Silylium-metallocenium dications derived from hydrosilyl-bridged metallocenes and roles in polymerization of polar and nonpolar vinyl monomers

Article abstract of DOI:10.1016/j.jorganchem.2010.02.030

Hydrosilyl-bridged half-metallocene (^HCGC)TiMe₂ (1) [^HCGC = Me(H)Si(η⁵-C₅Me₄)(η¹-N^tBu)] and ansa-metallocene rac-(^HSBI)ZrMe₂ (2) [^HSBI = Me(H)Si(η⁵-indenyl)₂] are doubly activated with 2 equiv of Al(C₆F₅)₃, through methide abstractions from both M-Me bonds, to generate homo-dications 1-Ti²⁺ and 2-Zr²⁺, or with 2 equiv of Ph₃CB(C₆F₅)₄, through unique simultaneous hydride and methide abstractions from the Si-H and M-Me bonds, to produce a novel class of silylium-metallocenium hetero-dications 1-Si⁺Ti⁺ and 2-Si⁺Zr⁺. Analogous chloride-derivatives of hetero-dications can also be produced starting from the dichloride precursors, but the chloride abstraction occurs through a transit silylium ion. An opposite activity trend has been observed for the influence of the hetero-dication formation on polymerizations of polar (MMA) and nonpolar (propylene) vinyl monomers. In MMA polymerization, the activity of the hetero-dications, especially the metallocene-based 2-Si⁺Zr⁺, is substantially lower than the corresponding monocations. In marked contrast, the propylene polymerization activity of hetero-dications has seen 100% and 40% enhancements for 1-Si⁺Ti⁺ and 2-Si⁺Zr⁺, respectively, over their respective monocations.

Full text of DOI:10.1016/j.jorganchem.2010.02.030

Journal of Organometallic Chemistry 695 (2010) 1464–1471
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Silylium–metallocenium dications derived from hydrosilyl-bridged metallocenes
and roles in polymerization of polar and nonpolar vinyl monomers
*
Yuetao Zhang, Eugene Y.-X. Chen
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA
a r t i c l e i n f o
a b s t r a c t
Hydrosilyl-bridged half-metallocene (HCGC)TiMe2 (1) [HCGC = Me(H)Si( 5-C5Me4)( 1-NtBu)] and ansa-
g g
Article history:
metallocene rac-(HSBI)ZrMe2 (2) [HSBI = Me(H)Si( 5-indenyl)2] are doubly activated with 2 equiv of
g
Received 11 February 2010
Accepted 28 February 2010
Available online 3 March 2010
Al(C6F5)3, through methide abstractions from both M–Me bonds, to generate homo-dications 1-Ti2+
and 2-Zr2+, or with 2 equiv of Ph3CB(C6F5)4, through unique simultaneous hydride and methide abstrac-
tions from the Si–H and M–Me bonds, to produce a novel class of silylium–metallocenium hetero-dica-
tions 1-Si+Ti+ and 2-Si+Zr+. Analogous chloride-derivatives of hetero-dications can also be produced
starting from the dichloride precursors, but the chloride abstraction occurs through a transit silylium
ion. An opposite activity trend has been observed for the influence of the hetero-dication formation on
polymerizations of polar (MMA) and nonpolar (propylene) vinyl monomers. In MMA polymerization,
the activity of the hetero-dications, especially the metallocene-based 2-Si+Zr+, is substantially lower than
the corresponding monocations. In marked contrast, the propylene polymerization activity of hetero-
dications has seen 100% and 40% enhancements for 1-Si+Ti+ and 2-Si+Zr+, respectively, over their respec-
tive monocations.
Keywords:
Dication
Double activation
Metallocene catalyst
MMA polymerization
Propylene polymerization
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
otherwise singly activated, cationic catalysts [4]. Reactions with
multiple equiv of the analogous borane B(C6F5)3 or trityl borate
Cationic group 4 metallocene and related complexes [1] have
been widely used as highly active catalysts for coordination-inser-
tion polymerization of nonpolar vinyl monomers (a-olefins in par-
Ph3CB(C6F5)4 did not generate the dicationic complexes. Most re-
cently, we revealed novel living/controlled methyl methacrylate
(MMA) [5] and acrylate [6] polymerizations using a combination
of a silyl ketene acetal (SKA) with a catalytic amount of the ubiqui-
tous olefin polymerization activator Ph3CB(C6F5)4, readily producing
PMMA of low to high number-average molecular weight (Mn > 105)
with narrow molecular weight distributions (MWDs = 1.04–1.12) at
ambient temperature [5]. An intriguing, ‘‘monomer-less” initiation
step involves oxidative activation of dimethylketene methyl tri-
methylsilyl acetal, Me2C@C(OMe)OSiMe3 (MeSKA), by Ph3CB(C6F5)4,
leading to the Me3Si+-activated (silylated) MMA derived from vinyl-
ogous hydride abstraction of MeSKA with Ph3C+; subsequent Michael
addition of MeSKA to the activated MMA affords the highly active,
bifunctional active species containing both nucleophilic SKA and
electrophilic silylium cation catalyst sites, Me3SiOC(O-
Me)@CMeCH2CMe2C(OMe)@Oꢁ ꢁ ꢁSiMe3][B(C6F5)4] [5]. The use of
the borane B(C6F5)3 did not effect hydride abstraction of SKA. A prop-
agation ‘‘catalysis” cycle in the MMA polymerization consists of a
fast step of recapturing the silylium catalyst from the ester group
of the growing polymer chain by the incoming MMA, followed by
a rate-determining step of the C–C bond formation via intermolecu-
lar Michael addition of the polymeric SKA to the silyl cation-acti-
vated MMA. Hence, both activation methodology and fundamental
steps involved in the chain initiation and propagation of the
ticular) [2] and coordination-addition polymerization of polar vinyl
monomers [3]. The continuing interest in the study of such cata-
lysts, typically in their cationic form, is attributed to their high cat-
alytic activity and remarkable ability to precisely control the
stereochemistry of the polymerization and the architecture of the
resulting polymers as well as the ability to produce new classes
of polymeric materials unattainable by other means of polymeriza-
tion [3].
In 2001, one of us discovered that double activation of group 4
metallocene and half-metallocene dimethyl complexes with two
equiv of the highly Lewis acidic alane, Al(C6F5)3, leads to formation
of doubly activated, dicationic metallocene complexes [4]. Signifi-
cantly, such doubly activated complexes show a considerably higher
activity in large-scale runs of ethylene and 1-ocetene copolymeriza-
tions than the singly activated (monocation) analogous. Another
remarkable polymerization feature of the doubly activated catalyst
is its much larger initial polymerization exothermicity (by ꢀ30 °C), a
desirable feature for continuous polymerization processes, than
* Corresponding author. Tel.: +1 970 491 5609; fax: +1 970 491 1801.
E-mail address: eugene.chen@colostate.edu (E.Y.-X. Chen).
0022-328X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.02.030

Y. Zhang, E.Y.-X. Chen / Journal of Organometallic Chemistry 695 (2010) 1464–1471
1465
silylium-catalyzed living polymerization are mechanistically different
from the classic group-transfer polymerization [7], commonly consid-
ered involving reductive activation and associative propagation.
In this contribution, we explored the possibility of generating
novel silylium–metallocenium hetero-dicationic catalysts based
on hydrosilyl-bridged [Me(H)Si<] ansa-metallocene and half-
metallocene dimethyl complexes. We reasoned that such com-
plexes bearing both M–Me and Si–H ligands can be activated by
Ph3CB(C6F5)4, which is known for its ability to abstract the methide
group from the metallocene M–Me bond [8] and the hydride from
the silyl Si–H bond [9]. The end result would be the formation of a
novel class of hetero-dicationic metallocene complexes consisting
of the Si+M+ (M = Ti, Zr) core fragment, generated through simulta-
neous hydride and methide abstractions by the potent abstractor
Ph3CB(C6F5)4. Overall, the motivation of this study is twofold: first,
we are driven to develop a new class of silylium–metallocenium
hetero-dicationic catalysts; second, we are interested in knowing
how such dications would affect polymerization of polar and non-
polar vinyl monomers, specifically MMA and propylene.
with the unsolvated form, because of its thermal and shock sensi-
tivity. Dimethylketene methyl trimethylsilyl acetal (MeSKA), 1,2,
3,4-tetramethyl-1,3-cyclopentadiene, diisopropylamine (P99%),
methyl isobutyrate, chlorotrimethylsilane, dichloromethylsilane,
and isopropyl isobutyrate were purchased from Aldrich and dried
over CaH2, followed by vacuum distillation, whereas ZrCl4, TiCl4
(in CH2Cl2), indene, nBuLi (1.6 M in hexanes), and MeMgBr (3.0 M
in diethyl ether) were purchased from Aldrich and used as re-
ceived. Literature procedures were employed for the preparation
of the following compounds and metallocene complexes:
ZrCl4ꢁ(THF)2 [15], (CGC)TiMe2 [16] [CGC = Me2Si(
g
5-C5Me4)
((g g
1-NtBu)], rac-(SBI)ZrMe2 [17] [SBI = Me2Si( 5-indenyl)2], rac-
(SBI)ZrMe+MeMðC6F5Þ3ꢂ (M = B, Al) [4], rac-(HSBI)ZrCl2 (4) [HSBI =
Me(H)Si(
5-C5Me4)( 1-NtBu)] [19], and (HCGC)TiMe2 (1) [19].
g
g
5-indenyl)2] [18], (HCGC)TiCl2 (3) [HCGC = Me(H)Si
(g
2.2. Preparation of rac-(HSBI)ZrMe2 (2)
In a glovebox, MeMgBr (0.48 mL, 3.0 M in Et2O, 1.44 mmol) was
added to a solution of rac-(HSBI)ZrCl2 (4) (0.25 g, 0.58 mmol) in
Et2O (25 mL) at room temperature. The mixture was stirred over-
night, after which all volatiles were removed in vacuo. Toluene
(ꢀ50 mL) was added to the residue, and the resulting precipitates
were filtered through a pad of Celite. The solvent of the filtrate
was removed in vacuo to afford 0.19 g (84 %) of the pure title com-
plex as a brown powder. Anal. Calc. for C21H22SiZr: C, 64.06; H,
5.63. Found: C, 63.78; H, 5.57%.
2. Experimental
2.1. Materials and methods
All syntheses and manipulations of air- and moisture-sensitive
materials were carried out in flamed Schlenk-type glassware on a
dual-manifold Schlenk line, on a high-vacuum line (typically from
10ꢂ5 to 10ꢂ7 Torr), or in an argon-filled glovebox (typically
<1.0 ppm oxygen). NMR-scale reactions (typically in a 0.02 mmol
scale) were conducted in Teflon-valve-sealed J. Young-type NMR
tubes. HPLC grade organic solvents were first sparged extensively
with nitrogen during filling 20 L solvent reservoirs and then dried
by passage through activated alumina (for Et2O, THF, and CH2Cl2)
followed by passage through Q-5 supported copper catalyst (for
toluene and hexanes) stainless steel columns. Benzene-d6, and tol-
uene-d8 were dried over sodium/potassium alloy and vacuum-dis-
tilled or filtered, whereas C6D5Br, CD2Cl2, and CDCl3 were dried
over activated Davison 4 Å molecular sieves. NMR spectra were re-
corded on either a Varian Inova 300 (FT 300 MHz, 1H; 75 MHz, 13C;
282 MHz, 19F) or a Varian Inova 400 spectrometer. Chemical shifts
for 1H and 13C spectra were referenced to internal solvent reso-
nances and are reported as parts per million relative to SiMe4,
whereas 19F NMR spectra were referenced to external CFCl3. Ele-
mental analyses were performed by Robertson Microlit Laborato-
ries, Madison, NJ.
1H NMR (CD2Cl2, 300 MHz, 23 °C): d 7.65–7.62 (m, 2H, C6 ring),
7.45–7.42 (m, 1H, C6 ring), 7.38–7.35 (m, 1H, C6 ring), 7.30–7.25
(m, 2H, C6 ring), 7.04–6.97 (m, 2H, C6 ring), 6.86 (d, J = 3.3 Hz,
1H, C5 ring), 6.82 (d, J = 3.3 Hz, 1H, C5 ring), 5.94 (d, J = 3.3 Hz,
1H, C5 ring), 5.93 (d, J = 3.3 Hz, 1H, C5 ring), 5.44 (q, J = 3.9 Hz,
1H, SiH), 1.01 (d, J = 3.9 Hz, 3H, SiMe), ꢂ1.41 (s, 3H, ZrMe), ꢂ1.42
(s, 3H, ZrMe). 1H NMR (C6D6, 400 MHz, 23 °C): d 7.38–7.55 (m,
2H, C6 ring), 7.23 (d, J = 8.4 Hz, 1H, C6 ring), 7.09–7.03 (m, 3H, C6
ring), 6.79–6.71 (m, 2H, C6 ring), 6.61 (d, J = 3.2 Hz, 1H, C5 ring),
6.58 (d, J = 3.2 Hz, 1H, C5 ring), 5.72 (d, J = 3.2 Hz, 1H, C5 ring),
5.57 (d, J = 3.6 Hz, 1H, C5 ring), 5.26 (q, J = 4.0 Hz, 1H, SiH), 0.50
(d, J = 4.0 Hz, 3H, SiMe), ꢂ1.04 (s, 3H, ZrMe), ꢂ1.05 (s, 3H, ZrMe).
13C NMR (C6D6, 100 MHz, 23 °C): d 131.1, 130.6, 128.5, 128.3,
126.8, 126.6, 126.5, 126.2, 125.5, 125.4, 125.0, 124.9 (C6 ring),
119.3, 117.5, 112.1, 111.5, 80.78, 80.22 (C5 ring), 38.27, 37.73
(ZrMe2), ꢂ5.22 (SiMe).
2.3. Activation of (HCGC)TiMe2 (1) with 1, 2, and 3 equiv of Al(C6F5)3
Methyl methacrylate (MMA; Aldrich Chemical Co.) was de-
gassed and dried over CaH2 overnight, distilled under reduced
pressure, titrated with neat tri(n-octyl)aluminum (Strem Chemi-
cals) to a yellow end point [10], and finally distilled under
reduced pressure. The purified monomer was stored in a brown
bottle inside a ꢂ30 °C glovebox freezer. Propylene (polymer purity,
Metheson) was purified by passage through the mixed stainless
steel column of the activated alumina and Q-5 supported copper
catalyst. Butylated hydroxytoluene (BHT-H, 2,6-di-tert-butyl-4-
methylphenol, Aldrich Chemical Co.) was recrystallized from hex-
anes prior to use. Tris(pentafluorophenyl)borane B(C6F5)3 [11]
and trityl tetrakis(pentafluorophenyl)borate Ph3CB(C6F5)4 [12]
were obtained as a research gift from Boulder Scientific Co.,
B(C6F5)3 was purified by recrystallization from hexanes at ꢂ30 °C,
whereas Ph3CB(C6F5)4 was used as received. Tris(pentafluorophe-
These reactions were carried out by the same manner as de-
scribed for the reaction with the dimethylsilyl derivative
(CGC)TiMe2 [4]. The reaction of 1 with 1 equiv of Al(C6F5)3 in tolu-
ene-d8 at room temperature cleanly generates the corresponding
l
-methyl bridged ion pair, (HCGC)TiMe( -Me)Al(C6F5)3 (1a-Ti+
l
and 1b-Ti+) as a yellow solution, which can be readily character-
ized by NMR as two isomers in a 2:3 ratio. The isolated product
in quantitative yield was spectroscopically and analytically pure.
Anal. Calc. for C34H31AlF15NSiTi: C, 48.53; H, 3.71; N, 1.66. Found:
C, 48.33; H, 3.56; N, 1.57%.
1H NMR (toluene-d8, 300 MHz, 23 °C): d 5.21, 5.14 (q, J = 3.0 Hz,
1H, SiH), 1.71, 1.75 (s, 3H, CpMe), 1.68, 1.44 (s, 3H, CpMe), 1.59,
1.55 (s, 6H, CpMe), 1.13, 1.12 (s, 9H, NtBu), 0.83, 0.80 (s, 3H, TiMe),
0.22, 0.32 (d, J = 3.0 Hz, 3H, SiMe), 0.08, 0.15 (s, br, 3H, Al-l F
-Me). 19
NMR (toluene-d8, 282 MHz, 23 °C): d ꢂ122.7 (m, 6F, o-F of C6F5),
nyl)alane Al(C6F5)3 [13], as a 0.5 toluene adduct Al(C6F5)3ꢁ(C7H8)0.5
,
ꢂ153.9 (t, J = 19.9 Hz, 3F, p-F of C6F5), ꢂ161.8 (m, 6F, m-F of
was prepared by the reaction of B(C6F5)3 and AlMe3 in a 1:3
toluene/hexanes solvent mixture in quantitative yield [14].
Although we have experienced no incidents when handling this
material, extra caution should be exercised, especially when dealing
C6F5). 13C NMR (toluene-d8, 100 MHz, 23 °C): d 150.3 (d, JC–F
=
235.0 Hz), 141.7 (d, JC–F = 249.5 Hz), 137.3 (d, JC–F = 236.8 Hz), and
116.5 (s, br) for C6F5 groups, 141.6, 141.1, 139.1, 138.7, 137.9,
137.8, 136.3, 135.8, 101.8, and 100.8 for C5Me4, 68.54, 66.26 (TiMe),

1466
Y. Zhang, E.Y.-X. Chen / Journal of Organometallic Chemistry 695 (2010) 1464–1471
62.47, 62.42 (NCMe3), 32.64 (NCMe3), 19.47 (s, br, Al-
14.15, 13.84, 12.68, 11.84, 11.73, 11.48, and 11.24 for C5Me4, 1.24,
1.08 (SiMe).
l
-Me), 15.57,
(HCGC)+TiMe+½BðC6F5Þ4ꢃ2ꢂ (1-Si+Ti+), a result of both methyl and hy-
dride abstractions from the Ti–Me and Si–H bonds. This reaction
was repeated directly in CD2Cl2, obtaining the same result. The iso-
lated dication is unstable at room temperature and thus not suit-
able for elemental analysis. 1H NMR for Ph3CH and Ph3CMe
(toluene-d8, 300 MHz, 23 °C): d 2.01 (s, 3H, Ph3CMe), 5.37 (s, 1H,
Ph3CH), 6.98–7.10 (m, 30H, Ph3CH and Ph3CMe). 1H NMR for
(CGC)+TiMe+½BðC6F5Þ4ꢃꢂ2 (CD2Cl2, 300 MHz, 23 °C): d 2.61 (s, 3H,
CpMe), 2.56 (s, 3H, CpMe), 2.49 (s, 3H, CpMe), 2.14 (s, 3H, CpMe),
1.50 (s, 9H, NtBu), 1.03 (s, 3H, TiMe), 0.91 (s, 3H, SiMe). 19F NMR
(CD2Cl2, 282 MHz, 23 °C): d ꢂ131.5 (d, J = 11.0 Hz, 8F, o-F of
C6F5), ꢂ161.7 (t, J = 20.3 Hz, 4F, p-F of C6F5), ꢂ165.6 (t,
J = 18.2 Hz, 8F, m-F of C6F5).
Activation of 1 with 2 equiv of Al(C6F5)3 cleanly affords the
corresponding dicationic complex, (HCGC)Ti[(
l-Me)Al(C6F5)3]2
(1-Ti2+), as an orange-yellow solution, which can be readily charac-
terized by NMR. The isolated dication is unstable at room temper-
ature and thus not suitable for elemental analysis. 1H NMR
(toluene-d8, 300 MHz, 23 °C): d 5.17 (q, J = 3.0 Hz, 1H, SiH), 1.67
(s, 3H, CpMe), 1.65 (s, 3H, CpMe), 1.60 (s, 3H, CpMe), 1.51 (s, 3H,
CpMe), 1.13 (s, 9H, NtBu), 0.53 (s, br. 3H, AlMe), 0.41 (s, br. 3H,
AlMe), 0.28 (d, J = 3.0 Hz, 3H, SiMe). 19F NMR (toluene-d8,
282 MHz, 23 °C): d ꢂ122.9 (m, 6F, o-F of C6F5), ꢂ152.4 (m, 3F, p-F
of C6F5), ꢂ161.3 (m, 6F, m-F of C6F5). 13C NMR (toluene-d8,
100 MHz, 23 °C):
d
150.2 (d, JC–F
=
234.9 Hz), 142.0 (d,
2.6. Activation of rac-(HSBI)ZrMe2 (2) with 1, 2, and 3 equiv of
JC–F = 249.5 Hz), 137.3 (d, JC–F = 249.6 Hz), and 114.6 (s, br) for
Al(C6F5)3
C6F5 groups, 141.2, 140.2, 137.9, 137.4, 101.6 for C5Me4, 62.87
(NCMe3), 32.64 (NCMe3), 32.24, 31.75 (s, br, Al-
13.64, 11.73, 11.46 for C5Me4, 1.10 (SiMe).
Activation of 1 with 3 equiv of Al(C6F5)3 forms the same dication
as above, leaving 1 equiv of Al(C6F5)3 unreacted. This result shows
that the alane does not abstract the hydride from the bridging
hydrosilyl group.
l
-Me), 14.61,
These reactions were carried out by the same manner as de-
scribed for the reaction with the dimethylsilyl derivative
rac-(SBI)ZrMe2 [4]. The reaction of 2 with 1 equiv of Al(C6F5)3 in
toluene-d8 at room temperature cleanly generates the correspond-
ing l l-Me)Al(C6F5)3 (2a-
-methyl bridged ion pair, rac-(HSBI)ZrMe(
Zr+ and 2b-Zr+)), as yellow solution, which can be readily
characterized by NMR as two isomers in a 1:1 ratio. The isolated
product in quantitative yield was spectroscopically and analyti-
cally pure. Anal. Calc. for C39H22AlF15SiZr: C, 50.81; H, 2.41. Found:
C, 51.05; H, 2.53%.
2.4. Activation of (HCGC)TiMe2 (1) with 1, 2, and 3 equiv of B(C6F5)3
These reactions were carried out by the same manner as de-
scribed for the reaction with the dimethylsilyl derivative
(CGC)TiMe2 [4]. The reaction of 1 with 1 equiv of B(C6F5)3 in tolu-
ene-d8 at room temperature cleanly generates the corresponding
1H NMR (toluene-d8, 300 MHz, 23 °C): d 7.50, 7.53 (d, 1H, C6
ring), 6.33–7.19 (m, 9H, Ind), 5.57, 5.66 (d, J = 3.3 Hz, 1H, C5 ring),
5.01, 5.16 (d, J = 3.3 Hz, 1H, C5 ring), 4.96, 5.13 (q, J = 3.9 Hz, 1H,
SiH), 0.36, 0.43 (d, J = 3.9 Hz, 3H, SiMe), ꢂ0.77, ꢂ0.76 (s, 3H, ZrMe),
ꢂ1.00, ꢂ0.99 (s, 3H, AlMe). 19F NMR (toluene-d8, 282 MHz, 23 °C): d
ꢂ122.7 (m, 6F, o-F of C6F5), ꢂ153.9 (m, 3F, p-F of C6F5), ꢂ161.7 (m,
6F, m-F of C6F5). 13C NMR (toluene-d8, 100 MHz, 23 °C): d 150.3 (d,
JC–F = 231.3 Hz), 141.5 (d, JC–F = 249.5 Hz), 137.2 (d, JC–F = 251.3 Hz),
and 117.3 (s, br) for C6F5 groups, 137.8, 137.2, 132.7, 132.3, 132.1,
131.8, 128.4, 127.6, 127.5, 127.3, 127.2, 126.8, 126.4, 126.3, 125.8,
125.6, 124.0, 123.5, 122.1, 120.0, 118.6, 117.1, 115.8, 114.7, 112.9,
112.2, 83.56, 82.96, 82.87, 82.36 for Ind, 49.65, 49.03 (ZrMe), 6.07
(Zr-Me-Al), ꢂ6.08, ꢂ7.01 (SiMe).
l
-methyl bridged ion pair, (HCGC)TiMe(
l-Me)B(C6F5)3, as a yel-
low-green solution, which can be readily characterized by NMR
as two isomers in a 2:3 ratio. The isolated product in quantitative
yield was spectroscopically and analytically pure. Anal. Calc. for
C34H31BF15NSiTi: C, 49.48; H, 3.79; N, 1.70. Found: C, 50.07; H,
3.81; N, 1.54%.
1H NMR (toluene-d8, 300 MHz, 23 °C): d 5.08, 5.17 (q, J = 3.0 Hz,
1H, SiH), 1.77, 1.72 (s, 3H, CpMe), 1.55, 1.53 (s, 3H, CpMe), 1.46,
1.49 (s, 3H, CpMe), 1.44, 1.43 (s, 3H, CpMe), 1.01, 0.98 (s, 9H, NtBu),
0.95, 0.93 (s, 3H, TiMe), 0.61, 0.68 (s, br. 3H, BMe), 0.16, 0.30 (d,
J = 3.0 Hz, 3H, SiMe). 19F NMR (toluene-d8, 282 MHz, 23 °C): d
ꢂ133.5 (m, 6F, o-F of C6F5), ꢂ159.1 (m, 3F, p-F of C6F5), ꢂ164.3
(m, 6F, m-F of C6F5). 13C NMR (toluene-d8, 100 MHz, 23 °C): d
148.73 (d, JC–F = 233.1 Hz), 139.6 (d, JC–F = 245.9 Hz), 137.5 (d,
JC–F = 235.5 Hz), and 123.1 (s, br) for C6F5 groups, 141.5, 140.7,
139.6, 139.3, 139.1, 138.9, 136.9, 136.4, 103.4, and 102.3 for
C5Me4, 68.77, 66.33 (TiMe), 63.33, 63.22 (NCMe3), 32.29, 32.26
(NCMe3), 15.72, 14.32, 13.39, 12.10, 11.90, 11.77, 11.46, 11.20 for
C5Me4, 1.19, 0.89 (SiMe).
Activation of 2 with 2 equiv of Al(C6F5)3 cleanly affords the cor-
responding dicationic complex, rac-(HSBI)Zr[(
l-Me)Al(C6F5)3]2 (2-
Zr2+), as a red solution, which can be readily characterized by
NMR as a single isomer. The isolated dication decomposes in
30 min at room temperature and is not suitable for elemental anal-
ysis. 1H NMR (toluene-d8, 300 MHz, 23 °C): d 7.33 (s, br. 1H, Ind),
6.85 (d, J = 8.7 Hz, 1H, Ind), 6.62–6.65 (m, 1H, Ind), 6.74 (d,
J = 8.7 Hz, 1H, Ind), 6.51–6.56 (m, 6H, Ind), 5.41 (s, br. 1H, C5 ring),
5.05 (q, J = 3.9 Hz, 1H, SiH), 0.40 (d, J = 3.9 Hz, 3H, SiMe), –0.80 (s,
6H, AlMe). 19F NMR (toluene-d8, 282 MHz, 23 °C): d ꢂ122.8 (m,
6F, o-F of C6F5), ꢂ152.1 (m, 3F, p-F of C6F5), ꢂ161.2 (m, 6F, m-F
of C6F5).
Activation of 1 with 2 or 3 equiv of B(C6F5)3 affords the same ion
pair as above, leaving 1 or 2 equiv of B(C6F5)3 unreacted. This result
shows that the borane abstracts neither the second Zr–Me group
from the mono-cation nor the hydride from the bridging hydrosilyl
group.
Activation of 2 with 3 equiv of Al(C6F5)3 forms the same dication
as above, leaving 1 equiv of Al(C6F5)3 unreacted. This result shows
again that the alone does not abstract the hydride from the bridg-
ing hydrosilyl group.
2.5. Activation of (HCGC)TiMe2 (1) with 2 equiv of Ph3CB(C6F5)4
The NMR reaction of 1 and Ph3CB(C6F5)4 in 0.7 mL of toluene-d8
in a 1:2 ratio (0.02 mmol scale) was carried out in a J-Young NMR
tube, the sample being loaded into the NMR tube in a glovebox.
The mixture was allowed to react for 20 min at room temperature
before the NMR spectra were recorded. A solution and red oily pre-
cipitates were observed immediately upon mixing of the reagents.
The NMR spectra of the solution show the formation of Ph3CH and
Ph3CMe, while the NMR spectra of the red oil upon dissolution in
CD2Cl2 indicate the formation of the dicationic complex
2.7. Activation of rac-(HSBI)ZrMe2 (2) with 2 equiv of Ph3CB(C6F5)4
The NMR reaction of 2 and Ph3CB(C6F5)4 in 0.7 mL of toluene-d8
in a 1:2 ratio (0.02 mmol scale) was carried out in a J-Young NMR
tube, the sample being loaded into the NMR tube in a glovebox.
The mixture was allowed to react for 20 min at room temperature
before the NMR spectra were recorded. A solution and red oily pre-
cipitates were observed immediately upon mixing of the reagents.
The NMR spectra of the solution show the formation of Ph3CH and

Y. Zhang, E.Y.-X. Chen / Journal of Organometallic Chemistry 695 (2010) 1464–1471
1467
Ph3CMe, and the repeated reaction in CD2Cl2 indicates the forma-
tion of the dicationic complex (HSBI)+ZrMe+½BðC6F5Þ4ꢃꢂ2 (2-Si+Zr+)),
a result of both methyl and hydride abstractions from the Zr–Me
and Si–H bonds. The isolated dication is unstable at room temper-
ature and thus not suitable for elemental analysis. 1H NMR (CD2Cl2,
300 MHz, 23 °C) for most diagnostic resonances due to limited sol-
ubility and stability: d 0.20 (s, ZrMe), 1.67 (s, SiMe), 2.17 (s,
Ph3CMe), 5.54 (s, Ph3CH), 7.07–7.33 (m, Ph3CH and Ph3CMe). 19F
NMR (CD2Cl2, 282 MHz, 23 °C): d ꢂ131.3 (s, br, 8F, o-F of C6F5),
ꢂ161.8 (t, J = 20.3 Hz, 4F, p-F of C6F5), ꢂ165.7 (t, J = 18.2 Hz, 8F,
m-F of C6F5).
val, the polymerization was quenched by addition of 5 mL of 5%
HCl-acidified methanol. The quenched mixture was precipitated
into 100 mL of methanol, stirred for 1 h, filtered, washed with
methanol, and dried in a vacuum oven at 50 °C overnight to a con-
stant weight.
Propylene polymerizations were performed in 100 mL oven-
and flame-dried Schlenk flasks that was charged with 20 mL
solvent (CH2Cl2 or toluene) and the desired amounts of the pre cat-
alyst (10 lmol) in glovebox and then interfaced to the high-vac-
uum line. The flask was degassed at ꢂ78 °C and warmed to room
temperature. The propylene feed (15 psi) was introduced into this
rapidly stirred flask with an external temperature bath equili-
brated at the desired polymerization temperature, followed by
addition of a solution of Ph3CB(C6F5)4 via a gas-tight syringe to
start the polymerization. The pressure of propylene during the
polymerization was regulated at 1.0 atm throughout the polymer-
ization. The polymerization was quenched and the polymer prod-
uct was isolated using the above procedure described for the
MMA polymerization.
2.8. Activation of (HCGC)TiCl2 (3) with Ph3CB(C6F5)4
NMR-scale reactions were carried out in J-Young NMR tubes,
the samples being loaded into the NMR tubes in a glove box after
mixing the two reagents in 0.7 mL of CD2Cl2 in a 1:0.5 ratio
(0.02 mmol scale). The mixture was allowed to react at room tem-
perature overnight before the NMR spectra were recorded. A yel-
low solution was observed, and the NMR data are consistent
with the formation of Ph3CH, (ClCGC)TiCl2 (5) [19], and
(HCGC)TiCl+[B(C6F5)4]ꢂ (3-Ti+). 1H NMR for 5 (CD2Cl2, 300 MHz,
23 °C): d 2.30 (s, 3H, CpMe), 2.28 (s, 3H, CpMe), 2.27 (s, 3H, CpMe),
2.18 (s, 3H, CpMe), 1.48 (s, 9H, NtBu), 1.07 (s, 3H, SiMe). 1H NMR for
3-Ti+ (CD2Cl2, 300 MHz, 23 °C): d 5.53 (d, J = 3.9 Hz, 1H, SiH), 2.59
(s, 3H, CpMe), 2.54 (s, 3H, CpMe), 2.46 (s, 3H, CpMe), 2.39 (s, 3H,
CpMe), 1.59 (s, 9H, NtBu), 1.06 (d, J = 3.9 Hz, 3H, SiMe). 19F NMR
(CD2Cl2, 282 MHz, 23 °C): d ꢂ131.3 (s, br, 8F, o-F of C6F5), ꢂ161.8
(t, J = 20.3 Hz, 4F, p-F of C6F5), ꢂ165.7 (t, J = 17.5 Hz, 8F, m-F of
C6F5).
The reaction of 3 and 1 equiv of Ph3CB(C6F5)4 was carried out in
the same manner as described in the above reaction. A light yellow
solution was observed and the NMR data are consistent with the
formation of Ph3CH, 5 [19], 3-Ti+, and (HCGC)+TiCl+½BðC6F5Þ4ꢃꢂ2 (3-
Si+Ti+). 1H NMR for 3-Si+Ti+ (CD2Cl2, 300 MHz, 23 °C): d 2.66 (s,
3H, CpMe), 2.61 (s, 3H, CpMe), 2.48 (s, 3H, CpMe), 2.42 (s, 3H,
CpMe), 1.67 (s, 9H, NtBu), 1.54 (s, 3H, SiMe). 19F NMR (CD2Cl2,
282 MHz, 23 °C): d ꢂ131.4 (s, br, 8F, o-F of C6F5), –161.9 (t,
J = 20.3 Hz, 4F, p-F of C6F5), ꢂ165.8 (t, J = 17.5 Hz, 8F, m-F of C6F5).
2.11. Polymer characterizations
Molecular weights and molecular weight distributions of PMMA
were measured by gel permeation chromatography (GPC) analyses
carried out at 40 °C and a flow rate of 1.0 mL/min, with CHCl3 as
the eluent on a Waters University 1500 GPC instrument equipped
with four 5 lm PL gel columns (Polymer Laboratories). The instru-
ment was calibrated with 10 PMMA standards, and chromato-
grams were processed with Waters Empower software (version
2002); number-average molecular weight (Mn) and molecular
weight distribution (Mw/Mn) of the polymers were given relative
to PMMA standards. 1H NMR spectra for the analysis of PMMA
microstructures were recorded in CDCl3 and analyzed according
to the literature [20].
3. Results and discussion
3.1. Mono- and di-cations from activation with E(C6F5)3 (E = B, Al)
The reaction of (HCGC)TiMe2 (1) with 1 equiv of Al(C6F5)3
cleanly generates the corresponding cationic complex as two ste-
reo-isomers, 1a-Ti+ and 1b-Ti+ (Scheme 1), a result of the asym-
metric substitution at the bridging Si. On the other hand, the
reaction of 1 with 2 equiv of Al(C6F5)3 affords the corresponding
2.9. Activation of rac-(HSBI)ZrCl2 (4) with Ph3CB(C6F5)4
NMR reactions were carried out in J-Young NMR tubes, the sam-
ples being loaded into the NMR tubes in a glovebox after mixing
the two reagents in 0.7 mL of CD2Cl2 in a 1:1 ratio (0.02 mmol
scale). The mixture was allowed to react at room temperature
overnight before the NMR spectra were recorded. A solution and
red precipitates were formed. Only the Ph3CH resonances can be
seen in the NMR due to the insolubility of the precipitates. 1H
NMR (CD2Cl2, 300 MHz, 23 °C): d 7.11–7.32 (m, 15H, Ph3CH), 5.55
(s, 1H, Ph3CH).
2Al(C6F5)3
Me
H
Me
H
Ti
Ti
Si
Si
Me
Me
Si
Si
MeAl(C6F5)3
MeAl(C6F5)3
N
N
2.10. General polymerization procedures
2+
1
1
-Ti
Al(C6F5)3
MMA polymerizations were performed either in 30 mL, oven-
dried glass reactor inside the glovebox, or in 25 mL oven- and
flame-dried Schlenk flasks interfaced to the dual-manifold Schlenk
line. Two different polymerization procedures were employed for
comparative studies. In the pre-activation methodology (method
A), the precatalyst (metallocene dimethyl complexes) and the acti-
vator were premixed in a predetermined ratio in toluene or CH2Cl2
and stirred for 10 min (to generate in situ the active species), fol-
lowed by addition of monomer to start the polymerization. In the
in-reactor activation methodology (method B), the precatalyst
and the monomer were premixed and the polymerization was
started by addition of the activator. After the measured time inter-
Me
H
Me
H
Ti
Ti
+
Me
MeAl(C6F5)3
N
N
Me
MeAl(C6F5)3
+
+
1a
1b
-Ti
-Ti
Scheme 1. Generation of (HCGC)Ti-based mono- and di-cations via activation with
Al(C6F5)3.

1468
Y. Zhang, E.Y.-X. Chen / Journal of Organometallic Chemistry 695 (2010) 1464–1471
dicationic complex 1-Ti2+ (Scheme 1). Addition of excess Al(C6F5)3
does not effect hydride abstraction from the hydrosilyl bridging
group in 1. Furthermore, unlike Al(C6F5)3, multiple equiv of
B(C6F5)3 does not effect the methide abstraction from the second
Ti–Me bond, producing only the monocations, analogous to 1a-
Ti+ and 1b-Ti+ (see Section 2). Overall, these activation chemistry
results are the same as those seen for the parent (CGC)TiMe2 acti-
vated with one or multiple equiv of E(C6F5)3, [4] except for the two
isomers observed for the monocation derived from 1.
2Ph3CB(C6F5)4
Me
H
Me
B(C6F5)4
Me
Ti
Ti
Si
Si
Me
Me
Si
B(C6F5)4
Ph CMe
3
-
-
Ph3CH
N
N
+
+
1
1
-Si Ti
Likewise, activation of (HSBI)ZrMe2 (2) with 1 equiv of Al(C6F5)3
cleanly produces the corresponding cationic complex also as two
isomers, 2a-Zr+ and 2b-Zr+, whereas activation with 2 equiv of
Al(C6F5)3 generates the corresponding dicationic complex 2-Zr2+
(Scheme 1). Activation of 2 with 1 or 2 equiv of B(C6F5)3 forms
the same monocation as two isomers, analogous to 2a-Zr+ and
2b-Zr+ (see Section 2). These results are also consistent with the
activation of the parent (SBI)ZrMe2 using 1 or multiple equiv of
E(C6F5)3 [4]. Again, addition of excess Al(C6F5)3 does not effect hy-
dride abstraction from the hydrosilyl bridging group in 2 (see
Scheme 2).
2Ph3CB(C6F5)4
Me
H
B(C6F5)4
Me
Me
Me
Me
Si
Zr
Zr
Ph CMe
3
-
-
Ph3CH
B(C6F5)4
+
+
2
2
-Si Zr
Scheme 3. Generation of silylium–metallocenium (Si+M+) dications with 2 equiv of
Ph3CB(C6F5)4.
[B(C6F5)4]ꢂ (3-Ti+, Scheme 4). As outlined in Scheme 4, the reaction
is proposed to proceed with the transit silyl cation, 3-Si+, with con-
comitant formation of Ph3CH, and subsequent chloride abstraction
from another 0.5 equiv of 3 by 3-Si+ gives both 5 and 3-Ti+. Accord-
ingly, the reaction of 3 with 1 equiv of Ph3CB(C6F5)4 further
converts 3-Ti+ to dication 3-Si+Ti+. Activation of rac-(HSBI)ZrCl2
(4) with Ph3CB(C6F5)4 in CD2Cl2 gave a solution containing Ph3CH
and red precipitates. The reaction is assumed to follow the
same pathway forming the corresponding dicationic complex,
but the insolubility of this dication hampered its NMR
characterizations.
3.2. Hetero-dications from activation with Ph3CB(C6F5)4
As strong Lewis acids E(C6F5)3 did not effect the hydride
abstraction from the bridging hydrosilyl group, next we examined
the oxidative cleavage pathway enabled by Ph3CB(C6F5)4 to poten-
tially generate hetero-dications on the bridging Si and the metal
center. Excitingly, treatment of 1 with 2 equiv of Ph3CB(C6F5)4 in
toluene-d8 resulted in spontaneous formation of a red solution
accompanied by red oily precipitates; the solution phase is readily
shown to be Ph3CH and Ph3CMe, and NMR data (see Experimental)
of the red precipitates, upon dissolution in CD2Cl2, indicate the for-
mation of the dicationic complex 1-Si+Ti+ (Scheme 3), a result of
both methide and hydride abstractions from the Ti–Me and Si–H
bonds in 1. The same products were formed when the reaction
was carried out directly in CD2Cl2. This methodology can be ex-
tended to hydrosilyl-bridged zirconocene 2, the reaction of which
with 2 equiv of Ph3CB(C6F5)4 likewise forms the corresponding
dicationic complex 2-Si+Zr+ (Scheme 3), with concomitant forma-
tion of Ph3CH and Ph3CMe co-products.
3.3. MMA polymerization
Table 1 summarizes the results of the MMA polymerization by
(HCGC)TiMe2 activated with 1 and 2 equiv of Ph3CB(C6F5)4 in a
fixed [MMA]:[catalyst] ratio = 400, but in different activation
methods (pre-activation and in-reactor activation), solvents (tolu-
ene and CH2Cl2), and temperature (25 °C and 80 °C). A control run
with the B(C6F5)3 activation at 25 °C in toluene achieved quantita-
tive monomer conversion in 40 h, producing PMMA with narrow
MWD = 1.10 and Mn = 4.10 ꢄ 104; this experimental Mn value is
nearly identical to the calculated Mn value, thereby giving a near
quantitative initiator efficiency of 97% (run 1). These results also
show living characteristics of the MMA polymerization by this cat-
alyst. The PMMA produced has a syndiotacticity (rr) of 75%, similar
to that observed for the parent CGC complex (CGC)TiMe2 activated
with B(C6F5)3 at room temperature [21]. The polymerization activ-
ity can be drastically increased by raising the reaction temperature,
which reduced the time to achieve near quantitative monomer
conversion from 80 h at 25 °C to 4 h at 80 °C while maintaining
the living characteristics of the polymerization (run 2); however,
the polymer syndiotacticity decreased from 75% rr at 25 °C to
ꢀ70% rr at 80 °C, which was also observed for the parent CGC cat-
alyst [21a].
Interestingly, activation of the dichloride complex (HCGC)TiCl2
(3) with 0.5 equiv of Ph3CB(C6F5)4 in CD2Cl2 produced a yellow
solution consisting of Ph3CH, (ClCGC)TiCl2 (5), and (HCGC)TiCl+
2Al(C6F5)3
Me
H
Me
H
MeAl(C6F5)3
MeAl(C6F5)3
Me
Me
Si
Si
Zr
Zr
2
2-Zr2+
Al(C6F5)3
Turning to the polymerizations by (HCGC)TiMe2 pre-activated
with Ph3CB(C6F5)4 in toluene, all polymerization characteristics
(activity, control, efficiency, and syndiotacticity) were lowered
when comparing the dication generated by 2 equiv of Ph3CB(C6F5)4
(run 4) to the monocation by 1 equiv Ph3CB(C6F5)4 (run 3). This
intriguing observation was repeated when either switching to
the in-reactor activation methodology (run 6 vs. 5) or raising the
temperature to 80 °C (run 8 vs. 7). Also noteworthy is the consid-
erably lower initiator efficiency by the catalyst generated with
the Ph3CB(C6F5)4 activation than the catalyst generated with the
B(C6F5)3 activation, reflecting a consequence of the instability of
MeAl(C6F5)3
Me
Me
H
Me
H
Me
Si
Si
Zr
Zr
+
MeAl(C6F5)3
2a-Zr+
2b-Zr+
Scheme 2. Generation of (HSBI)Zr-based mono- and di-cations with Al(C6F5)3.

Y. Zhang, E.Y.-X. Chen / Journal of Organometallic Chemistry 695 (2010) 1464–1471
1469
Ph3CB(C6F5)4
Me
H
Me
B(C6F5)4
Ti
Ti
Si
Cl
Cl
Si
N
Cl
N
B(C6F5)4
+
+
3
3
-Si Ti
Ph CH
3
-
Ph CH
3
-
3
Me
Cl
Me
Si
Me
B(C6F5)4
Ti
Ti
Ti
Si
+
Cl
Cl
Si
Cl
Cl
H
Cl
N
N
N
B(C6F5)4
+
+
5
3
-Ti
3
-Si
Scheme 4. Generation of Si+Ti+ dications with Ph3CB(C6F5)4.
Table 1
Selected MMA polymerization results by (HCGC)TiMe2 (1).a
b
c
*
d
b
b
b
Run No. Activator (method) Solvent Temp (°C) Time (h) Conv. (%) 10ꢂ4Mn (g/mol) MWDc (Mw/Mn)
I
(%) [mm] (%) [mr] (%) [rr] (%)
1
2
1 B(C6F5)3 (A)
1 B(C6F5)3 (A)
TOL
TOL
25
80
40
4
99
94
4.10
4.17
1.10
1.08
97
90
2.6
3.3
22.5
27.2
74.9
69.5
3
4
5
6
7
8
1TTPB (A)
2TTPB (A)
1TTPB (B)
2TTPB (B)
1TTPB (B)
2TTPB (B)
TOL
TOL
TOL
TOL
TOL
TOL
25
25
25
25
80
80
40
40
40
40
2
96
86
94
86
78
21
5.22
5.65
6.40
5.91
5.22
3.63
1.09
1.21
1.12
1.26
1.10
1.18
74
61
59
58
60
23
2.8
7.2
3.0
6.0
2.1
6.7
21.2
23.7
20.9
22.8
26.6
29.6
76.0
69.1
76.1
71.2
71.3
63.7
2
9
1TTPB (A)
2TTPB (A)
1TTPB (B)
2TTPB (B)
DCM
DCM
DCM
DCM
25
25
25
25
48
48
48
48
23
<1
100
47
6.01
5.51
6.94
4.40
1.28
1.12
1.12
1.33
15
<1
58
43
2.2
5.7
2.0
3.0
20.3
23.4
20.5
20.5
77.5
70.9
77.5
76.5
10
11
12
a
Carried out in 10 mL toluene (TOL) or CH2Cl2 (DCM) at 25 °C or 80 °C; [Zr] = 2.34 mM; [MMA]0/[Zr]0 = 400; activator = B(C6F5)3 or Ph3CB(C6F5)4 (TTPB); activation
method = pre-activation (the Ti complex and the activator were premixed and stirred for 10 min followed by addition of MMA, method A) or in-reactor activation (the Ti
complex and MMA were premixed before addition of the activator, method B).
b
Monomer conversion and tacticity (methyl triad distribution) determined by 1H NMR spectroscopy.
Mn and MWD determined by GPC relative to PMMA standards in CHCl3.
Initiator efficiency (I*) = Mn(calc)/Mn(exptl), where Mn(calc) = MW(MMA) ꢄ [MMA]0/[I]0 ꢄ conversion (%) + MW of chain-end groups.
c
d
Table 2
Selected MMA polymerization results by rac-(HSBI)ZrMe2 (2).a
*
Run No.
Activator (method)
Solvent
Time (h)
Conv. (%)
10ꢂ4Mn (g/mol)
MWD (Mw/Mn)
I
(%)
[mm] (%)
[mr] (%)
[rr] (%)
13
14
15
16
1TTPB (A)
2TTPB (A)
1TTPB (A)
2TTPB (A)
TOL
TOL
DCM
DCM
4
24
4
85
0
48
14
20.3
1.71
17
89.6
6.0
4.4
12.4
4.80
1.66
1.45
16
12
74.9
75.9
14.7
14.7
10.4
9.4
24
17
18
19
20
1TTPB (B)
2TTPB (B)
1TTPB (B)
2TTPB (B)
TOL
TOL
DCM
DCM
4
24
4
94
2
81
36
21.1
1.48
19
83.6
10.6
5.8
14.1
5.64
1.51
1.62
23
25
71.1
74.7
17.3
15.3
11.6
10.0
24
a
See footnotes in Table 1 for conditions and explanations.
the cations paired with the non-coordinating anion [B(C6F5)4]ꢂ, a
well-known phenomenon [8]. This instability issue associated with
the cation paired with [B(C6F5)4]ꢂ became more pronounced in the
polar solvent CH2Cl2. Thus, the polymerization with the pre-
formed catalyst in CH2Cl2 resulted in only 15% efficiency for the
monocation (run 9) and <1% efficiency for the even less stable dica-
tion (run 10), suggesting most (for the monocation) or nearly all
(for the dication) of the pre-formed catalyst have already decom-
posed before contacting the monomer. To back up this assertion,
the same polymerization with the in-reactor activation methodol-
ogy drastically improved their efficiencies to 58% for the monoca-
tion (run 11) and to 43% for dication (run 12).
Table 2 summarizes the results of the MMA polymerization by
rac-(HSBI)ZrMe2 activated with Ph3CB(C6F5)4. In toluene, the pre-
formed monocation exhibits good activity, achieving 85% mono-
mer conversion in 4 h. The PMMA produced has an isotacticity
(mm) of ꢀ90%, but the polymerization is ill-controlled with a low
initiator efficiency of only 17% (run 13), due to the slow initiation
by the Zr–Me species [3]. The pre-formed dication showed no
activity (run 14) because the dication 2-Si+Zr+ is insoluble in tolu-

1470
Y. Zhang, E.Y.-X. Chen / Journal of Organometallic Chemistry 695 (2010) 1464–1471
Table 3
Selected propylene polymerization results by metallocene catalysts.a
Run No.
Precatalyst (method)
[TTPB]/[precat]
Time (min)
Yieldb (g)
Productivity (10ꢂ6 g PP/mol h atm)
21
22
23
24
(CGC)TiMe2 (B)
(CGC)TiMe2 (B)
rac-(SBI)ZrMe2 (B)
rac-(SBI)ZrMe2 (B)
1
2
1
2
10
10
5
1.70
1.69
3.48
3.84
1.02
1.01
4.18
4.60
5
25
26
27
28
(HCGC)TiMe2 (B)
(HCGC)TiMe2 (B)
rac-(HSBI)ZrMe2 (B)
rac-(HSBI)ZrMe2 (B)
1
2
1
2
10
10
5
0.88
1.78
2.34
3.23
0.53
1.07
2.81
3.88
5
a
Carried out in 20 mL CH2Cl2 at 25 °C with 10
procedure.
lmol Zr or Ti catalyst under 1 atm propylene using Ph3CB(C6F5)4 (TTPB) as activator using the in-reactor activation
b
Average yield of 2–3 runs with typical errors of 65%.
ene. Switching to CH2Cl2, the cation instability drastically lowered
the polymerization activity for the monocation (run 15), but now
the cation solubility gave some activity for the dication (run 16).
Again, consistent with the instability of the cation paired with
[B(C6F5)4]ꢂ, the in-reactor activation methodology improved poly-
merization performances by such catalysts (runs 17 and 18 vs. runs
13 and 14), especially for polymerizations carried out in CH2Cl2
(runs 19 and 20 vs. runs 15 and 16). However, complications using
the in-reactor activation may arise from these slowly initiating cat-
ionic zirconocene alkyl catalysts due to a competing bimetallic
pathway when the neutral zirconocene dimethyl co-exists with
the cationic alkyl species, the consequence of which is the lowered
overall isotacticity due to the contribution of the syndiotactic poly-
mer produced by the bimetallic pathway [3].
Si–H and M–Me bonds by the potent abstractor Ph3CB(C6F5)4. On
the other hand, homo-dications (M2+) can be readily produced by
the activation with 2 equiv of Al(C6F5)3, but not with B(C6F5)4 or
Ph3CB(C6F5)4. Analogous chloride-derivatives of hetero-dications
can also be produced starting from the dichloride precursors, but
the chloride abstraction occurs through the transit silylium ion.
The formation of the silylium–metallocenium hetero-dications
has shown a substantial impact mostly on the activity of polymeri-
zation of MMA and propylene. In MMA polymerization, the activity
of the dications, especially the metallocene-based 2-Si+Zr+, is
substantially lower than the corresponding monocations. The insta-
bility and low solubility of the pre-formed cations paired with the
non-coordinating anion [B(C6F5)4]ꢂ are assumed to account for the
observations. In sharp contrast, the propylene polymerization activ-
ity of the dication 1-Si+Ti+, derived from activation of (HCGC)TiMe2
with 2 equiv of Ph3CB(C6F5)4, is twice of that for the monocation.
The propylene polymerization by rac-(HSBI)ZrMe2 activated with
2 equiv of Ph3CB(C6F5)4 is also seen a 40% activity enhancement
over the monocation. These considerably rate enhancements are
contributable to the more electrophilic metal centers rendered by
the silylium bridging within dications 1-Si+Ti+ and 2-Si+Zr+.
A currently unexplored interesting aspect of the polymerization
of polar vinyl monomers is to use the chiral silylium ion within
dications such as 1-Si+Ti+ and 2-Si+Zr+ as catalyst and the added
MeSKA as initiator for promoting stereoregulation in silylium-cata-
lyzed polymerization. However, the dications in their present form
would present complications due to the presence of two reactive
sites within the same complex (i.e., interplay between the silyli-
um-catalyzed bimolecular process and metallocenium-catalyzed
unimolecular process). The chloride-derivative 3-Si+Ti+ is seem-
ingly a more suitable candidate, but it co-exists with two other
species in a mixture. The work to explore this aspect of the poly-
merization and develop new approaches to access such isolable
dications is in progress.
3.4. Propylene polymerization
We first carried out several control runs to determine whether
the ‘‘trityl effect” [22] (i.e., enhanced activity with increasing trityl
borate activator concentration) applies to the current systems un-
der current conditions (0.5 mM catalyst in CH2Cl2 at 25°), or not.
Specifically, propylene polymerizations using (CGC)TiMe2 acti-
vated with 1 or 2 equiv of Ph3CB(C6F5)4 gave an identical activity
(run 21 vs. 22, Table 3), consistent with the formation of the same
monocationic catalyst with 1 or 2 equiv of the activator. Likewise,
the polymerization activity of rac-(SBI)ZrMe2 differed only slightly
between runs activated with 1 and 2 equiv of Ph3CB(C6F5)4 (run 23
vs. 24). Hence, both systems under the current conditions showed
no or non-attributable ‘‘trityl effect”, thereby establishing a basis
for judging effects of the trityl borate activator equivalency on
polymerization activity by catalyst structural changes (i.e., mono-
cation vs. dication).
Indeed, the propylene polymerization activity of the dicationic
catalyst 1-Si+Ti+ derived from activation of (HCGC)TiMe2 with
2 equiv of Ph3CB(C6F5)4 was twice of that achieved by the mono-
cationic catalyst (run 26 vs. 25). The propylene polymerization
by rac-(HSBI)ZrMe2 activated with 2 equiv of Ph3CB(C6F5)4 was also
seen a 40% activity enhancement over the monocation derived
from activation with 2 equiv of Ph3CB(C6F5)4 (run 28 vs. 27). These
consistent results argue that the silylium bridging within the dica-
tions 1-Si+Ti+ and 2-Si+Zr+ (Scheme 3) enhances the propylene
polymerization activity of the respective cationic Ti and Zr metal
centers by rendering the metal centers more electrophilic.
Acknowledgments
This work was supported by the National Science Foundation
(NSF-0848845). We thank Boulder Scientific Co., for the research
gifts of B(C6F5)3 and Ph3CB(C6F5)4.
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