Azacalix[6]arene hexamethyl ether: Synthesis, structure, and selective uptake of carbon dioxide in the solid state

Article abstract of DOI:10.1002/chem.200800502

To investigate dynamic solid-state complexation hitherto unexplored in nitrogen-bridged calixarene analogues, azacalix[6]arene hexamethyl ether has been prepared in three steps by applying a 5+1 fragment-coupling approach by using a Buchwald-Hartwig aryl amination reaction with the aid of our previously devised temporal N-silylation protocol. X-ray crystallographic analysis and NMR spectroscopic measurements have revealed that the azacalix[6]arene is well endowed with hydrogen-bonding ability, by which both the molecular and crystal structures are controlled. The azacalix[6]arene is conformationally flexible in solution on the NMR time scale,whereas it adopts a definite 1,2,3-alternate conformation with S₂ symmetry in the solid state as a result of intramolecular bifurcated hydrogen-bonding interactions. In the crystal, molecules of the azacalix[6]arene are mutually interacted by intermolecular hydrogen bonds to establish one-dimensional hexanefilled nanochannel crystal architecture. Although the single crystal was broken after desolvation, the resultant polycrystalline powder material was capable of selectively adsorbing CO₂ among the four main gaseous components of the atmosphere. In contrast, carbocyclic p-tert-butylcalix[6]arene hexamethyl ether, the crystal structure of which was also elucidated for the first time in the present study, gave rise to almost no uptake of CO₂. Additional solid-gas adsorption experiments for another three gases, such as N₂, O₂, and Ar, suggested that quadrupole/induceddipole interactions and/or hydrogenbonding interactions played an important role in permitting the observed selective uptake of CO₂ by this new azacalix[6]arene in the solid state.

Full text of DOI:10.1002/chem.200800502

FULL PAPER
DOI: 10.1002/chem.200800502
Azacalix[6]arene Hexamethyl Ether: Synthesis, Structure, and Selective
Uptake of Carbon Dioxide in the Solid State
Hirohito Tsue,* Koichi Ishibashi, Satoshi Tokita, Hiroki Takahashi, Kazuhiro Matsui, and
Rui Tamura[a]
Dedicated to Professor Yoshiteru Sakata on the occasion of his 70th birthday
Abstract: To investigate dynamic solid-
state complexation hitherto unexplored
in nitrogen-bridged calixarene ana-
logues, azacalix[6]arene hexamethyl
ether has been prepared in three steps
by applying a 5+1 fragment-coupling
whereas it adopts a definite 1,2,3-alter-
nate conformation with S2 symmetry in
the solid state as a result of intramolec-
ular bifurcated hydrogen-bonding inter-
actions. In the crystal, molecules of the
azacalix[6]arene are mutually interact-
ed by intermolecular hydrogen bonds
to establish one-dimensional hexane-
filled nanochannel crystal architecture.
Although the single crystal was broken
after desolvation, the resultant poly-
crystalline powder material was capa-
ble of selectively adsorbing CO2 among
the four main gaseous components of
the atmosphere. In contrast, carbocy-
clic p-tert-butylcalix[6]arene hexameth-
yl ether, the crystal structure of which
was also elucidated for the first time in
the present study, gave rise to almost
no uptake of CO2. Additional solid–gas
adsorption experiments for another
three gases, such as N2, O2, and Ar,
suggested that quadrupole/induced-
dipole interactions and/or hydrogen-
bonding interactions played an impor-
tant role in permitting the observed se-
lective uptake of CO2 by this new aza-
calix[6]arene in the solid state.
approach by using
a
Buchwald–
Hartwig aryl amination reaction with
the aid of our previously devised tem-
poral N-silylation protocol. X-ray crys-
tallographic analysis and NMR spectro-
scopic measurements have revealed
that the azacalix[6]arene is well endow-
ed with hydrogen-bonding ability, by
which both the molecular and crystal
structures are controlled. The azaca-
lix[6]arene is conformationally flexible
in solution on the NMR time scale,
Keywords: calixarenes · conforma-
tion analysis · structure elucidation ·
host–guest systems
adsorption
·
solid–gas
Introduction
heteroatoms can impart novel properties and functions to
the molecules. In recent years, nitrogen-bridged calixarene
analogues have emerged as a new calixarene family.[7] While
the diversity is still limited relative to those of carbon- and
sulfur-bridged calixarenes, intriguing complexation proper-
ties based on the introduction of nitrogen atoms as the
bridging units have thus far been disclosed.[8–10]
For instance, Wang reported that azacalix[m]arene[n]pyri-
dines exhibited much greater complexation ability for ful-
lerenes C60 and C70, relative to those of other mono-macro-
cyclic receptors reported to date.[8a,b,d] Wang further revealed
that azacalix[4]pyridine was able to self-regulate its confor-
mation to finely tune the cavity, in which guest species, such
as a diol and a zinc ion, were incorporated.[8c,e] Yamamoto
and Kanbara demonstrated that azacalix[n]pyridines be-
haved not only as a ligand for heavy-metal ions but also as
an organic superbase far superior to proton sponge.[9] Very
recently, we reported that azacalix[4]arene 1 exhibited selec-
Since the pioneering and systematic studies by Gutsche, ca-
lixarenes have been playing a significant role in supramolec-
ACHTREUNG
ular chemistry together with crown ethers and cyclodex-
trins.[1–3] As typified by thiacalixarenes, a variety of calixar-
ene analogues with heteroatoms as the bridging units have
thus far been reported.[3–7] Interest in these analogues comes
from the fact that replacement of the carbon bridges with
[a]Dr. H. Tsue, K. Ishibashi, S. Tokita, Dr. H. Takahashi, K. Matsui,
Prof. Dr. R. Tamura
Graduate School of Human and Environmental Studies
Kyoto University, Yoshida-nihonmatsu
Sakyo-ku, Kyoto 606-8501 (Japan)
Fax : (+81)757536722
E-mail: tsue@ger.mbox.media.kyoto-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800502.
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report the synthesis, molecular and crystal structures, and
solid–gas adsorption behavior of azacalix[6]arene 2.
Results and Discussion
Synthesis: For the preparation of azacalix[6]arene 2, three
typical synthetic strategies are applicable that are substan-
tially identical to those established in the carbocyclic calixar-
ene chemistry, that is, 1) single-step synthesis, 2) nonconver-
gent synthesis, and 3) convergent fragment-coupling synthe-
sis.[7] After our numerous attempts to prepare azacalix[6]ar-
ene 2, a convergent 5+1 fragment-coupling approach, as
shown in Scheme 1, was finally devised. The 5-fragment 5
tive complexation with the potassium ion due to its frozen
1,3-alternate conformation.[10] However, complexation be-
haviors of nitrogen-bridged calixarene analogues had been
studied principally in solution, despite the fact that X-ray
crystallographic analyses were performed to examine the
static structures of the complexes. To the best of our knowl-
edge, there is no precedent for the dynamic complexation
studies of nitrogen-bridged calixarene analogues in the solid
state, though Atwood and Barbour recently published their
significant findings that carbocyclic p-tert-butylcalix[n]arenes
(n=4 and 5) were capable of efficiently adsorbing gaseous
molecules even on the nonporous crystals under ambient
conditions,[11] in which the carbon-bridged calixarenes exhib-
ited a greater adsorption ability relative to those of other
new materials, such as metal-organic frameworks (MOF)
and carbon nanotubes.[11i]
Solid-state complexation by nitrogen-bridged calixarene
analogues is of great interest because the bridging nitrogen
atoms can act not merely as additional binding sites, but
also as conjugation sites with aromatic p-systems to increase
the electron density of the p-cloud, thereby boosting inter-
molecular interactions with guest species from a molecular
level, as verified by the above-mentioned complexation
studies in solution. To explore whether solid-state complexa-
tion by nitrogen-bridged calixarene analogues would be fea-
sible or not, we have synthesized azacalix[6]arene hexa-
A
cycle than our previously reported azacalix[4]arene 1[12] is
essential for gaining a deeper insight into the host-guest
chemistry of this molecular system. Among a variety of
solid-state complexation phenomena, solid–gas adsorption
behavior of 2 has been investigated in this study because it
is an old, but yet forefront research area for selective gas
recognition, storage, and separation, coupled with the envi-
ronmental and industrial demands. As an eventual outcome,
selective and rapid adsorption of CO2 among the four main
gaseous components of the atmosphere was observed on the
desolvated polycrystalline powder of 2, which was obtained
from the single crystals of the hexane clathrate of 2 with a
1:1 host/guest ratio. In contrast, azacalix[4]arene 1 and car-
bocyclic calix[6]arene 3 exhibited almost no uptake of gas-
eous molecules examined herein. More interestingly, adsorp-
tion capacity of the polycrystalline powder of 2 for CO2 was
comparable to, or somewhat poorer than, those of MOFs[13]
and zeolites,[14] which similarly exhibited the selective ad-
sorption of CO2. It has also been found from the X-ray crys-
tallographic analysis and NMR spectroscopic measurements
that azacalix[6]arene 2 with NH bridges is well endowed
with hydrogen-bonding ability, by which both the molecular
and crystal structures of 2 are controlled. In this paper, we
Scheme 1. Convergent 5+1 fragment-coupling synthesis of azacalix[6]ar-
ene 2. dba=E,E-dibenzylideneacetone.
was prepared in 92% yield by the full N-benzylation of liner
pentamer 4.[15] To a ring-closing reaction of 5-fragment 5
with 1-fragment 6,[16] a Buchwald–Hartwig aryl amination
reaction[17] with a temporal N-silylation protocol[18] was ap-
plied, which was demonstrated to be expedient for prepar-
ing the azacalixarene with no substituent on the bridging ni-
trogen atoms. From this macrocyclization reaction, regiose-
lectively tetrabenzylated azacalix[6]arene 7 was successfully
obtained in 32% yield through the in-situ N,N’-bissilylation
of 1-fragment
6
with tert-butyldimethylsilyl chloride
(TBDMSCl) in the presence of tBuONa, followed by the
one-pot Pd0-catalyzed aryl amination reaction with the 5-
ꢀ
fragment 5 and by the subsequent cleavage of the N Si
bond in the workup process. Final hydrogenolysis of the N-
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Azacalix[6]arene Hexamethyl Ether
FULL PAPER
Table 1. X-ray analytical data for azacalix[6]arene 2, calix[6]arene 3, and
linear trimer 8.
benzyl groups of 7 smoothly proceeded by using 10%
Pd(OH)2/C as a catalyst to give azacalix[6]arene 2 in 88%
1
yield. FDMS, H and 13C NMR spectroscopy, FTIR spectros-
Compound
2·hexane
3
8
copy, elemental analysis, and X-ray crystallographic analysis
fully characterized the structure of this new azacalix[6]arene
2.
formula
Mr
C72H104N6O6
1149.65
monoclinic
P21/a (#14)
10.6734(5)
11.9422(7)
14.1011(7)
82.442(2)
73.837(1)
70.986(2)
1630.5(2)
2
C72H96O6
1057.55
triclinic
C33H44Br2N2O3
676.53
monoclinic
C2/c (#15)
crystal system
space group
a [Š]13.7437(5)
b [Š]18.6121(8)
c [Š]15.4552(8)
a [8]–
¯
P1 (#2)
26.027(9)
9.851(3)
1
Molecular structure in solution: In the H NMR spectrum of
25.329(9)
azacalix[6]arene 2 (Figure 1a), only four sharp singlet signals
were observed at d=1.27, 3.26, 6.20, and 6.87 ppm for the
–
b [8]111.850(1)
g [8]–
92.05(3)
–
6490(3)
1
V [Š3]3669.4(3)
Z
8
1 [gcmꢀ3]1.040
m [cmꢀ1]0.656
T [8C]-140
no. of unique reflns
no. of variables
R1[a]
1.077
0.663
-140
8346
488
0.080
0.275
1.134
1.385
25.40
-100
6892
380
0.067
0.240
1.090
7424
362
0.045
0.130
1.044
2
wR2[b]
S
[a] R1=SjjFo jꢀjFc jj/SjFo j for I>2s(I) data. [b] wR2=[S[w
(Fo ꢀFc2)2]/
2
Sw
A
.
ternate conformation in the solid state, as shown in Figure 2.
Its ellipsoidal shape with S2 symmetry is in a striking con-
trast to the parallelogram-shaped 1,2,3-alternate conforma-
Figure 1. Temperature-dependent 1H NMR spectra of azacalix[6]arene 2
in CDCl3 at a) 25, b) ꢀ20, and c) ꢀ508C. Peaks marked with an asterisk
are due to water.
tert-butyl, methoxy, amino, and aromatic hydrogen atoms,
respectively. NH hydrogen atoms of 2 at d=6.20 ppm expe-
rience a downfield shift relative to that of diphenylamine at
d=5.6 ppm, which indicates that the NH hydrogen atoms
form hydrogen bonds with spatially adjacent methoxy
groups, as revealed by the X-ray crystallographic analysis
mentioned in the following section. The NMR spectroscopic
signals of 2 became broad when the temperature was de-
creased to ꢀ208C (Figure 1b), but no signal splitting was ob-
served even at ꢀ508C (Figure 1c), which indicates that aza-
calix[6]arene 2 was conformationally flexible in solution.
The larger ring size of 2 is responsible for its flexibility, in
contrast to azacalix[4]arene 1, which is demonstrated to be
conformationally inflexible in solution in the range of ꢀ80
to 808C.[10]
Figure 2. ORTEP drawing[20] of the hexane clathrate of azacalix[6]arene
2. The displacement ellipsoids are drawn at the 50% probability level.
Solvent molecules and all hydrogen atoms, except for the bridging NH
groups are omitted for clarity. Numbers indicate O···H distances [Š]of
intramolecular NH···OMe hydrogen bonds.
tion of the carbocyclic analogue 3 (Figure 3), of which the
crystal structure has also been elucidated for the first time
in the present study. Interestingly, the centrosymmetric
1,2,3-alternate conformation of 2 differs greatly from the
highly distorted 1,3-alternate conformation of the fully N-
methylated derivative of 2 (see Figure S2 in the Supporting
Information).[19]
The asymmetric unit of azacalix[6]arene 2 involves half
the molecule, and thus the aromatic units can be classified
into three types, designated as A, B, and C (Figure 2). These
Crystal structure: A single crystal suitable for X-ray crystal-
lographic analysis was obtained by slow crystallization from
hexane with a few drops of CH2Cl2. Azacalix[6]arene 2 crys-
tallizes with one molecule of hexane into a monoclinic form,
space group P21/a (Z=2); the fundamental crystal data and
experimental parameters for the structure determination are
summarized in Table 1. Contrary to the flexible conforma-
tion in solution, azacalix[6]arene 2 adopts a definite 1,2,3-al-
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H. Tsue et al.
Figure 3. ORTEP drawing[20] of calix[6]arene 3 with 50% displacement
ellipsoids. Hydrogen atoms are omitted for clarity.
Figure 5. Crystal structure of the hexane clathrate of azacalix[6]arene 2
viewed along the c axis. Molecules of azacalix[6]arene 2 and hexane are
represented by stick and space-filling models, respectively.
three aromatic rings point to almost the same direction due
to the presence of the two sets of intramolecular bifurcated
MeO···NH···OMe hydrogen bonds formed between the aro-
matic rings A and B, and between the rings B and C. As
shown in Figure 4, the same type of intramolecular bifurcat-
azacalix[6]arene 2 interacts with adjacent molecules by in-
termolecular NH/O hydrogen bonds (NH···O, 2.07 Š and
1738; N···O, 3.04 Š; see Figure 6 and Figure S4 in the Sup-
porting Information) formed between the bridging nitrogen
atoms N(3) and the methoxy groups of the aromatic ring B
belonging to the neighboring molecule. Propagation of the
intermolecular NH/O hydrogen-bonding interactions along
¯
the crystallographic [1 1 0]and [1 1 0]directions results in
the formation of a two-dimensional (2D) herringbone ar-
rangement of 2 on the ab plane, as shown in Figure 6 and
Figure S4 in the Supporting Information. In the 2D sheet
structure, each hydrogen-bonded assembly comprising of
four molecules of azacalix[6]arene 2 creates an interstitial
pore (ca. 200 Š3), in which one molecule of hexane with an
all-anti conformation is embedded by weak intermolecular
CH/O interactions (CH···O, 2.98 Š and 1498; C···O, 3.85 Š;
Figure 7) between the methylene group of hexane and the
methoxy group of the ring B. As a result, the hexane-filled
2D sheet structure is stacked along the c axis to form the
1D nanochannel crystal architecture of the hexane clathrate
of 2 with a 1:1 host-guest ratio, as shown in Figure 5 and
Figure S3 in the Supporting Information.
Figure 4. ORTEP drawing[20] of linear trimer 8 with 50% displacement el-
lipsoids. All hydrogen atoms, except for the bridging NH groups are
omitted for clarity. Numbers indicate O···H distances [Š]of intramolecu-
lar NH···OMe hydrogen bonds.
ed MeO···NH···OMe hydrogen bonds were observed for
linear trimer 8,[12] which adopted the almost coplanar ar-
rangement of the three aromatic rings. Accordingly, azaca-
Preparation of desolvated crystal: Prior to the preparation
of desolvated crystal, thermogravimetric analysis was carried
out to examine the thermal stability of the hexane clathrate
of 2. As shown in Figure 8, upon heating the single crystals
of the hexane clathrate, the weight of the crystals was gradu-
ally decreased to reach a constant value above 1408C. The
observed overall weight change of 7.4% was equivalent to
the loss of one molecule of hexane, which indicated the
complete escape of the molecules of hexane from the single
crystals of 2. Desolvation was also feasible by heating the
single crystals of the hexane clathrate of 2 at 608C for 13 h
under reduced pressure. After desolvation, however, the
hexane clathrate lost the single crystallinity and turned to
colorless polycrystalline powder 2P. The letter P is used
below to represent the solvent-free polycrystalline powder
lix[6]arene 2 can be viewed as a N(3)-bridged head-to-tail
dimer of two planar trimer units 8. NH hydrogen atoms of
the N(3) atoms of 2 are directed outward from the cavity
and take part in the formation of intermolecular hydrogen
bonds.
Another interesting point to note is the crystal packing
characterized by an infinite one-dimensional (1D) solvent-
filled nanochannel structure, as shown in Figure 5 and
Figure S3 in the Supporting Information. In the crystal, each
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Azacalix[6]arene Hexamethyl Ether
FULL PAPER
Figure 7. ORTEP drawing[20] of the hexane clathrate of azacalix[6]arene 2
with 50% displacement ellipsoids. All hydrogen atoms, except for hexane
and the bridging NH groups are omitted for clarity. Number indicates the
distance of the CH/O interaction between hexane and the methoxy
group of the ring B.
Figure 8. Thermogravimetric analysis of the hexane clathrate of azaca-
lix[6]arene 2 recorded at 58C minꢀ1
.
data of the hexane clathrate of 2 (Figure 9b) and also from
the imaginary crystal structure simply created by deleting
the atomic coordinates of hexane from the crystallographic
data of the hexane clathrate (Figure 9c). Although the crys-
tal structure of the polycrystalline powder 2P could not be
solved from the XRD pattern at this moment, FTIR mea-
surement of 2P revealed no appreciable change in the NH-
stretching, NH-bending, and CN-stretching bands, even after
the loss of single crystallinity (Figure 10), which suggested
that the hydrogen bonding network observed in the single
crystal (Figure 6 and Figure S4 in the Supporting Informa-
tion) remained almost unchanged in polycrystalline powder
2P. From these experimental facts, it is conceivable that,
upon heating the single crystals of the hexane clathrate of 2,
molecules of hexane escape through the 1D nanochannel
almost keeping the inter- and intramolecular hydrogen-
bonding interactions, though azacalix[6]arene 2 should alter
its 1,2,3-alternate conformation to some extent and/or un-
dergo slight translations in the solid state.
Figure 6. a) Ball-and-stick and b) schematic representations of the two-di-
mensional sheet structure of the hexane clathrate of azacalix[6]arene 2
on the ab plane. In panel (a), the carbon atoms are depicted by light-gray
circles, and gray circles stand for the nitrogen and oxygen atoms. In pan-
els (a) and (b), intermolecular NH···OMe hydrogen bonds are illustrated
by dotted lines.
material obtained from the single crystals of the hexane
clathrates of 2.
The powder XRD pattern of 2P (Figure 9a and Figure S1
in the Supporting Information) displays a similarity to the
XRD patterns simulated from the X-ray crystallographic
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H. Tsue et al.
Figure 10. FTIR spectra of a) polycrystalline powder 2P and b) the single
crystals of the hexane clathrate of azacalix[6]arene 2.
Figure 9. a) Powder XRD pattern of polycrystalline powder 2P. b) XRD
pattern simulated from the X-ray crystallographic data of the hexane
clathrates of 2. c) XRD pattern simulated from the imaginary crystal
structure simply created by deleting the atomic coordinates of hexane
from the crystallographic data of the hexane clathrate of 2.
Solid–gasadosrption behavior : To gain an insight into the
ability of polycrystalline powder 2P to function as a nano-
porous material, adsorption experiments for gaseous mole-
cules were carried out. According to the described proce-
dure,[11c] adsorption isotherms for the four main atmospheric
components, such as N2, O2, Ar, and CO2 were recorded at
room temperature and 195 K on desolvated colorless poly-
crystalline powder 2P, which was prepared by heating the
single crystals of the hexane clathrate of 2 at 608C for 13 h
under reduced pressure. The initial pressure was set to ca.
100 kPa in all adsorption experiments. At room temperature
(Figure 11a and Figure S5a in the Supporting Information),
no uptake was observed for all of the examined gases except
for CO2, which was slightly adsorbed on polycrystalline
Figure 11. Gas adsorption isotherms recorded at a) room temperature
and b) 195 K for CO2, N2, O2, and Ar by using polycrystalline powder 2P
as an adsorbent. Isotherms for N2, O2, Ar, and CO2 are shown by g,
a, d, and c, respectively.
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Azacalix[6]arene Hexamethyl Ether
FULL PAPER
powder 2P. In contrast, CO2 was rapidly and selectively ad-
sorbed on 2P at 195 K (Figure 11b and Figure S5b in the
Supporting Information), and the initial pressure of ca.
100 kPa was decreased within 10 min to reach equilibrium at
65 kPa, which corresponded to the uptake of ca. 1.8 mol of
CO2 per mol of 2 (i.e. 37 cm3 gꢀ1 at standard temperature
and pressure). It is interesting to note that the observed ad-
sorption capacity of 2P for CO2 is comparable to, or some-
what poorer than, those of MOFs[13] and zeolites[14] similarly
capable of selectively adsorbing CO2. On the whole, the ob-
served gas adsorption behavior of 2P at 195 K was correlat-
ed fairly well with the molecular sieve effect[21] based on the
difference in the kinetic diameters of the examined gases,
such as N2 (3.64 Š), O2 (3.46 Š), Ar (3.40 Š), and CO2
(3.3 Š).
As a control, adsorption experiments were performed for
azacalix[4]arene
1 and carbocyclic calix[6]arene 3, the
former of which was densely packed in the crystal solely by
intermolecular CH/p interactions to form a nonporous crys-
tal structure with no solvent molecule in the lattice.[12] As
shown in Figure 12 and Figure S6 in the Supporting Infor-
mation, carbocyclic calix[6]arene 3 also retains a nonporous
and nonsolvated crystal structure, which is characterized by
a 1D chain structure established by intermolecular CH/O in-
teractions (CH···O, 2.61 Š and 1268; C···O, 3.28 Š) between
the methoxy group of the aromatic ring C and that of ring B
belonging to the nearest molecules. As anticipated from
their densely-packed nonporous crystal structures, both aza-
calix[4]arene 1 and calix[6]arene 3 gave rise to almost no
uptake of CO2 even at 195 K, as shown in Figure 13 and
Figure S7 in the Supporting Information. Although gas sorp-
tion was reported to happen even by nonporous materi-
als,[11,22] the absence of interstitial void space enough to ac-
commodate gaseous molecules in the crystal lattices of 1
and 3 would be responsible for the observed non-uptake of
CO2 by these crystalline powder materials. In other words,
an infinite 1D nanochannel crystal structure observed in the
single crystal of the hexane clathrate of 2 would be retained,
though presumably imperfectly, in the desolvated polycrys-
talline powder 2P, as suggested by the XRD and FTIR
measurements (Figures 9 and 10).
Figure 12. a) Ball-and-stick and b) schematic representations of the one-
dimensional chain structure of calix[6]arene
3 along the b axis. In
panel (a), the carbon atoms are depicted by light-gray circles, and gray
circles stand for the oxygen atoms. In panels (a) and (b), intermolecular
CH/O interactions are illustrated by dottted lines.
Selective and rapid uptake of CO2 by polycrystalline
powder 2P at 195 K can be explained by considering 1) hy-
drogen-bonding interactions and/or 2) quadrupole/induced-
dipole interactions between 2 and CO2, in addition to the
molecular sieve effects mentioned above. The desolvation-
induced disintegration of the single crystals of the hexane
clathrate of 2 into polycrystalline powder 2P would break a
portion of the NH hydrogen bonds and change them into
dangling bonds, thereby leading to the formation of hydro-
gen bonds with CO2 molecules diffused into the 1D nano-
channel of 2P. This is parallel to the experimental fact that
azacalix[4]arene 1 and calix[6]arene 3 lead to almost no
uptake of CO2 (Figure 13 and Figure S7 in the Supporting
Information) because of the lack of hydrogen bonding sites,
together with their densely-packed crystal structures. As an
additional factor, quadrupole/induced-dipole interactions
between CO2 and the surrounding aromatic “walls” built
along the 1D nanochannel of 2P would also be involved in
the selective adsorption of CO2, judging from the greater
quadrupole moment (absolute value)[23] of CO2 (13.4”
1040 Cm2) than the other examined gases, such as N2 (4.7”
1040), O2 (1.3”1040), and Ar (0). This intermolecular interac-
tion must also contribute to the gas adsorption behavior of
carbocyclic calix[6]arene 3, which exhibits the slight uptake
of CO2 at 195 K (Figure 13b and Figure S7b in the Support-
ing Information). As a result, it is conceivable that hydro-
gen-bonding interactions and/or quadrupole/induced-dipole
interactions are responsible for the observed selective and
rapid uptake of CO2 by 2P. Nevertheless, these intermolecu-
lar interactions are insufficient to efficiently entrap and hold
CO2 molecules in the nanochannel space of 2P at ambient
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H. Tsue et al.
pole/induced-dipole interactions between 2 and CO2 in the
1D nanochannel of 2P. As an eventual outcome, the present
study has clearly demonstrated that the selective solid-state
complexation of gaseous CO2 by a nitrogen-bridged calixar-
ene analogue is feasible.
Because azacalix[6]arene 2 possesses electron-rich p-sys-
tems and the striking hydrogen-bonding ability to act as
both donor and acceptor, there appears a fair prospect that
azacalix[6]arene 2 may exhibit a greater gas adsorption ca-
pacity with the aid of crystal engineering. This legitimate an-
ticipation strongly compels us to further study the solid–gas
adsorption behavior of 2 and its homologues with smaller
and larger ring sizes. Investigations along this line are cur-
rently under progress to gain a deeper insight into the dy-
namic solid-state complexation of this molecular system.
Experimental Section
General: Melting points were determined on a Yanaco MP-J3 apparatus
and are uncorrected. NMR spectra were recorded on a JEOL JNM-A500
instrument by using tetramethylsilane (1H NMR spectra) and solvent res-
onance (13C NMR spectra) as internal standards. IR spectra were ob-
tained on a Shimadzu IRPrestige-21 spectrometer. Mass spectra were re-
corded at the GCMS and NMR Laboratory, Faculty of Agriculture, Hok-
kaido University (Japan). Elemental analyses were conducted at the Mi-
croanalytical Center, Kyoto University (Japan). Thermogravimetric anal-
ysis was performed with a Shimadzu TGA-50 instrument at the scanning
rate of 58Cminꢀ1. Powder XRD patterns were recorded by a Rigaku
RINT 2200 diffractometer by using CuKa radiation (l=1.54056 Š, 40 kV,
40 mA) with a graphite monochrometor at a step width of 0.018 2q and a
counting time of 2 sstepꢀ1. Flash column chromatography was performed
with Kanto Silica Gel 60N (40–50 mm). Toluene, CH2Cl2, and DMF were
refluxed over and then distilled from CaH2 under Ar before use. Palladi-
Figure 13. Gas adsorption isotherms recorded at 195 K for CO2, N2, O2,
and Ar by using crystalline powder materials of a) azacalix[4]arene 1 and
b) carbocyclic calix[6]arene 3 as adsorbents. Isotherms for N2, O2, Ar,
and CO2 are shown by g, a, d, and c, respectively.
temperature, as can be seen from the experimental results
shown in Figure 11 and Figure S5 in the Supporting Infor-
mation.
um bis(benzylideneacetone) ([Pd
(dba)2]),[24] calix[6]arene 3,[25] and com-
G
pounds 4[15], 6[16], and 8[12] were prepared according to the described pro-
cedure. Other chemicals were purchased from commercial suppliers and
used as received.
Conclusions
N,N’-Bis[3-(N’’-benzyl-3-bromo-5-tert-butyl-2-methoxyanilino)-5-tert-
butyl-2-methoxyphenyl]-N,N’-dibenzyl-5-tert-butyl-2-methoxy-1,3-phenyl-
enediamine (5): 60% NaH (481 mg, 12.0 mmol) was added to a solution
of 4[15] (2.06 g, 2.00 mmol) in anhydrous DMF (40 mL) at 08C under Ar.
After the reaction mixture had been stirred for 5 min, BnBr (1.1 mL,
9.0 mmol) was added. Stirring was continued at room temperature over-
night, and then Et2O was added to the reaction mixture. The organic
layer was washed with brine, dried over Na2SO4, filtered, and evaporated.
Flash column chromatography on silica gel (hexane/CH2Cl2 3:1) and sub-
sequent precipitation from Et2O/MeOH gave 5 (2.56 g, 92%) as a pale-
pink solid. M.p. 103–1058C; 1H NMR (500 MHz, CDCl3): d=7.28–7.08
The present study has demonstrated that azacalix[6]arene 2
can be prepared in a simple and efficient manner by apply-
ing a Pd0-catalyzed aryl amination reaction by using our pre-
viously devised temporal N-silylation protocol. X-ray crys-
tallographic analysis and NMR spectroscopic measurements
have clearly revealed that azacalix[6]arene 2 with NH
bridges is well endowed with hydrogen-bonding ability,
which controls both the molecular and crystal structures of
2. In fact, NH hydrogen atoms of 2 experience a downfield
(m, 20H; ArH), 7.06 (d, 4J
2.3 Hz, 2H; ArH), 6.70 (d, 4J
ArH), 6.59 (d, 4J
(H,H)=2.3 Hz, 2H; ArH), 4.81 (s, 4H; NCH2Ph), 4.80
A
N
AHCTREUNG
1
shift in the H NMR spectrum because of the presence of
ACHTREUNG
(s, 4H; NCH2Ph), 3.461 (s, 3H; OMe), 3.457 (s, 6H; OMe), 3.41 (s, 6H;
OMe), 1.11 (s, 18H; tBu), 1.03 (s, 18H; tBu), 1.02 ppm (s, 9H; tBu);
13C NMR (125 MHz, CDCl3): d=148.1, 147.4, 145.7, 145.23, 145.21, 145.0,
143.2, 142.7, 142.6, 139.4, 128.04, 127.99, 127.97, 127.9, 126.5, 126.2, 123.2,
119.9, 117.5, 117.1, 117.0, 116.3, 59.9, 59.13, 59.10, 56.9, 56.7, 34.4, 34.28,
34.27, 31.14, 31.13, 31.08 ppm; elemental analysis calcd (%) for
C83H98Br2N4O5: C 71.64, H 7.10, N 4.03; found: C 71.72, H 7.09, N 3.99.
the intramolecular hydrogen-bonding interaction, by which
azacalix[6]arene 2 adopts a 1,2,3-alternate conformation
with S2 symmetry in the solid state. In the crystal, molecules
of 2 are mutually interacted by intermolecular hydrogen
bonds to establish the infinite 1D nanochannel crystal archi-
tecture of the hexane clathrate with a 1:1 host/guest ratio.
After desolvation, the resultant polycrystalline powder ma-
terial 2P was capable of selectively and rapidly adsorbing
CO2 among the four main components of the atmosphere,
as a result of hydrogen-bonding interactions and/or quadru-
N,N’,N’’,N’’’-Tetrabenzyl-5,11,17,23,29,35-hexa-tert-butyl-
37,38,39,40,41,42-hexamethoxy-2,8,14,20,26,32-hexaazacalix[6]arene (7):
A solution of 5 (696 mg, 500 mmol), 6[16] (97.2 mg, 500 mmol), tBuONa
(240 mg, 2.50 mmol), and TBDMSCl (151 mg, 1.00 mmol) in anhydrous
toluene (100 mL) was stirred at 808C under Ar. After the reaction mix-
6132
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Chem. Eur. J. 2008, 14, 6125 – 6134

Azacalix[6]arene Hexamethyl Ether
FULL PAPER
ture had been stirred for 30 min, [Pd(dba)2](57.4 mg, 100 mmol) and
A
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10 wt% tBu3P in hexane (240 mL, 80 mmol) were added. The solution was
refluxed for 22 h and then cooled to room temperature. The reaction
mixture was passed through Celite and evaporated. Flash column chro-
matography on silica gel (hexane/CH2Cl2 1:1) and subsequent washing
with Et2O/MeOH gave 7 (227 mg, 32%) as a colorless solid. M.p. 256–
2578C (dec.); 1H NMR (500 MHz, CDCl3): d=7.36 (d, 3J
4H; ArH), 7.29 (d, 3J
(H,H)=7.3 Hz, 4H; ArH), 7.24–7.21 (m, 6H;
(H,H)=7.3 Hz,
C
ArH), 7.16–7.15 (m, 6H; ArH), 7.11 (brs, 2H; ArH), 7.07 (brs, 2H;
ArH), 6.97 (brs, 2H; ArH), 6.79 (brs, 2H; ArH), 6.54 (brs, 2H; ArH),
6.35 (brs, 2H; NH), 5.19–4.77 (m, 8H; NCH2Ph), 3.70 (brs, 3H; OMe),
3.28 (brs, 6H; OMe), 2.60 (brs, 6H; OMe), 2.24 (brs, 3H; OMe), 1.32
(brs, 9H; tBu), 1.30 (brs, 18H; tBu), 1.15 (s, 18H; tBu), 1.12 ppm (s, 9H;
tBu); 13C NMR (125 MHz, CDCl3): d=149.6, 146.5, 145.7, 145.5, 145.0,
142.0, 140.0, 139.6, 138.9, 138.2, 137.9, 136.5, 130.0, 128.4, 127.8, 127.6,
126.8, 126.5, 121.2, 117.2, 113.7, 112.5, 109.5, 100.9, 60.3, 60.0, 59.6, 59.5,
59.2, 58.9, 34.58, 34.56, 34.52, 34.4, 31.5, 31.40, 31.36, 31.1 ppm; IR (KBr):
3361 cmꢀ1 (nNH); elemental analysis calcd (%) for C94H114N6O6: C 79.29,
H 8.07, N 5.90; found: C 79.54, H 8.23, N 5.80.
5,11,17,23,29,35-Hexa-tert-butyl-37,38,39,40,41,42-hexamethoxy-2,8,14,20,
6]arene (2): A solution of 7 (427 mg, 300 mmol) and 10% Pd(OH)2/C
(204 mg) in cyclohexane (50 mL) was stirred at room temperature under
H2 (3.9 atm). After the reaction mixture had been stirred for 31 h, addi-
tional 10% Pd(OH)2/C (200 mg) was added, and then stirring was contin-
ued for another 14 h under H2 (4.0 atm). The reaction mixture was
passed through Celite and evaporated. Flash column chromatography on
silica gel (hexane/CH2Cl2 1:1 then 1:2) and recrystallization from CH2Cl2/
hexane gave 2 (280 mg, 88%) as a colorless solid. M.p. 295–2968C (dec.);
1H NMR (500 MHz, CDCl3): d=6.87 (s, 12H; ArH), 6.20 (s, 6H; NH),
3.26 (s, 18H; OMe), 1.27 ppm (s, 54H; tBu); 13C NMR (125 MHz,
CDCl3): d=146.5, 139.5, 137.6, 110.4, 59.6, 34.5, 31.4 ppm; IR (KBr)
3358, 3406 cmꢀ1 (nNH); MS (FD): m/z (%): 1063 (100) [M+], 531 (9) [M2+
], 354 (2) [M3+]; elemental analysis calcd (%) for C66H90N6O6: C 74.54,
H 8.53, N 7.90; found: C 74.33, H 8.58, N 7.83.
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Gasadosrption experiments : By reference to the literature of Atwood
and Barbour,[11c] essentially the same devise was constructed to record
solid–gas absorption isotherms of
1 (10.4 mg), 2P (19.7 mg), and 3
(19.4 mg) at room temperature and 195 K. The powder material was
placed in a sample chamber, and then both sample and reference cham-
bers (each volume=2.0 cm3) were evacuated at 1 kPa for 1 h. After the
chambers were pressurized to ca. 100 kPa by the desired gas, their inter-
nal pressures were monitored by using two pressure transducers (Swage-
lok PTI-S-MC.15–15AQ-A) attached to each chamber. Adsorption equi-
librium was reached within 20 min, and the gas adsorption capacity was
estimated from the changes in pressure.
X-ray crystallographic analyses: The X-ray data were collected on a
Rigaku RAXIS RAPID diffractometer with graphite-monochromated
MoKa radiation (l=0.71075 Š) to 2qmax of 54.98 for 2·hexane and of 55.08
for 3 and 8. All the crystallographic calculations were performed by
using
a crystallographic software package, CrystalStructure, Version
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by full-matrix least-squares. All non-hydrogen atoms were refined aniso-
tropically, and hydrogen atoms were refined by using the riding model.
The fundamental crystal data and experimental parameters for the struc-
ture determination are given in Table 1. CCDC-681115, 681116, and
681117 contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystal-
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research (no.
19550037 to H.T.) from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan. We are grateful to the GCMS and NMR
Laboratory, Faculty of Agriculture, Hokkaido University for FD MS
measurements.
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Received: March 18, 2008
Published online: May 28, 2008
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