Reverse shuttling in a fullerene-stoppered rotaxane

Article abstract of DOI:10.1021/ol062277v

(Chemical Equation Presented) The preparation and characterization of a solvent-switchable rotaxane that shuttles in the opposite direction to that expected are reported. The reverse shuttling is confirmed by NMR spectroscopy and can be monitored by cycli

Full text of DOI:10.1021/ol062277v

ORGANIC
LETTERS
2006
Vol. 8, No. 22
5173-5176
Reverse Shuttling in a
Fullerene-Stoppered Rotaxane
Aurelio Mateo-Alonso,*,† Giulia Fioravanti,‡ Massimo Marcaccio,‡
Francesco Paolucci,*,‡ Dhiredj C. Jagesar,§ Albert M. Brouwer,§ and
Maurizio Prato*,†
INSTM, unit of Trieste, and Dipartimento di Scienze Farmaceutiche, UniVersita` degli
Studi di Trieste, Piazzale Europa 1, 34127 Trieste, Italy, Dipartimento di Chimica “G.
Ciamician”, UniVersita` di Bologna, V. Selmi 2, 40126 Bologna, Italy, and Van’t Hoff
Institute for Molecular Sciences, Faculty of Science, UniVersity of Amsterdam, Nieuwe
Achtergracht 129, 1018 WS Amsterdam, The Netherlands
amateo@units.it; prato@units.it; francesco.paolucci@unibo.it
Received September 15, 2006
ABSTRACT
The preparation and characterization of a solvent-switchable rotaxane that shuttles in the opposite direction to that expected are reported. The
reverse shuttling is confirmed by NMR spectroscopy and can be monitored by cyclic voltammetry. The electrochemically generated anions on
the fullerene moiety are stabilized by the closer proximity of the macrocycle.
The usefulness of rotaxanes relies directly on their abacus-
like structure. The search for new and efficient manners for
shuttling the macrocycle along the thread, which can be
employed to vary the chemical and physical properties of
the molecule, is an important field of research. Several
groups, active in this field, have developed a wide variety
of switches to shuttle the macrocycle between two different
and well-defined parts of the thread, which include solvent,1-5
acid-base,6-9 chemical,10 electrochemical,11,12 allosteric,13
counterion,14,15 and light-based switches.16-21 Although elec-
trochemical and light switches are among the best for
triggering the translocation of the macrocycle for practical
technological applications, solvent-switchable rotaxanes have
been shown to be very useful when efficient switching
without the need of further chemical transformation is
required. In this light, solvent-switchable rotaxanes have been
(3) Leigh, D. A.; Morales, M. A. F.; Perez, E. M.; Wong, J. K. Y.; Saiz,
C. G.; Slawin, A. M. Z.; Carmichael, A. J.; Haddleton, D. M.; Brouwer, A.
M.; Buma, W. J.; Wurpel, G. W. H.; Leon, S.; Zerbetto, F. Angew. Chem.,
Int. Ed. 2005, 44, 3062-3067.
(4) Gong, C. G.; Gibson, H. W. Angew. Chem., Int. Ed. 1997, 36, 2331-
2333.
† Universita` degli Studi di Trieste.
(5) Onagi, H.; Rebek, J. Chem. Commun. 2005, 4604-4606.
(6) Keaveney, C. M.; Leigh, D. A. Angew. Chem., Int. Ed. 2004, 43,
1222-1224.
‡ Universita` di Bologna.
§ University of Amsterdam.
(1) Da Ros, T.; Guldi, D. M.; Morales, A. F.; Leigh, D. A.; Prato, M.;
Turco, R. Org. Lett. 2003, 5, 689-691.
(2) Hannam, J. S.; Lacy, S. M.; Leigh, D. A.; Saiz, C. G.; Slawin, A.
M. Z.; Stitchell, S. G. Angew. Chem., Int. Ed. 2004, 43, 3260-3264.
(7) Elizarov, A. M.; Chiu, S. H.; Stoddart, J. F. J. Org. Chem. 2002, 67,
9175-9181.
(8) Badjic, J. D.; Balzani, V.; Credi, A.; Silvi, S.; Stoddart, J. F. Science
2004, 303, 1845-1849.
10.1021/ol062277v CCC: $33.50
© 2006 American Chemical Society
Published on Web 10/07/2006

Scheme 1. Typical Behavior of Solvent-Switchable Molecular Shuttles
used for the preparation of solvent and pH sensors.3 Rotaxane
opens new possibilities in fullerene-containing rotaxanes.
Here, we describe the unexpected reverse shuttling of the
new fullerene rotaxane.
1 is a good example of a solvent-switchable rotaxane
equipped with a C60 stopper (Scheme 1).1 In solvents that
do not disturb hydrogen bonds, the macrocycle resides over
the glycylglycine (GlyGly) template by complementary
hydrogen bond recognition. Instead, in solvents that interfere
with hydrogen bonds strongly such as DMSO, the macro-
cycle is positioned over the alkyl chain. This behavior is
typical of this type of rotaxanes and is independent of the
stopper used.3,5 Fullerenes possess well-defined chemical,
photophysical, and electrochemical properties.22,23 Such
properties can be affected by the presence or the position of
the macrocycle,1,24 providing a useful way to monitor
shuttling. To further investigate the applicability of rotaxanes
in research fields where fullerenes play an active role (i.e.,
photovoltaics, nonlinear optics, self-organization25), we
decided to prepare an equivalent and more accessible
rotaxane based on building block 526-29 (Scheme 2) as the
starting point, which is easier to process than N-H fullero-
pyrrolidine, used in the synthesis of rotaxane 1. Surprisingly,
in the analogous rotaxane 7, the macrocycle switched in the
opposite direction to that expected and positioned next to
the fullerene stopper. The opportunity to have a third station
Thread 6 displays a chemical structure equivalent to that
of rotaxane 1. It is comprised of a fullerene stopper attached
to a succinamide template, functionalized with an alkyl chain
that is equipped with a non-fullerenic stopper at the other
end. Succinamide and GlyGly templates can be considered
equivalent because they have the same internal degrees of
freedom and bind weakly the macrocycle compared to other
templates such as fumaramides,30 which allows easier trans-
location of the macrocycle. 2,2′-Diphenylethylamine was
coupled with 11-Boc-aminoundecanoic acid, previously
activated using EDC/HOBt (Scheme 2). This was followed
by deprotection of the amine and nucleophilic ring opening
of succinic anhydride to give 4. Then, fulleropyrrolidine 5
was coupled with 4 in pyridine, leading to the corresponding
fullerene thread 6.
Rotaxane 7 was assembled by simultaneous addition of
isophthaloyl chloride and p-xylylenediamine to a solution
of thread 6 in the presence of triethylamine. The structure
of the rotaxane 7 was confirmed by NMR (1H and COSY)
MS, IR, and UV-vis-NIR. The low solubility of thread 6
Scheme 2. Synthesis of the Reverse Solvent-Switchable Rotaxane 7
5174
Org. Lett., Vol. 8, No. 22, 2006

and rotaxane 7 in organic solvents did not allow the
acquisition of 13C spectra. The position of the macrocycle
along the thread can be monitored by NMR spectroscopy
due to the anisotropy effect of the aromatic macrocycle over
the thread. In CDCl3 and THF-d8, protons K (the assignments
correspond to the lettering shown in Scheme 2) were shielded
by nearly 1.4 ppm relative to the thread, showing that the
macrocycle was located over the succinamide template11
(Figure 1). In DMSO-d6, the typical shielding of some
protons in the aliphatic region was expected due to the
location of the macrocycle over the alkyl chain. Surprisingly,
the protons associated with the aliphatic chain underwent
negligible shielding. Instead, the fulleropyrrolidine (D) and
adjacent protons (G and J) were shifted upfield by as much
as 0.8 ppm, which evidenced that the macrocycle was
preferentially located in that region. A progressive deshield-
ing of protons D, G, and J was observed together with the
increase of the temperature in 1H NMR spectra recorded in
DMSO-d6 up to 110 °C. Nevertheless, the rest of the signals
remained mainly unaltered, indicative of faster pirouetting
due to the higher temperatures.
The photophysical properties of thread 6 and rotaxane 7
were investigated by measurements centered on the fullerene
stopper, which showed the typical characteristics of fulle-
ropyrrolidines (see Supporting Information). However, al-
most identical features were observed for thread 6 and
rotaxane 7 in the ground- and excited-state spectra recorded
(9) Garaudee, S.; Silvi, S.; Venturi, M.; Credi, A.; Flood, A. H.; Stoddart,
J. F. ChemPhysChem 2005, 6, 2145-2152.
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Paolucci, F.; Slawin, A. M. Z.; Wong, J. K. Y. J. Am. Chem. Soc. 2003,
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Chem., Int. Ed. 2006, 45, 1385-1390.
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K. M.; Stoddart, J. F. J. Am. Chem. Soc. 2004, 126, 9884-9885.
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Lett. 2004, 6, 4167-4170.
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J. K. Y.; Zerbetto, F. Angew. Chem., Int. Ed. 2003, 42, 5886-5889.
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F. J. Am. Chem. Soc. 2004, 126, 12210-12211.
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Flood, A. H.; Stoddart, J. F. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 1178-
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1
Figure 1. 400 MHz H NMR spectra of (a) thread 6 in CDCl3,
(b) rotaxane 7 in CDCl3, (c) thread 6 in DMSO-d6, and (d) rotaxane
7 in DMSO-d6. The peaks highlighted with stars correspond to
residual solvent peaks.
in DCM, PhCN, and DMSO. Such observations suggest that
the macrocycle does not have a strong interaction with the
fulleropyrrolidine stopper.
The electrochemical properties of thread 6 and rotaxane
7 were investigated by cyclic voltammetry in THF as the
hydrogen-bonding phase and in DMSO as the hydrogen-bond
disrupting phase (see also Supporting Information). The
voltammograms displayed five waves that correspond to the
reduction of the fulleropyrrolidine stopper. The CV behavior
of rotaxane 7 differs from that of thread 6 for anodic shifts
of the half-wave potential values (E1/2) for the first three
cathodic processes (Table 1). In THF, E1/2 values of rotaxane
7 were slightly affected by the presence of the macrocycle,
by comparison with those of thread 6, revealing weak
interactions between the macrocycle and C60 that stabilizes
the electrogenerated fullerene anions. When the electro-
chemical measurements were carried out in DMSO, more
distinct shifts were observed, being especially substantial in
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Zerbetto, F. Angew. Chem., Int. Ed. 2003, 42, 2296-2300.
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Org. Lett., Vol. 8, No. 22, 2006
5175

strong interactions in the neutral state between the macro-
cycle and the fullerene stopper, and thus, reverse switching
is explained as a result of solvation and solvophobic
interactions.2,3 Solvation of the peptide and the macrocycle
by DMSO molecules results in decomplexation of the two
components. In rotaxane 7, the peptidic template is connected
to the fulleropyrrolidine by two methylene groups (J and G),
which allows shuttling in both directions. Weak interactions
between the macrocycle and the spheroid might be respon-
sible for the co-conformation where the macrocycle is
positioned over the fulleropyrrolidine ring. Such interactions
cannot be confirmed by changes in the UV-Vis absorption
spectra. However, the presence of the macrocycle next to
the fullerene spheroid does stabilize the electrochemically
generated anions.
Table 1. Half-Wave Potential Values (E1/2, V) for the First
Three Cathodic Processes in Different Solvents
a
b
wave
6a
7a
∆E1/2
6b
7b
∆E1/2
A
B
C
-0.603 -0.599
-1.132 -1.127
-1.757 -1.748
0.004
0.005
0.009
-0.395 -0.387
-0.828 -0.805
-1.432 -1.421
0.008
0.023
0.011
a THF (0.1 M TBAPF6). b DMSO (0.1 M TBAPF6).
the second reduction wave (23 mV), which is in agreement
with the proximity of the macrocycle to the fulleropyrrolidine
stopper (Figure 2). The ∆E1/2 values suggest that π-π
This effect and its applicability are currently under
investigation because it provides a new way to induce
shuttling that might be useful for the preparation of novel
multistation rotaxanes.
Acknowledgment. We thank Prof. David A. Leigh
(Department of Chemistry, University of Edinburgh, Scot-
land, U.K.) for helpful discussions. The European Union
(RTN-EMMMA), Universita` di Trieste, INSTM, MIUR
(PRIN 2004, prot. 2004035502 and prot. 2004035330, FIRB
prot. RBNE033KMA), and NWO (project 700.51.014) are
gratefully acknowledged for funding.
Figure 2. Cyclic voltammograms of thread 6 (s) and rotaxane 7
(- -) in DMSO (0.1 M TBAPF6) at 1 V/s.
Supporting Information Available: Full experimental
details and characterization, NMR and COSY spectra, and
photophysical and electrochemical characterization. This
material is available free of charge via the Internet at
http://pubs.acs.org.
interactions might take place between the negatively charged
fullerene and the macrocycle.
1
The H NMR, photophysical, and electrochemical char-
acterization of rotaxane 7 does not support the existence of
OL062277V
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Org. Lett., Vol. 8, No. 22, 2006