Methylene bridged binuclear bis(imino)pyridyl iron(II) complexes and their use as catalysts together with Al(i-Bu)3 for ethylene polymerization

Article abstract of DOI:10.1016/j.ica.2007.09.039

Three novel methylene bridged binuclear iron(II) complexes: {[2, 6 -R₂ -C₆ H₃ N {double bond, long} C (CH₃) C₅ H₃ N (CH₃) C {double bond, long} N (3, 5 -R₂^′) C₆ H₂ -CH₂ - (3, 5 -R₂^′) C₆ H₂ N {double bond, long} C (CH₃) C₅ H₃ N (CH₃) C {double bond, long} N (2, 6 -R₂) C₆ H₃] [FeCl₂]₂(R,R′ = i-C₃H₇ (6); R = i-C₃H₇, R′ = CH₃ (7); R,R′ = CH₃ (8))} have been synthesized. Activated by Al(i-Bu)₃, complex 6 shows very poor activity for the polymerization of ethylene at one bar ethylene pressure, whereas, 7 and 8 exhibit much higher activity than mononuclear iron catalysts {[ArN{double bond, long}C(Me)C₅H₃N(Me)C{double bond, long}NAr′]FeCl₂ (Ar,Ar′ = 2,6-C₆H₃-i-Pr (9); Ar = 2,6-C₆H₃-i-Pr₂, Ar′ = 2,6-C₆H₃-Me₂ (10); Ar,Ar′ = 2,6-C₆H₃-Me₂ (11))}. The molecular weight (M_w) of PE produced by 7 and 8 are in the range 13.2-46.0 × 10⁴ and much higher than those produced by mononuclear iron catalysts 9 and 10. GPC results demonstrate that 7 and 8 yield PE with a broad/bimodal molecular weight distribution (MWD). In contrast, 9 and 10 yield PE with relatively narrow and unimodal MWD (4.26 and 3.55). Elevating the temperature and Al/Fe molar ratio will narrow the MWD of PE.

Full text of DOI:10.1016/j.ica.2007.09.039

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 1843–1849
www.elsevier.com/locate/ica
Methylene bridged binuclear bis(imino)pyridyl iron(II) complexes
and their use as catalysts together with Al(i-Bu)3
for ethylene polymerization
*
Lincai Wang, Junquan Sun
Department of Chemical Engineering, Zhejiang University, Hangzhou 310027, PR China
Received 14 May 2007; received in revised form 18 July 2007; accepted 27 September 2007
Available online 5 October 2007
Abstract
Three novel methylene bridged binuclear iron(II) complexes: f½2; 6-R2-C6H3N@CðCH3ÞC5H3NðCH3ÞC@Nð3; 5-R02Þ-
C6H2-CH2-ð3; 5-R02ÞC6H2N@CðCH3ÞC5H3NðCH3ÞC@Nð2; 6-R2ÞC6H3ꢀ½FeCl2ꢀ (R,R0 = i-C3H7 (6); R = i-C3H7, R0 = CH3 (7);
2
R,R0 = CH3 (8))} have been synthesized. Activated by Al(i-Bu)3, complex 6 shows very poor activity for the polymerization of ethylene
at one bar ethylene pressure, whereas, 7 and 8 exhibit much higher activity than mononuclear iron catalysts {[ArN@C(Me)C5H3N-
(Me)C@NAr0]FeCl2 (Ar,Ar0 = 2,6-C6H3-i-Pr (9); Ar = 2,6-C6H3-i-Pr2, Ar0 = 2,6-C6H3–Me2 (10); Ar,Ar0 = 2,6-C6H3–Me2 (11))}. The
molecular weight (Mw) of PE produced by 7 and 8 are in the range 13.2–46.0 · 104 and much higher than those produced by mononu-
clear iron catalysts 9 and 10. GPC results demonstrate that 7 and 8 yield PE with a broad/bimodal molecular weight distribution
(MWD). In contrast, 9 and 10 yield PE with relatively narrow and unimodal MWD (4.26 and 3.55). Elevating the temperature and
Al/Fe molar ratio will narrow the MWD of PE.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Iron; Binuclear catalysts; Bis(imino)pyridyl complexes; Bimodal polyethylene; Ethylene polymerization
1. Introduction
reason maybe that Al(i-Bu)3 does not force to chain trans-
fer reactions like other alkyaluminum compounds [8,9].
As well known, catalyst structure is a key factor to influ-
ence catalytic activity and polymer properties (such as Mw
and MWD). Recently, Li [10] reported on the macrocycle
trinuclear bis(imino)pyridyliron catalyst 12 (Chart 1) which
displays higher activity and produces much higher molecu-
lar weight PE than its analogue mononuclear iron catalyst
9 (Chart 1) in the presence of Al(i-Bu)3. But also in this
case, the MWD of PE obtained is still relatively narrow.
Considerable attention has lately been focused on binu-
clear organometallic compounds, based on expectations
that their catalytic behavior may significantly differ from
that of analogous mononuclear compounds [11–13]. For
binuclear catalysts, there maybe a cooperative effect
between the two metal centers. The cooperative effect
between closely adjacent metal centers could potentially
modify catalytic performance or provide alternative means
The production of polyethylene with high molecular
weight and broad or bimodal molecular weight distribution
has been of growing interest in recent years. Gibson and
Brookhart independently reported in 1998 on the use of
well-known bis(imino)pyridyl iron complexes as highly effi-
cient catalysts [1–3], which activated by methylaluminox-
ane (MAO) [3] and some alkyaluminum compounds [4–9]
produced low molecular weight PE with broad or bimodal
MWD due to chain transfer reactions to aluminum. How-
ever, only PE with relatively narrow and unimodal MWD
was obtained when Al(i-Bu)3 was used as a cocatalyst. The
*
Corresponding author. Tel.: +86 571 87953159; fax: +86 571
87951227.
E-mail address: sunjunquan@zju.edu.cn (J. Sun).
0020-1693/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.09.039

1844
L. Wang, J. Sun / Inorganica Chimica Acta 361 (2008) 1843–1849
Esquire-LC_00075 and FAB-MS with High Resolution
Mass Spectrometer (MAT95XP). The IR spectra were
obtained on Nicolet E.S.P.560 FT-IR spectrometer. Ele-
mental analyses were performed by Perkin Elmer240. The
DSC measurements on Tm of PE were performed on a
Q100 V9.5 Build 288 differential scanning calorimeter
(DSC) from 30 ꢁC to 160 ꢁC at a rate of 10 ꢁC/min. The
molecular weights (Mw) and the molecular weight distribu-
tions (MWD) of the polymer samples were determined at
150 ꢁC on a PL-220 type high-temperature chromatograph
equipped with three PLgel 10 lm Mixed-B LS type col-
umns. 1,2,4-Trichlorobenzene (TCB) was employed as the
solvent at a flow rate of 1.0 mL/min. The calibration was
made by polystyrene standard EasiCal PS-1 (PL Ltd).
The average viscosity molecular weight (Mg) was deter-
mined by the viscosity correlation method according to
equation [19]: ½gꢀ ¼ 6:67 ꢁ 10ꢂ4Mg0:67 ðml=gÞ.
N
Fe
N
N
Cl
Cl
R'
R
N
N
N
Fe
Cl
Cl
Cl
Cl
Cl
N
N
N
Fe
R
Cl
Fe
R'
N
N
N
9: R=R'= i-Pr
10: R=i-Pr, R'=Me
11: R=R'=Me
12
Chart 1. Mononuclear iron(II) catalysts 9–11 and macrocyclic trinuclear
bis(imino)pyridyliron catalyst 12.
for substrate activation. In our previous work, the research
on binuclear organometallic catalysts revealed that binu-
clear catalysts could broaden the MWD of the resulting
PE [14–16].
The aim of our work is to develop new binuclear iron
catalysts to obtain higher activity and yield higher molecu-
lar weight polyethylene with broad MWD. Here we report
on three novel binuclear methylene bridged bis(imino)pyr-
idyl iron(II) catalysts for ethylene polymerization with Al(i-
Bu)3 as cocatalyst. Except for 6/Al(i-Bu)3 system, these
binuclear catalysts enhanced the catalytic activity and the
molecular weight of PE compared with mononuclear iro-
n(II) catalysts. At the same time, the MWD of PE obtained
was also broadened. Effects of catalyst structure modifica-
tion and polymerization parameters on catalysis behaviors
were investigated in detail.
2.3. Preparation of methylene bridged bis(imino)pyridyl
ligands and complexes
2.3.1. Preparation of ligands 3–5
Monoimine 1 (1.5 g, 4.65 mmol) was dissolved in 30 mL
dry toluene, and p-toluenesulfonic acid (0.05 g) and 4,40-
methylene-bis(2,6-diisopropylaniline) (0.85 g, 2.32 mmol)
were added. The solution was stirred under reflux for
15 h. A Dean and Stark apparatus was used to remove
the water generated during the reaction. The mixture was
concentrated and cooled to give a yellow solid which was
isolated by filtration and washed with ethanol several
times. After drying a yellow powder was obtained in high
yield (1.9 g, 83.7%). ESI-MS: m/z = 976.5 [M+]. 1H
NMR (400 MHz, DCCl3): d = 1.12–1.25 (m, 48H, CHMe),
2.27 (s, 6H, N@CMe), 2.29 (s, 6H, N@CMe), 2.75 (m, 8H,
CHMe2), 4.02 (s, 2H, –CH2–), 7.02–7.16 (m, 10H, Ar-H),
7.92 (t,J = 8.0 Hz, 2H, Py-Hp), 8.50 (d, J = 8.0 Hz, 4H,
Py-Hm). Anal. Calc. for C67H86N6 (975.4): C, 82.50; H,
8.89; N, 8.61. Found: C, 82.43; H, 8.91; N, 8.54%.
2. Experimental
All manipulations of air and/or moisture sensitive com-
pounds were performed under an argon atmosphere with
standard Schlenk techniques.
2.1. Materials
In a similar method described for 3, ligand 4 was pre-
pared using monoimine 1 (1.5 g, 4.65 mmol) and 4,40-meth-
ylene-bis(2,6-dimethylaniline) (0.58 g, 2.32 mmol) as a
yellow powder. Yield: 1.74 g (87.0%). ESI-MS: m/
Toluene was refluxed and distilled from sodium/benzo-
phenone under dry Ar. Triisobutylaluminum (Al(i-Bu)3)
and triethylaluminum (AlEt3) were purchased from Roth
company. All other chemicals including 4,40-methylene-
bis(2,6-dimethylaniline) and 4,40-methylene-bis(2,6-diiso-
propylaniline) were commercially available and used
without further purification. The mononuclear 2,6-bis-
(imino)pyridyl iron complexes 9, 10 and 11 (Chart 1)
[3,17], 1-{6-[(2,6-diisopropylphenyl)ethanimidoyl]-2-pyridi-
nyl}-1-thanone (1) [18] and 1-{6-[(2,6-dimethylphenyl)-eth-
animidoyl]-2-pyridinyl}-1-ethanone (2) [18] were synthesized
according to the known procedures.
1
z = 864.3 [M+]. H NMR (400 MHz, DCCl3): d = 1.12–
1.26 (m, 24H, CHMe), 2.04 (s, 12H, Ar-Me), 2.27 (s,
12H, N@CMe), 2.78 (m, 4H, CHMe2), 3.88 (s, 2H,
–CH2–), 6.94–7.12 (m, 10H, Ar-H), 7.92 (t, J = 10.8 Hz,
2H, Py-Hp), 8.50 (m, 4H, Py-Hm). Anal. Calc. for
C59H70N6 (863.2): C, 82.09; H, 8.17; N, 9.74. Found: C,
81.89; H, 8.23; N, 9.63%.
In a similar method described for ligand 3, ligand 5 was
prepared using monoimine 2 (1.5 g, 5.63 mmol) and 4,40-
methylene-bis(2,6-dimethylaniline) (0.72 g, 2.81 mmol) as
a yellow powder. Yield: 1.75 g (83.0%). ESI-MS: m/
z = 751.5 [M+]. 1H NMR (400 MHz, DCCl3): d = 2.04
(m, 24H, Ar-Me), 2.27 (m, 12H, N@CMe), 3.88 (s, 2H,
–CH2–), 6.95–7.09 (m, 10H, Ar-H), 7.92 (t, J = 10.0 Hz,
2H, Py-Hp), 8.50 (d, J = 8.4 Hz, 4H, Py-Hm). Anal. Calc.
2.2. Measurements
The 1H NMR data of the ligands were obtained on Bru-
ker Advance DMX400 instrument in DCCl3 with TMS as
standard. Mass spectra were recorded using ESI-MS on

L. Wang, J. Sun / Inorganica Chimica Acta 361 (2008) 1843–1849
1845
for C51H54N6 (750.4): C, 81.56; H, 7.25; N, 11.19. Found:
C, 81.48; H, 7.19; N, 11.25%.
was then filtered, washed repeatedly with ethanol and dried
in vacuum at 60 ꢁC to a constant weight.
2.5. Results and discussion
2.3.2. Preparation of complexes 6–8
Under an argon atmosphere, a solution of ligand 3
(1.00 g, 1.03 mmol) in THF was added dropwise to a solu-
tion of FeCl2 Æ 4H2O (0.41 g, 2.06 mmol) in THF to yield a
blue solution. After stirring under reflux for 10 min, the
reaction was allowed to cool to room temperature and then
stirred overnight. The reaction volume was concentrated,
and diethyl ether (30 mL) was added to precipitate the
product as a blue powder, which was subsequently washed
with diethyl ether (3 · 20 mL), filtered, and dried to afford
complex 6 as a blue powder. Yield: 1.18 g (93.6%). NMR:
not recorded due to the paramagnetic character of the com-
plex. IR (KBr, disk, cmꢂ1): 3065w, 2962vs, 2923w, 2867m,
1617w (mC@N), 1582s, 1458s, 1371s, 1267s, 1215s, 1178w,
1102s, 1057w, 935w, 813s, 777m, 739w, 717w. MS (FAB):
m/z 1229 [M+], 1192 [M+ꢂCl], 1103 [M+ꢂFeCl2], 976
[M+ꢂ2FeCl2]. Anal. Calc. for C67H86Cl4Fe2N6 (1228.9):
C, 65.48; H, 7.05; N, 6.84. Found: C, 65.38; H, 6.96; N,
6.79%.
According to the same procedure described above, using
ligand 4 (1.0 g, 1.16 mmol) and FeCl2 Æ 4H2O (0.46 g,
2.32 mmol) yielded complex 7 as a blue powder. Yield:
1.17 g (90.2%). NMR: not recorded due to the paramag-
netic character of the complex. IR (KBr, disk, cmꢂ1):
3064w, 2962vs, 2927w, 2867m, 1619w (mC@N), 1582s,
1463s, 1368s, 1264s, 1210s, 1173w, 1101s, 1042w, 935w,
803s, 773m, 739w, 717w. MS (FAB): m/z 1118 [M+],
1081 [M+ꢂCl], 954 [M+ꢂFeCl2]. Anal. Calc. for
C59H70Cl4Fe2N6 (1117.3): C, 63.46; H, 6.32; N, 7.53.
Found: C, 63.40; H, 6.28; N, 7.50%.
2.5.1. Synthesis of new ligands and iron complexes
Methylene bridged bis(imino)pyridyl ligands 3–5
were prepared by the reaction of bridged diamines with
1-{6-[(2,6-diisopropylphenyl)ethanimidoyl]-2-pyridinyl}-1-
ethanone (1) and 1-{6-[(2,6-dimethylphenyl)-ethanimi-
doyl]-2-pyridinyl}-1-ethanone (2) according to the known
procedure as shown in Scheme 1. During the reaction, p-
toluenesulfonic acid was added as catalyst and a Dean
and Stark apparatus was used to remove the water to accel-
erate the reaction. The reaction of the new ligands 3–5 with
iron(II) dichloride results in the formation of the blue com-
plexes 6–8 in nearly quantitative yields. The paramagnetic
character of the resulting catalyst makes NMR analysis
impossible. These three binuclear complexes 6–8 are char-
acterized by FT-IR spectra. The absorptions of C@N
bands at ca. 1617–1627 cmꢂ1 are present in all the deriva-
tives, which appear together with the other one intense
absorption in the range 1590–1580 cmꢂ1. Their composi-
tion could be proved by FAB-MS spectra as well as by ele-
mental analysis. The mass spectrometric data (FAB-MS)
reveal the molecule ion in all the cases and the fragments
after the loss of one chloride atom for 6 and 7. The frag-
mentation pattern with halide loss has been shown to occur
for analogue compounds 9 and 11 [3].
2.5.2. Comparison of the catalytic behavior with that of
corresponding catalysts 9–12
Table 1 demonstrates a dramatic change in the catalytic
activity of the new binuclear iron complexes 6–8 compared
with that of the mononuclear iron(II) complexes 9–11. We
observed that, compared with the corresponding mononu-
clear iron(II) catalysts, the binuclear iron(II) catalysts
except for the inactive catalyst 6 exhibited much higher cat-
alytic activity and produced much higher molecular weight
PE in the presence of Al(i-Bu)3. It is obviously due to the
introduction of the methylene bridge between the phenyl
groups of ligands in the structure of the catalysts, which
might lead to coorperative effect between the closely adja-
cent metal centers. Generally, it is believed that electronic
and steric ligand effects that make cationic active species
more stable or unstable can lead to the increase or decrease
of catalytic activity. For the binuclear iron catalysts 7 and
8, the electronic effect is taken into account for the modifi-
cation on catalytic performance. Such binuclear catalysts
have two similar parts around the central methylene group.
Each part together with the CH2 group can be considered
as a big para-substituent of the other phenyl group. The
big para-substituent in the ligand backbone is quite remote
from the active site but still within range to influence the
activity and selectivity of the active center [20]. In this case,
special substituents (containing one metal center) at the
para-position makes the phenyl ring more electron riched
According to the same procedure described above, using
ligand 5 (1.0 g, 1.33 mmol) and FeCl2 Æ 4H2O (0.53 g,
2.66 mmol) yielded 8 as a blue powder. Yield: 1.23 g
(92.1%). NMR: not recorded due to the paramagnetic
character of the complex. IR (KBr, disk, cmꢂ1): 3074w,
2962s, 2920s, 2867w, 1625w (mC@N), 1587s, 1473s, 1372s,
1263s, 1218s, 1170w, 1095s, 1037s 812s, 773s, 740w,
717w. FAB MS: m/z 1005 [M+], 1004 [M], 843
[M+ꢂFeCl2ꢂCl]. Anal. Calc. for C51H54Cl4Fe2N6
(1004.5): C, 60.98; H, 5.42; N, 8.37. Found: C, 60.85; H,
5.39; N, 8.29%.
2.4. General procedure for ethylene polymerization
The polymerization was carried out in a 50 ml glass
reactor. The reactor was filled with a proper amount of tol-
uene, Al(i-Bu)3(AlEt3) solution and saturated with a con-
tinuous flow of ethylene under one bar ethylene pressure.
The polymerization was initiated by injection of the cata-
lyst solution. The total volume of the solution is 25 mL.
The reaction mixture was stirred for an appropriate period
at the desired temperature. It was quenched by addition of
acidified ethanol containing 10% HCl. The precipitated PE

1846
L. Wang, J. Sun / Inorganica Chimica Acta 361 (2008) 1843–1849
NH2
R
HCOOH , 0 ºC
+
R
R
N
N
1: R=iPr
2: R=Me
CH OH
O
O
O
N
3
R=iPr, M e
R
R'
R'
NH2
R'
P-toluenesulfonicacid
H2N
CH2
drytoluene,reflux,-H O
R'
2
R'=iPr, M e
R
R
R
R'
R'
3: R=R'=iPr
4: R=iPr,R'=Me
5: R=R'=Me
N
N
N
CH2
N
N
N
R
R'
R'
•
THF
FeCl2 4H2O
Cl
Cl
R
R
6: R=R'=iPr
7: R=iPr,R'=Me
8: R=R'=Me
R'
R'
R'
Fe
N
N
N
CH2
N
N
Fe
N
R
R
R'
Cl
Cl
Scheme 1. The route for the preparation of ligands 3–5 and complexes 6–8.
Table 1
Results of ethylene polymerization by iron catalysts 6–11 activated with Al(i-Bu)3
Run
Cat
Al/Fe
Yield (g)
Activityd
Mw/104
Mn/104
MWD
Tm/ꢁC
1
6
6
7
7
7
8
9
10
11
12
1000
200
trace
1.77
1.79
2.08
0.69
2.19
0.42
0.23
trace
1.10
2a
5.32
5.36
6.24
2.07
6.57
1.26
069
29.3
46.0
21.7
13.2
20.3
4.15
3.21
0.50
1.07
0.76
0.45
0.50
0.97
0.90
58.0
43.2
28.7
29.4
40.9
4.26
3.55
131.3
132.2
131.1
130.5
131.5
127.1
126.5
3
4
5b
6
7
8
9
1000
2000
1000
1000
1000
1000
1000
1200
10 [10]c
2.20
21.9
5.53
3.96
Polymerization conditions: [Fe] = 0.4 · 10ꢂ4 mol/L, 0 ꢁC, 20 min, 1 bar ethylene pressure, 25 mL toluene.
a
AlEt3 as cocatalyst, Al/Fe = 200, 20 ꢁC.
b
Obtained at 40 ꢁC.
c
[10] [Fe] = 0.6 · 10ꢂ4 mol/L, Al/Fe = 1200, 0 ꢁC, 1 bar ethylene pressure, 10 min.
d
106 g PE/(molFe Æ h).
[21] and strengthens the coordination bond between N–Fe
to some extent, which probably influences the fundamental
steps of the polymerization process (olefin insertion and
chain termination) by modifying the coordination environ-
ment of the metal center, thus resulting in an increase in
catalytic activity and molecular weight [22].
Besides the electronic effects, steric effects also cause an
important influence on the catalytic activity of the bridged
binuclear iron catalytic systems. In view of the catalytic
behaviors of the mononuclear iron catalyst 9 and macrocy-
clic trinuclear iron catalyst 12 [10], binuclear iron(II) cata-
lyst 6 containing the same substituents (isopropyl) on the
phenyl rings is expected to exhibit high catalytic activity
for ethylene polymerization. However, the experimental
results showed that there is no doubt that only 6 is inactive
(Run1) in the presence of TIBA. To investigate the poten-
tial catalytic performance of 6, one polymerization experi-
ment was carried out, in which 6 was found to be an
effective catalyst for the polymerization of ethylene in the
presence of AlEt3. The explanation for this phenomenon
may be that the steric bulk around the active iron atoms
formed by 6 and Al(i-Bu)3 is too big so that the insertion
of ethylene monomer is prohibited. In addition, the results
also indicate that the introduction of methylene bridge into
the catalyst structure might increase the steric bulk around
the metal center. The combination of electronic and steric
effects maybe the reason for the different catalytic activities
of the binuclear iron(II) catalysts 6 and 7 and the mononu-
clear iron(II) catalysts 9–11, even if they had similar
substituents.

L. Wang, J. Sun / Inorganica Chimica Acta 361 (2008) 1843–1849
1847
2.5.3. Effects of polymerization conditions on ethylene
polymerization
7
6
5
4
3
2
1
0
40
35
30
25
20
15
10
5
Polymerization conditions have a great influence on the
activity of the catalytic system employed and the nature of
the PE produced. A series of experiments under various
conditions were carried out to study these effects with the
catalytic system 7/TIBA. The results of ethylene polymeri-
zation using various temperatures are displayed in Fig. 1.
As shown in Fig. 1, the catalytic system 7/TIBA is
highly sensitive to the polymerization temperature. The
activity decreases from 5.38 · 106 g PE/(molFe Æ h) at
0 ꢁC with an Al/Fe molar ratio of 1000:1 to 2.53 · 106 g
PE/(molFe Æ h) after raising the temperature to 20 ꢁC.
Above 0 ꢁC, a higher deactivation rate of the catalytic cen-
ters and a lower solubility of ethylene in toluene result in
low activity of the catalytic system. The molecular weight
of PE also decreases with rising polymerization tempera-
ture, slowly between 0 and 20 ꢁC and more drastically
between 20 and 40 ꢁC.
Fig. 2 shows the influence of the Al/Fe molar ratio on
the catalytic activity and molecular weight of PE. It is clear
that the catalytic activity is higher with increasing amounts
of Al(i-Bu)3 in the range studied. The low activity of the
catalytic system at low Al/Fe ratio might result from insuf-
ficient activation of the catalyst. When the Al/Fe molar
ratio is increased from 500 to 4000, the color of the poly-
merization system turned from orange-yellow to pink, indi-
cating there might be different kinds of active species
generated at different Al/Fe molar ratios [23,24]. Further-
more, the pink color does not disappear within the poly-
merization time. These results reveal that Al(i-Bu)3 as a
cocatalyst can not only activate iron centers but also stabi-
lize active iron centers effectively. In addition, a high Al/Fe
molar ratio will lead to such more active species formed
that are responsible for the preparation of low molecular
weight PE [24], which is also confirmed by the GPC char-
acterization of the PE sample.
Activity
M
M
0
500
2000
4000
1000
Al/Fe
Fig. 2. Effects of Al/Fe molar ratio on catalytic activity and Mg of PE.
Polymerization conditions: [Fe] = 0.4 · 10ꢂ4 mol/L, 0 ꢁC, 20 min, 1 bar
ethylene pressure, 25 mL toluene.
6
5
4
3
2
1
0
70
60
50
40
30
20
10
0
M
Activity
M
10
20
40
60
Time(min)
Fig. 3. Effects of reaction time on catalytic activity and Mg of PE.
Polymerization conditions: [Fe] = 0.4 · 10ꢂ4 mol/L, 0 ꢁC, Al/Fe = 1000,
1 bar ethylene pressure, 25 mL toluene.
stant up to 20 min and decreases gradually with the pro-
longing polymerization time. However, catalyst 9 displays
a very high activity in the initial stage followed by a rapid
decay [5,10]. This behavior indicates that the methylene
bridged ligand in 7 could effectively restrain the active cen-
ter from deactivation [10]. Additionally, the molecular
weight of PE always increases with prolonged reaction
time. This trend is in accordance with the literature [24].
The effect of reaction time on ethylene polymerization is
illustrated in Fig. 3. The activity of catalyst 7 keeps con-
6
5
4
3
2
1
0
40
35
30
25
20
15
10
5
2.5.4. Characterization of resultant PE
Activity
M
The molecular weight and MWD of the resultant PE
were determined by the GPC method and listed in Table
1. The results demonstrate that 7 and 8 produce PE with
higher molecular weight than those of PE produced by
the mononuclear iron(II) catalysts 9 and 10. Fig. 4 shows
the GPC plots of PE samples prepared by different iron cat-
alysts with TIBA as cocatalyst. The molecular weight dis-
tributions of PE prepared by 7 and 8 are very broad,
with the bimodal character in the case of 8. In contrast,
the mononuclear iron catalysts 9 and 10 yield PE with rel-
atively narrow MWDs (4.26 and 3.55) as a result of a miss-
ing chain transfer to aluminum in those iron/TIBA
systems. But the possible chain transfer reaction to alumi-
M
0
0
20
40
60
Temperature(°c)
Fig. 1. Effects of reaction temperature on catalytic activity and Mg of PE.
Polymerization conditions: [Fe] = 0.4 · 10ꢂ4 mol/L, Al/Fe = 1000,
20 min, 1 bar ethylene pressure, 25 mL toluene.

1848
L. Wang, J. Sun / Inorganica Chimica Acta 361 (2008) 1843–1849
8
7
N
N
N
N
N
N
Fe
6
Fe
Ar
Ar
Ar
Ar
R
Cl
R
R
R
R
3
Al
Al
a
b
Ar=2,6-R2C6H3; R=CH2CH(CH3)2
Chart 2. Possible active species.
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
logMw
iron(II) catalysts 9 and 10, which indicates that the PE pro-
duced is highly linear and highly crystalline [8,9].
Fig. 4. GPC curves of PE samples produced with 7–10/TIBA systems.
(numbers of the curves correspond to runs in Table 1) Polymerization
conditions: [Fe] = 0.4 · 10ꢂ4 mol/L, 20 min, 0 ꢁC, 1 bar ethylene pressure,
25 mL toluene.
3. Conclusions
Three novel methylene bridged binuclear iron(II) cata-
lysts were synthesized and successfully used to catalyze eth-
ylene polymerization in the presence of TIBA. Compared
with mononuclear iron(II) catalysts 9–11, binuclear iro-
n(II) catalysts demonstrate quite different catalytic perfor-
mance due to the combined electronic and steric effects
from the structure of the complexes. High molecular weight
PE with broad/bimodal MWD was easily obtained using
methylene bridged binuclear iron(II) catalysts 7 and 8.
num in the case of the catalytic active systems 7 and 8 can
not explain the bimodal behavior of the resulting PE. A
possible explanation for this maybe the existence of two
different kinds of catalytic active species, [LFe(II)Cl(l-
R)2AlR2] (a) and [LFe(II)R(l-R)2AlR2] (b) (L =
bis(imino)pyridyl ligand, R = CH2CH(CH3)) in the
bridged binuclear iron complexes [24,25]. Elevating the
temperature and the Al/Fe molar ratio will benefit the for-
mation of b, which mainly produces PE with low molecular
weight [24]. Therefore, changing the reaction temperature
and the Al/Fe molar ratio could increase or decrease the
high molecular weight fraction as shown in Fig. 5 (see
Chart 2).
Acknowledgements
The research was supported by the National Natural
Science Foundation of China (20374043). We thank Prof.
H. Schumann for helpful discussion.
Additionally, the last column of Table 1 shows that the
melting temperatures of the PE samples produced by the
binuclear iron(II) catalysts 7 and 8 are higher (above
130 ꢁC) than that of the PE produced by the mononuclear
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