Synthesis and characterization of weakly coordinating anion salts of a new, stable carbocationic reagent, the dibenzosuberenyl (dibenzotropylium) ion

Article abstract of DOI:10.1039/b804868a

The new, bright red compounds [DBS][B(C₆F₅) ₄], [DBS][BCl(C₆F₅)₃], [DBS][B(C₆F₅)₃-OH...H₂O- B(C₆F₅)₃] and [DBS][B(CH₂CMe ₃)(C₆F₅)₃] (DBS = C ₁₅H₁₁, dibenzosuberenyl, dibenzotropylium, dibenzo[a,e]tropylium or dibenzo[a,d]cycloheptenocarbonium) have been synthesized and characterized spectroscopically and, except for [DBS][B(CH ₂CMe₃)(C₆F₅)₃], crystallographically. The crystal structures show that the DBS⁺ carbocation is planar and suggest pronounced delocalization of the positive charge over part of the seven-membered ring, in particular the CH carbon and the carbon atoms α and β to it. The electrophilic DBS⁺ carbocation activates alkene polymerization catalyst precursors of the type Cp′₂ZrMe₂, its efficacy being comparable to that of the well known but sterically more hindered trityl carbocation, Ph ₃C⁺. As with the Ph₃C⁺ ion, DBS ⁺ does not initiate carbocationic polymerization of isobutene in dichloromethane directly, but rather by reacting with trace amounts of water to generate protons. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008.

Full text of DOI:10.1039/b804868a

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www.rsc.org/njc | New Journal of Chemistry
Synthesis and characterization of weakly coordinating anion salts of a
new, stable carbocationic reagent, the dibenzosuberenyl
(dibenzotropylium) ionw
Adina Cordoneanu, Mark J. Drewitt, Neda Bavarian and Michael C. Baird*
Received (in Gainesville, FL, USA) 26th March 2008, Accepted 8th May 2008
First published as an Advance Article on the web 18th July 2008
DOI: 10.1039/b804868a
The new, bright red compounds [DBS][B(C6F5)4], [DBS][BCl(C6F5)3],
[DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3] and [DBS][B(CH2CMe3)(C6F5)3]
(DBS = C15H11, dibenzosuberenyl, dibenzotropylium, dibenzo[a,e]tropylium or
dibenzo[a,d]cycloheptenocarbonium) have been synthesized and characterized spectroscopically
and, except for [DBS][B(CH2CMe3)(C6F5)3], crystallographically. The crystal structures show that
the DBS+ carbocation is planar and suggest pronounced delocalization of the positive charge
over part of the seven-membered ring, in particular the CH carbon and the carbon atoms a and b
to it. The electrophilic DBS+ carbocation activates alkene polymerization catalyst precursors of
the type Cp02ZrMe2, its efficacy being comparable to that of the well known but sterically more
hindered trityl carbocation, Ph3C+. As with the Ph3C+ ion, DBS+ does not initiate carbocationic
polymerization of isobutene in dichloromethane directly, but rather by reacting with trace
amounts of water to generate protons.
Cp2ZrMe2 to generate viable olefin polymerization catalysts, we
found that the solubility of [C7H7][B(C6F5)4] in most solvents is
sufficiently low that only suspensions of the salt can be used. We
Introduction
The highly electrophilic trityl carbocation, [Ph3C]+, is a
powerful carbanion abstracting reagent which, in combination
have therefore extended our interests to the dibenzosuberenyl
with the weakly coordinating counteranion [B(C6F5)4]ꢁ be-
haves as a very effective activator of dimethylmetallocenes for
carbocation (B)4 (alternatively dibenzotropylium, dibenzo-
[a,e]tropylium or dibenzo[a,d]cycloheptenocarbonium ion) as
the previously unknown salt [C15H11][B(C6F5)4], or [DBS]-
olefin polymerization.1 Thus treatment of dimethylzirco-
nocenes Cp02ZrMe2 (Cp0 = substituted cyclopentadienyl) with
[Ph3C][B(C6F5)4] results in abstraction of a methyl group to
[B(C6F5)4]. (Henceforth the atom labels shown for B will be
utilized to denote the various hydrogen and carbon atom
give Ph3CMe and the excellent alkene polymerization catalysts
positions during discussions of NMR spectra and X-ray crystal
structures.)
[Cp02ZrMe]+ as the [B(C6F5)4]ꢁ salts (eqn (1)).
Cp02ZrMe2 + [Ph3C][B(C6F5)4] -
[Cp02ZrMe][B(C6F5)4] + Ph3CMe
(1)
Intrigued with the possibility of developing alternative carbo-
cationic species as carbanion abstracting reagents (and
prompted by the occasional formation of oils during attempts
to synthesize crystalline [Ph3C][B(C6F5)4]), we earlier2a investi-
gated the use of the tropylium ion (A) as the salt
[C7H7][B(C6F5)4].2b–e The tropylium ion is a nonbenzenoid,
aromatic 6p-electron system with a thermodynamic stability
somewhat higher than that of the trityl ion.3 Unfortunately,
although we were able to demonstrate (NMR) that the [C7H7]+
ion does abstract a methyl group from e.g. Cp*TiMe3 and
Dibenzosuberenyl chloride, 1-ClC15H11, has found use as an
alkylating agent, i.e. as a source of the DBS carbocation,4a–d
but when desired free this cationic species has usually been
generated in strongly acidic solutions at low temperatures (e.g.
FSO3H–SO2ClF at ꢁ78 1C).4f,g Thus, although its gas phase
photochemistry has in fact been studied,4h the carbocation
itself has not been readily available for study in other media.
Thus a reliable synthesis of [DBS][B(C6F5)4] as a stable salt,
possibly for use as an alkylating agent superior to 1-ClC15H11,
would be of general interest.
Department of Chemistry, Queen’s University, Kingston, Canada
ON K7L 3N6
w Electronic supplementary information (ESI) available: Crystallo-
graphic details, including figures of [DBS][B(C6F5)4] (1),
[DBS][BCl(C6F5)3] (2) and [DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3]
(3), showing complete numbering schemes and thermal ellipsoid
figures, tables of positional and thermal parameters and bond lengths
and angles. CCDC reference numbers 687298–687300. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b804868a
The DBS carbocation is a derivative of the tropylium ion
+
and is thermodynamically destabilized relative to C7H7 by
the benzo-annulation.3 Although also thermodynamically
more stable than the trityl ion,3 the DBS carbocationic center
ꢂc
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is less sterically hindered than is that of the trityl ion and we
anticipated that the DBS+ cation would be capable of methyl
carbanion abstraction as in Scheme 1.
Canadian Microanalytical Services, Delta, B.C. X-Ray crys-
tallographic studies of [DBS][B(C6F5)4], [DBS][BCl(C6F5)3]
and [DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3] were carried out
by Dr. R. Wang on the departmental Bruker AXS SMART
CCD 1000 X-ray diffractometer with graphite-monochro-
mated Mo Ka radiation (l = 0.71073 A), controlled with a
Cryostream Controller 700. In a typical experiment, the
crystals were mounted on a glass fibre with epoxy glue. The
structures were solved using a Pentium PC operating a Bruker
AXS Windows NT and SHELXTL software (version 5.10).
No significant decay was observed during data collections.
Neutral atom scattering factors were taken from Cromer and
Waber.6 The raw intensity data were converted (including
corrections for scan speed, background, and Lorentz and
polarization effects) to structure amplitudes and their esd’s
using the program SAINT, which corrects for Lp and decay.
Absorption corrections were applied using SADABS program.
All non-hydrogen atoms were refined anisotropically. The
positions of all hydrogen atoms were calculated (unless other-
wise stated) and their contributions were included in the
structure factors and calculations. Crystallographic data are
summarized in Table 1, important bond lengths and angles in
Table 2.
Scheme 1
We find that the new compound [DBS][B(C6F5)4] is readily
synthesized as a bright red, thermally stable crystalline com-
pound, and describe here the preparation and both spectral
and crystallographic characterization of [DBS][B(C6F5)4]. We
have also, for purposes of comparisons of structures and
reactivities, prepared the analogous DBS salts of the anions
[BCl(C6F5)3]ꢁ,
[B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3]ꢁ
and
[B(CH2CMe3)(C6F5)3]ꢁ. We have verified that the DBS carbo-
cation does indeed activate metallocene catalyst precursors of
the type Cp02ZrMe2, as in Scheme 1, and we make a brief
comparison of [DBS][B(C6F5)4], [Ph3C][B(C6F5)4] and
[DBS][B(CH2CMe3)(C6F5)3] as initiators of metallocene
induced olefin polymerization processes.
Molecular weights of atactic polypropylene and polyisobu-
tene were determined at 75 1C on a Waters Associates model
GPC-2690 liquid chromatograph equipped with a Waters 2410
RI detector. The separation columns used were a series of
Styragel HR 5, HR 4E and HMW 7 columns. Gel permeation
chromatography analyses of the polypropylene and polyiso-
butene samples were done using filtered toluene HPLC grade
(99.99%) as eluent at a flow rate of 1.0 mL minꢁ1. Calibration
of the instrument was done using commercially available
polystyrene standards with molecular weights ranging from
770 to 6 500 000 g molꢁ1. Molecular weight determinations of
isotactic polypropylene samples were carried out at 145 1C in
1,2,4-trichlorobenzene using a Waters 150-C GPC.
Experimental
All manipulations were carried out using standard Schlenk
techniques and flame-dried glassware under dry, deoxygenated
nitrogen or argon, or in an MBraun Labmaster glove box
unless otherwise indicated. Dichloromethane was dried by
refluxing over calcium hydride, toluene by passage through a
column of activated alumina. The residual water content of the
solvents was less than 9 ppm as measured by Karl Fischer
titrations. NMR solvents were purchased from Cambridge
Isotope Laboratories Inc. or CDN Isotopes. CDCl3 was dried
using 4A molecular sieves, CD2Cl2 and C6D5CD3 by passing
through activated alumina. Ethylene, propylene and isobutene
(polymerization grades; 99+%) were purchased from Air
Products and were dried by passing through columns of
activated 3A or 4A molecular sieves which had been activated
under vacuum at B150 1C for B15 h prior to use. Dibenzo-
suberenol (Aldrich) was purified by recrystallization from hex-
anes, while Cp2ZrCl2 (Strem), Indenyl2ZrCl2 (Strem), SBIZrCl2
(Strem) and [Ph3C][B(C6F5)4] (Asahi Glass) were used as
Synthesis of [DBS][B(C6F5)4]
Addition of 0.2 g dibenzosuberenyl chloride (0.88 mmol) to a
solution of 0.5 g Li[B(C6F5)4] (0.73 mmol) in 100 mL dry
dichloromethane resulted in the instant formation of a deep
red color; the solution was stirred for 90 min under argon as
LiCl precipitated. The LiCl was removed by filtration and the
product was precipitated by the addition of 200 mL of dry
hexanes. The solvent volume was then reduced to 20 mL under
reduced pressure and the mixture was warmed to 70 1C while
being stirred for 10 min. The resulting red powder was isolated
by decantation, washed with 20 mL of hexanes and dried
under reduced pressure; yield 0.45 g (71%). Alternatively a
solution of 0.35 g [Ph3C][B(C6F5)4] (0.38 mmol) and 0.176 g
dibenzosuberenyl chloride (0.38 mmol) in 100 mL dry dichloro-
methane was stirred for 60 min under argon. The red product
was isolated as above; yield 0.22 g (67%). In order to obtain
very pure [DBS][B(C6F5)4], the warming and decantation steps
were carried out at least twice by adding fresh amounts of
30 mL hexanes each time. 1H NMR (CD2Cl2): (d 10.37 (s, 1H),
9.13 (s, 2H), 8.97 (d, 2H), 8.76 (m, 4H), 8.52 (m, 2H). 19F
NMR (CD2Cl2): d ꢁ134.2 (s, 8F, o-F), ꢁ164.6 (t, 4F, p-F),
received. The compounds B(C6F5)3,5a Li[B(C6F5)4],5b dibenzo-
5d
suberenyl chloride,5c Cp2ZrMe2,5d Indenyl2ZrMe2
and
5e
SBIZrMe2 were synthesized as described in the literature.
All 1H, NOESY, 13C and 19F NMR spectra were run on
Bruker Avance 400 or 500 NMR spectrometers, mass spectra
analyses on a Fisons Quatro Pro Quadrupole Mass Spectro-
meter or an Applied Biosystems/MDS Sciex QSTAR XL
QqTOF mass spectrometer in EIꢃ modes using dichloro-
methane as solvent. Elemental analyses were performed by
ꢂc
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Table 1 Crystallographic data for [DBS][B(C6F5)4] (1), [DBS][BCl(C6F5)3] (2) and [DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3] (3)a
1
2
3
Empirical formula
Mr
T/K
C39H11BF20
870.29
180(2)
C33H11BClF15
738.68
180(2)
C51H14B2F30O2
1250.24
180(2)
l/A
Crystal system
Space group
a/A
b/A
c/A
a/1
0.71073
Monoclinic
P21/n
9.779(2)
22.098(5)
15.386(4)
0.71073
Monoclinic
P21/c
10.3060(19)
7.4208(15)
36.756(7)
0.71073
Triclinic
ꢀ
P1
12.512(5)
13.248(6)
14.052(6)
81.676(7)
82.635(8)
76.977(8)
2234.4(16)
2
b/1
96.055(4)
93.601(3)
g/1
V/A3
3306.3(13)
4
1.748
0.180
1720
2805.5(9)
4
1.749
0.263
1464
Z
Dc/Mg mꢁ3
1.858
0.199
m/mmꢁ1
F(000)
1232
0.10 ꢄ 0.10 ꢄ 0.08
Crystal size/mm
y range for data collection/1
Index ranges, hkl
Reflections collected
Independent reflections
Rint
Completeness to y = 26.001 (%)
Max., min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I 4 2s(I)]
R indices (all data)
Largest diff. peak, hole/e Aꢁ3
0.40 ꢄ 0.30 ꢄ 0.30
0.50 ꢄ 0.30 ꢄ 0.20
1.62–26.00
ꢁ11 to 12, ꢁ26 to 27, ꢁ17 to 18
1.98–26.00
ꢁ11 to 12, ꢁ9 to 9, ꢁ43 to 45
1.47–26.001
ꢁ15 to 15, ꢁ16 to 16, ꢁ17 to 15
20843
6489
0.0211
100.0
14398
5486
0.0196
99.5
14087
8721
0.0927
99.2
1.000, 0.876
8721/1/796
0.723
R1 = 0.0672, wR2 = 0.0821
R1 = 0.2552, wR2 = 0.1069
0.281, ꢁ0.260
1.0000, 0.7728
6489/0/585
0.979
R1 = 0.0322, wR2 = 0.0741
R1 = 0.0521, wR2 = 0.0802
0.272, ꢁ0.193
0.9493, 0.8798
5486/0/495
1.073
R1 = 0.0414, wR2 = 0.0989
R1 = 0.0556, wR2 = 0.1061
0.647, ꢁ0.220
a
Details in common: absorption correction: empirical (Bruker SADABS), refinement method: full-matrix least-squares on F2.
ꢁ168.4 (s, 8F, m-F). 13C NMR for DBS carbocation
(CD2Cl2): d 172.0, 146.4, 144.1, 143.0, 141.6, 137.6, 135.5,
134.5. Anal. Calc. for C39H11BF20: C, 53.76; H, 1.26. Found:
C, 53.61; H, 1.12%. The identity of the compound was
confirmed (ES-MS) by observation of a peak at m/z 191.11
in the positive ion mode, corresponding to the [DBS]+ carbo-
cation, and m/z 679.02 in the negative ion mode, correspond-
ing to the [B(C6F5)4]ꢁ anion. In addition, recrystallization of
[DBS][B(C6F5)4] from dichloromethane–hexanes at room tem-
perature afforded red crystals suitable for X -ray diffraction.
(0.96 mmol) and quickly turned red. The solution was stirred
at room temperature for 3 h, and precipitation of a red powder
was induced by adding 50 mL of hexanes. The product was
isolated by decantation and then dried under reduced pressure.
The compound could not be purified satisfactorily for ele-
mental analyses, but a small number of crystals suitable for
X-ray crystallography were obtained through layering a solution
in dichloromethane with hexanes. 1H NMR (CD2Cl2): d 10.37
(s, 1H), 9.13 (s, 2H), 8.97 (d, 2H), 8.76 (dd, 4H), 8.52 (m, 2H),
8.16 (br s, 3H, H3O2). 19F NMR (CD2Cl2): d ꢁ137.1 (d, 6F,
o-F), ꢁ160.9 (t, 3F, p-F), ꢁ166.5 (br, t, 6F, m-F).
Synthesis of [DBS][BCl(C6F5)3]
A solution of 14 mg dibenzosuberenyl chloride (0.06 mmol)
and 75 mg B(C6F5)3 (0.12 mmol) in 50 mL of dried dichloro-
methane was stirred for 3 h under argon. A red powder was
then precipitated by the addition of 50 mL hexanes, and the
product was isolated by decantation and dried under reduced
pressure. The compound could not be purified satisfactorily
for elemental analyses, but a few red crystals suitable for X-ray
crystallography were obtained through layering a solution in
dichloromethane with hexanes. 1H NMR (CD2Cl2): d 10.37
(s, 1H), 9.13 (s, 2H), 8.97 (d, 2H), 8.76 (dd, 4H), 8.52 (m, 2H).
19F NMR (CD2Cl2): d ꢁ129.9 (s, 6F, o-F), ꢁ166.9 (br, s, 3F, p-F),
ꢁ160.5 (s, 6F, m-F).
Synthesis of [DBS][B(CH2CMe3)(C6F5)3]
A mixture of 0.2 g Li[B(CH2CMe3)(C6F5)3]7 (0.34 mmol) and
80 mg of dibenzosuberenyl chloride (0.34 mmol) in 50 mL of
hexanes was stirred for 10 h. The resulting red solid was
filtered, dissolved in dichloromethane, and filtered again to
remove LiCl and the solvent was then removed under reduced
pressure to give the red product. Alternatively a solution of
0.5 g Li[B(CH2CMe3)(C6F5)3] (0.85 mmol) and 0.19 g dibenzo-
suberenyl chloride (0.85 mmol) in 40 mL dichloromethane was
stirred for 15 min to give a red solution and white precipitate.
The latter was filtered off and the solvent was removed to give
the product as a red solid. Yields 65–70%. Attempted recrys-
tallizations resulted in apparent decomposition to
DBSCH2CMe3 (see below), but successful synthesis was con-
firmed by ES-MS (peaks at m/z 190.7 and 582.9 in the positive
and negative ion modes, respectively, corresponding to the
Synthesis of [DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3]
A solution of 0.1 g dibenzosuberenol (0.48 mmol) in 50 mL
dried dichloromethane was treated with 0.61 g B(C6F5)3
ꢂc
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Table 2 Important bond lengths (A) and angles (1) for DBS carbo-
cations of [DBS][B(C6F5)4] (1), [DBS][BCl(C6F5)3] (2) and
[DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3] (3)
NMR studies of the reaction of Cp2ZrMe2 with
[DBS][B(C6F5)4]
The reaction of Cp2ZrMe2 with [DBS][B(C6F5)4] was moni-
tored by NMR spectroscopy in CD2Cl2 at room temperature
and at ꢁ30 1C. In a typical experiment, a solution of 35 mg
[DBS][B(C6F5)4] (0.04 mmol) in 0.5 mL dried CD2Cl2 was
combined with a solution of 10 mg Cp2ZrMe2 (0.04 mmol) in
0.5 mL of the same deuterated solvent in an NMR tube. The
1
2
3
C(1)–C(2)
C(1)–C(15)
C(8)–C(9)
C(9)–C(10)
C(7)–C(8)
C(2)–C(7)
C(10)–C(15)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(6)–C(7)
C(10)–C(11)
C(11)–C(12)
C(12)–C(13)
C(13)–C(14)
C(14)–C(15)
1.412(3)
1.419(3)
1.355(3)
1.424(3)
1.418(3)
1.435(2)
1.431(2)
1.419(3)
1.353(3)
1.395(3)
1.367(3)
1.433(3)
1.416(3)
1.382(3)
1.394(3)
1.344(3)
1.421(3)
1.401(3)
1.402(3)
1.356(4)
1.427(4)
1.420(4)
1.444(3)
1.433(3)
1.418(3)
1.360(4)
1.401(4)
1.359(4)
1.412(3)
1.404(4)
1.364(4)
1.391(5)
1.362(4)
1.424(3)
1.393(8)
1.379(8)
1.331(8)
1.407(8)
1.414(8)
1.403(8)
1.419(8)
1.404(8)
1.366(8)
1.379(10)
1.364(9)
1.425(9)
1.391(8)
1.352(8)
1.391(8)
1.324(8)
1.423(8)
1
NMR tube was shaken and H and 19F NMR spectra were
acquired. Similar experiments were carried out at room
temperature in toluene-d8.
Ethylene polymerizations
Ethylene polymerization experiments were carried out using
Cp2ZrMe2 activated by [DBS][B(C6F5)4], [Ph3C][B(C6F5)4] or
[C7H7][B(C6F5)4]; the polymerizations were performed under
1 atm of ethylene in a 200 mL glass reactor which was flame-
dried under reduced pressure prior to use. In a typical proce-
dure, a solution of 0.04 mmol Cp2ZrMe2 dissolved in 5 mL of
dry toluene was added to a solution of 0.04 mmol co-catalyst
in 10 mL of dry toluene in the glass reactor. The mixture
solution was stirred for 2 min at room temperature, after
which dried ethylene was bubbled through the solution for
30 min. The solution immediately became hot and polyethylene
began to precipitate, causing the mixture to become very
viscous. The polymerization was quenched after 30 min with
10 mL acidified methanol and the polyethylene was collected
by filtration, washed with methanol and dried under reduced
pressure overnight. The results are reported in Table 3.
C(2)–C(1)–C(15)
C(8)–C(9)–C(10)
C(1)–C(2)–C(7)
C(2)–C(7)–C(8)
C(9)–C(10)–C(15)
C(7)–C(8)–C(9)
C(1)–C(15)–C(10)
C(10)–C(15)–C(14)
C(9)–C(10)–C(11)
C(10)–C(11)–C(12)
C(11)–C(12)–C(13)
C(12)–C(13)–C(14)
C(13)–C(14)–C(15)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(4)–C(5)–C(6)
C(5)–C(6)–C(7)
C(2)–C(7)–C(6)
C(1)–C(2)–C(3)
C(3)–C(2)–C(7)
C(6)–C(7)–C(8)
C(11)–C(10)–C(15)
C(1)–C(15)–C(14)
134.13(19)
131.46(19)
124.7(7)
127.7(7)
124.4(7)
130.6(2)
125.23(18)
118.68(17)
115.99(18)
121.3(2)
119.7(2)
121.1(2)
121.3(2)
121.8(2)
120.3(2)
120.5(2)
121.4(2)
117.24(18)
116.01(18)
118.68(18)
116.05(19)
117.79(17)
116.09(18)
134.2(2)
131.3(2)
125.9(2)
125.3(2)
125.4(2)
131.5(2)
126.2(2)
118.8(2)
117.2(3)
122.2(3)
120.5(3)
119.9(3)
121.1(3)
122.1(2)
119.4(3)
120.9(2)
122.0(2)
117.4(2)
115.8(2)
118.2(2)
117.2(2)
117.3(2)
115.0(2)
134.9(7)
133.5(7)
124.7(7)
127.7(7)
124.4(7)
128.7(7)
125.8(7)
118.0(7)
117.2(7)
122.0(7)
119.2(7)
121.7(7)
120.7(8)
120.5(7)
118.8(8)
122.9(8)
119.5(8)
117.4(7)
114.5(7)
120.8(7)
114.9(7)
118.3(7)
116.2(7)
Propylene polymerizations
Propylene polymerization experiments were carried out as
above but at 0 1C using 0.024 mmol of Cp2ZrMe2 activated
by 1.5 molar equivalents of [DBS][B(C6F5)4] or
[Ph3C][B(C6F5)4] in the presence of 0.048 mmol of A1(i-Bu)3
as scavenger. A series of complementary propylene polymeri-
zation experiments was also carried out using
[DBS][B(CH2CMe3)(C6F5)3] as the co-catalyst to activate
molecular ions of the carbocation and the anion, respectively)
and NMR spectroscopy (1H NMR (CD2Cl2): (d 10.35 (s, 1H),
9.10 (s, 2H), 8.97 (d, 2H), 8.75 (m, 4H), 8.51 (m, 2H), 1.48
(br s, 2H), 0.63 (s, 9H)).
5d
Cp2ZrMe2,5d (Indenyl)2ZrMe2 and SBIZrMe2.5e Polymeri-
zation conditions and polymer manipulations were similar to
those described above, and the results are reported in Table 3.
The polymerizations were carried out under continuous bub-
bling of propylene for 2–5 min, after which the reaction was
terminated by stopping the propylene flow and quenching with
10 mL of acidified methanol. The solvent was removed under
reduced pressure to give viscous oils of atactic polypropylene
(Cp2ZrMe2 and (Indenyl)2ZrMe2) or white, solid isotactic
polypropylene (SBIZrMe2). The atactic materials were puri-
fied by dissolving in hexanes and elution through a silica
column. The solvent was removed under reduced pressure,
and the polypropylenes obtained were characterized by NMR
spectroscopy and GPC. Isotactic polypropylene was washed
with methanol and acetone, dried under reduced pressure and
characterized by NMR spectroscopy.
Synthesis of DBSCH2CMe3
A solution of 1.45 g of dibenzosuberenyl chloride in 60 mL of
dichloromethane was added dropwise to 0.5 g of LiCH2CMe3
cooled in an ice bath. The mixture was stirred for 1 h,
becoming light pink and cloudy, and then filtered through
Celite to give a yellow solution which was taken to dryness
under reduced pressure. The resulting crude DBSCH2CMe3
was purified by eluting through an alumina column with
hexanes to give a clear yellow oil (1.22 g, 71% yield). 1H
NMR of DBSCH2CMe3 (CD2Cl2): d 0.66 (s, 9H, CH2CMe3),
1.70 (d, 2H, CH2CMe3), 4.18 (t, 1H, H(1) of DBS) 6.98–7.34
(m, 10H, C(2)–C(14)). To confirm the identity, a high resolu-
tion mass spectrum (TOF MS EI+) was obtained: observed
261.1648; calc. for C20H21 (DBSCH2CMe3–H)+ 261.1643.
Difference 1.7 ppm.
Isobutene polymerizations initiated by [DBS][B(C6F5)4]
Isobutene polymerizations initiated by [DBS][B(C6F5)4] were
carried out in methyl chloride/dichloromethane mixtures at
ꢂc
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Table 3 Alkene polymerization results
Initiator system
Alkene
Yield/g
10ꢁ3Mw
Mw/Mn
Cp2ZrMe2/[Ph3C][B(C6F5)4]a
Ethylene
Ethylene
Ethylene
Propylene
Propylene
Propylene
Propylene
Propylene
1.4
0.8
1.3
4.4
4.9
2.4
1.6
3.1
—
—
—
38
55
78
98
84
—
—
—
2.0
Cp2ZrMe2/[DBS][B(C6F5)4]a
Cp2ZrMe2/[C7H7][B(C6F5)4]a
Cp2ZrMe2/[DBS][B(C6F5)4]b
Cp2ZrMe2/[Ph3C][B(C6F5)4]b
2.0
Cp2ZrMe2/[DBS][B(CH2CMe3)(C6F5)3]c
Ind2ZrMe2/[DBS][B(CH2CMe3)(C6F5)3]c
SBIZrMe2/[DBS][B(CH2CMe3)(C6F5)3]c,d
1.99
1.76
1.75d
Initiator system
Alkene
Conversion (%)
10ꢁ3Mw
Mw/Mn
[DBS][BC6F5)4] (1.25 mM)e
[DBS][BC6F5)4] (1.25 mM)e,f
[DBS][BC6F5)4] (1.8 mM)e
[DBS][BC6F5)4] (1.8 mM)e,f
[DBS][BC6F5)4] (2.8 mM)e
[DBS][BC6F5)4] (3.7 mM)e,f
a
Isobutene
Isobutene
Isobutene
Isobutene
Isobutene
Isobutene
99
6.7
96
24
95
10
1.7
1.3
2.2
1.2
2.0
0.5
3.0
1.4
2.5
1.3
2.8
1.4
Catalyst : activator ratio = 1 : 1; 0.040 mmol catalyst and 0.040 mmol activator in 15 mL toluene; initiated at 25 1C; 1 atm ethylene; time =
b
30 min; terminated with methanol. In the presence of Al(i-Bu)3. Catalyst : activator : Al(i-Bu)3 ratio = 1 : 1.5 : 2; 0.024 mmol catalyst and
c
0.036 mmol cocatalyst in 50 mL toluene at 0 1C; 1 atm propylene; time = 2 min. In the presence of Al(i-Bu)3. Catalyst : activator : Al(i-Bu)3
d
ratio = 1 : 2 : 2; 0.024 mmol catalyst and 0.048 mmol cocatalyst in 50 mL toluene at 0 1C; 1 atm propylene; time = 2 min. mmmm =
e
f
95%. Temperature = ꢁ80 1C; time = 1 h; vol of isobutene = 10 mL; vol of CH3Cl = 13 mL; vol of dichloromethane = 2 mL. In the presence
of DTBP. Initiator : DTBP = 1 : 2.
ꢁ80 1C as described previously.8 Similar polymerization ex-
periments were also carried out in the presence of the proton
trap 2,6-di-tert-butylpyridine (ratio of DTBP : DBS = 2 : 1).
In all cases, 10 mL of isobutene were polymerized using
concentrations of initiator as indicated below. The results
are presented in Table 3.
of chloride ion from dibenzosuberenyl chloride by B(C6F5)3;
[DBS][BCl(C6F5)3] has not been reported previously although
Bochmann et al. have reported the crystal structure of the
analogous [Ph3C][BCl(C6F5)3].1f Similarly [DBS][B(C6F5)3–
OHꢀ ꢀ ꢀH2O–B(C6F5)3] was prepared inadvertently during a reac-
tion of DBSOH with B(C6F5)3 in an attempt to obtain
[DBS][B(C6F5)3(m-OH)B(C6F5)3]. Trace amounts of water pre-
sent in the solution resulted instead in the formation of a small
amount of the [B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3]ꢁ anion in
Results and discussion
The new, stable carbocationic salt [DBS][B(C6F5)4] was
synthesized by two methods. In the first, dibenzosuberenyl
chloride was reacted with Li[B(C6F5)4] in dichloromethane at
room temperature as in eqn (2).
which
a water molecule has been incorporated. While
[DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3] has not previously been
reported, the [B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3]ꢁ anion was
previously synthesized as the [C10H6(NMe2)2H]+ and
[NHEt3]+ salts.9 The compounds [DBS][BCl(C6F5)3] and
[DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3] could not be obtained
analytically pure, but the formation of a few small crystals of
each made crystallographic structure determinations possible
(see below).
C15H11Cl + Li[B(C6F5)4] - [DBS][B(C6F5)4] + LiCl (2)
In the second method [DBS][B(C6F5)4] was prepared by reac-
tion of [Ph3C][B(C6F5)4] with dibenzosuberenyl chloride as in
eqn (3).
We
have
previously
described
the
use
of
C15H11Cl + [Ph3C][B(C6F5)4] - [DBS][B(C6F5)4]
+ Ph3CCl
[B(CH2CMe3)(C6F5)3]ꢁ as a weakly coordinating anion during
olefin polymerizations,7 and we decided to investigate also the
DBS+ salt of the [B(CH2CMe3)(C6F5)3]ꢁ anion. The compound
[DBS][B(CH2CMe3)(C6F5)3] was accordingly synthesized via a
metathesis reaction involving Li[B(CH2CMe3)(C6F5)3]7 and
dibenzosuberenyl chloride, and the red salt was characterized
as such by NMR spectroscopy and mass spectrometry. Unfortu-
nately, all attempts to purify it resulted in conversion to what
(3)
The latter result is consistent with the above-mentioned
greater stability of the DBS cation.3
In both cases, the deep red color of the DBS carbocation
developed quickly and the ionic product of metathesis was
readily precipitated in good yield by the addition of hexanes.
Identification of [DBS][B(C6F5)4] was confirmed by elemental
analyses and 1H and 19F NMR spectroscopy and the structure
was determined crystallographically (see below). The DBS salt
is quite stable when stored in a dry environment, and the
procedure outlined here can readily be used to synthesize this
source of the DBS carbocation for use in other than strongly
acidic media at low temperatures.4f,g
1
appeared to be C15H11CH2CMe3 on the basis of its H NMR
spectrum. This would be the product of neopentyl anion transfer
from boron to the DBS carbocation, and it was therefore also
synthesized by treating dibenzosuberenyl chloride with neopentyl-
lithium. The 1H NMR spectrum of the resulting compound
was identical to that of the compound obtained from the
decomposition of [DBS][B(CH2CMe3)(C6F5)3] and also very
similar to that of C15H11Me, a known compound discussed
The analogous red compound [DBS][BCl(C6F5)3] was pre-
pared in small amounts during an investigation of the abstraction
ꢂc
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Fig. 1 Structure of [DBS][B(C6F5)4] (1).
below. Thus the nature of the thermal degradation of
[DBS][B(CH2CMe3)(C6F5)3] seems confirmed.
Fig. 3 Structure of [DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3] (3).
X-Ray quality crystals of the compounds [DBS][B(C6F5)4],
[DBS][BCl(C6F5)3] and [DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3]
were grown from dichloromethane solutions by layering with
hexanes and their crystal structures were determined. The
structures of [DBS][B(C6F5)4], [DBS][BCl(C6F5)3] and
[DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3] are shown in Fig. 1, 2
and 3, respectively, crystallographic data for all in Table 1 and
important bond lengths and angles for the cation of each in
Table 2. To our knowledge, these crystal structures are the first
reported for the DBS carbocation.
only the C(8)–C(9) bond lengths are sufficiently short to
approximate that of the tropylium ion (average B1.35 A).10
Indeed, the C(1)–C(2)/C(1)–C(15) and C(2)–C(7)/C(10)–C(15)
bonds are relatively long, implying an extensively delocalized,
conjugated structure as in C rather than the localized structure
implied by B.
As expected, the structures of the cations in the three salts
are all essentially planar and the carbon–carbon bond lengths
are also very similar although the relatively large esds for
[DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3] render comparisons
involving this salt tenuous. The average of the six C(1)–C(2)/
C(1)–C(15) bond lengths is B1.40 A, that of the six C(2)–C(7)/
C(10)–C(15) bond lengths is B1.43 A and that of the six
C(7)–C(8)/C(9)–C(10) bond lengths is B1.42 A while the
average of the three C(8)–C(9) bond lengths is B1.35 A. Thus
The structures of the anions are similar to those of
[Ph3C][BCl(C6F5)3]1f or the [C10H6(NMe2)2H]+ and [NHEt3]+
salts of [B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3].9
NMR spectra
The numbering scheme used to assign 1H NMR chemical shifts
1
is shown in B and C. The H NMR spectra (CD2Cl2) of the
DBS salts of [B(C6F5)4]ꢁ, [BCl(C6F5)3]ꢁ, [B(C6F5)3–
OHꢀ ꢀ ꢀH2O–B(C6F5)3]ꢁ and [B(CH2CMe3)(C6F5)3]ꢁ all exhib-
ited resonances at d 10.37 (s, 1H, H(1)), 9.13 (s, 2H, H(8,9)), and
8.97 (d, 2H), 8.76 (m, 4H) and 8.52 (m, 2H), attributed to
H(3–6, 11–14). These chemical shifts are similar to those
reported elsewhere for DBS+ generated in FSO3H–SO2ClF at
ꢁ78 1C (d 10.98 (H(1)), 9.65 (H(8,9)), 8.9–9.54).4f,g For pur-
poses of comparison, we note that the 1H chemical shift of
[C7H7][B(C6F5)4] is d 9.27 (CD3CN).2a
The 1H NMR spectrum of [DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–
B(C6F5)3] also exhibited a broad resonance at d 8.16, attri-
1
buted to the H3O2 moiety,9 while the H NMR spectrum of
[DBS][B(CH2CMe3)(C6F5)3] exhibited singlets at d 0.63 and
1.48, attributed to the Me3C and CH2 groups, respectively.7
The 13C NMR spectrum (CD2Cl2) of [DBS][B(C6F5)4] exhi-
bited resonances with chemical shifts (d 172.0, 146.4, 144.1,
143.0, 141.6, 137.6, 135.5, 134.5) very similar to those reported
previously for other DBS salts in FSO3H–SO2ClF at ꢁ78 1C
Fig. 2 Structure of [DBS][BCl(C6F5)3] (2).
ꢂc
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(d 170.7, 147.1, 144.9, 143.6, 142.5, 138.3, 136.2, 135.0),4f,g
while the 19F NMR spectra of all three salts were consistent
with the literature for the various anions.1f,9
Utilizing the atom labeling scheme of B and C,5c the multi-
1
plet at d 7.2–7.5 in the H NMR spectrum of Fig. 4 may be
assigned to H(3–6, 11–14) of the two isomers, the singlets at d
6.95 and 7.17 to H(8,9) of the two isomers, the quartets at d
3.50 (J 7.32 Hz) and 4.18 (J 7.32 Hz) to the methyne hydrogens
H(1) of the two isomers and the doublets at d 1.31 (J 7.32 Hz)
and 1.90 (J 7.32 Hz) to the methyl groups of the two isomers.
A NOESY experiment run in CD2Cl2 at ꢁ30 1C demonstrated
correlations between the resonances at d 1.90 and 3.50 and
between those at d 1.31 and 4.18. There was also a correlation
between the methyl resonance at d 1.90 and a resonance in the
aromatic region at d B7.5 and another between the methyne
resonance at d 4.18 and a resonance in the aromatic region at d
B7.3, suggesting that the resonances at d 1.90 and 4.18 are
both to be attributed to pseudo-equatorial groups. Therefore,
while neither of the olefinic resonances at d 6.95 and 7.17
correlated with any of the methyl or methyne resonances and
thus cannot be assigned to D or E, the resonances at d 1.31 and
4.18 can confidently be assigned to D, those at d 1.90 and 3.50
to E. These assignments have not previously been attempted.5c
A similar reaction of [DBS][B(C6F5)4] with Cp2ZrMe2 in
toluene-d8 resulted in the precipitation of an orange oil,
presumably of [Cp2ZrMe][B(C6F5)4] in view of the appearance
in the solution phase of the resonances of the two isomers, D
and E, of DBSMe.
Activation of dimethylzirconocene by [DBS][B(C6F5)4]
The reaction of Cp2ZrMe2 with [Ph3C][B(C6F5)4] at room
temperature has been shown to proceed as in eqn (1).1f In
order to assess and compare the ability of the DBS carbo-
cation to likewise abstract a methyl group, the room-tempera-
ture reaction of [DBS][B(C6F5)4] with Cp2ZrMe2 was
monitored by 1H NMR spectroscopy in CD2Cl2 and
1
toluene-d8. The H NMR spectrum of a representative reac-
tion mixture in CD2Cl2, run a few minutes after mixing of the
reactants, is shown in Fig. 4 and, as can be seen, the Cp and
Me resonances of Cp2ZrMe2 at d 6.13 and ꢁ0.38, respectively,
have disappeared and have been replaced by the Cp and Me
resonances of [Cp2ZrMe]+ at d 6.42 and 0.77, respectively.1f
Thus clean methyl abstraction as in eqn (4) has indeed
occurred.
Cp2ZrMe2 + [DBS][B(C6F5)4] - [Cp2ZrMe][B(C6F5)4]
+ DBSMe
(4)
Concomitantly with these changes, the resonances of
[DBS][B(C6F5)4] at d 10.37 (s, 1H), 9.13 (s, 2H), 8.97
(d, 2H), 8.76 (m, 4H), 8.52 (m, 2H) have disappeared and
have been replaced by a new set of resonances at d 7.2–7.5 (m),
6.95 (s) and 7.17 (s), 3.50 (q, J 7.32 Hz) and 4.18 (q, J 7.32 Hz),
1.31 (d, J 7.32 Hz) and 1.90 (d, J 7.32 Hz), which are very
similar to the resonances of DBSMe reported previously.5c
Thus, as hoped, the DBS carbocation does indeed readily
abstract a methyl carbanion from Cp2ZrMe2.
We note that the 1H NMR spectrum of DBSCH2CMe3,
obtained via both the thermal decomposition of
[DBS][B(CH2CMe3)(C6F5)3] and the reaction of Li[B(CH2CMe3)-
(C6F5)3] with dibenzosuberenyl chloride, exhibited a quartet
methyne resonance at d 4.18 but not one at d B3.50. On this
basis, it seems that DBSCH2CMe3 exists in solution with the
bulky neopentyl group in the pseudo-axial position.
That there are two sets of resonances in the spectrum of the
hydrocarbon product is a result of there being present in
solution almost equal amounts of two isomers in which the
methyl group exists in the pseudo-axial or the pseudo-
equatorial position as in D and E, respectively.
Ethylene, propylene polymerizations
Ethylene and propylene polymerization experiments were
carried out to compare the efficiency of [DBS][B(C6F5)4] as
cocatalyst with that of the well known [Ph3C][B(C6F5)4];
[C7H7][B(C6F5)4] was also investigated briefly. In the case of
ethylene, the precursor Cp2ZrMe2 was activated with
[DBS][B(C6F5)4], [C7H7][B(C6F5)4] and [Ph3C][B(C(C6F5)4] in
1 : 1 molar ratios. Polymerizations were carried out in flame-
dried glass reactors in 15 mL of dry toluene and were
terminated by methanol injection after 30 min. In a typical
polymerization run, ethylene was continuously bubbled
Fig. 4 1H NMR spectrum of a reaction of [DBS][B(C6F5)4] with Cp2ZrMe2 in CD2Cl2 at ꢁ30 1C.
ꢂc
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through the catalyst mixture throughout the whole course of
polymerization experiment in order to keep the ethylene
pressure constant at 1 atmosphere. Polymerization experi-
ments were carried out with the reactors immersed in a water
at 25 1C, although all became warmer during the polymeriza-
tions. In all cases polyethylene precipitated as the polymeriza-
tions proceeded, causing the solutions to become highly
viscous. Similar behavior has been reported previously for
similar types of metallocene polymerization catalysts,1f and it
is likely that monomer diffusion was subject to mass-transport
limitations and that the polymer yields obtained for ethylene
polymerization should be regarded as the lower limits. As can
be observed in Table 3, in which we report data which are
averages of results of two or three experiments, the polymer
yields obtained using [DBS][B(C6F5)4], [C7H7][B(C6F5)4] and
[Ph3C][B(C6F5)4] are probably not significantly different. Thus
the activity of [DBS][B(C6F5)4] is, as anticipated, comparable
to that of the much better studied [Ph3C][B(C(C6F5)4].
The results of the propylene polymerizations with
Cp2ZrMe2 activated by [DBS][B(C6F5)4] and, for comparison,
with [Ph3C][B(C6F5)4], in the presence of Al(i-Bu)3 as scaven-
ger at 0 1C are shown in Table 3. Again we report data which
are averages of results of two or three experiments. As in the
ethylene polymerizations, the reactions were carried out in
50 mL dry toluene in a flame-dried glass reactor and Cp2ZrMe2
was activated with [DBS][B(C6F5)4] or [Ph3C][B(C6F5)4] in a
1 : 1 molar ratio. In a typical experiment, the toluene solution
was saturated with propylene followed by addition of toluene
solutions of both Cp2ZrMe2 and [DBS][B(C6F5)4] or
[Ph3C][B(C6F5)4], respectively. Polymerizations were rapid at
1 atm propylene pressure and the reaction temperatures rose
somewhat. All reactions were terminated by injection with
methanol after 2 min. As with ethylene the yields and mole-
cular weights of the products varied somewhat but there
appears to be no significant difference between DBS and trityl
carbocations as initiators.
other initiators under the same conditions.8 The molecular
weight distributions were also rather broad,8 suggesting the
presence of more than one initiator species in solution.
Although cations such as DBS+ can in principal induce
carbocationic polymerization of isobutene directly,11a it has
been shown that isobutene polymerizations initiated by the
electrophilic species B(C6F5)3,11b Ph3C+,11b and Me3Si+11c
occur because of the release of protons formed via reaction
of these cations with trace amounts of water in the solvent
systems used. This would involve the reaction of eqn (5) for
DBS+, and, since dichloromethane is in fact very difficult to
dry rigorously, initiation by residual protons needed to be
considered.
[DBS][B(C6F5)4] + H2O - DBSOH + [H][B(C6F5)4] (5)
To assess the possibility that isobutene polymerization was
initiated by the protons arising from the partial hydrolysis of
[DBS][B(C6F5)4] by adventitious water present in the system,
polymerization experiments were carried out in the presence of
2,6-di-tert-butylpyridine (DTBP). The non-nucleophilic
DTBP generally behaves as a strong, sterically hindered base
which does not react with most electrophiles but is readily
protonated and thus behaves as a very effective proton
scavenger.11d
To ensure DTBP does not in fact react with the [DBS]+
1
cation, we carried out initially a H NMR study in which we
combined 0.015 mmol of [DBS][B(C6F5)4] with 0.03 mmol of
DTBP in CD2Cl2 in an NMR tube. As expected, the only
resonances observable were those of the [DBS]+ cation and of
DTBP and therefore [DBS][B(C6F5)4] does not react with
DTBP. DTBP was then added to several [DBS][B(C6F5)4]
solutions prior to initiation of isobutene polymerization and,
as can be seen from the results shown in Table 3, the addition
of DTBP significantly reduces the yield of the product formed.
Thus the number of active sites is indeed decreased by the
presence of DTBP and hydrolysis as in eqn (5) is indeed a
factor in these polymerization reactions.
Similar experiments were carried out in order to assess and
compare the utility of [DBS][B(CH2CMe3)(C6F5)3] as a co-
initiator of Cp2ZrMe2, Ind2ZrMe2 and SBIZrMe2. As can be
seen in Table 3, while the polymer yields obtained using the
Cp2ZrMe2/[DBS][B(CH2CMe3)(C6F5)3] and Ind2ZrMe2/
[DBS][B(CH2CMe3)(C6F5)3] catalyst systems appear to be
lower than those of Cp2ZrMe2/[DBS][B(C6F5)4] and In-
d2ZrMe2/[DBS][B(C6F5)4], the resulting polymers have signi-
ficantly higher molecular weights. The activities of these
[B(CH2CMe3)(C6F5)3]ꢁ-based catalyst systems are also com-
parable with activities reported previously for analogous
catalysts, as is the tacticity of the isotactic polypropylene
produced by the SBIZrMe2/[DBS][B(CH2CMe3)(C6F5)3]
catalyst system.7
Summary
The bright red compounds [DBS][B(C6F5)4], [DBS]-
[BCl(C6F5)3], [DBS][B(C6F5)3–OHꢀ ꢀ ꢀH2O–B(C6F5)3] and
[DBS][B(CH2CMe3)(C6F5)3] (DBS = dibenzosuberenyl or
dibenzotropylium or dibenzo[a,e]tropylium or dibenzo-
[a,d]cycloheptenocarbonium), all of them new, have been synthe-
sized and characterized spectroscopically and, for the first
three, crystallographically. The three crystal structures show
that the DBS+ carbocation is planar, as expected, and are
consistent with delocalization of the positive charge over part
of the seven-membered ring, in particular the CH carbon and
the carbon atoms a and b to it. The DBS+ carbocation is an
efficient carbanion abstractor and activates metallocene cata-
lyst precursors of the type Cp02ZrMe2, its efficacy being
comparable to that of the well known but sterically more
hindered trityl carbocation, Ph3C+. As with Ph3C+, DBS+
does not initiate carbocationic polymerization of isobutene in
dichloromethane directly, but rather by reacting with trace
amounts of water to generate protons.
Isobutene polymerizations initiated by [DBS][B(C6F5)4]
Polymerizations of isobutene were carried out in methyl
chloride–dichloromethane mixtures at ꢁ80 1C as described
previously,8 with typical results (all averages of 2–4 experi-
ments) shown in Table 3. As can be seen, conversions were
generally high, but molecular weights of the polymers ob-
tained were considerably lower than observed previously using
ꢂc
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Acknowledgements
We thank Queen’s University, the Natural Sciences and
Engineering Research Council of Canada (Discovery Grant
to M. C. B.), the Government of Ontario and Lanxess, Inc. for
financial support, Drs Andrew Carr and Kevin Kulbaba
(Lanxess) for helpful discussions and advice, and Dr Q. Wang
of NOVA Chemicals for molecular weight measurements of
the isotactic polypropylene samples.
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1898 | New J. Chem., 2008, 32, 1890–1898