Achiral Cs-symmetric half-sandwich scandium(III) complexes with imine-cyclopentadienyl ligands catalyze isotactic polymerization of 1-hexene

Article abstract of DOI:10.1021/om301026e

Unchelated scandium(III) trichloride complexes, 2-(ArN=CH)C ₆H₄Me₄CpScCl₃Li(THF)₄ (Ar = 2,6-ⁱPr₂C₆H₃ (1a), 2,6-Et ₂C₆H₃ (1b), 2,6-Me₂C ₆H₃ (1c)), were obtained from the reaction of ScCl ₃(THF)₃ with the lithium salt of the corresponding ligand, 2-(ArC₆H₃N=CH)C₆H₄Me ₄CpLi, in THF. After heating at 120 C under vacuum for 30 min, the attached LiCl and THF were removed from complexes 1 to give the chelated scandium(III) dichloride complexes 2-(ArN=CH)C₆H₄Me ₄CpScCl₂ ([Ar = 2,6-ⁱPr₂C ₆H₃ (2a), 2,6-Et₂C₆H₃ (2b), 2,6-Me₂C₆H₃ (2c)). Attempts to synthesize dialkyl scandium(III) complexes by the reaction of Sc(CH₂SiMe ₃)₃(THF)₂ with the corresponding free ligands were not successful. The scandium(III) trialkyl complex 2-[Li(THF) ₃(2,6-ⁱPr₂C₆H₃)N=CH] C₆H₄Me₄CpSc(CH₂SiMe ₃)₃ (3) was synthesized by a one-pot reaction of ScCl ₃(THF)₃ with 2-(2,6-ⁱPr₂C ₆H₃N=CH)C₆H₄Me₄CpLi and 3 equiv of Me₃SiCH₂Li in THF sequentially. The scandium(III) dialkyl complex 2-(2,6-ⁱPr₂C ₆H₃N=CH)C₆H₄Me ₄CpSc(CH₂SiMe₃)₂ (4) was obtained from the reaction of the dichloride complex 2a with 2 equiv of Me ₃SiCH₂Li in hexane. Complexes 1b,c were directly converted to complexes 2b,c without purification and characterization. All other scandium(III) complexes were characterized by ¹H and ¹³C NMR spectroscopy and elemental analyses. The structures of complexes 1a, 2c, 3, and 4 were determined by single-crystal X-ray crystallography, which indicates that the imine N atoms in complexes 1a and 3 do not coordinate to the central scandium atoms. Complexes 2a-c and 4 were found to exhibit moderate catalytic activity for propylene and 1-hexene polymerization upon activation with AlR ₃/Ph₃CB(C₆F₅)₄ or methylaluminoxane (MAO) and produce atactic polypropylene and isotactic poly(1-hexene). The effects of molecular structures and reaction conditions on the catalytic behavior of these complexes were examined and the possible catalytic mechanism was discussed.

Full text of DOI:10.1021/om301026e

Article
pubs.acs.org/Organometallics
Achiral Cs‑Symmetric Half-Sandwich Scandium(III) Complexes with
Imine−Cyclopentadienyl Ligands Catalyze Isotactic Polymerization
of 1‑Hexene
Xin Tao, Wei Gao, Hang Huo, and Ying Mu*
The State Key Laboratory for Supramolecular Structure and Materials, School of Chemistry, Jilin University, 2699 Qianjin Street,
Changchun 130012, People’s Republic of China
S
* Supporting Information
ABSTRACT: Unchelated scandium(III) trichloride com-
plexes, 2-(ArNCH)C6H4Me4CpScCl3Li(THF)4 (Ar =
2,6-iPr2C6H3 (1a), 2,6-Et2C6H3 (1b), 2,6-Me2C6H3 (1c)),
were obtained from the reaction of ScCl3(THF)3 with the
lithium salt of the corresponding ligand, 2-(ArC6H3N
CH)C6H4Me4CpLi, in THF. After heating at 120 °C under
vacuum for 30 min, the attached LiCl and THF were removed
from complexes 1 to give the chelated scandium(III)
dichloride complexes 2-(ArNCH)C6H4Me4CpScCl2 ([Ar = 2,6-iPr2C6H3 (2a), 2,6-Et2C6H3 (2b), 2,6-Me2C6H3 (2c)).
Attempts to synthesize dialkyl scandium(III) complexes by the reaction of Sc(CH2SiMe3)3(THF)2 with the corresponding free
ligands were not successful. The scandium(III) trialkyl complex 2-[Li(THF)3(2,6-iPr2C6H3)NCH]C6H4Me4CpSc-
(CH2SiMe3)3 (3) was synthesized by a one-pot reaction of ScCl3(THF)3 with 2-(2,6-iPr2C6H3NCH)C6H4Me4CpLi and 3
equiv of Me3SiCH2Li in THF sequentially. The scandium(III) dialkyl complex 2-(2,6-iPr2C6H3NCH)C6H4Me4CpSc-
(CH2SiMe3)2 (4) was obtained from the reaction of the dichloride complex 2a with 2 equiv of Me3SiCH2Li in hexane.
Complexes 1b,c were directly converted to complexes 2b,c without purification and characterization. All other scandium(III)
complexes were characterized by 1H and 13C NMR spectroscopy and elemental analyses. The structures of complexes 1a, 2c, 3,
and 4 were determined by single-crystal X-ray crystallography, which indicates that the imine N atoms in complexes 1a and 3 do
not coordinate to the central scandium atoms. Complexes 2a−c and 4 were found to exhibit moderate catalytic activity for
propylene and 1-hexene polymerization upon activation with AlR3/Ph3CB(C6F5)4 or methylaluminoxane (MAO) and produce
atactic polypropylene and isotactic poly(1-hexene). The effects of molecular structures and reaction conditions on the catalytic
behavior of these complexes were examined and the possible catalytic mechanism was discussed.
1-hexene under high pressures (up to 1000 MPa) with chiral
metallocene/MAO catalysts was reported to produce high-
molecular-weight stereospecific poly(1-hexene).6 Polymeriza-
tion of α-olefins initiated by the relatively bulky rac-Me2Si(1-
C5H2-2-CH3-4-tBu)2Zr(NMe2)2/Al(iBu)3/Ph3CB(C6F5)4 cata-
lyst was found to yield high-molecular-weight polymers without
the need for high pressures.7 In 2000, it was reported that a
class of bulky C1-symmetric cyclopentadienylzirconium acet-
amidinate complexes, Cp*ZrMe2[NR1C(Me)NR2], catalyze
the isospecific living polymerization of 1-hexene and produce
highly isotactic, high-molecular-weight polymers with low
polydispersities.8 In the same year, C2-symmetric non-metal-
locene zirconium complexes with [ONNO] ligands were also
reported as good catalysts for the isospecific living polymer-
ization of 1-hexene.9 These two classes of catalysts were further
investigated for their catalytic performance in the stereospecific
polymerizations of α-olefins in the following years.10 In recent
INTRODUCTION
■
The development of stereospecific homogeneous α-olefin
polymerization catalysts has been a major research field
involving many academic and industrial research groups in
the past two decades.1−14 Although most of the research efforts
have been focused on the development of new catalysts for
propylene polymerization,1 the design and synthesis of efficient
catalysts for the stereospecific polymerization of long-chain α-
olefins have also attracted extensive research interests.2−14 The
first single-component C2-symmetric catalyst system for the
stereospecific polymerization of long chain α-olefins, rac-Me2Si-
(2-SiMe3-4-tBu-C5H2)2YR, was reported by Bercaw’s group in
1992.2 These C2-symmetric yttrium catalysts show remarkably
high stereospecificities for the polymerization of α-olefins. The
rac-ethylene-1,2-bis(1-indenyl)ZrCl2/MAO catalyst system was
studied for the polymerization of α-olefins and found to be able
to polymerize 1-hexene to yield highly isotactic poly(1-hexene)
in various solvents.3,4 The polymers of long-chain α-olefins
obtained with these catalysts have low molecular weight. High-
molecular-weight poly(1-hexene) was obtained under high
pressures (over 100 MPa) with achiral zirconocene and
hafnocene/MAO catalysts.5 Stereospecific polymerization of
Special Issue: Recent Advances in Organo-f-Element Chemistry
Received: October 30, 2012
Published: February 6, 2013
© 2013 American Chemical Society
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Scheme 1. Synthetic Route for Complexes 1 and 2
years, types of C1-symmetric zirconium complexes with [NNN]
ligands11 and types of C2-symmetric zirconium complexes with
[OSSO] ligands12 were also reported to be good catalysts for
the isospecific polymerization of 1-hexene. In addition, isotactic
polymerization of 1-hexene catalyzed by C3-symmectric
dicationic Sc alkyl species was reported, and the stereo-
selectivity was believed to result from a chain-end control
mechanism according to a computational study.13 Although
most of the complexes that show good catalytic performance
for the stereospecific polymerizations of α-olefins possess C1 or
C2 symmetry, a few Cs-symmetric zirconium and hafnium
complexes with a pyridylamido ligand14a,b or a phenyl-
thiomethyl-substituted phenoxy ligand14c have been reported
to catalyze the isospecific polymerization of α-olefins, and the
stereoselectivity has been found to follow the enantiomorphic
site control mechanism14a,b in spite of the fact that the origin of
the chirality is unclear. Half-sandwich rare-earth complexes
have been studied as good catalysts for the stereo- and
regioselective (co)polymerizations of olefins.15 In particular, it
was found that a side arm on the Cp ring of the half-sandwich
rare-earth complexes significantly affects their catalytic
activity15b,g,h and stereoselectivity.15a,16 Inspired by these
aforementioned results, we have developed a number of new
Cs-symmetric imino−cyclopentadienyl scandium(III) com-
plexes and found that they can catalyze the isospecific
polymerization of 1-hexene upon activation with AlR3/
Ph3CB(C6F5)4 or methylaluminoxane (MAO) and produce
isotactic poly(1-hexene) with ultrahigh molecular weight at
room temperature. In this paper, we report the synthesis and
characterization of these complexes as well as their catalytic
properties for 1-hexene and propylene polymerization.
CH2Cl2 to remove insoluble impurities and recrystallization in a
CH2Cl2/hexane mixed solvent system.
Attempts to synthesize scandium(III) dialkyl complexes by
reactions of Sc(CH2SiMe3)3(THF)2 with the corresponding
free ligands or reactions of ScCl3(THF)3 with the correspond-
ing lithium salts of the ligands and 2 equiv of Me3SiCH2Li
sequentially in THF as reported in the literature15g,18 were
unsuccessful. No distinguishable product has been isolated
from these reactions. However, the scandium(III) trialkyl
complex 2-[Li(THF)3(2,6-iPr2C6H3)NCH]C6H4Me4CpSc-
(CH2SiMe3)3 (3) was obtained from a one-pot reaction of
ScCl3(THF)3 with LaLi and 3 equiv of Me3SiCH2Li in THF, as
shown in Scheme 2. Finally, the scandium(III) dialkyl complex
Scheme 2. Synthetic Route for Complex 3
2-(2,6-iPr2C6H3NCH)C6H4Me4CpSc(CH2SiMe3)2 (4) was
successfully synthesized by the reaction of the scandium(III)
dichloride complex 2a with 2 equiv of Me3SiCH2Li in hexane,
as seen in Scheme 3. On the basis of the above results, it seems
Scheme 3. Synthetic Route for Complex 4
RESULTS AND DISCUSSION
■
Synthesis of Complexes. The free ligands 2-(ArN
CH)C6H4Me4CpH (Ar = 2,6-iPr2C6H3 (LaH), 2,6-Et2C6H3
(LbH), 2,6-Me2C6H3 (LcH)) were synthesized as reported
previously.17 They can be deprotonated by treatment with BuLi
to give the corresponding lithium salts 2-(ArNCH)-
C6H4Me4CpLi (Ar = 2,6-iPr2C6H3 (LaLi), 2,6-Et2C6H3
(LbLi), 2,6-Me2C6H3 (LcLi)). The new scandium(III) chloride
complexes were synthesized by the reactions of ScCl3(THF)3
with the corresponding lithium salts of the ligands in THF at
room temperature, as illustrated in Scheme 1. The scandium-
(III) trichloride complexes 2-(ArNCH)C6H4Me4CpScCl3Li-
(THF)4 (Ar = 2,6-iPr2C6H3 (1a), 2,6-Et2C6H3 (1b), 2,6-
Me2C6H3 (1c)) were obtained directly from reactions in about
60% yields. After heating at 120 °C under vacuum for several
hours, the attached LiCl and THF were removed from
complexes 1 to give the chelated scandium(III) dichloride
complexes 2-(ArNCH)C6H4Me4CpScCl2 (Ar = 2,6-iPr2C6H3
(2a), 2,6-Et2C6H3 (2b), 2,6-Me2C6H3 (2c)). Pure complexes 2
were obtained in 60−65% isolated yields by extraction with
that the formation of the anionic half-sandwich scandium(III)
trialkyl or trichloride complexes with their imine N atom
uncoordinated to the central scandium atom is favorable for
this ligand system when the reactions are carried out in THF.
The structures of complexes 2c, 3, and 4 were also confirmed
by X-ray crystallography.
The new scandium(III) complexes 1a, 2a−c, 3, and 4 were
1
characterized by H and 13C NMR spectroscopy along with
elemental analyses. The structures of complexes 1a, 2c, 3, and 4
were determined by single -crystal X-ray crystallography, which
confirms that the imine N atom in complexes 1a and 3 does not
coordinate to the central scandium atom. The anionic
trichloride complexes 1b,c were directly converted to the
neutral dichloride complexes 2b,c without purification and
1
characterization. The H NMR spectra of these complexes all
show two sets of singlets for the CpCH3 protons and one set of
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singlets for the CHN protons. Complexes 1a and 3 show one
set of doublets for the methyl protons of the CH(CH3)2 group
(1.06 ppm for 1a and 1.16 ppm for 3) in their ligands, while
complexes 2a and 4 show two sets of doublets for the methyl
protons of the CH(CH3)2 group (1.04 and 1.36 ppm for 2a,
0.85 and 1.37 ppm for 4). On the other hand, the 13C NMR
spectra of complexes 2a and 4 show two signals for the two
methyl carbons (22.0 and 26.2 ppm for 2a, 22.5 and 26.4 ppm
i
for 4) of the Pr group. A similar phenomenon was also
observed for the resonances of the CH2CH3 protons in
complex 2b, which has two sets of multiplets for the methylene
protons (2.75−2.62 and 2.48−2.33 ppm). These observations
indicate that the rotation of the 2,6-R2C6H3 group about the
N−C (Ar group) bond is restricted in complexes 2 and 4, with
the imine N atom coordinating to the central scandium atom.
The 1H NMR spectrum of complex 3 shows a broad singlet for
the methylene protons of the CH2SiMe3 group at −0.16 ppm.
In contrast, the methylene protons of the same CH2SiMe3
group in complex 4 give a doublet of doublets (−0.48, −0.21
ppm, JH−H = 10.5 Hz). These results demonstrate that the
imine N atom in the ligand of complex 4 coordinates to the
central scandium atom, while that in complex 3 does not in
Figure 2. Perspective view of 2c with thermal ellipsoids drawn at the
30% probability level. Hydrogens and uncoordinated solvent are
omitted for clarity. Selected bond lengths (Å) and bond angles (deg):
Sc(1)−CCp(range) = 2.412(4)−2.459(5), Sc(1)−CCp(av) = 2.438,
Sc(1)−N(1) = 2.254(2), Sc(1)−Cp(ct) = 2.118, Sc(1)−Cl(1) =
2.3804(16), Sc(1)−Cl(2) = 2.3776(17), C(16)−N(1) = 1.285(6);
Cl(1)−Sc(1)−Cl(2) = 101.72(6), Cl(1)−Sc(1)−N(1) = 105.99(11),
Cl(2)−Sc(1)−N(1) = 104.00(6), C(16)−N(1)−Sc(1) = 135.9(3),
C(17)−N(1)−Sc(1) = 108.7(3), C(16)−N(1)−C(17) = 115.3(4),
N(1)−C(16)−C(15) = 127.9(4), Cp(ct)−C(1)−C(10) = 171.87,
Cp(ct)−Sc(1)−Cl(1) = 117.24, Cp(ct)−Sc(1)−Cl(2) = 117.79,
Cp(ct)−Sc(1)−N(1) = 108.75.
1
solution. In addition, the H NMR spectroscopic analysis on
complexes 1a and 3 reveals that these anionic complexes
convert to the neutral complexes 2a and 4 slowly in solution
i
with the resonance of the methyl protons in the Pr group
changing from one set of doublets to two sets of doublets over
several hours. As a result, the 13C NMR spectra of complexes
1a and 3 could not be obtained.
X-ray Crystallography Studies. Single crystals of
complexes 1a, 2c, 3, and 4 suitable for X-ray crystallographic
analysis were obtained and their structures were determined.
The molecular structures (in ORTEP form) of complexes 1a,
2c, 3 and 4, together with selected bond distances and angles of
them, are shown in Figures 1−4, respectively. The X-ray
diffraction analysis reveals that the central scandium atom in
these scandium complexes is coordinated by one Cp ring and
three other ligands in a three-legged piano-stool geometry with
a distorted-octahedral coordinating environment. Complexes
1a and 3 exist in an anionic form with an additional chloride or
alkyl ligand instead of the neutral imine N atom in their ligand
coordinating to the central metal atom. The lithium counter-
ions attached to 1a and 3 are coordinated by four THF
molecules or three THF molecules plus the imine N atom in a
distorted-tetrahedral geometry. In complexes 2c and 4, the
imine N atom coordinates to the central scandium atom to
form a six-membered chelating ring in a vertical position to the
cyclopentadienyl ring with the aryl group at the imine N atom
being placed close to the metal center, which constructs a
relatively crowded coordinating environment surrounding the
central scandium atom.
The Sc−Cp(ct) (ct = centroid) distances in the neutral
chelating complexes 2c (2.118 Å) and 4 (2.201 Å) are shorter
than those in their corresponding anionic complexes 1a (2.165
Å) and 3 (2.262 Å). These parameters are in agreement with
those reported for similar N-functionalized scandium cyclo-
pentadienyl complexes.15g,h,18,19 The Sc−CCp(av) (av =
average) distances are 2.475 Å for 1a, 2.438 Å for 2c, 2.564
Å for 3, and 2.510 Å for 4. In complex 1a, the Sc−C3 (2.438(7)
Å) and Sc−C4 (2.458(7) Å) distances are obviously shorter
than the remaining Sc−CCp distances (2.480(6), 2.494(6), and
2.504(6) Å), indicating that the central scandium atom is
located away from the position below the center of the Cp ring
due to the steric hindrance of the uncoordinated bulky side
arm. The Sc−Cl bond distances of complex 1a (2.392(2),
2.370(2), and 2.384(2) Å) are close to those of complex 2c
(2.3804(16) and 2.3776(17) Å). The Sc−C1 distances in
complexes 2c (2.412(4) Å) and 4 (2.443(2) Å) are shorter than
Figure 1. Perspective view of 1a with thermal ellipsoids drawn at the
30% probability level. Hydrogens are omitted for clarity. Selected bond
lengths (Å) and bond angles (deg): Sc(1)−CCp(range) = 2.438(7)−
2.504(6), Sc(1)−CCp(av) = 2.475, Sc(1)−Cp(ct) = 2.165, Sc(1)−
Cl(1) = 2.392(2), Sc(1)−Cl(2) = 2.370(2), Sc(1)−Cl(3) = 2.384(2),
C(16)−N(1) = 1.268(7); Cl(1)−Sc(1)−Cl(2) = 103.18(8), Cl(1)−
Sc(1)−Cl(3) = 102.48(8), Cl(2)−Sc(1)−Cl(3) = 101.97(9), C(16)−
N(1)−C(17) = 120.7(6), N(1)−C(16)−C(15) = 122.6(6), Cp(ct)−
C(1)−C(10) = 172.80, Cp(ct)−Sc(1)−Cl(1) = 114.44, Cp(ct)−
Sc(1)−Cl(2) = 116.07, Cp(ct)−Sc(1)−Cl(3) = 116.71.
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Figure 4. Perspective view of 4 with thermal ellipsoids drawn at the
30% probability level. Hydrogens are omitted for clarity. Selected bond
lengths (Å) and bond angles (deg): Sc(1)−CCp(range) = 2.443(2)−
2.558(2), Sc(1)−CCp(av) = 2.510, Sc(1)−N(1) = 2.3125(17), Sc(1)−
Cp(ct) = 2.201, Sc(1)−C(29) = 2.229(2), Sc(1)−C(33) = 2.211(2),
C(16)−N(1) = 1.288(6); C(29)−Sc(1)−C(33) = 108.66(9), C(29)−
Sc(1)−N(1) = 110.34(8), C(33)−Sc(1)−N(1) = 96.09(8), C(16)−
N(1)−Sc(1) = 127.64(14), C(17)−N(1)−Sc(1) = 116.46(12),
C(16)−N(1)−C(17) = 115.62(17), N(1)−C(16)−C(15) =
129.58(19), Cp(ct)−C(1)−C(10) = 177.73, Cp(ct)−Sc(1)−C(29)
= 115.37, Cp(ct)−Sc(1)−C(33) = 118.97, Cp(ct)−Sc(1)−N(1) =
105.42.
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 bond angles (deg): Sc(1)−CCp(range) = 2.555(5)−
2.570(5), Sc(1)−CCp(av) = 2.564, Sc(1)−Cp(ct) = 2.262, Sc(1)−
C(28) = 2.259(5), Sc(1)−C(29) = 2.287(6), Sc(1)−C(37) =
2.262(6), C(16)−N(1) = 1.277(7), Li(1)−N(1) = 2.138(11);
C(28)−Sc(1)−C(29) = 103.3(2), C(28)−Sc(1)−C(37) = 101.6(2),
C(29)−Sc(1)−C(37) = 106.6(2), C(16)−N(1)−Li(1) = 132.7(5),
C(17)−N(1)−Li(1) = 111.9(4), N(1)−C(16)−C(15) = 124.1(5),
Cp(ct)−C(1)−C(10) = 173.35, Cp(ct)−Sc(1)−C(28) = 116.83,
Cp(ct)−Sc(1)−C(29) = 113.68, Cp(ct)−Sc(1)−C(37) = 114.56.
the remaining Sc−CCp bond lengths (average 2.445 Å for 2c
and 2.515 Å for 4) due to the coordination of the imine N
atom. The Sc−N distance in the dichloride complex 2c
(2.254(4) Å) is shorter than that in the dialkyl complex 4
(2.3125(17) Å). They are longer than the Sc−N coordination
bonds found in related CpPN-Sc type complexes (2.188(2)−
2.214(3) Å).15i In addition, the Sc−Calkyl(av) bond length in
the trialkyl complex 3 (2.269 Å) is longer than that in complex
4 (2.220 Å). The imino CN bonds in these complexes retain
their double-bond character, being 1.268(7) Å for 1a, 1.285(6)
Å for 2c, 1.277(7) Å for 3, and 1.288(3) Å for 4.
precatalysts for the polymerization of propylene and 1-hexene
under different conditions. It was found that these complexes
show moderate catalytic activity for propylene and 1-hexene
polymerization upon activation with AlR3/Ph3CB(C6F5)4 (R =
Me, Et, iBu) or MAO. The polymerization results of propylene
and 1-hexene with 2a−c/AlR3/Ph3CB(C6F5)4 catalyst systems
are summarized in Table 1. Atactic polypropylene was obtained
from the propylene polymerization, as indicated by the 13C
NMR spectrum of a typical polypropylene sample shown in
Figure 5. It is expected for these catalyst systems to produce
syndiotactic or atactic polypropylene on the basis of their Cs-
symmetric structural feature. Similar Cs-symmetric constrained
geometry catalysts of group 4 metals have usually been reported
to produce atactic polypropylene.1b,20,21 It is very interesting
that ultrahigh-molecular-weight isotactic poly(1-hexene) was
obtained from the polymerization reactions of 1-hexene
catalyzed by 2a−c/AlR3/Ph3CB(C6F5)4 and 4/AliBu3/Ph3CB-
(C6F5)4 catalyst systems under conditions similar to those for
In the chelated complexes 2c and 4, the Cp(ct)−Sc−N
angles (108.75° for 2c and 105.42° for 4) are obviously
influenced by the bulkiness of the aryl groups at the imine N
atoms, while little similar effect on the Cp(ct)−Sc−Cl (Calkyl
)
angles (117.24 and 117.79° for 2c, 115.37 and 118.97° for 4)
can be seen. The Sc−N−C(17) (108.70°) and N−Sc−Cp(ct)
angles (108.75°) in complex 2c are almost the same, which
locates the aryl group at the imine N atom in a nearly vertical
direction to the cyclopentadienyl ring. In contrast, the Sc−N−
C(17) bond angle (116.46°) in complex 4 is obviously larger
than the N−Sc−Cp(ct) angle (105.42°) due to the relatively
1
the propylene polymerization. The H NMR spectrum of a
typical poly(1-hexene) sample shows no resonance for terminal
olefinic groups, demonstrating that either the obtained poly(1-
hexene) is of high molecular weight or the main polymer chain
termination step of the polymerization reaction is not the β-
hydride elimination process. GPC analysis indicates that the
obtained poly(1-hexene) samples indeed possess high molec-
ular weight with a weight-averaged molecular weight (Mw) up
to 1.48 × 106. On the basis of the yields of the poly(1-hexenes)
and their molecular weights, it can be seen that the precatalysts
i
large repulsion between the Pr2C6H3 aryl group and the two
Me3SiCH2 alkyl groups. The dihedral angles between the Cp
plane and the attached phenyl plane (79.97° for 2c and 52.40°
for 4) are less than 90°, indicating that the six-membered
chelating rings in these complexes are somewhat distorted in
their solid-state structures.
Polymerization of 1-Hexene and Propylene. The
scandium(III) dichloride complexes 2a−c have been tested as
are only partially activated (10% or lower) in these system. 13
C
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a
Table 1. Polymerization Results Catalyzed by Complexes 2 and 4/AlR3/Ph3CB(C6F5)4 Systems
b
b
entry
monomer
procatalyst
AlR3
temp (°C)
yield (g)
[mmmm] (%)
Mw
Mw/Mn
1
2
3
4
5
6
7
8
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
1-hexene
propylene
propylene
propylene
2a
2b
2c
2a
2a
2a
2a
4
AliBu3
AliBu3
AliBu3
AlEt3
20
20
20
20
20
40
0
0.61
0.33
0.25
0.32
0.21
0.82
0.16
0.56
0.58
0.54
0.45
0.38
90
1337
1268
1153
1214
1136
1195
1479
1315
755
1.38
1.59
1.56
1.47
1.78
4.13
2.15
1.42
5.08
2.17
2.24
2.35
51
39
21
AlMe3
AliBu3
AliBu3
AliBu3
13
41
95
20
20
20
20
20
88
c
9
2a
2a
2b
2c
95
d
10
AliBu3
AliBu3
AliBu3
atactic
atactic
atactic
159
d
11
126
d
12
104
a
Polymerization of 1-hexene was carried out with 5 μmol of precatalyst, 60 equiv of AlR3, and 1.1 equiv of Ph3CB(C6F5)4 in a mixture of 2 mL of
b
toluene and 5 mL of monomer for 48 h. IN units of kg/mol of polymer, determined by GPC analysis at 40 °C using polystyrene standards and
c
d
THF as the eluant. Activated by 500 equiv of MAO. Polymerization of propylene was carried out with 5 μmol of procatalyst, 60 equiv of AliBu3,
and 1.1 equiv of Ph3CB(C6F5)4 in 20 mL of toluene under 0.5 MPa propylene pressure at 20 °C for 48 h.
Figure 5. 13C NMR (100 MHz, o-C6D4Cl2, 130 °C) spectrum for polypropylene produced from entry 10 (Table 1).
Figure 6. 13C NMR (125 MHz, CDCl3, 25 °C) spectrum for poly(1-hexene) produced from entry 7 (Table 1).
NMR spectroscopic analysis of the obtained poly(1-hexenes)22
as shown in Figures 6 and 7 reveals that isotactic poly(1-
hexenes) with [mmmm] up to 95% can been produced by these
Cs-symmetric half-sandwich scandium(III) complexes. It has
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Figure 7. 13C NMR (100 MHz, o-C6D4Cl2, 130 °C) spectrum for poly(1-hexene) produced from entry 3 (Table 1).
been reported that atactic poly(1-hexenes) were obtained with
half-sandwich scandium(III) catalysts without a side arm.15a,16
By comparison of the polymerization results obtained under
similar conditions, it can be seen that the steric effect of the Ar
group at the imine N atom of these complexes on the
isotacticity of the produced poly(1-hexenes) is remarkable. The
stereoselectivity of complexes 2a−c decreases in the order 2a >
2b > 2c under similar conditions, which may be attributed to
the fact that a bulkier aryl group at the imine N atom would
make the coordination sphere of the catalytic active species
more crowded and therefore have more impact on the direction
of the coordinated 1-hexene. On the other hand, the
stereoselectivity of these catalyst systems also depends on the
type of AlR3 used and decreases in the order AliBu3 > AlEt3 >
AlMe3 with complex 2a as the precatalyst. In addition, the
MAO activated 2a catalyst system was found to produce
poly(1-hexene) with the highest isotacticity under similar
conditions.
The detailed mechanism for these catalyst systems in
promoting the isospecific polymerization of 1-hexene is not
clear. In principle, isotactic poly(1-hexene) can be formed by
either an enantiomorphic site control mechanism or a chain end
control mechanism.1b For the isotactic poly(1-hexenes)
produced by the enantiomorphic site control mechanism, 13C
NMR signals for the mrrm pentads caused by misinsertion can
usually be observed from samples with adequate isotactici-
ty.1b,13,22 We have clearly observed the mrrm pentads in 13C
NMR spectra of some isotactic poly(1-hexene) samples
produced by these Cs-symmetric half-sandwich scandium(III)
complexes, as shown in Figure 7 for a typical sample, implying
that the isotactic poly(1-hexene) may be produced by the
enantiomorphic site control mechanism for our present catalyst
systems. On the basis of the widely accepted olefin polymer-
ization mechanism, the chain migratory insertion entailing the
site isomerization was essential feature of Cs-symmetric
metallocene catalysts to produce atactic or syndiotactic
polymer.1b,20c,d Something must happen in our present catalyst
systems to cause the Cs-symmetric precatalysts to be trans-
formed into chiral active catalysts. Considering the fact that the
isotacticity of the produced poly(1-hexenes) is significantly
affected by the size of both the aryl group at the imine N atom
and the AlR3 (or MAO) cocatalyst, it is possible that a strong
interaction23 between the catalyst and the cocatalyst AlR3 or
MAO may exist in our catalyst systems, which creates a catalyst
having a C1-symmetric environment for the coordination and
insertion of the 1-hexene molecule and leads to the formation
of the isotactic poly(1-hexene).
CONCLUSION
■
A number of new half-sandwich scandium(III) dichloride, 2-
(ArNCH)C6H4Me4CpScCl2 (2a−c), and trichloride com-
plexes, 2-(ArNCH)C6H4Me4CpScCl3Li(THF)4 (1a−c),
have been synthesized in good yields. The dichloride complexes
were obtained by heating the corresponding trichloride
complexes at 120 °C under vacuum to remove the attached
LiCl and THF. The scandium(III) trialkyl complex 2-
[Li(THF)3(2,6-iPr2C6H3)NCH]C6H4Me4CpSc(CH2SiMe3)3
(3) was synthesized by a one-pot reaction of ScCl3(THF)3 with
2-(2,6-iPr2C6H3NCH)C6H4Me4CpLi and 3 equiv of
Me3SiCH2Li in THF sequentially, while the scandium(III)
dialkyl complex 2-(2,6-iPr2C6H3NCH)C6H4Me4CpSc-
(CH2SiMe3)2 (4) was obtained from the reaction of the
dichloride complex 2a with 2 equiv of Me3SiCH2Li in hexane.
X-ray crystallographic analysis of complexes 1a, 2c, 3 and 4
indicates that these half-sandwich scandium(III) complexes
adopt a three-legged piano-stool geometry with a distorted-
octahedral coordinating environment. Upon activation with
AliBu3/Ph3CB(C6F5)4 or MAO, the Cs-symmetric half-
sandwich scandium(III) dichloride complexes exhibit moderate
catalytic activity for propylene and 1-hexene polymerization
and produce atactic polypropylene and high-molecular-weight
isotactic poly(1-hexene).
EXPERIMENTAL SECTION
■
General Considerations. All manipulations for air- and water-
sensitive compounds were performed under an inert atmosphere of
nitrogen using standard Schlenk or glovebox techniques.24 Solvents
and 1-hexene were purified and dried by known procedures and
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Article
distilled under nitrogen prior to use.25 Polymerization grade proylene
was further purified by passage through columns of 5 Å molecular
sieves and MnO. ScCl3(THF)3, Ph3CB(C6F5)4,26 and 2-
(aryliminomethyl)phenyltetramethylcyclopentadiene17 ligands were
prepared according to published procedures. MAO, AlMe3, AlEt3,
AliBu3, 2,6-dimethylaniline, 2,6-diethylaniline, and 2,6-diisopropylani-
line were purchased from Aldrich or Acros and used as received. NMR
spectra were measured using a Varian Mercury-300 NMR
spectrometer, and elemental analyses were performed on a Perkin-
Elmer 2400 analyzer. 13C NMR spectra of polypropylene and poly(1-
hexene) samples were measured on a Varian Unity 400 MHz
spectrometer at 130 °C in o-C6D4Cl2 or on a Bruker AVANCEIII500
spectrometer at 25 °C in CDCl3. The molecular weights and
molecular weight distributions of the polymer samples were
determined by gel permeation chromatography on a Waters Breeze
HPLC with a Waters 2414 refractive index detector at 40 °C using
THF as an eluent at a flow rate of 1.00 mL/min against polystyrene
standards.
ArH), 7.35−7.13 (m, 4H, ArH), 2.75−2.62 (m, 2H, CH2CH3), 2.48−
2.33 (m, 2H, CH2CH3), 2.27 (s, 6H, CpCH3), 2.03 (s, 6H, CpCH3),
1.14 (t, J = 7.5 Hz, 6H, CH2CH3). 13C NMR (75 MHz, CDCl3, 298
K): δ 175.8, 144.5, 138.3, 137.8, 137.2, 135.5, 134.4, 131.1, 129.1,
128.6, 127.3, 127.0, 125.4, 125.2, 25.8, 15.1, 12.3 (2C) ppm.
Synthesis of Complex 2c. Complex 2c was synthesized in the
same manner as for complex 2a with 2-(2,6-Me2C6H3NCH)-
C6H4Me4CpH (643 mg, 1.95 mmol), n-BuLi (1.15 mL, 1.95 mmol),
and ScCl3(THF)3 (717 mg, 1.95 mmol) as starting materials. Pure
product 2c (533 mg, 1.20 mmol, 61.5%) was obtained as a yellow
powder. A single crystal suitable for X-ray crystallographic analysis was
obtained from a CH2Cl2/n-hexane solution ((1−2)/10 v/v). Anal.
Calcd for C24H26Cl2NSc (444.33): C, 64.87; H, 5.90; N, 3.15. Found:
1
C, 64.90; H, 5.95; N, 3.10. H NMR (300 MHz, CDCl3, 298 K): δ
8.28 (s, 1H, CHN), 7.79−7.68 (m, 2H, ArH), 7.62−7.55 (m, 1H,
ArH), 7.35 (d, J = 7.6 Hz, 1H, ArH), 7.18−7.07 (m, 3H, ArH), 2.28 (s,
6H, CpCH3), 2.24 (s, 6H, ArCH3), 2.03 (s, 6H, CpCH3). 13C NMR
(75 MHz, CDCl3, 298 K): δ 176.8, 145.9, 138.2, 137.9, 135.5, 134.3,
131.4, 131.3, 129.3, 128.7, 128.5, 128.0, 127.3, 125.3, 19.7, 12.3 (2C)
ppm.
Synthesis of Complex 1a. To a solution of the free ligand 2-
(2,6-iPr2C6H3NCH)C6H4Me4CpH (750 mg, 1.95 mmol) in 10 mL
of THF was added dropwise a solution of butyllithium (1.15 mL, 1.95
mmol) in THF at −78 °C. The reaction mixture was warmed to room
temperature and stirred for 1 h. The resulting solution was then added
to a suspension of ScCl3(THF)3 (717 mg, 1.95 mmol) in 30 mL of
THF at room temperature, and then the reaction mixture was stirred
for another 12 h. During the reaction period, the color of the reaction
mixture changed from white to yellow. After the solvent was removed
under vacuum, the residue was extracted with 15 mL of toluene to
remove the insoluble impurities. Oily product was obtained by
removing the toluene. Pure product 1a was obtained by recrystalliza-
tion from THF/n-hexane ((1−2)/10 v/v) as yellow crystals (989 mg,
1.19 mmol, 61.1%). Anal. Calcd for C44H66Cl3LiNO4Sc (831.26): C,
Synthesis of Complex 3. To a solution of free ligand 2-
(2,6-iPr2C6H3NCH)C6H4Me4CpH (750 mg, 1.95 mmol) in 10 mL
of THF was added dropwise a solution of butyllithium (1.15 mL, 2.40
mmol) in THF at −78 °C. The reaction mixture was warmed to room
temperature and stirred for 1 h. The resulting solution was then added
to a suspension of ScCl3(THF)3 (717 mg, 1.95 mmol) in 30 mL of
THF at room temperature. The reaction mixture was stirred for
another 1 h, and the color of the reaction mixture changed from white
to yellow. To the reaction mixture was slowly added LiCH2SiMe3 (551
mg, 5.85 mmol) at −40 °C. After the mixture was stirred at room
temperature for 30 min, the color of the reaction mixture changed
from yellow to red. The solvent was removed under vacuum, and the
residue was extracted with 15 mL of toluene to remove the insoluble
impurities. An oily product was obtained by removing the toluene.
Single crystals suitable for X-ray analysis were grown from a hexane
solution at −30 °C (822 mg, 0.90 mmol, 46.4%). Anal. Calcd for
C52H91LiNO3ScSi3 (914.44): C, 68.30; H, 10.03; N, 1.53. Found: C,
1
63.58; H, 8.00; N, 1.69. Found: C, 63.61; H, 8.04; N, 1.72. H NMR
(300 MHz, CDCl3, 298 K): δ 8.35 (d, J = 7.5 Hz, 1H, ArH), 7.82 (d, J
= 7.5 Hz, 1H, ArH), 7.59 (s, 1H, CHN), 7.56−7.39 (m, 2H, ArH),
7.15−7.00 (m, 3H, ArH), 3.81 (b, 12H, THF), 2.87 (m, 2H,
CH(CH3)2), 2.10 (s, 6H, CpCH3), 2.00 (s, 6H, CpCH3), 1.90 (b,
12H, THF), 1.06 (d, J = 6.8 Hz, 12H, CH(CH3)2).
1
68.35; H, 10.08; N, 1.50. H NMR (300 MHz, C6D6, 298 K): δ 8.78
Synthesis of Complex 2a. To a solution of the free ligand 2-
(2,6-iPr2C6H3NCH)C6H4Me4CpH (750 mg, 1.95 mmol) in 10 mL
of THF was added dropwise a solution of butyllithium (1.15 mL, 1.95
mmol) in THF at −78 °C. The reaction mixture was warmed to room
temperature and stirred for 1 h. The resulting solution was added to a
suspension of ScCl3(THF)3 (717 mg, 1.95 mmol) in 30 mL of THF at
room temperature, and then the reaction mixture was stirred for
another 3 h. During the reaction period, the color of the reaction
mixture changed from white to yellow. After the solvent was removed
under vacuum, the oily crude product was heated to 120 °C to remove
the THF which coordinated to the Li atom. The residue was extracted
with 15 mL of CH2Cl2 to remove the insoluble impurities. A yellow
powder was obtained by removing the CH2Cl2 and washing with
hexane (641 mg, 1.28 mmol, 65.4%). Anal. Calcd for C28H34Cl2NSc
(500.44): C, 67.20; H, 6.85; N, 2.80. Found: C, 67.25; H, 6.90; N,
(d, J = 7.7 Hz, 1H, ArH), 8.05 (s, 1H, CHN), 7.62 (d, J = 6.4 Hz,
1H, ArH), 7.36−6.99 (m, 5H, ArH), 3.56 (b, 12H, THF), 3.31−3.09
(m, 2H, CH(CH3)2), 1.98 (s, 6H, CpCH3), 1.85 (s, 6H, CpCH3), 1.40
(b, 12H, THF), 1.16 (d, J = 6.9 Hz, 12H, CH(CH3)2), 0.30 (s, 27H,
CH2Si(CH3)3), −0.16 (broad, 6H, CH2SiMe3).
Synthesis of Complex 4. To a suspention of complex 2a (500
mg, 1.00 mmol) in 10 mL of n-hexane was slowly added LiCH2SiMe3
(188 mg, 2.00 mmol) powder at room temperature. The orange
mixture was stirred at room temperature for 3 h, and then the LiCl was
removed by filtration. After reduction of the residual solution volume
to about 2 mL under reduced pressure, the oily residue was cooled at
−30 °C overnight to give 4 as red crystals (331 mg, 0.55 mmol, 54.7%
yield). Anal. Calcd for C36H55NScSi2 (603.96): C, 71.59; H, 9.35; N,
1
2.32. Found: C, 71.73; H, 9.22; N, 2.34. H NMR (300 MHz, C6D6,
298 K): δ 7.90 (s, 1H, CHN), 7.17−6.78 (m, 7H, ArH), 2.92 (m,
2H, CH(CH3)2), 2.32 (s, 6H, CpCH3), 2.09 (s, 6H, CpCH3), 1.37 (d,
J = 6.7 Hz, 6H, CH(CH3)2), 0.85 (d, J = 6.7 Hz, 6H, CH(CH3)2), 0.25
(s, 18H, CH2Si(CH3)3), −0.21 (d, J = 10.5 Hz, 2H, CH2Si(CH3)3),
−0.48 (d, J = 10.5 Hz, 2H, CH2Si(CH3)3). 13C NMR (75 MHz, C6D6,
298 K): δ 175.9, 147.5, 141.7, 139.7, 137.2, 134.7, 134.1, 131.2, 128.6,
127.1, 124.8, 123.2, 120.4, 119.2, 45.9, 29.1, 26.4, 22.5, 13.1, 12.4, 4.5
ppm.
Propylene Polymerization Procedures. A dry 250 mL steel
autoclave with a magnetic stirrer was charged with 18 mL of toluene
and saturated with propylene (1.0 bar) at 20 °C. The polymerization
reaction was started by injection of a mixture of AlR3 and a catalyst in
toluene (1 mL) together with a solution of Ph3CB(C6F5)4 in toluene
(1 mL). The vessel was repressurized to the desired pressure with
propylene immediately, and the pressure was maintained by
continuous feeding of propylene. After a certain period of time, the
polymerization was quenched by injecting acidified methanol (HCl (3
M)/methanol = 1/1). Then the polymer was collected by filtration,
1
2.75. H NMR (300 MHz, CDCl3, 298 K): δ 8.29 (s, 1H, CHN),
7.76 (t, J = 7.5 Hz, 1H, ArH), 7.68 (d, J = 7.7 Hz, 1H, ArH), 7.58 (t, J
= 7.5 Hz, 1H, ArH), 7.40−7.20 (m, 4H, ArH), 2.86 (m, 2H,
CH(CH3)2), 2.27 (s, 6H, CpCH3), 2.06 (s, 6H, CpCH3), 1.36 (d, J =
6.6 Hz, 6H, CH(CH3)2), 1.04 (d, J = 6.8 Hz, 6H, CH(CH3)2). 13C
NMR (75 MHz, CDCl3, 298 K): δ 175.7, 144.4, 142.0, 138.2, 137.7,
135.4, 134.4, 131.0, 128.9, 128.5, 127.5, 125.0, 124.84, 124.5, 29.4,
26.2, 22.0, 12.5, 12.3 ppm.
Synthesis of Complex 2b. Complex 2b was synthesized in the
same manner as for complex 2a with 2-(2,6-Et2C6H3NCH)-
C6H4Me4CpH (697 mg, 1.95 mmol), n-BuLi (1.15 mL, 1.95 mmol),
and ScCl3(THF)3 (717 mg, 1.95 mmol) as starting materials. Pure
product 2b (577 mg, 1.22 mmol, 62.6%) was obtained as a yellow
powder. Anal. Calcd for C26H30Cl2NSc (472.39): C, 66.11; H, 6.40; N,
2.97. Found: C, 66.14; H, 6.45; N, 2.92. 1H NMR (300 MHz, CDCl3,
298 K): δ 8.29 (s, 1H, CHN), 7.75 (td, J = 7.5, 1.5 Hz, 1H, ArH),
7.70 (dd, J = 7.7, 1.5 Hz, 1H, ArH), 7.58 (td, J = 7.6, 1.3 Hz, 1H,
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washed with water and methanol, and dried at 40 °C under vacuum to
a constant weight.
(9) Tshuva, E. Y.; Goldberg, I.; Kol, M. J. Am. Chem. Soc. 2000, 122,
10706.
1-Hexene Polymerization Procedures. In the glovebox, to a
mixture of toluene, 1-hexene, and procatalyst was added MAO (500
equiv) or a mixture of AlR3 (60 equiv)/Ph3CB(C6F5)4 (1.1 equiv) at
polymerization temperature. After it was stirred for about 48 h, the
reaction mixture was quenched by addition of acidified methanol and
the polymer was collected, washed with water and methanol, and dried
at 40 °C under vacuum to a constant weight.
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Crystal Structure Determination. The crystals were mounted on
a glass fiber using an oil drop. Data obtained with the ω−2θ scan
mode were collected on a Bruker SMART 1000 CCD diffractometer
with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The
structures were solved using direct methods,27 and further refinements
with full-matrix least squares on F2 were obtained with the SHELXTL
program package. All non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were introduced in calculated positions with the
displacement factors of the host carbon atoms. All calculations were
performed using the SHELXTL crystallographic software packages.28
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ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files giving X-ray crystallographic data for complexes 1a,
2c, 3, and 4. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*Tel: (86)-431-85168376. Fax: (86)-431-85193421. E-mail:
ymu@jlu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research work was supported by the National Natural
Science Foundation of China (Nos. 51173061, 21074043, and
21274050).
(20) (a) Chen, Y.-X.; Marks, T. J. Organometallics 1997, 16, 3649.
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