Formation of a pseudorotaxane, capable of sensing cations via dethreading molecular motion, from a cryptand and bipyridinium salts

Article abstract of DOI:10.1007/s10847-012-0280-z

A new type of cryptand-based pseudorotaxane was prepared, utilizing a novel cryptand incorporating two aromatic rings as a wheel component, combined with various bipyridinium salts as the axle component. These pseudorotaxanes exhibited charge-transfer absorption at approximately 380 nm due to π-electron interactions between the cryptand and the bipyridinium salt. Upon addition of one equivalent of sodium ion to the pseudorotaxane, this absorption was observed to disappear as a result of displacement of the bipyridinium salt by the sodium.

Full text of DOI:10.1007/s10847-012-0280-z

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-012-0280-z
ORIGINAL ARTICLE
Formation of a pseudorotaxane, capable of sensing cations
via dethreading molecular motion, from a cryptand
and bipyridinium salts
Masahiro Muraoka
Yuko Mizutani Mashio Takezawa
Ayuri Matsumoto Yohji Nakatsuji
•
Mamoru Ohta
•
•
•
•
Received: 25 September 2012 / Accepted: 21 December 2012
Ó Springer Science+Business Media Dordrecht 2013
Abstract A new type of cryptand-based pseudorotaxane
was prepared, utilizing a novel cryptand incorporating two
aromatic rings as a wheel component, combined with
various bipyridinium salts as the axle component. These
pseudorotaxanes exhibited charge-transfer absorption at
approximately 380 nm due to p-electron interactions
between the cryptand and the bipyridinium salt. Upon
addition of one equivalent of sodium ion to the pseudoro-
taxane, this absorption was observed to disappear as a
result of displacement of the bipyridinium salt by the
sodium.
goal, researchers have pursued the development of
chemosensors based on highly efficient host molecules (or
receptors), including crown ether derivatives.
Recently, various interlocked molecules, such as rotaxanes
and catenanes, have been investigated as both chemosensors
[4–14] and molecular machines [15–18]. These interlocked
molecules can bind specific guest molecules, including alkali
metal cations [7–10] and chloride and sulfate anions [11–14],
inside three-dimensional cavities. In addition, molecular
recognition can be achieved by utilizing pseudorotaxanes
[19–22] tosenseanalytesviaadisplacementtypeofmolecular
motion [23–25]. For example, Stoddart and coworkers [20,
21] reported that [2]pseudorotaxanes can be used to obtain an
optical response upon alkali metal binding, based on the un-
threading of the axle component from the wheel component.
Beer and coworkers [22] reported that sulfate anion com-
plexation by an isophthalamide macrocycle in a [2]pseud-
orotaxane resulted in displacement of a threaded molecule,
with a subsequent loss of color. Both of the above preliminary
resultsrequiredasubstantialexcessofthetriggeringspeciesin
order to achieve displacement of the thread. This suggests that
such displacement may be optimized when the non-covalent
wheel-axle interaction is relatively weak in the pseudorotax-
ane while the complexation ability of the wheel component
toward the trigger (or analyte) is proportionately strong.
Cryptands are promising candidates for the formation of
rotaxanes in conjunction with various threads [26–43].
Because cryptands have in the past been employed to achieve
strongbindingofmetalcationsandorganicmoleculeswithina
three-dimensional cavity [44–46], the displacement-type
molecular motion of rotaxanes composed of a bipyridinium
salt as the axle component and a cryptand as the wheel com-
ponent can be expected to occur upon addition of metal cat-
ions. Gibson and Huang et al. have performed pioneering
work in this area and reported that pseudorotaxanes formed
Keywords Pseudorotaxanes ꢀ Cryptands ꢀ Bipyridinium
salts ꢀ Charge-transfer interaction ꢀ Threading–dethreading
molecular motion
Introduction
To date, substantial effort has been devoted to the devel-
opment of chemosensors for both ionic and neutral species
[
1–3]. In the case of chemosensors utilizing molecular
recognition, it is particularly important that such systems
are capable of responding to analytes with a high degree of
both selectivity and sensitivity. In order to achieve this
Electronic supplementary material The online version of this
article (doi:10.1007/s10847-012-0280-z) contains supplementary
material, which is available to authorized users.
M. Muraoka (&) ꢀ M. Ohta ꢀ Y. Mizutani ꢀ M. Takezawa ꢀ
A. Matsumoto ꢀ Y. Nakatsuji
Department of Applied Chemistry, Faculty of Engineering,
Osaka Institute of Technology, Ohmiya, Asahi-ku,
Osaka 535-8585, Japan
e-mail: muraoka@chem.oit.ac.jp; nakatsuji@chem.oit.ac.jp
123

J Incl Phenom Macrocycl Chem
from cryptands can undergo switching upon the addition of
alkali metal cations [47–50].
Herein, we describe a highly sensitive alkali metal cat-
ion-switchable pseudorotaxane, newly designed and syn-
thesized by our research group, formed from the inclusion
complex of a bipyridinium salt with a cryptand. The
threading–dethreading molecular motion of this pseudoro-
taxane has been characterized by studies of the binding
between the cryptand and bipyridinium salt based on UV–
1
Vis and H NMR spectral changes.
Results and discussion
Scheme 2 Synthesis of macrocycle 1. Reagents and conditions: i
2 3 3
tri(ethylene glycol) ditosylate, K CO , CH CN, reflux, 68 h, 69 %; ii
Synthesis
cis-2,9-bis(bromomethyl)-2.9-dimethyl-15-crown-5, NaH, di(ethyl-
ene glycol) dimethyl ether, 120 °C, 79 h, 33 %
Based on work with monoazacryptands as alkali metal
cation carriers for liquid membrane transport, reported
previously by our group [51–53], the new cryptand 1 was
designed. The intent was to obtain a molecule containing
two p-electron-rich aromatic rings in order to stabilize the
p-electron-poor aromatic rings of bipyridinium salts
through p–p stacking (Scheme 1). Cryptand 1 was pre-
pared via the intermolecular [1?1] macrocyclization
reaction of cis-2,9-bis(bromomethyl)-2,9-dimethyl-15-
crown-5 [54] with a diol [55] under basic conditions [56] as
summarized in Scheme 2, resulting in a 33 % yield. The
salt 2 in acetone (Fig. 1c), a pale yellow coloration was
observed as a result of charge-transfer absorption at
approximately 380 nm, as evident in the UV–Vis spectrum
shown in Fig. 1b. This absorption can be attributed to
p-stacking between the bipyridinium and phenylene rings.
Further evidence of the pseudorotaxane structure was
1
provided by changes in H NMR spectra. The binding
behavior of the cryptand with bipyridinium salts was
examined via the addition of one molar equivalent of the
1
salt to a solution of the cryptand in acetone-d . The H
6
1
13
structure of 1 was ascertained by H NMR, C NMR, mass
spectrometry and elemental analysis.
NMR spectra of cryptand 1, dimethyl bipyridinium salt 2,
and an equimolar mixture of 1 and 2 (1 . 2) in acetone-d6
are shown in Fig. 2. Upon addition of dimethyl bipyridi-
nium salt 2, slight upfield shifts are observed in the signals
of the phenylene protons (d and e) of cryptand 1, implying
p–p interactions between the electron-rich benzene rings of
the macrocycle and the positively charged bipyridinium
salt. Thus, it appears that cryptand 1 forms a stable com-
plex with dimethyl bipyridinium salt 2 through p–p inter-
actions, indicating the formation of a [2]pseudorotaxane.
Pseudorotaxane formation
Initial evidence of pseudorotaxane formation based on a
combination of cryptand 1 with bipyridinium salts was
provided by UV–Vis spectroscopy as shown in Fig. 1.
When the colorless cryptand 1 solution (Fig. 1a) was
mixed with a colorless solution of dimethyl bipyridinium
a
b
O
O
O
O
O
O
c
B
A
C
+
+
N
O
O
O
O
O
O
H C
N
CH3
3
+
N
+
N
2
+
d
e
PF6-
H C
3
CH3
+
N
a
2
+
2
2
PF -
6
f
O
O
O
O
O
O
O
O
O
O
O
O
Na+
O
O
O
O Me
Me
Me
Me
Me
O
O Me
O
O
O
1
Scheme 1 Schematic representation of formation of a pseudorotaxane and dethreading molecular motion
1
23

J Incl Phenom Macrocycl Chem
0
0
0
0
0
.04
.03
.02
.01
.00
(
a)
(b)
3
.5
2
2
1
0
0
0.2
0.4
0.6
0.8
1
[
1] / ( [1] + [2] )
Fig. 3 Job plot showing the 1:1 stoichiometry of the complex
between 1 and 2 in acetone-d : [1] ? [2] = 5.0 mM; Dd = chemical
shift change for proton d of 1
.5
1
6
(b)
.5
(c)
(a)
0
3
00
350
400
Wavelength / nm
450
500
Fig. 1 Top: Photographs depicting the color change (acetone,
0 mM); bottom: partial UV–Vis spectra (acetone, 10 mM) of a 1,
b an equimolar mixture of 1 and 2, and c 2
1
a
e
d
f
c b
(a)
(b)
(c)
C
B
A
1
Fig. 2 Partial H NMR spectra (300 MHz, acetone-d
b an equimolar mixture (15 mM) of 1 and 2, and c 2
6
, 293 K) of a 1,
Fig. 4 Structure of the macrocycle 1 and the axles 2–4 for the
[
2]pseudorotaxanes
1
The job plot [57] presented in Fig. 3, based on H NMR
spectroscopic data measured in acetone-d , demonstrates
6
protons of the macrocycle (d) were monitored [58]. Mea-
suring the upfield shifts of these protons upon the addition
of bipyridinium salts 2–4 allowed the association constants
that the stoichiometry in solution of the complex 1 . 2
was 1:1. The binding properties of cryptand 1 were esti-
1
mated by quantitative H NMR titration experiments in
(K ) for the complexations to be determined. The associ-
a
acetone-d in which cryptand 1 was combined with dime-
6
ation constants thus derived for each complexation are
presented in Table 1. The association constants for the
complexation of cryptand 1 with all three bipyridinium
salts proved to be all on the same order of magnitude.
thyl bipyridinium salt 2 as well as bipyridinium salt
derivatives 3 and 4, as shown in Fig. 4. During these
1
titrations, the H NMR shift changes of the phenylene
123

J Incl Phenom Macrocycl Chem
-
) [M ] for complexes of macro-
1
Table 1 Association constants (K
cycle 1 with bipyridinium salt derivatives 2, 3 and 4 as determined by
a
color change was observed from pale yellow to colorless.
As shown in Fig. 6, the absorbance associated with the
charge transfer band at about 380 nm in the UV–Vis
1
6
H NMR titrations in acetone-d at 293 K
-1
)
Guest
K
a
(M
D
0
(ppm)
spectrum decreases as the level of NaPF increases, indi-
6
2
2
2
cating that the addition of the cation results in the bipy-
ridinium salt dethreading from the cryptand. Interestingly,
2
3
4
1.8 9 10
1.3 9 10
2.1 9 10
0.41
0.41
0.57
?
adding one equivalent of Na to the pseudorotaxane
appears to have induced complete displacement, due to
?
strong complexation of the Na by the cryptand via the
Error \ 8 %. Determined by chemical shift changes of macrocycle 1
proton d during titration experiments followed by non-linear least-
squares data treatment as reported by Hirose [58]. An initial host
concentration of 2.0 mM was used for the titrations
three dimensional electron-donating sidearms of the
1
5-crown-5 ring. Electrostatic repulsion between the bound
?
Na and the bipyridinium salt within the cavity of the
cryptand evidently bring about the disassembly of the
cryptand-based pseudorotaxane. It has therefore been
demonstrated that this cryptand-based pseudorotaxane may
act as a sensitive sensor for alkali metal cations via a color
change.
NOE experiments provided useful information and fur-
ther evidence of the pseudorotaxane formation, in which
bipyridinium salt 2 threads inside the three dimensional
cavity of cryptand 1. The NOESY spectrum of the pseud-
orotaxane 1 . 2 in acetone-d6 is presented in Fig. 5,
wherein clear correlations may be seen between phenylene
protons d and e of cryptand 1 and protons A and B of
bipyridinium salt 2, confirming that bipyridinium salt 2 was
incorporated within cryptand 1.
This cation-induced dethreading behavior was also
1
1
monitored by H NMR. Figure 7 presents the H NMR
spectra of cryptand 1 (Fig. 7a), the pseudorotaxane formed
from 1 with bipyridinium salt 2 (Fig. 7b), an equimolar
mixture of NaPF and the pseudorotaxane (Fig. 7c) as well as
6
Cation induced switching studies
an equimolar mixture of NaPF and 1 (Fig. 7d), all in ace-
6
tone-d . Upon addition of one equivalent (14 mM) of NaPF
6
6
Subsequent to synthesis of the pseudorotaxane, we under-
took investigations into the use of alkali metal cations to
induce dissociation and subsequent displacement of the
threaded component from the cryptand cavity. We antici-
pated that, upon addition of an alkali metal cation, the
cryptand would preferentially coordinate to the cation,
leading to displacement of the bipyridinium salt and a
consequent change in the absorption spectrum of the
complex.
to the pseudorotaxane, the chemical shifts corresponding to
(a)
(b)
(c)
When NaPF6 was added to an equimolar mixture
(
2 mM) of cryptand 1 and bipyridinium salt 2 in acetone, a
B
A
0.2
+
Na ]/[1 2]
⊃
[
0
0
1
1
2
.5
.0
.5
.0
0
0
.15
0.1
B - d
B - e
A - d
A - e
.05
0
320 340 360 380 400 420 440 460 480 500
Wavelength / nm
Fig. 6 Top: Photographs depicting the color of 10 mM solutions in
acetone of a 1, b an equimolar mixture of 1 and 2 and c solution
(
b) upon addition of 1 equiv. of NaPF ; bottom: partial UV–Vis
into an equimolar solution of 1 and 2
6
Fig. 5 Partial NOESY spectrum (300 MHz, acetone-d
equimolar mixture (50 mM) of 1 and 2
6
, 293 K) of an
spectra of the titration of NaPF
(acetone, 2 mM)
6
1
23

J Incl Phenom Macrocycl Chem
0
0
0
0
0
.15
.12
.09
.06
.03
0
a
K+
Na+
e d
f c b
(
a)
Li+
(
b)
c)
(
(d)
0
0.5
1
1.5
2
2.5
3
[G] / [H]
1
Fig. 8 H NMR titration curves for KPF
6
, NaPF
6 6
, and LiPF with an
equimolar mixture (5 mM) of 1 and 2, monitoring proton d in
acetone-d at 293 K
1
6
Fig. 7 Partial H NMR spectra (300 MHz, acetone-d
b an equimolar mixture (14 mM) of 1 and 2, c solution (b) after
addition of 1 equiv. of NaPF , and d an equimolar mixture (14 mM)
of 1 and NaPF
6
, 293 K) of a 1,
metal cations are capable of dethreading bipyridinium salt
6
6
2 in this displacement system.
phenylene protons d and e of cryptand 1 return to essentially
the same values as their pre-complexed chemical shifts, as
can be seen by comparing Fig. 7a–c, indicating that bipy-
ridinium salt 2 was completely dethreaded from cryptand 1.
Conclusions
This work accomplished both the design and synthesis of a
cryptand containing two electron-rich aromatic rings and
able to bind bipyridinium derivatives. This cryptand
structure has been shown to exhibit complexation behavior
towards various bipyridinium salts by p-electron interac-
tions and the resulting pseudorotaxane is capable of sens-
ing alkali metal cations via a displacement mechanism. As
a result, we have developed a simple synthetic strategy for
a pseudorotaxane employing bipyridinium salts and have
demonstrated the functioning of this pseudorotaxane as a
chemosensor for alkali metal cations through the use of
displacement assays.
The addition of NaPF to the pseudorotaxane also results in
6
chemical shifts corresponding to the oxyethylene protons of
the crown ring and the benzylic methylene protons (f) which
are practically identical to those obtained for the complex of
?
cryptand 1 with Na but without 2 in acetone-d , as shown in
6
?
Fig. 7c, d. This indicates that the Na was indeed captured
inside the cavity of cryptand 1 when added to the pseud-
orotaxane. These signal changes imply that the sodium cat-
ion becomes associated with the ether oxygen atoms in the
1
5-crown-5 ether moiety and consequently the bipyridinium
salt 2 is dethreaded due to electrostatic repulsion forces,
resulting in displacement-type molecular motion.
We are currently extending this principle to other
interlocked structures such as rotaxanes and catenanes
which may be capable of providing shuttling [15–18] and
rotating [59, 60] movements for molecular sensing appli-
cations, utilizing oxyethylene moieties as a binding site for
cationic as well as neutral sensing targets.
NMR titration experiments with the pseudorotaxane
were conducted to determine if the particular alkali metal
cation added had an effect on the extent to which the p
electron interactions between the cryptand and the bipy-
ridinium salt were disrupted. Solutions of KPF , NaPF , or
6
6
LiPF were titrated into a solution of the pseudorotaxane in
6
acetone-d with results as shown in Fig. 8. Interestingly,
6
Experimental
when either KPF or NaPF was titrated into the pseud-
6
6
orotaxane solution, signal changes for proton d reached
saturation upon addition of equimolar amounts of either
guest species. On the other hand, larger quantities of LiPF6
were required to achieve comparable degrees of replace-
ment of the bipyridinium salt by the alkali metal cation.
The dethreading process is therefore more effective when
All chemicals were commercially available reagent grade
and were used without further purification. The synthesis of
cis-2,9-bis(bromomethyl)-2,9-dimethyl-15-crown-5
has
been described previously [54]. The preparation of 1,8-bis[4-
(hydroxymethyl)phenoxy]-3,6-dioxaoctane was carried out
according to a previously reported procedure [55]. Bipyrid-
inium salt derivatives 2, 3, and 4 were prepared according to
?
?
?
either K or Na is used, as compared to Li , probably due
?
?
1
13
to selectivity toward K and Na by the crown ether
portion. Nevertheless, it is apparent that a variety of alkali
published procedures [61–63]. H and C NMR spectra
were obtained using a Varian Mercury 300 spectrometer
123

J Incl Phenom Macrocycl Chem
with SiMe as an internal standard. Mass spectra were
4
association constants, were determined by SOLVSTAT
[64].
obtained using a JEOL JMS-DX-303 mass spectrometer.
Elemental analyses were performed using a Yanaco CHN-
Corder MT-5 analyzer.
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