Synthesis, characterization and thermal properties of thiosalicylate ionic liquids

Article abstract of DOI:10.1007/s12039-013-0516-5

In an attempt to produce new functionalized ionic liquids, a series of thiosalicylate ionic liquids based on imidazolium, ammonium, phosphonium, choline and pyrrolidinium cations were synthesized. The compounds were characterized by Infra Red (IR), Nuclear Magnetic Resonance (NMR) and mass spectra (ESI-MS). Their glass-transition temperatures, melting points and decomposition temperatures have been measured. Physicochemical properties of ionic liquids are influenced by alkyl chain length and nature of the cation of ionic liquids. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-013-0516-5

c
J. Chem. Sci. Vol. 125, No. 6, November 2013, pp. 1511–1515. ꢀ Indian Academy of Sciences.
Synthesis, characterization and thermal properties of thiosalicylate
ionic liquids
CECILIA DEVI WILFRED∗ and FADWA BABIKER MUSTAFA
Fundamental and Applied Sciences Department, Universiti Teknologi PETRONAS,
Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia
e-mail: cecili@petronas.com.my
MS received 23 June 2013; revised 15 August 2013; accepted 21 August 2013
Abstract. In an attempt to produce new functionalized ionic liquids, a series of thiosalicylate ionic liquids
based on imidazolium, ammonium, phosphonium, choline and pyrrolidinium cations were synthesized. The
compounds were characterized by Infra Red (IR), Nuclear Magnetic Resonance (NMR) and mass spectra
(ESI-MS). Their glass-transition temperatures, melting points and decomposition temperatures have been mea-
sured. Physicochemical properties of ionic liquids are influenced by alkyl chain length and nature of the cation
of ionic liquids.
Keywords. Thiosalicylate; imidazolium; ammonium; phosphonium; choline; pyrrolidinium.
1. Introduction
Formation of ionic liquids were carried out by qua-
ternization of imidazole with haloalkane. Removal of
excess haloalkanes and reaction solvents is generally
not a problem, especially for the relatively volatile
shorter chain haloalkanes. This is followed by the
metathesis process which provides the desired anions
of ionic liquids. ILs were also be prepared by acid/base
neutralization reaction hydroxide ILs with thiosali-
cyalte acids. A greener method via methyl carbon-
ate precursors was applied and precursors then reacted
with acids to yield ionic liquids of interest. Seven
thiosalicylate ionic liquids were prepared and their
Ionic liquids (ILs) are organic salts that become liquids
usually below about 200◦C. They are salts where the
ions are poorly coordinated. At least one ion has a delo-
calized charge and one component is organic, which
prevents the formation of a stable crystal lattice, result-
ing in it being liquid below 200◦C. Properties such as
melting point, viscosity and solubility of starting mate-
rials and other solvents are determined by substituents
on the organic component and the counter ion. Ionic
liquids are non-volatile, have high thermal stability and
have recently gained interest in new solvent applica-
tions such as electrochemistry,1,2 separation,3–5 synthe-
sis6,7 and catalysis.8–12 Many ILs have been developed
for specific synthetic problems. The collection of ILs
based on the combinations of cations and anions has
dramatically increased, and constantly new salts13 and
solvent mixtures14 are prepared.
Our interest was kindled from the possibility of
preparing a new interesting class of ILs from sulphur-
containing anions. There is interest in the coordination
chemistry of deprotonated thiosalicylic acid and thios-
alicylate, since the combination of hard carboxylate and
soft thiolate donors makes it potentially able to form
complexes with a wide range of metal centres. The
examples in literature are limited to a few ionic liquids
and there is a need to synthesis different types of ionic
liquids attached to various cations.
1
molecular structures were confirmed using H NMR,
13C NMR, IR and mass spectroscopy. Their ther-
mal properties, i.e, decomposition temperatures, melt-
ing points and glass-transitions temperatures were
recorded.
2. Experimental
All reagents and solvents were pure analytical grade
materials purchased from commercial sources and were
used without further purification. Structures of eight
thiosalicylate functionalised ionic liquids are shown
here in figure 1.
2.1 Characterization of ionic liquids
Structures of the ionic liquids were analysed by IR,
NMR and ESI-MS. A Bruker Avance 300 Nuclear
Magnetic Resonance Spectrophotometers was used to
∗For correspondence
1511

1512
Cecilia Devi Wilfred and Fadwa Babiker Mustafa
O
O
O
O
N
N
N
N
SH
SH
II = [1-Propyl-3-methylimidazolium] thiosalicylate
I = [1-Butyl-3-methylimidazolium] thiosalicylate
O
N
N
O
SH
III = [1-Octyl-3-methylimidazolium] thiosalicylate
+
+
O
O
O
O
N
N
SH
SH
IV = [N,N-methylbutylpyrrolidinium] thiosalicylate
V = [Methyltributylammonium] thiosalicylate
+
O
O
CH2(CH2
P
)4CH3
+
O
HO
N
O
CH2(CH2
)4CH3
H3C(H2C)4H2
C
SH
CH2(CH2 12CH3
)
SH
VI = [P6,6,6,14] thiosalicylate
VII = [Choline] thiosalicylate
Figure 1. Structure of ionic liquids.
1
determine the H and 13C NMR spectra of ILs using using Mettler-Toledo STAR software. Measurement
DMSO-d6. Chemical shifts were reported in parts per was carried out at a scan rate of 5◦C min−1.
million (ppm) from tetramethylsilane, TMS reference
ppm units with use of δ scale. Multiplicities are abbrevi-
ated as s, singlet; d, doublet; t, triplet; and m, multiplet.
The IR spectra were recorded in a Shimadzu FTIR-
2.2 Preparation of thiosalicylate functionlized
ionic liquids
8400S Fourier Transform Infrared Spectrometer (FTIR) 2.2a [1-Butyl-3-methylimidazolium] thiosalicylate:
in the mid-region (4000–400 cm−1) using the Atten- 1-Methylimidazole (0.1 mol) and 1-chlorobutane
uated Total Reflectance (Miracle ATR) measurement (0.12 mol) was added directly to a 50 ml acetonitrile in
mode. ESI-MS spectra were obtained on a Jeol-Agilent a 250 ml round bottommed flask equipped with a reflux
system.
condenser and magnetic stirrer. The mixture was left
Thermal stability of the synthesized ILs was inves- to stir at reflux for 48 h. The top layer was decanted
tigated using a thermogravimetric analyzer (TGA, and the bottom phase was extracted thrice with ethyl
Perkin-Elmer, Pyris V-3.81) at temperatures range of actetate. The product was evaporated under reduced
50–500◦C, heating rate of 10◦C min−1 (with a tem- pressure and dried in vacuo at 70◦C for 24 h. Halide
perature accuracy better than 2◦C) and under a nitro- salts of 1-butyl-3-methylimidazolium was reacted
gen atmosphere supplied at the rate of 20 mL min−1. with equimolar amount of sodium thiosalicylates in a
Glass transition temperatures, Tg and melting points, Tm mixture of acetone–water for 12 h. The solvent was
were recorded on a Mettler Toledo Differential Scan- removed and methanol added to precipitate sodium
ning Calorimeter (DSC), model and data were evaluated halide salt. The salt was filtered out and the filtrate

Synthesis of thiosalicylate ionic liquids
1513
1
concentrated and dried in vacuo. Yield 76%. H NMR 153.0, 156.0, 172.2; IR (v, cm−1) 2964, 2876, 1640,
(300 MHz, DMSO-d6): δ 0.89 (3H, t), 1.20 (2H, t), 1576, 1541, 1441; ES+ mz 142 (100%, C9H20N+), ES-
1.69 (2H, t), 3.69 (3H, s), 4.15 (2H, d), 6.66–6.70 (2H, m/z 152.99 (100%, C7H5O2S+).
m), 6.71 (1H, m), 6.91 (1H, m), 7.78 (1H, m), 9.33
(1H, m); 13C NMR (75 MHz) 10.6, 12.1, 30.25, 35.5,
2.2e [Methyltributylammonium] thiosalicylate:
38.9, 126.2, 126.8, 121.0, 126.8, 126.9, 134.9, 138.1,
157.2, 169.1; IR (v, cm−1) 3396, 2960, 2360, 1750
1500, 1160; ES+ mz 139 (100%, C8H15N+), ES-m/z
153 (100%, C7H5O2S+).
Tributylamine was reacted with excess dimethyl car-
bonate in methanol in a microwave quartz reaction tube,
sealed and heated with magnetic stirring to 140◦C. The
unreacted amine was decanted and the bottom layer
washed with excess ethyl acetate. The IL-precursor was
then dried and brought to react with thiosalicylic acid
at rt for 4 h. The solvent was removed and the IL dried
in vacuo. Yield 76%. 1H NMR (300 MHz, DMSO-d6):
δ 0.78 (9H, t), 1.18 (6H, m), 1.51 (6H, m), 2.77 (3H, s),
3.03 (6H, m), 6.95–7.64 (4H, m); 13C NMR (75 MHz)
13.5, 19.7, 23.9, 48.1, 61.9, 124.6, 126.1, 129.9, 130.9,
136.5, 173.3; IR (v, cm−1) 2956, 2886, 1645, 1597,
1530, 1449; ES+ mz 200.24 (100%, C13H30N+), ES-
m/z 152.99 (100%, C7H5O2S+).
2.2b [1-Propyl-3-methylimidazolium] thiosalicylate:
Similar procedure as earlier was carried out between
1-methylimidazole and n-propyl chloride to give [1-
propyl-3-methylimidazolium] chloride. The halide salt
was subjected to metathesis reaction to give the desired
1
ionic liquid. Yield 83%. H NMR (300 MHz, DMSO-
d6): δ 0.85 (3H, t), 1.81 (2H, m), 3.87 (3H, s), 4.13 (2H,
t), 6.69 (1H, m), 6.93 (1H, m), 7.24 (1H, m), 7.75 (2H,
m), 7.87 (1H, m), 9.19 (1H, s); 13C NMR (75 MHz)
16.1, 19.8, 22.0, 25.3, 47.3, 62.9, 64.2, 123.3, 136.1,
149.1, 151.2, 167.5; IR (v, cm−1) 3138, 2965, 1567,
1459, 1169; ES+ mz 124 (100%, C7H12N+2 ), ES- m/z
153 (100%, C7H5O2S+).
2.2f [P6,6,6,14] thiosalicylate: [P6,6,6,14] thiosalicylate
chloride was added to equimolar sodium thiosalicy-
lates in a mixture of acetone–water for 12 h. The sol-
vent was removed and methanol added to precipitate
the sodium halide salt. The salt was filtered out and
the filtrate concentrated and dried in vacuo. Yield 72%.
1H NMR (300 MHz, DMSO-d6): δ 0.92 (12H, t), 1.62
(48H, m), 2.03 (9H, t), 6.60–7.80 (4H, m); 13C NMR
(75 MHz) 15.4, 18.6, 18.7, 22.4, 22.6, 36.8, 37.2, 53.4,
119.2, 120.1, 130.6, 138.4, 139.2, 158.4, 169.1; IR (v,
cm−1) 3394, 2900, 2360, 1650 ES+ mz 483 (100%,
C32H68P+), ES- m/z 153 (100%, C7H5O2S+).
2.2c [1-Octyl-3-methylimidazolium] thiosalicylate:
Similar procedure as earlier was carried out between
1-methylimidazole and n-octyl chloride to give [1-
octyl-3-methylimidazolium] chloride. The halide salts
underwent metathesis reaction to give the desired ionic
1
liquid. Yield 79%. H NMR (300 MHz, DMSO-d6): δ
0.88 (3H, t), 1.20 (2H, t), 1.68–1.76 (10H, m), 3.72
(3H, s), 4.18 (2H, d), 6.68–6.72 (2H, m), 6.75 (1H, m),
6.92 (1H, m), 7.80 (1H, m), 9.35 (1H, m); 13C NMR
(75 MHz) 10.1, 14.2, 16.5, 19.8, 22.0, 23.2, 25.4, 27.3,
47.5, 62.6, 65.2, 124.3, 137.2, 150.1, 151.5, 168.2; IR
(v, cm−1) 3140, 2960, 1580, 1450, 1164; ES+ mz 195
(100%, C12H23N+2 ), ES- m/z 153 (100%, C7H5O2S+).
2.2g [Choline] thiosalicylate: Equimolar concentra-
tion of choline hydroxide was reacted with thiosalicylic
acid in acetone at room temperature for 2 h. The solvent
was removed and the ionic liquid dried in vacuo. Yield
1
83%. H NMR (300 MHz, DMSO-d6): δ 3.16 (9H, s),
3.42 (2 H, m), 3.88 (2H, m), 6.80 (1H, m), 7.07 (1H, m),
7.42 (1H, m), 8.02 (1H, m); 13C NMR (75 MHz) 54.0,
55.3, 67.1, 120.3, 128.3, 130.7, 132.1, 136.1, 156.8,
170.7; IR (v, cm−1) 2924, 2842, 1560, 1472, 1365; ES+
mz 104.11 (100%, C5H14NO+), ES- m/z 153.00 (100%,
C7H5O2S+).
2.2d [N,N-methylbutylpyrrolidinium] thiosalicylate:
1-Butylpyrrolidine was reacted with excess dimethyl
carbonate in methanol in a microwave quartz reac-
tion tube, sealed and heated with magnetic stirring
to 130◦C. [N,N-methylbutylpyrrolidinium] methylcar-
bonate. The precursor was then brought to react with
thiosalicylic acid at room temperature for 4 hour. The
solvent was removed and the IL dried in vacuo. Yield
3. Results and discussion
1
84%. H NMR (300 MHz, DMSO-d6): δ 0.98 (3H, t),
3.1 Synthesis
1.20 (2 H, m), 1.46 (2H, m), 1.87 (4H, m), 2.72 (3H, s),
3.04 (2H, m), 3.14 (4H, m), 6.67 (1H, m), 6.89 (1H, m),
7.25 (1H, m), 7.80 (1H, m); 13C NMR (75 MHz) 12.6,
19.2, 20.9, 25.1, 53.7, 63.7, 120.3, 130.3, 131.0, 135.1,
Ionic liquids I, II and III were prepared via metathesis
of the halide salt of the precursors. The route is straight-
forward, however the task of removing sodium halides

1514
Cecilia Devi Wilfred and Fadwa Babiker Mustafa
Table 1. Thermal properties of thiosalicylate ionic liquids.
Ionic liquids
T
dec/◦C (5%)
Tg/◦C
I
II
III
IV
V
[1-Butyl-3-methylimidazolium] thiosalicylate
[1-Propyl-3-methylimidazolium] thiosalicylate
[1-Octyl-3-methylimidazolium] thiosalicylate
[N, N-methylbutylpyrrolidinium] thiosalicylate
[Methyltributylammonium] thiosalicylate
[P6,6,6,14] thiosalicylate
203.0
224.0
195.0
216.7
188.0
225.0
187.5
−61.95
−55.52, 91.22∗
−64.45
−54.8
−35.8
VI
−66.2
VII [Choline] thiosalicylate
−49.5
∗Melting point
proved to be tedious. This is because both the ionic gave water-miscible ionic liquids with the exception of
liquids containing sulphur and the inorganic salt have [P6,6,6,14] cation. It is predicted that hydrophobicity of
similar solubility properties in organic and aqueous an ionic liquid increases with the length of the alkyl
solutions. Through screening the compatible solvent for chain on the cation. The water-miscibility character is
precipitation, we found that sodium halides are less important as these ionic liquids could be applied as
soluble in methanol compared to imidazolium thiosali- water pollutant extractants.
cylates. Hence, we managed to remove the inorganic
salt by adding methanol to the reaction mixture which
gave the inorganic salt. The four protons of aromatic
3.2 Thermal properties
hydrogen atoms of these thiosalicylate ionic liquids had
chemical shifts at 6.66–7.87 ppm. FTIR spectra of I,
Thermal properties of ionic liquids are shown in table 1.
Thiosalicylates showed glass-transition tempera-
tures, from −66 to −35◦C. They are highly viscous
liquids, with the exception of II, which has a gel-
like feature at room temperature, and indicated a clear
melting point at 91◦C. [P6,6,6,14] thiosalicylate exhi-
bited lower glass-transition temperature than those of
imidazolium, pyrrolidinium and ammonium. 1-Octyl-
3-methylimidazolium thiosalicylate showed lower Tg
than its butyl and propyl chain cations. A longer chain
attached to a imidazolium cation is speculated to show
better liquidus properties than a shorter one.
II and III showed absorption bands at ∼1160, 1466
and 1562 which could be assigned to the stretching and
bending vibrations of the aromatic C-H group. For imi-
dazolium, the N-H, C-H and C-N ring stretching vibra-
tions were observed at ∼3156, 3116, and 1575 cm−1,
respectively. The C-H stretching vibration of CH2 was
observed at ∼2960 cm−1.
All the ESI-MS experiments results showed a single
peak at m/z = 153.2, which could be assigned to the
thiosalicylate anion.
The Tdec/◦C (5%) is the onset temperature which
was obtained from the intersection of the baseline
weight and the tangent of the weight vs temperature
curve obtained from TGA analysis as decomposition
occurs. From the TGA results, the imidazolium cation
is more thermally stable than ammonium cation. Ngo
et al.16 and Begg et al.17 reported that the imidazole
is resistant to ring fission in thermal exposure. In
addition, 1-propyl-3-methylimidazolium thiosalicylate
cation showed better thermal stability than its octyl and
butyl cations. It is suggested that the thermal stabil-
ity decreases as the organic content of the molecule
increases. [P6,6,6,14] thiosalicylate, VI showed the high-
est thermal stability due to its cation structure.
Compounds IV and V were prepared from their
methyl carbonate precursor as described in an earlier
finding.15 An excess of dimethyl carbonate solvent
with the base was heated in a microwave reactor. With-
out any work-up, the mixture was then brought to react
with thiosalicylic acid at room temperature. The route
proved easy and greener as no halide impurities would
be present in the ionic liquids, hence separation step is
unnecessary.
Compound VI was synthesized by exchanging the
chloride ion of [P6,6,6,14] with thiosalicylate ion. The
reaction was carried out in biphasic solution as the
ionic liquid produced was immiscible in water. Com-
pound VII was easily synthesized by neutralization of
choline hydroxide with thiosalicylic acid. This gave a
water-soluble ionic liquid.
All the thiosalicylate-based ionic liquids studied pre-
viously16 are immiscible with water. Hydrophilicity of
thiosalicyalte ionic liquids are very much dependent
on the nature of the cation. Findings from this study
4. Conclusion
Seven thiosalicylate ionic liquids have been syn-
thesized. Their glass-transition temperatures, melting

Synthesis of thiosalicylate ionic liquids
1515
points and decomposition temperatures have been
measured. We find a general trend in the influence of
alkyl chain length and nature of cation on physicoche-
mical properties of ionic liquids. With similar cations,
ionic liquids containing 1-propyl-3-methylimidazolium
cations exhibit higher melting points but lower water
solubilities than those ionic liquids containing octyl-
and butylimidazolium cations. On the other hand,
imidazolium cations exhibit greater stability than ionic
liquids containing ammonium cations. The most sta-
ble ionic liquids among the thiosalicylates were the
phosphonium cations.
6. Sheldon R 2001 Chem. Commun. 2399
7. Welton T 2004 Coord. Chem. Rev. 248 2459
8. Dupont J, De Souza R F and Suarez P A Z 2002 Chem.
Rev. 102 667
9. Gordon C M 2001 Appl. Catal. 222 101
10. Wilkes J S 2004 J. Mol. Catal. A 214 11
11. Huddleston J G, Visser A E, Reichert W M, Willauer
H D, Broker G A and Rogers R D 2001 Green Chem.
3 156
12. (a) Pandey S, Baker G A, Sze L, Pandev S, Kamath G,
Zhao H and Baker S N 2013 New J. Chem. 37 909;
(b) Cojocaru O A, Shamshina J L, Gurau G, Syguda A,
Praczyk T, Pernak J and Rogers R D 2013 Green Chem.
15 2110; (c) Katritzky A R, Singh S, Kirichenko K,
Holbrey J D, Smiglak M, Reichert W M and Rogers R D
2005 Chem. Commun. 868;(d) Fukumoto K, Yoshizawa
M and Ohno H 2005 J. Am. Chem. Soc. 127 2398; (e)
Carter E B, Culver S L, Fox P A, Goode R D, Ntai I,
Tickell M D, Traylor R K, Hoffman N W and Davis
J H 2004 Chem. Commun. 630;(f) Keglevich G, Baan Z,
Hermecz I, Novak T and Odinets I L 2007 Org. Chem.
11 107
13. (a) Imperato G, Eibler E, Niedermaier J and König B
2005 Chem. Commun. 5 1170; (b) Imperato G, König B
and Chiappe C 2007 Eur. J. Org. Chem. 1049
14. Wilfred C D, Holbrey J D, Rogers R D and Shukla S
2010 Green Chem. 12 407
15. Kogelnig D, Stojanovic A, Galanski M, Groessl M, Jirsa
F, Krachler R and Keppler B K 2008 Tetrahedron Lett.
49 2782
Acknowledgement
Authors thank the University Teknologi PETRONAS
for providing research and library facilities.
References
1. Endres F and Abedin S Z E 2002 Chem. Commun.
892
2. Endres F and Abedin S Z E 2006 Phys. Chem. Chem.
Phys. 8 2102
3. Huddleston J G, Willauer H D, Swatloski R P and Visser
A E 1998 Chem. Commun. 1765
4. Swatloski R P, Visser A E, Reichert W M and Mayton R 16. Ngo H L, LeCompte K, Hargens L and McEwen A B
2001 Chem. Commun. 135 2000 Thermochim. Acta 357 97
5. Zhao H, Xia S Q and Ma P S 2005 J. Chem. Technol. 17. Begg C G, Grimmett M R and Wethey P D 1973 Aust. J.
Biotechnol. 80 1089 Chem. 26 2435