Functionally rigid and degenerate molecular shuttles

Article abstract of DOI:10.1002/chem.200802096

The preparation and dynamic behavior of two functionally rigid and degenerate [2]rotaxanes (1·4PF₆ and 2·4PF₆) in which a π-electron deficient tetracationic cyclophane, cyclobis(para-quat-p-phenylene) (CBPQT⁴⁺) ring, shutt

Full text of DOI:10.1002/chem.200802096

FULL PAPER
DOI: 10.1002/chem.200802096
Functionally Rigid and Degenerate Molecular Shuttles
[b]
Il Yoon,[b] Diego Benꢀtez,[b] Yan-Li Zhao,[b] Ognjen S. Miljanic´, Soo-Young Kim,[b]
Ekaterina Tkatchouk,[c] Ken C.-F. Leung,[b] Saeed I. Khan,[b]
William A. Goddard, III,[c] and J. Fraser Stoddart*[a, b]
ˇ
Abstract: The preparation and dynamic
behavior of two functionally rigid and
degenerate [2]rotaxanes (1·4PF6 and
2·4PF6) in which a p-electron deficient
tetracationic cyclophane, cyclobis(para-
dumbbell-shaped components experi-
enced significant upfield shifts at low
temperatures, just as has been observed
in the flexible [2]rotaxanes. Interesting-
ly, the resonances for the same protons
did not exhibit a significant upfield
shift at 298 K, but rather only a modest
shift. This phenomenon arises from the
much reduced binding of the ethynyl-
NP unit compared to the 1,5-dioxy-NP
unit. This effect, in turn, increases the
shuttling rate when compared to the
1,5-dioxy-NP-based rotaxane systems
investigated previously. The kinetic and
thermodynamic data of the shuttling
behavior of the CBPQT4+ ring be-
tween the NP units were obtained by
variable-temperature NMR spectrosco-
py and using the coalescence method
to calculate the free energies of activa-
tion (DGc°) of 9.6 and 10.3 kcalmolꢀ1
for 1·4PF6 and 2·4PF6, respectively,
probed by using the rotaxaneꢀs a-bipyr-
idinium protons. The solid-state struc-
ture of the free dumbbell-shaped com-
pound (3) shows the fully rigid ethynyl-
PH-ethynyl linker with a length (8.1 ꢁ)
twice as long as that (3.8 ꢁ) of the bu-
tadiyne linker. Full-atomistic simula-
tions were carried out with the
DREIDING force field (FF) to probe
the degenerate molecular shuttling pro-
cesses, and afforded shuttling energy
barriers (DG° =10.4 kcalmolꢀ1 for
1·4PF6 and 2·4PF6) that are in good
agreement with the experimental
values (DGc° =9.6 and 10.3 kcalmolꢀ1
for 1·4PF6 and 2·4PF6, respectively,
probed by using their a-bipyridinium
protons).
quat-p-phenylene) (CBPQT4+
)
ring,
shuttles back and forth between two p-
electron-rich naphthalene (NP) stations
by making the passage along an ethyn-
yl-phenylene-(PH)-ethynyl or buta-
diyne rod, are described. The [2]ro-
taxanes were synthesized by using the
clipping approach to template-directed
synthesis, and were characterized by
NMR spectroscopic and mass spectro-
metric analyses. 1H NMR spectra of
both [2]rotaxanes show evidence for
the formation of mechanically inter-
locked structures, resulting in the up-
field shifts of the resonances for key
protons on the dumbbell-shaped com-
ponents. In particular, the signals for
the peri protons on the NP units in the
Keywords: molecular modeling
molecular shuttles rotaxanes
·
·
·
self-assembly
synthesis
·
template-directed
Introduction
[a] Prof. Dr. J. F. Stoddart
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
Fax : (+1)847-491-7713
Bistable [2]rotaxanes,[1] which have evolved over the past
couple of decades from degenerate molecular shuttles,[2] are
playing an emergent role in the development of molecular
nanotechnology.[3] To date, they have already shown impor-
tant applications in the fabrication of molecular electronic
devices (MEDs)[4] and in the construction of nanoelectrome-
chanical systems (NEMS).[3,4] p-Electron rich 1,5-disubstitut-
ed naphthalene (NP) ring systems encircled by the p-elec-
tron-deficient tetracationic cyclophane, cyclobis(paraquat-p-
phenylene) (CBPQT4+), constitute a well-known[5] donor–
acceptor recognition motif. Introduction of rigidity through
the incorporation of NP, monopyrrolotetrathiafulvalene
(MPTTF),[1a,6] and bispyrrolotetrathiafulvalene (BPTTF)[7]
ˇ
´
[b] Dr. I. Yoon, Dr. D. Benꢂtez, Dr. Y.-L. Zhao, Dr. O. S. Miljanic,
Dr. S.-Y. Kim, Dr. K. C.-F. Leung, Dr. S. I. Khan,
Prof. Dr. J. F. Stoddart
Department of Chemistry and Biochemistry
University of California, Los Angeles
405 Hilgard Avenue, Los Angeles, CA 90095 (USA)
[c] Dr. E. Tkatchouk, Prof. Dr. W. A. Goddard, III
Materials and Process Simulation Center
California Institute of Technology
Pasadena, CA 91125 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802096.
Chem. Eur. J. 2009, 15, 1115 – 1122
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1115

into the dumbbell-shaped components of bistable [2]ro-
taxanes and their degenerate molecular shuttle progenitors
limits the number of geometries that can be adopted in con-
densed media, as well as in the solution and crystalline
phases. This increased constitutional rigidity in the dumbbell
backbone of [2]rotaxanes is advantageous in the design of
switches and machines since it limits the different number of
geometries that a rotaxane can adopt in condensed media
which often constitutes device settings.[3,4] Furthermore, the
structural rigidity is also favorable for the fabrication of sur-
face-based machinery[8] because of the absence of back-fold-
ing[9] by the free recognition site. Recently, we reported[10]
the preparation of two [2]rotaxanes with rigid spacers be-
tween their two recognition sites either two NPs or an
MPTTF and an NP in which the rigid system simplifies both
the structures and functions with respect to related bistable
[2]rotaxanes[1] with flexible spacers.
ing the longer ethynyl-PH-ethynyl linker, by a clipping reac-
tion under high-pressure[13] through the macrocyclization of
the subunits of the macrocyclic component around the NP
station of the dumbbell-shaped component in 1·4PF6
(Scheme 2), which was characterized by 1H NMR spectro-
scopic and electrospray ionization mass spectrometric (ESI-
MS) analyses. Moreover, we have employed variable-tem-
perature (VT) 1H NMR spectroscopy to calculate the
switching rates of the molecular shuttles. The solid-state su-
perstructure of the dumbbell-shaped compound 3 has also
been obtained. Finally, probing of the degenerate shuttling
process using molecular dynamic (MD) simulations in both
molecular shuttles 1·4PF6 and 2·4PF6 was carried out by
using the DREIDING force field,[14] which was described[11]
recently along with the experimental and calculated solid-
state structure of 2·4PF6.
The degenerate NP/NP [2]rotaxane (2·4PF6 in Figure 1)
was synthesized[10] to probe the shuttling barrier (by dynam-
ic 1H NMR spectroscopy) for the movement of the
Results and Discussion
Synthesis: The route for the synthesis of the [2]rotaxane
1·4PF6 containing the ethynyl-PH-ethynyl linker between
the two NP units is shown in Scheme 1 and 2. The dumb-
bell-shaped compound 3 was prepared in 50% yield by com-
bining a 2:1 ratio of compound 5, a monotosylated diethyle-
neglycol carrying a 2,6-diisopropylphenyl ether at its other
end, with compound 7, the fully rigid backbone of the
dumbbell component, in the presence of K2CO3 in MeCN.
Tosylation of the compound 4, which was obtained following
a literature[10] procedure, afforded compound 5 in 87%
yield. Compound 7 was obtained by using the Sonogashira[15]
coupling reaction between 5-iodonaphthol[10] and 1,4-diethy-
nylbenzene in 75% yield (Scheme 1). The degenerate mo-
lecular shuttle 1·4PF6, was synthesized from the dumbbell-
[16]
shaped compound 3 and the precursors 8·2PF6 and p-di-
Figure 1. Structural formulas of the degenerate [2]rotaxanes 1·4PF6 and
2·4PF6.
CBPQT4+ ring along the rigid butadiyne spacer. The solid-
state structures of the [2]rotaxane 2·4PF6 and its free dumb-
bell compound have been reported previously.[11] From the
solid-state structure, the length of the rigid butadiyne linker
was found to be 3.8 ꢁ.[11] To investigate the shuttling behav-
ior of the [2]rotaxane with an extended rigid linker between
two NP units on its dumbbell backbone, a phenylene (PH)
unit between two ethynyl units on the dumbbell backbone
has now been introduced into the system, resulting (Figure 1
and Scheme 1) in the [2]rotaxane 1·4PF6 and its dumbbell-
shaped compound 3.
Here, we describe the template-directed[12] synthesis
(Scheme 1 and 2) of a rigid and degenerate molecular shut-
tle, namely the donor–acceptor [2]rotaxane 1·4PF6 contain-
Scheme 1. Synthesis of the dumbbell compound 3 containing NP units.
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Chem. Eur. J. 2009, 15, 1115 – 1122

Functionally Rigid and Degenerate Molecular Shuttles
FULL PAPER
bromoxylene in DMF using a template-directed clipping ap-
proach under high pressure (Scheme 2). The [2]rotaxane
1·4PF6 was isolated as a red solid in 10% yield after column
chromatography on silica gel using Me2CO/NH4PF6 (100:1
v/w) as the eluent followed by column chromatography on
alumina using MeOH/2m NH4Cl (9.5:0.5 v/w) as the eluent.
Mass spectrometric investigation: The [2]rotaxane 1·4PF6
was characterized by electrospray ionization mass spectrom-
etry (ESI-MS) which revealed (see Figure S1 and S2 in the
Supporting Information) peaks at m/z 1863, 859, and 524,
ꢀ
corresponding to the loss of one, two, and three PF6 coun-
terions, respectively. From the synthesis of the [2]rotaxane
1·4PF6, a [3]rotaxane was obtained as a by-product, which
was isolated and characterized by ESI-MS revealing (see
Figure S1 in the Supporting Information) the peaks at m/z
1409 and 891, corresponding to the loss of two and three
ꢀ
PF6 counterions, respectively.
1H NMR spectroscopic investigations: A good indication of
the formation of the [2]rotaxane containing NP units and
the CBPQT4+ ring are the signals for the two peri protons
(H4 and H8) on the NP unit, which are shifted upfield signif-
icantly at 298 K compared with those observed for the
ꢀ
dumbbell compound 3, on account of [C H···p] interac-
tions[19] between the encircled NP unit and the two p-xyly-
lene links in the CBPQT4+ ring. Usually, the signals of the
two peri protons are found at dꢁ2–4 ppm in flexible[20] ro-
taxanes incorporating polyethyleneglycol linkers between
the two NP units and in non-degenerate (bistable) rigid[10]
rotaxane systems incorporating an MPTTF and an NP unit
with phenylacetylene linker in their dumbbell components.
In the rigid and degenerate [2]rotaxanes 1·4PF6 and
2·4PF6, however, the two peri proton peaks for the
CBPQT4+-encircled NP unit were not found at dꢁ3 ppm at
298 K. Instead, they were located (Table 1 and Figure 2) at
dꢁ6 ppm, a chemical shift which was confirmed by 2D
COSY and ROESY NMR spectra (see Figures S3–S9 in the
Supporting Information). This observation is consistent with
a much weaker binding of an oxyethynyl-NP unit as com-
pared to a dioxy-NP unit. The enthalpic contribution to the
binding free energy is dwarfed by the entropic gain from the
higher degrees of freedom that the ring experiences when it
is not encircling an NP unit. Consequently, the two peri
proton peaks were found at dꢁ3 ppm in both [2]rotaxanes
1·4PF6 and 2·4PF6 with aid of 2D COSY and ROESY NMR
spectra at low temperatures. This observation suggests that
there is a thermodynamic balance between the enthalpic
(DH) and entropic (TDS) components to the free energy
where, at low temperatures, enthalpy takes over, whereas at
higher temperatures, entropy dominates. This situation rep-
Scheme 2. Synthesis of the degenerate [2]rotaxane 1·4PF6.
The synthesis of 2·4PF6 was carried out previously[10,11] by
two different methods. The first low-yielding (8%)[10] syn-
thesis of 2·4PF6 using a template-directed clipping approach
under high pressure reaction called for the development of
a higher yielding synthesis. Our subsequent success[17] in
making catenanes by employing Cu2+-mediated Eglinton[18]
coupling as the final step encouraged us to improve (18%
yield) the preparation of 2·4PF6 in which the crucial me-
chanical bond-forming step relies on the Eglinton dimeriza-
tion of the two half dumbbell molecules with terminal
alkyne functions.[11]
Table 1. 1H NMR (500 MHz) spectroscopic data for the [2]rotaxanes 1·4PF6 and 2·4PF6 and their chemical shift differences (Dd) compared with refer-
ence to their dumbbell components in CD3COCD3 at 298 K.
Compound 1·4PF6
CBPQT4+ component
Dumbbell component
Ha
Hb
HXy
HCH2
H4
H8
H3
H7
H2
H6
glycol
HiꢀPr
stopper
HPh
9.39
8.09
8.29
6.12
5.97
6.23
7.01
7.12
6.90
7.65
4.11–4.54
3.48 (0)
7.06–7.15
7.70
A
R
(+0.55)
E
G
(ꢀ1.83)
N
(ꢀ0.48)
(ꢀ0.95)
N
G
(+0.01)
R
Dumbbell component
Ha
9.42
Hb
8.02
HXy
8.49
HCH2
6.16
H4
7.95
H8
7.92
H3
7.63
H7
7.54
H2
7.14
H6
8.45
glycol
3.98–4.46
HiꢀPr
3.47(+0.02) 7.05–7.12
(ꢀ0.04) (ꢀ0.03~+0.02)
stopper
ACHTUNGTRENNUNG
A
A
A
A
A
A
A
A
A
A
ACHTUNGTRENNUNG
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J. F. Stoddart et al.
Table 2. Kinetic and thermodynamic data[a] for the shuttling behavior of
the CBPQT4+ ring between the NP units in the [2]rotaxane 1·4PF6 using
the coalescence method (Tc =coalescence temperature, Du=limiting
chemical shift difference, kex =rate constant).
Probe H
Tc
Du
kex
[sꢀ1
DGc°
[K][b]
[Hz]
G
]
[kcalmolꢀ1
]
a-BIPY2+[c]
208
208
160
54
355ꢂ1
119ꢂ1
9.6ꢂ0.1
bismethylene[d]
10.0ꢂ0.1
[a] 1H NMR spectrum recorded at 500 MHz in CD3COCD3 solution.
[b] Calibrated by using neat MeOH sample. [c] The a-BIPY2+ protons
on the CBPQT4+ ring used to probe the energy barrier. [d] The bis-
methylene protons on the glycol chains in the dumbbell-shaped compo-
nent used to probe the energy barrier.
1
The VT H NMR spectra of 1·4PF6 is shown in Figure 3
1
and the data are summarized in Table 2. The H NMR spec-
trum at 187 K features four broad peaks in the region asso-
ciated with a-bipyridinium (BIPY2+) protons (in pale blue
stripe) on the CBPQT4+ ring. With increasing temperature,
these broad peaks coalesce into one broad peak (d
ꢁ9.5 ppm) at 208 K. These spectral changes correspond to a
Figure 2. 1H NMR spectra of the [2]rotaxanes a) 1·4PF6 in CD3OD and
b) 2·4PF6 in CD3COCD3 at 298 K. The protons are defined alongside the
structural formula in Scheme 2.
resents a perfect scenario for a molecular thermal switch,
where the CBPQT4+ ring shuttles constantly from one sta-
tion to the other. At low temperatures, the ring spends more
of its time on the NP units, but as the temperature is in-
creased, the ring resides less and less of the time on the NP
units, effectively becoming a “roving ambassador” along the
whole length of the dumbbell component.
free energy of activation (DGc°)[21] of 9.6 kcalmolꢀ1,[10]
a
value which was found to be significantly lower than that
(13.0–15.0 kcalmolꢀ1) found previously[20] for other more
flexible molecular shuttles. Similarly, the bismethylene pro-
tons (in pale pink stripe) on the glycol chains were used to
probe the shuttling process and the coalescence data were
calculated, leading to a DGc° value of 10.0 kcalmolꢀ1. The
methine protons (in pale green stripe) on the isopropyl
groups were not used to probe the shuttling process because
there was no separation of their signals at the low tempera-
tures.
The 1H NMR spectra of the [2]rotaxanes 1·4PF6 (Fig-
ure 2a) in CD3OD and 2·4PF6 (Figure 2b) in CD3COCD3
recorded at 298 K provide overwhelming evidence for the
formation of the [2]rotaxanes. The signals for some key pro-
tons in both 1·4PF6 and 2·4PF6 recorded in CD3COCD3 are
shifted considerably as compared with those in the
CBPQT4+ ring and the dumbbell-shaped compounds
(Table 1). The two peri proton peaks on the NP units in
1·4PF6 were located at d=5.97 and 6.23 ppm and so are
shifted upfield (Dd=1.25 and 1.83 ppm, respectively) in
CD3COCD3 at 298 K with respect to the corresponding sig-
nals in the dumbbell. Similarly, the two peri proton peaks in
2·4PF6 were located at d=5.63 and 5.95 ppm and so are
shifted upfield (Dd=2.32 and 1.97 ppm, respectively) in
CD3COCD3 at 298 K, once again with respect to the dumb-
bell. Furthermore, the two peri proton peaks exhibit broad-
ening, while the other protons on the NP units produce
sharp peaks, a fact which may be attributable to the molecu-
lar shuttling of the CBPQT4+ ring between the two NP
units.
1
Figure 3. Partial VT H NMR spectra of 1·4PF6 in CD3COCD3.
Dynamic 1H NMR spectroscopy was performed in
CD3COCD3 to quantify the shuttling barrier for the
CBPQT4+ ring travelling along the rigid rod between the
two NP units in the [2]rotaxane 1·4PF6. This barrier plays
an important role in relation to the bistable rotaxaneꢀs per-
formance in device settings.[3,4] The kinetic and thermody-
namic parameters for the dynamic processes observed in
both 1·4PF6 and 2·4PF6 are listed in Tables 2 and 3, respec-
tively.
1
The VT H NMR spectra of 2·4PF6 is shown in Figure 4
and the relevant data are summarized in Table 3. The a-pro-
tons on the BIPY2+ unit of the CBPQT4+ ring, the methine
on the isopropyl stoppers, and the bismethylene protons on
the glycol chains in the dumbbell-shaped component in
2·4PF6 were used to probe the energy barrier, resulting in
similar free energies of activation (DGc°) of 10.3, 9.5, and
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Chem. Eur. J. 2009, 15, 1115 – 1122

Functionally Rigid and Degenerate Molecular Shuttles
FULL PAPER
Table 3. Kinetic and thermodynamic data[a] for the shuttling behavior of
the CBPQT4+ ring between the NP units in the [2]rotaxane 2·4PF6 using
the coalescence method (Tc =coalescence temperature, Du=limiting
chemical shift difference, kex =rate constant).
shaped backbone overall in which dihedral angles of 1348
and 348 are evident with respect to the central PH ring. It
would appear that stabilization of the molecule is achieved
ꢀ
by a combination of a) intermolecular [C H···O] interac-
Probe H
Tc
Du
kex
[sꢀ1
DGc°
tions[22] (see Figure S13 in the Supporting Information) be-
tween the three oxygen atoms in the diethyleneglycol chains
and appropriate protons in the diethyleneglycol chains and
[K][b]
[Hz]
G
]
[kcalmolꢀ1
]
a-BIPY2+[c]
methine[d]
225
196
214
211
38
59
469ꢂ1
85ꢂ1
10.3ꢂ0.1
9.5ꢂ0.1
10.3ꢂ0.1
bismethylene[e]
131ꢂ1
ꢀ
the PH centers, and b) intermolecular [C H···p] interactions
[a] 1H NMR spectrum recorded at 500 MHz in CD3COCD3 solution.
[b] Calibrated by using neat MeOH sample. [c] The a-BIPY2+ protons
on the CBPQT4+ ring used to probe the energy barrier. [d] The methine
protons on the isopropyl stoppers in the dumbbell-shaped component
used to probe the energy barrier. [e] The bismethylene protons in the
glycol chains in the dumbbell-shaped component used to probe the
energy barrier.
(see Figure S13 in the Supporting Information) between the
NP units and the protons in the diethyleneglycol chains of
the neighboring molecules, the NP units and the protons in
the NP units of the neighboring molecules, as well as the
benzene rings in the stoppers and the protons in the NP
units of the neighboring molecules.
The solid-state structures of the molecular shuttle 2·4PF6
and its free dumbbell-shaped molecule were reported[11] re-
cently. The structure of 2·4PF6 shows an interdigitated su-
perstructure with the molecules of this [2]rotaxane lining
themselves up in parallel p–p stacks[23] of alternating NP
ring systems and the BIPY2+ units, exhibiting a well-organ-
ized donor–acceptor recognition motif. In the structure of
the free dumbbell-shaped molecule, the total length of the
molecule is 3.1 nm and the distance covered by the buta-
diyne unit as the rod between the two NP units is 3.8 ꢁ,
which is less than half that in 3 containing the ethynyl-PH-
ethynyl rod.
Molecular simulations investigation: To understand the
nature of the weak forces that govern the extended and
highly ordered p–p stacking in the superstructure, as well as
in the shuttling exhibited by compounds 1·4PF6 and 2·4PF6,
we calculated their molecular structures utilizing the
DREIDING force field.[16] This force field has been used[24]
for the accurate description of structural and dynamic pa-
rameters in mechanically interlocked molecules, in conjunc-
tion with the charge equilibration (QEq) method,[25] as im-
plemented in the Lingraf[26] software suite [release 3.23].
The DREIDING force field is a full atomistic generic force
field. Figure 6 shows the overlay of the X-ray structure in
the darker colors and the DREIDING minimized (0 K)
structure of 2·4PF6 in the paler colors. The structure was ob-
tained by minimization of a periodic system using the crys-
tallographic data as input. There are similarities and differ-
ences between the experimental and calculated geometries
to the extent that those regions in the superstructure where
intramolecular forces at work are described with higher fi-
1
Figure 4. Partial VT H NMR spectra of 2·4PF6 in CD3COCD3.
10.3 kcalmolꢀ1, respectively. Interestingly, these DGc° values
match exactly those observed for the [2]rotaxane 1·4PF6.
Consequently, the different length of the linkers in the
dumbbell components of the two [2]rotaxanes are not re-
flected directly in the heights of the energy barriers.
X-ray crystallographic investigation: Slow evaporation of a
MeCN/CH2Cl2/C6H14/iPr2O solution of 3 yielded colorless
platelet-shaped, single crystals suitable for X-ray crystallog-
raphy. In the solid-state, 3 adopts (Figure 5) a non-centro-
symmetric geometry. The total length of the molecule is
3.6 nm and the distance covered by the ethynyl-PH-ethynyl
unit as the spacer between two NP units is 8.1 ꢁ, with an S-
Figure 5. Solid-state structure of the compound 3. The hydrogen atoms
are omitted for the sake of clarity. Color code: the ethynyl-PH-ethynyl
unit: gray; the NP units: red; the diethyleneglycol chains: pink; 2,6-diiso-
propylphenyl units: green.
Figure 6. Overlay of the calculated and experimental (solid-state) struc-
tures of 2·4PF6. The X-ray crystal structure is displayed in darker colors
and the DREIDING-minimized structure is overlayed in paler colors.
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J. F. Stoddart et al.
delity than those in which intramolecular forces dominate
the superstructure. The variance in the dihedral angle be-
tween the NP rings in 2·4PF6 is also a consequence of the
lack of parametrization for the extended p-delocalized buta-
diyne linking both aromatic systems. Next, we examined the
geometry of a single molecule in order to estimate the barri-
ers to shuttling of the CBPQT4+ ring in 1·4PF6 and 2·4PF6.
Using a manually steered[27] combination of MM and MD at
0 K and 300 K, respectively, we calculated[28] a shuttling
energy barrier of 10.4 kcalmolꢀ1 for both 1·4PF6 and 2·4PF6
which is in reasonably good agreement with the experimen-
tal values[10] of 9.6 and 10.3 kcalmolꢀ1 for 1·4PF6 and
2·4PF6, respectively. The calculated structure of 1·4PF6
using the DREIDING force field shown in Figure 7, adopts
encircled rigid and degenerate molecular shuttle containing
an enlarged spacer between the two NPs in the [2]rotaxane
formation.
since they are key signals which are shifted significantly up-
field at low temperatures. Interestingly, the same signals ex-
hibited a moderate upfield shift at 298 K, something which
has never been observed in this kind of molecular shuttle.
1
The VT H NMR spectroscopy experiments were performed
to obtain kinetic and thermodynamic data. The a-BIPY2+
,
methine, and bismethylene protons were used to probe the
shuttling behavior, resulting in very similar energy barriers
in the two molecular shuttles. The shuttling energy barriers
calculated by the coalescence method at low temperatures
are lower than those for other more flexible rotaxanes, as
well as non-degenerate (bistable) rigid rotaxanes. The
CBPQT4+ ring in the rigid molecular shuttles moves back
and forth between the two NP units in solution at a rate of
around 560000–570000 (0.6 at 199 K) times per second at
room temperature. The solid-state structure of the rigid
dumbbell-shaped compound 3 shows clearly the fully rigid
backbone which allows for the fast shuttling process in the
[2]rotaxane. The different lengths of the spacers between
two NP units in the two rotaxanes were not reflected direct-
ly in the height of the energy barrier between the two NP
sites. These results could be useful for the growth in the fun-
damental understanding of molecular shuttles in rigid mo-
lecular systems, as well as for developing devices having
structural and functional rigidity advantages in condensed
phases.
Figure 7. The DREIDING-minimized structure of 1·4PF6. The hydrogen
atoms are omitted for the sake of clarity. Color code: the ethynyl-PH-
ethynyl unit: gray; the NP units: red; the diethyleneglycol chains: pink;
the 2,6-diisopropylphenyl units: green; the CBPQT4+ ring: blue.
Experimental Section
The fact that DREIDING predicts virtually identical shut-
tling barriers for 1·4PF6 and 2·4PF6 is not surprising, since
the activation energy is associated with the slipping and de-
slipping of the CBPQT4+ ring from the highly organized ar-
rangement when situated on an NP unit. For both 1·4PF6
and 2·4PF6, while the CBPQT4+ ring is encircling around
the NP units, it experiences the same relative environment.
This observation suggests that the stereoelectronic charac-
teristics of only the first few linker atoms play a role in de-
termining the shuttling rate. This phenomenon has also been
observed[20] in the series of degenerate shuttles with 1,5-
dioxy-NP ring systems with highly varied intermediate link-
ers where the shuttling barrier did not vary more than
0.4 kcalmolꢀ1 away from 15.4 kcalmolꢀ1.
General: All reagents were purchased from Aldrich and used without
further purification. The rotaxane 2·4PF6 was prepared according to the
previous reports.[10,11] 2-(2-(2,6-Diisopropylphenoxy)ethoxy)ethanol (4),[10]
5-iodonaphthol (6),[10] 1,1’-[1,4-phenylenebis(methylene)]bis-4,4’-bipyridi-
nium bis(hexafluorophosphate) (8·2PF6),[16] and cyclobis(paraquat-p-
phenylene) tetrakis(hexafluorophosphate) (CBPQT·4PF6)[5] were pre-
pared according to literature procedures. High-pressure reactions were
carried out in a custom-made Teflon container under 12 kbar of pressure
generated by a Psika high-pressure apparatus. Thin layer chromatography
(TLC) was performed on silica gel 60 F254 (E. Merck). The plates were
inspected by UV light and, if required, developed in I2 vapor. Column
chromatography was performed on silica gel 60F (Merck 9385, 0.040–
0.063 mm) or alumina (J. T. Baker, 0.050–0.200 mm). Melting points were
recorded on an Electrothermal 9100 instrument in open capillary tubes
and are uncorrected. Routine nuclear magnetic resonance (NMR) spec-
tra were recorded at 258C on a Brꢄker Avance 500 spectrometer, with
working frequencies of 500 MHz for 1H, and 125 MHz for 13C nuclei.
Chemical shifts are quoted in ppm on the d scale and coupling constants
1
(J) are expressed in Hertz (Hz). Samples for H NMR spectroscopic stud-
Conclusions
ies were prepared using solvents purchased from Cambridge Isotope
Labs. VT-NMR spectra were recorder on a Brꢄker Avance 500 spectrom-
eter and temperatures were calibrated with a neat MeOH sample before
or after each experiment and assumed to remain constant during the ex-
periment. All spectra were referenced using the residual solvent peak.
The full assignment of the 1H NMR signals was performed using 2D
COSY (correlation spectroscopy) and ROESY (rotating-frame Over-
hauser enhancement spectroscopy) experiments. Matrix-assisted laser-de-
sorption/ionization time-off-light mass spectrometry (MALDI-TOF MS)
was performed on an IonSpec 4.7 tesla Ultima Fourier Transform mass
spectrometer, utilizing a 2,5-dihydroxybenzoic acid (DHB) matrix. Fast
atom bombardment mass spectra were obtained on a JEOL JMS-600H
high resolution mass spectrometer equipped with a FAB probe. Electro-
In conclusion, two rigid and degenerate molecular shuttles
have been investigated in considerable detail. The new
[2]rotaxane (1·4PF6) was synthesized by using the template-
directed protocol of clipping under high-pressure conditions,
and was characterized by a combination of NMR spectros-
copy and ESI-MS spectrometry. The formation of both
[2]rotaxanes was confirmed by the shifts in the proton sig-
nals of the [2]rotaxanes, relative to those in the free compo-
nents, especially the two peri proton signals on the NP units
1120
www.chemeurj.org
ꢃ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 1115 – 1122

Functionally Rigid and Degenerate Molecular Shuttles
FULL PAPER
spray ionization mass spectra (ESI-MS) were measured on an IonSpec
FTICR mass spectrometer with CH3COCH3 as the mobile phase.
4H), 7.61 (d, J=7.0 Hz, 2H), 7.11–7.07 (m, 6H), 6.97 (t, J=7.0 Hz, 2H),
6.87 (t, J=7.0 Hz, 2H), 6.84 (d, J=7.0 Hz, 2H), 5.96 (br, 2H), 5.90 (s,
8H), 5.72 (br, 2H), 4.52 (br, 4H), 4.27 (br, 4H), 4.17 (br, 4H), 4.11 (br,
4H), 3.42 (sep, J=7 Hz, 2H), 1.11 ppm (d, J=7.0 Hz, 24H); MS (ESI):
m/z: 1863 [MꢀPF6]+, 859 [Mꢀ2PF6]2+, 524 [Mꢀ3PF6]3+; HRMS: calcd
for C98H98F12N4O6P2 ([Mꢀ2PF6]2+) 858.8402, found 858.8830. Selected
2-[2-(2,6-Diisopropylphenoxy)ethoxy]ethyl
4-methylbenzenesulfonate
(5): 2-[2-(2,6-Diisopropylphenoxy)ethoxy]ethanol 4 (1.0 g, 3.75 mmol), p-
toluenesulfonyl chloride (0.86 g, 4.51 mmol), and dimethylaminopyridine
(DMAP) (100 mg) were dissolved in anhydrous CH2Cl2 (50 mL) and
Et3N (2 mL), and the reaction mixture was stirred for one day. The sol-
vent was evaporated, and extracted with CH2Cl2, washed with H2O
before the solvent was evaporated and the residue purified by column
chromatography (SiO2: EtOAc/hexane 1:5) to afford 5 as a white solid
(1.37 g, 87%). 5: m.p. 40–428C; 1H NMR (500 MHz, CDCl3, 258C,
TMS): d=7.83 (d, J=8 Hz, 2H), 7.35 (d, J=8 Hz, 2H), 7.10 (s, 3H), 4.23
(t, J=5 Hz, 2H), 3.86 (t, J=5 Hz, 2H), 3.82–3.78 (m, 4H), 3.33 (sep, J=
7 Hz, 2H), 2.44 (s, 3H), 1.21 ppm (d, J=7 Hz, 12H); 13C NMR
(100 MHz, CDCl3, 258C, TMS): d=152.9, 144.8, 141.8, 129.8, 129.1,
128.0, 124.7, 124.0, 73.7, 70.7, 69.3, 69.0, 26.2, 24.1, 21.6 ppm; HRMS:
calcd for C23H32O5S ([M]+) 420.1970, found 420.1980; MALDI-TOF MS:
m/z: 420 [M]+.
data for [3]rotaxane: MS (ESI): m/z: 1409 [Mꢀ2PF6]2+, 891 [Mꢀ3PF6]3+
;
HRMS: calcd for C134H130F30N8O6P5 ([Mꢀ2PF6]3+
) 890.9452, found
891.1596.
X-ray crystallographic analysis: CCDC-679354 contains the supplementa-
ry crystallographic data of 3 for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. The crystallographic data for
2·4PF6 and its free dumbbell-shaped compound are given in reference
[11]. The data were processed by using the program SAINT[29] to give the
structure factors. The structures were solved by direct methods and re-
fined by full-matrix least squares against jF2j. Absorption corrections
were based on multiple and symmetry-equivalent reflections in the data
sets using the SADABS[30] program. All non-hydrogen atoms were re-
fined anisotropically. Hydrogen atoms were treated as idealized contribu-
tions. Scattering factors and anomalous dispersion coefficients are con-
tained in the SHELXTL[31] 6.12 program library.
5-(2-{4-[2-(5-Hydroxynaphthalen-1-yl)ethynyl]phenyl}ethynyl)naphtha-
len-1-ol (7): 5-Iodonaphthol 6 (1.4 g, 5.41 mmol), 1,4-diethynylbenzene
(0.31 g, 2.46 mmol), [PdCl2ACHTNUTRGNE(UGN PPh3)2] (89 mg, 0.127 mmol), CuI (47 mg,
0.247 mmol), and PPh3 (60 mg, 0.229 mmol) were dissolved in Et3N
(30 mL) and MeCN (10 mL), causing a color change to a dark brown,
before being heated under reflux for 4 h. After extraction with CH2Cl2,
the solvent was evaporated and the residue was purified by column chro-
matography (SiO2: MeOH/CH2Cl2, 1:50) to afford 7 as a colorless solid
(0.90 g, 90%). 7: m.p. ꢁ2138C (decomp); 1H NMR (500 MHz,
CD3COCD3, 258C, TMS): d=9.26 (br, 2H), 8.31 (d, J=8 Hz, 2H), 7.95
(d, J=8 Hz, 2H), 7.79 (d, J=8 Hz, 2H), 7.73 (s, 4H), 7.48 (t, J=8 Hz,
2H), 7.46 (t, J=8 Hz, 2H), 7.01 ppm (d, J=8 Hz, 2H); 13C NMR
(125 MHz, CD3COCD3, 258C, TMS): d=153.5, 134.3, 131.6, 130.8, 127.4,
124.7, 124.0, 123.4, 123.2, 119.8, 116.9, 108.8, 93.4, 89.7 ppm; HRMS:
calcd for C30H18O2 ([M]+) 410.1307, found 410.1303.
Crystal data for 3: C62H66O6, Mr =907.15, monoclinic, space group P21/c,
a=23.130(2), b=11.874(1), c=18.840(1) ꢁ, b=98.656(1)8, V=
5115.3(6) ꢁ3, Z=4, 1calcd =1.178 gcmꢀ3
,
m
G
,
T=
100(2) K, colorless plate, 0.20ꢅ0.20ꢅ0.02 mm, R1 =0.0522 [I> 2 s(I)],
wR2 =0.1401 (all data), GOF=1.017.
Acknowledgements
This work was supported by the Microelectronics Advanced Research
Corporation (MARCO) and its Focus Center Research Program
(FCRP), the Center on Functional Engineered NanoArchitectonics
(FENA), the Center for Nanoscale Innovation for Defense (CNID), and
the National Science Foundation (NSF) (ECS-0609128).
1,4-Bis[2-(5-{2-[2-(2,6-diisopropylphenoxy)ethoxy]ethoxy}naphthalen-1-
yl)ethynyl]benzene (3): Compounds
5 (0.74 g, 1.76 mmol), 7 (0.3 g,
0.731 mmol), and K2CO3 (0.4 g, 2.89 mmol) were dissolved in anhydrous
MeCN (80 mL), before being heated under reflux for three days. The sol-
vent was evaporated, and extracted with CH2Cl2, washed with H2O
before the solvent was evaporated and the residue was purified by
column chromatography (SiO2: CH2Cl2/hexane 1:2) to afford 3 as a color-
less solid (0.33 g, 50%). 3: m.p. 108–1108C; 1H NMR (500 MHz, CDCl3,
258C, TMS): d=8.37 (d, J=8 Hz, 2H), 8.04 (d, J=8 Hz, 2H), 7.79 (d,
J=8 Hz, 2H), 7.67 (s, 4H), 7.52 (t, J=8 Hz, 2H), 7.46 (t, J=8 Hz, 2H),
7.11 (s, 6H), 6.94 (d, J=8 Hz, 2H), 4.40 (t, J=5 Hz, 4H), 4.13 (t, J=
5 Hz, 4H), 4.03–3.98 (m, 8H), 3.42 (sep, J=7 Hz, 4H), 1.21 ppm (d, J=
7 Hz, 24H); 13C NMR (125 MHz, CDCl3, 258C, TMS): d=154.8, 153.0,
141.8, 134.3, 131.5, 131.0, 126.8, 125.5, 124.6, 124.5, 124.0, 123.2, 123.2,
120.1, 118.6, 105.5, 93.8, 89.8, 73.9, 70.8, 70.0, 68.1, 26.2, 24.0 ppm;
HRMS: calcd for C62H67O6 ([M + H]+) 907.4938, found 907.4805, calcd
for C62H66KO6 ([M + K]+) 945.4496, found 945.4279; MALDI-TOF MS:
m/z: 906 [M]+.
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3 (100 mg,
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Received: October 9, 2008
Published online: December 22, 2008
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