A tuneable self-complexing molecular switch

Article abstract of DOI:10.1021/ol0602553

A new self-complexing donor-acceptor system has been synthesized that has the propensity to undergo intramolecular decomplexation under thermal and electrochemical perturbation and upon addition of a competitive guest for the cyclophane's cavity.

Full text of DOI:10.1021/ol0602553

ORGANIC
LETTERS
2006
Vol. 8, No. 7
1423-1426
A Tuneable Self-Complexing Molecular
Switch
Graeme Cooke,*,† Patrice Woisel,*,‡ Marc Bria,§ Francois Delattre,‡
¸
James F. Garety,† Shanika Gunatilaka Hewage,† Gouher Rabani,† and
|
Georgina M. Rosair
WestCHEM, Department of Chemistry, Joseph Black Building, UniVersity of Glasgow,
Glasgow G12 8QQ, U.K., Laboratoire de Synthe`se Organique et EnVironnement,
UniVersite´ du Littoral Coˆte d’Opale, Dunkerque 59140, France, CCM-RMN Lille 1,
UniVersite´ des Sciences et Techniques de Lille, VilleneuVe d’Ascq 59655, France, and
ScotCHEM, Chemistry, School of Engineering & Physical Sciences, Heriot-Watt
UniVersity, Edinburgh EH14 4AS, U.K.
graemec@chem.gla.ac.uk
Received January 30, 2006
ABSTRACT
A new self-complexing donor−acceptor system has been synthesized that has the propensity to undergo intramolecular decomplexation
under thermal and electrochemical perturbation and upon addition of a competitive guest for the cyclophane’s cavity.
Self-complexing systems fabricated using complementary
donor-acceptor interactions have received considerable
attention over the past few years.1 Supramolecules of this
type are of interest due to their intrinsic topology and ability
to undergo controllable intramolecular complexation, making
them possible candidates for constructing rudimentary mo-
lecular machines and devices.2 Donor-acceptor self-com-
plexing supramolecules have been prepared by attaching an
electron-rich arm containing either a 1,5-dioxynaphthalene
unit or a tetrathiafulvalene (TTF) unit to the electron-deficient
cyclophane cyclobis(paraquat-p-phenylene) (CBPQT4+). Here,
we report a new class of CBPQT4+-based self-complexing
system whereby the arm contains a pyrrole moiety as the
electron-rich unit. Furthermore, we report that the self-
complexing properties of this system can be disrupted
thermally, electrochemically, and by the addition of a more
effective guest for the cyclophane’s cavity.
Scheme 1 outlines the synthesis of 1. Reduction of 2,1a
using diisobutylaluminum hydride (DIBAL-H) as reducing
agent, afforded the corresponding alcohol 3 in good yield.
Subsequent esterification of 3 with acid 43 furnished
compound 5. Formation of the cyclophane was achieved
using the clipping methodology, by stirring 5 and salt 64 in
the presence of template 7 in DMF at room temperature for
10 days. Compound 1 was separated by column chroma-
tography (SiO2, MeOH/NH4Cl/MeNO2) and isolated, after
† University of Glasgow.
‡ Universite´ du Littoral Coˆte d’Opale.
§ Universite´ des Sciences et Techniques de Lille.
| Heriot-Watt University.
(1) For examples, see: (a) Ashton, P. R.; Ballardini, R.; Balzani, V.;
Boyd, S. E.; Credi, A.; Gandolfi, M. T.; Go´mez-Lo´pez, M.; Iqbal, S.; Philp,
D.; Preece, J. A.; Prodi, L.; Ricketts, H. G.; Stoddart, J. F.; Tolley, M. S.;
Venturi, M.; White, A. J. P.; Williams, D. J. Chem. Eur. J. 1997, 3, 152-
170. (b) Nielsen, M. B.; Nielsen, S. B.; Becher, J. Chem. Commun. 1998,
475-476. (c) Nielsen, M. B.; Hansen, J. G.; Becher, J. Eur. J. Org. Chem.
1999, 2807-2815. (d) Liu, Y.; Flood, A. H.; Stoddart, J. F. J. Am. Chem.
Soc. 2004, 126, 9150-9151. (e) Liu, Y.; Flood, A. H.; Moscowitz, R. M.;
Stoddart, J. F. Chem. Eur. J. 2005, 11, 369-385.
(2) Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and
Machines: A Journey into the Nano World; Wiley-VCH: Weinheim, 2003.
(3) Pickett, C. J.; Ryder, K. S. J. Chem. Soc., Perkin Trans. 1 1994,
2181-2189.
10.1021/ol0602553 CCC: $33.50
© 2006 American Chemical Society
Published on Web 03/09/2006

the π-electron-deficient bipyridinium units of the cyclophane
(Figure 2). The absorbance of 1 decreased linearly upon
Scheme 1
Figure 2. Selected UV-vis spectra of 1 recorded at the following
concentrations in CH3CN at 298 K: (a) 0.1 mmol, (b) 0.4 mmol,
(c) 0.8 mmol.
exchanging the Cl- counterions of the cyclophane with NH4-
PF6, as an orange-red solid (see the Supporting Information).
The pale orange/red color of 1 in the solid and solution
state prompted us to investigate whether the electron-rich
pyrrole unit complexes with the CBPQT4+ moiety. X-ray
quality crystals were obtained by slow evaporation of solvent
from a concentrated solution of 1 in acetonitrile/methanol
(Figure 1 and the Supporting Information). The structure
dilution (within the concentration range 4 × 10-3 to 5 ×
10-5 M, see the Supporting Information), suggesting the
presence of a intramolecular equilibrium and the absence of
higher order equilibria. Moreover, significant concentration
dependence was not observed in the 1H NMR spectrum of 1
recorded in CD3CN (10-2-10-4M, ∆δ < 0.1 ppm), further
suggesting the absence of intermolecular complexation in
the solution state (see the Supporting Information). Thus, it
is unlikely that supramolecular oligomers of 1 are formed
in solution to any significant extent under the conditions
examined, which is in contrast to its structure in the solid
state, where intermolecular complexation is clearly observed.
The most important features of the 1H NMR spectra of 1
recorded in CD3CN at 298 K are summarized in Figure 3.
Figure 1. X-ray crystal structure of 1.
clearly indicates that the system forms a dimer in the solid-
state, with the pyrrole units being located within the electron-
deficient cavity of the cyclophane moiety of another molecule
of 1.
To obtain insight into the structure of 1 in solution, UV-
vis spectroscopy, 1D NMR, and 2D NOESY experiments
were undertaken. The absorption spectrum of 1 recorded in
CH3CN at 298 K shows a broad band in the visible region
(λmax ) 433 nm, ꢀ ) 677 M-1cm-1) resulting from charge-
transfer (CT) interactions between the pyrrole moiety and
1
Figure 3. Partial H NMR spectra of (a) 8 and (b) 1 recorded in
CD3CN at 298 K, showing the relative position of protons HR and
Hâ of their pyrrole moieties.
First, the two pyrrole proton resonances (HR, Hâ) are centered
at unusually high fields of 3.90 and 3.48 ppm, respectively,
compared to those belonging to the “open” model 8 and
N-methylpyrrole, suggesting that the pyrrole ring is located
(4) Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Delgado,
M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philp, D.; Pietraszk-
iewicz, M.; Prodi, L.; Reddington, M. V.; Slawin, M. V.; Spencer, A. M.
Z.; Stoddart, J. F.; Vicent, C.; Williams, D. J. J. Am. Chem. Soc. 1992,
114, 193-218.
1424
Org. Lett., Vol. 8, No. 7, 2006

inside the cavity of the tetracationic cyclophane. Second, the
lack of “uncomplexed” pyrrole proton resonances between
6 and 7 ppm indicate a fast equilibrium, at least on 1H NMR
time scale, between the “self-complexed” and the “uncom-
plexed” forms of 1.
With self-complexation confirmed for 1, we next turned
our attention to whether complex formation could be
controlled using an external stimulus. The fast in-out
isomerism of the pyrrole arm has been characterized by
1
variable-temperature H NMR spectroscopy (see the Sup-
porting Information). Indeed, by comparison of spectra
recorded at 298 K, lowering the temperature of 1 to 203 K
causes a significant upfield shift in the chemical shift of the
pyrrole protons, indicating the displacement of the equilibria
toward the formation of the self-complexed conformation.
In contrast, increasing the temperature results in a downfield
shift of the pyrrole protons indicating decomplexation occurs.
This process could also be monitored spectrophotometrically
by recording the UV-vis spectra at different temperatures,
which showed that a gradual loss of the charge-transfer
absorption occurred as the temperature is increased from
ambient temperature. Thus, the data are consistent with
decomplexation occurring upon heating 1 above room
temperature.
The presence of the pyrrole moiety inside the cavity has
been confirmed using 2D NOESY spectroscopy recorded in
CD3COCD3 at 298 K (Figure 4). The spectrum exhibits
As an alternative strategy for disrupting self-complexation
of 1, we have exploited the well-documented ability of
reducing the CBPQT4+ unit to its diradical dicationic state
to induce pseudorotaxane dethreading.4 To investigate this,
we have compared the cyclic and square wave voltammo-
grams of separate solutions of model compound 9 and 1 in
acetonitrile (0.1 M Bu4NPF6). These studies indicated that
the first reduction wave of 1 occurred at a 32 mV more
negative potential than that of 9 (which lacks a pyrrole
moiety), presumably due to donor-acceptor interactions
between the pyrrole and cyclophane moieties stabilizing the
Figure 4. Partial 2D NOESY NMR spectra of 1 recorded in CD3-
COCD3 at 298 K and a representation of the possible geometry of
1 in its self-complexed form.
several cross-peaks between both the HR and Hâ protons of
the pyrrole moiety and either the phenylene protons (Hi, Hq)
or those belonging to the viologen moieties (He, Ho, Hf, Hg)
of 1, suggesting that the pyrrole moiety probably adopts an
average position close to the center of the cavity. This
hypothesis is also supported by the strongest dipolar cor-
relation observed between the Hâ and the Hf protons.
Moreover, the lack of dipolar correlations between the
protons H1 and H2 of the pendant arm and the protons of
the tetracationic macrocycle indicates that the pyrrole part
is not deeply included in the cyclophane’s cavity (Figure
4).
Figure 5. Square wave voltammograms of 9 (-) and 1 (‚‚‚). Step
potential ) 0.004 V; pulse amplitude ) 0.025 V; quiet time ) 2
s.
Org. Lett., Vol. 8, No. 7, 2006
1425

diradical dication state of the latter (Figure 5).5 The half-
wave potential of the second reduction wave is largely
unaffected, indicating the pyrrole unit dethreads when the
cyclophane is first reduced.
tent with the TTF unit disrupting self-complexation by
forming a pseudorotaxane with the CBPQT4+ moiety of 1.
In conclusion, we have shown that compound 1 self-
complexes in solution. Furthermore, by applying an external
stimuli in the form of heat, electrochemical reduction or the
addition of a completing guest for the cyclophane’s cavity,
we have shown that self-complexation can be disrupted.
Further work in our laboratory will investigate the complex
formation between CBPQT4+ and noncovalently linked
pyrrole units (and their oligomers) and the electropolymer-
ization of the pyrrole moiety of 1 to furnish surface-confined
electrochemically tuneable CBPQT4+ host units.
Finally, we have explored the addition of TTF to disrupt
self-complexation, as it is well established that TTF is an
effective guest for CBPQT4+-based cyclophanes.6 Addition
of aliquots of TTF to a cuvette containing 1 immediately
resulted in the appearance of an emerald-green solution
characteristic of TTF-CBPQT4+ complexes, resulting from
a charge-transfer band centered around 870 nm (Figure 6).
Acknowledgment. We thank the EPSRC for funding this
work.
Supporting Information Available: Full synthetic details
pertaining to the preparation of 1. Characterization of 1 using
single-crystal X-ray diffraction, UV-vis, and NMR spec-
troscopies and electrochemistry. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL0602553
(5) All electrochemical experiments were performed using a CH Instru-
ments 620A electrochemical workstation. The electrolyte solution (0.1 M)
was prepared from recrystallized Bu4NPF6 using spectroscopic grade CH3-
CN. A three-electrode configuration was used with a 2 mm diameter Pt
disc working electrode, a Ag/AgCl reference electrode, and a platinum wire
as the counter electrode. The solution was vigorously purged with nitrogen
prior to recording the electrochemical data. All voltammetry measurements
were recorded under a nitrogen atmosphere.
(6) For examples, see: (a) Devonport, W.; Blower, M. A.; Bryce, M.
R.; Goldenberg, L. M. J. Org. Chem. 1997, 62, 885-887. (b) 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. (c) Nielsen, M.
B.; Jeppesen, J. O.; Lau, J.; Lomholt, C.; Damgaard, D.; Jacobsen, J. P.;
Becher, J.; Stoddart, J. F. J. Org. Chem. 2001, 66, 3559-3563.
Figure 6. UV-vis spectra of 1 (-) (6.5 × 10-4 M) in CH3CN
and upon the addition of 1 equiv of TTF (-).
This process was also monitored using 1H NMR spectroscopy
and revealed a downfield shift for the HR and Hâ pyrrole
protons and a concomitant upfield shift for the TTF protons
(compared to the NMR spectra of TTF recorded in CD3CN)
upon addition of aliquots of the latter (see the Supporting
Information). Thus, the UV-vis and NMR data are consis-
1426
Org. Lett., Vol. 8, No. 7, 2006