Living 3,4-polymerization of isoprene by cationic rare earth metal alkyl complexes bearing iminoamido ligands

Article abstract of DOI:10.1021/om100971d

Treatment of rare earth metal trialkyl complexes Ln(CH₂SiMe ₃)₃(THF)₂ (Ln = Sc, Lu, and Y) with 1 equiv of α-diimine ligands 2,6-R₂C₆H₃N=CH-CH= NC₆H₃R₂-2,6 (R = ⁱPr, Me) affords straightforwardly monoanionic iminoamido rare earth metal dialkyl complexes [2,6-R₂C₆H₃N-CH₂-C(CH ₂SiMe₃)=NC₆H₃R₂-2,6] Ln(CH₂SiMe₃)₂(THF) (1: Ln = Sc, R = ⁱPr; 2: Ln = Lu, R = ⁱPr; 3: Ln = Y, R = ⁱPr; 4: Ln = Sc, R = Me; 5: Ln = Lu, R = Me; 6: Ln = Y, R = Me) in 65-85% isolated yields. X-ray analyses show these complexes have decreasing steric hindrance in the coordination spheres of the metal centers in the order 1 > 2 > 3 > 4 > 5 > 6. A mechanism involving intramolecular alkyl and hydrogen migration is supported on the basis of DFT calculations to account for ligand alkylation. Activated by [Ph₃C][B(C₆F₅) ₄], all of these iminoamido rare earth metal dialkyl complexes are active for living polymerization of isoprene, with activity and selectivity being significantly dependent on the steric hindrance around the metal center to yield homopolyisoprene materials with different microstructures and compositions. The sterically crowded complexes 1-3 give a mixture of 3,4- and trans-1,4-polyisoprenes (3,4-selectivities: 48-82%, trans-1,4-selectivities: 50-17%), whereas the less sterically demanding complexes 4-6 show high 3,4-selectivities (3,4-selectivities: 90-100%). In the presence of 2 equiv of AlⁱBu₃, the complexes 1-6/activator systems exhibit higher activities and 3,4-selectivities in the living polymerization of isoprene. A similar structure-reactivity relationship in polymerization catalysis can be also observed in these ternary systems. A possible mechanism of the isoprene polymerization processes is proposed on the basis of the DFT calculations.

Full text of DOI:10.1021/om100971d

160 Organometallics 2011, 30, 160–170
DOI: 10.1021/om100971d
Living 3,4-Polymerization of Isoprene by Cationic Rare Earth Metal
Alkyl Complexes Bearing Iminoamido Ligands
Gaixia Du,† Yanling Wei,† Lin Ai,‡ Yuanyuan Chen,† Qi Xu,† Xiao Liu,†
Shaowen Zhang,*,† Zhaomin Hou,*,§ and Xiaofang Li*,†
†Key Laboratory of Cluster Science of Ministry of Education, Beijing Institute of Technology,
5 South Zhongguancun Street, Haidian District, Beijing 100081, People’s Republic of China, ‡Department of
Chemistry, Beijing Normal University, 19 Xinjiekou Wai Street, Haidian Destrict, Beijing 100875,
§
People’s Republic of China, and Organometallic Chemistry Laboratory and Advanced Catalyst Research Team,
RIKEN Advanced Science Institute, Hirosawa 2-1, Wako, Saitama 351-0198, Japan
Received October 8, 2010
Treatment of rare earth metal trialkyl complexes Ln(CH2SiMe3)3(THF)2 (Ln = Sc, Lu, and Y) with 1
equiv of R-diimine ligands 2,6-R2C6H3NdCH-CHdNC6H3R2-2,6 (R = iPr, Me) affords straightfor-
wardly monoanionic iminoamido rare earth metal dialkyl complexes [2,6-R2C6H3N-CH2-C(CH2-
SiMe3)dNC6H3R2-2,6]Ln(CH2SiMe3)2(THF) (1: Ln = Sc, R=iPr; 2: Ln=Lu, R=iPr; 3: Ln = Y, R =
iPr; 4:Ln=Sc, R=Me;5:Ln=Lu, R=Me;6:Ln=Y, R=Me) in65-85% isolated yields. X-ray analyses
show these complexes have decreasing steric hindrance in the coordination spheres of the metal centers in
the order 1> 2> 3> 4> 5> 6. A mechanism involving intramolecular alkyl and hydrogen migration is
supported on the basis of DFT calculations to account for ligand alkylation. Activated by [Ph3C]-
[B(C6F5)4], all of these iminoamido rare earth metal dialkyl complexes are active for living polymerization
of isoprene, with activity and selectivity being significantly dependent on the steric hindrance around the
metal center to yield homopolyisoprene materials with different microstructures and compositions. The
sterically crowded complexes 1-3 give a mixture of 3,4- and trans-1,4-polyisoprenes (3,4-selectivities:
48-82%, trans-1,4-selectivities: 50-17%), whereas the less sterically demanding complexes 4-6 show
high 3,4-selectivities (3,4-selectivities: 90-100%). In the presence of 2 equiv of AliBu3, the complexes
1-6/activator systems exhibit higher activities and 3,4-selectivities in the living polymerization of isoprene.
A similar structure-reactivity relationship in polymerization catalysis can be also observed in these ternary
systems. A possible mechanism of the isoprene polymerization processes is proposed on the basis of the
DFT calculations.
Introduction
Despite these recent extensive efforts, however, a cationic alkyl
rare earth metal complex based on an iminoamido ligand is rare,
and little is known about the catalytic potential of these com-
plexes for olefin polymerization.20
Cationic alkyl rare earth (group 3 and lanthanide) metal
complexes are of current interest as potentially novel olefin
polymerization catalysts.1-19 A number of cationic alkyl rare
earth metal complexes supported by various ancillary ligands
such as cyclopentadienyls,2 deprotonated aza-crowns,3 benz-
amidinates,4 β-diketiminates,5 anilido-imines,6 triazacyclono-
nanes,7 amide-functionalized triazacyclononanes,8 phosphides,9
tris(oxazolyl)ethane,10 crown ethers,11 bis(phosphinophenyl)-
amido,12 indenyl- or fluorenyl-functionalized N-heterocyclic
carbene,13 iminophosphonamido,14 thiophene-NPN,15 in-
dolide-imine,16 pyrrolide,17 arylamido,18 and aminopyridinate19
have been synthesized and have had their reactivities studied.
R-Diimine ligands, which allow an extensive adjustment in
both the geometric and donor-acceptor properties by vary-
ing the substituents in the ligand frame and thereby a control
over properties of the prepared catalysts, represent one of
the most widely utilized groups of nitrogen donor chelating
ligands in coordination chemistry.21 In some cases, the
neutral R-diimine ligands can transform into monoanionic
iminoamido ligands via ligand alkylation by reacting with
alkyls and hydrides of main-group metals Al, Mg, and Zn
and transition metals Zr, Hf, and Ni.22 For example, reac-
tions of the substituted R-diimine compounds with AlEt2Cl
afford a mixture of products via an ethyl transfer from
aluminum to either the imine nitrogen atom (N-alkylation)
or the imine carbon atom (C-alkylation) with formation of
an enamine aluminum compound A or an iminoamido
aluminum compound B, respectively (Scheme 1).22g,h Treat-
ment of the nonsubstituted R-diimine ligand with AlEt2Cl
affords an iminoamido product C possibly resulting from the
C-alkylation followed by a 1,2-hydrogen shift (Scheme 1).22g,h
Recently, the rediscovery and nice extension of Keim’s work
*Corresponding authors. E-mail: xfli@bit.edu.cn; swzhang@bit.edu.cn;
houz@riken.jp.
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pubs.acs.org/Organometallics
Published on Web 12/06/2010
2010 American Chemical Society

Article
Organometallics, Vol. 30, No. 1, 2011 161
Scheme 1. Intramolecular Alkyl Transfer and Hydrogen Shift in
r-Diimine Aluminum Complexes
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162 Organometallics, Vol. 30, No. 1, 2011
Du et al.
transition metals.24 However, the incorporation of R-diimine
ligands into rare earth (group 3 and lanthanide) metal
complexes has never been reported.25 More recently, the
synthesis of the dialkyl scandium and yttrium complexes
based on R-diimine ligands and their performance in the
intramolecular hydroamination/cyclization of aminoalkenes
have been reported by Mashima et al. when this paper was
in progress.26 However, their catalytic activities in olefin
polymerization reactions are not known yet.
controlled polymerization of 1,3-conjugated dienes such as
isoprene (IP).27-29 Most of these catalysts serve as efficient
catalysts for the production of cis-1,4-2g,y,j,x,5c,11d,12,16,17,27 or
trans-1,4-polyisoprenes.15,28 In contrast, 3,4-polyisoprene,
far less studied, has attracted renewed interest recently because
of its peculiar mechanical properties such as wet-skid-resistant
and low-rolling-resistant tread.2m,13,14,18a,19c,29 However, few
cases of the catalytic systems show both high 3,4-selecitivity
(>90%) and living mode.13b Herein, we report the one-pot
synthesis and structural characterization of a new class of
cationic alkyl rare earth metal complexes bearing iminoamido
ligands based on R-diimine ligands. X-ray structures show that
these complexes have decreasing steric hindrance around the
metal centers in the order 1>2>3>4>5>6. Amechanism
involving intramolecular alkyl and hydrogen migration is sup-
ported on the basis of DFT calculations to account for the
observed ligand alkylation. Activated by an equivalent of
[Ph3C][B(C6F5)4], these complexes are active for living 3,4-
polymerization of isoprene, with activity and selectivity being
significantly dependent on the steric hindrance around the metal
center to yield homopolyisoprene materials with different mi-
crostructures and compositions. In the presence of 2 equiv of
AliBu3, these cationic catalytic systems show higher activities
and 3,4-selectivities for the living polymerization of isoprene. A
similar structure-reactivity relationship in polymerization catal-
ysis can also be observed in these ternary systems. A possible
mechanism of the isoprene polymerization processes is proposed
on the basis of DFT calculations.
Cationic alkyl rare earth metal complexes usually exhib-
it distinguished catalytic proformances in the precisely
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Results and Discussion
Synthesis and Structural Characterization of Monoanionic
Iminoamido Rare Earth Metal Dialkyl Complexes. Recently,
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based on R-diimine ligands and the structural characteriza-
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Article
Organometallics, Vol. 30, No. 1, 2011 163
˚
Table 1. Selected Bond Lengths [A] and Angles [deg] for
Scheme 2. Synthesis of Monoanionic Iminoamido Rare Earth
Metal Dialkyl Complexes Based on r-Diimine Ligands
Complexes 3-6
3
4
5
6
Ln = Y
Ln = Sc Ln = Lu Ln = Y
Ln-N1
Ln-N2
Ln-O
Ln-C1
Ln-C5
N1-C9
N2-C10
C9-C10
—N1-Ln-O
—N1-Ln-C1
—O-Ln-C1
—N1-Ln-C5
—O-Ln-C5
—C1-Ln-C5
—N1-Ln-N2
—O-Ln-N2
—C1-Ln-N2
—C5-Ln-N2
—N1-C9-C10
—N2-C10-C9
—C9-N1-Ln
—C10-N2-Ln
Ln-N1C9C10N2
C15-N1C9C10N2
C27-N1C9C10N2
C23-N1C9C10N2
N1LnN2-N1C9C10N2
2.213(5)
2.515(4)
2.357(4)
2.419(5)
2.429(6)
1.447(7)
1.300(7)
1.497(8)
90.4(2)
2.066(2)
2.350(2)
2.217 (2) 2.304(2)
2.182(3)
2.428(3)
2.222(2)
2.483(2)
2.352(2)
2.421(3)
2.415(3)
1.449(3)
1.292(3)
1.485(4)
89.8 (1)
122. 6(1)
92.1(1)
2.247(2)
2.256(2)
1.450(3)
1.291(3)
1.486(3)
90.1 (1)
2.365(4)
2.366(3)
1.453(4)
1.288(4)
1.485(5)
89.3(1)
123.2(2)
93.6 (2)
122.8(2)
86.8(2)
122.0 (1) 122.5(1)
92.0 (1)
124.1(1)
90.9 (1)
92.4(1)
122.7(1)
91.0(1)
122.0 (1)
91.4(1)
114.1(2)
70.3(2)
160.3 (1) 161.9 (1) 158.1(1)
113.7 (1) 114.8(1)
74.0 (1) 71.2 (1)
115.3 (1)
69.8(1)
156.8 (1)
relation of the catalysts, we then synthesized and structurally
characterized a series of monoanionic iminoamido rare earth
metal dialkyl complexes. The reaction between rare earth metal
trialkyl complexes Ln(CH2SiMe3)3(THF)2 (Ln = Sc, Lu, and
Y) and 1 equiv of R-diimine ligands 2,6-R2C6H3NdCH-CHd
NC6H3R2-2,6 (R = iPr, Me) in toluene at room temperature
gave straightforwardly monoanionic iminoamido rare earth
metal dialkyl complexes 1-6 [2,6-R2C6H3N-CH2-C(CH2Si-
Me3)dNC6H3R2-2,6]Ln(CH2SiMe3)2(THF) (1: Ln = Sc, R =
iPr, 85%; 2: Ln = Lu, R = iPr, 78%; 3: Ln = Y, R = iPr, 65%;
4:Ln=Sc, R=Me, 70%;5:Ln=Lu, R=Me, 65%;6:Ln=
Y, R = Me, 78%) in high yields (Scheme 2). All of these com-
plexes 1-6 are soluble in common organic solvents, such as
THF, toluene, and hexane, and gave well-resolved NMR
spectra in C6D6. 1H NMR spectra indicated a loss of symmetry
in the chelating ligands, implying that the target tris(trimethyl-
silylmethyl) products containing neutral R-diimine ligands
could not be obtained, whereas one singlet for the imino-
methylene group at 1.80 ppm integrating for two protons and
another singlet for the amino-methylene groups at 4.50 ppm
integrating for two protons were observed. Characteristic high-
field resonances for the metal alkyl group Ln-CH2Si(CH3)3
appeared around -0.58 ppm (s, 4H, CH2) and 0.22 ppm (s,
18H, Si(CH3)3), suggesting that only two trimethylsilylmethyl
groups coordinated with the metal center. Moreover, only one
singlet for the methylene protons of the Ln-alkyl groups indi-
cated that the metal alkyl species in complexes 1-6 are fluxional
in the solution state, which can freely rotate. One THF molecule,
which showed signals at 1.0 and 3.5 ppm, was also found in the
1H NMR spectra. In the 13C NMR spectra of complexes 1-6, a
downfield shift resonance assignable to the imine carbon of
N-CH2-CR0dN appeared around 190.2 ppm. Similar obser-
vations were also obtained by Mashima et al.26
100.0(2)
100.4(2)
114.5(4)
118.9(5)
119.9(3)
113.5(3)
103.6 (1) 106. 5(1) 107.9 (1)
91.0(1)
91.2 (1)
114.6(3)
117.2 (2) 117.6(3)
90.4 (1)
114.6(2)
117.9(2)
121.2(2)
116.3(2)
0.099(5)
113.8(2)
120.1(1)
114.7(1)
120.2(2)
116.3(2)
0.101(6)
0.454(10) 0.096(4)
-0.629(9) -0.212(4) -0.216(6) -0.226(4)
-0.229(9)
-0.188(4) -0.188(6) -0.180(5)
13.8(3)
3.26(2)
89.5(1)
3.1(3)
88.0 (2)
3.0(2)
86.7(1)
(C15-C20)-N1C9C10N2 88.6(2)
(C27-C32)-N1C9C10N2 82.0(2)
(C23-C28)-N1C9C10N2
78.4(1)
77.1(1)
76.1 (1)
the normal range of a CdN double bond, are shorter than those
˚
of N1-C9 (1.447(7)-1.453(4) A) in the normal range of a single
bond. Because of the ionic radius of the metal center in the trend
˚
˚
˚
Sc (0.89 A) < Lu (1.00 A) < Y (1.04 A), the bond distances of
the chelating Ln-N1, Ln-N2, Ln-O, Ln-C1, and Ln-C5
bonds increase in the order 4 < 5 < 3, 6. The bond distances of
˚
˚
the Ln-N1 (2.481(6) A), Ln-N2 (2.336(6) A), and Ln-O
(2.382(3) A) bonds in 4 are comparable with those found in a
˚
similar scandium complex reported by Mashima et al. recently.26
The angles between N1-Ln-N2 of complexes 3-6 are
very narrow, about 69.8(1)-74.0(1), maybe due to the bite of
the chelating iminoamido ligand.22f The N1-C9-C10-N2
(NCCN) portion of the iminoamido ligand is almost planar.
The rare earth metal center has a slightly folded geometry with
the NCCN plane. The yttrium center in the 2,6-diisopropylphe-
˚
nyl-substituted complex 3is lying 0.454(10) A out of the plane of
NCCN. In contrast, the distances of the metal centers to the
NCCN planes in the 2,6-dimethylphenyl-substituted complexes
Crystals of complexes 3-6 suitable for an X-ray crystal
structure determination were grown from a concentrated hexane
solution at -30 ꢀC. X-ray analyses revealed that complexes 4-6
are isostructural and isomorphous. Their selected bond lengths
and angles are summarized in Table 1, and the ORTEP drawings
of 3-6 are shown in Figure 1. Each of complex 3-6 adopts a
distorted trigonal-bipyramidal geometry, in which the rare earth
metal center is bonded with one monoanionic iminoamido unit
[2,6-R2C6H3N-CH2-C(CH2SiMe3)dNC6H3R2-2,6], two tri-
methylsilylmethyl alkyls (in cis position), and one THF. The
Ln-N bond with the formally negatively charged amido nitro-
˚
4-6 are much shorter, only about 0.10 A. The dihedral angle of
the best planes of N1-Ln-N2 and NCCN of complex 3 is
13.8(3)ꢀ, which is larger than those of the best planes of N1-
Ln-N1 and NCCN of complexes 4-6(av3.1ꢀ), consistent with
a planar five-membered metallacyclic ring. The distances of the
phenyl carbon atoms C15 and C27 to the NCCN plane of
the 2,6-diisopropylphenyl-substituted complex 3 (0.629(9) and
˚
0.229(9) A, respectively, located in a different direction from
the metal center) are longer than those of the corresponding
phenyl carbon atoms C15 and C23 to the NCCN planes in
˚
˚
gen N1 is significantly shorter at 2.066(2)-2.222(2) A than the
complexes 4-6 (av 0.218 and 0.185 A, respectively). Moreover,
neutral imino nitrogen Ln-N2 at 2.350(2)-2.515(4) A. The
the different steric hindrance of the 2,6-disubstitutes (iPr > Me)
on the iminoamido ligand and the different ionic radius of the
˚
˚
bond distances of N2-C10 (1.288(4)-1.300(7) A), which lie in

164 Organometallics, Vol. 30, No. 1, 2011
Du et al.
Figure 1. ORTEP drawings of complexes 3-6 with 30% thermal ellipsoids. The hydrogen atoms in 3-6 are omitted for clarity.
Scheme 3. Possible Mechanism Involving Intramolecular Alkyl and Hydrogen Migration for Formation of Complexes 1-6
Scheme 4. DFT Calculations for Possible Mechanism of Ligand Alkylation Reaction
metal center result in the different rotation angles of the 2,6-
disubstituted phenyl rings to NCCN plane. In contrast to deriv-
atives containing sterically unhindered phenyl substituents in
which one phenyl group is virtually coplanar with the AlNCCN
ring, the two aromatic rings of the iminoamido ligand are planar
(C15-C20 and C23(C27)-C28(C32)), the former plane is
almost perpendicular to the NCCN plane (av 88.2ꢀ), while the
latter forms a rotated angle decreasing in the order 82.0(2)ꢀ >
78.4(1)ꢀ > 77.1(1)ꢀ > 76.1(1)ꢀ. As a result, these complexes have
decreasing steric hindrance around the metal centers in the order
1 > 2 > 3 > 4 > 5 > 6.
high yields. In view of ligand alkylation via alkyl transfer in com-
bination with a hydrogen shift having been proposed previously
by Vrieze et al.,22a a mechanism involving intramolecular alkyl
transfer to an imine carbon atom followed by hydrogen migra-
tion was performed based on DFT calculations to account for
the observed isomerization of the R-diimine rare earth metal
trialkyl complex a (Schemes 3 and 4). The calculations were
done in the same conditions as in the experiments; namely, the
toluene solvation effects were included for the -CH2SiMe3
transfer and hydrogen migration. The 2,6-dimethylphenyl-
substituted iminoamido lutetium complex 5was used as a model.
The calculated structures and the relative Gibbs free energies
are summarized in Scheme 4. It is clear that the isomerization of
complex a involves two steps; in the first one, a -CH2SiMe3
group transfers from the lutetium to a carbon atom through a
Reaction Mechanism. The observation described above
demonstrated that the reaction between the R-diimine ligands
and the rare earth metal trialkyl complexes gave the mono-
anionic iminoamido rare earth metal dialkyl complexes 1-6 in

Article
Organometallics, Vol. 30, No. 1, 2011 165
Table 2. Isoprene Polymerization by Complexes 1-6/Borate
the 3,4-regioselectivity gradually increased from 48% to 99%
(Table 2, entries 3-8). Among them, the sterically crowded
complexes 1-3 gave a mixture of 3,4- and trans-1,4 poly-
isoprenes (3,4-selectivities: 48-82%; trans-1,4-selectivities:
50-17%), whereas the less sterically demanding complexes 4-6
showed high 3,4-selectivities (3,4-selectivities: 90-99%). For the
complex 5/[Ph3C][B(C6F5)4] system, with the increasing poly-
merization time, the molecular weight of the resulting polymer
increased linearly with the conversion of IP, while the molecular
weight distribution stayed less than 1.5, indicating that the IP
polymerization behavior by these binary systems has living
properties (Table 2, entries 5, 7-9; see Supporting Information).
When the isoprene polymerization by the 5/[Ph3C][B(C6F5)4]
system was carried out at the lower temperature, further higher
regioselectivity (3,4-selectivity: 100%) was observed, as shown
by the 1H and 13C NMR analyses (see Supporting Information)
(Table 2, entries 5, 10). If the anilinium borate [PhNHMe2]-
[B(C6F5)4] was used as activator, similar activity and selectivity
were obtained (Table 2, entry 11).
The 3,4-polyisoprenes all showed good solubilities in THF
and CHCl3. The 1H NMR spectra of the polyisoprenes
obtained by the complexes 1-3/borate systems in CDCl3
indicated a mixture of 3,4- and 1,4-microstructures (3,4-
PIP:1,4-PIP = 1:1-1:4), and the 13C NMR spectra showed
diagnostic signals for both a trans configuration (δ 16.2,
26.9, 39.9, 124.4, and 135.1 ppm) and a 3,4-configuration (δ
18.8, 37.6, 111.4, and 147.9 ppm). The 1H NMR spectra of
the polyisoprenes obtained by the complexes 4-6/borate sys-
tems in CDCl3 indicated a predominant 3,4-microstructure
(above 90%), and the 13C NMR spectra showed main reso-
nances assigned to 3,4-isoprene units. The GPC curves of the
resulting copolymers were all unimodal with moderate poly-
dispersities (Mw/Mn = 1.04-1.36), consistent with the predom-
inance of a homogeneous single-site catalytic species. The glass
transition temperature (Tg) of the polymers showed an almost
linear correlation with the 3,4-isoprene content.
Binary Systemsa
microstructuresb
c
yield cis-
1,4-
trans-
1,4-
Mn
103
/
Mw/
c
Mn
Tgd/ꢀ
C
entry cat. t/h (%)
3,4-
1
2
3
4
5
6
7
8
9
1(Sc)
2(Lu)
3(Y)
3
3
3
15
40
2
2
1
0
0
0
0
0
0
0
0
50
28
17
10
2
48
70
82
90
98
99
99
99
99
100
99
28
45
1.04
1.12
1.36
1.18
1.22
1.17
1.27
1.27
1.22
1.12
1.20
-36
-10
10
100
60
96
62
4(Sc) 1.5
25
37
5(Lu) 1.5 100
6(Y) 1.5 100
5(Lu) 0.25 20
110
100
62
1
39
38
1
5(Lu) 0.5
5(Lu)
10e 5(Lu) 24
48
91
10
1
82
40
39
1
1
110
20
0
40
40
11f 5(Lu) 1.5 100
1
107
a Conditions: 25 μmol of Ln complex, 25 μmol of [Ph3C][B(C6F5)4],
10 mL of toluene, 25 ꢀC. b Determined by 1H, 13C NMR spectra.
c Determined by GPC in THF at 40 ꢀC against polystyrene standard.
d Measured by DSC. e Reaction temperature is 0 ꢀC. f [PhNMe2H]-
[B(C6F5)4] served as activator.
transition state (TS1) with a barrier height of 16.32 kcal mol-1
,
3
forming an intermediate (b) with a free energy of -39.63 kcal
3
mol-1 relative to the reactant; in the second step, a hydrogen
atom shifts from the carbon atom connected with the tranferred
-CH2SiMe3 group to the nearby carbon atom through a
transition state (TS2) with a barrier height of 15.65 kcal mol-1
,
3
yielding the final product 5, which will pack into crystals as
determined in experiments, with a free energy of -55.77 kcal
3
mol-1 relative to the reactant. These results indicate that the
reaction process from the reactant to the product 5is exothermic
and can proceed automatically due to the low reaction barrier
heights. The driving force for the alkyl shift, which is restricted to
tertiary alkyl groups, is maybe due to the release of steric strain
in the five-membered NCCN rare earth metal chelate ring.
Polymerization of Isoprene (IP) by Complexes 1-6/Borate
Binary Systems. The cationic iminoamido rare earth metal
alkyl species in situ generated by the reaction of complexes 1-6
and 1 equiv of activator such as [Ph3C][B(C6F5)4] or [PhMe2-
NH][B(C6F5)4] were active for the living polymerization of
isoprene, with activity and selectivity being significantly depen-
dent on the steric hindrance around the metal center to yield
homopolyisoprene materials with different microstructures and
compositions. Some representative results are summarized in
Table 2. The neutral iminoamido rare earth metal dialkyl com-
plexes 1-6 showed no activities in the absence of an activator.
The trityl perfluorophenyl borate [Ph3C][B(C6F5)4] alone acted
as a cationic polymerization initiator, yielding amorphous
polyisoprene with a mixture of 3,4- and 1,4-cis/trans microstruc-
tures,2m while the ammonium borate [PhNHMe2][B(C6F5)4]
was inactive for IP polymerization under the same conditions.
In contrast, the combination of 1-6 and [Ph3C][B(C6F5)4]
exhibited good activity and regioselectivity for the polymeriza-
tion of IP at room temperature (Table 2, entries 1-6). More-
over, significant influences of the steric hindrance of the metal
centers on the polymerization activities and selectivities were
observed in these binary systems. With the decreasing steric hin-
drance of the metal center in the order 1 > 2 > 3 > 4 > 5 > 6,
Polymerization of Isoprene (IP) by the Complexes 1-6/
Borate/AliBu3 Ternary Systems. To thoroughly investigate
the structure-reactivity relationship of these cationic imi-
noamido rare earth metal catalysts-promoted isoprene poly-
merization, 2 equiv of AliBu3 was added to the complexes
1-6/activator binary system in the isoprene polymerization
under the same conditions. Representative results are sum-
marized in Table 3. In contrast with the complexes 1-6/
activator binary systems, the presence of 2 equiv of AliBu3
dramatically increased catalytic activities and regioselectiv-
ities under the same conditions (Table 3, entries 1-6). Only
8-15 min was needed for all of the complexes 1-6/[Ph3C]-
[B(C6F5)4]/AliBu3 ternary systems to convert quantitatively
600 equiv of isoprene into polyisoprenes at room tempera-
ture (Table 3, entries 1-6), which was much faster than those
of the complexes 1-6/activator binary systems. Significant
influences of the steric hindrance of the metal centers on the
polymerization activities and selectivities were also observed
in these ternary systems. As the steric hindrance of the metal
center decreased in 1 > 2 > 3 > 4 > 5 > 6, the content of the
3,4-isoprene unit in the resulting polyisoprene gradually
increased from 54% to 99% (Table 3, entries 1-6), which
was slightly higer than that of the corresponding complexes
1-6/activator binary systems. Similar to the complex 5/
borate binary system, the 5/[Ph3C][B(C6F5)4]/AliBu3 ternary
system also showed higher 3,4-selectivities at lower tempera-
tures (3,4-selectivities:100%) (Table3,entries10-11). Moreover,
the small amount of aluminum compounds had less effect on

166 Organometallics, Vol. 30, No. 1, 2011
Table 3. Isoprene Polymerization by Complexes 1-6/Borate/AliBu3 Ternary Systemsa
Du et al.
microstructuresb
Mnc/103
Mw/Mn
Tgd/ꢀC
c
entry
cat.
time/min
yield (%)
cis-1,4-
trans-1,4-
3,4-
1
2
3
4
5
6
7
8
1(Sc)
2(Lu)
3(Y)
4(Sc)
5(Lu)
6(Y)
5(Lu)
5(Lu)
5(Lu)
5(Lu)
5(Lu)
15
10
8
8
8
100
100
100
100
100
100
23
55
95
71
9
6
1
2
1
0
0
0
0
0
0
0
40
22
10
8
1
1
1
1
1
0
54
77
88
91
99
99
99
99
59
69
75
59
72
78
38
53
70
102
65
1.26
1.27
1.14
1.18
1.30
1.16
1.15
1.19
1.27
1.25
1.35
-30
-3
10
29
36
36
36
37
35
43
43
8
1
3
5
9
99
100
100
10e
11e
12f
42f
0
a Conditions: 25 μmol of Ln complex, 25 μmol of [Ph3C][B(C6F5)4], 50 μmol of AliBu3, 10 mL of toluene, 25 ꢀC. b Determined by 1H, 13C NMR
spectra. c Determined by GPC in THF at 40 ꢀC against polystyrene standard. d Measured by DSC. e Reaction temperature is 0 ꢀC and -20 ꢀC,
respectively. f The unit for reaction time is hour.
Figure 3. GPC curves of the PIPs obtained by the complex
5/[Ph3C][B(C6F5)4]/AliBu3 system in Table 3 (entries 5, 7-9).
Figure 2. Plot of yield versus molecular weight and molecular
weight distribution of the PIPs obtained by the complex
5/[Ph3C][B(C6F5)4]/AliBu3 system in Table 3 (entries 5, 7-9).
on the lanthanide center is dominated by the relative stability
of the different coordination isomers of isoprene with the
catalyst system (η4-cis-1,4, η4-trans-1,4, η2-cis-3,4, η2-trans-
3,4, η2-cis-1,2, η2-trans-1,2). In the present study, we found
only two coordination isomers, i.e., the cis-3,4,-mode and the
the living nature of the complexes 1-6/activator binary sys-
tems in the isoprene polymerization (Table 3, entries 1-9).
For the complex 5/borate/AliBu3 ternary system, with the
increasing isoprene conversion from 23% to 100%, the molec-
ular weight of the resulting polymer increased linearly, while the
molecular weight distribution stayed less than 1.5, indicating
that the IP polymerization behavior by these ternary systems is
still living (Table 3, entries 5, 7-9, Figures 2 and 3). The GPC
curves of these polymers were all unimodal with narrow molec-
ular weight distributions (Mw/Mn = 1.18-1.35), consistent with
the predominance of a single homogeneous catalytic species.
These results showed that the chain transfer reaction did not
occur in the presence of the small amount of AliBu3 compounds
(2 equiv) in these ternary systems.
Polymerization Mechanism. To gain more insight into the
influence of the steric hindrance around the rare earth metal
center on the isoprene polymerization processes, the DFT
calculations on the polymerization mechanism by cationic
species 5 [2,6-Me2C6H3N-CH2-C(CH2SiMe3)dNC6H3Me2-
2,6]Lu(CH2SiMe3)(THF)n][B(C6F5)4] were carried out. It has
been demonstrated that the selective polymerization of isoprene
trans-3,4-mode, and the latter is 2.73 kcal mol-1 more stable
3
than the former. It was revealed that the coordination of
isoprene to the metal center in an η2-trans-3,4 fashion is the
most favored fashion among six possible isoprene coordina-
tion modes. As described elaborately in an earlier paper,2w
the trans-3,4-mode coordination complex will selectively
form the 3,4-insertion product.
Therefore, the η2-trans-3,4-coordination (e.g., b in Scheme 5)
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 out the chain propa-
gation process from c (Scheme 5). One is the coordination
(in a trans-3,4-mode) and subsequent insertion 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 inter-
mediate (d) or that to the anti-η3-π-allylic intermediate (l) via
rotation of the C3-C2 single bond of the η1-σ-allyl inter-
mediate (k). These competitive pathways would result in

Article
Organometallics, Vol. 30, No. 1, 2011 167
Scheme 5. Possible Scenarios of Isoprene Polymerization Catalyzed by Cationic Iminoamido Rare Earth Metal Alkyl Species
different microstructures (3,4-, trans-1,4-, and cis-1,4-) in the
polymer chains. For the less sterically demanding catalysts
4-6, which bear 2,6-dimethylphenyl-substituted iminoamido
ligands, the more electropositive metal center and the suf-
ficient coordinative space around the metal center would
relatively favor the coordination of the next isoprene mono-
mer to the metal center. That is to say that isoprene co-
ordination is faster than the isomerization of the σ-allyl
intermediate c to the π-η3-allylic intermediates d and l.
Therefore, the coordination of an incoming isoprene mono-
mer to the σ-allyl intermediate c to give h led to formation of
the 3,4-polyisoprene sequences as a major part in the result-
ing polymers (cf. h-j, Scheme 5; see also Table 2). Regarding
a possible chain termination reaction, the steric hindrance of
the 2,6-dimethylphenyl-substituted iminoamido ligands and
the THF ligand in these catalysts would prevent β-hydrogen
elimination from a species like i. Therefore, a chain transfer
reaction would be difficult, thus accounting for the living
feature of these catalysts for the polymerization of isoprene.
For the more sterically demanding complexes 1-3, bear-
ing the 2,6-diisopropylphenyl-substituted iminoamido li-
gands, the less electropositive metal center and more steric
hindrance around the metal center hampered the next iso-
prene coordination to such an extent that the isomerization
reaction of the σ-allyl intermediate c to the less sterically
crowding syn-π-η3-allylic intermediates d became relatively
favored. In contrast, the isomerization reaction of the σ-allyl
intermediate c to the sterically overcrowding anti-π-η3-allylic
intermediates l via rotation of the C3-C2 single bond of the
η1-σ-allyl intermediate (k) became impossible because of the
steric hindrance around metal center. As a result, both the
next isoprene coordination and the isomerization of the
σ-allyl intermediate c to the syn-η3-π-allyl species d should
occur at the same time. The former process gave the 3,4-
polyisoprene sequences, while the latter process afforded
trans-1,4-polyisoprene sequences via the coordination of an
incoming isoprene monomer to the metal center of d (e)
followed by isoprene insertion (f). The steric hindrance of
both the 2,6-diisopropylphenyl-substituted iminoamido li-
gands and the THF ligands could account for the livingness
observed in isoprene polymerization, as described above in
the case of 4-6. These scenarios are in agreement with the
experimental results (see Table 2).
Conclusion
A series of rare earth metal dialkyl complexes bearing
monocationic iminoamino ligands (1-6) can be easily pre-
pared in high yields by reaction of the tris(trimethylsilyl-
methyl) scandium complexes with neutral R-diimine ligands.
Complexes 3-6 are fully characterized by 1H and 13C NMR
and X-ray analyses. The different steric hindrance of 2,6-
disubstitutes (iPr > Me) on the iminoamido ligand and the
˚
ionic radius of the metal center in the trend Sc (0.89 A) < Lu
˚
˚
(1.00 A) < Y (1.04 A) result in the different distance of the
metal center to the iminoamido plane (NCCN), the different
dihedral angle of the best planes of N1-Ln-N2 and the
iminoamido NCCN plane, and the different rotation angle
of the 2,6-disubstituted phenyl rings to the iminoamido
NCCN plane. As a result, these complexes have decreasing

168 Organometallics, Vol. 30, No. 1, 2011
Du et al.
steric hindrance around the metal center in the order 1 > 2 >
3 > 4 > 5 > 6. The DFT calculations determine whether the
possible mechanism process involving intramolecular alkyl
and hydrogen migration is exothermic and can proceed
automatically due to the low reaction barrier heights. Acti-
vated by borate such as [Ph3C][B(C6F5)4], complexes 1-6
show high catalytic activities for the living polymerization of
isoprene to afford mainly the 3,4-poly(IP)s (3,4-selectivities:
48-99%) with narrow molecular weight distributions. A
catalytic structure-reactivity relationship is observed in the
isoprene polymerization catalyzed by these cationic imino-
amido rare earth metal catalysts. The diisopropylphenyl-
substituted iminoamido complexes 1-3 give a mixture of
3,4- and trans-1,4-polyisoprenes (3,4-selectivities: 48-82%;
trans-1,4-selectivities: 50-17%), whereas the dimethylphe-
nyl-substituted iminoamido demanding complexes 4-6
show high 3,4-selectivities (3,4-selectivities: 90-100%). In
the presence of 2 equiv of AliBu3, the complexes 1-6/
activator systems exhibit higher activities and 3,4-selectiv-
ities in the living polymerization of isoprene. Significant
influences of the steric hindrance of the metal centers on
the polymerization activities and selectivities are also ob-
served in these ternary systems. A fundamental understand-
ing of the chelating ligand effects, and hence the elucidation
of the structure-reactivity relationship in polymerization
catalysis, has been obtained based on DFT calculation.
Further studies on the details of the polymerization mecha-
nism and the polymerization and copolymerization of other
monomers by the cationic monocationic iminoamino and
related rare earth catalysts are in progress.
Synthesis of [(2,6-iPr2C6H3N-CH2-C(CH2SiMe3)dNC6-
H3 Pr2-2,6)]Sc(CH2SiMe3)2(THF) (1). To a colorless toluene
solution (5 mL) of Sc(CH2SiMe3)3(THF)2 (1.35 g, 2.50 mmol)
i
was added a solution of 2,6-iPr2C6H3NdCH-CHdNC6H3 Pr2-
i
2,6 (1.13 g, 2.50 mmol) in toluene (5.0 mL) at room temperature.
The mixture was stirred at room temperature for 3.5 h. After
removal of all volatiles in vacuo, the residue was recrystallized from
hexane at -30 ꢀC to give 1 as light yellow crystals (1.60 g, 85%
1
yield). H NMR (C6D6, 25 ꢀC, δ/ppm): -0.58 (s, 4H, ScCH2Si-
Me3), -0.09 (s, 9H, N-CH2C(CH2Si(CH3)3)dN), 0.23 (s, 18H,
ScCH2Si(CH3)3), 1.11 (br, 4H, THF-β-CH2), 1.20 (d, 6H, CH-
(CH3)2), 1.39 (br, 12H, CH(CH3)2), 1.60 (d, 6H, CH(CH3)2), 1.80
(s, 2H, N-CH2C(CH2SiMe3)dN)), 3.32 (m, 2H, CH(CH3)2), 3.51
(br, 4H, THF-R-CH2), 3.96 (m, 2H, CH(CH3)2), 4.57 (s, 2H,
N-CH2C(CH2SiMe3)dN), 7.04-7.19 (m, 6H, ArH). 13C NMR
(C6D6, 25 ꢀC, δ/ppm): 0.1 (s, CH2Si(CH3)3), 4.2 (s, ScCH2SiCH3),
24.1 (s, CH2Si(CH3)3), 24.6 (s, THF), 25.1, 27.0, 27.6, 27.8, 28.0,
28.5 (m, CH(CH3)2, CH(CH3)2), 37.6 (s, ScCH2Si(CH3)3), 67.8 (s,
NCH2CdN), 71.3 (s, THF), 123.7, 123.9, 125.1, 127.1, 140.9,
143.4, 145.9, 153.3 (m, ArC), 190.5 (s, CdN). Anal. Calcd for
C42H77N2OSi3Sc: C, 66.79; H, 10.28; N, 3.71. Found: C, 66.30; H,
9.71; N, 3.14.
Synthesis of [2,6-iPr2C6H3N-CH2-C(CH2SiMe3)dNC6-
i
H3 Pr2-2,6)] Lu(CH2SiMe3)2(THF) (2). To a colorless hexane
solution (5 mL) of Lu(CH2SiMe3)3(THF)2 (1.16 g, 2.0 mmol) was
added a solution of 2,6-iPr2C6H3NdCH-CHdNC6H3 Pr2-2,6
i
(0.75 g, 2.00 mmol) in toluene (5.0 mL) at room temperature.
The pale yellow mixture was stirred for 3.5 h. After removal of all
volatiles in vacuo, the residue was recrystallized from hexane at
1
-30 ꢀC to give 2 as light yellow crystals (1.39 g, 78% yield). H
NMR (C6D6, 25 ꢀC, δ/ppm): -0.74 (s, 4H, LuCH2SiMe3), -0.09
(s, 9H, N-CH2C(CH2Si(CH3)3)dN), 0.22 (s, 18H, LuCH2Si-
(CH3)3), 1.08 (m, 4H, THF-β-CH2), 1.19 (d, 6H, CH(CH3)2),
1.37 (q, 12H, CH(CH3)2), 1.62 (d, 6H, CH(CH3)2), 1.80 (s, 2H,
N-CH2C(CH2SiMe3)dN)), 3.34 (m, 2H, CH(CH3)2), 3.53 (m, 4H,
THF-R-CH2), 3.97 (m, 2H, CH(CH3)2), 4.67 (s, 2H, N-CH2C-
(CH2SiMe3)dN), 7.10-7.21 (m, 6H, ArH). 13C NMR (C6D6,
25 ꢀC, δ/ppm): 0.0 (s, CH2Si(CH3)3), 4.6 (s, LuCH2SiCH3), 24.5 (s,
CH2Si(CH3)3), 24.7 (s, THF), 25.2, 26.7, 27.5, 27.6, 28.0, 28.6 (m,
CH(CH3)2, CH(CH3)2), 40.6 (s, LuCH2Si(CH3)3), 68.4 (s, NCH2-
CdN), 70.8 (s, THF), 123.5, 123.7, 124.9, 127.2, 140.8, 143.1, 146.3,
154.2 (m, ArC), 191.9 (s, CdN). Anal. Calcd for C42H77N2OSi3Lu:
C, 56.98; H, 8.77; N, 3.16. Found: C, 57.12; H, 9.21; N, 3.58.
Synthesis of [2,6-iPr2C6H3N-CH2-C(CH2SiMe3)dNC6-
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. Anhydrous THF, hexane, and toluene were refluxed and
distilled from sodium/benzophenone under dry nitrogen, degassed
by freeze-pump-thaw cycles, 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] and
[PhMe2NH][B(C6F5)4] were purchased from Strem and used with-
out purification. LnCl3 was purchased from Strem. LiCH2SiMe3
(1.0 M solution in pentane) was purchased from Aldrich and used
as received. AliBu3 (1.1 M solution in toluene) was purchased from
Acros and used as received. R-Diimine ligands [2,6-R2C6H3Nd
i
H3 Pr2-2,6)]Y(CH2SiMe3)2(THF) (3). To a colorless hexane
solution (5 mL) of Y(CH2SiMe3)3(THF)2 (1.48 g, 3.00 mmol)
was added a solution of 2,6-iPr2C6H3NdCH-CHdNC6H3 Pr2-
i
2,6 (1.12 g, 3.00 mmol) in toluene (5.0 mL) at room temperature.
The pale yellow mixture was stirred for 3.5 h. After removal of
all volatiles in vacuo, the residue was recrystallized from hexane
at -30 ꢀC to give 3 as light yellow crystals (1.07 g, 45% yield). 1H
NMR (C6D6, 25 ꢀC, δ/ppm): -0.58 (d, 4H, YCH2SiMe3), -0.09
(s, 9H, N-CH2C(CH2Si(CH3)3)dN), 0.23 (s, 18H, YCH2Si-
(CH3)3), 1.11 (br, 4H, THF-β-CH2), 1.20 (d, 6H, CH(CH3)2),
1.39 (q, 12H, CH(CH3)2), 1.60 (d, 6H, CH(CH3)2), 1.80 (s, 2H,
N-CH2C(CH2SiMe3)dN)), 3.32 (m, 2H, CH(CH3)2), 3.51 (m,
4H, THF-R-CH2), 3.96 (m, 2H, CH(CH3)2), 4.57 (s, 2H, N-CH2C-
(CH2SiMe3)dN), 7.04-7.19 (m, 6H, ArH). 13C NMR (C6D6, 25
ꢀC, δ/ppm): 0.0 (s, CH2Si(CH3)3), 4.5 (s, YCH2SiCH3), 24.7 (s,
CH2Si(CH3)3), 24.8 (s, THF), 25.2, 26.4, 27.2, 27.4, 28.1, 28.6 (m,
CH(CH3)2, CH(CH3)2), 33.9 (d, YCH2Si(CH3)3), 68.5 (s, NCH2-
CdN), 70.3 (s, THF), 123.5, 123.8, 125.0, 127.0, 140.6, 143.0, 146.6,
152.8 (m, ArC), 191.6 (s, CdN). Anal. Calcd for C42H77N2OSi3Y:
C, 63.12; H, 9.71; N, 3.51. Found: C, 62.88; H, 9.53; N, 3.21.
Synthesis of [2,6-Me2C6H3N-CH2-C(CH2SiMe3)dNC6H3-
Me2-2,6)]Sc(CH2SiMe3)2(THF) (4). To a colorless hexane solu-
tion (5 mL) of Sc(CH2SiMe3)3(THF)2 (1.29 g, 2.85 mmol) was
i
CH-CHdNC6H3R2-2,6] (R = Pr, Me)23 and Ln(CH2SiMe3)3-
(THF)2 were synthesized according to the literature.2w The deu-
terated solvents benzene-d6 (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.
General Methods. Samples of rare earth metal dialkyl com-
plexes for NMR spectroscopic measurements were prepared in
the glovebox using J. Young valve NMR tubes. The NMR (1H,
13C) spectra were recorded on an AVANCE 400 spectrometer at
45 ꢀC with CDCl3 or C6D6 as a solvent. Elemental analyses were
performed on an Elementar Vario MICRO CUBE (Germany).
The molecular weights and molecular weight distributions of
polyisoprenes were determined by gel permeation chromatog-
raphy on a Waters 2410 HPLC at 35 ꢀC using THF as an eluent
at a flow rate of 1 mL/min against polystyrene standards. The
DSC measurements were performed on a Q 2000 (TA Co.) at a
rate of 20 ꢀC/min. Any thermal history difference in the poly-
mers was eliminated by first heating the specimen to 200 ꢀC,
cooling at 5 ꢀC/min to -80 ꢀC, and then recording the second
DSC scan.
i
added a solution of 2,6-iMe2C6H3NdCH-CHdNC6H3 Me2-
2,6 (0.75 g, 2.85 mmol) in toluene (5.0 mL) at room temperature.
The pale yellow mixture was stirred at room temperature for

Article
Organometallics, Vol. 30, No. 1, 2011 169
1.5 h. After removal of all volatiles in vacuo, the residue was
recrystallized from hexane at -30 ꢀC to give 4 as reddish-brown
The isomer contents of the polyisoprene products were
calculated from the 1H and 13C NMR spectra according to the
following formulas (eqs 1-5):
1
crystals (1.00 g, 55% yield). H NMR (C6D6, 25 ꢀC, δ/ppm):
-0.23 (s, 4H, ScCH2SiMe3), -0.20 (s, 9H, N-CH2C(CH2Si-
(CH3)3)dN), 0.24 (s, 18H, ScCH2Si(CH3)3), 1.01 (br, 4H, THF-
β-CH2), 1.58 (s, 2H, N-CH2C(CH2SiMe3)dN)), 2.42 (s, 6H,
CH3), 2.50 (s, 6H, CH3), 3.68 (m, 4H, THF-R-CH2), 4.18 (s,
2H, N-CH2C(CH2SiMe3)dN), 6.87-7.16 (m, 6H, ArH). 13C
NMR (C6D6, 25 ꢀC, δ/ppm): -0.2 (s, CH2Si(CH3)3), 4.3 (s,
ScCH2SiCH3), 19.8 (s, CH2Si(CH3)3), 24.5 (s, THF), 20.1, 25.7
(m, CH3), 37.0 (s, ScCH2Si(CH3)3), 64.6 (s, NCH2CdN), 71.1 (s,
THF), 122.4, 126.0, 128.6, 129.4, 130.0, 146.0, 155.6 (m, ArC),
190.7 (s, CdN). Anal. Calcd for C34H61N2OSi3Sc: C, 63.50; H,
9.56; N, 4.36. Found: C, 63.46; H, 9.38; N, 4.02.
Mol 1, 4-IP% ¼ fIH1=ðIH1 þ 0:5IH2Þg ꢀ 100 ð1Þ
Mol 3, 4-IP% ¼ f0:5IH2=ðIH1 þ 0:5IH2Þg ꢀ 100 ð2Þ
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 unit) in the 1H NMR spectrum.
Mol cis-1, 4-IP% ¼ fIC1=ðIC1 þ IC2 þ IC3Þg ꢀ 100 ð3Þ
Synthesis of [2,6-Me2C6H3N-CH2-C(CH2SiMe3)dNC6H3-
Me2-2,6)]Lu(CH2SiMe3)2(THF) (5). To a colorless hexane solu-
tion (5 mL) of Lu(CH2SiMe3)3(THF)2 (0.43 g, 0.74 mmol) was
i
added a solution of 2,6-iMe2C6H3NdCH-CHdNC6H3 Me2-
Mol trans-1, 4-IP% ¼ fIC3=ðIC1 þ IC2 þ IC3Þg ꢀ 100 ð4Þ
Mol 3, 4-IP% ¼ fIC2=ðIC1 þ IC2 þ IC3Þg ꢀ 100 ð5Þ
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 Isoprene (IP) Polymerization by
Complex 5/[Ph3C][B(C6F5)4]/AliBu3 Ternary Systems (Table 3,
entry 5). In a glovebox, a toluene solution (5 mL) of
[Ph3C][B(C6F5)4] (0.023 g, 25 μmol) was added to a well-stirred
toluene solution (5 mL) of complex 5 (0.019 g, 25 μmol), AliBu3
(45 μL, 1.1 M, 50 μmol), and isoprene (1.022 g, 15 mmol) at
25 ꢀC in a 30 mL flask. The reaction mixture rapidly became
viscous. After 8 min, the flask was taken outside and the poly-
merization 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.
2,6 (0.20 g, 0.74 mmol) in toluene (5.0 mL) at room temperature.
The pale yellow mixture was stirred at room temperature for
0.5 h. After removal of all volatiles in vacuo, the residue was
recrystallized from hexane at -30 ꢀC to give 5 as reddish-brown
1
crystals (0.34 g, 60% yield). H NMR (C6D6, 25 ꢀC, δ/ppm):
-0.61 (s, 4H, LuCH2SiMe3), -0.06 (s, 9H, N-CH2C(CH2Si-
(CH3)3)dN), 0.40 (s, 18H, LuCH2Si(CH3)3), 1.14 (br, 4H, THF-
β-CH2), 1.68 (s, 2H, N-CH2C(CH2SiMe3)dN)), 2.53 (s, 6H,
CH3), 2.69 (s, 6H, CH3), 3.68 (br, 4H, THF-R-CH2), 4.47 (s,
2H, N-CH2C(CH2SiMe3)dN), 7.03-7.31 (m, 6H, ArH). 13
C
NMR (C6D6, 25 ꢀC, δ/ppm): -0.2 (s, CH2Si(CH3)3), 4.8 (s,
LuCH2SiCH3), 19.9 (d, CH2Si(CH3)3), 24.6 (s, THF), 20.0,
25.9 (m, CH3), 40.9 (s, LuCH2Si(CH3)3), 64.9 (s, NCH2-
CdN), 70.6 (s, THF), 122.0, 126.1, 128.6, 129.4, 129.8, 135.1,
145.5, 156.6 (m, ArC), 192.0 (s, CdN). Anal. Calcd for C34H61-
N2OSi3Lu: C, 52.82; H, 7.95; N, 3.62. Found: C, 53.04; H, 8.08;
N, 3.99.
Synthesis of [2,6-Me2C6H3N-CH2-C(CH2SiMe3)dNC6H3-
Me2-2,6)]Y(CH2SiMe3)2(THF) (6). To a colorless hexane solu-
tion (5 mL) of Y(CH2SiMe3)3(THF)2 (1.45 g, 2.90 mmol) was
i
added a solution of 2,6-iMe2C6H3NdCH-CHdNC6H3 Me2-
2,6 (0.78 g, 2.90 mmol) in toluene (5.0 mL) at room temperature.
The pale yellow mixture was stirred at room temperature for
0.5 h. After removal of all volatiles in vacuo, the residue was
recrystallized from hexane at -30 ꢀC to give 6 as reddish-brown
1
crystals (1.55 g, 78% yield). H NMR (C6D6, 25 ꢀC, δ/ppm):
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 CCD diffractometer with a CCD area
detector using graphite-monochromated Mo KR radiation
-0.60 (d, 4H, YCH2SiMe3), -0.21 (s, 9H, N-CH2C(CH2Si-
(CH3)3)dN), 0.26 (s, 18H, YCH2Si(CH3)3), 1.03 (m, 4H, THF-
β-CH2), 1.52 (s, 2H, N-CH2C(CH2SiMe3)dN)), 2.38 (s, 6H,
CH3), 2.51 (s, 6H, CH3), 3.49 (m, 4H, THF-R-CH2), 4.26 (s,
2H, N-CH2C(CH2SiMe3)dN), 6.88-7.16 (m, 6H, ArH). 13C
NMR (C6D6, 25 ꢀC, δ/ppm): -0.3 (s, CH2Si(CH3)3), 4.6 (s,
YCH2SiCH3), 19.9 (s, CH2Si(CH3)3), 24.7 (s, THF), 20.0 (m,
CH3), 25.5 (m, CH3), 33.3, 33.7 (YCH2Si(CH3)3), 65.2 (s,
NCH2CdN), 70.2 (s, THF), 121.9, 126.0, 128.7, 129.4, 129.6,
134.9, 145.3, 155.4 (m, ArC), 191.3 (s, CdN). Anal. Calcd for
C34H61N2OSi3Y: C, 59.44; H, 8.95; N, 4.08. Found: C, 59.55; H,
9.29; N, 4.26.
˚
(λ = 0.71073 A). The determination of crystal class and unit
cell was carried out by the SMART program package.30 The raw
frame data were processed using SAINT31 and SADABS32 to
yield the reflection data file. The structures were solved by using
the SHELXTL-97 program.33 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 hydro-
gen 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. Crystallographic data (excluding struc-
ture factors) have been deposited with the Cambridge Crystal-
lographic Data Centre as supplementary publication nos.
CCDC-789585 (3), -789586 (5), and -789587 (6). Crystal data
A Typical Procedure for Isoprene (IP) Polymerization by
Complex 5/[Ph3C][B(C6F5)4] Binary Systems (Table 2, entry 5).
In a glovebox, a toluene solution (5 mL) of [Ph3C][B(C6F5)4]
(0.023 g, 25 μmol) was added to a well-stirred toluene solution
(5 mL) of complex 5 (0.019 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 3 h, 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.
(30) SMART Software Users Guide, version 4.21; Bruker AXS, Inc.:
Madison, WI, 1997.
(31) SAINTþ, Version 6.02; Bruker AXS, Inc.: Madison, WI, 1999.
(32) Sheldrick, G. M. SADABS; Bruker AXS, Inc.: Madison, WI, 1998.
(33) Sheldrick, G. M. SHELXTL, Version 5.1; Bruker AXS, Inc.:
Madison, WI, 1998.

170 Organometallics, Vol. 30, No. 1, 2011
Du et al.
and data collection and processing parameters for the rare earth
metal dialkyl complexes 3-6 are summarized in the Supporting
Information. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Computational Details. The geometry optimizations and en-
ergy estimations were carried out with the M06 density func-
tional method34 with the sdd basis set,35 which is adopted in the
Gaussian 09 program.36 The toluene solvation effects were
included in all the calculations. Since there is no symmetry in
these complexes, the C1 symmetry point group was used
throughout all calculations, and no higher molecular symmetry
restriction was imposed. All calculations were performed utiliz-
ing the Gaussian 09 program.
Acknowledgment. This work was partly supported by
the National Natural Science Foundation of China (No.
20974014), Scientific Research Foundation for the Returned
Overseas Chinese Scholars, State Education Ministry (No.
2010173201), the 111 Project (B07012), Excellent Young
Scholars Research Fund of Beijing Institute of Technology
(No. 2 2050205), Basic Research Fund of Beijing Institute
of Technology (Nos. 20081742012 and 20091742015), Key
Laboratory of Cluster Science Foundation (Beijing Institute
of Technology), and Basic Research Fund of School of
Science, Beijing Insitute of Technology.
(34) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241.
(35) (a) Dunning, T. H., Jr.; Hay, P. J. Modern Theoretical Chemistry;
Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, pp 1-28.
(b) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chem. Acc. 1990, 77, 123–141. (c) Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chem. Acc. 1993, 85, 441–450.
Supporting Information Available: GPC, DSC, and NMR (1H
and 13C) spectra of representative polymer products. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(36) Frisch, M. J.; et al. Gaussian 09, Revision A.01; Gaussian, Inc.:
Wallingford, CT, 2009.