Na[B(hfip)4] (hfip = OC(H)(CF3)2): A weakly coordinating anion salt and its first application to prepare ionic liquids

Article abstract of DOI:10.1039/c1dt10722d

The fast, high yield synthesis and full characterization of Na[B(hfip) ₄] (hfip: OC(H)(CF₃)₂) from NaBH₄ and hexafluoroisopropanol (hfipH) is presented. By anion metathesis, five [B(hfip)₄]^- salts with classical/functionalized ionic liquid (IL) cations with melting points between 0 ([C₆MIM] ⁺[B(hfip)₄]^-) and 113 °C ([C ₄MMorph]⁺[B(hfip)₄]^-) were prepared. Four of these qualify as ILs and one as room temperature IL (RTIL). The properties of the borate anion [B(hfip)₄]^- and its aluminum analogue [Al(hfip)₄]^- were compared based on the available structural information from XRD. Viscosities (10.3 (90 °C) to 855 (0 °C) mPa s^-1) and conductivities (0.603 (30 °C) to 4.844 (90 °C) mS cm^-1) were measured between 0 and 90 °C, and described by the Vogel-Fulcher-Tammann (VFT) equations. The properties of the [B(hfip)₄]^- ILs were analyzed in the context of the anion-dependent molecular volume V_m-viscosity-/conductivity- correlations, also in comparison to ILs with [BF₄] ^-/[PF₆]^-, [N(CN)₂]^-, [Tf₂N]^- and [Al(hfip)₄]^- counterions. The viscosities and conductivities of [B(hfip)₄] ^- ILs are slightly inferior to [Al(hfip)₄]^- ILs, similar to/better than all other anions given above. According to the Walden plots, the ionicity of the [B(hfip)₄]^- ILs may at least be classified as "good". By sharp contrast to the [Al(hfip) ₄]^- ILs, the [B(hfip)₄]^- ILs have good stability against humidity/water. Thus, handling of [B(hfip) ₄]^- ILs in an open laboratory atmosphere over hours and days is allowed and further facilitates the use of this new IL class. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10722d

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PAPER
Na[B(hfip)4] (hfip = OC(H)(CF3)2): a weakly coordinating anion salt and its
first application to prepare ionic liquids†
Safak Bulut, Petra Klose and Ingo Krossing*
Received 20th April 2011, Accepted 2nd June 2011
DOI: 10.1039/c1dt10722d
The fast, high yield synthesis and full characterization of Na[B(hfip)4] (hfip: OC(H)(CF3)2) from
NaBH4 and hexafluoroisopropanol (hfipH) is presented. By anion metathesis, five [B(hfip)4]- salts with
classical/functionalized ionic liquid (IL) cations with melting points between 0 ([C6MIM]+[B(hfip)4]-)
and 113 ◦C ([C4MMorph]+[B(hfip)4]-) were prepared. Four of these qualify as ILs and one as room
temperature IL (RTIL). The properties of the borate anion [B(hfip)4]- and its aluminum analogue
[Al(hfip)4]- were compared based on the available structural information from XRD. Viscosities
(10.3 (90 ◦C) to 855 (0 ◦C) mPa s-1) and conductivities (0.603 (30 ◦C) to 4.844 (90 ◦C) mS cm-1) were
measured between 0 and 90 ◦C, and described by the Vogel–Fulcher–Tammann (VFT) equations. The
properties of the [B(hfip)4]- ILs were analyzed in the context of the anion-dependent molecular volume
Vm-viscosity-/conductivity-correlations, also in comparison to ILs with [BF4]-/[PF6]-, [N(CN)2]-,
[Tf2N]- and [Al(hfip)4]- counterions. The viscosities and conductivities of [B(hfip)4]- ILs are slightly
inferior to [Al(hfip)4]- ILs, similar to/better than all other anions given above. According to the Walden
plots, the ionicity of the [B(hfip)4]- ILs may at least be classified as “good”. By sharp contrast to the
[Al(hfip)4]- ILs, the [B(hfip)4]- ILs have good stability against humidity/water. Thus, handling of
[B(hfip)4]- ILs in an open laboratory atmosphere over hours and days is allowed and further facilitates
the use of this new IL class.
to the frequently used small [BF4]- anion is [B(CN)4]-,20,21 which
e.g. stabilizes a [C6F5Xe]+ salt20 and readily forms RTILs.22
Introduction
Salts with melting points below 100 ◦C are generally defined as
ionic liquids (ILs). They attracted much recent attention due to
their benefits in contrast of conventional solvent, i.e. extremely low
vapor pressures, interesting transport and electrochemical proper-
ties. Thus, ILs were considered as reaction media,1,2 as electrolytes
in batteries and super capacitors,3–7 in solar and fuel cells,8–13 for
electrochemical deposition of metals and semiconductors,14 as
additives for stabilizing proteins,15 for liquid–liquid extractions16
and in nanoscience.17–19 ILs usually consist of organic cations and
more inorganic anions, which are able to coordinate weakly with
each other. Ultimately, weak coordination with non-symmetrical
ions is thought after, as this reduces the lattice energies, increases
melting entropies of the salts, and thus allows for lower melting
points with formation of—favorably RT—ILs.
Exchanging the phenyl groups in [BPh4]-23,24 for fluorinated
groups as in [B(C6F5)4]-25 and [B(C6H3(CF3)2)4]-26,27 improves
performance in homogenous catalysis,28 but their use as IL-
counterions is precluded due to exceedingly high melting points.
Also modified [B(ArF)4]- borates are known,29–34 in addition
to CF3-substituted borate-WCAs: [F3C-BF3]-,35 [F-B(CF3)3]-36–39
and [B(CF3)4]-.40–42 Out of those, [B(CF3)4]- is very stable and
weakly coordinating: e.g., resistant against Na in liquid ammonia,
against F2 in anhydrous HF. It also stabilizes a [Ag(CO)4]+
cation.40 Alternatives to those alkyl- and arylborates, are alkoxy-
and aryloxy-borates: Catecholborate-based salts were reported
as lithium battery electrolytes.43–49 Also the difluoromono(1,2-
oxalato)borate [BF2Ox]- was described, e.g. for the synthesis
of imidazolium and ammonium based ILs.50 However, their
conductivities were lower than those of comparable [BF4]- ILs.
Hydrolytically unstable (glycerol)borate-based ILs with inorganic
(Li+, Na+, K+) and organic (ammonium, imidazolium) cations
were prepared.51
Several borate based weakly coordinating anions (WCAs) are
known, e.g. for stabilizing reactive cations, for IL synthesis or
in catalyst as well as in electrochemical applications. A relative
Albert-Ludwigs-Universita¨t Freiburg, Institut fu¨r Anorganische und All-
gemeine Chemie, Freiburg Institute of Advanced Studies (FRIAS), and
Freiburger Materialforschungszentrum (FMF), Albertstr., 21, D-
79104, Freiburg, Germany. E-mail: krossing@uni-freiburg.de; Web: http://
portal.uni-freiburg.de/molchem; Fax: (+49) 761 203 6001
† Electronic supplementary information (ESI) available. CCDC reference
numbers 820689 and 820690. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1dt10722d
In our group, [Al(ORF)4]- WCAs with fluorinated alkoxy
residues ORF = OC(CF3)3, OC(CH3)(CF3)2 or OC(H)(CF3)2 have
been used since 1999 to successfully stabilize reactive and other
cations.52–61 But the [Al(hfip)4]- aluminate forms—unexpectedly
for the large size of the anion—also RTILs using suitable cations
with very interesting physical properties such as low viscosity,
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1
11
by H-, 19F- and 11B-NMR (Figure S2 and S3†). The H{ B}-
(Figure S2b†) and 2D 11B, 1H HSQC- (Figure S3a†) NMR spectra
confirmed the coupling of both nuclei in the anion. When DME
was used, the solvent was removed by vacuum distillation after
completion of the reaction (NMR check), and the product was
dried at r.t. in vacuum (0.1 Pa) for 3 d leading to a material
high conductivity, high dielectric constant, very good ionicity,
very good hydrogen solubility62 and high electrochemical stability
(> 5.0 V vs. Li/Li+).63 A disadvantage of all [Al(hfip)4]- salts is their
water sensitivity; decomposition of the anion is already observed
in air due to humidity. We reasoned that a homologous anion
with boron as a central atom should be more stable against water,
because B–O bonds are shorter and less polar than Al–O bonds.
To the best of our knowledge, no ORF analogs (ORF as above)
with boron as the central atom were reported yet, probably due
to the large steric impediment introduced by binding four large
alkoxy groups to the small boron atom. In agreement with this,
the attempted synthesis of Na[B(hfip)4] from NaBH4 and hfipH
was earlier shown64 to proceed only up to Na+[H-B(hfip)3]-, while
[B(OR)4]- salts with smaller alkoxides like OCH3 or OCH2CF3
were straight forward to prepare.64 Despite these difficulties, since
1996 (USA)/2000 (worldwide), all [M(ORF)x]- WCAs (M = any
metal, x = any number) are protected by a patent from the Strauss
group.65,66 The synthesis of the related Lewis acid [B(hfip)3] and its
use in lithium battery electrolytes was also published.67–69
consistent with the formulation Na(DME)3 [B(hfip)4]- (NMR).
+
The DME content could be reduced to Na(DME)+[B(hfip)4]- (2;
also crystal structure) by heating to 40–45 ◦C in the vacuum (0.1
Pa) for 10 h (NMRs: Figure S4 and S5†). All NMR investigations
point to a clean synthesis of 2 in DME with no visible by- or
decomposition-products. 2 turned out to be the preferred starting
material for further reactions. In attempts to remove the DME
completely, the salt 2 was dried at 60 and 80 ◦C in the vacuum
(Figure S6 and S7†): upon drying at 60 ◦C in vacuum (0.1 Pa),
the amount of DME decreased on the expense of decomposition
evident from the 19F-NMR (Figure S6a†). By drying at 80 ◦C
at 0.1 Pa, the solvent DME was completely removed, but also
several decomposition signals were detected in the 1H-, 11B-
and 19F-NMR spectra (Figure S6b and Figure S7c–d†). Thus,
during vacuum drying of 2 at temperatures above 60 ◦C, the salt
probably decomposes with◦formation of the volatile known Lewis
acid [B(hfip)3] (b.p. 78–80 C),67 and the known solid Na+[hfip]-
alkoxide. The signals of the remaining Na+[hfip]- were observed
Here we present a straight forward synthesis of the hitherto
unknown Na[B(hfip)4] salt, as well as metathesis reactions leading
to [B(hfip)4]- ILs, the principal physical properties of which
were determined (m.p., conductivity, viscosity, decomposition
temperature).
1
19
at d H = 5.1–5.2 and d F = -74.7 ppm (Figure S6b and S7d†) in
about 23% of the total intensity.
Results and discussion
In conclusion to this section we note that it was impossible in
our hands to prepare the pure free Na[B(hfip)4] salt by vacuum
treatment at elevated temperatures and the best reproducible
access to sodium [B(hfip)4]- salts is heating the DME-residue to
40–45 ◦C in the vacuum (0.1 Pa) for 10 h yielding clean 2.
Synthesis and characterization of Na+[B(hfip)4]-·(solvent)x
The synthesis of Na+[B(hfip)4]- was investigated by several reaction
pathways out of which only a one pot reaction, starting from
NaBH4 and hfipH, was successful. The other investigated routes
are provided in the Supporting Information.†
Synthesis and characterization of ionic liquids with the [B(hfip)4]-
anion
(1)
Through anion metathesis in dry Et2O with stirring at r.t. for 24 h
as shown in Scheme 1, the sodium cation of the clean mono-
DME adduct 2 was exchanged for substituted imidazolium and
morpholinium cations. Then the mixture was filtered over a G4
Schlenk frit giving clear filtrates that did not show any indication
of AgX formation upon addition of aqueous AgNO3 solution.
By contrast to these difficulties, the synthesis of the related
Li+[Al(ORF)4]- (RF = C(H)(CF3)2, C(CH3)(CF3)2 or C(CF3)3)
works well using LiAlH4 and the fluorinated alcohol in refluxing
hexane or heptane suspension.70 Two distinct differences between
the synthesis of the aluminate and borate salts exist: i) the basicity
of the hydrides in [BH4]- is lower than that in [AlH4]-; ii) boron is
smaller than aluminum and thus a higher steric impediment can
be expected when binding four large alkoxy groups to a central
boron atom. Both differences are negative for the synthesis of
the Na+[B(hfip)4]- salt, and in agreement with earlier findings,64
incomplete conversions were observed in many cases. Usually the
first three hydrides reacted fast and completely (see deposited Table
S3†). But substituting the fourth hydride is difficult. Best results
were finally obtained in 1,2-dimethoxyethane (DME), in which
the conversion was complete within 4 h reflux (see the Supporting
Informationfor optimization procedures and details†). Depending
on the solvent used, THF or DME solvates crystallize from the
solutions. After complete conversion in THF (NMR-check), the
solvent was removed by vacuum distillation and the product was
dried in vacuum (0.1 Pa) at r.t. for 2 d. Single crystal X-ray
analysis revealed the coordination of two THF molecules giving
Scheme 1 Cations used: [a] [CnMIM]+ = 1-alkyl-3-methylimidazolium,
where n is the length of the alkyl chain. [b] [AllylMIM]+ = 1-allyl-
3-methylimidazolium. [c] [CnMMIM]+ = 1-alkyl-2,3-dimethylimidazolium,
where
n =
is the length of the alkyl chain. [d] [C4MMorph]+
N-butyl-N-methylmorpholinium.
From these filtrates, the solvent was removed by vacuum
distillation and the IL-salts were dried in vacuum (0.1 Pa) until no
+
Na(THF)2 [B(hfip)4]- (1). The sodium salt 1 was characterized
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Table 1 Melting points Tm, crystallization temperatures Tc and decom-
position temperatures Td of the [B(hfip)4]- salts investigated in this study
(average values of two or three determinations)
traces of DME were visible in the H-NMR spectra (5 to 24 h).
Typical yields of the purified product were 93 to 98%.
Purity considerations. No traces of decomposition were ob-
Salt
Tm (◦C)
Tc (◦C)
Td (◦C)
1
served in the H-, 11B- and 19F-NMR and IR or Raman spectra.
The 11B- and 19F-NMR spectra were similar to those of 2 and
included only the signals of the anion. 1H-NMR spectra were also
very clean and included all expected signals for the anion–cation
pair but no trace of DME. Exemplarily, elemental C, H, N analyses
for ILs 4, 5 and 7 were carried out and met the expectation within
0.36% (see Experimental Section).
Na+[B(hfip)4]-·(DME) (2)
[AllylMIM]+[B(hfip)4]- (3)
[C4MIM]+[B(hfip)4]- (4)
[C4MMIM]+[B(hfip)4]- (5)
[C4MMorph]+[B(hfip)4]- (6)
[C6MIM]+[B(hfip)4]- (7)
56
68
49
41
44
42
≥ 159
≥ 172
≥ 198
≥ 221
≥ 256
—
68
48
113a
102a
—
-25 < Tm < 0b
a The DSC investigation showed two endothermic signals during heating
(at 78 and 113 ◦C) and two exothermic signals during cooling (at 63
and 102 ◦C). The melting and crystallization temperatures from the DSC
analysis were confirmed by measuring the sample manually. Additional
signals, the endothermic (78 ◦C) and the exothermic one belong to the
first event (63 ◦C), indicate a phase transition or polymorphism of the salt
(see Fig. 2b). b Observed manually by using refrigerators with the indicated
cooling temperatures.
Thermal investigations by DSC. Melting points, crystallization
temperatures and decomposition temperatur◦es were determined
from DSC experiments with heating rates of 5 C◦min-1 for melting
and crystallization processes of all salts, but 6 C min-1 for the
decomposition processes of 2 and 3, 8 ◦C min-1 for 4, 5 and 6.
Also the mono-DME adduct of the sodium salt 2 melts at low
temperature (56 ◦C). Clear decomposition signals were observed
between 159 and 181 ◦C. Afterwards, no melting or crystallization
signals were visible, cf. Fig. 1.
Fig. 1 Melting, crystallization and decomposition of 2 in the DSC.
Melting point and crystallization temperature were observed repeatedly
at 56 and 41 ◦C. Decompositions were observed at 159, 171 and 181 ◦C.
The melting points of the other salts (3–7) lie between less than 0
(7) and 113 ◦C (6); four of them can be categorized as ILs and one
qualifies as a RTIL. The ILs studied in this work were heated to
300 ◦C during the DSC investigations. Clear decomposition signals
were observed between 172 and 256 ◦C (Table 1, Fig. 2). For 6, an
additional endothermic as well as exothermic signal was observed
in all three cycles, which points to a possible polymorphism of
the salt (Fig. 2b; only 6). The DSC traces of all other salts
only included melting, crystallization and decomposition signals
(Table 1).
Stability against humidity and water. To test the [B(hfip)4]- IL
stability against humidity and water, the [C4MIM]+[B(hfip)4]- IL
(4) was put into an open beaker and left in air for 1 d, after which
a NMR sample was prepared in CD2Cl2 and measured. After this
measurement, one drop of water was added intentionally and after
intense shaking the fate of the sample was monitored over time by
NMR. It should be noted that CD2Cl2 is not very miscible with
water and thus the contact of both phases in the NMR tube may
have been limited. The NMR spectra of this sample are collected
in Fig. 3.
Fig. 2 a) Typical DSC trace of the [B(hfip)4]- ILs as for 5: melting, crys-
tallization and decomposition. b) DSC trace of 6: including an additional
endothermic as well as exothermic signal (possibly polymorphism). Onset
points of all the corresponding signals are given in Table 1.
Possibly, the [B(hfip)4]- anion decomposes very slowly with
formation of boric acid and hfipH. Since boric acid is insoluble
in CD2Cl2, in solution only the remaining [B(hfip)4]- IL and the
upon hydrolysis-formed alcohol (hfipH) should be visible in the
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Fig. 4 Section of the solid-state structure of 1 at 113(2) K. Ther-
mal ellipsoids were drawn at the 25% probability level. The asym-
metric unit contains one entire cation with two THF molecules and
one entire anion. Selected distances and bond angles: d(B1–O1) =
148.7(2) pm, d(B1–O2) = 145.0(2) pm, d(B1–O3) = 146.7(2) pm,
d(B1–O4) = 147.2(2) pm, d(Na1–O1) = 234.36(15) pm, d(Na1–O4) =
252.82(16) pm, d(Na1–O5(THF)) = 225.89(17) pm, d(Na1–O6(THF)) =
228.52(16) pm, d(Na1–F13)
253.57(16) pm, <(O4–B1–O1)
=
246.07(15) pm, d(Na1–F19)
=
=
=
101.85(14)◦, <(O4–B1–O3)
109.93(14)◦, <(O1–B1–O3) = 110.58(14)◦, <(O4–B1–O2) = 114.30(15)◦,
<(O1–B1–O2) = 109.50(14)◦, <(O3–B1–O2) = 110.39(14)◦.
Fig. 3 1H- (left) and 19F- (right) NMR spectra of 4 in CD2Cl2 before and
after treatment with air and water. After water addition, in the 1H-NMR
spectra, the water signal at 4.76 ppm appears.
1H- and 19F-NMR investigations.‡ Fig. 3 shows that after leaving
the sample in the air, immediately after adding water as well as
8 h after the addition of water no trace of decomposition of 4
was observed. After 5 d a slight clouding (possibly hydrolysis
product boric acid) was observed in the NMR tube, which was
accompanied by the observation of small traces of hfipH (ca. 1%
by NMR; Fig. 3e). These investigations suggest that the [B(hfip)4]-
ILs may be safely handled in air, but not extracted for prolonged
times with water.
Solid-state structures
Fig. 5 Section of the solid-state structure of 2 at 100(2) K. Ther-
mal ellipsoids were drawn at the 25% probability level. The asym-
metric unit contains one entire cation with one DME molecule and
one entire anion. Selected distances and bond angles: d(B1–O1) =
149.13(18) pm, d(B1–O2) = 148.07(18) pm, d(B1–O3) = 146.36(19) pm,
d(B1–O4) = 145.67(19) pm, d(Na1–O1) = 230.83(13) pm, d(Na1–O2) =
248.24(13) pm, d(Na1–O5(DME)) = 228.54(14) pm, d(Na1–O6(DME)) =
For 1 and 2, crystals suitable for X-ray diffraction were formed
by sublimation during drying in vacuum (0.1 Pa) at r.t. (1) or
313–318 K (2). The diffraction measurements were carried out at
100 to 120 K in order to minimize rotation of the CF3 groups. In
Table 2, the most characteristic structural parameters of the anion,
B–O distances and the O–B–O and B–O–C bond angles, are listed.
The structural parameters of the anions are similar and indicate a
tetrahedral environment. In 1 and 2 (Figures 4 and 5), the O–B–
O angle (101.8◦ and 101.2◦) including both sodium coordinated
oxygen atoms, is ca. 7–8◦ more acute than the ideal tetrahedral
angle.
234.96(14) pm, d(Na1–F10)
252.24(14) pm, <(O4–B1–O1)
=
260.98(14) pm, d(Na1–F13)
=
=
=
110.41(12)◦, <(O4–B1–O3)
110.49(12)◦, <(O1–B1–O3) = 110.46(12)◦, <(O4–B1–O2) = 112.85(12)◦,
<(O1–B1–O2) = 101.20(11)◦, <(O3–B1–O2) = 111.09(12)◦.
We note in the structures of both 1 and 2, a disturbed octahedral
coordination for sodium, where sodium forms contacts with four
oxygens (two from the anion and two from the solvent/s) and two
fluorine atoms giving a distorted octahedral NaO4F2 unit each.
‡
11B-NMR spectra were not added to Fig. 3, because only the anion was
visible.
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Table 2 Structural information of the Na[B(hfip)4] salts (1 and 2) in comparison to the reported Li[Al(hfip)4] salt
(M = B, Al)
Na[B(hfip)4]·(THF)2 (1)
Na[B(hfip)4]·(DME) (2)
Li[Al(hfip)4]61
d(M–O) range (pm)
d(M–O) average (pm)
<(O–M–O) range (◦)
<(O–M–O) average (◦)
<(M–O–C) range (◦)
<(M–O–C) average (◦)
d(Na–O) range (pm)
d(Na–O) average (pm)
d(Na–F) range [pm]
d(Na–F) average (pm)
Dip. moment A- (D)a
145.0(2)–148.7(2)
146.9
101.85(14)–114.30(15)
109.42
120.82(14)–124.18(14)
122.81
225.89(17)–252.82(16)
235.4
145.67(19)–149.13(18)
147.3
101.20(11)–112.85(12)
109.42
121.30(11)–123.51(11)
122.56
228.54(14)–248.24(13)
235.6
169.0(2)–177.1(2)
174.6
94.49(9)–127.1(1)
109.23
124.1(2)–149.7(2)
134.13
—
—
—
246.07(15)–253.57(16)
249.8
3.136
252.24(14)–260.98(14)
256.6
3.375
—
4.407
a Dipole moments in Debye are taken from single point DFT calculations at the (RI)-BP86/SV(P) level starting with crystal structure coordinates of the
anion (A-) from the asymmetric unit.
Table 3 Temperature-dependent viscosities (h) and conductivities (s) of the [B(hfip)4]- (= [A]-) ILs and their molecular volumes (Vm)
h (mPa s)
[B(hfip)4]- ILs
Vm (nm3) 0 ◦C 10 ◦
C
20 ◦C 25 ◦C 30 ◦
C
35 ◦ 40 ◦C 45 ◦ 50 ◦ 55 ◦ 60 ◦ 65 ◦ 70 ◦ 75 ◦ 80 ◦ 90 ◦
C C C C C C C C C C
[C4MIM]+[A]- (4)
0.753
—
—
—
—
—
—
170
—
—
124
—
—
—
—
—
—
48.6 38.8 31.7 25.2 22.1 18.7 16
13.8 10.3
17 12.8
39.2 34.4 26.7 24.8 19.8 18.9 17.1 13.7
[C4MMIM]+[A]- (5) 0.775
—
—
—
—
33.8 27.7 21.6 20
[C6MIM]+[A]- (7)
0.800
855 348
94.6 73.2 58.9
s (mS cm-1)
Vm (nm3)
30 ◦
C
35 ◦
C
40 ◦
C
50 ◦
C
60 ◦
C
65 ◦
C
70 ◦
C
75 ◦
C
80 ◦
C
90 ◦
C
[C4MIM]+[A]- (4)
[C4MMIM]+[A]- (5)
[C6MIM]+[A]- (7)
0.753
0.775
0.800
—
—
0.603
—
—
0.807
—
—
1.096
—
—
1.491
2.848
—
2.014
3.17
—
2.183
3.342
—
2.435
3.677
2.575
2.517
3.994
3.388
2.774
4.844
4.012
3.371
Here, d(Na–F) is close to d(Na–O); the difference is only 14 to
21 pm indicating that also the fluorine atoms of the CF3-groups
may act as donor atoms in the absence of better donors (Table 2).
/ (T −T
)
h=Ah T ekh
0
(2)
T0 is often described as the ideal glass transition temperature
(corresponds to the temperature at which the viscosity is infinite),
kh is a constant and the product khR has been related to the
Arrhenius activation energy, Ah is the pre-exponential factor and
T as the temperature.75,76 Similarly, the VFT equation is used
to describe the temperature dependence of the conductivity of
[B(hfip)4]- ILs (eqn (3)):
Viscosities and conductivities
Temperature-dependent viscosity and conductivity measurements
are given in Table 3. The viscosities were measured between 0
◦
and 90 C in an atmosphere of dry air in a specifically purpose-
built glove box with a relative humidity <0.1%. All viscosity
measurements were performed as a function of speed and torque;
all of these [B(hfip)4]- ILs are Newtonian fluids independent of
speed and torque (torque-range from 1 to 99%). The viscosity
of some ILs could be measured at lower temperatures than their
1
/ (T −T
)
0
s =As
e-k
s
(3)
T
T0 is the same for the viscosity and conductivity. In eqn (3),
it corresponds to the temperature at which the conductivity is
zero. ks is a constant and the product ks R was related to the
Arrhenius activation energy, As is the pre-exponential factor and
T the temperature.
Using the data in Table 3, the temperature dependence of the
viscosity and conductivity of the [B(hfip)4]- ILs was described
by the VFT model with very good regressions (R2 ≥ 0.99). For
5, only very few conductivity data points were available, so it
was omitted. The constants Ah, kh, T0 as well as As and ks
as determined from the VFT fits (Fig. 6) are given in Table 4.
Since T0 is the temperature at which molecular or ionic migration
becomes impossible, for both viscosity and conductivity of each
IL identical T0 values have to be used in the VFT picture. We
decided to use T0 derived from VFT-fits of the viscosities, because
◦
melting point, because their freezing points are 7 to 20 C lower
than the melting temperatures (see Table 1). The conductivities
were measured between 30 and 90 ◦C in an argon filled glove box
with water and oxygen contents <1 ppm (Table 3). Due to their
high melting points, the viscosity and conductivity of 3 and 6 were
not measured.
Description of the temperature dependence of the viscosity and
conductivity
Previously, the temperature dependence of the viscosity and
conductivity of [Tf2N]- ILs71 and [Al(hfip)4]- ILs62 were discussed
in detail; they are best described with the three-parameter Vogel–
Fulcher–Tammann (VFT) equation72–74 (eqn (2)):
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Table 4 Parameters for the [B(hfip)4]- (= [A]-) ILs in the VFT equation of the viscosity (eqn (2)), determined from the VFT fits in Fig. 6a (columns 2–4);
and in the VFT equation of the conductivity (eqn (3)), determined from the VFT fits in Fig. 6b (columns 5–6)
IL
Ah (mPa s K-0.5
)
kh (K)
T0 (K)
As (mS cm-1 K-0.5
)
ks (K)
[C4MIM]+[A]- (4)
[C4MMIM]+[A]- (5)
[C6MIM]+[A]- (7)
0.0083 0.0057
0.0670 0.0862
0.0156 0.0012
689.8 177.4
233.6 200.8
633.0 13.3
199.1 16.6
262.8 32.4
195.1 0.9
1201.82 183.01
—
1226.18 167.66
424.06 22.66
—
-501.61 20.55
Fig. 6 Description of the temperature dependence a) of the viscosity (h), and b) of the conductivity (s) for [B(hfip)4]- ILs with the VFT behavior (eqn (2)
and eqn (3), respectively). The conductivity-data of 5 were not fitted, since only very few data points were available. The y-axes are scaled logarithmically.
Table 5 Scaled-calculated ionic volumes (Vion) of the ions investigated in
Recently we extended the known Vm-based correlations of
the viscosity and conductivity also to [Al(hfip)4]- ILs.62 The
correlations are a tool to investigate the anion–cation interactions
and allow for an understanding or even the prediction of the
physical properties of unknown ILs (some say up to 1012–1018 ILs
are possible82,83). Only the data of three [B(hfip)4]- ILs are available
(Table 3). Thus, it is not possible to make meaningful correlations
with small error bars that may be used for the predictions of the
physical properties of yet unknown salts. Nevertheless, we use
these data points to judge the overall performance of these WCAs
for IL synthesis: the slope of the regression line is connected to the
coordinative strengths of the anion and can be used to compare
to other IL-anions (see reference 62 for details). The higher the
coordinative ability of the anion, the steeper is the slope of the
viscosity–Vm regression line and the absolute magnitude of the
constant in the exponential term increases. The slope decreases
from [BF4]-/[PF6]- to [N(CN)2]-, [Tf2N]- and [Al(hfip)4]-.78 Fig.
7 shows Vm vs. viscosity and Vm vs. conductivity correlations for
[B(hfip)4]--ILs in combination with other known anions, where
this work (scaled BP86/TZVP/COSMO)77
Ion
Vion (nm3)
Ion
Vion (nm3)
[AllylMIM]+
[C4MIM]+
0.167
0.197
0.218
[C4MMorph]+
[C6MIM]+
0.222
0.244
0.556
[C4MMIM]+
[B(hfip)4]-
of their significantly higher number of data points in the fit. For
4 and especially 5, larger error bars of the VFT parameters were
observed due to the small number of data points available for these
ILs.
Correlations between physical properties and molecular volume
In earlier work, anion dependent correlations between Vm and
physical properties, e.g. viscosity, conductivity, density and heat ca-
pacity of ILs were reported77,78 with classical, non-functionalized
cations. Vm is a physical observable and is defined as the sum
of the ionic volumes of the constituent ions (Vion), which can
be determined experimentally from X-ray crystal structures or
by using a semi-empirical method through quantum chemical
calculations and correlations of calculated and experimental Vm
values.77 An advantage of the calculated Vm is its independence
of the temperature. Thus, the use of the calculated Vm through-
out an investigation is internally consistent in contrast to the
experimental Vm (see reference 62 for details). In Table 5 all
ionic volumes used in this work are shown. They were obtained
from gas phase structure optimizations at the (RI)-BP86/TZVP79
level, followed by COSMO80 calculations with TURBOMOLE81
(scaled-calculated volumes, see reference 71 and 77 for
details).
◦
three correlation-series include data at 20–2◦2 C ([BF4]-/[PF6]-,
[N(CN)2]- and [Tf2N]-), but two series at 80 C ([M(hfip)4]-, M =
B, Al). The slopes of the regression-lines at various temperatures
are comparable with each other, since it is known that the slopes
vary very little with temperature as shown elsewhere.62
As seen in Table 3, the viscosities increase with increasing size
(Vm) and decrease with increasing temperature (the reverse for
conductivities), which is also evident from Fig. 7. The absolute
magnitude of the slope of the regression lines for the [Al(hfip)4]-
and [B(hfip)4]- ILs is smaller than for all other counterions, with
those of [B(hfip)4]- ILs being slightly higher than those of the
[Al(hfip)4]- ILs (better with viscosity-correlations, Fig. 7a). This
suggests a slightly stronger coordination behavior of the [B(hfip)4]-
anion. More detailed discussions on the effect of WCAs with large
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Fig. 8 Walden plots for [B(hfip)4]- ILs studied in this work in comparison
with [Al(hfip)4]- ILs including the same cations. The thick line is the
ideal Walden line.12 The classification of the ILs in the diagram as super,
good, poor etc. can be found elsewhere.86–88 The temperature increases
from the lower left corner to the upper right corner. To compare with
other ILs, the Walden plot of the classical IL [C4MIM]+[Tf2N]- was also
included (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide,
labeled +). 1 Poise = 100 mPa s.
As seen from Fig. 8 the Walden plots of all ILs lie less than an
order of magnitude below the ideal line and hence all of these ILs
qualify at least as ILs with good ionicities.86,87 The [Al(hfip)4]-
ILs are slightly better than the [C4MIM]+[Tf2N]- reference IL
determined in our labs, whereas [B(hfip)4]- ILs are slightly inferior
than the [C4MIM]+[Tf2N]- IL, as already noticed from the Vm-
correlations above.
Conclusions
Fig. 7 Correlations between the molecular volume (Vm) and a) the
The synthesis of the solvated Na+[B(hfip)4]- salt from NaBH4 and
hexafluoroisopropanol was optimized in various solvents and is
best done in DME (4 h reflux). From the sodium borate 2, low
melting or even RTILs are straight forward to prepare. By contrast
to the [Al(hfip)4]- ILs that offer very interesting properties,62 but
are highly hydrolysis-sensitive, the [B(hfip)4]- ILs are much more
stable towards water and may be handled in moist air without
decomposition; even the addition of water led after 5 days only
to 1% hydrolysis products. The temperature-dependent viscosities
and conductivities of the non-functionalized [B(hfip)4]- ILs were
described on the basis of VFT-theory. By examination in the
context of the molecular volume, the coordinative strength of
[B(hfip)4]- followed as slightly higher than that of [Al(hfip)4]-, and
similar to/better than that of the [BF4]-/[PF6]-, [N(CN)2]-, [Tf2N]-
ions. According to the Walden plot, the [B(hfip)4]- ILs are at least
classified as “good ILs” and offer slightly inferior ionicities than
the corresponding [Tf2N]-/[Al(hfip)4]- ILs with the same cations.
However, the straight forward synthesis, the low cost of the starting
materials and their superior hydrolytic stability will facilitate the
wide spread use of these [B(hfip)4]- ILs.
viscosity (h) b) the conductivity (s). ᭡ [BF4]-/[PF6]- salts, [N(CN) ]-
᭹
2
salts, ᭜ [Tf2N]- salts, all correlations for these salts are for data obtained
◦
at 20–22 ◦C.78 ꢀ [Al(hfip)4]- salts at 80 C,62 ꢁ [B(hfip)4]- salts at 80 ◦C.
The y-axes are scaled logarithmically; x and y in the regression equations
are Vm and h or s, respectively.
fluorinated surfaces (such as [Al(hfip)4]-) on the physical properties
are given in reference 62.
Walden product
The empirical Walden rule84,85 is related to the Stokes–Einstein
and Nernst–Einstein equations and states that the product of
the molar conductivity Km and the viscosity h is approximately
constant.86,87 See reference 62 for a detailed literature-overview in
the context of the ionicity of ILs. If log (Km) is plotted against
log (h-1), the Walden-plot is obtained. Ideal behavior implies,
that all ions in the medium are dissociated, are independent of
each other and move individually due to the absence of any ion–
ion interactions. The resulting diagonal line is then the “ideal
line”; the position of which was established using aqueous KCl
solutions at high dilutions.87 Deviations from the ideal line imply
that the ions of a salt are not completely dissociated and thus
higher viscosities or lower conductivities can be observed (with
respect to the expectation according to their volumes). In Fig. 8,
the Walden plots of the [B(hfip)4]- based ILs in comparison with
[Al(hfip)4]- ILs are shown. The temperature-dependent densities
required here were calculated according to reference 77.
Experimental section
General procedures and starting materials
Due to the air- and moisture sensitivity of most materials all
manipulations were undertaken using standard vacuum and
Schlenk techniques (vacuum-lines with vacuum of 0.1 Pa) as well
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Table 6 Crystallographic details for 1 and 291
as a glove box with an argon atmosphere (H2O and O2 < 1 ppm).
All solvents and the fluorinated alcohol were dried by conventional
drying agents (usually P2O5 and CaH2) and distilled afterwards.
CH2Cl2 and Et2O were dried using a M. Braun Grubbs apparatus.
The solvents were distilled directly into air-tight flasks with teflon
or glass/teflon valves (Schott Produran or J. Young) and stored
in these. Trace water contents were routinely determined by Karl–
Fischer titration and solvents were used, if the moisture level was
below 10 ppm. The alcohol HO–C(H)(CF3)2 was purchased from
www.fluorine.ru. NaBH4 (obtained from Sigma-Aldrich Chemie
GmbH) was used as it was. The halide salts [Cat]+[X]- (X = Cl,
Br), which are often hygroscopic, were obtained from IoLiTec
and were dried prior to use in vacuum (0.1 Pa) for several days
(the solid salts were occasionally ground). The dried salts were
stored in the glove box after drying. Solution NMR spectra were
recorded in purified deuterated solvents on a BRUKER AC 250
or a BRUKER AVANCE 400 spectrometer; data are given in ppm
relative to 1% TMS solution in CDCl3 using the solvent signals
as a secondary reference (1H) or external CFCl3 (19F) and 15%
BF3. Et2O solution in CDCl3 (11B). NMR spectra were recorded
at r.t., if not mentioned otherwise. The Bruker Topspin software
package (version 2.1) was used for measuring and processing of
the spectra. IR spectra were recorded at r.t. on a Nicolet Magna-
IR 760 FT spectrometer using a Diamond ATR cell with the
OMNIC software package (a resolution of 4 cm-1 and extended
ATR correction with refractive index of 1.5 was used). Raman
spectra were recorded at r.t. in sealed NMR tubes or flame-
sealed capillaries on a BRUKER RAM II FT-Raman module
of the BRUKER VERTEX 70 IR spectrometer (equipped with
a Nd-YAG laser, 1064 nm radiation) with the OPUS software
package and using a liquid nitrogen cooled, highly sensitive Ge
detector (4 cm-1 resolution). Melting points were determined from
DSC experiments on a SETARAM DSC 131 instrument in 30
ml aluminum crucibles with an empty crucible of the same type
as reference (the crucibles were filled and closed in a glove box
and then stored under Argon). To infer the melting temperatures,
the onset points were taken into account. Additionally, melting
points were determined manually in flame-sealed capillaries on
a MEL-TEMP instrument (Laboratory Devices, Cambridge,
MASS. 02139). The viscosity was measured with a programmable
Brookfield rotation viscosimeter (RVDV-III UCP) in an atmo-
sphere of dry air in a specifically purpose-built glove box (relative
humidity below 0.1%, see photo in the Supporting Information†).
The samples were tempered with an external cryostat between
0 and 90 0.1 ◦C (if the samples permitted). The conductivity
1
2
Chemical formula
Formula weight (g mol-1)
T (K)
C20H20BF24NaO6 C16H14BF24NaO6
846.16
113(2)
792.07
100(2)
0.71073
Triclinic
˚
l (A)
0.71073
Triclinic
Crystal system
Space group
¯
¯
P1
P1
˚
a (A)
9.2805(19)
10.964(2)
15.890(3)
87.32(3)
10.491(2)
10.546(2)
13.212(3)
77.05(3)
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
85.69(3)
78.66(3)
1580.0(6)
2
1.779
0.227
Multiscan
840
80.40(3)
81.33(3)
1394.8(5)
2
1.886
0.250
Multiscan
780
0.4 ¥ 0.3 ¥ 0.2
3.19 to 27.46
-13 £ h £ 13
-13 £ k £ 13
-17 £ l £ 17
26660
6359
(Rint = 0.0279)
5219
6359/0/489
S = 1.085
R1 = 0.0323
wR2 = 0.0714
R1 = 0.0420
wR2 = 0.0792
0.0273/0.6890
3
˚
V (A )
Z
rc (mg m-3)
m (mm-1)
Absorption correction
F(000)
Crystal size (mm)
q range for data collection (◦)
Index ranges
0.3 ¥ 0.3 ¥ 0.1
3.03 to 27.48
-12 £ h £ 10
-14 £ k £ 14
-20 £ l £ 20
28052
Reflections collected
Independent reflections
7219
(Rint = 0.0398)
5263
7219/0/549
S = 1.069
R1 = 0.0417
wR2 = 0.0952
R1 = 0.0616
wR2 = 0.1051
0.0418/0.5437
Reflections with I > 2s(I)
data/restraints/parameters
Goodness-of-fit on F2
R indices [I > 2s(I)]
R indices (all data)
Weighting schemea x/y
Largest diff. peak and hole (e A ) 0.577 and -0.288 0.367 and -0.321
-3
˚
2
2
a w = 1/[s (Fo2) + (xP)2 + yP], in which P = (Fo + 2Fc2)/3.
isotropic displacement parameters and refined without positional
constraints. Complex neutral-atom scattering factors were used.
Synthesis of Na[B(hfip)4]. Due to the very high volatility and
low boiling point of HOC(H)(CF3)2 (59 ◦C), a double reflux
condenser connected to a cryostat was used and the temperature
of the cryostat-fluid was set to -10 ◦C.
a) in THF, and experimental data of 1. An equimolar amount
of finely ground NaBH4 (2 g, 52.86 mmol) was weighted into a two-
necked Schlenk flask, and 150 mL THF was added afterwards. 4.5
equivalents of hexafluoroisopropanol, HOC(H)(CF3)2 (39.97 g =
25 mL, 237.9 mmol) were slowly added over a period of 4 h into the
stirred solution of NaBH4 in THF. At first the reaction mixture was
stirred at r.t. for 48 h and then refluxed. The reaction was followed
continuously by NMR. Since the reaction did not completely
finish, more THF was added in order to dissolve the intermediates
better (altogether 450 mL THF). This reaction mixture was then
stirred and refluxed again. Up to completion, the reaction took
36 d, during which 40 h were refluxed and in the remaining time
the solution was stirred at r.t. After the conversion was complete
(NMR), the solvent was removed by vacuum distillation and the
product was dried in vacuum (0.1 Pa) at r.t. for 2 d; isolated yield:
◦
was measured between 30 and 90 0.1 C with a Metrohm 712
conductometer in an argon-filled glove box (water and oxygen
content below 1 ppm, see photo in the Supporting Information†).
Data collection for X-ray structure determinations (Table 6)
were performed on a Rigaku R-AXIS Spider IP area-detector
diffractometer, using graphite-monochromated Mo-Ka radiation
˚
(l = 0.71073 A). Single crystals were mounted in perfluoroether
oil on top of a glass fiber and measured at 100/113 K. All
calculations were performed using the SHELX9789,90 software
package. The structures were solved by direct methods and
refined by least squares on weighted F2 values for all reflections.
All non-hydrogen atoms were assigned anisotropic displacement
parameters and refined without positional constraints. Hydrogen
atoms were located in the electron density difference map, assigned
1
33.6 g (75.1%). H NMR (400.17 MHz, CDCl3/THF, 298 K):
1
d = 4.32 (m, w 2 = 24 Hz, 4H, CH). 19F NMR (376.53 MHz,
3
CDCl3/THF, 298 K): d = -75.6 (d, J(F, H) = 6.6 Hz, CF3). 11B
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3
NMR (128.39 MHz, CDCl3/THF, 298 K): d = 1.23 (quin, J(B,
H) = 3.1 Hz).
990, 1025, 1100, 1186, 1217, 1299, 1360, 1383, 1413, 1430, 1457,
1496, 1564, 1576, 1654, 2756, 2849, 2902, 2947, 2976, 3008, 3042,
3114, 3191 cm-1. m.p. (DSC): 68 ◦C.
b) in DME, and experimental data of 2. Finely ground NaBH4
(2 g, 52.86 mmol) was weighted into a two-necked Schlenk
flask, and 100 mL DME was added afterwards. 4.3 equivalents
hexafluoroisopropanol, HOC(H)(CF3)2 (38.19 g = 24 mL, 227.3
mmol) were added over a period of 1 h into the stirred solution
of NaBH4 in DME. The reaction mixture was refluxed for 4 h
afterwards. However, hydrogen evolution visually ended after 40
min. reflux. The solvent was then removed by vacuum distillation;
the product was dried at first at r.t. and then for 3 d in vacuum (0.1
Pa). After this drying initial process, NMR spectroscopy indicated
Experimental data for 4. The DME was completely removed
by drying at 50 ◦C for 5 h. Na[B(hfip)4].(DME) (5 g, 6.31 mmol),
[C4MIM]Cl (1.103 g, 6.31 mmol); isolated yield: 5.13 g (99%). 1H
NMR (400.17 MHz, CD2Cl2, 298 K): d = 0.98 (t, 3H, CH3), 1.38
(sex, 2H, CH2), 1.86 (quin, 2H, CH2), 3.92 (s, 3H, N(CH3)), 4.14
1
(t, 2H, N(CH2)), 4.71 (m, w 2 = 22 Hz, 4H, CH), 7.28 (m, 2H,
1
CH), 8.11 (s, w 2 = 4.4 Hz, 1H, CH). 19F NMR (376.53 MHz,
CD2Cl2, 298 K): d = -75.1 (d, 3J(F, H) = 6.6 Hz, CF3). 11B NMR
(128.39 MHz, CD2Cl2, 298 K): d = 1.64 (quin, 3J(B, H) = 3.0 Hz).
a solvent/anion ratio of 3 (־ Na(DME)3 [B(hfip)4]-). The DME
IR (diamond ATR): n = 421 (vw), 522 (w), 540 (m), 623 (m), 632
+
˜
content was reduced by further drying at 40–45 ◦C in vacuum (0.1
Pa) for 10 h (solvent/anion ratio: 1; ־ Na(DME)+[B(hfip)4]- (2),
the analytical data of which are given in following). Isolated yield:
(m), 655 (m), 684 (s), 742 (m), 755 (vw), 823 (m), 865 (m), 890 (m),
958 (m), 990 (m), 1019 (s), 1056 (m), 1093 (vs), 1149 (vs), 1164
(vs), 1180 (s), 1214 (s), 1263 (m), 1289 (m), 1381 (m), 1464 (vw),
1
-1
˜
39 g (93%). H NMR (400.17 MHz, CDCl3, 298 K): d = 3.41 (s,
1571 (w), 1602 (vw), 2948 (vw), 3179 (vw) cm . Raman: n = 416,
1
6H, DME-CH3), 3.59 (s, 4H, DME-CH2), 4.66 (m, w 2 = 21 Hz,
428, 498, 536, 550, 557, 603, 634, 658, 696, 721, 743, 756, 804,
830, 862, 881, 955, 990, 1025, 1057, 1103, 1112, 1120, 1181, 1204,
1257, 1294, 1318, 1336, 1359, 1383, 1418, 1432, 1446, 1464, 1496,
1516, 1548, 1571, 2706, 2754, 2887, 2951, 2976, 3120, 3162, 3185
4H, CH). 19F NMR (376.53 MHz, CDCl3, 298 K): d = -75.2 (d,
3J(F, H) = 5.7 Hz, CF3). 11B NMR (128.39 MHz, CDCl3, 298 K):
3
˜
d = 2.09 (quin, J(B, H) = 2.7 Hz). IR (diamond ATR): n = 421
◦
(vw), 438 (vw), 474 (vw), 520 (m), 527 (w), 536 (m), 551 (w), 564
(vw), 633 (m), 648 (m), 663 (m), 686 (vs), 721 (vw), 743 (m), 804
(w), 839 (m), 861 (s), 889 (m), 930 (m), 963 (m), 1008 (s), 1037
(m), 1078 (s), 1096 (vs), 1136 (s), 1156 (vs), 1166 (vs), 1178 (vs),
cm-1. m.p. (DSC): 49 C. Elemental analysis found (calc.): 29.72
(29.36)% C, 2.34 (2.34)% H, 3.36 (3.42)% N.
Experimental data for 5. The DME was completely removed
by drying at r.t. over night. Na[B(hfip)4].(DME) (5 g, 6.31 mmol),
[C4MMIM]Br (1.472 g, 6.31 mmol); isolated yield: 5.18 g (99%).
1H NMR (400.17 MHz, CD2Cl2, 298 K): d = 0.99 (t, 3H, CH3),
1.39 (sex, 2H, CH2), 1.80 (quin, 2H, CH2), 2.58 (s, 3H, CH3),
1209 (s), 1245 (m), 1267 (m), 1286 (m), 1293 (m), 1366 (m), 1378
-1
˜
(m), 1457 (vw), 1465 (vw), 1480 (vw), 2968 (vw) cm . Raman: n =
423, 522, 538, 553, 636, 649, 664, 690, 721, 746, 805, 839, 869, 893,
1019, 1035, 1103, 1120, 1165, 1180, 1202, 1249, 1266, 1286, 1367,
1380, 1416, 1455, 1479, 1504, 2735, 2757, 2806, 2864, 2887, 2917,
2969, 3027 cm-1. m.p. (DSC): 56 ◦C.
1
3.78 (s, 3H, N(CH3)), 4.03 (t, 2H, N(CH2)), 4.70 (m, w 2 = 22 Hz,
1
4H, CH), 7.17 (m, w 2 = 4 Hz, 2H, CH). 19F NMR (376.53 MHz,
CD2Cl2, 298 K): d = -75.1 (d, 3J(F, H) = 6.1 Hz, CF3). 11B NMR
(128.39 MHz, CD2Cl2, 298 K): d = 1.61 (quin, 3J(B, H) = 3.0 Hz).
General procedure for the synthesis of [Cat]+[B(hfip)4]- salts.
Equimolar amounts of Na[B(hfip)4]·(DME) and [Cat]X (X = Cl,
Br) were weighed into a Schlenk vessel in the glove box. Absolute
Et2O was added (approx. 10 mL solvent/1 g Na[B(hfip)4]·(DME)).
The mixture was stirred at r.t. for 24 h and filtered over a G4
Schlenk frit. From the filtrate, the solvent was removed by vacuum
distillation and the product was first dried at r.t. in vacuum (0.1
Pa). With NMR spectroscopy it was checked, whether the DME
was completely removed. If this was not the case, the product was
dried at the below indicated higher temperatures in vacuum (0.1
Pa) until complete removal of DME was achieved.
˜
IR (diamond ATR): n = 416 (vw), 523 (m), 540 (m), 631 (m), 657
(m), 671 (m), 684 (s), 729 (w), 742 (m), 762 (w), 794 (m), 808 (vw),
864 (m), 890 (m), 960 (m), 1020 (s), 1056 (m), 1096 (vs), 1164 (vs),
1185 (s), 1211 (s), 1261 (m), 1289 (m), 1364 (w), 1382 (m), 1426
(vw), 1444 (vw), 1471 (vw), 1539 (vw), 1592 (vw), 2945 (vw), 2983
-1
˜
(vw) cm . Raman: n = 432, 474, 502, 526, 536, 551, 561, 585, 633,
658, 689, 706, 722, 743, 762, 804, 829, 849, 865, 879, 918, 944, 962,
1051, 1076, 1099, 1177, 1194, 1210, 1290, 1297, 1341, 1358, 1383,
1445, 1454, 1468, 1516, 1593, 2758, 2885, 2948, 2979, 3168, 3199
◦
cm-1. m.p. (DSC): 68 C. Elemental analysis found (calc.): 30.43
(30.31)% C, 2.59 (2.54)% H, 3.35 (3.37)% N.
Experimental data for 3. The DME was completely re-
moved by drying at 50 ◦C for 4 h and at 90 ◦C for 20 h.
Na[B(hfip)4]·(DME) (5 g, 6.31 mmol), [AllylMIM]Cl (1.001 g,
6.31 mmol); isolated yield: 4.71 g (93%). 1H NMR (400.17 MHz,
CD2Cl2, 298 K): d = 3.92 (s, 3H, N(CH3)), 4.71 (m, 4H, CH), 4.73
Experimental data for 6. The DME was completely removed
by drying at 60 ◦C for 6 h. Na[B(hfip)4].(DME) (5 g, 6.31 mmol),
[C4MMorph]Br (1.504 g, 6.31 mmol); isolated yield: 4.95 g (94%).
1H NMR (400.17 MHz, CD2Cl2, 298 K): d = 1.03 (t, 3H, CH3), 1.46
(sex, 2H, CH2), 1.74 (quin, 2H, CH2), 3.09 (s, 3H, N(CH3)), 3.28
(m, 4H, N(CH2)), 3.38 (m, 2H, N(CH2)), 4.01 (m, 4H, O(CH2)),
(d, 3J(H, H) = 6.5 Hz, 2H, CH2), 5.55 (m, 2H, N(CH2)), 5.97 (m,
1
1H, CH), 7.30 (m, 2H, CH), 8.12 (m, w 2 = 4.2 Hz, 1H, CH). 19
F
1
NMR (376.53 MHz, CD2Cl2, 298 K): d = -75.1 (d, 3J(F, H) = 6.5
4.71 (m, w 2 = 22 Hz, 4H, CH). 19F NMR (376.53 MHz, CD2Cl2,
Hz, CF3). 11B NMR (128.39 MHz, CD2Cl2, 298 K): d = 1.77 (quin,
298 K): d = -75.0 (d, 3J(F, H) = 6.6 Hz, CF3). 11B NMR (128.39
3
3
˜
J(B, H) = 3.0 Hz). IR (diamond ATR): n = 406 (vw), 523 (m), 539
MHz, CD2Cl2, 298 K): d = 1.62 (quin, J(B, H) = 3.0 Hz). IR
˜
(m), 624 (m), 657 (m), 685 (s), 744 (m), 826 (m), 862 (m), 890 (m),
958 (m), 989 (m), 1016 (s), 1053 (m), 1097 (vs), 1153 (vs), 1209 (s),
(diamond ATR): n = 414 (vw), 522 (m), 539 (m), 632 (m), 657
(m), 684 (s), 743 (m), 803 (vw), 817 (vw), 864 (m), 890 (m), 914
(w), 958 (m), 1019 (s), 1059 (m), 1096 (vs), 1151 (vs), 1174 (vs),
1244 (m), 1260 (m), 1293 (m), 1381 (m), 1428 (w), 1457 (vw), 1563
-1
˜
(w), 1605 (vw), 2944 (vw), 3180 (vw) cm . Raman: n = 431, 499,
1212 (s), 1259 (m), 1289 (m), 1382 (m), 1431 (vw), 1479 (w), 2888
-1
˜
527, 537, 551, 562, 632, 657, 688, 721, 744, 757, 803, 878, 922, 957,
(vw), 2950 (w) cm . Raman: n = 419, 446, 502, 524, 536, 551, 560,
8122 | Dalton Trans., 2011, 40, 8114–8124
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634, 658, 689, 722, 743, 804, 819, 862, 880, 916, 948, 983, 997,
1018, 1041, 1065, 1096, 1122, 1177, 1239, 1291, 1308, 1358, 1381,
1454, 1482, 1496, 2757, 2852, 2888, 2952, 2991 cm-1. m.p. (DSC):
113 ◦C.
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Experimental data for 7. The DME was completely removed
by drying of the product in vacuum (0.1 Pa) at r.t. over night.
Na[B(hfip)4]·(DME) (8.99 g, 11.35 mmol), [C6MIM]Cl (2.301 g,
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1
11.35 mmol); isolated yield: 9.13 g (95.1%). H NMR (400.17
MHz, CD2Cl2, 298 K): d = 0.90 (t, 3H, CH3), 1.34 (m, 6H, CH2),
1.88 (quin, 2H, CH2), 3.91 (s, 3H, N(CH3)), 4.12 (t, 2H, N(CH2)),
1
4.72 (m, w 2 = 23 Hz, 4H, CH), 7.27 (m, 2H, CH), 8.09 (s, 1H,
CH). 19F NMR (376.53 MHz, CD2Cl2, 298 K): d = -75.1 (d, 3J(F,
H) = 6.2 Hz, CF3). 11B NMR (128.39 MHz, CD2Cl2, 298 K): d =
3
˜
1.66 (quin, J(B, H) = 3.1 Hz). IR (diamond ATR): n = 410 (w),
30 K. Fujiki, J. Ichikawa, H. Kobayashi, A. Sonoda and T. Sonoda, J.
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523 (w), 537 (m), 623 (m), 631 (m), 656 (m), 686 (s), 743 (m), 829
(w), 861 (m), 890 (m), 958 (m), 1019 (s), 1054 (m), 1096 (vs), 1161
(vs), 1176 (vs), 1213 (s), 1259 (m), 1290 (m), 1380 (m), 1463 (vw),
1568 (w), 1600 (vw), 2873 (vw), 2940 (w), 3176 (vw) cm-1. Raman:
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32 L. Jia, X. Yang, C. L. Stern and T. J. Marks, Organometallics, 1997, 16,
842.
˜
n = 430, 538, 553, 562, 601, 626, 634, 660, 690, 723, 745, 805, 879,
33 F. A. R. Kaul, G. T. Puchta, H. Schneider, M. Grosche, D. Mihalios
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36 E. Bernhardt, M. Finze, H. Willner, C. W. Lehmann and F. Aubke,
Angew. Chem., Int. Ed., 2003, 42, 2077.
960, 1027, 1069, 1100, 1123, 1183, 1229, 1266, 1292, 1314, 1341,
1363, 1388, 1420, 1436, 1448, 1464, 1528, 1575, 2744, 2881, 2913,
◦
2947, 2975, 3117, 3187 cm-1. m.p. (manual): -25 < m.p. < 0 C.
Elemental analysis found (calc.): 31.33 (31.23)% C, 2.73 (2.74)%
H, 3.30 (3.31)% N.
37 D. J. Brauer, H. Buerger, Y. Chebude and G. Pawelke, Inorg. Chem.,
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Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft
DFG (SPP 1191 and Normalverfahren) and the Albert-Ludwigs-
Universita¨t Freiburg, Germany.
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42 Germany Pat., EP 1205480, 2002.
43 J. Barthel, M. Wu¨hr, R. Buestrich and H. J. Gores, J. Electrochem. Soc.,
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8124 | Dalton Trans., 2011, 40, 8114–8124
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