Base stable quaternary ammonium ionic liquids

Article abstract of DOI:10.1039/c3ra45126g

Ionic liquids with the bis(2-ethylhexyl)dimethylammonium cation, [BEDMA]⁺, were prepared by a halide-free route starting from the readily available secondary amine bis(2-ethylhexyl)amine. The following anions were considered: chloride, bromide, iodide, nitrate, hydrogensulphate, dihydrogenphosphate, formate, acetate, propionate, trifluoroacetate, methyl sulphate, methanesulphonate, tosylate, isonicotinate, nicotinate and picolinate. Several of the compounds are room-temperature ionic liquids, albeit with a high viscosity. All ionic liquids are soluble in water. The ionic liquids are very stable in strongly alkaline medium. No signs of decomposition could be observed by contact of the chloride ionic liquid with 50 wt% sodium hydroxide, even after prolonged heating at 80 °C. The high stability against strong bases is attributed to the branched structure of the quaternary ammonium cation, which blocks the Hofmann elimination reaction.

Full text of DOI:10.1039/c3ra45126g

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Base stable quaternary ammonium ionic liquids†
*
Kallidanthiyil Chellappan Lethesh, Wim Dehaen and Koen Binnemans
Cite this: RSC Adv., 2014, 4, 4472
Ionic liquids with the bis(2-ethylhexyl)dimethylammonium cation, [BEDMA]+, were prepared by a halide-
free route starting from the readily available secondary amine bis(2-ethylhexyl)amine. The following
anions were considered: chloride, bromide, iodide, nitrate, hydrogensulphate, dihydrogenphosphate,
formate, acetate, propionate, trifluoroacetate, methyl sulphate, methanesulphonate, tosylate,
isonicotinate, nicotinate and picolinate. Several of the compounds are room-temperature ionic liquids,
albeit with a high viscosity. All ionic liquids are soluble in water. The ionic liquids are very stable in
strongly alkaline medium. No signs of decomposition could be observed by contact of the chloride ionic
liquid with 50 wt% sodium hydroxide, even after prolonged heating at 80 ꢀC. The high stability against
strong bases is attributed to the branched structure of the quaternary ammonium cation, which blocks
the Hofmann elimination reaction.
Received 14th September 2013
Accepted 2nd December 2013
DOI: 10.1039/c3ra45126g
www.rsc.org/advances
groups of these cations will also undergo deprotonation under
mild conditions.23 Jurcik and Wilhelm synthesized a new class
Introduction
of ionic liquids having a phenyl substituent on the C(2) carbon
position of the imidazolium ring.29 They successfully used these
ionic liquids as a reaction medium for the Baylis–Hillman
reaction between methyl acrylate and a variety of aromatic
aldehydes, with DABCO or quinuclidinol as the base. They also
showed that these ionic liquids could be used as a solvent for
reactions involving Grignard reagents. Stevens and coworkers
found that peralkylated imidazolium ionic liquids are stable
against organolithium reagents.30 Upon quenching of the
reaction mixtures with D2O, no deuterium exchange could be
observed. N-alkyl pyridinium ionic liquids with an ether func-
tionality have been used as solvents for Grignard reactions.31
Clyburne and coworkers reported that phosphonium-based
ionic liquids such as tetradecyl(trihexyl)phosphonium chloride,
bromide and bis(triuoromethylsulphonyl)imide are very stable
under alkaline conditions.32,33 They also showed that Grignard
reagents were stable in these ionic liquids, even aer one
month.34 Quaternary phosphonium salts can be hydrolysed in
alkaline aqueous medium to a phosphine oxide and an alkane,
but an organic solvent is required to accelerate the reaction.35
Quaternary ammonium salts will undergo Hofmann degrada-
tion in the presence of strong bases, leading to the formation of
an alkene and a tertiary amine.36,37 This elimination reaction
requires the presence of a b-hydrogen atom on the quaternary
ammonium salt. Base stable ionic liquids could be useful as
solvents for organic reactions with strongly basic reagents such
as Grignard reagents or organolithium compounds. These ionic
liquids could also be applied as extractants for removal of metal
ions from alkaline aqueous solutions by solvent extraction.38
In this paper, we present new base stable ammonium ionic
liquids with the bis(2-ethylhexyl)dimethylammonium cation,
[BEDMA]+, and with a variety of anions. All the new ionic liquids
Ionic liquids are solvents that consist entirely of ions, and they
have usual properties such as a negligible vapour pressure, a
high thermal stability, a wide electrochemical window and an
intrinsic electric conductivity.1–4 They are also excellent solvents
for many classes of inorganic and organic compounds. Ionic
liquids nd wide applications, including solvent for catalytic
reactions,5 electrolytes for electrodeposition of reactive
metals,6–9 electrolytes for Li-ion batteries10,11 and dye-sensitized
solar cells,12 as engineering uids,13,14 solvents for liquid–liquid
extraction processes,15–18 and for applications in hydrometal-
lurgy.19 Several ionic liquids are being used on an industrial
scale.20 Most ionic liquids are very stable against strong
oxidants and reductants, as well as against strong acids, but
they oen have a remarkably poor resistance against strong
bases.21 Imidazolium ionic liquids are the most intensively
studied class of ionic liquids, but they are not inert in alkaline
medium.22 The C(2) proton of the 1,3-dialkylimidazolium cation
is quite acidic and can be exchanged under mild conditions.23,24
Deprotonation of this acidic proton results in the formation of
carbenes, and hence in the loss of the ionic character of the
imidazolium ring. It is even possible to replace all hydrogen
atoms on the imidazolium ring by deuterium atoms in the
presence of bases.25 Efforts have been made to avoid this
unwanted deprotonation by incorporating a methyl or isopropyl
group in the C(2) position of the 1,3-dialkylimidazolium ionic
liquids.26–28 However it has been reported that C(2) methyl
KU Leuven, Department of Chemistry, Celestijnenlaan 200F, P.O. Box 2404, B-3001
Heverlee, Belgium. E-mail: Koen.Binnemans@chem.kuleuven.be; Fax: +32 16 32
7992; Tel: +32 16 32 7446
† Electronic supplementary information (ESI) available: Copies of the 1H NMR
and 13C NMR spectra of all ionic liquids. See DOI: 10.1039/c3ra45126g
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are uorine-free (with the exception of the triuoroacetate salt) (2-ethylhexyl)dimethylammonium 4-tert-butylphenolate (15 g,
and soluble in water.
35.74 mmol) in toluene (50 mL). The reaction mixture was
stirred vigorously at room temperature for half an hour. The
aqueous layer was separated and evaporated in vacuo to give
bis(2-ethylhexyl)dimethylammonium acetate as a colourless
liquid. Yield: (11.42 g, 97%). 1H NMR (300 MHz, D2O): d ¼ 0.85
(m, 12H), 1.28 (m, 16H), 1.88 (s, 3H), 1.92 (m, 2H), 2.92 (s, 6H),
3.06 (m, 4H). 13C NMR (75 MHz, D2O): d ¼ 9.74, 13.54, 21. 68,
22.34, 25.65, 27.82, 32.34, 33.32, 50.38, 69.66, 178.77. Elemental
analysis: found: C, 61.13; H, 12.35; N, 3.54. C20H43NO2$3.5H2O
requires C, 61.34; H, 12.61; N, 3.57%.
Experimental
General information
Elemental analyses (carbon, hydrogen, nitrogen) were carried
out on a CE Instruments EA-1110 elemental analyser. H and
1
13C NMR spectra were recorded on a Bruker Avance 300 spec-
trometer (operating at 300 MHz for 1H and at 75.5 MHz for 13C).
The water content of the ionic liquids was determined by a
coulometric Karl Fischer titrator (Mettler-Toledo coulometric
Karl Fischer titrator, model DL39). The density was measured by
a pycnometer. The viscosity of the ionic liquids was determined
with a Brookeld DV-II + Pro Cone/plate set-up viscometer, with
a thermostatted sample cell that is being purged with dry
nitrogen gas. Differential scanning calorimetry (DSC)
Bis(2-ethylhexyl)dimethylammonium
triuoroacetate,
[BEDMA][CF3COO]. Bis(2-ethylhexyl)dimethylammonium tri-
uoroacetate was prepared by the general procedure described
for [BEDMA][CH3COO] from bis(2-ethylhexyl)dimethylammo-
nium 4-tert-butylphenolate (15 g, 35.74 mmol) and triuoro-
acetic acid (6.11 g, 53.61 mmol), to give a colourless liquid.
Yield: (11.10 g, 81%).1H NMR (300 MHz, D2O): d ¼ 0.85 (m,
measurements were made on
a Mettler-Toledo DSC822e
12H), 1.29 (m, 16H), 1.92 (m, 2H), 3.00 (s, 6H), 3.20 (m, 4H) 13
C
module, at a scan rate of 10 ꢀC minꢁ1 in a helium atmosphere.
Chemicals were purchased from Acros Organics (Geel, Belgium)
or from Sigma-Aldrich (Bornem, Belgium). All chemicals were
used as received, without further purication.
NMR (75 MHz, D2O): d ¼ 9.74, 13.54, 22.34, 25.65, 27.82, 32.34,
33.32, 50.38, 69.66, 117. 98(q), 169.32(q). Elemental analysis:
found: C, 59.90; H, 10.67; N, 3.37. C20H40F3NO2$H2O requires C,
59.82; H, 10.54; N, 3.49%.
Bis(2-ethylhexyl)dimethylammonium formate, [BEDMA]-
[HCOO]. Bis(2-ethylhexyl)dimethylammonium formate was
Syntheses
Bis(2-ethylhexyl)dimethylammonium
methyl sulphate, prepared by the general procedure described for [BEDMA]
[BEDMA][CH3SO4]. To a suspension of potassium carbonate [CH3COO] from bis(2-ethylhexyl)dimethylammonium 4-tert-
(45.79 g, 331.13 mmol) in acetonitrile (250 mL), bis(2-ethyl- butylphenolate (15 g, 35.74 mmol) and formic acid (8.22 g,
hexyl)amine (20 g, 82.82 mmol) was added, followed by 178.70 mmol), to give a colourless liquid. Yield: (10.48 g, 93%).
dimethyl sulphate (20.89 g, 165.65 mmol). The reaction mixture 1H NMR (300 MHz, D2O): d ¼ 0.87 (m, 12H), 1.31 (m, 16H), 1.89
was reuxed for 48 hours. Potassium carbonate was removed by (m, 2H), 3.05 (s, 6H), 3.22 (m, 4H), 6.31(s, 1H). 13C NMR
ltration and the solvent removed in vacuo to yield bis(2-ethyl- (75 MHz, D2O): d ¼ 9.11, 12.84, 21.71, 25.37, 27.24, 31.90, 32.89,
hexyl)dimethylammonium methyl sulphate as a pale yellow 49.32, 69.81, 169.08. Elemental analysis: found: C, 60.32; H,
liquid. Yield: (26.84 g, 85%). 1H NMR (300 MHz, D2O): d ¼ 0.85 12.64; N, 3.71. C19H41NO2$3.5H2O requires C, 60.27; H, 12.79;
(m, 12H), 1.25 (m, 16H), 1.81(s, 2H), 3.02 (s, 6H), 3.20 (m, 4H), N, 3.69%.
3.66 (s, 3H). 13C NMR (75 MHz, D2O): d ¼ 8.72, 12.45, 21.33,
Bis(2-ethylhexyl)dimethylammonium propionate, [BEDMA]-
24.97, 26.86, 31.53, 32.48, 48.83, 54.54, 70.19. Elemental anal- [CH3CH2COO]. Bis(2-ethylhexyl)dimethylammonium propio-
ysis: found: C, 59.60; H, 11.24; N, 3.52. C19H43NO4S requires C, nate was prepared by the general procedure described for
59.80; H, 11.36; N, 3.67%.
[BEDMA][CH3COO] from bis(2-ethylhexyl)dimethylammonium
Bis(2-ethylhexyl)dimethylammonium 4-tert-butylphenolate, 4-tert-butylphenolate (15 g, 35.74 mmol) and propionic acid
[BEDMA][4-tBuPhen]. To a solution of bis(2-ethylhexyl)dime- (3.97 g, 53.61 mmol), to give a colourless liquid. Yield: (10.80 g,
1
thylammonium methyl sulphate (15 g, 39.34 mmol) in water 88%). H NMR (300 MHz, D2O): d ¼ 0.79 (m, 12H), 0.93 (t, 3H,
(100 mL) sodium-4-tert-butylphenolate (6.77 g, 39.34 mmol) was 7.64 Hz), 1.21 (m, 16H), 1.90 (m, 2H), 2.12 (m, 2H), 2.95 (s, 6H),
added, followed by dichloromethane (200 mL). The mixture was 3.09 (m, 4H). 13C NMR (75 MHz, D2O): d ¼ 9.49, 9.65, 22.14,
vigorously stirred at room temperature for ve hours. The 25.80, 27.67, 29.71, 32.34, 33.30, 49.70, 70.25, 183.46. Elemental
organic layer was separated and evaporated in vacuo to give analysis: found: C, 66.53; H, 13.23; N, 3.61. C21H45NO2$2H2O
bis(2-ethylhexyl)dimethylammonium 4-tert-butylphenolate as a requires C, 66.44; H, 13.01; N, 3.69%.
highly viscous liquid. Yield: (13.20 g, 80%). 1H NMR (300 MHz,
Bis(2-ethylhexyl)dimethylammonium methanesulphonate,
Bis(2-ethylhexyl)dimethylammonium
DMSO[D6]): d ¼ 0.85 (m, 12H), 1.30 (m, 25H), 1.41 (m, 2H), 3.01 [BEDMA][CH3SO3].
(s, 6H), 3.19 (m, 4H), 7.08 (d, 2H, 8.74 Hz), 7.15 (d, 2H, 8.74 methanesulphonate was prepared by the general procedure
Hz).13C NMR (75 MHz, DMSO[D6]): d ¼ 10.07, 13.89, 22.34, described for [BEDMA][CH3COO] from bis(2-ethylhexyl)dime-
25.71, 27.58, 31.24, 32.45, 33.55, 52.78, 68.92, 114; 68, 125.78, thylammonium 4-tert-butylphenolate (15 g, 35.74 mmol) and
140.55, 155.12. Elemental analysis: found: C, 72.88; H, 10.41; N, methanesulphonic acid (3.09 g, 32.17 mmol), to give a colour-
4.94. C28H53NO requires C, 72.94; H, 10.62; N, 5.00%.
less liquid. Yield: (11.23 g, 86%). 1H NMR (300 MHz, D2O):
Bis(2-ethylhexyl)dimethylammonium acetate, [BEDMA]- d ¼ 0.91 (m, 12H), 1.31 (m, 16H), 1.50 (m, 2H), 2.78 (s, 3H), 3.02
[CH3COO] (¼ general procedure). An aqueous solution (250 mL) (s, 6H), 3.20 (s, 4H). 13C NMR (75 MHz, D2O): d ¼ 9.54, 13.24,
of glacial acetic acid (6.43 g, 107.21 mmol) was added to bis- 22.10, 25.76, 27.64, 31, 17, 32.30, 38.39, 49.56, 70.19. Elemental
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analysis: found: C, 56.60; H, 11.85; N, 3.42. C19H43NO3S$2H2O analysis: found: C, 51.22; H, 11.35; N, 3.18. C18H41NO4S$3H2O
requires C. 56.82; H, 11.79; N, 3.49%.
requires C, 51.28; H, 11.24; N, 3.32%.
Bis(2-ethylhexyl)dimethylammonium tosylate, [BEDMA]-
Bis(2-ethylhexyl)dimethylammonium dihydrogenphosphate,
[TsO]. Bis(2-ethylhexyl)dimethylammonium tosylate was prepared [BEDMA][H2PO4]. Bis(2-ethylhexyl)dimethylammonium dihy-
by the general procedure described for [BEDMA][CH3COO] from drogenphosphate was prepared by the general procedure
bis(2-ethylhexyl)dimethylammonium 4-tert-butylphenolate (15 g, described for [BEDMA][CH3COO] from bis(2-ethylhexyl)dime-
35.74 mmol) and 4-methylbenzenesulphonic acid (5.84 g, 33.95 thylammonium 4-tert-butylphenolate (15 g, 35.74 mmol) and
mmol), to give a colourless solid. Yield: (12.63 g, 80%). mp: 78 ꢀC. phosphoric acid (85% solution, 4.12 mL, 3.50 g, 35.74 mmol), to
1H NMR (300 MHz, D2O): d ¼ 0.80 (m, 12H), 1.38 (m, 16H), 1.42 give a colourless liquid. Yield: (10.50 g, 80%). 1H NMR (300 MHz,
(m, 2H), 2.31 (s, 3H), 2.94 (s, 6H), 3.12 (m, 4H), 7.27 (d, 2H), 7.61 (d, D2O): d ¼ 0.86 (m, 12H), 1.28 (m, 16H), 1.45 (m, 2H), 3.02 (s, 6H),
2H). 13C NMR (75 MHz, D2O): d ¼ 9.45, 13.19, 20.37, 22.06, 25.68, 3.20 (m, 4H). 13C NMR (75 MHz, D2O): d ¼ 9.45, 13.29, 22.11,
27.58, 32.24, 33.20, 70.15, 125.24, 129.32, 139.32, 142.33. 25.69, 27.63, 32.23, 33.23, 49.99, 69.93. Elemental analysis:
Elemental analysis: found: C, 62.21; H, 18.31; N, 3.00. C25 found: C, 53. 46; H, 11.37; N, 3.60. C18H42NO4P$2H2O requires C:
H47NO3S$2H2O requires C, 62.29; H, 18.19; N, 2.91%.
53.57, H: 11.49, N: 3.47%.
Bis(2-ethylhexyl)dimethylammonium chloride, [BEDMA][Cl].
Bis(2-ethylhexyl)dimethylammonium nitrate, [BEDMA][NO3].
Bis(2-ethylhexyl)dimethylammonium chloride was prepared by Bis(2-ethylhexyl)dimethylammonium nitrate was prepared by
the general procedure described for [BEDMA][CH3COO] from the general procedure described for [BEDMA][CH3COO]
bis(2-ethylhexyl)dimethylammonium 4-tert-butylphenolate (15 g, from bis(2-ethylhexyl)dimethylammonium 4-tert-butylphenolate
35.74 mmol) and hydrochloric acid (37% solution, 3.52 mL, 1.30 (15 g, 35.74 mmol) and nitric acid (65% solution, 3.46 mL, 2.25 g,
g, 35.74 mmol), to give a colourless solid. Yield: (9.18 g, 84%). 35.74 mmol), to give a colourless liquid. Yield: (10.10 g, 85%). 1H
mp: 84 ꢀC. 1H NMR (300 MHz, D2O): d ¼ 0.87 (m, 12H), 1.35 (m, NMR (300 MHz, D2O): d ¼ 0.88 (m, 12H), 1.25 (m, 16H), 1.45 (m,
16H), 1.45 (m, 2H), 2.9 (s, 6H), 3.15 (m, 4H). 13C NMR (75 MHz, 2H), 3.02 (s, 6H), 3.20 (m, 4H). 13C NMR (75 MHz, D2O): d ¼ 9.45,
D2O): d ¼ 9.74, 13.54, 22.34, 25.65, 27.82, 32.34, 33.32, 50.38, 13.29, 22.11, 25.69, 27.63, 32.23, 33.23, 49.99, 69.93. Elemental
69.66. Elemental analysis: found: C, 58.74; 12.76; N, 3.81. analysis: found: C, 58.52; H, 12.10; N, 7.57. C18H40N2O3$2H2O
C18H40ClN$3.5H2O requires C, 58.58; H, 12.83; N, 3.79%.
requires C, 58.66; H, 12.03; N, 7.60%.
Bis(2-ethylhexyl)dimethylammonium bromide, [BEDMA][Br].
Bis(2-ethylhexyl)dimethylammonium isonicotinate, [BEDMA]-
Bis(2-ethylhexyl)dimethylammonium bromide was prepared by [Isonic]. Bis(2-ethylhexyl)dimethylammonium isonicotinate was
the general procedure described for [BEDMA][CH3COO] from prepared by the procedure described for [BEDMA][CH3COO] from
bis(2-ethylhexyl)dimethylammonium 4-tert-butylphenolate (15 bis(2-ethylhexyl)dimethylammonium 4-tert-butylphenolate (15 g,
g, 35.74 mmol) and hydrobromic acid (48% solution, 6.02 mL, 35.74 mmol) and isonicotinic acid (5.49 g, 44.68 mmol), to give a
2.89 g, 35.74 mmol), to give a colourless solid. Yield: (10.14 g, colourless solid. Yield: (11.50 g, 82%). mp: 266 ꢀC. 1H NMR (300
1
ꢀ
81%). mp: 90 C. H NMR (300 MHz, D2O): d ¼ 0.85 (m, 12H), MHz, D2O): d ¼ 0.82 (m, 12H), 1.27 (m, 12H), 1.85 (m, 2H), 3.01 (s,
1.32 (m, 16H), 1.45 (m, 2H), 2.93 (s, 6H), 3.14 (m, 4H). 13C NMR 6H), 3.19 (m, 4H),7.69 (d, 2H, 6.33 Hz), 8.55 (d, 2H, 6.33 Hz). 13
C
(75 MHz, D2O): d ¼ 9.74, 13.54, 22.34, 25.65, 27.82, 32.34, 33.32, NMR (75 MHz, D2O): d ¼ 9.50, 13.26, 22.15, 25.79, 27.68, 32.35,
50.38, 69.66. Elemental analysis: found: C, 58.55; H, 11.33; N, 32. 41, 55.41, 70.27, 124.24, 146.15, 148.76, 171.66. Elemental
3.80. C18H40BrN$H2O requires C, 58.67; H, 11.49; N, 3.80%.
analysis: found: C, 70.26; H, 11. 32; N, 6.75. C24H44N2O2$H2O
Bis(2-ethylhexyl)dimethylammonium iodide, [BEDMA][I]. requires C: 70.20, H: 11.29, N: 6.82%.
Bis(2-ethylhexyl)dimethylammonium iodide was prepared by
Bis(2-ethylhexyl)dimethylammonium nicotinate, [BEDMA]
the general procedure described for [BEDMA][CH3COO] from [Nic]. Bis(2-ethylhexyl)dimethylammonium nicotinate was
bis(2-ethylhexyl)dimethylammonium 4-tert-butylphenolate (15 prepared by the procedure described for [BEDMA][CH3COO]
g, 35.74 mmol) and hydrogen iodide (57% solution, 8.02 mL, from bis(2-ethylhexyl)dimethylammonium 4-tert-butylpheno-
4.57 g, 35.74 mmol), to give a colourless solid. Yield: (11.36 g, late (15 g, 35.74 mmol) and nicotinic acid (3.96 g, 32.16 mmol),
1
1
ꢀ
80%). mp: 73 C. H NMR (300 MHz, D2O): d ¼ 0.80 (m, 12H), to give a colourless solid. Yield: (11.22 g, 80%). mp: 224 ꢀC. H
1.22 (m, 16H), 1.42 (m, 2H), 2.96 (s, 6H), 3.13 (m, 4H). 13C NMR NMR (300 MHz, D2O): d¼ 0.77 (m, 12H), 1.26 (m, 16H), 1.70
(75 MHz, D2O): d ¼ 9.73, 13.54, 22.31, 25.60, 27.82, 32.34, 33.32, (m, 2H), 2.96 (s, 6H), 3.15 (m, 4H), 7.38 (m, 1H), 8.20 (d, 1H, 7.98
50.38, 69.66. Elemental analysis: found C, 40.12; H, 9.50; N, Hz), 8.48 (d, 1H, 7.14 Hz), 8.92 (s, 1H). 13C NMR (75 MHz, D2O):
2.40. C18H40IN requires: C, 40.18; H, 9.05; N, 2.47%.
d ¼ 9.50, 13.31, 22.14, 25.70, 27.65, 32.29, 33.27, 49.81, 70.07,
Bis(2-ethylhexyl)dimethylammonium hydrogensulphate, 125.82, 134.42, 142.89, 144.76, 145.57, 169.53. Elemental anal-
[BEDMA][HSO4]. Bis(2-ethylhexyl)dimethylammonium hydro- ysis: found: C, 67.15; H, 11. 10; N, 6.60. C24H44N2O2$2H2O
gensulphate was prepared by the general procedure described requires C, 67.25; H, 11.29; N, 6.54%.
for [BEDMA][CH3COO] from bis(2-ethylhexyl)dimethylammo-
Bis(2-ethylhexyl)dimethylammonium picolinate, [BEDMA]-
nium 4-tert-butylphenolate (15 g, 35.74 mmol) and sulphuric [Pic]. Bis(2-ethylhexyl)dimethylammonium picolinate was
acid (95% solution, 3.68 mL, 3.50 g, 35.74 mmol), to give a prepared by the procedure described for [BEDMA][CH3COO]
colourless liquid. Yield: (10.64 g, 81%). 1H NMR (300 MHz, from bis(2-ethylhexyl)dimethylammonium 4-tert-butylpheno-
D2O): d ¼ 0.89 (m, 12H), 1.30 (m, 16H), 1.43 (m, 2H), 3.00 late (15 g, 35.74 mmol) and picolinic acid (4.39 g, 35.74 mmol),
(s, 6H), 3.20 (m, 4H). 13C NMR (75 MHz, D2O): d ¼ 9.45, 13.26, to give a colourless liquid. Yield: (11.92 g, 85%). 1H NMR
22.11, 25.69, 27.63, 32.25, 33.23, 49.99, 69.90. Elemental (300 MHz, D2O): d ¼ 0.77 (m, 12H), 1.22 (m, 16H), 1.79 (m, 2H),
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2.95 (s, 6H), 3.13 (m, 4H), 7.81 (m, 1H), 8.13 (d, 1H, 7.88 Hz), successful, probably because of the steric hindrance by the ethyl
8.31 (m, 1H), 8.60 (d, 1H, 5.07 Hz). 13C NMR (75 MHz, D2O): group in the 2-position of bis(2-ethylhexyl)amine. It has been
d¼ 9.53, 13.26, 22.15, 25.82, 27.68, 32.35, 33.30, 49.60, 55.40, tried to react bis(2-ethylhexyl)amine with 1-bromoethane,
70.26, 125.53, 127.31, 143.43, 144.29, 148.19, 166.81. Elemental 1-bromopropane and 1-iodobutane. In all the cases, the
analysis: found: C, 63.51; H, 23.28; N, 6.11. C24H44N2O2$3H2O unreacted starting materials were recovered aer the reaction.
requires: C, 63.62; H, 23.14; N, 6.18%.
No evidence for formation of tertiary amines were observed in
the reaction mixture.
The second step was the synthesis of the phenolate
precursor, from which the desired anions were introduced by
ion-exchange reactions with Brønsted acids. This was achieved
by the extraction of the [BEDMA]+ cation from the aqueous
phase to the organic phase by stirring with sodium 4-tert-
butylphenolate. The phenolate precursor ionic liquid, [BEDMA]
[4-tert-butylphenolate] was obtained as a highly viscous liquid
with 85% yield. Sodium 4-tert-butylphenolate was prepared by
the reaction of 4-tert-butylphenol and sodium hydroxide in
ethanol. Initially, we tried to use sodium phenolate instead of
sodium 4-tert-butylphenolate for the extraction of the [BEDMA]+
cation from the aqueous phase to the organic phase. The intent
was to remove the phenol that was formed aer the ion-
exchange reaction with Brønsted acids in vacuo. Even though
extraction of [BEDMA]+ was achieved, the complete removal of
phenol from the ionic liquid formed aer the ion-exchange
reaction with Brønsted acids was not successful. The third step
was the introduction of different anions by ion exchange of the
[BEDMA][4-tert-butylphenolate] with different Brønsted acids to
obtain the desired ionic liquids. The following Brønsted acids
were used: formic acid, acetic acid, propionic acid, triuoro-
acetic acid, methanesulphonic acid, para-toluenesulphonic
acid, hydrochloric acid, hydrobromic acid, hydroiodic acid,
nitric acid, sulphuric acid, phosphoric acid, nicotinic acid,
isonicotinic acid and picolinic acid. Ion-exchange reaction was
carried out by adding an aqueous solution of Brønsted acid to a
solution of [BEDMA][4-tert-butylphenolate] in toluene. The
reaction mixture was vigorously stirred for 30 minutes. The
ionic liquid formed was transferred to the aqueous layer
because of its overall hydrophilic nature. The side product of
the ion exchange reaction was 4-tert-butylphenol, which stayed
in the organic layer. The hydrophilic ionic liquid was isolated by
separation of the aqueous layer from the organic phase and the
evaporation of water under reduced pressure. The 1H NMR
spectra showed the complete disappearance of the phenolate
anion from the nal ionic liquids. Due to the hydrophobic
nature of the 4-tert-butylphenol, the side product of the ion-
exchange reaction, it does not contaminate the hydrophilic
ionic liquids formed. It was possible to recover 4-tert-butyl-
phenol aer the ion-exchange reaction and it could be reused
for the preparation of sodium-4-tert-butylphenolate for the next
stage. The ion-exchange reaction is preferably performed in a
non-polar organic solvent to avoid the reduction in the yield of
the nal ionic liquids. Ion-exchange reactions in polar solvents
lead to lower yields, which are caused by the solubility of these
ionic liquids in polar solvents, i.e. a major portion of the ionic
liquids formed during the ion-exchange reaction would stay in
the organic phase together with the 4-tert-butylphenol. A higher
or lower relative integration of the protons of the anions was
Results and discussion
The bis(2-ethylhexyl)dimethylammonium cation [BEDMA]+ was
selected as cation for the base stable ionic liquids, because it is
accessible via the secondary amine bis(2-ethylhexyl)amine,
which is a widely available and cheap starting material. The
2-ethylhexyl group is an oen used branched alkyl chain, since
it is prepared from butyraldehyde, a product of the hydro-
formylation of propene. In the past, [BEDMA][Cl] has been used
as an anionic extractant for the liquid–liquid extraction of
cobalt(II), iron(III), indium(III) and gallium(III) from chloride
medium.39 However, the base stability of ionic liquids based on
the [BEDMA]+ cation has never been investigated.
The synthesis of the ionic liquids consists of the following
steps: (1) quaternization of the secondary amine bis(2-ethyl-
hexyl)amine by dimethyl sulphate; (2) transformation of the
dimethyl sulphate salt into the corresponding 4-tert-butylphe-
nolate salt; (3) reaction of the 4-tert-butylphenolate salt with a
Brønsted acid (Scheme 1). The synthetic route was based on the
so-called “phenolate platform”, that we have introduced
recently.40 It must be noted that the ionic liquids were prepared
via a halogen-free route, and that no detectable amounts of
halide are present, with the exception, of course, of the chloride,
bromide and iodide salts. The quaternisation of bis(2-ethyl-
hexyl)amine was performed by the reaction with two equivalents
of dimethyl sulphate, in acetonitrile, in the presence of potas-
sium carbonate. Aer 48 hours of heating under reux,
complete conversion of the starting amine was achieved. The
precipitated potassium salts were removed by ltration and the
solvent was removed in vacuo. The dimethyl sulphate salt
[BEDMA][CH3OSO3] was obtained as a highly viscous pale
yellow liquid in very good yield (96%). The product was char-
acterized by 1H NMR, 13C NMR spectroscopy and CHN
elemental analysis. Attempts to introduce alkyl groups other
than a methyl group, such as an ethyl or a butyl group, were not
Scheme 1 Three-step procedure for the synthesis of ionic liquids
derived from the bis(2-ethylhexyl)dimethylammonium cation, [BEDMA]+. observed in the 1H NMR spectra when stoichiometric amounts
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of Brønsted acids were used for the ion-exchange reaction. To were liquid at room temperature. The densities, viscosities
obtain a perfect 1 : 1 ratio between cation and anion, the and residual water content of the ionic liquids were measured
amount of Brønsted acid used had to be tuned for every specic (Table 2). By drying of the ionic liquids in high vacuum
acid. Ion-exchange reactions were carried out in other non-polar (10ꢁ2 to 10ꢁ3 mbar) for 48 hours at 75 C, the water content
ꢀ
solvents to study the efficiency of the process in other solvent was reduced to less than about 1000 ppm (determined by
systems. Different solvents such as cyclohexane, m-xylene, tert- coulometric Karl Fischer titraꢀtion). The viscosities of the ionic
butyl methyl ether, n-heptane were used for the ion-exchange liquids were measured at 25 C. The viscosities are high, and
reaction. There was not a considerable variation in the yields are ranging between 1450 cP and 4700 cP. All the ionic liquids
when different non-polar solvents were used. Hence, different with the bis(2-ethylhexyl)dimethylammonium cation are
non-polar solvents, which are immiscible with water, can be completely miscible with water, ethanol, DMSO, dichloro-
used for the ion-exchange reaction.
methane or chloroform. They form a biphasic mixture with
The chloride, bromide, iodide, nitrate and tosylate salts are diethylether or toluene. The miscibility tests were performed
solids at room temperature with a melting point between 73 ꢀC at room temperature and the ionic liquid to solvent volume
and 90 ꢀC (Table 1). An interesting behavior is observed for the ratio was about 1 : 1.
pyridinecarboxylates. The nicotinate (mp: 224 ꢀC) and the
The base stability of the [BEDMA]+ cation was studied using
isonicotinate (mp: 266 ꢀC) are high-melting solids, whereas 1H NMR spectroscopy. In a typical experiment, the reaction
the isomer with the picolinate anion is a room-temperature ask was thermostated at a selected temperature between room
ꢀ
ionic liquid. This large difference in the melting point may be temperature and 80 C and charged with 20 mL of 1,2-dichlo-
due to the asymmetry in the anion, or differences in the roethane solution of [BEDMA][Cl] (3.26 mmol) and 20 mL of a
interactions between cations and anions (for instance, 50 wt% NaOH solution. Stirring and time logging were started.
hydrogen bonding). More studies are needed to explain this Samples of (1–2 mL) of organic phase were withdrawn at various
observation, but no single crystals of a quality high enough for times by stopping the stirrer for 40 to 60 seconds to allow
1
X-ray diffraction could be obtained. All the other compounds adequate separation. The organic layer was evaporated and H
NMR spectra of the sample were recorded. The possible Hof-
mann degradation products of the [BEDMA]+ cation are 2-ethyl-
hex-1-ene and 2-ethyl-N,N-dimethylhexan-1-amine. Even aer a
Table 1 Melting points (Tm) of ionic liquids
long reaction time (120 hours), no evidence for the decompo-
Compounda,b
Tm (ꢀC)
c
sition of the starting material was observed at room tempera-
ture. Similar results were obtained when the reactions were
conducted at higher temperatures, i.e. at 40 ꢀC, 60 ꢀC and 80 ꢀC.
[BEDMA][Cl]
[BEDMA][Br]
[BEDMA][I]
84
90
73
1
The H NMR spectra contained only the peaks of the starting
material. These results indicate that the [BEDMA]+ cation is very
stable against strong bases at elevated temperatures. This is the
more remarkable when this stability is compared with that of
other quaternary ammonium salts. Landini et al. carried out a
systematic study on the stability of a series of quaternary
ammonium salts in aqueous/organic systems in the presence of
an aqueous alkaline solution as a function of alkyl chain
length.37 For instance, 50% of tetrahexylammonium chloride
was found to be decomposed aer 35 hours at 25 ꢀC. When the
[BEDMA][NO3]
[BEDMA][TsO]
[BEDMA][Isonic]
[BEDMA][Nic]
80
78
266
224
a
Abbreviations: TsO
¼
tosylate, Isonic
¼
isonicotinate, Nic
¼
b
nicotinate. All other ionic liquids were liquid at room temperature
(see Table 2). c The melting point was determined by DSC.
ꢀ
temperature was raised to 60 C, the half-life time dropped to
0.4 hours. The higher base stability of [BEDMA]+ ionic liquids
can be explained by the steric hindrance caused by the
branching, which limits the accessibility of the b-hydrogen by
the base.
Table 2 Physicochemical properties of the room temperature ionic
liquids
Densityc
Viscosityd
(cP)
Water contente
(ppm)
Compounda,b
(g cmꢁ3
)
[BEDMA][CH3COO]
[BEDMA]
0.98
1.01
1445
2500
610
720
Conclusions
[CH3CH2COO]
[BEDMA][CF3COO]
[BEDMA][HCOO]
[BEDMA][CH3SO3]
[BEDMA][Pic]
A range of ionic liquids based on the bis(2-ethylhexyl)dime-
thylammonium cation, [BEDMA]+, were prepared by a halide-
free route starting from the readily available secondary amine
bis(2-ethylhexyl)amine. Most of the ionic liquids prepared are
0.90
0.91
0.96
1.22
1.29
1500
2600
2850
N.d.
810
670
760
780
780
[BEDMA][HSO4]
4500
1
liquid at room temperature. The H NMR studies showed that
a
All the other ionic liquids are solids (see Table 1). b Abbreviation: Pic ¼
Viscosity measured at 25 ꢀC, n.d.: not determined, due to
this new class of ionic liquids is very stable under strongly
alkaline conditions. The ionic liquids are completely miscible
with water or polar organic solvents, but form a biphasic
mixture with diethylether or toluene.
c
picolinate (pyridine-2-carboxylic acid). Density measured at 19 ꢀC.
d
e
experimental difficulties (viscosity too high).
coulometric Karl Fischer titration.
Measured by
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RSC Advances
20 N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37,
123–150.
Acknowledgements
21 S. Chowdhury, R. S. Mohan and J. L. Scott, Tetrahedron, 2007,
63, 2363–2389.
22 V. K. Aggarwal, I. Emme and A. Mereu, Chem. Commun.,
2002, 1612–1613.
This project was supported by the KU Leuven (projects IDO/05/
005, GOA 08/05, IOF-KP CO2IL and a DBOF PhD grant to KCL),
by the IWT-Flanders (SBO-project “MAPIL”) and by the FWO-
Flanders (Research Community “Ionic Liquids”).
23 S. T. Handy and M. Okello, J. Org. Chem., 2005, 70, 1915–
1918.
24 E. Ennis and S. T. Handy, Curr. Org. Synth., 2007, 4, 381–389.
References
1 J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508–3576. 25 R. Giernoth and D. Bankmann, Eur. J. Org. Chem., 2008,
2 T. Welton, Chem. Rev., 1999, 99, 2071–2083.
2881–2886.
3 P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000, 26 S. T. Handy, J. Org. Chem., 2006, 71, 4659–4662.
39, 3772–3789.
27 J. C. Hsu, Y. H. Yen and Y. H. Chu, Tetrahedron Lett., 2004,
4 M. J. Earle and K. R. Seddon, Pure Appl. Chem., 2000, 72,
1391–1398.
45, 4673–4676.
28 E. Ennis and S. T. Handy, Molecules, 2009, 14, 2235–
5 V. I. Parvulescu and C. Hardacre, Chem. Rev., 2007, 107,
2615–2665.
2245.
29 V. Jurcik and R. Wilhelm, Green Chem., 2005, 7, 844–
6 F. Endres, ChemPhysChem, 2002, 3, 144–154.
848.
7 S. Z. El Abedin and F. Endres, ChemPhysChem, 2006, 7, 58– 30 C. Maton, N. De Vos, B. I. Roman, E. Vanecht, N. R. Brooks,
61.
K. Binnemans, S. Schaltin, J. Fransaer and C. V. Stevens,
ChemPhysChem, 2012, 13, 3146–3157.
31 L. Ford, F. Ate, R. D. Singer and P. J. Scammells, Eur. J. Org.
Chem., 2011, 942–950.
32 T. Ramnial, D. D. Ino and J. A. C. Clyburne, Chem. Commun.,
2005, 325–327.
33 T. Ramnial, S. A. Taylor, M. L. Bender, B. Gorodetsky,
P. T. K. Lee, D. A. Dickie, B. M. McCollum, C. C. Pye,
C. J. Walsby and J. A. C. Clyburne, J. Org. Chem., 2008, 73,
801–812.
8 M. C. Buzzeo, R. G. Evans and R. G. Compton,
ChemPhysChem, 2004, 5, 1106–1120.
9 A. P. Abbott and K. J. McKenzie, Phys. Chem. Chem. Phys.,
2006, 8, 4265–4279.
10 M. Armand, F. Endres, D. R. Macfarlane, H. Ohno and
B. Scrosati, Nat. Mater., 2009, 8, 621–629.
11 M. Galinski, A. Lewandowski and I. Stepniak, Electrochim.
Acta, 2006, 51, 5567–5580.
12 D. B. Kuang, C. Klein, Z. P. Zhang, S. Ito, J. E. Moser,
S. M. Zakeeruddin and M. Gratzel, Small, 2007, 3, 2094–2102. 34 T. Ramnial, S. A. Taylor, J. A. C. Clyburne and C. J. Walsby,
13 S. Werner, M. Haumann and P. Wasserscheid, Annu. Rev.
Chem. Biomol. Eng., 2010, 1, 203–230.
14 H. Zhao, Chem. Eng. Commun., 2006, 193, 1660–1677.
Chem. Commun., 2007, 2066–2068.
35 H. J. Cristau, A. Long and H. Christol, Tetrahedron Lett.,
1979, 349–352.
15 J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E. Visser 36 E. V. Dehmlow and V. Knunke, J. Chem. Res. -S, 1989, 224–
and R. D. Rogers, Chem. Commun., 1998, 1765–1766.
225.
16 X. Han and D. W. Armstrong, Acc. Chem. Res., 2007, 40, 1079– 37 D. Landini, A. Maia and A. Rampoldi, J. Org. Chem., 1986, 51,
1086.
3187–3191.
17 S. Wellens, B. Thijs and K. Binnemans, Green Chem., 2012, 38 D. C. Stepinski, G. F. Vandegri, I. A. Shkrob, J. F. Wishart,
14, 1657–1665.
K. Kerr, M. L. Dietz, D. T. D. Qadah and S. L. Garvey, Ind.
Eng. Chem. Res., 2010, 49, 5863–5868.
39 M. L. Good, F. F. Holland and S. C. Srivastava, Anal. Chim.
Acta, 1964, 31, 534–544.
18 T. Vander Hoogerstraete, S. Wellens, K. Verachtert and
K. Binnemans, Green Chem., 2013, 15, 919–927.
19 A. P. Abbott, G. Frisch, S. J. Gurman, A. R. Hillman,
J. Hartley, F. Holyoak and K. S. Ryder, Chem. Commun., 40 K. C. Lethesh, D. Parmentier, W. Dehaen and K. Binnemans,
2011, 47, 10031–10033.
RSC Adv., 2012, 2, 11936–11943.
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