Acid-Base Switchable [2]- and [3]Rotaxane Molecular Shuttles with Benzimidazolium and Bis(pyridinium) Recognition Sites

Article abstract of DOI:10.1002/asia.201601179

For the purpose of developing higher level mechanically interlocked molecules (MIMs), such as molecular switches and machines, a new rotaxane system was designed in which both the 1,2-bis(pyridinium)ethane and benzimidazolium recognition templating motifs were combined. These two very different recognition sites were successfully incorporated into [2]rotaxane and [3]rotaxane molecular shuttles which were fully characterized by ¹H NMR, 2D EXSY, single-crystal X-ray diffraction and VT NMR analysis. By utilizing benzimidazolium as both a recognition site and stoppering group it was possible to create not only an acid/base switchable [2]rotaxane molecular shuttle (energy barrier 20.9 kcal?mol^?1) but also a [3]rotaxane molecular shuttle that displays unique dynamic behavior involving the simultaneous motion of two macrocyclic wheels on a single dumbbell. This study provides new insights into the design of switchable molecular shuttles. Due to the unique properties of benzimidazoles, such as fluorescence and metal coordination, this new type of molecular shuttle may find further applications in developing functional molecular machines and materials.

Full text of DOI:10.1002/asia.201601179

CHEMISTRY
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Accepted Article
Title: Acid-Base Switchable [2]- and [3]Rotaxane Molecular Shuttles with Benzimi-
dazolium and Bis(pyridinium) Recognition Sites
Authors: Kelong Zhu; Nicholas Vukotic; Stephen J. Loeb
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Acid-Base Switchable [2]- and [3]Rotaxane Molecular Shuttles
with Benzimidazolium and Bis(pyridinium) Recognition Sites
Kelong Zhu*[a], V. Nicholas Vukotic[b] and Stephen J. Loeb*[b]
Abstract: For the purpose of developing higher level mechanically
In the last two decades, electron deficient pyridinium salts have
interlocked molecules (MIMs), such as molecular switches and
been broadly used as recognition templates for developing MIMs
machines, a new rotaxane system was designed in which both the
with crown ethers or cryptands due to their straightforward
1,2-bis(pyridinium)ethane
and
benzimidazolium
recognition
synthesis and strong binding ability.[11] In particular, we reported
that 1,2-bis(pyridinium)ethane dications (12+) could be used as
axles to form [2]pseudorotaxanes with 24-membered crown
ethers such as commercially available dibenzo[24]-crown-8
(DB24C8).[11c] This has also been shown to be a versatile motif
for the formation of [2]rotaxanes,[12] [3]rotaxanes,[13]
[3]catenanes,[14] molecular shuttles,[15] branched [n]rotaxanes (n
= 2–4)[16] and metal-organic rotaxane frameworks (MORFs).[9],[17]
Imidazolium and benzimidazolium cations have also been found
to complex macrocycle hosts such as crown ethers, cryptands
and pillararenes.[18] In this regard, we developed a series of
T-shaped benzimidazolium cations (e.g. [2-H]+) which act as an
efficient template for the formation of [2]pseudorotaxanes with
crown ethers. Based on this new recognition motif, we further
developed an H-shaped molecular shuttle[19] which could be
incorporated into a MOF showing for the first time that a molecular
shuttle could operate in the solid-state.[9i]
templating motifs were combined. These two very different recognition
sites were successfully incorporated into [2]rotaxane and [3]rotaxane
molecular shuttles which were fully characterized by 1H NMR, 2D
EXSY, single crystal X-ray diffraction and VT NMR analysis. By
utilizing benzimidazolium as both a recognition site and stoppering
group it was possible to create, not only, an acid/base switchable
[2]rotaxane molecular shuttle (energy barrier 20.9 kcal•mol‒1) but also
a [3]rotaxane molecular shuttle that displays unique dynamic behavior
involving the simultaneous motion of two macrocyclic wheels on a
single dumbbell. This study provides new insights into the design of
switchable molecular shuttles. Due to the unique properties of
benzimidazoles, such as fluorescence and metal coordination, this
new type of molecular shuttle may find further applications in
developing functional molecular machines and materials.
Introduction
For the purpose of developing higher level MIMs, such as
molecular switches and machines, a new rotaxane system was
designed in which both the 1,2-bis(pyridinium)ethane and
benzimidazolium recognition templating motifs are combined; see
Figure 1.[19b,19c] Due to the different binding strengths of the two
types of recognition sites, it was anticipated that it would be
possible to translocate macrocyclic wheels between the different
sites in a controlled manner. Herein, we report [2]- and
[3]rotaxanes constructed from a combination of both recognition
motifs and DB24C8. The shuttling dynamics of these molecular
shuttles were also examined with acid/base as the external
stimulus.
Molecular shuttles and switches derived from mechanically
interlocked molecules (MIMs) are attractive outcomes form
supramolecular chemistry.[1] They have proven to have potential
applications in, not only, biomimetic synthesis,[2] catalysis,[3]
sensors,[4] and drug delivery,[5] but also advanced materials
including liquid crystals,[6] smart polymers,[7] computer memory,[8]
and metal–organic frameworks (MOFs).[9] For the efficient
fabrication of MIMs, templated synthesis,[10] which involves
intermolecular recognition, is usually employed. Moreover, by the
judicious combination of different recognition motifs, one can
develop artificial molecular switches which operate under external
stimuli at the molecular level.
[a]
[b]
Prof. Dr. K. Zhu
School of Chemistry and Chemical Engineering
Sun Yat-Sen University
Guangzhou, P. R. China, 510275
E-mail: zhukelong@mail.sysu.edu.cn
Figure 1. Simple examples of [2]pseudorotaxanes based on
1,2-bis(pyridinium)ethane and benzimidazolium axles and DB24C8 wheels.
Anions are omitted for clarity.
Dr. V. N. Vukotic, Prof. Dr. S. J. Loeb
Department of Chemistry and Biochemistry
University of Windsor
Windsor, Ontario, Canada, N9B 3P4
E-mail: loeb@uwindsor.ca
Supporting information for this article is given via a link at the end of
the document.
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[2]Rotaxane Molecular Shuttles
comparison of the 1H NMR spectra of [2]rotaxane
Results and Discussion
A
[2]Rotaxanes
[7DB24C8]2+ and the dumbbell 72+ is shown in Figure 2. The
higher frequency chemical shifts of protons a ( = +0.40 ppm)
and b ( = +0.45 ppm) for 72+ likely result from hydrogen bonding
of these protons to oxygen atoms of the crown ether. Significant
upfield chemical shifts were observed for protons c ( = −0.35
ppm) and d ( = −0.32 ppm) which indicates -stacking of the
electron-rich benzo groups of the crown ether with the electron-
poor pyridinium rings of the dumbbell. All of this spectral evidence
indicates that 72+ and DB24C8 form structures similar to other
1,2-bis(pyridinium)ethane/crown ether rotaxanes, i.e. the crown
ether adopts an S-shaped conformation and is seated in the
middle of the 1,2-bis(pyridinium)ethane axle to maximize
noncovalent interactions.[12] [2]Rotaxane formation was also
confirmed by mass spectrometric analysis; the observed m/z of
The [2]rotaxanes [7DB24C8]2+ and [8DB24C8]2+ were
prepared as outlined in Scheme 1. 4-(4ʹ-Formylphenyl)pyridine,
3[20] was alkylated in 1,2-dibromoethane to give 1-bromoethyl-4-
(4ʹ-formylphenyl)pyridinium 4+ which was followed by another
alkylation to give the dialdehyde bispyridinium thread 52+. The
threading-followed-by-stoppering method was employed to
synthesize the [2]rotaxanes. Firstly, one equivalent of 52+ was
mixed with four equivalents of DB24C8 in solution to maximize
[2]pseudorotaxane formation and then the [2]pseudorotaxane
was condensed with 2.2 equivalents of 4,5-di-isopropoxy-1,2-
diamine, 6a in the presence of a catalytic amount of ZrCl4 to
produce the [2]rotaxane [7DB24C8]2+ in 66% yield. Similarly,
[2]rotaxane [8DB24C8]2+ was obtained in a yield of 32% by
using 1,2-diamino-3,6-di(4-t-butylphenyl)benzene, 6b[19c] as the
stopper. In both cases, the naked dumbbell 72+ or82+ was isolated
as the main by-product. The formation of [2]rotaxane was
confirmed by NMR spectroscopy and mass spectrometric
analysis for each.
625.3164 corresponds to [7DB24C8]2+
.
Figure 2. Partial 1H NMR spectra (500 MHz, CD3CN, 298K) for 72+ (top),
[7DB24C8]2+ (middle) , and DB24C8 (bottom). See Scheme 1 for labelling.
Single crystals of [7DB24C8][CF3SO3]2 suitable for X-ray
diffraction were obtained by slow evaporation of a methanol
-
solution of the compound; BF4- anions were replaced by CF3SO3
anions in order to grow suitable crystals of the cation. The solid-
state structure of the [2]rotaxane is shown in Fig 3 (top). The
crown ether adopts an S-shaped conformation and is located
around the 1,2-bis(pyridinium)ethane site. It is also clear from the
structure that the aromatic rings of the crown ether are involved
in significant -stacking interactions with the aromatic groups of
the pyridinium axle. The four ethylene protons of the -NCH2CH2N-
linkage form H-bonds with O-atoms of the crown ether which is
consistent with what was observed in the 1H NMR spectrum. Thus,
Scheme 1. Synthesis of [2]rotaxanes [7DB24C8]2+ and [8DB24C8]2+
.
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this structure can be considered as the static state with the lowest
energy conformation; i.e. no shuttling occurs.
downfield chemical shifts of the aromatic protons a and b were
observed for [7-H2DB24C8]4+. This was rationalized as being
due to a slight enhancement of the H-bonding of the now
protonated dumbbell [7-H2]4+ to the crown ether. No significant
chemical shift changes were observed for the crown ether protons.
These limited spectral changes indicate that the crown ether
remains exclusively at the 1,2-bis(pyridinium)ethane recognition
site even after protonation. To verify this interpretation, a simple
model, di-isopropoxy benzimidazolium 9a+ (see SI) was
synthesized and an association constant of 43 M‒1 measured for
[2]pseudorotaxane formation with DB24C8. This is much weaker
Since the readily incorporated benzimidazole stopper can be
protonated to give a benzimidazolium cation which can then act
as a secondary binding site, the acid/base switching ability
[7DB24C8]2+ was tested. After the addition of two equivalents of
tetrafluoroboric acid to an acetonitrile solution of [7DB24C8]2+,
only small changes were observed in the 1H NMR spectrum
(Figure S1). The biggest difference was the appearance of a
characteristic singlet at 13.1 ppm corresponding to the NH proton
of the benzimidazolium group. However, only very small
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than the value of 230 M‒1 previously reported for the parent
1,2-bis(4-phenylpyridinium)ethane dication (see Table S1).[21]
Thus, we can infer that for the protonated dumbbell [7-H2]4+, the
DB24C8 wheel favors the 1,2-bis(pyridinium)ethane recognition
site.
The H-shaped [2]rotaxane [8DB24C8]2+ was synthesized by
using the same approach as for [7DB24C8]2+ but including
larger and more rigid t-butylphenyl groups. Comparison of the 1H
NMR spectrum of [2]rotaxane [8DB24C8]2+ with the dumbbell
82+ is shown in Figure 4. The higher frequency chemical shifts of
protons a ( = +0.42 ppm) and b ( = +0.47 ppm) for 82+ are the
result of H-bonding of these protons to oxygen atoms on the
crown ether. Significant upfield chemical shifts were observed for
protons c ( = −0.38 ppm) and d ( = −0.36 ppm) indicative of
-stacking of the benzo group of the crown ether with the
pyridinium rings. Again, no change in the signal for the
benzimidazole NH proton was observed. This spectral evidence
be described as the state with the lowest energy conformation
where no shuttling occurs.
However, in stark contrast to [7DB24C8]2+, very different
spectral changes were observed upon protonation of
[8DB24C8]2+ to give [8-H2DB24C8]4+. As shown in Figure 4
(top), with two equivalents of acid, two sets of proton signals are
observed for [8-H2DB24C8]4+ which indicates an unsymmetrical
environment for the dumbbell. Two signals for the NH protons
were observed corresponding to both complexed and free
benzimidazolium sites. At the same time, proton a is upfield
shifted to 5.25 ppm which is close to the signal for free 82+
indicating
the
absence
of
the
wheel
at
the
1,2-bis(pyridinium)ethane site. Significant upfield chemical shifts
were also observed for proton f and benzo proton i indicating
-stacking of the benzo moieties on the benzimidazolium units.
These significant spectral changes infer that the ring has moved
from the central 1,2-bis(pyridinium)ethane site to the newly
formed benzimidazolium site. This is consistent with the fact that
the association constant for [2]pseudorotaxane formation of the
simple model 2,4,7-triphenylbenzamidazolium cation with
DB24C8, previously measured to be 1780 M‒1,[19a] is much
greater than the value of 230 M‒1 reported for the parent
1,2-bis(4-phenylpyridinium)ethane dication. [21] Furthermore, the
presence of two benzimidazolium recognition sites at the termini
of the dumbbell means that this protonated rotaxane could
potentially be operating as degenerate molecular shuttle. Indeed,
exchange signals for the complexed and free proton f were
observed in a 2D EXSY experiment which confirms that the
macrocyclic wheel is indeed shuttling between the two
benzimidazolium sites at a rate slower than the NMR timescale.[22]
At 298 K, a shuttling rate of 4.8 x 10 s‒1 (7.8 x 102 s‒1 at 338 K),
was measured which corresponds to an energy barrier of 20.9
kcal•mol‒1 (see SI). Further studies showed that the protonated
rotaxane can readily be neutralized back to the original dication
which has a static co-conformation where no shuttling occurs
(Figure S10). Thus, we can conclude that the combination of
T-shaped benzimidazolium and 1,2-bis(pyridinium)ethane
recognition sites allows for formation of an acid/base switchable
[2]rotaxane molecular shuttle; i.e two distinct cations with different
charges, linked by changes in pH represent ON (shuttling) and
OFF (no shuttling) states.
indicates that [8DB24C8]2+ has
[7DB24C8]2+ and this was verified by an X-ray crystal structure.
a structure similar to
f
f
f
f
[3]Rotaxane Molecular Shuttle
Encouraged by the successful formation of [2]rotaxanes from
the
combination
of
T-shaped
benzimidazolium
and
Figure 5. A diagram illustrating the shuttling of the wheel in [2]rotaxane
[8-H2DB24C8]4+ (top) and a 2D 1H−1H EXSY spectrum of [8-H2DB24C8]4+
(CD3CN, 300 MHz, 338K, τm= 0.5 s).
1,2-bis(pyridinium)ethane recognition sites, we attempted to
expand this novel chemistry into a more sophisticated system – a
[3]rotaxane which consists of one dumbbell 82+ with three
recognition sites and two DB24C8 wheels.
As shown in Figure 3 (bottom), the crown ether wheel adopts
an S-shaped conformation and is seated on the
1,2-bis(pyridinium)ethane site. The aromatic rings of the crown
ether are involved in significant -stacking interactions with the
aromatic groups of the pyridinium axle as designed. The four
central ethylene protons form H-bonds with O-atoms of the crown
ether which is consistent with observations from the 1H NMR
experiments. Again, similar to [7DB24C8]2+, this structure can
Accordingly, the threading-followed-by-stoppering method was
employed to synthesize the [3]rotaxane [8(DB24C8)2]2+. As
outlined in Scheme 2, firstly, the monoaldehyde [10-H]3+ was
obtained by condensation of an excess of dialdehyde axle 52+ with
the diamino compound 6b followed protonation with one
equivalent of tetrafluoroboric acid. After one equivalent of [10-H]3+
was mixed with four equivalents of DB24C8 in solution, the
resulting [3]pseudorotaxane was condensed with one equivalent
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of 6b in the presence of a catalytic amount of ZrCl4, followed by
The 1H NMR spectrum of [3]rotaxane [8(DB24C8)2]2+ at 298
K is shown in Figure 6 (middle). Only one set of signals for both
rings and dumbbell were observed which indicates either a static
state or a dynamic process faster than the NMR timescale. The
NH proton was found to have a chemical shift 0.27 ppm downfield
compared to the dumbbell which could be due to hydrogen
bonding of the NH to oxygen atoms on the macrocyclic wheel;
Figure 6 (top).[23] In addition, the ethylene proton signal a also
shifted downfield by 0.38 ppm which is characteristic for threading
of a DB24C8 wheel onto the 1,2-bis(pyridinium)ethane site.
Considering only one set of proton signal was observed, the
[3]rotaxane is most likely a degenerate molecular shuttle where
the two wheels are translating simultaneously. This molecular
shuttling behavior was further characterized by a VT NMR study.
Upon cooling a solution of [8(DB24C8)2]2+ in CD3CN, the proton
signal for the NH starts to broaden and eventually splits into two
signals at 233 K (Figure S8). In order to improve the solubility of
neutralization with proton sponge to produce the [3]rotaxane
[8(DB24C8)2]2+ in
a
yield of 42%. The [2]rotaxane
[8DB24C8]2+ and the dumbbell 82+ were found to be the main
by-products. The formation of [3]rotaxane [8(DB24C8)2]2+was
confirmed by NMR spectroscopic and mass spectrometry
analysis.(see Figure S5)
[8(DB24C8)2]2+ at low temperature,
a
solution of
[8(DB24C8)2]2+ in CD2Cl2 was used for the VT NMR study. As
shown in Figure 7, a coalescence temperature was accessible at
230 K. Two separate NH signals, at 12.13 and 10.48 ppm, were
obtained after cooling the sample to 204 K indicating a reduction
in symmetry for the chemical environments of the dumbbell due
to an uneven distribution of the crown ether wheels. This was
further supported by the observation of separate signals for the
benzo protons 1 and 2. The signal at 6.5 ppm is similar to the
chemical shift (6.6 ppm) observed for [2]rotaxane [8DB24C8]2+
in which the wheel resides exclusively on the central
1,2-bis(pyridinium)ethane recognition site. By using the
coalescence temperature method, an energy barrier of 9.9
kcal•mol‒1 and a rate of 3.4 × 105 s‒1 were estimated for the
simultaneous shuttling of both the wheels at 298 K.
Scheme 2. Synthesis of [3]rotaxane molecular shuttle [8DB24C8)2]2+
.
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Table 1. Proton chemical shift changes for the dumbbell protons of Conclusions
82+ upon formation of [2]rotaxane and [3]rotaxane.[a]
The combination of benzimidazolium/crown ether and
1,2-bis(pyridinium)ethane/crown ether recognition motifs has
been successfully exploited to prepare both [2]rotaxane and
[3]rotaxane molecular shuttles. It can be concluded from this new
design that: 1) formation of a benzimidazolium group to act as a
stopper simultaneously creates a second recognition site, 2) by
fine tuning the substituents on the benzimidazolium moiety, the
obtained [2]rotaxane can be further developed into an acid/base
switchable molecular shuttle, 3) a [3]rotaxane molecular shuttle
with unique shuttling dynamics can be produced from the same
approach and 4) shuttling of two macrocyclic wheels
simultaneously on one single dumbbell can be accomplished.
This study may provide new insight for the design of switchable
molecular shuttles. Owing to the unique properties of the
benzimidazole group, such as fluorescence and coordination to
metal ions, these new type molecular shuttles may found further
applications in developing functional molecular machines and
materials.
 (), ppm
[8DB24C8)2]2+
5.47 (+0.38)
8.95 (+0.34)
8.39 (+0.01)
8.03 (−0.09)
8.52 (+0.07)
11.15 (+0.27)
Proton
82+
5.09
8.61
8.38
8.12
8.45
10.88
[8DB24C8]2+
5.51 (+0.42)
9.08 (+0.47)
8.00 (−0.38)
7.76 (−0.36)
8.36 (−0.09)
10.87 (−0.01)
a
b
c
d
e
NH
[a] Solution concentration is 1.0 x 10-3 M and temperature 298 K unless
-
otherwise indicated. All anions are BF4 .
Again, in contrast to [2]rotaxane [8DB24C8]2+, after two
equivalents of acid were added to [8DB24C8)2]2+,
a
symmetrical spectrum was obtained; Figure 6 (bottom). A singlet
at 12.9 ppm was observed for the benzimidazolium NH proton
which is almost identical to the signal observed for the complexed
benzimidazolium NH of [8-H2DB24C8]4+. The upfield chemical
shifting of protons a, f, 1 and 2 further indicates the crown ether
wheels are now located on the two benzimidazolium sites of the
[3]rotaxane. In other words, the [3]rotaxane shuttling is switched
OFF by protonation of the benzimidazole sites.
Details of X-ray diffraction studies and EXSY experiments
including all NMR spectra are available in the Supplementary
information accompanying this article.
Experimental Section
General comments: All chemicals were purchased from Aldrich
Chemicals and used without further purification. Compounds 4-(4'-
formylphenyl)pyridine (3)[20] and 1,2-diamino-3,6-di(4′-t-butylphenyl)-
benzene (6b)[19c] were synthesized according to reported methods.
Deuterated solvents were obtained from Cambridge Isotope Laboratories
and used as received. Solvents were dried using an Innovative
Technologies Solvent Purification System. NMR spectra were recorded
either on a Brüker Advance 500 or Brüker Advance 300 spectrometer.
Chemical shifts are quoted in ppm relative to tetramethylsilane using the
residual solvent peak as
a reference standard. For the variable
temperature experiments, the low temperatures were calibrated using a
neat methanol sample. High resolution mass spectrometry (HR-MS)
experiments were performed on a Micromass LCT electrospray ionization
(ESI) time-of-flight (ToF) mass spectrometer. Solutions with
concentrations of 1.0 x 10-3 M were prepared in methanol and injected for
analysis at
a rate of 5 μL/min using a syringe pump. Elemental
compositions were determined on a PerkinElmer 2400 Series II Elemental
Analyzer. All single crystal X-ray data were collected on a Brüker APEX
diffractometer with a CCD detector operated at 50 kV and 30 mA with MoKα
radiation. Crystals were frozen in paratone oil inside a cryoloop and
reflection data were integrated from frame data obtained from hemisphere
scans. The SHELXTL library of programs[24], Bruker AXS APEX-III[25] and
OLEX2[26] software were used for data reduction, X-ray solution and
refinement while figures were drawn with the CrystalMaker software
package.[27] CCDC 1500508 and 1500509 contain the supplementary
crystallographic data for this paper. These data are provided free of charge
by the Cambridge Crystallographic Data Centre.
1-Bromoethyl-4-(4'-formylphenyl)pyridinium
tetrafluoroborate,
[4][BF4]: 4-(4'-Formylphenyl)pyridine (2.00 g, 0.0109 mol) was dissolved
in 1,2-dibromoethane (25 mL) and heated at 65 °C for 48 h. After cooling
to room temperature, the reaction mixture was added to CH2Cl2 (50 mL)
and stirred for 30 min. The solid was filtered, washed with CH2Cl2 then
Figure 7. Top, illustration of shuttling of the wheel in [3]rotaxane
[8DB24C8)2]2+ and bottom, variable temperature NMR spectra of
[8DB24C8)2]2+ in CD2Cl2.
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CH3CN, and air dried. After completely dissolving the solid in hot deionized
water (40 mL), a saturated sodium tetrafluoroborate solution (20 mL) was
added. The resulting solid was filtered, washed with deionized water, and
air dried. Yield: 1.7 g, 41%. MP: 187189 °C (Decomp.) 1H NMR (300 MHz,
CD3CN, 298 K)  = 10.13 (s, 1H), 8.79 (d, 2H, J = 6.9 Hz), 8.37 (d, 2H, J =
was removed on a rotary evaporator, the residue was re-dissolved in
dichloromethane (50 mL) and washed once with saturated sodium
tetrafluoroborate. After removal of the solvent, the residue was washed
with diethyl ether. The crude product was purified by flash column
chromatography (SiO2, dichloromethane/methanol, v/v = 10:1 to 1:1) to
give an orange solid. Yield: 94 mg, 66%. Single crystals were obtained by
slow diffusion of di-isopropyl ether into an acetonitrile solution of the
compound. MP: >250 °C (decomp). 1H NMR (500 MHz, CD3CN, 298 K) 
= 9.09 (d, 4H, J = 6.0 Hz), 8.25 (d, 4H, J = 7.5 Hz), 8.02 (d, 4H, J = 6.5
Hz), 7.78 (d, 4H, J = 8.0 Hz), 7.27 (s, 4H), 6.68 (m, 4H), 6.54 (m, 4H), 5.51
(s, 4H), 4.58 (m, 4H), 4.014.06 (m, 24H), 1.35 (d, 24H, J = 6.0 Hz). 13C
NMR (75 MHz, CD3CN, 298 K)  = 155.0, 148.0, 147.0, 145.7, 134.7, 132.2,
131.8, 129.1, 127.2, 124.0, 123.3, 121.3, 119.1, 112.4, 102.7, 72.3, 70.8,
70.3, 67.8, 57.8, 21.4. HR MS (ESI): m/z calcd for C74H86N6O12
[M2CF3SO3]2+: 625.3147; found: 625.3143.
6.9 Hz), 8.11 (m, 4H), 4.95 (t, 2H, J = 6.3 Hz), 3.98 (t, 2H, J = 7.2 Hz). 13
C
NMR (75 MHz, CDCl3, 298 K)  = 193.0, 156.6, 145.8, 139.6, 139.2, 131.1,
130.0, 126.5, 62.2, 31.2. Elemental analysis (%): Calculated for
C14H13NOBrBF4: C, 44.49; H, 3.47; N, 3.71. Found: C, 45.08; H, 3.45; N,
3.65.
2,6-Bis(4-(4'-formylphenyl)-1-pyridyl)ethane
tetrafluoroborate,
[5][BF4]2: 1-Bromoethyl-4-(4'-formylphenyl)pyridinium tetrafluoroborate
(1.00 g, 0.00265 mol) and 4-(4'-formylphenyl)pyridine (2.43 g, 0.0133 mol)
were dissolved in CH3CN (50 mL) and refluxed for 3 days. After the solvent
was removed, the residue was washed with CH2Cl2 (20 mL × 2) and air
dried. The solid was anion exchanged to the tetrafluoroborate salt by
heating the bromide salt in deionized water and adding NaBF4. The
resulting precipitate was collected by suction filtration, washed with
deionized water and air dried. Yield (1.03 g, 68 %). MP: >250 °C (decomp).
1H NMR (500 MHz, CD3CN, 298 K)  = 10.18 (s, 2H), 8.74 (d, 4H, J = 7.0
Hz), 8.42 (d, 4H, J = 7.0 Hz), 8.19 (d, 4H, J = 8.5 Hz), 8.15 (d, 4H, J = 8.5
Hz). 13C NMR (75 MHz, CD3CN) δ (ppm) = 192.9, 157.4, 146.0, 139.5,
139.5, 131.2, 130.0, 127.4, 60.0. HR MS (ESI): m/z calcd for
C27H22N2O5F3S [M-2BF4 + CF3SO3]+: 543.1196; found: 543.1191.
[7][BF4]2: The titled compound was isolated as a byproduct from the
synthesis of [2]rotaxane [7(DB24C8)][BF4]2. Yield: 22 mg, 23%. MP:
189192 °C (decomp). 1H NMR (500 MHz, CD3CN, 298 K)  = 8.63 (d, 4H,
J = 6.5 Hz), 8.37 (d, 4H, J = 6.5 Hz), 8.29 (d, 4H, J = 8.5 Hz), 8.10 (d, 4H,
J = 8.5 Hz), 7.18 (s, 4H), 5.11 (s, 4H), 4.55 (m, 4H), 1.33 (d, 24H, J = 6.0
Hz). 13C NMR (75 MHz, CD3NO2, 298 K)  = 157.2, 148.0, 144.8, 134.6,
132.6, 129.1, 127.4, 125.8, 123.1, 103.0, 72.6, 59.6, 21.1. HRMS (ESI):
Due to cleavage of the N-C pyridinium bond, the largest ion observed was
the protonated pyridine fragment, m/z calcd for C24H26N3O2: 388.2020;
found: 388.2025.
[10][BF4]2: [5][BF4]2 (503 mg, 0.886 mmol), 1,2-diamino-3,6-
di(4′-t-butylphenyl)benzene (6b) (110 mg, 0.295 mmol), and ZrCl4 (7.0 mg,
0.0300 mmol) were stirred in acetonitrile (30 mL) at room temperature for
24 h. After removal of the solvent on a rotary evaporator, the mixture was
extracted with hot tetrahydrofuran (30 mL × 2). Evaporation of the solvent
and washing with methanol gave the title compound as a yellow solid.
Yield: 166 mg, 61%. Mp 245–248 °C (decomp). 1H NMR (500 MHz,
CD3CN) δ (ppm) = 10.91 (s, 1H), 10.13 (br s, 1H), 8.72 (d, J = 7.0 Hz, 2H),
8.67 (d, J = 7.0 Hz, 2H), 8.43 (d, J = 8.6 Hz, 2H), 8.41–8.35 (m, 4H), 8.18–
8.03 (m, 8H), 7.73 (d, J = 7.5 Hz, 2H), 7.64 (d, J = 7.5 Hz, 2H), 7.60 (d, J
= 8.0 Hz, 2H), 7.55 (d, J = 7.5 Hz, 1H), 7.42 (d, J = 7.5 Hz, 1H), 5.13 (m,
4H), 1.41 (s, 18H). 13C NMR (125 MHz, CD3CN) δ (ppm) = 192.1, 156.9,
156.7, 150.7, 145.2, 144.9, 138.8, 138.7, 134.3, 134.2, 130.5, 129.2, 129.0,
128.8, 128.4, 128.1, 126.6, 126.1, 125.7, 125.3, 123.9, 121.9, 59.4, 59.1,
34.4, 30.6. HRMS (ESI): m/z calcd for C52H50N4O2+ [M-2BF4]2+: 373.1987;
found: 373.1973.
[8DB24C8][BF4]2: [5][BF4]2 (75.0 mg, 0.133 mmol) and dibenzo[24]-
crown-8 (240 mg, 0.532 mmol) were dissolved in acetonitrile (20 mL). After
the mixture was stirred at room temperature for 10 min, 1,2-diamino-3,6-
di(4′-t-butylphenyl)benzene (6b) (109 mg, 0.293 mmol) was added. After
the mixture was stirred for 30 min, ZrCl4 (6.2 mg, 0.0260 mmol) was added.
The mixture was allowed to stir open to the air overnight. After the solvent
was removed on a rotary evaporator, the residue was re-dissolved in
dichloromethane (50 mL) and washed with saturated sodium
tetrafluoroborate once. After removal of the solvent, the residue was
washed with diethyl ether. The crude product was purified by flash column
chromatography (SiO2, dichloromethane/acetone, v/v =3:1) to give a
yellow solid. Yield: 73 mg, 32%. Single crystals were obtained by slow
evaporation of an acetonitrile solution of the compound. MP: >250 °C
(decomp). 1H NMR (500 MHz, CD3CN, 298 K)  = 10.88 (s, 2H), 9.08 (d,
4H, J = 7.0 Hz), 8.36 (d, 4H, J = 8.5 Hz), 8.13 (d, 4H, J = 6.5 Hz), 8.00 (d,
4H, J = 7.0 Hz), 7.757.77 (m, 8H), 7.66 (d, 4H, J = 6.5 Hz), 7.62 (d, 4H,
J = 6.5 Hz), 7.57 (d, 2H, J = 8.0 Hz), 7.44 (d, 2H, J = 8.0 Hz), 6.69 (m, 4H),
6.57 (m, 4H), 5.51 (s, 4H), 4.004.08 (m, 24H), 1.44 (s, 18H), 1.43 (s, 18H).
13C NMR (125 MHz, CD3CN) δ = 155.4, 150.8, 146.9, 145.6, 135.5, 134.5,
133.6, 128.9, 128.6, 128.0, 124.0, 122.5, 121.2, 117.3, 112.3, 70.7, 70.2,
[10-H][BF4]3: Tetrafluoroborate diethyl ether complex (22 μL, 0.163 mmol)
was added to
a solution of [10][BF4]2 (150 mg, 0.163 mmol) in
dichloromethane (5.0 mL). After stirring for 5 min, the solvent was
evaporated under vacuum. Yield: 164 mg, quantitative. Mp 203–206 °C
(decomp). The product was used directly in the next step. 1H NMR (500
MHz, CD3CN) δ = 12.54 (br s, 2H), 10.12 (s, 1H), 8.83 (d, J = 6.9 Hz, 2H),
8.80 (d, J = 6.9 Hz, 2H), 8.47 (d, J = 6.9 Hz, 2H), 8.41 (d, J = 6.9 Hz, 2H),
8.30 (d, J = 8.5 Hz, 2H), 8.26 (d, J = 8.5 Hz, 2H), 8.13 (s, 4H), 7.75 (s, 2H),
7.71 (m, 8H), 5.20 (s, 4H), 1.42 (s, 18H). 13C NMR (125 MHz, CD3CN) δ =
192.2, 156.6, 156.0, 152.3, 150.2, 145.4, 145.3, 138.8, 138.7, 138.5, 132.4,
131.03, 130.4, 120.0, 129.2, 129.2, 128.6, 127.7, 127.4, 126.6, 126.5,
125.1, 117.4, 59.4, 59.3, 34.5, 30.6. HRMS (ESI): m/z calcd for
C52H50N4O2+ [M2BF4HBF4]2+: 373.1987; found: 373.1973. m/z calcd for
C54H54N5O3+ [M-3BF4+CH3CN]3+: 262.8104; found: 262.8055.
2+
67.7, 57.4, 34.3, 30.7. HRMS (ESI): m/z calcd for C102H110N6O8
[M2BF4]2+: 773.9203; found: 773.9353.
[8(DB24C8)2]2+: [6-H][BF4]3 (101 mg, 0.100 mmol) and dibenzo[24]-
crown-8 (270 mg, 0.600 mmol) were dissolved in nitromethane (4.0 mL)
and chloroform (8.0 mL). 1,2-Diamino-3,6-di(4′-t-butylphenyl)benzene (6b)
(41 mg, 0.110 mmol) was added followed by ZrCl4 (6.2 mg, 0.0260 mmol).
The mixture was allowed to stir open to the air overnight. After the solvent
was removed on a rotary evaporator, the residue was re-dissolved in
dichloromethane (50 mL) and washed once with a saturated sodium
tetrafluoroborate solution. After removal of the solvent, the residue was
washed with diethyl ether and air dried. Methanol (10 mL) was added and
the insoluble solid was filtered off. Upon adding trimethylamine (0.1 mL) to
the filtrate, a yellow solid was precipitated. The solid was filtered by
vacuum suction and washed with cold methanol and air dried. Slow
evaporation of a dichloromethane/ethyl acetate solution afforded the pure
product as a yellow solid. Yield: 91 mg, 42%. MP: >250 °C (decomp). 1H
[7DB24C8][BF4]2: [5][BF4]2 (57.0 mg, 0.100 mmol) and dibenzo[24]-
crown-8 (180 mg, 0.400 mmol) were dissolved in acetonitrile (15 mL). After
the mixture was stirred at room temperature for 10 min, 1,2-di-isopropoxy-
4,5-diaminobenzene (6a) (47.0 mg, 0.210 mmol) was added. After the
mixture was stirred for 30 min, ZrCl4 (4.9 mg, 0.0210 mmol) was added.
The mixture was allowed to stir open to the air overnight. After the solvent
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NMR (500 MHz, CD3CN) δ = 11.15 (s, 2H), 8.96 (d, J = 6.8 Hz, 4H), 8.52
(d, J = 8.3 Hz, 4H), 8.39 (d, J = 5.7 Hz, 4H), 8.17 (d, J = 8.3 Hz, 4H), 8.03
(d, J = 8.1 Hz, 4H), 7.71 (d, J = 8.1 Hz, 4H), 7.59 (d, J = 7.7 Hz, 2H), 7.55
(m, 8H), 7.43 (d, J = 7.7 Hz, 2H), 6.85 (m, 16H), 5.47 (s, 4H), 4.06 (m,
16H), 3.69 (s, 16H), 3.33 (m, 8H), 3.20 (m, 8H), 1.40 (s, 18H), 1.39 (s,
18H). 13C NMR (125 MHz, CD3CN, 298 K)  = 155.3, 151.3, 148.7, 147.4,
144.3, 134.3, 133.5, 129.2, 128.9, 125.9, 125.2, 125.0, 121.3, 121.2, 114.2,
112.0, 70.1, 69.6, 68.0, 57.4, 34.2, 30.6. HRMS (ESI): m/z calcd for
C126H143N6O163+ [M-2BF4+H]3+: 665.6859; found: 665.6862.
Leigh, V. Marcos, J. A. Morales-Serna, A. L. Nussbaumer, J. Am. Chem.
Soc. 2014, 136, 4905−4908.
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di(4′-t-butylphenyl)benzene (6b) (82 mg, 0.220 mmol) and ZrCl4 (4.7 mg,
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temperature overnight. The yellow precipitate was filtered and re-dissolved
in nitromethane (30 mL). After washing once with a saturated sodium
tetrafluoroborate solution, the solvent was removed. The crude product
was further purified by washing with methanol. Yield: 103 mg, 81%. MP:
223226 °C (decomp). 1H NMR (500 MHz, CD3CN, 298 K)  = 10.88 (s,
2H), 8.61 (d, 4H, J = 7.0 Hz), 8.45 (d, 4H, J = 8.5 Hz), 8.38 (d, 4H, J = 7.0
Hz), 8.108.13 (m, 8H), 7.74 (d, 4H, J = 8.5 Hz), 7.64 (d, 4H, J = 8.5 Hz),
7.60 (d, 4H, J = 7.0 Hz), 7.56 (d, 2H, J = 7.5 Hz), 7.43 (d, 2H, J = 7.5 Hz),
5.09 (s, 4H), 1.42 (s, 18H), 1.41 (s, 18H). 13C NMR (125 MHz, DMSO-d6,
298 K)  = 155.2, 151.3, 150.6, 150.2, 145.8, 142.5, 135.7, 135.5, 134.4,
134.2, 134.2, 130.4, 129.1, 128.9, 128.9, 128.6, 126.2, 125.6, 125.6, 125.1,
124.3, 121.9, 59.4, 34.8, 31.6. HRMS (ESI): m/z calcd for C78H78N6
[M-2BF4]2+: 549.3138; found: 549.3137.
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SJL is grateful for the awarding of NSERC of Canada Discovery
grants in support of this research. SJL also thanks the Canadian
Foundation for Innovation, the Ontario Innovation Trust and the
University of Windsor for support of the X-ray diffraction at the
University of Windsor. Additional support was provided to SJL
through the NSERC Canada Research Chair (CRC) program.
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Keywords: rotaxane • molecular shuttle • molecular switch •
benzimidazole • supramolecular chemistry
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Chemistry - An Asian Journal
10.1002/asia.201601179
FULL PAPER
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[3]Rotaxane molecular
shuttle: translocating
two macrocyclic
Kelong Zhu*[a] , V.
Nicholas Vukotic[b] and
Stephen J. Loeb*[b]
wheels simultaneously
on one single
Page No. – Page No.
dumbbell-shaped axle.
Acid-Base Switchable
[2]- and [3]Rotaxane
Molecular Shuttles with
Benzimidazolium and
Bis(pyridinium)
Recognition Sites
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