New switchable [2]pseudorotaxanes formed by pyridine N-oxide derivatives with diamide-based macrocycles

Article abstract of DOI:10.1039/c003118f

Pyridine N-oxide derivatives are capable of formation of stable [2]pseudorotaxanes with diamide-based macrocycles in solution and in the solid state, and their dethreading/rethreading movements can be easily controlled by acid-base stimuli.

Full text of DOI:10.1039/c003118f

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New switchable [2]pseudorotaxanes formed by pyridine N-oxide
derivatives with diamide-based macrocyclesw
Mujuan Chen, Shujuan Han, Lasheng Jiang,* Songgen Zhou, Fei Jiang,
Zhikai Xu, Jidong Liang and Suhui Zhang
Received 15th February 2010, Accepted 7th April 2010
First published as an Advance Article on the web 27th April 2010
DOI: 10.1039/c003118f
Pyridine N-oxide derivatives are capable of formation of stable
[2]pseudorotaxanes with diamide-based macrocycles in solution
and in the solid state, and their dethreading/rethreading
movements can be easily controlled by acid–base stimuli.
N-oxide derivatives 3 and 4 were synthesized from the
corresponding pyridine dicarboxy acids by a simple two-step
procedure (see ESIw). To simplify the spectroscopic analyses,
we also synthesized a pyridine-containing new macrocycle 2
(see ESIw).
Although the pyridine N-oxide derivatives 3 and 4 are
soluble in MeCN, Me2CO, Me2SO and MeOH, their solubility
is very limited in CHCl3 and CH2Cl2. However, the solubility
in these chlorinated solvents can be significantly improved by
addition of one or more molar equivalents of the macrocycle 1
or 2. This observation suggested that strong intermolecular
complexation of some type could occur with these systems. We
observed significant shifts in the 1H NMR spectra of an
equimolar pyridine N-oxide (3 or 4) and macrocycle (1 or 2)
in CDCl3. The most important features of 1H NMR spectrum
of an equimolar mixture of 1 with 3 in CDCl3 are presented in
Fig. 1.
Pseudorotaxanes are important building blocks for syntheses
of mechanically interlocked structures such as rotaxanes and
catenanes which are of potential applications in construction
of molecular level machines and switches.1 In the past two
decades, several elegant recognition systems with pseudorotaxane
structures have been developed. For example, crown ethers
form pseudorotaxanes with paraquat derivatives,2 secondary
ammonium compounds3 and 1,2-bis(pyridinium)ethane;4
cyclobis(paraquat-p-phenylene) forms charge-transfer com-
plexes with p-electron-rich molecules such as hydroquinone,5
tetrathiafulvalene (TTF),6 and 1,5-dioxynaphthalene derivatives.7
Recently, Beer and co-workers exploited the anion-templated
system of isophthalamide-based compounds in the formation
of interpenetrated and interlocked structures.8
The signals of protons Hf, Hg of the macrocycle 1 and that
0
of Hb Hc’ of 3 are shifted high field, and the peaks for Ha, Hb
and Hg of the ethylene glycol unit in 1 moved upfield (Fig. 1b),
suggesting that hydrogen bonding (N–Hꢀ ꢀ ꢀO) probably takes
place between the amide N–H of 1 and the oxygen atom of the
pyridine N-oxide 3 and between the amide N–H of 3 and the
oxygen atoms of the ethylene glycol unit in 1. All of these
changes were also observed for the equimolar mixture of 1 and
4, 2 and 3, 2 and 4 (see ESIw). This result implies that the
pseudorotaxane-like complexes would be formed between the
pyridine N-oxide and amide macrocycle.
On the other hand, pyridine N-oxide derivatives are
interesting compounds and have been widely used as com-
plexation ligands,9 asymmetric catalysts.10 Ivan Huc et al. have
reported their pioneering work on synthetic double helices
consisting of pyridine N-oxide units.11 Recently, use of pyridine
N-oxides for molecular recognition studies was also reported.12
However, we are unaware of any of pseudorotaxanes and
tunable molecular switches that have been exploited by utilizing
pyridine N-oxide as a component. In this communication, it is
demonstrated that pyridine N-oxide derivatives form stable
[2]pseudorotaxanes with diamide-based macrocycles, plus an
easily controlled dethreading/rethreading process of the resulting
pseudorotaxanes under acid–base stimuli. They are therefore
viable candidates for constructing interlocked supramolecular
structures and related molecular machines.
To quantify the affinity strength of this recognition
system, the association constants are determined to be
K1ꢀ3 = 2.2 ꢁ 0.6 ꢂ 103, K1ꢀ4 = 3.8 ꢁ 1.0 ꢂ 102, K2ꢀ3
=
5.5 ꢁ 0.4 ꢂ 103 and K2ꢀ4 = 1.8 ꢁ 0.2 ꢂ103 Mꢃ1 for 1ꢀ3, 1ꢀ4, 2ꢀ3
and 2ꢀ4, respectively, by NMR titration experiments in a
mixed solvent of CDCl3–CD3OD (3 : 1 v/v) (see ESIw). The
large values of Ka indicate strong interactions between the
pyridine N-oxide and amide macrocycle. Electrospray ioniza-
tion mass spectra (see ESIw) of the equimolar mixture of the
pyridine N-oxide and amide macrocycle confirmed not only
the complex formation, but also the 1 : 1 stoichiometry.
The X-ray structural analysis of 1ꢀ3 (see ESIw) clearly
reveals that the pyridine ring of 3 is located in the cavity of
the macrocycle 1 (Fig. 2), and is almost parallel to the planes
of phenoxy rings of 1 (angles: 6.5 and 6.01; centroid–centroid
distances: 3.71 and 3.73 A, respectively), indicating that p–p
stacking interaction may exist between the pyridine ring of 3
and the phenoxy rings of the host. As expected, the oxygen
atom of the N-oxide binds with the amide N–H and one
The pyridine N-oxide derivatives and macrocycles studied
in this investigation are shown in Scheme 1. The pyridine
School of Chemistry and Environment, South China Normal
University, Shipai, Guangzhou 510631, P. R. China.
E-mail: jianglsh@scnu.edu.cn; Fax: (+86) 20-39310187;
Tel: (+86) 20-39310039
w Electronic supplementary information (ESI) available: Experimental
details; 1H NMR spectra for 1 : 1 complexes and for acid–base
controlled dethreading/rethreading processes; association constants
determination; crystal data for complexes 1ꢀ3 and 2ꢀ3, and ESI-MS
spectra of the complexes. CCDC 762537 (2), 762538 (1ꢀ3) and 762539
(2ꢀ3). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c003118f
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Scheme 1 Formation of pyridine N-oxide-based pseudorotaxanes and the acid–base controlled dethreading/rethreading movements.
Fig. 1 Partial 1H NMR spectra of (a) macrocycle 1, (b) an equimolar mixture of 1 and 3, and (c) pyridine N-oxide 3 (4 mM, CDCl3, 400 MHz,
295 K).
aromatic C–H of the macrocycle 1 via hydrogen bondings
(distances of N–Hꢀ ꢀ ꢀO are 2.21, 2.27; C–Hꢀ ꢀ ꢀO is 2.20 A).
There is evidence for a second hydrogen bonding interaction
between the amide NꢃH of 3 and the polyether oxygens of the
macrocycle 1 (distances of N–Hꢀ ꢀ ꢀO are 2.31, 2.61 A, respec-
tively). These multiple and cooperative hydrogen bondings, in
addition to p–p stacking interactions, make the complexes
highly stable as evidenced by their K values. The resulting
X-ray crystal structure showed unambiguously that the
complexes are [2]pseudorotaxanes. The crystal structure of
complex 2ꢀ3 (see ESIw) further confirmed the [2]pseudorotaxane
formation of this recognition system.
To study the chemically driven dethreading and rethreading
movements in this system, an excess of trifluoroacetic acid
(TFA, 45 equiv) was added to an equimolar solution of 1 and 3
in CDCl3. Gratifyingly, the 1H NMR spectrum (Fig. 3c) of
the resulting mixture revealed that the protonation13 of the
Fig. 2 Ball-and-stick views of the X-ray crystal structure of complex
1ꢀ3. Macrocycle 1 is blue, pyridine N-oxide 3 is red, hydrogens are
pyridine N-oxide led to decomplexation, because the signals of
orange, oxygens are green, and nitrogen is red. Hydrogens except
the phenoxy ring protons shifted almost back to the original
those involved in hydrogen bonding have been omitted for clarity.
positions of the free macrocycle 1 (Fig. 3a). It is noted that
small differences were observed for pyridine ring protons in
the 1H NMR spectrum due to the protonation of the N-oxide.
complex, a [2]pseudorotaxane, was regenerated. This process
can be also realized for the systems of 1ꢀ4, and 2ꢀ4 by
This indicates that, upon protonation, the pyridine N-oxide no
sequential addition of TFA and TEA (see ESIw).
longer resides inside the cavity of the macrocycle. Deproton-
ation of the protonated pyridine N-oxide by adding an excess of
triethylamine (TEA, 50 equiv.) almost recovered the 1H NMR
spectrum (Fig. 3d), which is similar to that of the original
complex (Fig. 3b), suggesting a return of the pyridine N-oxide
unit back inside the cavity of the macrocycle, that is to say the
However, this TFA/TEA stimulus does not work for the
system of 2ꢀ3 (see ESIw), indicating that the complex 2ꢀ3 is too
stable to achieve the protonation of the pyridine N-oxide by
TFA. This is probably because the protonation of the pyridine
ring of 2 increased the acidity of the amide and enhanced
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Fig. 3 Partial 1H NMR spectra of (a) macrocycle 1, (b) an equimolar mixture of 1 and 3, (c) the mixture obtained after adding an excess of TFA
to the solution in (b), and (d) the mixture obtained after adding an excess of TEA to the solution in (c) (4 mM, CDCl3, 295 K, 400 MHz).
binding ability of the macrocycle. Thus, the pyridine N-oxide-
based pseudorotaxanes can be considered to be suitable
candidates for the development of acid–base-controllable14
molecular machines and switches.
A. Trabolsi, Y.-W. Yang, M. Elhabri, A.-M. Albrecht-Gary and
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In summary, we have demonstrated that pyridine N-oxide
derivatives, as new motifs, can form stable [2]pseudorotaxanes,
which are stabilized by multiple and cooperative hydrogen
bonding and p–p stacking interactions, with diamide-based
macrocycles in solution and in the solid state; and the
dethreading/rethreading process of the resulting [2]pseudo-
rotaxanes is reversible under acid–base control. These findings
may offer an alternative approach for constructing a variety of
new supramolecular structures with interlocked [n]rotaxanes
and [n]catenanes. In particular, it might be useful for develop-
ment of new nanoscale molecular machines and devices.
This study was supported by the National Natural
Science Foundation of China (No. 20672038) and Natural
Science Foundation of Guangdong Province of China
(No. 8151063101000015). We wish to thank Dr David E.
Finlow (Shawnee State University) for his help with correcting
and proofreading this manuscript.
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This journal is The Royal Society of Chemistry 2010
3934 | Chem. Commun., 2010, 46, 3932–3934