CO2 in supramolecular chemistry: Preparation of switchable supramolecular polymers

Article abstract of DOI:10.1002/chem.200306062

CO₂ gas was used to construct novel types of supramolecular polymers. Self-assembling nanostructures 11 and 13 were prepared, which employ both hydrogen bonding and dynamic, thermally reversible carbamate bonds. As precursors, calixarene ureas

Full text of DOI:10.1002/chem.200306062

FULL PAPER
CO2 in Supramolecular Chemistry: Preparation of Switchable
Supramolecular Polymers
Heng Xu and Dmitry M. Rudkevich*[a]
Abstract: CO2 gas was used to con-
struct novel types of supramolecular
polymers. Self-assembling nanostruc-
tures 11 and 13 were prepared, which
employ both hydrogen bonding and dy-
namic, thermally reversible carbamate
bonds. As precursors, calixarene ureas
1 and 2 were synthesized, which strong-
ly aggregate/dimerize (KD ꢀ106 mꢁ1 per
capsule) in apolar solution with the for-
mation of self-assembling capsules 7
and linear polymeric chains 8, respec-
tively, and also possess “CO2-philic”
primary amino groups on the periph-
ery. CO2 effectively reacts with mole-
cules 7 and 8 in apolar solvents and
cross-links them with the formation of
multiple carbamate salt bridges. Oligo-
meric aggregate 11 and three-dimen-
sional polymeric network 13 were pre-
pared and characterized by 1H and
13C NMR spectroscopy. The morpholo-
gy of supramolecular gel 13 was stud-
ied by scanning electron microscopy.
Addition of a competitive solvent de-
stroyed the hydrogen bonding in as-
sembling structures 11 and 13, but did
not influence the carbamate linkers;
carbamate salts 12 and 14, respectively,
were obtained. On the other hand,
thermal release of CO2 from 11 and 13
was easily accomplished (1h, 100 8C)
while retaining the hydrogen-bonding
capsules. Thus, three-dimensional poly-
meric network 13 was transformed
back to linear polymeric chain 8 with-
out breaking up. Encapsulation and
storage of solvent molecules by 11 and
13 was demonstrated. This opens the
way for switchable materials, which re-
versibly trap, store, and then release
guest molecules.
A
two-parameter
switch and control over hydrogen
bonding and CO2–amine adducts was
established.
Keywords: calixarenes
· carbon
dioxide fixation · self-assembly ·
supramolecular chemistry
Introduction
In this paper, application of CO2 in supramolecular chemis-
try will be demonstrated. CO2 circulates in the environment
extensively through a number of processes known as the
carbon cycle.[1] The development of novel methods of chem-
ical fixation and utilization of this gas is ongoing,[2] and car-
bamate chemistry offers much potential in this direction.
Generally unreactive, CO2 readily combines with amines at
ordinary temperatures and pressures to form carbamates, in
which two amine molecules are held together by the salt
bridge (Scheme 1).[3] The process is thermally reversible and
can be considered as dynamic, covalent self-assembly.[4]
With this in mind, we employed CO2 as a cross-linking
agent to build supramolecular polymeric materials. Supra-
molecular polymers represent a novel class of macromole-
Scheme 1. Reversible covalent chemistry between CO2 and amines: self-
assembly of molecular blocks.
cules, in which monomeric units are held together by rever-
sible forces.[5]
Supramolecular polymers are self-assembling polymers,
which form and dissipate by means of hydrogen bonds,
metal–ligand interactions, and van der Waals forces. Thus,
they combine features of conventional polymers with prop-
erties resulting from the bonding reversibility. Structural pa-
rameters of supramolecular polymeric materials, in particu-
lar their two- and three-dimensional architectures, can be
switched “on–off” through the main chain assembly–dissoci-
ation processes. On the other hand, their strength and
degree of polymerization relies on how tightly the mono-
meric units are aggregated. In this paper, we introduce a
strategy to build supramolecular polymers that utilize hydro-
gen bonding and take advantage of the dynamic, reversible
[a] H. Xu, Prof. Dr. D. M. Rudkevich
Department of Chemistry & Biochemistry
University of Texas at Arlington
Arlington, TX 76019-0065 (USA)
Fax : (+1)817-272-3808
E-mail: rudkevich@uta.edu
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DOI: 10.1002/chem.200306062
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chemistry between CO2 and amines. These polymers are
also functional and possess multiple self-assembling capsules
that may envelop guests. We demonstrate that subtle, two-
parameter control over hydrogen bonding and CO2–amine
chemistry leads to switchable materials, which reversibly
trap, store, and then release guest molecules. And finally,
using CO2, we convert linear supramolecular polymeric
chains into supramolecular, three-dimensional polymeric
networks. These are also switchable and can be transformed
back to the linear chains without breaking up. Indeed, while
supramolecular cross-linked polymers are known,[5] they
break upon dissociation of the noncovalent aggregates, of
which they are composed. Our materials are different in
that they only release CO2 and keep the hydrogen bonding
intact.[6]
works (Scheme 3). Addition of a competitive solvent breaks
the bonds formed from self-assembly but not the carbamate
linkers. On the other hand, thermal release of CO2 can be
easily accomplished, but it does not influence the noncova-
lent aggregates and the capsules do not dissociate.
In the design of monomeric units, we took advantage of
calixarenes as both self-assembling and cavity-forming mod-
ules.[11] Calix[4]arene tetraurea dimers were specifically
chosen as these are probably the most-studied class of cap-
sules.[11,12] Discovered almost ten years ago by Rebek[13] and
Böhmer,[14] these capsules form in apolar solution (KD ꢀ
106 mꢁ1) and are held together by a seam of sixteen intermo-
ꢁ
lecular C=O···H N hydrogen bonds at the upper rims. This
results in a rigid cavity of about 200 Š3, which reversibly en-
capsulates one solvent molecule or a benzene-sized guest.
When two calix[4]arene tetraurea compounds are covalently
linked at their lower rims, hydrogen bonding yields supra-
molecular polymeric capsules.[15,16]
Results and Discussion
For this study, calix[4]arene tetraurea compounds 1 and 2
were synthesized, which possess “CO2-philic” primary
amino groups on the periphery (Scheme 4). Specifically, ca-
lixarene 1 is functionalized with a hexamethyleneamine
fragment at its lower rim. In biscalix[4]arene 2, two calixar-
ene tetraurea moieties are linked with a dipeptide, di-l-
lysine chain. Calixarenes were attached to the e-NH2 ends
so that the di-lysine module orients them away from each
other, in roughly opposite directions.[17,18] According to ex-
tensive molecular modeling, this also prevents the intramo-
lecular assembly. The hexamethyleneamine chain was then
attached to the carboxylic side of the dipeptide. This and
the a-NH2 group of 2 can react with CO2, providing cross-
linking.
The key building block for the syntheses of 1 and 2 is ca-
lix[4]arene tetraurea acid 3. It was prepared from known ca-
lixarene precursors in five steps starting with the parent tet-
rakis-tert-butyl calix[4]arene (Scheme 5).[15] Calixarene
amine 1 was synthesized (as a TFA-salt; TFA=trifluoroace-
tic acid) from acid 3 and 1-N-Boc-protected 1,6-diaminohex-
ane (N,N’-dicyclohexylcarbidiimide (DCC), 1-hydroxybenzo-
triazole (HOBt), Et3N, DMF, 72%; Boc=tert-butyloxycar-
bonyl), followed by deprotection with TFA (THF, 93%).
Design and synthesis: The chemistry between CO2 and
amines is essentially an acid–base equilibrium, and the for-
mation of carbamate salts is thermally reversible.[3] CO2 can
typically be released by simple heating at ꢀ808C. This
property has been utilized in amine-based, reusable poly-
meric “CO2 scrubbers”.[7] Similarly, CO2 can be trapped by
amine-containing ionic liquids.[8] Thermally reversible carba-
mate chemistry has been recently employed for the prepara-
tion of organogels from long-chain alkyl amines.[9,10]
Our approach is sketched in Schemes 2 and 3 and introdu-
ces two generations of CO2-based self-assembling nanostruc-
tures. Monomeric units were designed, which a) strongly ag-
gregate/dimerize in apolar solution, b) possess “CO2-philic”
primary amino groups on the periphery, and c) form cap-
sules upon self-assembly. For cross-linking, two such mono-
meric units were covalently attached with the appropriate
orientation for linear, noncovalent polymerization
(Scheme 3). The “CO2-philic” amino groups were then in-
troduced perpendicular to the main chain. In apolar sol-
vents, once CO2 is involved, multiple carbamate salt bridges
should form resulting in either linear supramolecular aggre-
gates (Scheme 2) or three-dimensional supramolecular net-
Scheme 2. CO2 linking calixarene capsules into a linear supramolecular polymer.
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D. M. Rudkevich and H. Xu
FULL PAPER
Scheme 3. CO2 cross-links polymeric calixarene chains into a three-dimensional supramolecular network.
Self-assembly: As expected,[13–15] calixarene tetraurea 1 di-
merizes in apolar solution (1H NMR, ESI-MS) with the for-
mation of capsule 7 (Scheme 6). Due to the lack of symme-
try in 7, a multiple set of NH urea signals was recorded in
C6D6, CDCl3, and CDCl2CDCl2 between d=6.0 and 8.5 ppm
(for example, Figure 1a). These are characteristically shifted
down field (dꢀ2 ppm), compared with model, non-dimer-
ized ureas, showing the key features[13–15] of the capsule for-
mation. Statistically, both a proximal and a distal regioiso-
mer of 7 form, with respect to the orientation of the acet-
amide OCH2C(O)NH substituents at the lower rims of each
calixarene capsule of 1.[15] Moreover, the circular array of
hydrogen bonds can be arranged either clockwise or count-
erclockwise. Capsule 7 dissociates to form monomeric tet-
raurea 1 in a more competitive solvent, [D6]DMSO. This re-
sults in a much simpler 1H NMR spectrum, reflecting the
presence of a vertical symmetry plane in 1 (Figure 1b). For
example, three ArNHC(O) urea singlets in a ratio of 1:1:2
at d=8.05, 8.00, and 7.85 ppm and apparently three aromat-
ic CH singlets in a ratio of 2:2:4 at d=6.81, 6.79, and
6.61ppm are clearly seen in the down-field part of the spec-
trum.
Scheme 4. Calixarene building blocks for supramolecular polymers. The
“CO2-philic” sites are marked.
Having two calixarene modules for assembly, compound 2
and its immediate precursors 4 and 5 form linear supra-
molecular polymers 8–10 in apolar solution (Scheme 7).
Similar to 7, multiple sets of NH urea signals were seen in
the corresponding 1H NMR spectra in CDCl3 and C6D6.
These were characteristically shifted down field (ꢀ2 ppm;
for example, Figure 2a).
Biscalix[4]arene diamine 2 was prepared (as a TFA-salt)
from bis-N-Boc-protected dipeptide 4 (THF, TFA, >95%).
Compound 4 was obtained from calix dipeptide methyl ester
5 by basic hydrolysis of the ester (LiOH, H2O/THF, 91%),
followed by reaction with 1-N-Boc-protected 1,6-diamino-
hexane (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hy-
drochloride (EDCI), HOBt, DMF, 76%). Dipeptide 5 was
obtained by a conventional peptide coupling procedure
from 2 equivalents of acid 3 and 1equivalent of di- l-lysine 6
(EDCI, HOBt, DMF, 56%).[17] The amino groups in 1 and 2
were subsequently liberated from TFA by washing with
aqueous NaOH solution.
With the dimerization constant KD ꢀ106 mꢁ1 for each ca-
lixarene capsule,[19] an average degree of polymerization
(DP) of at least 102 can be theoretically estimated for struc-
tures 8–10 at the NMR concentration range.[20] During the
experiments, significantly increased viscosities were ob-
served for solutions of biscalixarenes 2, 4, and 5 in CHCl3
compared with the precursor 3. While the relative viscosity
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CO2 in Supramolecular Chemistry
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Scheme 5. a) DCC, HOBt, Et3N, DMF, 24 h, 72%. b) TFA, THF, 2 h, 93%. c) EDCI, HOBt, DMF, 24 h, 56%. d) LiOH, H2O, THF, 12 h, 91%. e) EDCI,
HOBt, Et3N, DMF, 24 h, 76%. b) TFA, THF, 4 h, >95%.
Scheme 6. Formation and dissociation of supramolecular aggregate 11.
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D. M. Rudkevich and H. Xu
FULL PAPER
Specific viscosities (hsp) of derivatives 1, 3, and 5 were
measured as a function of concentration; the double-loga-
rithmic plots are represented in Figure 3a. As expected, for
Figure 1. Downfield fragments of 1H NMR spectra (500 MHz, 295ꢃ1K)
of a) capsule 7 in C6D6, b) calixarene amine 1 in [D6]DMSO, c) salt 12,
prepared upon dissociation of aggregate 11 in [D6]DMSO; for this ex-
periment, 11 was obtained upon bubbling CO2 through a solution of 7 in
benzene and thus entrapping benzene (the benzene signal is shown by an
arrow), and d) aggregate 11, obtained from CO2 and 7 in CHCl3/hexanes,
1:2 solution and redissolved in CDCl3. The residual solvent signals are
*
Figure 3. Viscosity measurements with calixarenes 3 and 5 in CHCl3 and
1
and 11 in CHCl3/benzene, ꢂ2:1(295 ꢃ1K): a) specific viscosities
marked as
.
versus concentration (6–35mm range), double-logarithmic plot;
a
b) effect of the addition of 3 (mole fraction x) on the specific viscosity of
5 at 20mm. Viscosities of 1 and 11 in neat CHCl3 (not shown) are similar,
and comparable in value to calixarene 3.
calixarenes 1 and 3 the viscosities are low and the plot has a
slope of 1.1ꢃ0.1. Such a linear relationship between hsp and
concentration indicates that only small aggregates (e.g., cap-
sules) are formed, which are of constant size and apparently
do not interact with each other. In contrast, the double-loga-
rithmic relationship between hsp and concentration for bisca-
lixarene 5 exhibits a slope of ꢂ2; this implies the formation
of reversibly breakable polymers, the size of which increases
with concentration.[21] Due to steric restraints on the design
of dipeptide chains, unimolecular cyclization of two calixar-
ene tetraurea compounds in 2, 4, and 5 is not possible.
Addition of small quantities of calixarene 3 to a solution
of biscalixarene 5 in CHCl3 resulted in a dramatic decrease
in viscosity (Figure 3b). Acting as a chain stopper, 3 may
compete for hydrogen bonding with the calixarene frag-
ments in 5 and its polymeric chains. Based on these viscosity
measurements and using an approach developed by Meijer
and co-workers,[21] the DP value for biscalixarene 5 of
ꢂ2.8”102 was estimated at 20mm, which corresponds to the
average molar mass of ꢂ7.6”105 gmolꢁ1. When and
2 mol% of stopper 3 were used, the DP values dropped to
1.2”102 and 7.5”101, respectively. These observations once
1
Figure 2. Downfield portions of H NMR spectra (500 MHz, 295ꢃ1K) of
a) polymeric capsules 10 in CDCl3, b) biscalixarene 5 in [D6]DMSO; this
spectrum was obtained upon dissociation of 10 in [D6]DMSO. The residu-
*
al solvent signals are marked as
.
of 3 is similar to the solvent and does not apparently change
with changing the concentration, dramatic changes were de-
tected for the biscalixarenes (ꢀ5-fold, concentration range
from 5 to 40mm). Solutions of biscalixarene 2 were already
viscous at the NMR concentrations (ꢂ5mm) and had to be
diluted for further operations.
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CO2 in Supramolecular Chemistry
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Scheme 7. Formation and dissociation of linear supramolecular polymers 8–10 and cross-linked supramolecular material 13. 8: R2 =H, R3 =
NH(CH2)6NH2; 9: R2 =Boc, R3 =NH(CH2)6NHBoc; 10: R2 =Boc, R3 =OMe.
again confirm the reversibility of the described polymeriza-
tion processes, which occur through multiple-capsule forma-
tion. Similar supramolecular polymerization phenomena are
expected for structurally related biscalixarenes 2 and 4.
The interiors of polymeric capsules 8–10 are most proba-
bly filled with solvent. As expected, 8–10 fully dissociate to
monomeric units 2, 4, and 5 in polar [D6]DMSO. Similar to
1, this results in a much simpler H NMR spectrum, reflect-
ing the presence of the apparent vertical symmetry planes
(e.g., Figure 2b).
tion of carbamate-linked supramolecular material, 11. This
belongs to the first generation. The chains in 11 are held to-
gether by calixarene hydrogen bonds and carbamate CH2N+
H3···OꢁC(O)NHCH2 salt bridges (Scheme 6). Initially, one
molecule of an amine reacts with CO2 to form the corre-
sponding carbamic acid. It is highly unstable and rapidly
transfers the acidic proton to the second amine molecule,
thus producing a relatively robust carbamate salt.[4] Forma-
tion of the carbamate bridges was confirmed by 1H and
13C NMR spectroscopy. In the 1H NMR spectrum of 1 in
[D6]DMSO, the terminal -CH2NH2 protons were seen as a
triplet at d=2.53 ppm (J=6 Hz). For salt 12, which is
formed upon dissociation of polymer 11 in [D6]DMSO, the
1
Reactions with CO2—first generation: Bubbling CO2
through a solution of 7 in benzene caused a rapid precipita-
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D. M. Rudkevich and H. Xu
FULL PAPER
spectrum showed that these protons split in two 1:1 sets
(-CH2N+H3···OꢁC(O)NHCH2-): a triplet at d=2.58 ppm
(J=6.4 Hz) for the first, and an apparent multiplet at dꢂ
2.9 ppm for the second. These were assigned through NMR
experiments with model alkyl amines, COSY, and from the
literature.[4,10] A broad carbamate NH signal was detected at
dꢂ6 ppm (1H NMR, COSY). Resonance at dꢂ160 ppm in
exist (see for example, Scheme 3). Model experiments with
CO2 and simpler aliphatic amines[4,9,10] and e-N-CBz-protect-
ed lysine (CBz=phenylmethoxycarbonyl) showed that these
reactions readily occur.
Formation of the carbamate bridges was further con-
firmed by 13C NMR spectroscopy. To be certain, we used
13CO2 gas and prepared the 13C-labeled gel 13. In the
13C NMR spectrum of diamine 2 (in [D6]DMSO), prior to
the reaction four C=O carbonyl signals were clearly detect-
ed: three for the amide fragments at d=175.4, 171.7, and
169.4 ppm, and one, intense signal for the upper-rim urea
compounds at d=155.8 ppm (Figure 4a). In the spectrum of
the 13C NMR spectrum of 12 unambiguously identified the
ꢁ
ꢁ
carbamic carbon atom (-HN C(O)O ). Notably, when
amine 1 was treated with a large excess of CO2 in
[D6]DMSO, the corresponding free carbamic acid formed,
1
which was studied by H, 13C NMR, and COSY spectrosco-
ꢁ
py. For example, the HN COOH resonance was clearly
seen at d=158 ppm in the 13C NMR spectrum. Free carbam-
ic acids are still rare and elusive.[4,22,23]
Supramolecular material 11 is a colorless solid, soluble in
chlorinated solvents, and insoluble in aromatic solvents. It
was also obtained by the CO2-induced precipitation from
solutions of 7 in CHCl3/hexanes, 1:2. A multiple set of the
down-field NH urea signals of 11, recorded in CDCl3, clearly
indicate the hydrogen-bonding assembly of polymeric chains
(Figure 1d). At the same time, viscosities of capsules 7 and
material 11, obtained after the reaction with CO2, appeared
to be similar (CHCl3 and CHCl3/benzene, 2:1) (Figure 3a).
These viscosities were low, apparently concentration inde-
pendent (5–25mm range), and comparable to relative viscos-
ities of precursor 3. Evidently, 11 is not significantly aggre-
gated under these conditions.
The dimerization constant for each calixarene capsule of
11 is high,[19] and the carbamate–ammonium electrostatic in-
teractions are also very strong in apolar solution.[4,8–10] These
features do not allow the high concentrations of free end
groups in structure 11. On the other hand, carbamate–am-
monium electrostatics is not directional and may offer sig-
nificant flexibility to the resulting structures. We propose
that for 11, oligomeric rings rather than long polymeric
chains are formed upon reaction of 7 with CO2. The double-
logarithmic plots of specific viscosities hsp versus concentra-
tion obtained for monomer 1 and also polymer 11 in CHCl3
are low and show slopes of approximately 1(Figure 3a).
Such linear relationships indicate that aggregates of constant
size are formed, which do not interact with each other. Due
to the low viscosity, these rings may not be large; we are
currently studying their structure. We also noticed that
chain–ring equilibrium posed a typical problem for supra-
molecular polymers and had been thoroughly analyzed by
Meijer, Sijbesma, and co-workers.[21] The problem does not
exist for preformed, linear supramolecular polymer 8, for
which CO2 serves as a cross-linking agent.
Figure 4. Portions of 13C NMR spectra (125 MHz, [D6]DMSO, 295ꢃ1K)
of a) biscalixarene 2, b) carbamate salt 14 obtained upon dissociation of
13C-labeled gel 13. The gel was prepared from 2 and 13CO2 in CHCl3. The
carbamate 13C-labeled signals are marked. For the corresponding
1H NMR spectra, see Figure 5.
the 13C-labeled salt 14 (which is formed upon dissociation of
the 13C-labeled polymer 13 in [D6]DMSO), in addition to
these signals, two new singlets of high intensity appeared at
d=163.5 and 162.8 ppm (Figure 4b). We attribute these sin-
glets to the carbamate a-HN-13C(O)Oꢁ and (CH2)6HN-
13C(O)Oꢁ groups. Notably, these two signals disappeared
after heating solution 14 for 1h at ꢂ1008C and bubbling N2
through it.
1
The H NMR spectra of material 13 is difficult to obtain,
which is clearly due to the cross-linked structure and numer-
ous possibilities for forming carbamic bridges. However, the
same trend as for the simpler oligomer 11 can be clearly ob-
served (compare Figure 5 with Figures 1and 2). Rather sim-
ilar to capsule 7, multiple sets of NH urea signals were seen
in the corresponding 1H NMR spectra of precursor 2 in
CDCl3; viscous polymer 8 was formed (Figure 5a). These
NH signals were characteristically shifted down field. As ex-
Reactions with CO2—second generation: Bubbling the gas
through a solution of 8 in CHCl3 or benzene yielded materi-
al 13, which is a gel (Scheme 7). The main chains in 13 are
held together by a hydrogen-bonding assembly of capsules
and multiple carbamate -N+H3···OꢁC(O)NH- bridges cross-
link these chains. This is clearly a three-dimensional net-
work, as the side amine groups are oriented in all three di-
rections. Moreover, structure 8 possesses two types of amino
group, and several possibilities for the carbamate formation
pected,
8 fully dissociated to monomeric 2 in polar
[D6]DMSO (Figure 5b). Similar to 1, this resulted in a sim-
plified 1H NMR spectrum, reflecting the apparent vertical
symmetry plane in the molecule. Being insoluble in apolar
solvents, material 13 readily dissociated in DMSO to form a
mixture of carbamate salts of type 14. The corresponding
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CO2 in Supramolecular Chemistry
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So far, CHCl3 and benzene have been gelated, and we are
currently studying other solvents and guests.
To obtain visual insight into the aggregation mode and
morphology in 13, dry samples were prepared for scanning
electron microscopy (SEM) analysis. While its precursor 8
only shows the formation of negligible fibers, a three-dimen-
sional network is obvious for 13. Figure 6 displays typical
pictures obtained from the xerogel of 13.
1
Figure 5. Downfield portions of H NMR spectra (500 MHz, 295ꢃ1K) of
a) calixarene 2 in CDCl3 (e.g., polymeric chain 8), b) calixarene 2 in
[D6]DMSO, c) salt 14, prepared upon dissociation of polymeric gel 13 in
[D6]DMSO; for this experiment, polymer 13 was obtained upon bubbling
CO2 through a solution of 2 in CHCl3 (e.g., 8). The CHCl3 signal is
*
Figure 6. SEM pictures of xerogel 13 obtained upon bubbling CO2
through a solution of 8 in CHCl3(bar 20 mm).
marked as
.
1H NMR spectrum resembles those for carbamate salt 12
(Figure 5c and Figure 1c).
Conclusion
Properties: Self-assembling materials 11 and 13 exhibit
unique properties. They assemble and dissipate in a two-pa-
rameter fashion, upon changing either the solvent polarity
or temperature. The calixarene capsules completely dissoci-
ate in DMSO, so only carbamate salts 12 and 14 can be de-
tected (Figure 1c and Figure 5c, respectively). Salts 12 and
14 most probably undergo further solvolysis generating
In summary, CO2 can be used to build supramolecular poly-
mers and polymeric materials. These utilize hydrogen bond-
ing and take advantage of the dynamic, reversible chemistry
of CO2. Subtle, two-parameter control over noncovalent and
covalent forces can be achieved. This leads to switchable
materials. Their dynamics and structural characteristics can
be controlled on a molecular level. These polymers also pos-
sess multiple, self-assembling capsules that may envelop
guests. The most immediate applications are in encapsula-
tion, and we are currently testing the ability of our materials
to entrap, store, and release chemicals into reaction mix-
tures. We are also exploring supramolecular, three-dimen-
sional polymeric networks. Their morphology and mechani-
cal properties can be manipulated by reversible switching to
form the corresponding linear polymeric chains without
breaking hydrogen bonds by the simple thermal release of
CO2.[25] With synthetic variations of the polymeric chains,
particularly their geometry and stereochemistry and also
side-chain functionalization, more possibilities are open for
the use of CO2 in supramolecular chemistry and nanochem-
istry. Simultaneously, we are looking at supramolecular ap-
plications of other gases.[26]
ꢁ
loose ion pairs. The carbamate C N bonds are not broken
under these conditions, however, they can be dismantled
upon heating, thus releasing CO2. In the case of 12, in
apolar solution monomeric capsules of type 7 form, and in
DMSO free amine 1 is regenerated. For 14 in apolar so-
lution, linear hydrogen-bonded polymer 8 forms, and in
DMSO biscalixarene 2 is completely regenerated. In both
cases, carbamate polymers 11 and 13 can be reconstructed
by simply reintroducing CO2.
Another interesting feature of materials 11 and 13 is their
multiple capsules. These are already preformed in apolar
solutions, but then convert into solids/gels upon exposure to
CO2. Upon completing this CO2-initiated polymerization,
they trap guest molecules and transport them to the solid
state; this results in guest storing materials.
In a preliminary test, obtained from benzene and carefully
dried polymer (0.1mm Hg, RT, 24 h), 11 did not release
benzene when the capsules were intact. In a suspension of
11 in noncompetitive [D10]p-xylene, no trace of benzene was
detected (1H NMR, 500 MHz), but when [D6]DMSO was
used, polymeric capsules of type 11 dissociated and released
visible quantities of benzene, approximately one molecule
per capsule (Figure 1c). We fully expect similar behavior
from gel 13. However, in addition to being encapsulated,
guest/solvent molecules are entrapped within the gelꢁs
three-dimensional network[24] (see for example, Figure 5c).
Experimental Section
General: Melting points were determined on a Mel-Temp apparatus
(Laboratory Devices, Inc.) and are uncorrected. 1H, 13C NMR, and
COSY spectra were recorded at 295ꢃ18C on a JEOL Eclipse 500 MHz
spectrometer. Chemical shifts were measured relative to residual, non-
deuterated solvent resonances. FTIR spectra were recorded on a Bruker
Vector 22 spectrometer. ESI-MS spectra were obtained on a Finnigan
LCQ Ion Trap apparatus. MALDI-TOF mass spectra were recorded on a
delayed extraction MALDI-TOF mass spectrophotometer Voyager DE
Chem. Eur. J. 2004, 10, 5432 – 5442
www.chemeurj.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5439

D. M. Rudkevich and H. Xu
FULL PAPER
(Applied Biosystems). HRMS MALDI spectra were obtained on an Ion
Spec Ultima FTMS. Elemental analyses were performed on a Perkin-
Elmer 2400 CHN analyzer. SEM images were obtained by using a JEOL
35C microscope.
30.4, 27.6, 26.6, 26.2, 22.9, 22.7, 14.5, 10.7, 10.6 ppm; FTIR (KBr): n˜ =
3339, 2932, 2859, 1659, 1599, 1562, 1468, 1213 cmꢁ1; ESI-MS: m/z: calcd
for C75H115F3N10O11: 1389; found: 1389.
The TFA salt (0.50 g, 0.36 mmol) in CHCl3 (100 mL) was washed with
aqueous 10% NaOH (2”50 mL), then evaporated and dried in high
vacuo. 1H NMR ([D6]DMSO): d=8.18 (t, J=5 Hz, 1H), 8.05 (brs, 1H),
8.00 (brs, 1H), 7.85 (brs, 2H), 6.80 (2”s, 4H), 6.61 (s, 4H), 5.88 (m,
2H), 5.80 (t, J=5.5 Hz, 2H), 4.36 (s, 2H), 4.32 (d, J=12.8 Hz, 2H), 4.26
(d, J=12.8 Hz, 2H), 3.76 (t, J=7.8 Hz, 4H), 3.69 (t, J=7.8 Hz, 2H), 3.22
(m, 2H), 2.99 (m, 12H), 2.53 (t, J=6.0 Hz, 2H), 1.9–1.7, 1.6–1.4, 1.4–1.3,
1.3–1.1, 1.0–0.8 ppm (5”m, 67H); 13C NMR ([D6]DMSO): d=169.3,
155.8, 155.7, 151.2, 150.7, 150.4, 135.4, 135.1, 135.0, 134.5, 134.2, 134.1,
118.8, 118.7, 118.5, 118.47, 77.3, 76.5, 74.8, 31.6, 30.4, 26.7, 22.9, 22.7, 14.5,
All experiments with moisture- and/or air-sensitive compounds were car-
ried out under a dried nitrogen atmosphere. For column chromatography,
Silica Gel 60 Š was used (Sorbent Technologies, Inc.; 200–425 mesh).
Parent tetrahydroxycalix[4]arene[27] and other calixarene precursors[15]
were prepared according to the published procedures. Molecular model-
ing was performed using commercial MacroModel 7.1with Amber*
Force Field.
Calix[4]arene tetraurea acid (3): n-Hexyl isocyanate (1.78 mL,
12.25 mmol) was added to a solution of the previously described tetra-
aminocalix[4]arene[15] (2.0 g, 2.45 mmol) in dry CH2Cl2 (80 mL), and the
reaction mixture was stirred at RT for 4 h. The solvent was removed in
vacuo, and the residue was triturated with hexane to yield the tetraurea
ester as a tan powder (2.72 g, 2.25 mmol, 92%). 1H NMR ([D6]DMSO):
d=8.06 (s, 1H), 8.05 (s, 1H), 7.81 (s, 2H), 6.87 (s, 2H), 6.84 (s, 2H), 6.53
(m, 4H), 5.88 (m, 2H), 5.69 (t, J=5.0 Hz, 2H), 4.71(s, 2H), 4.50 (d, J=
13.3 Hz, 2H), 4.29 (d, J=12.37 Hz, 2H), 4.12 (q, J=7.3 Hz, 2H), 3.75 (t,
J=7.8 Hz, 2H), 3.68 (t, J=7.8 Hz, 2H), 3.62 (t, J=7.8 Hz, 2H), 3.15–2.90
(m, 12H), 1.92 (m, 2H), 1.82 (m, 4H), 1.5–0.80 ppm (6”m, 56H).
10.6 ppm; FTIR (KBr): n˜ =3344, 2930, 2858, 1654, 1559, 1475, 1213 cmꢁ1
;
ESI MS: m/z: calcd for C73H114N10O9: 1275; found: 1276 [M+H]+, 2552
[2M+2H]+.
Di-l-lysine (6):[28,18] To a stirred and ice-cooled solution of N-e-Cbz-l-
lysine TFA salt (1.0 g, 2.45 mmol) in DMF (30 mL), Et3N (0.34 mL,
2.45 mmol) was added. Then, after 15 min, acid N-a-Boc-N-e-Cbz-l-
lysine (0.93 g, 2.45 mmol), HOBt (0.66 g, 4.90 mmol), and DCC (1.01 g,
4.90 mmol) were successively added. The mixture was stirred for 30 min
at 08C and for 24 h at RT, then filtered, concentrated under reduced
pressure, diluted with EtOAc (200 mL), and washed successively with
NaHSO4 (1n, 4”50 mL), water (3”50 mL), NaHCO3 (1n, 4”50 mL),
and again with water (3”50 mL). The organic layer was then dried over
anhydrous Na2SO4 and evaporated. The residue was separated chromato-
graphically on silica gel eluting with THF/hexanes (2:3) to afford the de-
sired Cbz-protected dipeptide (1.14 g, 71%). 1H NMR ([D6]DMSO): d=
8.10 (d, J=7.3 Hz, 1H), 7.34 (m, 10H), 7.22 (t, J=5.5 Hz, 2H), 6.78 (d,
J=8.0 Hz, 1H), 4.99 (s, 4H), 4.20 (m, 1H), 3.90 (m, 1H), 3.59 (s, 3H),
2.96 (m, 4H), 1.32 (s, 9H), 1.8–1.1 ppm (4 m, 12H); 13C NMR (CDCl3):
d172.8, 172.7, 156.8, 156.0, 136.7, 136.6, 128.6, 128.57, 128.3, 128.2, 128.2,
80.1, 66.8, 66.7, 54.1, 52.4, 52.1, 40.5, 32.2, 31.6, 29.4, 29.2, 28.4, 22.6,
A mixture of the tetraurea ester (1.5 g, 1.2 mmol) and KOH (0.67 g,
12.0 mmol) in THF/H2O, (5:1, 60 mL) was placed under reflux overnight,
after which H2O (60 mL) was added, and the pH was adjusted to 2 with
aqueous HCl (1m). The product was extracted with CHCl3 (3”60 mL),
the organic layer was dried over Na2SO4, evaporated, and recrystallized
from MeOH to give tetraurea acid 3 as a yellowish powder (1.13 g,
80%). M.p. >3008C; 1H NMR ([D6]DMSO): d=8.07 (s, 1H), 8.03 (s,
1H), 7.83 (s, 2H), 6.87 (s, 4H), 6.59 (s, 4H), 5.88 (m, 2H), 5.72 (t, J=
5.0 Hz, 2H), 4.56 (s, 2H), 4.43 (d, J=12.6 Hz, 2H), 4.27 (d, J=12.6 Hz,
2H), 3.76 (t, J=7.8 Hz, 2H), 3.69 (t, J=7.8 Hz, 2H), 3.67 (t, J=7.8 Hz,
2H), 3.01 (m, 8H), 2.95 (m, 4H), 1.95–1,75(m, 6H), 1.5–1.1 (m, 32H),
1.0–0.8 ppm (m, 21H); 13C NMR ([D6]DMSO): d=171.5, 155.8, 151.1,
150.4, 150.2, 135.6, 135.3, 135.1, 134.8, 134.3, 134.1, 118.7, 77.5, 77.2, 71.2,
31.7, 30.4, 30.3, 26.7, 23.1, 23.0, 22.7, 14.4, 10.8, 10.5 ppm; FTIR (KBr):
22.3 ppm; FTIR (KBr): n˜ =3359, 3036, 2948, 1699, 1544, 1259 cmꢁ1
.
A solution of the Cbz-protected dipeptide (0.2 g, 0.30 mmol) in CH3OH
(10 mL) was treated with 10% Pd/C (20 mg) and stirred under an H2 at-
mosphere for 6 h. The mixture was filtered through Celite and concen-
trated under reduced pressure to give product 6 as an oil (0.11 g, 94%).
1H NMR ([D6]DMSO): d=8.28 (d, J=7.3 Hz, 1H), 6.86 (d, J=8.5 Hz,
1H), 4.22 (m, 1H), 3.93 (m, 1H), 3.62 (s, 3H), 2.72 (t, J=7.1Hz, 4H),
1.36 (s, 9H), 2.0–1.0 ppm (m, 12H); MALDI-TOF MS: m/z: calcd for
C18H36N4O5: 388.5; found: 388.8 [M]+.
n˜ =3376, 3333, 2961, 2931, 2858, 1761, 1654, 1558, 1478, 1213 cmꢁ1
;
MALDI-FTMS: m/z: calcd for C67H101N8O10
1177.7632 [M+H]+.
:
1177.7635; found:
Calixarene (1): N-Boc-1,6-diaminohexane (0.38 mL, 1.68 mmol), DCC
(0.35 g, 1.68 mmol), HOBt (0.23 g, 1.68 mmol), and Et3N (0.23 mL,
1.68 mmol) were added to a stirred and ice-cooled solution of 3 (1.0 g,
0.84 mmol) in DMF (30 mL). The mixture was stirred for 30 min at 08C
and for 24 h at RT, then filtered, concentrated in vacuo, diluted with
CHCl3, and washed successively with NaHSO4 (1n, 4”100 mL), water
(3”100 mL), NaHCO3 (1n, 4”100 mL), and again with water (3”
100 mL). The organic layer was then dried over anhydrous Na2SO4 and
evaporated. The residue was separated chromatographically on silica gel
eluting with CHCl3/CH3OH (95:5) to afford the N-Boc-protected amine
1 as a colorless solid (0.84 g, 72%). M.p. 1858C (decomp); 1H NMR
([D6]DMSO): d=8.17 (t, J=5.7 Hz, 1H), 8.02 (brs, 1H), 7.98 (brs, 1H),
7.83 (brs, 2H), 6.81(s, 2H), 6.79 (s, 2H), 6.76 (t, J=5.5 Hz, 1H), 6.62
(brs, 4H), 5.82 (m, 2H), 5.77 (t, J=5.3 Hz, 2H), 4.35 (s, 2H), 4.33 (d, J=
12.6 Hz, 2H), 4.27 (d, J=12.6 Hz, 2H), 3.76 (t, J=7.3 Hz, 4H), 3.72 (t,
J=7.3 Hz, 2H), 3.24 (m, 2H), 3.00 (m, 12H), 2.91 (m, 2H), 1.36 (s, 9H),
1.9–0.8 ppm (5”m, 71H); 13C NMR ([D6]DMSO): d=169.2, 156.1,
155.74, 155.7, 151.2, 150.7, 150.3, 135.4, 135.1, 135.0, 134.5, 134.2, 134.1,
118.7, 118.4, 77.8, 77.3, 76.5, 74.8, 31.6, 30.4, 28.83, 28.8, 26.7, 23.0, 22.9,
22.7, 14.5, 10.63, 10.6 ppm; FTIR (KBr): n˜ =3329, 2928, 2852, 1628, 1559,
Biscalixarene (5): Calixarene tetraurea acid 3 (1.0 g, 0.84 mmol), EDCI
(0.32 g, 1.68 mmol), and HOBt (0.23 g, 1.68 mmol) were added to a stir-
red and ice-cooled solution of dipeptide 6 (0.16 g, 0.42 mmol) in DMF
(30 mL). The mixture was stirred for 30 min at 08C and for 24 h at RT,
filtered, concentrated, diluted with CHCl3, and washed with water (3”
100 mL). The organic layer was then dried over anhydrous Na2SO4 and
evaporated. The residue was separated chromatographically on silica gel
eluting with CHCl3/CH3OH (9.5:0.5) to afford calix dipeptide 5 (0.64 g,
56%). M.p. >188C (decomp); 1H NMR ([D6]DMSO): d=8.19 (m, 2H),
8.12 (d, J=7.8 Hz, 1H), 8.02 (s, 1H), 8.01 (s, 1H), 7.97 (s, 1H), 7.96 (s,
1H), 7.81 (s, 2H), 7.80 (s, 2H), 6.79 (s, 2H), 6.78 (s, 4H), 6.77 (s, 2H),
6.59 (s, 4H), 6.58 (s, 4H), 5.84 (m, 4H), 5.76 (m, 4H), 4.33 (brs, 4H),
4.30 (d, J=12.4 Hz, 4H), 4.23 (d, J=12.4 Hz, 4H), 3.91 (m, 1H), 3.72 (t,
J=6.9 Hz, 8H), 3.68 (t, J=6.9 Hz, 4H), 3.58 (s, 3H), 3.20 (m, 4H), 3.08–
2.90 (m, 24H), 1.34 (s, 9H), 1.9–0.8 ppm (6”m, 130H); 13C NMR
([D6]DMSO): d=173.0, 172.9, 169.3, 155.74, 155.7, 151.25, 151.2, 150.7,
150.3, 135.3, 135.16, 135.1, 135.0, 134.5, 134.46, 134.2, 134.1, 134.0, 118.8,
118.72, 118.5, 118.4, 78.5, 77.3, 76.5, 74.8, 54.5, 52.3, 32.3, 31.6, 31.4, 31.2,
30.4, 30.1, 29.9, 28.7, 26.7, 23.6, 23.4, 23.0, 22.9, 22.7, 14.5, 10.6 ppm;
1476, 1213 cmꢁ1
.
A solution of the N-Boc-protected 1 (0.5 g, 0.36 mmol) in THF (15 mL)
was treated with TFA (5 mL) and stirred at RT for 2 h. The reaction mix-
ture was concentrated in vacuo to afford the pure TFA salt of 1 (0.47 g,
93%). 1H NMR ([D6]DMSO): d=8.18 (t, J=5.5 Hz, 1H), 8.05 (s, 1H),
7.99 (s, 1H), 7.84 (s, 2H), 6.80 (s, 2H), 6.63 (s, 2H), 6.60 (s, 2H), 5.88 (m,
2H), 5.78 (t, J=5.5 Hz, 2H), 4.37 (s, 2H), 4.32 (d, J=12.8 Hz, 2H), 4.26
(d, J=12.8 Hz, 2H), 3.75 (t, J=7.2 Hz, 4H), 3.71(t, J=7.2 Hz, 2H), 3.26
(m, 2H), 3.0 (m, 12H), 2.78 (m, 2H), 1.9–0.8 ppm (5”m, 70H);
13C NMR ([D6]DMSO): d=169.4, 155.8, 151.3, 150.7, 150.4, 135.3, 135.1,
135.0, 135.0, 134.5, 134.2, 134.0, 118.8, 118.54, 118.5, 77.3, 76.6, 74.8, 31.6,
FTIR (KBr): n˜ =3333, 2931, 2858, 1653, 1559, 1213, 1042, 965 cmꢁ1
;
MALDI-FTMS: m/z: calcd for
C152H232N20O23Na: 2728.7491; found:
2728.7671[ M+Na]+; ESI-MS: m/z: calcd for C152H232N20O23Cl: 2741;
found: 2743 [M+Cl]ꢁ.
Biscalixarene (4): A mixture of 5 (2 g, 0.74 mmol), THF (25 mL), and
aqueous LiOH (1n, 10 mL) was stirred overnight at RT, after which H2O
(30 mL) was added, and the pH was adjusted to 6 with aqueous 1m HCl.
The product was extracted with CHCl3 (3”60 mL). The organic layer
was dried over Na2SO4 and evaporated to give the tetraurea acid (1.81 g,
5440
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 5432 – 5442

CO2 in Supramolecular Chemistry
5432 – 5442
91%). M.p. >3008C; 1H NMR ([D6]DMSO): d=8.22 (brs, 2H), 8.09
(brs, 2H), 7.99 (brs, 2H), 7.93 (d, J=7.8 Hz, 1H), 7.83 (brs, 4H), 6.83 (s,
2H), 6.82 (s, 4H), 6.81(s, 2H), 6.62 (s, 4H), 6.61(s, 4H), 5.85 (m, 4H),
5.78 (m, 4H), 4.35 (brs, 4H), 4.33 (d, J=13.3 Hz, 4H), 4.22 (d, J=
13.3 Hz, 4H), 4.19 (m, 1H), 3.94 (m, 1H), 3.75 (t, J=6.9 Hz, 8H), 3.69 (t,
J=6.9 Hz, 4H), 3.22 (m, 4H), 3.10–2.90 (m, 24H), 1.34 (s, 9H), 1.9–
0.8 ppm (6”m, 130H); 13C NMR ([D6]DMSO): d=174.1, 172.7, 169.3,
155.8, 151.3, 151.2, 150.7, 150.3, 135.4, 135.2, 134.9, 134.4, 134.2, 134.0,
118.8, 118.5, 78.5, 77.3, 76.5, 74.8, 67.6, 54.7, 52.2, 31.7, 31.5, 31.2, 30.4,
30.1, 30.0, 28.7, 26.7, 25.7, 23.7, 23.4, 23.0, 22.9, 22.6, 14.4, 10.6 ppm;
reproducible results. Upon dissolution in DMSO, material 13 dissociated
to form carbamate salts of type 14. H NMR ([D6]DMSO): d=8.27 (brs),
1
8.22 (brs), 8.12–7.90 (brs), 6.90–6.70 (brs), 6.70–6.50 (brs), 6.14 (brs),
6.02 (brs), 4.36 (brs), 4.25 (m), 3.90–3.60 (m), 3.50–3.30 (m), 3.22 (m),
3.15–2.85 (m), 1.85–1.70 (m), 1.59–1.53 (m), 1.33 (m), 1.23 (m), 0.95–
0.85 ppm (m); 13C NMR ([D6]DMSO): d=175.5, 171.8, 169.4, 163.3,
162.9, 155.9, 151.4, 150.8, 150.3, 135.4, 135.2, 134.9, 134.4, 134.1, 133.8,
118.9, 118.6, 77.4, 76.3, 74.5, 31.6, 30.4, 26.7, 23.4, 23.1, 22.9, 22.7, 14.5,
10.6 ppm.
SEM: Samples of 8 and 13 were prepared by a conventional procedure,
previously described by Shinkai and co-workers.[29] The gel was placed in
a flask and frozen in liquid nitrogen. The frozen specimen was dried in
vacuo for 24 h and then coated with palladium-gold.
FTIR (KBr): n˜ =3349, 2930, 1664, 1560, 1472, 1367, 1216 cmꢁ1
.
N-Boc-1,6-diaminohexane (0.17 mL, 0.74 mmol), EDCI (0.14 g,
0.74 mmol), HOBt (0.10 g, 0.74 mmol), and Et3N (0.10 mL, 0.74 mmol)
were added to a stirred and ice-cooled solution of the above-mentioned
acid (1.0 g, 0.37 mmol) in DMF (20 mL). The mixture was stirred for
30 min at 08C and for 24 h at RT, filtered, concentrated, diluted with
CHCl3, and washed successively with NaHSO4 (1n, 3”80 mL), water (2”
80 mL), NaHCO3 (1n, 3”80 mL), and again with water (3”80 mL). The
organic layer was then dried over anhydrous Na2SO4 and evaporated.
The residue was separated chromatographically on silica gel eluting with
CHCl3/CH3OH (94:6) to afford 4 (0.81g, 76%). M.p. >1858C (decomp);
1H NMR ([D6]DMSO): d=8.20 (brs, 2H), 8.04 (brs, 2H), 8.00 (brs, 2H),
7.87 (brs, 1H), 7.82 (s, 4H), 7.73 (d, J=7.8 Hz, 1H), 6.98 (d, J=7.3 Hz,
1H), 6.82 (s, 2H), 6.81 (s, 4H), 6.80 (s, 2H), 6.73 (t, J=7.1Hz, 1H), 6.61
(s, 4H), 6.60 (s, 4H), 5.87 (m, 4H), 5.78 (m, 4H), 4.35 (s, 4H), 4.32 (d,
J=12.8 Hz, 4H), 4.26 (d, J=12.8 Hz, 4H), 3.87 (m, 1H), 3.74 (t, J=
6.9 Hz, 8H), 3.70 (t, J=6.9 Hz, 4H), 3.21(m, 4H), 3.00 (m, 24H), 2.86
(m, 4H), 1.36 (s, 9H), 1.34 (s, 9H), 1.9–0.8 ppm (6”m, 138H); 13C NMR
([D6]DMSO): d=172.4, 171.6, 169.3, 169.29, 156.1, 156.0, 155.8, 151.3,
151.2, 150.7, 150.3, 135.4, 135.2, 135.0, 134.4, 134.2, 134.0, 118.7, 118.5,
118.45, 78.6, 77.7, 77.3, 76.5, 74.8, 55.2, 52.9, 39.2, 39.0, 31.6, 31.5, 30.4,
30.0, 29.5, 28.8, 28.7, 26.7, 26.5, 23.8, 23.3, 23.0, 22.9, 22.7, 14.4, 10.6,
10.57 ppm; FTIR (KBr): n˜ =3325, 2929, 2864, 1659, 1556, 1471,
Viscosity: Viscosity measurements for calixarenes 1–5, 7, and 11 in
CHCl3 and CHCl3/benzene were performed in a standard glass viscome-
ter using conventional protocols.[30] All experiments were performed at
least twice showing good reproducibility. The DP values for biscalixarene
5 in the presence of chain stopper 3 were estimated using an earlier-de-
rived equation (see below),[21a] assuming that the dimerization constant
KD for a calixarene tetraurea capsule[19] was 106 mꢁ1
:
2ð½5Š þ ½3ŠÞ
DP ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
4KD
½3Šꢁ
½1ꢁ 1 þ 8KDð½3Š þ 2 ½5ŠÞŠ
Acknowledgement
We are grateful to Shelley Hampe for experimental assistance. We are
also thankful to the referee for the most valuable suggestions on the vis-
cosity measurements and analyses. Financial support was provided by the
Petroleum Research Fund, administered by the American Chemical Soci-
ety, and the Alfred P. Sloan Foundation. H. Xu is a Deanꢁs Excellence
Scholar of the University of Texas at Arlington.
1214 cmꢁ1
.
Biscalixarene (2): A solution of 5 (0.2 g, 0.07 mmol) in THF (20 mL) was
treated with TFA (20 mL) and then stirred at RT for 4 h. The reaction
mixture was concentrated in vacuo to afford the pure TFA-salt of 2. The
salt was then dissolved in CHCl3 (60 mL) and washed with 10% NaOH
(2”30 mL). The organic layer was dried over anhydrous Na2SO4 and
evaporated to give free amine 2 (0.18 g, 96%). 1H NMR ([D6]DMSO):
d=8.22 (brs, 4H), 8.17 (brs, 2H), 8.01 (brs, 1H), 7.94 (brs, 5H), 6.82 (s,
2H), 6.81(s, 4H), 6.80 (s, 2H), 6.56 (s, 4H), 6.55 (s, 4H), 6.07 (brs, 4H),
5.97 (brs, 4H), 4.35 (s, 4H), 4.33–4.19 (m, 8H), 4.14 (m, 1H), 3.80–3.60
(m, 12H), 3.22 (m, 4H), 3.10–2.90 (m, 26H), 1.90–0.80 ppm (5”m,
138H); 13C NMR ([D6]DMSO): d=175.4, 171.7, 169.4, 155.9, 155.8,
151.3, 150.7, 150.3, 135.4, 135.2, 135.0, 134.4, 134.2, 133.9, 133.89, 118.9,
118.8, 118.6, 77.3, 76.5, 74.8, 55.3, 52.6, 31.6, 30.4, 29.6, 28.7, 26.9, 26.7,
23.3, 23.1, 22.7, 22.6, 14.5, 10.6 ppm; FTIR (KBr): n˜ =3340, 2928, 2863,
[1] a) W. M. Stigliani, T. G. Spiro, Chemistry and the Environment, 2nd
ed, Prentice Hall, New Jersey, 2003, pp. 3–178; b) D. S. Schimel, J. I.
House, K. A. Hibbard, P. Bousquet, P. Ciais, P. Peylin, B. H. Bras-
well, M. J. Apps, D. Baker, A. Bondeau, J. Canadell, G. Churkina,
W. Cramer, A. S. Denning, C. B. Field, P. Friedlingstein, C. Goodale,
M. Heimann, R. A. Houghton, J. M. Melillo, B. Moore, III, D. Mur-
diyarso, I. Noble, S. W. Pacala, I. C. Prentice, M. R. Raupach, P. J.
Rayner, R. J. Scholes, W. L. Steffen, C. Wirth, Nature 2001, 414,
169–172; c) C. V. Cole, J. Duxbury, J. Freney, O. Heinemeyer, K.
Minami, A. Mosier, K. Paustian, N. Rosenberg, N. Sampson, D. Sau-
erbeck, Q. Zhao, Nutr. Cycling Agroecosyst. 1997, 49, 221–228.
[2] a) X. Xiaoding, J. A. Moulijn, Energy Fuels 1996, 10, 305–325;
b) N. H. Batjes, Biol. Fertil. Soils 1998, 27, 230–235. For recent re-
views on supercritical CO2 see: c) W. Leitner, Acc. Chem. Res. 2002,
35, 746–756; d) B. Subramanian, C. J. Lyon, V. Arunajatesan, Appl.
Catal. B 2002, 37, 279–292.
[3] a) D. B. DellꢁAmico, F. Calderazzo, L. Labella, F. Marchetti, G.
Pampaloni, Chem. Rev. 2003, 103, 3857–3898; b) W. D. McGhee, D.
Riley, K. Christ, Y. Pan, B. Parnas, J. Org. Chem. 1995, 60, 2820–
2830; c) T. E. Waldman, W. D. McGhee, J. Chem. Soc. Chem.
Commun. 1994, 957–958; d) R. N. Salvatore, S. I. Shin, A. S. Nagle,
K. W. Jung, J. Org. Chem. 2001, 66, 1035–1037; M. Aresta, E. Quar-
anta, Tetrahedron 1992, 48, 1515–1530.
[4] a) E. M. Hampe, D. M. Rudkevich, Tetrahedron 2003, 59, 9619–
9625; b) E. M. Hampe, D. M. Rudkevich, Chem. Commun. 2002,
1450–1451.
[5] a) J.-M. Lehn, Polym. Int. 2002, 51, 825–839; b) L. Brunsveld,
B. J. B. Folmer, E. W. Meijer, R. P. Sijbesma, Chem. Rev. 2001, 101,
4071–4097; c) C. Schmuck, W. Wienand, Angew. Chem. 2001, 113,
4493–4499; Angew. Chem. Int. Ed. 2001, 40, 4363–4369; d) A. T.
ten Cate, R. P. Sijbesma, Macromol. Rapid Commun. 2002, 23,
1094–1112; e) U. S. Schubert, C. Eschbaumer, Angew. Chem. 2002,
114, 3016–3050; Angew. Chem. Int. Ed. 2002, 41, 2892–2926.
1657, 1557, 1470, 1213 cmꢁ1
;
MALDI-TOF: m/z: calcd for
C152H236N20O2ONa: 2712.8; found: 2712.0 [M+Na]+; ESI-MS: m/z: calcd
for C152H237N20O20: 2691; found: 2692 [M+H]+.
Supramolecular oligomer (11) and salt (12): Calixarene
1 (0.50 g,
0.39 mmol) in benzene (6 mL) was placed in a glass tube (13”100 mm)
and dry CO2 was then bubbled through the solution for 5 min at 358C.
Oligomer 11 quantitively precipitated, was filtered off, and dried under
vacuum at RT. The experiment was performed at least five times giving
reproducible results. Upon dissolution in DMSO, material 11 dissociated
to give carbamate salt 12. M.p. >1408C (decomp); 1H NMR
([D6]DMSO): d=8.18 (2”brs, 4H), 8.11 (brs, 2H), 7.96 (brs, 4H), 6.89
(2”s, 8H), 6.66 (s, 8H), 6.00 (brs, 2H), 5.95 (brs, 2H), 5.91(brs, 4H),
5.80 (brs, 1H), 4.42 (s, 4H), 4.37 (d, J=12.4 Hz, 4H), 4.29 (d, J=
12.4 Hz, 4H), 3.77 (m, 12H), 3.26 (m, 4H), 3.04 (m, 26H), 2.58 (t, J=
6.4 Hz, 2H), 1.9–1.7, 1.7–1.5, 1.4–1.3, 1.3–1.1, 1.0–0.8 ppm (5”m, 134H);
13C NMR ([D6]DMSO): d=169.3, 159.8, 155.8, 151.3, 150.8, 150.3, 135.5,
135.2, 135.1, 134.5, 134.2, 134.0, 134.0, 118.8, 118.6, 77.3, 76.5, 74.8, 31.7,
31.5, 30.4, 27.1, 26.7, 23.0, 22.9, 22.7, 14.4, 10.6, 10.59 ppm.
Supramolecular polymer (13) and carbamate salts (14): Diamine 2 (0.2 g,
0.07 mmol) in CHCl3 (5 mL) was placed in a glass tube (13”100 mm)
and dry CO2 (13CO2) was then bubbled through the solution for 3 min at
RT. Material 13 formed as a gel, which was then dried under high
vacuum at RT. The experiment was performed at least five times giving
Chem. Eur. J. 2004, 10, 5432 – 5442
www.chemeurj.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5441

D. M. Rudkevich and H. Xu
FULL PAPER
[6] For the preliminary communication of these studies, see: H. Xu,
E. M. Hampe, D. M. Rudkevich, Chem. Commun. 2003, 2828–2829.
[7] a) T. Yamaguchi, L. M. Boetje, C. A. Koval, R. D. Noble, C. N.
Bowman, Ind. Eng. Chem. Res. 1995, 34, 4071–4077; b) T. Yamagu-
chi, C. A. Koval, R. D. Nobel, C. Bowman, Chem. Eng. Sci. 1996,
51, 4781–4789; c) E. Sada, H. Kumazawa, Z. Han, Chem. Eng. J.
1985, 31, 109–115; d) A. S. Kovvali, K. K. Sirkar, Ind. Eng. Chem.
Res. 2001, 40, 2502–2511.
[19] R. K. Castellano, S. L. Craig, C. Nuckolls, J. Rebek, Jr., J. Am.
Chem. Soc. 2000, 122, 7876–7882.
[20] R. B. Martin, Chem. Rev. 1996, 96, 3043–3064.
[21] a) R. P. Sijbesma, F. H. Beijer, L. Brunsveld, B. J. B. Folmer,
J. H. K. K. Hirschberg, R. F. M. Lange, J. K. L. Lowe, E. W. Meijer,
Science 1997, 278, 1601–1604; b) B. J. B. Folmers, R. P. Sijbesma,
E. W. Meijer, J. Am. Chem. Soc. 2001, 123, 2093–2094; c) S. H. M.
Söntjens, R. P. Sijbesma, M. H. P. van Genderen, E. W. Meijer, Mac-
romolecules 2001, 34, 3815–3818; d) A. T. ten Cate, H. Kooijman,
A. L. Spek, R. P. Sijbesma, E. W. Meijer, J. Am. Chem. Soc. 2004,
126, 3801–3808.
[22] M. Aresta, D. Ballivet-Tkatchenko, D. B. DellꢁAmico, M. C. Bonnet,
D. Boschi, F. Calderazzo, R. Faure, L. Labella, F. Marchetti, Chem.
Commun. 2000, 1099–1100, and references therein.
[23] a) K. Wittmann, W. Wisniewski, R. Mynott, W. Leitner, C. L. Krane-
mann, T. Rische, P. Eilbracht, S. Kluwer, J. M. Ernsting, C. J. Elsevi-
er, Chem. Eur. J. 2001, 7, 4584–4589; b) A. Fꢂrstner, L. Ackermann,
K. Beck, H. Hori, D. Koch, K. Langemann, M. Liebl, C. Six, W.
Leitner, J. Am. Chem. Soc. 2001, 123, 9000–9006.
[24] Reviews on organogels: a) D. J. Abdallah, R. G. Weiss, Adv. Mater.
2000, 12, 1237–1247; b) J. H. van Esch, B. L. Feringa, Angew. Chem.
2000, 112, 2351–2354; Angew. Chem. Int. Ed. 2000, 39, 2263–2266.
[25] For pH-switchable supramolecular polymers, see ref. [17]. For tuna-
ble cyclic/linear chain in self-assembling polymers: a) M. S. Vollmer,
T. D. Clark, C. Steinem, M. R. Ghadiri, Angew. Chem. 1999, 111,
1703–1706; Angew. Chem. Int. Ed. 1999, 38, 1598–1601 (photo
switchable); see ref. [21b] (temperature control), see also ref. [5d].
[26] Review on supramolecular chemistry of gases: D. M. Rudkevich,
Angew. Chem. 2004, 116, 568–581; Angew. Chem. Int. Ed. 2004, 43,
558–571.
[27] a) C. D. Gutsche, M. Iqbal, Org. Synth. 1990, 68, 234–237; b) C. D.
Gutsche, J. A. Levine, P. K. Sujeeth, J. Org. Chem. 1985, 50, 5802–
5806.
[28] a) M. Sisido, Y. Imanishi, Macromolecules 1986, 19, 2187–2193;
b) A. M. Bray, D. P. Kelly, T. K. Lim, Aust. J. Chem. 1991, 44, 1649–
1658.
[8] E. D. Bates, R. D. Mayton, I. Ntai, J. H. Davis, Jr., J. Am. Chem.
Soc. 2002, 124, 926–927.
[9] a) M. George, R. G. Weiss, J. Am. Chem. Soc. 2001, 123, 10393–
10394; b) M. George, R. G. Weiss, Langmuir 2002, 18, 7124–7135;
c) E. Carretti, L. Dei, P. Baglioni, R. G. Weiss, J. Am. Chem. Soc.
2003, 125, 5121–5129; d) M. George, R. G. Weiss, Langmuir 2003,
19, 1017–1025.
[10] M. George, R. G. Weiss, Langmuir 2003, 19, 8168–8176, (NMR
studies).
[11] a) D. M. Rudkevich in Calixarene 2001 (Eds.: Z. Asfari, V. Böhmer,
J. Harrowfield, J. Vicens), Kluwer Academic, Dordrecht, 2001,
pp. 155–180; b) D. M. Rudkevich, Bull. Chem. Soc. Jpn. 2002, 75,
393–413.
[12] a) J. Rebek, Jr., Chem. Commun. 2000, 637–643; b) V. Böhmer,
M. O. Vysotsky, Aust. J. Chem. 2001, 54, 671–677; c) F. Hof, S. L.
Craig, C. Nuckolls, J. Rebek, Jr., Angew. Chem. 2002, 114, 1556–
1578; Angew. Chem. Int. Ed. 2002, 41, 1488–1508.
[13] K. D. Shimizu, J. Rebek, Jr., Proc. Natl. Acad. Sci. USA 1995, 92,
12403–12407.
[14] a) O. Mogck, V. Böhmer, W. Vogt, Tetrahedron 1996, 52, 8489–
8496; b) O. Mogck, E. F. Paulus, V. Böhmer, I. Thondorf, W. Vogt,
Chem. Commun. 1996, 52, 2533–2534, (X-ray structure).
[15] a) R. K. Castellano, D. M. Rudkevich, J. Rebek, Jr., Proc. Natl.
Acad. Sci. USA 1997, 94, 7132–7137; b) R. K. Castellano, J. Re-
bek, Jr., J. Am. Chem. Soc. 1998, 120, 3657–3663.
[16] a) R. K. Castellano, C. Nuckolls, S. H. Eichhorn, M. R. Wood, A. J.
Lovinger, J. Rebek, Jr., Angew. Chem. 1999, 111, 2764–2768;
Angew. Chem. Int. Ed. 1999, 38, 2603–2606; b) R. K. Castellano, R.
Clark, S. L. Craig, C. Nuckolls, J. Rebek, Jr., Proc. Natl. Acad. Sci.
USA 2000, 97, 12418–12421.
[29] J. H. Jung, Y. Ono, S. Shinkai, Chem. Eur. J. 2000, 6, 4552–4557.
[30] R. J. Sime, PhysicalChemistry—Methods, Techniques and Experi-
ments, Saunders, Philadelphia, 1990, pp. 522–527.
[17] H. Xu, S. P. Stampp, D. M. Rudkevich, Org. Lett. 2003, 5, 4583–
4586.
[18] H. Xu, G. R. Kinsel, J. Zhang, M. Li, D. M. Rudkevich, Tetrahedron
2003, 59, 5837–5848.
Received: December 23, 2003
Revised: June 17, 2004
Published online: September 23, 2004
5442
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 5432 – 5442