Alternating and random copolymerization of isoprene and ethylene catalyzed by cationic half-sandwich scandium alkyls

Article abstract of DOI:10.1021/ja9056213

The acid-base reactions between the scandium trialkyl complex Sc(CH ₂SiMe₃)₃(THF)₂ and 1 equiv of Cp′-H afforded straightforwardly the corresponding mono(cyclopentadienyl) scandium dialkyl complexes Cp′Sc(CH

Full text of DOI:10.1021/ja9056213

Published on Web 09/03/2009
Alternating and Random Copolymerization of Isoprene and
Ethylene Catalyzed by Cationic Half-Sandwich Scandium
Alkyls
Xiaofang Li,†,‡ Masayoshi Nishiura,† Lihong Hu,§ Kyouichi Mori,† and
Zhaomin Hou*,†
Organometallic Chemistry Laboratory, RIKEN AdVanced Science Institute, 2-1 Hirosawa, Wako,
Saitama 351-0198 Japan, Department of Chemistry, Beijing Institute of Technology, Beijing
100081, P. R. China, and Department of Chemistry and Centre of Theoretical and
Computational Physics, UniVersity of Hong Kong, Hong Kong, P. R. China
Received July 8, 2009; E-mail: houz@riken.jp
Abstract: The acid-base reactions between the scandium trialkyl complex Sc(CH2SiMe3)3(THF)2 and 1
equiv of Cp′-H afforded straightforwardly the corresponding mono(cyclopentadienyl)scandium dialkyl
complexes Cp′Sc(CH2SiMe3)2(THF) (Cp′ ) C5H5 (1), C5MeH4 (2), C5Me4H (3), C5Me5 (4), C5Me4SiMe3 (5))
in 65-80% isolated yields. The analogous half-sandwich complexes having a heteroatom-containing side
arm, (C5Me4R)Sc(CH2SiMe3)2 (R ) CH2CH2PPh2 (6), C6H4OMe-ꢀ (7)), were obtained by the one-pot
metathetical reactions of ScCl3(THF)3 with 1 equiv of the potassium salts of the ligands and 2 equiv of
LiCH2SiMe3. The similar reactions of ScCl3(THF)3 with KC5Me4(C6H4NMe2-ꢀ) and LiCH2SiMe3 gave a
methylene-bridged binuclear complex [{C5Me4(ꢀ-C6H4N(Me)CH2-µ}Sc(CH2SiMe3)]2 (8). Complexes 1-8 were
1
fully characterized by H, 13C NMR, X-ray, and microelemental analyses. The reactions of 5 and 7 with 1
equiv of [PhMe2NH][B(C6F5)4] in THF afforded quantitatively the structurally characterizable cationic
monoalkyl complexes [(C5Me4SiMe3)Sc(CH2SiMe3)(THF)2][B(C6F5)4] (10) and [(C5Me4C6H4OMe-
ꢀ)Sc(CH2SiMe3)(THF)2][B(C6F5)4] (11), respectively. In the presence of an activator such as [Ph3C][B(C6F5)4],
[PhMe2NH][B(C6F5)4], or B(C6F5)3, all of the half-sandwich dialkyl complexes 1-7 were active for isoprene
polymerization and isoprene-ethylene copolymerization, with the activity and selectivity being significantly
dependent on the substituents at the cyclopentadienyl ligands to yield the corresponding homo- and
copolymer materials with different microstructures and compositions. In the homopolymerization of isoprene,
the less sterically demanding complexes 1 and 2 showed high cis-1,4 selectivity (up to 95%), whereas the
more sterically demanding complexes 3-5 yielded 3,4-polyisoprene (51-65%) as a major product. The
ether side arm coordinated complex 7 preferred trans-1,4-polyisoprene formation (60-79%), whereas the
phosphine analogue 6 showed high cis-1,4 selectivity (84-90%) under the same conditions. In the
copolymerization of isoprene and ethylene, complexes 1 and 2 afforded the random copolymers with high
isoprene contents (85-92 mol %) and predominant cis-1,4-microstructures (up to 90%), thus constituting
the first example of cis-1,4-selective copolymerization of isoprene with ethylene. In contrast, the
copolymerization of isoprene and ethylene by 3, 4, 6, and 7 gave, for the first time, almost perfect alternating
isoprene-ethylene copolymers. Possible mechanisms of the polymerization and copolymerization processes
were proposed on the basis of the DFT calculations.
Introduction
resistance, low environmental stress cracking, and good rubber
compatibility. Further functionalization of the remaining C-C
The precise control of the copolymerization of conjugated
dienes (butadiene, isoprene) with ethylene is an attractive
research subject and has drawn continued attention from both
academic and industrial researchers for more than half a century,
because of the large availability and low cost of the starting
monomers and the high potential of industrial application of
the resulting copolymer products.1-7 The copolymers of con-
jugated dienes and ethylene are expected to show various novel
properties, such as high tensile and tear strength, high impact
double bonds in the copolymers could also be possible to afford
a broad range of new functionalized polymers with improved
properties. However, the copolymerization of conjugated dienes
(1) For examples of alternating copolymerization of ethylene with
butadiene, see: (a) Natta, G.; Zambelli, A.; Pasquon, I.; Cianpelli, F.
Makromol. Chem. 1964, 79, 161. (b) Furukawa, J. J. Polym. Sci. 1972,
10, 3027. (c) Arnold, M.; Wohlfahrt, L.; Frank, W.; Reinhold, G.
Makromol. Chem. 1991, 192, 1017. (d) Pragliola, S.; Costabile, C.;
Magrino, M.; Napoli, M.; Longo, P. Macromolecules 2004, 37, 238.
(e) Llauro, M. F.; Monnet, C.; Barbotin, F.; Monteil, V.; Spitz, R.;
Boisson, C. Macromolecules 2001, 34, 6304. (f) Thuilliez, J.; Monteil,
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4573.
† RIKEN Advanced Science Institute.
‡ Beijing Institute of Technology.
§ University of Hong Kong, Hong Kong.
9
13870 J. AM. CHEM. SOC. 2009, 131, 13870–13882
10.1021/ja9056213 CCC: $40.75  2009 American Chemical Society

Copolymerization of Isoprene and Ethylene
A R T I C L E S
Scheme 1. Polymerization of Isoprene Leading to Polymers with
Various Microstructures
with ethylene is usually rather difficult, because these two classes
of monomers show very different reactivity characteristics for
a given catalyst and in some cases conjugated dienes could act
as poisons for the catalysts designed for ethylene or higher olefin
polymerization.1f,3a,8 So far, many studies have been carried out
in this area, but most of them were limited to the copolymer-
ization of butadiene (the smallest, most reactive 1,3-conjugated
diene) with ethylene.1-4 In contrast, the copolymerization of
the more sterically demanding isoprene (2-methyl-1,3-butadiene)
with ethylene has hardly been investigated.6,7 In particular, the
precise control of the isoprene content and microstructure of
the isoprene-ethylene copolymers has remained a challenge
to date.
Isoprene is a very attractive and interesting conjugated diene.
It is industrially available for a low price, and moreover, the
methyl substituent on the conjugated diene could force it to be
polymerized in unusual reaction paths, leading to new polymeric
structures. Generally, the homopolymerization of isoprene can
yield polyisoprenes with various microstructures which show
different chemical, physical, and mechanical properties (cf.
Scheme 1).9-12 cis-1,4-Polyisoprene is a major component of
natural rubber and acts as an excellent elastomer, whereas trans-
1,4-polyisoprene and iso-3,4-polyisoprene are crystalline poly-
mers. 1,2-Polyisoprene is usually not available in coordination
polymerization because of the steric reason. In consideration
of the limited supply of natural rubber and the increasing
demands for various high-performance synthetic rubbers, the
development of high-quality elastomers via (co)polymerization
of isoprene has grown in importance. So far, a large number of
catalysts has been reported for the homopolymerization of
isoprene.9-12 However, catalysts suitable for the copolymeri-
zation of isoprene with ethylene remained scarce. Very recently,
Carpentier and co-workers described briefly the copolymeriza-
tion of isoprene with ethylene by an ansa-neodymocene allyl
catalyst,whichyieldedanisoprene-rich(76mol%)isoprene-ethylene
copolymer with predominant trans-1,4-polyisoprene microstruc-
tures.6 Visseaux and co-workers reported the incorporation of
some amount of R-olefins (6-10 mol %) or nonconjugated
dienes (4-8 mol %) into trans-1,4-polyisoprene by a Sm/Li
bimetallic allyl catalyst (Me2CC5H4)2Sm(C3H5)2Li(DME) (DME
) MeOCH2CH2OMe).7 The cis-1,4- or 3,4-selective copolym-
erization of isoprene with ethylene has not been reported
previously, and neither has the alternating copolymerization of
these two monomers, as far as we are aware. The search for
(2) For examples of block copolymerization of ethylene with butadiene,
see: (a) Cucinella, S.; De Chirico, A.; Mazzei, A. Eur. Polym. J. 1976,
12, 65. (b) Bruzzone, M.; Carbonaro, A.; Corno, C. Makromol. Chem.
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Bull. 1982, 8, 473. (e) Sun, L.; Lu, Z.; Lin, S. J. Polym. Sci., Part B
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Symp. 1986, 4, 103. (b) Barbotin, F.; Monteil, V.; Llauro, M.-F.;
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C.; Monteil, V.; Thuilliez, J.; Spitz, R.; Monnet, C; Llauro, M. F.;
Barbotin, F.; Robert, P. Macromol. Symp. 2005, 226, 17.
(4) For examples of cyclocopolymerization of ethylene with butadiene,
see: (a) Galimberti, M.; Albizzati, E. Makromol. Chem. 1991, 192,
2591. (b) Pragliola, S.; Milano, G.; Guerra, G.; Longo, P. J. Am. Chem.
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G. J. Am. Chem. Soc. 2003, 125, 4799. (d) Choo, T. N.; Waymouth,
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(10) For examples of cis-1,4-polymerization of isoprene, see: (a) Scott,
K. W. Rubber Plast. Age 1961, 42, 175. (b) Shen, Z.; Ouyang, J.;
Wang, F.; Hu, Z.; Yu, F.; Qian, B. J. Polym. Sci. Polym. Chem. Ed.
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Ed. 2004, 43, 2234. (g) Kaita, S.; Doi, Y.; Kaneko, K.; Horiuchi, A. C.;
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Chem., Int. Ed. 2007, 46, 6508. (k) Zhang, L.; Suzuki, T.; Luo, Y.;
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(5) For examples of copolymerization of R-olefin with butadiene, see:
(a) Kaulbach, R.; Gebauer, U.; Gehrke, K.; Lechner, M. D; Hummel,
K. Angew. Makromol. Chem. 1995, 226, 101. (b) Kawasaki, A.;
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earth coordination catalysts in stereospecific polymerization). (b) Porri,
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Dormond, A.; Pardes, C.; Richard, P.; Visseaux, M.; Zhu, C. Eur.
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9
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A R T I C L E S
Li et al.
Scheme 2. Synthesis of Half-Sandwich Scandium Dialkyl
Complexes Bearing Mono(cyclopentadienyl) Ligands
more efficient, selective catalysts for the copolymerization of
isoprene with ethylene is therefore of significant importance and
interest.
We have recently reported that the cationic scandium alkyl
species, generated by the reaction of the half-sandwich dialkyl
precursors such as (C5Me4SiMe3)Sc(CH2SiMe3)2(THF) with 1
equiv of a borate compound such as [Ph3C][B(C6F5)4], can serve
as excellent catalysts for the polymerization and copolymeri-
zation of a wide range of olefin monomers, such as syndiospe-
cific copolymerization of styrene with isoprene (or ethylene),
alternating copolymerization of norbornene (or dicyclopenta-
diene) with ethylene, and terpolymerization of norbornene (or
dicyclopentadiene), ethylene, and styrene.13,14 During these
studies, we found that the scandium catalysts of this type are
also active for the copolymerization of isoprene with ethylene
to afford the corresponding copolymers unavailable previously.15
To gain more insight into the structure-activity relation of the
catalysts and to develop more efficient, selective catalysts for
the copolymerization of isoprene and ethylene, we then syn-
thesized a new series of half-sandwich scandium dialkyl
complexes bearing the cyclopentadienyl (Cp) ligands with
different substituents. In this article, we report a detailed,
systematic study on the copolymerization of isoprene with
ethylene catalyzed by the cationic half-sandwich scandium alkyl
species generated from these dialkyl precursors. By modifying
the Cp ligands, both random and alternating isoprene-ethylene
copolymerization have been achieved for the first time to afford
a new family of isoprene-ethylene copolymers with a wide
range of isoprene contents and controlled microstructures. The
isolation and structural characterization of some of the cationic
scandium alkyl species and DFT calculations on the coordination
and insertion of isoprene at the scandium metal center are also
carried out, which shed new light on the mechanistic aspects
of the polymerization processes.
Scheme 3. Synthesis of Half-Sandwich Scandium Dialkyl
Complexes Bearing Cyclopentadienyl Ligands with a Phosphine or
Ether Side Arm
Formation of a bis(cyclopentadienyl) compound of type
“Cp′2ScR” was not observed even in the case of the smallest
C5H5 ligand.16 In contrast, the analogous reaction of
Sc(CH2SiMe3)3(THF)2 with cyclopentadiene derivatives having
a heteroatom-containing side arm such as HC5Me4CH2CH2PPh2
and HC5Me4(C6H4OMe-ꢀ) was very slow. Alternatively, the
one-pot metathetical reactions of ScCl3(THF)3 with 1 equiv of
the potassium salts of the ligands followed by addition of 2
equiv of LiCH2SiMe3 in THF at 25 °C afforded easily the
corresponding half-sandwich scandium dialkyl complexes
(C5Me4R)Sc(CH2SiMe3)2 (6: R ) CH2CH2PPh2, 86%; 7: R )
C6H4OMe-ꢀ, 84%) (Scheme 3). The similar reactions of
ScCl3(THF)3 with KC5Me4(C6H4NMe2-ꢀ) and LiCH2SiMe3 gave
amethylene-bridgedbinuclearcomplex[{C5Me4(ꢀ-C6H4N(Me)CH2-
µ}Sc(CH2SiMe3)]2 (8), probably through dehydrogenation of a
methyl group of the NMe2 unit by a Sc-CH2SiMe3 unit in the
initially formed half-sandwich dialkyl species followed by
intermolecular interaction between the resulting methylene unit
and the Sc atom (Scheme 4). The reactions of KC5Me4(C6H4OiPr-
ꢀ) with ScCl3(THF)3 and LiCH2SiMe3 yielded unidentified
insoluble precipitates under the similar conditions probably due
to the similar subsequent C-H bond cleavage reactions.
Complexes 1-8 were fully characterized by 1H and 13C NMR,
X-ray, and microelemental analyses. The ORTEP drawings of
1-8 are shown in Figure 1. Their selected bond lengths and
angles are summarized in Table 1. Despite the large difference
in steric hindrance of the Cp′ ligands in these complexes,
complexes 1-7 adopt a similar overall structure, in which the
Sc metal center is boned to one Cp′ unit, two alkyls, and one
Lewis base. In 1-5, THF serves as a Lewis base ligand, whereas
in 6 and 7 a heteroatom (P or O) in the side arm is coordinated
to the metal center instead of THF. Complex 8 adopts a dimeric
Results and Discussion
Synthesis and Structures of Half-Sandwich Scandium
Dialkyl Complexes with Different Cyclopentadienyl Ligands.
The acid-base reactions between the scandium trialkyl complex
Sc(CH2SiMe3)3(THF)2 and 1 equiv of cyclopentadiene or its
methyl- and trimethylsilyl-substituted derivatives gave straight-
forwardly the corresponding mono(cyclopentadienyl)scandium
dialkyl complexes Cp′Sc(CH2SiMe3)2(THF) (1: Cp′ ) C5H5,
80%; 2: Cp′ ) C5MeH4, 76%; 3: Cp′ ) C5Me4H, 77%; 4: Cp′
) C5Me5, 65%; 5: Cp′ ) C5Me4SiMe3, 78%) (Scheme 2).
(13) (a) Luo, Y.; Baldamus, J.; Hou, Z. J. Am. Chem. Soc. 2004, 126, 13910.
(b) Li, X.; Baldamus, J.; Hou, Z. Angew. Chem., Int. Ed. 2005, 44,
962. (c) Li, X.; Hou, Z. Macromolecules 2005, 38, 6767. (d) Hou, Z.;
Luo, Y.; Li, X. J. Organomet. Chem. 2006, 691, 3114. (e) Li, X.;
Baldamus, J.; Nishiura, M.; Tardif, O.; Hou, Z. Angew. Chem., Int.
Ed. 2006, 118, 8364. (f) Li, X.; Nishiura, M.; Mori, K.; Mashiko, T.;
Hou, Z. Chem. Commun. 2007, 4137. (g) Zhang, H.; Luo, Y.; Hou,
Z. Macromolecules 2008, 41, 1064. (h) Yu, N.; Nishiura, M.; Li, X.;
Xi, Z.; Hou, Z. Chem. Asian J. 2008, 3, 1406. (i) Nishiura, M.;
Mashiko, T.; Hou, Z. Chem. Commun. 2008, 2019.
(14) Selected reviews on the cationic rare earth metal complexes: (a)
Zeimentz, P. M.; Arndt, S.; Elvidge, B. R.; Okuda, J. Chem. ReV.
2006, 106, 2404, and references therein. (b) Hou, Z.; Luo, Y.; Li, X
J. Organomet. Chem. 2006, 691, 3114. (c) Arndt, S.; Okuda, J. AdV.
Synth. Catal. 2005, 347, 339. (d) Gromada, J.; Carpentier, J. F.;
Mortreux, A. Coord. Chem. ReV. 2004, 248, 397. (e) Mountford, P.;
Ward, B. D Chem 2003, 1797. (f) Piers, W. E.; Emslie, D. J. H. Coord.
Chem. ReV. 2002, 233-234, 131. (g) Hou, Z.; Wakatsuki, Y. Coord.
Chem. ReV. 2002, 231, 1.
(16) Mono(cyclopentadienyl) rare earth dialkyl complexes of type “CpMR2”
tend to undergo ligand redistribution to give “Cp2MR” and MR3. For
example, see: Hultzsch, K. C.; Spaniol, T. P.; Okuda, J. Angew. Chem.,
Int. Ed. 1999, 38, 227.
(15) A portion of this work was reported at the 88th Annual Meeting of
Chemical Society of Japan, March 26-30, 2008. See:Li, X.; Nishiura,
M.; Mori, K.; Hou, Z. Abstract, 2F1-14.
9
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Copolymerization of Isoprene and Ethylene
A R T I C L E S
Scheme 4. Synthesis of a Binuclear Scandium Alkyl Complex bearing Mono(cyclopentadienyl) Ligand with an Amine Side Arm
structure, in which the two metal centers are bridged by two
aminomethylene units and each metal center is also bonded to
one η5-Cp unit and one η1-CH2SiMe3 ligand. A crystallographic
inversion center exists at the center of the whole molecule of
8.
bigger in 3 (Cp′ ) C5Me4H), the orientation of the THF ligand
is changed to parallel to the Cp′ ring, and only one CH2SiMe3
group is in a supine fashion, whereas the other CH2SiMe3 group
adopts a prone form. In the case of 4 and 5, which bear further
sterically demanding persubstituted ligands C5Me5 and
C5Me4SiMe3, respectively, both CH2SiMe3 groups adopt a prone
fashion, together with a parallel THF orientation to the Cp′ ring.
In 6 and 7, the CH2SiMe3 groups adopt a prone fashion similar
to those in 4 and 5.
The bond distances of the Sc-Cp′ bonds as well as the
Sc-CH2SiMe3 bonds in 1-7 are comparable among each other
(Table 1). The bond distance of the chelating Sc-O bond in 7
(2.208(1) Å) is longer than those of the Sc-O (THF) bonds
found in 1-5 (2.139(1)-2.168(1) Å). The Sc-P bond distance
in 6 (2.794(1) Å) is much longer than that of the Sc-O bond
in 7 (2.208(1) Å), but shorter than that of the Sc-PMe3 bond
found in [Me2Si(C5Me4)(NtBu)Sc(µ-H)(PMe3)]2 (2.996(1) Å).17
Among 1-5, differences in the orientation of the THF and
CH2SiMe3 ligands were observed, possibly as a result of the
steric influence of the Cp′ ligands. In 1 and 2, which bear the
relatively small C5H5 and C5MeH4 ligands, respectively, the THF
ring plane is oriented almost vertically toward the Cp′ ring plane,
and the two CH2SiMe3 groups adopt a supine configuration
(Figure 1). When the steric hindrance of the Cp′ ligand becomes
All complexes 1-8 are soluble in common organic solvents
1
such as THF, toluene, and hexane and gave well resolved H
NMR spectra in C6D6, without ligand redistribution being
observed. Complexes 1-3 each showed one singlet for the
methylene protons of the CH2SiMe3 groups at -0.22, -0.21,
and -0.33 ppm, respectively. In contrast, the methylene protons
of the CH2SiMe3 groups in 4 and 5 gave two doublets (4: -0.40,
-0.33 ppm, JH-H ) 15.0 Hz; 5: -0.30, -0.25 ppm, JH-H
)
11.5 Hz). The methylene groups of the CH2SiMe3 units in 7
appeared as a singlet at -0.18 ppm, while those in 6 showed
two doublets at 0.02 and 0.16 ppm with a germinal H-H
Figure 1. ORTEP drawings of 1-8 with thermal ellipsoids at 30% probability. Hydrogen atoms have been omitted for clarity.
Table 1. Selected Bond Distances (Å) and Angles (deg) of Complexes 1-8
1
2
3
4
5
6
7
8
Sc-Cpav.
2.486(2)
2.220(2)
2.211(2)
2.486(2)
2.204(2)
2.215(2)
2.487(3)
2.242(3)
2.221(3)
2.493(3)
2.239(3)
2.221(3)
2.498(2)
2.240(2)
2.206(2)
2.491(4)
2.222(4)
2.217(4)
2.470(2)
2.221(2)
2.243(2)
2.496(2)
2.228(2)
2.400(2)
2.432(2)
2.246(2)
96.57(8)
37.93(7)
116.21(9)
95.49(9)
141.73(13)
-
Sc-C1
Sc-C5
Sc-C5′
Sc-O/P/N
2.145(1)
97.02(6)
100.48(7)
102.50(7)
2.139(1)
100.09(6)
93.64(6)
2.168(2)
96.29(9)
96.40(9)
2.166(2)
95.26(9)
105.64(8)
105.19(10)
2.158(1)
95.88(6)
106.36(10)
101.78(8)
2.794(1)
96.91(11)
103.05(12)
113.88(15)
2.208(1)
104.73(6)
98.51(7)
O/P/N-Sc-C1
O/P/N-Sc-C5
C5-Sc-C1
C5′-Sc-C1
Si1-C1-Sc
Si2-C5-Sc
Sc-CpCent(1)
CpCent(1)-Sc-O/P/N
CpCent(1)-Sc-C1
CpCent(1)-Sc-C5/C5′
103.95(7)
107.90(12)
111.69(8)
127.69(10)
128.46(11)
2.186
113.6
118.8
131.05(9)
127.48(10)
2.182
114.7
120.1
124.61(15)
126.91(15)
2.177
112.8
117.7
125.08(14)
133.10(13)
2.188
115.5
115.3
124.09(10)
147.3(2)
2.189
112.2
118.8
126.6(2)
149.2(3)
2.180
98.2
118.7
128.27(10)
125.07(11)
2.155
100.4
118.3
2.186
101.5
119.7
112.8/113.9
120.5
119.5
120.8
117.2
119.0
119.5
118.8
9
J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009 13873

A R T I C L E S
Li et al.
Scheme 5. Synthesis of the Cationic Half-Sandwich Scandium
Alkyl Complexes 9-11
Table 2. Selected Bond Distances (Å) and Angles (deg) of
Complexes 5, 7, 10 and 11
5
10
7
11
Sc-C(Cp)(av.)
Sc-C1(av.)
Sc-O (av.)
O1-Sc-C1
O2-Sc-C1
O1-Sc-O2
O3-Sc-C1
O1-Sc-O3
O2-Sc-O3
Si1-C1-Sc
Sc-CpCent(1)
2.498(2)
2.240(2)
2.158(1)
2.454(3)
2.193(3)
2.136(2)
2.470(2)
2.221(2)
2.208(1)
2.486(7)
2.115(9)
2.255(5)
87.7(3)
110.9(3)
77.66(16)
86.6(3)
151.25(17)
78.13(16)
154.2(5)
2.174
97.2, 109.4,
125.4
95.88(6) 106.22(11)
104.73(6)
95.96(12)
99.15(10)
-
-
-
124.09(10) 127.12(17)
2.189
CpCent(1)-Sc-O 112.2
128.27(10)
2.155
2.136
116.3, 114.6 100.4
CpCent(1)-Sc-C1 118.8
120.7 118.3
123.1
the [B(C6F5)4]- anion has no bonding interaction with the
scandium center. In each complex, the Sc metal center is bonded
to two THF, one Cp′, and one alkyl ligands. In 11, an additional
coordination of the oxygen atom of the side arm in the Cp′
ligand is also present, and therefore 11 has a higher coordination
number than that of 10 and that of its neutral precursor 7 as
well. Because of the higher electron deficiency of the cationic
metal center in 10, the bond distances of the Sc-Cp′ bonds
(av. 2.454(3) Å) are significantly shorter than those found in
its neutral precursor 5 (av. 2.498(2) Å), and so are the
Sc-CH2SiMe3 (10: 2.193(3) Å, 5: 2.240(2) Å) and Sc-O (10:
2.136(2) Å, 5: 2.158(1) Å) bond distances. The average bond
length of the Sc-CH2SiMe3 bond in 11 (2.115(9) Å) is also
shorter than that of its neutral precursor 7 (2.221(2) Å).
However, the bond distances of the Sc-Cp′ bonds (av. 2.485(7)
Å) and Sc-O bonds (av. 2.255(4) Å) in 11 are longer than those
found in the neutral precursor 7 (av. 2.470(2) Å, 2.208(1) Å,
respectively), probably because of the higher coordination
number in 11.18 Complexes 10 and 11 represent rare examples
of structurally characterized cationic half-sandwich rare earth
metal complexes with an η1-alkyl ligand.19
Figure 2. ORTEP drawings of the cation part of 10 (left) and 11 (right)
with thermal ellipsoids at 30% probability. Hydrogen atoms have been
omitted for clarity.
coupling constant of 11.1 Hz. These results may suggest that
free rotation of the CH2SiMe3 groups in 1-3 and 7 could be
possible, whereas the CH2SiMe3 groups in 4-6 are fixed to
some extent at the NMR time scale. The 1H NMR spectrum of
the binuclear complex 8 showed two doublets at -1.29 and
-0.51 ppm with a germinal H-H coupling constant of 10.6
Hz for the methylene protons of the CH2SiMe3 groups, indicative
of a rather rigid structure.
Isolation and Structural Characterization of the Cationic
Half-Sandwich Scandium Alkyl Complexes. 1H NMR monitor-
All complexes 9-11 showed well-resolved 1H NMR spectra
in C6D5Cl or THF-d8, which are in agreement with their solid
structure. No ligand redistribution was observed even in the case
of the C5H5-ligated complex 9.
ing of the reactions of 1-7 with
1
equiv of
[PhMe2NH][B(C6F5)4] in C6D5Cl at 25 °C suggested instant
formation of PhNMe2 and SiMe4, but an unambiguous identi-
fication of the resulting cationic scandium species was difficult
because of rapid decomposition. However, when the reactions
were carried out in THF, much better resolved 1H NMR spectra
of the cationic scandium species were observed. Complexes 1,
5, and 7 were then chosen for further detailed investigations,
as representative examples with different steric hindrances in
this series of half-sandwich complexes. Thus, the reactions of
1, 5, and 7 with 1 equiv of [PhMe2NH][B(C6F5)4] in THF
afforded quantitatively the corresponding bis(THF)-coordinated,
cationic half-sandwich scandium monoalkyl complexes 9-11,
respectively (Scheme 5). Single crystals of 9-11 were obtained
from a THF/hexane solution. The crystal structures of 10 and
11 were unambiguously determined by X-ray diffraction studies.
In the case of 9, the overall structure was identified to be similar
to those of 10 and 11, but a well-refined structure was not
obtained because of the disorder problems.
Homopolymerization of Isoprene (IP). To gain fundamental
information on the ligand influences on the catalyst activity and
selectivity, the homopolymerization of isoprene was first
examined by use of complexes 1-8. [Ph3C][B(C6F5)4] (A),
[PhMe2NH][B(C6F5)4] (B), and B(C6F5)3 (C) were used as
activators to generate the corresponding cationic scandium alkyl
active species. Representative results are summarized in Table
3.
Generally, the trityl and anilinium borate activators A and B
showed similar influences on the polymerization activity and
(18) A higher coordination usually results in longer bond distances around
the metal center. See: Shannon, R. D. Acta Crystallogr. 1976, A32,
751.
(19) The isolation of a cationic half-sandwich rare earth metal complex
with an η1-alkyl ligand is usually difficult because of facile ligand
scrambling. Only two examples were reported previously: (a) Robert,
D.; Spaniol, T. P.; Okuda, J. Eur. J. Inorg. Chem. 2008, 2801. (b)
Henderson, L. D.; Maclnnis, G. D.; Piers, W. E.; Parvez, M. Can.
J. Chem. 2004, 82, 162. For η2-aminobenzyl and η3-allyl cationic half-
sandwich Sc complexes, see (c) Li, X.; Nishiura, M.; Mori, K.;
Mashiko, T.; Hou, Z. Chem. Commun. 2007, 4137. (d) Yu, N.;
Nishiura, M.; Li, X.; Xi, Z.; Hou, Z. Chem. Asian. J. 2008, 3, 1406.
Cationic rare earth alkyls with multidentate non-Cp ligands are
relatively easier to isolate.14
The ORTEP drawings of 10 and 11 are shown in Figure 2.
Their selected bond lengths and angles are summarized in Table
2. These complexes are solvent-separated ion pairs, in which
(17) Shapiro, P. J.; Bunel, E. E.; Schaefer, W. P.; Bercaw, J. E.
Organometallics 1990, 9, 867.
9
13874 J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009

Copolymerization of Isoprene and Ethylene
A R T I C L E S
Table 3. Isoprene Polymerization by Using the Complexes 1-8/Activator Systemsa
microstructure (%)c
trans-1,4-
entry
comp.
act.b
t (min)
yield (%)
cis-1,4-
3,4-
Mnd (× 103)
Mw/Mnd
Tge (°C)
Eff.f (%)
1
2
3
4
5
6
7
8
1
1
1
2
2
2
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
B
C
A
5
5
240
5
5
240
60
100
100
68
100
100
50
100
100
100
100
100
95
100
100
91
100
100
100
100
100
100
8
95
92
82
91
87
69
41
39
30
25
23
7
20
17
3
90
90
84
29
28
10
77
0
3
13
0
4
22
8
10
29
20
22
61
15
18
57
0
5
5
5
9
9
120
85
25
122
105
30
67
70
72
75
66
57
77
69
59
73
77
107
77
69
114
7
2.39
1.64
2.20
2.26
1.63
1.65
1.10
1.08
1.09
1.07
1.07
1.11
1.07
1.07
1.08
1.12
1.07
1.14
1.05
1.08
1.11
1.37
-57
-60
-64
-60
-62
-62
-32
-33
-44
-32
-27
-50
-18
-21
-43
-59
-60
-58
-61
-63
-65
-44
0.3
0.5
1.1
0.3
0.4
0.7
0.6
0.6
0.6
0.5
0.6
0.7
0.5
0.6
0.6
0.6
0.5
0.4
0.5
0.6
0.4
0.5
9
51
51
41
55
55
32
65
65
40
10
10
16
11
11
11
23
60
60
9
10
11
12
13
14
15
16
17
18
19
20
21
22
180
180
180
180
180
180
120
120
120
120
120
120
120
0
0
60
61
79
0
a Conditions: 25 µmol of Sc, 25 µmol of activator, 15 mmol of isoprene, 10 mL of C6H5Cl, 25 °C. b Activator: A ) [Ph3C][B(C6F5)4], B )
[PhMe2NH][B(C6F5)4], C ) B(C6F5)3. c Determined by 1H and 13C NMR. d Determined by GPC in THF at 45 °C. e Measured by DSC. f Catalyst
efficiency ) Mn(Calculated)/Mn(Measured).
selectivity, while the neutral borane compound C showed lower
activity and gave polyisoprene with higher trans-1,4-configu-
ration (cf. Table 3, entries 1-3, 4-6, 10-12, and 13-15).
Significant influences of the Cp′ ligands on the polymerization
activity and selectivity were observed. Among these complexes,
1 and 2, which bear the smallest Cp′ ligands (C5H5 and C5MeH4,
respectively), showed the highest activity, converting quanti-
tatively 600 equiv of isoprene into polyisoprene within 5 min
in the presence of 1 equiv of A or B at room temperature (Table
3, entries 1-2 and 4-5). High cis-1,4 selectivity (up to 95%)
was observed. As the steric hindrance of the Cp′ ligands
increased in 3 (C5Me4H), 4 (C5Me5) and 5 (C5Me4SiMe3), the
polymerization became slower and much longer reaction time
was required for completion. The formation of 3,4-polyisoprene
(51-65%) was preferred in the case of 3-5 under the same
conditions (Table 3, entries 7-8, 10-11, and 13-14). The
phosphine side arm coordinated complex 6 showed the similar
activity as that of the ether analogue 7. However, a distinct
difference in selectivity was observed between these two
complexes. Complex 6 showed high cis-1,4 selectivity (84-90%),
whereas 7 preferred trans-1,4-polyisoprene formation (60-79%)
(Table 3, entries 16-18 and 19-21). The binuclear complex 8
showed much lower activity than those of the mononuclear
complexes 1-7 under the same conditions (Table 3, entry 22).
The neutral complexes 1-8 alone were inactive under the same
conditions, suggesting that the generation of a cationic metal
alkyl species is essential for the present polymerization. The
isolated, THF-solvated cationic complexes 9-11 were inactive
under the same conditions, obviously due to the strongly
coordinating THF ligands.
with high isoprene contents and predominant cis-1,4-micro-
structures (up to 90%) (Table 4 entries 1-8). As the isoprene
feed was raised gradually from 30 to 60 mmol under 1 atm of
ethylene, the copolymerization activity (or the copolymer yield)
increased significantly (from 456 to 1598 kg/mol-Sc ·h for 1
and 744 to 1234 kg/mol-Sc ·h for 2) and the isoprene content
in the resulting random copolymers also increased gradually in
the range of 80-90 mol %. These results suggest that an
increase of isoprene feed under 1 atm of ethylene could
accelerate the copolymerization of these two monomers. When
the more sterically demanding complexes 3-5/A were used as
catalysts, the copolymerization activity generally increased at
first and then decreased after reaching a peak value, as the
isoprene feed was raised in the range of 30-60 mmol under 1
atm of ethylene (Table 4, entries 9-20). The copolymers
produced by 3,4/A possessed almost perfect alternating
isoprene-ethylene sequences with 3,4-rich polyisoprene mi-
crostructures (isoprene content: 47-51 mol % for 3 and 38-50
mol % for 4; 3,4-selectivity: 58-68%) (Table 4, entries 9-20),
whereas the copolymers produced by 5/A adopted a random
microstructure with lower isoprene incorporation (isoprene
content: 29-40 mol %; 3,4-selectivity: 55%). In all the cases,
the copolymerization activity was much higher than that of the
homopolymerization of isoprene. Such “co-monomer effect” was
observed previously in the alternating copolymerization of
cycloolefins (norbornene, dicyclopentadiene) with ethylene
catalyzed by 5/A.13b,c
Similarly, the side arm-coordinated complexes 6 and 7 also
exhibited high activity for the copolymerization of isoprene with
ethylene (Table 4, entries 21-34), although their activity for
the homopolymerization of isoprene was rather low under the
similar conditions. Predominant alternating isoprene-ethylene
copolymers with high molecular weights and narrow molecular
weight distributions were obtained (isoprene content around 50
mol %, Mn up to 4.6 × 105, Mw/Mn ) 1.12-1.87) (Table 4,
entries 21-34). It is also noteworthy that the isoprene units in
all the copolymers produced by 6 and 7 adopted predominantly
the 3,4-configuration (68-82%). This is distinctly different from
Copolymerization of Isoprene (IP) with Ethylene (E). The
copolymerization reactions were carried out by feeding varying
amounts of isoprene under an atmosphere of ethylene (1 atm)
in C6H5Cl at 25 °C, in the presence of a scandium complex
and an activator. Representative results are shown in Table 4.
In consistence with the selectivity observed in the homopo-
lymerization of isoprene, the copolymerization of isoprene with
ethylene by 1/A or 2/A afforded the corresponding copolymers
9
J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009 13875

A R T I C L E S
Li et al.
Table 4. Copolymerization of Isoprene with Ethylene by Using the Complexes 1-7/Activator Systemsa
entry
comp.
Ab
IP (mmol)
yield (g)
act.c
IP cont.d(mol %)
3,4-IPd(%)
1,4-IPd(%)(C/T)e
Mnf (103)
Mw/Mnf
Tgg (°C)
1
2
3
4
5
6
7
8
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
5
6
6
6
6
6
6
6
7
7
7
7
7
7
7
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
B
C
C
A
B
B
B
B
B
C
30
40
50
60
30
40
50
60
30
40
50
60
30
40
50
60
30
40
50
60
30
40
50
60
40
40
50
40
20
30
40
50
60
40
0.95
1.83
2.73
3.33
1.55
1.78
2.00
2.57
1.18
1.66
2.01
1.58
1.24
2.42
2.15
1.99
1.26
1.39
1.52
1.42
1.35
1.69
2.13
2.09
1.90
2.03
1.56
1.95
1.59
2.17
2.66
2.71
2.15
2.22
456
878
1310
1598
744
854
960
1234
566
797
965
758
595
1162
1032
955
605
667
730
682
648
811
1022
1003
912
974
749
82
83
87
90
80
86
88
89
47
49
50
51
38
44
48
50
29
32
36
40
43
46
52
55
52
49
50
50
46
48
49
49
50
47
11
10
8
89 (85/4)
90 (86/4)
92 (90/2)
92 (90/2)
83 (78/5)
85 (n.d.)h
85 (80/5)
85 (n.d.)
32 (n.d.)
32 (15/17)
32 (14/18)
32 (11/21)
43 (n.d.)
42 (7/35)
42 (n.d.)
43 (4/39)
45 (n.d.)
45 (6/39)
45 (5/40)
45 (n.d.)
32 (5/27)
29 (2/27)
19 (2/17)
19 (2/17)
25 (1/24)
19 (1/18)
20 (1/19)
25 (n.d.)
19 (n.d.)
19 (n.d.)
19 (12/7)
19 (12/7)
19 (12/7)
18 (11/7)
86
150
249
261
110
181
227
281
440
256
429
352
294
595
423
310
303
327
256
238
296
407
487
483
413
309
283
356
346
417
363
393
462
267
1.40
1.42
1.33
1.56
1.48
1.31
1.48
1.56
1.13
1.25
1.11
1.30
1.55
1.38
1.40
1.31
1.29
1.22
1.25
1.26
1.14
1.19
1.12
1.12
1.12
1.22
1.37
1.87
1.24
1.34
1.40
1.25
1.45
1.48
-61
-62
-62
-62
-61
-59
-60
-60
-37
-38
-38
-38
-51
-52
-52
-50
-53
-51
-50
-48
-52
-31
-20
-22
-24
-24
-24
-30
-35
-31
-31
-31
-30
-30
8
17
15
15
15
68
68
68
68
57
58
58
57
55
55
55
55
68
71
81
81
75
81
80
75
81
81
81
81
81
82
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
936
763
1042
1277
1301
1032
1066
a Conditions: 25 µmol of Sc, 25 µmol of activator, 1 atm of ethylene, 40 mL of C6H5Cl, 25 °C, 5 min. b Activator: A ) [Ph3C][B(C6F5)4], B )
[PhMe2NH][B(C6F5)4], C ) B(C6F5)3. c Kilograms of copolymer/mol-Sc ·h·atm. d Determined by 1H NMR. e Determined by 13C NMR, C ) cis, T )
trans. f Determined by GPC in o-dichlorobenzene at 145 °C. g Measured by DSC. h n.d. ) not detected.
what was observed in the homopolymerization of isoprene,
where either cis-1,4- (6: 84-90%) or trans-1,4-configuration
(7: 60-79%) was predominant (see Table 3).
or amidinate ligand, which had shown excellent regio- and
stereoselectivity for the homopolymerization of isoprene,10k,l
yielded only a mixture of the homopolymers under the same
conditions, while an isoprene-ethylene copolymer product was
not observed. These results may demonstrate that the half-
sandwich scandium systems are unique for the copolymerization
of isoprene and ethylene.
The reactivity ratios (rE ) kEE/kEIP and rIP ) kIPIP/kIPE) for
the 3/A system (rE ) 0.4 and rIP ) 0.003) and the 7/B system
(rE ) 0.7 and rIP ) 0.007) were obtained from the Fineman-Ross
plots at very low monomer conversion (<5%).20 These results
suggest that the ethylene-isoprene alternating insertion should
be much faster than successive insertion of either monomer in
the present catalytic systems. The reactivity ratios for 5/A were
found to be rE ) 6.7 and rIP ) 0.020, indicating a preference
for the coordination/insertion of ethylene over those of isoprene
in this catalyst system. The product of the reactivity ratios (rErIP
) 0.134) demonstrates a tendency to form random copolymers,
in agreement with the microstructures of the resulting copoly-
mers determined by 13C NMR analysis. Attempts to obtain the
reactivity ratios for 1 or 2 from the Fineman-Ross plot were
not successful because the copolymerization reaction in these
cases was too fast.
As far as we are aware, the catalyst system based on 1 or 2
represents the first example of a catalyst for the cis-1,4-selective
copolymerization of isoprene with ethylene, and those based
on 3, 4, 6, and 7 are the first examples for the alternating
copolymerization of isoprene with ethylene. It should also be
noted that the use of the analogous nonmetallocene Sc or Y
complexes bearing an ancillary (phosphinophenyl) amido (PNP)
Characterization of Isoprene-Ethylene Copolymers. The
copolymers obtained above are white elastomers (no gel
formation) and have good solubility in C2H2Cl4 and C6H4Cl2.
GPC curves revealed that these copolymers possess high
molecular weights (Mn ) 86,000-595,000 g/mol) and unimodal
molecular weight distributions (1.11-1.87), indicative of the
predominance of a homogeneous single-site catalyst species.
Differential scanning calorimetry (DSC) demonstrated that the
Tg values of these polymers vary in the range of -64 to -22
°C depending on the relative amount of the 3,4-isoprene units
in the copolymers. An endothermic peak corresponding to the
melting point of polyethylene expected in the temperature range
of 100-130 °C was not observed even in the samples with high
ethylene contents.
Because very little NMR information on the isoprene-ethylene
copolymers could be found in the literature,6 we then carried
1
out the full characterization of our copolymers by the H, 13C,
DEPT-13C, 1H-1H, and 1H-13C NMR analyses (see Supporting
Information) to establish the comonomer incorporation and
distribution in the copolymer chains. The aliphatic parts of the
13C NMR spectra of some representative copolymers are shown
(20) Fineman, M.; Ross, S. D. J. Polym. Sci. 1949, 2, 259.
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Copolymerization of Isoprene and Ethylene
A R T I C L E S
via rotation of the C3-C2 single bond of the η1-σ-allyl
intermediate (k). These competitive pathways would result in
different microstructures (3,4-, trans-1,4- and cis-1,4-) in the
polymer chains. The DFT calculations showed that the π-η3-
allylic intermediates (d and l) have lower energies than that of
the σ-allyl intermediate (c), and the anti-η3-allylic intermediate
(l) is further more stable (ca. 1.5-2.0 kcal/mol) than the syn-
η3-allylic intermediate (d). These energy differences could
provide a driving force for the σ-π and syn-anti isomerizations
among these allyl intermediates, especially when the complexes
have small steric hindrance around the metal center. Therefore,
for the less steric catalysts 1 and 2, all these processes should
be faster and the equilibrium would shift to the thermodynami-
cally more stable anti π-η3-allylic intermediate (l), and thus
afford the cis-1,4-polyisoprene sequences as a major component
(cf. m-p, Scheme 6). This scenario is in agreement with the
experimental results (see Table 3). As to a possible chain
termination reaction, the growing-polymer-chain-containing
species (similar to o) could have enough space for ꢁ-hydrogen
elimination. The resulting olefin/hydride species would be
susceptible to ligand exchange with an incoming isoprene
monomer, thus leading to chain termination/transfer.
Figure 3. Aliphatic parts of the 13C NMR spectra of some representative
In the case of the THF-free, phosphine side arm coordinated
complex 6, the chain propagation process could follow those
in the case of 1 and 2 to give cis-1,4-predominant polyisoprene
sequences (c, k, l-p, Scheme 6) (see also Table 3).21 However,
the larger steric hindrance of the Cp′ ligand in 6 could make its
polymerization speed slower than those of 1 and 2. For the
analogous THF-free complex 7, which bears a more strongly
coordinating ether side arm on the Cp ring, the isomerization
of the σ-allyl intermediate c to the anti-η3-π-allyl species l via
the η1-σ-alkyl intermediate k, would be kinetically less favored
compared with that to the syn-η3-π-allyl species d because of
the steric hindrance around metal center. Therefore, the coor-
dination of an incoming isoprene monomer to the metal center
of d would give a primary σ-allyl species such as e, which after
isoprene insertion should afford f, and finally lead to formation
of trans-1,4-polyisoprene sequences as a major component in
the resulting polymers. Regarding a possible chain termination
reaction, the steric hindrance of the bulky Cp′ ligands in these
catalysts would prevent ꢁ-hydrogen elimination from a species
like f or o. Therefore, a chain transfer reaction would be difficult,
thus accounting for the living feature of these catalysts (6 and
7) for the polymerization of isoprene.
In the case of the further sterically demanding catalysts such
as 3-5, which contain both a bulky multisubstituted cyclopen-
tadienyl ligand and a five-membered ring THF ligand, all the
reaction processes would be retarded because of steric hindrance.
In particular, the isomerization reactions of the σ-allyl inter-
mediate c to the π-η3-allylic intermediates d and l could be
hampered to such an extent that the coordination of an incoming
isoprene monomer to the σ-allyl intermediate c to give h would
become relatively favored, thus leading to formation of the 3,4-
polyisoprene sequences as a major part in the resulting polymers
(cf. h-j, Scheme 6) (see also Table 3). The steric hindrance of
both the Cp′ and the THF ligands could account for the
livingness observed in isoprene polymerization, as described
above in the case of 6 and 7.
copolymers obtained in Table 4.
in Figure 3. The 13C NMR spectra of the random copolymers
(IP content ) 82-90 mol %) prepared by 1,2/A are very simple,
consisting mainly of cis-1,4-PIP sequences and small amounts
of cis-1,4-IP-alt-E, isolated 3,4-IP and isolated trans-1,4-IP units.
In contrast, the copolymers prepared by 3 (e.g., Table 4, entry
12, IP content ) 51 mol %) and 7 (e.g., Table 4, entry 33, IP
content ) 50 mol %) showed almost perfect alternating
microstructures, in which the 3,4-IP-alt-PE sequence is domi-
nant, with small amounts of cis-1,4-IP-alt-E and trans-1,4-IP-
alt-E sequences (Figure 3). The PE units (at 29.4 ppm) are
negligible in these copolymers. In the alternating copolymers
produced by 6, a significant amount of (3,4-IP)(3,4-IP)EE
sequences was observed in addition to 3,4-IP-alt-E and trans-
1,4-IP-alt-E sequences. In the ethylene-rich copolymers prepared
by 5 (Table 4, entry 18, IP content ) 32 mol %), the signals of
the 3,4-IP-alt-PE units decreased, while those of PE and trans-
1,4-IP-alt-E increased compared with those found in the
polymers prepared by 3 (Figure 3).
Polymerization Mechanism. To gain more insight into the
influence of the steric hindrance around the scandium center
on the polymerization processes, the DFT calculations on four
cationic species [Cp′Sc(CH2SiMe3)(THF)n][B(C6F5)4] (1a: Cp′
) C5H5, n ) 1; 4a: Cp′ ) C5Me5, n ) 1; 6a: Cp′ )
C5Me4CH2CH2PPh2, n ) 0; 7a: Cp′ ) C5Me4C6H4OMe, n )
0) were carried out. In all the cases, it was revealed that the
coordination of isoprene to the metal center in an η2-trans-3,4
fashion is the most favored among four possible isoprene
coordination modes (η4-cis-1,4, η4-trans-1,4, η2-cis-3,4, η2-trans-
3,4) (See Supporting Information). Therefore, the η2-trans-3,4-
coordination (e.g., b in Scheme 6) might be prevailing in the
chain initiation process of the present catalyst systems. The
subsequent 3,4-isoprene insertion into the Sc-alkyl bond could
lead to the formation of an η3-σ-allyl intermediate (c) as a
secondary growing chain. There could be three competitive
pathways to carry through the chain propagation process from
c (Scheme 6). One is the coordination (in a trans-3,4-mode) of
another molecule of isoprene at the metal center of c to give h,
others are the isomerization of c to give the syn-η3-π-allylic
intermediate (d) or that to the anti-η3-π-allylic intermediate (l)
When the neutral borane compound B(C6F5)3 (C) was used
as an activator,
a contact ion pair such as Cp′Sc-
(21) The loosely coordinated phosphine side arm could probably be
dissociated from the metal center upon coordination or insertion of
isoprene, thus making the isomerization of c to l possible.
9
J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009 13877

A R T I C L E S
Li et al.
Scheme 6. Possible Scenarios of Isoprene Polymerization Catalyzed by Cationic Half-Sandwich Scandium Species
(CH2SiMe3)(THF)n(CH2SiMe3)B(C6F5)3 (n ) 0, 1) would be
formed, in which a relatively strong bonding interaction between
the metal center and the resulting anionic borate unit
[B(CH2SiMe3)(C6F5)3] could exist.19b Therefore, the cationic
metal center would be more crowded and less electropositive
compared to those generated by use of [Ph3C][B(C6F5)4] (A) or
[PhMe2NH][B(C6F5)4] (B) as an activator. As in the case of 7,
the larger steric hindrance around metal center could account
for the higher content of the trans-1,4-polyisoprene unit in the
polymers obtained when B(C6F5)3 (C) was used as an activator
(cf. Table 3).
and 5, which bear both a persubstituted cyclopentadienyl ligand
and a five-membered ring THF ligand at the metal center, would
favor the insertion of the smaller ethylene monomer, to give
more ethylene-ethylene sequences in the resulting copolymers
(cf. G and H, Scheme 7). The coordination and insertion of an
isoprene monomer at c followed by successive insertion of
ethylene could explain the formation of some amount of IPIPEE
sequences observed in the case of 6 (cf. h, i, and I, Scheme 7).
The coordination and insertion of ethylene at the metal center
of the syn-η3-π-allyl intermediate d followed by those of
isoprene would lead to formation of the alternating trans-1,4-
isoprene-ethylene units, as observed in the case of 3-6 (cf. d,
J, K, and L, Scheme 7).
For the copolymerization of isoprene with ethylene, whether
the initiation step is the insertion of isoprene or ethylene, the
copolymerization reaction should proceed through the allyl
intermediates similar to those formed in the homopolymerization
of isoprene, such as c, l, and d (Schemes 6 and 7). In the case
of the less sterically demanding catalysts 1 and 2, the copo-
lymerization could take place through the thermodynamically
more stable anti-η3-π-allyl intermediate l as in the case of
isoprene homopolymerization, to afford the copolymers with
high cis-1,4-polyisoprene sequences (Scheme 7). In the case of
the more sterically demanding catalysts 3-7, the coordination
of the smaller ethylene molecule to the metal center of c could
be possible before the isomerization of c to d or l, thus leading
to formation of the copolymers with alternating 3,4-
isoprene-ethylene units as main sequences via the intermediates
D-F.22 Among these catalysts, the most bulky complexes 4
Concluding Remarks
A series of structurally well-defined half-sandwich scandium
dialkyl complexes such as 1-7, which bear mono(cyclopenta-
dienyl) ligands with various substituents including those having
a heteroatom side arm, have been synthesized either by the
alkane elimination reactions between the trialkyl complex
Sc(CH2SiMe3)3(THF)2 and the neutral cyclopentadiene ligands
or by the one-pot metathetical reactions of ScCl3(THF)3 with
the potassium salts of the ligands and LiCH2SiMe3. The
reactions of the dialkyl complexes with one equiv of a borate
(22) For similar alternating copolymerization of ethylene with sterically
demanding cyclic olefins, see: (a) Li, X.; Baldamus, J.; Hou, Z. Angew.
Chem., Int. Ed. 2005, 44, 962. (b) Li, X.; Hou, Z. Macromolecules
2005, 38, 6767.
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Copolymerization of Isoprene and Ethylene
A R T I C L E S
Scheme 7. Possible Scenarios of Isoprene-Ethylene Copolymerization Catalyzed by Cationic Half-Sandwich Scandium Speciesa
a Structures shown represent the micro-structural units in the copolymer products (e.g., PE represents polyethylene units/blocks in the copolymers, rather
than ethylene homopolymer).
compound such as [Ph3C][B(C6F5)4] or [PhMe2NH][B(C6F5)4]
have afforded easily the corresponding cationic monoalkyl
species, some of which (e.g., 10 and 11) have been structurally
characterized by X-ray diffraction studies. In the presence of
an activator such as [Ph3C][B(C6F5)4], [PhMe2NH][B(C6F5)4]
or B(C6F5)3, all the dialkyl complexes 1-7 are active for the
polymerization of isoprene and the copolymerization of isoprene
with ethylene, with the activity and selectivity being significantly
dependent on the ancillary ligands, to yield a new family of
polymer materials with different microstructures and composi-
tions. In the homopolymerization of isoprene, the less sterically
demanding complexes 1 and 2 showed high cis-1,4 selectivity
(up to 95%), while the ether side arm containing complex 7
yielded trans-1,4-polyisoprene (60-79%) as a major product.
In the case of the more sterically demanding complexes 3-5,
the formation of 3,4-polyisoprene (51-65%) was preferred. In
the copolymerization of isoprene and ethylene, complexes 1 and
2 afforded the random copolymers with high isoprene contents
(85-92 mol %) and predominant cis-1,4-microstructures (up
to 90%), thus constituting the first example of cis-1,4-selective
copolymerization of isoprene with ethylene. In contrast, the
copolymerization of isoprene and ethylene by 3, 4, 6, and 7
gave,forthefirsttime,almostperfectalternatingisoprene-ethylene
copolymers. The DFT calculations have offered unprecedented
insight into the mechanistic aspects of the polymerization
processes. This work has demonstrated that the activity and
selectivity of the cationic half-sandwich rare earth alkyl catalysts
can be fine-tuned by modifying the substituents on the cyclo-
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A R T I C L E S
Li et al.
THF-R-CH2), 6.34 (s, 5H, C5H5). 13C NMR (C6D6, δ/ppm): 4.2 (s,
CH2Si(CH3)3), 25.2 (s, THF), 40.1 (br, CH2SiMe3), 72.2 (s, THF),
112.5 (s, C5H5). Anal. calcd for C17H35OScSi2: C, 57.26; H, 9.89.
Found: C, 57.20; H, 9.71.
pentadienyl ligands. Further studies on the polymerization and
copolymerization of other monomers by the cationic half-
sandwich and related rare earth catalysts are in progress.
Synthesis of (C5MeH4)Sc(CH2SiMe3)2(THF) (2). To a colorless
hexane solution (5 mL) of Sc(CH2SiMe3)3(THF)2 (3.268 g, 7.25
mmol), was added C5H5Me (0.581 g, 7.25 mmol) at room
temperature. The pale-yellow mixture was stirred for 3 h. After
reduction of the solution volume (1 mL) under reduced pressure,
the oily residue was cooled at -30 °C overnight to give 2 as
colorless crystals (2.038 g, 5.50 mmol, 76% yield). 1H NMR (300
MHz, C6D6, 22 °C, δ/ppm): -0.21 (s, 4H, CH2Si(CH3)3), 0.32 (s,
18H, CH2Si(CH3)3), 1.06 (m, 4H, THF-ꢁ-CH2), 2.21 (s, 3H,
C5H4CH3), 3.41 (m, 4H, THF-R-CH2), 6.19 (s, 4H, C5MeH4). 13C
NMR (75 MHz, C6D6, 22 °C, δ/ppm): 4.0 (s, CH2Si(CH3)3), 15.3
(s, C5H4CH3), 24.9 (s, THF), 40.0 (br, CH2Si(CH3)3), 71.7 (s, THF),
111.8 (s, C5H4CH3), 113.0 (s, C5MeH4), 124.8 (s, C5MeH4). Anal.
calcd for C18H37OScSi2: C, 58.33; H, 10.06. Found: C, 58.61; H,
10.27.
Experimental Section
Materials. All manipulations of air and moisture-sensitive
compounds were performed under a dry argon atmosphere by use
of standard Schlenk techniques or a nitrogen-filled Mbraun glove-
box. Argon, nitrogen, and ethylene (Takachiho Chemical Industrial
Co., Ltd.) were purified by being passed through a Dryclean column
(4 Å molecular sieves, Nikka Seiko Co.) and a Gasclean GC-XR
column (Nikka Seiko Co.). Solvents were purified by an Mbraun
SPS-800 Solvent Purification System and dried over fresh Na chips
in the glovebox. Isoprene was purchased from TCI, dried over CaH2,
vacuum-transferred, and degassed by two freeze-pump-thaw
cycles prior to polymerization experiments. [Ph3C][B(C6F5)4],
[PhMe2NH][B(C6F5)4], and B(C6F5)3 were purchased from Tosoh
Finechem Corporation and used without purification. ScCl3 was
purchased from Strem. LiCH2SiMe3 (1.0 M solution in pentane),
C5Me4H2, C5Me5H, and C5Me4H(SiMe3) were purchased from
Aldrich, and used as received. C5H6 and C5MeH5 were obtained
by cracking of dicyclopentadiene and methylcyclopentadiene dimer
at 180 °C, respectively. Sc(CH2SiMe3)3(THF)2,23 C5Me4(CH2CH2-
PPh2)Li,24 C5Me4H(C6H4OMe-o),25 and C5Me4H(C6H4NMe2-o)26
were synthesized according to literature. The deuterated solvents
benzene-d6 (99.6 atom % D), THF-d8 (99.6 atom % D), CDCl3
(99.8 atom % D), and 1,1,2,2-tetrachloroethane-d2 (99.6 atom %
D) were obtained from Cambridge Isotope.
Synthesis of (C5Me4H)Sc(CH2SiMe3)2(THF) (3). To a colorless
hexane solution (5 mL) of Sc(CH2SiMe3)3(THF)2 (3.268 g, 7.25
mmol) was added C5H2Me4 (0.886 g, 7.25 mmol) at room
temperature. The pale-yellow mixture was stirred for 3 h. After
reduction of the solution volume (1 mL) under reduced pressure,
the oily residue was cooled at -30 °C overnight to give 3 as
colorless crystals (2.299 g, 5.57 mmol, 77% yield). 1H NMR (300
MHz, C6D6, 22 °C, δ/ppm): -0.33 (s, 4H, CH2SiMe3), 0.33 (s,
18H, CH2Si(CH3)3), 1.15 (br, 4H, THF-ꢁ-CH2), 1.95 (s, 6H,
C5H(CH3)4), 2.10 (s, 6H, C5H(CH3)4), 3.52 (m, 4H, THF-R-CH2),
5.96 (s, 1H, C5H(CH3)4). 13C NMR (75 MHz, C6D6, 22 °C, δ/ppm):
4.3 (s, CH2Si(CH3)3), 11.6 (s, C5H(CH3)4), 13.7 (s, C5H(CH3)4),
24.9 (s, THF), 38.9 (s, CH2SiMe3), 71.1 (s, THF), 111.3 (s,
C5Me4H), 120.1 (s, C5Me4H), 120.5 (s, C5Me4H). Anal. calcd for
C21H43OScSi2: C, 61.12; H, 10.50. Found: C, 60.56; H, 10.34.
Synthesis of (C5Me5)Sc(CH2SiMe3)2(THF) (4). To a colorless
hexane solution (5 mL) of Sc(CH2SiMe3)3(THF)2 (1.470 g, 3.26
mmol) was added C5Me5H (0.444 g, 3.26 mmol) at room temper-
ature. The pale yellow mixture was stirred at 50 °C for 48 h. After
reduction of the solution volume (1 mL) under reduced pressure,
the dark brown oily residue was cooled at -30 °C overnight to
give 4 as colorless plate-like crystals (0.900 g, 2.11 mmol, 65%
yield). 1H NMR (C6D6, 22 °C, δ/ppm): -0.40 (d, 2H, CH2Si(CH3)3,
2JH-H ) 15 Hz), -0.33 (d, 2H, CH2Si(CH3)3, 2JH-H ) 15 Hz), 0.32
(s, 18H, CH2Si(CH3)3), 1.13 (m, 4H, THF-ꢁ-CH2), 2.01 (s, 15H,
C5(CH3)5), 3.53 (m, 4H, THF- R-CH2). 13C NMR (C6D6, 22 °C,
δ/ppm): 4.4 (s, CH2Si(CH3)3), 11.9 (s, C5(CH3)5), 24.8 (s, THF),
38.8 (s, CH2SiMe3), 71.2 (s, THF), 119.0 (s, C5(CH3)5). Anal. calcd
for C22H45OScSi2: C, 61.92; H, 10.63. Found: C, 61.71; H, 9.92.
Synthesis of (C5Me4SiMe3)Sc(CH2SiMe3)2(THF) (5). To a
colorless hexane solution (10 mL) of Sc(CH2SiMe3)3(THF)2 (1.366
g, 3.03 mmol) was added C5Me4H(SiMe3) (0.589 g, 3.03 mmol) at
room temperature. The pale-yellow mixture was stirred at room
temperature for 2 h. After reduction of the solution volume under
reduced pressure, the oily residue was cooled at -30 °C overnight
to give 5 as colorless cubic crystals (1.145 g, 2.36 mmol, 78%
yield). 1H NMR (C6D6, 22 °C, δ/ppm): -0.30 (d, 2H, CH2Si(CH3)3,
JH-H ) 11.5 Hz), -0.25 (d, 2H, CH2Si(CH3)3, JH-H ) 11.5 Hz),
0.29 (s, 18H, CH2Si(CH3)3), 0.43 (s, 9H, C5(CH3)4Si(CH3)3), 1.19
(m, 4H, THF-ꢁ-CH2), 1.91 (s, 6H, C5(CH3)4), 2.22 (s, 6H,
C5(CH3)4), 3.63 (m, 4H, THF-R-CH2). 13C NMR (C6D6, 22 °C,
δ/ppm): 2.8 (s, C5Me4Si(CH3)3), 4.6 (s, CH2Si(CH3)3), 12.1 (s,
C5(CH3)4(SiMe3)), 15.4 (s, C5(CH3)4), 24.9 (s, THF), 40.4 (s,
CH2SiMe3), 71.5 (s, THF), 116.6 (s, ipso-C5Me4(SiMe3), 124.2 (s,
C5Me4(SiMe3)), 128.0 (s, C5Me4(SiMe3)). Anal. calcd for
C24H51OScSi3: C, 59.45; H, 10.60. Found: C, 59.20; H, 10.19.
Synthesis of (C5Me4(C2H4PPh2))Sc(CH2SiMe3)2 (6). ScCl3
(0.378 g, 2.50 mmol) was refluxed in 10 mL of THF at 80 °C for
12 h, the solution was cooled down with stirring to room
General Methods. Samples of rare earth metal complexes for
NMR spectroscopic measurements were prepared in the glovebox
using J. Young valve NMR tubes. The NMR (1H, 13C, 31P, 11B,
19F) spectra were recorded on a JEOL JNM-EX 300, JNM-EX 400
or an ECA-600 spectrometer. The 31P, 19F and 11B NMR spectra
were referenced to external 85% H3PO4, CFCl3 and BF3 · Et2O,
respectively. Elemental analyses were performed on MICRO
CORDER JM10(J-SCIENCE LAB Co.). The NMR data of the
polymers and copolymers were obtained on a JNM-EX 300 (FT,
1
300 MHz for H; 75 MHz for 13C) spectrometer at 45 or 120 °C
with CDCl3 or 1,1,2,2-C2D2Cl4 as a solvent, respectively. The
molecular weights and molecular weight distributions of polyiso-
prenes were determined by gel permeation chromatography on
TOSOH HLC-8220 GPC (Column: Super HZM-H × 3) at 40 °C
using THF as an eluent at a flow rate of 0.35 mL/min against
polystyrene standards. The molecular weights and molecular weight
distributions of the copolymer samples were determined at 145 °C
by high temperature gel permeation chromatography (HT-GPC) on
a HLC-8121GPC/HT apparatus (Tosoh Co.). 1,2-Dichlorobenzene
(DCB) was employed as an eluent at a flow rate of 1.0 mL/min.
The calibration was made by polystyrene standard. The DSC
measurements were performed on a DSC6220 (SII Co.) at a rate
of 20 °C/min. Any thermal history difference in the polymers was
eliminated by first heating the specimen to 200 °C, cooling at 10
°C/min to -120 °C, and then recording the second DSC scan.
Synthesis of (C5H5)Sc(CH2SiMe3)2(THF) (1). To a colorless
hexane solution (5 mL) of Sc(CH2SiMe3)3(THF)2 (0.225 g, 0.50
mmol) was added a solution of cyclopentadiene (0.033 g, 0.50
mmol) in hexane (1.0 mL) at room temperature. The mixture was
stirred at room temperature for 3 h. After removal of all volatiles
in vacuo, the residue was recrystallized from hexane at -30 °C to
give 1 as light yellow crystals (0.143 g, 0.40 mmol, 80% yield).
1H NMR (C6D6, 22 °C, δ/ppm): -0.22 (s, 4H, CH2SiMe3), 0.30
(s, 18H, CH2Si(CH3)3), 1.07 (m, 4H, THF-ꢁ-CH2), 3.40 (m, 4H,
(23) Lappert, M. F.; Pearce, R. J. Chem. Soc., Chem. Commun. 1973, 126.
(24) Krutko, D. P.; Borzov, M. V.; Veksler, E. N.; Kirsanov, R. S.;
Churakov, A. V. Eur. J. Inorg. Chem. 1999, 1973.
(25) Xu, J.; Gao, W.; Zhang, Y.; Li, J.; Mu, Y. J. Organomet. Chem. 2007,
692, 1505.
(26) Enders, M.; Ludwig, G.; Pritzkow, H. Organometallics 2001, 20, 827.
temperature to give a white suspension of ScCl3(THF)3.
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Copolymerization of Isoprene and Ethylene
A R T I C L E S
[C5Me4(CH2CH2PPh2)]Li (0.851 g, 2.50 mmol) in THF (5 mL) was
added slowly and stirred at room temperature for 1 h. A THF
solution (3 mL) of Me3SiCH2Li (0.471 g, 5.00 mmol) was then
added slowly. After the mixture was stirred at room temperature
for 30 min, the solvent was removed in vacuum, and the residue
was extracted with toluene. The toluene extract was dried in vacuum
and washed with cold hexane to give 6 (1.189 g, 2.15 mmol, 86%
yield) as white micro crystals. Single crystals suitable for X-ray
(aromatics and Cp ring carbons). Anal. calcd for C48H78N2Sc2Si2
(8·C6H14): C, 69.52; H, 9.48; N, 3.38. Found: C, 69.52; H, 9.48;
N, 3.59.
Synthesis of [(C5H5)Sc(CH2SiMe3)(THF)2][B(C6F5)4] (9). To
a THF solution (5 mL) of (C5H5)Sc(CH2SiMe3)2(THF) (1) (0.007
g, 21 µmol) was added 1 equiv of [PhNMe2H][B(C6F5)4] (0.017 g,
21 µmol) in THF (5 mL) at room temperature. The pale-yellow
mixture was stirred for 10 min. The solvent was removed in
vacuum. The residue was washed with hexane several times and
dried in vacuum to give 9 (0.021 g, 20.8 µmol, 99% yield) as white
micro crystals. Single crystals for X-ray analysis were grown from
a mixed solvent of THF/hexane (5/20 mL/mL) at -30 °C.
Unfortunately, the X-ray structure of 9 could not be unambiguously
determined completely because of the disorder of the cyclopenta-
1
analysis were grown from a hexane solution at -30 °C. H NMR
(300 MHz, C6D6, r.t., δ/ppm): 0.02 (d, 2H, J ) 11.1 Hz, CH2SiMe3),
0.16 (d, 2H, J ) 11.1 Hz, CH2SiMe3), 0.34 (s, 18H, CH2Si(CH3)3),
1.87 (s, 6H, C5(CH3)4), 2.19 (s, 6H, C5(CH3)4), 2.45 (br, 4H,
C2H4PPh2), 7.00 (m, 6H, C2H4P(C6H5)2), 7.39 (m, 4H,
C2H4P(C6H5)2). 13C NMR (75 MHz, C6D6, r.t., δ/ppm): 4.5 (s,
CH2Si(CH3)3), 12.1 (s, C5(CH3)4), 12.2 (s, C5(CH3)4), 21.9 (s,
CH2CH2PPh2), 31.8 (br, CH2SiMe3), 47.3 (s, CH2CH2PPh2), 120.5,
121.4, 124.6, 129.2, 130.7, 133.1 (aromatics and Cp ring carbons).
31P NMR (75 MHz, C6D6, r.t., δ/ppm): -7.2. Anal. calcd for
C31H48PScSi2: C, 67.35; H, 8.75. Found: C 67.48; H 8.88.
1
dienyl ligand. H NMR (C4D8O, 22 °C, δ/ppm): -0.45 (s, 2H,
CH2SiMe3), 0.01 (s, 9H, CH2Si(CH3)3), 1.69-1.73 (br, 4H, THF-
R-CH2), 3.55-3.58 (br, 4 H, THF-ꢁ-CH2), 6.47 (s, C5H5). 13C NMR
(C4D8O, 22 °C, δ/ppm): 4.2 (s, CH2Si(CH3)3), 25.2 (s, THF), 40.1
(br, CH2SiMe3), 72.2 (s, thf), 112.5 (s, C5H5), 125.4 (m, C6F5-1),
1
1
137.2 (d, JCF ) 245.7 Hz, C6F5-3), 139.1 (d, JCF ) 242.0 Hz,
Synthesis of (C5Me4(C6H4OMe-o))Sc(CH2SiMe3)2 (7). ScCl3
(0.076 g, 0.50 mmol) was refluxed in 10 mL of THF at 80 °C for
12 h, the solution was cooled down with stirring to room
temperature to give a white suspension of ScCl3(THF)3. A THF
solution (5 mL) of [C5Me4(C6H4OMe-o)]K (0.133 g, 0.50 mmol),
which was prepared by the reaction of C5Me4H(C6H4OMe-o) with
KH, was added slowly and stirred at room temperature for 1 h. A
THF solution (3 mL) of Me3SiCH2Li (0.094 g, 1.00 mmol) was
then added slowly. After the mixture was stirred at room temper-
ature for 30 min, the solvent was removed in vacuum, and the
residue was extracted with toluene. The toluene extract was dried
in vacuum and washed with cold hexane to give 7 (0.188 g, 0.42
mmol, 84% yield) as white micro crystals. Single crystals suitable
for X-ray analysis were grown from a hexane solution at -30 °C.
1H NMR (300 MHz, C6D6, r.t., δ/ppm): -0.16 (s, 4H, CH2SiMe3),
0.28 (s, 18H, CH2Si(CH3)3), 1.88 (s, 6H, C5(CH3)4), 2.19 (s, 6H,
C5(CH3)4), 3.60 (s, 3H, OCH3), 6.26 (dd, J ) 8.4, 1.1 Hz, 1H,
-C6H4OMe), 6.83 (dt, J ) 7.3, 1.1 Hz, 1H, -C6H4OMe), 6.91 (dt,
J ) 7.9, 1.8 Hz, 1H, -C6H4OMe), 7.04 (dd, J ) 7.3, 2.0 Hz, 1H,
-C6H4OMe). 13C NMR (75 MHz, C6D6, r.t., δ/ppm): 4.1 (s,
CH2Si(CH3)3), 11.8 (s, C5(CH3)4), 11.9 (s, C5(CH3)4), 38.6 (br,
CH2SiMe3), 58.9 (s, OCH3), 110.5, 120.5, 120.7, 122.0, 124.4,
126.9, 128.5, 132.5, 160.0 (aromatics and Cp ring carbons). Anal.
calcd for C24H41OScSi2: C, 64.53; H, 9.25. Found: C 64.57; H 9.17.
1
C6F5-4), 149.3 (d, JCF ) 241.5 Hz, C6F5-2). Anal. calcd for
C41H32BF20O2ScSi: C, 48.25; H, 3.16. Found: C, 49.53; H, 3.66.
Synthesis of [(C5Me4SiMe3)Sc(CH2SiMe3)(THF)2][B(C6F5)4]
(10). To a 5 mL of THF solution of complex 5 (C5Me4SiMe3)-
Sc(CH2SiMe3)2(THF) (0.012 g, 25 µmol) was added 1 equiv of
[PhNMe2H][B(C6F5)4] (0.020 g, 25 µmol) in THF (5 mL) at room
temperature. The pale yellow mixture was stirred for 10 min. The
solvent was removed in vacuum. The residue was washed with
hexane several times and dried in vacuum to give 10 (0.028 g,
24.7 µmol, 99% yield) as white micro crystals. Single crystals
suitable for X-ray analysis were grown from a mixed solvent of
THF/hexane (5/20 mL/mL) at -30 °C. 1H NMR (400 MHz,
o-C6D4Cl2, 23 °C, δ/ppm): -0.09 (s, 9H, CH2Si(CH3)3), -0.02 (s,
2H, CH2SiMe3), 0.24 (s, 9H, C5Me4Si(CH3)3), 1.82 (s, 6H,
C5(CH3)4), 1.92 (br s, 8H, THF), 2.09 (s, 6H, C5(CH3)4), 3.95 (m,
8 H, THF). 13C NMR (99 MHz, o-C6D4Cl2, 23 °C, δ/ppm): δ 1.9
(s, C5Me4Si(CH3)3), 3.4 (s, CH2Si(CH3)3), 11.6 (s, C5(CH3)4), 15.0
(s, C5(CH3)4), 25.5 (s, THF), 50.9 (b, CH2SiMe3), 73.8 (s, THF),
124.4 (m, C6F5-1), 125.5 (s, ipso-C5Me4(SiMe3)), 129.6 (s,
1
C5Me4(SiMe3)), 134.1 (s, C5Me4(SiMe3)), 137.0 (d, JCF ) 240.7
1
1
Hz, C6F5-3), 138.8 (d, JCF ) 244.0 Hz, C6F5-4), 149.0 (d, JCF
)
240.5 Hz, C6F5-2). 19F NMR (o-C6D4Cl2, 564 MHz, 25 °C, CFCl3):
-132.4 (s, o-C6F5), -162.8 (t, p-C6F5) -166.8 (s, m-C6F5). 11B
NMR (o-C6D4Cl2, 192 MHz, 25 °C, BF3 ·Et2O): -16.8. Anal. calcd
for C48H48BF20O2ScSi2: C, 50.18; H, 4.21. Found: C, 50.50; H, 4.61.
Synthesis of [C5Me4{o-C6H4N(Me)CH2-µ}Sc(CH2SiMe3)]2
(8). ScCl3 (0.454 g, 3.00 mmol) was refluxed in 20 mL of THF at
80 °C for 12 h, the solution was cooled down with stirring to room
temperature to give a white suspension of ScCl3(THF)3. A THF
solution (15 mL) of [C5Me4(C6H4NMe2-o)]K·0.5Et2O (0.950 g, 3.00
mmol), which was prepared by the reaction of C5Me4H(C6H4NMe2-
o) with KH, was added slowly and stirred at room temperature for
1 h. A THF solution (5 mL) of Me3SiCH2Li (0.565 g, 6.00 mmol)
was then added slowly. After the mixture was stirred at room
temperature for 30 min, the solvent was removed in vacuum, and
the residue was extracted with toluene. The solution was kept at
-30 °C for 12 h, and was decanted. The solvent was removed in
vacuum. The residue was washed with hexane and diethyl ether to
give 8 (0.839 g, 1.13 mmol, 75% yield) as white powder. Single
crystals (with one molecule of hexane as a lattice solvent) suitable
for X-ray analysis were grown from a hexane solution at -30 °C.
1H NMR of anti-complex (400 MHz, C6D6, r.t., δ/ppm): -1.29 (d,
J ) 10.6 Hz, 2H, CH2SiMe3), -0.51 (d, J ) 10.6 Hz, 2H,
CH2SiMe3), 0.07 (s, 18H, CH2Si(CH3)3), 1.44 (d, J ) 13.3 Hz,
2H, NCH2), 1.54 (d, J ) 13.1 Hz, 2H, NCH2), 1.89 (s, 6H,
C5(CH3)4), 2.00 (s, 6H, C5(CH3)4), 2.15 (s, 6H, C5(CH3)4), 2.36 (s,
Synthesis of [{C5Me4(C6H4OMe)}Sc(CH2SiMe3)(THF)2]-
[B(C6F5)4] (11). To a THF solution (5 mL) of {C5Me4(C6H4OMe)}-
Sc(CH2SiMe3)2 (7) (0.011 g, 25 µmol) was added 1 equiv of
[PhNMe2H][B(C6F5)4] (0.020 g, 25 µmol) in THF (5 mL) at room
temperature. The pale-yellow mixture was stirred for 10 min. The
solvent was removed in vacuum. The residue was washed with
hexane several times and dried in vacuum to give 11 (0.029 g,
24.7 µmol, 99% yield) as white micro crystals. Single crystals
suitable for X-ray analysis were grown from a mixed solvent of
THF/hexane (5/20 mL/mL) at -30 °C. 1H NMR (C4D8O, 396 MHz,
23 °C, δ/ppm): -0.17 (s, 2H, CH2SiMe3), -0.06 (s, 9H,
C5Me4Si(CH3)3), 1.78 (br s, 8H, THF), 2.01 (s, 6H, C5(CH3)4), 2.31
(s, 6H, C5(CH3)4), 3.62 (m, 8H, THF), 4.20 (s, 3H, OCH3), 7.24
(d, 2H, -C6H4OMe), 7.34 (d, 1H, -C6H4OMe), 7.47 (m, 1H,
-C6H4OMe). 13C NMR (C4D8O, 99 MHz, 23 °C): δ 4.0 (s,
C5Me4SiMe3), 12.0 (s, C5Me4), 14.5 (s, C5Me4), 26.4 (s, THF), 32.6
(s, CH2SiMe3), 60.2 (s, OMe), 68.3 (s, THF), 112.2, 113.6, 117.7,
123.5, 124.9, 129.7, 130.3, 132.3, 159.0 (aromatics and Cp ring
1
carbons), 125.7 (m, C6F5-1), 137.2 (d, JCF ) 251.0 Hz, C6F5-3),
6H, C5(CH3)4), 2.68 (s, 6H, NCH3), 6.95-7.15 (m, 8H, C6H4). 13
C
139.3 (d, JCF ) 241.1 Hz, C6F5-4), 149.3 (d, JCF ) 238.7 Hz,
C6F5-2). 19F NMR (C4D8O, 564 MHz, 25 °C, CFCl3): -132.7 (s,
o-C6F5), -165.0 (t, p-C6F5) -168.5 (s, m-C6F5). 11B NMR (C4D8O,
192 MHz, 25 °C, BF3 · Et2O): -16.6. Anal. calcd for
C52H46BF20O3ScSi: C, 52.81; H, 3.92. Found: C, 53.42; H, 4.25.
1
1
NMR (75 MHz, C6D6, 50 °C, δ/ppm): 4.4 (s, CH2Si(CH3)3), 12.2
(s, C5(CH3)4), 13.0 (s, C5(CH3)4), 13.1 (s, C5(CH3)4), 13.4 (s,
C5(CH3)4), 39.0 (br, CH2SiMe3), 48.5 (NCH3), 72.8 (NCH2), 117.0,
119.3, 120.0, 120.5, 122.3, 123.5, 126.6, 128.7, 131.6, 133.9, 159.3
9
J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009 13881

A R T I C L E S
Li et al.
A Typical Procedure for Isoprene (IP) Polymerization
(Table 3, entry 1). In a glovebox, a chlorobenzene solution (5 mL)
of [Ph3C][B(C6F5)4] (0.023 g, 25 µmol) was added to a well-stirred
chlorobenzene solution (5 mL) of complex 1 (0.009 g, 25 µmol)
and isoprene (1.022 g, 15 mmol) at 25 °C in a 30-mL flask. The
reaction mixture became viscous rapidly. After 5 min, the flask
was taken outside and the polymerization was quenched by addition
of methanol (50 mL, containing 5% butylhydroxytoluene (BHT)
as a stabilizing agent). Then the mixture was poured into methanol
(200 mL) to precipitate the polymer product. The precipitated
polymer was dried under vacuum at 60 °C to a constant weight
(1.020 g, 100% yield). The resulting polymer is soluble in THF
and chloroform at room temperature.
in which IH5 is the integration of the broad signals at 1.3 ppm as
the total methylene protons in both 3,4-isoprene units and ethylene
units in the H NMR spectrum.
1
X-Ray Crystallographic Analysis. A crystal was selected and
sealed in a thin-walled glass capillary under a microscope in a
glovebox. Data collections were performed at -100 °C on a Bruker
SMART APEX diffractometer with a CCD area detector using
graphite-monochromated Mo KR radiation (λ ) 0.71069 Å). The
determination of crystal class and unit cell was carried out by
SMART program package.27 The raw frame data were processed
using SAINT28 and SADABS29 to yield the reflection data file.
The structures were solved by using SHELXTL program.30
Refinements were performed on F2 anisotropically for all the non-
hydrogen atoms by the full-matrix least-squares method. The
analytical scattering factors for neutral atoms were used throughout
the analysis. The hydrogen atoms were placed at the calculated
positions and were included in the structure calculation without
further refinement of the parameters. The residual electron densities
were of no chemical significance. The methyl carbon and silicon
atoms of CH2SiMe3 (Si2/Si3, C7/C9) in 6 were disordered (50:50
occupancy). Crystal data, data collection and processing parameters
for compounds 1-8, 10, and 11 are summarized in Supporting
Information. Crystallographic data (excluding structure factors) have
been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication nos. CCDC-738570 (1), -738573 (2),
-738574 (3), -738575 (4), -256379 (5), -738576 (6), -738577
(7), -738578 (8), -738571 (10), and -738572 (11). These data
can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
The isomer contents of the polyisoprene products were calculated
from the 1H and 13C NMR spectra according to the following
formula (eqs 1-5):
Mol 1,4-IP% ) {IH1/(IH1 + 0.5IH2)} × 100
Mol 3,4-IP% ) {0.5IH2/(IH1 + 0.5IH2)} × 100
(1)
(2)
in which IH1 is the integration of the resonance at 5.13 ppm (one
vinyl proton of the 1,4-isoprene unit), and IH2 is the integration of
the resonance at 4.72 ppm (two vinyl protons of the 3,4-isoprene
1
unit) in the H NMR spectrum.
Mol cis-1,4-IP% ) {IC1/(IC1+IC2+IC3)} × 100
Mol trans-1,4-IP% ) {IC3/(IC1+IC2+IC3)} × 100
Mol 3,4-IP% ) {IC2/(IC1+IC2+IC3)} × 100
(3)
(4)
(5)
Computational Details. The geometry optimizations and energy
estimations were carried out with B3LYP density functional
method,31 which is adopted in Gaussian 03 program.32 The
6-31G(D) basis set was considered for the entire coordinate
complexes. The initial structures of active species were from their
crystal structures. Since there is no symmetry in these complexes,
A C1 symmetry point group was used throughout all calculations,
and no higher molecular symmetry restriction was imposed. All
calculations were performed utilizing the Gaussian 03 program.
in which IC1 is the integration of the signals at 23.2 ppm assigned
as the methyl carbon of the cis-1,4-isoprene unit, and IC2 is the
integration of the signals at 18.5 ppm assigned as the methyl carbon
of the 3,4-isoprene unit, while IC3 is the integration of the signals
at 15.9 ppm assigned as the methyl carbon of the trans-1,4-isoprene
unit in the 13C NMR spectrum.
A Typical Procedure for the Copolymerization of Isoprene
(IP) with Ethylene (E) (Table 4, entry 32). In a glovebox, a
chlorobenzene solution (30 mL) of isoprene (3.406 g, 50 mmol)
was charged into a two-necked flask with a magnetic stir bar. The
flask was taken outside, set in a water bath (25 °C), and connected
to a well-purged Schlenk ethylene line and a mercury-sealed stopper
by use of a three-way cock. Ethylene (1 atm) was introduced into
the system and was saturated in the solution by stirring for 2 min.
Acknowledgment. This work was partly supported by a Grant-
in-Aid for Scientific Research on Priority Areas (No. 18065020,
“Chemistry of Concerto Catalysis”) from MEXT of Japan and a
Grant-in-Aid for Scientific Research (S) (No. 21225004) from JSPS.
A
chlorobenzene solution (10 mL) of (C5Me4(C6H4OMe-
Supporting Information Available: Detailed experimental
procedures and spectra of representative polymer products.
Complete ref 32. This material is available free of charge via
the Internet at http://pubs.acs.org.
o))Sc(CH2SiMe3)2 (7) (0.011 g, 25 µmol) and [PhNMe2H][B(C6F5)4]
(0.020 g, 25 µmol) was then added through a syringe under vigorous
stirring. The reaction mixture became viscous rapidly. The polym-
erization was quenched after 5 min by addition of methanol (300
mL, containing 5% butylhydroxytoluene (BHT) as a stabilizing
agent). The polymer product was collected by filtration, washed
with methanol, and then dried in Vacuum at room temperature to a
constant weight (2.71 g, 1.3 × 103 kg of copolymer/mol-Sc ·h·atm).
The resulting polymer is soluble in dichlorobenzene and 1,1,2,2-
tetrachloroethane.
JA9056213
(27) SMART Software Users Guide, Version 4.21; Bruker AXS, Inc.:
Madison, WI, 1997.
(28) SAINT+, Version 6.02; Bruker AXS, Inc.: Madison, WI 1999.
(29) Sheldrick, G. M. SADABS; Bruker AXS, Inc.: Madison, WI, 1998.
(30) Sheldrick, G. M. SHELXTL, Version 5.1; BUKER AXS, INC.:
Madison, WI, 1998.
(31) (a) Beck, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (b) Lee, C., T.;
Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(32) Frisch, M. J.; et al. Gaussian 03, ReVision E.01; Gaussian, Inc.:
Wallingford, CT, 2004. (see Supporting Information for all authors).
1
The IP contents in the copolymers were calculated from the H
and 13C NMR spectra according to eq 6, whereas the isoprene
isomer units in the copolymer chains were calculated according to
eqs 1-5 as described above.
Mol IP% ) {(IH1 + 0.5IH2)/(IH1 + 0.5IH2 + 0.25(IH5-
IH2))} × 100 (6)
9
13882 J. AM. CHEM. SOC. VOL. 131, NO. 38, 2009