Catalyst retention utilizing a novel fluorinated phosphonium ionic liquid in Heck reactions under fluorous biphasic conditions

Article abstract of DOI:10.1016/j.jfluchem.2017.06.014

A novel ionic liquid based on a highly fluorinated phosphonium cation was synthesized and its physicochemical properties were compared to its semi- and non-fluorinated analogue. The fluorinated ionic liquid was found to show a thermomorphic mixing behavio

Full text of DOI:10.1016/j.jfluchem.2017.06.014

Journal of Fluorine Chemistry 200 (2017) 115–122
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Catalyst retention utilizing a novel fluorinated phosphonium ionic liquid in
Heck reactions under fluorous biphasic conditions
a,⁎
a
a,b
Daniel Rauber , Frederik Philippi , Rolf Hempelmann
a
Institute of Physical Chemistry, Saarland University, Campus B 2.2, 66123 Saarbrücken, Germany
Korea Institute of Science and Technology Europe, Campus E 7.1, 66123 Saarbrücken, Germany
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ionic liquids
Fluorous biphasic catalysis
Heck reaction
Catalyst retention
A novel ionic liquid based on a highly fluorinated phosphonium cation was synthesized and its physicochemical
properties were compared to its semi- and non-fluorinated analogue. The fluorinated ionic liquid was found to
show a thermomorphic mixing behavior with organic solvents so that it could be applied as a substitute for
volatile perfluorinated solvents in fluorous biphasic catalysis to achieve the recovery of perfluoro-tagged cata-
lysts. Efficient immobilization of a perfluoro-tagged palladium catalyst in the fluorous ionic liquid phase was
demonstrated for the Heck reaction as a model reaction of the widely applied Pd-catalysed CeC coupling re-
actions. The reaction of iodobenzene and methyl acrylate resulted in 83% yield after 20 runs proofing the
efficient immobilization in the fluorophilic ionic liquid.
1. Introduction
serious environmental problems [11,12]. The use of highly fluorinated
ionic liquids [13] combines the advantages of fluorous solvents for ef-
Ionic liquids (ILs) and fluorous solvents are both regarded as
neoteric solvents” in the context of a greener, more sustainable
ficient extraction and immobilization with non-volatility and a high
degree of flexibility (through the task-specific and tailored designs) of
ionic liquids. In this way it is possible to combine the benefits of these
two neoteric solvents classes for effective and environmentally friendly
reagent immobilization strategies. The use of fluorous groups in ionic
liquids allows the implementation of fluorophilicity to the broad field of
possible modifications beside the choices for anion and cation. This
task-specific tuning allows new possible applications or the improve-
ment of existing techniques [14]. It thus becomes feasible to modify the
properties of liquid multiphasic systems in a wide range using non-
volatile, modifiable fluorinated ILs as fluorous phase. The search for
effective and reusable catalyst systems for multiphasic homogenous
catalysis is an important issue in green chemistry since it saves en-
ergetic and chemical input [15,16]. ILs with perfluoroalkyl substituents
have found numerous applications in many scientific fields like the use
as surfactants [17], coating material [18] or liquid crystals [19,20],
highly hydrophobic extraction agents [21], as well as for gas separation
and gas dissolution [22,23] to name only a few. Furthermore fluori-
nated ILs are intensively investigated because of their unique micro-
scopic heterogeneous structuring leading to a triphilic organization in
ionic, hydrocarbon and fluorous domains driven by solvophobic inter-
actions comparable to the macroscopic solvent behavior observed in
limited miscibility and phase separation [24–27].
“
chemistry since they share the potential to facilitate recycling proce-
dures and reduce waste material as well as energy consumption [1–4].
However their physicochemical properties vary considerably. While
ionic liquids have a negligible vapor pressure, polar structural motifs
and are often hydrophilic, fluorous solvents show a significant vapor
pressure and are highly apolar. Furthermore perfluoroalkyl groups
show an even greater hydrophobicity than hydrocarbons through the
reduced polarizability resulting from the high electronegativity of the
fluorine atoms. The behavior of the perfluorocarbons of being hydro-
and lipophobic at the same time is referred to as flurorophilicity [5].
Many fluorous and organic solvents are known to show a thermo-
morphic mixing behavior, forming a biphasic system at room tem-
perature and a homogenous phase at elevated temperatures. This
thermoregulated effect allows the specific separation and immobiliza-
tion of components in liquid–liquid multiphasic systems and orthogonal
chemistry in the separated phases [6]. A famous concept for thermo-
regulated chemistry is the fluorous biphasic catalysis (FBC) to achieve
the recovery of perfluoro-tagged transition metal catalysts [7–10].
Important drawbacks of conventional fluorous solvents are their limited
tuneability and relatively high vapor pressure, which restricts the use-
able temperature range. Furthermore bioaccumulation, persistence and
the global warming potential of the volatile fluorous solvents can pose
In this work we describe a new highly fluorinated phosphonium
⁎
Corresponding author.
E-mail address: daniel.rauber@uni-saarland.de (D. Rauber).
http://dx.doi.org/10.1016/j.jfluchem.2017.06.014
Received 3 April 2017; Received in revised form 28 June 2017; Accepted 28 June 2017
Available online 29 June 2017
0022-1139/ © 2017 Elsevier B.V. All rights reserved.

D. Rauber et al.
Journal of Fluorine Chemistry 200 (2017) 115–122
ionic liquid [P(Rf6
)
3
R
f4][NTf
2
] (1) having four perfluoroalkyl chains
2.2. Thermal properties
(
(
“
R
fx = (CH
trifluoromethanesulfonyl)imide (NTf
building-block”-approach [28] for the synthesis of perfluoroalkyl
2 2 2 x
) (CF ) F; x = 4, 6) in the cation combined with the bis
−
2
) anion. The application of this
Melting, glass transition and cold crystallization points were de-
termined by differential scanning calorimetry (DSC) using a DSC 1
STARe System (Mettler Toledo, Gießen, Germany) equipped with liquid
nitrogen cooling. For the measurements about 20 mg sample were
weighted into aluminum crucibles and hermetically sealed. The tem-
perature program started with a heating rate of +5 °C/min to 100 °C
followed by 30 min isothermal step at 100 °C to remove thermal his-
tory. Afterwards a dynamic step with cooling rate of −1 °C/min to
‐120 °C was applied. The temperature was held constant at −120 °C for
10 min, followed by a heating step (+1 °C/min) to 120 °C and then
returned to the initial conditions. Decomposition temperatures were
determined by thermogravimetric analysis (TGA) using a TG 209 F1 Iris
substituted ILs allows the synthesis under laboratory conditions with
easily adjustable structures. To investigate the effect of fluorination on
important physicochemical properties and mixing behavior with or-
ganic solvents, IL 1 was compared to the non-fluorinated analogues
hexyltrioctylphosphonium bis(trifluoromethanesulfonyl)imide [P(oc-
t)
monly
used 1-butyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide
BMIM][NTf ]. The highly fluorinated IL 1 shows a low solubility and
3 2 3 2
hex][NTf ], the semifluorinated IL [P(oct) Rf4][NTf ] and the com-
[
2
thermomorphic phase-mixing behavior with many organic solvents as it
is commonly found for perfluorinated molecular solvents. The combi-
nation of the perfluorinated IL 1 and DMF was found to be an effective
liquid–liquid system for immobilization of a palladium catalyst with
highly fluorinated ligands [29] in Heck reactions [30]. Van Koten and
coworkers already reported the use of a highly fluorinated IL for the
retention of a perfluoro-tagged rhodium catalyst in a hydrosilylation
2 3
(Netzsch, Selb, Germany) and Al O crucibles. The temperature pro-
gram was set from 20 °C to 600 °C applying a heating rate of 10 °C/min
and a nitrogen flow of 20 mL/min. Decomposition temperatures were
determined as extrapolated onset temperatures.
2.3. Polarity determination using the solvatochromic dye Nile Red
[
13]. Since the palladium catalyzed CeC bond formation is of high
interest for laboratory and industrial preparations we wanted to de-
monstrate the application of this concept to the cross coupling reactions
utilizing a Pd-catalyst immobilized in a fluorous ionic liquid phase. As a
model reaction for palladium catalysis we chose the widely used Heck
reaction for the cross coupling reaction of iodobenzene with either
methyl acrylate or styrene. The Heck reaction is one of the most im-
portant reactions for C‐C-bond formation, but in most reports the cat-
alyst is lost during work-up and cannot be recovered. Creating an ef-
ficient recycling protocol for the Heck cross coupling is found to be a
tough challenge because of the instable catalytic cycle and the use of
multiple reagents and products which are changing the system prop-
erties and may have a disturbing or inactivating effect on the catalyst
To 0.5 g of the ILs was added a solution of 0.5 mL Nile Red (NR) in
−
3
−1
acetone (c(NR) = 3.0 × 10 mol L ) and stirred until the solution
became homogenous. In case of IL 1 an additional milliliter acetone was
added to ensure full dissolution. The solvent was then removed by
drying in oil-pump vacuum for two days and the Nile Red solutions of
the ILs were transferred to a quartz cuvette of 1 mm diameter to record
a transmission UV/Vis spectrum. In case of 1 the UV/Vis-measurement
was performed on a thin film of the sample in supercooled state as this
IL showed a widely suppressed crystallization (see DCS results in
Section 3.1). For each IL ten measurements were performed, the wa-
velength of the absorption maxima detected and the results averaged.
The ENR values were calculated after Eq. (1).
[
31].
−1
× 10−6
E
NR = hcN
A
λ
max
(1)
with:
2. Experimental
h: Planck’s constant
c: speed of light
2.1. Synthesis of the ionic liquids
N
A
: Avogadro’s constant
λ
max: wavelength of the maximum absorbance (in nm)
The investigated ILs were synthesized by modified quaternization
reactions of the corresponding phosphines with 1‐bromohexane or
H,1H,2H,2H-perfluorohexyl iodide in acetonitrile or DMF under inert
2.4. Crystal structure determination of IL 1
1
gas atmosphere at elevated temperatures [32]. Tris(1H,1H,2H,2H-per-
fluorooctyl)phosphine was synthesized following literature protocols
Single crystal X-ray diffraction data were collected on a X8 Apex II
diffractometer (Bruker AXS, Karlsruhe, Germany) at −121 °C using Mo-
[
18,33]. The halide ILs were then converted to the corresponding NTf
2
-
Kα radiation (λ = 0.71073 Å) in theta range from 0.607° to 27.811°
+
ILs by anion metathesis reaction with LiNTf in acetone. The exemplary
2
collecting 88389 reflections in total. Crystal data:
C
0 16 48
H F P
,
3
−
synthesis of the fluorous IL 1 via quarternization of the fluorinated
phosphine followed by anion metathesis is sketched in Scheme 1. Purity
of the obtained ILs was checked by multinuclear NMR-spectroscopy and
ESI–MS. The absence of halide ions was confirmed using methanolic
C F NO S
:
r
M = 1599.55, triclinic, P-1, a = 9.1235(6) Å,
2
6
4
2
b = 9.6920(6) Å, c = 33.677(2) Å, α = 86.357(3)°, β = 85.533(4)°,
3
−3
γ = 63.392(3)°, V = 2653.0(3) Å , Z = 2, ρ
r
= 2.002 g cm . The re-
2
finement using full-matrix least-squares on F resulted in 12220 in-
dependent reflections, 381 restraints, 822 parameters, R[F > 2σ(F )]
2
2
AgNO
conditions and multinuclear NMR-spectra are given in the Supporting
information. [BMIM][NTf ] was prepared following literature reports
34].
3
solution. Further details about the applied materials, synthesis
2
= 0.1245, wR(F ) = 0.3897.
2
[
2.5. Determination of ionic liquid solubility/miscibility in organic solvents
Solubilities or miscibilities of the phosphonium ILs with a range of
organic solvents were determined by preparing saturated solutions of
the ILs and 3.000 mL of purified and dried organic solvents under argon
by stirring for at least 3 h in a thermostated bath keeping the tem-
perature at 25 °C until a two phase system remained. Then the solution
was allowed to settle for 16 h after which a completely homogenous
upper phase was observed. A 2.000 mL sample of the organic phase was
carefully taken and the solvent removed in oil pump vacuum by drying
for at least one day until the mass remained constant. By weighting the
remaining IL the solubility per liter could be calculated. If no second
Scheme 1. Synthesis of the fluorinated ionic liquid 1 via quaternization of ternary
phosphine and 1H,1H,2H,2H-perfluorohexyl iodide followed by anion metathesis reac-
tion with LiNTf .
2
116

D. Rauber et al.
Journal of Fluorine Chemistry 200 (2017) 115–122
phase was formed with varying the amount of organic solvent the IL
was stated as miscible. Experiments were performed in triple and the
results averaged. The phase diagram of IL 1 and DMF was constructed
by inserting a molten sample of the IL into a graduated cylinder. Known
volumes of DMF were added, the cylinder sealed and slowly heated
with stirring until the phases became homogenous. The temperature at
this point is the mixing temperature Tmix for the given volume fraction.
After cooling to ambient temperature further volumes of DMF were
added and the process repeated to construct the phase diagram.
pump vacuum and the residue weighted. This process was repeated
three times and the results averaged.
3. Results and discussion
3.1. Physical properties
The fluorous IL 1 is a thermal, air and water stable white solid if
crystallized, forming a slightly yellow liquid above its melting point or in
the presence of small amounts of organic solvents. In supercooled state
the IL is observed as highly viscous, slightly yellow liquid. To investigate
the effect of increasing fluorination in the cation on the physicochemical
properties a comparison with the semi-fluorinated and non-fluorinated
analogue was made. To investigate the influence of the anion choice the
2.6. Heck reactions under fluorous biphasic conditions using the fluorous IL
All reactions were carried out under inert atmosphere of argon.
1.00 g of IL 1 was degassed in vacuum and flushed with argon for 3
2
results were also compared to the widely used IL [BMIM][NTf ] which
times. Then the IL-phase was saturated with DMF (about 0.2 mL) to
incorporated the same anion. DSC measurements were performed to in-
vestigate the phase transition behavior. Low scan speeds of ± 1 °C/min
were chosen to ensure the suppression of supercooling through thermal
quenching which can result in solely glass formation and thereby in-
complete determination of the phase transition points [35]. Thermal
stability was examined by means of dynamic TGA. As method for the
empirical determination of solvent polarity the solvatochromic dye Nile
Red [36] was applied which was also used for polarity determination for
a range of other ionic liquids [37]. The more commonly used Reichardt’s
Dye [38] was found to be insoluble in the fluorous IL 1. The empirical
polarity determination is used to gain insight into the interactions on
molecular level instead of using macroscopic parameters like relative
permittivity, dipole moment or refractive index [39]. The polarities of ILs
are of special interest as they allow conclusions about miscibility and
phase behavior along with comparison to molecular solvents. The results
of the transition and decomposition temperatures along with the ENR for
empirical polarity are summarized in Table 1. The DSC traces of the
phosphonium ILs with different degree of fluorination are shown in
obtain a slightly viscous, fluorous phase. In case of the cross coupling
6 4
reactions performed at 85 °C, 8.0 mg of the catalyst [Pd(P(m-C H -
−3
(
CH
2 2 2 8 3 2 2
) -(CF ) ‐F) ) Cl ] (2.37 × 10 mmol, 0.80 mol%) was added.
For the coupling reaction carried out at 70 °C 10.0 mg of the catalyst
−
3
(
2.96 × 10 mmol, 1.00 mol%) was added. The catalyst was then
−
1
dissolved under slight heating and 0.400 mL of a 0.740 mol L
iodo-
benzene stock solution in DMF (0.296 mmol 1.00 eq.), 0.400 mL of a
−
1
0
0
.888 mol L olefin stock solution in DMF (0.355 mmol, 1.20 eq.) and
−1
.200 mL of a 1.78 mol L
stock solution of triethylamine in DMF
(
0.355 mmol, 1.20 eq.) were added. The mixture was stirred and heated
to 85 °C resp. 70 °C for 16 h (unless noted otherwise) and then cooled
down to −18 °C. The upper organic phase was carefully removed and
the fluorous phase extracted with 0.2 mL of DMF. The combined DMF-
phases were filled up to 100 mL with acetonitrile and a sample was
analyzed with high performance liquid chromatography (HPLC). The
HPLC system consisted of ASI‐100 automated sample injector, Degasys
DG-1210 solvent degasser, P680 HPLC Pump, TCC100 Thermostated
Column Compartment and UVD3404 UV/Vis detector (all from Dionex,
Sunnyvale, USA). The used column was a Hibar 150 × 4 mm C18/5 μm
Fig. 1. Upon cooling, the fluorous IL 1 exhibits only a glass transition T
g
at ‐18 °C and no crystallization peak which is a commonly observed
phenomenon in ILs [40]. In the heating cycle a cold crystallization Tcc is
(
Merck, Darmstadt, Germany). The column temperature was controlled
by a column oven at 25 °C. Gradient program with a total flow of
.000 mL/min using water and acetonitrile started with 100% water at
observed at 14 °C followed by a melting T
that although the fluorous IL 1 is not a room-temperature ionic liquid
RTIL), it can be observed in the liquid state at ambient temperature. This
m
at 69 °C. These results show
1
t = 0 min to 100% MeCN at t = 20 min. Wavelength for UV/Vis de-
tection was set to 254 nm. Retention times of the fully resolved peaks
were 13.8 min for methyl cinnamate, 17.8 min for stilbene, 15.8 min
for iodobenzene and 15.0 min for styrene. The amount of product was
calculated by integration of the product peak in comparison with cali-
bration curves. For each sample at least three runs were performed and
the results averaged. Facile product isolation for six times was proven in
a separate experiment for the reaction of iodobenzene with methyl
acrylate at reaction times of 16 h and a temperature of 70 °C. For this
purpose six equivalents of water were added to the DMF-phase after the
reaction and the resulting suspension was extracted with 3 mL hexane
three times. The solvent of the combined organic phases was removed
by rotary evaporation and the residue dried in vacuum yielding pure
(
is due to supercooling and suppressed crystallization which is an intrinsic
property of most ILs [41], resulting from anti-crystal engineering [42]. A
similar behavior is observed for the non-fluorinated sample [P(oct)
NTf ] and [BMIM][NTf ] [43] as example of a widely applied imida-
zolium IL. For these salts also T , Tcc and T were observed in the heating
curve and only T in the cooling step. In contrast for the semi-fluorinated
f4][NTf ] only T is observed for both heating and
3
hex]
[
2
2
g
m
g
ionic liquid [P(oct)
cooling.
3
R
2
g
Table 1
g m d
Glass transition T , cold crystallization Tcc, melting T and decomposition T tempera-
tures along with the polarity values ENR determined by the solvatochromic shift of Nile
Red for the investigated ILs.
1
methyl cinnamate in high isolated yield as confirmed by H- and
1
3
1
C{1H}-NMR spectroscopy (see Supporting information; H NMR
3
Property
IL 1 [P
[P(oct)
[NTf
3
R
f4
]
[P(oct)
3
hex]
2
[BMIM][NTf ]
(
400 MHz, CDCl
3
): δ[ppm] = 7.70 (d,
J
HH = 16.0 Hz, 1H), 7.59–7.46
3
(Rf6
)
3
R
]
f4
]
2
]
2
[NTf ]
(
m, 2H), 7.43–7.35 (m, 3H), 6.45 (d, JHH = 16.1 Hz, 1H), 3.81 (s, 3H);
[
NTf
2
1
3
1
C{ H} NMR (101 MHz, CDCl
30.36, 128.95, 128.14, 117.84, 51.76.). Turnover number (TON) for
the reaction was calculated using Eq. (2).
3
): δ[ppm] = 167.47, 144.93, 134.42,
cation
highly
semi-
non-
non-
1
composition
fluorinated
fluorinated
fluorinated
fluorinated,
aromatic
TON = n(product)/
n
(catalyst)
(2)
T
T
T
T
λ
E
g
/°C
cc/°C
/°C
/°C
max/nm
NR/kJ mol
−18
14
69
345
546.6
218.8
−75
–
–
374
548.2
218.2
−89
−40
−18
395
548.6
218.1
−86 (−87)a
−56
The cumulative TON is the summation of the individual TON per
run after separation of the product phase and addition of new reagents.
In further experiments leaching of the fluorous IL 1 was investigated by
combining 1.00 g with 1.00 mL DMF without further reagents stirring
at 70 °C for 30 min and cooling down to −18 °C. A sample of 0.500 mL
was taken from the upper organic phase, the solvent removed in oil
m
−2 (−3)a
431 (427)a
548.8
d
−1
218.0 (218.0)b
a
b
Values taken from Ref. [44].
Value taken from Ref. [37].
117

D. Rauber et al.
Journal of Fluorine Chemistry 200 (2017) 115–122
[
47]. Phosphonium ILs containing one fluorous tail and other anions
−
−
like BF
4
or OTf were furthermore found to have decomposition
temperatures above 300 °C [48].
The polarity values ENR determined by the solvatochromic shift of
Nile Red are increasing with the degree of fluorination but show only
minor overall differences. Higher ENR-values are related to lower sol-
vent polarities. As the combination of other anions with the thoroughly
investigated [BMIM] cation show similar ranges of deviations, the main
contribution to the ENR-values for the ILs investigated here seems to
−
result from the NTf
2
-anion. This is in accordance with literature re-
ports that the dominating contribution to the solvatochromic shift of
Nile Red is the hydrogen-bond ability of the cation towards the dye
[
49]. Protons of significant acidity are intrinsically absent in quar-
ternary phosphonium cations due to their molecular structure in con-
trast to dialkylimidazolium or protic ILs. The application of Reichardt’s
Dye to other phosphonium-based ILs revealed much larger differences
in the empirical polarity towards imidazolium based samples [38].
Molecular solvents of similar ENR-values than these ILs are ethanol
−
1
−1
(
E
E
NR = 218.2 kJ/ mol ) or 1-butanol (ENR = 218.5 kJ/ mol ). The
NR value reported for comparable IL [P(hex) 29][NTf
NR = 218.7 kJ mol ) is quite similar to the values obtained [50]. A
slightly lower polarity was found for an IL based on [BMIM] and a
a
C
3 14
H
2
]
‐
1
(
E
‐
1
highly fluorinated borate anion (ENR = 220.7 kJ mol ) [13].
Fig. 1. DSC thermograms of the highly fluorinated IL 1 (1), the semi-fluorinated [P
(
3 2 3 2
oct) Rf4][NTf ] (2) and the non-fluorinated [P(oct) hex][NTf ] (3) with indicated
3.2. Crystal structure of the fluorous ionic liquid
transition temperatures. Exothermal transitions are plotted as positive peaks.
The crystalline structure of 1 was investigated by means of single
The absence of a melting point in the semi-fluorinated IL can be
rationalized by symmetry-breaking effects [45] induced by the fluori-
nated chain leading to a destabilized crystalline structure due to
packing effects. For IL 1 and the tetraalkylphosphonium IL a higher
grade of symmetry is incorporated as these samples have four sub-
crystal X-ray diffraction. Suitable single crystals could be obtained by
slowly evaporating saturated solution IL 1 in benzotrifluoride. Crystal
growth occurred in the form of colorless needles and plates. Although a
lot of crystallization methods and solvents were investigated, benzo-
trifluoride was the only solvent applicable for crystal growth.
Evaporation or slow cooling of a saturated solution of IL 1 in other
solvents resulted in the formation of liquid biphasic systems. Owing to
the nature of the substitution pattern, the shorter perfluoroalkyl-chain
stituents of similar chemical composition. The thermal stability of the
−
phosphonium ILs containing the NTf
2
-anion was found to be lower
than for the imidazolium-cation with the same anion. The obtained
TGA curves are shown in Fig. 2. It is well known that the decomposition
temperature of ILs is mainly related to the anion-cation choice and
functional groups incorporated. Elongation of attached chains of similar
chemical structure is reported to have only minor effects on the thermal
stability [46]. By comparing the thermal stability of ILs with the same
anion it is ensured that the determined decomposition is mainly related
to the thermal stability of the cation. The decomposition temperatures
of the phosphonium compounds decreases upon increasing number of
attached perfluoroalkyl-chains. However the observed decomposition
temperatures are still comparatively high with the lowest value being
Rf4 and one of the slightly longer chains Rf6 occupied alternating po-
sitions leading to disordering. Furthermore the packing of the interac-
tions between the perfluorinated molecular fragments driven by weak
van der Waals forces lead to disordering in the periphery of the mole-
cule. These effects overall lead to comparable high value for the final R-
indices. Obtaining a suitable single crystal for XRD from ILs was found
to be rather difficult even for symmetrical ions [51] as ILs are designed
to contain destabilized crystalline structures. The highly fluorinated IL
crystallizes in the triclinic, centrosymmetric P-1 space group showing
two formula units per elemental cell. The unit cell and packing diagram
−
345 °C for IL 1. For the fluorous IL [P(Rf8
)
2
Rf6CH
2
CH(CH
3
)CH
2
C(CH
3
)
3
]
2
are shown in Fig. 3. The NTf -anions adopted a trans-configuration in
[
NTf ] bearing three fluorinated chains, a T
2
d
of 407 °C was reported
the solid state as it is reported for the majority of measured IL samples
in literature [52]. The fluorinated IL 1 showed structuring in domains
of ionic moieties consisting of the phosphonium core and the anion on
the one hand and the perfluoroalkyl-segments on the other. This or-
ganization into blocks of molecular segments of the same polarity
driven by solvophobic interactions was also reported for imidazolium
ionic liquids with perfluorinated anions [19]. Similar structural motifs
and self-assembled nanostructures resulting from solvophobic interac-
tions were reported for ILs with perfluoro-groups in bulk state leading
to tricontinuous nanostructures of ionic, alkyl and fluorinated domains
[
25,53].
3.3. Miscibility and phase behavior towards organic solvents
In order to find an appropriate solvent for the fluorous biphasic
catalysis and to demonstrate the fluorophilic characteristics of IL 1 the
miscibility of the phosphonium ILs with a range of commonly applied
solvents was investigated. The solvents investigated ranged from sol-
vents of high polarity such as water to the highly nonpolar, fluorophilic
perfluoroheptane. Mixing behavior and miscibility of the three
Fig. 2. TGA traces of the investigated ionic liquids at 10 °C/min heating rate.
118

D. Rauber et al.
Journal of Fluorine Chemistry 200 (2017) 115–122
Fig. 3. Crystalline structure of the fluorous IL 1. a) centrosymmetric
elemental cell containing two formula units and b) packing diagram
showing the composition of ionic and perfluorinated domains.
Table 2
slight water solubility when combined with a hydrophilic cation [54].
Upon combination with the highly hydrophobic cations which in-
corporate long hydrophobic alkyl or even fluorous chains, the solubility
drops below the detection limit. The non-fluorinated and semi-fluori-
nated phosphonium ionic liquids show a similar phase behavior and
were found to be completely miscible at ambient temperature with a
wide range of organic solvents, ranging from polar methanol to non-
polar toluene. Only with the nonpolar aliphatic hydrocarbon hexane a
limited solubility was found, being slightly higher for the semi-fluori-
Solubility of the investigated phosphonium ILs with common molecular solvents at 25 °C.
−
1
Solvent
IL 1/g L
[P(oct)
L
3
R
f4][NTf
2
]/g
3 2
[P(oct) hex][NTf ]/g
L
−1
−1
perfluoroheptane
hexane
toluene
1.76
immiscible
13.76
immiscible
12.30
immiscible
1.6
miscible
miscible
miscible
miscible
miscible
miscible
miscible
miscible
immiscible
miscible
miscible
miscible
miscible
miscible
miscible
miscible
miscible
immiscible
Et
THF
CF
CH
2
O
8.3
20.7
4.7
0.65
274.5
15.6
76.6
3
C
5
H
6
3 2
nated [P(oct) Rf4][NTf ]. Both ILs are immiscible with the fluorous
2
Cl
2
solvent perfluoroheptane. The miscibility behavior towards organic
solvents of a wide polarity range can be rationalized by the amphiphilic
character of these ILs consisting of polar ionic and nonpolar alkyl mo-
tifs. These results show that a higher degree of fluorination is demanded
to ensure a limited miscibility of ILs towards organic solvents at am-
bient temperature. For the highly fluorinated compound 1 the situation
is indeed very different. The fluorous IL shows only limited solubility
with the investigated organic solvents. The highest solubility was found
acetone
DMF
methanol
H
2
O
immiscible
phosphonium ILs with common solvents at 25 °C are listed in Table 2.
The criteria to find an appropriate second solvent for fluorous biphasic
catalysis are thermomorphic mixing behavior towards the fluorous
solvent and minimal solubility at ambient temperature or the tem-
perature at which the second solvent is extracted. The first criterion is
crucial to ensure mixing of fluorous and organic phase and homogenous
reaction kinetics. In general the FBC could also be performed if the
catalyst with perfluorinated ligands shows a highly increased solubility
in the organic phase at elevated temperatures so that a homogenous
catalysis can be performed. The second criterion is demanded to avoid
leaching of the fluorous phase accompanied with catalyst leaching. By
this way an efficient catalyst retention and stable catalytic performance
can be guaranteed. Furthermore it is in general preferable to apply high
boiling solvents for both the fluorinated and organic phase as this
minimizes losses through evaporation and allows higher critical mixing
temperatures associated with lower miscibilities at moderate tempera-
tures. All investigated ILs were found to be completely insoluble in
−1
for acetone reaching 274.5 g L . Solubilities for the other molecular
solvents were overall significantly lower but slightly higher for oxygen
containing liquids like methanol, DMF, THF and Et O. Similar limited
2
solubilities were obtained for the highly fluorinated IL described by van
Koten and coworkers. where a solubility of 580 g/L of the fluorous IL in
acetone was found [13]. For organic solvents without oxygen the so-
lubility of IL 1 per liter was even lower. In the aliphatic hexane the
fluorinated IL was found to be insoluble whereas it displayed a slight
solubility in the fluorous solvent perfluoroheptane. In summary the IL 1
showed a miscibility behavior that is comparable to molecular fluorous
solvents, showing limited solubilities with organic solvents at ambient
temperature. However because of the additional ionic groups the so-
lubility with the conventional fluorous liquids was also very limited.
Following these results we investigated the thermomorphic mixing
behavior of 1 with DMF in dependence on the volume fraction to de-
termine the phase diagram. The thermoregulated mixing behavior and
the obtained phase diagram are displayed in Fig. 4.
water. This results from the combination of a hydrophobic anion with a
hydrophobic cation. Although ILs with the NTf
to form a biphasic liquid system upon addition of water they display a
−
2
-anion are well known
The aprotic DMF is a commonly applied solvent for the Heck cross
119

D. Rauber et al.
Journal of Fluorine Chemistry 200 (2017) 115–122
Fig. 4. a) Phase behavior of 1 (lower phase) with
DMF (upper phase). Photographs: 1) formation of a
biphasic mixture at room temperature; 2) heating
above mixing temperature (T > Tmix); 3) cloud
point upon cooling (T < Tmix); d) ongoing phase
separation of fluorous and organic phase. b) Phase
diagram: mixing temperatures Tmix in dependence of
the volume fraction φDMF
.
coupling reaction as it shows a comparatively high polarity capable to
dissolve a wide range of educts and the base which is essential for the
catalytic cycle in the reaction mechanism by base-induced reductive
elimination [55]. It was found that IL 1 and DMF form a biphasic
mixture at room temperature when the volume fraction of DMF (φDMF
exceeded 0.2 and had an upper critical solution temperature UCST of
2 °C at φDMF = 0.57. Above the respective mixing temperatures a
homogenous monophasic system was found. The phase diagram
showed the characteristic progression for liquid–liquid phase diagrams
with upper critical solution temperature. As indicated in the phase
diagram, slight amounts of DMF are miscible with 1 at room tem-
perature resulting in a moderately viscous fluorine rich phase which
remained liquid and homogeneous even at −18 °C.
)
8
Scheme 2. Reaction scheme of iodobenzene and methyl acrylate or styrene applying IL 1
under fluorous biphasic catalysic conditions.
Table 3
Catalyst recycling using the thermomorphic biphasic system applying IL 1 as fluorous
a
phase .
b
R = Phc
R = CO Me
2
d
d
run
yield /% TON cumulative TON yield /% TON cumulative TON
3
.4. Catalyst retention in Heck reactions
blanke 99
124
124
124
123
124
121
120
120
110
95
–
94
93
94
93
92
94
93
90
86
75
61
85
118
116
118
116
115
118
116
113
108
94
–
1
2
3
4
5
6
7
8
99
99
98
99
97
96
96
88
76
67
92
124
248
371
495
616
736
855
965
1060
1144
1259
116
234
350
465
583
699
812
920
1014
1090
1196
To investigate the performance of the fluorinated IL 1 as fluorous
phase for the immobilization of a fluorous catalyst we investigated the
Heck cross coupling as a model reaction for palladium catalysis.
Homogenous palladium catalysis is a very important, widespread and
versatile method utilized in chemical laboratories as well as on in-
dustrial scale [56]. One important drawback of conventional Pd-cata-
lysis is the loss of the relative expensive catalyst that cannot or hardly
be recovered [57]. Among other immobilization strategies the catalyst
retention in a fluorous phase seems to be a promising way to overcome
this drawback and to provide overall more sustainable processes [58].
Fluorophilic catalysts can be obtained by simply attaching per-
fluorinated alkyl-groups in the periphery of catalyst that are known to
show high activity and stability [59]. By using highly fluorinated ionic
liquids is should become possible to further eliminate the drawbacks of
the volatile molecular perfluorinated solvents that were found to have a
range of negative impacts on the environment. The Heck reaction al-
lows the selective formation of CeC bonds between aryl halides and
activated alkenes but an efficient catalyst recovery was found to be
difficult [31]. This results from the usage of multiple reagents along
with different obtained products or side products that disturb the cat-
alytic cycle or lead to inactivation of the catalyst. The catalyst with
9
10
11
84
115
76
106
f
a
Conditions: 1.0 g 1, 0.296 mmol iodobenzene (1.0 eq.), 0.355 mmol olefine (1.2 eq.)
and 0.355 mmol NEt
stated otherwise).
3
(1.2 eq.) in 1.00 mL DMF, 8.0 mg catalyst (0.8 mol%), 16 h (unless
b
Trans only.
Trans/cis.
c
d
Determined by HPLC.
No fluorinated IL added; catalyst not recovered.
t = 32 h.
e
f
the yield determined by means of HPLC. To proof the efficient catalyst
retention fresh reagents were added and the catalytic process repeated.
A blank run using only fluorous catalyst in DMF showed a similar
thermoregulated mixing behavior and was able to efficiently catalyze
the Heck reaction. However the recovery was not possible under these
conditions. The catalyst recovery using the fluorous IL 1 was found to
be efficient for seven runs without significant decrease in yield for both
the coupling to methyl cinnamate and stilbene being 90% and more.
The yields for methyl cinnamate were found to be slightly higher than
those for stilbene which is also found for other multiphasic systems
investigating the Heck reaction [60]. For the runs eight to ten a de-
crease in the catalytic activity was found. By doubling the reaction time
for the eleventh run it was possible to increase the yield again whereby
the obtained yields remained lower than for the initial runs.
6 4 2 2 2 8 3 2 2
highly fluorinated ligands [Pd(P(m-C H -(CH ) -(CF ) F) ) Cl ] [29]
used for the cross coupling reactions was found to be insoluble in pure
DMF and many other common organic solvents at ambient tempera-
−
2
−1
tures. However concentrations of at least 1.2 × 10 mol L could be
obtained in IL 1 saturated with DMF. We first investigated a thermo-
regulated biphasic system consisting of 1.00 g of IL 1 and 1.00 mL DMF
at 85 °C as this temperature is above the upper critical solution tem-
perature UCST. Scheme 2 shows the reactions and conditions applied
for the first experiments coupling iodobenzene with either methyl ac-
rylate or styrene. The results for the yield, TON and cumulative TON
are given in Table 3.
After each run the upper organic phase was carefully removed and
These results already show the potential of the fluorous IL for the
120

D. Rauber et al.
Journal of Fluorine Chemistry 200 (2017) 115–122
4. Conclusion
We successfully synthesized a novel highly fluorinated ionic liquids
and showed the effect of fluorination on the physicochemical properties
in comparison to the non-fluorinated and semi-fluorinated analogue.
The fluorous IL was found to have a higher melting point, high thermal
stability and a slightly lower polarity investigated by means of the
solvatochromic dye Nile Red. Single crystal X-ray analysis showed the
formation of fluorinated and ionic domains driven by solvophobic in-
teractions. The fluorinated IL was found to have only limited solubility
in a wide range of investigated organic liquids comparable to molecular
perfluorinated solvents. The non- and semi-fluorinated analogues
showed completely different behavior being completely miscible in
polar as well nonpolar organic solvents. The thermoregulated mixing
behavior towards DMF was investigated in detail. The fluorinated IL
could be successfully applied as a substitute for volatile fluorocarbon
compounds that pose environmental threats but are still commonly
applied solvents in the fluorous biphasic catalysis. Thus the in-
corporation of perfluoroalkyl-groups adds fluorophilicity as possible
modification to the widely tuneable properties of ILs. By investigating
the Heck reaction as exemplary reaction for palladium catalyzed CeC
bond formation we were able to proof the applicability of the fluorous
ionic liquid to the very widely used Pd-catalyzed reactions. The highly
fluorinated IL was found to efficiently immobilize a perfluoro-tagged
catalyst which allowed repetitive usage for up to 14 runs with only
minor loss in catalytic activity and up to 21 runs with yields still being
83% and higher. By this multiphasic catalysis reaction conditions the
activity of the expensive Pd-catalyst can be preserved for multiple re-
action runs thus leading to more economic and environmental friendly
processes. In further studies we will investigate the expansion of the
fluorous IL to other catalytic systems and reactions as well as the ap-
plication of catalytic systems that show increased thermal stability.
Fig. 5. Yield of methyl cinnamate per run and cumulative TON for the overall catalysis.
Table 4
Isolated mass of the pure methyl cinnamate and corresponding yield per run.
run
mass isolated methyl cinnamate/mg
yield/%
1
2
3
4
5
6
46.09
45.62
46.23
46.31
44.93
45.47
96
95
96
96
94
95
retention of the fluorous catalyst. We believe that the loss in catalytic
activity results mainly from thermal degradation of the catalyst rather
than leaching to the organic phase. This was indicated by a color
change of the solutions of the blank run and the IL 1 in the thermo-
morphic catalysis to orange-red which was reported to result from the
formation of nanoparticles in solution [61]. Inactivation of the same
catalyst after prolonged exposure to higher temperatures was also re-
ported before [62]. We therefore investigated the performance of the
catalyst in the biphasic system at lower temperatures to reduce the
extend of thermal catalyst degradation. Hence we performed the cross-
coupling reactions at 70 °C, increased the catalyst concentration to 1
mol% and kept the same conditions for the coupling of iodobenzene
and methyl acrylate to methyl cinnamate. The results for the TON and
cumulative TON for this reaction conditions are plotted in Fig. 5.
By using these conditions the fluorous catalyst showed nearly
quantitative yields for 14 runs and only a slight drop remaining 83%
yield for run 20. Nearly quantitative yield of 96% could be restored for
the 21. run by increasing the reaction time to 32 h. To demonstrate
facile product isolation and removal of side product triethylammonium
iodide from the reductive elimination an extraction process was per-
formed for the first six runs in an additional experiment. For this pur-
pose six equivalents of water were added to the carefully separated
DMF phase after the reaction and the resulting suspension was ex-
tracted with 3 mL hexane three times. After solvent removal from the
combined organic phases the residue was dried in oil-pump vacuum
yielding pure methyl cinnamate in high isolated yield. Purity of the
Acknowledgements
The authors gratefully acknowledge the Deutsche Bundesstiftung
Umwelt (DBU) (31925-41) for financial support and thank Reiner
Wintringer for performing the HPLC measurements and technical help.
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