Mono- and di-nuclear nickel(II) complexes bearing 3-aryliminomethyl-2- hydroxybenzaldehydes: Synthesis, structures and vinyl polymerization of norbornene

Article abstract of DOI:10.1016/j.jorganchem.2012.06.024

A series of mono- and di-nuclear nickel(II) complexes bearing 3-aryliminomethyl-2-hydroxybenzaldehyde ligands were synthesized and characterized by FT-IR and elemental analysis along with single-crystal X-ray diffraction. The X-ray diffraction analysis re

Full text of DOI:10.1016/j.jorganchem.2012.06.024

Journal of Organometallic Chemistry 716 (2012) 222e229
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Mono- and di-nuclear nickel(II) complexes bearing 3-aryliminomethyl-2-
hydroxybenzaldehydes: Synthesis, structures and vinyl polymerization of
norbornene
1
*
Yintian Guo, Pengfei Ai, Lingqin Han , Lin Chen, Bo-Geng Li, Suyun Jie
State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of mono- and di-nuclear nickel(II) complexes bearing 3-aryliminomethyl-2-hydroxybenzaldehyde
ligands were synthesized and characterized by FT-IR and elemental analysis along with single-crystal X-ray
diffraction. The X-ray diffraction analysis revealed that the dinuclear nickel complexes formed with two
bridged ligands and one bromide around each nickel center and the mononuclear nickel complexes bearing
two centrosymmetrical ligands exhibited a distorted square-planar geometry. Upon activation with meth-
ylaluminoxane (MAO), all these nickel complexes displayed high catalytic activities up to 9.88 ꢀ 107 g(PNB)
molꢁ1(Ni) hꢁ1 (6b) for the vinyl polymerization of norbornene, yielding the vinyl-type PNBs with high
molecular weights and narrow molecular weight distributions. The steric and electronic factors of ligands
and reaction parameters had influences on the catalytic properties.
Received 22 May 2012
Received in revised form
22 June 2012
Accepted 27 June 2012
Keywords:
Nickel
Norbornene
Vinyl polymerization
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
nickel complexes [32e37] were all found to be highly active for the
vinyl polymerization of norbornene, affording the vinyl-type PNBs
Bicyclo[2.2.1]hept-2-ene, better known by its trivial name nor-
bornene (NB) and its derivatives can be polymerized in three
different pathways, including ring-opening metathesis polymeri-
zation (ROMP) [1e4], cationic or radical polymerization [5e8], and
vinyl or addition polymerization [9e13]. Polynorbornenes (PNBs),
via vinyl polymerization, with two constrained rings in each unit,
have been widely used as high-performance materials in the
microelectronics industry because of their unique properties, such
as good UV resistance, low dielectric constant, excellent trans-
parency, low water uptake and good heat resistivity [14e18].
Vinyl polymerization of NB was first described in the early 1960s
by using TiCl4-based Ziegler catalyst [19]. The first example of
nickel complexes for the vinyl polymerization of norbornene was
reported in 1993 [20], and in the recent years, nickel complexes,
especially with bidentate [NO] ligands [13,21], have drawn much
attention since the discovery of neutral salicylaldiminato nickel
complexes [22] used as active catalysts for ethylene polymeriza-
tion. Among all these nickel complexes containing [NO] ligands,
neutral salicylaldiminato nickel complexes [23e27], bis(salicy-
with high molecular weights. However, the examples of nickel
complexes bearing tridentate or multidentate ligands used in the
vinyl polymerization of norbornene were much less and their
catalytic activities were also relatively lower than those of nickel
complexes with bidentate ligands. Nickel complexes bearing tri-
dentate ligands, such as phenoxy-imino ligands with an hydroxyl
or alkoxyl group sidearm ([ONO]) [38,39], phenoxyl-imino (quin-
oline) or (imino)pyridyl alcohol ligands ([NNO]) [40,41], bis(imino)
pyridine ligands ([NNN]) [42e45], pyrrole-imine ligand with ortho-
PPh2 group ([PNN]) [46] and anilidoeimino ligand with a sidearm
group containing an S donor atom ([NNS]) [47], have been reported
to be used as precatalysts for the vinyl polymerization of norbor-
nene. Very recently, nickel complexes with multidentate ligands,
mainly tetradentate [NNNN] ligands [48e51], were reported to
afford moderate to high catalytic activities for the vinyl polymeri-
zation of norbornene upon activation with MAO or MMAO.
The majority of above nickel complexes bearing bi- or tri-
dentate ligands were mononuclear, but there were only few
samples of dinuclear complexes used in the polymerization of
norbornene. Dinuclear nickel complexes bearing two bidentate
[NN] ligands displayed the catalytic activity in the range of
105e106 g(PNB) molꢁ1(Ni) hꢁ1 [52e54]; whereas the catalytic
activity of reaching 108 g(PNB) molꢁ1(Ni) hꢁ1 was obtained by
a dinuclear nickel complex with two 2-methyl-8-(diphenylphos-
phino)quinoline ligands ([PN]) [55]. In addition, dinuclear nickel
laldiminato) nickel complexes [28e31], and bis(b-ketoiminato)
* Corresponding author. Tel.: þ86 571 87951515; fax: þ86 571 87951612.
E-mail address: jiesy@zju.edu.cn (S. Jie).
1
Present address: 200 Swanton St, Apt 205, Winchester, MA 01890, USA.
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.06.024

Y. Guo et al. / Journal of Organometallic Chemistry 716 (2012) 222e229
223
complexes bearing bis(pyridyl-imine) [56] or bis(salicylaldiminato)
[57] ligands, activated with MAO or MMAO, generated PNBs with
moderate or very high activity.
condensation reaction of 4-substituted 2,6-diformylphenols with
one equivalent of appropriate aniline in refluxing ethanol (Scheme
1), according to our previous procedure and further determined by
FT-IR and 1H NMR spectra [58,60].
In order to explore the effective late-transition metal catalysts for
the polymerization of olefins or conjugated diolefins, tridentate
ligands and dinuclear complexes are of great interest for us. We have
found that dinuclear cobalt complexes bearing 3-aryliminomethyl-
2-hydroxybenzaldehyde ligands ([NOO]) were highly active and
selective for the polymerization of 1,3-butadiene with ethyl-
aluminum sesquichloride (EASC) as cocatlyst [58] and now the
corresponding mono- and di-nuclear nickel(II) complexes were
synthesized and used as highly active catalysts for the vinyl poly-
merization of norbornene. Herein we will report the synthesis and
structures of mono- and di-nuclear nickel(II) complexes bearing 3-
aryliminomethyl-2-hydroxybenzaldehyde ligands and their cata-
lytic properties for the vinyl polymerization of norbornene.
The dinuclear nickel complexes 1ae6a were readily prepared by
the reaction of the corresponding ligand with one equivalent of
NiBr2 in absolute ethanol (Scheme 1). All the resulting dinuclear
complexes were isolated as yellow or orange powders in good
yields and high purities except for complex 5a. The structures of
these complexes were determined by FT-IR spectra and elemental
analysis. All these nickel complexes showed a dinuclear cen-
trosymmetrical structure with two bridged phenolate ligands and
two bromides, as was established by single-crystal X-ray diffrac-
tion. In the IR spectra, the stretching vibration bands of C]O and
C]N double bonds of these dinuclear nickel complexes
(1644e1653 and 1623e1632 cmꢁ1, respectively) shifted to lower
wave numbers and the peak intensities greatly reduced, as
compared to the corresponding free ligands (1672e1686 cmꢁ1 and
1624e1630 cmꢁ1, respectively), indicating the coordination inter-
action between both the carbonyl-O and imino-N atoms and the
metal centers. The elemental analysis results revealed that the
components of these complexes were in accordance with the
2. Results and discussion
2.1. Synthesis and characterization
The starting materials, 2,6-diformylphenols, were prepared
from the corresponding 4-substituted phenols via the Dull reac-
tion [59]. The titled 3-aryliminomethyl-2-hydroxybenzaldehyde
ligands (L1HeL6H) were synthesized through the Schiff-base
formula of (m-L)2Ni2Br2. The molecular structure of complex 1a was
further confirmed by the single-crystal X-ray diffraction analysis
(Fig. 1).
Scheme 1. Synthesis of mono- and di-nuclear nickel complexes.

224
Y. Guo et al. / Journal of Organometallic Chemistry 716 (2012) 222e229
Fig. 2. Molecular structure of complex 2b. All the hydrogen atoms have been omitted
for clarity.
Fig. 1. Molecular structure of complex 1a showing one of two independent molecules
in the asymmetric unit. All the hydrogen atoms have been omitted for clarity.
The coordination geometry at the nickel center can be best
described as distorted square pyramidal, with Br1 occupying the
apical position. The four basal atoms (N1, O1, O1A, O2A) are co-
planar to within 0.0039 Å, with the nickel atom lying 0.3742 Å
out of this plane. All the bond angles in the basal plane are close to
a right angle (O1eNi1eO1A, 80.2(2)ꢂ; O1eNi1eN1, 88.8(2)ꢂ;
O1AeNi1eO2A, 87.8(2)ꢂ; O2AeNi1eN1, 95.2(2)ꢂ). The NieBr bond
(2.4364(16) Å) is noticeably longer than those of basal counterparts
showing slight difference from 2.004(5) Å to 2.017(5) Å; whereas it
is shorter than the distance between Ni and bridged Br atoms in
tetranickel complexes [61]. There is no apparent delocalization of
the imino and carbonyl double bonds into the phenyl ring system,
with the C9eN1 and C1eO2 bond lengths being 1.284(7) and
1.256(8) Å, respectively and both of them are slightly longer than
The reaction of sodium phenolates, which were derived from
the reaction of ligands and NaH in THF, with half equiv. of NiBr2 or
NiCl2$6H2O afforded the corresponding mononuclear nickel
complexes as green powders (Scheme 1). The NiCl2$6H2O salt gave
much higher yields than NiBr2, which could be probably ascribed
that the insolubility of NiBr2 in the solvents resulted in the slow
reaction rate. The synthesized mononuclear nickel complexes were
also characterized by FT-IR and elemental analysis. Comparing the
IR spectra of mononuclear nickel complexes with free ligands, the
variation of C]N double bonds was similar to the corresponding
dinuclear complexes; however, the stretching vibration bands of
carbonyl groups kept very strong and the wave numbers didn’t
change greatly (1660e1677 cmꢁ1), which demonstrated that the
ligands in the mononuclear complexes acted as bidentate [NO]
ligands with the non-coordinated carbonyl groups, similar to the
neutral arylnickel(II) phosphine complexes [60]. The molecular
structure of complex 2b was further confirmed by the single-crystal
X-ray diffraction analysis (Fig. 2).
those in the corresponding dinuclear cobalt complex (m-L)2Co2(m-
OAc)2 [58]. The basal plane is nearly coplanar to phenoxy plane
with the dihedral of 17.4ꢂ and perpendicular to 2,6-
diisopropylphenyl ring with the dihedral angle of 87.3ꢂ. The intra-
molecular Ni1/Ni1A distance of 3.071 Å is slightly shorter than
those reported for other nitrogen-containing nickel(II) dimeric
complexes [62,63], but it’s not short enough to form a metalemetal
bond.
2.2. Crystal structures
Green crystals of 2b suitable for X-ray diffraction analysis were
obtained by slow diffusion of n-pentane into its dichloromethane
solution. As shown in Fig. 2, complex 2b shows a centrosymmetric
structure and contains two ligands around the nickel center with
Yellow crystals of 1a suitable for X-ray structure determination
were grown from a layered dichloromethane/n-hexane (1:2) solu-
tion. In the structure of 1a, halves of two independent molecules
exist in the unit cell, which give slightly different bond lengths and
bond angles (Table 1); only one of them is shown in Fig. 1 and
described in detail as below. Complex 1a displays a dinuclear
centrosymmetrical structure, in which each nickel metal center is
coordinated to [N,O] atoms of one ligand, [O,O] atoms of the other
ligand, and one terminal bromide. The phenoxo atoms of two
ligands bridge the two nickel atoms in such a way that a planar
a
slightly distorted square-planar coordination geometry, as
observed for some related nickel complexes [28e31,64,65]. In the
structure, the nickel center is coordinated to [N,O] atoms from two
ligands, forming two fused six-membered rings with the free
carbonyl groups away from the nickel center. The distance of NieO1
(1.8391(16) Å) is shorter than that of NieN1 (1.9125(19) Å), because
nickel has strong oxophilicity. The imine C9eN1 bond length is
1.289(3) Å with the typical character of a C]N double bond. The
C1eO2 bond lengths (1.208(3) Å) of the free carbonyl groups are
obviously shorter than the coordinated ones (1.256(8) Å) in 1a. All
the bond angles around the nickel center are closely right angles:
86.20(8)ꢂ for O1eNieN1 and 93.80(8)ꢂ for O1eNieN1A.
Ni2(m-O)2 core is formed and the two bromide atoms centrosym-
metrically locate in either side of this plane, respectively. The
dihedral angles between Ni2(m-O)2 plane and phenoxy plane, N-
aryl ring are 23.9ꢂ and 83.9ꢂ, respectively, while the dihedral angle
between the phenoxy plane and N-aryl ring is 71.5ꢂ.

Y. Guo et al. / Journal of Organometallic Chemistry 716 (2012) 222e229
225
Table 1
solutions increased from 20 mL to 30 or 40 mL, the conversion of
norbornene and catalytic activity decreased dramatically along
with the decrease of molecular weight (entries 3e5 in Table 2).
Therefore, all other polymerization reactions were carried out with
Selected bond lengths (Å) and bond angles (ꢂ) of complexes 1a and 2b.
1a
2b
Bond lengths
Ni1eO1
Ni1eO1A
Ni1eO2A
Ni1eN1
Ni1eBr1
C1eO2
C9eN1
Ni1/Ni1A
Bond angles
O1eNi1eO1A
O1eNi1eO2A
O1eNi1eN1
O1AeNi1eO2A
O1AeNi1eN1
O2AeNi1eN1
Br1eNi1eO1
Br1eNi1eO1A
Br1eNi1eO2A
Br1eNi1eN1
2.004(5)
2.012(4)
2.017(5)
2.009(6)
2.4364(16)
1.256(8)
1.284(7)
3.071
NieO1
NieN1
C9eN1
C1eO2
1.8391(16)
1.9125(19)
1.289(3)
5
mmol of Ni and NB/Ni molar ratio of 10,000 in 20 mL of toluene.
The polymerization behaviors of complex 1a/MAO catalytic
system were influenced strongly by the Al/Ni molar ratio and reac-
tion temperature. With the increase of Al/Ni molar ratio from 250 to
1500, both the conversion of norbornene and the catalytic activity
initially increased and then decreased (entries 3 and 6e10 in Table 2
and Fig. 3). The highest catalytic activity up to 1.55 ꢀ 107 g(PNB)
molꢁ1(Ni) hꢁ1 was obtained under the Al/Ni molar ratio of 1000.
However, it was observed that the molecular weight of the resulting
polymers increased graduallyand the molecular weight distribution
(1.91e2.20) was slightly affected by the variation of Al/Ni molar
ratio. When the polymerization reactions were carried out over the
temperature range from 0 to 100 ꢂC under the Al/Ni molar ratio of
1000, the conversion of norbornene and catalytic activity decreased
remarkably from 64.4% (1.82 ꢀ 107 g(PNB) molꢁ1(Ni) hꢁ1) to 3.1%
(0.88 ꢀ 106 g(PNB) molꢁ1(Ni) hꢁ1), respectively (entries 3 and 11e14
inTable 2 and Fig. 4). Although the molecular weight of the resultant
PNBs also decreased as the polymerization temperature was
elevated, their molecular weight distributions kept narrow in the
range of 1.74e2.20.
1.208(3)
80.2(2)
157.3(2)
88.8(2)
O1eNieN1
O1eNieN1A
O1eNieO1A
N1eNieN1A
86.20(8)
93.80(8)
180.00(14)
180.00(16)
87.8(2)
157.1(2)
95.2(2)
100.39(14)
95.07(14)
106.10(18)
100.54(17)
2.3. Norbornene polymerization
The dinuclear nickel complexes (1ae6a) with different ligand
environments were applied in the polymerization in order to
examine the steric and electronic effects on catalytic activity and
properties of resulting polymers (Table 3). For some cases, the
polymerization solution resulted in a white solid mass that was
difficult to be stirred in very short time, therefore only the catalytic
activity was concerned for comparison in this part. When
comparing the complexes bearing the same substituents on the
imino-N aryl ring, complexes 5a and 6a with the electron-
withdrawing Cl group at the 4-position of phenol (R ¼ Cl)
showed higher catalytic activities than the corresponding
complexes 1a, 2a (R ¼ Me) and 3a, 4a (R ¼ t-Bu) with the electron-
donating methyl and tert-butyl groups, respectively; and the
molecular weight of PNBs had similar trend (orders of activities and
molecular weights: 5a > 1a, 3a; 6a > 2a, 4a). Concerning the
complexes bearing the bulkier isopropyl groups on the imino-N
aryl ring, 1a with methyl group gained much higher activity than
3a with tert-butyl group at 4-position of phenol; whereas for the
complexes bearing the methyl groups on the imino-N aryl ring, 4a
with tert-butyl group was slightly more active than 2a with methyl
group at 4-position of phenol. As for the complexes with the same R
group at the 4-position of phenol, the Me-substituted complex 1a
with isopropyl groups displayed slightly higher catalytic activity
than 2a with methyl groups on the imino-N aryl ring; nevertheless,
the t-Bu- and Cl-substituted complexes 3a and 5a with isopropyl
groups were less active than the corresponding 4a and 6a with
methyl groups on the imino-N aryl ring, respectively. To sum up,
among all the dinuclear nickel complexes, 3a with the bulkier
isopropyl groups on the imino-N aryl ring and tert-butyl group at
the 4-position of phenol showed lowest catalytic activity (entries 3
and 7 in Table 3); however, 6a with methyl groups on the imino-N
aryl ring and electron-withdrawing Cl group at the 4-position of
phenol was most active for norbornene polymerization
(6.46 ꢀ 107 g(PNB) molꢁ1(Ni) hꢁ1, entry 10 in Table 3). Whether for
the electron-donating or electron-withdrawing groups at the 4-
position of phenol, the isopropyl groups on the imino-N aryl ring
led to higher molecular weights of resulting PNBs than methyl
groups (orders of molecular weights: 1a > 2a; 3a > 4a; 5a > 6a).
The PNBs produced by all the dinuclear nickel complexes had
relatively narrow molecular weight distributions in the range of
1.71e2.20.
All the synthesized mono- and di-nuclear nickel complexes
were evaluated for the vinyl polymerization of norbornene. In order
to investigate the influences of reaction parameters on the catalytic
properties, dinuclear nickel complex 1a was typically studied as
a model precatalyst by varying various reaction conditions and the
detailed results were summarized in Table 2. Since trialkyl
aluminum could not induce the formation of the active center [41],
methylaluminoxane (MAO) was chosen to be cocatalyst. Upon
activation with MAO, the effect of monomer concentration was
firstly investigated. With the fixed amount of nickel precursor
(5 mmol of Ni) and the same volume of solutions, the increase in the
norbornene concentration led to an increase in the conversion of
norbornene and catalytic activity during the same polymerization
time (entries 1e3 in Table 2). The highest conversion of norbornene
(54.7%) and catalytic activity (1.55 ꢀ 107 g(PNB) molꢁ1(Ni) hꢁ1
)
were obtained with the NB/Ni molar ratio of 10,000. In addition, the
higher monomer concentration resulted in the formation of
PNBs with higher molecular weights in the range of 3.78e6.77
ꢀ 105 g/mol although their molecular weight distributions had
slight difference (2.12e2.20). When the total volume of reaction
Table 2
Polymerization of norbornene with complex 1a/MAOa.
c
c
Entry Al/Ni NB/Ni
T
V
Conv. Activityb Mw
Mw/Mn
(ꢂC) (mL) (%)
(105 g/mol)
1
2
3
4
5
6
7
8
1000
1000
5000
7500
25 20
25 20
25 20
25 30
25 40
25 20
25 20
25 20
25 20
25 20
28.3
4.00
8.97
15.45
7.91
3.78
5.91
6.77
5.64
4.25
5.45
6.32
6.75
6.86
7.05
7.19
5.74
5.53
2.89
2.17
2.12
2.20
2.02
1.90
2.14
1.97
1.91
2.00
2.18
1.74
1.86
2.09
1.95
42.3
54.7
28.0
17.9
37.2
40.0
47.4
48.4
31.1
64.4
34.6
19.6
3.1
1000 10,000
1000 10,000
1000 10,000
250 10,000
500 10,000
750 10,000
1250 10,000
1500 10,000
1000 10,000
1000 10,000
1000 10,000
5.07
10.51
11.29
13.38
13.67
8.80
18.18
9.76
5.53
9
10
11
12
13
14
0
20
50 20
75 20
1000 10,000 100 20
0.88
a
General conditions: 5
mmol of Ni; cocatalyst: MAO; solvent: toluene; reaction
time: 2 min.
b
In units of 106 g(PNB) molꢁ1(Ni) hꢁ1
Determined by GPC.
.
c

226
Y. Guo et al. / Journal of Organometallic Chemistry 716 (2012) 222e229
16
14
12
10
8
7.2
6.8
6.4
6.0
5.6
Table 3
Polymerization of norbornene with complexes 1ae6a, 1b, 2b, 4b, 6b/MAOa.
Activityb
Mw (105 g/mol)
Mw/Mn
c
c
Entry
Cat.
t (min)
Conv. (%)
1
2
3
4
5
6
7
8
1a
2a
3a
4a
1a
2a
3a
4a
5a
6a
1b
2b
4b
6b
2
2
2
2
1
1
1
1
0.25
0.25
2
0.75
2
54.7
50.6
27.6
54.1
40.8
36.5
19.6
43.4
22.6
28.6
43.9
62.9
54.0
43.7
1.55
1.43
0.78
1.53
2.30
2.06
1.11
2.45
5.11
6.46
1.24
4.73
1.53
9.88
6.77
5.55
6.40
4.51
5.87
5.69
5.66
5.34
6.37
5.88
5.84
5.72
6.68
5.06
2.20
1.92
1.76
2.08
1.84
1.80
1.78
1.95
1.71
1.71
1.82
2.31
2.00
2.15
9
10
11
12
13
14
0.25
250
500
750
1000
1250
1500
a
Al/Ni molar ratio
General conditions: 5 mmol of Ni; NB: 50 mmol; cocatalyst: MAO; Al/Ni: 1000;
solvent: toluene; total volume: 20 mL; reaction temperature: 25 ꢂC.
b
In units of 107 g(PNB) molꢁ1(Ni) hꢁ1
Determined by GPC.
.
Fig. 3. The plots of Al/Ni molar ratio vs catalytic activity (-) and molecular weight (:)
of PNBs obtained by complex 1a/MAO systems (entries 3 and 6e10 in Table 2).
c
In order to compare the difference between mono- and di-
nuclear nickel complexes, the synthesized mononuclear
complexes (1b, 2b, 4b and 6b) were also employed in the poly-
merization of norbornene. Under the similar polymerization
conditions, mononuclear complex 1b obtained relatively lower
catalytic activity than dinuclear complex 1a bearing the same
ligand; 4b and 4a showed comparable activities; however, 2b and
6b were much more active than the corresponding dinuclear nickel
complexes 2a and 6a, respectively. This indicated that the different
active centers formed during the polymerization despite of the
same ligands on the mono- and di-nuclear nickel complexes. For
example, the conversion of norbornene with complex 6b/MAO
system reached 43.7% in 15 s and the catalytic activity was up to
9.88 ꢀ 107 g(PNB) molꢁ1(Ni) hꢁ1 (entry 14 in Table 3). For the
mononuclear complexes with the same group at the 4-positon of
phenol, the bulkier isopropyl groups (1b) resulted in lower catalytic
activity and relatively higher molecule weight of PNBs than methyl
groups (2b), which was different from the dinuclear complexes.
When the complexes bearing the same methyl groups on the
imino-N aryl ring were compared, complex 6b with the electron-
withdrawing Cl group at the 4-position of phenol (R ¼ Cl) exhibi-
ted higher activity than the corresponding complexes 2b (R ¼ Me)
and 4b (R ¼ t-Bu) with the electron-donating methyl and tert-butyl
groups, respectively; furthermore, the bulkier tert-butyl group led
100
80
60
40
20
0
0
-12
-24
-36
-48
0
200
400
600
800
Temperature (oC)
Fig. 5. TG/DTG curves of PNB obtained by complex 1a/MAO system (entry 3 in Table 2).
to lower activity than methyl group (that is, activity order:
6b > 2b > 4b). The molecular weights and molecular weight
distributions of PNBs obtained by the mono- and di-nuclear nickel
complexes didn’t have a large difference.
The FT-IR spectra proved no existence of double bonds appear-
ing at 1620e1680 cmꢁ1, which confirmed the occurrence of vinyl-
type polymerization rather than ring-opening metathesis poly-
merization (ROMP) [66]. The PNB samples were thermally stable up
to 450 ꢂC under an atmosphere of nitrogen and the TG/DTG curves
of PNB obtained by complex 1a/MAO system was shown in Fig. 5 as
a representative example. The attempts to determine the glass
transition temperature (Tg) of PNBs were unsuccessful, and the DSC
curves did not show an endothermic signal upon heating the
samples up to the decomposition temperature. Therefore, it is very
different to determine the Tg of vinyl-type PNBs.
7.2
20
6.0
15
4.8
10
3.6
3. Conclusions
5
In summary,
a series of mono- and di-nuclear nickel(II)
2.4
complexes bearing 3-aryliminomethyl-2-hydroxybenzaldehyde
ligands have been synthesized and characterized. The X-ray
diffraction analysis revealed that in the dinuclear complex (1a),
each nickel atom is surrounded by two bridged ligands and one
bromide to form a distorted square pyramidal geometry around
each nickel center and a distorted square-planar geometry fromed
0
0
25
50
75
100
o
Temperature ( C)
Fig. 4. The plots of reaction temperature vs catalytic activity (C) and molecular weight
(<) of PNBs obtained by complex 1a/MAO systems (entries 3 and 11e14 in Table 2).

Y. Guo et al. / Journal of Organometallic Chemistry 716 (2012) 222e229
227
in the mononuclear complex (2b) bearing two centrosymmetrical
ligands. Upon activation with methylaluminoxane (MAO), these
mono- and di-nuclear nickel complexes showed high catalytic
activities for the vinyl polymerization of norbornene. Among all
these nickel complexes, the highest catalytic activities of up to
6.46 ꢀ 107 g(PNB) molꢁ1(Ni) hꢁ1 and 9.88 ꢀ 107 g(PNB) molꢁ1(Ni)
1501, 1463, 1409, 1361, 1243, 1219, 1039, 1018, 849, 800, 732. Anal.
Calcd. For C48H60Br2N2Ni2O4 (1006.2): C, 57.30; H, 6.01; N, 2.78.
Found: C, 57.18; H, 6.18; N, 2.64.
Complex 4a. Obtained as a light orange powder in 67.7% yield.
FT-IR (KBr disk, cmꢁ1): 3405, 2964, 2920, 1652 (
nC]O), 1625 (nC]N),
1530, 1406, 1357, 1326, 1288, 1240, 1218, 1040, 1018, 850, 769, 731,
586. Anal. Calcd. For C40H42Br2Cl2N2Ni2O4 (922.04): C, 54.71; H,
5.25; N, 3.04. Found: C, 54.53; H, 5.12; N, 3.21.
h
ꢁ1 have been achieved by dinuclear complex 6a and mononuclear
complex 6b, respectively. With the increase of Al/Ni molar ratio, the
molecular weight of PNBs increased gradually and the highest
catalytic activity was obtained under the Al/Ni molar ratio of 1000.
The catalytic activity and molecular weight of PNBs decreased
remarkably as the polymerization temperature was elevated. The
narrow molecular weight distributions (in the range of 1.71e2.31)
indicated the presence of single active species in the polymeriza-
tion process whether for the mononuclear or dinuclear nickel
complexes.
Complex 5a. Obtained as an orange powder in 39.2% yield. FT-IR
(KBr disk, cmꢁ1): 3389, 2960, 2923, 2856, 1653 (
n
C]O), 1628
(nC]N), 1582, 1536, 1459, 1403, 1348, 1255, 1220, 1174, 1102, 1030,
992, 783, 722. Anal. Calcd. For C40H42Br2Cl2N2Ni2O4 (962.87): C,
49.90; H, 4.40; N, 2.91. Found: C, 49.85; H, 4.42; N, 2.80.
Complex 6a. Obtained as a yellow powder in 92.2% yield. FT-IR
(KBr disk, cmꢁ1): 3399, 2919, 2848, 1650 (
nC]O), 1632 (nC]N), 1529,
1482, 1329, 1258, 1220, 1029, 859, 777, 717. Anal. Calcd. For
C34H30Br2Cl2N2Ni2O4 (878.71): C, 46.47; H, 3.44; N, 3.19. Found: C,
46.31; H, 3.57; N, 3.35.
4. Experimental
4.1. General considerations
4.2.2. Synthesis of mononuclear nickel complexes
A solution of the ligand (0.5 mmol) in THF (10 mL) was added
into sodium hydride (0.5 mmol) in a Schlenk tube. After the reac-
tion mixture was stirred at room temperature for 1 h, all the
volatiles were removed under reduced pressure. And then a solu-
tion of NiCl2$6H2O (0.25 mmol) in absolute ethanol (20 mL) was
added to the residue. The reaction mixture was stirred at room
temperature overnight and the solvent was removed. The residue
was dissolved in dichloromethane and filtered through a pad of
Celite eluted with dichloromethane. The resulting solution was
concentrated and the resulting product was washed with diethyl
ether and dried in vacuo to give the desired product.
All manipulations of air- or moisture-sensitive compounds were
carried out under an atmosphere of nitrogen using standard
Schlenk techniques. FT-IR spectra were recorded on
kineElmer FT-IR 2000 spectrometer by using KBr disks in the
range of 4000e400 cmꢁ1 1H NMR spectra were recorded on
a Per-
.
a Bruker DMX-400 instrument with TMS as the internal standard.
Elemental analysis was performed on a Flash EA1112 micro-
analyzer. The molecular weight and molecular-weight distribution
of PNBs were measured at 150 ꢂC in 1,2,4-trichlorobenzene using
a PL-GPC220 coupled with an in-line capillary viscometer. TGA data
were collected with a PerkineElmer Pyris 1 TGA instrument under
a nitrogen atmosphere up to 800 ꢂC at heating rate of 10 ꢂC/min.
Toluene and THF were refluxed over sodium-benzophenone and
distilled under nitrogen prior to use. Methylaluminoxane (MAO,
1.46 M in toluene) was purchased from Akzo Nobel Corp. All the
anilines, sodium hydride and NiBr2 were obtained from Acros
Chemicals and directly used without further purification. Norbor-
nene (from Alfa Aesar) was purified by distillation over sodium and
used as a solution in toluene. All other chemicals were obtained
commercially and used without further purification unless other-
wise stated.
Complex 1b. Obtained as a green powder in 79.2% yield. FT-(KBr
disk, cmꢁ1): 3451, 2963, 2924, 2868, 1673 (
nC]O), 1621 (nC]N),
1594, 1545, 1451, 1381, 1341, 1305, 1233, 1176, 1103, 1043, 974, 853,
756. Anal. Calcd. for C42H48N2NiO4 (703.53): C, 71.70; H, 6.88; N,
3.98. Found: C, 72.06; H, 6.80; N, 3.85.
Complex 2b. Obtained as a green powder in 60.4% yield. FT-IR
(KBr disk, cmꢁ1): 3404, 2973, 2916, 2854, 1660 (
n
C]O), 1621
(nC]N), 1593, 1545, 1480, 1449, 1382, 1344, 1307, 1239, 1197, 1142,
1045, 974, 854. Anal. Calcd. for C36H36N2NiO4 (619.38): C, 69.81; H,
5.86; N, 4.52. Found: C, 70.18; H, 5.92; N, 4.45.
Complex 4b. Obtained as a green powder in 78.0% yield. FT-IR
(KBr disk, cmꢁ1): 3452, 2975, 2916, 1663 (
nC]O), 1616 (nC]N),
4.2. Synthesis of nickel complexes
1541, 1479, 1440, 1394, 1338, 1302, 1222, 1194, 1141, 1020, 848, 708.
Anal. Calcd. for C42H48N2NiO4 (703.53): C, 71.70; H, 6.88; N, 3.98.
Found: C, 71.50; H, 7.12; N, 3.91.
4.2.1. Synthesis of dinuclear nickel complexes
To a stirred solution of the ligand (0.5 mmol) in 20 mL of
absolute ethanol at room temperature, solid NiBr2 (0.5 mmol) was
added. The reaction mixture was stirred at 50e60 ꢂC for 12 h.
Thereafter the solution was concentrated to ca. 2 mL and diethyl
ether was added. The resulting precipitate was filtered, washed
with diethyl ether and dried in vacuo to afford the desired product.
All the complexes were prepared in good yields in this manner.
Complex 1a. Obtained as a light yellowish orange powder in
Complex 6b. Obtained as a yellowish green powder in 44.8%
yield. FT-IR (KBr disk, cmꢁ1): 3423, 2961, 2915, 2866, 1677 (
n
C]O),
1618 (nC]N), 1541, 1509, 1479, 1453, 1396, 1362, 1313, 1247, 1231,
1196, 1144, 1039, 987, 856, 725. Anal. Calcd. For C34H30Cl2N2NiO4
(660.21): C, 61.85; H, 4.58; N, 4.24. Found: C, 61.79; H, 4.58; N, 4.14.
4.3. Procedure for norbornene polymerization
98.8% yield. FT-IR (KBr disk, cmꢁ1): 3404, 2964, 1650 (
C]N), 1534, 1464, 1333, 1285, 1227, 1058, 996, 801, 719. Anal.
n
C]O), 1623
Polymerizations were performed in a 100 mL glass reactor
equipped with a seal septum and a magnetic stirrer. The reactor
(n
Calcd. For C42H48Br2N2Ni2O4 (922.04): C, 54.71; H, 5.25; N, 3.04.
Found: C, 54.85; H, 5.23; N, 3.08.
was charged with the desired amounts of precatalyst (5 mmo1 of
nickel) and cocatalyst (MAO) solutions. The polymerization was
initiated by adding the desired amount of a solution of norbornene
in toluene with a syringe through a rubber septum, and the reaction
temperature was controlled by an ice or oil bath and kept constant
during the polymerization. After the desired period of time, the
polymerization was terminated by adding HCl-acidic ethanol (5%).
The precipitated polynorbornenes were isolated by filtration,
adequately washed with ethanol, and dried under vacuum at 100 ꢂC
Complex 2a. Obtained as a light yellowish organe powder in
90.3% yield. FT-IR (KBr disk, cmꢁ1): 3405, 2920, 1644 (
n
C]O), 1623
(nC]N), 1536, 1481, 1407, 1348, 1233, 1202, 1043, 995. Anal. Calcd.
For C36H36Br2N2Ni2O4 (837.88): C, 51.60; H, 4.33; N, 3.34. Found: C,
51.45; H, 4.10; N, 3.37.
Complex 3a. Obtained as a yellow powder in 66.6% yield. FT-IR
(KBr disk, cmꢁ1): 3400, 2965, 2926, 1652 (
nC]O), 1624 (nC]N), 1531,

228
Y. Guo et al. / Journal of Organometallic Chemistry 716 (2012) 222e229
Table 4
free of charge from the Cambridge Crystallographic Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
Crystallographic data and refinement parameters for complexes 1a and 2b.
Data
1a
2b
Formula
C42H48Br2N2Ni2N2O4
922.06
293(2)
0.71073
Triclinic
C36H36N2NiO4
619.38
293(2)
0.71073
Monoclinic
P2(1)/n
11.3278(4)
11.1542(4)
12.2331(4)
90.00
91.900(3)
90.00
1544.83(9)
2
1.332
References
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
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13.301(4)
15.317(5)
65.616(6)
88.483(6)
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2052.4(11)
2
b (Å)
c (Å)
a
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g
(ꢂ)
(ꢂ)
(ꢂ)
Volume (Å3)
Z
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944
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F(000)
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ꢁ13 ꢃ h ꢃ 13, ꢁ15
ꢃ k ꢃ 8, ꢁ18 ꢃ l ꢃ 17
8254
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3.02e26.99
ꢁ14 ꢃ h ꢃ 11, ꢁ14
ꢃ k ꢃ 11, ꢁ15 ꢃ l ꢃ 13
6817
q
Limiting indices
No. of rflns collected
No. of unique rflns
No. of obsd rflns
(I > 2
Completeness to
Absorption correction
No. of parameters
Goodness-of-fit on F2
Final R indices
[I > 2s(I)]
R indices (all data)
7009
3015
3332
2458
s(I))
q
(%)
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otherwise stated, the total reaction volume of vinyl polymerization
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4.4. X-ray crystallography measurements
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l
¼ 0.71073 Å). Cell parameters were
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Crystallographic data and processing parameters for complexes 1a
and 2b are summarized in Table 4.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (Grant No. 21006085) and the State Key
Laboratory of Chemical Engineering (Grant No. SKL-ChE-11D03).
Appendix A. Supplementary material
CCDC 880595 and 880596 contain the supplementary crystal-
lographic data for complexes 1a and 2b. These data can be obtained

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