Synthesis and catalytic activity of neutral salicylaldiminato nickel(II) complexes bearing a single N-heterocyclic carbene ligand

Article abstract of DOI:10.1016/j.jorganchem.2008.03.005

Neutral salicylaldiminato Ni(II) complexes bearing a single N-heterocyclic carbene (NHC) ligand [3,5-^tBu₂-2-(O)C₆H₂CH{double bond, long}NAr]Ni(C{RNCHCHNⁱPr})Ph [Ar = 2,6-ⁱPr₂C

Full text of DOI:10.1016/j.jorganchem.2008.03.005

Journal of Organometallic Chemistry 693 (2008) 2047–2051
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Journal of Organometallic Chemistry
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Synthesis and catalytic activity of neutral salicylaldiminato nickel(II) complexes
bearing a single N-heterocyclic carbene ligand
*
*
Wan-Fei Li, Hong-Mei Sun , Mu-Zi Chen, Qi Shen , Yong Zhang
The Key Lab of Organic Synthesis of Jiangsu Province, College of Chemistry and Chemical Engineering, Suzhou University, Suzhou 215123, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Neutral salicylaldiminato Ni(II) complexes bearing
a single N-heterocyclic carbene (NHC) ligand
[3,5-tBu2-2-(O)C6H2CH@NAr]Ni(C{RNCHCHNiPr})Ph [Ar = 2,6-iPr2C6H3, R = Bn (1); Ar = 2,6-iPr2C6H3,
R = iPr (2)], have been synthesized via a one-pot procedure in high yield. The X-ray structure analysis
reveals that both of 1 and 2 adopt distorted square-planar coordination geometry and NHC carbon
(Ccarbene) is trans to the ketimine nitrogen. Preliminary study indicates that complex 1 is inert toward
the insertion of ethylene, however, it can catalyze the dimerization of ethylene in the presence of mod-
ified methylaluminoxane (MMAO) with a moderate activity of 3.05 Â 104 g(mol Ni)À1 h À1 atmÀ1 in a
highly selective fashion.
Received 7 November 2007
Received in revised form 5 March 2008
Accepted 6 March 2008
Available online 18 March 2008
Keywords:
N-Heterocyclic carbene
Neutral nickel complex
Catalyst
Ó 2008 Elsevier B.V. All rights reserved.
Ethylene
Dimerization
1. Introduction
cially the ethylene oligomerization [6,11,12]. The quite different
behaviors in olefin polymerization showed by NHC-based systems,
In recent years, N-heterocyclic carbenes (NHCs) are being used
increasingly in late transition metal-based organometallic chemis-
try as a valuable alternative to traditional phosphine ligands [1].
Due to their strong donor properties, easily tuned in electronic
and steric effect by the diversity of nitrogen substituents, the use
of NHCs ligands has opened new opportunities in late transition
metal-based catalysis [2]. In particular, nickel-based NHC systems
have been widely employed in a series of reactions such as C–F
bond [3] or C–H bond activation [4], and various coupling reactions
for the formation of C–C bond [5] or C–N [5a] bond and so on. How-
ever, the application of such kind of NHC complexes in olefin poly-
merization has been relatively limited [6–12]. Up to date, only
several kinds of NHC complexes of Ni(II) have been tested, mainly
including: the bis-NHC dihalides [6], the picolyl-functionalized
NHC complexes [7], the enol-functionalized NHC complexes [8],
the salicylaldiminato-functionalized NHC complexes [9], and a sin-
gle NHC co-supported cylcopentadienylnickel(II) [10], indenylnick-
el(II) [11] or allylnickel(II) [12] complexes. Furthermore, most of
them are predominantly capable of catalyzing olefin oligomeriza-
tions [6,8,11,12] rather than olefin polymerization [7,9,10], espe-
as compared with their phosphine analogues, may be attributed to
the facile decomposition of the active species stabilized by NHC via
the hydrocarbyl-imidazolium elimination [6a]. In line with this no-
tion are recent reports of several NHC-based Ni(II) complexes
showing somewhat good selectivity control either in ethylene olig-
omerization [6,11] or in styrene oligomerization [12].
On the other hand, the salicylaldimine framework (N–O), as a
common motif in organometallic chemistry, has been extensively
applied in Ni(II)-based olefin polymerization and oligomerization
catalysts [13]. The leading example is Grubbs’ neutral salicylaldim-
inato Ni(II) complexes of the type (N–O)Ni(R)(L) (R = phenyl or
methyl, L = triphenylphosphine or acetonitrile) (Scheme 1) [13a]
for the polymerization of ethylene. In general, the catalytic activity
and selectivity, i.e. ethylene polymerization vs. ethylene oligomer-
ization, are greatly affected by steric and electronic properties of
salicylaldimine framework as well as the nature of the L ligand.
Very recently, neutral Ni(II) complexes supported by enolate che-
lating NHC ligand (4) (Scheme 2) have been reported to be capable
of polymerizing ethylene in a highly linear fashion [8a]. This
prompted us to develop novel neutral Ni(II) complexes bearing a
single NHC ligand (instead of a phosphine ligand) and to under-
stand further their catalytic behaviors for ethylene polymerization.
Herein, we describe the synthesis, structure and catalytic behav-
iors of two neutral salicylaldiminato Ni(II) complexes stabilized
* Corresponding authors. Tel.: +86 512 65880306; fax: +86 512 65880305.
E-mail addresses: sunhm@suda.edu.cn (H.-M. Sun), qshen@suda.edu.cn
(Q. Shen).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.03.005

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W.-F. Li et al. / Journal of Organometallic Chemistry 693 (2008) 2047–2051
H), 7.52 (2H, d, Ph-H), 6.91–7.11 (10H, Ph-H), 6.59–6.81 (2H, Ph-H),
R'
L
3: R = Ph, L = PPh3
O
6.13 (1H, s, NCH), 6.18 (1H, s, NCH), 5.47 (1H, m, NCH(CH3)2), 4.32
(2H, m, NCH2), 0.94–1.60 (38H, CH(CH3)2, NCH(CH3)2, CH(CH3)2 or
C(CH3)3). 13C NMR: d 22.9, 23.4, 24.2, 26.3, 26.6, 29.5, 30.3, 32.0,
32.2, 35.9, 52.6, 55.3, 116.3, 120.5, 121.9, 123.3, 125.5, 126.2,
128.3, 128.5, 128.7, 129.0, 129.4, 139.0, 141.1, 141.3, 168.0. Anal.
Calc. for C46H59N3NiO: C, 75.82; H, 8.16; N, 5.76. Found: C, 75.80;
H, 8.17; N, 5.73%.
Ni R
R' = tBu, Ar = 2,6-iPr2C6H3
N
Ar
Scheme 1. Reported salicylaldimine framework.
Ph
O
Ni
N
2.4. Synthesis of 2
4: Ar = 2,6-iPr2C6H3
Ph
N
N
Following the procedure similar to the synthesis of 1, 1,3-diiso-
propylimidazolium bromide (0.19 g, 1.0 mmol) was added to the
Schlenk flask instead of 1-benzyl-3-isopropylimidazolium bro-
mide. After workup, complex 2 was obtained as orange crystals
in ca. 82% yield (0.56 g), which was suitable for X-ray diffraction
studies and elemental analysis. 1H NMR (400 MHz, CDCl3, 25 °C):
d 8.01 (s, 1H, N@CH), 7.67 (1H, s, Ph-H), 7.50 (2H, d, Ph-H), 7.04
(1H, s, Ph-H), 6.91 (2H, m, Ph-H), 6.61 (4H, m, Ph-H), 6.22 (2H, s,
NCH), 4.40 (2H, m, NCH(CH3)2), 1.34–1.50 (32H, CH(CH3)2,
CH(CH3)2 or C(CH3)3), 1.10 (12H, m, CH(CH3)2). 13C NMR: d 23.2,
28.6, 29.4, 30.7, 32.2, 51.9, 52.4, 115.5, 116.3, 119.2, 122.5, 123.0,
124.6, 125.1, 127.8, 128.0, 128.2, 129.0, 138.2, 138.6, 140.7,
151.5, 165.0, 167.7, 174.0. Anal. Calc. for C42H59N3NiO: C, 74.11;
H, 8.74; N, 6.17. Found: C, 74.08; H, 8.70; N, 6.14%.
Ar
Scheme 2. Reported enolate chelating NHC framework.
by a single NHC ligand (1 and 2). To our best acknowledge, this is
the first example of neutral salicylaldiminato Ni(II) phenyl com-
plexes bearing a single NHC ligand.
2. Experimental
2.1. General procedures
All manipulations were performed under pure argon with rigor-
ous exclusion of air and moisture using standard Schlenk tech-
niques. Solvents were distilled from Na/benzophenone ketyl
under pure argon prior to use. Ni(PPh3)2PhCl [14]. 3,5-tBu2-2-
(HO)C6H2CH@NAr (Ar = 2,6-iPr2C6H3) [15] and 1,3-diisopropylimi-
dazolium bromide (iPr Á HBr) [16] were prepared by published
methods. All other chemicals were obtained commercially and
used as received unless stated otherwise. Elemental analysis was
performed by direct combustion on a Carlo-Erba EA-1110 instru-
ment. NMR (CDCl3) spectra were measured on a Unity Inova-400
spectrometer at 25 °C.
2.5. X-ray structural determination of 1 and 2
Suitable single crystals of 1 and 2 were sealed in a thin-walled
glass capillary for X-ray structural analysis. Diffraction data were
collected on a Rigaku Mercury CCD area detector at 153(2) K for
1 and 193(2) K for 2, respectively. The structure was solved by di-
rect methods and refined by full-matrix least-squares procedures
based on F2. All non-hydrogen atoms were refined with anisotropic
displacement coefficients. Hydrogen atoms were treated as ideal-
ized contributions. The structures were solved and refined using
SHELXS-97 and SHELXL-97 programs, respectively. Crystal data and col-
lection and main refinement parameters are given in Table 1. Se-
lected bond lengths ( Å) and angles (°) for 1 and 2 are given in
Table 2.
2.2. Synthesis of 1-benzyl-3-isopropylimidazolium bromide
(Bn-iPr Á HBr)
To a THF solution (30 mL) of 1-isopropyl-imidazole (2.34 g,
20 mmol) was added equivalent of benzyl bromide (2.38 mL,
20 mmol). After stirring 3 h, the white powder was collected
through filtration, washed with ether and dried in vacuo. The yield
is almost quantitative (3.77 g, 95%). 1H NMR (400 MHz, D2O) d:
8.77 (s, 1H, NCHN), 7.49 (s, 1H, Ph-H), 7.32–7.39 (6H, Ph-H and
NCH), 5.29 (s, 2H, NCH2), 4.52 (m, 1H, NCH(CH3)2), 1.43 (m, 6H,
2CH(CH3)2). Anal. Calc. for C13H17BrN2: C, 55.53; H, 6.09; N, 9.96.
Found: C, 55.18; H, 6.49; N, 10.34%.
2.6. A typical procedure for ethylene dimerization
In a typical run, a Schlenk flask was charged with toluene
(30 mL, saturated by ethylene (1 atm)) and MMAO (500 equiv.), a
toluene solution (5 mL) of complex 1 (5.0 lmol) was added via syr-
inge under N2 at room temperature. After 60 min the ethylene was
bled off and the reaction quenched with 10% HCl at À20 °C. The tol-
uene phase was analyzed by GC–MS with n-heptane (0.8 mL) as an
internal standard.
2.3. Synthesis of 1
3. Results and discussion
A Schlenk flask was charged with 1-benzyl-3-isopropylimidazo-
lium bromide (0.28 g, 1.0 mmol), 3,5-tBu2-2-(HO)C6H2CH@NAr
(Ar = 2,6-iPr2C6H3) (0.39 g, 1.0 mmol), THF (20 mL) and a stir bar.
To this suspension was added dropwise the solution of NaN-
(SiMe3)2 (1.0 M, 2.0 mL) in THF at 0 °C. The reaction was stirred
for 2.5 h at 0 °C and gradually warmed to room temperature for
additional 30 min. The resulted mixture was added slowly to the
solution of Ni(PPh3)2PhCl (0.70 g, 1.0 mmol) in 20 mL toluene un-
der stirring at room temperature. The solution was stirred over-
night, filtered, and evaporated to dryness. The residue was
extracted with toluene and recrystallized from toluene/DME at
À10 °C yielded yellow crystals (0.58 g, 80%) suitable for X-ray dif-
fraction studies and elemental analysis. Melt point: 227–228 °C. 1H
NMR (400 MHz, CDCl3, 25 °C): d8.02 (s, 1 H, N@CH), 7.82 (1H, d, Ph-
3.1. Synthesis and characterization of 1 and 2
The procarbene ligand, 1-benzyl-3-isopropylimidazolium
bromide was prepared in quantitative yield by the alkylation reac-
tion of the 1-isopropyl-imidazole with benzyl bromide. The forma-
tion of 1-benzyl-3-isopropylimidazolium bromide was supported
by its 1H NMR spectrum, particularly by the characteristic reso-
nance at 8.77 ppm for the imidazolium proton.
Up to date, the substitution of phosphine ligands in Ni(II)
halides by NHC ligands is one of the most common route to pre-
pare NHC complexes of Ni(II) [5c]. Herein, a one-pot procedure
was developed for the preparation of complexes 1 and 2 in high

W.-F. Li et al. / Journal of Organometallic Chemistry 693 (2008) 2047–2051
2049
R
N
NaN(SiMe3)2
iPr
Br
H
N
N R
R
N
iPr
Ph
N
N
iPr
THF, 0 °C
iPr
Ni
O
N
Ni(PPh3)2PhCl
tBu
iPr
iPr
iPr
toluene / THF, r.t.
NaN(SiMe3)2
ONa N
tBu
N
tBu
OH
tBu
tBu
tBu
iPr
THF, 0 °C
iPr
R = Bn (1), 80%
R = iPr (2), 82%
Scheme 3. Synthesis of 1 and 2.
yield (Scheme 3). This procedure has the advantage to avoid the
preparation of the phosphine-based neutral salicylaldiminato Ni(II)
complexes beforehand. Thus, the mixture of 3,5-tBu2-2-(HO)-
C6H2CH@NAr (Ar = 2,6-iPr2C6H3), Bn-iPr Á HBr and NaN(SiMe3)2
(1:1:2 molar ratio), which had stirred at 0 °C for 2.5 h, was directly
reacted with an equivalent of Ni(PPh3)2PhCl at room temperature.
After workup, the target product 1 was isolated as yellow crystals
with analytical purity in ca. 80% yield. The similar one-pot
procedure employing the imidazolium salt iPr Á HBr afforded the
complex 2 as orange crystals in ca. 82% yield. The formation of 1
or 2 was initially supported by elemental analysis and NMR
spectroscopy. The 1H NMR spectra of complexes 1 and 2 exhibit
the characteristic resonances of iso-propyl and tert-butyl protons,
meanwhile disappear the characteristic resonances of the imidazo-
lium proton. In their 13C NMR spectra, the singlet NHC carbon
(Ccarbene) signals appear at d 168.0 ppm for 1 and d 174.0 ppm for
2, respectively. These chemical shifts of Ccarbene signals in 13C
NMR spectra are in good agreement with those reported for
monocarbene Ni(II) complexes [11]. Further determination of the
molecular structures of 1 and 2 were obtained by X-ray structural
analysis. Complexes 1 and 2 represent one of only two examples
of a stable phenyl Ni(II) complex bearing a single NHC ligand pub-
lished. The other one is Ni(tmiy)2(o-tolyl)Br which was synthesized
via the oxidative addition of Ni(tmiy)2 with o-tolyl bromide
(tmiy = 1,3,4,5-tetramethylimidazol-2-ylidene) [17].
Fig. 1. The crystal structure of 1 showing 20% probability ellipsoids. The hydrogen
atoms are omitted for clarity.
In fact, it should be noted that the Ni(II) complexes presented
in this work are quite stable, which is very similar to Ni(tmiy)2(o-
tolyl)Br [17]. For example, complexes 1 and 2 are thermally stable
and will melt (not decompose) at 227–228 °C and 232–234 °C,
respectively. Besides, both of 1 and 2 are showed greatly en-
hanced air-stability in solid-state, as compared with their phos-
phine analogous, and even can remain unchanged for several
days in open dry air. However, they are sensitive to water, espe-
cially in solution. For example, the color of their THF solution
changed immediately during the addition of water. Complexes 1
and 2 are soluble in CHCl3, THF, DME, toluene and diethyl ether,
sparingly soluble in hexane.
3.2. Crystal structures of 1 and 2
Crystals of 1 and 2 suitable for an X-ray crystal structure
determination were grown from hexane/DME for 1 and toluene/
DME for 2 at À10 °C. The solid structures for 1 and 2 were shown
in Figs. 1 and 2, respectively. The crystallographic data and se-
lected bond distances and angles are listed in Tables 1 and 2,
respectively.
The X-ray structural analysis revealed complexes 1 and 2 have
the similar solid-state structure. The center nickel atom of each
complex coordinated to phenyl carbon (Cphenyl), O, N and Ccarbene
to form a distorted square-planar geometry with a sum of the four
Fig. 2. The crystal structure of 2 showing 30% probability ellipsoids. The hydrogen
atoms are omitted for clarity.

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W.-F. Li et al. / Journal of Organometallic Chemistry 693 (2008) 2047–2051
Table 1
length between 1/2 and 3, i.e. Ni–Cphenyl and Ni–O bond, indicates
X-ray crystallographic data for 1 and 2
that the PPh3 and NHC ligand presented herein has little effect on
the Ni–Cphenyl and Ni–O interaction in the neutral salicylaldiminato
Ni(II) phenyl complexes of the type (N–O)Ni(Ph)(L), whereas the
slightly difference of Ni–N bond length between the two kinds of
neutral Ni(II) complexes might be due to the stronger trans influ-
ence exerted by PPh3 than by the NHC moiety [5d] in the salicyl-
aldimine framework. Otherwise, the different substituents on the
carbene ring of 1 and 2 showed only little effect on their solid
structure, which might be due to the rotation of the carbene ring.
1
2
Empirical formula
Formula weight
Temperature (K)
k (Mo Ka) (Å)
Crystal system
Space group
a (Å)
C
46H59N3NiO
C42H59N3NiO
680.63
153(2)
728.67
193(2)
0.71070
Monoclinic
P21/n
15.2641(18)
17.519(2)
15.7000(17)
94.460(3)
4185.7(8)
4
0.71070
Monoclinic
P21/n
12.3136(9)
14.0031(9)
22.8218(17)
99.167(2)
3884.9(5)
4
b (Å)
c (Å)
b (°)
3.3. The catalytic activity of 1 for the dimerization of ethylene
V (Å3)
Z
Dcalc (g/cm3)
1.156
0.500
1568
1.164
0.533
1472
Since complexes 1 and 2 have very similar solid structure, only
complex 1 was tested as a catalyst for ethylene polymerization.
Surprisingly, toluene solution of complex 1 is inert toward the
insertion of ethylene (1–7 bar ethylene, 25–60 °C) in the absence
of a co-catalyst such as MMAO, which is quite different from the
PPh3-based analogues of 3 [13a]. Then, we studied the same poly-
merization reaction in the presence of MMAO. However, complex 1
still showed very low reactivity at room temperature under 1 bar
ethylene when the molar ratio of MMAO to 1 varied from 1500
to 500. After 1 h only trace dimmer of ethylene was detected.
When the pressure of ethylene being increased to 7 bar and the
reaction temperature being increasing to 60 °C, complex 1 showed
a modest activity of 3.05 Â 104 g(mol Ni)À1 hÀ1 atmÀ1 after 60 min
under a 500 molar ratio of MMAO to 1 (the oligomerization condi-
tions were not optimized). Interestingly, GC/MS analysis of the fi-
nal reaction solution showed the products to be unique 1-butene,
indicating that b-H elimination from the putative Ni–Bu intermedi-
ate is too fast to chain propagation. It is worthy noticing that the
catalytic system of 1 and MMAO is quite stable and no decomposi-
tion of 1, i.e. the appearance of black powder in the reaction solu-
tion, was observed during the reaction process. Considering the
greatly difference of catalytic behaviors for ethylene polymeriza-
tion between 1 and 3, it might be reasonable to assume that the
resting state of the present system should be a NHC complex. We
note that a recent report also assumed that the NHC ligand coordi-
nated to the active species for the styrene oligomerization initiated
by cationic allyl complexes of Ni(II) bearing a single NHC ligand,
which also be studied as mimicking the analogous phosphine-
based system [12].
Absorption coefficient (mmÀ1
F(000)
)
Crystal size (mm)
2hmax(°)
Number of reflections collected
Number of independent reflections(Rint
Goodness-of-fit on F2
0.39 Â 0.20 Â 0.05 0.28 Â 0.20 Â 0.18
50.7
40850
50.7
37589
7107 (0.0760)
1.124
0.0607
)
7641 (0.0904)
1.198
0.0783
Final R indices [I > 2r(I)]
Table 2
Selected bond distances (Å) and angles (°) for 1 and 2
1
Ni(1)–C(28)
Ni(1)–O(1)
C(28)–Ni(1)–C(41)
C(41)–Ni(1)–O(1)
C(41)–Ni(1)–N(1)
1.861(4)
1.905(2)
86.9(2)
171.3(1)
95.6(1)
Ni(1)–C(41)
Ni(1)–N(1)
C(28)–Ni(1)–O(1)
C(28)–Ni(1)–N(1)
O(1)–Ni(1)–N(1)
1.891(4)
1.927(3)
85.1(1)
176.2(1)
92.6(1)
2
Ni(1)–C(28)
Ni(1)–O(1)
C(28)–Ni(1)–C(37)
C(37)–Ni(1)–O(1)
C(37)–Ni(1)–N(1)
1.875(3)
1.905(2)
86.1(1)
170.5(1)
93.5(1)
Ni(1)–C(37)
Ni(1)–N(1)
C(28)–Ni(1)–O(1)
C(28)–Ni(1)–N(1)
O(1)–Ni(1)–N(1)
1.889(3)
1.924(2)
87.8(1)
173.7(1)
93.4(1)
bond angles at Ni of 360.2° (1) and 360.8° (2). Steric congestion
around the Ni center is relieved by rotation of the carbene ring,
the phenyl ring and the arylamine ring. Dihedral angles of the
above rings relative to the coordination plane are 72.5°, 64.1°
and 78.4° in turn for 1 and 88.9°, 81.5° and 80.7° in turn for 2.
The more perpendicular dihedral angles presented in 2 reflect a
bigger steric hindrance induced by the iso-propyl substituent on
the carbene ring than by the benzyl moiety. The NHC ligand is trans
to the arylamine group with a nearly linear Ccarbene–Ni–N angle of
176.1° for 1 or 173.7° for 2, which is close to the reported value
(172.2° for 3) [13a]. The Ni–Ccarbene bond length of 1.861 Å for 1
or 1.875 Å for 2 is within the range for normal Ni–Ccarbene bonds.
For example, both of them are shorter than that of 1.911–1.919 Å
observed in Ni(tmiy)2(o-tolyl)Br [17], and slightly longer than that
of 1.848 Å observed in the phenyl Ni(II) complexes bearing enolate
chelating NHC ligand (4) [8a]. The Ni–N bond length of 1.927 Å for
1 and 1.924 Å for 2 is very close to the reported value of 1.937 Å for
PPh3-based neutral salicylaldiminato Ni(II) phenyl complex 3
[13a]. The phenyl group attached to Ni(II) lies approximately trans
to phenoxide oxygen with a O–Ni–Cphenyl angle of 171.3° for 1 or
170.5° for 2, which is closer to linearity than that of 166.2° ob-
served in 3 [13a]. The Ni–Cphenyl bond length of 1.891 Å for 1 or
1.889(3) Å for 2 is almost the same as that of 1.895 Å observed
in 3 [13a] and 1.891 Å in 4 [8a]. By the way, the same Ni–O bond
length of 1.905 Å is observed in 1 and 2, which is also just the same
as 1.910 Å found in 3, whereas a slightly shorter Ni–O bond of
1.891 Å was observed in 4 [8a]. The similarity of the typical bond
4. Conclusion
In summary, we have prepared two neutral salicylaldiminato
Ni(II) complexes bearing a single NHC ligand via a convenient
one-pot procedure in high yield. Both of them show obviously en-
hanced thermal and air-stability as compared with their phosphine
analogues. The preliminary study showed that complex 1 could
catalyze the dimerization of ethylene in the presence of MMAO
with a moderate activity of 3.05 Â 104 g(mol Ni)À1 hÀ1 atmÀ1 in a
highly selective fashion. Although complex 1 could not be used
as single-component catalyst for the ethylene polymerization, the
present work provide a directly characteristic comparison between
NHC ligands and phosphine ligands, and provide an useful hypoth-
esis for guiding future nickel-based catalyst construction, i.e. tun-
ing the degree of olefin polymerization and so on.
Acknowledgements
We are grateful to the Department of Education of Jiangsu Prov-
ince, the National Natural Science Foundation of China (Grants
20632040 and 20772089) and Key Laboratory of Organic Chemis-
try, Jiangsu Province, for the financial support.

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Organometallics 24 (2005) 5925.
CCDC 665468 and 665469 contain the supplementary crystallo-
graphic data for complexes 1 and 2. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
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