Supramolecular features of calixarene-based synthetic nanotubes

Article abstract of DOI:10.1002/anie.200500057

(Figure Presented) Even better than the real thing? As host molecules, long synthetic nanotubes may be reasonable alternatives to single-walled nanotubes. For example, calixarene-based nanotubes effectively pack into infinite tubular bundles in the solid

Full text of DOI:10.1002/anie.200500057

Angewandte
Chemie
Host–Guest Chemistry
Supramolecular Features of Calixarene-Based
Synthetic Nanotubes**
Voltaire G. Organo, Alexander V. Leontiev,
Valentina Sgarlata, H. V. Rasika Dias,* and
Dmitry M. Rudkevich*
Filling single-walled carbon nanotubes (SWNTs) with foreign
species is a quickly emerging research area.[1] The major goal
is to enforce filling materials to adopt one-dimensional
morphology for nanowiring, transport, and information
flow. Other potential applications include the use of SWNTs
[*] V. G. Organo, Dr. A. V. Leontiev, Dr. V. Sgarlata, Prof. Dr. H. V. R. Dias,
Prof. Dr. D. M. Rudkevich
Department of Chemistry & Biochemistry
The University of Texas at Arlington
Arlington, TX 76019-0065 (USA)
Fax: (+1)817-272-3808
E-mail: dias@uta.edu
rudkevich@uta.edu
[**] V.G.O. and A.V.L. contributed equally to this work. Special thanks to
Dr. Iris Thondorf for preliminary calculations. Financial support
from the National Science Foundation (awards CHE-0350958 to
D.M.R and CHE-0314666 to H.V.R.D.), the Alfred P. Sloan
Foundation (D.M.R.), and the Robert A. Welch Foundation (grant Y-
1289 to H.V.R.D.) is acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 3043 –3047
DOI: 10.1002/anie.200500057
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3043

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as reaction vessels and gas-storage cyl-
inders.[2] However, far-from-trivial pro-
tocols for the chemical opening of
SWNTs, still not fully understood mech-
anisms of their filling, as well as identi-
fication of the encapsulated material are
some of the problems faced in this area.
Physical measurements within the inte-
riors are also a challenge. Synthetic
analogues of SWNTs have recently
been introduced.[3,4] Organic synthesis
offers variety of the sizes and shapes.
However, most of these synthetic nano-
tubes are formed through self-assembly
and, thus, are stable only under specific
conditions. Furthermore, the stability of
their encapsulation complexes is gener-
ally weak, which diminishes their capa-
bilities as storage and transport devices
and materials.
Herein, we demonstrate novel and
unique features of synthetic, calixarene-
based nanotubes. These nanotubes are
covalently built and robust and can be
prepared by conventional organic
chemistry protocols. The length of the
nanotubes can be controlled precisely
and easily, and they effectively pack into
infinite tubular bundles in the solid
state. These nanotubes can also be
easily filled and form kinetically and
thermodynamically stable encapsula-
tion complexes, and they can be emp-
tied at will in a nondestructive manner.
Taken together, we propose reasonable
synthetic alternatives to SWNTs for
filling.
Figure 1. Left: Synthetic calixarene-based nanotubes 1a,b for NO2/N2O4 fixation and nitroso-
nium ion (NO+) storage. Right: X-ray crystal structures of 1a,b (from CHCl3/MeOH; side and
top views; O red, C gray). Hydrogen atoms are omitted for clarity.
Earlier we reported that simple calix[4]arenes reversibly
interact with NO2/N2O4 and entrap reactive nitrosonium
(NO+) cations—NO+ is generated upon disproportionation of
N2O4—within their p-electron-rich interiors, one per cavity.[5]
Very high association constants Ka @ 106 mÀ1 (DG295 @ 8 kcal
molÀ1) for these processes were determined, and the com-
plexes were kinetically stable. These particular properties
have been used in the design of calixarene-based nanotubes
1a,b (Figure 1).
and NOESY experiments, mass spectrometry, and X-ray
crystallography.[7] X-ray data show that in nanotubes 1a and
1b, 1,3-alternate calix[4]arenes are rigidly linked with dieth-
ylene glycol bridges to form hollow cylinders with cavities of
diameters of approximately 6 ꢀ and lengths of 17 and 26 ꢀ,
respectively (Figure 1). The inner tunnels are defined by two
cofacial pairs of aromatic rings oriented orthogonally along
the cavity axis.
Nanotubes 1a,b can be easily filled, as each calixarene
unit can accommodate one NO+ guest. Addition of excess
NO2/N2O4 to 1a,b in tetrachloroethane in the presence of
Lewis acids such as SnCl4 or BF3·Et2O resulted in quantitative
formation of nitrosonium complexes 6a,b (Figure 2). Similar
complexes formed when nanotubes 1a,b were mixed with
The synthesis of tubes 1a,b is modular (see Scheme 1 and
Supporting Information). Calixarene building blocks were
linked by simple, Williamson-type alkylation procedures.
Alkylation of calix[4]arene diol 2a (R = nPr) with ditosylate
3
resulted in tube 1a.[6] Reaction of diol 2b (R =
nitrosonium salt NO+SbF6 in tetrachloroethane. Adducts
À
CH2CH2OBn) with 3 led to the formation of biscalixarene
tube 4, which has two terminal hydroxyl groups at one end.
When 4 was coupled with another equivalent of ditosylate 3,
triscalixarene nanotube 1b was isolated in 26% yield.
Alternatively, nanotube 1b could be isolated in modest
yield ( ꢀ 10%) from the one-step reaction of two equivalents
of 3a with tetrakisalcohol 5. Nanotubes 1a,b were charac-
1
6a,b were identified by UV/Vis and H NMR spectroscopy
and exhibit typical features of earlier described, simpler
calix[4]arene–NO+ species.[5,8] Of particular importance is the
characteristic deep purple color. The broad charge-transfer
absorption bands that account for the color are observed at
lmax ꢀ 550 nm in the UV/Vis spectra. Charge transfer only
occurs when NO+ guests are tightly entrapped inside the
1
terized by high-resolution H NMR spectroscopy and COSY
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Scheme 1. a) NaH, DMF, 808C, 48 h, 15–20%; b) NaH, THF, reflux, 24 h, 32%; c) H2, 10% Pd/C, THF/AcOH, 82%; d) NaH, THF, reflux, 24 h,
26%; e) NaH, THF, reflux, 3 days, 7%. Ts=p-toluenesulfonyl; Bn=benzyl; DMF=N,N-dimethylformamide.
and location of the guests inside nanotubes 6a,b can be
deducted from conventional NMR analysis. Structural frag-
ments that are involved in the complexation process were
identified by 1H NMR spectroscopy, COSY, and NOESY
experiments. Although entrapped NO+ species cannot be
directly seen, chemical shifts of the methylene protons from
Ar-O-CH2 and, to a lesser extent, Ar-O-CH2CH2 and the
aromatic protons are very sensitive to the encapsulation.
Indeed, besides the charge transfer, strong cation–dipole
interactions between the oxygen atoms of the calixarenes and
the entrapped NO+ take place.
The three signals for the protons for the propyloxy groups
in 6a,b appeared significantly downfield (Dd ꢀ 1 ppm) rela-
tive to empty tubes 1a,b (Figure 2). This implies that two NO+
cations are located at the ends of the nanotubes and occupy
the terminal calixarene compartments. The middle calixarene
in the longer tube 6b is most probably filled as well.
Downfield shifts (Dd > 1 ppm) of the corresponding signals
for the protons of Ar-O-CH2 and CH2-O-CH2 were observed
for this fragment. Higher stoichiometries of NO+ were ruled
out; there is simply no room to accommodate a larger number
of electrostatically repulsive cations.
According to molecular modeling studies, the nitroso-
nium-filled nanotubes adopt somewhat shrunken structures,
Figure 2. Partial 1H NMR spectra (500 MHz, (CDCl2)2, 295 K) of
a) nanotube 1a, b) filled nanotube 6a, c) nanotube 1b, and d) filled
nanotube 6b. The residual solvent signals are marked with filled
À
with all-gauche conformations about the glycol C C bonds.
Such folding brings closer the aromatic rings from the
neighboring calixarene units. This behavior was confirmed
by NOESY for the nitrosonium-filled nonsymmetrical O,O’-
dimethylated derivative of 4. Consequently, the aromatic
protons CHc of tube 6a, and CHc and CHm in tube 6b, appear
shielded and are seen somewhat upfield at d ꢀ 6.2–6.4 ppm
(Figure 2).
*
circles ( ). The assignments were performed by COSY and NOESY
experiments and supported by molecular modeling (MacroModel 7.1).
calixarene cavities.[5,8,9] Accordingly, the filling process can be
monitored visually.
Upon stepwise addition of NO2/N2O4 or NO+SbF6 in
Judging from the H NMR spectra at room temperature,
À
1
[D2]tetrachloroethane, signals for the protons of 1a,b and
6a,b can be seen separately and in slow exchange by 1H NMR
spectroscopy. This is typical for host–guest complexes with
high exchange DGꢀ barriers (> 15 kcalmolÀ1) and/or high
values of association constants (Ka > 106 mÀ1). The presence
complexation with NO+ does not influence the symmetry of
6a,b relative to empty 1a,b. Although in both cases the
nonequivalence of the CH2 protons of the calixarene meth-
ylene bridges becomes more pronounced, the number of
1H NMR signals for the propyl ArOCH2, glycol CH2OCH2
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and ArOCH2, and aromatic groups for 6a,b
does not change and implies that the NO+
guests, with van der Waals dimensions of
approximately 2 ꢀ, freely rotate along the N-
O axis and also tumble within the cavity.
To determine the mechanism of filling is a
difficult task[1] and it remains a great chal-
lenge for SWNTs, but conventional NMR
spectroscopy provides useful insights for
synthetic tubes such as those prepared here.
Modeling suggests that the first NO+ ion can
enter the nanotube through either its neck or
one of the middle gates between the calixar-
enes. Upon addition of NO+SbF6À to tube 1a,
the intermediate half-filled 1:1 complex
1a·NO+ was initially observed and isolated,
and the guest was found to be located
between the calixarenes, within the electron-
rich oxygen atoms of the crown ether.[6] The
signals for the protons of the propyl Ar-O-Pr
group in this complex were not shifted, which
indicates that the ends of the nanotube were
not occupied with NO+. Likewise upon
Figure 3. Side and top views of supramolecular packing of nanotube 1b into infinite
nanocylinders and the parallel stacking of neighboring nanotubes. View (d) shows the
continuous tunnel of four columns of nine 1b molecules.
addition of NO+SbF6 to longer nanotube
À
1b (at À208C), the intermediate partially
filled species were observed. They exhibit distinct signals of
all groups of protons, but the signals for the propyl Ar-O-Pr
protons were not shifted downfield. These intermediate
complexes were further converted into 6a,b simply by
addition of more NO2/N2O4 or NO+SbF6À. To avoid electro-
static repulsions, the next NO+ ion should instead enter
through the neck thus further pushing the first guest towards
the end of the tube.
conventional, shorter calixarenes. The unique linear nano-
structures maximize the intermolecular van der Waals
interactions in the crystal through the overall shape simpli-
fication.
In conclusion, besides SWNTs, synthetic nanotubes are
now available that pack in tubular bundles and can be
reversibly filled with guest molecules. At this stage only
certain guests can fill the interiors, but they are charged,
which is important for the design of nanowires. Given the
ability of calixarenes to react with NOx gases even in the solid
state,[5,11] we will look at the flow of nitrosonium ions along
the infinite nanocylinders in the solid-state bundles of 1a,b.
Not only would the charge be transported and detected
through the changes in conductivity, but also the color of the
crystals would serve as an indicator of such transport
processes.[12]
Filled nanotubes 6a,b are stable in anhydrous solution at
room temperature for hours, but can be readily destroyed
with H2O to quantitatively regenerate free 1a,b (according to
1H NMR and UV/Vis spectral analysis). We found that the
encapsulated NO+ species can also be removed by simple
addition of 18-crown-6; it is known that crown ethers form
stable complexes with NO+.[10] When 18-crown-6
( ꢀ 10 equiv) was added to solutions of 6a,b in [D2]tetra-
chloroethane, empty nanotubes 1a,b were regenerated within
minutes (NMR, UV/Vis), and the deep purple color dis-
appeared. This observation is important, as in this case
foreign species are removed without decomposition and
without the need to change the polarity of the solution.
Nanotube units 1a and 1b pack head-to-tail in straight
rows to result in infinitely long cylinders. Chloroform
molecules occupy the vacant spaces between the tubes.
Nanotube 1b shows a particularly appealing supramolecular
structure (Figure 3) with the neighboring nanocylinders
aligned parallel to each other. In each nanocylinder, mole-
cules 1b are twisted by 908 relative to each other, and the Ar-
O-Pr propyl groups effectively occupy the voids between the
adjacent molecules. In such an arrangement, the intermolec-
ular distance between two neighboring tubes in the nano-
cylinder is around 6 ꢀ, and the nanocylinders are separated
from each other by about 9 ꢀ. This supramolecular order
comes with the tube length and is without precedent for
Received: January 6, 2005
Published online: April 21, 2005
Keywords: calixarenes · nanotubes · nitrogen oxides ·
.
solid-state structures · supramolecular chemistry
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[7] Crystals of suitable quality of 1a and 1b for X-ray studies were
obtained from CHCl3/CH3OH solutions at room temperature.
The X-ray intensity data were measured at 100(2) K on a Bruker
SMART APEX CCD area detector system equipped with a
Oxford Cryosystems 700 Series cooler, a graphite monochro-
mator, and a MoKa fine-focus sealed tube (l = 0.710 73 ꢀ). The
data frames were integrated with the Bruker SAINT-Plus
(version 6.45) software package. Structures were solved and
refined using Bruker SHELXTL (version 6.14) software pack-
age. X-ray data for 1a·2CHCl3; C78H86Cl6O10, Monoclinic, Space
group P21/n; a = 17.9825(7), b = 10.6205(4) ꢀ, c = 19.8479(7) ꢀ,
b = 111.7390(10)8, V= 3521.0(2) ꢀ3, Z = 2, 1calcd = 1.317 MgmÀ3
,
the hydrogen atoms were placed in idealized positions and
included as riding atoms. All non-hydrogen atoms were refined
anisotropically. R1, wR2 (I > 2s(I)) = 0.0397, wR2 = 0.1024; R1,
wR2 (all data) = 0.0465, 0.1077, GOF = 1.040. X-ray data for
¯
1b·4.5CHCl3; C116.5H124.5Cl13.5O16, Triclinic, Space group P1, a =
14.7634(6) ꢀ, b = 15.2376(6) ꢀ, c = 26.4844(11), ꢀ, a =
75.9360(10)8,
b = 78.0270(10)8,
g = 80.0890(10)8,
V=
5606.9(4) ꢀ3, Z = 2, 1calcd = 1.338 MgmÀ3, the hydrogen atoms
were placed in idealized positions and included as riding atoms.
All the chloroform molecules show severe disorder. One of the
n-propyl groups is also disordered. These disorders were
modeled reasonably well. All non-hydrogen atoms were refined
anisotropically, R1, wR2 (I > 2s(I)) = 0.0828, 0.1667; R1, wR2
(all data) = 0.1100, 0.1783, GOF = 1.105. CCDC 259477 (1a) and
259478 (1b) 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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