New chromium(III) complexes with imine-cyclopentadienyl ligands: Synthesis, characterization, and catalytic properties for ethylene polymerization

Article abstract of DOI:10.1021/om1005935

A series of new half-sandwich chromium(III) complexes chelated with (2-((arylimino)methyl)phenyl)tetramethylcyclopentadienyl ligands, 2-(ArN=CH)C₆H₄Me₄CpCrCl₂ (Ar = 2,6-Me₂C₆H₃ (

Full text of DOI:10.1021/om1005935

Organometallics 2011, 30, 433–440 433
DOI: 10.1021/om1005935
New Chromium(III) Complexes with Imine-Cyclopentadienyl
Ligands: Synthesis, Characterization, and Catalytic Properties
for Ethylene Polymerization
Lei Zhang, Wei Gao, Xin Tao, Qiaolin Wu, Ying Mu,* and Ling Ye
The State Key Laboratory for Supramolecular Structure and Materials, School of Chemistry,
Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China
Received June 17, 2010
A series of new half-sandwich chromium(III) complexes chelated with (2-((arylimino)methyl)phenyl)-
tetramethylcyclopentadienyl ligands, 2-(ArNdCH)C6H4Me4CpCrCl2 (Ar = 2,6-Me2C6H3 (1), 2,
6-Et2C6H3 (2), 2,6-iPr2C6H3 (3), 4-MeC6H4 (4)), have been synthesized from the reaction of CrCl3 with
the lithium salt of the corresponding ligand 2-(ArNdCH)C6H4Me4CpLi (Ar = 2,6-Me2C6H3 (LiL1),
2,6-Et2C6H3 (LiL2), 2,6-iPr2C6H3 (LiL3), 4-MeC6H4 (LiL4)). Free ligands HL1-HL4 were prepared
by the condensation reaction of 2-(tetramethylcyclopentadienyl)benzaldehyde with 2,6-dialkylani-
1
line. The free ligands were characterized by H NMR spectroscopy, while the chromium(III)
complexes were characterized by elemental analyses and single-crystal X-ray crystallography. The
X-ray crystallographic analysis indicates that the imine N atom in these complexes coordinates to the
central chromium atom. Upon activation with AlR3 and Ph3CB(C6F5)4, these complexes exhibit high
catalytic activity for ethylene polymerization and produce polyethylene with moderate to high molec-
ular weights. These chromium(III) complexes can also be activated with AlR3 alone. In the latter case,
they show slightly lower catalytic activity for ethylene polymerization in comparison to the AlR3/
Ph3CB(C6F5)4 activated catalyst systems. The effects of ligand structure, polymerization tempera-
ture, AlR3, and Al/Cr molar ratio on the catalytic behavior of these complexes were examined.
Introduction
chromium-based catalysts with various ligands, monocyclo-
pentadienyl chromium(III) complexes have been extensively
studied for ethylene homo- or copolymerization.8-24 We
Heterogeneous chromium-based catalysts are an impor-
tant class of catalysts for producing polyethylenes in indus-
try. The silica-supported chromium-based Phillips1 and
Union Carbide catalysts2 have been used for the industrial
production of high-density polyethylene for decades. The
development of homogeneous chromium-based catalysts for
olefin polymerization has also attracted intensive research
interest in the past decade.3-7 Among the homogeneous
(8) Emrich, R.; Heinemann, O.; Jolly, P, W.; Kruger, C.; Verhovnik,
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*To whom correspondence should be addressed. Tel: (86)-431-
85168376. Fax: (86)-431-85193421. E-mail: ymu@jlu.edu.cn.
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2010 American Chemical Society
Published on Web 12/31/2010
pubs.acs.org/Organometallics

434 Organometallics, Vol. 30, No. 3, 2011
Zhang et al.
Figure 1. Functionalized cyclopentadienyl chromium(III) complexes with a pendant N-donor group.
have recently focused our research attention on the devel-
opment of new half-sandwich chromium(III) catalysts.25 Of
the monocyclopentadienyl chromium(III) complexes, the so-
called constrained-geometry monocyclopentadienyl chromium-
(III) complexes with a pendant nitrogen donor, such as
dialkylamino (a),9 isopropylimino (b),9 N,N-dimethylanilinyl
(c),22 alkylamido (d),17 pyridyl (e),18 quinolinyl (f),13 and imida-
zolin-imino (g),23 as shown in Figure 1, have attracted par-
ticular attention due to their structural features. Monocyclopen-
tadienyl chromium(III) complexes with a dialkylamino group
attached to the Cp ring were reported to show good catalytic
activity for ethylene polymerization and ethylene/R-olefin
copolymerization.8,9,11 Similar chromium(III) complexes with
a pyridyl-cyclopentadienyl,18 quinolinyl-cyclopentadienyl13
or N,N-dimethylanilinyl-cyclopentadienyl22 ligand were also
found to be efficient catalysts for ethylene polymeriza-
tion. The typical constrained-geometry complex d was
reported to show only moderate catalytic activity for
ethylene polymerization when activated with MAO or
fluorinated borate cocatalyst.26 Imidazolin-imino group
functionalized cyclopentadienyl chromium(III) complexes
g were recently reported to show moderate catalytic
activity for ethylene polymerization as well.23 The iso-
propylimino functionalized cyclopentadienyl chromium-
(III) complex b was reported as having been synthesized
and tested as an ethylene polymerization catalyst, but its
Scheme 1. Synthetic Route for Complexes 1-4
structure cannot be confirmed in the absence of any crystal-
lographic data.9 Considering that many late-transition-metal
complexes with a diimine ligand show good catalytic perfor-
mance for ethylene polymerization and their catalytic proper-
ties can be easily tuned by changing the substituent on the
imine groups,4,27 it would be interesting to systematically
synthesize imino group functionalized cyclopentadienyl
chromium(III) complexes similar to complex b and investi-
gate their catalytic properties. In comparison to the pyridyl
and dialkylamino functional groups, the imino group at-
tached to the cyclopentadienyl ligand should have the super-
iority of relatively strong coordination ability to the central
metal and easily modified bulkiness of the substituent on the
N atom. On the basis of these considerations and the experi-
ences accumulated from previous research work, we have
developed a number of new chromium(III) complexes of the
type h, as shown in Figure 1, and found that these complexes
exhibit high catalytic activity for ethylene polymerization
upon activation with AlR3 (R=Me, Et, iBu) alone or AlR3
and Ph3CB(C6F5)4 together. In this paper, we report the syn-
thesis and characterization of these complexes as well as their
catalytic properties for ethylene polymerization reactions.
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Huang, Y.; Yu, W.; Jin, G. Organometallics 2009, 28, 4598.
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Chem. 2007, 692, 1505. (b) Xu, T.; Mu, Y.; Gao, W.; Ni, J.; Ye, L.; Tao, Y.
J. Am. Chem. Soc. 2007, 129, 2236.
(26) (a) Okuda, J.; Eberle, T. In Metallocenes; Togni, A., Halterman,
R. L., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol. 1, p 415. (b)
Chum, P. S.; Kruper, W. J.; Guest, M. J. Adv. Mater. 2000, 12, 1759. (c)
Cano, J.; Kunz, K. J. Organomet. Chem. 2007, 692, 4411.
(27) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000,
100, 1169. (b) Mecking, S. Angew. Chem., Int. Ed. 2001, 40, 534. (c) Park,
S.; Han, Y.; Kim, S. K.; Lee, J.; Kim, H. K.; Do, Y. J. Organomet, Chem.
2004, 689, 4263.

Article
Organometallics, Vol. 30, No. 3, 2011 435
Figure 3. Perspective view of 2 with thermal ellipsoids drawn at
the 30% probability level. Hydrogens are omitted for clarity.
Figure 2. Perspective view of 1 with thermal ellipsoids drawn at
the 30% probability level. Hydrogens are omitted for clarity.
Results and Discussion
1. Synthesis of Ligands and Complexes. The synthetic route
for the free ligands (2-((arylimino)methyl)phenyl)tetramethyl
cyclopentadiene, 2-(ArNdCH)C6H4Me4CpH (Ar = 2,
6-Me2C6H3 (HL1), 2,6-Et2C6H3 (HL2), 2,6-iPr2C6H3 (HL3),
4-MeC6H4 (HL4)) and their chromium(III) complexes
((2-((arylimino)methyl)phenyl)tetramethylcyclopentadienyl)-
chromium(III) dichloride, 2-(ArNdCH)C6H4Me4CpCrCl2
(Ar = 2,6-Me2C6H3 (1), 2,6-Et2C6H3 (2), 2,6-iPr2C6H3 (3),
4-MeC6H4 (4)) is illustrated in Scheme 1. The free ligands
HL1-HL4 were prepared by the condensation reaction of
2-(tetramethylcyclopentadienyl)benzaldehyde with the corre-
sponding aniline derivatives in methanol in the presence of
˚
formic acid and 4 A molecular sieves. These free ligands were
found to be sensitive to water and moisture. To obtain them in
high yields (80-90%), it is necessary to carry out the con-
densation reaction under an inert atmosphere with anhydrous
reagents and solvents. We have also tried to synthesize
(2-((alkylimino)methyl)phenyl)tetramethylcyclopentadiene
derivatives from the condensation reaction of 2-(tetra-
methylcyclopentadienyl)benzaldehyde with alkylamines.
However, these attempts were not successful, due probably
Figure 4. Perspective view of 3 with thermal ellipsoids drawn at
the 30% probability level. Hydrogens are omitted for clarity.
soluble in CH2Cl2, less soluble in toluene, and almost insoluble
in common alkanes. They form green solutions in CH2Cl2 and
blue solutions in toluene, due probably to the difference in
coordination ability of the solvent molecules. All complexes
were characterized by means of elemental analyses and infra-
red spectroscopy, and the molecular structures of complexes
1-3 were determined by X-ray diffraction. Crystals suitable
for an X-ray structural determination were obtained by re-
crystallization from n-hexane/CH2Cl2 mixed solvent systems.
2. Crystal Structures of Complexes 1-3. The molecular
structures of complexes 1-3 with the atom-numbering
schemes are shown in Figures 2-4, and selected bond lengths
and angles are given in Table 1. Crystallographic data
indicate that crystals of complexes 1 and 3 belong to the
monoclinic system and P21/n space group, while complex 2
crystallizes in the orthorhombic system and P212121 space
group. All three complexes possess a three-legged piano-
stool geometry with a distorted-octahedral coordination
environment around the central chromium atom. As can
be seen from their crystal structures, the coordination of the
1
to the poor stability of the alkylimine products. H NMR
spectroscopic analysis of the free ligands indicates that they
all exist as mixtures of three isomers, owing to the variation
in the locations of the two CdC double bonds in the
cyclopentadienyl ring. These free ligands were obtained in
about 95% purity and were used without further purification.
Treatment of the free ligands with 1.05 equiv of n-butyllithium
in THF at -78 °C led to the deprotonation of their cyclopen-
tadiene rings to form the lithium salts of the ligands
LiL1-LiL4, which was accompanied by a color change from
pale yellow to brown. The chromium(III) complexes 1-4 were
synthesized in high isolated yields (60-65%) by the reaction of
CrCl3(THF)3 with the corresponding lithium salt of the ligand
in THF at room temperature. During the period of the
reaction, the color of the reaction mixture changes from purple
to deep green, indicating the formation of the chromium(III)
complexes. These complexes are found to be thermally stable
even heated to melting temperature under an inert atmosphere,
but are air and moisture sensitive. These complexes are fairly

436 Organometallics, Vol. 30, No. 3, 2011
Table 1. Selected Bond Lengths (A) and Bond Angles (deg) for Complexes 1-3
Zhang et al.
˚
1
2
3
Cr(1)-N(1)
2.128(5)
2.293(2)
2.303(2)
2.197(5)-2.274(6)
2.238(9)
1.886
2.117(2)
2.1313(15)
Cl(1)-Cr(1)
2.2928(9)
2.2877(9)
2.185(2)-2.284(3)
2.237(3)
1.886
1.279(3)
2.2840(7)
2.2755(8)
2.195(2)-2.269(2)
2.238(2)
1.884
1.289(2)
Cl(2)-Cr(1)
Cr(1)-CCp (range)
Cr(1)-CCp (av)
Cr(1)-Cp(ct)
C(16)-N(1)
1.290(7)
Cl(1)-Cr(1)-Cl(2)
N(1)-Cr(1)-Cl(1)
N(1)-Cr(1)-Cl(2)
Cp(ct)-Cr-N(1)
C(16)-N(1)-Cr(1)
C(17)-N(1)-Cr(1)
C(16)-N(1)-C(17)
N(1)-C(16)-C(15)
94.87(9)
98.53(15)
99.18(13)
118.7
130.9(4)
116.1(3)
113.1(4)
127.6(5)
96.89(4)
96.54(7)
99.09(6)
119.3
129.05(17)
117.83(16)
113.1(2)
128.4(2)
96.72(4)
95.86(5)
98.48(5)
120.6
126.11(13)
120.54(11)
113.33(15)
130.13(17)
imine nitrogen atom to the central metal in these complexes
builds a six-membered chelating ring in a vertical position to
the cyclopentadienyl ring with the aryl group at the imine
nitrogen atom being placed close to the metal center, which
constructs a relatively crowded coordinating environment
surrounding the central chromium atom. The Cr-N dis-
between the Cp ring and the attached phenyl ring (77.9° for 1,
73.8° for 2, and 75.3° for 3) are less than 90°, indicating that
the six-membered chelating rings in these complexes are
somewhat distorted. The angles between the Cp ring and
the aryl ring at the imine N atom (87.7° for 1, 87.8° for 2, and
84.6° for 3) are near 90°. The bulkiness and location of the
aryl group in these complexes may be important factors for
affecting their catalytic performance by influencing the rate
of ethylene molecule coordination to the chromium atom
and insertion into the growing polyethylene chain, as well as
the rate of polymer chain termination.28
˚
tances in complexes 1-3 are 2.128(5), 2.117(2), and 2.131(2) A,
respectively, being shorter than the corresponding Cr-N
distances found in complexes a (2.17 A), f (2.17 A), and
9
13
˚
˚
22
c (2.25 A) but longer than those in complexes d (1.920 A),
17
˚
˚
18
23
9
˚
˚
˚
e (2.108 A), g (2.088 A), and b (2.04 A) shown in Figure 1.
Complex 3 has the longest Cr-N bond distance among the
complexes 1-3, which is reasonable since the aryl group at
the imine N atom in complex 3 is the largest, 2,6-iPr2Ph. The
3. Ethylene Polymerization. The chromium(III) com-
plexes 1-4 have been tested as precatalysts for ethylene
polymerization and copolymerization with 1-hexene under
different conditions, and it was found that these complexes
show catalytic activity only for ethylene polymerization
˚
Cr-N bond length in complex 2 being about 0.01 A shorter
than that in complex 1 is abnormal, which may be caused by
i
˚
upon activation with either AlR3 (R = Me, Et, Bu) alone
packing forces. The Cr-Cl bond distances (2.276-2.303 A)
in these complexes are in agreement with those observed for
related amino- or pyridyl-functionalized cyclopentadienyl
chromium dichloride complexes9,17,18,22 and change in the
order 1 > 2 > 3. The Cr-Cp(ct) distances (ct = centroid)
or AlR3 and Ph3CB(C6F5)4 together. The polymerization
results of ethylene catalyzed by 1-4/AlR3 systems and 1-4/
AlR3/Ph3CB(C6F5)4 systems are summarized in Tables 2 and 3,
respectively. Upon activation with AlR3, complexes 1-4 all
exhibit moderate to high catalytic activities for ethylene
polymerization. Their catalytic activities ((0.5-3.0) ꢀ 106 g
of PE (mol of Cr)-1 h-1) are obviously higher than those
reported for most N-containing group functionalized cyclo-
pentadienyl chromium complexes,10,14,18,19 except for the N,
N-dimethylanilinyl functionalized cyclopentadienyl chro-
mium complexes.22 By comparison of the polymerization
results under similar conditions in Table 2, it can be seen that
the steric effect of the Ar group at the imine N atom of these
complexes and the R group in the AlR3 activator on their
catalytic properties is remarkable. For example, the catalytic
activity of complexes 1-4 decreases in the order 1 > 2 > 4 >
3 under similar conditions on activation with AlMe3, while
the catalytic activity is in the order 1> 4 >2 > 3and 4 >1 > 2
> 3 on activation with AlEt3 and AliBu3, respectively. The
decrease in the catalytic activity of complexes 1-3 with an
increase in the bulkiness of the aryl group may be attributed
to the fact that a bulkier aryl group at the imine N atom
would make the coordination sphere of the catalytic active
species in these systems more crowded and thus slow down
the ethylene coordination and insertion rate relatively. The
fact that complex 4, with the smallest aryl group, does not
show the highest catalytic activity in the AlMe3 and AlEt3
˚
are close to each other for complexes 1-3 at 1.886 A for 1 and
˚
2 and 1.884 A for 3. Similarly, the Cr-CCp(av) (av =
average) distances (2.238 A for 1 and 3 and 2.237 A for 2)
˚
˚
are also close to each other. The individual Cr-CCp bond
˚
distances range from 2.185 to 2.284 A, with the Cr-C3 and
Cr-C4 distances being obviously longer than the remaining
Cr-CCp bond lengths, indicating that the central chromium
atom is not located exactly below the center of the Cp ring
due to the coordination of the imine N atom. The imino
CdN bonds in these complexes retain their double-bond
˚
˚
character, being 1.290(7) A for 1, 1.279(3) A for 2, and
˚
1.289(2) A for 3.
The N-Cr-Cp(ct) angles of complexes 1 (118.7°), 2
(119.3°), and 3 (120.6°) are obviously influenced by the
bulkiness of the aryl groups at the imine N atoms. Similar
effects on the N-Cr-Cl angles (98.53 and 99.18° for 1, 96.54
and 99.09° for 2, 95.86 and 98.48° for 3) can also be observed.
However, little influence from the bulkiness of these aryl
groups on the Cl-Cr-Cp(ct) (120.4 and 120.1° for 1, 120.5
and 119.5° for 2, 120.0 and 119.7° for 3) and Cl-Cr-Cl
(94.87° for 1, 96.89° for 2, and 96.72° for 3) angles can be
seen. The Cr-N-C(17) angles (116.1° for 1, 117.8° for 2, and
120.5° for 3) are slightly less than the corresponding
N-Cr-Cp(ct) angles of these complexes, which causes the
aryl group at the imine N atom to be located in a nearly
vertical direction to the cyclopentadienyl ring. The angles
(28) (a) Alt, H. G.; Koppl, A. Chem. Rev. 2000, 100, 1205. (b)
McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587.

Article
Organometallics, Vol. 30, No. 3, 2011 437
Table 2. Polymerization Results of Ethylene Catalyzed by Complex 1-4/AlR3 Systemsa
c
d
entry
cat./amt (μmol)
AlR3
Al/Cr
temp (°C)
yield (g)
activityb
10-4Mη
10-4Mn
Mw/Mn
Tme (°C)
Xcf (%)
1
2
3
4
5
6
7
8
1
2
3
4
1
2
3
4
1
1
1
1
1
1
1
1
1
1
2
2
3
3
4
4
1
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlEt3
60
60
60
60
120
120
120
120
30
180
240
300
400
120
120
120
120
120
120
120
120
120
120
120
120
20
20
20
20
20
20
20
20
20
20
20
20
20
0
40
60
20
20
20
20
20
20
20
20
20
2.21
1.49
0.65
0.81
2.75
1.80
0.97
1.19
trace
2.80
2.81
2.74
2.70
3.04
1.21
0.55
2.55
0.70
1.42
0.58
0.75
0.48
1.43
0.85
1.52
2210
1490
650
26.3
38.9
45.7
114.1
19.8
23.1
26.9
77.7
17.6
25.4
29.3
2.37
2.47
2.55
133.3
134.5
135.3
138.8
131.8
133.2
134.3
137.1
64.7
65.9
60.8
68.4
62.0
67.4
67.9
65.7
810
2750
1800
970
14.1
16.2
17.9
2.34
2.38
2.36
1190
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25g
2800
2810
2740
2700
3040
1210
550
2550
700
1420
580
750
16.7
13.2
11.6
10.9
21.2
17.3
13.0
28.6
58.3
31.9
104.1
59.1
114.8
23.2
36.1
10.3
11.7
8.4
7.5
6.8
15.6
11.3
8.2
2.35
2.33
2.31
2.34
2.41
2.76
3.23
2.38
131.5
130.8
130.5
129.6
132.2
131.0
130.0
133.7
136.6
134.2
138.4
137.3
138.9
132.2
134.4
130.1
61.6
68.6
64.5
63.2
66.5
58.8
66.7
60.2
69.5
68.7
70.5
68.8
54.5
67.2
69.5
65.6
18.1
AliBu3
AlEt3
19.4
2.43
AliBu3
AlEt3
AliBu3
AlEt3
480
1430
850
9120
15.6
23.8
6.3
2.42
2.48
2.29
AliBu3
AlMe3
a Polymerization conditions: solvent 60 mL of toluene, ethylene pressure 5 bar, amount of catalyst 2 μmol. b In units of kg of PE (mol of Cr)-1 h-1
.
c Measured in decahydronaphthalene at 135 °C. d Measured by GPC analysis. e Determined by DSC at a heating rate of 10 °C min-1. f Crystallinity Xc =
ΔHf/ΔHf0; ΔHf0 = 273 J/g for polyethylene. g Polymerization time 5 min.
Table 3. Polymerization Results of Ethylene Catalyzed by Complex 1-4/AlR3/Ph3CB(C6F5)4 Systemsa
activityb
10-4Mη
10-4Mn
Mw/Mn
Tme (°C)
Xcf (%)
c
d
entry
cat.
AlR3
Al/Cr
temp (°C)
time (min)
yield (g)
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
2
3
4
1
1
1
1
1
1
1
2
2
3
3
4
4
AlMe3
AlEt3
120
120
120
30
20
20
20
20
20
20
20
20
20
20
60
40
0
20
20
20
20
20
20
20
20
20
20
30
30
30
30
30
30
30
30
30
30
30
30
30
5
10
20
60
30
30
30
30
30
30
2.50
1.46
0.59
trace
1.47
2.12
2.08
1.41
0.68
1.71
0.55
1.65
2.55
1.30
1.77
2.25
2.72
0.91
0.58
0.65
0.32
2.64
0.93
5000
2920
1180
31.3
45.5
106.5
19.2
28.7
2.41
2.59
134.3
135.3
138.1
64.2
62.4
68.5
AliBu3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlEt3
60
2940
4240
4160
2820
1360
3420
1100
3300
5100
15600
10620
6750
2720
1820
580
46.2
29.8
21.3
31.9
34.2
19.7
21.4
27.6
39.8
10.2
19.3
28.5
34.6
69.3
236.7
76.7
114.2
11.8
267.3
29.2
18.6
15.4
18.8
22.5
13.9
12.8
16.3
25.6
6.1
2.54
2.35
2.33
2.38
2.42
2.37
3.24
2.88
2.31
2.34
2.36
2.39
2.47
135.7
133.8
132.8
134.6
134.8
131.1
132.5
133.3
135.2
130.3
131.8
135.2
134.7
136.5
140.6
137.8
138.2
129.6
140.8
63.5
68.6
67.0
68.4
65.1
57.5
62.1
72.7
64.0
62.0
66.6
68.1
67.4
62.3
64.5
67.8
62.1
71.5
61.1
180
240
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
13.3
17.7
20.4
AliBu3
AlEt3
650
320
5280
1860
AliBu3
AlEt3
AliBu3
a Polymerization conditions: solvent60 mL of toluene, ethylene pressure 5 bar, B/Cr molar ratio 1.2/1, amount of catalyst1 μmol. b In units of kg of PE
(mol of Cr)-1 h-1. c Measured in decahydronaphthalene at 135 °C. d Measured by GPC analysis. e Determined by DSC at a heating rate of 10 °C min-1
f Crystallinity Xc = ΔHf/ΔHf0; ΔHf0 = 273 J/g for polyethylene.
.
activated systems seems to imply that, except for the afore-
mentioned steric effects, the electronic effects of the aryl
group may also play an important role in these systems. With
the more bulky AliBu3 cocatalyst, the 4/AliBu3 system shows
the highest catalytic activity in the 1-4/AliBu3 systems,
indicating that the steric effect of the aryl group plays a
dominant role in controlling the catalytic activity of the
AliBu3 activated catalyst systems. Upon activation with
different AlR3, the catalytic activity of complexes 1-3
decreases in the order AlMe3>AlEt3 > AliBu3. The varia-
tion of catalytic activity caused by trialkylaluminum may be
attributed to the interaction between the chromium catalyst
and the aluminum cocatalyst molecules by forming a
bridged heterobimetallic chromium-aluminum complex, as
reported previously for similar catalyst systems.29 The effect
(29) (a) Mani, G.; Gabba, F. P. Angew. Chem., Int. Ed. 2004, 43, 2263.
(b) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.;
McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem.
Commun. 1998, 849. (c) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.;
Kimberley, B. S.; Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw,
€
C.; Solan, G. A.; Stromberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem.
Soc. 1999, 121, 8728.

438 Organometallics, Vol. 30, No. 3, 2011
Zhang et al.
of Al/Cr molar ratio on the catalytic activity was also
examined for the complex 1/AlMe3 system. It was found
that the catalytic activity increases rapidly with the increase
in Al/Cr molar ratio from 30 to 120, while further increase in
the Al/Cr molar ratio from 120 to 400 results in limited
impact on the catalytic activity. The effect of polymerization
temperature on the catalytic activity of the complex 1/AlMe3
system was studied as well (entries 5 and 14-16 in Table 2).
The catalytic activity of these reactions was found to de-
crease remarkably with an increase in polymerization tem-
perature from 0 to 60 °C, which is indicative of the weak
thermal stability of the catalytically active species formed in
this catalyst system. In addition, as observed for most of the
olefin polymerization catalyst systems, the catalytic activity
of this catalyst system also decreases obviously with an
increase in polymerization time.
30 min, although the catalytic activity decreases quickly with
time (entries 1 and 14-17 in Table 3).
The molecular weights of the polyethylenes produced by
these catalyst systems are quite different ((10-267) ꢀ 104
Da), depending on the catalyst structure and polymerization
conditions. By examination of the viscosity-average molec-
ular weight (Mη) data, it can be seen that the molecular
weight of the polyethylene is affected by several factors. In
general, catalysts with a bulky Ar group seem to produce
polyethylenes with relatively high molecular weight (on
comparison of the Mη data of the samples from catalysts
1-3), while polymerization reactions carried out at high
temperatures or with high Al/Cr molar ratios give polyethy-
lenes with relatively low molecular weight. These results
are normal for metallocene catalyst systems,32 and similar
results were also observed for related half-sandwich
chromium(III) catalysts.9,18 However, there are more factors
affecting the polyethylene molecular weight in the studied
catalyst systems worth noting. One is that the polyethylenes
produced from the AlR3/Ph3CB(C6F5)4 activated catalyst
systems generally possess higher molecular weight than the
polyethylenes obtained from the AlR3 activated catalyst
systems under similar conditions, which could result from
the higher catalytic activity of the former systems. The other
is that the molecular weight of the obtained polyethylenes is
strongly dependent on the alkylaluminum cocatalyst and
decreases in the order AliBu3 > AlEt3 > AlMe3 for catalysts
1-3, which might be attributed to the fact that both the
polymer chain transfer to the alkylaluminum cocatalyst and
the β-H elimination reaction taking place at the catalytically
active species in a bulkier AlR3 activated catalyst system
should be slower, considering the interaction between the
catalyst and cocatalyst molecules as discussed above. An-
other thing worth noting is that the molecular weight of the
polyethylene produced by these catalysts increases with time
up to about 60 min, although it increases more rapidly in the
beginning 20 min of the reaction (entries 1 and 14-17 in
Table 3), which implies that at least a part of the polymer
chains in these catalyst systems grow over a long period of
time.
The polyethylene samples with low molecular weight have
been analyzed by gel permeation chromatography (GPC).
The molecular weight distribution is basically unimodal and
narrow, being characteristic for metallocene polyolefins. All
polyethylene samples were analyzed by differential scanning
calorimetry (DSC), and the melting transition temperature
and crystallinity data are summarized in Tables 2 and 3.
Most of the samples show a sharp endothermic melting peak
in the range of 129-140 °C in their DSC diagrams with
crystallinity in the range of 60-80%. 13C NMR spectra of
the polyethylene samples obtained at 130 °C in C6D4Cl2
show no signals for branches,33 which indicates that the
polyethylenes are linear. Powder X-ray diffraction (XRD)
analysis on some typical polyethylene samples was also
carried out, and the observed diffraction peaks in the 2θ
region of 19-25° in the XRD diagrams can be assigned to the
polyethylene’s orthorhombic (110 and 200) reflections at
When complexes 1-4 were activated with AlR3 and
Ph3CB(C6F5)4 together, it was found that these chromium
complexes show much higher catalytic activity for ethylene
polymerization than the AlR3 activated systems under simi-
lar conditions. The high catalytic activity of these AlR3/
Ph3CB(C6F5)4 activated systems should be attributed to the
fact that the catalytically active species in these systems are
positively charged.30 Attempts to isolate or observe the
cationic species from stoichiometric reactions have not been
1
successful so far. However, Ph3CCH3 was observed by H
NMR from the reactions of complexes 1-4 with AlMe3 and
Ph3CB(C6F5)4, which may indirectly demonstrate the for-
mation of the catalytically active cationic species in these
systems. In a way similar to that for the AlR3 activated
systems, the catalytic activity of complexes 1-4 decreases in
the order 1 > 4 > 2 > 3 under similar conditions on
activation with AlMe3/Ph3CB(C6F5)4, while the catalytic
activity is in the order 4 > 1 > 2 > 3 on activation with
AlEt3/Ph3CB(C6F5)4 and AliBu3/Ph3CB(C6F5)4. For com-
plexes 1-3, their catalytic activity was found to decrease in
the order AlMe3 > AlEt3 > AliBu3 upon activation with
different AlR3/Ph3CB(C6F5)4 cocatalysts. As discussed
above, such results might result from the interaction between
the chromium catalyst and AlR3 cocatalyst. As noted above
in the AlR3 activated catalyst system, the catalytic activity of
complex 4 also changes in the order AlEt3 > AlMe3 >
AliBu3 on activation with different AlR3/Ph3CB(C6F5)4
cocatalysts. Similar to the case for the AlR3 activated
catalyst systems, the catalytic activity of the AlR3/Ph3CB-
(C6F5)4 activated catalyst systems was found to increase with
an increase in Al/Cr molar ratio from 30 to 120, while a
further increase in the Al/Cr molar ratio from 120 to 240
results in a decrease in the catalytic activity. It is under-
standable that excess AlR3 would slow down the ethylene
coordination rate, and probably consume some Ph3CB-
(C6F5)4 cocatalyst as well.31 The catalytic activity of the
AlR3/Ph3CB(C6F5)4 activated catalyst systems was also
found to decrease with an increase in polymerization tem-
perature from 0 to 60 °C, which is similar to the case for the
AlR3 activated catalyst systems. In addition, it was noticed
that the lifetime of these catalyst systems is relatively long at
20 °C and the polymerization reaction can last for more than
(32) Zhang, Y.; Mu, Y.; Lu, C.; Li, G.; Xu, J.; Zhang, Y.; Zhu, D.;
Fen, S. Organometallics 2004, 23, 540.
(33) (a) Wang, W.; Yan, D.; Zhu, S.; Hamielec, A. E. Macromolecules
(30) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994,
116, 10015. (b) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991,
113, 3623. (c) Bochmann, M.; Lancaster, S. J. Angew. Chem., Int. Ed. 1994,
33, 1634. (d) Chen, E. Y.; Marks, T. J. Chem. Rev. 2000, 100, 1391.
(31) Bochmann, M.; Sarsfield, M. J. Organometallics 1998, 17, 5908.
1998, 31, 8677. (b) Kooko, E.; Lehmus, P.; Leino, R.; Luttikhedde, H. J. G.;
€
€ €
Ekholm, P.; Nasman, J. H.; Seppala, J. V. Macromolecules 2000, 33, 9200.
€
€ €
(c) Malmberg, A.; Kokko, E.; Lehmus, P.; Lofgren, B.; Seppala, J. V.
Macromolecules 1998, 31, 8448.

Article
Organometallics, Vol. 30, No. 3, 2011 439
21.32 and 23.72° and monoclinic (001, 200, and 201) reflec-
tions at 19.22, 22.94, and 24.96°.34
spectrometer at 130 °C in o-C6D4Cl2. Powder XRD was per-
formed on a Rigaku D/Max-2550 diffractometer using Cu KR
radiation operating at 50 kV and 200 mA, and the data were
collected in the 2θ range of 15-30° with a scanning rate of
Conclusion
1 ° min-1
.
Synthesis of 2-(2,6-Me2C6H3NdCH)C6H4Me4CpH (HL1).
To solution of 2-(tetramethylcyclopentadienyl)benzyl-
aldehyde (4.37 g, 19.3 mmol) in dried methanol (80 mL) was
A series of new constrained-geometry chromium(III) com-
plexes with chelated (2-((arylimino)methyl)phenyl)tetramethyl-
cyclopentadienylligands have been synthesized in good yields
by the reaction of CrCl3(THF)3 with the lithium salt of the
corresponding ligand in THF. X-ray crystallographic anal-
ysis indicates that these half-metallocene chromium(III)
complexes adopt a pseudo-octahedral coordination environ-
ment with the imine N coordinated to the Cr metal center.
a
˚
added 2,6-dimethylaniline (2.57 g, 21.2 mmol), 4 A molecular
sieves (3 g), and formic acid (4 drops). After the reaction mixture
was stirred for 8 h, the molecular sieves were filtered off and the
solution was concentrated under reduced pressure to give the
crude product, which was purified by column chromatography
on silica gel using petroleum ether/ethyl acetate (98/2) as the
eluent to afford the product (5.70 g, 17.3 mmol, 89.6%) in about
i
Upon activation with AlR3 (R = Me, Et, Bu) or AlR3/
1
95% purity as an orange oil. H NMR (300 MHz, CDCl3): δ
Ph3CB(C6F5)4, these complexes exhibit good to highcatalytic
activity for ethylene polymerization and produce polyethy-
lene with high molecular weight under mild conditions. The
catalytic activity of these complexes and the molecular weight
of the produced polyethylene vary in a broad range with
variation in the Ar group and the AlR3 cocatalyst. Catalysts
with a bulkier Ar group show lower catalytic activity and
produce polyethylene with higher molecular weight. In addi-
tion, the AlR3/Ph3CB(C6F5)4 activated catalyst systems show
higher catalytic activity in comparison to the AlR3 activated
catalyst systems.
8.37 (s, 1H, ArHCdN), 8.12 (d, J = 5.2 Hz, 1H, ArH),
7.50-7.37 (m, 2H, ArH), 7.16 (d, J = 7.8 Hz, 1H, ArH), 7.05
(d, J = 7.5 Hz, 2H, ArH), 6.90-6.97 (m, 1H, ArH), 3.29-3.11
(m, 1H, CpH), 2.09 (s, 6H, ArCH3), 1.87 (s, 3H, CpCH3), 1.79 (s,
3H, CpCH3), 1.68 (s, 3H, CpCH3), 0.89 (d, J = 6.9 Hz, 3H,
CpCH3). MS: m/z 330 [M þ H].
Synthesis of 2-(2,6-Et2C6H3NdCH)C6H4Me4CpH (HL2).
Compound HL2 was synthesized in the same manner as HL1
with 2,6-diethylaniline (3.13 g, 21.0 mmol) as starting material.
The product (6.41 g, 17.9 mmol, 85.4%) was obtained in about
1
95% purity as an orange oil. H NMR (300 MHz, CDCl3): δ
8.38 (s, 1H, ArHCdN), 8.16 (d, 1H, ArH), 7.52-7.42 (m, 2H,
ArH), 7.22-7.14 (m, 1H, ArH), 7.09 (m, 2H, ArH), 6.92 (m, 1H,
ArH), 3.29-3.10 (m, 1H, CpH), 2.52-2.43 (m, 4H,CH2CH3),
1.88 (s, 3H, CpCH3), 1.80 (s, 3H, CpCH3), 1.70 (s, 3H, CpCH3),
1.17-1.04 (m, 6H, CH2CH3), 0.90 (d, J = 6 Hz, 3H, CpCH3).
MS: m/z 358 [M þ H].
Experimental Section
General Considerations. All manipulations for air- and water-
sensitive compounds were performed under an inert atmosphere
of nitrogen using standard Schlenk or glovebox techniques.
Solvents were purified and dried by known procedures and
distilled under nitrogen prior to use. Polymerization grade
ethylene was further purified by passage through columns of 5
Synthesis of 2-(2,6-iPr2C6H3NdCH)C6H4Me4CpH (HL3).
Compound HL3 was synthesized in the same manner as HL1
with 2,6-diisopropylaniline (3.73 g, 21.0 mmol) as starting
material. The product (6.53 g, 16.9 mmol, 80.7%) was obtained
A molecular sieves and MnO. CrCl3(THF)3,35 Ph3CB(C6F5)4,36
˚
1
in about 95% purity as an orange oil. H NMR (300 MHz,
and 2-(tetramethylcyclopentadienyl)benzaldehyde37 were pre-
pared according to published procedures. AlMe3, AlEt3, AliBu3,
2,6-dimethylaniline, 2,6-diethylaniline, and 2,6-diisopropylani-
line were purchased from Aldrich or Acros and used as received.
NMR spectra were measured using a Varian Mercury-300
NMR spectrometer, and elemental analysis was performed on
a Perkin-Elmer 2400 analyzer. The intrinsic viscosity, [η], was
measured in decahydronaphthalene at 135 °C using an Ubbe-
lohde viscometer. Viscosity average molecular weight (Mη)
values of polyethylenes were calculated by the following
equation:38 [η] = (6.77 ꢀ 10-4)Mη0.67. Molecular weight and
molecular weight distribution of the low-molecular-weight
polymer samples were measured on a PL-GPC 220 at 140 °C
with 1,2,4-trichlorobenzene as the solvent. The melting transi-
tion temperature (Tm) and crystallinity (Xc) of the polymers
were measured with a Model 204 differential scanning calori-
meter (DSC). The samples (5-10 mg) were heated from 35 to
160 °C at a rate of 10 °C/min, and the data from the second
heating cycle were used. 1H and 13C NMR spectra of polyethy-
lene samples were measured on a Varian Unity 400-MHz
CDCl3): δ 8.39 (s, 1H, ArHCdN), 8.15 (d, J = 8.4 Hz, 1H,
ArH), 7.50-7.43 (m, 3H, ArH), 7.22-7.01 (m, 3H, ArH),
3.20-3.10 (m, 1H, CpH), 2.99-2.91 (m, 2H, CH3CH), 1.82 (s,
3H, CpCH3), 1.68 (s, 3H, CpCH3), 1.53 (s, 3H, CpCH3), 1.11 (d,
J = 7.2 Hz, 12H, CHCH3), 0.86 (d, J = 7.2 Hz, 3H, CpCH3).
MS: m/z 386 [M þ H].
Synthesis of 2-(4-MeC6H3NdCH)C6H4Me4CpH (HL4).
Compound HL4 was synthesized in the same manner as HL1
with 4-methylaniline (2.25 g, 21.0 mmol) as starting material.
The product (5.82 g, 18.5 mmol, 87.9%) was obtained in about 95%
purity as an orange oil. 1H NMR (300 MHz, CDCl3): δ 8.35 (s, 1H,
ArHCdN), 8.25 (d, 1H, ArH), 7.97 (d, 1H, ArH), 7.60-7.55 (m,
1H, ArH), 7.46-7.42 (m, 1H, ArH), 7.18-7.15 (d, 2H, ArH),
7.06-7.03 (d, 2H, ArH), 3.23-3.17 (m, 1H, CpH), 2.36 (s, 3H,
ArCH3), 1.93 (s, 3H, CpCH3), 1.82 (s, 3H, CpCH3), 1.72 (s, 3H,
CpCH3), 0.95 (d, J = 6 Hz, 3H, CpCH3). MS: m/z 316 [M þ H].
Synthesis of Complex 1. To a solution of free ligand HL1 (750
mg, 2.28 mmol) in 20 mL of THF was added dropwise a solution
of butyllithium (1.50 mL, 2.40 mmol) in THF at -78 °C. The
reaction mixture was warmed to room temperature and stirred
for 1 h. The resulting solution was then added to a suspension of
CrCl3(THF)3 (900 mg, 2.40 mmol) in 30 mL of THF at -78 °C.
The reaction mixture was warmed to room temperature and
stirred for another 3 h. During the reaction, the color of the
reaction mixture changed from purple to deep blue. After the
solvent was removed under vacuum, the residue was extracted
with 15 mL of dichloromethane to remove the insoluble impu-
rities. Pure product 1 was obtained by recrystallization from
CH2Cl2/n-hexane ((1-2)/10 v/v) as blue crystals (680 mg, 1.38
(34) (a) Russell, K. E.; Hunter, B. K.; Heyding, R. D. Polymer 1997,
38, 1409. (b) Bartczak, Z.; Galeski, A. Polymer 1996, 37, 2113. (c) Vickers,
M. E. Polymer 1995, 36, 2667. (d) Joo, Y. L.; Han, O. H.; Lee, H. K.; Song,
J. K. Polymer 2000, 41, 1355. (e) Baker, A. M. E.; Windle, A. H. Polymer
2001, 42, 667.
(35) Boudjouk, P.; So, J.-H. Inorg. Synth. 1992, 29, 108.
(36) (a) Massey, A. G.; Park, A. J. J. Org. Chem. 1962, 2, 245. (b)
Massey, A. G.; Park, A. J. J. Organomet. Chem. 1966, 5, 218. (c) Chein,
J. C. W.; Tsai, W. M.; Rasch, M. D. J. Am. Chem. Soc. 1991, 113, 8570.
(37) Matharu, D. S.; Morris, D. J.; Kawamoto, A. M.; Clarkson,
G. J.; Wills, M. J. Org. Chem. 2005, 7, 5489.
mmol, 60.4%). Anal. Calcd for C24H26NCrCl2 0.5CH2Cl2-
3
(493.82): C, 59.59; H, 5.51; N, 2.84. Found: C, 59.51; H, 5.58;
N, 2.88. MS: m/z 452, 474 [M þ H, M þ Na].
(38) Francis, P. S.; Cooke, R. C.; Elliott, J. H. J. Polym. Sci. 1958,
31, 453.

440 Organometallics, Vol. 30, No. 3, 2011
Zhang et al.
˚
Synthesis of Complex 2. Complex 2 was synthesized in the
same manner as complex 1 with compound HL2 (820 mg, 2.29
mmol), n-BuLi (1.50 mmol), and CrCl3(THF)3 (900 mg, 2.40
mmol) as starting materials. Pure product 2 (670 mg, 1.44 mmol,
62.8%) was obtained as dark green crystals. Anal. Calcd for
C26H30NCrCl2 (479.42): C, 65.14; H, 6.31; N, 2.92. Found: C,
65.09; H, 6.37; N, 2.89. MS: m/z 480, 502 [M þ H, M þ Na].
Synthesis of Complex 3. Complex 3 was synthesized in the
same manner as complex 1 with compound HL3 (880 mg, 2.28
mmol), n-BuLi (2.40 mmol) ,and CrCl3(THF)3 (900 mg, 2.40
mmol) as starting materials. Pure product 3 (750 mg, 1.48 mmol,
64.7%) was obtained as dark green crystals. Anal. Calcd for
C28H34NCrCl2 (507.48): C, 66.27; H, 6.75; N, 2.76. Found: C,
66.24; H, 6.79; N, 2.74. MS: m/z 508, 530 [M þ H, M þ Na].
Synthesis of Complex 4. Complex 4 was synthesized in the
same manner as complex 1 with compound HL4 (720 mg, 2.28
mmol), n-BuLi (2.40 mmol), and CrCl3(THF)3 (900 mg, 2.40
mmol) as starting materials. Pure product 4 (620 mg, 1.42 mmol,
62.2%) was obtained as dark green crystals. Anal. Calcd for
C23H24NCrCl2 (437.34): C, 63.16; H, 5.53; N, 3.20. Found: C,
63.08; H, 5.47; N, 3.15. MS: m/z 508, 530 [M þ H, M þ Na].
X-ray Crystallographic Studies. Single crystals of complexes
1-3 suitable for X-ray structure analysis were obtained from the
CH2Cl2/n-hexane mixed solvent system. The data were collected
at 293 K on a Rigaku R-AXIS PAPID IP diffractometer with
Mo KR radiation (λ = 0.710 73 A). All structures were solved by
direct methods39 and refined by full-matrix least squares on F2.
All non-hydrogen atoms were refined anisotropically, and the
hydrogen atoms were included in idealized positions. All calcu-
lations were performed using the SHELXTL crystallographic
software packages.40
Ethylene Polymerization Procedure. A dry 250 mL steel auto-
clave with a magnetic stirrer was charged with 60 mL of toluene
and saturated with ethylene (1.0 bar) at 20 °C. The polymeriza-
tion reaction was started by injection of a mixture of AlR3 and a
catalyst in toluene (10 mL) (together with a solution of Ph3CB-
(C6F5)4 in toluene (10 mL) for the AlR3/Ph3CB(C6F5)4 activated
polymerization). The vessel was repressurized to the needed
pressure with ethylene immediately, and the pressure was main-
tained by a continuous feed of ethylene. After a certain period of
time, the polymerization was quenched by injecting acidified
methanol (HCl (3 M)/methanol 1/1). The polymer was then
collected by filtration, washed with water and methanol, and
dried at 60 °C in vacuo to a constant weight.
Acknowledgment. This research work was supported
by the National Natural Science Foundation of China
(Nos. 20772044 and 21074043).
Supporting Information Available: A table giving crystal data
and structure refinement details and CIF files giving X-ray
crystallographic data for complexes 1-3. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(39) SHELXTL PC; Siemens Analytical X-ray Instruments, Madison,
WI, 1993.
(40) Sheldrick, G. M. SHELXTL Structure Determination Programs,
Version 5.0 PC; Siemens Analytical Systems, Madison, WI, 1994.