Highly cis-1,4 selective polymerization of dienes with homogeneous Ziegler-Natta catalysts based on NCN-pincer rare earth metal dichloride precursors

Article abstract of DOI:10.1021/ja711146t

The first aryldiimine NCN-pincer ligated rare earth metal dichlorides (2,6-(2,6-C₆H₃R₂N=CH)₂-C ₆H₃)LnCl₂(THF)₂ (Ln = Y, R = Me (1), Et (2), ⁱPr (3); R = Et, Ln = La (4), Nd (5), Gd (6), Sm (7), Eu (8), Tb (9), Dy (10), Ho (11), Yb (12), Lu (13)) were successfully synthesized via transmetalation between 2,6-(2,6-C₆H₃-R ₂N=CH)₂-C₆H₃Li and LnCl ₃(THF)_1-3.5. These complexes are isostructural monomers with two coordinating THF molecules, where the pincer ligand coordinates to the central metal ion in a κC:κN: κN′ tridentate mode, adopting a meridional geometry. Complexes 1-6, 9-11, and 13 combined with aluminum tris(alkyl)s and [Ph₃C][B(C₆F₅) ₄] established a homogeneous Ziegler-Natta catalyst system, which exhibited high activities and excellent cis-1,4 selectivities for the polymerizations of butadiene (T_p = 25°C, 99.9%; 0°C, 100%) and isoprene (T_p = 25°C, 98.8%). Remarkably, such high cis-1,4 selectivity almost remained at elevated polymerization temperatures up to 80°C and did not vary with the type of the central lanthanide element, however, which was influenced obviously by the ortho substituent of the N-aryl ring of the ligands and the bulkiness of the aluminum alkyls. The Ln-Al bimetallic cations were considered as the active species. These results shed new light on improving the catalytic performance of the conventional Ziegler-Natta catalysts for the specific selective polymerization of dienes.

Full text of DOI:10.1021/ja711146t

Published on Web 03/14/2008
Highly cis-1,4 Selective Polymerization of Dienes with
Homogeneous Ziegler-Natta Catalysts Based on NCN-Pincer
Rare Earth Metal Dichloride Precursors
Wei Gao†,‡ and Dongmei Cui*,†
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Chemistry School,
Jilin UniVersity, Changchun 130012, China
Received December 16, 2007; E-mail: dmcui@ciac.jl.cn
Abstract: The first aryldiimine NCN-pincer ligated rare earth metal dichlorides (2,6-(2,6-C6H3R2NdCH)2-
C6H3)LnCl2(THF)2 (Ln ) Y, R ) Me (1), Et (2), iPr (3); R ) Et, Ln ) La (4), Nd (5), Gd (6), Sm (7), Eu (8),
Tb (9), Dy (10), Ho (11), Yb (12), Lu (13)) were successfully synthesized via transmetalation between
2,6-(2,6-C6H3-R2NdCH)2-C6H3Li and LnCl3(THF)1∼3.5. These complexes are isostructural monomers with
two coordinating THF molecules, where the pincer ligand coordinates to the central metal ion in a κC:κN:
κN′ tridentate mode, adopting a meridional geometry. Complexes 1-6, 9-11, and 13 combined with
aluminum tris(alkyl)s and [Ph3C][B(C6F5)4] established a homogeneous Ziegler-Natta catalyst system, which
exhibited high activities and excellent cis-1,4 selectivities for the polymerizations of butadiene (Tp ) 25 °C,
99.9%; 0 °C, 100%) and isoprene (Tp ) 25 °C, 98.8%). Remarkably, such high cis-1,4 selectivity almost
remained at elevated polymerization temperatures up to 80 °C and did not vary with the type of the central
lanthanide element, however, which was influenced obviously by the ortho substituent of the N-aryl ring of
the ligands and the bulkiness of the aluminum alkyls. The Ln-Al bimetallic cations were considered as the
active species. These results shed new light on improving the catalytic performance of the conventional
Ziegler-Natta catalysts for the specific selective polymerization of dienes.
Introduction
outstanding characteristics such as excellent abrasion and
cracking resistance, raw polymer strength, and high tensile
cis-1,4 selective polymerization of dienes is a very important
process in the chemical industry to afford products that are
among the most significant and widely used rubbers.1 It is
believed that a slight increase in the cis-1,4 regularity of the
product leads to a great improvement in the elastic properties.2
Thus, the investigation of new catalyst systems that are
homogeneous, well-defined, straightforward and high yielding
in synthesis, thermostable, and providing over 98% cis-1,4
selectivity has been one of the most fascinating and challenging
subjects in both academic and industrial fields. The transition
metal η3-allyl derivatives,3 the homogeneous Ziegler-Natta
catalyst systems composed of lanthanide (Ln) carboxylates
(including phosphates)4 or alkoxides5 and co-activators alumi-
num alkyls or aluminum alkyl chlorides, have been investigated
extensively and industrially applied.6 Among these catalysts,
the Ln-based mixtures are superior with respect to the activity
and the cis-1,4 selectivity (>97%) to provide polymers with
strength of the vulcanizates. For these industrial recipes, little
is known about the structures of the precatalysts; however, it is
commonly accepted that the active rare earth metal center
involves the formation of an active Ln-alkyl or Ln-H species
and cationation via Al-to-Ln chloride transfer.7 Therefore, well-
defined Ln-C bond containing complexes, for instance, lan-
thanide-based metallocene alkyl complexes,8 lanthanocene
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1838. (b) Dong, W.; Endo, K.; Masuda, T. Macromol. Chem. Phys. 2003,
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† Chinese Academy of Sciences.
‡ Jilin University.
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4984
J. AM. CHEM. SOC. 2008, 130, 4984-4991
10.1021/ja711146t CCC: $40.75 © 2008 American Chemical Society

NCN-Pincer Rare Earth Metal Dichloride Precursors
A R T I C L E S
Scheme 1. Synthesis of Aryldiimino NCN-Pincer Rare Earth Metal Dichlorides via Transmetalation
aluminates,9 and alkyl bridged lanthanide carboxylates,10 were
developed. With these systems, more controllable or even living
polymerization of butadiene can be achieved with a cis-1,4
selectivity within 97-99% or as high as 99.9% if performing
the polymerization at low temperatures. These systems are
strongly metal-dependent, showing intrinsic Nd7b,10 or Sm and
Gd8,9 effects. Recently, yttrium alkyl dications have been
reported to be highly active and showed a 97% cis-1,4 selectivity
for the polymerization of isoprene.11 Meanwhile, another
cationic system based on lanthanide alkyls bearing the PNP-
type auxiliary ligand12 has received an upsurge in research
interest, providing the living polymerization of dienes and an
over 99% cis-1,4 selectivity, whereas the central metals are
restricted to the late lanthanide elements.
Ziegler-Natta catalysts based on Ti, Co, Ni, and Nd metal
chlorides, first innovated to initiate the polymerization of
butadiene in the early 1960s, are highly active to give polymers
with 98% cis-1,4 selectivity but are heterogeneous, leading to
gel formation.13,14 The addition of an electron donor to lan-
thanide trichlorides improves the catalytic activity and selectivity
slightly, whereas the system is still heterogeneous.4a,15 Modifying
the lanthanide chloride species with cylopentadienyl or silylene-
bridged cyclopentadienyl fluorenyl moieties generates an inert
system for the polymerization of butadiene, which is active for
copolymerization with ethylene, affording trans-1,4-butenyl
units.8,16
We have been interested in pincer ligated complexes17 in a
general formula of [(ECE)M] (E ) PR2, SR, NR2) because the
symmetrically coordinating geometry and the electron donating
nature of such a ligand are anticipated to induce specific
selectivity to the metal center. Pincer complexes based on the
Ru, Os,18 Ni,19 Pd,20 Pt,21 and Rh22 etc. transition metals have
attracted widespread interest in catalysis on Heck coupling and
C-C bond formation reactions and material science,23 whereas
the rare earth metal counterparts have been exploited less. The
only reported rare earth metal complexes are stabilized by an
aryldiamino NCN-pincer ligand that is not suitable for preparing
the corresponding analogues based on lanthanide elements with
a large ionic radius,24 and their chemistry is unknown.
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Organometallics 2004, 23, 5653.
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127, 12273. (b) Hossain, M.; Lucarini, S.; Powell, D.; Bowman-James, K.
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20, 4437.
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R. Macromolecules 2006, 39, 6811. (b) Meermann, C.; To¨rnroos, K.;
Nerdal, W.; Anwander, R. Angew. Chem., Int. Ed. 2007, 46, 6508. (c)
Fischbach, A.; Perdih, F.; Herdtweck, E.; Anwander, R. Organometallics
2006, 25, 1626. (d) Fischbach, A.; Klimpel, M.; Widenmeyer, M.;
Herdtweck, E.; Scherer, W.; Anwander, R. Angew. Chem., Int. Ed. 2004,
43, 2234.
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Chem., Int. Ed. 2005, 44, 7473.
(12) (a) Zhang, L.; Suzuki, T.; Luo, Y.; Nishiura, M.; Hou, Z. Angew. Chem.,
Int. Ed. 2007, 46, 1909. The non-metallocene ligated Nd amido or lithium
ate complexes have been reported to be cis-1,4 selective, see: (b) Monteil,
V.; Spitz, R.; Boisson, C. Polym. Int. 2004, 53, 576. (c) Sugiyama, H.;
Gambarotta, S.; Yap, G.; Wilson, D.; Thiele, S. Organometallics 2004,
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Organometallics 2004, 23, 4550. (b) Oakley, S.; Coogan, M.; Arthur, R.
Organometallics 2007, 26, 2285.
(23) For reviews on pincer complexes, see: (a) Albrecht, K.; van Koten, G.
Angew. Chem., Int. Ed. 2001, 40, 3750. (b) van der Boom, M. E.; Milstein,
D. Chem. ReV. 2003, 103, 1759. (c) Singleton, J. T. Tetrahedron 2003, 59,
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9
J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008 4985

A R T I C L E S
Gao and Cui
Herein, we report the first successful synthesis of aryldiimino
NCN-pincer stabilized rare earth metal dichlorides via trans-
metalation methodology. These complexes in combination with
aluminum alkyls and organoborate generated a new type of
homogeneous Ziegler-Natta catalyst system that displayed high
activities and distinguished cis-1,4 selectivities for the poly-
merizations of butadiene and isoprene. These excellent catalytic
performances did not show an obvious metal-type dependence
except for the inert Yb, Sm, and Eu counterparts but were
influenced significantly by the spacial sterics of the auxiliary
ligand and the aluminum alkyl. Remarkably, the high cis-1,4
selectivity remained at elevated polymerization temperatures up
to 80 °C. These results are in striking contrast to the conven-
tional Ziegler-Natta systems in the specific selective polym-
erization of dienes. The living species and the probable
mechanism are also presented.
Figure 1. Perspective view of 1 with thermal ellipsoids drawn at the 30%
probability level. Hydrogens are omitted for clarity. Selected bond lengths
(Å) and angles (deg): Y1-C1 2.393(3), Y1-N1 2.640(3), Y1-N2 2.649-
(3), Y1-Cl1 2.6152(9), Y1-Cl2 2.6105(10), Y1-O1 2.392(2), Y1-O2
2.436(2), Cl1-Y1-Cl2 174.98(3), Cl1-Y1-O1 92.07(6), Cl2-Y1-O1
84.54(6), Cl1-Y1-O2 83.92(6), Cl2-Y1-O2 91.69(6), Cl1-Y1-N1
90.23(7), Cl1-Y1-N2 90.82(6), Cl2-Y1-N1 92.52(7), Cl2-Y1-N2
90.47(7), O1-Y1-O2 76.64(8), and N1-Y1-N2 131.77(9).
Results and Discussion
Synthesis and Characterization of Aryldiimino NCN-
Pincer Rare Earth Metal Dichlorides. The straightforward
synthetic route for preparing pincer complexes is metal intro-
duction, including direct cyclometalation, oxidative addition,
transmetalation, as well as transcyclometalation,23a which is
suitable for small metal ions and aryldiamino NCN-pincer
ligands,25 whereas most aryldiimino pincer compounds are
unstable under metalation conditions, leading to CdN double
bond addition26 or metalation at the undesired 3,5-positions.27
Thus, complexes bearing an aryldiimino pincer ligand have been
limited to Pd complexes prepared via ligand introduction,20a Pt
complexes via direct platination,26 and Ir and Rh halides via
oxidative addition.22 The title rare earth metal complexes bearing
aryldiimino pincer ligands were successfully prepared via
sequential lithiation and transmetalation. Keeping in mind that
the lithiation of such aryldiimino pincer compounds swiftly
induced the imino CdN bond addition, we performed the
Figure 2. Perspective view of 2 with thermal ellipsoids drawn at the 30%
probability level. Hydrogens are omitted for clarity. Selected bond lengths
(Å) and angles (deg): Y1-C1 2.411(4), Y1-N1 2.671(2), Y1-Cl1 2.6072-
(7), Y1-O1 2.457(2), Cl1-Y1-Cl1(A) 173.47(3), Cl1-Y1-O1 85.97-
(5), Cl1-Y1-N1 95.88(5) O1-Y1-O1(A) 75.08(9), and N1-Y1-N1(A)
130.24(10).
n
reaction of (2,6-dialkyl)isophthalaldimine-2-bromide28 with -
BuLi at a very low temperature (-78 °C) for 1 h and then at
-40 °C for another 2 h. The lithium salt 2,6-(2,6-C6H3-R2Nd
CH)2-C6H3Li was isolated selectively without visible CdN bond
addition. The transmetalation took place immediately upon the
addition of rare earth metal trichlorides, LnCl3(THF)1∼3.5, to
the THF solution of the lithium salt at -40 °C and was stirred
for another 12 h at room temperature. This reaction did not show
restriction to the size of the metal ion; thus, complexes based
on varied lanthanide elements, [2,6-(2,6-C6H3R2NdCH)2-C6H3]-
LnCl2(THF)2 (Ln ) Y, R ) Me (1), Et (2), iPr (3); R ) Et, Ln
) La (4), Nd (5), Gd (6), Sm (7), Eu (8), Tb (9), Dy (10), Ho
(11), Yb (12), Lu (13)), were isolated in 50-80% yield (Scheme
1). No metalation at the 3,5-positions was observed. This
represented the first example of preparing aryldiimino NCN-
pincer metal compounds via transmetalation methodology.
δ 8.35-8.45 ppm assigned to the CHdN proton. The 13C NMR
spectra confirmed further the formation of the Ln-σ-C bond,
which showed typical downfield shifts around δ 194.15 -200.13
ppm.29 Complexes 1-3, 5-7, 9, 10, 12, and 13 were character-
ized by X-ray diffraction analysis as isostructural monomers
with two coordinating THF molecules (Figures 1-4 for 1-3
and 6). The monoanionic NCN-pincer ligand coordinates to the
central metal ion in a κC:κN:κN′ tridentate mode to form a
meridional conformation. The two N-aryl rings dispose the
vertical positions against the NCN plane, which are parallel to
the two cis-located THF rings, respectively. The chloride groups
arrange in trans-positions to form a large Cl-Ln-Cl angle (av.
176.25(4)° in 1-3) bisected by the NCN plane. The Ln3+ ion
is essentially coplanar with the NCN plane except in the cases
of 1 and 3 with deviations of 0.0927(2) and 0.0739(1) Å,
respectively. The solid-state structures of these complexes have
C2 symmetry, adopting a pentagonal bipyramidal geometry
around the central metal. In complexes 1-3 and 6, the Ln-C
bond lengths varying from 2.393(3) to 2.420(4) Å fall in a
reasonable range for a linkage between a lanthanide ion and a
Complexes 1-4 and 13 were characterized by 1H NMR
spectra that displayed a similar topology, giving singlets within
(25) Jung, I.; Son, S.; Park, K.; Chung, K.; Lee, J.; Chung, Y. Organometallics
2003, 22, 4715.
(26) Fossey, J.; Richards, C. Organometallics 2002, 21, 5259. (b) Fosssey, J.;
Richards, C. Tetrahedron Lett. 2003, 44, 8773.
(27) (a) Steenwinkel, P.; Gossage, R.; Maunula, T.; Grove, D.; von Koten, G.
Chem.sEur. J. 1998, 4, 763. (b) Fossey, J.; Richards, C. Organometallics
2004, 23, 367.
(28) Wille, E.; Stephenson, D.; Capriel, P.; Binsch, G. J. Am. Chem. Soc. 1982,
104, 405.
(29) (a) Liu, B.; Cui, D.; Ma, J.; Chen, X.; Jing, X. Chem.sEur. J. 2007, 13,
834. (b) Wang, D.; Cui, D.; Miao, W. Li, S.; Huang, B. Dalton Trans.
2007, 4576.
9
4986 J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008

NCN-Pincer Rare Earth Metal Dichloride Precursors
A R T I C L E S
replaced by the more bulky isopropyl in the case of complex 3,
a much more obvious decrease in the catalytic activity was
observed (68% in 1 h) (Table 1, entries 1-3). These results
indicated that the spacial sterics of the ancillary ligands greatly
influenced the catalytic activity of the corresponding precursors.
The catalytic activity was also strongly affected by the type of
aluminum alkyls in the trend AliBu3 . AlEt3 > AlMe3. When
AlMe3 or AlEt3 was employed instead of AliBu3 to activate 2,
the polymerization became sluggish to reach a medium yield
in 1 h (Table 1, entries 18 and 19).
To evaluate the role of the central metal type in this system,
complexes based on various lanthanide elements bearing the
o-ethyl substituent of the N-aryl ring of the pincer ligand were
examined. The Nd (5), Gd (6), Tb (9), Dy (10), and Ho (11)
precursors exhibited superior activity to reach complete conver-
sion within 15 min at room temperature. Even the complexes
attached by the early La (4) or the late Lu (13) lanthanide
elements showed notable catalytic activities, albeit slightly lower
than their Y analogue 2 (Table 1, entries 4-6, 9-11, and 13).
To date, only a few examples of diene polymerization catalysts
based on La have been reported. In contrast, all precursors
attached by lanthanide elements Sm (7), Yb (8), as well as Eu
(12) were demonstrated to be inert, owing to their reducible
nature9e (Table 1, entries 7, 8, and 12).
Figure 3. Perspective view of 3 with thermal ellipsoids drawn at the 30%
probability level. Hydrogens are omitted for clarity. Selected bond lengths
(Å) and angles (deg): Y1-C1 2.398(3), Y1-N1 2.680(3), Y1-N2 2.680-
(3), Y1-Cl1 2.6286(9), Y1-Cl2 2.5941(9), Y1-O1 2.427(3), Y1-O2
2.413(2), Cl1-Y1-Cl2 173.55(3), O1-Y1-O2 76.71(9), and N1-Y1-
N2 131.49(9).
When 2 was chosen as the precursor to initiate the polym-
erization of butadiene under various monomer-to-initiator ratios
varying from 500 to 5000, the molecular weight of the resultant
PB was found to increase almost linearly when the ratio was
below 4000, which reached up to 133.0 × 104 (Mn) at a ratio
of 5000; meanwhile, the molecular weight distribution did not
change obviously or became even narrower (Mw/Mn ) 2.40-
1.79) (Table 1, entries 2 and 14-17). Thus, this system was
probably a single site to induce controllable polymerization. It
should be noted that all these molecular weights of the measured
values were essentially 3 times higher than the calculated ones
(Table 1, entries 14-16), leading to about 30% catalytic
efficiency (Mcalcd/Mnmeasd), consistent with the previously re-
ported homogeneous single-site catalyst systems or even the
living systems,10b,12a which had been attributed to fast propaga-
tion as compared to the initiation, the aggregation of the
active species, or the heterogeneous nature of some catalyst
systems.
Figure 4. Perspective view of 6 with thermal ellipsoids drawn at the 30%
probability level. Hydrogens and uncoordinated solvent molecules are
omitted for clarity. Selected bond lengths (Å) and angles (deg): Gd1-C1
2.421(4), Gd1-N1 2.682(2), Gd1-Cl1 2.6213(7), Gd1-O1 2.481(2), O1-
Gd1-Cl1 91.06(5), N1-Gd1-N1A 130.80(10), Cl1-Gd1-Cl1A 176.25-
(4), and O1-Gd1-O1A 75.63(10).
carbon atom;30 the C-Nimine bond lengths (av. 1.284(3) Å) are
comparable to those in the analogous Rh and Pt complexes,22
while the Ln-Clterminal bond length with an average of 2.619 Å
is close to 2.60 Å (av) for Y-Clterminal given in the literature.31
The N-Ln-N angle of 131.07° (av) is much smaller than 158°
(av) in the related transition metal complexes owing to the
crowded environment of the metal center arising from the bulky
ligands, which may contribute greatly to the high selectivity of
the complexes.
Catalytic Activity. The combination of complexes 1-13 with
AliBu3 and [Ph3C][B(C6F5)4] established a Ziegler-Natta
catalyst system that was homogeneous, providing varied cata-
lytic activities toward the polymerization of butadiene. The
selected polymerization data were summarized in Table 1.
Complex 2 bearing the o-ethyl substituent of the N-aryl ring of
the ligand was highly active to reach complete conversion within
1 h, while complex 1 with the o-methyl was less active
comparatively than that under the same conditions: only 80%
conversion could be attained. When the o-substituent was
cis-1,4 Selectivity. Remarkably, this new type of Ziegler-
Natta catalytic system provided extremely high cis-1,4 selectivity
at room temperature, varying within a narrow range of 99.3-
99.9% despite the central metal type (Table 1, entries 2, 4-6,
9-11, and 13). These results were in contrast to the conventional
Ziegler-Natta systems such as LnCl3/EtOH/AlEt3, Ln(O-
COCl3)3/AliBu3/AlEt2Cl, and the modified Ziegler-Natta system
i
[Me2Al(O2CC6H2 Pr3-2,4,6)2]2Ln[(µ-Me)2AlMe2]/R2AlCl, where
Nd-based mixtures were the mostly active and selective (intrinsic
Nd effect).3-5,10,32 These results were also quite different from
the system composed of Cp*2Ln[(µ-Me)2AlMe2(µ-Me)]2LnCp*2
and AlEt2Cl, where the cis-1,4 selectivity showed a strong
dependence on the central metal type, changing from 38.8%
for Ce to 97.3% for Gd.9c,e
(30) (a) Yang, Y.; Liu, B.; Lv, K.; Gao, W.; Cui, D.; Chen, X.; Jing, X.
Organometallics 2007, 26, 4575. (b) Shang, X.; Liu, X.; Cui, D. J. Polym.
Sci., Part A: Polym. Chem. 2007, 45, 5662. (c) Liu, B.; Yang, Y.; Cui,
D.; Tang, T.; Chen, X.; Jing, X. Dalton Trans. 2007, 4252. (d) Wang, B.;
Wang, D.; Cui, D.; Gao, W.; Tang, T.; Chen, X.; Jing, X. Organometallics
2007, 26, 3167.
(31) (a) Kuo, P.; Chang, J.; Lee, W.; Lee, H. M.; Huang, J. J. Organomet. Chem.
2005, 690, 4168. (b) Xue, M.; Yao, Y.; Shen, Q.; Zhang, Y. J. Organomet.
Chem. 2005, 690, 4685. (c) Glazier, M. J.; Levason, W.; Matthews, M. L.;
Thornton, P. L.; Webster, M. Inorg. Chim. Acta 2004, 357, 1083.
(32) (a) Shen, Z.; Gong, C.; Ouyang, J. Polym. Commun. 1965, 7, 193. (b) Jin,
Y.; Zhang, X.; Pei, F.; Wu, Y. Chin. J. Polym. Sci. 1990, 8, 121. (c) Taube,
R.; Sylvester, G. In Applied Homogenous Catalysis with Organometallic
Compounds; Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH: Weinheim,
Germany, 2002; p 280.
9
J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008 4987

A R T I C L E S
Gao and Cui
Table 1. Polymerization of Butadiene under Various Conditionsa
micros tructure (%)b
c
entry
complex
[BD]/[Ln]
Tp
(°
C)
time (min)
yield (%)
Mn
(×
104)
Mw/Mn
cis-1,4
trans-1,4
1,2-
1
2
YMe(1)
YEt(2)
500
500
500
500
500
500
500
500
500
500
500
500
500
1000
2000
4000
5000
500
500
500
500
500
500
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
0
60
60
60
60
15
10
120
120
10
10
15
120
60
60
60
180
180
60
60
30
5
80
100
68
3.96
8.60
2.63
7.01
18.00
32.21
1.47
2.23
1.31
1.59
2.08
2.18
98.5
99.7
95.3
99.3
99.5
99.7
1.3
0.3
4.2
0.5
0.4
0.3
0.2
3
YiPr(3)
LaEt(4)
NdEt(5)
GdEt(6)
SmEt(7)
EuEt(8)
TbEt(9)
DyEt(10)
HoEt(11)
YbEt(12)
LuEt(13)
YEt(2)
0.5
0.2
0.1
4
87
5
100
100
0
6
7
8
0
9
100
100
100
0
21.00
26.00
14.22
2.43
2.24
2.44
99.7
99.4
99.4
0.3
0.6
0.6
10
11
12
13
14
15d
16e
17f
18g
19h
20
21
22
23
90
9.75
18.63
39.50
79.20
133.00
31.88
17.40
33.10
21.35
11.70
10.23
2.48
2.31
2.40
2.13
1.79
2.11
1.68
1.89
2.03
1.87
1.95
99.3
99.3
99.4
99.7
99.9
66.5
51.5
100
0.6
0.7
0.1
100
93
YEt(2)
0.6
YEt(2)
90
0.3
YEt(2)
85
0.1
YEt(2)
43
31.7
47.5
1.8
1.0
YEt(2)
65
GdEt(6)
GdEt(6)
GdEt(6)
GdEt(6)
85
40
60
80
100
100
80
99.3
97.6
96.9
0.7
2.1
2.6
5
0.3
0.5
5
a C6H5Cl (5 mL), complex (20 µmol), [Ln]0/[AliBu3]0/[B]0 ) 1:20:1 (B ) [Ph3C][B(C6F5)4]. b Determined by 13C NMR spectrum of polybutadiene.
c Determined by GPC with respect to a polystyrene standard. d C6H5Cl (15 mL). e C6H5Cl (20 mL). f C6H5Cl (30 mL). g [Y]0/[AlMe3]0/[B]0 ) 1:20:1. h [Y]0/
[AlEt3]0/[B]0 ) 1:20:1.
The cis-selectivity of this system was found to be influenced
by the ortho substituent of the N-aryl ring of the ligands,
reaching as high as 99.7% for 2 bearing o-ethyl, which dropped
slightly for 1 bearing o-methyl (98.5%) and even more in the
case of 3 with the bulky o-isopropyl (95.3%) (Table 1, entries
1-3). This might be ascribed to the different steric environment
of the center metal that was pinched by the two ortho-
substituted N-aryl rings. The dihedral angle formed by the aryl
rings was 118.56(13)° in 1, 109.9° in 2, and 133.28(15)° in 3,
respectively. The smaller the dihedral angle is, the bulkier the
metal center is, leading to the higher cis-1,4 selectivity.
Meanwhile, the cis-selectivity also showed that it was sensitive
to the steric demands of the aluminum alkyl. When applying
AlMe3 as the activator, the catalyst system based on precursor
2 exhibited only medium cis-selectivity (66.5%) to afford PB
with an as high as 31.7% trans-1,4 regularity. Switching to
AlEt3, the system showed almost an equal cis-1,4 (51.5%) and
trans-1,4 (47.5%) selectivity (Table 1, entries 18 and 19). Both
the sterics of the ligand and the aluminum alkyls influenced
the catalytic performances, suggesting that the active living
species of this new system should contain an Ln-Al bimetallic
unit that was not cleaved during the polymerization process (vide
infra).
More strikingly, this system exhibited a distinguished toler-
ance toward the polymerization temperature. With the Gd-based
precursor 6, when the polymerization was performed at 40 °C,
a cis-1,4 selectivity of 99.3% could be obtained that was
comparable to 99.7% at 25 °C, which decreased slightly to
96.9% when the temperature was increased up to 80 °C,
suggesting that the active species did not decompose under these
conditions (Table 1, entries 6 and 21-23). Lowering the
polymerization temperature to 0 °C, a perfect cis-1,4 selectivity
(100%) could be achieved with an amazing activity (a 85%
conversion in 30 min) (Table 1, entry 20). No 1,2- and trans-
1,4 regularities were detectable from the 13C NMR spectrum
of the resultant PB (Figures S11 and S12.). These results
demonstrated for the first time, as far as we are aware, that an
excellent cis-1,4 selectivity for the polymerization of butadiene
could be maintained at elevated temperatures, in contrast to
many previous systems where the selectivity varied drastically
with the polymerization temperature. Moreover, this present
system displayed a similar catalytic behavior for the polymer-
ization of isoprene to that for butadiene, which need not be
discussed further.33
Active Species. The active center of the conventional
Ziegler-Natta catalyst system was generated in a two-step
activation sequence involving the formation of a reactive Ln-
alkyl or Ln-hydride bond and the Al-to-Ln chlorotransfer
(cationization).34 Anwander et al.36 proved that preformed Ln/
Al heterobimetallic complexes such as [LnAl3Me8(O2CC6H2-
iPr3-2,4,6)4], [Ln(OR)3(AlMe3)n] (R ) 2,6-R*2C6H3, R* )
tBu, iPr), and [Ln(AlMe4)3] are the alkylated intermediates that
upon further activation with Et2AlCl give highly efficient
initiators for isoprene polymerization. [Me2LnCl]n and [MeLnCl2]n
are considered as the actual initiating species,10 while the
mechanistic scenarios elucidated by Taube et al., Okuda et al.,
Hou et al., and Anwander et al. involve cationic initiators [Nd-
(C3H5)(C4H6)n]2+ 35 [YMe(sol)6]2+ 11
, [(PNP)Y(CH2SiMe3)-
,
(thf)x]+,12a or [{[(C5Me5)La{(µ-Me)2AlMe(C6F5)}][Me2Al-
(C6F5)2]}2].36 To obtain information about the active species of
this system, the pincer yttrium dichlorides (2) were first treated
with aluminum alkyls in C6D5Cl. The alkylation, we noted, did
not take place at room temperature within 12 h, suggesting that
(33) The high cis-1,4 regulated (98.8%) polyisoprene (Mn ) 7.78 × 104, Mw/
Mn ) 1.76) was obtained by using a system composed of 2, AliBu3, and
[Ph3C][B(C6F5)4] at room temperature for 2 h, yielding 100%. The selected
polymerization data are listed in Table S1 of the Supporting Information.
(34) (a) Porri, L.; Ricci, G.; Shubin, N. Macromol. Symp. 1998, 128, 53. (b)
Oehme, A.; Gebauer, U.; Gehrke, K.; Beyer, P.; Hartmann, B.; Lechner,
M. Macromol. Chem. Phys. 1994, 195, 3773. (c) Wilson, D. Macromol.
Chem., Macromol. Symp. 1993, 66, 273.
(35) (a) Maiwald, S.; Weissenborn, H.; Windisch, H.; Sommer, C.; Mu¨ller, G.;
Taube, R. Macromol. Chem. Phys. 1997, 198, 3305. (b) Maiwald, S.;
Sommer, C.; Mu¨ller, G.; Taube, R. Macromol. Chem. Phys. 2001, 202,
1446.
(36) For the active species [{[(C5Me5)La{(µ-Me)2AlMe(C6F5)}][Me2Al(C6F5)2]}2]
for trans-1,4 polymerization of isoprene, see: Zimmermann, M.; To¨rnroos,
K.; Anwander, R. Angew. Chem., Int. Ed. 2008, 47, 775.
9
4988 J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008

NCN-Pincer Rare Earth Metal Dichloride Precursors
A R T I C L E S
Scheme 2. Probable Active Species
the Ln-Cl bond in 2 was strengthened by the chelating NCN-
pincer donor as compared to LnCl3. Instead, an intermediate A
([NCN]LnCl2‚(Al(iBu)3)2) of the coordination adduct of AliBu3
to 2 was generated (Scheme 2).10a The methine proton of the
CHdN group of the NCN moiety shifted to δ 8.17 in A as
compared to δ 8.38 in 2, while the methylene protons of the
group CH2CH(CH3)2 gave a doublet around δ 0.06 shifting
upfield as compared to δ 0.35 in free Al(CH2CH(CH3)2)3 (C6D5-
Cl).37 It is reasonable that A could not induce the polymerization
of butadiene at room temperature. The addition of aluminum
alkyl chloride Et2AlCl as the weak cationizing cocatalyst to A
did not improve its catalytic activity. When activated with
[Ph3C][B(C6F5)4], A was swiftly transferred into the ion pair
B, [NCN]Ln[(µ-Cl)2(Al(iBu)2)]+[B(C6F5)4]-,38 with releasing
Ph3CH (δ 5.52), CH2C(CH3)2 (δ 4.48, 1.70), and minor Ph3Ci-
Bu39 via [Ph3C][B(C6F5)4] electrophilic attack of the iBu group
of A. Correspondingly, the methylene protons of the o-ethyl
group gave one multiple resonance at the upfield region (δ 2.20)
different from the two discrete multiplets (δ 2.91 and 3.19) in
A. B was still inert to the polymerization. When adding one
more equivalent Al(iBu)3 to B, a heteroleptic alkyl/chloride
bridged bimetallic intermediate C, [NCN]Ln[(µ-Cl)(µ-iBu)(Al-
(iBu)2)]+[B(C6F5)4]-, was generated.40 The signals for the
bridged isobutyl group shifted to a much higher region (δ
-0.04), while the terminal isobutyl groups did not change. No
visible polymerization was found with C. Under the presence
of excess Al(iBu)3, C initiated the polymerization of butadiene
(isoprene) immediately. Thus, we suggest that the homoleptic
alkyl bridged D might be formed and that it acted as the actual
active species.41
The mode of the diene monomer enchainment via 1,4
insertion (i.e., the formation of cis-1,4 or trans-1,4 monomeric
units in the polymer) is determined by the structure, anti or
syn, of the last inserted monomeric unit in the growing chain.
The anti form of the η3-butyl group gives rise to the formation
of a cis-1,4 monomeric unit, whereas the syn form leads to a
trans-1,4 unit. Thus, the exclusive cis-1,4 selectivity of this
present Ziegler-Natta system suggested that the anti form was
favored, which could be ascribed to the concert influences of
the pincer-like geometry of the ancillary ligand and the steric
demands of aluminum alkyl (Scheme S1). Increasing the
polymerization temperature gave a rise in trans-1,4 content as
the isomerization to the thermally stable syn π-allylic form
became faster.42
Conclusion
The first aryldiimine NCN-pincer stabilized rare earth metal
dichlorides were successfully synthesized via transmetalation
methodology and were well-defined, which in combination with
aluminum alkyls and borate established a new type of Ziegler-
Natta catalyst system. This system provided extremely high
activity and perfect cis-1,4 selectivity for the polymerization
of butadiene (cis-1,4 99.9%) and isoprene (cis-1,4 98.8%). The
spacial sterics of the ortho substituent of the N-aryl ring of the
NCN-pincer ligand and the bulkiness of aluminum alkyls played
significant roles in controlling the catalytic activity and selectiv-
ity. In contrast, the central metal type had almost no effect on
the specific selectivity but slightly influenced the catalytic
activity. Remarkably, such distinguished catalytic performances
remained under broad ranges of monomer-to-initiator ratios
(500-5000) and polymerization temperatures (25-80 °C). The
alkyl bridged Ln-Al bimetallic cations were proven to be the
actual active species. These results were in striking contrast to
the conventional Ziegler-Natta catalysts and the other previ-
ously reported systems, which might shed light on to a new
and more applicable procedure for highly cis-1,4 selective
catalysts.
(37) 1H NMR spectrum of A: 1H NMR (400 MHz, C6D5Cl, 25 °C): δ 0.06 (d,
JH-H ) 8 Hz, 12H, Al-CH2CH(CH3)2), 1.09 (d, JH-H ) 4 Hz, 36H, Al-
CH2CH(CH3)2), 1.14 (t, JH-H ) 8 Hz, 12H, CH2CH3), 1.42 (b, 16H, THF),
1.97 (m, 6H, Al-CH2CH(CH3)2), 2.91 (m, 4H, CH2CH3), 3.19 (m, 4H, CH2-
CH3), 3.70 (b, 16H, THF), 6.97-7.11 (m, 6H, N-C6H3, 1H, p-Y-C6H3)
7.27 (d, JH-H ) 8 Hz, 2H, m-Y-C6H3), 8.17 (s, 2H, CHdN).
(38) 1H NMR spectrum of B: 1H NMR (400 MHz, C6D5Cl, 25 °C): δ 0.06 (d,
JH-H ) 4 Hz, 4H, Al-CH2CH(CH3)2), 0.95 (t, JH-H ) 8 Hz, 12H, CH2CH3),
1.09 (d, 12H, JH-H ) 4 Hz, Al-CH2CH(CH3)2), 1.54 (b, 16H, THF), 1.61
(s, 6H, CH2dC(CH3)2), 1.98 (m, 2H, Al-CH2CH(CH3)2), 2.20 (m, 8H, CH2-
CH3), 3.62 (b, 16H, THF), 4.78 (s, 2H, CH2dC(CH3)2), 5.43 (s, 1H, Ph3CH),
Experimental Procedures
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
6.97-7.20 (m, 6H, N-C6H3, 1H, p-Y-C6H3, 15H, Ph3C), 7.31 (d, JH-H
8 Hz, 2H, m-Y-C6H3), 8.06 (s, 2H, CHdN).
)
(39) Dealkylation of the iBu group of aluminum alkyls of A by [Ph3C][B(C6F5)4]
took place swiftly to give Ph3C-iBu as the normal product; meanwhile,
Ph3C+ could also abstract a proton from the iBu group to afford Ph3CH
and CH2CHiBu2 as the main products according to the resonances. In the
case of AlMe3 as the coactivator, the NMR monitoring result showed that
Ph3CMe was given exclusively, showing a singlet resonance at δ 2.03
(CH3).
1
MBraun SPS system. H and 13C NMR spectra were recorded on a
Bruker AV400 (FT, 400 MHz for 1H; 100 MHz for 13C) spectrometer.
NMR assignments were confirmed by 1H-1H COSY and 1H-13C
HMQC experiments when necessary. The molecular weight and
molecular weight distribution of the polymers were measured by
(40) 1H NMR spectrum of C: 1H NMR (400 MHz, C6D5Cl, 25 °C): δ -0.04
(b, 2H, Y-CH2CH(CH3)2), 0.05 (d, JH-H ) 4 Hz, 4H, Al-CH2CH(CH3)2),
0.95 (t, JH-H ) 8 Hz, 12H, CH2CH3), 1.09 (m, 6H, Y-CH2CH(CH3)2, 12H,
Al-CH2CH(CH3)2), 1.54 (b, 16H, THF), 1.61 (m, 6H, CH2dC(CH3)2), 1.98
(m, 2H, Al-CH2CH(CH3)2), 2.20 (m, 8H, CH2CH3), 2.44 (m, 1H,
Y-CH2CH(CH3)2), 3.62 (b, 16H, THF), 4.78 (s, 2H, CH2dC(CH3)2), 5.43
(s, 1H, Ph3CH), 6.97-7.20 (m, 6H, N-C6H3, 1H, p-Y-C6H3, 15H, Ph3C),
7.31 (d, JH-H ) 8 Hz, 2H, m-Y-C6H3), 8.06 (s, 2H, CHdN).
(41) 1H NMR spectrum of the species D/(excess AliBu3) was not informative
owing to the strong signals from the free AliBu3, whereas washing off the
free AliBu3 with hexane gave an unknown inert species, most likely due to
the decomposition of D at the absence of AliBu3.
(42) Tobisch, S. Acc. Chem. Res. 2002, 34, 96.
9
J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008 4989

A R T I C L E S
Gao and Cui
TOSOH HLC 8220 GPC at 40 °C using THF as an eluent against
polystyrene standards. Elemental analyses were performed at the
National Analytical Research Centre of the Changchun Institute of
Applied Chemistry (CIAC). 2,6-Dimethylaniline, 2,6-diethylaniline, and
2,6-diisopropylaniline was obtained from Aldrich and purified by
distillation before use.
situ adding YCl3(THF)3.5 (0.67 g, 1.5 mmol) yielded 3 (0.41 g, 55%).
Yellow crystals for X-ray analysis grew from the mixture of THF and
hexane at -30 °C within several days. 1H NMR (400 MHz, CDCl3, 25
°C): δ 1.08 (d, 6H, JH-H ) 6.8 Hz, CH(CH3)2), 1.30 (d, JH-H ) 6.8
Hz, 6H, CH(CH3)2), 1.75 (b, 8H, THF), 3.76 (m, 4H, CH(CH3)2), 3.92
(b, 8H, THF), 7.11 (m, 6H, Ph), 7.24 (m, 1H, Ph), 7.46 (d, JH-H ) 6
Hz, 2H, Ph), 8.40 (s, 2H; CHdN). 13C NMR (100 MHz, CDCl3, 25
°C): δ 22.14 (s, 4C, THF), 25.19 (s, 8C, CH(CH3)2) 27.46 (s, 4C,
CH(CH3)2), 70.58 (s, 4C, THF), 123.23 (s, 2C, p-N-C6H3), 125.76 (s,
4C, m-N-C6H3), 126.41 (s, 1C, p-Y-C6H3), 132.19 (s, 2C, m-Y-C6H3),
140.93 (s, 4C, o-N-C6H3), 143.30 (s, 2C, o-Y-C6H3), 149.27 (s, 2C,
CdNsC), 177.57 (s, 2C, CdNsC), 194.25 (d, JC-Y ) 41 Hz, 1C,
C-Y). Anal. calcd for C40H53Cl2N2O2Y: C, 63.74; H, 7.09; N, 3.72.
Found: C, 63.64; H, 6.98; N, 3.63.
X-ray Crystallographic Studies. Crystals for X-ray analysis
were obtained as described in the following preparations. The crystals
were manipulated in a glovebox. Data collections were performed at
-86.5 °C on a Bruker SMART APEX diffractometer with a CCD
area detector, using graphite monochromated Mo KR radiation
(λ ) 0.71073 Å). The determination of crystal class and unit cell
parameters was carried out by 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. For crystallographic data and refinement
of complexes 1-3, 5-7, 9, 10, 12, and 13, see the Supporting
Information.
Synthesis of 4-13. Following the procedure described previously,
4-13, [bis(N-2,6-diethylphenyl)isophthalaldimin-2-yl]LnCl2(THF)2 (Ln
) La, Sm, Eu, Nd, Ho, Dy, Gd, Tb, Yb, Lu), were synthesized from
LnCl3(THF)x: Sm, yellow microcrystals, yield 62.1%; Eu, purple
microcrystals, yield 35.2%; Nd, yellow microcrystals, yield 65.8%; Ho,
yellow microcrystals, yield 54.3%; Dy, yellow microcrystals, yield
64.2%; Tb, yellow microcrystals, yield 67.7%; Yb, yellow microcrys-
tals, yield 54.6%. The NMR spectra of these complexes were not
available due to paramagnetism. La, yellow microcrystals, yield 54.2%.
[Bis(N-2,6-dimethylphenyl)isophthalaldimine-2-yl]YCl2(THF)2
n
(1). Under a nitrogen atmosphere, BuLi (1.6 M in hexane, 0.66 mL,
1.05 mmol) was added dropwise to a THF solution (25 mL) of
2,6-(2,6-Me2sC6H3NdCH)2sC6H3s1-Br (0.42 g, 1.00 mmol) at
-78 °C and was stirred for 1 h. The reaction solution was warmed
to -40 °C to react for another 2 h and then was added to a THF
suspension (15 mL) of YCl3(THF)3.5 (0.67 g, 1.5 mmol). The re-
action mixture was allowed to warm to room temperature gradually
and was stirred for 12 h. Removal of volatiles under reduced pres-
sure, extracting the residue with toluene, and evaporating the toluene
to dryness afforded 1 as yellowish powder (0.53 g, 82.6%). Single
crystals for X-ray analysis grew from the mixture of THF and
hexane at -30 °C within several days and were yellow blocks. 1H NMR
(400 MHz, CDCl3, 25 °C): δ 1.44 (b, 8H, THF), 2.47 (s, 12H,
CH3), 3.73 (m, 8H, THF), 7.00 (t, JH-H ) 7.2 Hz, 2H, p-N-C6H3),
7.06 (d, JH-H ) 7.2 Hz, 4H, m-N-C6H3), 7.23 (t, JH-H ) 7.2 Hz,
1H, p-Y-C6H3), 7.45 (d, JH-H ) 7.2 Hz, 2H, m-Y-C6H3), 8.35
(d, JH-H ) 2 Hz, 2H, CHdN). 13C NMR (100 MHz, CDCl3, 25 °C):
δ 19.16 (s, 4C, CH3), 24.84 (s, 4C, THF), 70.83 (b, 4C, THF), 124.95
(s, 2C, p-N-C6H3), 126.32 (s, 1C, p-Y-C6H3), 127.73 (s, 4C, m-N-C6H3),
130.24 (s, 2C, CdNsC), 132.04 (s, 2C, m-Y-C6H3), 143.61 (s, 2C,
o-Y-C6H3), 151.39 (s, 4C, o-N-C6H3), 178.12 (s, 2C, CdNsC),
194.15 (d, JC-Y ) 43 Hz, 1C, C-Y) ppm. Anal. calcd for C32H37-
Cl2N2O2Y (%): C, 59.92; H, 5.81; N, 4.37. Found: C, 59.83; H, 5.74;
N, 4.31.
NMR data: 1H NMR (400 MHz, CDCl3, 25 °C): δ 1.14 (t, JH-H
)
7.5 Hz, 12H, CH2CH3), 1.53 (b, 8H, THF), 2.73 (q, JH-H ) 7.5 Hz,
4H, CH2CH3), 2.92 (q, JH-H ) 7.5 Hz, 4H, CH2CH3), 3.56 (b, 8H,
THF), 6.92 (m, 6H, N-C6H3), 7.30 (t, JH-H ) 7.5 Hz, 1H, p-La-C6H3),
7.40 (d, JH-H ) 7.5 Hz, 2H, m-La-C6H3) 8.36 (s, 2H, CHdN); 13C
NMR (100 MHz, CDCl3, 25 °C): δ 15.26 (s, 4C, CH2CH3), 25.08 (s,
4C, H2CH3), 25.62 (s, 4C, THF), 70.25 (s, 4C, THF), 125.68 (s, 2C,
p-N-C6H3), 125.96 (s, 4C, m-N-C6H3), 126.30 (s, 1C, p-La-C6H3),
132.30 (s, 2C, m-La-C6H3), 136.24 (s, 4C, o-N-C6H3), 144.86 (s, 2C,
o-Lu-C6H3), 150.54 (s, 2C, CdNsC), 179.49 (s, 2C, CdNsC), 206.89
(s, 1C, La-C) ppm; Lu, pale yellow microcrystals, yield 59.4%, NMR
data: 1H NMR (400 MHz, CDCl3, 25 °C): δ 1.16 (t, JH-H ) 8 Hz,
12H, CH2CH3), 1.56 (b, 8H, THF), 2.84 (q, JH-H ) 8 Hz, 4H, CH2-
CH3), 3.10 (q, JH-H ) 8 Hz, 4H, CH2CH3), 3.72 (b, 8H, THF), 7.12
(m, 6H, N-C6H3), 7.24 (t, JH-H ) 8 Hz, 1H, p-Lu-C6H3), 7.50 (d, JH-H
) 8 Hz, 2H, m-Lu-C6H3) 8.44 (s, 2H, CHdN). 13C NMR (100 MHz,
CDCl3, 25 °C): δ 15.10 (s, 4C, CH2CH3), 24.57 (s, 4C, CH2CH3),
25.06 (s, 4C, THF), 69.91 (s, 4C, THF), 125.39 (s, 2C, p-N-C6H3),
125.91 (s, 4C, m-N-C6H3), 126.24 (s, 1C, p-Lu-C6H3), 132.30 (s, 2C,
m-Lu-C6H3), 136.27 (s, 4C, o-N-C6H3), 143.51 (s, 2C, o-Lu-C6H3),
150.66 (s, 2C, CdNsC), 175.27 (s, 2C, CdNsC), 200.13 (s, 1C, Lu-
C) ppm.
Polymerization of Butadiene. A typical procedure for the polym-
erization was as follows (Table 1, entry 2): in a glovebox, a
chlorobenzene solution of butadiene (2 mL, 10 mmol), 5 mL of
chlorobenzene, and 400 µmol of AlR3 were added into a 25 mL flask.
Then, 20 µmol of 2 and equimolar borate ([Ph3C][B(C6F5)4]) were added
to initiate the polymerization. After a designated time, methanol was
injected into the system to quench the polymerization. The mixture
was poured into a large quantity of methanol to precipitate the white
solids. Filtered and dried under vacuum at 40 °C for 24 h, polybutadiene
resulted at a constant weight (0.54 g, 100%).
[Bis(N-2,6-diethylphenyl)isophthalaldimin-2-yl]YCl2(THF)2 (2).
Following the same procedure described for the formation of 1, the
treatment of 2,6-(2,6-Et2-C6H3NdCH)2-C6H3-1-Br (0.48 g, 1.00 mmol
in 25 mL of THF) with nBuLi (1.6 M in hexane, 0.66 mL, 1.05 mmol)
and then in situ adding YCl3(THF)3.5 (0.67 g, 1.5 mmol) gave 2 in an
85.9% yield (0.60 g). Yellow crystals for X-ray analysis grew from
1
the mixture of THF and hexane at - 30 °C within several days. H
NMR (400 MHz, CDCl3, 25 °C): δ 1.17 (t, JH-H ) 8 Hz, 12H,
CH2CH3), 1.36 (b, 8H, THF), 2.84 (m, 4H, CH2CH3), 3.07 (m, 4H,
CH2CH3), 3.69 (b, 8H, THF), 7.13 (m, 6H, N-C6H3), 7.23 (t, JH-H
)
8 Hz, 1H, p-Y-C6H3), 7.46 (d, JH-H ) 8 Hz, 2H, m-Y-C6H3) 8.38 (s,
2H, CHdN). 13C NMR (100 MHz, CDCl3, 25 °C): δ 15.08 (s, 4C,
CH2CH3), 24.62 (s, 4C, CH2CH3), 24.77 (s, 4C, THF), 71.34 (s, 4C,
THF), 125.39 (s, 2C, p-N-C6H3), 125.88 (s, 4C, m-N-C6H3), 126.39 (s,
1C, p-Y-C6H3), 132.11 (s, 2C, m-Y-C6H3), 136.18 (s, 4C, o-N-C6H3),
143.49 (s, 2C, o-Y-C6H3), 150.34 (s, 2C, CdNsC), 177.77 (s, 2C,
CHdNsC), 194.20 (d, JC-Y ) 43 Hz, 1C, C-Y) ppm. Anal. calcd for
C36H45Cl2N2O2Y: C, 61.98; H, 6.50; N, 4.02. Found: C, 61.92; H,
6.47; N, 4.09.
Acknowledgment. We thank The National Natural Science
Foundation of China (Projects 20571072 and 20674081), The
Ministry of Science and Technology of China (Project
2005CB623802), and the Hundred Talents Program of the
Chinese Academy of Sciences for financial support.
Supporting Information Available: 1H, 13C, 1H-1H COSY,
[Bis(N-2,6-diisopropylphenyl)isophthalaldimin-2-yl]YCl2-
(THF)2 (3). Following the same procedure described for the formation
of 1, treatment of 2,6-(2,6-iPr2-C6H3NdCH)2-C6H3-1-Br (0.53 g, 1.00
mmol) with nBuLi (1.6 M in hexane, 0.66 mL, 1.05 mmol) and then in
1
1
and H-13C HMQC NMR spectra of 1; H and 13C NMR
spectra of 2; 1H NMR spectra of 3 and 13; 1H NMR spectra of
intermediates A-C; 1H-1H COSY of A; selected data for the
9
4990 J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008

NCN-Pincer Rare Earth Metal Dichloride Precursors
A R T I C L E S
high cis-1,4 polymerization of isoprene; 1H and 13C NMR
spectra of selected polybutadiene and polyisoprene samples;
ORTEP drawings for molecular structures of 5, 7, 9, 10, 12,
and 13; scheme for the probable mechanism; and X-ray
crystallographic data and refinement for 1-3, 5-7, 9, 10, 12,
and 13 in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA711146T
9
J. AM. CHEM. SOC. VOL. 130, NO. 14, 2008 4991