Convenient synthesis of imidazolium based dicationic ionic liquids

Article abstract of DOI:10.1007/s11164-015-2388-4

A series of imidazolium based Dicationic Ionic Liquids (ILs) have been prepared and characterized successfully. Derivatives are prepared keeping the alkyl chain of alkylimidazole as methyl, ethyl, and propyl groups and the linker dibromoalkane is varied as an ethyl, propyl and butyl group. The variations of anions like Br^?, BF₄ ^? and PF₆ ^? impact their morphological appearance like amorphous powder to low melting white solids or a hygroscopic nature. Refluxing the alkylimidazoles and dibromoalkanes in toluene for 24?h and an easy workup procedure makes this methodology more time saving for detailed exploration of Dicationic ILs. The formation of dicationic ILs with their anions like Br^?, BF₄ ^?, PF₆ ^? has been confirmed with ¹H, ¹³C, ³¹P and ¹⁹F NMR spectroscopy as well as the High Resolution Mass Spectroscopy Electron Impact technique.

Full text of DOI:10.1007/s11164-015-2388-4

Res Chem Intermed
DOI 10.1007/s11164-015-2388-4
Convenient synthesis of imidazolium based dicationic
ionic liquids
Sainath Babaji Aher1 Pundlik Rambhau Bhagat1
•
Received: 28 October 2015 / Accepted: 7 December 2015
Ó Springer Science+Business Media Dordrecht 2016
Abstract A series of imidazolium based Dicationic Ionic Liquids (ILs) have been
prepared and characterized successfully. Derivatives are prepared keeping the alkyl
chain of alkylimidazole as methyl, ethyl, and propyl groups and the linker dibro-
moalkane is varied as an ethyl, propyl and butyl group. The variations of anions like
Br-, BF-4 and PF-6 impact their morphological appearance like amorphous powder
to low melting white solids or a hygroscopic nature. Refluxing the alkylimidazoles
and dibromoalkanes in toluene for 24 h and an easy workup procedure makes this
methodology more time saving for detailed exploration of Dicationic ILs. The
formation of dicationic ILs with their anions like Br-, BF4-, PF6- has been confirmed
1
with H, 13C, 31P and 19F NMR spectroscopy as well as the High Resolution Mass
Spectroscopy Electron Impact technique.
Keywords Alkylimidazoles Á Bromide Á Dibromoalkanes Á Dicationic ILs Á
Hexafluorophosphate Á Tetrafluoroborate
Introduction
Dicationic ILs have proved to be vital in the fields of catalysis [1], solar cells [2],
lubricants [3], and several other fields. However, they are also called organic ionic
plastic crystals due to their solid state electrolyte property [4, 5]. This typical
property makes them applicable in fuel cells [6] and batteries [7]. Dicationic ILs are
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2388-4)
contains supplementary material, which is available to authorized users.
&
Pundlik Rambhau Bhagat
drprbhagat111@gmail.com
1
Organic Chemistry Division, School of Advanced Sciences, VIT University, Vellore,
Tamilnadu 632014, India
123

S. B. Aher, P. R. Bhagat
studied in detail with improved photovoltaic properties for dye-sensitized solar cells
[8, 9]. The property of being sufficiently stable at melting points above 200 °C gives
them additional superior lubrication properties, which enable them to be used in
high temperature lubricants [10]. They have shown promising results in separating
aromatic fractions from extracts, thus, replacing aromatic solvents [11]. Moreover,
they have been successfully studied in applications of zeolite beta, which produced
promising results [12–14].
Though dicationic ILs are more advantageous than monocationic ILs, still they
are not explored in detail with various substitutions. These compounds can be more
appealing in the study of an extensive range of applications owing to the presence of
a dication and two anions. The chain length is of such importance that it can
influence conductivity [15], lipophilicity [16], and morphological appearance in
dicationic ILs [17].
Many researchers reported synthesis of imidazolium based dicationic ionic
liquids by various methods with their advantages and limitations. After observing
various methods, the major limitation is time consumed in synthesis and tedious
work up procedure [18, 19]. So here we reported a modified system for synthesis of
N-alkylimidazoles and their corresponding ILs.
Experimental
1H, 13C, 31P, 19F NMR spectra for all synthesized compound were recorded in D2O,
CDCl3 and DMSO-d6 on a Bruker Spectrophotometer operating at 400 MHz. Melting
points were recorded on a GUNA melting point apparatus and are uncorrected.
Materials
All chemicals used were of reagent grade and used without further purification as
otherwise specified. Imidazole, 1-methylimidazole, sodium tetrafluoroborate, and
potassium hexafluorophosphate were purchased from Sigma-Aldrich and used as
received. Sodium hydride (60 % suspension in paraffin oil), activated charcoal
purchased from SD Fine chemicals, 1-bromoethane, 1-bromopropane, 1,2-dibro-
moethane, 1,3-dibromopropane, 1,4-dibromobutane, Toluene (Specially dried),
Tetrahydrofuran (Specially dried), Acetonitrile (Specially dried), Dichlorometha-
ne (DCM), and sodium sulphate anhydrous were purchased from Avra chemicals.
N-Functionalized imidazoles (Scheme 1)
A THF solution of imidazole (1 equivalent) is added to a suspension of NaH (1.1
equivalents) (60 % in paraffin oil) previously stirred in n-hexane for 15 min. The
imidazole solution is added dropwise over a period of 25–30 min at 5–10 °C to
avoid vigorous liberation of H2 gas. Once H2 gas evolution ceased, the suspension
of imidazole sodium is stirred at RT for 2–3 h to favor complete formation of NaIm
salt. To this suspension, corresponding R–Br (0.90 equivalents), was added
dropwise over a period of 30 min and further stirred for 3 h or longer to achieve
123

Convenient synthesis of imidazolium based dicationic…
Scheme 1 Synthesis of N-functionalized imidazoles
uniform mixing indicated by vertex formation. The thoroughly stirred reaction
mixture then was refluxed overnight at 60–65 °C. The reaction mass was then
allowed to cool and filtered to remove NaBr salt. THF is removed under a rotary
evaporator and DCM is added to the resultant brownish liquid followed by the
addition of activated charcoal and anhydrous Na2SO4, stirred for 2–3 h and filtered
over Celite. A Celite bed was washed with DCM and DCM removed under vacuum
leading to a pale yellowish liquid. The NMR spectra checked in CDCl3 and matches
with literature reports [20].
Dicationic ionic liquids (Scheme 2)
A solution of N-alkylimidazole (10 mmol, 2 equivalents) and Dibromoalkane
(5 mmol, 1 equivalent) was refluxed in toluene (20 mL) for 24–30 h. The reaction
mixture was allowed to cool and toluene was decanted leaving a sticky solid behind.
The sticky solid was washed three times with dry THF and finally with diethyl ether
once. Solvent was removed under vacuum to get a white amorphous hygroscopic
powder (88–94 % yield). NMR spectra were recorded in D2O and/or DMSO-d6.
General anion exchange procedure (metathesis) (Scheme 3)
Br to BF4 exchange
Dicationic ILs are dissolved in deionized water and aq. NaBF4 (2.5 equivalents) was
added dropwise under stirring at RT. The reaction mixture was stirred overnight and
water was removed under vacuum. To the resulting solid, acetonitrile was added and
filtered to remove NaBr, acetonitrile was removed under reduced pressure to get a
fine powder and washed once with diethyl ether. NMR spectra were recorded in D2O.
Br to PF6 exchange
Ionic liquids dissolved in methanol and methanolic KPF6 (2.5 equivalents) were
added dropwise resulting in off-white powdered suspension. The resultant
Scheme 2 Synthesis of dicationic ionic liquids
123

S. B. Aher, P. R. Bhagat
Scheme 3 General anion exchange procedure
suspension was stirred 3–4 h, filtered, and the solid washed with methanol to
remove unreacted KPF6. NMR spectra were recorded in DMSO-d6.
Results and discussion
N-Functionalized imidazoles or N-alkylimidazoles (R = C2H5, C3H7) were suc-
cessfully prepared as per the procedure mentioned above. As the reaction of NaIm
and Bromoalkane is a solid–liquid reaction, slow addition and stirring for enough
time at RT before heating achieves stable mixing indicated by vertex formation.
Quick addition and subsequent heating may lead toa risky situation like hot spots or
runaway reaction conditions [21, 22].
The dry solvents like THF and DCM utilized in this reaction can be recycled and
reused easily after drying. The alkylimidazoles possess very low vapor pressure and,
can be easily stored at RT with properly packed conditions (Table 1).
Dicationic ILs can be easily prepared by a modified procedure. Existing
procedures involve refluxing of N-alkylimidazole and Dibromoalkane in 1:0.5
equivalents using acetonitrile as a solvent. This needs 3 days reflux, which is time
consuming and the subsequent procedure has the same disadvantage about the time
required [23]. Also, in the case of acetonitrile, the solvent need to be removed and
further washings done with THF. In the current development, we have replaced
acetonitrile by toluene and refluxing for 24–30 h leads to time saving. As ILs are
insoluble in toluene, it develops an insoluble sticky mass inside the flask. We can
easily decant toluene avoiding compound loss where starting materials will go away
with toluene. Further washing the sticky mass 3 times THF and removing the
solvent gives pure IL as an off-white powder (Table 2).
Table 1 Yields of N-alkylimidazoles
S. no.
Product (N-Alkylimidazole)
MW
Eq. of R–X (L. R.)
% Yield
1.
2.
1-Ethylimidazole
1-Propylimidazole
96.13
110.16
0.90
0.90
90
86
L.R. Limiting reagent
123

Convenient synthesis of imidazolium based dicationic…
Table 2 Quaternization and metathesis of Bis (alkylimidazolium) salts
S. no.
Code
R
n
Quaternization
yield (%, X = Br)
Anion exchange
X
% Yield
1.
S1
S2
S3
S4
S5
S6
S7
S8
S9
CH3
2
3
4
2
3
4
2
3
4
90
94
92
88
86
88
86
86
84
BF4
PF6
BF4
PF6
BF4
PF6
BF4
PF6
BF4
PF6
BF4
PF6
BF4
PF6
BF4
PF6
BF4
PF6
90
94
92
95
90
92
93
94
95
96
92
94
93
95
92
95
94
95
2.
3.
4.
5.
6.
7.
C2H5
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
C3H7
1H NMR studies
1H NMR spectra of prepared alkylimidazoles are recorded in CDCl3 and matched
with the literature. 1-ethylimidazole: 1H NMR (400 MHz, CDCl3, TMS), d:
7.38 ppm (singlet, 1H, C2–H), 6.94 (singlet, 1H, C4–H), 6.83 (singlet, 1H, C5–H),
1
3.89 (quartet, 2H, –CH2), 1.33 (triplet, 3H, –CH3). 1-propylimidazole: H NMR
(400 MHz, CDCl3, TMS), d: 7.456 ppm (singlet, 1H, C2–H), 7.044 (singlet, 1H,
C4–H), 6.90 (singlet, 1H, C5–H), 3.89 (triplet, 2H, –CH2), 1.807 (multiplet, 2H,
–CH2), 0.917 (triplet, 3H, –CH3)
In the case of dicationic ILs, the chemical shift values are relatively varying in
different media: d 8.8 (D2O) versus 9.4 (DMSO-d6) for C2–H in the case of 3,30-
(ethane-1,2-diyl)bis(1-ethyl-1H-imidazol-3-ium)dibromide. This variation is mainly
attributed to solvating properties of the solvents. In D2O, due to its protic nature, the
imidazolium cations as well as the bromide anions are well solvated. This ability of
D2O results in decrease of H bonding of proton H2 with Br- anion and thereby the
signal is shifted to a higher field. At the same time, signals for proton H4 and H5
appear to be closely coupled. However, in the case of DMSO-d6, whose aprotic
nature doesn’t show any sort of H-bonding, the proton H2 appears to be more
downfield i.e., at d 9.4 ppm. It also shows well defined splitting of signals associated
with protons H4 and H5 as observed in Fig. 1.
123

S. B. Aher, P. R. Bhagat
Fig. 1 Comparison of d values of IL protons in D2O and DMSO-d6 at 400 MHz and 25 °C
1,2-Bis[N-(N0-methylimidazolium)]ethane 2Br- (S1)
1
White hygroscopic powder, Quaternization yield 90 %. mp 195–197 °C. H NMR
(400 MHz, D2O, 25 °C): d 3.90 (s, 6H), 4.77 (s 4H), 7.45 (s, 2H), 7.51 (s, 2H). 13
C
NMR (100 MHz, 25 °C): 36.08, 48.78, 122.16, 124.59, 136.06. 2BF24 : H NMR
(400 MHz, D2O, 25 °C): d 3.962 (s, 6H), 4.816 (s, 4H), 7.487 (s, 2H), 7.571 (, 2H),
8.775 (s, 2H). 13C NMR (100 MHz, 25 °C): d 35.99, 48.77, 122.14, 124.64, 136.62.
1
1
19F NMR (376 MHz, 25 °C): d -150.49, -150.44. 2PF-6 : H NMR (400 MHz,
DMSO-d6, 25 °C): d 3.829 (s, 6H), 4.636 (s, 4H), 7.558 (s, 2H), 7.699 (s, 2H), 8.947
(s, 2H). 13C NMR (100 MHz, DMSO-d6, 25 °C): d 35.92, 48.42, 122.32, 123.99,
137.05. 31P NMR (162 MHz, 25 °C): d -152.99, -148.60, -144.21, -139.82,
-135.43. 19F NMR (376 MHz, 25 °C): d -71.07, -69.19. HR–MS (EI) m/
z molecular weight: 482.06, found: 482.0741.
1,3-Bis[N-(N0-methylimidazolium)]propane 2Br- (S2)
1
White hygroscopic powder, Quaternization yield 94 %. mp 201–204 °C. H NMR
(400 MHz, DMSO-d6, 25 °C): d 2.4 (t, 2H), 3.87 (s, 6H), 4.273 (t, 4H), 7.76 (s, 2H),
7.87 (s, 2H), 9.41 (s, 2H). 13C NMR (100 MHz, 25 °C): d 29.48, 35.82, 45.61,
122.18, 123.66, 136.96. 2BF24 : H NMR (400 MHz, D2O, 25 °C): d 2.52 (m, 2H),
1
3.9 (s, 6H), 4.32 (t, 4H), 7.47 (s, 2H), 7.51 (s, 2H), 8.79 (s, 2H). 13C NMR
(100 MHz, D2O, 25 °C): d 29.71, 35.79, 46.24, 122.08, 123.91, 136.18. 19F NMR
1
(376 MHz, 25 °C): d -150.45, -150.55. 2PF-6 : H NMR (400 MHz, DMSO-d6,
25 °C) 2.41 (m, 2H), 3.874 (s, 6H), 4.27 (t, 4H), 7.76 (s, 2H), 7.87(s, 2H), 9.411 (s,
2H). 13C NMR (100 MHz, DMSO-d6, 25 °C): d 29.48, 35.82, 45.61, 122.18,
123

Convenient synthesis of imidazolium based dicationic…
123.66, 136.96. 31P NMR (162 MHz, DMSO-d6 25 °C): d -157.20, -152.81,
-148.42, -144.03, -139.64, -135.25, -130.86 19F NMR (376 MHz, DMSO-d6,
25 °C): d -71.02, -69.14.
1,4-Bis[N-(N0-methylimidazolium)]butane 2Br- (S3)
1
White hygroscopic powder, Quaternization yield 88 %. mp 205–206 °C. H NMR
(400 MHz, DMSO-d6, 25 °C): d 1.812 (s, 4H), 3.8 (s, 6H), 4.26 (s, 4H), 7.75 (s,
2H), 7.84 (s, 2H), 9.3 (s, 2H). 13C NMR (100 MHz, 25 °C): d 26.00, 35.82, 47.91,
1
122.22, 123.59, 136.60. 2BF-4 : H NMR (400 MHz, D2O, 25 °C): d 1.95 (t, 4H),
3.94 (s, 6H), 4.29 (s, 4H), 7.48 (s, 2H), 7.50 (s, 2H), 8.71 (s, 2H). 13C NMR
(100 MHz, 25 °C): d 26.19, 35.70, 48.77, 122.11, 123.77, and 136.01. 19F NMR
(376 MHz, 25 °C): d -150.53, -150.48. 2PF-6 : H NMR (400 MHz, DMSO-d6,
1
25 °C): d 1.84 (t, 2H), 3.91 (s, 6H), 4.3 (s, 4H), 7.78 (s, 2H), 7.87 (s, 2H), 9.345 (s,
2H). 13C NMR (100 MHz, 25 °C): d 25.96, 35.78, 47.87, 12.18, 123.55, 136.55. 31
P
NMR (162 MHz, 25 °C): d -157.37, -152.98, -148.59, -144.20, -139.81,
-135.42, -131.03. 19F NMR (376 MHz, 25 °C): d -71.08, -69.19.
1,2-Bis[N-(N0-ethylimidazolium)]ethane 2Br- (S4)
1
White amorphous powders, Quaternization yield 86 %. mp 205–207 °C. H NMR
(400 MHz, DMSO-d6, 25 °C): d 1.4 (t, 6H), 4.2 (m, 4H), 4.73 (s, 4H), 7.69 (s, 2H),
7.84 (s, 2H), 9.28 (s, 2H). 13C NMR (100 MHz, 25 °C): d 14.98, 44.44, 48.52,
1
122.50, 122.58, 136.24. 2BF-4 : H NMR (400 MHz, D2O, 25 °C): d 1.51 (t, 6H),
4.26 (m, 4H), 4.78 (s, 4H), 7.4 (s, 2H), 7.63 (s, 2H), 8.81 (s, 2H). 13C NMR
(100 MHz, D2O, 25 °C): d 14.37, 45.23, 48.81, 122.20, 123.16, and 135.56. 19F
NMR (376 MHz, 25 °C): d -150.45, -150.40. 2PF-6 : 1H NMR (400 MHz, DMSO-
d6, 25 °C): d 1.39 (t, 6H), 4.16 (m, 4H), 4.64 (s, 4H), 7.59 (s, 2H), 7.81 (s, 2H), 9.05
(s, 2H). 13C NMR (100 MHz, 25 °C): d 14.95, 44.41, 48.49, 122.48, 122.55, 136.22.
31P NMR (162 MHz, 25 °C): d -57.38, -152.98, -148.60, -144.20, -139.81,
-135.42, -131.03. 19F NMR (376 MHz, 25 °C): d -71.07, -69.18. HR–MS (EI)
m/z, molecular weight: 510.10, found: 510.24.
1,3-Bis[N-(N0-ethylimidazolium)]propane 2Br- (S5)
1
White amorphous powder, Quaternization yield 88 %. mp 205–207 °C. H NMR
(400 MHz, D2O, 25 °C): d 1.50 (t, 6H), 2.54 (m, 2H), 4.24 (m, 4H), 4.33 (t, 4H),
7.52, (s, 2H), 7.54 (s, 2H), 8.864 (s, 2H). 13C NMR (100 MHz, 25 °C): d 14.45,
1
26.28, 44.97, 48.81, 122.23, 122.29, 134.99. 2BF-4 : H NMR (400 MHz, D2O,
25 °C): d 1.513 (t, 6H), 2.549 (m, 2H), 4.22 (t, 4H), 4.32 (t, 4H), 7.53 (s, 2H), 7.55
(s, 2H), 8.872 (s, 2H). 13C NMR (100 MHz, 25 °C): d 12.19, 24.06, 42.79, 46.67,
120.07, 120.08, and 132.80. 19F NMR (376 MHz, 25 °C): d -150.73, -150.68.
2PF-6 : 1H NMR (400 MHz, DMSO-d6, 25 °C): d 1.434 (t, 6H), 2.40 (m, 2H), 4.202
(m, 8H), 7.756 (s, 2H), 7.82 (s, 2H), 9.16 (s, 2H). 13C NMR (100 MHz, 25 °C): d
14.88, 26.02, 44.23, 48.07, 122.18, 122.31, 135.68. 31P NMR (162 MHz, 25 °C): d
123

S. B. Aher, P. R. Bhagat
-157.36, -152.98. -148.59, -144.20, -139.81, -135.41, -131.03. 19F NMR
(376 MHz, 25 °C): d -71.08, -69.19.
1,4-Bis[N-(N0-ethylimidazolium)]butane 2Br- (S6)
1
White amorphous powder, Quaternization yield 88 %. mp 206–208 °C. H NMR
(400 MHz, D2O, 25 °C): d 0.818 (t, 6H), 1.76 (m, 4H), 4.11 (t, 4H), 4.71 (s, 4H),
7.67 (s, 2H), 7.80 (s, 2H), 9.12 (s, 2H). 13C NMR (100 MHz, 25 °C): d 10.29, 22.65,
1
48.42, 50.50, 122.50, 122.84, 136.51. 2BF-4 : H NMR (400 MHz, D2O, 25 °C): d
0.82 (t, 6H), 1.73 (m, 4H), 4.14 (t, 4H), 4.72 (s, 4H), 7.65 (s, 2H), 7.67 (s, 2H), 9.09
(s, 2H). 13C NMR (100 MHz, 25 °C): d 14.40, 29.66, 45.05, 46.44, 122.21, 122.46,
and 135.19. 19F NMR (376 MHz, 25 °C): d -150.75, -150.70. 2PF-6 : H NMR
1
(400 MHz, DMSO-d6, 25 °C): d 1.42 (t, 6H), 1.78 (s, 4H), 4.17 (m, 8H), 7.74 (s,
2H), 7.79 (s, 2H), 9.13 (s, 2H). 13C NMR (100 MHz, 25 °C): d 14.88, 29.27, 44.29,
45.87, 122.25, 122.31, 135.89. 31P NMR (162 MHz, 25 °C): d -157.36, -152.98,
-148.59, -144.19, -139.81, -135.41, -131.03. 19F NMR (376 MHz, 25 °C): d
-71.07, -69.19.
1,2-Bis[N-(N0-propylimidazolium)]ethane 2Br- (S7)
1
Hygroscopic powder, Quaternization yield 88 %. H NMR (400 MHz, DMSO-d6,
25 °C): d 0.81 (t, 6H), 1.77 (m, 4H), 4.12 (t, 4H), 4.77 (s, 4H), 7.74 (s, 2H), 7.84 (s,
2H), 9.28 (s, 2H). 13C NMR (100 MHz, 25 °C): d 10.29, 22.65, 48.42, 50.50,
1
122.50, 122.84, 136.51. 2BF-4 : H NMR (400 MHz, D2O, 25 °C): d 0.96 (s, 6H),
1.95 (d, 4H), 4.25 (s, 4H), 4.87 (s, 4H), 7.58 (s, 2H), 7.68 (s, 2H), 8.83 (s, 2H). 13C
NMR (100 MHz, 25 °C): d 9.85, 22.79, 48.87, 51.55, 122.35, 123.66, 135.92. 19F
NMR (376 MHz, 25 °C): d -150.19, -150.14. 2PF26 : 1H NMR (400 MHz,
DMSO-d6, 25 °C): d 0.80 (t, 6H), 1.76 (m, 4H), 4.10 (t, 4H), 4.7 (s, 4H), 7.66 (s,
2H), 7.79 (s, 2H), 9.11 (s, 2H). 13C NMR (100 MHz, 25 °C): d 10.42, 22.78, 48.55,
50.62, 122.63, 122.97, 136.64. 31P NMR (162 MHz, 25 °C): d -157.36, -152.98,
-148.59, -144.19, -139.81, -135.41, -131.03. 19F NMR (376 MHz, 25 °C): d
-71.07, -69.19. HR–MS (EI) m/z molecular weight: 538.13.06, found: 538.1310.
1,3-Bis[N-(N0-propylimidazolium)]propane 2Br- (S8)
1
White sticky powder, Quaternization yield 88 %. H NMR (400 MHz, DMSO-d6,
25 °C): d 0.867 (t, 6H), 1.834 (m, 4H), 2.44 (t, 2H), 4.15 (t, 4H), 4.28 (t, 4H), 7.85 (s,
2H), 7.87 (s, 2H), 9.38 (s, 2H). 13C NMR (100 MHz, 25 °C): d 10.47, 22.74, 29.40,
1
45.88, 50.40, 122.40, 122.54, 136.27. 2BF-4 : H NMR (400 MHz, D2O, 25 °C): d
1.521 (t, 6H), 4.26 (m, 4H), 4.74 (s, 6H), 7.46 (s, 2H), 7.62 (s, 2H), 8.77 (s, 2H) 13
C
NMR (100 MHz, 25 °C): d 22.19, 24.06, 42.79, 46.67, 120.07, 120.08, and 132.8. 19
F
NMR (376 MHz, 25 °C): d. 2PF-6 : 1H NMR (400 MHz, DMSO-d6, 25 °C): d 0.865
(t, 6H), 1.81 (m, 4H), 2.45 (m, 2H), 4.14 (t, 4H), 4.275 (t, 4H), 7.845 (s, 2H), 7.865 (s,
2H), 9.367 (s, 2H). 13C NMR (100 MHz, 25 °C): d 10.50, 22.77, 29.44, 45.93, 50.44,
122.44, 122.58, 136.29. 31P NMR (162 MHz, 25 °C): d -158.2, -152.9, -148.1,
-139.2, -131.1. 19F NMR (376 MHz, 25 °C): d -71.1, -69.2.
123

Convenient synthesis of imidazolium based dicationic…
1,4-Bis[N-(N0-propylimidazolium)]butane 2Br- (S9)
White sticky powder, Quaternization yield 88 %. 1H NMR (400 MHz, D2O, 25 °C):
1.5 (t, 6H), 2.05 (m, 4H), 2.51 (m, 4H), 4.25 (t, 4H), 4.34 (t, 4H), 7.52 (s, 2H), 7.54
(s, 2H), 7.85 (s, 2H). 13C NMR (100 MHz, 25 °C): d 11.2, 21.1, 24.5, 28.6, 118.7,
1
129.2, 129.4, 137.2. 2BF-4 : H NMR (400 MHz, D2O, 25 °C): d 1.38 (t, 6H), 1.95
(m, 4H), 2.39 (m, 4H), 4.26 (t, 4H), 4.32 (t, 4H), 7.51 (s, 2H), 7.54 (s, 2H), 8.86 (s,
2H). 13C NMR (100 MHz, 25 °C): d 15.2, 20.8, 30.1, 45.1, 47.2, 124.3, 124.5,
138.2. 19F NMR (376 MHz, 25 °C): d -150.5, -150.7. 2PF-6 : 1H NMR (400 MHz,
DMSO-d6, 25 °C): d 1.41 (t, 6H), 1.74 (m, 4H), 2.08 (m, 4H), 4.18 (m, 8H), 7.55 (s,
2H), 7.57 (s, 2H), 8.86 (s, 2H). 13C NMR (100 MHz, 25 °C): d 15.1, 22.1, 26.5,
44.6, 48.5, 122.3, 122.5, 136.3. 31P NMR (162 MHz, 25 °C): d -164.2, -159.6,
-155.2, -150.1, -146.0, -141.0, -138.3. 19F NMR (376 MHz, 25 °C): d -75.1,
-73.05.
Conclusion
A series of alkylene 1,2-bis[N-(N-alkylimidazolium)] salts with Br-, BF4- and PF6-
1
anions was prepared and investigated successfully using H, 13C, 31P, 19F NMR
techniques. The hygroscopic nature of bromide and tetrafluoroborate is due to the
hydrophilic nature of both of these anions. The successful synthesis of dicationic ILs
containing various anions and characterization by NMR spectroscopy and HRMS
makes their synthesis more convenient and time saving. Application of this
methodology for synthesis of dicationic ILs with novel moieties and their
applications in various fields are under investigation in our laboratory.
Acknowledgments We gratefully acknowledge SIF DST-VIT-FIST, VIT University, Vellore for
providing NMR data as well as other required facilities.
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