Synthesis and characterization of a series of new lanthanide derivatives supported by silylene-bridged diamide ligands and their catalytic activities for the polymerization of methyl methacrylate

Article abstract of DOI:10.1021/om060055v

The synthesis and structures of a series of new lanthanide complexes supported by silylene-bridged diamide ligands are described. The complex {[Me₂Si(NPh)₂]YbCl(TMEDA)}₂ (1) can be synthesized by YbCl₃ with PhLiNSiMe₂NLiPh in 1:1 molar ratio in good yield. Treatment of {[Me₂Si(NPh)₂]LnCl-(THF) n}₂ in situ and Cp′Na in 1:2 molar ratio afforded a series of new anionic complexes: {[Me₂Si-(NPh)₂] LnCp′₂}{Li(DME)₃} [Cp′ = C₅H ₅, Ln = Yb (2); Sm(3); Cp′ = MeC₅H₄, Ln = Yb(4)]. Complexes 2 and 4 can also be synthesized by Cp′₂YbCl with PhLiNSiMe₂NLiPh in 1:1 molar ratio in good yield. The complex 1 reacted with PhLiNSiMe₂NLiPh in 1:1 molar ratio to yield an anionic amide complex, {[Me₂Si(NPh)₂]₂Yb(THF) ₂Li(THF) (6). These complexes were characterized by elemental analysis, IR, and ¹H NMR. The molecular structures of 2-4 and 6 were further determined by X-ray diffraction techniques. These anionic complexes showed high activity for the polymerization of methyl methacrylate (MMA) at rt, giving syndiotactic-rich polymers with high molecular weights (M_n > 10⁴) and relatively narrow molecular weight distributions (M_w/M_n = 1.54-1.85).

Full text of DOI:10.1021/om060055v

2880
Organometallics 2006, 25, 2880-2885
Synthesis and Characterization of a Series of New Lanthanide
Derivatives Supported by Silylene-Bridged Diamide Ligands and
Their Catalytic Activities for the Polymerization of
Methyl Methacrylate
Liying Zhou,† Yingming Yao,† Cheng Li,† Yong Zhang,† and Qi Shen*,†,‡
Department of Chemistry and Chemical Engineering, Suzhou UniVersity, Suzhou 215006,
People’s Republic of China, and State Key Laboratory of Organometallic Chemistry, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, People’s Republic of China
ReceiVed January 18, 2006
The synthesis and structures of a series of new lanthanide complexes supported by silylene-bridged
diamide ligands are described. The complex {[Me2Si(NPh)2]YbCl(TMEDA)}2 (1) can be synthesized by
YbCl3 with PhLiNSiMe2NLiPh in 1:1 molar ratio in good yield. Treatment of {[Me2Si(NPh)2]LnCl-
(THF)n}2 in situ and Cp′Na in 1:2 molar ratio afforded a series of new anionic complexes: {[Me2Si-
(NPh)2]LnCp′2}{Li(DME)3} [Cp′ ) C5H5, Ln ) Yb (2); Sm(3); Cp′ ) MeC5H4, Ln ) Yb(4)]. Complexes
2 and 4 can also be synthesized by Cp′2YbCl with PhLiNSiMe2NLiPh in 1:1 molar ratio in good yield.
The complex 1 reacted with PhLiNSiMe2NLiPh in 1:1 molar ratio to yield an anionic amide complex,
{[Me2Si(NPh)2]2Yb(THF)2Li(THF) (6). These complexes were characterized by elemental analysis, IR,
1
and H NMR. The molecular structures of 2-4 and 6 were further determined by X-ray diffraction
techniques. These anionic complexes showed high activity for the polymerization of methyl methacrylate
(MMA) at rt, giving syndiotactic-rich polymers with high molecular weights (Mn > 104) and relatively
narrow molecular weight distributions (Mw/Mn ) 1.54-1.85).
large ionic radius and the demand of a high coordination number
of the lanthanide element. However, only a few papers are
concerned with the catalytic behavior of anionic complexes.
Anionic Sm(II) complexes of [(C5Me5)(THF)xSm(ER)(C5Me5)K-
(THF)y]n (ER ) OAr, SAr, NR2, SiH3, CHR2) were found to
be active for the homopolymerization of ethylene and styrene
and their block copolymerization.16 The analogous complexes
of Sm(II) [(C5Me5)(THF)Sm(PHAr)(KC5Me5)(THF)]n and (C5-
Me5)[(SiMe3)2N]Sm[(C5Me5)Na(THF)3] are active for the po-
Introduction
The application of organolanthanide complexes as single-
component catalysts in the polymerization of polar monomers
has attracted much attention since H. Yasuda et al. first reported
that lanthanide hydride is a highly active catalyst for living
syndiotactic polymerization of methyl methacrylate (MMA) in
1992.1 Many kinds of lanthanide derivatives have been explored
to perform polymerization of MMA to give high syndiotactic
or high isotactic polymers. Most of the complexes published
are the neutral trivalent complexes containing Ln-C, Ln-N,
and Ln-H σ bonds as well as divalent complexes.2-15 The
anionic complexes of lanthanides are often isolated, especially
for the case where a less bulky ligand was used, due to the
(7) (a) Knjazhanski, S. Y.; Elizalde, L.; Cadenas, G.; Bulychev, B. M.
J. Polym. Sci. A: Polym. Chem. 1998, 36, 1599. (b) Knjazhanski, S. Y.;
Elizalde, L.; Cadenas, G.; Bulychev, B. M. J. Organomet. Chem. 1998,
568, 33.
(8) Lee, M. H.; Hwang, J. W.; Kim, Y.; Kim, J.; Han, Y.; Do, Y.
Organometallics 1999, 18, 5124.
* Corresponding author. Tel: +86-512-6588-0306. Fax: +86-512-6588-
0305. E-mail: qshen@suda.edu.cn.
(9) (a) Qian, C. T.; Nie, W. L.; Sun, J. Organometallics 2000, 19, 4134.
(b) Qian, C. T.; Zou, G.; Chen, Y. F.; Sun, J. Organometallics 2001, 20,
3106. (c) Qian, C. T.; Zou, G.; Jiang, W. H.; Chen, Y. F.; Sun, J.; Li, N.
Organometallics 2004, 23, 4980.
† Suzhou University.
‡ Shanghai Institute of Organic Chemistry.
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J. Am. Chem. Soc. 1992, 114, 4908.
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J. F. Organometallics 2003, 22, 4467.
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mura, A.; Miyake, S.; Kai, Y.; Kanehisa, N. Macromolecules 1993, 26,
7134. (b) Ihara, E.; Morimoto, M.; Yasuda, H. Macromolecules 1995, 28,
7886. (c) Yasuda, H.; Ihara, E.; Nitto, Y.; Kakehi, T.; Morimoto, M.;
Nodono, M. ACS Symp. Ser. 1998, 704, 149. (d) Ihara, E.; Adachi, Y.;
Yasuda, H.; Hashimoto, H.; Kanehisa, N.; Kai, Y. J. Organomet. Chem.
1998, 569, 147. (e) Tanaka, K.; Furo, M.; Ihara, E.; Yasuda, H. J. Polym.
Sci. A: Polym. Chem. 2001, 39, 1382. (f) Yasuda, H. J. Polym. Sci. Part
A: Polym Chem. 2001, 39, 1955.
(3) Giardello, M. A.; Yamamoto, Y.; Bard, L.; Marks, T. J. J. Am. Chem.
Soc. 1995, 117, 3276.
(4) Nakayama, Y.; Shibahara, T.; Fukumoto, H.; Nakamura, A.; Mash-
ima, K. Macromolecules 1996, 29, 8014.
(11) Zhou, S. L.; Wang, S. W.; Yang, G. S.; Liu, X. Y.; Sheng, E. H.;
Zhang, K. H.; Cheng, L.; Huang, Z. X. Polyhedron 2003, 22, 1019.
(12) (a) Woodman, T. J.; Schormann, M.; Bochmann, M. Organome-
tallics 2003, 22, 2938. (b) Woodman, T. J.; Schormann, Hughes, M., D.;
Bochmann, L. M. Organometallics 2003, 22, 3028. (c) Woodman, T. J.;
Schormann, M.; Hughes, D. L.; Bochmann, M. Organometallics 2004, 23,
2972.
(13) Yuan, F. G.; Wang, H. Y.; Zhang, Y. Chin. J. Chem.2005, 23, 409.
(14) Luo, Y. J.; Yao, Y. M.; Li, W. J., Chen, J. L.; Zhang, Z. Q.; Zhang,
Y.; Shen, Q. J. Organomet. Chem. 2003, 679, 125.
(15) Simpson, C. K.; White, R. E.; Carlson, C. N.; Wrobleski, D. A.;
Kuehl, C. J.; Croce, T. A.; Steele, I. M.; Scott, B. L.; Young, V. G.; Hanusa,
T. P.; Sattelberger, A. P.; John, K. D. Organometallics 2005, 24, 3685.
(16) (a) Hou, Z. M.; Tezuka, H.; Zhang, Y. G.; Yamazaki, H.; Wakatsuki,
Y. Macromolecules 1998, 31, 8650. (b) Hou, Z. M.; Zhang, Y. G.; Tezuka,
H.; Xie, P.; Tardif, O.; Koizumi, T.; Yamazaki, H.; Wakatsuki, Y. J. Am.
Chem. Soc. 2000, 122, 10533. (c) Hou, Z. M.; Zhang, Y. G.; Tardif, O.;
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10.1021/om060055v CCC: $33.50 © 2006 American Chemical Society
Publication on Web 04/27/2006

Silylene-Bridged Diamide Ligands
Scheme 1
Organometallics, Vol. 25, No. 11, 2006 2881
Scheme 3
Scheme 2
Scheme 4
lymerization of ethylene.16b Recently, the “ate” allyl complexes
of lanthanides K[Ln{C3H3(SiMe3)2}4 and [Ln{(C3H3SiMe3)2-
SiMe2}2{µ-K(THF)}(THF)x]∞ were reported to exhibit much
higher activity for the polymerization of MMA than the
corresponding neutral one, Ln[C3H3(SiMe3)2]3, and to work as
a single-component catalyst, not the combined catalyst of two
fragments; even K[C3H3(SiMe3)2] itself shows very high activ-
ity.15
Scheme 5
To further understand the catalytic behavior of anionic
lanthanide complexes, we synthesized the anionic complexes
{[Me2Si(NPh)2]LnCp′2}[Li(DME)3] (Cp′ ) C5H5, Ln ) Yb,
Sm; Cp′ ) MeC5H4, Ln ) Yb), {[Me2Si(NPh)2]2Yb(THF)2Li-
(THF), and Cp4YbLi(THF)n. In these complexes only the
ancillary ligands of Cp′ and chelating diamido groups, which
were recently developed as alternatives to Cp in organometallic
chemistry, existed around the central metals. The study on their
catalytic activity demonstrated that all the anionic complexes
synthesized exhibit high activity for the polymerization of MMA
at 30 °C to yield the syndiotactic-rich polymers (53-59% rr)
with high molecular weights and moderate polydispersity (Mw/
Mn 1.54-1.85). In contrast, 1, Cp3Ln, CpLi, and PhLiNSiMe2-
NLiPh are almost inactive under the same conditions. Here we
would like to report the results.
When PhLiNSiMe2NLiPh reacts with Cp2YbCl and (MeC5H4)2-
YbCl in 1:1 molar ratio, respectively, the same complexes 2
and 4, not the bimetallic complex 5 (Scheme 4), were obtained.
The results indicated that the anionic lanthanide complexes
are much more stable, as the less bulky ligand provides a larger
coordination sphere around the lanthanide atom.
Complex 1 reacts with PhLiNSiMe2NLiPh in 1:1 molar ratio
to yield the expected anionic amide complex {[Me2Si(NPh)2]2Yb-
(THF)2Li(THF) (6) in high yield (Scheme 5).
All these complexes are air- and moisture-sensitive. They are
soluble in tetrahydrofuran (THF) and dimethylethyl ether
(DME), but hardly so in toluene and hexane.
Results and Discussion
Crystal Structure Analyses. The molecular structures of
complexes 2-4 and 6 were determined by X-ray diffraction.
Crystals of complexes 2-4 suitable for X-ray diffraction were
grown from DME, while 6 was grown from toluene. Because
complexes 2-4 have an analogous solid structure and the same
monoclinic space groups, only the crystal structure of 2 is
depicted in Figure 1 as a representation. The molecular structure
of 6 is shown in Figure 2. Selected bond lengths and angles are
given in Tables 1, 2, and 3, and details of the crystallographic
data in Table 4.
As shown in Figure 1, the molecule contains the crystallo-
graphically independent anion {[Me2Si(NPh)2]LnCp′2}- and the
cation [Li(DME)]+. The lanthanide ion is ligated by two nitrogen
atoms of the chelating diamide ligand and two cyclopentadienyl
groups in η5-fashion. The coordination geometry around the
lanthanide ion is best described as a pseudodistorted tetrahedron.
The lithium atom in the cation is coordinated to six oxygen
atoms from three DME molecules and adopts a distorted
octahedral geometry.
Synthesis and Characterization of Lanthanide Complexes.
To synthesize the desired anionic complex in high yield, the
less bulky bridged diamide ligand is chosen. Thus, the silylene-
bridged diamide ligand PhNHSi(Me)2NHPh was synthesized
according to the literature procedure,17 and the corresponding
lithium salt was prepared by its reaction with n-BuLi in hexane.
The reaction of YbCl3 with PhLiNSiMe2NLiPh in THF at rt
followed by crystallization from toluene with tetramethyleth-
yldiamine (TMEDA) yields the corresponding chloride {[Me2-
Si(NPh)2]YbCl(TMEDA)}2 (1) as red crystals in 85% yield
(Scheme 1).
The anhydrous lanthanide trichloride reacts with PhLiNSiMe2-
NLiPh, then Cp′Na in 1:1:2 molar ratio to afford the corre-
sponding anionic complexes {[Me2Si(NPh)2]LnCp′2}{Li(DME)3}
[Cp′ ) C5H5, Ln ) Yb (2); Sm(3); Cp′ ) MeC5H4, Ln ) Yb
(4)] in high yields (Scheme 2). The cation in each complex is
Li(DME)3, not Na(DME)3.
Even when the reaction was carried out in the 1:1 molar ratio
of complex 1 to CpNa, the anionic complex 2 is still isolated
in reasonable, lower yield, instead of the neutral one (Scheme
3).
The average lanthanide-carbon (ring) bond lengths for 2, 3,
and 4 are 2.628, 2.727, and 2.640 Å, respectively. These values
are comparable when the difference in the ionic radii between
Sm and Yb is considered. The Ln-N bond lengths in complexes
2 and 4 are similar (2.250(4), 2.232(7) Å for 2 and 2.242(4),
2.254(4) Å for 4), which are somewhat longer than those in
(17) Patent U.S. Nat. Aeronautics and Space Administration, U.S.
3433818.

2882 Organometallics, Vol. 25, No. 11, 2006
Zhou et al.
Figure 1. Molecular structure of complex 2.
Table 2. Selected Bond Angles (deg) for Complexes 2, 3, and
4
2
3
4
N(2)-Ln(1)-N(1)
N(2)-Si(1)-N(1)
N(2)-Si(1)-Ln(1)
N(1)-Si(1)-Ln(1)
Si(1)-N(1)-Ln(1)
Si(1)-N(2)-Ln(1)
O(1)-Li(1)-O(2)
O(3)-Li(1)-O(4)
O(5)-Li(1)-O(6)
70.0(2)
96.5(3)
48.6(2)
48.0(2)
96.3(2)
97.0(3)
78.5(6)
76.0(5)
77.4(5)
67.13(15)
97.5(2)
48.61(15)
48.93(15)
97.33(19)
98.0(2)
76.5(4)
77.1(4)
78.5(4)
70.17(17)
97.0(2)
48.66(15)
48.29(15)
96.5(2)
96.4(2)
78.1(4)
79.6(6)
73.8(6)
Table 3. Selected Bond Distances (Å) and Angles (deg) for 6
Bond Distances
Yb(1)-N(2)
Yb(1)-N(1)
Yb(1)-O(1)
Yb(1)-Li(1)
Yb(1)-Si(1)
2.281(2)
2.300(2)
2.3361(19)
2.965(7)
3.0455(8)
Si(1)-Li(1)
O(2)-Li(1)
N(1)-Li(1)
C(1)-Li(1)
3.139(4)
1.847(8)
2.093(5)
2.575(3)
Figure 2. Molecular structure of complex 6.
Bond Angles
170.66(12) N(2)-Si(1)-N(1)
Table 1. Selected Bond Distances (Å) for Complex 2, 3, and
4
N(2)#1-Yb(1)-N(2)
N(2)#1-Yb(1)-N(1)
N(2)-Yb(1)-N(1)
N(1)-Yb(1)-N(1)#1
O(2)-Li(1)-Yb(1)
Si(1)#1-Li(1)-Si(1)
96.28(11)
104.43(8)
68.59(8)
N(1)-Li(1)-Yb(1) 50.57(17)
2
3
4
N(2)-Yb(1)-Li(1) 85.33(6)
89.31(11) N(1)-Yb(1)-Li(1) 44.65(6)
180.0
119.6(2)
Ln(1)-N(1)
Ln(1)-N(2)
Ln(1)-C(15)
Ln(1)-C(16)
Ln(1)-C(17)
Ln(1)-C(18)
Ln(1)-C(19)
Ln(1)-C(20/25)
Ln(1)-C(21)
Ln(1)-C(22)
Ln(1)-C(23)
Ln(1)-C(24)
O(1)-Li(1)
O(2)-Li(1)
O(3)-Li(1)
O(4)-Li(1)
O(5)-Li(1)
O(6)-Li(1)
2.250(4)
2.232(7)
2.65(1)
2.64(1)
2.641(9)
2.67(1)
2.66(1)
2.59(1)
2.60(1)
2.61(1)
2.610(9)
2.610(8)
2.11(1)
2.07(2)
2.21(1)
2.11(1)
2.08(2)
2.14(1)
2.340(4)
2.332(4)
2.745(6)
2.720(6)
2.733(6)
2.742(6)
2.741(6)
2.731(7)
2.726(7)
2.707(8)
2.703(7)
2.722(8)
2.180(12)
2.107(10)
2.087(11)
2.148(13)
2.133(11)
2.073(11)
2.242(4)
2.254(4)
2.661(6)
2.635(7)
2.637(7)
2.628(7)
2.639(7)
2.622(8)
2.644(8)
2.632(7)
2.630(8)
2.611(8)
2.178(14)
2.121(11)
2.254(17)
1.969(14)
2.163(17)
2.034(14)
N(1)-Yb(1)-Si(1)
34.66(6)
Li(1)-N(1)-Yb(1) 84.78(17)
Si(1)-Yb(1)-Si(1)#1 125.90(3)
Si(1)-N(2)-Yb(1)
Si(1)-N(1)-Yb(1)
98.15(10)
96.75(10)
N(2)-Si(1)-Yb(1)
N(1)-Si(1)-Yb(1)
47.84(8)
48.58(7)
Yb(1)-Li(1)-Si(1) 59.78(12)
diamide ligand provides a more open coordination environment.
The corresponding N-Ln-N angle in 3 (67.13(15)°) is smaller
than those in 2 and 4, resulting from the difference in ionic
radii between Sm and Yb.
As shown in Figure 2, the space group of complex 6 is
orthorhombic. The six-coordinate lanthanide center is ligated
by four nitrogen atoms of two chelating diamide ligands and
two oxygen atoms from two THF molecules and adopts a
distorted octahedral geometry. The lithium atom is coordinated
to two nitrogen atoms of two chelating diamide ligands and
one oxygen atom from the THF molecule, and the structure
adopts a trigonal geometry. The three coordinated lithium atom
is scarcely found.
The Yb-N bond lengths in complex 6 are 2.281(2) and
2.300(2) Å, which are somewhat longer than those in complexes
2 and 4. The angle of N-Si-N is 96.28(11)°, which is almost
equal to that in complex 2 (96.5(3)°). The two N-Yb-N angles
in complex 6 are equal to 68.59(2)°, which is somewhat smaller
than those in 2 and 4 (70.0(2)° and 70.17(17)°), indicating that
two bridged diamide ligands make the central metal very
[N(C6H3-iPr2-2,6)(SiMe3)]2LnCH3(µ-CH3)Li(THF)3 (2.224(6)
and 2.227(6) Å).14 The corresponding Ln-N bond lengths in
complex 3 are 2.340(4) and 2.332(4) Å; these values are
comparable to those of 2 and 4, when the difference in the ionic
radii between the two metals is considered. The angles of
N-Si-N bonds are 96.5(3)° for 2, 97.5(2)° for 3, and 97.0(2)°
for 4, respectively, which are almost equal. The N-Ln-N
angles in complexes 2 (70.0(2)°) and 4 (70.17(17)°) are almost
the same and smaller than that of [N(C6H3-iPr2-2,6)(SiMe3)]2-
LnCH3(µ-CH3)Li(THF)3 (121.3(2)°),14 indicating that the bridged

Silylene-Bridged Diamide Ligands
Organometallics, Vol. 25, No. 11, 2006 2883
Table 4. Details of the Crystallographic Data of Complexes 2, 3, 4, and 6
2
3
4
6
empirical formula
fw
C36H56LiN2O6SiYb
820.91
C36H56LiN2O6SiSm
798.21
C38H60LiN2O6SiYb
848.95
C40H56LiN4O3Si2Yb
877.05
cryst color
temperature (K)
wavelength (Å)
size (mm)
cryst syst
space group
a (Å)
red
193.1
0.7107
yellow
193(2)
0.7107
red
193(2)
0.7107
yellow
193(2)
0.7107
0.70 × 0.31 × 0.27
orthorhombic
pbcn
12.4130(5)
18.8362(8)
17.8812(7)
90.00
90.00
90.00
4180.9(3)
4
1.393
2.334
1796
3.01-27.48
43 594
0.38 × 0.35 × 0.55
monoclinic
Pn (#7)
17.038(1)
15.6124(10)
16.9013(13)
90.00
117.499(3)
90.00
3987.9(5)
4
1.367
2.417
1684.00
3.0-27.5
88 030
0.54 × 0.48 × 0.30
monoclinic
Pn
16.8941(16)
15.6116(14)
17.1362(19)
90.00
117.361(2)
90.00
4014.0(7)
4
1.321
1.534
1652
3.0-25.3
39 202
13 781
849
0.32 × 0.30 × 0.12
monoclinic
P21/c
17.4946(18)
15.9091(13)
16.973(2)
90.00
118.801(3)
90.00
4139.7(7)
4
1.362
2.331
1748
3.0-25.3
40 642
7564
417
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D
calc (g cm-3
)
absorp coeff (mm-1
F(000)
)
θ (deg)
no. of reflns collected
no. of indep reflns
no. of variables
R [I > 2σ(I)]
Rw
9450
959
0.029[I > 3σ(I)]
0.065
4802
235
0.0291
0.0644
0.0368
0.0861
1.063
0.0499
0.1117
1.116
GOF on F2
1.001
1.117
Scheme 6
for 3 (entries 2 and 5, Table 5). The increasing order of Sm <
Yb in activity is observed, which is just the opposite of the
ionic radii, Sm (1.079 Å) > Yb (0.985 Å). The activity order
contrasts that published for the catalyst systems of lanthanocene
methyl complexes.2a The reason for this is still not well
understood. The substituent on the cyclopentadienyl ligand
exerts a dramatic influence. A methyl substituent on the
cyclopentadienyl ligand led to a decrease in the activity (entries
2, 3, 8, and 9).
Table 5. Polymerization of MMA Initiated by Anionic
Lanthanide Complexesa
tacticityd (%)
The PMMAs obtained with these complexes have high
molecular weights (Mn > 104) and relatively narrow molecular
weight distributions (Mw/Mn ) 1.54-1.85), which is comparable
with that obtained with Bochmann’s anionic initiators of
[Li(OEt2)(THF)3]+[Ln{3-(η3-C3H3SiMe3-1)2SiMe2}2]- (1.85)12
and better than that of the anionic complex [{(3,6-tBu2C13H6)-
SiMe2NtBu}2La]-[Li(THF)4]+ (2.8).10 In all cases, gel perme-
ation chromatography shows a unimodal pattern, indicating only
one kind of active species existed in the polymerization systems.
Triad microstructural analysis of the resulting polymers was
c
init [M]/[I] convb (%) Mn (× 104) Mw/Mn mm rm
rr
1
2
3
4
5
6
7
8
2
2
2
3
3
3
4
4
4
6
7
500:1
1000:1
1500:1
500:1
1000:1
1500:1
500:1
1000:1
1500:1
500:1
96.8
83.4
72.0
100
69.9
44.1
3.40
4.65
5.37
3.50
4.30
5.39
3.11
3.82
4.21
4.44
3.23
1.54
1.57
1.60
1.70
1.78
1.85
1.58
1.70
1.77
1.45
1.50
6.1 34.6 59.3
15.6 30.9 53.5
12.9 34.3 52.9
14.8 31.0 54.3
100
13.5 30.5 56.0
13.5 32.6 54.0
69.9
48.4
49.3
73.0
9
10
11
1
500:1
carried out using HNMR spectra in CDCl3 according to the
reported triad.18 The present polymerization systems all yielded
the syndiotactic-rich PMMA, and the amount of syndiotacticity
ranges from 53 to 59%.
a Polymerization conditions: sol/MMA ) 2v/v, THF, 30 °C, 3 h. b Conv
) weight of polymer obtained/weight of monomer used. c Measured by
GPC calibrated with standard polystyrene samples. d Carried out using 1H
NMR spectra in CDCl3 at 25 °C.
Among all these complexes used here, complex 2 exhibits
the highest catalytic activity. Thus, the effects of the solvent
on both the activity and stereoselectivity were examined by using
complex 2 as the initiator. The activity in THF solvent is much
higher than that in toluene (entries 1 and 3, Table 6), and the
polymerization in THF yielded the syndiotactic polymer, while
the atactic polymer occurred in toluene. The change of ste-
reospecificity may contribute to the coordination of THF, which
favors the polymers with syndiotacticity. Such a solvent effect
on the microstructure of PMMA is often observed in the anionic
polymerization of MMA.9a,11,12c
congested. Each chelating diamide ligand binds to Yb through
two nitrogen atoms to yield a planar tetragon (sum of angles,
359.77°).
Catalytic Activity of 2-4 for the Polymerization of MMA.
The catalytic activity of 2-4 for the polymerization of MMA
(Scheme 6) was first examined, and various conditions were
explored.
Interestingly, all the complexes are able to initiate the
polymerization of MMA in high activity under mild conditions
as shown in Table 5. For example, the conversion reaches 96.8%
in the case of [M]/[I] ) 500 at 30 °C in 3 h using 2 as the
initiator (entry 1, Table 5).
The central metal has a great influence on the activity. For
example, the conversion reaches 83.4% in the loading of [M]/[I]
) 1000 at 30 °C in 3 h using 2 as the initiator, while it is 69.9%
To understand further the possible polymerization mechanism,
the polymerizations of MMA with the complexes Cp3Yb, 1,
CpLi, and PhLiNSi(Me)2NLiPh, respectively, were first con-
ducted under the same conditions (sol/MMA ) 2v/v, [M]/[I]
(18) Ferugusan, R. C.; Overnall, D. W. Polym. Prepr. 1985, 26, 182.

2884 Organometallics, Vol. 25, No. 11, 2006
Zhou et al.
Table 6. Effect of Solvent on Polymerization of MMAa
ketyl prior to use. The silylene-bridged diamide ligand PhNHSi-
19
(Me)2NHPh17 and anhydrous LnCl3 were prepared according to
tacticityb (%)
the literature procedures. Melting points were determined in sealed
Ar-filled capillary tubes. Carbon, hydrogen, and nitrogen analyses
were performed by direct combustion on a Carlo-Erba EA-1110
instrument. Analysis of Ln was carried out by complexometric
titration of EDTA. The IR spectra were recorded on a Magna-IR
550 spectrometer. Molecular weight and molecular weight distribu-
tions were determined against a polystyrene standard by gel
permeation chromatography (GPC) on a waters 1515 apparatus with
three HR columns (HR-1, HR-2, and HR-4); THF was used as an
eluent at 30 °C. 1H NMR spectra were measured on a Unity Inova-
400 spectrometer in C6D6 or CDCl3.
solv
THF
THF:toluene
) 1:1
toluene
conv (%) Mn (× 104) Mw/Mn mm
rm
rr
1
2
96.8
100
3.40
2.70
1.54
1.60
6.1 34.6 59.3
6.7 36.8 56.5
3
15.7
2.59
1.45
29.0 41.7 29.3
a Polymerization conditions: complex 2, sol/MMA ) 2v/v, [M]/[I] )
500, 30 °C, 3 h. b Carried out using H NMR spectra in CDCl3 at 25 °C.
1
) 500, THF, 30 °C, 3 h). The results show that all these
complexes show no or very low activity (yield: 0, 0, 6.5, and
10.8%, respectively). Then, the anionic complex Cp4YbLi-
(THF)n (7) was synthesized by the reaction of YbCl3 with 4
equiv of CpLi in THF. The polymerization of MMA with the
anionic complexes 6 and 7, respectively, was performed under
the above conditions. The two complexes both can initiate the
polymerization of MMA with moderate activities, and the
activity of 7 is higher than that of 6 (entries 10 and 11, Table
5). The results tell us that the polymerization might be initiated
by either the amide or the Cp ligand present in the anionic
complex. If the reaction goes by the former way, the end group
of the polymer should be connected with the amide group, while
with Cp group by the latter one. To answer the question, an
oligomer of MMA was prepared from the oligomerization of
MMA with a [MMA]/[initiator] mole ratio of 10 and character-
ized by 1H NMR (CDCl3, without TMS). A single peak at 0.06
ppm is assigned to the signal of the silylene group, while
multipeaks around 6.7-7.6 ppm might belong to the protons
of the benzene of the amino group. According to the value of
the molecular weights of the resulting polymer, it could be
supposed that the polymerization of MMA is initiated by one
Ln-N bond of the chelating diamide group. The postulated
anionic polymerization of MMA is shown in Scheme 7. In the
initiation step, the N of the amide attacks the CH2 group of
MMA to generate a transient species, and then the incoming
MMA molecule may participate in a 1,4-addition to afford an
eight-membered-ring intermediate. In the propagation step,
another MMA molecule may attack the growing end, liberating
the coordinated ester group. The polymerization should proceed
by repeating these pathways. Further study on the reactivity of
anionic complexes of lanthanides is proceeding.
{[Me2Si(NPh)2]YbCl(TMEDA)}2 (1). A Schlenk flask was
charged with PhHNSi(Me)2NHPh (1.00 g, 4.10 mmol), THF (15
mL), and a stir bar. The solution was cooled to 0 °C, and n-BuLi
(8.20 mL, 8.2 mmol, 1.00 M in hexane) was added. The solution
was slowly warmed to room temperature and stirred for 1 h. Then
this solution was added slowly to a pale gray slurry of YbCl3 (1.15
g, 4.12 mmol) in 15 mL of THF. The resulting red solution was
then stirred for another 24 h and removed under vacuo. The residue
was extracted with toluene, and LiCl was removed by centrifuga-
tion. After the extracts were concentrated and TMEDA added, red
crystals of {[Me2Si(NPh)2]YbCl(TMEDA)}2‚3C7H8 (2.45 g, 85%)
were obtained at -30 °C for few days. Mp: 165-166 °C (dec).
1
The H NMR spectrum of this compound displayed very broad
resonances and proved uninformative because of the paramagnetism
of Yb metal. Anal. Calc for C61H88Cl2N8Si2Yb2: C, 52.09; H, 6.31;
N, 7.97; Yb, 24.60. Found: C, 52.01; H, 6.51; N, 7.85; Yb, 24.76.
IR (KBr pellet, cm-1): 3036 (w), 2963 (w), 2832 (w), 2782 (w),
2627 (w), 2458 (w), 1605 (s), 1501 (s), 1397 (m), 1242 (s), 1157
(s), 1096 (m), 1026 (m), 976 (w), 802 (m), 756 (m), 694 (m), 625
(w), 556 (w), 505 (m).
{[Me2Si(NPh)2]YbCp2}{Li(DME)3} (2). Path A. A Schlenk
flask was charged with YbCl3 (0.78 g, 2.80 mmol, 15 mL of THF)
and PhNLiSi(Me)2NLiPh (2.80 mmol, 20 mL of THF), and the
solution was stirred for 4 h at room temperature. Then CpNa (5.60
mL, 5.60 mmol, 1.00 M in THF) was added. The resulting solution
was then stirred for another 48 h and removed under vacuo. The
residue was extracted with DME, and LiCl was removed by
centrifugation. Concentration of the extracts and crystallization at
rt for a few days gave 1 as red crystals (1.73 g, 75%). Mp: 127 °C
1
(dec). H NMR (400 MHz, C6D6, 25 °C): δ 6.37-7.22 (br, 20H,
H-Ph, H-Cp); 2.00-4.00 (br, 30H, OCH2, OCH3); 0.18-0.36 (br,
6H, SiCH3). Anal. Calc for C36H56LiN2O6SiYb: C, 52.67; H, 6.88;
N, 3.41; Yb, 21.08. Found: C, 52.34; H, 6.95; N, 3.29; Yb, 20.85.
IR (KBr pellet, cm-1): 2959 (w), 2928 (w), 2859 (w), 1628 (s),
1609 (s), 1389 (m), 1262 (m), 1084 (m), 1026 (m), 910 (w), 802
(m), 752 (w), 694 (w), 469 (w).
Conclusion
In summary, a series of anionic lanthanocene complexes
supported by silylene-bridged diamide ligand have been suc-
cessfully synthesized for the first time. The structural features
of 2-4 and 6 have been determined by an X-ray diffraction
study. These anionic complexes can serve as single-component
catalysts to initiate the polymerization of MMA at rt with high
activity. The polymerization systems yield syndiotactic-rich
polymers with high molecular weights (Mn > /104) and relatively
narrow molecular weight distributions (Mw/Mn ) 1.54-1.85).
The results indicated that the anionic complexes of lanthanides
should have potential in homogeneous catalysis. Further study
on the reactivity of anionic complexes of lanthanides is
proceeding in our laboratory.
Path B. A Schlenk flask was charged with a pale gray slurry of
YbCl3 (0.63 g, 2.26 mmol) in 15 mL of THF, and then CpNa (4.52
mL, 4.52 mmol, 1.00 M in THF) was added by a syringe. The
solution was stirred for 4 h at room temperature, and then the fresh
dilithium PhLiNSiMe2NLiPh (2.26 mmol) was added. The resulting
solution was then stirred for another 48 h and removed under vacuo.
Treatment as path A and crystallization from DME yielded red
crystals (1.35 g, 73%).
{[Me2Si(NPh)2]SmCp2}{Li(DME)3} (3). Following a procedure
similar to the synthesis of complex 2 (path A), using 2.65 mmol of
PhNLiSi(Me)2NLiPh, 0.68 g of SmCl3 (2.65 mmol), 5.30 mmol of
CpNa (5.30 mL, 1.00 M), and 45 mL of THF, followed by
crystallization from DME, yielded yellow crystals of 3 (1.65 g,
1
78%). Mp: 120 °C (dec). H NMR (400 MHz, C6D6, 25 °C): δ
7.15-7.20 (m, 10H, H-Ph); 6.74-6.82 (m, 10H, H-Cp); 3.17 (m,
12H, OCH2); 3.04 (m, 18H, OCH3), 0.17 (s, 6H, SiCH3). Anal.
Calc for C36H56LiN2O6SiSm: C, 54.17; H, 7.07; N, 3.51; Sm, 18.84.
Experimental Section
General Procedures. All manipulations were performed under
pure Ar with rigorous exclusion of air and moisture using standard
Schlenk techniques. Solvents were distilled from Na/benzophenone
(19) Taylor, M. D.; Carter, C. P. J. Inorg. Nucl. Chem. 1962, 24, 387.

Silylene-Bridged Diamide Ligands
Organometallics, Vol. 25, No. 11, 2006 2885
Scheme 7. Possible Polymerization Mechanism
Found: C, 54.01; H, 6.90; N, 3.63; Sm, 18.75. IR (KBr pellet,
cm-1): 3041 (w), 2956 (w), 2925 (w), 1729 (w), 1605 (s), 1497
(s), 1389 (m), 1289 (s), 1173 (w), 1104 (w), 1089 (w), 1027 (w),
911 (m), 834 (w), 795 (m), 756 (m), 695 (w), 664 (w), 471 (w).
{[Me2Si(NPh)2]Yb(CpMe)2}{Li(DME)3} (4). Path A. Follow-
ing a procedure similar to the synthesis of complex 2, path A, using
2.80 mmol of PhNLiSi(Me)2NLiPh, 0.78 g of YbCl3 (2.80 mmol),
5.60 mmol of MeCpNa (5.09 mL, 1.10 M) and 50 mL of THF,
followed by crystallization from DME, yielded red crystals of 4
syringe. After about 24 h, the solvent was removed under vacuo,
and the residue was extracted with 30 mL of toluene to remove
the LiCl. The resulting solution was concentrated and cooled at 0
°C. Then the dark green crystals (2.45 g, 85%) were obtained. The
1H NMR spectrum of this compound displayed very broad
resonances and proved uninformative because of the paramagnetism
of ytterbium metal. IR (KBr pellet, cm-1): 2956 (m), 2933 (m),
1628 (m), 1513 (s), 1412 (s), 1258 (w), 1158 (w), 1088 (w), 1019
(w), 895 (w), 664 (s), 617 (s).
X-ray Structure Determination. Owing to air- and moisture-
sensitivity, suitable single crystals of complexes 2-4 and 6 were
each sealed in thin-walled glass capillaries. Intensity data were
collected on a Rigaku Mercury CCD equipped with graphite-
monochromatized Mo KR (λ ) 0.71070 Å) radiation. Details of
the intensity data collection and crystal data are given in Tables
1-4. The crystal structures of these complexes were solved by
direct methods and expanded by Fourier techniques. Atomic
coordinates and thermal parameters were refined by full-matrix
least-squares analysis on F2. All the non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were all generated geo-
metrically with assigned appropriate isotropic thermal parameters.
The structures were solved and refined using the SHELXS-97 and
SHELXL-97 programs.
Typical Polymerization Procedure. All polymerizations were
carried out in a 50 mL Schlenk flask under a dry Ar atmosphere.
A typical polymerization reaction is given below (entry 1, Table
5). A 50 mL Schlenk flask equipped with a magnetic stir bar was
charged with a solution of 1 mL of MMA in THF (0.14 mL). To
this solution was added 1.86 mL of a solution of complex 2 in
THF (1.0 × 10-2 M, 1.86 × 10-3 mmol) using rubber septum and
syringe. The contents of the flask were then vigorous stirred for 3
h at 30 °C. The polymerization was quenched by ethanol with 5%
HCl, precipitated from ethanol, dried under vacuum, and weighed.
1
(1.62 g, 68%). Mp: 167-168 °C. H NMR (400 MHz, C6D6, 25
°C): δ 6.75-7.20 (m, 20H, H-Ph, H-Cp); 3.37 (m, 12H, OCH2);
3.16 (m, 18H, OCH3); 1.40 (s, 6H, CpCH3); 0.17 (s, 6H, SiCH3).
Anal. Calc for C38H62LiN2O6SiYb: C, 53.64; H, 7.34; N, 3.29;
Yb, 20.33. Found: C, 53.49; H, 7.28; N, 3.46; Yb, 20.13. IR (KBr
pellet, cm-1): 3041 (m), 2964 (m), 1605 (s), 1497 (s), 1389 (m),
1289 (s), 1081 (w), 1027(w), 996 (w), 911 (s), 834 (m), 795 (m),
756 (m), 695 (m), 656 (w), 563 (w), 463 (w).
Path B. Following a procedure similar to the synthesis of
complex 2, path B, using a solution of (MeCp)2YbCl (3.00 mmol),
PhNLiSi(Me)2NLiPh (3.00 mmol), and 45 mL of THF, followed
by crystallization from DME, yielded complex 4 as red crystals
(1.79 g, 70%).
{[Me2Si(NPh)2]2Yb(THF)2Li(THF) (6). A Schlenk flask was
charged with YbCl3 (1.01 g, 3.60 mmol, 15 mL of THF) and
PhNLiSi(Me)2NLiPh (3.60 mmol, 20 mL of THF), and the solution
was stirred for 4 h at rt. Then PhNLiSi(Me)2NLiPh (3.60 mmol,
15 mL of THF) was added. The resulting yellow solution was then
stirred for another 48 h and removed in vacuo. The residue was
extracted with toluene, and LiCl was removed by centrifugation.
Concentrating the extracts at rt for few days gave 6 as yellow
crystals (2.54 g, 80%). Mp: 114-115 °C (dec). The 1H NMR
spectrum of this compound displayed very broad resonances and
proved uninformative because of the paramagnetism of ytterbium
metal. Anal. Calc for C40H56LiN4O3Si2Yb: C, 54.78; H, 6.44; N,
6.39; Yb, 19.73. Found: C, 54.33; H, 6.46; N, 6.28; Yb, 20.05. IR
(KBr pellet, cm-1): 2963 (w), 2932 (w), 2890 (w), 2168(w), 1632
(s), 1570 (m), 1385 (m), 1265 (m), 1076 (m), 899 (w), 868 (w),
802 (m), 690 (m), 617 (w), 478 (w).
Acknowledgment. We are indebted to the Chinese National
Natural Science Foundation for financial support.
Supporting Information Available: X-ray crystallographic data
for the structure determination of 2-4 and 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
Cp4YbLi(THF)n (7). A Schlenk flask was charged with a pale
gray slurry of YbCl3 (0.84 g, 3.00 mmol) in 15 mL of THF, and
then CpLi (24 mL, 12.00 mmol, 0.50 M in THF) was added by a
OM060055V