Perfluoroalkyltricyanoborate and Perfluoroalkylcyanofluoroborate Anions: Building Blocks for Low-Viscosity Ionic Liquids

Article abstract of DOI:10.1002/chem.201703685

The potassium perfluoroalkyltricyanoborates K[C_nF_{2 n+1}B(CN)₃] [n=1 (1 d), 2 (2 d)] and the potassium mono(perfluoroalkyl)cyanofluoroborates K[C_nF_{2 n+1}BF(CN)₂] [n=1 (1 c), 2 (2 c)] and [C_nF_{2 n+1}BF₂(CN)]^? [n=1 (1 b), 2 (2 b), 3 (3 b), 4 (4 b)] are accessible with perfect selectivities on multi-gram scales starting from K[C_nF_{2 n+1}BF₃] and Me₃SiCN. The K⁺ salts are starting materials for the preparation of salts with organic cations, for example, [EMIm]⁺ (EMIm=1-ethyl-3-methylimidazolium). These [EMIm]⁺ salts are hydrophobic room-temperature ionic liquids (RTILs) that are thermally, chemically and electrochemically very robust, offering electrochemical windows up to 5.8 V. The RTILs described herein, exhibit very low viscosities with a minimum of 14.0 mPa s at 20 °C for [EMIm]1 c, low melting points down to ?57 °C for [EMIm]2 b and extraordinary high conductivities up to 17.6 mS cm^?1 at 20 °C for [EMIm]1 c. The combination of these properties makes these ILs promising materials for electrochemical devices as exemplified by the application of selected RTILs as component of electrolytes in dye-sensitised solar cells (DSSCs, Gr?tzel cells). The efficiency of the DSSCs was found to increase with a decreasing viscosity of the neat ionic liquid. In addition to the spectroscopic characterisation, single crystals of the potassium salts of the anions 1 b–d, 2 d, 3 b and 4 c as well as of [nBu₄N]2 c have been studied by X-ray diffraction.

Full text of DOI:10.1002/chem.201703685

A Journal of
Accepted Article
Title: Perfluoroalkyltricyanoborate and Perfluoroalkylcyanofluoroborate
Anions: Building Blocks for Low-viscosity Ionic Liquids
Authors: Johannes Landmann, Jan A. P. Sprenger, Philipp T. Hennig,
Rüdiger Bertermann, Matthias Grüne, Frank Würthner, Nikolai
V. Ignat'ev, and Maik Finze
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To be cited as: Chem. Eur. J. 10.1002/chem.201703685
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Perfluoroalkyltricyanoborate and Perfluoroalkylcyanofluoro-
borate Anions: Building Blocks for Low-viscosity Ionic Liquids
Johannes Landmann,[a] Jan A. P. Sprenger,[a] Philipp T. Hennig,[a] Rüdiger Bertermann,[a]
Matthias Grüne,[b] Frank Würthner,[b] Nikolai V. Ignat’ev,[a,c] and Maik Finze*[a]
Abstract: Potassium perfluoroalkyltricyanoborates K[CnF2n+1B(CN)3]
(n = 1 (1d), 2 (2d)) and potassium mono(perfluoroalkyl)cyanofluoro-
borates K[CnF2n+1BF(CN)2] (n = 1 (1c), 2 (2c)) and [CnF2n+1BF2(CN)]−
(n = 1 (1b), 2 (2b), 3 (3b), 4 (4b)) are accessible with perfect
selectivities on multi-gram scales starting from K[CnF2n+1BF3] and
Me3SiCN. The K+ salts are starting materials for the preparation of
salts with organic cations, e.g. [EMIm]+ (EMIm = 1-ethyl-3-methyl-
imidazolium). These [EMIm]+ salts are hydrophobic room tempera-
ture ionic liquids (RTILs) that are thermally, chemically, and
electrochemically very robust, offering electrochemical windows of
up to 5.8 V. The RTILs described exhibit very low viscosities with a
minimum of 14.0 mPa·s at 20 °C for [EMIm]1c, low melting points of
down to –57 °C for [EMIm]2b, and extraordinary high conductivities
of up to 17.6 mS·cm–1 at 20 °C for [EMIm]1c. The combination of
properties makes these ILs promising materials for electrochemical
devices as exemplified by the application of selected RTILs as
component of electrolytes in dye-sensitized solar cells (DSSCs,
Grätzel cells). The efficiency of the DSSCs was found to increase
with decreasing viscosity of the neat ionic liquid. In addition to the
spectroscopic characterization, single crystals of the potassium salts
of 1b–d, 2d, 3b and 4c as well as of [nBu4N]2c have been studied
by X-ray diffraction.
Introduction
[EMIm][BH(CN)3] provide the best compromise in hydrophobicity,
low melting point, low viscosity, and chemical, thermal as well as
electrochemical stability.[1] These properties are requirements for
ILs used in electrochemical devices. The [EMIm]+ salts of
[BF2(CN)2]– and [BH2(CN)2]– have lower melting points and
viscosities than [EMIm][BX(CN)3] (X = F, H) but they are more
reactive and less stable and their hydrophobic nature is less
distinct.[1,10] In addition to applications in ILs, cyanoborate anions
are used as counteranions for metal and main group element
cations. E.g. redox couples[11] and luminescent materials[12]
based on the TCB anion have been developed, Ph3CTCB was
used in zirconium chemistry[13] and HTCB[14] is a superacid.[15]
Cyanoborates are starting materials for further borates, for
Cyanoborate anions are an emerging class of building
blocks especially for room temperature ionic liquids (RTILs).[1]
RTILs are of growing importance for fundamental and applied
science ranging from pharmaceutics, biochemistry, chemistry to
physics, and engineering.[2] The archetype cyanoborate anion is
the tetracyanoborate anion [B(CN)4]– (TCB, Figure 1),[3-5] which
gives low-melting and low-viscosity ionic liquids that are
chemically and electrochemically very stable and thermally
robust.[1,5,6] Many physicochemical properties of TCB-ILs were
studied experimentally and by theoretical methods.[1,7] For
example, the RTILs [EMIm]TCB (EMIm = 1-ethyl-3-methylimida-
zolium) and [BMPL]TCB (BMPL = 1-butyl-1-methylpyrrolidinium)
have been used in electrolyte compositions for electrochemical
devices such as dye-sensitized solar cells (DSSCs, Grätzel
cells).[8,9] Similarly, mixed cyanofluoro- [BF4–x(CN)x]– and cyano-
example salts of [(CF3)3B(CN)]– (I, Figure 1),[16] [B(CF3)4]– (II,
[18]
Figure 1)[16,17] and [B(CO2H)4]–
were prepared from tetra-
2–
cyanoborates. B(CN)3 has become accessible from salts of
[BX(CN)3]– (X = CN (TCB),[19] F (MFB),[20] H[21]) and [B2(CN)6]2–
from MFB-salts.[22]
hydridoborate anions [BH4–x(CN)x]– (x
= 1–3) have been
successfully applied in RTILs that have even lower melting
points and lower viscosities than the corresponding TCB-IL,
which is mostly due to the lower symmetry and the reduced
molecular weight of the heteroleptic borate anions.[1] In the
series [EMIm][BF4–x(CN)x] and [EMIm][BH4–x(CN)x] (x = 0–4) the
tricyanoborate-ILs [EMIm][BF(CN)3] ([EMIm]MFB, Figure 1) and
[a]
J. Landmann, Dr. J. A. P. Sprenger, P. T. Hennig, Dr. R.
Bertermann, Dr. Nikolai V. Ignat’ev, Prof. Dr. M. Finze
Julius-Maximilians-Universität Würzburg, Institut für Anorganische
Chemie, Institut für nachhaltige Chemie & Katalyse mit Bor (ICB)
Am Hubland, 97074 Würzburg, Germany
Figure 1. Selected cyano- and perfluoroalkylborate anions and most relevant
fields of applications.
E-mail: maik.finze@uni-wuerzburg.de
https://go.uniwue.de/finze-group
[b]
[c]
Dr. M. Grüne, Prof. Dr. F. Würthner
Perfluoroalkylborate anions[23-25] are another important class
of anions that are of interest for catalysis, materials science and
organic synthesis. The tetrakis(trifluoromethyl)borate anion
[B(CF3)4]− (II, Figure 1)[16,17,26] is a very weakly coordinating
anion[24,27] that was used for the stabilization of reactive cations
Julius-Maximilians-Universität Würzburg, Institut für Organische
Chemie & Center for Nanosystems Chemistry, Am Hubland, 97074
Würzburg, Germany
Dr. Nikolai V. Ignat’ev
Consultant, Merck KGaA, Frankfurter Straße 250, 64293 Darmstadt,
Germany
+ [30]
such as [Ag(CO)4]+,[17] [Au(NCMe)2]+,[28] [Co(CO)5]+,[29] N5 ,
NO+,[31] and [H(OEt2)2]+.[13] Furthermore, the [B(CF3)4]− anion is
electrochemically very stable and solutions of its Li+ salt reveal
Supporting information for this article is given via a link at the end of
the document.
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high Li-ion conductivity.[32] Anion II is the only tetrakis(perfluoro-
alkyl)borate anion known, to date.[23-25] In contrast, a substantial
number of borate anions with one, two and three perfluoroalkyl
groups have been described.[23-25] Especially, mono(perfluoro-
alkyl)borate anions with three fluoro or alkoxy groups have been
studied.[23,33,34] The physicochemical properties of RTILs with
perfluoroalkyltrifluoroborate anions like [C2F5BF3]– (2a, Figure 1)
have been reported,[35,36,37] and some anions have been studied
as component in Li-ion electrolytes.[38,39] Perfluoroalkyltrialkoxy-
borates are valuable starting materials for perfluoroalkyl transfer
reactions in organic synthesis[40] and for the preparation of the
corresponding perfluoroalkyltrifluoroborates.[23,37,39,41-43]
Little is known on borate anions containing both, cyano
and perfluoroalkyl groups, in general. The tris(trifluoromethyl)-
cyanoborate anion [(CF3)3B(CN)]– (I, Figure 1)[16,44,45] was used
in coordination[28,46,47] and main group element chemistry.[13,15,47]
The high thermal and chemical stability of KI points towards
potential applications of mixed perfluoroalkylcyanoborates. Lan-
thanide frameworks with the pentafluoroethyltricyanoborate
anion [C2F5B(CN)3]– (2d) are the only further examples for
compounds containing mixed perfluoroalkylcyanoborate anions.
However, no details on the synthesis of anion 2d were given.[48]
Here, we report on the synthesis and spectroscopic as well as
structural properties of potassium perfluoroalkyltricyanoborates
and potassium mono(perfluoroalkyl)cyanofluoroborates that in
part have been described in two patent applications, earlier.[49,50]
The potassium salts were used as starting materials for the
preparation of salts with organic cations. The [EMIm]+ salts are
RTILs with promising properties that were characterized by
spectroscopic, thermal and electrochemical methods, in detail.
Figure 2. Selected optimized syntheses of K[C2F5BF3‒x(CN)x] (x = 1: K2b; 2:
K2c; 3: K2d) and molecular structures of selected cyanoborate anions
(displacement ellipsoids at the 50% probability level for K1b, K1d,
K3b·0.33Me2CO or at the 25% probability level for K1c, K2d, [nBu4N]2c and
K4c·Me2CO).
salts of the [BF(CN)3]− anion starting from alkali metal tetra-
fluoroborates.[51] Another synthesis towards K2b started from
K2a, LiCl, and KCN. The solids were pestled and heated to
180 °C for 20 hrs resulting in a mixture of 82% of 2b and 18% of
further borate anions (see Supporting Information). This method
is similar to the synthesis of KTCB using K[BF4], KCN and LiCl.[5]
In case of potassium mono(perfluoroalkyl)dicyanofluoro-
borates K[RFBF(CN)2] harsher conditions had to be applied to
achieve the exchange of a second fluorine substituent against a
cyano group. K2c was obtained in neat Me3SiCN at 180 °C in a
closed reaction vessel after 5 hrs (entry A). The conversion of
2a into 2c was achieved at 150 °C, as well, but with a reaction
time of 42 h. Attempted synthesis of K2c in an open vessel
under reflux remained unsuccessful, which is rationalized by (i)
the lower reaction temperature (Tbp(Me3SiCN) = 117.2 °C[52]) and
(ii) by loss of the volatile by-product Me3SiF (Tbp(Me3SiF) =
16 °C[52]). Me3SiF promotes the transformation as a weak Lewis
acid as demonstrated by reactions of K[C6F5BF3] with and with-
out trimethylsilyl fluoride (Figure S1 in the Supporting Informa-
tion). The addition of Me3SiCl resulted in a significant reduction
of the temperature, for example K2a was transformed into K2c
already at 80 °C (entry B). During the synthesis of salts of
perfluoroalkyldicyanofluoroborate anions, the corresponding per-
fluoroalkylcyanofluoroisocyanoborate anions were identified as
intermediates, which is a result of the relatively low reaction
temperature that is not sufficient for the isomerization of the
isocyano- to the cyanoborate. Figure S7 in the Supporting Infor-
mation shows representative 11B, 13C, and 19F NMR spectra of a
reaction mixture that contained minor amounts of the anions
[C2F5BF2(CN)]– (2b) and [C2F5BF(CN)2]– (2c) along with the
major component [C2F5BF(NC)(CN)]–, which at higher tempera-
tures isomerizes to the dicyanoborate anion 2c. Earlier, the
formation of isocyanoborate intermediates was described for the
synthesis of [BF(CN)3]−, earlier.[51] The potassium salt of anion 2c
was prepared via a microwave assisted reaction in neat Me3Si-
CN, as well (entry C). Although the microwave-assisted synthesis
Results and Discussion
Synthetic Aspects
Anion synthesis. Potassium perfluoroalkyltricyanoborates and
potassium mono(perfluoroalkyl)cyanofluoroborates were obtain-
ed in excellent yield, with perfect selectivity, in high purity and on
a large scale (up to 0.15 mol) starting from the respective per-
fluoroalkyltrifluoroborate and Me3SiCN. The trimethylsilylcyanide
can conveniently be synthesized in situ and used without purifi-
cation for the preparation of the cyanoborates. The trimethylsilyl-
cyanide served as reagent and solvent and the unreacted
material was recovered and reused in subsequent syntheses. In
Figure 2 the most important synthetic strategies are exemplified
for the preparation of K[C2F5BF2(CN)] (K2b), K[C2F5BF(CN)2]
(K2c) and K[C2F5B(CN)3] (K2d) (Figure 2). The unprecedentedly
high selectivity of the successive exchange of fluorine against
cyano substituents is demonstrated by the 11B, 19F, and 13C
NMR spectra of the four salts in Figure 3 and it is important to
note that the 11B and 19F NMR spectra have been recorded on
samples of the reaction mixtures.
The perfluoroalkylcyanodifluoroborates K[RFBF2(CN)] were
obtained either in neat Me3SiCN by heating or microwave
irradiation (entries A and C) or in a mixture of Me3SiCN and
Me3SiCl (entry B). The trimethylsilylchloride acts as Lewis acid
catalyst and enables lower reaction temperatures. E.g. the addi-
tion of ca. 7 vol-% of Me3SiCl resulted in full conversion of 2a
into 2b within minutes at room temperature, whereas more than
50 °C were necessary for reactions without Me3SiCl. A related
Me3SiCl-catalysed cyanation was established for the synthesis of
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Figure 3. 11B, 19F and 13C NMR spectra of the anions [C2F5B(CN)3]− (2d), [C2F5BF(CN)2]− (2c), [C2F5BF2(CN)]− (2b), [C2F5BF3]− (2a), as well as the 11B NMR
spectrum of the anion [B(CN)4]− (TCB).
is by far the fastest reaction, the Me3SiCl-catalysed process is
the method of choice as it easily enables reactions on larger
scales.
potassium salts after precipitation from e.g. a solution in THF by
the addition of dichloromethane. The resistance of the cyano-
borates against peroxides exemplifies their chemical robustness.
In addition, the salts K[RFBF3–x(CN)x] (x = 1–3) are indefinitely
stable at room temperature in air.
Ionic liquid (IL) synthesis. The potassium salts of the per-
fluoroalkyltricyanoborate and mixed mono(perfluoroalkyl)cyano-
fluoroborate anions have been converted into salts with organic
cations in moderate to excellent yields, mostly >80%. The
metatheses were performed in aqueous solutions and the
hydrophobic organic salts were isolated either by (i) phase
separation, (ii) filtration, or (iii) extraction with dichloromethane.
The [EMIm]+ salts are RTILs that have been washed with
deionized water in order to remove traces of halides. The ILs
were dried in a vacuum overnight at slightly elevated tempe-
ratures to remove traces of water. The water content was below
50 ppm as assessed by Karl-Fischer titration.
The exchange of the last fluorine substituent at boron to
yield the respective perfluoroalkyltricyanoborates K[RFB(CN)3]
required even harsher conditions (Figure 2). The most conve-
nient entry towards K[RFB(CN)3] is again the Me3SiCl-catalyzed
reaction that was performed at 205 °C in an autoclave as exem-
plified for K2d (entry B). The related synthesis without Me3SiCl
started at temperatures higher than 250 °C but the mixture
contained a number of unidentified side- and decomposition
products. Thus, the procedure was not optimized and is not
included in Figure 2. A technical disadvantage is the high
pressure of the synthesis in Me3SiCN with Me3SiCl. This dis-
advantage was circumvented by the use of high-boiling Et3SiCN
and Et3SiCl (entry D; Tbp(Et3SiCN) = 180–182 °C,[53] Tbp(Et3SiCl) =
144–146 °C,[54] Tbp(Et3SiF) = 109 °C[55]). However, Et3SiCN is
commercially not available and Et3SiCl is more expensive than
Me3SiCl. An alternative approach is again a microwave assisted
synthesis in neat Me3SiCN that gave pure K[C2F5B(CN)3] (K2d)
but high pressure has to be considered for this transformation
(entry C).
Structural and spectroscopic characterization
X-ray diffraction. Colourless single crystals of K1b, K1c, K1d,
K2d∙Me2CO, [nBu4N]2c, K3b·0.33Me2CO, and K4c·Me2CO
suitable for X-ray diffraction were obtained from acetone solu-
tions by slow evaporation of the solvent (Figure 2, Table 1,
Table S6 and S7 in the Supporting Information). The bond
parameters of the new anions are similar to values reported for
other perfluoroalkylborate and cyanoborate anions, e.g. [RFBF3]–
(RF = CF3 (1a),[56] C2F5 (2a)[57]) [(CF)3B(CN)]− (I),[44] [B(CN)4]−
(TCB)[3] and [BF(CN)3]− (MFB).[51]
The crude potassium salts that have been obtained after
removal of all volatiles, contain small amounts of polymeric
organic impurities. Especially, tricyanoborates are brownish to
black, which is due to the harsh reaction conditions required.
However, these impurities are easily removed by treatment with
hydrogen peroxide (up to 30% v/v) and K2CO3 to give the
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Table 1. Selected experimental and calculated[a] bond parameters and spectroscopic data of [CF3BF3–x(CN)x]– (1a–d).[b]
[CF3BF3]– (1a)
[CF3BF2(CN)]– (1b)
[CF3BF(CN)2]– (1c)
[CF3B(CN)3]– (1d)
exptl
139.1(5)
–
calcd
exptl
calcd
exptl
calcd
exptl
–
calcd
d(B–F)
[pm]
[pm]
141.2
–
139.7(2)
163.0(2)
141.2
162.4
139.8(3)
161.0(3)
142.0
160.6
–
d(B–CN)
160.1(2)
159.1
d(B–CF3)
d(C≡N)
d(C–F)
[pm]
[pm]
[pm]
[cm–1
162.5(6)
–
165.7
–
163.0(2)
114.3(2)
135.7(2)
2209
165.2
115.7
137.1
2301
162.3(4)
114.2(3)
133.3(3)
2214
164.7
115.6
136.8
2306
163.2(2)
114.7(2)
135.3(2)
2223
–32.0
–
164.5
115.6
136.5
2314
–36.4
–
134.3(8)
–
137.6
–
ν(̃
CN)
]
d(11B)
d(19F) BF
d(19F) CF3
d(13C) CN
d(13C) CF3
1J(19F,11B)
1J(13C,11B) B–CN
1J(13C,11B) B–CF3
1J(19F,13C)
2J(19F,13C) F–B–CN
2J(19F,13C) F–B–CF3
[ppm]
[ppm]
[ppm]
[ppm]
[ppm]
[Hz]
–1.3
–156.6
–76.1
–
–2.0
–195.5
–98.4
–
–3.8
–5.7
–12.8
–219.9
–74.0
127.8
130.1
49.1
–16.3
–256.1
–95.9
132.9
144.1
–105.6
77.0
–169.2
–77.5
129.8
130.0
48.8
–203.9
–100.0
134.6
144.1
–108.2
82.9
–66.4
123.6
130.0
–
–87.3
129.0
144.1
–
131.4
39.6
–
145.5
–101.5
–
[Hz]
75.3
71.3
69.4
80.6
301.7
–
72.8
80.2
–373.0
–
[Hz]
110.1
310.8
–
112.5
–385.6
–
97.8
98.4
88.3
87.9
[Hz]
308.1
48.7
–382.1
45.2
305.4
35.7
–377.5
32.1
[Hz]
[Hz]
66.7
64.4
43.3
41.0
31.8
28.6
–
–
[a] B3LYP/6-311++G(d); NMR parameters: B3LYP/6-311++G(2d) using the geometries calcd at the B3LYP/6-311++G(d) level of theory. [b] NMR solvent:
(CD3)2CO; IR data: neat [EMIm]+ salt; bond distances and NMR data: K+ salts.
According to the X-ray diffraction data, d(B–F) increases and
d(B–CN) decreases with increasing content of cyano groups in
the series of trifluoromethylborate anions [CF3BF3–x(CN)x]– (x =
0–3; 1a–d; Table 1). Although the differences are small and not
all are significant (<3s) they are in excellent agreement to theo-
retical data. Both trends indicate stronger B‒CN and weaker B–
F bond(s) with increasing number of cyano groups. The experi-
mentally determined distances d(B–CF3), d(C–F), and d(C≡N)
do not show any explicit trend but DFT calculations predict
shorter bonds with increasing number of cyano groups
The pentafluoroethylborate anions 2a–d reveal an analo-
gous behaviour for the bond lengths as the ones discussed for
the trifluoromethylborate anions 1a–d (Table S7 in the Suppor-
ting Information). Furthermore, an increasing length of the
perfluoroalkyl chain does not significantly influence the B–CN
and B–F bond distances. The B‒CF2/3 distance is marginally
elongated for anions with longer perfluoroalkyl chains (1c:
162.3(4), 2c: 163.6(6), and 4c: 165.4(4) pm).
cy of the CF2/3 unit that is bonded to boron shows the reverse
trend with [CF3BF3]– (1a) being the only exception. The
difference in d(19F) for the fluorine atoms at the CF2/3 units that
are not directly attached to boron is much less pronounced as
evident from the signal of the CF3 unit of the pentafluoroethyl
groups of anions 2a–d in Figure 3.
The assignment of the strongly coupled 13C NMR spectra is
aided by 19F and 11B decoupling experiments as exemplified by
the spectra of the pentafluoroethyl derivatives 2a–d depicted in
Figure 3. d(13C) of the cyano groups follows a definite trend to
smaller resonance frequencies with increasing number of CN
groups (Table 1 and Figure 3). In contrast, the 13C NMR signals
of the perfluoroalkyl groups are close and no obvious tendency
is observed. In the 11B coupled spectra of the anions [C2F5BF2-
(CN)]– (2b) and [C2F5BF(CN)2]– (2c) the signals of the CN and
CF2 group(s) are much broader than the respective signals of
the anion [C2F5B(CN)3]– (2d). Similarly, the signal of the CF2
group of [C2F5BF3]– (2a) is less broadened. The larger line width
is paralleled by a distortion of the quartet that is due to
1J(13C,11B) coupling. Both effects, broad lines and distorted
multiplet, are a result of the interaction with the quadrupolar
nucleus 11B, which increases with decreasing local symmetry at
boron.[44,59] The 19F NMR signals of the F atoms and CF2/3 unit
bonded to boron are affected by quadrupolar coupling in a
similar way as the 13C signals. The line width of the 11B signals
shows the same behaviour, as well, but without any distortion of
the multiplets.[44,59]
Vibrational spectroscopy. The slightly shorter C≡N
distance predicted by DFT calculations is an indication for a
stronger C≡N bond that is supported by n(CN) of the EMIm-ILs
by increasing wavenumbers in the same order. The larger n
reflect the ascending Lewis acidity of the corresponding boranes
CF3BF2–x(CN)x (x = 0–2, vide infra). Thus, n(CN) of [(CF3)3B-
̃(CN)
̃
(CN)]– (I) is the highest value with 2244 cm–1 that has been
reported for a cyanoborate anion, so far.[44] Again, this high
wavenumber correlates with d(B‒CN) of [(CF)3B(CN)]− of
158.9(3) pm[44] that is significantly shorter than the one of 1d
(160.1(2) pm). Both findings display the high Lewis acidity of
(CF3)3B,[24,58] which leads to a strengthening of the B–CN and
C≡N bond, alike.
Most of the coupling constants reveal definite trends (Table
1
1). For example, J(13C,11B) of the B–CN and B–CF3 moieties in
1a–d decreases with increasing number of cyano groups.
Thermal properties
NMR spectroscopy. All new perfluoroalkylcyanoborate
anions [RFBF3‒x(CN)x]− (x = 1–3) and the corresponding trifluoro-
borate anions were characterized by multinuclear NMR spec-
troscopy (Table S4 in the Supporting Information). d(11B)
decreases in the series 1a–d (Table 1) and 2a–d (Figure 3) with
increasing number of cyano groups. d(19F) of the BF moiety
reveals an analogous trend whereas the 19F resonance frequen-
EMIm-ILs. All salts [EMIm][RFBF3–x(CN)x] (x = 1–3) are RTILs
(Table 2). The melting points (Tmp) are mostly lower than the
ones of [EMIm][RFBF3] and all are below those of [EMIm][BF4]
(15 °C)[35] and [EMIm]TCB (8 °C).[1] [EMIm]II is a solid that melts
at 125 °C. The lowest melting points for [EMIm][RFBF3–x(CN)x]
have been found for the ILs with the most unsymmetrical anions
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Table 2. Thermal properties of the K+ and [EMIm]+ salts.[a]
with minima for [EMIm]2b and [EMIm]3b with –57 and –53 °C,
respectively (Figure 4a). Noticeably, the length of the perfluoro-
alkyl chain has different effects on Tmp for [EMIm][RFBF3] and
[EMIm][RFBF2(CN)] as evident from Figure 4a. The EMIm-ILs
show supercooling behaviour with freezing points (Tfp) of up to
–97 °C for [EMIm]1b (Table 2, Figure 4b).
anion
K+ salt[b]
[EMIm]+ salt[c]
Tmp [°C] Tdec [°C] Tfp [°C]
Tg [°C] Tmp [°C] Tdec [°C]
1a[d]
1b
1c
-
270
200
300
320
270
260
310
350
296[h]
180
300[h]
220
–
−72[e]
−97[e]
−72
−53
−54
−86
−74[e]
−57
–
–
−21
−34
−30
−7
265
-
−123
215
-
–
270
The salts [EMIm][RFBF3–x(CN)x] (x = 0–3) are thermally very
robust as shown by their decomposition temperatures (Table 2,
Figure 4c). As a consequence of the low melting points and the
high thermal stabilities, the new RTILs based on mono(per-
fluoroalkyl)cyanofluoroborate anions provide large liquid ranges.
The EMIm-monocyanoborates [EMIm][RFBF2(CN)] possess the
lowest thermal stabilities in the series and the stability increases
with increasing number of cyano groups. Thus, the perfluoro-
alkyltricyanoborates are the most stable ILs and their stability
exceeds those of the respective perfluoroalkyltrifluoroborates, as
well. Decomposition of the mixed perfluoroalkylcyanofluoro-
borates starts with dismutation of the cyano and fluorine substi-
tuents at boron. E.g. a sample of [EMIm]2b was found to slowly
dismutate to give [EMIm]2a and [EMIm]2c at 245 °C, a value
slightly below Tdec determined by TGA (260 °C). Related dismu-
tation reactions have been described for cyanofluoroborates,
earlier.[5] The gaseous decomposition products of the thermo-
gravimetric analyses were studied by IR spectroscopy and mass
spectrometry. All EMIm-cyanoborates studied herein decom-
pose under evolution of HCN (IR). All fluoroborates as well as
[EMIm][RFB(CN)3] lose fluorine as indicated by mass spectrome-
try. The mass spectra of the gaseous decomposition products of
the pentafluoroethylborates showed the formation of tetrafluoro-
ethylene C2F4. CF3CF=CF2 was found during the decomposition
of [EMIm]3b. Furthermore, decomposition of [EMIm]1a and
[EMIm]1b gave some tetrafluoroethylene, as well, but not the
degradation of [EMIm]1c and [EMIm]1d.
1d
2a[f]
2b
2c
200
–
330
-
–
1
290[g]
260[g]
330
-
–
−57
−21
−7
-
−110
2d
3a
190
-
–
375
–
8[i]
304[i]
265[g]
277[i]
260[g]
420[i]
420
3b
4a
138
-
−86[e]
−112
–
−53
−4[i]
−36
15[i]
8[l]
-
4b
[BF4]−
TCB
I
190
570[j]
430[k]
365[m,n]
–
−67[e]
−60[i]
−55
n.o.[o]
–
–
−93[i]
510[k]
370[n]
320[p]
–
n.o.
–
n.o.
125[q]
n.o.
II
325
[a] All temperatures are onset values (rounded down for the decomposition
temperature (Tdec), except for literature values); Tfp = freezing point; Tmp
=
melting point; Tg = glass transition temperature. [b] Tmp and Tdec determined
by DSC and confirmed visually. [c] Phase transition temperatures determined
by DSC; Tdec determined by TGA. [d] Literature data for [EMIm]1a: Tg
=
–117 °C; Tfp = –80 °C; Tmp = –20 °C; Tdec = 246 °C.[35] [e] Only part of the IL
crystallizes at the given temperature upon cooling, the remainder crystallizes
upon heating at approximately the same temperature. [f] Literature data for
K2a: Tdec = 287 °C;[39] for [EMIm]2a: Tmp = 1 °C; Tdec = 305 °C.[35] [g] Onset of
a continuous loss of mass. [h] Ref. [39]. [i] Ref. [35]. [j] Ref. [60]. [k] Ref. [5].
[l] Ref. [1]. [m] Melts under decomposition. [n] Ref. [44]. [o] n.o. = not observ-
ed. [p] Ref. [17]. [q] Phase transition at 81 °C.
The thermal stabilities of [EMIm][RFBF3–x(CN)x] (x = 1–3)
parallel the B–F and B–CN bond dissociation energies DH
calculated for the gas phase, which correspond to the fluoride
and cyanide ion affinities of the Lewis acids RFBF2–x(CN)x
(x = 0–2) (Tables S10 and S11 in the Supporting Information).
According to the calculated fluoride and cyanide ion affinities,
the B–CN bond(s) is(are) weaker than the B–F bond(s) for all
mixed mono(perfluoroalkyl)cyanofluoroborate anions. Further-
more, loss of F– at the a-carbon atom under formation of a di- or
a monofluorocarbene borane complex, which is well document-
ed for trifluoromethyl- and pentafluoroethylborate anions,[58] is
thermodynamically the least favoured decomposition pathway
for the borate anions.
The EMIm-trifluoromethylborates are less stable than
[EMIm]+ salts of the pentafluoroethylborate anions with the same
F/CN substitution scheme (Table 2, Figure 4c). The small
change in Tdec of the [EMIm]+ salts of 2b, 3b and 4b shows that
the influence of longer perfluoroalkyl chains is negligible. The
instability of CF3-substituted borates compared to borates with
longer perfluoroalkyl chains is rationalized by the F– affinity of
the corresponding fluorocarbene complexes of boron: difluoro-
carbene complexes are easier formed than monofluorocarbene
complexes such as CF3CFB(CN)3 (Table S11 in the Supporting
Information).
Figure 4. a) Melting point (Tmp), b) freezing point (Tfp) and c) decomposition
temperature (Tdec) of [EMIm][CnF2n+1BF3–x(CN)x] (n = 1–4; x = 0–3).
K[RFBF3–x(CN)x] (x = 0–2). The trends in thermal stability of the
potassium salts K[RFBF3–x(CN)x] (x = 0–2) are similar to the ones
discussed for the EMIm-ILs (Table 2). The low stabilities of K3b
and K4b are two exceptions. K3b and K4b decompose at
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Table 3. Physical and electrochemical properties[a] of [EMIm][RFBF3–x(CN)x] (x = 0–3; RF = perfluoroalkyl), [EMIm][BF4], and [EMIm][B(CN)4] ([EMIm]TCB) at
20 °C (values in italics: 25 °C).
anion
c
D+
D−
[10−11·m2·s−1
I
Epc
Epa
h
r
s
[mS·cm−1
13.1
LNMR
Limp
DE
[mPa∙s]
[g·cm−3
]
[mol·L–1
]
]
]
[cm2·S·mol−1
]
[-]
[V]
−2.5[b]
[V]
2.1[b]
[V]
4.6[b]
1a
32.5
16.5
14.0
17.8
32.2
17.6
16.4
21.8
32
1.34
1.26
1.20
1.15
1.42
1.34
1.28
1.24
1.49
1.40
1.55
1.46
1.28
1.04
5.42
4.95
4.58
4.27
4.75
4.39
4.11
3.87
4.28
3.95
3.89
3.59
6.46
4.60
5.6
3.8
7.6
8.4
6.3
3.5
6.6
6.1
5.0
n.d.
4.8
n.d.
3.4
n.d.
5.4
3.59
2.42
3.23
3.84
2.69
2.19
3.48
3.19
2.53
n.d.
0.67
0.50
0.55
0.50
0.63
0.59
0.59
0.57
n.d.
1b
9.3
16.0
6.42
7.01
5.34
3.47
5.89
5.37
4.47
n.d.
−2.5
−2.5
−2.4
−2.5[b]
−2.6
−2.4
−2.4
−2.6
−2.5
−2.6
−2.5
−2.5
–2.4[1]
2.6
2.3
2.4
2.1[b]
3.2
2.4
2.1
2.1
2.8
2.1
3.0
2.0
2.0[1]
5.1
4.8
4.8
4.6[b]
5.8
4.8
4.5
4.7
5.3
4.7
5.5
4.5
4.4[1]
1c
10.0
7.7
17.6
11.5
10.4
15.3
13.1
9.8
1d
2a
5.6
2b
8.8
2c
8.0
2d
3a[c],[35]
6.7
n.d.[d]
8.6
3b
22.2
38
6.9
10.4
5.2
4.48
n.d.
2.63
n.d.
0.59
n.d.
4a[c],[35]
4b
[BF4]− [c],[35] 42[e]
TCB
21.8[f]
n.d.
5.1
30.8
7.1
3.26
n.d.
1.98
n.d.
0.61
n.d.
n.d.
13.6
6.3
12.1
4.47
2.64
0.59
[a] Dynamic viscosity h; density r; concentration c; diffusion coefficients of cations (D+) and anions (D−); specific conductivity s measured via impedance
spectroscopy; molar conductivities calcd. by LNMR = (D++D−)·NA·e2·k−1·T−1 and Limp = s·M·r−1; ionicity I = Limp·LNMR−1; cathodic and anodic limits Ec and Ea;
electrochemical window DE = Epa – Epc. [b] Data reported earlier: [EMIm]1a: Epc = –2.49, Epa = 2.14, DE = 4.63 V; [EMIm]2a: Epc = –2.50, Epa = 2.15, DE = 4.65 V.
[c] Published data for: viscosity, density and conductivity at 25 °C; for cyclic voltammetry experiments no temperature was given.[35] [d] n.d. = not determined.
[e] 67 mPa·s at 20 °C.[1] [f] The viscosity differs slightly from a value published, earlier (h = 22.2 mPa·s),[1] which is due to the different methods employed.
180 and 220 °C whereas K2b is stable up to 260 °C. K3b and
K4b, which are both liquids at these temperatures, are prone to
dismutation more easily, which is evident from the DSC curves.
K2b does not melt. So, dismutation of K2b is slow and it takes
several hours until significant amounts were converted into other
borates such as K2a and K2c.
The decomposition temperatures of the perfluoroalkyltri-
cyanoborates K1d and K2d of 320 and 350 °C, respectively, are
similar to Tdec of K[(CF3)3B(CN)] (370 °C)[44] and K[B(CF3)4]
(320 °C)[17] but lower than Tdec reported for K[B(CN)4] (510 °C).[5]
Furthermore, K1d and K2d are the sole potassium trifluoro-
methyl- and pentafluoroethylcyanoborates that melt (200 and
190 °C, respectively) before decomposition occurs.
Viscosities, self-diffusivities and conductivities of EMIm-ILs
Dynamic viscosities. The [EMIm]+ salts based on the new
mono(perfluoroalkyl)cyanofluoroborate anions [CnF2n+1BF3–x
-
(CN)x]– (n = 1–4; x = 0–3) are low-viscosity RTILs (Table 3,
Figures 5 and 6). [EMIm][CF3BF(CN)2] (h = 14.0 mPa∙s) has the
lowest dynamic viscosity at 20 °C of the new [EMIm]+ salts pre-
sented, herein, which is much lower than those of [EMIm][BF4]
(67 mPa∙s) and [EMIm]TCB (21.8 mPa∙s) and close to the one
of [EMIm]MFB (12.6 mPa∙s).[1] The viscosity strongly decreases
with increasing temperature and their differences become
smaller (Figure 5). The decrease in viscosity with higher tempe-
rature is less pronounced for the EMIm-perfluoroalkylcyano-
(fluoro)borates than for [EMIm]TCB.
The unsymmetrical anions of the type [CnF2n+1BF2(CN)]– and
[CnF2n+1BF(CN)2]– result in ILs with the lowest viscosities in the
respective series of perfluoroalkylborates. This effect is similar to
the one found for the melting and freezing points (Table 2,
Figure 4). In addition to this symmetry-related effect, an increas-
ing number of cyano groups has a beneficial influence with
regard to lower viscosities. This is rationalized by the more
efficient charge delocalization due to the strong electron-with-
drawing cyano groups.[1] So, lower viscosities are observed (i)
for [EMIm][CnF2n+1B(CN)3] compared to the respective [EMIm]-
[CnF2n+1BF3] and (ii) for [EMIm][CnF2n+1BF(CN)2] compared to the
corresponding [EMIm][CnF2n+1BF2(CN)]. The interplay of symme-
try and content of CN groups makes [EMIm]1c and [EMIm]2c to
the ILs having the lowest viscosity in the series of trifluoro-
methyl- and pentafluoroethylborates, respectively.
The viscosity increases with longer perfluoroalkyl chains as
exemplified by the series [EMIm][CnF2n+1BF2(CN)] (n = 1–4, 1b–
4b; Figure 6). This trend basically parallels the one reported for
the corresponding trifluoroborates, earlier, with one exception,
the viscosity of [EMIm]1a (32.5 mPa∙s) and [EMIm]2a (32.2
mPa∙s) that are almost the same (Figure 6).[35] The difference
between the trifluoromethyl- and the related pentafluoroethyl-
borate is small, in general, but it increases with an increasing
number of cyano groups (Figure 6). The higher viscosities for
anions with longer perfluoroalkyl chains may be explained by
Electrochemical stabilities of EMIm-ILs
The electrochemical stability of the neat EMIm-ILs was assess-
ed by cyclic voltammetry at 20 °C. The cathodic (Epc) and anodic
limits (Epa) and the electrochemical windows are listed in Table 3
and the respective voltammograms are shown in Figure S4 in
the Supporting Information. The cathodic limit of all [EMIm]+ salts
is similar with approximately –2.4 to –2.6 V and it is determined
by the stability of the [EMIm]+ cation. In contrast, the anodic limit
is strongly dependent on the borate anion and therefore the
electrochemical windows differ. [EMIm][RFBF2(CN)] provide the
largest electrochemical windows (>5 V) with a maximum of 5.8 V
for [EMIm]2b. The monoperfluoroalkyl)dicyanofluoroborates
[EMIm][RFBF(CN)2] are electrochemically very stable, too, but
the values (~4.8 V) are significantly smaller than the ones of
[EMIm][RFBF2(CN)]. The lowest electrochemical stability was
found for the monoperfluoroalkyltrifluoro- and -tricyanoborates
although their stability is still high. The influence of the length of
the perfluoroalkyl chain is small, which is in agreement to an
earlier report on EMIm-ILs with the [RFBF3]– anions.[35] The
biggest deviation is found for [EMIm]1b and [EMIm]2b with 5.1
and 5.8 V, respectively. The electrochemical windows of the
[EMIm]+ salts of the anions [RFB(CN)3]– are similar to the
stabilities reported for [EMIm][RFBF3],[35] [EMIm][BF4][35] and
[EMIm]TCB.[1]
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Figure 7. Walden plot for [EMIm][CnF2n+1BF3–x(CN)x] (x = 0–3) and [EMIm]TCB.
Figure 5. Temperature dependence of the dynamic viscosity (h) of [EMIm]-
[CnF2n+1BF3–x(CN)x] (n = 1–4; x = 0–3) and [EMIm]TCB.
measurements performed at 20 °C, the specific conductivities
were studied at 40, 60, and 80 °C and for higher temperatures
higher conductivities were observed (Table S1 in the Supporting
Information).
The specific conductivity of an ionic liquid is governed by its
dynamic viscosity and density as well as the molecular mass
and size of the ions.[63] The density of the IL and the molecular
mass and size of ions, which determine the concentration (c),
are a measure for the number of carrier ions. On the contrary,
the dynamic viscosity describes the particle mobility. A combina-
tion of high concentration of the carrier ions and low viscosity
results in a high specific conductivity. So, it is rationalized that
longer perfluoroalkyl chains result in lower specific conductivities
for each series of EMIm-ILs. The successive exchange of fluo-
rine against CN is accompanied by decreasing concentrations
(c) and dynamic viscosities, which both have opposing effects
on s. Since the EMIm-ILs based on the mixed perfluoroalkyl-
cyanofluoroborate anions [CnF2n+1BF3–x(CN)x]– (x = 1, 2; n = 1, 2)
have very low viscosities but still high concentrations (c), their
specific conductivities are extraordinary high (Table 3). The res-
pective [EMIm][CnF2n+1B(CN)3] have higher viscosities and lower
densities than the EMIm-ILs with the less symmetrical anions
[CnF2n+1BF3–x(CN)x]– (x = 1, 2; n = 1, 2). This explains the signifi-
cantly lower specific conductivities of [EMIm][CnF2n+1B(CN)3].
This gives even lower s for [EMIm][CnF2n+1B(CN)3] compared to
[EMIm][CnF2n+1BF3].
Figure 6. Dynamic viscosity (h) of [EMIm][CnF2n+1BF3–x(CN)x] (n = 1–4; x =
0–3) at 20 °C.
stronger van der Waals interactions[35] and the higher mass of
the anions.[61] The exception discussed for [EMIm]1a and
[EMIm]2a reflects the strong gain in charge delocalization of a
C2F5 versus a CF3 group. This effect becomes less important the
more CN groups are present at boron because of their efficient
charge delocalization. This results in (i) significantly lower
viscosities for [EMIm][CF3BF3–x(CN)x] compared to [EMIm]-
[C2F5BF3–x(CN)x] (x = 1–3) and (ii)
a steady increase in
D[h([EMIm][C2F5BF3–x(CN)x]) – h([EMIm][CF3BF3–x(CN)x])] with
increasing x.
Densities. The densities of the EMIm-ILs based on the
anions [CnF2n+1BF3–x(CN)x]– (n = 1–4; x = 0–3) are higher than
the density of [EMIm]MFB (1.07 g∙cm–3). The density increases
with longer perfluoroalkyl chains and it decreases with
increasing number of cyano groups (Table 3). Analogous trends
have been reported for [EMIm][CnF2n+1BF3] (n = 1–4)[35] and
[EMIm][BF4–x(CN)x] (x = 0–4).[1]
Conductivities. The EMIm-ILs with the mixed perfluoroalkyl-
cyanofluoroborate anions [CnF2n+1BF3–x(CN)x]– (x = 1, 2; n = 1, 2)
exhibit extraordinary high specific conductivities (s) of up to
17.6 mS·cm–1 for [EMIm]1c at 20 °C (Table 3). These conduc-
tivities are much higher than s of EMIm-perfluoroalkyltrifluoro-
borates.[35] Furthermore, specific conductivities of non-protic ILs
are usually smaller than 10 mS·cm–1 at 20 °C,[62] e.g.
9.6 mS·cm–1 for [EMIm](CF3CO2).[63] In addition to the impedance
In contrast to the specific conductivity, the molar conductivity
Limp mostly depends on the dynamic viscosity. So, [EMIm]-
[CnF2n+1B(CN)3] reveal higher Limp than the respective [EMIm]-
[CnF2n+1BF3] (Table 3). The other dependencies found for Limp
are basically the same as those discussed for s. The Walden
plot depicted in Figure 7 shows that the EMIm-ILs based on the
anions [CnF2n+1BF3–x(CN)x]– (x = 0–3) and on the TCB anion can
be classified as good ILs.[64]
Self-diffusion coefficients were extracted for the anions (D–)
and cations (D+) from diffusion-ordered spectroscopy (DOSY)
NMR measurements performed at 20, 40 and 60 °C (Table 3
and S1 in the Supporting Information). The diffusion coefficients
of [EMIm][CnF2n+1BF3–x(CN)x] were derived from 1H (D+) and 19
F
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borates and the largest for [EMIm]TCB that has the maximum
number of four cyano groups at boron. For the VFT and Litovitz
fittings the opposite trend was found, although the absolute
standard deviations are very small for both models. An
Arrhenius behaviour is indicative for non-associating electro-
lytes,[67] whereas Litovitz behaviour points towards associating
electrolytes.[68] Hence, the present results indicate a slight
change in the behaviour from non-associating to more
associating electrolytes with an increasing number of cyano
groups. The good fitting according to the VFT approach is in
accordance to the observation of glass transitions for some of
the RTILs by DSC (Table 2).
Dye-sensitized solar cells (DSSCs) with
EMIm-cyanoborate-ILs
Selected EMIm-ILs with the new perfluoroalkylcyano(fluoro)-
borate anions were employed as component of the electrolyte of
dye-sensitized solar cells (DSSCs, Grätzel cells[69]).[50] [EMIm]-
TCB was introduced as a non-volatile component of electrolytes
for DSSCs, earlier,[8] and it was employed in the same setup as
the new RTILs as a reference.[50] According to the DSSC data
collected in Table 4, the four EMIm-ILs with mixed perfluoro-
alkylcyanofluoroborate anions 1b, 1c, 2b and 2c result in higher
efficiencies than [EMIm]TCB. The highest efficiency was
achieved with [EMIm][CF3BF(CN)2] that has the lowest viscosity
within the investigated series of ILs. In general, the efficiency
decreases with increasing viscosity, which explains the inferior
performance of the cell that contained [EMIm]TCB.
Figure 8. Dynamic viscosities (h) and molar conductivities (L) as a function of
the number of CN groups of the perfluoroalkylborate anions and the TCB
anion (top) and of the length of the perfluoroalkyl group of the
mono(perfluoroalkyl)difluorocyanoborate anions (bottom), respectively (20 °C).
DOSY NMR experiments (D–). For selected EMIm-ILs 11B DOSY
NMR studies were performed, as well, that gave identical D–.
1
Table 4. DSSC efficiencies with electrolytes based on EMIm-cyanoborate
The diffusion coefficients of [EMIm]TCB were determined by H
RTILs at 25 °C[a] and dynamic viscosities of the neat RTIL at 20 °C.[50]
(D+), 11B (D–) and 13C DOSY NMR measurements (D+ and D–).
D+ and D– extracted from the 13C DOSY NMR study were
salt
DSSC efficiency[a]
(25 °C, AM[b] 1.5)
5.8 ± 0.8%
h(20 °C)
[mPa∙s]
16.5
1
basically identical to D+ and D– derived from H and 11B experi-
[EMIm]1b
[EMIm]1c
[EMIm]2b
[EMIm]2c
[EMIm]TCB
ments, respectively. For all EMIm-ILs, the diffusion coefficient of
the cation is slightly larger than D– of the anion.
6.0 ± 0.8%
14.0
5.5 ± 0.3%
17.6
The molar conductivity LNMR was calculated from the
diffusion coefficients D+ and D– using the Nernst-Einstein equa-
tion (Table 3). LNMR is much higher than Limp for all ILs and both
molar conductivities LNMR and Limp follow basically the same
trends (Figure 8). For example, the mixed EMIm-perfluoroalkyl-
cyanofluoroborates [EMIm][C2F5BF3–x(CN)x] (x = 1 (2b), 2 (2c))
have higher LNMR compared to [EMIm]2a and [EMIm]2d. The
ratio of Limp to LNMR that is termed ionicity (I) can be regarded as
a measure for the ion-pairing of an IL (Table 3, Figure S2).[65]
The ionicities are in the range of 0.50 to 0.67 for the EMIm-ILs
discussed herein and no trend concerning I is evident.
5.4 ± 0.3%
16.4
5.2 ± 0.5%
22.2
[a] Composition of the electrolyte (molar ratio): 36 [MMIm]I (MMIm = 1,3-
dimethylimidazolium), 36 [EMIm]I,
5
I2, 48 [EMIm][RFBF3–x(CN)x] or
[EMIm]TCB, 2 [C(NH2)3]SCN, 10 N-butylbenzimidazole. [b] AM = air mass
coefficient.
Summary and Conclusions
Potassium salts of perfluoroalkyltricyanoborate and mono-
(perfluoroalkyl)cyanofluoroborate anions have become available
in high purity and on a large scale via different synthetic
approaches. These potassium salts have been found to be well-
suited for metathesis reactions giving, e.g. [EMIm]+ salts that are
room-temperature ionic liquids (RTILs). These RTILs possess
very low viscosities, extraordinary high specific conductivities
and low melting and freezing points. The highest conductivities,
lowest viscosities and melting as well as freezing points were
Temperature-dependent fitting of
and . The experi-
h s
mental dynamic viscosities measured at different temperatures
were fitted with the Vogel-Fulcher-Tammann (VFT) ansatz (Eq.
1 in the Supporting Information), which is often used to describe
the thermal behaviour of glass-forming liquids.[66] The fits give
very good results for all RTILs (R2 ≥ 99.99%) and the fit
parameters are given in Table S2 and the fitted curves are
shown in Figure S5 in the Supporting Information.
The conductivities were fitted using a VFT,[66] Arrhenius,[67]
and Litovitz[68] ansatz (Eq. 2–4 in the Supporting Information). All
three equations were found to be well suited for the description
of the temperature dependence of the specific conductivities (R2
≥ 99.6%). The standard deviation of the Arrhenius fitting
increases with increasing number of CN groups bonded to boron.
Hence, relatively large deviations were found for the tricyano-
found for the most unsymmetrical borate anions [CnF2n+1BF3–x
-
(CN)x]– (n = x = 1, 2). Especially the salts of the anions [RFBF-
(CN)2]– and [RFB(CN)3]– reveal very high thermal stabilities and
they are chemically and electrochemically very robust. In
summary, EMIm-ILs based on the mono(perfluoroalkyl)dicyano-
fluoroborate anions [CF3BF(CN)2]– (1c) and [C2F5BF(CN)2]– (2c)
provide a desirable combination of properties required for ILs
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C4F9I,[43] respectively. K[B(CF3)4] was synthesized as described
earlier.[16,17] Me3SiCN and Et3SiCN were synthesized according to a
modified literature procedure reported for Me3SiCN.[75]
used for example in electrochemical devices. A first demonstra-
tion is given by their application in dye-sensitized solar cells
(DSSCs, Grätzel cells[69]) and it has to be expected that others
will follow. In addition, with the synthesis of first lanthanide
frameworks with the [C2F5B(CN)3]– anion, the value of the mono-
(perfluoroalkyl)cyanoborate anions for coordination chemistry
and luminescent materials was already demonstrated.[48]
Syntheses of Potassium Salts
K[CF3BF2(CN)] (K1b): K1a (3.0 g, 17.0 mmol) was stirred with Me3SiCN
(6.0 mL, 45.0 mmol) at room temperature for 20 hrs. All volatiles were
removed under reduced pressure and most of the excess of the Me3SiCN
(3.0 mL, 22.5 mmol) was recovered by fractional distillation. The crude
product was taken up into THF (3 mL) and addition of CH2Cl2 (150 mL)
resulted in K1b as a colourless solid, which was isolated by filtration and
subsequent drying in a vacuum. Yield: 3.00 g (16.4 mmol, 96%). 13C
NMR: d = 130.0 (qt, 1C, 1J(19F, 13C) = 308.1 Hz, 2J(19F, 13C) = 43.3 Hz,
CF3), 129.8 ppm (t, 1C, 2J(19F, 13C) = 48.7 Hz, CN). 11B NMR: d = –3.8
ppm (tq, 1B, 1J(19F,11B) = 48.8 Hz, 2J(19F,11B) = 34.6 Hz). 19F NMR: d = –
77.5 (qt, 3F, 2J(19F,11B) = 34.6 Hz, 3J(19F,19F) = 1.9 Hz, CF3), –169.2 ppm
(qq, 2F, 1J(19F,11B) = 48.8 Hz, 3J(19F,19F) = 1.9 Hz, BF2). Raman: 2233,
2226 cm‒1 (n(C≡N)). MALDI-MS: m/z (isotopic abundance) calcd. for 1b
([C2BF5N]–): 144(100), 143(25), 145(2); found: 144(100), 143(27), 145(5).
Anal. calcd. for Elemental analysis calcd (%) for C2BF5KN: C 13.13, N
7.66; found: C 13.29, N 7.62.
Experimental Section
General: Reactions involving air-sensitive compounds were performed
either in 100 or 250 mL round bottom flasks or in 20 or 60 mL glass tubes
equipped with valves with PTFE stems (Young, London) under argon
using standard Schlenk line techniques. For reactions under high
pressure (>5 bar) an autoclave BR-300 (Co. Berghof, 72800 Eningen,
Germany; capacity 350 mL) equipped with an anchor agitator, an internal
temperature control unit as well as a manometer was used. Some
reactions were performed in glass fingers equipped with stems with
PTFE valves under microwave irradiation (CEM Discover S-Klasse Plus
(SP)). 11B, 19F, and 13C NMR spectra were recorded at 25 °C in
(CD3)2CO on a Bruker Avance I 500 spectrometer. 13C{11B} NMR spectra
were recorded on a Bruker Avance III HD 300 spectrometer. For the 13C
NMR spectra concentrated solutions (100‒500 mg in 0.7 mL (CD3)2CO)
were used. The concentrated samples were diluted to ensure small line
widths in the 11B and 19F spectra (30‒100 mg in 0.7 mL (CD3)2CO). The
13C{19F} NMR spectra were recorded on the Bruker Avance I 500 spec-
trometer equipped with a Prodigy-Cryoprobe using a simple zgig pulse
sequence. The broadband fluorine decoupling was achieved with the
adiabatic decoupling scheme p5m4sp180.2 using a smoothed chirp
shape pulse with 500 µs pulse length to ensure a decoupling bandwidth
of 85 kHz or 180 ppm for 19F. The NMR signals were referenced against
TMS (1H and 13C), BF3·OEt2 in CDCl3 with X(11B) = 32.083974%, and
CFCl3 with X(19F) = 94.094011% as external standards.[70] 1H and 13C
chemical shifts were calibrated against the residual solvent signal and
the solvent signal, respectively (d(1H): (CD2H)(CD3)CO 2.05 ppm; d(13C):
(CD3)2CO 29.84 and 206.26 ppm).[71] IR spectra were recorded at room
temperature on an Excalibur FTS 3500 spectrometer (Digilab, Germany),
on a Nicolet 380 spectrometer or on a Bruker Alpha spectrometer. IR
spectra were measured in the attenuated total reflection (ATR) mode in
K[CF3BF2(CN)] (K1b) with Me3SiCl: K1a (6.00 g, 34.1 mmol) was
suspended in Me3SiCN (60 mL, 450 mmol). Upon addition of Me3SiCl
(4.0 mL, 31 mmol) the suspension immediately turned into
a clear
solution that was stirred for 10 minutes. All volatiles were removed under
reduced pressure and most of the excess Me3SiCN was recovered by
fractional distillation. The slightly coloured residue was dissolved in water
and several drops of aqueous H2O2 (30% v/v) and K2CO3 (1 g) were
added. After stirring for 5 hrs, the mixture was concentrated under
reduced pressure until K2CO3 began to precipitate. The mixture was
extracted with THF (3 × 50 mL), the combined organic layers were dried
with K2CO3, filtered, and concentrated to a volume of 10 mL. CH2Cl2
(100 mL) was added and a colourless precipitate formed that was collect-
ed by filtration and dried in a vacuum. Yield: 5.80 g (31.7 mmol, 93%).
K[CF3BF(CN)2] (K1c) using MW irradiation: Trimethylsilyl cyanide
(17.1 mL, 127.5 mmol) and K1a (6.0 g, 34.1 mmol) were stirred at room
temperature for 12 days to give K1b according to NMR spectroscopy.
Most of the trimethylsilyl fluoride, which had formed in the course of the
reaction, was removed under reduced pressure. The reaction mixture
was irradiated using a microwave oven (CEM Discover; 200 W, Tmax
=
90 °C) for 20 minutes. All volatiles were removed under reduced pressu-
re and most of the remaining Me3SiCN (6.3 mL, 47.2 mmol) was recover-
ed by fractional distillation. The solid brownish residue was taken up into
aqueous H2O2 (50 mL, 30% v/v) and was stirred for 1 hr. The pH of the
solution was adjusted to 1 by the addition of conc. hydrochloric acid. Tri-
n-propylamine (7.0 mL, 36.8 mmol) was added while stirring and the
crude [nPr3NH]1c was extracted with CH2Cl2 (3 × 50 mL). The combined
organic fractions were dried with MgSO4, filtered and a solution of KOH in
water (6 g, 30 mL) was added. The organic layer was removed and
extracted with an aqueous solution of KOH (6 g, 30 mL). The collected
aqueous KOH solutions were extracted with THF (3 × 50 mL). The three
THF phases were combined, dried with K2CO3 and filtered. The volume
of the THF solution was reduced until a viscous solution was obtained.
Pure K1c precipitated upon addition of CH2Cl2 (150 mL). The salt was
filtered off and dried in a vacuum. Yield: 5.0 g (26.3 mmol, 77%). 13C
NMR: d = 130.1 (qd, 1C, 1J(19F, 13C) = 305.4 Hz, 2J(19F, 13C) = 31.8 Hz,
CF3), 127.8 ppm (dq, 2C, 2J(19F, 13C) = 35.8 Hz, 3J(19F, 13C) ≈ 2 Hz, CN).
11B NMR: d = –12.8 ppm (dq, 1B, 1J(19F,11B) = 49.1 Hz, 2J(19F,11B) =
35.7 Hz). 19F NMR: d = –74.0 (qd, 3F, 2J(19F,11B) = 35.7 Hz, 3J(19F,19F) =
8.9 Hz, CF3), –219.9 ppm (qq, 1F, 1J(19F,11B) = 49.1 Hz, 3J(19F,19F) =
8.9 Hz, BF). Raman: 2228 cm‒1 (n(C≡N)). MALDI-MS: m/z (isotopic
abundance) calcd. for 1c ([C3BF4N2]–): 151(100), 150(25), 152(4); found:
151(100), 150(24), 152(3). Elemental analysis calcd (%) for C3BF4KN2: C
18.97, N 14.75; found: C 19.08, N 14.58
the region of 4000–530 cm−1 with an apodized resolution of 4 cm−1
.
Raman spectra were recorded using the 1064 nm excitation line of a
Nd/YAG laser on crystalline samples contained in melting point capilla-
ries in the region of 3500–80 cm−1 at room temperature either on an
Excalibur FTS 3500 spectrometer (Digilab, Germany) with an apodized
resolution of 4 cm−1 or on a Bruker IFS-120 spectrometer with an apodiz-
ed resolution of 1 cm−1, respectively. Matrix-assisted laser desorption/-
ionization (MALDI) mass spectra in the negative-ion mode were recorded
on a Bruker Ultraflex TOF spectrometer. DSC measurements were per-
formed with a Netzsch DSC 204F1 Phoenix in the temperature range of
25 to −180 °C with a cooling and heating rate of 5 K·min–1 and DTA
measurements were performed with a Netzsch STA 449 C in the
temperature range of 32 to 500 °C with a heating rate of 10 K·min–1. The
water content of the ILs was determined by Karl-Fischer titration using a
831 KF Coulometer (Metrohm). Viscosities were measured on samples
with a minimum volume of 3.5 mL using a SVM 3000 Stabinger visco-
meter (Anton Paar, Austria). Elemental analyses (C, H, N) were perform-
ed either with a Euro EA3000 instrument (HEKA-Tech, Germany) or a
Perkin-Elmer EA240 instrument.
Chemicals: All standard chemicals were obtained from commercial
sources. Solvents were dried according to standard protocols[72] and
stored in flasks equipped with valves with PTFE stems (Young, London)
over molecular sieves (4 Å) under an argon atmosphere. K[CnF2n+1BF3]
(n = 1‒4) were obtained from M[CnF2n+1B(OMe)3] (M = Li, K; n = 1–4) and
anhydrous HF as reported for K[C2F5BF3].[73] M[CnF2n+1B(OMe)3] was
synthesized according to known procedures starting from B(OMe)3 with
CF3SiMe3,[74] nBuLi, and C2F5H,[41] tBuLi and C3F7H,[33] or Mg and
K[CF3BF(CN)2] (K1c) with Me3SiCl: K1c was prepared similar to K1b
with Me3SiCl starting from K1a (6.00 g, 34.1 mmol), Me3SiCN (60 mL,
450 mmol), and Me3SiCl (4.0 mL, 31 mmol). The reaction mixture was
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stirred at 90 °C for 3 days and an off-white powder was obtained after
work-up. Yield: 6.00 g (31.6 mmol, 93%).
K[C2F5BF2(CN)] (K2b) with Me3SiCl: The procedure used for the
preparation of K2b was similar to the one described for the synthesis of
K1b in the presence of Me3SiCl using K2a (15.0 g, 66.4 mmol), Me3SiCN
(100 mL, 0.75 mol), and Me3SiCl (10 mL, 79 mmol). The crude product
was a colourless oil. The crude product was dissolved in THF and the
addition of CH2Cl2 resulted in K2b as a powder. The off-white hygros-
copic powder was filtered off and stored under an inert atmosphere.
Yield: 14.4 g (61.8 mmol, 93%).
K[CF3B(CN)3] (K1d) using MW irradiation. K1a (0.50 g, 2.8 mmol) and
Me3SiCN (10.0 mL, 74.9 mmol) were stirred at 100 °C for 2 hrs. The
Me3SiF was continuously allowed to evaporate from the reaction mixture.
The closed reaction vessel was placed into a microwave oven and
irradiated for 95 minutes (300 W, Tmax = 120 °C). Subsequently, all
volatiles were removed under reduced pressure and most of the excess
Me3SiCN (8.0 mL, 59.9 mmol) was recovered by fractional distillation.
The solid residue was dissolved in aqueous H2O2 (5.0 mL, 30% v/v) and
K2CO3 was added. The mixture was stirred for 1 hr and evaporated to
dryness at a rotary evaporator. The solid remainder was extracted with
ether (3×50 mL). The volume of the combined ethereal layers was
reduced to 5 mL and colourless K1d precipitated upon addition of CH2Cl2
(100 mL). The solid was filtered off and dried in a vacuum. Yield: 0.53 g
(2.7 mmol, 95%). 13C NMR: d = 130.0 (q, 1C, 1J(19F, 13C) = 301.7 Hz,
CF3), 123.6 ppm (q, 3C, 3J(19F, 13C) ≈ 3 Hz, CN). 11B NMR: d = –32.0
ppm (q, 1B, 2J(19F,11B) = 36.3 Hz). 19F NMR: d = –66.4 ppm (q, 1F,
2J(19F,11B) = 36.3 Hz, CF3). Raman: 2237, 2231 cm‒1 (n(C≡N)). MALDI-
MS: m/z (isotopic abundance) calcd. for 1d ([C4BF3N3]–): 158(100),
157(25), 159(6); found: 158(100), 157(24), 159(5). Elemental analysis
calcd (%) for C4BF3KN3: C 24.39, N 21.33; found: C 24.49, N 21.22.
K/Li[C2F5BF2(CN)] (K/Li2b): K2a (100 mg, 0,44 mmol), LiCl (180 mg,
4,2 mmol), and KCN (260 mg, 3.99 mmol) were pestled and mixed inside
a glove box and placed into a cylindrically reaction flask equipped with a
glass valve and a PTFE stem (Young, London). The reaction mixture was
heated to 180 °C for 20 hrs in a vacuum. According to the 11B NMR
spectroscopic analysis, the reaction mixture contained the following
borate anions: [C2F5BF2(CN)]− (2b, 82%), [BF2(CN)2]− (5%), [BF(CN)3]−
(MFB, 4%), [B(CN)4]− (TCB, 6%), and an unknown borate anion (d(11B) =
– 20.5 ppm, 3%).
K[C2F5BF(CN)2] (K2c): K2a (1.0 g, 4.4 mmol) was dissolved in Me3SiCN
(10.0 mL, 74.9 mmol). The solution was heated to 110 °C and stirred for
1 hr. During this period, the Me3SiF that had formed was allowed to
evaporate from the reaction mixture. The cylindrical reaction flask was
closed and the mixture was heated to 180 °C for 30 min, cooled to room
temperature, and most of the Me3SiF that had formed was removed
under reduced pressure. This procedure was repeated nine times until all
the starting material was converted into the dicyanoborate according to
11B and 19F NMR spectroscopy. All volatiles were distilled off and most of
the excess of Me3SiCN (7.1 mL, 53.0 mmol) was recovered and purified
via distillation. The solid residue was dissolved in THF (3 mL) and
addition of CH2Cl2 (100 mL) gave off-white K2c. The salt was filtered off
and dried in a vacuum. Yield: 0.97 g (4.0 mmol, 92%). 13C NMR: d =
127.8 ppm (dt, 2C, 2J(19F,13C) = 35.8 Hz, 3J(19F,13C) ≈ 4 Hz, CN), 121.2
ppm (qt, 1C, 1J(19F, 13C) = 284.9 Hz, 2J(19F, 13C) = 31.9 Hz, CF3), 117.3
ppm (tqd, 1C, 1J(19F, 13C) = 258.7 Hz, 2J(19F, 13C) = 36.8 Hz, 30.3 Hz,
CF2). 11B NMR: d = –12.0 ppm (dt, 1B, 1J(19F,11B) = 51.4 Hz, 2J(19F,11B) =
25.1 Hz). 19F NMR: d = –82.6 (dt, 3F, 4J(19F,19F) = 6.3 Hz, 3J(19F,19F) =
1.0 Hz, CF3), –132.0 (qd, 2F, 2J(19F,11B) = 25.0 Hz, 3J(19F,19F) ≈ 5 Hz,
CF2), –219.3 ppm (qqt, 1F, 1J(19F,11B) = 51.4 Hz, 3J(19F,19F) ≈ 4J(19F,19F)
= 5‒6 Hz, BF). Raman: 2229, 2219 cm‒1 (ν(C≡N)). MALDI-MS: m/z
(isotopic abundance) calcd. for 2c ([C4BF6N2]–): 201(100), 200(25),
202(5); found: 201(100), 200(26), 202(6). Elemental analysis calcd (%)
for C4BF6KN2: C 20.02, N 11.67; found: C 20.09, N 11.64.
K[CF3B(CN)3] (K1d) with Me3SiCl in an autoclave: K1a (17.5 g, 99.5
mmol) was placed into an autoclave and Me3SiCN (210 mL, 1.56 mol)
and Me3SiCl (15.0 mL, 119 mmol) were added. The reaction mixture was
heated to 205 °C and stirred for 8 hrs, including the heating and cooling
periods. The maximum pressure was approximately 14 bar. All volatiles
were removed under reduced pressure and the black residue was dried
in a vacuum overnight. The residue was dissolved in H2O (30 mL),
filtered through a plug of Celite, and treated with aqueous H2O2 (3×20 mL,
30% v/v) and K2CO3 (20 g). After stirring overnight, the mixture was
concentrated under reduced pressure until K2CO3 began to precipitate.
The mixture was extracted with THF (3×100 mL), the combined organic
layers were dried with K2CO3, filtered, and concentrated to a volume of
30 mL. Slow addition of CH2Cl2 (300 mL) yielded two crops of
(semi)solids. The first minor fraction was a black slime that was discard-
ed. The second crop was an off-white solid that was collected by filtration
and dried in a vacuum. Yield: 14.0 g (76.2 mmol, 77%).
K[CF3B(CN)3] (K1d) with Me3SiCl: K1d was prepared similar to K1b
with Me3SiCl starting from K1a (3.00 g, 17.1 mmol), Me3SiCN (10.0 mL,
112 mmol), and Me3SiCl (2.0 mL, 15.7 mmol). The reaction mixture was
stirred in a closed vessel at 130 °C for 2 hrs. Additional Me3SiCN (10 ml,
74.9 mmol) and Me3SiCl (1 mL, 7.86 mmol) were added and the reaction
mixture was stirred for 3 weeks at 140 °C. The addition of CH2Cl2
(30 mL) resulted in a brownish slimy solid at the bottom of the flask that
was removed from the colourless liquid. Colourless K1d precipitated
during the addition of CH2Cl2 (150 mL) that was filtered off and dried in a
vacuum. Yield: 1.90 g (9.65 mmol, 56%).
K[C2F5BF(CN)2] (K2c) using MW irradiation: Me3SiCN (25.0 mL,
187.5 mmol) and K2a (8.0 g, 35.4 mmol) were stirred at room
temperature for 3 days. After removal of most of the Me3SiF, the closed
reaction vessel was placed into a microwave oven (CEM Discover) and
irradiated for 15 minutes (200 W, Tmax = 78 °C). All volatiles were
removed under reduced pressure and most of the excess Me3SiCN
(13.5 mL, 101.5 mmol) was recovered by fractional distillation. The solid
residue was taken-up into aqueous H2O2 (20 mL, 30% v/v) and K2CO3
(5 g) was added. The solution was stirred for 1 hr, evaporated to dryness
at a rotary evaporator with a bath temperature of 70 °C, and the solid
remainder was extracted with ether (7×50 mL). The combined ethereal
layers were dried with K2CO3, filtered and the solvent was removed. The
solid residue was dissolved in acetone (5 mL) and colourless K2c preci-
pitated upon addition of CH2Cl2 (150 mL). The solid was filtered off and
dried in a vacuum. Yield: 7.8 g (32.5 mmol, 92%).
K[C2F5BF2(CN)] (K2b): A mixture of K2a (15.8 g, 69.9 mmol) and tri-
methylsilyl cyanide (35.0 mL, 262.5 mmol) was stirred at 100 °C. The
Me3SiF that had formed was continuously distilled off during the reaction.
All volatiles were removed under reduced pressure and most of the
excess Me3SiCN (21.3 mL, 159.9 mmol) was recovered by fractional
distillation. The solid residue of the reaction mixture was dissolved in
THF (5 mL). Neat, colourless K2b was precipitated by addition of CH2Cl2
(200 mL) that filtered off and dried in a vacuum. Yield: 15.8 g (67.8 mmol,
97%). 13C NMR: d = 129.9 (t, 1C, 2J(19F, 13C) = 49.1 Hz, CN), 121.7 (qt,
1C, 1J(19F, 13C) = 284.6 Hz, 2J(19F, 13C) = 31.8 Hz, CF3), 117.4 ppm (ttq,
1C, 1J(19F, 13C) = 258.8 Hz, 2J(19F, 13C) = 39.7 Hz, 36.5 Hz, CF2). 11B
NMR: d = –2.7 ppm (tt, 1B, 1J(19F,11B) = 51.0 Hz, 2J(19F,11B) = 23.4 Hz).
19F NMR: d = –83.2 (tt, 3F, 4J(19F,19F) = 5.2 Hz, 3J(19F,19F) = 0.9 Hz, CF3),
K[C2F5BF(CN)2] (K2c) with Me3SiCl: K2c was prepared according to a
modified synthesis applied for K1b with Me3SiCl using K2a (38.4 g, 170
mmol), Me3SiCN (220 mL, 1.66 mol), and Me3SiCl (25 mL, 198 mmol).
The mixture was stirred at 80 °C for 7 hrs and K2c was isolated as an off-
white sticky material. Yield: 34.3 g (143 mmol, 84%). Colourless powdery
K2c was obtained from THF (50 mL) by addition of CH2Cl2 at –30 °C and
subsequent filtration. The powder is hygroscopic and was stored under
an inert atmosphere.
–136.3 (q, 2F, 2J(19F,11B)
= 23.3 Hz, CF2), –167.1 ppm (qq, 2F,
1J(19F,11B) = 51.0 Hz, 4J(19F,19F) ≈ 5 Hz, BF2). Raman: 2233 cm‒1
(n(C≡N)). MALDI-MS: m/z (isotopic abundance) calcd. for 2b ([C3BF7N]–):
194(100), 193(25), 195(4); found: 194(100), 193(28), 195(3). Elemental
analysis calcd (%) for C3BF7KN: C 15.47, H 0.00, N 6.01; found: C 15.39,
H 0.55, N 6.01.
K[C2F5B(CN)3] (K2d) using MW irradiation: K2a (0.50 g, 2.2 mmol) and
Me3SiCN (10.0 mL, 74.9 mmol) were stirred at room temperature for 2
hrs. The cylindrical reaction vessel was closed, placed into a microwave
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10.1002/chem.201703685
Chemistry - A European Journal
FULL PAPER
oven (CEM Discover), and irradiated for 60 minutes (300 W, Tmax
=
260.4 Hz, BCF2CF2), 110.2 ppm (tqt, 1C, 1J(19F,13C)
= 267.0 Hz,
120 °C). All volatiles were removed under reduced pressure and most of
the excess of Me3SiCN (6.9 mL, 52.0 mmol) was recovered by fractional
distillation. The solid residue was dissolved in aqueous H2O2 (10.0 mL,
30% v/v) and K2CO3 (2 g) was added. The mixture was stirred for 1 hr
and then evaporated to dryness at a rotary evaporator at a bath
temperature of 70 °C. The solid remainder was extracted with ether
(4×50 mL). The combined ethereal layers were dried with K2CO3, filtered,
and the solvent was removed. The volume of the solution was reduced to
5 mL and colourless K2d precipitated upon addition of CH2Cl2 (150 mL).
The solid was filtered off and dried in a vacuum. Yield: 0.48 g (1.9 mmol,
88%). 13C NMR: d = 123.8 (qt, 3C, 1J(13C,11B) = 69.2 Hz, 3J(19F, 13C) =
5.3 Hz, CN), 121.3 (qt, 1C, 1J(19F,13C) = 285.5 Hz, 2J(19F,13C) = 32.1 Hz,
CF3), 118.4 ppm (tqq, 1C, 1J(19F, 13C) = 259.6 Hz, 1J(13C,11B) = 63.8,
2J(19F, 13C) = 36.9 Hz, CF2). 11B NMR: d = –32.0 ppm (t, 1B, 2J(19F,11B) =
25.2 Hz). 19F NMR: d = –82.3 (s, 3F, CF3), –124.2 ppm (q, 2F, 2J(19F,11B)
= 25.2 Hz, CF2). Raman: 2236, 2232 cm‒1 (ν(C≡N)). MALDI-MS: m/z
(isotopic abundance) calcd. for 2d ([C5BF5N3]–): 208(100), 207(24),
209(7); found: 208(100), 207(25), 209(5). Elemental analysis calcd (%)
for C5BF5KN3: C 24.32, N 17.01; found: C 24.53, N 16.79.
2J(19F,13C) = 37.9 Hz, 34.8 Hz, CF2CF3). 11B NMR: d = –2.6 (tt, 1B,
1J(19F,11B) = 51.1 Hz, 2J(19F,11B) = 23.3 Hz). 19F NMR: d = –82.0 (tt, 3F,
3J(19F,19F) = 9.8 Hz, 4J(19F,19F) = 3.8 Hz, CF3), –124.2 (m, 2F, CF2),
–127.0 (tm, 2F, 3J(19F,19F) = 12.7 Hz, CF2), –133.2 (m, 2F, BCF2), –166.0
(q, 2F, 1J(19F,11B)
=
51.0 Hz, BF2). Raman: 2236 cm‒1 (ν(C≡N)).
Elemental analysis calcd (%) for C5BF11KN: C 18.04, N 4.21; found: C
18.86, N 4.68.
K[C4F9BF(CN)2] (K4c): The procedure used for the preparation of K4c
was similar to the one described for the synthesis of K2c. The yield was
not determined. 13C NMR: d = 128.1 (qd, 2C, 1J(13C,11B) ≈ 72 Hz,
2J(19F,13C) = 35.7 Hz, CN), 120.6 (tqtd, 1C, 1J(19F,11B) ≈ 261 Hz,
1J(13C,11B) ≈ 72 Hz, 2J(19F,11B) = 36.0 Hz, 30.7 Hz, BCF2), 118.5 (qt, 1C,
1J(19F,13C) = 287.2 Hz, 2J(19F,13C) = 33.7 Hz, CF3), 113.3 (tm, 1C,
1J(19F,13C) = 261.1 Hz, BCF2CF2), 110.0 ppm (tqt, 1C, 1J(19F,13C) =
267.4 Hz, 2J(19F,13C) = 38.3 Hz, 34.4 Hz, CF2CF3). 11B NMR: d =
–11.8 ppm (dt, 1B, 1J(19F,11B) = 51.5 Hz, 2J(19F,11B) = 25.1 Hz). 19F NMR:
d = –82.1 (tt, 3F, 3J(19F,19F) = 9.8 Hz, 4J(19F,19F) = 3.8 Hz, CF3), –122.6
(m, 2F, CF2), –127.0 (tm, 2F, 3J(19F,19F) = 13.5 Hz, CF2), –128.7 (m, 2F,
BCF2), –218.0 (q, 1F, 1J(19F,11B) = 51.3 Hz, BF).
K[C2F5B(CN)3] (K2d) with Et3SiCN: K2a (2.60 g, 11.5 mmol) was
dissolved in Et3SiCN (25.0 mL, 140 mmol). Et3SiCl (2 mL, 11.9 mmol)
was added and the reaction mixture was degassed. The vessel was
closed and heated to 190 °C for 16 hrs. All volatiles were removed under
reduced pressure and most of the excess of Et3SiCN (15 mL, 86.7 mmol)
was recovered by fractional distillation. The solid residue was taken-up
into aqueous H2O2 (15 mL, 30% v/v) and K2CO3 (3 g) was added. The
solution was stirred for several hours, evaporated to dryness at a rotary
evaporator, and the solid remainder was extracted with acetone
(4×50 mL). The volume of the combined organic layers was concentrated
to 5 mL and addition of CH2Cl2 (100 mL) gave off-white K2d. The salt
was filtered off and dried in a vacuum. Yield: 2.1 g (8.5 mmol, 74%).
Salt Metathesis
Standard protocol: The potassium borate was dissolved in deionized
water (10–50 mL) and an aqueous solution that contains a slight excess
of [Kat]Hal (1.1–1.2 equivalents; [Kat]+ = [EMIm]+, [nBu4N]+, [Ph4P]+, …;
Hal− = Cl−, Br−, I−) was added, slowly. The IL was either separated as a
liquid that usually separated at the bottom of the flask or it was obtained
via extraction using CH2Cl2 (3× 20–50 mL). The IL or a solution thereof in
an organic solvent was washed with deionized water (5×1–5 mL). All
volatiles were removed under reduced pressure to yield the IL that was
dried in a vacuum at 40–60 °C overnight. Solid salts were separated by
filtration, washed with water and dried in a vacuum.
Experimental details including spectroscopic data for cyanoborates
with organic cations are given in the Supporting Information.
K[C2F5B(CN)3] (K2d) with Me3SiCl using an autoclave: The procedure
applied for the preparation of K2d was similar to the one described for
the synthesis of K1d (Me3SiCl/autoclave). K2a (41.0 g, 182 mmol),
Me3SiCN (220 mL, 1.65 mol), and Me3SiCl (15.0 mL, 119 mmol) were
used as starting materials. The maximum pressure during the reaction
was approximately 20 bar. Off-white K2d was obtained by addition of
CHCl3 (200 mL) to a solution in acetone (10 mL). Yield: 38.3 g (155 mmol,
85%).
Diffusion-ordered Spectroscopy (DOSY): 1H, 19F, 11B, and 13C DOSY
measurements were performed at 20, 40, and 60 °C (13C only at 20 °C)
in 5 mm NMR tubes on a Bruker Avance III HD 600 spectrometer
equipped with a 5 mm BBFO probe (1H/X with X = 15N–31P and 19F) with
z axis gradient coil capable of producing pulsed magnetic field gradients
of 50 G/cm. The pure ILs were investigated, locking and shimming were
executed by use of a coaxial inlet filled with DMSO-d6 and centred in the
NMR tube. Temperature calibration was performed with standard
samples of 0.2% CH3OH in CD3OD and 80% ethylene glycole in DMSO-
d6. Data were acquired and processed using the Bruker software
Topspin 3.2. The DOSY data were recorded with the stimulated echo
BPP-LED pulse sequence[76] (longitudinal eddy current delay sequence
with bipolar gradient pulse pairs for diffusion and additional spoil
gradients after the second and fourth 90° pulse). The diffusion time D
was kept constant in each DOSY experiment while the ‘smoothed square’
diffusion gradients were incremented from 2 to 98% of maximum gradient
strength in 32 linear steps (except for 13C DOSY, due to limited
signal/noise ratio only 16 steps were used). No decoupling was used to
avoid heating of the sample. In samples with more than one 19F signal,
19F DOSY measurements were performed for each signal individually to
avoid experimental errors due to non-uniform excitation in the large 19F
chemical shift range. One component fittings of the gradient strength
dependence of the signal intensities were performed by a Levenberg-
Marquardt algorithm incorporated in the Bruker software Topspin 3.2.
The experimental intensities show no systematic and only very small
random deviations from a Gaussian decay curve. Since the viscosity of
the ILs is high as compared with organic solvents and no cryoprobe was
used it was not expected that the DOSY results were influenced by
convection effects. To verify this, for all of the samples (except for
[EMIm]TCB) the DOSY measurements at 60 °C were performed with
K[C2F5B(CN)3] (K2d) starting from Na2a in an autoclave: The pro-
cedure used for the preparation of K2d was similar to the one described
for the synthesis of K1d with Me3SiCl in an autoclave starting from Na2a
(25.0 g, 119 mmol), Me3SiCN (210 mL, 1.58 mol), and Me3SiCl (15.0 mL,
119 mmol). After 9 hrs the reaction was completed and the maximum
pressure was approximately 15 bar. Yield: 27.0 g (109 mmol, 92%).
K[C3F7BF2(CN)] (K3b): The procedure used for the preparation of K3b
was similar to the one described for the synthesis of K1b with Me3SiCl.
K3a (2.72 g, 9.86 mmol), Me3SiCN (20.0 mL, 150 mmol), and Me3SiCl
(1.0 mL, 7.86 mmol) were used as starting compounds. Yield: 2.60 g
(9.19 mmol, 93%). 13C NMR: d = 130.5 (qt, 1C, 1J(13C,11B) = 76.8 Hz,
2J(19F,13C) = 49.0 Hz, CN), 120.0 (tqtt, 1C, 1J(19F,13C) = 261.2 Hz,
1J(13C,11B) = 77.3 Hz, 2J(19F,13C) = 41.6 Hz, 34.1 Hz, BCF2), 119.5 (qt,
1C, 1J(19F,13C) = 287.1 Hz, 2J(13C,19F) = 35.3 Hz, CF3), 111.9 ppm (tqt,
1C, 1J(19F,13C) = 259.5 Hz, 2/3J(19F,13C) = 36.3 Hz, 31.3 Hz, CF2). 11B
NMR: d = –2.6 ppm (tt, 1B, 1J(19F,11B) = 51.1 Hz, 2J(19F,11B) = 23.4 Hz).
19F NMR: d = –81.7 (tt, 3F, 3J(19F,19F) = 9.5 Hz, 4J(19F,19F) = 2.9 Hz, CF3),
–127.9 (tm, 2F, 3J(19F,19F) ≈ 6 Hz, CF2), –133.8 (m, 2F, BCF2), –166.3
ppm (q, 2F, 1J(19F,11B) = 51.0 Hz, BF2). Raman: 2235, 2226, 2219 cm‒1
(n(C≡N)). Elemental analysis calcd (%) for C4BF9KN: C 16.98, N 4.95;
found: C 18.62, N 5.17.
K[C4F9BF2(CN)] (K4b): K4b was synthesized similar to K1b with Me3SiCl
starting from K4a (2.85 g, 8.74 mmol), Me3SiCN (20.0 mL, 150 mmol)
and Me3SiCl (1.0 mL, 7.86 mmol). Yield: 2.57 g (7.7 mmol, 88%). 13C
NMR: d = 130.6 (qt, 1C, 1J(13C,11B) = 76.7 Hz, 2J(19F,13C) = 49.1 Hz, CN),
different diffusion times
D or by use of the corresponding double
stimulated echo pulse sequence[77] (with spoil gradients after the second,
fourth, and sixth 90° pulse) in which the double stimulated echo results in
1
1
120.7 (qm, 1C, J(13C,11B) = 77.1 Hz, BCF2), 118.8 (qt, 1C, J(19F,13C) =
287.2 Hz, 2J(19F,13C) = 33.9 Hz, CF3), 113.6 (tm, 1C, 1J(19F,13C) =
a
compensation of possible flow effects. No dependence of the
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10.1002/chem.201703685
Chemistry - A European Journal
FULL PAPER
experimentally determined diffusion coefficient on the diffusion time D
and identical results for the stimulated and the double stimulated echo
sequence were observed.
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diffractometer equipped with an EOS detector, and crystals of
K3b·0.33Me2CO and K4c·Me2CO were studied on a Bruker X8-Apex II
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isotropic displacement parameters were kept equal to 150% for H atoms
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Keywords: borates • perfluoroalkylborates • ionic liquids •
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Entry for the Table of Contents
FULL PAPER
Johannes Landmann, Jan A. P.
Sprenger, Philipp T. Hennig, Rüdiger
Bertermann, Matthias Grüne, Frank
Würthner, Nikolai V. Ignat’ev, and
Maik Finze*
Page No. – Page No.
Unique perfluoroalkylcyanoborate ionic liquids exhibiting high thermal, chemical and
electrochemical stabilities and providing low viscosities and high conductivities were
developed.
Perfluoroalkyltricyanoborate and
Perfluoroalkylcyanofluoroborate
Anions: Building Blocks for Low-
viscosity Ionic Liquids
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