A novel pentiptycene bis(crown ether)-based [2](2)rotaxane whose two DB24C8 rings act as flapping wings of a butterfly

Article abstract of DOI:10.1021/ol500149k

A novel [2](2)rotaxane based on pentiptycene-derived bis(crown ether) can be efficiently synthesized via a click chemistry method and the subsequent N-methylation. Due to the different affinities of DB24C8 with the ammonium and triazolium stations, the wing-flapping movement of the DB24C8 wings in the [2](2)rotaxane can be easily achieved by acid/base stimulus.

Full text of DOI:10.1021/ol500149k

Letter
pubs.acs.org/OrgLett
A Novel Pentiptycene Bis(crown ether)-Based [2](2)Rotaxane Whose
Two DB24C8 Rings Act as Flapping Wings of a Butterfly
Ying-Xian Ma, Zheng Meng, and Chuan-Feng Chen*
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
S
* Supporting Information
ABSTRACT: A novel [2](2)rotaxane based on pentiptycene-derived
bis(crown ether) can be efficiently synthesized via a “click chemistry”
method and the subsequent N-methylation. Due to the different affinities
of DB24C8 with the ammonium and triazolium stations, the wing-flapping
movement of the DB24C8 “wings” in the [2](2)rotaxane can be easily
achieved by acid/base stimulus.
ith the aim of mimicking nature, artificial molecular
the [2](2)rotaxane based on host H. Especially, it is found that
the shuttle process of two DB24C8 moieties of the host
between the ammonium and N-methyltriazolium stations can
be easily achieved by acid−base stimulus, which makes the
opening−closing movement of the two DB24C8 moieties act as
the flapping motions of a butterfly (Figure 1).20
W
machines (AMMs)1 based on mechanically interlocked
molecules (MIMs)2 with special properties and functions have
been representing a hot issue and major challenge in
supramolecular chemistry. During the past decade, various
highly complicated and well-defined MIMs, such as molecular
shuttles,3 molecular elevators,4 molecular cable cars,5 molecular
information ratchets,6 and so on,7 have been designed and
constructed. Among numerous building blocks for MIMs, the
ammonium ion−crown ether recognition pair has been widely
used in the construction of acid−base responsive MIMs.7f,8 In
2008, Coutrot and Busseron9 reported the first [2]rotaxane
shuttle based on an anilinium and triazolium station through a
two-step sequence templated approach, containing “click
chemistry”10 and subsequent N-methylation. Recently, Coutrot
and his co-workers11 also reported a pH-sensitive double-lasso
molecular machine from an ends-activated [c2]daisy chain. So
far, some [2]rotaxanes based on ammonium and triazolium
stations have been obtained for a broadening range of
applications, due to their clean and clear shuttle process
between the two stations in response to the acid and base
stimuli.9,12
Figure 1. Cartoon representation of the flapping motions of the [2]
(2)rotaxane, and proton designations of the host and the guest.
First, we synthesized diammonium salt 1 containing two
terminal azido groups. Starting from [1,1′-biphenyl]-4,4′-
diyldimethanamine, the corresponding reversible dynamic
imine could be obtained via the condensation with the 4-(2-
azidoethoxy)benzaldehyde. Then reduction of the imine with
NaBH4 in CH3OH could afford the kinetically stable amine.
The target diammonium salt 1 could be obtained in an ideal
total yield (73%) through further protonation with 10 equiv of
HCl and the counterion exchange with saturated NH4PF6
solution, subsequently.
Pentiptycene,13 as a common member of the iptycene family,
has specific applications in low dielectric constant materials,14
fluorescent chemosensors,15 molecular machines,16 and so on,
due to its unique rigid, aromatic, and H-shaped scaffold.
Recently, we17 reported a series of pentiptycene-derived hosts
by the combination of a rigid pentiptycene moiety and flexible
crown ether chains. Since pentiptycene-derived bis(crown
ether) H contains two DB24C8 moieties in the cis position,
we deduce that it can show strong interactions with two
ammonium cations18 and two triazolium salts simultaneously,
which thus provides an opportunity to construct a novel
interlocked molecule of [2](2)rotaxane19 by the ammonium
template moiety through alkyne−azide “click chemistry” and
subsequent N-methylation. Herein, we report the synthesis of
The complexation between host H and diammonium salt 1
1
in solution was then studied by the H NMR spectroscopic
method. We mixed the host H (3.0 mM) with 1.0 equiv of 1 in
Received: January 16, 2014
Published: March 17, 2014
© 2014 American Chemical Society
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Scheme 1. Synthesis of the [2](2)Rotaxane Rota-M·4PF6
1:1 (v/v) CD3CN/CDCl3, and the 1H NMR spectrum21 of the
1:1 mixture exhibited a great difference from those of free host
H and diammonium salt 1 with only one set of peak signals,
which suggested that the complexation of host H with
diammonium salt 1 was a corresponding fast complexation
process in solution. The signals for the benzylic methylene
protons Hc and Hd adjacent to the NH2 centers showed the
large downfield shift of 0.71 and 0.20 ppm, respectively.
Meanwhile, the signals of the protons in crown rings also
exhibited significant changes, due to the effect of the
diammonium axle. These observations suggested that diammo-
nium salt 1 as an axle ran through two crown ether rings of H.
In addition, all the protons Ha, Hb, He, and Hf in diammonium
salt 1 obviously shifted upfield to different degrees, which may
be due to the shielding effect. It revealed that the diphenyl
section of 1 was included in the shielding area between the
DB24C8 moieties of H. All of the signal changes mentioned
above confirmed that the novel pentiptycene-based [2]pseudo-
rotaxane was formed, as we supposed.
aromatic protons (Ha, Hb, He, and Hf) of the axle section
shifted toward upfield obviously; especially, for the protons Ha
and Hb of the diphenyl moiety, the upfield shifts could reach up
to 0.51 and 0.56 ppm, respectively (Figure 2). These
1
Figure 2. Partial H NMR spectra (300 MHz, CDCl3/CD3CN = 1:1,
v/v, 298 K) of (a) free G, (b) Rota-M·4PF6, and (c) free H.
Resonance protons are labeled in Figure 1
With this [2]pseudorotaxane containing two terminal azido
groups in hand, we further performed a copper(I)-catalyzed
alkyne−azide 1,3-dipolar cycloaddition22 with 1,3-dimethyl-5-
(prop-2-yn-1-yloxy) benzene 2 in DCM. As shown in Scheme
1, the mixture of host H and 1.0 equiv of diammonium salt 1
was stirred for 2 h at room temperature to form a
[2]pseudorotaxane, which was then reacted with alkyne 2 via
the “CuAAC click chemistry” in the presence of a
stoichiometric amount of [Cu(CH3CN)4]PF6 to give the target
[2]rotaxane Rota·2PF6 in 45% yield. Rota·2PF6 was sub-
sequently treated with an excess of iodomethane in dry CH3CN
at 50 °C for 48 h, followed by anion exchange with NH4PF6 to
afford our designed [2](2)rotaxane Rota-M·4PF6 containing
two stations. We assigned most of the resonance protons of [2]
observations suggested that the axle threads through the two
DB24C8 moieties, and the diphenyl moiety was located in the
stronger shielding zone between the two DB24C8 rings.
Further evidence for the formation of the [2](2)rotaxane came
from the ESI-HRMS, in which the three high-intensity signals
at m/z 1262.0077, 793.0169, and 558.5218 corresponding to
the ion masses of [M − 2PF6]2+, [M − 3PF6]3+, and [M −
4PF6]4+, respectively, were observed.21
We also carried out a 2D ROESY spectral experiment for the
[2](2)rotaxane Rota-M·4PF6. The cross-peaks between Hc
(Hd) and the protons in crown ether units of H could be
found. This observation revealed that the DB24C8 moieties in
Rota-M·4PF6 still encircled the ammonium station.21 Actually,
there were ammonium and triazolium complexation stations in
the axle for DB24C8 moieties, so we further tested the
response of Rota-M·4PF6 to acid and base (Scheme 2). When
we added 2.2 equiv of 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) to a solution of Rota-M·4PF6 in CD3CN/CHCl3
1
(2)rotaxane Rota-M·4PF6 with the help of H−1H COSY and
ROESY 2D NMR spectroscopic experiments.21 It was found
that the characteristic signal for proton Hi corresponding to the
triazolium moiety emerged at 8.50 ppm. Compared to free axle
G, we noted that the benzylic methylene protons Hc and Hd
adjacent to the NH2 centers showed great downfield shifts (Δδ
= −0.69 for Hc and Δδ = −0.19 ppm for Hd), which revealed
that two NH2 stations were located in the cavities of two crown
ether rings, respectively. At the same time, the proton signals of
DB24C8 moieties also exhibited the corresponding changes,
due to the effect of the NH2 station. Additionally, all of the
1
(1:1, v/v), the obvious changes could be observed in its H
NMR spectrum. As shown in Figure 3, proton Hi of the
triazolium moiety moved downfield with a shift of 0.31 ppm.
Moreover, the signals of the methylene protons adjacent to the
ammonium center (Hc and Hd) also showed dramatic changes
that included their multiple splitting pattern disappearing, along
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relatively sensitive to pH stimulus. Moreover, the relational
signals in H NMR spectrum showed the cyclic shift by the
Scheme 2. Representing the Wing-Flapping Motions
1
successive addition of DBU-TFA-DBU-TFA (Figure S17)
without any obvious difference. Thus, it demonstrated that
the wing-flapping movement of the DB24C8 “wings” in Rota-
M·4PF6 could cycle several times under acid/base control.
In conclusion, we have constructed a novel [2](2)rotaxane
based on a pentiptycene-derived bis(crown ether) host, which
is an extension of the Coutrot’s system of molecular machinery
via a “click chemistry” method and the subsequent N-
methylation. Since the [2](2)rotaxane contained ammonium
and triazolium stations with a different affinity for DB24C8,
subsquently, the wing-flapping movement of the DB24C8
“wings” in the [2](2)rotaxane could be achieved efficiently and
chemically controlled by the addition of acid and base. The
results presented here represent the first example of a
mechanically interlocked molecular switch based on the
pentiptycene-derived synthetic host. We believe that this kind
of mechanically interlocked molecules could be useful for
further construction of more complicated and well-defined
mechanically interlocked supramolecular systems based on the
pentitycene scaffold and serve as inspiration for the develop-
ment of smart bionic machines.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, compounds characterization, 1H−1H
COSY and ROESY 2D NMR spectra, and other materials.
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
Figure 3. Partial H NMR spectra (600 MHz, CD3CN/CDCl3 = 1:1,
v/v, 298 K) of (a) free Rota-M·4PF6, (b) to the solution of a were
added 2.2 equiv of DBU, and (c) to the solution of b were added 4.0
equiv of TFA. [Rota-M·4PF6]0 = 3.0 mM. DCM peaks are marked
with •.
AUTHOR INFORMATION
■
Corresponding Author
with the great upfield shifts (Δδ = 1.07 ppm for Hc and Δδ =
0.70 ppm for Hd). While the methylene protons (Hp and Hq)
attached to the triazolium station moved upfield with great
shifts. The Ha and Hb of the diphenyl moiety moved back
downfield, which emerged at 7.53 and 7.42 ppm. These signal
changes revealed that the successful deprotonation of the
ammonium ion, and subsequently the movement of DB24C8
moieties to triazolium stations, occurred. The two triazolium
stations of the encircled axle were too distant; the pure bending
of the axle without any torsion of the host seems to be too
difficult to achieve these signal changes. Thus, we suggested
these changes might come from the opening of the DB24C8
moieties, just like a butterfly spreading its wings from the amine
center to the triazolium station. The corresponding cross-peaks
between the protons close to the triazolium station and the
ones of crown ether rings could be observed in the 2D ROESY
spectrum, which further revealed this “spread-wing” process.21
The introduction of 4.0 equiv of trifluoroacetic acid (TFA)
to the above mixture solution resulted in a recovery of the
proton signals compared to that of the [2](2)rotaxane Rota-M·
4PF6 in the absence of any additive. Host H closed its DB24C8
“wings” back to the ammonium station with the reformation of
the NH2 centers. These 1H NMR spectroscopic measurements
demonstrated that the variation of pH could induce the motion
of the two DB24C8 “wings” of [2](2)rotaxane Rota-M·4PF6,
which was reminiscent of the butterfly wing-flapping move-
ment. In addition, we noted that only an approximate
equivalent amount of DBU to ammonium ions could make
the DB24C8 moieties spread to the triazolium station. This
observation revealed that this [2](2)rotaxane Rota-M·4PF6 was
*E-mail: cchen@iccas.ac.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(91127009, 21332008) and the National Basic Research
Program (2011CB932501) of China for financial support.
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