A Highly Fluorous Room-Temperature Ionic Liquid Exhibiting Fluorous Biphasic Behavior and Its Use in Catalyst Recycling

Article abstract of DOI:10.1021/ol026700l

(Matrix Presented) A novel fluorous room-temperature ionic liquid, 1-butyl-3-methyl-imidazolium tetrakis[p-{dimethyl(1H,1H,2H,2H-perfluorooctyl)silyl}phenyl]-borate (1), was used as a solvent for the homogeneous hydrosilylation of 1-octene catalyzed by a

Full text of DOI:10.1021/ol026700l

ORGANIC
LETTERS
2
002
Vol. 4, No. 22
851-3854
A Highly Fluorous Room-Temperature
Ionic Liquid Exhibiting Fluorous
Biphasic Behavior and Its Use in
Catalyst Recycling
3
†
†
,†,‡
Joep van den Broeke, Ferry Winter, Berth-Jan Deelman,* and
Gerard van Koten
†
Department of Metal-Mediated Synthesis, Debye Institute, Utrecht UniVersity,
Padualaan 8, 3584 CH Utrecht, The Netherlands, and ATOFINA Vlissingen B.V.,
P.O. Box 70, 4380 AB Vlissingen, The Netherlands
berth-jan.deelman@atofina.com
Received August 8, 2002
ABSTRACT
A novel fluorous room-temperature ionic liquid, 1-butyl-3-methyl-imidazolium tetrakis[p-{dimethyl(1H,1H,2H,2H-perfluorooctyl)silyl}phenyl]-
borate (1), was used as a solvent for the homogeneous hydrosilylation of 1-octene catalyzed by a fluorous version of Wilkinson’s catalyst. The
catalyst was recycled by biphasic separation with an average retention of catalyst activity of 94%. As opposed to other ionic liquids, 1 exhibits
high miscibility with apolar compounds such as alkenes and resembles fluorous solvents in its phase behavior with organic solvents.
The use of air- and water-stable imidazolium-based ionic
liquids as a solvent for transition metal catalysts has received
limited solubility of substrates and catalysts can result in
reduction or even complete loss of activity when compared
with truly homogeneous conditions.
The nature of ionic liquids, being composed of discrete
anions and cations, makes it possible to fine-tune both
lipophilicity and polarity through the choice of suitable
cation-anion combinations. In view of the considerations
above, we reasoned that it would be interesting to design
ionic liquids with improved solvating properties for apolar
compounds, as this might allow more efficient catalysis with
1
3a,5
growing attention since their preparation was first reported
2
in 1992. This interest was particularly sparked by the fact
that these ionic liquids are highly stable and show limited
miscibility with most of the common organic solvents,
offering potential for efficient catalyst recovery by facile
1
phase separation. This approach has been successfully
applied in a wide range of catalytic processes, for example,
3
a,b
3a
3c
hydrogenation,
hydroformylation, Heck reactions,
3
d
3e
3f
oligomerizations, epoxidations, and polymerizations. A
serious limitation for the application of ionic liquids is their
poor solubility of apolar organic substrates, especially
saturated hydrocarbons and in some cases neutral metal
catalysts (vide infra).1 As in aqueous biphasic systems, the
(3) (a) Chauvin, Y.; Mussmann, L., Olivier, H. Angew. Chem., Int. Ed.
Engl. 1995, 34, 2698-2700. (b) Suarez, P. A. Z.; Dullius, J. E. L.; Einloft,
S.; de Souza, R. F.; Dupont, J. Polyhedron 1996, 15, 1217-1219. (c)
Carmichael, A. J.; Earle, M. J.; Holbrey, J. D.; McCormac, P. B.; Seddon,
K. R. Org. Lett. 1999, 1, 997-1000. (d) Chen, W.; Xu, L.; Chatterton, C.;
Xiao, J. Chem. Commun. 1999, 1247-1248. (e) Song, C. E.; Roh, E. J.
Chem. Commun. 2000, 837-838. (f) Carmichael, A. J.; Haddleton, D. M.;
Bon, S. A. F.; Seddon, K. R. Chem. Commun. 2000, 1237-1238.
(4) (a) Bonh oˆ te, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram,
K.; Gr a¨ tzel, M. Inorg. Chem. 1996, 35, 1168-1178. (b) Koel, M. Proc.
Estonian Acad. Sci. Chem. 2000, 3, 145-155.
,4
†
Utrecht University.
ATOFINA Vlissingen B.V..
‡
(1) Recent reviews and references therein: (a) Wasserscheid, P.; Keim,
W. Angew. Chem., Int. Ed. 2000, 39, 3772-3789. (b) Sheldon, R. Chem.
Commun. 2001, 2399-2407. (c) Welton, T. Chem. ReV. 1999, 99, 2071-
2
9
083. (d) Olivier, H. J. Mol. Catal. A 1999, 146, 285-289.
(5) (a) Einloft, S.; Dietrich, F. K.; de Souza, R. F.; DuPont, J. Polyhedron
1996, 15, 3257-3259. (b) Dyson, P. J.; Ellis, D. J.; Parker, D. G.; Welton,
T. Chem. Commun. 1999, 25-26.
(
2) Wilkes, J. S.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1992,
65-967.
1
0.1021/ol026700l CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/01/2002

apolar substrates. In this respect, the introduction of per-
fluoroalkyl moieties as lipophilic groups could be of special
interest, as this could lead to ionic liquids exhibiting fluorous
biphasic behavior, i.e., biphasic conditions at lower temper-
atures and monophasic at higher temperatures. Such a
fluorous ionic liquid could well be an alternative to perfluo-
roalkane-based fluorous biphasic systems (FBSs), the fluo-
rous phase of which appears to be less suitable for the
immobilization of ionic catalysts.6
Recently, we reported the synthesis of highly fluorous
6a
tetraphenylborate anions. We now present the use of these
anions in the preparation of imidazolium borate ionic liquids
and their performance in fluorous biphasic catalyst recycling.
Synthesis and Physical Properties of [BMIm][B{C
SiMe CH CH ]. The imidazolium salt [BMIm]-
B{C
6 4
H -
(
[
2
2
2 6 4
C F13)-p}
6
H
4
(SiMe
2 2 2 6
CH CH C F
13)-p}
4
] (1) (BMIm ) 1-butyl-
3-methylimidazolium) (Scheme 1) was obtained by metathesis
Figure 1. ENR values for [C
organic solvents. ENR ) (hcN
constant, c is the speed of light, N
max is the wavelength at maximum absorbance (nm). [BArf] )
B{C (SiMe CH CH 13)-p} ].
n
-MIm]X ionic liquids and common
A
6
/λmax) × 10 , where h is Planck’s
Scheme 1
A
is Avogadro’s number, and
λ
[
H
6 4
2
2
2
C
6
F
4
Table 1. Solubility of
BMIm][B{C (SiMe
Solvents
[
H
6 4
2 2 2 6 4
CH CH C F13)-p} ] (1) in Various
7
6a
of [BMIm]I and Na[B{C
6
H
4
(SiMe
2
CH
2
CH
2
C
6
F
13)-p}
4
] in
2 2
CH Cl . This salt is an air-stable, nonhygroscopic viscous
dielectric
constant (ꢀr)a
yellow liquid (F ) 1.38 g/mL) at 25 °C that is transformed
into a glasslike substance at subambient temperatures. No
distinct phase transition enthalpy could be determined using
DSC; however, during repeated heating cycles, a slight
change in the slope of the DSC trace was observed, pointing
to a possible glass transition at -10 °C. The conductivity of
neat 1 is 1.9 × 10 S m at 25 °C, and it increases as
viscosity decreases at elevated temperatures (1.1 × 10
at 95 °C). These values are low for room temperature
ionic liquids, which usually have conductivities in the range
solvent
DNa
solubility (g/L)b
1.6c
2.3
1.9
1.8
2.4
4.2
20.6
32.7
3.7 × 10
-1
FC-72
benzene
hexane
pentane
toluene
Et2O
acetone
methanol
methanol:H2O (1:1)
0.1
0.0
8.0
1
1.9 × 10
1
2.7 × 10
1
0.1
19.2
17.0
30.0
4.1 × 10
-
5
-1
2
2
1
5.1 × 10
5.8 × 10
3.3 × 10
2.5
-
3
S
-1
m
-
1 8
2
H O
18.0
78.4
<0.2
of 0.1-1.0 S m . The relative polarity of the ionic liquid
was assessed using the solvatochromatic dye Nile Red.
Measurement of λabsmax of Nile Red dissolved in 1 and
9
a
b
Donor number, taken from ref 24. Solubility in g/L of pure solvent
at 25 °C. c A mixture of perfluorohexanes, the ꢀ given is that of perfluoro-
r
n-hexane, a major component of FC-72.
determination of the molar transition energy (ENR) for this
dye in the solution show that 1 can be considered as a polar
solvent (Figure 1).
The solubility properties of 1 were investigated; it was
found to be insoluble in water and very soluble in polar and
to some extent in apolar organic solvents (Table 1). As
alkenes are important substrates in various catalytic pro-
cesses, the behavior of various mixtures of 1-alkenes and 1
was also studied. Solubilities of 1-hexene (16.7 mol/mol, 9.3
are much higher than those reported for closely related ionic
10
6
liquids [BMIm][PF ] (1-octene: 0.026 mol/mol) and [MeN-
11
(
n-Hex) ]Tos (1-octene: 1.5 mol/mol). Furthermore, mix-
3
tures of 1-hexene or 1-octene and 1 (φv(1-alkene) ) 0.63) give
a biphasic system at room temperature but display low
consolute temperatures (T ) 60 and 84 °C, respectively).
They transform into clear homogeneous liquids above these
temperatures. The phase behavior of 1 with toluene was
2
2
×
10 g/L) and 1-octene (4.7 mol/mol, 3.5 × 10 g/L) in 1
c
investigated more thoroughly; a critical temperature (T ) of
(6) (a) van den Broeke, J.; Lutz, M.; Kooijman, H.; Spek, A. L.; Deelman,
B.-J.; van Koten, G. Organometallics 2001, 20, 2114-2117. (b) de Wolf,
E.; Spek, A. L.; Kuipers, B. W. M.; Philipse, A. P.; Meeldijk, J. D.; Bomans,
P. H. H.; Frederik, P. M.; Deelman, B.-J.; van Koten, G. Tetrahedron 2002,
62 °C at a toluene volume fraction (φv(toluene)) of 0.90 was
found (Figure 2).
5
8, 3911-3922..
(
7) Lucas, P.; El Mehdi, N.; Ho, H. A.; Belanger, D.; Breau, L. Synthesis
(10) Brasse, C. C.; Englert, U.; Salzer, A. Organometallics 2000, 19,
3818-3823.
(11) Waffenschmidt, H. Ph.D. Thesis, Rheinisch-Westfalische Technische
Hochschule Aachen, Aachen, Germany, 2000.
2
000, 9, 1253-1258.
(8) Hagiwara, R.; Ito, Y. J. Fluorine Chem. 2000, 105, 221-227.
9) Deye, J. F.; Berger, T. A. Anal. Chem. 1990, 62, 615-622.
(
3852
Org. Lett., Vol. 4, No. 22, 2002

of 1 resemble that of both an ionic liquid and a fluorous
solvent. The low critical temperature with toluene confirms
1
4
this observation.
Recycling of a Fluorous Wilkinson’s Catalyst Dissolved
in the Fluorous Ionic Liquid. To assess the suitability of 1
for use as a catalyst immobilization medium, rhodium-
15
catalyzed hydrosilylation of 1-octene was studied. Wilkin-
son’s complex, [RhCl(PPh ], a well-known catalyst for this
reaction, is known to form stable solutions in [BMIm][BF
3 3
)
4
]
3
b
6
and [BMIm][PF ]. However, as Wilkinson’s complex
displayed a higher affinity for the organic rather than for
the ionic phase, the latter ionic liquids turned out to be
unsuccessful for the immobilization of the catalyst. Surpris-
ingly, whereas [RhCl(PPh
3 3
) ] proved to be insoluble in 1, a
lightly fluorous derivative of Wilkinson’s catalyst, RhCl-
1
6
[
6 4 2 2 2 6 3 3
P{C H (SiMe CH CH C F13)-p} ] (2), exhibited interest-
-
2
ing solubility in 1; concentrations of at least 1.4 × 10
M
were attainable.
Figure 2. Phase diagram of [BMIm][B{C
p} ] and toluene.
4
6 4 2 2 2 6
H (SiMe CH CH C F13)-
The hydrosilylation reaction of 1-octene with dimethyl-
silane catalyzed by 2 in 1 afforded dimethylphenyloctylsilane
(Table 2), with addition of the silane occurring in a selective
The physical properties of 1 show that it differs consider-
ably from the ionic liquids reported previously. Whereas
other [BMIm]-based compounds display polarities close to
that of methanol and have considerable solubility in water
Table 2. Comparison of the Hydrosilylation of 1-Octene Using
Either Non-Fluorous or Fluorous Wilkinson’s Catalysts in Ionic
Liquidsa
12
3
e,4
(
exceptions are [BMIm][PF
6
] and [BMIm][(CF
3
SO
2
)
2
N])
1a,4
and essentially no solubility in alkanes, the polarity and
solubility of 1 correspond with those for less polar solvents.
It is well-known that an increase in the size of the anion is
1
2a
accompanied by a decrease in the polarity of ionic liquids,
and our results correspond with this trend, 1 being less polar
than [BMIm][PF ] or [C10MIm][BF ] (C10MIm ) 1-decyl-
-methyl-imidazolium). The determined ENR shows a polarity
catalyst
solvent
benzene
cycle
TOFb (h-1
)
rc
6
4
3 d
RhCl(PPh3)3
RhCl(PPh3)3
1.8 × 10
3
[BMIm][BF4]
4.0 × 102
for 1 that is comparable to that of acetone and diethyl ether,
which corresponds with the data in Table 1; the solubility
of 1 is highest in these solvents. Furthermore, large ion size
results in high viscosity, which in turn causes low conductiv-
e
2
2
2
2
2g
1
1
1
1
1
1
<1
2
1
2
3
4.0 × 10
2
3.1 × 10
0.77
0.91
〈0.94〉 f
2
2.8 × 10
2
15
1.3 × 10
8
ity, and both of these properties were observed for 1.
2
3.2 × 10
The fact that 1 is insoluble in water, dissolves best in
solvents with a high dielectric constant and a high donor
number, and exhibits some solubility in hydrocarbons is
remarkable. Ionic liquids, in general, are highly soluble in
a
Conditions: 17 µmol (0.2 mol %) of catalyst, 1.2 mL of ionic liquid,
.75 mmol of 1-octene, 7.85 mmol of dimethylphenylsilane, t ) 60 min, T
8
)
84 °C (unless stated otherwise). b Average turnover frequency (TOF)
determined after 1 h and defined as mol of silane per mol of initial Rh per
hour. c Retention of catalyst activity ) activity in cycle n/activity in cycle
4a
polar solvents, especially in MeOH, and virtually insoluble
n - 1). Reaction was 99% complete after 15 min. Blank run. f Average
d
e
(
4
g
in apolar solvents. The high solubility in a solvent capable
retention per cycle for cycles 4-15. T ) 100 °C, homogeneous conditions.
2
of acting as a hydrogen bridge donor (e.g., H O, MeOH) is
predominantly the result of interactions between the anions
1
3
of the ionic liquid and the solvent. However, as the strength
of the hydrogen bonding decreases, the solubility of the ionic
liquid will be reduced as well. Because the fluorous anion
in 1 offers no possibility for hydrogen bonding interactions,
low solubility in water and, to some extent, methanol results.
On the other hand, the lipophilic anion enhances solubility
in apolar solvents, indicating that the perfluoroalkyl groups
moderate the ionic character of 1. This makes the behavior
2 2
anti-Markovnikov fashion. Formation of PhMe SiSiMe Ph
was not observed, but some isomerization of the remaining
1-octene (ca. 30% by GC) took place. Similar behavior was
17
(
14) Hildebrand, J. H.; Cochran, D. R. F. J. Am. Chem. Soc. 1949, 71,
2-25.
15) de Wolf, E.; Speets, E. A.; Deelman, B.-J.; van Koten, G.
2
(
Organometallics 2001, 20, 3686-3690. The authors used 1-hexene as an
olefinic substrate.
(16) (a) Richter, B.; de Wolf, E.; Deelman, B.-J.; van Koten, G. PCT
Int. Appl. WO 0018444, 2000. (b) Richter, B.; Spek, A. L.; van Koten, G.;
Deelman, B.-J. J. Am. Chem. Soc. 2000, 122, 3945-3951.
(17) The formation of disilanes is catalyzed by Wikinson’s complex but
can be suppressed by using an excess of olefin: (a) Brown-Wensley, K. A.
Organometallics 1987, 6, 1590-1591. (b) Chang, L. S.; Corey, J. Y.
Organometallics 1989, 8, 1885-1893.
(
12) (a) Carmichael, A. J.; Seddon, K. R. J. Phys. Org. Chem. 2000, 13,
91-595. (b) Fletcher, K. A.; Storey, I. A.; Hendricks, A. E.; Pandey, S.;
Pandey, S. Green Chem. 2001, 3, 210-215.
13) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Phys.
5
(
Chem. Chem. Phys. 2001, 3, 5192-5200.
Org. Lett., Vol. 4, No. 22, 2002
3853

reported for Wilkinson’s catalyst in conventional solvents.18
When the reaction was performed at 84 °C, the temperature
at which a mixture of 1-octene (φv(1-octene) ) 0.63) in 1
becomes homogeneous, the reaction mixture was still an
emulsion. Although it did not affect the catalytic activity,
raising the reaction temperature to 100 °C resulted in fully
homogeneous conditions. This suggests that neither the
concentration of the substrates in the ionic phase nor their
phase transfer into the ionic phase are rate limiting at 84
The drop in conversion (Table 2) is most likely caused
by catalyst leaching. After separation from the ionic liquid,
the product layers were slightly orange, suggesting some
leaching of the rhodium catalyst. ICP-AAS analyses of the
product phases of cycles 2 and 3 confirmed this view. Both
21
-
5
-5
phosphine (1.5 × 10 wt %) and rhodium (3.6 × 10 wt
%) were found in the product layer, corresponding to a loss
of 4% of initial rhodium and 2% of initial phosphine per
2
2
cycle. Phosphine oxidation, caused by traces of oxygen,
which can never be completely excluded in experiments on
this scale, might be an additional source of activity loss.23
Previously, 2 was used in the hydrosilylation of 1-hexene
under fluorous biphasic conditions; Rh leaching was 12%
°
(
C. Furthermore, as the catalytic activities of both RhCl-
PPh in [BMIm][BF ] and 2 in 1 are similar, albeit lower
than that of RhCl(PPh in benzene, it can be concluded
3
)
3
4
3 3
)
that the perfluoro tail substitution of both the ionic liquid
and the catalyst does not influence the reaction rates and
selectivity.
1
5
per cycle under these conditions, and Wilkinson’s catalyst
in the conventional ionic liquid [BMIm][BF ] could not be
4
recycled efficiently (because the catalyst dissolved prefer-
entially in the product phase). Therefore, the use of fluorous
catalyst 2 in fluorous ionic liquid 1 is the most efficient
combination thus far for recycling of a Wilkinson-type
catalyst in the hydrosilylation of olefins.
Whereas the turnover frequencies (TOF) observed do not
make a strong case for the use of 1 in hydrosilylation
catalysis, the total turnover numbers (TON) obtained after
multiple cycles clearly do. Reasonably efficient catalyst
recycling was possible by phase separation at 0 °C. A single
aliquot of precatalyst was used 15 times with a retention of
catalyst activity of 92% per cycle (94% when the first cycle
In conclusion, we have developed a fluorous ionic liquid
with characteristics of both conventional ionic liquids and
fluorous solvents. It was shown that this fluorous ionic liquid
can be used in fluorous biphasic catalyst recycling and that
it offers a higher solubility of apolar substrates compared
with existing ionic liquids. Along with the fact that it forms
a homogeneous phase with organic solvents above the
consolute temperature, this makes this solvent especially
attractive for use in catalytic processes that suffer from phase
transfer limitation in other biphasic solvent systems. Non-
volatile fluorous ionic liquids of this kind could also be an
alternative for the more commonly used perfluorocarbons
in fluorous biphasic catalyst recycling.
19
is disregarded, Figure 3). This resulted in a TON of 4.0 ×
Acknowledgment. This work was financially supported
by ATOFINA Vlissingen B.V. and the Dutch Ministry of
Economic Affairs (Senter/BTS).
Supporting Information Available: Experimental pro-
cedures and characterization data for compound 1. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Figure 3. Turnover numbers achieved in hydrosilylation of
-octene by catalyst 2 in fluorous ionic liquid 1. The lightly shaded
box represents the common turnover number achieved with RhCl-
PPh . Darkly shaded boxes represent the cumulative turnover
number achieved with 2 after each cycle.
1
(
3 3
)
OL026700L
(21) ICP-AAS analysis showed the presence of 147 mass ppm of boron
in the product layer, which corresponds with a leaching of 4 wt % of ionic
liquid per cycle.
(22) The ratio of Rh and P observed was 1:1.5 but is known to be 1:2
in the spent catalyst (ref 15). This can be explained by phosphine oxidation,
resulting in the formation of rhodium particles and phosphine oxides. The
latter will have a higher affinity for the fluorous phase, resulting in a
relatively higher leaching of the metal.
1
03 mol per mol of catalyst, which is significantly higher
than corresponding values reported for the conventional
system under monophasic conditions (first bar in Figure
3
1
5,20
).
(23) Substrate quality also proved to be important for the observed
activity. Freshly distilled 1-octene resulted in increased conversions relative
to aged material. Formation of epoxides, which is known to decrease the
activity of Wilkinson’s catalyst in hydrogenation, is a likely cause for this
lower activity.
(24) Marcus, Y. in The Properties of SolVents; Fogg, P. G. T., Ed.; Wiley
& Sons: Chicester, UK, 1998.
(
18) (a) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16-21.
(
b) Chalk, A. J. J. Organomet. Chem. 1970, 21, 207-213.
(
19) The drop in activity after the first recycle is well above average.
(20) Yoshida, J.-I.; Itami, K.; Mitsudo, K.; Suga, S. Tetrahedron Lett.
1
999, 40, 3403-3406.
3854
Org. Lett., Vol. 4, No. 22, 2002