Diverse pathways of activation and deactivation of half-sandwich aryloxide titanium polymerization catalysts

Article abstract of DOI:10.1021/om0507272

A series of half-sandwich aryloxide titanium complexes, [CpTi(OAr)Me ₂] (Cp = C₅H₅; OAr = OC₆H ₃Me₂-2,6, OC₆H₃Et₂-2,6, OC₆H₃ⁱPr₂-2,6, OC₆H ₃^tBu₂-2,6, and OC₆HPh ₄-2,3,5,6), have been synthesized. These compounds react with B(C₆F₅)₃ to give thermally unstable complexes [CpTi(OAr)Me][MeB(C₆F₅)₃]. Two different deactivation pathways have been identified within the series. The tetraphenylphenoxide cationic methyl compound decomposes cleanly at room temperature to give [CpTi(OC₆HPh₄-2,3,5,6)(C ₆F₅){CH₂B(C₆F₅) ₂}] and methane with a first-order rate constant of 7.6(2) × 10^-4 s^-1 at 25°C. For relatively smaller aryloxide ligands, OAr = OC₆H₃ⁱPr₂-2,6, OC₆H₃^tBu₂-2,6, a Me/C ₆F₅ exchange takes place, yielding CpTi(OAr)Me(C ₆F₅) and MeB(C₆F₅)₂. The cationic titanium complexes are shown to be active for the polymerization of 1-hexene. At -20 and 0°C, first-order dependence on the concentration of 1-hexene is observed. The rate of polymerization decreases with increasing steric hindrance of aryloxides except for OAr = OC₆HPh ₄-2,3,5,6.

Full text of DOI:10.1021/om0507272

214
Organometallics 2006, 25, 214-220
Diverse Pathways of Activation and Deactivation of Half-Sandwich
Aryloxide Titanium Polymerization Catalysts
Khamphee Phomphrai,*,†,‡ Andrew E. Fenwick,† Shalini Sharma,† Phillip E. Fanwick,†
James M. Caruthers,§ W. Nicholas Delgass,§ Mahdi M. Abu-Omar,*,† and Ian P. Rothwell†,
Department of Chemistry and Center for Catalyst Design and School of Chemical Engineering and
Center for Catalyst Design, Purdue UniVersity, 560 OVal DriVe, West Lafayette, Indiana 47907-2084
ReceiVed August 22, 2005
A series of half-sandwich aryloxide titanium complexes, [CpTi(OAr)Me2] (Cp ) C5H5; OAr ) OC6H3-
i
t
Me2-2,6, OC6H3Et2-2,6, OC6H3 Pr2-2,6, OC6H3 Bu2-2,6, and OC6HPh4-2,3,5,6), have been synthesized.
These compounds react with B(C6F5)3 to give thermally unstable complexes [CpTi(OAr)Me][MeB(C6F5)3].
Two different deactivation pathways have been identified within the series. The tetraphenylphenoxide
cationic methyl compound decomposes cleanly at room temperature to give [CpTi(OC6HPh4-2,3,5,6)-
(C6F5){CH2B(C6F5)2}] and methane with a first-order rate constant of 7.6(2) × 10-4 s-1 at 25 °C. For
i
t
relatively smaller aryloxide ligands, OAr ) OC6H3 Pr2-2,6, OC6H3 Bu2-2,6, a Me/C6F5 exchange takes
place, yielding CpTi(OAr)Me(C6F5) and MeB(C6F5)2. The cationic titanium complexes are shown to be
active for the polymerization of 1-hexene. At -20 and 0 °C, first-order dependence on the concentration
of 1-hexene is observed. The rate of polymerization decreases with increasing steric hindrance of aryloxides
except for OAr ) OC6HPh4-2,3,5,6.
homogeneous catalysts supported by non-Cp ancillary ligands.6-10
These include constrained geometry,11 half-sandwich complexes
bearing pendant electron donors12,13 and related diamide7a-d,i,j
Introduction
A great degree of success has been reported in the area of
group 4 metallocenes as olefin polymerization catalyst precur-
sors.1,2 The cationic complexes [Cp2MCH3]+ (M ) Ti, Zr, Hf),3
generated from the activation of dihalide or dialkyl complexes
with a varieties of activators4 such as MAO, perfluoroaryl
boranes, B(C6F5)3, or [Ph3C][B(C6F5)4], have been shown to
be catalytically active in olefin polymerization. In contrast to
the heterogeneous Ziegler-Natta catalysts, a well-defined
coordination environment in the homogeneous metallocenes
afforded in many instances “single-site” catalysts, which led to
an efficient control of polymer properties such as molecular
weight, molecular weight distribution, stereochemical micro-
structure, and comonomer incorporation, through systematic
ligand modification.1,5
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* Corresponding authors. E-mail: sckpp@mahidol.ac.th; mabuomar@
purdue.edu.
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T. P.; Kang, H.-C.; Massa, W.; Okuda, J. Organometallics 1996, 15, 5069.
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† Department of Chemistry and Center for Catalyst Design.
‡ Present address: Department of Chemistry, Faculty of Science, Mahidol
University, Rama 6 Road, Bangkok 10400, Thailand.
§ School of Chemical Engineering and Center for Catalyst Design.
This paper is dedicated to the memory of our mentor, colleague, and
friend Ian P. Rothwell, Richard B. Moore Distinguished Professor of
Chemistry.
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10.1021/om0507272 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 11/25/2005

Aryloxide Ti Polymerization Catalysts
Organometallics, Vol. 25, No. 1, 2006 215
based catalyst systems. Other catalysts based on aryloxide8,9,14,15
and alkoxide16 ligands have also received some attention
recently. The aryloxide ligand can adopt a bonding motif
isolobal with that of the cyclopentadienyl ligand, potentially
bonding in a σ2π4 fashion. A key property of these ligands is
their tunability, as a wide collection of phenols offering a unique
and diverse set of steric and electronic features are commercially
available or can be readily synthesized. We have reported on a
series of bisaryloxide complexes (ArO)2MR2 (M ) Ti, Zr;
R ) Me, CH2Ph).17 Although the bisaryloxide complexes can
be activated with B(C6F5)3, giving compounds [(ArO)2MR][RB-
(C6F5)3], they showed very moderate catalytic activity for the
polymerization of ethylene, propylene, and 1-hexene.17b On the
other hand, the polymerization activity is dramatically enhanced
when one aryloxide ligand is replaced by one cyclopentadienyl
ligand. From a series of studies, Nomura and co-workers have
shown that mixed Cp/aryloxide titanium complexes CpTi(OAr)-
Cl2 are highly active for 1-hexene, ethylene, styrene, and
propylene polymerization when activated with methylalumi-
noxane (MAO).15
Scheme 1
While the dihalide derivatives, CpTi(OAr)X2, have been
studied extensively with a large excess of MAO in olefin
polymerization, studies on the dialkyl analogues CpTi(OAr)R2
until now have been very minimal.18,19 Activation of CpTi(OAr)-
R2 with 1 equiv of activator such as B(C6F5)3 or [Ph3C]-
[B(C6F5)4] would eliminate the need for excess MAO or
trialkylaluminum as required for the dihalide complexes, thus
making the catalyst system less complicated and amenable to
quantitative rate measurements. In this study, we report on the
synthesis of a series of dialkyl CpTi(OAr)R2 complexes along
with their activation, deactivation, and olefin polymerization
kinetics.
t
(3), OC6H3 Bu2-2,6 (4), and OC6HPh4-2,3,5,6 (5), have been
synthesized. We previously reported the syntheses of compounds
1, 3, and 5.18 Starting from CpTiCl3, [CpTiMe3] was prepared
in situ by reacting with 3 equiv of LiMe at -78 °C, followed
by addition of the corresponding phenol, giving 1-3 and 5 in
high yield and purity (Scheme 1, method A).18 However, the
t
deprotonation of HOC6H3 Bu2-2,6 by [CpTiMe3] was not
successful due to the steric bulk of tBu groups. Thus, compound
4 was prepared from the reaction of CpTiCl3 with LiOC6H3-
t
tBu2-2,6 to give CpTi(OC6H3 Bu2-2,6)Cl2,20 followed by meth-
ylation with 2 equiv of LiMe (Scheme 1, method B). Spectro-
scopic characterization of 1-5 did not reveal remarkable
features that were not anticipated. H NMR of Ti-Me groups
shift downfield (δ 0.91, 0.91, 0.94, and 1.02 ppm) with
increasing alkyl size from Me to tBu in 1-4. For relatively small
aryloxide ligands, compounds 1-3 are not stable at room
temperature, where a slow decomposition was observed over
days to produce CpTi(OAr)2Me.18 Compounds 4 and 5 are stable
at room temperature over 1 month in a drybox.
1
Results and Discussion
Synthesis and Characterization of Dimethyl Compounds.
A series of compounds CpTi(OAr)Me2, where Cp ) C5H5;
i
OAr ) OC6H3Me2-2,6 (1), OC6H3Et2-2,6 (2), OC6H3 Pr2-2,6
(14) (a) Thorn, M. G.; Fanwick, P. E.; Chesnut, R. W.; Rothwell, I. P.
Chem. Commun. 1999, 2543. (b) Turner, L. E.; Thorn, M. G.; Fanwick, P.
E.; Rothwell, I. P. Chem. Commun. 2003, 1034. (c) Thorn, M. G.; Parker,
J. R.; Fanwick, P. E.; Rothwell, I. P. Organometallics 2003, 22, 4658. (d)
Turner, L. E.; Swartz, R. D.; Thorn, M. G.; Chesnut, R. W.; Fanwick, P.
E.; Rothwell, I. P. Dalton Trans. 2003, 4580. (e) Turner, L. E.; Thorn, M.
G.; Fanwick, P. E.; Rothwell, I. P. Organometallics 2004, 23, 1576.
(15) (a) Nomura, K.; Okumura, H.; Komatsu, T.; Naga, N. Macromol-
ecules 2002, 35, 5388-5395. (b) Nomura, K.; Oya, K.; Imanishi Y. J. Mol.
Catal. A 2001, 174, 127-140. (c) Nomura, K.; Fudo, A. Catal. Commun.
2003, 4, 269-274. (d) Nomura, K.; Tsubota, M.; Fujiki, M. Macromolecules
2003, 36, 3797-3799. (e) Nomura, K.; Fudo, A. Inorg. Chim. Acta 2003,
345, 37-43. (f) Nomura, K.; Okumura, H.; Komatsu, T.; Naga, N.; Imanishi,
Y. J. Mol. Catal. A 2002, 190, 225-234. (g) Nomura, K.; Fudo, A. J. Mol.
Catal. A 2004, 209, 9-17. (h) Nomura, K.; Hatanaka, Y.; Okumura, H.;
Fujiki, M.; Hasegawa, K. Macromolecules 2004, 37, 1693-1695. (i) Byun,
D. J.; Fudo, A.; Tanaka, A.; Fujiki, M.; Nomura, K. Macromolecules 2004,
37, 5520-5530. (j) Wang, W.; Tanaka, T.; Tsubota, M.; Fujiki, M.;
Yamanaka, S.; Nomura, K. AdV. Synth. Catal. 2005, 347, 433-446. (k)
Wang, W.; Fujiki, M.; Nomura, K. J. Am. Chem. Soc. 2005, 127, 4582-
4583. (l) Nomura, K.; Naga, N.; Miki, M.; Yanagi, K. Macromolecules
1998, 31, 7588-7597.
Heating a solution of CpTi(OC6HPh4-2,3,5,6)Cl2 and the
dimethyl compound 5 in benzene gave the mixed monomethyl/
monochloro CpTi(OC6HPh4-2,3,5,6)(Cl)(Me), 6, in quantitative
yield (Scheme 1). A similar comproportionation of the dichloride
and dimethyl titanium complexes to give mixed monomethyl/
monochloro titanium complexes has been documented.21 The
preparation, an ORTEP drawing of the molecular structure of
6, and selected bond distances (Å) and angles (deg) are given
in the Supporting Information.
Synthesis and Deactivation of Cationic Titanium Methyl
Complexes. Addition of 1 equiv of B(C6F5)3 to the dimethyl
compounds 1-5 in benzene or toluene led to the immediate
formation of the thermally unstable (vide infra) cationic methyl
compounds 7-11 as indicated by 1H NMR and an instant color
1
change from yellow to red (Scheme 2). At -25 °C, H NMR
of compounds 7-11 showed a single set of Cp and OAr
resonances along with resolved Ti-Me (sharp) and Ti-Me-B
(broad) signals. The Ti-Me proton signals shifted ∆δ 0.60-
0.74 ppm downfield from their neutral parent dimethyl com-
plexes. The Ti-Me proton signals also shifted downfield (δ 1.51,
1.57, 1.61, and 1.76 ppm) with increasing size of alkyl groups
(16) Mack, H.; Eisen, M. S. J. Chem. Soc., Dalton Trans. 1998, 917-
921.
(17) (a) Thorn, M. G.; Etheridge, Z. C.; Fanwick, P. E.; Rothwell, I. P.
Organometallics 1998, 17, 3636. (b) Thorn, M. G.; Etheridge, Z. C.;
Fanwick, P. E.; Rothwell, I. P. J. Organomet. Chem. 1999, 591, 148.
(18) Fenwick, A. E.; Phomphrai, K.; Thorn, M. G.; Vilardo, J. S.; Trefun,
C. A.; Hanna, B.; Fanwick, P. E.; Rothwell, I. P. Organometallics 2004,
23, 2146-2156.
(19) Virginia, A.; Andre´s, R.; Jesu´s, E. d.; Mata, F. J.; Flores, J. C.;
Go´mes, R.; Go´mes-Sal, P.; Turner, J. F. C. Organometallics 2005, 24,
2331-2338.
(20) Willoughby, C. A.; Duff, R. R.; Davis, W. M.; Buchwald, S. L.
Organometallics 1996, 15, 472-475.
(21) Zhang, S.; Piers, W. E.; Gao, X. L.; Parvez, M. J. Am. Chem. Soc.
2000, 122, 5499-5509.

216 Organometallics, Vol. 25, No. 1, 2006
Phomphrai et al.
Scheme 2
at ortho positions from 7-10, respectively. The Ti-Me carbon
atoms of 7-10 appeared around 74.7 ppm in the 13C spectrum,
significantly downfield from their neutral parent dimethyl
complexes (δ 54.0-61.3 ppm). Horton and co-workers sug-
gested that the difference between meta and para fluorine
chemical shifts (∆(m,p-F)) in the 19F NMR of the [RB(C6F5)3]-
anion can be used to determine the strength of the d0 metal-
anion interaction, where values of 3-6 ppm indicate coordina-
tion and values less than 3 ppm indicate noncoordination.7k The
large ∆(m,p-F) value in the 19F NMR of 5.2-5.4 ppm in 7-10
suggested strong association of the [MeB(C6F5)3]- anion with
titanium. At higher temperature, the methyl signals broadened
and began to coalesce (ranging from -10 to 15 °C), which was
attributed to boron exchange between the two methyl groups
on titanium. However, thermal instability of compounds 7-10
prevented the detection of a complete coalescence. Only
complex 11 was stable enough at higher temperature, and the
free energy of activation for this boron exchange was estimated
to be 14.4(5) kcal mol-1 at 10 °C.18
Figure 1. (A) Plot of ln([9]/[9]0) vs time of the deactivation of
i
[CpTi(OC6H3 Pr2-2,6)Me][MeB(C6F5)3], 9, at 10 °C. (B) Plot of
t
ln([10]/[10]0) vs time of the deactivation of [CpTi(OC6H3 Bu2-2,6)-
Me][MeB(C6F5)3], 10, at 25 °C. Initial concentrations of 9 and 10
were 0.040 M.
counteranion to the metal.3,17b,22-24 The deactivation process was
1
monitored by H NMR at 10 and 25 °C following the disap-
The stability of the cationic compounds is highly dependent
on the size of the aryloxide ligand. Larger ligands stabilize the
cationic species. For example, a significant deactivation (ca.
20%) of compound 7 (OAr ) OC6H3Me2-2,6) was observed
(1H NMR) in 15 min at temperatures higher than -20 °C. In
contrast, complexes 8-11 showed deactivation comparable to
pearance of the Cp signal of 9 or 10, respectively (Figure 1).
Despite fairly clean deactivation product, the plot of ln([9]/[9]0)
vs time was rather complicated and deviated significantly from
first-order kinetics (Figure 1A). A closer look at the products
from the deactivation reaction surprisingly revealed another
minor product, Me2B(C6F5). From the deactivation of compound
9 prepared from 1 equiv of 3 and 1 equiv of B(C6F5)3, MeB-
(C6F5)2 and Me2B(C6F5) were found at approximately 80 and
20%, respectively. The fact that Me2B(C6F5) was detected in
the deactivation reaction implied that MeB(C6F5)2 (the primary
deactivation product) was also capable of activating CpTi(OAr)-
Me2 to give [CpTi(OAr)Me][Me2B(C6F5)2]. The secondary
deactivation via Me/C6F5 exchange then led to compound 12
and Me2B(C6F5). To determine if Me2B(C6F5) was also capable
of activating CpTi(OAr)Me2, an excess of compound 3 was
allowed to react with B(C6F5)3 for several days. Only compound
12 and Me3B were detected in addition to the starting compound
3. Formation of Me3B implied that a similar process in the
activation/deactivation of MeB(C6F5)2 also occurred for Me2B-
(C6F5), giving compound 12 and Me3B as final products. Taking
into account that B(C6F5)3 is a stronger Lewis acid than MeB-
(C6F5)2 and Me2B(C6F5), reactions of 3 with MeB(C6F5)2 and
Me2B(C6F5) may not be classified as activation to form cationic
species but rather a scrambling of the terminal Me and C6F5. A
possible reaction scheme is proposed in Scheme 3.
1
that observed by H NMR in 15 min at -10, 10, 10, and 30
°C, respectively. Hence, the order of stability of the cationic
complexes is 7 < 8 < 9 ≈ 10 < 11.
The products from the deactivation of complexes 7-11 were
identified. Rather than one dominant deactivation pathway (as
the case usually is), two deactivation pathways were observed
for the same mixed Cp/aryloxide ligand framework (Scheme
2). The first involves exchange of the bridging methyl group
(Ti-Me-B) with C6F5 on boron leading to the neutral complex
CpTi(OAr)(Me)(C6F5) and MeB(C6F5)2. The second is a result
of σ-bond metathesis in which a proton on the bridging methyl
group (Ti-Me-B) migrates to the terminal methyl (Ti-Me),
evolving methane and followed by C6F5 transfer to Ti. This
deactivation yields complex CpTi(OAr)(C6F5)(CH2B(C6F5)2) as
the thermodynamic product. Up to date and to the best of our
knowledge, the only example having both deactivation pathways
in one complex was reported by Andre´s et al. in a related
compound, Cp*Ti(OSiR3)Me2, activated with B(C6F5)3.19
Complexes 9 and 10 deactivated rapidly at room temperature,
leading to the neutral monomethyl titanium complexes 12 and
13, respectively, and MeB(C6F5)2 via C6F5 transfer from the
The deactivation of complex 10 at 25 °C was more straight-
forward, giving only the neutral complex 13 and MeB(C6F5)2

Aryloxide Ti Polymerization Catalysts
Organometallics, Vol. 25, No. 1, 2006 217
Scheme 3
Figure 2. Eyring plot for the deactivation of complex 11 at various
temperatures.
Table 1. Polymerization of 100 equiv of 1-Hexene Using
as products. From proton NMR, the formation of Me2B(C6F5),
found in the deactivation of 10, was negligible (<1%). The plot
of ln([10]/[10]0) vs time gave a first-order dependence on [10]
with a short induction period (Figure 1B). A deactivation rate
constant of 6.0(5) × 10-3 s-1 at 25 °C was obtained. Preparation
of 12 and 13 on a synthetic scale from the reactions of 9 and
10, respectively, with B(C6F5)3 was not successful. Clean
products were not recoverable during isolation attempts. The
deactivation analysis of cationic compounds 7 and 8 was less
conclusive compared to that for compounds 9 and 10 due to
multiple deactivation products. However, MeB(C6F5)2 was
detected in the reaction mixtures, suggesting that the Me/C6F5
exchange was one of the deactivation pathways leading to CpTi-
(OAr)(Me)(C6F5).
Catalyst Precursors 7-11a
cat.
temp (°C)
k
obs (× 10-4 s-1
)
act.b
Mn
PDI
7
-20
0
25
-20
0
25
-20
0
25
-20
0
N/Ac
N/Ac
N/Ad
4.4
4.7
14
9.5
4.1
13
24
3.6
18
4100
5200
2500
4600
5500
4600
6800
6600
5000
1.46
1.57
1.91
1.36
1.49
1.81
1.24
1.36
1.60
8
N/Ac
N/Ad
3.5
9
21
N/Ad
17
10
11
trace
trace
trace
50
252
504
25
-20
0
52
12 600
12 500
12 100
1.09
1.10
1.14
N/Ae
N/Ae
The second deactivation pathway was observed in the reaction
of complex 5 and B(C6F5)3. The reaction generated rapidly (1H
NMR) the cationic compound 11, which deactivated to give
compound 1418 and methane (Scheme 2) via σ-bond meta-
25
a Conditions: Polymerization was carried out using 1.0 M 1-hexene (100
equiv) and 0.010 M [CpTi(OAr)Me][MeB(C6F5)3] (1 equiv) in toluene-d8.
b Activity: (g polyhexene) (mmol Ti)-1 h-1 measured when polymerization
1
thesis.7c,18,21,25,26 The deactivation was monitored by H NMR
c
ceased or at >95% completion. kobs cannot be determined due to a catalyst
decomposition. d Polymerization ceased at about 60% completion. e Polym-
erization was complete too fast to obtain a reliable kobs. Only activity was
reported.
following the disappearance of the Cp signal of 11. The rate of
deactivation at various temperatures followed first-order de-
pendence on [11]. From the Eyring plot (Figure 2), the activation
parameters ∆Hq ) 15.8(5) kcal mol-1 and ∆Sq ) -22(5) cal
mol-1 K-1 were obtained. These numbers are comparable to
the deactivation of the [Cp*Ti(NdCtBu2)Me][MeB(C6F5)3]
system reported by Piers and co-workers, where the deactivation
was developed from the contact ion pair rather than two neutral
reactants.21
Polymerization Studies with 1-Hexene. Polymerizations of
100 equiv of 1-hexene (1.0 M) were conducted using the catalyst
precursors 1-5 activated with B(C6F5)3 in toluene-d8 at various
temperatures. The disappearance of 1-hexene was monitored
by 1H NMR following the integrals of the H2CdCH-nBu proton
against CH2Ph2 as internal standard. At 25 °C, the cationic
titanium complexes 7-9 and 11 were active for 1-hexene
polymerization. However, thermal instability of complexes 7-9
prevented the polymerization from going to completion. The
polymerization using 7-9 stopped after 40 min at about 60%
conversion of monomer, which is in agreement with the
observed decomposition of compounds 7-9 in the absence of
olefin at temperatures higher than 10 °C. Polyhexenes with Mn
(PDI) values of 2500 (1.91), 4600 (1.81), and 5000 (1.60) were
isolated from the polymerization using 7-9, respectively (Table
1). Compound 10, on the other hand, generated only a trace
amount of polyhexene at any given temperature, possibly as a
result of a higher steric encumbrance of the tert-butyl groups.
Compound 11 rapidly consumed 100 equiv of 1-hexene at 25
°C in 2 min, giving polyhexene with a Mn of 12 100 and a low
PDI of 1.14. The fact that the bulky compound 11 is highly
t
active while the Bu analogue 10 is inactive seems, at first,
puzzling. One possible explanation is that the enhanced reactiv-
ity of compound 11 is a result of electron donation (interaction)
from the ortho-phenyl rings to the titanium center.27 This
interaction effectively reduces the energy barrier required to
separate the titanium cation and [MeB(C6F5)3]- anion, facilitat-
ing monomer insertion.
At 0 °C, compounds 7-9 were sufficiently stable for
complete polymerization of 100 equiv of 1-hexene, giving
polyhexene with a Mn (PDI) of 5200 (1.57), 5500 (1.49), and
(22) Metcalfe, R. A.; Kreller, D. I.; Tian, J.; Kim, H.; Taylor, N. J.;
Corrigan, J. F.; Collins, S. Organometallics 2002, 21, 1719-1726.
(23) Woodman, T. J.; Thornton-Pett, M.; Bochmann, M. Chem. Commun.
2001, 329-330.
(24) Guerin, F.; Stewart, J. C.; Beddie, C.; Stephan, D. W. Organome-
tallics 2000, 19, 2994-3000.
(25) Gomez, R.; Gomez-Sal, P.; del Real, P. A.; Royo, P. J. Organomet.
Chem. 1999, 588, 22-27.
(26) Thorn, M. G.; Vilardo, J. S.; Fanwick, P. E.; Rothwell, I. P. Chem.
Commun. 1998, 2427-2428.
(27) (a) Lentz, M. R.; Vilardo, J. S.; Lockwood, M. A.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 2004, 23, 329-343. (b) Lentz, M. R.;
Fanwick, P. E.; Rothwell, I. P. Organometallics 2003, 22, 2259-2266. (c)
Manz, T. A.; Thomson, K. T., unpublished work.

218 Organometallics, Vol. 25, No. 1, 2006
Phomphrai et al.
Figure 3. Plot of ln([1-hexene]/[1-hexene]0) vs time for polym-
erization of 100 equiv of 1-hexene (1.0 M) in toluene-d8 using
[CpTi(OC6H3 Pr2-2,6)Me][MeB(C6F5)3], 9, at -20, 0, and 25 °C.
Figure 4. Plot of kobs vs [Ti] of 1-hexene (1.0 M) polymerization
at -10 °C using complex 9 at concentrations of 4.0, 6.0, 8.0, and
10.0 mM at 0 °C in toluene-d8.
i
6600 (1.36), respectively (Table 1). Although the polymerization
using 7 and 8 proceeded to completion, the plot of ln([Hex]/
[Hex]0) vs time deviated from linearity, suggesting catalyst
deactivation during the course of reaction, which agreed with
the observed instability of 7 and 8 at 0 °C. As illustrated in
Figure 3, the catalyst generated from complex 9 at 0 °C was
persistent during the polymerization reaction, giving pseudo-
first-order dependence on [1-hexene] with a kobs of 2.1(5) ×
10-3 s-1. Compound 11 still consumed 100 equiv of 1-hexene
rapidly at 0 °C in 4 min, giving polyhexene with a Mn of 12 500
and a low PDI of 1.10.
At -20 °C, polyhexene with Mn (PDI) values of 4100 (1.46),
4600 (1.36), 6800 (1.24), and 12 600 (1.09) was isolated from
the polymerization using 7-9 and 11, respectively (Table 1).
Only the polymerization using 8, 9, and 11 was pseudo-first-
Figure 5. 1H NMR (CDCl3, 300 MHz) spectra of the vinyl region
of polyhexene made from (a) complexes 7-9 and (b) complex 11.
order dependent on [1-hexene] with a kobs of 4.4(5) × 10-4
,
3.5(5) × 10-4, and 5.2(5) × 10-3 s-1, respectively, in agreement
with their stability at -20 °C. With the exception of complex
11, the polymerization rate decreased with increasing bulk of
the aryloxide ligand. With a given catalyst, higher PDI and
higher polymerization rate were observed with increasing
temperature. At a given temperature, higher PDI was observed
in the order 11 < 9 < 8 < 7 and higher Mn in the reverse order
7 < 8 < 9 < 11. This is understandable in terms of steric
hindrance at titanium, where steric encumbrance and lower
temperature suppress chain termination/transfer processes such
as â-H elimination.
one of the advantages of using B(C6F5)3 is a lower activity
catalyst that is amenable to kinetic investigations.
On the basis of 13C{1H} NMR in CDCl3, the polyhexene
produced from every catalyst precursor at every temperature
was atactic polyhexene.28,29 1H NMR of polyhexene is very
informative especially in the range δ 4.50-5.50 ppm.30 The
1H NMR spectra of polyhexene produced from 7-9 were very
similar in both shape and intensity. Therefore, only NMR data
from 9 are shown in Figure 5a. There were only two signals
observed in this region at about 4.70 and 5.35 ppm, which
corresponded to the vinylidene and vinylene end groups,
respectively. These unsaturated end groups are common in olefin
polymerization using group 4 metal complexes. The vinylidene
and vinylene end groups are typically formed during the
polymerization as a result of â-H elimination following a 1,2-
and 2,1-insertion of 1-hexene, respectively. The presence of both
end groups in polyhexene produced from 7-9 suggested that
â-H elimination occurred after both 1,2- and 2,1-insertion of
1-hexene. As shown in Figure 5a, the vinylene end group was
formed about 10 times more than the vinylidene end group.
The relative amount of the vinylidene and vinylene end groups
did not change significantly in the polymerization at -20, 0,
or 25 °C. The proton NMR of polyhexene produced from the
bulkier complex 11 is also shown in Figure 5b. There was only
Polymerizations of 1-hexene (1.0 M) were carried out at -10
°C in toluene-d8 using various concentrations of 9. The
polymerizations were first-order dependent on [1-hexene] for
[Ti] ) 4.0, 6.0, 8.0, and 10.0 mM. A plot of kobs vs Ti
concentrations revealed a linear dependence on catalyst, in
agreement with the rate law: -d[1-hexene]/dt ) kp[Ti][1-
hexene] for the case where there was no catalyst decomposition
(Figure 4). From the plot, kp ) 7.3(7) × 10-2 M-1 s-1 at -10
°C was obtained.
By comparison to the related Cp′Ti(OAr)Me2 and Cp′Ti-
(OAr)Cl2 catalyst systems investigated by Nomura and co-
workers, our CpTi(OAr)Me2/B(C6F5)3 system is less reactive
for 1-hexene polymerization by an order of magnitude.15e,l The
important contributor to the activity difference is the activator/
cocatalyst (such as MAO, AlR3, [Ph3CB][(C6F5)4], or [Me2-
PhNHB][(C6F5)4]) used in Nomura’s system. These activators
generally yield highly efficient olefin polymerization catalysts,
while B(C6F5)3 tends to afford less effective catalysts.4 However,
(28) Asakura, T.; Demura, M.; Nishiyama, Y. Macromolecules 1991,
24, 2334-2340.
(29) Murray, M. C.; Baird, M. C. Can. J. Chem. 2001, 79, 1012-1018.
(30) Liu, Z.; Somsook, E.; White, C. B.; Rosaaen, K. A.; Landis, C. R.
J. Am. Chem. Soc. 2001, 123, 11193-11207.

Aryloxide Ti Polymerization Catalysts
Organometallics, Vol. 25, No. 1, 2006 219
t
CpTi(OC6H3 Bu2-2,6)Me2, 4. LiMe (3.2 mL, 1.6 M solution in
diethyl ether, 5.1 mmol) was added to a chilled solution (10 °C) of
one signal observed at about 5.35 ppm, which corresponded to
the vinylene end group. This suggested that the polymer chain
termination occurred only after the 2,1-misinsertion, but not after
the regular 1,2-insertion. The more hindered aryloxide and the
interaction of phenyl groups with the titanium center in 11 could
be responsible for the prevention of chain termination after
regular 1,2-insertion of 1-hexene. This is in agreement with the
observed high Mn and low PDI in polyhexene produced from
11. The vinylene end group was observed exclusively in the
polymerization using 11 at every measured temperature.
t
CpTi(OC6H3 Bu2-2,6)Cl2 (1.00 g, 2.57 mmol) in 20 mL of benzene.
The mixture was slowly warmed to room temperature in 30 min
and stirred for 2 h. The solvent was then removed under vacuum,
and hexane was added to the solid residue. The suspension was
filtered through a plug of Celite over fritted glass to remove the
lithium salts. The filtrate was then evacuated to dryness, giving a
dark yellow solid (0.81 g, 91%). Anal. Calcd for C21H32OTi: C,
72.46; H, 9.19. Found: C, 72.21; H, 9.13. 1H NMR (C6D6, 25 °C):
δ 7.30 (d, J ) 7.8 Hz, 2H, m-H); 6.90 (t, J ) 7.8 Hz, 1H, p-H);
5.90 (s, 5H, Cp); 1.41 (s, 18H, tBu); 1.02 (s, 6H, Ti-Me). 13C NMR
(C6D6, 25 °C): δ 166.5 (Ti-O-C); 139.8, 125.5, 121.0 (Ar-C);
114.9 (Cp); 61.3 (Ti-Me); 35.5 (CMe3); 31.6 (CMe3).
Conclusions
We have demonstrated herein the use of aryloxides as
ancillary ligands in the study of activation, deactivation, and
1-hexene polymerization with titanium dimethyl complexes. We
have identified two different deactivation pathways. The cationic
titanium complexes have been shown to be active for 1-hexene
polymerization at -20, 0, and 25 °C, giving atactic polyhexene.
The titanium cationic catalyst stability and polymerization rates
depend strongly on the aryloxide ligand. Bulkier aryloxide is
more stable but less active for 1-hexene polymerization. The
exception is OC6HPh4-2,3,5,6, where the ortho-phenyl rings
facilitate monomer coordination, giving rise to an increased
polymerization rate. This effect is being scrutinized in more
detail.
NMR-Scale Synthesis of [CpTi(OC6H3Me2-2,6)Me][MeB-
(C6F5)3], 7. The following synthesis of 7 was also used to synthesize
complexes 8-10. An NMR tube with a rubber-septum screw cap
was charged with CpTi(OC6H3Me2-2,6)Me2 (10 mg, 40 µmol) and
0.30 mL of toluene-d8. A solution of B(C6F5)3 in toluene-d8 (0.20
mL, 0.20 M, 40 µmol) was added to the NMR tube through the
rubber septum at -25 °C, giving a bright red solution of compound
7. 1H NMR (C7D8, -25 °C): δ 6.68 (m, 3H, m, p-H); 5.50 (s, 5H,
Cp); 1.67 (s, 6H, o-Me); 1.51 (s, 3H, Ti-Me); 0.64 (br, 3H, B-Me).
Selected 13C NMR (C7D8, -25 °C): δ 165.2 (Ti-O-C); 119.1 (Cp);
75.2 (Ti-Me). 19F NMR (C7D8, -25 °C): δ -134.6 (d, 6F, o-F);
-159.4 (t, 3F, p-F); -164.8 (m, 6F, m-F).
[CpTi(OC6H3Et2-2,6)Me][MeB(C6F5)3], 8. 1H NMR (C7D8,
-25 °C): δ 6.84 (t, J ) 6.9 Hz, 1H, p-H); 6.77 (d, J ) 6.9 Hz,
2H, m-H); 5.55 (s, 5H, Cp); 2.08 (quartet, J ) 7.5 Hz, 4H, CH2-
CH3); 1.57 (s, 3H, Ti-Me); 0.95 (t, J ) 7.5 Hz, 6H, CH2CH3); 0.77
(br, 3H, B-Me). Selected 13C NMR (C7D8, -25 °C): δ 164.4 (Ti-
O-C); 119.3 (Cp); 75.2 (Ti-Me). 19F NMR (C7D8, -25 °C): δ
-134.8 (d, 6F, o-F); -159.4 (t, 3F, p-F); -164.8 (m, 6F, m-F).
Experimental Section
General Details. All operations were carried out under dry
nitrogen atmosphere using standard Schlenk techniques. Hydro-
carbon solvents were purified using an Innovative Technologies
solvent purification system and were stored over sodium ribbons
under nitrogen until use. LiMe (Aldrich), B(C6F5)3 (Strem), and
2,6-diethylphenol (Ethyl Corp.) were used as received. Compounds
i
[CpTi(OC6H3 Pr2-2,6)Me][MeB(C6F5)3], 9. 1H NMR (C7D8,
-25 °C): δ 6.81 (m, 3H, m, p-H); 5.60 (s, 5H, Cp); 2.63 (m, J )
6.6 Hz, 2H, CHMe2); 1.61 (s, 3H, Ti-Me); 1.01 (d, J ) 7.2 Hz,
6H, CHMe2); 0.88 (d, J ) 7.2 Hz, 6H, CHMe2); 0.84 (br, 3H,
B-Me). Selected 13C NMR (C7D8, -25 °C): δ 162.9 (Ti-O-C);
119.2 (Cp); 75.2 (Ti-Me). 19F NMR (C7D8, -25 °C): δ -134.8
(d, 6F, o-F); -159.3 (t, 3F, p-F); -164.7 (m, 6F, m-F).
t
20
1, 3, 5,18 and CpTi(OC6H3 Bu2-2,6)Cl2 were prepared according
to literature procedures. H and 13C NMR spectra were recorded
1
on a Varian Associates Gemini-200 or Inova-300 spectrometer and
referenced to protio impurities of commercial benzene-d6 (C6D6),
chloroform-d (CDCl3), or toluene-d8 (C7D8) as internal standards.
Elemental analyses and X-ray crystallography were obtained
through Purdue in-house facilities. Gel permeation chromatography
(GPC) was performed using a Waters 1515 isocratic HPLC pump
running at a THF flow rate of 1 mL/min at 35 °C and a Waters
2414 refractive index detector to determine molecular weights and
molecular weight distributions of polymer samples with respect to
polystyrene standards.
t
[CpTi(OC6H3 Bu2-2,6)Me][MeB(C6F5)3], 10. 1H NMR (C7D8,
-25 °C): δ 7.01 (d, J ) 6.9 Hz, 2H, m-H); 6.73 (t, J ) 6.9 Hz,
1H, p-H); 5.70 (s, 5H, Cp); 1.76 (s, 3H, Ti-Me); 0.98 (br, 18H,
tBu); 0.86 (br, 3H, B-Me). Selected 13C NMR (C7D8, -25 °C): δ
168.6 (Ti-O-C); 120.8 (Cp); 75.2 (Ti-Me). 19F NMR (C7D8, -25
°C): δ -134.4 (d, 6F, o-F); -159.6 (t, 3F, p-F); -164.8 (m, 6F,
m-F).
i
NMR-Scale Synthesis of CpTi(OC6H3 Pr2-2,6)Me(C6F5), 12.
CpTi(OC6H3Et2-2,6)Me2, 2. LiMe (9.0 mL, 1.6 M solution in
diethyl ether, 14 mmol) was added dropwise to a precooled
suspension of CpTiCl3 (1.00 g, 4.56 mmol) in 30 mL of Et2O at
-78 °C. After the mixture was stirred for approximately 4 h at
-78 °C, a solution of 2,6-diethylphenol (0.684 g, 4.56 mmol) in
10 mL of Et2O was added dropwise. The mixture was slowly
warmed to room temperature and stirred overnight. The solvent
was then removed under vacuum, and benzene was added to the
solid residue. The suspension was filtered through a plug of Celite
over fritted glass to remove the lithium salts. The filtrate was
evacuated to dryness, yielding a dark yellow liquid. Upon standing
at room temperature for a few hours, the liquid solidified, giving a
dark yellow solid (1.04 g, 78%). The solid was stored at -30 °C
to prevent decomposition. Anal. Calcd for C17H24OTi: C, 69.91;
Compound 9 was generated in situ from compound 3 and B(C6F5)3
in an NMR tube. The NMR tube was left at room temperature for
3 h, during which time the color slowly changed from red to yellow,
giving compounds 12, MeB(C6F5)2, and Me2B(C6F5). 1H NMR
(C7D8, 25 °C): δ 6.50-7.20 (m, 3H, m, p-H); 6.12 (s, 5H, Cp);
5
3.44 (m, J ) 6.9 Hz, 2H, CHMe2); 1.47 (m, JHF ) 1.9 Hz, Ti-
Me); 1.15 (d, J ) 6.9 Hz, 6H, CHMe2); 1.12 (d, J ) 6.9 Hz, 6H,
CHMe2). Selected 13C NMR (C7D8, 25 °C): δ 162.5 (Ti-O-C);
117.1 (Cp); 74.7 (Ti-Me). 19F NMR (C7D8, 25 °C): δ -117.5 (m,
2F, o-F); -155.2 (m, 1F, p-F); -163.0 (m, 2F, m-F).
t
NMR-Scale Synthesis of CpTi(OC6H3 Bu2-2,6)Me(C6F5), 13.
Compound 10 was generated in situ from compound 4 and B(C6F5)3
in an NMR tube. The NMR tube was left at room temperature for
3 h, during which time the color slowly changed from red to yellow,
1
H, 8.22. Found: C, 69.26; H, 8.14. H NMR (C6D6, 25 °C): δ
1
7.03 (d, J ) 7.5 Hz, 2H, m-H); 6.93 (t, J ) 7.5 Hz, 1H, p-H); 5.87
(s, 5H, Cp); 2.59 (quartet, J ) 7.5 Hz, 4H, CH2CH3); 1.21 (t, J )
7.5 Hz, 6H, CH2CH3); 0.91 (s, 6H, Ti-Me). 13C NMR (C6D6, 25
°C): δ 163.1 (Ti-O-C); 134.1, 127.2, 122.5 (Ar-C); 114.4 (Cp);
54.5 (Ti-Me); 24.7 (CH2CH3); 15.5 (CH2CH3).
giving compounds 13 and MeB(C6F5)2. H NMR (C7D8, 25 °C):
δ 7.13 (d, J ) 7.0 Hz, 2H, m-H); 6.79 (t, J ) 7.0 Hz, 1H, p-H);
6.13 (s, 5H, Cp); 1.58 (m, 3H, Ti-Me); 1.18 (s, 18H, tBu). 19F NMR
(C7D8, 25 °C): δ -115.7 (m, 2F, o-F); -154.9 (m, 1F, p-F); -163.3
(m, 2F, m-F).

220 Organometallics, Vol. 25, No. 1, 2006
Phomphrai et al.
Deactivation Studies of Compounds 9-11. The following
representative deactivation study of 9 was also used for 10 and 11.
Ph2CH2 (2 µL) was added to a solution of 3 in toluene-d8 (0.30
mL, 0.067 M, 20 µmol) in a screw-cap NMR tube with a PTFE/
silicone septum. This solution was then cooled to 10 °C using a
cooler attached to the NMR spectrometer. An NMR spectrum was
acquired. This is taken as the spectrum at time ) 0 s. The sample
was taken out of the spectrometer and submerged in a 10 °C water
bath. A solution of B(C6F5)3 in toluene-d8 (0.20 mL, 0.10 M, 20
µmol) was then added through the septum. The NMR tube was
shaken vigorously and placed back immediately into the spectrom-
eter. The initial concentrations of 9 and Ph2CH2 were 0.040, and
This is the spectrum at time ) 0 s. A solution of B(C6F5)3 in
toluene-d8 (50.0 µL, 0.10 M, 5.0 µmol) was then added through a
septum at 0 °C. The NMR tube was shaken vigorously and quickly
placed back into the spectrometer. The initial concentrations of
1-hexene, [CpTi(OAr)Me][MeB(C6F5)3], and Ph2CH2 were 1.0,
0.010, and 0.12 M, respectively. The conversion of 1-hexene was
monitored by proton NMR integration against the internal standard
Ph2CH2. After the polymerization proceeded to over 95% comple-
tion, the solution was poured into excess methanol to precipitate
the polymer. The excess methanol was decanted, and the polymer
was dried under vacuum overnight.
1
0.024 M, respectively. The formation of 12 was monitored by H
Acknowledgment. Support for this research was provided
by the U.S. Department of Energy, Office of Basic Energy
Sciences, through the Catalysis Science Grant No. DE-FG02-
03ER15466.
NMR with integration against the internal standard Ph2CH2.
Deactivation studies of 10 were performed similarly at 25 °C. A
deactivation study of 11 was performed at -5, 10, 25, and 35 °C.
NMR-Scale Polymerization of 1-Hexene. The following rep-
resentative polymerization of 1-hexene at 0 °C was used for all
catalysts at -20, 0, and 25 °C. A solution of the catalyst precursor
in toluene-d8 (50.0 µL, 0.10 M, 5.0 µmol) was added to a mixture
of 1-hexene (62.5 µL, 0.500 mmol) and Ph2CH2 (10 µL) in 0.34
mL of toluene-d8 in a screw-cap NMR tube with a PTFE/silicone
septum. This solution was then cooled to 0 °C using a cooler
attached to the NMR spectrometer. An NMR spectrum was taken.
Supporting Information Available: Synthesis, ORTEP draw-
ing, and X-ray crystallographic data (CIF file) for compound 6.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM0507272