C5Me5/ER-Ligated samarium(II) complexes (ER=OAr, SAr, NR1R2): A new type of olefin polymerization catalysts Presented at the 22nd Rare Earth Research Conference, Argonne, July 10-15, 1999 (Abstract No. E-2)

Article abstract of DOI:10.1016/S0925-8388(00)00654-X

The synthesis and polymerization reactions of a series of C₅Me₅/ER-ligated lanthanide(II) complexes (ER = OAr, SAr, NR¹R²) are described. In sharp contrast with the previously known metallocene complexes, this type of heteroleptic samarium(II) complex acts as a unique catalytic system, which cannot only polymerize styrene and ethylene, but also copolymerize them into block styrene-ethylene copolymers in the presence of both monomers.

Full text of DOI:10.1016/S0925-8388(00)00654-X

Journal of Alloys and Compounds 303–304 (2000) 75–82
L
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C5Me5 /ER-Ligated samarium(II) complexes (ER5OAr, SAr, NR1R2):
a new type of olefin polymerization catalysts
Presented at the 22nd Rare Earth Research Conference, Argonne,
July 10–15, 1999 (Abstract No. E-2)
*
Zhaomin Hou , Yasuo Wakatsuki
The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako, Saitama 351-0198, Japan
Abstract
The synthesis and polymerization reactions of a series of C5Me5 /ER-ligated lanthanide(II) complexes (ER5OAr, SAr, NR1R2) are
described. In sharp contrast with the previously known metallocene complexes, this type of heteroleptic samarium(II) complex acts as a
unique catalytic system, which cannot only polymerize styrene and ethylene, but also copolymerize them into block styrene–ethylene
copolymers in the presence of both monomers.
2000 Elsevier Science S.A. All rights reserved.
Keywords: Chemical synthesis; Copolymerization; Crystal structure; Lanthanides; Pentamethylcyclopentadienyl; Thiolate
1. Introduction
2. Synthesis of lanthanide(II) complexes with mixed
C5Me5 /ER ligands
There has been intense recent interest in the use of
well-defined group 4 and group 3/lanthanide metal com-
plexes as catalysts or precatalysts for polymerization of
olefins [1–7]. Among these catalytic systems, the lantha-
nide (including group 3) metal-based catalysts have a
potential advantage over group 4 metal analogs, since they
do not require a co-catalyst to show high reactivity.
However, most of the lanthanide catalysts reported so far
have been based on complexes which bear two sterically
demanding C5Me5 ligands [4–6]. These metallocene com-
plexes are active for ethylene polymerization, but not able
Our approaches to a lanthanide(II) complex with mixed
C5Me5 /ER ligands started with the metathetical reac-
tion of the samarium(II) bis(aryloxide) complex
(ArO)2Sm(THF)3 (Ar5C6Ht2Bu2-2,6-Me-4) [15] with 1
equiv of KC5Me5 (Scheme 1) [16]. The final product
obtained in this reaction was not the expected simple
C5Me5 /OAr-ligated samarium(II) complex A, but its
KC5Me5 adduct 1a (Scheme 1). Complex 1a could also
be obtained more efficiently by the reaction of
(C5Me5)2Sm(THF)2 with 1 equiv of KOAr, as shown in
Scheme 1 [16]. This reaction is quite general and can be
extended to the synthesis of similar lanthanide(II) com-
plexes (1b–f) bearing other anionic monodentate ligands,
such as thiolates and amides (Scheme 2) (Z. Hou, Y.
Wakatsuki, unpublished result) [17]. Complexes 1a–f all
can be viewed as a C5Me5 /ER-ligated Ln(II) complex
1
to polymerize substituted olefins [8–14].
We present herein a new type of lanthanide(II) complex
with mixed C5Me5 /ER-ligands (ER5OAr, SAr, NR1R2),
which was recently developed in our laboratories. This
type of complex, particularly that of samarium(II), acts as
a very unique catalytic system which not only polymerizes
but also copolymerizes styrene and ethylene.
(Ln5Sm
or Yb)
coordinated
by
a
neutral
‘C5Me5K(THF)n’ ligand [16]. These complexes all adopt a
similar polymeric structure via ‘intermolecular’ interaction
between the K atom and a C5Me5 ligand. X-ray structures
of 1b and 1c are shown in Figs. 1 and 2, respectively (Z.
Hou, Y. Wakatsuki, unpublished result).
*Corresponding author.
1Polymerization of a-olefins by group 3 or lanthanide metal complexes
with modified cyclopentadienyl ligands has been recently reported. For
examples, see Refs. [12–14].
The K-free, C5Me5 /OAr-ligated Sm(II) complex 2,
which adopts a dimeric structure via m-OAr bridges, could
0925-8388/00/$ – see front matter
PII: S0925-8388(00)00654-X
2000 Elsevier Science S.A. All rights reserved.

76
Z. Hou, Y. Wakatsuki / Journal of Alloys and Compounds 303–304 (2000) 75–82
Scheme 1.
be synthesized in high yields by the reaction of
(C5Me5)2Sm(THF)2 with equiv of ArOH or
sible, if the neutral ‘C5Me5K(THF)n’ ligand in 1a–f is
dissociated from the lanthanide atom.
1
(ArO)2Sm(THF)3 in toluene, as shown in Scheme 3 [16].
Reaction of 2 with 2 equiv of KC5Me5 yields 1a, while
treatment of 2 with 4 equiv of hexamethylphosphoric
triamide (HMPA) affords the monomeric complex 3 in
high yields (Scheme 3) [16].
Based on this assumption, the reactivity of this new type
of lanthanide(II) complex towards some olefin monomers
was examined.
The reaction of the polymeric 1a or 1e with 2 equiv of
HMPA (per Sm) yielded almost quantitatively the K-free,
monomeric complexes 3 or 4, respectively (Scheme 4) (Z.
Hou, Y. Wakatsuki, unpublished result) [16], which again
demonstrates that the ‘C5Me5K(THF)n’ unit in 1a–f can
be viewed as a neutral ligand. These results also suggested
that generation of a sterically unsaturated, highly reactive
Ln(II) species, such as ‘(C5Me5)Ln(ER)’, could be pos-
3. Polymerization of styrene
The results of the polymerization of styrene by several
types of lanthanide(II) complexes are summarized in Table
1.
The reaction of the unsolvated samarium(II) complex
(C5Me5)2Sm with an excess of styrene has been reported
to yield a stable bimetallic complex, (C5Me5)2Sm(m-
Scheme 2.

Z. Hou, Y. Wakatsuki / Journal of Alloys and Compounds 303–304 (2000) 75–82
77
Fig. 1. X-ray structure of 1b. /Sm(1)–O(1)–C(21): 165(1)8.
CH2CHPh)Sm(C5Me5)2 [11], which is inert toward poly-
merization of styrene [11,18]. The solvated homoleptic
1a–e, which contain the neutral ‘C5Me5K(THF)n’ ligand,
all showed a high activity for the polymerization of
styrene. In addition, ER-ligand dependence of the activity
was also observed. Complex 1b, which bears a less bulky
OC6Hi3Pr2-2,6 ligand, is more active than the 4-Me-2,6-
tBu2C6H2O-ligated complex 1a (runs 4 and 5, Table 1).
The more electron-donating amide complexes 1d,e showed
a higher activity than the aryloxide and thiolate complexes
samarium(II)
complexes
(C5Me5)2Sm(THF)2,
Sm(OAr)2(THF)3, or Sm(N(SiMe3)2)2(THF)2 did not
react with styrene at room temperature (runs 1–3, Table 1).
Neither did the dimeric complex 2 or the HMPA-coordi-
nated complex 3 (runs 10–11, Table 1).
In contrast, the C5Me5 /ER-ligated Sm(II) complexes
Fig. 2. X-ray structure of 1c. /Sm(1)–S(1)–C(21): 124.0(8)8.

78
Z. Hou, Y. Wakatsuki / Journal of Alloys and Compounds 303–304 (2000) 75–82
Scheme 3.
Scheme 4.
Table 1
Polymerization of styrene by lanthanide(II) complexesa
b
b
Run
Cat
Run time
Yield (%)
Mn (31024
)
Mw /Mn
1
2
(C5Me5)2Sm(THF)0
Sm(OAr)2(THF)3
1 day
1 day
0
0
–
–
–
–
or
2
3
Sm(N(SiMe3)2)2(THF)2
1 day
0
–
–
4
5
6
7
8
9
10
11
1a
1b
1c
1d
1e
1f
2
30 min
30 min
150 min
20 min
10 min
1 day
89
100
79
100
100
0
34.7
24.5
17.1
14.4
8.2
–
1.73
1.93
1.45
2.16
2.45
–
1 day
1 day
0
0
–
–
–
–
3
a Conditions: Sm(II) compound, 0.05 mmol; styrene, 4 ml; toluene, 10 ml; room temperature.
b Determined at 1358C against polystyrene standard by GPC.

Z. Hou, Y. Wakatsuki / Journal of Alloys and Compounds 303–304 (2000) 75–82
79
Table 2
5, Table 2). The ytterbium(II) complex 1f was again inert
for polymerization of ethylene (run 6, Table 2).
Polymerization of ethylene by lanthanide(II) complexes bearing mixed
C5Me5 /ER ligandsa
b
b
Run
Cat
Run time
Yield (g)
Mn (31024
)
Mw /Mn
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
20 min
10 min
10 min
10 min
30 min
2 h
3.00
1.00
0.99
0.48
0.55
0
45.0
33.0
58.0
49.7
309.9
–
2.75
2.49
1.79
2.90
2.83
–
5. Block copolymerization of styrene and ethylene
The C5Me5 /ER-ligated samarium(II) complexes 1a–d
can not only polymerize but also copolymerize styrene and
ethylene. Of particular interest is that the copolymer
products obtained under the coexistence of both monomers
are styrene–ethylene block-copolymers as confirmed by
NMR spectroscopic studies [17]. The thiolate complex 1c
is particularly useful for this copolymerization, which
produces block styrene–ethylene copolymers with selec-
tivity of higher than 90% (runs 6–9, Table 3). The styrene
content in the copolymer products increased as the feeding
amount of styrene monomer was increased under an
atmosphere of ethylene, and could therefore be easily
controlled (runs 2–9, Table 3).
a Reaction conditions: a precatalyst, 0.05 mmol; ethylene, 1 atm;
toluene, 15 ml; room temperature.
b Determined at 1358C against polystyrene standard by GPC.
1a and 1c (runs 4, 6, 7 and 8, Table 1). The ytterbium(II)
complex 1f is not active for the polymerization of styrene,
showing that the reducing power of the lanthanide(II) ions
plays a critically important role in the present polymeri-
2
zation reaction.
The selective formation of block ethylene–styrene co-
polymers in the present reactions are in sharp contrast with
what has been previously observed in group 4 metal-
catalyzed reactions, in which random or alternating ethyl-
ene–styrene copolymers were always obtained under the
4. Polymerization of ethylene
3
The samarium(II) complexes 1a–e also showed high
activity for ethylene polymerization, giving linear poly-
ethylene with Mn up to 33106 (based on polystyrene
standard) (Table 2). Among these catalysts, the 4-Me-2,6-
tBu2C6H2O-ligated complex 1a showed the highest activi-
ty (run 1, Table 2), while the (Me3Si)2N-ligated complex
1e gave the highest molecular weight of polyethylene (run
coexistence of both monomers [19–33]. The present
reactions are also in sharp contrast with those promoted by
samarocene(II) complexes such as (C5Me5)2Sm and
(C5Me5)2Sm(THF)2, in which the maximum incorporation
of styrene into polyethylene was only two molecules per
chain due to the steric hindrance of the bulky bis(pen-
tamethylcyclopentadienyl) ligand set (C5Me5)2 [9]. In
Table 3
Block copolymerization of ethylene and styrene by samarium(II) complexes bearing mixed C5Me5 /ER ligandsa
c
c
Run
Cat
Styrene (ml)
Yield (g)
Ps cont
Mn (31024
)
Mw /Mn
(mol%)b
THF-sol
(PS)d
Tol-sol (,1088C)
Tol-insol (1088C)
(PES)d
(PE)d
1
2
3
4
5
6
7
8
1a
1b
1b
1b
1b
1c
1c
1c
1c
1d
1e
5
3
5
7
10
3
5
7
10
3
5
0.10
0.46
1.29
1.37
2.40
0.06
0.23
0.27
0.34
1.89
4.55
1.60
2.14
2.83
2.94
3.99
1.31
2.55
2.74
2.82
1.21
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
38
34
48
68
81
13
37
43
60
36
–
7.8
15.9
14.6
15.1
13.1
11.3
10.7
13.7
14.6
5.7
2.36
1.97
1.82
1.92
1.84
2.21
2.01
1.73
1.66
3.75
–
9
10
11
–
a Conditions: a precatalyst, 0.05 mmol; ethylene, 1 atm; total volume of styrene and toluene, 25 ml; room temperature, 30 min, unless otherwise noted.
b Polystyrene content in the copolymers determined by 13C NMR in ortho-dichlorobenzene/CDCl2CDCl2 at 1258C.
c Determined at 1358C against polystyrene standard by GPC.
d PS, atactic polystyrene; PES, block ethylene–styrene copolymer; PE, polyethylene.
2It is well known that Yb(II) is less reducing than Sm(II), cf.
3For examples of group 4 metal-catalyzed random or alternating
Eo(Sm31 /Sm21)521.55 V, Eo(Yb31 /Yb21)521.15
medium.
V in aqueous
copolymerization of ethylene with styrene (see Refs. [19–33]).

80
Z. Hou, Y. Wakatsuki / Journal of Alloys and Compounds 303–304 (2000) 75–82
Scheme 5.
separate experiments it was established that Sm(ER)2,
KC5Me5, or KER are not active for the copolymerization
of ethylene with styrene under the same conditions. These
interaction between an ER ligand and the samarium metal
remains in a catalytic species during the polymerization
reaction. The formation of a yellow solution at the
beginning of the reactions and the narrow molecular
weight distribution of the resulting polymers are consistent
with the fact that the active species in the present poly-
merization reactions is a homogeneous Sm(III) species.
It is well known that the samarocene(II) complexes
(C5Me5)2Sm(THF)n (n50, 2) can reductively dimerize
ethylene to produce the corresponding Sm(III) species
(C5Me5)2Sm(CH2CH2)2Sm(C5Me5)2 which is active for
polymerization (Scheme 6) [9,10,37]. In the present sys-
tems, similar Sm(III) species which are ligated by the
heteroleptic C5Me5 /ER ligands could be formed (Scheme
7). The more open ligand sphere provided by the C5Me5 /
ER ligand set could explain why the present systems are
more active than the corresponding metallocene complex-
es. The selective formation of block ethylene–styrene
copolymers in the present systems strongly suggests that
the reactivity of the propagation center of polyethylene
unit is critically different from that of polystyrene unit, one
being able to incorporate both ethylene and styrene, while
the other only the identical monomer. The nature of the
metals and the ancillary ligands seem to be an important
factor in determining the behavior of the propagation
results
strongly
suggest
that
the
heteroleptic
‘(C5Me5)Sm(ER)’ unit in 1a–d plays a very important
role in the present copolymerization reactions.
In the case of complexes 2 and 3 the lack of activity for
the polymerization reactions is probably due to the stable
m-OAr bridging in 2 and the strong coordination of the
HMPA ligands in 3 [34,35], which prevent access of an
olefin monomer to the metal center. In fact, when a more
reactive electrophile, 9-fluorenone, was allowed to react
with 3, the C5Me5 /OAr-ligated samarium(III) ketyl com-
plex 5 was isolated in 91% yield (Scheme 5) [36], which
well demonstrates that the mixed C5Me5 /OAr ligand
system is indeed able to stabilize both Sm(II) and Sm(III)
species.
6. Mechanistic consideration on the polymerization
reactions by C5Me5 /ER-ligated Sm(II) complexes
As shown in Tables 1–3, dependence of the poly-
merization reaction on the ER ligands in 1a–e has been
observed in all cases, which strongly suggests that bonding
Scheme 6.

Z. Hou, Y. Wakatsuki / Journal of Alloys and Compounds 303–304 (2000) 75–82
81
Scheme 7.
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propagation center of the polystyrene unit has some extent
of anionic character which could prevent insertion of
ethylene monomer, and thus the copolymerization reaction
might probably be initiated by polymerization of ethylene
followed by incorporation of styrene (Scheme 7). The ER
ligand dependence of the copolymerization reactions and
the relatively narrow molecular weight distributions of the
resulting copolymers exclude the possibility that the
copolymerization process is a radical reaction.
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Acknowledgements
This work was supported by the President’s Special
Research Grant of The Institute of Physical and Chemical
Research (RIKEN) and grant-in-aid from the Ministry of
Education of Japan. We are grateful to all our co-workers
who have effectively participated in the work described
here and whose names are shown in the list of references.
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