A bifunctionalized [3]rotaxane and its incorporation into a mechanically interlocked polymer

Article abstract of DOI:10.1039/c0cc00999g

A novel bifunctionalized [3]rotaxane based on a triptycene-derived macrotricyclic host was conveniently synthesized. On the basis of the [3]rotaxane, a linear main-chain poly[3]rotaxane was further obtained by the highly efficient Huisgen 1,3-dipolar cycloaddition. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0cc00999g

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A bifunctionalized [3]rotaxane and its incorporation into a mechanically
interlocked polymerw
Yi Jiang,ab Jia-Bin Guoab and Chuan-Feng Chen*a
Received 18th April 2010, Accepted 27th May 2010
First published as an Advance Article on the web 24th June 2010
DOI: 10.1039/c0cc00999g
A novel bifunctionalized [3]rotaxane based on a triptycene-
derived macrotricyclic host was conveniently synthesized. On
the basis of the [3]rotaxane, a linear main-chain poly[3]rotaxane
was further obtained by the highly efficient Huisgen 1,3-dipolar
cycloaddition.
Rotaxanes,1 mechanically linked molecules, have recently
attracted great interest for not only their aesthetic structures
but also their potential applications in nanotechnology and
molecular machines.2 Especially, rotaxane-based molecular
machines, controlled by external stimuli, seem to be parti-
cularly important in the field of information storage and
processing.3 During the past two decades, various rotaxanes
have been synthesized in high efficiency approaches since
Fig.
1 Graphical representation of (a) macrotricyclic host 1,
transition metal templates4 or organic templates5 have been
applied. However, the synthesis of high order [n]rotaxanes
remains a considerable challenge for supramolecular chemists.
The mechanical (noncovalent) bond is the distinguishing
characteristic of mechanically interlocked molecule-incorporated
polymers. It could be deduced that the incorporation of
flexible mechanical bonds would have repercussions for the
polymer-chain behaviour, and subsequently produce new
types of polymers with specific properties in both solution
and the solid state.6 Despite significant attention paid to
synthesize these types of macromolecules, few successful
examples7 exist, on account of considerable challenges in the
synthesis. Recently, Stoddart and Grubbs described an
efficient strategy for the polymerization of a bifunctionalized
[c2]daisy-chain monomer,8 which was obviously different from
a template-directed self-assembly for the polymerization step.9
Triptycene,10 with its unique three-dimensional rigid
structure and electron-rich property, has been found to be a
useful building block for the construction of novel receptors.11
As a result, we recently reported a triptycene-based macro-
tricyclic host 1 (Fig. 1a), and found that the host could
self-assemble with two dibenzylammonium salts to form
[3]pseudorotaxanes (Fig. 1b).12 On the basis of this work, we
herein report the facile synthesis of a novel bifunctionalized
[3]rotaxane and its further incorporation into a mechanically
interlocked polymer (Fig. 1).
(b) [3]pseudorotaxane, (c) [3]rotaxane, and (d) poly[3]rotaxane.
The synthesis of the functionalized dibenzylammonium salt
2-HÁPF6 is outlined in Scheme 1. Reaction of 4-cyanophenol 5
with tosylated 6 in acetonitrile in the presence of K2CO3, and
then reduction of the cyano group by LiAlH4 in THF, gave the
primary amine 4. Condensation of amine 4 with aldehyde 3
gave the corresponding reversible dynamic imine, which was
then reduced by NaBH4 in MeOH to give the kinetically stable
amine. Protonation of the free amine with an excess of HCl
and subsequent counterion exchange with a saturated NH4PF6
solution afforded the unsymmetrical dibenzylammonium salt
2-HÁPF6 in 62% total yield for the four steps. The salt 2-HÁPF6
has good solubility in acetonitrile, but poor solubility in
chloroform.
A mixture of the triptycene-based macrotricyclic host 1
(1 mM) and 2-HÁPF6 (2 mM, 2.0 equiv) in 1 : 1 CD3CN:CDCl3
at room temperature provided a complicated 1H NMR
spectrum after one day (see ESIw),13 with corresponding rates
a Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Molecular Recognition and Function, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
E-mail: cchen@iccas.ac.cn; Fax: 8610-62554449
b Graduate School, Chinese Academy of Sciences, Beijing 100049,
China
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for the bifunctionalized
[3]rotaxane and poly[3]rotaxane. See DOI: 10.1039/c0cc00999g
Scheme 1 Synthesis of the dibenzylammonium salt 2-HÁPF6.
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of association and/or dissociation slower than the NMR
spectroscopic time scale under the tested conditions. However,
the signals of protons H5 and H6 of the guest obviously shifted
upfield, while a downfield shift of the aromatic proton H1 of
host 1 was also observed. These observations indicated that
complexation between host 1 and the guest occurred, and
that complex 1Á(2-HÁPF6)2 thus formed. The electrospray
ionization mass spectrum (ESI-MS) provided more evidence
for the formation of the 1 : 2 complex 1Á(2-HÁPF6)2 when a
strong peak at m/z 1059.48 for [1Á(2-H)2]2+ was observed.13
The formation of complex 1Á(2-HÁPF6)2 provides a chance to
further synthesize a [3]rotaxane.
It was known that the 3,5-dimethyl phenyl group could be
used as an efficient stopper for 24-crown-8.14 Thus, we tried to
prepare the [3]rotaxane, 3-2HÁ2PF6, using two 3,5-dimethyl
phenyl groups as the stoppers. As shown in Scheme 2, after the
macrotricyclic host 1 and 2-HÁPF6 were mixed and stirred at
room temperature in a 1 : 1 CH3CN–CHCl3 solution for 24 h,
the mixture was treated with 3,5-dimethylbenzoic anhydride in
the presence of a catalytic amount of tri(n-butyl)phosphine to
afford 3-2HÁ2PF6 in 37% yield. The partial proton NMR
spectrum of the [3]rotaxane 3-2HÁ2PF6 in 1 : 1 CD3CN:CDCl3
is shown in Fig. 2b. The aromatic proton H1 of 1 and
proton H4 of 2-HÁPF6 in 3-2HÁ2PF6 moved downfield,
while the aromatic protons H5 and H6 shifted upfield greatly
(Dd = 1.65 and 1.58 ppm, respectively), which might be due
to the strong shielding effect of the aromatic rings in host 1.
The ESI-HRMS of 3-2HÁ2PF6 revealed a high-intensity
signal at m/z = 1191.1187 corresponding to the ion mass of
Fig. 2 Partial 1H NMR spectra (300 MHz, 1 : 1 CD3CN:CDCl3,
298 K) of (a) 2-HÁPF6 (2 mM), (b) the [3]rotaxane 3-2HÁ2PF6 (1 mM),
and (c) host 1 (1 mM). Resonance protons are labeled in Scheme 2.
À
[3-2H]2+ (see ESIw),13 that is, the loss of 2PF6 from the
[3]rotaxane.
There are two terminal propargyl groups in the [3]rotaxane,
which could provide an alternative means to further synthesize
mechanically interlocked polymers. The highly efficient
Cu-catalyzed, Huisgen 1,3-dipolar cycloaddition15 was chosen
Scheme 3 Synthesis of the poly[3]rotaxane 4-2mHÁ2mPF6.
for the polymerization reaction on account of its mild nature
and high conversion. The synthesis of the linear main-chain
poly[3]rotaxane 4-2mHÁ2mPF6 is outlined in Scheme 3. When
the terminally bifunctionalized [3]rotaxane 3-2HÁ2PF6 was
subjected to a step-growth polymerization with 1 equiv of
the diazide 2 in DMF in the presence of a stoichiometric
amount of CuI, the linear main-chain poly[3]rotaxane
4-2mHÁ2mPF6 could be conveniently obtained. The structure
of the poly[3]rotaxane was confirmed by FT-IR, 1H NMR,
MALDI-TOF MS and size exclusion chromatography
(SEC).13 In the FT-IR spectrum of 4-2mHÁ2mPF6, an
absorption peak at 1062 cmÀ1, corresponding to a triazole
group,16 was observed, while the signals at 3303 cmÀ1
(RC–H) and 2091 cmÀ1 (—N3) were also found, which were
caused by the remaining terminal unreacted alkyne and azide
groups. The MALDI-TOF MS spectrum revealed signals at
m/z = 2740 corresponding to the ion mass of [4-H]+ (m = 1),
and 5918 corresponding to the ion mass of [4-4HÁ3PF6]+
1
(m = 2). In the H NMR of 4-2mHÁ2mPF6, the signals for Ha,
corresponding to 1,2,3-triazole, and Hb emerged at 7.82 and
5.11 ppm, respectively. All the above results indicated the
Scheme 2 Synthesis of the [3]rotaxane 3-2HÁ2PF6.
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occurrence of the polymerization reaction and the formation
of the poly[3]rotaxane. Size exclusion chromatography (SEC)
analysis of 4-2mHÁ2mPF6 was also performed using pure
DMF as the eluent, which showed a major peak in the
chromatograph. The calculated average molecular weight
(Mm) is ca. 43 kDa with a polydispersity index (PDI) of
1.42. The Mm indicates that each polymer chain is composed
of ca. 14 repeating units. Moreover, differential scanning
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a
characteristic glass transition process at 108 1C.
In conclusion, we have shown the facile synthesis of a novel
bifunctionalized [3]rotaxane based on a triptycene-derived
macrotricyclic host. On the basis of the [3]rotaxane, we have
also synthesized a linear main-chain poly[3]rotaxane by the
highly efficient Cu-catalyzed, Huisgen 1,3-dipolar cyclo-
addition. Our further work will focus on the construction of
the stimuli-responsive [3]rotaxanes and the corresponding
poly[3]rotaxanes.
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We thank the National Natural Science Foundation of
China (20625206, 20772126), the National Basic Research
Program (2007CB808004) and the Chinese Academy of
Sciences for financial support.
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