Reversible 2D pseudopolyrotaxanes based on cyclodextrins and cucurbit[6]uril

Article abstract of DOI:10.1021/jo0617159

In this paper, a pseudorotaxane (2) was synthesized by reaction of cucurbit [6]uril with 6-[(6-aminohexyl)amino]-6-deoxy-β-cyclodextrin chloride. Subsequently, pseudorotaxanes 2 were further assembled to form a 2D pseudopolyrotaxane (3) through an α,ω-PPG2000 diamino polymer threading the cavities of cyclodextrins in 2, and the resulting pseudopolyrotaxane was comprehensively characterized by FT-IR, NMR, TG-DTA, elemental analysis, and transmission electron microscopy. Significantly, the 2D pseudopolyrotaxane can turn into a main-chain pseudopolyrotaxane in the presence of base, and then the addition of α-cyclodextrins may result in a reversible switch between two different 2D pseudopolyrotaxanes.

Full text of DOI:10.1021/jo0617159

of a polymer (entitled “side chain pseudopolyrotaxanes”),3-5
are especially fascinating due to their unusual architectures and
the different properties from conventional polymers. Therefore,
designing and synthesizing pseudopolyrotaxanes with novel
topologies is desirable for both polymer science and supramo-
lecular chemistry. Recently, we successfully constructed not only
a series of pseudopolyrotaxanes based on â-cyclodextrins (â-
CDs)6 and modified â-CDs,7 but also bis(pseudopolyrotaxane)s
based on bridged bis(â-CD)s.8 Here, we report a simple way to
prepare 2D pseudopolyrotaxane 3 in which cyclic molecules
are threaded onto both the polymeric main chain and its side
chains (Scheme 1). Furthermore, the 2D pseudopolyrotaxane
can turn into a main-chain pseudopolyrotaxane in the presence
of base because of the cyclic cucurbit[6]uril (CB[6]) molecules
dethreading from the side chain of 3, and vice versa. Addition
of R-CDs may result in a reversible switch between two different
2D pseudopolyrotaxanes by acid-base control.
Reversible 2D Pseudopolyrotaxanes Based On
Cyclodextrins and Cucurbit[6]uril
Yu Liu,* Chen-Feng Ke, Heng-Yi Zhang, Wei-Jia Wu, and
Jun Shi
Department of Chemistry, State Key Laboratory of
Elemento-Organic Chemistry, Nankai UniVersity,
Tianjin 300071, P. R. China
yuliu@nankai.edu.cn
ReceiVed August 17, 2006
To obtain the 2D pseudopolyrotaxane 3, pseudorotaxane 2
was first synthesized by reaction of CB[6] with 6-[(6-amino-
hexyl)amino]-6-deoxy-â-CD chloride (1) in 81% yield. The 1:1
1/CB[6] complexation in 2 was confirmed by the results of
elemental analysis, MS, and 1H NMR spectroscopy. In the ESI-
MS spectrum of pseudorotaxane 2, a signal was observed at
m/z (M2+) ) 1115.8, indicating the formation of the pseudoro-
In this paper, a pseudorotaxane (2) was synthesized by
reaction of cucurbit [6]uril with 6-[(6-aminohexyl)amino]-
6-deoxy-â-cyclodextrin chloride. Subsequently, pseudoro-
taxanes 2 were further assembled to form a 2D pseudopoly-
rotaxane (3) through an R,ω-PPG2000 diamino polymer
threading the cavities of cyclodextrins in 2, and the resulting
pseudopolyrotaxane was comprehensively characterized by
FT-IR, NMR, TG-DTA, elemental analysis, and transmission
electron microscopy. Significantly, the 2D pseudopolyro-
taxane can turn into a main-chain pseudopolyrotaxane in the
presence of base, and then the addition of R-cyclodextrins
may result in a reversible switch between two different 2D
pseudopolyrotaxanes.
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of rotaxanes and catenanes but also viewed as the most
interesting prototypes for molecular machines because of their
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In this respect, pseudopolyrotaxanes, in which many cyclic
molecules are threaded onto either a polymeric main chain
(entitled “main chain pseudopolyrotaxanes”) or the side chains
* To whom correspondence should be addressed. Fax: +86-22-23503625.
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10.1021/jo0617159 CCC: $37.00 © 2007 American Chemical Society
Published on Web 12/06/2006
280
J. Org. Chem. 2007, 72, 280-283

SCHEME 1. Preparation Processes of 2D Polypseudorotaxane 3
taxane 2. In the 1H NMR experiments, the resonance signals of
which is attributed to the strong binding ability between hexane-
1,6-diamine and CB[6] (K ) 4.49 × 108 M-1) in aqueous
solution.10 Cross-peaks of protons of CH3 of PPG and H-3/H-5
of CD in 2D ROESY spectrum of 3 also reveals that PPG thread
into the CDs cavities.
Ha/a′, Hb/b′, and Hc/c′ of CD in 2 shift upfield ca. 0.04, 1.02, and
0.91 ppm, respectively, relative to the free 1, as shown in Figure
1. On the other hand, the chemical shift of Hx protons of the
Pseudopolyrotaxane 3 is more thermally stable than both
pseudorotaxane 2 and its precursor 1. The TGA analysis (Figure
2) shows that 1-3 exhibit small initial weight loss corresponding
FIGURE 2. TGA curves of (a) CB[6], (b) 1, (c) 2, and (d) 3.
FIGURE 1. 1H NMR spectra of (a) 1, (b) 2, and (c) CB[6] in D2O
containing 0.2M Na2SO4. Symbols [ and B indicate the solvent and
MeOH resonances, respectively.
to hydration before 230 °C. And then, 1 loses 94.0% of its
original weight at 507 °C, corresponding to the loss of CDs
backbone. Remarkably, 2 and 3 exhibit a similar change profile
after 298 °C, but the decomposition temperatures of 3 in every
step are ca. 60 °C higher than those of 2, suggesting that the
threading of PPG increases the stability of 3. Similar increases
in thermal stability were also observed in main-chain3c and side-
chain3d pseudopolyrotaxanes incorporating CB[6].
CB[6] molecule in 2 shifts downfield accompanying its signal
to split to quadruple peaks from its original double peaks, which
should be attributed to the asymmetry guest penetrating into
the cavity of CB[6].9 All of these observations suggest that the
methylene groups of CD moiety must be included to the cavity
of CB[6].
Pseudopolyrotaxane 3 was prepared by adding amino-
terminated PPG2000 to a saturated aqueous solution of 2 in
21% yield. From H NMR data of 3 in D2O (Figure 4a), we
could calculate the number of â-CD units by comparing the
integral of signals at 0.51 ppm (Hc/c′ of â-CD) or 4.34 ppm (Hy
of CB[6]) with those at 1.15 ppm (CH3 of PPG). The obtained
values were 11.8 ( 0.7, which means that PPG2000 could thread
about 12 pseudorotaxanes 2 to form pseudopolyrotaxane 3. The
consistency between both values calculated form Hc/c′ and Hy
indicates that no CB[6]s dethread off during this procedure,
TEM was performed to provide further insight into the size
and shape of pseudopolyrotaxane 3. From Figure 3a, we may
note that there exist some sticklike nanowires on the substrate.
The lengths of these nanowires are in the range of 14-25 nm,
which is consistent with those of PPG2000. To obtain their width
data, we also tried to further magnify the samples to obtain
higher resolution TEM images, but unfortunately, these samples
melt. Hence, their widths are just roughly estimated from Figure
3a. They are about 3 nm. As is well-known, the diameter of
the â-CD’s periphery is about 1.5 nm, and the height of CB[6]
1
(9) Wheate, N. J.; Buck, D. P.; Day, A. I.; Collins, J. G. Dalton Trans.
2006, 451-458.
(10) Liu, S.; Ruspic, C.; Mukhopadhyay, P.; Chakrabarti, S.; Zavalij, P.
Y.; Isaacs, L. J. Am. Chem. Soc. 2005, 127, 15959-15967.
J. Org. Chem, Vol. 72, No. 1, 2007 281

FIGURE 3. TEM images of (a) free 3 and (b) 3 in the presence of
NaOH. Nanowires were highlighted by the white ellipses.
is 0.9 nm, so the extra width of the nanowires in excess of 1.5
nm should be attributed to the CB[6] molecules threading the
side chain of CDs. That is, these nanowires in the TEM image
belong on pseudopolyrotaxane 3.
Interestingly, while the sample of 3 for TEM experiments
was prepared in the presence of NaOH, the width of nanowires
became thinner, as shown in Figure 3b. The observation should
be attributed to CB[6]s dethreading off the side chain of
pseudopolyrotaxane 3.3d 1H NMR spectra of 3-D2O solutions
in the presence and absence of NaOD also confirm unambigu-
ously this process.
FIGURE 4. 1H NMR spectra of (a) 3 (0.33 mM); (b) 3 (0.33 mM) +
NaOD (39.6 mM); (c) 3 (0.32 mM) + NaOD (39.1 mM) + excess
DCl; (d) 3 (0.33 mM) + R-CD (22.6 mM); (e) 3 (0.32 mM) + R-CD
(22.6 mM) + NaOD (39.4 mM). Symbols [ and B indicate the solvent
and MeOH resonances, respectively.
When R-CDs exist in the above systems, two interconvertible
2D pseudopolyrotaxanes could be obtained by acid-base
control. In the case of acidity, R-CDs do not interact with the
protonated 1,6-hexanediamino residue of CD (Figure 4d), so
the structure of 2D pseudopolyrotaxane 3 is stable. Upon the
addition of NaOD, the original protonated 1,6-hexanediamino
residues first become neutral side chains resulting in CB[6]s
dethreading, and then R-CDs thread on the bare side axle to
form a new 2D pseudopolyrotaxane, as shown in Figure 5d.
As can been seen from Figure 4a,b, upon the addition of
NaOD, the resonance signals of Hb/b′ and Hc/c′ of CD in 3 shift
downfield (ca 0.8 ppm) and those of Ha/a′ shift upfield (ca. 0.3-
0.4 ppm). Meanwhile, the signal of Hx in CB[6] turned to double
peaks from the original quadruple peaks. These phenomena
completely contrast with those of CB[6] threading the cavity
of CD in 1, indicating that the dethreading process of CB[6]s
from their axle is due to the deprotonation of 1,6-hexane diamino
1
This process was recorded by H NMR spectroscopy. As can
been seen from Figure 4e, upon addition of base, the resonance
signals of Ha/a′ in 3 shift upfield, and those of Hb/b′ and Hc/c′
shift downfield, suggesting that CB[6]s dethread from the side
chains of â-CDs. It is noted that these resonance signals
corresponding to the side chains of â-CDs are different from
those in the absence of R-CDs (B and E in Figure 4), which
might be attributed to the inclusion of 1,6-hexanediamino
residues into the cavities of R-CD molecules. In all NMR
spectra, the chemical shift of the methyl protons in PPG is
invariable, indicating that the structure of the main chain
pseudopolyrtoaxane is stable.
1
units. From the H NMR spectrum of 3 in the presence of
NaOD, 100% CB[6] molecules dethread, while CDs were still
threaded on PPG2000. That is, the 2D pseudopolyrotaxane 3
decomposed to a “main-chain pseudopolyrtoaxane”, as il-
lustrated in Figure 5a,b. Furthermore, when the solution pD
value was adjusted to acidity by adding DCl, all CB[6]s
rethreaded on the side chain of 1 to re-form the 2D pseudopoly-
rotaxane (Figure 4c). This process may be repeated at least four
times upon addition of base or acid.
282 J. Org. Chem., Vol. 72, No. 1, 2007

the pan was recorded, while the temperature was increased from
room temperature to 700 °C at a heating rate of 10 °C/min.
NMR spectra were recorded in D2O. All chemical shifts were
referenced to the internal 0.05 M MeOH signal at 3.341 ppm, which
had been determined using sodium 2,2-dimethyl-2-silapentane-
5-sulfonate (DSS) as the standard.12
Transmission Electron Microscopic (TEM) Measurements.
A drop of sample solution was dropped on a carbon-coated copper
grid. The grid was then air-dried. The samples were examined by
a high-resolution transmission electron microscope (TEM) operating
at an accelerating voltage of 200 kV.
6-[(6-Aminohexyl)amino]-6-deoxy-â-CD Chloride (1). 6-[(6-
Aminohexyl)amino]-6- deoxy-â-CD13 (500 mg) was dissolved in
water (8 mL) at room temperature, and then 1 mL of HCl (aq, 2
M) was added in under stirring. After 30 min, an additional 40 mL
of acetone was added to form a white precipitate. Stirring was
continued for another 30 min, and the precipitate was collected by
filtration with 20 mL of acetone washing and dried under vacuum
to obtain 1 with a yield of 99%. 1H NMR (300 MHz, D2O, ppm):
δ 4.97 (d, 7H), 3.83-3.64 (m, 42H), 2.71(q, 4H), 1.58 (m, 4H),
1.31 (s, 4H). FTIR (KBr): ν 3342, 2923, 1699, 1655, 1559, 1456,
FIGURE 5. Schematic interconvertible processes between 2D
pseudopolyrotaxane 3 and the main chain pseudopolyrtoaxane (a and
b) and two 2D pseudopolyrotaxanes (c and d).
1332, 1154, 1079, 1034, 943, 849, 756, 706, 608, 581, 533 cm-1
.
Pseudorotaxane 2. Compound 1 (500 mg) was dissolved in 20
mL of water with stirring at room temperature; after that, CB[6]
(650 mg) was added in portions. Unsolved CB[6] was removed by
centrifugation after 12 h, and the solution was dried under reduced
pressure. Product was collected with a yield of 81%. 1H NMR (300
MHz, D2O, ppm): δ 5.58 (tert, 12H), 5.40 (s, 12H), 4.17 (d, 12H),
3.81-3.18(m, 42H), 2.76 (d, 4H), 0.59 (s, 4H), 0.31 (s, 4H). ESI-
MS: m/z 1115.87, (M2+). FTIR (KBr): ν 3344, 2925, 1736, 1474,
1420, 1374, 1324, 1294, 1235, 1190, 1147, 1028, 964, 919, 816,
The experiments of microcalorimetric titrations further con-
firmed the inclusion of hexanediamine side chain in R-CD. We
measured the binding constant of R-CD with 6-[(6-aminohexyl)-
amino]-6-deoxy-â-CD by microcalorimetric titrations under the
basic conditions at 298 K. The value of the binding constant
obtained is 46 M-1. Considering each 2D pseudopolyrotaxane
bearing 12 hexanediamino residue units, therefore, the concen-
tration of 6-[(6-aminohexyl)amino]-6-deoxy-â-cyclodextrin should
be about 3.84 mM, and the concentration of R-CD is 22.6 mM.
As a result, we can calculate the ratio of the inclusion of
hexanediamine side chain in R-CD. It is about 49%. That is to
say, almost half of R-CDs are threaded on pseudopolyrotaxane.
We have synthesized a 2D pseudopolyrotaxane 3 through
R,ω-PPG2000 diamino polymer threading the cavities of CDs
in pseudorotaxane 2. This novel 2D pseudopolyrotaxane can
change to a familiar main-chain pseudopolyrotaxane in the
presence of base. Furthermore, R-CDs can be threaded onto the
side chains of the main-chain pseudopolyrotaxane, forming
another 2D pseudopolyrotaxane. The two 2D pseudopolyrotax-
anes can interconvert by acid-base stimuli. The topological
structures of the pseudopolyrotaxanes and their reversible
behavior would not only be useful for designing novel supramo-
lecular polyrotaxanes and multicatenanes but also afford the
opportunity to design and develop molecular switches and
machines based on the pseudopolyrotaxane structure as scaf-
folds.
800, 758, 673, 629, 495, 445 cm-1. Anal. Calcd for (C84H122
-
Cl2N26O46)(H2O)14: C, 39.49; H, 5.92; N, 14.25. Found: C, 39.45;
H, 6.07; N, 14.21.
Pseudopolyrotaxane 3. In a solution of 2 (800 mg in 5 mL of
water), PPG 2000 was added at room temperature. The mixture
was put in a supersonic bath for 2 h and then stirred for another 24
h. Then, the resulting solution was washed with ethyl acetate (10
mL × 3) to remove the unreacted PPG2000 and dried under reduce
pressure. Crude product was purified with Sephadex G-25 to afford
a white solid in 21% yield. 1H NMR (300 MHz, D2O, ppm): 5.68
(tert, 11 × 12H of CB[6]), 5.50 (s, 11 × 12H of 1), 4.27 (d, 11 ×
12H of 1), 3.86-3.31 (m, 11 × 42H of 1, 34 × 3H of -CH2-
CHO- of PPG), 2.85 (s, 11 × 4H of 1), 1.07 (d, 34 × 3H of
-CH3 of PPG), 0.61 (s, 11 × 4H of 1), 0.39 (s, 11 × 4H of 1).
FTIR (KBr): ν ) 3342, 2929, 1735, 1558, 1474, 1420, 1374, 1324,
1294, 1236, 1190, 1147, 1029, 964, 923, 816, 800, 758, 673, 630,
445 cm-1. Anal. Calcd for (C84H122Cl2N26O46)12(C102H204O34N2H4)-
(H2O)180: C, 40.61; H, 6.22; N, 13.36. Found: C, 40.31; H, 6.41;
N, 13.37.
Acknowledgment. This work is supported by NNSFC (Nos.
90306009, 20421202, 20572052, and 20372038) and the Tianjin
Natural Science Foundation (No. 05YFJMJC06500), which are
gratefully acknowledged.
Experimental Section
General Methods. R,ω-Diaminopolypropylene glycols (PPG,
average molecular weight of 2000) and â-cyclodextrin (CD) were
purchased from commercial suppliers. Cucurbit[6]uril (CB[6]) was
prepared according to previous reports.11 FT-IR spectra were
recorded on a Shimadzu Bio-Rad FTS 135 instrument. Thermo-
gravimetric (TG) and differential thermal analyses (DTA) were
obtained by using a RIGAKU standard-type spectrometer. The
samples were put into platinum pans, which were hung in the
heating furnace. The weight percentage of material remaining in
JO0617159
(12) The widely used internal standard for NMR studies in aqueous is
DSS. However, DSS can interact with R-CDs or â-CDs diminishing its
value in our experiment. Herein, MeOH is a good internal standard under
our conditions with wide range pH changes. (a) Funasaki, N.; Ishikawa,
S.; Neya, S. J. Phys. Chem. B 2003, 107, 10094. (b) Matsui, Y.; Tokunaga,
S.; Bull. Chem. Soc. Jpn. 1996, 69, 2477. (c) Funasaki, N.; Ishikawa, S.;
Neya, S. Bull. Chem. Soc. Jpn. 2002, 75, 719-723. (d) Funasaki, N.;
Ishikawa, S.; Neya, S. J. Phys. Chem. B 2004, 108, 9593-9598.
(13) 1H NMR (300 MHz, D2O, ppm): δ 4.96 (d, 7H), 3.79-3.62 (m,
42H), 2.66 (m, 4H), 1.44-1.33 (m, 4H), 1.19 (t, 4H). ESI-MS: m/z 1233.60
(M + H+). Yang, X.-L.; Wang, Q.; Xu, H.-B. Carbohydr. Res. 2002, 337,
1309-1312.
(11) (a) Freeman, W. A.; Mock, W. L.; Shih, N. Y. J. Am. Chem. Soc.
1981, 103, 7367-7368. (b) Day, A.; Arnold, A. P.; Blanch, R. J.; Snushall,
B. J. Org. Chem. 2001, 66, 8094-8100.
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