A molecular shuttle driven by fullerene radical-anion recognition

Article abstract of DOI:10.1002/chem.201201264

We describe a electrochemically driven molecular shuttle, in which shuttling takes place by means of fullerene radical-anion recognition that results in a very low operation potential (E_1/2=-0.580 V vs. decamethylferrocene). This has been achieved by introducing positive charges on the macrocycle, which strengthen the existing π-π interactions between the macrocycle and the electrogenerated fullerene radical anion by means of an electrostatic component. In addition, the synthesis of such a molecular shuttle has been accomplished by developing a new synthetic approach that exploits the controlled translocation of the macrocycle as a selective protecting group. Copyright

Full text of DOI:10.1002/chem.201201264

FULL PAPER
DOI: 10.1002/chem.201201264
A Molecular Shuttle Driven by Fullerene Radical-Anion Recognition
Francesco Scarel,[a] Giovanni Valenti,[b] Sudhakar Gaikwad,[a] Massimo Marcaccio,[b]
Francesco Paolucci,*[b] and Aurelio Mateo-Alonso*[a, c, d]
Abstract: We describe a electrochemi-
cally driven molecular shuttle, in which
shuttling takes place by means of full-
erene radical-anion recognition that re-
sults in a very low operation potential
(E1/2 =À0.580 V vs. decamethylferro-
cene). This has been achieved by intro-
ducing positive charges on the macro-
cycle, which strengthen the existing p–
p interactions between the macrocycle
and the electrogenerated fullerene rad-
ical anion by means of an electrostatic
component. In addition, the synthesis
of such a molecular shuttle has been
accomplished by developing a new syn-
thetic approach that exploits the con-
trolled translocation of the macrocycle
as a selective protecting group.
Keywords: electrochemistry · full-
erenes · molecular shuttles · rotax-
anes · supramolecular chemistry
Introduction
chemically switchable molecular shuttles have been shown
to be very useful components in nanotechnology and have
been applied to prepare artificial molecular muscles,[14]
memory devices,[15,16] and nanovalves for drug release,[17]
among others. In such electrochemically driven molecular
shuttles, easily accessible redox potentials are key to ensure
a low-potential operation.[9] Recently, we have reported the
use of C60 stoppers[18–23] in rotaxanes as units to switch the
macrocycle reversibly by exploiting the existing p–p interac-
tions between electrochemically generated fullerene anions
and the macrocycle. Indeed, C60 can be reduced reversibly
with up to 6 electrons and possess easily accessible reduc-
tion potentials.[24] However, because p–p interactions are
rather weak, the complete translocation of the macrocycle
requires the formation of the trianion and thus the applica-
tion of a large potential (E1/2 =À1.750 V vs. decamethylfer-
rocene).
Bistable rotaxanes or molecular shuttles are a unique plat-
form to perform precise and complex switching operations
derived from sub-molecular motion.[1] On this type of mo-
lecular interlocked systems, which display the same structur-
al features of an abacus, switching consists of a position
change of the ring component (macrocycle) along two differ-
ent parts of the linear component (thread). A strategy to de-
velop practical applications based on molecular shuttles fo-
cuses on the use of electrons to control the position of the
macrocycle,[2–13] since electrons can be easily introduced or
removed by applying an electric or an electrochemical po-
tential and also allow the interfacing of such molecular sys-
tems with existing electronic technologies. In fact, electro-
Although there is limited data or theory that explains the
attracting interactions between p-receptors and anions[25] or
radicals[26] with which to develop recognition motifs for elec-
trogenerated fullerene anions, it seemed plausible that a
more suitable macrocycle able to bind effectively fullerene
radical-anions, will provide not only a low-potential operat-
ed molecular shuttle but also an insight into the p–p interac-
tions with radical anions. Herein, we describe a fullerene-
stoppered molecular shuttle (rotaxane 3, Scheme 1) in which
shuttling is triggered by applying a very low reduction po-
tential (E1/2 =À0.580 V vs. decamethylferrocene). This has
been achieved by introducing two positive charges on the
macrocycle, which strengthen the existing p–p interactions
between the macrocycle and the electrogenerated fullerene
radical anion by means of an electrostatic component, which
results in an excellent recognition motif for fullerene radical
anions. Most importantly the synthesis of rotaxane 3 has
been accomplished by applying a new synthetic approach
that exploits the controlled translocation of the macrocycle
as a protecting group. In other words, the reversible transla-
[a] F. Scarel, S. Gaikwad, Prof. A. Mateo-Alonso
Freiburg Institute for Advanced Studies (FRIAS)
School of Soft Matter Research
Albert-Ludwigs-Universitꢀt Freiburg
Albertstr. 19, 79104 Freiburg im Breisgau (Germany)
and
Institut fur Organische Chemie und Biochemie
Albert-Ludwigs-Universitꢀt Freiburg
Albertstr. 21, 79104 Freiburg im Breisgau (Germany)
E-mail: amateo@frias.uni-freiburg.de
[b] G. Valenti, Dr. M. Marcaccio, Prof. F. Paolucci
Dipartimento di Chimica “G. Ciamician”, Universitꢁ di Bologna
Via Selmi 2, 40126 Bologna (Italy)
E-mail: francesco.paolucci@unibo.it
[c] Prof. A. Mateo-Alonso
POLYMAT, University of the Basque Country UPV/EHU
Avenida de Tolosa 72, E-20018 Donostia-San Sebastian (Spain)
E-mail: amateo@polymat.eu
[d] Prof. A. Mateo-Alonso
Ikerbasque, Basque Foundation for Science
E-48011 Bilbao (Spain)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201264.
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[D6]DMSO the hydrogen bonds
between the diamide station
AHCTUNGTRENNUNG
and the macrocycle are weak-
ened and this allows the macro-
cycle to switch to the other end
of the thread and p-stack to the
fullerene (translational isomer
2B, Scheme 1). This is con-
firmed by up-field shifts of pro-
tons E of the rotaxane 2 with
respect to thread 1 (Figure 1d
and e).
In principle, the alkylation of
the pyridine groups on the mac-
rocycle of rotaxane 2 should
provide the desired rotaxane 3.
However, it is known that the
fulleropyrrolidine ring can also
be alkylated to give fulleropyr-
rolidinium salts.[29] We foresaw
that the encapsulation of the
fulleropyrrolidine by the mac-
rocycle (translational isomer
2B) could act as a protective
group inhibiting alkylation of
the fulleropyrrolidine, since
there are a examples in which
encapsulation have been used
to protect threaded substrates
Scheme 1. Synthesis of 2 and 3. i) Dinicotinoyl chloride, p-xylylene diamine, NEt3, CHCl3, room temperature,
32%; ii) dodecyl iodide, [D6]DMSO, 608C, 48 h, 8–14%.
from
chemical
degrada-
tion.[22,29–32] In addition, since
tion of the macrocycle over a reactive site has been used to
inhibit an undesired reaction by means of selective encapsu-
lation.
the macrocycle is only mechanically bonded the protection
should be perfectly reversible by changing the position of
the macrocycle, providing new perspectives for mechanically
interlocked molecules in organic synthesis. The reaction of
thread 1 with dodecyl iodide[30] in DMSO yields fulleropyr-
rolidinium salt 4 (Scheme 1). Conversely, when rotaxane 2
was allowed to react with an excess of dodecyl iodide (3, 27
and 45 equivalents) in DMSO, only rotaxane 3 was obtained
after chromatographic purification and the formation of the
Results and Discussion
Rotaxane synthesis—shuttling as a protecting group: Rotax-
ane 3 was synthesised in two steps from thread 1, as de-
A
corresponding trialACTHNGUTRENNUkG ylACHTNUGTRENaNUGN tion by-product was not detected.
thread 1[18] following the methodology introduced by Leigh
et al.,[27] in which the diamide residue of thread 1 acts as
template in the synthesis of benzylic amide macrocycles.
The reaction of thread 1 with dinicotinoyl chloride and p-xy-
lylene diamine yielded rotaxane 2 with two pyridyl rings on
the macrocycle. Rotaxane 2 displays solvent-switchable be-
haviour similar to other fullerene rotaxanes.[18–20] In fact,
NMR experiments confirmed the existence of rotaxane 2 in
two solvent-dependent translational isomers. In solvents
with low hydrogen-bond basicity, such as CDCl3, the macro-
cycle stays preferentially on the diamide station by hydro-
gen-bond interactions between the macrocycle and the di-
To ensure that the formation of the trialkylation by-prod-
uct is truly inhibited and thus that the encapsulation of the
macrocycle is acting as a protective group, the same reac-
tions (with 3, 27 and 45 equivalents of dodecyl iodide) were
carried out in [D6]DMSO and the reaction mixtures were di-
rectly analysed by NMR spectroscopy at different times and
compared with the spectrum of thread 4. The NMR spec-
trum of thread 4 shows a distinctive AB system at d=
5.5 ppm that corresponds to protons E of the alkylated full-
eropyrrolidine (Scheme 1 and Figure S1 in the Supporting
Information). Therefore, by monitoring the d=5.5 ppm
region of the reaction mixtures, it is possible to detect and
quantify the alkylation (if any) of the fulleropyrrolidine of
rotaxane 2. As a matter of fact, when the experiments were
carried out on rotaxane 2, no traces of the characteristic AB
system of the alkylated fulleropyrrolidine were observed
ACHTUNGTRENNUNGamide (translational isomer 2A, Scheme 1). NMR confirms
this, since protons C and G in rotaxane 2 appear shielded
with respect to thread 1 because of the anisotropic effect of
the macrocycle (Figure 1a and b). Conversely, in
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Fullerene Radical-Anion Recognition
FULL PAPER
(Figure S1 in the Supporting Information), even after pro-
longed reaction times, confirming that the macrocycle is
indeed inhibiting the alkylation reaction of the fulleropyrro-
lidine.[35] The low yields of the reaction are associated with
the diminished stability of rotaxane 2 in polar solvents,
which has been previously noted for similar rotaxanes even
at room temperature.[19] This is in agreement with the NMR
spectra of the crude reaction mixture that displays the for-
mation of multiple new peaks that we were unable to assign
(Figure S1 in the Supporting Information), and also with the
fact that no traces of rotaxane 2 were recovered after chro-
matography.
Rotaxane 3 displays the same solvent behaviour as rotax-
ane 2, which is consistent with the ability of the macrocycle
to inhibit the alkylation of the fulleropyrrolidine by dodecyl
iodide. In [D6]DMSO, the macrocycle sits on top of the full-
eropyrrolidine (translational isomer 3B, Scheme 1), as con-
firmed by the upfield shifts of protons E of rotaxane 3 with
respect to thread 1 (Figure 1d and f). Conversely, in sol-
vents, such as CDCl3 or [D8]THF, the macrocycle binds di-
AHCTUNGTREGaNNUN mide by hydrogen bonding (translational isomer 3 A,
Scheme 1), as confirmed by the upfield shifts observed for
proton signals C and G of rotaxane 3 with respect to thread
1 (Figure 1a and c).
Shuttling by fullerene radical-anion recognition: The ability
of rotaxanes 2 and 3 to switch electrochemically has been
studied by cyclic voltammetry. The voltammograms showed
three reduction waves centred on the fullerene (I, II and III,
Figure 2 left), which correspond to the fullerene radical
anion, dianion and trianion, respectively. Half-wave poten-
tial shifts (DE1/2) to less negative potentials are observed in
all the redox waves of rotaxane 2 when compared with
those of thread 1 (Figure 2 left), which was used a reference.
Such anodic shifts provide evidence that p–p stacking inter-
actions between the macrocycle and the fullerene stopper
are taking place, which are a direct indication of electro-
chemically induced shuttling, as previously described.[8,10,18]
Figure 2. Cyclic voltammograms of rotaxane 2 (gray solid line, left) and 3
(gray solid line, right), shown with thread 1 (black solid line, left and
right) and iodo N-dodecyl-3,5-dicarboxypyridinium bisbenzylamide S2
(black dashed line, right, see also the Supporting Information for synthe-
sis and characterisation). 0.5 mm in a 0.07m NBu4PF6, THF. Working
electrode Pt disk 125; scan rate=1 VsÀ1; T=258C. Potential vs. decame-
thylferrocene.
Figure 1. Partial NMR spectra of thread 1, rotaxane 2 and rotaxane 3 in
CDCl3 (a, b, and c, respectively) and DMSO (d, e, and f, respectively).
The assignments correspond to the lettering shown in Scheme 1.
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F. Paolucci, A. Mateo-Alonso et al.
2A and 2B in the equilibrium
(Keq =1). After the second and
third reductions, the interac-
tions between the negatively
charged fullerene and the mac-
rocycle become more apACHTUNGTRENNUNGparACHTUNGTRENNUNGent,
which favour the formation of
2B over 2A both for the dia-
nion (Keq =10) and the trianion
(Keq =80), respectively.
The same experiments per-
formed on rotaxane 3 revealed
a different behaviour. Besides
the reduction waves of the full-
erene (I, II and III), an addi-
tional reduction wave was ob-
served (Im), which falls at a sim-
ilar potential as the fullerene
dianion (Figure 2 right). This
reduction process (Im) was un-
doubtedly assigned to the re-
duction of macrocycle by com-
parison with iodo N-dodecyl-
3,5-dicarboxypyridinium bisben-
zylamide (compound S2, see
the Supporting Information),
which was synthesised and used
as a reference. The reduction
wave of the radical anion of ro-
taxane 3 is shifted to even less
negative potentials (DE1/2
=
À41 mV) than for rotaxane 2
(DE1/2 =À30 mV) when com-
pared with thread 1. This is not
only a direct indication of elec-
trochemically induced shuttling,
but also, most importantly, an
illustration of more efficient
shuttling at the radical-anion
state.
Simulations were carried out
only on the first reduction wave
due to the simultaneous reduc-
Scheme 2. Square scheme mechanism describing the behaviour of rotaxane 2.
tion of the macrocycle and the
fullerene upon the formation of
Simulation of the voltammograms using the square scheme
mechanism[36] set out in Scheme 2 allows for the reproduc-
tion of the voltammograms and the estimation of the Keq be-
tween translational isomers 2A and 2B for each redox state.
As supported by both NMR spectroscopy and simulations,
translational isomer 2A dominates in the neutral state
(Keq =10À1).[37]
Consecutive increase of the negative charge shifts the
equilibrium towards 2B. Upon the first reduction, the inter-
action between the fullerene radical anion and the macrocy-
cle is sufficiently strong to compete with the hydrogen-bond-
ing station (Scheme 2), with equimolar concentrations of
the dianion. As expected, simulations of the first reduction
wave of rotaxane 3 using the square scheme mechanism set
out in Scheme 3, revealed that 3A dominates in the neutral
state (Keq =10À1),[37] but upon reduction of the fullerene
stopper to the radical anion (E1/2 =À0.580 V vs. decamethyl-
ferrocene), the equilibrium is completely shifted and domi-
nated by 3B (Keq =5). This illustrates the large affinity be-
tween the fullerene radical anion and the macrocycle, which
is able to overcome four hydrogen bonds to switch.
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Overall we have developed a new type of fullerene-driven
molecular shuttle, in which shuttling is achieved by an effi-
cient strategy based on fullerene radical-anion recognition
that results in
a very low operation potential (E1/2 =
À0.580 V vs. decamethylferrocene). Molecular shuttles oper-
ated under such mild conditions represent a promising step
forward towards applications in many fields of nanotechnol-
ogy. Also, the fact that only one electron is required to oper-
ate the shuttle paves the way for developing light-driven sys-
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electron donors upon irradiation, which is currently being
explored in our laboratory. In addition, we have described
the first synthetic approach that exploits the reversible
translation of the macrocycle over a reactive site to inhibit
an undesired reaction by means of selective encapsulation.
The use of the macrocycle as a protective group reported
here provides new perspectives for mechanically interlocked
molecules in organic synthesis.
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Acknowledgements
This work was carried out with support of the Freiburg Institute for Ad-
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K
eq =10À1 on the simulations provides the same values of Keq for the
charged redox states. However, the deviations between the experi-
mental and the calculated E1/2 become more apparent when using
K
eq =10À2. For this reason and taking into account the structural dif-
ferences on the macrocycle with respect to those systems previously
reported,[8,9,11,18] we have used Keq =10À1 since it provides a better
fitting with the experimental measurements. This might also indicate
that the presence of dinicotinamide rings instead of diisophthala-
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Received: April 13, 2012
Revised: June 5, 2012
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Fullerene Radical-Anion Recognition
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Molecular Recognition
F. Scarel, G. Valenti, S. Gaikwad,
M. Marcaccio, F. Paolucci,*
A. Mateo-Alonso* ........... &&&& — &&&&
A Molecular Shuttle Driven by Full-
erene Radical-Anion Recognition
Molecular shuttles: An electrochemi-
cally driven molecular shuttle, in which
shuttling takes place by means of full-
erene radical-anion recognition that
results in a very low operation poten-
tial (E1/2 =À0.580 V vs. decamethylfer-
rocene) is described (see scheme). This
has been achieved by introducing posi-
tive charges on the macrocycle, which
strengthen the existing p–p interac-
tions between the macrocycle and the
electrogenerated fullerene radical
anion by means of an electrostatic
component.
A fullerene-driven molecular
shuttle…
…in which shuttling is achieved by
implementing a strategy based on
fullerene radical-anion recognition
is reported. The introduction of
positive charges on the macrocycle
strengthened the existing p–p
interactions between the macro-
cycle and the electrogenerated full-
erene radical anion by means of an
electrostatic component. For more
details, see the Full Paper by A.
Mateo-Alonso et al. on page &
& ff.
Chem. Eur. J. 2012, 00, 0 – 0
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!