Soluble gold and palladium complexes heterogenized on MCM-41 are effective and versatile catalysts

Article abstract of DOI:10.1002/ejic.200700955

Chiral Schiff base-gold and -palladium complexes were immobilized on ordered mesoporous silica supports (MCM-41), and their catalytic hydrogenation ability is studied and compared with that of their homogeneous counterparts. The high accessibility introduced by the structure of the supports allows the preparation of highly efficient immobilized catalysts with TOFs of up to 6000 h^-1 for the hydrogenation of diethyl itaconate. The easily recoverable immobilized catalysts duplicate the activity of their homogeneous analogues, and no deactivation of the catalysts is observed after repeated recycling. Gold(III) homogeneous or heterogenized complexes also catalyze the homocoupling of arylboronic acids or alkynes to afford symmetrical biaryls, whereas the respective gold(I) and palladium(II) complexes catalyze the corresponding cross-coupling reaction. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200700955

FULL PAPER
DOI: 10.1002/ejic.200700955
Soluble Gold and Palladium Complexes Heterogenized on MCM-41 Are
Effective and Versatile Catalysts
Camino González-Arellano,[a,b] Avelino Corma, Marta Iglesias,* and Félix Sánchez*
[c]
[a]
[b]
Keywords: Gold / Palladium / Hydrogenation / C-C coupling / Supported catalysts / Schiff bases
Chiral Schiff base–gold and –palladium complexes were im-
mobilized on ordered mesoporous silica supports (MCM-41),
deactivation of the catalysts is observed after repeated re-
cycling. Gold(III) homogeneous or heterogenized complexes
and their catalytic hydrogenation ability is studied and com- also catalyze the homocoupling of arylboronic acids or al-
pared with that of their homogeneous counterparts. The high
accessibility introduced by the structure of the supports al-
lows the preparation of highly efficient immobilized catalysts
with TOFs of up to 6000 h– for the hydrogenation of diethyl
itaconate. The easily recoverable immobilized catalysts du-
plicate the activity of their homogeneous analogues, and no
kynes to afford symmetrical biaryls, whereas the respective
gold(I) and palladium(II) complexes catalyze the correspond-
ing cross-coupling reaction.
1
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
tions on supports are generally slower and less predictable
than their solution counterparts and commonly require ex-
cesses of reagents, higher temperatures and longer reaction
The use of gold compounds in homogeneously and het-
erogeneously catalysed organic reactions has been underval-
ued for many years due to the preconceived idea that gold
is chemically inert. However, recent reports have changed
this assessment, and gold compounds have been shown to
[
12]
times.
III
We have shown recently that Au –phenolic Schiff-base
complexes catalyze the homocoupling of arylboronic acids
but are unable to catalyze the Suzuki cross-coupling.
However, Au , which has the same d electronic configura-
tion as Pd , is an active catalyst for the Suzuki and Sonoga-
[13]
display high catalytic activity.[ Indeed, Au can act as a
Lewis acid catalyst for a large variety of reactions.[ Solid
gold catalysts can be recycled, and when prepared in the
form of gold nanoparticles they are highly active and selec-
tive for reactions such as CO oxidation,[ chemoselective
reduction of nitro-substituted aromatic compounds by
1]
III
I
10
2,3]
0
[
14]
shira cross-coupling reactions.
We have also reported
that stable unsymmetrical N-heterocyclic carbene (NHC)–
gold(I) complexes (both soluble and heterogenized) are ef-
fective catalysts for Suzuki cross-coupling and hydrogena-
4]
[
5]
[6]
H , selective oxidation of alcohols and some C–C bond-
2
[15]
tion reactions.
Herein we report the detailed synthesis
[
7]
forming reactions. Moreover, the use of Au complexes in
homogeneous catalysis has undergone a renaissance, and
spectacular achievements have been reported recently.
Remarkable developments in homogeneous catalysis
have taken place over the past two decades,[ and there have
been many approaches to “heterogenizing” homogeneous
catalysts by attaching well-defined molecular species to
I
III
II
and characterisation of Au , Au and Pd complexes with
Schiff-base ligands derived from 1,1-binaphthyl-2,2Ј-di-
amine and a comparison of the different behaviours shown
[
8]
I
III
II
by these Au , Au and Pd complexes with respect to cata-
lyzing the hydrogenation of olefins and Suzuki and Sonoga-
9]
I
II
shira C–C coupling reactions. Au and Pd catalyze Suzuki
and Sonogashira C–C cross-coupling, while Au com-
III
[10]
polymeric supports.
Despite the development of some
plexes with the same ligands can only catalyze the homo-
coupling of arylboronic acids or alkynes. Moreover, by
properly choosing the support it is possible to achieve an
important increase in activity with respect to the homogen-
eous counterpart due to a cooperative effect of the support,
which may stabilize the charged reaction transition state.
very effective supported systems,[ a general strategy for
11]
mimicking solution behaviour is yet to be developed. Reac-
[
a] Instituto de Ciencia de Materiales de Madrid, CSIC,
C/ Sor Juana Inés de la Cruz 3,
Cantoblanco 28049 Madrid, Spain
Fax: +34-91-372-0623
E-mail: marta.iglesias@icmm.csic.es
[
[
b] Instituto de Química Orgánica, CSIC,
C/ Juan de la Cierva 3, 28006 Madrid, Spain
E-mail: felix-iqo@iqog.csic.es
c] Instituto de Tecnología Química, UPV-CSIC,
Avda. de los Naranjos s/n, 46022 Valencia, Spain
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Results and Discussion
Synthesis of Ligands and Complexes
All soluble ligands and their respective palladium and
gold complexes (reference systems) relevant for this work
Eur. J. Inorg. Chem. 2008, 1107–1115
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1107

C. González-Arellano, A. Corma, M. Iglesias, F. Sánchez
FULL PAPER
Scheme 1. Synthesis of ligands and complexes.
were obtained in high yields by a method similar to that and FT IR and 13C NMR spectroscopy. The COH carbon
described previously (Scheme 1).[
16]
signal appears at δ ≈ 157–158 ppm in the C NMR spectra
13
and the imine carbon atoms signals appear at δ ≈ 162–
167 ppm.
Preparation of Immobilized Ligands and Complexes
In the last few years we have developed a modular system
for combining functionalised ligands with different sup-
Synthesis and Characterisation of Heterogenized Complexes
ports and linkers in order to have a systematic access to a
The heterogenized ligands were treated with palladi-
variety of immobilized chiral catalysts.[ We have applied um(II) acetate or tetrachloroauric acid to give the corre-
this methodology here (Scheme 1) to immobilize Schiff-base sponding anchored Pd and Au complexes, which were used
ligands on a mesoporous silica support such as MCM-41 subsequently for catalytic reactions. The catalytic activity of
17]
2
–1
[
=
BET surface area = 1030 m g ; micropore surface (t-plot) the anchored complexes was also compared to their homo-
2
–1
0 m g ; external (or mesoporous) surface area = geneous analogs (2/3-Pd and 2/3-Au), which were synthe-
2
–1
1030 m g ]. MCM-41 is a short-range amorphous material sized from 2/3 as shown in Scheme 1. The catalysts pre-
containing a large number of silanol groups available for pared in this way have a metal loading of 0.1–0.30 mmol of
grafting. However, MCM-41 also presents a long-range or- metal per gram of support, as determined by atomic ab-
dering with hexagonal symmetry with regular monodirec- sorption analysis. The solids were characterised by FT IR,
tional channels of 3.5 nm in diameter. An amorphous silica DFTR and 13C NMR spectroscopy.
2
–1
support (BET surface area = 750 m g ) was also used as a
support but the results were poor.
Infrared and Electronic Spectra
All solids were functionalised in the same manner ac-
Peaks due to the support dominate the spectra. These
cording to the procedure shown in Scheme 1.[ Supported include the O–H vibration in the range 3700–3300 cm .
precursors were obtained by refluxing a mixture of the pre- Some of the characteristic bands of the complexes could,
cursor and the respective support in toluene for 16 h. These however, be distinguished. Major zeolite framework bands
18]
–1
–1
anchored aldehydes were treated with an equimolar amount appear around 1140, 1040, 960, 785 and 740 cm , and the
of (S)-1,1Ј-binaphthyl-2,2Ј-diamine to afford the supported FT IR spectra are characteristic of the binding of imine
chiral Schiff-base ligands 2/3-(support) as fine powdered so- nitrogen atoms. The bands around 1600 cm–1 can be as-
lids. These materials were characterised by microanalysis signed to C=C and azomethine C=N vibrations, which are
1108
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Eur. J. Inorg. Chem. 2008, 1107–1115

Soluble Au and Pd Complexes Heterogenized on MCM-41
shifted to lower wavenumbers (relative to the free ligands) 40 °C). The possibility of recovering the catalysts, the influ-
due to N-coordination of the imine.[ The IR spectra of ence of the nature of the support, and the comparison of
the palladium complexes also show strong bands at 1290 the activity and stability of supported catalysts with their
19]
–
1
and around 1500 cm due to the symmetric and asymmet- homogeneous counterparts were all studied. Table 1 sum-
ric ν(COO) vibrations, respectively, in agreement with those marises the catalytic results obtained with homogeneous
[
20]
expected for monocoordinated acetate ligands.
New Schiff-base complexes immobilized on different supports.
–
1
bands in the 500–600 cm region are ascribed to ν(M–O), The results show that the homogeneous Pd and Au systems
–
1
and ν(Au–Cl) appears at about 350 cm .
lead to quantitative conversion of olefins under these hy-
II
III
The chiral Schiff-base–Pd or –Au complexes immobi- drogenation conditions and that the order of reactivity can
lized on supports were characterised by diffuse-reflectance be explained on the basis of the steric factors.
UV spectroscopy. The complexes show bands due to intrali-
As Table 1 shows, the heterogenized catalysts generally
have higher activities than their homogeneous counterparts
gand πǞπ* and nǞπ* transitions in the aromatic ring and
[
21]
azomethine-group and charge-transfer transitions.
The
(Figure 1) as, depending on the substrate studied, the TOFs
bands in the 400–450 nm region correspond to the d–d
transitions expected for planar complexes[ and MLCT
bands. The diffuse-reflectance spectra of the complexes are
almost identical before and after heterogenization, thereby
indicating that there is no significant distortion in the ge-
ometry and electronic environment of these complexes
upon heterogenization.
are higher than those observed for the homogeneous cata-
lysts. These results indicate that either the support or the
metal complexes have a considerable effect on the catalytic
process that results from the stability of the active metallic
species formed. The fact that the heterogenized catalysts
have higher reaction rates is one of the expected advantages
of using heterogenized catalysts and is in good agreement
22]
[
18]
with our previous results.
The higher reaction rates ob-
served for heterogenized samples in different systems can
be explained by the site isolation effect in these systems,
which means that the complex is not subject to self-degra-
dation as it is present in a molecularly dispersed form. After
prolonged storage (3–6 months) at room temperature, our
heterogenized catalysts show the same activity and selectiv-
ity as the freshly prepared catalysts in the standard hydro-
genation reaction. We used our less conventional imine hy-
drogenation catalysts based on Pd– and Au–Schiff bases to
carry out the hydrogenation reaction as they are economi-
cally attractive compared to those based on Rh, Ru or Ir.
The reaction conditions chosen were also milder (P Յ
NMR Spectra
In all cases the spectra show the simultaneous occurrence
of two sets of signals attributable to the substituted benzal-
dimine entity and to the aliphatic part of the ligand. The
1
3
C NMR spectra show signals assigned to the C=N carbon
atom, which are shifted to high field, and to the C1 atom
at δ ≈ 162 ppm, which are shifted downfield, thereby con-
firming that metallation has occurred.
Catalytic Hydrogenation
With these structurally well-defined PdII and AuIII com- 10 bar) than those reported previously.
plexes available, we explored their ability to catalyse the hy-
The results presented in Table 1 show that the Pd and Au
drogenation of alkenes (diethyl itaconate, diethyl 2-benzyl- complexes described here are very active in the homogen-
–1
idenesuccinate, diethyl 2-naphthylidenesuccinate) and im- eous phase, with TOFs of about 5000 h . We checked that
ines (5-phenyl-3,4-dihydro-2H-pyrrole) under standard con- the catalytic activity observed is due to the complex and
ditions (EtOH as the solvent, 4 atm of hydrogen pressure, not to any metal formed by decomposition of the complex
Table 1. Turnover rates (TOF)[a] for the catalytic hydrogenation of diethyl itaconate, diethyl 2-benzylidenesuccinate, diethyl 2-naphthylid-
II
III
[b]
enesuccinate and 5-phenyl-3,4-dihydro-2H-pyrrole with Pd and Au catalysts, with the ee values (%) given in parentheses.
Eur. J. Inorg. Chem. 2008, 1107–1115
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1109

C. González-Arellano, A. Corma, M. Iglesias, F. Sánchez
FULL PAPER
dition of catalyst several times and found that both yield
and activity were retained. A portion of the supported cata-
lyst was analysed after each experiment to determine the
concentration of metal still present on the support. The fil-
trate was used in a new reaction and was not found to cata-
lyze the hydrogenation reaction.
[
18]
We have shown in a previous paper
that the rate of
hydrogenation can be increased by introducing some acidity
on the support due to stabilisation of the transition state by
+
the H in the reaction medium. In light of these findings we
prepared an MCM-41 containing tetrahedrally coordinated
framework Al. An increase in the reaction rate with respect
to catalysts on non-acidic supports was observed when hy-
drogenation was performed with Pd– and Au–Schiff-base
complexes grafted onto this acidic MCM-41 (Si/Al ≈ 15;
Table 1).
We were unable to achieve high enantioselectivity with
these catalysts. Indeed, despite their very high activity, the
Pd and Au catalysts gave enantioselectivities of only 10–
15%. We believe that the low enantioselectivity of the com-
plexes for hydrogenation of prochiral substrates could be
due to the planarity of the metal complex. The intermediate
species also has a square-planar structure, which means that
the approximation of the substrate to only one face of the
catalytic centre is not sufficiently favoured in this case.
We studied the mechanism of the reaction in detail by a
combination of kinetic experiments and theoretical calcula-
[
24]
tions
and propose a reaction pathway based on hetero-
lytic hydrogen cleavage on the basis of these experimental
results. The mechanism of the reaction involves the hetero-
lytic cleavage of H , which is the controlling step of the
2
reaction, as demonstrated by the presence of an induction
period in the kinetic curve, as the first step. The correct
and heterogenized catalysts in the hydrogenation of 5-phenyl-3,4- choice of solvent plays a critical role in lowering the acti-
Figure 1. Comparison of the catalytic activities of homogeneous
dihydro-2H-pyrrole.
vation barrier of this step. After the induction period, which
corresponds to the formation of the active catalytic species,
in the reaction media by applying the methodology de- the new controlling step of the reaction is the insertion of
scribed in the literature.[ The presence of bulk metal was the olefin. Supporting the Au complexes on a polar sup-
eliminated as follows: the reaction mixtures were passed port increases the catalytic activity rather than decreasing
through a micropore filter, the reactor was washed with eth- it. This increase is even larger when the surface of the car-
anol, and the washings were filtered. The filter did not show rier contains Brönsted acid sites.
traces of bulk metal (microscope, 100ϫ magnification). An-
23]
III
other problem that can affect the results may arise from the
unperceived presence of colloidal metal in “homogeneous”
catalysts. Such colloidal particles can be catalytically active
Catalytic C–C Coupling Reactions
and may be responsible for all of the observed catalytic ac-
tivity even though they may not subsequently precipitate. It ganic synthesis, with Heck,
is not sufficient, therefore, to verify merely that the solu- couplings
tions are clear, nor to show that any bulk metal that does tions due to their mild reaction conditions. The coupling
precipitate is catalytically inactive. products are mostly used as intermediates for polymers,
Carbon–carbon coupling reactions are ubiquitous in or-
[
25]
[26]
Suzuki
and Sonosgahira
[
27]
occupying a special place among such reac-
It appears from the experiments described above that natural products and bioactive compounds. The most com-
Au– and Pd–Schiff-base complexes are stable under these monly used catalytic systems for C–C coupling reactions
reaction conditions and are very active hydrogenation cata- include [PdCl (PPh ) ], PdCl /PPh and [Pd(PPh ) ], to-
2
3 2
2
3
3 4
lysts. The activity of these catalysts increases when they are gether with CuI as a co-catalyst (Sonogashira reaction), and
supported on mesoporous MCM-41. We carried out the large amounts of amines as the solvents or co-solvents.[
heterogeneous asymmetric hydrogenation of olefins until Most of the ligands used are air- and moisture-sensitive,
completion, filtered and washed the heterogenized catalyst, require multistep syntheses and are difficult to separate
then added fresh substrate and solvent without further ad- from the final product, all of which makes supported cata-
28]
1110
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 1107–1115

Soluble Au and Pd Complexes Heterogenized on MCM-41
lysts an attractive option. We therefore decided to investi- the boronic acid on the reactivity can be seen in Table 2. In
II
I
III
gate the use of our Pd , Au and Au complexes, sup- general, moderate yields and good selectivity for the cross-
I
ported or not on silica-based mesoporous solids, for the C– coupling product are found with Au complexes.
C coupling reactions.
Table 3. Yields for the AuIII-catalysed homocoupling of arylboronic
acids under Suzuki reaction conditions.[
a]
Suzuki Coupling
Suzuki coupling has become increasingly popular due to
its compatibility with a variety of functional groups, the
stability of the organoboron precursors and the ease of
RЈ
2/3-AuIII
2/3-AuIII-(MCM-41)
workup of the reaction mixture. Furthermore, a wide vari-
ety of arylboronic acids are now available commercially.
Herein we present a very easy and widely applicable ap-
Ph
95/97
90/96
93/95
97/99
94/97
92/95
4
6 4
-MeOC H
3-BrC H
6 4
II
I
proach using Pd and Au complexes, supported or not on
silica-based mesoporous solids, as catalysts for the Suzuki [a] Arylboronic acid (1.5 equiv.), catalyst (15–20 mol-%) and
I
3 4
K PO (2.0 equiv.) in xylene at 130 °C. Yields determined by GC
analysis after 24 h and by isolating samples from the reaction mix-
ture.
reaction (Table 2). The Au complex was synthesised ac-
cording to Equation (1). Thus, a solution of [AuCl(PPh3)]
1 mmol) in thf was added to a solution of the phenolate
ligand 2 (1 mmol) in thf (20 mL) at 40 °C and the resultant
(
Complexes 2-PdII were also tested under the same reac-
mixture stirred for 2 h. It was then cooled to room tempera- tion conditions for comparison purposes (Table 2). The re-
I
ture, filtered and concentrated under vacuum. The residue sults show that the activity and selectivity of 2-Au and 2-
II
was extracted with dichloromethane and precipitated with Pd complexes are similar. Nevertheless, it is interesting to
I
10
pentane, and the precipitate was washed several times, fil- note that Au , which has the same d electronic structure
0
tered and dried to afford a yellow-brown, air-stable solid in as Pd , is able to catalyze the cross-coupling reaction with
high yields.
high selectivity, and that simply changing the charge on the
III
gold atom (2-Au ) gives a complex that can selectively cat-
II
I
[13]
Table 2. Pd - and Au -catalysed cross-coupling under Suzuki reac- alyze the homocoupling reaction (Table 3).
[
a]
tion conditions. .
Sonogashira Coupling Reaction
As gold(I) has been successfully used to perform the
same role as Pd during carbon–carbon bond formation un-
[
14a]
10
II
II
2-AuI
der Suzuki–Miyaura conditions,
has the same d elec-
2
/3-Pd
2/3-Pd /(MCM-41)
[b]
S[c] Conv.
[b]
[c]
Conv.[b] S[c]
I
Aryl
Conv.
S
tronic structure as Cu , and can easily and properly interact
[
29]
with an acetylenic group,
it appeared to us that gold
Ph
-MeOC
80/87
50/67
30/60
68
–
97/100
28/100
22/95
–
100
20
89
61
42
–
86
95
100
100
4
6
H
4
4
100
100
100
100
100
100
–
could catalyze a copper-free Sonogashira cross-coupling re-
action. We therefore studied the homogeneous complexes
as catalysts for the Sonogashira cross-coupling of PhI with
a series of alkynes containing electron-donating and -with-
drawing substituents. The standard reaction conditions
3
-BrC
-MeC
-OHCC
6
H
4
4
6
H
4
–
–
6
H
4
39
–
[
(
a] Arylboronic acid (15 mmol), aryl halide (10 mmol), catalyst
30 mol-%), and K PO (20 mmol) in xylene at 130 °C. [b] Reaction
time: 3 h. [c] Cross-coupling/conversion.
3
4
0
were applied with the catalysts [Pd(PPh ) ] (Pd ), 2-Pd
3
4
I
II
(
Pd ) and [AuCl(PPh )] and 2-Au (both Au ); the conver-
sions are presented in Table 4. When Au complexes were
tested as catalysts for several of these reactions, the product
3
III
32 30 2 3
(C H N )OK + 3 AuCl(PPh ) Ǟ
[(C32
H
30
N
2
)O{Au(PPh
3
)
3
}] + KCl (1) observed contained 10% of the alkyne homocoupling prod-
uct along with a very small amount of the iodobenzene
The standard reaction conditions were applied to a series
of catalysts, and the conversions are tabulated in Tables 2
and 3. We chose K PO as a mild base because longer reac-
II
I
Table 4. Pd - and Au -catalysed cross-coupling under Sonogashira
conditions.
[a]
3
4
tion times were necessary with K CO to obtain reasonable
2
3
conversions. The general utility of the reaction conditions
with a variety of arylboronic acid substrates (aryl = Ph, 4-
MeOC H , 3-BrC H , 4-MeC H , 4-OHCC H ) and aryl
6
4
6
4
6
4
6
4
Catalyst
Pd(PPh
RЈ = Ph
RЈ = CH
2
2 2
CH(CO Me)
halides (BrPh, IPh) was studied. The optimal reaction con-
ditions were applied to a series of catalysts, and conversions
of more than 80% were obtained after 24 h. Complete con-
[
3
II
)
4
]
13
17
54
50
25
15
10
5
2-Pd
[
3
AuCl(PPh )]
I
version was obtained with longer reaction times. Au com-
-AuI
2
plexes are ineffective with phenyl bromide as they form the
homocoupling compound as the principal product. The ef-
[
a] Alkyne (15 mmol), aryl halide (10 mmol), catalyst (20 mol-%)
and K PO (20 mmol) in xylene at 130 °C. Yields (%) calculated
3
4
fect of electron-donating and -withdrawing substituents in after 24 h.
Eur. J. Inorg. Chem. 2008, 1107–1115
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1111

C. González-Arellano, A. Corma, M. Iglesias, F. Sánchez
FULL PAPER
homocoupling product. PdII and [Pd(PPh ) ] complexes Recycling of Heterogenized Catalysts
3
4
were also prepared and tested under the same reaction con-
ditions for comparison purposes (Table 4). The results show
that the activity of these Pd complexes is similar to that of
gold and that their selectivity for the Sonogashira reaction
is also high.
The major advantage of using heterogenized metal com-
plexes as catalysts is the ease with which they can be reco-
vered as stable species from the reaction mixture (by simple
filtration) and then reused. We carried out the hetero-
geneous asymmetric hydrogenation of olefins until comple-
tion, filtered and washed the heterogenized catalyst, then
added fresh substrate and solvent without further addition
Heck Reaction
I
III
II
We studied the performance of Au , Au and Pd com- of catalyst several times and found that both yield and ac-
plexes in Heck reactions using a biphasic mixture of toluene tivity are retained. A portion of supported catalyst was ana-
and ethylene glycol as solvent in the presence of potassium lyzed after each experiment to determine the concentration
[
30]
acetate.
Catalysis in two-phase liquid/liquid systems is of metal still present on the support. The filtrate was used
generally referred to as biphasic catalysis as the catalyst is in a new reaction and was found not to catalyze the hydro-
present in one phase while the reactants and products are genation. The catalysts were re-used three times (to test
present in the other phase. In our case the term “biphasic” their recyclability) and the results show that reproducibility
refers to the mixture of two immiscible solvents, with the is high. Hot filtration experiments and ICP (inductively
heterogenized catalyst being a third phase.
coupled plasma) measurements were carried out indepen-
The reactivity and recyclability of these systems for the dently to rule out the possibility of leaching.
Heck reaction of iodobenzene with n-butyl acrylate under
The recycling experiments for Heck reactions were per-
phosphane-free conditions was studied (Table 5). The com- formed by separating the ethylene glycol phase and reusing
plexes are not sensitive to oxygen or moisture, and no this together with fresh toluene and base. Since the stability
change in their activity was observed when the reaction was of supported complexes towards dissolution during the re-
carried out in an open system. This catalytic system is stable action is a common problem for most anchored catalysts,[
and recyclable, and the reaction products, catalyst and base and considering the leaching observed with palladium sup-
can easily be separated. In this biphasic vinylation system, ported on polymer beads, quantitative analysis by atomic
the toluene phase contains the reactants and products while absorption spectroscopy (AAS) was employed to determine
the metal complex and potassium acetate are present in the the amount of metal in the final reaction solution. We ob-
ethylene glycol phase. The reaction should therefore occur served that the catalyst could be recycled at least three times
in the ethylene glycol phase and at the ethylene glycol/tolu- without significant loss of activity (Table 5).
31]
ene interface, where the base is partly soluble.
In order to check the stability of the metal complexes
supported on the solid matrix, we characterised the solid
before and after the reaction. As can be seen from the IR,
Table 5. Heck reaction of iodobenzene with n-butyl acrylate using
13
homogeneous and supported palladium catalysts.[
a]
C NMR and UV/Vis spectra, the nature of supported spe-
cies is very similar and the most important signals for the
Schiff-base ligands appear in the same position after the
reaction.
Catalyst
Conv. [%][b]
Conclusions
2
-Pd
-Pd-(MCM-41) (1st run)
-Pd-(MCM-41) (2nd run)
7
2
10
53
43
69
15
60
75
Palladium and gold catalysts supported on mesoporous
silica have been successfully prepared by sequential grafting
of organic functional groups. The catalysts are active in hy-
drogenation and C–C coupling reactions (Suzuki, Sonoga-
shira and Heck) and, more importantly, are stable under
the reaction conditions employed and can be recycled with-
out loss of activity. Au – and Pd –Schiff-base complexes
are very active hydrogenation catalysts, and their activity
increases when they are supported on MCM-41. Gold(III)
homogeneous or heterogenized complexes catalyze the
2
2
-Pd-(MCM-41) (3rd run)
3-Pd
3
3
3
-Pd-(MCM-41) (1st run)
-Pd-(MCM-41) (2nd run)
-Pd-(MCM-41) (3rd run)
III
II
[
a] Olefin (10 mmol), aryl halide (10 mmol), catalyst (20 mol-%)
and KOAc (11 mmol) in toluene/ethylene glycol at 140 °C. [b]
Yields (%) after 24 h.
Table 5 shows the catalytic performance of various sup- homocoupling of arylboronic acids or alkynes to afford
ported complexes for the Heck reaction. The reaction was symmetrical biaryls, whereas the respective gold(I) and pal-
successful at low metal catalyst loadings (2–5 mol-%), al- ladium(II) complexes catalyze the corresponding cross-
though the reactions did not proceed at concentrations be- coupling reaction. The fact that no inert gas and no ad-
III
low 2 mol-%. Analogous Au complexes and gold colloids ditional ligands or co-catalyst are necessary, their easy sepa-
do not catalyse this reaction under the same reaction condi- ration, and their easy availability make the supported cata-
tions. No palladium black was observed after more than lysts an interesting alternative to homogeneous catalysts,
72 h when using the palladium complexes as catalysts.
and we have shown here that it is possible to obtain a selec-
1112
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 1107–1115

Soluble Au and Pd Complexes Heterogenized on MCM-41
tive catalytic system for a specific reaction by choosing the corresponding anchored ligands. The heterogenized ligands 3-
(
1
MCM-41) were synthesised as follows. A stirred slurry containing
mmol of the corresponding anchored hydroxy aldehyde was pre-
appropriate combination of metal, ligand and support.
pared in acetonitrile (15 mL) under argon at room temperature.
After 10 min, 125 mg (1.2 mmol) of the amine was added and the
mixture stirred at reflux for 1 h. 3-tert-Butyl-5-methylsalicylalde-
hyde (1 mmol) was then added and the mixture stirred for 3 h. The
mixture was cooled, and the deep yellow solid was filtered off and
washed thoroughly with ethanol and diethyl ether.
Experimental Section
General Remarks: All preparations of metal complexes were carried
out under nitrogen using conventional Schlenk-tube techniques.
Solvents were carefully degassed before use. The silylating agent
OCN(CH
2
)Si(OEt
3
) was obtained from ABCR. C,H,N analyses
–1
2
(
3
-(MCM-41): Found C 13.9, H 2.4, N 1.8 (0.32 mmolg ). IR
were carried out by the analytical department of the Instituto de
Química Orgánica (CSIC) with a Lecco apparatus. Metal contents
were analysed by atomic absorption spectroscopy using a Perkin–
Elmer AAnalyst 300 atomic absorption apparatus and plasma ICP
Perkin–Elmer 40 instrument. IR spectra were recorded with a
–1
KBr): ν˜ = 1646, 1630, 1557 (C=N, C=O, C=C) cm . UV/Vis: λ =
13
47, 265, 224 nm. C NMR (solid): δ = 159.43 (CH=N); 154.96
NHCONH); 152.62 (COH); 137.25, 128.37, 119.58, 110.27
arom); 42.48 (CH CH CH Si); 33.68 [C(CH ]; 27.57 [C(CH ];
5.78 (CH CH Si); 8.64 (CH Si) ppm.
(
(
C
2
2
2
3
)
3
3 3
)
1
2
2
2
–
1
Bruker IFS 66v/S spectrophotometer (range 4000–200 cm ) for
1
13
2-(MCM-41) (Si/Al ≈ 15): Found
(
C
20.3,
0.55 mmolg ). IR (KBr): ν˜ = 1647, 1630 (C=N, C=O, C=C) cm .
UV/Vis: λ = 346, 264, 222 nm.
H 2.8, N 3.1
KBr pellets. H and C NMR spectra were recorded with Varian
XR300 and Bruker 200 spectrometers. Chemical shifts are quoted
relative to tetramethylsilane as internal standard. High-resolution
–
1
–1
13
C MAS or CP/MAS NMR spectra of powdered samples, which
–1
3
(
2
1
2
-(MCM-41): Found C 15.6, H 2.5, N 2.3 (0.41 mmolg ). IR
in some cases also included a Toss sequence to eliminate the spin-
ning side-bands, were recorded at 100.63 MHz (6 µs 90° pulse
width, 2 ms contact time and 5–10 s recycle delay) with a Bruker
MSL 400 spectrometer equipped with an FT unit. The spinning
frequency at the magic angle (54°44Ј) was 4 kHz.[ Optical rota-
tion values were measured at the sodium-D line (589 nm) with a
Perkin–Elmer 241 MC polarimeter. GC analysis was performed
with a Hewlett–Packard 5890 instrument. The enantiomeric excess
was measured by HPLC using a chiralcel OD chiral column. The
inorganic supports for anchoring were purely siliceous MCM-41[33]
and Al-containing MCM-41 (Si/Al ≈ 15).
–1
KBr): ν˜ = 1642–1599 (C=N, C=O, C=C) cm . UV/Vis: λ = 349,
13
82, 235 nm. C NMR (solid): δ = 160.30 (NHCONH, CH=N),
28.71–120.26 (Carom), 43.58 (CH CH CH Si), 34.36 [C(CH ],
8.73 [C(CH ], 23.78 (CH CH CH Si, CH ), 9.85 (CH Si) ppm.
2
2
2
3 3
)
3
)
3
2
2
2
3
2
32]
3
-(MCM-41) (Si/Al ≈ 15): Found 20.8, 3.1, 3.2
C
H
N
–
1
–1
(
0.56 mmolg ). IR (KBr): ν˜ = 1639 (C=N, C=O, C=C) cm . UV/
Vis: λ = 348, 264, 222 nm.
Synthesis of Metal Complexes: The homogeneous chiral complexes
were synthesised and used as catalysts to compare the catalytic ac-
tivities and to evaluate the relationship between the structural fea-
tures of the ligands and the activity of the complexes. Soluble com-
plexes were obtained as follows. An ethanolic solution of Pd-
Synthesis of Ligands: The soluble ligand 2 was synthesised in high
[16]
yield by a similar method to that described previously. Ligand 3
was obtained by reported methods.[
34]
2 4 2
(OAc) or HAuCl ·3H O (0.5 mmol/15 mL) was added to a solu-
tion of the ligand (50 mg, 1 mmol) in EtOH (15 mL) at room tem-
perature. The resulting mixture was stirred under reflux for 4 h,
then cooled to room temperature and concentrated under vacuum.
The residue was washed several times with diethyl ether, dried and
filtered to afford the respective complexes in almost quantitative
yields.
(
R)-2-[(2Ј-Amino-1,1Ј-binaphthyl-2-ylimino)methyl]-6-tert-butyl-4-
methylphenol (2): 1,1-Binaphthyl-2,2Ј-diamine (568 mg, 2 mmol)
was added to a solution of 3-tert-butyl-5-methylsalicylaldehyde (1;
3
stirred under reflux for 10 h. The solvent was then removed under
vacuum and the residue purified by flash chromatography (hexane)
to give an orange solid. Yield: 690 mg (75%). M.p. 52–55 °C.
84 mg, 2 mmol) in acetonitrile (20 mL), and the mixture was
2-Pd: Brick-red solid. Yield: 61 mg (90%). M.p. Ͼ 230 °C. [α]2
–92 (c = 0.02, MeOH). C34
5
=
D
2
5
[α]
D
= –134 (c = 0.6, CHCl
3
). C32
H
30
N
2
O (458): calcd. C 83.8, H
H
32
N
2
O
3
Pd (623.04): calcd. C 65.5, H
6
3
2
.6, N 6.1; found C 83.0, H 6.4, N 5.9. IR (KBr): ν˜ = 3468 (OH); 5.2, N 4.5, Pd 17.1; found C 65.2, H 5.6, N 4.3, Pd 16.9. IR (KBr):
–1
–1
381 (NH); 1618, 1578 (C=C + C=N) cm . UV/Vis: λ = 360, 325,
95, 232 nm. 1H NMR (CDCl
): δ = 8.59 (s, CH=N), 8.03–6.85
), 2.22 (s, CH ), 1.27 [s, C(CH
): δ = 167.43 (CH=N); 158.36 (COH); (Harom), 2.32 (s, 3 H, OCH
ν˜ = 1620, 1584 (CO); 1522 (C=N); 552 (Pd–O) cm . UV/Vis: λ =
1
467, 326, 299, 229, 214 nm. H NMR (CDCl
3
): δ = 8.04–7.91 (m,
3
(
m, Harom), 3.70–3.56 (br., NH
2
3
3
)
3
]
1 H, CH=N), 7.60–7.50 (Harom), 7.47–7.17 (Harom), 7.00–6.74
), 2.13 (s, 3 H, CH ), 1.46 [s, 9 H,
] ppm. C NMR (CDCl ): δ = 196.97 (OCOCH ); 164.62
13
ppm. C NMR (CDCl
3
3
3
1
3
1
1
1
1
37.17, 134.59, 133.95, 133.31, 132.67, 131.17, 130.92, 130.04,
C(CH
3
)
3
3
3
29.74, 129.48, 129.31, 128.33, 128.19, 128.00, 127.17, 126.76,
26.44, 125.93, 123.92, 122.05, 118.59, 118.30, 118.16, 118.11,
(COPd); 162.68 (CH=N); 146.90, 141.40, 135.35, 134.33, 133.23,
132.81, 131.41, 130.82, 130.22, 128.35, 127.03, 126.03, 125.94,
18.02 (Carom); 34.59 [C(CH
3
)
3
]; 29.08 [C(CH
3
)
3
]; 20.57 (CH
3
)
3 3
125.81, 124.58, 123.43, 123.24 (Carom); 35.77 [C(CH ) ]; 29.75
ppm. MS (EI): m/z (%) = 458 (100) [M], 443 (82) [M – 15], 267
53) [M – 191].
[C(CH
OAc; Pd].
3
)
3
]; 20.51 (CH
3
); 20.25 (OCH
3
) ppm. MS: m/z = 564 [M –
1
06
(
5
Synthesis of Heterogenized Ligands: The heterogenized ligands 2-
MCM-41) were synthesised in high yields according to a similar
2-Au: Dark brown. Yield: 68 mg (91%). M.p. Ͼ 230 °C. [α]2
D
= –92
(
(c = 0.05, MeOH). C32 28AuClN
H
2
O (689.01): calcd. C 55.8, H 4.1,
Au 28.6, N 4.1; found C 55.6, H 4.0, Au 28.3, N 3.8. IR (KBr): ν˜
= 1615, 1555 (C=N) cm . UV/Vis: λ = 482, 345, 328, 230 nm. H
NMR (CD OD): δ = 7.89–6.37 (Harom, CH=N), 1.96 (s, 3 H, CH ),
1.20 [s, 9 H, C(CH OD): δ = 167.34
(COAu); 156.00 (CH=N); 135.46, 134.96, 134.53, 133.52, 132.87,
[18]
method to that described previously. Thus, a stirred slurry con-
taining 1 mmol of the corresponding anchored hydroxy aldehyde
was prepared in acetonitrile (10 mL) under argon at room tempera-
ture. After 10 min, 125 mg (1.1 mmol) of the amine was added and
the mixture stirred for 16 h. It was then cooled and the deep yellow
solid filtered off and washed thoroughly with ethanol and diethyl
ether. The resultant solid was dried under vacuum to afford the
–
1
1
3
3
1
3
3 3 3
) ] ppm. C NMR (CD
132.35, 131.54, 131.32, 130.63 (Carom); 35.22 [C(CH
3
)
3
]; 30.04
[C(CH
3
)
3
]; 20.00 (CH
3
) ppm. MS: m/z = 654 [M – Cl].
Eur. J. Inorg. Chem. 2008, 1107–1115
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1113

C. González-Arellano, A. Corma, M. Iglesias, F. Sánchez
FULL PAPER
-Pd: Brick-red solid. Yield: 176 mg (92%). M.p. Ͼ 230 °C. [α]2
289 (c = 0.36, CH Cl ). C44 Pd (737.23): calcd. C 71.7,
H 5.7, N 3.8, Pd 14.4; found C 71.5, H 5.6, N 3.7, Pd 14.1. IR
5
=
Hydrogenation Reactions: The catalytic properties (alkenes: diethyl
succinates; imine: 5-phenyl-3,4-dihydro-2H-pyrrole) of the Pd and
Au complexes were examined under conventional conditions for
batch reactions in a 100 mL reactor (Autoclave Engineers) at 40 °C
3
+
D
2
2
42 2 2
H N O
–
1
(
KBr): ν˜ = 1607, 1504 (C=N, C=C) cm . UV/Vis: λ = 438, 318,
1
2
98, 231 nm. H NMR (CDCl
arom, CH=N), 6.74–6.63 (Harom), 2.09 (s, 3 H, CH
H, C(CH Cl): δ = 169.34 (COPd); 158.00
] ppm. 13C NMR (CD
CH=N); 135.63, 134.89, 134.65, 133.73, 133.50, 132.25, 131.97,
3
): δ = 8.20–7.78 (Harom), 7.60–6.82
2
and 4 atm of H pressure with a metal/substrate molar ratio of
(
H
3
), 1.35 [s, 9 1:1000. Formation of the hydrogenated product was monitored by
3
)
3
3
GC.
(
Recycling Experiments: At the end of the hydrogenation process,
the reaction mixture was filtered, and the zeolite-containing cata-
lyst residue was washed to completely remove any remaining prod-
ucts and/or reactants and used again. A portion of supported cata-
lyst was analysed after each experiment to determine the concentra-
tion of metal still present on the support. The filtrate was also used
1
2
31.65, 130.58, 129.28 (Carom); 35.42 [C(CH
0.10 (CH
) ppm. MS: m/z = 737 [M; 106Pd].
3
)
3
3 3
]; 30.09 [C(CH ) ];
3
-Au: Black solid. Yield: 197 mg (88%). M.p. Ͼ 230 °C. [α]2
42AuClN
Au 22.8, N 3.2; found C 61.0, H 4.6, Au 22.6, N 3.4. IR (KBr): ν˜
5
= –35
3
D
(c = 0.06, MeOH). C44
H
2 2
O (863): calcd. C 61.2, H 4.9,
–1
=
1638 (C=N), 555 (Au–O) cm . UV/Vis: λ = 493, 343, 326, in a new reaction and was found to not catalyze hydrogenation.
1
227 nm. H NMR (CD
3
OD): δ = 8.01–7.15 (Harom), 2.00 (s, 3 H,
C–C Coupling Reactions
CH
3
), 1.25 [s, 9 H, C(CH
3
)
3
] ppm. 13C NMR (CD
3
OD): δ = 162.41
Suzuki Reaction: The reaction was carried out in a 25 mL vessel at
30 °C for a reaction time of 20–180 min. In a typical run, a mix-
(
[
COAu); 159.98 (CH=N); 123.19, 119.35, 112.44 (Carom); 35.40
C(CH ]; 29.72 [C(CH ]; 20.34 (CH ) ppm. MS: m/z = 820 [M –
Cl].
1
3
)
3
3
)
3
3
ture of aryl halide (10 mmol), boronic acid (15 mmol), aqueous
potassium phosphate (20 mmol) and catalyst (0.3 mmol) in 3 mL
of o-xylene was stirred for the desired time. The solution was co-
oled, and a 1:1 mixture of diethyl ether/water (20 mL) was added.
The organic layer was washed, separated, further washed again
with another 10 mL portion of diethyl ether, dried with anhydrous
Synthesis of Metal Complexes: The heterogenized complexes were
synthesised as follows. An ethanolic solution of Pd(OAc) or
HAuCl ·3H O (0.5 mmol/15 mL) was added to a suspension of het-
2
4
2
erogenized ligand in toluene (20 mL) at room temperature, and the
resulting mixture was stirred under reflux for 12 h, then cooled to
room temperature and filtered. The solid was washed several times
with ethanol, dried and filtered to afford the respective heterogen-
ized complexes in almost quantitative yields.
4
MgSO and filtered. The solvent and volatiles were removed com-
pletely under vacuum to give the crude product, which was sub-
jected to column chromatographic separation to give pure com-
pounds. The reaction was monitored by GC-MS. At the end of the
process, the reaction mixture was filtered and the zeolite-containing
catalyst residue washed to completely remove any remaining prod-
ucts and/or reactants and used again. A portion of supported cata-
lyst was analysed after each experiment to determine the concentra-
tion of metal still present on the support.
2
-Pd-(MCM-41): Found
C
11.0,
H 2.1, N 1.7, Pd 10.6
–1
(1.0 mmolg ). IR (KBr): ν˜ = 1634 (CO), 1576 (C=N), 586 (Pd–O)
–1
13
cm . UV/Vis: λ = 432, 346, 263, 217 nm. C NMR (solid): δ =
1
1
2
1
97.40 (OCOCH
30.70–118.41 (Carom), 44.30 (CH
7.80 [C(CH ], 24.00 (CH CH CH
0.20 (CH Si) ppm.
3
), 160.14 (NHCONH, COPd), 155.58 (CH=N),
2
CH
2
CH
2
Si), 33.90 [C(CH
3
)
3
],
3
)
3
2
2
2
Si, CH
3
), 20.66 (OCOCH
3
),
Sonogashira Reaction: The reaction was carried out in a 25 mL ves-
sel at 130 °C for 24 h. In a typical run, a mixture of aryl halide
2
(
10 mmol), alkyne (15 mmol), aqueous potassium carbonate or
2
-Pd-(MCM-41) (Si/Al ≈ 15): Found C 18.0, H 2.5, N 2.7, Pd 1.7
–
1
–
1
phosphate (20 mmol) and catalyst (0.5 mmol) in 3 mL of o-xylene
was stirred for the desired time.
(0.49 mmolg ). IR (KBr): ν˜ = 1649, 1568 (C=N, CO, C=C) cm .
UV/Vis: λ = 346, 264, 222 nm.
Heck Reaction: In a typical experiment, the catalyst (0.03 mmol,
5%) was suspended in a mixture of ethylene glycol and toluene
(1:1, 6 mL), then iodobenzene (1 mmol), n-butyl acrylate (1 mmol)
and potassium acetate (1.14 mmol) were added with stirring. The
mixture was refluxed, and the reaction progress was monitored by
GC. The recycling experiments were performed by separating the
ethylene glycol phase and reusing this together with fresh toluene
and base.
2
-Au-(MCM-41): Found
C
7.5,
H 1.7, Au 0.6, N 0.7
–1
(0.21 mmolg ). IR (KBr): ν˜ = 1635, 1580 (C=N, C=C); 556 (Au–
–1
13
O) cm . UV/Vis: λ = 526, 389, 302 nm. C NMR (solid): δ =
67.68 (NHCONH, COAu), 153.67 (CH=N), 138.60–113.00
arom), 44.00 (CH CH CH Si), 30.86 [C(CH ], 23.14
CH CH CH Si), 22.10 (CH ), 8.78 (CH Si) ppm.
1
(C
2
2
2
3 3
)
(
2
2
2
3
2
2-Au-(MCM-41) (Si/Al ≈ 15): Found C 11.8, H 1.9, Au 2.5, N 1.9
–
1
–
1
(0.48 mmolg ). IR (KBr): ν˜ = 1644, 1557 (C=N, C=C) cm . UV/
Supporting Information (see also the footnote on the first page of
this article): Representative experimental procedures and charac-
terization for all new compounds.
Vis: λ = 507, 349, 264, 221 nm.
3-Pd-(MCM-41): Found
C
12.2,
H 2.2, N 1.7, Pd 1.2
–1
(
(
0.32 mmolg ). IR (KBr): ν˜ = 1637, 1574 (CO, C=N, C=C); 558
–1
Pd–O) cm . UV/Vis: λ = 700, 448, 322, 285 nm.
-Pd-(MCM-41) (Si/Al ≈ 15): Found C 17.8, H 2.3, N 2.8, Pd 5.9
Acknowledgments
3
–1
–1
(0.52 mmolg ). IR (KBr): ν˜ = 1637, 1572 (C=N, CO, C=C) cm .
Financial support from the Dirección General de Investigación Ci-
entífica y Técnica of Spain (project nos.: MAT2006-14274-C02-01
and MAT2006-14274-C02-02) is gratefully acknowledged. C. G.-A.
UV/Vis: λ = 445, 349, 270, 224 nm.
3-Au-(MCM-41): Found 11.0, H 2.1, Au 0.3, N 1.2
C
–1
(
(
0.31 mmolg ). IR (KBr): ν˜ = 1646, 1589 (CO, C=N, C=C); 561 thanks the I3P program for financial support.
–1
Au–O) cm . UV/Vis: λ = 518, 424, 348, 264, 212 nm.
[
1] a) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180; b) A. S. K.
Hashmi, G. J. Hutchings, Angew. Chem. Int. Ed. 2006, 45, 7896;
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Imperial College Press, London, 2006; d) G. Dyker, Angew.
Chem. Int. Ed. 2000, 39, 4237 and references cited therein.
3
-Au-(MCM-41) (Si/Al ≈ 15): Found C 11.3, H 1.9, Au 4.7, N 2.5
–1
–1
(0.26 mmolg ). IR (KBr): ν˜ = 1633, 1540 (C=N, C=C) cm . UV/
Vis: λ = 512, 346, 262, 223 nm.
Catalytic Experiments
1114
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 1107–1115

Soluble Au and Pd Complexes Heterogenized on MCM-41
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Published Online: January 8, 2008
Eur. J. Inorg. Chem. 2008, 1107–1115
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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