A permanent mesoporous organic cage with an exceptionally high surface area

Article abstract of DOI:10.1002/anie.201308924

Recently, porous organic cage crystals have become a real alternative to extended framework materials with high specific surface areas in the desolvated state. Although major progress in this area has been made, the resulting porous compounds are restricted to the microporous regime, owing to the relatively small molecular sizes of the cages, or the collapse of larger structures upon desolvation. Herein, we present the synthesis of a shape-persistent cage compound by the reversible formation of 24 boronic ester units of 12 triptycene tetraol molecules and 8 triboronic acid molecules. The cage compound bears a cavity of a minimum inner diameter of 2.6 nm and a maximum inner diameter of 3.1 nm, as determined by single-crystal X-ray analysis. The porous molecular crystals could be activated for gas sorption by removing enclathrated solvent molecules, resulting in a mesoporous material with a very high specific surface area of 3758 m² g^-1 and a pore diameter of 2.3 nm, as measured by nitrogen gas sorption. Big boronic ester cages: A shape-persistent cuboctahedron can be almost quantitatively formed by a 48-fold one-pot condensation of 12 molecules of a triptycene tetrol with 8 molecules of triboronic acid. The desolvated crystalline material of this cage has a specific surface area of 3758 m² g^-1 and a maximum pore size of 2.3 nm, thus making it mesoporous by the IUPAC definition. Copyright

Full text of DOI:10.1002/anie.201308924

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DOI: 10.1002/anie.201308924
Porous Organic Cage Compounds
A Permanent Mesoporous Organic Cage with an Exceptionally High
Surface Area**
Gang Zhang, Oliver Presly, Fraser White, Iris M. Oppel, and Michael Mastalerz*
Abstract: Recently, porous organic cage crystals have become
a real alternative to extended framework materials with high
specific surface areas in the desolvated state. Although major
progress in this area has been made, the resulting porous
compounds are restricted to the microporous regime, owing to
the relatively small molecular sizes of the cages, or the collapse
of larger structures upon desolvation. Herein, we present the
synthesis of a shape-persistent cage compound by the reversible
formation of 24 boronic ester units of 12 triptycene tetraol
molecules and 8 triboronic acid molecules. The cage com-
pound bears a cavity of a minimum inner diameter of 2.6 nm
and a maximum inner diameter of 3.1 nm, as determined by
single-crystal X-ray analysis. The porous molecular crystals
could be activated for gas sorption by removing enclathrated
solvent molecules, resulting in a mesoporous material with
a very high specific surface area of 3758 m2 gÀ1 and a pore
diameter of 2.3 nm, as measured by nitrogen gas sorption.
correction”, often accompanied by low overall yields in the
cyclization step. Recently, the use of reversible formations of
covalent bonds has been applied to synthesize organic cage
compounds from small and readily accessible precursors in
reasonable to very high yields.[3]
It has been demonstrated that some of those organic cage
compounds are highly porous in the crystalline or amorphous
state, when desolvated in vacuum,[4] with reported Brunauer–
Emmett–Teller (BET) surface areas (SAs) of up to
2071 m2 gÀ1.[5] The solubility of porous organic molecules
offers some advantages in comparison to insoluble network
materials:[6] porous molecules are miscible in solution ena-
bling adjustment of the properties of the material;[7] an
exhaustive post-functionalization by chemical reactions in the
inner cavities of the cage molecules allows the functional
groups on the surface, and hence the gas sorption properties
of the porous materials, to be changed.[8] Furthermore,
processing of porous molecules into functional devices such
as quartz crystal microbalances for the sensing of volatile
analytes has been reported,[9] as has embedding them into
membranes.[10]
When aiming for organic cages with very large pores,
especially in the mesopore regime (> 2 nm), the prevention of
a structural collapse of the molecules upon desolvation is one
of the major obstacles one has to face.[11] There are very few
reports on organic cages with cavity diameters > 2 nm, and
none of these was reported as being permanently porous.[12]
For example, Warmuth and co-workers synthesized organic
cage compounds with solvodynamic diameters of approx-
imately 4 nm, according to the diffusion coefficients mea-
sured by DOSY-NMR spectroscopy.[12a] However, the result-
ing cage compounds were not investigated in terms of
porosity and no additional structural proof by single-crystal
X-ray analysis could be provided. Cages of larger sizes, which
were characterized in the solid state by X-ray diffraction of
single-crystals were synthesized by the Cooper group,[12b] the
Severin group,[12c] and Gawronski et al.[12d] (see also the
Supporting Information, Figure S27). Whereas the Cooper
cage had an outer diameter of 2.9 nm and inner diameter of
1.5 nm, which is similar to a previously reported [4+6] cage
compound,[4c] the Gawronski cage was larger, especially when
the inner cavity dimensions were taken into account (outer
diameter: 3.0 nm, inner diameter: 1.7 nm). By a combination
of metallasupramolecular chemistry and dynamic imine bond
formation, Severin and co-workers prepared a cage with outer
and inner diameters of 2.7 nm and 1.9 nm, respectively.
Unfortunately, these larger cages failed to remain porous
upon desolvation of the crystals: mainly nonporous com-
pounds were observed, as has been verified by gas sorption
measurements.
T
he self-assembly of smaller molecular subunits into larger
supramolecular entities with the shape of Platonic or Archi-
median bodies has been the focus of several studies in recent
years, especially when the coordination of metal ions has been
used for binding motifs.[1] By using the metal coordination
motif, it was even possible to synthesize near-spherical
coordination cage compounds with outer diameters of
approximately five nanometers from rather simple molecular
building blocks and metal cations.[2] In comparison to
coordination cages, purely organic cages are much rarer,
probably due to the fact that most covalent bonds are formed
by irreversible reactions that do not allow a structural “self-
[*] Dr. G. Zhang, Prof. Dr. M. Mastalerz
Organisch-Chemisches Institut, Ruprecht-Karls-Universitꢀt
Heidelberg, Im Neuenheimer Feld 270
69120 Heidelberg (Germany)
E-mail: michael.mastalerz@oci.uni-heidelberg.de
Dr. O. Presly, Dr. F. White
Agilent Technologies, Yarnton (UK)
Prof. Dr. I. M. Oppel
Anorganische Chemie, RWTH Aachen
Landoltweg 1, 52056 Aachen (Germany)
[**] We would like to thank the German Research Council (Deutsche
Forschungsgemeinschaft, DFG) and the “Fond der Chemischen
Industrie” (FCI) for generous financial support. G.Z. would like to
thank the Alexander-von-Humboldt foundation for a postdoctoral
fellowship. Conny Egger and Samuel Blessing (both Ulm University)
are acknowledged for performing nitrogen sorption measurements
and collecting PXRD data. We are grateful to Benjamin Eberle
(Heidelberg University) for TGA measurements.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308924.
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Herein, we demonstrate that an organic cage compound
with an average inner pore diameter of 2.8 nm can be
synthesized in one step from 20 rigid precursor molecules. The
shape-persistent voids of the cage molecules stay intact upon
desolvation, resulting in a permanent porous material with an
extraordinarily high BET SA of 3758 m2 gÀ1 and a measured
pore diameter of 2.3 nm, according to the Dubinin–Radush-
kevich equation, which is defined by IUPAC as mesoporos-
ity.[13]
We started our investigation with tetraol precursor 1a (for
the synthesis, see Scheme S1) as a molecular building block.
This is expected to give cuboctahedral cage compound 3a,
with cavities of a minimum inner diameter > 2 nm owing to
the molecular geometry in condensation with triboronic acid
2 (Figure 1a). However, all condensation attempts (changing
solvent, concentration, and reaction temperature) with the
triboronic acid 2 gave no distinct signals in the 1H NMR
spectrum of the crude product. This assumes that intermedi-
ates of the desired cage compound are already insoluble,
leading to no further reaction. We found that adding two
additional ethyl chains in the 13- and 16-positions of the
triptycene precursor (1b) is sufficient to drive the reaction to
completion, resulting in full conversion of the precursors into
the corresponding cage 3b by a 48-fold condensation of
twelve triptycene molecules 1b and eight molecules of the
triboronic acid 2. The synthesis was performed by dispersing
the insoluble reactants in chloroform and heating the result-
ing suspension to reflux until the reaction mixture became
a clear solution. The solution was concentrated by evapo-
ration of solvent on a rotary evaporator and precipitated with
n-hexane. The white precipitate was collected on a Bꢀchner
funnel to give the product in quantitative yield. However,
once the compound had precipitated it was difficult to
redissolve. The characteristic peaks in the IR-spectrum of
3b indicated the presence of catechol boronic ester units (see
the Supporting Information). Furthermore, the measured
elemental analysis was in good agreement to the calculated
one (see the Supporting Information).
To characterize the compound in solution, we added
deuterated tetrachloroethane to the chloroform solution and
removed the chloroform on a rotary evaporator. The
1H NMR spectrum of cage compounds 3b is relatively
simple, and showed only three peaks in the aromatic region
(d = 8.83 ppm, 7.42 ppm and 6.82 ppm) and one signal that
can be clearly assigned to the bridgehead protons of the
triptycene units (d = 5.72 ppm; Figure 2a). By the addition of
an internal standard (mesitylene) we calculated that the cage
Figure 1. a) One-step 48-fold condensation reaction of twelve mole-
cules of triptycene tetraol 1a or 1b with eight molecules of the
triboronic acid 2 to the cuboctahedral [12+8] cages 3a and 3b,
respectively. b) The scaffold of the cuboctahedral cage with selected
dimensions: outer diameter (3.99 nm), inner maximum diameter
(3.03 nm), and open quadric windows of 1.52 nm. Edges of the
cuboctahedral geometry are highlighted in red. The shown dimensions
are based on an AM1-optimized model (see the Supporting Informa-
tion).
Figure 2. a) Comparison of the aromatic and bridgehead regions of the
1H NMR spectra of reactant 1b (heterogeneous), model compound
M3b, and cage compound 3b in C2D2Cl4. For full spectra, see the
Supporting Information. b) DOSY NMR spectrum of cage compound
3b in C2D2Cl4. The hash (#) symbols mark residual non-deuterated
tetrachloroethane. The asterisks mark residual chloroform.
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compound had formed in 94% in solution. Comparing the
NMR spectrum with that of the model compound M3b, we
assumed that all diol units had been turned into the
corresponding catechol boronic ester units, which is also
suggested by the integrals of the peaks. In principle, from the
1H NMR spectrum we can only conclude that the correct ratio
(2:3) of molecular reactants is found in the product formed,
which could be any [3n(1b) + 2n(2)] condensation product.
Unfortunately, no mass spectrum could be recorded showing
the molecular peak, which may be due to the fact that the
molecule bears 24 catechol boronic ester units, each of which
is relatively easily chemically cleaved. Therefore, we recorded
a DOSY NMR spectrum of compound 3b to get more
structural information for the molecule (Figure 2b). The
spectrum showed only one trace of signals with a diffusion
coefficient of D = 1.33 ꢁ 10À6 cm2 sÀ1, corresponding to a sol-
vodynamic radius of 1.27 nm, which is in good agreement with
the minimum inner diameter of 2.6 nm of the sphere of
a [12+8] cage compound as depicted in Figure 1a.[14]
The structure of cage compound 3b was also determined
by single-crystal X-ray diffraction. Single-crystals of 3b were
grown by vapor diffusion of n-hexane into a saturated
solution of 3b in chloroform. Owing to the porous nature of
this material, the crystals
rings of the triboronic ester units and 2.9–3.1 nm between the
bridgehead atoms of two opposite triptycene units (Figure 3).
The pore window to the cavity is approximately 1.7 nm in
diameter. The molecules pack through p–p interactions along
the crystallographic c axis, which is reflected in a staggered
coplanar assembling of boronic ester units of adjacent
molecules at a distance of 3.2–3.4 ꢂ. Along other axes, the
packing interactions are rather weak, which can mainly be
assigned as CH–p interactions. For p–p interactions, 3.2 ꢂ is
relatively short, thus revealing that this interaction is rela-
tively strong in comparison to existing CH–p interactions.
As mentioned above, shape-persistency during evacuation
for molecules of such dimensions has not been reported thus
far. Because of the geometrical shape of an Archimedian
body, we expected that it should be possible for it to remain as
a stable porous material after removing enclathrated solvent
molecules from 3b. To prevent the crystalline material from
thermal stress, we activated it by immersing (12 h ꢁ 6) in n-
pentane before evacuating at room temperature, then meas-
uring the nitrogen sorption of the material; as has been
previously described for a hydrogen-bonded network with
very high specific surface area.[17] The measured isotherm
(Figure 4) can best be described as a type I isotherm.[13] The
contain large amounts
(72% of the unit cell) of
disordered solvent, and
therefore the structure
was treated using the
SQUEEZE function of
PLATON to remove the
electron density of the sol-
vate molecules.[15] Addi-
tionally, it was necessary
to use constraints to con-
trol the geometry of aro-
matic rings and restraints
to enforce chemically sen-
sible bond lengths and
angles in the ethyl chains
in particular. Vibrational
restraints were also used
to control atomic displace-
ment parameters across the
molecule, which was high,
presumably due to the
large and spherical nature
of the molecule allowing
considerable motion. The
compound crystallizes in
the triclinic space group
P-1 with one molecule in
the unit cell.[16] The cavity
of the molecule is very
large in comparison with
other porous cage com-
pounds, with an inner
diameter of 2.4–2.6 nm
Figure 3. Single-crystal X-ray structure of solvated cage 3b. Enclathrated solvent molecules were disordered
and could not be refined. a) Space-filling model. C gray, H white, O red, B yellow. b) Molecular packing of cage
3b in solid state. One cage is depicted as a stick representation. c) Packing in the solid state. The yellow
between two opposite aryl surface represents a solvent-accessible surface with a probe radius of 1.8 ꢁ.
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of the sorption isotherms is similar to that reported for MIL-
101, and the pore dimensions and surface areas of the cage
compound 3b are indeed comparable to MIL-101 (Vpore
=
2.0 cm3 gÀ1; BET SA = 4100 m2 gÀ1).[20] All data are readily
reproducible, as has been demonstrated by the synthesis and
measurement of
a second batch sample (BET SA =
3475 m2 gÀ1). The specific surface area of 3b is higher than
for COF-102 (BET SA = 3472 m2 gÀ1),[21] and is the highest
reported thus far for any porous cage compound. To the best
of our knowledge, 3b also represents the first single-crystal X-
ray structure of any reported crystalline porous organic
material based on the formation of boronic esters.
In conclusion, we have shown that it is possible to
synthesize a shape-persistent, discrete mesoporous organic
cage compound by dynamic covalent formation of 24 boronic
ester units in one step. The resulting BET specific surface area
of 3758 m2 gÀ1 is the highest reported thus far for any porous
organic molecules. Currently, we are working on the synthesis
of even larger porous cages to gain further insight into the
scope and limitations of this new and fascinating class of
porous materials.
Received: October 13, 2013
Published online: January 8, 2014
Keywords: boronic acids · cage compounds ·
.
dynamic covalent chemistry · porosity · triptycene
Figure 4. a) Nitrogen gas sorption isotherm at 77 K for cage 3b.
Closed symbols represent adsorption, open symbols represent desorp-
tion. b) The NLDFT pore-size distribution.
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