Entrapment of an organometallic complex within a metal: A concept for heterogeneous catalysis

Article abstract of DOI:10.1021/ja804112z

A novel family of composite materials, organically doped metals, has been recently introduced. Here, we demonstrate their use as a new platform for heterogeneous catalysis, namely the doping of a metal with a catalytic organometallic complex. Specifically, a rhodium(I) catalyst, (RhCl(COD)(Ph₂P(C₆H₄SO₃Na))), ([Rh]), was physically entrapped within silver, thus creating a new type of catalytic material: [Rh]@Ag. Several aspects were demonstrated with the development of this heterogeneous catalyst: a metal can be used as a support for heterogenizing a homogeneous catalyst; the homogeneous catalyst is stabilized by the entrapment within the metal; the products of the composite catalyst are different compared to those obtained from the homogeneous one; and the adsorption of [Rh] on the surface of Ag and its entrapment are very different processes-only the latter provided appreciable catalytic activity. Thus, while homogeneous [Rh] was entirely destroyed after converting styrene to ethylbenzne at 50%, [Rh]@Ag remained active after effecting the same reaction to a yield of 85% (compared to only 7% for [Rh] adsorbed on Ag), and while homogeneous [Rh] hydrogenated diphenylacetylene to bibenzyl (and was completely deactivated after one cycle) with no trace of cis-stilbene, [Rh]@Ag afforded that compound as the main product and could be reused. Copyright

Full text of DOI:10.1021/ja804112z

Published on Web 08/15/2008
Entrapment of an Organometallic Complex within a Metal: A Concept for
Heterogeneous Catalysis
Itzik Yosef, Raed Abu-Reziq, and David Avnir*
Institute of Chemistry, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel
Received June 1, 2008; E-mail: david@chem.ch.huji.ac.il
We have recently introduced a new family of materials,
Scheme 1. Synthesis of the Rh(I) Catalyst
metal-organic composites, in which organic molecules, small or
1
-7
polymeric, are entrapped within metals.
Besides the basic
8
motivation, namely that these composites have been unknown, the
practical motivation is based on the potential ability to tailor metals
with any of the properties of the vast library of organic molecules,
on the potential to affect the properties of the organic dopant by
virtue of immersing it in a sea of electrons, on the potential of
creating materials that are “in-between” metal and, say, plastics,
and on the potential of the emergence of new properties which are
not found in either of the components. Proof-of-concept for these
expectations is now rapidly being accumulated. For instance, one
can induce unorthodox properties such as having silver which acts
14
on styrene hydrogenation with silica entrapped RhCl(PPh
3
)
3
,
on
diphenylacetylene hydrogenation with a Rh complex incorporated
in a siloxane network, on the coentrapment of a Rh complex with
Pd in a silica sol-gel matrix, and the anchoring of a Rh complex
15
16
to a silica surface containing dispersed Pd.
4
as an acid or having right-handed and left-handed enantiomers of
Entrapment was carried out according to (redox balanced): [Rh](aq)
6
chiral gold and silver. Two principal methods were developed for
+
2AgNO
3
3 2
(aq) + Zn(s) f [Rh]@(2Ag)(s) + Zn(NO ) (aq). In
the synthesis of these composites, both of which are based on the
reduction of the metal cation in the presence of the molecule to be
entrapped. In one method, the process is homogeneous, in that all
components, including the reducing agent, are soluble. In the second
method, employed in this report, the metal cation is reduced by a
order to ascertain that the entrapped Rh-complex ([Rh]) remained
intact, its extraction from [Rh]@Ag was needed. It was found that
water, in which [Rh] is soluble and which was therefore used as a
solvent for the preparation of the doped metal, could not extract
the entrapped complex. This is one of several indications that the
entrapment process is a completely different process compared to
adsorption, and we shall see more indications for this below. DMSO
was found to be a good solvent for the purpose of extraction. As
seen in Figure 1, spectral analysis of the DMSO-extracted catalyst
proves that the complex was kept intact during the entrapment
process. Of course, this does not exclude the possibility that, in its
entrapped form, the complex is a precatalyst to the actually active
species. Nitrogen adsorption-desorption isotherms reveal that the
composite is mesoporous with an average pore size of 13 nm and
1
heterogeneous dispersion of a sacrificial reducing metal. For
instance, solutions of silver and copper salts were treated with
metallic zinc powder, and gold salt solution was treated with copper
powder in the presence of the dopants, either the polymer Nafion
or the dye thionin, thus creating various Nafion@metals and
thionin@metals. An important property of these composites, of
relevance to this report, is that they are porous and that the dopant
2
is accessible for reaction by substrates diffusing into the composite.
Yet despite being accessible, the dopant does not leach out (even
in solvents it is normally soluble in), or leaches out to a negligible
2
a consequent low surface area of 1.5 m /g. Still, these values are
larger than those obtained for pure Ag synthesized under similar
conditions: 5 nm and 0.3 m /g. The organometallic complex seems
1
,2
extent.
The proven ability to entrap an organic molecule within a metal
while retaining its accessibility opens up interesting questions: First,
traditionally, heterogenization of a homogeneous catalyst has been
2
9
-12
done on two major types of supports: inorganic oxides
and
1
3
organic polymers; can metals be used for that purpose? And
second, will a metal as a heterogenizing environment affect the
activity of the homogeneous catalyst? We report here a feasibility
study which has provided positive answers to these two questions,
thus opening, we believe, a new direction in heterogeneous catalysis.
Specifically, we have synthesized (Scheme 1) and entrapped a
2 6 4 3
COD-rhodium(I) complex (RhCl(COD)(Ph P(C H SO Na))) in
silver ([Rh]@Ag) and tested its ability to catalyze the hydrogenation
of styrene and diphenylacetylene. With these reactions we have
shown that a metal can act as a heterogeneous carrier of a
homogeneous complex, the entrapped complex is stabilized by the
metallic matrix, and the nature of the products is affected by the
entrapment. We have also shown that adsorption of [Rh] on Ag
and entrapment are entirely different processes and only by
entrapment could these observations have been made. For comple-
tion of these introductory notes it is relevant to recall the reports
Figure 1. Spectra of the rhodium(I) catalyst in DMSO before entrapment
(dotted line) and after extraction from [Rh]@Ag (smooth line).
1
1880 9 J. AM. CHEM. SOC. 2008, 130, 11880–11882
10.1021/ja804112z CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2. Catalytic Hydrogenation Reactions
a
Table 1. Hydrogenation of Styrene to Ethylbenzene
b
conversion (%)
catalyst
1st cycle
2nd cycle
3rd cycle
[
Rh]@Ag
85
50
7
65
-
0
45
-
0
c
pure [Rh]
adsorbed [Rh]
pure Ag
0
0
0
a
b
Reaction conditions: 24 h, 80 °C, 200 psi. Conversion determined
c
by GC. Cannot be recycled.
Figure 2. High resolution SEM images of [Rh]@Ag, ((a): bar ) 1 µ, (c):
bar ) 500 nm), and, for comparison, [Rh] adsorbed on silver ((b): bar )
a
Table 2. Hydrogenation of Diphenylacetylene
1
µ), (d): bar ) 500 nm).
Figure 3. High resolution SEM images for [Rh]@Ag sample - the oozing
out of the organic dopant due to the high energy of the electron beam (see
circled area): (a) First taken, and (b) after a few seconds.
a
b
Reaction conditions: 6 h, 80 °C, 400 psi. Conversion determined
c
by GC. After ∼3 h: cis - 12%, trans - 0%, bibenzyl - 20%.
d
to interfere with the growth process of the silver crystals (as indeed
observed in the SEM images; see below). It should be noted that
the surface area value was obtained by using the BET equation,
the compliance to which was excellent; such compliance is usually
associated with high surface homogeneity of the adsorbing material.
High-resolution SEM further underlines the differences between
the doped silver and the pure one: HR-SEM images of [Rh]@Ag
are shown in Figure 2 and contrasted with [Rh] adsorbed on silver.
Smaller crystalline particles can be seen in the doped silver (around
Cannot be recycled.
that ethylbenzene is the only product of the styrene hydrogenation.
Table 1 gives the hydrogenation activities of the [Rh]@Ag
heterogeneous catalyst, the dissolved homogeneous rhodium com-
plex catalyst, the pure Ag metal, and the rhodium complex catalyst
adsorbed on silver. It is seen that the pure Ag is inactive as a
catalyst, adsorbed [Rh] on silver is only marginally active, and under
homogeneous conditions [Rh] affects a 50% conversion but is
destroyed after one cycle. This deterioration is due to reductive
formation of metallic rhodium deposit sa black depositswhich
is inactive in this reaction. [Rh]@Ag performs better than all of
these blanks: The first cycle conversion is the highest (85%), and
the easily separated catalyst can be used again, losing however some
of its activity. UV-vis and NMR analyses of the liquid phase that
was separated from the reaction mixture showed that no detectable
[Rh] catalyst had leached into the liquid phase. On the other hand,
DMSO extraction of [Rh]@Ag proves that [Rh] remained intact.
The [Rh]@Ag-catalyzed hydrogenation of diphenylacetylene was
1
50 nm compared to ∼360 nm for the [Rh] adsorbed on silver).
1
1a
This demonstrates again that entrapment and adsorption are different
processes: Silver on which [Rh] was adsorbed is affected to a much
lesser extent and resembles the pure silver (Figure 1S, Supporting
Information). Another visible distinction is that [Rh]@Ag is more
sheetlike while in the adsorbed case the particles are more spherical.
Higher resolution is provided in Figure 3, where a very interesting
observation and proof of the composite nature of the material is
shown: the organic dopant is seen oozing out of the composite due
to the high energy electron beam focused on the sample.
Adsorption measurements of hydrogen on [Rh]@Ag did not
reveal any chemisorption, in agreement with the known fact that
2
carried out under a somewhat higher pressure of 400 psi of H and
was stopped after 6 h at 60% conversion. GC and NMR analyses
revealed cis-stilbene to be the main product with trans-stilbene and
bibenzyl as the secondary ones (Table 2; the preference for the
kinetically dictated cis product in heterogeneous catalysis has been
1
7
hydrogen is not soluble in silver; typically, at 820 mmHg and 77
K, only 0.03 wt % of H was adsorbed (see Supporting Information
2
for details and Figure 2S there). This indicates that the observed
effects of the entrapment are not due to synergism between the
entrapped complex and the metal but that the metal is acting here
mainly as a protecting matrix which provides a confined environ-
ment around the [Rh] molecules.
18
proposed to be due to syn-addition ). No cis-stilbene was obtained
under homogeneous [Rh] catalysis conditions, bibenzyl being the
main product in this case (At shorter times, cis-stilbene could be
detected as an intermediate, e.g., 12% after ∼3 h). The homoge-
neous reaction proceeds to the final reduction product, but the
conversion is lower, due to the gradual deterioration of [Rh];
[Rh]@Ag, in contrast, can be reused. And again, the adsorbed
The [Rh]@Ag-catalyzed hydrogenation of styrene (Scheme 2)
was carried out in 1,2-dichloroethane under 200 psi of H
2
at 80 °C
and stopped after 24 h at 85% conversion. GC analyses showed
J. AM. CHEM. SOC. 9 VOL. 130, NO. 36, 2008 11881

C O M M U N I C A T I O N S
Scheme 3. Suggested Mechanism of Heterogeneous Entrapment
meets the basic requirements of a heterogeneous carrier: Stability
to the reaction conditions (temperature and pressure), nonleach-
ability, inertness to the reactants and products, and porosity which
is open enough for the transport of the substrates in and out. Since
a metal and a dopant can each be catalysts, we believe that a
potential for interesting exploration of synergism has been opened.
Acknowledgment. The development of these new composite
materials is supported by the U.S. Air Force (Award No. FA9550-
0
0
6-1-0227) and by the Israel Science Foundation (Grant No. 494/
5).
configuration gives much lower conversions and becomes inactive
after one cycle, and pure silver is inactive in this reaction. When
the reaction time is extended up to full conversion (90 h),
cis-stilbene remains the main product, with an increase in the
percentage of bibenzyl (Table 1S, Supporting Information). Yet
another proof that entrapment and adsorption provide different
catalytic materials is obtained by the following experiment:
Supporting Information Available: Experimental details and
additional results. This material is available free of charge via the
Internet at http://pubs.acs.org.
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Conclusions
1
In conclusion, we have shown that a homogeneous catalyst can
be heterogenized by entrapment within a metal. The silver matrix
JA804112Z
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1882 J. AM. CHEM. SOC. 9 VOL. 130, NO. 36, 2008