Post-modification of the interior of porous shape-persistent organic cage compounds

Article abstract of DOI:10.1002/anie.201208156

Interior decorating: A post-synthetic method allows porous organic cage compounds to be prepared with functionalized interior cavities. The approach produces modified cage compounds in quantitative yield and opens the possibility of preparing organic alloys with different functionality. The solution-based technique shows the advantage of solubility, an inherent property of porous materials derived from discrete organic molecules. Copyright

Full text of DOI:10.1002/anie.201208156

Angewandte
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DOI: 10.1002/anie.201208156
Porous Organic Cage Compounds
Post-Modification of the Interior of Porous Shape-Persistent Organic
Cage Compounds**
Markus W. Schneider, Iris M. Oppel, Alexandra Griffin, and Michael Mastalerz*
Cage compounds are fascinating molecules for several
reasons.[1] They are defined molecular reaction vessels for
“uncommon” products,[2] or used to stabilize highly reactive
species in their interior.[3–5] Recently, materials made from
purely organic cage compounds showed remarkable perma-
nent porosities, with very high surface areas and good gas-
sorption properties, both, in crystalline as well as amorphous
phases.[6–8]
A feature that distinguishes the porous materials derived
from cage compounds from those derived from extended
network structures (such as metal organic frameworks
(MOFs) and covalent organic frameworks (COFs)) is, that
the intrinsically porous building units (the cage molecules)
are soluble.[9] This creates several possibilities, which cannot
be easily achieved for extended network structures or are
even impossible. For instance, Cooper et al. reported on the
co-crystallization of organic cage compounds in a binary or
even ternary fashion to create porous organic alloys.[10,11] Very
recently, this property was exploited to grow microporous
cage crystals in mesoporous silica.[12] Another example of
“processable” porosity has been demonstrated by our
group:[13] various cage compounds, which are highly porous
in the bulk, can be deposited as thin films on quartz crystal
microbalances (QMBs) by spray-coating. The modified
Scheme 1. Synthesis of cavity-modified cage compounds 5a–5e by two
different approaches. Method A: direct route by 12-fold imine conden-
sation. Method B: Post-synthetically modification by sixfold Williamson
ether formation. For reaction conditions, results, and yields, see
QMBs showed very good affinities for several aromatic
analytes.
In 2008, we introduced the one-pot synthesis of the endo-
functionalized [4+6] cage compound 3 by reacting four
Table 1 and Supporting Information.
molecules of triamine 1 and six molecules of salicyldialdehyde
2 (Scheme1).[14] What distinguishes this type of cage com-
pounds from others, is that it bears six hydroxy groups
pointing to the center of the cage cavity. This structural motif
is very rare for organic cage compounds, and the functional-
ization of the cages interiors through reaction at the hydroxy
groups was to date unsuccessful. Herein we present a facile
method for the post-synthetic modification of the intrinsic
voids in the cage compound, which allows the pore structure
of the resulting material to be “fine-tuned” in the solid state.
Initially, we attempted to directly synthesize 5a by the
reaction of O-methylated salicyldialdehyde 4a with triamine
1 (Method A in Scheme 1).[14] Unfortunately, only a small
amount of cage 5a was detected in the crude product by the
MALDI-TOF mass spectroscopy.[14] However, cage 5a could
not be isolated by chromatographic methods.[15] Further
optimization of the reaction conditions did not offer
a decent route to synthesize 5a in reasonable yield (never
exceeding 17%). 1H NMR analysis of the crude product
(Figure 1b) exemplifies the high complexity of the resulting
mixture. Similarly, reactions of triamine 1 with other salicyl-
dialdehyde ethers 4b–4d failed too. These unsuccessful
experiments forced us to switch to an indirect approach (see
[*] Dipl.-Chem. M. W. Schneider, Dr. M. Mastalerz
Ulm University
Institute of Organic Chemistry II & Advanced Materials
Albert-Einstein-Allee 11, 89081 Ulm (Germany)
E-mail: michael.mastalerz@uni-ulm.de
Prof. Dr. I. M. Oppel
Inorganic Chemistry Department, RWTH Aachen
Landoltweg 1, 52056 Aachen (Germany)
Dr. A. Griffin
Oxford Diffraction (Agilent Technologies) Yarnton (UK)
[**] We thank the German Research Foundation (Deutsche For-
schungsgemeinschaft) for funding (project MA4061/5-1) and the
“Fonds der Chemischen Industrie” for the generous financial
support. We thank C. Egger (Ulm University) for measuring the
nitrogen sorption isotherms, and S. Blessing (Ulm University) for
recording the PXRD data.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208156.
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other alkyl halide reagents. With this method we were able to
introduce propyl (5b), allyl (5c), and benzyl (5d) substituents
in high yields of 63 to 81% (see Supporting Information).
However, by using 4-nitrobenzyl bromide, only 12% yield of
5e was isolated, most probably because of the products low
stability.
Cage compounds 5a–5d were investigated by nitrogen
sorption at 77 K to examine the impact of the introduced
moieties on the specific surface areas determined by the
Brunauer-Emmett-Teller (BET) model. All samples were
degassed at 2008C and 6 ꢀ 10À2 mbar for 3 h, prior to the
measurement. As expected, with increasing bulkiness of the
introduced moieties, the specific surface area decreases. For
instance, the propylated cage 5b gave a lower BET surface
area (491 m2 gÀ1) than the methylated cage 5a (824 m2 gÀ1).
The allylated cage 5c showed a slightly lower specific surface
(333 m2 gÀ1) than 5b. If the bulky benzyl substituent is
introduced, almost all of the voids interior is “filled”, which
results in a BET surface area of 119 m2 gÀ1. Investigating the
materials after performing the gas sorption experiments by
powder X-ray diffraction (PXRD) revealed that all the
samples are mainly amorphous (Supporting Information). It
was reported before that the permanent porosity of periphery
substituted [4+6] cage compounds is strongly depending on
the morphology,[7d,16] which encouraged us to obtain crystal-
line material of the post-synthetic modified cage compounds.
We grew suitable single-crystals of 5a for X-ray diffraction
analysis by slow evaporation of solvent from a concentrated
THF solution (Figure 2, left). Six molecules of 5a crystallized
in the unit cell with trigonal space group R3.[17] Since some
THF molecules located inside the crystal voids could not be
refined satisfyingly, the electron-density of disordered solvent
molecules (64% of the unit cell) was removed with the
SQUEEZE function to elucidate the structure.[18] All the
hydroxy protons in 5a have been substituted by methyl
groups. In contrast to the crystal structure of 3, this
substitution led to a repulsion of the imine nitrogen lone
pairs and the oxygen lone pairs, which forces all the imine
Figure 1. Partial 1H NMR spectra of a) cage compound 3, measured in
[D6]DMSO at 373 K; b) crude product of the reaction of 4a and
1 (Method A); the # indicates aromatic protons of mesitylene, which
was added as internal standard to quantify the amount of cage
compound 5a formed. c) Pure cage compound 5a (Method B). b) and
c) were measured in [D8]THF. For full spectra and peak assignment,
see Supporting Information.
Method B in Scheme 1): modifying the cage interior by
introducing substituents after the cage scaffold is formed.
We started the investigation with methyl-group introduc-
tion by a six-fold Williamson ether synthesis. After optimizing
the reaction conditions (1.2 equiv. MeI, DMF, K2CO3, 708C,
16 h), we were able to isolate 5a in 82% yield. The 1H NMR
spectrum of pure 5a (see Figure 1c) clearly shows a full
conversion of all six hydroxy groups by a characteristic signal
of the methoxy-group protons, resonating at d = 4.09 ppm
with an integral of 18H. The compounds purity and integrity
was further confirmed by mass spectrometry, IR-, and
13C NMR-spectroscopy, elemental analysis (see Supporting
Information), and ultimately, single-crystal X-ray diffraction
(see below).
To explore the scope of the post-synthetic modification by
Williamson etherification, several alkyl halides were tested in
the reaction (see Scheme 1, Table 1). However, we found that
DMSO is a better solvent than DMF for reactions with all the
Table 1: Yields and specific surface areas of post-synthetically modified
cage compounds 5a–5e.
[c]
[d]
Compound
Yield[a]
method A [%]
Yield[b]
method B [%]
SABET
Vpore
[m2 gÀ1
]
[cm3 gÀ1
]
5aam
17
–
–
81
–
–
63
76
81
12
824
741
1700
494
333
119
0.43
0.38
0.71
5acr(200)[e]
5acr(rt)[f]
5b
[g]
24
23
38
–
[g]
5c
–
–
[g]
5d
5e[h]
–
–
–
[g]
[g]
[g]
[a] Determined by integration of characteristic peaks in the 1H NMR
spectrum of the crude product against mesitylene as internal standard.
[b] Yield of isolated product. [c] BETsurface area determined by nitrogen
sorption at 77 K. [d] Determined by NLDFT. [e] Activated at 2008C
[f] Activated at room temperature. [g] Not determined. [h] Compound
decomposes within a few hours.
Figure 2. Single-crystal X-ray structures of 3 (right) and 5a (left). a)
and b) C3-symmetric molecule. c) and d) packing. In (a) and (b):
gray C, white H, blue N, red O. In (c) and (d) the molecules are
colored to highlight the packing.
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nitrogen atoms to change their orientation directing with their
lone pairs out from the cavity, which remains shape-persis-
tent. Most interestingly, there are some similarities to the
crystal structure of 3.[7d] First of all the molecular symmetry of
C3 is the same and the molecular dimensions are similar. The
inner triptycene bridgehead protons build a slightly distorted
tetrahedron with an average edge length of 11.4 ꢁ (for 3 it is
10.4 ꢁ). The six methyl carbon atoms form a distorted
octahedron with an average edge length of 8.53 ꢁ (The
oxygen atoms form an octahedron with edge length of
9.70 ꢁ). As mentioned, 5a crystallized in the trigonal space
group R3, whereas 3 crystallized in the trigonal space group
prevented by a complete exchange of solvents.[20] We applied
this method to crystalline 5a to desolvate the material at
room temperature (to give 5acr(rt)) and indeed, with
1700 m2 gÀ1, the specific BET surface area is nearly double
that of the amorphous material. It is worth mentioning, that
this value is higher than for as-synthesized cage compound 3
(1377 m2 gÀ1) but lower than for crystalline 3 (2071 m2 gÀ1) and
consequently the second highest reported for intrinsically
porous materials derived from organic molecules.[6] PXRD
after the gas sorption measurement revealed that the sample
remained crystalline. The pore-size distribution has now
a sharp maximum at 10.3 ꢁ, which is comparable to that of
3.[7d]
ꢀ
R3. However the dimensions of the unit cells are comparable.
For 3 the length of the a and b axis is 29.15 ꢁ, for 5a it is
30.37 ꢁ. Most interestingly, the packing of the molecules is
very similar (see Figure 2c and d). In a very simplified way,
removing the yellow as well as the upper green and orange
highlighted molecules in Figure 2d, would give a similar
pattern to that depicted in Figure 2c.
It is established that the adsorption of gas molecules is
dominated by dispersion interactions, especially when the
adsorbent is a covalent organic material.[21] Consequently,
surface area and pore-size have a major effect for the
adsorption of non-polar gases, such as methane. As expected,
the curve progression for the heat of adsorptions of methane
is very similar for both 3 and 5acr(rt) with DHads(3) =
40.7 kJmolÀ1 and DHads(5a) = 36.1 kJmolÀ1 (Table 2). This
Consequently the unit cell of 3 contains the double
number of molecules as found for 5a and the c axis (58.52 ꢁ)
is double the length as that for 5a (25.85 ꢁ).The calculated
density of the framework after in silico removal of residual
solvent molecules is at 0.55 gcmÀ3 exceptionally low for
a solid compound, indicating that the material is potentially
very porous.[19] For 3 the density is comparable, 0.51 gcmÀ3.
Crystalline compound 5a was investigated by nitrogen
sorption at 77 K. If crystalline 5a was treated analogously to
the as-synthesized 5a (2008C and 6 ꢀ 10À2 mbar for 3 h)
before measuring the nitrogen sorption isotherms, no signifi-
cant difference in isotherm shape or the resulting specific
surface areas, micropore volumes, and pore-size distributions
is found (see Figure 3 and Table 1). PXRD of the crystalline
material after gas sorption clearly showed that it became
amorphous, which explains the similar sorption data.
Table 2: Gas sorption data.
Compound
Adsorbed volume (wt%) at
1 bar [cm3 gÀ1
CO2 CH4
DHads [kJmolÀ1
CO2 CH4
]
]
CO2
CH4
273 K
283 K
263 K
273 K
3
60.2
(11.8)
47.4
(9.3)
21.1
(1.5)
15.8
(1.1)
60.6[a] 40.7[c]
12.2[b] 36.1[d]
5acr(rt)
56.3
43.1
17.4
14.6
(11.0)
(8.4)
(1.2)
(1.0)
[a] At an adsorbed volume of 0.14 cm3 gÀ1. [b] At an adsorbed volume of
0.16 cm3 gÀ1. [c] At an adsorbed volume of 0.27 cm3 gÀ1. [d] At an
adsorbed volume of 0.20 cm3 gÀ1
.
The loss of crystallinity during the desolvation of porous
molecular crystals is often accompanied by the loss of
porosity, especially for extrinsically porous crystals. Recently,
we have demonstrated that this loss of porosity can be
suggests that only dispersion interactions are responsible and
the material with the higher specific surface area (3,
2071 m2 gÀ1)[7d] shows a slightly better performance.
However, the surface environment plays an additional
role for the adsorption of polar gases. Since the pore surface
of 3 with its hydroxy groups is more polar than that in 5a with
its methoxy groups, polarity might have a small but recog-
nizable effect on the sorption of CO2. Indeed, both com-
pounds show very different heats of adsorption curves for
CO2 (see Supporting Information). For the more polar
compound 3 the heat of adsorption at low adsorbed volume
(V= 0.14 cm3 gÀ1) is very high (DHads = 60 kJmolÀ1) and with
increasing adsorbed volume decreases to DHads = 25 kJmolÀ1
at V= 0.8 cm3 gÀ1, before leveling out between 25 and
27 kJmolÀ1. For the less-polar compound 5acr(rt), the heat
of adsorption at V= 0.16 cm3 gÀ1 is with DHads = 12 kJmolÀ1
significantly lower than for 3. With increasing volumes, DHads
reaches a level of approximately 22 kJmolÀ1.
We assume that for the adsorption of CO2 by 3 at low
adsorbed volume, an additional interaction of the hydroxy
moieties with the CO2 molecules can occur through hydrogen
bonding,[22] which would explain the high DHads value of
Figure 3. Nitrogen sorption isotherms of cage compound 5a at 77 K.
Closed symbols: adsorption; open symbols: desorption. Circles:
amorphous material. Squares: crystals desolvated at 2008C; triangles:
crystals activated at room temperature after solvent exchange.
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60 kJmolÀ1 at low surface coverage. In contrast, for com-
pound 5acr(rt), there is no possibility for such hydrogen bonds,
which is reflected in the small initial value for DHads of
12 kJmolÀ1. These examples clearly show that by the method
presented herein the pores cannot only be adjusted in terms
of specific surface area and pore volume but also in terms of
function.
In summary, we have shown for the first time that the
interiors of porous organic cage compounds can be post-
synthetically modified in high yields. The numerous reaction
sites can be transformed quantitatively, which greatly sim-
plifies the purification procedure. This method provides
a rapid way to enrich the organic-cage-compound family
and opens opportunities, for example, to form porous organic
alloys with versatile functionality.[11] It is worth mentioning
that the post-synthetic functionalization of extended non-
soluble porous frameworks by the formation of covalent
bonds still is a difficult task.[23,24] In contrast to our approach,
in most cases conversion of the interior functional groups is
incomplete and very seldom surpasses 90%.[25] Once more,
this clearly underlines the advantage of solubility, which is an
inherent property of porous materials derived from discrete
organic molecules.
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[9] J. R. Holst, A. Trewin, A. I. Cooper, Nat. Chem. 2010, 2, 915 –
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[10] a) J. T. A. Jones, T. Hasell, X. Wu, J. Basca, K. E. Jelfs, M.
Schmidtmann, S. Y. Chong, D. J. Adams, A. Trewin, F. Schiff-
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[14] M. Mastalerz, Chem. Commun. 2008, 4756 – 4758.
[15] It has been reported, that a similar type of cage compound is
even stable in boiling water: T. Hasell, M. Schmidtmann, C. A.
Stone, M. W. Smith, A. I. Cooper, Chem. Commun. 2012, 48,
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chromatography.
Received: October 10, 2012
Revised: December 19, 2012
Published online: February 19, 2013
Keywords: cage compounds · gas sorption · porous materials ·
.
post-synthetic modification · supramolecular chemistry
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[17] Data were collected on an Oxford Diffraction SuperNova Atlas
Dual system with a (Cu) microfocus source and focusing
multilayer mirror optics. The structure was solved by direct
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anisotropic thermal parameters. X-ray data have been deposited
at the Cambridge Crystallographic Data Centre (CCDC 904717
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.) Crystal Data for 5a: T= 100(2) K, C158H140N12O6,
Mr = 2302.82, trigonal space group R3, a = b = 30.374(4), c =
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