Contorted conformations of 1,4-butylidenedipyridinium and 1,10-decylidenedipyridinium cationic guests in a cucurbit[8]uril host

Article abstract of DOI:10.1002/ejoc.201001504

1,4-Butylidenedipyridinium (C₄DP²⁺) and 1,10-decylidenedipyridinium (C₁₀DP²⁺) cationic guests form highly stable 1:1 inclusion complexes in aqueous solution with a cucurbit[8]uril (Q[8]) host. Single-crystal structure analysis of the inclusion complexes revealed that the alkyl chain of the C_nDP²⁺ (n = 4, 10) cationic guest adopts an unconventional contorted conformation, which is attributed to favorable host-guest interactions (including charge-dipole and hydrophobic interactions) when bound within the cavity of the Q[8] host. Moreover, their crystal structures suggest that the alkyl chain and the aromatic group can be encapsulated into the Q[8] host simultaneously, and the alkyl chain is more favorably encapsulated into the Q[8] host than the aromatic group. The alkyl chains of two organic cationic guests adopt unconventional contorted conformations, which is attributed to favorable host-guest interactions, including charge-dipole and hydrophobic interactions, when bound within the cavity of a Q[8] host. Copyright

Full text of DOI:10.1002/ejoc.201001504

FULL PAPER
DOI: 10.1002/ejoc.201001504
Contorted Conformations of 1,4-Butylidenedipyridinium and
,10-Decylidenedipyridinium Cationic Guests in a Cucurbit[8]uril Host
1
[
a]
[a]
[b]
[a]
[a]
[a]
Xin Xiao, Qian Wang, Yi-Hua Yu, Zhi-You Xiao, Zhu Tao,* Sai-Feng Xue,
[a]
[c]
[c]
Qian-Jiang Zhu, Jing-Xin Liu,* and Xin-Hua Liu
Keywords: Host–guest systems / Inclusion compounds / Structure elucidation / Conformation analysis
1
,4-Butylidenedipyridinium (C
4
DP2+) and 1,10-decylidenedi-
pyridinium (C10DP ) cationic guests form highly stable 1:1
host–guest interactions (including charge–dipole and hydro-
phobic interactions) when bound within the cavity of the Q[8]
2+
inclusion complexes in aqueous solution with a cucur- host. Moreover, their crystal structures suggest that the alkyl
bit[8]uril (Q[8]) host. Single-crystal structure analysis of the
inclusion complexes revealed that the alkyl chain of the
C DP (n = 4, 10) cationic guest adopts an unconventional
n
chain and the aromatic group can be encapsulated into the
Q[8] host simultaneously, and the alkyl chain is more favor-
ably encapsulated into the Q[8] host than the aromatic group.
2
+
contorted conformation, which is attributed to favorable
Introduction
helical conformations when bound within the cavities of
[
10]
two Q[8] hosts.
We noticed that the alkyl chain section
Alkyl chains are well known to adopt fully extended con-
formations in the free space of solutions, which minimizes
steric interactions and energies.[1] Helical or folding confor-
mations occur frequently in natural biochemical structures,
such as in proteins and nucleic acids.[ However, several
synthetic hydrophobic host systems in which the alkyl
of each guest is situated within the cavity of the Q[8] host,
whereas the aromatic groups are located outside the portals
of the Q[8] host.
2]
chains adopt contorted conformations have been observed
in the past decade.[
3–6]
In particular, Rebek Jr. and co-
workers synthesized and studied a series of cylindrical cap-
[
6]
sules in which multifarious alkanes are encapsulated.
Their results clearly show that alkyl chains can be com-
pressed to fit within the synthetic cylindrical capsules when Figure 1. Molecular structure of cucurbit[n]urils (n = 5–8, 10).
their internal strains are compensated.
It is well known that aromatic groups participate in many
Cucurbit[n]urils[
7,8]
(n = 5–8, 10, hereafter abbreviated
kinds of chemical reactions, and the cavity of Q[8] is large
Q[n]; Figure 1), a class of organic macrocyclic cavitands
that are barrel shaped with two identical portals fringed
by carbonyl groups, have received increased interest. For
example, the Kim group discovered a U-shaped conforma-
tion of the alkyl chains embedded in Q[8].[ We recently
discovered that the long alkyl chain of N,NЈ-bis(4-dimethyl-
aminobenzyl)dodecane-1,12-diamine adopts different chiral
[11]
enough to accommodate two aromatic molecules.
Is it
possible to have one alkyl chain and one aromatic group
residing in the cavity of Q[8]? With this idea in mind, we
2+
synthesized 1,4-butylidenedipyridinium (C DP ) dibro-
4
9]
[12]
2+
mide and 1,10-decylidenedipyridinium (C DP ) dibro-
10
mide (Experimental Section). Each of the two organic di-
cationic guests contains an alkylene group and two aro-
matic (pyridinium) groups. In the present work, we investi-
gated the formation of a 1:1 host–guest inclusion complex
[
a] Key Laboratory of Macrocyclic and Supramolecular Chemistry
of Guizhou Province, Guizhou University and Institute of
Applied Chemistry, Guizhou University,
Guiyang 550025, P. R. China
2+
1
between a Q[8] host and a C DP guest (n = 4, 10) by H
n
NMR and UV spectroscopy. We also obtained the X-ray
Fax: +86-851-3620906
2
+
crystal structures of the complexes formed between C DP
E-mail: gzutao@263.net
4
2+
[
[
b] Key Laboratory of Optical and Magnetic Resonance
Spectroscopy of the East China Normal University, Ministry
of Education of China,
or C DP and Q[8], in which the alkyl chains of the guests
10
adopt an unconventional contorted conformation inside the
Q[8] cavity. More importantly, we found that the Q[8] cavity
can accommodate the alkyl chain and the aromatic group
simultaneously, and if the alkyl chain is long enough, the
aromatic group will be squeezed out of the Q[8] cavity.
Shanghai 200062, P. R. China
c] College of Chemistry and Chemical Engineering, Anhui
University of Technology,
Maanshan 243002, P. R. China
E-mail: jxliu411@ahut.edu.cn
2366
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Contorted Conformations of Dipyridinium Guests in a Cucurbit[8]uril Host
Results and Discussion
1
Description of H NMR and UV Spectroscopy of the Two
Inclusion Complexes
1
As confirmed by H NMR spectroscopy, the Q[8] host
2
+
forms a 1:1 host–guest inclusion complex with the C DP
n
1
2+
cationic guest. The H NMR spectra of C DP in the ab-
4
sence and presence of Q[8] (1.0 equiv.) in neutral D O solu-
2
tion are shown in Figure 2, in which separate signals for
each methylene and methylidyne group of the guest are
clearly seen. In the presence of the Q[8] host (1.0 equiv.), all
carbon atoms of the guest showed distinct upfield-shifted
proton signals (Δδ ≈ 0.36–0.60 ppm, Figure 2b), which indi-
cates that the whole guest is deeply buried inside the cavity
of the Q[8] host. Closer inspection revealed that the
8
.8ϫ9.1 Å (equatorial widthϫdepth) cavity of Q[8] can ac-
DP2+ complex in D
4 2
Figure 3. NOESY spectrum of the Q[8]·C O
commodate no more than six linearly arranged atoms in a
fully extended conformation. We conclude that folding must
occur during encapsulation of the C DP guest. However,
no correlation through space between the host and the
guest in the NOESY spectrum of the Q[8]·C DP complex
at 20 °C.
2
+
4
1
(
Figure 5). Although H NMR spectroscopy suggests that
2+
2+
the alkyl chains of C DP and C DP adopt a folding
2
+
4
10
4
conformation when bound within the cavity of Q[8], the
precise dimensional conformations of these two inclusion
complexes are still unknown.
2
+
indicates that the encapsulated C DP undergoes chemical
exchange with the free C DP (Figure 3).
4
2
+
4
1
O) of C10DP2+ in the (a)
Figure 4. H NMR spectra (400 MHz, D
2
absence and (b) presence of Q[8] (1.0 equiv.).
1
DP2+ in the (a)
Figure 2. H NMR spectra (400 MHz, D
2
O) of C
4
absence and (b) presence of Q[8] (1.0 equiv).
Theoretically, the cavity of Q[8] is not large enough to
2
+
accommodate the whole C DP guest even in a contorted
1
0
conformation. To our surprise, however, in the presence of
2
+
the Q[8] host, all the C DP protons experience consider-
1
0
able upfield shifts (Figure 4), of which the alkane protons
C5–C8) exhibit a large upfield shift (Δδ ≈ 0.44–1.10 ppm),
(
whereas the pyridinium protons (C1–C3) and the methylene
protons (C4) attached to the pyridinium group show a
smaller upfield shift (Δδ ≈ 0.25–0.34 ppm), indicating the
formation of an inclusion complex. This observation can be
explained as follows: The chemical exchange of the folding
2
+
C DP in and out of the cavity of Q[8] is slow but near
1
0
the intermediate rate on the NMR timescale, which leads
to broadening of the signals for the included methylene and
methylidyne moieties of the guest. No correlation through
2
+
space between Q[8] and C DA in the corresponding
2+
1
0
Figure 5. NOESY spectrum of the Q[8]·C DP complex in D O
1
0
2
NOESY spectrum could further confirm this explanation at 20 °C.
Eur. J. Org. Chem. 2011, 2366–2371
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Z. Tao J.-X. Liu et al.
FULL PAPER
To better understand the interaction between Q[8] and
As depicted in Figure 8a, the interaction between Q[8]
2
+
2+
C DP , we carried out UV titration experiments. As and C DP showed a similar UV spectrum. According to
n
10
2
+
shown in Figure 6, the UV spectrum of C DP has a peak the experimental data and the mol ratio method mentioned
4
with λmax = 259 nm. As the concentration of Q[8] is in- above, we obtained the curve of absorbance (A) vs. the ratio
creased in a solution containing a fixed concentration of of the number of mol of host and guest (NQ[8]/N ; Fig-
g2
2
+
C DP , a slow decrease in absorbance is observed. Ac- ure 8b), which suggests the formation of a 1:1 host–guest
4
2+
cording to the mol ratio method, the experimental data of inclusion complex between the Q[8] host and the C DP
10
the absorbance (A) vs. the ratio of the number of moles of guest in aqueous solution. Again, the corresponding UV
2
+
Q[8] host to C DP guest (NQ[8]/N ) can be fitted to a 1:1 spectrum data were used to determine the equilibrium
4
g1
host–guest inclusion model. For this 1:1 inclusion complex, association constant value, which was found to be
5
–1
the UV spectra data yield an equilibrium association con- 1.46ϫ10 m . Clearly, the UV behavior of these two in-
4
–1
stant of 1.87ϫ10 m , which indicates that the inclusion clusion complexes is consistent with the results obtained by
1
complex is quite stable. In addition, continuous variation
Job’s plot was constructed (Figure 7), which shows that the
UV spectrum data fit well to the same 1:1 stoichiometry of
the host–guest inclusion complex.
H NMR spectroscopy.
Figure 8. (a) UV titration of C10DP2+ dibromide with increasing
concentrations of Q[8] and (b) absorbance (A) vs. the ratio of the
number of mol of host and guest NQ[8]/Ng2
.
Figure 6. (a) UV titration of C
concentration of Q[8] and (b) absorbance (A) vs. the ratio of the
number of mol of host and guest NQ[8]/Ng1
DP2+ dibromide with increasing
4
.
Description of Crystal Structures of the Two Inclusion
Complexes
To confirm the exact dimensional conformation of the
2+
2+
two Q[8]·C DP and Q[8]·C DP inclusion complexes,
4
10
we determined their X-ray crystal structures. Slow evapora-
tion of a hydrochloric acid aqueous solution containing a
2+
1
:1 mixture of C DP dibromide and Q[8] at room tem-
4
perature resulted in the formation of crystals of the type
2
+
¯
[13]
Q[8]·C DP , which crystallized in the space group R3.
4
2+
The X-ray crystal structure of Q[8]·C DP
clearly shows that the whole C DP guest is buried deep
(Figure 9)
4
2+
4
inside the cavity of the Q[8] host to form an inclusion com-
1
plex, as suggested by H NMR and UV spectroscopy. Two
2
+
included terminal pyridinium rings of the C DP guest are
Figure 7. Continuous variation Job’s plot for Q[8] and the guest on
the basis of UV spectra.
4
neither aspectant nor parallel to each other, which may be
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Contorted Conformations of Dipyridinium Guests in a Cucurbit[8]uril Host
attributed to unfavorable electrostatic repulsions between conformation when bound within the Q[8] cavity. At the
them. It is noticeable that the butylidene chain of the same time, the Q[8] host also experience an ellipsoidal de-
2
+
C DP guest adopts a folded conformation when bound formation, which can be attributed to the internal pressure
4
within the Q[8] cavity. The shortest distance between the of the alkane. It is interesting that the decylidene chain in
2+
hydrogen atoms of the butylidene chain and the hydrogen the C DP cation is so long, much longer than the butyli-
10
2+
atoms of the methylene of the Q[8] host is 3.46 Å. Most dene chain in the C DP cation, that the cavity of Q[8] is
4
2
+
2+
interestingly, the folding of the C DP guest inside the cav- not large enough to accommodate the whole C DP cat-
4
10
ity of Q[8], just as a contorted leaf sprig is confined to a ionic guest and one of the aromatic groups is squeezed out
small space, leads to severe ellipsoidal deformation of the of the Q[8] cavity. It seems as though encapsulation of the
Q[8] host. Thus, in order to accommodate the leaf-sprig- alkyl chain favored over that of the aromatic group. It must
2
+
2–
shaped C DP guest and to provide a better complemen- be noted that there are many tetrahedral [ZnCl4] counteri-
4
tary shape, the Q[8] host undergoes severe ellipsoidal defor- ons between these inclusion complexes, and they form a
mation spontaneously during the encapsulation process. In complicated hydrogen-bonding network.
2
+
the crystal structure of Q[8]·C DP , the opened molecular
4
capsule is surrounded by water molecules and chloride and
bromide anions, and they interact to form a complicated
hydrogen-bonding network.
Figure 10. X-ray crystal structure of the inclusion complex Q[8]·
2
+
C
10DP . Solvate water molecules and anions are omitted for clar-
ity.
2
+
To better understand the formation of the Q[8]·C DP
4
2+
and Q[8]·C DP complexes, we carried out ITC experi-
10
Figure 9. X-ray crystal structure of the inclusion complex Q[8]· ments. The result is shown in Figure 11, indicating that the
2
+
2+
C DP . Solvate water molecules and anions are omitted for clarity. complexation of C DP with Q[8] is driven by both en-
4
4
–1
thalpy and entropy (ΔH° = –26.222 kJmol , TΔS° =
11.742 kJmol ), whereas the formation of the Q[8]·
C DP complex is almost exclusively enthalpy (ΔH° =
39.753 kJmol , TΔS° = –0.339 kJmol ) driven. Obvi-
At first glance, the inclusion complex Q[8]·C DP2 is
structurally very similar to its counterpart Q[8]·C DA ,
a previously reported host–guest complex of a U-shaped
bolaamphiphile embedded in Q[8].[ However, at least two
significant differences exist between the inclusion complex
+
–1
4
2+
2+
12
10
–1
–1
–
9c]
2
+
2+
Q[8]·C DP and its counterpart Q[8]·C DA . Firstly, in
the inclusion complex Q[8]·C DP , the leaf-sprig-shaped
4
12
2
+
4
guest is entirely encapsulated into the cavity of Q[8]. The
inclusion complex somewhat resembles an opened molecu-
2+
lar capsule. In the host–guest complex Q[8]·C DA , two
1
2
nitrogen atoms of the U-shaped guest lie above the mean
oxygen plane of the portal of the Q[8] host. Secondly, the
leaf-sprig-shaped guest of the inclusion complex Q[8]·
2
+
C DP is accumbent, whereas the U-shaped guest of the
4
2
+
host–guest complex Q[8]·C DA is upstanding.
1
2
2+
The crystal of the inclusion complex Q[8]·C DP was
1
0
obtained by the same method as that illustrated for the in-
2
+
clusion complex Q[8]·C DP . The structure of Q[8]·
4
2
+
C DP (Figure 10) clearly reveals that one pyridinium
group and the whole alkyl chain of the C DP cationic
1
0
2
+
1
0
guest were encapsulated into the hydrophobic cavity of the
Q[8] host, whereas another pyridinium group resides out-
side the portal and is almost parallel with the mean oxygen
plane of the portal. It should be noted that the decylidene
Figure 11. (a) ITC profile of Q[8] with C
4
DP2+ at 298.15 K: ΔH°
–1
–1
=
–26.222 kJmol , TΔS° = 11.742 kJmol ; (b) ITC profile of Q[8]
with C10DP at 298.15 K: ΔH° = –39.753 kJmol , TΔS° =
2+
–1
2
+
–1
chain of the C DP guest adopts a severely contorted –0.339 kJmol .
1
0
Eur. J. Org. Chem. 2011, 2366–2371
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2369

Z. Tao J.-X. Liu et al.
FULL PAPER
ously, the contorted conformation of the alkyl chains in
these inclusion complexes creates sterically unfavorable
C
4
DP2+ Dibromide: The guest was prepared by heating a mixture
of 1,4-dibromobutane (1.0 equiv.), pyridine (3.0 equiv.), and poly-
ethylene glycol (0.01 equiv.) at reflux for 4 h. The resulting precipi-
tate was filtered, and the residue was collected, washed with diethyl
ether, and air dried. The end product was obtained in 81.8% yield
based on 1,4-dibromobutane. H NMR (400 MHz,D
[
13]
gauche interactions.
However, their equilibrium associa-
tion constant values, determined by UV titration experi-
ments, suggest that the two inclusion complexes are highly
stable. What causes the assemblies of the two inclusion
complexes and what causes the high enthalpy? At least two
driving forces can be identified. Strong charge–dipole inter-
actions between the guest and the carbonyl oxygen atom at
the portals of the Q[8] host appear to be the major driving
force. In addition, the reciprocal conformational changes in
the host–guest partners maximize hydrophobic interactions
1
2
O): δ = 8.74
(
d, J = 6 Hz, 4 H), 8.44 (t, J = 8 Hz, 2 H), 7.96 (t, J = 7 Hz, 4 H),
4.57 (s, 4 H), 2.01 (s, 4 H) ppm. MS: m/z (%) = 52 (58) [CH
3
CH
2
+
+
+
Na ], 79 (100) [C
5 5
H N] .
C
10DP2+ Dibromide: The guest was prepared by heating a mixture
of 1,10-dibromodecane (1.0 equiv.), pyridine (3.0 equiv.), and poly-
ethylene glycol (0.01 equiv.) at reflux for 4 h. The resulting precipi-
tate was filtered, and the residue was collected, washed with diethyl
between the surfaces of the folded guest and the inner wall ether, and air dried. The end product was obtained in 68.2% yield
1
of the Q[8] host. Because the flexible alkyl chain can be based on 1,10-dibromodecane. H NMR (400 MHz, D
2
O): δ = 8.68
compressed to fit within the Q[8] cavity and conform to its (d, J = 6 Hz, 4 H), 8.39 (t, J = 8 Hz, 2 H), 7.91 (t, J = 7 Hz, 4 H),
4
.45 (t, J = 7 Hz, 4 H), 1.85 (t, J = 7 Hz, 4 H), 1.161 (s, 8 H), 1.099
shape, it is not surprising that the aromatic group is prefer-
entially squeezed out of the Q[8] cavity.
+
(s, 4 H) ppm. MS: m/z (%) = 149.1 (100) [M/2 ], 379.1 (10) [M +
–
Br] .
Q[8]·C
4
DP2+: To a solution of C DP2+ dibromide (0.075 g,
4
0
.20 mmol) dissolved in H O (35 mL) and HCl (6 m, 5 mL) was
2
Conclusions
added Q[8] (0.27 g, 0.20 mmol). The mixture was stirred and heated
at 50 °C for 10 min and then filtered. Slow evaporation of the fil-
trate over a period of five weeks provided rod-shaped colorless
crystals.
Q[8]·C10DP2+: To a solution of C10DP2+ dibromide (0.092 g,
0
1
In summary, through H NMR and UV spectroscopy,
we have demonstrated that a Q[8] host can accommodate
2
+
2+
C DP and C DP cationic guests to form highly stable
4
10
1:1 inclusion complexes. The X-ray crystal structures of
.20 mmol) and ZnCl
2 2
(0.014 g, 0.20 mmol) dissolved in H O
2
+
these two inclusion complexes, Q[8]·C DP
and Q[8]·
4
(40 mL) and HCl (6 m, 5 mL) was added Q[8] (0.27 g, 0.20 mmol).
2
+
C DP , clearly show that the alkyl chains of the cationic
1
0
The mixture was stirred and heated at 50 °C for 10 min and then
filtered. Slow evaporation of the filtrate over a period of four weeks
provided rod-shaped colorless crystals.
guests adopt contorted conformations, and internal strains
are compensated by favorable host–guest interactions (in-
cluding charge–dipole and hydrophobic interactions), when
bound within the cavity of Q[8]. More importantly, we
found that the Q[8] cavity can accommodate the alkyl chain
and the aromatic group simultaneously. In comparison with
Single-Crystal X-ray Crystallography: Diffraction data for the com-
2+
2+
plexes Q[8]·C
a Bruker SMART Apex-II CCD diffractometer using graphite-mo-
nochromated Mo-K radiation (λ = 0.71073 Å). Absorption correc-
4
DP and Q[8]·C10DP were collected at 173 K with
α
the aromatic group, the alkyl chain is more favorably encap- tions were applied by using the multiscan program SADABS.
sulated into the Q[8] cavity. The main reason is that the Structural solution and full-matrix least-squares refinement based
2
on F were performed with the SHELXS-97 and SHELXL-97 pro-
alkyl chain is flexible and can be compressed to fit inside
the Q[8] cavity.
gram package,[ respectively. All non-hydrogen atoms were refined
with anisotropic displacement parameters. The carbon-bound hy-
drogen atoms were introduced at calculated positions. All hydrogen
atoms were treated as riding atoms with an isotropic displacement
14]
Experimental Section
parameter equal to 1.2 times that of the parent atom. For the com-
plexes Q[8]·C DP and Q[8]·C10DP , no hydrogen atoms are
4
given for all isolated water molecules, as it is difficult and unneces-
sary.
2
+
2+
Materials and Methods: 1,4-Dibromobutane, 1,10-dibromodecane,
pyridine, and solvents employed were used as supplied without fur-
ther purification. Q[8] was synthesized according to literature
[
7]
Crystal Data for Complex Q[8]·C DP2+: (C H N O ) ·
methods. The NMR spectra were recorded at 20 °C with a Varian
INOVA-400 spectrometer. UV absorption spectra of the host–guest
complex were recorded with an Agilent 8453 spectrophotometer at
room temperature.
4
48 48 32 16 6
(
C
14
H
18
N
2 2 6 2 2 r
Cl ) ·(H O)91·(KBr) , M = 11563.80, hexagonal, space
¯
3
group R3, a = 29.4138(12) Å, c = 52.9469(16) Å, V = 39671(3) Å ,
Z = 3, Dcalcd. = 1.452 gcm , F(000) = 18138, GoF = 1.187, R
–
3
1
=
0
.1090 [IϾ2σ(I)], wR = 0.3079 (all data).
2
Isothermal Titration Calorimetry: Microcalorimetric experiments
were performed by using an isothermal titration calorimeter Nano
ITC 2G (TA, USA). Each experiment consisted of 25 consecutive
Crystal Data for Complex Q[8]·C10DP2+
Cl )·(H O) ·ZnCl , M = 1924.89, orthorhombic, space
group Pna2 , a = 21.793(6) Å, b = 18.812(5) Å, c = 20.983(6) Å, V
:
48 32
(C48H N O16)·
(
C
20
H
30
N
2
2
2
5
2
r
2+
2+
1
injections (10 μL) of a guest solution (C
4
DP or C10DP dibro-
3
–3
mide: 1ϫ10–3 m) into the microcalorimetric reaction cell (1 mL)
= 8602(4) Å , Z = 4, D
calcd. = 1.486 gcm , F(000) = 4000, GoF =
= 0.2407 (all data).
–4
1.089, R = 0.0872 [IϾ2σ(I)], wR
1
2
charged with a solution of Q[8] (1ϫ10 m). The heat of reaction
was corrected for the heat of dilution of the guest solution deter-
mined in the separate experiments. All solutions were degassed
prior to titration experiment by sonication. Computer simulations
CCDC-787351 (for Q[8]·C
DP2+) and -798915 (for Q[8]·C10DP2+
)
4
contain the supplementary crystallographic data 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.
(curve fitting) were performed using the Nano ITC analysis soft-
ware.
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Eur. J. Org. Chem. 2011, 2366–2371

Contorted Conformations of Dipyridinium Guests in a Cucurbit[8]uril Host
Am. Chem. Soc. 2009, 131, 13190–13191; i) J. Rebek Jr., Acc.
Chem. Res. 2009, 42, 1660–1668.
7] a) J. Kim, I. S. Jung, S. Y. Kim, E. Lee, J. K. Kang, S. Sakam-
Acknowledgments
[
We are grateful to the National Natural Science Foundation of
China (NSFC, No. 20961002 and 20971002), the “Chun-Hui”
Fund of the Chinese Ministry of Education, the Science and Tech-
nology Fund of Guizhou Province (No. 2010-2005), and the Inter-
national Collaborative Project Fund of Guizhou province for finan-
cial support of this work.
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Published Online: March 4, 2011
Eur. J. Org. Chem. 2011, 2366–2371
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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