A versatile tripodal host with cylindrical conformation: Solvatomorphism, inclusion behavior, and separation of guests

Article abstract of DOI:10.1002/chem.201002246

Tripodal host 2,4,6-tris(1-phenyl-1H-tetrazolylsulfanylmethyl)mesitylene (TPTM) has been synthesized through a facile procedure. As expected, it adopts an all-syn cylindrical configuration, thereby delimiting an inner cavity. To explore the solvatomorphis

Full text of DOI:10.1002/chem.201002246

FULL PAPER
DOI: 10.1002/chem.201002246
A Versatile Tripodal Host with Cylindrical Conformation: Solvato
Inclusion Behavior, and Separation of Guests
ACHTUNGTRENNmUNG orphism,
Wei Wei,[a, b] Guo Wang,[b] Ying Zhang,[b] Feilong Jiang,[a] Mingyan Wu,[a] and
Maochun Hong*[a]
Abstract: Tripodal host 2,4,6-tris(1-
phenyl-1H-tetrazolylsulfanylmethyl)-
mesitylene (TPTM) has been synthe-
sized through a facile procedure. As
expected, it adopts an all-syn cylindri-
cal configuration, thereby delimiting an
inner cavity. To explore the solvato-
morphism and inclusion behavior of
TPTM, a series of organic and inorgan-
ic species were employed as guests to
afford 17 inclusion compounds (1, 2,
3a–3 f, 4a–4i) that can be classified
into four distinct forms (forms I–IV),
under similar conditions. These com-
pounds were characterized by single-
crystal and powder X-ray diffraction,
and 1H NMR studies. In compound 1
with form I, one foot of a TPTM mole-
cule inserts into the cavity of an oppo-
site TPTM molecule to form a dimeric
“hand-shake” motif with one acetoni-
trile molecule occupying the void.
through further superposition to form
open hexagonal channels. From the ex-
perimental and theoretical results, the
intrinsic properties of guest molecules,
such as size, shape, and self-interaction,
can be regarded as the main factors
leading to these solvatomorphism phe-
nomena and the subtle inclusion be-
havior of TPTM. Thermogravimetric
analyses show that the encapsulated
guest molecules in these compounds
can be evacuated at relatively high
temperatures, and this demonstrates
the outstanding inclusion capability of
TPTM. In addition, for compound 4a
with benzene molecules in the chan-
nels, reversible exchange of toluene
and separation of xylene isomers on
single crystals have been observed.
Compound
2 with form II contains
three types of capsule-shaped dimers,
each of which holds a CH2Cl2 molecule
as the guest. In compounds 3a–3 f with
form III, each pair of TPTM molecules
interdigitates to form a capsule-shaped
dimeric unit accommodating a guest
molecule in the endo-cavity. In com-
pounds 4a–4i with form IV, each
TPTM molecule makes contact with
three nearby TPTM molecules in a
“self-including” manner to generate a
graphite-like
organic
layer,
and
Keywords: host–guest systems
inclusion compounds
morphism · tripodal hosts
·
·
solvato-
AHCTUNGTRENNUNG
Introduction
mers,[3] storage of gases[4] and toxic substances,[5] stabiliza-
tion of reactive compounds,[6] and control of reaction path-
ways within a confined space.[7] In recent decades, the devel-
opment of this field has inspired modern supramolecular
design of more elaborate organic host molecules with vari-
ous structural motifs, such as tri-o-thymotide (TOT),[8] cyclo-
triveralene (CTV),[9] calixarenes,[10] and a series of tripodal
host molecules.[11–13]
One attractive feature of the tripodal host molecule is
that the rigid central platform can provide some degree of
preorganization with all three binding feet projected in one
direction and afford a thermodynamically favored cavity.
On the basis of this model, a variety of organic hosts have
been developed by using trialkylbenzene cores as rigid plat-
forms connected with straight pendant groups, either rigid[12]
or flexible.[13] However, the small spacing between the three
feet for each trialkylbenzene core precludes large guests
from occupying the cavities and limits their applications as
Solid-state inclusion compounds, which essentially involve
physical immobilization of guest molecules in host cavities
or voids between hosts in crystalline form,[1] have been of
importance in the separation of analogues[2] and enantio-
[a] Dr. W. Wei, Prof. Dr. F. Jiang, Dr. M. Wu, Prof. Dr. M. Hong
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
Fuzhou, Fujian 350002 (China)
Fax : (+86)591-8379-4946
E-mail: hmc@fjirsm.ac.cn
[b] Dr. W. Wei, Dr. G. Wang, Y. Zhang
Department of Chemistry, Capital Normal University
Beijing 100048 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem..201002246.
Chem. Eur. J. 2011, 17, 2189 – 2198
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2189

molecular hosts. Instead of using straight pendant groups,
we envisioned that longer bent ones would expand the dis-
tance between the three binding feet and also the volume of
the predesigned cavities. Moreover, the newly introduced
bent pendant groups could be adjusted to a position perpen-
dicular to the central ring plane by selecting proper linker
atoms, and thus lead to a structural transformation of the
tripodal host from the traditional conical conformation to a
cylindrical one (Scheme 1). This modulation would remarka-
bly strengthen the inclusion capability towards larger guests.
Results and Discussion
Synthesis and configuration of TPTM: Treatment of tris-
(bromomethyl)mesitylene with 1-phenyl-1H-tetrazole-5-
thiol in the presence of base under reflux afforded TPTM in
high yield. Crystals of 1–4 suitable for single-crystal X-ray
analysis were obtained by crystallization from MeCN (1) or
dichloromethane (2–4). As predicted, X-ray structure analy-
sis reveals that the TPTM molecules adopt an all-syn u,u,u
configuration in all cases (u=up and d=down; there are
only two conformations: u,u,uꢀd,d,d and u,u,dꢀu,d,d), and
the pendant phenyl groups are directed towards the same
side. To explain the conformational preference, we calculat-
ed the relative energies of different conformers at the PBE/
6-31G* and B3LYP/6-31G* levels. The results indicate that
u,u,d conformation lies about 0.7 kcalmolÀ1 higher than the
most stable u,u,u configuration. Despite this relatively slight
energy difference, host–guest interactions and close packing
may also account for the stabilization of all-syn conforma-
tion, which can be concluded from diverse crystal structures
of TPTM molecules.
Scheme 1. Steric configuration of the substituents of 1,3,5-trialkylbenzene
derivatives with straight and bent pendant groups (gray balls represent
the volumes of the preorganized cavities).
The phenomenon of polymorphism is attracting significant
attention in the chemical and pharmaceutical industries for
various reasons.[14] In particular, some versatile organic host
molecules have a pronounced tendency to undergo cocrys-
tallization with a variety of relatively small solvent guest
molecules, which generates different forms of inclusion com-
pounds.[10a] This solvate diversity of a particular host com-
pound is known as solvatomorphism.[15] Therefore, it is in-
structive to understand the relationship between the forms
of inclusion compounds and the nature of solvent guest mol-
ecules, such as size, shape, and self-interaction, for new in-
sights into the inclusion behavior of organic hosts.
As an extension of our work on tridentate thioether com-
pounds,[16] we chose to attach 1-phenyl-1H-tetrazole-5-thiol
pendant groups to a mesitylene core to prepare an expanded
tripodal host molecule, namely, 2,4,6-tris(1-phenyl-1H-tetra-
zolylsulfanylmethyl)mesitylene (TPTM).[17] Due to the hy-
drophobic nature of the protruding phenyl groups and mesi-
tylene core, some cyclic hydrocarbon molecules are regard-
ed as appropriate guests for packing into the cavities or lat-
tices of this new host molecule. Herein we study the de-
pendence of the solvatomorphism of TPTM on the nature
of the guest molecules, and present 17 inclusion compounds
(1, 2, 3a–3 f, 4a–4i) with four distinct forms I–IV, along with
their thermal behavior. Besides these results, reversible
guest exchange and selective separation of xylene isomers
on single crystals of 4a were also studied.
X-ray crystallography and solvatomorphism: In the presence
of different guest molecules, we obtained four diverse crys-
tal forms, denoted forms I–IV.
Form I: Recrystallization of TPTM from MeCN affords
large amounts of colorless block crystals of TPTM2·MeCN
(1), and single-crystal X-ray analysis reveals that 1 crystalli-
¯
zes in the triclinic space group P1 (form I). As shown in
Figure 1, there are two TPTM molecules and one aceto-
AHCTUNGTREGnNNUN itrile molecule in the asymmetric unit. One foot of a
TPTM molecule inserts into the preorganized cavity of the
other and thus forms a dimeric “hand-shake” motif. The dis-
tances between the hydrogen atoms on the feet (the “plug”)
and the centers of the mesitylene rings (the “socket”) are
2.81 and 3.06 ꢁ (the closest contacts are ca. 2.99 and
2.83 ꢁ), and clearly indicate CH···p interactions. These
offset dimers are arranged by close stacking in the crystal
structure, and the small acetonitrile molecule fills the void
among them with an interesting p···p interaction between
the CN group and the tetrazole ring (C···centroid and
N···centroid distances are 3.22 and 3.29 ꢁ, respectively).
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Chem. Eur. J. 2011, 17, 2189 – 2198

A Tripodal Host with Cylindrical Conformation
FULL PAPER
half-molecules of toluene in the asymmetric unit. Thus, two
TPTM molecules and their corresponding symmetry-related
molecules in compound 3a interdigitate to form two similar
capsule-shaped dimers containing a toluene in each endo-
cavity. Despite the similarity of the dimeric units to those in
2, the pendant phenyl groups of TPTM in 3a are almost per-
pendicular to the central core, and the whole molecule
adopts a more precisely cylindrical configuration than in 2.
As shown in Figure 3, the toluene molecule is sealed in by
the hydrophobic walls and interacts with phenyl groups
through perceptible CH···p interactions (2.79 and 2.84 ꢁ).
Figure 1. View of the homodimeric hand-shake dimer of compound 1.
The CH···p and p···p interactions are represented as dashed lines.
Form II: Slow evaporation of a solution of TPTM in di-
chloromethane in air afforded colorless hexagonal crystals
of TPTM2·CH2Cl2 (2), which crystallizes in the trigonal
¯
space group R3 (form II). In the structure of 2, two TPTM
molecules are arranged face-to-face to form a capsule-
shaped dimer with an interior cavity holding a CH2Cl2 mole-
cule as the guest (Figure 2). The unit cell contains three
Figure 3. View of the dimeric units of TPTM with cylindrical configura-
tion in 3a. The encapsulated toluene molecule is shown as a space-filling
model and the two positions of its disordered methyl group each with
50% occupancy are represented.
Form IV: Surprisingly, distinct crystalline phase TPTM·C6H6
¯
(4a) in trigonal space group P3c1 was obtained simply by
replacing toluene with benzene. Single-crystal analysis re-
veals only one-third of a TPTM molecule in the asymmetric
unit, and it can be expanded to a whole molecule by a three-
fold rotation axis. As indicated by Figure 4a and b, TPTM
molecules are offset in a face-to-face arrangement, and
three phenyl feet from opposite diverse TPTM molecules
are inserted in the preorganized cavity of each host mole-
cule, in what can be described as a “self-including” manner.
Because of its contact with three nearby TPTM molecules,
each TPTM molecule can be regarded as a three-connected
topological node, resulting in a graphite-like organic layer
with hexagonal grids, and further superposition of these or-
ganic layers forms hexagonal channels in the presence of
guest benzene molecules (Figure 4c). Crystallographic struc-
ture solution revealed that one benzene molecule is located
at the center of a channel on a threefold rotoinversion axis,
and adopts a horizontal mode. Besides, TG analysis and
1H NMR spectroscopy determine one more highly disor-
dered benzene molecule (Figures S4 and S5 in the Support-
Figure 2. View of the three kinds of dimeric units in compound 2. Only
one set of disordered fragments of the encapsulated CH2Cl2 molecules is
shown as space-filling models for clarity.
types of dimeric units in which all of the TPTM molecules
adopt a gauche conformation with deviation from the strict
cylindrical form by rotation of the pendant groups to various
extents to accommodate a CH2Cl2 guest molecule. Despite
the similar inclusion modes, the three types of dimers lying
on different crystallographic positions have diverse thermal
stabilities, as supported by thermogravimetric analysis (see
below).
Form III: When one equivalent of toluene was added to a
solution of TPTM in dichloromethane, block crystals of
TPTM2·C7H8 (3a) were obtained. Single-crystal X-ray analy-
sis showed two independent molecules of TPTM and two
Chem. Eur. J. 2011, 17, 2189 – 2198
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M. Hong et al.
Table 1. The crystal forms of inclusion compounds 1–4 with diverse guest
molecules.
Guest
Form
1
2
CH3CN
CH2Cl2
I
II
3a, b
III
R1 =H (3a), CH3 (3b)
3c
3d
III
III
3e
3 f
III
III
I2
4a, b
IV
X=CH (4a), N (4b)
4c–h
IV
X=F (4c), Cl (4d), Br (4e),
I (4 f), CF3 (4g), CN (4h)
Cp2Fe
4i
IV
Figure 4. a) Axial and b) side view of the offset face-to-face arrangement
of TPTM in a “self-including” manner. c) Graphite-like layer with hexag-
onal grids occupied by benzene molecules (hexagonal rings).
ing Information), probably within the solvent-free area of
organic layers.
In an attempt to gain a better understanding of what
properties of the guest molecules are responsible for the
packing mode of crystals, many other guest molecules were
employed to explore the inclusion behavior of TPTM. The
crystal forms of inclusion compounds with diverse guest
molecules are listed in Table 1, the structures of which were
characterized by single-crystal X-ray diffraction (Table 2) or
Figure 5. View of the dimeric units of TPTM in a) 3b, b) 3c, c) 3d, and
d) 3 f. The encapsulated guest molecules are p-xylene, phenylethylene,
tetrahydrofuran, and iodine, respectively. Only one disordered fragment
of the guest molecules is shown as a space-filling model for clarity.
1
X-ray powder diffraction (XRPD) and H NMR spectrosco-
py (see the Supporting Information).
Figure S3 in the Supporting Information); this phenomenon
reveals the versatile inclusion capability of TPTM.
As shown in Table 1, crystals 3a–3e with form III were
obtained when cyclic hydrocarbons such as toluene (C7H8),
p-xylene (PX), phenylethylene (PE), and cyclohexane
(C6H12), as well as THF, were added as guests. X-ray single
crystal diffraction and XRPD analyses determined that they
are all comprised of capsular dimeric units with correspond-
ing guests and have similar crystal cell parameters
(Figure 5). Interestingly, the inorganic, diatomic iodine mol-
ecule (I2) can also be encapsulated in the cavity of a dimeric
unit to afford crimson inclusion compound TPTM2·I2 (3 f,
In contrast, other small organic analogues of very similar
size, such as benzene (C6H6), pyridine (py), halobenzenes
(C6H5X; X=F, Cl, Br, I), a,a,a-trifluorotoluene (TFT), and
ferrocene (Cp2Fe), induced formation of crystal form IV
(Figure 6a). In contrast to the horizontal arrangement of the
benzene molecules in 4a, the halobenzene molecules are all
arranged along the threefold axes in an approximately up-
right fashion in 4c–4h. These halobenzene molecules all
show threefold disorder due to their location on threefold
symmetry axes; we chose one position of the guest for the
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A Tripodal Host with Cylindrical Conformation
FULL PAPER
Table 2. Crystal data and structure refinements for all the complexes.
1
2
3a
3b
3c
3d
3 f
formula
Mr
C68H63N25S6
1422.79
C335H310N120S30Cl5
7333.32
C73H68N24S6
1473.87
C74H70N24S6
1487.90
C74H68N24S6
1485.88
C70H68N24OS6
1453.84
C66H60N24S6I2
1635.54
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
triclinic
P1
trigonal
R3
21.8938(5)
21.8938(5)
66.196(3)
90
90
120
27479.4(16)
3
1.329
triclinic
P1
triclinic
P1
triclinic
P1
triclinic
P1
triclinic
P1
¯
¯
¯
¯
¯
¯
¯
13.815(4)
15.133(5)
18.047(6)
78.186(11)
75.196(9)
75.301(9)
3488.7(19)
2
13.228(8)
13.625(8)
22.478(13)
91.279(5)
94.493(10)
108.253(9)
3831(4)
2
13.0908(13)
13.5630(13)
22.536(2)
90.560(2)
95.241(2)
108.149(2)
3783.2(6)
2
13.0761(6)
13.6229(6)
22.4652(10)
90.4990(10)
95.2880(10)
108.7250(10)
3770.8(3)
2
13.198(5)
13.304(5)
22.199(8)
92.334(6)
93.410(3)
108.509(5)
3682(2)
2
13.2137(6)
13.3168(6)
22.1910(10)
92.285(2)
93.507(2)
108.6050(10)
3686.5(3)
2
a [8]
b [8]
g [8]
V [A3]
Z
1calcd [MgmÀ3
]
1.354
0.258
1.278
0.237
1.306
0.241
1.309
0.242
1.311
0.247
1.473
1.081
mcalcd [mmÀ1
]
0.318
measured reflns
unique reflns
Rint
27121
15708
0.0277
49066
15190
0.0345
28628
17019
0.0320
13509
7585
0.0268
51996
17774
0.0592
28727
16639
0.0230
50755
17259
0.0895
GOF
1.085
1.088
1.056
1.069
1.154
1.030
0.905
R1, wR2 [I>2s(I)]
R1, wR2 (all data)
0.0614, 0.1407
0.0933, 0.1617
0.1150, 0.3345
0.1982, 0.4366
0.0704, 0.1840
0.0960, 0.2079
0.0973, 0.3029
0.1306, 0.3834
0.1129, 0.3257
0.2245, 0.4036
0.0697, 0.1824
0.0992, 0.2133
0.1099, 0.2998
0.2583, 0.3756
4a
4d
4e
4 f
4h
4i
4k
4a’’
formula
Mr
crystal system
space group
a [ꢁ]
C13H12N4S
256.33
C39H35N12S3Cl C39H35N12S3Br C39H35N12S3I
C40H35N13S3
793.99
C76H70N24S6Fe C35H32.50N12S3
C13H12N4S
256.33
803.42
847.88
894.87
1567.80
trigonal
P3c1
717.41
trigonal
trigonal
P3c1
trigonal
P3c1
trigonal
P3c1
trigonal
trigonal
trigonal
¯
¯
¯
¯
¯
¯
¯
¯
P3c1
P3c1
P3c1
P3c1
13.0587(3)
13.0587(3)
27.2749(10)
4028.0(2)
12
13.0884(2)
13.0884(2)
27.2076(9)
4036.40(16)
4
13.0859(2)
13.0859(2)
27.2294(9)
4038.09(16)
4
13.1135(19)
13.1135(19)
27.249(5)
4058.0(12)
4
13.1288(6)
13.1288(6)
27.313(2)
4077.1(4)
4
13.0209(2)
13.0209(2)
27.3029(8)
4008.86(15)
2
13.0958(2)
13.0958(2)
27.2611(9)
4048.91(16)
4
13.0346(4)
13.0346(4)
27.2151(16)
4004.4(3)
12
b [ꢁ]
c [ꢁ]
V [A3]
Z
1calcd [MgmÀ3
]
1.268
0.229
1.322
1.395
1.465
1.294
0.229
1.299
1.177
1.276
0.230
mcalcd [mmÀ1
]
0.295
1.219
0.989
0.403
0.223
measured reflns
unique reflns
Rint
23176
3336
0.0243
1.105
52011
3397
0.0791
1.058
24025
3356
0.0310
1.041
51911
3406
0.0546
1.087
23203
3434
0.0283
1.057
32234
3197
0.0953
1.075
19724
3351
0.0203
1.150
21591
3177
0.0321
1.094
GOF
R1, wR2 [I>2s(I)] 0.0628, 0.1812 0.0702, 0.2078 0.0921, 0.2784 0.1485, 0.4782 0.0530, 0.1530 0.0808, 0.2067 0.0616, 0.1846 0.0562, 0.1646
R1, wR2 (all data) 0.0754, 0.1963 0.1165, 0.2404 0.1137, 0.3041 0.1777, 0.5340 0.0654, 0.1667 0.1239, 0.2328 0.0705, 0.1958 0.0711, 0.1789
sake of clarity in Figure 6b. As illustrated in Figure 6b, the
adjacent phenyl groups of guests inside the channels interact
with each other through offset p–p stacking, whereas their
neighboring halogen atoms exhibit some halogen···halogen
contacts.[18] Compared with the conventional isotropic van
der Waals radii, which are about 3.52 ꢁ for chlorine, 3.70 ꢁ
for bromine, and 3.96 ꢁ for iodine,[19] these interactions
(dCl···Cl =4.09, dBr···Br =3.72, and dI···I =3.89 ꢁ) appear to be
quite weak. Besides these halogen-containing guests, the or-
ganometallic compound ferrocene also can be included in
the one-dimensional channels as a guest to form crystal 4i
(Figure 6c), which shows a characteristic golden color (Fig-
ure S3 in the Supporting Information).
delicate balance between the structural alternatives of form
III with dimeric units and form IV with open channels. To
aid in understanding the subtle inclusion behavior of TPTM,
we calculated the bonding energies of thr corresponding
guests and capsule-shaped dimeric unit as described for 3a
at the 6-31G(d) level (see Table 3).
Primarily, the size of the guest molecules remarkably af-
fects the packing of TPTM. For example, in 1 the CH3CN
molecule is too small to accommodate in capsule-shaped
dimers by effective packing compared with the larger
CH2Cl2 or cyclic hydrocarbons, and thus only occupies the
voids among offset TPTM dimers. Another convincing case
is mesitylene, which is larger than other benzene derivatives
and cannot be packed into the cavities of dimeric units to
cocrystallize with the host molecules. Moreover, the calcu-
lated bonding energy of 39.45 kcalmolÀ1 for mesitylene is
much higher than that of toluene (À5.38 kcalmolÀ1), which
also agrees with the considerable destabilization of the
former.
When m-xylene (MX), o-xylene (OX), or mesitylene was
used as guests in dichloromethane, we did not obtain inclu-
sion compounds with TPTM and only obtained crystals of
compound 2 containing CH2Cl2 molecules.
Discussion of inclusion behavior: In general, the above ex-
perimental results have shown that for most guests there is a
Chem. Eur. J. 2011, 17, 2189 – 2198
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M. Hong et al.
The shape of the guest is another crucial factor that can
determine the final form of the product. For instance, many
cyclic compounds, such as THF, cyclohexane, toluene, and
even the larger p-xylene, can all be encapsulated by the cap-
sular dimer (form III). However, benzene unexpectedly in-
duced formation of crystal form IV with open channels, al-
though its size falls in the range of the above cyclic mole-
cules. Although there is no perceptible directional interac-
tion in this host–guest system, the hexagonal shape of ben-
zene is most probably responsible for the formation of the
hexagonal channels in form IV because of the perfect match
of their shapes. This hypothesis can also be supported by
formation of the same crystal form when using pyridine,
which has the same hexagonal shape. Another example of
the influence of guest shape is xylene, for which only the
linear p-xylene can cocrystallize with TPTM in form III,
whereas nonlinear o- and m-xylene molecules cannot be en-
veloped. This fact can be further interpreted by calculated
results that the energy of dimeric unit with p-xylene is favor-
able (À1.68 kcalmolÀ1), whereas dimeric units with m-
xylene and o-xylene are energetically unstable (2.64 and
3.02 kcalmolÀ1, respectively).
As shown in Table 3, theoretical calculations support the
existence of stable dimeric units with the halobenzene mole-
cules (the binding energies of C6H5X are À6.83, À5.01, and
À10.50 kcalmolÀ1 for X=F, Cl, and Br, respectively), which
have comparable sizes and shapes to toluene and p-xylene.
Significantly, these halobenzenes co-crystallize with TPTM
in form IV and cannot be encapsulated by dimers of for-
m III. Further investigations suggest that these different
crystal forms are most likely attributable to the distinct self-
interaction of guest molecules. In contrast to the repelling
effect between methyl groups of toluene and p-xylene, the
above-mentioned halogen···halogen contacts between halo-
carbon molecules, such as halobenzenes and a,a,a-trifluoro-
toluene, can offer an attractive effect to make guests adopt
an upright array and connect linearly, which leads to forma-
tion of open channels in form IV. To examine this, we used
benzonitrile as a pseudohalogen-substituted guest to see
which crystal form would be formed. Because the CN group
can interact with halogen atoms,[20] benzonitrile should
induce formation of form IV similarly to halobenzenes. On
the other hand, the benzonitrile molecule with its eight non-
hydrogen atoms and an additional unsaturated bond has an
obvious structural similarity in size and shape to phenylethy-
lene, which can be packed into the dimeric unit of form III
(3c). The final result of formation of form IV (TPTM·BN,
4h) undoubtedly indicates that in this situation self-interac-
tion of the guest is the decisive factor for inclusion behavior,
rather than the guest structure.
Figure 6. a) View of the hexagonal channels occupied by guest molecules
in form IV. Schematic representations of arrangement of b) (pseudo)-
halogen-substituted aromatics (X=F, Cl, Br, I, and CN) and c) ferrocene
in the hexagonal channels of form IV. The interactions are represented as
dashed lines.
Table 3. Theoretical calculation of bonding energy and charge on guests.
Guest
Structure
Bonding energy
[kcalmolÀ1
Charge
]
benzene
toluene
p-xylene
À6.183003495
À5.38308826
À1.682965505
À0.00249
À0.03151
À0.00414
m-xylene
o-xylene
2.637200509
3.021773777
À0.01447
À0.01111
mesitylene
39.450753
À0.0159
fluorobenzene
chlorobenzene
bromobenzene
pyridine
À6.831962414
À5.006966296
À10.51756333
À6.541004852
À7.268890725
0.00019
As a general trend, it has been observed that guest mole-
cules containing five to eight light non-hydrogen atoms (C,
N, or O) are readily encapsulated in the cavities of the mo-
lecular dimer to generate form III, which results from a fa-
vorable energetic effect between host and guest with appro-
priate sizes. However, shape and self-interaction of guests
may also play a decisive role in controlling the final struc-
À0.00533
À0.00544
À0.00184
0.00257
tetrahydrofuran
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Chem. Eur. J. 2011, 17, 2189 – 2198

A Tripodal Host with Cylindrical Conformation
FULL PAPER
ture, and the distinct form IV with open channels is ob-
tained in some situations. Therefore, solvatomorphism of
host molecules largely depends on the intrinsic properties of
guest molecules, such as size, shape, and self-interaction,
and these conclusions give new insight into the inclusion be-
havior of the tripodal host molecules.
mained (Figure S5 in the Supporting Information). Unfortu-
nately, resultant opaque material 4x is devoid of single crys-
tallinity, so the use of single-crystal X-ray diffraction for
structural identification is precluded. This solid–solid phase
transformation of molecular material is very interesting be-
cause it may form an active phase that rapidly adsorbs gases
under ambient conditions, especially for a synthetic host
with a permanent cavity.[4a,21] Further investigation into the
structure and application in gas adsorption of 4x are under-
way.
Thermogravimetric analysis (TGA): The thermal stabilities
of 1–4 were studied by TGA on crystalline samples under
an N2 atmosphere with a heating rate of 108CminÀ1 between
room temperature and 5008C. All TG plots of these inclu-
sion compounds exhibited an abrupt weight loss from about
200 to 2158C, that is, the TPTM host is stable up to about
2008C (see Figure S4 in the Supporting Information).
Reversible exchange and separation of guests on single crys-
tals: Compounds 4 have about 24% solvent-accessible
volume per unit cell,[22] which is occupied by diverse guest
molecules. Additionally, the networks of compounds 4 are
expected to have flexibility to some extent because of the
large open channels assembled from purely organic mole-
cules only by interdigitated packing. On the basis of these
considerations, we tested whether the guests can be ex-
changed or selectively adsorbed within the channels in the
vapor phase under mild conditions.
For compound 1, a gradual weight loss of approximately
5.74% from 50 to 1208C corresponds to the loss of the
MeCN molecules (calcd 5.60%), which suggests that the
packing is not close and guest molecules tend to escape
from the voids of the crystal lattice. The TG analysis of 2 in-
dicated three sequential steps of weight loss before collapse
of the host: a gradual weight loss of approximately 2.56%
initially occurs from 86 to 1708C, and then weight losses of
1.36 and 2.25% in the ranges of 171–178 and 182–1908C
correspond to evacuation of the other two kinds of CH2Cl2
molecules (calcd 2.32, 1.16, 2.32%). These stepwise weight
losses of diverse thermal stabilization may correspond to the
three categories of crystallographic positions of dimers in 2.
For compounds 3 with form III, the weight loss up to about
1908C can be assigned to the release of the corresponding
encapsulated guest molecules (calcd for 3a: 6.24%, found:
6.02%; calcd for 3b: 7.12%, found: 7.07%; calcd for 3d:
4.95, found: 4.91%; loss of guest and collapse of the TPTM
host overlap for 3e). For these guest molecules packed in di-
meric capsules of 2 and 3, the escape processes all require
relatively high temperatures and the obvious evacuation
usually occurs after about 1508C. Particularly for 2, the en-
capsulated CH2Cl2 has a low boiling point of 408C but ex-
ceedingly high escape temperatures of about 170 and 1808C.
These results are in good agreement with the structures of
compounds 2 and 3 and demonstrate the outstanding inclu-
sion capability of the TPTM host molecule.
When single crystals of 4a containing benzene molecules
were exposed to toluene vapor at 308C for about 20 d, they
retained single crystallinity, and single-crystal X-ray analysis
gave crystal cell parameters similar to those of 4a. However,
1
the H NMR spectrum indicates that the benzene has been
completely replaced by toluene in a molar host/guest ratio
of approximately 2:1 to generate TPTM·0.5C7H8 (4j; Fig-
ure S6 of the Supporting Information). It is reasonable that
toluene cannot afford a compound with 1:1 host/guest ratio
as observed in compounds 3c–3h (Figure 6b) due to the re-
pelling effect between methyl groups of toluene. Interesting-
ly, the crystal transformation between 4a and 4j is reversi-
ble. Exposing the same batch of crystals 4j to benzene
vapor for about 20 d led to re-adsorption of benzene to
afford 4a’ (Figure S6 of the Supporting Information).
Because the channels in 4 can accommodate substituted
benzenes in an upright fashion along the channels, separa-
tion of linear and branched hydrocarbons, such as xylene
isomers, can be envisioned. To explore the separation of
xylene isomers in the vapor phase, crystals 4a were exposed
to an equimolar mixed vapor of xylene isomers for 20 d at
408C. The 1H NMR spectrum indicated that only the p-
xylene is adsorbed into the channels, displacing the benzene
to generate TPTM·0.25PX (4k) with an approximately 4:1
molar ratio of host/guest (Figure S6 of the Supporting Infor-
mation). Also, the X-ray structural analysis confirms guest
exchange in the channels and the single-crystallinity of 4k.
The crystal transformation between 4a and 4k is reversible,
and compound 4a’’ re-obtained in benzene vapor was char-
acterized by X-ray structural analysis, which indicated that
the guest exchange process could take place in a reversible
single-crystal to single-crystal manner. Apparently, this se-
lective adsorption is due to the diameter of the hexagonal
channel, which can accommodate linear benzene derivatives
and exclude nonlinear ones (Figure 7). This property is rem-
iniscent of the inclusion behavior of urea and thiourea clath-
For compounds 4 with open channels, all guest molecules
are completely removed before collapse of the host (calcd
for 4a: 5.07 and 5.07%, found: 5.17 and 5.01%; calcd for
4e: 18.51, found: 19.10%; calcd for 4g: 17.44, found:
18.21%). As shown for 4a in Figure S4 of the Supporting
Information, TGA indicates that there are two separate
steps of weight loss before collapse of the host at about
2008C, corresponding to loss of two kinds of benzene mole-
cules at 100–123 and 175–1958C. When as-prepared 4a is
treated under vacuum for 24 h, the sample undergoes the
first loss of benzene molecules at 908C, but retains the origi-
nal structure and the second type of benzene molecules. In
contrast, the structure of 4a obviously changes after only 4 h
above 1058C, affording a new unknown phase 4x character-
1
ized by XRPD patterns, although the H NMR spectrum re-
vealed that the second type of benzene molecules still re-
Chem. Eur. J. 2011, 17, 2189 – 2198
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2195

M. Hong et al.
and these studies will give new insight into the design and
application of molecular materials.
Experimental Section
General: All chemicals were of reagent grade, obtained from commercial
sources, and used without further purification. XRPD patterns were col-
lected in sealed glass capillaries by using a Rigaku DMAX 2500 power
diffractometer with ultra 18 kW Cu radiation. The thermal behavior was
analyzed by TGA by using
a NETSCH STA-449C thermoanalyzer.
1H NMR data were collected by using Bruker AVANCE III 400 and 600
spectrometers. ESI-MS was performed by using a Finigan LCQDECA
XP MAX ion trap mass spectrometer.
2,4,6-Tris(1-phenyl-1H-tetrazolsulfanylmethyl)mesitylene
(TPTM):
TPTM was prepared by a modified literature method.[23] A solution of 1-
phenyl-1H-tetrazole-5-thiol (2.851 g, 16.00 mmol) and potassium carbon-
ate (2.211 g, 16.00 mmol) in MeCN (40 mL) was stirred for 15 min, and
tris(bromomethy1)mesitylene (1.995 g, 5.00 mmol) in dichloromethane
(10 mL) was then added slowly. The mixture was stirred vigorously at RT
for 24 h and then heated to reflux for 2 h. After evaporation to dryness
under vacuum, the solid was added to deionized water (100 mL), and the
resultant white precipitate was collected by filtration, washed several
times until the pH of the filtrate was 7, dried under vacuum at 908C, and
recrystallized from MeCN to give TPTM (2.473 g, ca. 71.6% yield).
1H NMR (CDCl3, 258C): d=7.54–7.59 (m, 15H), 4.77 (s, 6H), 2.52 ppm
(s, 9H); ESI-MS: m/z (%): 691.6 (10) [M+H]+, 713.6 (100) [M+Na]+,
729.3 (35) [M+K]+ (see the Supporting Information).
Figure 7. Schematic representation of the separation of xylene isomers on
single crystals of 4a with hexagonal channels.
rates with appropriately sized hexagonal channels, and has
potential applications in the separation of linear and
branched hydrocarbons in the petroleum industry.[2b]
TPTM2·MeCN (1): Slow evaporation of a solution of TPTM (69.1 mg,
0.1 mmol) in MeCN (10 mL) in air afforded a large number of colorless
crystals after 48 h (yield: 95%).
Conclusion
TPTM2·CH2Cl2 (2): TPTM (69.1 mg, 0.1 mmol) was dissolved in CH2Cl2
(10 mL), and slow evaporation of the solution in the air gave a large
amount of colorless hexagonal crystals after 24 h (yield: ca. 99%).
By using the trialkylbenzene core as a rigid platform, new
tripodal host molecule TPTM has been synthesized by a
facile procedure. Instead of using straight pendant groups,
we have introduced longer bent pendant groups to expand
the volume of the predesigned cavity. As expected, this mol-
ecule adopts an all-syn cylindrical configuration in the solid
state, as determined by X-ray crystallographic analyses of 17
inclusion compounds classified into four different forms I–
IV. Herein we have focused on form III, comprised of cap-
sule-shaped dimeric units, and form IV with open hexagonal
channels. From the experimental and theoretical results, the
intrinsic properties of these guest molecules, such as size,
shape, and self-interaction, can be regarded as the main fac-
tors influencing the solvatomorphism and inclusion behavior
of TPTM.
TPTM2·C7H8 ACHTUNTRGNEUNG(3a): TPTM (69.1 mg, 0.1 mmol) was dissolved in CH2Cl2/
C7H8 (1:1; 10 mL), and colorless block crystals were obtained after slow
evaporation of the mixture in air for 24 h (yield: ca. 94%).
TPTM2·PX (3b): The synthesis was similar to that of 3a, except that tolu-
ene was replaced by p-xylene (PX). Colorless block crystals were ob-
tained after slow evaporation for 24 h (yield: ca. 92%).
TPTM2·PE (3c): The synthesis was similar to that of 3a, except that tolu-
ene was replaced by phenylethylene (PE). Colorless block crystals were
obtained after slow evaporation for 24 h (yield: ca. 97%).
TPTM2·THF (3d): The synthesis was similar to that of 3a, except that
toluene was replaced by THF. Colorless block crystals were obtained
after slow evaporation for 24 h (yield: ca. 95%).
TPTM2·C6H12 ACTHNUGTRNEUG(N 3e): The synthesis was similar to that of 3a, except that
toluene was replaced by cyclohexane (C6H12). Colorless block crystals
were obtained after slow evaporation for 24 h (yield: ca. 95%).
The capsule-shaped dimeric unit of form III can accom-
modate a variety of guest molecules from aromatic hydro-
carbons to the inorganic iodine molecule, which all have
proper structures and no obvious self-interactions. In addi-
tion, TGA reveals that the escape of guest molecules en-
closed in the dimers of form III requires very high tempera-
tures, which demonstrates the outstanding inclusion capabili-
ty of TPTM. Consequently, the versatile and fine inclusion
capabilities make this tripodal form III host an attractive
candidate as a molecular flask for storing labile molecules
or a reactor for single-molecule reactions.
TPTM2·I2 ACTHUNGTRENNU(G 3 f): TPTM (69.1 mg, 0.1 mmol) and I2 (126.9 mg, 0.5 mmol)
were dissolved in CH2Cl2 (10 mL). After evaporation to dryness, the re-
sultant solid was washed three times with ethanol to remove residual
iodine, affording crimson block crystals (yield: ca. 88%).
TPTM·C6H6 ACTHNUTRGNE(NUG 4a): The synthesis was similar to that of 3a, except that tol-
uene was replaced by benzene (C6H6). Colorless hexagonal crystals were
obtained after slow evaporation for 24 h (yield: ca. 96%).
TPTM·py (4b): The synthesis was similar to that of 3a, except that tolu-
ene was replaced by pyridine (py). Colorless hexagonal crystals were ob-
tained after slow evaporation for 24 h (yield: ca. 96%).
TPTM·C6H6F (4c): The synthesis was similar to that of 3a, except that
toluene was replaced by fluorobenzene (C6H6F). Colorless hexagonal
crystals were obtained after slow evaporation for 24 h (yield: ca. 93%).
On the other hand, form IV with open channels (for 4a)
exhibited interesting properties of reversible exchange of
toluene and separation of xylene isomers on single crystals,
TPTM·C6H6Cl (4d): The synthesis was similar to that of 3a, except that
toluene was replaced by chlorobenzene (C6H6Cl). Colorless hexagonal
crystals were obtained after slow evaporation for 24 h (yield: ca. 95%).
2196
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Chem. Eur. J. 2011, 17, 2189 – 2198

A Tripodal Host with Cylindrical Conformation
FULL PAPER
TPTM·C6H5Br (4e): The synthesis was similar to that of 3a, except that
toluene was replaced by bromobenzene (C6H5Br). Colorless hexagonal
crystals were obtained after slow evaporation for 48 h (yield: ca. 96%).
responsible for the weak interaction. The charges on guests are also
listed in Table 3.
TPTM·C6H5I (4 f): The synthesis was similar to that of 3a, except that
toluene was replaced by iodobenzene (C6H5I). Colorless hexagonal crys-
tals were obtained after slow evaporation for 48 h (yield: ca. 98%).
Acknowledgements
TPTM·TFT (4g): The synthesis was similar to that of 3a, except that tol-
uene was replaced by a,a,a-trifluorotoluene (TFT). Colorless hexagonal
crystals were obtained after slow evaporation for 48 h (yield: ca. 94%).
This study was supported by the National Natural Science Foundation of
China (Project No. 21001076 and 20801004), the Natural Science Founda-
tion of Fujian Province, and Funding Project for Academic Human Re-
souces Development in Institutions of Higher Learning Under the Juris-
diction of Beijing Municipality (PHR20100718). Special thanks to Dr.
Yanqing Xu for structural measurements and solutions.
TPTM·BN (4h): The synthesis was similar to that of 3a, except that tolu-
ene was replaced by benzonitrile (BN). Colorless hexagonal crystals were
obtained after slow evaporation for one week (yield: ca. 95%).
TPTM·0.5Cp2Fe (4 i): The synthesis was similar to that of 3 f, except that
I2 was replaced by ferrocene (Cp2Fe). Colorless hexagonal crystals were
obtained after slow evaporation for 48 h (yield: ca. 86%).
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TPTM·0.5C7H8 ACHTUNGTRENNUNG(4j): Single crystals of 4a were exposed to toluene vapor
at 308C for about 20 days, and the resultant crystals that retained single
crystallinity were collected (yield: 100%).
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TPTM·0.25PX (4k): Single crystals of 4a were exposed to an equimolar
mixed vapor of p-xylene, m-xylene, and o-xylene at 408C for about
20 days, and the resultant crystals that retained single-crystallinity were
collected (yield: 100%).
X-ray data collection and structure determination: Data for 1, 3a, and
3d were collected by using a Rigaku Mercury CCD-based diffractometer
with graphite-monochromated MoKa radiation (l=0.71073 ꢁ) by using
the w-scan mode at 293 K. All other data were collected by using a
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squares techniques by using the SHELXTL-97 program package.[25] The
ordered atoms in each structure were refined with anisotropic displace-
ment parameters, whereas hydrogen atoms were placed in idealized posi-
tions and allowed to ride on the relevant carbon atoms. Crystallographic
data for 1–4 are summarized in Table 2. CCDC-787266 (1), -787267 (2),
-787268 (3a), -787269 (3b), -787270 (3c), -787271 (3d), -787272 (3 f),
-787273 (4a), -787275 (4d), -787276 (4e), -787277 (4 f), -787278 (4h),
-787279 (4i), -787280 (4k), and -787274 (4a’’) 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.
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Because of the very large thermal motion and disorder of the guests in
the cavity and lattice of host molecules, the diffuse residual electron den-
sity was usually difficult to model accurately and thus a number of re-
straints were applied to these molecules. Therefore, no attempt was made
to locate the hydrogen atoms of guest molecules, and the disordered non-
hydrogen atoms were refined isotropically. Also, large differences in the
Ueq values between neighboring non-hydrogen atoms and some irregular
bond lengths could be ascribed to the mobility of the guests. For com-
pound 3c, some of the non-hydrogen atoms of TPTM were refined iso-
tropically due to the relatively poor quality of the crystal. For compounds
4a, 4a’’, and 4k, there was significant void space and diffuse residual
electron density that could not be accurately assigned in a chemically
meaningful way.
Theoretical calculations: We performed a density functional study to in-
vestigate the interactions between the TPTM host and the guests. The
capsular dimeric unit of two host molecules and one guest molecule were
extracted from the crystal structure of 3a for modeling. The calculation
was carried out by using the PBE[26] density functional at the 6-31G(d)
level implemented in Gaussian 03.[27] The skeleton of the host was fixed
to represent the environment of the crystal. All the structural parameters
of the guests and the hydrogen atoms in the host were optimized to give
the binding energies Eb, listed in Table 3. Most of the binding energies
are negative, that is, most of the interactions between host and guest are
energetically favorable. However, the interactions for m-xylene, o-xylene,
and especially mesitylene are endothermic. There is no noticeable orbital
interaction between host and the guest, and the weak charge transfer is
Chem. Eur. J. 2011, 17, 2189 – 2198
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M. Hong et al.
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Received: August 5, 2010
Published online: January 17, 2011
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