Nickel(II) complexes chelated by 2-quinoxalinyl-6-iminopyridines: Synthesis, crystal structures and ethylene oligomerization

Article abstract of DOI:10.1016/j.jorganchem.2007.04.036

A series of nickel(II) complexes ligated by tridentate ligands of 2-quinoxalinyl-6-iminopyridines was synthesized and characterized by elemental and spectroscopic analysis as well X-ray diffraction analysis. X-ray crystallographic analysis revealed the nickel complexes as five-coordinated distorted trigonal bipyramidal geometry. In the presence of Et₂AlCl, these complexes displayed high catalytic activity for ethylene oligomerization and the dimmers were produced as main products. The nickel dibromide complexes exhibited relative higher activity than their dichloride analogues. Both elevation of the ethylene pressure and addition of auxiliary ligand have catalytic enhancement effects on all the complexes.

Full text of DOI:10.1016/j.jorganchem.2007.04.036

Journal of Organometallic Chemistry 692 (2007) 3532–3541
www.elsevier.com/locate/jorganchem
Nickel(II) complexes chelated by 2-quinoxalinyl-6-iminopyridines:
Synthesis, crystal structures and ethylene oligomerization
a
a
a
a
Sherrif Adewuyi , Gang Li a,b, Shu Zhang , Wenqing Wang , Peng Hao ,
a,
b
c
Wen-Hua Sun *, Ning Tang , Jianjun Yi
a
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, China
b
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
Petrochemical Research Institute, PetroChina Company Limited, No. 20 Xueyuan Road, Beijing 100083, China
c
Received 1 February 2007; received in revised form 20 April 2007; accepted 24 April 2007
Available online 1 May 2007
Abstract
A series of nickel(II) complexes ligated by tridentate ligands of 2-quinoxalinyl-6-iminopyridines was synthesized and characterized by
elemental and spectroscopic analysis as well X-ray diffraction analysis. X-ray crystallographic analysis revealed the nickel complexes as
five-coordinated distorted trigonal bipyramidal geometry. In the presence of Et2AlCl, these complexes displayed high catalytic activity
for ethylene oligomerization and the dimmers were produced as main products. The nickel dibromide complexes exhibited relative higher
activity than their dichloride analogues. Both elevation of the ethylene pressure and addition of auxiliary ligand have catalytic enhance-
ment effects on all the complexes.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Nickel complexes; 2-Quinoxalinyl-6-iminopyridines; Ethylene oligomerization
1. Introduction
[5,6]; 8-iminoquinoline [7] and anilinotropine [8] ligands.
More importantly, as a follow up to independent discovery
of bis(arylimino)pyridyl Fe(II) and Co(II) dihalides by
Brookhart [9] and Gibson [10], the zeal for designing five-
coordinated nickel(II) dihalides has been on the rise. By
careful modification of the ligand backbones, Ni(II) com-
plexes bearing tridentate chelates of PꢁNꢁN [11] and
NꢁNꢁN [12] models have been developed. Remarkably,
the ethylene oligomerization productivities of these models
varied from moderate to high.
In exploring new catalyst for ethylene oligomerization
in our laboratory with tridentate chelated model, nickel
(II) complexes were designed and investigated for their
catalytic performance bearing ligands such as carboxy-
late ester substituted monoiminopyridine (OꢁNꢁN) [13],
diphenylphosphinoanilines (PꢁNꢁP) [14], diarylphosphino-
quinoline-8-amines (PꢁNꢁN) [14], 2,9-bis(imino)-1,10-phe-
nanthroline (NꢁNꢁN) [15], 2-imino-1,10-phenanthroline
The selection of metal–ligand combinations suitable as
catalyst for ethylene oligomerization and/or polymeriza-
tion has been at the heart of the discovery process and
one of the most noteworthy advances in this regard has
been the increased prominence of late transition metal sys-
tems. After the pioneering work of Keim that formed the
basis of SHOP process [1]; the interest resurgence period
of Brookhart group (1995) that culminated diimine Ni-cat-
alyst system [2], many research efforts have centered on
four coordinated nickel (II) complexes bearing bidentate
ligands. Some examples in this regard include: diimine
[3]; imino-pyridine [4]; imino-phenol [5]; salicylaldiimine
*
Corresponding author.
E-mail address: whsun@iccas.ac.cn (W.-H. Sun).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.04.036

S. Adewuyi et al. / Journal of Organometallic Chemistry 692 (2007) 3532–3541
3533
(NꢁNꢁN) [16] and 2-benzimidazolylpyridines (NꢁNꢁN)
[17]. All these catalysts exhibited considerably good to
high activity for ethylene oligomerization upon activation
either with MAO and/or Et2AlCl. In furtherance of our
research in this field, synthesis and ethylene reactivity
study of new metal halides bearing 2-quinoxalinyl-6-imi-
nopyridine analogues with various ortho-positioned sub-
stituents on the aryl ring is being embarked upon. Their
iron complexes gave good catalytic activities for ethylene
reactivity (oligomerization and polymerization) upon
activation with methylaluminoxane (MAO), while their
cobalt analogues showed moderate activities toward ethyl-
ene oligomerization with modified methylaluminoxane
(MMAO) [18]. Successively, the nickel analogues were
synthesized and characterized. Owing to the large p-sys-
tem present in the ligand backbone, this is proposed to
increase the Lewis-acidity of the coordinated nickel center
by strong ability to accommodate negative charge. The
net effect is the stabilization of the catalyst active center
and consequently, rendering a positive impact on the cat-
alytic activity. Herein, we report the synthesis, molecular
structures and ethylene oligomerization capability of
Ni(II) complexes containing monoiminopyridine-quinox-
alinyl extended ligands.
spectra of the ligands show that the C@N stretching fre-
quencies of the ligands appear in the range 1638–
1646 cmÀ1 with strong intensities, however, in the com-
plexes (1–12), the C@N stretching frequencies shifted
toward lower values, between 1606 and 1631 cmÀ1 with
weak intensities. The results reveal the coordination inter-
action between the imino nitrogen atom and the nickel cen-
ter. In addition, the unambiguous molecular structures of
complexes 1, 3, 8, 9 and 12 were confirmed by the single-
crystal X-ray diffraction studies.
2.2. Molecular structures
Crystals of complexes 1, 3, 8, 9 and 12 suitable for X-ray
structural determination were obtained by crystallization
through the slow diffusion of diethyl ether into their dichlo-
romethane solution. Analysis of X-ray crystallography
reveals that all complexes are five-coordinated with the
geometry of distorted trigonal bipyramidal. Their selected
bond lengths and angles are collected in Table 1.
In the solid state of 1 (Fig. 1), the pyridyl nitrogen atom
and two chlorine atoms form the equatorial plane with the
equatorial plane angles range between 102.23(1)ꢂ and
129.15(8)ꢂ. The two axial Ni–N bonds subtend an angle
of 155.51(2)ꢂ, a distortion that is a consequence of satisfy-
ing the tridentate chelating constrains of the ligand. The
axial plane is oriented almost orthogonally (ca. 89.4ꢂ) to
the equatorial plane. All the atoms of pyridine ring and
the N3, C14 of the imino group as well as N1, C8 of the
quinoxalinyl group make an almost perfect plane with
2. Results and discussion
2.1. Synthesis and characterization
The compounds used as ligands L1–L6 were efficiently
synthesized according to our reported procedure [18]. The
dichloride nickel complexes (1–6) were synthesized by reac-
tions of NiCl2 Æ 6H2O with the ligands L1–L6 in ethanol
(Scheme 1). The mixture of NiCl2 Æ 6H2O with the corre-
sponding ligand was stirred at room temperature for
12 h, the resulting solution was concentrated and diethyl
ether was thereafter added to precipitate the product. The
precipitate was collected, washed several times with diethyl
ether and dried in vacuum. Meanwhile the dibromide
nickel complexes (9–12) were synthesized through the reac-
tion of ligands L1–L6 with (DME) NiBr2 in THF under
nitrogen atmosphere. All of these complexes were formed
by this method in excellent yields, and their structures were
confirmed by IR spectra and elemental analysis. The IR
˚
the mean deviation of 0.0457 A and the nickel atom lie
˚
0.1397 A out of this plane. The dimethylphenyl ring is ori-
ented almost orthogonally (82.7ꢂ) to this plane. The
˚
two axial Ni–N bond lengths, 2.144(4) A and 2.163(4)
˚
A are longer than that of the pyridyl nitrogen atom
˚
(1.965(4) A), a geometry similar to that observed in the
2-imino-1,10-Phenanthroline nickel complexes [16]. There
is a significant difference in the two Ni–Cl linkages, the
˚
Cl1 being about 0.081 A shorter than that of Cl2, which
could be attributed to the bipyramidal geometry where
the longer Ni–Cl bond was associated with the apical site.
There is no evidence of any bond delocalization involving
the imines, the C@N bond having typical double-bond
˚
character (1.283(6) A). In the solid state, complex 1 dis-
R1
.
NiCl2 6H2O in Ethanol
N
N
N
R1
or (DME)NiBr2 in THF
N
N
N
N
N
R1
R2
Ni
X
X
R1
R2
L1 - L6
1 - 12
1
2
3
4
5
6
7
8
9
10 11 12
R1 Me Et i-Pr Me F Cl Me Et i-Pr Me
F
Cl
H
R2
X
H
H
H
Me H
H
H
H
H
Me
H
Cl Cl Cl Cl Cl Cl Br Br Br Br Br Br
Scheme 1. Synthesis of the nickel complexes 1–12.

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S. Adewuyi et al. / Journal of Organometallic Chemistry 692 (2007) 3532–3541
Table 1
˚
Selected bond lengths (A) and angles (ꢂ) for 1, 3, 8, 9 and 12
1
3
8
9
12
Bond lengths
Ni–N(1)
Ni–N(2)
Ni–N(3)
Ni–X(1)
2.144(4)
1.965(4)
2.163(4)
2.2206(2)
2.3014(2)
1.283(6)
2.186(3)
1.978(3)
2.157(3)
2.2802(1)
2.2427(9)
1.281(4)
2.219(4)
1.983(4)
2.150(5)
2.3675(1)
2.3996(1)
1.272(7)
2.199(4)
1.975(4)
2.172(4)
2.4255(9)
2.3733(9)
1.283(7)
2.141(4)
1.968(4)
2.191(4)
2.3746(8)
2.4142(8)
1.289(6)
Ni–X(2)
N(3)–C(14)
Bond angles
N(1)–Ni–N(2)
N(1)–Ni–N(3)
N(2)–Ni–N(3)
N(1)–Ni–X(1)
N(1)–Ni–X(2)
N(2)–Ni–X(1)
N(2)–Ni–X(2)
N(3)–Ni–X(1)
N(3)–Ni–X(2)
X(1)–Ni–X(2)
78.55(2)
77.40(1)
76.88(2)
77.33(2)
78.74(2)
155.15(4)
76.59(2)
96.82(1)
90.27(1)
130.78(1)
99.81(1)
96.97(1)
96.78(1)
129.36(3)
155.51(2)
77.20(2)
92.84(1)
90.44(1)
128.13(1)
102.23(1)
99.36(1)
98.00(1)
129.15(8)
154.55(1)
77.53(1)
86.35(8)
99.55(8)
99.09(8)
136.63(8)
101.87(8)
95.63(8)
124.08(4)
153.42(2)
76.99(2)
102.42(1)
90.20(1)
147.68(1)
100.50(1)
97.06(1)
99.22(1)
111.83(4)
154.53(2)
77.71(2)
85.91(1)
99.76(1)
98.11(1)
137.76(1)
102.60(1)
95.26(1)
123.91(4)
Table 2
Hydrogen-bond interactions for complex 1
D–HÁ Á Á A
D–H
HÁ Á ÁA
DÁ Á ÁA
\DHA (ꢂ)
C7–H7AÁ Á ÁCl2
C10–H10AÁ Á ÁCl2
C15–H15AÁ Á ÁN4
0.930
0.930
0.960
2.682
2.806
2.586
3.605
3.697
3.472
171.85
160.68
153.58
plays intermolecular hydrogen bonding interactions (Fig. 2
and Table 2). As a consequence of these hydrogen bonding
interactions, this complex exhibits a 2D supramolecular
structure via C–HÁ Á ÁCl and C–HÁ Á ÁN hydrogen bonds
along the [1,0,1] and [À1,0,1] directions, respectively
(Fig. 2).
As shown in Fig. 3, the geometry in complex 3 is similar
with that in complex 2. The pyridyl nitrogen atom and two
chlorine atoms form the equatorial plane while the axial
Fig. 1. Molecular structure of complex 1 with thermal ellipsoids at the
30% probability level. Hydrogen atoms have been omitted for clarity.
Fig. 2. The 2-D chain formed by intermolecular hydrogen bonding interactions in complex 1.

S. Adewuyi et al. / Journal of Organometallic Chemistry 692 (2007) 3532–3541
3535
plane includes the other two nitrogen atoms and the nickel
center. The nickel atom is almost coplanar with the equa-
torial plane with the deviation of 0.0532 A. The equatorial
ular dichloromethane attach to the polymer chain by
hydrogen bond (Fig. 4).
˚
The nickel dibromide complex 8 (Fig. 5) with 2,6-diethyl-
phenyl and complex 9 (Fig. 7) with 2,6-diisopropyl almost
have the same structural characters. One nitrogen atom of
the pyridyl group and two chlorides form the equatorial
angles are 99.09(8)ꢂ, 124.08(4)ꢂ and 136.63(8)ꢂ, respec-
tively, which showed great difference with that in complex
1. The difference could be attributed to the different substi-
tuent at the ortho-position of the phenyl ring. The axial Ni–
N bonds form a N1–Ni–N3 angle of 154.55(1). Similar with
˚
plane, while the nickel atom lies 0.0024 and 0.0569 A out
of the equatorial plane in 8 and 9, respectively. Due to the
different substituent on the phenyl ring, these two com-
plexes show different equatorial angles. The equatorial
angles are 100.50(1)ꢂ, 111.83(4)ꢂ and 147.68(1)ꢂ for 8 and
98.11(1)ꢂ, 123.91(4)ꢂ and 137.76(1)ꢂ for 9. The equatorial
planes of these two complexes are nearly orthogonal to
the pyridyl plane, with dihedral angles of 92.0ꢂ in 8 and
93.4ꢂ in 9. The dihedral angle between the phenyl plane
on the imino-N group and the pyridyl plane is 88.7ꢂ in 8,
which is much smaller than that in 9 (100.2ꢂ). Similar to
the nickel dichloride analogues, the Ni–N bonds in the
˚
complex 1, the two axial Ni–N bond lengths, 2.186(3) A
˚
and 2.157(3) A are longer than that of the pyridyl nitrogen
˚
atom (1.978(3) A). The equatorial plane and the phenyl
ring on the imino-C are nearly perpendicular to the pyri-
dine plane with the dihedral angles of 93.4ꢂ and 99.1ꢂ
respectively. The two Ni–Cl bond distances show a differ-
˚
ence of 0.037 A, which is much smaller than the aryl
methyl-substituted complex 1. In complex 3, hydrogen
bonds between two neighboring molecules linked them to
1-D infinite polymer chain. In addition, the solvent molec-
Fig. 5. Molecular structure of complex 8 with thermal ellipsoids at the
30% probability level. Hydrogen atoms have been omitted for clarity.
Fig. 3. Molecular structure of complex 3 with thermal ellipsoids at the
30% probability level. Hydrogen atoms and solvent have been omitted for
clarity.
Fig. 4. (a) The 1-D chain formed by intermolecular hydrogen bonding interactions (Cl1Á Á ÁH15B) in complex 3. (b) The spacefill structure of the polymer
chain (the black ones are dichloromethane molecules).

3536
S. Adewuyi et al. / Journal of Organometallic Chemistry 692 (2007) 3532–3541
Fig. 8. Molecular structure of complex 12 with thermal ellipsoids at the
30% probability level. Hydrogen atoms and isolated water molecules have
been omitted for clarity.
Fig. 6. A view of the crystal packing for complex 8.
As shown in Fig. 8, in the solid state of complex 12 with
2,6-dichlorophenyl, the nitrogen atom of pyridyl and two
bromides form the equatorial plane with the nickel atom
equatorial plane are longer than that in the axial plane. The
difference in bond distance is about 0.03 A in 8 and 0.05 A
˚
˚
˚
slightly deviating by 0.0274 A from this plane. The three
in 9.The two imino C@N bonds have distinctive double-
equatorial angles are respectively 99.81(1)ꢂ, 130.78(1)ꢂ
and 129.36(3)ꢂ, with a larger Br–Ni–Br angle than its 2,6-
dialkylphenyl analogues, while the axial N1–Ni–N3 angles
is 155.15(4)ꢂ. Both the equatorial plane and the phenyl
plane link to imino-N are almost perpendicular to the pyr-
idyl plane, with the dihedral angles of 90.6ꢂ and 84.1ꢂ,
respectively. Similar to its analogues, the two axial Ni–N
˚
bond character, with C–N distance of 1.272(7) A (8) and
˚
1.283(7) A (9). Comparing nickel dibromide complex 9 with
dichloride 3 bearing the same ligand, no obvious influence
on the molecular geometry was observed. This conclusion
is based on the similar bond lengths and angles of these
two complexes. In the solid state, complex 8 displays inter-
molecular hydrogen bonding interactions. The hydrogen
˚
˚
bond lengths, 2.141(4) A and 2.191(4) A, are longer than
˚
bond C10–H10AÁ Á ÁBr1 (2.982 A) linked different molecules
˚
that of Ni–N2 in the equatorial plane 1.968(4) A. Further-
more, the two Ni–Br show a difference of about 0.04 A.
together to form a 1-D infinite chain. In addition, as shown
in Fig. 6, four chains were nest together by two different
˚
The imino C14–N3 bond has typical C@N double-bond
˚
kinds of hydrogen bond: C15–H15BÁ Á ÁBr2 (2.826 A) and
˚
character, with a bond length of 1.289(6) A, which is
˚
C4–H4BÁ Á ÁBr2 (2.974 A).
slightly longer than that of the analogues with 2,6-dialkyl-
phenyl substituents.
2.3. Ethylene oligomerization
2.3.1. The selection of co-catalyst
Initial scanning of trimethylaluminum (Me3Al), methyl-
aluminoxane (MAO) and its modified form (MMAO) as
activators of the titled Ni(II) complexes for ethylene oligo-
merization at ambient pressure of ethylene yielded low
activities in the order of À104 g molÀ1 (Ni) hÀ1. However,
a comparable response was obtained from the catalyst acti-
vator, diethylaluminumchloride (Et2AlCl), with improved
oligomerization activity of 5.7 · 104 g molÀ1 (Ni) hÀ1
.
The catalysts productivities further improved at elevated
ethylene pressure. The 1/Et2AlCl system showed higher
catalytic activity than the other systems activated with
MAO and Me3Al (entries 2, 4 and 5, Table 3). Therefore,
further investigations were carried out at elevated ethylene
pressure and Et2AlCl as the choice co-catalyst. The
obtained results are presented in Table 3.
Fig. 7. Molecular structure of complex 9 with thermal ellipsoids at the
30% probability level. Hydrogen atoms and isolated water molecules have
been omitted for clarity.

S. Adewuyi et al. / Journal of Organometallic Chemistry 692 (2007) 3532–3541
3537
Table 3
ther investigations were carried out using toluene as
solvent.
Ethylene oligomerization with complexes 1–12/Et2AlCla
Entry Complex Al/Ni P (atm)
T
Activity
Oligomers
Apparently, the ethylene concentration significantly
affects the catalytic behavior of the complexes. The cata-
lytic systems 1/Et2AlCl and 7/Et2AlCl were investigated
under different ethylene pressure. The activities of these
complexes rose sharply with increasing ethylene pressure
and at 30 atmospheres, values of 1.37 and 1.86 · 106 g
molÀ1 (Ni) hÀ1were obtained respectively (Entries 9 and
19, Table 3). Variation of the reaction temperature also
affects the catalytic performance with little or no influence
on the distribution of low-order C4 and C6 oligomers pro-
duced (Entries 2, 10 and 11, Table 3). Complex 1 displayed
higher oligomerization activity at lower temperature and at
elevated temperatures of 30 ꢂC while at 40 ꢂC, a significant
reduction in activity was observed compared with that at
20 ꢂC. The prominent reason might be unfavorable active
sites and lower ethylene solubility at higher temperature.
(ꢂC) (105 g molÀ1 distribution
(Ni) hÀ1
)
(%)
C4
C6
5.8
1
2
3
4b
5c
6d
7e
8
1
1
500
700
1000
700
700
700
700
700
700
700
700
700
700
700
700
700
700
700
700
700
700
700
700
700
10
10
10
10
10
10
10
20
30
10
10
10
10
10
10
10
10
20
30
10
10
10
10
10
20
20
20
20
20
20
20
20
20
30
40
20
20
20
20
20
20
20
20
20
20
20
20
20
2.1
6.9
3.5
3.8
0.4
3.7
9.7
7.9
13.7
1.6
1.4
6.1
5.3
5.0
1.1
5.6
7.4
8.5
18.6
6.3
5.2
2.2
2.3
5.9
94.2
96.7
>99
3.3
1
1
100
100
1
1
99.2
98.3
0.8
1.7
1
1
87.7 12.3
61.8 38.2
9
10
1
1
97.5
97.0
2.5
3.0
11
12
13
14
15
16
17
18
19
20
21
22
23
24
a
1
2
89.6 10.4
85.9 14.1
3
4
92.1
99.1
97.7
7.9
0.9
2.3
5
6
2.3.3. Effect of the ligand environments
7
84.2 15.8
87.5 12.5
The dependence of catalytic activities of all the com-
plexes [(L)NiCl2 (1–6) and (L)NiBr2 (7–12)] on halide
anion linkage to the nickel center was monitored. In most
cases, the nickel dibromide precursors showed better activ-
ity than their corresponding dichloride precursors. This
observation is in concord with the reported pyrazolylimi-
nophosphorane based Ni(II) analogue [11b] and the effect
is attributable to higher solubility of the dibromides in
the toluene solvent medium than the dichlorides counter-
parts. In addition, with the increased ethylene pressure,
the nickel dichloride 1 produced more trimers (Entries 2,
8–9, Table 3). However, the opposite trend was observed
for the nickel dibromide 7 (Entries 17–19, Table 3). A
noticeable effect of the alkyl substituents on the aryl ring
of the complexes is also worth mentioning. For the nickel
dichloride complexes, a reduction of steric bulk at the
ortho-aryl position resulted in increase activity of the com-
plexes. Hence, complexes 1 with methyl groups in the
ortho-positions of the aryl ring showed better activities of
6.9 · 105 g molÀ1(Ni) hÀ1 at 10 atmospheres ethylene.
The introduction of substituent at the para-position of
the phenyl group resulted in decreased activity (Entry 14,
Table 3). Complex 6, which bear 2,6-dichlorophenyl group
showed comparable catalytic activity than its analogues
bearing 2,6-dialkylphenyl groups. However, complex 5,
which contain 2,6-diflorophenyl group, showed relative
lower activity. The nickel dibromide complexes followed
the same trend as the nickel dichloride complexes.
7
7
97.6
89.1 10.9
97.4 3.6
77.7 22.3
2.4
8
9
10
11
12
98.5
97.8
1.5
2.2
General conditions: 5 lmol cat.; 100 mL toluene as solvent; 30 min.
Cocat: MAO.
Cocat: Me3Al.
Solvent: heptane (100 mL).
Solvent: dichloromethane (100 mL).
b
c
d
e
2.3.2. Effect of reaction parameters
As depicted in Table 3, the reaction parameters includ-
ing the ratio of Et2AlCl to nickel complex, reaction temper-
ature and ethylene pressure grossly affected their catalytic
performances. The influences of the Al/Ni molar ratio
and the reaction temperature were investigated in detail
with complex 1. The results indicate a high influence of var-
ied Al/Ni ratio on the oligomerization activity. The cata-
lytic activity reached optimum value of 6.9 · 105 g molÀ1
(Ni) hÀ1 with the ratio of 700. Subsequently, the activity
sharply dropped and the value of 3.5 · 105 g molÀ1 (Ni)
hÀ1 was obtained with the Al/Ni ratio of 1000 (Entries
1–3, Table 3). A similar trend was observed in our previous
work [16]. As initially proposed, a possible reason could be
that a threshold amount of Et2AlCl as co-catalyst was
needed to efficiently activate the catalyst precursor, but,
large amount might cause such deactivation [19].
Changing the solvent by using the heptane (Entry 6,
Table 3), lower activity of ethylene oligomerization was
observed. However, when the dichloromethane was used
as solvent for ethylene oligomerization, relative higher
activity was obtained. But from the GC chart, some new
compounds were observed, which could be attributed the
reaction of dichloromethane and Et2AlCl. In this case, fur-
2.3.4. Effect of the auxiliary ligand on catalyst activity
The application of triphenylphosphine (PPh3) as an aux-
iliary ligand with nickel-based catalyst for ethylene oligo-
merization has proved worth while due to the high
enhancement of the catalyst activity [13,15,19]. In order
to examine this effect on the quinoxalin-based iminopyri-
dine nickel halides, ethylene oligomerization with all the
titled complexes was carried out in the presence of 10

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S. Adewuyi et al. / Journal of Organometallic Chemistry 692 (2007) 3532–3541
Table 4
ligand have catalytic enhancement effects on all the
complexes.
Ethylene oligomerization with complexes 1–12/Et2AlCl and 10 equiv of
PPha3
Entry Complex P (atm) Activity
Oligomers distribution (%)
4. Experimental
(105 g molÀ1
C4
C6
(Ni) hÀ1
)
4.1. General considerations
1
2
3
4
5
6
7
8
9
10
11
12
13b
14b
a
1
2
1
1
1
1
1
1
1
1
1
1
1
1
10
10
7.2
5.9
2.3
1.3
2.4
1.5
8.5
6.1
2.0
4.8
3.4
2.2
10.4
12.0
96.8
97.1
85.0
97.7
98.6
94.6
96.2
97.6
82.7
97.0
97.1
98.7
95.2
95.6
3.2
2.9
15.0
2.3
1.4
5.4
3.8
2.4
17.3
3.0
2.9
1.3
4.8
4.4
All manipulations of air and/or moisture sensitive com-
pounds were performed under nitrogen atmosphere using
standard Schlenk techniques. Solvents were dried by the
appropriate drying reagents and distilled under nitrogen
prior to use. Et2AlCl (1.90 mol lÀ1) solution in hexane
was purchased from Acros Chemicals. All other reagents
were purchased from Aldrich and were directly used with-
out further purification unless otherwise stated. IR spectra
were recorded on a Perkin–Elmer system 2000 FT-IR spec-
trometer. Elemental analysis was performed on a Flash EA
1112 microanalyser. GC analyses were performed with a
VARIAN CP-3800 gas chromatograph equipped with
a flame ionization detector and a 30 m (0.2 mm i.d.,
0.25 lm film thickness) CP-Sil 5 CB column. The yield of
oligomers was calculated by referencing with the mass of
the solvent based on the prerequisite that the mass of each
fraction is approximately proportional to its integrated
areas in the GC trace.
3
4
5
6
7
8
9
10
11
12
1
7
General conditions: 5 lmol catalyst; Al/Ni = 700; 30 mL toluene as
solvent; reaction temperature: 20 ꢂC; 30 min.
b
100 mL toluene as solvent.
equivalent amount of PPh3. As shown in Table 4, the cat-
alytic activities of the complexes were much more improved
at ambient ethylene pressure, in which, a low performance
in the order of À104 g molÀ1 (Ni) hÀ1 was initially
recorded. Also, complexes 5 and 9 with the best catalytic
performance at 10 atmosphere ethylene, behaved in a sim-
ilar manner in the presence of PPh3 at ambient ethylene
pressure with activities of 7.2 and 8.5 · 105 g molÀ1 (Ni)
hÀ1 respectively (Entries 1 and 7, Table 4). The oligomeri-
zation products of the complexes as in the case without
PPh3 are mainly low-order carbon number oligomers (C4
and C6) with high preference for C4. Increasing ethylene
pressure to 10 atmospheres, complexes 5 and 9 further gave
improved activities of 1.04 and 1.2 · 106 g molÀ1 (Ni) hÀ1
respectively in the presence of PPh3 auxiliary ligand
(Entries 13 and 14, Table 4). The plausible explanation,
so far, offered for the activity enhancement role of PPh3
is simultaneous association and dissociation with nickel
metal core and consequent activation and protection of
the active sites [15]. We are presently embarking on further
research to unravel the mechanistic details of this effect.
4.2. Synthesis of LNiX2 (1–12; X = Cl or Br)
The complexes 1–12 were synthesized by the reaction of
NiCl2 Æ 6H2O in ethanol or (DME)NiBr2 in THF with the
corresponding ligands. A typical synthetic procedure for
complex 1 is described as the following. The ligand L1
(0.070 g, 0.2 mmol) and NiCl2 Æ 6H2O (0.047 g, 0.2 mmol)
was added to a Schlenk tube together with 10 ml of freshly
distilled ethanol. The reaction mixture was then stirred for
12 h at room temperature after which the solvent volume
was reduced to 4 ml. Absolute diethyl ether (10 ml) was
added to precipitate the complex and the mixture was
allowed to stand for 30 min. The supernatant was removed
and the precipitate was washed with diethyl ether twice and
dried in vacuum to obtain a brownish powder in 93.0%
(0.09 g) yield. IR (KBr, cmÀ1): 1621 (w, mC@N), 1589,
1375, 1215, 759, 744, 417. Anal. Calc. (C23H20Cl2NiN4):
N, 11.62; C, 57.31; H, 4.18. Found: N, 11.37; C, 56.90;
H, 4.10%. Data of complex 2 are as follow: Yield: 84.9%
(0.085 g), IR (KBr, cmÀ1): 1609 (w, mC@N), 1591, 1480,
1445, 1375, 1276, 1129, 1095, 818, 774, 415. Anal. Calc.
(C25H24Cl2NiN4): N, 10.98; C, 58.87; H, 4.74. Found: N,
11.01; C, 59.10; H, 4.83%. Data of complex 3 are as follow:
Yield: 87.6% (0.094 g), IR (KBr, cmÀ1): 1606 (w, mC@N),
1590, 1541, 1458, 1373, 1265, 1208, 1095, 810. Anal. Calc.
(C27H28Cl2NiN4): N, 10.41; C, 60.26; H, 5.24. Found: N,
10.52; C, 59.82; H, 5.26%. Data of complex 4 are as follow:
Yield: 96.3% (0.099 g), IR (KBr, cmÀ1): 1614 (w, mC@N),
1593, 1484, 1370, 1219, 1095, 816, 775, 418. Anal. Calc.
(C24H22Cl2NiN4): N, 11.29; C, 58.11; H, 4.47. Found: N,
11.57; C, 57.86; H, 4.23%. Data of complex 5 are as follow:
3. Conclusion
The synthesis and characterization of new Ni(II) halides
complexes with asymmetrical 2-quinoxalinyl-6-iminopyri-
dine ligands have been successfully achieved. Upon activa-
tion with Et2AlCl, all the complexes reveal moderate to
good catalytic activity for the dimerization and trimeriza-
tion of ethylene at low temperature and 10 atmospheres
ethylene. Due to higher solubility of the nickel bromide
precursors in toluene solvent medium, their catalytic per-
formance for ethylene oligomerization are slightly higher
than the corresponding nickel chloride precursors. Both
elevation of the ethylene pressure and addition of auxiliary

S. Adewuyi et al. / Journal of Organometallic Chemistry 692 (2007) 3532–3541
3539
Yield: 62.0% (0.06 g), IR (KBr, cmÀ1): 1624 (w, mC@N),
1593, 1483, 1426, 1370, 1262, 1225, 1096, 803, 405. Anal.
Calc. (C21H14Cl2F2NiN4): N, 11.44; C, 51.48; H, 2.88.
Found: N, 11.56; C, 51.74; H, 3.11%. Data of complex 6
are as follow: Yield: 67.0% (0.07 g), IR (KBr, cmÀ1):
1625 (w, mC@N), 1590, 1480, 1436, 1371, 1262, 1095, 1028,
803, 405. Anal. Calc. (C21H14Cl4NiN4): N, 10.72; C,
48.24; H, 2.70. Found: N, 10.88; C, 47.96; H, 2.54%. Data
of complex 7 are as follow: Yield: 96.4% (0.110 g), IR
(KBr, cmÀ1): 1626 (w, mC@N), 1590, 1479, 1373, 1211,
1097, 773, 417. Anal. Calc. (C23H20Br2NiN4): N, 9.81; C,
48.39; H, 3.53. Found: N, 9.56; C, 48.14; H, 3.55%. Data
of complex 8 are as follow: Yield: 89.0% (0.106 g), IR
(KBr, cmÀ1): 1631 (w, mC@N), 1583, 1436, 1257, 1082,
852, 709, 396. Anal. Calc. (C25H24Br2NiN4): N, 9.35; C,
50.13; H, 4.04. Found: N, 10.10; C, 49.91; H, 4.01%. Data
of complex 9 are as follow: Yield: 92.8% (0.125 g),
IR (KBr, cmÀ1): 1614 (w, mC@N), 1592, 1502, 1482,
1371, 1277, 1212, 1093, 817, 772, 417. Anal. Calc.
(C27H28Br2NiN4): N, 8.94; C, 51.72; H, 4.50. Found: N,
8.12; C, 52.30; H, 5.08%. Data of complex 10 are as follow:
Yield: 97.0% (0.130 g), IR (KBr, cmÀ1): 1618 (w, mC@N),
1593, 1503, 1484, 1370, 1277, 1219, 1095, 816, 775, 418.
Anal. Calc. (C24H22Br2NiN4): N, 9.58; C, 49.28; H, 3.79.
Found: N, 9.26; C, 50.10; H, 3.53%. Data of complex 11
are as follow: Yield: 62.0% (0.06 g), IR (KBr, cmÀ1):
1624 (w, mC@N), 1593, 1483, 1426, 1370, 1262, 1225, 1096,
803, 405. Anal. Calc. (C21H14Cl2F2NiN4): N, 11.44; C,
51.48; H, 2.88. Found: N, 11.56; C, 51.74; H, 3.11%. Data
of complex 12 are as follow: Yield: 79.5% (0.10 g), IR
(KBr, cmÀ1): 1619 (w, mC@N), 1591, 1471, 1372, 1280,
1226, 1076, 776, 580. Anal. Calc. (C21H14Cl2Br2NiN4): N,
9.16; C, 41.23; H, 2.31. Found: N, 9.01; C, 40.71; H, 2.42%.
4.3. Procedure for ethylene oligomerization
Ethylene oligomerization at 1 atm of ethylene was car-
ried out as follows: Complex (5 lmol) was added to a
Schlenk flask under nitrogen. The flask was back-filled
three times with N2 and twice with ethylene and thereafter
charged with toluene and co-catalyst solutions in turn
under ethylene atmosphere. The reaction solution was vig-
orously stirred under 1 atm of ethylene at the set tempera-
ture. At the end of the desired period of time, the reaction
was quenched by 5% aqueous hydrogen chloride and the
products were analyzed by GC.
The oligomerization at higher ethylene pressure was per-
formed in a 0.25 L stainless steel autoclave equipped with
mechanical stirrer, a temperature controller and gas ballast
through a solenoid clave for continuous feeding of ethylene
Table 5
Crystal data and structure refinement for compounds 1, 3, 8, 9 and 12
Data
1
3 Æ CH2Cl2
8
9 Æ 2H2O
12 Æ 3H2O
Formula
C23H20Cl2N4Ni
C28H30Cl4N4Ni
C25H24Br2N4Ni
C27H28Br2N4Ni O2
C21H14Br2Cl2
N4NiO3
Fw
482.04
623.07
599.01
659.06
659.79
Color
Brown
Brown
Brown
Brown
Brown
Temperature (K)
Crystal system
Space group
293(2)
Orthorhombic
Fdd2
31.1073(7)
32.1295(9)
8.6723(2)
90
8667.6(4)
16
293(2)
Rhombohedral
R-3
26.7441(3)
26.7441(3)
22.7376(6)
90
14084.2(4)
18
293(2)
Monoclinic
P2(1)/c
9.6064(19)
14.944(3)
17.090(3)
98.22(3)
2428.1(8)
4
293(2)
Rhombohedral
R-3
27.0801(6)
27.0801(6)
23.1749(1)
90
14718.0(8)
18
293(2)
Monoclinic
P2(1)/n
8.80010(1)
18.4102(3)
16.3100(3)
104.2220(1)
2561.42(7)
4
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢂ)
3
˚
V (A )
Z
˚
Wavelength (A)
0.71073
1.478
0.71073
1.322
0.71073
1.639
0.71073
1.338
0.71073
1.711
Dcalc (mg mÀ3
)
Crystal size (mm)
h Range (ꢂ)
Limiting indices
0.32 · 0.23· 0.16
1.82–28.27
À41 6 h 6 33,
À37 6 k 6 41,
À11 6 l 6 11
0.35 · 0.25 · 0.20
1.52–28.28
À35 6 h 6 35,
À35 6 k 6 35,
À26 6 l 6 29
0.40 · 0.30 · 0.30
1.82–25.01
À11 6 h 6 11,
0 6 k 6 17,
0 6 l 6 20
0.40 · 0.30 · 0.30
1.50–28.32
À35 6 h 6 36,
À36 6 k 6 36,
À29 6 l 6 30
0.30 · 0.20 · 0.20
1.70–28.33
À11 6 h 6 11,
À24 6 k 6 24,
À21 6 l 6 15
Reflections collected/unique
R(int)
11990/5133
0.0457
68949/7712
0.0231
8360/4244
0.0401
66303/8104
0.0562
24529/6344
0.0323
Data/restraints/parameters
Absorption coefficient (mmÀ1
F(000)
5133/1/271
1.160
3968
7712/0/334
0.984
5796
4244/0/284
4.110
1200
8104/0/325
3.063
5976
6344/18/299
4.115
1296
)
Final R indices
[(I) > 2r(I)]
R indices (all
data)
Goodness of fit
R1 = 0.0573,
wR2 = 0.1377
R1 = 0.0852,
wR2 = 0.1484
1.099
R1 = 0.0656,
wR2 = 0.2174
R1 = 0.0787,
wR2 = 0.2342
1.076
R1 = 0.0589,
wR2 = 0.1217
R1 = 0.0841,
wR2 = 0.1323
1.051
R1 = 0.0639,
wR2 = 0.2224
R1 = 0.1160,
wR2 = 0.2612
1.063
R1 = 0.0509,
wR2 = 0.1704
R1 = 0.0735,
wR2 = 0.1863
1.056
Largest difference in peak and hole
0.621 and À0.422
1.762 and À1.295
0.693 and À0.408
1.756 and À0.804
1.604 and À0.629
À3
˚
(e A
)

3540
S. Adewuyi et al. / Journal of Organometallic Chemistry 692 (2007) 3532–3541
(c) W. Keim, A. Behr, B. Limba¨cker, C. Kruger, Angew. Chem., Int.
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at desired pressure. 100 ml of toluene containing the cata-
lyst precursor was transferred to the fully dried reactor
under nitrogen atmosphere. The required amount of
Et2AlCl was then injected into the reactor using a syringe.
At the prescribed temperature, the reactor was pressurized
to corresponding ethylene pressure and mechanically stir-
red for 30 min. The reaction was subsequently quenched
and products were analyzed employing same method as
described above with 1 atm ethylene.
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4.4. X-ray Crystal structural determination of 1, 3, 8, 9 and
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Intensity data for complexes 1, 3, 9 and 12 were col-
lected on a Brucker SMART 1000 CCD diffractometer
with graphite-monochromated Mo Ka radiation (k =
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˚
radiation (k = 0.71073 A). Unit cell dimensions were
obtained with least-squares refinements. Intensities were
corrected for Lorentz and polarization effects and empirical
absorption. The structures were solved by direct methods,
and refined by full-matrix least-square on F2. Each hydro-
gen atom was placed in a calculated position, and refined
using a riding model. All non-hydrogen atoms were refined
anisotropically. Structure solution and refinement were
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Acknowledgements
This work was supported by NSFC No. 20674089 and
MOST 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.
(b) G.J.P. Britovsek, M. Bruce, V.C. Gibson, B.S. Kimberley, P.J.
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Appendix A. Supplementary material
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4895.
CCDC 639011, 639012, 639013, 639014 and 639015 con-
tain the supplementary crystallographic data for 1, 3, 8, 9
and 12. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.jorganchem.2007.04.036.
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