Unique regioselective binding of permethylated β-cyclodextrin with azobenzene derivatives

Article abstract of DOI:10.1002/ejoc.200800829

The binding behavior of permethylated β-cyclodextrin (PM-β-CD) upon complexation with 4-hydroxyazobenzene (4-HAB) and 4-aminoazobenzene (4-AAB) was investigated by X-ray crystallography, circular dichroism, 2D NMR spectra, and isothermal titration calorim

Full text of DOI:10.1002/ejoc.200800829

FULL PAPER
DOI: 10.1002/ejoc.200800829
Unique Regioselective Binding of Permethylated β-Cyclodextrin with
Azobenzene Derivatives
Jun Shi,[a] Dong-Sheng Guo,[a] Fei Ding,[a] and Yu Liu*[a]
Keywords: Host–guest systems / Cyclodextrins / Inclusion compounds / Thermodynamics
The binding behavior of permethylated β-cyclodextrin (PM-
β-CD) upon complexation with 4-hydroxyazobenzene (4-
HAB) and 4-aminoazobenzene (4-AAB) was investigated by
X-ray crystallography, circular dichroism, 2D NMR spectra,
and isothermal titration calorimetry. In comparison to the re-
ported β-cyclodextrin (β-CD) complexes with 4-HAB and 4-
AAB, PM-β-CD complexes 3 and 4 present not only unique
binding regioselectivity with azobenzene guests but also
have distinct spatial arrangements, that is, β-CDs arrange in
a head-to-head manner, whereas PM-β-CDs arrange in a
head-to-tail manner. The results obtained from circular di-
chroism and 2D NMR spectra indicate that the binding
modes of complexes 3 and 4 in solution are in accordance
with those in the solid state. Furthermore, the binding abili-
ties and thermodynamic parameters of β-CD and PM-β-CD
upon complexation with azobenzene derivatives were dis-
cussed from the viewpoints of the flexibilities of the frame-
works of the hosts and driving forces.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
media,[12a] whereas β-CD only present around 103 –1 for
complexation with porphyrins in aqueous solution.[12c]
However, the crystallographic investigations on the PMCD
complexes are still not as mature as those of CD up to
now.[14] Harata and coworkers analyzed the crystal struc-
tures of PMCD complexes with some optically active guests
such as mandelic acid[14a] and flurbiprofen.[14b] Petit and
coworkers[14c] obtained a pair of PM-β-CD complexes with
racemic 1-(p-bromophenyl) by successive recrystallization
and discussed the crystal structures and chiral discrimi-
nation mechanisms of PM-β-CD. Direct comparison of the
complex structures between CD and PMCD with the same
guests is especially reported less frequently,[14d] which is sig-
nificant to help us comprehend the differences of these two
kinds of CD hosts, including binding geometries, driving
forces of complexation, further aggregation topologies, and
so on.
In a previous study, we reported two inclusion complexes
of β-CD with 4-hydroxyazobenzene (4-HAB, 1) and 4-ami-
noazobenzene (4-AAB, 2), including their binding and as-
sembly behavior both in aqueous solution and the solid
state. The obtained results indicate that the positions of the
substituents in the azobenzenes play an important role in
inclusion complexation and molecular assembly.[8d] Herein,
we further obtained two inclusion complexes of PM-β-CD
with 4-HAB (3) and 4-AAB (4) shown in Scheme 1, and
their binding and assembly behaviors were studied by X-
ray crystallography, circular dichroism, 2D NMR spectra,
and isothermal titration calorimetry (ITC). To obtain
deeper insight on the respective characteristics of CD and
PMCD, their complexation structures and binding thermo-
dynamics were systemically analyzed and discussed. More-
Introduction
Supramolecular architectures constructed by cyclodex-
trin (CD) blocks, such as rotaxane,[1–3] molecular shuttles,[4]
molecular tube,[5] and so on prove to be of more interest
in chemistry and materials science nowadays due to their
fascinating properties including water solubility, biological
consistency, and economics endowed by CD blocks upon
their remarkable ability to include various guest molecules
either in solution or in the solid state.[6] A number of crys-
tallographic studies on the CD-based inclusion complexes
have been performed to obtain direct evidence for the for-
mation of the inclusion complexes that can be subsequently
used as building blocks to construct supramolecular aggre-
gates.[7,8] Moreover, among the CD derivatives, permethyl-
ated cyclodextrins (PMCDs), representing a new family of
CD hosts, where all of the hydroxy groups of the CDs are
methylated, are widely used as the solubilizing agent and
as the hosts for encapsulating various organic molecules,
including drugs, because of their patulous cavity,[9] flexible
framework,[10] and better solubility in both aqueous and
organic solution.[7a,9] It is also interesting that PMCDs ex-
hibit distinguishable inclusion complexation behaviors for
some special substrates from CDs.[11–13] For example, PM-
β-CDs can afford binding constants up to 105 to 106 –1
with charged porphyrins in aqueous or aqueous/organic
[a] Department of Chemistry, State Key Laboratory of Elemento-
Organic Chemistry, Nankai University,
Tianjin 300071, P. R. China
Fax: +86-22-2350-3625
E-mail: yuliu@nankai.edu.cn
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over, it is also our interest to explore how the disparities of 0.0159 Å (or 0.0136 Å). However, in PM-β-CD, seven gly-
substituents of azobenzene guests affect the inclusion com- cosidic oxygen atoms comprise a coplane with a mean devi-
plexation and aggregate behavior of PMCD.
ation of 0.4014 Å (or 0.2835 Å). This indicates that the
framework of PM-β-CD is more flexible than that of β-
CD, as the methylation of all hydroxy groups destroys the
intrinsic hydrogen-bonding network of β-CD. The binding
structures of the β-CD and PM-β-CD complexes are there-
fore distinct from each other. In complexes 1 and 2, the 4-
HAB and 4-AAB guests penetrate through the cavity of β-
CD and the hydrophobic azo group[15] is located in the cen-
ter of the cavity, whereas in complexes 3 and 4, one of the
aromatic rings of the guests enters into the middle area of
the PM-β-CD cavity and the azo group is located at the
narrow torus rim. This result should be attributed to the
diverse hydrophobic characteristics owned by β-CD and
PM-β-CD. As is well known, β-CD has a truncated cone
structure with a hydrophilic exterior and a hydrophobic in-
ner cavity, whereas the most hydrophobic region is no
longer located at the central cavity but at the narrow/broad
torus rims made up by the 2-OMe, 3-OMe, and 6-OMe
groups once all of the hydroxy groups are methylated.[10a]
In addition, the elliptical cavity of PM-β-CD should prefer
to accommodate the planar guests such as aromatic rings.
It is also noteworthy that there is an obvious difference in
the binding geometries of PM-β-CD complexes 3 and 4. In
complex 3, the phenol group of 4-HAB is embedded in the
PM-β-CD cavity, forming three C–H···π interactions (C5d–
H···centroid of aromatic ring 3.530 Å, 133.4°; C6a–
H···centroid of aromatic ring 3.744 Å, 121.3°, C2g–
H···centroid of aromatic ring 3.756 Å, 162.5°), and the al-
ternative benzene group of 4-HAB is exposed out of the
narrow port of the PM-β-CD cavity with two C–H···π inter-
actions (C9e–H···centroid of aromatic ring 3.670 Å, 143.2°;
C6e–H···centroid of aromatic ring 3.556 Å, 141.5° where a–
g are defined for the labels of seven glucose units of the
PM-β-CD cavity). In complex 4, the benzene group of 4-
AAB is embedded in the PM-β-CD cavity with two C–H···π
interactions (C5c–H···centroid of aromatic ring 3.860 Å,
145.3°; C5e–H···centroid of aromatic ring 3.268 Å, 128.0°),
whereas the aniline group of 4-AAB is located outside the
narrow port of the PM-β-CD cavity, forming a C–H···π in-
teraction (C9f–H···centroid of aromatic ring 2.889 Å,
142.1°). Obviously, β-CD provides a similar binding geome-
Scheme 1. Schematic representation of the structures of complexes
1–4.
Result and Discussion
Crystal Structures
Crystals 3 and 4 were synthesized as the complexes of
PM-β-CD with 4-HAB and 4-AAB in satisfactory yields
(see Experimental Section). Their crystal data and the ex-
perimental and refinement parameters are shown in
Table 2. Complexes 3 and 4 crystallize in the orthorhombic
system and the structural solutions were performed in the
space group P212121. The asymmetric units contain the fol-
lowing: one PM-β-CD, one 4-HAB, and six water molecules
for 3; one PM-β-CD, one 4-AAB, and one water molecule
for 4.
Figure 1 show the host–guest structures of complexes 3
and 4, in which PM-β-CD assumes a distorted elliptical
macrocyclic ring, differing from β-CD in 1 and 2 that main-
tains the round shape with an approximate sevenfold axis.
4
Every glucose residue of β-CD has a C1 chair conforma-
tion, and seven glycosidic oxygen atoms are coplanar within
Figure 1. The inclusion structures of complexes 3 (a) and 4 (b).
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Binding of Permethylated β-Cyclodextrin with Azobenzene Derivatives
try with the azobenzene guests, whereas PM-β-CD displays cent β-CD units emerge to be the dominating factor. On
certain regioselectivity upon complexation with 4-HAB and the contrary, PM-β-CD aggregates with in a head-to-tail
4-AAB. One possible explanation for the regioselectivity is manner in complexes 3 and 4, which is in accordance with
the different electronegativities of the hydroxy and amino the reported results of most PM-β-CD complexes.[7a,14] This
groups, which can be illuminated from the thermodynamic pronounced difference between β-CD and PM-β-CD
results mentioned below. Moreover, it is noticeable that 4- should be attributed to the remarkable reduction of hydro-
AAB is included into the host cavity at a more shallow gen bonds among adjacent PM-β-CD torus rims.
depth than that of 4-HAB, and two N–H···O hydrogen
Figure 2 shows the extended structures of complex 3
bonds (N1···O1f 2.439 Å, N1···O1d 2.503 Å) imply that the viewed from different orientations. Running along the axial
amidogen of 4-AAB is inclined to be included into the adja- direction of the PM-β-CD cavity, one weak noncovalent C–
cent PM-β-CD cavity. This binding geometry of complex 4 H···O hydrogen bond (C3 of HAB···O4f 2.514 Å) makes the
is favorable to form channel-type aggregation of PM-β-CD host–guest complex units arrange in a 1D linear array (Fig-
complexes, which will be discussed below with the packing ure 2a). Otherwise, the orientation of two adjacent PM-β-
structures.
CD units is different, and thus the linear array is with the
In contrast, it is well known that there are several water zigzag shape. In contrast, although the hydroxy groups of
molecules in the inner cavities of CDs.[7a] For example, 6.5 two torus rims of the CD cavity are all methylated, which
included water molecules were found in the cavity of β-CD inhibits the formation of axial hydrogen bonds among adja-
dodecahydrate.[16] However, in complexes 1 and 2, the water cent PM-β-CDs; PM-β-CDs form multiple C–H···O hydro-
molecules are all located outside the β-CD cavities, which gen bonds (C3b···O5e 2.358 Å, C6b···O3e 2.559 Å,
indicates that the inclusion of the azobenzene guests expels C9c···O5f 2.487 Å, C3e···O2b 2.557 Å, C4e···O5c 2.479 Å,
the original high-energy water molecules from the inner C3d···O5g 2.560 Å), which makes the exterior walls of PM-
cavities. Otherwise, no water molecule is located in the PM- β-CDs connected to form a regular plane along the equato-
β-CD cavity even without any inclusion of guests, according rial direction of the cavity (Figure 2b). Therefore, in the
to the previous reports of PM-β-CD crystals.[9,10b] There is overall view of complex 3 (Figure 2c), we prefer the layer-
also no water molecule inside the PM-β-CD cavity in com- type packing structure.
plexes 3 and 4, which demonstrates that the release of high-
Although both amino and hydroxy groups are able to
energy water molecules does not occur during the inclusion form hydrogen bonds, no O–H···O hydrogen bond is ob-
process of PM-β-CD with the azobenzene guests. Therefore, served between the hydroxy group of HAB and the oxygen
we can primarily judge that the driving forces and thermo- atoms of PM-β-CD in complex 3. Instead, there is one O–
dynamics of the inclusion with guests should be different H···O hydrogen bond (O1 of HAB···O2 of water 2.773 Å)
between β-CD and PM-β-CD.
between the hydroxy group of HAB and a water molecule.
In the further extended structures, complexes 1 and 2 Much differently, the amino group of 4-AAB in complex 4
both present a channel-type arrangement of β-CD in a is further included into the adjacent PM-β-CD cavity, form-
head-to-head manner, in which the intermolecular multiple
hydrogen bonds between the hydroxy groups of two adja-
Figure 2. (a) View of the arrangement along the axial direction of
the PM-β-CD cavity in complex 3; (b) view showing the hydrogen-
bonding network among the exterior walls of PM-β-CDs in com-
plex 3; (c) view of the overall packing structure of complex 3.
Figure 3. (a) View of the arrangement along the axial direction of
the PM-β-CD cavity in complex 4; (b) view showing the hydrogen-
bonding network among the exterior walls of PM-β-CDs in com-
plex 4; (c) view of the overall packing structure of complex 4.
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ing two strong N–H···O hydrogen bonds. As a result, a typi- plexes in aqueous solution, which represents two effective
cal channel arrangement along the axial direction of the means used to analyze the solution structures of CD deriva-
CD cavity is presented in complex 4 (Figure 3a). Moreover, tives and complexes.[17–19] As can be seen from Figure 4, the
a C–H···O hydrogen bond (C9g···O3a 2.425 Å) also partici- achiral guests (4-HAB and 4-AAB) gave two induced circu-
pates in the axial aggregation. In comparison to the chan- lar dichroism (ICD) signals around their corresponding
nels fabricated in β-CD complexes 1 and 2, the channel in transition band upon inclusion into the chiral microenvi-
complex 4 exhibits poorer compactness with a distance be- ronment of the PM-β-CD cavity. In a previous report, nei-
tween the unrepeatable units of 10.7 Å. It is easily accept- ther 4-HAB nor 4-AAB displayed any appreciable circular
able that methylation of the hydroxy groups elongates the dichroism signals in the wavelength range from 200–
depth of the CD cavity and simultaneously leads to the lack
of hydrogen bonds between torus rims of the CD cavities.
In the equatorial direction of the PM-β-CD cavity, some C–
H···O hydrogen bonds (C8a···O5e 2.322 Å, C3b···O3e
2.564 Å, C4f···O5c 2.594 Å, C7g···O5d 2.584 Å, C9g···O3g
2.455 Å) also connect the exterior walls of the PM-β-CD
cavities (Figure 3b). However, unlike the case of complex 3,
complex 4 does not display the regular layer arrangement,
but the interlaced channel-type array in the overall view
(Figure 3c).
Binding Modes in Solution
Circular dichroism and 2D NMR spectra of complexes
Figure 4. Circular dichroism spectra (a) and UV/Vis absorption
spectra (b) of complexes 3 and 4 (a: 2.0ϫ10–4 moldm–3 and b:
3 and 4 were performed to comprehensively investigate the
structure feature and assembly process of PM-β-CD com- 1.0ϫ10–5 moldm–3) in aqueous solution at 25 °C.
Figure 5. (a) ROESY spectrum of complex 3 (2.6ϫ10–3 moldm–3) in D2O at 25 °C with a mixing time of 250 ms, (b) possible conforma-
tion of 3 in D2O.
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Binding of Permethylated β-Cyclodextrin with Azobenzene Derivatives
500 nm.[8d] Both 3 and 4 show one positive Cotton effect inner H3 and H5 protons, the substituent methyl groups
peak as well as one negative Cotton effect peak (complex (CH3-2, CH3-3, and CH3-6) can also provide valuable infor-
3: ∆ε
–2.95 dm–3 mol–1 cm–1 at 431 nm; complex 4: ∆ε
4.15 dm–3 mol–1 cm–1
at 376 nm and ∆ε
=
3.42 dm–3 mol–1 cm–1 at 348 nm and ∆ε
=
=
=
mation about the complex structures by analyzing the cor-
relations between the protons of the methyl groups and the
guests. As shown in the ROESY spectrum of complex 3
–4.94 dm–3 mol–1 cm–1 at 446 nm), which is in accordance (Figure 5), several cross-peak signals between PM-β-CD
with π–π* and n–π* absorbance bands of the azobenzene and 4-HAB protons are observed in peaks A and B, sug-
group, respectively. Therefore, according to the empirical gesting that the 4-HAB guest is included into the cavity of
rules on the ICD phenomena of CD complexes,[20] these PM-β-CD. Peak B represents the correlations between the
ICD signals can result from two possible inclusion orienta- Ha protons and the inner PM-β-CD protons including H5,
tions: (1) The N=N group is included into the chiral micro- H3, or H6 CH3-3 protons. In peak A, multiple correlation
environment of the central cavity with its π–π* transition signals of the Hb and Hc protons with H5, H3, or H6, CH3-
band nearly parallel to the axial direction of PM-β-CD. (2) 3 protons of the PM-β-CD cavity as well as a weak corre-
The N=N group is located outside the chiral central cavity lation with the CH3-6 protons are observed. Moreover, the
and likely to stay in the region of the achiral torus rims Hd protons of the benzene group show correlations only
with its π–π* transition band perpendicular to the axial di- with the CH3-3 protons of the wide rim of the PM-β-CD
rection of PM-β-CD.
cavity. It can be inferred from these cross-peaks that the
Further detailed information about the binding geome- phenol moiety of 4-HAB is embedded into the central cav-
tries of complexes 3 and 4 comes from 2D ROESY experi- ity from the narrow side of the PM-β-CD cavity. Although
ments. ROESY cross-peaks are indicative of specific prox- considering the superposition of the H3 and H6, H6Ј peaks,
imity relationships between adjacent protons (the distance the correlations between the Hc protons and H3 of PM-β-
between protons is generally in the region of 4–5 Å).[21] CD are also possible, which makes it difficult to estimate if
Therefore, once the guest molecule is included into the β- the benzene group of 4-HAB is also included into the cen-
CD cavity, the NOE correlation signals between the protons tral cavity. However, the correlation peaks between the aro-
of the guest and the interior protons of the β-CD cavity matic protons Hd of the benzene group and H5 or H3 of
(H3 and H5) can be observed. For PM-β-CDs, besides the the inner PM-β-CD are not found. Thus, the hypothesis of
Figure 6. (a) ROESY spectrum of complex about 4 (3.3ϫ10–3 moldm–3) in D2O at 25 °C with a mixing time of 250 ms, (b) possible
conformation of 4 in D2O.
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J. Shi, D.-S. Guo, F. Ding, Y. Liu
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the benzene group in the central cavity should be excluded. bility constants (KS) and thermodynamic parameters (∆H°
Furthermore, the only correlation signals of the Hd protons and ∆S°) are listed in Table 1. In a previous report,[8d] UV
of 4-HAB and the CH3-3 protons of PM-β-CD illustrate spectral titrations were performed in aqueous buffer solu-
that the 4-HAB guest also has interactions with the wide tion, giving KS values of 840 –1 for 4-HAB and 2130 –1
side of the adjacent PM-β-CD cavity.
for 4-AAB with β-CD, respectively. It is acceptable that the
As shown in Figure 6, multiple correlation signals are present KS values of β-CD with azobenzene guests decrease
also observed in the 2D ROESY spectrum of complex 4. to 339 –1 for 4-HAB and 461 –1 for 4-AAB owing to the
Peak A exhibits the correlation of the Hg or Hf protons of addition of ethanol in solution.[12a,23]
4-AAB with H5, H3, or H6, CH3-3, and CH3-6 of PM-β-
CD. Moreover, the Hh protons show a correlation signal
only with CH3-3 protons of PM-β-CD. Peak B represents
correlations between the He protons of 4-AAB and CH3-3
and CH3-6 of PM-β-CD. From these correlation peaks, it
can be deduced that the benzene moiety of the 4-AAB guest
is included into the PM-β-CD cavity. However, because of
the superposition of the signals of the Hg and Hf protons,
it is also possible that the Hf protons are included into the
central cavity. However, the He protons of the aniline group
have no correlation with any protons of the central cavity
of PM-β-CD, excluding the hypothesis of the aniline group
in the central cavity. Furthermore, the He protons of the
aniline group show correlations with the CH3-3 and CH3-6
protons of PM-β-CD, suggesting that the aniline moiety of
4-AAB is exposed between the torus rims of two adjacent
Figure 7. Raw data of the calorimetric titrations of PM-β-CD with
4-AAB in 0.1  phosphate aqueous buffer solution (pH 7.2) in the
PM-β-CD cavities. By taking the results of the ICD experi-
ments into account, it can be deduced that the azo groups
of 4-HAB and 4-AAB prefer to be located at the narrow
torus rim of the PM-β-CD cavity with their π–π* transition
bands perpendicular to the axial direction of PM-β-CD,
and the phenol moiety of 4-HAB and benzene moiety of 4-
AAB are included into the central CD cavity.
presence of ethanol (20 vol.-%) at 25 °C. For sequential 10 µL injec-
tions of CD solution (20.0 m) into 4-AAB solution (0.93 m).
Table 1. Complex stability constants (KS) and standard enthalpy
(∆H°) and entropy changes (T∆S°) for the inclusion complexation
of β-CD and PM-β-CD with azobenzene derivatives in 0.1  phos-
phate aqueous buffer solution (pH 7.2) in the presence of ethanol
(20 vol.-%) at 25 °C.
Hosts
Guests
KS /
–∆H° /
T∆S° /
Complexation Thermodynamics
–1
kJmol–1
kJmol–1
To provide quantitative and further thermodynamic in-
sight into the inclusion complexation behaviors of 4-HAB
or 4-AAB with different CD hosts, the ITC experiments
were performed at atmospheric pressure in phosphate aque-
ous buffer solution (pH 7.2) in the presence of ethanol
(20 vol.-%) at 25 °C by using a titration microcalorimeter.
The added ethanol is used to solve the poor water solubility
of guests. Typical titration curves are shown in Figure 7.
According to the ROESY experimental results above, com-
β-CD
4-HAB
4-AAB
840[a]
339Ϯ6
–
–
–9.8
–
–5.1
0.6
–2.5
–24.2Ϯ0.2
–
–20.4Ϯ0.1
–18.2Ϯ0.5
–20.8Ϯ0.4
2130[a]
461Ϯ5
1987Ϯ123
1586Ϯ87
PM-β-CD
4-HAB
4-AAB
[a] Determined by UV/Vis spectral titrations in a phosphate aque-
ous buffer solution (pH 7.2).[8d]
The binding processes of β-CD and PM-β-CD with azo-
plexes 3 and 4 show n:n binding stoichiometry in aqueous benzene guests are all driven by favorable enthalpy changes
solution with high concentrations up to 2.6 and 3.3 m. (–∆H° Ͻ 0), which should be attributed to the significant
However, the ITC experiment was performed in an water/ contribution of hydrophobicity, hydrogen bonds, and
ethanol solution, where the concentrations of 4-HAB or 4- van der Waals interactions, as well as C–H···π interactions
AAB guests were in the region of about 1.0 m. The pres- between host and guest molecules.[24] Especially, β-CD
ence of ethanol and the decrease in concentration exert an shows comparative or even more favorable enthalpy
appreciable negative influence over the aggregation of the changes upon complexation with azobenzene guests than
obtained complexes. Therefore, under the ITC conditions, PM-β-CD, although the KS values of β-CD are 3–5 times
PM-β-CD may just form 1:1 simple inclusion complexes lower than those of PM-β-CD. This could result from the
with 4-HAB and 4-AAB, but not the n:n aggregates. Actu- pronounced release of high-energy water molecules from
ally, we also found that the present host–guest complex- the β-CD cavity during the hydrophobic process, which
ation could be well fitted by using the one set of binding contributes to the favorable enthalpy changes during the
sites model in the ITC experiments with the 1:1 model, course of the complexation of β-CDs; the same does not
which implied that the stoichiometry of these two com- occur for PM-β-CDs. In contrast, the entropy changes of
plexes should be 1:1.[22] The obtained data of complex sta- PM-β-CD with azobenzene guests are much more favorable
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Binding of Permethylated β-Cyclodextrin with Azobenzene Derivatives
than those of β-CD, which could be attributed to two and the benzene group of 4-AAB are immersed into the
reasons: (1) PM-β-CD possesses a more flexible framework central cavity of PM-β-CD, and the azo groups are located
than β-CD; thus, its loss of conformational degrees of free- at the narrow torus rim. Viewed along the axis of the CD
dom upon complexation with guests is certainly less than cavity, PM-β-CDs are arranged in a contrary head-to-tail
β-CD. (2) The extended hydrophobic cavity of PM-β-CD manner. The aggregation structures of complexes 3 and 4
can lead to more desolvation effects of the guests. As a re- are also different from each other, which originate from the
sult, although the host–guest complexation is enthalpy considerable complexation regioselectivity of PM-β-CD
driven, the large enthalpy changes do not always mean high with two azo guests: layer-type packing structure in com-
complex stability, and the molecular selectivity is mainly plex 3 and channel-type aggregation in complex 4. More-
governed by the entropy term. Moreover, it is notable that over, further ITC investigations indicate that PM-β-CD can
only the PM-β-CD complex with the hydroxy derivative form more stable complexes with two azobenzene guests
guest exhibits a favorable ∆S value of 0.6 kJmol–1, suggest- than β-CD, which mainly originates from the more favor-
ing that more electrostatic interactions exist between PM- able entropy changes during the course of complexation of
β-CD and the hydroxy group owned by the 4-HAB guest. PM-β-CD with azobenzene guests. The present results will
However, by switching the substituent from a hydroxy serve us to further understand the inclusion and aggrega-
group to an amino group, the entropy becomes less favor- tion behaviors of PM-β-CD and the intrinsic differences be-
able, which indicates that there are fewer electrostatic effects tween the β-CD and PM-β-CD cavities, which is beneficial
in this host–guest system. This result also probably reflects in the design and construction of diverse supramolecular
the regioselective complexation of two PM-β-CD com- assemblies based on CDs.
plexes, suggesting that the hydroxy group is caged into the
cavity, resulting in positive entropy gain as a result of elec-
Experimental Section
trostatic interactions, whereas the amino group is outside
the cavity and does not contribute to the positive electro-
static gain (Figure 8).
Materials: Both 4-hydroxyzazobenzene (4-HAB) and 4-aminoazo-
benzene (4-AAB) were commercially available and used without
further purification. Permethylated β-cyclodextrin (PM-β-CD) was
synthesized according to a previous literature report.[25] All other
chemicals used in the reactions were reagent grade and used with-
out further purification. Disodium hydrogen phosphate dodecahy-
drate and sodium dihydrogen phosphate were dissolved in deion-
ized water to make a 0.1  phosphate aqueous buffer solution of
pH 7.2, which was measured by pH electrode for calorimetric ti-
tration.
Instruments: Circular dichroism and UV/Vis spectra were recorded
with a JASCO J-715S spectropolarimeter and a Shimadzu UV3600
spectrophotometer in a conventional quartz cell (light path 10 mm)
in aqueous solution at 25 °C. 1H and 2D ROESY NMR spectra
were obtained in D2O with a Varian Mercury VX300 instrument
with a mixing time of 250 ms. Elemental analysis was performed
with a Perkin–Elmer 2400C instrument. The X-ray intensity data
for 3 and 4 were collected with a Rigaku MM-007 rotating anode
diffractometer equipped with a Saturn CCD Area Detector System
by using monochromated Mo-Kα radiation at T = 113(2) K. Data
collection and reduction were performed with the use of the Crys-
talclear[26] program. The structures were solved by using direct
methods and refined by employing full-matrix least-squares on F2
(CrystalStructure, SHELXTL-97).[27] Table 2 summarizes the crys-
tal data and experimental and refinement parameters of 3 and 4.
Figure 8. (a) Heat effects of dilution and of complexation of PM-
β-CD with 4-AAB for each injection during titration of micro-
calorimetric experiment; (b) “net” heat effect obtained by sub-
tracting the heat of dilution from the heat of reaction, which was
analyzed by computer simulation with the use of the “one set of
binding sites” model.
CCDC-691287 (for 3) and -691288 (for 4) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Conclusions
We obtained two complexes, 3 and 4, of PM-β-CD with
azobenzene guests, and their structures were identified by
X-ray crystallography, circular dichroism, and 2D NMR
spectroscopy and also compared with previous complexes 1
and 2 of β-CD. β-CD includes 4-HAB and 4-AAB guests
in the same mode with the azo groups accommodated in
the center of the hydrophobic cavity, and further, complexes
1 and 2 present a 1D channel-type arrangement in a head-
to-head manner. Conversely, PM-β-CD includes the azo-
The complex stability constants (KS) of β-CD and PM-β-CD with
4-HAB and 4-AAB were determined by the method of isothermal
titration calorimetry (ITC). The ITC experiments were performed
with a Microcal VP-ITC titration microcalorimeter at atmospheric
pressure in 0.1  phosphate aqueous buffer solution (pH 7.2) in
the presence of ethanol (20 vol.-%) at 25 °C by using a titration
microcalorimeter. The ITC instrument was calibrated chemically
by measurement of the complexation reaction of β-CD with cyclo-
benzene guests regioselectively; the phenol group of 4-HAB hexanol, and the obtained thermodynamic data were shown to be
Eur. J. Org. Chem. 2009, 923–931
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
929

J. Shi, D.-S. Guo, F. Ding, Y. Liu
FULL PAPER
tration experiments were carried out to afford self-consistent
thermodynamic parameters.
Table 2. Crystal data and experimental and refinement parameters
of complexes 3 and 4.
Complex 3: An ethanol solution of 4-HAB (19.8 mg, 0.1 mmol,
5 mL) was added dropwise to an aqueous solution of PM-β-CD
(142.8 mg, 0.1 mmol, 15 mL). The mixture was stirred at 50 °C for
12 h. After removing the insoluble substances by filtration, the re-
sultant solution was kept at 50 °C for several days, and orange com-
plex 3 was collected along with its mother liquor for X-ray crystal-
lographic analysis. Yield: 107.6 mg, 62%. UV/Vis (H2O): λ (ε,
Complex 3
Complex 4
Molecular formula
Mr / gmol–1
Crystal system
Space group
Z
a / Å
b / Å
c / Å
α / °
β / °
C75H134N2O42
1735.84
orthorhombic
P212121
4
14.8450(8)
22.1705(12)
27.5443(17)
90
90
90
9065.4(9)
1.272
3736
C75H134N3O36
1644.78
orthorhombic
P212121
4
10.6993(4)
27.3627(12)
29.1666(13)
90
90
90
8538.9(6)
1.279
3536

–1 cm–1) = 348 (2.07ϫ104) nm. 1H NMR (300 MHz, D2O): δ =
7.65 (d, 2 H, Ar-H), 7.54 (d, 2 H, Ar-H), 7.44–7.24 (m, 3 H, Ar-
H), 6.90 (d, 2 H, Ar-H), 5.05 (s, 7H 1-H), 3.72–3.08 (m, 105 H),
3.40 (21 H, 3-OMe), 3.31 (21 H, 2-OMe), 3.18 (18 H, 6-OMe) ppm.
C75H122O36N2·6H2O (1735.87): calcd. C 51.89, H 7.78, N1.61;
found C 51.42, H 7.64, N 1.52.
γ / °
V / Å3
ρcalcd. / gcm–3
F(000)
Complex 4: Prepared from 4-AAB and PM-β-CD according to a
procedure similar to that described above. Yield: 111.8 mg, 68%.
UV/Vis (H2O): λ (ε, –1 cm–1) = 379 (1.95ϫ104) nm. 1H NMR
(300 MHz, D2O): δ = 7.64–7.56 (d, 4 H, Ar-H), 7.53–7.37 (m, 3 H,
Ar-H), 6.79 (d, 2 H, Ar-H), 5.05 (s, 7H 1-H), 3.72–3.08 (m, 105
H), 3.40 (21 H, 3-OMe), 3.31 (21 H, 2-OMe), 3.18 (18 H, 6-OMe)
ppm. C75H123O35N3·H2O (1644.81): calcd. C 54.77, H 7.66, N 2.55;
found C 54.74, H 7.60, N 2.58.
T / K
293(2)
0.104
113(2)
0.102
µ (Mo-Kα) / mm–1
Crystal size / mm3
θ Range / °
0.16ϫ0.14ϫ0.14 0.26ϫ0.20ϫ0.18
1.48–25.01
1.58–25.00
64982
15017
0.0598
1.078
0.0822
0.2175
0.0931
0.2267
No. of reflections collected 68276
No. of unique reflections
Rint
GOF
Final R indices
[IϾ2σ(I)]
R1 = R indices
(all data)
15829
0.1139
1.181
0.0983
0.1984
0.1126
0.2059
Acknowledgments
This work was supported by the 973 Program (2006CB932900), the
National Natural Science Foundation of China (20572052,
20673061, and 20703025), and Special Fund for Doctoral Program
from the Ministry of Education of China (20050055004), which are
gratefully acknowledged.
in good agreement (error Ͻ 2%) with the literature data. Each
solution was degassed and thermostatted by a ThermoVac access-
ory before titration experiment. Twenty-five successive injections
were made for each titration experiment. A constant volume
(10 µL/injection) of host solution (20.04–20.32 m) in a 0.250 mL
syringe was injected into the reaction cell (1.4227 mL) charged with
guest molecules solution (0.85–1.00 m) in the same buffer solu-
tion. A representative titration curve is shown in Figure 7. Each
titration of PM-β-CD into the sample cell gave an apparent reac-
tion heat, caused by the formation of the inclusion complex be-
tween the host and guest. The reaction heat decreases after each
injection of host molecules because less and less guest molecules
are available to form inclusion complexes. A control experiment
was carried out in each run to determine the dilution heat by in-
jecting host solution into the same buffer solution containing no
guest molecules. The dilution heat determined in these control ex-
periments was subtracted from the apparent reaction heat mea-
sured in the titration experiments to give the net reaction heat.
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930
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Received: August 29, 2008
Published Online: January 2, 2009
Eur. J. Org. Chem. 2009, 923–931
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