Pillar[5]arene Based Pseudo[1]rotaxane Operating as Acid/Base-Controllable Two State Molecular Shuttle

Article abstract of DOI:10.1002/ejoc.201900075

A ethylene glycol bridged pyridine and pillar[5]arene based mechanically selflocked pseudo[1]rotaxane was constructed successfully. The structure and selflocked conformation of pseudo[1]rotaxane were confirmed by ¹H, 2D NMR spectrum and HR-ESI-MS. It was found that in dilute solution the pseudo[1]rotaxane could be operated as an acid/base-controllable two state molecular shuttle while In concentrated solution, the pseudo[1]rotaxane existed as a dimer and could be operated from shrinking state to extension state.

Full text of DOI:10.1002/ejoc.201900075

DOI: 10.1002/ejoc.201900075
Communication
Rotaxane | Very Important Paper |
Pillar[5]arene Based Pseudo[1]rotaxane Operating as Acid/Base-
Controllable Two State Molecular Shuttle
[
a]
[a]
[a]
[a]
[a]
[a]
Qian Zhao, Yuan Chen, Baobao Sun, Cheng Qian, Ming Cheng,* Juli Jiang,*
[
a]
[a,b]
Chen Lin, and Leyong Wang
Abstract: A ethylene glycol bridged pyridine and pillar[5]arene pseudo[1]rotaxane could be operated as an acid/base-control-
based mechanically selflocked pseudo[1]rotaxane was con- lable two state molecular shuttle while In concentrated solu-
structed successfully. The structure and selflocked conformation tion, the pseudo[1]rotaxane existed as a dimer and could be
1
of pseudo[1]rotaxane were confirmed by H, 2D NMR spectrum operated from shrinking state to extension state.
and HR-ESI-MS. It was found that in dilute solution the
Introduction
in CHCl3.[10] Recently, our group prepared series of pillar[5]-
arene-based pseudo[1]rotaxanes and the relationships between
the self-locked behaviors and lengths of bridging chains were
The 2016 Nobel Prize in Chemistry was awarded to Jean-Pierre
Sauvage, J. Fraser Stoddart and Bernard L. Feringa for their out-
standing contributions in the design and synthesis of molecular
[
11]
investigated.
However, although numerous pillar[5]arene
based pseudo[1]rotaxane have been constructed so far, it is still
a big challenge to fabricate the controllable pillar[5]arene based
pseudo[1]rotaxane due to the lack of enough way to regulate
[
1]
machines. Particularly, Jean-Pierre Sauvage took the first step
[
2]
towards the synthesis of molecular machine in 1983, from
then on, various artificial molecular machines based on me-
chanically interlocked molecules (MIMs) were fabricated in the
[
3]
past decades.
Among them, rotaxanes based molecular
shuttles represent the fundamental archetypes of molecular
machines and have attracted increasing public attention due to
their diverse functions and great applications in smart materi-
als, information storage, sensors and biological science and so
[
4]
on. Pseudorotaxanes, in which the axle components could
threaded into/out rings reversibly, play an important role in
MIMs based molecular machines. Thus, different kinds of macro-
cyclic hosts have been employed to construct pseudorotax-
ane.[
5]
Pillararenes, as a kind of macrocyclic hosts, were firstly re-
[
6]
ported in 2008. After that, pillararenes have been widely used
[
7]
to fabricate pseudo[1]rotaxanes with different guests, not
only due to the highly symmetrical and rigid structure, but also
[
8]
for the electron-donating cavities. For example, Xue et al. fab-
ricated the first pillar[5]arene-based pseudo[1]rotaxane by com-
[
9]
bining C–H···π and ion-pair interactions together in 2014.
Yang et al. reported a pillar[5]arene-based pseudo[1]rotaxane
which could be used as a catalyst for the Knoevenagel reaction
[
[
a] Jiangsu Key Laboratory of Advanced Organic Materials,
School of Chemistry and Chemical Engineering, Nanjing University,
1
63 Xianlin Avenue, Nanjing 210023, China
E-mail: ming.cheng2015@foxmail.com
jjl@nju.edu.cn
hysz.nju.edu.cn/supramol
b] School of Petrochemical Engineering, Changzhou University,
Changzhou, 213164, China
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201900075.
Scheme 1. Synthesis and shuttling process of pseudo[1]rotaxane 1.
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host–guest interaction between pillar[5]arene and different ton signals of the protonated pyridine shifted upfield obviously
guests. Therefore, the introduction of stimuli responsive units due to aromatic shielding effect of the MeP5A cavity, suggest-
into pillar[5]arene based pseudo[1]rotaxanes would greatly ing the formation of host–guest complex MeP5A⊃protonated
expand the application and development prospects of pillar[5]- pyridine. Moreover, after addition of N,N-diisopropylethylamine
arene based pseudo[1]rotaxanes.
(iPr NEt, 3.0 equiv.) to the above solution, in order to remove
2
1
Considering that, acid/base-controllable two state pseudo- proton from the protonated pyridine, a H NMR spectrum (Fig-
[
1]rotaxane 1 were designed (Scheme 1), which incorporated ure 2c) similar to 1:1 mol ratio mixture of pyridine and MeP5A
ethylene glycol, pyridine in the thread and 1,4-dimethoxy- was obtained, indicating that the guest pyridine had move out
pillar[5]arene (MeP5A) as the macrocycle. By addition of acid/ of MeP5A's cavity and the host–guest complex between MeP5A
base, pseudo[1]rotaxane 1 exists in two possible states, the eth- and pyridine had dethreaded. All the above results demon-
ylene glycol bridged pyridine thread could shrink or expand in strated that the threading/dethreading processes between pyr-
the pillar[5]arene cavity as an acid/base-controllable two state idine and MeP5A could be well-regulated by the addition of
molecular shuttle (Scheme 1).
acid/base.
Results and Discussion
The first step to design pillar[5]arene based acid/base-controlla-
ble molecular shuttle is the appropriate choice of acid/base-
[
12]
controllable guest unit. According to previous report,
elec-
tron poor pyridinium salts could form very stable host–guest
complexes with pillar[5]arenes. Thus, we chose pyridine as the
acid/base-controllable guest unit and the host–guest properties
1
between pyridine and pillar[5]arene were investigated by
H
NMR. As shown in Figure 1, upon addition of MeP5A (1.0 equiv.)
to a solution of pyridine, no obvious proton signal changes
could be observed. In contrast, upon addition of MeP5A
(
1.0 equiv.) to a solution of protonated pyridine, proton signals
of the protonated pyridine are shifted upfield significantly
ΔδH1 = –0.21 ppm, ΔδH2 = –0.40 ppm, ΔδH3 = –0.44 ppm)
(
due to strong shielding effects of MeP5A cavity, indicating the
formation of a stable host–guest complex between protonated
pyridine and MeP5A in acidic condition.
1
Figure 2. Partial HNMR spectra (400 MHz, CDCl
10.00 m TFA; (b) 5.00 m pyridine + 5.00 m
pyridine + 5.00 m MeP5A + 10.00 m
pyridine + 5.00 m MeP5A.
3
, 298 K): (a) 5.00 m
MeP5A + 10.00 m
TFA + 15.00 m iPr
2
M
pyridine
TFA;
NEt;
+
M
M
M
M
(
c) 5.00 m
M
M
M
M
(
d) 5.00 m
M
M
On the basis of above host–guest investigation, we designed
pillar[5]arene based pseudo[1]rotaxane 1. The association con-
stant of complexation between protonated pyridine and MeP5A
–
1
in CDCl was determined to be 57.0 ± 1.3
M
at 25 °C using a
3
1
nonlinear curve-fitting analysis based on the H NMR titration
experiments (see SI, section 4 in detail). Considering the associ-
ation constant was not high as expected, so we chose ethylene
glycol chain rather than alkyl chain as a linker connected pyr-
[
13]
idine and pillar[5]arene.
As shown in Scheme 1, by reacting
monohydroxy pillar[5]arene 2 with ethylene glycol bridged pyr-
idine 3, pseudo[1]rotaxane 1 was synthesized successfully in
2
0 % yields in anhydrous DMF (Scheme 1). The detailed route
Figure 1. Partial 1HNMR spectra (400 MHz, CDCl
ine; (b) 5.00 m pyridine + 5.00 m MeP5A; (c) 5.00 m
0.00 m TFA + 5.00 m MeP5A; (d) 5.00 m pyridine + 10.00 mM
, 298 K): (a) 5.00 m
pyridine +
TFA.
M pyrid-
3
was displayed in supporting information section 2. Pseudo[1]-
rotaxane 1 was further characterized by H, C, 2D NMR spec-
trum, high-resolution electrospray ionization mass spectrum
M
M
M
1
13
1
M
M
M
Then, we investigated the acid/base-controllable threading/ (HR-ESI-MS). In the HR-ESI-MS of the 1 (see SI, Fig S6 ), peaks
dethreading processes of complex between pyridine and were found at m/z 946.4378 and 968.4192 corresponding to
MeP5A. As shown in Figure 2, Upon addition of trifluoroacetic [1 + H]+ (calcd. 946.4372) and [1 + Na]+ (calcd. 968.4192) re-
acid (TFA, 2.0 equiv.) to a 1:1 mol ratio mixture of MeP5A and spectively, which preliminarily supported the formation of 1. In
1
1
pyridine, the H NMR spectrum was recorded (Figure 2b). Pro- the H NMR spectrum (see SI, Fig S4), a broad peak in upfield
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1
3 2
Figure 3. Partial HNMR spectra (400 MHz, CDCl , 298 K): (a) 10.00 mM 1; (b) 10.00 mM 1 + 20.00 mM TFA; (c) 10.00 mM 1 + 20.00 mM TFA + 30.00 mM iPr NEt.
chemical shift (δ = 0.19) were observed which was caused by ure 4, the diffusion coefficients of pseudo[1]rotaxane 1 at both
the strong shielding effects of pillar[5]arene cavity, indicating low concentration and high concentration were recorded. For
ethylene glycol chain stay in the cavity of pillar[5]arene and the pseudo[1]rotaxane 1 at both low concentration (10 m
M) and
formation of selflocked structure. Besides, NOE correlations high concentration (100 m ), only one set of signals was ob-
M
were observed clearly between ethylene glycol chain protons served, indicating only one type of assembly exist in both con-
H , H and pillararene protons H , H , H from 2D NOESY analysis dition. Then, the radius of pseudo[1]rotaxane 1 in both concen-
e
d
i
j
k
[
14]
(
see SI, Fig S11), which further confirmed the formation of self- tration were calculated by Stokes–Einstein relationship.
As
locked pseudo[1]rotaxane 1.
shown in Table 1, the calculated radius in high concentration
The shuttling properties of pseudo[1]rotaxane 1 among eth- (100 m
M
, 4.70 Å) is about twice of low concentration (10 m
M
,
1
ylene glycol chain and pyridine unit were investigated by H 2.86 Å), indicating pseudo[1]rotaxane 1 performed as selflocked
NMR. Firstly, we tested the response of pseudo[1]rotaxane 1 to monomer 1 in low concentration and interlocked-dimer 4 in
acid. Upon addition of TFA, 2.0 equiv. to a solution of high concentration (Scheme 2).
pseudo[1]rotaxane 1. The signals belonging to ethylene glycol
chain (Figure 3a and 3b) showed significant downfield shifts
(
ΔδHd,He = 0.88 ppm and ΔδHc = 0.36 ppm), due to the aromatic
shielding effect of the macrocycle weaken, indicating that part
of the ethylene glycol chain moved out from the cavity of pil-
lar[5]arene. In contrast, the signals belonging to pyridine shifted
upfield obviously (ΔδHh = –1.12 ppm, ΔδHg = –0.21 ppm) due
to aromatic shielding effect of the pillar[5]arene cavity, suggest-
ing protonated pyridine moved into the cavity of pillar[5]arene.
Furthermore, when addition of 3.0 equiv. iPr NEt to the
2
above solution, which was used as base to remove proton from
the protonated pyridine. A spectrum (Figure 3c) similar to the
original pseudo[1]rotaxane 1 was obtained, suggesting that de-
protonated pyridine unit left the cavity of pillar[5]arene and
ethylene glycol chain back to the cavity of pillar[5]arene. Conse-
quently, all above results confirmed that the switchable process
between ethylene glycol chain and pyridine unit have been re-
alized by acid/base control. Therefore, our pseudo[1]rotaxane 1
could be operated as acid/base-controllable two state molec-
ular shuttle.
Figure 4. Partial DOSY spectra (600 MHz, CDCl
3
, 298 K): (a) 10.00 mM 1;
(b) 100.00 m 1; (c) 10.00 m 1 + 200.00 m TFA. Notice that only one set
M
M
M
of signals could be observed from the DOSY spectrum and full spectrum was
given in SI.
1
In addition, diffusion-ordered H NMR spectroscopy (DOSY)
experiments were performed to investigate self-assembly be-
havior and aggregation sizes of pseudo[1]rotaxane 1 in both
Moreover, after further addition of TFA (2.0 equiv.) to the
low concentration (10 m
M
), high concentration (100 mM) and above high concentration solution, the calculated radii extend
acidic condition (100 m
M
+ 200.0 m TFA). As shown in Fig- from 4.70 Å to 5.30 Å (Table 1), indicating that after the proto-
M
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Table 1. The hydrodynamic radius calculated from the DOSY experiments.
added to the above mixture and continued stirred at room temper-
ature for 24 h. The reaction was quenched by water. The DMF was
removed in vacuo and the residue was dissolved in CH Cl . The
Sample
lg D (lg m2 s–1
)
D (10–10 m2 s–1
)
r [Å]
2
2
1
4
[
–8.858
–9.072
–9.125
1.390
0.847
0.750
2.86
4.70
5.30
organic phase was washed with water, dried with anhydrous
Na SO and concentrated under vacuum. The residue was purified
2
4
4 + 2H]2+
by silica-gel column chromatography (CH Cl /MeOH = 50:1) to af-
2
2
ford pseudo[1]rotaxane 1 (0.26 g, 20 %) as a white solid. Mp. 142–
1
1
43 °C. H NMR (400 MHz, CDCl ) δ = 7.12 (s, 1 H), 7.04 (s, 1 H),
3
6.95–6.92 (m, 3 H), 6.83 (s, 1 H), 6.79 (s, 1 H), 6.65 (s, 1 H), 6.61 (s, 1
H), 6.42 (s, 1 H), 6.17 (s, 2 H), 5.18 (s, 2 H), 4.18 (s, 2 H), 3.94–3.59
m, 38 H), 3.43 (s, 2 H), 2.84 (s, 2 H), 0.19 (s, 4 H). 13C NMR (100 MHz,
CDCl , 298 K) δ = 178.1, 151.2, 150.9, 150.8, 150.6, 150.4, 150.1,
(
3
1
1
1
5
2
9
50.1, 149.9, 149.4, 149.3, 141.2, 129.1, 129.0, 128.9, 128.7, 128.5,
27.9, 127.8, 127.7, 127.2, 116.5, 115.4, 115.2, 114.5, 114.5, 114.2,
13.6, 113.5, 113.1, 112.8, 112.2, 70.9, 69.9, 69.4, 66.6, 65.6, 56.5,
6.4, 56.4, 56.1, 55.8, 55.7, 55.6, 55.4, 54.6, 51.7, 30.5, 30.5, 29.7, 28.8,
+
8.0, 27.8. HR-MS (ESI): calcd. for [1 + H] : 946.4372, found m/z =
+
46.4378; calcd. for [1 + Na] : 968.4192, found m/z = 968.4192.
Acknowledgments
We gratefully thank the financial support of the National Natu-
ral Science Foundation of China (Nos. 21871135, 21871136).
Keywords: Self-assembly · Pseudo[1]rotaxanes · Acid/base-
control · Molecular shuttle
[
[
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Received: January 15, 2019
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Rotaxane
Q. Zhao, Y. Chen, B. Sun,
C. Qian, M. Cheng,* J. Jiang,*
C. Lin, L. Wang .................................... 1–6
Pillar[5]arene Based Pseudo[1]-
rotaxane Operating as Acid/Base-
Controllable Two State Molecular
Shuttle
A ethylene glycol bridged pyridine and base-controllable two state molecular
pillar[5]arene based mechanically self- shuttle. In concentrated solution, the
locked pseudo[1]rotaxane was suc- pseudo[1]rotaxane existed as a dimer
cessfully constructed. It was found that and could be operated from shrinking
in dilute solution the pseudo[1]rotax- state to extension state.
ane could be operated as an acid/
DOI: 10.1002/ejoc.201900075
Eur. J. Org. Chem. 0000, 0–0
www.eurjoc.org
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