Chiral and Redox-Active Room-Temperature Ionic Liquids Based on Ferrocene and l-Proline

Article abstract of DOI:10.1002/ejic.201600585

The syntheses of room-temperature ionic liquids (RTILs) combining the naturally occurring amino acid l-proline and ferrocene (Fc) building blocks are reported. After quaternization of ({[(2S)-N-methylpyrrolidine-2-yl]methyleneoxy}carbonyl)ferrocene (1) with alkyl iodides and anion exchange, the resulting diastereomeric (1S,2S)- and (1R,2S)-[(ferrocenylcarbonyl)oxy]methylene-N,N-dialkylpyrrolidine-1-ium RTILs are redox-active and air- and water-stable. They are also thermally stable up to 263 °C. The electrochemical Fe^II/Fe^IIIpotential is shifted to +0.28 V versus Fc/Fc⁺. Before anion exchange, several iodide derivatives were obtained as crystalline products, and their crystal structures are reported. According to the NMR spectroscopic data cation–anion aggregates are present in the non-coordinating solvent CDCl₃. In contrast, in the polar solvent [D₆]dimethyl sulfoxide ([D₆]DMSO), the ion pairs are separated.

Full text of DOI:10.1002/ejic.201600585

DOI: 10.1002/ejic.201600585
Full Paper
Ionic Liquids
Chiral and Redox-Active Room-Temperature Ionic Liquids Based
on Ferrocene and -Proline
L
[a]
[a]
Carola B. Bouvet and Harald Krautscheid*
II
III
Abstract: The syntheses of room-temperature ionic liquids stable up to 263 °C. The electrochemical Fe /Fe potential is
+
(
RTILs) combining the naturally occurring amino acid
L
-proline shifted to +0.28 V versus Fc/Fc . Before anion exchange, several
and ferrocene (Fc) building blocks are reported. After quater- iodide derivatives were obtained as crystalline products, and
nization of ({[(2S)-N-methylpyrrolidine-2-yl]methyleneoxy}- their crystal structures are reported. According to the NMR
carbonyl)ferrocene (1) with alkyl iodides and anion exchange, spectroscopic data cation–anion aggregates are present in the
the resulting diastereomeric (1S,2S)- and (1R,2S)-[(ferrocenyl- non-coordinating solvent CDCl . In contrast, in the polar solvent
3
carbonyl)oxy]methylene-N,N-dialkylpyrrolidine-1-ium RTILs are [D ]dimethyl sulfoxide ([D ]DMSO), the ion pairs are separated.
6
6
redox-active and air- and water-stable. They are also thermally
Introduction
could serve as solvents and at the same time act as electron
donors or acceptors and control the stereochemistry of the re-
action.
Since the discovery of salts with low melting points by Walden
in 1914, a variety of low-melting salts, so-called ionic liquids
ILs with a cationic pyrrolidinium unit are rather rare. How-
ILs), have been found and characterized.[ As they consist most
1]
(
ever,
rally occurring chiral amino acid. The first pyrrolidinium-based
L-proline is an interesting starting material, as it is a natu-
commonly of bulky organic cations and polyatomic inorganic
anions, their various combinations offer a whole new branch of
liquids. Owing to their strong Coulombic interactions, they are
proposed as solvents for chemical syntheses, catalysis, and bio-
catalysis and in the fields of electrochemistry and analytics.
Even their use as heat-transfer media or extractants has been
[
10]
ILs were reported by MacFarlane et al. in 1999.
Herein, we report the synthesis and characterization of
pyrrolidinium ILs containing a ferrocenyl group as a redox-
active center. Although ferrocenoyl hydroxyproline derivatives
[
11]
[
2]
have been reported, to the best of our knowledge, this is the
first time that RTILs have been prepared by linking ferrocene to
a pyrrolidine ring through an ester bridge. These RTILs are
formed by quaternization of the pyrrolidine ring with alkyl
discussed. Through the introduction of a redox-active center
in the cation or anion, the IL can act not only as a solvent but
also as an electron donor or acceptor with possible applications
in dye-sensitized solar cells,[3] supercapacitors, and batteries.[4]
iodides and subsequent exchange of the iodide anion by bis(tri-
Examples from the literature for redox-active cations are ferro-
cene-based derivatives such as (ferrocenylmethyl)imidazolium
room-temperature ionic liquids (RTILs) in which the oxidation
–
fluoromethanesulfonyl)imide (NTf ).
2
II
state of the iron atom can be changed reversibly from Fe to
III[5]
Fe
or the viologen derivative 1-butyl-1′-heptyl-4,4′-bipyridin- Results and Discussion
[6]
ium bis(trifluoromethanesulfonyl)imide.
An example of a
–
redox-active anion in an IL is SCN , which was included in 1-
alkyl-3-methylpyridinium ILs, for example. A typical synthesis
Reaction Sequence
[
7]
route to ILs is the quaternization of nitrogen- or phosphorus- The ferrocene (Fc) ester ({[(2S)-N-methylpyrrolidine-2-yl]meth-
containing heterocycles. Most commonly, imidazolium-based yleneoxy}carbonyl)ferrocene (1) was synthesized by reaction of
[
8]
ILs are synthesized and already available industrially. Al- [(2S)-1-methylpyrrolidine-2-yl]methanol and ferrocene mono-
though imidazolium-based ILs with chiral pyrrolidine side carboxylic acid chloride, FcCOCl, in tetrahydrofuran (THF,
groups have been investigated for their catalytic activity in aldol Scheme 1). [(2S)-1-Methylpyrrolidine-2-yl]methanol was
[
9]
reactions, chirality in combination with redox activity in the obtained from L-proline, formic acid, acetic anhydride, and
[
12]
FcCOCl[13] was formed by the reaction of
with oxalyl chloride in the presence of catalytic
same IL could lead to new applications in synthesis. Such ILs LiAlH
FcCOOH
.
4
[
14,15]
[
a] Universität Leipzig, Fakultät für Chemie und Mineralogie,
amounts of N,N-dimethylformamide (DMF). The N atom of the
pyrrolidine ring was quaternized with alkyl iodides to give the
Johannisallee 29, 04103 Leipzig, Germany
E-mail: krautscheid@rz.uni-leipzig.de
https://anorganik.chemie.uni-leipzig.de/anorganik/krautscheid/
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201600585.
desired
(2S)-[(ferrocenylcarbonyl)oxy]methylene-N,N-dialkyl-
pyrrolidine-1-ium iodides. To generate salts with lower melting
points, anion exchange with NTf2– was performed. The resulting
Eur. J. Inorg. Chem. 0000, 0–0
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Full Paper
NTf2– salts 7–11 (Table 1) are liquids at room temp. and show decreasing reactivity of the alkyl halides with increasing alkyl
[
18]
good stability towards air, light, and moisture. Details concern- chain length.
The yields and the 1S,2S/1R,2S product ratios
ing the product characterization are given in the Supporting are reported in Table 1. The diastereomeric ratios were deter-
1
Information.
mined by comparing the integrals of the H NMR signals of the
methyl groups of both diastereomers. The 1S,2S diastereomer
is always the minor product in a ratio close to 43:57. Within the
1
error of the H NMR signal integration, only a slight preference
for the 1R,2S diastereomer can be determined.
Melting Points and Decomposition Processes
The melting points and decomposition temperatures (begin-
ning of weight loss) of 1–11 were determined by simultaneous
thermal analysis (STA) and are summarized in Table 1. The vola-
tile decomposition products were analyzed by mass spectrome-
try and gas-phase IR spectroscopy. The melting point of the
methylated iodide 2 is 175 °C, whereas the glass transition tem-
perature of 6 (R = pentyl) is below –25 °C and its decomposition
temperature of 166 °C is slightly lower than those of the other
–
–
iodides (184–194 °C). The change of the anion from I to NTf
2
affects the melting points significantly: all NTf2– salts 7–11 are
RTILs with melting points below –25 °C. Compounds 7–11 show
the same decomposition behavior between 180 and 263 °C.
With a liquid range from below –25 to 263 °C, 10 shows the
highest thermal stability and the widest liquid range.
Crystal Structures
Single-crystal structure analyses were performed for the IL pre-
cursors FcCOCl, 2, (1S,2S)-3, (1R,2S)-3, (1R,2S)-4, and (1R,2S)-5
(
3
Table 2). On the basis of their different crystal shapes, (1S,2S)-
and (1R,2S)-3 (Figure 1) were separated manually, and the
crystal structures of both diastereomers of 3 were determined.
The chiral compounds crystallize in chiral space groups, mono-
Scheme 1. Synthetic route to (2S)-2-[(ferrocenylcarbonyl)oxy]methylene-N,N-
clinic P2 for 2 (Z = 4) and (1R,2S)-3 (Z = 2) and orthorhombic
–
1
dialkylpyrrolidine-1-ium iodides 2–6 and NTf
2
salts 7–11. Compounds 3–6
P2 2 2 for (1S,2S)-3, (1R,2S)-4, and (1R,2S)-5 (Z = 4). Although
and 8–11 were obtained as diastereomeric mixtures.
1 1 1
the molecules are closely related, the free rotation of the pyrrol-
As the chain length of the alkyl group R in the alkyl iodides idinium ring around the C12–C13 bond, ring puckering, the ori-
increases, the quaternization yield decreases. This is due to the entation of the alkyl groups at the nitrogen atom, and the ori-
Table 1. Reaction yields of 1–11 and 1S,2S/1R,2S ratios. The ratios were determined by integration of the signals of the methyl groups of 1H NMR spectra of
the compounds in CDCl
3 6 m g
or [D ]DMSO. The melting points (T ), glass transition (T ), and decomposition (Tdec) temperatures were determined by simultaneous
thermal analysis (STA) experiments of the diastereomeric mixtures.
R
X
Yield[a] [%]
1S,2S/1R,2S ratio
T
m
and (T
g
) [°C]
Tdec [°C]
Ferrocene
–
–
–
–
–
173[16]
<r.t.
>460[17]
170
194
191
190
184
166
180
245
209
263
251
1
2
3
4
5
6
7
8
9
74
91
94
62
37
30
64
42
67
97
76
–
[b]
methyl
ethyl
propyl
butyl
pentyl
methyl
ethyl
propyl
butyl
pentyl
I
(155) 175
142
–
[c]
I
45:55, 42:58
–
[d]
[e]
I
–
–
–
–
[e]
I
I
43:57
45:55
–
–
(<–25)
(<–70)
(<–25)
(<–25)
(<–25)
(<–25)
–
–
–
–
–
NTf
NTf
NTf
NTf
NTf
2
2
2
2
2
[c]
42:58
40:60
[d]
1
1
0
1
–
46:54
[
a] The yield refers to the last step in the synthesis. [b] Phase transition. [c] In [D
]DMSO. [d] The integrals of the 1H NMR signals could not be analyzed
6
because of overlap with other signals. [e] No melting point was observed before decomposition.
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Table 2. Crystal-structure data of 2, (1R,2S)-3, (1S,2S)-3, (1R,2S)-4, and (1R,2S)-5.
2
(1R,2S)-3
orange plates
26FeINO
(1S,2S)-3
(1R,2S)-4
orange needles
28FeINO
(1R,2S)-5
Color, habit
Formula
orange needles
C
orange needles
orange plates
C H30FeINO
21 2
18
H
24FeINO
2
C
19
H
2
C
19
H26FeINO
2
C
20
H
2
Formula weight [g/mol]
Temperature [K]
Diffractometer
Crystal system
Space group
a [pm]
469.13
180
STOE IPDS 2T
monoclinic
483.16
213
STOE IPDS I
monoclinic
483.16
213
STOE IPDS I
497.18
213
STOE IPDS I
511.21
180
STOE IPDS 2T
orthorhombic
orthorhombic
P2
orthorhombic
P2
P2
1
P2
1
1
2
1
2
1
1
2
1
2
1
1 1 1
P2 2 2
1191.7(2)
1245.2(2)
1244.2(2)
96.78(2)
1833.5(5)
4
755.5(2)
1255.4(2)
1011.7(3)
94.72(2)
956.2(4)
2
834.2(1)
999.0(2)
2365.9(5)
90
1971.6(7)
4
806.3(3)
1044.5(3)
2446.0(7)
90
2060(1)
4
819.76(8)
1056.4(1)
2484.5(3)
90
2151.6(4)
4
b [pm]
c [pm]
ꢀ
[°]
V [10 pm3]
6
Z
3
Calcd. density [g/cm ]
1.699
2.515
1.678
2.414
1.628
2.341
1.603
2.243
1.578
2.150
μ [mm–1
]
θ range [°]
Unique reflections (Rint
Reflections with I > 2σ(I)
1.0–24.0
5652 (0.0564)
3450
2.3–28.0
4414 (0.0531)
3576
1.6–25.0
3461 (0.1662)
1790
1.6–22.5
2694 (0.2046)
1186
1.0–25.0
3728 (0.0514)
2071
)
Parameters
419
219
194
128
237
R
1
[I > 2σ(I)]
wR (all reflections)
Flack parameter
Largest difference peak and hole [e × 10 pm–3
0.0422
0.0679
–0.03(4)
0.59 and –0.45
0.0296
0.0546
–0.05(2)
0.87 and –0.43
0.0538
0.1101
–0.1(1)
1.22 and –0.50
0.0553
0.1028
–0.01(8)
0.60 and –0.69
0.0353
0.0495
0.02(4)
0.62 and –0.37
2
–
6
]
entation of the ferrocene backbones lead to different conforma- which has two independent molecules per asymmetric unit,
tions and to different packings in the unit cell (Figures 2 and and (1R,2S)-3, these torsion angles are 60(2)° in 2 and 58.2(6)°
S20–S24). The basic crystallographic data are listed in Table 2.
in (1R,2S)-3 for O2–C12–C13–N1 and –89(2) and –75(2)° in 2
and –76.2(6)° in (1R,2S)-3 for C12–C13–N1–C18. For small sub-
stituents (R = Me, Et) at the N atom, the pyrrolidine N atom is
orientated towards the unsubstituted ferrocene cyclopentadi-
enyl ring. For the bulkier substituents such as R = Pr and Bu,
the N atom points outwards (Figure 2).
Figure 1. Crystals of the (1S,2S)-3 and (1R,2S)-3 diastereomers (polarized
light).
Although the cell parameters of the orthorhombic structures
are similar, only (1R,2S)-4 and (1R,2S)-5 are isostructural. The
unit cell of (1R,1S)-5 is slightly larger owing to the butyl group
instead of the propyl group in (1R,2S)-4. As depicted in Figure 2,
the orientation of the propyl and butyl groups in (1R,2S)-4 and
(
1R,2S)-5 with respect to the pyrrolidinium ring and ferrocenyl–
Figure 2. Structures of the crystallographically independent [(ferrocenyl-
carbonyl)oxy]methylene-N,N-dialkylpyrrolidine-1-ium cations of 2, (1S,2S)-3,
ester backbone are the same. This is in agreement with similar
torsion angles for O2–C12–C13–N1 [–84(2)° in (1R,2S)-4 and
–
4
(
1R,2S)-3, (1R,2S)-4, and (1R,2S)-5. The thermal displacement ellipsoids are
78.3(8)° in (1R,2S)-5] and C12–C13–N1–C18 [–80(2)° in (1R,2S)-
and –87.1(8)° in (1R,2S)-5]. In (1S,2S)-3, the unit-cell parame-
shown at the 50 % probability level. The atom labeling follows the scheme
shown for (1R,2S)-5.
ters and the positions of the molecules in the unit cell are very
similar to those of (1R,2S)-4 and (1R,2S)-5. However, the rotation
The main difference between the 1R,2S and 1S,2S diastereo-
of the pyrrolidinium group in (1S,2S)-3 around the C12–C13 mers of 3 is the relative position of the bulkier alkyl group in
bond results in torsion angles O2–C12–C13–N1 of 76(2)° and relation to the ferrocene–ester part on the pyrrolidine ring; for
C12–C13–N1–C18 of –32(2)°. In the monoclinic structures of 2, all 1R,2S diastereomers, the longer alkyl group (ethyl, propyl, or
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Table 3. Puckering parameters[21]
atom N1 followed by C13, C14, C15, and C16.
q
2
and ꢁ
as well as the z
coordinates of the pyrrolidinium-ring-containing structures. The calculations were started with
2
j
q
2
[Å]
ꢁ
2
[°]
Conformation
5T
T2
1T
z
j
2
2
– mol I
– mol II
0.37(1)
0.40(1)
0.37(2)
0.404(5)
0.36(2)
0.42(1)
164(3)
192(2)
18(3)
177(1)
251(3)
250(1)
1
z
z
z
z
z
z
2
2
2
2
2
2
= 0.14(1), z
= 0.232(9), z
= –0.22(1), z
= 0.199(3), z
= 0.19(1), z
= 0.220(5), z
3
= –0,01(1), z
= –0.13(1), z
= 0.14(1), z
= –0.066(4), z
= –0.23(1), z = 0.18(1), z
= –0.266(6), z = 0.210(6), z
5
4
= –0.13(1), z
= –0.03(1), z
= –0.00(1), z
5
= 0.22(1)
1
3
4
5
= 0.17(1)
(1S,2S)-3
(1R,2S)-3
(1R,2S)-4
(1R,2S)-5
2
3
4
5
= –0.14(1)
E
E
E
1
3
4
= –0.092(4), z
= –0.07(1)
= –0.074(6)
5
= 0.215(4)
3
3
4
5
3
3
4
butyl) and the ferrocene–ester group are both arranged Clearly, the powder contains both diastereomers. The diastereo-
pseudoequatorially around the pyrrolidine ring (trans confor- meric ratio of (1S,2S)-3/(1R,2S)-3 of 43(5):57(3), as determined
mation). The only structure analysis of a 1S,2S diastereomer by a Rietveld fit (Figure S5), fits well to the results of the NMR
[
(1S,2S)-3] shows a pseudoequatorial arrangement of the ethyl spectroscopy study (43:57, Table 1). Rietveld phase analyses
group and a pseudoaxial positioning of the ferrocene ester could not be performed for the diastereomeric mixtures of 4
group (cis conformation). For steric reasons, free rotation and 5 as no single crystals of the (1S,2S) diastereomers could
around the C12–C13 bond is less likely in the 1S,2S diastereo- be obtained. On the basis of the reflections that are different
mer than in the 1R,2S diastereomer. The smaller methyl group from those of the (1R,2S) diastereomer in the X-ray powder pat-
(C17) makes rotation around the C12–C13 bond possible. This terns (Figures S7 and S8), for example, at 2θ = 18.88, 20.68, and
higher degree of rotational freedom might be a reason for the 24.36° for 4, the fraction of the crystalline (1S,2S) diastereomer
marginal preference for the formation of the 1R,2S diastereo- decreases from 3 to 5. The temperature-dependent powder X-
mers.
ray diffraction patterns of the manually separated diastereo-
In all structures, the O1–C11–O2 group is in the plane of the mers of 3 and their mixture are shown in Figure S6. In combina-
substituted cyclopentadienyl ring consisting of atoms C6 to C10 tion with the STA experiment (Figure S10), the melting point of
and provides a rigid ferrocene–ester backbone. The C6–C11– this powder mixture was demonstrated to be ca. 140 °C.
O1 (124–128°), C6–C11–O2 (108–114°), and O2–C12–C13 angles
(
107–112°) are in the expected ranges and similar to those in
[
19]
FcCOOH (C6–C11–O1: 122°; C6–C11–O2: 116°) and the ferro-
cene ester 3-{4-[(4-methoxyphenoxy)carbonyl]phenoxy}propyl
ferrocenecarboxylate (C6–C11–O1: 125°; C6–C11–O2: 111°; O2–
[
20]
C12–C13: 110°).
In the crystal structures of 2–5, the pyrrolidinium ring is in
the twist [2, (1S,2S)-3] or envelope [(1R,2S)-3, (1R,2S)-4, and
(
1R,2S)-5] conformation. In Table 3, the Cremer–Pople puckering
parameters for the five-membered pyrrolidinium rings are
[
21]
listed.
In the propyl and butyl derivatives (1R,2S)-4 and
(
1R,2S)-5, C14 is out of the plane in the envelope conformation,
in contrast to the methyl (2) and ethyl [(1S,2S)-3, (1R,2S)-3] de-
rivatives, in which we observe twist [2, (1S,2S)-3] as well as en-
velope [E for (1R,2S)-3] conformations. The total puckering am-
1
plitudes q for these pyrrolidinium rings are between 0.36 and
2
Figure 3. X-ray powder diffraction pattern of the mixture of (1S,2S)-3 and
1R,2S)-3 (top) in comparison with their simulated patterns from the single-
crystal structure data; λ(Cu-Kα1) = 154.060 pm. The high background intensity
0.42 Å, similar to the puckering amplitudes of other 2S pyrrol-
(
idinium rings with a quaternized N atom (q = 0.36 Å for CCDC-
2
7
7
71076, q = 0.37 Å for CCDC-771077, q = 0.41 Å for CCDC-
2 2
is due to fluorescence effects.
[
21,22]
69510, and q = 0.38 Å for CCDC-769512).
Only (1R,2S)-3
2
is close to the “pure envelope” conformation (with ꢁ = 0 or
2
1
80°) and also fulfills the requirements for the z values, that is,
[
21]
z = z and z = z . Although the energy barrier for the rota- Cyclic Voltammetry Studies
2
5
3
4
tion of the cyclopentadienyl rings is low, the cyclopentadienyl
rings of the ferrocenyl group in all crystal structures reported
herein are eclipsed.
II
III
The Fe /Fe redox potential in ferrocene derivatives is influ-
enced by the substituents of the cyclopentadienyl rings. For
example, the electron-releasing substituent in tert-butyl ferro-
+
cene shifts the redox potential to –0.061 V versus Fc/Fc ,
whereas ferrocene carbaldehyde with an electron-withdrawing
X-ray Powder Diffraction
substituent shows its redox activity at +0.415 V versus Fc/Fc+
The powder diffraction pattern of a mixture of the diastereo- [high-speed channel electrode with a Pt mesh counter and a
mers (1S,2S)-3 and (1R,2S)-3 is shown in Figure 3 together with silver wire quasireference electrode, (Bu N)ClO as supporting
4
4
[
23]
their simulated patterns from the single-crystal structure data. electrolyte in acetonitrile at 21 °C].
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The formal potentials E0 in acetonitrile versus a Ag/AgNO3 and the ferrocenyl units, as observed in CV experiments of sub-
f,Fc
[
33]
electrode are summarized in Table 4. The biggest shift relative stituted ferrocenium triiodides in ILs, were not observed; the
to the potential of ferrocene (E
0
+
= 0 V) was observed for Fc/Fc formal potentials of the iodides 2–6 are the same as
f,Fc
0
–
FcCOCl (E
= +0.40 V, Figure S25) owing to the electron-with- those of the NTf2 salts 7–11 (Table 4).
f,Fc
drawing properties of the carboxylic acid chloride group. Ferro-
cene monocarboxylic acid and the synthesized esters have simi-
lar E0 values in the range +0.24 to +0.28 V. The quaternization
f,Fc
of the N atom and the type of anion do not influence the elec-
trochemical potential of the ferrocenyl group significantly (Fig-
ure 4). In the iodide salts 2–6, the iodide anions are also redox-
active. The cyclic voltammogram of tetra-n-butylammonium
0
iodide (TBAI) shows two peaks at E
(I1) = –0.18 V and
f,Fc
0
E
(I2) = +0.22 V. These peaks originate in the reactions given
f,Fc
[
26,31]
in Equations (1) and (3).
In 2–6 with the combination of
iodide and the ferrocenyl group, the I2 and Fc peaks overlap to
form one single peak. However, interactions between iodine
and ferrocene species leading to modified electrochemical
[
32,33]
properties have been reported.
In this work, shifts of the
redox potentials owing to interactions between the iodine atom
Table 4. Formal potentials E0f,Fc in acetonitrile referenced to Fc/Fc . For 2–6,
the second peak pair I2 overlaps with the ferrocene peak and, therefore,
could not be analyzed separately (marked as I2+Fc). The estimated uncer-
tainty of these measurements is ca. ±0.02 V.
+
Figure 4. Cyclic voltammograms (E0f,Fc) of TBAI (–0.18, +0.22 V), 1 (+0.27 V),
E0f,Fc [V]
3 (–0.18 V, +0.27 V), and 8 (+0.28 V) in acetonitrile; scan rate 0.1 V/s. The
Anion
Literature values [V]
–1.33[a,b]
–
cyclic voltammograms of 2, 4, 5, and 6 are similar to that of 3. The NTf
2
RTILs
Ferrocene
–
0
7, 9, 10, and 11 show the same electrochemical behavior as 8. Conditions
for all CV experiments: 1 mmol/L analyte, 0.1 mol/L TBAPF6 versus a Ag/
–
Cobaltocene PF
6
–1.33
–0.18 (I1), 0.22 (I2)
–
[c]
TBAI
I
–0.10 (I1), 0.22 (I2),
AgNO3 electrode ([AgNO ] = 0.01 mol/L, 0.1 mol/L tetra-n-butylammonium
3
[d]
–
0.13 (I1), 0.17 (I2)
perchlorate; scan rate 0.1 V/s) with a Pt-wire counter electrode (d = 0.5 mm)
[
28]
[29]
2
FcCOOH
FcCOCl
1
–
–
–
0.24
0.40
0.27
0.17 , 0.25
and a Pt-disc working electrode (A = 0.0222 cm , r = 0.8 mm), 23 °C.
–
2
7
–6
–11
I
–0.18 (I1), 0.27 (I2+Fc)
0.28
–
NTf
Cl
Ag/AgNO
3
2
NMR Spectroscopy Studies
[
a] Ref.[24]; in CH
2
. [b] Ref.[25]; in MeCN. [c] Ref.[26]; calculated according to
2
[30]
→ Fc/Fc+ –0.09 V. [d] Ref.[27]; calculated accord-
+
The carbon atoms C13, C16, C17, and C18 (see atom-labeling
scheme in Figure 2) and the attached protons are expected to
ref. from 0.01
M
[
30]
ing to ref.
from saturated calomel electrode (SCE) → Fc/Fc –0.38 V.
Table 5. 1H and 13C NMR chemical shifts [ppm] of atoms at the 13-, 16-, 17-, and 18-positions of 2–11 in CDCl
and [D
]DMSO (in parentheses).
3
6
R
X–
13
16
17
18
1H
13C
1H
13C
1H
13C
41.4
53.5 (51.9)
49.6 (47.9)
44.4 (42.3)
50.3
44.8
50.2
44.7
50.2
1H
13C
1
2
–
–
2.64
64.2
2.35, 3.17
4.12–4.31 (3.69)
57.6
68.2 (66.7)
2.53
–
–
–
methyl
ethyl
ethyl
propyl
propyl
butyl
butyl
pentyl
pentyl
methyl NTf
ethyl
ethyl
propyl
propyl
I
4.53–4.74 (4.06) 74.0 (72.6)
3.68 (3.30)
3.49 (3.18)
3.20 (3.00)
3.54
3.22
3.50
3.17
3.45
3.15
3.32 (3.29)
3.16 (3.17)
2.97 (3.00)
3.05–3.27
2.99
3.19
2.98
3.20
2.99
3.33 (3.06)
47.1 (45.3)
53.0 (51.2)
60.4 (59.0)
59.0
66.4
57.4
–
(
(
(
(
(
(
(
1S,2S)-3
1R,2S)-3
1S,2S)-4
1R,2S)-4
1S,2S)-5
1R,2S)-5
1S,2S)-6
I
4.94 (4.14)
4.64 (4.08)
4.79
4.66
4.83
4.63
4.73
4.55
4.07 (4.06)
4.14 (4.11)
4.07 (4.07)
4.09–4.25
4.09–4.25
4.22
4.22
4.16
4.11
75.0 (73.8)
73.1 (71.8)
75.2
73.5
75.0
73.6
75.0
73.5
74.1 (72.7)
75.2 (73.7)
73.1 (71.6)
75.2
73.4
75.3
73.6
75.2
73.4
3.79, 4.03 (3.50, 3.71) 63.3 (61.3)
3.49, 3.79 (3.36)
3.89 (3.45, 3.71)
3.30, 3.46
3.64–3.83
3.22–3.44
–
I
3.89, 4.09 (3.61)
3.83–4.01, 4.19
3.83–4.01, 4.19
3.77–3.97, 4.23
3.77–3.97, 4.23
3.80
3.86, 4.13
3.71 (3.68)
3.25–3.81 (3.60)
3.25–3.81 (3.60)
3.57–3.80
64.8 (63.1)
64.0
65.4
63.5
65.5
–
I
–
I
–
I
–
I
3.60–3.77, 3.77–3.97 64.9
–
I
63.8
3.24, 3.41
3.68
3.06 (3.06)
57.7
65.0/ 65.3
46.1 (45.3)
–
[a]
[a]
(1R,2S)-6
I
65.0/ 65.3
68.5 (66.8)
62.9 (61.8)
60.7 (63.7)
63.6
64.9
63.8
65.5
63.6
44.4
–
7
2
53.5 (51.9)
49.3 (47.7)
43.0 (42.0)
49.9
43.6
50.1
43.5
49.9
43.4
–
(
(
(
(
1S,2S)-8
1R,2S)-8
1S,2S)-9
1R,2S)-9
NTf
2
3.25–3.81 (3.39, 3.70) 52.4 (51.8)
3.25–3.81 (3.39, 3.70) 64.5 (59.6)
–
–
–
–
–
–
–
NTf
NTf
NTf
NTf
NTf
NTf
NTf
2
2
3.05–3.27
3.33, 3.51
3.06–3.39
3.42–3.83
3.16
58.7
66.7
57.3
65.5
57.4
65.6
2
3.57–3.80
3.42–3.83
3.42–3.83
3.46–3.78
(
(
(
(
1S,2S)-10 butyl
1R,2S)-10 butyl
1S,2S)-11 pentyl
2
2
2
1R,2S)-11 pentyl
2
3.46–3.78
65.0
3.31, 3.46–3.78
[
a] These signals could not be assigned unambiguously because of their proximity.
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
be influenced mostly by the newly created stereocenter at the
quaternized N atom. With the help of 2D NMR spectroscopy
1
1
[
H– H COSY, heteronuclear single quantum coherence (HSQC),
and HMBC], it is possible to distinguish between the 1R,2S and
S,2S diastereomers. The NMR spectra of manually separated
1
crystals of the diastereomers (1S,2S)-3 and (1R,2S)-3 were re-
corded (Figures S17–S19). The chemical shifts of C13 and C17
are characteristic of the different diastereomers (Table 5). The
chemical shift of atom C13 of the (1R,2S) diastereomer in CDCl3
is 73 ppm, and that of the (1S,2S) diastereomer is 75 ppm. For
C17, the signals of the (1S,2S) diastereomer are in the range δ =
4
9.3–52.9 ppm, and those of the (1R,2S) diastereomers are at
1
δ = 43.0–44.8 ppm. The H NMR spectra of 1 and both dia-
stereomers of 3 are presented in Figures S2 and S4. Atoms H7–
H10 were assigned in accordance with the study by Richards et
[
34]
al.
1
Figure 5. H NMR chemical shifts for 13-H, 16-H, 17-H, and 18-H of the neutral
In Table 5, the H and 13C NMR chemical shifts of atoms C13,
1
–
compound 1, the iodide salts 2–6, and the NTf
2 3
salts 7–11 in CDCl .
C16, C17, C18 and the attached protons of 1–11 are summa-
rized. The assignment of atoms H17/C17 (trans to the ferro-
cenyl–ester part) and H18/C18 (cis position) of 2 and 7 is in
accordance with studies by Faure et al. and Blunden and Crabb
[
35]
1
et al.
The difference of 0.29–0.35 ppm between the H sig-
nals of the methyl protons H17 of the 1S,2S and 1R,2S diastereo-
mers as well as that for the dimethyl compounds 2 and 7 be-
tween methyl protons H17 and H18 fits well to Δδ = 0.3 ppm
for the diastereotopic methyl groups in N,N-dimethyl-2-methyl-
[
36]
pyrrolidine-1-ium iodides. In reports about N-alkyl-N-methyl-
-organylpyrrolidinium iodides (with alkyl = methyl, ethyl and
2
organyl = methyl, phenyl), the signal of the methyl group trans
to the organyl group is observed at higher δ values than that
[
37]
for the cis position.
from our investigation.
In Figures 5 and 6, the H and C NMR chemical shifts of 1–
These results are consistent with those
1
13
1
1
1 in CDCl are visualized. For the H NMR chemical shifts, it is
Figure 6. 13C NMR chemical shifts for C13, C16, C17, and C18 of the neutral
compound 1, the iodide salts 2–6, and NTf2 salts 7–11 in CDCl3.
3
–
clear that the change from the neutral molecule 1 to the qua-
ternary ammonium salts 2 or 7 causes downfield shifts by ca.
ures 5 and 6, the type of anion makes no difference to the
chemical shift of C13. The only influence on C13 is the stereo-
chemical configuration. In contrast, the chemical shifts of C16,
C17, and C18 show not only differences between the 1S,2S and
1
ppm for H16 and H17 and ca. 2 ppm for H13. The influence
of the chain length of the alkyl substituents decreases with in-
creasing alkyl chain length.
Chloroform as a non-coordinating and relatively apolar sol-
vent leads to a closer anion–cation contact in solution; there-
fore, the type of anion in the IL influences the chemical shifts
1
R,2S diastereomers but also partly between the two different
anions. In contrast, the NMR spectra of the compounds in
D ]DMSO show a different behavior: the ions are solvated, and
[
6
1
13
of some H and C nuclei in the cation. In the iodide salts, the
the chemical shifts are independent of the nature of the anions.
Howarth et al. reported similar observations for ferrocenylimid-
azolium salts. They discovered C–H···X hydrogen bonds of 1,3-
1
H NMR signals are shifted downfield relative to those of the
–
NTf2 salts; a significant difference in the chemical shifts of 13-
H, 16-H, 17-H, and 18-H exists between the iodide salt 2 and
–
–
–
di(ferrocenylmethyl)imidazolium salts in CDCl with Cl , Br , I ,
–
3
the NTf2 salt 7 in CDCl . This chemical-shift difference between
3
–
–
–
[38]
NO , HSO , and PF anions.
On the basis of the work by
the iodide and NTf2– salts is observed for all 13-H and 16-H
3
4
6
Howarth et al. and Dupont et al., the differences are attributed
atoms of 3–11. The 17-H signals of the 1S,2S iodide diastereo-
mers (δ = 3.45 and 3.54 ppm) and the 1R,2S NTf2 diastereo-
mers (δ ≈ 2.9 ppm) are separated clearly from those of the
to the presence of a contact ion pair structure in CDCl in con-
–
3
[39]
For the NTf2–
trast to solvent-separated ions in [D ]DMSO.
6
1
9
salts, the F NMR chemical shift of –79.2 ppm is in agreement
–
–
1R,2S I and 1S,2S NTf2 diastereomers, which overlap. Owing
[40]
with values reported previously.
to peak overlap for 18-H, the assignment to distinct diastereo-
mers is possible only for the 1R,2S diastereomers of the iodides.
An explanation might be the localization of the negative charge
Conclusions
at only one atom for the iodide versus its distribution over the The syntheses of a series of six new RTILs based on ferrocene
–
NTf2 anion and its proximity to the cation. As shown in Fig- units coupled to a pyrrolidinium ring through an ester bridge
Eur. J. Inorg. Chem. 0000, 0–0
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6
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
are reported. One RTIL has an iodide anion, and the other five 3.9 mmol) was dissolved in THF (25 mL). [(2S)-1-Methylpyrrolidine-
–
2-yl]methanol (0.5 mL) was added, and the solution was heated
under reflux for 1 h. A yellow solid precipitated from the red solu-
tion. The organic phase was cooled to room temp., and water
have NTf2 anions. The quaternization of the chiral pyrrolidine
units with alkyl iodides results in diastereomeric mixtures, and
the two diastereomers 1S,2S and 1R,2S are obtained in product
ratios of 43:57 according to NMR spectroscopic data and
3(5):57(3) according to PXRD experiments for (1S,2S)-3 and
1R,2S)-3. Thus, there is only a slight preference for the 1R,2S
(40 mL) was added. After deprotonation with NaOH pellets (0.5 g),
1
was extracted three times with Et O (10 mL each). The combined
2
4
(
organic phases were dried with MgSO , and the solvent was re-
4
moved under reduced pressure to leave 1 as an orange oil, yield
diastereomer.
1.03 g (74 %).
In their NMR spectra, both diastereomers differ in their
Iodides 2–6: Compound 1 (0.70 g, 2.0 mmol) was dissolved in an
excess of alkyl iodide (2 mL), and the mixture was stirred at room
temp. for 1 h. After 3 h at 70 °C, a yellow precipitate (2–5) or red
1
3
chemical shifts; the C atoms C13 (2S stereocenter) and C17
methyl group attached to the 1S/1R stereocenter) of the 1S,2S
diastereomers are more deshielded than those in the 1R,2S dia-
(
oil (6) was separated, washed with Et O, and dried. Single crystals
2
stereomers. The NMR spectra of the compounds in CDCl show were obtained by recrystallization from MeOH/Et O (1:1) at –22 °C.
3
2
different chemical shifts for the cation depending on the anion,
Bis(trifluoromethylsulfonyl)imides 7–11: The syntheses of the
–
–
I or NTf . In the more polar solvent [D ]DMSO, this effect van-
–
[43r]
2
6
NTf2 salts were performed by a literature procedure.
LiNTf2
ishes. Therefore, a contact ion pair structure in CDCl is pro-
3
(2 mmol) dissolved in water (10 mL) was added to the iodide salt
posed in contrast to solvent-separated ions in [D ]DMSO.
(2–6, 1 mmol) in water (10 mL). Immediately, a yellow emulsion
6
The synthesized RTILs based on ferrocene, pyrrolidinium, and formed, and an orange-brown oil separated from the aqueous
NTf2– show thermal stability up to 263 °C (10). Of the iodide
derivatives, only the pentyl compound 6 is liquid at room temp.
The decomposition of the iodide ILs starts between 166 and
phase after a few seconds. The suspension was stirred for a further
1
6 h at room temp. The oil was separated and extracted with CH Cl
2 2
(
10 mL). The combined organic phases were dried with MgSO , and
4
the CH Cl was removed under reduced pressure. The resulting red
2
2
194 °C. We are aware that the compounds might remain liquid
–
2
oils were dried at 10 mbar and 40–50 °C for 2 h.
even below their melting point because of hindered crystalliza-
tion. For 6–11, we could not observe the formation of a solid.
The single-crystal structure analyses of the IL precursors
FcCOCl, 2, (1S,2S)-3, (1R,2S)-3, (1R,2S)-4, and (1R,2S)-5 reveals
that their crystal structures are closely related. However, only
Supporting Information (see footnote on the first page of this
article): Details on the synthetic procedures and characterization of
1
4
–11; crystal structure data of FcCOCl, 2, (1S,2S)-3, (1R,2S)-3, (1R,2S)-
, and (1R,2S)-5; NMR spectra of 1, (1S,2S)-3, and (1R,2S)-3; (temper-
ature-dependent) X-ray powder diffraction patterns of 2–5; Rietveld
refinement of the mixture of (1S,2S)-3 and (1R,2S)-3; STA data of 1–
(
1R,2S)-4 and (1R,2S)-5 are isostructural. The pyrrolidine ring ap-
pears in the twist [2 and (1S,2S)-3] or envelope conformation
(1R,2S)-3, (1R,2S)-4, and (1R,2S)-5]. The main difference be-
11; IR spectra of 1–3, 7, and 8.
[
tween the structures of the 1R,2S and 1S,2S diastereomers in
the crystalline state is the arrangement of the longer alkyl chain
Acknowledgments
(
e.g., ethyl, propyl, and butyl) in relation to the ferrocenyl–ester The authors thank Shiyang Dong for his help in the synthesis
group. For 1R,2S, both groups are arranged pseudoequatorially of some of the compounds reported herein. Professor Rudolf
trans conformation) around the pyrrolidinium ring, whereas the Holze (Technische Universität Chemnitz) is gratefully acknowl-
ethyl group is pseudoequatorial and the ferrocenyl–ester group edged for valuable discussions of the cyclic voltammetry data.
(
is pseudoaxial (cis conformation) for 1S,2S.
The authors thank the University of Leipzig (PbF-1), Germany
Cyclic voltammetry studies show Fe /Fe redox potentials for support.
II
III
+
of ca. +0.28 V versus Fc/Fc for the [(ferrocenylcarbonyl)-
oxy]methylene-N,N-dialkylpyrrolidine-1-ium salts. This potential
Keywords: Ionic liquids · Metallocenes · Sandwich
complexes · Redox chemistry · Cyclic voltammetry
is independent of alkyl chain length and type of anion. There-
fore, the chemical behavior can be changed by further anion
exchange and adapted to special needs, but the redox potential
will remain the same.
[
[
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Ferrocene monocarboxylic acid chloride
and [(2S)-1-methyl-
pyrrolidine-2-yl]methanol[ were prepared according to literature
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[
CCDC 1452807 (for 2), 1452808 [for (1R,2S)-3], 1452809 [for (1S,2S)-
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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[
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Published Online: ■
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Ionic Liquids
C. B. Bouvet, H. Krautscheid* .......... 1–9
Chiral and Redox-Active Room-Tem-
perature Ionic Liquids Based on Fer-
rocene and -Proline
L
The syntheses of the first ferrocene- The RTILs are reversibly redox-active
II
and pyrrolidinium-based room-tem- through the Fe metal center as well
perature ionic liquids (RTILs) linked as chiral and thermally stable up to
through ester bridges are reported. 263 °C.
DOI: 10.1002/ejic.201600585
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim