Utilization of a Crown Ether/Amine-Type Rotaxane as a Probe for the Versatile Detection of Anions and Acids by Thin-Layer Chromatography

Article abstract of DOI:10.1002/asia.202000746

A crown ether/amine-type [2]rotaxane was synthesized and utilized as a probe for the detection of acids and anions. The addition of acids to the amine-type [2]rotaxane solution generated corresponding crown ether/ammonium-type [2]rotaxanes, which were purified by silica gel column chromatography as ammonium salts. The isolated yields of the [2]rotaxanes, possessing a variety of anions, depended on the acidity and polarity of the counter anions. The behaviours of the ammonium-type [2]rotaxanes on thin-layer chromatography (TLC) silica gel reflected the properties of the counter anions. The treatment of the amine-type [2]rotaxane with acids afforded the corresponding ammonium-type [2]rotaxanes bearing several different anions. The ammonium-type [2]rotaxanes behaved similarly to the purified [2]rotaxanes on the TLC silica gel. Furthermore, we succeeded in the analysis of anions using mixtures of the amine-type [2]rotaxane and salts in an appropriate solvent. We demonstrated the detection of anions by the combination of TLC and the utilization of the [2]rotaxane probe.

Full text of DOI:10.1002/asia.202000746

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Utilization of a Crown Ether/Amine-Type Rotaxane as a Probe
for the Versatile Detection of Anions and Acids by Thin-Layer
Chromatography
Authors: Shinobu Miyagawa, Masaki Kimura, Shin Kagami, Tsuneomi
Kawasaki, and Yuji Tokunaga
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10.1002/asia.202000746
Chemistry - An Asian Journal
FULL PAPER
Utilization of a Crown Ether/Amine-Type Rotaxane as a Probe for
the Versatile Detection of Anions and Acids by Thin-Layer
Chromatography
Shinobu Miyagawa,[a] Masaki Kimura,[a] Shin Kagami,[a] Tsuneomi Kawasaki,[b] Yuji Tokunaga*[a,c]
[a]
[b]
Dr. S. Miyagawa, Dr. M. Kimura, S. Kagami, Prof. Dr. Y. Tokunaga
Department of Materials Science and Engineering, Faculty of Engineering, University of Fukui, Bunkyo, Fukui 910-8507 (Japan)
E-mail: tokunaga@u-fukui.ac.jp
Prof. Dr. T. Kawasaki
Department of Applied Chemistry
Tokyo University of Science
Kagurazaka, Shinjuku-ku, Tokyo, 162-8601 (Japan)
[c]
Prof. Dr. Y. Tokunaga
Research and Education Center for Regional Environment, University of Fukui, Bunkyo, Fukui 910-8507 (Japan)
Supporting information for this article is given via a link at the end of the document.
Abstract: In this study, a crown ether/amine-type [2]rotaxane was
synthesized and utilized as a probe for the detection of acids and
anions. The addition of acids to the amine-type [2]rotaxane solution
generated corresponding crown ether/ammonium-type [2]rotaxanes,
which were purified by silica gel column chromatography as
ammonium salts. The isolated yields of the [2]rotaxanes, possessing
a variety of anions, depended on the acidity and polarity of the
counter anions. The behaviours of the ammonium-type [2]rotaxanes
on thin-layer chromatography (TLC) silica gel reflected the properties
of the counter anions. The treatment of the amine-type [2]rotaxane
with acids afforded the corresponding ammonium-type [2]rotaxanes
bearing several different anions. The ammonium-type [2]rotaxanes
behaved similarly to the purified [2]rotaxanes on the TLC silica gel.
Furthermore, we succeeded in the analysis of anions using mixtures
of the amine-type [2]rotaxane and salts in an appropriate solvent.
We demonstrated the detection of anions by the combination of TLC
and the utilization of the [2]rotaxane probe.
inexpensive analytical method for anions, which can be operated
by any person, is still required.
Generally, ammonium salts are easily and rapidly neutralized
in the presence of bases to produce deprotonated amines.
Stoddart’s group discovered the crown ether/sec-ammonium-
type rotaxane,[16] and they experienced difficulty in neutralizing
the ammonium salt because of the unprecedented strong
hydrogen bonds between the crown ether and the ammonium
ion.[17] Takata’s group investigated the kinetics and the
thermodynamic stability of the ammonium moiety in crown
ether/ammonium-type rotaxanes.[18,19] Further, we regulated the
acidity of the ammonium protons by changing the size of the
crowns in the rotaxanes, and developed a five-state molecular
shuttling system comprising a pair of [2]rotaxanes.[20]
In 2013, Loeb’s group directly utilized a crown-encircled
amine as
a
frustrated Lewis base.[21] They treated the
deprotonated amine rotaxane, consisting of aniline and a crown
ether, with hydrogen gas (4 atm) in the presence of B(C6F5)3.
The rotaxane acted as a Brønsted–Lowry base to produce a
−
crown/ammonium rotaxane bearing HB(C6F5)3 as a counter ion
Introduction
without the corresponding amine-B(C6F5)3 complex. Although
the strong basicity of the encircled amine has been fully
elucidated, the straightforward utilization of this property has not
been sufficiently investigated, except for Loeb’s example.
Considering the critical role of anions in living organisms, the
environment, and medicine, anion receptors based on small
molecules have been developed for specific and/or selective
anion sensing.[1] Many supramolecular structures have been
synthesized to achieve strong anion binding through different
kinds of noncovalent interactions.[2,3] Firstly, the size and shape
of the binding sites, as well as the functionalities of anion
receptors, have been designed specifically for the effective
binding of target anions.[4−12] Secondly, the changes in the
optical (absorbance or fluorescence)[1c,2a,3,4,5,6,8,9a,11,12a,13] and
electrochemical properties[1c,2a,3,4,6b,11,12a] of receptors complexed
with anions have been investigated for anion sensing.
Generally, in the detection of primary, secondary, and tertiary
ammonium salts by thin-layer chromatography (TLC) on silica
gel, the ammonium salt is deprotonated (the acid of the
ammonium salt is absorbed by the silica gel), and a spot
corresponding to the resulting amine is detected on the TLC
plate. In contrast, since the encapsulated ammonium in the
crown ether/ammonium-type rotaxane is extremely stable owing
to the strong hydrogen bonds between both components, as
described above, the rotaxane, accompanied by a counter anion,
might exist as an ammonium salt under the TLC-on-silica-gel
conditions. Therefore, the Rf values of the rotaxanes could be
affected by the counter anion (Figure 1). Herein, we report the
versatile detection of acids and anions by the combination of a
Ion chromatography[14] and capillary electrophoresis[15] are
powerful tools for the qualitative and quantitative analyses of
many anions simultaneously. However,
a
simple and
1
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10.1002/asia.202000746
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crown ether/amine-type rotaxane as a probe and TLC on silica
gel.
because the hydrogen bond between the aniline and crown
ether is stronger than that between the amine and the crown
ether, owing to the high acidity of aniline-NH (Scheme 1). In the
1H NMR spectrum (500 MHz, CDCl3) of 1/DB24C8 (Figure 2a),
the signals indicative of the benzylic protons, Hd and He,
significantly shifted upfield, relatively to those indicative of
1H/DB24C8[X] (Figure 2h), which is consistent with the
deprotonation of the dialkylammonium group and the loss of the
deshielding effects of DB24C8. In contrast, the characteristic
signal of another benzylic proton, Hh, shifted to the low field
region, due to the deshielding effects of DB24C8.
Z
H
N
HZ
H2
N
TLC plate
(NaZ)
rotaxane probe
1/DB24C8
1H/DB24C8[Z]
TLC
analysis
HY
(NaY)
HX
(NaX)
Y
X
H2
N
H2
N
1H/DB24C8[Z]
1H/DB24C8[Y]
1H/DB24C8[X]
N
N
H
1H/DB24C8[X]
1H/DB24C8[Y]
H
H2
H
N
NaOH
N
PF6
1H/DB24C8[PF6]
1/DB24C8
Figure 1. Concept of detection of acids and anions by the combination of a
crown ether/amine-type rotaxane as a probe and thin-layer chromatography
(TLC).
O
O
O
O
O
O
O
O
Results and Discussion
DB24C8
Isolation of the deprotonated rotaxane (1/DB24C8)
Scheme 1. Synthesis of crown/amine-type rotaxane 1/DB24C8.
For this investigation, the rotaxane structure of 1H/DB24C8[X]
(deprotonated form: 1/DB24C8) is shown in Scheme 1.
Dialkylammonium and aniline stations are present in the
dumbbell-like axle component for dibenzo[24]crown-8 as the
macrocyclic component.[22] The rotaxane bearing two stations in
the axle component might be suitable for isolation as a crown
ether/amine-type rotaxane, because the second station holding
DB24C8 can stabilize the deprotonated form. First, we
attempted to isolate 1/DB24C8 by treating
1H/DB24C8[X] in dichloromethane with
a
solution of
a
basic aqueous
solution (aq. NaOH), and the organic phase was concentrated to
afford the rotaxane mainly in its deprotonated form, 1/DB24C8.
However, 1/DB24C8 was not reproducibly isolated. Although the
treatment of a solution of 1H/DB24C8[X] with potassium tert-
butoxide and Bu4NF,[19] using several solvents, afforded
1/DB24C8 in situ, we could not completely isolate the rotaxane
in its deprotonated form (a mixture of the deprotonated and the
protonated rotaxanes was isolated) after workup. Takata et al.
reported that the stability of the ammonium group in the
crown/ammonium-type rotaxane is dependent on the counter ion
−
and that PF6 is the most suitable counter ion for stabilizing the
protonated rotaxane.[19] Finally, we succeeded in completely
isolating 1/DB24C8. A suspension of 1H/DB24C8[X] in ether
and an excess amount of aqueous NaOH were vigorously stirred
for 54 h, after which the suspension transformed into a two-
phase solution. Separation of the organic phase followed by
concentration gave the deprotonated rotaxane 1/DB24C8. Since
Figure 2. 1H NMR spectra (500 MHz, CDCl3) of the deprotonated rotaxane
1/DB24C8, mixtures of 1/DB24C8 (10 mM) and acetic acid (0–6 eq),
1H/DB24C8[PF6], and axle molecule 1. (a) 1/DB24C8, (b) 1/DB24C8 and
AcOH (0.5 eq), (c) 1/DB24C8 and AcOH (1.0 eq), (d) 1/DB24C8 and AcOH
(1.5 eq), (e) 1/DB24C8 and AcOH (2.0 eq), (f) 1/DB24C8 and AcOH (4.0 eq),
(g) 1/DB24C8 and AcOH (6.0 eq), (h) 1H/DB24C8[PF6], (i) 1, and (j) 1 and
AcOH (97 eq).
−
the counter anion, PF6 , is required to stabilize the ammonium-
type rotaxane, the removal of the PF6 species is important for
isolating 1/DB24C8. Since both PF6 salts (1H/DB24C8[PF6] and
−
NaPF6) are insoluble in ether, PF6 completely migrates to the
aqueous phase as NaPF6 during the treatment of
1H/DB24C8[X] in the presence of aqueous NaOH. The reaction
time was reduced when toluene was used as the organic solvent.
The presence of 1/DB24C8 was confirmed by nuclear magnetic
resonance (NMR) spectroscopy (Figures 2a and S10).
As previously reported,[22] the DB24C8 unit predominantly
encircled the aniline moiety under the deprotonation conditions
As similar deprotonated rotaxane bearing dialkylamine and
aniline moieties in its axle component exists as a mixture of
translational isomers in solution,[23] VT NMR experiments
(CD2Cl2) were performed to reveal the translational isomerism of
2
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1/DB24C8 (Figure S3). Although significant signals based on the
isomers were not observed above –90 °C, small signals (ca 3%),
which can be assigned to the benzyl protons in translational
isomer were detected at 4.16, 4.35, and 4.50 ppm at –50 °C. At
–70 °C, Hi and Hj signals broadened, and Hj signals were split in
two at –90 °C. This observation suggests slow rotation of
DB24C8 around aniline moiety in the axle component.
isolated in its deprotonated form after purification. After the
treatment of 1/DB24C8 with several acids (2.0 equiv), we
employed chromatography to isolate the rotaxanes as mono-
ammonium salts, 1H/DB24C8[X], (Table 1). When AcOH was
employed,
1H/DB24C8[CH3CO2].
we
could
not
As
isolate
the
rotaxane,
above,
described
1H/DB24C8[CH3CO2] is not sufficiently stable as a salt under
the chromatographic conditions because of the low acidity of
AcOH. For chloroacetic acid and dichloroacetic acid (weak
acids), the yields of the corresponding 1H/DB24C8[ClCH2CO2]
and 1H/DB24C8[Cl2CHCO2] were moderate (49% and 57%),
and these rotaxanes included 1.5 and 1.0 equiv of the
corresponding acids, respectively (runs 1 and 2), as these
organic acids could not be completely absorbed under the
chromatographic conditions. In contrast, the utilization of strong
acids afforded the corresponding mono-protonated rotaxanes
(1H/DB24C8[X]) in good yields (runs 3, 5, 6, 8 and 9). However,
the isolated yield of 1H/DB24C8[HSO4] was quite low,
presumably owing to the high polarity of H2SO4 and/or
1H/DB24C8[HSO4] (run 7), as high polar acid can be absorbed
by silica gel easily. The potential stability (pKa value) and the
affinity for silica gel might be related to the isolated yield. A
strong diacid (ethane disulfonic acid) was employed (run 10),
and a 2:1 salt (1H/DB24C8[(CH2SO3)2]1/2) was isolated in 60%
yield after treatment with 1 equiv of ethane disulfonic acid and
chromatographic purification. At first, a 1:1 salt was mainly
formed in the presence of 1 equiv of the diacid. In consideration
of the yield and basicity of aniline, silica gel promotes the
deprotonation of anilinium and captures the highly polar
disulfonic acid. Finally, a 2:1 salt of 1H/DB24C8[(CH2SO3)2]1/2
was isolated. These isolated rotaxanes (1H/DB24C8[X]) were
validated by 1H NMR spectroscopy (Figure S11).
Purification of the axle molecule (2H0X0) using preparative
TLC on silica gel after treatment with acids
First, after the axle molecule 1 was treated with hydrochloric
acid and toluenesulfonic acid (TsOH), separately (Scheme 2).
The generated protonated axle molecules (1H1-2[Cl]1-2 and 1H1-2
[OTs] 1-2]) were subjected to preparative TLC (CHCl3/MeOH,
10:1 and toluene/THF, 3:2) to afford the deprotonated axle
1
molecule, 1, in all cases. All the products were validated by H
NMR spectroscopy (Figures S1 and S2). The results show that
the hydrochloric acid and TsOH were completely removed
(absorbed) by the silica gel.
N
N
H
H
H
H2
HX
N
N
X
silica gel
1
1H[X]
Scheme 2. Interconversion between the amine 1 and its ammonium salt 1H[X].
Formation of 1H/DB24C8[X]: NMR titration experiments of
rotaxane 1/DB24C8 with acids
To investigate the formation of the ammonium rotaxanes
(1H/DB24C8[X]), 1/DB24C8 was treated with acids, including
acetic acid (AcOH) (pKa: 4.8), chloroacetic acid (pKa: 2.9),
trifluoroacetic acid (TFA) (pKa: 0.23), and methanesulfonic acid
(MsOH) (pKa: −1.9). Figure 2 displays the 1H NMR spectra of
1/DB24C8 (10 mM) in the presence of AcOH (0–6.0 equiv) in
CDCl3. Upon increasing the amount of AcOH, the Hh signal (0
equiv of AcOH: 4.67 ppm) moved to the high field region, and
the Hd and He signals (0 equiv of AcOH: 3.65 and 3.60 ppm)
shifted down field. These shifts were observed until the addition
of 4.0 equiv of AcOH (Figure 2f). The results revealed that the
formation of the mono-ammonium salt (1H/DB24C8[X]) was
gradual and that the process was completed after 4.0 equiv of
Table 1. The isolated yields of mono-protonated rotaxanes after silica gel
column chromatography.[a]
run
acid
pKa
Isolated yield
(%)
1
2
ClCH2CO2H
Cl2CHCO2H
TFA
2.86
1.29
−0.25
−2.6
−2.8
−1.3
−3.0
−8.0
−10
49[b]
57[c]
96
3
4
MsOH
52
AcOH was added.
5
TsOH
94
In the cases of chloroacetic acid (Figure S4), TFA (Figure
S5), and MsOH (Figure S6), ca 1.5, 1.0–1.5 and 1.0 equiv of the
acids, respectively, were enough for the mono-ammonium salt
formation under these conditions. The strong acid [pKa: < 0.2
(TFA)] was effective for the quantitative mono-protonation of
1/DB24C8. Furthermore, the addition of excess amounts of
strong acids (TFA and MsOH) caused the protonation of the
aniline moiety of [2]rotaxane.[24]
6
HNO3
<100
9
7
H2SO4
8
HCl
<100
97
9
HClO4
[d]
10
(CH2SO3H)2
−1.46
−2.06
60[e]
Purification of rotaxanes 1H/DB24C8[X] using silica gel
column chromatography after the treatment of 1/DB24C8
with acids and TLC analysis of the ammonium rotaxanes
1H/DB24C8[X]
The purification of 1H/DB24C8[X] was performed using silica gel
column chromatography. Similar to the case of the protonated
axle molecule (1H1-2[X]1-2), if silica gel absorbs the acid from the
protonated rotaxane (1H1-2/DB24C8[X]1-2), the rotaxane will be
[a] After treatment of deprotonated rotaxane 1/DB24C8 with 2 eq of acids, the
salts were purified by silica gel column chromatography. [b] The product
(1H/DB24C8[ClCH2CO2]) including 1.5 eq of chloroacetic acid. [c] The product
(1H/DB24C8[Cl2CHCO2]) including 1.0 eq of dichloroacetic acid. [d] The
rotaxane 1/DB24C8 was treated with 1.0 eq of (CH2SO3H)2. [e] Isolated as a
1H/DB24C8[(CH2SO3)2]1/2
.
3
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Next, we subjected these isolated salts, mono-ammonium
rotaxanes (1H/DB24C8[X]) and the deprotonated rotaxane
(1/DB24C8), to TLC analysis (Figure 3). As expected, all the
ammonium rotaxanes (1H/DB24C8[X]) and the deprotonated
rotaxane (1/DB24C8) exhibited different behaviours on the TLC
plate (eluent: CH2Cl2/acetone/H2O, 3:16:1).
the spectrum of 1/DB24C8 in only CD3OD was quite similar to
that of 1H/DB24C8[X] (Figure S8; benzylic protons appeared at
4.23, 4.52, and 4.59 ppm).[25] We suspect that the two spots on
the TLC plates correspond to 1H/DB24C8[X] and
1H/DB24C8[MeO] in the presence of the weak acids (HX).
Figure 3. TLC analysis of the protonated rotaxanes 1H/DB24C8[X] and
deprotonated rotaxane 1/DB24C8 (eluent: CH2Cl2/acetone/H2O, 3:16:1) under
UV irradiation. (a) 1H/DB24C8[ClCH2CO2], (b) 1H/DB24C8[Cl2CHCO2], (c)
1H/DB24C8[CF3CO2], (d) 1H/DB24C8[MsO], (e) 1H/DB24C8[TsO], (f)
Figure 4. TLC analysis of the deprotonated rotaxane 1/DB24C8 in the
presence of acids (eluent: CH2Cl2/acetone/H2O, 3:16:1) under UV irradiation.
Deprotonated rotaxane 1/DB24C8 in the presence of (a) ClCH2CO2H, (b)
Cl2CH2CO2H, (c) TFA, (d) MsOH, (e) TsOH, (f) (CH2SO3H)2, (g) HClO4, (h)
HCl, (i) H2SO4, and (j) HNO3. (k) Deprotonated rotaxane 1/DB24C8.
1H/DB24C8[(CH2SO3)2]1/2, (g) 1H/DB24C8[ClO4], (h) 1H/DB24C8[Cl], (i)
1H/DB24C8[HSO4], (j) 1H/DB24C8[NO3], and (k) 1/DB24C8.
Next, acid-competitive TLC analysis was performed. After
treatment of a solution of the deprotonated rotaxane (1/DB24C8)
in the presence of TsOH (2.0–0 equiv) and dichloroacetic acid
(0–2.0 equiv), each solution was subjected to TLC analysis
(Figure 5). Upon increasing the ratio of either of the two acids, a
spot corresponding to 1H/DB24C8[X] became noticeable. Even
though the acidity of TsOH is higher than that of dichloroacetic
acid, 1H/DB24C8[Cl2CHCO2] could be detected by TLC.
Detection of acids using the deprotonated rotaxane
(1/DB24C8) as a probe
With the knowledge that the behaviours of each salt
(1H/DB24C8[X]) on the TLC plate depend on the anion, we
directly conducted TLC analysis of the mixtures of the
deprotonated rotaxane (1/DB24C8) and several acids,
separately. After mixing 1/DB24C8 (5 mM) in CHCl3 (0.25 mL)
and the acids (100 or 250 µM) in H2O, CHCl3, or THF (12.5 or 5
µL), a small amount of the organic layer was utilized for the TLC
analysis using CH2Cl2/acetone/H2O (3:16:1) as the eluent
(Figure 4). The TLC behaviours of all the mixtures, except for
the sulfuric acid mixture, were identical to those of the
corresponding salts, which were purified by column
chromatography on silica gel (Figure 3), individually. As
described above, the deprotonation of the anilinium moiety
proceeded under the TLC conditions even though 1/DB24C8
was treated with excess amounts of the acids. In the case of
sulfuric acid (Figure 4i), excess amounts appear to disturb the
predominant formation of the mono-protonated rotaxane, as
silica gel cannot sufficiently absorb the sulfuric acid under these
conditions. The detection of the acids is completed by the
combination of the rotaxane probe and TLC analysis under the
acidic conditions.
In contrast, when CHCl3/MeOH (10:1) was used as the
eluent in the TLC analysis (Figure S7), two spots were observed
in the absence and presence of chloroacetic acid and TFA
(Figure S7k, S7a, and S7c) on the TLC plate, and one Rf values
of them were quite similar even without any acid. We suspected
that excess MeOH might promote the protonation of 1/DB24C8
to produce 1H/DB24C8[MeO]. 1H NMR experiments of
1/DB24C8 in CD3OD without an acid were performed. When
mixtures of CD3OD/CDCl3 were used as solvents,
1H/DB24C8[MeO] partially formed, as increasing the ratio of
CD3OD, the ratio of 1H/DB24C8[MeO] was increased. Finally,
Figure 5. TLC analysis of the deprotonated rotaxane 1/DB24C8 in the
presence of TsOH and Cl2CH2CO2H under UV irradiation. (a) 1/DB24C8 +
TsOH (2 eq), (b) 1/DB24C8 + TsOH (1.75 eq) and Cl2CH2CO2H (0.25 eq), (c)
1/DB24C8 + TsOH (1.5 eq) and Cl2CH2CO2H (0.5 eq), (d) 1/DB24C8 + TsOH
(1.0 eq) and Cl2CH2CO2H (1.0 eq), (e) 1/DB24C8 + TsOH (0.5 eq) and
Cl2CH2CO2H (1.5 eq), (f) 1/DB24C8 + TsOH (0.25 eq), and Cl2CH2CO2H (1.75
eq), (g) 1/DB24C8 + Cl2CH2CO2H (2 eq).
Detection of the salt anions using rotaxane 1/DB24C8 as a
probe
After spotting solutions or suspensions of 1/DB24C8 (5 mM,
0.152 mL) and the salts (250 µM) in aqueous MeOH (12.5 µL),
4
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the TLC plate was dried in vacuo to remove water. Thereafter,
the analysis was performed using the mixed solvent system
(CH2Cl2/acetone/H2O, 3:16:1) as an eluent. The results of the
TLC analysis of the mixtures of 1/DB24C8 and several salts are
shown in Figure 6. The behaviours of the salts on the TLC plate
depend on the corresponding anions. These behaviours are
similar to those of the mixtures of 1/DB24C8 and the
corresponding conjugated acids. For examples, TFA (Figures
4c) and CF3CO2Na (Figure 6a), HClO4 (Figure 4g) and NaClO4
(Figure 6g), and HCl (Figure 4h) and NaCl (Figure 6h). However,
the shapes of the spots were not partly identical. Probably, the
excess salt, which was spotted at the same time, influenced the
behaviours of each 1H/DB24C8[X], as some of the spots that
were not detected in the acid detection experiments (Figure 4)
appeared to push up the spots of 1H/DB24C8[X], in the
presence of CF3CO2Na, NaBr, and NaNO3 (Figure (B) 6a, 6c
and 6j).
The protonation of 1/DB24C8 is not promoted in the
presence of neutral salts in comparison with the experiments in
the presence of acids (Figure 4). However, the spots of the
corresponding salts were detected. As described above, MeOH
promotes the protonation from the solvent, and the exchange of
the counter ion afforded the protonated rotaxane with the
removal of the solvent.
1H/DB24C8[PF6], by deprotonation, as a probe for acid and
anion detection, in cooperation with TLC. At first, mono-
protonated rotaxanes (1H/DB24C8[X]) were isolated by silica
gel column chromatography, and their Rf values reflected the
corresponding counter anions. Secondly, we confirmed that the
TLC behaviours of the mixtures of 1/DB24C8 and the acids
coincided with those of the corresponding isolated mono-
protonated rotaxanes 1H/DB24C8[X]. In the presence of two
acids, both protonated rotaxanes 1H/DB24C8[X] could be
detected by TLC. Finally, the treatment of 1/DB24C8 with salts
produced
the
corresponding
protonated
rotaxanes
(1H/DB24C8[X]), which were directly detected by TLC.
In this study, we used 1H/DB24C8+ as a counter ion for the
anion detection. A quaternary ammonium cation could be
employed in the same manner, since the differences seems to
be negligible. The advantage of this method is that rotaxane
1/DB24C8 has no anion in the initial stage; therefore,
1H/DB24C8[X] can be formed directly. As 1H/DB24C8[MeO]
was detected when MeOH was used as a solvent, protic
solvents may form similar species. However removal of the
solvent helps revive original deprotonated rotaxane. In contrast,
the disadvantage is that the counter anion, i.e., the conjugate
base of the weak acid, could not be adapted for this method.
Experimental Section
Materials and General Methods
The rotaxane 1H/DB24C8[PF6] was prepared using modified
literature procedure.[22] All solvents and commercially available
chemicals were used as received, except for dichloroethane,
which was dried over 4Å molecular sieves. 1H and 13C NMR
spectra were recorded using ECX-500II and ECA-600II
spectrometers, with TMS as the internal standard. Mass spectra
were recorded using JMS-700T (FAB) spectrometer,
respectively. Infrared spectra were recorded using a Shimadzu
FTIR-8600PC spectrometer. All reactions were performed under
a positive atmosphere of dry N2. All solvents were removed
through rotary evaporation under reduced pressure. Thin-layer
chromatography was performed using Merck Kieselgel 60PF254
.
Silica gel column chromatography was performed using Kanto
Chemical silica gel 60N.
Rotaxane 1/DB24C8
A suspension of rotaxane 1H/DB24C8[PF6] (509 mg, 0.534
mmol) in toluene (50 mL) and 10% NaOH aq. (50 mL) was
vigorously stirred for 24 h at room temperature. The organic
phase was separated, washed with H2O and sat NaCl aq., and
dried (Na2SO4). After evaporation of the solvent, the residue was
washed with hexane to afford the rotaxane 1/DB24C8 (0.350 g,
81%) as a white powder. IR ν max (NaCl) cm-1: 3402, 2922,
Figure 6. TLC analysis of the deprotonated rotaxane 1/DB24C8 in the
presence of salts, (A) under UV irradiation and (B) after treatment of
12MoO3·H3PO4 in EtOH. Deprotonated rotaxane 1/DB24C8 in the presence of
(a) CF3CO2Na, (b) NaI, (c) NaBr, (d) NaH2PO4, (e) NH4PF6, (f) C2H5SO3Na, (g)
NaClO4, (h) NaCl, (i) Na2SO4, and (j) NaNO3. (k) Deprotonated
rotaxane1/DB24C8.
1
2868, 1601, 1506, 1252, 1215, 1125, 1055, 953. H-NMR (500
MHz, CDCl3) δ: 7.54–7.46 (m, 2H), 6.97–6.90 (m, 4H), 6.88 (s,
1H), 6.86-6.75 (m, 8H), 6.54 (s, 2H), 6.15 (s, 1H), 5.26 (br s, 1H),
4.67 (s, 2H), 4.13–4.04 (m, 8H), 3.87–3.77 (m, 4H), 3.73–3.63
(m, 4H), 3.64 (s, 2H), 3.60 (s, 2H), 3.34 (br s, 8H), 2.30 (s, 6H),
2.05 (s, 6H). 13C NMR (125 MHz, CDCl3) δ: 149.8, 148.3, 140.5,
139.6, 137.8, 137.5, 137.4, 128.8, 128.4, 127.1, 126.0, 120.4,
116.6, 111.6, 69.9, 69.3, 67.8, 53.0, 52.9, 46.6, 21.4, 21.3.
Conclusion
We utilized the deprotonated rotaxane (1/DB24C8), which was
+
HRMS (FAB): m/z calcd. for C49H63N2O8 [M+H]+ 807.4584,
prepared
from
the
corresponding
ammonium
salt,
found 807.4573.
5
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10.1002/asia.202000746
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6
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10.1002/asia.202000746
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FULL PAPER
Entry for the Table of Contents
A crown ether/amine-type rotaxane was utilized as a probe for the detection of acids and anions. The treatment of the ‘amine-type’
[2]rotaxane with acids or salts afforded the corresponding ‘ammonium-type’ [2]rotaxanes bearing the corresponding anions. The
behaviours of the ammonium-type [2]rotaxanes on thin-layer chromatography (TLC) silica gel reflected the properties of the counter
anions.
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