Synthesis of palladium(II) pyridyl acylhydrazone complexes and application to homo- and copolymerization of cyclic olefins

Article abstract of DOI:10.1016/j.inoche.2007.11.024

A series of new palladium(II)/pyridyl acylhydrazone complexes were synthesized through the reaction of PdBr₂(NCMe)₂ and pyridyl acylhydrazone. The X-ray crystallographic analysis revealed that the palladium center is coordinated by two bromide atoms and two nitrogen atoms of pyridyl acylhydrazone ligand. This coordination mode of pyridyl acylhydrazone ligand forms a striking contrast to the corresponding nikel(II)/pyridyl acylhydrazone complexes. These new palladium complexes showed high catalytic activity for homopolymerization of cyclic olefins such as norbornene and tetracyclododecene, and for copolymerization of tetracyclododecene with norbornene in the presence of MAO.

Full text of DOI:10.1016/j.inoche.2007.11.024

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Inorganic Chemistry Communications 11 (2008) 215–219
www.elsevier.com/locate/inoche
Synthesis of palladium(II) pyridyl acylhydrazone complexes
and application to homo- and copolymerization of cyclic olefins
Kenichi Ogata *, Atsuo Watanabe, Tomoshige Yunokuchi, Akinori Toyota *
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho,
Koganei-shi, Tokyo 184-8588, Japan
Received 16 November 2007; accepted 30 November 2007
Available online 10 January 2008
Abstract
A series of new palladium(II)/pyridyl acylhydrazone complexes were synthesized through the reaction of PdBr2(NCMe)2 and pyridyl
acylhydrazone. The X-ray crystallographic analysis revealed that the palladium center is coordinated by two bromide atoms and two
nitrogen atoms of pyridyl acylhydrazone ligand. This coordination mode of pyridyl acylhydrazone ligand forms a striking contrast to
the corresponding nikel(II)/pyridyl acylhydrazone complexes. These new palladium complexes showed high catalytic activity for hom-
opolymerization of cyclic olefins such as norbornene and tetracyclododecene, and for copolymerization of tetracyclododecene with nor-
bornene in the presence of MAO.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Palladium complex; Polymerization; Cyclic olefin; Tetracyclododecene; Norbornene
Late-transition-metal-catalyzed polymerization of ole-
fins has attracted much attentions over the past decade
since Brookhart’s discovery of the nickel and palladium
catalysts bearing a-diimine ligands [1]. In particular, there
were numerous reports dealing with group 10 catalytic sys-
tems for olefin polymerization and a number of nickel and
palladium complexes bearing a bidentate nitrogen ligand
have been reported as a catalyst for the olefin polymeriza-
tion such as diimine [2], pyridylimine [3] and bipyridine [4]
complexes.
Cyclic olefin polymers and cyclic olefin copolymers have
been recognized as an advanced material with unique phys-
ical properties such as good mechanical strength, optical
transparency, and low birefringence [5]. Norbornene has
been used as popular cyclic olefin monomer for synthesis
of cyclic olefin polymers and cyclic olefin copolymers. On
the other hand, to modify the physical properties of these
polymers, other cyclic olefins have been also used as mono-
mer [6]. For example, the use of tetracyclododecene as a
monomer has been attempted because of its unique bulky
structure. However, comparing with norbornene, the poly-
merization of tetracyclododecene is hard to proceed pre-
sumably due to its steric hindrance [7]. In the course of
our studies directed toward the late-transition-metal-cata-
lyzed polymerization of tetracyclododecene, we have
recently reported the first examples of late-transition-metal
catalyzed homo- and copolymerization reactions involving
tetracyclododecene by using a previously reported nicke-
l(II)/pyridyl acylhydrazone complex 2 [NiBr2(PyCH@
NNHCOPh)2] [8] in the presence of MAO (Scheme 1) [9].
On the other hand, not only nickel(II) complexes but also
a number of palladium complexes have been employed as
efficient catalysts for vinyl-type homopolymerization of
norbornene [10]. Accordingly, we turned our attention to
the synthesis of new palladium(II)/pyridyl acylhydrazone
complexes for the polymerization of tetracyclododecene.
In this communication, we report the synthesis of new
palladium(II)/pyridyl acylhydrazone complexes 3 showing
*
Corresponding authors. Tel./fax: +81 42 388 7162.
E-mail addresses: kogata@cc.tuat.ac.jp (K. Ogata), a-toyota@cc.tuat.
ac.jp (A. Toyota).
1387-7003/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.11.024

216
K. Ogata et al. / Inorganic Chemistry Communications 11 (2008) 215–219
Ph
HN
Ph
NH
O O
Ni
NiBr2(DME)
N
N
CH
HC
Br Br
N
N
N
NH
O
C
H
N
2
R
O
1
CH
N
Pd
PdBr2(NCMe)2
N
N
R
H
Br
Br
3
Scheme 1. Synthesis of nickel(II) and palladium(II) pyridyl acylhydrazone complexes.
the different coordination mode compared to the corre-
sponding nickel(II)/pyridyl acylhydrazone complexes 2
(Scheme 1), and their catalytic activities for the polymeriza-
tion of tetracyclododecene as well as norbornene in the
presence of MAO.
The palladium complexes 3a–c bearing ligands 1a–c
were prepared as shown in Scheme 2. The pyridyl acylhyd-
razone 1a–c were prepared by the reaction of 2-pyridine-
carbaldehyde with substituted benzoyl hydrazides in the
presence of acetic acid [11]. Palladium complexes 3a–c
bearing ligands 1a–c were obtained in 52–67% yields by
the treatment of PdBr2(NCMe)2 with 1a–c in CH2Cl2 at
room temperature. The structures of complexes 3a–c were
determined by NMR spectra and elementary analysis,
and the crystal structure of 3a could be obtained by X-
ray analysis [12]. ORTEP drawing of 3a is shown in
Fig. 1 along with selected bond distances and angles [13].
The palladium center is coordinated by two bromide atoms
O
CH
N
C
H
N
N
N
NH
O
N
R
PdBr2(NCMe)2
+
R
H
Pd
CH2Cl2, r.t.
Br
Br
1a: R = H 57%
1b: R = F 42%
1c: R = tBu 85%
3a: R = H 52%
3b: R = F 67%
3c: R = tBu 67%
Scheme 2. Preparation of palladium complex 3a–c.
Fig. 1. ORTEP drawing of complex 3a with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond
˚
distances (A) and angles (°): Pd(1)–Br(1), 2.4035(6); Pd(1)–Br(2), 2.4214(5); Pd(1)–N(1), 2.040(3); Pd(1)–N(2), 2.050(4); N(2)–C(6), 1.294(5); N(2)–N(3),
1.363(5); N(3)–C(7), 1.382(7); O(1)–C(7), 1.227(5); Br(1)–Pd(1)–Br(2), 90.996(19); N(1)–Pd(1)–N(2), 79.69(15); Br(1)–Pd(1)–N(1), 95.36(12); Br(2)–Pd(1)–
N(2), 94.01(9).

K. Ogata et al. / Inorganic Chemistry Communications 11 (2008) 215–219
217
Table 2
and by two nitrogen atoms of pyridyl acylhydrazone ligand
1a. This coordination mode of pyridyl acylhydrazone
ligand to palladium(II) forms a striking contrast to the cor-
responding nickel(II) complex, in which identical ligand 1a
coordinated to nickel(II) by imine-N and carbonyl-O
atoms (Scheme 1) [8]. Coordination geometry at palladium
center is square planar. The sum of angles around palla-
dium atom is 360.1°. The Pd–N(imine) and Pd–N(pyridine)
Copolymerization of norbornene with tetracyclododecene catalyzed by
3a–c/MAOa
Entry Catalyst Time
(min)
Yield
(%)b
Activityc Mn(10À4 d
)
MW/
Mn
d
1
2
3
3a
3b
3c
3
5
5
91
97
77
7981
6240
4066
8.1
11.0
2.5
2.3
1.7
2.0
a
Polymerization conditions: Total volume, 25 mL; Solvent, toluene;
˚
bond distances are 2.050 (4) and 2.040(3) A, respectively.
Temperature, r.t.; Pd complex, 7.4 lmol; Al/Pd, 330; NB/TD, 1; Mono-
Theses bond distances are the same values of previously
reported pyridylimine palladium complexes [3]. In 1H
NMR measurements of complexes 3a–c, the N–H proton
was observed at 12.1–12.2 ppm as a broad singlet,
mer/Pd, 3400.
b
Calculated from amounts of obtained polymers (g) based on total
amounts of starting monomer (g).
c
Activity: kg(poly)mol(Pd)À1hÀ1
Determined by high-temperature GPC (o-dichlorobenzene at 140 °C).
.
d
t
respectively, and the Bu proton of 3c was observed at
1.31 ppm as a singlet.
The catalytic activities of complexes 3a–c for the poly-
merization of norbornene and tetracyclododecene were
examined in the presence of MAO as a cocatalyst (Table 1)
[14]. We were pleased to find that these palladium(II)/
pyridyl acylhydrazone complexes 3a–c exhibit significantly
higher catalytic activities for the polymerization of both
norbornene and tetracyclododecene than previously
reported nickel(II)/pyridyl acylhydrazone complexes [9].
Importantly, the para-substituent of the benzoyl group of
complex 3 significantly affected the catalytic activity. Com-
paring with the catalytic activity of complex 3a (entry 1),
complex 3b bearing the electron-withdrawing fluorine sub-
stituent increased the catalytic activity (entry 2), but com-
plex 3c bearing the electron-donating tBu substituent
dramatically decreased the catalytic activity (entry 3). On
the other hand, in the case of the polymerization of tetra-
cyclododecene, complex 3a showed higher catalytic activity
than complexes 3b and 3c (entries 4–6). The norbornene
polymerization activities of complexes 3a–c were compara-
ble with analogue palladium complexes bearing a pyridyli-
mine ligand [3e].
The present palladium(II) complex 3a–c/MAO system
could also catalyzed a copolymerization of tetracyclodode-
cene and norborenene with high activity as shown in Table 2.
Although homopolymers of norbornene and tetracyclod-
odecene were insoluble in common organic solvents, the
resulting copolymers were soluble in o-dichlorobenzene at
high temperature. Thus, these copolymers could be ana-
lyzed by high-temperature GPC. In general, molecular
weight distribution of these copolymers showed rather nar-
row value (Mw/Mn = 1.7–2.3). Among the catalysts exam-
ined (entries 1–3), 3b showed the highest yield, and the
resulting copolymer was the highest molecular weight
(Mn = 11.0 Â 104) and the most narrow molecular weight
distribution with unimodal (Mw/Mn = 1.7) (entry 2).
In conclusion, we have synthesized new palladium(II)/
pyridyl acylhydrazone complexes showing the different
coordination mode compared to the corresponding nicke-
l(II)/pyridyl acylhydrazone complexes. Their catalytic
activities for the polymerization of norbornene and tetracy-
clododecene were examined, which revealed that these
palladium(II)/pyridyl acylhydrazone complexes exhibit sig-
nificantly higher catalytic activities than previously
reported nickel(II)/pyridyl acylhydrazone complexes. We
have also determined that a copolymerization of tetracy-
clododecene and norborenene could also proceed by using
the same palladium complexes with high catalytic activity.
The present new palladium-based catalyst system exhibits
the highest catalytic activity for the polymerization of
tetracyclododecene. Further applications of these new pal-
ladium(II)/pyridyl acylhydrazone complexes to various
polymerization reactions are currently under investigation
in our laboratory.
Table 1
Homopolymerization of norbornene and tetracyclododecene catalyzed by
3a-c/MAOa
Entry
Catalyst
Monomer
Yield (%)
Activityb
1
2
3
4
5
6
a
3a
3b
3c
3a
3b
3c
NB
NB
NB
TD
TD
TD
53
77
19
22
13
8.2
2050
2900
690
1400
850
520
Polymerization conditions: Total volume, 25 mL; Solvent, toluene;
Temperature, r.t.; Time, 5 min; Pd complex, 7.4 lmol; Al/Pd, 330;
Monomer/Pd, 3400.
b
Activity: kg(poly) mol(Pd)À1 hÀ1.
Acknowledgements
The authors are grateful to Ms. Keiko Yamada (RI-
KEN) for the assistance in elemental analyses. We thank
Dr. Noriyuki Suzuki and Dr. Daisuke Hashizume (RI-
KEN) for their help in X-ray crystallographic analysis.
Norbornene
Tetracyclododecene
(TD)
(NB)

218
K. Ogata et al. / Inorganic Chemistry Communications 11 (2008) 215–219
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Appendix A. Supplementary material
CCDC 650032 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Cen-
tre via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-
tary data associated with this article can be found, in the
online version, at doi:10.1016/j.inoche.2007.11.024.
(k) F. Bao, X. Lu, Y. Chen, Polym. Bull. 58 (2007) 495.
¨
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0.77 mmol), 1a (354 mg, 1.57 mmol) and dichloromethane (80 mL)
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H, 2.26; N, 8.55. Found: C, 31.57; H, 2.25; N, 8.53. Complex3b
(428 mg, 0.78 mmol, 67%) was prepared from PdBr2(NCMe)2
(276 mg, 0.79 mmol), 1b (388 mg, 1.59 mmol) and dichloromethane
(80 mL) in the same manner as that for 3a. 3b: 1H NMR (CDCl3): d
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t
1H NMR (CDCl3): d 1.31 (s, 9H, Bu), 7.4–9.5 (m, 9H, N@CH, Ph
and Py), 12.1 (s, 1H, NH).
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dimensions
0.40 Â 0.40 Â 0.05 mm,
space
group
˚
P-1(#2),
˚
˚
a = 7.5174(7) A, b = 8.3849(7) A, c = 11.9226(10) A, a = 95.724(2)°,
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3
˚
b = 94.709(3)°, c = 106.340(2)°, V = 712.77(11) A , Z = 2, Dcalc
=
2.290 g/cm3, l(MoKa) = 69.227 cmÀ1
measured 6999, Independent reflection (Rint) 3227 (0.088), number
of reflections to calculated R1 2785 [I > 2.0r(I)], R1 = 0.0498; RW
,
2hmax = 55.0°, Reflection
=
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¨
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˚
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structure was solved by the direct method and expanded using
Fourier techniques. All non-hydrogen atoms were refined anisotrop-
ically and all hydrogen atoms were refined using riding model. The
calculations were performed using the CrystalStructure crystallo-
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4582;

K. Ogata et al. / Inorganic Chemistry Communications 11 (2008) 215–219
219
using SHELXL-97. Atomic coordinates, thermal parameters, and
bond distances and angles have been deposited at the Cambridge
Crystallographic Data Center.
palladium complex was added at room temperature. After a desired
time, the polymerization was quenched with acidified methanol. The
precipitated polymer was filtered off, and washed with methanol, and
then dried under vacuum for several hours at 60 °C. In case of cyclic
olefin copolymerization, the copolymer was extracted with boiling o-
dichlorobenzene.
[14] Polymerization procedure: Cycloolefin were placed in a 50 mL of
flask containing a magnetic stirrer bar, and then solvent and MAO
were added. To the cycloolefin and MAO solution, toluene solution of