A rapidly shuttling copper-complexed [2]rotaxane with three different chelating groups in its axis

Article abstract of DOI:10.1002/anie.200903311

Fast and furious: The mobile ring of a copper-complexed [2]rotaxane incorporates an endocyclic but nonsterically hindering bidentate chelate. The rotaxane axis contains three different chelates (see picture), and both terminal coordination sites are separ

Full text of DOI:10.1002/anie.200903311

Communications
DOI: 10.1002/anie.200903311
Molecular Devices
A Rapidly Shuttling Copper-Complexed [2]Rotaxane with Three
Different Chelating Groups in Its Axis**
Jean-Paul Collin, Fabien Durola, Jacques Lux, and Jean-Pierre Sauvage*
Molecular machines are particularly promising in relation to
potential applications in the fields of information storage and
processing, imaging, and nanoscale electro- or photochemi-
cally driven mechanical devices, as illustrated by recent
spectacular results.[1–10] Catenanes and rotaxanes[11–14] consti-
tute an important subclass of such systems, and our research
group has been particularly interested in transition-metal-
complexed interlocking compounds.[15–22] Most of these pre-
viously reported systems were two-geometry compounds,
which were able to switch between a stable five-coordinate
copper(II) complex where the copper(II) center is coordi-
nated to a bidentate and a tridentate chelate and a second
form, which is a four-coordinate copper(I) complex. In the
latter form, the copper(I) center is coordinated to two
bidentate ligands. A rare example of a three-geometry
system was reported in 1996, and was based on a [2]catenane
that consisted of two identical rings, each ring incorporating
two different chelating units (a bi- and a tridentate ligand).
The copper center could thus be four-, five-, or six-coordina-
te.[17b] Molecular “shuttles”[23–30] represent the archetype of
molecular machines and, equally importantly, they are often
used in the fabrication of real devices.[9] The shuttle-like
[2]rotaxanes reported to date are two-station systems. These
rotaxanes consist of a mobile ring threaded by an axis that
incorporates two distinct functional groups, which are able to
interact with the ring. To the best of our knowledge, no
molecular shuttles with three distinct stations have been
reported to date. However, catenanes have been described in
which one or two rings (considered as mobile) are threaded
through a larger ring that incorporates three different func-
tional groups, which are able to interact with the mobile
ring(s).[31] A particularly elegant compound that belongs to
this family of catenane-based molecular machines was
reported in 2003.[3] This molecule was the first example of a
catenane-based rotary motor, that is, a machine that displays
controlled directionality during the dynamic process.[3] Inci-
dentally, non-interlocking rotary machines had already been
reported by other research groups,[32,33] and continue to
attract much attention.[34,35]
Herein, we describe the synthesis and electrochemical
behavior of a rotaxane that acts as an electrochemically
driven molecular shuttle over a long distance. The rotaxane
consists of a coordinating ring threaded by an axis that
incorporates three different chelates. It was expected that by
introducing an intermediate “station” between the two
terminal chelating groups, the gliding motion of the metal-
complexed ring would be much faster than the analogous
motion without the intermediate chelating group, as the
distance between the terminal stations is the same for the two
systems.
In the present system, the distance between the two
terminal coordination sites is approximately 23 ꢀ. Without a
relay between the two end-chelates of the axis, the shuttling
motion between these two stations would be expected to be
very slow. It has already been shown that the presence of an
aromatic spacer between the end-stations slows the shuttling
motion significantly.[36–37] The two forms of the rotaxane are
shown in Scheme 1.
As discussed below, the introduction of a 2,2’-bipyridine
(bipy) between the two end-chelates of the thread facilitates
the gliding process. The translational motion over 23 ꢀ is as
fast as the related motion in a two-station rotaxane that
incorporates a 2,9-diphenyl-1,10-phenanthroline (dpp) unit
and a 2,2’,6’,2’’-terpyridine (terpy) chelate, that is, the same
groups as the terminal chelates of the present system, but over
a distance of less than 10 ꢀ. The coordinating units on the axis
are 1) a dpp chelate, 2) a bipy chelate and 3) terpy, which is a
tridentate ligand. The ring incorporates an 8,8’-diphenyl-3,3’-
biisoquinoline (dpbiiq) bidentate ligand. This endocyclic but
nonsterically hindering chelate is a key component, which
favors fast translational or rotational motions within shuttle-
like rotaxanes or pirouetting systems, respectively.[20b,30] The
principle of the electrochemically driven motion relies on the
relative stabilities of the various copper(I) and copper(II)
complexes formed with the various ligands. Within the
following sequence, the thermodynamic stability of the
copper(I) complexes increases from [Cu(terpy)(dpbiiq)]+ to
[*] Prof. J.-P. Collin, Dr. F. Durola, J. Lux, Prof. J.-P. Sauvage
Laboratoire de Chimie Organo-Minꢀrale, Institut de Chimie, LC3
UMR 7177 du CNRS
[Cu(dpp)(dpbiiq)]+:
[Cu(terpy)(dpbiiq)]+ < [Cu(bipy)-
(dpbiiq)]+ < [Cu(dpp)(dpbiiq)]+. The stability sequence is
reversed for CuII complexes: [Cu(dpp)(dpbiiq)]2+ < [Cu-
(bipy)(dpbiiq)]2+ < [Cu(terpy)(dpbiiq)]2+.
Universitꢀ de Strasbourg
4 rue Blaise Pascal, 67070 Strasbourg Cedex (France)
Fax: (+33)3-9024-1368
These relative stabilities of the complexes within these two
sequences reflect the electrochemical properties of the
models listed above and, in particular, their redox potentials.
The CuII/CuI redox potentials of the threaded model
complexes (see the Supporting Information) and those of the
E-mail: sauvage@chimie.u-strasbg.fr
Homepage: http://www-chimie.u-strasbg.fr/~lcom/
[**] Funding from the CNRS and Rꢀgion Alsace is acknowledged
(fellowships to F.D. and J.L.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903311.
+
two-station shuttle 5(4) as well as their chemical structures,
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Chemie
as those shown in Figure 1.
The cyclic voltammograms
(CVs) of 1(4)+, of models
+
2(4)+, 3(4)+, 4(5)2+, and of 5(4)
are shown in Figure 2. The
+
synthesis of 1(4) and the
reference compounds is de-
scribed in the Supporting
Information. The CV of
+
2(4) shows a quasi-reversi-
ble redox couple at 0.34 V
(DEp = 160 mV). An addi-
tional reversible redox
couple at 0.63 V (DEp =
100 mV) is unambiguously
assigned[17a] to [Cu(dap)2]+
(dap = 2,9-dianisyl-1,10-
phenanthroline). The CV of
+
the reference complex 3(4)
displays a quasi-reversible
redox couple at 0.18 V
(DEp = 170 mV). This low
redox potential reflects the
high stabilization of the CuII
oxidation state by two 2,2’-
bipyridine-type ligands. The
Scheme 1. Structures of the four-coordinate CuI (1(4)+) and five-coordinate CuII (1(5)2+) molecular shuttles in
their thermodynamically stable forms. The subscripts 4 and 5 indicate the coordination number of the copper
center, excluding possible solvent molecules or counterions.
2+
CV of 4(5) and of the two-
station shuttle 5(4)+[30b–c] also
indicate a high stabilization
of the copper(II) ion in a
five-coordinate
environ-
are given in Scheme 2. Each complex is almost identical to the
corresponding form of the three-station [2]rotaxane (four-,
five-, or six-coordinate). The redox potential values are thus
ment, with redox couples at À0.03 (4(5)2+/4(5)+) and 0.02 V
(5(5)2+/5(5)+) respectively. For complex 5(4)+, the partially
reversible redox couple at 0.37 V is assigned to the couple
+
expected to be very close for the models and for the shuttling
5(4)2+/5(4)+. Interestingly, the CV of the molecular shuttle 1(4)
+
.
[2]rotaxane 1(n)
displays only two redox signals, which are located at 0.38 and
By oxidizing or reducing a given thermodynamically
stable state, the complex switches from one form to another
so as to afford the most stable form of the compound—either
[Cu(dpp)(dpbiiq)]+ for the reduced state (four-coordinate) or
[Cu(terpy)(dpbiiq)]2+ (five-coordinate) for the oxidized state.
The general principle of the shuttling rotaxane is represented
in Figure 1.
À0.02 V and can be easily assigned by comparison with the
+
2+
model complexes to the redox couples 1(4)2+/1(4) and 1(5)
/
1(5)+, respectively. This pattern suggests that the two motion
steps of the copper(II)-complexed ring from the dpp site to
the bipy site and from the bipy site to the terpy site are fast on
the CV timescale. This hypothesis is supported by the
observation that the concentration of the species involving
the intermediate bipy ligand is negligible. By varying the
potential scan rate, the rate constant for the rearrangement of
As previously described for related systems,[17,30] cyclic
voltammetry is well-adapted to the study of systems that
display coupled electron transfer and chemical reactions such
2+
2+
the four-coordinate 1(4) to the five-coordinate 1(5) can be
Scheme 2. The three models 22+/+, 32+/+, 42+/+, the two-station shuttle 52+/+, and their respective redox potentials versus the standard calomel
electrode (SCE).
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Communications
estimated to be 0.4 sÀ1 [38,39]
.
An upper value of 50 sÀ1 is
+
estimated for the conversion of five-coordinate 1(5) to four-
coordinate 1(4)+. The rates of these translational motion over a
distance of 23 ꢀ compare favorably with those of other
related shuttles that involve shorter distances.[30b–c] This
observation illustrates the importance of intermediate
energy states provided by properly chosen ligands.
In conclusion, a long-distance but rapidly moving copper-
based molecular shuttle, as well as threaded model com-
pounds have been prepared and studied by electrochemical
techniques. The presence of an intermediate 2,2’-bipyridine
group interspersed between the two terminal chelating groups
apparently has a dramatic influence on the shuttling process
rate. Although it is still not certain whether the bipy group
behaves as a real “station”, with a certain residence time of
the mobile part on this station, or if its function is simply to
modify the nature of the axis and stabilize a coordinatively
unsaturated copper centre during the motion, the present
observations pave the way to long-distance copper-based
shuttles and related molecular machines that are able to
undergo linear motion.
Figure 1. Principle of the electrochemically driven molecular shuttle
based on copper(I) and copper(II) coordination.
Received: June 18, 2009
Published online: October 1, 2009
Keywords: chelates · coordination modes · copper ·
.
molecular devices · rotaxanes
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