Self-assembly of [2]pseudorotaxanes based on pillar[5]arene and bis(imidazolium) cations

Article abstract of DOI:10.1039/c0cc03575k

A simple bis(imidazolium) dication, 1,4-bis[N-(N′-hydroimidazolium)] butane, can act as a new template for formation of [2]pseudorotaxane with pillar[5]arene, in which the dethreading/rethreading process can be controlled by addition of base and acid. The effect on the association constant of both the solvent and counterion is also described.

Full text of DOI:10.1039/c0cc03575k

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Self-assembly of [2]pseudorotaxanes based on pillar[5]arene
and bis(imidazolium) cationsw
Chunju Li,*a Liu Zhao,a Jian Li,a Xia Ding,b Songhui Chen,a Qiaolin Zhang,a Yihua Yub and
Xueshun Jia*a
Received 31st August 2010, Accepted 20th October 2010
DOI: 10.1039/c0cc03575k
A simple bis(imidazolium) dication, 1,4-bis[N-(N0-hydroimidazo-
lium)]butane, can act as a new template for formation of [2]pseu-
dorotaxane with pillar[5]arene, in which the dethreading/
rethreading process can be controlled by addition of base and
acid. The effect on the association constant of both the solvent and
counterion is also described.
Rotaxanes have been attracting considerable attention,1 not
only for their topological importance but also due to their
potential applications in preparation of molecular devices and
machines.2 Pseudorotaxanes3 are the supramolecular
precursors of rotaxanes, and also viewed as prototypes of
simple molecular machines. The ongoing search for new
macrocyclic hosts (the ‘wheels’) and linear guest molecules
(the ‘axles’) that template pseudorotaxane construction could
increase the options available for the development of
rotaxanes, catenanes, ‘muscles’,4 switches,2 and machines.2
Pillar[5]arene (P5A), firstly synthesized by Ogoshi’s group,5
was a symmetrical calixarene analogue made up of five
hydroquinone units linked by methylene (–CH2–) bridges.
Later, the family of pillar[n]arene hosts has been expanded to
a larger-cavity homologue, pillar[6]arene, by Cao, Meier and
co-workers.6 More recently, Huang et. al7 synthesized
copillar[5]arene (P5A containing different repeating units).
Being different from the conventional calixarene’s ‘‘basket’’
structure, pillararene forms the symmetrical pillar architecture.
Pillararenes’ structural characteristics and p-rich cavities make
the hosts suitable to develop pseudorotaxanes with linear
cationic molecules. Searching new axles for the new
supramolecular building blocks is thus very interesting. Our
previous work8 has reported the formation of a series of 1 : 1
[2]pseudorotaxanes and 2 : 1 host–guest complexes between
P5A with dicationic 1,4-bis(pyridinium)butanes and alkyl-
substituted paraquat derivatives. An essential contribution to
the formation of these complexes is the cation–p-electron
interactions.8,9 We reasoned that by the substitution of
the pyridinium on the 1,4-bis(pyridinium)butane dications
Scheme 1 Structure of pillar[5]arene (P5A) and bis(imidazolium)
guests.
could construct a new templating axle that would bind
with the P5A host through cation–p interactions, and
simultaneously, the resulting pseudorotaxane could be
reversibly switched off (and back on) by deprotonation (and
protonation) of the N3 atoms on the axle (Scheme 1).
Herein, we report the self-assembly of [2]pseudorotaxanes based
on P5A and 1,4-bis(imidazolium)butane cations ([1-2H]2+–32+
)
that function as templating motifs, and the use of acid/base
reaction to switch between complexed/uncomplexed states.
Moreover, the solvent and counteranion effects have also
been investigated to look at how they affect the association
strength during the course of host–guest complexation.
Fig. 1 shows the 1H NMR spectra of [1-2H]ꢀ2PF6 in acetone-
d6 recorded in the absence and in the presence of about 1
equivalent (eq.) amount of the P5A host. The peaks for the
methylene protons of [1-2H]ꢀ2PF6 exhibit substantial upfield
shifts and broadening effects compared to the free axle (Dd =
ꢁ0.96 and ꢁ1.25 ppm for Hd and He, respectively) as a
consequence of inclusion-induced shielding effects.10 At the
same time, the signal corresponding to the a-protons of the
imidazolium N1 atom (Ha and Hc) exhibits a pronounced
upfield shift, while no obvious changes were observed for the
b-protons (Hb). In the control experiments, under identical
conditions no NMR changes of [1-2H]ꢀ2PF6 occurred upon
for
a
similar sized imidazolium, to get 1,4-bis[N-(N0-
hydroimidazolium)]butane ([1-2H]2+, see Scheme 1), we
a Department of Chemistry, Shanghai University, Shanghai, 200444,
P. R. China. E-mail: cjli@shu.edu.cn, xsjia@mail.shu.edu.cn;
Fax: +86 21 66132408; Tel: +86 21 66132408
b Shanghai Key Laboratory of Magnetic Resonance, Department
of Physics, East China Normal University, Shanghai, 200062,
P. R. China
w Electronic supplementary information (ESI) available: Synthesis,
determination of the association constants, additional NMR spectra,
job plots and ESI-MS spectra. See DOI: 10.1039/c0cc03575k
Fig. 1 1H NMR spectra (500 MHz) of (a) [1-2H]ꢀ2PF6, (b) P5A +
[1-2H]ꢀ2PF6, and (c) P5A in acetone-d6 at 5.0–5.3 mM.
c
9016 Chem. Commun., 2010, 46, 9016–9018
This journal is The Royal Society of Chemistry 2010

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addition of hydroquinone, i.e., the monomeric unit of the P5A
host. Hence, the P5A-induced upfield shifts and broadening
effects on the imidazolium Ha and Hc protons and methylene
(Hd and He) protons reveal that the P5A wheel is fully threaded
by the axle and the main binding site for the host is the
methylene linker. Meanwhile, part of the imidazolium ring
(N1+, C2 and C5) is also included in the host cavity. From 2D
NOESY analysis (Fig. S18, ESIw), NOE correlations were
observed between Ha, Hc, Hd and He of the axle and
aromatic protons (H2) of P5A. While no correlations
between H2 and Hb were observed. These results also
confirm the threading binding mode. This inclusion complex
can be considered to have a 1 : 1 [2]pseudorotaxane structure.
For comparison purpose, we also selected for our study
two substituted bis(imidazolium) cations where the H proton
in [1-2H]2+ was replaced by –CH3 and –CH2COOtBu
(22+ and 32+) (Scheme 1, for synthesis, see ESIw). Although
2ꢀ2PF6 has good solubility in acetone-d6, precipitation
occurred immediately when mixed with P5A, which itself
signals an interaction between the host and guest. Therefore,
we chose 3 : 2 (v : v) acetone-d6 and DMSO-d6 as solvent for
Table 1 Association constants15 (Ka/Mꢁ1) for 1 : 1 complexation of
P5A and [1-2H]ꢀ2PF6 and 2ꢀ2PF6 in different solvents at 25 1C
Solvent (v : v)
[1-2H]ꢀ2PF6
2ꢀ2PF6
DMSO
(5.5 ꢃ 0.2) ꢂ 10
(1.2 ꢃ 0.4) ꢂ 102
(2.7 ꢃ 0.3) ꢂ 102
(4.6 ꢃ 0.6) ꢂ 102
(3.1 ꢃ 0.5) ꢂ 103
(1.4 ꢃ 0.2) ꢂ 102
(3.3 ꢃ 0.4) ꢂ 102
(1.0 ꢃ 0.2) ꢂ 103
3 : 7 (CD3)2CO : DMSO
3 : 2 (CD3)2CO : DMSO
(CD3)2CO
a
—
a
—
1 : 1 (CD3)2CO : CDCl3
a
Could not be determined due to the poor solubility of the P5A–2ꢀ
2PF6 complex in these solvents.
should be the important driving forces8,9,11 during the course
of complexation of P5A with these positively charged guests
and they are dramatically affected by the solvent polarity. In
order to assess counterion effects for the new synthetic receptor
P5A, the [1-2H]2+ guest was synthesized with
a
a
noncoordinating counterion, and
[1-2H]ꢀ2ClO4,
coordinating counterion, [1-2H]ꢀ2Cl, and the dependence of
the association constants of [1-2H]2+ with P5A on the anion
type in acetone-d6 and 3 : 2 acetone-d6 : DMSO-d6 was
determined (Table 2). It’s well known that ion-pairing effects
hamper the complexation of charged species by neutral
receptors.12 In a lower polarity acetone-d6 solvent, when the
counterion of [1-2H]2+ changed from PF6ꢁ to a little strongly
coordinating anion, ClO4ꢁ, the Ka value decreases from 4.6 ꢂ
1
the H NMR experiments (Fig. S13, ESIw). Upon addition of
the P5A host, the NMR response of 2ꢀ2PF6 in this mixed
solvent was fast and the signal changes were similar to that of
[1-2H]ꢀ2PF6 (Fig. 1b and 3b). In contrast, no obvious signal
changes were observed for 3ꢀ2PF6 in acetone-d6. Since the
bulky tBu unit was unable to thread the cavity of P5A, the
P5A–3ꢀ2PF6 inclusion complex did not form.
102 to 3.6
ꢂ
102 Mꢁ1
. The counterion effects of
pseudorotaxane formation between P5A and [1-2H]2+ in
acetone-d6 were similar to those of crown ether complexes in
lower polarity solvent systems.13 On the other hand, no
counterion effects were found in 3 : 2 acetone-d6 : DMSO-d6,
since the Ka values between P5A and [1-2H]2+ with different
Further evidence of 1 : 1 host–guest complex was obtained
by the ESI mass experiments. For example, in the ESI mass
spectrum of an eq. mixture of 2ꢀ2PF6 and P5A (Fig. S23,
counterions (PF6ꢁ, ClO4 and Clꢁ) were almost the same
ꢁ
ESIw), only two intense peaks for
a 1 : 1 complex
were observed, one for [2–P5A]2+ (m/z 415.8), and one for
[2ꢀPF6–P5A]+ (m/z 975.4). On the other hand, job plots based
on proton NMR data also demonstrated that the complexes
between [1-2H]ꢀ2PF6 and 2ꢀ2PF6 with P5A were of 1 : 1
(Table 2). This is reasonable because the counterion effects
were overruled by the relatively high polarity of DMSO and
the threads would be expected to be nearly completely ionized
in the 3 : 2 acetone-d6 : DMSO-d6 solvent. That is to say, P5A
can form pseudorotaxane complexes with bis(imidazolium)
threads containing strongly coordinating anions (such as
Clꢁ) in high polarity solvents (such as 3 : 2 acetone-
d6 : DMSO-d6), which is very rare in the calix[n]arene and
large crown ether series.14
1
stoichiometry (Fig. S24, ESIw). Combined with the H NMR
experiments, we can unambiguously conclude the formation of
[2]pseudorotaxane-type complex, as shown in Fig. 2.
We have also explored the effect on the association constants
of the solvent and counterion. It can be seen from Table 1 that
the solvent effects are very pronounced on the formation of
P5A–[1-2H]ꢀ2PF6 and P5A–2ꢀ2PF6 inclusion complexes since
the Ka values significantly increased when the solvent polarity
was reduced. For example, the P5A–[1-2H]ꢀ2PF6 Ka values in
With the aim of investigating the ability of the new
[2]pseudorotaxane to perform dethreading/rethreading
movement by pH control, we chose 3 : 2 (v : v) acetone-
d6 : DMSO-d6 as solvent due to the lower solubility of the
deprotonated [1-2H]ꢀ2PF6 (1) in pure acetone-d6. [1-2H]ꢀ2PF6
and P5A form a [2]pseudorotaxane with Ka = 2.7 ꢂ 102 M–1 in
the mixed solvent, while the neutral compound 1 is unable to
associate with P5A. 1H NMR spectra of a solution of P5A and
[1-2H]ꢀ2PF6 following the addition of B2.2 eq. of n-Bu3N,
3 : 7 acetone-d6 : DMSO-d6 (1.2 ꢂ 102
M
ꢁ1), 3 : 2 acetone-
d6 : DMSO-d6 (2.7 ꢂ 102 Mꢁ1), pure acetone-d6 (4.6 ꢂ 102 Mꢁ1
)
and 1 : 1 acetone-d6 : CDCl3 (3.1 ꢂ 103 Mꢁ1) are enhanced by
factors of 2.2, 4.9, 8.4 and 56.4 compared with that of pure
DMSO-d6. It is reasonable that cation–p-electron interactions
Table 2 Association constants15 (Ka/Mꢁ1) for 1 : 1 complexation of
P5A and [1-2H]2+ with different counterions at 25 1C
Xꢁ
Acetone-d6
3 : 2 (v : v) acetone-d6 : DMSO-d6
(4.6 ꢃ 0.6) ꢂ 102
(2.7 ꢃ 0.3) ꢂ 102
(2.5 ꢃ 0.4) ꢂ 102
(2.6 ꢃ 0.4) ꢂ 102
ꢁ
ꢁ
PF6
ClO4
Clꢁ
(3.6 ꢃ 0.3) ꢂ 102
a
—
Fig. 2 Formation of [2]pseudorotaxanes between [1-2H]ꢀ2PF6 and
P5A and the base–acid controlled dethreading/rethreading movements.
a
Could not be determined due to the poor solubility of the salt.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 9016–9018 9017

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on the binding interaction, indicating that P5A can form a
[2]pseudorotaxane-type complex with bis(imidazolium) thread
containing strongly coordinating anions (such as Clꢁ) in high
polarity solvents (such as 3 : 2 acetone-d6 : DMSO-d6).
The availability of the simple components, the controllable
assembly and disassembly, the ability to easily tune the
association strength, and the generality of the solvent and
the type of counterion imply potential applications of this
motif in the fabrication of interlocked structures and molecular
devices.
This work was supported by NNSFC (Nos: 20902057 and
20872087) and Leading Academic Discipline Project of
Shanghai Municipal Education Commission (No: J50101).
Notes and references
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Fig. 3 1H NMR spectra (500 MHz) of (a) P5A, (b) P5A + [1-2H]ꢀ
2PF6, (c) [1-2H]ꢀ2PF6, (d) [1-2H]ꢀ2PF6 + n-Bu3N, (e) P5A + [1-2H]ꢀ
2PF6 + n-Bu3N, (f) P5A + [1-2H]ꢀ2PF6 + n-Bu3N + CF3COOH,
in 3 : 2 acetone-d6 : DMSO-d6. The concentrations of P5A and [1-2H]ꢀ
2PF6 were 5.9–6.4 mM; the concentrations of n-Bu3N and CF3COOH
were 12.6–13.8 mM.
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together with only [1-2H]ꢀ2PF6 in the absence and the presence
of n-Bu3N, are shown in Fig. 3. In this acetone-d6 and DMSO-
d6 mixed solvent, the changes of proton resonance bands of
[1-2H]ꢀ2PF6 observed upon P5A addition are similar to those
when using acetone-d6 as solvent (Fig. 3a–c), indicating
the formation of [2]pseudorotaxane. Among the protons in
[1-2H]2+, methylenes Hd and He exhibit the most remarkable
complexation-induced broadening effects because their signals
can’t be observed in the 1H NMR spectrum (Fig. 3c). After the
addition of B2.2 eq. of n-Bu3N, the resonances associated with
P5A–[1-2H]ꢀ2PF6 completely disappears, and only the
resonances of P5A and the deprotonated [1-2H]2+ are
observed (Fig. 3d and e). This is an unambiguous
confirmation of the dethreading process. The rethreading
process can be reversed quantitatively by the addition of
B2.2 eq. of CF3COOH (Fig. 3f), which is attributed to the
protonation of 1 restoring the original equilibrium between
[1-2H]2+ and P5A. DOSY results further confirm the pH-
controllable dethreading/rethreading process definitely (Fig.
S19, ESIw). The diffusion coefficient of [1-2H]2+ (Dguest
)
decreases from 6.61 ꢂ 10ꢁ10 to 5.81 ꢂ 10ꢁ10 m2 sꢁ1 upon
addition of P5A, indicating the host–guest complexation.
The Dguest value increases to 9.77 ꢂ 10ꢁ10 m2 sꢁ1 (resembling
the value of free guest) upon addition of n-Bu3N, showing
that guest 1 has been released from the cavity of P5A.
Upon addition of CF3COOH again, the Dguest value (5.84 ꢂ
10ꢁ10 m2
s
ꢁ1) restores the original value, indicating the
rethreading process. To the best of our knowledge, the
present pH-controlled assembly–disassembly system is
the first pillararene-based molecular switch (Fig. 2).
In summary, we have presented that a simple bis(imidazo-
lium) dication, 1,4-bis[N-(N0-hydroimidazolium)]butane, is
able to thread through the cavity of pillar[5]arene to construct
stable [2]pseudorotaxane, and the dethreading/rethreading
process can be reversibly controlled by acid–base stimulus.
We have explored the effect of both the solvent and counterion
13 L. Li and G. J. Clarkson, Org. Lett., 2007, 9, 497–500.
14 K. Zhu, S. Li, F. Wang and F. Huang, J. Org. Chem., 2009, 74,
1322–1328.
15 The Ka values in acetone-d6 or 1 : 1 acetone-d6 : CDCl3 should be
taken as approximate because they do not take into account the
extent of ion pair dissociation on the observed binding interaction
with P5A. For a detailed discussion, see ref. 12c.
c
9018 Chem. Commun., 2010, 46, 9016–9018
This journal is The Royal Society of Chemistry 2010