A transition metal containing rotaxane in motion: Electrochemically induced pirouetting of the ring on the threaded dumbbell

Article abstract of DOI:10.1002/(SICI)1521-3765(19991105)5:11<3310::AID-CHEM3310>3.0.CO;2-R

In this work we describe a rotaxane in which a new type of motion, pirouetting of the wheel around its axle, can be electrochemically triggered. The rotaxane was synthesized by using the three-dimensional effect of copper(I). It incorporates both a hetero

Full text of DOI:10.1002/(SICI)1521-3765(19991105)5:11<3310::AID-CHEM3310>3.0.CO;2-R

FULL PAPER
A Transition Metal Containing Rotaxane in Motion: Electrochemically
Induced Pirouetting of the Ring on the Threaded Dumbbell
Laurence Raehm, Jean-Marc Kern,* and Jean-Pierre Sauvage*[a]
Abstract: In this work we describe a
rotaxane in which a new type of motion,
pirouetting of the wheel around its axle,
can be electrochemically triggered. The
rotaxane was synthesized by using the
three-dimensional effect of copper(i). It
incorporates both a hetero-biscoordinat-
ing ring (containing both 2,9-diphenyl-
1,10-phenanthroline and 2,2':6',2''-ter-
pyridine units) and a molecular string
which contains only one 2,9-diphenyl-
1,10-phenanthroline. Both ends of the
string are functionnalized by a bulky
stopper (tris(p-tert-butylphenyl)(4-hy-
droxyphenyl)methane). Large molecu-
lar motions have been induced electro-
chemically in this molecule and were
detected by using cyclic voltammetry.
The driving force of the two rearrange-
ment processes observed is the high
stability of two markedly different coor-
dination environments for copper(i) and
copper(ii) ions. In the copper(i) state, two
phenanthroline units (one of the ring,
one of the string) interact with the metal
the terpyridine of the ring. The kinetic
rate constants of the two molecular
motion processes (from CuI(5)
)
to
CuI(4)) and fom CuII(4)) to CuII(5))) have
been determined and both rates are
much faster than the ones in previously
studied analoguous systems. In addition,
the rate of the pirouetting motion de-
pends greatly on the copper oxidation
state. The divalent four-coordinate cop-
per complex (CuII(4))) rearranges in tens
of seconds, whereas the monovalent
five-coordinate copper system leads to
the four-coordinate complex in the milli-
second time scale.
in
a
tetrahedral geometry (CuI(4)),
whereas in the copper(ii) state the five-
coordinate geometry (CuII(5)) is due to
the phenanthroline of the string and to
Keywords: copper ´ molecular mo-
tions ´ phenanthroline ´ rotaxanes ´
supramolecular chemistry
Introduction
Molecular motors of various kinds are very common in
biology.[1] Roughly, biolological motors can be classified in
two families: linear and rotary motors. The most classical
examples of linear motors are the myosin ± actin complex[2±5]
present in muscles or the kinesin-containing system.[6, 7]
Rotary motors have recently attracted much attention since
the extremely common enzyme ATP synthase[8±12] has been
shown to behave in a fashion reminiscent of a rotary motor. A
¹shaftª (g protein) rotates inside a hollow stator (a3b3
aggregate of proteins, Figure 1) and this spinning motion
induces formation of ATP from ADP and inorganic phos-
phate (3 moles per round). Another example of a rotary
motor is that of bacterial flagella,[13] which are responsible for
bacteria mobility.
Figure 1. A schematic representation of ATP synthase, a biological rotary
motor. g (and c) are mobile whereas the aggregate a3b3 is fixed to the
membrane and constitutes the stator of the motor.
The number of synthetic molecular ensembles whose
dynamic behavior is reminiscent of biological motors is
presently very limited. In order for an object to be regarded
as a motor, several basic requirements have to be fulfilled.
Even without trying to apply a strict thermodynamic defi-
nition, the system will have to convert a certain type of energy
into another form of energy, while undergoing some kind of
continuous motion.
[a] Prof. J.-M. Kern, Dr. J.-P. Sauvage, L. Raehm
Â
Institut Le Bel, Universite Louis Pasteur
4 rue Blaise Pascal, F-67070 Strasbourg (France)
Fax: (‡33)3 88 60 73 12
Very generally, creating molecules which can change shape,
change position in space, or whose certain parts can be set in
E-mail: sauvage@chimie.u-strasbg.fr
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Chem. Eur. J. 1999, 5, No. 11

3310±3317
uation corresponding to a stable complex, the system is set out
of equilibrium. The relaxation process of the compound
implies a large amplitude motion which will bring the system
back to its new equilibrium
position (Figure 2).
In previous work, gliding mo-
tion of a ring around another
one (Figure 3 a) in the case of a
catenane[44] or translation mo-
tion along a molecular thread
(Figure 3 b) in the case of a
rotaxane[43] were studied. Pres-
ently, we would like to describe
a rotaxane in which a new mo-
tion, pirouetting of the wheel
around its axle (Figure 3 c), can
be electrochemically triggered.
The driving force of this mo-
Figure 3. Molecules in motion:
tion is here again based on
different geometrical preferen-
ces for CuI and CuII. The wheel
of the rotaxane is a bis-coordi-
nating macrocycle containing
both a bidentate moiety, a 1,10-phenanthroline, and a
terdentate unit, a 2,2',6',2¹-terpyridine (terpy). The axle
incorporates only one bidentate moiety.
The three-dimensional template effect of CuI, introduced
fifteen years ago, made catenanes, rotaxanes, and related
systems reasonably accessible from a preparative point of
view.[45±47] The strategy leading to prepare catenanes is based
on a precursor consisting of two phenanthroline-type ligands
entwined around a CuI center. In the case where one of these
ligands is macrocyclic, the structure of the precursor is
particular: the acyclic ligand is threaded through the macro-
cycle. Reacting the end functions of the acyclic ligand of this
complex with an appropriate difunctionalized molecular
fragment leads to the formation of a catenane, whereas
reacting them with two monofunctionalized and bulky frag-
ments leads to a rotaxane.
a) gliding of
a ring around
another one; b) translation of
a ring along its axle;[21] c) wheel
pirouetting around its axle.
Figure 2. Principle of the electrochemically induced molecular motions in
copper complex rotaxane. The stable four-coordinate monovalent
a
complex is oxidized to an intermediate tetrahedral divalent species. This
compound undergoes a rearrangement to afford the stable five-coordinate
copper(ii) complex. Upon reduction, the five-coordinate monovalent state
is formed as transient. Finally, the latter undergoes the reorganization
process that regenerates the starting complex (the black circle represents
CuI and the white circle represents CuII).
motion at will under the action of an external signal, is a
challenging target in relation with natural systems mimics but
also as components of long-term potential information
storage or processing devices.
Results and Discussion
General principle, design, and synthesis
Artificial molecular machines have been described[14±17]
Ð
among them the remarkable work achieved by Stoddart,
Balzani, Kaifer and their groups[18±21]Ðand special interest
was focused on transition metal containing systems.[22±41]
Indeed, the stereoelectronic requirements of a metal center
can strongly depend on its ox-
An alternative strategy is to use a monostoppered species,
to form the prerotaxane intermediate and to add the second
stopper afterwards (Figure 4). The advantage of this method,
idation state. Thus, varying the
redox state of the metal can
lead to a large rearrangement
of the surrounding ligands. This
is particularly true for copper
complexes: CuI is most of the
time low-coordinate (coordina-
tion number ꢀ4), whereas CuII
is preferably square planar or
higher coordinate (coordina-
tion numberˆ 5 or 6). In the
systems previously made and
investigated in our group,[42, 43]
‡
the same general principle is
used: by reducing or oxidizing
the copper center, from a sit-
Figure 4. Principle of the synthesis of rotaxane 10(4) (see Figure 5): i) formation of a monostoppered axle; ii)
threading of the axle through the wheel; iii) dethreading of the macrocycle is prevented by adding a second
blocking group.
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J.-M. Kern, J.-P. Sauvage, and L. Raehm
FULL PAPER
which was chosen in the present work, is to limit dethreading
of the macrocycle during the stoppering reaction.
‡
The rotaxane 10(4) (the nomenclature of the rotaxanes
n‡
described in this work is 10(N) , where N refers to the
coordination number of the metal (4 or 5) and n to its charge)
synthesized herein is composed of two subunits: a macrocycle
and a molecular thread (Figure 5bottom). The macrocycle[44]
1 (Figure 5top) contains two different coordinating sites: a
terpyridine moiety and a 1,10-phenanthroline one. A phenan-
throline moiety in which the a-positions of the coordinating
atoms are substituted with phenyl groups (2,9-diphenyl-1,10-
phenanthroline, ¹dppª) was used. Indeed, it was shown that
the low oxidation state of dpp-based transition metal com-
plexes are strongly stabilized. This is particularly true for
copper complexes.[48] The two units are joined through two C3
chains which are linked to the 5,5''-positions of the terpyridine
on one side and to the 4,4'-positions of the phenanthroline-
attached phenyl goups through an oxygen atom each on the
other side. The resulting macrocycle 1 has a 33-membered ring.
The molecular thread contains the phenanthroline biden-
tate unit (dpp). The end groups of the thread, that is the
stoppers of the rotaxane, are tetraarylmethane derivatives.
These last species were selected as blocking groups since they
are large enough to prevent dethreading of the 33-membered
ring 1. In order to lower solubility problems, lipophilic groups
(tert-butyl fragments) were introduced on the para positions
of three of the aryl groups. The stoppers and the coordinating
site are linked through bisethoxy-ether spacers.
The tetraarylmethane derivative 7 was prepared in four
steps starting from compound 3 (tris(p-tert-butylphenyl)(4-
hydroxyphenyl)methane) whose synthesis has been described
by Gibson et al.[49] Compound 3 was treated with a small
excess of THP-protected 2-(2'-iodoethoxy)ethanol (THP ˆ
tetrahydropyran-2-yl) to afford 4 in high yield. Transforma-
tion of 4 into bromo derivative 7 was done by treatment of the
mesylate ester 6 with NaBr in acetone, 6 being prepared by
esterification of the alcohol 5, obtained by hydrolysis of 4. The
monostoppered thread 8 was obtained by reacting the
electrophile 7 with a large excess of 2,9-di(p-hydroxyphen-
yl)-1,10-phenanthroline[50] (2) in basic (K2CO3) DMF medi-
um. Following this procedure, 8 was isolated in 50% yield, and
most of the excess of diphenol 2 could be recovered.
‡
Pre-rotaxane 9 was prepared in the following manner:
[51]
Cu(CH3CN)4BF4 was added to a solution of 1 in CH2Cl2/
CH3CN. The resulting orange-red solution was then treated
with 8 to give almost quantitatively a brown-red solution of
‡
semi-rotaxane [9 ]BF4. For the preparation of the fully
‡
‡
blocked rotaxane 10(4) , 9 and 7 were dissolved in DMF,
during which the temperature was maintained between 55 and
608C, and the base Cs2CO3 was added portionwise over 4 h.
Indeed, it was observed that dethreading, which occurs in hot
and basic medium, is limited using this procedure. The
reaction was followed by counterion exchange (KPF6) to give
‡
[10(4) ]PF6. The overall yield of these two last steps is 30%.
‡
The rotaxane 10(4) was characterized by FAB-MS, 1H NMR
and UV spectroscopy. FAB-MS revealed the threaded
structure of the compound; intense peaks corresponding to
the loss of the counterion (m/z 2255.2) and to the loss of the
counterion and the thread (m/z 740.2) are observed.
‡
Figure 5. Molecular representation of the metalated rotaxanes 10(4)
,
‡
2‡
2‡
10(5) , 10(4) , 10(5) , of the metal-free rotaxane 11 and of their precursors
‡
1 ± 9 .
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Pirouetting Motion of Rotaxanes
3310±3317
1H NMR spectroscopy (Figure 6a) confirmed the threaded
structure of 10(4) and the entwining of the two dpp subunits
around the metal. Indeed, it shows the usual upfield shifts of
the aromatic protons of the phenoxy moieties attached to the
phenanthrolines in such intertwined structures.[52] For in-
stance, a shielding of d ˆ 1.11 ppm is observed from the meta
protons of the free ring 1 to the corresponding protons (Hm) in
protons (Ho, H'o, Hm, H'm) of the phenoxy moieties attached
to the phenanthrolines is observed, indicating that the
intertwined structure of the two dpp fragments has disap-
eared. The chemical shift values observed for 11 are in the
same range as the ones for the free subunits 1 and 8.
‡
Transformation of the free rotaxane 11 into the CuII
2‡
complex 10(5) was quantitatively achieved by mixing 11 with
a CH3CN/CH2Cl2 solution of CuII(BF4)2. In this case, the
structure was confirmed by FAB-MS and the five-coordinate
state of the metal was evidenced by UV/Vis spectroscopy: lmax
is observed at 618 nm, and this wavelength range is character-
istic of CuII polyimine complexes.[51] The very low value of the
extinction coefficient (e ˆ 104) indicates that the metal is five-
2‡
coordinate[42, 44] (which means in the case of 10(5) that the
copper is surrounded by one dpp and one terpy ligand). For
CuII(dpp)2 complexes where the metal can not reasonably be
more than tetracoordinate, lmax is around 680 nm and the
extinction coefficient is significantly higher[51] (around 800).
Electrochemical study of the pirouetting motion
The electrochemical behavior of tetracoordinate CuI com-
plexes that is, Cu(dpp)2-based cores, is well established.[51, 53]
The reversible redox potential for the CuII/CuI transition is
around 0.6 ± 0.7 V vs. SCE. This relatively high potential
underlines the stability of the four-coordinate CuI complexes
versus the corresponding CuII ones. The redox potential of
pentacoordinate copper complexes[42, 43] is observed in a much
more cathodic range. For example, for the five-coordinate
complex Cu(1, dap)2‡/‡ (dap ˆ 2,9-di-p-anisyl-1,10-phenan-
throline), in which the terpy fragment of the ring is bound
to the metal, the redox potential is 0.035 V. This potential
shift when going from tetracoordinate to pentacoordinate
copper systems is due to the better stabilization of the CuII
state thanks to the presence in the coordination sphere of five
donor atoms.
Figure 6. 1H NMR spectra (field: 400 MHz, solvent: CD2Cl2) of a) the CuI
rotaxane 10(4) and b) the corresponding metal-free rotaxane 11, in the
‡
‡
a) Electrochemical behaviour of chemically isolated 10(4) and
2‡
‡
aromatic region. The protons of the two different dpp units undergoing the
10(5) :The electrochemical behavior of 10(4) in a CH2Cl2/
CH3CN solution has been studied by cyclic voltammetry (CV)
and is represented Figure 7a. A reversible signal appears at
0.545 V. The difference of potential DEp between the anodic
and the cathodic peak is 70 mV for this signal. A very weak
redox peak at 0.15 V can also be observed.
‡
most significant shift when going from 10(4) to 11 are labeled; the proton
numbering is indicated in Figure 5.
‡
10(4) . A similar shift happens for the meta protons of the
thread: the string precursor 8 contains two types of such
protons resonating at d ˆ 7.12 (H'm1) and d ˆ 6.98 (H'm2),
‡
In the rotaxane 10(4) , where the metal is tetracoordinate,
‡
respectively. For 10(4) , one unique signal at d ˆ 6.02 corre-
the signal occurring at 0.54 V corresponds to the tetracoordi-
nate CuII/CuI couple. The ratio of the intensities of the anodic
and cathodic peaks ipc/ipa is 0.95 (ipc and ipa are the intensity
of the cathodic and anodic peaks, respectively), showing that
no transformation or reorganization of the coordination
sphere of the tetracoordinate CuII complex occurs in the time
scale of the measurements (sweep rate of the potential :
sponds to the four meta protons (H'm) of the string.
As expected, the high MLCT characteristic of the absorp-
tion band in the visible range for CuI(dpp)2-based complexes
‡
is observed for 10(4) (lmax ˆ 438 nm, e ˆ 2830) .
‡
The CuI complex rotaxane 10(4) could be easily demeta-
lated[50] by reaction with cyanide under mild conditions,
leading to rotaxane 11, with free coordination sites. The FAB-
MS of the metal-free 11 shows an intense peak at m/z 2192.2
1
100 mVs ). We checked that the signal at 0.15 V and the
main redox signal are not related. It is probably due to the
‡
2‡
corresponding to the molecular peak (‡H ), and two peaks
presence of small amounts of 10(5) (vide infra).
corresponding to the loss of the ring and to the loss of the
The cyclic voltammetry behavior of the CuII rotaxane
10(5) (Figure 7 b), is very different from that of 10(4) . The
potential sweep for the measurement was started at ‡0.9 V, a
1
2‡
‡
thread (m/z 1513.8, m/z 678.3, respectively). In the H NMR
spectrum (Figure 6b), a downfield shift of the aromatic
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J.-M. Kern, J.-P. Sauvage, and L. Raehm
FULL PAPER
peared, corresponding to the reduction of this tetracoordinate
species.
These two complementary cyclic voltammetry experiments
confirm that in this rotaxane, like in previously studied related
systems, the tetracoordinate CuI state is more stable than the
pentacoordinate one and the pentacoordinate CuII state is
more stable than the tetracoordinate one. Moreover, it was
observed that the rearrangement rates from the less to the
most stable geometries are drastically different for the two
oxidation states of the metal.
2‡
The irreversibility of the reduction peak of 10(5) , com-
bined with the appearance of a reversible peak corresponding
to tetracoordinate copper, suggests that the reorganization of
‡
the rotaxane in its pentacoordinate form 10(5) (that is, with
the copper being coordinated to terpy and to dpp units), to its
‡
tetracoordinate form (10(4) , where the copper is surrounded
by two dpp units) occurs within the time scale of the cyclic
voltammetry. CV measurements at high potential sweep rates
is therefore a well-suited method to study the reorganization
kinetics of such systems. On the other hand, Figure 7a
indicates the inertness of the reorganization of the tetracoor-
dinate CuII rotaxane, and cyclic voltammetry is thus inappro-
priate to study the kinetic of such a system. Therefore, a total
‡
2‡
conversion of tetracoordinate 10(4) to tetracoordinate 10(4)
was performed by preparative electrolysis and the subsequent
2‡
rearrangement of tetracoordinate 10(4) into pentacoordinate
10(5) was monitored by an amperometric method.
Figure 7. Cyclic voltammetry curves recorded using a Pt working electrode
2‡
1
at a 100 mVs sweep rate (CH3CN/CH2Cl2 (4:1), supporting electrolyte:
tetrabutylammonium tetrafluoroborate, 0.1 molL 1, Ag wire pseudo-refer-
‡
2‡
ence). a) Compound 10(4) , b) chemically prepared 10(5) . Curve ii) refers
to a second potential sweep following immediatly the first one (i).
b) Determination of the kinetic rate constant k for the
transformation of pentacoordinate CuI into tetracoordinate
‡
‡
CuI: 10(5) !10(4) : Figure 8 illustrates the evolution of the
potential at which no electron transfer should occur, regard-
less of the nature of the surrounding of the central CuII center
(penta- or tetracoordinate). Curve i) (first scan, recorded at
2‡
cyclic voltammetry response of pentacoordinate 10(5) when
scanning the potential at different sweep rates, going from v ˆ
1
1
1
500 mVs to v ˆ 8 Vs . It was observed that the system
100 mVs ) shows two cathodic peaks: a very small one,
became progressively reversible.
located at ‡0.53 V followed by an intense one at 0.13 V.
Only one anodic peak at 0.59 V appears during the reverse
sweep. If a second scan ii) follows immediately the first one i),
the intensity of the cathodic peak at 0.53 V increases notice-
ably.
The behavior of the I/E (current/potential) curves at
different sweep rates is typical for the case where an electron
transfer is followed by an irreversible chemical reaction
‡
(transformation of pentacoordinate 10(5) into tetracoordi-
‡
nate 10(4) ). This qualitative observation was confirmed by the
The weak peak at 0.53 V (i, Figure 7b) is due to the
‡
numeric exploitation of the I/E responses, following the
method described by Nicholson and Shain.[54] For each curve,
the ipa/ipc ratio was measured as well as E1/2 (half-wave
potential). Thus, parameter t could be calculated from
Equation (1), where v ˆ sweep rate and El ˆ inversion poten-
tial.
presence of small quantities of 10(4) . The main cathodic peak
at 0.15 V is characteristic of pentacoordinate CuII. Thus, in
2‡
10(5) prepared from the free rotaxane by metalation with
CuII ions, the central metal is coordinated to the terdentate
terpyridine of the wheel and to the bidentate dpp of the axle.
On the other hand, the irreversibility of this peak means that
the pentacoordinate CuI species formed in the diffusion layer
when sweeping cathodically is transformed very rapidly and in
any case before the electrode potential becomes again more
t ˆ(El E1/2)/v
(1)
‡
anodic than the potential of the pentacoordinate Cu2‡/Cu
Using the working curve established by the authors, the
value of log(kt) could be determined and thus, k extracted. In
order to obtain an accurate k value, several curves were
studied, varying both v and the inversion potential. An
average value of 17 Æ 9 s 1 was found for k. In other words, in
this medium (acetonitrile/dichloromethane (4/1), Bu4NBF4
(10±1 molL ), room temperature), the half-life time of
pentacoordinate CuI rotaxane is 56 Æ 28 ms (t1/2 ˆ ln2/k).
redox system. The irreversible character of the wave at
0.15 Vand the appearance of an anodic peak at the value of
‡0.53 V indicates that the transient species, formed by
2‡
reduction of 10(5) , has undergone a complete reorganization,
which leads to a tetracoordinate copper rotaxane. The second
scan (ii) which follows immediatly the first one (i) confirms
this assertion. Indeed, a cathodic peak (‡0.53 V) has ap-
1
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Pirouetting Motion of Rotaxanes
3310±3317
half-life time of tetracoordi-
nate CuII rotaxane is 120 Æ 50 s.
d) Conclusion: These experi-
ments underline the noticeable
difference of the kinetic rate
constants k and k' for the
reorganization processes lead-
‡
‡
ing from 10(5) to 10(4) and
2‡
2‡
from 10(4) to 10(5) , respec-
tively. Indeed, the ratio k/k' is
about 3000:1. In the analogous
systems already studied,[42, 43]
an important difference be-
tween the related rate con-
stants had also been observed.
Nevertheless, for the systems
based on a copper [2]cate-
nate,[42, 44] where one of the
macrocycles is monochelating
and the other one hetero-bis-
2‡
Figure 8. Illustration of the evolution of the CV curves of 10(5) when increasing the potential sweep rate (same
conditions as indicated in Figure 7). i) v ˆ 500 mVs 1, ii) v ˆ 3000 mVs 1, iii) v ˆ 8000 mVs
.
1
c) Determination of the kinetic rate constant k' for the
transformation of tetracoordinate CuII into pentacoordinate
2‡
2‡
CuII: 10(4) !10(5) : As illustrated by the cyclic voltammetry
curve represented in Figure 7a and discussed above, the
tetracoordinate CuII rotaxane generated in the diffusion layer
does not undergo any detectable rearrangement during the
reverse potential scanning. Indeed, at small sweep rates
1
1
(100 mVs or 50 mVs ), the ratio ipa/ipc remains near to
one and no increase of the cathodic peak at 0.15 V occurs.
The E1/2 values of 10(4)2‡/10(4) and 10(5)2‡/10(5) are ‡0.55 V
and 0.1 V, respectively. Thus, when polarizing the working
electrode at a potential between these two values, under
stationnary conditions (by using a rotating disk electrode, for
example), the cathodic current observed is representative of
‡
‡
2‡
the presence and concentration of 10(4) , the tetracoordinate
Figure 9. Representation of the logarithmic decay of the current intensity
CuII complex only. This remains true even if the electrolytic
solution contains pentacoordinate 10(5)
electrochemically silent in the potential range used.
A solution of 10(4) was anodically electrolyzed into the CuII
2‡
‡
versus time of a solution of 10(4) obtained by anodic electrolysis of 10(4)
.
2‡
The working electrode is a rotating Pt disk electrode (w ˆ 6300 rads 1),
polarized at ‡0.3 V vs. Ag wire pseudo-reference.
which will be
‡
state as quickly as possible, using an electrolytic cell with an
high S/V value (S ˆ electrode surface, Vˆ volume of the
electrolytic solution). A rotating disk electrode was then
introduced in the solution and polarized at ‡0.3 V. Current
versus time was recorded and an exponential decrease of the
intensity of the current was observed. When the current was
near to 0, a new cyclic voltammetry curve of the solution was
measured, leading to a voltammogram similar to the one
represented Figure 7b. This confirms that the electrogener-
chelating, the two processes are much slower. In addition,
reproducibility problems made the accurate measurement of
the corresponding rate constants difficult, especially for the
II
rearrangement from Cu(4) to Cu(5)II. The reorganization
around CuI takes about 1 s and the one around CuII takes as
much as several days, depending on the medium. For the
rotaxane[43] where the axle is a hetero-bischelating molecular
thread and the wheel a monochelating macrocycle, the
reorganization processes which imply a translation of the
ring along the string, the values determined for k and k' are
ated tetracoordinate CuII rotaxane has undergone a rear-
2‡
1
rangement to form the pentacoordinate CuII rotaxane 10(5)
.
10 2 and 1.5 Â 10 4 s , respectively. Thus, by comparison with
The kinetic constant of this transformation can be extracted
from the variation of the cathodic current. Indeed, the
logarithm of this current intensity vs. time is linear (Figure 9),
as expected for a monomolecular process. The value of k' is
given by the slope of this straight line. The average value of k'
the rate constant values determined for the rotaxane studied
1
in this work (k ˆ 17 and k' ˆ 0.007 s ), it appears that
pirouetting of a macrocycle around its axle induced by
changing the redox state of the central metal is also faster
than the translation of a macrocycle along a molecular thread,
using the same triggering process.
1
in this rearrangement is 0.007 Æ 0.003 s . In other words, the
Chem. Eur. J. 1999, 5, No. 11
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0947-6539/99/0511-3315 $ 17.50+.50/0
3315

J.-M. Kern, J.-P. Sauvage, and L. Raehm
FULL PAPER
1H NMR (200 MHz, CDCl3): d ˆ 7.24 (d, J ˆ 8.6 Hz, 8H; Hb,c), 7.07 (d, J ˆ
8.6 Hz, 6H; Hd), 6.78 (d, J ˆ 8.8 Hz, 2H; Ha), 4.40 (m, 2H; Ha), 4.10 (m,
2H; Hd), 3.9 ± 3.8 (m, 4H; Hb,g), 3.04 (s, 3H; CH3), 1.32 (s, 27H; CH3); MS
(MALDI-TOF MS): m/z: 670.64 ([M ‡ H] , 670.95).
Synthesis of 7: After being degassed, a solution of 6 (900 mg, 1.34 mmol)
and LiBr (780 mg, 8.96 mmol) in acetone (30 mL) was refluxed for 3 h
under argon (708C). Acetone was removed and the residue was taken up
into H2O/CH2Cl2. Extraction with CH2Cl2 and drying over Na2SO4, left,
after the solvent was removed, a white powder (857 mg) whose purity was
sufficient (>95% by NMR spectroscopy) to be used without further
purification. White powder; m.p. 1838C; 1H NMR (200 MHz, CDCl3): d ˆ
7.24 (d, J ˆ 8.6 Hz, 8H; Hb,c), 7.09 (d, J ˆ 8.6 Hz, 6H; Hd), 6.77 (d, J ˆ
8.8 Hz, 2H; Ha), 4.12 (m, 2H; Hd), 4 ± 3.8 (m, 4H; Hb,g), 3.46 (m, 2H; Ha),
1.31 (s, 27H; CH3); elemental analysis calcd (%) for C41H51O2Br: C 75.10,
H 7.84; found C 75.39, H 8.09.
These different types of molecular motion: gliding, trans-
lating and pirouetting are possible thank to the kinetic lability
of copper complexes. As mentioned above, the k/k' ratio is
high in all the cases studied so far, which means that the
reorganization process around CuI is much faster than around
CuII. Both rearrangements require a decoordination step of
one of the chelates, followed by recomplexation by the other
chelate. The activation barrier of this decoordination step
might be higher for the tetracoordinate CuII to pentacoordi-
nate CuII process than for the pentacoordinate CuI to
tetracoordinate CuI one, due to the higher electronic require-
ments of CuII. Thus, the difference of molecular motion rates
induced by changing the redox state of the metal could be
partially attributed to the ligand field effect.
‡
Synthesis of 8: Compound 7 ( 100 mg, 1.49 mmol) and 2,9-di(p-hydrox-
yphenyl)-1,10-phenanthroline (271 mg, 0.75 mmol) were dissolved in DMF
(50 mL) in a 100-mL round-bottomed flask equipped with a condenser. The
solution was then degassed and K2CO3 (513 mg, 3.7 mmol) was added. The
mixture was heated at 608C for 18 h under argon. DMF was removed and
the residue was taken up in H2O/CH2Cl2. Extraction with CH2Cl2 and
drying over Na2SO4, left, after the solvent was removed, a bright orange
powder which was purified by flash chromatography (SiO2, eluent CH2Cl2/
MeOH 1%) to afford 8 (70 mg, 0.73 mmol) in 49% yield. Bright orange
solid; 1H NMR (200 MHz, CDCl3): d ˆ 8.40 (d, J ˆ 8.8 Hz, 2H; HO1'), 8.29
(d, J ˆ 8.8 Hz, 2H; HO2'), 8.26 (d, J ˆ 8.4 Hz, 1H; H7'), 8.25 (d, J ˆ 8.6 Hz,
1H; H4'), 8.06 (d, J ˆ 8.6 Hz, 1H; H8'), 8.04 (d, J ˆ 8.4 Hz, 1H; H3'), 7.75 (s,
2H; H5',6'), 7.26 (d, J ˆ 8.6 Hz, 8H; Hb,c), 7.12 (d, J ˆ 8.6 Hz, 2H; Hm1'), 7.12
(d, J ˆ 8.4 Hz, 6H; Hd), 6.98 (d, J ˆ 8.6 Hz, 2H; Hm2'), 6.82 (d, J ˆ 8.8 Hz,
2H; Ha), 4.27 (m, J ˆ 4.3 Hz, 2H; Ha), 4.17 (m, J ˆ 4.3 Hz, 2H; Hd), 4 ± 3.9
(m, 4H; Hb,g), 1.32 (s, 27H; CH3); MS (MALDI-TOF MS): m/z: 940.14
Conclusion
The presently described system represents a first step towards
the elaboration of rotary motors at the molecular level. At the
present stage, the bifunctional ring can only oscillate between
two positions on the axle on which it is threaded. This process
is now well controlled and reasonnably fast, especially for the
monovalent copper(i) complex rearrangement. In order to
make real rotary motors, it will be necesssary to introduce
directionality in the system by using, among other possibil-
ities, a ring containing three different coordination sites and a
directed axle. The synthesis of such multicomponent com-
pounds is presently under way.
‡
([M ‡ H] , 939.25).
‡
Synthesis of 9 : A solution of Cu(CH3CN)4BF4 (57.3 mg, 0.182 mmol) in
degassed acetonitrile (5 mL) was added by canula to a stirred degassed
solution of 1 (123.5 mg, 0.182 mmol) in degassed CH2Cl2 (10 mL) at room
temperature. A brown-orange coloration appeared instantaneously. After
Experimental Section
stirring for 30 min at room temperature,
a solution of 8 (158 mg,
0.166 mmol) in degassed CH2Cl2 (10 mL) was added to the previous
mixture by the canula transfer technique and the solution immediatly
turned dark red. The solution was stirred for one hour under argon. After
the solvents were removed, a dark red solid of crude 9 was obtained in a
nearly quantitative yield (285 mg). The purity of 9 was sufficient (>95%
by NMR spectroscopy) to be used without further purification. Dark red
solid; 1H NMR (400 MHz, CD2Cl2): d ˆ 8.64 (d, J ˆ 8.4 Hz, 2H; t3), 8.57 (d,
2H; t6), 8.43 (d, J ˆ 8.6 Hz, 2H; t3'), 8.37 (d, J ˆ 8 Hz, 2H; H4,7), 8.29 (d, J ˆ
7.8 Hz, 2H; H4',7'), 8.06 (t, J ˆ 7.6 Hz, 1H; t4'), 7.92 (s, 2H; H5,6), 7.72 (s, 2H;
H5',6'), 7.9 ± 7.6 (m, 6H; H3, 8, H3', 8', t4), 7.36 (d, J ˆ 8.5 Hz, 4H; Ho), 7.31 (d,
J ˆ 8.5 Hz, 4H; Ho1',o2'), 7.18 (d, J ˆ 8.6 Hz, 8H; Hb,c), 7.07 (d, J ˆ 8.6 Hz,
6H; Hd), 6.80 (d, J ˆ 8.8 Hz, 2H; Ha), 6 ± 5.9 (m, 8H, H m, Hm1', m2'), 4.04 (m,
2H; Hd), 3.84 (m, 2H; Ha), 3.65 (m, 4H; Hb,g), 3.25 (m, 4H; Hx), 2.90 (m,
4H; Hz), 2.10 (m, 4H; Hy), 1.32 (s, 27H; CH3).
Synthesis of 4: Compound 3 (1 g, 1.98 mmol) and 2-(2-iodoethoxy)ethyl
2-tetrahydro-2H-pyran ether (740 mg, 2.46 mmol) were dissolved in DMF
(50 mL) in a 100-mL round-bottomed flask equiped with a condenser.
Potassium carbonate (1.4 g, 10 mmol) was added and the solution was
degassed before being heated at 608C under argon for 17 h. DMF was
removed and the residue was taken up into H2O/CH2Cl2. Extraction with
CH2Cl2 and drying over Na2SO4 left, after the solvent was removed, a white
powder whose purity was sufficient (>95% by NMR) to be used without
‡
‡
1
further purification. White powder; H NMR (200 MHz, CDCl3): d ˆ 7.25
(d, J ˆ 8.6 Hz, 8H; Hb,c), 7.07 (d, J ˆ 8.6 Hz, 6H; Hd), 6.78 (d, J ˆ 8.8 Hz,
2H; Ha), 4.65 (t, 1H; Ha'), 4.12 (m, 2H; Hd), 3.8 ± 3.4 (m, 12H; Ha,b,g
‡Hb',c',d',e'), 1.31 (s, 27H; CH3).
Synthesis of 5: Compound 4 (1.32 g, 1.96 mmol) was dissolved in CHCl3
(15 mL) in a 100-mL round-bottomed flask equipped with a condenser.
EtOH (20 mL) was added and the solution was heated at 808C. HCl 36%
(1 mL, catalytic) was then added and the mixture was refluxed for 18 h. The
solvents were removed and the residue was taken up into H2O/CH2Cl2. The
opaque organic layer was washed with a saturated solution of K2CO3 in
H2O and then with pure H2O to give a translucent organic layer. Drying
over Na2SO4 left, after the solvent was removed, a white powder which was
purified by flash column chromatography (SiO2, eluent CH2Cl2) to afford 5
in 82% yield (944 mg, 1.61 mmol). White powder; 1H NMR (200 MHz,
CDCl3): d ˆ 7.24 (d, J ˆ 8.6 Hz, 8H; Hb,c), 7.08 (d, J ˆ 8.6 Hz, 6H; Hd), 6.79
(d, J ˆ 8.8 Hz, 2H; Ha), 4.12 (m, 2H; Hd), 3.87 (m, 2H; Ha), 3.8 ± 3.6 (m,
4H; Hb,g), 1.32 (s, 27H; CH3).
‡
‡
Synthesis of 10(4) : Complex 9 (285 mg, 0.16 mmol) and 7 (128.5 mg,
0.19 mmol) were dissolved in DMF (16 mL) in a 100-mL Schlenk flask. The
solution was degassed and heated at 588C. Cs2CO3 (520 mg, 1.6 mmol),
maintained in suspension in DMF (5 mL) in an addition funnel thanks to
bubbling argon, was added dropwise over 4 h under argon. The solution
was then stirred under argon for an additional 15 h. DMF was removed and
the residue was taken up in H2O/CH2Cl2 and dried over Na2SO4. Before
purification, the counterions were exchanged from BF4 to PF6, following a
well-known procedure. The product was then purified by column chroma-
‡
tography (SiO2, eluent CH2Cl2/MeOH 2%) to afford 10 (120 mg,
0.048 mmol) in 30% yield. Dark red solid. 1H ROESY NMR (400 MHz,
CD2Cl2): d ˆ 8.75 (d, J ˆ 8.1 Hz, 2H; t3), 8.62 (d, 2H; t6), 8.40 (d, J ˆ 7.9 Hz,
2H; t'3), 8.39 (d, J ˆ 8.4 Hz, 2H; H4',7'), 8.37 (d, J ˆ 8.4 Hz, 2H; H4), 8.04 (t,
J ˆ 7.8 Hz, 1H; t4'), 7.93 (s, 2H; H5,6), 7.75 (d, J ˆ 8.4 Hz, 2H; H3), 7.71 (s,
2H; H5',6'), 7.68 (d, J ˆ 8.4 Hz, 2H; H3',8'), 7.66 (dd, J ˆ 8.1 Hz, 2H; t4), 7.32
(d, J ˆ 8.8 Hz, 4H; Ho), 7.27 (d, J ˆ 8.8 Hz, 4H; Ho'), 7.21 (d, J ˆ 8.6 Hz,
12H; Hc) 7.15 (d, J ˆ 8.8 Hz, 4H; Hb), 7.11 (d, J ˆ 8.6 Hz, 12H; Hd), 6.81 (d,
J ˆ 9 Hz, 4H; Ha), 6.01 (d, J ˆ 8.8 Hz, 4H; Hm'), 5.94 (d, J ˆ 8.9 Hz, 4H;
Hm), 4.13 (m, 4H; Hd), 3.84 (m, 4H; Hg), 3.74 (m, 4H; Hb), 3.67 (m, 4H;
Synthesis of 6: Mesyl chloride (0.62 mL, 7.97 mmol) dissolved in CH2Cl2
(8 mL) was added over 40 min to a solution of 5 (944 mg, 1.59 mmol) in
CH2Cl2 (30 mL) at
58C containing triethylamine (2.8 mL, 2.03 g,
20 mmol). After stirring for 3 h at 58C, the solution was extracted with
H2O/CH2Cl2 and dried over Na2SO4. The solvents were removed and the
crude product was filtered by flash chromatography (SiO2, eluent CH2Cl2)
to afford 6 (900 mg, 1.33 mmol) in 84% yield. White powder; m.p. 2318C;
3316
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0947-6539/99/0511-3316 $ 17.50+.50/0
Chem. Eur. J. 1999, 5, No. 11

Pirouetting Motion of Rotaxanes
3310±3317
Ha), 3.19 (m, 4H; Hz), 2.90 (m, 4H; Hx), 2.10 (m, 4H; Hy), 1.26 (s, 54H;
CH3); MS (FAB-MS): m/z: 2255.2 ([M PF6] , 2255.5).
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118, 12487 ± 12494.
‡
‡
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Synthesis of 11: Complex 10 (40 mg, 16.6 mmol) was dissolved in CH2Cl2
(5 mL). KCN (26 mg; 0.38 mmol) was dissolved in water (1 mL) and added
to the former solution. CH3CN (2 mL) was added to the mixture and the
solution was stirred for 30 min before more CH3CN (2 mL) was added. The
solution was stirred until the initially dark red color had totally dissapeared
(about 1 h). The mixture was then extracted with CH2Cl2 and the resulting
organic layer was carefully washed with water and dried over Na2SO4. The
water was discarded by pouring it into a solution of sodium hypochlorite.
CH2Cl2 was removed to afford the demetalated rotaxane 11 as a colorless
solid (36 mg) quantitaively. Colorless solid; d ˆ 8.68 (d, J ˆ 8.1 Hz, 2H; t3),
8.53 (d, 2H, J ˆ 1.8 Hz; t6), 8.36 (d, J ˆ 7.9 Hz, 2H; t3'), 8.25 (d, J ˆ 8.9 Hz,
4H; Ho'), 8.21 (d, J ˆ 8.6 Hz, 2H; H4), 8.19 (d, J ˆ 8.4 Hz, 2H; H4',7'), 8.16 (d,
J ˆ 8.9 Hz, 4H; Ho), 7.98 (d, J ˆ 8.6 Hz, 2H; H3',8'), 7.96 (d, J ˆ 8.6 Hz, 2H;
H3), 7.90 (t, J ˆ 8, 2.2 Hz, 1H; t4'), 7.71 (s, 4H; H5,6, H5',6'), 7.61 (dd, J ˆ 8 Hz,
2H; t4), 7.20 (d, J ˆ 8.6 Hz, 12H; Hc), 7.07 (d, J ˆ 8.6 Hz, 12H; Hd), 7.00 (dd,
J ˆ 8.6 Hz, 4H; Hb), 6.94 (d, J ˆ 8.8 Hz, 4H; Hm), 6.92 (d, J ˆ 8.8 Hz, 4H;
Hm'), 6.65 (d, J ˆ 8.8 Hz, 4H; Ha), 4.06 (m, 4H; Ha), 4.00 (m, 4H; Hd), 3.79
(m, 12H; Hz, Hb,g), 2.86 (m, 4H; Hx), 2.08 (m, 4H; Hy), 1.28 (s, 54H; CH3);
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2‡
Synthesis of 10(5) : Complex 11 (15.56 mg, 7 mmol) in CH2Cl2 (3 mL) was
introduced in a 10-mL flask and 1 mL of a 7mm pale blue solution of
CuII(BF4)2 in CH3CN (i.e. 7 mmol) was added. The solution turned pale
green and the volume was brought up to 10 mL with CH3CN. This solution
was used for both UVand CVexperiments. Part of it was evaporated to give
the mass characterization. Pale green solid; MS (FAB-MS): m/z: 2255.1
‡
([M PF6] , 2255.5).
Electrochemistry: Electrochemical experiments were carried out using a
EGG Princeton 273 A model potentiostat. All experiments were run at
room temperature. For analytical experiments, a standard three-electrode
cell was used. Potentials are referenced to an Ag wire pseudo-reference
1
electrode with 0.1 mol
L
tetrabutylammonium tetrafluoroborate as
supporting electrolyte in an acetonitrile/dichloromethane (4/1) mixture as
solvent. A platinum disc electrode (2 mm diameter) was used for CV
experiments. A platinum wire (0.6 mm diameter, 52 cm long) was used as
working electrode for electrolysis at controlled potential.
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Received: March 29, 1999 [F 1703]
Chem. Eur. J. 1999, 5, No. 11
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0511-3317 $ 17.50+.50/0
3317