Structure and properties of N, N-alkylene bis(Na?2-alkylimidazolium) salts

Article abstract of DOI:10.1021/jp102370j

A series of N,N-alkylene bis(Na?2-alkylimidazolium) salts with various anions was prepared and characterized. The hydrogen-bonding abilities and ion-pairing strengths of the salts in solution were varied by changing the solvent and anion. Qualitatively, the extent of ion pairing of the 1,2-bis[N-(Na?2-butylimidazolium)]ethane salts with different anions was determined in acetone-d₆ by ¹H NMR spectroscopy. Thermal properties of the imidazolium salts were related not only to the nature of anions but also to the spacer length between imidazolium cations. Exceptionally high melting points of 1,2-bis[N-(Na?2-alkylimidazolium)]ethane bis(hexafluorophosphate)s can be explained by multiple hydrogen bonds observed in the X-ray crystal structures. Moreover, a trans conformation of the ethylene spacer of 1,2-bis[N-(Na?2-alkylimidazolium)]ethane bis(hexafluorophosphate)s allows good stacking structure in the crystals. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp102370j

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7312
J. Phys. Chem. B 2010, 114, 7312–7319
Structure and Properties of N,N-Alkylene Bis(N′-Alkylimidazolium) Salts
Minjae Lee, Zhenbin Niu, Carla Slebodnick, and Harry W. Gibson*
Department of Chemistry, Virginia Polytechnic Institute & State UniVersity, Blacksburg, Virginia 24061
ReceiVed: March 15, 2010; ReVised Manuscript ReceiVed: April 22, 2010
A series of N,N-alkylene bis(N′-alkylimidazolium) salts with various anions was prepared and characterized.
The hydrogen-bonding abilities and ion-pairing strengths of the salts in solution were varied by changing the
solvent and anion. Qualitatively, the extent of ion pairing of the 1,2-bis[N-(N′-butylimidazolium)]ethane salts
1
with different anions was determined in acetone-d6 by H NMR spectroscopy. Thermal properties of the
imidazolium salts were related not only to the nature of anions but also to the spacer length between imidazolium
cations. Exceptionally high melting points of 1,2-bis[N-(N′-alkylimidazolium)]ethane bis(hexafluorophosphate)s
can be explained by multiple hydrogen bonds observed in the X-ray crystal structures. Moreover, a trans
conformation of the ethylene spacer of 1,2-bis[N-(N′-alkylimidazolium)]ethane bis(hexafluorophosphate)s allows
good stacking structure in the crystals.
Introduction
cells,30,31 and dicationic imidazolium ILs connected with fluo-
roalkyl, ethyleneoxy, and phenylene units were synthesized as
high-temperature lubricants.32,33
Imidazolium salts are well-known N-heterocyclic carbene
(NHC) precursors in organometallic chemistry1-11 and homo-
geneous catalysis.12-15 The formation of NHC is due to the
acidic proton on the 2-position of the imidazolium unit; its pKa
is 16-24, which depends on the nature of the substituents on
the two imidazolium nitrogens.16 Anions of the imidazolium
salts also have a strong influence on the H/D exchange reaction
at the 2-position; for example, the 1-butyl-3-methyl dicyanimide
salt undergoes deuterium exchange even in the absence of any
base.17 Transition-metal complexes with bis(alkylimidazolium)
salts containing a (CH2)n linker were studied; the length of the
linker controls physical properties and effects the chelating
ability.2,5
However, we are surprised that no one has reported a detailed
property study of dicationic imidazolium salts with an ethylene
spacer. Therefore, here, we report the structure-property
relationships of alkylene bis[N-(N′-alkylimidazolium)] salts with
with a C2 (ethylene) spacer and the comparison of these salts
with the analogous salts with C3 and C4 spacers with various
lengths of N-alkyl side chains. We also studied the counterion
effects on the properties. Some interesting solid-state structures
were found for the imidazolium salts, which showed unique
stacking patterns in X-ray crystal structures.
Experimental Section
Imidazolium salts are also popular as ionic liquids (ILs)
because of their unique characteristics, such as low volatility,
nonflammability, large electrochemical window, and high ionic
conductivity.18-23 As a result, many new ILs have been
synthesized, and applications of ILs have been also expanded
in diverse areas, such as electroactive devices.24-27 Demand for
new ILs with designed properties is growing because of the
expansion of their applications. As one trend for new imida-
zolium ILs, systems with more than one imidazolium unit per
molecule have been studied to obtain better properties and wider
applications. Armstrong et al. studied the structure-property
relationships of thermally stable dicationic ILs; the geminal
imidazolium dicationic salts, in which two imidazolium units
are connected by C3-C12 alkylenes, are more thermally stable
than monoimidazolium ILs.28 Also, the thermal properties and
ionic conductivity of imidazolium dicationic di[bis(trifluo-
romethanesulfonyl)imide] (2Tf2N-) salts were investigated by
Pitawala et al.29 They compared DSC transitions and ionic
conductivities of 2H- (unsubstituted at C2 of imidazolium),
2-methyl-, and 2-phenyl-imidazolium dicationic salts with
various spacers, with monoimidazolium Tf2N- salts. The
applications of imidazolium dicationic salts were also reported;
for example, dicationic imidazolium iodide salts with alkylene
or ethyleneoxy spacers were used in dye sensitized solar
NMR spectra for all synthesized compounds and X-ray
crystallographic data are presented in the Supporting Informa-
tion.
Materials. Acetone for the quaternization reactions was dried
with anhydrous CaSO4 and then distilled. Acetonitrile (MeCN)
was dried with anhydrous K2CO3 and then distilled. All other
chemicals and solvents were used as received.
1
Instruments. H and 13C NMR spectra were obtained on
Varian Inova 400 MHz and Unity 400 MHz spectrometers.
High-resolution electrospray ionization time-of-flight mass
spectrometry (HR ESI TOF MS) was carried out on an Agilent
6220 Accurate Mass TOF LC/MS Spectrometer in positive ion
mode. DSC results were obtained on a TA Instrument Q2000
differential scanning calorimeter at a scan rate of 5 or 10 °C/
min heating under N2 purge. TGA results were obtained on a
TA Instrument Q500 Thermogravimetric Analyzer at a heating
rate of 10 °C/min under N2 purge. Melting points were observed
on a Bu¨chi B-540 apparatus at a 2 °C/min heating rate.
General Anion-Exchange Procedure A. Into a solution of
bromide salt (1 equivalent) in deionized water, KPF6 (or NaBF4
or LiTf2N, 2-4 equivalents) was added with stirring, which
continued for 1-2 h at room temperature. The precipitate was
filtered and washed with deionized water twice. Drying in a
-
-
vacuum oven gave the pure imidazolium PF6 (or BF4 or
* Corresponding author. E-mail: hwgibson@vt.edu; FAX: 540-231-8517;
Tel: 540-231-5902.
Tf2N-) salt.
10.1021/jp102370j  2010 American Chemical Society
Published on Web 05/06/2010

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N,N-Alkylene Bis(N′-Alkylimidazolium) Salts
J. Phys. Chem. B, Vol. 114, No. 21, 2010 7313
General Anion-Exchange Procedure B. Into a solution of
bromide salt (1 equivalent) in deionized water, AgTfO (or
AgNO3 or AgTFA, 2.06 equivalents) was added with stirring,
which continued for 3-4 h at 50 °C. Insoluble AgBr was
removed by filtration, and the water in the filtrate was removed
under vacuum. Further drying in a vacuum oven gave the
water (10 mL), colorless crystalline 3BF4 (0.91 g, 88%), mp
98.6-100.1 °C, was isolated. 1H NMR (400 MHz, acetone-d6,
23 °C): δ 0.94 (t, J ) 7, 6H), 1.36 (m, 4H), 1.92 (m, 4H), 4.34
(t, J ) 7, 4H), 4.97 (s, 4H), 7.74 (t, J ) 2, 2H), 7.80 (t, J ) 2,
2H), 9.02 (s, 2H). 13C NMR (100 MHz, acetone-d6, 23 °C): δ
13.7, 20.0, 32.4, 49.7, 50.5, 123.8, 124.1, 137.7. HRMS (ESI):
m/z 363.2363 ([M-BF4]+, calcd for C16H28N4BF4 363.2343,
error 5.5 ppm).
1,2-Bis[N-(N′-butylimidazolium)]ethane Bis(triflate) (3TfO).
Anion-exchange procedure B was used; from 3Br (0.30 g, 0.69
mmol) and AgTfO (0.364 g, 1.42 mmol) in deionized water
(15 mL), colorless solid 3TfO (0.384 g, 97%), mp 90.1-93.2
°C, resulted. 1H NMR (400 MHz, acetone-d6, 23 °C): δ 0.94 (t,
J ) 7, 6H), 1.38 (m, 4H), 1.92 (m, 4H), 4.35 (t, J ) 7, 4H),
5.03 (s, 4H), 7.82 (m, 4H), 9.20 (t, J ) 2, 2H). 13C NMR (100
MHz, acetone-d6, 23 °C): δ 13.6, 20.0, 32.5, 49.8, 50.5, 123.9,
124.1, 137.8. HRMS (ESI): m/z 425.1845 ([M-TfO]+, calcd
for C17H28F3N4O3S 425.1834, error 2.6 ppm).
-
corresponding TfO- (or NO3 or TFA-) salt.
1,2-Bis[N-(N′-methylimidazolium)]ethane Salts (1Br and
1PF6). A solution of 1-methylimidazole (2.49 g, 30 mmol) and
1,2-dibromoethane (2.82 g, 15 mmol) in MeCN (20 mL) was
refluxed for three days. The precipitate was filtered after cooling
and washed with tetrahydrofuran (THF) three times. Drying in
a vacuum oven gave colorless crystalline 1Br (4.54 g, 86%),
mp 231.1-233.8 °C (lit. mp ) 230-234 °C).4 1PF6 was
obtained by following general anion-exchange procedure A;
from 1Br (4.50 g), KPF6 (5.52 g, 30 mmol), and deionized water
(40 mL), colorless crystalline solid 1PF6 (5.26 g, 73% based
1
on 1,2-dibromoethane), mp 213.8-214.9 °C, was isolated. H
NMR (400 MHz, CD3CN, 23 °C): δ 3.85 (s, 6H), 4.62 (s, 4H),
7.33 (t, J ) 2, 2H), 7.42 (t, J ) 2, 2H), 8.40 (s, 2H). 13C NMR
(100 MHz, CD3CN, 23 °C): δ 37.2, 49.7, 123.5, 125.6, 137.6.
HRMS (ESI): m/z 337.1022 ([M-PF6]+, calcd for C10H16N4PF6
337.1017, error 1.5 ppm).
1,2-Bis[N-(N′-butylimidazolium)]ethane Bis(nitrate) (3NO3).
Anion-exchange procedure B was used; from 3Br (0.30 g, 0.69
mmol) and AgNO3 (0.234 g, 1.42 mmol) in deionizedwater (15
mL), 3NO3 was isolated as a colorless viscous liquid (0.277 g,
1
97%). H NMR (400 MHz, acetone-d6, 23 °C): δ 0.94 (t, J )
1,4-Bis[N-(N′-methylimidazolium)]butane Salts (2Br and
2PF6). The same procedure as for 1Br was used. 1-Methylimi-
dazole (2.46 g, 30 mmol) and 1,4-dibromobutane (3.24 g, 15
mmol) in MeCN (20 mL) produced colorless crystalline 2Br
(3.04 g, 53%), mp 131.7-133.8 °C. 1H NMR (400 MHz, D2O,
23 °C): δ 1.89 (m, 4H), 3.88 (s, 6H), 4.23 (m, 4H), 7.43 (t, J
7, 6H), 1.35 (m, 4H), 1.91 (m, 4H), 4.35 (t, J ) 7, 4H), 5.02 (s,
4H), 7.78 (t, J ) 2, 2H), 7.89 (t, J ) 2, 2H), 9.57 (t, J ) 2,
2H). 13C NMR (100 MHz, acetone-d6, 23 °C): δ 13.7, 20.0,
32.5, 49.8, 50.4, 123.8, 124.0, 138.6. HRMS (ESI): m/z
338.2203 ([M-NO3]+, calcd for C16H28N5O3 338.2192, error
3.3 ppm).
1,2-Bis[N-(N′-butylimidazolium)]ethane Bis(trifuoroace-
tate) (3TFA). Anion-exchange procedure B was used; from 3Br
(0.30 g, 0.69 mmol) and AgTFA (0.348 g, 1.42 mmol) in
deionized water (15 mL), yellow crystalline 3TFA (0.348 g,
91%), mp 131.7-134.6 °C, resulted. 1H NMR (400 MHz,
acetone-d6, 23 °C): δ 0.93 (t, J ) 7, 6H), 1.33 (m, 4H), 1.91
(m, 4H), 4.32 (t, J ) 7, 4H), 5.12 (s, 4H), 7.75 (t, J ) 2, 2H),
8.14 (t, J ) 2, 2H), 10.09 (t, J ) 2, 2H). 13C NMR (100 MHz,
acetone-d6, 23 °C): δ 13.6, 19.9, 32.5, 49.2, 50.3, 123.3, 124.3,
139.1. HRMS (ESI): m/z 389.2173 ([M-CF3CO2]+, calcd for
C18H28F3N4O2 389.2164, error 2.3 ppm).
1
) 2, 2H), 7.46 (t, J ) 2, 2H), 8.72 (s, 2H). The H NMR
spectrum was identical to that in the literature.34 2PF6 was
obtained by following the general anion-exchange procedure
A; from 2Br (2.10 g, 5.5 mmol), KPF6 (3.04 g, 16.5 mmol),
and deionized water (40 mL), colorless crystalline 2PF6 (2.59
g, 92% from 2Br), mp 111.7-114.2 °C, resulted. 1H NMR (400
MHz, acetone-d6, 23 °C): δ 2.05 (t, J ) 7, 4H), 4.01 (s, 6H),
4.41 (s, 4H), 7.66 (t, J ) 2, 2H), 7.68 (t, J ) 2, 2H), 8.86 (s,
2H). 13C NMR (100 MHz, acetone-d6, 23 °C): δ 26.7, 35.9,
49.0, 122.6, 124.2, 136.7. HRMS (ESI): m/z calcd 365.1344
([M-PF6]+, calcd for C12H20N4PF6 365.1330, error 3.8 ppm).
1,2-Bis[N-(N′-butylimidazolium)]ethane Dibromide (3Br).
The same procedure as for 1Br was used. From 1-butylimidazole
(6.21 g, 50 mmol) and 1,2-dibromoethane (4.69 g, 25 mmol)
in MeCN (25 mL), colorless crystalline 3Br (9.64 g, 88%), mp
166.5-168.2 °C, was produced. 1H NMR (400 MHz, D2O, 23
°C): δ 0.90 (t, J ) 7, 6H), 1.25 (m, 4H), 1.83 (m, 4H), 4.22 (t,
J ) 7, 4H), 4.80 (s, 4H), 7.52 (t, J ) 2, 2H), 7.62 (t, J ) 2,
1,3-Bis[N-(N′-butylimidazolium)]propane Bis(hexafluoro-
phosphate) (4PF6). The same procedure as for 1Br was used,
except the product could not be filtered because it was a viscous
oil. From 1-butylimidazole (3.73 g, 30 mmol) and 1,3-
dibromopropane (3.03 g, 15 mmol) in MeCN (20 mL), a light
brown viscous liquid 4Br (5.81 g, 86%) was isolated. 1H NMR
(400 MHz, D2O, 23 °C): δ 0.92 (t, J ) 7, 6H), 1.31 (m, 4H),
1.84 (m, 4H), 2.54 (m, 2H), 4.20 (t, J ) 7, 4H), 4.33 (t, J ) 7,
2H), 7.53 (m, 4H), 8.86 (s, 2H). Anion-exchange procedure A
was used for 4PF6; from 4Br (5.00 g, 11.1 mmol) and KPF6
(6.20 g, 33 mmol) in deionized water (40 mL) colorless
crystalline solid 4PF6 (6.25 g, 97% from 4Br), mp 96.4-97.7
°C, resulted.1H NMR (400 MHz, acetone-d6, 23 °C): δ 0.94 (t,
J ) 7, 6H), 1.38 (m, 4H), 1.94 (m, 4H), 2.74 (m, 2H), 4.36 (t,
1
2H), 8.85 (s, 2H). The H NMR result corresponds to the
literature spectrum (in CDCl3).5 HRMS (ESI): m/z 355.1508
([M-Br]+, calcd for C16H28N4Br 355.1497, error 3.1 ppm).
1,2-Bis[N-(N′-butylimidazolium)]ethane Bis(hexafluoro-
phosphate) (3PF6). Anion-exchange procedure A was used;
from 3Br (1.30 g, 3.0 mmol) and KPF6 (1.65 g, 9 mmol) in
deionized water (10 mL), colorless crystalline 3PF6 (1.54 g,
91%), mp 181.6-182.5 °C, was isolated. 1H NMR (400 MHz,
acetone-d6, 23 °C): δ 0.94 (t, J ) 7, 6H), 1.33 (m, 4H),
1.78-1.85 (m, 4H), 4.14 (m, 4H), 5.04 (s, 4H), 7.39 (m, 4H),
8.42 (s, 2H). 13C NMR (100 MHz, acetone-d6, 23 °C): δ 13.5,
19.8, 32.4, 49.9, 50.6, 123.6, 124.2, 137.5. HRMS (ESI): m/z
421.1966 ([M-PF6]+, calcd for C16H28N4PF6 421.1950, error
3.2 ppm).
J ) 7, 4H), 4.55 (t, J ) 7, 2H), 7.81 (m, 4H), 9.12 (s, 2H). 13
C
NMR (100 MHz, acetone-d6, 23 °C): δ 13.6, 20.0, 31.5, 32.6,
47.5, 50.4, 123.6, 123.9, 136.9. HRMS (ESI): m/z 435.2143
([M-PF6]+, calcd for C17H30N4PF6 435.2112, error 7.1 ppm).
1,4-Bis[N-(N′-butylimidazolium)]butane Bis(hexafluoro-
phosphate) (5PF6). The same procedure as for 4Br was used;
from 1-butylimidazole (3.73 g, 30 mmol) and 1,4-dibromobutane
(3.24 g, 15 mmol) in MeCN (20 mL), 5Br was obtained as a
light brown viscous liquid (5.71 g, 82%). Anion-exchange
procedure A was used for 5PF6; from 5Br (5.70 g, 2.3 mmol)
1,2-Bis[N-(N′-butylimidazolium)]ethane Bis(tetrafluorobo-
rate) (3BF4). Anion-exchange procedure A was used; from 3Br
(1.0 g, 2.3 mmol) and NaBF4 (0.55 g, 5 mmol) in deionized

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7314 J. Phys. Chem. B, Vol. 114, No. 21, 2010
Lee et al.
and KPF6 (5.56 g, 30 mmol) in deionized water (40 mL) 5PF6
was isolated as an off-white viscous liquid (7.05 g, 96% from
5Br) which solidified in a vacuum oven, mp 50.4-52.4 °C.1H
NMR (400 MHz, CD3CN, 23 °C): δ 0.94 (t, J ) 7, 6H), 1.34
(m, 4H), 1.78-1.85 (m, 8H), 2.74 (m, 4H), 4.14 (m, 8H), 7.38
(m, 4H), 8.42 (s, 2H). 13C NMR (100 MHz, CD3CN, 23 °C): δ
13.7, 20.0, 27.2, 32.5, 49.8, 50.4, 123.4, 123.6, 136.3. HRMS
(ESI): m/z 449.2304 ([M-PF6]+, calcd for C18H32N4PF6 449.2269,
error 7.9 ppm).
(m, J ) 7, 4H), 4.36 (t, J ) 8, 4H), 5.08 (s, 4H), 7.73 (s, 2H),
7.96 (s, 2H), 9.11 (s, 2H). 13C NMR (100 MHz, acetone-d6, 22
°C): δ 14.1, 23.0, 26.3, 30.4, 31.7, 49.8, 50.7, 123.6, 124.2,
137.3. HRMS (ESI): m/z 612.2155 ([M-Tf2N]+, calcd for
C22H36F6N5O4S2 612.2113, error 4.2 ppm).
1-Dodecylimidazole. To a solution of imidazole (6.81 g, 100
mmol) in NaOH (50%) solution (8.80 g, 110 mmol), 1-bromo-
dodecane (24.92 g, 100 mmol) and THF (30 mL) were added.
The mixture was refluxed for three days. After the mixture had
cooled, THF was removed on a rotoevaporator. The residue was
extracted with dichloromethane/water three times. The combined
organic layer was washed with water and then dried over
Na2SO4. The drying agent was filtered off, and the filtrate
solution was concentrated. Column chromatography through a
short silica-gel column with THF gave a clear yellow oil (20.51
g, 87%). 1H NMR (400 MHz, CDCl3, 22 °C): δ 0.88 (t, J ) 7,
3H), 1.29 (m, 18H), 1.76 (m, J ) 7, 2H), 3.91 (t, J ) 7, 4H),
6.90 (s, 1H), 7.05 (s, 1H), 7.45 (s, 1H). 13C NMR (100 MHz,
CDCl3, 22 °C): δ 14.0, 22.6, 26.4, 29.0, 29.2, 29.3, 29.4, 29.5,
1-Hexylimidazole. To a solution of imidazole (1.361 g, 20
mmol) in NaOH (50%) solution (0.19 g, 24 mmol), 1-bromo-
hexane (3.31 g, 20 mmol) and THF (15 mL) were added. The
mixture was refluxed for three days. After the mixture had
cooled to room temperature, THF was removed by a rotoevapo-
rator. The residue was extracted with dichloromethane/water
three times. The combined organic layer was washed with water
and then dried over Na2SO4. The drying agent was filtered off,
and the filtrate solution was concentrated. Drying in a vacuum
oven gave a yellow oily product, 2.64 g (87%). 1H NMR (400
MHz, CDCl3, 22 °C): δ 0.88 (t, J ) 7, 3H), 1.29 (s (br), 6H),
1.77 (m, 2H), 3.92 (t, J ) 7, 2H), 6.90 (s, 1H), 7.05 (s, 1H),
1
31.0, 31.8, 46.9, 118.6, 129.2, 136.9. The H NMR spectrum
was exactly identical to the literature result.35
1
1,2-Bis[N-(N′-dodecylimidazolium)]ethane Salts (7Br, 7PF6
and 7TFSI). A solution of 1-dodecylimidazole (3.78 g, 16
mmol) and 1,2-dibromoethane (1.50 g, 8 mmol) in MeCN (15
mL) was refluxed for three days. After the mixture had cooled,
the volatile materials were removed by vacuum. The residue
was dispersed in THF (30 mL), and the insoluble bromide salt
was filtered. The filter cake was washed with THF three times.
Drying in a vacuum oven gave a colorless crystalline solid 7Br
(4.54 g, 86%), mp 249.3-252.9 °C (dec). 1H NMR (400 MHz,
DMSO-d6, 23 °C): δ 0.86 (t, J ) 7, 6H), 1.25 (m (br), 36H),
1.74 (t, J ) 7, 4H), 4.13 (t, J ) 7, 4H), 4.71 (s, 4H), 7.69 (s,
2H), 7.85 (s, 2H), 9.15 (s, 2H). HRMS (ESI): m/z 499.4739
([M-H-2Br]+, calcd for C32H59N4 499.4740, error 0.2 ppm),
m/z 250.7419 ([M-2Br+H]2+, calcd for half mass of C32H60N4
250.7443, error 9.4 ppm). 7PF6 was obtained by following
general anion-exchange procedure A, except that the mixture
was stirred at 50 °C for 24 h; from 7Br (2.00 g, 3.03 mmol),
KPF6 (1.56 g, 8.5 mmol), and deionized water (60 mL), colorless
solid 7PF6 (2.33 g, 97%), mp 217.4-223.0 °C (dec.), was
7.46 (s, 1H). The H NMR spectrum was exactly identical to
the literature result.35
1,2-Bis[N-(N′-hexylimidazolium)]ethane Dibromide (6Br).
The same procedure as for 1Br was used. From 1-hexylimida-
zole (1.52 g, 10 mmol) and 1,2-dibromoethane (0.940 g, 5
mmol) in MeCN (10 mL), colorless crystalline 6Br (2.14 g,
1
87%) was isolated, mp 225.5-229.3 °C (dec.). H NMR (400
MHz, D2O, 23 °C): δ 0.71 (t, J ) 7, 6H), 1.12 (m, 12H), 1.68
(m, J ) 7, 4H), 4.04 (t, J ) 8, 4H), 4.79 (overlapped with
solvent residual peak), 7.34 (s, 2H), 7.45 (s, 2H), 8.68 (s, 2H).
13C NMR (100 MHz, D2O, 23 °C): δ 13.4, 21.9, 25.1, 29.0,
30.4, 49.0, 50.2, 123.8, 124.0, 136.0. HRMS (ESI): m/z
411.2132 ([M-Br]+, calcd for C20H36N4Br 411.2123, error 2.2
ppm).
1,2-Bis[N-(N′-hexylimidazolium)]ethane Bis(tetrafluorobo-
rate) (6BF4). Anion-exchange procedure A was used; the
precipitated oil was washed with water five times and then dried
in a vacuum oven. From 6Br (1.0 g, 2.0 mmol) and NaBF4
(0.66 g, 6 mmol) in deionized water (10 mL), 6BF4 was isolated
1
isolated. H NMR (400 MHz, acetone-d6, 23 °C): δ 0.87 (t, J
1
as a colorless viscous material (0.80 g, 78%). H NMR (400
) 7, 6H), 1.27-1.36 (m (br), 36H), 1.95 (t, J ) 7, 4H), 4.35
(t, J ) 7, 4H), 5.03 (s, 4H), 7.73 (t, J ) 2, 2H), 7.85 (t, J ) 2,
2H), 9.03 (s, 2H). 13C NMR (100 MHz, acetone-d6, 23 °C): δ
14.3, 23.3, 26.8, 29.7, 30.0, 30.1, 30.30, 30.35, 30.36, 32.6, 50.1,
50.9, 123.8, 124.4, 137.4. HRMS (ESI): m/z 645.4465
([M-PF6]+, calcd for C32H60N4PF6 645.4460, error 0.8 ppm).
For 7TFSI, the same procedure as for 7PF6 was used. From
7Br (2.70 g, 6.4 mmol), LiTf2N (4.32 g, 15 mmol), and
deionized water (60 mL), crystalline 7TFSI (3.89 g, 97% from
MHz, acetone-d6, 23 °C): δ 0.87 (t, J ) 7, 6H), 1.33 (m, 12H),
1.97 (m, J ) 7, 4H), 4.33 (t, J ) 8, 4H), 4.94 (s, 4H), 7.72 (s,
2H), 7.76 (s, 2H), 9.00 (s, 2H). 13C NMR (100 MHz, acetone-
d6, 22 °C): δ 14.1, 22.9, 26.3, 30.3, 31.8, 49.9, 50.8, 123.6,
124.1, 137.1. HRMS (ESI): m/z 419.2991 ([M-BF4]+, calcd
for C20H36N4BF4 419.2969, error 5.2 ppm).
1,2-Bis[N-(N′-hexylimidazolium)]ethane Bis(hexafluoro-
phosphate) (6PF6). Anion-exchange procedure A was used;
from 6Br (0.630 g, 1.3 mmol) and KPF6 (0.56 g, 3.0 mmol) in
deionized water (5 mL), colorless crystalline 6PF6 (0.810 g,
91%), mp 197.5-198.8 °C, was isolated. 1H NMR (400 MHz,
acetone-d6, 22 °C): δ 0.87 (t, J ) 7, 6H), 1.33 (m, 12H), 1.94
(m, J ) 7, 4H), 4.34 (t, J ) 8, 4H), 5.01 (s, 4H), 7.74 (s, 2H),
7.87 (s, 2H), 9.04 (s, 2H). 13C NMR (100 MHz, acetone-d6, 22
°C): δ 14.1, 23.0, 26.4, 30.5, 31.8, 49.9, 50.8, 123.7, 124.3,
137.3. HRMS (ESI): m/z 477.2603 ([M-PF6]+, calcd for
C20H36N4PF6 477.2582, error 4.4 ppm).
1
7Br), mp 45.5-47.7 °C, was isolated. H NMR (400 MHz,
CDCl3, 23 °C): δ 0.87 (t, J ) 7, 6H), 1.25-1.31 (m (br), 36H),
1.86 (t, J ) 7, 4H), 4.15 (t, J ) 7, 4H), 4.74 (s, 4H), 7.33 (t, J
) 2, 2H), 7.60 (t, J ) 2, 2H), 8.84 (s, 2H). 13C NMR (100
MHz, CDCl3, 23 °C): δ 14.2, 22.8, 26.3, 29.0, 29.4, 29.5, 29.6,
29.7, 32.0, 48.5, 50.7, 123.0, 123.3, 136.0. HRMS (ESI): m/z
780.3997 ([M-Tf2N]+, calcd for C34H60F6N5O4S2 780.3985,
error 0.8 ppm).
1,2-Bis[N-(N′-hexylimidazolium)]ethane Bis[bis(trifluo-
romethanesulfonyl)imide] (6TFSI). The same procedure as for
6BF4 was used. From 6Br (0.492 g, 1.0 mmol) and LiTf2N (0.86
g, 3.0 mmol) in deionized water (5 mL), 6TFSI was isolated
as a colorless viscous oil (0.810 g, 91%). 1H NMR (400 MHz,
acetone-d6, 22 °C): δ 0.87 (t, J ) 7, 6H), 1.32 (m, 12H), 1.97
Results and Discussion
Various alkylene bis[N-(N′-alkylimidazolium)] salts were
synthesized in two steps (Scheme 1): the coupling reaction of
the 1-alkylimidazole (2 mol equiv) with a dibromoalkane (1
mol equiv) and anion exchange in water. Noncommercial

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N,N-Alkylene Bis(N′-Alkylimidazolium) Salts
J. Phys. Chem. B, Vol. 114, No. 21, 2010 7315
SCHEME 1: Synthesis of Alkylene bis[N-(N′-
alkylimidazolium)] Salts
results of the quaternizaton and anion-exchange reactions for
the alkylene bis[N-(N′-alkylimidazolium)] salts are summarized
in Table 1.
1H NMR Studies. In our 1H NMR studies of the new bis[N-
(N′-alkylimidazolium)] salts, noteworthy chemical-shift changes
were observed with different solvents and counterions. The 1H
NMR spectra of the bromide salt of 3 (3Br) in D2O and acetone-
d6 are shown in Figure 1. The imidazolium protons appear at
different positions in these solvents: δ 7.5, 7.6, and 8.9 in D2O
but δ 7.7, 8.4, and 10.4 in acetone-d6. The peaks of H4 and H5
are close to each other in D2O, but they are widely separated in
acetone-d6. These chemical-shift changes are due to the solvent
properties: the better solvation of the ions in D2O versus acetone-
d6. This higher solvation level isolates the bromide anions in
solution, and the positive charge on the resultant free cation is
delocalized. As a result of the delocalization of positive charge,
the chemical shifts of protons H4 and H5 are close to each other.
However, poorer ion solvation in acetone-d6 results in relatively
tight ion pairs; therefore, the positive charge tends to localize
on a specific nitrogen atom. The localized positive charge leads
to different environments for protons H4 and H5, and hence,
their peaks occur at different positions.
The chemical-shift change of proton H2 is quite large in
different media: δ 8.9 (D2O) versus 10.4 (acetone-d6). This can
be explained by the different solvating powers of the solvents.
In D2O, the imidazolium cations and bromide anions are well
solvated, and hydrogen bonding of proton H2 with the bromide
anion is decreased, thus moving its signal to higher field.
However, hydrogen bonding occurs in the less polar acetone-
d6, in which the ions are rather tightly paired; therefore, in
acetone-d6, the proton H2 appears downfield. The chemical-shift
changes of bromide salts of (mono)imidazolium ILs in different
solvents were reported previously,36 and those results are
consistent with our study of bis(imidazolium) bromide 3Br.
TABLE 1: Quaternization and Ion-Exchange Reactions for
Alkylene Bis[N-(N′-alkylimidazolium)] Salts
quaternization yield
ion exchange
R
n
(%, X ) Br)
X
yield (%) appearance
Me
2
4
86
PF6
PF6
92
92
Crst. Solid
Crst. Solid
53a
n-Bu
2
2
2
2
2
3
4
88
PF6
BF4
TfO
NO3
CF3CO2
PF6
91
88
97
96
97
96
96
Crst. Solid
Crst. Solid
Crst. Solid
RTILb
Yellow Solid
Crst. Solid
Sticky Solid
86
82
PF6
n-Hex
2
2
2
87
PF6
BF4
Tf2N
91
88
90
Solid
RTILb
RTILb
n-Dodecyl
2
2
82
PF6
Tf2N
97
89
Crst. Solid
Crst. Solid
a The yield was lower than that for other reactions because of a
higher solubility of 2Br in MeCN during the workup procedure.
b RTIL ) room temperature IL.
1-alkylimidazoles were first prepared before the quaternization;
1-hexylimidazole and 1-dodecylimidazole were prepared from
imidazole and corresponding bromoalkanes with NaOH in high
yields.10
There is a counteranion effect on the chemical shifts of
the imidazolium protons as shown in Figure 2. The hydrogen-
bonding abilities of the different anions influence the NMR
chemical shifts. In this series, the bromide salt (3Br) showed
Most of the alkylene N-(N′-alkylimidazolium) bromide salts
were very hygroscopic; they are either crystalline solids or waxy
materials. The counteranion exchange was performed in aqueous
conditions with NaBF4, KPF6, or (CF3SO2)2NLi (denoted Tf2NLi
or LiTFSI), and the resulting products (BF4-, PF6-, and Tf2N-
-
-
the strongest hydrogen bonds, and the PF6 and BF4 salts
(3PF6 and 3BF4) showed the least hydrogen bonding in
acetone-d6. The chemical shift of H2 (a) increased with
-
increasing anion basicity and hydrogen bonding ability (PF6
-
-
≈ BF4 < TfO- < NO3 , Br-).37 The different ion-pairing
strengths38-40 of the anions were also observed from the
chemical-shift changes of H4 and H5. The most ion-paired
salt (3Br) showed the largest difference in the chemical shifts
of these protons, whereas the least ion-paired salts gave more
closely spaced (3TFA, 3NO3, 3BF4, 3PF6) or merged peaks
(3TfO). These changes are the result of changes in the charge
-
salts) precipitated, as solids or liquids. The BF4-, PF6 , and
Tf2N- salts are not hygroscopic because of their hydrophobic
anions. The triflate (TfO-), nitrate, and trifluoroacetate (TFA)
salts were prepared from the corresponding Ag salts in water;
AgBr precipitation provided the driving force to complete the
ion-exchange reactions. The triflate, nitrate, and TFA salts are
water-soluble and as hygroscopic as the bromide salts. The
1
Figure 1. Partial 400 MHz H NMR spectra of 3Br in D2O (upper) and acetone-d6 (lower).