Isoprene polymerization with yttrium amidinate catalysts: Switching the regio- and stereoselectivity by addition of AlMe

Article abstract of DOI:10.1002/anie.200705120

(Chemical Equation Presented) Aluminum switches: The addition of AlMe ₃ to the isoprene polymerization catalyst 1/[Ph₃C] [B(C₆F₅)₄] switches the regio- and stereoselectivity from 3,4-isospecific to 1,4-cis selective (see scheme). A heterotrinuclear Y/Al complex obtained from the reactionof 1 with AlMe ₃ shows 1,4-cis selectivity under similar conditions inthe presence of [Ph₃C][B(C₆F₅)₄]. PI = polyisoprene.

Full text of DOI:10.1002/anie.200705120

Communications
DOI: 10.1002/anie.200705120
Polymerization Catalysts
Isoprene Polymerization with Yttrium Amidinate Catalysts: Switching
the Regio- and Stereoselectivity by Addition of AlMe3**
Lixin Zhang, Masayoshi Nishiura, Masahiro Yuki, Yi Luo, and Zhaomin Hou*
The preparation of polymers with desired microstructures and
properties by controlling the regio- and stereoselectivity of
olefin polymerization is an important research area.
Approaches toward this goal have, to date, mainly involved
modifying the ancillary ligands of metal catalysts. The use of
chelating amidinate ligands as an alternative to cyclopenta-
dienyl ligands in the development of rare-earth-metal
(Group 3 and lanthanide) based polymerization catalysts
has received considerable attention.[1,2] Although the majority
of amidinate-containing rare-earth-metal complexes reported
to date contain two or three amidinate ligands, recent work by
Hessen and co-workers has demonstrated that benzamidi-
nates with bulky substituents at the nitrogen atoms, such as
N,N’-bis(2,6-diisopropylphenyl)benzamidinate
polymerization of isoprene in the presence of one equivalent
of [Ph3C][B(C6F5)4]. More remarkably, however, the regio-
and stereoselectivity of this catalyst system can be switched
from 3,4-isospecific to 1,4-cis selective simply by adding an
alkylaluminum compound, such as AlMe3. Although the
polymerization of isoprene by various catalyst systems has
been studied extensively,[4d,f,5] such a dramatic switching of the
regio- and stereoselectivity is, to our knowledge, unprece-
dented.[6] Isolation of the heterotrinuclear Y/Al complex
[(NCNdipp)Y{(m-Me)2AlMe2}2] (2) from the reaction of 1 with
AlMe3 and its performance in the polymerization of isoprene
are also described.
Treatment of the tris(aminobenzyl)yttrium complex [Y(o-
CH2C6H4NMe2)3][7] with one equivalent of the amidine ligand
N,N’-bis(2,6-diisopropylphenyl)benzamidine (NCNdippH)[8] in
THF or toluene at room temperature overnight affords the
corresponding mono(amidinate) bis(aminobenzyl) complex 1
in 85% yield (Scheme 1). The reaction can be completed in
3 h if it is carried out at 708C. Complex 1 was fully
[PhC(NC6H4iPr2-2,6)2]À, can serve as excellent ancillary
ligands for a series of mono(amidinate)/dialkyl or cationic
mono(amidinate)/alkyl rare-earth-metal complexes.[2d,e] How-
ever, despite the extensive interest in using amidinate rare-
earth-metal complexes as polymerization catalysts, the poly-
merization chemistry reported to date for these complexes
has been limited mainly to that of ethylene and polar
monomers; studies on the polymerization of higher olefins
remain scarce.[1,2] In particular, the use of an amidinate-
ligated rare-earth-metal catalyst for the polymerization of a
conjugated diene, such as isoprene, has not been reported to
date.
We recently found that cationic rare-earth alkyl com-
plexes can serve as excellent catalysts for the polymerization
and copolymerization of various olefins.[3,4] During these
studies, we became interested in the polymerization of
isoprene by cationic rare-earth alkyl complexes bearing a
single amidinate ligand, and report herein that the amidinate-
ligated yttrium complex [(NCNdipp)Y(o-CH2C6H4NMe2)2] (1;
NCNdipp = PhC(NC6H4iPr2-2,6)2) is a unique catalyst precur-
sor for the polymerization of isoprene. Thus, complex 1 shows
extremely high activity and excellent 3,4-isospecificity for the
1
characterized by H and 13C NMR spectroscopy, elemental
analysis, and X-ray crystallography (Figure 1).[9] The NCNdipp
unit in 1 is bonded to the Y center through its two N atoms, as
observed in other amidinate complexes.[2] The two amino-
benzyl groups are bonded to the Yatom in a chelating fashion
through both the N atom and the benzyl carbon atom.
Intramolecular coordination of the amino group means that
complex 1 does not possess a THF co-ligand, in contrast with
the THF-containing CH2SiMe3 analogue [(NCNdipp)Y-
(CH2SiMe3)2(thf)].[2d] Complex
1 is slightly soluble in
hexane but highly soluble in toluene and THF.
The neutral complex 1 does not catalyze the polymeri-
zation of isoprene but it becomes extremely active in the
presence of one equivalent of [Ph3C][B(C6F5)4], whereby it
converts 750 equivalents of isoprene quantitatively into
polyisoprene in 2 min at room temperature. This reaction
proceeds with high 3,4-regioselectivity (91%) and some
degree of isotacticity (mm ꢀ 50%) (Table 1, entry 3).[10]
When the polymerization is carried out at low temperature
(ꢁ À108C), an even higher regio- and stereoselectivity is
[*] Dr. L. Zhang, Dr. M. Nishiura, Dr. M. Yuki, Dr. Y. Luo, Prof. Dr. Z. Hou
Organometallic Chemistry Laboratory RIKEN (The Institute of
Physical and Chemical Research)
Hirosawa 2-1, Wako, Saitama 351-0198 (Japan)
Fax: (+81)48-462-4665
E-mail: houz@riken.jp
[**] This work was partly supported by a Grant-in-Aid for Scientific
Research on Priority Areas (grant no. 18065020, “Chemistry of
Concerto Catalysis”) from the Ministry of Education, Culture,
Sports, Science and Technology of Japan, and a Grant-in-Aid for
Scientific Research (A) (grant no. 18205010) from the Japan Society
forthe Promotion of Science.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Synthesis of the amidinate-ligated bis(aminobenzyl)yttrium
complex.
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Angew. Chem. Int. Ed. 2008, 47, 2642 –2645

Angewandte
Chemie
the 1/[Ph3C][B(C6F5)4] system changed the regio- and stereo-
selectivity of the polymerization dramatically from 3,4-
isospecific to 1,4-cis selective (Table 1, entries 8–11). In the
presence of five equivalents of AlMe3 at À108C or below, the
1,4-cis selectivity is as high as 98% and the reaction produces
1,4-cis-polyisoprene with an Mn of up to 4.01 ” 105 and an Mw/
Mn ratio of around 1.7 (Table 1, entries 13 and 14). The
polymerization system remains 3,4-selective in the presence
of two or fewer equivalents of AlMe3 (Table 1, entries 12). For
comparison, the combination 1/AlMe3 does not show catalytic
activity for isoprene polymerization in the absence of [Ph3C]
[B(C6F5)4] (Table 1, entries 15), while the combination
AlMe3/[Ph3C][B(C6F5)4] yields polyisoprene containing 1,4-
trans units as the main component (Table 1, entry 16).
To gain a better understanding of the effect of AlMe3
observed in the above polymerizations, the reaction of 1 with
AlMe3 was examined further. Thus, complex 1 reacts rapidly
with five equivalents of AlMe3 in toluene at room temper-
ature to give the heterotrinuclear Y/Al complex 2 in 80%
yield (Scheme 2).[12] As shown by an
Figure 1. ORTEP structure of 1 (thermal ellipsoids set at 30% proba-
bility). Hydrogen atoms have been omitted for clarity. Selected bond
À
À
À
lengths [Š] and angles [8]: Y1 N1 2.577(3), Y1 N2 2.610(4), Y1 N3
À
À
À
À À
2.369(3), Y1 N4 2.377(3), Y1 C1 2.439(4), Y1 C10 2.413(4); C1 Y1
À
À
À
À
C10 128.29(14), N3 Y1 N4 55.62(10), N3 C19 N4 111.1(3).
X-ray diffraction study, complex 2
can formally be viewed as a combi-
Table 1: Polymerization of isoprene by amidinate-ligated yttrium catalysts.[a]
nation of one molecule of
[(NCNdipp)YMe2] with two mole-
cules of AlMe3 (Figure 2).[9] The Y
atom and each of the two Al atoms
[b]
Entry Complex
AlR3
Tp
t
Yield
Mn
Mw/
Mn
3,4/1,4-cis/1,4- Tg[d]
in this complex are bridged by two
Me groups, and a crystallographic
twofold axis passes through the Y
atom and the central carbon atom
of the NCNdipp amidinate unit. The
eight methyl groups in 2 exhibit
only one singlet at around d =
(equiv)[a] [8C] [min] [%]
(”104)
trans[c]
[8C]
[b]
1[e] 1 or 2
–
–
–
–
–
25 120
25 120
0
80
–
–
–
–
2
3
4
5
6
7
8
9
10
11
12
13
14
15[e]
16
17
18
–
1
1
1
1
1
1
1
1
1
1
1
1
1
–
2
2
0.3
13.7
18.3
16.5
9.9
6.1
6.6
6.3
7.0
7.1
9.2
17.6
40.1
–
7.9
1.3
1.3
1.4
1.3
1.2
1.6
1.5
1.6
1.5
1.5
1.6
1.7
–
5/15/80
91/9/0
–
23
25
À10
À20
25
2
20
20
10
10
10
10
10
10
10
60
100
100
100
100
100
100
100
100
100
100
82
100
0
60
100
100
99/1/0
24
99.5[f]/0.5/0
96/3.9/0.1
96/4/0
22[g]
21
AliBu3 (5)
AlEt3 (5)
AlMe3 (5)
AlMe3 (10)
AlMe3 (4)
AlMe3 (3)
AlMe3 (2)
1
0.02 ppm in the H NMR spectrum
25
25
25
25
25
25
22
in [D8]toluene at both room tem-
perature and À608C, thereby sug-
gesting a rapid exchange between
the bridging and terminal methyl
groups in solution.[13]
Complex 2 alone is not an active
isoprene polymerization catalyst.
However, in the presence of one
equivalent of [Ph3C][B(C6F5)4] at
room temperature it rapidly produ-
ces polyisoprene with around 78%
3/91/6
3/91/6
4/90/6
9/90/1
86/14/0
1/98/1
1/>98/<1
–
À63
À63
À59
À59
21
AlMe3 (5) À10
À66
À67
–
AlMe3 (5) À20 960
AlMe3 (5)
AlMe3 (5)
–
25 120
25 120
0.8
6.8
7.5
4.7
1.8
1.6
7/8/85
–
25
25
10
10
22/77.8/0.2
6/92/2
À57
AlMe3 (1)
À59
[a] Conditions: C6H5Cl (10 mL); 1 or 2 (Y, 20 mmol); isoprene (1.022 g); [Y]0/[Ph3C][B(C6F5)4]0 =1:1,
equiv=[AlR3]0/[Y]0, unless otherwise noted. [b] Determined by GPC with respect to a polystyrene
standard. [c] Determined by 1H and 13C NMR spectroscopy. [d] Determined by DSC. [e] Without [Ph3C]
[B(C6F5)4]. [f] mm=100%, mmmm=99%. [g] Tm =1428C was also observed.
1,4-cis
selectivity
(Table 1,
entry 17), which contrasts with the
3,4-selectivity of the aluminum-free
achieved and almost perfectly isotactic 3,4-polyisoprene is
obtained (3,4-selectivity up to 99.5%, mmmm up to 99%;
Table 1, entries 4 and 5).
In a subsequent reaction AlR3 (R = iBu, Et, Me) was
added to the catalyst system to see whether alkylaluminum
compounds have any influence on the polymerization. The
addition of AliBu3 or AlEt3 had little effect on the polymer-
ization (Table 1, entries 6 and 7, respectively).[11] To our
surprise, however, addition of AlMe3 ([AlMe3]0/[1]0 ꢂ 3) to
Scheme 2. Synthesis of the heterotrinuclear Y/Al complex.
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Communications
Figure 2. ORTEP structure of 2 (thermal ellipsoids set at 30% proba-
bility). Hydrogen atoms have been omitted for clarity. Selected bond
À
À
À
lengths [Š] and angles [8]: Y1 N1 2.3157(15), Y1 C1 2.542(2), Y1 C2
À
À
À
À
2.523(2), Al1 C1 2.071(2), Al1 C2 2.041(3), Al1 C3 1.963(2), Al1 C4
À
À
À
À
À À
1.967(2); C1 Y1 C2 81.94(8), C1 Y1 C1* 165.04(10), C1 Y1 C2*
89.71(8), C1 Y1 N1 96.02(6), C1 Y1 N1* 97.09(6), C2 Y1 C1*
89.71(8), C2 Y1 C2* 112.18(11), C2 Y1 N1 95.20(7), C2 Y1 N1*
152.57(7), N1 Y1 N1* 57.51(8), N1 C5 N1* 112.0(2).
À
À
À
À
À À
À
À
À
À
À À
À
À
À À
system 1/[Ph3C][B(C6F5)4] (Table 1, entry 3). Addition of one
equivalent of AlMe3 to the 2/[Ph3C][B(C6F5)4] system further
raises the 1,4-cis selectivity to 92% (Table 1, entry 18), which
is close to that (ca. 91% 1,4-cis selectivity) obtained in the
1/[Ph3C][B(C6F5)4]/AlMe3 ([AlMe3]0/[1]0 ꢂ 3) three-compo-
nent systems (Table 1, entries 8–11). These results suggest
that the active species for the 1,4-cis polymerization of
isoprene in the present catalyst systems is likely to be a
cationic Y/Al heteronuclear species.
Scheme 3. A possible mechanism forthe isospecific 3,4-polymerization
of isoprene by 1/[Ph3C][B(C6F5)4].
A possible mechanism for the isospecific 3,4-polymeri-
zation of isoprene by 1/[Ph3C][B(C6F5)4] can be proposed on
the basis of the results observed above and those reported
previously (Scheme 3).[2d,e,4h] Thus, the reaction of 1 with
[Ph3C][B(C6F5)4] gives a cationic aminobenzyl species such as
1a.[2d,e,4h,14] The steric hindrance and C2 symmetry of the
(NCNdipp)Y unit means that coordination of isoprene to the Y
center in 1a takes place preferably in a 3,4-h2 fashion through
the si face to give 1b.[4d,15] Subsequent migratory addition of
the aminobenzyl group to the coordinated isoprene in 1b
affords 1c, followed by multiple subsequent 3,4-coordination
and isoprene insertion steps, leads to the formation of
isotactic 3,4-polyisoprene.
The reaction of 1/AlMe3 or 2 with [Ph3C][B(C6F5)4]
probably first yields a cationic, heterotrinuclear Y/Al species,
such as 2a (Scheme 4).[12,16] The coordination of isoprene
probably takes place at a heterobimetallic Y/Al unit in a m-cis-
1,4-h4 fashion by replacement of an AlMe3 unit in 2a to give
2b. The 1,4-addition of a methyl group to the coordinated
isoprene gives a m-p-anti-allyl species, such as 2c, which could
be in an equilibrium with the m-s-allyl species 2d. Subsequent
repeated isoprene coordination and insertion would yield 1,4-
cis-polyisoprene.
Scheme 4. A possible mechanism forthe 1,4- cis-polymerization of
isoprene by 2/[Ph3C][B(C6F5)4] or 1/[Ph3C][B(C6F5)4]/AlMe3.
In summary, we have demonstrated that an amidinate-
ligated aminobenzylyttrium complex such as 1, in combina-
tion with [Ph3C][B(C6F5)4], is an excellent catalyst system for
the isospecific 3,4-polymerization of isoprene. More remark-
ably, the regio- and stereoselectivity of this polymerization
system can be dramatically switched from 3,4-isospecific to
1,4-cis selective by addition of AlMe3. Isolation of the
heterotrinuclear Y/Al complex 2 from the reaction of 1
with AlMe3, and the 1,4-cis specificity of the 2/[Ph3C]
[B(C6F5)4] system for the polymerization of isoprene, suggests
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Organometallic Compounds, Vol. 1 (Eds.: B. Cornils, W. A.
that this selectivity switching from 3,4-isospecific to 1,4-cis
selective could be due to the involvement of a cationic,
heterobimetallic Y/Al catalyst species. Although alkylalumi-
num compounds have often been used as additives or co-
catalysts in various isoprene polymerization catalyst systems,
this is the first report to unambiguously show that the
incorporation of an alkylaluminum species can dramatically
change the regio- and stereoselectivity of a catalyst system.
Herrmann), Wiley-VCH, Weinheim, 1996, pp. 280 – 318; d) S.
Kaita, Y. Doi, K. Kaneko, A. C. Horiuchi, Y. Wakatsuki,
Macromolecules 2004, 37, 5860 – 5862; e) S. Arndt, K. Beckerle,
P. M. Zeimentz, T. P. Spaniol, J. Okuda, Angew. Chem. 2005, 117,
7640 – 7644; Angew. Chem. Int. Ed. 2005, 44, 7473 – 7477; f) W. J.
Evans, T. M. Champagne, D. G. Giarikos, J. W. Ziller, Organo-
metallics 2005, 24, 570 – 579; g) W. J. Evans, T. M. Champagne,
D. G. Giarikos, J. W. Ziller, Chem. Commun. 2005, 5925 – 5927;
h) A. Fischbach, C. Meermann, G. Eickerling, W. Scherer, R.
Anwander, Macromolecules 2006, 39, 6811 – 6816; i) A. Fisch-
bach, F. Perdih, E. Herdtweck, R. Anwander, Organometallics
2006, 25, 1626 – 1642; j) N. Ajellal, L. Furlan, C. M. Thomas,
O. L. Casagrande, Jr., J. F. Carpentier, Macromol. Rapid
Commun. 2006, 27, 338 – 343; k) C. Meermann, K. W. Törnroos,
W. Nerdal, R. Anwander, Angew. Chem. 2007, 119, 6628 – 6633;
Angew. Chem. Int. Ed. 2007, 46, 6508 – 6513; l) M. Zimmermann,
N. Š. Frøystein, A. Fischbach, P. Sirsch, H. M. Dietrich, K. W.
Törnroos, E. Herdtweck, R. Anwander, Chem. Eur. J. 2007, 13,
8784 – 8800; m) Y. Yang, B. Liu, K. Lv, W. Gao, D. Cui, X. Chen,
X. Jing, Organometallics 2007, 26, 4575 – 4584.
Received: November 6, 2007
Revised: December 6, 2007
Published online: February 27, 2008
Keywords: aluminum · homogeneous catalysis · N ligands ·
.
polymerization · yttrium
[1] Selected reviews: a) Z. Hou, Y. Wakatsuki, Coord. Chem. Rev.
2002, 231, 1 – 22; b) W. E. Piers, D. J. H. Emslie, Coord. Chem.
Rev. 2002, 233, 131 – 155; c) F. T. Edelmann, D. M. M. Freck-
mann, H. Schumann, Chem. Rev. 2002, 102, 1851 – 1896; d) J.
Gromada, J. F. Carpentier, A. Mortreux, Coord. Chem. Rev.
2004, 248, 397 – 410; e) Y. Nakayama, H. Yasuda, J. Organomet.
Chem. 2004, 689, 4489 – 4498; f) P. M. Zeimentz, S. Arndt, B. R.
Elvidge, J. Okuda, Chem. Rev. 2006, 106, 2404 – 2433.
[6] Switching the selectivity of isoprene polymerization from 1,4-cis
(75%) to 1,4-trans (90%) by replacing the MAO activator with
AlEt3 or AliBu3 in an yttrium-based catalyst system has been
reported previously.[5j] A similar reaction was also observed for
butadiene polymerization: R. Taube, H. Windisch, S. Maiwald,
H. Hemling, H. Schumann, J. Organomet. Chem. 1996, 513, 49 –
61.
[2] a) K. B. Aubrecht, K. Chang, M. A. Hillmyer, W. B. Tolman, J.
Polym. Sci. Part A: Polym. Chem. 2001, 39, 284 – 293; b) Y. Luo,
Y. Yao, Q. Shen, J. Sun, L. Weng, J. Organomet. Chem. 2002, 662,
144 – 149; c) Y. Luo, Y. Yao, Q. Shen, K. Yu, L. Weng, Eur. J.
Inorg. Chem. 2003, 318 – 323; d) S. Bambirra, D. van Leusen, A.
Meetsman, B. Hessen, J. H. Teuben, Chem. Commun. 2003, 522 –
523; e) S. Bambirra, M. Bouwkamp, A. Meetsman, B. Hessen, J.
Am. Chem. Soc. 2004, 126, 9182 – 9183; f) C. Villiers, P. Thuery,
M. Ephritikhine, Eur. J. Inorg. Chem. 2004, 4624 – 4632; g) A. A.
Trifonov, E. A. Fedorova, G. K. Fukin, M. N. Bochkarev, Eur. J.
Inorg. Chem. 2004, 4396 – 4401; h) M. L. Cole, P. C. Junk, Chem.
Commun. 2005, 2695 – 2697; i) S. Bambirra, A. Meetsma, B.
Hessen, Organometallics 2006, 25, 3454 – 3462; j) S. Bambirra,
H. Tsurugi, D. van Leusen, B. Hessen, Dalton Trans. 2006, 1157 –
1161; k) A. A. Trifonov, G. G. Skvortsov, D. M. Lyubov, N. A.
Skorodumova, G. K. Fukin, E. V. Baranov, V. N. Glushakova,
Chem. Eur. J. 2006, 12, 5320 – 5327; l) C. Li, Y. Wang, L. Zhou, H.
Sun, Q. Shen, J. Appl. Polym. Sci. 2006, 102, 22 – 28.
[7] S. Harder, Organometallics 2005, 24, 373 – 379.
[8] S. Ogata, A. Mochizuki, M.-A. Kakimoto, Y. Imai, Bull. Chem.
Soc. Jpn. 1986, 59, 2171 – 2177.
[9] CCDC-657217 (1) and CCDC-657218 (2) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[10] The use of the THF-containing CH2SiMe3 analogue
[(NCNdipp)Y(CH2SiMe3)2(thf)] instead of
1 gave a similar
result in terms of selectivity but with a lower activity. The
polymerization in toluene also showed a similar selectivity,
although the activity was somewhat lower than that in chlor-
obenzene because of lower solubility.
[11] The molecular weight of the resulting polymers decreased when
AlR3 was added, probably a result of chain transfer to the
aluminum atom.
[12] The reaction of 1 with AliBu3 or AlEt3 was much slower than
that with AlMe3. This could be a reason why AliBu3 or AlEt3
have little effect on the polymerization selectivity.
[13] For similar complexes, see: a) R. Anwander, M. G. Klimpel,
H. M. Dietrich, D. J. Shorokhov, W. Scherer, Chem. Commun.
2003, 1008 – 1009; b) H. M. Dietrich, C. Zapilko, E. Herdtweck,
R. Anwander, Organometallics 2005, 24, 5767 – 5771.
[14] Formation of the cationic species [(NCNdipp)Y(-o-
CH2C6H4NMe2)(thf)x][B(C6F5)4] was confirmed by 1H NMR
spectroscopy in [D8]THF: d = 7.18–6.59 (m, 15H; aromatic),
3.33 (m, 4H; -CHMe2), 2.51 (s, 6H; -o-CH2C6H4NMe2), 2.07 (s,
2H; -o-CH2C6H4NMe2), 1.30 (d, J = 3 Hz, 3H; -CHMe2),
0.80 ppm (d, J = 3 Hz, 3H; -CHMe2). The THF-free analogue
is unstable.
[15] Computational studies suggest that only the 3,4-coordination of
isoprene to the metal center in 1a is permitted as the 1,4-
coordination mode (either cis or trans) is not accessible. See the
Supporting Information for details.
[16] Attempts to isolate a cationic heterobimetallic Y/Al species
from either the reaction of 2 with [Ph3C][B(C6F5)4] or the
reaction of 1/[Ph3C][B(C6F5)4] with AlMe3 have not yet been
successful because of its instability.
[3] For a review, see: Z. Hou, Y. Luo, X. Li, J. Organomet. Chem.
2006, 691, 3114 – 3121.
[4] a) Y. Luo, J. Baldamus, Z. Hou, J. Am. Chem. Soc. 2004, 126,
13910 – 13911; b) X. Li, J. Baldamus, Z. Hou, Angew. Chem.
2005, 117, 984 – 987; Angew. Chem. Int. Ed. 2005, 44, 962 – 965;
c) X. Li, Z. Hou, Macromolecules 2005, 38, 6767 – 6769; d) L.
Zhang, Y. Luo, Z. Hou, J. Am. Chem. Soc. 2005, 127, 14562 –
14563; e) X. Li, J. Baldamus, M. Nishiura, O. Tardif, Z. Hou,
Angew. Chem. 2006, 118, 8364 – 8368; Angew. Chem. Int. Ed.
2006, 45, 8184 – 8188; f) L. Zhang, T. Suzuki, Y. Luo, M.
Nishiura, Z. Hou, Angew. Chem. 2007, 119, 1941 – 1945;
Angew. Chem. Int. Ed. 2007, 46, 1909 – 1913; g) Y. Luo, M.
Nishiura, Z. Hou, J. Organomet. Chem. 2007, 692, 536 – 544;
h) X. Li, M. Nishiura, K. Mori, T. Mashiko, Z. Hou, Chem.
Commun. 2007, 4137 – 4139.
[5] For examples, see: a) Z. Shen, J. Ouyang in Handbook on the
Physics and Chemistry of Rare Earths (Eds.: K. A. Gschneid-
ner, Jr., L. Fleming), Elsevier Science Publishers, Dordrecht,
1987, chap. 61; b) L. Porri, A. Giarrusso in Comprehensive
Polymer Science, Vol. 4 (Eds.: G. C. Eastmond, A. Ledwith, S.
Russo, P. Sigwalt), Pergamon, Oxford, 1989, pp. 53 – 108; c) R.
Taube, G. Sylvester in Applied Homogeneous Catalysis with
Angew. Chem. Int. Ed. 2008, 47, 2642 –2645
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www.angewandte.org
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