The formation and polymerization behavior of (pentafluorophenyl)-cyclopentadienyl titanium compounds

Article abstract of DOI:10.1016/S0022-328X(99)00742-1

[C₅H₄(C₆F₅)]CpTiCl₃ (2) and [C₅H₄(C₆F₅)]₂TiCl₂ (4) have been synthesized and are compared with [C₅H₄(C₆F₅)]₂ZrCl₂ (5), CpTiCl₃, Cp₂TiCl₂ and Cp₂ZrCl₂ in their ability to polymerize olefins. The half-sandwich complex 2 is formed by reaction of a trimethylsilyl intermediate with TiCl₄. Similarly, the metallocene complex 4 is formed by reaction of a trimethyltin intermediate with TiCl₄. EPR measurements of 2/methylaluminoxane (MAO) and 4/MAO in toluene solution indicate facile reduction to Ti(III) species occurs. It is suggested that the titanium complexes 2 and 4 with electron-withdrawing pentafluorophenyl substituents favor reduction during polymerizations. This would account for the high activities and stereoregularities observed with 2 for styrene, while poor activities are seen with 4 for ethylene. In general for zirconium compounds, the reduction potential is much lower compared with that of titanium compounds and may be the reason higher activities are observed with 5.

Full text of DOI:10.1016/S0022-328X(99)00742-1

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 599 (2000) 107–111
The formation and polymerization behavior of
(pentafluorophenyl)-cyclopentadienyl titanium compounds
Richard J. Maldanis, James C.W. Chien, Marvin D. Rausch *
Department of Chemistry, Lederle Graduate Research Center, Uni6ersity of Massachusetts, Amherst, MA 01003, USA
Received 12 August 1999; received in revised form 30 November 1999
Abstract
[C5H4(C6F5)]CpTiCl3 (2) and [C5H4(C6F5)]2TiCl2 (4) have been synthesized and are compared with [C5H4(C6F5)]2ZrCl2 (5),
CpTiCl3, Cp2TiCl2 and Cp2ZrCl2 in their ability to polymerize olefins. The half-sandwich complex 2 is formed by reaction of a
trimethylsilyl intermediate with TiCl4. Similarly, the metallocene complex 4 is formed by reaction of a trimethyltin intermediate
with TiCl4. EPR measurements of 2/methylaluminoxane (MAO) and 4/MAO in toluene solution indicate facile reduction to
Ti(III) species occurs. It is suggested that the titanium complexes 2 and 4 with electron-withdrawing pentafluorophenyl
substituents favor reduction during polymerizations. This would account for the high activities and stereoregularities observed
with 2 for styrene, while poor activities are seen with 4 for ethylene. In general for zirconium compounds, the reduction potential
is much lower compared with that of titanium compounds and may be the reason higher activities are observed with 5. © 2000
Elsevier Science S.A. All rights reserved.
Keywords: Polymerizations; Metallocenes; Titanium; Half-sandwich complexes; Electron-withdrawing; Olefins
parent complexes CpTiCl3, Cp2TiCl2 and Cp2ZrCl2 in
order to measure the effect the electron-withdrawing
pentafluorophenyl moiety has on the polymerization of
olefins.
1. Introduction
The polymerization behavior of metallocenes on
olefins is strongly dependent on the ligand environment.
Modification of the steric environment has resulted in
higher catalytic activities and improved stereoregulari-
ties of the resulting polymer [1]. Recently, research has
been geared toward tailoring subsituents on a metal-
locene to determine the effect of changing electron
density at the metal center and its overall effect on the
polymerization of olefins [2]. In many cases, it was
unclear if a steric or an electronic influence of a given
substituent was a factor in the polymerization. Recently
Deck and Jackson reported the formation of (pen-
tafluorophenyl)-cyclopentadiene and its zirconium
derivative [C5H4(C6F5)]2ZrCl2 (5) [3]. The purpose of
this present study was to synthesize the titanocene
derivative [C5H4(C6F5)]2TiCl2 (4) and its half-sandwich
complex [C5H4(C6F5)]TiCl3 (2) and to examine their
ability to polymerize styrene and ethylene. Compounds
2 and 4 as well as 5 have been compared with their
2. Results and discussion
2.1. Synthesis of catalyst precursors
Trimethylsilyl derivatives of cyclopentadiene have
been shown to be very effective intermediates in the
formation of half-sandwich complexes [4]. (Pen-
tafluorophenyl-cyclopentadienyl) trimethylsilane (1)
was synthesized in a 66% yield from a reaction of
[C5H4(C6F5)]−Na+ and chlorotrimethylsilane in THF
* Corresponding author. Fax: +1-413-5454490.
E-mail address: rausch@chem.umass.edu (M.D. Rausch)
Scheme 1.
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00742-1

108
R.J. Maldanis et al. / Journal of Organometallic Chemistry 599 (2000) 107–111
We found that the titanocene [C5H4(C6F5)]2TiCl2 (4)
could be synthesized in good yield from the reaction of
the trimethyltin intermediate 3 with TiCl4. Compound 3
was prepared from the reaction of [C5H4(C6F5)]−Na+
with trimethyltin chloride in THF for 24 h (Scheme 3).
Compound 3 was obtained in 31% yield as a clear oil
by distillation. Treatment of 3 with titanium tetrachlo-
ride in refluxing toluene for 16 h subsequently afforded
4 in 51% yield after recrystallization from hot toluene
(Scheme 4).
Scheme 2.
Compound 4 is a dichroic complex that changes from
a dark-green to a violet color on exposure to light. This
compound was also characterized by 1H-NMR and
elemental analysis and was found to be elementally
pure. Initially, we tried to form 4 via the trimethylsilyl
route as described above using an excess of 1. However,
even refluxing the reaction for 24 h in toluene only led
to the formation of the half-sandwich complex
[C5H4(C6F5)]TiCl3 (2). It is clear that the trimethyltin
route is more effective in generating metallocenes of
this type.
Scheme 3.
2.2. Polymerization results
Group IV half-sandwich catalysts are known to give
high activity for styrene when activated by methylalu-
minoxane [5]. Compound 2 was examined as a styrene
polymerization catalyst and was compared with the
parent compound CpTiCl3 (Table 1).
It can be seen that compound 2 shows a significantly
higher activity, percentage syndiotacticity and Tm than
CpTiCl3. The activity is a factor of ten higher at 50°C
and the Tm is more than ten degrees higher at both
temperatures. The current mechanism for styrene poly-
merization indicates that a multihapto coordination
occurs between the cationic center, the styrene
monomer and the propagating chain [6a]. Studies have
proposed that a Ti(III) oxidation state is active for
Scheme 4.
for 15 h (Scheme 1). Compound 1 was purified by
distillation and was found to be elementally pure. It
was then treated with titanium tetrachloride in
methylene chloride to give the half-sandwich complex
[C5H4(C6F5)]TiCl3 (2) in 80% yield (Scheme 2).
Table 1
a
Polymerization of styrene with 2 and CpTiCl3
b
c
Catalyst
2 e
Tp (°C)
PS (g)
A
(×10−7
)
Tm (°C)
s-PS d (%)
20
20
50
50
20
20
50
50
0.55
0.51
1.62
1.59
0.33
0.35
0.76
0.75
1.0
0.95
9.1
275
277
262
264
95
98
94
96
2 e
8.1
f
f
f
f
CpTiCl3
CpTiCl3
CpTiCl3
CpTiCl3
0.61
0.65
1.4
1.4
a Polymerization conditions: MAO used as the co-catalyst; 4000/1=Al/Ti; 50 mM catalyst solution in toluene; 5 ml of styrene as monomer.
b A (activity)=g PS/[(mol Ti) [Styrene](h)].
c Tm, melting temperature of polymer.
d % s-PS=[g of 2-butanone insoluble polymer)/(g of bulk polymer)]×100.
e 10 min polymerization.
f 30 min polymerization.

R.J. Maldanis et al. / Journal of Organometallic Chemistry 599 (2000) 107–111
109
Table 2
splitting pattern with g=1.988 with a splitting of 7.2
G. Both EPR spectra are consistent with the proposed
Ti(III) reductive species. Additional satellites are also
seen in the spectra but are less pronounced.
a
Polymerization of ethylene with 5, 4, Cp2TiCl2 and Cp2ZrCl2
b
c
Catalyst
Tp (°C)
PE (g)
A
(×10−6
)
Tm (°C)
133.2
135.1
138.3
137.2
115.9
119.0
135.8
134.5
5 d
20
20
50
50
20
20
50
50
20
20
50
50
20
20
50
50
2.15
2.04
4.05
4.08
6.42
6.18
5.92
5.61
0.388
0.372
0.091
0.11
0.85
0.77
0.74
0.75
3.0
2.9
31
Preactivation was also attempted with 2 as a catalyst
under the same conditions as above. However, there
was only a slight increase in activity observed. This
result suggests that there is a very short induction time
to form the alkylated species, the resulting cation after
methide abstraction, and reduction of the catalyst spe-
cies to Ti(III) by MAO. This is probably due to the
electron-withdrawing pentafluorophenyl groups increas-
ing the electrophilicity and reactivity of the catalyst.
Ethylene polymerizations were carried out using 4
and [C5H4(C6F5)]2ZrCl2 (5). These complexes were
compared to Cp2ZrCl2 and Cp2TiCl2 (Table 2).
Low-temperature polymerizations of ethylene with 5,
4, Cp2TiCl2, and Cp2ZrCl2 were also carried using trityl
tetrakis(pentafluorophenyl borate, [CPh3]+ B[C6F5]4−
(TIBA) [8] as the co-catalyst. The polymerization re-
sults are given in Table 3.
As seen in Tables 2 and 3, 4 shows much lower
activity and Tm than Cp2TiCl2. Studies have shown that
reduction of the catalytic species can occur during
polymerizations and it is believed that ethylene is poly-
merized with low productivity when the catalyst is in
the Ti(III) oxidation state [6c,7]. Titanium complexes
have a high reduction potential and are easily reduced
to the Ti(III) state. We have confirmed via EPR mea-
surements that both 2 and 4 are readily reduced to a
Ti(III) species by excess MAO in toluene solutions. We
believe this electronic effect is observed in our catalyst
system because reduction in the presence of MAO or
trityl/TIBA is more favorable with a highly elec-
trophilic center. With compound 4, the rate of reduc-
tion of the catalyst species should increase as the
temperature increases and this may explain why lower
activities are observed at higher temperatures. Zirco-
nium compounds are more difficult to reduce to the
Zr(III) state and this may explain why there is no
dramatic difference in the activities of 5 and Cp2ZrCl2
compared to 2 and Cp2TiCl2.
Recently, Rausch and co-workers performed poly-
merizations of ethylene with titanocene dichloride, and
its bis(trifluoromethyl) and bis(N,N-dimethylamino)
derivatives with MAO and trityl/TIBA co-catalysts [2e].
In these systems, they found that the electron-with-
drawing bis(trifluoromethyl) derivative [C5H4(CF3)]2-
TiCl2 gave lower activities at both low and elevated
temperatures compared with the parent compound
Cp2TiCl2. It was concluded that reduction of the cata-
lytic species could have occurred, resulting in lower
activities.
5 d
5 e
5 e
32
42
40
38
f
f
f
f
Cp2ZrCl2
Cp2ZrCl2
Cp2ZrCl2
Cp2ZrCl2
4 g
33
0.42
0.40
0.14
0.16
11.1
9.9
13
4 g
4 g
4 g
h
h
h
h
Cp2TiCl2
Cp2TiCl2
Cp2TiCl2
Cp2TiCl2
13
a Polymerization conditions: MAO used as co-catalyst, 4000/1:
Al/Ti or Al/Zr; 1 ml of catalyst solution inject into polymerization
bottle.
b A (activity)=g PE/[(mol Ti) [ethylene] (h)].
c Tm, melting temperature of polymer.
d 38 mM catalyst solution in toluene, 1 h polymerization.
e 38 mM catalyst solution in toluene, 15 min polymerization.
f 50 mM catalyst solution in toluene, 10 min polymerization.
g 50 mM catalyst solution in toluene, 1 h polymerization.
h 50 mM catalyst solution in toluene, 5 min polymerization.
Table 3
a
Polymerization of ethylene with 5, 4, Cp2TiCl2, and Cp2ZrCl2
b
c
Catalyst
PE (g)
A
(×10−6
)
Tm (°C)
5 d
0.97
0.91
0.96
0.99
0.28
0.31
1.7
3.7
3.6
3.7
3.8
0.52
0.53
6.3
6.0
124
124
121
130
5 d
d
d
Cp2ZrCl2
Cp2ZrCl2
4 e
4 e
d
Cp2TiCl2
Cp2TiCl2
d
1.6
a Polymerization conditions: polymerization performed at 0°C; 20/
1: TIBA/Ti or TIBA/Zr; 1:1 trityl/catalyst; 50 mM catalyst solution in
toluene; 1 ml of catalyst solution injected in polymerization bottle.
b A (activity)=g PE/[(mol Ti) [ethylene](h)].
c Tm, melting temperature of polymer.
d 15 min polymerization.
e 1 h polymerization.
styrene polymerization [6] due to this mechanism. It has
been shown that a typical co-catalyst causes reduction
of metallocenes and half-sandwich complexes during
polymerizations [6c,7]. EPR spectroscopy was per-
formed using 2/methylaluminoxane (MAO) and 4/
MAO in toluene solutions. Both solutions rapidly
changed to a light-green color on exposure to MAO.
An EPR spectrum recorded after 5 min show a well-
defined doublet at g=2.0045 with a splitting of 7.4 G
for 2/MAO. In addition, 4/MAO showed a similar
We have shown that 2 is an excellent catalyst precur-
sor for the polymerization of styrene while 4 is a
relatively poor catalyst for ethylene. EPR studies

110
R.J. Maldanis et al. / Journal of Organometallic Chemistry 599 (2000) 107–111
confirm that the highly electrophilic titanium complexes
2 and 4 in toluene solutions undergo facile reduction to
the Ti(III) species in the presence of MAO. Reduction
is not favorable in 5 due to its much lower reduction
potential than that of titanium.
3.3. Synthesis of (pentafluorophenyl-cyclopentadienyl)
trimethyltin (3)
To a solution of [C5H4(C6F5)]−Na+ (5.1 g, 20 mmol)
in 100 ml THF, trimethyltin chloride (4.3 g 22 mmol)
was added as a solid and the solution changed from a
deep-violet to an orange color. The resulting solution
was allowed to stir overnight. The solvent was removed
and the resulting red oil was extracted with 100 ml of
hexane. After removal of the hexane the red oil was
distilled at 100°C/0.01 mmHg to give 2.68 g of pure 3 in
31% yield as a mixture of isomers. 1H-NMR (CDCl3): l
5.72–7.21 (m, 3H), 3.12–3.45 (m, 1H), −0.021–0.5
(m, 9H).
3. Experimental
Reactions were carried out under an argon atmo-
sphere using standard Schlenk techniques. MAO was
purchased from Akzo Nobel. THF, pentane, hexane
and toluene were distilled from Na/K alloy under ar-
gon. Methylene chloride was distilled from CaH2.
Styrene was distilled from CaH2 in vacuo prior to use.
Dicyclopentadiene was cracked prior to use.
[C5H4(C6F5)]2ZrCl2 (5) and [C5H4(C6F5)]−Na+ were
prepared by literature procedures [3]. Trityltetrakis(pen-
tafluorophenyl)borate, [CPh3]+B[C6F5]4−, was synthe-
sized by literature procedure [8]. All other chemicals
were purchased from Aldrich and used without further
purification. 1H-NMR spectra were recorded on a
Bruker-200 spectrometer. EPR measurements were per-
formed on a Bruker-300 MHz spectrometer. Elemental
analyses were performed by the Microanalytical Labo-
ratory, University of Massachusetts, Amherst, MA.
3.4. Synthesis of [C5H4(C6F5)]2TiCl2 (4)
To a solution of TiCl4 (0.27 ml, 2.5 mmol) in 50 ml
of
toluene,
(pentafluorophenyl-cyclopentadienyl)
trimethyltin (3) (2.0 g, 5.0 mmol) (dissolved in 5 ml of
hexane) was added at 0°C and the resulting solution
was warmed to r.t. and refluxed overnight. The result-
ing red solution was cooled and the solvent was re-
moved in vacuo. The resulting red oil was triturated
and washed with several portions of hexane. Toluene
(20 ml) was added and the suspension was then heated
and quickly filtered via cannula while hot to give 0.72 g
of pure 4 in 51% yield after cooling and removal of the
solvent. 1H-NMR (CDCl3): l 7.15 (m, 4H), 6.67 (t,
4H). Anal. Calc. for C22H8Cl2F10Ti: C, 45.47; H, 1.39.
Found: C, 45.64; H, 1.63%.
3.1. Synthesis of (pentafluorophenyl-cyclopentadienyl)
trimethylsilane (1)
To a solution of [C5H4(C6F5)]−Na+ (5.1 g, 20 mmol)
in 100 ml of THF, chlorotrimethylsilane (slight excess)
(3.1 ml, 23 mmol) was added and the solution changed
from a deep-violet to a light-orange color. The resulting
solution was allowed to stir overnight. The solvent was
removed and the resulting oil was extracted with 100 ml
of hexane. After removal of the hexane, the resulting oil
was distilled at 70°C/0.01 mmHg to give 4.01 g of 1 as
a mixture of isomers in 66% yield. 1H-NMR (CDCl3): l
6.62–7.3 (m, 3H), 3.21–3.8 (m, 1H), −0.0162–0.21
(m, 9H). Anal. Calc. for C14H13F5Si: C, 55.25; H, 4.30.
Found: C, 55.08; H, 4.17%.
3.5. Polymerizations
Polymerizations were carried out in 250 ml crown-
capped glass pressure reactors containing 50 ml of
freshly distilled toluene and the desired monomer. For
ethylene polymerizations, the system was saturated with
ethylene at 15 psig. Styrene polymerizations were car-
ried out using 5 ml of freshly distilled styrene from
CaH2. The system was then injected with the desired
amount of MAO and was left to stir for 10 min. The
polymerizations were initiated by adding the desired
catalyst in a solution of toluene to the reactors. The
catalyst was preactivated by injecting 1 ml of MAO to
the catalyst solution and this was left to stir for 15 min.
Trityl/TIBA polymerization were performed by adding
a 20-fold excess of TIBA to the toluene solution of
monomer, followed by the desired amount of catalyst
and finally [CPh3]+B[C6F5]4−. The polymerizations
were terminated by adding 2% HCl–methanol solution.
The polymers were then filtered, dried and weighed.
The bulk polymer in the case of polystyrene was ob-
tained by using a Soxhlet extractor for 24 h in 2-bu-
tanone to determine the percentage s-PS. The melting
points of the polymers were obtained by DSC.
3.2. Synthesis of [C5H4(C6F5)]TiCl3 (2)
To a solution of (pentafluorophenyl-cyclopentadi-
enyl) trimethylsilane (1) (2.00 g, 6.6 mmol) in 50 ml of
CH2Cl2, TiCl4 ( 0.71 ml, 6.6 mmol) was added at 0°C.
The mixture was warmed to room temperature (r.t.)
and allowed to stir overnight. The solvent was removed
and the resulting orange solid was sublimed at 70°C/
0.01 mmHg to give 2.02 g of pure 2 as orange crystals
1
in 80% yield. H-NMR (CDCl3): l 7.46 (m, 2H), 7.17
(t, 2H). Anal. Calc. for C11H4Cl3F5Ti: C, 34.28; H,
1.05. Found: C, 34.21; H, 1.07%.

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