Chloride anion triggered motion in a bis-imidazolium rotaxane

Article abstract of DOI:10.1039/c1dt10186b

We report the first bis-imidazolium-containing rotaxane, synthesised via anion templated self-assembly. Its co-conformation is controlled by a chloride anion recognition mechanism, thus demonstrating the viability of this protocol as a stimulus for shuttl

Full text of DOI:10.1039/c1dt10186b

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COMMUNICATION
Chloride anion triggered motion in a bis-imidazolium rotaxane†
Christopher J. Serpell, Ricky Chall, Amber L. Thompson and Paul D. Beer*
Received 2nd February 2011, Accepted 1st March 2011
DOI: 10.1039/c1dt10186b
We report the first bis-imidazolium-containing rotaxane,
synthesised via anion templated self-assembly. Its co-
conformation is controlled by a chloride anion recognition
mechanism, thus demonstrating the viability of this protocol
as a stimulus for shuttling molecular motion.
in organometallic chemistry20 but scarcely examined under the
guise of anion coordination21–24), and synthesised by application
of anion templated self-assembly. Furthermore, the resulting
system was found to exhibit anion-controlled switching be-
haviour with respect to the position of the macrocycle on the
axle.
The formation of anion-templated pseudorotaxane assemblies
using bis-imidazolium units were investigated as precursors to in-
terlocked systems. Threads were prepared via alkylation of 2,6-bis-
imidazolylpyridine25 1 (Scheme 1). The shorter dicationic thread
2 was obtained by reaction of 1 with propargylbenzenesulfonate
under microwave irradiation with catalytic tetrabutylammonium
iodide. Anion exchange via precipitation from water using ammo-
nium hexafluorophosphate gave 2 in 73% yield.
Precise and controllable determination of the co-conformation of
mechanically bonded molecules such as catenanes and rotaxanes
is a major area of current interest, with important applications
in materials,1,2 drug delivery,3,4 biological imaging,5 and data
storage.6 In spite of this, the use of molecular recognition to
effect such changes is underdeveloped. This is surprising, given
that interlocked molecules contain topologically unique three
dimensional cavities which may approximate those found in the
active sites of enzymes. Molecular switches and machines also
present a new range of mechanisms for signal transduction and
resultant macroscopic responses2 beyond the commonly employed
optical and electrochemical reporting systems.
Although cations have been used to induce molecular machine-
like switchable/shuttling behaviour between different states,7
anion mediated changes in co-conformation of interlocked com-
ponents are rare, and operate through displacement of the
macrocycle from a binding site by an anionic guest,8–10 cooperative
recognition,11 or creation of a negative charge on one component,
thus altering the conformation.12,13 However, the use of true anion
coordination chemistry is almost unknown. We have therefore
pursued this line of enquiry, having produced a [2]catenane in
which anion coordination acts as a locking mechanism,14 and a
heteroditopic ion-pair [2]rotaxane receptor which displays anion
recognition and cation-controlled shuttling.15
The imidazolium motif is well known as a charge-assisted C–
H hydrogen bond donor in acyclic16 and macrocyclic16,17 anion
receptors,18 and within this group we have demonstrated its capac-
ity to act as an anion-pair axle in pseudorotaxane formation.19
In this communication we report the first bis-imidazolium-
based rotaxane, making use of an isophthalamide macrocycle and
2,6-bis-imidazolium-pyridine-cored thread (a system ubiquitous
Chemistry Research Laboratory, Department of Chemistry, University of
Oxford, Mansfield Road, Oxford, OX1 3TA, United Kingdom. E-mail:
paul.beer@chem.ox.ac.uk; Fax: +44 (0) 1865 272609; Tel: +44 (0) 1865
285142
† Electronic supplementary information (ESI) available: Full details of
synthesis and characterisation. CCDC reference number 811551 (2). For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10186b
Scheme 1 Synthesis of bis-imidazolium thread 2 and [2]rotaxane 7.
12052 | Dalton Trans., 2011, 40, 12052–12055
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©

For the longer thread 4 (Scheme 2, S1 in the ESI†), the
alkynyl-polyether-tosylate 3 was prepared initially by reaction of
tetraethylene glycol with propargyl benzenesulfonate, with sodium
hydroxide acting as a base (46% yield), and subsequent tosylation
of the remaining terminal hydroxyl group using tosyl chloride in
dichloromethane with triethylamine and DMAP gave 3 in 91%
yield. This was then appended to the 2,6-bisimidazolyl pyridine 1
via the same protocol as given above for 2. The hexafluorophos-
phate salt 4 was obtained as a viscous oil coincidentally also
in 91% yield. Both were characterised by 1H and 13C NMR,
high resolution electrospray mass spectrometry and elemental
analysis and crystals of 2 suitable for X-ray structural analysis
were obtained and examined (Fig. S2†). Macrocycle 5 was used as
the host system, containing an isophthalamide unit for hydrogen-
bonded anion binding and electron rich hydroquinone units for
p–p stacking with the positively charged p-surfaces found in the
threads.19
Evidence for anion templated penetration of macrocycle 5 by
1
threads 2 and 4 was obtained by H NMR titration studies in
acetone-d6 (Fig. 1). Firstly, one equivalent of chloride, as its
non-coordinating tetrabutylammonium (TBA) salt was added
to macrocycle 5. A characteristic downfield shift of the amide
and interior isophthalamide proton resonances was observed,
consistent with chloride anion binding. Subsequent portionwise
addition of threads 2 or 4 resulted in a slight upfield movement of
the macrocycle amide proton resonances due to competition for
coordination of the halide anion, and importantly a greater upfield
perturbation, accompanied by splitting, was observed for the
hydroquinone peaks. This is indicative of threading, the shielding
effect arising from the influence of the bis-imidazolium-pyridine
ring currents.19 Monitoring the hydroquinone macrocycle protons
gave titration data which were analysed by WinEQNMR,17 giving
apparent association constant values of greater than 104 mol-1 dm3
in either acetone-d6 or acetonitrile-d3. Importantly, when the same
experiments were conducted in the absence of the chloride anion,
no association was observed, demonstrating the crucial role of
the chloride template. The interpenetrative assembly was further
characterised by 1H ROESY spectroscopy in acetone-d6 (Fig S5†),
which revealed through-space interactions between 5 and 2 or 4,
confirming the presence of the threads in the macrocycle cavity.
Having established the formation of robust pseudorotaxane
assemblies, azide-functionalised stopper groups were prepared for
the synthesis of rotaxanes using 1,3-dipolar Huisgen cycloaddition
chemistry (Scheme S2†). This was achieved by reaction of a
tetraaryl amine with bromoacetylbromide and then sodium azide,
giving the azide stopper 6. Rotaxanes were synthesised by stirring
of equimolar mixtures of thread 2 or 4, macrocycle 5, and TBA
chloride with 2.2 equivalents of the azide stopper in acetone in the
presence of Cu(MeCN)4PF6 and 2,6-lutidine (Scheme 1). This gave
the [2]rotaxane 8 in an isolated yield of 12% after purification by
preparative TLC eluted with 17 : 2 : 1 acetonitrile/water/KNO3
(aq, sat). Anion exchange by precipitation from water with
Scheme 2 Synthesis of bis-imidazolium thread 4 and [2]rotaxane 8.
Fig. 1 1H NMR spectra of macrocycle 5, and equimolar mixtures 5·TBACl, and 5·TBACl·2 (293 K, acetone-d6).
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Dalton Trans., 2011, 40, 12052–12055 | 12053
©

-
Fig. 2 1H NMR spectra of rotaxane 8 as its 2PF6 salt and changes upon addition of one equivalent of TBACl (acetone-d6, 293 K).
NH4PF6 was used to procure the hexafluorophosphate salt. The
has been shown that amide-amide hydrogen bonding is the major
feature of catenane host systems for anions in the absence of a
coordinating anion.28 The preservation of symmetry in the 1D
spectrum indicates the dynamic nature of the system on the NMR
timescale.
analogous procedure using short thread 2 gave [2]rotaxane 7,
1
which was observed by H NMR but could not be isolated due
to decomposition (diagnosed by degradation of the NMR sample
over the course of a week).
1
-
The H NMR spectrum of 8 as its PF6 salt (Fig. 2, S1) was
assigned and displayed all the peaks corresponding to macrocycle,
thread, and stopper components, as well as the appearance of
the triazole proton at 8.17 ppm which verifies the success of the
click reaction. Importantly, the hydroquinone protons are split and
shifted upfield, confirming interpenetration. The doubly charged
Upon addition of one equivalent of TBA chloride, the 1H
ROESY spectrum was again recorded (Fig. S7). Additional cross-
peaks arose between the imidazolium proton 5 and the macrocycle
isophthalamide (b and c) and alkyl protons f. The coupling
between protons f and 16 (a- to the triazole group) was also lost.
This is consistent with movement of the macrocycle component
to the centre of the axle, with the periphery of the axle encircling
parent ion peak [M - 2PF6 ]2+ was observed in the HR-ESMS
-
1
at m/z = 1232.6777, corroborating that the two components are
interlocked.
the core. The 1D H NMR spectrum (Fig. 2) displayed changes
with respect to both components which are typical of anion
binding: significant downfield shifts of macrocycle amide (d)
and isophthalamide (c) and thread imidazolium C2 protons (3).
Additionally, an upfield shift of the thread amide proton (17)
bears witness to a decrease in hydrogen-bonding at this site, and a
further upfield movement along with splitting of the hydroquinone
protons (g and h) demonstrates that the degree of interaction
with the thread has increased due to p–p stacking effects. This
combination of results lead to the conclusion that the concerted
binding of the halide anion by both components of the axle more
than compensates for the loss of amide-amide hydrogen bonding,
resulting in a movement of the macrocycle along the axle, thus
providing a pseudotetrahedral array of hydrogen bond donors to
encapsulate the chloride anion.
In conclusion, we herein report the first bis-imidazolium-
containing rotaxane, synthesised through an anion templated self-
assembly route. In the hexafluorophosphate salt, the macrocycle
resides primarily around the axle amide close to the terphenyl stop-
per through N–H ◊ ◊ ◊ O(carbonyl) hydrogen bonding interactions.
However, upon addition of chloride the macrocycle is observed
to move to the axle’s central bis-imidazolium position due to the
complementarity of the resulting cleft for the templating anion.
This result demonstrates the viability of anion recognition as
1H ROESY spectroscopy in acetone-d6 was then used to gauge
the interactions between the macrocycle and thread in 8·2PF6
(Fig. S6†). Three cross-peaks between the two components were
observed: between the macrocycle amide protons (d) and the alkyl
protons adjacent to the thread amide group, near the stopper
(16); between d and the closer stopper protons 18; and between
a macrocycle alkyl proton f and 18. This indicates that the
macrocycle resides primarily at the terminus, a position which is
favourable due to the reciprocal amide-amide hydrogen bonding
possible at this location (Fig. 2). This co-conformation would
allow the macrocycle amide protons to hydrogen bond with
the carbonyl oxygen on the axle. Such motifs are common in
the solid state and have been used by Leigh and co-workers in
the manufacture of high-yielding catenanes26 and rotaxanes.27
Some p–p stacking of the triazole linking motif between the
hydroquinone units is also possible in this co-conformation, with
the hydroquinone peaks in acetonitrile-d3 being 0.019 ppm further
upfield in the rotaxane than the free macrocycle, while the axle
amide proton (17) may interact with the macrocycle polyether
chain. This arrangement is favourable when the rotaxane is the
hexafluorophosphate salt, since the anion is non-coordinating
and cannot fit within the interlocked binding cavity – indeed it
12054 | Dalton Trans., 2011, 40, 12052–12055
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The Royal Society of Chemistry 2011
©

a stimulus for molecular motion, and the further construction
of molecular machines with mechanically-mediated signalling
responses for anion sensing. Research into further elaboration
of these systems is ongoing in our group.
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