A reverse donor-acceptor bistable [2]catenane

Article abstract of DOI:10.1021/ol800931z

(Figure Presented) A [2]catenane, composed of a π-electron-rich bis-1,5-dioxynaphthalene[38]crown10 (BDNP38C10) ring, mechanically interlocked with a large macrocycle containing two disubstituted tetraarylmethane speed bumps and two different π-electron-deficient units - namely, naphthalene dimide (NpI) and bipyridinium (BIPY²⁺) units - has been synthesized from a [2]rotaxane, containing the former recognition unit, after performing two sequential Cu(I)-catalyzed azide-alkyne cycloadditions with a linker containing the latter recognition unit. The product, which exists as a single co-conformer, wherein the BDNP38C10 ring encircles the NpI unit, undergoes equilibration to give approximately equal amounts of the other co-conformer in which the BDNP38C10 ring encircles the BIPY²⁺ unit.

Full text of DOI:10.1021/ol800931z

ORGANIC
LETTERS
2008
Vol. 10, No. 15
3187-3190
A Reverse Donor-Acceptor Bistable
[2]Catenane
Ali Coskun,†,‡ Sourav Saha,† Ivan Aprahamian,† and J. Fraser Stoddart*,†,‡
California NanoSystems Institute and Department of Chemistry and Biochemistry,
UniVersity of California, Los Angeles, 405 Hilgard AVenue, Los Angeles, California 90095,
and Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road, EVanston,
Illinois 60208
stoddart@northwestern.edu
Received April 22, 2008
ABSTRACT
A [2]catenane, composed of a π-electron-rich bis-1,5-dioxynaphthalene[38]crown10 (BDNP38C10) ring, mechanically interlocked with a large
macrocycle containing two disubstituted tetraarylmethane “speed bumps” and two different π-electron-deficient unitssnamely, naphthalene
dimide (NpI) and bipyridinium (BIPY2+) unitsshas been synthesized from a [2]rotaxane, containing the former recognition unit, after performing
two sequential Cu(I)-catalyzed azide-alkyne cycloadditions with a linker containing the latter recognition unit. The product, which exists as a
single co-conformer, wherein the BDNP38C10 ring encircles the NpI unit, undergoes equilibration to give approximately equal amounts of the
other co-conformer in which the BDNP38C10 ring encircles the BIPY2+ unit.
Mechanically interlocked molecules,1 in particular bistable
[2]catenanes and [2]rotaxanes,2 constitute some of the most
intriguing candidates to serve as nanoscale switches and
machines in the fields of molecular electronic devices
(MEDs)3 and nanoelectromechanical systems4 (NEMS). The
advantages of using such bistable molecules stem from their
abilities to display controllable relative mechanical motions
in response to external stimuli. In a bistable [2]catenane, for
example, the competitive associations of one (degenerate)
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† University of California, Los Angeles.
‡ Northwestern University.
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10.1021/ol800931z CCC: $40.75
Published on Web 07/09/2008
2008 American Chemical Society

ring with two recognition sites grafted into the other
(nondegenerate) ring can be employed to stimulate the
circumrotation of the first ring around the second one.
Thus, a large-amplitude molecular mechanical motion can
be realized within a mechanically interlocked molecules
be it a [2]catenane or a [2]rotaxanesprovided it exhibits
bistability within the time frame of the experiment in
progress. Further necessary conditions for generating highly
controllable molecular actuations in a bistable [2]rotaxane,
for example, are (i) an appreciable difference in the binding
affinities of the ring component for the two different
recognition sites on the dumbbell component and (ii) a means
of altering completely the binding affinities in a wholly
reversible manner. A free energy difference of greater than
1.2 kcal mol-1 between the two translational isomers of a
bistable rotaxane ensures that, at room temperature, at least
90% of the molecules have the ring residing around one site
in preference to the other one. In order to induce motion of
the ring along the dumbbell component within such bistable
rotaxanes, a variety of stimulise.g., chemical, electrochemi-
cal, and photochemicalshave been employed3,4 with con-
siderable success. In the case of MEDs,3 successful device
performance has also been asscociated with a free energy
barrier in excess of 21 kcal mol-1 between the translational
isomers where the so-called metastable state co-conformation
(MSCC) and the ground-state co-conformation (GSCC)
corresponding to the ON and OFF states, respectively of the
switch.
advent more recently6 of the copper(I)-catalyzed azide-alkyne
cycloaddition7 (CuAAC) has made it possible to synthesize
(Scheme 1) a bistable [2]catenane 1·2PF6 with a much larger
Scheme 1. Synthesis of the [2]Catenane 1·2PF6
In the past, bistable donor-acceptor [2]catenanes have been
sythesized by a template-directed protocol5 in which the
recognition sites in one ring act as a template for the
formation of the mechanically interlocked second ring. The
(4) (a) Collin, J.-P.; Dietrich-Buchecker, C.; Gavina˜, P.; Jimenez-Molero,
M. C.; Sauvage, J.-P. Acc. Chem. Res. 2001, 34, 477–487. (b) 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. (c) Lioubashevski, O.; Chegel, V. I.;
Patolsky, F.; Katz, E.; Willner, I. J. Am. Chem. Soc. 2004, 126, 7133–
7143. (d) Berna, J.; Leigh, D. A.; Lubomska, M.; Mendoza, S. M.; Perez,
E. M.; Rudolf, P.; Teobaldi, G.; Zerbetto, F. Nat. Mater. 2005, 4, 704–
710. (e) Liu, Y.; Flood, A. H.; Bonvallet, P. A.; Vignon, S. A.; Northrop,
B. H.; Tseng, H.-R.; Jeppesen, J. O.; Huang, T. J.; Brough, B.; Baller, M.;
Maganov, S.; Solares, S. D.; Goddard, W. A.; Ho, C. M.; Stoddart, J. F
J. Am. Chem. Soc. 2005, 127, 9745–9759. (f) Balzani, V.; Clemente-Leo´n,
M.; Credi, A.; Fetter, B.; Venturi, M.; Flood, A.; Stoddart, J. F. Proc. Natl.
Acad. Sci. U.S.A. 2006, 103, 1178–1183. (g) Brough, B.; Northrop, B.;
Schmidt, J. J.; Tseng, H.-R.; Houk, K. N.; Stoddart, J. F.; Ho, C.-M. Proc.
Natl. Acad. Sci. U.S.A. 2006, 103, 8583–8588. (h) Serreli, V.; Lee, C. F.;
Kay, E. R.; Leigh, D. A. Nature 2007, 445, 523–527. (i) Saha, S.; Stoddart,
J. F. Chem. Soc. ReV. 2007, 36, 77–92. (j) Saha, S.; Flood, A. H.; Stoddart,
J. F.; Impellizzeri, S.; Silvi, S.; Credi, A. J. Am. Chem. Soc. 2007, 109,
12159–12171. (k) Patel, K.; Angelos, S.; Dichtel, W. R.; Coskun, A.; Yang,
Y.-W.; Zink, J. I.; Stoddart, J. F J. Am. Chem. Soc. 2008, 130, 2382–2383.
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free energy barrier between its MSCC and GSCC, while
hopefully retaining the free energy difference between the
two translational isomers. This communication describes the
synthesis, employing CuAAC, of 1·2PF6 and the character-
ization of the bistable [2]catenane using 1H NMR spectros-
copy and cyclic voltammetry in (CD3CN/MeCN) solution.
The synthesis of 1·2PF6, which is composed of a π-electron
rich bis-1,5-dioxynaphthalene[38]crown-10 (BDNP38C10)
ring mechanically interlocked with a second macrocycle
containing two π-electron-deficient recognition sitessnamely,
a naphthalene diimide (NpI) unit and a 4,4′-bipyridinium
(BIPY2+) oneswhich are separated by two bulky tetraaryl-
methane residues, designed to act as “speed bumps”, is
outlined in Scheme 1. The tetraarylmethane derivative 2,
carrying a tosyl group at one end and a hydroxyl group at
the other, was prepared according to the literature.8 The NpI
derivative 3 was isolated in 82% yield, after carrying out a
Mitsunobu reaction between NpI and 2 in the presence of
(5) (a) Templated Organic Synthesis; Diederich, F., ; Stang, P. J., Eds.;
Wiley-VCH, Weinheim, 2000. (b) Busch, D. H.; Stephenson, N. A. Coord.
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C. A.; Mayers, D. C. Tetrahedron 1999, 55, 5265–5293. (e) Hubin, T. J.;
Busch, D. H. Coord. Chem. ReV. 2000, 200, 5–52. (f) Stoddart, J. F.; Tseng,
H.-R. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4797–4800. (g) Blanco, M.-
J.; Chambron, J.-C.; Jimenez, M. C.; Sauvage, J.-P. Top. Stereochem. 2003,
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J.; Leung, K. C.-F.; Stoddart, J. F. Proc. Natl. Acad. Sci. U.S.A. 2007, 104,
17266–17271. (j) Griffiths, K. E.; Stoddart, J. F. Pure. Appl. Chem. 2008,
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ˇ
(6) Miljanic´, O. S.; Dichtel, W. R.; Aprahamian, I.; Rohde, R. D.;
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1165–1174.
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Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–3064.
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and 1·2PF6 confirms that the BDNP38C10 ring still
encircles the NpI unit in the [2]catenane at ambient
temperature, establishing its structural integrity. Heating
1·2PF6 in CD3CN at 70 °C during 60 h results in the
equilibration of the two possible co-conformationssthe
one in which the BDNP38C10 ring still encircles the NpI
unit and another in which the crown ether encircles the
BIPY2+ unit as a consequence of its circumrotation over
the bulky tetraarylmethane groups, or “speed bumps”. The
equilibration of the two co-conformers was monitored at
1
70 °C by running H NMR spectra (Figure 2) at suitable
time intervals at room temperature. At the outset, the
chemical shifts for (i) the methylene protons adjacent to
the triazole rings, (ii) the triazole ring protons, (iii) the
NpI protons, and the ꢀ-protons on the BIPY2+ units
resonate at 4.96, 7.55, 8.17, and 8.20 ppm, respectively.
As time passed, however, a second set of peaks emerged
downfield at 5.06, 7.82, 8.51, and 8.57 ppm for (i), (ii),
Figure 1.
1H NMR spectra (500 MHz, room temperature) of (a)
the NpI diazide 4 in CDCl3, (b) the corresponding [2]rotaxane 5 in
CDCl3, and (c) the [2]catenane 1·2PF6 in (CD3)2CO. Annotations
are shown in color. Blue: HR, Hꢀ and the methylene protons
neighboring BIPY2+. Purple: NpI protons. Red: Aromatic protons
of BDNP38C10.
diethyl azodicarboxylate (DEAD) and PPh3. The resulting
ditosylate 3 was then converted quantitatively to the corre-
sponding diazide 4 by treating 3 with NaN3 at 80 °C in DMF
for 12 h. The [2]rotaxane 5 was obtained in 15% yield by
slippage9 of the BDNP38C10 ring over the bulky tetraaryl-
methane stoppers onto the dumbbell 4 at 70 °C in CHCl3/
MeOH. The bistable [2]catenane was isolated in 25% yield,
following reaction of 5 under CuAAC conditions with the
bisalkyne 6·2PF6 obtained from bipyridine.10
1
Comparison (Figure 1) of the H NMR spectra of the
dumbbell 4, the [2]rotaxane 5, and the [2]catenane 1·2PF6
provides evidence for the encirclement of the NpI unit
by the BDNP38C10 ring in the [2]rotaxane as well as in
the [2]catenane, a situation which is evident from the
upfield shifts of the signals for the four protons on the
NpI unit from 8.75 ppm in 4 to 8.26 ppm in 5 and 8.22
ppm in 1·2PF6. The fact that there is a very small
difference in the δ values for the four NpI protons in 5
Figure 2.
1H NMR spectra (500 MHz, CD3CN, room temperature)
of the [2]catenane 1·2PF6 after heating the solution at 70 °C for (a)
0 h, (b) 20 h, (c) 40 h, and (d) 60 h. The spectrum (a) recorded
before heating the sample shows the presence of only one
co-conformation in which BDNP38C10 ring encircles the NpI unit.
The thermal activation produces the other co-conformation with
the crown ether ring encircling the BIPY2+ unit, as observed from
the emergence of the corresponding peaks in (b), (c), and (d).
Annotations are shown in color. Green line: HR of BIPY2+. Blue
line (indicating the co-conformation in which BIPY2+ is encircled
by BDNP38C10): downfield shifts in Hꢀ of BIPY2+, NpI unit and
triazole protons. Purple line (indicating the co-conformation in
which NpI is encircled by BDNP38C10): Hꢀ of BIPY2+, NpI, and
triazole protons.
ˇ
(8) Dichtel, W. R.; Miljanic´, O. S.; Spruell, J. M.; Heath, J. R.; Stoddart,
J. F. J. Am. Chem. Soc. 2006, 128, 10388–10390.
(9) (a) Kaiser, G.; Jarrosson, T.; Otto, S.; Ng, Y.-F.; Sanders, J. K. M.
Angew. Chem., Int. Ed. 2004, 43, 1959–1962. (b) Iijima, T.; Vignon, S. A.;
Tseng, H.-R; Jarrosson, T.; Sanders, J. K. M.; Marchioni, F.; Venturi, M.;
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Org. Lett., Vol. 10, No. 15, 2008
3189

(iii), and (iv), respectively, assignable to the co-conformer
of 1·2PF6 in which the BDNP38C10 ring encircles the
BIPY2+ unit. After 60 h, the ratio of the two sets of peaks
was 60:40 in favor of 1·2PF6. Although the energy barrier
to circumrotation is calculated to be 28.5 ( 0.3 kcal mol-1,
the free energy difference between the two co-conformers
is less than 0.3 kcal mol-1. Thus, one of our objectives
has been reached at the expense of the other. Although
potential which is identical with that for the first reduction
potential of a free BIPY2+ unitswhile the other ca. 40%
is reduced subsequently at more negative potentials (-0.47
to -0.55 V), suggesting that the BIPY2+ unit is encircled
by the π-electron rich BDNP38C10 ring in this minor
isomeric form of the bistable [2]catenane. The second
reduction of the BIPY•+ unit to the neutral species (BIPY0)
and the reduction of the NpI unit are all shifted toward
more negative potentials, observations that are com-
mensurate with both BIPY2+ and NpI units being encircled
by the BDNP38C10 ring. Neither of the isomers of the
bistable [2]catenane could be switched, however, to the
other one by electrochemical inputs, presumably because
of relatively high energy barrier to their inconversion.
The synthesis of a nondegenerate bistable [2]catenane
under kinetic control using Cu(I)-catalyzed azide-alkyne
cycloadditions turns out to be reasonably efficient, employing
an interaction which may be augmented by some templation
between the diazide 5 and the bisalkyne 6·2PF6. It is not
unlikely, however, that this noncovalent interaction lives on
within [2]catenane afterward helping to equalize the propor-
tion of the two translational isomers. As so often happens
in science, one finds the answer to one question only to raise
another two questions to be answered. It is after this fashion
that our knowledge base is enhanced and progress is made.11
the magnitude of the energy barrier makes 1·2PF6
a
suitable candidate for molecular flash memory in MEDs,
the almost equal populations of the two states of the
molecular switch undermines its candidature somewhat.
Cyclic voltammetry (CV) of the product, after warming
1·2PF6 in MeCN at 70 °C for 60 h, in addition to the CVs
of the dumbbell 4 and the bisalkyne 6·2PF6, confirm
(Figure 3) the presence of both translation isomers in a
Acknowledgment. This work was supported by the
Microelectronics Advanced Research Corporation (MARCO)
and its focus center on Functional Engineered NanoArchi-
tectonics (FENA) and the Center for Nanoscale Innovative
Defense (CNID).
Supporting Information Available: Experimental details
and spectral characterization data of all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL800931Z
(11) In bistable catenanes and rotaxanes, with degeneracy present in
the π-donating cyclophanesnamely, in cyclobis(paraquat-p-phenylene)
(CBPQT4+) sand bistability present in the π-donating crown ether or the
acyclic polyether component of a dumbbell, where the recognition sites
are commonly tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP)
units, the free energy barriers associated with the cyclophane travelling from
the metastable co-conformation with CBPQT4+ encircling DNP back to
the ground state co-conformation in which CBPQT4+ encircles TTF are
in the range 16-23 kcal mol-1, depending upon the structure of the bistable
molecule and the medium in which it finds itself. (Choi, J. W.; Flood, A. H.;
Steuerman, D. W.; Nygaard, S.; Braunschwieg, A. B.; Moonen, N. N. P.;
Laursen, B. W.; Luo, Y.; DeIonno, E.; Peters, A. J.; Jeppesen, J. O.; Xu,
K.; Stoddart, J. F.; Heath, J. R. Chem. Eur. J. 2006, 12, 261–279. ). When
this recognition motif is reversedsas it is in the case 12+sthe free energy
barrier is expected to be very much lower, commonly in the region of only
10 kcal mol-1. (Ashton, P. R.; Belohradsky, M.; Philp, D.; Spencer, N.;
Stoddart, J. F J. Chem. Soc., Chem. Commun. 1993, 1274–1277. ). This
∆Gq value is much too low for most device-oriented applications, and so
it was for this reason that we incorporated speed bumps between the NpI
and BIPY2+ units in 12+. The ∆Gq value of 28.5 kcal mol-1 indicates that
the speed bumps in this bistable [2]catenane are considerably higher than
we would wish in relation to this compound being used in a device setting.
This is the bad news. The good news is that we can, in principle, lower the
∆Gq value by “pruning” methyl groups off the tertiary butyl groups attached
to the tetraarylmethane cores. This exercise is already underway.
Figure 3. Cyclic voltammograms (0.1 M TB·APF6/MeCN, vs SCE,
100 mVs-1) of (a) the BIPY2+ derivative 6·2PF6 (2 mM), (b) the
NpI derivative 4 (2 mM), and (c) the [2]catenane 1·2PF6 (1 mM)
after heating at 70 °C for 60 h, conditions which generated a 60:
40 mixture of the two co-conformations.
roughly 1:1 ratio. The CV of the equilibrated bistable
[2]catenane shows that ca. 60% of the BIPY2+ units are
reduced to the radical cation (BIPY•+) at -0.33 Vsa
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Org. Lett., Vol. 10, No. 15, 2008