Poised on the brink between a bistable complex and a compound

Article abstract of DOI:10.1021/ol0171362

(figure presented) An enmeshed supramolecular complex, based on a semi-dumbbell-shaped component containing an asymmetrically substituted tetrathiafulvalene site and a 1,5-dioxynaphthalene site for encirclement by a cyclobis(paraquat-p-phenylene) ring component and with a speed bump in the form of an thiomethyl group situated between the two recognition sites, has been self-assembled. This complex is a mixture in acetone solution of two slowly interconverting [2]pseudorotaxanes, one of which is on the verge of being a [2]rotaxane at room temperature.

Full text of DOI:10.1021/ol0171362

ORGANIC
LETTERS
2
002
Vol. 4, No. 4
57-560
Poised on the Brink between a Bistable
Complex and a Compound
5
Jan Oskar Jeppesen,*,†,‡ Jan Becher, and J. Fraser Stoddart*
‡
,†
Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles,
405 Hilgard AVenue, Los Angeles, California 90095-1569, and Department of
Chemistry, Odense UniVersity (UniVersity of Southern Denmark),
CampusVej 55, DK-5230, Odense M, Denmark
stoddart@chem.ucla.edu
Received November 28, 2001
ABSTRACT
An enmeshed supramolecular complex, based on a semi-dumbbell-shaped component containing an asymmetrically substituted tetrathiafulvalene
site and a 1,5-dioxynaphthalene site for encirclement by a cyclobis(paraquat-p-phenylene) ring component and with a “speed bump” in the
form of an thiomethyl group situated between the two recognition sites, has been self-assembled. This complex is a mixture in acetone
solution of two slowly interconverting [2]pseudorotaxanes, one of which is on the verge of being a [2]rotaxane at room temperature.
The advent of supramolecular chemistry has aroused the
interest of chemists of many different persuasions in
compounds such as catenanes and rotaxanes. The synthetic
use of noncovalent interactions in the synthesis of catenanes
and rotaxanes.
1
A [2]rotaxane is a molecule composed of a ring and
1
guidance provided by noncovalent bonds has transformed
these interlocked molecular compounds from chemical
curiosities into a vibrant area of modern-day research. They
are now prime candidates for the construction of artificial
dumbbell-shaped component. The ring encircles the linear
portion of the dumbbell-shaped component and is trapped
mechanically around it by two bulky stoppers. By contrast,
1
in a [2]pseudorotaxane, at least one of the stoppers on the
2
molecular machines and the fabrication of molecular
dumbbell-shaped component is absent with the consequence
that dissociation into its two components can occur sponta-
neously, i.e., it behaves like a 1:1 complex. Slipping of
macrocycles over the bulky stoppers of dumbbell-shaped
electronic devices.2 Much effort has been devoted during
b,3
the past two decades to trying to understand and control the
†
University of California, Los Angeles.
Odense University (University of Southern Denmark).
4
‡
components has been used sparingly to self-assemble
rotaxanes in solution under thermodynamic control. Judicious4e
choice of the constitutions and the sizes of the slippage
stoppers and the macrocycle’s cavity is essential in order to
reach that fine balance between the system being capable of
slippage and not. When such a balance is achieved, the
macrocyclic component will possess sufficient thermal
energy in solution just above room temperature to permit
its slow passage over the slippage stopper. In this regard, it
(
1) (a) Amabilino, D. B.; Stoddart, J. F. Chem. ReV. 1995, 95, 2725-
2
9
5
828. (b) J a¨ ger, R.; V o¨ gtle, F. Angew. Chem., Int. Ed. Engl. 1997, 36, 930-
44. (c) Breault, G. A.; Hunter, C. A.; Mayers, P. C. Tetrahedron 1999,
5, 5265-5293. (d) Molecular Catenanes, Rotaxanes and Knots; Sauvage,
J.-P., Dietrich-Buchecker, C. O., Eds.; VCH-Wiley: Weinheim, 1999.
(2) (a) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew.
Chem., Int. Ed. 2000, 39, 3348-3391. (b) Pease, A. R.; Jeppesen, J. O.;
Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath J. R. Acc. Chem. Res. 2001,
3
4, 433-444. (c) Ballardini, R.; Balzani, V.; Credi, A.; Gandolfi, M. T.;
Venturi, M. Acc. Chem. Res. 2001, 34, 445-455. (d) Harada, A. Acc. Chem.
Res. 2001, 34, 456-464. (e) Schalley, C. A.; Beizai, K.; V o¨ gtle, F. Acc.
Chem. Res. 2001, 34, 465-476. (f) Collin, J.-P.; Dietrich-Buchecker, C.;
Gavina, P.; Jim e´ nez-Molero M. C.; Sauvage, J.-P. Acc. Chem. Res. 2001,
5
has been concluded that a well-defined cutoff between
3
4, 477-487. (g) Amendola, V.; Fabbrizzi, L.; Mangano, C.; Pallavicini,
P. Acc. Chem. Res. 2001, 34, 488-493.
3) (a) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.;
Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999,
85, 391-394. (b) Wong, E. W.; Collier, C. P.; Belohradsky, M.; Raymo,
F. M.; Stoddart, J. F.; Heath, J. R. J. Am. Chem. Soc. 2000, 122, 5831-
5840. (c) Collier, C. P.; Mattersteig, G.; Wong, E. W.; Lou, Y.; Beverly,
K.; Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000,
289, 1172-1175. (d) Collier, C. P.; Jeppesen, J. O.; Luo, Y.; Perkins, J.;
Wong, E. W.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc. 2001, 123,
12632-12641. (e) Service, R. F. Science 2001, 294, 2442-2443.
(
2
1
0.1021/ol0171362 CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/19/2002

6
rotaxanes and pseudorotaxanes does not exist. In this Letter,
we present a rare example of “piggy-back” supramolecular
assistance leading to the formation in solution of two
nonidentical [2]pseudorotaxanes, one of which is on the brink
7
of becoming a [2]rotaxane. We have reported previously
that slow “internal” passage of the macrocyclic ring com-
4+
ponent, cyclobis(paraquat-p-phenylene) CBPQT , over an
SMe group occurs (Figure 1a) in the amphiphilic bistable
4
+
[
2]rotaxane 1 . Here, we record the results of kinetic and
thermodynamic studies between semi-dumbbell-shaped com-
pound 2 and CBPQT4+ (Figure 1b). Compound 2 contains
two different recognition sitessa tetrathiafulvalene (TTF)
unit and a 1,5-dioxynaphthalene (DNP) ring systemsfor
4+
CBPQT along with a “speed bump” in the form of an SMe
group situated on the rod section between the two recognition
sites.
The inclusion of TTF derivatives inside the cavity of
4
+
8
CBPQT is well documented and leads to the formation
1
of pseudorotaxanes under thermodynamic control upon
mixing of their acyclic and cyclic components in solution.
The occurrence of the threading process is evidenced by the
1
8,9
H NMR and absorption spectra. As a model system for
‚CBPQT‚4PF , we chose to investigate the complexation
Figure 2) of CBPQT4+ with 3 containing only a monopy-
2
(
6
rrolo-TTF unit on its rod section. Mixing equimolar propor-
tions of the semi-dumbbell-shaped compound3 3 and the
d,10
(
4) The “slippage” methodology was first exploited for the rotaxane
syntheses that were conducted in a statistical manner. For examples, see:
a) Harrison, I. T. J. Chem. Soc., Chem. Commun. 1972, 231-232. (b) Schill,
G.; Beckmann, W.; Schweikert, N.; Fritz, H.; Chem. Ber. 1986, 119, 2647-
655. The first template-directed successful synthesis of rotaxanes utilizing
(
2
slippage was reported in 1993. See: (b) Ashton, P. R.; Belohradsky, M.;
Philp, D.; Stoddart, J. F. J. Chem. Soc., Chem. Commun. 1993, 1269-
1
274. For other example, see: (c) Ashton, P. R.; Ballardini, R.; Balzani,
V.; Belohradsky, M.; Gandolfi, M. T.; Philp, D.; Prodi, L.; Raymo, F. M.;
Reddington, M. V.; Spencer, N.; Stoddart, J. F.; Venturi, M.; Williams, D.
J. J. Am. Chem. Soc. 1996, 118, 4931-4951. (d) H a¨ ndel, M.; Plevoets, M.;
Gestermann, S.; V o¨ gtle, F. Angew. Chem., Int. Ed. Engl. 1997, 36, 1199-
Figure 1. (a) Hindered interconversion in a “slow” bistable
amphiphilic [2]rotaxane. (b) Self-assembly of the bistable [2]-
pseudorotaxane 2‚CBPQT‚4PF
6
‚GREEN and its slow interconver-
1
201. (e) Raymo, F. M.; Houk, K. N.; Stoddart, J. F. J. Am. Chem. Soc.
998, 120, 9318-9322. (f) Fyfe, M. C. T.; Raymo, F. M.; Stoddart, J. F.
sion to 2‚CBPQT‚4PF ‚RED.
6
1
In Stimulating Concepts in Chemistry; Shibasaki, M., Stoddart, J. F., V o¨ gtle,
F., Eds.; VCH-Wiley: Weinheim, 2000; pp 211-220.
1
1
4+
(5) Ashton, P. R.; Baxter, I.; Fyfe, M. C. T.; Raymo, F. M.; Spencer,
2
tetracationic cyclophane CBPQT in Me CO leads to the
N.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1998,
20, 2297-2307.
6) The distinction between rotaxanes and pseudorotaxanes is far from
4
+
formation of the [2]pseudorotaxane 3‚CBPQT , as shown
by the immediate formation of a green-colored solution and
the appearance of a broad charge transfer (CT) band centered
1
(
being a straightforward one. When size-complementarity between the
stoppers and the macrocyclic component is achieved, certain “rotaxanes”
behave as pseudorotaxanes and can dissociate into their constituent
components under appropriate conditions. Thus, a species which is a
rotaxane at ambient temperature might well be a pseudorotaxane at elevated
temperatures. Even a solvent change can turn a rotaxane into a pseudoro-
taxane at the same temperature. See: (a) Raymo, F. M.; Stoddart, J. F.
Chem. ReV. 1999, 99, 1643-1663. (b) Chiu, S.-H.; Rowan, S. J.; Cantrill,
S. J.; Glink, P. T.; Garrell, R. L.; Stoddart, J. F. Org. Lett. 2000, 2, 3631-
on 805 nm in the UV-vis spectrum (Me
situation which is characteristic of superstructures containing
2
CO, 298 K), a
8
3
634.
7) Jeppesen, J. O.; Perkins, J.; Becher J.; Stoddart J. F. Angew. Chem.,
Int. Ed. 2001, 40, 1216-1221.
8) (a) Devonport, W.; Blower, M. A.; Bryce, M. R.; Goldenberg, L. M.
(
(
J. Org. Chem. 1997, 62, 885-887. (b) 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.
(9) (a) Ashton, P. R.; Ballardini, R.; Balzani, V.; Boyd, S. E.; Credi, A.;
Gandolfi, M. T.; G o´ mez-L o´ 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) Credi, A.;
Montalti, M.; Balzani, V.; Langford, S. J.; Raymo, F. M.; Stoddart, J. F.
New J. Chem. 1998, 22, 1061-1065.
(
10) Jeppesen, J. O.; Perkins, J.; Becher J.; Stoddart J. F. Org. Lett. 2000,
Figure 2. Complexation of 3 by CBPQT‚4PF
6
.
2
, 3547-3550.
558
Org. Lett., Vol. 4, No. 4, 2002

4
+
of 3 by CBPQT4+ in CD
The associated ∆G° value is -3.2 kcal mol at this
temperature.
a TTF unit located inside CBPQT . A UV-vis dilution
3
COCD
3
at 170 K was obtained.
1
2
-1
experiment was carried out to determine the binding
constant for the 1:1 complexation of CBPQT with 3 in
2
CO at 298 K. The complexation of 3 by CBPQT was
followed by correlating the maximum absorptions of the CT
4+
4
+
Me
Mixing equimolar amounts of the semi-dumbbell-shaped
7
4+
compound 2 and CBPQT in Me
Figure 1b) of 2‚CBPQT‚4PF ‚GREEN, as evidenced by the
spontaneous production of a green-colored solution. The
UV-vis spectrum (Figure 4a) recorded at 296 K in Me CO
2
CO leads to the formation
8
b
bands with the absolute concentrations of the components.
(
6
The binding constant (K
(
a
), which was calculated to be 1300
200 M (ꢀ ) 1310 L mol-1 cm ) in Me
-
1
-1
CO at 298 K,
2
2
12
corresponds to a free energy of complexation (-∆G°) of
.2 kcal mol . In the case of the 1:1 complex, exchange
-
1
4
between the complexed and uncomplexed species occurs
1
3 3
rapidly (Figure 3a) on the H NMR time scale (CD COCD ,
Figure 4. Absorption spectra recorded in Me
:1 mixture of 2 and CBPQT‚4PF immediately after their mixture
trace a), after 2.5 h (trace b), and after 24 h (trace c).
2
CO at 296 K on a
1
6
(
of this solution showed, immediately after its preparation, a
broad band centered on 805 nm as a result of the CT
4
+
8
1
interactions that occur when CBPQT encircles the TTF
unit. Allowing the green solution to stand for 24 h at room
temperature produced a brown solution. The UV-vis
spectrum recorded (Figure 4c) on this solution revealed a
CT band centered on 805 nm, together wth the appearance
of a shoulder at 540 nm, a feature which results from the
Figure 3. Partial (cyclophane region) H NMR spectra (500 MHz)
of the complex formed between CBPQT4 and the semi-dumbbell
in CD COCD at (a) 300 K, (b) 255 K, and (c) 170 K. The
3 3
+
3
descriptions c and uc refer to complexed and uncomplexed
CBPQT4 components, respectively. Spectra were recorded on a
+
-3
4+
1
.73 × 10 M solution of CBPQT (equal to that of 3). The ratio
4+
between the integrals of complexed CBPQT and uncomplexed
CBPQT4 was 4:1 in favor of the complexed CBPQT at 170 K.
+
4+
13
DNP ring system being located inside the cyclophane. It
indicates that a partial interconversion of 2‚CBPQT‚4PF
6
‚
GREEN into 2‚CBPQT‚4PF ‚RED has taken place. The
6
5
00 MHz) at 300 K. Thus, the chemical shifts of the observed
4+
kinetics of the movement of CBPQT from the TTF to the
DNP recognition site were investigated using UV-vis
spectroscopy. Immediately after mixing equimolar amounts
resonances are the average values between those for the
uncomplexed and those for the complexed species. On
cooling the CD
enter the regime of slow exchange and both complexed and
uncomplexed species can be observed (Figure 3c) in the H
NMR spectrum. The local asymmetry present in the monopy-
rrolo-TTF unit is responsible for the desymmetrization of
the complexed CBPQT4 ring. It results, for example, in four
doublets (J ) 6-7 Hz) being observed for the R-bipyri-
dinium protons in 3‚CBPQT . At 255 K, fast exchange is
reestablished (Figure 3b) between the complexed and un-
complexed species. Figure 3c reveals that the cyclophane
protons show significant shifts in their resonances upon
complexation, an observation which makes it possible to
3
COCD
3
solution down to 170 K, the kinetics
of CBPQT and 2, a UV-vis spectrum was recorded
4+
4+
(
Figure 4a) and the movement of CBPQT from the TTF
1
to the DNP recognition site was followed at 296 K using
4+
the TTF-CBPQT CT band (805 nm) as the probe. After
24 h, the system had reached equilibrium and no perceptible
+
changes were observed in UV-vis spectra recorded subse-
quently. The experimental data were subject to a first-order
4+
12
-5 -1
analysis and a rate constant (k ) 2.4 × 10 s ) was
4+
obtained for the passage of CBPQT over the SMe group
in the direction from 2‚CBPQT‚4PF ‚GREEN to 2‚CBPQT‚
6
(13) (a) Ashton, P. R.; Ballardini, R.; Balzani, V.; Boyd, S. E.; Credi,
determine the K
a
value at 170 K using the single-point
A.; Gandolfi, M. T.; G.-L o´ 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) Wolf, R.;
Asakawa, M.; Ashton, P. R.; G o´ mez-L o´ pez, M.; Hamers, C.; Menzer, S.;
Parsons, I. W.; Spencer, N.; Stoddart, J. F.; Tolley, M. S.; Williams, D. J.
Angew. Chem., Int. Ed. 1998, 37, 975-979. (c) Balzani, V.; Ceroni, A.;
Credi, A.; G o´ mez-L o´ pez, M.; Hamers, C.; Stoddart J. F.; Wolf, R. New J.
Chem. 2001, 25, 25-31.
method.12 A K
-1
a
value of 11 500 M for the complexation
(
11) Asakawa, M.; Dehaen, W.; L’abb e´ , G.; Menzer, S.; Nouwen, J.;
Raymo, F. M.; Stoddart, J. F.; Williams, D. J. J. Org. Chem. 1996, 61,
591-9595.
12) For details, see Supporting Information.
9
(
Org. Lett., Vol. 4, No. 4, 2002
559

4
PF
6
‚RED in Me
2
CO at 296 K. The corresponding free
recognition site of 2‚CBPQT‚4PF
UV-vis spectroscopy. Immediately after isolation of 2‚
CBPQT‚4PF ‚RED, a UV-vis spectrum was recorded and
6
were investigated using
12
‡
energy of activation (∆G ) for this co-conformational
change is 24 kcal mol .
-
1
6
4
+
Thin-layer chromatography (TLC) of the equilibrated
the passage of CBPQT from the DNP to the TTF
brown solution containing 2‚CBPQT‚4PF
CBPQT‚4PF ‚RED showedsbesides a yellow and a colorless
spot arising from 2 and CBPQT , respectivelysonly a red
spot, thus indicating that 2‚CBPQT‚4PF ‚RED may be
isolated as a rotaxane-like complex, while 2‚CBPQT‚4PF
GREEN dissociates into its components (i.e., 2 and CB-
PQT ) on the time scale of the TLC experiment performed
on silica gel. By employing flash column chromatography,
it was possible to isolate the red co-conformation 2‚CBPQT‚
6
‚GREEN and 2‚
recognition site was followed (Figure 5a-c) at 296 K using
4
+
6
the DNP-CBPQT CT band (540 nm) as probe. After 24
h, the system reached equilibrium and no perceptible changes
were observed in UV-vis spectra recorded subsequently.
As a consequence of the spectroscopic behavior, the color
of the solution goes from red to brown. On carrying out a
4+
6
6
‚
4+
12
-5
first-order kinetic analysis, a rate constant (k ) 2.5 × 10
-
1
4+
s ) for the passage of CBPQT over the SMe group in the
direction from 2‚CBPQT‚4PF
GREEN can be obtained. The ∆G value for this co-
conformational change is 24 kcal mol .
6
‚RED to 2‚CBPQT‚4PF ‚
6
‡
12
6
4PF ‚RED. The UV-vis spectrum (Figure 5a) of 2‚CBPQT‚
-
1
In summary, a bistable supramolecular complex, based on
a ring component which threads onto a semi-dumbbell-
shaped component containing two different recognition sites
in a fast step to form initially a kinetically labile [2]-
pseudorotaxane, before progressing more slowly onto form-
ing a second more kinetically stable [2]pseudorotaxane, has
been characterized. A “speed bump” in the shape of a
thiomethyl group has made it possible to isolate the second
of the two [2]pseudorotaxanes, establishing that it has some
[2]rotaxane character to it. The unique properties of this
bistable complex make it an attractive candidate for incor-
poration into redox-controllable, piston-like, motor mol-
ecules, as well as other kinds of artificial molecular
Figure 5. Absorption spectra recorded in Me
2
CO at 296 K on
‚RED immediately after its isolation (trace a), after
h (trace b), and after 24 h (trace c).
2,14
machinery.
2
4
‚CBPQT‚4PF
6
Acknowledgment. This research was funded by the
Defense Advanced Research Projects Agency in the United
States and by the Carlsbergfondet in Denmark.
6
4PF ‚RED reveals a CT band in the form of a shoulder at
5
40 nm. It results from the DNP ring system being located
Supporting Information Available: Experimental pro-
13
4+
inside CBPQT . Furthermore, no absorption band is
observed in the region 750-850 nm for a CT interaction
that would result from the TTF unit being located inside the
cyclophane, supporting the conclusion that the DNP ring
system in the semi-dumbbell-shaped component is encircled
exclusively by CBPQT . Allowing the red solution of 2‚
CBPQT‚4PF ‚RED to stand for 24 h at 296 K results in a
return to the “original” spectrum (Figure 5c) as a conse-
quence of the passage of CBPQT from the DNP to the
TTF recognition site. The first-order kinetics of this passage
wherein CBPQT4 moves from the DNP to the TTF
cedures for the syntheses of 2‚CBPQT‚4PF
6
and 3‚CBPQT‚
4
PF and details on the calculations of binding constants and
6
kinetic and thermodynamic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
8
4+
OL0171362
6
(14) (a) Leigh, D. A.; Murphy, A. Chem. Ind. 1999, 178-183. (b) Ward,
M. D. Chem. Ind. 2000, 22-26. (c) Schalley, C. A.; Beizai, K.; V o¨ gtle, F.
Acc. Chem. Res. 2001, 34, 465-476. (d) Shinkai, S.; Ikeda, M.; Sugasaki,
A.; Takeuchi, M. Acc. Chem. Res. 2001, 34, 494-503. (e) Feringa, B. L.
Acc. Chem. Res. 2001, 34, 504-513. (f) Kelly, T. R. Acc. Chem. Res. 2001,
34, 514-522.
4+
+
560
Org. Lett., Vol. 4, No. 4, 2002