Synthesis, characterization and ethylene oligomerization studies of nickel complexes bearing 2-benzimidazolylpyridine derivatives

Article abstract of DOI:10.1021/om070049e

A series of nickel complexes ligated by 2-(2-benzimidazole)-6- methylpyridine, 2-(1-methyl-2-benzimidazole)-6-acetylpyridine, and 2-(1-methyl-2-benzimidazole)-6-(1-aryliminoethyl)pyridine was synthesized and examined by IR spectroscopic and elemental analysis. Their molecular structures were determined by single-crystal X-ray diffraction analysis. On activation with diethylaluminum chloride (Et₂AlCl), all the nickel complexes exhibited good catalytic activities for ethylene oligomerization, and the nickel(II) complexes bearing 2-(1-methyl-2-benzimidazole)-6-(1-aryUminoethyl) pyridines showed good activities up to 5.87 × 105 g mol^-1(Ni) h^-1 atm^-1. The various reaction parameters were investigated in detail, and the results revealed that both the steric and electronic effects of ligands strongly affect the catalytic activities of their nickel complexes as well as different coordination style.

Full text of DOI:10.1021/om070049e

Organometallics 2007, 26, 2439-2446
2439
Synthesis, Characterization and Ethylene Oligomerization Studies of
Nickel Complexes Bearing 2-Benzimidazolylpyridine Derivatives
Peng Hao,† Shu Zhang,† Wen-Hua Sun,*,† Qisong Shi,† Sherrif Adewuyi,†
Xiaoming Lu,‡ and Peizhou Li‡
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and Department
of Chemistry, Capital Normal UniVersity, Beijing 100037, People’s Republic of China
ReceiVed January 18, 2007
A series of nickel complexes ligated by 2-(2-benzimidazole)-6-methylpyridine, 2-(1-methyl-2-
benzimidazole)-6-acetylpyridine, and 2-(1-methyl-2-benzimidazole)-6-(1-aryliminoethyl)pyridine was
synthesized and examined by IR spectroscopic and elemental analysis. Their molecular structures were
determined by single-crystal X-ray diffraction analysis. On activation with diethylaluminum chloride
(Et2AlCl), all the nickel complexes exhibited good catalytic activities for ethylene oligomerization, and
the nickel(II) complexes bearing 2-(1-methyl-2-benzimidazole)-6-(1-aryliminoethyl)pyridines showed good
activities up to 5.87 × 105 g mol-1(Ni) h-1 atm-1. The various reaction parameters were investigated in
detail, and the results revealed that both the steric and electronic effects of ligands strongly affect the
catalytic activities of their nickel complexes as well as different coordination style.
challenges of nickel complexes as catalysts toward ethylene
oligomerization and/or polymerization include their coordination
modes, catalytic activities, and selectivity toward R-olefins along
with controlled molecular weights of products (oligomers and
polymers). The nickel complexes are commonly four-coordi-
nated nickel dihalides containing bidentate ligands such as
P∧O,1,4 P∧N,5 N∧N,3,6 and N∧O7. Intensive research, however,
has shown moderate to high catalytic activities for conversion
of ethylene by the five-coordinated nickel halides incorporating
tridentate ligands of N∧N∧O,8 N∧P∧N,9 P∧N∧P10, P∧N∧N,6n,10
and N∧N∧N.11 Some of the nickel catalytic systems are worthy
of further investigation and are promising for both industrial
and academic development.
1. Introduction
The oligomerization of ethylene is currently the major
industrial process for the production of linear R-olefins, which
are important reactants used in the preparation of detergents,
lubricants, plasticizers, oil field chemicals, and monomers for
copolymerization. Current industrial processes employ catalysts
including either alkylaluminum compounds or a combination
of alkylaluminum compounds and early transition metal com-
pounds or nickel(II) complexes and bidentate monoanionic [P,O]
ligands (the SHOP process).1 In the past decade, late-transition
metal complexes as homogeneous catalysts toward ethylene
activation have attracted great attention in both academic and
industrial research.2 In contrast to early-transition metal com-
plexes, late-transition metal complexes were less investigated
because of the competition of â-hydrogen elimination with chain
propagation, and the discovery of diimino cationic nickel
complexes as effective catalysts for ethylene oligomerization
and polymerization renewed interest in designing new late-
transition metal complexes as catalysts.3 The important progress
of ethylene activation promoted by nickel complexes is reflected
by recent review articles.2 Owing to fundamental research points
from an organometallic chemistry perspective, the recent major
(4) (a) Keim, W. Angew. Chem., Int. Ed. Engl. 1990, 29, 235. (b)
Heinicke, J.; He, M.; Dal, A.; Klein, H.-F.; Hetche, O.; Keim, W.; Flo¨rke,
U.; Haupt, H.-J. Eur. J. Inorg. Chem. 2000, 431.
(5) (a) Keim, W.; Killat, S.; Nobile, C. F., Suranna, G. P.; Englert, U.;
Wang, R.; Mecking, S.; Schro¨der, D. L. J. Organomet. Chem. 2002, 662,
150. (b) Sun, W.-H.; Li, Z.; Hu, H.; Wu, B.; Yang, H.; Zhu, N.; Leng, X.;
Wang, H. New J. Chem. 2002, 26, 1474. (c) Speiser, F.; Braunstein, P.;
Saussine, L.; Welter, R. Organometallics 2004, 23, 2613. (d) Speiser, F.;
Braunstein, P.; Saussine, L. Organometallics 2004, 23, 2625. (e) Speiser,
F.; Braunstein, P.; Saussine, L. Organometallics 2004, 23, 2633. (f) Speiser,
F.; Braunstein, P.; Saussine, L.; Welter, R. Inorg. Chem. 2004, 43, 1649.
(g) Weng, Z.; Teo, S.; Hor, T. S. A. Organometallics 2006, 25, 4878.
(6) (a) Killian, C. M.; Johnson, L. K.; Brookhart, M. Organometallics
1997, 16, 2005. (b) Svejda, S. A.; Brookhart, M. Organometallics 1999,
18, 65. (c) Meneghetti, S. P.; Lutz, P. J.; Kress, J. Organometallics 1999,
18, 2734. (d) Laine, T. V.; Lappalainen, K.; Liimatta, J.; Aitola, E.; Lo¨fgren,
B.; Leskela¨, M. Macromol. Rapid Commun. 1999, 20, 487. (e) Laine, T.
V.; Piironen, U.; Lappalainen, K.; Klinga, M.; Aitola, E.; Leskela¨, M. J.
Organomet. Chem. 2000, 606, 112. (f) Li, Z.; Sun, W.-H.; Ma, Z.; Hu, Y.;
Shao, C. Chin. Chem. Let. 2001, 12, 691. (g) Lee, B. Y.; Bu, X.; Bazan, G.
C. Organometallics 2001, 20, 5425. (h) Shao, C.; Sun, W.-H.; Li, Z.; Hu,
Y.; Han, L. Catal. Commun. 2002, 3, 405. (i) Tang, X.; Sun, W.-H.; Gao,
T.; Hou, J.; Chen, J.; Chen, W. J. Organomet. Chem. 2005, 690, 1570. (j)
Jie, S.; Zhang, D.; Zhang, T.; Sun, W.-H.; Chen, J.; Ren, Q.; Liu, D.; Zheng,
G.; Chen, W. J. Organomet. Chem. 2005, 690, 1739. (k) Nelkenbaum, E.;
Kapon, M.; Eisen, M. S. J. Organomet. Chem. 2005, 690, 2297. (l) Benito,
J. M.; de Jesu´s, E.; de la Mata, F. J.; Flores, J. C.; Go´mez, F.; Go´mez-Sal,
P. Organometallics 2006, 25, 3876. (m) Song, C.-L; Tang, L.-M.; Li, Y.-
G.; Li, X.-F.; Chen, J.; Li, Y.-S. J. Polym. Sci., Part A: Polym. Chem.
2006, 44, 1964. (n) Zhang, C.; Sun, W.-H.; Wang, Z.-X. Eur. J. Inorg.
Chem. 2006, 23, 4895. (o) Meinhard, D.; Reuter, P.; Rieger, B. Organo-
metallics 2007, 26, 751.
* Corresponding author. Tel: +86 10 62557955. Fax: +86 10 62618239.
E-mail: whsun@iccas.ac.cn.
† Key Laboratory of Engineering Plastics and Beijing National Laboratory
for Molecular Sciences.
‡ Capital Normal University.
(1) (a) Keim, W.; Kowaldt, F. H.; Goddard, R.; Kru¨ger, C. Angew. Chem.,
Int. Ed. Engl. 1978, 17, 466. (b) Keim, W.; Behr, A.; Limba¨cker, B.; Kru¨ger,
C. Angew. Chem., Int. Ed. Engl. 1983, 22, 503.
(2) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283.
(c) Speiser, F.; Braustein, P.; Saussine, L. Acc. Chem. Res. 2005, 38, 784.
(d) Mecking, S. Angew. Chem., Int. Ed. 2001, 40, 534. (e) Zhang, W.; Zhang,
W.; Sun, W.-H. Prog. Chem. 2005, 17, 310. (f) Jie, S.; Zhang, S.; Sun,
W.-H. Petrochem. Tech. (Shiyou Huagong) 2006, 35, 297. (g) Sun, W.-H.;
Zhang, D.; Zhang, S.; Jie, S.; Hou, J. Kinet. Catal. 2006, 47, 278.
(3) (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414. (b) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am.
Chem. Soc. 1996, 118, 267. (c) Killian, C. M.; Tempel, D. J.; Johnson, L.
K.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 11664.
10.1021/om070049e CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/30/2007

2440 Organometallics, Vol. 26, No. 9, 2007
Hao et al.
Scheme 1. Reaction of NiCl2‚6H2O with
2-(Benzimidazolyl)pyridine Derivatives
Figure 1. Molecular structure of Me-PBID-MeNiCl2. Thermal
ellipsoids are shown at 30% probability. Hydrogen atoms have been
omitted for clarity.
and R-olefins were produced with high selectivity.12 Subse-
quently, to distinguish them from former nickel catalysts, their
nickel analogues as a new series of catalysts for ethylene
activation were also synthesized by reactions of nickel halides
with the synthesized 2-benzimidazolylpyridine derivatives. In
the presence of diethylaluminum chloride (Et2AlCl) as cocata-
lyst, all the nickel complexes showed good catalytic activities
toward ethylene oligomerization, and the N∧N∧N nickel com-
plexes (L1NiCl2-L5NiCl2) showed high activities of ethylene
oligomerization up to 5.87 × 105 g mol-1(Ni) h-1 atm-1. Herein,
the synthesis and characterization of these nickel complexes are
reported with their catalytic properties for ethylene activation
investigated under various reaction conditions.
In our group, catalytic systems ligated by tridentate 2,9-bis-
(imino)-1,10-phenanthroline derivatives have been reported,11a
in which the nickel complexes showed better activities than their
iron and cobalt analogues. Furthermore, the nickel complexes
ligated by 2-imino-1,10-phenanthroline derivatives exhibited
even higher activity for ethylene oligomerization, up to 106 g
mol-1(Ni) h-1 atm-1 11d
Their catalytic activity and product
.
distributions were somehow controlled through changing the
steric and electronic properties of the ligand backbones. Inspired
by the above promising results, our subsequent exploration
involves tridentate N∧N∧N ligands in new types of metal
complexes as catalysts for ethylene oligomerization and po-
lymerization. Very recently, a new family of iron(II) and cobalt-
(II) complexes bearing 2-(2-benzimidazole)pyridine derivatives
was found to form highly active catalysts for ethylene oligo-
merization and polymerization upon activation with methyla-
luminoxane (MAO) or modified methylaluminoxane (MMAO),
2. Results and Discussion
2.1. Synthesis and Characterization of Nickel Complexes
Bearing Various 6-Substituted 2-Benzimidazolylpyridines.
As observed in our previous work on nickel complexes
containingbidentateortridentateligandsforethyleneactivation,6h-j,10,11f,13
the nickel complexes based on the 6-substituted 2-benzimida-
zolylpyridine derivatives, 2-(1-R′-2-benzimidazole)-6-R′′-pyri-
dine (R′ ) H and R′′ ) Me: H-PBID-Me; R′ ) R′′ ) Me:
Me-PBID-Me; R′ ) H and R′′ ) carboethoxyl: H-PBID-
ester; R′ ) Me and R′′ ) carboethoxyl: Me-PBID-ester; R′
) Me and R′′ ) acetyl: Me-PBID-acetyl) were synthesized
in proper yields by equimolar reactions of the synthesized
2-benzimidazolylpyridine derivatives with NiCl2‚6H2O in etha-
nol (Scheme 1).
These complexes were characterized by IR spectroscopic and
elemental analysis. They showed high stability in both solution
and solid state. Compared with the IR spectrometry of the free
organic ligands, the CdN pyridine and CdO absorption bands of
the complexes were shifted to lower frequencies, indicating
strong coordination effects. The unambiguous molecular struc-
tures of complexes Me-PBID-MeNiCl2, Me-PBID-esterNiCl2,
and Me-PBID-acetylNiCl2 were confirmed by single-crystal
X-ray diffraction analysis. Their molecular structures are shown
in Figures 1-3, and selected bond lengths and angles are
collected in Table 1.
(7) (a) Wang, C.; Friedrich, S.; Younkin, T. R.; Li, R. T.; Grubbs, R.
H.; Bansleben, D. A.; Day, M. W. Organometallics 1998, 17, 3149. (b)
Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs,
R. H.; Bansleben, D. A. Science 2000, 287, 460. (c) Carlini, C.; Isola, M.;
Liuzzo, V.; Galletti, A. M. R.; Sbrana, G. Appl. Catal., A 2002, 231, 307.
(d) Wang, L.; Sun, W.-H.; Han, L.; Li, Z.; Hu, Y.; He, C.; Yan, C. J.
Organomet. Chem. 2002, 650, 59. (e) Wu, S.; Lu, S. Appl. Catal., A 2003,
246, 295. (f) Zhang, D.; Jie, S.; Zhang, T.; Hou, J.; Li, W.; Zhao, D.; Sun,
W.-H. Acta Polym. Sin. 2004, 5, 758. (g) Sun, W.-H.; Zhang, W.; Gao, T.;
Tang, X.; Chen, L.; Li, Y.; Jin, X. J. Organomet. Chem. 2004, 689, 917.
(h) Hu, T.; Tang, L.-M.; Li, X.-F.; Li, Y.-S.; Hu, N.-H. Organometallics
2005, 24, 2628. (i) Chen, Q.; Yu, J.; Huang, J. Organometallics 2007, 26,
617.
(8) Yang, Q.-Z.; Kermagoret, A.; Agostinho, M.; Siri, O.; Braunstein,
P. Organometallics 2006, 25, 5518.
(9) Speiser, F.; Braunstein, P.; Saussine, L. J. Chem. Soc., Dalton Trans.
2004, 1539.
(10) Hou, J.; Sun, W.-H.; Zhang, S.; Ma, H.; Deng, Y.; Lu, X.
Organometallics 2006, 25, 236.
(11) (a) Wang, L.; Sun, W.-H.; Han, L.; Yang, H.; Hu, Y.; Jin, X. J.
Organomet. Chem. 2002, 658, 62. (b) Kunrath, F. A; De Souza, R. F;
Casagrande, O. L., Jr.; Brooks, N. R; Young, V. G., Jr. Organometallics
2003, 22, 4739. (c) Ajellal, N.; Kuhn, M. C. A.; Boff, A. D. G.; Ho¨rner,
M.; Thomas, C. M; Carpentier, J.-F.; Casagrande, O. L., Jr. Organometallics
2006, 25, 1213. (d) Sun, W.-H.; Zhang, S.; Jie, S.; Zhang, W.; Li, Y.; Ma,
H.; Chen, J.; Wedeking, K.; Fro¨hlich, R. J. Organomet. Chem. 2006, 691,
4196. (e) Sun, W.-H.; Jie, S.; Zhang, S.; Zhang, W.; Song, Y.; Ma, H.;
Chen, J.; Wedeking, K.; Fro¨hlich, R. Organometallics 2006, 25, 666. (f)
Jie, S.; Zhang, S.; Sun, W.-H.; Kuang, X.; Liu, T.; Guo, J. J. Mol. Catal.
A: Chem. 2007, 269, 85. (g) Al-Benna, S.; Sarsfield, M. J.; Thornton-Pett,
M.; Ormsby, D. L.; Maddox, P. J.; Bre´s, P.; Bochmann, M. J. Chem. Soc.,
Dalton Trans. 2000, 4247.
(12) Sun, W.-H.; Hao, P.; Zhang, S.; Shi, Q.; Zuo, W.; Tang, X.; Lu, X.
2-(Benzimidazole)-6-(1-aryliminoethyl)pyridyl Complexes of Iron(II) and
Cobalt(II) as Catalysts for Ethylene Oligomerization and Polymerization:
Synthesis, Characterization, and Their Catalytic Behaviors. Organometallics,
in press.
(13) Zhang, W.; Sun, W.-H.; Wu, B.; Zhang, S.; Ma, H; Li, Y.; Chen,
J.; Hao, P. J. Organomet. Chem. 2006, 691, 4759.

Ethylene Oligomerization by N∧N∧N Nickel Complexes
Organometallics, Vol. 26, No. 9, 2007 2441
Scheme 2. Synthesis of Nickel Complexes with
2-(1-Methyl-2-benzimidazole)-6-(1-aryliminoethyl)pyridines
is significantly longer than that of Ni-N1benzimidazole (2.0571(2)
Å). It is noteworthy that the benzimidazole ring and phenyl ring
are not strictly coplanar, with a dihedral angle of 24.3°.
Single crystals of Me-PBID-esterNiCl2 were obtained by
diffusing the diethyl ether layer into its solution mixture of
ethanol and DMF. The molecular structure of six-coordinated
Me-PBID-esterNiCl2 can be described as a distorted octahedron
geometry in which the nickel atom is surrounded by one ligand,
two chlorides, and one DMF molecule through linkage by its
oxygen atom (Figure 2). The ester-substituted pyridylbenzimi-
dazole ligand coordinates the nickel center as a tridentate
N∧N∧O ligand. The plane of the benzimidazole ring is nearly
oriented coplanar to the pyridine plane (8.0°). The distance
between oxygen (O1) and the nickel atom is 2.305(2) Å, which
is substantially shorter than the value observed in the 2-(eth-
ylcarboxylato)-6-iminopyridylnickel(II) complexes (2.342(3)-
2.402(5) Å).6i Compared with the nickel complex Me-PBID-
MeNiCl2, the Ni-N2pyridine bond is remarkably shorter, which
might be attributed to the strong interaction between the nickel
and carbonyl oxygen atom and formation of a five-membered
chelate ring. Also, the distance between the nickel and the trans-
disposed chloride is significantly shorter than the distance of
the chloride in the cis position.
Figure 2. Molecular structure of Me-PBID-esterNiCl2. Thermal
ellipsoids are shown at 30% probability. Hydrogen atoms and one
EtOH molecule have been omitted for clarity.
Figure 3. Molecular structure of Me-PBID-acetylNiCl2. Thermal
ellipsoids are shown at 30% probability. Hydrogen atoms have been
omitted for clarity.
In the molecular structure of Me-PBID-acetylNiCl2 (Figure
3), the geometry around the nickel center can also be described
as a distorted octahedron. The nickel atom is surrounded by
one ligand, one chloride, and two methanol molecules. The
pyridine ring was nearly coplanar with the benzimidazole ring,
and the dihedral angle is 5.0°. The distance between oxygen
(O1) and the nickel atom is 2.243(2) Å, and it is shorter than
that in Me-PBID-esterNiCl2, which indicates a much stronger
interaction between these two atoms. In addition, the bond length
of Ni-N2 in Me-PBID-acetylNiCl2 is noticeably shorter than
that in the former two nickel complexes. Furthermore, the
distance of Ni-Cl1 is much shorter than cis Ni-Cl distances.
2.2. Synthesis and Characterization of the Nickel Com-
plexes Containing 2-(1-Methyl-2-benzimidazole)-6-(1-arylim-
inoethyl)pyridines. The 2-(1-methyl-2-benzimidazole)-6-(1-
aryliminoethyl)pyridines (L1-L5) were prepared according to
our reported procedures.12 The nickel complexes (L1NiCl2-
L5NiCl2) were obtained by treating the ethanol solution of NiCl2‚
6H2O with the corresponding ligand (L1-L5) at room temper-
ature in satisfying yields (57%-88%) (Scheme 2). These
complexes showed high stability in both solution and solid state
and were identified by FT-IR and elemental analysis. The IR
spectra of the ligands L1-L5 gave CdNimino stretching frequen-
Table 1. Selected Bond Lengths and Angles for Complexes
Me-PBID-MeNiCl2, Me-PBID-esterNiCl2, and
Me-PBID-acetylNiCl2
Me-PBID-
Me-PBID-
Me-PBID-
MeNiCl2
esterNiCl2
acetylNiCl2
Bond Lengths (Å)
Ni-N1
Ni-N2
Ni-Cl1
Ni-Cl2
N1-C1
N1-C7
N2-C8
N2-C12
C7-C8
Ni-O1
2.0571(2)
2.2144(2)
2.3880(6)
2.3841(6)
1.378(3)
1.323(2)
1.357(2)
1.341(3)
1.469(3)
2.1198(2)
2.065(2)
2.045(2)
2.3196(7)
2.3890(8)
1.373(3)
1.340(3)
1.341(3)
1.327(4)
1.473(4)
2.305(2)
2.051(3)
2.027(3)
2.2887(1)
1.389(4)
1.333(4)
1.339(4)
1.334(4)
1.473(4)
2.243(2)
Bond Angles (deg)
N2-Ni-N1
N2-Ni-Cl1
N1-Ni-Cl1
Cl1-Ni-Cl2
N1-Ni-Cl2
N2-Ni-Cl2
N2-Ni-O1
N1-Ni-O1
78.35(6)
85.00(5)
95.23(5)
174.46(2)
89.75(5)
93.66(5)
170.96(6)
93.10(6)
78.28(8)
173.72(6)
104.52(6)
96.05(3)
92.81(7)
89.38(6)
74.55(8)
152.73(8)
78.71(1)
176.14(8)
104.74(8)
cies in the range 1639 to 1650 cm-1 12
while the CdNimino
,
74.54(1)
153.24(1)
stretching vibrations of the complexes L1NiCl2-L5NiCl2 shifted
toward lower frequencies between 1594 and 1595 cm-1 with
weak intensities. This indicates the effective coordination
between the imino nitrogen atom and the nickel center.
Moreover, single crystals of complexes L1NiCl2, L2NiCl2,
and L3NiCl2 suitable for X-ray diffraction were individually
obtained by slow diffusion of diethyl ether into their methanol
solutions or their saturated solution in a methanol-DMF solvent
Crystals of complex Me-PBID-MeNiCl2 (Figure 1) were
grown from a methanol solution layered with diethyl ether. The
nickel center coordinates with two methanol molecules, and the
coordinate geometry around the metal center could be described
as a distorted octahedron. Perhaps due to higher basicity of
benzimidazole, the bond length of Ni-N2pyridine (2.2144(2) Å)

2442 Organometallics, Vol. 26, No. 9, 2007
Hao et al.
Figure 4. Molecular structure of L1NiCl2. Thermal ellipsoids are
shown at 30% probability. Hydrogen atoms and one DMF molecule
have been omitted for clarity.
Figure 5. Molecular structure of L2NiCl2. Thermal ellipsoids are
shown at 30% probability. Hydrogen atoms have been omitted for
clarity.
Table 2. Selected Bond Lengths and Angles for Complexes
L1NiCl2, L2NiCl2, and L3NiCl2
L1NiCl2
L2NiCl2
L3NiCl2
Bond Lengths (Å)
Ni-N1
Ni-N2
Ni-N3
Ni-Cl1
Ni-Cl2
N1-C1
N1-C7
N2-C8
N2-C12
N3-C13
N3-C15
2.0701(2)
2.0100(1)
2.1809(2)
2.2954(6)
2.2662(5)
1.379(2)
1.328(2)
1.346(2)
1.337(2)
1.277(2)
1.438(2)
2.091(2)
2.002(2)
2.171(2)
2.2516(9)
2.2759(9)
1.381(3)
1.320(4)
1.351(4)
1.335(3)
1.284(4)
1.439(4)
2.1012(1)
2.0140(1)
2.1964(1)
2.2815(5)
2.2561(5)
1.379(2)
1.3234(2)
1.3435(2)
1.3328(2)
1.283(2)
1.4480(2)
Bond Angles (deg)
Figure 6. Molecular structure of L3NiCl2. Thermal ellipsoids are
shown at 30% probability. Hydrogen atoms have been omitted for
clarity.
N2-Ni-N1
N2-Ni-N3
N1-Ni-N3
N2-Ni-Cl1
N1-Ni-Cl1
N3-Ni-Cl1
Cl1-Ni-Cl2
N1-Ni-Cl2
N2-Ni-Cl2
N3-Ni-Cl2
77.78(6)
76.21(6)
152.60(6)
100.38(5)
94.34(5)
98.66(4)
110.17(2)
149.45(5)
99.31(4)
98.66(4)
77.66(9)
76.42(9)
152.17(9)
150.06(8)
98.49(7)
98.90(7)
114.00(4)
95.93(7)
95.01(7)
97.54(7)
77.27(5)
75.90(5)
150.50(5)
97.30(4)
95.80(4)
99.56(4)
108.02(2)
154.68(4)
99.33(4)
99.64(4)
and 0.0052 Å out of the equatorial plane in L2NiCl2 and L3-
NiCl2, respectively. The axial Ni-N bonds subtend an angle
of 152.17(9)° (L2NiCl2) and 150.50(5)° (L3NiCl2) (N(1)-Ni-
N(3)). The dihedral angles between the phenyl ring and the
pyridyl plane are 85.6° (L2NiCl2) and 83.2° (L3NiCl2), respec-
tively. The equatorial planes of these two complexes are nearly
perpendicular to the benzimidazole ring, with dihedral angles
of 91.6° in L2NiCl2 and 92.6° in L3NiCl2. Similar to L1NiCl2,
the Ni-N(1) and Ni-N(3) bond lengths are noticeably longer
than that of Ni-N(2). The imino N(3)-C(13) bond lengths are
1.284(4) Å (L2NiCl2) and 1.283(2) Å (L3NiCl2) with the typical
character of a CdN double bond, but slightly longer than that
of L1NiCl2 (1.277(2) Å). The difference is probably due to the
less bulky methyl at the ortho-position of the phenyl ring in
L1NiCl2.
2.3. Ethylene Oligomerization. 2.3.1. Ethylene Oligomer-
ization Promoted by N∧N or N∧N∧O Coordinated Nickel
Complexes. Different functional groups and coordination
environments of the complexes exert varied influences on their
reactivity with ethylene. The above presented nickel complexes
with bidentate N∧N and tridentate N∧N∧O ligands were
investigated for their use in ethylene oligomerization in the
presence of various organoaluminum cocatalysts. At ambient
pressure of ethylene, these systems showed low catalytic
activities. Therefore, 20 atm pressure of ethylene was employed
for the catalytic reaction. Considerable catalytic activities of
the nickel complexes were obtained with cocatalysts such as
MAO, MMAO. and Et2AlCl. As a matter of fact, the catalytic
activities were not largely different in the systems with various
mixture. An X-ray crystallographic study of the nickel complex
L1NiCl2 revealed the coordination geometry around the metal
center as a distorted trigonal bipyramid with the pyridyl nitrogen
atom and the two chlorides forming an equatorial plane; the
molecular structure is shown in Figure 4 with selected bond
lengths and angles in Table 2. In the structure of L1NiCl2, the
nickel atom is nearly in the equatorial plane with equatorial
angles ranging between 100.38(5)° and 149.45(5)°, deviating
from 120°. The two nitrogen atoms (N1 and N3) occupy the
axial coordination sites with a bond angle of 152.60(6)° for N1-
Ni-N3. This benzimidazole ring and the pyridyl ring are nearly
coplanar, with a dihedral angle of 2.9°. The dihedral angle
between the phenyl ring and the plane of pyridine is 84.8°. The
phenyl ring lies almost perpendicular to the plane formed by
the coordinated nitrogen atoms (80.8°), which is similar to the
2-imino-1,10-phenanthrolinyl nickel(II) complexes.11d The two
Ni-Cl bond lengths show a slight difference between the Ni-
Cl(1) (2.2954(6) Å) and Ni-Cl(2) (2.2662(5) Å) bonds.
Furthermore, the imino N(3)-C(13) bond in L1NiCl2 displays
clear double-bond character (1.277(2) Å), and it is 0.015 Å
longer than that in the free ligand L1.
Similarly, its analogues, nickel complexes L2NiCl2 and L3-
NiCl2 (Figure 5 and Figure 6), also show a distorted trigonal-
bipyramidal geometry. The nickel atom slightly resides 0.0045

Ethylene Oligomerization by N∧N∧N Nickel Complexes
Organometallics, Vol. 26, No. 9, 2007 2443
Table 3. Ethylene Oligomerization by Nickel Complexes Ligated with H-PBID-Me, Me-PBID-Me, H-PBID-ester,
Me-PBID-ester, and Me-PBID-acetyl and Their Oligomer Distributiona
oligomer distributionc
entry
complex
cocat.
Al/Ni
activityb
C4/∑C
C6/∑C
1
2
3
4
5
6
7
8
9
H-PBID-MeNiCl2
H-PBID-MeNiCl2
Me-PBID-MeNiCl2
H-PBID-esterNiCl2
H-PBID-esterNiCl2
H-PBID-esterNiCl2
H-PBID-esterNiCl2
Me-PBID-esterNiCl2
Me-PBID-acetylNiCl2
Et2AlCl
MAO
Et2AlCl
Et2AlCl
Et2AlCl
MAO
MMAO
Et2AlCl
Et2AlCl
200
1000
200
500
200
1000
1000
200
3.15
2.95
0.83
0.88
0.70
1.04
1.15
0.29
0.73
95.5
95.1
88.7
44.6
31.7
47.3
74.4
72.6
61.5
4.5
4.9
11.3
55.4
68.3
52.7
25.6
27.4
38.5
200
c
a Conditions: 5 µmol of catalyst; 30 min; toluene (100 mL); 20 atm; 20 °C. b 104 g mol-1(Ni) h-1 atm-1. Determined by GC.
Table 4. Ethylene Oligomerization of the H-PBID-MeNiCl2/
Et2AlCl Systema
Table 5. Ethylene Catalytic Activity with L1NiCl2 Using
Different Cocatalysta
oligomer distributionc
oligomer distributionc
entry
Al/Ni
P/atm
T/°C
activityb
C4/∑C
C6/∑C
entry
cocat.
Al/Ni
activityb
C4/∑C
C6/∑C
1
2
3
4
5
6
7
8
50
100
200
500
1000
500
500
500
500
500
20
20
20
20
20
30
10
20
20
30
20
20
20
20
20
20
20
40
60
20
0.36
0.52
3.15
3.66
11.2
4.13
1.85
0.86
0.64
1.65
85.5
81.1
95.5
94.5
93.8
93.1
89.8
85.8
84.6
89.8
14.5
18.9
4.5
5.5
6.2
1
2
3
MAO
MMAO
Et2AlCl
1000
1000
200
6.50
1.82
4.97
95.2
93.3
91.8
4.8
6.7
8.2
a Conditions: 5 µmol of precatalyst, 30 atm of ethylene, 0.5 h, 100 mL
of toluene. b 104 g mol-1(Ni) h-1 atm-1. Determined by GC.
c
6.9
10.2
14.2
15.4
10.2
(entries 4, 8, and 9 in Table 4), which might be attributed to
the instability of the active species or lower concentration of
ethylene in the reaction solution. Commonly, lower catalytic
activity was observed at lower pressure, and only butenes were
produced at ethylene pressures lower than 10 atm (entries 4, 6,
and 7 in Table 4). Previous studies reported that nickel catalysts
incorporating PPh3 into their catalytic systems resulted in higher
activity.6i,7c,g,11d,16 Therefore, the oligomerization reaction with
the H-PBID-MeNiCl2/Et2AlCl system was carried out in the
presence of 10 equiv of PPh3 (entry 10 of Table 4). However,
a sharp decrease in catalytic activity was observed. The reason
might be that the stronger Ni-P bond in this system prevented
the coordination of ethylene on the active metal center.
2.3.2. Ethylene Oligomerization Behavior of the N∧N∧N
Coordinated Nickel Complexes. The influences of various
cocatalysts on the ethylene activation were evaluated with the
catalytic systems formed from L1NiCl2 in the presence of
methylaluminoxane (MAO), modified methylaluminoxane
(MMAO), or Et2AlCl. Because of the low reactivity at ambient
pressure of ethylene, it was difficult to evaluate the influence
of cocatalyst; hence an elevated pressure of ethylene at 30 atm
was used. The optimum conditions were determined for various
cocatalysts in different molar ratios to L1NiCl2. The results
compiled in Table 5 indicate that the L1NiCl2/MAO or Et2AlCl
systems showed remarkable and similar catalytic activity on the
order of 105 g mol-1(Ni) h-1 atm-1. However, the Al/Ni ratio
is only 200 for Et2AlCl; therefore, it would be better to use
Et2AlCl on the basis of economical consideration.
9
10d
a Conditions: 5 µmol of catalyst; 30 min; toluene (100 mL). b 104
g
mol-1(Ni) h-1 atm-1. Determined by GC. 10 equiv of Ph3P.
c
d
cocatalysts (Table 3). Et2AlCl requires the smallest Al/Ni ratio
and therefore is chosen for a detailed investigation.
Comparing with the nickel complexes ligated by Me-PBID-
Me as catalysts (entries 3 in Table 3), the catalytic systems of
complexes H-PBID-MeNiCl2 had relatively higher reactivity
(entries 1 in Table 3), and the same phenomenon was also
observed in the nickel complexes ligated by H-PBID-ester and
Me-PBID-ester (entry 5 vs 8 in Table 3). These results can be
attributed to the deprotonation of the N-H group to give anionic
amide ligands when activated by a cocatalyst of organoalumi-
num to form N-Al species with increased catalytic activ-
ity.12,14,15 Furthermore, the nickel complexes H-PBID-MeNiCl2
and Me-PBID-MeNiCl2, containing N∧N ligands, exhibited
higher catalytic reactivity than their analogues containing
N∧N∧O ligands H-PBID-ester, Me-PBID-ester, and Me-PBID-
acetyl. When the nickel complexes with ester groups were used
as catalysts, the C6 proportion in the products was higher than
the C4 proportion and the C6 proportion in the N∧N catalyst
hints to slower elimination or higher stability of the organo-
metallic intermediate in the catalytic cycle. When it was
activated by MMAO, the C6 proportion decreased sharply
because of the inhibition of the isobutyl group in the cocatalyst.
Different reaction temperature, amount of cocatalyst, and
pressure of ethylene also affected their catalytic reactivity. Their
influences were investigated in detail using the catalytic system
of H-PBID-MeNiCl2/Et2AlCl, and the results are summarized
in Table 4. The catalytic activity was increased when the Al/Ni
molar ratio was varied from 50 to 1000. However, the optimum
Al/Ni molar ratio was fixed at 500 considering the economy
(entries 1-5 in Table 4). Elevating the reaction temperature
from 20 °C to 60 °C resulted in decreased catalytic activity
On fixing the Al/Ni ratio of Et2AlCl at 200, all nickel
complexes ligated by 2-(1-methyl-2- benzimidazole)-6-(1-
aryliminoethyl)pyridines were examined for ethylene oligomer-
ization with changing pressure of ethylene and reaction tem-
perature. The results are summarized in Table 6.
2.3.2.1. Effect of Ligand Enviroment. According to Table
6 (entries 1-5), the oligomerization activities decreased in the
order L3NiCl2> L2NiCl2 > L1NiCl2 with decreasing size of
the alkyl substituents on the aryl group, and likewise for L5-
(14) Zhang, W.; Sun, W.-H.; Zhang, S.; Hou, J.; Wedeking, K.; Schultz,
S.; Fro¨hlich, R.; Song, H. Organometallics 2006, 25, 1961.
(15) McGuinness, D. S.; Wasserscheid, P.; Morgan, D. H.; Dixon, J. T.
Organometallics 2005, 24, 552.
(16) (a) Jenkins, J. C.; Brookhart, M. Organometallics 2003, 22, 250.
(b) Tang, X.; Zhang, D.; Jie, S.; Sun, W.-H.; Chen, J. J. Organomet. Chem.
2005, 690, 3918.

2444 Organometallics, Vol. 26, No. 9, 2007
Hao et al.
Table 6. Oligomerization of Ethylene with L1NiCl2-L5NiCl2
Table 7. Influence of Different Al/Ni ratios on Complex
L4NiCl2/Et2AlCl Catalytic Activitya
and their Oligomer Distributiona
oligomer distributionc
oligomer distributionc
entry complex P/atm T/°C activityb C4/∑C C6/∑C C8/∑C
entry
Al/Ni
activityb
C4/∑C C6/∑C C8/∑C
1
2
3
4
5
6
7
8
9
L1NiCl2
L2NiCl2
L3NiCl2
L4NiCl2
L5NiCl2
L4NiCl2
L4NiCl2
L4NiCl2
L4NiCl2
L4NiCl2
L3NiCl2
L4NiCl2
30
30
30
30
30
10
20
30
30
30
30
30
20
20
20
20
20
20
20
40
60
80
20
20
0.50
1.00
1.18
3.06
3.40
2.82
1.66
5.87
4.00
0.29
91.8
94.9
95.1
93.1
94.3
92.4
91.8
89.5
82.4
74.6
85.3
88.6
8.2
5.1
4.9
6.6
5.5
7.6
8.2
9.6
16.4
21.3
14.5
10.8
1
2
3
4
5
6
7
8
5
10
50
100
200
300
500
1000
0.25
0.53
1.54
3.80
3.06
2.17
1.91
0.80
87.0
92.4
92.8
92.5
93.1
93.7
92.5
92.8
13.0
7.6
7.2
7.0
6.6
6.3
7.5
7.2
0.3
0.2
0.5
0.3
0.9
1.2
4.1
0.2
0.6
a Conditions: 5 µmol of precatalyst; 30 atm of ethylene; 20 °C;
10
11d
12d
0.5 h; 100 mL of toluene. b 105 g mol-1(Ni) h-1 atm-1. Determined by
c
13.7
18.8
GC.
a Conditions: 5 µmol of precatalyst; 0.5 h; 100 mL of toluene; cocat.:
proportions were recorded, implying that the rate of chain
propagation increases more rapidly than that of â-hydrogen
elimination.
c
200 equiv of Et2AlCl. b 105 g mol-1(Ni) h-1 atm-1. Determined by GC.
d10 equiv of PPh3.
NiCl2 > L4NiCl2 with decreasing size of halide subtituents. The
catalytic activity for L5NiCl2 is up to 3.40 × 105 g mol-1(Ni)
h-1 atm-1. The lower activity for less bulky substituents can
be explained by the exposure of the active sites of the catalyst
to not only ethylene but also other reactive species, and the
latter actually led to the deactivation of the active species.
Indeed, Gibson has proved that the catalytic intermediates
formed from the precursors containing pyridyldiimine ligands
are more easily deactivated through the interaction with alky-
laluminum reagents if they lack sufficient steric bulk in the aryl
ring.17 In our experiments, the major products are C4 and the
minor products are C6 by using L3NiCl2, L2NiCl2, or L1NiCl2,
while the catalytic systems of L4NiCl2 and L5NiCl2 also
produced a very small amount of C8. When the nickel complexes
contain halides as the electron-withdrawing substituents on the
aryl ring, the reactivity is much higher than with alkyl
substituents (entries 4 and 5 show higher activities than entries
1-3). The electron-withdrawing groups at the ortho-position
of the aryl rings make the central metal more positive, so the
ethylene can easily coordinate to nickel species, which finally
led to higher catalytic activity.
2.3.2.2. Effects of Ethylene Pressure and Reaction Tem-
perature. Oligomerization experiments with L4NiCl2 catalysts
were conducted under different pressures of ethylene as shown
in Table 6. It is observed that the catalytic activities were
improved with increasing ethylene pressure, attributable to the
higher monomer concentration around active nickel centers at
higher pressure. In addition, at 10 or 20 atm of ethylene, there
is no C8 in the oligomer products, and the higher monomer
concentration speeds up not only the ethylene reactivity
(coordination and insertion) but also the rate of alkyl-chain
propagation.
2.3.2.3. Effects of Auxiliary Ligand. The influence of
auxiliary ligand on the nickel catalyst was also investigated for
this system. Upon the addition of 10 equiv of PPh3 to the
catalytic system, the reactivity was strongly increased. It became
about 5 times higher than that without PPh3. Moreover, larger
amounts of C6 and C8 are observed (entries 11 and 12 in Table
6). This phenomenon was attributed to the fact that the active
nickel species are coordinated with auxiliary PPh3 on the vacant
coordination sites when lacking ethylene monomers. This
prevents deactivation by impurities of active reactants in the
catalytic species, while the incoming molecules of ethylene
easily replace PPh3 for further reactions. This is consistent with
previous observations,11d,e,g,15b and a series of active species
containing PPh3 was once isolated in our group.7g
2.3.2.4. Effects of Al/Ni Molar Ratio. The influence of
different molar ratios of Et2AlCl to nickel complexes on ethylene
activation was investigated in detail with L4NiCl2. When the
Al/Ni molar ratio was varied from 5 to 1000, the oligomerization
activities initially increased and then decreased (Table 7).
Increasing the Al/Ni molar ratio from 5 to 100 led to a
predominantly increased reactivity up to 3.80 × 105 g mol-1(Ni)
h-1 atm-1, indicating that L4NiCl2 can be easily activated. On
increasing the Al/Ni ratio further from 100 to 1000, the activity
gradually decreased. A small amount of C8 was found in the
oligomers when the catalytic activity is at the high stage. In
addition, employing a very small amount of Et2AlCl such as
with an Al/Ni ratio of 5 or 10, the ethylene oligomerization
with this catalyst system is still promising, with an activity of
about 104 g mol-1(Ni) h-1 atm-1
.
3. Conclusion
A series of nickel complexes ligated by 2-(2-benzimidazole)-
6-methylpyridine, 2-(1-methyl-2-benzimidazole)-6-acetylpyri-
dine, and 2-(1-methyl-2-benzimidazole)-6-(1-aryliminoethyl)-
pyridine was synthesized and characterized carefully. The X-ray
crystallographic studies on all 2-(1-methyl-2-benzimidazole)-
6-(1-aryliminoethyl)pyridylnickel chlorides revealed a distorted
trigonal-bipyramidal geometry, while other nickel complexes
displayed a distorted octahedral coordination geometry with
incorporation of solvent molecules. These complexes could be
easily activated and exhibited good to high activities for ethylene
oligomerization in the presence of Et2AlCl as cocatalyst, even
though the amount of cocatalyst was very small and the main
products were dimers and trimers of ethylene. The complexes
ligated by tridentate N∧N∧N ligands, 2-(1-methyl-2-benzimi-
dazole)-6-(1-aryliminoethyl)pyridines, more favored the inser-
As the ethylene oligomerization is a highly exothermic
reaction, the reaction temperature significantly affects the
catalytic activity. To estimate this influence, the catalytic system
of L4NiCl2 with 200 equiv of Et2AlCl at 30 atm of ethylene
was investigated with varied reaction temperature (entries 4
and 8-10 in Table 6). Increasing the temperature from 20 to
80 °C, the optimum catalytic activity was shown at 40 °C. At
80 °C, its catalytic activity was 1/20 of that at 40 °C because
of the lower stability of nickel active species with lower
coordination rate of ethylene. Interestingly, on raising the
temperature from 40 to 80 °C, increased C6 and C8 oligomer
(17) Britovsek, G. J. P.; Mastroianni, S.; Solan, G. A.; Baugh, S. P. D.;
Redshaw, C.; Gibson, V. C.; White, A. J. P.; Williams, D. J.; Elsegood, M.
R. J. Chem.-Eur. J. 2000, 6, 2221.

Ethylene Oligomerization by N∧N∧N Nickel Complexes
Organometallics, Vol. 26, No. 9, 2007 2445
Table 8. Crystal Data and Structure Refinement for Compounds
Me-PBID-Me-
Me-PBID-ester-
Me-PBID-acetyl-
NiCl2
NiCl2
NiCl2
L1NiCl2
L2NiCl2
L3NiCl2
empirical
formula
fw
C16H21Cl2N3NiO2
C21H28Cl2N4NiO4
C17H21Cl2N3NiO3
C26H29Cl2N5NiO
C25H26Cl2N4Ni
C27H30Cl2N4Ni
416.97
530.08
444.98
557.13
512.11
540.16
cryst color
temp (K)
wavelength (Å)
cryst syst
space group
a (Å)
green
293(2)
0.71073
triclinic
P1h
9.9010(2)
10.8544(2)
11.0362(2)
62.3650(1)
69.4580(1)
63.2600(1)
923.10(3)
2
green
293(2)
0.71073
triclinic
P1h
8.0306(2)
12.5947(3)
13.3400(3)
71.778(2)
86.4870(1)
86.6650(1)
1278.09(5)
2
green
293(2)
brown
293(2)
0.71073
triclinic
P1
8.6398(3)
12.7231(4)
12.8188(4)
70.6870(1)
80.1010(1)
86.3540(1)
1309.98(7)
2
brown
293(2)
brown
293(2)
0.71073
monoclinic
P2(1)/c
7.8096(4)
12.9994(8)
19.3219(1)
90
93.425(3)
90
1958.06(2)
4
0.71073
monoclinic
P2(1)/c
9.5439(3)
13.9544(4)
18.5858(6)
90.0
94.433(2)
90.0
2467.84(1)
4
0.71073
monoclinic
P2(1)/c
9.6969(2)
14.9381(3)
18.2266(4)
90.0
91.9400(1)
90.0
2638.67(1)
4
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
volume (Å3)
Z
D
calc (g cm-3
)
1.500
1.353
432
1.377
1.000
552
1.509
1.285
920
1.412
0.973
580
1.378
1.022
1064
1.360
0.960
1128
µ (mm-1
F(000)
)
cryst size (mm)
0.42 × 0.32 ×
0.35 × 0.25 ×
0.35 × 0.24 ×
0.30 × 0.10 ×
0.30 × 0.20 ×
0.35 × 0.10 ×
0.10
0.14
0.13
0.05
0.10
0.05
θ range (deg)
limiting
indices
2.12-28.37
-13ehe10,
-14eke14,
-14ele14
14 501
1.61-28.30
-10ehe10,
-16eke16,
-17ele16
19 074
1.89-28.30
-9ehe10,
-17eke17,
-25ele25
20 918
1.70-28.34
-11ehe7,
-16eke16,
-17ele16
16 294
1.83-28.33
-12ehe12,
-18eke18,
-23ele24
21 881
1.76-28.31
-12ehe12,
-19eke19,
-23ele24
26 953
no. of reflns
collected
no. of unique
reflns
4582
6290
4833
6436
6135
6520
completeness
to θ (%)
absorp corr
no. of params
goodness-of-fit
on F2
99.2 (θ)28.37°)
98.9 (θ)28.30°)
99.2 (θ)28.30°)
98.6 (θ)28.34°)
99.6 (θ)28.33°)
99.4 (θ)28.31°)
empirical
225
1.123
empirical
289
0.961
empirical
243
0.818
empirical
316
1.108
empirical
271
1.088
empirical
307
1.034
final R indices
[I > 2σ(I)]
R indices
R1 ) 0.0333,
wR2 ) 0.0990
R1 ) 0.0450,
wR2 ) 0.1028
0.455 and -0.390
R1 ) 0.0435,
wR2 ) 0.1232
R1 ) 0.0767,
wR2 ) 0.1406
0.545 and -0.460
R1 ) 0.0415,
wR2 ) 0.0881
R1 ) 0.1429,
wR2 ) 0.1084
0.289 and -0.333
R1 ) 0.0344,
wR2 ) 0.1004
R1 ) 0.0441,
wR2 ) 0.1046
0.408 and -0.351
R1 ) 0.0522,
wR2 ) 0.1594
R1 ) 0.0721,
wR2 ) 0.1713
1.087 and -1.389
R1 ) 0.0305,
wR2 ) 0.0875
R1 ) 0.0416,
wR2 ) 0.0930
0.292 and -0.311
(all data)
largest diff
peak and hole
(e Å-3
)
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. The yield of oligomers was calculated by
referencing with the mass of the solvent on the basis of the
prerequisite that the mass of each fraction was approximately
proportional to its integrated areas in the GC trace. All the organic
compounds used as ligands for the nickel complexes were prepared
by employing our developed procedure.12
4.2. Synthesis of Nickel Complexes. General Procedure: A
solution of the ligand in ethanol was added dropwise to an
equimolar amount (1 mmol) of NiCl2‚6H2O in ethanol solution.
Immediately, the color of the solution changed and some precipitate
formed. The reaction mixture was stirred at room temperature for
6 h, then diluted with diethyl ether. The resulting precipitate was
collected, washed with diethyl ether, and dried in vacuum. All of
the complexes were prepared in high yield in this manner.
4.2.1. Preparation of the Nickel Complexes with Precursor
Compounds. 2-(2-Benzimidazole)-6-methylpyridine NiCl2 (H-
PBID-MeNiCl2): yellow powder, 91% yield. IR (KBr; cm-1):
3396, 3061, 1609, 1577, 1477, 1460, 1322, 1251, 1040, 806, 748.
Anal. Calcd for C13H11Cl2N3Ni: C, 46.08; H, 3.27; N, 12.40.
Found: C, 45.74; H, 2.99; N, 12.61.
2-(1-Methyl-2-benzimidazole)-6-methylpyridine NiCl2 (Me-
PBID-MeNiCl2): yellow powder, 97% yield. IR (KBr; cm-1):
3346, 2926, 1605, 1477, 1491, 1442, 1384, 1336, 1160, 1102, 801,
755. Anal. Calcd for C14H13Cl2N3Ni: C, 47.65; H, 3.71; N, 11.91.
Found: C, 47.29; H, 3.83; N, 11.58.
tion of ethylene and led to better catalytic activities than those
bearing bidentate N∧N or tridentate N∧N∧O ligands. In the
tridentate N∧N∧N catalytic system, the complexes containing
large steric substituents or electron-withdrawing groups on the
N-aryl ring had higher ethylene insertion activity, and especially
the electronic effect was very obvious. Good activities (5.87 ×
105 g mol-1(Ni) h-1 atm-1) were observed at higher pressures
of ethylene, with optimum conditions of the Al/Ni molar ratio
and reaction temperature. In addition, the catalytic system
containing the auxiliary ligand PPh3 showed better catalytic
activities toward ethylene oligomerization because of the
protection of the phosphine group.
4. Experimental Section
4.1. General Considerations. All manipulations of air- and
moisture-sensitive compounds were performed under a nitrogen
atmosphere using standard Schlenk techniques. Toluene was
refluxed over sodium-benzophenone and distilled under argon prior
to use. Methylaluminoxane (MAO, a 1.46 M solution in toluene)
and modified methylaluminoxane (MMAO, 1.93 M in heptane)
were purchased from Akzo Nobel Corp. Diethylaluminum chloride
(Et2AlCl, 1.7 M in toluene) was purchased from Acros Chemicals.
Other reagents were purchased from Aldrich or Acros Chemicals.
IR spectra were recorded on a Perkin-Elmer System 2000 FT-IR
spectrometer. Elemental analysis was carried out using a Flash EA
1112 microanalyzer. GC analysis was performed with a Carlo Erba

2446 Organometallics, Vol. 26, No. 9, 2007
Hao et al.
2-Carboethoxy-6-(2-benzimidazolyl)pyridine NiCl2 (H-PBID-
esterNiCl2): green powder, 77% yield. IR (KBr; cm-1): 2977,
1694, 1605, 1469, 1422, 1375, 1329, 1275, 1157, 1007, 856, 837,
765. Anal. Calcd for C15H13Cl2N3NiO2: C, 45.39; H, 3.30; N, 10.59.
Found: C, 45.24; H, 3.47; N, 10.40.
2-Carboethoxy-6-(1-methyl-2-benzimidazolyl)pyridine NiCl2
(Me-PBID-esterNiCl2): green powder, 98% yield. IR (KBr; cm-1):
3302, 1684, 1600, 1487, 1408, 1380, 1332, 1285, 1157, 1013,
825, 758. Anal. Calcd for C16H15Cl2N3NiO2: C, 46.77; H, 3.68;
N, 10.23. Found: C, 47.11; H, 3.29; N, 10.12.
2-(1-Methyl-2-benzimidazole)-6-acetylpyridine NiCl2 (Me-
PBID-acetylNiCl2): green powder, 84% yield. IR (KBr; cm-1):
3207, 1666, 1570, 1488, 1423, 1404, 1335, 1277, 1251, 1189, 1089,
814, 758. Anal. Calcd for C15H13Cl2N3NiO: C, 47.30; H, 3.44; N,
11.03. Found: C, 47.51; H, 3.31; N, 11.08.
to the fully dried reactor under a nitrogen atmosphere. The required
amount of cocatalyst (MAO, MMAO, or Et2AlCl) was then injected
into the reactor via a syringe. At the reaction temperature, the reactor
was sealed and pressurized to high ethylene pressure, and the
ethylene pressure was maintained during feeding of ethylene. After
the reaction mixture was stirred for the desired period, the pressure
was released and a small amount of the reaction solution was
collected, which was then analyzed by gas chromatography (GC)
to determine the composition and mass distribution of the oligomers
obtained. Then the residual reaction solution was quenched with
5% hydrochloride acid ethanol. The precipitated low molecular
weight waxes were collected by filtration, washed with ethanol and
water, and dried in a vacuum until constant weight.
4.4. X-ray Crystallographic Studies. Single-crystal X-ray
diffraction studies for Me-PBID-MeNiCl2, Me-PBID-esterNiCl2,
Me-PBID-acetylNiCl2, L1NiCl2, L2NiCl2, and L3NiCl2 were carried
out on a Bruker SMART 1000 CCD diffractometer with graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å). 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.18
Crystal data and processing parameters for Me-PBID-MeNiCl2,
Me-PBID-esterNiCl2, Me-PBID-acetylNiCl2, L1NiCl2, L2NiCl2,
and L3NiCl2 are summarized in Table 8.
4.2.2. Preparation of the Title Nickel Complexes. Data for
these complexes are as follows.
L1NiCl2: yellow powder, 57% yield. IR (KBr; cm-1): 1595 (VCd
N); 1490; 1402; 1368; 1334; 1307; 1282; 1258; 1212; 1157; 1098;
796; 752. Anal. Calcd for C23H22Cl2N4Ni: C, 57.07; H, 4.58; N,
11.57. Found: C, 56.77; H, 4.83; N, 11.22.
L2NiCl2: yellow powder, 87% yield. IR (KBr; cm-1): 1595 (VCd
N); 1489; 1446; 1404; 1370; 1330; 1283; 1257; 1203; 1164; 1091;
1030; 794; 758. Anal. Calcd for C25H26Cl2N4Ni: C, 58.63; H, 5.12;
N, 10.94. Found: C, 58.55; H, 5.01; N, 10.68.
L3NiCl2: yellow powder, 88% yield. IR (KBr; cm-1): 1595 (VCd
N); 1490; 1461; 1424; 1404; 1370; 1329; 1306; 1283; 1255; 1201;
1105; 1028; 794; 760. Anal. Calcd for C27H30Cl2N4Ni: C, 60.04;
H, 5.60; N, 10.37. Found: C, 59.85; H, 5.75; N, 10.08.
L4NiCl2: yellow powder, 73% yield. IR (KBr; cm-1): 1595 (VCd
N); 1489; 1434; 1403; 1365; 1308; 1282; 1259; 1192; 1159; 1090;
1027; 867; 790; 748. Anal. Calcd for C21H16Cl4N4Ni: C, 48.05;
H, 3.07; N, 10.67. Found: C, 47.86; H, 3.33; N, 10.46.
L5NiCl2: yellow powder, 87% yield. IR (KBr; cm-1): 3377;
3072; 1594(VCdN); 1488; 1431; 1368; 1281; 1259; 1228; 1188;
1159; 1025; 927; 816; 789; 748. Anal. Calcd for C21H16Cl2Br2-
NiN4: C, 41.09; H, 2.63; N, 9.13. Found: C, 40.72; H, 2.57; N,
9.07.
Acknowledgment. This work was supported by NSFC No.
20473099 and MOST No. 2006AA03Z553. S.A. is grateful to
the Chinese Academy of Science (CAS) and the Academy of
Science for The Developing World (TWAS) for the Postgraduate
Fellowships.
Supporting Information Available: X-ray crystallographic data
(CIF) for Me-PBID-MeNiCl2, Me-PBID-esterNiCl2, Me-PBID-
acetylNiCl2, L1NiCl2, L2NiCl2, and L3NiCl2. This material is
available free of charge via the Internet at http://pubs.acs.org.
4.3. Procedure for Ethylene Oligomerization. Ethylene oli-
gomerization was performed in a stainless steel autoclave (0.5 L
capacity) equipped with a gas ballast through a solenoid clave for
continuous feeding of ethylene at constant pressure. A 100 mL
amount of toluene containing the catalyst precursor was transferred
OM070049E
(18) Sheldrick, G. M. SHELXTL-97, Program for the Refinement of
Crystal Structures; University of Gottingen: Germany, 1997.