Exclusion complexes of the HCl salts of benzidine and bis(4-aminophenyl) methane with two methyl-substituted cucurbiturils

Article abstract of DOI:10.1039/b908490h

The interaction between two partially methyl-substituted cucurbiturils, a sym-tetramethyl-substituted cucurbit[6]uril (TMeQ[6]) and a meta-hexamethyl- substituted cucurbituril (m-HMeQ[6]), with the hydrochloride salt of benzidine (g1·HCl) and the analogue

Full text of DOI:10.1039/b908490h

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Exclusion complexes of the HCl salts of benzidine and bis(4-aminophenyl)
methane with two methyl-substituted cucurbiturilsw
Ying Yan,a Sai-Feng Xue,*a Hang Cong,a Jian-Xing Zhang,b Yun-Qian Zhang,a
Qian-Jiang Zhua and Zhu Taoa
Received (in Victoria, Australia) 6th May 2009, Accepted 18th June 2009
First published as an Advance Article on the web 25th August 2009
DOI: 10.1039/b908490h
The interaction between two partially methyl-substituted cucurbiturils, a sym-tetramethyl-
substituted cucurbit[6]uril (TMeQ[6]) and a meta-hexamethyl-substituted cucurbituril (m-HMeQ[6]),
with the hydrochloride salt of benzidine (g1ꢀHCl) and the analogue bis(4-aminophenyl) methane
1
(g2ꢀHCl) was investigated by single crystal X-ray diffraction determination, H NMR
spectroscopy, electronic absorption spectroscopy and fluorescence spectroscopy. Single crystal
X-ray diffraction determination showed the two guest compounds were excluded at the portals of
1
the partial methyl-substituted cucurbiturils in the solid state. The H NMR spectroscopic analysis
in aqueous solution supported the crystallographic results in which an excluding or portal
interaction occurs between the host and guest. Aqueous absorption spectrophotometric and
fluorescence spectroscopic analysis defined the stability of the host–guest exclusion complex at
pH 5.6 with a host : guest ratio of 1 : 1, which forms quantitatively as B105 L molꢁ1 for the
TMeQ[6]–g1 system. The host : guest ratio of 2 : 1 forms quantitatively as B1010 L2 molꢁ2 for
the m-HMeQ[6]–g2 system. The experimental results are in good agreement with HF and B3LYP
computational approaches with a moderate-sized basis set.
the ability to catalyze 1,3-dipolar cycloadditions in a regio-
selective fashion.6 More recently, Q[8] was used as a template
Introduction
for [2 + 2] and [4 + 4] photodimerizations in water.7
Since the structure of the cucurbituril Q[6] was first determined
and reported by Mock and Freeman in 1981,1 a number of
The potential of cucurbit[n]urils for tuning the properties of
homologues and analogues of Q[6] have been reported in the
fluorescent materials may lead to new applications, which
last decade.2 The hydrophobic cavity and the polar carbonyl
include increased fluorescence intensity, brightness, enhanced
group periphery surrounding the portals are characteristic
photostability, protection towards fluorescence quenching, and
the extension of fluorescence lifetimes of fluorescent materials.3i,8
features for all cucurbiturils. These functional groups can
interact with various organic or inorganic species through a
Most of the potential applications of cucurbit[n]urils are
combination of different non-covalent interactions, such as
established on the basis of the inclusion of certain organic
dipole–ion interaction, hydrogen bonding, hydrophobic
guests in cucurbit[n]urils. On the other hand, Fedin and
cavity interactions and so on, to form a series of self-
co-workers demonstrated a series of supramolecular chains
assembled supramolecular entities with novel structures and
constructed of Q[6] and metal ions as well as their complexes
properties. This has led to a dramatic development in
and clusters formed through dipole–ion interaction and hydrogen
Q[n] chemistry.3 Based on the formation of inclusion
bonding.9 Recently, we first demonstrated the formation of
complexes of cucurbit[n]urils, Kim and co-workers demonstrated
some molecular bracelets in which alkyl-substituted Q[5]s are
linked directly by coordinated metal ions.10 The metal ions or
elegant networks constructed of ‘‘molecular necklace rings’’
in which a number of Q[6] beads are threaded onto a large
their complexes are located at the portals of the cucubit[n]urils
ring consisting of organic species and metal ions through
coordination bonding.4 Macromolecular encapsulation into
thereby forming exclusion complexes with the host Q[n]s. Only
a few works have focused on exclusion complexes of Q[n]s and
organic compounds such as mono- or di-ammonium ions.11
Q[n] could improve drug stability, control the release of a
potential drug, and increase drug activation or solubility in
Benzidine (4,40-diaminobiphenyl) and benzidine derivatives,
drug delivery applications.5 A remarkable feature of Q[n]s is
which are potentially carcinogenic aromatic amines, have been
widely used in the past as intermediates in the manufacture of
a Key Laboratory of Macrocyclic and Supramolecular Chemistry of
dyes and pigments for cloth, paper, and leather. Benzidine and
Guizhou Province, Guizhou University, Guiyang 550025,
its derivatives are discharged by the dye industry and are often
P. R. China. E-mail: gzutao@263.net
b Key Laboratory of Chemistry for Natural Products of Guizhou
Province, Guiyang 550002, P. R. China
w Electronic supplementary information (ESI) available: Variation in
absorbance and fluorescence intensity with host–guest ratio. CCDC
reference numbers 718845 and 718846. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b908490h
released in effluent waste water leading to their presence
in lakes, rivers and soils.12 Buschmann and Schollmeyer
investigated the decolorization, or selective removal, of
solutions containing different dyes with water-insoluble Q[6]
precipitated on a carrier material or dissolved in formic acid in
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solution was mixed thoroughly and allowed to stand at room
temperature. Crystals suitable for X-ray diffraction formed
after several days. Anal. Calcd. for C55H64N26O12Cl2: C,
33.93; H, 6.73; N, 18.71. Found: C, 33.55; H, 6.58; N, 18.88%.
The solvent molecules in the crystals of Q[n]s and their
host–guest complexes are generally easily lost, which results
in crystal deterioration. Although single crystals of the
compounds 1 and 2 were of poor quality, they produced
suitable X-ray reflection and allowed for orientation matrix
determination in both cases. Experimental data were collected
for the compounds 1 and 2 on a Bruker APEX2 CCD
diffractometer [graphite monochromatized MoKa-radiation,
o and f scan mode, 10 s frameꢁ1].
Fig. 1 Structures of the host and the guests used in this work.
the 1990s, but few structural details of the Q[6]–dyes were
reported due to the poor solubility of the normal Q[6].13
In the present work, we used two water soluble partially
methyl-substituted cucurbiturils, a sym-tetramethyl-substituted
cucurbit[6]uril (TMeQ[6])2i and a meta-hexamethyl-substituted
cucurbituril (m-HMeQ[6])2h (shorted as SQ[6]s), to investigate
the interaction details of the hydrochloride salts of benzidine
(g1ꢀHCl) and its analogue, bis(4-aminophenyl) methane
(g2ꢀHCl) by using various methods, including X-ray crystal
structure analysis, 1H NMR spectroscopy, electronic absorption
spectroscopy and fluorescence spectroscopy (Fig. 1).
The experimental results revealed that both TMeQ[6]–g1
and m-HMeQ[6]–g2 systems formed host–guest exclusion
complexes. No host–guest inclusion complexes were observed
despite the potential ability of the cavity to encapsulate the
aromatic ring of the guest.11
Structures of the compounds 1 and 2 were solved and
refined with anisotropic thermal parameters used for all
non-hydrogen atoms. Hydrogen atoms of the host and guest
were placed in calculated positions and further refined using
the riding model.w
Crystal data for the compound 1. 2(TMeQ[6]–g1)ꢀCl4ꢀ25H2O,
M = 3086.29, monoclinic, a = 23.9121(19) A, b =
17.6841(14) A, c = 16.8216(13) A, b = 99.475(2)1, V =
7016.2(10) A3, T = 223.0(2) K, space group P21/c (no. 14),
Z = 2, l(Mo-Ka) = 0.71073 A, m(Mo-Ka) = 0.166 mmꢁ1
,
12 318 reflections measured, 6528 unique (Rint = 0.0639)
which were used in all calculations. The final R1 and wR2
were 0.1170 and 0.3501 for I 4 2s(I), 0.1602 and 0.3773 for
all data.
Crystal data for the compound 2. m-HMeQ[6]–g2ꢀCl2ꢀ
33H2O, M = 1944.23, triclinic, a = 16.897(9) A, b =
18.037(9) A, c = 18.475(10) A, a = 68.835(8)1, b =
69.689(7)1, g = 73.900(8)1, V = 4849(4) A3, T = 223.0(2)
Experimental
Materials
ꢀ
TMeQ[6] and m-HMeQ[6] were prepared and purified according
to methods from the literature2h and our laboratory.2i The
guests benzidine (g1) and bis(4-aminophenyl) methane (g2)
were obtained from Shanghai Chongming Chemical Co., Ltd.
and were of analytical grade and were used without further
purification. The corresponding HCl salts of these guests were
prepared by dissolving them in concentrated HCl followed by
removal of excess HCl by using a rotary evaporator. The pure
TMeQ[6] and m-HMeQ[6] were used in all processes in
this work.
K, space group P1 (no. 2), Z = 2, l(Mo-Ka) = 0.71073 A,
m(Mo-Ka) = 0.121 mmꢁ1, 15 535 reflections measured, 5499
unique (Rint = 0.0833) which were used in all calculations. The
final R1 and wR2 were 0.0966 and 0.2334 for I 4 2s(I), 0.1706
and 0.2653 for all data.
All the refinements were performed using SHELXTL-Plus
software.
1H NMR titrations. The titration experiments were performed
in D2O solution at 25 1C. The concentration of the host was
fixed about B1 ꢃ 10ꢁ2 mol Lꢁ1, and the guest concentration
was increased, starting from 5.0 ꢃ 10ꢁ3 mol Lꢁ1. The highest
guest/host concentration ratios investigated were B7.0.
X-Ray crystallography
Preparation of 2(TMeQ[6]–g1)ꢀCl4ꢀ25H2O (1). A single
crystal of the TMeQ[6] adduct with g1 was obtained by
dissolving TMeQ[6] (0.20 g, 0.19 mmol) in a solution
of g1ꢀHCl (0.051 g, 0.20 mmol) in water (5 mL). The
final solution was mixed thoroughly and allowed to stand
at room temperature. Crystals suitable for X-ray
diffraction were formed after several days. Anal. Calcd. for
Absorption and fluorescence studies. UV-visible (UV-vis)
absorption spectra of the host–guest complexes were recorded
on an Agilent 8453 spectrophotometer at room temperature.
Fluorescence spectra of the host–guest complexes were
recorded on a Varian RF-540 fluorescence spectrophotometer.
Aqueous solutions of HCl salts of the guests were prepared
with a concentration of 1 ꢃ 10ꢁ3 mol Lꢁ1. Aqueous
solutions of the hosts were prepared with a concentration of
2 ꢃ 10ꢁ3 mol Lꢁ1. These stock solutions were combined to
give solutions with a guest : SQ[6] ratio of 0, 0.2 : 1, 0.4 : 1,
1 : 1, 1.5 : 1, 2 : 1. . . for both absorption spectra
and fluorescence spectra determination. A model SA 720
C
104H168N52O50Cl4: C, 40.44; H, 5.48; N, 23.58. Found: C,
40.42; H, 5.36; N, 23.75%.
Preparation of m-HMeQ[6]–g2ꢀCl2ꢀ33H2O (2). A single
crystal of the m-HMeQ[6] adduct with g2 was obtained by
dissolving m-HMeQ[6] (0.20 g, 0.19 mmol) in a solution of
g2ꢀHCl (0.054 g, 0.20 mmol) in water (5 mL). The final
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(Aolilong) pH meter was used for accurate adjustment of the
pH of the samples with HCl and NaOH.
Computational methods
All calculations were processed on an Intel Pentium 3.0G PC
with the Gaussian 03W (Revision C.02) software package.14
The initial geometries of all structures were constructed with
the aid of the Hyperchem package, release 7.5215 and based on
the corresponding crystal structures. Becke’s three-parameter
hybrid function with the correlation function of Lee, Yang,
and Parr (B3LYP)16 was used for energy calculations with a
STO-3G basis set.17
Results and discussion
Crystal structures of the exclusion complexes TMeQ[6]–g1ꢀHCl
and m-HMeQ[6]–g2ꢀHCl
The Q[n] hosts exhibit remarkably tight and selective inclusion
binding of organic species, particularly organic ammonium
ions in water.18 During the last three decades, the inclusion
binding characteristics of the Q[n] hosts were extensively
studied in catalyzing special cycloadditions,6,7 creating novel
molecular shuttles or switches,19 and constructing elegant
supramolecular entities.4 Little attention, however, has
focused on the exclusion binding characteristics of the Q[n]
hosts toward organic species and consequently the resultant
novel supramolecular entities. Generally, the Q[6]s have a
strong tendency to include five or six-membered aromatic
rings, such as pyridyl, phenyl, and so on.7b,20 Herein, we
introduce two supramolecular entities constructed of two
water soluble hosts, TMeQ[6] and m-HMeQ[6] with the guests
g1 and g2, respectively. Although the guests contain both
aromatic rings and protonated amines, which look the ideal
moieties to be included by the host SQ[6]s, the resultant
products were unexpectedly the exclusion complexes in which
portal binding was observed.
Fig. 2 Crystal structure of (a) a 1D supramolecular chain constructed
of alternating TMeQ[6] and g1, (b) the interaction between the
neighboring TMeQ[6] and g1, (c) the crossed supramolecular chains
in the compound 1.
N26 of the guests and the portal carbonyl oxygens of the hosts
(with an average distance of 4.036 A for N25, and 3.602 A for
N26), hydrogen bonding between N25–Hꢀ ꢀ ꢀO11 (3.003 A),
N25–Hꢀ ꢀ ꢀO12 (2.770 A), N26–Hꢀ ꢀ ꢀO5 (2.835 A), and
N26–Hꢀ ꢀ ꢀO6 (2.553 A) is an additional force in the formation
of the 1D supramolecular chain. Fig. 2c shows that the 1D
supramolecular chains cross each other in the crystal structure
of the compound 1.
In contrast, the crystal structure obtained from m-HMeQ[6]
and the hydrochloride salt of g2 also presents supramolecular
chains consisting of alternating g2 dications and m-HMeQ[6]
molecules through both ion–dipole and hydrogen bonding
interactions. Other ion–dipole and hydrogen bonding inter-
actions link two 1D supramolecular chains in pairs (Fig. 3a).
Unlike the case in 1, each sandwiched m-HMeQ[6] molecule is
not centrosymmetric, two end amine nitrogens N25 and N26
belonging to the two neighboring guest g2 are excluded at the
two portals of the m-HMeQ[6] molecule or an excluded guest
g2 and a m-HMeQ[6] are a repeated unit in the 1D supra-
molecular chain (Fig. 3a). A close inspection of the 1D
supramolecular chain reveals that a g2 guest is excluded at
the portal of a host m-HMeQ[6], and sandwiched by two
m-HMeQ[6] molecules. The distance of N25 to the plane
(O1, O2, O3, O4, O5, O6) is 0.230 A, whereas the distance
of N26 to the plane (O7, O8, O9, O10, O11, O12) is 1.961 A.
The formation of a 1D supramolecular chain is attributed to a
combination of the ion–dipole interaction between the proto-
nated amine nitrogens N25 or N26 of guests with the portal
carbonyl oxygens of the hosts and the hydrogen bonding
between N25–Hꢀ ꢀ ꢀO2 (2.960 A), N25–Hꢀ ꢀ ꢀO3 (3.029 A),
N25–Hꢀ ꢀ ꢀO4 (2.894 A), N26–Hꢀ ꢀ ꢀO10 (2.816 A), and
N26–Hꢀ ꢀ ꢀO11 (2.955 A). The formation of the 1D supra-
molecular chain pair is attributed to a combination of spare
ion–dipole and hydrogen bonding interactions between the
The crystals obtained from an aqueous solution of TMeQ[6]
and the hydrochloride salt of g1 had a stoichiometry of
2(TMeQ[6]–g1)ꢀCl4ꢀ25ꢀH2O (1). In the crystal structure of
the compound 1, each host TMeQ[6] molecule was sandwiched
by two guest dications. Each guest molecule was sandwiched
by two host TMeQ[6] molecules through both of ion–dipole
and hydrogen-bonding interactions. This resulted in the
formation of a supramolecular chain consisting of alternating
g1 dications and TMeQ[6] molecules (Fig. 2a). A close
inspection of the structure reveals that the distances of the
two terminal amine nitrogens, N25 and N26, of a guest to the
two neighboring portals of the two neighboring TMeQ[6]
molecules are different (Fig. 2b). The distance of N25 to the
plane (O7, O8, O9, O10, O11, O12) is 1.730 A, whereas the
distance of N26 to the plane (O1, O2, O3, O4, O5, O6) is
0.866 A. This means that the sandwiched guest g1 is not
centrosymmetric, and the two phenyl rings in the guest are
twisted away from each other by 26.031. Each sandwiched
TMeQ[6] molecule is centrosymmetric, and the same two end
amine nitrogens N25 or N26 are excluded at both the portals
of the TMeQ[6] molecule (Fig. 2a). Besides the ion–dipole
interaction between the protonated amine nitrogens N25 or
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Fig. 4 Variation in the 1H NMR spectra of TMeQ[6]–g1ꢀHCl system
with increasing concentration of TMeQ[6].
Fig. 3 Crystal structure of (a) a 1D supramolecular chain pair
constructed of alternating m-HMeQ[6] and g2, (b) the interaction
between the neighboring m-HMeQ[6] and g2, (c) the arrangement of
the supramolecular chain pairs in the compound 2.
protonated amine nitrogens in a chain pair with the portal
carbonyl oxygens of m-HMeQ[6] in a chain pair, such as the
interaction between the atoms N26 and O7 (2.936 A) (Fig. 3b).
Fig. 3c shows that the supramolecular chain pairs in 2 are
arranged in such a way as to produce a structure with linear,
tetragonal channels extending along the b axis. The supra-
molecular chain pairs are surrounded by four neighboring
chains. The mean diameter of the channels is about 10 A, and
the channels are filled with water molecules that form a
complicated hydrogen bonding network. No organic
molecules are found in the channels.
1H NMR spectra analysis of the interaction between the
SQ[n]s with gꢀHCl
Fig. 5 Variation in the 1H NMR spectra of m-HMeQ[6]–g2ꢀHCl
system with increasing concentration of m-HMeQ[6].
Above, we demonstrated the two exclusion complexes of
two partially methyl-substituted SQ[6]s with two guests,
containing both phenyl groups and protonated amines, in
the solid state where the guests did not move into the cavity
of the host SQ[6]s. Fig. 4 and Fig. 5 show the 1H NMR
titration spectra of the HCl salts of the guest recorded in the
absence, and in the presence of various equivalents, of the host
SQ[6]s in aqueous solution. One can see that the spectra are
unexpectedly simple without obvious chemical shifts of the
proton resonances for either the guest or the host at the
different host/guest ratios for the TMeQ[6]–g1 system. For
the m-HMeQ[6]–g2 system, resonances of the protons on the
aromatic ring experience different chemical environment, and
experience different deshielding extents by the carbonyl portal
of m-HMeQ[6]. The resonance of the proton Hx experiences a
downfield shift of 0.24 ppm, while the resonance of the proton
Hy experiences a downfield shift of only 0.05 ppm when the
ratio of Nm-HMeQ[6]/Ng2 is up to 6.6. Therefore, one can see
that the two separated resonances of the protons Hx and Hy
overlap together and then separate again with an increase in
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the ratio of Nm-HMeQ[6]/Ng2, but the positions of Hx and Hy
are swapped. All the above information suggests that the guest
is not included by the host in solution and an exclusion
interaction could be occurring between the host and the guest.
However, the detailed interaction between the host and the
guest is still unclear in solution.
of the guest g1ꢀHCl exhibit
a
progressive increase in
fluorescence intensity with a slightly blue shift as the ratio
NTMeQ[6]/Ng1ꢀHCl is increased. The fluorescence intensity change
(DIf) vs. ratio of moles of [Ng1ꢀHCl/(NTMeQ[6] + Ng1ꢀHCl)] data
can be fitted to a 1 : 1 binding model for the TMeQ[6]–g1HCl
system (Fig. 6f), while the emission spectra of the guest g2ꢀHCl
exhibit a progressive decrease in fluorescence intensity as the
ratio Nm-HMeQ[6]/Ng2ꢀHCl is increased. The fluorescence intensity
change (DIf) vs. ratio of moles of [Ng2ꢀHCl/(Nm-HMeQ[6] + Ng2ꢀHCl)]
data are also fitted to a 2 : 1 binding model for the
m-HMeQ[6]–g2ꢀHCl system (Fig. 6h). This behavior is
consistent with the results from the absorption spectro-
photometric analysis.
Spectrophotometric analysis on the interaction between
SQ[6]s and gꢀHCl
To further quantify the interaction between the methyl
substituted SQ[6]s and g1–2ꢀ2HCl in solution, a ratio-
dependent study was pursued by monitoring electronic
absorption and fluorescence spectra at pH 5.6. Usually, the
two hosts SQ[6]s show no absorbance at l 4 210 nm, and the
free HCl salt of the two guest shows a maximum absorption at
lmax 281 nm for g1ꢀHCl and 242 nm for g2ꢀHCl, respectively.
Fig. 6(a) and (c) show the variation in the UV spectra obtained
with aqueous solutions containing a fixed concentration of
g1ꢀHCl or g2ꢀHCl (32 mM) and variable concentrations of
SQ[6]s, respectively. The absorption band of the guests
exhibits a progressively lower absorbance with a slight blue
shift as the ratio NSQ[6]/NgꢀHCl is increased. The absorbance
change (DA) vs. ratio of moles of [Ng1ꢀHCl/(NTMeQ[6] + Ng1ꢀHCl)]
data can be fitted to a 1 : 1 binding model for the TMeQ[6]–g1ꢀHCl
system at lmax (Fig. 6b). Interestingly, the absorbance change
(DA) vs. ratio of moles of [Ng2ꢀHCl/(NmꢁHMeQ[6] + Ng2ꢀHCl)]
data can be fitted to a 2 : 1 binding model for the
m-HMeQ[6]–g2ꢀHCl system at lmax (Fig. 6d).
Generally, the binding constants can be calculated based on
the absorbance or fluorescence intensity vs. ratio of moles of
the host SQ[6] and guest (NSQ[6]/Nguest) data. However, for
these two typical host–guest interaction systems, both the
absorbance and fluorescence intensity data are almost linear
as the ratio NSQ[6]/Nguest is increased and are not suitable for
calculating the related binding constants (see the figures in the
ESIw). Based on the corresponding Job plots (Fig. 6b,d,f,h),
the binding constants (K) can be estimated for the two
exclusion host–guest systems. They are 7.59 ꢃ 105 L molꢁ1
and 7.16 ꢃ 105 L molꢁ1 for the TMeQ[6]–g1 system,
4.43 ꢃ 1010 L2 molꢁ2 and 5.43 ꢃ 1010 L2 molꢁ2 fot the
m-HMeQ[6]–g2 system, respectively.
Molecular mechanics calculations
Using fluorescence spectroscopy, similar experiments were
performed. The two free host SQ[6]s are non-fluorescent
compounds and the maximum fluorescence emission wave-
lengths of the guests g1ꢀHCl and g2ꢀHCl are 411 nm and
353 nm, respectively. Fig. 6e, g show the variation in the
emission spectra obtained with aqueous solutions containing a
fixed concentration of g1–2ꢀHCl (32 mM) and variable
concentrations of SQ[6]s, respectively. The emission spectra
Above, we have demonstrated two host–guest systems which
exhibit unexpected behaviors and properties. Although the
guests have protonated aniline moieties, which are typical
species to form inclusion host–guest complexes with the
SQ[6]s,20 the exclusion host–guest complexes of the SQ[6]s
with g1 or g2ꢀHCl were observed. When compared to a
number of known inclusion host complex systems,3d,18
the quantification of the interaction between the methyl
Fig.
6
(a, c) Electronic absorption spectrum for the TMeQ[6]–g1ꢀHCl and m-HMeQ[6]–g2ꢀHCl systems, (b, d) the corresponding
DA–NgꢀHCl/(NSQ[6] + NgꢀHCl) curves. (e.g.) Fluorescence emission spectra for the TMeQ[6]–g1ꢀHCl and m-HMeQ[6]–g2ꢀHCl systems, (f, h) the
corresponding DIf–NgꢀHCl/(NSQ[6] + NgꢀHCl) curves.
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substituted SQ[6]s and g1–2ꢀ2HCl in solution revealed that the
formed exclusion host–guest complexes presented unexpected
stability. Although the two guests used in this work have
similar structures, the absorption spectrophotometric
and fluorescence spectroscopic analysis in aqueous solution
indicated the formation of the host–guest exclusion complex
with a host : guest ratio of 1 : 1 for the TMeQ[6]–g1 system,
and a host : guest ratio of 2 : 1 for the m-HMeQ[6]–g2 system.
In addition, a novel 1D supramolecular construction based on
the exclusion host–guest complexes was observed in the
solid state.
hydrogen bonding at the calculated minimum B. A combination
of the hydrophobic cavity interaction, ion–dipole interaction
and hydrogen bonding could lead to the formation of such a
partial inclusion host–guest complex. However, the structure
corresponding to the lowest minimum (D) shows that the guest
moves out of the cavity of the host completely at a distance of
about 8.0 A which is consistent with the structural parameters
presented in the crystal structure of the compound 1. In the
presence of ion–dipole interactions and hydrogen bonding, the
exclusion host–guest complex formed shows unexpected
stability, which is confirmed by the calculations, absorption
spectrophotometry, and fluorescence spectroscopy. As for the
two maxima A and C in Fig. 7, the corresponding structures
have the common feature that the aromatic ring(s) is (are)
included in the portal(s), but not the cavity of the host. This
causes high strain on the portal(s) and consequently the
inclusion host–guest complexes have higher potential energies.
In general, the cavity of a Q[6] favors inclusion of a single
aromatic ring, such as phenyl or pyridyl and so on which can
fit comfortably in the cavity. In our previous work, the
demonstration of interaction between Q[6]s with 2,20-dipyridyl
and its derivatives revealed that one pyridyl ring of the guest
was contained within the cavity of Q[6] and that the other
pyridyl ring of the guest protruded from one portal of the host
To understand these unexpected behaviors and properties,
we have selected the B3LYP/STO-3G method implemented in
Gaussian 03. All simulations were done in the gas phase in the
presence of host and guest only. We assumed that the
pathways for ingression and egression are the same
(microscopic reversibility) and have therefore only modeled
the process of guest egression. The activation energies for
guest egression from the inner cavity of Q[6] were calculated
by defining a complexation coordinate coinciding with the
Q[6] rotational symmetry axis, along which the guest could be
forced in 0.5 A increments from the center of the cavity (0 A)
through the portals (ca. 3 A) into free space (10 A). To achieve
this, the position of a dummy atom was fixed at the center of
cavity of the cucurbiturils.21 The distance between host and
guest was then varied and calculations provided the potential
energy at each step.
Q[6], and
a 1 : 1 unsymmetrical host–guest complex
was formed, where the protonated N atom on the pyridyl
further strengthened the interaction of the host (Q[6]) and
2,20-dipyridyl through hydrogen bonding and ion–dipole
interactions between the protonated N atom and the portal
carbonyl oxygens.22 However, in this case, no such extra
interaction exists between the host and guest, and in addition,
the neighboring phenyl ring sits at the portal area, and
increases the geometric strain.
Fig. 7 shows the profile of the relative potential energy to
the point of the lowest potential energy at dmin 8.0 A (E, a.u.)
vs. the distance between the geometric center of the guest g1
and the dummy atom (d, A) and the structures corresponding
to the energy maxima. Overall, the energy of TMeQ[6]–g1ꢀ2H+
appears to increase with increasing distance. A close inspection
of the profile reveals a wave-like curve in the distance range
0–8.0 A. There are two wave minima (B and D) and two
maxima (A and C) (Fig. 7, inset). It is understandable that
only one of the aromatic rings of guest g1 is included in the
cavity of the host TMeQ[6], as shown in Fig. 7B. In addition,
the protonated amine group on the aromatic ring just inside a
portal of the TMeQ[6] host could interact with the carbonyl
groups of the portal through ion–dipole interactions and
In summary, both the experimental results and the theoretical
analysis suggest that the guest g1 with two neighboring
aromatic rings does not favor the formation of a stable
inclusion host–guest complex due to the geometric strain caused
by two neighboring aromatic rings in the cavity of a Q[6].
Fig. 8 shows the profile of the relative potential energy
(to the point of the lowest potential energy at dmin 8.0 A) vs.
the distance (d) between the geometric center of the guest g2
Fig. 8 The profile of potential energy (E, a.u.) vs. distance (d, A), and
the corresponding structures at the peaks for the m-HMeQ[6]–g2
system.
Fig. 7 The profile of potential energy (E, a.u.) vs. distance (d, A), and
the corresponding structures at the peaks for the TMeQ[6]–g1 system
ꢂc
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and the dummy atom. Typical structures corresponding to the
peak points in the profile are included. The calculated results
revealed that the potential energies of the inclusion models are
always higher than those of the exclusion models. The profile
also shows a minimum at dmin 8.0 A (Fig. 8, inset), and the
corresponding structure is consistent with the crystal structure
in the compound 2. Looking into the structure of the guest g2,
a bridged methylene orients the two neighboring phenyl rings
at 113.761 (see the CIF file of compound 2w), which causes a
more strained geometry when the guest moves into the cavity
of the host Q[6] and consequently leads to the higher potential
energy of the inclusion model.
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Conclusion
The single crystal X-ray diffraction determination of the
interaction complexes obtained from methyl-substituted
cucurbiturils (TMeQ[6] and m-HMeQ[6]) and guests containing
protonated aniline groups (hydrochloride salts of benzidine
hydrochloride and bis(4-aminophenyl) methane) showed the
formation of two host–guest exclusion complexes in which the
guest molecules were excluded at the portals of the host
cucurbiturils. In the solid state, each guest or host was
sandwiched between neighboring hosts or guests leading to
the formation of 1D supramolecular chains. Analysis of the 1H
NMR spectra showed no obvious change in the proton
chemical shifts of either the host or guest molecules thereby
suggesting that an inclusion interaction was not present
between TMeQ[6] with g1ꢀHCl in the solution state, while a
downfield shift of the resonances of the aromatic protons of g2
clearly suggested the formation of the exclusion complex for
the m-HMeQ[6]–g2 system. Spectroscopic analysis not only
revealed unexpectedly high formation constants B105 L molꢁ1
for the TMeQ[6]–g1 system and B1010 L2 molꢁ2 for the
m-HMeQ[6]–g2 system in aqueous solution at pH 5.6, but
also revealed that the interaction ratio of the host : guest was
1 : 1 for the TMeQ[6]–g1 system and 2 : 1 for the TMeQ[6]–g1
system. For the systems studied here, the experiments are
in good agreement with HF and B3LYP computational
approaches using moderately-sized basis sets. Overall, guests
with neighboring aromatic rings favor formation of the
exclusion host–guest complexes due to the geometric strain
in the Q[6] cavity.
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Acknowledgements
Support from the National Natural Science Foundation
of China (NSFC; No. 20662003 and 20767001) and the
‘‘Chun-Hui’’ Funds of Chinese Ministry of Education is
gratefully acknowledged.
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