Synthesis and characterization of task-specific ionic liquids based on peroxydisulfate and their application in oxidation reactions

Article abstract of DOI:10.1002/ejic.200901134

Four new task-specific ionic liquids based on the peroxydisulfate ion were synthesized by the ion exchange method and characterized by FTIR and ¹H NMR spectroscopy and elemental analysis. The physicochemical characteristics of these ionic liquids such as density, thermal, stability, conductivity, viscosity, and solubility in common solvents have been investigated in detail. The experimental results testify that the ionic liquids prepared show excellent performance as both oxidant and medium in given oxidation reactions of alcohols and thiols under mild reaction conditions.

Full text of DOI:10.1002/ejic.200901134

FULL PAPER
DOI: 10.1002/ejic.200901134
Synthesis and Characterization of Task-Specific Ionic Liquids Based on
Peroxydisulfate and Their Application in Oxidation Reactions
Shenyi Shi,[a] Aiguo Kong,[a] Xinhua Zhao,[a] Qiying Zhang,[a] and Yongkui Shan*[a]
Keywords: Ionic liquids / Peroxydisulfate salts / Imidazolium salts / Oxidation / Alcohols
Four new task-specific ionic liquids based on the peroxydi-
sulfate ion were synthesized by the ion exchange method
and characterized by FTIR and 1H NMR spectroscopy and
elemental analysis. The physicochemical characteristics of
these ionic liquids such as density, thermal stability, conduc-
tivity, viscosity, and solubility in common solvents have been
investigated in detail. The experimental results testify that
the ionic liquids prepared show excellent performance as
both oxidant and medium in given oxidation reactions of
alcohols and thiols under mild reaction conditions.
Oxidation reactions are key steps in the industrial syn-
thesis of chemicals. The catalytic oxidation of hydrocarbons
accounts for approximately one quarter of the catalytic pro-
cesses worldwide.[9] Although environmentally benign pro-
cesses to prepare chemicals by using molecular oxygen or
hydrogen peroxide as the oxidant have been presented,[10]
inorganic oxidants (such as permanganate, peroxydisulfate,
and high-valent oxochromium species) and conventional
volatile organic solvents are still widely utilized in the man-
ufacture of fine chemicals and pharmaceuticals. Ecological
and economic concerns associated with chemical synthesis
have posed stringent and compelling demands for greener,
more sustainable technologies. Obviously, in the develop-
ment of various oxidation processes, fine-tuning the reactiv-
ity of common oxidants, expanding the application of
“new” oxidants, avoiding the use of conventional solvents,
applying solvent-free reactions, and recycling reaction me-
dia are of prime importance to dramatically reduce energy
consumption, material cost, waste, and risk.
In this paper, we describe the synthesis and characteriza-
tion of a new series of TSILs with high oxidizing abilities
based on peroxydisulfate, 1-alkyl-3-methylimidazolium per-
oxydisulfate, denoted as [Cnmim]2[S2O8] {n = 4 (TSIL-1), 6
(TSIL-2), 8 (TSIL-3), 10 (TSIL-4)} (in which alkyl = butyl,
hexyl, octyl, decyl, respectively), and their performance as
both reactant and reaction medium in oxidation reactions
of alcohols and thiols.
Introduction
Task-specific ionic liquids (TSILs) attract enormous
interest due to the potential to modulate their physicochem-
ical properties through appropriate structure modification
or component change and to tailor the best ionic liquid for
each specific application.[1] Intelligent choice of the anion
and cation or the introduction of functional groups on the
components of ionic liquids can create a TSIL that has wide
anodic or cathodic limits (or both), more or less hydro-
phobic behavior, chiral character, acidic and/or basic prop-
erties, etc. A large number of TSILs have been created in
recent years because of this flexibility in their design. In
general, hydrophilic properties will be tuned by anion ex-
change of the ionic liquid moiety. For example, 1-butyl-3-
methylimidazolium ([bmim]) tetrafluoroborate ([BF4]) is a
hydrophilic ionic liquid, whereas its hexafluorophosphate
([PF6]) analogue is hydrophobic.[2] The basic ionic liquid
[bmim]OH can be obtained by the reaction of 1-butyl-3-
methylimidazolium bromide with potassium hydroxide,[3]
whereas an imidazole cation that bears an alkane sulfonic
acid group can behave as a Brønsted acid.[4] In an ionic
liquid designed for CO2 capture, 1-butyl-3-aminoprop-
ylimidazolium tetrafluoroborate, the cation core is as-
sembled by the reaction of 1-butylimidazole with 3-bromo-
propylamine hydrobromide in ethanol.[5] Ionic liquids bear-
ing thiol groups on the imidazolium cation can stabilize
nanomaterials in homogeneous fluids.[6] Although TSILs
have been investigated since their first introduction in
2001,[7] little work has concentrated on their intrinsic redox
behavior.[8]
Results and Discussion
Preparation
Four task-specific ionic liquids were prepared by the me-
tathesis reaction of 1-alkyl-3-methylimidazolium chloride
with potassium peroxysulfate, as shown in Scheme 1. These
TSILs are air- and water-stable.
[a] Department of Chemistry, East China Normal University,
Shanghai, 200062, China
Fax: +86-21-62233503
E-mail: ykshan@chem.ecnu.edu.cn
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S. Shi, A. Kong, X. Zhao, Q. Zhang, Y. Shan
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decomposition of the alkylimidazolium and sulfate ions. In
general, for strong oxidants such as peroxydisulfate salts, a
lower decomposition temperature means a higher reactivity.
Thus, the lower decomposition temperatures of TSILs con-
taining peroxydisulfate indicate that these task-specific
ionic liquids may possess high reactivities.
Scheme 1. Preparation of [Cnmim]2[S2O8] by ion exchange
([Cnmim] = 1-alkyl-3-methylimidazolium, n = 4, 6, 8, 10).
There were variations in the methods of preparation of
the TSILs because of their different hydrophobicity. TSIL-
4, which is a hydrophobic ionic liquid, was prepared in
aqueous solution, because the reactants are soluble in
water. The product spontaneously separated from the aque-
ous solution, which further promoted the formation of the
product and led to higher yields. TSIL-1, -2, and -3, which
are hydrophilic ionic liquids, were prepared in acetone solu-
tion, because the byproduct, KCl, is insoluble in acetone.
KCl spontaneously separated from the reaction system as a
precipitate, which also promoted the formation of the ionic
liquid and led to higher yields. Using these preparation
methods, four new task-specific ionic liquids were obtained
in high yield. The chloride content in all of the ionic liquids
produced was less than 0.1% w/w. Some physical character-
istics of the ionic liquids are listed in Table 1.
Table 1. Some physical characteristics of the ionic liquids produced.
Ionic liquid Density[a] Conductivity[a] Viscosity[a] Tm
(gcm–3)
(10–4 Scm–1)
(mPas)
(°C)
TSIL-1
TSIL-2
TSIL-3
TSIL-4
1.17
1.15
2.80
2.00
700
960
1450
–
–
–
25
48
1.12
1.40
1.10[b]
1.70[b]
[a] Measured at 40.0 °C. [b] Measured at 50.0 °C.
Thermal Properties
Figure 1. TG/DTA analysis for (1) [C6mim]2[S2O8] (TSIL-2) and
(2) [C8mim]2[S2O8] (TSIL-3). ∆T is the difference between sample
and reference temperatures.
Melting points (Tm) measured by the capillary tube
method are shown in Table 1. The fluid ionic liquids
showed a strong tendency to form a supercooled state when
they were cooled, especially [C4mim][S2O8] (TSIL-1), which
remains fluid even on cooling to –20 °C. The melting point
of an ionic liquid is governed by many factors, including
Coulombic, van der Waals, and H-bonding interactions.
The ionic liquids prepared here contain the same anion and
cation cores; therefore, the only difference between them is
the different alkyl chain length. The melting points of these
TSILs become higher with the increased alkyl chain length
on the imidazolium ring. This may be attributed to en-
hanced van der Waals interactions and is in agreement with
previous reports.[11] The thermal behavior of the ionic li-
quids was investigated by means of thermogravimetry/
differential thermal analysis (TG/DTA) measurements. In
the analysis of the samples, a prominent exothermal peak
at 95 °C combined with a little weight loss (about 3%) was
observed (Figure 1). This was attributed to the decomposi-
tion of the peroxydisulfate anion, resulting in the release
of oxygen. Further degradation and obvious mass loss that
Conductivity
Figure 2 shows the temperature dependence of the ionic
conductivity (δ) of the TSILs. The ionic conductivity de-
pends strongly on temperature and increases exponentially
with the rise in temperature. This can be ascribed to fast
ion transfer at high temperatures. The length of the alkyl
chain on the cation also has an influence on the conductivi-
ties of the TSILs: as the length of the alkyl chain increases,
the cation size increases, and the ionic transfer rate de-
creases, which results in a lower conductivity. Moreover,
Arrhenius-type plots of δ show convex upward curves for
TSIL-1 and TSIL-2 (Figure 2), whose ion-conducting be-
havior was analyzed with the Vogel–Tammann–Fulcher
(VTF) equation [Equation (1)].[12]
occurred between 300 and 360 °C may be attributed to the δ = δ0T–1/2exp[–B/(T – T0)]
(1)
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Task-Specific Ionic Liquids Based on Peroxydisulfate
ical window of about 1.2 V at 25 °C. Such redox stability
of the ionic liquid must be imposed by the reduction poten-
tial of the imidazolium cation and the oxidation potential
of the peroxydisulfate anion. The reduction reaction involv-
ing the imidazolium cation begins at approximately –0.6 V.
Although the standard oxidation potential of the peroxydis-
ulfate ion is 2.01 V in aqueous solution, its reduction pro-
cess on a Pt electrode begins at about 0.6 V in an ionic
liquid and is an irreversible electrode reaction. Its narrow
electrochemical window is related to its strong oxidizing
ability. The electrochemical window of an ionic liquid is an
important indicator of its ability to act as both the reactant
and the medium in a solvent-free reaction system.
Figure 2. Temperature dependence of ionic conductivity (δ) for
[Cnmim]2[S2O8] (n = 4, 6, 8, 10).
In Equation (1), δ0 is a pre-exponential constant pro-
portional to the number of carrier ions at infinitely high
temperature, B is the pseudo-activation energy for ion con-
duction, and T0 is the ideal glass transition temperature.
Straight lines obtained from this analysis are depicted in
Figure 3. These results illustrate that the temperature de-
pendencies of the ionic conductivity for these TSILs do not
obey the Arrhenius equation; however, they do obey the
VTF equation. The migration of the carrier ions was domi-
nated by a cooperative transport mechanism in the tem-
perature range studied.[13] For TSIL-3 and TSIL-4, the ex-
perimental data result in a straight line when plotted in
Arrhenius coordinates as shown in Figure 2. This demon-
strates that the temperature dependence of their ionic con-
ductivities obeys the Arrhenius equation, which implies that
migration of the carrier ions is dominated by an activated
jump mechanism in the temperature range studied.[13c,14]
Figure 4. Cyclic voltammogram of [C6mim]2[S2O8] at 25 °C; scan
rate: 0.1 Vs–1.
Dynamic Viscosity
Figure 5 shows the temperature dependence of dynamic
viscosity (η) for [Cnmim]2[S2O8] (n = 4, 6, 8) in the tempera-
ture range 298.15–333.15 K under atmospheric pressure. In
general, the viscosity of an ionic liquid is strongly governed
by Coulomb forces, van der Waals interactions, and hydro-
gen bonding.[15] Compared with common ionic liquids,
these TSILs have a high relative viscosity [700–1450 mPas
(25 °C), Figure 5]. A possible reason for this is the enhance-
ment of Coulombic interactions between the ionic species
as a result of the introduction of the divalent peroxydisulf-
ate anion. The increase in alkyl chain length also makes
the TSILs more viscous due to increased van der Waals
interactions. This may be the reason why the viscosity of
[C8mim]2[S2O8] is higher than those of [C4mim]2[S2O8] and
[C6mim]2[S2O8] (Figure 5) and comparable to that of
[C2mim][BF4].[16] Deviations from the Arrhenius law are
observed in the temperature range studied for TSIL-1 and
TSIL-2 (Figure 6). Arrhenius-type plots of η show convex
downward curves, which suggests that these data can be
fitted by the empirical VFT equation [Equation (2)].[12c]
Figure 3. VTF plots of the ionic conductivity of TSIL-1 (δ0 = 0.048
SK1/2 cm–1, B = 77.6 K, T0 = 278.8 K) and TSIL-2 (δ0 = 0.037
SK1/2 cm–1, B = 77.6 K, T0 = 277.4 K).
Cyclic Voltammetry
The electrochemical behavior of the TSILs was investi-
gated by cyclic voltammetry. Figure 4 shows the cyclic vol-
tammogram of [C6mim]2[S2O8] (TSIL-2) in an electrochem- η = η0T–1/2exp[B/(T – T0)]
(2)
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S. Shi, A. Kong, X. Zhao, Q. Zhang, Y. Shan
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materials preparation.[18] Most of these applications corre-
late closely with the solubility of ionic liquids in solvents or
the solubility of solutes in ionic liquids. In the present study,
the solubility of the ionic liquids in water and common or-
ganic solvents was investigated. The water solubility of the
ionic liquids was strongly dependent on the length of the
alkyl chain on the cation and followed the order [C4mim]
Ͼ [C6mim] Ͼ [C8mim] Ͼ [C10mim], which is similar to that
of [Cnmim][BF4] ionic liquids.[12] On the contrary, the solu-
bility in trichloromethane followed the order [C10mim] Ͼ
[C8mim] Ͼ [C6mim] Ͼ [C4mim]. All the ionic liquids
studied here were immiscible with nonpolar solvents such
as toluene, benzene, ethyl ether, and hexane, and were com-
pletely miscible with polar solvents such as acetone, aceto-
nitrile, and ethanol due to their high polarity (see Table 2).
The solubility of other substances in these ionic liquids was
also investigated, and phenol and amines (triethylamine,
aniline, etc) were found to be soluble. This solvent property
could make these ionic liquids an ideal new medium for
chemical reactions.
Figure 5. Temperature dependence of viscosity for [Cnmim]2[S2O8]
(n = 4, 6, 8).
Table 2. Miscibility of new ionic liquids in some common solvents
at room temperature (M: miscible, I: immiscible).
Ionic liquids H2O EtOH Acetone CH3CN CHCl3 Et2O Toluene
TSIL-1
TSIL-2
TSIL-3
TSIL-4
M
M
M
I
M
M
M
M
M
M
M
M
M
M
M
M
I
I
M
M
I
I
I
I
I
I
I
I
Figure 6. VTF plots of the viscosity of TSIL-1 (η0 = 1603 mPas
K1/2, B = 86.5 K, T0 = 269.4 K) and TSIL-2 (η0 = 2101 mPas Oxidation Reactions
K1/2, B = 90.2 K, T0 = 269.5 K).
The selective oxidation of cysteine to cystine is a funda-
mentally important laboratory and commercial procedure
and is conventionally carried out with stoichiometric
amounts of metallic complex ions such as [FeIII(bpy)2-
In Equation (2), η0 (mPas) is a pre-exponential constant
proportional to the viscosity coefficient, B (K) is the
pseudo-activation energy for viscous behavior, and T0 (K)
is the ideal glass transition temperature.
(CN)2]+,[19] [IrIVCl6]2–,[20] [CoIIIW12O40]5–,[21] [FeIII
-
(CN)6]3–,[22] and [Co2(CN)10(O)2]5–[23] in volatile organic
solvents. From both an economic and environmental point
of view, developing a green process for converting cysteine
to cystine on an industrial scale has been an important ob-
jective for chemists. In order to search for an appropriate
oxidant and medium for the development of a cost-effective
and environmentally benign synthesis, the ability of the
ionic liquids prepared to act as both reactive oxidant and
medium was scrutinized in the oxidation reaction of thiols.
The experimental results are shown in Table 3. The reactive
system comprises the ionic liquid and cysteine or 1-pen-
tanethiol as the substrate. The reaction takes place in the
homogeneous phase and has the advantages of being a sol-
vent- and catalyst-free system. The product of the reaction
with cysteine was isolated in 92% yield with 100% selectiv-
ity under moderate reaction conditions (see footnotes in
Table 3). At the end of the reaction, the ionic liquid was
easily recovered by extraction. Its regeneration was realized
by the electrolytic method,[24] because the hydrogen ion has
However, the temperature dependence of the viscosity of
TSIL-3 shown in Figure 3 is in agreement with Equa-
tion (3).
η = η0exp[Eη/RT]
(3)
In Equation (3), η0 is a constant, T is the absolute tem-
perature, and Eη is the energy of activation of viscous flow.
The data for TSIL-3 (Figure 5) fit Equation (3) well (r =
0.996) and yield a value for Eη of 46.2 kJmol–1. This is
larger than that of conventional molecular liquids but com-
parable to those of other room-temperature ionic liquids.[17]
It should be noted here that the sequence of viscosity in
this series of TSILs has close similarity with that of Tm (see
Table 1) and may be governed by the van der Waals force
and the electrostatic interaction force.
Solubility
Ionic liquids have attracted considerable attention for a higher reduction potential on the Pt electrode and the
their wide application in organic synthesis, catalysis, and imidazolium ring is inactive to oxidation at the anode.
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Task-Specific Ionic Liquids Based on Peroxydisulfate
Pt working electrode was used with Ag as quasi-reference electrode
and platinum as the counter electrode. Dynamic viscosities of these
ionic liquids were measured with a NDJ-79 rotating viscometer.
GC–MS analysis was completed with a CP-3900 GC/Saturn 2100
MS system. The product yields were determined with a GC-14C
instrument. The conditions for the GC run are listed in Table 4.
Melting points were measured with a SGW X4 microscopic melting
point instrument. The content of chlorine ion was determined by
an ICS-5200 Ion Chromatography System (Dionex IonPac AS11-
HC, 2ϫ 250 mm). Constant potential electrolysis of the spent ionic
Furthermore, by using this reactive system, benzyl alcohol
was converted to benzyl aldehyde in 96% yield and 100%
selectivity at room temperature after 1 h (Table 3). Second-
ary alcohols were oxidized to ketones with a lower yield,
whereas aliphatic primary alcohols (such as n-hexyl and n-
heptyl alcohol) were not oxidized to the corresponding al-
dehydes under the same conditions (Table 3). Unlike con-
ventional methods, this solvent- and catalyst-free reaction
system has many advantages including reduced pollution,
lower costs, and simplicity in process and handling. These liquids was performed with a CHI760 electrochemistry workstation
equipped with Pt as cathode and anode. The cell was put in a water
bath to avoid overheating during the electrolysis.
factors are especially important in industry.
Table 3. Oxidation of substrates by ionic liquids.[a]
Entry Substrate
Product
Ionic liquid Selectivity Yield[b]
Table 4. The conditions and retention times for the GC runs.
(%)
(%)
Samples
Column
Column
Detector
Retention
1
2
3
PhCH2OH
PhCH2OH
PhCH2OH
PhCHO
PhCHO
PhCHO
TSIL-1
TSIL-2
TSIL-3
TSIL-3
TSIL-3
TSIL-3
TSIL-3
94%
95%
96%
Ͻ1%
Ͻ1%
15%
40%
85%
100%
94%
95 %
96%
Ͻ1%
Ͻ1%
15%
40 %
85%
92%[d]
temp. (°C) temp. (°C) time (min)
PhCH2OH
PhCHO
RTX-1
120
250
250
250
250
250
320
4.05
3.64
6.18
4.64
8.55
6.49
4.73
3.32
7.34
5.80
3.44
9.76
4
5
n-C5H11CH2OH n-C5H11CHO
n-C6H13CH2OH n-C6H13CHO
n-C5H11CH2OH
n-C5H11CHO
n-C6H13CH2OH
n-C6H13CHO
Isopropyl alcohol
Acetone
Cyclohexanol
Cyclohexanone
1-Pentanethiol
Dipentyl disulfide
DB-WAX 120
DB-WAX 120
DB-WAX 50
DB-WAX 120
6
7
2-propanol
cyclohexanol
1-pentanethiol
cysteine
acetone
cyclohexanone
dipentyl disulfide TSIL-3
cystine TSIL-3
8
9[c]
[a] Molar ratio of substrate to ionic liquid = 1:2; reaction time =
1 h; temperature = 80 °C. [b] Yields determined by GC based on
the substrate. [c] Room temperature. [d] Isolated yield.
RTX-1
120–300[a]
[a] Programmed temperature.
Conclusions
Synthesis
Four oxidizing ionic liquids based on peroxydisulfate
anions were synthesized and characterized. These ionic li-
quids have the desired oxidizing properties for given oxi-
dation reactions of alcohols and thiols and perform excep-
tionally well as both oxidant and medium in a solvent- and
catalyst-free reaction system. They make the oxidation pro-
cesses more environmentally benign and cost-effective. This
research work will enrich the knowledge of ionic liquid
chemistry and extend the range of applications of ionic li-
quids.
1-Alkyl-3-methylimidazolium chloride compounds ([Cnmim][Cl], n
= 4, 6, 8, 10) were prepared by an adaptation of a reported pro-
cedure[25] and were characterized by 1H NMR and IR spectroscopy.
1-Alkyl-3-methylimidazolium peroxydisulfate ([Cnmim]2[S2O8])
compounds were synthesized by the metathesis reaction of the ap-
propriate 1-alkyl-3-methylimidazolium chloride with potassium
peroxydisulfate in acetone solution or water (Scheme 1).
[C4mim]2[S2O8] (TSIL-1): This compound was prepared by the re-
action of [C4mim][Cl] (0.1 mol, 17.5 g) with K2S2O8 (0.055 mol,
14.8 g) in acetone (150 mL). The solution was stirred for 24 h at
room temperature and then filtered. The solvent was removed un-
der reduced pressure at room temperature. The product was further
dried under high vacuum for 24 h. The product was obtained as a
light-yellow liquid in 82% yield. The same method was used to
prepare [C6mim]2[S2O8] (TSIL-2; yield: 88%) and [C8mim]2[S2O8]
(TSIL-3; yield: 88%).
Experimental Section
Chemicals and Analysis: 1-Chlorodecane, 1-chlorooctane, 1-chlo-
rohexane, and 1-chlorobutane (Shanghai Reagent Co. China) were
distilled before use. 1-Methylimidazole was vacuum-distilled prior
to use. Reagent grade ethyl acetate, acetone, dichloromethane, ace-
tonitrile, absolute ethanol, alcohols, and K2S2O8 were obtained
from Sinopharm Chemical Reagent Co. Ltd. China and used with-
out further purification. 1H NMR spectra were recorded with a
Bruker DRX-500 spectrometer in CDCl3 or D2O relative to a TMS
internal standard. IR spectra were recorded as liquids or as solid
KBr disks with a NEXUS 670 FTIR infrared spectrometer (Nico-
let). Elemental analysis was performed with a Vario EL III ele-
mentar instrument. The thermal stability of the ionic liquids was
investigated with a TG/DTA851 thermogravimetric analyzer at a
[C10mim]2[S2O8] (TSIL-4): This ionic liquid was prepared by the
reaction of a solution of K2S2O8 (0.055 mol, 14.8 g) in water
(150 mL), which was added dropwise to a rapidly stirring solution
of 1-decyl-3-methylimidazolium chloride (0.1 mol, 25.8 g) in water
(100 mL) at 0 °C over 30 min. The reaction mixture separated into
two phases; the lower phase was collected, washed with water (2ϫ
100 mL), and dried under high vacuum at room temperature for
24 h. After drying, [C10mim]2[S2O8] was obtained as a white solid
in 80% yield (25.5 g).
heating rate of 10 °Cmin–1 with nitrogen as the purge gas. Conduc- TSIL-1: C16H30N4O8S2 (470.57): calcd. C 40.9, H 6.4, N 11.9, S
tivity was measured with a DDS-11 conductivity meter. Electro-
chemical stability was analyzed by cyclic voltammetry, in which a
13.6; found C 41.2, H 6.1, N 12.0, S 12.8. IR: ν = 3150, 3098, 2930,
2859, 1659, 1612, 1384, 1285, 1167, 1135, 1066, 1002, 728, 622,
˜
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S. Shi, A. Kong, X. Zhao, Q. Zhang, Y. Shan
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591, 561 cm–1. 1H NMR (500 MHz, D2O, 25 °C): δ = 0.80 [t, 3JH,H
of adsorbed water in samples. It indicates that storage in vacuo was
required to obtain the ionic liquid containing little or no water.
The 1H NMR spectrum of TSIL-3 (Figure 8) revealed nine very
strong, well-resolved resonance lines. The resonances with the
chemical shifts δ = 7.26, 7.36, and 9.25 ppm were attributed to
the protons in the imidazolium ring. The singlet at δ = 4.24 ppm
corresponds to the protons of the methyl group on the imidazolium
nitrogen atoms (NCH3). The chemical shifts of protons of the octyl
group on the imidazolium ring were δ = 0.86 (3 H), 1.24–1.28 (m,
3
3
= 7.5 Hz, 3 H], 1.20 (q, JH,H = 7.5 Hz, 2 H), 1.73 (m, JH,H
7.5 Hz, 2 H), 3.77 (s, 3 H), 4.08 (t, JH,H = 7.5 Hz, 2 H), 7.30 (s, 1
=
3
H), 7.35 (s, 1 H), 8.58 (s, 1 H) ppm.
TSIL-2: C20H38N4O8S2 (526.67): calcd. C 45.6, H 7.2, N 10.6, S
12.2; found C 46.2, H 6.7, N 11.0, S 12.3, IR: ν = 3150, 3095, 2925,
˜
2855, 1636, 1573, 1384, 1283, 1170, 1064, 723, 620, 591, 561 cm–1.
3
1H NMR (500 MHz, D2O, 25 °C): δ = 0.76 (t, JH,H = 6.5 Hz, 3
H), 1.20 (m, 6 H), 1.78 (m, 3JH,H = 6.5 Hz, 2 H), 3.80 (s, 3 H), 4.10
1
10 H), and 1.84 ppm (t, 2 H), similar to the H NMR spectrum of
3
(t, JH,H = 7 Hz, 2 H), 7.33 (s, 1 H), 7.38 (s, 1 H), 8.60 (s, 1 H)
1-octyl-3-methylimidazolium tetrafluoroborate.[27]
ppm.
TSIL-3: C24H46N4O8S2 (582.78): calcd. C 49.5, H 7.9, N 9.6, S
11.0; found C 49.2, H 7.7, N 9.4, S 10.8, IR: ν = 3146, 3090, 2926,
˜
2855, 1636, 1575, 1466, 1298, 1169, 1062, 704, 621, 592, 562 cm–1.
3
1H NMR (500 MHz, CDCl3, 25 °C): δ = 0.87 (t, JH,H = 8 Hz, 3
3
H), 1.24–1.29 (m, 10 H), 1.85 (t, JH,H = 7 Hz, 2 H), 4.02 (s, 3 H),
3
4.26 (t, JH,H = 7.5 Hz, 2 H), 7.20 (s, 1 H), 7.28 (s, 1 H), 9.34 (s, 1
H) ppm.
TSIL-4: C28H54N4O8S2 (638.89): calcd. C 52.6, H 8.5, N 8.8, S
10.0; found C 51.7, H 7.9, N 9.1, S 9.5. IR: ν = 3147, 3091, 2926,
˜
2855, 1572, 1467, 1295, 1169, 1117, 1063, 704, 622, 592, 562 cm–1.
3
1H NMR (500 MHz, CDCl3, 25 °C): δ = 0.87 (t, JH,H = 7 Hz, 3
3
H), 1.24–1.28 (m, 14 H), 1.84 (t, JH,H = 7 Hz, 2 H), 3.99 (s, 3 H),
3
4.24 (t, JH,H = 7 Hz, 2 H), 7.26 (s, 1 H), 7.36 (s, 1 H), 9.25 (s, 1
H) ppm.
IR and 1H NMR Spectra: The infrared spectra of all the TSILs
prepared are similar. A typical IR spectrum for a TSIL is shown
in Figure 7. The characteristic bands of the imidazolium cation[26]
and peroxydisulfate anion are clearly observed. In the FTIR spec-
tra of TSIL-4, the bands at 1295, 1063, and 704 cm–1 were ascribed
to the characteristic bands of the [S2O8] anion. The bands at 3147
and 3091 cm–1 were attributed to a C–H stretching vibration of the
imidazolium ring, whereas C–H stretching bands of the alkyl chain
on the imidazolium ring were observed at 2926 and 2855 cm–1. This
characteristic spectrum is similar to those of other ionic liquids
containing an imidazolium ring.[27] The absorption band in the
3500–3800 cm–1 region of the infrared spectra shows the presence
Figure 7. FTIR spectra of (1) K2S2O8 and (2) [C10mim]2[S2O8].
Oxidation of Benzyl Alcohol and Cysteine
Benzyl alcohol (5 mmol, 0.54 g) and [C8mim]2[S2O8] (10 mmol,
5.82 g) were combined in a 50 mL three-necked flask equipped with
a reflux condenser and a magnetic stirrer. The solution was heated
under reflux for 1 h at 80 °C. The reaction mixture was cooled to
room temperature, and the product was extracted into diethyl ether
(3ϫ 10 mL). The yield of the reaction was measured by GC with-
out further purification. All products were identified by GC–MS
by comparison with the retention time of authentic samples.
1
Figure 8. H NMR spectrum of [C8mim]2[S2O8].
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Eur. J. Inorg. Chem. 2010, 2283–2289

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Acknowledgments
We thank the key project of the Shanghai Science and Technology
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Received: November 22, 2009
Published Online: April 20, 2010
Eur. J. Inorg. Chem. 2010, 2283–2289
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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