Novel highly active binuclear neutral nickel and palladium complexes as precatalysts for norbornene polymerization

Article abstract of DOI:10.1016/j.molcata.2006.03.008

A series of binuclear neutral nickel and palladium complexes [(XC₆H₂CH{double bond, long}NC₆H₃-iPr₂)MRL]₂ 4b-f (X = NO₂, M = Ni, R = Ph, L = PPh₃, 4b; X = H, M = Pd, R = Me, L = PPh₃, 4c; X = H, M = Pd, R = Me, L = Py, 4d; X = NO₂, M = Pd, R = Me, L = PPh₃, 4e; X = NO₂, M = Pd, R = Me, L = Py, 4f) and [(C₁₀H₇CH{double bond, long}NC₆H₃-iPr₂)MRL]₂ 8a-c (M = Ni, R = Ph, L = PPh₃, 8a; M = Pd, R = Me, L = PPh₃, 8b; M = Pd, R = Me, L = Py, 8c) have been synthesized and characterized. The structures of complexes 4e and 8b have also been confirmed by X-ray crystallographic analysis. With modified methylaluminoxane (MMAO) as cocatalysts, these complexes and complex [(C₆H₃CH{double bond, long}NC₆H₃-iPr₂)NiPh(PPh₃)]₂ 4a are capable of catalyzing the addition polymerization of norbornene (NBE) with the high activity up to 2.3 × 10⁸ g PNBE/(mol_M h). The structure of complexes affects considerably catalytic activity towards norbornene polymerization. The polymers obtained with nickel complexes are soluble, while those obtained with palladium complexes are insoluble. Palladium complexes 4c, 4e and 8b bearing PPh₃ ligands exhibit much higher activities than the corresponding complexes 4d, 4f and 8c bearing pyridine ligands under the same conditions.

Full text of DOI:10.1016/j.molcata.2006.03.008

Journal of Molecular Catalysis A: Chemical 253 (2006) 155–164
Novel highly active binuclear neutral nickel and palladium complexes
as precatalysts for norbornene polymerization
Tao Hu a,b, Yan-Guo Lia,b, Yue-Sheng Lia,∗, Ning-Hai Huc
a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China
b Graduate School of Chinese Academy of Sciences, Beijing, China
c State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, China
Received 17 October 2005; received in revised form 28 February 2006; accepted 7 March 2006
Available online 24 April 2006
Abstract
A series of binuclear neutral nickel and palladium complexes [(XC6H2CH NC6H3-iPr2)MRL]2 4b–f (X = NO2, M = Ni, R = Ph, L = PPh3, 4b;
X = H, M = Pd, R = Me, L = PPh3, 4c; X = H, M = Pd, R = Me, L = Py, 4d; X = NO2, M = Pd, R = Me, L = PPh3, 4e; X = NO2, M = Pd, R = Me, L = Py,
4f) and [(C10H7CH NC6H3-iPr2)MRL]2 8a–c (M = Ni, R = Ph, L = PPh3, 8a; M = Pd, R = Me, L = PPh3, 8b; M = Pd, R = Me, L = Py, 8c) have
been synthesized and characterized. The structures of complexes 4e and 8b have also been confirmed by X-ray crystallographic analysis. With
modified methylaluminoxane (MMAO) as cocatalysts, these complexes and complex [(C6H3CH NC6H3-iPr2)NiPh(PPh3)]2 4a are capable of
catalyzing the addition polymerization of norbornene (NBE) with the high activity up to 2.3 × 108 g PNBE/(molM h). The structure of complexes
affects considerably catalytic activity towards norbornene polymerization. The polymers obtained with nickel complexes are soluble, while those
obtained with palladium complexes are insoluble. Palladium complexes 4c, 4e and 8b bearing PPh3 ligands exhibit much higher activities than the
corresponding complexes 4d, 4f and 8c bearing pyridine ligands under the same conditions.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Binuclear nickel complex; Binuclear palladium complex; Norbornene; Addition polymerization
1. Introduction
reported by Novak and co-workers [20], Sen and co-workers
[21] and our group [22].
For the past several years, interest in neutral late transition
metal complexes as olefin polymerization catalysts has been on
the rise because neutral complexes are less electrophilic than
the corresponding cationic ones [1–8]. Early in the research, the
modified SHOP catalysts were developed to convert ethylene to
high molecular weight polymers [9]. Recently, Grubbs and co-
workersfoundthatneutralsalicylaldimine-basednickel(II)com-
plexes with bulky O-ortho substituents are highly active single-
component catalysts towards ethylene polymerization [10–12],
which greatly stimulated the researches in this area. Subse-
quently, some neutral nickel(II) complexes with novel ligands
were developed to catalyze ethylene polymerization [13–19].
In addition, the neutral palladium(II) catalysts have also been
During the past few years, we have found that the neutral
nickel complexes, based on pyrrole-imine and salicyladimine
ligands, exhibit very high activities for vinylic norbornene
polymerization in the presence of modified methylaluminox-
ane (MMAO) [23,24]. Recently, several mononuclear nickel(II)
complexes [25–29] and binuclear nickel and palladium com-
plexes [30–32] were utilized to catalyze norbornene poly-
merization by means of activation with different co-catalysts,
which has greatly deepened this research field. More recently,
we have developed a novel binuclear Ni(II) complex based
on the bis(salicylaldimine) ligand, which displays higher
activity towards ethylene polymerization and produces the
polyethylenes with broader molecular weight distribution than
mononuclear salicylaldimine–Ni(II) complexes [33]. This urges
us to check if there are analogous trends when the binuclear and
mononuclear salicylaldimine-based neutral nickel or palladium
complexes are used to catalyze norbornene polymerization.
∗
Corresponding author. Fax: +86 431 5685653.
E-mail address: ysli@ciac.jl.cn (Y.-S. Li).
1381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.03.008

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T. Hu et al. / Journal of Molecular Catalysis A: Chemical 253 (2006) 155–164
Here, we report the syntheses and characterizations of a series
of binuclear neutral nickel(II) and palladium(II) complexes
4b–f and 8a–c based on bis(salicylaldimine)-type ligands. For
these complexes, each salicylaldimine derivative unit serves as
a bulky group at C-3 position of the other unit. The behav-
ior of these complexes towards norbornene polymerization is
investigated.
pyridine (Py) gave a yellow complex Py2PdMeCl (confirmed by
X-ray analysis and 1H NMR) without the bis(salicylaldimine)-
type ligand, instead of 4d. Therefore, we believe that it is
critical that the sodium salts 3a and 3b should be added to
(COD)PdMeCl and be stirred for a while, followed by the
addition of pyridine or PPh3, in view of the strong binding
strength of pyridine or PPh3 with palladium atom. In addition,
for the syntheses of 4e and 4f, CH2Cl2 was used as solvent in
place of benzene due to the insolubility of sodium salt 3b in
benzene.
2. Results and discussion
2.1. Synthesis and structure of complexes
The molecular structures of complexes 4e and 8b were con-
firmed by single-crystal X-ray structure analyses. Crystals suit-
able for X-ray diffraction analysis were obtained by slow diffu-
sion of pentane into a concentrated solution of 4e in CH2Cl2 and
8b in benzene, respectively. The crystallographic data together
with the collection and refinement parameters are summarized
in Table 1, and their ORTEP diagrams are shown in Figs. 1 and 2.
The selected bond lengths and angles for 4e and 8b are given
in Tables 2 and 3. In the solid state, they adopt geometries
best described as square planar about each palladium center,
having slight distortions from idealized geometry. The devi-
ations of two Pd atoms from the plane of their ligands are
The synthetic routes for binuclear complexes 4b–f and
8a–c are shown in Schemes 1 and 2, respectively. The free
ligands 2a, 2b and 6 were synthesized via the condensa-
tion reaction of excess 2,6-diisopropylaniline and the cor-
responding compounds 1a, 1b and 5, respectively, in good
yields (2a, 73%; 2b, 92%; 6, 99%). The deprotonation of
the ligands 2a, 2b and 6 readily proceeded with excess
sodium hydride in anhydrous THF at room temperature. The
reaction of isolated sodium salts 3a, 3b and 7 with two
equivalent of trans-NiCl(Ph)(PPh3)2, (COD)PdMeCl/PPh3 or
(COD)PdMeCl/pyridine for 14 h afforded corresponding binu-
clear complexes 4b–f and 8a–c, respectively.
˚
˚
˚
approximately 0.016 A (4e-Pd1) and 0.011 A (4e-Pd2), 0.053 A
˚
(8b-Pd1) and 0.025 A (8b-Pd2), respectively. The bulky 2,6-
In the syntheses of palladium complexes, the sequence of
addition of reactants is crucial. Addition of a solution of sodium
salt 3b in benzene to the solution containing (COD)PdMeCl and
diisopropylbenzimine occupies the position trans to the triph-
enylphosphine ligand with a nearly linear P Pd N angle. The
CH3 group attached to the Pd atom lies trans to the O atom
Scheme 1.

T. Hu et al. / Journal of Molecular Catalysis A: Chemical 253 (2006) 155–164
157
Scheme 2.
Table 2
◦
˚
Selected bond distances (A) and angles ( ) for 4e
Pd(1)–C(1)
Pd(1)–O(1)
Pd(1)–N(1)
Pd(1)–P(1)
Pd(2)–C(76)
Pd(2)–O(4)
Pd(2)–N(3)
2.041(4)
2.080(3)
2.088(3)
2.2328(11)
2.046(4)
2.086(3)
P(1)–C(20)
P(1)–C(26)
P(1)–C(14)
N(1)–C(32)
N(1)–C(2)
N(2)–O(2)
N(2)–C(37)
1.823(4)
1.824(5)
1.829(4)
1.291(5)
1.455(5)
1.229(5)
1.443(5)
Table 1
Crystal data and structure refinement for complexes 4e and 8b
4e · CH2Cl2
8b · 4.5C6H6
109N2O2P2Pd2
Empirical formula
fw
Cryst syst
Space group
C
77H78Cl2N4O6P2Pd2 C111H
2.103(3)
1501.07
Monoclicic
P21/n
22.4715(13)
104823(6)
32.8045(19)
7496.0(7)
1777.74
Triclinic
P¯ı
Pd(2)–P(2)
2.2302(11)
176.38(19)
91.18(16)
89.62(12)
85.36(13)
94.16(8)
173.75(11)
172.77(17)
92.85(15)
88.40(12)
85.37(13)
C(1)–Pd(1)–O(1)
C(1)–Pd(1)–N(1)
O(1)–Pd(1)–N(1)
C(1)–Pd(1)–P(1)
O(1)–Pd(1)–P(1)
N(1)–Pd(1)–P(1)
C(76)–Pd(2)–O(4)
C(76)–Pd(2)–N(3)
O(4)–Pd(2)–N(3)
C(76)–Pd(2)–P(2)
O(4)–Pd(2)–P(2)
N(3)–Pd(2)–P(2)
C(20)–P(1)–C(26)
C(20)–P(1)–Pd(1)
C(32)–N(1)–C(2)
C(32)–N(1)–Pd(1)
C(2)–N(1)–Pd(1)
O(2)–N(2)–O(3)
O(2)–N(2)–C(37)
94.10(8)
173.89(10)
107.8(2)
112.80(15)
113.8(3)
122.3(3)
123.3(2)
122.2(4)
119.5(3)
˚
a (A)
14.4910(9)
15.1753(9)
22.5290(14)
4576.2(5)
73.4640(10)
74.6710(10)
83.4240(10)
2
˚
b (A)
˚
c (A)
3
˚
V (A )
α (◦)
90
β (◦)
104.0520(10)
90
4
1.330
0.646
γ (◦)
Z
Dcalc (Mg/m3)
Abs coeff. (mm−1
F(0 0 0)
1.290
0.480
1854
)
3088
Table 3
Crystal size (mm)
θ range (◦)
No. of indep rflns
Abs corr
0.50 × 0.15 × 0.08
1.92 to 26.03
14633 (Rint = 0.0316)
Semi-empirical from
equivalents
0.9477 and 0.7304
Full-matrix
least-squares on F2
R1 = 0.0565,
wR2 = 0.1503
R1 = 0.0673,
wR2 = 0.1600
0.43 × 0.24 × 0.14
1.46 to 26.04
17544 (Rint = 0.0188)
Semi-empirical from
equivalents
0.9367 and 0.8205
Full-matrix
least-squares on F2
R1 = 0.0540,
wR2 = 0.1304
R1 = 0.0613,
wR2 = 0.1348
2.735 and
◦
˚
Selected bond distances (A) and angles ( ) for 8b
Pd(1)–C(24)
Pd(1)–O(1)
Pd(1)–N(1)
Pd(1)–P(1)
Pd(2)–C(66)
Pd(2)–N(2)
Pd(2)–O(2)
C(24)–Pd(1)–O(1)
C(24)–Pd(1)–N(1)
O(1)–Pd(1)–N(1)
C(24)–Pd(1)–P(1)
O(1)–Pd(1)–P(1)
N(1)–Pd(1)–P(1)
C(66)–Pd(2)–N(2)
2.041(4)
2.091(3)
2.094(3)
2.2314(10)
2.032(4)
2.094(3)
Pd(2)–P(2)
P(1)–C(37)
P(1)–C(25)
P(1)–C(31)
O(1)–C(3)
N(1)–C(1)
N(1)–C(12)
C(66)–Pd(2)–O(2)
C(37)–P(1)–C(25)
C(37)–P(1)–Pd(1)
C(3)–O(1)–Pd(1)
C(1)–N(1)–C(12)
C(1)–N(1)–Pd(1)
C(12)–N(1)–Pd(1)
2.2515(10)
1.815(4)
1.825(4)
1.828(4)
1.308(4)
1.285(5)
1.450(4)
Max. and min. transmn
Refinement method
2.094(3)
Final R indices [I > 2σ(I)]
R indices (all data)
174.70(16)
89.86(14)
88.81(11)
86.50(12)
94.76(8)
173.7(2)
108.67(19)
110.09(13)
122.3(2)
114.6(3)
121.6(2)
123.9(2)
Largest diff. peak and hole 3.557 and
−3
−3
˚
˚
−0.924e A
−1.268e A
176.31(9)
89.57(15)
Type of diffractometer
Monochromator
Bruker SMART
Graphite
Bruker SMART
Graphite

158
T. Hu et al. / Journal of Molecular Catalysis A: Chemical 253 (2006) 155–164
Fig. 1. ORTEP view of complex 4e (A CH2Cl2 molecules of crystallization are omitted).
with an O Pd C angle higher than 172.5◦. For 4e, two planes
defined by the atoms of the salicylideneiminato fragment form
an approximately perpendicular angle (101.2◦). Comparatively,
the corresponding angle is only 40.2◦ for 8b.
2.2. Polymerization of norbornene
Preliminary experiments have indicated that these binu-
clear neutral (salicylaldimine)Ni(II) and (salicylaldimine)Pd(II)
Fig. 2. ORTEP view of complex 8b (4.5 benzene solvent molecules of crystallization are omitted).

T. Hu et al. / Journal of Molecular Catalysis A: Chemical 253 (2006) 155–164
159
Table 4
Norbornene polymerization results catalyzed by 4a–f and 8a–c/MMAO
Time (min) Temperature (◦C) Al:metal
(mol ratio)
Polymer (g) Yield (%) 10−7 Activitya 10−6 Mw
PDIb Tg (◦C)c
b
Entry Catalyst Norbornene
feeds (g)
1
2
3
4
5
6
7
8
9
4a
4b
8a
4c
4c
4c
4d
4e
4f
3.31
3.31
3.31
4.14
3.31
3.31
4.14
4.14
4.14
4.14
4.14
5
5
5
2
5
5
2
2
2
2
2
20
20
20
50
20
40
50
50
50
50
50
2000
2000
2000
3000
2000
2000
3000
3000
3000
3000
3000
3.12
3.22
3.05
3.78
1.33
3.04
0.42
3.64
0.47
3.44
0.14
94
97
92
91
40
92
10
88
11
83
7.49
7.73
7.31
1.84
1.60
1.76
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
3.72
3.36
3.51
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
324
314
329
325
330
330
320
319
321
318
325
22.7
3.19
7.29
2.52
21.8
2.82
20.6
0.84
10
11
8b
8c
3.4
All the polymerizations run using 0.25 ␮mol catalyst with Vtotal = 30 ml.
a
In units of g PNBE/(molM h).
Determined by GPC and n.d. means the sample was not determined.
Tg is determined by DSC.
b
c
complexes are not able to catalyze norbornene polymeriza-
tion without cocatalysts. As we all know, MMAO alone is
also inactive for norbornene polymerization [23,24]. Inter-
estingly, in combination with MMAO, these neutral (sal-
icylaldimine)M(II) (M = Ni, Pd) complexes 4a–f and 8a–c
exhibit extremely high catalytic activity for norbornene poly-
merization under moderate conditions. Especially for 4c and
4e, norbornene can be rapidly converted into white polynor-
bornene (PNBE) powder with higher than 88% yield within
2 min. The typical results are summarized in Table 4. The
bis(salicylaldimine)Ni(II)/MMAO systems display higher activ-
ities (7.31–7.73 × 107 g PNBE/(molNi h), Entries 1–3 in Table 4)
than the mononuclear salicylaldimine–Ni-based complexes with
or without substituents on the phenoxy fragment (5.11 × 107 g
PNBE/(molNi h)) [23] under similar conditions. Although there
is a sharp difference in the steric structures of complexes 4a
and 8a, the catalytic activities of the two complexes are very
close. This indicates that the dimension of substituents on the
phenoxy fragment have little influence on the catalytic activ-
ity towards norbornene polymerization, which is different from
ethylene polymerization [10–12].
For palladium complexes, bis(salicylaldimine)-based 4c and
4e show high catalytic activities up to 2.3 × 108 g PE/(molPd h)
(Entries 4 and 8 in Table 4). To well understand the difference
between binuclear neutral palladium complex and the corre-
sponding mononuclear one, we synthesized salicylaldimine-
based palladium complex {␩2-1-[CH N(2,6-iPr2C6H3)]-2-O-
C6H4}-Pd(CH3)PPh3 and evaluated its catalytic behavior under
the same conditions. This mononuclear complex displays much
lower activity (6.6 × 107 g PNBE/(molPd h)) than the corre-
sponding binuclear complexes 4c and 4e. Obviously, bulky
groups in the three-position of the salicylaldiminato fragment
are responsible for the high catalytic activities of the binuclear
palladium complexes. It is noteworthy that 4c displays a slightly
higher activity than 8b (Entries 4 and 10 in Table 4), which seems
toindicatethatverybulkysubstituentsinthethree-positionofthe
salicylaldiminato fragment are not advantageous to norbornene
polymerization. A possible reason for this is that very bulky
steric hindrance restricts the insertion of cyclic norbornene to
Pd center and the growth of polynorbornene chain. Additionally,
the NO2 group on the phenoxy group slightly raises the catalytic
activities of the Ni complexes (Entries 1 and 2 in Table 4), which
is similar to the mononuclear neutral Ni complexes catalyzing
ethylene polymerization [10–12]. However, for the palladium
complexes 4e and 4f (Entries 8 and 9 in Table 4), this effect of
the NO2 group does not appear.
Surprisingly, palladium complexes with pyridine as a neu-
tral ligand exhibit much lower activities (Entries 7, 9 and 11 in
Table 4) than those (Entries 4, 8 and 10 in Table 4) with PPh3 as
a neutral ligand. When the polymerization time was prolonged
from 2 to 20 min, the PNBE yield only slightly increased for
the palladium complexes with the pyridine ligand. As a matter
of fact, the PPh3 ligand has stronger binding strength with Pd
than pyridine. On the basis of the need to dissociate the donor
ligands to free a coordination site for monomer, it is reason-
able to expect that complexes bearing the pyridine ligand have
reasonably higher activity than those bearing the PPh3 ligand.
Therefore, we speculate that this difference is due to the fact
that the bulkly PPh3 dissociates more easily than small pyridine
under the activation with MMAO.
Up to now, the active species in MAO- or B(C6F5)3-activated
nickel or palladium complex for norbornene polymerization
have not been clarified. Interestingly, the “naked” Pd2+ cations
were considered highly active species of the norbornene poly-
merization [34]. However, in our cases, the distinct difference
between binuclear neutral palladium complex and the corre-
sponding mononuclear one in catalytic behavior suggests that
the bulky groups in the three-position of the salicylaldiminato
fragment substantially affect the catalytic activities of the pal-
ladium complexes, which indirectly indicates the [N,O]-chelate
ligands remain bound to the palladium center upon activation.
About the actual active species and the catalytic mechanism, the
relevant research is underway.
To further investigate the norbornene polymerization behav-
ior of these catalyst precursors, polymerizations were also
investigated under varied reaction conditions. Although similar

160
T. Hu et al. / Journal of Molecular Catalysis A: Chemical 253 (2006) 155–164
Fig. 3. Plot of activity and Mv vs. Al/Ni (molar ratio); 20 ◦C, 0.25 ␮mol 4a,
3.31 g of norbornene feeds, Vtotal = 30 ml, 5 min polymerization run.
Fig. 5. Plot of activity and Mv vs. reaction temperature; 0.25 ␮mol 4a, 3.31 g of
norbornene feeds, Al/Ni = 2000, Vtotal = 30 ml, 5 min polymerization run.
As shown in Fig. 5, with the increase in the reaction tem-
perature, both the catalytic activities and Mv of the polymers
decrease apparently. At 10 ◦C, the highest catalytic activity up to
7.68 × 107 g PNBE/(molNi h) and Mv up to 116.8 × 104 g/mol
are obtained. However, for palladium precatalyst 4c, with the
increase in the reaction temperature, a significant increase in the
catalytic activities is observed and the highest catalytic activity
up to 2.38 × 108 g PNBE/(molPd h) appears at 70 ◦C (Fig. 4(b)).
As shown in Fig. 6, the concentration of norbornene also con-
siderably affects the polymerization. For nickel precatalyst 4a,
the catalytic activity and Mv of the PNBEs display a nearly linear
increase with the increase in the concentration of norbornene.
For palladium precatalyst 4c, the catalytic activity also increases
with the increase in the concentration of norbornene; however,
the monomer conversion increases initially and then gradually
decreases (Fig. 7).
investigations were done before [35,36], for a new class of
precatalysts, it is necessary to investigate the effects of reac-
tion conditions on the polymerization behavior. Variation of the
molar ratio of MMAO to precatalyst 4a, which is expressed
here as the Al/Ni ratio, showed effects on the catalytic activ-
ity and polymer molecular weights. As shown in Fig. 3,
the catalytic activity slightly increases from 6.58 × 107 to
7.63 × 107 g PNBE/(molNi h) when the mole ratio of Al/Ni
increases from 500/1 to 2500/1. In contrast, the viscosity-
average molecular weight (Mv) of the polymers decreases
sharply with the increase in the Al/Ni ratio due to the chain
transfer to MMAO. As for palladium precatalyst 4c, the catalytic
activity slowly increases with the increase in the molar ratio of
MMAO to 4c (Al/Pd ratio) and reaches the highest value at Al/Pd
ratio = 2500, and then turns to decrease (Fig. 4(a)).
The reaction temperature also affects the catalytic activity of
precatalyst 4a and molecular weights of the polynorbornenes.
Since the effects of temperature on the polymerization per-
formance show the different trends between the nickel com-
plexes and palladium complexes, the comparative experiments
of their catalytic activities were performed at 20–40 ◦C. As
Fig. 4. Plot of activity vs. Al/Pd (molar ratio) and reaction temperature;
0.25 ␮mol 4c, 4.14 g of norbornene feeds, Vtotal = 30 ml, 2 min polymerization
run, Al/Pd = 2000 for (ꢀ) and 50 ◦C for (ꢀ).
Fig. 6. Plot of activity and Mv vs. concentration of norobornene; 20 ◦C,
0.25 ␮mol 4a, Al/Ni = 2000, Vtotal = 30 ml, 5 min reaction run.

T. Hu et al. / Journal of Molecular Catalysis A: Chemical 253 (2006) 155–164
161
Fig. 8. Solid-state 13C NMR spectra of polynorbornenes produced by precata-
lysts 4c (a) and 4a (b).
Fig. 7. Plot of activity and conversion vs. concentration of norobornene; 50 ◦C,
0.25 ␮mol of 4c, Al/Pd = 2000, Vtotal = 30 ml, 2 min polymerization run.
tion, the PNBEs are exo enchained because no resonances appear
in the 20–24 ppm region, which has been taken as evidence of
endo enchainment on the basis of model studies [38]. Despite
these similarities, the 13C NMR spectra of the PNBEs are dis-
tinctly different in appearance, which indicates that there are
differences in the microstructure of the PNBEs. Therefore, it
is the high stereoregularity that leads to the insolubility of the
PNBEs produced by palladium complexes.
It is noteworthy that the PNBEs obtained by using the
binuclear nickel complexes display a slightly broader molec-
ular weight distribution (PDI > 3.2, Entries 1–3 in Table 4)
than the PNBEs (PDI < 1.6) obtained by using mononuclear
salicylaldimine–Ni(II) complexes [23] under similar conditions.
Presumably, the two different nickel units of complexes 4a, 4b
or 8a in structure create the two different active species, which
directly leads to the broadening of the molecular weight distri-
bution.
shown in Table 4 (Entries 1, 5 and 6), precatalyst 4a has higher
activity than precatalyst 4c (7.49 × 107 g PNBE/(molNi h) ver-
sus 3.19 × 107 g PNBE/(molPd h)) at 20 ◦C and the activ-
ity of precatalyst 4c and precatalyst 4a are compara-
ble at 40 ◦C (7.29 × 107 g PNBE/(molPd h) versus 7.01 ×
107 g PNBE/(molNi h)). Therefore, precatalyst 4a has higher
activity than precatalyst 4c at low temperature. In contrast, the
activity of precatalyst 4c is higher at high temperature.
2.3. Analyses of polynorbornene
Norbornene can be polymerized by three means: ring-
opening metathesis polymerization (ROMP), cationic polymer-
ization and vinyl addition polymerization [37]. ROMP yields
polynorbornene containing unsaturated double bond in the back-
bone. Cationic polymerization typically results in the formation
of low molecular weight polymers with rearranged norbornene
units in the backbone. Vinyl addition polymerization yields a
completely saturated polynorbornene. As shown in Table 4,
these polynorbornenes bear high glass transition temperature
ranging from 316.0 to 325.0 ◦C. FTIR analyses display that no
absorption is observed in the range of 1500–1600 cm−1, which
is characteristic of carbon–carbon double bonds; therefore these
polynorbornenes are vinyl addition products in nature.
All PNBEs produced by employing these nickel com-
plexes are soluble in chlorobenzene, o-dichlorobenzene and
dichloroethylene, which indicates that they have low stereo-
regularities. However, the PNBEs produced by these palla-
dium complexes are insoluble in usual solvents such as cholo-
form, chlorobenzene, dichlorobenzene, DMF, THF and DMSO.
Therefore, the molecular weights of the polynorbornenes cannot
be determined. For the purposes of comparison, the solid-state
CPMAS-13C NMR spectra of polynorbornenes produced by pal-
ladium precatalyst and nickel complex are presented in Fig. 8(a
and b), respectively. Both polynorbornenes again proved to be
vinyl addition polymers in nature since no olefinic resonances
are observed, which is in accord with the FTIR results. In addi-
3. Conclusion
We have synthesized a series of binuclear salicylaldimine-
based neutral nickel and palladium complexes and character-
ized their structures. For these complexes, each salicylaldimine
derivative unit plays a role of a bulky group at C-3 position of
the other unit. With MMAO as cocatalysts, these well-defined
complexes and 4a displayed higher catalytic activities than those
of mononuclear salicylaldimine-based complexes reported pre-
viously. For salicylaldimine-based nickel complexes, the steric
effect of substituents in the ortho position of the salicylaldimi-
nato fragment is negligible in norbornene polymerization. How-
ever, this steric effect is noticeable for the corresponding pal-
ladium complexes. For salicylaldimine–palladium complexes,
appropriately bulky groups in the ortho position of the sali-
cylaldiminato fragment are responsible for their high catalytic
activity. In addition, these palladium complexes with the pyri-
dine ligand display much lower activities than their counterparts
withthePPh3 ligand. FTIR and 13CNMR analysesofthePNBEs
show that norbornene polymerization proceeds with vinyl addi-
tion polymerization.

162
T. Hu et al. / Journal of Molecular Catalysis A: Chemical 253 (2006) 155–164
4. Experimental
2b: To a stirred solution of compound 1b (1.66 g,
5 mmol) in dried methane dichloride (30 ml) were added 2,6-
diisopropylaniline (2.22 g, 12.5 mmol) and formic acid (0.1 ml)
as a catalyst. The mixture was refluxed and stirred for 24 h. The
evaporation of solvent gave a yellow solid powder as the crude
product. Recrystallization from CHCl3-EtOH or chloroform-
4.1. General remarks and materials
All manipulations of air- and/or water-sensitive compounds
were performed under a dry nitrogen atmosphere using stan-
dard Schlenk techniques. The NMR data of the ligands and
the complexes were obtained on a Bruker 300 MHz spec-
trometer at ambient temperature, with CDCl3 or C6D6 as
the solvents. The 13C NMR spectra of the polynorbornenes
obtained with nickel precatalysts were recorded on a Varian
Unity-400 MHz spectrometer with o-dichlorobenzene as the sol-
vent at ambient temperature. Solid-state 13C CP/MAS (cross-
polarization with magic angle spinning) NMR spectra of the
polynorbornenes obtained with palladium precatalysts were
obtained on a Varian Unity Plus NMR spectrometer operating at
100 MHz. The IR spectra were recorded on a Bio-Rad FTS-135
spectrophotometer. The DSC measurements were performed
on a Perkin-Elmer Pyris 1 differential scanning calorimeter
at a heating rate of 10 ◦C/min. The weight-average molec-
ular weights (Mw) and the polydispersity indices (PDI) of
the polynorbornene samples were determined via high tem-
perature GPC according to the procedure reported previously
[39]. Elemental analyses were performed on a Perkin-Elmer
Series II CHN/O analyzer 2400. Viscosity-average molecular
weights were calculated from the intrinsic viscosity by using the
Mark–Houwink coefficients: α = 0.56, K = 5.97 × 10−4 dl g−1
[40].
Benzene, n-pentane and tetrahydrofuran were refluxed with
sodium/benzophenone ketyl and were distillated under nitrogen
prior to use. Chlorobenzene and dichloromethane were dried
over CaH2 and distilled before use. Triphenyphosphine, sodium
tetrachloropalladate(II) and tetramethyltin were obtained from
Aldrich and used without further purification. Pyridine was dried
over sodium hydroxide and distilled prior to use. Norbornene
was purchased from Aldrich and dried over sodium before use.
Modified methylaluminoxane (MMAO, 7% aluminum in hep-
tane solution) was purchased from Akzo Nobel Chemical Inc.
2,6-Diisopropylaniline and NaH were purchased from Acros.
trans-[Ni(PPh3)2PhCl] [41] and (1,5-cycloioctadene)PdMeCl
[(COD)PdMeCl] [42,43] were prepared according to the analo-
gous methods reported, respectively. Compounds 1a, 5 [44] and
complex 4a [33] were synthesized according to the literature
procedures.
1
ethanol gave 3.0 g (92%) of 2b as yellow needle crystals H
3
NMR (CDCl3): δ 1.24 (d, J = 6.6 Hz, 24H, CH3), 2.99 (sept,
3J = 6.6 Hz, 4H, CH), 7.27 (s, 6H, Ar–H), 8.45 (d, J = 2.7 Hz,
3
2H, Ar–H), 8.47 (s, 2H, CH N), 8.66 (d, 3J = 2.7 Hz, 2H, Ar–H),
15.1 (s, 2H, OH). Anal. calcd. for C38H42N4O6: C, 70.13; H,
6.51; N, 8.61. Found: C, 69.83; H, 6.48; N, 8.57.
Ligand 6: Ligand 6 was obtained as yellow powder in 99%
yield, by the method analogous to that for the synthesis of ligand
2b. 1H NMR (CDCl3): δ 1.18 (d, 3J = 6.8 Hz, 12H, CH3), 1.19
(d, 3J = 6.8 Hz, 12H, CH3), 3.09 (m, 3J = 6.8 Hz, 4H, CH), 7.20
(s, 6H, Ph–H), 7.42 (m, 6H, Ar–H), 7.95 (d, 2H, Ar–H), 8.18 (s,
2H, Ar–H), 8.60 (s, 2H, CH N), 12.7 (s, 2H, OH). Anal. calcd.
for C46H48N2O2: C, 83.60; H, 7.32; N, 4.24. Found: C, 83.37;
H, 7.27; N, 4.18.
4.3. Synthesis of complexes 4b and 8a
4b: A solution of 2b (0.49 g, 0.76 mmol) in anhydrous THF
(15 ml) was added to sodium hydride (46 mg, 1.9 mmol). The
resultant mixture was stirred at room temperature for 4 h, then
filtered and evaporated. The resultant solid residue and trans-
[Ni(PPh3)2PhCl] (1.06 g, 1.52 mmol) were dissolved in THF
(40 ml) in an Schlenk flask and stirred at room temperature
for 14 h. The resultant mixture was filtered, and the filtrate was
concentrated in vacuum to ca. 6 ml. Pentane (30 ml) was then
added. A brown solid precipitated from solution and was iso-
lated by filtration to yield 0.37 g (34%) of 4b. 1H NMR (C6D6):
δ 1.26 (d, JHH = 6.7 Hz, 12H, CH3), 1.38 (d, JHH = 6.6 Hz, 12H,
CH3), 4.02 (m, 4H, CH), 6.77–7.89 (m, 46H, Ar–H), 8.14
(d, 2H, JHP = 9.3 Hz, N CH), 8.35 (d, 4H, Ar–H); 13C NMR
(C6D6): δ 23.1, 23.7, 25.3, 25.6, 28.4, 29.1, 116.0, 117.4, 118.9,
119.3, 120.6, 124.4, 126.6, 128.9, 129.8, 132.8, 134.1, 135.8,
136.0, 138.8, 140.5, 142.9, 148.4, 150.7, 168.3. Anal. calcd. for
C86H80N4Ni2O6P2: C, 71.49; H, 5.58; N, 3.88. Found: C, 71.85;
H, 5.55; N, 3.91.
8a: Complex 8a was obtained as red-brown powder in
37% yield, following the method analogous to that for the
synthesis of 4b. 1H NMR (300 MHz, C6D6): δ 1.29 (d,
JHH = 6.5 Hz, 12H, CH3), 1.36 (d, JHH = 6.5 Hz, 12H, CH3), 4.13
(m, 4H, CH), 6.12–7.94 (m, 54H, Ar–H), 8.07 (s, 2H, Ar–H),
8.18 (s, 2H, N CH); 13C NMR (C6D6): δ 23.8, 25.9, 28.4,
114.2,118.9,121.4, 123.2, 124.7, 125.4, 129.5,131.1, 133.0,
133.8, 135.3, 136.1, 136.9, 137.6, 138.3, 141.1, 144.6, 150.6,
164.3, 168.6. Anal. calcd. for C94H86N2Ni2O2P2: C, 77.59; H,
5.96; N, 1.93. Found: C, 77.29; H, 5.93; N, 1.98.
4.2. Synthesis of compounds 1b and ligands 2b, 6
1b: Compound 1a (2.42 g, 10 mmol) was added cautiously
with cooling in ice and salt, to a mixture of concentrated sul-
phuric acid (2 ml) and concentrated nitric acid (1 ml). After
30 min the nitration mixture was poured on ice, and the yel-
low product was washed with ice-water, dried and crystallized
from THF, giving 2.4 g (72%) of product as a pale yellow pow-
der. 1H NMR (DMSO): δ 10.4 (s, 2H, CHO), 8.43 (d, 3J = 3 Hz,
4.4. Synthesis of binuclear palladium complexes 4c–f and
8b and 8 c
3
1
2H, Ar–H), 8.50 (d, J = 3 Hz, 2H, Ar–H), 4.04 (br, H, OH).
MS(EI): m/z 332(M+). Anal. calcd. for C14H8N2O8: C, 50.61;
H, 2.43; N, 8.43. Found: C, 50.34; H, 2.47; N, 8.38.
A solution of 2a (0.28 g, 0.5 mmol) in anhydrous THF (15 ml)
was added to sodium hydride (48 mg, 2 mmol). The resultant

T. Hu et al. / Journal of Molecular Catalysis A: Chemical 253 (2006) 155–164
163
JHP = 11.3 Hz, N CH); 13C NMR (C6D6): δ 2.97(d, JCP = 42 Hz,
Pd CH3), 23.3, 23.1, 25.7, 25.1( CH3), 28.7 ( CH), 119.8,
123.7, 124.0, 125.3, 126.6, 126.8, 129.0, 129.8, 132.1, 132.7,
134.5 (d, JCP = 45 Hz), 138.5, 139.4, 141.3, 141.9, 148.8, 168.9
(N C). Anal. calcd. for C84H82N2O2P2Pd2: C, 70.73; H, 5.79;
N, 1.96. Found: C, 70.49; H, 5.83; N, 2.03.
mixture was stirred at room temperature for 4 h, then filtered and
evaporated. The solid residue was dissolved in 15 ml of benzene
to form a clear orange solution. The solution was transferred to
a Schlenk flask containing 0.265 g of (COD)PdMeCl (1 mmol)
and 20 ml of benzene. The mixture was stirred at room tempera-
ture for 15 min and then 0.262 g PPh3 (1 mmol) was added. The
resulting mixture was stirred at room temperature overnight and
filtrated. The filtrate was concentrated in vacuum to ca. 6 and
30 ml of pentane was slowly added. 0.54 g of 4c was given as
deep yellow flakes in 83% yield. The other palladium complexes
4d–f, 8b and 8c were prepared via similar procedures.
8c (81%): Pink powder. 1H NMR (C6D6): δ 0.29 (s,
6H, Pd CH3), 1.04 (d, JHH = 6.8 Hz, 6H, CH3), 1.18 (d,
JHH = 6.8 Hz, 6H, CH3), 1.33 (d, JHH = 6.8 Hz, 6H, CH3),
1.37 (d, JHH = 6.8 Hz, 6H, CH3), 3.71 (m, 4H, CH), 6.29 (t,
4H, Py–H), 6.71 (t, 2H, Py–H), 6.93 (m, 4H, Py–H), 7.24 (s, 6H,
Ph–H), 7.70 (t, 6H, Ar–H), 8.06 (s, 2H, CH N), 8.35 (dd, 4H,
1.2 Hz, Ar–H); 13C NMR (C6D6): δ −0.86 (s, Pd CH3), 21.5,
21.6, 23.5, 23.6, 26.6, 26.8, 118.8, 122.2, 123.4, 123.8, 125.6,
127.7, 134.6, 135.3, 138.6, 139.8, 148.0, 150.7, 166.7 (N C).
Anal. calcd. for C58H62N4O2Pd2: C, 65.72; H, 5.90; N, 5.29.
Found: C, 65.86; H, 5.86; N, 5.24.
4c: 1H NMR (C6D6): δ 0.01 (d, JPH = 3.6 Hz, 6H, Pd CH3),
1.01 (d, JHH = 6.9 Hz, 12H, CH3), 1.12 (d, JHH = 6.9 Hz, 12H,
CH3), 2.95 (m, 4H, CH), 6.56–7.96 (m, 42H, Ar–H), 8.02
(s, 2H, N CH); 13C NMR (C6D6): δ 3.61 (d, Pd CH3), 23.2,
23.7, 25.1, 25.6( CH3), 28.4, 29.1 ( CH), 118.8, 121.6, 123.5,
129.1, 127.8, 128.6, 129.1, 129.9, 130.4, 132.0, 133.6, 134.1,
137.0, 137.4, 143.5, 143.5, 167.9 (N C). Anal. calcd. for
C76H78N2O2P2Pd2: C, 68.83; H, 5.93; N, 2.11. Found: C, 69.07;
H, 5.88; N, 2.16.
4.5. Typical polymerization procedure
To a 100 ml Schlenk flask with a mechanical stirrer were
added the following materials under inert gas atmosphere in
the order given: a solution of norbornene (44 mmol, 4.14 g)
in C6H5Cl (6 ml), 4c (0.25 ␮mol) dissolved in C6H5Cl (1 ml)
and 22.3 ml of C6H5Cl. The reaction was started by the addi-
tion of 0.75 ml of a MMAO solution (1.5 mmol in heptane) at
50 ◦C. After vigorous stirring for 2 min, the reaction mixture
was poured into 200 ml of acidic EtOH (EtOH–HClconc = 50/1).
The polymer was isolated by filtration, washed with EtOH and
dried under vacuum at 80 ◦C for 10 h.
4d (78%): The reaction was carried out according to the
same procedure as for 4c, except that pyridine (5 equivalents)
was used instead of triphenylphosphine. H NMR (C6D6): δ
0.19 (s, 6H, Pd CH3), 0.90 (d, JHH = 6.6 Hz, 24H, CH3), 2.85
(m, 4H, CH), 6.66–7.85 (m, 22H, Ar–H + Py–H), 8.21 (d, 2H,
N CH); 13C NMR (C6D6): δ 1.63 (Pd CH3), 23.1, 24.7, 25.4
( CH3), 28.2, 29.1 ( CH), 120.9, 121.7, 122.5, 125.9, 127.3,
126.4, 136.6, 139.8, 146.3, 150.3, 152.4, 169.4 (N C). Anal.
calcd. for C50H58N4O2Pd2: C, 62.56; H, 6.09; N, 5.84. Found:
C, 62.94; H, 6.04; N, 5.76.
1
4e (74%): The reaction was carried out according to the same
procedure as for 4c, except that CH2Cl2 was used instead of ben-
zene. 1H NMR (C6D6): δ 0.22 (d, JPH = 3.3 Hz, 6H, Pd CH3),
1.21 (d, JHH = 6.8 Hz, 12H, CH3), 1.42 (d, JHH = 6.7 Hz, 12H,
CH3), 3.73 (m, 4H, CH), 7.18–7.79 (m, 40H, Ar–H), 8.13 (d,
2H, JHP = 3 Hz, N CH); 13C NMR (C6D6): δ 3.88 (d, Pd CH3),
23.1, 23.7, 24.9, 25.2 ( CH3), 28.7, 29.5 ( CH), 119.4, 123.6,
123.9, 130.1, 130.8, 130.9, 131.1, 131.6, 132.6, 134.0, 134.6
(d, JCP = 15 Hz), 135.1, 141.0, 141.4, 147.5, 147.7, 164.2, 166.8
(N C). Anal. calcd. for C76H76N4O6P2Pd2: C, 64.45; H, 5.41;
N, 3.96. Found: C, 64.28; H, 5.37; N, 3.94.
Acknowledgements
The authors are grateful for subsidy provided by the National
Natural Science Foundation of China (Nos. 20334030 and
50525312), and by the National Basic Research Program of
China (No. 2005CB623801).
Appendix A. Supplementary material
Crystallographic data for the structural analysis have been
deposited with the Cambridge Crystallographic Data Cen-
ter, CCDC Nos. 286028 and 286029 for complexes 4e and
8b, respectively. Copies of this information may be obtained
free of charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223 336033; email:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
4f (87%): The reaction was carried out according to the
same procedure as for 4e, except that pyridine (5 equivalents)
1
was used instead of triphenylphosphine. H NMR (C6D6): δ
0.42 (s, 6H, Pd CH3), 1.11 (d, JHH = 6.9 Hz, 12H, CH3), 1.48
(d, JHH = 6.9 Hz, 12H, CH3), 3.69 (m, 4H, CH), 6.47–8.33
(m, 20H, Ar–H + Py–H), 8.72 (d, 2H, JHP = 3.1 Hz, N CH);
13C NMR (C6D6): δ 1.02 (Pd CH3), 23.1, 24.9, 25.1 ( CH3),
28.4, 29.2 ( CH), 123.9, 124.3, 124.6, 127.9, 128.2, 128.5,
137.9, 140.8, 147.5, 151.3, 152.1, 167.7 (N C). Anal. calcd. for
C50H56N6O6Pd2: C, 57.20; H, 5.38; N, 8.00. Found: C, 57.38;
H, 5.32; N, 7.93.
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