Rhodium bis-phosphine catalysts on mesoporous silica supports: New highly efficient catalysts for the hydrogenation of alkenes

Article abstract of DOI:10.1039/b102355c

A bidentate rhodium phosphine complex anchored onto a mesoporous molecular sieve was shown to be an active catalyst for the hydrogenation of alkenes, with the activity dependent on the method of grafting, and in general, exceeding that of related homogeneous catalysts.

Full text of DOI:10.1039/b102355c

Rhodium bis-phosphine catalysts on mesoporous silica supports: new
highly efficient catalysts for the hydrogenation of alkenes†‡
Cathleen M. Crudden,* Daryl Allen, Michael D. Mikoluk and Jian Sun
Department of Chemistry, University of New Brunswick, Fredericton, New Brunswick, P.O. Box 45222,
Canada, E3B 6E2. E-mail: Cruddenc@unb.ca
Received (in Corvallis, OR, USA) 9th March 2001, Accepted 27th April 2001
First published as an Advance Article on the web 7th June 2001
A bidentate rhodium phosphine complex anchored onto a
mesoporous molecular sieve was shown to be an active
catalyst for the hydrogenation of alkenes, with the activity
dependent on the method of grafting, and in general,
exceeding that of related homogeneous catalysts.
Remarkable developments in homogeneous catalysis have
taken place over the past two decades.1 Unfortunately, the
recovery of the precious metals and expensive ligands from the
reaction medium is often not feasible. For this reason, there
have been many approaches to ‘heterogenizing’ homogeneous
catalysts by attaching well defined molecular species to
polymeric supports.2 Despite the development of some very
effective supported systems,3 a general strategy for mimicking
solution phase behaviour is yet to be developed. Reactions on
Scheme 1 Synthesis of Rh catalyst 1.
supports are generally slower and less predictable than their
solution counterparts, commonly requiring excesses of rea-
gents, higher temperatures and longer reaction times.4
to isolation. Compound 4 was purified by flash chromatography
The discovery of mesoporous molecular sieves has provided
and then deprotected with morpholine. Treatment with
new possibilities in catalysis.5 These mesoporous materials
[Rh(COD)(THF)2]+BF42§ generated the homogeneous Rh
have unique structures characterized by high surface areas,
large pores, and high long-range order; properties that make
complex 1. This was spectroscopically characterized and then
grafted onto the surface of a molecular sieve (SBA-15-type)5 at
them ideal catalyst supports. MCM-41-type silicates have been
25, 110 and 165 °C (method A). Grafting was also carried out in
used as supports for a variety of metals and metal complexes.6
the presence of an inert spacer, EtSi(OEt)3 (method B).10 The
Recently, Shyu et al. described a supported analogue of
resulting catalysts were characterized by MAS NMR, TEM, and
nitrogen adsorption.
Wilkinson’s catalyst, prepared by treating MCM-41 with
(EtO)3Si-(CH2)3-PPh2 and then RhCl(PPh3)3.7 Our approach to
31P MAS NMR analysis of these materials revealed that as
the preparation of a Rh complex supported on a mesoporous
the grafting temperature was increased, the resonance at 14 ppm
molecular sieve involves the use of a bidentate phosphine in
which corresponded to the homogeneous complex 1 (Fig. 2A)
order to minimize leaching, and a cationic complex to prevent
decreased while a new peak at 35 ppm, attributable to a P(V)
dimerization through chloride bridges8 (Fig. 1). The catalysts
species, appeared.11 When the complex was grafted in refluxing
thus obtained are highly active for the hydrogenation of olefins,
even more than the corresponding homogeneous complexes.
mesitylene, the resulting material (1-mps-165A) was composed
entirely of the P(V) species (Fig. 2B). Grafting in the presence
The magnitude of the activity and the nature of the grafted
of an inert spacer (method B) proved to be optimal (Fig. 2B).
species is strongly dependent on the method of grafting. To the
The various Rh complexes were examined for their ability to
best of our knowledge, this is the first report of a bidentate
catalyze the hydrogenation of isosafrole (5), Table 1.¶ With the
exception of 1-mps-25A, all the mesoporous catalysts tested are
more active than the corresponding homogeneous catalysts
rhodium phosphine complex grafted onto a mesoporous
molecular sieve.
(entries 1, 2).
Catalyst 1-mps-25B, prepared in the presence of EtSi(OEt)3,
has the highest activity (3300 turnovers h21, entry 6). These
results are consistent with the results of Shyu,7 who found that
their MCM-41-modified catalyst was ca. 33 more active than
Wilkinson’s catalyst. Since 1-mps-25B also had the lowest
loading on support, the increased activity may be due to site
isolation.12 The least active catalyst of those examined, 1-mps-
25A, also has the highest Rh loading of 2.0%, consistent with
this postulate. Studies are currently underway to measure the
proximity of the Rh complexes on support.12 Simple mixing of
[Rh(COD)2]+BF42 with the mesoporous support in the absence
of any phosphine also led to an active hydrogenation catalyst
(entry 7). The presence of an intermediate heteropoly acid
species to aid in adsorption was not found to be necessary in this
case.13
Fig. 1 Supported Rh catalyst.
Catalyst 1 is prepared as shown in Scheme 1.9 Alkylation of
diethyl malonate with allyl bromide (89% yield) and reduction
of the ester groups with LiAlH4 (69% yield) provides diol 2.
Treatment of 2 with PPh3 in CCl4 followed by hydrosilylation
of the pendant olefin using H2PtCl6 yields 3. The phosphine
substituents were introduced using Ph2PLi, and protected prior
† Electronic supplementary information (ESI) available: NMR data. See
http://www.rsc.org/suppdata/cc/b1/b102355c/
‡ Presented in part: 80th Canadian Society for Chemistry conference,
Whistler, B.C., June, 1998.
Retention of Rh on the surface was also a function of the
grafting method. Supported systems with lower loadings of Rh
were found to be more prone to leaching as determined by ICP-
1154
Chem. Commun., 2001, 1154–1155
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b102355c

Dr Robert Young and Merck & Company are thanked for
their generous support of this research, along with the Natural
Sciences and Engineering Research Council of Canada
(NSERC) and the Canada Foundation for Innovation (CFI).
Michael Lumsden of Dalhousie University is thanked for the
MAS NMR data. Professor Stephen Bergens is thanked for
helpful discussions and an anonymous reviewer is thanked for
very valuable suggestions.
Notes and references
§ Prepared by treatment of [Rh(COD)Cl]2 in THF with AgBF4 and filtration
of the resulting solution through Celite-545 (not acid washed).
¶ Isosafrole was purified by column chromatography, distillation, deoxy-
genated with three freeze, pump, thaw cycles, and purified immediately
prior to use by passage through a short plug of dry alumina. The catalyst was
found to be extremely sensitive to trace amounts of oxygen and deactivates
with time, even when stored in a glove box.
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Fig. 2 31P NMR of homogeneous (A) supported (B) Rh complexes.
Table 1 Hydrogenation of 5a
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Loading
Turnover
Entry
Catalystb
on supportc Frequencyd
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1
2
3
4
5
6
7
Rh+/DPPP
1
1-mps-110A
1-mps-25A
1-mps-165A
1-mps-25B
Rh+/mps
n.a.
n.a.
350
750
> 1600
200
1030
3300
2300
1.4%
2.0%
1.4%
0.34%
3.3%
a All reactions were carried out at rt, 750 psi of H2, [substrate]init = 0.06 M
in dry, deoxygenated, distilled THF. Reaction times were between 30 and
90 min. Solutions were prepared in a glove box under argon. b mps =
mesoporous silicate. c Wt%, determined by ICP MS analysis. d Turnovers
per hour.
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MS analysis of the reaction mixture after filtration through a
nylon filter. Catalyst 1-mps-25B (0.3% Rh by weight) lost a
significant amount of Rh after the first use (21–36%) and less
(ca. 5%) in subsequent runs. Catalyst 1-mps-110A (1.4% Rh by
weight) was much more robust, and lost only 2–3% Rh during
the first run.
In conclusion, we have demonstrated that rhodium com-
plexes heterogenized on mesoporous molecular sieves are
highly active catalysts for the hydrogenation of olefins. With
one exception (1-mps-25A), the supported complexes are all
more active than the corresponding homogeneous catalysts.
Chem. Commun., 2001, 1154–1155
1155