Controlling Dual Molecular Pumps Electrochemically

Article abstract of DOI:10.1002/anie.201803848

Artificial molecular machines can be operated using either physical or chemical inputs. Light-powered motors display clean and autonomous operations, whereas chemically driven machines generate waste products and are intermittent in their motions. Herein, we show that controlled changes in applied electrochemical potentials can drive the operation of artificial molecular pumps in a semi-autonomous manner—that is, without the need for consecutive additions of chemical fuel(s). The electroanalytical approach described in this Communication promotes the assembly of cyclobis(paraquat-p-phenylene) rings along a positively charged oligomeric chain, providing easy access to the formation of multiple mechanical bonds by means of a controlled supply of electricity.

Full text of DOI:10.1002/anie.201803848

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Accepted Article
Title: Controlling Dual Molecular Pumps Electrochemically
Authors: Cristian Pezzato, Minh T Nguyen, Dong Jun Kim, Ommid
Anamimoghadam, Lorenzo Mosca, and Fraser Stoddart
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803848
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10.1002/anie.201803848
Angewandte Chemie International Edition
COMMUNICATION
Controlling Dual Molecular Pumps Electrochemically
Cristian Pezzato, Minh T. Nguyen, Dong Jun Kim, Ommid Anamimoghadam, Lorenzo Mosca, and J.
Fraser Stoddart*
Abstract: Artificial molecular machines can be operated using
either physical or chemical inputs. Light-powered motors display
clean and autonomous operations, whereas chemically-driven
machines generate waste products and are intermittent in their
motions. Herein, we show that controlled changes in applied
electrochemical potentials can drive the operation of artificial
molecular pumps in a semi-autonomous manner – that is, without
the need for consecutive additions of chemical fuel(s). The
electroanalytical approach described in this communication
promotes the assembly of cyclobis(paraquat-p-phenylene) rings
along a positively charged oligomeric chain, providing easy
access to the formation of multiple mechanical bonds by means
of a controlled supply of electricity.
Coulombic barrier (PY+) and a steric speed-bump (IPP) which is
attached, via a triazole linker (T), to a collecting-chain for the rings.
We have shown[14b] that the Mark II version operates much more
rapidly thanks to the well-tuned noncovalent bonding interactions
exerted between the PY+ and IPP units, allowing V2+ to attract and
then quickly repel the CBPQT4+ rings unidirectionally upon redox
switching.
a
PY+
IPP
V2+
Mark II pump constitution
T
CBPQT4+
b
c
Zn(PF6)2
Zn(s)
+3e−
Control over the operation of artificial molecular machines
(AMMs) has been achieved in the past by employing[1] either
physical or chemical inputs. In concert with the rise[2] of
overcrowded alkene-containing molecular motors, which operate
autonomously by absorbing light, AMMs based on mechanical
bonds[3] have experienced a paradigm shift in design principles[4]
– i.e., from switches to ratchets[5] – as well as in the way external
stimuli[6] – e.g., light irradiation, pH or redox changes – are
transduced into motion or other functions.[7] Early examples of
bistable mechanically interlocked molecules (MIMs) in the form of
simple [2]rotaxanes[8] and [2]catenanes[9], followed by molecular
elevators[10] and muscles[11], have led to more sophisticated[12]
rotary and linear molecular motors[13] and pumps[14] capable of
controlling the directional movements of their component parts on
the molecular scale. Recent advances[15] have drawn attention to
establishing autonomous AMMs which work as a result of the
consumption of chemical fuels, a typical property of biomolecular
machines. In closed systems, however, the capability for AMMs
to operate without generating waste products is seldom
observed[16] and remains an exclusive feature of light-driven[12b]
machines. Here, we describe how AMMs can be operated semi-
autonomously by controlling the supply of electrical current, thus
avoiding the accumulation of waste products during repetitive
cycles.
G
−3e−
Zn
NOPF6
or
or
−0.7 V
+1.4 V
NOPF6
G
NO(g)
Figure 1. a) Graphical representations of structural formulas for the Mark II pump
constitution and CBPQT4+. b) Schematic illustration of the pump’s stroke. Upon
reduction (either using Zn dust or electrochemically), V•+ and CBPQT2(•+) radical cations
associate to form a trisradical tricationic complex V•+  CBPQT2(•+), whereas upon re-
oxidation (either using NOPF6 or electrochemically), V2+ and CBPQT4+ coexist as a
metastable co-conformation with CBPQT4+ located alongside the IPP unit, which
subsequently rearranges into the corresponding rotaxane through energy dissipation. Grey
labels = chemical fuels; black labels = electrochemical inputs. c) Energetic landscapes of
the two redox states, illustrating the energy ratchet mechanism.
The ratchet mechanism of these pumps was operated by the
consecutive addition of redox reagents – Zn dust for the reduction
of V2+ and CBPQT4+ to their corresponding radical cations V•+ and
CBPQT2(●+), and NOPF6 for the reverse oxidation reactions
(Figure 1b and c, grey labels). It turned out to be difficult, however,
to perform multiple redox cycles on a solution of starting materials,
most likely because of interferences originating from the waste
products, namely Zn(PF6)2 and NO(g). Additionally, Zn dust has
been shown to over-reduce[17] viologen-based compounds to their
neutral forms, a flaw which could decrease the pumping efficiency.
Herein, we show that cycles of controlled-potential electrolysis
(CPE) can drive (Figure 1b and c, black labels) repetitively the
operation of the Mark II pump in solution. Moreover, we
demonstrate the double addition[7] of CBPQT4+ rings to a chain by
locating two Mark II pumps at both ends of a dicationic oligomer.
First of all, we focused our attention on designing an
oligomeric dual pump – referred hereafter to as DP8+ – which
comprises (Scheme 1) two Mark II pumps attached, by means of
a triazole linker, to a 36-carbon atom chain incorporating two
dimethylammonium centers. This chain is expected to act as the
axle for the collection of multiple CBPQT4+ rings. We chose this
Previously, we have reported Mark I[14a] and II[14b] versions of
artificial molecular pumps – i.e., AMMs capable of driving the
confinement
of
multiple
cyclobis(paraquat-p-phenylene)
(CBPQT4+) rings onto a short oligomethylene chain. These pumps
comprise (Figure 1a) a viologen unit (V2+) positioned between a
[*]
Dr C. Pezzato, Dr. M. T. Nguyen, Dr. D. J. Kim, Dr. O.
Anamimoghadam, Dr. L. Mosca, Professor J. F. Stoddart
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
E-mail: Stoddart@northwestern.edu
Professor J. F. Stoddart
Institute of Molecular Design and Synthesis, Tianjin University,
Nankai District, Tianjin 300072 (China)
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201803848
Angewandte Chemie International Edition
COMMUNICATION
quaternary ammonium-based chain for three main reasons: they
are (i) facile synthesis, (ii) high solubility in MeCN of the
corresponding PF6¯ salt and (iii) the possibility of investigating[18]
the enthalpically and entropically unfavorable assembly of
polycationic species. The axle was prepared (Scheme 1) in good
overall yield (67%) in three steps, starting from 1,12-
dibromododecane by reacting it with 1 to give 2•2Br, which was
converted into 3•2PF6 after reaction with NaN3 and counterion
exchange (NH4PF6 / H2O). DP•8PF6 was isolated in satisfactory
yield (120 mg, 78%), following a click reaction between 3•2PF6
and the pump precursor[14b] 4•2PF6. See the Supporting
Information for more details.
total charge (Q) transferred as a function of time: ca. 7 C were
consumed by the solution upon reduction, which is in good
agreement with the expected value (ca. 6 C), assuming the
concomitant monoreduction of CBPQT4+ and DP8+.[21] In this case,
we observed a product distribution of (22 : 78) ± 9 in favor of the
[3]R16+, an observation which confirms that the kinetics of
association between the two CBPQT2(●+) rings and DP(2●)(6+) is
indeed the rate-determining step.
a
b
POTENTIOSTAT
RE WE CE
+
+
+
T
−0
10 min
20 min
N2
Me2CO
NaN3
+
6
6
6
6
Reflux
6
6
MeCN
6
6
1
2•2Br
3•2PF6
89%
78%
Me2NH
THF / −78 °C
Cu(MeCN)4PF6
TBTA / Me2CO
97%
77%
4•2PF6
+1 4
10 min
10 min
6
6
6
+
+
DP•8PF6 (DP8+
)
Mark II pump
quaternary ammonium-based chain
Mark II pump
Scheme 1. Synthetic route for the preparation of the dual molecular pump DP8+, which
is isolated as its PF6¯ salt. A graphical representation of DP8+ is provided below the
2
0
c
structural formula
.
-2
Next, we performed CPE experiments (Figure 2a) on a
solution of DP•8PF6 (50 μM) and CBPQT•4PF6 (1.0 mM) in dry
and degassed MeCN (30 mL) maintained at 40 °C. Our approach
entailed the alternation of two constant potentials, namely (i) –0.7
V for the reduction[19] of bipyridinium dications to radical cations,
and (ii) +1.4 V for the reverse oxidation[20] process. In a first
experiment, we carried out a 20-min cycle where reduction (10
min) at –0.7 V was followed immediately by re-oxidation (10 min)
at +1.4 V. 1H NMR Spectroscopic analysis of the reaction mixture
revealed the presence of a third species, along with CBPQT4+ and
DP8+. It was shown to be a [3]rotaxane, hereafter referred to as
[3]R16+, that is DP8+ < (CBPQT4+)2. The product distribution,
however, was only (75 : 25) ± 6 for DP8+ : [3]R16+, as calculated
-4
-6
-8
-10
0
0
300
600 0
300
600
10 30
40
time / min
DP8+
[3]R16+
[5]R24+
100
75
50
25
0
d
1
from H NMR integrations. See Supporting Information. Bearing
in mind that, after re-oxidation, the stroke of the Mark II pump is
very rapid[14b] at 40 °C, we ascribed this result to kinetically slow
complex formation between the CBPQT2(●+) rings and DP(2●)(6+)
.
Thus, we modified (Figure 2b) the CPE protocol introducing
“resting” periods of 20 and 10 min after reduction and re-oxidation,
respectively, which allow for the completion of (i) complex
formation before re-oxidation and (ii) co-conformational
isomerization before starting a second cycle. In a repetition of the
experiment, we subjected the same solution firstly to a potential
of –0.7 V for 10 min, followed by 20 min of off-current, and then
to a potential of +1.4 V for 10 min, followed by 10 min of off-current.
The redox processes were followed (Figure 2c) by monitoring the
1a
1b
2b
# of cycles
Figure 2. a) Representative photograph of the CPE setup used in this work. b) Schematic
illustration of the electrochemically controlled pump operation. c) Total charge (Q)
consumed and released by the working solution during reduction (− 0.7 V) and re-
oxidation (+1.4 V), respectively. d) Product distribution obtained after one or two cycles
as determined by 1H NMR integrations. Experimental conditions (CPE steps), 1a: − 0.7
V for 10 min followed by +1.4 V for 10 min; 1b and 2b: − 0.7 V for 10 min followed 20
min of off-current, and +1.4 V for 10 min followed by 10 min of off-current.
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10.1002/anie.201803848
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COMMUNICATION
13
15
CH2
30
14
7
9
18 20 22 24 26 28
32 34 36
5
17
16
β
1
α
12
11
6
8
19 21 23 25 27 29 31 33 35
4
10
2
Xyl
2
3
H2O
x
3
15
20-27
30
4/5
16
33-36
6/9
8
10
19
7
1
17
29/31
14
18
13 12
2
28/32
11
a
Xyl'
CH2'
β'
α'
19/27
26 20 25 21 24 23 22
b
c
Xyl''
CH2'/CH2''
β''
α''
19
27
26
20
25 21 24
23
22
10
9
8
7
6
5
4
3
2
1
0
-1
-2
δ / ppm
Figure 3. Full 1H NMR spectra (500 MHz, CD3CN, 298 K) of (a) DP•8PF6, (b) [3]R•16PF6 and (c) [5]R•24PF6 with all the proton resonances assigned. Each proton in the structural
formulas of the dumbbell and ring components is labelled to aid rapid interpretation of the 1H NMR spectra.
When we subjected the same solution to two consecutive redox
cycles, H NMR spectroscopic analysis revealed a decrease in
Coulombic repulsions between them and the central portion of
[3]R24+. Notwithstanding the low concentration employed to
compensate for the adsorption phenomena[23] at the working
electrode, an average pumping efficiency of close to 70% was
attained. In addition, no formation of [2]- or [4]rotaxanes was
observed, indicating that the two pumps work independently.
After each experiment, [3]R16+ and [5]R24+ were isolated as
their PF6¯ salts after reverse-phase chromatography and
counterion exchange (NH4PF6 / EtOH). The constitutions of
[3]R•16PF6 and [5]R•24PF6 can be appreciated at a glance from
1
the intensities of the resonances associated with [3]R16+ in favor
to a new species which was found to be a [5]rotaxane, hereafter
referred to as [5]R24+, that is DP8+ < (CBPQT4+)4. Repetition of this
experiment gave an average product distribution of (53 : 47) ± 13
for [3]R16+ : [5]R24+, indicating a small decrease (from 78 to 60%)
in the efficiency of the second pumping cycle. This observation
can be ascribed to the relatively short length of the 36-carbon
atom chain which, along with two CBPQT4+ rings, can facilitate (i)
the folding of radical cationic form of [3]R16+ sustained by radical-
radical interactions[22] developing amongst the two rings and the
two pumps, thus limiting the association of subsequent rings even
under reducing conditions and/or (ii) the dissociation of the
additional CBPQT4+ rings upon re-oxidation, because of
1
Figure 3, which compares the respective H NMR spectra with
that of DP•8PF6 (Figure 3a). In the case of [3]R•16PF6, inspection
of the 1H NMR spectrum (Figure 3b) reveals one new set of four
proton resonances – labelled ', ', Xyl' and CH2' – suggesting a
symmetrical situation where the two CBPQT4+ rings added to the
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10.1002/anie.201803848
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COMMUNICATION
chain are homotopic and reside on the oligomethylene chain the chain of the dumbbell. We envision that applying this
adjacent to the triazole linkers. This conclusion is supported by approach in polymer science could lead to the emergence of
the progressive upfield shifts exhibited by resonances for protons materials with unprecedented physicochemical properties.
on C-19 up to C-27, an observation which contrasts with the Currently, we are targeting the synthesis and investigation of
almost unchanged chemical shifts for protons on C-33 up to C-36. polycationic poly[n]rotaxanes.
In particular, the significant shielding of the protons on C-22
defines the relative location (co-conformation) of the rings on
[3]R16+
.
This predominant co-conformation for [3]R16+ is
supported by the upfield shifts for protons on C-16, C-17, C-18
and the concomitant downfield shifts for those on C-28 up to C-
32, all of which suggest shuttling by the CBPQT4+ rings in the
regions between the IPP units and the quaternary ammonium
centers. The 1H NMR spectrum (Figure 3c) of [5]R•24PF6
displays only a new set of four proton resonances – labelled '',
'', Xyl'' and CH2'' – indicating the presence of two distinct pairs of
CBPQT4+ rings symmetrically distributed along the chain. The
dramatic upfield shift for the singlet arising from the proton on C-
17, in keeping with the substantial upfield shifts observed for the
resonances associated with protons on C-11, C-14, C-16 and C-
18, suggests that each of the two additional rings encircles the
triazole linkers adjacent to the IPP units. Homonuclear Correlation
spectroscopy (COSY) indicates that the progressive shielding,
experienced by the methylene protons across the IPP units and
the quaternary ammonium centers, follows the same pattern as
that observed in the case of [3]R16+. This observation, together
with (i) the small downfield shifts and (ii) the signal broadening for
protons in the vicinity of C-30, suggests that the CBPQT4+ rings
are not prevented from shuttling across the two quaternary
ammonium centers. Compelling evidence for the localization of
the two homotopic pairs of heterotopic (inner as opposed to outer)
rings emerge from Nuclear Overhauser Effect spectroscopy
(NOESY), which shows through-space interactions between the
protons of the oligomethylene chains and ', and between the
protons at C-15 and both '' and Xyl''. See Supporting Information.
In order to obtain more insights about the shuttling mechanism we
carried out variable temperature (VT) 1H NMR spectroscopy. In a
temperature range from –40 °C to +40 °C, the symmetry of [5]R24+
remains intact. Although the resonances associated to ', ', Xyl'
and CH2', together with those for the protons in the vicinity of the
quaternary ammonium centers, broaden, they do not give rise to
separate signals even at –40 °C. The resonances do, however,
sharpen at higher temperature, presumably as a result of the fast
exchange of their protons on the 1H NMR timescale. All these
observations support a shuttling (Figure 3c) of the rings along the
chain in a manner that preserves molecular symmetry.
Experimental Section
General Procedure. DP•8PF6 (4 2 mg, 1 5 μmol) and CBPQT•4PF6 (33
mg, 30 μmol) were dissolved in MeCN (30 mL) containing
tetrabutylammonium hexafluorophosphate TBAPF6 (0.1 M) as the
supporting electrolyte, and transferred into a BASi® bulk electrolysis cell
under N2 atmosphere. The solution was maintained at 40 °C and vigorous
stirring (1000 rpm) while recording Coulometric responses during the CPE
cycle(s) (Figure 2b). The resulting mixture was concentrated by rotary
evaporation and washed with CH2Cl2 (3 × 40 mL) and EtOH (3 × 40 mL)
to remove excess TBAPF6. The obtained crude product was purified by
reverse-phase chromatography (C18-capped SiO2, H2O/MeCN 0.1%
CF3CO2H 0-100%). Fractions containing the product were kept to dryness
by N2 flux and a saturated solution of NH4PF6 in EtOH was added till
precipitation was complete (ca. 20 mL). The product was collected,
washed with EtOH (3 × 20 mL) and dried overnight under vacuum. See
Supporting Information for the full characterization.
Acknowledgements
This work was supported by National Science Foundation (NSF;
CHE-1308107) and Northwestern University (NU). The authors
acknowledge the Integrated Molecular Structure Education and
Research Center (IMSERC) at NU for providing access to
equipment for the experiments. We would like to thank Professors
D. R. Astumian and M. Frasconi for numerous helpful discussions.
Keywords: electrochemistry • molecular machines • rotaxanes •
radicals • supramolecular chemistry
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10.1002/anie.201803848
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Cristian Pezzato, Minh T. Nguyen, Dong
Jun Kim, Ommid Anamimoghadam,
Lorenzo Mosca, and J. Fraser Stoddart*
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G
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Page No. – Page No.
OX
(+1.4 V)
RED
(−0.7 V)
Controlling Dual Molecular Pumps
Electrochemically
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