An unsymmetrical iron(II) bis(imino)pyridyl catalyst for ethylene polymerization: Effect of a bulky ortho substituent on the thermostability and molecular weight of polyethylene

Article abstract of DOI:10.1021/om9010356

Three new iron(II) 2,6-bis(imino)pyridyl complexes bearing bulky and unsymmetrical substituted aniline groups, [2-(CH(CH₃)(C ₆H₅))-4-R₁-6-R₂-C₆H ₂N - C(CH₃)]₂C₅H ₃NFeCl₂ (1, R₁ = H, R₂ = methyl; 2, R₁ = methyl, R₂ = methyl; 3, R₁ = H, R ₂ = methoxy), were synthesized. The unsymmetrical bis(imino)pyridyl ligand L2 contains three isomers that can be detected by ¹H NMR and ¹³C NMR spectroscopy. However, the corresponding complex 2 exhibits only one isomer in solution. X-ray diffraction of 2 confirms that the iron complex adopts a C_s configuration. These complexes activated by methylaluminoxane (MAO) have high catalytic activities for ethylene polymerization and produce linear polyethylenes with bimodal or broad molecular weight distribution. The steric and electronic effects of the ortho substituents on the aniline moiety distinctly affect the molecular weight of the obtained polyethylene. In comparison with 1 and 3, having a single ortho substituent on the aryl rings, and the typical complex 4, containing o-diisopropyl substitution, complex 2 has better thermal stability and produces much higher molecular weight polyethylene. Even at 70 °C, the 2/MAO system still keeps high activity and relatively stable kinetics.

Full text of DOI:10.1021/om9010356

2118 Organometallics 2010, 29, 2118–2125
DOI: 10.1021/om9010356
An Unsymmetrical Iron(II) Bis(imino)pyridyl Catalyst for Ethylene
Polymerization: Effect of a Bulky Ortho Substituent on the
Thermostability and Molecular Weight of Polyethylene
Li-hua Guo,†,‡ Hai-yang Gao,*,† Ling Zhang,† Fang-ming Zhu,† and Qing Wu*,†,‡
†DSAPM Lab, Institute of Polymer Science, School of Chemistry and Chemical Engineering and
‡PCFM Lab, OFCM Institute, Sun Yat-Sen (Zhongshan) University, Guangzhou 510275, China
Received December 1, 2009
Three new iron(II) 2,6-bis(imino)pyridyl complexes bearing bulky and unsymmetrical substituted aniline
groups, [2-(CH(CH3)(C6H5))-4-R1-6-R2-C6H2NdC(CH3)]2C5H3NFeCl2 (1, R1 = H, R2 = methyl; 2,
R1 = methyl, R2 = methyl; 3, R1 = H, R2 = methoxy), were synthesized. The unsymmetrical bis(imino)-
pyridyl ligand L2 contains three isomers that can be detected by 1H NMR and 13C NMR spectroscopy.
However, the corresponding complex 2 exhibits only one isomer in solution. X-ray diffraction of 2 confirms
that the iron complex adopts a Cs configuration. These complexes activated by methylaluminoxane (MAO)
have high catalytic activities for ethylene polymerization and produce linear polyethylenes with bimodal or
broad molecular weight distribution. The steric and electronic effects of the ortho substituents on the aniline
moiety distinctly affect the molecular weight of the obtained polyethylene. In comparison with 1and 3,having
a single ortho substituent on the aryl rings, and the typical complex 4, containing o-diisopropyl substitution,
complex 2 has better thermal stability and produces much higher molecular weight polyethylene. Even at
70 °C, the 2/MAO system still keeps high activity and relatively stable kinetics.
Introduction
Scheme 1
Important progress in the area of late-transition-metal
catalysts using iron(II) bis(imino)pyridyl for ethylene poly-
merization has been achieved in the past few decades.1 In
contrast to the nickel and palladium R-diimine catalysts,
which typically produced highly or moderately branched
polyethylene,2 tridentate iron(II) bis(imino)pyridyl cata-
lysts, in spite of their higher activity, produced highly linear
polyethylene with low molecular weight and broad mole-
cular weight distribution.
polymerization. Generally, bulkiness on imine carbon can
increase catalytic activity for ethylene polymerization and
the molecular weight of the polyethylenes (Fe/ketimine >
Fe/aldimine),1a,3b while the steric hindrance of the 2,6-sub-
stitution on the aniline moiety can decrease catalytic activity
(Me > Et > i-Pr) but give an obvious increase in the mole-
cular weight of the products (Me < Et < i-Pr).1a-c,3 Iron(II)
catalysts lacking ortho alkyl substituents only produced
oligomers or trace polymer.3b,4b,e One drawback of these
Since the initial studies by Brookhart1a and Gibson,1b,c
a
great deal of research has been involved in the design of new
ligands,1d,e especially on changes of the backbone substituents
on the carbon atoms of imine groups and replacement of
the aniline moiety and the resultant influence on ethylene
*To whom correspondence should be addressed. E-mail: ceswuq@
mail.sysu.edu.cn (Q.W.); gaohy@mail.sysu.edu.cn (H.G.).
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Chem. Commun. 1998, 849. (c) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.;
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pubs.acs.org/Organometallics
Published on Web 04/05/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 9, 2010 2119
Scheme 2. Synthesis of Bis(imino)pyridyl Ligands and Corresponding Iron Complexes
catalysts is their poor thermal stability. The activity and
molecular weight dropped rapidly as the reaction tempera-
ture increased.1 A bulky macrocyclic iron(II) bis(imino)-
pyridyl complex can restrain the catalytically active iron
center from deactivation and produce higher molecular
weight polymers.5
Moreover, a great deal of work has been focused on
unsymmetrical catalyst precursors as follows: a change of
substituents at the ketimine nitrogen atoms (I)6 and a
change in the alkylphenyl moiety in the 2,6-positions (II)
(Scheme 1).1a,c,f,g,3b,4 Catalysts derived from such Fe com-
plexes I with the alkyl group R were still very active but
typically gave ethylene oligomers.6b The complexes II with
unsymmetric ortho substitution of various alkyl/aryl groups
on aniline produced highly linear polymers after activation
by MAO. Despite all this, modification of the ligand of iron
bis(imino)pyridyl catalysts to improve their thermostability
and produce higher molecular weight polyethylene remains a
challenge.
Previous studies have shown that a bulky R-diimine com-
plex with sec-phenethyl substituents on the aniline moiety
adopts a C2-symmetric configuration and can effectively
shield the axial space.4d,f The objective of the present study
is to synthesize new iron(II) 2,6-bis(imino)pyridyl complexes
bearing unsymmetrical and bulky o-sec-phenethyl-substituted
aryl groups and to obtain high-molecular-weight polyethyl-
enes at elevated temperature. The syntheses of three new
iron(II) complexes containing o-sec-phenethylaniline groups
and their catalytic properties in ethylene polymerization are
reported herein. It was found that an appropriate ortho sub-
stitution on aniline moieties can promote the thermostability
of the catalyst and the control of molecular weight more
efficiently than typical iron(II) o-diisopropyl-substituted
bis(imino)pyridyl analogues.
Results and Discussion
Syntheses and Characterization of Ligands and Complexes.
The procedures in the preparation of ligands and complexes
are shown in Scheme 2. Reaction of the 2,4-disubstituted
anilines with styrene at elevated temperature (160 °C) in the
presence of CF3SO3H catalyst resulted in the corresponding
o-sec-phenethylanilines rac-A1-A3. The tridentate 2,6-bis-
(imino)pyridine ligands L1-L3 were synthesized by the
Schiff base condensation of 2,6-diacetylpyridine with 2 equiv
of the o-sec-phenethylanilines in toluene at 80-110 °C using
p-toluenesulfonic acid (p-TsOH) as catalyst and molecular
sieves as the water adsorbent7 and were verified by elemental
analysis, 1H NMR, and 13C NMR.
1
The H NMR spectrum of ligand L2 at 20 °C shows the
presence of three isomers in the ratio 5.6/2.3/1. As shown in
Figure 1, three quartets at 4.08, 4.03, and 3.74 ppm are
clearly observed, corresponding to methine protons of the
o-sec-phenethyl substituents. The origin of the isomers may
be S,S, S,R, and R,R diastereomers of chiral carbon8 or E,Z,
Z,Z, and E,E configurations of the CdN double bond of
imine.9 rac/meso diastereoisomers can be safely ruled out,
because the ratio of these three isomers is temperature
dependent. The ratio of peak intensities of the three isomers
changes from 5.6/2.3/1 to 6.4/3/1 with an increase in temp-
erature from 20 to 60 °C. Therefore, there are many pos-
sibilities for E,Z, Z,Z, and E,E configurational isomers for
the ligand. Previous reports have also noted that the E/Z
isomerization is reversible via an inversion at the imine
nitrogen.9c,d
Treatments of metal salt with the ligands L1-L3 in THF
for 8 h afforded the complexes 1-3. It has been reported3f,10
that the CdN stretching frequencies of the analoguous free
ligands appeared at 1638-1645 cm-1 in FTIR spectra. For
complexes 1-3, the CdN stretching vibrations are shifted to
(5) Liu, J. Y.; Li, Y. S.; Liu, J. Y.; Li, Z. S. Macromolecules 2005, 38,
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Passaglia, E.; Gragnoli, T. Organometallics 2004, 23, 6087. (d) Bianchini,
C.; Gatteschi, D.; Giambastiani, G.; Rios, I. G.; Ienco, A.; Laschi, F.; Mealli,
C.; Meli, A.; Sorace, L.; Toti, A.; Vizza, F. Organometallics 2007, 26, 726.
(e) Sun, W. H.; Hao, P.; Zhang, S.; Shi, Q.; Zuo, W.; Tang, X.; Lu, X.
Organometallics 2007, 26, 2720. (f) Bianchini, C.; Giambastiani, G.; Rios,
I. G.; Meli, A.; Oberhauser, W.; Sorace, L.; Toti, A. Organometallics 2007,
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2003, 2566.
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Benedix, R. Recl. Trav. Chim. Pays-Bas 1994, 113, 88. (b) Gates, D. P.;
~
Svejda, S. A.; Onate, E.; Killian, C. M.; Johnson, L. K.; White, P. S.;
Brookhart, M. Macromolecules 2000, 33, 2320. (c) Britovsek, G. J. P.;
Gibson, V. C.; Hoarau, O. D.; Spitzmesser, S. K.; White, A. J. P.; Williams, D.
J. Inorg. Chem. 2003, 42, 3454. (d) Orrell, K. G.; Osborne, A. G.; Sik, V.;
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26, 5066. (g) Wallenhorst, C.; Kehr, G.; Luftmann, H.; Erohlich, R.; Erker, G.
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Organometallics 2008, 27, 6547.
Y. Y.; Hu, Y. L. J. Mol. Catal. A: Chem. 2005, 230, 1.

2120 Organometallics, Vol. 29, No. 9, 2010
Guo et al.
Figure 1. 1H NMR spectra of ligand L2 measured at 20 and 60 °C.
Figure 2. 1H NMR spectra of complex 2 at 25 °C at different times (500 MHz, CD2Cl2 (peak indicated by an asterisk), 20 °C): (0 h)
determined immediately after fresh solution was prepared from the precipitated solid; (36 h) determined after the solution was stored
for 36 h.
lower frequencies around 1602-1620 cm-1, indicating an effi-
cacious coordination interaction between the imino nitrogen
atoms and the metal ions. The synthesis of the typical complex 4
was performed according to the literature method.1a-c
This result also further confirms that the isomers of ligand L2
cannot come from chiral carbons. Reyes12 previously re-
ported that there is an equilibrium between the Cs and C2
diastereoisomers of a iron(II) bis(imino)pyridyl complex
with unsymmetrical 2-methyl-6-isopropyl substitution on
aniline. In contrast, iron complexes 1-3 have a single iso-
mer in solution. This obvious difference is a result of the
more bulky o-sec-phenethyl substituen, which can effecti-
vely prohibit the rotations of C-N bonds. An observation
that the conversion rate of the isomers depends on the
steric bulkiness of ortho substituents has been reported by
Pellecchia.11
1H NMR spectra showed that the iron(II) bis(imino)-
pyridyl complexes individually exhibit only one isomer.
A fresh powder sample of complex 2 was typically chosen
to study time dependence by 1H NMR analysis in CD2Cl2.
Figure 2 clearly shows only one set of signals corresponding
to a variety of 12 hydrogen atoms of complex 2 and no
changes in the spectra can be observed within 36 h, revealing
that no isomerization occurs on the NMR time scale.4a,11
ꢀ
´
guez-Delgado, A.; Campora, J.; Naz, A. M.; Palma, P.;
Reyes, M. L. Chem. Commun. 2008, 5230.
(11) Pappalardo, D.; Mazzeo, M.; Antinucci, S.; Pellecchia, C.
Macromolecules 2000, 33, 9483.
(12) Rodrı

Article
Organometallics, Vol. 29, No. 9, 2010 2121
Table 1. Results of Ethylene Polymerizations Catalyzed by 1-4/MAO at 30 °Ca
b
d
activity
(105g/((mol of Fe) h))
Mw
(kg/mol)
Mpk1/Mpk2
(kg/mol)
entry
complex
yield (g)
PDIc
Tme (°C)
1
2
3
4
1
2
3
4
2.21
2.50
1.86
2.55
7.37
8.33
6.20
8.50
9.3
1190.5
108.9
2.8
167.6
21.5
4.3
125.0
135.4
130.1
132.2
969.8/2.7
30.1/1.7
608.6/2.7
553.1
88.5
a General conditions: 3 μmol of iron complex; Al/Fe = 1500; 30 °C; 30 mL of toluene; 60 min; 1 bar of ethylene pressure. b Weight-average molecular
weight based on the whole GPC trace. c Based on the whole polymer sample. d Molecular weight of the peaks in the GPC trace. e Determined by DSC.
Figure 3. Molecular structure of complex 2. Hydrogen atoms
have been omitted for clarity. Selected bond lengths (A) and angles
(deg): Fe(1)-N(1), 2.284(3); Fe(1)-N(2), 2.083(3); Fe(1)-N(3),
2.248(3); Fe(1)-Cl(1), 2.3275(14); Fe(1)-Cl(2), 2.2597(13);
Cl(2)-Fe(1)-Cl(1), 128.14(5); N(1)-Fe(1)-Cl(1), 95.90(10);
N(3)-Fe(1)-Cl(1), 94.61(9); N(2)-Fe(1)-Cl(1), 95.50(10);
N(2)-Fe(1)-Cl(2), 136.34(11); N(3)-Fe(1)-N(1), 146.71(12);
N(2)-Fe(1)-N(1) 73.55(13).
Figure 4. GPC curves of the polyethylenes prepared by 1-3/
MAO at 30 °C.
A single crystal suitable for X-ray diffraction analysis was
obtained by slow diffusion of hexane into a dichloromethane
solution of complex 2. Fortunately, a crystal of pure complex
2 could be obtained and the ORTEP diagram is shown in
Figure 3. Unlike the unsymmetrical bulky R-diimine Ni(II)
complexes which typically adopt a C2 configuration,4a,d,13,14
iron complex 2 possesses a near Cs-symmetric configuration,
and both sec-phenethyl groups are oriented above the bis-
(imino)pyridyl plane. The origin of this structural difference
is that the iron complex has more open steric space due to its
N-N-N coordination geometry, as compared to R-diimine
nickel complexes with N-N coordination geometry. There-
fore, it is reasonably concluded that the iron complexes have
a single isomer with a Cs-symmetric configuration both in the
solid state and in solution. The phenyl rings attached on
imine groups are nearly perpendicular to the coordination
plane, with the dihedral angles being 84.36° for the ring
bonded to N(1) and 82.19° for the ring bonded to N(3) for 2,
respectively. The axial sites for the metal center are almost
blocked by the o-sec-phenethyl substituents, which will play
a critical role in maintaining high activity and obtaining
high-molecular-weight polymers at elevated temperatures.
Ethylene Polymerization. Effect of Ligand Structure. The
results from ethylene polymerizations conducted at 30 °C
and 1 bar by using complexes 1-4 as catalyst precursors in
Figure 5. GPC curves of the polyethylenes prepared by 2/MAO
and 4/MAO at 30 °C.
the presence of MAO are shown in Table 1. A striking feature
observed in the data is that the complexes with only one
ortho substituent on the aniline moieties do not yield high-
Mpk polymer even though the substituent is bulky. Among
the o-sec-phenethyl substitution analogues 1-3, complex 2,
containing methyl substituted for H on another ortho posi-
tion, exhibits slightly better catalytic activity and produces
much higher molecular weight polymer than the single-
ortho-substituent analogues 1 and 3. As shown in Figure 4,
the polymers produced by the catalysts exhibit broad or
bimodal molecular weight distributions that are in accor-
dance with the previous observation from other iron bis-
(imino)pyridyl catalysts.1 The broad or bimodal molecular
weight distributions can be attributed to the coexistence of
two chain-transfer processes in the polymerizations: β-H
transfer to monomer or metal and chain transfer to alkyl-
aluminum.1a,c The results obtained herein indicate that the
(13) Park, S.; Takeuchi, D.; Osakada, K. J. Am. Chem. Soc. 2006,
128, 3510.
ꢀ
(14) Rosa, V.; Aviles, T.; Aullon, G.; Covelo, B.; Lodeiro, C. Inorg.
Chem. 2008, 47, 7734.

2122 Organometallics, Vol. 29, No. 9, 2010
Table 2. Results of Ethylene Polymerizations under Various Conditionsa
Guo et al.
b
d
e
temp
(°C)
Al/Fe
(mol/mol)
yield
(g)
activity
(105g/((mol of Fe) h))
Mw
(kg/mol)
Mpk1/Mpk2
(kg/mol)
Tm
(°C)
entry
complex
PDIc
5
1
6
7
8
2
9
10
11
12
13
3
14
15
16
4
1
1
1
1
2
2
2
2
2
2
3
3
3
3
4
4
4
4
0
30
50
70
0
30
50
70
30
30
0
30
50
70
0
30
50
70
1500
1500
1500
1500
1500
1500
1500
1500
1000
2000
1500
1500
1500
1500
1500
1500
1500
1500
2.31
2.21
2.15
0.94
2.25
2.50
3.31
2.74
1.03
3.59
2.32
1.86
2.31
1.27
1.50
2.55
2.03
1.04
7.70
7.37
7.17
3.13
7.50
8.33
11.03
9.13
0.34
11.97
7.73
6.20
7.70
4.23
5.00
8.50
6.77
3.48
38.6
9.3
6.3
2.8
855.8
1190.5
1121.6
23.3
1499.2
1054.1
152.3
108.9
28.2
9.3
2.8
8.8
4.3
2.6
2.0
905.2/3.7
969.8/2.7
796.5/2.5
9.0
1222.2/3.1
1122.1/1.6
45.0/1.9
30.1/1.7
5.6
127.8
125.0
122.8
106.4
134.6
135.4
134.7
127.2
136.1
135.7
131.6
130.1
125.5
123.0
131.7
132.2
131.2
124.5
2.2
f
140.2
167.6
131.7
5.7
155.8
257.1
26.0
21.5
8.5
6.5
2.7
4.5
483.1
553.1
269.1
8.6
64.2
88.5
26.5
4.3
723.5/3.2
608.6/2.7
321.3/2.5
3.8
17
18
a General conditions: 3 μmol of iron complex; 30 mL of toluene; 60 min; ethylene pressure 1 bar. b Weight-average molecular weight based on the
whole GPC trace. c Based on the whole polymer sample. d Molecular weight of the peaks in the GPC trace. e Determined by DSC. f The molecular weight
is too low to get a correct PDI value.
bulky ortho disubstitution on the aniline can efficiently sup-
press the chain transfer reactions, especially for β-H transfer.
For the catalysts with a single ortho substituent, a change
of para substituent from methyl (1) to the more strongly
electron-donating methoxy (3) results in a slight decrease of
catalytic activity from 7.37 ꢀ 105 to 6.20 ꢀ 105 g/((mol of Fe)
h) but a significant increase of Mpk1 of the obtained polymer
from 4.3 to 30.1 kg/mol (Table 1, entries 1 and 3). The result
suggests that the electron-donating effect on para substitu-
tion can also decrease the rate of chain transfer relative to the
rate of chain propagation, although the electron-donating
effect may not be so strong, as compared with the steric
hindrance on ortho substitution as mentioned above.
In particular, in contrast to the case for the typical com-
plex 4, which contains o-diisopropyl sbustituents on the ani-
Figure 6. Time dependences of the catalytic activities of com-
line moiety, complex 2 affords ca. 1.5-fold increased Mpk1
plexes 2 and 4 at 0 and 70 °C. Polymerization conditions:
value for polyethylene with a higher melting point. As shown
in Figure 5, by using complexes 2 and 4, both polyethylenes
ethylene pressure 1 bar; Al/Fe = 1500.
display similar bimodal molecular weight distributions.
bimetallic species,15 the low-molecular-weight fraction is inevi-
However, the high-molecular-weight fraction of the polymer
transfer to alkylaluminum by formation of Fe-Al hetero-
obtained by complex 2 accounts for 58% more than that by
complex 4 (46%).
table but can be decreased by the adequate steric hindrance of
the ligands.
Effect of Reaction Conditions. The polymerization results
under various conditions are summarized in Table 2. These
As investigated by Brookhart1a and Gibson,1c the low-
molecular-weight fraction is a result of chain transfer to
alkylaluminum while the high-molecular-weight fraction is
complexes have different sensitivities to polymerization
produced mainly by β-H transfer. When all of the above results
are combined, it is clear that the bulky o-sec-phenethyl sub-
stituent is a crucial factor in protecting the metal center and
substitution on the aniline moiety are practically constant
increasing the molecular weight of the polymer. Ortho disub-
stitutions, just as in complex 2, can provide adequate steric
temperature. With elevated temperature, the catalytic acti-
vities of complexes 1 and 3 containing single o-sec-phenethyl
from 0 to 50 °C and then drop rapidly at 70 °C and the Mpk
values of the obtained polymer decrease continuously. Com-
plex 4, containing o-diisopropyl substitution on the aniline,
hindrance for the metal center and significantly reduce β-H transfer
exhibits a maximum activity of 8.5 ꢀ 105 g/((mol of Fe) h)
reaction; thus, high-Mpk1 polyethylenes and even superhigh-
molecular-weight polymers can be obtained. Due to the chain
and produces the highest molecular weight polymer (Mpk1
=
608.6 kg/mol) at 30 °C. It is worth noting that complex 2,
containing 2-sec-phenethyl-6-methylaniline, reaches its maxi-
mum activity of 11 ꢀ 105 g/((mol of Fe) h) at 50 °C and
still retains a high activity of 9.1 ꢀ 105 g/((mol of Fe) h) at 70 °C
(15) (a) Talsi, E. P.; Babushkin, D. E.; Semikolenova, N. V.; Zudin,
V. N.; Panchenkov, V. N.; Zakharrow, V. A. Macromol. Chem. Phys.
2001, 202, 2046. (b) Bryliakov, K. P.; Semikolenova, N. V.; Zakharrov, V. A.;
Talsi, E. P. Organometallics 2004, 23, 5375. (c) Komiya, S.; Katoh, M.;
Ikariya, T.; Grubbs, R. H.; Yamamoto, T.; Yamamoto, A. J. Organomet.
Chem. 1984, 260, 115. (d) Wang, S. B.; Liu, D. B.; Huang, R. B.; Zhang,
Y. D.; Mao, B. Q. J. Mol. Catal. A: Chem. 2006, 245, 122. (e) Bryliakov,
K. P.; Talsi, E. P.; Semikolenova, N. V.; Zakharov, V. A. Organometallics
2009, 28, 3225.
and affords superhigh-molecular-weight polymers (Mpk1
=
797-970 kg/mol) at 30-50 °C. It is quite evident that complex
2 has better thermostable properties for ethylene polymeriza-
tion than do its analogues.

Article
Organometallics, Vol. 29, No. 9, 2010 2123
Table 3. Results of Ethylene Polymerizations under 10 bar of Ethylene Pressurea
b
d
e
temp
(°C)
yield
(g)
activity
(105g/((mol of Fe) h))
Mw
(kg/mol)
Mpk
(kg/mol)
Tm
(°C)
entry
complex
PDIc
19
20
21
22
23
24
25
26
2
2
2
2
4
4
4
4
0
30
50
70
0
30
50
70
3.46
3.52
4.12
10.9
2.86
2.95
3.38
2.75
138
141
165
436
114
118
135
110
ndf
nd
nd
nd
nd
nd
nd
nd
nd
5.8
4.1
2.8
nd
nd
nd
nd
136.6
136.3
136.5
136.3
134.8
135.4
134.7
127.2
nd
nd
828.5
566.6
16.0
617.3
226.2
10.1
a General conditions: 0.5 μmol of iron complex; Al/Fe = 1500; 250 mL of toluene; 30 min. b Weight-average molecular weight based on the whole
polymer sample. c Based on the whole polymer sample. d Molecular weight of the peaks in the GPC trace. e Determined by DSC. f Not determined due
to insolubility of the polymers.
Figure 6 shows the time dependences of catalytic activities
of complex 2 and 4 for ethylene polymerizations at 0 and
70 °C. For these two complexes, with elevating polymerization
temperature, catalytic activities increase but the rate profiles
exhibit decay kinetics. Though the rate profile of complex 2
exhibits decay kinetics as the temperature increased, it still
displays higher activity and slower decay at 70 °C relative to
that of complex 4. The half-lifetime of the catalytic activity of
complex 4 at 70 °C is only 10 min, while the half-lifetime of
complex 2 is much longer: up to 25 min at the same tempera-
ture. The result indicates that the bulky and unsymmetrical
substituted ligand can restrain the catalytically active iron
centers from deactivation and effectively prolong the catalyst
lifetime.
To further examine the thermal stability of the complexes,
we carried out ethylene polymerizations by using 2 and 4
activated with MAO under 10 bar of pressure. High ethylene
pressure significantly increases the catalytic activities and
molecular weights of the products. The catalytic activity of 2
increases consistently on an increase in temperature from
0 to 70 °C under 10 bar of pressure (entries 19-22 in Table 3),
and very high activity up to 4.4 ꢀ 107 g/((mol of Fe) h) can be
achieved at 70 °C. The obtained products are linear PEs
having high melting points (Tm ≈ 136 °C) and are insoluble in
TCB. Therefore, it is difficult to determine their molecular
weight by GPC using TCB. The superhigh molecular weight
is the only possible explanation for the insolubility of these
PEs. Although 4 also shows an increased activity under 10
bar of pressure, it exhibits the maximun activity (1.3 ꢀ 107
g/((mol of Fe) h)) at 50 °C. Therefore, the results further
confirm the better thermal stability of complex 2 for ethylene
polymerization.
As mentioned above, the coexistence of two chain transfer
pathways results in a bimodal molecular weight distribution
for iron 2,6-bis(imino)pyridyl catalysts; thus, the influence of
Al/Fe ratio on these two chain-transfer processes is obvious.
Complex 2 was chosen to examine the effect of Al/Fe ratio,
and the results are shown in Table 2 (entries 2, 11, and 12).
On raising the Al/Fe ratio from 1000 to 2000, an increase in
catalytic activity and PDI of the whole polymer sample and a
decrease in the peak value of the low-molecular-weight
fraction (Mpk2) can be observed. As shown in Figure 7, with
an increase of Al/Fe ratio, the high-molecular-weight frac-
tion is essentially unchanged in the GPC traces, while the
low-molecular-weight fraction increases and Mpk2 moves to
lower molecular weight in the GPC traces. The percentage of
the low-molecular-weight fraction in the whole polymer
mass increases from 39% to 53% as the Al/Fe ratio increases
from 1000 to 2000. The results further demonstrate that β-H
Figure 7. GPC curves of the polyethylenes prepared by 2/MAO
at different Al/Fe ratios.
transfer is restrained by the bulky steric hindrance of com-
plex 2, so that the rate of β-H transfer is relatively reduced
and the chain transfer to alkylaluminum can become com-
petitive or even dominant at a high Al/Fe ratio.
Conclusion
In summary, three new unsymmetrical iron(II) bis(imino)-
pyridyl complexes with bulky o-sec-phenethyl-substituted
aryl groups have been successfully synthesized and charac-
terized. Though the bis(imino)pyridyl ligands may contain
E and Zconfiguration, all the iron complexes exhibit only
one isomer in the solid state and solution. The substituent of
the ortho positions on the aniline moiety plays a crucial role
in promoting the thermal stability of the catalyst and control
of molecular weight of polymer for ethylene polymerization.
Complex 2, containing 2-methyl-6-sec-phenethyl substitu-
ents on the aniline moiety, exhibits a better activity and
produces much higher molecular weight polyethylene as
compared to the singly ortho-substituted analogues 1 and 3
and the typical o-diisopropyl-substituted complex 4. Even at
70 °C, catalyst 2 still maintains a high activity and relati-
vely stable kinetics. The polymers obtained by the unsym-
metrical and bulky alkyl-substituted catalysts appear as a
bimodal molecular weight distribution due to the coexistence
of two chain transfer pathways, and the content of the low-
molecular-weight fraction increases with an increase in the
Al/Fe ratio.
Experimental Section
General Considerations. All manipulations of air- and/or
moisture-sensitive compounds were performed by means of

2124 Organometallics, Vol. 29, No. 9, 2010
Guo et al.
standard high-vacuum Schlenk and cannula techniques under a
N2 atmosphere. Toluene, hexane, and THF were refluxed over
metallic sodium for 48 h and distilled under a nitrogen atmo-
sphere before use. Dichloromethane was distilled from P2O5.
MAO was prepared by the controlled reaction of trimethylalu-
3.69-4.13 (m, 2H, CHCH3), 2.20-2.41 (m, 6H, aryl-p-CH3),
1.93-2.11 (m, 6H, aryl-o-CH3), 1.52-1.63 (m, 6H, CHCH3),
1.35-1.48 (m, 6H, NdCMe). 13C NMR (CDCl3, δ (ppm)):
168.77, 167.63, 167.44, 155.14, 155.02, 154.91, 146.89, 146.74,
145.95, 145.73, 145.51 136.55, 133.92, 133.76, 133.63, 129.01,
128.14, 127.99, 127.94, 127.64, 125.86, 125.59, 125.43, 124.95,
124.90, 124.77, 122.12, 121.97, 121.85, 40.66, 40.56, 39.63, 22.48,
22.29, 21.16, 21.01, 20.56, 18.02, 17.73, 17.66, 16.61, 16.15, 16.01.
Anal. Calcd for C41H43N3: C, 85.23; H, 7.50; N, 7.27. Found: C,
84.48; H, 7.41; N, 7.01. EI-MS (m/z): 578.9 [M]þ.
minum (TMA) with H2O from Al2 (SO4)3 18H2O dispersed in
3
toluene at 0-60 °C for several hours. The initial [H2O]/[TMA]
molar ratio was 1.3. CF3SO3H was used from a freshly opened
ampule. Other commercially available reagents were purchased
and used without purification.
Synthesis of 4-Methyl-2-(sec-phenethyl)aniline (A1). A1 was
prepared according to the literature procedure.16
Synthesis of ArNdC(C5H3N)CdNAr (L3; Ar = 4-Methoxy-
2-(sec-phenethyl)phenyl). A solution of 2,6-diacetylpyridine
(0.24 g, 1.47 mmol), aniline A3 (0.68 g, 2.99 mmol), p-toluene-
sulfonic acid (0.10 g), and molecular sieves 4 A (3.0 g) in toluene
(30 mL) was stirred at 80-110 °C for 24 h. Then the reaction
mixture was filtered, and the molecular sieves were washed with
CH2Cl2 several times. The solvents of the combined filtrates
were removed under vacuum. Anhydrous ethanol (30 mL) was
added to the residue. A yellow solid was filtered off to give L3
Synthesis of 4,6-Dimethyl-2-(sec-phenethyl)aniline (A2). 2,4-
Dimethylaniline (15 mL, 0.12 mol), styrene (20 mL, 0.17 mol),
and CF3SO3H (3 mL, 0.034 mol) in 25 mL of xylenes were
heated in a sealed tube at 160 °C for 30 h. Volatile materials were
then removed by rotary evaporation, and the residue was
recrystallized in a petroleum ether/ethyl acetate (9/1) mixed
solvent to give pure A2 as white crystals with a yield of 8.43 g
1
1
(31%). H NMR (300 MHz, CDCl3, δ (ppm)): 7.16-7.30 (m,
with a yield of 0.425 g (51%). H NMR (300 MHz, CDCl3, δ
5H, J = 6.9 Hz, aryl-H), 6.99 (s, 1H, aryl-H), 6.84 (s, 1H, aryl-
H), 4.12 (q, 1H, J = 6.9 Hz, CHCH3), 2.30 (s, 3H, p-CH3), 2.13
(s, 3H, o-CH3), 1.62 (d, 3H, J = 6.9 Hz, CHCH3). EI-MS (m/z):
(ppm)): 8.33 (d, 2H, J = 7.8 Hz, Py-Hm), 7.87 (t, 1H, J = 7.5 Hz,
Py-Hp), 7.01-7.10 (m, 12H, aryl-H), 6.77 (d, 2H, J = 8.4 Hz,
aryl-H), 6.58 (d, 2H, J = 8.4 Hz, aryl-H), 4.28 (q, 2H, J = 6.9
Hz, CHCH3), 3.85 (s, 6H, p-OCH3), 1.88 (d, 6H, J = 8.1 Hz,
CHCH3), 1.63 (d, 6H, J = 6.9 Hz, NdCMe). 13C NMR (CDCl3,
δ (ppm)): 167.80, 156.54, 155.59, 146.26, 142.87, 137.73, 136.65,
128.92, 128.29, 127.95, 127.66, 125.81, 122.12, 119.50, 113.77,
111.19, 55.78, 40.64, 21.65, 16.26. Anal. Calcd for C39H39N3O2:
C, 80.52; H, 6.76; N, 7.22. Found: C, 80.65; H, 7.12; N, 6.77. EI-
MS (m/z): 582.8 [M]þ.
226.2 [M]þ
.
Synthesis of 4-Methoxy-2-(sec-phenethyl)aniline (A3). 4-Methoxy-
aniline (15.7 g, 0.11 mol), styrene (13 mL, 0.11 mol), and CF3SO3H
(2 mL, 0.023 mol) in 25 mL of xylenes were heated in a sealed
tube at 160 °C for 30 h. Volatile materials were then removed by
rotary evaporation, and the residue was typically purified by
chromatography on silica gel (petroleum ether/ ethyl acetate 5/1)
to give the desired product as a light red oil. The oil was recry-
stallized in a petroleum ether/ethyl acetate (9/1) mixed solvent to
produce pure A3 as white crystals with a yield of 3.19 g (11%). 1H
NMR (300 MHz, CDCl3, δ (ppm)): 7.18-7.30 (m, 5H, aryl-H),
6.91 (s, 1H, aryl-H), 6.58-6.69 (m, 2H, aryl-H), 4.11 (q, 1H, J =
7.2 Hz, CHCH3), 3.80 (s, 3H, p-OCH3), 3.18 (2H, NH2), 1.61 (d,
3H, J = 7.2 Hz, CHCH3). EI-MS (m/z): 228.5 [M]þ.
Synthesis of ArNdC(C5H3N)CdNAr (L1; Ar = 4-Methyl-
2-(sec-phenethyl)phenyl). A solution of 2,6-diacetylpyridine
(1.05 g, 6.44 mmol), aniline A1 (3.12 g, 14.8 mmol), p-toluene-
sulfonic acid (0.20 g), and molecular sieves 4 A (3.0 g) in toluene
(40 mL) was stirred at 80-110 °C for 24 h. Then the reaction
mixture was filtered, and the molecular sieves were washed with
CH2Cl2 several times. The solvents of the combined filtrates
were removed under vacuum. Anhydrous ethanol (30 mL) was
added to the residue. A yellow solid was filtered off to give L1 in
a 45% yield (1.50 g). 1H NMR (300 MHz, CDCl3, δ (ppm)): 8.30
(d, 2H, J = 7.8 Hz, Py-Hm), 7.87 (t, 1H, J = 7.5 Hz, Py-Hp),
7.02-7.23 (m, 14H, aryl-H), 6.53 (d, 2H, J = 7.5 Hz, aryl-H),
4.21 (q, 2H, J = 6.9 Hz, CHCH3), 2.40 (s, 6H, p-CH3), 1.84 (d,
6H, J = 7.5 Hz, CHCH3), 1.63 (d, 6H, J = 6.9 Hz, NdCMe).
13C NMR (CDCl3, δ (ppm)): 167.55, 155.57, 147.11, 146.70,
136.74, 135.73, 133.25, 128.34, 128.07, 127.85, 127.50, 125.79,
122.25, 118.69, 40.59, 21.82, 21.68, 16.31. Anal. Calcd for
C39H39N3: C, 85.21; H, 7.15; N, 7.64. Found: C, 84.74; H,
7.15; N, 7.64. EI-MS (m/z): 551.1 [M]þ.
Synthesis of (ArNdC(C5H3N)CdNAr)FeCl2 (1; Ar = 4-Methyl-
2-(sec-phenethyl)phenyl). L1 (0.56 g, 1.02 mmol) was added to a
solution of FeCl2 4H2O (0.19 g, 0.96 mmol) in THF (35 mL) at
3
room temperature with violent stirring. The solution turned deep
blue immediately, and the blue product precipitated from the
solution after several minutes. After the mixture was stirred for
8 h at room temperature, the supernatant liquid was removed and
the product was washed three times with 3 ꢀ 5 mL of Et2O and
dried under vacuum. The desired product was isolated as a light
blue powder in 84% yield (0.58 g). 1H NMR (500 MHz, CD2Cl2,
δ (ppm)): 76.65 (s, 2H, Py-Hm), 37.3 (s, 1H, Py-Hp), 22.93 (s, 4H,
Ar-Ho), 21.56 (s, 2H, aryl-Ho), 20.83 (s, 2H, aryl-Hm), 16.81 (s, 4H,
Ar-Hp), 11.01 (s, 2H, aryl-Hm), 9.42 (s, 6H, p-CH3), -3.82 (s, 2H,
Ar-Hp), -5.52 (s, 6H, CHCH3), -16.09 (s, 2H, CHCH3), -24.41
(s, 6H, NdCMe). Anal. Calcd for C39H39Cl2FeN3: C, 69.24; H,
5.81; N, 6.21. Found: C, 68.84; H, 5.68; N, 5.99. IR (KBr, cm-1):
1620 (νCdN), 1586, 1451, 1371, 1266, 1217, 1029, 817, 763. FAB-
MS (m/z): 675 [M]þ.
Synthesis of (ArNdC(C5H3N)CdNAr)FeCl2 (2; Ar = 4,6-
Dimethyl-2-(sec-phenethyl)phenyl). By the above procedure,
2 was isolated as a deep blue powder in 67% yield. H NMR
1
(500 MHz, CD2Cl2, δ (ppm)): 79.48 (s, 2H, Py-Hm), 49.16 (s, 1H,
Py-Hp), 23.07 (s, 4H, Ar-Ho), 17.07, 14.54 (s, 4H, aryl-Hm), 8.80
(s, 6H, aryl-o-CH3), 8.66 (s, 4H, Ar-Hm), 3.24 (s, 2H, CHCH3),
2.15 (s, 6H, aryl-p-CH3), -4.05 (s, 2H, Ar-Hp), -8.46 (s, 6H,
CHCH3), -23.41 (s, 6H, NdCMe). Anal. Calcd for C41H43-
Cl2FeN3: C, 69.89; H, 6.15; N, 5.96. Found: C, 69.52; H, 6.45; N,
5.77. IR (KBr, cm-1): 1616 (νCdN), 1581, 1470, 1371, 1261, 1214,
Synthesis of ArNdC(C5H3N)CdNAr (L2; Ar = 4,6-Dimethyl-
2-(sec-phenethyl)phenyl). A solution of 2,6-diacetylpyridine (1.15 g,
7.05 mmol), aniline A2 (4.27 g, 18.9 mmol), p-toluenesulfonic acid
(0.25 g), and molecular sieves 4 A (3.0 g) in toluene (40 mL) was
stirred at 80-110 °C for 24 h. Then the reaction mixture was
filtered, and the molecular sieves were washed with CH2Cl2 several
times. The solvents of the combined filtrates were removed under
vacuum. Anhydrous ethanol (30 mL) was added to the residue.
A yellow solid was filtered off to give L2 in 47% yield (1.86 g).
1H NMR (300 MHz, CDCl3, δ (ppm)): 8.43-8.48 (m, 2H,
Py-Hm), 8.23-8.27 (m, 1H, Py-Hp), 6.94-7.97 (m, 14H, aryl-H),
1028, 817, 764. FAB-MS (m/z): 703 [M] þ
.
Synthesis of (ArNdC(C5H3N)CdNAr)FeCl2 (3; Ar = 4-Methoxy-
2-(sec-phenethyl)phenyl). By the above procedure, 3 was isolated
as a light brown powder in 91% yield. 1H NMR (500 MHz,
CD2Cl2, δ (ppm)): 81.62 (s, 2H, Py-Hm), 60.1 (s, 1H, Py-Hp), 23.74
(s, 4H, Ar-Ho), 18.35 (s, 4H, Ar-Hm), 16.02 (s, 2H, aryl-Ho), 15.45,
11.39 (s, 4H, aryl-Hm), 5.96 (s, 6H, p-OCH3), -1.07(s, 4H, Ar-Hp),
-3.87 (s, 6H, CHCH3), -6.13 (s, 2H, CHCH3), -18.60 (s, 6H,
NdCMe). Anal. Calcd for C39H39Cl2FeN3O2: C, 66.11; H, 5.55;
N, 5.93. Found: C, 65.92; H, 5.71; N, 5.67. IR (KBr, cm-1): 1602
(νCdN), 1480, 1372, 1289, 1214, 1029, 806, 773. FAB-MS (m/z):
707 [M]þ.
(16) Cherian, A. E.; Domski, G. J.; Rose, J. M.; Lobkovsky, E. B.;
Coates, G. W. Org. Lett. 2005, 7, 5135.

Article
Synthesis of (ArNdC(C5H3N)CdNAr)FeCl2 (4; Ar = 2,6-Diiso-
propylphenyl). Complex 4 was prepared according to a published
Organometallics, Vol. 29, No. 9, 2010 2125
recorded on a Nicolet NEXUS-670 FT-IR spectrometer. DSC
analysis was conducted with a PerkinElmer DCS-7 system. The
DSC curves were recorded at a heating rate of 10 °C/min to
160 °C. The cooling rate was 10 °C/min. Gel permeation
chromatography (GPC) analyses of the molecular weight and
molecular weight distribution of the polymers were performed
on a Waters 135C instrument using standard polystyrene as
reference and with 1,2,4-trichlorobenzene (TCB) as the solvent
at a flow rate of 1.0 mL/min. The samples were obtained by
removing the aluminum cocatalyst through the addition of HCl-
acidified ethanol and subsequent filtration of the polymeriza-
tion products.
Crystal Structure Determination. Crystals were mounted
on glass fibers using the oil drop scan method.17 Data obtained
with the ω-2θ scan mode was collected on a Bruker SMART
1000 CCD diffractometer with graphite-monochromated Mo
KR radiation (λ = 0.710 73 A) at 293 K. The structures were
solved using direct methods, while further refinement with full-
matrix least squares on F2 was obtained with the SHELXTL
program package.18,19 All non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were introduced in calculated positions
with the displacement factors of the host carbon atoms.
procedure.1a-c
Polymerization of Ethylene under 1 bar of Pressure. The
polymerization runs were carried out under an extra-pure-grade
nitrogen atmosphere in a 100 mL glass flask equipped with a
magnetic stirrer. MAO in toluene (30 mL) was injected into the
reactor, and then the solution was saturated under 1 bar of
ethylene pressure. The solution of iron(II) complex in toluene
was added into the reactor. After 1 h, the polymerizations were
stopped by quenching with ethanol containing 10% HCl. The
resulting polymer was separated by filtration and dried under
vacuum at 50 °C to constant weight.
Polymerization of Ethylene under 10 bar of Ethylene Pressure.
A mechanically stirred 500 mL Parr reactor was heated over-
night to 150 °C under vacuum and then cooled to room temp-
erature. The autoclave was pressurized to 10 bar of ethylene and
vented three times. The autoclave was then charged with 240 mL
of a solution of MAO in toluene under 1 bar of ethylene at
initialization temperature. The system was maintained by con-
tinuous stirring for 30 min, and then 10 mL of a solution of the
iron complex in toluene was charged into the autoclave under
1 atm of ethylene. The ethylene pressure was raised to the speci-
fied value (10 bar), and the reaction was carried out for a certain
time. Polymerization was terminated by addition of acidic
ethanol after releasing ethylene pressure. The resulting precipi-
tated polymers were collected and treated by filtering, washing
with ethanol several times, and drying under vacuum at 50 °C to
a constant weight.
Acknowledgment. Financial support by the NSFC
(Projects 20734004, 20974125, and 20674097), NSFG
(Project 8251027501000018), and the Ministry of Educa-
tion of China (Foundation for Ph.D. Training) are grate-
fully acknowledged.
Measurements. Elemental analyses were performed on a
Vario EL microanalyzer. Mass spectra were obtained using
either fast atom bombardment (FAB) LCQ DECA XP or
Supporting Information Available: A CIF file and a table
giving details of the crystallographic data of complex 2 and
figures giving NMR spectra of ligands L1-L3. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
electron impact (EI) LCMS-2010A. H NMR (300 MHz) and
13C NMR (75 MHz) spectra were recorded in CDCl3 on a
Varian Unity Inova 300 spectrometer for the ligands, and H
1
(17) Kottke, T.; Stalke, D. J. Appl. Crystallogr. 1993, 26, 615.
(18) SHELXTL, Version 5.1; Bruker AXS, Madison, WI, 1998.
(19) Sheldrick, G. M. SHELXL-97, Program for X-ray Crystal
NMR spectra were carried out on an INOVA 500 MHz spectro-
meter in CD2Cl2 solution for the complexes. 1H and 13C NMR
chemical shifts were referenced to TMS and to the 13C NMR
signals of the deuterated solvents, respectively. IR spectra were
€
€
Structure Solution and Refinement; University of Gottingen, Gottingen,
Germany, 1998.