β-diketiminato rare-earth metal complexes. Structures, catalysis, and active species for highly cis -1,4-selective polymerization of isoprene

Article abstract of DOI:10.1021/om100100r

Lithiation of the β-diketimines (2,6-C₆H₃R ₂)NH - C(Me)CH - C(Me)N(2,6-C₆H₃R₂) (R = Me (HL¹), Et (HL²)) by nBuLi was followed by metathesis reaction with LnCl₃(THF)_x and Y(BH ₄)₃(THF)₂ to afford the corresponding complexes L¹LnCl₂(THF)₂ (Ln = Gd (1), Nd (3), Dy (4), Er (5), Y (6)), L²GdCl₂(THF)₂ (2), and L ¹Y(BH₄)₂(THF) (8), respectively. Treatment of neutral HL¹ with Y(CH₂SiMe₃) ₃(THF)₂ generated the bis(alkyl) complex 7, L ¹Y(CH₂SiMe₃)₂(THF). Upon activation with [PhNHMe₂][B(C₆F₅)₄] and AliBu₃, complex 6 showed the highest cis-1,4 selectivity (99.3%, T_p = 0 °C) toward the polymerization of isoprene, while complex 7 had a comparatively low cis-1,4 selectivity, and in contrast, complex 8 was completely inert. The influences of the ortho substituents of the N-aryl rings of the ligands, the types of central metals and cocatalysts, and addition sequence of the catalyst components had been thoroughly investigated. By means of X-ray diffraction and ¹H NMR spectroscopy analyses, the intermediates arising from the stoichiometric reactions among the catalyst components and the probable active species were elucidated, which facilitates further investigation of the mechanism for diene polymerization.

Full text of DOI:10.1021/om100100r

2186 Organometallics 2010, 29, 2186–2193
DOI: 10.1021/om100100r
β-Diketiminato Rare-Earth Metal Complexes. Structures, Catalysis, and
Active Species for Highly cis-1,4-Selective Polymerization of Isoprene
Danfeng Li,†,‡ Shihui Li,‡ Dongmei Cui,*,† and Xuequan Zhang†
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and ‡Graduate School of the
Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
Received February 9, 2010
Lithiation of the β-diketimines (2,6-C6H3R2)NHdC(Me)CHdC(Me)N(2,6-C6H3R2) (R =
Me (HL1), Et (HL2)) by nBuLi was followed by metathesis reaction with LnCl3(THF)x and
Y(BH4)3(THF)2 to afford the corresponding complexes L1LnCl2(THF)2 (Ln = Gd (1), Nd (3),
Dy (4), Er (5), Y (6)), L2GdCl2(THF)2 (2), and L1Y(BH4)2(THF) (8), respectively. Treatment of
neutral HL1 with Y(CH2SiMe3)3(THF)2 generated the bis(alkyl) complex 7, L1Y(CH2SiMe3)2-
(THF). Upon activation with [PhNHMe2][B(C6F5)4] and AliBu3, complex 6 showed the highest cis-
1,4 selectivity (99.3%, Tp = 0 °C) toward the polymerization of isoprene, while complex 7 had a
comparatively low cis-1,4 selectivity, and in contrast, complex 8 was completely inert. The influences
of the ortho substituents of the N-aryl rings of the ligands, the types of central metals and cocatalysts,
and addition sequence of the catalyst components had been thoroughly investigated. By means of
X-ray diffraction and 1H NMR spectroscopy analyses, the intermediates arising from the stoichio-
metric reactions among the catalyst components and the probable active species were elucidated,
which facilitates further investigation of the mechanism for diene polymerization.
Introduction
Al-Mg-Ln (Ln = rare-earth metal) allyl and MAO systems
and the molecular lanthanide-aluminates as well as the single-
site cationic lanthanocenes or non-cyclopentadienyl rare-earth
metal alkyl complexes4-8 provide highly cis-1,4-selective poly-
merization in a controllable manner.
Highly cis-1,4-selective polymerization of isoprene has
attracted much attention, because the resultant polymer
has wide applications as tires and other elastic materials
and especially as an alternative to natural rubber if it has an
over 98% cis-1,4 regularity and a high molecular weight.1
Thus the catalyst systems for the polymerization of isoprene
should possess distinguished cis-1,4 selectivity and provide
controlled molecular weight or even a living mode from an
academic point of view. Meanwhile they should be low cost,
straightforward, and high-yield in synthesis and thermo-
stable to satisfy industrial requirements. The first innovated
Ziegler-Natta catalysts based on neodymium carboxylates
are easy to prepare and cheap and have been applied in
industrial scale for producing polyisoprene, although with
less than 98% cis-1,4 tacticity and poorly controlled molec-
ular weight owing to the multiactive species of the systems;2
however, those based on titanium encounter the problems of
providing cross-linked products.3 The subsequently developed
modified Ziegler-Natta catalysts such as the combination of
(4) Ajellal, N.; Furlan, L.; Thomas, C. M.; Casagrande, J. O. L.;
Carpentier, J.-F. Macromol. Rapid Commun. 2006, 27, 338.
(5) (a) Fischbach, A.; Meermann, C.; Eickerling, G.; Scherer, W.;
Anwander, R. Macromolecules 2006, 39, 6811. (b) Fischbach, A.; Perdih,
F.; Herdtweck, E.; Anwander, R. Organometallics 2006, 25, 1626. (c) Friebe,
L.; Nuyke, O.; Obrech, W. Adv. Polym. Sci. 2006, 204, 47, and references
therein.
(6) (a) Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091.
(b) Jeske, G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks, T. J.
J. Am. Chem. Soc. 1985, 107, 8103. (c) Yasuda, H.; Yamamoto, H.; Yokota,
K.; Miyake, S.; Nakamura, A. J. Am. Chem. Soc. 1992, 114, 4908.
(d) Yasuda, H.; Furo, M.; Yamamoto, H.; Miyake, S.; Kibino, N. Macro-
molecules 1992, 25, 5115. (e) Yamashita, M.; Ihara, E.; Yasuda, H. Macro-
molecules 1995, 29, 1798.
(7) (a) Kaita, S.; Takeguchi, Y.; Hou, Z.; Nishiura, M.; Doi, Y.;
Wakatsuki, Y. Macromolecules 2003, 36, 7923. (b) Kaita, S.; Hou, Z.;
Nishiura, M.; Doi, Y.; Kurazumi, J.; Akira, C.; Horiuchi, A. C.; Wakatsuki, Y.
Macromol. Rapid Commun. 2003, 24, 179. (c) Kaita, S.; Doi, Y.; Kaneko,
K.; Horiuchi, A. C.; Wakatsuki, Y. Macromolecules 2004, 37, 5860.
(d) Zhang, L.; Suzuki, T.; Luo, Y.; Nishiura, M.; Hou, Z. Angew. Chem.,
Int. Ed. 2007, 46, 1909. (e) Zhang, L.; Nishiura, M.; Yuki, M.; Luo, Y.; Hou,
Z. Angew. Chem., Int. Ed. 2008, 47, 2642. (f) Tardif, O.; Kaita, S. Dalton
*Corresponding author. Fax: þ86-431-85262773. E-mail: dmcui@
ciac.jl.cn.
€
(1) (a) Wilson, D. Makromol. Chem. Symp. 1993, 66, 273. (b) Lauretti,
E.; Miani, B.; Misttrali, F. Rubber World 1994, 210, 34. (c) Zhao, J.;
Ghebremeskel, G. N. Rubber Chem. Technol. 2001, 74, 409.
(2) (a) Shen, Z.; Gong, C.; Chung, C.; Ouyang, J. Sci. Sin. 1964, 13,
1339. (b) Shen, Z.; Ouyang, J.; Wang, F.; Hu, Z.; Yu, F.; Qian, B. J. Polym.
Sci., Polym. Chem. Ed. 1980, 18, 3345. (c) Yang, J.; Hu, J.; Pan, S.; Xie, D.;
Zhong, C.; Ouyang, J. Sci. Sin. 1980, 23, 734.
(3) (a) Porri, L.; Giarrusso, A. Conjugated Diene Polymerisation. In
Comprehensive Polymer Science; Pergamon Press: Oxford, 1989; Vol. 4, p
53. (b) Hsieh, H.; Yeh, H. Rubber Chem. Technol. 1985, 58, 117.
(c) Longiave, C.; Castelli, R. J. Polym. Sci. Part C: Polym. Symp. 1963,
4, 387.
Trans. 2008, 2531. (g) Rastatter, M.; Muterle, R.; Roesky, P.; Thiele, S.
Chem.-Eur. J. 2009, 15, 474.
(8) (a) Duchateau, R.; van Wee, C. T.; Meetsma, A.; Teuben, J. H.
J. Am. Chem. Soc. 1993, 115, 4931. (b) Aubrecht, K. B.; Chang, K.;
Hillmyer, M. A.; Tolman, W. B. J. Polym. Sci., Part A: Polym. Chem. 2001,
39, 284. (c) Giesbrecht, G. R.; Whitener, G. D.; Arnold, J. Dalton Trans.
2001, 923. (d) Yang, Y.; Liu, B.; Lv, K.; Gao, W.; Cui, D.; Chen, X.; Jing, X.
Organometallics 2007, 26, 4575. (e) Yang, Y.; Wang, Q.; Cui, D. J. Polym.
Sci., Part A: Polym. Chem. 2008, 46, 5251. (f) Arndt, S.; Beckerle, K.;
Zeimentz, P. M.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2005, 44,
7473. (g) Robert, D.; Abinet, E.; Spaniol, T. P.; Okuda, J. Chem.-Eur. J.
2009, 15, 11937.
r
pubs.acs.org/Organometallics
Published on Web 04/12/2010
2010 American Chemical Society

Article
We have reported recently an unconventional Ziegler-
Organometallics, Vol. 29, No. 9, 2010 2187
Scheme 1. Synthesis of Complexes 1-8
Natta system composed of NCN-pincer-type ligand stabi-
lized rare-earth metal chlorides, organoborate, and AlR3.9
This system exhibits extremely high activity and perfect
cis-1,4 selectivity for the polymerization of conjugated dienes
(99.9% cis-1,4 polybutadiene vs 98.8% cis-1,4 polyisoprene),
which is mainly attributed to the C2 geometry of the ligand.
Intrigued by this result, we wondered whether the quasi
C2-geometric β-diketiminato ligands can be applied instead
of the NCN-pincer ligands to endow similar catalytic per-
formances to the attached rare-earth metal chlorido com-
plexes, because β-diketimines can be conveniently prepared
by condensation of β-diketones with amines and their steric
and electronic effects can be swiftly tuned via choosing
appropriate starting agents. Although β-diketiminate
lanthanide complexes10-13 have been isolated and exhibit
versatile chemistry such as luminescent property and
catalytic activity toward ring-opening polymerizations of
ε-carprolactone or lactides,12f,g,13c methyl methacrylate,12b,g
and ethylene11c and copolymerization of cyclohexene oxide
and CO2,13e,f etc., whether they possess catalytic activity in the
polymerization of isoprene is not known yet.
Herein we report the synthesis and full characterization of
β-diketiminato-ligated rare-earth metal dichlorido com-
plexes, as well as the alkyl and borohydrido counterparts.
In addition, we show that a ternary catalytic system com-
posed of the chlorido complexes, [PhNHMe2][B(C6F5)4],
and aluminum tris(alkyl)s shows for the first time good
activity and excellent cis-1,4 selectivity (99.3%) for the
polymerization of isoprene in a controllable manner. In
addition, the formation of cationic initiation active species
arising from the ternary system is described, which involves
the abstraction of ligand, alkylation of metal-chloride, and
probable β-H transfer of the aluminum-isobutyl moiety.
Results and Discussion
(9) Gao, W; Cui, D. J. Am. Chem. Soc. 2008, 130, 4984.
(10) (a) Hitchcock, P. B.; Lappert, M. F.; Tian, S. Dalton Trans. 1997,
1945. (b) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002,
102, 3031. (c) Eisenstein, O.; Hitchcock, P. B.; Khvostov, A. V.; Lappert, M.
F.; Maron, L.; Perrin, L.; Protchenko, A. V. J. Am. Chem. Soc. 2003, 125,
10790. (d) Avent, A. G.; Hitchcock, P. B.; Khvostov, A. V.; Lappert, M. F.;
Protchenko, A. V. Dalton Trans. 2003, 1070. (e) Avent, A. G.; Caro, C. F.;
Hitchcock, P. B.; Lappert, M. F.; Li, Z.; Wei, X. H. Dalton Trans. 2004, 1567.
(f) Avent, A. G.; Hitchcock, P. B.; Khvostov, A. V.; Lappert, M. F.;
Protchenko, A. V. Dalton Trans. 2004, 2272. (g) Hitchcock, P. B.; Lappert,
M. F.; Protchenko, A. V. Chem. Commun. 2005, 951.
(11) (a) Lee, L. W. M.; Piers, W. E.; Elsegood, M. R. J.; Clegg, W.;
Parvez, M. Organometallics 1999, 18, 2947. (b) Hayes, P. G.; Piers, W. E.;
Lee, L. W. M.; Knight, L. K.; Parvez, M.; Elsegood, M. R. J.; Clegg, W.
Organometallics 2001, 20, 2533. (c) Hayes, P. G.; Piers, W. E.; McDonald,
R. J. Am. Chem. Soc. 2002, 124, 2132. (d) Hayes, P. G.; Piers, W. E.; Parvez,
M. J. Am. Chem. Soc. 2003, 125, 5622. (e) Knight, L. K.; Piers, W. E.;
Fleurat-Lessard, P.; Parvez, M.; McDonald, R. Organometallics 2004, 23,
β-Diketiminato Rare-Earth Metal Complexes. β-Diketi-
mines (2,6-C6H3R2)NHdC(Me)CHdC(Me)N(2,6-C6H3R2)
(R=Me (HL1), Et (HL2)) were first treated with nBuLi and
then reacted with LnCl3(THF)x via metathesis at 0 °C. The
reaction mixture was stirred at 50 °C for 24 h to afford
complexes L1LnCl2(THF)2 (Ln = Gd (1), Nd (3), Dy (4),
Er (5), Y (6)) and L2GdCl2(THF)2 (2) in 55.4-87% yields
(Scheme 1). The molecular structures of gadolinium com-
plexes 1 and 2 (Figure 1) in the solid state confirmed by X-ray
analysis were two-THF-solvated isostructural monomers,
similar to the other β-diketiminato rare-earth metal dichlor-
ides reported previously,12d adopting octagonal geometry
with the two chloride atom apexes. Complex 1, bearing
ortho-methyl substituents of the N-aryl rings, is highly sym-
metric, containing two perpendicular mirror planes across the
central metal ion, while complex 2 contains ortho-ethyl sub-
stituents of the N-aryl rings, adopting C2 geometry. When the
ortho substituents are isopropyl groups (HL3), the corre-
sponding gadolinium complex L3GdCl2(THF)2 could not be
obtained following a similar synthetic procedure, owing to the
overcrowded environment around the central metal arising
from the ligand.
€
2087. (f) Lauterwasser, F.; Hayes, P. G.; Brase, S.; Piers, W. E.; Schafer, L. L.
Organometallics 2004, 23, 2234. (g) Hayes, P. G.; Piers, W. E.; Parvez, M.
Organometallics 2005, 24, 1173. (h) Hayes, P. G.; Piers, W. E.; Parvez, M.
Chem.-Eur. J. 2007, 13, 2632. (i) Kenward, A. L.; Piers, W. E.; Parvez, M.
Organometallics 2009, 28, 3012. (j) Kenward, A. L.; Ross, J. A.; Piers, W. E.;
Parvez, M. Organometallics 2009, 28, 3625. (k) Kenward, A. L.; Piers,
W. E.; Parvez, M. Organometallics 2009, 28, 6228.
(12) (a) Yao, Y. M.; Zhang, Y.; Shen, Q.; Yu, K. B. Organometallics
2002, 21, 819. (b) Yao, Y. M.; Zhang, Y.; Zhang, Z. Q.; Shen, Q.; Yu, K. B.
Organometallics 2003, 22, 2876. (c) Yao, Y. M.; Xue, M. Q.; Luo, Y. J.; Jiao,
R.; Zhang, Z. Q.; Shen, Q.; Wong, W. T.; Yu, K. B.; Sun, J. J. Organomet.
Chem. 2003, 678, 108. (d) Yao, Y. M.; Luo, Y. J.; Jiao, R.; Shen, Q.; Yu, K. B.;
Weng, L. H. Polyhedron 2003, 22, 441. (e) Zhang, Z. Q.; Yao, Y. M.; Zhang,
Y.; Shen, Q.; Wong, W. T. Inorg. Chim. Acta 2004, 357, 3173. (f) Xue, M. Q.;
Yao, Y. M.; Shen, Q.; Zhang, Y. J. Organomet. Chem. 2005, 690, 4685.
(g) Yao, Y. M.; Zhang, Z. Q.; Peng, H. M.; Zhang, Y.; Shen, Q.; Lin, J. Inorg.
Chem. 2006, 45, 2175.
The straightforward dealkylationofY(CH2SiMe3)3(THF)2
by the neutral β-diketimine HL1 gave the bis(alkyl) complex7,
L1Y(CH2SiMe3)2(THF) (Scheme 1). The 1H NMR spectrum
exhibited that the amino proton of the ligand originally
centered at δ 12.33 disappeared, suggesting the completeness
of the reaction, while a doublet at δ -0.52 for the Y-CH2
(13) (a) Basuli, F.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J.
Organometallics 2003, 22, 4705. (b) Cui, C. M.; Shafir, A.; Schmidt, J. A. R.;
ꢀ
Oliver, A. G.; Arnold, J. Dalton Trans. 2005, 1387. (c) Sanchez-Barba, L. F.;
3
Hughes, D. L.; Humphrey, S. M.; Bochmann, M. Organometallics 2005, 24,
3792. (d) Vitanova, D. V.; Hampel, F.; Hultzsch, K. C. Dalton Trans. 2005,
1565. (e) Vitanova, D. V.; Hampel, F.; Hultzsch, K. C. J. Organomet. Chem.
2005, 690, 5182. (f) Zhang, Z.; Cui, D.; Liu, X. J. Polym. Sci., Part A:
Polym. Chem. 2008, 46, 6810.
group appeared with a JY-H coupling constant of 3.2 Hz.
High-quality crystals grown from a hexane solution were
characterized by X-ray diffraction analysis (Figure 2, left).
The yttrium ion coordinates to the N,N-bidentate ligand, two

2188 Organometallics, Vol. 29, No. 9, 2010
Li et al.
Figure 1. Molecular structures of complexes 1 (left) and 2 (right). Thermal ellipsoids are set at 35% probability, and hydrogen atoms
are not shown for clarity.
Figure 2. Molecular structures of complexes 7 (left) and 8 (right). Thermal ellipsoids are set at 35% probability, and hydrogen atoms
are not shown for clarity.
alkyl groups arranging in trans positions against the “nacnac”
plane, and a THF molecule, to generate a distorted trigonal-
bipyramidal geometry different from the analogous com-
plexes 1 and 2. The two N-aryl rings are perpendicular
to the “nacnac” plane. The bond distances of N(1)-C(2),
C(2)-C(3), C(3)-C(4), and C(4)-N(2) are intermediate
between the corresponding single-bond and double-bond
distances, indicating the electrons delocalizing within the
backbone of the β-diketiminato ligand.
Although lanthanocene borohydrido compounds have
been reported extensively,14-16 non-cyclopentadienyl (non-
Cp)-ligated analogues still remain scarce, which might be
ascribed to the sterically less demanding environment of the
borohydrido ligand.17 In this work we successfully isolated
a β-diketiminato yttrium bis(borohydride), 8, L1Y(BH4)2-
(THF), by means of a one-pot reaction of the β-diketimine
ligand (HL1), nBuLi, and Y(BH4)3(THF)2 (Scheme 1). The
1H NMR spectrum of 8 displayed a broad signal around δ
1.36-0.89 assigned to BH4- protons, while the signal around
δ 5.23 arose from the NCCH moiety shifting downfield
compared with that in free HL1. X-ray diffraction analysis
revealed 8 as a monomer of a THF solvate, adopting a
trigonal-bipyamidal geometry (Figure 2, right). The THF
(14) (a) Marks, T. J.; Kolb, J. R. Chem. Rev. 1977, 77, 263. (b) Segal,
B. G.; Lippard, S. J. Inorg. Chem. 1978, 17, 844. (c) Lappert, M. F.; Singh,
A.; Atwood, J. L.; Hunterb, W. E. Chem. Commun. 1983, 206. (d) Fryzuk,
M. D.; Giesbrecht, G. R.; Rettig, S. J. Can. J. Chem. 2000, 78, 1003.
(15) (a) Ephritikhine, M. Chem. Rev. 1997, 97, 2193. (b) Cendrowski-
Guillaume, S. M.; Nierlich, M.; Lance, M.; Ephritikhine, M. Organometal-
lics 1998, 17, 786. (c) Cendrowski-Guillaume, S. M.; Gland, G. L.; Nierlich,
M.; Ephritikhine, M. Organometallics 2000, 19, 5654. (d) Cendrowski-
Guillaume, S. M.; Gland, G. L.; Lance, M.; Nierlich, M.; Ephritikhine, M.
C. R. Chim. 2002, 5, 73. (e) Barros, N.; Schappacher, M.; Dessuge, P.;
Maron, L.; Guillaume, S. M. Chem.-Eur. J. 2008, 14, 1881.
(16) (a) Bonnet, F.; Visseaux, M.; Barbier-Baudry, D.; Hafid, A.;
Vigier, E.; Kubicki, M. M. Inorg. Chem. 2004, 43, 3682. (b) Bonnet, F.;
Visseaux, M.; Pereira, A.; Barbier-Baudry, D. Macromolecules 2005, 38,
3162. (c) Barbier-Baudry1, D.; Bouyer1, F.; Bruno, A. S. M.; Visseaux, M.
Appl. Organomet. Chem. 2006, 20, 24. (d) Zinck, P.; Visseaux, M.;
Mortreux, A. Z. Anorg. Allg. Chem. 2006, 632, 1943. (e) Visseaux, M.;
Mainil, M.; Terrier, M.; Mortreux, A.; Roussel, P.; Mathivet, T.; Destarac, M.
Dalton Trans. 2008, 4558. (f) Bonnet, F.; Violante, C. D. C.; Roussel, P.;
Mortreux, A.; Visseaux, M. Chem. Commun. 2009, 3380.
(17) (a) Duchateau, R.; Wee, C. T.; Meetsma, A.; Duijnen, P. T.;
Teuben, J. H. Organometallics 1996, 15, 2279. (b) Bonnet, F.; Hillier, A. C.;
Collins, A.; Dubberley, S. R.; Mountford, P. Dalton Trans. 2005, 421.
Meyer, N.; Jenter, J.; Roesky, P. W.; Eickerlingc, G.; Scherer, W. Chem.
Commun. 2009, 4693. (c) Yuan, F.; Zhu, Y.; Xiong, L. J. Organomet. Chem.
2006, 691, 3377. (d) Trifonov, A. A.; Skvortsov, G. G.; Lyubov, D. M.; Fukin,
G. K.; Fedorova, E. A.; Bochkarev, M. N. Russ. Chem. Bull. Int. Ed. 2005,
54, 2511. (e) Skvortsov, G. G.; Yakovenko, M. V.; Fukin, G. K.; Cherkasov,
A. V.; Trifonov, A. A. Russ. Chem. Bull. Int. Ed. 2007, 56, 1742.

Article
Organometallics, Vol. 29, No. 9, 2010 2189
Table 1. Polymerization of Isoprene under Various Conditionsa
run cat. borate AlR3 [IP]/[Ln] Tp (°C) time (min) conv (%) cis-1,4 (%)b trans-1,4 (%)b 3,4 (%)b Mcaled (104)c Mn (104)d PDId
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
1
1
1
1
1
6
6
6
6
6
6
6
6
6
C
C
C
C
C
C
C
C
A
A
A
B
C
C
C
C
C
C
C
C
C
C
C
iBu
iBu
iBu
iBu
iBu
iBu
iBu
iBu
Me
Et
iBu
iBu
Me
Et
iBu
iBu
iBu
iBu
iBu
iBu
iBu
iBu
iBu
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
500
2000
3000
4000
5000
1000
1000
1000
1000
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
0
20
20
60
20
60
60
10
24 h
24 h
24 h
30
24 h
60
60
60
60
60
100
100
100
100
100
100
100
0
0
53.1
100
0
98.6
98.5
98.7
98.5
98.5
98.6
97.5
0
0
0
0
0
0
1.1
1.4
1.5
1.3
1.5
1.5
1.4
1.4
6.8
6.8
6.8
6.8
6.8
6.8
6.8
23.2
48.9
9.33
18.4
16.1
12.2
39.8
2.6
1.8
2.0
1.9
1.7
1.5
1.2
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
98.4
98.6
0
0
1.6
1.4
3.6
6.8
8.1
29.7
2.5
2.3
64.9
98.4
98.4
98.3
98.6
98.6
98.7
98.7
99.3
98.5
97.8
96.0
0
0
0.9
0
0
0
0
0
1.6
1.6
0.8
1.4
1.4
1.3
1.3
0.7
0.9
1.4
1.7
4.4
6.8
3.4
13.6
20.4
27.2
34.0
4.3
49.5
25.6
8.8
40.2
44.1
52.1
73.6
15.9
10.4
9.9
2.0
2.3
1.7
1.7
1.7
1.6
1.5
2.3
1.4
2.2
2.4
100
100
100
100
100
100
62.9
100
100
60
60
300
30
20
20
40
60
80
0.6
0.8
2.3
6.8
6.8
5.0
73.5
11.4
a General condition: toluene 5 mL, catalyst 10 μmol, [Ln]0/[B]0/[AliBu3]0 = 1:1:10, A: [Ph3C][B(C6F5)4], B: B(C6F5)3, C: [PhNHMe2][B(C6F5)4]. b 1
H
and 13C NMR spectroscopy in CDCl3. c Mcaled = ([IP]/[Ln]) ꢀ 68.12 ꢀ X (X = conversion). d Measured by SEC calibrated with standard polystyrene
samples.
molecule is about perpendicular to the “nacnac” plane, while
the borohydrido groups act as tridentate ligands in accor-
could be observed in 24 h by employing AlMe3 as the
activator. Switching to AlEt3, 53.1% conversion could be
obtained under the same conditions. Using AliBu3 instead of
AlEt3, completeness was achieved within 30 min. With the
combination of 1 and AliBu3, the addition of organoborate
or borane showed an influence on the catalytic activity of the
resultant ternary system in the order [PhNHMe2][B(C6F5)4 >
[Ph3C][B(C6F5)4] . B(C6F5)3 (Table 1, runs 1, 11, 12). In
contrast, the cis-1,4 selectivity of these systems seemed not to
be affected, which was in agreement with the “chloride
determine the selectivity” phenomenon in the conventional
Ziegler-Natta system.19
With the catalytic system of 6/[PhNHMe2][B(C6F5)4]/
AliBu3, the polymerization of isoprene showed a controlla-
ble nature. Increase of the monomer-to-lanthanide ratios
from 500 to 5000 led to a proportional increase of the
molecular weight from 8.8 ꢀ 104 to 73.6 ꢀ 104, in good
correlation with the theoretic values, while the molecular
weight distribution and the cis-1,4 selectivity stayed constant
(Table 1, runs 6, 15-19).
dance with a short Y-B bond length of 2.566(4) A,14b which
˚
are trans-located to form a large B(1)-Y-B(2) angle
(116.23(15)°) bisected by the “nacnac” plane. The coordi-
˚
nated B-H bond distances (av 2.302 A) are significantly
longer than the noncoordinated B-H bond distances (av
1.05 A). The “nacnac” backbone coordinates to the central
˚
metal ion to generate a six-membered metallacycle that is
twisted into a boat conformation in complexes 7 and 8 but is
planar in 1 and 2.
Isoprene Polymerization. All these complexes were evalu-
ated in the initiation of polymerization of isoprene activated
by [PhNHMe2][B(C6F5)4] and AliBu3 (Table 1, runs 1-8).
The alkyl complex 7 showed the highest activity, reaching a
complete conversion in 10 min with a 97.5% cis-1,4 selectiv-
ity. The gadolinium chlorido complexes 1 and 2 and the
dysprosium counterpart 4 came in next, with 20 min to attain
completeness. Comparatively, the analogous yttrium (6),
neodymium (3), and erbium complexes (5) were less active.
Remarkably all these chlorido complexes exhibited the
similarly distinguished cis-1,4 selectivity (>98%) despite
the central metal ions and the steric bulkiness of the ligands.
However, the borohydrido complex 8 was surprisingly inert.
These results showed that the axial ligands (Ln-X, X = Cl,
BH4, CH2SiMe3) exerted a more significant influence than
the other ligands on both the activity and selectivity. The
central metal affected only the activity, which was in contrast
to the conventional Ziegler-Natta systems, having an “in-
trinsic Nd effect”.5,18
In 6/[PhNHMe2][B(C6F5)4]/AliBu3, the polymerization
temperature exerted a great influence on the catalytic per-
formances (Table 1, runs 6, 20-23). When the polymeriza-
tion was performed at 60 °C, full conversion required 20 min
as compared with 60 min at 20 °C, which however was
accompanied by a slight drop of cis-1,4 selectivity. When
the temperature was increased to 80 °C, the yield was 73.5%
(19) (a) Evans, W. J.; Giarikos, D. G.; Ziller, J. W. Organometallics
2001, 20, 5751. (b) Porri, L.; Ricci, G.; Shubin, N. Macromol. Symp. 1998,
128, 53. (c) Quirk, R.; Kells, A.; Yunlu, K. M.; Cuif, J.-P. Polymer 2000, 41,
5903. (d) Oehme, A.; Gebauer, U.; Gehrke, K.; Lechner, M. D. Angew.
Makromol. Chem. 1996, 235, 121. (e) Kwag, G.; Lee, H.; Kim, S.
Macromolecules 2001, 34, 5367. (f) Koboyashi, E.; Hayashi, N.; Aoshima,
S.; Furukaya, J. J. Polym. Sci., Polym. Chem. 1998, 36, 1707. (g) Pross, A.;
Marquardt, P.; Reichert, K.-H.; Nentwig, W.; Knauf, T. Angew. Makromol.
Chem. 1993, 211, 89. (h) Ricci, G.; Italia, S.; Cabassi, F.; Porri, L. Polym.
Commun. 1987, 28, 223. (i) Iovu, H.; Hubca, G.; Racoti, D.; Hurst, J. S. Eur.
Polym. J. 1999, 35, 335. (j) Boisson, C.; Barbotin, F.; Spitz, R. Macromol.
Chem. Phys. 1999, 200, 1163. (k) Evans, W. J.; Giarikos, D. G. Macro-
molecules 2004, 37, 5130.
Besides the influence of the type of Ln-X (X = Cl, BH4,
CH2SiMe3) groups, the types of cocatalysts played a sig-
nificant role in governing the catalyst activity (Table 1, runs
1, 9-14). When complex 1 was chosen as the precursor and
[Ph3C][B(C6F5)4] as the cationizing agent, no polymerization
€
(18) (a) Meermann, C.; Tornroos, K.; Nerdal, W.; Anwander, R.
Angew. Chem., Int. Ed. 2007, 46, 6508. (b) Fischbach, A.; Perdih, F.;
Herdtweck, E.; Anwander, R. Organometallics 2006, 25, 1626.

2190 Organometallics, Vol. 29, No. 9, 2010
Li et al.
because of isoprene volatilizing partly at this high tempera-
ture. At 0 °C, the polymerization reaction became sluggish
but gave a rise in cis-1,4 selectivity to 99.3% compared
with 96.0% when the polymerization was performed at
80 °C. This phenomenon followed the theory that the
increase of trans-1,4 selectivity was through thermodynamic
control.20
Active Species. The isolation and characterization of active
species for any polymerization are an important topic, which
facilitates further investigation of the mechanisms and mod-
ification of the catalytic performances of the existent catalyst
systems and design of new ones. However, this is a challen-
ging and very tough task, as the active species is usually
highly active, less stable, and sometimes a mixture. For the
polymerization of dienes with the conventional Ziegler-
Natta catalytic systems based on lanthanide carboxylate,
the actual initiating species are still ambiguous; however, it is
commonly accepted that the active species were generated in
a two-step activation sequence involving the formation of a
reactive Ln-alkyl bond or Ln-hydride bond and then the
Al-to-Ln chloro transfer (cationization).1a,19b,21 Recently
our group reported a NCN-pincer-type ligand stabilized
rare-earth metal dichloride, which in combination with
AlR3 and [Ph3C][B(C6F5)4] gave an unconventional Ziegler-
Natta catalyst system. The active species was characterized as
the cationic binuclear LLn-R-Al (L = ligand), and the
selectivity of this system was governed by the geometry of the
metal center, which was strongly dependent on the substitu-
ents of the ligands. Obviously our present catalyst systems
behaved quite differently, as displayed by the above poly-
merization results, although the involved ligands possessed
similar C2 symmetry. Thus the active species might be
different. Surprisingly to us the catalytic activity of the
resultant ternary catalyst system was strongly depen-
dent on the addition order of the components. Although
such a phenomenon was observed in many conventional
Ziegler-Natta catalyst systems, especially those based on
neodymium carboxylate (Nd(OCOR)3) and neodymium
ispropoxides (Nd(OiPr)3),22,23 the rational evidence and de-
tailed explanations were not given. In this work we found that
when the addition order followed LLnCl2(THF)2/10AliBu3/
borate ([Ph3C][B(C6F5)4] or [PhNMe2H][B(C6F5)4]), only a
trace amount of polyisoprene was isolated. In striking con-
trast, when the order was changed to LLnCl2(THF)2/borate
([Ph3C][B(C6F5)4] or [PhNMe2H][B(C6F5)4])/10AliBu3, a
100% yield was achieved in a short time with an over 98%
cis-1,4 selectivity.
Figure 3. Molecular structure of ion pair 9a. Thermal ellipsoids
are set at 35% probability. Hydrogen atoms and four N-aryls
are not shown for clarity.
place immediately, evidenced by the resonances of the
methine proton CCHdC of the ligand backbone shifting
to δ 5.16 compared with δ 5.27 in 6. The coordinated THF
resonances appeared at δ 3.48 compared with δ 3.79, in-
dicating that THF was abstracted by AliBu3 from 6 to
generate AliBu3(THF)x.24 Fortunately, an ion pair [L1 Y3Cl5]-
3
[Cl(AliBu3)2] (9a) was successfully isolated from the above
reaction mixture. X-ray diffraction analysis revealed the
cation [L1 Y3Cl5]þ, featuring a five-coordinate yttrium center,
3
a bidentate L1 ligand, two μ2-Cl, and two μ3-Cl. The anion
a chloride-bridged two-tri(isobutyl
[Cl(AliBu3)2]- was
aluminum) unit (Figure 3). Then, [Ph3C][B(C6F5)4] was added
to the above C6D6 solution to give a new ion pair, [L2 Y3Cl5]-
3
[B(C6F5)4] (10a), by releasing Ph3CH (δ 5.53) and CH2C-
(CH3)2 (δ 4.86 and 1.71) (SFigure 1). This was confirmed
further when complex 2, L2GdCl2(THF)2, was employed
instead of 6 and sequentially treated with AliBu3 and
[Ph3C][B(C6F5)4], to give [L2 Gd3Cl5][B(C6F5)4] (10b). The
3
molecular structure of 10b was determined by X-ray diffrac-
tion analysis to possess a similar cationic part to that of 9a
albeit with poor quality (Figure 4). Therefore the reaction
pathway for this addition order could be depicted as
Scheme 2,25 in which no active Ln-alkyl or Ln-H species
was generated, and the resultant cationic intermediates 10 were
inert owing to the strong Ln-Cl bonds. This could explain
why the ternary system generated in this way did not initiate
the polymerization of conjugated dienes.
In order to investigate the actual initiating species of the
second addition order of LLnCl2(THF)2/borate/10AliBu3,
we tried to isolate single crystals from the system but failed,
unfortunately. To our delight, monitoring the reaction mix-
ture by 1H NMR spectroscopy provided useful information
about the formation of probable active species, as shown in
Scheme 3. First, complex 6, L1YCl2(THF)2, reacted with an
equimolar amount of Brønsted acid [PhNMe2H][B(C6F5)4]
in C6D6 to afford a light yellow oil deposited at the bottom of
the NMR tube and a supernatant clear solution. The oil was
Thus the step by step investigation of the two addition
sequences was carried out by means of a 1H NMR spectrum
monitoring technique. Complex 6, L1YCl2(THF)2, was first
treated with 5 equiv of AliBu3 in C6D6. The reaction took
(20) (a) Kuran, W. Principles of Coordination Polymerisation; John
Wiley and Sons Ltd.: New York, 2001; p 305. (b) Zinck, P.; Barbier-Baudry,
D.; Loupy, A. Macromol. Rapid Commun. 2005, 26, 46.
(21) Oehme, A.; Gebauer, U.; Gehrke, K.; Beyer, P.; Hartmann, B.;
Lechner, M. Macrol. Chem. Phys. 1994, 195, 3773.
(24) (a) Li, S.; Miao, W.; Tang, T.; Dong, W.; Zhang, X.; Cui, D.
Organometallics 2008, 27, 718. (b) Arndt, S.; Spaniol, T. P.; Okuda, J.
Angew. Chem., Int. Ed. 2003, 42, 5075.
(22) (a) Oehme, A.; Gebauer, U.; Gehrke, K.; Lechner, M. D. Angew.
Makromol. Chem. 1996, 235, 121. (b) Dong, W.; Endo, K.; Masuda, T.
Macromol. Chem. Phys. 2003, 204, 104. (c) Jenkins, D.; Ansell, P. J. Chem.
Abstr. 1990, 113, 154135. (d) Fischbach, A.; Anwande, R. Adv. Polym. Sci.
2006, 204, 155, and references therein.
(23) (a) Quirk, R. P.; Kells, A. M. Polym. Int. 2000, 49, 751. (b) Quirk,
R. P.; Kells, A. M.; Yunlu, K.; Cuif, J. P. Polymer 2000, 41, 5903. (c) Ansell,
P. J.; Williams, H. D. Chem. Abstr. 1993, 119, 119251. (d) Steinhauser, N.;
Obrecht, W. Chem. Abstr. 2002, 136, 136085. (e) Balducci, A.; Righi, S.
Chem. Abstr. 2004, 141, 54809.
(25) When complex 2 (L2GdCl2(THF)2) reacted with AlEt3 and
[Ph3C][B(C6F5)4] in toluene sequentially, the green crystal 10c grew
from the toluene at -30 °C. Its unit cell parameters are the same as 10b,
indicating that 10b and 10c were analogues formulated as [L23Ln3Cl5]-
[B(C6F5)4]. However, when AlMe3 was used, the different ion pair 10d,
[L22Gd2Cl3(THF)2][B(C6F5)4] (SFigure 3), was obtained. So, the ability
to abstract THF from dichloride by AlMe3 was weaker than AlEt3 and
AliBu3.

Article
Organometallics, Vol. 29, No. 9, 2010 2191
Scheme 3. Reaction Pathway for the Addition Order of
LLnCl2(THF)2/[PhNMe2H][B(C6F5)4]/AliBu3
Figure 4. Molecular structure of ion pair 10b. Hydrogen atoms,
ethyl groups, and anion part [B(C6F5)4]- are not shown for
clarity. The quality of this crystal is not good enough, so the ball-
and-stick model diagram is drawn instead of the ellipsoid
diagram.
by giving L1Al(CH2CH(CH3)2)2 simultaneously.10b This pro-
cess was clearly recorded by 1H NMR spectrum monitoring
technique, as the typical NH peak (12.33 ppm) of HL1
disappeared. Thus the true active species should be generated
via the reaction between AliBu3 and 11 ([YCl2(THF)2]þ). 11
reacted with AliBu3 to form, probably, a new species,
Scheme 2. Reaction Pathway for the Addition order of LLnCl2-
(THF)2/AliBu3/[Ph3C][B(C6F5)4]
þ
[(iBu)YClþ]n (12), which could partly transform into
[(H)YCl]n (13) by releasing isobutene. This might be the
only way to afford isobutene. Both 12 and 13 could be
cationic, although it was not clear whether they existed as a
cluster or a kind of AliBu3 adduct. The 19F NMR spectrum
proved further there was no coordination between the anion
[B(C6F5)4]- and the central lanthanide ion regardless of the
presence of AliBu3 (SFigure 5).1a,19b,22a The polymerization
behavior changed little with variation of the ligand from HL1
to HL2 (Table 1, runs 1, 2), which also supported the ligand
abstraction during the formation of the cationic active species.
These findings supported the theory that the active species
involved the formation of Ln-alkyl or Ln-H bonds and were
also inconsistent with the neutral active species [Me2LnCl]n
and [MeLnCl2]n of the modified Ziegler-Natta systems com-
prising the molecular lanthanide aluminates.5,18 We also
believe that the Y-Cl bond of the active species governed
the cis-1,4 selectivity, because the very similar catalyst
system Nd{N(SiMe3)2}3/[PhNMe2H][B(C6F5)4]/AliBu3 gave
the cationic species bearing a Ln-C bond but without a
Ln-Cl moiety, leading to a low cis-1,4 selectivity (86.5%).28
identified as the ion pair 11, [YCl2(THF)2][B(C6F5)4],26 on
the basis of the generation of free HL1 (δNH = 12.33 ppm,
δCCHC = 4.39 ppm, δCH3(Ar) = 2.25 ppm, δCH3 = 1.66 ppm)
and free PhNMe2 (δHotho = 6.75 ppm, δHmeta = 7.35 ppm,
δHpara = 6.91 ppm, δCH3 = 2.63 ppm) detected in the
supernatant C6D6 solution (SFigure 4). Thus other reaction
27
pathways giving 110 and 110011k were excluded. This meant
that the anionic ligand L1 of complex 6 was abstracted by
[PhNMe2H][B(C6F5)4] to recover the neutral HL1 and
PhNMe2. We noted that upon addition of 5 equiv of AliBu3
obvious gas extrusion was observed. The gas was a mixture
of isobutane (CH3 = 1.88 ppm, CH = 2.01 ppm) and
isobutene (CH3 = 1.71 ppm, CH2 = 4.86 ppm). Isobutane
was generated from the reaction of the above-mentioned
HL1 with AliBu3 via deprotonation, which was accompanied
Conclusion
We have demonstrated that the β-diketiminato rare-earth
metal dichlorides, conveniently synthesized and thermally
stable, combined with [PhNHMe2][B(C6F5)4] and AliBu3
form the ternary catalytic systems that can initiate the
polymerization of isoprene. The Ln-X (X = Cl, BH4,
CH2SiMe3) groups other than the ligands influence signifi-
cantly the catalytic performances. The rare-earth metal alkyl
complex (Ln-C) is the most active, whereas the borohydrido
(26) The yttrium trichloride YCl3(THF)3 reacted with Li[B(C6F5)4] in
THF at 80 °C for 3 days following extraction with toluene to give ion pair
[YCl2(THF)2][B(C6F5)4], which can be used to mimic the formation of
complex 11. The final product might be the cluster [YxCl3x-1(THF)y]-
[B(C6F5)4], where x and y were difficult to determine. When x and y were
deemed to be 1 and 2, respectively, and 10 equiv of AliBu3 (0.1 mmol)
was used to activate it, the resultant system could polymerize isoprene
(0.68 g) with high cis-1,4 selectivity (98.4%) and high activity (1 h, yield
100%).
€
(27) Kohn, R. D.; Smith, D.; Mahon, M. F.; Prinz, M.; Mihan, S.;
Kociok-Kohn, G. J. Organomet. Chem. 2003, 683, 200.
(28) Monteil, V.; Spitz, R.; Boisson, C. Polym. Int. 2004, 53, 576.

2192 Organometallics, Vol. 29, No. 9, 2010
Li et al.
analogue (Ln-BH4) is inert. Strikingly, both high activity
and distinguished cis-1,4 selectivity (99.3%) are achieved by
the Ln-Cl-based precursor. Moreover, the addition order of
the three catalyst components of LLnCl2(THF)2/borate/
AliBu3 is crucial to form an efficient catalyst system. The
cationic active species are generated via the ligand elimina-
tion, alkylation of metal-chloride, and probable β-H trans-
fer of the aluminum-isobutyl moiety. However, another
addition sequence, LLnCl2(THF)2/AliBu3/borate, provides
an inert system owing to the formation of intermediates
bearing strong Ln-chloride bonds.
GdCl3(THF)4 (0.552 g, 1 mmol) afforded complex 2 (0.58 g,
79%). Green single crystals for X-ray analysis grew from the
toluene at room temperature within several days. Anal. Calcd
for C33H49Cl2N2O2Gd: C, 54.01; H, 6.73; N, 3.82. Found: C,
53.68; H, 6.81; N, 3.53.
β-Diketiminato Rare-Earth Metal Dichloride L1LnCl2(THF)2
(3-6). Following the procedure described previously, 3-6,
L1LnCl2(THF)2 (Ln = Nd, Dy, Er, Y), were synthesized from
LnCl3(THF)x: Nd, a green microcrystal was detected by X-ray
analysis, yield 77.4%; Dy, yellow crystals, yield 69.7%; Er, pink
crystals, yield 55.4%. The NMR spectra of these complexes were
not available due to paramagnetism. Complex 6, off-white
crystals determined by X-ray analysis, yield 80.1%. NMR data:
1H NMR (300 MHz, C6D6, 25 °C): δ 7.10 (d, 3JH-H = 6.6 Hz,
4H, m-2,6-Me2-C6H3), 7.00 (tr, 3JH-H = 6.6 Hz, 2H, p-2,6-Me2-
C6H3), 5.27 (s, 1H, HC(C(CH3)NAr)2), 3.70 (m, 12H, THF)
2.74 (s, 12H, 2,6-CH3Ar), 1.72 (s, 6H, CH3), 1.36 (m, 12H, THF)
ppm. 13C NMR (100 MHz, C6D6, 25 °C): δ 166.91 (2C, HC-
(C(CH3)NAr)2), 146.93 (2C, ipso-N-C6H3), 135.48 (4C, o-C6H3),
129.21 (4C, m-C6H3), 125.35 (2C, p-C6H3), 99.19 (1C, HC-
(C(CH3)NAr)2), 71.95 (6C, THF), 25.75 (6C, THF), 24.35 (2C,
HC(C(CH3)NAr)2), 20.36 (4C, 2,6-(CH3)2-C6H3) ppm. Anal.
Calcd for complex 6, C29H41Cl2N2O2Y: C, 57.15; H, 6.78; N,
4.60. Found: C, 57.57; H, 7.03; N, 4.22. Anal. Calcd for complex
3, C29H41Cl2N2NdO2: C, 52.39; H, 6.22; N, 4.21. Found: C, 52.75;
H, 6.89; N, 4.01. Anal. Calcd for complex 4, C29H41Cl2N2O2Dy:
C, 50.99; H, 6.05; N, 4.10. Found: C, 60.21; H, 6.21; N, 3.96. Anal.
Calcd for complex 5, C29H41Cl2N2O2Er: C, 50.64; H, 6.01; N,
4.07. Found: C, 50.82; H, 6.33; N, 3.94.
Experimental Section
General Methods. All reactions were carried out under a dry
and oxygen-free argon atmosphere by using Schlenk techniques
or under a nitrogen atmosphere in a glovebox. All solvents were
purified from an MBraun SPS system. 1H and 13C NMR spectra
were recorded on a Bruker AV300, Bruker AV400, or Bruker
AV600 spectrometer. NMR assignments were confirmed by
1
1H-1H COSY and H-13C HMQC experiments when neces-
sary. 19F NMR spectra were recorded on a Bruker AV300
spectrometer and used 1-fluorobenzene as internal standard.
The molecular weight and molecular weight distribution of the
polymers were measured by TOSOH HLC 8220 GPC at 40 °C
using THF as eluent (the flow rate was 0.35 mL/min) against
polystyrene standards. The ligands HL (2,6-C6H3R2)NHd
C(Me)CHdC(Me)N(2,6-C6H3R2) (R = Me (HL1), Et (HL2),
iPr(HL3)) were prepared by literature procedures.29 All other
materials were obtained from Sigma-Aldrich and purified ac-
cording to standard procedures.
β-Diketiminato Rare-Earth Metal Alkyl Complex L1Y-
(CH2SiMe3)2(THF) (7). To a stirred solution of ligand HL1
(0.306 g, 1 mmol) in hexane (10 mL) was added Y(CH2SiMe3)3-
(THF)2 (0.496 g, 1 mmol) in hexane (5 mL) at 25 °C, and the
mixture was reacted until a clear solution was obtained in 0.5 h.
Then, the solution was concentrated to half volume and cooled
to -30 °C. Bright yellow single crystals of complex 7 were
isolated after several hours (0.52 g, 87%). 1H NMR (400 MHz,
C6D6, 25 °C): δ 7.07 (d, 3JH-H = 1.8 Hz, 4H, m-2,6-Me2-C6H3),
7.02 (br, 2H, p-2,6-Me2-C6H3), 5.15 (s, 1H, HC(C(CH3)NAr)2),
X-ray Crystallographic Studies. Crystals for X-ray analysis
were obtained as described in the preparations. The crystals
were manipulated in a glovebox. Data collections were per-
formed at -86.5 °C on a Bruker SMART APEX diffractometer
with a CCD area detector, using graphite-monochromated Mo
˚
KR radiation (λ = 0.71073 A). The determination of crystal
class and unit cell parameters was carried out using the SMART
program package. The raw frame data were processed using
SAINT and SADABS to yield the reflection data file. The
structures were solved by using the SHELXTL program.
Refinement was performed on F2 anisotropically for all non-
hydrogen atoms by the full-matrix least-squares method. The
hydrogen atoms were placed at the calculated positions and
were included in the structure calculation without further
refinement of the parameters. The crystallographic data and
the refinement of complexes 1, 2, 6, 7, 8, 9a, 10b, and 10d are
summarized in STables 1 and 2.
3.93 (m, 4H, THF), 2.37 (s, 12H, 2,6-CH3Ar), 1.63 (s, 6H, CH3),
3
1.23 (m, 4H, THF), 0.28 (s, 18H, SiCH3), -0.52 (d, JH-H
=
3.2 Hz, 4H, CH2SiMe3) ppm. 13C NMR (100 MHz, C6D6, 25 °C):
δ 166.64 (2C, HC(C(CH3)NAr)2), 148.75 (2C, ipso-N-C6H3),
133.29 (4C, o-C6H3), 125.4 (6C, m,p-C6H3), 98.62 (1C, HC-
(C(CH3)NAr)2), 70.95 (2C, THF), 37.58 (1C, CH2SiCH3),
37.18 (1C, CH2SiCH3), 25.45 (2C, THF), 24.51 (2C, HC-
(C(CH3)NAr)2), 20.36 (4C, 2,6-(CH3)2-C6H3), 4.91 (6C, Si(CH3)3)
ppm. Anal. Calcd for C33H55N2OSi2Y: C, 61.85; H, 8.65; N, 4.37.
Found: C, 62.19; H, 8.71; N, 4.19.
Synthesis and Character of Complexes 1-10. β-Diketiminato
Rare-Earth Metal Dichloride L1GdCl2(THF)2 (1). Under a
nitrogen atmosphere, nBuLi (1.6 M in hexane, 0.66 mL,
1.05 mmol) was added dropwise to a THF solution (25 mL) of
HL1 at 0 °C (0.31 g, 1 mmol), and the mixture remained
stirring for 1 h. Then the reaction solution was added to a THF
suspension (15 mL) of GdCl3(THF)4 (0.552 g, 1 mmol) at
0 °C. The mixture was allowed to warm to room temperature
gradually. Removal of THF in vacuo was followed by addition of
toluene. The reaction mixture was stirred for 24 h at 50 °C, then
hot-filtered to remove LiCl. Green crystals were obtained from
the concentrated toluene at rt (0.59 g, 87%). Anal. Calcd for
C29H41Cl2N2O2Gd: C, 51.39; H, 6.10; N, 4.13. Found: C, 51.06;
H, 6.23; N, 4.54.
β-Diketiminato Rare-Earth Metal Bisborohydrido Complex
L1Y(BH4)2(THF) (8). The synthesis of complex 8 was carried
out by the procedure for the synthesis of complex 1, but Y(BH4)3-
(THF)2 (0.28 g, 1 mmol) was usedinplaceofGdCl3(THF)4. Light
yellow crystals were obtained from concentrated toluene at
1
-30 °C (0.26 g, 62.4%). H NMR (400 MHz, C6D6, 25 °C): δ
7.05 (br, 4H, m-2,6-Me2-C6H3), 6.89 (br, 2H, p-2,6-Me2-C6H3),
5.23 (s, 1H, HC(C(CH3)NAr)2), 3.28 (s, 4H, THF), 2.39 (s, 12H,
2,6-CH3Ar), 1.57 (s, 6H, CH3), 1.07 (m, 4H, THF), between 1.36
and 0.89 (v br, overlap with THF, 8H) ppm. 13C NMR (75 MHz,
C6D6, 25 °C): δ 167.16 (2C, HC(C(CH3)NAr)2), 148.62 (2C, ipso-
N-C6H3), 132.76 (4C, o-C6H3), 125.24 (6C, m, p-C6H3), 99.27
(1C, HC(C(CH3)NAr)2), 71.75 (2C, THF), 24.60 (2C, THF),
24.25 (2C, HC(C(CH3)NAr)2), 19.59 (4C, 2,6-(CH3)2-C6H3)
ppm. Anal. Calcd for C25H41B2N2OY: C, 60.52; H, 8.33; N,
5.65. Found: C, 60.37; H, 8.61; N, 5.15.
β-Diketiminato Rare-Earth Metal Dichloride L2GdCl2(THF)2
(2). Following the same procedure as that for the preparation
of 1, the reaction of HL2 (0.36 g, 1 mmol) with 1 equiv of
[L1 Y3Cl5][Cl(AliBu3)2] (9a). A 50 mL round-bottom flask
3
was charged with complex 6, L1YCl2(THF)2 (0.138 g, 0.2 mmol),
and toluene (20 mL). Ten equivalents of isoprene (0.136 g,
2 mmol) and 5 equiv of AliBu3 (0.198 g, 1 mmol) were slowly
dropped into the flask successively at room temperature. The
(29) (a) Budzelaar, P. H. M.; Gelder, R.; Gal, A. W. Organometallics
1998, 17, 4121. (b) Cheng, M.; Moore, D. R.; Reczek, J. J.; Chamberlain,
B. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 8738.

Article
Organometallics, Vol. 29, No. 9, 2010 2193
reaction mixture was stirred for 0.5 h, during which time the color
changed from colorless to light yellow. The toluene solution was
condensed and hexane was added. Colorless crystals were
grown at -30 °C after several days. The yield was 41%. 1H
NMR (600 MHz, C6D5Cl, 25 °C): δ 7.04 (m, 18H, m,p-Ar),
4.80 (s, 3H, HC(C(CH3)NAr)2), 3.60 (s, 4H, THF), 2.19 (s, 36H,
2,6-CH3-C6H3), 1.56 (s, 24H, HC(C(CH3)NAr)2 and Al(CH2CH-
(CH3)2)3), 1.60 (s, 4H, THF), 1.22 (m, 36H, Al(CH2CH(CH3)2)3),
0.81 (m, 12H, Al(CH2CH(CH3)2)3) ppm. 13C NMR (125 MHz,
C6D5Cl, 25 °C): δ 159.75 (6C, HC(C(CH3)NAr)2), 141.35
(6C, ipso-C6H3), 138.82 (12C, o-C6H3), 136.75 (12C, m-C6H3),
123.65 (6C, p-C6H3), 93.21 (3C, HC(C(CH3)NAr)2), 66.76 (2C,
THF), 30.91 (12C, Al(CH2CH(CH3)2)3), 27.57 (6C, Al(CH2CH-
(CH3)2)3), 24.88 (2C, THF), 20.43 (12C, 2,6-(CH3)2-C6H3), 16.82
(6C, HC(C(CH3)NAr)2), 16.52 (6C, Al(CH2CH(CH3)2)3) ppm.
Anal. Calcd for C87H129Al2Cl6N6Y3: C, 58.30; H, 7.25; N, 4.69.
Found: C, 59.02; H, 7.52; N, 4.50.
that for 10a, with AlEt3 and AlMe3 used instead of AliBu3,
respectively. After a few weeks, green crystals of 10c (41%) and
10d (38%) were obtained suitable for X-ray structure analysis.
10c was an isomer of 10b, while 10d was a new ion pair (SFigure 3).
Anal. Calcd for 10d, C82H82BCl3F20Gd2N4O2: C, 50.07; H, 4.20;
N, 2.85. Found: C, 51.01; H, 4.42; N, 3.79.
Procedure for Isoprene Polymerization. A typical polymeriza-
tion procedure was as follows (Table 1, run 1): In a glovebox, a
toluene solution (4.0 mL) of 1 (0.01 mmol) was added into a
25 mL flask. Then, 1 equiv of [PhNHMe2][B(C6F5)4] (8.0 mg,
0.01 mmol) dissolved in toluene (1.0 mL), 10 equiv of AliBu3
(0.2 mL, 0.5 mol/L), and isoprene (0.68 g, 10 mmol) was added
to the flask. After stirring for 20 min, methanol was injected to
terminate the polymerization. The viscous mixture was poured
into a large quantity of methanol. The obtained white polymer
was filtered and then dried under vacuum at 40 °C to a constant
weight.
[L2 Gd3Cl5][B(C6F5)4] (10b). A 50 mL round-bottom flask
3
was charged with complex 2, L2GdCl2(THF)2 (0.22 g, 0.3 mmol),
and toluene (20 mL), to which 5 equiv of AliBu3 (0.30 g,
1.5 mmol) was slowly dropped at roomtemperature. The reaction
mixture was stirred for 0.5 h. Then [Ph3C][B(C6F5)4] (0.28 g,
0.3 mmol) was dissolved in chlorobenzene (5 mL) and added into
the flask. The color became brown from orange. After 0.5 h, the
reaction mixturewas condensed, filtered, andcooled at-30 °C. A
few weeks later, a green crystal was obtained, suitable for X-ray
structure analysis. The yield was 32%. Anal. Calcd for
C99H99BCl5F20Gd3N6: C, 49.28; H, 4.14; N, 3.48. Found: C,
49.97; H, 4.41; N, 3.34.
Acknowledgment. We thank the National Natural Science
Foundation of China (No. 20934006) and the Ministry of
Science and Technology of China (Nos. 2005CB623802 and
2009AA03Z501) for financial support.
Supporting Information Available: 1H NMR spectra of inter-
mediates 9a and 10a; 1H, 19F, 1H-1H COSY NMR spectra of
intermediates 11, 12, and 13; ORTEP drawings for molecular
structures of 6 and 10d; X-ray crystallographic data and refine-
ment for 1, 2, 6, 7, 8, 9a, 10b, and 10d in CIF format. These
materials are available free of charge via the Internet at http://
pubs.acs.org.
[L2 Gd3Cl5][B(C6F5)4] (10c) and [L2 Gd2Cl3(THF)][B(C6F5)4]
3
2
(10d). The synthetic processes of 10c and 10d were similarly to