Ionic liquids of chelated orthoborates as model ionic glassformers

Article abstract of DOI:10.1021/jp034548e

Ionic liquids based on various chelated orthoborate anions of different N-containing onium cations have been synthesized using an economic synthesis strategy. Most orthoborates do not crystallize. They are found to have much higher glass transition temperatures and room-temperature viscosities than those with perfluorinated anions such as TFSI^-, BF₄^-, and CF₃SO₃^- (Tf^-), as predicted from anion polarizability arguments. The ambient conductivities of the new ionic liquids are low relative to those with perfluorinated anions. The transport properties all show that cohesion in these liquids increases, and ionic mobilities decrease, as anion size increases, implying that van der Waals interactions, not Coulomb interactions, have become the controlling influence. In view of their resistance to crystallization, the large range of temperature over which these liquids can be studied, their hydrophobic properties, and their high fragilities, these liquids may provide good model systems for fundamental liquid state investigations and interesting solvents for large-molecule dissolution.

Full text of DOI:10.1021/jp034548e

J. Phys. Chem. B 2003, 107, 11749-11756
11749
Ionic Liquids of Chelated Orthoborates as Model Ionic Glassformers
Wu Xu, Li-Min Wang, Ronald A. Nieman, and C. Austen Angell*
Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604
ReceiVed: March 3, 2003; In Final Form: August 4, 2003
Ionic liquids based on various chelated orthoborate anions of different N-containing onium cations have been
synthesized using an economic synthesis strategy. Most orthoborates do not crystallize. They are found to
have much higher glass transition temperatures and room-temperature viscosities than those with perfluorinated
-
-
-
-
4 3 3
anions such as TFSI , BF , and CF SO (Tf ), as predicted from anion polarizability arguments. The ambient
conductivities of the new ionic liquids are low relative to those with perfluorinated anions. The transport
properties all show that cohesion in these liquids increases, and ionic mobilities decrease, as anion size increases,
implying that van der Waals interactions, not Coulomb interactions, have become the controlling influence.
In view of their resistance to crystallization, the large range of temperature over which these liquids can be
studied, their hydrophobic properties, and their high fragilities, these liquids may provide good model systems
for fundamental liquid state investigations and interesting solvents for large-molecule dissolution.
1
. Introduction
All were used as received. The chemical structures of the cations
and anions of the synthesized ionic liquids are shown in
Ionic liquids, also known as room-temperature molten salts,
Schemes 1 and 2, respectively. It is noticed, from the literature,
that molten salts with the n-butyl side group usually have lower
melting points than those with the ethyl side group.9
Therefore in this work the butyl group was chosen over the
ethyl group despite the attendant sacrifices in viscosity (higher)
and conductivity (lower). Indeed the melting points obtained
were so low, relative to liquid viscosities, that crystallization
was not observed at all in most cases.
have been receiving much attention for their potential applica-
tions, not only in electrochemistry but also as alternative solution
and reaction media in separation sciences and chemical syn-
theses. They have attractive physical properties such as negli-
gible vapor pressure even at elevated temperatures, miscibility
with organic solvents, excellent thermal and chemical stability,
,12,22,23
1-6
high conductivities, and wide electrochemical windows. The
applications in electrochemistry include electroplating, batteries,
electrochemical capacitors, and photoelectrochemical cells.
2.1. Characterization. The intermediate compounds and the
7
-23
final products were characterized by NMR spectroscopy in
Many new ionic liquids have been reported in recent years.
1
13
deuterated dimethyl sulfoxide (DMSO-d6). H and C NMR
A characteristic of many commonly reported ionic liquids is
the presence of anions, known to be weakly coordinating, such
spectra were obtained on a Varian Gemini 300 NMR spectrom-
11
7
23
19
-
eter with TMS as internal reference. B, Li, Na, and F NMR
spectra were collected using a Varian Inova 400, with BF3‚
Et2O, LiNO3, and NaF as external references, respectively.
The melting points of onium chloride compounds prepared
in this work were measured using a simple differential scanning
analysis (DTA) instrument, which was described in our previous
as bis(trifluoromethanesulfonyl)imide (TFSI ), bis(perfluoro-
-
-
ethanesulfonyl)imide (BETI ), tetrafluoroborate (BF4 ), hexaflu-
-
-
orophosphate (PF6 ), trifluoromethanesulfonate (Tf ), and
-
tetrachloroaluminate (AlCl4 ), although the presence of thio-
-
-
cyanates (SCN ) and dicyanoamides [N(CN)2 ], among most
fluid examples, shows that inability to coordinate is not a
requirement.
Recently we have reported a new anion, bis(oxalato)borate
BOB ), which is also very weakly coordinating. Its lithium
2
6
publication. After appropriate calibration with melting point
standards this simple instrument is able to define melting points
with an accuracy and precision of (1 °C.
24
-
(
2
.2. Preparation of Li or Na Orthoborate Salt. The
salt, LiBOB, showed high ionic conductivity and also electro-
chemical stability in nonaqueous electrolytic solutions. Practical
orthoborate salts (MBXB), such as LiBOB, sodium bis(oxalato)-
borate (NaBOB), lithium bis(malonato)borate (LiBMB), lithium
bis(salicylato)borate (LiBScB), and lithium bis(2-methyllactato)-
borate (LiBMLB), were prepared by complete evaporation of
water from the aqueous solutions of the corresponding organic
acid (i.e., oxalic acid, malonic acid, salicylic acid, and 2-meth-
yllactic acid, respectively), boric acid, and lithium or sodium
hydroxide in a molar ratio of 2:1:1. The white crude solid
products were refluxed with acetonitrile, followed by filtration
and drying in a vacuum oven at 100 °C for 2 days.
cell studies using LiBOB in carbonate electrolytes25 showed
excellent charge/discharge and cycling performance relative to
the industry standard LiPF6. The advantage was especially
notable at elevated temperatures.
Here we report the physical properties and electrochemical
properties of a series of room-temperature molten salts made
by combining various organic cations with orthoborate anions.
We find that orthoborate ionic liquids are too viscous for most
practical applications but have many attractive properties as
model systems for fundamental liquid state studies.
2. Experimental Section
The chemical compounds used in this work were all from
Aldrich except LiTFSI, which was obtained gratis from 3M.
where X is oxalato, malonato, salicylato, and 2-methyllactato,
respectively.
*
Corresponding author. E-mail: caa@asu.edu.
1
0.1021/jp034548e CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/27/2003

11750 J. Phys. Chem. B, Vol. 107, No. 42, 2003
Xu et al.
SCHEME 1: Chemical Structures of the Cations of the
Ionic Liquids Synthesized in This Worka
SCHEME 2: Chemical Structures of the Anions of Ionic
Liquids Synthesized in This Work
a
a
BMI is 1-n-butyl-3-methylimidazolium, RNM
2
E+ is alkyl dimeth-
+
+
yl ethyl ammonium, P14 is N-methyl-N-n-butyl pyrrolidinium, and
BPy is N-n-butylpyridinium.
+
a
-
-
-
-
BOB , BMB , BScB , and BMLB represent the anions of
bis(oxalato)borate, bis(malonato)borate, bis(salicylato)borate, and bis(2-
methyllactato)borate, respectively.
1
LiBOB, 1: white solid. NMR in DMSO-d6: H no signal
except for solvent; 13C δ 158.20 ppm; B δ 12.20 ppm; Li δ
11
7
3
3
H, J ) 7.5 Hz); 13C δ 135.75, 123.52, 122.25, 48.31, 35.63,
1.33, 18.70, 13.24 ppm.
1
.01 ppm.
1
NaBOB, 2: white solid. NMR in DMSO-d6: H no signal
except for solvent; 1 C δ 158.21 ppm; 11B δ 12.25 ppm; 23Na
3
BPyCl, 7. BPyCl was prepared by refluxing pyridine and
excess 1-chlorobutane for 3 days. After decanting the unreacted
-chlorobutane, the solid product was washed with diethyl ether
to yield a white solid, which was dried in high vacuo at ca. 90
C for 3 days. Yield: 37.2%. Mp: 98.0 °C. NMR in DMSO-
δ 1.65 ppm.
1
1
LiBMB, 3: white solid. NMR in DMSO-d6: H δ 3.40 ppm
1
3
11
7
(
s); C δ 166.01 and 38.67 ppm; B δ 8.10 ppm; Li δ 1.02
ppm.
LiBScB, 4: white solid. NMR in DMSO-d6: 1H δ 7.71 (d,
H, J ) 7.5 Hz), 7.40 (t, 2H, J ) 7.7 Hz), 6.84 ppm (m, 4H);
°
1
d6: H δ 9.20 (d, 2H, J ) 6.8 Hz), 8.61 (t, 1H, J ) 7.8 Hz),
8
2
1
.16 (t, 2H, J ) 7.1 Hz), 4.65 (t, 2H, J ) 7.4 Hz), 1.89 (m,
2
1
3
H), 1.27 (m, 2H), 0.90 ppm (t, 3H, J ) 7.3 Hz); C δ 145.40,
1
3
C δ 163.76, 159.05, 134.66, 129.25, 118.82, 118.04, 115.40
44.78, 128.01, 60.34, 32.61, 18.63, 13.22 ppm.
1
1
7
ppm; B δ 8.29 ppm; Li δ 1.07 ppm.
LiBMLB, 5: white solid. NMR in DMSO-d6: H δ 1.13 ppm
P14Cl, 8. P14Cl was synthesized by refluxing a solution of
N-methylpyrrolidine and excess 1-chlorobutane for 3 days. The
solid product was thoroughly dried in a vacuum oven at ca. 90
1
13
7
(
1
s); C δ 181.46, 75.07 26.84 ppm; 11B δ 13.67 ppm; Li δ
.08 ppm.
.3. Preparation of Onium Chloride Salts. The chlorides
of 1-n-butyl-3-methylimidazolium (BMI), n-butylpyridinium
BPy), alkyl dimethyl ethylammonium (RNM2E), and N-methyl-
°
C for 2 days. Yield: 38.7%. Mp: 86.4 °C. NMR in DMSO-
2
1
d6: H δ 3.50 (m, 2 × 2H), 3.36 (m, 2H), 3.00 (s, 3H), 2.06 (s,
2
× 2H), 1.66 (m, 2H), 1.29 (m, 2H), 0.91 ppm (t, 3H, J ) 7.4
(
13
Hz); C δ 63.21, 62.61, 47.31, 24.84, 20.96, 19.23, 13.40 ppm.
N-n-butylpyrrolidinium (P14) were synthesized by the following
Alkyl dimethyl ethylammonium chloride (RNM2ECl) was
prepared by reacting the solution of alkyl chloride and excess
dimethylethylamine in tetrahydrofuran (THF) at moderate
temperature for 1-4 days according to the activity of alkyl
chloride. All crude products were washed with fresh THF to
get solids, which were dried in high vacuo at ca. 90 °C for 4
days.
reactions.
+
-
NR R R + R-Cl f RN R R R Cl
1
2
3
1 2 3
BMICl, 6. BMICl was synthesized by refluxing 1-methylimi-
dazole and excess 1-chlorobutane for 2 days, followed by
decanting most of the unreacted 1-chlorobutane, evaporating
the rest on a rotavapor at reduced pressure, and finally drying
it in a high-vacuum oven at 90 °C for 4 days to yield a light
yellow solid. Yield: 84.7%. Mp: 61.5 °C. NMR in DMSO-d6:
MOMNM2ECl, 9. This compound was obtained from the
reaction in THF at 0 °C first for 3 h and then at room
temperature overnight: white solid. Yield: 91.7%. Mp: 135.0
1
°C. NMR in DMSO-d6: H δ 4.71 (s, 2H), 3.58 (s, 3H), 3.34
1
13
H δ 9.57 (s, 1H), 7.89 (s, 1H), 7.81 (s, 1H), 4.18 (t, 2H, J )
.1 Hz), 3.86 (s, 3H), 1.74 (m, 2H), 1.22 (m, 2H), 0.86 ppm (t,
(m, 2H), 2.96 (s, 2 × 3H), 1.20 ppm (t, 3H, J ) 7.4 Hz);
C
7
δ 90.60, 60.57, 55.52, 45.84, 7.42 ppm.

Ionic Liquids of Chelated Orthoborates
J. Phys. Chem. B, Vol. 107, No. 42, 2003 11751
7
2
EOMNM2ECl, 10. This compound was obtained with the
Li δ 1.01 ppm (very weak signal, when LiBOB was used) or
Na no signal (when NaBOB was used).
3
same mehod as MOMNM2ECl: white solid. Yield: 89.3%.
1
Mp: 134.7 °C. NMR in DMSO-d6: H δ 4.84 (s, 2H), 3.84
BMI-BMB, 15: yellow viscous liquid. Yield: 38.0%. NMR
1
(
3
m, 2H), 3.39 (m, 2H), 3.01 (s, 2 × 3H), 1.19 ppm (m, 2 ×
in DMSO-d6: H δ 9.28 (s, 1H), 7.76 (s, 1H), 7.69 (s, 1H),
4.14 (t, 2H, J ) 7.0 Hz), 3.82 (s, 3H), 3.35 (s, 2H), 1.72 (m,
2H), 1.20 (m, 2H), 0.83 ppm (t, 3H, J ) 7.2 Hz); C δ 165.90,
136.49, 123.47, 122.15, 48.50, 38.60, 35.77, 31.42, 18.83, 13.30
ppm; B δ 8.09 ppm; Li δ 1.08 ppm (very weak signal).
BMI-BScB, 16: yellow-brown solid. Yield: 85.6%. NMR in
DMSO-d6: H δ 9.08 (s, 1H), 7.73 (s, 1H), 7.71 (d, 2H), 7.67
(s, 1H), 7.41 (t, 2H, J ) 7.8 Hz), 6.87 (t, 2H, J ) 7.6 Hz), 6,81
(d, 2H, J ) 8.0 Hz), 4.13 (t, 2H, J ) 7.2 Hz), 3.82 (s, 3H),
3.36 (s, 2H), 1.74 (m, 2H), 1.23 (m, 2H), 0.87 ppm (t, 3H, J )
7.2 Hz); C δ 163.52, 158.81, 136.62, 134.51, 129.08, 123.48,
122.13, 118.68, 117.90, 115.23, 48.50, 35.74, 31.35, 18.79,
13.29 ppm; B δ 8.28 ppm; Li δ 1.02 ppm (very weak signal).
BMI-BMLB, 17: yellow viscous liquid. Yield: 51.9%. NMR
in DMSO-d6: H δ 9.35 (s, 1H), 7.80 (s, 1H), 7.72 (s, 1H),
1
3
H); C δ 89.09, 68.54, 55.33, 45.65, 15.05, 7.49 ppm.
MOENM2ECl, 11. This compound was obtained from the
1
3
reaction in an autoclave at ca. 75 °C for 2 days; white solid.
1
11
7
Yield: 77.3%. Mp: 85.7 °C. NMR in DMSO-d6: H δ 3.73
(
3
6
m, 2H), 3.55 (t, 2H, J ) 4.8 Hz), 3.43 (m, 2H), 3.27 (s, 3H),
13
1
.05 (s, 2 × 3H), 1.21 ppm (t, 3H, J ) 7.3 Hz); C δ 65.34,
1.56, 59.43, 58.04, 49.96, 7.87 ppm.
EOENM2ECl, 12. This compound was obtained from the
reaction in an autoclave at 80 °C for 4 days: white solid.
1
13
Yield: 47.2%. Mp: 59.4 °C. NMR in DMSO-d6: H δ 3.76
(
3
)
m, 2H), 3.52 (t, 2H, J ) 5.0 Hz), 3.45 (m, 2H), 3.39 (m, 2H),
.05 (s, 2 × 3H), 1.21 (t, 3H, J ) 7.3 Hz), 1.10 ppm (t, 3H, J
7.0 Hz); 13C δ 65.49, 63.24, 61.63, 59.46, 50.01, 14.78, 7.84
ppm.
BNM2ECl, 13. This compound was obtained from the reaction
11
7
1
4.15 (t, 2H, J ) 7.0 Hz), 3.83 (s, 3H), 1.72 (m, 2H), 1.20 (m,
in an autoclave at 80 °C for 7 days: yellowish white solid.
2H), 1.09 (d, 12H, J ) 6.0 Hz), 0.83 ppm (t, 3H, J ) 7.4 Hz);
1
13C δ 181.20, 136.47, 123.42, 122.12, 75.01, 48.42, 35.70, 31.38,
Yield: 25.5%. Mp: 99.9 °C. NMR in DMSO-d6: H δ 3.32
1
1
7
(
q, 2H), 3.19 (m, 2H), 2.97 (s, 2 × 3H), 1.61 (m, 2H), 1.31 (m,
26.78 (d), 18.77, 13.26 ppm; B δ 13.67 ppm; Li δ 1.08 ppm
(very weak signal).
2
H), 1.23 (t, 3H, J ) 7.2 Hz), 0.90 ppm (t, 3H, J ) 7.0 Hz);
C δ 62.38, 58.61, 49.44, 23.67, 19.17, 13.36, 7.71 ppm.
1
3
BMI-TFSI, 18: yellow liquid. Yield: 89.3%. NMR in DMSO-
d6: 1H δ 9.05 (s, 1H), 7.70 (s, 1H), 7.64 (s, 1H), 4.12 (t, 2H,
J ) 7.2 Hz), 3.81 (s, 3H), 1.73 (m, 2H), 1.22 (m, 2H), 0.86
2
.4. Synthesis of Orthoborate Ionic Liquids. In this work
we made ionic liquids by substitution of orthoborate anions for
chloride ions using an economic but effective metathesis
1
3
ppm (t, 3H, J ) 7.4 Hz); C δ 136.49, 123.58, 122.23, 121.65,
19
3,27,28
117.38, 48.53, 35.68, 31.33, 18.73, 13.11 ppm; F δ 46.11 ppm;
reaction, which has been widely used for the preparations.
7
Li δ 1.01 ppm (very weak signal).
The insolubility of sodium chloride in anhydrous acetonitrile,
and particularly in dichloromethane, is sufficient enough that
when these solvents are used in successive steps of the synthesis
and refinement, the NMR spectrum of the final product in
DMSO-d6 shows no sodium signal. In most cases even an
AgNO3 solution test for trace chloride anion proves negative.
This level of purity is quite sufficient for the purposes of the
ionic liquid evaluations that we have elected to make in this
study. The obvious advantage of this method is that the
quantities of product needed to make additional measurements
such as viscosities (often not reported in other studies) are not
excluded for economic reasons.
The onium chloride salt was reacted with excess orthoborate
salt in anhydrous acetonitrile under refluxing for 1-3 days to
allow the chloride to react completely. After cooling, the alkali
halide precipitates were filtered off and the solvent in the filtrate
was evaporated on a rotavapor at reduced pressure. The residue
was dried in a vacuum oven at 100 °C for 1 day and then
dissolved in a large amount of dichloromethane (CH2Cl2). The
solution was allowed to stand at room temperature overnight
in order for the dissolved lithium or sodium orthoborate salt to
precipitate in CH2Cl2. After filtration and evaporation of the
solvent from the filtrate, the residue was further dried in a
vacuum oven at 90 °C for 2 days to yield the orthoborate ionic
liquids.
BMI-BF4, 19: yellow liquid. Yield: 83.6%. NMR in DMSO-
d6: 1H δ 9.04 (s, 1H), 7.72 (s, 1H), 7.66 (s, 1H), 4.15 (t, 2H,
J ) 7.2 Hz), 3.84 (s, 3H), 1.76 (m, 2H), 1.25 (m, 2H), 0.89
1
3
ppm (t, 3H, J ) 7.4 Hz); C δ 136.46, 123.58, 122.23, 48.52,
1
1
19
35.68, 31.32, 18.75, 13.20 ppm; B δ 3.63 ppm; F δ -23.46
2
3
ppm; Na no signal.
BMI-Tf, 20: yellow liquid. Yield: 82.8%. NMR in DMSO-
d6: 1H δ 9.19 (s, 1H), 7.77 (s, 1H), 7.70 (s, 1H), 4.16 (t, 2H,
J ) 7.2 Hz), 3.85 (s, 3H), 1.75 (m, 2H), 1.24 (m, 2H), 0.89
1
3
ppm (t, 3H, J ) 7.4 Hz); C δ 136.58, 123.58, 122.86, 122.26,
19
118.59, 48.59, 35.67, 31.37, 18.76, 13.09 ppm; F δ 46.98 ppm;
7
Li δ 0.95 ppm (very weak signal).
BPy-BOB, 21: yellow viscous liquid. Yield: 75.6%. NMR
1
in DMSO-d6: H δ 9.06 (d, 2H, J ) 7.0 Hz), 8.59 (t, 1H, J )
7.8 Hz), 8.13 (t, 2H, J ) 7.4 Hz), 4.61 (t, 2H, J ) 7.5 Hz),
1.90 (m, 2H), 1.28 (m, 2H), 0.86 ppm (t, 3H, J ) 7.3 Hz); 13
C
δ 158.40, 145.54, 144.79, 128.19, 60.83, 32.75, 18.82, 13.23
1
1
23
ppm; B δ 12.25 ppm; Na no signal.
P14-BOB, 22: yellow viscous liquid. Yield: 74.9%. NMR in
1
DMSO-d6: H δ 3.47 (m, br, 2 × 2H), 3.29 (m, 2H), 2.98 (s,
3H), 2.09 (s, 2 × 2H), 1.68 (m, 2H), 1.30 (m, 2H), 0.89 ppm
1
3
(t, 3H, J ) 7.3 Hz); C δ 158.35, 63.63, 63.19, 47.68, 25.00,
1
1
23
21.14, 19.31, 13.35 ppm; B δ 12.08 ppm; Na no signal.
MOMNM E-BOB, 23: yellow viscous liquid. Yield: 69.6%.
2
1
For comparison, the 1-n-butyl-3-methylimidazolium salts of
NMR in DMSO-d6: H δ 4.59 (s, 2H), 3.59 (s, 3H), 3.30 (m,
-
-
-
13
TFSI , BF4 , and Tf were also prepared by reacting BMICl
and the corresponding lithium (LiTFSI anf LiTf) or sodium salt
2H), 2.94 (s, 2 × 3H), 1.21 ppm (m, 3H); C δ 158.37, 91.07,
11
23
60.67, 55.96, 46.20, 7.45 ppm; B δ 12.07 ppm; Na no signal.
(
NaBF4) in acetonitrile, following the procedures as above. BMI-
EOMNM E-BOB, 24: yellow viscous liquid. Yield: 71.2%.
2
1
6
TFSI was extracted from the aqueous solution with CH2Cl2.
NMR in DMSO-d : H δ 4.63 (s, 2H), 3.81 (m, 2H), 3.29 (m,
Their properties are being reported elsewhere.26
13
2H), 2.92 (s, 2 × 3H), 1.19 ppm (m, 3H); C δ 158.30, 89.56,
1
1
7
BMI-BOB, 14: yellow viscous liquid. Yield: 76.8%. NMR
68.83, 55.83, 46.07, 14.98, 7.50 ppm; B δ 12.05 ppm; Li δ
1.02 ppm (very weak signal).
1
in DMSO-d6: H δ 9.07 (s, 1H), 7.73 (s, 1H), 7.66 (s, 1H),
4
2
1
.15 (t, 2H, J ) 7.2 Hz), 3.85 (s, 3H), 1.76 (m, 2H), 1.25 (m,
MOENM2E-BOB, 25: yellow viscous liquid. Yield: 92.0%.
NMR in DMSO-d6: H δ 3.71 (s, 2H), 3.48 (t, 2H, J ) 4.8
Hz), 3.39 (m, 2H), 3.27 (s, 3H), 3.03 (s, 2 × 3H), 1.23 ppm (t,
13
1
H), 0.87 ppm (t, 3H, J ) 7.4 Hz); C δ 158.16, 136.49, 123.60,
11
22.23, 48.53, 35.77, 31.37, 18.78, 13.19 ppm; B δ 12.05 ppm;

1
1752 J. Phys. Chem. B, Vol. 107, No. 42, 2003
Xu et al.
1
3
3
7
H, J ) 7.3 Hz); C δ 158.27, 65.41, 61.92, 59.93, 58.11, 50.22,
for the perfluorinated anions. In this work, since the lithium or
sodium orthoborate salt was used in large excess with long
refluxing time, the onium chlorides should have reacted
completely.
11
7
.86 ppm; B δ 12.06 ppm; Li δ 1.02 ppm (very weak signal).
EOENM2E-BOB, 26: yellow viscous liquid. Yield: 76.0%.
1
NMR in DMSO-d6: H δ 3.76 (s, 2H), 3.47 (m, 2 × 2H), 3.38
(
(
5
m, 2H), 3.02 (s, 2 × 3H), 1.23 (t, 3H, J ) 7.2 Hz), 1.11 ppm
When the ionic liquids were synthesized from lithium salts,
t, 3H, J ) 7.0 Hz); 1 C δ 158.15, 65.61, 63.25, 61.87, 59.74,
3
7
7
small Li NMR signals were obtained. Li NMR signals could
be observed only after signal-averaging for long periods (more
than 1 h), whereas spectra are normally acquired within several
minutes for a lithium salt. This indicates that the amount of
lithium salt residual in the sample is very small. These traces
of lithium salts are either from the starting materials or from
LiCl formed during reactions. Evidently Li salts are slightly
soluble in CH2Cl2 in the presence of the soluble ionic liquids,
hence cannot be removed completely by our method.
0.12, 14.79, 7.84 ppm; 11B δ 12.27 ppm; Na no signal.
23
BNM2E-BOB, 27: yellow viscous liquid. Yield: 73.5%. NMR
in DMSO-d6: 1H δ 3.31 (m, 2H), 3.20 (m, 2H), 2.97 (s, 2 ×
3
H), 1.62 (m, 2H), 1.31 (m, 2H), 1.23 (t, 3H), 0.90 ppm (t, 3H,
13
J ) 7.3 Hz); C δ 158.30, 62.43, 58.68, 49.54, 23.71, 19.18,
3.38 7.76 ppm; 11B δ 12.27 ppm; Na no signal.
23
1
2
.5. Physical Properties of the Ionic Liquids. The thermal
properties of the ionic liquids were determined using a Perkin-
Elmer DSC-7 differential scanning calorimeter in the temper-
ature range -150 to 80 °C. The instrument temperature scale
was calibrated with crystal-crystal transition of cyclopentane
-151.16 °C) and melting of indium (+156.60 °C). Samples
were sealed in aluminum pans, purged with helium gas, and
heated at a rate of 20 K min . For the orthoborate compounds
only a glass transition endotherm (Tg) could be observed, unless
the samples were stored for several days before scanning;
melting points (Tm) could then be observed in the rescans. In
the cases of TFSI and Tf compounds, devitrification tem-
perature (Tc) and melting peaks (Tm) could also be observed in
the initial scan.
Approximate density values at different temperatures, accurate
to 0.5%, were obtained simply by measuring the weight of the
sample filling a 2.00 mL volumetric flask within a VAC drybox.
The flasks were maintained in a heating block at the measured
temperature for half an hour until the temperature was steady
before measurement.
On the other hand, when the ionic liquids were obtained using
23
sodium salts (NaBOB and NaBF4), no Na NMR signal was
detected, indicating that these ionic liquids are pure. Even if
residual alkali were present in the Li salt preparation, the amount
is too little to influence the properties of the liquids. For instance,
the thermal and ionic conductance properties of the liquid BMI-
BOB formed from each of the LiBOB and NaBOB starting
materials are identical.
(
-
1
3
.2. Hydrophobic Behavior. The orthoborate ionic liquids
-
-
of this study dissolve readily in nearly all the common polar
organic solvents. However, when added to water, round oil
beads form at the bottom of the container, suggesting that the
orthoborate liquids have low solubility in water. On standing,
the beads develop a coating of white solid on the surface, which
is the result of a hydrolysis reaction. The hydrolysis is the
reverse of the reaction that produces the orthobrate species by
dehydration of oxalic acid-boric acid-metal hydroxide mix-
29
tures in the first place. It cannot be established whether the
Kinematic viscosities of ionic liquids were measured using
Cannon-Ubbelohde viscometers of appropriate viscometer con-
stants, in the temperature range between ambient and 130 °C.
CaCl2 drying tubes were used to protect the samples from
moisture in the air. A uniform temperature environment was
provided by a tall aluminum temperature smoothing block with
slots to permit meniscus observation. The temperature of the
sample was maintained for half an hour before measurement.
The precision of measurement with Cannon-Ubbelohde vis-
cometers is controlled by the reproducibility of flow times, and
accuracy is controlled by accuracy of calibration constants and
by temperature measurement. Precision was limited at the
highest temperatures (above 100 °C) by the short flow times
apparent hydrophobicity is a result of pre-emptive hydrolysis
at the ionic liquid-water interface or whether it is due to the
nature of the physical interaction between solvent water and
the ionic liquid. Hydrophobic behavior is also suggested by the
apparent stability in air. Changes in weight that occurred when
the orthoborate ionic liquids were exposed to air for 3 days
were barely detectable. The samples showed no change in
appearance.
3
.3. Density and Molar Volume. Figure 1 shows the
+
temperature dependence of density of the BMI ionic liquids.
The densities show the usual linear variation with temperature.
30
The data for these and other compounds of this study are
collected, as parameters of the linear equation, in Table 1. For
(<10 s) consequent on our using only a single viscometer for
+
the orthoborate ionic liquids of the BMI cation, the densities
each sample. The flow times were reproducible: the standard
deviation was (0.2 s. For temperatures below 40 °C the run
times are typically 200 s or longer; hence the reading error is
only 0.1% of the efflux time.
-
-
-
-
fall in the order BScB > BOB > BMB > BMLB . Apart
-
from the BScB , the density of the orthoborate ionic liquids
decreases with the increase of anion size. However, when
compared with data for salts of fluorinated anions, the density
The conductivities of the ionic liquids were measured by ac
impedance spectroscopy using a HP 4192A LF impedance
analyzer in the frequency range from 5 to 13 MHz. We used
dip-type cells containing two parallel platinum disks. Measure-
ments were made during slow continuous cooling from 120 to
-
-
-
at room temperature follows the order TFSI > BOB > Tf
-
>
BF4 . It seems that the density increases with anion size,
perhaps because packing becomes more efficient as the alternat-
ing positive and negative species become more even in size.
-
Keeping the anion constant (BOB ), the aromatic N-hetero
-50 °C. The cell constants lie between 0.5 and 0.7 and were
+
+
cations (BPy and BMI ) show higher density than the aliphatic
determined with 0.2% precision, by a 0.1 m KCl aqueous
solution calibration.
+
+
N-hetero cations (P14 and BNM2E ). The densities are in the
+
+
+
+
+
order BPy > BMI > P14 > BNM2E (Table 1). For BPy ,
the densest, and also the most fragile case, there is some
indication of deviations from the linear temperature dependence.
This will be investigated using a more precise method of
measurement.
3
. Results and Discussion
.1. Characterization. The ionic liquids have all the H and
1
3
1
3
C NMR signals of the corresponding N-containing onium
cations as well as the corresponding anions. 1 B NMR spectra
indicate the sole singlet peak for the orthoborate (BXB ) and
BF4 anions, and F NMR spectra show the sole singlet peak
1
-
+
-
For the RNM2E -BOB liquids, the pure alkyl group-
containing salts show lower density than the same length of
-
19

Ionic Liquids of Chelated Orthoborates
J. Phys. Chem. B, Vol. 107, No. 42, 2003 11753
TABLE 2: Calorimetric Properties of Ionic Liquids from
DSC Measurements
ionic liquid
T
g
/°C
T
c
/°C
m
T /°C
BMI-BOB
-29.2
-33.1
21.5
BMI-BMB
BMI-BScB
BMI-BMLB
BMI-TFSI
-39.4
-85.9
-85.3
-81.6
-20.5
-37.8
-32.4
-40.2
-35.9
-44.8
-44.4
-16.0
-4.9
BMI-BF
BMI-Tf
4
2.9
16.4
BPy-BOB
14-BOBa
MOMNM
P
b
b
b
b
b
b
2
E-BOB
50.8
46.7
48.3
EOMNM
MOENM
2
E-BOB
E-BOB
2
EOENM
2
E-BOB
BNM E-BOB
2
a
MacFarlane et al. (private communication) have succeeded in
crystallizingP14-BOB. b TheT
andT ofMOMNM E-BOB,EOMNM E-
BOB and MOENM E-BOB cannot be measured in the same scan. The
g
m
2
2
2
samples had to be stored at room temperature for a few days to allow
the formation of crystals. The melting point was determined from an
initial scan, and the glass temperature from a repeat scan. The
crystallized samples were cooled and measured during upscans to 80
°C. After that they were cooled to -120 °C and once again measured
Figure 1. Densities of BMI+ ionic liquids with different anions, in
relation to temperature. See also Table 1.
during upscans until 80 °C. The first measurement gives only T
m
, while
TABLE 1: Density Equation Parameters and Molar
Volumes of Ionic Liquids
the second only T . It is indicated that the samples are not easy to
crystallize.
g
-
3
density equation d ) b - aT/g cm
tautological basis of the “2/3 rule” by showing that substances
that would violate the rule on the low side, never get tested,
because no crystals are available to determine Tm. It is already
known from hyperquenching studies that substances that violate
the rule in the other direction usually do not get tested because
they do not vitrify in the first place. The rule should be
reformulated to read “substances that may readily be observed
in both glassy and crystalline states are those that have glass
temperatures about 2/3 of the melting points”.
(
where T in °C)
Vm at 25 °C/
3
-1
ionic liquid
BMI-BOB
BMI-BMB
BMI-BScB
BMI-BMLB
BMI-TFSI
BMI-BF4
BMI-Tf
a
b
R2
cm mol
-
4
4
7 × 10
7 × 10
1.31
1.26
n.m.
0.994
0.972
n.m.
254
286
322
310
302
191
237
247
265
234
251
246
267
261
-
a
a
a
n.m.
-
-
-
-
-
-
-
-
-
-
-
4
4
4
4
4
4
4
4
4
4
4
5 × 10
7 × 10
5 × 10
6 × 10
7 × 10
5 × 10
1.12
1.41
1.21
1.24
1.33
1.25
1.32
1.29
1.32
1.27
1.23
0.971
0.995
0.991
0.996
0.933
0.989
0.976
0.993
0.992
0.993
0.968
BPy-BOB
P14-BOB
The glass temperatures for orthoborate melts are much higher
than for the corresponding salts of the fluorinated anions. Table
MOMNM2E-BOB 7 × 10
EOMNM2E-BOB 7 × 10
MOENM2E-BOB 7 × 10
2
shows that for the same N-containing onium cation, e.g.,
+
BMI , the Tg of the ionic liquids with different anions decreases
in the order BScB > BOB > BMB > BMLB . Tf >
BF4 ∼ TFSI . The ionic liquid of BScB has the highest Tg
among the four orthoborate anions, presumably as a consequence
of the phenyl ring component of this anion structure.
EOENM2E-BOB
BNM2E-BOB
8 × 10
5 × 10
-
-
-
-
-
-
-
-
a
n.m. means not measured.
+
+
-
alkoxyalkyl group-containing salts, i.e., BNM2E < EOMNM2E
For the same BOB anion, the aromatic N-hetero cations
+
+
+
<
MOENM2E . On the other hand, the longer the alkoxyalkyl
(BPy and BMI ) show higher Tg than the aliphatic N-hetero
cations (P14 and BNM2E ). The order is BPy > BMI >
P14 > BNM2E . For RNM2E -BOB ionic liquids, the longer
the alkoxyalkyl group R, the lower the Tg of the ionic
liquid, i.e., MOMNM2E > MOENM2E > EOMNM2E
EOENM2E . However, the presence of the ether group in the
+
+
+
+
+
group R, the lower the density of the liquid, i.e., MOMNM2E
+
+
+
+
+
+
∼
MOENM2E > EOMNM2E > EOENM2E .
The molar volumes of orthoborate ionic liquids at ambient
temperature, obtained from density data, are also presented in
Table 1.
+
+
+
>
+
3
.4. Thermal Behavior: Melting Points Tm, Glass Tem-
side chain raises Tg relative to salts with only alkyl side groups,
+
+
+
peratures Tg, and the “2/3 Rule”. Table 2 summarizes the
calorimetric properties of the ionic liquids according to DSC
measurements. None of the orthoborate ionic liquids crystallize
on fast cooling in liquid nitrogen, nor do most of them crystallize
on long standing at -30 °C (in a freezer). In three cases, crystals
formed after standing some days at room temperature. For these,
the melting points were measured and found to be only ca. 1.4Tg
compared with the normal ca. 1.5Tg for glass-formers. Those
that do not crystallize presumably have even lower Tm/Tg values.
Efforts to induce crystallization of these may be worthwhile
for the purpose of demonstrating the tautological origin of the
the order being BNM2E < EOMNM2E < MOENM2E .
In general, the glass temperatures of these liquids increase
with the ion size. However, as seen in Figure 2, there is no
simple correlation with the molar volumes. Since all our salts
have the same charge per mole of ions, it is clear from Figure
2 that, in these ionic liquids, the cohesive energy is not being
determined by the Coulomb energy since the latter must decrease
with increasing volume. The orthoborate class of “ionic liquid”
therefore has an interesting feature: the properties of the liquid
are more under the control of the van der Waals interactions
than of the electrostatic interactions. Similar behavior was found
for ionic liquids with increasingly large quaternary ammonium
3
1
3
2
well-known “2/3” rule for glass-formers (Tg/Tm ≈ 2/3). It
would be interesting to show that the difficult-to-crystallize cases
have even lower Tg/Tm ratios. It would help establish the
1
6
cations by Sun et al. The structures of such liquids may show
interesting differences from those normally associated with

11754 J. Phys. Chem. B, Vol. 107, No. 42, 2003
Xu et al.
Figure 2. Variation of T
liquids, compared with that for BMI ILs with fluorinated anions. The
dashed line shows the behavior of T with molar volume for a large
g
with molar volume for orthoborate ionic
+
g
number of ionic liquids in which the anions were all of low
polarizability and indicates that the dominance of coulomb interactions
is limited to the salts of smaller ions.
Figure 4. Fluidities of BOB- ionic liquids of different types of
cations: (a) aromatic pyridine-based, aromatic imadazole-based, cyclic
but nonaromatic, and noncyclic nonaromatic; (b) different nonaromatic,
noncyclic cations.
+
ing, as BMI has a very low and dispersed electrostatic charge.
The reason must be the same as given above for the high glass
temperature, namely, the strong van der Waals interactions
associated with its multiatom unfluorinated character. Much of
the difference in ambient temperature viscosities is removed
with the increase of temperature to 100 °C because of the much
larger temperature dependence of the BMI-BOB liquid.
Figure 3. Fluidities of BMI+ ionic liquids, shown as Arrhenius
functions of temperature, showing the much higher viscosity of the
orthoborate salt BMI-BOB, notwithstanding the weakly coordinating
-
character of BOB established by solutions studies.
molten salts. Indeed such differences are manifested in the
existence of interesting types of liquid crystals reported for ionic
-
For a given anion, BOB , the salts with n-butyl-containing
33
+
+
+
liquids with chlorozincate anions reported by Martin et al. Our
findings suggest that similar complex structures, driven by van
der Waals interactions between the anions, might be generated
from orthoborate type ionic liquids, although we have no
evidence for such structures from the present study.
aromatic and aliphatic N-hetero cations, BPy , BMI , P14 , and
+
BNM2E , show similar viscosity especially at high temperatures.
At room temperature the BPy-BOB ionic liquid is the most
viscous among the four ionic liquids, as shown in Figure 4a.
2
6
The pyridine ring-based cations have earlier been found to be
associated with the highest cohesion. This must be a reflection
of the importance of the van der Waals interactions in determin-
ing the fluid properties, as discussed in the previous section.
3.5. Dynamic Viscosity and Fragility. As indicated by their
high Tg values, the orthoborate ionic liquids are much more
-
viscous than those with perfluorinated anions such as TFSI ,
-
-
+
BF4 , and Tf . Here we present the data in the inverse form,
fluidity, because this gives the same shape as obtained for the
conductivity, and it is this form that we will use in Walden
plot comparisons below.
For the RNM2E -BOB ionic liquids, the viscosity shows the
+
+
+
+
order MOENM2E > MOMNM2E ∼ BNM2E ∼ EOMNM2E
+
+
> EOENM2E (Figure 4b). The reason that the MOENM2E
is the most viscous is not clear. The lowest viscosity at room
temperature is 1331 cP for EOENM2E-BOB.
In Figure 3 we compare the dynamic inverse viscosities and
+
26
their temperature dependences for BMI-BOB with other BMI
It was noted previously that BPy-BOB was outstanding
ionic liquids. BMI+ with the BOB anion has a much higher
-
among ionic liquids for its rate of change of viscosity with
temperature, i.e., its high fragility, which was at the high limit
viscosity than seen with the fluorinated anions. This is surpris-

Ionic Liquids of Chelated Orthoborates
J. Phys. Chem. B, Vol. 107, No. 42, 2003 11755
Figure 6. Arrhenius plot of conductivities of BMI+ ionic liquids of
different types of orthoborate anions, compared with those of fluorinated
anions.
-
salt. Indeed, BOB salts were found to provide the most fragile
of a group of ionic liquids involving eight different anions.2
6
-
Considering the effect of different cations with the BOB
+
+
+
anion, BMI , P14 , and BNM2E all have similar conductivities,
while BPy , the most viscous, has a lower value (Figure 7a).
+
Within the quaternary ammonium cation series the order of
-
+
+
conductivity with BOB is MOENM2 < MOMNM2E
<
+
+
+
BNM2E ∼ EOENM2E < EOMNM2E . However, the latter
-
5
-1
is only 8.7 × 10 S cm at room temperature. Because of the
-
unusual rate, per K, at which BOB salts gain fluidity with
increasing temperature, it is possible that, if paired with a
perfluorinated cation that will lower Tg, BOB-based ionic liquids
could be highly fluid and conductive at ambient temperatures.
However such cations have yet to be described.
-
Figure 5. T
g
-scaled Arrhenius plot of fluidities of BOB ionic liquids
3
.7. General: Conductivity-Viscosity Relations and the
of different types of cations, compared with that for the least fragile
case in ref 26, i.e., MOMNM E-BF . The plot shows that the BOB
2 4
anion leads to more fragile liquids and appears to distinguish aromatic
from nonaromatic cations.
Walden Plot. It was found that the equivalent conductivity could
-
generally be predicted from the relative viscosities of ionic
26
liquids, but with some interesting and unexpected exceptions.
For example, a specific “docking” interaction could occur in
some compounds, leading to a loss of conductivity and an
among recorded liquids. A comparison is made in Figure 5 using
the usual Tg-reduced Arrhenius plot.34 Here we note that the
high fragility seems to be characteristic of all the orthoborate
anion cases. For orthoborate ionic liquids with the same cation,
the orthoborate with the largest viscosity is the one with the
largest anion, i.e., BScB > BMLB > BMB > BOB . This
confirms the increasing importance of van der Waals over
Coulomb attractions since the latter are diminishing with
increasing size of ions.
2
6
increase in vapor pressure.
To determine if such exceptions exist among the ionic liquids
with orthoborate anions, we present the data in Walden plot
form in Figure 8. Figure 8 shows that, while the majority of
orthoborate ionic liquids conform to the normal pattern, there
are indeed some cases in which the degree of association is
high. One of them involves the same quaternary ammonium
-
-
-
-
+
cation MOMNM2E that was found exceptional when combined
-
with the BF4 anion. Thus the association leading to low
With a glass temperature near room temperature, an excep-
tionally high fragility, no propensity for crystallization, and only
weak sensitivity to moisture, the BPy BScB salt could become
a good model system for future studies of glass-former
phenomenology.
conductivity seems to be cation-driven. The fact that the second-
most associated case should also involve a symmetrical oxy
+
-
+
side chain (EOE in EOENM2E ) may be significant, but the
mechanism is not clear to us.
3
.6. Conductivity. A comparison of the temperature depen-
4. Concluding Remarks
Since the molecular weights of the orthoborate anions are
comparable with, or smaller than, that of TFSI , and since the
+
dence of ionic conductivity of the BMI ionic liquids is made
in Figure 6. As expected from their high glass temperatures
and viscosities, the orthoborate ionic liquids are much less
conductive than their perfluorinated anion analogues. Of the
orthoborates the most conductive is the smallest one with the
-
charge dispersion across the anion is also comparable (according
to the extent of dissociation in dilute solutions of lower dielectric
24
constant solvents ), it seems that the source of superior fluidities
of the TFSI-containing salts must lie in the weak van der Waals
interactions that are characteristic of molecules with outermost
fluorine atoms. If this is indeed the case, then the only way to
-
-
anion BOB . Interestingly, BOB has a higher conductivity than
-
-
the BMB or BMLB salts, even though it has the higher Tg.
This must be a consequence of a higher fragility32 for the BOB
-

11756 J. Phys. Chem. B, Vol. 107, No. 42, 2003
Xu et al.
recently described.26 The possible benefit of incorporating
smaller anions is exemplified by the low viscosity of salts of
the dicyanamide anion reported by MacFarlane et al. recently.23
We will report other examples of the success of this approach
in future publications.
A possible advantage of the orthoborate type systems may
lie in their strong van der Waals interactions. It is possible that
these will empower them to serve as solvents for high molecular
weight organic substances while remaining, themselves, soluble
in polar solvents. Investigations of their utility for such purposes
will be reported elsewhere.
Acknowledgment. This work was supported by Mitsubishi
Chemical Corporation of Japan. We also acknowledge the U.S.
NSF Grant CHE-9808678 for NMR measurements.
References and Notes
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