2,2,6,6-Tetramethylpiperidine-1-yloxyl bound to the imidazolium ion by an acetamido group for investigation of ionic liquids

Article abstract of DOI:10.1016/j.tetlet.2009.11.124

New spin probes bearing the 2,2,6,6-tetramethylpiperidine-1-yloxyl covalently bound to the imidazolium ion via a methylene spacer and an amide group are synthesized. If the anion is bis(trifluoromethylsulfonylimide) instead of iodide, the new spin probe has a similar structure as that of an ionic liquid. Nevertheless, the new spin probes are useful tools to investigate ionic liquids.

Full text of DOI:10.1016/j.tetlet.2009.11.124

Tetrahedron Letters 51 (2010) 747–750
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Tetrahedron Letters
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2,2,6,6-Tetramethylpiperidine-1-yloxyl bound to the imidazolium ion
by an acetamido group for investigation of ionic liquids
a
b
Veronika Strehmel a, , Hans Rexhausen , Peter Strauch
*
a University of Potsdam, Institute of Chemistry, Applied Polymer Chemistry, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam-Golm, Germany
b University of Potsdam, Institute of Chemistry, Inorganic Chemistry, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam-Golm, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
New spin probes bearing the 2,2,6,6-tetramethylpiperidine-1-yloxyl covalently bound to the imidazoli-
Received 5 October 2009
Revised 25 November 2009
Accepted 27 November 2009
Available online 2 December 2009
um ion via
a methylene spacer and an amide group are synthesized. If the anion is bis(trif-
luoromethylsulfonylimide) instead of iodide, the new spin probe has a similar structure as that of an
ionic liquid. Nevertheless, the new spin probes are useful tools to investigate ionic liquids.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Spin probes
ESR spectroscopy
Nitroxides
Ionic liquids
Ionic liquids have received increased attention as solvents in inor-
ganic, organic, and polymer chemistry.1–4 Furthermore, they possess
a high potential for application in batteries, fuel cells, and solar
cells.5–7 The mobility of both the individual ions of the ionic liquid
andthedissolvedspeciesisimportantinthesepotentialapplications.
Stable radicals, so called spin probes, have been successfully used as
model compounds to describe the mobility of reactive species in the
ionic liquids.8–20 Furthermore, the isotropic hyperfine coupling con-
stants of the stable radicals give information about micropolarity of
the surrounding matrix.13,16,20–22 2,2,6,6-Tetramethylpiperidine-1-
yloxyl derivatives are examples for spin probes, which have been
successfully applied for the investigation of ionic liquids.13,14,16–20
Recently, 2,2,6,6-tetramethylpiperidine-1-yloxyl derivatives have
been modified by various substituents to investigate the interactions
between model radicals and ionic liquids.17–20 Thus, cationic or anio-
nic substituents are bound at the 4 position with respect to the
nitroxyl group resulting in additional ionic interactions with either
the anion or the cation of the ionic liquid. They are useful spin probes
for the investigation of ionic liquids. It is shown that the ionic substi-
tuent at the4 position with respect to thenitroxylgroup significantly
reduces the mobility of these spin probes in the ionic liquids caused
by the additional ionic interactions between the ionic substituent
and the individual ions of the ionic liquids.13,14,16,20 Recently, a pyr-
rolidine-1-yloxyl derivative covalently bound via a spacer group to
the imidazolium ion was described in the literature as further new
spin probe for the investigation of ionic liquids.21
In this Letter, we describe the synthesis of a 2,2,6,6-tetrameth-
ylpiperidine-1-yloxyl derivative that is covalently bound to the
imidazolium ion. Covalent bonding of a 2,2,6,6-tetramethylpiperi-
dine-1-yloxyl derivative to the imidazolium ion gives more de-
tailed information about the mobility of the cation of the ionic
liquid. Furthermore, anion metathesis of the iodide by bis(trif-
luoromethylsulfonylimide) results in a new spin probe bearing
the same anion as the ionic liquid and a radical structure cova-
lently bound to the cation of the ionic liquid. In particular, the
new spin probe has a similar structure as 1-butyl-3-methylimida-
zolium bis(trifluoromethylsulfonylimide).
4-(2-Iodacetamido)-2,2,6,6-tetramethylpiperidine-1-yloxyl (1)
was selected for the covalent bonding of 2,2,6,6-tetramethylpiperi-
dine-1-yloxyl to the imidazolium ion because this compound reacts
with 1-alkylimidazole at room temperature. Reaction of 1 with either
1-methylimidazole (2) or1-butylimidazole (3) results inthe new spin
probe 1-(2-(4-(2,2,6,6-tetramethyl-piperidinyl-1-yloxyl)amino)-2-
oxoethyl)-3-methylimidazolium iodide (4) or 1-(2-(4-(2,2,6,6-tetra-
methyl-piperidinyl-1-yloxyl)amino)-2-oxoethyl)-3-butylimidazo-
lium iodide (5) bearing a radical structure covalently bound to the
imidazolium ion as described in the Schemes 1 and 2.23,24 Anion
metathesis of iodide by bis(trifluoromethylsulfonylimide) using sil-
ver bis(trifluoromethylsulfonylimide) results in the new spin probe
6 (Scheme 3), which bears the same anion as the 1-butyl-3-methyl-
imidazolium bis(trifluoromethylsulfonylimide).25 The silver iodide
formed in the course of the reaction precipitates from the reaction
mixture and can be therefore easily separated from 1-(2-(4-(2,2,6,
6-tetramethyl-piperidinyl-1-yloxyl)amino)-2-oxoethyl)-3-buty-
limidazolium bis(trifluoromethylsulfonylimide) (6), which remains
* Corresponding author. Tel.: +49 331 977 5224; fax: +49 331 977 5036.
E-mail address: vstrehme@uni-potsdam.de (V. Strehmel).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.11.124

748
V. Strehmel et al. / Tetrahedron Letters 51 (2010) 747–750
Scheme 1. Synthesis of 1-(2-(4-(2,2,6,6-tetramethyl-piperidinyl-1-yloxyl)amino)-
2-oxoethyl)-3-methylimidazolium iodide (4).
Scheme 2. Synthesis of 1-(2-(4-(2,2,6,6-tetramethyl-piperidinyl-1-yloxyl)amino)-
2-oxoethyl)-3-butylimidazolium iodide (5).
Scheme 3. Preparation of 1-(2-(4-(2,2,6,6-tetramethyl-piperidinyl-1-yloxyl)
amino)-2-oxoethyl)-3-butylimidazolium bis(trifluoromethylsulfonylimide) (6).
Figure 1. ESR spectra of the new spin probes. (a) 1: Aiso
4: Aiso = 0.8 ns, and (c) 5: Aiso
14N) = 15.7 G, 14N) = 15.7 G,
dimethylsulfoxide at room temperature.
(
14N) = 15.7 G,
= 0.8 ns dissolved in
s = 0.6 ns; (b)
dissolved in the organic solvent. The new spin probe 6 remains as or-
ange oil after evaporation of the solvent in vacuo.
(
s
(
s
The new model radicals 4, 5, and 6 contain the 2,2,6,6-tetra-
methylpiperidine-1-yloxyl structure covalently bound to the imi-
dazolium ion via an amide group and
a methylene spacer.
Therefore, the spin probe mobility is strongly influenced by the
mobility of the imidazolium ion of the ionic liquid. Similar ESR
spectra are obtained in the case of 1, 4, and 5 dissolved in dimeth-
ylsulfoxide (Fig. 1). The isotropic hyperfine coupling constants re-
lated to nitrogen (Aiso
correlation time ( ) are similar for the three spin probes in dimeth-
(
14N)) and the average rotational
Scheme 4. Chemical structure of 1-butyl-3-methylimidazolium tetrafluoroborate
(IL-BF4) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonylimide)
(IL-NTf2).
s
ylsulfoxide (Fig. 1). The latter were determined by the method of
Budil et al.26 This shows that no significant differences exist in
the mobility of 1, 4, and 5 in dimethylsulfoxide. Furthermore, these
spin probes detect the same value for micropolarity, which is sim-
ilar to the micropolarity of dimethylsulfoxide.
Moreover, the new spin probes were investigated in 1-butyl-3-
methylimidazolium tetrafluoroborate (IL-BF4), which is an exam-
ple for a widely investigated ionic liquid (Scheme 4).27 Although
4 has a methyl substituent at one nitrogen atom of the imidazoli-
um ring, the new spin probe 5 bears a butyl group at this nitrogen
atom. Therefore, the structure of 5 is comparable with the cation of
IL-BF4.
In contrast to the similar ESR spectra obtained for 1, 4, and 5 in
dimethylsulfoxide, significant differences are observed in the ESR
spectra if these spin probes are dissolved in the ionic liquid
IL-BF4 (Fig. 2). The three lines of the ESR spectra of 1, 4, and 5 show
a significant line broadening and differences in the habitus of the
spectra in comparison with the ESR spectra of these spin probes
dissolved in dimethylsulfoxide (Fig. 1). The strong immobilization
of the spin probe mobility in IL-BF4 is caused by the strong inter-
actions between these spin probes and the ionic liquid. Further-
more, covalent bonding of the spin probe via an amido function
and a methylene spacer to the imidazolium ion in the case of 4
and 5 results in a further reduction of the spin probe mobility (Figs.
2b and 2c) in comparison with the spin probe mobility in the case
of 1 (Fig. 2a). Although the amido function is also present in 1,
stronger immobilization is found in the case of 4 and 5 in the ionic
liquid in comparison with 1. Covalent binding of the radical struc-
ture to the imidazolium ion causes this effect. However, the ESR
spectra are similar for the spin probes 4 and 5 although the alkyl
substituent at one nitrogen atom of the imidazolium ring is a
methyl group in the case of 4 and a butyl group in the case of 5.
The rotational correlation time of the spin probes is significantly
increased in IL-BF4 in comparison with dimethylsulfoxide (Figure

V. Strehmel et al. / Tetrahedron Letters 51 (2010) 747–750
749
Figure 3. ESR spectra of the new spin probe 6 dissolved in (a) dimethylsulfoxide
(Aiso = 1.0 ns), and in the ionic liquids; (b) IL-BF4 (Aiso
14N) = 15.5 G, 14N) = 16.2 G,
= 10.4 ns), and (c) IL-NTf2 (Aiso = 10.0 ns) at room temperature.
14N) = 16.0 G,
Figure 2. ESR spectra of (a) 1: (Aiso
probes covalently bound at the imidazolium ion; (b) 4: (Aiso
= 9.8 ns), and (c) 5: (Aiso = 10.2 ns) dissolved in IL-BF4 at room
14N) = 16.5 G,
temperature.
(
14N) = 15.9 G,
s
= 7.1 ns) and of the new spin
14N) = 16.6 G,
(
s
(
(
s
(
s
s
(
s
liquid. The results show that the mobility of radicals covalently
bound to the imidazolium ion is significantly reduced in an ionic
liquid compared to the mobility of a radical without additional
charged group. Furthermore, these model radicals give information
about the mobility of radicals formed in the course of polymeriza-
tion of ionic liquid monomers.
captions of Figs. 1 and 2). Although the hyperfine coupling con-
stants of 1 are similar in dimethylsulfoxide and IL-BF4, differences
exist for the covalently bound spin probes 4 and 5 in these sol-
vents. This may be attributed to the strong immobilization of the
spin probe mobility caused by the interactions with the ionic
liquid. Therefore, spin probes covalently bound to the imidazolium
ion are not useful for the discussion of micropolarity of ionic liq-
uids if their mobility is strongly immobilized.
Acknowledgments
The new spin probe 6 dissolved in dimethylsulfoxide also shows
three narrow lines in the ESR spectrum at room temperature
(Fig. 3a). Significant line broadening and differences in the habitus
of the ESR spectra are also observed if 6 is dissolved in the ionic li-
quid IL-BF4 or IL-NTf2 (Figs. 3b and 3c).27 As discussed for 4 and 5,
the spin probe 6 also shows similar values to 1 for the hyperfine
coupling constant and the rotational correlation time if dimethyl-
sulfoxide is used as the solvent although higher values are ob-
tained if an ionic liquid is used as the solvent (Fig. 3). The strong
immobilization of the spin probe mobility in the ionic liquid is
caused by the strong interactions of 6 with the individual ions of
the ionic liquid. Because 6 possesses the same anion as IL-NTf2, this
new spin probe is preferred for the investigation of imidazolium
bis(trifluoromethylsulfonylimides).
The authors gratefully acknowledge A. Laschewsky (University
of Potsdam and Fraunhofer Institute for Applied Polymer Research)
for the use of laboratory equipment and the group of E. Kleinpeter
(University of Potsdam), especially I. Starke and S. Fürstenberg for
mass spectrometric investigation, and H. Wetzel (Fraunhofer Insti-
tute for Applied Polymer Research) for analysis of the water con-
tent in the ionic liquids using Karl–Fischer analysis. Furthermore,
V.S. and H.R. gratefully acknowledge the DFG for financial support
within the priority program SPP 1191.
References and notes
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2008, 10, 2189–2199.
In general, the new spin probes 4, 5, and 6 reflect the mobility of
reactive species bound to the cation of the ionic liquid in an ionic

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V. Strehmel et al. / Tetrahedron Letters 51 (2010) 747–750
2. Paape, N.; Wei, W.; Bösmann, A.; Kolbeck, C.; Maier, F.; Steinrück, H.-P.;
precipitation of the new spin probe 4 bearing the spin probe covalently bound
to the imidazolium ion (50% yield, mp 179–187 °C, decomp.). Mass
spectrometric analysis gives 294.2076 Da in the TOF MS ES+ mode (Calcd:
294.21 Da) and 127 Da in the TOF MS ESÀ mode (Calcd: 126.91 Da); IR
1685 cmÀ1 amido group (shift of the IR vibration from 1643 cmÀ1 in the case of
1).
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24. Twenty
milligrams
of
4-[(2-iodoacetyl)amino]2,2,6,6-tetramethyl-1-
piperidine-1-yloxyl (0.059 mmol) (1) are dissolved in 5 ml of a tert-butyl
methyl ether/acetone mixture (Vol:Vol = 2:8). Then, 7.3 mg of 1-
butylimidazole (3) dissolved in 1 ml tert-butyl methyl ether are added to the
solution. The resulting mixture is stirred under nitrogen at room temperature
for 14 days and kept at room temperature for further 2 days resulting in
precipitation of a slightly yellow precipitate. The solution was removed by
decanting and the precipitate was washed with 0.2 ml tert-butyl methyl ether.
After drying the precipitate at room temperature under vacuum (4 mbar) for
24 h, the new spin probe 5 (mp 82–87 °C) is obtained in 74% yield. Mass
spectrometric analysis gives 336.2517 Da in the TOF MS ES+ mode (Calcd:
336.25 Da) and 127 Da (Calcd: 126.91 Da) in the TOF MS ESÀ mode; IR
1684 cmÀ1 amido group (shift of the IR vibration from 1643 cmÀ1 in the case of
1).
25. Twenty-eight milligrams of 1-(2-(4-(2,2,6,6-tetramethyl-piperidinyl-1-
yloxyl)amino)-2-oxoethyl)-3-butylimidazolium iodide (5) are dissolved in
acetone. Then, a solution of 25 mg silver bis(trifluoromethylsulfonylimide)
dissolved in 5 ml acetone is added under nitrogen and stirred at room
temperature for 30 min. The reaction mixture is kept at room temperature for
1 h, and then at 4 °C for 12 h. The precipitated silver iodide is filtrated using a
0.2 lm polytetrafluoroethylene syringe filter, which results in a clear solution.
16. Stoesser, R.; Herrmann, W.; Zehl, A.; Laschewsky, A.; Strehmel, V. Z. Phys. Chem.
2006, 220, 1309–1342.
The solvent is evaporated under vacuo at room temperature. The resulting
orange oil-like product 6 is dried at room temperature in vacuo (1–4 mbar) for
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4 h. The new spin probe
6
(Tg = À10 °C) is obtained in 85% yield. Mass
spectrometric analysis gives 336,252 Da in the TOF MS ES+ mode (Calcd:
336,25 Da) and 280 Da (Calcd: 279,92 Da) in the TOF MS ESÀ mode.; IR
characteristic vibrations for S–N–S (asymmetric vibration) at 1059 cmÀ1, –
SO2À (symmetric vibration) at 1135 cmÀ1
, , –SO2À
CF3 at 1198 cmÀ1
(asymmetric vibration) at 1352 cmÀ1, amido group at 1691 cmÀ1 (shift of the
IR vibration from 1684 cmÀ1 in the case of 5). The absence of iodide in the spin
probe 6 is checked by washing with water and by adding silver nitrate solution
to the washing water. No precipitation is observed.
23. Twenty-five milligrams of 4-[(2-iodoacetyl)amino]2,2,6,6-tetramethyl-1-
piperidine-1-yloxyl (1) are dissolved in 5 ml of a tert-butyl methyl ether/
acetone mixture (Vol:Vol = 1:9). Then, 6 mg of 1-methylimidazole (2) dissolved
in 1 ml tert-butyl methyl ether are added to the solution. The resulting mixture
is stirred under nitrogen at room temperature for 7 days resulting in the
26. Budil, D. E.; Lee, S.; Saxena, S.; Freed, J. H. J. Magn. Reson., Ser. A 1996, 120, 155–
189.
27. The water content is 0.2% in the case of IL-BF4 and 0.03% in the case of IL-NTf2
determined by Karl–Fischer analysis. No halogenide was detected in the ionic
liquids using silver nitrate solution.