Novel neutral arylnickel(II) phosphine catalysts containing 2-oxazolinylphenolato N-O chelate ligands for ethylene oligomerization and propylene dimerization

Article abstract of DOI:10.1016/j.jorganchem.2004.05.021

A series of new neutral arylnickel(II) phosphine complexes 1 bearing 2-oxazolinylphenolato ligands [2-(4-R¹-5-R²-C₃H₂NO) -C₆H₄O]Ni (2-R⁴-4-R³-C₆H₃ (PPh₃) were synthesized by reactions of sodium salts of 2-(4,5-dihydro-2-oxazolyl)phenol derivatives with trans-Ni(Ar)(Cl (PPh₃)₂ or by direct reactions of the ligands with trans-Ni(Ar)(Cl)(PPh₃)₂ in the presence of NEt₃. These neutral Ni(II) complexes 1 exhibited high activities and selectivities in ethylene oligomerization and propylene dimerization. The catalytic activities and the product distributions were dependent on the selection of various organoaluminum cocatalysts and phosphine scavenger (Ni(COD)₂). The effects of various reaction conditions on ethylene oligomerization were also examined. The highest activity of 5.51×10⁵ g oligomers/(mol Ni · h) and 83% selectivity of C₆ internal olefins were obtained in 1a/MAO catalytic system in ethylene oligomerization. The oligomers consisted mainly of lower carbon olefins in the range of C₄-C₈. Complexes 1 showed the moderate tolerance of polar additives in ethylene oligomerization. The highest activity of 1a/MAO in propylene dimerization reached to 1.32×10⁵ g oligomers/(mol Ni · h).

Full text of DOI:10.1016/j.jorganchem.2004.05.021

Journal of Organometallic Chemistry 689 (2004) 2614–2623
www.elsevier.com/locate/jorganchem
Novel neutral arylnickel(II) phosphine catalysts containing
2-oxazolinylphenolato N–O chelate ligands for
ethylene oligomerization and propylene dimerization
*
Wei Zhao, Yanlong Qian, Jiling Huang , Jianjun Duan
Laboratory of Organometallic Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
Received 28 January 2004; accepted 18 May 2004
Available online 6 July 2004
Dedicated to Professor Yanlong Qian
Abstract
A series of new neutral arylnickel(II) phosphine complexes 1 bearing 2-oxazolinylphenolato ligands [2-(4-R1-5-R2-C3H2NO)–
C6H4O]Ni(2-R4-4-R3-C6H3)(PPh3) were synthesized by reactions of sodium salts of 2-(4,5-dihydro-2-oxazolyl)phenol derivatives
with trans-Ni(Ar)(Cl)(PPh3)2 or by direct reactions of the ligands with trans-Ni(Ar)(Cl)(PPh3)2 in the presence of NEt3. These neu-
tral Ni(II) complexes 1 exhibited high activities and selectivities in ethylene oligomerization and propylene dimerization. The cata-
lytic activities and the product distributions were dependent on the selection of various organoaluminum cocatalysts and phosphine
scavenger (Ni(COD)2). The effects of various reaction conditions on ethylene oligomerization were also examined. The highest ac-
tivity of 5.51·105 g oligomers/(mol NiÆh) and 83% selectivity of C6 internal olefins were obtained in 1a/MAO catalytic system in
ethylene oligomerization. The oligomers consisted mainly of lower carbon olefins in the range of C4–C8. Complexes 1 showed the
moderate tolerance of polar additives in ethylene oligomerization. The highest activity of 1a/MAO in propylene dimerization
reached to 1.32·105 g oligomers/(mol NiÆh).
ꢀ 2004 Published by Elsevier B.V.
Keywords: Nickel complexes; Oxazoline ligand; Ethylene oligomerization; Propylene dimerization
1. Introduction
metal complexes did. Nickel and palladium complexes
containing a-diimine ligands as olefin polymerization
catalysts had been reported by Brookhart and
co-workers [5]. These complexes could not only poly-
merize ethylene to high-branched polymers but also
copolymerize ethylene and polar-functionalized a-olefins
[6]. Grubbs and co-workers [7,8] described the neutral
salicylaldiminato nickel complexes, which exhibited
high catalytic activity and good tolerance of polar
functional group in ethylene polymerization [9]. In
addition, Brookhart [10] and Gibson [11] groups,
reported highly active cationic iron and cobalt olefin
polymerization catalysts with bulky pyridine diimine
Late transition metal catalysts attracted consider-
able attentions in both scientific and commercial fields
in transforming a-olefins to more valuable oligomers
or polymers [1–4]. Because of their less oxophilic na-
ture, late transition metal complexes generally had
more tolerance of polar media than early transition
*
Corresponding author. Tel.: +86-21-6425-3519; fax: +86-21-5428-
2375.
E-mail address: qianling@online.sh.cn (J. Huang).
0022-328X/$ - see front matter ꢀ 2004 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2004.05.021

W. Zhao et al. / Journal of Organometallic Chemistry 689 (2004) 2614–2623
2615
ligands independently, whose activities were up to 107
TOF. At the same time, Mecking [12–14] and Claverie
2. Results and discussion
ˆ
[15–17] groups, successfully applied Ni(P O), Ni(N O),
ˆ
2.1. Synthesis and structure characterization
ˆ
and Pd(N N) chelating complexes in aqueous emulsion
ethylene polymerization with the coming out of the
stable polymer latices.
Two synthetic routes of neutral nickel complexes 1
are shown in Scheme 1. In route b, similarly to the
synthesis of salicylaldiminato nickel complexes [7], the
reaction of the sodium salt of 2-(5-methyl-4,5-dihydro-
2-oxazolyl)phenol or 2-(4-phenyl-4,5-dihydro-2-oxazolyl)-
phenol with trans-Ni(Ar)(Cl)(PPh3)2 results in the
formation of complexes 1b, 1e and 1f. However, the sol-
ubility of sodium salt obtained by the reaction of 2-(4,5-
dihydro-2-oxazolyl)phenol with NaH is very poor,
which leads to lower yield. In view of the high acidity
of the hydroxyl proton, route a is adopted to obtain
target complexes 1a, 1c and 1d in the presence of
NEt3. Although there are some by-products of bis(2-ox-
azolinylphenolato)nickel complexes 2, the separation of
these two kinds of complexes 1 and 2 is very easy ac-
cording to their different solubility in toluene.
The IR spectra of the free oxazoline ligands show a
very strong absorption band at 1642 cmÀ1 attributed
to mC‚N, while the mC‚N of complexes 1 is in 1608–
1617 cmÀ1. The shift of the C‚N stretching absorption
band to lower frequency indicates the decrease of the
C‚N stretching force constant as a consequence of
the coordination through the nitrogen atom, which is
in agreement with the results of X-ray diffraction [41].
The strong absorption bands in about 1435 (mP–C) and
693 (mNi–P) cmÀ1 can confirm the existence of PPh3
[42]. The weak absorption bands of 1 in 572–576 cmÀ1
and 460–464 cmÀ1 may be attributed to mNi–N and
Furthermore, late transition metal complexes could
act as better candidates for olefin oligomerization be-
cause of their facile b-hydrogen elimination [18]. The
researches could go back to the prominent SHOP cat-
alytic systems of a-keto-ylide nickel catalysts
ˆ
[(P O)Ni(R)(L)] reported by Keim and co-workers
[19–21]. Later, many neutral and cationic late transi-
tion metal complexes with chelating ligands have been
successfully applied in the field of olefin oligomeriza-
tion [22]. Recently, Brookhart and co-workers [23–
25] reported that nickel and iron complexes bearing
diimine ligands with MMAO together showed high ac-
tivities in ethylene oligomerization by decreasing the
steric hindrance of aniline. He [26] and Qian [27]
groups, revealed the catalytic property of diimine pyr-
idyl iron complexes when EAO or MMAO were em-
ployed in ethylene oligomerization. Bianchini et al.
[28] also reported that cobalt complexes with pyridyli-
mino ligands could act as efficient catalysts for ethyl-
ene oligomerization. More recently, Carlini and
co-workers [29–34] investigated that many olefin olig-
omerization catalytic systems based on neutral nickel
ˆ
ˆ
complexes bearing anionic [N O] and [O O] chelate li-
gands showed high activities and selectivities for ethyl-
ene or propylene oligomerization. So, the design and
selection of ligandꢀs backbone of catalysts were of cur-
rent interest in the field of olefin oligomerization and
polymerization by late transition metal catalysts.
2-Oxazolinylphenolato ligands had extensive appli-
cations in the catalytic reaction fields [35–40]. In con-
trast to salicyaldimine ligands, the basicity of the
nitrogen atom of 2-oxazolinylphenolato ligand was a
litter stronger, because the oxygen in the oxazoline
ring was conjugated with C‚N. So, it could be de-
duced that the strong coordination of the nitrogen
with metal centers was in existence. If the ligands were
introduced to arylnickel phosphine complexes, it was
speculated that the neutral nickel complexes might
serve as olefin oligomerization catalysts. With this in
mind, we designed and synthesized neutral nickel com-
plexes 1 bearing 2-oxazolinylphenolato ligands. We
have reported the short communication about the syn-
thesis of complex 1a and the application in catalyzing
olefin oligomerization [41]. In this paper, the synthesis
of oxazoline–nickel complexes 1a–1f and their
catalytic behavior in ethylene and propylene oligomer-
izations were studied in detail when complexes 1 were
activated by methylaluminoxane (MAO), diisobutyl-
aluminum hydride (DIBAL-H), triethylaluminum
(AlEt3), and di(1,5-cyclooctadiene)nickel (Ni(COD)2).
m
Ni–O, respectively [43]. The X-ray diffraction reveals
that complexes 1 have a square-planar arrangement
and the ligand, phenyl, triphenylphosphane coordinate
with nickel atom, and triphenylphosphane is in the
trans- position to the N atom [41].
2.2. Ethylene oligomerization
The neutral nickel complexes 1 with 2-oxazolinylphe-
nolato ligands exhibit considerable activities and selec-
tivities in ethylene oligomerization. The highest
activity up to 5.51·105 g oligomers/(mol NiÆh) can be
obtained by 1a/MAO catalytic system (Table 1). The
products provided by complexes 1 consist mainly of di-
mer, trimer and tetramer. For example, in 1/MAO, only
low-molecular-weight oligomers (C4–C8 olefins) are
formed, and the proportion of the trimer (C6 olefins)
is relatively high. No other oligomers such as C10 and
higher carbon olefins are detected by GC analysis. In or-
der to study the effects of the cocatalysts and phosphine
scavenger on the catalytic properties, another two sets of
experiments are performed: one using DIBAL-H and
AlEt3 as organoaluminum cocatalysts, whose results
are listed in Tables 2 and 3; and the other using

2616
W. Zhao et al. / Journal of Organometallic Chemistry 689 (2004) 2614–2623
R2
R1
Route a
OH
O
O
trans-Ni(C6H3R3R4)(Cl)(PPh3)2
NEt3
R2
R1
R2
N
O
O
N
O
N
O
N
+
Ni
R1
Ni
R1
O
Ph3P
R4
R3
2
R
2
1
1a: R1 = R2 = R3 = R4 = H;
1c: R1 = R2 = R4 = H, R3 = CH3;
1d: R1 = R2 = R3 = H, R4 = CH3;
Route b
OH
O
2
2
trans-Ni(C6H3R3R4)(Cl)(PPh3)2
2
O
N
R
O
R
R
NaH
R
R
O
N
1
1
1
N
R
Ni
ONa
Ph P
3
3
R4
R
1
1b: R1 = R3 = R4 = H, R2 = CH3;
1e: R1 = Ph, R2 = R3 = R4 = H;
1f: R1 = Ph, R2 = R4 = H, R3 = CH3
Scheme 1. Synthetic routes of complexes 1.
Table 1
Ethylene oligomerization with complexes 1/MAOa
Run
Reaction conditions
Activity (·105
g
Distribution of
oligomers (%)
Distribution of
C4 (%)
Distribution of C6 (%)
(mol NiÆh)À1
)
Catalyst
Cocatalyst
Al:Ni
C4
9.0
C6
C8
1
1a
1b
1c
1d
1e
1f
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
MAO
1000
1000
1000
1000
1000
1000
1000
1000
1000
5.51
4.59
4.42
3.65
5.43
4.57
1.82
1.47
2.09
82.9
73.8
80.9
77.9
83.0
87.3
27.3
21.9
48.1
8.1
100
100
100
100
100
100
100
100
100
17.5
21.1
14.0
15.9
19.1
21.4
72.4
100
20.3
27.1
62.2
51.8
50.0
43.6
51.8
48.1
27.6
Trace
68.8
2
13.9
7.9
12.3
11.2
16.7
10.0
5.7
3
36.0
40.5
4
5.4
7.0
5
29.1
30.5
6
7.0
7b
8c
9d
1a
1a
1a
45.8
78.1
28.0
26.9
Trace
23.9
Trace
Trace
Trace
31.2
a
b
c
Conditions: 1.5 lmol of complexes 1; solvent: toluene; total volume: 30 ml; ethylene pressure: 12 atm; temperature: 25 ꢁC; reaction time: 0.5 h.
Polar additive: 1.5 ml Et2O.
Polar additive: 1.5 ml THF.
Polar additive: 1.5 ml CH2Cl2.
d
Ni(COD)2 as phosphine scavenger, whose results are
listed in Table 4.
the methyl substituent on Ni-aryl (ortho- or para-posi-
tion). Especially, 1d with the ortho-substituent on the
aryl shows the lowest catalytic activity among all six
complexes 1 when activated by any one of three organo-
aluminum cocatalysts, because of the effects of Ni-aryl
steric bulkiness on the formation of Ni-hydride active
species. In contrast, no oligomers are detected by GC
analysis when cocatalysts or phosphine scavenger are
absent in the catalytic system (Table 4, run 24).
It is noted that oxazoline–nickel complexes 1 are
highly active catalysts for ethylene oligomerization when
activated by MAO, DIBAL-H, and AlEt3 (Tables 1–3).
The activities of 1/organoaluminum catalytic systems
are almost 10 times as high as those of 1/Ni(COD)2 (Ta-
ble 4). In any organoaluminum cocatalytic system, higher
activities can be shown by complex 1a, which has no any
substituent on its oxazoline ring and Ni-aryl. In 1/MAO
and 1/AlEt3, 1c, 1d and 1f show slightly lower activities
than the corresponding complexes 1a and 1e, because of
Keim and Schulz [21] summarized that the limitation
of ligand backbone flexibility could bring higher catalytic
ˆ
activities in the known Ni(P N) complexes. In this

W. Zhao et al. / Journal of Organometallic Chemistry 689 (2004) 2614–2623
2617
Table 2
Ethylene oligomerization with complexes 1/DIBAL-Ha
Run
Reaction conditions
Activity (·105
g
Distribution of
oligomers (%)
Distribution of
C4 (%)
Distribution of C6 (%)
(mol NiÆh)À1
)
Catalyst
Cocatalyst
Al:Ni
C4
C6
C8
10
11
12
13
14
15
1a
1b
1c
1d
1e
1f
DIBAL-H
DIBAL-H
DIBAL-H
DIBAL-H
DIBAL-H
DIBAL-H
500
500
500
500
500
500
4.87
3.28
5.10
2.40
2.95
3.41
54.0
56.5
49.8
33.0
57.7
53.1
28.7
32.0
31.6
59.9
33.8
35.2
17.3
11.5
18.6
7.1
54.8
46.5
36.7
100
45.2
53.5
63.3
Trace
55.3
57.9
25.5
24.2
18.1
14.8
26.3
25.7
31.1
43.4
Trace
26.5
30.0
75.8
55.4
55.2
73.7
74.3
8.5
11.7
44.7
42.1
Trace
Trace
a
Conditions: 1.5 lmol of complexes 1, solvent: toluene, total volume: 30 ml, ethylene pressure: 12 atm, temperature: 25 ꢁC, reaction time: 0.5 h.
Table 3
Ethylene oligomerization with complexes 1/AlEt3
a
Run
Reaction conditions
Activity (·105
(mol NiÆh)À1
g
Distribution of
oligomers (%)
Distribution
of C4 (%)
Distribution of C6 (%)
)
Catalyst
Cocatalyst
Al:Ni
C4
C6
C8
16
17
18
19
20
21
1a
1b
1c
1d
1e
1f
AlEt3
AlEt3
AlEt3
AlEt3
AlEt3
AlEt3
500
500
500
500
500
500
3.64
1.50
1.09
1.36
2.35
2.00
61.8
60.6
47.5
79.8
68.1
75.9
11.1
13.8
36.9
5.1
27.1
25.6
15.6
15.1
17.6
17.6
100
100
100
100
100
100
46.0
22.7
22.1
36.9
48.4
33.9
28.5
47.8
66.9
32.9
35.7
38.3
25.5
29.4
11.0
30.2
15.9
27.8
14.3
6.5
a
Conditions: 1.5 lmol of complexes 1, solvent: toluene, total volume: 30 ml, ethylene pressure: 12 atm, temperature: 25 ꢁC, reaction time: 0.5 h.
Table 4
Ethylene oligomerization with complexes 1/Ni(COD)2
a
Run
Reaction conditions
Activity (·105
g
Distribution of oligomers (%)
(mol NiÆh)À1
)
Catalyst
Phosphine scavenger
Ni(COD)2:catalyst
C4
C6
C8
22
23
24
1a
1b
1a
Ni(COD)2
Ni(COD)2
–
2
2
–
0.56
0.62
0
–
–
–
15.1
47.2
–
84.9
52.8
–
a
Conditions: 1.5 lmol of complexes 1, solvent: toluene, total volume: 30 ml, ethylene pressure: 12 atm, temperature: 25 ꢁC, reaction time: 0.5 h.
system, the oxygen atom in the oxazoline ring is conju-
gated with C‚N, so it is possible that the p-conjugated
system is composed of the phenyl ring and the oxazoline
ring, which makes the structure of ligands more rigid.
Thus, the structural factor makes the ethylene insertion
into the active center accelerated and the catalytic activ-
ities improved.
Tables 1–3 also shows that the selectivities of 1/
MAO, 1/DIBAL-H, and 1/AlEt3 are excellent in obtain-
ing only lower carbon olefins (C4–C8 olefins). As reported
in cationic Ni a-diimine catalytic system, the axial steric
hindrance on metal has a more pronounced effect on the
selectivity of ethylene polymerization and oligomeriza-
tion [23–25,44]. In order to obtain lower carbon oligo-
mers, lower steric demand should be satisfied in the
molecular structure of complexes 1 to speed up the oc-
currence of chain transfer (b-H elimination).
Most of ethylene oligomerization catalytic systems
reported show the good selectivity of linear a-olefins.
Surprisingly, in 1/MAO, the oligomers formed are exclu-
sive branched C6 internal olefins ranging from 74% to
87% (wt%). The major components of C6 olefins are
cis-4-methyl-2-pentene, cis-3-methyl-2-pentene, and
trans-3-methyl-2-pentene. Babik and Fink [45] reported
that higher steric demand of bisiminepyridine iron com-
plexes could promote the formation of linear a-olefins.
As to the 1/MAO catalytic system, lower steric hin-
drance of oxazoline ligands may cause the formation

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W. Zhao et al. / Journal of Organometallic Chemistry 689 (2004) 2614–2623
of internal olefins, especially of 2-olefins. Svejda and
Brookhart [24] proposed the mechanism of chain isom-
erization and chain transfer with diimine nickel com-
plexes in ethylene oligomerization. By analogy, we
suppose the possible mechanism of ethylene oligomeri-
zation with complexes 1 (Scheme 2). After ethylene in-
sertion and chain propagation occurs to (Scheme 2,
Path A), if chain transfer (b-H elimination) takes place,
a-olefins can be formed. On the contrary, if chain isom-
erization takes place, the formation of internal olefins is
inevitable (Scheme 2, Path B).
Upon activation with both DIBAL-H and AlEt3,
complexes 1 show the excellent selectivities to C4 frac-
tion. In 1/DIBAL-H, the content of C4 olefins composed
by 1-butene and 2-butene is about 51% (wt%); while in
1/AlEt3, 1-butene is about 66% (wt%) of the total
amount of oligomers produced. For both cases above,
three kinds of C6 olefins are obtained, which are 2-meth-
ylpentene, 1-hexene, and 2-hexene. We note that the de-
pendence of the selectivity on the nature of the
cocatalysts seems to correlate with the bulk of cocata-
lysts in ethylene oligomerization [46]. The MAO mole-
cule is much larger than AlEt3 and is favor to the
formation of C6 fraction. It is possible that these activa-
tors acting as ligands attach to the nickel center, making
chain propagation faster than chain transfer (b-H elim-
ination) and affecting the catalytic performance. In re-
cent studies, the XAS spectroscopy supports this
assumption that the alkylaluminum activator bonds to
the metal active center [47,48].
known that phosphine scavengers, such as Ni(COD)2,
plays a important role of the removal of PPh3 in the
course of olefin oligomerization or polymerization
[7,49]. In combining with Ni(COD)2, SHOP-type oligo-
merization catalysts could also convert ethylene to poly-
ethylene with molecular weights over 1,000,000.
Interestingly, when phosphine scavenger is applied to
the neutral oxazoline–nickel catalytic system, the molec-
ular weights of oligomers slightly increase, and polymer
can not be obtained. Although 1/Ni(COD)2 show mod-
erate activity to produce olefin oligomers, being a non-
organoaluminum activator, Ni(COD)2 plays a crucial
role in the stabilization of the catalytic active site.
In view of higher activity of 1a/MAO catalytic sys-
tem, some further studies are performed aiming at eval-
uating the influence of ethylene pressure, reaction
temperature, and Al/Ni molar ratio as well as oligomer-
ization in the presence of polar additives on the activity
and selectivity.
In 1/MAO, Fig. 1 demonstrates clearly that the higher
ethylene pressure, the higher the catalytic activity. For
example, the enhancement of ethylene pressure (from 4
to 12 atm) gives a proportional promotion of the activ-
ities from 2.32·105 to 5.51·105 g oligomers/(mol NiÆh)
in 1a/MAO. In the meantime, the selectivity of trimer
(C6 fraction) increases and the selectivity of dimer (C4
fraction) decrease (Fig. 2). However, the enhancement
of ethylene pressure has no dramatic effect either on
the distribution of C4 olefin (100% 1-butene) or on the
distribution of C6 olefins. The other five complexes
1b–1f follow the same general tendency in activity and
distribution as observed on 1a.
In analogy with 1/organoaluminum catalytic system,
lower carbon olefins, C6 and C8 olefins, are also ob-
tained in 1/Ni(COD)2. However, C8 fraction is major
product rather than C6 and C4 fractions. It is well
Higher ethylene pressure means to the enhancement
of ethylene concentration in toluene. As explained
above, Scheme 2 illustrates that ethylene concentration
affects not only on the steps of ethylene insertion and
nickel-alkyl species
R1
O
N
7
6
O
N
Path B
R1
Ni
Ni
H
H
Path A
1a,12atm
1e,12atm
1f,12atm
Path C
Path C
R2
5
4
3
2
1
0
1b,12atm
1c,12atm
internal olefins
and
branched olefins
O
N
Path D
R1
1-Olefins
Path C
Ni
H
1d,12atm
nickel-hydride
species
8
8
8
Path C
8
4
8
8
Path A
4
4
4
R2
O
O
N
R2
Ni
Ni
N
H
H
4
Path B
4
nickel-alkyl species
Path A: ethylene insertion and chain propagation
Path B: chain isomerization
Path C: chain transfer (β-Helimination)
Complexes 1 and ethylene pressure (atm)
Path D: the reinsertion of previously formed olefins
Scheme 2. Proposed mechanism of ethylene oligomerization with
complexes 1.
Fig. 1. The effects of ethylene pressure on activities with complexes 1/
MAO.

W. Zhao et al. / Journal of Organometallic Chemistry 689 (2004) 2614–2623
2619
proportion of C6 olefins decreases from 82.9% to
34.9%. However, when temperature is further increased,
the activities go down smoothly. Furthermore, tempera-
ture has almost no effect on the distribution of trimers,
in analogy with the results of the influence of ethylene
pressure.
100
90
80
70
60
50
40
1f,12atm
1e,12atm
1a,12atm
1b,12atm
1c,12atm
1d,12atm
8
8
In 1a/MAO, the effects of Al/Ni molar ratio on ethyl-
ene oligomerization are investigated and the results are
shown in Fig. 4. The catalytic activities are enhanced
dramatically as the ratio of Al/Ni (mol/mol) increase
from 250 to 1000. However, when the Al/Ni ratio im-
proves further, the activities go down smoothly. The ac-
tivity of 1a/MAO reaches a maximum at 700–1000 Al/Ni
molar ratio under 25 ꢁC, 12 atm ethylene pressure. The
percentage of C6 olefins also follows the rule. When the
ratio is lower than 700, the product distribution tends to
be dimer (C4 olefin). When the ratio is higher than 1000,
the proportion of C8 olefin slightly increases. The fact
that a large excess of MAO does not significantly in-
crease the activity suggests that catalytic active species
(Ni-alkyl species) could be possibly deactivated by the
reaction with excess MAO, or the envelope of excess
MAO on the surface of active species [26].
To test the compatibility of complexes 1 toward func-
tional groups, the effects of polar additive on 1a-cata-
lyzed ethylene oligomerization are also studied. Besides
catalytic activity decreasing in 1a/MAO, runs 7–9 (Table
1) also shows that ethylene oligomerization can be car-
ried out in the presence of three kinds of 5% polar addi-
tives, which make early transition metal catalysts
deactivate, e.g., Et2O, THF, and CH2Cl2. The polar ad-
ditives affect not only the catalytic activities but also the
product distribution. The percentage of dimer (C4 ole-
fin) increases, while the percentage of trimer (C6 olefins)
decreases. The most remarkable are that the amount of
cis-4-methyl-2-pentene is strikingly higher than that of
8
8
8
4
4
4
4
4
8
4
Complexes 1 and ethylene pressure (atm)
Fig. 2. The effects of ethylene pressure on product distribution with
complexes 1/MAO.
chain propagation (Path A) but also on the step of chain
transfer (Path C) [50]. Therefore, in principle, higher
ethylene pressure improves the catalytic activities and
the content of C6 fraction. However, as shown in Path
B, ethylene pressure should have less influence on the
step of chain isomerization. As a result, the distribution
of both C6 olefins and C4 olefin does not be changed.
In 1a/MAO, Fig. 3 shows that the catalytic activities
decrease with the enhancement of reaction temperatures
for ethylene oligomerization. As explained the effects of
ethylene pressure, the solubility of ethylene in toluene
decreases dramatically while increasing the temperature
[51], similar conclusions can be drawn. When tempera-
ture is raised from 25 to 40 ꢁC, with ethylene pressure
held constant at 12 atm and Al/Ni molar ratio being
1000, the activities of 1a/MAO rapidly decrease from
5.51·105 to 1.71·105 g oligomers/(mol NiÆh), and the
7
8
7
6
5
4
3
2
1
0
100
90
80
70
60
50
40
30
20
10
80
6
70
Activity
Percentage
5
60
4
50
3
40
2
Activity
Percentage
30
1
0
20
10
20
30
40
50
60
70
400
800
1200
1600
2000
Temperature (˚C
)
Al/Ni molar ratio
Fig. 3. The effects of reaction temperature with complex 1a/MAO.
Fig. 4. The effects of Al/Ni molar ratio with complex 1a/MAO.

2620
W. Zhao et al. / Journal of Organometallic Chemistry 689 (2004) 2614–2623
Table 5
Propylene dimerization with complexes 1a
Run
Reaction conditions
Activity (·104
g
Distribution of dimers (%)
(mol NiÆh)À1
)
Catalyst
Cocatalyst
Cocatalyst:catalyst
DMBb
2MPb
HEXb
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
1a
1b
1c
1d
1e
1f
MAO
MAO
1000
1000
1000
1000
1000
1000
500
500
500
500
500
500
500
500
500
500
500
500
2
13.24
9.82
8.08
2.37
6.36
2.78
8.68
9.26
6.25
8.41
11.13
9.76
3.81
3.26
1.82
1.61
3.08
1.42
2.43
Trace
11.9
48.0
27.7
39.1
43.8
41.5
34.0
19.9
19.1
15.3
16.3
18.5
16.9
Trace
4.1
52.0
60.4
60.9
56.2
49.2
66.0
80.1
80.9
84.7
83.7
81.5
83.1
26.5
39.3
38.7
44.7
43.0
39.4
24.0
MAO
MAO
Trace
Trace
9.3
MAO
MAO
Trace
Trace
Trace
Trace
Trace
Trace
Trace
73.5
1a
1b
1c
1d
1e
1f
DIBAL-H
DIBAL-H
DIBAL-H
DIBAL-H
DIBAL-H
DIBAL-H
AlEt3
1a
1b
1c
1d
1e
1f
AlEt3
AlEt3
56.6
45.8
15.5
16.2
18.4
15.7
76.0
AlEt3
AlEt3
39.1
38.6
AlEt3
Ni(COD)2
44.9
Trace
1a
a
Conditions: 1.5 lmol of complexes 1, solvent: toluene, total volume: 30 ml, propylene pressure: 1 atm, temperature: 25 ꢁC, reaction time: 0.5 h.
DMB, 2,3-dimethyl-1-butene; 2MP, 2-methyl-1-pentene; HEX, hexene; 4MP, 4-methyl-pentene.
b
3-methyl-2-pentene when employing Et2O and THF as
additives.
and 1 atm of propylene pressure, when three organoalu-
minum cocatalysts and phosphine scavenger are em-
ployed. The results are summarized in Table 5.
2.3. Proylene dimerization
Firstly, complexes 1 exhibit high activities and excel-
lent selectivities in propylene dimerization. Only dimers
(C6 olefins) are produced, higher carbon olefins are not
found by GC analysis. Secondly, 1a shows the highest ac-
The catalytic performance of neutral nickel complexes
1 is also evaluated in propylene oligomerization at 25 ꢁC
Path A
Path B
2, 1-insersion
1, 2-insersion
O
N
O
N
O
Ni
N
Ni
Ni
Ni
Ni
4MP
1
DMB
2
2, 1-insersion
12
9
3
4
O
N
6
7
H
11
10
isomerization
products
isomerization
products
5
8
2MP
HEX
1, 2-insersion
O
N
O
N
O
N
Ni
Ni
1, 2-insersion
Path C
2, 1-insersion
Path D
Scheme 3. Proposed mechanism of propylene dimerization with complexes 1.

W. Zhao et al. / Journal of Organometallic Chemistry 689 (2004) 2614–2623
2621
tivity up to 1.32·105 g oligomers/(mol NiÆh) in 1/MAO.
The conclusion of propylene dimerization is similar to
that of ethylene oligomerization aforementioned. Under
identical reaction conditions, 1c, 1d and 1f with the methyl
group on Ni-aryl show slightly lower activities than the
corresponding complexes with no substituent on Ni-aryl.
Thirdly, 1/organoaluminum catalytic systems have the
excellent selectivities on hexene (HEX), and the content
of HEX is 80–85% in 1/DIBAL-H. However, in 1a/Ni-
(COD)2, the content of 2-methyl-1-pentene (2MP) is up
to 76% of the product mixtures.
Babik and Fink [45] proposed the mechanism of pro-
pylene oligomerization involving the iron alkyl activated
species. They believed that the propylene oligomeriza-
tion catalytic system was a very complicated process of
insertion and subsequent elimination. By analogy, a
possible mechanism proposed in this case is shown in
Scheme 3.
In 1/MAO and 1/DIBAL-H, because of the unique
mechanism of propylene dimerization in which the reg-
iochemistry of propylene insertion changes from 1,2 to
2,1 between the first and second steps (Scheme 3, Path
D), the major products are HEX comprising up to
85% of the product mix. Another product in the reaction
is 2MP resulting from two successive 1,2 insertions fol-
lowed by chain termination (Scheme 3, Path C). In ad-
dition, trace product is 2,3-dimethyl-1-butene (DMB),
which is formed by the route of 2,1 and 1,2 insertions
(Scheme 3, Path B). In 1a/Ni(COD)2, the major product
is 2MP, which can be formed by two successive 1,2 in-
sertions and subsequent b-H elimination. Because of
the lower steric hindrance of 2-oxazolinylphenolato li-
gands, the first propylene insertion step tends to under-
go 1,2 insertion pathway. Thus, the proportion of HEX
and 2MP is higher than that of DMB and 4MP in cata-
lytic systems. Recently, Fink found that such relation
also existed in the iron complexes with bisiminopyridine
ligands. The more bulky the ligand, the more frequent
2,1 propylene insertion, and thus the higher molecular
mass of the oligomers.
using KBr pellets were collected on a Nicolet Magna-
IR 550 spectrophotometer. EI-MS and EI-HRMS (m/
z, 70 eV) were recorded on a Micromass GCT mass
spectrometer. GC analysis of oligomers was performed
on Shimadazu GC-14B equipped with a Simplicity-1
column (30 m·0.25 mm).
3.2. Synthesis of complexes 1a, 1c and 1d
To a suspension of trans-Ni(Ar)(Cl)(PPh3)2 (1.5
mmol) and NEt3 (1.78 mmol) in toluene (20 ml) was
dropwise added the toluene solution (10 ml) of 2-(4,5-di-
hydro-2-oxazolyl)phenol (1.66 mmol). This reaction
mixture was allowed to stir for 20 h at ambient temper-
ature. After filtered, the reaction products were separated
into two parts: the dark red filtrate and the green solid.
The filtrate was reduced to 5 ml under vacuum, and n-
hexane (30 ml) was added to it. A yellow solid precipi-
tated from solution was isolated by filtration. The crude
product was recrystallized from toluene and n-hexane to
give brown–yellow needle crystals as complexes 1 in 57–
66% yields. The green solid separated from the reaction
mixture was washed with toluene (3·10 ml), and recrys-
tallized from THF and n-hexane to give green solid as
complex 2 [55].
1a – M.p.: 149–150 ꢁC. 1H NMR (d6-C6D6, 500
MHz): d=2.30 (s, 1.5H, CH3 Ph), 2.89 (t, J=9.5 Hz,
2H, NCH2), 3.40 (t, J=9.5 Hz, 2H, OCH2), 6.61 (d,
J=8.5 Hz, 1H, Ar–H), 6.73 (t, J=7.4 Hz, 1H, Ar–H),
6.90–6.96 (m, 3H, Ar–H), 7.15–7.24 (m, 10H, Ar–H),
7.25–7.32 (m, 2.5H, Ar–H), 7.67 (d, J=7.4 Hz, 2H,
Ar–H), 7.92 (t, J=9.9 Hz, 6H, Ar–H), 8.10 (dd,
J=8.0, 1.6 Hz, 1H, Ar–H). 13C NMR (d6-C6D6, 300
MHz): d=55.0 (NC2), 65.9 (OC2), 110–138 (Ar–C). 31P
NMR (d6-C6D6, 300 MHz): d=31.4 (PPh3). IR (KBr
pellet): 3044m, 2986m, 2908m, 1617vs, 1586m, 1539m,
1475vs, 1433s, 1399m, 1349m, 1260s, 1242m, 1153m,
1096m, 1075m, 928m, 851m, 733s, 695vs, 572w, 463w
cmÀ1
.
MS (m/z, %): 262 ([PPh3]+Å, 100), 220
(M+Å ÀPPh3 ÀPh, 1), 185 ([PPh2]+, 8), 183 ([
]+,
62), 154 ([Ph–Ph]+, 89). Anal. Calc. for 2C33H28NNiO2-
PÆC6H5CH3: C, 72.30; H, 5.32; N, 2.31; Found: C,
72.58; H, 5.33; N, 2.15%. HRMS calcd for
C33H28NNiO2P: 559.1211, found: 559.1162.
3. Experimental
3.1. General methods
1c – M.p.: 148–149 ꢁC. 1H NMR (d6-C6D6, 500 MHz):
d=2.36 (s, 3H, CH3PhNi), 2.92 (t, J=9.5 Hz, 2H,
NCH2), 3.41 (t, J=9.5 Hz, 2H, OCH2), 6.61 (d, J=8.3
Hz, 1H, Ar–H), 6.73 (t, J=7.4 Hz, 1H, Ar–H), 6.78 (d,
J=7.7 Hz, 1.5 H, Ar–H), 6.91–6.95 (m, 1H, Ar–H),
7.17–7.26 (m, 9H, Ar–H), 7.27–7.29 (m, 1H, Ar–H),
7.52 (d, J=6.7 Hz, 1.5H, Ar–H), 7.92 (t, J=9.9 Hz,
6H, Ar–H), 8.10 (dd, J=8.0, 1.8 Hz, 1H, Ar–H). IR
(KBr pellet): 3055m, 2960m, 2906m, 1615vs, 1585s,
1539s, 1471vs, 1436s, 1397s, 1355s, 1257s, 1242m,
1153m, 1093m, 1074m, 927m, 745m, 693s, 573w, 460w
All operations were carried out under argon atmo-
sphere using standard Schlenk techniques. All solvents
were distilled from sodium wire prior to use, and chem-
icals were obtained from commercial suppliers. The
2-(4,5-dihydro-2-oxazolyl)phenol derivatives, trans-
Ni(Ar)(Cl)(PPh3)2, and Ni(COD)2 were prepared ac-
cording to the literatures [52–54]. 1 H NMR spectra were
recorded on a Bruker Advance 500 spectrometer in
d6-C6D6. Elemental (C, H, N) analysis data were ob-
tained with an EA-1106 spectrometer. Infrared spectra
cmÀ1
.
MS (m/z, %): 262 ([PPh3]+Å, 100), 220

2622
W. Zhao et al. / Journal of Organometallic Chemistry 689 (2004) 2614–2623
(M+Å ÀPPh3 ÀPh, 1), 185 ([PPh2]+, 6), 183 ([
]+, 64).
1e – M.p.: 152–153 ꢁC; 1H NMR (d6-C6D6, 500
MHz): d=3.74 (dd, J=8.4, 3.1 Hz, 1H, OCH2), 3.82
(t, J=8.5 Hz, 1H, NCH), 4.33 (dd, J=9.0, 3.0 Hz, 1H,
OCH2), 6.44 (s, 1H, Ar–H), 6.65 (d, J=8.4 Hz, 2H,
Ar–H), 6.76 (t, J=7.2 Hz, 1H, Ar–H), 6.85 (t, J=7.3
Hz, 1H, Ar–H), 6.94 (s, 1H, Ar–H), 7.11–7.19 (m,
12H, Ar–H), 7.25–7.27 (m, 3H, Ar–H), 7.30–7.32 (m,
1H, Ar–H), 7.85 (t, J=10.0 Hz, 6H, Ar–H), 8.18 (dd,
J=8.0, 1.8 Hz, 1H, Ar–H). IR (KBr pellet): 3050m,
3022m, 2960w, 2909w, 1612vs, 1578s, 1536s, 1469s,
1435s, 1399m, 1355m, 1267s, 1234m, 1156m, 1095m,
Anal. Calc. for C34H30NNiO2 P: C, 71.11; H, 5.27; N,
2.44; Found: C, 71.24; H, 5.20; N, 2.43%.
1d – M.p.: 152–153 ꢁC; 1H NMR (d6-C6D6, 500
MHz): d=2.79À2.90 (m, 2H, NCH2), 3.00 (s, 3H,
CH3PhNi), 3.36–3.47 (m, 2H, OCH2), 6.57 (d, J=8.6
Hz, 1H, Ar–H), 6.73 (t, J=7.5 Hz, 1H, Ar–H), 6.81
(d, J=7.3 Hz, 1H, Ar–H), 6.99 (t, J=7.2 Hz, 1H, Ar–
H), 7.15–7.19 (m, 6H, Ar–H), 7.21–7.24 (m, 3H, Ar–
H), 7.27 (t, J=7.8 Hz, 1H, Ar–H), 7.84 (s, 1H, Ar–H),
7.87 (t, J=7.3 Hz, 1H, Ar–H), 7.88 (t, J=8.9 Hz, 6H,
Ar–H), 8.10 (dd, J=8.1, 1.3 Hz, 1H, Ar–H). IR (KBr
pellet): 3046m, 2973m, 2901m, 1612vs, 1584s, 1540m,
1471s, 1435s, 1401m, 1351m, 1260m, 1245s, 1154m,
1083m, 931m, 858m, 737s, 693vs, 576w, 463w cmÀ1
.
MS (m/z, %): 296 (M+Å ÀPPh3 ÀPh, 1), 262 ([PPh3]+Å,
54), 185 ([PPh2]+, 4), 183 ([
]+, 33), 154 ([Ph–Ph]+,
1095m, 1077m, 928w, 740m, 693s, 574w, 460w cmÀ1
.
100). Anal. Calc. for C39H32NNiO2P: C, 73.61; H,
5.07; N, 2.20; Found: C, 73.62; H, 5.13; N, 2.05%.
1f – M.p.: 150–151 ꢁC; 1H NMR (d6-C6D6, 500
MHz): d=2.31 (s, 3H, CH3), 3.75 (dd, J=8.2, 3.0Hz,
1H, OCH2), 3.83 (t, J=8.8 Hz, 1H, NCH), 4.37 (dd,
J=8.9, 2.9 Hz, 1H, OCH2), 6.44 (s, 1H, Ar–H), 6.66
(t, J=8.5 Hz, 2H, Ar–H), 6.76 (t, J=7.7 Hz, 1H, Ar–
H), 6.85 (t, J=7.2 Hz, 1H, Ar–H), 6.95 (s, 1H, Ar–H),
7.11–7.21 (m, 12H, Ar–H), 7.25–7.27 (m, 3H, Ar–H),
7.85 (t, J=9.5 Hz, 6H, Ar–H), 8.18 (dd, J=8.1, 1.7
Hz, 1H, Ar–H). IR (KBr pellet): 3052m, 3024m,
2960w, 2910w, 1612vs, 1577s, 1535s, 1468s, 1444m,
1435s, 1398m, 1355m, 1266s, 1234m, 1156m, 1094m,
MS (m/z, %): 262 ([PPh3]+Å, 100), 220 (M+Å ÀPPh3 ÀPh,
Ph, 2), 185 ([PPh2]+, 6), 183 ([
]+, 75). Anal. Calc. for
C34H30NNiO2 P: C, 71.11; H, 5.27; N, 2.44; Found: C,
71.10; H, 5.48; N, 2.27%.
3.3. Synthesis of complexes 1b, 1e and 1f
To a suspension of sodium hydride (3 mmol) in THF
(5 ml) was added a solution of 2-(5-methyl-4,5-dihydro-
2-oxazolyl)phenol or 2-(4-phenyl-4,5-dihydro-2-oxazol-
yl)- phenol (1.8 mmol) in THF (10 ml). The resulting
mixture was stirred for 2 h at ambient temperature. Af-
ter filtered, the filtrate was reduced in vacuum. The solid
residue was washed with n-hexane (10 ml) and dried in
vacuum. The salt was immediately used in the next step
without further purification.
The sodium salt of ligands and trans-
Ni(Ar)(Cl)(PPh3)2 (1.4 mmol) were dissolved in toluene
(30 ml) and stirred for 8 h at room temperature. After
filtered, the filtrate was reduced in vacuum to 5 ml,
and n-hexane (30 ml) was added to the solution. A yel-
low solid precipitated from solution was isolated by fil-
tration. The crude product was recrystallized from
toluene and n-hexane to give brown–yellow needle crys-
tals as complexes 1 in 65–73% yields.
1081m, 930m, 857m, 737s, 692vs, 575w, 464w cmÀ1
.
MS (m/z, %): 296 (M+Å ÀPPh3 ÀPh, 10), 262 ([PPh3]+Å,
33), 185 ([PPh2]+, 3), 183 ([
]+, 20). Anal. Calc. for
C40H34NNiO2P: C, 73.87; H, 5.27; N, 2.15; Found: C,
73.45; H, 5.07; N, 2.10%.
3.4. Oligomerization procedure
A 100 ml autoclave, equipped with a magnetic stir
bar, was preheated at 100 ꢁC under vacuum for 30
min and then cooled to the required temperature. Tolu-
ene was injected into the reactor and pressured with eth-
ylene (or propylene) to 1 atm. After equilibrating for 20
min, the appropriate volume of catalysts solution and
cocatalysts were injected to start the reaction. The ethyl-
ene pressure was kept constant during the reaction. Af-
ter the desired run time, the reactor was vented and the
reaction mixture was terminated by ethanol. n-Heptane
was added to the mixture as internal standard for GC
analysis. An upper-layer clear solution was separated
from the reaction mixture to analyze and quantify the
soluble components by GC. The individual products
of oligomerization were identified by GC–MS.
1b – M.p.: 134–135 ꢁC. 1H NMR (d6-C6D6, 500
MHz): d=0.79 (d, J=6.2 Hz, 3H, CH3), 2.30 (s, 3H,
CH3C6H5), 2.76 (dd, J=13.9, 8.1 Hz, 1H, NCH2),
3.19 (dd, J=13.9, 9.4 Hz, 1H, NCH2), 3.99–4.03 (m,
1H, OCH), 6.61 (d, J=8.6 Hz, 1H, Ar–H), 6.73 (t,
J=7.5 Hz, 1H, Ar–H), 6.93–6.96 (m, 3H, Ar–H),
7.16–7.24 (m, 12H, Ar–H), 7.25–7.32 (m, 3H, Ar–H),
7.71 (t, J=6.7 Hz, 2H, Ar–H), 7.93 (t, J=9.0 Hz, 6H,
Ar–H), 8.12 (dd, J=8.1, 1.4 Hz, 1H, Ar–H). IR (KBr
pellet): 3047m, 2975m, 1608vs, 1582s, 1536m, 1471vs,
1435s, 1352m, 1242m, 1153m, 1096m, 1075m, 1059m,
734s, 693vs, 575w, 463w cmÀ1. MS (m/z, %): 262
([PPh3]+Å, 100), 234 (M+Å ÀPPh3 ÀPh, 1), 185 ([PPh2]+,
Acknowledgement
6), 183 ([
]+, 52). Anal. Calc. for C34H30NNiO2-
PÆC6H5CH3: C, 73.89; H, 5.75; N, 2.10; Found: C,
73.73; H, 5.62; N, 1.92%.
We gratefully acknowledge financial support from
the Special Funds for Major State Basic Research

W. Zhao et al. / Journal of Organometallic Chemistry 689 (2004) 2614–2623
2623
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