Iron(III) complexes bearing 2-(benzimidazole)-6-(1-aryliminoethyl)pyridines: Synthesis, characterization and their catalytic behaviors towards ethylene oligomerization and polymerization

Article abstract of DOI:10.1016/j.jorganchem.2009.09.032

A series of iron(III) complexes ligated by 2-(benzimidazole)-6-(1-aryliminoethyl)pyridines was synthesized and examined by ¹H NMR, ESI-MS, IR spectroscopic, elemental analysis and X-ray photoelectron spectroscopy (XPS). Activated with methylaluminoxane (MAO), all ferric complexes exhibited good activities (up to 5.38 × 10⁶ g mol^-1(Fe) h^-1) of ethylene oligomerization and polymerization, and resultant oligomers and polyethylene waxes showed high α-olefin feature, meanwhile the distribution of oligomers mostly resembled Schulz-Flory rules. The various reaction parameters were investigated in detail, and the less bulky and electron-withdrawing substituents of ligands could enhance the catalytic activities of their ferric complexes. The observations explain the cause for unstable activities performed by stored iron(II) complexes.

Full text of DOI:10.1016/j.jorganchem.2009.09.032

Journal of Organometallic Chemistry 695 (2010) 90–95
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
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Iron(III) complexes bearing 2-(benzimidazole)-6-(1-aryliminoethyl)pyridines:
Synthesis, characterization and their catalytic behaviors towards ethylene
oligomerization and polymerization
Peng Hao a, Yanjun Chen a, Tianpengfei Xiao a, Wen-Hua Sun a,b,
*
a Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, 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 iron(III) complexes ligated by 2-(benzimidazole)-6-(1-aryliminoethyl)pyridines was synthe-
sized and examined by 1H NMR, ESI-MS, IR spectroscopic, elemental analysis and X-ray photoelectron
spectroscopy (XPS). Activated with methylaluminoxane (MAO), all ferric complexes exhibited good activ-
ities (up to 5.38 Â 106 g molÀ1(Fe) hÀ1) of ethylene oligomerization and polymerization, and resultant
Received 8 April 2009
Received in revised form 20 September
2009
Accepted 25 September 2009
Available online 3 October 2009
oligomers and polyethylene waxes showed high
a-olefin feature, meanwhile the distribution of oligo-
mers mostly resembled Schulz–Flory rules. The various reaction parameters were investigated in detail,
and the less bulky and electron-withdrawing substituents of ligands could enhance the catalytic activities
of their ferric complexes. The observations explain the cause for unstable activities performed by stored
iron(II) complexes.
Keywords:
Ferric complex
2-(Benzimidazole)-6-(1-
aryliminoethyl)pyridine
Ethylene oligomerization and
polymerization
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
1,10-phenanthroline [40–42], 2-benzimidazole-6-iminopyridines
[43–45], 6-(quinoxalin-2-yl)-2-iminopyridines [46], 2-benzoxazol-
Ethylene oligomerization is a major industrial process for pro-
ducing linear -olefins in the range of C6–C20 with about six mil-
yl-6-[1-(arylimino)ethyl]pyridines [47], 2-methyl-2,4-bis(6-imino-
pyridin-2-yl)-1H-1,5-benzodiazepines [48,49], 2-benzimidazole-
1,10-phenanthrolines [50], 2-oxazoline/benzoxazole-1,10-phen-
anthrolines [51] and N-((pyridin-2-yl)methylene)-quinolin-8-
amine derivatives [52] provided alternative models performing
high catalytic activities [53]. Despite great interest in designing
new iron(II) based catalysts, unstable catalytic activities of stored
iron(II) catalysts were often observed, however, commonly within
one-order or less than ten times. The part oxidation of ferrous into
ferric was imaged, and the nature of the active species as iron(III)
or iron(II) were not settled [54–57]. Recently ferric pre-catalysts li-
gated by bis(imino)pyridines have generated from iron(II) precur-
sors and shown the catalytic behavior towards ethylene
oligomerization [17,21,58,59]. The detail investigation of ferric
complexes will extend novel catalytic model for ethylene oligo-
merization and polymerization and explain the cause in various
activities of stored iron(II) catalysts.
In this paper, a series of ferric complexes based on 2-(benzimid-
azole)-6-(1-aryliminoethyl)pyridines was synthesized and fully
investigated for their catalytic behaviors in ethylene reactivity.
Good catalytic activities toward ethylene oligomerization and
polymerization could be observed in the presence of methylalumi-
noxane (MAO), which exhibited similar trend in catalytic behaviors
but a little lower reactivity than that of their ferrous analogues
a
lions of tons productivities annually, which provide basic
feedstocks in the preparation of detergents, plasticizers and copo-
lymerization with ethylene for the linear low-density polyethylene
(LLDPE) [1–3]. The iron and cobalt catalysts showed unique prop-
erties for the high selectivity of vinyl-type oligomers and polyole-
fins (polyolefin waxes). The pioneering work was reported by
Gibson [4] and Brookhart groups [5] employing bis(imino)pyridyl
iron and cobalt complexes as highly active catalyst for ethylene
polymerization and oligomerization [6–17]. Extensive research
on bis(imino)pyridyl iron complexes has been focused on under-
standing the active species or intermediates [18–25], controlling
the products of polymers or oligomers as well as improving the
catalytic activities through adapting steric and electronic charac-
teristics of the complexes [26–33]. Apart from ferrous catalysts
ligated by bis(imino)pyridines, a few models of iron catalysts were
reported [34–39], but the iron complexes bearing 2-imino-
* Corresponding author. Address: Key Laboratory of Engineering Plastics and
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, No. 2, Beiyijie, Zhongguancun, Haidian District, Beijing
100190, China. Tel.: +86 10 62557955; fax: +86 10 62618239.
E-mail address: whsun@iccas.ac.cn (W.-H. Sun).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.09.032

P. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 90–95
91
[43]. The catalytic reaction parameters and the steric and elec-
tronic effects of ligands were investigated in detail on the influence
of their catalytic activities.
C25H26Cl3N4Fe: C, 55.12; H, 4.81; N, 10.29. Found: C, 55.23; H,
5.02; N, 10.02%.
2.2.1.3. L3FeCl3. Obtained as yellow powder in 79% yield. 1H NMR
(300 MHz, DMSO-d6): d 8.52 (1H, –Py), 8.41 (1H, –Py), 8.21 (1H,
–Ph), 7.74 (m, 2H), 7.35 (2H, –Ph), 7.20 (2H, –Ph), 7.10 (1H, –Ph),
4.35 (3H, –CH3), 2.27 (3H, –CH3), 2.10 (2H, –CH), 1.14 (12H,
–CH3). ESI-MS: 537.1 (MÀClÀ), 411.5 (MÀFeCl3+H+). IR (KBr;
cmÀ1): 3423, 2964, 1594 (vC@N); 1492; 1463; 1423; 1402; 1369;
1330; 1307; 1284; 1258; 1197; 1105; 1025; 792; 753. Anal. Calc.
for C27H30Cl3N4Fe: C, 56.62; H, 5.28; N, 9.78. Found: C, 56.86; H,
5.44; N, 9.58%.
2. Experimental
2.1. General considerations
All manipulations of air- and moisture-sensitive compounds
were performed under a nitrogen atmosphere using standard
Schlenk techniques. All the organic compounds used as ligands for
the ferric complexes were prepared by employing our developed
procedure [43,60]. Toluene was refluxed over sodium-benzophe-
none and distilled under argon prior to use. Methylaluminoxane
(MAO, a 1.46 M solution in toluene) and modified methylaluminox-
ane (MMAO, 1.93 M in heptane) were purchased from Akzo Nobel
Corp., diethylaluminum chloride (Et2AlCl, 1.7 M in hexane) was pur-
chased from Acros Chemicals. Other reagents were purchased from
Aldrich or Acros Chemicals. 1H NMR spectra were recorded on JOEL
300 NMR spectrometer with TMS as internal standard. IR spectra
were recorded on a Perkin–Elmer System 2000 FT-IR spectrometer.
Elemental analysis was carried out using a Flash EA 1112 microana-
lyzer. Mass spectra were obtained using a Bruker Esquire ion trap
mass spectrometer in positive ion mode. X-ray photoelectron spec-
troscopy (XPS) data were obtained with an ESCALab220i-XL electron
2.2.1.4. L4FeCl3. Obtained as brown powder in 69% yield. 1H NMR
(300 MHz, DMSO-d6): d 8.57 (1H, –Py), 8.37 (1H, –Py), 8.24 (1H,
–Ph), 7.74 (m, 2H), 7.29 (m, 4H), 6.87 (1H, –Ph), 4.35 (3H, –CH3),
2.54 (3H, –CH3). ESI-MS: 487.9 (MÀClÀ), 363.4 (MÀFeCl3+H+). IR
(KBr; cmÀ1): 3406, 3048, 1593 (vC@N); 1491; 1424; 1405; 1368;
1308; 1284; 1254; 1183; 1154; 1094; 1025; 873; 782; 749. Anal.
Calc. for C21H16Cl3F2N4Fe: C, 48.08; H, 3.07; N, 10.68. Found: C,
47.93; H, 3.33; N, 10.37%.
2.2.1.5. L5FeCl3. Obtained as brown powder in 71% yield. 1H NMR
(300 MHz, DMSO-d6): d 8.56 (1H, –Py), 8.42 (1H, –Py), 8.24 (1H,
–Ph), 7.65 (m, 4H), 7.37 (m, 2H), 7.20 (1H, –Ph), 4.35 (3H, –CH3),
2.40 (3H, –CH3). ESI-MS: 397.2 (MÀFeCl3+H+), 418.5
(MÀFeCl3+Na+). IR (KBr; cmÀ1): 3432, 3069, 1596 (vC@N); 1483;
1433; 1370; 1306; 1254; 1226; 1188; 1158; 1024; 931; 802;
783; 745. Anal. Calc. for C21H16Cl5FeN4: C, 45.24; H, 2.89; N,
10.05. Found: C, 45.10; H, 2.99; N, 10.03%.
spectrometer from VG Scientific using 300 W Al Ka radiation. The
base pressure was about 3 Â 10À9 mbar. The binding energies were
referenced to the C1s line at 284.8 eV from adventitious carbon. GC
analysis was performed with a Carlo Erba Strumentazione gas chro-
matograph equipped with a flame ionization detector and a 30 m
(0.25 mm i.d., 0.25 lm film thickness) DM-1 silica capillary column.
2.2.1.6. L6FeCl3. Obtained as yellow powder in 73% yield. 1H NMR
(300 MHz, DMSO-d6): d 8.57 (1H, –Py), 8.45 (1H, –Py), 8.24 (1H,
–Ph), 7.76 (m, 4H), 7.36 (m, 2H), 7.07 (1H, –Ph), 4.36 (3H, –CH3),
2.35 (3H, –CH3). ESI-MS: 485.1 (MÀFeCl3+H+), 507.1
(MÀFeCl3+Na+). IR (KBr; cmÀ1): 3425, 3067, 1594 (vC@N); 1492;
1429; 1368; 1282; 1259; 1228; 1186; 1160; 1025; 928; 830;
779; 751. Anal. Calc. for C21H16Cl3Br2FeN4: C, 39.02; H, 2.49; N,
8.67. Found: C, 39.13; H, 2.67; N, 8.31%.
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.
2.2. Synthesis of title iron(III) complexes
2.2.1. General procedure
2.3. Procedure for oligomerization and polymerization with 1 atm of
ethylene
A solution of the ligand in THF was added dropwise to an equi-
molar amount of FeCl3 in THF solution. Immediately, the color of
the solution changed and some precipitate formed. The reaction
mixture was stirred at room temperature for 8 h then diluted with
diethyl ether. The resulting precipitate was collected, washed with
diethyl ether and dried in vacuum. All of the complexes were pre-
pared in high yield in this manner. Data for these complexes are as
follows.
Ethylene oligomerization and polymerization were carried out
as follows: the catalyst precursor (Fe(III) complex) was dissolved
in toluene in a 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 add-
ing 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% hydrochloric
acid in an ice-water bath in accordance with the oligomers pro-
duced. An analysis by gas chromatography (GC) was carried out
to determine the distribution of oligomers obtained. The remaining
solution was quenched with hydrochloric acid in ethanol (5%), and
the precipitated polyethylene was collected by filtration, washed
with ethanol, dried under vacuum at 60 °C to constant weight,
weighed and finally characterized.
2.2.1.1. L1FeCl3. Obtained as brown powder in 82% yield. 1H NMR
(300 MHz, DMSO-d6): d 8.50 (1H, –Py), 8.43 (1H, –Py), 8.21 (1H,
–Ph), 7.76 (m, 2H), 7.53 (2H, –Ph), 7.12 (2H, –Ph), 6.96 (1H, –Ph),
4.34 (3H, –CH3), 2.23 (3H, –CH3), 2.01 (s, 6H, –CH3). ESI-MS:
481.0 (MÀClÀ), 355.3 (MÀFeCl3+H+). IR (KBr; cmÀ1): 2968, 1594
(vC@N); 1489; 1403; 1368; 1332; 1305; 1282; 1256; 1205; 1156;
1100; 793; 750. Anal. Calc. for C23H22Cl3N4Fe: C, 53.47; H, 4.29;
N, 10.84. Found: C, 53.86; H, 4.68; N, 10.34%.
2.2.1.2. L2FeCl3. Obtained as yellow powder in 79% yield. 1H NMR
(300 MHz, DMSO-d6): d 8.51 (1H, –Py), 8.42 (1H, –Py), 8.20 (1H,
–Ph), 7.74 (m, 2H), 7.34 (2H, –Ph), 7.14 (2H, –Ph), 7.03 (1H, –Ph),
4.34 (3H, –CH3), 2.35 (3H, –CH3), 2.25 (4H, –CH2), 1.10 (6H,
–CH3). ESI-MS: 383.4 (MÀFeCl3+H+), 421.2 (MÀFeCl3+K+). IR (KBr;
cmÀ1): 3428, 2964, 1593 (vC@N); 1491; 1455; 1403; 1371; 1332;
1283; 1258; 1200; 1160; 1091; 1025; 791; 749. Anal. Calc. for
2.4. Procedure for oligomerization and polymerization at higher
ethylene pressure
Ethylene oligomerization and polymerization were performed
in a 250-mL autoclave stainless steel reactor equipped with a
mechanical stirrer and a temperature controller. A 100 mL amount
of toluene containing the catalyst precursor was transferred to the

92
P. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 90–95
Table 1
fully dried reactor under an ethylene atmosphere. The required
amount of cocatalyst (MAO, MMAO or Et2AlCl) was then injected
into the reactor via a syringe. At the reaction temperature, the reac-
tor was sealed and pressurized to high ethylene pressure, and the
ethylene pressure was maintained with feeding of ethylene. After
the reaction mixture was stirred for the desired period, the pressure
was released and a small amount of the reaction solution was col-
lected, which was then analyzed by gas chromatography (GC) for
determining the composition and mass distribution of oligomers
obtained. Then the residual reaction solution was quenched with
5% hydrochloric acid in ethanol. The precipitated low-molecular-
weight waxes were collected by filtration, washed with ethanol
and water and dried under vacuum to constant weight.
Fe 2p3/2 binding energies from XPS analysis.
Ligand
Fe 2p3/2 (eV)
Fe(III) complexes
Fe(II) complexes
L1
L2
L3
L4
L5
711.1
711.1
711.2
711.2
710.6
710.6
710.6
710.8
708.7, 710.8
710.2
3.2. Ethylene activation behaviors of the N^N^N coordinated iron(III)
complexes
3. Results and discussion
3.2.1. Cocatalysts selection
The influences of various cocatalysts on the ethylene activation
were evaluated with the catalytic systems formed from L1FeCl3 at
10 atm of ethylene pressure in the presence of methylaluminoxane
(MAO), modified methylaluminoxane (MMAO) or diethylalumi-
num chloride (Et2AlCl) as cocatalyst. The results summarized in
Table 2 indicated that L1FeCl3/MAO system showed remarkable
catalytic activity in the order of 105 g molÀ1(Fe) hÀ1, which was
nearly 13 times and 30 times higher than that of L1FeCl3/MMAO
system and L1FeCl3/Et2AlCl system, the distribution of oligomers
3.1. Synthesis and characterization of the iron(III) complexes
containing 2-(1-methyl-2-benzimidazole)-6-(1-
aryliminoethyl)pyridines
The 2-(1-methyl-2-benzimidazole)-6-(1-aryliminoethyl)pyri-
dines (L1–L6) were prepared according to our reported procedures
[43,60]. The iron(III) complexes (L1FeCl3–L6FeCl3) were obtained by
treating the THF solution of FeCl3 with an equimolar corresponding
ligand (L1–L6) at room temperature in satisfying yields (69–82%)
(Scheme 1). These complexes showed high stability in both solu-
tion and solid state, such behaviors were different from their iro-
n(II) complexes [43,60]. These iron(III) complexes were identified
by 1H NMR, ESI-MS, FT-IR and elemental analysis. From 1H NMR,
the chemical shift to a higher field for the pyridyl ring was ob-
served because of the coordination with iron center. The IR spectra
of the ligands L1–L6 gave the C@Nimino stretching frequencies in the
range of 1639–1650 cmÀ1 [43,60], while the C@Nimino stretching
vibrations of the complexes L1FeCl3–L6FeCl3 shifted toward lower
frequencies between 1593 and 1596 cmÀ1 with weak intensities
which indicates the effective coordination between the imino
nitrogen atom and the iron center. It has been tried hard in getting
the single crystals of those iron(III) complexes in different solution
parameters for their absolute molecular structures, however, likely
crystals are not suitable for X-ray diffraction and the differences of
both ionic radius and electronic effects of ferrous and ferric cores
would be considered. Further investigation for direct molecular
structures of ferric complexes is still worthy to study.
was ranged from C4 to C28 with high a-olefin selectivity.
3.2.2. Ethylene oligomerization at ambient pressure
L1FeCl3 and L3FeCl3 were investigated for their ethylene cata-
lytic behaviors at ambient pressure and the results were collected
in Table 3. Under different conditions, the oligomer products were
obtained in moderate catalytic activity with high a-olefin selectiv-
ity. The amount of cocatalyst was found to have significant influ-
ences on the catalytic behaviors of L1FeCl3/MAO system. When
the Al/Fe molar ratio was varied from 300 to 1500, the catalytic
activities initially increased and then decreased (entries 1–4 in Ta-
ble 3). Notably, the catalytic activity increased sharply from the Al/
Fe molar ratio of 300–500, which may be attributed that MAO
scavenged adventitious water and impurities in the solvent at
the Al/Fe ratio of 300 and the iron catalysts require more cocatalyst
to be active. At the Al/Fe ratio of 500, the catalytic activity was at a
peak (entries 2 in Table 3). Increase the Al/Fe molar ratio to 1000
and 1500 led to lower activity (entries 3–4 in Table 3). This obser-
vation could be traced to the impurities in commercial MAO such
as alkyl aluminum, which led to the deactivation of active catalytic
sites [48]. If the reaction temperature was elevated to 40 °C, it re-
sulted in a sharp decrease of activity (entry 5 in Table 3), which is
possible due to the decomposition of active species and lower eth-
ylene solubility at higher temperature [7,21], but the oligomer dis-
tribution was not changed evidently.
In order to confirm the oxidation state of Fe(III) core in the title
complexes, XPS experiments for the title Fe(III) complexes and
their iron(II) analogues are carried out to observe their binding
energies of Fe 2p3/2 (Table 1). According to the results, the obvious
differences indicate the difference between the Fe(III) complexes
and their Fe(II) analogues. The Fe(III) complexes clearly show the
characteristic peaks coinciding with literature data of Fe(III) [61].
In addition, the significant difference for Fe(II) complex bearing
6-difluoro-N-(1-(6-(1-methyl-1H-benzo[d]imidazol-2-yl)pyridin-2-
yl)ethylidene)benzenamine (L4) is caused by different coordination
3.2.3. Ethylene oligomerization at 10 atm of ethylene
3.2.3.1. Effect of ethylene pressure. Ethylene activation with iron(III)
catalysts were conducted at 10 atm of ethylene as shown in Table 4.
mode in resulting its molecular structure as L4FeÁFeCl4 [60].
2
N
N
R
N
R
N
FeCl3
THF
N
R
N
N
R
N
Fe
Cl
Cl
Cl
L1: R = Me; L2: R = Et; L3: R = i-Pr
L1FeCl3; L2FeCl3; L3FeCl3
L4: R = F; L5: R = Cl; L6: R = Br
L4FeCl3; L5FeCl3; L6FeCl3
Scheme 1. Synthesis of iron(III) complexes with 2-(1-methyl-2-benzimidazole)-6-(1-aryliminoethyl)pyridines.

P. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 90–95
93
L1FeCl3 and L4FeCl3, and the results were collected in Table 4. For
complex L1FeCl3, when the Al/Fe molar ratio was enhanced from
100 to 1000, the oligomerization activity initially increased and
then decreased (entries 1–5 in Table 4), the highest activity was
obtained at Al/Fe = 500 (entry 3 in Table 4). For the Al/Fe molar ra-
tio was 100, the activity was very low and only 1-butene was pro-
duced (entry 1 in Table 4). The distribution of the oligomers was
nearly as a constant with different Al/Fe molar ratio from 300 to
1000, which could be observed from the K value (0.68–0.71) (en-
tries 2–5 in Table 4). The polymerization activity was changed with
the similar trend but got the peak value at Al/Fe = 300 (entry 2 in
Table 4). Similar to the effects of the molar ratio of Al/Fe on L1FeCl3,
L4FeCl3 showed the same trend and the activity was also peaked at
Al/Fe = 500 (entry 10 in Table 4). Differently, the K value was de-
creased gradually with the increase of cocatalyst molar ratio from
300 to 1000 probably due to the fast b-hydrogen elimination (en-
tries 9–12 in Table 4).
As the oligomerization and polymerization of ethylene are
highly exothermic reactions, the reaction temperature significantly
affects the catalytic activity. To understand such influence, the cat-
alytic system of L1FeCl3 with 500 eq. MAO at 10 atm of ethylene
was investigated with different reaction temperature (entries 3,
15 and 16 in Table 4). Elevation of the reaction temperature from
20 °C to 60 °C resulted in a sharp decrease of productivity sepa-
rated as oligomerization and polymerization activity, which can
be explained as catalyst decomposition and lower ethylene solubil-
ity at higher temperature [7,21]. Moreover, the oligomers distribu-
tion became narrower along with the decreased reactivity. The
proportion of C4 and other short-chain oligomers were greatly in-
creased at higher temperature because of the faster b-hydrogen
elimination than the rate of ethylene propagation and the distribu-
tion of obtained oligomers did not resemble Schulz–Flory rules
(entry 16 in Table 4). At 60 °C, only 1-butene and 1-hexene could
be obtained with very low activity (entry 16 in Table 4). This phe-
nomenon is in consistent with our previous observations [43].
Table 2
Selection of the suitable cocatalyst based on L1FeCl3.
Entry
Cocatalyst
Al/Fe
Oligomer activitya
Oligomer distribution
1
2
3
MAO
MMAO
Et2AlCl
1000
1000
500
21.8
1.64
0.75
C4–C28
C4–C28
C4
Condition: 5
lmol catalyst; 10 atm ethylene; 30 min; 20 °C; 100 mL toluene.
a
Unit of activity: 104 g molÀ1(Fe) hÀ1
.
Table 3
Oligomerization of ethylene with L1FeCl3, L3FeCl3/MAO at 1 atm.
Entry Catalyst Al/Fe
T
Activitya -olefinb Oligomer
a
(°C)
(%)
distribution (%)
C4/
R
C
C6/R
C
CP8/RC
1
2
3
4
5
6
L1FeCl3 300
L1FeCl3 500
20
20
2.78
8.37
5.19
5.01
3.62
4.08
>99
>99
>99
>99
>99
>99
77.0
60.2
40.6
77.2
55.5
73.6
4.9
8.4
20.2
13.0
5.5
18.1
31.4
39.2
9.8
39.0
17.7
L1FeCl3 1000 20
L1FeCl3 1500 20
L1FeCl3 500
L3FeCl3 500
40
20
8.7
Condition: 5
lmol catalyst; 1 atm ethylene; 30 min; 30 mL toluene.
a
Unit of activity: 103 g molÀ1(Fe) hÀ1
Percent a-olefin content determined by GC and GC–MS.
.
b
It is observed that the catalytic activities were dramatically im-
proved along with increasing ethylene pressure attributable to
the higher monomer concentration around active iron centers.
The product properties were also significantly affected. Much long-
er carbon chain oligomers and polymer wax could be obtained at
high ethylene pressure. Comparing with the catalytic behavior at
ambient pressure (Table 3), the activities were increased by nearly
two order of magnitude at 10 atm of ethylene and the distribution
of oligomers resembled Schulz–Flory rules, the chain propagation
is represented with the constant K, where K = rate of propaga-
tion/(rate of propagation + rate of chain transfer) = (moles of
3.2.3.3. Effect of the ligand environments. The natural catalytic
behaviors of these tridentate ferric complexes are rooted in the
characteristics of ligands due to their different substituents, which
greatly affected their catalytic behaviors. Fixed the reaction condi-
tion, complexes L1FeCl3–L6FeCl3 showed high activities in ethylene
oligomerization (entries 3, 6, 7, 10, 13, 14 in Table 4). Based on
these data, their oligomerization activity varied in the orders of
C
n + 2)/(moles of Cn), the K values are determined by the molar ratio
of C12 and C14 fractions [62–65].
3.2.3.2. Effects of the molar ratio of Al/Fe and reaction tempera-
ture. The influences of different molar ratio of MAO to iron(III)
complex on catalytic activities were investigated in detail with
Table 4
Oligomerization and polymerization of ethylene with L1FeCl3–L6FeCl3/MAO at 10 atm.
Entry
Catalyst
Al/Fe
T (°C)
K
a
-olefinb (%)
Activitya
Oligomer
Oligomer distribution (%)
Polymer
C4/R
C
C6/R
C
CP8/RC
1
2
3
4
5
6
7
8
L1FeCl3
L1FeCl3
L1FeCl3
L1FeCl3
L1FeCl3
L2FeCl3
L3FeCl3
L4FeCl3
L4FeCl3
L4FeCl3
L4FeCl3
L4FeCl3
L5FeCl3
L6FeCl3
L1FeCl3
L1FeCl3
100
300
500
700
1000
500
500
100
300
500
700
1000
500
500
500
500
20
20
20
20
20
20
20
20
20
20
20
20
20
20
40
60
–
>99
99
98
98
96
98
97
>99
99
99
99
99
99
99
89
>99
0.06
6.56
17.5
4.83
2.18
10.3
7.95
0.05
18.2
53.8
35.9
8.65
19.1
8.07
0.49
0.04
–
100
–
–
0.71
0.68
0.71
0.70
0.59
0.46
–
0.45
0.43
0.40
0.38
0.65
0.75
0.70
–
1.18
0.69
0.23
–
–
–
–
–
–
–
–
–
–
0.14
–
17.2
16.9
15.7
18.1
28.9
42.5
100
50.5
49.6
54.4
51.6
21.0
10.1
34.9
77.3
15.8
17.0
15.8
17.3
21.6
25.7
–
28.6
29.1
28.5
28.3
19.4
11.4
21.6
22.7
67.0
66.1
68.5
64.6
49.5
31.8
–
20.9
21.3
17.1
20.1
59.6
78.5
43.5
–
9
10
11
12
13
14
15
16
Condition: 5
l
mol catalyst; 30 min; 10 atm; 100 mL toluene.
a
Unit of activity: 105 g molÀ1(Fe) hÀ1
.
b
Percent a-olefin content determined by GC and GC–MS.

94
P. Hao et al. / Journal of Organometallic Chemistry 695 (2010) 90–95
45
40
35
30
25
20
15
10
5
highest catalytic activity was up to 5.38 Â 106 g molÀ1(Fe) hÀ1 in
producing
a-olefins (C4–C28) with high selectivity. These iron(III)
1FeCl3
complexes showed the same trend in catalytic behaviors in con-
trast to the relative iron(II) analogues [43]. The catalysts containing
less bulky substituents of ligands enhance ethylene insertion and
show better activity. In some cases, the polyethylene waxes have
been obtained and confirmed to be linear long-chain vinyl-end al-
kenes. On the base of current results, iron(III) and iron(II) com-
plexes are assumed to form the same active species for ethylene
oligomerization and polymerization.
L
2FeCl3
L
3FeCl3
L
Acknowledgment
This work was supported by NSFC No. 20674089.
0
References
C4 C6 C8 C10 C12 C14 C16 C18 C20 C22 C24 C26
Carbon number
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