Modular construction of a porous organometallic network based on rhodium olefin complexation

Article abstract of DOI:10.1021/ja305482a

We describe the rational design and synthesis of the first member of a new class of microporous materials. It is built from rhodium and a polyolefinic ligand featuring a rigid tetraphenylsilane backbone via metal olefin complexation, creating a truly orga

Full text of DOI:10.1021/ja305482a

Communication
pubs.acs.org/JACS
Modular Construction of a Porous Organometallic Network Based on
Rhodium Olefin Complexation
Ulrich Stoeck,† Georg Nickerl,† Ulrich Burkhardt,‡ Irena Senkovska,† and Stefan Kaskel*,†
†Department of Inorganic Chemistry, Dresden University of Technology, Bergstr. 66, 01062 Dresden, Germany
‡Max Planck Institute for Chemical Physics of Solids, Nothnitzer Str. 40, 01187 Dresden, Germany
̈
S
* Supporting Information
In this paper, we present the rational design and synthesis of
ABSTRACT: We describe the rational design and
a porous material generated from transition-metal−carbon
synthesis of the first member of a new class of
microporous materials. It is built from rhodium and a
polyolefinic ligand featuring a rigid tetraphenylsilane
backbone via metal olefin complexation, creating a truly
organometallic network. The resulting framework, denoted
as DUT-37 (Dresden University of Technology no. 37)
exhibits considerable porosity and unprecedented stability
under ambient conditions. Furthermore, it is catalytically
active in transfer hydrogenation.
bonds (olefin π-complexes), thus creating a truly organo-
metallic framework. Although some examples of 3D coordina-
tion polymers utilizing metal−carbon bonds are known in the
literature, they are neither based exclusively on a organometallic
connecting motif nor permanently porous.5
Olefin ligands are ideal candidates to stabilize low oxidation
states of transition metals. In particular, chelating diolefin
ligands are highly stable. Since coordination of an olefin to a
metal center leads to pyramidalization of the carbon atoms,
pyramidalized molecules, such as highly strained bicyclo[2.2.1]-
heptadienes, bind strongly to the metal centers.6 Taking these
considerations into account, we chose bicyclo[2.2.1]heptadiene
(norbornadiene) as the connecting motif. Since a late transition
metal such as rhodium will link only two norbornadiene
moieties in a linear fashion, our objective was to design a
polytopic and accordingly polyolefinic linker with a tetrahedral
backbone. Starting from commercially available norbornadiene
(1), we developed a short synthesis sequence that afforded a
polyolefinic ligand with a tetraphenylsilane backbone (Scheme
1).
he design and synthesis of microporous materials (d < 2
Tnm) has been a subject of intense research in recent years
because of the wide variety of possible areas of application, such
as gas storage, separation, and catalysis.1 In the last 20 years,
decisive achievements in the development of new microporous
materials incorporating organic molecules have been made.
Linking single (transition) metal atoms or clusters through
mostly rigid polyfunctional donor-heteroatom-containing or-
ganic molecules results in crystalline 3D connected micro-
porous structures named porous coordination polymers
(PCPs) or metal−organic frameworks (MOFs).2 This modular
approach has been translated into polymer chemistry by linking
rigid organic molecules through multiple coupling reactions to
create porous purely organic polymers.3 A synthetic concept
bridging the aforementioned approaches has been used to
connect main-group elements such as tin and silicon through
rigid organic molecules by means of strong covalent bonds
yielding element−organic frameworks.4
Readily available 1 was functionalized using CuBr and tert-
butyl peroxybenzoate, resulting in 7-norbornadienyl tert-butyl
ether (2). Although the yield appeared to be quite poor, the
Scheme 1. Synthesis of the TNPS Ligand
As highly crystalline solids, MOFs are highly aesthetic and
beneficial for detailed structural analyses. However, the rigid
network architectures are often detrimental for catalytic
applications because changes in coordination of the nodes
cause network collapse. In this sense, to date it has been
impossible to make use of the high catalytic activity of many
organometallic complexes in extended networks such as MOFs.
Moreover, the nodes in MOFs are traditional Werner-type
complexes, and thus, their catalytic potential is limited
compared with organometallic catalysts. Our interest was to
generate such truly organometallic networks with intrinsic
microporosity in order to integrate catalytically active nodes
into an open porous structure. The latter requires a more
flexible amorphous matrix that allows for local structural
changes.
Reagents and conditions: (I) tert-butyl peroxybenzoate, CuBr,
benzene, reflux, 2 h, 26%; (II) benzene, reflux, 2 days, 60%; (III) t-
BuLi, THF, −90 °C, SiCl4, 80%.
Received: June 11, 2012
Published: October 12, 2012
© 2012 American Chemical Society
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Journal of the American Chemical Society
Communication
reaction could be performed easily on a several hundred gram
scale, thus setting no restrictions on the synthesis of the target
molecule. Ether 2 was cleaved using p-dibromobenzene-derived
Grignard reagent 3 to form 4-bromophenyl-substituted
norbornadiene 4. Careful lithiation of the aforementioned
species and subsequent reaction with SiCl4 generated the target
molecule, tetra(4-(7-norbornadienyl)phenyl)silane (TNPS, 5).
The composition and structure of the ligand were confirmed by
NMR data and single-crystal X-ray analysis [see section 2 in the
Supporting Information (SI)].
The new organometallic network DUT-37 (Dresden
University of Technology no. 37) was obtained in a ligand
exchange reaction (Figures S18−S20 in the SI) using a suitable
molecular precursor such as bis(norbornadiene)rhodium(I)
tetrafluoroborate (6). A solution of the TNPS linker in dry
dichloromethane (DCM) was combined with a solution of 6 in
the same solvent at room temperature (Scheme 2). Upon
nonordered porous polymers and is usually assigned to swelling
induced by condensed adsorbate.7 Despite its limited specific
surface area, DUT-37 stored 1 wt % hydrogen at 77 K and 1 bar
pressure (Figure S9). Attempts to synthesize a crystalline
network have failed to date, probably as a result of the
nonlinearity of the (7-norbornadienyl)phenyl moiety and the
rotational flexibility of the norbornadienyl group due to the sp3
hybridized bridge carbon atom.
In contrast to the air- and moisture-sensitive organometallic
precursor complex 6, the DUT-37 material showed unprece-
dented stability under ambient conditions. According to
thermogravimetric analysis, DUT-37 was stable up to 493 K
in air (Figure S2). When stored under normal laboratory
conditions in an open jar for 1 month, no significant change in
porosity could be found. Furthermore, the appearance did not
change during that time. Formation of rhodium black or
rhodium oxide species could not be observed visually or in
powder X-ray diffraction patterns.
Scheme 2. Synthesis of DUT-37
For a quantitative analysis of the water affinity, water vapor
physisorption isotherms were measured (Figure S8). The
degree of pore filling by water (calculated by comparison of the
total pore volumes obtained from the nitrogen physisorption
data and the pore volumes calculated from water physisorp-
tion) was only 30%. This figure characterizes the material as
fairly hydrophobic, which is essential for stabilization against
hydrolysis.
The rhodium content of the material varied between 9.7 and
11.4 wt % (based on a data set of 17 independently prepared
samples; see Table S2 in the SI). The Rh/Si ratio varied from
2.1 to 3. The values for an ideal infinite polymeric framework
would be 19.12 wt % for the rhodium content and 2 for the
Rh/Si ratio (Table S3). On the other hand, the Rh/B ratio was
equal to 1, the same as in the ideal framework, supporting the
assumed idealized reaction in Scheme 2.
Reagents and conditions: DCM, room temperature, 2 h.
mixing, an orange precipitate was formed (Figure S12). When
the precipitate was collected by filtration and dried in air, it
suffered a drastic loss of volume. Subsequent physisorption
experiments revealed no significant adsorption of either
hydrogen or nitrogen at 77 K.
However, when the sample was dried using supercritical
carbon dioxide, the resulting solid exhibited considerable
porosity (Figure 1). The DUT-37 polymer had Brunauer−
Emmett−Teller (BET) surface areas of up to 470 m2/g, a
micropore volume of 0.20 cm3/g, and a total pore volume of
0.38 cm3/g, which are comparable to textural data for zeolites.
The nitrogen physisorption isotherm showed hysteresis over
the entire range of relative pressures, which is typical for
The connecting motif of DUT-37 in Scheme 2 (see Figure
S12) is a 16-electron complex, leaving an open coordination
site. When the material was exposed to a solvent with a
coordinating donor atom (e.g., alcohols, nitriles, ethers), the
color of the as-made material changed from red-orange to
yellow (Figure S11), suggesting coordination to the free site
and leading the way to a potential application as an indicator
material. In addition, a carbon monoxide sorption experiment
at 298 K using the activated material revealed coordination of at
least one molecule of CO to the rhodium center, as confirmed
by IR spectroscopy (Figure S3). Further support for this CO
chemisorption mechanism was obtained from the shape of the
sorption isotherm at 298 K (Figure S10). To get a more
thorough picture of the actual binding situation in amorphous
DUT-37, X-ray absorption spectroscopy (XAS) measurements
of the rhodium K-edge on as-made samples dried under argon
were performed. Fitting the measured data on a molecular
model complex led to acceptable quality factors (e.g., R factor
of 2.34%). The data suggest a mononuclear rhodium species
surrounded by two norbornadiene ligands, thus supporting our
hypothesis depicted in Scheme 2.
These observations demonstrate the accessibility of the metal
and the fact that the norbornadiene groups have both relatively
low dissociation energies and high flexibility caused by the sp3-
hybridized bridge carbon atom, which make the material a good
candidate for heterogeneous catalysis. DUT-37 catalyzes
transfer hydrogenation reactions (Figure 2); in this case,
cyclohexanone was reduced to cyclohexanol. Almost complete
conversion was achieved within 12 h with a yield of 96%. To
●
Figure 1. Nitrogen physisorption isotherm of DUT-37 at 77 K: ( )
adsorption; ( ) desorption.
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Figure 2. Formation of cyclohexanol (red ), conversion of
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rule out homogeneous side reactions and prove the
heterogeneity, 1 mL of the reaction mixture was removed
after 6 h using a syringe and filtered using a syringe filter (pore
width 200 nm). The clear solution was subjected to the same
conditions as the initial reaction mixture. No further conversion
was observed, thus confirming the heterogeneous mechanism
of the reaction. This proves the concept of truly organometallic
networks and their use in catalytic conversions. The lack of
crystallinity is advantageous in guaranteeing node accessibility
without causing framework collapse. Further catalytic tests are
currently under investigation to demonstrate broader applic-
ability of DUT-37 as a first representative of porous
organometallic networks.
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In summary, we have reported the first truly organometallic
microporous network, DUT-37. The material displays an
unprecedented stability under ambient conditions. Moreover,
DUT-37 is catalytically active in transfer hydrogenation and a
promising candidate for further catalytic studies.
1871−1876. (d) Weber, J.; Schmidt, J.; Thomas, A.; Bohlmann, W.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis and general characterizations (additional adsorption
data, TGA curve, and EA data), crystallographic data of 5 in
CIF format, and additional structural figures. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Stefan.Kaskel@chemie.tu-dresden.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful for financial support from the German
Research Foundation (SPP 1362) and thank Dominik Munz
for quantum-chemical calculations on the model complex used
as the reference for XAS and Dr. Ralf Biedermann for
determining the crystal structure of TNPS.
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