Three novel anions based on pentafluorophenyl amine combined with two new synthetic strategies for the synthesis of highly lipophilic ionic liquids

Article abstract of DOI:10.1039/b823153b

The three new fluorinated anions BPFPA, PFTFSI, and PFNFSI for highly hydrophobic, hydrolytically stable ionic liquids are introduced, together with a strategy allowing a combinatorial approach for the synthesis of imidazolium and phosphonium based ionic

Full text of DOI:10.1039/b823153b

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Three novel anions based on pentafluorophenyl amine combined with two
new synthetic strategies for the synthesis of highly lipophilic ionic
liquidswz
¨
Thomas Linder and Jorg Sundermeyer*
Received (in Cambridge, UK) 24th December 2008, Accepted 10th March 2009
First published as an Advance Article on the web 1st April 2009
DOI: 10.1039/b823153b
The three new fluorinated anions BPFPA, PFTFSI, and PFNFSI
for highly hydrophobic, hydrolytically stable ionic liquids are
introduced, together with a strategy allowing a combinatorial
approach for the synthesis of imidazolium and phosphonium based
ionic liquids of these relatives of bistriflylimide (BTFSI).
idene-imidazolines5 (also known as N,N-ketenediacetals or
ene-1,1-diamines) and trialkyl phosphonium ylides, next to
N-heterocyclic carbenes.
On the anion precursor side bis(2,3,4,5,6-pentafluorophenyl)-
amine HN(C6F5)2 is easily synthesized on a large scale from
the cheap starting materials LiNH2 and C6F6.6 We developed
a better synthesis for the known acid N-(2,3,4,5,6-pentafluoro-
7
Highly hydrophobic, non-hygroscopic and water immiscible
ILs are promising candidates for two-phase (water–IL) catalysis
or as solvents for lithium electrolytes in electrochemical
devices.1,2 Most known hydrophobic ILs consist of imidazo-
lium or phosphonium cations in combination with anions such
as tetrafluoroborate (BF4ꢀ), hexafluorophosphate (PF6ꢀ) and,
in particular BTFSI, which is more stable with respect to
phenyl)-N-trifluoromethylsulfonamide
HN(C6F5)SO2CF3
and its new derivative N-(2,3,4,5,6-pentafluorophenyl)-N-non-
afluorobutylsulfonamide HN(C6F5)SO2C4F9 (Scheme 1).z
On the cation precursor side, we developed a method to
deprotonate 1,2-dimethyl-3-alkyl imidazolium salts by strong
bases such as NaNH2, KH or KO-tert-Bu and to purify the so
far unknown asymmetrically substituted air-sensitive
1-methyl-2-methylidene-3-alkylimidazolines (alkyl = ethyl,
n-butyl, n-octyl) by distillation. Furthermore the ylide
hydrolysis. In
a simplified model, the strong electron
withdrawing properties of the fluorine atoms give rise to
delocalization of the negative anion charge, and as a consequence
low coulombic interactions between anion and cation, and by
that means low lattice energies and melting points.
8
n-Bu3P-CH2 is easily synthesized from technically available
and cheap Bu3P, distilled and stored as an air-sensitive liquid
on a large scale. In a combinatorial approach combining three
bases with three acids in an inert solvent (hexane or ether) nine
new ILs formed almost quantitatively. Their melting or glass
temperatures as well as decomposition temperatures were
determined by DSC/TGA (Scheme 2).
For ILs used in electrochemical devices synthetic procedures
have to be provided that guarantee the absence of any oxidiz-
able anion impurities, such as chloride, bromide or iodide.
Unfortunately the most common procedure for IL synthesis,
ion metathesis, has the disadvantage of ending up in halide
exchange equilibria that have to be shifted more or less to one
side. Direct synthesis of ILs from volatile anion and cation
precursors that are easily purified via distillation is a highly
competitive alternative, realized for example in the direct
synthesis of imidazolium triflates via methylation of N-alkyl-
imidazoles by methyl triflate or in the deprotonation of
Brønsted acids by N-heterocyclic carbenes reported by Seddon
et al.3 and a patent by BASF AG.4
The thermal stability of this set of ILs is mainly dependent
on the nature of the anion. Whereas BPFPA derived ILs show
a mass loss at temperatures between 170 1C and 190 1C, triflyl
and nonaflyl derivatives with PFTFSI and PFNFSI anions
exhibit higher thermal stabilities of up to 290 1C. Some of
them,
namely
[BMIM][PFTFSI],
[BMIM][PFNFSI],
[BMMIM][PFNFSI], [Bu3PMe][PFTFSI] and [Bu3PMe][PFNFSI],
are liquids at room temperature (RTIL). It is worth mention-
ing that, on the cation side, 2-H substituted imidazolium salts
are in general less stable towards thermal decomposition or the
formation of carbene complexes when used as solvents for
metal catalysts9 than the 2-alkyl imidazolium and tetraalkyl
Here we describe the favourable combination of two innova-
tions in IL design and synthesis: introduction of three new
fluorinated anions related to the prominent parent BTFSI
via stepwise substitution of alkylsulfonyl groups by electron-
withdrawing pentafluorophenyl groups and the use of two
types of uncommon volatile zwitterionic and strongly basic
cation precursors as synthons for IL synthesis, namely 2-alkyl-
Fachbereich Chemie der Philipps-Universitat Marburg,
¨
Hans-Meerwein-Str., 35032 Marburg, Germany.
E-mail: jsu@staff.uni-marburg.de; Fax: +49 6421 28-25711;
Tel: +49 6421 28-25693
w Dedicated to Professor Helmut Werner on the occasion of his 75th
birthday.
z Electronic supplementary information (ESI) available: Experimental
details. CCDC 715077–715078. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b823153b
Scheme 1 Preparation of the anion precursors.
ꢁc
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Scheme 2 General procedure for the synthesis of the ILs (Xꢀ = Clꢀ, Brꢀ or Iꢀ; HA = BPFPA-H, PFTFSI-H or PFNFSI-H).
phosphonium based counterparts, accessible by the general
strategy reported here.
not of X-ray quality. Therefore a 1,3-diisopropyl-2H-imida-
zolium salt [iPiPIM][BPFPA] was chosen as reference for
specific anion–cation interactions. The results of the XRD
analyses are shown in Fig. 1 and 2 and Table 1. In both
structures all proton positions could be localized and were
refined isotropically.
The most powerful method for studying specific anion–
cation interactions is single crystal XRD analysis. Interestingly,
independent of the cation, all PFTFSI and PFNFSI salts
reveal good solubility in Et2O whereas BPFPA salts show
limited solubility in Et2O, which served as the solvent for
growing single crystals of [BMMIM][BPFPA]. In order to
directly compare this 1-methyl-2-methyl-3-butylimidazolium
with a 1-methyl-3-butylimidazolium salt with the same anion,
[BMIM][BPFPA] was crystallized; however the crystals were
[BMMIM][BPFPA] consists of a quasi naked bis(pentafluoro-
phenyl)amide anion packed into the lattice of imidazolium
cations. As the imidazole ring is substituted by a methyl group
at the 2-position (C13), the closest contact of the amide
nitrogen atom N1 to the most acidic CH bond of the cation
is defined by N1–H2 248(2) pm or N1–C14 336.4(2) pm,
whereas the shortest distance to the 2-H imidazolium cation
of [iPiPIM][BPFPA] is defined by N1–H1 220(2) pm or
N1–C13 312.2(3) pm. In both lattice structures there are
Table 1 Crystallographic data
[BMMIM][BPFPA]
Empirical formula
Formula weight
Crystal system
C21H17F10N3
501.38
Triclinic
Unit cell dimensions
a = 9.8625(11) A
b = 10.3192(12) A
c = 10.8965(12) A
Volume
a = 81.843(13)1
b = 80.651(13)1
g = 75.228(13)1
1052.2(2) A3
Temperature
Space group
193(2) K
ꢀ
P1
Fig. 1 ORTEP-plot of the molecular structure of [BMMIM][BPFPA].
Z
2
10 424
Thermal ellipsoids are shown for a probability of 30%.
Reflections collected
Independent reflections
Final R indices [I 4 2s(I)]
R indices (all data)
CCDC no.
3839 [R(int) = 0.0573]
R1 = 0.0345, wR2 = 0.0943
R1 = 0.0433, wR2 = 0.0984
715077
[iPiPIM][BPFPA]
Empirical formula
Formula weight
Crystal system
C21H17F10N3
501.38
Monoclinic
Unit cell dimensions
a = 14.1670(16) A
b = 11.0290(10) A
c = 13.6160(14) A
Volume
a = 901
b = 91.0680(10)1
g = 901
2127.1(4) A3
193(2) K
C2/c
Temperature
Space group
Z
4
10 551
Reflections collected
Independent reflections
Final R indices [I 4 2s(I)]
R indices (all data)
CCDC no.
2269 [R(int) = 0.0338]
R1 = 0.0355, wR2 = 0.0969
R1 = 0.0507, wR2 = 0.1020
715078
Fig. 2 ORTEP-plot of the molecular structure of [iPiPIM][BPFPA].
Thermal ellipsoids are shown for a probability of 30%.
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Table 2 Absorption maxima and polarity parameters of the novel [BMIM]-ILs
N
ET
lmax/nm (Reichardt)
lmax/nm (Spange 1)
b
lmax/nm (Spange 2)
p*
a
[BMIM][NTf2]
[BMIM][PFTFSI]
[BMIM][PFNFSI]
—
560.8
557.5
0.644
0.626
0.635
585.5
602.0
597.0
0.42
0.52
0.49
630.0
629.5
626.0
0.92
0.91
0.86
0.64
0.61
0.67
additional long anion cation contacts via very weak F–HC
hydrogen bridges.z The amide anion of [iPiPIM][BPFPA]
reveals crystallographically imposed C2 symmetry. The
N1–C1 distance 136.3(2) pm well between the ideal C–C single
and double bond length and the C1–N1–C10 angle of 121.7(2)1
are indicators of an sp2 character of the amide nitrogen atom
and a perfect charge delocalisation into the C6F5 rings. In the
non-fluorinated diphenylamide [Ph3PEt][NPh2] contact ion
pair, N–C bond distances of 138.0(5) pm, 136.5(3) pm and
an angle of 121.5(3)1 was found.10 In this BPFPA salt
accumulation of negative charge at the para fluorine ring
atoms is also indicated by a shift of dF (D3CCN) values
from ꢀ166.53 ppm (free acid) to ꢀ187.19 ppm ([BMIM][BPFPA])
upon deprotonation. However, perfect conjugation within the
anion is hampered by steric factors. The interplanar angle of
both pentafluorophenyl rings is 57.71.
To evaluate the hydrophobic nature of ILs containing the
new perfluorinated anions the maximum water uptake of the
RTILs [BMIM][PFTFSI] and [BMIM][PFNFSI] was examined
using the Karl-Fischer titration method. Probes of both ILs
were equilibrated with deionized water and stirred at room
temperature for five hours. After phase separation the water
content of the IL phase was determined. [BMIM][PFTFSI]
exhibits a maximum water uptake of 2.7 mass% and
[BMIM][PFNFSI] an uptake of 2.8 mass%. These values are
slightly higher than the reported values for the RTIL
[BMIM][BTFSI] (2.0 mass%) and lie in the same range as
the value observed for the IL [BMIM][PF6] (2.7 mass%).11
Furthermore the polarity displays a significant characteristic
for the hydrophobic nature of a liquid. To achieve comparable
values for the novel ILs UV/VIS spectra with a set of
whereas the dipolarity/polarizability is nearly identical for all
investigated RTILs. As expected the a values, representing the
hydrogen-bridge-donation ability, is nearly identical for all
investigated ILs since this value is mainly influenced by the
cations. The lower p* value of the PFNFSI-based compared to
PFTFSI-based ILs is in accord with delocalisation of negative
charge by a larger perfluoroalkyl substituent. [BMIM][PFNFSI]
possesses the lowest p* value among all known and
investigated ILs.
Our results hopefully will encourage the science community
to use highly basic zwitterionic cation precursors, namely
1-methyl-2-alkylidene-3-alkylimidazolines, tetraalkylphospho-
nium ylides, next to N-heterocyclic carbenes as synthons for
highly efficient IL syntheses and in combinatorial approach.
We also hope that the three new fluorinated anions BPFPA,
PFTFSI, and PFNFSI related to the most prominent parent
BTFSI will provide some impulse for further investigations
with respect to their use in highly hydrophobic, water immiscible
and hydrolytically stable ILs or their use as electrolyte anions
such as in lithium salts.
This
work
was
supported
by
the
Deutsche
Forschungsgemeinschaft, Priority Program 1191, Ionic Liquids.
Notes and references
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solvatochromic dyes were recorded. Reichardt’s betaine dye
N 12
provides the normalized polarity solvent scale values ET
,
and the two dyes recently developed by Spange allowed a
direct determination of the Kamlet–Taft parameters b (a measure
of anion H-bond acceptor ability) and p* (a measure of
polarizability).13 Structures of the dyes are depicted in the
ESI.z The last parameter a (a measure of the cation H-bond
donator ability) can be calculated according to an equation
developed by Marcus.14 The obtained absorption maxima and
the resulting parameters are summarized in Table 2.
6 R. Koppang, Acta Chem. Scand., 1971, 25, 3067–3071.
7 R. D. Trepka, J. K. Harrington, J. W. McConville,
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11 J. Jaquemin, P. Husson, A. A. H. Padua and V. Majer, Green
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12 C. Reichardt, Chem. Soc. Rev., 1992, 147–153; C. Reichardt, Pure
Appl. Chem., 2004, 76, 1903–1919.
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533–536.
14 Y. Marcus, Chem. Soc. Rev., 1993, 22, 409–416.
15 R. Lungwitz and S. Spange, New J. Chem., 2008, 32, 392–394.
The values obtained are similar to the values obtained for
the prototype of a hydrophobic IL [BMIM][BTFSI].12,15 The
N
IL incorporating the PFTFSI anion possesses the lowest ET
value of all investigated ILs with a value of 0.626. Both our ILs
tend to have a higher ability to accept hydrogen bridges,
represented by the b value, compared to [BMIM][BTFSI],
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This journal is The Royal Society of Chemistry 2009
2916 | Chem. Commun., 2009, 2914–2916