A shape-persistent organic molecular cage with high selectivity for the adsorption of CO2 over N2

Article abstract of DOI:10.1002/anie.201001517

Carbon capture in an organic cage: A shape-persistent, organic prismatic molecular cage (see structure) was synthesized in one step and high yield from readily accessible starting materials through dynamic covalent chemistry. The resulting cage molecule exhibited high selectivity for the adsorption of CO2 over N2 and thus shows promise as a carbon-capture material. Copyright

Full text of DOI:10.1002/anie.201001517

Communications
DOI: 10.1002/anie.201001517
CO2 Adsorption
A Shape-Persistent Organic Molecular Cage with High Selectivity for
the Adsorption of CO2 over N2**
Yinghua Jin, Bret A. Voss, Richard D. Noble, and Wei Zhang*
The efficient separation of CO2 from N2, particularly at
ambient temperature and pressure, is a key factor in the
capture of carbon[1] from flue gas. In the past decade, the
emergence of metal–organic frameworks (MOFs)[2] repre-
sented a milestone in the development of promising materials
for adsorption-based gas separation. The pore size of MOFs
can be controlled well, and certain functionalities can be
introduced. Purely organic materials[3,4] and covalent organic
frameworks (COFs)[5] have also been considered as candidate
porous materials for gas storage and separation. The advan-
tages of COFs are their light weight, thermal stability, and
chemical robustness. Although MOFs and COFs show great
promise, the exploitation of new absorbents is still essential to
the advancement of cost-effective and environmentally
benign gas separation. In contrast to the growing body of
adsorption data for MOFs and COFs, only a few organic
molecules (i.e. calixarenes,[6] cucurbit[6]uril,[7] tris-o-phenyl-
enedioxycyclotriphosphazene,[8] imine-linked cages[9]) have
been explored as porous materials for gas storage and
separation. Porous organic molecules have the aforemen-
tioned advantages of COFs and are usually also highly soluble
in a variety of solvents. Thus, they are solution-processable
and can be purified by simple chromatography or recrystal-
lization.
one-pot synthesis of a shape-persistent 3D organic prismatic
cage molecule. This new porous cage compound showed
extraordinarily high ideal selectivity (73:1, v/v) for the
adsorption of CO2 over N2 under ambient conditions (1 bar,
293 K), as well as high thermal stability up to 703 K. Control
experiments indicated that this high, selective gas uptake was
due to a significant extent to the porous nanostructure of the
compound.
Although molecular trigonal-prismatic cages are among
the simplest 3D constructs, they are relatively uncommon.
The shape-persistent molecular prism 5 was prepared in a
one-pot process from the readily accessible triamine 3 (as the
top and bottom panels) and dialdehyde 4 (as the three lateral
edges) through DCC (Scheme 1). Triamine 3 was synthesized
from 1,3,5-trihexyl-2,4,6-triiodobenzene[13] (1) by Suzuki
coupling followed by reductive hydrogenation. Imine forma-
tion[14] between the two building units 3 and 4 was explored in
various solvents (CHCl3, CH2Cl2, 1,2,4-trichlorobenzene,
1,2-dichloroethane) and at different temperatures (from
room temperature up to 808C). The highest yield was attained
by stirring the solution of 3 and 4 in a 2:3 stoichiometric ratio
in chloroform for 16 h at room temperature under the
catalysis of scandium(III) triflate. Because of the dynamic
nature of imine metathesis, the condensation reaction
between two equivalents of triamine 3 and three equivalents
of dialdehyde 4 proceeds under thermodynamic control.
Among all possible products, the desired 3D molecular prism
5 is enthalpy-favored owing to a lack of angle strain; it is also
entropy-favored, as it consists of the minimum number of
building units. Therefore, 5 is generated as the major product
at equilibrium. The trigonal prism 5 was converted by hydride
Among organic molecules with internal voids,[10] shape-
persistent, rigid, three-dimensional, covalent cage molecules
are of particular interest because of their well-defined pore
dimensions and chemically and thermally robust backbone
structures. Until now, shape-persistent 3D molecules have
mainly been constructed by self-assembly processes driven by
metal–ligand coordination.[11] The synthesis of covalent,
shape-persistent 3D molecules that have higher thermal and
chemical stability than metal-assisted self-assembled mole-
cules is more challenging. Herein, we report the use of
dynamic covalent chemistry (DCC)[12] in a highly efficient,
À
reduction into compound 6, which contains more robust C N
single bonds. The product 6 was isolated in 74% yield after
column chromatography and was fully characterized by
1H NMR and 13C NMR spectroscopy as well as gel perme-
ation chromatography and MALDI MS analysis.
The overall geometry of the proposed prismatic structure
was confirmed by single-crystal X-ray analysis. Single crystals
were obtained from a solution of 6 in a 1:1:4 mixture of
dichloromethane, ethyl acetate, and hexane. X-ray diffraction
analysis showed that 6 had the expected trigonal-prismatic
structure, with six amine bonds and a large central cavity
(Figure 1). The two trigonal panels are twisted about 228 with
respect to each other; thus, the phenyl arms are not eclipsed
but rotated toward a staggered geometry to minimize steric
interactions. The distance between the top and bottom panels
in this prism structure is about 5.6 ꢀ, and the diameter at the
widest point is 2.4 nm. To our knowledge, these dimensions
determined by X-ray crystallography place compound 6
among the largest prismatic molecules constructed from
[*] Dr. Y. Jin, Prof. Dr. W. Zhang
Department of Chemistry and Biochemistry
University of Colorado, Boulder, CO 80309 (USA)
Fax: (+1)303-492-5894
E-mail: wei.zhang@colorado.edu
B. A. Voss, Prof. Dr. R. D. Noble
Department of Chemical and Biological Engineering
University of Colorado, Boulder, CO 80309 (USA)
[**] We thank Dr. Bruce Noll from Bruker AXS Inc. for single-crystal X-ray
data collection, Robert Kerr for powder XRD analysis, and Prof. D. L.
Gin for helpful discussions. W.Z. is grateful for a CRCW Junior
Faculty Development Award and thanks the University of Colorado
for funding.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001517.
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adsorption of these two
gases. High selectivity
and reversible loading are
highly desired characteris-
tics for a good adsorbent.
A solvent-free sample of 6
was obtained by drying the
crystal sample under high
vacuum. Powder X-ray
diffraction (PXRD) anal-
ysis showed that desolva-
tion under vacuum at
room temperature for a
prolonged time period
(4 days) results in a slight
loss of crystallinity, and
heating to 808C acceler-
ates the process. The
adsorption isotherms of
CO2 and N2 at 208C were
measured with a desol-
vated sample under high
vacuum at room temper-
ature (Figure 3). The
uptake value of CO2 was
4.46 cm3 gÀ1 (amount of
gas at standard tempera-
ture and pressure (STP:
Scheme 1. Covalent assembly of the cage molecule 6 from triamine 3 and dialdehyde 4. MS=molecular sieves,
Tf =trifluoromethanesulfonyl.
208C, 1 bar) per gram of compound 6), which corresponds to
0.20 mmolgÀ1, whereas N2 was hardly adsorbed at all
(0.061 cm3 gÀ1). Although the CO2-adsorption capacity at
1 bar is lower than that of some other recently reported
organic cage molecules (1.2–3.0 mmolgÀ1 at 275 K and
1.12 bar),[9] the material showed an excellent ideal CO2/N2
adsorption selectivity of 73 at STP.[15] One reason for the low
gas uptake is presumably the presence of six hexyl chains for
each cage molecule. These groups occupy a large portion of
void space and thus decrease the accessible pore volume. A
similar effect—a decrease in CO2 adsorption due to the
presence of side chains—was observed previously for
MOFs.[16] The loading was completely reversible: when we
applied a vacuum to the sample at room temperature and
repeated the adsorption test multiple times, we observed a
similar loading capacity (see Figure S5 in the Supporting
Information). To our knowledge, no purely organic porous
materials with such high CO2/N2 adsorption selectivity have
been described previously.
It seems that the gas-adsorption selectivity can be
attributed to the combination of a well-defined, porous 3D
cage structure and a strong chemical interaction of CO2 with
secondary amine groups in the pores through reversible
carbamate formation.[17] The porous nanostructure not only
ensures the highly regular 3D porosity, but also promotes the
favored interaction between amine groups and CO2 mole-
cules. As a control experiment, compound 5 was used instead
of cage 6 as the adsorbent. The ideal CO2/N2 adsorption
selectivity at 1 bar and 293 K decreased from 73 to 39,
presumably as a result of the weaker interaction of CO2 with
Figure 1. X-ray crystal structure of compound 6. Left: ellipsoid model
as viewed down the C3 axis. Top right: ellipsoid model as viewed down
the C2 axis. Hydrogen atoms are omitted for clarity. Bottom right:
space-filling model as viewed down the C2 axis.
purely organic building blocks. Compounds of this type thus
have great potential for studies of host–guest interactions and
supramolecular chemistry.
The packing diagram shows that the molecular prisms
stack on top of each other along the [100] direction in an
offset fashion and that the layers do not interpenetrate. A
layered structure results (Figure 2). The distance between the
two layers, as defined by the distance between the top and
bottom panels of neighboring cages, is 4.6 ꢀ; the distance
reaches 10.4 ꢀ at the widest point, which is defined by two
anthracene side moieties. In addition to the internal voids, the
cage material has a one-dimensional pore channel running
between the cages. The pore channel is shown as an orange
Connolly surface in Figure 2.
To demonstrate the potential of the cage molecule 6 in
CO2/N2 gas separation, we measured its ideal selectivity in the
À
= À
the imine groups ( CH N ) than with secondary amines (see
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Figure 3. CO2- and N2-adsorption isotherms at 208C for compound 6.
by subjecting the top panel 3 to reaction with 1-naphthalde-
hyde (3 equiv). The lower CO2/N2 adsorption selectivity of 38
observed for 7 indicated that the 3D cage structure is
responsible for the high selectivity of 6 in the adsorption of
CO2 over N2 (see Table S1 in the Supporting Information).
In summary, we efficiently synthesized a shape-persistent,
prismatic molecular cage 6 in one step from readily accessible,
solely organic building blocks. The material exhibited high
selectivity in the adsorption of CO2 over N2 and thus shows
great potential for gas-separation applications. The cage
molecule showed excellent solubility in a variety of solvents
and thus also holds great promise for the fabrication of
membranes or thin films for gas-separation purposes. A range
of molecular cages with controllable pore sizes and shapes as
well as surface functionalities can be synthesized readily by
selection of the appropriate building blocks. This ready
accessibility of a range of structures opens many new
possibilities for the use of such shape-persistent molecular-
cage-based porous materials in gas adsorption, catalysis, and
chemical sensing. The structure–property relationships and
detailed mechanism of gas-adsorption selectivity of this class
of materials are under investigation.
Figure 2. Crystal-packing diagram of 6. The 1D pore channel between
the cages is shown as an orange Connolly surface (probe radius=
1.82 ꢀ).
Table S1 in the Supporting Information). To further inves-
tigate the importance of the intrinsic porosity of the
molecular cage structure for gas-adsorption selectivity, we
synthesized compound 7, which is the noncage analogue of 6,
Experimental Section
A solution of Sc(OTf)3 (32 mg, 0.087 mmol) in CH3CN (10 mL) was
added dropwise to a solution of 1,3,5-trihexyl-2,4,6-tris(4-amino-
phenyl)benzene (3; 350 mg, 0.58 mmol) and 1,8-diformylanthracene
(4; 204 mg, 0.87 mmol) in chloroform (193 mL). The yellow solution
was stirred at room temperature for 16 h, and the resulting red
solution was concentrated to give 5. Fresh chloroform (50 mL) was
added, followed by NaBH(OAc)3 (3.69 g, 17.4 mmol), and the
resulting yellow suspension was stirred at room temperature for 5 h.
The reaction was quenched by the addition of saturated NaHCO3
(50 mL), and the mixture was extracted with CHCl3 (3 ꢁ 75 mL). The
combined organic extracts were dried over Na2SO4 and concentrated
to give the crude product. Purification by flash column chromato-
graphy (with CH2Cl2 as the eluent) yielded the molecular cage 6
(387 mg, 74%) as a yellow solid.
5: 1H NMR (crude product, 500 MHz, CDCl3): d = 11.67 (s, 3H),
9.65 (s, 6H), 8.59 (s, 3H), 8.26 (d, J = 6.9 Hz, 6H), 8.19 (d, J = 8.3 Hz,
6H), 7.81 (d, J = 7.1 Hz, 6H), 7.66 (app t, J = 7.7 Hz, 6H), 7.59 (d, J =
8.0 Hz, 6H), 7.53 (d, J = 8.7 Hz, 6H), 7.30 (d, J = 8.0 Hz, 6H), 2.30–
2.22 (m, 12H), 1.26–1.17 (m, 12H), 0.95–0.87 (m, 12H), 0.84–0.76 (m,
24H), 0.57 ppm (t, J = 7.2 Hz, 18H).
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Angewandte
Chemie
6: 1H NMR (400 MHz, CDCl3): d = 9.07 (s, 3H), 8.51 (s, 3H), 8.01
(d, J = 8.5 Hz, 6H), 7.56 (d, J = 6.6 Hz, 6H), 7.46 (dd, J = 8.5, 6.7 Hz,
6H), 7.19 (dd, J = 8.2, 1.7 Hz, 6H), 6.94–6.86 (m, 10H), 6.66 (dd, J =
8.1, 2.2 Hz, 6H), 4.82 (br s, 12H), 3.80 (s, 6H), 2.24–2.28 (m, 12H),
1.25 (m, 12H), 1.07–0.99 (m, 12H), 0.94–0.85 (m, 24H), 0.68 ppm (t,
J = 7.2 Hz, 18H); 13C NMR (CDCl3, 101 MHz): d = 146.6, 139.7,
135.6, 132.2, 131.9, 130.4, 119.4, 114.5, 111.3, 47.3, 32.3, 31.4, 31.3, 29.9,
22.7, 14.4 ppm; MS (MALDI): m/z calcd for C132H134N6: 1814.15 [M]+;
found: 1814.74.
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Keywords: cage compounds · CO2 adsorption ·
dynamic covalent chemistry · porous materials · Schiff bases
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