Push-pull [2]pseudorotaxanes. Electronic control of threading by switching on/off an intramolecular charge transfer

Article abstract of DOI:10.1021/ol0610338

Addition of acid and base allows switching between two different structures in a push-pull bis(pyridinium)ethane axle, controlling its ability to form [2]pseudorotaxanes with a dibenzo-24-crown-8 ether wheel.

Full text of DOI:10.1021/ol0610338

ORGANIC
LETTERS
2006
Vol. 8, No. 16
3421-3424
Push−Pull [2]Pseudorotaxanes.
Electronic Control of Threading by
Switching ON/OFF an Intramolecular
Charge Transfer
Sarah J. Vella, Jorge Tiburcio, James W. Gauld, and Stephen J. Loeb*
Department of Chemistry and Biochemistry, UniVersity of Windsor,
Windsor, ON, Canada N9B 3P4
loeb@uwindsor.ca
Received April 28, 2006
ABSTRACT
Addition of acid and base allows switching between two different structures in a push−pull bis(pyridinium)ethane axle, controlling its ability
to form [2]pseudorotaxanes with a dibenzo-24-crown-8 ether wheel.
Control over the relative position and motion of components
in interpenetrated or interlocked molecules can impart ma-
chine-like properties at the molecular level. Examples include
threading and unthreading of a [2]pseudorotaxane,1 transla-
tion of the macrocycle in a [2]rotaxane molecular shuttle,2
rotation of the rings in a [2]catenane,3 or reorientation
(flipping, pirouetting) of the cyclic wheel in [2]rotaxanes.4
Previously, we demonstrated the synthetic value of 1,2-
bis(pyridinium)ethane cations as templates for the construc-
tion of interpenetrated and interlocked molecules, such as
[2]pseudorotaxanes,5 [2]rotaxanes,6 [n]polyrotaxanes,7 and
[3]catenanes.8 The interaction between a cationic 1,2-bis-
(pyridinium)ethane axle and a dibenzo-24-crown-8 ether
(DB24C8) wheel occurs by three sets of complementary
interactions: (i) ion-dipole interactions between the N+-pyri-
dinium and the oxygen atoms on the crown ether, (ii) a set
of eight weak C-H‚‚‚O hydrogen bonds between the R-N+
hydrogen atoms and the oxygen atoms on the crown ether,
and (iii) π-stacking between the electron-rich catechol rings
of the crown ether and the electron-poor pyridinium rings
of the axle.
During a systematic study of [2]pseudorotaxane formation
between 1,2-bis(pyridinium)ethane axles and crown ether
wheels, we noted that the presence of an electron-donating
(2) (a) Jeppesen, J. O.; Nygaard, S.; Vignon, S. A.; Stoddart, J. F. Eur.
J. Org. Chem. 2005, 1, 196-220. (b) Leigh, D. A.; Perez, E. M. Chem.
Commun. 2004, 2262-2263. (c) Badjie`, J.; Balzani, V,; Credi, A.; Silva,
S.; Stoddart, J. F. Science 2004, 303, 1845-1849. (d) Vignon S. A.;
Jarrosson T.; Iijima T.; Tseng H-R. Sanders J. K. M.; Stoddart J. F. J.
Am. Chem. Soc. 2004, 126, 9884-9885. (e) Keaveney, C. M.; Leigh, D.
A. Angew. Chem., Int. Ed. 2004, 43, 1222-1224. (f) Loeb, S. J.; Wisner,
J. A. Chem. Commun. 2000, 1939-1940. (g) Balzani, V.; Credi, A.; Raymo,
F. M.; Stoddart, J. F. Angew. Chem., Int. Ed. 2000, 39, 3348-3391. (h)
Sauvage, J.-P. Acc. Chem. Res. 1998, 31, 611-619. (i) Ashton, P. R.;
Ballardini, R.; Balzani, V.; Baxter, I.; Credi, A.; Fyfe, M. C. T.; Gandolfi,
M. T.; Gomez-Lopez, M.; Martinez-Diaz, M.-V.; Piersanti, A.; Spencer,
N.; Stoddart, J. F.; Venturi, M.; White, A. J. P.; Williams, D. J. J. Am.
Chem. Soc. 1998, 120, 11932-11942.
(1) (a) Clemente-Leon, M.; Pasquini, C.; Hebbe-Viton, V.; Lacour, J.;
Dalla Cort, A.; Credi, A. Eur. J. Org. Chem. 2006, 1, 105-112. (b) Credi,
A.; Balzani, V.; Langford, S. J.; Stoddart, J. F. J. Am. Chem. Soc. 1997,
119, 2679-2681. (c) Montalti, M. Chem. Commun. 1998, 1461-1462. (d)
Ballardini, R.; Balzani, V.; Clemente-Leo´n, M.; Credi, A.; Gandolfi, M.
T.; Ishow, E.; Perkins, J.; Stoddart, J. F.; Tseng, H.-R.; Wenger, S. J. Am.
Chem. Soc. 2002, 124, 12786-12795. (e) Huang, F.; Switek, K. A.; Gibson,
H. W. Chem. Commun. 2005, 3655-3657. (f) Sambrook, M. R.; Beer, P.
D.; Wisner, J. A.; Paul, R. L.; Cowley, A. R.; Szemes, F.; Drew, M. G. B.
J. Am. Chem. Soc. 2005, 127, 2292-2302. (g) Kim, K.; Jeon, W. S.; Kang,
J.-K.; Lee, J. W.; Jon, S. Y.; Kim, T.; Kim, K. Angew. Chem., Int. Ed.
2003, 42, 2293-2296.
10.1021/ol0610338 CCC: $33.50
© 2006 American Chemical Society
Published on Web 07/08/2006

NH2 group at the 4-position of the pyridinium group dra-
matically reduced the observed association constant.9 This
was attributed to a reduction in the acidity of the participating
hydrogen bonding groups on the axle but also to a reduction
in the charge at the pyridinium nitrogen due to contributions
from an unfavorable resonance form. We reasoned that it
should be possible to fine-tune the strength of a [2]pseudo-
rotaxane interaction and control the threading-unthreading
process using these different resonance structures.
pyridine obtained by a Suzuki coupling between 4-N,N-
dimethylaminophenylboronic acid and 4-bromopyridine. The
protonated version 24+ was generated by the addition of
CF3SO3H; see Scheme 1.
Scheme 1. Synthesis of Axles 12+ and 24+
To this end, we designed the new axle molecule 12+, which
can be represented by two possible resonance forms having
dramatically different structures and charge distributions; see
Figure 1. This molecule has the structure of an organic D-π-
Figure 1. Dicationic axle 12+ can be represented by two possible
resonance forms.
The X-ray crystal structures of 12+ and 24+ reveal that
there are significant differences in their solid-state structures;
see Figure 2.10 There is evidence that axle 12+ adopts a
A-π-D chromophore with two terminal donor groups (N,N-
dimethylamino) and an inner acceptor group (bis-pyridini-
um). In principle, this should give rise to an intramolecular
charge transfer (ICT) observable in the electronic spectrum,
and we reasoned that it should be possible to turn OFF the
ICT by addition of a Lewis acid, such as BF3 or H+.
The new axle 12+ was synthesized from the reaction of
1,2-dibromoethane with 4′-(N,N-dimethylamino) phenyl-4-
(3) (a) Leigh, D. A.; Wong, J. K. Y.; Dehez, F.; Zerbetto, F. Nature
2003, 424, 174-179. (b) Ceccarelli, M.; Mercuri, F.; Passerone, D.;
Parrinello, M. J. Phys. Chem. B 2005, 109, 17094-17099. (c) Amabilino,
D. B.; Anelli, P.-L.; Ashton, P. R.; Brown, G. R.; Cordova, E.; Godinez,
L. A.; Hayes, W.; Kaifer, A. E.; Philp, D.; Slawin, A. M. Z.; Spencer. N.;
Stoddart. J. F.; Tolley, M. S.; Williams, D. J. J. Am. Chem. Soc. 1995,
117, 11142-11170. (d) Liu, Y,; Bonvallet, P, A.; Vignon, S, A.; Khan, S,
I.; Stoddart, J. F. Angew. Chem., Int. Ed. 2005, 44, 3050-3055. (e)
Yamamoto, T.; Tseng, H.-R.; Stoddart, J. F.; Balzani, V.; Credi, A.;
Marchioni, F.; Venturi, M. Collect. Czech. Chem. Commun. 2003, 68, 1488-
1514. (f) Raymo, F. M.; Stoddart, J. F. In Molecular Switches; Feringa, B.
L., Ed.; Wiley-VCH: Germany, 2001; pp 219-248. (g) Ashton, P. R.;
Baldoni, V.; Balzani, V.; Credi, A.; Hoffmann, H. D. A.; Martinez-Diaz,
M.-V.; Raymo, F. M.; Stoddart, J. F.; Venturi, M. Chem. Eur. J. 2001, 7,
3482-3493.
(4) (a) Sauvage, J.-P. Chem. Commun. 2005, 1507-1510. (b) Loeb, S.
J.; Tiburcio, J.; Vella, S. J. Chem. Commun. 2006, 1598-1600.
(5) Loeb, S. J.; Wisner, J. A. Angew. Chem., Int. Ed. 1998, 37, 2838-
2840.
(6) (a) Loeb, S. J.; Wisner, J. A. Chem. Commun. 1998, 2757-2758.
(b) Davidson, G. J. E.; Loeb, S. J.; Parekh, N. A.; Wisner, J. A. Dalton
Trans. 2001, 3135-3136. (c) Georges, N.; Loeb, S. J.; Tiburcio, J.; Wisner,
J. A. Org. Biomol. Chem. 2004, 2, 2751-2756.
Figure 2. X-ray structures of 12+ (top) and 24+ (bottom) showing
the atom labeling scheme and bond distances.
pseudo-quinoid form with, for example, reduced N1-C3 and
C8-C9 distances (1.367(3), 1.466(4) Å, respectively) and a
small dihedral angle between aromatic rings of 18.4°
supporting delocalization of the positive charge. In contrast,
the bonding parameters for 24+ are indicative of the more
common bis(pyridinium)ethane form with longer N1-C3 and
C8-C9 distances (1.483(5), 1.473(5) Å, respectively) and a
more substantial dihedral angle of 55.4°; see Table 1.
For both 12+ and 24+, DFT (B3LYP) calculations localized
the HOMO on the aniline ring and the LUMO on the
(7) (a) Tiburcio, J.; Davidson, G. J. E.; Loeb, S. J. Chem. Commun. 2002,
1282-1283. (b) Davidson, G. J. E.; Loeb, S. J. Angew. Chem., Int. Ed.
2003, 42, 74-77. (c) Loeb, S. J.; Tramontozzi, D. A. Org. Biomol. Chem.
2005, 3, 1393-1401. (d) Loeb, S. J. Chem. Commun. 2005, 1511. (e)
Hoffart, D. J.; Loeb, S. J. Angew. Chem., Int. Ed. 2005, 44, 901-904.
(8) Hubbard, A. L.; Davidson, G. J. E.; Patel, R. H.; Wisner, J. A.; Loeb,
S. J. Chem. Commun. 2004, 138-139.
(9) Loeb, S. J.; Tiburcio, J.; Vella, S. J.; Wisner, J. A. Org. Biomol.
Chem. 2006, 4, 667-680.
(10) DIAMOND 3.1; CRYSTAL IMPACT, Postfach 1251, D-53002,
Bonn, Germany 2005.
3422
Org. Lett., Vol. 8, No. 16, 2006

by the addition of 2 equiv of acid to yield 24+, which can
then act as a standard 1,2-bis(pyridinium)ethane axle for [2]-
pseudorotaxane formation. A substantial increase in associa-
tion constant was observed for 24+ (∼5-fold) to a value (156
M-1) that is comparable to that observed for 32+ (172 M-1)
for which there is no substituent on the phenyl ring and no
ICT.11
To further assess this system, an analogous series of
axle molecules, 42+, 54+, and 62+, were synthesized with the
N,N-dimethylaniline, N,N-dimethylanilinium, and phenyl
groups moved to the 3-position of the pyridinium ring.
Complexation with DB24C8 in CD3CN was studied, and
although axle 42+ containing the N,N-dimethylaniline group
has a lower association constant, the effect is far less pro-
nounced. Table 2 summarizes the association constants for
Table 1. Comparison of Bond Distances in 12+ and 24+ a
bond
distance for 12+ (Å)
distance for 22+ (Å)
N1-C3
C1-C2
C2-C3
C3-C6
C6-C7
1.367(3)
1.376(4)
1.415(4)
1.406(4)
1.374(4)
1.403(4)
1.466(4)
1.401(4)
1.365(4)
1.348(3)
1.349(3)
1.365(4)
1.410(4)
1.471(3)
1.483(5)
1.381(5)
1.382(5)
1.368(5)
1.380(5)
1.388(5)
1.473(5)
1.390(5)
1.372(5)
1.337(5)
1.344(5)
1.363(5)
1.396(5)
1.491(4)
C7-C8
C8-C9
C9-C10
C10-C11
C11-N2
N2-C12
C12-C13
C9-C13
N2-C14
a The dihedral angle between the pyridine ring and the aniline ring is
18.4° for 12+ and 55.4° for 24+. Both molecules have a crystallographically
imposed center of symmetry.
Table 2. Association Constants, Ka, and ∆G° Values for
12+-62+ with DB24C8 in CD3CN (2 × 10-3 M) at 298 K
a
axle
Ka (M-1
)
∆G (kJ mol-1
)
12+
24+
32+
42+
54+
62+
34
156
172
91
116
125
8.7
12.5
12.8
11.2
11.8
12.0
pyridinium ring, with important contributions from the N
atoms in each case (see Supporting Information). Figure 3
a Values were determined using the single point method by direct
integration of the appropriate resonances. Error estimated at <5%.
axles 12+-62+ with DB24C8, and these results are depicted
graphically in Figure 4.
Figure 3. DFT calculated (B3YLP) electron distributions for (top
to bottom) the dication bis(4-phenylpyridinium)ethane 32+, axle 12+
,
and protonated axle 24+
.
shows a comparison of the electron distribution in 12+ and
24+ along with the unsubstituted axle 1,2-bis(4-phenylpyri-
dinium)ethane, 32+. It is clear that the protonated axle 24+
and the model compound 32+ have a similar electron-poor
distribution (dark blue color) at the central recognition site,
whereas 12+ is quite different (lighter blue), which supports
the notion of different resonance forms for these axles.
Mixing 1 equiv of axle with 1 equiv of DB24C8 yields a
[2]pseudorotaxane that undergoes slow exchange on the
NMR time scale. The new push-pull axle, 12+, binds
DB24C8 in CD3CN solution with a very small association
constant (34 M-1). This OFF scenario can be switched ON
Figure 4. Comparison of the association constants for the formation
of [2]pseudorotaxanes between DB24C8 and the six axles in this
study (blue ) 4-position, purple ) 3 position).
The difference in association constants, the X-ray metrical
parameters, and the DFT calculated structures all support
(11) CD3CN was used for this study because of solubility limitations.
Measurements for 12+ and 24+ in CD3NO2 resulted in larger association
constants (65 and 251 M-1) but show that the relative 5-fold increase holds.
Thus it is likely this difference will be a consistent factor regardless of
solvent.
Org. Lett., Vol. 8, No. 16, 2006
3423

the idea that it is a major contribution from a pseudo-quinoid
resonance form for axle 12+ that results in its anomalously
poor ability to interpenetrate DB24C8 and form significant
noncovalent interactions with the crown ether. Moreover,
protonation of 12+ to give 24+ restores the pyridinium
character of the axle and significantly enhances its ability
to interact noncovalently and form a [2]pseudorotaxane. This
feature can then be used as a methodology to control
threading and unthreading by turning ON and OFF the
interaction.
As predicted, the D-π-A-π-D nature of axle 12+ provides
a chromophore with two terminal donor groups and two inner
acceptor groups and results in an ICT band with λmax at 447
nm in CH3CN solution. The resulting intense yellow-orange
coloration of the naked axle can be completely eliminated
upon the addition of H+ to give the axle 24+; see Figure 5.
reduction of the association constant with DB24C8 in
CD3CN compared to 12+ and 32+ or 42+, 54+, and 62+ and is
an effective OFF state for [2]pseudorotaxane formation.12
When a Lewis acid such as H+ is added, protonation of
the aniline nitrogen atom occurs and the resulting 1,2-bis-
(pyridinium)ethane axle threads through DB24C8 and con-
verts to the ON state. The linking of this mechanical action
of [2]pseudorotaxane formation to a significant color change
can be described as a NOT logic gate since the threading of
the two components to form the interpenetrated molecule is
signaled by the loss of the bright orange color. We have
successfully used alternating equivalents of trifluoromethane-
sulfonic acid and triethylamine to cycle this protonation-
deprotonation in the presence of DB24C8 five times with
no evidence of decomposition or loss of color intensity.
Incorporation of this switchable component into other
mechanically linked systems is in progress.
Figure 6. Schematic representation of the threading and dethread-
ing process initiated by alternating acid and base.
Figure 5. UV-vis absorption spectra of [2]pseudorotaxanes
recorded in CD3CN solution at a concentration of 1.0 × 10-3 M.
Acknowledgment. S.J.L. and J.W.G. thank NSERC of
Canada for support of this research in the form of Discovery
Grants.
The colorless, protonated axle 24+ can then form a [2]pseu-
dorotaxane with DB24C8, which gives rise to a pale yellow
coloration due to a weak charge-transfer interaction between
the electron-rich catechol rings of DB24C8 and the electron-
poor pyridinium rings of the axle.
In summary, the push-pull effect reduces the ability of
the bis(pyridinium)ethane unit to act as a recognition site
toward DB24C8, by decreasing the positive charge on the
pyridinium nitrogen atom, reducing the acidity of the
hydrogen atoms adjacent the pyridinium N+, and increasing
the electron density in the pyridinium rings. This induces a
Supporting Information Available: Experimental de-
tails, NMR spectra, and depictions of HOMO and LUMO,
including files in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL0610338
(12) For a recent ICT example, see: Das, S.; Nag, A.; Goswami, D.;
Bharadwaj, P. K. J. Am. Chem. Soc. 2006, 128, 402-403.
3424
Org. Lett., Vol. 8, No. 16, 2006