Novel [2]pseudorotaxanes constructed by self-assembly of bis-urea-functionalized pillar[5]arene and linear alkyl dicarboxylates

Article abstract of DOI:10.1039/c2cc34049f

A bis-urea-functionalized pillar[5]arene has been synthesized and shown to form [2]pseudorotaxanes spontaneously with linear alkyl dicarboxylates in highly polar solvent DMSO, in which the hydrogen bonding interactions between the bis-urea hydrogens and dicarboxylate oxygens play an important role in stabilizing the novel [2]pseudorotaxanes alongside C-H...π interactions.

Full text of DOI:10.1039/c2cc34049f

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COMMUNICATION
Novel [2]pseudorotaxanes constructed by self-assembly of
bis-urea-functionalized pillar[5]arene and linear alkyl dicarboxylatesw
Qunpeng Duan, Wei Xia, Xiaoyu Hu, Mengfei Ni, Juli Jiang, Chen Lin, Yi Pan and
Leyong Wang*
Received 6th June 2012, Accepted 2nd July 2012
DOI: 10.1039/c2cc34049f
A bis-urea-functionalized pillar[5]arene has been synthesized and
shown to form [2]pseudorotaxanes spontaneously with linear alkyl
dicarboxylates in highly polar solvent DMSO, in which the
hydrogen bonding interactions between the bis-urea hydrogens
and dicarboxylate oxygens play an important role in stabilizing
the novel [2]pseudorotaxanes alongside C–Hꢀ ꢀ ꢀp interactions.
(see Fig. 1), and then novel [2]pseudorotaxanes could be
constructed by self-assembly of the above macrocyclic host
(wheel) with linear alkyl dicarboxylates (axles) through
N–Hꢀ ꢀ ꢀO hydrogen bonding interactions.
Herein we present the host–guest complexation of a bis-urea-
functionalized pillar[5]arene (H) with linear alkyl dicarboxylates
(Gn) (Gn, n: carbon number, n = 5, 7, 9, 11–15, 20) (Fig. 1). By
self-assembly of H and Gn, [2]pseudorotaxanes were formed in
highly polar solvent DMSO. To the best of our knowledge, it is
the first example of the design and preparation of such
pillar[5]arene-based pseudorotaxanes by self-assembly through
the hydrogen bonding interactions between the bis-urea hydrogens
and dicarboxylate oxygens.
One of the most important aspects of self-assembly is the formation
of threaded structures, such as rotaxanes, pseudorotaxanes and
catenanes, due to their potential applications in nanotechnology
and molecular machines.1 As the basic threaded structures,
pseudorotaxanes2 have been a topic of great interest in recent
years because they are not only the fundamental precursors for
the preparation of novel supramolecular species, such as
rotaxanes and catenanes,3 but also the prototypes of simple
molecular machines.1c,4 Therefore, the design and preparation
of pseudorotaxanes with novel macrocyclic hosts (the ‘wheels’) and
linear guests (the ‘axles’) to form the templated-pseudorotaxane is
highly desirable for both supramolecular chemistry and the
fabrication of molecule-based devices. Pillar[5]arenes, firstly
synthesized by Ogoshi’s group,5 are a new class of macrocyclic
hosts,6 which have a unique, rigid and symmetrical pillar
architecture, and have been widely used as macrocyclic hosts
(wheels) in the construction of pseudorotaxanes with various
guests (axles) from cationic molecules5,7 to neutral compounds.8
However, the linear anionic guest recognition based on pillararenes
is rarely reported.9 Therefore, searching for new linear anionic
guests as axles to pillararene-based hosts is very fascinating. On the
other hand, the urea functional group has been widely studied as
an anion receptor moiety,10 and urea-based heteroditopic receptors
have been reported to act as wheels to form pseudorotaxane-type
complexes with the dialkylviologen salts as axles.11 Hence, we
envisioned that by introducing two urea groups into pillar[5]arene,
a bis-urea-functionalized pillar[5]arene (H) could be obtained
The bis-urea-functionalized pillar[5]arene (H) was prepared
from diamino functionalized pillar[5]arene12 with p-tolyliso-
cyanate in excellent yield (Scheme S1, ESIw). The studies of
host–guest complexation of H with Gn were carried out by
1H NMR in DMSO-d6. All dicarboxylates were employed as
their bis-tetrabutylammonium (TBA) salts. Fig. 2b shows the
1H NMR titration result of G13 into a solution of H in
DMSO-d6. With the addition of G13, four new signals
1
(marked with 1, 2, 3 and 4, respectively, in H NMR titration
spectra, Fig. 2b) were observed (for example, their chemical
shifts are d: ꢁ2.01, ꢁ1.41, ꢁ0.35 and 0.53 ppm, respectively, in
the presence of 1.0 equivalent of G13) in the upfield area, which
are corresponding to the seven methylene groups (identified by
integration and symmetrically connected to the central methylene
group, marked with 1, 2, 3 and 4, respectively, Fig. 2a) of the linear
alkyl chain of G13. Because only the strong inclusion-induced
shielding effects by the bis-urea-functionalized cyclophane could
lead to such large upfield shifts of those seven methylene groups of
G13,13 it provided the direct evidence that these seven methylene
groups were threaded within the cavity of H. Meanwhile, large
Key Laboratory of Mesoscopic Chemistry of MOE,
Center for Multimolecular Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093,
P. R. China. E-mail: lywang@nju.edu.cn; Fax: +86 025 83597090;
Tel: +86 025 83592529
w Electronic supplementary information (ESI) available: Synthesis,
2D NOESY, additional NMR spectra, ESI-MS spectra, Job plot
and determination of the association constants. See DOI: 10.1039/
c2cc34049f
Fig. 1 Graphical representation of the formation of [2]pseudorotaxanes
from H and Gn.
c
8532 Chem. Commun., 2012, 48, 8532–8534
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signals of the urea protons Ha and Hb of H shifted downfield
notably (Fig. S21, ESIw). However, the signals of the middle
methylene units of G5 and G7 did not show obvious shift,
indicating that they did not thread within the cavity of H. This
should be due to that the chain lengths of both G5 and G7
were too short to be fitted for site separation of the bis-urea
groups. The substantial downfield shifts of the urea protons
Ha and Hb can be attributed to the non-encapsulated formation
of external hydrogen bonds between the bis-urea hydrogens of H
and dicarboxylate oxygens of G5 and G7, respectively. However,
in the case of G9, G11, G12, G14 and G15 with a longer alkyl
chain, bis-urea-functionalized pillar[5]arene inclusion-induced
shielding effects were observed in DMSO-d6 (Fig. 3 and
Fig. S22–S26 (ESIw)), which are similar to G13CH. Remarkably,
longer Gn of up to 20 carbons could also thread through the
cavity of H in DMSO-d6 (Fig. 3 and Fig. S27 (ESIw)). These
inclusion complexes can be considered to have [2]pseudorotaxane
structures.
In the above [2]pseudorotaxane-type complexes, the four
N–Hꢀ ꢀ ꢀO hydrogen bonds may be the main driving force for
the formation of the interpenetrated complexes. Additionally,
C–Hꢀ ꢀ ꢀp interactions15 may also play an important role in
stabilizing the inclusion complexes. The evidence for the
presence of N–Hꢀ ꢀ ꢀO hydrogen bonds upon complexation
was the large (>1 ppm) changes in the chemical shifts of urea
NH protons with the added guest.
Fig. 2 (a) Graphical representation of the formation of [2]pseudo-
rotaxane from H and G13. (b) Partial 1H NMR spectra (400 MHz,
DMSO-d6, 298 K) of H at a concentration of 10 mM with different
equivalents of G13. (a0 and b0: signals of urea protons from interpene-
trated H; a00 and b00: signals of urea protons from non-interpenetrated
H; 1–4: signals from methylene protons of encapsulated Gn; the peaks
Regardless of which Gn (n = 9, 11–15, 20) was employed, a
1 : 1 complexation stoichiometry was determined by direct
integration of the urea NH peaks of interpenetrated H and the
encapsulated methylene peaks exhibited in Fig. 3. Electrospray
ionization mass spectrometry (ESI-MS) provided further evidence
for this 1 : 1 binding stoichiometry. In all cases, two significant
m/z peaks were observed – one corresponding to [GnCH–TBA]ꢁ
and the other one to [Gn C H–2TBA]2ꢁ ions (Fig. S28–S34 and
also see ESIw, Table S1). While no peaks with other complexation
stoichiometries were found.
marked with
are ascribed to chloroform.)
downfield shift changes for the host ureido protons Ha and Hb
were also observed compared to the free host, which suggests
their involvement in hydrogen bonding. Furthermore, peaks
of Ha and Hb of the free H were further split into two
separated peaks (Fig. 2b, a0, a00, b0 and b00) after the addition
of 0.2 equiv. of G13, corresponding to the urea protons
that bind to encapsulated dicarboxylate G13 (a0 and b0) and
non-encapsulated dicarboxylate G13 (a00 and b00), respectively.
The above results indicated that H and G13 formed either a
pseudorotaxane-type inclusion complex or external complicated
complexes (Scheme S2, ESIw), both of such complexes are based
on the hydrogen bonding interactions between the bis-urea
hydrogens and dicarboxylate oxygens, and it also revealed that
the formation of the pseudorotaxane is a slow exchange process
on the NMR spectroscopy timescale. However, different from
the general slow exchange process,7a,h,14 no uncomplexed
signals were observed. From 2D NOESY analysis (Fig. S19,
ESIw), NOE correlations were observed between the seven
methylene protons (H1, H2, H3 and H4) of G13 and the
aromatic protons of H, which also confirmed the interpenetrated
geometry. In addition, the Job plot based on proton NMR data
demonstrated that the complex was of 1 : 1 stoichiometry (Fig. S20,
ESIw). All of these results suggested that the linear alkyl
dicarboxylate G13 was threaded through the cavity of H to
form a [2]pseudorotaxane, in which the carboxylate groups of
G13 bound to the bis-urea groups of H via four hydrogen
bonds, and the middle seven methylene groups of G13
(marked with 1, 2, 3 and 4, respectively, Fig. 2a) were
encapsulated in the cavity of H.
To quantitatively assess the inclusion complexation behavior
1
of these compounds, H NMR spectra of the 1 : 1 solutions
of H and Gn (n Z 9) in DMSO-d6 were recorded (Fig. 3).
Fig. 3 Partial 1H NMR spectra (400 MHz, 298 K) of H (10 mM) and
its mixture with Gn (10 mM) in DMSO-d6. (a0 and b0: signals of urea
protons from interpenetrated H; a00 and b00: signals of urea protons from
non-interpenetrated H; : signals from methylene protons of encapsulated
With respect to the equimolar solutions of both G5 and G7
1
with H in DMSO-d6, the H NMR spectra showed that the
Gn; the peaks marked with
are ascribed to chloroform.)
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 8532–8534 8533

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Table 1 Association constantsa (Ka) for 1 : 1 inclusion complexation
of Gn with H determined by H NMR in DMSO-d6 at 298 K
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1
Bis-TBA salts of dicarboxylates [Gn]
Ka (Mꢁ1
)
b
Glutarate (G5)
Pimelate (G7)
Azelate (G9)
—
—
2006, 2105; (d) K. D. Hanni and D. A. Leigh, Chem. Soc. Rev.,
2010, 39, 1240.
¨
b
c
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Undecanedioate (G11)
Dodecanedioate (G12)
Tridecanedioate (G13)
Tetradecanedioate (G14)
Pentadecanedioate (G15)
Eicosanedioate (G20)
2.8 ꢂ 101 (0.6)
6.9 ꢂ 101 (0.8)
7.1 ꢂ 101 (2.5)
1.4 ꢂ 102 (4.9)
1.5 ꢂ 102 (4.5)
7.6 ꢂ 101 (2.0)
a
Average values are shown with standard deviations from three
independent measurements by the single-point method using a 10 mM
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concentration in DMSO-d6. No binding of G5 and G7 inside the inner
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c
cavity of the host. The Ka value was too small to be calculated.
The spectra of the mixture of H with Gn (n Z 9) displayed
two sets of signals corresponding to two types of compounds
(pseudorotaxane-type inclusion complexes and external complexes).
From integration of the urea NH peaks of interpenetrated
H and non-interpenetrated H exhibited in Fig. 3, binding
constants (Ka) can be calculated using the single point
method.16 The Ka data are listed in Table 1. It was found that
the chain length of Gn was a dominant factor for the binding
affinities of encapsulated Gn and H. It may be attributed to
structural complementarity between the dianionic guest and
the bis-urea-functionalized host in which the two binding
subunits cooperate for guest binding. The terminal anionic
groups of the dicarboxylate would each interact with a urea
unit of the host, the polymethylene chain stretching through
the cavity of the host. Highest stability of the threaded complex
corresponds to the best fit between guest length and site separation
of the bis-urea groups, as schematically represented by Fig. 1.
Guests that are either too short or too long lead to formation
of less stable interpenetrated complexes.
In summary, we have demonstrated that ureido functional
groups can easily be introduced into pillar[5]arene to yield
bis-urea-functionalized pillar[5]arene, which can be utilized as
a novel wheel to form a series of [2]pseudorotaxanes with axles
derived from linear alkyl dicarboxylates containing nine to
twenty carbons in highly competitive solvent DMSO. This novel
binding motif affords the first example of pseudorotaxanes
stabilized mainly by the hydrogen bonding interactions between
the bis-urea hydrogens and dicarboxylate oxygens alongside
C–Hꢀꢀꢀp interactions, which might be applicable to the future
fabrication of interlocked structures and molecular devices.
This work was supported by the National Natural Science
Foundation of China (No. 20932004, 21072093, 21102073), the
National Basic Research Program of China (2011CB808600),
and Jiangsu Provincial Natural Science Foundation of China
(BK2011055, BK2011551).
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8534 Chem. Commun., 2012, 48, 8532–8534
This journal is The Royal Society of Chemistry 2012