over sodium/benzophenone and distilled under nitrogen
prior to use. Methylaluminoxane (MAO, a 1.46 M solution in toluene)
and modified methylaluminoxane (MMAO, 1.93 M in heptane, 3A)
were purchased from Akzo Nobel Corp. Diethylaluminum chloride
(Et2AlCl, 1.7 M in toluene) and triethyl aluminum (AlEt3, 2 M in
hexane) were purchased from Acros Chemicals. Other reagents were
purchased from Beijing Chemicals. FT-IR spectra were recorded on a
Perkin-Elmer System 2000 FT-IR spectrometer. Elemental analysis was
carried out using an HPMOD 1106 microanalyzer. For measuring
magnetic moments by the Evans method, a solution of the chromium
complex in C6H12ꢀCD3OD solution (3% cyclohexane in CD3OD, v/v)
was prepared along with the reference C6H12ꢀCD3OD solution (3%
cyclohexane in CD3OD, v/v).27 The ESI-MS spectrum was obtained
2-(1H-Benzo[d]imidazol-2-yl)-N-(2,6-diethylphenyl)quinoline-8-car-
boxamide CrCl3 (C2). Obtained as a green powder in 94.5% yield. FT-IR
(KBr, cmꢀ1): 3416m, 3198m, 3065w, 2970m, 1623s, 1600s, 1565s,
3006
dx.doi.org/10.1021/om2001124 |Organometallics 2011, 30, 3001–3009

Organometallics
ARTICLE
1532m, 1501m, 1457w, 1340w, 1233w, 1141m, 1012m, 860m, 760m.
m/z 538.1, [M ꢀ 2Cl ꢀ H þ CH3OH]þ (ESI-MS). μeff. = 3.73 μB. Anal.
Calcd for C27H24Cl3CrN4O: C, 56.02; H, 4.18; N, 9.68. Found: C,
55.98; H, 4.09; N, 9.65.
obtained by slow diffusion of diethyl ether into methanol or DMF
solutions, respectively. Single-crystal X-ray diffraction studies for C2
3
CH3OH and C3 DMF were carried out on a Rigaku RAXIS Rapid IP
3
diffractometer with graphite-monochromated Mo KR radiation (λ =
0.71073 Å). Cell parameters were obtained by global refinement of the
positions of all collected reflections. Intensities were corrected for
Lorentz and polarization effects and empirical absorption. The struc-
tures were solved by direct methods and refined by full-matrix least-
squares on F2. All non-hydrogen atoms were refined anisotropically. All
hydrogen atoms were placed in calculated positions. Structure solution
and refinement were performed by using the SHELXL-97 package.36
Crystal data and processing parameters for C2 CH3OH and C3 DMF
2-(1H-Benzo[d]imidazol-2-yl)-N-(2,6-dimethylphenyl)quinoline-8-
carboxamide CrCl3 (C3). Obtained as a green powder in 97.0% yield.
FT-IR (KBr, cmꢀ1): 3428m, 3185m, 3064w, 2925m, 1624s, 1601s,
1566s, 1538m, 1474w, 1440w, 1341w, 1303w, 1228w, 1150m, 1092m,
857m, 761m. m/z 510.1, [M ꢀ 2Cl ꢀ H þ CH3OH]þ (ESI-MS). μeff.
=
3.91 μB. Anal. Calcd for C25H20Cl3CrN4O: C, 54.51; H, 3.66; N, 10.17.
Found: C, 54.50; H, 3.33; N, 10.10.
2-(1H-Benzo[d]imidazol-2-yl)-N-(2,4,6-trimethylphenyl)quinoline-
8-carboxamide CrCl3 (C4). Obtained as a green powder in 96.2% yield.
FT-IR (KBr, cmꢀ1): 3399m, 3256m, 3057w, 2971m, 1625s, 1602s,
1570s, 1534m, 1476w, 1441w, 1422w, 1378w, 1337w, 1223w, 1147m,
1045m, 853m, 763m. m/z 524.1, [M ꢀ 2Cl ꢀ- H þ CH3OH]þ (ESI MS).
μeff. = 3.95 μB. Anal. Calcd for C26H22Cl3CrN4O: C, 55.29; H, 3.93;
N, 9.92. Found: C, 54.97; H, 3.77; N, 9.68.
3
3
are summarized in Table 6.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF file giving X-ray crystal
b
structural data of nickel complexes C2 and C3. This material is
2-(1H-Benzo[d]imidazol-2-yl)-N-phenylquinoline-8-carboxamide
CrCl3 (C5). Obtained as a green powder in 91.0% yield. FT-IR
(KBr, cmꢀ1): 3408m, 3201m, 3022w, 2951m, 1620s, 1602s, 1567s,
1533m, 1476w, 1440w, 1429w, 1366w, 1325w, 1221w, 1177m, 1042m,
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
851m, 760 m. m/z 482.1, [M ꢀ 2Cl ꢀ H þ CH3OH]þ (ESI-MS). μeff.
=
Corresponding Author
4.09 μB. Anal. Calcd for C23H16Cl3CrN4O: C, 52.84; H, 3.09; N, 10.72.
Found: C, 52.57; H, 3.08; N, 10.53.
*Tel: þ86-10-62557955. Fax: þ86-10-62618239. E-mail: whsun@
iccas.ac.cn (W-H.S.). Tel: þ44 (0)1603 593137;. Fax: þ44
(0)1603 592003. E-mail: carl.redshaw@uea.ac.uk (C.R.).
2-(1H-Benzo[d]imidazol-2-yl)-N-(2,6-difluorophenyl)-8-carboxa-
mide CrCl3 (C6). Obtained as a green powder in 91.5% yield. FT-IR
(KBr, cmꢀ1): 3448m, 3257m, 3067w, 1618s, 1601m, 1571s, 1541m,
1474s, 1441m, 1422w, 1341w, 1324w, 1227w, 1148m, 1050w, 1016s,
998m, 856m, 763m. m/z 518.0, [M ꢀ 2Cl ꢀ H þ CH3OH]þ (ESI-MS).
μeff. = 4.22 μB. Anal. Calcd for C23H14Cl3CrF2N4O: C, 49.44; H, 2.53;
N, 10.03. Found: C, 49.21; H, 2.39; N, 9.95.
’ ACKNOWLEDGMENT
This work was supported by MOST 863 program No.
2009AA033601. C.R. wishes to thank the EPSRC for an overseas
travel grant (EP/H031855/1).
4.3. Procedure for Oligomerization and Polymerization
with 1 atm of Ethylene. Polymerizations were carried out as follows:
the catalyst precursor (chromium complex) was dissolved in toluene in a
250 mL Schlenk tube stirred with a magnetic stirrer under an ethylene
atmosphere (1 atm), and the reaction temperature was controlled by a
water bath. The reaction was initiated by adding the desired amount of
cocatalyst. After the desired period of time, a small amount of the
reaction solution was collected with a syringe and was quenched by the
addition of 5% aqueous HCl. An analysis by gas chromatography (GC)
was carried out to determine the distribution of oligomers obtained. The
remaining solution was quenched with HCl-acidified ethanol (5%), and
the precipitated polyethylene was collected by filtration, washed with
ethanol, and dried under vacuum at 60 °C to constant weight.
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