Uniform porous nanospheres of discrete shape-persistent organic cage compounds

Article abstract of DOI:10.1039/c2jm16051j

By mixing solutions of the reactants for shape-persistent cage compounds in a binary solvent mixture, spheres in the nanometre regime with controlled sizes can be generated, which show permanent porosities.

Full text of DOI:10.1039/c2jm16051j

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COMMUNICATION
Uniform porous nanospheres of discrete shape-persistent organic cage
compounds†
a
Markus W. Schneider, Lorenz G. Lechner and Michael Mastalerz
b
a
*
Received 21st November 2011, Accepted 27th February 2012
DOI: 10.1039/c2jm16051j
Mirkin and Oh first showed that instead of the typical crystalline
ICPs, defined amorphous globular assemblies can be generated from
the same molecular building units.14 In comparison to their crystalline
counterparts, nanospheric porous ICPs have e.g. good dispersibility,
which is advantageous for processing the ICPs as thin films.11
As already mentioned above, research activities in the field of
discrete organic compounds as precursors for porous materials are
still in a nascent state and need to be studied in a broader sense,
including how morphology influences the porosity of the materials.
To date, only a few examples of investigations on different crystalline
polymorphs of cage-compounds were reported.10,15,16 For example,
Cooper et al. revealed that the porosities of certain [4+6] cage
compounds can be switched on and off by generating various poly-
morphs from different solvents.15a Very recently, it was shown that
the shape and porosities of particles of racemic mixtures of such cages
can be controlled by defined mixing of the two enantiomeric cage
compounds.16 However, to the best of our knowledge, no controlled
method to generate smooth nanospheric assemblies of shape-persis-
tent cage compounds during the synthesis was described to date.
Here, we communicate our first results on a synthetic procedure of
[4+6] cage compounds, which form micro- or nanospheric particles
by use of a well-balanced binary solvent mixture. Furthermore, we
compare the nitrogen sorption and the derived surface areas of the
amorphous nanospheric compounds with their crystalline
counterparts.10
By mixing solutions of the reactants for shape-persistent cage
compounds in a binary solvent mixture, spheres in the nanometre
regime with controlled sizes can be generated, which show perma-
nent porosities.
In recent years, the synthesis and investigation of porous materials
have gained more and more attention due to the fact that they show
various interesting properties, e.g. for separation or storage of gases1
or as highly active homogeneous catalysts.2 Porous materials became
more diverse and can be divided into four main classes: (i) purely
inorganic materials, such as zeolites;3 (ii) hybrids of organic linker
molecules and metal clusters, so-called metal–organic frameworks
(MOFs) or infinite coordination polymers (ICPs);4 (iii) purely organic
materials, such as crystalline covalent organic frameworks (COFs)5
or amorphous conjugated microporous polymers (CMPs);6 and (iv)
porous materials deriving from discrete molecules.7
The last one can be seen as a relatively new class of porous
materials, because the specific surface areas did not exceed the value
of 300 m2 gꢀ1 till 2009.8 We and others have demonstrated that shape-
persistent organic cage-compounds9 are superior within this class of
permanent porous crystalline materials having specific surface areas
of up to 2071 m2 gꢀ1 (Brunauer–Emmett–Teller model, BET).10
For polymeric porous compounds, it is known that morphology
has a high impact on the physical properties of the material.11 To
adjust such properties, controlling the morphology and size of the
material particles in a defined manner is desired. For instance,
Kitagawa and co-workers reported that the particle sizes of crystal-
line ICPs can be controlled by the addition of a modulator. Gas
adsorption behaviour was evidently related to the particle size of the
crystals.12 In a similar fashion, the addition of ammonium salts to the
reaction mixture led to the control of crystal-size and -shape, which
influences the material properties dramatically when in the nano-
sized regime.13
Recently, we introduced the one-pot synthesis of endo-function-
alized adamantoid [4+6] cage compounds by the condensation of
triptycene triamine and various salicyldialdehydes.10,17
Previously, we reported that the cage compounds with bulkier
groups in the periphery (3a and 3b, Scheme 1) had to be precipitated
through the addition of n-pentane to the THF solution after the
reaction mixture reached the thermodynamic equilibrium (here, we
call it method A), to obtain the pure products 3aA and 3bA.10 Material
properties of 3a and 3b revealed a strong dependency on the
morphology of the material. The as-synthesized materials are
amorphous with BET surface areas of approx. 700 m2 gꢀ1 (3aA and
3bA). When the compounds were crystalline, the specific surface areas
were the lower, the bulkier the peripheral substituents are (SA ¼
22 m2 gꢀ1 for 3bcr; SA ¼ 309 m2 gꢀ1 for 3acr). Method A was not
sufficient for the synthesis of another cage compound, with an n-butyl
chain in the periphery (3c). For this cage compound, we identified by
screening of several conditions that a binary solvent mixture of THF/
MeCN (1.6 : 1 v/v) was the ideal reaction medium to get the product
aInstitute of Organic Chemistry II & Advanced Materials, Ulm University,
Albert-Einstein-Allee 11, Ulm, Germany. E-mail: michael.mastalerz@
uni-ulm.de; Fax: +49 731-40-22840; Tel: +49 731-40-22844
bCentral Facility of Electron Microscopy, Electron Microscopy Group of
Materials Science, Ulm University, Albert-Einstein-Allee 11, D-89069
Ulm, Germany
† Electronic supplementary information (ESI) available: Synthetic
procedures, full characterization and nitrogen sorption measurements
for compounds 3c–e. See DOI: 10.1039/c2jm16051j
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(see ESI†). Investigation of the morphology by means of powder
X-ray diffraction revealed that the material is amorphous. Surpri-
singly, by scanning electronic microscopy (SEM) imaging, we found
that nearly homodisperse spheres were formed with an average
diameter of 374 nm ꢁ 76 nm (s.d., n ¼ 60) (Fig. 1a and b). Since this
cage compound with n-butyl groups is a new derivative in the row of
endo-functionalized [4+6] cage compounds,9b,10 it was not clear if the
formation of the nanospheres can be deduced as a unique property to
the less bulky peripheral n-butyl substituents of the molecular struc-
ture, or if the nanospheres derive from method B. Therefore, 1 and 2a
as well as 1 and 2b were reacted under conditions similar to method
B, to figure out, whether it is possible to control the morphology of
synthesized cage compounds, independently from their molecular
structure. To our delight, cage compounds 3aB and 3bB could be
synthesized as nanospheres from THF/MeCN (1 : 1.36 v/v and 1 : 1
v/v) according to the procedure described above for 3cB. By SEM
imaging (Fig. 1e and f), nanospheric assemblies can be detected for
both compounds 3aB and 3bB with average sphere diameters of 142 ꢁ
43 nm (s.d., n ¼ 60) for 3aB and 274 ꢁ 70 nm (s.d., n ¼ 60) for 3bB.
This is in contrast to the amorphous material generated by protocol
A, where the resulting particles are also amorphous, but much less
structured.10 Analysis of the compounds by 1H NMR-, IR- and mass-
spectroscopy are in agreement to those synthesized before by method
A, showing that the molecular composition is identical,10 although
the morphology is different.
Scheme 1 One-pot synthesis of shape-persistent adamantoid cage
compounds 3a–3c. (i) Reaction conditions: THF, reflux, three days, then
precipitation with n-pentane (3aA and 3bA);10 reaction in THF/acetoni-
trile, reflux, 2–3 days (3aB–3cB).
It is known for ICPs that the amorphous nanospheric assemblies
are less porous than the crystalline ones.1 To analyze the porosities of
the generated nanospheres 3cB by N2-sorption, we investigated the
thermal stability by thermogravimetrical analysis (TGA) first, which
suggested that the material is stable up to 400 ꢂC (see ESI†). A sample
of the compound was degassed at 200 ꢂC and 6 ꢃ 10ꢀ2 mbar for 23 h
as an orange powder in high yields. The authenticity and purity of 3cB
(here, we term the binary solvent approach method B) was proved by
NMR-, IR-, and mass-spectroscopy as well as by elemental analysis
Fig. 1 Scanning electron microscopic (SEM) images of nanospheres of 3cB (a and b) having an average diameter of 374 nm ꢁ 76 nm (s.d., n ¼ 60),
crystallized 3ccr (c and d) by slow diffusion of acetonitrile into a saturated THF-solution, and nanospheres of 3aB (e) and 3bB (f) with average diameters of
142 ꢁ 43 nm (s.d., n ¼ 60) and 274 ꢁ 70 nm (s.d., n ¼ 60). The white frames in (a) and (c) mark the zoomed-in sections depicted in (b) and (d),
respectively.
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is that the small hysteresis between adsorption and desorption is
located in the region between P/P0 ¼ 0.4 and 1.0, whereas for the
amorphous material 3cB the hysteresis is located in the low P/P0
regime below 0.4. As reported for similar cages (amorphous/crystal-
line) the micropore contribution in the crystalline material is higher
than in the amorphous porous material. From the t-plot method,
a micropore contribution of 48% on the overall surface can be
calculated, which is expected for a more ordered structure and
complements trends described before for similar pairs of poly-
morphs.10 The calculated specific surface area is with 174 m2 gꢀ1
(BET-model) or 196 m2 gꢀ1 (Langmuir-model) comparable to that of
the amorphous material of 3cB. This strengthens the observation that
there is no general ‘‘rule of thumb’’ for cage compounds that crys-
talline or amorphous materials will have higher specific surface areas.
We also measured the specific surface areas of nanospheres 3aB and
3bB. First investigations of the material by nitrogen sorption revealed
specific surface areas of 38 m2 gꢀ1 (BET) and 42 m2 gꢀ1 (Langmuir)
for 3aB, and 36 m2 gꢀ1 (BET) and 39 m2 gꢀ1 (Langmuir) for 3bB. Those
values are substantially lower than those measured for amorphous
3aA and 3bA.10 It is worth mentioning that the surface area of crys-
talline 3bcr was in the same range (22 m2 gꢀ1) as that of the nano-
spheres.10 If we compare bulk amorphous, nanospheric amorphous
and bulk crystalline materials of those compounds, it seems for now
that the nanospheric materials properties are closer to the bulk
crystalline ones. This finding for adamantoid [4+6] cage compounds
complements those published recently for the racemic mixtures of the
Cooper cages.16 However, these first results have to be proofed
further by a more detailed study, which will be published in due
course.
Fig. 2 Nitrogen sorption isotherms of 3c at 77 K. Top: amorphous,
nanospheric material 3cB; bottom: crystalline material 3ccr. Filled
symbols: adsorption isotherms; open symbols: desorption isotherms.
before N2-sorption measurements were performed. The sorption
isotherm (Fig. 2, top) of nanospheric 3cB can be classified as a hybrid
of a type-II and a type-I isotherm, revealing that the material is
mainly macroporous. The pore volume is calculated to be 0.11 cm3
gꢀ1. From the t-plot analysis, a micropore contribution of only 11%
of the overall surface is estimated. The calculated specific surface area
is 148 m2 gꢀ1 (BET-model) or 182 m2 gꢀ1 (Langmuir-model)
(Table 1).
To conclude, we introduced a new procedure to control the
morphology of shape-persistent organic cage compounds during
their syntheses to generate uniform spheres in the nanometre regime,
which were shown to be permanently porous. Most interestingly, for
3b and 3c, the sorption behaviour for nitrogen gas seems not to differ
whether the material is nanospheric or bulk crystalline.
As mentioned above, the specific surface area of ICP compounds
was shown to depend strongly on the morphology with usually lower
surface areas of the amorphous materials. This is not necessarily the
same trend for porous cage compounds.10,16 To compare crystalline
and nanospheric amorphous cage compound 3c, we crystallized cage
compound 3ccr as red crystals by slow diffusion of MeCN into
a saturated THF-solution. As can be seen by SEM imaging (Fig. 1c
and d), there are still some nanospheres left, sitting on top of the
crystals. The crystalline material was treated the same as the nano-
spheric one (200 ꢂC and 6 ꢃ 10ꢀ2 mbar for 23 h) prior to N2-sorption
measurements. Most interestingly, the isotherm is very similar to that
of the amorphous polymorph (Fig. 2, bottom). One minor difference
These types of nanospheres are, to the best of our knowledge, not
described to date for one-component materials of discrete cage
compounds16 and open the opportunity to further study the materials
properties, e.g. for processing the cage structures into thin films.
Furthermore, the influence of the reaction parameters (e.g. medium
and reaction temperatures, concentration and mixing rates) on shape
and porosity of such nanoparticles will be studied in more detail.
We would like to thank the ‘Deutsche Forschungsgemeinschaft’
DFG for funding the projects MA4061/4-1 and MA4061/5-1 as well
as the ‘Fond der Chemischen Industrie’ (FCI) for the financial
support. Solvay Fluor GmbH is acknowledged for the donation of
chemicals. We thank C. Egger for gas sorption, S. Blessing for PXRD
and G. Weber for TGA measurements.
Table 1 Comparison of selected materials properties of 3a–3c derived
from various synthetic methods
Notes and references
BET surface area/m2 gꢀ1
Average
Amorphous Nanospheres nanosphere
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