Cationic alkyl rare-earth metal complexes bearing an ancillary bis(phosphinophenyl)amido ligand: A catalytic system for living cis-1,4-polymerization and copolymerization of isoprene and butadiene

Article abstract of DOI:10.1002/anie.200604348

(Chemical Equation Presented) Excellent activator: Living cis-1,4-polymerization and copolymerization of isoprene (IP) and butadiene (BD) are achieved for the first time in the presence of an in situ formed cationic alkyl yttrium species (see scheme). A related cationic alkyl lutetium complex, which provides a well-defined structural model for the true catalytically active species, is isolated and structurally characterized by X-ray crystallography.

Full text of DOI:10.1002/anie.200604348

Angewandte
Chemie
DOI: 10.1002/anie.200604348
Living Polymerization
Cationic Alkyl Rare-Earth Metal Complexes Bearing an Ancillary
Bis(phosphinophenyl)amido Ligand: A Catalytic System for Living cis-
1,4-Polymerization and Copolymerization of Isoprene and Butadiene**
Lixin Zhang, Toshiaki Suzuki, Yi Luo, Masayoshi Nishiura, and Zhaomin Hou*
The creation of new polymer materials with well-controlled
microstructures and desired properties relies on the develop-
ment of new generations of polymerization catalysts. cis-1,4-
Regulated polyisoprene (PIP)and polybutadiene (PBD)are
among the most important elastomers used for tires and other
elastic materials. As the demand for high-performance
synthetic rubbers has increased, the development of high-
quality elastomers by polymerization of isoprene and buta-
diene has grown in importance. In addition, the limited supply
of natural rubber has promoted the need for improved
synthetic polyisoprene.[1]
tems, such as [(C5Me5)2Sm(m-Me)2AlMe2]/Al(iBu)3/[Ph3C]
[B(C6F5)4],[4b] have been found to afford polymers with both
high cis-1,4 selectivity (up to 99%)and a narrow molecular-
weight distribution, or “livingness” (Mw/Mn = 1.20–1.23), in
the polymerization of butadiene under appropriate condi-
tions,[4] however, the polymerization of isoprene has not been
achieved in a “living” fashion with such catalyst systems.[4f]
Very recently, the combination of a Nd/Mg heterotrimetallic
allyl complex with methylaluminoxane (MAO)was reported
to show both high cis-1,4 selectivity (95–99%)and a relatively
narrow molecular-weight distribution (Mw/Mn = 1.3–1.7)for
the polymerization of isoprene.[5,6]
To date a variety of catalyst systems have been reported
for the polymerization of butadiene and isoprene, among
which catalysts based on rare-earth metals have attracted
special attention because of their high activity and high cis-1,4
selectivity.[2] The catalysts utilized industrially for the cis-1,4
polymerization of butadiene and isoprene are heterogeneous
Ziegler–Natta-type multicomponent systems that consist
typically of a rare-earth metal (e.g., Nd)carboxylate, ethyl-
aluminum chloride, and isobutylaluminum hydride.[2a–c]
To mimic and improve the industry catalyst systems, and
to gain a better understanding of the mechanistic aspects of
these heterogeneous catalysts, various discrete rare-earth
metal carboxylate and alkoxide complexes as well as rare-
earth metal/aluminum heterometallic alkyl complexes have
been investigated.[3] Some of these molecular systems show
very high cis-1,4 selectivity (up to 99%)when combined with
a chloride additive, such as Et2AlCl. However, similar to the
heterogeneous industrial catalysts, such binary or ternary
catalyst systems generally lack “livingness” and yield poly-
mers with rather broad molecular-weight distributions. On
the other hand, lanthanide metallocene-based catalyst sys-
A common feature in the reported catalyst systems is that
they all require an aluminum additive, such as AlR2Cl, AlR3,
or MAO, to show high activity and high cis-1,4 selectivity,
which makes it difficult to identify the true catalytic species
and to understand the mechanistic aspects of the polymeri-
zation process.[2–8] Recently, cationic methylyttrium species,
such as [YMe2Àn(solv)x]n+1 (n = 0, 1; solv = solvent), have been
reported to show activity for the polymerization of butadiene
and isoprene in the absence of an aluminum additive, but the
use of a bulky alkyl aluminum additive, such as Al(iBu)3, was
again essential to achieve significant cis-1,4 selectivity (67%
vs. 90% for isoprene polymerization with and without
Al(iBu)3, respectively).[9] A related aluminum-free cationic
alkyl yttrium species bearing silylene-linked cyclopentadie-
nylphosphido ligands shows exclusive 3,4-selectivity for the
polymerization of isoprene, while a cationic alkyl yttrium
species bearing the C5Me4SiMe3 ligand yields a mixture of 3,4-
and 1,4-polyisoprenes under similar conditions.[10] The search
for more selective, simpler, and better-controlled catalyst
systems for the cis-1,4 polymerization and copolymerization
of isoprene and butadiene is therefore of interest and
importance.
[*] Dr. L. Zhang, Dr. T. Suzuki, Dr. Y. Luo, Dr. M. Nishiura,
Prof. Dr. Z. Hou
Herein we report a new catalyst system based on a
cationic alkyl rare-earth metal species bearing an ancillary
bis(phosphinophenyl)amido (PNP) ligand. This catalyst
system provides extremely high cis-1,4 selectivity (99%)
and excellent livingness (Mw/Mn = 1.05–1.13)with no need for
an aluminum additive to show high activity and high
selectivity for the polymerization of isoprene and butadiene,
and can also be applied to the copolymerization of isoprene
and butadiene in a living cis-1,4 fashion. The isolation and
structural characterization of a related cationic alkyl lutetium
Organometallic Chemistry Laboratory
RIKEN (The Institute of Physical and Chemical Research)
Hirosawa 2-1, Wako, Saitama 351-0198 (Japan)
and
PRESTO
Japan Science and Technology Agency (JST) (Japan)
Fax: (+81)48-462-4665
E-mail: houz@riken.jp
[**] This work was partly supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology of Japan. Computational resources were provided
by the RIKEN Advanced Center for Computing and Communication.
complex [(PNPPh)Lu(CH2SiMe3)(thf)2][B(C6F5)4] (PNPPh
=
{2-(Ph2P)C6H4}2N)is also described as this is a suitable
structural model complex for the true active species and for
the mechanistic understanding of the polymerization process.
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2007, 46, 1909 –1913
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1909

Communications
Our interest in the PNP-ligated cationic alkyl rare-earth
metal species stems from our recent studies on the use of
cationic half-sandwich (mono-cyclopentadienyl)rare-earth
metal complexes in olefin polymerization.[10,11] Although late
transition-metal complexes containing PNP ligands have
received much interest recently,[12] studies on PNP-ligated
rare-earth metal complexes remain very limited.[13] Analo-
gous to the synthesis of half-sandwich rare-earth metal
[10,11]
bis(alkyl)complexes,
the acid/base reaction of the tris-
(alkyl)complexes [Ln(CH 2SiMe3)3(thf)2] with one equivalent
of (2-(Ph2P)C6H4)2NH[12a] gave the corresponding bis(alkyl)
complexes [(PNPPh)Ln(CH2SiMe3)2(thf)x] (1: Ln = Sc, x = 0;
2: Ln = Y, x = 1; 3: Ln = Lu, x = 1)in a straightforward
manner in high yields (Scheme 1). The Y and Lu complexes 2
Figure 1. ORTEP structure of 3 with thermal ellipsoids set at 30%
probability. Hydrogen atoms and the solvent molecule have been
À
omitted for clarity. Selected bond lengths [Š] and angles [8]: Lu1 N1
À
À
À
À
2.342(3), Lu1 P1 2.9096(9), Lu1 P2 2.9765(9), Lu1 C1 2.354(3), Lu1
À
À
À
À
À
C5 2.385(3), Lu1 O1 2.278(2); C1 Lu1 C5 103.50(12), N1 Lu1 P1
À
À
À
À
65.42(7), N1 Lu1 P2 66.23(7), P1 Lu1 P2 102.76(2).
in which the [(PNPPh)Lu(CH2SiMe3)(thf)2]+ ion adopts a
distorted octahedral structure similar to that of its neutral
precursor 3 (Figure 2).[14] The Lu P (av. 2.888(2)Š)and Lu
À
À
N bond lengths (2.258(7)Š)in 6 are significantly shorter than
those in 3 (2.9431(9)and 2.342(3)Š, respectively,) thus
indicating that the interactions between the PNPPh ligand and
the metal center in the cationic species 6 are stronger than
those in the neutral precursor 3.
Scheme 1. Synthesis of neutral and cationic rare-earth-metal alkyl
complexes bearing a bis(2-diphenylphosphinophenyl)amido (PNPPh)
ligand.
and 3 contain a thf ligand, while the smaller Sc complex 1 is
thf-free, as confirmed by 1H NMR spectroscopy and elemen-
tal analysis. The structure of the Lu complex 3 was further
confirmed by an X-ray crystallographic analysis, which
revealed that 3 adopts a distorted octahedral structure in
which the tridentate PNPPh ligand is arranged in a facial
fashion (Figure 1).[14] This structural feature is in contrast to
the situation observed in related late transition-metal com-
plexes, where the PNPPh ligand usually adopts a meridional
form.[12b,c,e,j] The 31P{1H} NMR spectrum of complex 2 in C6D6
and [D8]THF shows a doublet at d = À11.60 (JY,P = 38 Hz)and
À11.63 ppm (JY,P = 31 Hz), respectively, which suggests that
Figure 2. ORTEP structure of 6 with thermal ellipsoids set at 30%
probability. Hydrogen atoms, the [B(C6F5)4] anion, and solvent mole-
cules have been omitted for clarity. Selected bond lengths [Š] and
À
the Y P bonds in 2 are retained in both solvents.
À
À
À
À
angles [8]: Lu1 N1 2.258(7), Lu1 P1 2.871(2), Lu1 P2 2.904(2), Lu1
Monitoring the reactions of 1–3 with one equivalent of
1
À
À
À
À
C1 2.336(8), Lu1 O1 2.294(6), Lu1 O2 2.255(6); C1 Lu1 O1 95.4(3),
[PhMe2NH][B(C6F5)4] in C6D5Cl at 258C by H NMR spec-
À
À
À
À
À
À
O1 Lu1 N1 111.0(2), N1 Lu1 P2 68.03(18), P2 Lu1 C1 109.0(2),
P2 Lu1 P1 106.67(7).
troscopy showed the instant disappearance of the signals for
1–3, quantitative formation of SiMe4 and PhMe2N, and the
appearance of new signals assignable to [(PNPPh)Ln-
(CH2SiMe3)(thf)x]+ (x = 0, 1). However, such cationic alkyl
species are unstable and decompose rapidly in C6D5Cl. When
the reactions were carried out in THF, however, the
corresponding bis(thf)-coordinated cationic alkyl complexes
[(PNPPh)Ln(CH2SiMe3)(thf)2][B(C6F5)4] (Ln = Sc (4) , Y 5(),
Lu (6)) could be isolated (Scheme 1). These cationic alkyl
species are more stable, and single crystals of the Lu complex
6 could be grown from chlorobenzene/hexane at À308C. An
X-ray diffraction study revealed that 6 is a separated ion pair
À
À
The cationic alkyl species generated in situ upon treat-
ment of 1–3 with one equivalent of [PhMe2NH][B(C6F5)4] in
C6D5Cl show excellent activity for the polymerization of
isoprene, although they are not stable enough to be isolated.
Addition of 600 equivalents of isoprene to a mixture of 2 and
[PhMe2NH][B(C6F5)4] in C6H5Cl at room temperature
afforded polyisoprene quantitatively in one hour (Table 1,
run 3). More remarkably, the resultant polymer showed a very
1910
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Angewandte
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Table 1: Living cis-1,4-polymerization of isoprene (IP).[a]
Mn[b] (”105)
Mw/Mn
Structure[c]
Tg[d] [8C]
Eff.[e] [%]
[b]
Run
Cat.
Tp [8C]
t [min]
Yield [%]
cis-1,4-
3,4-
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1–3
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
0
60
60
60
60
60
60
10
15
20
40
60
180
10
20
2
0
100
100
100
79
0
40
90
100
150
200
100
90
188
66
100
1/A[f]
2/A[f]
3/A[f]
2/B[f]
2/C[f]
2/A[g]
2/A[g]
2/A[g]
2/A[g,h]
2/A[g,h]
2/A[g]
2/A[i]
2/A[i,j]
2/A[i]
2/A[i]
1.6
2.3
1.9
3.2
1.10
1.10
1.09
1.11
96.5
99.3
97.1
99.3
3.5
0.7
2.9
0.7
À66
À69
À67
À69
26
18
22
10
0.5
1.0
1.2
1.8
2.3
1.3
0.7
1.4
0.6
0.8
1.05
1.05
1.07
1.08
1.08
1.06
1.05
1.05
1.05
1.05
99.3
99.3
99.3
99.3
99.3
99.6
98.7
98.7
98.5
98.5
0.7
0.7
0.7
0.7
0.7
0.4
1.3
1.3
1.5
1.5
À69
À69
À69
À69
À69
À69
À68
À68
À68
À68
33
37
34
34
36
31
53
55
45
51
50
50
80
80
5
[a] Conditions: C6H5Cl (10 mL); Ln (25 mmol); [IP]0/[Ln]0 =600; [Ln]0/[activator]0 =1:1 (activator=[PhMe2NH][B(C6F5)4] (A), [Ph3C][B(C6F5)4] (B),
B(C6F5)3 (C)), unless otherwise noted. [b] Determined by gel permeation chromatography (GPC) with respect to a polystyrene standard.
[c] Determined by 1H and 13C NMR spectroscopy. [d] Measured by differential scanning calorimetry (DSC). [e] Catalyst efficiency = Mn(calculated)/
Mn(measured). [f] In runs 2–6, IP was added to a mixture of 2 and the activator. [g] In runs 7–12, a C6H5Cl solution of activator A was added to a
solution of the neutral complex and IP. [h] After polymerization of 1.022 g (600 equiv) of IP for 30 min, another 1.022 g of IP was added and the mixture
was stirred for a further 10 (run 10) or 30 min (run 11). [i] In runs 13–16, a mixture of 2 and IP in C6H5Cl (3 mL) was added to a preheated C6H5Cl
solution (7 mL) of A. [j] [IP]0/[2]0 =1200.
high cis-1,4 content (99.3%)and very narrow molecular-
weight distribution (Mw/Mn = 1.10). No evidence for trans-1,4
polyisoprene was observed. Similarly, the Sc and Lu com-
plexes 1 and 3 also show high activity and excellent livingness
for the polymerization of isoprene, albeit with slightly lower
cis-1,4 selectivity (96.5–97.1%; Table 1, runs 2 and 4). In
contrast, complexes 1–3 in the absence of activator and the
bis(thf)-coordinated cationic complexes 4–6 are not active
under the same conditions. These results suggest that the true
active species in the present catalyst systems should be a
cationic alkyl species similar to [(PNPPh)Ln(CH2SiMe3)-
(thf)x]+ (x ꢀ 1)with one or fewer thf ligands per metal center.
Complex 2 was then examined further as a catalyst
precursor for the polymerization of isoprene under various
conditions. When the polymerization was carried out by
adding the activator [PhMe2NH][B(C6F5)4] to a mixture of the
neutral complex 2 and isoprene, the catalyst efficiency and
activity were found to be almost twice as high as with the
polymerization reaction that involves addition of isoprene to
a pre-generated cationic species, while the molecular-weight
distribution of the resultant polymers became even narrower
(Mw/Mn = 1.05–1.07; Table 1, runs 7–9). As the isoprene
conversion increased, the molecular weight of the resulting
polymers also increased linearly (Table 1, runs 7–11). Addi-
tion of another 600 equivalents of isoprene to a completed
polymerization mixture yielded a polymer with almost double
the molecular weight and sustained high cis-1,4 selectivity
(99.3%)and narrow molecular-weight distribution ( Mw/Mn =
1.07–1.08; Table 1, runs 9 and 11). More remarkably, the
excellent “livingness” and the high cis-1,4 selectivity of the
present polymerization system are maintained even at high
temperatures (Mw/Mn = 1.05–1.08, cis-1,4 selectivity > 98.5%,
at 50–808C; Table 1, runs 13–16). As far as we are aware, this
is the first example of well-defined, living cis-1,4 polymeri-
zation of isoprene, and also a rare example of a stereospecific,
living polymerization of any olefin monomer at high temper-
ature. These results suggest that although the PNPPh-sup-
ported cationic alkyl species generated by the reaction of 2
with [PhMe2NH][B(C6F5)4] in C6H5Cl is not stable and might
decompose rapidly in the absence of isoprene, the corre-
sponding PNPPh-supported cationic h3-p-butenyl species
formed by the reaction of the alkyl species with isoprene is
thermally stable.
The present catalyst system was also used for the living
cis-1,4 polymerization of butadiene and living cis-1,4 copoly-
merization of butadiene and isoprene to give the correspond-
ing polymers with extremely high cis-1,4 content
(99%)and very narrow molecular weight distribution ( Mw/
Mn ꢀ 1.13), as shown in Scheme 2.[15]
To gain insight into the origin of the exclusive cis-1,4
selectivity exhibited by the present polymerization
system,[9,10] the cationic alkyl scandium species [(PNPPh)Sc-
(CH2SiMe3)]+ was chosen as a model for a computational
study, which showed that coordination of isoprene to the Sc
center in a cis-1,4 fashion is around 5–7 kcalmolÀ1 more
favored than that in a 3,4-fashion; trans-1,4 coordination is
Ph
not accessible. The geometry (or sterics)of the PNP
ancillary ligand therefore seems to play an important role in
determining the regio- and stereoselectivity.[16]
In summary, we have demonstrated that a cationic rare-
earth metal alkyl species bearing a PNP ancillary ligand,
generated by treatment of the corresponding dialkyl com-
Angew. Chem. Int. Ed. 2007, 46, 1909 –1913
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1911

Communications
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Scheme 2. Living cis-1,4-polymerization of butadiene (BD) and its
block copolymerization with isoprene (IP).
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such as [PhMe2NH][B(C6F5)4], can serve as an excellent
catalyst for the living cis-1,4 polymerization of both isoprene
and butadiene as well as for the living cis-1,4 copolymeriza-
tion of isoprene and butadiene. This system affords, for the
first time, polyisoprene, polybutadiene, and poly(isoprene-b-
butadiene)with both an extremely high cis-1,4 content (ca.
99%)and an extremely narrow molecular-weight distribution
(Mw/Mn = 1.13). Such excellent selectivity and livingness can
be maintained even at elevated temperatures (up to 808C),
which is a rare, although not the first, example of stereospe-
cific, living polymerization at high temperature. The isolation
and structural characterization of the cationic alkyl lutetium
complex 6 have provided a suitable structural model for the
active species and for understanding the mechanistic aspects
of the polymerization process. The present catalyst system is
thus the best defined system for the exclusive cis-1,4
polymerization and copolymerization of isoprene and buta-
diene, as far as we are aware. Studies on the polymerization
and copolymerization of dienes and other olefin monomers
by this, and related, catalyst systems are in progress.
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Received: October 24, 2006
Revised: December 7, 2006
Published online: February 2, 2007
[7] An alkyl lithium compound alone can initiate anionic polymer-
ization of butadiene and isoprene, but the cis-1,4 selectivity is
usually poor. For examples, see: A. F. Halasa, D. N. Schulz, D. P.
Tate, V. D. Mochel, Adv. Organomet. Chem. 1980, 18, 55 – 97.
See also ref. [2c].
Keywords: alkyl ligands · block copolymers · lanthanides ·
P ligands · polymerization
.
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broad, uncontrollable molecular-weight distributions (Mw/Mn >
3).
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Angew. Chem. Int. Ed. 2007, 46, 1909 –1913
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