Synthesis, characterization, and ethylene oligomerization of nickel complexes bearing N-((pyridin-2-yl)methylene)quinolin-8-amine derivatives

Article abstract of DOI:10.1021/om700440v

A series of nickel complexes ligated by N-((pyridin-2-yl)methylene) quinolin-8-amine derivatives were synthesized by one-pot reaction of 8-aminoquinolines, 1-(pyridine-2-yl) ketones, and nickel halides and characterized by elemental and spectroscopic analyses along with X-ray diffraction analyses. These nickel complexes exhibit two kinds of structures, namely, dimeric with octahedral-coordinated geometry and monomeric with distorted trigonal-bipyramidal geometry, in their solid states. Activated by Et₂AlCl, these Ni(II) complexes exhibited good to high catalytic activities for ethylene oligomerization, and activity up to 4.9 × 10 ⁷ g mol^-1(Ni) h^-1 was observed in the catalytic system with Cy₃P as auxiliary ligand. The steric and electronic influence of the substituents as well as the influence of different catalytic conditions was carefully investigated. In particular, the correlations between reactivity and auxiliary R₃P ligands were examined.

Full text of DOI:10.1021/om700440v

Organometallics 2007, 26, 4781-4790
4781
Synthesis, Characterization, and Ethylene Oligomerization of Nickel
Complexes Bearing N-((Pyridin-2-yl)methylene)quinolin-8-amine
Derivatives†
Wen-Hua Sun,* Kefeng Wang, Katrin Wedeking, Dongheng Zhang, Shu Zhang,
Jingjing Cai, and Yan 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
ReceiVed May 4, 2007
A series of nickel complexes ligated by N-((pyridin-2-yl)methylene)quinolin-8-amine derivatives were
synthesized by one-pot reaction of 8-aminoquinolines, 1-(pyridine-2-yl) ketones, and nickel halides and
characterized by elemental and spectroscopic analyses along with X-ray diffraction analyses. These nickel
complexes exhibit two kinds of structures, namely, dimeric with octahedral-coordinated geometry and
monomeric with distorted trigonal-bipyramidal geometry, in their solid states. Activated by Et2AlCl,
these Ni(II) complexes exhibited good to high catalytic activities for ethylene oligomerization, and activity
up to 4.9 × 107 g mol-1(Ni) h-1 was observed in the catalytic system with Cy3P as auxiliary ligand. The
steric and electronic influence of the substituents as well as the influence of different catalytic conditions
was carefully investigated. In particular, the correlations between reactivity and auxiliary R3P ligands
were examined.
ment for the coordination of the olefin to the metal center.
Besides the well-explored SHOP catalyst, a neutral Ni complex
ligated by P∧O4 ligands in a bidentate manner, numerous Ni
complexes ligated by bidentate N∧O,5 P∧N,6 and N∧N,2,7 and
tridentate ligands such as N∧N∧O,8 N∧P∧N (A),9 P∧N∧P (B),10a
P∧N∧N (C10a and D10b), and N∧N∧N (E11 and F12) have been
reported (Scheme 1). Although these catalysts still have some
disadvantages for commercialization in a short period, some
nickel catalytic systems showed very good activities for ethylene
reactivity, and further investigation will be promising for both
industrial and academic developments.
1. Introduction
The modern chemical industry has much reliance on R-ole-
fins, and development of new catalysts for oligomerization and
polymerization of ethylene is of great interest to academic and
industrial pursuits. One major industrial process in ethylene
oligomerization, the Shell higher olefin process (SHOP),
employs nickel complexes as catalyst.1 Over the past decade,
research into nickel complexes as catalysts for olefin reactivity
has attracted much attention due to the substantial breakthrough
by the group of Brookhart with the development of highly active
catalysts for ethylene polymerization.2 The important progress
of ethylene reactivity promoted by nickel complexes is reflected
by the recent review articles.3 Ethylene reactivity includes two
competitive reactions, oligomerization and polymerization, with
the mechanistic difference of chain termination and propagation.
Recent challenges in the application of nickel complexes in
ethylene oligomerization are improving their catalytic activity
and selectively for the formation of R-olefins and controlling
the polymer properties of produced polymers. From an academic
view, the fundamental points in designing novel complexes as
good homogeneous catalysts are based on designing and
synthesizing suitable ligands to provide a conductive environ-
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† Dedicated to Prof. Heinz Berke on the occasion of his 60th birthday.
* Corresponding author. Tel: +86 10 62557955. Fax: +86 10 62618239.
E-mail: whsun@iccas.ac.cn.
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10.1021/om700440v CCC: $37.00 © 2007 American Chemical Society
Publication on Web 08/11/2007

4782 Organometallics, Vol. 26, No. 19, 2007
Sun et al.
lenged by the success of our recently reported N∧N∧N triden-
tate nickel complexes, we expect that successive exploration
of nickel catalysts, keeping a similar coordination environment
and the conjugated electronic system, can further improve
catalytic activity and product distribution. Therefore, it would
be interesting to substitute the ligand backbone with N-((pyridin-
2-yl)methylene)quinolin-8-amine derivatives.
Scheme 1. Nickel Catalysts Bearing Tridentate Ligands
Moreover, during our research toward new tridentate nickel
complexes for ethylene oligomerization we often observed a
very distinct increase in activity in the previously described
system, under the influence of Ph3P during the oligomeri-
zation.5g,7h,11f,12a,13a Although the influence of different ancillary
phosphines was studied in a ruthenium-based metathesis cata-
lyst18 and SHOP-type catalysts,4h it is still unclear about the
influence of different R3P on the catalytic behavior of tridentate
nickel systems. Therefore, the correlations between catalytic
activities with various R3P were investigated in the present
paper.
The preparation of N-((pyridin-2-yl)methylene)quinolin-8-
amine derivatives, however, was not successful due to their
instability. Fortunately, we could use a one-pot reaction to
synthesize the herein reported nickel complexes. The resultant
nickel complexes showed good to high activities up to 4.9 ×
107 g mol-1(Ni) h-1 for ethylene oligomerization when activated
by Et2AlCl. All these nickel complexes have been carefully
characterized along with their unambiguous molecular structures
and confirmed by single-crystal X-ray crystallographic analysis.
In the following, we report the synthesis and characterization
of the nickel complexes along with their catalytic properties
for ethylene oligomerization.
Nickel complexes containing bidentate ligands have been
extensively studied in our group. With the E model catalyst,
the nickel complexes incorporating an additional imino group
(R′) unusually showed even better activities than those with their
iron and cobalt analogues;11b however, the tridentate nickel
complexes showed high catalytic activity toward ethylene
oligomerization up to the order of 107 g mol-1(Ni) h-1 11f
The
.
coordination environments of all these well-defined catalysts
can be systematically varied by changing the substituents of
the ligand backbones, which provides an opportunity to control
their catalytic behaviors including activity and product distribu-
tion. Encouraged by these results, alternative models of tridentate
nickel complexes were explored for their catalytic behaviors
toward ethylene reactivity, and the metal complexes (F) ligated
by 2-(1-methyl-2-benzimidazole)-6-(1-aryliminoethyl)pyridines
showed high catalytic activity,12 and good activity was obtained
by complexes bearing 2-quinoxalinyl-6-iminopyridines.13 Chal-
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2006, 44, 1964-1974.
2. Results and Discussions
2.1. Synthesis and Characterization of the Complexes. The
N-((pyridin-2-yl)methylene)quinolin-8-amine derivatives can be
formed from the condensation reaction of 8-aminoquinolines
with the corresponding pyridine ketones or aldehydes. To
introduce bulky substituents such as i-Pr, t-Bu, and Cy to the
framework of the ligand, the substituted 8-nitroquinoline deriva-
tives were synthesized according to literature procedures14a,b and
(8) Yang, Q.-Z.; Kermagoret, A.; Agostinho, M.; Siri, O.; Braunstein,
P. Organometallics 2006, 25, 5518-5527.
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Ethylene Oligomerization by Tridentate Ni Complexes
Organometallics, Vol. 26, No. 19, 2007 4783
Scheme 2. Synthesis of Tridentate Nickel Complexes
Figure 1. Molecular structure of 3b, with thermal ellipsoids at
the 30% probability level. Hydrogen atoms and solvent are omitted
for clarity.
play important roles in forming solid-state structures of the
complexes. The catalytic properties of nickel complexes have
been considered as a driving force in this work, but interestingly
their molecular structures show some unique properties of
coordination. Therefore, some discussions on their molecular
structures are useful in coordination chemistry.
Fortunately, crystal structures of some of the complexes (3b,
4a, 6a, 6b, 10a, 10b, 11a, and 12a) aided the understanding of
the driving force for the formation of monomeric and dimeric
structures. When the steric demand of R1 and R3 is minor, the
molecular geometry prefers to build up an octahedral-coordi-
nated type. This coordination can be realized through the
formation of dimers, using bridged halogen atoms (3b, 12a) or
an additional solvent coordination to the metal (11a). Com-
plexes bearing bulkier substituents R1 and R3 display a dis-
torted trigonal-bipyramidal geometry; this is realized through a
monomeric structure, in which the nickel is coordinated by
the tridentate ligand and two halides (4a, 6a, 6b, 10a, and
10b).
Crystals of 3b, showing a dimeric structure in the solid state,
were grown from a methanol solution through slow evaporation
of the solvent at room temperature. The molecular structure is
shown in Figure 1, and selected bond lengths and angles are
listed in Table 1. The coordination of the nickel centers can be
described as distorted octahedron geometry. Each nickel atom
is coordinated by three N atoms (N(1), N(2), and N(3)) of the
ligand, one terminal bromide (Br(1)), and two bridging bromides
(Br(2) and Br(2A)). The N(2)-C(11) bond distance is 1.24(2)
Å, which is in accordance with a CdN bond, confirming the
formation of a Schiff base in the one-pot method employed.
The distance between the nickel and the terminal bromide
(Ni(1)-Br(1)) is 2.533(2) Å, whereas, the bond lengths between
the nickel and the two bridging bromides differ significantly
(2.505(2) and 2.638(2) Å). The bridge is planar, leading to a
Ni(1)-Ni(1A) distance of 3.811 Å, suggesting that there is no
direct metal-metal interaction. Each nickel forms two five-
membered chelate rings with three nitrogen atoms; the bite
angles of N(2)-Ni(1)-N(1) and N(2)-Ni(1)-N(3) are 79.1-
(5)° and 77.2(5)°, respectively. The Ni(1) atom deviates by 0.044
Å from the plane of the Br(1)-N(2)-Br(2) atoms and deviates
by 0.066 Å from the plane of the N(1)-N(2)-N(3) atoms. The
plane of Br(1)-N(2)-Br(2) lies almost perpendicular to the
plane formed by N(1)-Ni(1)-N(3) with the dihedral angle of
87.9°, and the quinoline plane is nearly coplanar with the
then reduced by iron powder in acidic solution to form
8-aminoquinolines.14c All 8-nitroquinoline and 8-aminoquinoline
derivatives were confirmed by NMR spectroscopy.14a
A typical Schiff-base reaction was carried out by reacting
8-aminoquinolines with the corresponding pyridine ketones or
aldehydes, leading to the preparation of N-((pyridin-2-yl)-
methylene)quinolin-8-amine derivatives. Much effort was spent
isolating the Schiff-base compounds; however, these compounds
tend to decompose when separated by column chromatography,
and pure products could not be obtained.15 As generally applied
to either unstable or low-yielding ligands, the one-pot synthesis
technique was therefore employed to prepare the metal com-
plexes in good yields.7l,10a,16 The central cationic nickel was
introduced in situ along with the condensation reaction of
pyridine-2-aldehyde or ketone and 8-aminoquinolines. Stoichio-
metric amounts of the 8-aminoquinoline derivative, pyridine-
2-aldehyde, or ketone and the nickel dihalides were mixed and
refluxed in acetic acid for 4 h to afford the corresponding
complexes (Scheme 2). The resulting products were precipitated
from the reaction solution, collected by filtration, washed with
diethyl ether, and dried under vacuum. The complexes were
separated as air-stable powders in moderate to high yields. All
complexes were characterized by elemental analysis and IR
spectroscopy. The IR spectra of complexes 1a-12a and
1b-12b show a strong and sharp band in the range 1595-
1630 cm-1 that can be ascribed to the stretching vibration of
CdN; however, their elemental analysis revealed that some of
them contained water or solvent in their solid state. The NMR
data were not obtained due to the paramagnetic nature of these
nickel complexes. Their unambigious molecular structures were
confirmed by single-crystal X-ray crystallography.
2.2. Crystal Structures. X-ray crystallographic analyses
revealed that these nickel complexes could form either dimeric
or monomeric structures in the solid state. Using various
substituents of the same framework, nickel complexes with
monomeric and dimeric structures were reported a long time
ago for a bidentate N∧N system.17 In that case, dimeric structures
were observed for the chlorides, while both dimeric and
monomeric structures were observed for the bromides. These
results point out that steric and electronic effects of the ligands

4784 Organometallics, Vol. 26, No. 19, 2007
Sun et al.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the
Dimeric Complex 3b
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Complex 11a
3b (X ) Br)
Bond Lengths
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-N(3)
Ni(1)-O(200)
2.046(2)
2.066(2)
2.049(2)
2.117(2)
Ni(1)-O(100)
Ni(1)-Cl(1)
N(2)-C(11)
2.124(2)
2.346(2)
1.291(3)
Bond Lengths
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-N(3)
N(2)-C(11)
Ni(1)-Br(1)
Ni(1)-Br(2)
Ni(1)-Br(2)#1
2.08(2)
2.02(2)
2.22(2)
1.24(2)
2.533(2)
2.638(2)
2.505(2)
Bond Angles
80.27(8) N(3)-Ni(1)-O(100)
159.04(8) O(200)-Ni(1)-O(100) 174.69(7)
78.77(8) N(1)-Ni(1)-Cl(1)
90.23(8) N(2)-Ni(1)-Cl(1)
87.34(7) N(3)-Ni(1)-Cl(1)
88.34(8) N(3)-Ni(1)-Cl(1)
93.12(8) O(200)-Ni(1)-Cl(1)
89.17(7) O(100)-Ni(1)-Cl(1)
N(1)-Ni(1)-N(2)
N(1)-Ni(1)-N(3)
N(3)-Ni(1)-N(2)
N(1)-Ni(1)-O(200)
N(2)-Ni(1)-O(200)
N(3)-Ni(1)-O(200)
N(1)-Ni(1)-O(100)
N(2)-Ni(1)-O(100)
87.05(8)
100.04(6)
178.77(6)
100.92(6)
88.4(3)
93.84(5)
89.63(5)
Bond Angles
N(2)-Ni(1)-N(1)
N(2)-Ni(1)-N(3)
N(1)-Ni(1)-N(3)
N(2)-Ni(1)-Br(2)#1
N(2)-Ni(1)-Br(2)
N(1)-Ni(1)-Br(2)#1
N(1)-Ni(1)-Br(2)
N(3)-Ni(1)-Br(2)#1
N(3)-Ni(1)-Br(2)
Br(2)#1-Ni(1)-Br(1)
Br(1)-Ni(1)-Br(2)
N(1)-Ni(1)-Br(1)
N(2)-Ni(1)-Br(1)
N(3)-Ni(1)-Br(1)
Br(2)#1-Ni(1)-Br(2)
Ni(1)#1-Br(2)-Br(1)
79.1(5)
77.2(5)
156.1(5)
166.4(3)
84.4(3)
93.3(4)
90.7(4)
110.5(3)
89.7(3)
95.88(7)
178.05(8)
91.2(4)
95.6(3)
88.4(3)
84.40(7)
95.60(7)
Table 3. Selected Bond Lengths (Å) and Angles (deg) for the
Monomeric Complexes 4a, 6a, and 10a
4a (X ) Cl)
6a (X ) Cl)
10a (X ) Cl)
Bond Lengths
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-N(3)
Ni(1)-Cl(1)
Ni(1)-Cl(2)
N(2)-C(11)
2.112(4)
1.980(4)
2.140(4)
2.274(2)
2.295(2)
1.272(5)
2.147(3)
2.000(2)
2.167(3)
2.279(2)
2.284(2)
1.279(4)
2.100(3)
1.990(3)
2.128(3)
2.289(2)
2.294(2)
1.272(5)
Bond Angles
N(2)-Ni(1)-N(1)
N(2)-Ni(1)-N(3)
N(1)-Ni(1)-N(3)
N(1)-Ni(1)-Cl(1)
N(2)-Ni(1)-Cl(1)
N(3)-Ni(1)-Cl(1)
N(1)-Ni(1)-Cl(2)
N(2)-Ni(1)-Cl(2)
N(3)-Ni(1)-Cl(2)
Cl(1)-Ni(1)-Cl(2)
79.7(2)
78.5(2)
158.0(2)
94.7(2)
110.4(2)
95.3(2)
95.2(2)
113.2(2)
91.0(2)
136.38(6)
79.02(9)
78.36(9)
157.4(1)
93.61(7)
110.58(8)
93.65(7)
96.39(7)
118.26(8)
94.89(7)
131.15(4)
80.0(2)
79.0(2)
159.0(2)
94.41(9)
110.9(1)
92.5(1)
93.7(1)
110.4(1)
94.2(1)
pyridine ring, the dihedral angle being 4.3°. In addition, complex
12a (R1 ) R3 ) H; R2 ) Ph) shows the same structural features
as 3b.
Complex 11a demonstrates another way to form distorted
octahedral coordination geometry around the Ni center. Crystals
of 11a were obtained from a methanol solution through slow
evaporation of the solvent. The molecular structure is shown
in Figure 2, and selected bond lengths and angles are listed in
Table 2. The octahedral geometry around the nickel is formed
by three N atoms of the ligand, two O atoms of coordinated
methanol solvent, and one Cl atom, which is clearly different
from the observed case in 3b. The other Cl atom is displaced
from its coordinated state and found separated as Cl-. The Ni
atom deviates 0.076 Å out of the O(100)-N(2)-O(200) plane
and deviates 0.010 Å out of the N(1)-N(2)-N(3) plane. The
dihedral angle between the quinoline plane and the pyridine
plane is measured to be 8.4°. The dihedral angle between
O(100)-N(2)-O(200) and the N(1)-Ni(1)-N(3) plane is 89.0°.
Crystals of 4a suitable for X-ray structural determination were
grown from a methanol solution through slow evaporation of
138.69(5)
the solvent, whereas crystals of 6a, 6b, 10a, and 10b were grown
from a slow diffusion of diethyl ether into a methanol solution
of the corresponding complex. Related bond lengths and angles
of these comparable complexes are shown in Table 3.
When the steric bulk of the substituents R1 and R3 is
increased, these complexes form monomeric structures. X-ray
analysis reveals that each nickel atom is coordinated with three
N atoms (N(1), N(2), and N(3)) of the ligand and two halogens.
The geometry around the nickel atom displays a distorted
trigonal bipyramid in which an equatorial plane is formed by
the imine nitrogen (N(2)) and the two halogens, while the
nitrogen (N(1)) of the quinolyl group and the nitrogen (N(3))
of the pyridyl group are located in axial positions. For these
five monomeric structures, two features in common could be
observed. One is that the distances between nickel and coor-
dinated nitrogens have the same trend: Ni(1)-N(3) >
Ni(1)-N(1) > Ni(1)-N(2); the other is that the angles
N(1)-Ni(1)-N(2) and N(2)-Ni(1)-N(3) are hardly effected
by the halogen and R1 substituents.
The molecular structure of 4a is shown in Figure 3. Although
the Me substituent in complex 4a is not very bulky, a monomeric
structure could also be observed. The N(2)-C(11) bond distance
is 1.272(5) Å, displaying clear CdN double-bond character. The
Ni-N bonds are different from each other. The shortest bond,
at 1.980(4) Å, is found for the imine-N (Ni(1)-N(2)), followed
by the quinoline-N with a Ni(1)-N(1) of 2.112(4) Å and the
pyridine-N with a Ni(1)-N(3) of 2.140 Å. This trend is observed
for all following monomeric structures. The Ni center deviates
by 0.032 Å out of the equatorial plane. The axial plane is almost
vertical to the equatorial plane, with a dihedral angle of 88.6°.
Like the previously discussed structures, the ligand backbone
Figure 2. Molecular structure of 11a. Thermal ellipsoids are drawn
at the 30% probability level. Hydrogen atoms are omitted for clarity.

Ethylene Oligomerization by Tridentate Ni Complexes
Organometallics, Vol. 26, No. 19, 2007 4785
Figure 3. Molecular structure of 4a, with thermal ellipsoids at
the 30% probability level. Hydrogen atoms and solvent are omitted
for clarity.
Figure 5. Molecular structure of 10a, with thermal ellipsoids at
the 30% probability level. Hydrogen atoms and solvent are omitted
for clarity.
Table 4. Oligomerization of Ethylene with 6a/Et2AlCla
oligomer distribution(%)
T
P
activity (105 g
entry Al/Ni (°C) (atm)
C4
R-C4
C6
mol-1(Ni) h-1
)
1
2
3
4
5
6
7
8
9
200
500
700
1000
500
500
500
500
500
20
20
20
20
20
20
40
60
80
30
30
30
30
10
20
30
30
30
96.9
92.9
96.7
97.9
96.8
95.0
94.9
93.3
97.9
96.2
91.2
93.4
54.9
87.6
94.1
91.2
92.3
80.0
3.1
7.1
3.3
2.1
3.2
5.0
5.1
6.7
2.1
8.3
33.9
14.0
9.2
3.7
17.6
15.6
12.7
6.8
a General conditions: 5 µmol of complex; 100 mL of toluene; 20 min.
2.3. Ethylene Oligomerization. The nickel(II) complexes
1a-12a and 1b-12b were systematically investigated for
ethylene oligomerization. Various aluminum activators were
scanned as cocatalysts to activate the title nickel complexes for
ethylene reactivity. The best results were obtained using
diethylaluminum chloride (Et2AlCl) as cocatalyst with catalytic
activities on the order of 105-106 g mol-1(Ni) h-1. Therefore,
Et2AlCl was used as an activator for further investigations.
Dimerization with moderate to high selectivity for R-C4
dominates along with minor amounts of trimers. Compared with
a typical catalyst (Cy3P)2NiCl23d for ethylene dimerization, the
current catalytic systems herein showed much higher selectivity
for R-C4. There were several isomers of hexenes detected with
low content of 1-hexene (<15%). Because of the low amount
of C6 (<10% of total oligomers), the individual isomers of
hexenes could not be quantified accurately. According to the
mechanism proposed by Braunstein,8 the formation of various
hexenes could be inferred by reincorporation of butene.
2.3.1. Effects of Reaction Parameters on Catalytic Behav-
ior. Considering the ethylene reactivity, the reaction parameters
including the ratio of Et2AlCl to nickel complex, pressure of
ethylene, and reaction temperature affect the catalytic activities.
The influence of different reaction parameters on ethylene
oligomerization was investigated in detail with complexes 6a
and 1b. The oligomerization results using complexes 6a and
1b are summarized in Tables 4 and 5, respectively. As revealed
in Tables 4 and 5, the reaction parameters significantly affect
the catalytic activity; however, no obvious influence on the
product distribution could be observed.
Figure 4. Molecular structure of 6a, with thermal ellipsoids at
the 30% probability level. Hydrogen atoms are omitted for clarity.
of 4a is not planar, having a dihedral angle of 7.3° between
quinoline and the pyridine ring.
The molecular structure of 6a is displayed in Figure 4.
Comparing bond lengths and angles of these complexes (Table
3 and Supporting Information), the influence of the halide
seems to be rather minimal. The Ni-N bonds show the same
trend as observed for 4a, and i-Pr makes the Ni-N(1) bond
slightly longer than 4a. The angle between the two axially
positioned N atoms (N(1)-Ni(1)-N(3)) shows only a small
deviation (157.4(1)°). The axial plane is almost vertical to
the equatorial plane, with the dihedral angle of 90.4°. The
dihedral angle measured between the pyridine and the quinoline
is 3.6°. Its bromide analogue 6b was also determined with
single-crystal X-ray analysis and showed the same structural
features as 6a.
The molecular structure of 10a is displayed in Figure 5. With
further increase in the steric bulk of the substituent at the R1
position the same trend in bond lengths and angles can be
observed as in 4a and 6a (Table 3). For 10a, however, the
Ni-N(1) bond is even shorter than 4a, and a chair conformation
could be observed for the cyclohexyl group. The ligand is not
perfectly coplanar, with a dihedral angle between the pyridine
and the quinoline ring of 2.5°. The Ni atoms deviate slightly
out of the equatorial plane spanned by X(1)-N(2)-X(2) (0.015
Å, 10a). The axial plane is almost vertical to the equatorial
plane, with a dihedral angle of 89.4°. The single-crystal X-ray
analysis revealed that 10b has the same structural features as
its analogue 6a.
With complex 6a, the amount of Et2AlCl played an important
role in the catalytic properties. Increasing the Al/Ni molar ratio

4786 Organometallics, Vol. 26, No. 19, 2007
Sun et al.
Table 5. Oligomerization of Ethylene with 1b/Et2AlCla
Table 6. Ethylene Oligomerization with 1a-12a and
1b-12b/Et2AlCla
oligomer distribution (%)
T
P
activity (105 g
mol-1(Ni) h-1
)
oligomer distribution (%)
activity (105 g
entry Al/Ni (°C) (atm)
C4
R-C4
C6
entry
cat.
C4
R-C4
C6
mol-1(Ni) h-1
)
1
2
3
4
5
6
7
8
9
200 20
500 20
700 20
1000 20
500 20
500 20
500 40
500 60
500 80
30
30
30
30
10
20
30
30
30
96.9
95.9
96.3
95.6
95.8
95.2
93.4
92.7
92.9
90.5
93.8
70.9
84.5
90.3
72.1
86.7
69.1
64.1
3.1
4.1
3.7
4.4
4.2
4.8
6.6
7.3
7.1
16.0
47.2
23.5
15.4
6.6
30.1
25.7
19.1
6.9
1
2
3
4
5
6
7
8
1a
2a
3a
4a
5a
6a
7a
8a
97.5
98.6
96.2
98.3
96.3
92.9
96.4
98.0
97.9
95.9
95.2
94.8
96.7
95.2
97.6
96.5
94.8
96.8
96.3
96.6
95.2
96.3
97.7
96.6
82.4
92.9
89.4
85.6
91.4
91.2
83.3
94.4
88.5
94.1
83.7
89.2
93.8
89.6
91.8
88.0
92.4
91.9
91.1
91.7
92.9
90.6
92.7
94.8
2.5
1.4
3.8
1.7
3.7
7.1
3.6
2.0
2.1
4.1
4.8
5.2
3.3
4.8
2.4
3.5
5.2
3.2
3.7
3.4
4.8
3.7
2.3
3.4
41.2
20.3
6.8
7.7
19.2
33.9
9.2
20.1
5.0
9
9a
a General conditions: 5 µmol of complex; 100 mL of toluene; 20 min;
Et2AlCl.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
10a
11a
12a
1b
2b
3b
4b
5b
6b
7b
8b
9b
10b
11b
12b
14.1
21.4
10.7
47.2
26.6
15.2
13.7
22.1
21.9
25.8
22.9
15.1
13.8
35.0
18.5
from 200 to 500 (entries 1 and 2 in Table 4), the highest activity
(33.9 × 105 g mol-1(Ni) h-1, entry 2 in Table 4) was observed
at an Al/Ni ratio of 500. However, when the Al/Ni molar ratio
was further increased (entries 2-4 in Table 4), the activity
decreased.
Ethylene oligomerization of complex 6a was also carried out
at different pressures (entries 1, 5, and 6 in Table 4). As
expected, the catalytic activities predominantly decreased at
lower ethylene pressure, due to lower monomer concentration
at lower pressure.
The reaction temperature significantly affects the catalytic
activity. Elevation of the temperature from 20 °C to 80 °C
(entries 2 and 7-9 in Table 4) led to a significant decline in
activity, probably due to the decomposition of the catalysts and
lower ethylene solubility at higher temperature.
A similar influence was also observed with complex 1b.
Different reaction temperature, amount of cocatalyst, and
pressure of ethylene also affected its catalytic reactivity. The
Al/Ni molar ratio was varied from 200 to 1000. However, the
optimum Al/Ni molar ratio for ethylene oligomerization was
fixed at 500. Elevating the reaction temperature from 20 °C to
80 °C resulted in decreased catalytic activity, and lower catalytic
activity was observed at lower pressure.
2.3.2. Effect of the Ligand Environment on the Catalytic
Behavior. Complexes 1a-12a and 1b-12b were examined for
oligomerization of ethylene at 30 atm, and the results are
summarized in Table 6. The substitution pattern of the ligand
plays a major role in the performance of the catalysts.
The effect of the substitution pattern of the R2 substituent on
the imino-C of the ligands is significant. Varying the electron-
donating properties of R2 (entries 1, 11, 12, X ) Cl, and 13,
23, 24, X ) Br, in Table 6), the highest activities were observed
when R2 ) H (entries 1 and 13 in Table 6). However, with R2
) Me, (entries 11 and 23 in Table 6) the observed activities
decreased. On changing R2 to a stronger electron-donating
phenyl group (entries 12 and 24 in Table 6), the activity further
decreased.
a General conditions: 5 µmol of complex; 100 mL of toluene; 20 min;
20 °C; Et2AlCl, Al/Ni ) 500; 30 atm.
bulky substituents, leading to a decline in activity, while on
the other hand, these bulky substituents are protecting the active
center against decomposition, affording higher activities. Hence,
the overall effect depends on the type of substituent.
As to the product distribution, it is surprising to observe that
the substituents have a minimal influence. In the total amount
of olefins formed in the oligomerization reactions, C4 con-
tent varied from 92.9% (entry 6 in Table 6) to 98.6% (entry
2 in Table 6) when different complexes were evaluated
under identical reaction conditions. For other tridentate
N∧N∧N Ni complexes, similar results had been observed
before.11d,f,12a
The influence of the halide anion linked to the nickel center
could be evaluated by comparing entries 1 to 12 with 13 to 24.
In most cases, the dibromonickel complex shows higher activity
than their dichloronickel analogue, and it could be explained
by the better solubility of the bromo compound than the chloro
analogue.
2.3.3. Effects of R3P as Auxiliary Ligand on the Catalytic
Behavior. For a long time the influence of the auxiliary
phosphine ligand in SHOP-type catalysts on activity and
selectivity was believed to be minor. The phosphine was thought
to be necessary only to stabilize the nickel center.4a In fact,
phosphine ligands play an important role in different catalytic
systems. One of the initiating steps in the oligomerization of
ethylene catalyzed by SHOP-type catalysts is the generation of
the active species via the dissociation of the phosphine ligand
from the hydride species, forming an unstable 14 e- species
that acts as the active species for ethylene oligomerization. Chain
growth can occur via coordination and insertion of ethylene.
The Braunstein group systematically investigated the effect of
different phosphines on the catalytic activity of the SHOP
catalyst, and similar observations concerning the influence on
the generation of the active species were made.4b The nature of
the phosphine mainly influences activity and length of the
resulting oligomers. Weakly coordinating phosphines such as
Ph3P and Tol3P produce higher molecular weight oligomers,
whereas catalysts containing strongly coordinating phosphines
The substituents R1 and R3 have considerable effects on the
catalytic behaviors. The highest activities of 41.2 × 105 (entry
1 in Table 6) and 47.2 × 105 g mol-1(Ni) h-1 (entry 13 in Table
6) were obtained with the least bulky complexes 1a and 1b.
However, increasing the steric bulk (complexes 2a to 4a, 2b to
4b) at the Me substituent resulted in a decreased activity (entries
2-4 and 14-16 in Table 6). A possible explanation is that the
active site of the catalyst is blocked through bulky substituents,
leading to a decline in activities.
Furthermore, on changing the R1 substituent from Me to i-Pr,
t-Bu, or Cy while keeping R3 ) Me (entries 4, 6, 8, and 10 and
16, 18, 20, and 22 in Table 6), higher activities were observed.
A possible explanation could be found in two opposing effects.
One is that the active site of the catalyst is blocked through

Ethylene Oligomerization by Tridentate Ni Complexes
Organometallics, Vol. 26, No. 19, 2007 4787
Table 7. Oligomerization of Ethylene with R3Pa
Table 8. Oligomerization of Ethylene with the 1b/Et2AlCl/
R3P Systema
oligomer distribution (%)
t
activity (107 g
mol-1(Ni) h-1
)
oligomer distribution (%)
activity (104 g
entry cat.
R3P
1a Ph3P
1b Ph3P
10a Ph3P
12a Ph3P
1a Cy3P
1b Cy3P
1b Cy3P
(min)
C4
R-C4
C6
entry
R3P
Ph3P
C4
R-C4
C6
mol-1(Ni) h-1
)
1
2
3
4
5
6
7
8
9
20
20
20
20
20
20
60
91.3
90.2
91.2
93.3
86.5
91.7
95.4
95.5
94.1
20.5
13.8
18.4
17.1
86.2
75.9
34.6
82.4
85.8
8.7
9.8
8.8
6.7
13.5
8.3
4.6
4.5
5.9
2.9
2.5
2.3
3.1
4.9
4.7
2.2
0.2
0.1
1b
2
100
97.3
61.5
6.2
6.3
70
2.7
3
4
Ph2MeP
PhMe2P
100
100
4.4
39.2
5.1
1.2
a General conditions: 5 µmol of complex; 1 atm; 30 mL of toluene; 20
b
min; 20 °C; Et2AlCl, Al/Ni ) 500; 25 equiv of R3P. R3P was not used.
1a (C6F5)3P 20
1b (C6F5)3P 20
Table 9. Oligomerization of Ethylene with the 1b/Et2AlCl/
R3P Systema
a General conditions: 5 µmol of complex; 100 mL of toluene; 20 °C;
Et2AlCl, Al/Ni ) 500; 30 atm; 25 equiv of R3P.
oligomer distribution (%)
activity (105 g
entry
R3P
t (min)
C4
R-C4
C6
mol-1(Ni) h-1
)
such as Me3P and PhMe2P produce low molecular weight
oligomers with significantly lower activities. By using phosphine
scavengers such as Ni(COD)2 even the production of polyeth-
ylene is possible. In the mechanism proposed by Braunstein,
recoordination of phosphine leads to chain termination via
â-hydride elimination and re-formation of the initial 16 e-
hydride species.4b Matt and Monteiro recently suggested that a
second termination mechanism is likely to occur.4f Matt
proposed a five-coordinated 18 e- ethylene/R3P intermediate
that undergoes chain termination via â-hydrogen transfer to the
coordinated olefin. Carlini reported a bis(salicylaldiminate)-
nickel(II) system that in the presence of MAO and auxiliary
phosphines shows higher activities for ethylene oligomerization
compared to the phosphine-free system, whereas the product
distribution turns to lower oligomers and the selectivity for
R-olefin decreases.5c However, to the best of our knowledge,
the influence of different R3P as auxiliary ligands on tridentate
nickel systems has not been reported. To get a better under-
standing of the actual influence, in our system the following
phosphines have been used: Cy3P, Ph3P, (C6F5)3P, Ph2MeP,
and PhMe2P. These phosphines differ in their electronic
properties, as well in their steric properties.
Using 25 equiv of Ph3P, the activities of the complexes in-
creased significantly, reaching a high activity level of 107 g
mol-1(Ni) h-1, which is much higher than the catalytic systems
without Ph3P (entries 1-4 in Table 7). For example, with com-
plex 12a more than 29 times higher activity was observed (3.1
× 107 g mol-1(Ni) h-1). However, the selectivity for R-C4
decreased sharply. This is consistent with previous reports by
other groups4d,5c and could be attributed to a higher isomerization
rate.
1
2
3
4
5
6
7
8
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
Cy3P
Cy3P
Cy3P
Cy3P
Cy3P
Cy3P
5
10
20
40
60
90
5
10
20
40
60
90
100
98.8
37.6
17.1
6.2
5.4
5.0
0
1.2
2.7
4.1
5.5
5.8
4.9
9.4
11.7
12.2
13.7
14.3
3.0
9.6
7.0
5.0
3.5
2.4
6.1
9.8
10.7
7.4
5.4
3.6
97.3
95.9
94.5
94.2
95.1
90.6
88.3
87.8
86.3
85.7
5.0
78.5
39.8
15.9
10.6
8.4
9
10
11
12
7.1
a General conditions: 5 µmol of complex; 30 mL of toluene; 20 °C;
Et2AlCl, Al/Ni ) 500; 1 atm; 25 equiv of R3P.
in Table 8. The results we got for Ph3P, Ph2MeP, and PhMe2P
are in line with the observations made by Braunstein4b and
Keim4d for the SHOP catalyst and by Grubbs for the metathesis
catalyst.18 It was concluded that the coodination of the phosphine
ligand with higher basicity to the metal center becomes so strong
that the initiation step, the dissociation of the phosphine,
becomes difficult and only small amounts of active species are
formed. For our N∧N∧N nickel system, the declining activities
of the complexes were observed with increasing the basicity of
their phosphine ligands; Ph2MeP and PhMe2P showed less
activity compared with a phosphine-free system (entries 1-4
in Table 8), and a stronger binding of the more basic phosphines
to the nickel center blocked coordination of ethylene on the
active species. Although Cy3P itself shows a more basic property
(pKa is 9.7), better catalytic activities were obtained (entries
5-7 in Table 7). It could be assumed that the bulkiness of the
Cy3P ligand has a major influence due to the nonplanar ring
Cy group and the existing tridentate N∧N∧N ligand, the
combined effect resulting in a relative weak bonding of Cy3P
with the nickel center along with the tendency for stronger
bonding of the phosphine to the nickel center. The presence of
Cy3P could stabilize active species and prepare space for
ethylene coordination; therefore, the catalytic activities were
increased. Similar observations were also obtained by another
group.4g
By replacing Ph3P with PCy3 even higher activities could be
obtained (entries 5-7 in Table 7). For complex 1a, its activity
is as high as 4.9 × 107 g mol-1(Ni) h-1, one of the highest
results ever reported. For complex 1b, a similar trend is also
observed here when comparing the data between entry 2 and
entry 6, and 1b still remains active even after 1 h (entry 7 in
Table 7). The reason could be attributed to the bulkiness of the
Cy group, which results in easy dissociation of the metal-
phosphorus bond.4d,e,g
Furthermore, the effect of auxiliary ligand on the catalyst life-
time was also investigated. The selectivity and catalyst activity
were monitored versus time in the 1b/Et2AlCl/Ph3P and
1b/Et2AlCl/Cy3P systems at 1 atm, and the results are sum-
marized in Table 9. For both cases, activities remained high
over a 5-20 min period, indicating a rather long lifetime and
good thermal stability of the catalytic species. Moreover, the
content of C6 increased gradually along with reaction time due
to plausible reincorporation of 1-butene. However, the selectivity
for R-C4 decreased with time, indicating a high isomerization
rate.
The results we got by using electron-deficient (C6F5)3P as
ancillary ligands are not what we expected. Due to its electron
deficiency, the basicity of (C6F5)3P is low and supposed to result
in much higher activities; however, the system hardly exerts
an acceleration effect for the oligomerization of ethylene (entries
8 and 9 in Table 7). Perhaps (C6F5)3P contains electron-
withdrawing fluoro substituents; it is difficult for (C6F5)3P to
donate its electrons to coordinate with the metal center.
To evaluate the influence of basicity of R3P, a series of
phosphines such as Ph3P, Ph2MeP, and PhMe2P were employed
in a catalytic reaction at 1 atm, and the results are summarized

4788 Organometallics, Vol. 26, No. 19, 2007
Sun et al.
1630, 1597, 1584, 1504, 1463, 1401, 1380, 1321, 1254, 1135, 1082,
1035, 1005, 834, 787.
3. Conclusions
A series of tridentate (N∧N∧N) nickel complexes of N-((py-
ridin-2-yl)methylene)quinolin-8-amine derivatives have been
synthesized and fully characterized. Due to the instability of
the ligands, the complexes were synthesized through a metal-
induced template reaction. X-ray crystallographic analysis
revealed that these nickel complexes could form dimeric or
monomeric structures. It was further confirmed that the sub-
stituents, instead of the halides, determine the formation of
dimeric or monomeric structures. Upon activation with Et2AlCl,
all nickel complexes exhibited considerably high catalytic
activity for ethylene oligomerization with dimers and trimers
as products. The selectivity for R-C4 is moderate to high. Both
substituents and catalytic reaction parameters significantly affect
the catalytic activity. In the presence of 500 equiv of Et2AlCl,
complex 1b showed a high activity of 4.7 × 106 g mol-1(Ni)
h-1. In addition, the influence of different R3P was investigated,
and their electronic and steric properties have a comparable
effect on the catalytic behavior. When Ph3P or Cy3P was
employed as an auxiliary ligand, the activities increased up to
Complex 4a was obtained as a brown powder in 88.4% yield.
Anal. Calcd for C17H15Cl2N3Ni‚2H2O: C, 51.05; H, 4.03; N, 10.51.
Found: C, 50.79; H, 4.00; N, 10.33. IR (KBr; cm-1): 3363, 3190,
1621, 1593, 1562, 1504, 1469, 1428, 1376, 1317, 1256, 1214, 1146,
1104, 1069, 1009, 834, 787.
Complex 5a was obtained as a brown powder in 51.5% yield.
Anal. Calcd for C18H17Cl2N3Ni‚2.5H2O: C, 48.04; H, 4.93; N, 9.34.
Found: C, 48.13; H, 4.96; N, 9.23. IR (KBr; cm-1): 3366, 2966,
1620, 1598, 1572, 1506, 1471, 1375, 1223, 1149, 1042, 1016, 843,
770.
Complex 6a was obtained as a brown powder in 47.6% yield.
Anal. Calcd for C19H19Cl2N3Ni: C, 54.47; H, 4.57; N, 10.03.
Found: C, 54.97; H, 4.59; N, 10.08. IR (KBr; cm-1): 3438, 3021,
2967, 2883, 1621, 1593, 1573, 1504, 1461, 1383, 1253, 1217, 1098,
1037, 861, 844, 774.
Complex 7a was obtained as a purple powder in 88.1% yield.
Anal. Calcd for C19H19Cl2N3Ni‚3H2O: C, 48.24; H, 5.33; N, 8.88.
Found: C, 47.82; H, 5.06; N, 8.65. IR (KBr; cm-1): 3350, 2963,
1602, 1507, 1475, 1443, 1366, 1233, 1132, 1014, 848, 766.
Complex 8a was obtained as a purple powder in 84.9% yield.
Anal. Calcd for C20H21Cl2N3Ni‚H2O: C, 53.26; H, 5.14; N, 9.32.
Found: C, 52.79; H, 4.84; N, 8.86. IR (KBr; cm-1): 3396, 2963,
1623, 1597, 1561, 1503, 1470, 1365, 1255, 1163, 1127, 1012, 844,
768.
be 107 g mol-1(Ni) h-1
.
4. Experimental Section
4.1. General Considerations. All manipulations of air- and/or
moisture-sensitive compounds were carried out under an atmosphere
of nitrogen using standard Schlenk techniques. Solvents were
refluxed over an appropriate drying agent, distilled, and degassed
before using. An Et2AlCl (1.90 mol L-1) solution in toluene was
purchased from Acros Chemicals. [(DME)NiBr2]19 and substituted
8-nitroquinolines14 were prepared according to literature procedures.
High-purity ethylene was purchased from Beijing Yansan Petro-
chemical Co. and used as received. Other reagents were purchased
from Aldrich, Acros, or local suppliers. Elemental analyses were
performed on a Flash EA 1112 microanalyzer. IR spectra were
recorded on a Perkin-Elmer System 2000 FT-IR spectrometer using
KBr discs in the range 4000-400 cm-1. The 1H NMR spectra were
recorded on a Bruker DMX-300 instrument with TMS as the
internal standard. GC analysis was performed with a Varian CP-
3800 gas chromatograph equipped with a flame ionization detector
and a 30 m (0.25 mm i.d., 0.25 µm film thickness) CP-Sil 5 CB
column.
4.2. Synthesis of Complexes 1a-12a and 1b-12b. Complexes
1a-12a were prepared in a similar manner by using NiCl2‚6H2O.
A suspension of pyridine-2-aldehyde or ketone (1.00 mmol),
8-aminoquinoline (1.00 mmol), and NiCl2‚6H2O (1.00 mmol) in
glacial acetic acid (10 mL) was refluxed for 4 h. The precipitate
was collected by filtration and washed with diethyl ether (3 × 5
mL). Then the collected solid was redissolved in methanol,
concentrated, and precipitated with diethyl ether. After washing
with diethyl ether the collected solid was dried under vacuum.
Complex 1a was obtained as a yellow powder in 55.1% yield.
Anal. Calcd for C15H11Cl2N3Ni‚1.5H2O: C, 46.21; H, 3.62; N,
10.78. Found: C, 46.15; H, 3.51; N, 10.85. IR (KBr; cm-1): 3323,
3127, 1627, 1598, 1584, 1506, 1474, 1402, 1380, 1323, 1136, 1080,
1031, 1009, 831, 769.
Complex 9a was obtained as a brown powder in 81.3% yield.
Anal. Calcd for C21H21Cl2N3Ni‚1.5H2O: C, 53.43; H, 5.12; N, 8.90.
Found: C, 53.66; H, 5.02; N, 9.02. IR (KBr; cm-1): 3327, 2927,
2850, 1620, 1596, 1571, 1502, 1475, 1446, 1375, 1299, 1259, 1221,
1150, 1104, 1025, 997, 839, 769, 607, 560, 500, 423, 375.
Complex 10a was obtained as a brown powder in 65.4% yield.
Anal. Calcd for C22H23Cl2N3Ni‚MeOH: C, 56.25; H, 5.54; N, 8.56.
Found: C, 56.59; H, 5.64; N, 8.53. IR (KBr; cm-1): 3434, 2926,
2851, 1620, 1594, 1570, 1503, 1445, 1382, 1255, 1160, 1032, 996,
838, 794, 768.
Complex 11a was obtained as a brown powder in 74.6% yield.
Anal. Calcd for C16H13Cl2N3Ni‚2H2O: C, 46.54; H, 4.15; N, 10.18.
Found: C, 46.87; H, 3.85; N, 9.83. IR (KBr; cm-1): 1612, 1594,
1498, 1468, 1439, 1367, 1324, 1307, 1256, 838, 781.
Complex 12a was obtained as a brown powder in 61.2% yield.
Anal. Calcd for C21H15Cl2N3Ni‚2H2O: C, 53.10; H, 4.03; N, 8.85.
Found: C, 53.52; H, 3.81; N, 8.76. IR (KBr; cm-1): 1613, 1592,
1497, 1465, 1441, 1383, 1324, 1266, 1247.
Complexes 1b-12b were prepared in a similar manner by using
(DME)NiBr2. A suspension of pyridine-2-aldehyde or ketone (1.00
mmol), 8-aminoquinoline (1.00 mmol), and (DME)NiBr2 (1.00
mmol) in glacial acetic acid (20 mL) was refluxed for 4 h. The
precipitate was collected by filtration and washed with diethyl ether
(3 × 5 mL). Then the collected solid was redissolved in methanol,
concentrated, and precipitated with diethyl ether. After washing
with diethyl ether the collected solid was dried under vacuum.
Complex 1b was obtained as a yellow powder in 83.2% yield.
Anal. Calcd for C15H11Br2N3Ni: C, 39.88; H, 2.45; N, 9.30.
Found: C, 39.56; H, 2.64; N, 8.95. IR (KBr; cm-1): 3039, 1622,
1593, 1562, 1501, 1472, 1426, 1392, 1318, 1256, 1214, 1134, 1101,
1069, 1014, 830, 776.
Complex 2a was obtained as a yellow powder in 74.0% yield.
Anal. Calcd for C16H13Cl2N3Ni‚2.5H2O: C, 45.55; H, 4.30; N, 9.96.
Found: C, 45.54; H, 4.02; N, 10.02. IR (KBr; cm-1): 3361, 3181,
1622, 1598, 1584, 1504, 1468, 1428, 1389, 1372, 1317, 1260, 1238,
1212, 1143, 1103, 1066, 1009, 834, 787.
Complex 2b was obtained as a yellow powder in 89.1% yield.
Anal. Calcd for C16H13Br2N3Ni‚H2O: C, 39.72; H, 3.13; N, 8.69.
Found: C, 39.70; H, 3.27; N, 8.62. IR (KBr; cm-1): 3361, 3181,
1622, 1598, 1584, 1504, 1468, 1428, 1389, 1372, 1317, 1260, 1238,
1212, 1143, 1103, 1066, 1009, 840, 773.
Complex 3a was obtained as a brown powder in 87.6% yield.
Anal. Calcd for C16H13Cl2N3Ni‚2H2O: C, 46.54; H, 4.15; N, 10.18.
Found: C, 46.21; H, 4.15; N, 10.17. IR (KBr; cm-1): 3313, 3198,
Complex 3b was obtained as a yellow powder in 79.2% yield.
Anal. Calcd for C16H13Br2N3Ni‚H2O: C, 39.72; H, 3.13; N, 8.69.
Found: C, 39.89; H, 3.26; N, 8.75. IR (KBr; cm-1): 3105, 1624,
1594, 1568, 1503, 1471, 1427, 1398, 1319, 1252, 1213, 1136, 1110,
1076, 1006, 828, 779.
(19) Cotton, F. A. Inorg. Synth. 1971, 13, 160-164.

Ethylene Oligomerization by Tridentate Ni Complexes
Organometallics, Vol. 26, No. 19, 2007 4789
Table 10. Summary of Crystallographic Data for 3b, 4a, 6a, 6b, 10a, 10b, 11a, and 12a
3b‚3H2O
4a‚H2O
6a
6b
empirical formula
fw
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C32H26Br4N6Ni2O6
1027.65
monoclinic
P2(1)/n
11.344(2)
11.117(2)
15.401(3)
90
99.16(3).
90
1917.6(7)
2
C17H15Cl2N3NiO
406.93
triclinic
P1h
7.4690(15)
9.6646(19)
12.041(2)
98.98(3)
95.52(3)
100.84(3)
836.1(3)
2
C19H19Cl2N3Ni
418.98
monoclinic
P2(1)/n
11.434(2)
13.312(3)
13.038(3)
90
111.92(3)
90
1841.0(6)
4
C19H19Br2N3Ni
507.9
monoclinic
P2(1)/n
11.781(2)
13.462(3)
13.021(3)
90
111.39(3)
90
1922.7(7)
4
Z
D
calcd (g cm-3
)
1.780
5.198
296(2)
1008
1.616
1.488
296(2)
416
1.512
1.350
296(2)
864
1.755
5.172
296(2)
1008
µ (mm-1
T (K)
)
F(000)
θ range (deg)
no. of rflns collected
2.08-25.01
3175
1.73-27.48
6659
2.28-27.47
8165
2.40-27.48
4409
no. of unique rflns
3175
1.038
3617
1.02
4206
1.061
4409
0.965
goodness-of-fit on F2
R
Rw
0.0955
0.2570
0.0548
0.1376
0.0467
0.109
0.0593
0.1105
10a‚MeOH
10b
11a‚2MeOH
12a‚2MeOH
empirical formula
fw
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C23H26Cl2N3NiO
490.08
triclinic
P1h
9.964(2)
10.764(2)
13.397(3)
97.85(3)
105.45(3)
117.28(3)
1173.5(4)
2
C22H23Br2N3Ni
547.96
orthorombic
Pbca
15.456(3)
14.705(3)
19.167(4)
90
C18H21Cl2N3NiO2
440.99
triclinic
P1h
7.667(3)
9.898(4)
13.088(5)
86.279(12)
84.640(10)
72.493(8)
942.3(6)
2
C44H36Cl4N6Ni2O2
940.01
monoclinic
P2(1)/n
8.7471(17)
21.063(4)
12.361(5)
90
112.42(2)
90
2105.3(10)
2
90
90
4356.3(15)
8
1.671
4.572
296(2)
2192
Z
D
calcd (g cm-3
)
1.387
1.073
296(2)
510
1.554
1.331
296(2)
456
1.483
1.193
296(2)
964
µ (mm-1
T (K)
)
F(000)
θ range (deg)
no. of rflns collected
2.23-27.47
8289
2.19-27.48
4990
2.16-27.48
7349
2.63-27.32
8055
no. of unique rflns
5275
1.092
4990
0.77
4253
1.075
4610
1.096
goodness-of-fit on F2
R
Rw
0.0624
0.1645
0.0328
0.0392
0.0387
0.0758
0.1069
0.2664
Complex 4b was obtained as a brown powder in 80.5% yield.
Anal. Calcd for C17H15Br2N3Ni: C, 42.55; H, 3.15; N, 8.76.
Found: C, 42.41; H, 3.21; N, 8.61. IR (KBr; cm-1): 3367, 3193,
1619, 1593, 1562, 1504, 1469, 1428, 1376, 1317, 1256, 1214, 1146,
1104, 1069, 1009, 848, 764.
Complex 5b was obtained as a brown powder in 72.3% yield.
Anal. Calcd for C18H17Br2N3Ni: C, 43.78; H, 3.47; N, 8.51.
Found: C, 43.38; H, 3.62; N, 8.46. IR (KBr; cm-1): 3367, 3051,
2962, 1619, 1597, 1571, 1505, 1374, 1223, 1040, 841, 769.
Complex 6b was obtained as a brown powder in 79.7% yield.
Anal. Calcd for C19H19Br2N3Ni: C, 44.93; H, 3.77; N, 8.27.
Found: C, 45.46; H, 3.96; N, 8.18. IR (KBr; cm-1): 3394, 2962,
1614, 1600, 1556, 1506, 1474, 1327, 1183, 789.
Complex 7b was obtained as a purple powder in 83.2% yield.
Anal. Calcd for C19H19Br2N3Ni‚MeOH: C, 44.49; H, 4.29; N, 7.78.
Found: C, 44.34; H, 4.68; N, 8.19. IR (KBr; cm-1): 3363, 2966,
1623, 1600, 1577, 1467, 1369, 1233, 1140, 749.
Complex 8b was obtained as a purple powder in 89.4% yield.
Anal. Calcd for C20H21Br2N3Ni: C, 46.03; H, 4.06; N, 8.05.
Found: C, 46.35; H, 4.29; N, 7.71. IR (KBr; cm-1): 3394, 2962,
1614, 1600, 1556, 1506, 1474, 1327, 1163, 789.
2931, 2855, 1619, 1596, 1571, 1506, 1477, 1448, 1380, 1298, 1260,
1225, 1146, 1106, 1014, 998, 839, 768.
Complex 10b was obtained as a brown powder in 81.6% yield.
Anal. Calcd for C22H23Br2N3Ni: C, 48.22; H, 4.23; N, 7.67.
Found: C, 47.85; H, 4.25; N, 7.69. IR (KBr; cm-1): 2927, 2851,
1620, 1595, 1505, 1472, 1446, 1382, 1319, 1255, 1215, 1168, 1147,
1023, 997, 958, 842, 767.
Complex 11b was obtained as a yellow powder in 81.0% yield.
Anal. Calcd for C16H13Br2N3Ni‚H2O: C, 39.72; H, 3.13; N, 8.69.
Found: C, 39.42; H, 3.21; N, 8.68. IR (KBr; cm-1): 3273, 3062,
1595, 1500, 1473, 1445, 1369, 1262, 1248, 1021, 835, 759, 765.
Complex 12b was obtained as a yellow powder in 83.4% yield.
Anal. Calcd for C21H15Br2N3Ni‚1.5H2O: C, 45.46; H, 3.27; N, 7.57.
Found: C, 45.59; H, 3.22; N, 7.69. IR (KBr; cm-1): 3394, 3050,
1610, 1595, 1499, 1467, 1386, 1329, 1267, 1249, 839, 792, 769,
708.
4.3. Procedure for Oligomerization of Ethylene. Ethylene
oligomerization at 1 atm of ethylene pressure was carried out as
follows: the catalyst precursor was dissolved in toluene in a Schlenk
tube, and the reaction solution was stirred with a magnetic stir bar
under ethylene atmosphere (1 atm) with the reaction temperature
being controlled by a water bath. Et2AlCl and the solution of R3P
were added by a syringe. After the reaction was carried out for the
required period, the reactor was cooled in an ice-water bath. A
Complex 9b was obtained as a yellow powder in 89.3% yield.
Anal. Calcd for C21H21Br2N3Ni‚0.5H2O: C, 46.46; H, 4.08; N, 7.74.
Found: C, 46.72; H, 4.05; N, 7.68. IR (KBr; cm-1): 3342, 3058,

4790 Organometallics, Vol. 26, No. 19, 2007
Sun et al.
small amount of the reaction solution was collected and terminated
by the addition of 10% aqueous hydrogen chloride. The organic
layer was analyzed by gas chromatography (GC) for determining
the composition and mass distribution of oligomers obtained.
High-pressure ethylene oligomerization was performed in a
stainless steel autoclave (0.5 L capacity) equipped with 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 to the fully dried reactor under
a nitrogen atmosphere. The required amount of cocatalyst (Et2AlCl)
was then injected into the reactor using a syringe. As the prescribed
temperature was reached, the reactor was pressurized to the desired
pressure. After the reaction mixture was stirred for the desired
period of time, the reaction was stopped without ethylene inputting.
The autoclave was cooled in an ice-water bath, then the pressure
was released. A small amount of the reaction solution was collected
and terminated by the addition of 10% aqueous hydrogen chloride.
The organic layer was analyzed by gas chromatography (GC) for
determining the composition and mass distribution of oligomers
obtained.
Mo KR radiation (λ ) 0.71073 Å) at 296(2) K. Cell parameters
were obtained by global refinement of the positions of all col-
lected 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.20 Crystal data and processing parameters for complexes
3b, 4a, 6a, 6b, 10a, 10b, 11a, and 12a are summarized in Table
10 while molecular structures and selected bond lengths and angles
of complexes 6b, 10b, and 12a are given in the Supporting
Information.
Acknowledgment. This project was supported by NSFC No.
20473099. K.W. was supported by a DAAD postdoctoral
fellowship.
Supporting Information Available: Crystallographic data for
3b, 4a, 6a, 6b, 10a, 10b, 11a, and 12a along with the descriptions
of molecular structures of 6b, 10b, and 12a. This material is
available free of charge via the Internet at http://pubs.acs.org.
4.4. X-ray Crystallographic Studies. Single-crystal X-ray
studies for complexes 3b, 4a, 6a, 6b, 10a, 10b, and 12a were carried
out on a Rigaku R-AXIS Rapid IP diffractometer with graphite-
monochromated Mo KR radiation (λ ) 0.71073 Å) at 296(2) K.
Intensity data for crystals of 11a were collected on a Bruker
SMART 1000 CCD diffractometer with graphite-monochromated
OM700440V
(20) Sheldrick, G. M. SHELXTL-97, Program for the Refinement of
Crystal Structures; University of Go¨ttingen: Germany, 1997.