Nickel(II) complexes chelated by 2-arylimino-6-benzoxazolylpyridine: Syntheses, characterization, and ethylene oligomerization

Article abstract of DOI:10.1021/om800647w

2-(2-Benzoxazolyl)-6-(1-(arylimino)-ethyl)pyridines (L) were designed as ligands, and their nickel complexes LNiX₂ (X = Cl, 1a-7a; X = Br, 1b-7b) were formed. All ligands were fully characterized by NMR, FT-IR spectra, and elemental analyses, w

Full text of DOI:10.1021/om800647w

Organometallics 2008, 27, 5641–5648
5641
Nickel(II) Complexes Chelated by
2-Arylimino-6-benzoxazolylpyridine: Syntheses, Characterization,
and Ethylene Oligomerization
Rong Gao,† Min Zhang,† Tongling Liang,† Fosong Wang,*,† and Wen-Hua Sun*,†,‡
Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute
of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China, and State Key
Laboratory for Oxo Synthesis and SelectiVe Oxidation, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou 730000, People’s Republic of China
ReceiVed July 9, 2008
2-(2-Benzoxazolyl)-6-(1-(arylimino)-ethyl)pyridines (L) were designed as ligands, and their nickel
complexes LNiX2 (X ) Cl, 1a-7a; X ) Br, 1b-7b) were formed. All ligands were fully characterized
by NMR, FT-IR spectra, and elemental analyses, while the nickel complexes were examined by FT-IR
spectra and elemental analyses. The molecular structures of nickel complexes were established by single-
crystal X-ray diffraction. Complex 6a possesses coordination number 6 by coordination of one ligand,
two moles of methanol, and one chloride anion, whereas complexes 1b and 5b possess coordination
number 5 with distinct distortions, for 1b toward trigonal-bipyramidal geometry and for 5b toward square-
pyramidal geometry. The nickel complex 1a was evaluated in the catalytic oligomerization of ethylene
with different alkylaluminums as cocatalysts, and diethylaluminum chloride (Et2AlCl) proved to be the
most effective. Upon activation with Et2AlCl, all nickel complexes showed outstanding thermal stability
and good catalytic activity toward ethylene dimerization with high selectivity for R-C4.
of bis(arylimino)pyridyl Fe(II) and Co(II) dihalides by
Brookhart12 and Gibson13 generated enthusiasm for designing
pentacoordinate nickel(II) complexes incorporating O∧N∧N,14
N∧P∧N,15 P∧N∧N,16 and N∧N∧N17,18 tridentate ligands, which
1. Introduction
In the past dozen years we have witnessed great progress in
the use of late-transition metal complexes as catalysts for
reactions of ethylene in both academic and industrial research.1,2
The SHOP process employing nickel complexes as catalysts
has been commercialized,3 and it was recognized by the
Brookhart group that diimine nickel complexes have high
activities toward ethylene in catalytic reactions.4 The progress
of ethylene activation by nickel complexes was summarized in
recent review articles.2 As far as efficient catalytic systems are
concerned, four-coordinate nickel dihalides bearing chelating
P∧P,5 N∧O,6,7 P∧N,8,9 and N∧N10,11 bidentate ligands have been
commonly considered. The discovery of the catalytic activity
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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.
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Y.-S.; Hu, N.-H. Organometallics 2005, 24, 2628.
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Jie, S.; Zhang, T.; Hou, J.; Li, W.; Zhao, D.; Sun, W.-H. Acta Polym. Sin.
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Chang, F.; Hao, P.; Sun, W.-H. C. R. Chim. 2007, 10, 1200.
* To whom correspondence should be addressed. Tel: +86 10 62557955.
Fax: +86 10 62618239. E-mail: whsun@iccas.ac.cn (W.-H.S.)
† Key laboratory of Engineering Plastics and Beijing National Laboratory
for Molecular Science.
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Wang, R.; Mecking, S.; Schro¨der, D. L. J. Organomet. Chem. 2002, 662,
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Wang, H. New J. Chem. 2002, 26, 1474.
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Organometallics 2004, 23, 2625. (c) Speiser, F.; Braunstein, P.; Saussine,
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S.; Sun, W.-H.; Chen, J. J. Organomet. Chem. 2005, 690, 3918. (f) Weng,
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‡ State Key Laboratory for Oxo Synthesis and Selective Oxidation.
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10.1021/om800647w CCC: $40.75
2008 American Chemical Society
Publication on Web 10/04/2008

5642 Organometallics, Vol. 27, No. 21, 2008
Gao et al.
Scheme 1. Synthetic Procedure
display good to high activity in ethylene oligomerization and
polymerization. In exploring alternative tridentate ligands bear-
ing benzene-fused heterocycles for nickel complexes in ethylene
reactivity,,18b,c metal complexes ligated by 2-(2-benzimidazole)-
6-(1-aryliminoethyl)pyridines showed high catalytic activity
toward ethylene oligomerization.19 However, for 2,6-bis(2-
benzimidazolyl)pyridine, only its chromium(III) complexes were
synthesized and showed high activity for ethylene oligomer-
ization and polymerization.20 Specifically it would be interesting
to replace the benzimidazole with other N-heterocycles. With
this in mind, 2-quinoxalinyl-6-aryliminopyridines were designed,
and the good catalytic activities of their complexes were
confirmed for ethylene oligomerization.21 In fact, the latter
model did not show catalytic activity as high as those containing
2-(2-benzimidazole)-6-(1-aryliminoethyl)pyridines.19 Since the
benzoxazole is considered a replacement for benzimidazole, new
models of 2-benzoxazolylpyridine nickel(II) complexes22 were
synthesized and found to present high activity toward ethylene
oligomerization. In extension, the N∧N∧N tridentate nickel(II)
complexes bearing 2-(2-benzoxazole)-6-(1-aryliminoethyl) py-
ridine were synthesized and fully characterized. Upon activation
with Et2AlCl, all nickel complexes showed good catalytic
activity toward ethylene dimerization with high selectivity for
R-C4. The influence of reaction parameters and the nature of
the complexes on catalytic activities were investigated in detail.
Herein the syntheses and characterization of organic compounds
and their nickel complexes are reported and discussed along
with the catalytic behavior of nickel complexes toward ethylene
oligomerization.
2. Results and Discussion
2.1. Syntheses and Characterization of the Ligands
and Complexes. Following the synthetic procedure for 2-(2-
benzimidazolyl)-6-acetylpyridine starting from the reaction of
2,6-lutidine with o-phenylenediamine,19a 2-(2-benzoxazolyl)-
6-acetylpyridine was synthesized in acceptable yield via a
modified reaction with o-aminophenol instead of o-phenylene-
diamine. The reaction of 2-(2-benzoxazolyl)-6-acetylpyridine
with anilines forms 2-(2-benzoxazolyl)-6-(1-aryliminoethyl)py-
ridines (Scheme 1), and the product yields depended on the
aniline substituents. Better yields were achieved with electron-
donating substituents, whereas electron-withdrawing substituents
gave lower yields. All ligand identities were confirmed by FT-
IR and 1H and 13C NMR spectroscopy as well as by elemental
analyses.
The nickel complexes were prepared by mixing NiCl2 · 6H2O/
(DME)NiBr2 with 1 equiv of the corresponding ligand in
ethanol/tetrahydrofuran (THF), stirring at room temperature for
12 h (Scheme 1). The resultant nickel complexes precipitated
from their reaction solution and were collected by filtration,
washed with diethyl ether, and dried under vacuum. All nickel
complexes were isolated as air-stable gray powders in moderate
to high yields and characterized by FT-IR spectra and elemental
analyses, which were in agreement with the formula (L)NiX2
(L ) ligand).
In the FT-IR spectra, the CdN stretching frequencies shifted
to lower values between 1610 and 1616 cm-1 with weaker
intensity compared with those of the corresponding free ligands.
Such features are consistent with the existence of coordination
between the imino nitrogen atom and the nickel center. The
molecular structures of the representative complexes 6a, 1b,
and 5b were unambiguously confirmed by single-crystal X-ray
diffraction studies.
2.2. Crystal Structures. Single crystals of 6a, 1b, and 5b
were grown by diffusing a diethyl ether layer into their methanol
solutions. Their molecular structures are shown in Figures 1,
2, and 3, respectively, along with selected bond lengths and
angles.
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Eur. J. Inorg. Chem. 2006, 4895.
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J. Inorg. Chem. 2007, 3816. (c) Zhang, S.; Sun, W.-H.; Kuang, X.; Vystorop,
I.; Yi, J. J. Organomet. Chem. 2007, 692, 5307. (d) Sun, W.-H.; Wang, K.;
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Lu, X. Organometallics 2007, 26, 2720. (b) Hao, P.; Zhang, S.; Sun, W.-
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Complex 6a possesses distorted octahedral coordination
geometry around the Ni center, as shown in Figure 1, which
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2008, DOI: 10.1039/b807604a.

Ni(II) Complexes Chelated by Arylimino-benzoxazolylpyridine
Organometallics, Vol. 27, No. 21, 2008 5643
Figure 3. Molecular structure of 5b. Thermal ellipsoids are shown
at 30% probability. Hydrogen atoms and solvent are omitted for
clarity. Selected bond lengths (Å) and angles (deg) are as follows:
Ni(1)-N(1), 2.003(4); Ni(1)-N(2), 2.128(4); Ni(1)-N(3), 2.135(4);
Ni(1)-Br(1), 2.3791(9); Ni(1)-Br(2), 2.4535(1); N(2)-C(6),
1.312(6);O(1)-C(6),1.356(5);N(3)-C(12),1.293(6);N(1)-Ni(1)-N(2),
77.81(2); N(2)-Ni(1)-N(3), 152.28(2); N(1)-Ni(1)-N(3), 76.66(2);
N(1)-Ni(1)-Br(1), 159.33(1); N(2)-Ni(1)-Br(1), 100.48(1);
N(3)-Ni(1)-Br(1),99.59(1);N(1)-Ni(1)-Br(2),92.20(1);N(2)-Ni(1)-Br(2),
91.43(1); N(3)-Ni(1)-Br(2), 100.14(1); Br(1)-Ni(1)-Br(2),
108.46(4).
Figure 1. Molecular structure of complex 6a. Thermal ellipsoids
are shown at 30% probability. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg) are as follows:
Ni(1)-N(1), 2.020(3); Ni(1)-N(2), 2.151(3); Ni(1)-N(3), 2.153(3);
Ni(1)-Cl(1), 2.3043(1); Ni(1)-O(2) 2.111(3); Ni(1)-O(3), 2.109(3);
N(2)-C(6), 1.311(4); O(1)-C(6), 1.365(4); N(3)-C(12), 1.291(4);
N(1)-Ni(1)-N(2),77.49(1);N(2)-Ni(1)-N(3),154.21(1);N(1)-Ni(1)-N(3),
76.84(1); N(1)-Ni(1)-Cl(1), 176.61(8); N(2)-Ni(1)-Cl(1),
100.65(8); N(3)-Ni(1)-Cl(1), 105.12(8); N(1)-Ni(1)-O(2),
84.25(1); N(1)-Ni(1)-O(3), 87.42(1); O(3)-Ni(1)-O(2), 171.46(1);
N(2)-Ni(1)-O(3),87.61(1);N(2)-Ni(1)-O(2),88.75(1);N(3)-Ni(1)-O(2),
91.10(1); N(3)-Ni(1)-O(3), 88.82(1); O(3)-Ni(1)-Cl(1), 95.35(7);
O(2)-Ni(1)-Cl(1), 92.91(7).
dazolyl)-6-acetylpyridine19b and N-(1-(pyridin-2-yl)ethylidene-
)quinolin-8-amine.18d
Complex 1b was confirmed as nickel dibromide bearing the
tridentate ligand 2,6-dimethyl-N-(1-(6-(benzo[d]oxazol-2-yl)py-
ridin-2-yl)ethylidene)benzenamine (Figure 2). The nickel center
is five-coordinated in an essentially distorted trigonal-bipyra-
midal manner. N(1) and two bromine atoms form the equatorial
coordination plane, and the nickel center slightly deviates from
the equatorial plane by 0.025 Å. The other two nitrogen atoms
(N(2) and N(3)) occupy the axial positions and with the nickel
core form an angle of 152.29(2)° while simultaneously making
a dihedral angle of 97.5° with the equatorial plane. The 2,6-
dimethylphenyl group is oriented almost orthogonally to the
coordination plane with a dihedral angle of 94.7°. In the
molecular structure, the N(2)-C(6) bond distance is 1.291(6)
Å, which is shorter than C(6)-O(1) at 1.360(6) Å and is clearly
a CdN double bond. This coordination geometry is essentially
similar to those commonly observed in nickel complexes co-
ordinatedwith2-(benzimidazolyl)-6-(1-(arylimino)ethyl)pyridines.19b,d
In contrast to complexes 6a and 1b, the coordination geometry
of complex 5b (Figure 3) can be best described as a distorted
square pyramid with the basal plane composed by N(1), N(2),
N(3), and Br(1) (Figure 3). The nickel atom deviates from the
plane formed by N(1), N(2), and N(3) by 0.0862 Å, while the
bromine atom Br(1) is almost coplanar with a deviation of
0.0502 Å. The other bromine atom, Br(2), deviates 0.8332 Å
from the basal plane at the apex of the pyramid. The plane of
thechelatingringNi(1)-N(2)-C(6)-C(7)-N(1)-C(11)-C(12)-N(3)
is nearly perpendicular to the imino-aryl ring (89.1°). The
dihedral angle between the imino-aryl ring and the pyridine ring
is 88.2°. The Ni-N bond lengths display the trend Ni(1)-N(3)
> Ni(1)-N(2) > Ni(1)-N(1), which was similarly observed
in nickel(II) complexes bearing 2-(benzimidazolyl)-6-(1-
(arylimino)ethyl)pyridines.19b,d The two Ni-Br bond lengths
are significantly different from each other, which is also seen
in 1b. The N(2)-C(6) bond distance is 1.312(6) Å, shorter than
C(6)-O(1) of 1.356(5) Å, displaying clear CdN double bond
character. Compared with the structure of 1b, the repulsion of
three lone electron pairs on bromine would make it come close
to a structure similar to 1b.
Figure 2. Molecular structure of complex 1b. Thermal ellipsoids
are shown at 30% probability. Hydrogen atoms have been omitted
for clarity. Selected bond lengths (Å) and angles (deg) are as
follows: Ni(1)-N(1), 2.000(4); Ni(1)-N(2), 2.149(4); Ni(1)-N(3),
2.139(4);Ni(1)-Br(1),2.3623(1);Ni(1)-Br(2),2.4277(1);N(2)-C(6),
1.291(6);O(1)-C(6),1.360(6);N(3)-C(12),1.281(6);N(1)-Ni(1)-N(2),
77.31(2); N(2)-Ni(1)-N(3), 152.29(2); N(1)-Ni(1)-N(3), 76.73(2);
N(1)-Ni(1)-Br(1), 147.80(1); N(2)-Ni(1)-Br(1), 96.61(1);
N(3)-Ni(1)-Br(1), 100.17(1); N(1)-Ni(1)-Br(2), 95.29(1);
N(2)-Ni(1)-Br(2), 91.10(1); N(3)-Ni(1)-Br(2), 100.76(1);
Br(1)-Ni(1)-Br(2), 116.59(4).
also gives selected bond lengths and angles. The octahedral
geometry around the Ni center is formed by three N atoms of
the ligand, two O atoms of coordinated methanol solvent, and
one Cl atom. The other Cl atom has been displaced from its
coordinated site and is found as a free Cl- anion. The Ni atom
deviates by 0.0200 Å out of the N(1)-N(2)-N(3) plane, while
the chlorine atom Cl(1) is almost coplanar with a deviation of
0.0411 Å. Two O atoms of coordinated methanol solvent (O(2)
and O(3)) occupy the axial positions and with the nickel core
form an angle of 171.46(1)°. The O atoms are nearly perpen-
dicular to the equatorial plane with a dihedral angle of 90.3°.
The benzoxazole ring is almost coplanar with the pyridine ring
(0.9°). Its coordination geometry is similar to that observed in
nickel dichloride complexes ligated by 2-(1-methyl-2-benzimi-
2.3. Ethylene Oligomerization. When complex 1a was
activated with methylaluminoxane (MAO) or its modified form

5644 Organometallics, Vol. 27, No. 21, 2008
Gao et al.
Table 1. Selection of a Suitable Cocatalyst Based on 1aa
Table 3. Oligomerization of Ethylene with 1a-7a/Et2AlCla
oligomer distribution (%)c
C4/∑C R-C4 C6/∑C
93.6
oligomer distribution (%)c
entry
cocat.
Al/Ni
activityb
C4/∑C
R-C4
C6/∑C
entry
complex
activityb
1
2
3
Et2AlCl
MAO
MMAO
200
1000
1000
3.12
0.21
0.04
100
98.5
100
100
93.7
100
0
1.5
0
1
2
3
4
5
6
7
1a
2a
3a
4a
5a
6a
7a
3.69
4.46
14.5
12.5
5.43
6.14
10.4
98.0
97.5
94.6
94.3
97.3
97.4
97.8
2.0
2.5
5.4
5.7
2.7
2.6
2.2
90.3
77.9
77.7
89.4
86.5
89.2
a Reaction conditions: 5 µmol of Ni; 20 °C; 30 min; 10 atm of
ethylene; 100 mL of toluene. b Activity, 105
c Determined by GC.
g .
mol-1(Ni) h-1
Table 2. Ethylene Oligomerization of the 3a/Et2AlCl Systema
a Conditions: 5 µmol of Ni; Et2AlCl/Ni ) 200; 50 °C; reaction time,
30 min; 10 atm of ethylene; 100 mL of toluene. b Activity, 105
mol-1(Ni) h-1 c Determined by GC.
g
oligomer distribution (%)c
.
entry Al/Ni T (°C) activityb C4/∑C
R-C4
C6/∑C
1.2
temperature higher than room temperature;24 however, nitrogen-
multidentate coordinated cationic nickel(II) catalysts17,18 usually
present higher activity at room temperature than improved
temperature. To the best of our knowledge, this catalyst exhibits
best activity at 50 °C (entry 8 in Table 2) and represents a unique
characteristic among all of the cationic nickel complexes used
in ethylene oligomerization. This phenomenon might be at-
tributed to the singular thermal stability of the catalysts formed.
With an increase of reaction temperature, the catalytic reaction
rate and the reversible ꢀ-H elimination rate both increase, which
results in lower selectivity for R-C4 along with minor amounts
of trimers. Complex 3a exhibited an activity of 1.45 × 106 g
mol-1(Ni) h-1 at 50 °C with an Al/Ni molar ratio of 200 under
10 atm of ethylene. By further increasing the temperature from
50 to 70 °C, a decreased activity and lower R-C4 selectivity
were observed (entries 8-10 in Table 2).
1
2
3
4
5
6
7
8
100
200
300
400
600
200
200
200
200
200
200
20
20
20
20
20
30
40
50
60
70
50
0.67
3.43
3.14
2.97
1.44
5.52
6.77
14.5
9.49
3.87
98.8
97.0
97.5
97.9
100
97.1
3.0
2.5
2.1
0
3.8
3.7
5.4
6.5
0
96.4
95.9
93.8
89.8
88.1
77.9
77.5
69.1
100
96.2
96.3
94.6
93.5
100
94.5
9
10
11d
71.9
8.30
5.5
a Conditions: 5 µmol of Ni; 30 min; 10 atm of ethylene; 100 mL of
toluene. b Activity, 105 g mol-1(Ni) h-1 c Determined by GC. d 20
.
equiv of PPh3 as auxiliary ligand. ∑C denotes the total amount of
oligomers.
MMAO for ethylene oligomerization, the catalytic systems
showed low activities on the order of 104 g mol-1(Ni) h-1
.
However, when diethylaluminum chloride (Et2AlCl) was used
as the activator, the catalytic activity was an order of magnitude
higher (Table 1).
Including PPh3 as an auxiliary ligand in the nickel catalyst
system has led to great improvements in the catalytic activities
and lifetimes.7c,9e,17a,e,19b,25 The plausible role of PPh3 is to
stabilize the catalytically active species while being labile
enough to provide a site for ethylene coordination.3c8e For
complex 3a, the addition of 20 equiv of PPh3 resulted in
increased catalytic activity, but a drastic decrease in the
selectivity for R-butene was observed (entry 11 in Table 2).
2.3.2. Effect of the Nature of the Complexes on Their
Catalytic Activities. The nature of the nickel complexes is
dependent upon the ligands and the halide and affects their
catalytic behavior in ethylene oligomerization. The nickel
complexes would be better separated into two groups based upon
the coordinated halide [(L)NiCl2 (1a-7a) and (L)NiBr2
(1b-7b)]. Employing the optimum reaction conditions selected
by using complex 3a (molar ratio of Et2AlCl/Ni 200, temper-
ature 50 °C and 10 atm ethylene), all nickel complexes displayed
good to high catalytic activities. The results for the nickel
chlorides (L)NiCl2 (1a-7a) are collected in Table 3, and the
results for the nickel bromides (L)NiBr2 (1b-7b) in Table 4.
On the basis of these results with complex 1a, Et2AlCl was
selected as a practical activator for further investigation. All
nickel complexes would mainly dimerize ethylene with good
selectivity for R-C4 along with minor amounts of trimers (see
below). Compared with a typical good nickel catalyst for
ethylene dimerization, such as (Cy3P)2NiCl2,23 the described
catalytic nickel systems showed much higher selectivities for
R-C4.
2.3.1. Effects of the Ratio of Et2AlCl and Temperature
on Catalytic Behavior. The influence of the reaction parameters
on the catalytic activities, including the molar ratio of Et2AlCl
to nickel complex and reaction temperature, was investigated
with complex 3a. The detailed results are summarized in Table
2. Both the ratio of activator to nickel complex and the reaction
temperature significantly affected the catalytic activity, whereas
there is no obvious influence on the distribution of oligomers
produced.
Comparing the two sets of data, the effect of the ligands on
catalytic activities is clearly reflected. Since excessive amounts
of Et2AlCl were used in these catalytic systems, the function
of halide anions of complexes should become very weak.
However, the nickel chloride complexes (Table 3) generally
showed higher catalytic activities than their bromide analogues
(Table 4). The bonding strength of the Al-Cl is higher than
that of the Al-Br; therefore the trend for the chloride anion to
Initial experiments tested a Al/Ti molar ratio over the range
100-600 at 20 °C. The catalytic activity reached its optimum
value of 3.43 × 105 g mol-1(Ni) h-1 with a ratio of 200 (entry
2 in Table 2). Upon further increasing the Al/Ni ratio, a slight
decrease of the catalytic activity was observed. Compared to
the effect of the Al/Ni ratio, the reaction temperature had a
dramatic impact on the catalytic activity and selectivity.
Increasing the reaction temperature from 20 to 50 °C (entries 2
and 6-8 in Table 2) led to a significant increase in activity.
The SHOP catalysts present higher activity at the appropriate
(24) (a) Klabunde, U.; Ittel, S. D. J. Mol. Catal. 1987, 41, 123. (b)
Klabunde, U.; Mulhaupt, R.; Herskovitz, T.; Janowicz, A. H.; Calabrese,
J.; Ittel, S. D. J. Polym. Sci., Part A: Polym. Chem. 1987, 25, 1989. (c)
Pietsch, J.; Braunstein, P.; Chauvin, Y. New J. Chem. 1998, 467. (d) Kuhn,
P.; Se´meril, D.; Jeunesse, C.; Matt, D.; Neuburger, M.; Mota, A. Chem.-
Eur. J. 2006, 12, 5210.
(23) Speiser, F.; Braunstein, P.; Saussine, L. Acc. Chem. Res. 2005, 38,
784.
(25) Jenkins, J. C.; Brookhart, M. Organometallics 2003, 22, 250.

Ni(II) Complexes Chelated by Arylimino-benzoxazolylpyridine
Organometallics, Vol. 27, No. 21, 2008 5645
Table 4. Oligomerization of Ethylene with 1b-7b/Et2AlCla
theory (DFT),27 integrated molecular orbital-molecular me-
chanics (IMOMM) method,28 low-temperature 1H NMR
spectroscopy,4a,29 and in situ UV-vis spectroscopy.30 An
alkylated nickel active species was isolated as an intermediate
in the related nickel catalytic system.7c Tridentate Ni(II) catalysts
used for ethylene activation have been reported,14-18 and the
distribution of products was determined by rates of chain
propagation and chain transfer rates. In our system, the nickel
catalyst precursors showed high activity and selectivity in
forming C4, and a similar phenomenon has been observed in
other N∧N∧N tridentate nickel(II) complexes.17,18 According
to the mechanism proposed by Braunstein,14 the formation of
various hexenes could be inferred by reincorporation of butene.
That the title catalysts showed high selectivity for forming R-C4
might be attributed to the fact that rates of chain transfer are
much higher than chain propagation rates.
Compared with nickel complexes bearing 2-benzimidazole-
6-(1-aryliminoethyl)pyridines,19b,d the title complexes containing
a 2-benzoxazole substituent present much higher catalytic
activities as well as better thermal stability. The benzoxazole
substituent on the ligand backbone is helpful in increasing the
net charge on the metal center, thus having a positive impact
on the catalytic activity and the stability of the active center.
All nickel complexes show good activity even at relatively high
temperatures. These results also are in line with the computa-
tional conclusions on the relationship of catalytic activity to
the net charge of the metal center in late-transition metal
complexes, in that the stronger the electron-donating ability of
the ligand, the lower the net charge on the metal, which results
in lower catalytic activities.26
oligomer distribution (%)c
entry
complex
activityb
C4/∑C
97.7
R-C4
91.5
90.6
81.7
87.9
90.3
91.2
90.2
C6/∑C
2.3
1
2
3
4
5
6
7
1b
2b
3b
4b
5b
6b
7b
1.74
3.48
8.31
5.67
4.03
3.12
6.24
95.1
94.6
94.2
95.6
97.3
98.8
4.9
5.4
5.8
4.4
2.7
1.2
a Conditions: 5 µmol of Ni; Et2AlCl/Ni ) 200; 50 °C; reaction time,
30 min; 10 atm of ethylene; 100 mL of toluene. b Activity, 105
mol-1(Ni) h-1 c Determined by GC.
g
.
be lost is easier than that of the bromide anion in their nickel
complexes, and then the net positive charge on the nickel center
bearing two chloride anions is higher than the ones bearing two
bromide anions. This results in higher catalytic activities for
the nickel complexes bearing two chloride anions. These results
also agree with our previous computational conclusion on the
relationship of catalytic activity with the net charge of late-
transition metal complexes.26 We have also chosen to investigate
how the nature of the substituents as well as the substitution
pattern on the imino-phenyl group of the 2-(2-benzoxazolyl)-
6-(1-aryliminoethyl)pyridine ligands affects the catalytic activity
of the nickel complexes. With 2,6-dialkyl substituents (entries
1-3 in Table 3 and 1-3 in Table 4), the bulkier substituents
led to increases in the catalytic activity of the nickel complexes.
The active sites in the systems with less bulky substituents are
exposed to not only ethylene but also other reactants (impurities),
and this leads to the deactivation of the active species. In
addition, the bulkier substituents increase the solubility of the
complexes.
3. Conclusions
A series of nickel(II) complexes bearing 2-benzoxazolyl-6-
(1-aryliminoethyl)pyridines has been synthesized and fully
characterized. Upon treatment with Et2AlCl, these nickel (II)
complexes showed high catalytic activities of up to 1.45 × 106
g mol-1(Ni) h-1 at 50 °C, which is a higher activity and due to
better thermostability compared to the related cationic nickel
catalysts.19b,d,21b The nickel chloride catalyst showed better
activity than the bromide analogues. The R-substituents on the
N-aryl ring influenced the catalytic activities. The bulkier the
alkyl substituents the ligand has, the higher catalytic activity
the complex shows; complexes with additional substituents at
the 4-position of the aryl group had even higher activity. When
PPh3 was employed as an auxiliary ligand, the catalytic activity
The catalytic activities of complexes with 2,6-dihalo-
substituted imino-phenyl groups (entries 4, 5 in Tables 3 and
4) are comparable, but better activities for the chloro-substituted
complexes were observed. Though the halogen atoms are far
from the nickel atom, the net charge of active species would
still be partly affected, and the more positive the net charge of
nickel is, the more active the species.26 Surely, we could not
figure out that the less steric hindrance of chlorine than bromine
could provide more appropriate space for ethylene to insert and
result in better activity. The complexes bearing 2,6-dihalo-
substituted imino-phenyl groups did not show better activities
than complexes bearing 2,6-diisopropyl substituents. A possible
reason could be the better solubility of complexes bearing 2,6-
diisopropyl substituents. In the same manner, by placing a
substituent at the 4-position of the aryl ring, it is possible to
increase the solubility of the nickel complex. This would explain
the higher catalytic activities of complexes 6a and 6b containing
a methyl group at the 4-position (entry 6 in Tables 3 and 4)
and 7a and 7b with a bromo group at the 4-position of aryl
group (entry 7 in Tables 3 and 4), when compared to their 2,6-
dimethyl analogues 1a and 1b (entry 1 in Tables 3 and 4).
Moreover, complexes 7a and 7b showed the highest activity.
increased to up to 7.19 × 106 g mol-1(Ni) h-1
.
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. 2-(2-Benzoxazole)-
6-acetylpyridine was prepared using a modified procedure according
to the literature.19a Toluene was refluxed over sodium-benzophe-
The mechanism of ethylene oligomerization and polymeri-
zation by traditional Brookhart-type Ni(II) and Pd(II) diimine
catalysts and tridentate bis(imino)pyridine Fe(II) and Co(II)
catalysts has been extensively studied by using density functional
(27) (a) Margl, P.; Deng, L.; Ziegler, T. Organometallics 1999, 18, 5701.
(b) Deng, L.; Margl, P.; Ziegler, T. J. Am. Chem. Soc. 1999, 121, 6479.
(28) Froese, R. D. J.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc.
1998, 120, 1581.
(29) (a) Johnson, L. K.; Mecking, S.; Brookhart, M. J. Am. Chem. Soc.
1996, 118, 267. (b) Svejda, S.; Johnson, L.; Brookhart, M. J. Am. Chem.
Soc. 1999, 121, 10634. (c) Leatherman, M. D.; Svejda, S. A.; Johnson,
L. K.; Brookhart, M. J. Am. Chem. Soc. 2003, 125, 3068.
(30) (a) Peruch, F.; Cramail, H.; Deffieux, A. Macromolecules 1999,
32, 7977. (b) Luo, H.; Yang, Z.; Mao, B.; Yu, D.; Tang, R. J. Mol. Catal.
A: Chem. 2002, 177, 195. (c) Schmidt, R.; Das, P.; Welch, M.; Knudsen,
R. J. Mol. Catal. A: Chem. 2004, 222, 27.
(26) (a) Guo, D.; Han, L.; Zhang, T.; Sun, W.-H.; Li, T.; Yang, X.
Macromol. Theory Simul. 2002, 11, 1006. (b) Sun, W.-H.; Tang, X.; Gao,
T.; Wu, B.; Zhang, W.; Ma, H. Organometallics 2004, 23, 5037. (c) Zhang,
T.; Guo, D.; Jie, S.; Sun, W.-H.; Li, T.; Yang, X. J. Polym. Sci., Part A:
Polym. Chem. 2004, 42, 4765.

5646 Organometallics, Vol. 27, No. 21, 2008
Gao et al.
none and distilled under argon prior to use. Methylaluminoxane
(MAO, a 1.46 M solution in toluene) and modified methylalumi-
noxane (MMAO, 1.93 M in heptane, 3A) were purchased from
Akzo Nobel Corp. Diethylaluminum chloride (Et2AlCl2, 1.7 M in
toluene) was purchased from Acros Chemicals. Other reagents were
purchased from Aldrich or Acros Chemicals. 1H and 13C NMR
spectra were recorded on a Bruker DMX 300 or 400 MHz
instrument at ambient temperature using TMS as an internal
standard. FT-IR spectra were recorded on a Perkin-Elmer System
2000 FT-IR spectrometer. Elemental analyses were carried out using
a Flash EA 1112 microanalyzer. 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 µm film thickness)
CP-Sil 5 CB column. The yields of oligomers were calculated by
referencing with the mass of the solvent, on the basis of the
prerequisite that the mass of each fraction was approximately
proportional to its integrated areas in the GC trace. Selectivity for
the R-C4 was defined as (amount of R-C4)/(total amounts of C4) in
percent.
4.2. Syntheses and Characterization. 2,6-Dimethyl-N-(1-(6-
(benzo[d]oxazol-2-yl)pyridin-2-yl)ethylidene)benzenamine (L1). A
solution of 2-(2-benzoxazole)-6-acetylpyridine (0.95 g, 4 mmol),
2,6-dimethylaniline (0.67 g, 4.8 mmol), and a catalytic amount of
p-toluenesulfonic acid in toluene (50 mL) was refluxed for 24 h.
After solvent evaporation, the crude product was purified with
petroleum ether/ethyl acetate (v/v ) 20:1) on an alumina column.
The second part to elute was collected and concentrated to give a
yellow powder (0.70 g, 51% yield). Mp: 136-137 °C. FT-IR (KBr
disk, cm-1): 3069, 1644 (νCdN), 1581, 1552, 1453, 1365, 1245,
1208, 1155, 1110, 1092, 1075, 823, 762. 1H NMR (400 MHz,
CDCl3, TMS): δ 8.56 (d, 1H, J ) 7.9, Py, Hm), 8.45 (d, 1H, J )
7.7, Py, Hm), 8.02 (t, 1H, J ) 7.9, Py, Hp), 7.88-7.86 (m, 1H, Ar
H), 7.70-7.68 (m, 1H, Ar H), 7.45-7.39 (m, 2H, Ar H), 7.09 (d,
2H, J ) 7.5, Ar H), 6.96 (t, 1H, J ) 7.4, Ar H), 2.36 (s, 3H, CH3),
2.06 (s, 6H, 2 × CH3). 13C NMR (300 MHz, CDCl3, TMS): δ
167.0, 156.9, 151.1, 148.6, 145.2, 141.9, 137.4, 128.0, 126.0, 125.3,
124.9, 124.7, 123.2, 120.8, 111.2, 18.0, 16.7. Anal. Calcd for
C22H19N3O (341.41): C, 77.40; H, 5.61; N, 12.31. Found: C, 77.21;
H, 5.64; N, 12.42.
126.0, 124.9, 124.7, 123.7, 123.2, 123.0, 120.7, 111.2, 28.3, 23.2,
22.9, 17.3. Anal. Calcd for C26H27N3O (397.51): C, 78.56; H, 6.85;
N, 10.57. Found: C, 78.32; H, 6.67; N, 10.54.
2,6-Dichloro-N-(1-(6-(benzo[d]oxazol-2-yl)pyridin-2-yl)ethyl-
idene)benzenamine (L4). A solution of 2-(2-benzoxazole)-6-
acetylpyridine (0.72 g, 3 mmol), 2,6-dichoroaniline (0.64 g, 3.9
mmol), and a catalytic amount of p-toluenesulfonic acid in tetraethyl
silicate in toluene solvent (3 mL) was refluxed for 24 h. After
solvent evaporation, the crude product was purified with petroleum
ether/ethyl acetate (v/v ) 20:1) on an alumina column. The second
part to elute was collected and concentrated to give a white powder
in 12.0% yield. Mp: 170-171 °C. FT-IR (KBr disk, cm-1): 3066,
1646 (νCdN), 1585, 1550, 1454, 1434, 1367, 1245, 1225, 1155,
1116, 1077, 786, 767, 745. 1H NMR (400 MHz, CDCl3, TMS): δ
8.56 (d, 1H, J ) 7.8, Py, Hm), 8.48 (d, 1H, J ) 7.7, Py, Hm), 8.03
(t, 1H, J ) 7.8, Py, Hp), 7.89-7.86 (m, 1H, Ar H), 7.70-7.68 (m,
1H, Ar H), 7.46-7.40 (m, 2H, Ar H), 7.37 (d, 2H, J ) 8.0, Ar H),
7.02 (t, 1H, J ) 8.0, Ar H), 2.48 (s, 3H, CH3). 13C NMR (300
MHz, CDCl3, TMS): δ 170.5, 160.7, 155.4, 150.4, 144.8, 144.6,
141.2, 137.0, 127.6, 125.4, 124.6, 124.3, 123.9, 123.8, 123.2, 120.1,
110.5, 17.1. Anal. Calcd for C20H13Cl2N3O (382.24): C, 62.84; H,
3.43; N, 10.99. Found: C, 62.72; H, 3.44; N, 11.02.
2,6-Dibromo-N-(1-(6-(benzo[d]oxazol-2-yl)pyridin-2-yl)ethyl-
idene)benzenamine (L5). Using the same procedure as for the
synthesis of L4, L5 was obtained as a white powder in 43% yield.
Mp: 169-170 °C. FT-IR (KBr disk, cm-1): 3063, 1647 (νCdN),
1583, 1549, 1454, 1432, 1402, 1368, 1243, 1223, 1153, 1120, 1093,
1076, 853, 762. 1H NMR (400 MHz, CDCl3, TMS): δ 8.58 (d,
1H, J ) 7.8, Py, Hm), 8.49 (d, 1H, J ) 7.8, Py, Hm), 8.04 (t, 1H,
J ) 7.8, Py, Hp), 7.89-7.87 (m, 1H, Ar H), 7.71-7.68 (m, 1H, Ar
H), 7.59 (d, 2H, J ) 8.1, Ar H), 7.46-7.40 (m, 2H, Ar H), 6.88 (t,
1H, J ) 8.0, Ar H), 2.47 (s, 3H, CH3). 13C NMR (400 MHz, CDCl3,
TMS): δ 171.1, 161.5, 156.1, 151.2, 148.1, 145.5, 142.0, 137.8,
132.1, 126.2, 125.6, 125.4, 125.1, 124.0, 120.9, 113.6, 111.3, 18.0.
Anal. Calcd for C20H13Br2N3O (471.14): C, 50.99; H, 2.78; N, 8.92.
Found: C, 50.84; H, 2.84; N, 8.69.
2,4,6-Trimethyl-N-(1-(6-(benzo[d]oxazol-2-yl)pyridin-2-yl)-
ethylidene)benzenamine (L6). Using the same procedure as for the
synthesis of L1, L6 was obtained as a yellow powder in 57% yield.
Mp: 150-151 °C. FT-IR (KBr disk, cm-1): 3069, 1645(νCdN),
1550, 1479, 1453, 1364, 1245, 1215, 1146, 1116, 1074, 850, 822,
2,6-Diethyl-N-(1-(6-(benzo[d]oxazol-2-yl)pyridin-2-yl)ethyl-
idene)benzenamine (L2). Using the same procedure as for the
synthesis of L1, L2 was obtained as a yellow powder in 69% yield.
Mp: 123-124 °C. FT-IR (KBr disk, cm-1): 2966, 1651 (νCdN),
1584, 1569, 1551, 1455, 1403, 1364, 1326, 1244, 1200, 1156, 1115,
1074, 855, 752. 1H NMR (300 MHz, CDCl3, TMS): δ 8.55 (d,
1H, J ) 7.9, Py, Hm), 8.45 (d, 1H, J ) 7.8, Py, Hm), 8.01 (t, 1H,
J ) 7.9, Py, Hp), 7.87-7.85 (m, 1H, Ar H), 7.72-7.66 (m, 1H, Ar
H), 7.46-7.38 (m, 2H, Ar H), 7.13 (d, 2H, J ) 7.6, Ar H),
7.08-6.96 (m, 1H, Ar H), 2.42 (q, 2H, J ) 7.8, CH2), 2.37 (s, 3H,
CH3), 2.34 (q, 2H, CH2), 1.15 (t, 6H, J ) 6.5, 2 × CH3). 13C NMR
(400 MHz, CDCl3, TMS): δ 166.2, 161.1, 156.4, 150.6, 147.2,
144.7, 141.4, 137.0, 130.6, 127.1, 125.5, 124.5, 124.2, 123.1, 122.6,
120.3, 117.8, 110.7, 24.2, 16.6, 13.3. Anal. Calcd for C24H23N3O
(369.46): C, 78.02; H, 6.27; N, 11.37. Found: C, 77.82; H, 6.24;
N, 11.43.
1
744. H NMR (400 MHz, CDCl3, TMS): δ 8.55 (d, 1H, J ) 7.9,
Py, Hm), 8.44 (d, 1H, J ) 7.6, Py, Hm), 8.00 (t, 1H, J ) 7.8, Py,
Hp), 7.88-7.86 (m, 1H, Ar H), 7.70-7.68 (m, 1H, Ar H),
7.46-7.39 (m, 2H, Ar H), 6.91 (s, 2H, Ar H), 2.35 (s, 3H, CH3),
2.30 (s, 3H, CH3), 2.02 (s, 6H, 2 × CH3). 13C NMR (400 MHz,
CDCl3, TMS): δ 167.3, 161.7, 157.2, 151.2, 146.2, 145.3, 142.0,
137.6, 132.5, 129.0, 128.8, 126.1, 125.3, 125.1, 124.8, 123.3, 120.9,
111.3, 20.9, 18.0, 16.8. Anal. Calcd for C23H21N3O (355.43): C,
77.72; H, 5.96; N, 11.82. Found: C, 77.64; H, 6.02; N, 11.88.
4-Bromo-2,6-dimethyl-N-(1-(6-(benzo[d]oxazol-2-yl)pyridin-
2-yl)ethylidene)benzenamine (L7). Using the same procedure as
for the synthesis of L1, L7 was obtained as a yellow powder in
56% yield. Mp: 170-171 °C. FT-IR (KBr disk, cm-1): 2924, 1642
(νCdN), 1583, 1554, 1453, 1365, 1244, 1207, 1154, 1108, 1074,
1
2,6-Diisopropyl-N-(1-(6-(benzo[d]oxazol-2-yl)pyridin-2-yl)-
ethylidene)benzenamine (L3). Using the same procedure as for
the synthesis of L1, L3 was obtained as a yellow powder in 50%
yield. Mp: 159-160 °C. FT-IR (KBr disk, cm-1): 2964, 1651
(νCdN), 1586, 1550, 1456, 1366, 1324, 1242, 1190, 1116, 1076,
766, 751, 670. 1H NMR (400 MHz, CDCl3, TMS): δ 8.56 (d, 1H,
J ) 7.9, Py, Hm), 8.45 (d, 1H, J ) 7.8, Py, Hm), 8.01 (t, 1H, J )
7.9, Py, Hp), 7.89-7.86 (m, 1H, Ar H), 7.70-7.67 (m, 1H, Ar H),
7.46-7.38 (m, 2H, Ar H), 7.21-7.18 (m, 2H, Ar H), 7.14-7.09
(m, 1H, Ar H), 2.81-2.74 (m, 2H, 2 × CH), 2.40 (s, 3H, CH3),
1.18-1.15 (m, 12H, 4 × CH3). 13C NMR (400 MHz, CDCl3, TMS):
δ 166.7, 161.5, 156.8, 151.0, 146.2, 145.1, 141.8, 137.5, 135.7,
835, 744. H NMR (400 MHz, CDCl3, TMS): δ 8.52 (d, 1H, J )
7.9, Py, Hm), 8.45 (d, 1H, J ) 7.6, Py, Hm), 8.01 (t, 1H, J ) 7.8,
Py, Hp), 7.88-7.86 (m, 1H, Ar H), 7.70-7.67 (m, 1H, Ar H),
7.46-7.39 (m, 2H, Ar H), 7.23 (s, 2H, Ar H), 2.35 (s, 3H, CH3),
2.02 (s, 6H, 2 × CH3). 13C NMR (400 MHz, CDCl3, TMS): δ
167.9, 161.6, 156.7, 151.2, 147.8, 145.4, 142.0, 137.6, 130.7 × 3,
127.8 × 2, 126.2, 125.0, 123.3, 120.9, 115.9, 111.3, 17.9 × 2, 17.0.
Anal. Calcd for C22H18BrN3O (420.30): C, 62.87; H, 4.32; N, 10.00.
Found: C, 62.47; H, 4.54; N, 10.38.
4.3. Synthesis of Nickel Complexes. General Procedure: A
solution of NiCl2 · 6H2O in ethanol or (DME)NiBr2 in THF was
added dropwise to a solution of the corresponding ligand in ethanol

Ni(II) Complexes Chelated by Arylimino-benzoxazolylpyridine
Organometallics, Vol. 27, No. 21, 2008 5647
(THF). The reaction mixture was stirred at room temperature for
12 h, forming a precipitate. The resulting precipitate was collected,
washed with diethyl ether, and dried under vacuum. All of the
complexes were prepared in high yield in this manner.
1a: gray powder in 84% yield. FT-IR (KBr disk, cm-1): 2916,
1615 (νCdN), 1595, 1544, 1449, 1377, 1280, 1216, 1173, 1101,
1029, 812, 792, 768. Anal. Calcd for C22H19Cl2N3NiO (471.01):
C, 56.10; H, 4.07; N, 8.92. Found: C, 55.75; H, 3.98; N, 8.70.
2a: gray powder in 70% yield. FT-IR (KBr disk; cm-1): 2972,
1611 (νCdN), 1594, 1571, 1544, 1449, 1377, 1278, 1211, 1195,
1104, 1028, 814, 788, 769, 750. Anal. Calcd for C24H23Cl2N3NiO
(499.06): C, 57.76; H, 4.65; N, 8.42. Found: C, 57.36; H, 4.71; N,
8.27.
1029, 814, 791, 765. Anal. Calcd for C22H19Br2N3NiO (559.91):
C, 47.19; H, 3.42; N, 7.50. Found: C, 46.89; H, 3.53; N, 7.10.
2b: gray powder in 88% yield. FT-IR (KBr disk; cm-1): 2975,
1610 (νCdN), 1594, 1571, 1543, 1448, 1378, 1278, 1211, 1194,
1172, 1103, 1028, 812, 788, 764, 750. Anal. Calcd for
C24H23Br2N3NiO (587.96): C, 49.03; H, 3.94; N, 7.15. Found: C,
48.96; H, 4.07; N, 6.98.
3b: gray powder in 86% yield. FT-IR (KBr disk; cm-1): 2960,
1612 (νCdN), 1594, 1543, 1448, 1378, 1278, 1209, 1193, 1103,
1028, 816, 788, 765. Anal. Calcd for C26H27Br2N3NiO (616.01):
C, 50.69; H, 4.42; N, 6.82. Found: C, 50.75; H, 4.68; N, 6.57.
4b: gray powder in 72% yield. FT-IR (KBr disk; cm-1): 2904,
1616 (νCdN), 1596, 1543, 1439, 1378, 1280, 1233, 1175, 1088,
1030, 855, 783, 770. Anal. Calcd for C20H13Br2Cl2N3NiO (600.74):
C, 39.99; H, 2.18; N, 6.99. Found: C, 39.60; H, 2.33; N, 6.90.
5b: gray powder in 82% yield. FT-IR (KBr disk; cm-1): 3060,
1612 (νCdN), 1593, 1541, 1430, 1377, 1279, 1229, 1174, 1088,
1026, 854, 788, 758, 730. Anal. Calcd for C20H13Br4N3NiO
(689.65): C, 34.83; H, 1.90; N, 6.09. Found: C, 34.65; H, 1.92; N,
5.84.
6b: gray powder in 87.2% yield. FT-IR (KBr; cm-1): 2918, 1610
(νCdN), 1592, 1538, 1448, 1370, 1276, 1217, 1172, 1026, 814, 772,
744. Anal. Calcd for C23H21Br2NiN3O (573.93): C, 48.13; H, 3.69;
N, 7.32. Found: C, 48.24; H, 3.77; N, 7.42.
7b: gray powder in 85.1% yield. FT-IR (KBr disk, cm-1): 2909,
1612 (νCdN), 1593, 1540, 1450, 1379, 1279, 1212, 1030, 1002, 867,
760. Anal. Calcd for C22H18Br3NiN3O (638.80): C, 41.36; H, 2.84;
N, 6.58. Found: C, 41.62; H, 2.88; N, 6.74.
4.3. X-ray Crystallographic Studies. Single crystals of 6a, 1b,
and 5b suitable for X-ray diffraction studies were obtained by
diffusing a diethyl ether layer into their methanol solutions. Single-
crystal X-ray diffraction studies for 6a, 1b, and 5b were carried
out on a Rigaku RAXIS Rapid IP diffractometer with graphite-
monochromated Mo KR radiation (λ) 0.71073 Å). Cell parameters
were obtained by global refinement of the positions of all collected
3a: gray powder in 62% yield. FT-IR (KBr disk; cm-1): 3066,
2962, 2869, 1613 (νCdN), 1594, 1544, 1449, 1377, 1278, 1210,
1193, 1102, 1027, 817, 788, 769, 746. Anal. Calcd for
C26H27Cl2N3NiO (527.11): C, 59.24; H, 5.16; N, 7.97. Found: C,
59.53; H, 5.23; N, 7.98.
4a: gray powder in 62% yield. FT-IR (KBr disk; cm-1): 3051,
1617 (νCdN), 1595, 1544, 1439, 1378, 1280, 1234, 1176, 1089,
1030, 814, 773. Anal. Calcd for C20H13Cl4N3NiO (511.84): C, 46.93;
H, 2.56; N, 8.21. Found: C, 46.63; H, 2.45; N, 8.14.
5a: gray powder in 76% yield. FT-IR (KBr disk; cm-1): 3046,
1616 (νCdN), 1595, 1546, 1431, 1377, 1279, 1230, 1177, 1089,
1029, 773, 730. Anal. Calcd for C20H13Br2Cl2N3NiO (600.74): C,
39.99; H, 2.18; N, 6.99. Found: C, 39.73; H, 2.25; N, 6.93.
6a: gray powder in 89.7% yield. FT-IR (KBr; cm-1): 2917, 1611
(νCdN), 1592, 1541, 1448, 1373, 1277, 1219, 1173, 1028, 815, 772,
745. Anal. Calcd for. C23H21Cl2NiN3O (485.03): C, 56.95; H, 4.36;
N, 8.66. Found: C, 56.77; H, 4.32; N, 8.57.
7a: gray powder in 83.3% yield. FT-IR (KBr disk, cm-1): 2911,
1618 (νCdN), 1595, 1548, 1453, 1374, 1276, 1211, 1023, 995, 865,
772. Anal. Calcd for C22H18BrCl2NiN3O (549.90): C, 48.05; H, 3.30;
N, 7.64. Found: C, 47.86; H, 3.43; N, 7.76.
1b: gray powder in 88% yield. FT-IR (KBr disk; cm-1): 3410,
1614 (νCdN), 1594, 1543, 1449, 1377, 1279, 1215, 1173, 1101,
Table 5. Crystal Data and Structure Refinement Details for 1a, 1b, and 5b
6a · 3CH3OH
1b
5b · CH3OH
formula
fw
C23H21Cl2N3NiO · 3CH3OH
581.16
C22H19Br2N3NiO
559.93
C20H13Br4N3NiO · CH3OH
721.67
T (K)
173(2)
173(2)
173(2)
wavelength (Å)
cryst syst
space group
a (Å)
b (Å)
c (Å)
0.71073
monoclinic
P2(1)/c
0.71073
0.71073
monoclinic
P2(1)/c
11.110(2)
15.427(3)
15.714(6)
90
121.36(2)
90
2299.8(1)
4
2.084
7.817
1392
triclinic
j
P1
9.984(2)
9.2389(2)
9.817(2)
11.656(2)
91.31(3)
98.99(3)
95.83(3)
4643.87(1)
2
1.791
12.774(3)
22.118(4)
90
95.56(3)
90
2807.6(1)
4
1.375
0.917
1216
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Dcalc (Mg m-3
)
µ (mm-1
F(000)
)
4.803
556
cryst size (mm)
θ range (deg)
limiting indices
0.52 × 0.40 × 0.33
1.84 - 25.00
-11 e h e 11,
-15 e k e 15,
-26 e l e 26
9471
0.42 × 0.21 × 0.11
2.24 - 25.00
-10 e h e 10,
-11 e k e 11,
-13 e l e 13
6732
3646
99.9 (θ ) 25.00)
multiscan
0.28 × 0.23 × 0.22
2.15 - 25.00
-13 e h e 13,
-18 e k e 18,
-18 e l e 18
7498
no. of rflns collected
no. of unique rflns
completeness to θ (%)
abs corr
data/restraints/params
goodness of fit on F2
final R indices (I >2σ(I))
4935
4053
99.8 (θ ) 25.00)
multiscan
4935/0/338
1.166
R1 ) 0.0497,
wR2 ) 0.1003
R1 ) 0.0619,
wR2 ) 0.1052
0.537 and -0.569
99.8 (θ ) 25.00)
multiscan
4053/1/296
1.102
R1 ) 0.0418,
wR2 ) 0.0654
R1 ) 0.0630,
wR2 ) 0.0693
0.584 and -0.518
3646/0/262
1.139
R1 ) 0.0532,
wR2 ) 0.0900
R1 ) 0.0760,
wR2 ) 0.0957
0.628 and -0.634
R indices (all data)
largest diff peak and hole (e Å-3
)

5648 Organometallics, Vol. 27, No. 21, 2008
Gao et al.
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.31
Crystal data and processing parameters for 6a, 1b, and 5b are
summarized in Table 5.
ethylene atmosphere. When the desired reaction temperature was
reached, ethylene at the desired pressure was introduced to start
the reaction. After the desired time, the reaction was stopped. A
small amount of the reaction solution was collected; the reaction
was terminated by the addition of 5% aqueous hydrogen chloride
and then analyzed by gas chromatography (GC) to determine the
distribution of oligomers obtained.
4.4. General Procedure for Ethylene Oligomerization. Eth-
ylene oligomerization at 10 atm ethylene pressures was carried out
in a 500 mL stainless steel autoclave equipped with a mechanical
stirrer and a temperature controller. The catalyst (and PPh3) was
dissolved in toluene in a Schlenk tube with stirring, and 50 mL of
toluene and the desired amount of EtAlCl2 were added. Then the
mixture and 50 mL of toluene were added into the autoclave under
Acknowledgment. The project was supported by NSFC
No. 20674089. We are grateful to Dr. Steven Schultz for
proofreading the manuscript.
Supporting Information Available: Crystallographic data for
6a, 1b, and 5b are available free of charge via the Internet at
http://pubs.acs.org.
(31) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal Structure; Universita¨t of Go¨ttingen, Go¨ttingen: Germany, 1997.
OM800647W