A copper-complexed rotaxane in motion: Pirouetting of the ring on the millisecond timescale

Article abstract of DOI:10.1039/b315080a

A new bistable rotaxane, consisting of a 2,2′-bipyridine-containing thread and a ring incorporating both a bidentate chelate and a tridentate fragment, has been prepared; this complex undergoes an electrochemically driven pirouetting motion of the ring around the axis which takes place on the millisecond timescale, i.e. several orders of magnitude faster than the other copper-based machines previously described.

Full text of DOI:10.1039/b315080a

A copper-complexed rotaxane in motion: pirouetting of the ring on the
millisecond timescale†
Ingo Poleschak, Jean-Marc Kern and Jean-Pierre Sauvage
Laboratoire de Chimie Organo-Minérale, UMR 7513 du CNRS, Université Louis Pasteur, Faculté de
Chimie, 4, rue Blaise Pascal, 67070 Strasbourg Cedex, France
Received (in Cambridge, UK) 21st November 2003, Accepted 15th December 2003
First published as an Advance Article on the web 23rd January 2004
A new bistable rotaxane, consisting of a 2,2A-bipyridine-
containing thread and a ring incorporating both a bidentate
chelate and a tridentate fragment, has been prepared; this
complex undergoes an electrochemically driven pirouetting
motion of the ring around the axis which takes place on the
millisecond timescale, i.e. several orders of magnitude faster
than the other copper-based machines previously described.
rotaxane whose ring can pirouette between two positions around an
axle.10 It is represented in Fig. 1 (compound 1 ).
+
n
The rotaxane 1n+ has two stable configurations: the 4-coordinate
copper(
) complex (14+) and the 5-coordinate copper(II) species
I
(152+) – the subscripts refer to the coordination number of the metal
centre. The interconversion between these two states is performed
electrochemically. Each interconversion process involves two
steps: (i) an electron transfer step (oxidation of Cu( ) or reduction of
I
Triggering and controlling large amplitude molecular motions is
particularly challenging, in relation to electro-, chemo- or photo-
chemical mechanical devices and molecular switches at the
nanometre level.1 Many artificial molecular machines and motors
involve catenanes and rotaxanes2 but several other systems have
also been proposed, either consisting of purely organic compounds3
or based on transition metal complexes.4 The rate of the motion is
an important factor which has been determined in a limited number
of examples. Depending on the nature of the movement, it can
range from microseconds, as in the case of organic rotaxanes acting
as light-driven molecular shuttles,5 to seconds or even minutes in
other systems involving threading–dethreading reactions6 or transi-
tion metal-centred redox processes.7 Metal hopping between two
distinct sites can be relatively fast (hundreds of milliseconds) when
the motion is triggered by a pH-change8 but it seems to be much
slower when it involves ligand exchange following a redox
process.9 The main weak point of our molecular machines based on
Cu(II) either chemically or, better, electrochemically) and (ii) a
rearrangement reaction corresponding to the pirouetting of the ring
around the axle from a position to the other. The rate of the motion
depends strongly on the oxidation state of the copper centre:
(1)
(2)
The 5-coordinate copper ( ) complex rearranges relatively fast
I
( ~ 50 ms) but it takes minutes for the 4-coordinate divalent copper
complex to find its stable form. These rates are obviously far too
low if one wants to elaborate practical systems (switches or
mechanical devices) based on rotaxanes containing the same
fragments as 1.
In order to improve the rate of the motions, we reasoned that
lowering steric hindrance and thus making the metal centre as
accessible as possible should certainly be very favourable since
ligand exchange within the coordination sphere of the copper centre
must be facilitated as much as possible. It is very likely that the rate
the Cu(II)/Cu( ) couple is certainly the long response time of the
I
system.7 Till now, the fastest system we had proposed was a
2+
+
limiting step of each motion (14 ? 152+ and 15 ? 14+) involves
† Dedicated to the memory of Jean-Marc Kern. He passed away in the tragic
airplane accident over Sharm el-Sheikh on the 3rd of January 2004.
decoordination of the metal centre. To verify this hypothesis, the
Fig. 1 Copper(I
)-complexed rotaxanes in motion. The subscripts 4 and 5 indicate the coordination number of the copper centre; 1n+ was described a few years
ago.10
474
C h e m . C o m m u n . , 2 0 0 4 , 4 7 4 – 4 7 6
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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2+
new bistable rotaxane 2n+ was prepared. Its two forms, 24+ and 25
,
Transfer (MLCT) band in the visible, providing an orange-red
+
are depicted in Fig. 1. The molecular axis contains a 2,2A-bipyridine
motif, which is less bulky than a 1,10-phenanthroline (phen)
fragment and thus is expected to spin more readily within the cavity
of the ring. In addition, and probably more importantly, the bipy
chelate does not bear substituents in the a-position to the nitrogen
colour to solutions of 24 .
+
As shown in Fig. 1, it is expected that 24 rearranges to the
2+
5-coordinate species 25 after oxidation and vice versa. The
electrochemically driven motions were studied by cyclic voltam-
metry (CV), which turned out to be the technique of choice to set
the molecule in motion, to monitor the movements and to measure
their rates. Representative CV data are collected in Fig. 3.
atoms in contrast with the corresponding phen fragment of 1n+
.
The new rotaxane has been prepared as the 4-coordinate
monovalent copper species, applying the threading approach
The cyclic voltammograms of 24+, recorded at two different scan
rates, e.g. at 100 mV s21 and at 3000 mV s21 are given in Fig. 3.
By starting the CV at a potential value of 20.4 V (vs. a silver quasi-
previously used in our group to make numerous copper(
complexed rotaxanes and catenanes.11 Since the stoppering reac-
tion of the threaded copper( ) complex intermediates is generally a
I)-
+
I
reference electrode), no current is observed since 24 is electro-
relatively low yielding step, especially when both stoppers are
introduced at the same time, we selected a two-step procedure, as
indicated in Fig. 2.
chemically inactive below the oxidation potential at which Cu( )
I
starts to be oxidized. On increasing the potential towards anodic
values, an oxidation peak at +0.45 V (at 100 mV s21) is observed,
+
Compound 312 was first reacted with an excess of 413 (NaH,
THF, 45 °C, 48 h) to afford the asymmetric bipy 5 in 73% yield,
with one of its ends already attached to a stopper. 5 was
as expected for the 24 ? 242+ redox process. By comparison with
the potential values found for related Cu(II)/Cu( ) couples with
I
+
analogous ligands,15,16 a significant cathodic shift for the 24
?
subsequently reacted with the copper(
I) complex of 6, this complex
242+ process is observed. After the peak potential, Cu(II) is obtained
and the current intensity decreases. By investigating the scan
potential in the reverse direction, starting from 0.9 V to the cathodic
being generated quantitatively in situ from the macrocyclic
compound and [Cu(CH3CN)4]BF4. The threading reaction mon-
1
itored by H-NMR spectroscopy leading to 7+ was also quantita-
region, it is expected that the Cu(II) species be reduced to Cu(
I). If
tive. 7+ is a moderately stable reddish complex which was subjected
to the stoppering reaction (4, K2CO3, dibenzo-18-crown-6,
the Cu(II) complex is still 4-coordinate, the return wave should be
2+
+
observed around 0.4 V, corresponding to the 24 ? 24 process.
By contrast, if the pirouetting motion is faster than the potential
sweep, the return wave corresponding to the reduction of the
4-coordinate Cu(II) complex 242+ is not observed any more. Instead,
as shown in Fig. 3a, it is replaced by another wave corresponding
+
CH2Cl2, 35 °C, 48 h) leading to 24 in a yield of 21%. The
+
1
copper(
and mass spectroscopy.14 Its UV–Vis spectrum (lmax = 418 nm, e
= 3420) is characteristic of copper( ) complexes of aromatic
diimine ligands,15 with a relatively intense Metal-to-Ligand Charge
I
)-complexed rotaxane 24 was characterized by H-NMR
I
2+
to the reduction of the rearranged complex 25 at a slightly
Fig. 2 Synthesis of the 4-coordinate copper(
I
) complexed-rotaxane 24+. i: 4, NaH, THF, 45 °C, 48 h; j: 4, K2CO3, dibenzo-18-crown-6, CH2Cl2, 35 °C,
48 h.
C h e m . C o m m u n . , 2 0 0 4 , 4 7 4 – 4 7 6
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negative potential (Ep = 20.04 V).7,16 A second scan between
20.4 V and +0.9 V (not shown on Fig. 3b) will allow estimation of
We thank the CNRS for its financial support. We would also like
to acknowledge the contribution of the European Commission for a
Marie Curie Fellowship to I. P. and for its financial support.
the rate of the 5-coordinate copper( ) complex rearrangement: the
I
2+
reoxidation wave expected after reduction of 25 should be
observed around 0 V7 if the pirouetting process is slow but at a
+
substantially higher potential, corresponding to 24 ? 242+, if this
Notes and references
process is fast. Fig. 3a corresponds to a series of 20 scans. In this
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+
case as well as in other studies at higher scan rates, 25 is never
observed. This is a clear demonstration that the 5-coordinate
copper( ) complex rearranges rapidly. A lower limit for the rate
I
constant of the process can be estimated using the procedure
reported by Shain and Nicholson:17
3 T. R. Kelly, H. de Silva and R. A. Silva, Nature, 1999, 401, 150; N.
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Using this k-value it can be calculated that t < 2 ms (t = k21).
By applying the same treatment on the wave observed around 0.5
V, an estimate of the rearrangement rate for the slower 4-coordinate
Cu(II) complex is obtained:
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2+
2+
242+) for the 24 ? 25 process shows that 2n+ is nearly three
orders of magnitude faster to rearrange than its sterically hindered
parent compound 1n+. These results also confirm that Cu(
I)
complexes are substitutionally much more labile than Cu(II
species.
)
In conclusion, the use of a non sterically hindering chelate in the
rotaxane axis allows fast motion. Clearly, subtle structural factors
can have a very significant influence on the general behaviour (rate
and reversibility, in particular) of artificial molecular machines. It
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14 1H-NMR (300 MHz, CD2Cl2): 8.74 (d, J 7.8 Hz, 2H, A3), 8.65 (d, J 1.7
Hz, 2H, A6), 8.59 (d, J 8.4 Hz, 2H, G3/4), 8.44 (d, J 7.8 Hz, 2H, D3), 8.09
(s, 2H, H5), 7.98 (d, J 8.4 Hz, 2H, G3/4), 7.96 (t, J 7.8 Hz, 1H, D4), 7.95
(d, J 8.4 Hz, 2H, E6), 7.78 (dd, J 7.8, 1.7 Hz, 2H, A4), 7.75 (d, J 8.2 Hz,
2H, E3), 7.66 (dd, J 8.2, 1.8 Hz, 2H, E4), 7.43 (d, J 8.8 Hz, 4H, F2), 7.31
(d, J 8.2 Hz, 12H, C3), 7.26 (d, J 9.0 Hz, 4H, B3), 7.20 (d, J 8.2 Hz, 12H,
C2), 6.81 (d, J 9.0 Hz, 4H, B2), 6.22 (d, J 8.8 Hz, 4H, F3), 4.81 (s, 4H,
benzyl), 3.55 (m, 4H, CH1), 3.00 (m, 4H, CH3), 2.22 (m, 4H, CH2), 1.31
(s, 52H, t-butyl); MS (FAB-ESI) : 1931.2 (M+); UV/Vis [l(e)] in
CH2Cl2: 418 (3480), 280 (57200), 245 (76800), 229 (82700).
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Fig. 3 Cyclic voltammograms of 2n+ in MeCN with 0.1 mol L21 Bu4NBF4
at two different scan rates. a) represents 20 cycles and shows that the various
CV curves obtained are superimposable. The potentials are referenced
versus a silver quasi-reference electrode.
16 Note: The potentials are referenced in the article against a silver quasi-
2+/+
2+/+
reference electrode. The E1/2 values of 24
MeCN are +0.3 V and 20.1 V respectively.
17 R. S. Nicholson and I. Shain, Anal. Chem., 1964, 36, 706.
and 25
vs. SCE in
476
C h e m . C o m m u n . , 2 0 0 4 , 4 7 4 – 4 7 6
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