Synthesis, characterization and ethylene oligomerization and polymerization of 2-(1H-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridylchromium chlorides

Article abstract of DOI:10.1016/j.poly.2009.06.037

A series of N^N^N tridentate chromium complexes (C1-C6) bearing 2-(1H-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridine derivatives was synthesized and characterized by elemental and spectroscopic analysis along with single-crystal X-ray crystallography. X-ray crystallographic analyses reveal chromium complex C1 as a distorted six-coordinated octahedral geometry. On treatment with modified methylaluminoxane (MMAO), the chromium complexes exhibited high activities for ethylene oligomerization (up to 1.50 × 10⁶ g mol^-1 (Cr) h^-1) and polymerization (up to 2.06 × 10⁶ g mol^-1 (Cr) h^-1) at 10 atm ethylene pressure. Various reaction parameters were investigated in detail, and less steric hindrance and electron-withdrawing substituents of ligands enhance the catalytic activities of their chromium complexes.

Full text of DOI:10.1016/j.poly.2009.06.037

Polyhedron 29 (2010) 142–147
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization and ethylene oligomerization and polymerization
of 2-(1H-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridylchromium chlorides
b
b
Liwei Xiao a, , Min Zhang , Wen-Hua Sun
*
a Department of Chemistry, Langfang Teacher’s College, Langfang 065000, China
b Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of N^N^N tridentate chromium complexes (C1–C6) bearing 2-(1H-2-benzimidazolyl)-6-(1-(aryli-
mino)ethyl)pyridine derivatives was synthesized and characterized by elemental and spectroscopic anal-
ysis along with single-crystal X-ray crystallography. X-ray crystallographic analyses reveal chromium
complex C1 as a distorted six-coordinated octahedral geometry. On treatment with modified methylalu-
minoxane (MMAO), the chromium complexes exhibited high activities for ethylene oligomerization (up
to 1.50 Â 106 g molÀ1 (Cr) hÀ1) and polymerization (up to 2.06 Â 106 g molÀ1 (Cr) hÀ1) at 10 atm ethylene
pressure. Various reaction parameters were investigated in detail, and less steric hindrance and electron-
withdrawing substituents of ligands enhance the catalytic activities of their chromium complexes.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 27 April 2009
Accepted 17 June 2009
Available online 23 June 2009
Keywords:
Chromium complex
N^N^N tridentate ligand
Ethylene oligomerization
Ethylene polymerization
1. Introduction
dine (C) [60] and 2-(1-isopropyl-2-benzimidazolyl)-6-(1-(arylimi-
no)ethyl)pyridines (D) [61] showed good catalytic activities
Chromium is the key element in the silica-supported Phillips [1]
and Union Carbide [2–4] catalytic systems commercially used for
the polymerization of olefins. The importance of these catalysts
has drawn much attention in developing Cr(III) complexes ligated
by a variety of ligands, including both cyclopentadienyl-based
[5–16] and non-cyclopentadienyl-based systems [17–31]. Cp-free
complexes are quite prone to steric and electronic modifications,
chromium complexes are capable of producing oligomers and/or
polymers with different properties [32–34]. Various chromium
complexes coordinated by anionic ligands have been explored such
as monodentate Cr–pyrrolyl complexes [35] and different multi-
chelate models such as N^O [23–25], N^N [19–22,36–38], N^N^N
[26]. Moreover, chromium complexes containing various P^P or
P^N^P [39–46], P^P^P [47], S^N^S [41,42,47], C^N^C [48], N^N^O
[31,49], N^S^N [50] and N^N^N^N [51] ligands have also been
investigated. Recently, chromium complexes ligated with neutral
N^N^N ligands, such as triazacyclohexane [26,27], bis(imino)pyri-
dines [30,31,52], (2-pyridylmethyl)amines [53], bis(oxazoli-
nyl)pyridine [54], bis(benzimidazolyl)pyridines [55,56], tridentate
pyrazolyl ligand [57] and 2,6-bis(azolylmethyl)pyridine [58] have
attracted great attentions due to their highly catalytic activities
in ethylene oligomerization and polymerization. In our course of
developing new chromium complexes as catalysts for ethylene
reactivity, chromium complexes containing N^N^N ligands
(Scheme 1) such as 2,6-bis(2-benzimidazolyl)pyridine (A) [55], 2-
imino-1,10-phenanthrolines (B) [59], 2-quinoxalinyl-6-iminopyri-
towards ethylene oligomerization or/and polymerization. Regard-
ing to catalytic behaviors showed by other metal complexes, the
metal complexes bearing 2-(1-methyl-2-benzimidazolyl)-6-(1-
(arylimino)ethyl)pyridine (E) [62,63] showed better activities than
their analogues with 1-isopropyl group (D) [64]. Recently, the
iron(II) and cobalt(II) complexes bearing 2-(1H-2-benzimidazol-
yl)-6-(1-(arylimino)ethyl)pyridyl derivatives [65] showed better
activities than their alkylated analogues (D and E) [62,64]. These
results indicated that the alkyl groups on benzimidazole of ligands
greatly affect the catalytic performance of metal complexes. Com-
paring with chromium complexes containing 1-isopropyl group on
benzimidazole of ligands [61], a series of chromium (III) complexes
bearing
(2-(1H-2-benzimidazolyl)-6-(1-(arylimino)-ethyl)pyri-
dines is synthesized and characterized. The investigation of their
catalytic activities towards ethylene reactivity shows better results
than the performance by their analogues [61]. Herein we report the
synthesis and characterization of the chromium complexes
bearing 2-(1H-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines.
Furthermore, their catalytic behaviors towards ethylene oligomer-
ization and polymerization are investigated in detail under
different reaction conditions.
2. Experimental
2.1. General considerations
All manipulations of air- and moisture-sensitive compounds
were performed under a nitrogen atmosphere using standard
Schlenk techniques. Toluene was refluxed and distilled over
* Corresponding author. Tel.: +86 316 2188617; fax: +86 10 62618239.
E-mail address: xiaoliwei2000@sina.com (L. Xiao).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.06.037

L. Xiao et al. / Polyhedron 29 (2010) 142–147
143
R3
N
R3
N
N
R
N
N
N
R1
R1
N
N
N
N
N
N
Ar
Ar
R2
R2
A
B
C
H
N
N
N
N
N
N
N
N
N
N
N
N
Ar
Ar
Ar
D
E
This work
Scheme 1. Model ligands for effective chromium catalysts.
sodium and benzophenone, while dichloromethane was dried and
distilled over CaH2. Methylaluminoxane (MAO) was purchased
from Lanzhou Research Center of PetroChina, while modified
methylaluminoxane (MMAO) and diethylaluminumchloride
(Et2AlCl) were purchased from Akzo Nobel Corp. and Acros Chem-
icals, respectively. FT-IR spectra were recorded on a Perkin–Elmer
System 2000 FT-IR spectrometer. Elemental analysis was carried
out using an HPMOD 1106 microanalyzer. GC analysis was per-
formed with a VARIAN CP-3800 gas chromatograph equipped with
C3: Green powder, 91% yield. IR (KBr; cmÀ1): 3392(br m),
3067(m), 2967(s), 2926(w), 1591(s), 1494(m), 1417(s), 1367(s),
1322(m), 1300(w), 1275(w), 1234(w), 1132(w), 1056(w), 855(w),
815(m), 806(m), 754(vs). Anal. Calc. for C26H28Cl3CrN4 (554.88):
C, 56.28; H, 5.09; N, 10.10. Found: C, 56.34; H, 5.14; N, 10.04%.
C4: Green powder, 83% yield. IR (KBr; cmÀ1): 3406(br m),
3065(m), 2953(w), 2915(w), 1586(s), 1481(vs), 1419(vs),
1371(m), 1321(m), 1277(w), 1216(m), 1154(m), 1038(w),
853(m), 816(w), 752(vs). Anal. Calc. for C23H22Cl3CrN4 (512.8): C,
53.87; H, 4.32; N, 10.93. Found: C, 53.82; H, 4.29; N, 10.99%.
C5: Green powder, 82% yield. IR (KBr; cmÀ1): 3388(br m),
3067(m), 2973(m), 1589(s), 1482(s), 1435(vs), 1369(m), 1322(m),
1278(w), 1230(m), 1097(w), 860(w), 817(m), 786(m), 750(vs).
Anal. Calc. for C20H14Cl5CrN4 (539.61): C, 44.52; H, 2.62; N, 10.38.
Found: C, 44.57; H, 2.58; N, 10.32%.
a flame ionization detector and a 30 m (0.2 mm i.d., 0.25
thickness). The selectivity for the linear -olefin was defined as
(amount of linear -olefin of all fractions)/(total amount of oligo-
lm film
a
a
mer products) in percentage. The resultant solution of ethylene
reactivity was quenched with 5% HCl–acidic ethanol. The precipi-
tated polyethylene (PE) was collected by filtration, washed with
ethanol, dried under vacuum at 60 °C to constant weight, weighed
and characterized. Melting points of PEs were measured on a Per-
kin–Elmer DSC-7 differential scanning calorimetry (DSC) analyzer.
Under a nitrogen atmosphere, a sample of about 4 mg was heated
from 20 to 160 °C at a rate of 10 °C/min and kept for 5 min at
160 °C in order to remove the thermal history, then cooled at a rate
of 10 °C/min to 20 °C. The DSC trace and the melting points of the
samples were obtained from the second scanning run.
C6: Green powder, 85% yield. IR (KBr; cmÀ1): 3570(br w),
3062(m), 2972(w), 1601(m), 1482(m), 1427(s), 1369(s), 1320(w),
1225(m), 1113(w), 858(w), 820(w), 763(m), 747(vs). Anal. Calc.
for C20H14Br2Cl3CrN4 (628.52): C, 38.22; H, 2.25; N, 8.91. Found:
C, 38.16; H, 2.20; N, 8.95%.
2.3. Procedure for oligomerization and polymerization at 1 atm
ethylene
2.2. Preparation of chromium(III) complexes C1–C6
The catalyst precursor was dissolved in 30 ml toluene in a
Schlenk tube stirred under nitrogen atmosphere with a magnetic
stirrer, and the reaction temperature was controlled by a water
bath. The reaction was initiated by adding the desired amount of
cocatalyst under ethylene atmosphere. After the desired time, a
small amount of the reaction solution was collected with a syringe
and was quenched by the addition of 5% aqueous HCl. Analysis by
gas chromatography (GC) was carried out to determine the distri-
bution of oligomers formed. The remaining solution was quenched
with 5% HCl–acidic ethanol, and the precipitated polyethylene was
collected by filtration, washed with ethanol, dried under vacuum at
60 °C to constant weight, weighed, and finally characterized.
The ligands L1–L6 were prepared according to our previous
work [65]. Complexes C1–C6 were synthesized by the reaction of
CrCl3(THF)3 with the corresponding ligands in dichloromethane.
A typical synthetic procedure for C1 is as follows: ligand L1
(0.15 g, 0.44 mmol) prior dissolved in a minimum volume of
CH2Cl2 was added to a solution of CrCl3(THF)3 (0.16 g, 0.44 mmol)
in CH2Cl2 (10 ml), and the mixture was stirred at room tempera-
ture for 9 h. The solvent was then removed in vacuo to afford a
green solid, which was washed with hexane and dried in vacuo.
The complex C1 was obtained as green powder (0.20 g, 0.40 mmol)
in 91%. IR (KBr; cmÀ1): 3386(br m), 3056(s), 2952(w), 2911(w),
1591(s), 1480(s), 1416(s), 1373(m), 1322(m), 1300(w), 1276(w),
1216(w), 1096(m), 1038(m), 857(w), 793(m), 754(vs). Anal. Calc.
for C22H20Cl3CrN4 (498.78): C, 52.98; H, 4.04; N, 11.23. Found: C,
53.03; H, 4.06; N, 11.18%.
2.4. Procedure for oligomerization and polymerization at 10 atm
ethylene
These reactions were carried out in a 250 ml stainless steel
autoclave reactor equipped with a mechanical stirrer and a tem-
perature controller. The desired amount of cocatalyst, a toluene
solution of the chromium complex (30 ml), and toluene (70 ml)
were added to the reactor in this order under an ethylene atmo-
sphere. When the reaction temperature was achieved, ethylene
at the desired pressure (10 atm) was introduced to start the
C2: Green powder, 87% yield. IR (KBr; cmÀ1): 3386(br m),
3062(s), 2966(s), 2909(w), 1590(s), 1480(s), 1456(s), 1417(vs),
1371(s), 1322(m), 1301(w), 1277(w), 1236(w), 1037(w), 985(w),
858(w), 815(m), 791(m), 753(vs). Anal. Calc. for C24H24Cl3CrN4
(526.83): C, 54.72; H, 4.59; N, 10.63. Found: C, 54.67; H, 4.53; N,
11.68%.

144
L. Xiao et al. / Polyhedron 29 (2010) 142–147
Table 1
lated positions; the H4N was located from difference map and
Crystal data and structure refinement for C1.
freely refined. Structure solution and refinement were performed
by using the SHELXL-97 package [66]. Crystal data and processing
parameters for complexes C1 are summarized in Table 1.
C1
Formula
C22H20Cl3CrN4ÁDMF
571.87
133(2)
0.71073
monoclinic
P2(1)/c
9.3944(2)
13.999(2)
19.920(3)
90
97.825(3)
90
2595.4(8)
4
1.464
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
3. Results and discussion
3.1. Synthesis and characterization of the chromium complexes C1–C6
b (Å)
c (Å)
2-(1H-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridylchromi-
um(III) trichlorides C1–C6 were prepared through the treatment of
2-(1H-2-benzimidazolyl)-6-(1-(arylimino)ethyl)pyridyl deriva-
tives with 1 equiv. of CrCl3(THF)3 in CH2Cl2 at room temperature
(Scheme 2). The resultant products were isolated as air-stable
green powders in good yields and characterized by IR spectra
and elemental analysis.
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
Dcalcd (g mÀ3
)
l
(mmÀ1
)
0.778
The identity of the complexes C1–C6 was established based on
IR spectroscopic and element analysis. Compared to the IR spectra
of the free ligands L1–L6 [65], the C@N stretching vibrations in
complexes C1–C6 were shifted to lower frequencies in the range
of 1586–1601 cmÀ1, indicating an effective coordination interac-
tion between the imino nitrogen atom and the chromium center.
In addition, crystals of C1 have been subject to single-crystal
X-ray diffraction studies.
F (0 0 0)
Crystal size (mm)
h range (°)
Limiting indicates
1180
0.17 Â 0.13 Â 0.05
3.09–25.50
À11 6 h 6 11
À16 6 k 6 16
À23 6 l 6 24
18 237
4825
0.0433
99.7% (h = 25.50°)
empirical
325
Number of reflections collected
number of unique reflections
Rint
Completeness to h (%)
Absorption correction
Number of parameters
Goodness-of-fit (GOF) on F2
3.2. Molecular structure
1.064
Final R indices [I > 2
r
(I)]
R1 = 0.0511
wR2 = 0.1306
R1 = 0.0579
wR2 = 0.1351
0.324, –0.334
Crystals of complex C1 suitable for X-ray structure determina-
tion were grown from its N,N-dimethylformamide (DMF) solutions
layered with diethyl ether. The molecular structure of complex C1
is depicted in Fig. 1. In the molecular structure of complex C1
(Fig. 1), the crystal structure of C1 confirms its geometry as a dis-
torted octahedron and the three coordinated nitrogen atoms and
the three chlorides are situated around the chromium center in a
meridional manner. The N1, N3, Cl1, and Cl3 atoms could be lo-
cated in the equatorial plane with a mean deviation of 0.1835 Å,
and the two axial bonds nearly form a straight line through the
metal center (N(2)–Cr(1)–Cl(2), 178.38(8)°). All the bond angles
R indices (all data)
Largest difference in peak and hole (e ÅÀ3
)
reaction. After 30 min, a small amount of the reaction solution was
collected. The reaction was terminated by the addition of 5% aque-
ous HCl and the mixture was analyzed by the gas chromatography
(GC) to determine the distribution of oligomers obtained. The
remaining solution was quenched with HCl–acidified ethanol
(5%) The precipitated polymer was collected by filtration, washed
with ethanol, dried under vacuum at 60 °C to constant weight,
weighed, and finally characterized.
in the equatorial plane are very close to
a
right angle
(N(3)–Cr(1)–Cl(1), 93.35(7)°; N(3)–Cr(1)–Cl(3),
90.35(7)°;
N(1)–Cr(1)–Cl(1), 86.62(8)°; N(1)–Cr(1)–Cl(3), 87.67(8)°). The axial
Cr(1)–N(2) bond length (2.016(3) Å) is shorter by 0.036 Å than the
Cr(1)–N(1) bond length (2.052(3) Å) and by 0.1379 Å than the
Cr(1)–N(3) bond length (2.154(2) Å), which is in accordance with
some other chromium complexes [57,58]. The Cr–Cl bond lengths
2.5. X-ray crystallographic studies
Single-crystal X-ray diffraction measurement for C1 was carried
span
a relatively narrow range with the Cr(1)–Cl(2) bond
out Rigaku AFC10 Saturn 724+ diffractometer with graphite-mono-
(2.2787(9) Å) located at a trans position to N(2) being slightly
shorter than the Cr(1)–Cl(1) (2.3321(9) Å) and Cr(1)–Cl(3)
(2.3157(9) Å) bonds in the cis-position. A similar phenomenon
was observed with the reported bis(imino)pyridine chromium
complexes [30,31,52]. The imino N(3)–C(13) bond length is
1.291(4) Å with the typical character of a C@N double bond. The
chromated Mo Ka radiation (k = 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 structures were solved by di-
rect methods and refined by full-matrix least squares on F2. All
hydrogen atoms except of H4N (bond to N4) were placed in calcu-
H
N
H
N
R1
N
R1
N
CrCl3(THF)3
CH2Cl2
N
N
N
N
Cr
Cl
R1
Cl
R2
Cl
R2
R1
C1 C2 C3 C4 C5 C6
L1_ L6
R1
R2
Me Et i-Pr Me Cl Br
H
H
H
Me
H
H
Scheme 2. Preparation of chromium complexes C1–C6.

L. Xiao et al. / Polyhedron 29 (2010) 142–147
145
organoaluminium compounds such as methylaluminoxane
(MAO), modified methylaluminoxane (MMAO) or diethylalumini-
um chloride (Et2AlCl), it showed different catalytic performance
and the results were collected in Table 2. At ambient pressure,
the catalytic system activated with Et2AlCl only produced butenes
in low activity (Entry 1, Table 2). Treatment of complex C1 with
MAO catalyzed ethylene to obtain both oligomers and polymers
(Entry 2, Table 2). The activities for both oligomerization and poly-
merization were further increased when MMAO was employed
(Entry 3, Table 2).
3.3.2. Effect of reaction parameters
The influences of the Al/Cr molar ratio and reaction temperature
on ethylene reactivity were studied with the C1/MMAO system
and the results are listed in Table 3. As shown in Table 3, increasing
the Al/Cr molar ratio from 500 to 1500 led to great enhancement of
both ethylene oligomerization and polymerization activities (En-
tries 1–3, Table 3). A further increase in the Al/Cr molar ratio to
2000 resulted in decreased oligomerization and polymerization
activity (Entry 4, Table 3). However, the distribution of oligomers
Fig. 1. ORTEP drawing complex C1 with thermal ellipsoids at the 50% probability.
Hydrogen atoms and solvent (DMF) have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): Cr(1)–N(1) = 2.052(3), Cr(1)–N(2) = 2.016(3), Cr(1)–
N(3) = 2.154(2), Cr(1)–Cl(1) = 2.3321(9), Cr(1)–Cl(2) = 2.2787(9), Cr(1)–Cl(3) =
2.3157(9), N(1)–C(1) = 1.385(4), N(1)–C(7) = 1.329(4), N(2)–C(8) = 1.343(4),
N(2)–C(12) = 1.346(4), N(3)–C(13) = 1.291(4), N(3)–C(15) = 1.462(4); N(1)–Cr(1)–
N(2) = 78.28(1), N(1)–Cr(1)–N(3) = 154.61(1), N(2)–Cr(1)–N(3) = 76.39(1), N(1)–
Cr(1)–Cl(1) = 86.62(8), N(2)–Cr(1)–Cl(1) = 86.52(7), N(3)–Cr(1)–Cl(1) = 93.35(7),
and the selectivity for
the Al/Cr ratio. At the ratio of Al/Cr 1500, both the productivity
and the selectivity for -olefins reached satisfied results. Therefore,
a-olefins were not significantly affected by
Cl(1)–Cr(1)–Cl(2) = 92.07(3),
N(1)–Cr(1)–Cl(2) = 100.85(8),
N(2)–Cr(1)–Cl(2) =
178.38(8), N(3)–Cr(1)–Cl(2) = 104.52(7), N(2)–Cr(1)–Cl(3) = 88.98(7), N(1)–Cr(1)–
Cl(3) = 87.67(8), N(3)–Cr(1)–Cl(3) = 90.35(7), Cl(2)–Cr(1)–Cl(3) = 92.35(3), and
Cl(3)–Cr(1)–Cl(1) = 173.34(4).
a
the further experiments were performed with the Al/Cr molar ratio
of 1500. It was noteworthy that the oligomer distribution showed
an unprecedented small amount of C6, which makes the distribu-
tion of oligomers doesn’t follow the Schulz-Flory or Poisson rules.
This is similar to the reported observation [61], in which the chro-
mium system showed much lower activity towards ethylene
trimerization.
The catalytic system of C1 with 1500 equiv. of MMAO under
1 atm ethylene was further investigated under different reaction
temperatures. As shown in Table 3, it can be found that the activ-
ities of both oligomerization and polymerization decreased sharply
with the increase of the temperature from 20 to 60 °C, which can
be explained by the decomposition of some active species and low-
er ethylene solubility at higher temperature. In addition, elevation
of the temperature from 20 to 60 °C led to a slightly decreased
atoms of the pyridine ring and C(13), N(3) of the imino group as
well as the chromium atom form a plane. The mean deviation is
0.028 Å with the largest deviation from the plane being 0.0628 Å
at Cr(1). The phenyl ring linking to the imino group is oriented
nearly perpendicularly to the coordination plane with a dihedral
angle of 98.4°.
3.3. Ethylene oligomerization and polymerization
3.3.1. The selection of cocatalysts
Complex C1 was used as model catalyst to select suitable reac-
tion parameters. When the catalyst was combined with various
selectivity for
uted to the faster isomerization of
at elevated temperature, and internal olefins are generally more
stable than -olefins. A similar influence of temperature on the
a
-olefins from 97.5% to 88.7%. This may be attrib-
a-olefins into internal olefins
Table 2
Effect of cocatalysts on ethylene reactivitya.
a
oligomers produced with chromium-based systems has been re-
ported [55,60,61].
Entry
Cocatalyst
Al/Cr
Oligomer
Polymer
Activityc
Distributionb
Activityc
1
2
3
Et2AlCl
MAO
MMAO
200
1000
1000
C4–C6
C4–C32
C4–C32
5.0
3.5
8.4
no
12.5
17.5
3.3.3. Effect of the ligand environment
The alkyl or halogen substituents (R1) on the imino-N aryl ring
had great influence on catalytic performance. For complexes
C1–C3, somewhat reduced catalytic activity was observed for the
sterically bulkier catalysts. This was clearly revealed by comparing
the 2,6-diisopropyl substituted C3 with 2,6-dimethyl substituted
a
Reaction conditions: 5
l
mol C1; 1 atm ethylene; 30 min; and 30 mL toluene.
Oligomers distribution determined by GC.
In units of 104 g molÀ1 (Cr) hÀ1
b
c
.
Table 3
Selection the optimum condition with C1/MMAO at 1 atm ethylenea.
Entry
Al/Cr
T (°C)
Oligomers distributionb
Polyethylene
Tm (°C)d
c
c
C4/R
C
C6/R
C
C8/R
C
PC
10/R
C
a-Olefin%
Ao
Ap
1
2
3
4
5
6
500
1000
1500
2000
1500
1500
20
20
20
20
40
60
17.8
16.9
19.0
22.8
19.7
35.3
4.8
4.9
6.8
8.4
6.5
6.4
11.5
6.2
10.9
5.4
4.5
5.2
65.7
72.0
63.3
62.3
69.3
43.1
92.8
91.4
97.5
91.6
92.3
88.7
4.5
7.2
10.6
8.2
7.1
5.0
8.1
16.7
20.0
12.7
8.3
122.3
122.4
125.0
119.7
122.4
124.3
5.1
a
Reaction conditions: 5 lmol C1; MMAO as cocatalyst, 1 atm ethylene; and 30 mL of toluene, 30 min.
b
c
Oligomers distribution determined by GC.
In units of 104 g molÀ1 (Cr) hÀ1
Determined by DSC.
.
d

146
L. Xiao et al. / Polyhedron 29 (2010) 142–147
Table 4
Polymerization and oligomerization with C1–C6/MMAO at 1 atm ethylenea.
Entry
Catalyst
Oligomer distributionb
Polyethylene
Tm (°C)d
c
c
C4/R
C
C6/R
C
C8/R
C
PC
10/R
C
a-Olefin%
Ao
Ap
1
2
3
4
5
6
C1
C2
C3
C4
C5
C6
19.0
17.3
22.6
13.8
23.9
21.8
6.8
7.3
7.0
7.8
8.9
6.6
10.9
7.9
9.8
12.7
10.7
9.3
63.3
67.5
60.6
65.7
56.5
60.3
97.5
95.4
95.5
95.4
96.1
94.5
10.8
6.5
5.8
10.6
18.6
13.9
20.0
14.7
11.4
26.2
23.7
20.6
125.0
124.6
127.6
119.3
116.8
122.3
a
Reaction conditions: 5 lmol Cr complex; 1 atm ethylene, the ratio of MMAO to chromium complex is 1500, 30 mL toluene, 30 min, 20 °C.
b
c
Oligomers distribution determined by GC.
In units of 104 g molÀ1 (Cr) hÀ1
Determined by DSC.
.
d
C1 or 2,6-diethyl substituted C2 (Entry 3 versus Entry 1 or 2,
Table 4). This can perhaps be attributed to the fact that more bulky
isopropyl groups at ortho-positions of the imino-N aryl ring on
imino-C may prevent the insertion of ethylene to catalytic sites,
therefore leading to lower catalytic activity. Complexes C5 and
C6, containing 2,6-dihalogen substituted ligands, exhibited rela-
tively higher oligomerization activities than complexes C1–C4
bearing alkyl groups (compare Entry 5 and 6 with Entries 1–4,
Table 4). The improved catalytic activity can be ascribed to the more
electrophilic nature of the chromium center after the introduction
of the electron-withdrawing substituents on the ortho-positions of
phenyl group. The result is similar to our previous work [61].
As shown in Table 4, the Tm peaks of the resultant polymers ob-
tained at ambient pressure ranged from 116.8 to 127.6 °C, which
were lower than those catalyzed by 2-quinoxalinyl-6-iminopyri-
dine chromium complexes [60], which can be mainly ascribed to
the lower molecular weights resulted from 2-benzimidazole-6-
iminopyridine chromium complexes. Furthermore, the high con-
behavior at ambient pressure (Table 4), the higher catalytic activi-
ties along with longer chain oligomers up to the range of C4–C36
could be obtained with increased ethylene pressure. The catalytic
activities for both oligomerization and polymerization increased
by nearly one order of magnitude at 10 atm ethylene.
The ligand environment has exerted similar influences on cata-
lytic behaviors as those found at ambient pressure. For example, for
complexes C1–C3 bearing alkyl-substituted (Entries 1–3, Table 5),
more bulky substituents at the ortho-positions of the imino-N
aryl ring led to decrease of activity. Similar with that of result
found at ambient pressure, 2,6-dihalogensubstituted complex C5
and C6 showed higher productivity than the 2,6-dialkylsubstituted
analogues C1, C2 and C3 (Entries 5 and 6 vs Entries 1–3, Table 5) at
10 atm ethylene pressure.
At 10 atm ethylene, the Tm peak of the resultant polymers ob-
tained at 10 atm ranged from 118.4 to 126.2 °C and the effects of
the ligand environments on the Tm values were in accorded with
the results at ambient pressure, which indicates the increased eth-
ylene pressure didn’t show obvious influence on the thermal prop-
erties of PEs obtained (compare Table 4 with Table 5).
It is notable that this series of chromium complexes containing
N–H bond on benzimidazole group showed higher activities than
those of N–iPr analogues [61]. For instance, under 1 atm ethylene,
complexes C1 containing N–H bond showed combined catalytic
productivity up to 3.06 Â 105 g molÀ1 (Cr) hÀ1 (Entry 1:
1.06 Â 105 g molÀ1 (Cr) hÀ1 and 2.00 Â 105 g molÀ1 (Cr) hÀ1 for
oligomerization activity and polymerization activity, respectively,
Table 4), which is higher than that of its analogue containing iso-
propyl substituted [61] (5.1 Â 104 g molÀ1 (Cr) hÀ1, Entry 3, Table 3
in Ref. [61]). Tentatively, these results suggest that N–H function-
ality is essential for high activity and selectivity with this ligand
system, which could be caused by their deprotonation to give an-
ionic amide ligands when activated by MMAO. The anionic amide
ligands could be free or form N–Al species (anion–cation pair) to
increase their catalytic activity. These results are in agreement
with previous literatures [42,55].
tents of
a-olefins of oligomers obtained partly indicate that the
PEs produced may terminate with C@C end group. Due to both
oligomers and PEs formed in catalytic system, the GPC measure-
ments for molecular weights and molecular weight distributions
of PEs might be not meaningful. It would be next challenge to find
suitable reaction conditions for either oligomers or PE in the cata-
lytic system. Regarding to molecular weights of resultant PEs, it is
common to accept higher molecular weights corresponded to high-
er melting points. The PEs obtained by the complexes (C1–C3) con-
taining 2,6-dialkyl-substituted possessed higher Tm than those by
the complexes (C5 and C6) containin 2,6-dihalogen-substituted.
3.3.4. Ethylene oligomerization and polymerization at 10 atm ethylene
The results of catalytic behavior employing C1–C6 under 10 atm
ethylene pressure are summarized in Table 5. The data indicated
that the ethylene pressure significantly affected the catalytic
behavior of all complexes including catalytic activities, oligomer
distributions and PE properties. Comparing with the catalytic
Table 5
Polymerization and oligomerization with C1–C6/MMAO at 10 atm ethylenea.
Entry
Catalyst
Oligomer distributionb
Polyethylene
Tm (°C)d
c
c
C4/R
C
C6/R
C
C8/R
C
PC
10/R
C
a-Olefin%
Ao
Ap
1
2
3
4
5
6
C1
C2
C3
C4
C5
C6
18.7
17.8
21.1
12.7
17.1
22.4
7.9
6.9
13.2
6.7
9.4
8.2
10.6
9.5
11.1
10.2
7.5
62.8
65.8
54.6
73.9
66.0
56.9
95.3
97.7
95.7
96.5
96.9
96.1
9.2
7.1
6.8
7.8
15.0
12.5
16.1
12.1
8.6
20.6
20.1
14.5
120.2
121.1
126.2
122.7
118.4
123.8
12.5
a
Reaction conditions: 5 lmol cat., 10 atm ethylene, Al/Cr = 1500, 100 mL toluene, 30 min, 20 °C.
b
c
Oligomers distribution determined by GC.
In units of 105 g molÀ1 (Cr) hÀ1
Determined by DSC.
.
d

L. Xiao et al. / Polyhedron 29 (2010) 142–147
147
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Supplementary data
CCDC 729547 contains the supplementary crystallographic data
for C1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgement
This work was supported by NSFC No. 20674089.
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