Entropy- and hydrolytic-driven positional switching of macrocycle between imine- and hydrogen-bonding stations in rotaxane-based molecular shuttles

Article abstract of DOI:10.1021/ja804888b

The construction and switching properties of a novel class of molecular shuttles 1 with imine-bonding stations for macrocyclic diamine parts are reported. Studies on dithioacetalized [2]rotaxane 4 with two hydrogen-bonding stations and a masked imine-bonding station showed that protonation of a macrocycle increases the shuttling barrier due to hydrogen-bond formation between NH₃⁺ groups and the TEG-stations. Hydrolysis of the imine-bonds of the imine-bridged molecular shuttles 1b,c with TEG-stations could exclusively give the [2]rotaxane 2b,c·2H²⁺, with the macrocycle hydrogen-bonded with the TEG-station. In contrast, 1a without TEG-stations gave an equilibrated mixture of 1a, monoimine 3a·H ⁺, and 2a·2H²⁺ under similar acidic hydrolytic conditions. The equilibrium between 1b,c and 2b,c·2H²⁺ to control the position of the macrocycle could be successfully switched to either side by applying acidic hydrolytic or dehydrating conditions. Furthermore, the equilibrium was largely biased to [2]rotaxane 2b,c·2H²⁺ under acidic hydrolytic conditions and could be reversed in favor of bis-imine 1b,c just by heating. This is a successful example of a molecular shuttle exhibiting entropy-driven translational isomerism with remarkable positional discrimination. An examination of thermodynamic parameters showed that imine-bond hydrolyses and the formation of hydrogen bonds between the macrocycle and the station are thermodynamically matched processes, because both processes are enthalpically favored and accompanied by a loss of entropy. The combination of imine-bonding and hydrogen-bonding station in a rotaxane system is the key to realizing the clear entropy-driven positional switching of the macrocycle observed.

Full text of DOI:10.1021/ja804888b

Published on Web 09/26/2008
Entropy- and Hydrolytic-Driven Positional Switching of
Macrocycle between Imine- and Hydrogen-Bonding Stations
in Rotaxane-Based Molecular Shuttles
Takeshi Umehara,† Hidetoshi Kawai,*,†,‡ Kenshu Fujiwara,† and Takanori Suzuki†
Department of Chemistry, Faculty of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan,
and PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi,
Saitama 332-0012, Japan
Received June 26, 2008; E-mail: kawai@sci.hokudai.ac.jp
Abstract: The construction and switching properties of a novel class of molecular shuttles 1 with imine-
bonding stations for macrocyclic diamine parts are reported. Studies on dithioacetalized [2]rotaxane 4 with
two hydrogen-bonding stations and a masked imine-bonding station showed that protonation of a macrocycle
increases the shuttling barrier due to hydrogen-bond formation between NH3+ groups and the TEG-stations.
Hydrolysis of the imine-bonds of the imine-bridged molecular shuttles 1b,c with TEG-stations could
exclusively give the [2]rotaxane 2b,c·2H2+, with the macrocycle hydrogen-bonded with the TEG-station.
In contrast, 1a without TEG-stations gave an equilibrated mixture of 1a, monoimine 3a·H+, and 2a·2H2+
under similar acidic hydrolytic conditions. The equilibrium between 1b,c and 2b,c·2H2+ to control the position
of the macrocycle could be successfully switched to either side by applying acidic hydrolytic or dehydrating
conditions. Furthermore, the equilibrium was largely biased to [2]rotaxane 2b,c·2H2+ under acidic hydrolytic
conditions and could be reversed in favor of bis-imine 1b,c just by heating. This is a successful example
of a molecular shuttle exhibiting entropy-driven translational isomerism with remarkable positional
discrimination. An examination of thermodynamic parameters showed that imine-bond hydrolyses and the
formation of hydrogen bonds between the macrocycle and the station are thermodynamically matched
processes, because both processes are enthalpically favored and accompanied by a loss of entropy. The
combination of imine-bonding and hydrogen-bonding station in a rotaxane system is the key to realizing
the clear entropy-driven positional switching of the macrocycle observed.
bond-formation,9,10 as well as the kind of macrocycle motion
that is regulated.11
Introduction
Imine bonds12 can be equilibrated with an amine-aldehyde
pair under hydrolytic conditions. The use of this dynamic
covalent bond13 to link components seems to be promising as
a mean to control the on/off switching of subunit mobility or
to change the position of the macrocycle on the axle in
rotaxanes. We recently reported in a preliminary communication
the successful preparation of a novel type of rotaxane 1a by
threading an axle molecule bearing two formyl groups into a
macrocyclic diamine, cross-linking to form imine bonds,14 and
attaching bulky end-groups.10 In that report, [2]rotaxane 2a was
successfully generated in equilibrium with 1a in an acidic
medium (Scheme 1), from which 1a could be completely
regenerated under dehydrating conditions. Thus, we demon-
strated the translocation of a macrocycle by the application of
H2O/H+. In addition, the independent mobility of the axle and
macrocycle components in the rotaxane can be regulated on
the basis of the controllable reversibility of the dynamic covalent
bond.
The incorporation of multiple recognition motifs into mo-
lecular structures is a prerequisite for controlling component
and substrate motion in future generations of molecular-level
machines.1 Stimuli-responsive molecular shuttles2-11 are prom-
ising candidates, where translocation of the macrocycle between
different sites (“stations”) on an axle of rotaxanes is controlled
by an external trigger, such as a redox process,3 pH change,4
ion exchange,5 polarity,6 light,7 temperature,8 or covalent
† Hokkaido University.
‡ PRESTO.
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However, the low equilibrium ratio of 2a under acidic
hydrolytic conditions (∼23% at 233 K in 0.08% TFA/CDCl3)
accompanied by a considerable amount of intermediary
monoimine 3a (∼40%) was inappropriate for achieving efficient
switching, with which a stimuli-responsive molecular shuttle
can be converted to another form. Therefore, we sought to
(2) (a) Anelli, P.-L.; Spencer, N.; Stoddart, J. F. J. Am. Chem. Soc. 1991,
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9
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 13981–13988 13981

A R T I C L E S
Umehara et al.
Scheme 1. Hydrolytic Control of Positional Switching of Macrocycle
in Rotaxane-Based Molecular Shuttles with/without
Hydrogen-Bonding Stations
Scheme 2. Thermodynamic Interplay of Imine- and
Hydrogen-Bonding in a Rotaxane-Based Molecular Shuttle
Exhibiting Entropy-Driven Translational Isomerism
design novel imine-bridged molecular shuttles that can be
completely transformed to [2]rotaxane under hydrolytic condi-
tions. Although the dynamic covalent properties of imine bonds
(formation/cleavage) are generally compatible with various types
of noncovalent interactions such as hydrogen-bonding, π-π
interaction, and metal-coordination, as proven by the successful
construction of many supramolecular architectures,15-17 we
envisaged that imine-bond cleavage under dynamic acidic
ing of the macrocycle in a rotaxane structure (Scheme 2):
hydrogen-bond formation is an enthalpically favored interaction
that is accompanied by a loss of entropy, whereas imine-bond
formation is an entropically favored process in a rotaxane
structure, since we reported10 that 1a was generated as an
entropically preferred species under acidic hydrolytic condi-
tions in the equilibrated mixture containing 2a and 3a (see
below). Giuseppone and Lehn have also reported the equi-
librium ratio of imine form in dynamic combinatorial libraries
(DCLs) under low acidic conditions was increased with
increasing temperature.12e Thus, imine-bond cleavage and
hydrogen-bond formation are thermodynamically matched pro-
cesses (enthalpically favored and entropically disfavored pro-
cesses). The reverse processes, that is, hydrogen-bond cleavage
and imine-bonding, have also to be matched processes (enthal-
pically disfavored and entropically favored processes). Accord-
ingly, it is expected that the incorporation of both imine-bridging
and hydrogen-bonding stations into a rotaxane axle would give
a novel rotaxane-based molecular shuttle that exhibits the
entropy-driven translational isomerism.8a
hydrolytic conditions would be assisted by the formation of
+
hydrogen-bonds by the resulting primary ammoniums (NH3
)
with suitable newly attached hydrogen-bonding acceptors.18
Therefore, we designed novel imine-bridged molecular shuttles
1b,c attached to triethylene glycol (TEG)-stations, which could
be completely transformed to hydrolyzed [2]rotaxane under
hydrolytic conditions due to hydrogen-bond formation between
the macrocycle and the TEG-station.
Furthermore, from the viewpoint of thermodynamic contribu-
tions, the combination of imine-bonding and hydrogen-bonding
has to work synergistically for entropy-driven positional switch-
(3) Recent examples of molecular shuttles controlled by redox: (a) Choi,
J. W.; Flood, A. H.; Steuerman, D. W.; Nygaard, S.; Braunschweig,
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. (b) Ikeda, T.; Saha, S.; Aprahamian, I.; Leung,
K. C.-F.; Williams, A.; Deng, W.-Q.; Flood, A. H.; Goddard, W. A.;
Stoddart, J. F. Chem. Asian J. 2007, 2, 76–93. (c) Green, J. E.; Choi,
J. W.; Boukai, A.; Bunimovich, Y.; Johnston-Halperin, E.; DeIonno,
E.; Luo, Y.; Sheriff, B. A.; Xu, K.; Shin, Y. S.; Tseng, H.-R.; Stoddart,
J. F.; Heath, J. R. Nature 2007, 445, 414–417. (d) Nguyen, T. D.;
Liu, Y.; Saha, S.; Leung, K. C.-F.; Stoddart, J. F.; Zink, J. I. J. Am.
Chem. Soc. 2007, 129, 626–634. (e) Nygaard, S.; Leung, K. C.-F.;
Aprahamian, I.; Ikeda, T.; Saha, S.; Laursen, B. W.; Kim, S.-Y.;
Hansen, S. W.; Stein, P. C.; Flood, A. H.; Stoddart, J. F.; Jeppesen,
J. O. J. Am. Chem. Soc. 2007, 129, 960–970. (f) Sindelar, V.; Silvi,
S.; Parker, S. E.; Sobransingh, D.; Kaifer, A. E. AdV. Funct. Mater.
2007, 17, 694–701. (g) Aprahamian, I.; Dichtel, D. R.; Ikeda, T.; Heath,
J. R.; Stoddart, J. F. Org. Lett. 2007, 9, 1287–1290. (h) Mateo-Alonso,
A.; Fioravanti, G.; Marcaccio, M.; Paolucci, F.; Rahman, G. M. A.;
Ehli, C.; Guldi, D. M.; Prato, M. Chem. Commun. 2007, 1945–1947.
(i) Durola, F.; Sauvage, J.-P. Angew. Chem., Int. Ed. 2007, 46, 3537–
3540. (j) Saha, S.; Flood, A. H.; Stoddart, J. F.; Impellizzeri, S.; Silvi,
S.; Venturi, M.; Credi, A. J. Am. Chem. Soc. 2007, 129, 12159–12171.
(k) Altobello, S.; Nikitin, K.; Stolarczyk, J. K.; Lestini, E.; Fitzmaurice,
D. Chem.-Eur. J. 2008, 14, 1107–1116. (l) Nikitin, K.; Lestini, E.;
Stolarczyk, J. K.; Mu¨ller-Bunz, H.; Fitzmaurice, D. Chem.-Eur. J.
2008, 14, 1117–1128. (m) Jagesar, D. C.; Hartl, F.; Buma, W. J.;
Brouwer, A. M. Chem.-Eur. J. 2008, 14, 1935–1946. (n) Fioravanti,
G.; Haraszkiewicz, N.; Kay, E. R.; Mendoza, S. M.; Bruno, C.;
Marcaccio, M.; Wiering, P. G.; Paolucci, F.; Rudolf, P.; Brouwer,
A. M.; Leigh, D. A. J. Am. Chem. Soc. 2008, 130, 2593–2601. (o)
Zhao, Y.-L.; Aprahamian, I.; Trabolsi, A.; Erina, N.; Stoddart, J. F.
J. Am. Chem. Soc. 2008, 130, 6348–6350.
We report here the construction of a novel class of molecular
shuttles 1b,c with imine-bridging and hydrogen-bonding stations.
Their shuttling properties were compared to those of derivatives
with or without a hydrogen-bonding station (1a-c) or an imine-
(4) Recent examples of molecular shuttles controlled by pH change: (a)
Schmidt-Scha¨ffer, S.; Grubert, L.; Grummt, U. W.; Buck, K.; Abraham,
W. Eur. J. Org. Chem. 2006, 378–398. (b) Badjic´, J. D.; Ronconi,
C. M.; Stoddart, J. F.; Balzani, V.; Silvi, S.; Credi, A. J. Am. Chem.
Soc. 2006, 128, 1489–1499. (c) Cheng, K. W.; Lai, C. C.; Chiang,
P. T.; Chiu, S. H. Chem. Commun. 2006, 2854–2856. (d) Choi, H. S.;
Hirasawa, A.; Ooya, T.; Kajihara, D.; Hohsaka, T.; Yui, N. ChemP-
hysChem 2006, 7, 1671–1673. (e) Ooya, T.; Inoue, D.; Choi, H. S.;
Kobayashi, Y.; Loethen, S.; Thompson, D. H.; Ko, Y. H.; Kim, K.;
Yui, N. Org. Lett. 2006, 8, 3159–3162. (f) Leigh, D. A.; Thomson,
¨
A. R. Org. Lett. 2006, 8, 5377–5379. (g) Tuncel, T.; Ozsar, O.; Tiftika,
H. B.; Salih, B. Chem. Commun. 2007, 1369–1372. (h) Vella, S. J.;
Tiburcio, J.; Loeb, S. J. Chem. Commun. 2007, 4752–4754. (i)
Crowley, J. D.; Leigh, D. A.; Lusby, P. J.; McBurney, R. T.; Perret-
Aebi, L.-E.; Petzold, C.; Slawin, A. M. Z.; Symes, M. D. J. Am. Chem.
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L.; Leigh, D. A.; Symes, M. D.; Teobaldi, G.; Zerbetto, F. Angew.
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Entropy- and Hydrolytic-Driven Molecular Shuttle
A R T I C L E S
Scheme 3. Synthesis of Imine-Bridged Rotaxanes 1a-c, Hydrogen-Bonded [2]Rotaxanes 2a-c · 2H2+, and Dithioacetalized
[2]Rotaxanes 4a,c
Scheme 4. Shuttling Control of Dithioacetalized [2]Rotaxane 4c by
Protonation
bonding station (4a,c). The equilibrium between 1b,c and 2b,c
gives position-control for the macrocycle, which could be
sufficiently biased toward either side by applying acidic
hydrolytic or dehydrating conditions. Furthermore, the equilib-
rium that was biased toward the side of [2]rotaxane 2b,c under
acidic hydrolytic conditions could be reversed in favor of bis-
imine 1b,c simply by heating. This is a successful example of
molecular shuttles exhibiting an entropy-driven translational
isomerism8 with remarkable positional discrimination.19
Results and Discussion
Synthesis. Imine-bridged molecular shuttles 1a-c with xy-
lylene (XYL) units as a non-hydrogen-bonding station or
triethylene glycol ether (TEG) units as a hydrogen-bonding
station were prepared by introducing end-groups 6a,b with a
XYL- or TEG-unit into imine-bridged pseudorotaxane 5,10
which was prepared from a hydrindacene axle and a macrocyclic
diamine through the threading method directed by imine-bond
formation (Scheme 3). Neutral dithioacetalized [2]rotaxanes 4a,c
were obtained by treating the solution of 1a,c under acidic
hydrolytic conditions with ethanedithiol.
bridging station were investigated along with their protonated
species to evaluate the effectiveness of the TEG-station as a
hydrogen-bonding station toward ammoniums (Scheme 4). In
1
the H NMR spectra of neutral [2]rotaxanes 4a,c in CDCl3
(Figure 1), the two XYL- (Hq,t) or TEG-stations (Hi-n) and their
linking phenylene groups (Ho and Hh) resonated at a higher field
than did those of the axle molecules 7a,c. The two stations
appeared as one set of signals and did not exhibit any peak-
splitting, even at low temperatures (188-298 K in CD2Cl2 for
4a and 223-298 K in CDCl3 for 4c, respectively). The central
Shuttling Control of Dithioacetalized [2]Rotaxane 4 by
Protonation of the Macrocycle. First, the shuttling properties
of dithioacetalized [2]rotaxanes 4a,c with a protected imine-
9
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A R T I C L E S
Umehara et al.
station is an effective hydrogen-bonding station for ammonium
groups, whereas stabilization by the XYL-station is negligible
in the present molecular-shuttle system. The dithioacetalized
[2]rotaxane 4c is yet another example of a pH-responsive
molecular shuttle.4
Generation of [2]Rotaxanes 2a-c by Hydrolysis of Imine-
Bonds in Imine-Bridged Rotaxanes 1a-c. We next investigated
the dynamic equilibrium under acidic hydrolytic conditions
between 1b,c and 2b,c · 2H2+ with TEG-stations to examine
whether the preferred position of a macrocycle could switch
from the imine-bridging hydrindacene-station to the TEG-station
on the rotaxane axle (Scheme 5).
As reported in the preliminary communication, hydrolytic
control with the use of 1a is incomplete due to the lack of
hydrogen-bonding stations. Upon the addition of TFA to a
solution of 1a in wet CDCl3, two sets of resonances of
hydrolyzed [2]rotaxane 2a·2H2+ and monoimine 3a·H+ ap-
peared as a dynamic equilibrated mixture with 1a (Figure
2a,b).20 These signals were fully assigned to the structures of
1a, 3a·H+, and 2a·2H2+ based on ROESY, respectively (Figure
S2). Although both sets of signals of 2a·2H2+ and 3a·H+
enhanced in intensity at the expense of those from 1a as the
recording temperature was lowered, bis-imine 1a was still the
major species in the equilibrated mixture. The equilibrium ratio
of 1a:3a·H+:2a·H2+ was 42:35:23 at 233 K and 81:9:1 at 313
K (see below). The signals of the two XYL-stations in 2a·2H2+
in the equilibrated mixture appeared as a single set at a higher
field than those of 1a and 3a·H+, indicating that the macrocycle
of 2a·2H2+ rapidly shuttles between the two XYL-stations.
In contrast to the case of 1a, where three species coexist under
acidic conditions, upon the addition of TFA to a solution of 1b
Figure 1. 1H NMR spectra (300 MHz, CDCl3, at 298 K) of (a) axle 7a,
(b) neutral [2]rotaxane 4a, (c) axle 7c, (d) neutral [2]rotaxane 4c, and (e)
protonated [2]rotaxane 4c·2H2+ generated upon the addition of TFA to a
solution of 4c. The lettering corresponds to the assignments shown in
Scheme 3.
hydrindacene part (δ ) 3.3-3.5 for Hb,c and 7.0-7.1 ppm for
Hd) and dithioacetal moieties (δ ) 4.8 ppm for Ha) were slightly
magnetically shielded as compared to those of the axles 7a,c.
These results indicate that the macrocycle in neutral [2]rotaxane
4a,c rapidly shuttles between the two XYL- or TEG-stations.
Upon the addition of trifluoroacetic acid (TFA) to the solution
of 4c in CDCl3, the signals around TEG-stations (Hh-o) and
the dithioacetal moieties (Ha) of 4c·2H2+ split into two sets of
signals at 298 K (Figure 1e), where one set appeared at a high
field and another set appeared at a low field similar to the signals
in bis-imine 1c (Figure 2c). These split signals clearly show
that the shuttling is slower than the NMR-time scale, although
they coalesced upon heating above 328 K (Figure S1). In
contrast, the addition of TFA to the solution of 4a did not cause
any peak-splitting. These observations indicate that the proto-
nated [2]rotaxane 4c·2H2+ has an increased shuttling-barrier
for the macrocycle as compared to neutral 4c and protonated
4a·2H2+, due to the thermodynamic stabilization of 4c·2H2+
by hydrogen-bond formation between the ammonium groups
of the macrocycle and -OCH2CH2O- groups of the TEG-
station. Accordingly, the results demonstrated that the TEG-
1
with a TEG- and an XYL-station in wet CDCl3, the H NMR
spectrum showed the disappearance of signals derived from the
bis-imine 1b and the appearance of a new set of signals
assignable to a single species including CHO signals (Ha,a′) at
(7) Recent examples of molecular shuttlescontrolled by light: (a) Balzani,
V.; Clemente-Leo´n, M.; Credi, A.; Semeraro, M.; Venturi, M.; Tseng,
H.-R.; Wenger, S.; Saha, S.; Stoddart, J. F. Aust. J. Chem. 2006, 59,
193–206. (b) Balzani, V.; Clemente-Leo´n, M.; Credi, A.; Ferrer, B.;
Venturi, M.; Flood, A. H.; Stoddart, J. F. Proc. Natl. Acad. Sci. U.S.A.
2006, 103, 1178–1183. (c) Cheetham, A. G.; Hutchings, M. G.;
Claridge, T. D. W.; Anderson, H. L. Angew. Chem., Int. Ed. 2006,
45, 1596–1599. (d) Tsuda, S.; Aso, Y.; Kaneda, T. Chem. Commun.
2006, 3072–3074. (e) Qu, D.-H.; Ji, F.-Y.; Wang, Q.-C.; Tian, H. AdV.
Mater. 2006, 18, 2035–2038. (f) Serreli1, V.; Lee1, C.-F.; Kay, E. R.;
Leigh, D. A. Nature 2007, 445, 523–527. (g) Abraham, W.; Buck,
K.; Orda-Zgadzaj, M.; Schmidt-Scha¨ffer, S.; Grummt, U.-W. Chem.
Commun. 2007, 3094–3096. (h) Zhou, W.; Chen, D.; Li, J.; Xu, J.;
Lv, J.; Liu, H.; Li, Y. Org. Lett. 2007, 9, 3929–3932. (i) Li, Y.; Li,
H.; Li, Y.; Liu, H.; Wang, S.; He, X.; Wang, N.; Zhu, D. Org. Lett.
2007, 9, 4835–4838. (j) Raiteri, P.; Bussi, G.; Cucinotta, C. S.; Credi,
A.; Stoddart, J. F.; Parrinello, M. Angew. Chem., Int. Ed. 2008, 47,
3536–3539.
(5) Recent examples of molecular shuttles controlled by ion exchange: (a)
Marlin, D. S.; Cabrera, D. G.; Leigh, D. A.; Slawin, A. M. Z. Angew.
Chem., Int. Ed. 2006, 45, 77–83. (b) Marlin, D. S.; Cabrera, D. G.;
Leigh, D. A.; Slawin, A. M. Z. Angew. Chem., Int. Ed. 2006, 45, 1385–
1390. (c) Tokunaga, Y.; Nakamura, T.; Yoshioka, M.; Shimomura,
Y. Tetrahedron Lett. 2006, 47, 5901–5904. (d) Lin, C.-F.; Lai, C.-C.;
Liu, Y.-H.; Peng, S.-M.; Chiu, S.-H. Chem.-Eur. J. 2007, 13, 4350–
4355. (e) Huang, Y.-L.; Hung, W.-C.; Lai, C.-C.; Liu, Y.-H.; Peng,
S.-M.; Chiu, S.-H. Angew. Chem., Int. Ed. 2007, 46, 6629–6633. (f)
Zhou, W.; Li, J.; He, X.; Li, C.; Lv, J.; Li, Y.; Wang, S.; Liu, H.;
Zhu, D. Chem.-Eur. J. 2008, 14, 754–763. (g) Chen, N.-C.; Lai, C.-
C.; Liu, Y.-H.; Peng, S.-M.; Chiu, S.-H. Chem.-Eur. J. 2008, 14, 2904–
2908.
(8) Examples of molecular shuttles controlled by temperature: (a) Bottari,
G.; Dehez, F.; Leigh, D. A.; Nash, P. J.; Pe´rez, E. M.; Wong, J. K. Y.;
Zerbetto, F. Angew. Chem., Int. Ed. 2003, 42, 5886–5889. (b) Choi,
H. S.; Yamamoto, K.; Ooya, T.; Yui, N. ChemPhysChem 2005, 6,
1081–1086. (c) Kidowaki, M.; Zhao, C.; Kataoka, T.; Ito, K. Chem.
Commun. 2006, 4102–4103. (d) Kataoka, T.; Kidowaki, M.; Zhao,
C.; Minamikawa, H.; Shimizu, T.; Ito, K. J. Phys. Chem. B 2006,
110, 24377–24383.
(6) Recent examples of molecular shuttles controlled by polarity: (a)
Mateo-Alonso, A.; Fioravanti, G.; Marcaccio, M.; Paolucci, F.; Jagesar,
D. C.; Brouwer, A. M.; Prato, M. Org. Lett. 2006, 8, 5173–5176. (b)
Tsukagoshi, S.; Miyawaki, A.; Takashima, Y.; Yamaguchi, H.; Harada,
A. Org. Lett. 2007, 9, 1053–1055. (c) Mateo-Alonso, A.; Ehli, C.;
Rahman, G. M. A.; Guldi, D. M.; Fioravanti, G.; Marcaccio, M.;
Paolucci, F.; Prato, M. Angew. Chem., Int. Ed. 2007, 46, 3521–3525.
(d) Mateo-Alonso, A.; Iliopoulos, K.; Couris, S.; Prato, M. J. Am.
Chem. Soc. 2008, 130, 1534–1535.
(9) Examples of molecular shuttles controlled by covalent bonding: (a)
Cao, J.; Fyfe, M. C. T.; Stoddart, J. F.; Cousins, G. R. L.; Glink,
P. T. J. Org. Chem. 2000, 65, 1937–1946. (b) Hannam, J. S.; Lacy,
S. M.; Leigh, D. A.; Saiz, C. G.; Slawin, A. M. Z.; Stitchell, S. G.
Angew. Chem., Int. Ed. 2004, 43, 3260–3264. (c) Leigh, D. A.; Pe´rez,
E. M. Chem. Commun. 2004, 2262–2263. (d) Tachibana, Y.; Kawasaki,
H.; Kihara, N.; Takata, T. J. Org. Chem. 2006, 71, 5093–5104.
(10) Kawai, H.; Umehara, T.; Fujiwara, K.; Tsuji, T.; Suzuki, T. Angew.
Chem., Int. Ed. 2006, 45, 4281–4286.
9
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Entropy- and Hydrolytic-Driven Molecular Shuttle
A R T I C L E S
Figure 2. 1H NMR spectra (300 MHz, wet CDCl3) of (a) imine-bridged rotaxane 1a, (b) the equilibrated mixture of 1a, monoimine 3a·H+, and protonated
[2]rotaxane 2a·2H2+ generated upon the addition of TFA to a solution of 1a at 253 K, (c) imine-bridged rotaxane 1b, (d) protonated [2]rotaxane 2b·2H2+
generated upon the addition of TFA to a solution of 1b at 298 K, (e) imine-bridged rotaxane 1c, (f) protonated [2]rotaxane 2c·2H2+ generated upon the
addition of TFA to a solution of 1c at 295 K, and (g) the same sample as in (f) at 215 K. The lettering corresponds to the assignments shown in Scheme
3 and in the Supporting Information. The superscript in the letterings in (b) corresponds to the compound number.
Scheme 5. Transformation between Imine-Bridged Rotaxanes 1a-c and [2]Rotaxanes 2a-c under Acidic Hydrolysis/Dehydration Conditions
δ ) 9.4 ppm even at 298 K (Figure 2c,d). The signals around
the TEG-station (Hi-n, Ho, and Hh) of the newly generated
species appeared at a higher field than those of the bis-imine
1b, whereas those around the XYL-station (Hh′,o′ and Hq,t)
remained unchanged. Thus, the newly generated species was
assigned as a [2]rotaxane 2b·2H2+ in which the macrocycle
was located at the TEG-station on the axle due to hydrogen-
bond formation. The ROESY spectra of 2b·2H2+ also provided
evidence for the localization of the macrocycle at the TEG-
station (Figure S3). Only trace amounts of the bis-imine 1b and
the monoimine 3b·H+ were observed at 298 K.
Similarly, the exclusive generation of [2]rotaxane 2c·2H2+
from 1c was observed under acidic hydrolytic conditions at 295
K (Figure 2e,f). In the 1H NMR spectrum, the two TEG-stations
9
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A R T I C L E S
Umehara et al.
of 2c·2H2+ appeared as a single set of broader signals at 295
K, which indicated that shuttling of the macrocycles between
two TEG-stations was slower than in the case of 2a·2H2+
between XYL-stations. With a decrease in temperature, these
signals split into two sets of distinct broad signals (Figure 2g),
where one set appeared at a high field similar to the signals of
the TEG-station of 2b·2H2+ and another set appeared at a low
field similar to the signals of the TEG-stations of bis-imine 1c.21
These observations indicate that the macrocycle of 2c·2H2+
shuttled between two TEG-stations on the axle at a speed
comparable to the NMR-time scale. The rate and the energy
barrier for the shuttling of the macrocycle of 2c·2H2+ were
determined by the coalescence method on the variable-temper-
ature (VT) NMR spectra: k273 ) 574 s-1 and ∆Gq ) 12.5 (
0.2 kcal mol-1 at 273 K (Tc). The larger shuttling-barrier in
2c·2H2+ than in 2a·2H2+ must be due to hydrogen-bond
formation between the macrocycle and the TEG-station.
When these acidic solutions containing 2b,c·2H2+ were
subjected to dehydrating conditions, bis-imines 1b,c could be
quantitatively regenerated as in the case of 1a.22 Thus, we
demonstrated that the incorporation of hydrogen-bonding TEG-
stations into imine-bridged rotaxanes could realize novel mo-
lecular shuttles where a macrocycle can be translocated revers-
ibly between the imine-bridging station and the TEG-station
by hydrolytic control with a complete positional discrimina-
tion.19
(11) (a) Jiang, L.; Okano, J.; Orita, A.; Otera, J. Angew. Chem., Int. Ed.
2004, 43, 2121–2124. (b) Wang, Q.-C.; Qu, D.-H.; Ren, J.; Chen, K.;
Tian, H. Angew. Chem., Int. Ed. 2004, 43, 2661–2665. (c) Ghosh, P.;
Federwisch, G.; Kogej, M.; Schalley, C. A.; Haase, D.; Saak, W.;
Lu¨tzen, A.; Gschwind, R. M. Org. Biomol. Chem. 2005, 3, 2691–
2700. (d) Le´tinois-Halbes, U.; Hanss, D.; Beierle, J. M.; Collin, J.-P.;
Sauvage, J.-P. Org. Lett. 2005, 9, 5753–5756. (e) Hirose, K.; Ishibashi,
K.; Shiba, Y.; Doi, Y.; Tobe, Y. Chem. Lett. 2007, 36, 810–811. (f)
Horie, M.; Sassa, T.; Hazhizume, D.; Suzaki, Y.; Osakada, K.; Wada,
T. Angew. Chem., Int. Ed. 2007, 46, 4983–4986. (g) Hirose, K.; Shiba,
Y.; Ishibashi, K.; Doi, Y.; Tobe, Y. Chem.-Eur. J. 2008, 14, 981–
986. (h) Hirose, K.; Shiba, Y.; Ishibashi, K.; Doi, Y.; Tobe, Y. Chem.-
Eur. J. 2008, 14, 3427–3433. (i) Hirose, K.; Ishibashi, K.; Shiba, Y.;
Doi, Y.; Tobe, Y. Chem.-Eur. J. 2008, 14, 5803–5811.
Entropy-Driven Translational Isomerism between Imine-
Bridged Rotaxane and Hydrogen-Bonded [2]Rotaxane. In
addition to the successful switching to bis-imines 1 and
[2]rotaxanes 2·2H2+ by hydrolytic control, we found that
switching between 1 and 2·2H2+ could also be realized by
simply changing the temperature under acidic hydrolytic condi-
tions. In particular, 1b,c/2b,c·2H2+ with TEG-stations could
be completely switched from one station to another as a function
of temperature, which demonstrates the entropy-driven positional
switching of the macrocycle in a molecular shuttle.8
(12) (a) Schiff, H. Ann. 1864, 131, 118. (b) Layer, R. W. Chem. ReV. 1963,
63, 489–510. (c) The Chemistry of the Amino Group; Patai, S., Ed.;
John Wiley & Sons: London, 1968; Chapter 7. (d) Giuseppone, N.;
Schmitt, J.-L.; Schwartz, E.; Lehn, J.-M. J. Am. Chem. Soc. 2005,
127, 5528–5539. (e) Giuseppone, N.; Lehn, J.-M. Chem.-Eur. J. 2006,
12, 1715–1722.
(13) (a) Lehn, J.-M. Chem.-Eur. J. 1999, 5, 2455–2463. (b) Rowan, S. J.;
Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.; Stoddart, J. F.
Angew. Chem., Int. Ed. 2002, 41, 899–952. (c) Corbett, P. T.; Leclaire,
J.; Vial, L.; West, K. R.; Wietor, J.-L.; Sanders, J. K. M.; Otto, S.
Chem. ReV. 2006, 106, 3652–3711. (d) Lehn, J.-L. Chem. Soc. ReV.
2007, 36, 151–160.
When a solution of [2]rotaxane 2b·2H2+ in C6D5Br under
acidic hydrolytic conditions was heated above 298 K, new sets
of signals assignable to the bis-imine 1b were enhanced in
intensity at the expense of those from 2b·2H2+ (Figure 3g to b
and Figure 5a). Bis-imine 1b finally became an exclusive species
above 343 K, consistent with the fact that the bis-imine 1b was
an entropically favored species. When the solution was cooled
to 273 K, 2b·2H2+ was completely regenerated. The monoimine
3b·H+ was not observed in either heating or cooling. Also, in
the case of 2c·2H2+, reversible switching was observed between
1c and 2c·2H2+ with a change in the temperature (Figure S4).
The release of water molecules accompanied by intramolecular
imine-bond formation from 2b,c·2H2+ to 1b,c seems to
contribute to the gain in entropy under heating (see below).
Thus, the equilibrium ratio of 1b,c/2b,c, which determines the
preferred position of the macrocycle between the imine-bridging
and hydrogen-bonding stations, could be completely reversed
(14) Other rotaxane syntheses based on covalent bond methods: (a) Hiratani,
K.; Suga, J.; Nagawa, Y.; Houjou, H.; Tokuhisa, H.; Numata, M.;
Watanabe, K. Tetrahedron Lett. 2002, 43, 5747–5750. (b) Kameta,
N.; Hiratani, K.; Nagawa, Y. Chem. Commun. 2004, 466–467. (c)
Hirose, K.; Nishihara, K.; Harada, N.; Nakamura, Y.; Masuda, D.;
Araki, M.; Tobe, Y. Org. Lett. 2007, 9, 2969–2972.
(15) (a) Meyer, C. D.; Joiner, C. S.; Stoddart, J. F. Chem. Soc. ReV. 2007,
36, 1705–1723. (b) Borisova, N. E.; Reshetova, M. D.; Ustynyuk, Y. A.
Chem. ReV. 2007, 107, 46–79.
(16) Recent examples of interlocked molecules assembled by combination
of imine bonds with noncovalent interactions: (a) Northrop, B. H.;
Arico´, F.; Tangchiavang, N.; Badjic´, J. D.; Stoddart, J. F. Org. Lett.
2006, 8, 3899–3902. (b) Williams, A. R.; Northrop, B. H.; Chang, T.;
Stoddart, J. F.; White, A. J. P.; Williams, D. J. Angew. Chem., Int.
Ed. 2006, 45, 6665–6669. (c) Pentecost, C. D.; Chichak, K. S.; Peters,
A. J.; Cave, G. W. V.; Cantrill, S. J.; Stoddart, J. F. Angew. Chem.,
Int. Ed. 2007, 46, 218–222. (d) Nitschke, J. R. Acc. Chem. Res. 2007,
40, 103–112. (e) Yates, C. R.; Ben´ıtez, D.; Khan, S. I.; Stoddart, J. F.
Org. Lett. 2007, 9, 2433–2436. (f) Leung, K. C.-F.; Arico´, F.; Cantrill,
S. J.; Stoddart, J. F. Macromolecules 2007, 40, 3951–3959. (g)
Haussmann, P. C.; Khan, S. I.; Stoddart, J. F. J. Org. Chem. 2007,
72, 6708–6713. (h) Wu, J.; Leung, K. C.-F.; Stoddart, J. F. Proc. Natl.
Acad. Sci. U.S.A. 2007, 104, 17266–17271.
as a function of temperature (the ratios of 1b,c/2b,c·H2+
)
<5/95 at 273 K; >95/5 at 373 K, respectively). There are very
few examples of entropy-driven translational isomerism,8 and
thus the complete positional discrimination in our molecular
shuttle system is noteworthy.
(17) Recent examples of giant structures assembled by imine bonds: (a)
Hui, J. K.-H.; MacLachlan, M. J. Chem. Commun. 2006, 2480–2482.
(b) Liu, X.; Warmuth, R. J. Am. Chem. Soc. 2006, 128, 14120–14127.
(c) Giuseppone, N.; Schmitt, J.-L.; Lehn, J.-M. J. Am. Chem. Soc.
2006, 128, 16748–16763. (d) Hartley, C. S.; Elliot, E. L.; Moore, J. S.
J. Am. Chem. Soc. 2007, 129, 4512–4513. (e) Chow, C.-F.; Fujii, S.;
Lehn, J.-M. Angew. Chem., Int. Ed. 2007, 46, 5007–5010. (f) Liu, Y.;
Liu, X.; Warmuth, R. Chem.-Eur. J. 2007, 13, 8953–8959. (g) Xu,
S.; Giuseppone, N. J. Am. Chem. Soc. 2008, 130, 1826–1827. (h)
Giuseppone, N.; Schmitt, J. L.; Allouche, L.; Lehn, J.-M. Angew.
Chem., Int. Ed. 2008, 47, 2235–2239. (i) Ulrich, S.; Lehn, J.-M. Angew.
Chem., Int. Ed. 2008, 47, 2240–2243. (j) Xu, D.; Warmuth, R. J. Am.
Chem. Soc. 2008, 130, 7520–7521.
(20) When a small amount of TFA (0.08% v/v) was added to the solution
of 1a in wet CDCl3, the dynamic equilibrium in Figure 2b was
observed. However, when a large excess of TFA (>1% v/v) was added,
hydrolysis of the imine bonds was not observed. This observation
resembles the results observed in DCLs by Giuseppone and Lehn.12e
(21) As the exclusive generation of 2c·2H2+ at low temperature region
(215-295 K) was also evidenced from the integrated intensity of the
formyl proton (Ha), the observed split of the signals is due to the slow
shuttling of the macrocycle of 2c·2H2+ but not due to the chemical
exchange between 1c and 2c·2H2+
.
(22) Thanks to the increased equilibrium ratios of the [2]rotaxane
2b,c·2H2+ under the acidic hydrolysis conditions, neutral [2]rotaxane
2b,c was obtained with a small amount of bis-imine 1b,c by treating
the acidic solution of 2b,c·2H2+ with aqueous NaHCO3 or by passing
the solution through a short column of aluminum oxide. On the other
hand, the similar treatment of 1a/2a·2H2+ quantitatively gave bis-
imine 1a.
(18) Chiu, S.-H.; Stoddart, J. F. J. Am. Chem. Soc. 2002, 124, 4174–4175.
(19) A Boltzmann distribution at 298 K requires a ∆∆E (or ∆∆G) value
between translational co-conformers of about 2 kcal mol-1 for 95%
occupancy of one station in a positionally bistable shuttle: Altieri, A.;
Bottari, G.; Dehez, F.; Leigh, D. A.; Wong, J. K. Y.; Zerbetto, F.
Angew. Chem., Int. Ed 2003, 42, 2296–2300.
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Entropy- and Hydrolytic-Driven Molecular Shuttle
A R T I C L E S
Figure 3. 1H NMR spectra (300 MHz, wet C6D5Br) of (a) imine-bridged
Figure 4. 1H NMR spectra (600 MHz, wet CDCl3) of (a)-(e) a hydrolyzed
mixture containing 1a, 3a·H+, and 2a·2H2+: (a) measured at 313 K, (b)
293 K, (c) 273 K, (d) 253 K, and (e) 233 K. The lettering corresponds to
the assignments shown in Scheme 3 and Figure S1.
rotaxane 1b, and (b)-(g) a hydrolyzed mixture containing 1b and 2b·2H2+
:
(b) measured at 373 K, (c) 343 K, (d) 328 K, (e) 313 K, (f) 293 K, and (g)
283 K. The lettering corresponds to the assignments shown in Scheme 3.
kcal mol-1 and ∆S ) -12.3 ( 0.6 cal mol-1 K-1) and from
3a·H+ to 2a·2H2+ (∆H ) -3.1 ( 0.3 kcal mol-1 and ∆S )
-13.8 ( 1.2 cal mol-1 K-1) have similar values and the steps
are noncooperative (see Supporting Information). Thus, without
the extra stabilization of 2 by hydrogen-bonding, a considerable
amount of intermediary 3a·H+ coexists during the intercon-
In contrast, in the case of 1a/3a/2a with two XYL-stations,10
1H NMR spectra in CDCl3 or C6D5Br under acidic hydrolytic
conditions showed the coexistence of bis-imine 1a, monoimine
3a, and [2]rotaxane 2a as an equilibrated mixture over the range
of 243-378 K, although the equilibrium ratio shows moderate
temperature dependence (1a:3a:2a ) 42:36:22 at 243 K to 81:
18:∼1 at 313 K in CDCl3, Figures 4, 5b, and S5).
version of 1a and 2a·2H2+
.
On the other hand, the changes in both enthalpy and entropy
from 1b to 2b·2H2+ (∆H ) -16.7 ( 0.7 kcal mol-1 and ∆S
) -52.2 ( 2.1 cal mol-1 K-1) were larger than those in the
change from 1a to 2a·2H2+, as shown by the steeper regression
line for the plot of 1b/2b·2H2+ (Figure 5c). This drastic
difference in the enthalpy and entropy changes originates from
the incorporation of a TEG-station in place of a XYL-station:
that is, the additional enthalpic stabilization and loss of entropy
due to hydrogen-bonding of the macrocycle with the TEG-
station in 2b·2H2+ (∆∆Hb-a ) -10.7 kcal mol-1 and ∆∆Sb-a
) -26.2 cal mol-1 K-1 for hydrogen-bond formation with a
TEG-station). The conformational restriction of TEG-moiety
accompanied by hydrogen-bonding might contribute to the
additional negative ∆S term.23 Consequently, the incorporation
of a TEG-station with hydrogen-bonding ability into imine-
bridged rotaxanes not only biased the equilibrium between 1
and 2b · 2H2+ under acidic hydrolytic conditions in favor of
hydrolyzed [2]rotaxane, but also enables entropy-driven posi-
tional switching8 of the macrocycle by supplying additional
enthalpic and entropic changes. Furthermore, these thermody-
namic parameters show that both imine-bond hydrolysis and
hydrogen-bond formation between the macrocycle and the
station in the rotaxane system are thermodynamically matched
processes, that is, enthalpically favored processes accompanied
by a loss of entropy, whereas both hydrogen-bond cleavage and
If we compare the temperature dependence of the equilibrium
ratio of 1b/2b with that of 1a/3a/2a in C6D5Br (Figure 5a,b),
we see that incorporation of a TEG-station brings about two
interesting thermodynamic effects on the equilibrium between
1 and 2: (1) a TEG-station biases the equilibrium in favor of
[2]rotaxane due to hydrogen-bond formation with the macro-
cycle under acidic hydrolytic conditions; and (2) the equilibrium
ratio in the molecular shuttle 1b,c/2b,c·2H2+ with TEG-stations
changes within a narrower temperature range than in the case
of 1a/2a·2H2+ without TEG-station. These effects can be
explained in terms of thermodynamic parameters for the
hydrolysis of 1 to 2·2H2+, which was estimated from a van’t
Hoff plot of the temperature-dependent equilibrium ratios
(Figure 5c and Table 1).
The enthalpy and entropy differences between 1a and
2a·2H2+ are both negative (∆H ) -6.0 ( 0.4 kcal mol-1 and
∆S ) -26.0 ( 1.4 cal mol-1 K-1), indicating that the hydrolysis
of imine-bonds is an enthalpically favored and entropically
disfavored process (Table 1, Scheme 2). The negative entropy
change for imine hydrolysis indicates that the entropy loss
by addition of two water molecules more than offset the entropy
gain by mobilization of the macrocycle in the rotaxane structure.
For further consideration, the thermodynamic parameters of
imine-hydrolysis were determined by considering the intermedi-
ary monoimine 3a·H+. The van’t Hoff plots of each step in
the imine-hydrolysis of 1a revealed that differences in both
enthalpy and entropy from 1a to 3a·H+ (∆H ) -2.9 ( 0.2
(23) Meot-Ner, M. J. Am. Chem. Soc. 1983, 105, 4912–4915.
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A R T I C L E S
Umehara et al.
Figure 5. (a) Temperature dependence of the equilibrium ratio of 1b and 2b·2H2+ in the hydrolyzed mixture in C6D5Br, and (b) temperature dependence
of the equilibrium ratio of 1a, 3a·H+, and 2a·2H2+ in the hydrolyzed mixture in C6D5Br. (c) van’t Hoff plots for the hydrolysis of 1b to 2b·2H2+ and 1a
to 2a·2H2+
.
Table 1. Thermodynamic Parameters for the Conversions of
Bis-imine 1 to Monoimine 3·H+ and [2]Rotaxane 2·2H2+ in
C6D5Br
The capability of the primary amine/ammonium group for both
dynamic imine- and hydrogen-bond formation is the key feature
for realizing entropy-driven translational isomerism in the novel
molecular shuttle. This interplay could play an important role
in the design of entropy-driven molecular machines that exhibits
a sharp transition in response to small temperature changes. The
further application of this interplay to molecular shuttles and
switches is a subject of current interest.
∆G273, ∆G373
(kcal mol-1
compounds
∆H (kcal mol-1
)
∆S (cal mol-1 K-1
)
)
1a to 2a·2H2+
[1a to 3a·H+]
-6.0 ( 0.4
-2.9 ( 0.2
-3.1 ( 0.3
-16.7 ( 0.7
-26.0 ( 1.4
-12.3 ( 0.6
-13.7 ( 1.2
-52.2 ( 2.1
+1.1, +3.7
+0.5, +1.7
+0.6, +2.0
-2.4, +2.8
[3a·H+ to 2a·2H2+
1b to 2b·2H2+
]
Acknowledgment. H.K. acknowledges support by JSPS KAK-
ENHI (No. 18750024), the Global COE Program (Project No. B01:
Catalysis as the Basis for Innovation in Materials Science) from
MEXT, and JST PRESTO project. T.U. gratefully acknowledges a
JSPS Research Fellowship for Young Scientist. We are grateful to
K. Watanabe of the GC-MS and NMR Laboratory (Hokkaido
University) for the mass spectrometric analyses and to Dr. Y.
Kumaki of the High-Resolution NMR Laboratory (Hokkaido
University) for the ROESY measurements.
imine-bond formation are entropically favored processes (Scheme
1). Consequently, the competitive combination of imine-bonding
and hydrogen-bonding stations in a rotaxane system enables the
entropy-driven positional switching of the macrocycle to give
novel molecular shuttles with a sharp transition by synergistic
effects of entropy terms, as demonstrated here.
Conclusion
We have shown that the incorporation of a hydrogen-bonding
TEG-station into imine-bridged rotaxane can lead to novel
molecular shuttles, in which the position of the macrocycle can
be switched between imine- and hydrogen-bonding stations with
complete positional discrimination by dehydration/hydrolysis
or simply by changes in temperature under hydrolytic conditions.
Supporting Information Available: Experimental details and
additional spectroscopic data for the rotaxanes. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA804888B
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13988 J. AM. CHEM. SOC. VOL. 130, NO. 42, 2008