Mononuclear titanium compounds based on the silyl-linked bis(amidinate) ligand: Synthesis, characterization and ethylene polymerization

Article abstract of DOI:10.1039/c1dt10365b

The mononuclear silyl-linked bis(amidinate) titanium compound 2 was prepared by salt metathesis of 1 and TiCl₄(thf)₂. Alkylation of 2 with methyllithium gave analogue 3. Both 2 and 3 exhibited a configuration similar to ansa-metalloc

Full text of DOI:10.1039/c1dt10365b

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COMMUNICATION
Mononuclear titanium compounds based on the silyl-linked bis(amidinate)
ligand: synthesis, characterization and ethylene polymerization†
Sheng-Di Bai,*a Fei Guan,a Min Hu,a Shi-Fang Yuan,a Jian-Ping Guob and Dian-Sheng Liu*a
Received 3rd March 2011, Accepted 30th March 2011
DOI: 10.1039/c1dt10365b
The mononuclear silyl-linked bis(amidinate) titanium com-
pound 2 was prepared by salt metathesis of 1 and TiCl4(thf)2.
Alkylation of 2 with methyllithium gave analogue 3. Both 2
and 3 exhibited a configuration similar to ansa-metallocene
and showed good activity towards ethylene polymerization
after activation with MAO.
Since the landmark discovery of Ziegler–Natta catalysts in early
1950s, innumerous attentions have been paid to this exciting
area in order to obtain excellent catalyst systems. Among the
successive progresses, Kaminsky and Sinn found systems of metal-
locene (I) activated with methylaluminoxane (MAO) were efficient
Fig. 1 Cyclopentadienyl and amidinate group 4 metal compounds.
catalysts.1,2 Subsequently, the stereorigid ansa-metallocene (II)
complexes reported by Brintzinger3,4 also showed impressively
high efficiency. This opened a broad range of research work in
stereospecific polymerization of a-olefins to polymers with stere-
coordination chemistry is well established.18,19 The amidinate
complexes were promising catalysts for oligomerization and
polymerization of olefins,20–29 hydroamination, intramolecular
hydroamination/cyclization,30 and hydrosilylation.31 Moreover,
some of them, such as the bis(amidinate) group 4 metal com-
plexes V, showed high performance in activation and reduction
of dinitrogen or other molecules.32–34 A class of silyl-linked
oregular microstructures.5 These groundbreaking developments
pushed Cp (cyclopentadienyl) to be ubiquitous for over half a
century in organometallic chemistry.6,7 In that surge of research,
both Erker’s and Liu’s groups developed a class of allyl-bridged
group 4 metallocene complexes.8,9 As a great deal of effort was
made to seek Cp alternatives such as N-centered donor ligands
in recent decades, the half-sandwich species with a mixed-ligand
environment were generated, including the commercialized con-
strained geometry compounds (CGC) (III10–12 and IV13–16 in Fig.
1). In recent years, our research interest was focused on the small
conjugated framework with N-centered donor ligands. Group
4 metal compounds supported with the monovalent bidentate
2-
bis(amidinate) ligands {SiMe2[NC(Ph)N(R)]2} were developed
in our recent research.35,36 The special bianionic ligands had the
advantage of affording both binuclear complexes and mononu-
clear complexes.37–41 The group 4 metal compounds based on them
were therefore synthesized and investigated. The mononuclear
species adopting the configuration of VI were presumed to be
more rigid in contrast to V, which was comparable to the
upgrade from I to II. On the other hand, in terms of the bridged
3
aminosilylanilido ligand and the further derived h -azaallyl ligand
5
5
biscyclopentadienyl h : h ligands, the silyl-linked bis(amidinate)
have been prepared and show good activities toward ethylene
(co)polymerization.17
Amidinate ligands are easily modified by variation of sub-
stituents on the conjugated N–C–N backbone to meet the
requirements of different metal centers. The corresponding
3
3
ligands can be considered as an h : h alternative system. Since the
latter is more electron-deficient than the former, it is supposedly
helpful to enhance the electrophilic behavior of the metal center
and then improve its activity for olefin polymerization. Herein the
syntheses and characterization of the silyl-linked bis(amidinato)
titanium are reported as well as the catalytic properties for ethylene
polymerization.
aInstitute of Applied Chemistry, Shanxi University, Taiyuan, Shanxi, 030006,
P. R. China. E-mail: sdbai@sxu.edu.cn, dsliu@sxu.edu.cn; Fax: 086-351-
7011743; Tel: 086-351-7017948
The ligand transfer reagents, with general formula
bSolid wastes resource utilization and energy saving building materials state
key laboratory, No. 69 Jingding North Road, Shijingshan District, Beijing,
100041, P. R. China
Li2{SiMe2[NC(Ph)N(R)]2} (R
= phenyl, 2,6-Me2C6H3, 2,6-
iPr2C6H3 and tBu), were prepared according to the reported
methods.35 Regarding those with aromatic terminal groups,
the titanium derivatives like VI could not be obtained when
reacting with titanium tetrachloride. If metal salts coordinated
with Cp in advance were used instead, it may lead to a series of
† Electronic supplementary information (ESI) available: Experimental
details and crystal structure determination. CCDC reference numbers
804643 and 804644. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1dt10365b
7686 | Dalton Trans., 2011, 40, 7686–7688
This journal is
The Royal Society of Chemistry 2011
©

versatile products, some involving ligand transformation process.
The related mechanism was presumed.39 It was interesting that
t
replacing the aromatic groups with Bu at the terminal positions
could prevent the ligand rearrangement and give product in the
expected fashion VI. As shown in Scheme 1, treatment of lithium
compound 1 with titanium tetrachloride tetrahydrofuran solvate
in molar ratio of 1 : 2 gave the titanium compound 2 as red crystals
in good yield up to 80%. Subsequently, compound 2 was alkylated
by two equivalents of methyl lithium in diethyl ether solution.
Compound 3 was obtained as yellow crystals in moderate yield
after extraction and crystallization with dichloromethane. Both 2
and 3 were characterized by NMR, elemental analysis as well as
X-ray crystallography.‡
Fig. 3 Comparison of titanocene complexes and titanium amidinates.
Structural comparison was made to a series of titanium com-
pounds including 2 (shown in Fig. 3) in order to demonstrate some
remarkable differences resulting from the ligand-changing. In the
titanocene reported by Clearfield et al., the angle between the two
Cp (cyclopentadienyl) planes is 51.73◦.42 The Ti center exhibits
tetrahedral geometry composed of two pairs of Ti–Cpcentroid and
Ti–Cl bonds. The Cpcentroid–Ti–Cpcentroid and Cl–Ti–Cl angles are
130.91 and 94.43◦, respectively. In the ansa-titanocene, two Cps
are pulled together by the silyl bridge and forced to adopt the
same orientation toward the silyl bridge, resulting in the widened
related angle of the Cp planes.43 Correspondingly, the Cpcentroid
–
Ti–Cpcentroid angle is decreased and the Cl–Ti–Cl angle is enlarged.
By comparing two titanocene complexes, closure of the Cp side
in ansa-titanocene leads the Ti center to be more “open” in some
degree. This phenomenon could be observed more obviously in the
following two titanium amidinates. Bis(benzamidinate) titanium
dichloride demonstrates the cis-Cl2 configuration as do most group
4 metal L2MX2 complexes with two bidentate ligands, and the
molecule obeys C2 symmetry along the midnormal line across
Ti of triangular [TiCl2].44 Two independent amidinate backbone
planes are perpendicular to each other and interestingly, they
meet across the line of two outer nitrogen atoms. The Cl–Ti–
Cl angle is 98.66◦. The six-coordinate Ti center has a distorted
octahedral configuration. Compound 2 also bears C2 symmetry
along the Ti–Si axis. Two amidinate units are linked by the silyl
bridge and are closely coplanar. Deviations from four nitrogen
atoms to both sides of the plane [Ti–C5–Si1–C5A] are very small,
distances of the terminal and inner N atoms to the plane being
Scheme 1 Synthesis of compounds 2 and 3.
The crystal structure of 2 is illustrated in Fig. 2 with selected
bond lengths and angles. The tetradentate ligand fixes the Ti ion in
the central position. The molecule exhibits C2 rotational symmetry
along the Ti1–Si1 axis. Each amidinate unit bites the metal center
with a N–Ti–N angle of 61.33(12)◦. The distances between Ti and
˚
˚
N are 2.247(3) A and 1.981(3) A, respectively. The weaker affinity
of Ti with the terminal N atoms implies the constrained force of
bending the N–C–N–Si–N–C–N seven-membered chain to adopt
the half circle fashion. Compound 3 is isostructural to 2 except
for the replacement of Ti–CH3 bonds for Ti–Cl bonds. It makes
the affinity between Ti and the N-donor ligand in 3 marginally
smaller.
˚
0.093 and 0.159 A, respectively. The two terminal N and Ti are
nearly linear, which is similar to that in above bis(benzamidinate)
titanium dichloride. While the inner N atoms are pulled so close
to the horizontal plane due to linkage, two Cl atoms show the
increasing tendency of occupying the vertical positions. The Cl–
Ti–Cl angle is released in large degree to be 133.19(8)◦. It displays
a rare transitional configuration between the cis-Cl2 and trans-
3
3
Cl2.12 The silyl-linked bis(amidinate) h : h ligand imposes upon
the metal center a closely planar coordinate environment, which
3
is far less shielding than when there are two separate amidinate h
ligands. It makes compound 2 present an “open” Ti center similar
to ansa-titanocene.
Catalytic properties of both 2 and 3 for ethylene polymerization
in the presence of MAO were investigated. Their activities were
found to be highly dependent on pressure. At normal pressure,
they were inactive at 30 ◦C and only trace amount of polymer could
be observed when heating the reaction system to 50 ◦C. Increasing
both the ethylene pressure from 1 to 10 atm and the Al : Ti
molar ratio led to the abrupt magnification of the polymerization
activities. Catalytic efficiency of 2 climbed to 46 kg (PE) mol-1
Fig. 2 Molecular structure of compound 2. Thermal ellipsoids are
set at 30% probability and hydrogen atoms are omitted for clarity.
◦
˚
Selected bond distances (A) and angles ( ): Ti1–N1 = 2.247(3), Ti1–N2 =
1.981(3), Ti1–Cl1 = 2.2735(12), N1–C5 = 1.303(5), N2–C5 = 1.353(5),
N2–Si1 = 1.697(4); N1–Ti1–N2 = 61.33(12), N1–Ti1–N1A = 168.24(16),
N2–Ti1–N2A = 70.3(2), Cl1–Ti1–Cl1A = 133.19(8), N1–C5–N2 = 109.4(3),
N2–Si1–N2A = 84.5(2).
◦
(Ti) h-1 at 50 C and the corresponding value of 3 was doubled.
Further increase of Al : Ti molar ratio and reaction temperature
would promote their activities in large degree, demonstrating high
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7686–7688 | 7687
©

values of 1020 kg (PE) mol-1 (Ti) h-1 at 100 ◦C for 2 and 1614
kg (PE) mol-1 (Ti) h-1 at 80 ◦C for 3, respectively. These activities
are much higher than the Cp2ZrCl2 standard (627 kg (PE) mol-1
h-1). It was noteworthy that high concentration of MAO did not
suppress or even deactivate their performance.45
In summary, among the silyl-linked bis(amidinate) ligands, only
the alkyl-ended one could support a titanium center ascribed
to the matched electronic property. Both 2 and 3 show high
activities for ethylene polymerization and the methylated species
3 displayed higher performance than chloride compound 2. They
were presumed to be Al : Ti molar ratio, pressure and temperature-
promoted procatalysts. It indicates the derivatives of this family
are promising catalysts.
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Acknowledgements
The authors thank the financial support of grants from the Natural
Science Foundation of China (20702029, S.-D. Bai) (20872084, J.-
P. Guo) (20772074, D.-S. Liu) and the Natural Science Foundation
of Shanxi Province (2008011024). We are grateful to Prof W.-H.
Sun for helpful advices about polymerizaiton reaction.
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‡ Crystal data for 2: C24H34Cl2N4SiTi, M = 525.44, orthorhombic, space
3
˚
˚
group Fdd2, a = 22.585(5), b = 26.749(6), c = 9.093(2) A, V = 5493(2) A ,
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˚
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˚
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7688 | Dalton Trans., 2011, 40, 7686–7688
This journal is
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©