Synthesis, crystal structure and catalysis for polyethylene of a paramagnetic chromium complex [(η5-CpSi)Cr(μ-Cl)Cl]2 [CpSi = (C5H4)Si(CH3) 2(C6H5)]

Article abstract of DOI:10.1016/S0277-5387(02)00866-5

The synthesis of the substituted silyl cyclopentadiene ligand (C₅H₅)Si(CH₃) ₂(C₆H₅) (Cp^SiH) (1) is described. Deprotonation of (1) gives (Cp^SiLi) (2) which reacts with CrCl₃ to produce the chloro-bridged complex [(η⁵-Cp^Si)Cr(μ-Cl)Cl]₂ (3). Single-crystal X-ray structure analysis reveals that a centrosymmetric dimer exists in the solid state. When activated with methylaluminoxane (MAO), complex (3) exhibits several interesting features as a catalyst for ethylene polymerization. With the help of a semi-empirical MO calculation, a cationic sandwich intermediate structure in the catalytic process was proposed.

Full text of DOI:10.1016/S0277-5387(02)00866-5

Polyhedron 21 (2002) 867Á872
/
www.elsevier.com/locate/poly
Synthesis, crystal structure and catalysis for polyethylene
of a paramagnetic chromium complex
[(h5-CpSi)Cr(m-Cl)Cl]2 [CpSiꢀ
/
(C5H4)Si(CH3)2(C6H5)]
Ying-Kai Xu a,b,*, Ying Mu b, Hui Sun b, Zheng-Wang Qu a, Fan Zhang b,
Yue-Tao Zhang b, Yang Liu a
a State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Center for Molecular Science,
the Chinese Academy of Sciences, North 1st Street No. 2, Zhongguancun, Beijing 100080, People’s Republic of China
b Department of Chemistry, Jilin University, Changchun 130023, People’s Republic of China
Received 5 September 2001; accepted 9 January 2002
Abstract
The synthesis of the substituted silyl cyclopentadiene ligand (C5H5)Si(CH3)2(C6H5) (CpSiH) (1) is described. Deprotonation of (1)
gives (CpSiLi) (2) which reacts with CrCl3 to produce the chloro-bridged complex [(h5-CpSi)Cr(m-Cl)Cl]2 (3). Single-crystal X-ray
structure analysis reveals that a centrosymmetric dimer exists in the solid state. When activated with methylaluminoxane (MAO),
complex (3) exhibits several interesting features as a catalyst for ethylene polymerization. With the help of a semi-empirical MO
calculation, a cationic sandwich intermediate structure in the catalytic process was proposed. # 2002 Elsevier Science Ltd. All
rights reserved.
Keywords: Chromium; Crystal structures; Homogeneous catalyst; Electron spin resonance; Polyethylene
1. Introduction
obtained by NMR spectroscopy [17]. It is also notice-
able that the active centers in homogeneous Kaminsky
The area of homogeneous metallocene-catalyzed ole-
fin polymerization recently has received much attention,
in part because ligand modification is an effective means
of optimizing catalyst activity as well as polymer
properties such as molecular weight, stereoregularity
catalyst systems are cationic [18Á20], and chromium
/
proved easy to coordinate phenyl [21,22]. Therefore, we
prepared a novel chromium-based paramagnetic homo-
geneous catalyst that bears a phenyl ring bound to the
Cp group via a silicon bridge in the hope that the cation
produced by methylaluminoxane (MAO) might give
higher activity of polyethylene, which could be regarded
and microstructure, etc. [1Á
/
7]. Our most recent efforts
were in this area [8,9].
Chromium has played a prominent role among the
transition metals that catalyze the polymerization of
as evidence for phenylÁchromium interactions.
/
olefins [10Á/12]. Recently, a class of chromium-based
homogeneous catalysts has been synthesized [13Á
/15],
2. Results and discussion
and some of them are cationic species, which are
somewhat similar to the cationic Group 4 zirconocene
complexes [16]. In some cationic zirconocene complexes,
evidence for the coordination of a phenyl group to the
2.1. Ligand synthesis
Grignard reaction of Mg with bromobenzene in Et2O
followed by addition to (CH3)2SiCl2 gives
zirconium center via agostic CÃ
/
HÃ/M interaction was
(CH3)2SiCl(C6H5) as a light yellow oil in near quanti-
tative yield (Scheme 1). Its isolation is possible, but not
necessary since in situ addition of either NaCp or LiCp
in Et2O produces a yellow solution. This yellow solution
* Corresponding author. Tel.: ꢁ86-10-6257-1074; fax: ꢁ86-10-
6255-9373.
E-mail address: xuyingkai@yahoo.com (Y.-K. Xu).
0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 0 8 6 6 - 5

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Y.-K. Xu et al. / Polyhedron 21 (2002) 867Á
/872
Scheme 1.
was hydrolyzed, extracted by Et2O, and then purified by
column chromatography. The major yellow oil is the
desired substituted cyclopentadiene (C5H5)Si(CH3)2-
(C6H5) (CpSiH) (1) as identified by 1H NMR, IR spectra
and MS (Scheme 1).
identified as dimeric by X-ray diffraction. We also tried
to isolate the green intermediate product, but it was
easily lost Et2O in vacuum so that it could not be
obtained. A similar process was described by S. K. Noh
[25], so we speculate that the following equilibrium may
exist (Eq. (3)).
1
The H NMR of (1) displays two broadened Cp ring
resonances and we attribute this to rapid sigmatropic
shifts of either the methine proton, the silyl group or
both. Sigmatropic rearrangements are well known in
related silyl substituted cyclopentadienyl systems
[23,24]. Compound (1) is readily deprotonated by nBuLi
in hexane to give the base-free lithium salt
Li(C5H4)Si(CH3)2(C6H5) (2) (CpSiLi) in good yield as
an air sensitive white solid that is insoluble in aliphatic
and aromatic hydrocarbons and soluble in ethers (Eq.
(1)). Exposure of (2) to moisture produces (1) as
ð3Þ
This hypothesis is further supported by the observa-
tion that if in the reaction there are other coordinatively
stronger reagents such as PPh3 or pyridine, the solution
color is always green and we cannot obtain the dark blue
crystals for X-ray diffraction.
1
observed by H NMR.
2.3. Study of electron spin resonance
ð1Þ
The ESR spectrum of (3) displayed in Fig. 1 shows
that complex (3) has unpaired electrons and reveals that
Cr (III) is located at a highly symmetric site.
2.2. Chromium complex synthesis
The reaction of 1 equiv. of (2) with 1 equiv. of CrCl3
in Et2O produces [(h5-CpSi)Cr(m-Cl)Cl]2 (3) which is
extracted and recrystallized in CH2Cl2 with high yield as
an air sensitive dark blue crystal that is soluble in most
common organic solvents (Eq. (2)). Complex (3) was
identified by IR spectra, elemental analysis, MS, ESR
and single-crystal X-ray analysis. Since complex (3) is
1
paramagnetic, it cannot be identified by H NMR.
ð2Þ
In the process of the above reaction, we observed that
the solution is firstly dark green, then gradually turns to
green-blue, and finally becomes blue. Only this blue
solution gives the dark blue crystals that can be
Fig. 1. ESR spectrum of complex (3) in toluene solution at room
temperature; gꢀ1.986; microwave frequency: 9.77 GHz; microwave
power: 20 dB, 2 mW; cavity: TE 104.

Y.-K. Xu et al. / Polyhedron 21 (2002) 867Á
/872
869
Fig. 2. Molecular structure of (3) with the atom-numbering scheme.
2.4. X-ray crystal structure analysis
ethylene polymerization. Upon activation with MAO,
(3) shows moderate catalytic activity. The polymeriza-
tion results are summarized in Table 2. As can be seen
from the results, (3) shows several interesting features as
a catalyst for ethylene polymerization. First, a low Al:
Cr ratio (500:1) is needed to obtain full activity in
contrast to the value of approximately 104:1 (Al:Zr)
reported for the MAO-activated ansa-zirconocene cat-
alysts, and this is similar to the amino-substituted
cyclopentadienyl chromium catalyst [28]. Second, the
catalytic activity of (3) does not decrease very much
when the Al: Cr ratio changes from 500 to 1000. Third,
the catalytic activity of (3) and the molecular weights of
polyethylene are much higher than the reported chro-
mium-based cationic half-sandwich catalysts [13]. It is
obvious that these observed features could be explained
The molecular structure of (3) is shown in Fig. 2.
Selected bond distances and angles are given in Table 1.
The complex (3) is a dimer located at a crystallographic
inversion center. Each chromium atom is bonded to one
CpSi ligand, one terminal chlorine atom and two
bridging chlorine atoms, thus completing the familiar
pseudooctahedral coordination environment of a three-
legged piano stool. The Cp ring is bonded to the Cr
atom in the h5 manner and can be thought of as a
tridentate ligand occupying one of the trigonal faces of
the octahedron. This view is certainly consistent with its
ESR behavior. Each chromium center has a 15-electron
configuration, so it is slightly electronic unsaturated.
˚
Cr (1A) distance of 3.336 A is too long to form
The CrÃ
/
a metalÃ
/
metal bond [25Á
/
27]. There are no short
on the basis of phenylÁchromium interactions, which
/
intermolecular contacts between dimers. The distance
˚
from the chromium to the Cp ring center is 1.861 A. The
could weaken the interaction between the cationic
chromium center and the anionic cocatalyst and, there-
fore, efficiently prevent the active catalyst from bimo-
lecular deactivation. On the other hand, this interaction
could effectively lower the rate of chain termination by
b-H elimination. So less MAO is required to keep the
catalyst in an active state and a relatively long poly-
ethylene chain can be formed. As a result, the catalytic
activity and lifetime of the catalyst increase, and the
angle between the Cp ring plane and the phenyl ring
plane is 78.78
2.5. Polymerization
In order to further our understanding of the structure
of complex (3), it was tested for catalytic activity in
Table 2
Ethylene polymerization results obtained with complex (3) activated
Table 1
˚
Selected bond lengths (A) and angles (8) for complex (3)
a
by MAO
Bond lenghts
d
Number Al:Cr Activity bꢂ10ꢃ5
Mh cꢂ10ꢃ5
Tm (8C)
CrÃC (4)
CrÃC (3)
CrÃC (1)
CrÃC (5)
SiÃC (7)
SiÃC (6)
2.209(3)
2.212(3)
2.217(3)
2.218(3)
1.852(3)
1.851(3)
CrÃC (2)
CrÃCl (2)
CrÃCl (1)
SiÃC (8)
SiÃC (3)
2.222(3)
2.273(1)
2.3679(9)
1.871(3)
1.881(3)
1
2
3
4
5
80:1
300:1
500:1
0.279
0.976
3.81
0.66
1.4
133.77
134.32
134.03
134.43
132.77
1.47
0.93
0.699
1000:1 3.40
2000:1 1.11
Bond angles
a
Cl (2)ÃCrÃCl (1A)
Cl (2)ÃCrÃCl (1)
C (7)ÃSiÃC (8)
C (8)ÃSiÃC (3)
C (6)ÃSiÃC (3)
96.56(4)
95.79(4)
111.2(2)
Cl (1A)ÃCrÃCl (1)
C (7)ÃSiÃC (6)
C (8)ÃSiÃC (6)
90.40(4)
111.3(2)
110.5(2)
Polymerization conditions: 50 ml of toluene, catalyst 1.5 mmol,
time 2 h, temp. 16 8C, Pi (C2H4)ꢀ1.5 atm. MAO: methylaluminox-
ane.
b
g PE (mol Cr)ꢃ1 hꢃ1
.
105.19(13) C (7)ÃSiÃC (3)
109.56(14)
108.93(14)
c
Measured in decahydronaphthalene at 135 8C.
d
Determined with DSC at a heating rate of 10 8C minꢃ1
.

870
Y.-K. Xu et al. / Polyhedron 21 (2002) 867Á872
/
molecular weight of polyethylene increases correspond-
ingly. In a word, there must be a sterically crowded
environment around the metal center [8,9].
Tetrahydrofuran (THF), diethyl ether (Et2O) and
toluene were refluxed over sodium benzophenone ketyl
and distilled before use under nitrogen. Hexane and
dichloromethane (CH2Cl2) were refluxed and distilled
from CaH2 under nitrogen. CrCl3 was prepared accord-
ing to a published route [29,30]. Polymerization grade
ethylene was further purified by passage through
For understanding the catalytic process we calculated
the cationic intermediate structure by a semi-empirical
ZINDO/1 MO method. Two stable structures were
obtained by geometry optimization (Fig. 3). The energy
of the sandwich structure was 80.4 kcal molꢃ1 lower
than that of the half-sandwich structure, which means
the sandwich is so stable that it is the only possible
structure in the catalytic process.
So we speculate that in the process of polymerization
the dimer (3) first depolymerizes and then loses a
chlorine to form a cationic sandwich complex in which
the phenyl coordinates to the Cr(III) in the presence of
MAO. This proposed structure is consistent with the
experimental results. This process is illustrated in
Scheme 2.
˚
columns of 10 A molecular sieves and MnO. Methyla-
luminoxane (MAO, 10 wt.% solution in toluene, Mnꢀ
/
800 g molꢃ1, Alꢀ
/
5.3 wt.%) was purchased from Witco.
nBuLi (1.6 M in hexanes) was purchased from Aldrich.
Elemental analyses were performed on American PE-
240 C elemental analyzer at the Center of Analysis and
Test of Jilin University. The MS was obtained by
Finningan GCÁ
/
MSÁDS MAT4510. The ESR spectrum
/
was acquired by Bruker ER2000 EPR. NMR spectra
were recorded on a Varian Unity-400 spectrometer and
the X-ray structure on a Siemens P4 diffractometer.
Viscosity-average molecular weights of the polyethylene
were determined in decahydronaphthalene at 135 8C
using a Schott Gerate model AVS/T2 Ubbelohde
viscosimeter. Melting transition temperatures (Tm) of
the polyethylene were determined by DSC (Du Pont 910
differential scanning calorimeter) at a heating rate of
3. Experimental
3.1. General procedures and material
10 8C minꢃ1
.
All reactions with moisture- and air-sensitive com-
pounds were carried out under dry argon atmosphere
(ultra-high purity) using standard Schlenk techniques.
All reagents are commercially available and were used as
received unless otherwise stated.
3.2. Synthesis of CpSiH (1)
To a solution of Me2SiCl2 (8ml, 66.33 mmol) in Et2O
(40 ml) was added dropwise a solution of PhMgBr (50
ml, 1.00 N, 50.03 mmol) in Et2O (10 ml) at ꢃ50 8C and
/
stirred for 1 h at room temperature (r.t.). Then solvent
and excess Me2SiCl2 were removed in vacuo. The
residue was redissolved in Et2O (40 ml), and CpNa (26
ml, 1.1919 N, 49.9 mmol) in THF was added dropwise
slowly to the above solution at ꢃ
/
78 8C over 40 min,
and stirred at r.t. for 5Á6 h to give a pale yellow
/
solution. The reaction was hydrolyzed and was ex-
tracted with Et2O. The Et2O layer was dried with
MgSO4. After filtration, the solvent was evaporated
under vacuum and left behind a yellow oil. The product
was purified by column chromatogram over silica gel
(hexane-CH2Cl2, 3:1). Yield 83%. 1H NMR (CDCl3): d
0.255 (s, 6H, Si (CH3)2); 3.135(s, 1/3H, methine);
3.676(br, 2/3H, methine); 6.580Á
ring); 7.425Á7.645(m, 5H, phenyl). IR (KBr):
3070.12(m), 2960.20(m), 1625.70(m), 1427.07(w),
1253.50(s), 1114.65(m), 1062.59(w), 975.80(m),
/6.696(br, 4H, Cp
/
9530.85(m), 829.24(s), 802.24(s), 732.82(s), 700.03(s).
Mass spectrum (m/e): 400.6[(Me2PhSiCpH)2ꢁ],
200.3(Mꢁ).
3.3. Synthesis of CpSiLi(2)
Fig. 3. Geometrically optimized structures of the cationic intermediate
in the catalytic process by semi-empirical ZINDO/1 MO method. A:
half-sandwich structure; B: sandwich structure.
nBuLi (14.5 ml) (1.6 M, 23.2 mmol) was injected
slowly into a solution of CpSiH (4.5043 g, 22.48 mmol)

Y.-K. Xu et al. / Polyhedron 21 (2002) 867Á
/872
871
Scheme 2.
in hexane (30 ml) at ꢃ78 8C. The reaction was allowed
to warm to r.t. and left stirring for 5 h. A white
precipitate was observed. The precipitate was cannula
/
Table 3
Crystal data and structure refinement for complex (3)
Empirical formula
Formula weight
Temperature (K)
C13H15Cl2CrSi
322.24
filtered, washed with 2ꢂ
/
30 ml of hexane and dried
under vacuum. Yield 70%.
293(2)
0.71073
˚
Wavelength (A)
Crystal system
Space group
monoclinic
P21/c
3.4. Synthesis of [(h5-CpSi)Cr(m-Cl)Cl]2 (3)
˚
a (A)
12.242(2)
9.621(2)
12.811(3)
101.60(3)
1478.1(5)
4
CpSiLi powder (0.6367 g, 3.1 mmol), CrCl3 powder
(0.4946 g, 3.1 mmol) and 30 ml Et2O was added to a dry
100 ml Schlenk flask under argon. The suspension was
stirred at r.t. for 15 days. In this process the colorless
˚
b (A)
˚
c (A)
b (o)
3
˚
V (A )
Z
Absorption coefficient (mmꢃ1
Crystal size (mm)
)
1.192
0.50ꢂ0.42ꢂ0.28
2.67 to 25.01
solution gradually turns into green, then greenÁblue,
/
and finally blue. The precipitate was filtered off and the
solvent was removed in vacuo. The residue was ex-
tracted three times with CH2Cl2. The combined frac-
tions were filtered and the solution was concentrated in
u Range for data collection (8)
Scan type
Reflections collected
v Á2u
/
3481
Independent reflections
Max./min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I ꢀ2s(I)]
R indices (all data)
2594 (Rintꢀ0.0238)
0.35464, 0.24856
2594/0/154
0.705
vacuo. Recrystallization at ꢃ10 8C afforded (3) as a
/
dark blue crystal. Yield 67.4%. IR: 3093.26(w),
3014.19(w), 2960.20(w), 2915.84(w), 1635.34(m),
1425.14(w), 1402.00(w), 1249.65(m), 1172.51(m),
R1ꢀ0.0369, wR2ꢀ0.0949
R1ꢀ0.0504, wR2ꢀ0.1023
) 0.396 and ꢃ0.646
1110.80(m),
1014.37(m),
833.10(s),
784.89(m),
ꢃ3
˚
Largest difference peak and hole (e A
736.67(m), 700.03(m). MS (m/e): 644.5(Mꢁ). Anal.
Calc. for (Me2PhSiCpCrCl2)2: C, 48.46; H, 4.69; Found:
C, 45.01; H, 4.42%.
was injected (total volume of the polymerization solu-
tion is 50 ml after the addition of catalyst). The vessel
was repressurized to the required pressure with ethylene
as soon as the catalyst was injected and the pressure was
maintained by continuously feeding monomer. After the
mixture was stirred for the desired reaction time, the
reactor was vented and the polymerization was termi-
nated by injecting acidified methanol [HCl (3 M)Á
methanolꢀ1:1]. The polymer was stirred in acidified
methanol overnight, washed with water and methanol,
dried in vacuo at 60 8C.
3.5. X-ray structure determination
Data of (3) were collected on a Siemens P4 diffract-
ometer with a graphite monochromator using Mo Ka
radiation. The structure was solved by direct methods
and refined by full-matrix least-squares methods on F2.
All calculations were performed using SHELX-93 [31].
Hydrogen atoms were treated as idealized contributions.
Crystallographic data and parameters are summarized
in Table 3.
/
/
3.6. Polymerizations
3.7. Semi-empirical MO calculation
A dry 250 ml steel reactor equipped with a magnetic
stirrer was evacuated and pressurized with ethylene to
1.5 atm. Toluene and an appropriate amount of MAO
were added to reactor through a septum port via
syringe. After the solution was saturated with ethylene
for 30 min, a solution of catalyst (1.5 mmol) in toluene
The cationic intermediate structure in the catalytic
process was calculated by a semi-empirical ZINDO/1
MO method of the HYPERCHEM 6.0 program [32]. The
parameters are the following, Total charge: 1; spin
multiplicity: 4; convergence limit: 0.01; RMS gradient
of: 0.01 kcal molꢃ1
.

872
Y.-K. Xu et al. / Polyhedron 21 (2002) 867Á872
/
[8] F. Zhang, Y. Mu, J. Wang, Z. Shi, W. Bu, S. Hu, Y. Zhang, S.
Feng, Polyhedron 19 (2000) 1941.
4. Supplementary material
[9] F. Zhang, Y. Mu, L. Zhao, Y. Zhang, W. Bu, C. Chen, H. Zhai,
H. Hong, J. Organomet. Chem. 613 (2000) 68.
Crystallographic data (excluding structure factors) for
the structure reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre,
CCDC No. 163491. Copies of the data can be obtained
free of charge from The Director, CCDC, 12 Union
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