Cyclic thiouronium ionic liquids: Physicochemical properties and their electronic structure probed by X-ray induced photoelectron spectroscopy

Article abstract of DOI:10.1002/chem.201200971

Radiography of a structure skeleton: Cyclic thiouronium ionic liquids have been synthesized and characterized in such a way that the straightforward introduction of three different alkyl substituents at the cation has been realized. X-ray induced photoelectron spectroscopy studies indicated a significant electronic contribution of the free electrons at the sulfur to stabilize the cationic charge, leading to structures with peculiar physicochemical properties (see figure). Copyright

Full text of DOI:10.1002/chem.201200971

DOI: 10.1002/chem.201200971
Cyclic Thiouronium Ionic Liquids: Physicochemical Properties and their
Electronic Structure Probed by X-Ray Induced Photoelectron Spectroscopy
Nicola Taccardi,[a] Inga Niedermaier,[b] Florian Maier,[b] Hans-Peter Steinrꢀck,[b] and
Peter Wasserscheid*[a]
In the last 20 years, ionic liquids (ILs) have attracted
growing interest as fluids for a broad range of applications,
including catalysis,[1] electrochemistry,[2] separation technolo-
gies,[3] molecular materials,[4] or performance chemicals.[5]
The main part of this research has been carried out using
ILs based on 1,3-dialkylimidazolium cations. This is due to
the particularly attractive physicochemical properties (e.g.
melting points, viscosities, thermal stabilities) of imidazoli-
um salts compared with their ammonium, phosphonium,
pyridinium, and sulfonium counterparts. In addition, the 1,3-
dialkylimidazolium structural motif offers attractive options
to modify the steric properties of the ionic head group.
Starting from differently alkyl functionalized imidazoles,[6] it
is synthetically straight forward to form the ionic liquid
cation by means of an alkylation reaction introducing other
alkyl groups, eventually with additional functionalities.
Though there are numerous publications describing a broad
range of imidazolium salts variously substituted at the 1-
and 3-position, reports dealing with the introduction of alkyl
or even substituted alkyl groups in the 2-position of the
same type of cation are rather scarce. It is well known that
the hydrogen attached to the C2-position of an 1,3-dialkyl-
ylimidazolium salts. Kempter and Kirchner have rational-
ized this important and general observation by molecular
dynamics studies.[10] Only a few approaches have been pro-
posed for functionalizing the imidazolium 2-position apart
from methyl. Ennis and Handy reacted 1,3-dialkylimidazoli-
um halide salts with NaH followed by alkylation of the
in situ formed carbene with alkylhalides.[11] Lee and co-
workers[12] introduced a carbonyl group in the 2-position by
reacting 1,3-dialkylimidazolium salts with formaldehyde.
Rogers and co-workers[13] have developed a synthetic strat-
egy based on imidazolyl-2-carboxylates obtained via alkyla-
tion of 1-alkylimidazole with dimethyl carbonate. Recently,
a report of the Rogersꢀ group has demonstrated the carbe-
noid nature of 1,3-dialkylimidazolium acetates by reacting
the latter with chalcogenides, such as sulfur.[14] Based on this
inspiring contribution, in the following we present a func-
tionalization strategy that involves the formal attachment of
a thio-alkyl group at the 2-position of the imidazolium ring
resulting in ILs with cyclic thiouronium cations exhibiting
a remarkable electronic structure of the cations as witnessed
by X-ray induced photoelectron spectroscopy (XPS).
Basically, the applied synthetic approach based on using
imidazolyl-2-thiones as starting material for a consecutive
alkylation reaction with alkyl iodides, followed by anion
metathesis with lithium bis(trifluoromethyl sulfonyl)imide
ACHTUNGTRENNUNG
imidzazolium ion is relatively acidic.[7] This offers a way to
exploit such a reactive site for new synthetic approaches, for
example, via the corresponding metal–carbene complexes.[8]
On the other hand, this fact limits the stability of 1,3-di-
(LiACTHNUGRTENUNG[Tf2N]). The adopted three-step synthetic sequence is de-
A
picted in Scheme 1. Note that the chemistry of such system
ternatively, 1,2-dimethyl-3-alkylimidazolium salts have been
proposed to overcome such complication. The latter are
easily accessible by alkylation of the commercial 1,2-dime-
thylimidazole but typically display significantly higher melt-
ing points compared to their corresponding 1-alkyl-3-meth-
has been pioneered by Merck KGaA.[15] However, the spe-
[a] Dr. N. Taccardi,+ Prof. Dr. P. Wasserscheid
Lehrstuhl fꢁr Chemische Reaktionstechnik
Universitꢂt Erlangen-Nꢁrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
Fax : (+49)9131-8527421
E-mail: Wasserscheid@crt.cbi.uni-erlangen.de
[b] I. Niedermaier,+ Dr. F. Maier, Prof. Dr. H.-P. Steinrꢁck
Lehrstuhl fꢁr Physikalische Chemie II
Universitꢂt Erlangen-Nꢁrnberg
Egerlandstrasse 3, 91058 Erlangen (Germany)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200971.
Scheme 1. Synthesis of cyclic thiouronium ionic liquids.
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Chem. Eur. J. 2012, 18, 8288 – 8291

COMMUNICATION
cific examples of cyclic thiouronium [Tf2N] ionic liquids
were not reported, being the object of this investigation.
The approach revealed to be general and allowed for the
synthesis of a variety of cyclic thiouronium imides by vary-
ing either the alkylating agent or the starting thione struc-
ture.
More in detail, the 1-alkyl-3-methylimidazolyl-2-thiones
were obtained by reacting the respective 1,3-dialkylimidazo-
lium halide salts with sulfur, under basic condition in reflux-
ing methanol. Subsequently, the thiones were alkylated with
alkyl iodides. The obtained iodide were not isolated, but di-
rectly extracted with water followed by metathesis with
alkyl substituents at the cation caused the suppression of
a clear melting temperature, and only glass transitions were
observable (rows 4,6,7 in Table 1). Moreover, comparing the
melting point of 2a (glass transition at À788C) with the one
of [C6mim]ACTHUNGTRENNUNG
the cyclic thiouronium head group decreases the tendency
for crystallization. To assess the thermal stability of the pre-
pared cyclic thiouronium salts, a TG analysis at 10 KminÀ1
heating ramp was carried out. Despite the fact that this
method tends to overestimate the thermal stability and is
not able to distinguish between thermal decomposition and
ionic liquid evaporation,[18] the obtained data indicate suffi-
cient thermal stability for most typical IL applications.
As mentioned at the beginning, XPS was employed for
electronic structure characterization of the thiouronium ILs,
particularly of IL 1a. XPS has been found to be an extreme-
ly powerful tool for studying ILs under well-defined ultra-
high vacuum conditions,[19] and we applied this methodology
to evaluate the core-level binding energy shifts that reflect
differences in electronic structure. The aim of this study was
to understand to which extent the sulfur of the thio-alkyl
moiety is involved in the delocalization of the positive
charge originally present on the imidazolium core. For the
purpose of this study, we compared the ILs shown in
Scheme 2.
LiACHTUNGTRENNUNG[Tf2N]. The final thiouronium [Tf2N] salts were obtained
in yield ranging from 62% for 1d to 95% for 2a (see the
Supporting Information). They were all room temperature
ionic liquids. A number of key properties is summarized in
Table 1. For comparison, also the properties of the related
and well known ILs 1-methyl-3-ethylimidazolium [Tf2N]
Table 1. Physicochemical data of selected cyclic thiouronium imide ILs
in comparison to [C2mim]ACTHNUGTREN[NUNG Tf2N] and [C2C1mim]ACHTGNUTRENNUNG
Ionic liquid
m.p.
[8C]
Decomposition
Density[b]
[gcmÀ3
point[a] [8C]
G
N
]
A
R
À17[c]
439
441
368
310
282
320
314
36
77
114
81
88
121
184
1.52[c]
1.48[c]
1.52
1.44
1.44
E
ACHTUNGTRENNUNG
25[c]
1a
1b
1c
1d
2a
À13
3 is a structural isomer of 1a, containing the sulfur atom
as part of a thioether group within the 1-alkyl side chain,
and [C2mim]ACTHNUGTRENUNG[Tf2N] (4) can be considered as the parent com-
pound of 1a, with the C2 position remaining unsubstituted.
In Figure 1, the C 1 s, N 1 s, and S 2p1/2, 3/2 XP spectra of ILs
1a, 3, and 4 are shown. All three compounds contain the
À84[d]
11
À85[d]
1.32
1.39
À78[d]
[a] Off-set temperature, heating rate 10 KminÀ1. [b] At 208C. [c] Data ac-
cording to Ref. [16]. [d] Glass transition.
([C2mim]
ACHTUNGTRENNUNG[Tf2N]) and 1,2-di-
methyl-3-ethylimidazolium
[Tf2N] ([C2C1mim]
reported.
ACHTUNGTRENNUNG[Tf2N]) are
The methyl substituted thio-
uronium IL 1a has a higher vis-
cosity compared with its 1,3-di-
alkylimidazolium counterpart
(rows 1 and 3 in Table 1). In
fact, [C2mim]ACHTUNGTRENNUNG[Tf2N] has a viscos-
ity of 36 MPas at 208C, versus
a viscosity of 114 MPas mea-
Scheme 2. Structures and marked atoms of the ionic liquid used for XPS-investigations.
sured for 1a. Interestingly, by
attaching a C3 alkyl chain to
the thiouronium sulfur atom (ILs 1b and 1c) the viscosity
drops and was comparable to the methyl substituted salt
and even for the octyl substituent the viscosity remains as
low as 121 MPas. Remarkably, the introduction of the thio-
alkyl moiety had only a minor effect on the melting points
of the resulting ILs. In sharp contrast to the increase in
melting point induced by methylating the C2-position of the
imidazolium cation (see entries 1 and 2 in Table 1), the at-
tachment of the S-CH3 group in the same position causes
almost no change in melting point. Elongating one of the
[Tf2N]À anion, giving rise to Canion, Nanion, and Sanion peaks at
identical binding energies (BE) of 292.7, 399.3 and 168.8 eV
(S 2p3/2), respectively. These values serve as internal BE ref-
erence with 0.1 eV accuracy over the whole BE range
(within each region, cation- and anion-derived BEs, for ex-
ample, Nimid relative to Nanion, can be derived within 0.03 eV
accuracy). The most remarkable finding is the BE of sulfur
attached in 2-position of thiouronum 1a: its BE is shifted by
1.0 eV towards higher BE as compared to the thioether S 2p
signal of the IL 3 cation. This difference is very large for an
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8289

P. Wasserscheid et al.
sulfur species, and, thus, to the remarkable electronic struc-
ture of 1a.
Remarkably, the electronic nature of the thiouronium
sulfur atom (as witnessed by XPS) was also reflected in our
preliminary reactivity tests. In fact, attempts to oxidize this
sulfur using conditions that have been found effective for
the transformation of aryl-methyl thioether to the corre-
sponding sulfoxide (H2O2 30%, AgNO3 as catalyst in reflux-
ing methanol)[22] did not result in any conversion of the
cation.
In summary, we have synthesized and characterized cyclic
thiouronium imide salts using a method that allows the
straight forward introduction of three different alkyl sub-
stituents at the cation. The method expands the possibilities
for cation design starting from standard imidazolium ionic
liquids. XPS studies of the obtained salts indicate a signifi-
cant electronic contribution of the free electrons at the thio-
uronium sulfur to the stabilization of the cationic charge
leading to peculiar physicochemical properties of these liq-
uids.
Acknowledgements
We would like to acknowledge the German Science Foundation for sup-
porting this research through the Erlangen Cluster of Excellence “Engi-
neering of Advanced Materials” and through the SPP 1191 “Ionic Liq-
uids”.
Figure 1. C1s, N1s and S2p XP spectra of IL 1a (solid), 3 (dashed), and 4
(dash-dotted); spectra are offset for clarity. C1s spectra include a decon-
volution (gray) into the different carbon species C2, Calkyl (i.e., all C
atoms bonded to C and H), and Chetero (i.e., all C atoms bonded to N or
S, excluding C2) of the cationic head group of IL 1a and 4.
Keywords: cyclic thiouronium · electronic structure · ionic
liquids · thiones · X-ray photoelectron spectroscopy
environment of neighbors with identical electronegativity.
The unusually high S 2p BE for the IL 1a cation indicates
a charge transfer between the imidazolium ring and the
thioalkyl group in 2-position, leading to a significantly re-
duced electron density at the sulfur. As a result of this
effect, an enhanced electron density within the imidazolium
ring is expected. Indeed, the XP spectra of ILs 1a, 3, and 4
in Figure 1 (C 1 s of IL 3 is not shown here because of the
more complicated overlap of the C 1 s signals) reveal that
the N 1 s signal and the dominating C 1 s signals of the imi-
dazolium ring in IL 1a exhibit a significant shift of about
0.1 eV towards lower BE, as compared to the other two ILs.
This behavior corroborates the assumed increase in electron
density in the imidazolium ring of IL 1a. Only the C2
carbon atom of 1a exhibits a shift of 0.3 eV towards higher
BE, which is most likely due to the fact that the neighboring
S is more electronegative than hydrogen in IL 3 and 4 (elec-
tronegativity values according to the Pauling scale C: 2.55,
S: 2.58, H: 2.20).[20] It should be noted that the C2 carbon in
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Cyclic Thiouronium Ionic Liquids
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Received: March 21, 2012
Published online: June 8, 2012
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