Nickel (II) complexes bearing 2-ethylcarboxylate-6-iminopyridyl ligands: Synthesis, structures and their catalytic behavior for ethylene oligomerization and polymerization

Article abstract of DOI:10.1016/j.jorganchem.2004.12.027

A series of nickel (II) complexes (L)NiCl₂ (7-9) and (L)NiBr₂ (10-12) were prepared by the reactions of the corresponding 2-carboxylate-6-iminopyridine ligands 1-6 with NiCl₂·6H ₂O or (DME)NiBr₂ (DME = 1,2-dimethoxyethane), respectively. All the complexes were characterized by IR spectroscopy and elemental analysis. Solid-state structures of 7, 8, 10, 11 and 12 were determined by X-ray diffraction. In the cases of 7, 8 and 10, the ligands chelate with the nickel centers in tridentate fashion in which the carbonyl oxygen atoms coordinate with the metal centers, while the carbonyl oxygen atoms are free from coordinating with the nickel centers in 11 and 12. Upon activation with methylaluminoxane (MAO), these complexes are active for ethylene oligomerization (up to 7.97 × 10⁵ g mol^-1 (Ni) h^-1 for 11 with 2 equivalents of PPh₃ as auxiliary ligand) and/or polymerization (1.37 × 10⁴ g mol^-1 (Ni) h^-1 for 9). The ethylene oligomerization activities of 7-12 were significantly improved in the presence of PPh₃ as auxiliary ligands. The effects of the coordination environment and reaction conditions on the ethylene catalytic behaviors have been discussed.

Full text of DOI:10.1016/j.jorganchem.2004.12.027

Journal of Organometallic Chemistry 690 (2005) 1570–1580
www.elsevier.com/locate/jorganchem
Nickel (II) complexes bearing
2-ethylcarboxylate-6-iminopyridyl ligands: synthesis, structures
and their catalytic behavior for ethylene oligomerization and
polymerization
a
a,
a
a
Xiubo Tang , Wen-Hua Sun *, Tielong Gao , Junxian Hou ,
b
Jiutong Chen , Wei Chen
c
a
Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou 350002, China
b
c
National Polyolefin Engineering and Research Center, Beijing Research Institute of Chemical Industry,
China Petroleum & Chemical Corporation, Beijing 100013, China
Received 7 October 2004; revised 9 December 2004; accepted 21 December 2004
Available online 12 January 2005
Abstract
A series of nickel (II) complexes (L)NiCl2 (7–9) and (L)NiBr2 (10–12) were prepared by the reactions of the corresponding 2-car-
boxylate-6-iminopyridine ligands 1–6 with NiCl2 Æ 6H2O or (DME)NiBr2 (DME = 1,2-dimethoxyethane), respectively. All the
complexes were characterized by IR spectroscopy and elemental analysis. Solid-state structures of 7, 8, 10, 11 and 12 were deter-
mined by X-ray diffraction. In the cases of 7, 8 and 10, the ligands chelate with the nickel centers in tridentate fashion in which
the carbonyl oxygen atoms coordinate with the metal centers, while the carbonyl oxygen atoms are free from coordinating with
the nickel centers in 11 and 12. Upon activation with methylaluminoxane (MAO), these complexes are active for ethylene oligomer-
ization (up to 7.97 · 105 g molÀ1 (Ni) hÀ1 for 11 with 2 equivalents of PPh3 as auxiliary ligand) and/or polymerization (1.37 · 104 g
molÀ1 (Ni) hÀ1 for 9). The ethylene oligomerization activities of 7–12 were significantly improved in the presence of PPh3 as aux-
iliary ligands. The effects of the coordination environment and reaction conditions on the ethylene catalytic behaviors have been
discussed.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Iminopyridine ligand; Nickel complexes; Ethylene oligomerization; Polymerization
1. Introduction
industrial catalysts include either alkylaluminum com-
pound or a combination of alkylaluminum compound
with early transition metal compounds or nickel (II)
with bidentate monoanionic [P,O] ligands (the SHOP
process) [2]. Various nickel complexes containing chelat-
ing ligands such as diimine [N,N] [3], salicylaldimine
[N,O] [4], pyridylimine [N,N] [5], and [P,N] [6] ligands
serve as highly active catalysts for ethylene oligomeriza-
tion or polymerization. The coordination environments
of these well-defined catalysts can be systematically
varied by changing the substituents in the ligand
The oligomerization of ethylene as the major indus-
trial process provides linear a-olefins, which are exten-
sively used as comonomer of polyolefin, surfactants
and lubricants. The linear a-olefins were originally man-
ufactured by the Ziegler (Alfen) process [1], and current
*
Corresponding author. Tel.: +86 10 62557955; fax: +86 10
62618239.
E-mail address: whsun@iccas.ac.cn (W.-H. Sun).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.12.027

X. Tang et al. / Journal of Organometallic Chemistry 690 (2005) 1570–1580
1571
procedures [8]. The dichloride nickel complexes 7–9
were obtained by stirring the ethanol solution of Ni-
Cl2 Æ 6H2O with the ligands 1–3 at room temperature,
while the dibromide complexes 10–12 were prepared
by the reaction of (DME)NiBr2 (DME = 1,2-dimeth-
oxyethane) with the corresponding ligands 4–6
(Scheme 1). These complexes were characterized by
IR spectroscopic and elemental analysis. The para-
magnetic nature of the nickel complexes precludes
the NMR characterization. These complexes show
high stability in both its solution and solid state.
The IR spectra of the ligands show that the C@N
stretching frequencies appear in the range of 1640–
1655 cmÀ1 [8], while the C@N stretching vibrations
shift toward lower frequencies between 1619 and
1625 cmÀ1 with weak intensities for complexes 9–12.
The results indicate the coordination interaction be-
tween the imino nitrogen atom and the nickel center.
Moreover, the C@O stretching vibrations in IR spec-
tra show slight red shift by ca. 10–20 cmÀ1 for the
nickel complexes.
R
EtO
N
O
N
Ni
X
X
R
7: R = Me, X = Cl;
8: R = Et, X = Cl;
10: R = F, X = Br.
EtO2C
N
R
N
R
R
1: R = Me;
3: R = i-Pr;
5: R = Cl;
2: R = Et;
4: R = F;
6: R = Br.
N
N
Ni
EtOOC
X
X
R
9: R = i-Pr, X = Cl;
11: R = Cl, X = Br;
12: R = Br, X = Br.
Scheme 1. The synthesis of the complexes 7–12.
backbones, which provides an opportunity to control
their ethylene catalytic behaviors, including the activities
and the product distributions. In addition, the late-metal
catalysts have the advantages such as less oxophilicity
and the tolerance of expanded functional groups, which
might open a door to copolymerization of ethylene and
polar monomers [7]. Recently, we reported a new family
of iron (II) and cobalt (II) complexes bearing carboxyl-
ate ester-substituted iminopyridine ligands, which were
verified to perform well as the catalysts for ethylene olig-
omerization and polymerization upon activation with
methylaluminoxane (MAO), and a-olefins are the resul-
tant products [8]. We extended the research with same
ligands into nickel complexes. Owning to the bonding
versatility of the carboxylate substituent in the ligands,
the coordination fashions of these nickel complexes
are different in solid state. In the presence of MAO as
cocatalyst, the Ni (II) complexes showed the favorable
ethylene oligomerization versus polymerization. The
selectivity for a-olefins by nickel complexes is relative
lower than its ferrous analogues, and the resultant poly-
ethylene is branched. The substituents in the ligand
backbones of the complexes are systemically varied from
the electron-donating alky groups to electron-withdraw-
ing halogen group, which provide an insight into both
electric and steric effects on their ethylene oligomeriza-
tion and polymerization performance. The ethylene olig-
omerization activities increase by one order of
magnitude when additional PPh3 is employed as auxil-
iary ligand.
2.2. Solid-state structures of the pyridylimine nickel (II)
complexes
It is notable that these complexes display different
colors in solid state. Complexes 7, 8 and 10 are bright
yellow, while 9, 11 and 12 are red brown, which sug-
gests that these complexes might adopt different coor-
dination fashions. The single crystal X-ray diffraction
determination provides direct evidences of the individ-
ual structures, and fortunately the single crystals of 7,
8, 10, 11 and 12 suitable for X-ray diffraction were ob-
tained. Selected bond lengths and bond angles for com-
plexes 7, 8, 10 and 11, 12 are listed in Tables 1 and 2,
and their crystallographic data and refinement are gi-
ven in Table 3. Crystals of 7 were grown from its eth-
anol solution layering with diethyl ether. As shown in
Fig. 1, the coordination geometry around the nickel
center of 7 is a pseudo-square-pyramid. The ester-
substituted pyridylimine ligand coordinates with the
nickel center as a tridentate ligand through nitrogen
atoms of imine and pyridine as well as carbonyl oxygen
atom. The plane of the dimethyl-substituted aryl ring is
essentially oriented orthogonally to the coordination
plane (85.1ꢁ).
Crystals of 8 were grown from a dichloromethane
solution layering with pentane, and its molecular struc-
ture is shown in Fig. 2. The carbonyl oxygen atom O1
also coordinates with the nickel center, therefore the li-
gand chelates in NꢂNꢂO tridentate fashion. The geom-
etry around the nickel atom can be described as a
distorted octahedron, with an adventitious water mole-
cule incorporating into the coordination sphere during
the crystal growth. The Ni–N (pyridyl) bond distance
2. Results and discussion
2.1. Synthesis of pyridylimine nickel (II) complexes
The ester-substituted pyridylimine ligands 1–6 [2-
CO2Et-6-(2,6-R2C6H3N@CCH3)C5H3N (1: R = CH3,
2: R = Et, 3: R = i-Pr, 4: R = F, 5: R = Cl, 6:
R = Br)] were prepared with the previously reported
˚
˚
(Ni–N1, 2.005 (3) A) is about 0.12 A shorter than the

1572
X. Tang et al. / Journal of Organometallic Chemistry 690 (2005) 1570–1580
Table 1
Selected bond lengths and bond angles for complex 7, 8 and 10
Complex 7
Complex 8
Complex 10
˚
Bond lengths (A)
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–Cl(1)
Ni(1)–Cl(2)
N(2)–C(6)
2.006(4)
2.094(4)
2.2133(18)
2.272(2)
1.285(6)
2.402(5)
4.280
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–Cl(1)
Ni(1)–Cl(2)
N(2)–C(6)
Ni(1)–O(1)
Ni(1)Á Á ÁO(2)
Ni(1)–O(3)
2.005(3)
2.122(3)
2.3618(12)
2.3096(12)
1.281(5)
2.342(3)
4.284
Ni–N(1)
Ni–N(2)
Ni–Br(1)
Ni–Br(2)
N(2)–C(9)
Ni–O(1)
NiÁ Á ÁO(2)
Ni–O(3)
2.016(4)
2.100(4)
2.4854(8)
2.5013(9)
1.287(7)
2.409(4)
4.346
Ni(1)–O(1)
Ni(1)Á Á ÁO(2)
2.178(3)
2.147(4)
Bond angles (ꢁ)
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–O(1)
N(2)–Ni(1)–O(1)
N(1)–Ni(1)–Cl(1)
N(2)–Ni(1)–Cl(1)
N(1)–Ni(1)–Cl(2)
N(2)–Ni(1)–Cl(2)
Cl(1)–Ni(1)–O(1)
Cl(2)–Ni(1)–O(1)
Cl(1)–Ni(1)–Cl(2)
77.89(15)
73.89(16)
149.66(17)
146.84(17)
103.38(11)
93.19(13)
103.27(15)
94.37(13)
89.35(16)
118.04(8)
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–O(1)
N(2)–Ni(1)–O(1)
N(1)–Ni(1)–O(3)
N(2)–Ni(1)–O(3)
N(1)–Ni(1)–Cl(1)
N(1)–Ni(1)–Cl(2)
N(2)–Ni(1)–Cl(1)
N(2)–Ni(1)–Cl(2)
O(1)–Ni(1)–O(3)
O(1)–Ni(1)–Cl(1)
O(1)–Ni(1)–Cl(2)
O(3)–Ni(1)–Cl(1)
O(3)–Ni(1)–Cl(2)
Cl(1)–Ni(1)–Cl(2)
78.06(12)
74.60(10)
152.30(10)
85.68(11)
92.60(12)
88.33(8)
N(1)–Ni–N(2)
N(1)–Ni–O(1)
N(2)–Ni–O(1)
N(1)–Ni–O(3)
N(2)–Ni–O(3)
N(1)–Ni–Br(1)
N(1)–Ni–Br(2)
N(2)–Ni–Br(1)
N(2)–Ni–Br(2)
O(1)–Ni–O(3)
O(1)–Ni–Br(1)
O(1)–Ni–Br(2)
O(3)–Ni–Br(1)
O(3)–Ni–Br(2)
Br(1)–Ni–Br(2)
77.99(16)
73.65(15)
150.80(15)
88.19(17)
89.88(15)
174.78(13)
88.99(13)
103.83(12)
96.10(12)
82.51(14)
103.85(9)
90.25(10)
86.92(11)
172.72(10)
95.64(3)
171.77(9)
98.15(9)
104.16(9)
81.11(11)
85.47(7)
102.33(7)
166.34(9)
86.30(8)
99.11(4)
Table 2
Selected bond lengths and bond angles for complex 11 and 12
Complex 11
Complex 12
˚
Bond lengths (A)
Ni–N(1)
Ni–N(2)
Ni–Br(1)
Ni–Br(2)
N(2)–C(6)
Ni–O
2.061(6)
2.078(6)
2.4990(11)
2.4656(12)
1.302(9)
2.096(6)
2.559
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–Br(1)
Ni(1)–Br(2)
N(2)–C(7)
1.974(6)
2.087(6)
2.3672(15)
2.3296(15)
1.275(9)
2.436
Ni(1)Á Á ÁO(1)
Ni(1)Á Á ÁO(2)
NiÁ Á ÁO(1)
4.330
NiÁ Á ÁO(2)
4.458
Bond angles (ꢁ)
N(1)–Ni–N(2)
N(1)–Ni–Br(1)
N(2)–Ni–Br(1)
N(1)–Ni–Br(2)
N(2)–Ni–Br(2)
Br(1)–Ni–Br(2)
N(1)–Ni–O
78.7(2)
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–Br(1)
N(2)–Ni(1)–Br(1)
N(1)–Ni(1)–Br(2)
N(2)–Ni(1)–Br(2)
Br(1)–Ni(1)–Br(2)
78.8(2)
174.96(17)
101.58(17)
89.63(17)
99.83(17)
95.25(4)
88.2(2)
108.65(17)
104.19(18)
125.13(18)
104.47(17)
122.56(5)
N(2)–Ni–O
O–Ni–Br(1)
O–Ni–Br(2)
97.4(2)
86.78(15)
161.90(16)
˚
Ni–N (imino) bond distance (Ni–N2, 2.122(3) A). The
plane of the phenyl ring is oriented approximately
orthogonal to the coordination plane with the angle of
82.5ꢁ. Deviation of the nickel center from the plane
tion layering with hexane. The molecular structure is de-
picted in Fig. 3. The coordination geometry around
nickel center of 10 is similar to that of 8, despite two
bromo atoms, instead of chloro atoms, coordinating
with the central nickel, as well as the presence of the
fluoro atoms instead of the ethyl groups in the ortho
˚
formed by the coordinated N1, N2 and O1 is 0.038 A.
Crystals of 10 were grown from a dichloromethane solu-

X. Tang et al. / Journal of Organometallic Chemistry 690 (2005) 1570–1580
1573
Table 3
Crystallographic data and refinement for 7, 8, 10, 11 and 12
7
8
10
11
12
Formula
C18H20Cl2
N2NiO2
425.97
Orthorhombic
Pca2(1)
13.706(3)
9.866(2)
14.247(3)
90
C20H27Cl2
N2NiO3.5
481.05
C16H16Br2
F2N2NiO3
540.84
Monoclinic
C2/c
C16H16Br2
Cl2N2NiO3
573.74
C16H14Br4N2
NiO2 Æ CH2Cl2
729.57
Formula weight
Crystal system
Space group
Triclinic
ꢀ
Monoclinic
C2/c
Triclinic
ꢀ
P1
P1
˚
a (A)
7.997(3)
9.907(3)
14.630(5)
92.516(6)
97.226(5)
101.403(5)
1124.3(7)
2
25.4776(6)
10.2440(3)
15.575
90
27.79620(10)
10.3562(3)
19.1319(3)
90
8.0963(16)
9.1394(18)
16.844(3)
103.70(3)
97.17(3)
94.46(3)
1194.0(4)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
90
90
90.7610(10)
90
133.1160(10)
90
3
˚
V (A )
1926.5(7)
4
1.469
4044.32(15)
8
1.766
4020.22(13)
8
1.896
Z
Dcalcd (g cmÀ3
)
1.421
1.125
502
2.209
7.744
700
Abs coefficient, l (mmÀ1
F(0 0 0)
)
1.298
880
4.964
2128
5.225
2256
h range (ꢁ)
2.06–27.48
2284
2284
1.21–26.47
6477
4548
1.61–24.92
6312
3495
2.01–25.06
5736
3488
2.31–27.48
5149
2900
Number of data collected
Number of unique data
R (%)
0.0436
0.1122
1.065
0.0514
0.1259
1.076
0.0442
0.1028
1.193
0.0561
0.1229
1.162
0.0732
0.1698
0.985
Rw (%)
Goodness of fit
Fig. 1. The molecular structure of 7. Hydrogen atoms are omitted for clarity.
Fig. 2. The molecular structure of 8. Hydrogen atoms are omitted for clarity.
positions of the phenyl ring. It is notified that the
˚
Ni–O1 bonding lengths of 7 (2.402 A), 8 (2.342(3)
the weak interactions between nickel and carbonyl
oxygen.
˚
A) and 10 (2.409(4) A) are longer than the previously
reported Ni–O bond length values [9], which indicates
˚
Crystals of 11 were grown through diffusing hexane
layer into its dichloromethane solution. X-ray structure

1574
X. Tang et al. / Journal of Organometallic Chemistry 690 (2005) 1570–1580
determination indicates that the carbonyl oxygen atom
O2 deviates away from the central nickel instead of
coordinating and the distance between Ni and O2 is
dron. The dibromophenyl ring lies nearly perpendicular
to the coordination plane (the dihedral angle is 92.0ꢁ).
The distance between the nickel atom and carbonyl
˚
˚
4.458 A. One adventitious water molecule incorporates
oxygen O1 (2.436 A) in 12 is slightly longer than the
into the coordination sphere. The geometry of the five-
coordinated nickel complex 11 can be better described
as distorted square pyramidal with the four atoms
N1, O, Br1 and Br2 forming the bottom plane and
the N2 atom occupying the apical position (Fig. 4).
The dichlorophenyl ring lies almost perpendicular to
the coordination plane (80.1ꢁ). It should be pointed
out that the water molecule is involving into the com-
plexes in 8, 10 and 11 during the crystal growth, and
the powders used for ethylene catalytic reaction might
have no water.
Crystals of 12 were grown through diffusing hexane
layer into its dichloromethane solution, and the molecu-
lar structure is shown in Fig. 5. X-ray structure charac-
terization conforms that the complex 12 adopts the
NꢂN coordination fashion. One dichloromethane mole-
cule is included in the crystal lattice without binding
interaction with the complex. The geometry around
the metal center can be described as a distorted tetrahe-
Ni–O(1) bond lengths in 7, 8 and 10, indicative of the
presence of weaker interaction between the carbonyl
oxygen and the nickel atom. The coordination geometry
around the nickel center of 7–12 was affected by the
backbones of their ligand, however, there is no unam-
biguous explanation yet.
2.3. Ethylene oligomerization and polymerization
The catalytic activities of precatalysts 7–12 for ethyl-
ene oligomerization and polymerization have been car-
ried out in the presence of methylaluminoxane (MAO)
as cocatalyst. In all cases, these catalysts generate bu-
tenes and hexenes as main oligomeric product and the
distribution of oligomers does not follow Schulz–Flory
rules. No odd carbon number oligomers were detected
in the GC and GC/MS analysis. For the bulky substi-
tuted catalysts, small amount of polyethylene is also
produced.
Fig. 3. The molecular structure of 10. Hydrogen atoms are omitted for clarity.
Fig. 4. The molecular structure of 11. Hydrogen atoms are omitted for clarity.

X. Tang et al. / Journal of Organometallic Chemistry 690 (2005) 1570–1580
1575
Fig. 5. The molecular structure of 12. Hydrogen atoms and solvent molecule (CH2Cl2) are omitted for clarity.
2.3.1. Effects of ligands environment
b-hydrogen elimination. Thus, the higher carbon num-
ber oligomers are favorable for the precatalysts with
bulkier substituents [a]. The effect of the substituents
in the ligands on the ethylene oligomerization activities
is not so definite as its on the product distribution (en-
try 1–6 in Table 4).
It is worth noting that complex 10 containing the
difluoro-substituted ligand shows the lowest activity un-
der the identical reaction conditions, which may be
attributed to the readily deactivation of the active spe-
cies formed from 10 due to the small size of the difluoro
substituents. Similar result was obtained in our recent
study on ethylene catalytic activities for the ferrous
and cobaltous bearing pyridylimine ligands [8]. Gibson
has shown that the catalytic intermediates formed from
precursors containing pyridyldiimine ligands lacking
sufficient steric bulk in the ortho-aryl positions are more
easily deactivated through an interaction with alkylalu-
minum reagent [10].
The results of ethylene oligomerization and poly-
merization with the nickel complexes as precatalysts
at 1 atm are collected in Table 4. It can be observed
that the ligand environment has considerable effects
on their catalytic behaviors, such as activity and prod-
uct distribution. The complex 9 with bulky diisopropyl-
substituted ligand produces oligomers with higher
order carbon number olefins (CP8 over 40%) and more
polyethylene than 7 and 8 under the identical reaction
conditions (entry 3 vs entry 1 and 2, Table 4). Simi-
larly, for complexes 10–12 with halogen substituents
in the ligand backbone, the dibromo-substituted 12
generates polyethylene in low activity, while the less
bulky substituted 10 and 11 produce only oligomers
(entry 6 vs entry 4 and 5 in Table 4). These observa-
tions are well consistent with the established rules that
the bulky substitutes in the ortho-positions will provide
efficient steric blocking in the axial sites and retard the
Table 4
Results of ethylene oligomerization and polymerization with 7–12/MAO at 1 atma
Entry
Precatalyst
Al/Ni
Temperature (ꢁC)
Time (min)
Oligom distributionb (%)
Activity 104
molÀ1 (Ni) hÀ1
g
C4/RC
C6/RC
CP8/RC
Linear a-olefin
Oligomb
Polym
1
2
7
8
1000
1000
1000
1000
1000
1000
500
15
15
15
15
15
15
15
15
15
0
30
30
30
30
30
30
30
30
30
30
30
30
60
80.63
77.30
48.53
87.86
84.28
70.15
87.76
66.85
80.50
75.00
71.24
73.67
84.35
5.19
5.67
14.17
17.02
42.74
10.48
5.47
8.9
14.7
7.9
8.88
9.17
5.47
4.08
12.10
9.04
8.71
8.96
5.52
7.81
21.30
2.30
12.40
0.308
trace
1.37
no
3
9
8.73
1.66
4
10
11
12
12
12
12
12
12
12
12
40.4
11.1
51.0
21.5
26.1
28.6
33.0
49.1
53.1
6.4
5
10.25
21.89
2.50
no
6
7.96
9.74
0.30
0.42
0.20
trace
0.74
0.26
trace
0.168
7
8
1500
2000
1000
1000
1000
1000
7.61
5.37
25.54
14.13
17.85
9.00
9
10
11
12
13c
a
7.15
40
60
15
19.76
3.17
12.15
23.16
3.50
General conditions: 5 lmol precatalyst, 30 ml toluene as solvent.
Determined by GC. RC denotes the total amounts of oligomers.
Determined by GC–MS.
b
c

1576
X. Tang et al. / Journal of Organometallic Chemistry 690 (2005) 1570–1580
2.3.2. Effects of reaction parameter
butene (major) while C6 fraction contains 1-hexene
(minor), 2-hexene (major) and a small amount of 2-
methyl-2-pentene. In general, the selectivity for linear
a-olefins is lower than their iron analogues [8]. The
low selectivity for linear a-olefin with the nickel com-
plexes 7–12 can be attributed to their ability to revers-
ibly eliminate b-H after ethylene insertion and reinsert
the olefin with the opposite regiochemistry and give 2-
butene after chain transfer or to lead to isomerization
of 1-butene by a re-uptake mechanism [6i]. The ability
of Ni(II) complexes to isomerizes a-olefins was also ob-
served in previous studies [6c,6h].
To probe the effects of reaction parameters on the
ethylene oligomerization and polymerization behaviors,
the catalyst precursor 12 was typically investigated via
changing the amounts of MAO, the reaction tempera-
ture and the reaction time.
The oligomerization activity values show slight frus-
tration when the Al/Ni molar ratio is varied in the range
of 1000–1500. Further increase the Al/Ni ratio to 2000
leads to noticeable decrease in oligomerization activity.
The decrease in the oligomerization activity with exces-
sive MAO might be caused by the impurities such as al-
kyl aluminum in commercial MAO, which will lead
to the deactivation of the active catalytic sites. In addi-
tion, the polymerization activities remarkably decease
with the increase of the Al/Ni molar ratio. The correla-
tion between the polymer productivity and the Al/Ni
molar ratio suggests that the chain transfer to aluminum
might occur in the polymerization cases. Reaction tem-
perature remarkably affects the oligomerization activity,
and the highest oligomerization activity is observed at
the temperature of 40 ꢁC. The precatalyst might not be
fully activated at lower temperature. Further elevating
the reaction temperature (60 ꢁC) leads to sharp reduc-
tion in oligomerization activities (Table 4, entry 12),
which may be ascribed to the decomposition of the ac-
tive catalytic sites and lower ethylene solubility at higher
temperature. In addition, the PE productivity decreases
with the increase of the reaction temperature, which sug-
gests the increase of the rate of chain transfer relative to
the rate of chain propagation at the higher temperature.
Higher oligomerization activity is observed for 60 min
versus 30 min (Table 4, entry 6 vs 13), which indicates
the existence of induction period of the catalyst.
2.3.3. Effects of ethylene pressure
The ethylene oligomerization and polymerization
behaviors for precatalysts 9 and 12 were also investi-
gated at 10 atm, and the results are shown in Table
5. The results show that increase of ethylene concentra-
tion has a little effect on the catalytic activities and
product distributions for 9 and 12. This is in sharp
contrast with the results for their iron analogues, for
which the oligomer distributions and productivities
are significantly impacted by the increase of the ethyl-
ene pressure [8]. GPC analysis reveals that the molecu-
lar weight of the PE produced by 9 and 12 is relative
low with the Mn value of 5500 and 2000, respectively.
As shown by the GPC traces in Fig. 6, the PE samples
display bimodal (PE produced by 9) or trimodal (PE
produced by 12) behaviors, which might be attributed
to the formation of different type of active species after
activated by MAO and the difference of the rate of
chain transfer relative to chain propagation of these ac-
tive species. With literatures explained, chain transfer
to aluminum also might result in broad multimodal
distribution of the PE [7b,7c]. The corresponding
DSC curves show multimodality as well. The PE sam-
It should be pointed out that the selectivities for lin-
ear a-olefins of oligomers are varied with the ligand
environments and reaction parameters, however, no
unambiguous trend can be summarized according to
the values obtained. In general, the selectivities for
a-olefins are relative low. For example, as determined
by GC–MS for the oligomers obtained in entry 13 in
Table 4, C4 fraction contains 1-butene (minor) and 2-
1
ple produced by 9 at 10 atm was selected for H and
13C NMR characterization. Less useful information
on the microstructure of the PE is obtained from the
1H NMR spectrum because of the resonance overlap.
The 13C NMR analysis (Fig. 7) reveals that the PE
sample contains small amount of n-butyl branches.
Table 5
Results of ethylene oligomerization and polymerization with 9 and 12 at 10 atma
Activity, 104 g molÀ1
(Ni) hÀ1
Mn (·103)
Linear a-olefin (%)b
PDIc
Tm (ꢁC)
c
d
Entry
Precatalyst (lmol)
Polym yield (g)
Oligomb
Polym
1
2
a
9 (56.0)
12 (43.8)
0.85
0.18
4.88
27.10
1.50
0.40
5.5
2.0
15.9
29.5
63.2
25.4
128
123
General conditions: Al/Ni = 1000, 15 ꢁC, 1 h, 700 ml toluene.
Determined by GC.
Determined by GPC.
b
c
d
Determined by DSC.

X. Tang et al. / Journal of Organometallic Chemistry 690 (2005) 1570–1580
1577
degree were performed according to the previously re-
ported methods [11].
9
12
2.3.4. Effects of auxiliary ligand (PPh3)
For catalyst precursors 9–12, oligomerization activ-
ities were remarkably enhanced when PPh3 was used
as the auxiliary ligand, and the results are summarized
in Table 6. Similar effect of PPh3 was observed in the
previous reports [12]. Contrarily, the decrease of ethyl-
ene polymerization activity was also reported due to
the addition of 5 equiv of PPh3 for the neutral nickel
(II) catalyst [13]. GC analysis indicates that the pres-
ence of PPh3 in the reaction system affects the oligo-
mer distributions as well. Low-order carbon number
oligomers are produced without polyethylene due to
the incorporation of PPh3 in the reaction system. Var-
iation of auxiliary ligand amount affects the oligomer-
ization activities of the catalyst precursor 12, and 2
equiv of PPh3 is optimal (Table 6, entries 6, 7 and
8). The turnover number (TON) keeps increasing
when the reaction time is increased from 30 to
90 min (13,000, 24,000 and 25,000 for 30, 60 and
90 min, respectively), although the activity value de-
creases, which indicates that the active catalytic spe-
cies are still alive after 60 min.
2
3
4
5
6
7
log M
Fig. 6. GPC traces of the PE samples generated by 9 and 12 at 10 atm.
The mechanism of the acceleration phenomena of
ethylene oligomerzation induced by the addition of
the auxiliary ligand PPh3 is not well disclosed. We
tentatively attributed it to the temporary coordination
of PPh3 with the vacant site formed by the action of
MAO, thus the active catalytic species are protected.
Remarkable induction period was observed during
the oligomerization reaction in the presence of PPh3,
which might reflect the coordination of PPh3 with
the vacant site. The PPh3 group may dissociate again
upon approach and coordination of ethylene mono-
mer. Further studies to understand the effects of the
auxiliary ligand on the ethylene oligomerization are
in progress.
40
ppm
35
30
25
20
15
10
140
ppm
120
100
80
60
40
20
Fig. 7. 13C NMR spectra of the polyethylene sample generated by 9 in
entry 1, Table 4.
The branching degree is about 2n-butyls per 1000
methylenes in the main chain. Assignments of the 13C
NMR chemical shifts and calculation of the branching
Table 6
Ethylene oligomerization with 7–12/MAO in presence of PPh3 at 1 atma
Entry
Precatalyst
Al/Ni/PPh3
Time (min)
Oligom distribution (%)b
Activityb 105 g molÀ1 (Ni) hÀ1
C4/RC
C6/RC
Linear a-olefin
1
2
7
8
1000:1:2
1000:1:2
1000:1:2
1000:1:2
1000:1:2
1000:1:1
1000:1:2
1000:1:4
1000:1:2
1000:1:2
30
30
30
30
30
30
30
30
60
90
86.00
76.35
76.49
86.10
98.12
84.78
81.68
81.62
78.39
76.74
12.98
21.86
21.62
13.90
0.47
9.7
9.3
8.8
7.9
9.8
10.6
8.7
7.9
2.2
2.0
6.66
6.50
6.52
5.83
7.97
4.91
7.92
5.10
6.69
4.46
3
9
4
10
11
12
12
12
12
12
5
6
14.56
17.09
17.50
21.48
23.15
7
8
9
10
a
General conditions: 5 lmol of precatalyst, 30 ml toluene as solvent, temp: 15 ꢁC.
Determined by GC. RC denotes the total amounts of oligomers.
b

1578
X. Tang et al. / Journal of Organometallic Chemistry 690 (2005) 1570–1580
3. Conclusion
4.2. Synthesis of (L)NiCl2 (7–9; L = 1–3)
A series of nickel complexes containing 2-carboxyl-
ate-6-iminopyridine ligands were synthesized. X-ray
determination reveals that these complexes adopt differ-
ent coordination fashions. Upon activation with MAO,
these complexes show considerable to high catalytic
activities for ethylene oligomerization with C4–C8 olefins
as the main products. Complexes 7, 8, 9 and 12 with
bulky substituents in the ortho-positions of the aryl ring
in the ligand backbone display low ethylene polymeriza-
tion activities simultaneously. Addition of PPh3 as aux-
iliary ligand leads to higher catalytic activity.
7–9 were prepared by similar method, thus, only the
synthesis of 7 is detailed described. To ligand 1 (0.286 g,
1 mmol) and NiCl2 Æ H2O (0.237 g, 1 mmol), was added
fresh distilled ethanol (5 ml) at the room temperature.
The solution turned to orange immediately. After stirred
for 10 h, hexane (20 ml) was added to precipitate the com-
plex. After filtrated and washed with hexane, the product
was dried in vacuo. The desired complex was obtained as
bright yellow powder in 90.0% yield. IR (KBr): 1695
(mC@O); 1624 (mC@N); 1592; 1469; 1375 cmÀ1. Anal. Calc.
for C18H20Cl2N2NiO2: C, 50.75; H, 4.73; N, 6.58. Found:
C, 50.48; H, 4.81; N, 6.46%. 8 was obtained as bright
yellow powder in 92.3% yield. IR (KBr): 1689 (mC@O);
1621 (mC@N); 1590; 1467; 1377 cmÀ1. Anal. Calc. for
C20H24Cl2N2NiO2: C, 52.91; H, 5.33; N, 6.17. Found:
C, 52.89; H, 5.35; N, 5.90%. 9 was obtained as red brown
powder in 89.0% yield. IR (KBr): 1694 (mC@O); 1619
4. Experimental
4.1. General considerations
All manipulations of air- and/or moisture-sensitive
were performed under nitrogen atmosphere using stan-
dard Schlenk techniques. Solvents were dried by the lit-
erature methods. Methylaluminoxane (MAO) was
purchased from Albemarle as a 1.4 M solution in tolu-
ene. Other reagents were purchased from Aldrich or
Acros Chemicals. IR spectra were recorded on a Per-
kin–Elmer system 2000 FT-IR spectrometer. Elemental
analysis was carried out using HP-MOD 1106 microan-
alyzer. GC analysis was performed with a Carlo Erba
Strumentazione gas chromatograph equipped a flame
ionization detector and a 30 m (0.25 mm i.d., 0.25 lm
film thickness) DM-1 silica capillary column. GC–MS
analysis was performed with HP 5890 SERIES II and
HP 5971 SERIES mass detector. The yield of oligomer
was calculated by referencing with the mass of the used
solvent based on the prerequisite that the mass of each
fraction is approximately proportional to its integrated
areas in the GC trace. Selectivity for linear a-olefin is
defined as: (amounts of linear a-olefin of all frac-
tions/total amounts of oligomer products)%. 1H and
13C NMR spectra of the PE samples were recorded
on Bruker DMX-400 MHz instrument at 135 ꢁC in
1,2-dichlorobenzene-d4 using TMS as internal stan-
dard. Molecular weight and polydispersity index
(PDI) of PE was determined by a PL-GPC220 at
150 ꢁC with 1,2,4-trichlorobenzene as an eluent. Melt-
ing points of the polymers were obtained on a Per-
kin–Elmer DSC-7 in the standard DSC run mode.
The instrument was initially calibrated for melting
point of an indium standard at a heating rate of
10 ꢁC/min. The polymer sample was first equilibrated
at 0 ꢁC and then heated to 160 ꢁC at a heating rate
of 10 ꢁC/min to remove the heat history. The sample
was then cooled to 0 ꢁC at a rate of 10 ꢁC/min. A sec-
ond heating cycle was used for collected DSC thermo
gram data at a ramping rate of 10 ꢁC/min.
(mC@N); 1591; 1445; 1376 cmÀ1
. Anal. Calc. for
C22H28Cl2N2NiO2: C, 54.81; H, 5.85; N, 5.81. Found:
C, 54.80; H, 5.87; N, 5.70%.
4.3. Synthesis of (L)NiBr2 (10–12; L = 4–6)
10–12 were synthesized by the same method, thus,
only the synthesis of 10 was described. Ligand 4
(0.100 g,
0.32 mmol),
(DME)NiBr2
(0.1159 g,
0.32 mmol) and dichloromethane (10 ml) were added
to the flask at the room temperature. The solution
turned to orange immediately. After stirred for 10 h,
the solution was concentrated to about 4 ml, and then
hexane (20 ml) was added to precipitate the complex.
After filtrated and washed with hexane, the product
was dried in vacuo. The desired complex was obtained
as bright yellow powder in 91.1% yield. IR (KBr):
1686 (mC@O); 1625 (mC@N); 1592; 1473; 1375 cmÀ1. Anal.
Calc. for C16H14Br2F2N2NiO2 Æ H2O: C, 35.53; H, 2.98;
N, 5.18. Found: C, 35.14; H, 2.93; N, 5.08%. 11 was ob-
tained as red brown powder in 84.0% yield. IR (KBr):
1693 (mC@O); 1623 (mC@N); 1592; 1436; 1375 cmÀ1. Anal.
Calc. for C16H14Br2Cl2N2NiO2 Æ H2O: C, 33.50; H, 2.81;
N, 4.88. Found: C, 33.20; H, 2.61; N, 4.62%. 12 was ob-
tained as red brown powder in 93.0% yield. IR (KBr):
1695 (mC@O); 1622 (mC@N); 1592; 1429; 1374 cmÀ1. Anal.
Calc. for C16H14Br4N2NiO2 Æ CH2Cl2: C, 27.96; H, 2.27;
Br, N, 3.84; Found: C, 28.01; H, 2.27; N, 3.82%. For
complexes 10, 11 and 12, X-ray crystal structure deter-
mination also confirms the existence of water molecule
or dichloromethane molecule.
4.4. Procedure for 1 atm ethylene oligomerization and
polymerization
Precatalyst (5 lmol) was added to a fully dried
Schlenk flask under nitrogen. The flask was back-filled

X. Tang et al. / Journal of Organometallic Chemistry 690 (2005) 1570–1580
1579
three times with N2 and twice with 1 atm ethylene, and
then charged with toluene and MAO solution in turn.
Under prescribed temperature, the reaction solution
was vigorously stirred under 1 atm ethylene for the de-
sired period. The polymerization reaction was quenched
by addition 60 ml 10% HCl solution. About 1 ml of or-
ganic solution was dried by anhydrous Na2SO4 for GC
or GC–MS analysis. The remained mixture was poured
into 100 ml of ethanol to precipitate the polymer. The
ployethylene was isolated via filtration and dried at
60 ꢁC to constant weight in a vacuum oven.
Data Centre, CCDC 247174, 247173, 247175, 247172
and 247171, respectively. Copies of this information
may be obtained free of charge from CCDC, 12 Union
Road, Cambridge, CB2 1 EZ, UK (fax: +44-1223-
336033; e-mail: deposit@ccdc.cam.ac.uk or http://
www.ccd.cam.ac.uk).
Acknowledgements
We are grateful to the National Natural Science
Foundation of China for financial supports under
Grants No. 20272062 and 20473099 along with National
863 Project (2002AA333060). We thank Dr. Jinkui Niu
for the English Language Corrections.
4.5. Procedure for 10 atm ethylene oligomerization and
polymerization
High-pressure ethylene polymerization was performed
in a stainless steel autoclave (2 l capacity) equipped with
gas ballast through a solenoid clave for continuous feed-
ing of ethylene at constant pressure. 700 ml toluene con-
taining the catalyst precursor was transferred to the fully
dried reactor under nitrogen atmosphere. Then the re-
quired amount of cocatalyst (MAO) was injected into
the reactor using a syringe. As the prescribed temperature
was reached, the reactor was pressurized to 10 atm. After
stirring for the desired reaction time, the reaction was
quenched and worked up using the similar method de-
scribed above for 1 atm reaction.
References
[1] D. Vogt, in: B. Cornils, W.A. Herrmann (Eds.), Applied Homo-
geneous with Organometallic Compounds, vol. 1, VCH, New
York, 1996, pp. 245–258.
[2] (a) W. Keim, F.H. Kowaldt, R. Goddard, C. Kruger, Angew.
Chem. Int. Ed. Engl. 17 (1978) 466;
(b) M. Peuckert, W. Keim, Organometallics 2 (1983) 594;
(c) W. Keim, A. Behr, B. Limbacker, C. Kruger, Angew. Chem.
Int. Ed. Engl. 22 (1983) 503;
(d) W. Keim, A. Behr, G. Kraus, J. Organomet. Chem. 251 (1983)
377;
(e) M. Peuckert, W. Keim, J. Mol. Catal. 22 (1984) 289;
(f) W. Keim, R.P. Schulz, J. Mol. Catal. 92 (1994) 21;
(g) W. Keim, B. Hoffmann, R. Lodewick, M. Peuckert, G.
Schmitt, J. Fleischhauer, U. Meier, J. Mol. Catal. 6 (1979)
79;
4.6. X-ray crystal structure determination
The single-crystal X-ray diffraction for 8 was carried
out on a Bruker SMART 1000 CCD diffractometer with
graphite monochromated Mo Ka radiation (k =
(h) U. Klabunde, S.D. Itten, J. Mol. Catal. 41 (1987) 123;
(i) P. Grenouillet, D. Neibecker, I. Tkatchenko, J. Organo-
met. Chem. 243 (1983) 213.
˚
0.71073 A). Intensity data of crystal 7 and 12 were
collected on a Rigaku RAXIS Rapid IP diffractometer
[3] (a) L.K. Johnson, C.M. Killian, M. Brookhart, J. Am. Chem.
Soc. 117 (1995) 6414;
with graphite monochromated Mo-Ka radiation
˚
(b) L.K. Johnson, S. Mecking, M. Brookhart, J. Am. Chem. Soc.
118 (1996) 267;
(k = 0.71073 A). Intensity data of 10 and 11 were collected
on a Siemens SMART CCD diffractometer with graphite
(c) C.M. Killian, D.J. Tempel, L.K. Johnson, M. Brookhart, J.
Am. Chem. Soc. 118 (1996) 11664;
˚
monochromated Mo Ka radiation (k = 0.71073 A). Cell
parameters were obtained by global refinement of the
positions of all collected reflections. Intensities were cor-
rected for Lorentz and polarization effects and empirical
absorption. The structures were solved by direct methods
and refined by full-matrix least-squares on F2 Each H
atom was placed in a calculated position and refined
anisotropically. Structure solution and refinement were
performed by direct methods using the SHELXL-97 Pack-
age. All the intensity data of the crystals were collected at
293(2) K. Crystal data and processing parameters for
complexes 7, 8, 10, 11 and 12 are summarized in Table 6.
(d) J. Feldman, S.J. McLain, A. Parthasarathy, W.J. Marshall,
J.C. Calabrese, S.D. Arthur, Organometallics 16 (1997) 1514;
(e) M. Helldo¨rfer, J. Backhaus, W. Milius, H.G. Alt, J. Mol.
Catal. A 193 (2003) 59;
(f) D.H Camacho, E.V. Salo, J.W. Ziller, Z. Guan, Angew.
Chem. Int. Ed. 43 (2004) 1821;
(g) B.Y. Lee, G.C. Bazan, J. Vela, Z.J.A. Komon, X. Bu, J. Am.
Chem. Soc. 123 (2001) 5352.
[4] (a) C. Wang, S. Friedrich, T.R. Younkin, R.T. Li, R.H. Grubbs,
D.A. Bansleben, M.W. Day, Organometallics 17 (1998) 3149;
(b) T.R. Younkin, E.F. Connor, J.I. Henderson, S.K. Friedrich,
R.H. Grubbs, D.A. Bansleben, Science 287 (2000) 460;
(c) E.F. Connor, T.R. Younkin, J.I. Henderson, A.W. Waltman,
R.H. Grubbs, Chem. Commun. (2003) 2272;
(d) L. Wang, W.-H. Sun, L. Han, Z. Li, Y. Hu, C. He, C. Yan, J.
Organomet. Chem. 650 (2002) 59.
5. Supplementary material
[5] (a) S.P. Meneghetti, P.J. Lutz, J. Kress, Organometallics 18
(1999) 2734;
Crystallographic data of complexes 7, 8, 10, 11 and
12 were deposited with the Cambridge Crystallographic
(b) T.V. Laine, K. Lappalainen, J. Liimatta, E. Aitola, B. Loefgren,
M. Leskelae, Macromol. Rapid Commun. 20 (1999) 487;

1580
X. Tang et al. / Journal of Organometallic Chemistry 690 (2005) 1570–1580
[8] W.-H. Sun, X. Tang, T. Gao, B. Wu, W. Zhang, H. Ma,
(c) T.V. Laine, U. Piironen, K. Lappalainen, M. Klinga, E. Aitola,
M. Leskel, J. Organomet. Chem. 606 (2000) 112;
Organometallics 23 (2004) 5037.
[9] (a) F.A. Hicks, M. Brookhart, Organometallics 20 (2001)
3217;
(d) B.Y. Lee, X. Bu, G.C. Bazan, Organometallics 20 (2001) 5425.
[6] (a) H.-P. Chen, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Organometallics
22 (2003) 4893;
(b) J.M. Malinoski, M. Brookhart, Organometallics 22 (2003)
5324;
(b) F. Speiser, P. Braunstein, L. Saussine, R. Welter, Inorg.
Chem. 43 (2004) 1649;
(c) C.B. Shim, Y.H. Kim, B.Y. Lee, Y. Dong, H. Yun,
Organometallics 22 (2003) 4272.
(c) W.-H. Sun, Z. Li, H. Hu, B. Wu, H. Yang, N. Zhu, X. Leng,
H. Wang, New J. Chem. 26 (2002) 1474;
(d) P.-Y. Shi, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Organometallics
21 (2002) 3203;
[10] G.J.P. Britovsek, S. Mastroianni, G.A. Solan, S.P.D. Baugh, C.
Redshaw, V.C. Gibson, A.J.P. White, D. Williams, M.R.J.
Elsegood, Chem. Eur. J. 6 (2000) 2221.
(e) O. Daugulis, M. Brookhart, Organometallics 21 (2002) 5926;
(f) O. Daugulis, M. Brookhart, P.S. White, Organometallics 21
(2002) 5935;
[11] (a) G.B. Galland, R.F. deSouza, R.S. Mauler, F.F. Nunes,
Macromolecules 32 (1999) 1620;
(b) E. Kokko, P. Lehmus, R. Leino, H.J.G. Luttikhedde, P.
Ekholm, J.H. Nsman, J.V. Seppa¨la¨, Macromolecule 33 (2000)
9200;
(g) F. Speiser, P. Braunstein, L. Saussine, R. Welter, Organomet-
allics 23 (2004) 2613;
(h) F. Speiser, P. Braunstein, L. Saussine, Organometallics 23
(2004) 2625;
(c) G.B. Galland, R. Quijada, R. Rojas, G. Bazan, Z.J.A.
Komon, Macromolecule 35 (2002) 339.
(i) F. Speiser, P. Braunstein, L. Saussine, Organometallics 23
(2004) 2633.
[12] (a) C. Carlini, M. Isola, V. Liuzzo, A.M.P. Galletti, G. Sbrana,
Appl. Catal. A Gen. 231 (2002) 307;
[7] (a) G.J.P. Britovsek, V.C. Gibson, D.F. Wass, Angew. Chem. Int.
Ed. Engl. 38 (1999) 428;
(b) F.A. Hicks, J.C. Jenkins, M. Brookhart, Organometallics 22
(2003) 3533;
(b) S.D. Ittel, L.K. Johnson, M. Brookhart, Chem. Rev. 100
(2000) 1169;
(c) V.C. Gibson, S.K. Spitzmesser, Chem. Rev. 103 (2003) 283.
(c) W.-H. Sun, W. Zhang, T. Gao, X. Tang, Y. Li, X. Jin, J.
Organomet. Chem. 689 (2004) 917.
[13] J.C. Jenkins, M. Brookhart, Organometallics 22 (2003) 250.