A stopperless tetrathiafulvalene based [2]pseudorotaxane molecular shuttle

Article abstract of DOI:10.1016/S0040-4039(01)00627-X

The first stopperless TTF containing molecular shuttle is described.

Full text of DOI:10.1016/S0040-4039(01)00627-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 4223–4226
A stopperless tetrathiafulvalene based [2]pseudorotaxane
molecular shuttle
a
b,
a
b
Martin R. Bryce, Graeme Cooke, * Wayne Devonport, Florence M. A. Duclairoir and
c
Vincent M. Rotello
a
Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK
b
Department of Chemistry, Heriot-Watt University, Riccarton, Edinburgh EH14 4AS, UK
c
Department of Chemistry, University of Massachusetts at Amherst, Amherst, MA 01002, USA
Received 19 February 2001; accepted 11 April 2001
Abstract—The first stopperless TTF containing molecular shuttle is described. © 2001 Elsevier Science Ltd. All rights reserved.
An important recent development in supramolecular
chemistry has been the construction of molecular
machines. Molecular shuttles based upon [2]rotaxanes
are an important representative class. These mechani-
cally linked molecules display switching properties
resulting from the relative positioning of a macrocyclic
p-deficient macrocyclic wheels based upon cyclobis-
(paraquat-p-phenylene) 3. More recently, the reversible
multi-stage redox properties and good electron donor
1
4
properties of tetrathiafulvalene (TTF) have been used
to create donor–acceptor based [2]rotaxanes, in which
this electron rich unit forms multiple stations along the
5
‘
wheel’ component over recognition ‘stations’ situated
axle for the cyclic acceptor 3. Remarkably, analogous
along a dumbbell shaped ‘axle.’ To prevent the wheel
from becoming detached from the axle, the latter mus2t
be terminated by bulky stoppers (Fig. 1a).
[2]pseudorotaxane molecular shuttles, which involve the
shuttling of macrocyclic wheels between multiple TTF
units situated along the axle, have not been described
(Fig. 1c). Here, we exploit for the first time the effective
binding properties between the TTF unit and acceptor
[
2]Pseudorotaxane based shuttle-like systems have also
been described; however, the shuttling process in
supramolecules of this type has been restricted to
threading/dethreading of the axle and wheel compo-
nents under chemical, photochemical and electrochemi-
3,
to
fabricate
the
first
TTF
containing
6
[2]pseudorotaxane based molecular shuttle.
1
,3
cal control (Fig. 1b). The majority of [2]rotaxane
based molecular shuttles have traditionally been fabri-
cated from p-rich polyhydroquinol containing axles and
In order to produce a TTF containing axle suitable for
a [2]pseudorotaxane molecular shuttle, we opted for a
†
structure based upon 2. In this system, the axle is
(b)
(c)
(a)
Figure 1. Rotaxane and pseudorotaxane molecular shuttles.
*
Corresponding author. Fax: 0131 451 3180; e-mail: g.cooke@hw.ac.uk
†
Selected data for 2: mp 154–156°C. Analysis found: C, 48.90; H, 2.60%; required for C28H18O S : C, 48.70; H, 2.60%; m/z (DCI) 691
5
8
+
(
M ); l [(CD ) CO] 8.04 (4H, d, J=9.0 Hz), 7.17 (4H, d, J=9.0 Hz), 6.80 (2H, s), 6.59 (4H, s), 5.13 (4H, s).
H 3 2
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00627-X

4
224
M. R. Bryce et al. / Tetrahedron Letters 42 (2001) 4223–4226
1
terminated by two TTF units that are linked by an
electron rich bisphenylether moiety. This linker was
included to help coax the macrocycle 3 to shuttle
intramolecularly between the two TTF units, thereby
discouraging the dethreading of 3 from 2. The axle was
synthesised by reacting alcohol 1 with half an equiva-
H NMR spectroscopic data performed at ambient
clearly suggest the
temperature on complex
4
[2]pseudorotaxane to be an inclusion complex, with a
TTF unit inserted into the cavity of cyclophane 3. Both
of the TTF units of axle 2 appear to be ‘occupied’ by
the macrocyclic guest 3 with 1:1 stoichiometry, indicat-
ing a rapid shuttling of 3 back and forth along 2 on the
NMR timescale. There are no signals in the spectrum
arising from an unoccupied TTF unit, thereby arguing
against a dethreading of the wheel from the axle. The
changes in the chemical shifts are in accordance with
those reported for the complex of TTF and 3, indicat-
ing a similar binding geometry for underivatised TTF
to that of the substituted TTF derivative of the axle.
The displacements, Dl, in the H NMR spectra of 4 are
collated in Table 1. The largest changes in chemical
shift are experienced by the C H of the cyclophane, the
4%,5%-TTF protons and the phenyl ether protons. The
upfield shift of the TTF protons indicates a complex in
which the TTF ring system is approximately coplanar
with the bipyridinium (bipy) aromatic rings. Larger
chemical shift displacements occur for the TTF protons
2
lent of 4,4%-oxybis(benzenecarbonyl chloride) in CH Cl2
at room temperature for 18 h, and the resulting mixture
was purified using column chromatography (silica gel,
CH Cl /acetone 50:50 v/v) to afford 2 in 73% yield
2
2
2
‡
(
Scheme 1). The [2]pseudorotaxane 4 was prepared by
mixing 2 with an equimolar proportion of 3 in acetone,
which resulted in the immediate formation of an emer-
ald green-coloured solution as a result of the appear-
ance of a charge-transfer (C-T) band in the visible
region of the spectra centred at around 854 nm. FAB
mass spectral analysis revealed peaks at m/z 1645 and
7
1
+
2+
1
500 corresponding to (M−PF6) and (M−2PF ) ,
6 4
6
−
resulting from the loss of one and two PF6 counteri-
ons, respectively, from 4. The association constant (Ka),
obtained using the dilution method in acetone, for 4
was found to be 3300 M , corresponding to a free
energy of complexation of 4.73 kcal mol .
−
1
−
1
O
O
Cl
Cl
O
O
O
S
S
S
S
S
S
S
S
S
S
S
S
OH
O
O
2
O
CH Cl , RT, 18 h.
2
2
1
73 %
2
N
1
equivalent of 2
O
O
N
S
S
S
S
S
N
N
N
O
O
S
O
S
S
4
PF6
N
N
N
4
3
4PF6
Acetone
N
N
O
O
N
N
0.5 equivalent of 2
S
S
O
O
S
S
S
S
O
S
S
N
N
5
N
N
4
PF6
4PF6
Scheme 1.
Table 1. Chemical shift displacements Dl (ppm) for 4 at 20°C in the 400 MHz NMR resonances of guest 3 with host 2
after their admixture in CD COCD
3
3
Unit
aCH(Ph)
bCH(bipy)
C H
NCH2
4-TTF
4%,5%-TTF
TTF CH2
aCH(Ph)
bCH(Ph)
6
4
3
2
+0.07
–
−0.09
–
+0.17
–
−0.03
–
–
–
–
–
–
−0.08
−0.12
+0.03
+0.15
+0.11
‡
Selected data for [2]pseudorotaxane 4: m/z (FAB) 1645 (M−PF ); lH [(CD ) CO] 9.50 (8H, s), 8.50 (8H, s), 8.24 (4H, d, J=8.4 Hz), 7.92 (8H,
6
3 2
s), 7.23 (4H, d, J=8.8 Hz), 6.72 (2H, s), 6.46 (4H, s), 6.11 (8H, s), 5.17 (4H, s).

M. R. Bryce et al. / Tetrahedron Letters 42 (2001) 4223–4226
4225
1
Figure 2. Variable temperature H NMR (400 MHz) data for pseudorotaxanes 4 (a) and 5 (b) recorded at +40 to −80°C in
CD COCD .
3
3
of 2 than for the biphenyl ether protons, implying that
complexation is more associated with a TTF unit.
However, the phenyl ether protons are shifted
downfield indicating that this unit is located close to the
edge of the bipy units, and outside the cavity.
[
2]Pseudorotaxane 4 displayed the temperature-depen-
2
dent behaviour expected of a molecular shuttle. The
variable temperature (VT) H NMR spectra are pro-
1
vided in Fig. 2. Upon cooling a sample of 4, the signals
corresponding to 2 broaden. At −80°C, these signals
split into two sets of signals of equal intensity, arising
from the occupied and unoccupied TTF moieties of 2.
Therefore, the data are consistent with the macrocycle
becoming ‘frozen’ around one of the TTF units at low
Figure
2]pseudorotaxane molecular shuttle 4 [(a) plan view, (b) side
view].
3. Energy
minimised
structure
of
the
[
1
temperature. We have also obtained VT H NMR data
for [3]pseudorotaxane 5, produced by adding two molar
equivalents of 3 to 2. In this system, the shuttling of 3
along axle 2 is prevented by Coulombic repulsion
between the two macrocycles. As a result, any observed
shuttling phenomena would be restricted to threading/
dethreading processes. At ambient temperatures the
NMR spectrum of 5 clearly show both of the TTF units
being occupied by two molecules of 3. Reducing the
temperature to −80°C, the NMR spectrum is consistent
with TTF stations remaining occupied, thereby suggest-
ing the association constants between the components
of 4 and 5 are sufficiently strong to discourage
dethreading, even when intracomplex Coulombic repul-
sion is introduced into the pseudorotaxane.
In conclusion, we have produced the first TTF based
2]pseudorotaxane molecular shuttle, by exploiting the
ability of the TTF units to form effective inclusion
complexes with 3. We are in the process of synthesising
axle units containing TTF moieties with disparate ioni-
sation potentials to furnish electrochemically address-
able shuttles, and this will be reported in due course.
[
Acknowledgements
We have modelled the [2]pseudorotaxane 4 using proce-
6a
We thank the EPSRC, The Royal Society and the
Royal Society of Chemistry for financial support for
this research. We thank Mrs. C. F. Heffernan for VT
NMR spectra.
dures previously described. The energy minimised
structure is shown in Fig. 3, and clearly predicts that
one of the TTF moieties of 2 is deeply inserted into the
cavity of 3 which further supports the NMR and the
UV–vis data described above. Furthermore, it is appar-
ent from the modelled structure that the phenyl ether
ring of 2 is in close proximity to the C H ring of
References
6
4
cyclophane 3, thereby accounting for the large changes
in chemical shift observed for these moieties in the
NMR spectrum of 4.
1. Balzani, V.; Gomez-Lopez, M.; Stoddart, J. F. Acc. Chem.
Res. 1998, 31, 405–414.

4
226
M. R. Bryce et al. / Tetrahedron Letters 42 (2001) 4223–4226
2
3
. Lucio, A. P.; Spencer, N.; Stoddart, J. F. J. Am. Chem.
Soc. 1991, 113, 5131–5133.
. Ashton, P. R.; Balzani, V.; Becher, J.; Credi, A.; Fyfe, M.
C. T.; Mattersteig, G.; Menzer, S.; Nielsen, M. B.; Raymo,
F. M.; Stoddart, J. F.; Venturi, M.; Williams, D. J. J. Am.
Chem. Soc. 1999, 121, 3951–3957.
ton, P. R.; Bissell, R. A.; Clavier, G.; G o´ rski, R.; Kaifer,
A.; Langford, S. J.; Mattersteig, G.; Menzer, S.; Philp, D.;
Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Tolley, M.
S.; Williams, D. J. Chem. Eur. J. 1997, 3, 1113–1135; (c)
Li, Z.-T.; Stein, P. C.; Becher, J.; Jensen, D.; Mørk, P.;
Svenstrup, N. Chem. Eur. J. 1996, 2, 624–633.
4
. For reviews on TTF in supramolecular and materials
chemistry, see: (a) Nielsen, M. B.; Lomholt, C.; Becher, J.
Chem. Soc. Rev. 2000, 29, 153–164; (b) Bryce M. R. J.
Mater. Chem. 2000, 10, 589–598.
6. (a) Bryce, M. R.; Cooke, G.; Duclairoir, F. M. A.;
Rotello, V. M. Tetrahedron Lett. 2001, 42, 1143–1145; (b)
Devonport, W.; Blower, M. A.; Bryce, M. R.; Goldenberg,
L. M. J. Org. Chem. 1997, 62, 885–887.
5
. (a) Damgaard, D.; Nielsen, M. B.; Lau, J.; Jensen, K. B.;
Zubarev, R.; Levillain, E.; Becher, J. J. Mater. Chem.
7. Philp, D.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.;
Williams, D. J. J. Chem. Soc., Chem. Commun. 1991,
1584–1586.
2000, 10, 2249–2258; (b) Anelli, P.-L.; Asakawa, M.; Ash-
.