Redox behavior of ferrocene-containing rotaxane: Transposition of the rotaxane wheel by redox reaction of a ferrocene moiety tethered at the end of the axle

Article abstract of DOI:10.1021/ol049817d

Matrix presented. A rotaxane with a ferrocene moiety at the axle terminus was prepared. The redox potential of the ferrocene moiety decreased by ca. 80 mV when the rotaxane had a crown ether wheel capable of moving on the axle. Thus, the stabilization of

Full text of DOI:10.1021/ol049817d

ORGANIC
LETTERS
2004
Vol. 6, No. 11
1693-1696
Redox Behavior of Ferrocene-Containing
Rotaxane: Transposition of the
Rotaxane Wheel by Redox Reaction of a
Ferrocene Moiety Tethered at the End of
the Axle
,†
Nobuhiro Kihara,* Makiko Hashimoto, and Toshikazu Takata*
Department of Applied Chemistry, Graduate School of Engineering,
Osaka Prefecture UniVersity, Gakuen-cho, Sakai, Osaka 599-8531, Japan
kihara@chem.osakafu-u.ac.jp
Received January 30, 2004
ABSTRACT
A rotaxane with a ferrocene moiety at the axle terminus was prepared. The redox potential of the ferrocene moiety decreased by ca. 80 mV
when the rotaxane had a crown ether wheel capable of moving on the axle. Thus, the stabilization of the oxidized state of the ferrocene moiety
is assumed to accompany the transposition of the wheel component on the axle toward the ferrocene moiety.
Interlocked molecules such as rotaxanes and catenanes are
characterized by the relative movements of a component
against its counterpart.1 Because these movements are similar
to basic mechanical motions, much attention has been paid
to the construction and the motion of interlocked-molecule-
based molecular machines, which are driven by an inter-
component interaction.2 The redox system is one of the most
promising methods to control this interaction because of its
compatibility to electric circuits.3 Sauvage et al. demonstrated
the redox reactions of copper ions to control the intercom-
ponent metal-ligand interaction, thus inducing drastic con-
formational changes of the interlocked compounds.4 Stoddart
et al. demonstrated that the reduction of viologen or oxidation
of TTF resulted in a change in intercomponent CT interaction
or electrostatic repulsion to induce the transposition of the
wheel component on the counterpart axle.3,5 Their studies
prompted us to investigate the ferrocene-based redox system
for the control of component transposition, given that
† Present address: Department of Organic and Polymeric Materials,
Tokyo Institute of Technology, Ookayama, Meguro, Tokyo 152-8552, Japan.
(1) (a) Sauvage, J.-P.; Dietrich-Buchecker, C. Molecular Catenanes,
Rotaxanes and Knots; Wiley-VCH: Weinheim, 1999. (b) Alteri, A.; Gatti,
F. G.; Kay, E. R.; Leigh, D. A.; Martel, D.; Paolucci, F.; Slawin, A. M. Z.;
Wong, J. K. Y. J. Am. Chem. Soc. 2003, 125, 8644 and references therein.
(2) (a) Stoddart, J. F. Acc. Chem. Res. 2001, 34, 410. (b) Balzani, V.;
Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39,
3349. (c) Ashton, P. R.; Ballardini, R.; Balzani, V.; Credi, A.; Dress, K.
R.; Ishow, E.; Kleverlaan, C. J.; Kocian, O.; Preece, J. A.; Spencer, N.;
Stoddart, J. F.; Venturi, M.; Wenger, S. Chem. Eur. J. 2000, 6, 3558. Very
recently, a supramolecular machine based upon the secondary ammonium
ion/crown ether interaction was shown to be driven by the redox properties
of a ferrocene unit in a system similar to that reported here; see: Horie,
M.; Suzaki, Y.; Osakada, K. J. Am. Chem. Soc. 2004, 126, 3684.
(3) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.;
Heath, J. R. Acc. Chem. Res. 2001, 34, 433.
(4) (a) Collin, J.-P.; Dietrich-Buchecker, C.; Gavin˜a, P.; Jimenez-Molero,
M. C.; Sauvage, J.-P. Acc. Chem. Res. 2001, 34, 477. (b) Collin, J.-P.;
Gavin˜a, P.; Heitz, V.; Sauvage, J.-P. Eur. J. Inorg. Chem. 1998, 1.
(5) (a) Ballardini, R.; Balzani, V.; Credi, A.; Gandolfi, M. T.; Venturi,
M. Acc. Chem. Res. 2001, 34, 445. (b) Segura, J. L.; Mart´ın, N. Angew.
Chem., Int. Ed. 2001, 40, 1372. (c) Jeppesen, J. O.; Nielsen, K. A.; Perkins,
J.; Vignon, S. A.; Di Fabio, A.; Ballardini, R.; Gandolfi, M. T.; Venturi,
M.; Balzani, V.; Becher, J.; Stoddart, J. F. Chem. Eur. J. 2003, 9, 2982.
10.1021/ol049817d CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/27/2004

Scheme 1
reversible redox systems between ferrocene and ferrocenium
cation are well-documented and that the ferrocene group can
be a bulky end-cap group.6 Therefore, we have designed a
novel ferrocene-containing rotaxane (1) with a crown ether
wheel as electron-rich component that may interact with the
ferrocenium cation.
1
Figure 1. Partial H NMR spectra of (a) 1a (270 MHz, CDCl3)
Rotaxane 1a was prepared as shown in Scheme 1.7 The
secondary ammonium salt 2 with a hydroxy group at the
terminus was treated with ferrocenecarboxylic anhydride and
a catalytic amount of tributylphosphine in the presence of
dibenzo-24-crown-8 (DB24C8) to afford 1a in 87% yield.8
The successful preparation of 1a indicates that the ferrocene
moiety is bulky enough to prevent unthreading of DB24C8.
The X-ray crystallographic structure of 1a as a benzene
adduct clearly confirmed the rotaxane structure of 1a
(Supporting Information). Because there is a strong inter-
component hydrogen-bonding interaction between the DB24C8
component and the ammonium group in this type of
rotaxanes, the transposition of the crown ether component
on the axle was not expected. Therefore, 1a was treated with
excess triethylamine and acetic anhydride to convert it to
the corresponding nonionic rotaxane 3a,9 in which both
components are free from hydrogen-bonding interaction. The
X-ray crystallographic structure of 3a reveals that the weak
and (b) 3a (270 MHz, CDCl3). Asterisk (*) denotes the residual
CHCl3 or CH2Cl2.
intercomponent interaction became working after the erasure
of the strong hydrogen-bonding interaction (Supporting
Information). The crown ether component was placed around
the p-phenylene group, and there was CH/π interaction
between the γ-methylene groups of the DB24C8 component
1
and the p-phenylene group.9 The H NMR spectrum of 3a
is shown in Figure 1. The considerable downfield shift of
the p-phenylene group and the split of the γ-methylene
groups in the DB24C8 component clearly indicate the
presence of CH/π interaction even in the solution state.
Because the CH/π interaction is rather weak, the DB24C8
component is probably loosely bound on the p-phenylene
group.
Cyclic voltammetry analyses of 1a and related compounds
were performed in acetonitrile to determine their formal
redox potentials (E1/2, vs Ag/AgCl). Reversible voltammmo-
grams corresponding to the ferrocene moiety were observed
for each ferrocene derivative. The results are summarized
in Table 1. Rotaxane 1a showed a E1/2 slightly higher than
that of benzyl ferrocenecarboxylate and model compound 4
because of the cationic character of 1a. Because the
hydrogen-bonding interaction between the DB24C8 com-
ponent and the ammonium group prevented the access of
the wheel component to the ferrocene moiety of 1a, the
rotaxane structure hardly affected the E1/2 of the ferrocene
moiety. On the other hand, nonionic rotaxane 3a showed a
(6) (a) Kaifer, A. E. Acc. Chem. Res. 1999, 32, 62. (b) Isnin, R.; Kaifer,
A. E. Pure Appl. Chem. 1993, 65, 495. (c) Benniston, A. C.; Harriman, A.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1459. (d) Isnin, R.; Kaifer, A. E. J.
Am. Chem. Soc. 1991, 113, 8188. (e) Liu, J.; Gomez-Kaifer, M.; Kaifer, A.
E. Mol. Machines Motors 2001, 99, 141.
(7) (a) Amabilino, D. B.; Stoddart, J. F. Chem. ReV. 1995, 95, 2725. (b)
Hubin, T. J.; Busch, D. H. Coord. Chem. ReV. 2000, 200-202, 5. (c) Takata,
T.; Kihara, N. ReV. Heteroatom Chem. 2000, 22, 197. (d) Kihara, N.; Takata,
T. J. Synth. Org. Chem. Jpn. 2001, 59, 206.
(8) (a) Kawasaki, H.; Kihara, N.; Takata, T. Chem. Lett. 1999, 1015.
(b) Kihara, N.; Nakakoji, N.; Takata, T. Chem. Lett. 2002, 924. (c)
Watanabe, N.; Yagi, T.; Kihara, N.; Takata, T. Chem. Commun. 2002, 2720.
(9) Kihara, N.; Tachibana, Y.; Kawasaki, H.; Takata, T. Chem. Lett. 2000,
506.
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Org. Lett., Vol. 6, No. 11, 2004

Table 1. Redox Potentials of Ferrocene-Containing
Table 2. Redox Potentials of Ferrocene Derivatives with
Compounds
Rotaxane Structure
a
ferrocene derivative
E1/2 (mV)
benzyl ferrocenecarboxylate
331
335
335
353
271
4b
4 + DB24C8b,c
1a
3a
a Versus Ag/Ag+ with glassy carbon electrode at 298 K in 0.1 mol/L
tetrabutylammonium perchlorate in acetonitrile (0.5 mmol/L). b With 0.1
mol/L tetraethylammonium perchlorate in acetonitrile (5 mmol/L). c One
equivalent of DB24C8 was added to the solution of 4.
ammonium
rotaxane
N-acylated
a
a
E1/2 (mV)
rotaxane
E1/2 (mV) ∆E1/2 (mV)
1a
1b
1c
1d
353
347
355
341
3a
3b
3c
3d
271
267
291
297
82
80
64
44
E1/2 ca. 80 mV lower than that of 1a. The DB24C8
component in 3a obviously stabilized the oxidized state to
reduce its E1/2. Because the E1/2 of 4 did not change by the
addition of DB24C8, it was concluded that the stabilization
was induced by the very weak crown ether-ferrocenium
cation interaction that occurs when the approach of the crown
ether component to the ferrocene moiety is enhanced by the
rotaxane structure and feeble intercomponent interaction.
To demonstrate the interaction between the wheel com-
ponent and the ferrocene group, the electronic spectra of 3a,
4, and their oxidized states were measured. One-electron
oxidation was chemically carried out with silver tetrafluo-
roborate in ether under an argon atmosphere. Figure 2 shows
the electronic spectra of 3a and 3a+. The absorption of 3a
a Versus Ag/Ag+ with glassy carbon electrode at 298 K in 0.1 mol/L
tetrabutylammonium perchlorate in acetonitrile (0.5 mmol/L).
at 444 nm corresponding to the ferrocene moiety disappeared
by oxidation, while the absorption at 632 nm, which was
assigned to the ferrocenium cation moiety,10 appeared. The
reversible redox behavior of 3a was easily confirmed: the
electronic spectrum of the solution of 3a+ readily returned
to that of 3a by exposure to air or water. Although the
electronic spectrum of 4 was identical to that of 3a in the
visible region, λmax of the ferrocenium cation of 4+ was
observed at 629 nm. The small but nonetheless significant
red shift of the ferrocenium cation of 3a+ as compared to
4+ indicates the presence of attractive interaction between
the crown ether and the ferrocenium cation. The ferrocenium
cation was obviously stabilized by this interaction. Because
there is no perturbation in the reduction state, it can be
deduced that the ferrocene group attracted the wheel
component by the interaction that emerged during the one-
electron oxidation.
What part of the wheel component contributes to the
reduction of E1/2? Does the wheel component really move
on the axle? To answer these questions, some ferrocene
derivatives with rotaxane structures were prepared, and their
E1/2 values were measured. The results are summarized in
Table 2. Rotaxanes 1b and 3b with dicyclohexano-24-
crown-8 as wheel component were prepared to examine the
effect of the electron-rich benzene rings of DB24C8. Since
the E1/2 of 3b was 80 mV lower than that of 1b, as also
observed for 1a and 3a, the ferrocenium cation interacted
with the ether oxygen but not with the benzene ring. Further,
rotaxanes 1c, 1d, 3c, and 3d having axles with longer spacers
were prepared to confirm the transposition of the wheel
1
Figure 2. Electronic spectra of 3a (solid line) and 3a+ (dashed
line) in CH2Cl2 (5.4 × 10-4 mol/L) at room temperature. Although
the electronic spectrum of 4 is identical to that of 3a in the visible
region, λmax of the ferrocenium cation 4+ was observed at 629 nm.
component on the axle. The H NMR spectra of 1d and 3d
are shown in Figure 3. The presence of CH/π interaction
(10) (a) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877. (b)
Guillon, C.; Vierling, P. J. Organomet. Chem. 1994, 464, C42.
Org. Lett., Vol. 6, No. 11, 2004
1695

ferrocene groups. The large ∆E1/2 cannot be explained
without the transposition of the wheel component on the axle
that leads it to interact with ferrocenium cation because the
p-phenylene moiety acts as the station of the wheel. The
rather small ∆E1/2 for 3c and 3d presumably came from the
difference in structure of the p-phenylene groups, dialkyl-
benzene for 3a and 3b and alkylalkoxybenzene for 3c and
3d.
In the oxidized state, an attractive interaction between the
DB24C8 component and the ferrocenium cation moiety is
expected to develop. At present, it is not clear if the
interaction is a simple ion-dipole interaction or the elec-
trostatic interaction that accompanies electron donation from
crown ether oxygen to the ferrocenium cation. When the
wheel component was pinned up on the axle by strong
interaction such as an intramolecular hydrogen-bonding
interaction with the ammonium group in the case of 1a, weak
crown ether-ferrocenium cation interaction did not induce
the transposition. However, when the wheel component was
labile on the axle, the transposition occurred even by weak
attractive intercomponent interaction. As a consequence, the
present work demonstrates that rotaxanes with a weak
intercomponent interaction such as 3 constitute an excellent
scaffold for the construction of a sensitive molecular device.
Acknowledgment. We acknowledge financial support
from the Grant-in-Aid for Scientific Research from the
Ministry of Education, Science, Sports and Culture, Iketani
Science and Technology Foundation, and Yazaki Memorial
Foundation for Science and Technology.
1
Figure 3. Partial H NMR spectra of (a) 1d (270 MHz, CDCl3)
and (b) 3d (270 MHz, CDCl3). Asterisk (*) denotes the residual
CHCl3.
between the wheel and the axle in 3d is evident from the
downfield shift of the p-phenylene group and the split of
the γ-methylene groups in the DB24C8 component, indicat-
ing that the DB24C8 wheel is located at the p-phenylene
group. The E1/2 of 3c was ca. 65 mV lower than that of 1c,
and the E1/2 of 3d was ca. 45 mV lower than that of 1d. A
considerable decrease of E1/2 was observed even in 3d, which
has a dodecamethylene spacer between the p-phenylene and
Supporting Information Available: Experimental pro-
cedures, spectral data, X-ray crystalographic analyses of 1a
and 3a, cyclic voltammogram of 1, 3, and 4, and electronic
spectra of 4 and 4+. This material is available free of charge
via the Inernet at http://pubs.acs.org.
OL049817D
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Org. Lett., Vol. 6, No. 11, 2004