Chromium complexes ligated by 2-carbethoxy-6-iminopyridines: Synthesis, characterization and their catalytic behavior toward ethylene polymerization

Article abstract of DOI:10.1016/j.molcata.2006.10.024

The titled chromium complexes (C1-C6) were prepared by the reaction of CrCl₃(THF)₃ with the corresponding 2-carbethoxy-6-iminopyridines (L1-L6) in dichloromethane. All the complexes were characterized by IR spectroscopy and elemental analysis. The unambiguous solid-state structures of C1, C3 and C5 were determined by X-ray crystallography. The chromium core was found to be coordinated by a molecule of 2-carbethoxy-6-iminopyridine and three atoms of chlorine to form a distorted octahedral coordination geometry. Activated with ethylaluminum dichloride (EtAlCl₂), these complexes showed notable catalytic activities for ethylene polymerization. The nature of the ligands and reaction parameters affected the properties of the resultant polyethylenes.

Full text of DOI:10.1016/j.molcata.2006.10.024

Journal of Molecular Catalysis A: Chemical 265 (2007) 159–166
Chromium complexes ligated by 2-carbethoxy-6-iminopyridines:
Synthesis, characterization and their catalytic behavior
toward ethylene polymerization
Wenjuan Zhanga, Wen-Hua Suna,∗, Xiubo Tanga, Tielong Gaoa,
Shu Zhanga, Peng Haoa, Jiutong Chenb
a Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, China
b State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou 350002, China
Received 13 July 2006; received in revised form 11 October 2006; accepted 15 October 2006
Available online 20 October 2006
Abstract
The titled chromium complexes (C1–C6) were prepared by the reaction of CrCl3(THF)3 with the corresponding 2-carbethoxy-6-iminopyridines
(L1–L6) in dichloromethane. All the complexes were characterized by IR spectroscopy and elemental analysis. The unambiguous solid-state
structures of C1, C3 and C5 were determined by X-ray crystallography. The chromium core was found to be coordinated by a molecule of
2-carbethoxy-6-iminopyridine and three atoms of chlorine to form a distorted octahedral coordination geometry. Activated with ethylaluminum
dichloride (EtAlCl2), these complexes showed notable catalytic activities for ethylene polymerization. The nature of the ligands and reaction
parameters affected the properties of the resultant polyethylenes.
© 2006 Elsevier B.V. All rights reserved.
Keywords: 2-Carbethoxy-6-iminopyridines; Chromium complexes; Ethylene polymerization; Ethylaluminum dichloride
1. Introduction
The discovery of metallocene not only provided highly active
catalysts [5] but also opened the way to trace the processes of
industrial engineering back to the concepts of their academic sci-
ence about the active sites and catalytic mechanism [6]. Along
with the development of measuring equipments, organometallic
chemists have speeded up their research works in order to inves-
tigate the catalytic sites and design single site catalysts. Their
extension in polyolefins has fast driven the application of tran-
sition metal complexes as catalysts in olefin polymerization [7].
There are many reports published concerning various transition
metal complexes as catalysts [8] and the design of functional cat-
alysts based on new ligands [9]. Chromium complexes bearing
various ligands of NˆN, NˆNˆN, NˆO and NˆSˆN as homoge-
nous catalysts have attracted considerable attention [10,11];
however, there are only few reports on the mechanism and
active sites [12]. In our earlier works, we studied the 2,6-bis(2-
benzimidazolyl)pyridyl chromium chlorides as catalysts for
ethylene reactivity, and also reported that nickel, iron and cobalt
complexes bearing 2-carbethoxy-6-iminopyridines exhibited
promising results exhibited for ethylene reactivity [13]. There-
Catalysts consisted of CrOx/SiO2 are well-known practical
catalysts in polyolefin industry, and those catalysts produce one-
third of the worldwide commercial high-density polyethylenes
(HDPE) [1]. The resultant polyethylenes are known to display
unique properties due to the ultra-broad molecular weight dis-
tribution, short and long chain branches and are suitable for
the blow-molding process [2]. In spite of their practical impact
for the past 50 years, the states of the active species and the
polymerization mechanism of these catalysts are still unclear
[3]. In addition, the heterogeneous nature of these systems is
responsible for certain heterogeneities in the produced poly-
mers, which might not be desirable for some applications [4].
∗
Corresponding author at: Key Laboratory of Engineering Plastics, Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100080, China.
Tel.: +86 10 62557955; fax: +86 10 62618239.
E-mail address: whsun@iccas.ac.cn (W.-H. Sun).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.10.024

160
W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 159–166
fore, wepreparedchromiumcomplexesligatedby2-carbethoxy-
6-iminopyridines (NˆNˆO ligands) and carefully explored their
reactivity toward ethylene in the presence of various cocatalysts.
During the course of our study, Small et al. reported an analo-
gous chromium complex containing 2-acetyl-6-iminopyridines,
which afforded low molecular weight polyethylene waxes and
polyethylene upon activation with MAO [12d].
We herein present the syntheses and identification of a
series of chromium complexes containing 2-carbethoxy-6-
iminopyridines. We, recently, found that the reaction with ethy-
lene in the presence of MAO afforded oligomers, whereas
polyethylene could be obtained when the reaction is performed
in the presence of Et2AlCl [13a]. We, thus, explored factors
affecting the catalysts toward their performance in ethylene
polymerization and/or oligomeriztion in the presence of Al
cocatalysts under various conditions.
2.2.2. 2-Carbethoxy-6-[1-((2,6-diethylphenyl)imino)
ethyl]pyridine chromium(III) complex (C2)
Using the procedure described for C1, complex C2 (0.429 g,
0.89 mmol) was obtained as green powder in yield 89.1%.
IR (KBr): 3428; 2965; 1645; 1587; 1458; 1406; 1380; 1341;
1284; 1094; 850; 802; 767; 397 cm−1. Anal. Calcd. for
C20H24Cl3CrN2O2: C, 49.76; H, 5.01; N, 5.80. Found: C, 49.54;
H, 4.81; N, 6.03%.
2.2.3. 2-Carbethoxy-6-[1-((2,6-diisopropylphenyl)imino)
ethyl]pyridine chromium(III) complex (C3)
Using the procedure described for C1, complex C3 (0.433 g,
0.85 mmol) was obtained as green powder in yield 85.0%. IR
(KBr): 3430; 2974; 1649; 1587; 1463; 1343; 1284; 849; 763;
398 cm−1. Anal. Calcd. for C20H24Cl3CrN2O2: C, 51.73; H,
5.52; N, 5.48. Found: C, 51.58; H, 5.53; N, 5.21%.
2. Experimental
2.2.4. 2-Carbethoxy-6-[1-((2,6-diflurophenyl)imino)
ethyl]pyridine chromium(III) complex (C4)
2.1. General procedures
Using the procedure described for C1, complex C4 (0.428 g,
0.93 mmol) was obtained as green powder in yield 92.6%.
IR (KBr): 1641; 1589; 1475; 1383 cm−1. Anal. Calcd. for
C16H14Cl3CrF2N2O2: C, 41.54; H, 3.05; N, 6.06. Found: C,
41.17; H, 3.29; N, 5.98%.
All manipulations of the moisture-sensitive compounds were
carried out under an atmosphere of nitrogen using standard
Schlenk techniques. The IR spectra were obtained on a Perkin-
Elmer FT-IR 2000 spectrophotometer using the KBr discs in
the range of 4000−400 cm−1. Elemental analyses were per-
formed on a Flash EA 1112 micro analyzer. The distribution
of oligomers obtained was measured on a Varian VISTA 6000
2.2.5. 2-Carbethoxy-6-[1-((2,6-dichloridephenyl)imino)
ethyl]pyridine chromium(III) complex (C5)
Using the procedure described for C1, complex C5
(0.461 g, 0.93 mmol) was obtained as green powder in yield
92.8%. IR (KBr): 1644; 1586; 1436; 1411. Anal. Calcd. for
C16H14Cl5CrN2O2: C, 38.78; H, 2.85; N, 5.65. Found: C, 38.50;
H, 3.01; N, 5.33%.
1
GC spectrometer and a HP 5971A GC–MS detector. H and
13C NMR spectra of the PE samples were recorded on a Bruker
DMX-300 MHz instrument at 135 ◦C in 1,2-dichlorobenzene-
d4 using TMS as an internal standard. Molecular weights and
polydispersity indices (PDI) of PE were determined by a PL-
GPC220 at 150 ◦C with 1,2,4-trichlorobenzene as eluent. Melt-
ing points of the polymers were obtained on a Perkin-Elmer
DSC-7 in the standard DSC run mode. The instrument was
initially calibrated for melting point of an indium standard at
a heating rate of 10 ◦C/min. The polymer sample was first
equilibrated at 10 ◦C and then heated to 160 ◦C at a rate of
10 ◦C/min to remove thermal history. The sample was then
cooled to 10 ◦C at a rate of 10 ◦C/min. A second heating cycle
was used for collecting DSC thermogram data at a ramping rate
of 10 ◦C/min.
2.2.6. 2-Carbethoxy-6-[1-((2,6-dibromophenyl)imino)
ethyl]pyridine chromium(III) complex (C6)
Using the procedure described for C1, complex C6
(0.575 g, 0.99 mmol) was obtained as green powder in yield
98.5%. IR (KBr): 1645; 1584; 1412; 1379. Anal. Calcd. for
C16H14Br2Cl3CrN2O2: C, 32.88; H, 2.41; N, 4.79. Found: C,
32.68; H, 2.56; N, 4.45%.
2.3. X-ray crystallography measurement
The single-crystal X-ray diffraction for complexes C1 and C5
wascarriedoutonaBrukerSmart1000CCDdiffractometerwith
2.2. Synthesis of complexes (Scheme 1)
2.2.1. 2-Carbethoxy-6-[1-((2,6-dimethylphenyl)imino)
ethyl]pyridine chromium(III) complex (C1)
Freshly distilled dichloromethane was added to ligand L1
(0.296 g, 1 mmol) and CrCl3(THF)3 (0.372 g, 1 mmol) at the
room temperature and the mixture was stirred for 12 h. The
green precipitate was filtered and washed with diethyl ether. The
desired complex C1 (0.390 g, 0.86 mmol) was obtained as green
powder in yield 86.0%. IR (KBr): 1642; 1587; 1467; 1407; 1381;
1341; 1291; 1217; 1097; 992; 851; 768 cm−1. Anal. Calcd. for
C18H20Cl3CrN2O2: C, 47.54; H, 4.43; N, 6.16. Found: C, 47.44;
H, 4.60; N, 6.33%.
Scheme 1. Synthesis of complexes C1–C6.

W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 159–166
161
Table 1
Crystallographic data and refinement for C1, C3 and C5
C1
C3
C5
Formula
fw
Crystallographic system
Space group
C19H22Cl5CrN2O2
539.64
C23H30Cl5CrN2O2
595.74
Monoclinic
P2(1)/c
17.872(4)
10.724(2)
15.887(3)
90
111.25(3)
90
2838.0(10)
4
1.394
0.896
1228
1.22–25.01
4386
C16H14Cl5CrN2O2
495.54
Monoclinic
P2(1)/c
8.320(3)
14.360(4)
19.810(7)
90
101.360(2)
90
2320.4(13)
4
1.545
1.087
1100
2.10–27.48
17698
Monoclinic
P2(1)/c
8.3797(2)
27.6092(8)
9.6062(2)
90
115.141(1)
90
2011.91(9)
4
1.636
1.246
996
1.48–25.06
6138
˚
a (A)
˚
b (A)
˚
c (A)
α (◦)
β (◦)
γ (◦)
3
˚
V (A )
Z
Dcalcd (g cm−3
)
Absolute coefficient, μ (mm−1
F(0 0 0)
)
θ range (◦)
No. of data collected
No. of unique data
R (%)
Rw (%)
Goodness of fit
5293
0.0629
0.1673
0.945
4386
3527
0.0672
0.1272
1.194
0.0600
0.1399
0.895
toluene were added to the reactor in this order under ethylene
atmosphere. Ethylene (10 atm pressures) was introduced at the
reaction temperature to commence the reaction and was stopped
after the desired time range. A small amount of the reaction solu-
tion was collected, terminated by the addition of 5% aqueous
hydrogen chloride and then analyzed by gas chromatography
(GC) for the distribution of obtained oligomers. The residual
solution was quenched with HCl-acidified ethanol (5%). The
precipitated polymer was collected by filtration, washed with
ethanol, dried in vacuum at 60 ◦C until constant weight, weighed
and finally characterized.
˚
graphite monochromated Mo K␣ radiation (λ = 0.71073 A) at
293(2) K, respectively. Intensity data of complexes C3 were col-
lected on a Rigaku RAXIS Rapid IP diffractometer at 293(2) K
˚
withgraphitemonochromatedMoK␣radiation(λ = 0.71073 A).
Cell parameters were obtained by global refinement of the posi-
tions of all collected reflections. Intensities were corrected. The
structures were solved by direct methods and refined by full-
matrix least-squares on F2. Each H atom was placed in a calcu-
lated position. Structure solution and refinement were performed
using the SHELXL-97 Package. Crystal data and processing
parameters are summarized in Table 1. Crystallographic data of
complexes C1, C3 and C5 have been deposited within the Cam-
bridge Crystallographic Data Center, CCDC 617982, 617983
and 617984, respectively.
3. Results and discussion
3.1. Synthesis of pyridylimine chromium(III) complexes
2.4. General procedure for ethylene polymerization at
1 atm ethylene pressure
The ester-substituted pyridylimine ligands L1–L6 [2-CO2Et-
6-(2,6-R2C6H3N CCH3) C5H3N (1: R = CH3; 2: R = Et; 3:
R = i-Pr; 4: R = F; 5: R = Cl; 6: R = Br)] were prepared accord-
ing to the reported procedures [13b]. The chromium complexes
(C1–C6) were obtained by treating the dichloromethane solu-
tion of CrCl3(THF)3 with the corresponding ligand (L1–L6) at
room temperature. These complexes showed high stability in
both solution and solid and could be identified by FT-IR and
elemental analysis; however, the NMR spectrum could not be
obtained due to their paramagnetic nature. The IR spectra of the
ligands showed that the C N stretching frequencies appeared in
the range of 1640–1655 cm−1 [13b], while the C N stretching
vibrations shifted toward lower frequency bands between 1619
and 1625 cm−1 with weak intensities for complexes C1–C6. The
results indicate the coordination interaction between the imino
nitrogen atom and the chromium center. Moreover, the C O
stretching vibrations in IR spectra showed a slight red shift by
ca. 10–20 cm−1 for the chromium complexes.
A flame dried three-neck round flask was loaded with
the complex C1–C6 and vacuum-filled three times by nitro-
gen. Then ethylene was charged together with freshly distilled
toluene and stirred for 10 min. MAO was added by a syringe.
The reaction mixture was stirred under 1 atm ethylene pressure
for a required time interval, and the catalytic reaction was ter-
minated with acidified water. An aliquot of the reaction mixture
was analyzed by GC and GC–MS.
2.5. General procedure for ethylene polymerization at
10 atm ethylene pressure
Ethylene polymerization was carried out in a 250 ml auto-
clave stainless steel reactor equipped with a mechanical stirrer
and a temperature controller. The desired amount of EtAlCl2,
30 ml toluene solution of chromium complex and 70 ml of

162
W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 159–166
Table 2
Selected bond lengths and bond angles for complexes C1, C3 and C5
C1 C3 C5
˚
Bond lengths (A)
Cr–N(1)
Cr–N(2)
Cr–O(1)
Cr–Cl(1)
Cr–Cl(2)
Cr–Cl(3)
2.012(3)
2.102(3)
2.160(3)
2.3120(11)
2.2814(11)
2.3124(12)
2.024(3)
2.152(4)
2.168(3)
2.3191(15)
2.2949(15)
2.2982(15)
2.024(4)
2.133(4)
2.122(4)
2.3126(17)
2.2861(16)
2.2935(18)
Bond angles (◦)
N(1)–Cr–N(2)
N(2)–Cr–O(1)
N(1)–Cr–O(1)
N(1)–Cr–Cl(1)
N(2)–Cr–Cl(1)
O(1)–Cr–Cl(1)
N(1)–Cr–Cl(2)
N(2)–Cr–Cl(2)
O(1)–Cr–Cl(2)
N(1)–Cr–C1(3)
N(2)–Cr–C1(3)
O(1)–Cr–C1(3)
Cl(1)–Cr–Cl(2)
Cl(1)–Cr–Cl(3)
C1(2)–Cr–C1(3)
76.87(12)
153.09(11)
76.50(11)
88.93(9)
93.52(9)
82.36(8)
177.62(9)
100.81(9)
105.85(8)
86.22(9)
95.84(9)
86.05(8)
91.78(5)
168.20(5)
93.50(5)
76.31(16)
153.62(13)
77.32(14)
86.21(11)
95.22(11)
83.76(10)
176.89(13)
100.61(10)
105.76(9)
87.42(10)
93.72(11)
84.34(10)
93.68(6)
75.89(16)
152.74(16)
76.85(15)
88.45(12)
92.70(12)
86.48(13)
178.18(13)
102.29(12)
104.96(11)
88.00(13)
93.74(12)
85.39(13)
91.80(6)
Fig. 1. The molecular structure of complex C1. The hydrogen atoms and the
solvated CH2Cl2 are omitted for clarity.
167.49(5)
93.26(6)
171.68(7)
91.98(7)
tate fashion. The geometry around the chromium atom could
be described as a distorted octahedron. The Cr–N (pyridine)
Fig. 2. The molecular structure of complex C3. The hydrogen atoms and the
solvated CH2Cl2 are omitted for clarity.
˚
˚
bond distance (2.012(3) A) is about 0.09 A shorter than the
˚
Cr–N(imino) bond distance (Cr–N2, 2.102(3) A). The plane
3.2. Structure determinations
of the phenyl ring is oriented approximately orthogonal to
the coordination plane with the angle of 77.7◦. It is worth-
while to mention, that the bond angle of Cl(1)–Cr–Cl(2) and
Cl(1)–Cr–Cl(3) are 91.77(5)◦ and 168.21◦, respectively, and
the distances between the chromium and the trans-disposed
chlorides are significantly different from the distance found for
the chloride in the mer position. The bond length of Cr–Cl(2)
Single crystals of the complexes C1, C3 and C5 for X-
ray diffraction were obtained and their molecular structures are
depicted in Figs. 1–3, respectively. Their selected bond lengths
and bond angles are listed in Table 2. There is a solvated CH2Cl2
in the crystal lattice of C1 and C3 without bond interaction
between the solvent molecule and the chromium atom. Because
the coordination geometry of the complexes C1, C3 and C5 are
highly similar, only the structure of C1 is described in detail.
Crystals of C1 were grown from a dichloromethane solution
layering with hexane. Fig. 1 shows that the carbonyl oxygen
atom O(1) also coordinates with the chromium center and,
therefore, the ligand chelates the metal core in NˆNˆO triden-
˚
(2.2814(11) A) is shorter than the bond length of Cr–Cl(1)
˚
˚
(2.3120(11) A) and Cr–Cl(3) (2.3124(12) A).
Crystals of C3 were grown from a dichloromethane solution
layeringwithhexane. ItsmolecularstructureisdepictedinFig. 2.
The coordination geometry around the chromium center of C3
is similar to that of C1 in spite of the presence of isopropyl group
in place of the methyl groups in the ortho-positions of the phenyl
ring. The plane of the phenyl ring is also oriented approximately
orthogonal to the coordination plane making an angle of 95.4◦,
which is much larger than that of complex C1 (77.7◦). Like the
structure of complex C1, two chlorides are trans-disposed and
the other chloride is in the mer position.
Crystals of C5 were obtained by diffusing hexane layer into
itsdichloromethanesolution. X-raystructure(asshowninFig. 3)
determination indicates that its structure is quite similar to those
of C1 and C3. The coordination geometry around the chromium
isfoundtobeadistortedoctahedronandthebondlengths(Cr–N)
are close to those of complexes C1 and C3. The dihedral angle
between the plane of the benzene ring and the coordination plane
◦
˚
is 83.0 . However, the Cr–O bonding length of C5 (2.122 (4) A)
Fig. 3. The molecular structure of complex C5. The hydrogen atoms are omitted
for clarity.
˚ ˚
is much shorter than that of C1 (2.160(3) A) or C3 (2.168(3) A),

W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 159–166
163
which may be due to electron-withdrawing effect of the ortho-
group in the benzene ring.
Therefore, EtAlCl2 is selected as the cocatalyst for the further
study and discussion.
3.3. Ethylene oligomerization and polymerization
3.3.2. The effect of Al/Cr on ethylene reactivity and
properties of resulting polymers
All of the complexes were tested for ethylene reactivity with
various cocatalysts in toluene under both ambient pressure and
10 atm.
To probe the effects of reaction parameters on the ethylene
reactivity, polymerization behavior was typically investigated
via changing the amounts of EtAlCl2, reaction temperature,
reaction solvent, reaction time and ethylene pressure.
The effect of the Al/Cr ratio on the polymerization activity
was investigated with C1 and the results are shown in Table 3.
With Al/Cr molar ratios in the range of 50–300, the system
demonstrated fairly high catalytic activity. The increase in the
Al/Cr ratio to 1000 led to noticeable increase in polymeriza-
tion activity; however, further increase led to a slight decrease
in the activity. This correlation between the polymer productiv-
ity and the Al/Cr molar ratio suggests that the chain transfer to
aluminum might occur with a consequence effect of ceased poly-
mer chain pregnancy [15]. In addition, The Mw value decreased
rapidly and the narrower molecular distribution was observed
upon increase in the Al/Cr molar ratio for complex C1 (entries
9, 11, 12 in Table 3), also corroborating to some extent the occur-
rence of chain transfer to Al. The Al/Cr ratio had little effect on
the Tm of resultant polyethylene.
3.3.1. The effect of different cocatalysts on ethylene
reactivity
The chromium complex C1 was used to select suitable reac-
tion parameters. It was found to be inactive for ethylene poly-
merization without cocatalyst. When combined with various
organo-aluminum compounds such as modified methylalumi-
noxane (MMAO), diethylaluminum chloride (DEAC), ethyla-
luminum dichloride (EADC) and triethylaluminum (TEA) in
toluene, C1 showed different behavior for ethylene polymer-
ization. The C1/MMAO, C1/EADC, C1/MAO systems showed
considerable activity for ethylene polymerization at 1 atm ethy-
lene, whilethe C1/DEACandC1/TEAsystemwerenoteffective
for ethylene polymerization under the same conditions (Table 3).
Moreover, among the active systems, the C1/EtAlCl2 system
showed the highest activity for ethylene polymerization, and
the other two catalytic systems produced polymers along with
some oligomers. The difference of the catalytic activity when
different cocatalysts are employed may imply that the initial
formation of active Cr species is involved with alkylation by
alkylaluminum cocatalyst, which was discussed previously [14].
3.3.3. The effect of reaction temperature on ethylene
reactivity and properties of resultant polymers
The effect of temperature on polymerization activity was
investigated with C3 and the results are shown in Table 4.
Table 3
The result of C1 with different cocatalysts for ethylene polymerizationa
Entry
Cocatalyst
Al/Cr
Oligomer activityb,c
Polymer
d
d
Activityb
Mw (×104)
Mw/Mn (×104)
Tm (◦C)e
f
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Et2AlCl
Et2AlCl
Et2AlCl
MMAO
MMAO
MMAO
EtAlCl2
EtAlCl2
EtAlCl2
EtAlCl2
EtAlCl2
EtAlCl2
Et3Al
50
150
500
–
–
–
–
41.3
24.8
42.2
–
–
–
–
–
Trace
Trace
1.26
2.16
2.14
Trace
Trace
1.18
2.90
3.61
2.64
Trace
Trace
Trace
Trace
1.28
500
n.d.g
n.d.
n.d.
n.d.
n.d.
n.d.
130.1
129.0
128.7
1000
1500
50
150
300
7.06
n.d.
5.07
2.38
5.46
n.d.
5.21
3.84
132.9
132.0
132.4
131.5
500
1000
1500
100
300
500
–
–
Et3Al
Et3Al
Et3Al
MAO
3.29
25.3
–
1000
1000
4.37
n.d.
n.d.
134.2
a
General condition: 5 ␮mol complex; ethylene pressure 1 atm; temperature 25 ◦C; 0.5 h; toluene 30 ml.
b
c
d
e
f
104 g mol−1(Cr) h−1
.
Determined by GC and GC–MS.
Determined by GPC.
Determined by DSC.
No activity.
g
Not determined.

164
W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 159–166
Table 4
The result of complexes with EtAlCl2 for polymerization of ethylene at 1 atma
Entry
Complex
◦C
t (min)
Polymerization
Activityb
c
d
Mw (×104)
Mw/Mn
Tm (◦C)
1
2
3
4
5
6
7
8
9
10
11
12g
13h
14i
C1
C2
C3
C4
C5
C6
C3
C3
C3
C3
C3
C3
C3
C3
25
25
25
25
25
25
0
40
60
25
25
25
25
25
30
30
30
30
30
30
30
30
30
60
120
30
30
30
3.69
4.88
5.46
5.11
5.60
7.4
2.40
5.22
1.78
6.60
6.00
Trace
3.0
5.07
3.12
3.01
2.87
4.08
6.82
10.6
0.54
0.10
n.d.
5.21
5.46
4.19
5.21
5.00
15.92
3.12
4.67
1.91
n.d.
132.4
131.0
132.4
131.4
131.6
131.5
134.1
107.2, 125.9e
77.3, 105.4e
n.d.f
132.7
n.d.
134.9
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
Trace
a
General conditions: 5 ␮mol complex; 1 atm ethylene pressure; 30 ml toluene.
104 g mol−1(Cr) h−1
Determined by GPC.
Determined by DSC.
Broad melting peak.
Not determined.
Solvent dichloromethane.
Solvent chlorobenzene.
Solvent hexane.
b
c
d
e
f
.
g
h
i
Comparing the data (entries 3, 7–9 in Table 4), we observed
that the catalytic activities increased gradually to maximum
value and then decreased rapidly with increasing reaction
temperature, which is very different from our previous result
[13c]. This phenomenon can be explained as follows: at lower
temperature, the pre-catalyst might not be fully activated, while
the catalytic species are possibly unstable and solubility of
ethylene in toluene is lower at higher temperature.
However, the reaction temperature had a remarkable effect on
the Tm, molecular weight and molecular weight distribution of
the resultant polyethylenes. Increasing the temperature from 0
to 60 ◦C, the Tm of the resultant polyethylene decreased rapidly
and the DSC curves gave broader melting peaks. Meanwhile,
the molecular weight of the polymers also decreased sharply
from (10.6 to 0.10) × 104. The GPC traces (as shown in Fig. 4)
clearly displayed the decreasing trends of molecular weights,
which can be explained by frequent ␤-hydride elimination at
elevated temperature. At 40 ◦C, the molecular weight trace of
produced PE displayed bimodal behavior with broad molecular
weightdistribution, whichmightbeattributedtotheformationof
different active species after activation by EtAlCl2 [16]. Some
literature also reported that chain transfer to aluminum might
result in broad multimodal distribution of PE [17].
polymerization in toluene (entry 3) than in other solvents such
as dichloromethane and chlorobenzene (entries 12–14).
3.3.5. The effect of ethylene pressure on ethylene reactivity
and properties of resulting polymers
The results of polymerizations performed under 10 atm
pressure employing complexes C1–C6 are summarized
in Table 5. The data indicated that the ethylene pres-
sure considerably influenced the yields, molecular weights
and molecular weight distributions of the resultant poly-
mers. First, significant increment in the productivities of
the catalysts were observed (C5: 5.60 × 104 g mol−1(Cr) h−1
3.3.4. The effect of reaction time and solvent on ethylene
reactivity and properties of resulting polymers
Highercatalyticactivityisobservedatreactiontimeof60 min
compared with 30 min (Table 4, entries 3 and 10), which indi-
cates the existence of the induction period. The effect of solvent
on the catalytic activity was studied with C3 and the results
(Table 4) showed that C3 exhibited higher activity of ethylene
Fig. 4. GPC curves of the polyethylene obtained by C3 at different temperatures
(entries 3, 7–9 in Table 4).

W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 159–166
165
Table 5
The result of complexes with EtAlCl2 for polymerization of ethylene at 10 atma
Entry
Complex
Ethylene pressure
Polymerization
Activityb
Mw (×104)
Mw/Mn
Tm (◦C)
d
c
1
2
3
4
5
6
7
C1
C2
C3
C4
C5
C5
C6
10
10
10
10
10
5
1.47
1.52
2.08
1.73
2.33
1.76
1.70
12.1
n.d.
10.7
n.d.
8.10
4.54
n.d.
3.57
n.d.
3.24
n.d.
2.87
3.36
n.d.
133.9
134.5
134.6
134.1
134.4
133.9
134.4
10
n.d., not determined.
a
General conditions: 5 ␮mol complex; temperature 25 ◦C; reaction time 0.5 h; 100 ml toluene.
b
c
d
105 g mol−1(Cr) h−1
.
Determined by GPC.
Determined by DSC.
at 1 atm; 1.76 × 105 g mol−1(Cr) h−1 at 5 atm; 2.33 × 105
g mol−1(Cr) h−1 at 10 atm). When the corresponding data in
Tables 4 and 5 are compared, higher Mw and narrower distri-
bution were observed for most complexes at higher pressure.
The GPC traces shown in Fig. 5 clearly illustrated that higher
pressure led to higher molecular weight, which is attributed to
the faster rates of ethylene insertion and alkyl chain propagation
at a higher ethylene concentration (pressure) [16]. In addition,
the Tm of the resultant polymers at 10 atm ranged from (133.9
to 134.6) ◦C; these Tms are higher than those of polyethylenes
at 1 atm (range from 131.5 to 132.4 ◦C).
dependent on the size of the ortho-substituents of the imino-aryl
ring. Contrary to the results reported by Small et al. [12d], the
polymerization activities varied in the orders of C1 < C2 < C3
and C4 < C5 < C6 (either at 1 atm or at 10 atm), which may con-
tribute to the more readily deactivation of the active species due
to the smaller size of the substituents on the ortho-position of
the benzene ring. Gibson reported that the catalytic precursors
containing the pyridylimine ligands lacking sufficient steric bulk
in the ortho-aryl positions are more easily deactivated through
an interaction with alkylaluminum reagents [18]. In general, the
complexes containing the halides showed slightly higher activ-
ity than complexes with alkyl-substituents, which may be due to
a more electrophilic chromium center induced by the electron-
withdrawing substituents. This caused an enhancement of the
interaction between the chromium atom and the ␲-electrons of
the ethylene monomer, which resulted in accelerated ethylene
insertion in the growing step [19].
The ligand environment also affected the Mw and Mw/Mn of
the obtained polymer. The methyl-substituted catalyst produced
higher molecular weight (Mw) than the isopropyl-substituted
system (both in 1 and 10 atm). This is surprising in compari-
son with previous reports that demonstrated a higher molecular
weight for polyethylenes produced with bulkier ligands. How-
ever, in our study, repeated ethylene polymerizations demon-
strated consistent results for both the catalyst activity and the
molecular weight of the obtained polyethylenes, which is in
agreement with the results obtained with its iron analogue [13b].
Especially for C6 (entry 6 in Table 3), the Mw and Mw/Mn are
much higher than those of the other complexes. The real rea-
son is unclear. In addition, under the same condition, the ligand
environment showed only little effect on the Tm of the resulting
polymer.
3.3.6. The effect of ligands environment on ethylene
reactivity and properties of resulting polymers
The data listed in Table 4 (at 1 atm) and 5 (at 10 atm)
showed that the structure of the ligands considerably affected
their polymerization activities and the properties of the result-
ing polyethylenes. These complexes provided an insight into the
steric and electronic effects on the catalyst behavior. Catalytic
activity for the NˆNˆO coordinated chromium systems is largely
4. Conclusions
The octahedral chromium(III) complexes ligated by 2-
carbethoxy-6-iminopyridines were synthesized and character-
ized. The polymerization behavior of these chromium com-
plexes was highly dependent on the cocatalysts employed and
reaction conditions. Notable catalytic activity was observed in
Fig. 5. GPC curves of the polyethylene obtained by both C3 and C5 at different
pressures.

166
W. Zhang et al. / Journal of Molecular Catalysis A: Chemical 265 (2007) 159–166
the presence of EtAlCl2 and the resultant polyethylenes were
obtained with different molecular weights ((0.9–12) × 104) and
broaddistributions(1.91–15.92). Thepolymerizationconditions
and catalysts greatly affected the catalytic activity and the prop-
erties for the resultant polyethylenes. The molecular weight can
be thus controlled by choosing the suitable reaction conditions
and catalyst.
(b) B. de Bruin, E. Bill, E. Bothe, T. Weyhermu¨ller, K. Weighardt, Inorg.
Chem. 39 (2000) 2936;
(c) S. Nu¨ckel, P. Burger, Organometallics 20 (2001) 4345;
(d) A.S. Abu-Surrah, K. Lappalainen, U. Piironen, P. Lehmus, T. Repo, M.
Leskel a¨, J. Organomet. Chem. 648 (2002) 55.
[10] (a) F.J. Feher, R.L. Blanski, J. Chem. Soc., Chem. Commun. (1990) 1614;
(b) M. Motevalli, M. Sanganee, P.D. Savage, S. Shah, A.C. Sullivan, J.
Chem. Soc., Chem. Commun. (1993) 1132;
(c) M.P. Coles, C.I. Dalby, V.C. Gibson, W. Clegg, M.R.J. Elsegood, J.
Chem. Soc., Chem. Commun. (1995) 1709;
(d)H.C.L. Abbenhuis, M.L.W. Vorstenbosch, R.A.v. Santen, W.J.J. Smeets,
A.L. Spek, Inorg. Chem. 36 (1997) 6431.
Acknowledgements
The project was supported by NSFC no. 20473099. We thank
Prof. A. Varada Rajulu of Sri Krishnadevaraya University, India
for the English corrections. Prof. Takashi Tatsumi is gratefully
acknowledged for his kindly editorial and English corrections.
[11] (a) W.-K. Kim, M.J. Fevola, L.M. Liable-Sands, A.L. Rheingold, K.H.
Theopold, Organometallics 17 (1998) 4541;
(b) V.C. Gibson, P.J. Maddox, C. Newton, C. Redshaw, G.A. Solan, A.J.P.
White, D.J. Williams, Chem. Commun. (1998) 1651;
(c) V.C. Gibson, C. Newton, C. Redshaw, G.A. Solan, A.J.P. White, D.J.
Williams, J. Chem. Soc., Dalton Trans. (1999) 827;
(d) V.C. Gibson, S. Mastroianni, C. Newton, C. Redshaw, G.A. Solan, A.J.P.
White, D.J. Williams, J. Chem. Soc., Dalton Trans. (2000) 1969;
(e) R.D. Ko¨hn, M. Haufe, S. Mihan, D. Lilge, Chem. Commun. (2000)
1927;
References
[1] (a) J.P. Hogan, R.L. Banks, US Patent 2,825,721 (1958);
(b) E. Groppo, C. Lamberti, S. Bordiga, G. Spoto, A. Zecchina, Chem. Rev.
105 (2005) 115.
(f) J.-Y. Liu, Y.-S. Li, J.-Y. Liu, Z.-S. Li, J. Mol. Catal. A: Chem. 244 (2006)
99.
[2] (a) T.J. Pullukat, R.E. Hoff, Catal. Rev. Sci. Eng. 41 (1999) 389;
´
(b) A. Razavi, C.R. Acad. Sci. Paris, Serie IIc, Chimie/Chem. 3 (2000) 615.
[12] (a) E. Groppo, C. Prestipino, F. Cesano, F. Bonino, S. Bordiga, C. Lamberti,
P.C. Thu¨ne, J.W. Niemantsverdriet, A. Zecchina, J. Catal. 230 (2005) 98;
(b) M.A. Esteruelas, A.M. Lo´pez, L. Me´ndez, M. Oliva´n, E. On˜ate,
Organometallics 22 (2003) 395;
[3] (a) B. Liu, H. Nakatani, M. Terano, J. Mol. Catal. A: Chem. 201 (2003)
187;
ˇ
(b) B. Liu, P. Sinadela´ˇr, M. Terano, J. Mol. Catal. A: Chem. 238 (2005)
142;
(c) A.K. Tomov, J.J. Chirinos, D.J. Jones, R.J. Long, V.C. Gibson, J. Am.
Chem. Soc. 127 (2005) 10166;
(d) B.L. Small, M.J. Carney, D.M. Holman, C.E. O’Rourke, J.A. Halfen,
Macromolecules 37 (2004) 4375;
(e) A. Bollmann, K. Blann, J.T. Dixon, F.M. Hess, E. Killian, H. Maumela,
D.S. McGuinness, D.H. Morgan, A. Neveling, S. Otto, M. Overett, A.M.Z.
Slawin, P. Wasserscheid, S. Kuhlmann, J. Am. Chem. Soc. 126 (2004)
14712.
(c) B. Liu, H. Nakatani, M. Terano, J. Mol. Catal. A: Chem. 184 (2002)
387.
[4] (a) P. Pino, R. Mu¨lhaupt, Angew. Chem., Int. Ed. Engl. 19 (1980) 867;
(b) G.G. Hlatky, Chem. Rev. 100 (2000) 1347;
(c) G. Fink, B. Steinmetz, J. Zechlin, B. Tesche, Chem. Rev. 100 (2000)
1377.
[5] (a) A. Do¨hring, J. Go¨hre, P.W. Jolly, B. Kryger, J. Rust, G.P.J. Verhownik,
Organometallics 19 (2000) 388;
[13] (a) W. Zhang, W.-H. Sun, S. Zhang, J. Hou, K. Wedeking, S. Schultz, R.
Fro¨hlich, H. Song, Organometallics 25 (2006) 1961;
(b) W.-H. Sun, X. Tang, T. Gao, B. Wu, W. Zhang, H. Ma, Organometallics
23 (2004) 5037;
(c) X. Tang, W.-H. Sun, T. Gao, J. Hou, J. Chen, W. Chen, J. Organomet.
Chem. 690 (2005) 1570.
[14] C. Huang, J. Ahn, S. Kwon, J. Kim, J. Lee, Y. Han, H. Kim, Appl. Catal.
A: Gen. 258 (2004) 173.
(b) V.R. Jensen, K. Angermund, P.W. Jolly, K.J. Børve, Organometallics
19 (2000) 403;
(c) M. Enders, P. Ferna´ndez, G. Ludwig, H. Pritzkow, Organometallics 20
(2001) 5005;
(d) A. Do¨hring, V.R. Jensen, P.W. Jolly, W. Thiel, J.C. Weber,
Organometallics 20 (2001) 2234;
(e) Y. Liang, G.P.A. Yap, A.L. Rheingold, K.H. Theopold, Organometallics
15 (1996) 5284;
(f) B.J. Thomas, S.K. Noh, G.K. Schulte, S.C. Sendlinger, K.H. Theopold,
J. Am. Chem. Soc. 113 (1991) 893;
(g) G. Bhandari, Y. Kim, J.M. McFarland, A.L. Rheingold, K.H. Theopold,
Organometallics 14 (1995) 738;
(h) P.A. White, J. Calabrese, K.H. Theopold, Organometallics 15 (1996)
5473.
[15] X. Tang, D. Zhang, S. Jie, W.-H. Sun, J. Chen, J. Organomet. Chem. 690
(2005) 3918.
[16] S.A. Svejda, M. Brookhart, Organometallics 18 (1999) 65.
[17] (a) F.A. Hicks, M. Brookhart, Organometallics 20 (2001) 3217;
(b) F.M. Bauers, S. Mecking, Angew. Chem., Int. Ed. 40 (2001) 3020.
[18] (a) T.R. Younkin, E.F. Connor, J.I. Henderson, S.K. Friedrich, R.H. Grubbs,
D.A. Bansleben, Science 287 (2000) 460;
[6] (a) K.H. Theopold, Eur. J. Inorg. Chem. (1998) 15;
(b) T.K. Fireman, T. Ziegler, J. Organomet. Chem. 635 (2001) 153;
(c) R. Emrich, O. Heinemann, P.W. Jolly, C. Kru¨ger, G.P.J. Verhownik,
Organometallics 16 (1997) 1511.
[7] (a) B.L. Small, M. Brookhart, A.M.A. Bennett, J. Am. Chem. Soc. 120
(1998) 4049;
(b) C.B. Shim, Y.H. Kim, B.Y. Lee, Y. Dong, H. Yun, Organometallics 22
(2003) 4272;
(c) F.M. Bauers, S. Mecking, Angew. Chem., Int. Ed. 40 (2001) 3020;
(d) W.-H. Sun, H. Yang, Z. Li, Y. Li, Organometallics 22 (2003) 3678;
(e) L. Wang, W.-H. Sun, L. Han, Z. Li, Y. Hu, C. He, C. Yan, J. Organomet.
Chem. 650 (2002) 59;
(b) G.P.J. Britovsek, V.C. Gibson, B.S. Kimberley, P.J. Maddox, S.J.
McTavish, G.A. Solan, A.J.P. White, D.J. Williams, Chem. Commun.
(1998) 849;
(c) G.J.P. Britovsek, S. Mastroianni, G.A. Solan, S.P.D. Baugh, C. Red-
shaw, V.C. Gibson, A.J.P. White, D.J. Williams, M.R.J. Elsegood, Chem.
Eur. J. 6 (2000) 2221.
(f) W.-H. Sun, W. Zhang, T. Gao, X. Tang, L. Chen, Y. Li, X. Jin, J.
Organomet. Chem. 689 (2004) 917;
(g) L. Chen, J. Hou, W.-H. Sun, Appl. Catal. A 246 (2003) 11;
(h) F.A. Hicks, J.C. Jenkins, M. Brookhart, Organometallics 22 (2003)
3533;
(i) G.J.P. Britovsek, S. Mastroianni, G.A. Solan, S.P.D. Baugh, C. Redshaw,
V.C. Gibson, A.J.P. White, D.J. Williams, M.R.J. Elsegood, Chem. Eur. J.
6 (2000) 2221.
[8] G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem., Int. Ed. 38 (1999)
429.
[9] (a) B.C. Etinkaya, C.E. Etinkaya, M. Brookhart, P.S. White, J. Mol. Catal.
A: Chem. 142 (1999) 101;
[19] K. Oouchi, M. Hayakawa, T. Yamada, Macromol. Chem. Phys. 197 (1996)
1545.