Metal-assisted salphen organic frameworks (MaSOFs) with high surface areas and narrow pore-size distribution

Article abstract of DOI:10.1039/c1cc14805b

The one-pot three component synthesis of metal containing microporous organic polymers with high BET surface areas is presented. The metal salphen units were built during the formation of the porous polymers. Selective gas adsorption depending on the metal ions is discussed.

Full text of DOI:10.1039/c1cc14805b

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COMMUNICATION
Metal-assisted salphen organic frameworks (MaSOFs) with high surface
areas and narrow pore-size distributionw
Michael Mastalerz,*a Hans-Jochen S. Hauswaldb and Raphael Stollb
Received 3rd August 2011, Accepted 20th October 2011
DOI: 10.1039/c1cc14805b
The one-pot three component synthesis of metal containing
microporous organic polymers with high BET surface areas
is presented. The metal salphen units were built during the
formation of the porous polymers. Selective gas adsorption
depending on the metal ions is discussed.
Since the introduction of covalent organic frameworks (COFs),1
porous organic polymers2 are recognized in a broader sense as a
Scheme 1 One-pot three component reaction of salicylaldehyde 1,
o-phenylene diamine 2 and metal(II) ions to metal salphen complexes 3a
and 3b, respectively. (i) Ethanol, 95 1C, 1d; yields: 82% (3a); 69% (3b).
new alternative class of porous materials besides metal–organic
frameworks (MOFs)3 or zeolites.4 These porous organic com-
We envisioned the introduction of metal salphen units into
pounds are usually synthesized from one or two components
the porous material by constructing the frameworks and the
either in reversible1,5,6 or irreversible reactions.7–11 High
metal salphen in the same synthetic step. Therefore, we exploited
the well-known Schiff base reaction of salicylaldehydes20 (e.g. 1)
surface areas were achieved with reversible as well as irrever-
sible methods.1,8
with o-phenylene diamine 2 in the presence of metal acetate to
Till date, metal complexes of those porous organic polymers
give the corresponding metal salphens 3a or 3b (Scheme 1),
are rare and are either created by the introduction of metal
ions via post functionalization,12 or by the use of predefined
metal-assisted salphen organic frameworks (MaSOFs, Scheme 2).
which can be seen as sectional parts of our polymeric porous
metal phthalocyanine or porphyrine building blocks as reactants.13
We started to investigate the reaction of rigid tetrahedral
tetrakis salicylaldehyde 420 with o-phenylene diamine 2 and
Nevertheless, ‘‘metal-doped’’ porous organic frameworks become
more and more into the focus of interest, because free metal sites
evidently influence gas adsorption in a positive manner.14 Further-
various solvents for the reaction such as ethanol, acetonitrile,
nickel acetate tetrahydrate, to give MaSOF-1. First we tested
more, those metal sites can act as catalytic centers.15
N,N-dimethylformamide (DMF) and toluene at elevated
To the best of our knowledge, there is only one type of porous
temperatures. Exclusively in DMF at 95 1C a voluminous reddish
brown homogeneous precipitate was formed in 98% yield.
covalent organic framework described, where the metal ions are
incorporated into the network through the polymerization of the
Thermogravimetric analysis (TGA) of the freshly prepared,
bis(phthalodinitriles), having a template as well as a catalytic
air-dried compound in a nitrogen atmosphere shows three major
effect on the formation of the covalent bonded network.16 These
steps of mass loss: at low temperatures (o80 1C) the compound
loses 10% of the mass, suggesting that this is loosely bound
porous networks have BET surface areas between 489 m2 gÀ1
and 895 m2 gÀ1. It was also demonstrated that those networks
solvent from the outer surface of the particles; second step is
had catalytic properties.17
assigned to the loss of solvent bound inside the pores (15% at
Salphens and salens are versatile tetradentate ligands and
270 1C); and third step between 270 and 400 1C (4% mass
their corresponding metal complexes are well-known for their
loss), which corresponds probably to the removal of solvent
excellent catalytic activities for a number of transformations.18
molecules bound on the microporous surface through weak
There are a few examples of porous materials containing
interactions with the generated nickel salphen sites. A TGA
salphen metal units within the pores. For instance, salens are
measurement of the same material in an oxygen atmosphere
introduced as linkers for the construction of zinc-MOFs,19
showed a very sharp mass reduction at 300 1C, remaining
16.8% of mass, even after further heating to 1000 1C. Assum-
ing that in an oxygen atmosphere exclusively nickel(II)oxide
which are catalytically active.
a Institute of Organic Chemistry II & Advanced Materials,
Ulm University, 89081 Ulm, Germany.
remains, a nickel content of 13.2% of the as-synthesized material
can be calculated. In comparison, the activated material (see
below) remains 20.4% of mass, which corresponds to 15.8% of
nickel content. This result fits the expected value of 15.6%,
calculated from the ideal molecular formula of MaSOF-1. In
the FT-IR of the as-synthesized, air-dried material a sharp peak
E-mail: michael.mastalerz@uni-ulm.de; Fax: +49 731-50-22840;
Tel: +49 731-50-22855
b RUBiospek, Ruhr-University of Bochum, 44780 Bochum, Germany
w Electronic supplementary information (ESI) available: Synthetic
details and analytical data of 3a, 3b, MaSOF-1 and MaSOF-2. See
DOI: 10.1039/c1cc14805b
c
130 Chem. Commun., 2012, 48, 130–132
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for 5 h (Fig. 2). The isotherm can be classified as a type I
isotherm, which is typical for a microporous material.21
The isotherm shows no hysteresis between adsorption and
desorption, providing the information that the network is rather
rigid than flexible. A total amount of 398 cm3 gÀ1 nitrogen was
adsorbed at P/P0 = 0.95. The measured surface areas are
647 m2 gÀ1 (BET-model) and 741 m2 gÀ1 (Langmuir-model),
respectively. Although the powder-X-ray diffraction (PXRD)
shows no defined signals—suggesting that the material is
amorphous—the compound shows a narrow pore-size distri-
bution with maxima at 5.0 A (Horvath–Kawazoe-model) or
4.2 A (NLDFT-model), which were calculated from adsorp-
tion at low P/P0 (see Fig. 2, inset).
The metal ion seems to play a crucial role in the formation
of the porous solid: in the absence of nickel acetate no precipitate
is formed at all under similar conditions (same concentration of
reactants, 95 1C reaction temperature). The solution stays clear
even after several days. Also the addition of trifluoroacetic acid
(TFA) does not induce any precipitation, again suggesting that
nickel(II) ions probably do not act as Lewis acidic centers to
accelerate the imine bond formation, but rather as a template.
We assume that the formation of the metal salphen is irreversible
(or less reversible), which would further explain the amorphous
nature of the material.
Scheme 2 Synthesis of metal-assisted salphen covalent organic frame-
works (MaSOFs).
at 1612 cmÀ1 is observed, which can be assigned to the CQN-
stretching in nickel salphen compounds. No band correspond-
ing to a CQO stretching of the precursor salicylaldehyde at
1658 cmÀ1 was detected any more, revealing a full conversion of
the salicylaldehyde functional groups (see Fig. 1). Additionally,
the stretching band of the DMF carbonyl bond can clearly be
assigned at 1666 cmÀ1. We exchanged the DMF by washing the
material extensively with ethanol and ‘‘activated’’ the material at
200 1C in vacuum for several hours. Besides the results of FT-IR
spectroscopy (the band from DMF is not detected any more, see
Fig. 1c), the 13C MAS NMR spectrum revealed that the material
consists certain expected characteristic signals, referred to the
13C NMR spectrum of model compound 3a. For instance, a peak
at d = 61 ppm can be assigned to the central quarternary carbon
nuclei of the tetraphenylmethane nodes. Furthermore, a peak of
the imine carbon resonates at d = 166 ppm, which is very similar
to the signals (172 ppm) of the model compound 3a in DMSO-d6
solution. Additionally, no peaks at approx. 200 pm are recognized,
which is again an evidence for the full conversion of all aldehyde
moieties.
We also tested to incorporate zinc(II) instead of nickel(II)
ions during the framework formation by using zinc(II) acetate
dihydrate as reagent. After completion of the reaction a bright
yellow powder (MaSOF-2) was formed. Similar to MaSOF-1,
the FT-IR spectrum of the as-synthesized material shows a
characteristic CQN stretching band at 1615 cmÀ1, which is
comparable to those of typical zinc-salphen complexes.20 Again,
remaining DMF can be detected as a peak at ca. 1660 cmÀ1. This
peak disappears when the compound was washed with ethanol
and dried in vacuum at 150 1C for 18 hours. The measured BET
surface area was 630 m2 gÀ1, which is similar to the surface area
of MaSOF-1. The compound shows a narrow pore-size distribu-
tion with maxima at 5.5 A (HK-model) or 4.2 A (NL-DFT-
model). Again, FT-IR and 13C MAS NMR spectroscopy revealed
a full conversion of all aldehyde moieties.
To investigate the porosity of the material, we measured
the nitrogen sorption of a sample, dried in vacuum at 200 1C
The relative small pore-sizes of 4.2 to 5.5 A and the presence
of charged metal centers, which, in the ideal case, should
contain accessible, free coordination sites, forced us to test
gas adsorption of various analytes (H2, CO2, CH4) that perhaps
Fig. 1 Comparison of FT-IR spectra of (a) model compound 3a,
(b) MaSOF-1, as-synthesized and (c) MaSOF-1, ‘‘activated’’. The orange
bar marks the CQN stretching bonds.
Fig. 2 Nitrogen sorption measurement of MaSOF-1 at 77.35 K.
Filled circles (adsorption), open circles (desorption). The inset frame
depicts the NL-DFT derived pore-size distribution.
c
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Chem. Commun., 2012, 48, 130–132 131

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Fig. 3 Adsorption of H2 at 77 K and P0 = 1 bar (triangles), CO2 (circles)
and methane (squares) at 273 K and P0 = 1 bar of MaSOF-1 (red) and
MaSOF-2 (black).
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interact with the metal centers. MaSOF-1, with nickel ions, adsorbs
at 77 K and 1 bar 4.46 mmol gÀ1 H2 and 3.49 mmol gÀ1 at 87 K,
which is equal to 0.90 wt% and 0.70 wt%, respectively (Fig. 3). The
heat of adsorption via the Clausius–Clapeyron equation is esti-
mated to be À6.8 kJ molÀ1 22
.
MaSOF-2 with zinc ions adsorbs
similar amounts under the same conditions (1 bar, 77 K:
4.14 mmol gÀ1, 0.84 wt%; at 87 K: 3.27 mmol gÀ1, 0.66 wt%).
Here, the calculated heat of adsorption is À7.8 kJ molÀ1
,
about 1 kJ molÀ1 lower than for MaSOF-1. This suggests that
the nature of the metal ion influences the adsorption proper-
ties. It is worth mentioning that the heat of adsorption for
both materials decreases with increasing surface coverage
(see Fig. S25 in the ESIw), which, again, can be seen as a hint
that first the metal interacts with the adsorbent. Methane is
adsorbed at 273 K and 1 bar by both MaSCOFs in lower
amounts: 0.43 mmol gÀ1 (0.69 wt%) for MaSOF-1 and
0.61 mmol gÀ1 (0.98 wt%) for MaSOF-2. For the adsorption
of CO2 at 273 K and 1 bar, a bigger difference can be detected:
MaSOF-1 adsorbs 1.83 mmol gÀ1 (8.1 wt%), whereas MaSOF-2
takes up more CO2 (2.23 mmol gÀ1, 9.8 wt%). This is a hint that
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In conclusion, we described a one-pot three component reaction
of metal salphen containing porous organic polymers, with high
BET surface areas, narrow pore-size distribution and selective gas
adsorption properties. To the best of our knowledge, this is
the first three-component synthesis of a metal-functionalized
porous organic framework. Several MaSOFs, with a broad
variety of incorporated metal ions, will be synthesized to study
the influence of charged, free metal sites in similar porous
networks.24 Furthermore, homochiral MaSOFs will be synthe-
sized, to combine the extraordinary catalytic properties of metal
salens18 with porous materials in heterogeneous catalysis.15,25
The authors like to thank the ‘‘Deutsche Forschungsge-
meinschaft’’ and the ‘‘Fond der Chemischen Industrie’’ for finan-
cial support. M. M. likes to thank C. Egger for nitrogen sorption,
S. Blessing for PXRD, and G. Weber (all Ulm University) for
TGA measurements.
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132 Chem. Commun., 2012, 48, 130–132
This journal is The Royal Society of Chemistry 2012