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Physical Chemistry Chemical Physics
Page 6 of 8
DOI: 10.1039/C8CP00311D
ARTICLE
Journal Name
-
neg. ion mode (C2F6NO4S2 ): 279.92 [M-]; MS, found: pos. ion
+
-
1-Butyl-3-methylimidazolium iodide (IL-H-I). Synthesis was
adapted from the literature.19,20 In a typical synthesis, 8 ml 1-
methylimidazole (0.100 mol) were dissolved in 30 ml
mode (C8H15N2 ): 139.118 [M+]; neg. ion mode (C2F6NO4S2 ):
279.924 [M-]).
Synthesis of 1-butyl-2,3-dimethylimidazolium iodide (IL-Me-I).
8.87 ml (0.100 mol) of 1,2-dimethylimidazole were dissolved in
DCM (30 ml), and 13.7 ml (0.120 mol, 1.2 eq.) iodobutane
were added. The mixture was stirred at 60 °C over night. On
the next day the solvent was removed at reduced pressure,
and the crude product was dried in vacuum (10-3 mbar) at
room temperature to reveal a solid product (26.45 g,
0.094 mol, 94 % yield) 1H NMR (400 MHz, chloroform-d) δ 7.62
(d, J = 2.1 Hz, 1H), 7.46 (d, J = 2.1 Hz, 1H), 4.20 (t, J = 7.5 Hz,
2H), 4.01 (s, 3H), 2.84 (s, 3H), 1.89 – 1.80 (m, 2H), 1.43 (dq, J =
14.8, 7.4 Hz, 2H), 0.98 (t, J = 7.3 Hz, 3H). (MS, calcd.: pos. ion
dichloromethane (DCM), and 13.7 ml iodobutane (0.120 mol,
1.2 eq.) were added while the mixture was stirred. The
reaction was heated to reflux (50 °C oil bath temperature) and
stirred overnight. On the following day the reaction mixture
was monitored with NMR-spectroscopy until no imidazole
peak was visible and then concentrated en vacuo. The so
obtained crude product (28.34 g) was used for further
reactions without purification (quant. yield). 1H NMR
(400 MHz, chloroform-d) δ 9.98 (d, J = 1.7 Hz, 1H), 7.56 (dt, J =
33.5, 1.7 Hz, 4H), 4.32 (t, J = 7.4 Hz, 2H), 4.10 (s, 3H), 1.94 –
1.84 (m, 3H), 1.41 – 1.29 (m, 4H), 0.94 (dd, J = 8.2, 6.4 Hz, 5H).
+
mode (C9H17N2 ): 153.14 [M+]; neg. ion mode (I-): 126.91 [M-];
(MS, calcd.:‡ pos. ion mode (C8H15N2 ): 139.12 [M+]; neg. ion
MS, found: pos. ion mode (C9H17N2 ): 153.138 [M+]; neg. ion
+
+
mode (I-): 126.91 [M-]; MS, found: pos. ion mode (C8H15N2 ):
mode (I-): 126.903 [M-]).
+
139.108 [M+]; neg. ion mode (I-): 126.907 [M-]).
Synthesis of 1-butyl-2,3-dimethylimidazolium
1-Butyl-3-methylimidazol-2-ylidene borane (ZIL). In a typical
synthesis 13.32 g (0.05 mol) of IL-H-I were dispersed in 50 ml
toluene. To the mixture, 2.27 g (0.06 mol, 1.2 eq.) sodium
borohydride were added, and the mixture was heated to
105 °C and stirred overnight. On the next day, the hot toluene
bis(trifluoromethanesulfonyl)imide (IL-Me-TFSI). 5.35 g
(0.019 mol) of IL-Me-I were dissolved in 40 ml water. A mixture
of 6.86 g (0.024 mol 1.26 eq.) lithium
bis(trifluoromethanesulfonyl)imide in 20 ml water was added,
and the mixture was stirred overnight. Two phases formed and
was decanted into an Erlenmeyer flask and from that flask into were separated on the next day. The (lower) ionic liquid
a round bottom flask.§ The decantation was repeated twice
containing layer was washed with water until testing with
with fresh toluene, which was always heated to 105 °C prior to silver nitrate did not show precipitation anymore. After drying
decantation. The collected toluene phase was left to settle for
one day to allow for precipitation of residual sodium
borohydride and then filtered. The toluene was removed by
rotary evaporation, and the crude product was dissolved in
dichloromethane to allow for further precipitation of sodium
borohydride and other solid impurities. After decantation and
concentration en vacuo, 2.54 g (0.017 mol, 34 % yield) of the
product were obtained as a clear liquid. 1H NMR (400 MHz,
chloroform-d) δ 6.82 (q, J = 2.0 Hz, 2H), 4.14 – 4.09 (m, 2H),
3.74 (s, 3H), 1.77 (tt, J = 8.8, 6.8 Hz, 2H), 1.36 (dt, J = 14.8, 7.4
Hz, 2H), 0.96 (t, J = 7.4 Hz, 3H). (MS, calcd. (C8H17BN2Na+,
[M+Na+]): pos. ion mode: 175.1377 [M+Na+], found 175.1376
[M+Na+]).
in high vacuum at 60 °C overnight a liquid product was
obtained (6.64 g, 0.015 mol, 79 % yield). 1H NMR (400 MHz,
chloroform-d) δ 7.23 (d, J = 2.1 Hz, 1H), 7.19 (d, J = 2.1 Hz, 1H),
4.07 (t, J = 7.5 Hz, 2H), 3.83 (s, 3H), 2.63 (s, 3H), 1.85 – 1.75 (m,
2H), 1.40 (dq, J = 14.7, 7.4 Hz, 2H), 0.99 (t, J = 7.4 Hz, 3H). (MS,
+
calcd.: pos. ion mode (C9H17N2 ): 153.14 [M+]; neg. ion mode
-
+
(C2F6NO4S2 ): 279.92 [M-]; MS, found: pos. ion mode (C9H17N2 ):
-
153.137 [M+]; neg. ion mode (C2F6NO4S2 ): 279.917 [M-]).
Conclusions
With significant enhancement of the electrochemical reductive
stability similar to C2-methylation and decrease of the
1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide
(IL-H-TFSI). 14.13 g (0.053 mol) IL-H-I were dissolved in 20 ml
distilled water, and 18.3 g (0.064 mol, 1.2 eq.) lithium
bis(trifluoromethanesulfonyl)imide dissolved in 30 ml distilled
water were added. The mixture was stirred overnight at room
temperature. The resulting phases were separated, and the
hydrophobic phase was washed with water until testing the
water phase with silver nitrate did not form a visible
precipitate and then one more time after that. The
viscosity similar to anion exchange by
a bulky anion,
zwitterionic liquids with borohydride groups may be an
interesting alternative to C2-methylated and TFSI-containing
imidazolium ILs. While physical properties are similar, they
have several distinct advantages. For example the molecular
weight is much lower (152 g mol-1 for 1-butyl-3-
methylimidazol-2-ylidene borane, vs. 433 g mol-1 for 1-butyl-
2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide),
making high loading with salts in an electrolyte more likely.
The zwitterionic liquid is also far more environmentally benign
(not fluorinated), much cheaper, and easy to synthesize.
Thermal and oxidative stabilities are lower, owing to the
nucleophilicity of the BH3 group, but sufficient at or around
room temperature or moderately increased temperature.
Hence, in applications which do not require heating to high
temperatures or exposure to strong oxidizers, the presented
hydrophobic phase was concentrated en vacuo to yield 16.74 g
(0.040 mol, 75.5 % yield) of the ionic liquid product. 1H NMR
(400 MHz, chloroform-d) δ 8.59 (d, J = 2.0 Hz, 1H), 7.31 (dq, J =
7.7, 1.8 Hz, 2H), 4.13 – 4.08 (m, 2H), 3.87 (d, J = 1.4 Hz, 3H),
1.84 – 1.74 (m, 2H), 1.36 – 1.23 (m, 2H), 0.89 (td, J = 7.4, 1.4
+
Hz, 3H). (MS, calcd.: pos. ion mode (C8H15N2 ): 139.12 [M+];
6 | J. Name., 2012, 00, 1-3
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