Communications
observation shows that the shuttling of the macrocycle is
apparently hampered by the blockade in its path. Treatment
of the complex (0.005 mmol) with a suspension of ion-
exchange resin (Murochelate B-1,[9] which is equivalent to
Dowex A-1; 400 mg) in acetone at pH 5.5 led to complete
decomplexation in 15 minutes and resulted in the original
1
color as well as the H NMR spectrum being restored as 1a
was recovered. Adjusting the pH value of the commercially
available resin (pH 8.5) to 5.0–6.0 was crucial for this
process.[10] At higher pH values the quaternary pyridinium–
carbon bond to the central bipyridine unit in 1a was cleaved,
while at lower pH values decomplexation took place,but the
released 1a could not undergo further complexation because
of protonation on the bipyridine unit.
The same behavior towards the CuI ion was essentially
observed with rotaxane 1b,in which the central 22, ’-bipyr-
idine unit is substituted at the 5,5’-positions.[6] The solution of
1b changed from orange to dark-red upon addition of the
copper salt. However,rotaxane 1c with a central 2,2’-
bipyridine unit substituted at the 6,6’-positions failed to give
a 2:1 complex because of the steric congestion resulting from
the side arms located in proximity to the nitrogen atoms.[11]
With these results in hand,we turned our attention to
devise a system that would enable alternate complexation and
decomplexation. Figure 3a shows the apparatus used for this
purpose. A glass cylinder was separated into two compart-
ments by a filter. The ion-exchange resin (pH 5.5,400 mg) was
added to one compartment (A) while a solution of 1a
(17.2 mg,0.006 mmol) in acetone (3.0 mL) was introduced
into the other (B). A solution of [Cu(CH3CN)4]PF6 (1.1 mg,
0.003 mmol) in acetone (0.2 mL) was added through the inlet
connected to compartment B and the color of the solution
changed instantaneously from orange to dark red (Figure 3b).
The apparatus was then inverted to allow the solution to
transfer to compartment A.[12] The color of the solution
gradually changed and in 15 minutes became the original
orange color (Figure 3c). Inverting the apparatus again
resulted in the solution returning to compartment B,where
further addition of the CuI solution regenerated the complex.
These operations were repeated 10 times and resulted in
alternating color changes (Table 1). Longer times were
understandably required for decomplexation as the repetition
number increased because of reduction of the active sites on
the resin surface. The accumulation of copper salts was
apparent from the gradual green coloration of the resin.
Although the switching process could be readily judged by the
color change of the solution,the formation of complex 5a in
compartment B followed by regeneration of free rotaxane 1a
in compartment A was further confirmed on the basis of
1H NMR spectra obtained by analysis after the fifth and tenth
runs. The spectra could be entirely superimposed on those of
the respective pure species,thus indicating that perfect
switching was realized between the two states. It should also
be noted that the rotaxane was recovered quantitatively
(ca. 100%) after the tenth run,and is thus indicative that no
loss of the materials occurs during the complexation and
decomplexation process or on filtration. The switching events
also occurred with 1b,and the times required for decom-
plexation are also given in Table 1.[13]
Figure 1. Schematic representation of a dynamic/static binary mode.
by introduction of a bulky tosylimino group on the sulfur
atom.[3] Reduction of the resulting sufinylimine with P4S10
regenerated the original sulfide to re-start the shuttling
process. However,no mention was made of whether these
events could be repeated.[4] Herein,we report on novel
molecular shuttles which can be converted repeatedly
between dynamic and static states through alternating
intermolecular complexation and decomplexation processes.
We designed degenerate [2]rotaxanes 1 containing a
central bipyridine moiety for effective formation of a chelate
and bipyridinium units to act as stations for a crown ether
bead. The requisite molecules were prepared according to the
slippage method[5] as shown in Scheme 1. [2]Rotaxane 1a
bearing a central 2,2’-bypyridine moiety substituted at the
4,4’-positions was obtained in 38% yield together with
[3]rotaxane 4a (25%) by heating a solution of thread 2a
and bead 3 in acetonitrile at 558C for 6 days.[6] The 1H NMR
spectrum of 1a exhibited a pattern consistent with a
centrosymmetric molecular structure at room temperature
(Figure 2a). Thus,the crown ether bead moves back and
forward between the two equivalent bipyridinium stations at
a rate which cannot be followed on the NMR time scale.
Addition of a solution of [Cu(CH3CN)4]PF6 (0.5 equiv) in
acetone to a solution of 1a (1.0 equiv) in the same solvent
resulted in the solution immediately turning from orange to
dark red.[7] Evaporation of the solvent afforded the 2:1
complex 5a,which is formed by chelation of the central
bipyridine ligands to a CuI ion. The stoichiometry of the
complex was confirmed by MALDI-TOF mass spectrome-
1
try.[6,8] The H NMR spectrum of this complex in (CD3)2CO
showed a desymmetrization of the original pattern of 1a,and
is most evident for the ring protons of the bipyridinium
moieties (Figure 2b). Each of the four relevant proton signals
was split into two parts with equal intensity: one encircled by
the crown ether ring (Ha’,Hb ’,Hc ’,or Hd ’) and the other
bearing no crown ether (Ha’’,Hb ’’,Hc ’’,or Hd ’’). This
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2004, 43, 2121 –2124