The amido-bridged zirconocene's reactivity and catalytic behavior for ethylene polymerization

Article abstract of DOI:10.1039/b914458g

Reaction of the amido-bridged zirconium complex (CpSiMe ₂NSiMe₂Cp)ZrCH₃ (1) (Cp = C₅H ₄) with half an equivalent of B(C₆F₅) ₃ or Ph₃CB(C₆F₅)₄ afforded the binuclear zirconium complexes [(CpSiMe₂NSiMe ₂Cp)Zr)₂(μ-CH₃)][RB(C₆F ₅)₃] (2a, R = CH₃, 2b, R = C₆F ₅) with a methyl group as the bridge between the two zirconium atoms. In the presence of one equivalent of B(C₆F₅)₃ or Ph₃C(C₆F₅)₄, 1 was transformed to the zwitterionic complexes [(CpSiMe₂NSiMe₂Cp)Zr] [RB(C₆F₅)₃] (3a, R = CH₃, 3b, R = C₆F₅) which are free of a metal-bound σ-alkyl ligand. 2b is stable with Me₃Al while 3b combined with Me ₃Al to form a hetero-binuclear complex [(CpSiMe₂NSiMe ₂Cp)Zr(μ-CH₃)]Al(CH₃)₂][B(C ₆F₅)₄] (4) as shown by NMR spectroscopy at room temperature. Treatment of 2a or 3a with an excess of Me₃Al led to (CpSiMe₂NSiMe₂Cp)Zr(C₆F₅) (5) through a group exchange process. 2b, 3a and 5 have been characterized by X-ray diffraction studies. 2a, 2b, 3a and 3b were highly active catalysts for ethylene polymerization and copolymerization with 1-octene in the presence of trialkylaluminium, but the binuclear zirconium complexes (2a and 2b) showed higher activities than their mononuclear counterparts 3a and 3b. Polymerization activities varied with the trialkylaluminiums and increased with the trialkylaluminium concentration applied in the system. The product existed mainly in the form of Al(PE)₃ with polymeric chains, and its molecular weight and distribution were greatly influenced by the type and amount of trialkylaluminium applied in the catalytic system.

Full text of DOI:10.1039/b914458g

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PAPER
www.rsc.org/dalton | Dalton Transactions
The amido-bridged zirconocene’s reactivity and catalytic behavior for
ethylene polymerization†
Cun Wang,* Martin van Meurs, Ludger P. Stubbs, Boon-Ying Tay, Xiang-Jie Tan, Srinivasulu Aitipamula,
Jacob Chacko, He-Kuan Luo, Pui-Kwan Wong and Suming Ye
Received 17th July 2009, Accepted 5th October 2009
First published as an Advance Article on the web 11th November 2009
DOI: 10.1039/b914458g
Reaction of the amido-bridged zirconium complex (CpSiMe2NSiMe2Cp)ZrCH3 (1) (Cp = C5H4) with
half an equivalent of B(C6F5)3 or Ph3CB(C6F5)4 afforded the binuclear zirconium complexes
[(CpSiMe2NSiMe2Cp)Zr)2(m-CH3)][RB(C6F5)3] (2a, R = CH3, 2b, R = C6F5) with a methyl group as
the bridge between the two zirconium atoms. In the presence of one equivalent of B(C6F5)3 or
Ph3C(C6F5)4, 1 was transformed to the zwitterionic complexes [(CpSiMe2NSiMe2Cp)Zr][RB(C6F5)3]
(3a, R = CH3, 3b, R = C6F5) which are free of a metal-bound s-alkyl ligand. 2b is stable with Me3Al
while 3b combined with Me3Al to form a hetero-binuclear complex [(CpSiMe2NSiMe2Cp)Zr(m-CH3)]-
Al(CH3)2][B(C6F5)4] (4) as shown by NMR spectroscopy at room temperature. Treatment of 2a or 3a
with an excess of Me3Al led to (CpSiMe2NSiMe2Cp)Zr(C6F5) (5) through a group exchange process. 2b,
3a and 5 have been characterized by X-ray diffraction studies. 2a, 2b, 3a and 3b were highly active
catalysts for ethylene polymerization and copolymerization with 1-octene in the presence of
trialkylaluminium, but the binuclear zirconium complexes (2a and 2b) showed higher activities than
their mononuclear counterparts 3a and 3b. Polymerization activities varied with the trialkylaluminiums
and increased with the trialkylaluminium concentration applied in the system. The product existed
mainly in the form of Al(PE)3 with polymeric chains, and its molecular weight and distribution were
greatly influenced by the type and amount of trialkylaluminium applied in the catalytic system.
cobalt complexes described by the groups of Gal, Gibson, and
Introduction
Erker.6 In the bimetallic complex model proposed by Chen et al.,
alkyl transfer from an R–[B]- or R–[Al]- counteranion to the
activated ethylene may represent the possible pathway for the
polymer chain growth.5a Activation of the precatalyst by lithium
cations is also suggested for the cobalt(I) system.6c
The field of olefin polymerization catalysts has experienced a
phenomenal acceleration in research activity over the past 20 years,
with many academic and industrial research laboratories engaging
in the design of organometallic precatalysts for the controlled
synthesis of polyolefin products.1 The silicon bridged Cp/amido
systems Me2Si(C5Me4)(t-BuN)MX2 (M = Ti, Zr, X = Cl, alkyl),
especially the titanium complexes, have attracted great interest
for their excellence in ethene/1-alkene copolymerization.2 Most
of the ‘CpSiNR’ type ligands are dianionic, and with or without
additional neutral donors, and doubly bridged analogous group
4 metal complexes with trianionic ligands have not been fully
developed, but they show an interesting reactivity.3 It is com-
monly assumed that homogeneous Ziegler–Natta polymerization
systems require the presence of a reactive transition metal-bound
s-alkyl ligand and an open coordination site according to the
Cossee–Arlman mechanism.4 However, a few examples from the
groups of Royo, Chen and Cuenca show that coordinatively unsat-
urated group 4 metal cations can catalyze ethylene polymerization
even if they are free of a metal-bound s-alkyl ligand.3b,5 This
behaviour is also found in a few chelate bis(iminoaryl)pyridine
We recently prepared the methyl zirconium complex 1 (Fig. 1)
with an amido-bridged trianionic ligand CpSiNSiCp which
was active toward ethylene polymerization in the presence of
methylaluminoxane (MAO).7 The active zirconium cation was
also thought to be free of a metal-bound s-alkyl group. In
our further research using boron-containing activators such as
B(C6F5)3 or Ph3CB(C6F5)4, we found that trialkylaluminiums play
a very important role in the polymerization system, and the
major products were long-chain aluminium alkyls in the form
of Al(PE)3. In this contribution, we report on further reactions of
1 with boron-containing activators B(C6F5)3 and Ph3CB(C6F5)4
with or without the presence of trialkylaluminium, and the
Institute of Chemical and Engineering Sciences, A*STAR (Agency for
Science, Technology and Research), Singapore, 1 Pesek Road, Jurong Island,
Singapore 627833. E-mail: wang_cun@ices.a-star.edu.sg; Fax: +(65)-6316-
6182; Tel: +(65)-6796-3838
† Electronic supplementary information (ESI) available: NMR spectra and
GPC traces. CCDC reference numbers 707932, 707933 and 707934. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b914458g
Fig. 1 The zirconium complex 1 with a trianionic ligand CpSiNSiCp.
Dalton Trans., 2010, 39, 807–814 | 807
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thus-formed intermediates’ catalytic behavior in subsequent ethy-
lene polymerizations as a route to long-chain aluminium alkyls.
Results and discussion
Synthesis and characterization of zirconium complexes
To search for possible reactive intermediates in this catalytic
system, we treated the neutral methyl zirconium complex 1 with
various ratios of B(C6F5)3 or Ph3CB(C6F5)4 with or without the
presence of Me3Al (Scheme 1). Treatment of 1 with half an equiv.
of B(C6F5)3 in C6D6/1,2-C6H4F2 at RT gave quantitatively the
homo-binuclear methyl-bridged zirconium complex 2a. 2a is an
oily product in toluene and no crystals of it were obtained. In
1
1
its H NMR spectrum, there are four sets of H NMR signals
of the diastereotopic pairs of C5H4 hydrogens (d 6.18, 6.12, 6.08,
5.48) and one pair of that of Si–(CH3)2 (d 0.20, 0.16). The bridged
methyl group shows a sharp singlet at d = -0.52, and a broad
signal at d = 1.28 is assigned to the methyl group attached to
the boron atom. 2b was formed analogously by the treatment
of 1 with half an equiv. of Ph3CB(C6F5)4 and exhibits similar
NMR behavior to 2a. Single crystals for X-ray structure analysis
were obtained by the slow diffusion of pentane vapor into a
Fig. 2 View of◦ the molecular structure of 2b. Selected bond lengths
˚
(A) and angles ( ): Zr(1)–C(1) 2.458(5), Zr(2)–C(1) 2.413(5), Zr(1)–N(1)
2.113(5), Zr(2)–N(2) 2.119(5), Si(1)–N(1) 1.727(5), Si(2)–N(1) 1.724(4),
Si(3)–N(2) 1.715(4), Si(4)–N(2) 1.741(4), Zr(2)–C(1)–Zr(1) 173.8(2),
Si(3)–N(2)–Si(4) 140.6(3), Si(3)–N(2)–Zr(2) 105.11(19), Si(4)–N(2)–Zr(2)
104.4(2), Si(1)–N(1)–Si(2) 139.2(3), Si(1)–N(1)–Zr(1) 105.22(19),
Si(2)–N(1)–Zr(1) 105.8(3).
◦
solution of 2b in toluene/1,2-difluorobenzene at -20 C (Fig. 2,
Table 1). The bridged carbon is attached to the two zirconium
˚
˚
atoms and the two Zr–C distances (2.458(5) and 2.413(5) A) are
C6D6/1,2-C6H4F2 with 5 equiv. of Me3Al, and there is no exchange
process between them up to a temperature of 25 ◦C.
longer than that of a normal terminal Zr–CH3 (2.255(4) A) and
˚
are comparable with the reported value of that (2.453(4) A) for
The zwitterionic complex 3a was formed quantitatively at room
temperature by the reaction of 1 with one equiv. of B(C6F5)3 in
toluene and precipitated as a white solid. Single crystals for X-ray
structure analysis were obtained by the slow diffusion of pentane
vapor into a solution of 3a in 1,2-difluorobenzene at -20 ◦C (Fig. 4,
a bridged zirconium complex.8 2a and 2b are stable in C6D6/1,2-
C6H4F2 at ambient temperature. In the presence of excess Me3Al,
2a was transformed into 5 through a group exchange process
between boron, aluminium and zirconium.9 Compound 5 has been
characterized by NMR spectroscopy and an X-ray diffraction
study (Fig. 3, Table 1). The pentafluorophenyl group bisects the
molecule and cannot rotate freely, and five 19F NMR signals were
detected even at 60 ◦C. Compared to 2a, 2b is quite stable in
˚
Table 1). The bond distances of Zr(1)–C(33) (2.594(3) A) and
10
˚
C(33)–B(1) (1.670(4) A) are comparable with reported values.
Further treatment of 3a with one equiv. or excess of Me3Al led
to the formation of 5. 3b was formed similarly by the reaction of
Scheme 1 Reactions of the complex 1 with different activators.
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Table 1 Summary of crystallographic data for compounds characterized by X-ray diffraction study
2b
3a
5
Mol. formula
Mol. weight
Temperature/K
Space group
C53H43BF20N2Si4Zr2
1393.50
110(2)
C20H20F5NSi2Zr
516.77
180(2)
P2(1)/n
Monoclinic
11.491(2)
15.672(3)
16.284(3)
90
100.20(3)
90
2135.8(7)
4
1.607
0.676
16465
5269
266
1.146
0.0370
0.0593
0.1300
0.799 and -1.001
C33H25BF15NSi2Zr
878.75
180(2)
P-1
P-1
Crystal system
Triclinic
12.554(3)
15.672(3)
16.284(3)
100.12(3)
103.26(3)
111.69(3)
2774.8(10)
2
Triclinic
11.245(2)
13.098(3)
13.383(3)
77.57(3)
67.01(3)
74.56(3)
1735.4(6)
2
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
Volume/A
Z
Dc/g cm-3
1.668
0.569
18307
13522
752
1.094
0.0488
0.0708
1.682
0.494
24966
8511
570
1.134
0.0270
0.0561
m/mm-1
Reflection collected
Unique reflections
Parameters varied
Goodness-of-fit
R (int)
R1 (I >2s(I))
wR2 (all data)
0.2743
1.471 and -1.519
0.1479
0.695 and -0.847
-3
˚
Largest diff. peak and hole e A
˚
Fig. 3 View of the molecular structure of 5. Selected bond lengths (A) and
angles (◦): Zr(1)–C(1) 2.367(4), Zr(1)–N(1) 2.106(3), Si(2)–N(1) 1.723(3),
Si(1)–N(1) 1.719(3), N(1)–Zr(1)–C(1) 104.13(13), Zr(1)–N(1)–Si(2)
105.47(16), Zr(1)–N(1)–Si(1) 105.34(15), Si(2)–N(1)–Si(1) 142.3(2).
˚
Fig. 4 View of the molecular structure of 3a. Selected bond lengths (A)
and angles (◦): Zr(1)–C(33) 2.594(3), C(33)–B(1) 1.670(4), Zr(1)–N(1)
2.098(3), Si(1)–N(1) 1.733(3), Si(2)–N(1) 1.732(3), Zr(1)–C(33)–B(1)
169.4(2), N(1)–Zr(1)–C(33) 99.30(10), Zr(1)–N(1)–Si(1) 105.71(13),
Zr(1)–N(1)–Si(2) 105.50(13), Si(1)–N(1)–Si(2) 141.78(16).
1 with one equiv. of Ph3CB(C6F5)4, and a trace of 2b was also
detected. Due to the very weak coordinating anion [B(C6F5)4]-, 3b
has a fluctuating behavior at room temperature, and in its 1H NMR
spectrum recorded in C6D5CD3/1,2-C6H4F2, broad signals were
found for the hydrogens of both Si–(CH3)2 and C5H4. Lowering
the temperature to -20 ◦C led to a well resolved spectrum in which
two sharp singlets at d = 0.16 (ZrCH3Al) and = -0.62 (Al(CH3)2).
4 is stable for a few hours and does not show fluctuating behavior
in C6D6/1,2-C6H4F2 up to a temperature of 25 ◦C. In addition,
there is no evidence of an exchange process between 4, 2b, and
excess Me3Al.
Attempts to isolate and characterize the analogous complexes
from the reaction of 3b with Et3Al or (i-Bu)3Al failed, and
only decomposed products were found, possibly due to the b-H
elimination process. There was also no reaction between 3b and
(Me3SiCH2)3Al due to its high steric demand at room temperature.
Interestingly, about 30% of 3b was converted to 2b by adding
one equivalent of (Me3SiCH2)2AlMe in situ at room temperature
in C6D6/1,2-C6H4F2 (Fig. S7†). Immediate near quantitative
1
four sets of H NMR signals of the diastereotopic pairs of C5H4
hydrogens (d 6.12, 5.65, 5.17, 5.04) and one pair of that of Si–
(CH3)2 (d 0.27, 0.12) were found (ESI, Fig. S4†). 3b decomposed
slowly in C6D5CD3/1,2-C6H4F2, but was still detected after a
weekend at room temperature. In the presence of one equiv. or
excess of Me3Al, 3b was directly converted to the hetero-binuclear
complex 4. 4 was an oily product in toluene and no crystals of
1
1
it could be obtained. In its H NMR spectrum, four sets of H
NMR signals of the diastereotopic pairs of C5H4 hydrogens (d
6.25, 6.15, 5.79, 5.71) and one pair of that of Si–(CH3)2 (d 0.25,
0.04) were found. The bridged and terminal methyl groups showed
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conversion of 3b to 2b was found if the aluminium Schiff base
complexes (6 and 7) were used (Scheme 2, ESI Fig. S8–9†).
7–9). The increase of the activity with the concentration of Et3Al
suggested that Et3Al may be involved in the formation of active
species or propagation and a chain transfer process took place
between the active zirconium species and aluminium. We also
noticed that the polymer molecular weights increased with the
bulkiness of the trialkylaluminiums applied (entries 5–7). This is
quite evident when 2b was used in the presence of (Me3SiCH2)3Al,
and a polyethylene of Mn = 3.6 ¥ 105 was obtained, even though
the catalyst was only active for the first minute, by monitoring the
consumption of ethylene (entry 10). Lowering the polymerization
temperature resulted in a lower activity but a higher molecular
weight of product with a narrower PDI value when Me3Al was
applied with 2b at 23 ◦C (entries 6 and 11).
Scheme 2 Conversion of the complex 3b to 2b.
Ethylene polymerization behavior of the zirconium complexes
Analysis of polymer or oligomer products
Preliminary studies showed that 1, 2a, 2b, 3a, and 3b themselves
were not active toward ethylene polymerization under a pressure
of 2 bar in toluene at 60 ◦C. The combination of 1 and
triethylaluminium did not yield any polymer either. A strong Lewis
acid was needed to activate the precatalyst 1 while the presence
of a trialkylaluminium was a prerequisite for the initiation of
the polymerization. For a better understanding of this catalytic
reaction, we have done a series of ethylene polymerizations using
different zirconium complexes and trialkylaluminiums. The results
are summarized in Table 2. In the presence of a large excess of
MAO, precatalyst 1 gave a polyethylene product of low molecular
weight with a high PDI value (Table 2, entry 1). The bimodal
molecular weight distribution of the GPC trace may explain the
comparatively high PDI value due to two possible active species
for the polymer formation. Compared with MAO, B(C6F5)3 and
Ph3CB(C6F5)4 were better activators, as higher activities and
lower PDI values were obtained using 3a and 3b as the catalysts
(entries 2–3). As a general trend, the highest activities and best
molecular weight control were obtained with Ph3CB(C6F5)4 due
to the non-coordinating anion and its high stability. The binuclear
zirconium complexes 2a and 2b showed higher activities compared
with their mononuclear counterparts 3a and 3b (entries 2–5). Of
the three trialkylaluminiums we used, Et3Al yielded the highest
activity and narrowest molecular weight distribution (entries
5–7). Notably, the activities were improved greatly and molecular
weight distributions narrowed by increasing the concentration
of Et3Al in the reaction system using 2b as the catalyst (entries
End group analysis of the products also gave very useful informa-
tion about the mechanism. When Me3Al was used as the initiator,
the product obtained was mainly linear polyethylene with a vinyl
group at one end. In the cases of bulkier trialkylaluminiums such
as Et3Al, the major product was saturated polyethylene with –
CH3 as both the end groups. When (i-Bu)3Al was used instead, the
product analysis showed that the isobutyl group was incorporated
into polyethylene chain ends in all cases (ESI, Fig. S10–18†). The
major part of the product existed in the form of Al(PE)3 with
polymeric chains which were released during the treatment with a
protic reagent. When D2O was used to terminate the reaction, we
obtained the end-deuterated polymer which was easily confirmed
with the help of 13C NMR spectroscopy showing a triplet peak
due to the coupling of D and 13C (Fig. 5).
Another feature of this catalytic ethylene polymerization is that
the molecular weight of the PE can be controlled by variation
of the reaction time and the concentration of trialkylaluminiums
(for example Et3Al). This offers the opportunity for GC analysis
of the oligomers formed in the presence of a large amount of
trialkylaluminium in a short reaction time. In a typical experiment,
1460 equiv. Et3Al to 2b (13.7 mmol) were used in 100 mL of toluene
under 2 bar of ethylene at 60 ◦C. Liquid samples were taken
at one, two and three minutes of reaction time, hydrolyzed and
analyzed by GC (Fig. 6(a)). The obtained oligomers consisted
of linear alkanes and alkenes of an even number of carbons
in a Schulz–Flory distribution with an average k value of
0.86. Contrary to the b-H elimination process suggested by the
Table 2 Ethylene polymerization results applying different zirconium complexes and trialkylaluminiumsa
Entry
[Zr]
Amount of catalyst/mmol
R3Al
Al/Zr
g(PE)
Activityb
Mn/kDac
PDIc
1
2
3
4
5
6
7
8
9
1
27.5
27.5
27.5
13.7
13.7
13.7
13.7
13.7
13.7
13.7
13.7
MAO
613
36
36
36
36
36
36
18
54
36
36
4.8
3.6
7.9
5.2
9.7
7.7
19.5
4.6
21.4
1.1
1.2
350
390
860
570
1050
840
2130
500
2330
245
1.60
5.58
7.6
13.0
5.09
2.02
4.39
3.98
3.75
356
49
3a
3b
2a
2b
2b
2b
2b
2b
2b
2b
(i-Bu)3Al
(i-Bu)3Al
(i-Bu)3Al
(i-Bu)3Al
Me3Al
Et3Al
Et3Al
Et3Al
(Me3SiCH2)3Al
Me3Al
8.4
4.9
10
4.4
15
3.0
16
2.8
3.9
2.5
10
11d
130
950
a Conditions: 100 mL of toluene, 2 bar of ethylene, 60 ◦C, 10 min. b Catalyst activities in g(PE)/((mmol of Zr) h bar). c Determined by GPC. d 23 ◦C.
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Copolymerization of ethylene with 1-octene
We also studied the amido-bridged zirconium complexes for their
catalytic copolymerization of ethylene with 1-octene. Only two
representative systems (1 with MAO and 2b with Et3Al) were tested
according to their performance in the ethylene polymerization.
The results are summarized in Table 3. The copolymers obtained
were viscous oils with the content of 1-octene up to 30% based
on NMR analysis. The amido-bridged zirconocene complexes’
capacity of incorporation of 1-octene can be compared with that
of the titanium “constrained-geometry” system.12 The molecular
weight of the product obtained with 1 activated by MAO is
relatively low (Mn = 1700) while its distribution is narrow
(Mw/Mn = 1.5). The result may imply that the presence of 1-octene
accelerates the chain transfer or termination. Indeed, end group
analysis showed a considerable amount of unsaturated double
bonds existed as the end groups due to b-H elimination and part
of the chains ended with 1-octene as the last inserted monomer
(ESI, Fig. S21†). Surprisingly, saturated copolymer was obtained
as the major product when 2b was used as the catalyst in the
presence of Et3Al (ESI, Fig. S22†). The copolymer’s relatively
high molecular weight could be due to the slow b-H elimination
termination process during chain propagation.
Fig. 5 13C NMR spectrum of polyethylene obtained by using 2b and
Et3Al, and D2O as the terminating reagent (conditions: 27.5 mmol of 1,
13.7 mmol of Ph3CB(C6F5)4, 1140 equiv. Et3Al, 100 mL of toluene, 2 bar
of ethylene, Tp = 60 ◦C, tp = 1 min).
Cossee–Arlman mechanism, we ascribe this product distribution
mainly to a chain exchange process between the active species and
the large excess of triethylaluminium in the reaction medium. This
can be easily seen from the concentration increase of saturated
alkane oligomers with reaction time while the concentration
increase of alkene products from the b-H elimination process
is negligible. The chain transfer process cannot be very fast,
otherwise we should have obtained a Poisson distribution of the
products as described by the groups of Mortreux and Gibson.11
When Me3Al was used in a two-minute reaction with other
parameters unchanged, the composition of the oligomer product
changed greatly. Although alkanes of an even number of carbons
and alkenes of both even and odd number of carbons were also
present, the major parts were alkene oligomers of an even number
of carbons following a Schulz–Flory distribution with a k value of
0.80 (Fig. 6(b)). b-H elimination was the main chain termination
process.
Concluding remarks
In summary, several new amido-bridged zirconium complexes
were obtained by the treatment of 1 with B(C6F5)3 and
Ph3CB(C6F5)4. They were active for ethylene polymerization and
exhibited good performance in the incorporation of 1-octene, but
trialkylaluminium played a very important role in the catalytic
system. The major products existed as long-chain aluminium
alkyls in the form of Al(PE)3 if the binuclear zirconium complexes
Table 3 Ethylene/1-octene copolymerization using the amido-bridged zirconium complexesa
Amount of
catalyst/mmol
Entry
[Zr]
R3Al
Al/Zr
g(copol)
Activityb
O/O+E (%)c
Mn/kDad
PDId
1
2
1
2b
27.5
13.7
MAO
Et3Al
1300
36
14.6
10.1
3180
2200
30
29
1.7
3.8
1.5
6.5
a Conditions: 50 mL of toluene, 50 mL of 1-octene, 2 bar of ethylene, 60 ◦C, 5 min. b Catalyst activities in g(copolymer)/((mmol of Zr) h bar). c Determined
by 1H NMR spectroscopy. d Determined by GPC.
Fig. 6 Alkane and alkene distributions of oligomers from chain growth reactions. (a) Et3Al was used as the initiator; (b) Me3Al was used as the initiator.
n = carbon number. — solid line represents alkanes, and --- dashed line represents alkenes. Alkenes and alkanes of the same carbon number more than
25 showed one single peak in the GC chromatograph and are omitted here.
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2a and 2b were used as the catalyst. Applying bulkier and more
nucleophilic aluminium such as (i-Bu)3Al with 3a or 3b also led
mainly to saturated polyethylene which was released from the
formed long-chain aluminium alkyls. Although 2a and 2b showed
higher activities than their mononuclear counterparts 3a or 3b,
and 2b was formed from 3b in the presence of (Me3SiCH2)2AlMe,
it would be arbitrary to conclude they are the true active species
at this stage. Anyway, 2a and 2b could be the intermediates and
transformed to the active species in the presence of aluminium
alkyls and an ethylene atmosphere. Consistent with the findings
by Mu¨lhaupt et al.,13 dimers of propagating species might be in a
dormant, resting state, 2a and 2b were possibly responsible for the
formation of long-chain aluminium alkyls.
gas chromatograph with an HP-5 column (30 m ¥ 0.32 mm, film
thickness 0.25 mm) using FID as the detector. Agilent Chemstation
from Agilent Technologies was used to analyze the collected data.
Synthesis of complexes
[((g5,g1,g5-C5H4SiMe2NSiMe2C5H4)Zr)2(l-CH3)][MeB(C6F5)3]
(2a). A solution of B(C6F5)3 (7.0 mg, 13.7 mmol) in 0.4 mL of
C6D6 was added into a solution of 1 (10.0 mg, 27.5 mmol) in
0.2 mL of 1,2-C6H4F2 and the combined solution was transferred
1
to an NMR tube. H NMR (400 MHz, C6D6/1,2-C6H4F2, 298
K): d 6.18, 6.12, 6.08, 5.48 (each m, each 4H, CpH), 1.28 (bs,
3H, BCH3), 0.20, 0.16 (each s, each 12H, Si(CH3)2), -0.52 (s, 3H,
1
ZrCH3Zr). 13C{ H} NMR (100 MHz, C6D6/1,2-C6H4F2, 298 K):
d 121.8, 117.8, 115.3, 114.2, 112.7 (Cp), 0.34, 0.29 (Si(CH3)2),
-9.9 (ZrCH3Zr). 11B NMR (128 MHz, C6D6/1,2-C6H4F2, 298 K):
d -13.99.
Experimental section
General considerations
[((g5,g1,g5 -C5H4SiMe2NSiMe2C5H4)Zr)2 (l-CH3)][B(C6F5)4]
(2b). A solution of Ph3CB(C6F5)4 (12.5 mg, 13.7 mmol) in 0.2 mL
of 1,2-C6H4F2 was added to a solution of 1 (10 mg, 27.5 mmol)
in 0.4 mL of C6D6. The combined solution was transferred to an
All manipulations were carried out under a dry argon atmosphere
using standard Schlenk techniques or in a glovebox. Solvents were
dried and purified with an MBRAUN solvent purification system.
MAO, B(C6F5)3 and Ph3CB(C6F5)4 were purchased and used as
received. 1, 6, 7 and (Me3SiCH2)3Al were prepared according
to the literature procedures.7,14 NMR spectra were recorded on
a Bruker 400 MHz spectrometer. Chemical shifts are reported
1
NMR tube. H NMR (400 MHz, C6D6/1,2-C6H4F2, 298 K): d
6.19, 6.14, 6.04, 5.59 (each m, each 4H, CpH), 0.25, 0.21 (each s,
1
each 12H, Si(CH3)2), -0.51 (s, 3H, ZrCH3Zr). 13C{ H} NMR
(100 MHz, C6D6/1,2-C6H4F2, 298 K): d 122.0, 117.7, 115.6, 113.9,
112.8 (Cp), 0.20, 0.15 (Si(CH3)2), -9.6 (ZrCH3Zr). 11B NMR
(128 MHz, C6D6/1,2-C6H4F2, 298 K): d -15.89. The treatment
of 1 (20 mg, 55 mmol) with Ph3CB(C6F5)4 (25 mg, 27.5 mmol) in
1.0 mL of toluene followed by the addition of 3.0 mL of pentane
led to the precipitation of 2b as white powder which was isolated
by filtration, washed with pentane and dried in vacuum. Yield:
27 mg, 19.4 mmol, 70%. Anal. Calcd. for C53H43BF20N2Si4Zr2: C
45.68, H 3.11, N 2.01. Found: C 45.47, H 3.16, N 1.78%. Single
crystals for X-ray structure analysis were obtained by the slow
diffusion of pentane vapor into a solution of 2b in toluene/1,2-
difluorobenzene at -20 ◦C.
1
in d, referenced to H (of residual protons) and 13C signals of
deuterated solvents as internal standards. Elemental analysis was
performed on a EuroEA3000 series CHNS analyzer. GC analysis
was performed on an Agilent 6890 N chromatographer using FID
as the detector.
X-Ray crystal structure analyses
Data sets were collected with a Rigaku Saturn 70 CCD diffrac-
tometer at 110 K for 2b and 180 K for 3a and 5 with graphite
˚
monochromated Mo Ka radiation (l = 0.71070 A). Absorption
correction was done using the multi-scan technique. Data were
collected and processed using CrystalClear (Rigaku) software.
The structure was solved by direct methods (SHELXS-97) and
refined on F2 against all reflections (SHELXL-97).15 A summary
of crystallographic data can be found in Table 1. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms of C1 in 2b
and all hydrogen atoms in 3a were located from the difference
Fourier maps. Other hydrogen atoms in 2b and 5 were generated
geometrically and allowed to ride on their parent heavy atoms and
refined isotropically. ORTEP diagrams of all the three structures
were drawn with thermal ellipsoids at 30% probability.
[(g5,g1,g5-C5H4SiMe2NSiMe2C5H4)Zr][MeB(C6F5)3] (3a).
A
solution of B(C6F5)3 (256 mg, 0.5 mmol) in 2 mL of toluene
was added into a solution of 1 (182 mg, 0.5 mmol) in 2 mL of
toluene, The product precipitated immediately and was isolated
by filtration and dried under vacuum to give a white powder,
Yield: 360 mg, 0.41 mmol, 82%. Single crystals for X-ray structure
analysis were obtained by the slow diffusion of pentane vapor into
a solution of 3a in 1,2-difluorobenzene at -20 ◦C. Anal. Calcd.
for C33H23BF15NSi2Zr: C 45.21, H 2.64, N 1.60. Found: C 44.75,
H 3.09, N 1.85. 1H NMR (400 MHz, C6D6, 298 K): d 6.23, 5.90,
5.65, 4.94 (each m, each 2H, CpH), 0.49 (bs, 3H, BCH3), 0.12,
1
-0.081 (each s, each 6H, Si(CH3)2). 13C{ H} NMR (100 MHz,
Characterization of polymers
C6D6, 298 K): d 123.4, 119.2, 117.7, 115.4, 113.0 (Cp), 0.45,
-1.0 (Si(CH3)2), the signals of aromatic carbons and the carbon
attached to boron could not be detected because of their com-
plicated coupling with neighboring atoms. 11B NMR (128 MHz,
1H and 13C-NMR experiments of polymers were carried out
in d2-tetrachloroethane at 110 ◦C for polyethylene and in d6-
benzene at room temperature for copolymer on a Bruker 400 MHz
spectrometer. Molecular weights and polydispersities of polymers
were obtained by GPC analysis (Varian PL-GPC 220 equipped
with a triple detection system) with two PLgel Olexis columns
(300 ¥ 7.5 mm, particle si◦ze: 13 mm) with a flow rate of 1 mL
of TCB per minute at 160 C. The measured data were analyzed
with Cirrus program, using polystyrene standards as references.
GC analysis of oligomers was performed on an Agilent 6890A
1
d6-benzene, 298 K): d -13.81. 19F{ H} NMR (376 MHz, C6D6,
298 K): d -133.3, -159.2, -164.3.
[(g5,g1,g5-C5H4SiMe2NSiMe2C5H4)Zr][B(C6F5)4] (3b). 10 mg
of 1 (27.5 mmol) was treated with Ph3CB(C6F5)4 (25 mg, 27.5 mmol)
in 0.4 mL of toluene and 1.0 mL of pentane. The product 3b
formed as an oil at the bottom. The upper layer of solvent was
812 | Dalton Trans., 2010, 39, 807–814
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1
removed and the remaining oil was dissolved in C6D5CD3/1,2-
C6H4F2 and transferred to an NMR tube. 1H NMR spectra were
recorded at 296, 273, and 253 K (see ESI†). 1H NMR (400 MHz,
C6D5CD3/1,2-C6H4F2, 253 K): d 6.12, 5.65, 5.17, 5.04 (each m,
was transferred to an NMR tube and a H NMR spectrum was
recorded. About 30% of 3b was converted to 2b based on a
calculation of the integrations.
The reaction of 3b with 6. 3b was formed similarly by the
addition of 1 (10 mg, 27.5 mmol) in 0.4 mL of C6D6 to a solution
of Ph3CB(C6F5)4 (25.0 mg, 27.5 mmol) in 0.2 mL of 1,2-C6H4F2
followed by stirring at room temperature for 10 min. One equiv.
of 6 was then added. The brown solution decolored immediately
and was then transferred to an NMR tube for 1H NMR spectrum
recording. Nearly all of 3b was converted to 2b. Other aluminium
complexes were also found, but not isolated and identified.
1
each 2H, CpH), 0.27, 0.12 (each s, each 6H, Si(CH3)2). 13C{ H}
NMR (100 MHz, C6D6/1,2-C6H4F2, 253 K): d 121.9, 118.3, 117.8,
116.5, 112.5 (Cp), -0.40, -1.8 (Si(CH3)2). 11B NMR (128 MHz,
C6D6/1,2-C6H4F2, 298 K): d -16.06.
[((g5,l,g1,g5 -C5H4SiMe2NSiMe2C5H4)Zr)(l-CH3)Al(CH3)2]-
[B(C6F5)4] (4). A solution of 1 (10 mg, 27.5 mmol) in 0.4 mL
of C6D6 was added to a solution of Ph3CB(C6F5)4 (25.0 mg,
27.5 mmol) and Me3Al (55 mmol) in 0.2 mL of 1,2-C6H4F2; the
combined solution was transferred to an NMR tube. A trace of 2b
was also detected. The product was precipitated as an oil by adding
pentane, and traces of 2b and Ph3CCH3 were removed by decanting
the upper layer of solution. The oily product was dissolved
in C6D6/1,2-C6H4F2 and an additional one equiv. of Me3Al in
The reaction of 3b with 7. 3b was formed similarly by the
addition of 1 (10 mg, 27.5 mmol) in 0.4 mL of C6D6 to a solution
of Ph3CB(C6F5)4 (25.0 mg, 27.5 mmol) in 0.2 mL of 1,2-C6H4F2
followed by stirring at room temperature for 10 min. Half an equiv.
of 7 was then added. The brown solution decolored immediately
and was then transferred to an NMR tube for 1H NMR spectrum
recording. Nearly all of 3b was converted to 2b.
1
toluene was added. H NMR (400 MHz, C6D6/1,2-C6H4F2, 298
K): d 6.25, 6.15, 5.79, 5.71 (each m, each 2H, CpH), 0.25, 0.04
(each s, each 6H, Si(CH3)2), 0.16 (S, 3H, ZrCH3Al), -0.62 (s, 6H,
1
Al(CH3)2). 13C{ H} NMR (100 MHz, C6D6/1,2-C6H4F2, 298 K):
Catalytic olefin polymerization
d 120.9, 115.8, 115.7, 114.2, 111.1 (Cp), 48.9 (ZrCH3Al), 14.2
(Al(CH3)2). 4.6, 4.2 (Si(CH3)2). 11B NMR (128 MHz, C6D6/1,2-
C6H4F2, 298 K): d -15.85.
Ethylene polymerization. A 300 mL Parr reactor was evacu-
ated and filled with argon three times before it was charged with
100 mL of toluene. Triethylaluminium (2.0 M in toluene, 0.5 mL,
1.0 mmol) was then added. The stirrer was started (600 U min-1)
and the autoclave was thermostated at 60 ◦C while the solution was
saturated with ethylene (2 bar). After the solution was saturated
with ethylene, a certain amount of catalyst in 2 mL of CH2Cl2 or
toluene prepared in a glovebox was introduced directly into the
autoclave by syringe. The polymerization reaction was stopped by
terminating the transfer of ethylene and quenching with methanol.
The precipitated polyethylene was treated with aqueous HCl in
methanol, collected by filtration, and washed with methanol. The
final product was dried in vacuum overnight to constant weight at
50 ◦C.
[(g5,g1,g5-C5H4SiMe2NSiMe2C5H4)Zr(C6F5)](5). The methyl
complex 1 (266 mg, 0.73 mmol) and B(C6F5)3 (187 mg, 0.37 mmol)
were combined in 2 mL of toluene and Me3Al (2.0 M in toluene,
1.8 mL) was then added. The mixture was stirred for 20 min at
room temperature and a clear yellow solution was obtained. All
the volatiles were removed under vacuum and the solid remaining
was washed with pentane (2 ¥ 2 mL) and dried under vacuum
to give a white powder, yield: 312 mg, 0.61 mmol, 83%. Single
crystals for X-ray structure analysis were obtained by the slow
diffusion of pentane vapor into a solution of 5 in toluene at
-20 ◦C. Anal. Calcd. for C20H20BF5NSi2Zr: C 46.68, H 3.90, N
2.71. Found: C 47.06, H 4.12, N 2.75%. 1H NMR (400 MHz, d6-
benzene, 298 K): d 6.13 (m, 4H, CpH), 5.63, 5.58 (each m, each
Copolymerization of ethylene with 1-octene. A 300 mL Parr
reactor was evacuated and filled with argon three times before
it was charged with 50 mL of toluene and 50 mL of 1-octene.
Triethylaluminium (2.0 M in toluene, 0.5 mL, 1.0 mmol) was then
added (30 mL of toluene, 50 mL of 1-octene, 20 mL of MAO in
toluene if 1 was used). The stirrer was◦started (600 U min-1) and
the autoclave was thermostated at 60 C while the solution was
saturated with ethylene (2 bar). After the solution was saturated
with ethylene, 2b (13.7 mmol) was formed in situ by reaction of
10 mg of 1 and 12.5 mg of Ph3[B(C6F5)4] in 5 mL of toluene in a
glovebox and was introduced directly into the autoclave using a
syringe. The polymerization reaction was stopped by terminating
the transfer of ethylene and quenching with methanol after a
polymerization time of 5 min. The reaction mixture was treated
with aqueous HCl and water. The organic layer was separated and
dried in vacuum to constant weight at 50 ◦C.
1
2H, CpH), 0.34, 0.19 (each s, each 6H, Si(CH3)2). 13C{ H} NMR
(100 MHz, d6-benzene, 298 K): d 121.0, 115.1, 114.2, 113.5, 111.1
(Cp), 1.08, 0.47 (Si(CH3)2), the signals of the aromatic carbons
could not be detected because of their complicated coupling with
1
neighboring atoms. 19F{ H} NMR (376 MHz, d6-benzene, 298
1
K): d -106.8, -111.6, -156.9, -160.6, -162.7. 19F{ H} NMR
(376 MHz, d6-benzene, 333 K): d -107.0, -111.8, -157.1, -161.1,
-162.9. The pentafluorophenyl group cannot rotate freely and five
fluoro signals were detected.
(Me3SiCH2)2AlMe. This aluminium compound was formed
directly by the combination of (Me3SiCH2)3Al (0.75 g, 2.6 mmol)
with Me3Al (0.65 mL, 2.0 M in toluene, 1.3 mmol). This resulted
in a 2.6 M toluene solution with a density of 0.835 g ml-1 at room
temperature. 1H NMR (400 MHz, d6-benzene, 298 K): d 0.14 (s,
18H, Si(CH3)3), - 0.03 (s, 3H, AlCH3), - 0.57 (s, 4H, AlCH2).
The reaction of 3b with (Me3SiCH2)2AlMe. 3b was formed by
the addition of 1 (10 mg, 27.5 mmol) in 0.4 mL of C6D6 to a solution
of Ph3CB(C6F5)4 (25.0 mg, 27.5 mmol) in 0.2 mL of 1,2-C6H4F2
followed by stirring at room temperature for 10 min. One equiv.
of (Me3SiCH2)2AlMe was then added. The combined solution
Acknowledgements
We thank the Institute of Chemical and Engineering Sciences
(ICES 05-111004) for financial support. We also greatly appreciate
the help from Dr C.-H. Wang for X-ray crystallographic analysis.
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814 | Dalton Trans., 2010, 39, 807–814
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The Royal Society of Chemistry 2010
©