A neutral naphthalene diimide [2]rotaxane

Article abstract of DOI:10.1021/ol3022963

A neutral donor-acceptor [2]rotaxane, which has been synthesized using click chemistry, has had its solid-state structure and superstructure elucidated by X-ray crystallography. Both dynamic ¹H NMR spectroscopy and electrochemical investigations have been employed in an attempt to shed light on both geometrical reorganization and redox-switching processes that are occurring or can be induced within the [2]rotaxane.

Full text of DOI:10.1021/ol3022963

ORGANIC
LETTERS
2012
Vol. 14, No. 20
5188–5191
A Neutral Naphthalene Diimide
[2]Rotaxane
Henri-Pierre Jacquot de Rouville, Julien Iehl, Carson J. Bruns, Psaras L. McGrier,
Marco Frasconi, Amy A. Sarjeant, and J. Fraser Stoddart*
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208-3113, United States
stoddart@northwestern.edu
Received August 17, 2012
ABSTRACT
A neutral donorÀacceptor [2]rotaxane, which has been synthesized using click chemistry, has had its solid-state structure and superstructure
elucidated by X-ray crystallography. Both dynamic 1H NMR spectroscopy and electrochemical investigations have been employed in an attempt to
shed light on both geometrical reorganization and redox-switching processes that are occurring or can be induced within the [2]rotaxane.
During the past 30 years, the advent of the mechanical
bond1 has provided a fresh incentive to chemical synthesis
and opened up a whole new field of scientific research,
which we have referred to as mechanostereochemistry.2
Countless mechanically interlocked molecules2,3 (MIMs)
havebeensynthesizedby exploitingmolecularrecognition4
and self-assembly5 processes in their template-directed6
synthesis. For the class of MIMs templated by aromatic
donorÀacceptor interactions, X-ray crystal structures of
[2]catenanes7 are commonplace,8 whereas solid-state struc-
tures of donorÀacceptor [2]rotaxanes are far more rare
since they have proven to be, with a few exceptions,9 much
harder to crystallize on account of the flexibility of their
dumbbell components.
(1) (a) Frisch, H.; Martin, I.; Mark, H. Monatsch. Chem. 1953, 84,
250–256. (b) Schill, G. Catenanes, Rotaxanes and Knots; Academic Press:
New York, 1971. (c) Amabilino, D. B.; Stoddart, J. F. Chem. Soc. Rev. 1995,
95, 2725–2828. (d) MolecularCatenanes, Rotaxanes and Knots:A Journey
through the World of Molecular Topology; Sauvage, J.-P., Dietrich-
Buchecker, C., Eds.; Wiley-VCH: Weinheim, 1999. (e) Stoddart, J. F. Chem.
Soc. Rev. 2009, 38, 1521–1529 and 1802À1820. (f) Forgan, R. S.;
Sauvage, J.-P.; Stoddart, J. F. Chem. Rev. 2011, 111, 5434–5464.
(g) Bruns, C. J.; Stoddart, J. F. Top. Curr. Chem. 2012, 323, 19–72.
(2) Olson, M. A.; Botros, Y. Y.; Stoddart, J. F. Pure Appl. Chem.
2010, 82, 1569–1574.
The Sanders group has developed and popularized a
family of neutral donorÀacceptor catenanes10 comprising
(6) (a) Busch, D. H.; Stephenson, N. A. Coord. Chem. Rev. 1990, 100,
119–154. (b) Anderson, S.; Anderson, H. L.; Sanders, J. K. M. Acc.
Chem. Rev. 1993, 26, 469–475. (c) Raymo, F. M.; Stoddart, J. F. Pure
Appl. Chem. 1996, 68, 313–322. (d) Breault, G. A.; Hunter, C. A.;
Mayers, P. C. Tetrahedron 1999, 55, 5265–5293. (e) Griffiths, K. E.;
Stoddart, J. F. Pure Appl. Chem. 2008, 80, 485–506.
(3) Deng, H.; Olson, M. A.; Stoddart, J. F.; Yaghi, O. M. Nat. Chem.
2010, 2, 439–443.
(4) (a) Cram, D. J.; Cram, J. M. Science 1974, 183, 803–809. (b) Lehn,
J.-M. Science 1985, 227, 849–856. (c) Lehn, J.-M. Angew. Chem., Int. Ed.
Engl. 1990, 29, 1304–1319. (d) Lehn, J.-M. Supramolecular Chemistry:
Concepts and Perspectives; Wiley-VCH: New York, NY, 1995. (e) Rebek, J.
Chem. Soc. Rev. 1996, 25, 255–264. (f) Spruell, J. M. Pure Appl. Chem.
2010, 82, 2281–2294.
(5) (a) Baxter, P.; Lehn, J.-M.; DeCian, A.; Fisher, J. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 69–72. (b) Chichak, K. S.; Cantrill, S. J.; Pease,
A. R.; Chiu, S. H.; Cave, G. W. V.; Atwood, J. L.; Stoddart, J. F. Science
2004, 304, 1308–1312. (c) Ayme, J.-F.; Beavis, J. E.; Leigh, D. A.;
McBurney, R. I.; Rissanen, K.; Schultz, D. Nat. Chem. 2012, 4, 15–20.
(d) Sun, Q. F.; Sato, S.; Fujita, M. Nat. Chem. 2012, 4, 330–333. (e)
Belowich, M. E.; Valente, C.; Smaldone, R. A.; Friedman, D. C.; Thiel,
J.; Cronin, L.; Stoddart, J. F. J. Am. Chem. Soc. 2012, 134, 5243–5261.
(7) Not only are there a limited number of degrees of (co)-conforma-
tional freedom that the molecules of donorÀacceptor [2]catenanes can
adopt, but they can also, and do commonly, pack as extended donorÀ
acceptor πÀπ stacks at the supramolecular level in the crystal.
(8) Zhao, Y.-L.; Shveyd, A. K.; Stoddart, J. F. Tetrahedron Lett.
2011, 52, 2044–2047.
ꢁ
ꢀ
(9) (a) Yoon, I.; Miljanic, O. S.; Benıtez, D.; Khan, S. I.; Stoddart,
J. F. Chem. Commun. 2008, 38, 4561–4563. (b) Slater, B. J.; Davies, E. S.;
Argent, S. P.; Nowell, H.; Lewis, W.; Blake, A. J.; Champness, N. R.
Chem.;Eur. J. 2011, 17, 14746–14757.
r
10.1021/ol3022963
Published on Web 10/11/2012
2012 American Chemical Society

1,5-dinaphtho[38]crown-1011 (DN38C10) rings interlocked
with relatively rigid macrocycles containing naphthalene
diimide (NpI) units,12 which are prepared by Eglinton
couplings of terminal acetylenes. Since Eglinton couplings
fall short of the ideal of kinetic covalent chemistry13 (KCC)
in MIM production;an approach requiring that the cou-
pling reaction is all but quantitative, particularly when it is
employed more than once;the modification of Huisgen’s
1,3-dipolar cycloaddition14 between an azide and an alkyne
in the context of the copper(I)-catalyzed azideÀalkyne
cycloaddition15 (CuAAC) represents an attractive alterna-
tive. The CuAAC reaction, which embodies the kind of
‘click chemistry’ envisioned16 by Sharpless, provides a highly
efficient means17 of producing rotaxanes and catenanes and
has led18 to the production of a wide range of MIMs in good
yields from readily available starting materials.
Scheme 1. Synthesis of the [2]Rotaxane 5 and Its Dumbbell 4
According to a Convergent Approach Using the Huisgen
1,3-Dipolar Cycloaddition
Here, we report (i) the synthesis, employing click chem-
istry, of a neutral NpI-based [2]rotaxane which, following
(ii) its solid-state characterization by X-ray crystallogra-
phy, is investigated for (iii) its solution-state properties by
1H NMR spectroscopy and electrochemical experiments.
In the synthesis (Scheme 1) of the [2]rotaxane 5,
DN38C10 (2) was stirred in PhMe with 119 and an
immediate change in color was observed from colorless
to purple, marking the formation of the 1:1 inclusion
complex 1 ⊂ 2. After complete solubilization of 1 by 2,
2.2 equiv of 319 were added to the reaction mixture,20
followed by a catalytic amount of Cu(MeCN)4PF6 and
diisopropylethylamine (DIPEA). The pure [2]rotaxane 5
and its corresponding free dumbbell 4 were isolated by
column (SiO2: CH2Cl2/MeOH, 97:3) and gel permeation
chromatography (Biobeads SX-1: CH2Cl2) in 20 and 26%
yields, respectively.
Single crystals suitable for X-ray analysis21 were ob-
tained by slow diffusion of MeOH into a solution of 5
in C6H6. The solid-state structure (Figure 1) of 5 reveals
that it has a center of symmetry. While the NpI unit in the
dumbbell component of the rotaxane is encircled by the
DN38C10 ring in such a manner that the 1,5-dioxy-
naphthalene (DNP) units of the crown ether are πÀπ
stacking with it at the short plane-to-plane separation of
3.29 A, the polyether loops of the DN38C10 ring are
(10) (a) Hamilton, D. G.; Davies, J. E.; Prodi, L.; Sanders, J. K. M.
Chem.;Eur. J. 1998, 4, 608–620. (b) Cougnon, F. B. L.; Jenkins, N. A.;
Pantos-, D. G.; Sanders, J. K. M. Angew. Chem., Int. Ed. 2012, 51, 1443–
1447.
interacting through [CÀH O] close contacts of 3.33 A
3 3 3
for the shortest [C O] distance. In the superstructure (see
3 3 3
Supporting Information (SI)), the macrocycles are packed
in a herringbone arrangement in a plane along the median
of the a and b axes of the unit cell with the dumbbells
oriented orthogonally therein, and the rotaxanes stack on
top of each other with a slight offset along the b axis.
The 1H NMR spectra (Figure 2) of both the dumbbell 4
and the [2]rotaxane 5 can best be interpreted in terms of
their molecules exhibiting averaged C2v symmetry. In the
case of 5, this averaging of its molecular symmetry has to
involve the rapid reorientation of the two DNP units in the
DN38C10 ring at room temperature, most likely as a result
of the crown ether forsaking its πÀπ stacking interactions
with the NpI unit in order to execute the necessarily rapid
pedaling motions, on the 1H NMR time scale, around the
central CÀC bonds of both DNP units. In support of the
πÀπ stacking of the NpI acceptor unit with the two DNP
donor units in the [2]rotaxane, the chemical shift (δ) of the
(11) Bruns, C. J.; Basu, S.; Stoddart, J. F. Tetrahedron Lett. 2008, 51,
983–986.
(12) DN38C10 and NpI exhibit a strong association in apolar and
polar aprotic solvents. See: Hamilton, D. G.; Montalti, M.; Prodi, L.;
Fontani, M.; Zanello, P.; Sanders, J. K. M. Chem.;Eur. J. 2000, 6, 608–
617.
(13) Ke, C.; Smaldone, R. A.; Kikuchi, T.; Li, H.; Davis, A. P.;
Stoddart, J. F. Angew. Chem., Int. Ed. 2012, 10.1002/anie.201205087.
€
(14) (a) Huisgen, R.; Szeimies, G.; Mobius, L. Chem. Ber. 1967, 100,
2494–2507. (b) Huisgen, R. Pure Appl. Chem. 1989, 61, 613–628.
(15) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless,
K. B. Angew. Chem., Int. Ed. 2002, 41, 2596–2599. (b) Tornøe, C. W.;
Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–3064.
(16) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int.
Ed. 2001, 40, 2004–2021.
€
(17) Aucagne, V.; Hanni, K. D.; Leigh, D. A.; Lusby, P. J.; Walker,
D. B. J. Am. Chem. Soc. 2006, 128, 2186–2187. (b) Dichtel, W. R.;
ꢁ
ꢀ
Miljanic, O. S.; Health, J. R.; Stoddart, J. F. J. Am. Chem. Soc. 2006,
128, 10388–10390.
€
(18) (a) Hanni, K. D.; Leigh, D. A. Chem. Soc. Rev. 2010, 39, 1240–
1251. (b) Fahrenbach, A. C.; Stoddart, J. F. Chem.;Asian J. 2011, 6,
2660–2669.
(19) See the Supporting Information: all compounds were character-
ized fully by 1H and 13C NMR spectroscopies and mass spectrometry.
The assignments of 1H and 13C resonances were made as a result of 2D
NMR experiments, i.e., COSY, HSQC, and HMBC.
(20) It is noteworthy that the reaction mixture had to be degassed to
prevent the poisoning of the catalyst and side reactions: in the presence
of O2 the oxidation of Cu(I) to Cu(II) can take place, giving rise to the
polymerization of 1 as a result of multiple Eglington couplings.
(21) Crystal data for [2]rotaxane (5): C36H44O10, C98H104N8O6, pink
plate, crystal size 0.328 Â 0.293 Â 0.04 mm3, monoclinic, space group,
P21/c, a = 22.5961(6) A, b = 18.2223(5) A, c = 14.5264(5) A, R =
90.00°, β = 96.378(2)°, γ = 90.00°, V = 5944.3(3) A3, Z = 2, Fcalcd
1.188, T = 100(2) K, R1(F2 > 2σF2) = 0.1508, wR2 = 0.4090.
=
Org. Lett., Vol. 14, No. 20, 2012
5189

An approximate calculation reveals that this temperature
dependent behavior, which is associated with a free energy
barrier of 11.3 kcal molÀ1, can be attributed to the slowing
1
down, on the H NMR time scale, of the degenerate
reorganization of the DNP rings in DN38C10 such that
they impose their local C2 symmetry on the NpI units.
Figure 1. Solid state structureof the [2]rotaxane 5. (a) Perspective
drawing of the [2]rotaxane (hydrogens have been omitted for
clarity). (b) The DN38C10 ring and NpI interact through πÀπ
stacking and [CÀH O] close contacts between the polyether
3 3 3
loops and the NpI protons.
Figure 3. Cyclic voltammograms of the dumbbell 4 (top) at a
scan rate of 100 mV sÀ1 and rotaxane 5 (bottom) at scan rates
3
resonances for the NpI and triazole protons in 5 are shifted
upfield by 0.48 ppm and downfield by 0.24 ppm, respec-
tively, with respect to their δ values in the free dumbbell 4,
while the signals for H2/6, H3/7, and H4/8 in the DN38C10
ring are shifted upfield by ca. 0.5 ppm with respect to their
δ values in the free crown ether. Variable temperature
1H NMR experiments were also performed in CD2Cl2 in
order to assess the reorientation of the DNP units in the
DN38C10 ring (see SI). Upon cooling the solution down
to 199 K, the singlet observed at δ = 8.27 ppm for the NpI
protons at ambient temperature broadens (Tc = 227 K),
while also moving to higher field, and then separates out
into an AB system with δA = 8.14 and δB = 8.11 ppm.
of 50 mV sÀ1 in dark red and 500 mV sÀ1 in orange (c = 1 Â
3
À31
10À3 mol LÀ1 in DMF with 0.1 mol L of TBAPF6). All scans
have been normalized to the square root of the scan rate.
3
3
Electrochemical experiments, namely, cyclic voltamme-
try (CV) and differential pulse voltammetry (DPV), were
performed at 298 K in Ar-purged DMF and CH2Cl2 solu-
tions with 0.1 mol LÀ1 TBAPF6 as the supporting elec-
3
trolyte. All potentials are referenced against a Ag/AgCl
electrode (SSCE). A comparison between the CV (Figure 3)
of the free dumbbell 4 and that of the [2]rotaxane 5 in
DMF allows for elucidation of the electrochemically in-
duced shuttling mechanism.22 At 50 mV sÀ1 in DMF, the
3
first reduction (I, NpI fNpI•À) of 5 is dramatically shifted
to higher potential (ca. À0.70 V) compared to the first reduc-
tion wave of 4 at À0.39 V. This shift can be explained by the
interaction between the DN38C10 ring and the NpI unit, since
the electron density transferred from the DNP donors makes
it more difficult to reduce NpI. On the contrary, compounds
4 and 5 share virtually identical second reduction waves
(II, NpI•ÀfNpI2À) near À0.90 V, suggesting that the ring
moves away from its station after reduction and does not
associate with the radical anion NpI•À. Likewise, the first
reoxidation step (III, NpI2ÀfNpI•À) is essentially iden-
tical for the dumbbell and the rotaxane on account of the
ring having no association with NpI2À. The final reoxida-
tion (IV, NpI•ÀfNpI) to regenerate the neutral species
occurs at a lower potential than would be expected for a
reversible process. The irreversibility of this redox process
at I and IV is not unexpected, since the ring resides around
NpI at I but is absent at IV, meaning the rotaxane does not
Figure 2. Partial 1H NMR spectra (500 MHz) recorded at room
temperature in CDCl3 of the DN38C10 ring 2 (top), the rotaxane
5 (middle), and the corresponding dumbbell 4 (bottom).
(22) Several scans were performed on the same sample of 5 in order to
confirm that the voltammograms remain unchanged, establishing the
stability of the [2]rotaxane and the repeatability of the redox process.
5190
Org. Lett., Vol. 14, No. 20, 2012

return to the ground state by the same path it followed at
the outset.
Scheme 2. Proposed Mechanism for the Electrochemically
Induced Shuttling of the [2]Rotaxane 5
Variable scan-rate CV of 5 revealed that only process
IV is scan-rate dependent in DMF, where the oxida-
tion potential shifts closer to that of the free dumbbell at
500 mV sÀ1. This phenomenon could be interpreted as
3
a result of the ring passing over the NpI•À station as it
shuttles back and forth along the entire dumbbell, with
less likelihood of ‘catching’ the ring on the NpI•À station
at fast scan rates. By contrast, translational motion has
enough time to equilibrate between data points at a slow
scan rate, making the redox process appear more rever-
sible. Similar redox behavior was observed in CH2Cl2. See
Figure S3aÀc in the SI. Although the irreversibility of the
CV precludes an accurate calculation23 of the equilibrium
constant K1 governing the ground state distribution of
translational isomers, we can estimate a lower limit K1 >
10000 MÀ1 in DMF based on the magnitude of the
dramatic shift of I and IV in rotaxane 5 with respect to
dumbbell 4, indicating that the ring’s association with the
triazole units is extremely small. Thus, the rotaxane can be
described as a ‘push button’24 with two states (on/off)
since no metastable translational isomer persists for a
significant period of time following redox-stimulated
switching events.
NpI and either one of the triazole units, with the vast
majority of its time spent around the NpI unit. On the
contrary, when NpI is reduced to NpI•À, the ring shuttles
along the length of the dumbbell with little association for
any site in particular.
In summary, we have described the synthesis, solid-state
structure, and electrochemically driven ‘push-button’
switching mechanism of the neutral [2]rotaxane 5. The
neutral, rigid structure of the rotaxane renders it a suitable
candidate for high resolution imaging by scanning tunnel-
ing microscopy in future studies.
Both 4 and 5 were investigated (see SI) by spectroelec-
trochemistry (SEC) in CH2Cl2 at 1 mmol LÀ1. The optical
3
spectrum of 5 shows a characteristic absorption band at
500 nm arising from a charge-transfer (CT) interaction
between the NpI and DNP units in the DN38C10 ring.
This observation agrees with the reported wavelength in
the literature25 and supports the claim that the ring resides
primarily on the NpI unit in the neutral form. At an
applied potential of À1.2 V, the reduced species 42À and
52À are generated, giving rise to similar spectra with two
bandsat 680 and 750 nm thatare attributed tothe diradical
dianion NpI2À, accompanied by a disappearance of the CT
absorption band in the case of 5. Following reoxidation
at a potential of 0.0 V, the original spectra of the neutral
compounds are restored, indicating that no significant
decomposition occurs during the reduction cycle.
Acknowledgment. We acknowledge support from the
Non-Equilibrium Energy Research Center (NERC), which
is an Energy Frontier Research Center (EFRC) funded by
the U.S. Department of Energy, Office of Basic Sciences
(DOE-BES) under Award DE-SC0000989. J.F.S. and
C.J.B. were supported by the WCU Program (NRF
R-31-2008-000-10055-0) funded by the Ministry of Educa-
tion, Science and Technology, Korea. C.J.B. acknowledges
support from an NSF Graduate Research Fellowship.
P.L.M. acknowledges the NSF and Georgia Tech Facil-
itating Academic Careers in Engineering and Science
(FACES) committee for a postdoctoral fellowship.
The proposed shuttling mechanism of the [2]rotaxane 5,
which is based on the CV and SEC experiments, is depicted
in Scheme 2. In the neutral form, the ring shuttles between
(23) For a detailed discussion of our approach to estimating ground
state distributions in MIMs by CV, see: (a) Fahrenbach, A. C.; Barnes,
J. C.; Li, H.; Benıtez, D.; Basuray, A. N.; Fang, L.; Sue, C.-H.; Barin, G.;
Dey, S. K.; Goddard, W. A., III; Stoddart, J. F. Proc. Natl. Acad. Sci.
U.S.A. 2011, 108, 20416–20421. (b) Fahrenbach, A. C.; Bruns, C. J.;
Cao, D.; Stoddart, J. F. Acc. Chem. Res. 2012, 45, 1581–1592.
(24) Spruell, J. M.; Paxton, W. F.; Olsen, J.-C.; Benıtez, D.; Tkatchouk,
E.; Stern, C. L.; Trabolsi, A.; Friedman, D. C.; Goddard, W. A., III;
Stoddart, J. F. J. Am. Chem. Soc. 2009, 131, 11571–11580.
Supporting Information Available. Experimentaldetails.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(25) Dey, S. K.; Coskun, A.; Fahrenbach, A. C.; Barin, G.; Basuray,
A. N.; Trabolsi, A.; Botros, Y. Y.; Stoddart, J. F. Chem. Sci. 2011, 2,
1046–1053.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 20, 2012
5191