Synthesis, characterization, and ethylene oligomerization and polymerization of [2,6-bis(2-benzimidazolyl)pyridyl]chromium chlorides

Article abstract of DOI:10.1021/om050805l

A series of [2,6-bis(2-benzimidazolyl)pyridyl]chromium chlorides have been synthesized and characterized by elemental analysis and IR spectroscopy, along with X-ray diffraction analysis for the structures of Cl and C7. When methylaluminoxane (MAO) was employed as the cocatalyst, the chromium complexes showed high activity for ethylene oligomerization and polymerization. Oligomers were produced with high selectivity for α-olefins, and polyethylenes were generated with extremely broad molecular weight distributions. In the presence of diethylaluminum chloride (Et₂AlCl), these chromium complexes showed moderate activity for ethylene polymerization and produced high-molecular-weight linear polyethylene.

Full text of DOI:10.1021/om050805l

Organometallics 2006, 25, 1961-1969
1961
Synthesis, Characterization, and Ethylene Oligomerization and
Polymerization of [2,6-Bis(2-benzimidazolyl)pyridyl]chromium
Chlorides
Wenjuan Zhang,† Wen-Hua Sun,*,† Shu Zhang,† Junxian Hou,† Katrin Wedeking,‡
Steven Schultz,‡ Roland Fro¨hlich,‡ and Haibin Song§
Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100080, People’s Republic of China, Organisch-Chemisches Institut der UniVersita¨t Mu¨nster,
Corrensstrasse 40, 48149 Mu¨nster, Germany, and State Key Laboratory of Elemento-organic Chemistry,
Nankai UniVersity, Tianjin 300071, People’s Republic of China
ReceiVed September 17, 2005
A series of [2,6-bis(2-benzimidazolyl)pyridyl]chromium chlorides have been synthesized and character-
ized by elemental analysis and IR spectroscopy, along with X-ray diffraction analysis for the structures
of C1 and C7. When methylaluminoxane (MAO) was employed as the cocatalyst, the chromium complexes
showed high activity for ethylene oligomerization and polymerization. Oligomers were produced with
high selectivity for R-olefins, and polyethylenes were generated with extremely broad molecular weight
distributions. In the presence of diethylaluminum chloride (Et2AlCl), these chromium complexes showed
moderate activity for ethylene polymerization and produced high-molecular-weight linear polyethylene.
attention, with the expectation of being highly effective catalysts
for industrial application and yielding an understanding of the
catalytic mechanism.2,4d Tridentate chromium complexes ligated
with S∧N∧S,6 P∧N∧P,4g O∧N∧N,7 or N∧N∧N4d,8 have shown
great potential in ethylene oligomerization and polymerization.
For example, chromium complexes coordinated with N∧N∧N
species such as bis(imino)pyridine,9 (2-pyridylmethyl)amines,10
and â-diketimate and pyrrolide-imine ligands11 have been
reported to polymerize or oligomerize ethylene when activated
by methylaluminoxane (MAO). The 2,6-bis(2-benzimidazolyl)-
pyridine ligand derivatives have been reported to coordinate to
transition metals in order to prepare various complexes for
structure and property investigations,12-14 but without consid-
eration of their catalytic properties in ethylene oligomerization
1. Introduction
In ethylene oligomerization and polymerization, increasing
interest has been focused on the exploration and development
of homogeneous transition-metal catalysts.1 Chromium catalysts
have played important roles in both ethylene polymerization
and oligomerization.2 The Cr-based heterogeneous catalysts
(Phillips catalyst) for ethylene polymerization produce poly-
ethylenes with broad molecular weight distributions as a unique
property, which are significantly more useful than other poly-
olefins or their blends produced by Ziegler-Natta catalysts for
application in blow-molding products.3 Meanwhile, the Cr-based
catalysts also play a central role in the oligomerization of
ethylene such as trimerization4 and tetramerization.5 In addition,
chromium complexes as homogeneous catalysts for ethylene
oligomerization and polymerization have recently drawn much
(6) (a) McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Hu, C.; Englert,
U.; Dixon, J. T.; Grove, C. Chem. Commun. 2003, 334. (b) Agapie, T.,
Schofer, S. J., Labinger, J. A., Bercaw, J. E. J. Am. Chem. Soc. 2004, 126,
1304.
(7) Jones, D. J.; Gibson, V. C.; Green, S. M.; Maddox, P. J. Chem.
Commun. 2002. 1038
(8) (a) Wills, A. R.; Edwards, P. G. J. Chem. Soc., Dalton Trans. 1989,
1253. (b) Wasserscheid, P.; Grimm, S.; Ko¨hn, R. D.; Haufe, M. AdV. Synth.
Catal. 2001, 343, 814. (c) Ru¨ther, T.; Cavell, K. J.; Braussaud, N. C.;
Skelton, B. W.; White, A. H. Dalton Trans. 2002, 4684.
(9) (a) Small, B. L.; Carney, M. J.; Holman, D. M.; O’Rourke, C. E.;
Halfen, J. A. Macromolecules 2004, 37, 4375. (b) Esteruelas, M. A.; Lo´pez,
A. M.; Me´ndez, L.; Oliva´n, M.; On˜ate, E. Organometallics 2003, 22, 395.
(c) Nakayama, Y.; Sogo, K.; Yasuda, H.; Shiono, T. J. Polym. Sci., Part
A: Polym. Chem. 2005, 43, 3368.
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Rheingold, A. L. Organometallics 2004, 23, 6184.
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G. A.; Andew, J. P. W.; Williams, D. J. Chem. Commun. 1998, 1651. (b)
Kim, W.-K.; Fevola, M. J.; Liable-Sands, L. M.; Rheingold, A. L.; Theopold,
K. H. Organometallics 1998, 17, 4541.
(12) (a) Breslow, R.; Hunt, J. T.; Smiley, R.; Tarnowski, T. J. Am. Chem.
Soc. 1983, 105, 5337. (b) Beer, R. H.; Tolmann, W. B.; Bott, S. G.; Lippard,
S. J. Inorg. Chem. 1989, 28, 4557. (c) Wu, F.-J.; Kurtz, D. M., Jr.; Hagen,
K. S.; Nyman, P. D.; Debrunner, P. G.; Vankai, V. A. Inorg. Chem. 1990,
29, 5174. (d) Kimblin, C.; Allen, W. E.; Parkin, G. J. Chem. Soc., Chem.
Commun. 1995, 1813. (e) Sorrell, T. N.; Allen, W. E.; White, P. S. Inorg.
Chem. 1995, 34, 952.
† Chinese Academy of Sciences.
‡ Organisch-Chemisches Institut der Universita¨t Mu¨nster.
§ Nankai University.
(1) (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428. (b) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem.
ReV. 2000, 100, 1169. (c) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV.
2003, 103, 283.
(2) Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A. Chem.
ReV. 2005, 105, 115.
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(4) (a) Briggs, J. R. J. Chem. Soc., Chem. Commun. 1989, 674. (b)
Emrich, R.; Heinemann, O.; Jolly, P. W.; Krueger, C.; Verhovnik, G. P. J.
Organometallics 1997, 16, 1511. (c) Ko¨hn, R. D.; Haufe, M.; Mihan, S.;
Lilge, D. Chem. Commun. 2000, 1927. (d) Ko¨hn, R. D.; Haufe, M.; Kociok-
Ko¨hn, G.; Grimm, S.; Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed.
2000, 39, 4337. (e) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.;
Scutt, J.; Wass, D. F. Chem. Commun. 2002, 858. (f) McGuinness, D. S.;
Wasserscheid, P.; Keim, W.; Morgan, D.; Dixon, J. T.; Bollmann, A.;
Maumela, H.; Hess, F.; Englert, U. J. Am. Chem. Soc. 2003, 125, 5272. (g)
McGuinness, D. S.; Wasserscheid, P.; Morgan, D. H.; Dixon, J. T.
Organometallics 2005, 24, 552. (h) Doehring, A.; Jensen, V. R.; Jolly, P.
W.; Thiel, W.; Weber, J. C. Organometallics 2001, 20, 2234.
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Maumela, H.; McGuinness, D. S.; Morgan, D. H.; Neveling, A.; Otto, S.;
Overett, M.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann, S. J. Am. Chem.
Soc. 2004, 126, 14712.
10.1021/om050805l CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/14/2006

1962 Organometallics, Vol. 25, No. 8, 2006
Zhang et al.
Scheme 1. Synthesis of 2,6-Bis(2-benzimidazolyl)pyridine Derivatives L1-L8 and Chromium Complexes C1-C8
and polymerization. As an extension of our investigation of
transition-metal complexes with N∧N∧N ligands,15 bis(ben-
zenimidazolyl)pyridine derivatives were prepared and coordi-
nated to iron(II) chloride. However, the bis(benzimidazolyl)-
pyridyl ferrous complexes showed low activity for ethylene
polymerization. Therefore, chromium complexes ligated by bis-
(benzimidazolyl)pyridine were synthesized (Scheme 1). Very
recently, the Gibson group reported a mechanism study of
ethylene oligomerization by C1.16 In this study, the chromium
complexes ligated by bis(benzimidazolyl)pyridine derivatives
showed good activities for the combined oligomerization and
polymerization of ethylene with MAO as cocatalyst, while the
ethylene polymerization only proceeds in the presence of
diethylaluminum chloride (Et2AlCl). Here the synthesis and
characterization of chromium complexes bearing bis(benzimi-
dazolyl)pyridine ligands are reported along with single-crystal
X-ray structural analyses. The ethylene reactivity and their
product properties have also been investigated.
alkylation of L1-L3 in acetone at room temperature,19 while
the alkylation for L8 proceeds at higher temperature (60 °C).
The stretching frequencies of all ligands fall within the following
ranges: ν(N-H) is about 3055 cm-1 for L1-L3, ν(CdC) is
between 1584 and 1600 cm-1, and ν(CdN) is between 1567
and 1572 cm-1. All of the 2,6-bis(2-benzimidazolyl) pyridine
derivatives synthesized were confirmed by elemental analysis
and NMR spectra.
2.2. Synthesis and Characterization of the Chromium
Complexes C1-C8. The six-coordinated [2,6-bis(2-benzimi-
dazolyl)pyridyl]chromium(III) trichlorides C1-C8 were pre-
pared in high yields through the treatment of 2,6-bis(2-
benzimidazolyl)pyridine derivatives with 1 equiv of CrCl3(THF)3
in CH2Cl2 at room temperature (Scheme 1). According to their
IR spectra, the typical stretching frequency of ν(N-H) shift to
a band at around 3064 cm-1 for C1-C3, and the frequencies
of ν(CdC) and ν(CdN) shift to bands between 1603 and
1613 cm-1 and between 1568 and 1590 cm-1, respectively. On
the other hand, the strong ligand bands at around 1313 cm-1
(L1-L3) and 1325 cm-1 (L4-L8) shift to higher frequencies
around 1323 cm-1 (C1-C3) and 1335 cm-1 (C4-C8), indicat-
ing coordination through the pyridine nitrogen. All of the
complexes were consistent with their elemental analyses.
Complexes C1 and C7 were further characterized by X-ray
crystallography. Crystals of complex C1 suitable for an X-ray
structure determination were grown from an N,N-dimethylfor-
mamide (DMF) solution layered with diethyl ether. It is the
first crystal structure determined by X-ray crystallography of a
chromium(III) complex bearing 2, 6-bis(2-benzimidazolyl)-
pyridine. The molecular structure is shown in Figure 1, and
selected bond lengths and angles are given in Table 1. The
geometry around the six-coordinated chromium atom could be
described as a distorted octahedron with a tridentate N∧N∧N
ligand and three chlorines. The Cl-Cr-Cl and N-Cr-N angles
are 77.55(10), 77.56(10), 155.09(10)° and 92.07(4), 93.17(4),
174.71(4)° respectively. The Cr-N(pyridine) bond distance
(Cr1-N3 ) 2.040(2) Å) is about 0.03 Å shorter than the
Cr-N(imidazole) bond distance ((2.072(3) Å (Cr1-N1) and
2.084(3) Å (Cr1-N4)), with the formal double-bond character
2. Results and Discussion
2.1. Synthesis and Characterization of 2,6-Bis(2-benzimi-
dazolyl)pyridine Derivatives (L1-L8). The 2,6-bis(2-benz-
imidazolyl)pyridine derivatives L1-L3 were prepared by
condensation reactions of the corresponding 1,2-phenylenedi-
amines and pyridine-2,6-dicarboxylic acid in the presence of
phosphoric acid (PPA) in the microwave oven (Scheme 1).
Among these compounds, L1 has been synthesized by refluxing
the corresponding reactants in solution for several hours.17 To
significantly shorten the reaction time, the condensation reac-
tions were carried out under microwave radiation.18 The ligands
L4-L7 were prepared in high yields through simple N-
(13) (a) Chatlas, J.; Kaizaki, S.; Kita, E.; Kita, P.; Sakagami, N.; Van
Eldik, R. J. Chem. Soc., Dalton Trans. 1999, 91. (b) Winter, J. A.; Caruso,
D.; Shepherd, R. E. Inorg. Chem. 1988, 27, 1086. (c) Bocarsly, J. R.; Chiang,
M. Y.; Bryant, L.; Barton, J. K. Inorg. Chem. 1990, 29, 4898.
(14) Ceniceros-Go´meza, A. E.; Barba-Behrensa, N.; Quiroz-Castro, M.
E.; Berne`s, S.; No¨th, H.; Castillo-Blum, S. E. Polyhedron 2000, 19, 1821.
(15) (a) Wang, L.; Sun, W.-H.; Han, L.; Yang, H.; Hu, Y.; Jin, X. J.
Organomet. Chem. 2002, 658, 62. (b) Ma, Z.; Sun, W.-H.; Li, Z.-L.; Shao,
C.-X.; Hu, Y.-L. Chin. J. Polym. Sci. 2002, 20, 205.
(16) Tomov, A. K.; Chirinos, J. J.; Jones, D. J.; Long, R. J.; Gibson, V.
C. J. Am. Chem. Soc. 2005, 127, 10166.
(17) (a) Addison, A. W.; Burke, P. J. J. Heterocycl. Chem. 1981, 18,
803. (b) Muller, G.; Bu1nzli, J.-C. G.; Schenk, K. J.; Piguet, C.; Hopfgartner,
G. Inorg. Chem. 2001, 40, 2642.
(18) (a) Cui, Y.; Tang, X.; Shao, C.; Li, J.; Sun, W.-H. Chin. J. Chem.
2005, 23, 589. (b) Yang, H.; Sun, W.-H.; Li, Z.; Wang, L. Synth. Commun.
2002, 32, 2395.
(19) Kikugawa, Y. Synthesis 1981, 124.

[2,6-Bis(2-benzimidazolyl)pyridyl]chromium Chlorides
Organometallics, Vol. 25, No. 8, 2006 1963
Figure 2. Molecular structure of C7. Thermal ellipsoids are shown
at 30% probability; hydrogen atoms and solvent have been omitted
for clarity.
Figure 1. Molecular structure of complex C1. Thermal ellipsoids
are shown at 30% probability; hydrogen atoms and solvent have
been omitted for clarity.
Table 2. Effect of Cocatalyst and Solvent on Ethylene
Reactivitya
Table 1. Selected Bond Lengths and Bond Angles for
Complexes C1 and C7
oligomer
polymer
C1
C7
entry com-plex cocat. Al/Cr solvent activityb distribnc activityb
Bond Lengths (Å)
2.040(2)
1
2
3
4
5
6
7
C1
C1
C1
C1
C1
C1
C1
MAO
MMAO 1000 toluene
AlEt3 1000 toluene
AlMe3 1000 toluene
1000 toluene
32.0
C4-C28
7.45
2.28
trace
trace
trace
trace
1.50
Cr1-N3
Cr1-N1
Cr1-N4
Cr1-Cl3
Cr1-Cl2
Cr1-Cl1
N4-C13
N4-C19
N1-C7
Cr1-N18
2.044(4)
2.050(4)
2.069(4)
2.3003(16)
2.3036(14)
2.3717(16)
1.340(6)
1.383(6)
1.329(6)
1.380(6)
8.49 C4-C20
7.95 C4
6.76 C4
4.14 C4, C6
3.25 C4-C16
0
2.072(3)
2.084(3)
2.2945(10)
2.3270(11)
2.3488(11)
1.334(4)
1.386(4)
1.337(4)
1.397(4)
Cr1-N1
Cr1-N38
Cr1-Cl1
Cr1-Cl2
Cr1-Cl3
N18-C10
N18-C17
N38-C30
N38-C37
MAO
MAO
1000 C2H4Cl2
1000 CH2Cl2
Et2AlCl 150 toluene
a General conditions: 5 µmol of complex; ethylene pressure 1 atm;
temperature 20 °C; 1 h; volume 30 mL. b In units of 104 g (mol of Cr)-1
h-1 c Determined by GC and GC-MS.
.
N1-C1
Bond Angles (deg)
N3-Cr1-N1
N3-Cr1-N4
N1-Cr1-N4
N3-Cr1-Cl3
N1-Cr1-Cl3
N4-Cr1-Cl3
N3-Cr1-Cl2
N1-Cr1-Cl2
N4-Cr1-Cl2
Cl3-Cr1-Cl2
N3-Cr1-Cl1
N1-Cr1-Cl1
N4-Cr1-Cl1
Cl3-Cr1-Cl1
Cl2-Cr1-Cl1
77.55(10)
77.56(10)
155.09(10)
178.70(8)
103.53(8)
101.37(7)
88.65(8)
89.10(8)
89.47(8)
92.07(4)
86.11(8)
89.00(8)
90.18(8)
93.17(4)
174.71(4)
N18-Cr1-N1
77.28(15)
154.24(14)
77.16(15)
88.94(12)
93.28(11)
89.36(12)
100.99(11)
173.53(12)
104.77(11)
92.92(6)
89.46(12)
85.04(11)
91.49(12)
177.89(6)
88.73(6)
(6)°. The bond distance Cr1-Cl2 ) 2.3036(14) Å is as long as
the distance Cr1-Cl1 (2.3003(16) Å), which is much shorter
than the bond length of Cr1-Cl3 (2.3717(16) Å). The two
benzimidazole rings are slightly distorted from the coordination
plane by three nitrogen atoms with dihedral angles of 5 and 3°.
The average deviations from the plane of the benzimidazole
C10-N11-C12-C13-C14-C15-C16-C17-N18 and the
other benzimidazole ring are 0.0224 and 0.0141 Å, respectively.
The dihedral angle between these two planes is 15.7°, which is
much larger than that in C1 (3.1°), indicating that the greater
substitution leads to more distortion of the benzimidazole ring.
2.3. Ethylene Oligomerization and Polymerization. 2.3.1.
Selecting a Suitable Solvent and Cocatalyst. The effects of
various cocatalysts on the productivity for ethylene reactivity
were first studied in detail with C1, and the results are collected
in Table 2. The results indicate that the ethylene reactivity is
strongly influenced by the chosen cocatalysts. The C1/MAO
system shows remarkably higher catalytic activity than the other
systems activated with MMAO, AlEt3, AlMe3, and Et2AlCl.
Among them, the distribution of oligomers with MAO and
MMAO activation resembles Schulz-Flory rules.20 The lower
activity with MMAO could be traced to the presence of
triisobutylaluminoxane (TIBAO), and the species generated
therefrom hinder the insertion reaction of ethylene, due to their
steric bulkiness.21 When C1 is activated by Et2AlCl, ethylene
polymerization is observed without oligomerization in the
process. Under the same reaction parameters, the C1/MAO
N18-Cr1-N38
N1-Cr1-N38
N18-Cr1-Cl1
N1-Cr1-Cl1
N38-Cr1-Cl1
N18-Cr1-Cl2
N1-Cr1-Cl2
N38-Cr1-Cl2
Cl1-Cr1-Cl2
N18-Cr1-Cl3
N1-Cr1-Cl3
N38-Cr1-Cl3
Cl1-Cr1-Cl3
Cl2-Cr1-Cl3
of the imidazole linkages (N4-C13 (1.334(4) Å) and N1-C7
(1.337(4) Å)) having been retained. The two benzimidazole rings
and the pyridine ring in the ligand are nearly coplanar: the
average deviations from the plane C13-N4-C19-C18-C17-
C10-C15-C4-N5 and the plane N1-C7-N2-C6-C5-C4-
C3-C2-C1 are 0.09 and 0.0088 Å, respectively, and the
dihedral angle between these two benzimidazole planes is 3.1°.
It is worthwhile to mention that the bond lengths between the
chromium and the mutually trans-disposed chlorine atoms are
significantly different. The bond length of Cr1-Cl2 (2.3270-
(11) Å) is shorter than the bond length of Cr1-Cl1 (2.3488-
(11) Å), while the bond length of Cr1-Cl3 (2.2945(10) Å) is
the shortest of the three Cr-Cl bonds.
Crystals of complex C7 suitable for an X-ray structure
determination were grown by diffusion of diethyl ether into a
dichloromethane solution. Its molecular structure is shown
in Figure 2, and selected bond lengths and angles are given
in Table 1. Its structure is very similar to that of C1. The
basal plane consists of the three nitrogen atoms from the pyri-
dine and imidazole and one equatorial chloride, with the
remaining chloride occupying the apical position. In this
geometry, the Cr1-N1(pyridine) distance (2.050(4) Å) is about
the same as the Cr1-N(imino) bond length (Cr1-N38 ) 2.069-
(4) Å, Cr1-N18 ) 2.044(4) Å) and Cl1-Cr1-Cl3 ) 177.89-
(20) (a) Flory, P. J. J. Am. Chem. Soc. 1940, 62, 1561. (b) Schulz, G. V.
Z. Phys. Chem., Abt. B 1935, 30, 379. (c) Schulz, G. V. Z. Phys. Chem.,
Abt. B 1939, 43, 25. (c) Meurs, M. V.; Britovsek, G. J. P.; Gibson, V. C.;
Cohen, S. A. J. Am. Chem. Soc. 2005, 127, 9913. (d) Britovsek, G. J. P.;
Cohen, S. A.; Gibson, V. C.; Meurs, M. V. J. Am. Chem. Soc. 2004, 126,
10701.
(21) (a) Chen, E. Y.; Marks, T. J. Chem. ReV. 2000, 100, 1391. (b)
Karam, A. R.; Catar´ı, E. L.; Lo´pez-Linares, F.; Agrifoglio, G.; Albano, C.
L.; D´ıaz-Barrios, A.; Lehmann, T. E.; Pekerar, S. V.; Albornoz, L. A.;
Atencio, R.; Gonza´lez, T.; Ortega, H. B.; Joskowics, P. Appl. Catal. A:
Gen. 2005, 280, 165.

1964 Organometallics, Vol. 25, No. 8, 2006
Table 3. Oligomerization and Polymerization under 1 atm of Ethylene with C1-C8/MAOa
oligomer distribnc (%)
Zhang et al.
entry
complex
Al/Cr
T (°C)
C4/∑C
C6/∑C
gC8/∑C
R-olefin (%)
productivityb
oligomers (wt %)
% PE (wt %)
1
2
3
4
5
6
7
8
C1
C2
C3
C4
C5
C6
C7
C8
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
500
500
500
500
500
500
500
500
100
20
20
20
20
20
20
20
20
20
20
20
20
20
0
34.7
40.4
56.6
89.7
59.9
100
86.7
74.9
100
17.6
15.4
13.9
12.5
47.9
9.4
13.9
12.0
8.5
3.1
11.8
51.4
47.6
34.9
7.2
>99.9
98.6
98.5
100
95.4
100
90.4
97.1
100
96.4
94.5
97.2
95.5
100
2.80
0.96
0.61
0.53
0.36
0.38
0.71
0.43
0.56
2.45
3.52
3.94
4.62
1.12
5.59
9.26
14.3
10.3.
3.02
78.6
91.1
96.0
21.4
8.9
4.0
28.3
100
100
100
100
5.01
11.6
8.3
13.5
9
100
10
11
12
13
14
15
16
17
18
19d
300
700
13.2
16.7
16.0
16.5
9.6
14.9
12.5
13.2
14.4
13.3
69.3
67.8
70.1
71.0
42.5
75.7
80.1
80.7
72.0
75.0
83.3
80.1
81.1
83.5
65.5
72.4
75.8
72.2
97.1
77.8
16.7
19.9
18.9
16.5
34.5
27.6
24.2
27.8
2.8
1000
1500
1000
1000
1000
1000
1000
1000
40
50
60
80
20
91.4
82.1
77.8
66.8
95.0
7.4
6.1
13.6
11.7
22.1
a General conditions: 5 µmol of complex; 30 mL of toluene; 1 h; MAO cocatalyst. b In units of 105 g (mol of Cr)-1 h-1 c Determined by GC and
.
GC-MS. d Reaction time 2 h.
shows better ethylene reactivity in toluene (entry 1) than in
dichloromethane or 1,2-dichloroethane (entries 5 and 6). It has
been suggested that chloro solvents act as effective chain
termination agents.22 Therefore, further detailed investigations
have been carried out in toluene while changing the catalytic
parameters with cocatalysts of MAO and Et2AlCl.
2.3.2. Ethylene Oligomerization and Polymerization with
MAO as Cocatalyst. The chromium complexes were studied
in detail for their catalytic activities in ethylene oligomerization
and polymerization using methylaluminoxane (MAO) as co-
catalyst. The results at 1 atm of ethylene are summarized in
Table 3.
2.3.2.1. Effect of the Ligand Environment. These complexes
exhibit moderate catalytic activities for ethylene oligomerization
and polymerization at 1 atm of ethylene pressure. The products
obtained by C1-C3 with MAO are comprised of oligomers
with a distribution that closely resembles Schulz-Flory rules20
Figure 3. Oligomer molar distribution obtained with C1 at different
and waxlike polyethylene. However, complexes C4-C8 pro-
duced the major product butene and a trace of polymer. The
ethylene reactivity greatly depends on the ligand environment.
It is observed that, for the same reaction conditions, the
productivity decreased in the orders C1 > C2 > C3 and C4 >
C5, which indicates that the more substituted the phenyl group,
the lower the ethylene reactivity. This can perhaps be attributed
to the increasing nucleophilicity of the metal center by the
greater number of methyl groups on the phenyl ring, which
weakens the interaction between the chromium atom and the
π-electrons of the ethylene monomer and decreases the rate of
ethylene insertion in the chain-growth steps.23
Al/Cr ratios at 1 atm.
C1-C3, containing N-H groups, show relatively high catalytic
activities compared with other analogues. This could be possibly
caused by their deprotonation to give anionic amide ligands
when activated by MAO (or residual AlMe3). The anionic amide
ligands could be free or form N-Al species (anion-cation pair)
to increase their catalytic activity. Though the mechanism is
not clear, the fact that complexes C4-C8, containing N-alkyl
groups, have low catalytic activities indicates that alkylation of
N-H groups plays a negative role in ethylene reactivity. These
results are in agreement with the literature.4g
On the other hand, the incorporation of an alkyl group on
the N atom of imidazole into the complexes leads to a dramatic
decrease in ethylene reactivity and greatly affects the distribution
of the oligomers. As shown in Table 3, the complexes C4-C8
showed lower productivity than C1-C3, especially in their
polymer yields which decrease to traces. For example, the
productivity of C1 is much higher than that of C4 (Table 3,
entries 1 and 4), and the same is true for the other complexes
in a similar order of C2 > C5 and C3 > C6. The complexes
2.3.2.2. Effects of Al/Cr Molar Ratio and Reaction
Temperature. The influences of the Al/Cr molar ratio and the
reaction temperature on ethylene activation were investigated
in detail with C1. Increasing the Al/Cr molar ratio from 100 to
1500 led to a significant increase of the productivity (entries 1
and 9-13 in Table 3). When the ratio of Al/Cr is 1500, the
catalytic system displayed high productivity of up to 4.62 ×
105 g (mol of Cr)-1 h-1. However, the molar distribution of
oligomers and the selectivity for R-olefins were not strongly
affected by Al/Cr ratio, which can be observed in Figure 3 and
is verified by the small variability in their selectivities, with all
values higher than 94.5% (entries 9-13 in Table 3), individually.
In particular, when the Al/Cr ratio is 100, the productivity was
(22) Robertson, N. J.; Carney, M. J.; Halfen, J. A. Inorg. Chem. 2003,
42, 6876.
(23) Oouchi, K.; Mitani, M.; Hayakawa, M.; Yamada, T. Macromol.
Chem. Phys. 1996, 197, 1545.

[2,6-Bis(2-benzimidazolyl)pyridyl]chromium Chlorides
Organometallics, Vol. 25, No. 8, 2006 1965
Table 4. Polymerization and Oligomerization of 10 atm of
Ethylene with C1-C8/MAOa
T
R-olefinc
(%)
oligomer
PE
entry complex (°C)
K
productivityb (wt %) (wt %)
1d
2
3
4
5
6
7
8
9
C1
C1
C1
C2
C3
C4
C5
C6
C7
C8
75
80
40
40
40
40
40
40
40
40
91.7
93.6 0.70
95.8
100
100
98.5 0.67
100
94.5 0.70
97.0
100
13.00
11.60
23.70
8.20
2.77
3.82
0.56
1.40
0.67
0.58
65.7
65.8
59.0
78.6
78.0
84.5
84.7
93.6
93.2
91.7
34.3
34.2
41.0
21.3
22.0
15.4
15.3
6.4
0.62
0.67
6.8
8.3
10
0.75
a General conditions: 2 µmol of complex; 100 mL of toluene; 1 h; MAO
cocatalyst, Al/Cr ) 1000. b In units of 106 g (mol of Cr)-1 h-1
.
c Determined
by GC and GC-MS. d 5 µmol of complex.
very low, which may be attributed to consumption of the MAO
by impurities in the solvent.
Figure 4. Oligomer distribution obtained with C1 at different
ethylene pressures (entry 15 in Table 3 and entry 3 in Table 4).
Elevating the reaction temperature from 0 to 60 °C sped up
the combined catalytic reactions of oligomerization and polym-
erization. A sharp increase in the production of oligomers and
polymer was observed. However, a further increase of the
reaction temperature to 80 °C results in a sharp decrease in
productivity due to the deactivation of some active centers.24
At the same time, increasing the temperature results in a decrease
of PE proportion (see Table 3, entries 14-18), which is due to
an increase in the rate of the â-hydrogen elimination relative
to the rate of propagation at higher temperature.25,26 In addition,
increasing the temperature leads to a rapid decrease in the
selectivity for R-olefins. For example, the selectivity for
R-olefins decreased from 100% at 0 °C to 66.8% at 80 °C.
The catalyst lifetime is one significant factor in industrial
considerations. The effect of reaction time on catalytic activity
was also studied using the C1/MAO system with a Al/Cr molar
ratio of 1000/1 at 20 °C. The productivity obviously decreases
with prolonged reaction time (compare entry 12 with entry 19,
Table 3), suggesting that the active species of C1 were
deactivated as the time of the catalytic reaction increased.
2.3.2.3. Ethylene Pressure Effects. Ethylene oligomerization
and polymerization with complexes C1-C8 were also studied
at 10 atm of ethylene with MAO as cocatalyst, and the results
are listed in Table 4. The results show that ethylene pressure
significantly affects the catalytic behavior of the complexes.
This observation of increased productivity at elevated ethylene
pressure is in accordance with the previously reported results
for a homogeneous chromium catalyst.27 Among those precur-
sors, C1 shows the highest activity of up to 2.37 × 107 g (mol
of Cr)-1 h-1, which is more than 40 times greater than the result
at 1 atm of ethylene (entry 3 in Table 4 vs entry 15 in Table 3).
At 10 atm of ethylene, the effect of the ligand environments on
the productivity activity is similar to the results at 1 atm.
In comparison with the results with 1 atm of ethylene, the
distribution of oligomers formed at 10 atm of ethylene pressure
shifts to higher carbon number olefins. Especially for complexes
C4-C8, bearing an N-alkyl group, the percentages of PE
increase remarkably, which can be explained by the rate of
propagation being faster than that of hydrogen elimination at
higher pressure. The oligomer distribution obtained by C1
showed a similar trend on changing the ethylene pressure (as
Figure 5. Oligomer distributions obtained in entries 3-6 in
Table 4.
shown in Figure 4), which is in accord with the result of the
Gibson group.16 Most of the oligomer distributions closely
resemble the Schulz-Flory rules, and the corresponding K values
are given in Table 4. Among them, the distributions obtained
by C1, C5, and C7 deviated more from the Schulz-Flory
distribution; perhaps their catalytic systems did not follow a
simple chain growth reaction. The results indicated that the
ligand environment had little effect on the chain growth reaction.
The substituents, located far away from the active site due to
the planar ligand structure in the complexes, will impose a slight
steric influence on the ethylene reactivity. The oligomer
distributions achieved by C1-C4 were shown in Figure 5, while
the imitations of Schulz-Flory distributions for C2 (K ) 0.62),
C3 (K ) 0.67) and C4 (K ) 0.67) fit well with their
experimental results (other oligomer distributions were included
in the Supporting Information). In addition, GC and GC-MS
analysis of the oligomers indicates that the selectivity for linear
R-olefins is higher than that at 1 atm for complexes C1-C8.
2.3.2.4. Characterization of Polyethylenes. Regardless of
oligomers, all catalysts at 10 atm of ethylene produced
significant quantities of polyethylene coproduct. The GPC traces
of polyethylenes (Figure 6) showed their bimodal and even
multimodal behavior, which were also observed in the catalytic
systems of the pyridinebis(imino)chromium9a and [bis(pyridyl-
methyl)amine]chromium complexes.10
(24) Svejda, S. A.; Brookhart, M. Organometallics 1999, 18, 65.
(25) Huang, C.; Ahn, J.; Kwon, S.; Kim, J.; Lee, J.; Han, Y.; Kim, H.
Appl. Catal. A: Gen. 2004, 258, 173.
(26) Chen, Y.; Qian, C.; Sun, J. Organometallics 2003, 22, 1231.
(27) Bluhm, M. E.; Walter, O.; Doring, M. J. Organomet. Chem. 2005,
690, 713.
The GPC curves showed that the molecular weights of the
polymers produced by these complexes are largely dependent

1966 Organometallics, Vol. 25, No. 8, 2006
Zhang et al.
Figure 7. 13C NMR spectrum of the PE sample obtained by
C1/MAO in entry 3 in Table 4.
Figure 6. GPC traces of PE samples obtained by entries 3-10 in
Table 4.
Table 5. Polymerization of Ethylene with Et2AlCl as
Cocatalysta
e
pressure
Tm
on the ligand environment. Precursors C1-C4 produced mainly
waxlike polyethylenes with low molecular weights (Mw of the
largest peak ranges from 927 to 1657). The steric bulk is not
sufficient to prevent associative displacement of the growing
oligomeric chain, and the electron affinity of the chromium
center facilitates the chromium-induced abstraction of a â-hy-
dride from the growing alkyl chain; these combined reasons
result in a rapid chain transfer process and low-molecular-weight
polymer chains.23,28 In contrast, precatalysts C5-C8 produced
mainly high-molecular-weight polymer along with some low-
molecular-weight polymer. The average molecular weights (Mw)
range from 6.0 × 104 to 14.5 × 104, which are much higher
than those of C1-C3. Perhaps the incorporation of substituents
on the nitrogen contributes to the increase in the molecular
weight, but more study is needed to fully assess these molecular
weight trends.
f
f
f
entry complex (atm) Al/Cr activityb (°C) 104Mn 104Mw Mw/Mn
1c
2
3
C2
C2
C2
C2
C2
C1
C2
C3
C4
C5
C6
1
1
1
1
1
10
10
10
10
10
10
100
150
200
250
500
250
250
250
250
250
250
2.41 136.3 1.65 27.8
2.88 136.7 1.63 28.7
16.8
17.7
nd
28.0
nd
8.1
7.55
8.26
2.68 nd
nd
nd
4
2.63 136.2 1.44 40.4
5
1.05 nd
3.98 nd
nd
nd
6d
7
2.23 18.0
3.52 136.7 4.82 36.4
3.19 nd 3.03 25.0
2.66 137.4 5.57 169.4 30.4
4.07 134.2 1.05 14.1
6.87 139.4 1.62 39.3
8
9
10
11
13.4
24.3
a General conditions: 5 µmol of complex; temperature 20 °C; reaction
time 1 h. b In units of 104 g (mol of Cr)-1 h-1 c 30 mL of toluene for
.
entries 1-5. d 100 mL of toluene for entries 6-11. e Determined by DSC.
f Determined by GPC; nd ) not determined.
the polymerization activity (entries 1-5 in Table 5). However,
when a ratio of 500 was used, the polymerization activity
decreased significantly. Meanwhile, increasing the ethylene
pressure resulted in a slight change in polymerization activities
(entries 4 and 7, Table 5), which is not what was observed with
MAO as cocatalyst. In addition, there was no regulation of the
environment effect on the polymerization activity, which differs
from the results of polymerization with MAO as cocatalyst.
2.3.3.1. Characterization of Polyethylenes. The GPC curves
(Figure 8) showed that all polyethylenes obtained with
AlEt2Cl activator have broad molecular weight distributions.
By using precursors C1-C3, the molecular weights of poly-
ethylenes with Et2AlCl are much higher than those with MAO.
From the Mn and Mw/Mn data in Table 5, polyethylene with a
lower molecular weight (Mn) and broader molecular weight
distribution is formed at higher Al/Cr ratios (entries 1, 2, and 4
in Table 5). This may be due to chain transfer to aluminum
and more active species generated in these systems. The effect
of environment on the polymerization activity is not regular;
however, the Mw/Mn values of polyethylenes produced by
complexes C4-C6 are much higher than those for C1-C3
(entries 6-11).
As characterized by its IR spectrum recorded using a KBr
disk in the range of 4000-400 cm-1, the polymer can be
confirmed to be mainly linear R-olefins from the characteristic
vibration absorption bands of various C-H and CdC bonds.
The NMR spectra verified that all the polyethylenes are similar
higher order linear R-olefins. The 13C NMR spectrum of the
PE obtained by complex C1 (see Figure 7) at 10 atm of ethylene
pressure demonstrated that the PE sample is strictly linear with
the presence of end vinyl groups.29
2.3.3. Ethylene Polymerization with Et2AlCl as Cocatalyst.
The initial study of ethylene polymerization showed that, with
Et2AlCl as activator, complexes C1-C3 showed special be-
havior in ethylene polymerization, which differs from the result
with MAO. It has been reported that chromium complexes
produce high-molecular-weight PE with noticeably higher
activities when Et2AlCl was used in place of MAO as cocata-
lyst.30 We performed a detailed study of chromium complexes
with the activator Et2AlCl for ethylene polymerization.
The polymerization results are shown in Table 5. With the
activator Et2AlCl, complexes C1-C6 showed moderate activity
for the polymerization of ethylene. At 1 atm of ethylene,
increasing the Al/Cr ratio from 100 to 250 has little effect on
Although the ethylene pressure has little effect on the polym-
erization activity, it significantly affects the Mw and Mw/Mn
(28) Cowdell, R.; Davies, C. J.; Hilton, S. J.; Mare´chal, J.-D.; Solan, G.
A.; Thomas, O.; Fawcett, J. Dalton Trans. 2004, 3231.
(29) (a) Kokko, E.; Lehmus, P.; Leino, R.; Luttikhedde, H. J. G.; Ekholm,
P.; Na¨sman, J.; Seppa¨la¨, J. V. Macromolecules 2000, 33, 9200. (b) Galland,
G. B.; Quijada, R.; Bazan, G.; Komon, Z. J. A. Macromolecules 2002, 35,
339.
(30) (a) Kim, W. K.; Fevola, M. J.; Liable-Sands, L. M.; Rheingold, A.
L.; Thepold, K. H. Organometallics 1998, 17, 4541. (b) Gibson, V. C.;
Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J.
Eur. J. Inorg. Chem. 2001, 1895.

[2,6-Bis(2-benzimidazolyl)pyridyl]chromium Chlorides
Organometallics, Vol. 25, No. 8, 2006 1967
System 2000 FT-IR spectrometer. Elemental analyses were per-
formed on a Flash EA 1112 instrument. GC analysis was performed
with a Carlo Erba Strumentazione gas chromatograph equipped with
a flame ionization detector and a 30 m (0.25 mm i.d., 0.25 µm
film thickness) DM-1 silica capillary column. GC-MS analysis was
performed with a HP 5890 Series II and a HP 5971 Series mass
detector. The yields of oligomers were calculated by referencing
with the mass of the solvent on the basis of the prerequisite that
the mass of each fraction is approximately proportional to its
integrated areas in the GC trace. The selectivity for linear R-olefin
is defined as (amounts of linear R-olefin of all fractions)/(total
amounts of oligomer products) × 100. 1H and 13C NMR spectra of
the PE samples were recorded on a Bruker DMX 400 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 instrument at 150
°C with 1,2,4-trichlorobenzene as the eluent. Melting points of the
polymers were obtained on a Perkin-Elmer DSC-7 instrument in
the standard DSC run mode. The instrument was initially cali-
brated for the melting point of an indium standard at a heating
rate of 10 °C/min. The polymer sample was first equilibrated at 0
°C and then heated to 160 °C at a rate of 10 °C/min to remove
thermal history. The sample was then cooled to 0 °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. A microwave
oven, the Midea PJ21B-A (800 W, 21 L), was used for microwave-
assisted condensation reactions.18
Figure 8. GPC traces of samples obtained by entries 6-11 in
Table 5.
values. A higher Mn value and a narrower molecular weight
distribution (Mw/Mn) were observed when the ethylene pressure
increased from 1 to 10 atm (compare entry 4 with entry 7 in
Table 5), which suggests that the chain transfer process was
depressed due to the faster insertion of ethylene because of the
higher monomer concentration.
The melting points (Tm) of the resulting PEs were determined
by differential scanning calorimetry analysis (DSC), and the
results are given in Table 5. For different catalysts and various
catalytic conditions, the melting point ranges between 134 and
139 °C with enthalpies between 210 and 225 J/g. These values
are in line with values given for HDPE.31
Toluene was refluxed over sodium-benzophenone until the
purple color appeared and distilled under nitrogen prior to use.
Dichloromethane was distilled under nitrogen from CaH2. Methyl-
aluminoxane (MAO) was purchased from Albemarle as a 1.4 M
solution in toluene. Modified methylaluminoxane (MMAO, 1.93
M in heptane) was purchased from Akzo Corp. Trimethylaluminum
(2 M in toluene) and diethylaluminum chloride (Et2AlCl, 1 M in
hexane) were purchased from Acros Chemicals. Other reagents were
purchased from Aldrich or Acros Chemicals. All other chemicals
were obtained commercially and used without further purification
unless stated otherwise.
NMR analyses of the polyethylenes also confirmed the high
linearity of polymers obtained with Et2AlCl as cocatalyst. This
is based upon the observation of only one signal from the
1
methylene group (-CH2-) in the H and 13C NMR spectra,
4.2. Synthesis of Ligands. 4.2.1. 2,6-Bis(benzimidazol-2-yl)-
pyridine (L1). Pyridine-2,6-dicarboxylic acid (0.720 g, 4.28
mmol), o-phenylenediamine (0.931 g, 8.58 mmol), and phosphoric
acid (8 mL) were mixed together and irradiated in the micro-
wave oven (450 W) three times for 2 min. The dark green solu-
tion of the reactants was poured into the ice-water mixture after
cooling, and a green solid precipitated. A NaHCO3 solution was
added to adjust the mixture pH value to 9-10, and the precipi-
tate was filtrated and washed with water. After drying, it was
recrystallized from an ethanol solution, and a yellow powder (0.631
g, 4.03 mmol) was obtained.17a Yield: 47.0%. IR (KBr; cm-1):
3186, 3055 (νN-H), 1599 (νCdC), 1573 (νCdN), 1458 (δN-H), 1436,
1318, 741 (δN-H). 1H NMR (300 MHz, acetone-d6): δ (ppm)
11.11-11.17 (2H, N-H), 8.48 (d, 2H, J ) 7.57 Hz, Py H), 8.20
(t, 1H, J ) 7.45 Hz, Py H), 7.75 (d, 4H, J ) 7.80 Hz, Ph H), δ
7.32 (s, 4H, Ph H).
which is clear evidence of the high linearity of the polymer.
3. Conclusion
In summary, the described chromium(III) complexes reveal
differences in their behavior as precatalysts for oligomerization
and polymerization of ethylene. When using MAO as the co-
catalyst, the complexes show high activity for ethylene oligo-
merization and polymerization, and the resulting polyethylenes
obtained at 10 atm have broad molecular weight distributions.
They were verified to be long linear olefins with vinyl end
groups by 1H NMR and 13C NMR. The complexes with N-alkyl
substitution show lower activities than the complexes with N-H.
Under activation by Et2AlCl, the chromium complexes show
moderate activity for polymerization of ethylene, and the
polyethylene obtained was a highly linear polymer with a high
molecular weight.
4.2.2. 2,6-Bis(5′-methylbenzimidazol-2′-yl)-pyridine (L2). Us-
ing the same procedure as for the synthesis of L1, L2 (1.08 g,
3.21 mmol) was obtained as a yellow powder by the reaction of
pyridine-2,6-dicarboxylic acid (0.800 mg, 4.79 mmol) and 4-methyl-
o-phenylenediamine (1.20 g, 9.84 mmol) in a yield of 67.0%. IR
(KBr; cm-1): 3196, 3056 (νN-H), 1599 (νCdC), 1572 (νCdN), 1457
4. Experimental Details
4.1. General Considerations. All manipulations of air- and/or
moisture-sensitive compounds were performed under a nitrogen
atmosphere using standard Schlenk techniques. Melting points (mp)
were measured with a digital electrothermal apparatus without
1
(δN-H), 1315, 737 (δN-H). H NMR (300 MHz, CDCl3): δ (ppm)
11.11-11.17 (2H, N-H), 8.34 (d, 2H, J ) 9.00 Hz, Py H), 7.82
(d, 1H, J ) 6.02, Ph H), 7.75 (s, 2H, Ph H), 7.12 (d, 4H, J ) 9.00
Hz, Ph H), 2.46 (s, 6H, CH3). 13C NMR (75.45 MHz, CDCl3): δ
(ppm) 149.5, 145.9, 137.6, 136.5, 135.7, 131.9, 124.7, 119.5, 113.9,
112.8, 20.3. Anal. Calcd for C21H17N5: C, 74.32; H, 5.05; N, 20.63.
Found: C, 74.44; H, 5.60; N, 20.33.
1
calibration. H and 13C NMR spectra were recorded on a Bruker
DMX 300 MHz instrument at ambient temperature using TMS as
an internal standard. IR spectra were recorded on a Perkin-Elmer
(31) Elias, H. G. An Introduction to Polymer Science; VCH: Weinheim,
Germany, 1997.

1968 Organometallics, Vol. 25, No. 8, 2006
Zhang et al.
4.2.3. 2,6-Bis(5′,6′-dimethylbenzimidazol-2′-yl)-pyridine (L3).
Using the same procedure as for the synthesis of L1, L3 (0.741 g,
2.02 mmol) was obtained as a yellow powder by the reaction of
pyridine-2,6-dicarboxylic acid (0.600 g, 3.59 mmol) and 4,5-
dimethylbenzene-1,2-diamine (1.00 g, 7.35 mmol) in a yield of
56.5%. IR (KBr; cm-1): 3200, 3052 (νN-H), 1599 (νCdC), 1572
(νCdN), 1457 (δN-H), 1313, 815, 736 (δN-H). 1H NMR (300 MHz,
acetone-d6): δ (ppm) 11.11-11.17 (2H, N-H), 8.31 (d, 2H, J )
6.00 Hz, Py H), 8.07 (t, 1H, J ) 6.01 Hz, Py H), 7.42 (s, 4H, Ph
H), 2.46 (s, 12H, CH3). 13C NMR (75.45 MHz, CDCl3): δ (ppm)
150.0, 147.5, 138.1, 132.8, 120.8, 119.2, 112.2, 20.6. Anal. Calcd
for C23H21N5: C, 75.18; H, 5.76; N, 19.06. Found: C, 74.92; H,
5.80; N, 18.65.
Anal. Calcd for C27H29N5: C, 76.56; H, 6.90; N, 16.53. Found: C,
76.80; H, 6.89; N, 16.19.
4.2.8. 2,6-Bis(1′-benzyl-5′,6′-dimethylbenzimidazol-2′-yl)py-
ridine (L8). The same procedure as for the synthesis of L6 was
used, except that the reaction temperature was 60 °C. L8 (0.490 g,
0.900 mmol) was obtained as a yellow powder by the reaction of
L3 (0.369 g, 1 mmol), powdered potassium hydroxide (0.280 g,
4.98 mmol), and benzyl chloride (1.50 mL, 13.0 mmol) in a yield
of 90.0%. Mp: 294-296 °C. IR (KBr; cm-1): 3028 (νC-H), 2966
(νC-H), 1600 (νCdC), 1570 (νCdN), 1455, 1324, 821, 737 (δC-H).
1H NMR (300 MHz, CDCl3): δ (ppm) 8.30 (d, 2H, J ) 7.80 Hz,
Py H), 7.97 (t, 1H, J ) 8.10 Hz, Py H), 7.59 (s, 2H, Ph H), 7.19
(d, 6H, J ) 6.61 Hz, Ph H), 6.98 (s, 2H, Ph H), 6.82 (d, 4H, J )
7.20 Hz, Ph H), 5.50 (s, 4H, CH2), 2.40 (s, 12H, CH3). 13C NMR
(75.45 MHz, CDCl3): δ (ppm) 149.8, 149.4, 141.5, 138.1, 137.4,
135.2, 133.3, 132.1, 128.8, 127.3, 126.2, 125.3, 120.2, 110.9, 47.8,
20.8, 20.4. Anal. Calcd for C37H33N5: C, 81.14; H, 6.07; N, 12.79.
Found: C, 81.10; H, 6.01; N, 12.48.
4.3. Synthesis of (L)CrCl3 (C1-C8; L ) L1-L8). 4.3.1.
Complex C1. Complexes C1-C8 were synthesized by the reaction
of CrCl3(THF)3 with the corresponding ligands in dichloromethane.
A typical synthetic procedure, for C1, is as follows. A solution of
CrCl3(THF)3 (0.132 g, 0.350 mmol) and L1 (0.109 g, 0.350 mmol)
in dichloromethane was stirred at room temperature for 24 h, giving
a green suspension. The reaction volume was reduced, diethyl ether
was added, and a green solid was obtained, which was washed
repeatedly with diethyl ether and dried under vacuum. The green
powder (0.122 g, 0.260 mmol) was obtained in a yield of 75.0%.
IR (KBr; cm-1): 3073 (νN-H), 1609 (νCdC), 1590 (νCdN), 1497,
1468 (δN-H), 1321, 1147, 998, 826, 754 (δN-H). Anal. Calcd for
C19H12Cl3CrN5: C, 48.59; H, 2.79; N, 14.91. Found: C, 48.30; H,
2.80; N, 14.54.
4.3.2. Complex C2. Analogous to the procedure for C1, ligand
L2 (0.200 g, 0.591 mmol) and CrCl3(THF)3 (0.223 g, 0.590 mmol)
reacted to form 0.282 g (0.570 mmol) of a green solid in a yield of
97.1%. IR (KBr; cm-1): 3064 (νN-H), 2956, 1612 (νCdC), 1573
(νCdN), 1471 (δN-H), 1323, 1021, 812, 748 (δN-H), 682. Anal. Calcd
for C21H17Cl3CrN5: C, 50.67; H, 3.44; N, 14.07. Found: C, 51.17;
H, 3.70; N, 13.92.
4.3.3. Complex C3. Analogous to the procedure for C1, ligand
L3 (0.232 g, 0.630 mmol) and CrCl3(THF)3 (0.231 g, 0.624
mmol) reacted to form 0.194 g (0.360 mmol) of a green solid in a
yield of 56.5%. IR (KBr; cm-1): 3064 (νN-H), 1613 (νCdC), 1573
(νCdN), 1471 (δN-H), 1324, 815, 748 (δC-H), 683. Anal. Calcd for
C23H21Cl3CrN5: C, 52.54; H, 4.03; N, 13.32. Found: C, 52.31; H,
3.70; N, 13.52.
4.3.4. Complex C4. Analogous to the procedure for C1, ligand
L4 (0.330 g, 0.971 mmol) and CrCl3(THF)3 (0.361 g, 0.970
mmol) reacted to form 0.462 g (0.931 mmol) of a green solid in a
yield of 95.8%. IR (KBr; cm-1): 3061, 3024, 1603 (νCdC), 1589
(νCdC), 1572 (νCdN), 1482, 1346, 764, 749. Anal. Calcd for
C21H17Cl3CrN5: C, 50.67; H, 3.44; N, 14.07. Found: C, 50.31; H,
3.70; N, 13.84.
4.3.5. Complex C5. Analogous to the procedure for C1, ligand
L5 (0.261 g, 0.710 mmol) and CrCl3(THF)3 (0.262 g, 0.710
mmol) reacted to form 0.313 g (0.590 mmol) of a green solid in a
yield of 83.0%. IR (KBr; cm-1): 3423, 3101, 2919, 1603 (νCdC),
1566 (νCdN), 1485, 1344, 807, 746. Anal. Calcd for C23H21Cl3-
CrN5: C, 52.54; H, 4.03; N, 13.32. Found: C, 52.09; H, 4.40; N,
13.47.
4.3.6. Complex C6. Analogous to the procedure for C1, ligand
L6 (0.343 g, 0.861 mmol) and CrCl3(THF)3 (0.320 g, 0.860
mmol) reacted to form 0.351 g (0.640 mmol) of a green solid in a
yield of 74.2%. IR (KBr; cm-1): 3033, 2922, 1602 (νCdC), 1566
(νCdN), 1520, 1483, 1320, 1261, 860, 806, 743. Anal. Calcd for
C25H25Cl3CrN5: C, 54.21; H, 4.55; N, 12.64. Found: C, 53.95; H,
4.84; N, 12.65.
4.2.4. 2,6-Bis(1′-methylbenzimidazol-2′-yl)pyridine (L4). Pow-
dered potassium hydroxide (0.280 g, 4.98 mmol) was added to a
stirred suspension of L1 (0.311 g, 1.00 mmol) in acetone. After a
few minutes, methyl iodide (1.00 mL, 16.0 mmol) was added
to the reaction mixture with vigorous stirring. After 6 h, the mix-
tures were added to the water, the precipitate was filtered and
was recrystallized from methanol, and yellow crystals (0.26 g,
0.77 mmol) were obtained in a yield of 76.7%. Mp: 196-197 °C
(lit.17b mp 197 °C). IR (KBr; cm-1): 3050 (νC-H), 2941 (νC-H),
1585 (νCdC), 1571 (νCdN), 1420, 1328, 745 (δC-H). 1H NMR
(300 MHz, CDCl3): δ (ppm) 8.42 (d, 2H, J ) 8.01 Hz, Py H),
8.07 (t, 1H, J ) 7.83 Hz, Py H), 7.88 (d, 2H, J ) 7.82, Ph H),
7.44 (d, 2H, J ) 7.56, Ph H), 7.38 (m, 4H, Ph H), 4.26 (s, 6H,
N-CH3).
4.2.5. 2,6-Bis(1′-methyl-5′-methylbenzimidazol-2′-yl)pyridine
(L5). Using the same procedure as for the synthesis of L4, L5
(0.300 g, 0.83 mmol) was obtained as a yellow powder by the
reaction of L2 (0.340 g, 1 mmol), powdered potassium hydroxide
(0.280 g, 4.98 mmol), and methyl iodide (1.00 mL, 16.0 mmol) in
a yield of 83.0%. IR (KBr; cm-1): 3019 (νC-H), 2942 (νC-H), 1585
(νCdC), 1568 (νCdN), 1455 (δC-H), 1330, 739 (δC-H). 1H NMR (300
MHz, CDCl3): δ (ppm) 8.37 (d, 2H, J ) 7.82 Hz, Py H), 8.02 (t,
1H, J ) 7.82 Hz, Py H), 7.75 (d, 1H, J ) 8.41 Hz, Ph H), 7.65 (s,
1H, Ph H), 7.35 (d, 1H, J ) 8.10 Hz, Ph H), 7.26 (s, 1H, Ph H),
7.19 (t, 2H, J ) 8.10 Hz, Ph H), 4.22 (s, 6H, N-CH3), 2.55 (d,
6H, J ) 8.12 Hz, CH3). Anal. Calcd for C23H21N5: C, 75.18; H,
5.76; N, 19.06. Found: C, 74.87; H, 5.80; N, 18.65.
4.2.6. 2,6-Bis(1′-methyl-5′,6′-dimethylbenzimidazol-2′-yl)py-
ridine (L6). Using the same procedure as for the synthesis of L4,
L6 (0.273 g, 0.68 mmol) was obtained as a yellow powder by the
reaction of L3 (0.371 g, 1 mmol), powdered potassium hydroxide
(0.281 g, 4.98 mmol), and methyl iodide (1.00 mL, 16.2 mmol) in
a yield of 67.8%. IR (KBr; cm-1): 3019 (νC-H), 2966 (νC-H), 2939
(νC-H), 1585 (νCdC), 1567 (νCdN), 1481, 1323, 998, 828, 743, 609.
1H NMR (300 MHz, CDCl3): δ (ppm) 8.35 (d, 2H, J ) 7.80 Hz,
Py H), 8.00 (t, 1H, J ) 7.82 Hz, Py H), 7.62 (s, 2H, Ph H), 7.22
(s, 2H, Ph H), 4.20 (s, 6H, N-CH3), 2.43 (d, 12H, J ) 8.11 Hz,
CH3). 13C NMR (75.45 MHz, CDCl3): δ (ppm) 149.8, 149.6, 141.3,
137.9, 135.9, 133.1, 131.9, 124.8, 120.1, 110.1, 32.5, 20.8, 20.5.
Anal. Calcd for C25H25N5: C, 75.92; H, 6.37; N, 17.71. Found: C,
75.99; H, 6.01; N, 17.42.
4.2.7. 2,6-Bis(1′-ethyl-5′,6′-dimethylbenzimidazol-2′-yl)pyri-
dine (L7). Using the same procedure as for the synthesis of L6,
L7 (0.371 g, 0.880 mmol) was obtained as a yellow powder by the
reaction of L3 (0.371 g, 1 mmol), powdered potassium hydroxide
(0.280 g, 4.98 mmol), and bromoethane (1.00 mL, 13.3 mmol) in
a yield of 88.0%. Mp: 226-228 °C. IR (KBr; cm-1): 3026 (νC-H),
2967 (νC-H), 2940 (νC-H), 1584 (νCdC), 1568 (νCdN), 1481, 1325,
999, 829, 745 (δC-H), 708. 1H NMR (300 MHz, CDCl3): δ (ppm)
8.30 (d, 2H, J ) 7.73 Hz, Py H), 8.00 (t, 1H, J ) 7.72 Hz, Py H),
7.62 (s, 2H, Ph H), 7.24(s, 2H, Ph H), 4.76 (s, 4H, -CH2-), 2.43
(d, J ) 7.00 Hz, 12H, Ph CH3), 1.34 (t, 6H, J ) 6.80 Hz, CH3).
13C NMR (75.45 MHz, CDCl3): δ (ppm) 150.2, 149.3, 141.7, 137.9,
134.7, 132.9, 131.8, 125.2, 120.2, 110.4, 39.8, 20.8, 20.4, 15.5.

[2,6-Bis(2-benzimidazolyl)pyridyl]chromium Chlorides
Organometallics, Vol. 25, No. 8, 2006 1969
Table 6. Crystallographic Data and Refinement Details for
C1 and C7
4.3.7. Complex C7. Analogous to the procedure for C1, ligand
L7 (0.350 g, 0.830 mmol) and CrCl3(THF)3 (0.312 g, 0.830 mmol)
reacted to form 0.252 g (0.420 mmol) of a green solid in a yield of
51.0%. IR (KBr; cm-1): 3416, 2974, 1604 (νCdC), 1568 (νCdN),
1516, 1484, 1457, 1335, 1275, 1094, 851, 809. Anal. Calcd for
C27H29Cl3CrN5: C, 55.73; H, 5.02, N, 12.04. Found: C, 55.44; H,
5.12; N, 11.75.
C1‚2DMF
C7‚CH2Cl2
formula
fw
cryst syst
space group
a (Å)
C25H25.50Cl3CrN7O2.25
618.37
triclinic
P1h
10.9720(15)
11.0483(16)
13.5496(19)
88.558(2)
86.707(2)
60.655(2)
1429.4(3)
2
C28H31Cl5CrN5
666.83
orthorhombic
Pbca
18.633(1)
15.712(1)
20.742(1)
90
90
90
6072.5(6)
8
1.459
4.3.8. Complex C8. Analogous to the procedure for C1, ligand
L8 (0.400 g, 0.731 mmol) and CrCl3(THF)3 (0.272 g, 0.730 mmol)
reacted to form 0.463 g (0.651 mmol) of a green solid in a yield of
88.8%. IR (KBr; cm-1): 3391, 1603 (νCdC), 1568 (νCdN), 1475,
1451, 1331, 1265, 1003, 733. Anal. Calcd for C37H33Cl3CrN5: C,
62.94; H, 4.71; N, 9.92. Found: C, 62.56; H, 5.02; N, 10.0.
4.4. Procedure for Oligomerization and Polymerization with
1 atm of Ethylene. Polymerizations were carried out as follows:
the catalyst precursor (chromium complex) was dissolved in toluene
in a Schlenk tube stirred with a magnetic stirrer under an ethylene
atmosphere (1 atm), and the reaction temperature was controlled
by a water bath. The reaction was initiated by adding the desired
amount of methylaluminoxane (MAO). After the desired period of
time, a small amount of the reaction solution was collected with a
syringe and was quenched by the addition of 5% aqueous HCl. An
analysis by gas chromatography (GC) was carried out to determine
the distribution of oligomers obtained. The remaining solution was
quenched with HCl-acidified ethanol (5%), and the precipitated
polyethylene was collected by filtration, washed with ethanol, dried
under vacuum at 60 °C to constant weight, weighed, and finally
characterized.
4.5. Procedure for Oligomerization and Polymerization with
10 atm of Ethylene. These reactions were carried out in a 250 mL
stainless steel autoclave reactor equipped with a mechanical stirrer
and a temperature controller. The desired amount of MAO, a toluene
solution of the chromium complex (30 mL), and toluene (70 mL)
were added to the reactor in this order under an ethylene
atmosphere. When the reaction temperature was achieved, ethylene
at the desired pressure (10 atm) was introduced to start the reaction.
After 1 h, the reaction was stopped. A small amount of the reaction
solution was collected, the reaction was terminated by the addition
of 5% aqueous HCl, and the mixture was then analyzed by gas
chromatography (GC) to determine the distribution of oligomers
obtained. The remaining solution was quenched with HCl-acidified
ethanol (5%). The precipitated polymer was collected by filtration,
washed with ethanol, dried under vacuum at 60 °C to constant
weight, weighed, and finally characterized.
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D
calcd (g cm-3
)
1.437
0.717
635
abs coeff, µ (mm-1
F(000)
)
0.844
2744
θ range (deg)
2.56-26.22
1.96-24.99
10 122
5333
no. of data collected
8040
no. of unique data
5688
R
Rw
0.0414
0.1015
1.070
0.0647
0.1181
1.025
goodness of fit
collected with a Nonius Kappa CCD diffractometer, where the
monochromated Mo KR radiation (λ ) 0.710 73 Å) is generated
by a rotating anode generator. Cell parameters were obtained by
global refinement of the positions of all collected reflections.
Intensities were corrected for Lorentz and polarization effects and
empirical absorption. The structures were solved by direct methods
and refined by full-matrix least squares on F2. All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms were placed
in calculated positions. Structure solution and refinement were
performed by using the SHELXL-97 package. Crystal data and
processing parameters for complexes C1 and C7 are summarized
in Table 6.
Acknowledgment. This project was supported by NSFC
Grant Nos. 20272062 and 20473099 along with the National
863 Project (Grant No. 2002AA333060).
Supporting Information Available: CIF files giving crystal-
lographic data for C1 and C7 and text and figures giving NMR
measurements of polyethylenes obtained by C4 with activation by
MAO and C6 under the activation of Et2AlCl at 10 atm pressure
of ethylene and oligomer molar distributions and their fitting
analysis by Schulz-Flory rules. This material is available free of
charge via the Internet at http://pubs.acs.org.
4.6. X-ray Crystallographic Studies. Single-crystal X-ray
diffraction measurements for C1 were carried out on a Bruker
SMART 1000 CCD diffractometer with graphite-monochromated
Mo KR radiation (λ ) 0.710 73 Å). Intensity data for C7 were
OM050805L