Exo-functionalized shape-persistent [2+3] cage compounds: Influence of molecular rigidity on formation and permanent porosity

Article abstract of DOI:10.1002/chem.201200032

Exo-functionalized [2+3] cage compounds were synthesized by using two different bissalicylaldehydes as molecular precursors in the reaction with triptycene triamine to study the influence of the rigidity of the molecular structure of [2+3] cages on their

Full text of DOI:10.1002/chem.201200032

DOI: 10.1002/chem.201200032
Exo-Functionalized Shape-Persistent [2+3] Cage Compounds: Influence of
Molecular Rigidity on Formation and Permanent Porosity
[
a]
[b]
[a]
Markus W. Schneider, Iris M. Oppel, and Michael Mastalerz*
[
7–9]
It was demonstrated recently that shape-persistent organic
cage compounds with defined cavities can form porous ma-
amines, respectively, to give [2+3] cage compounds.
2
À1
However, the reported BET surface areas (10 m g and
[1–9]
2
À1
terials in the solid state.
to 2071 m g , these new materials complement existing
systems of polymeric porous structures, for example, metal–
organic frameworks (MOFs),
works (COFs), and amorphous porous organic polymers.
With specific surface areas of up
99 m g ) of these [2+3] cage compounds are significantly
lower than for the [4+6] cage compounds, though a good se-
2
À1 [5]
[
7,8]
lectivity for the adsorption of CO over N was reported.
2
2
[10]
covalent organic frame-
The described [2+3] cage compounds contain flexible link-
ing units within the molecules that allow the molecular
structure of the cage compounds to twist so that the trigonal
planar subunits preferentially interact through p–p stacking
and almost close the cavities, which leaves insufficient space
inside the cage compound interior for gas sorption. This
effect might be the reason for the low measured BET sur-
face areas.
[11]
One method to synthesize organic cage compounds in
high yields is the reversible formation of multiple imine
[
12,13]
bonds.
Nevertheless, only some of those cage com-
[14]
pounds have been reported to be permanently porous.
One of the first examples was introduced by the Cooper
group, who synthesized T -symmetric cage compounds by
d
condensation of four molecules of 1,3,5-triformylbenzene
and six molecules of various 1,2-diamines (such cage com-
pounds are usually termed [4+6] cages). The Brunauer–
To study the influence of the rigidity of the molecular
structure of [2+3] cages on their formation and gas sorption
properties, we synthesized exo-functionalized [2+3] cage
compounds by using two different bissalicylaldehydes as mo-
lecular precursors in the reaction with triptycene triamine
[3]
Emmett–Teller (BET) surface areas of these first systems
2
À1
were 624 m g . These materials could selectively adsorb
various gases, for example, H2, CO , or CH . Very recently,
[16]
1.
2
4
the same group synthesized a porous [4+6] cage compound
The more rigid of the two resulting cage compounds, 3a,
2
À1
with a much higher specific BET surface area (1333 m g )
was synthesized by a sixfold imine condensation of tripty-
[6]
[17]
by enlarging the trialdehyde used.
We used a complementary approach to synthesize [4+6]
cene triamine 1 and bissalicylaldehyde 2a
under reflux
1
conditions (Scheme 1). The H NMR spectrum of the crude
[15]
[16]
cage compounds.
Triptycene triamine 1
was reacted
product in [D ]THF suggests that the main product is de-
8
with salicyldialdehydes to give endo-functionalized adaman-
toid cage compounds. These cage compounds show perma-
nent porosity in the crystalline phases, with highly accessible
sired cage compound 3a, formed in approximately 69%
yield (see Figure 1a). The byproducts could be removed
through crystallisation from THF/Et O to give pure, crystal-
2
2
À1 [4,5]
BET surface areas of up to 2071 m g .
These cage com-
line cage compound 3a (Figure 1b). The other, more flexible
cage compound, 3b, was synthesized under similar condi-
pounds could also selectively adsorb CO (9.4 wt%) over
2
[4]
[18]
methane (0.98 wt%). The above-mentioned unique sys-
tems tend to have shape-persistent cavities due to the intrin-
sic rigidity of the tetrahedral molecular scaffold.
tions by using bis-salicylaldehyde 2b as a reactant instead
1
of 2a. Note that the H NMR spectrum of the crude product
of 3b shows many more as-yet undefined byproducts than
the spectrum of crude 3a does (Figure 1).
Recently, another type of connection was used to generate
porous shape-persistent molecules through an imine conden-
sation of trisamines and dialdehydes or trisaldehydes and di-
By integration of typical regions (around d=9 ppm) at
which the imine protons resonate, we estimated that the
cage compound was formed in approximately 33% of the
collected precipitate, which corresponds to about 14% of
the whole product library. It is assumed that the higher de-
grees of rotational freedom of the two salicylaldehydic units
connected by an ethylene bridge causes a higher number of
reasonable possibilities in the virtual combinatorial li-
[
a] Dipl.-Chem. M. W. Schneider, Dr. M. Mastalerz
Institute of Organic Chemistry II & Advanced Materials
Ulm University
Albert-Einstein-Allee 11, 89081 Ulm (Germany)
Fax : (+49)731-50-22840
E-mail: michael.mastalerz@uni-ulm.de
[
b] Prof. Dr. I. M. Oppel
[19]
brary, which results in a higher number of mismatched oli-
Institute of Inorganic Chemistry, RWTH Aachen
Landoltweg 1, 52074 Aachen (Germany)
gomeric and polymeric byproducts. From this observation, it
is concluded that rigid precursors with directed functional
groups are beneficial for cage formation. This finding com-
plements suggestions for other organic cage compounds,
Supporting information for this article (including experimental de-
tails) is available on the WWW under http://dx.doi.org/10.1002/
chem.201200032.
4156
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4156 – 4160

COMMUNICATION
Scheme 1. Synthesis of [2+3] cage compounds 3a and 3b.
which are synthesized by imine bond formations. Cage com-
pound 3b was purified by crystallization from DMF/MeOH
to give the pure compound in 10% yield. As well as NMR
spectroscopy, the structural integrities were verified by FT-
IR spectroscopy, MALDI-TOF mass spectroscopy, and ele-
mental analysis for cage compounds 3a and 3b.
1
Figure 1. Comparison of H NMR spectra of a) crude cage compound 3a;
b) recrystallized cage compound 3a; c) crude cage compound 3b; and
d) recrystallized cage compound 3b. Depicted are only regions with typi-
cal peaks. For full spectra and assignment of the signals, see the Support-
ing information.
Crystals of both cage compounds were suitable for single-
Again, the cage compound itself is shape-persistent with an
extended cavity compared with that of 3a. Here, the intra-
molecular distance between two triptycene bridgehead pro-
tons is 9.81 ꢁ. The cage molecules self-assemble through p–
p and CH–p interactions, but in different patterns than cage
molecules 3a. From the crystal structure it can be estimated
that the intermolecular space is interconnected in three di-
mensions, although no continuous channels along any direc-
tion can be deduced. Again, the diffracting power of the
crystals was weak and no solvent molecules could be refined
satisfyingly. Therefore, the SQUEEZE program was again
used to correct the observed intensities for the influence of
the disordered molecules. The resulting calculated density is
[20]
crystal X-ray diffraction. Cage compound 3a crystallizes
¯
in the triclinic space group P1 and the asymmetric unit con-
tain two 3a molecules and 13 molecules of THF (Figure 2).
The cavity is shape-persistent, with distances of 7.81 or
.66 ꢁ between the two interior triptycene bridgehead hy-
7
drogen atoms. The cage compounds self-assemble mainly
through p–p and CH–p interactions to form a three-dimen-
sional porous system with enclathrated THF molecules.
Viewing along the crystallographic b axis, slit-like pores can
clearly be found (Figure 2b), whereas it is difficult to see the
porous channels along the other crystallographic axes (Fig-
ure 2c). The THF molecules can be considered to form
a three-dimensional interwoven network, by assuming the
Connolly surface of the THF molecules with a probe radius
of the van der Waals diameter of the atoms in THF (Fig-
ure 2d). After correction of the data by using the
À3
0.764 gcm , which is even lower than the density of com-
pound 3a.
Because the crystallographic data of both compounds ful-
À3
fill McKeownꢂs criteria (density <0.9 gcm , aromatic subu-
[21]
[22]
SQUEEZE routine of PLATON, the calculated density of
nits, and pores <10 ꢁ)
for potential permanent porous
À3
the solvent-free crystal is 0.868 gcm , which is in the range
crystals, we investigated the possibility of removing the en-
clathrated solvents from the pores without collapsing the
of potential permanent porous organic crystals, as described
[22]
[14]
by McKeown and co-workers.
Cage compound 3b crystallizes in the trigonal space
structure to achieve permanent porous organic crystals.
Initially we analyzed both materials by using thermogravi-
metric analysis (TGA). Both crystalline compounds lose sol-
¯
group R3 with six cage molecules in the unit cell (Figure 3).
Chem. Eur. J. 2012, 18, 4156 – 4160
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4157

M. Mastalerz et al.
3
À1
of nitrogen gas was 286 cm g at P/P =0.95. The calculated
0
2
À1
BET surface area is 744 m g and the corresponding Lang-
2
À1
muir surface area is 835 m g . This is, to the best of our
knowledge, the highest surface area for a [2+3] imine cage
compound and even higher than those reported for most of
the smaller [4+6] cage compounds. The pore volume
Vpore) is 0.26 cm g . The contribution of the micropore sur-
[3]
3
À1
(
face area on the total surface area was calculated to be 81%
by using the t-plot method. By using NL-DFT methods, the
pore-size distribution was found to be very narrow and
showed a maximum at 4.2 ꢁ.
In contrast to cage compound 3a, 3b seems to be of low
porosity: A sample of 3b treated as described above ad-
3
À1
sorbed only 25 cm g N2 at P/P =0.95. The determined
0
2
À1
specific surface area was 30 m g
04 m g (Langmuir model), which is much lower than for
(BET model) or
2
À1
2
cage compound 3a. From a t-plot analysis we concluded
that this measured surface area is basically not derived from
micropores, but rather from the bulk surface, which indi-
cates that the pores are very small and the adsorption of ni-
trogen at 77 K is kinetically hindered.
Figure 2. Single-crystal X-ray structure of 3a. a) Asymmetric unit. One
molecule is depicted as a space-filling model, the other one is shown as
a capped-stick model. The systematic disorder of one aromatic ring is
highlighted in yellow. Grey: carbon, white: hydrogen, red: oxygen, and
blue: nitrogen. b) Packing of a 2ꢃ2ꢃ2 unit cell viewed along the crystal-
lographic b axis. c) The same 2ꢃ2ꢃ2 unit cell viewed along a non-specif-
ic crystallographic axis. d) Connolly surface area of enclathrated THF
molecules exclusively of a 3ꢃ3ꢃ3 unit cell showing the three-dimension-
al channel system. In a), b), and c), enclathrated THF molecules are
omitted to show the channels and voids. In b) and c) the solvent-accessi-
ble surface area (probe radius 1.8 ꢁ) is colored in yellow.
We measured the powder X-ray diffraction (PXRD) pat-
terns of both compounds before and after nitrogen sorption
(see Figure 5). The PXRD pattern of 3a is almost identical
to the pattern calculated from the single-crystal X-ray struc-
ture before and after gas sorption, whereas the PXRD pat-
tern of 3b is different from the calculated one. This suggests
that the structure of the material is not stable under the
aforementioned evacuation conditions and undergo a phase
[3c]
transition to another crystalline polymorph, which is indi-
cated by different, sharp peaks in the PRXD pattern.
For a better comparison with other organic cage com-
pounds, we investigated both materials for their adsorption
properties of CO and methane at 298 K and 1.0 bar (see
2
[24]
also Table 1).
The more rigid compound 3a adsorbs
3
À1
6
2 cm g CO at 1.0 bar and 298 K, which corresponds to
2
1
1.9 wt%. This is significantly higher than described for
other [2+3] cage compounds and even higher than reported
for a bigger endo-functionalized [4+6] cage compound at
[
4]
2
73 K (9.4 wt%). In contrast to the adsorption of CO2,
3
À1
only 7.6 cm g (1.1 wt%) of methane is adsorbed at the
same temperature and pressure.
Most interestingly, cage compound 3b, which seemed to
Figure 3. Single crystal X-ray structure of cage compound 3b. a) View
along the crystallographic a* axis, b) view along the crystallographic
b axis. The color code is the same as for Figure 2. c) Packing of cage mol-
ecules in the unit cell viewed along the crystallographic c axis. d) View
along a non-specified axis. In c) and d) the solvent-accessible surface
area (probe radius 1.8 ꢁ) is colored in yellow.
be of low porosity from the results of N sorption at 77 K,
2
shows even a higher uptake of CO at 298 K than cage com-
2
3
À1
pound 3a, adsorbing 74 cm g , which corresponds to
4.5 wt%. Furthermore, the relative adsorbed amount of
1
3
À1
methane (7.6 cm g , 0.5 wt%) is significantly lower than for
cage compound 3a, which results in a high selectivity of 10
vent upon initial heating and start to decompose at approxi-
(mmol/mmol) or 29 (wt%/wt%) preferentially for CO over
2
mately 4008C. A sample of 3a was then treated at 3008C
and 6ꢃ10 mbar for 3 h before measuring the sorption of
methane.
À2
To summarize, we demonstrated that exo-functionalized
shape-persistent [2+3] cage compounds are accessible
through the dynamic formation of imine bonds. Herein, the
rigidity or flexibility of the aldehydic precursor plays an im-
portant role in the formation of the cage compound. Fur-
thermore, we found that the crystalline material with the
nitrogen at 77 K. The isotherm shows almost no hysteresis
between adsorption and desorption (see Figure 4) and can
be classified as a type I isotherm. This is typical for micro-
porous systems, which do not change their structural consti-
tution during gas uptake and release. The adsorbed amount
[23]
4158
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Chem. Eur. J. 2012, 18, 4156 – 4160

Exo-Functionalized Shape-Persistent [2+3] Cage Compounds
COMMUNICATION
more flexible introduced linker
underwent
a
phase change
during evacuation to give low
[25]
permanent porosity.
With
a stiffer linker unit, we were
able to remove solvent and pre-
serve the three-dimensional
pore structure, which resulted
in high BET surface areas of
2
À1
7
44 m g . This value is signifi-
cantly higher than reported for
other [2+3] imine cages and is
even comparable with many of
the [4+6] cages described
before.
Currently, we are systemati-
cally investigating the reaction
of triamine 1 with bissalicylal-
dehydes with various degrees of
molecular rotational freedom to
get a deeper insight in the mo-
Figure 4. Nitrogen sorption isotherms for cage compound 3a (*, *) and 3b (^, ^) at 77 K. Filled symbols are
adsorption, open symbols are desorption isotherms.
lecular needs for both an efficient cage formation and the
efficient construction of permanent porous materials from
discrete cage compounds. Moreover, the exo-directing sali-
cylimine units of 3a and 3b suggest their use as bidentate li-
gands to form secondary building units by complexing vari-
ous metal ions, which would result in new types of cage-
[26]
MOFs that will be studied in due course.
Acknowledgements
We would like to thank the German Research Council (Deutsche For-
schungsgemeinschaft, DFG) for funding (project MA4061/4-1) and the
Fonds der Chemischen Industrie for generous financial support. We
thank M. Ceni c´ for preliminary investigations of the synthesis of bissali-
cylaldehyde 2a. Prof. G. Raabe and Dr. I. L. Atodiresei (both RWTH,
Aachen) are acknowledged for helpful assistance in obtaining the X-ray
data. We are grateful to C. Egger, S. Blessing, and G. Weber (all Ulm
University) for measuring nitrogen sorption, performing PXRD, and per-
forming TGA experiments, respectively. Furthermore, we thank Solvay
Fluor GmbH for the donation of chemicals.
Figure 5. Comparison of the PXRD of cage compounds 3a (top) and 3b
bottom) before (c and f) and after (d and g) nitrogen sorption measure-
(
Keywords: cage compounds
· microporous materials ·
ment with the calculated pattern from the single-crystal structure analy-
ses; a) with enclathtated THF molecules, b) without enclathrated THF
molecules, and e) without solvate molecules.
salicylaldehyde · schiff bases · triptycene
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SALangmuir
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2
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4
CH [mmolg ] at
2
À1
2
À1
[m g
]
[m g
]
298 K,1.0 bar
298 K, 1.0 bar
[
[
3
3
CC6
a
b
744
30
99
835
204
n.d.
n.d.
2.7
3.3
0.9
0.67
0.34
4
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1
Chem. Eur. J. 2012, 18, 4156 – 4160
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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T=100(2) K; C82
H
52
N
6
O
6
; M=1217.30 gmol ; triclinic; space group
¯
P1; a=21.990(1), b=22.320(1), c=23.800(1) ꢁ; a=94.648(3), b=
116.848(3), g=110.901(3)8; V=9316.6(9) ꢁ ; Z=4;
[
[
[
3
6
calcd
1 =
À3
À1
0.868 gcm ; m=0.441 mm ; 2.178<V<66.008; reflections collect-
ed/unique 167668/31894, [R(int)=0.0736]; data(observed)/restraints/
parameters 24027/0/1779; GOF 1.083; final R[xI>2s(I)] R
wR
Crystal data for 3b: T=100(2) K; C88
A
H
U
G
R
N
U
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U
T
E
N
N
1
=0.0644,
À3
2
(all data)=0.1802, residual density 0.358 and À0.273 eA
.
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À1
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N
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O
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4
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Received: January 4, 2012
Published online: March 5, 2012
4160
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Chem. Eur. J. 2012, 18, 4156 – 4160