Preparation of a new clay-immobilized highly stable palladium catalyst and its efficient recyclability in the Heck reaction

Article abstract of DOI:10.1039/b204911b

The immobilization of a series of bis-carbene-pincer complexes of palladium(II) on montmorillonite K-10 affords the preparation of effective catalysts for the C-C coupling in a standard Heck reaction. The supported catalysts so obtained, show catalytic activity similar to their homogeneous counterparts. The determination of Pd content on the supported catalysts by XPS and elemental analysis, before and after each catalytic reaction, shows that leaching is negligible. Once the reaction conditions were optimized, we were able to recycle the catalyst at least ten times, without significant loss of activity.

Full text of DOI:10.1039/b204911b

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Preparation of a new clay-immobilized highly stable palladium
catalyst and its efficient recyclability in the Heck reaction
a
Macarena Poyatos, Francisco Marquez, Eduardo Peris,* Carmen Claver and
a
a
b
´
Elena Fernandez*b
a
´
Dpto. de Quımica Inorganica y Organica, Universitat Jaume I, 12080, Castellon, Spain.
E-mail: eperis@qio.uji.es
´
´
´
b
´
Dpto. de Quımica Fısica e Inorganica, Universitat Rovira i Virgili, 43005, Tarragona, Spain.
E-mail: elenaf@quimica.urv.es
´
´
Received (in Montpellier, France) 20th May 2002, Accepted 23rd September 2002
First published as an Advance Article on the web 16th December 2002
The immobilization of a series of bis-carbene-pincer complexes of palladium(II) on montmorillonite K-10
affords the preparation of effective catalysts for the C–C coupling in a standard Heck reaction. The supported
catalysts so obtained, show catalytic activity similar to their homogeneous counterparts. The determination of
Pd content on the supported catalysts by XPS and elemental analysis, before and after each catalytic reaction,
shows that leaching is negligible. Once the reaction conditions were optimized, we were able to recycle the
catalyst at least ten times, without significant loss of activity.
imply either immobilization of Pd catalysts on polymers,9 or
Introduction
on modified mesoporous silica gel.10 However, although some
of these preformed heterogenized catalytic systems, such as the
palladium bis-carbene complex immobilized in a functiona-
lized polystyrene support prepared by Herrmann and co-
workers,9a can be efficiently recycled in a Heck reaction, they
suffer from the disadvantage of their preparation which
requires multistep syntheses. Supported liquid phase catalysts,
in which thin liquid film containing Pd-sulfonate phosphines
complexes are supported on a porous silica, have provided
good performance on C–C coupling, but leaching of the
palladium species into the solvent questions whether the active
species are attached to the solid support or whether they are
dissolved palladium complexes.6,11
In the last years, we have focused on immobilizing catalytic
systems in clay structures via adsorption or cationic exchange,
as an easy and clean alternative for heterogenization. We have
satisfactorily used montmorillonite K-10 in the preparation
of heterogenized catalysts for asymmetric hydroboration,12
reductive alkylation of primary amines,13 and hydrogenation
of imines.14–16
Over the last few years, there has been increasing interest in
designing new synthetic methods that combine chemical effi-
ciency with environmentally benign procedures. In this sense,
one of the most challenging efforts is to find ways of catalyst
recycling, since this would undoubtedly achieve both com-
bined purposes.
Homogeneous catalysis affords a large number of routes to
most transformations of organic substrates. However, the cat-
alysts used are mainly unstable species with low lifetimes under
the reaction conditions, this preventing the use of high tem-
peratures (higher conversions, faster reactions) and catalyst
recovering. The use of phosphine ligands in most catalysts
seems to afford a route to catalyst deactivation since it is
proposed that P–C bond cleavage may be implicated in the
process.1
In the search of new phosphine-free robust catalysts, we
previously described the synthesis of palladium complexes
with tridentate pincer bis-carbene ligands, which have shown
efficient catalytic activity towards C–C coupling.2–4 The high
stability of these catalysts is due to a combination of the che-
late effect imposed by the tridentate ligand, and the thermal
stability of late transition metal heterocyclic carbenes. These
complexes, displayed high catalytic activity at higher tempera-
tures (184 ꢀC) than those normally used in Heck couplings,
affording the highest turnover numbers yet reported for cou-
plings with aryl chlorides.3
Based on our previous findings, we now report the catalytic
activity of clay-heterogenized pincer-bis-carbene complexes of
Pd in the coupling of phenyl halides with styrene.
Results and discussion
Heck reactions are a tough challenge for the design of
recyclable systems. The reaction needs at least three reagents
(halide, olefin, base) besides the catalyst itself. The salt gener-
ated by neutralization of HX and base in the last step of the
catalytic cycle is accumulated in the reaction media, leading
to an unexpected influence on the catalytic process such as
degradation of the catalyst5 or promotion of the reaction
rate.6,7 Due to the change of reaction mixture composition,
the Heck process always works in a varying environment.
Despite all these problems, some approaches to support8 and
heterogenizing Heck catalysts have given encouraging results.
Common heterogenization procedures of Pd catalytic systems
Preparation of the catalysts
According to our previous reported data, we found that pin-
cer-heterocyclic-carbene ligands are good alternatives to phos-
phines in the design of new catalysts in C–C coupling reactions
by the Heck method.2–4 The compounds obtained, not only
showed high efficiency in C–X activation (X ¼ I, Br, Cl), but
also showed an extremely high temperature stability which
allowed the design of reactions at temperatures much higher
than those often used in the Heck reaction. With this in mind,
we thought that these catalysts could be excellent candidates
for designing new recyclable C–C coupling catalysts, and
DOI: 10.1039/b204911b
New J. Chem., 2003, 27, 425–431
425
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Scheme 1
we prepared a new series of complexes in order to achieve this
purpose.
(bulk, by elemental analysis). These data are in agreement with
the lower surface area of Na+–M (53 m2 ꢁ1) compared to
g
Scheme 1 shows the general procedure for the synthesis of
the catalysts used. The preparation method is similar to the
one reported in our previous papers for [PdBr(CNC-Bu2)]Br,
1-Br.2–4 Compounds [PdBr(CNC-Bu2)]PF6 , 1-PF6 [PdBr-
(CNC-Bz2)]X, 2-X, and [PdBr(CNC-PhNO2)]X, 3-X (X ¼ Br
and PF6), are new, and their preparation and characterization
are described in the Experimental section.
MK-10. It is known that although both montmorillonite K-
10 and bentonite are the same montmorillonite clay material,
they show significant differences in crystallinity.18 In fact,
MK-10 is prepared from Na+–M by acid treatment, which
partially destroys the bentonite layer structure giving a disor-
dered and increased surface area.
It has been reported that the mechanism of grafting the ionic
complexes to MK-10, and Na+–M, is different.12,16 In the first
case the interaction between counter cation and counter anion
to MK-10 is likely hydrogen bonding, and in the second case
the electrostatic attractions between the counter cation and
the bentonite layers are the significant features of this immobi-
lization (Scheme 2). The different entrapment of these catalysts
on the clays could also imply differences in the activities when
they are used as catalytic systems in the Heck reaction.
The immobilization process of the organometallic complexes
1-Br, 2-X and 3-X onto the clays montmorillonite K-10 (MK-
10) and bentonite (Na+-M), was carried out by the solvent
impregnation method previously described.17 Thus, coloured
solutions of the ionic complexes in anhydrous dichloro-
methane were stirred under N2 with the solid support for 24
h. Both clays (MK-10 and Na+-M), were previously dried at
100 ꢀC, for 24 h to eliminate the small amount of adsorbed
water that they may contain, in order to guarantee the maxi-
mum immobilization.12 The amount of metal complex
adsorbed by the clay was determined by XPS and atomic
absorption spectroscopy (AAS). XPS was also used to charac-
terize the chemical state of palladium present in the supported
catalysts, before and after each catalytic reaction. The maxi-
mum palladium load achieved is 1.7% (weight) when deter-
mined by XPS analysis, and 0.8% for the elemental analysis
of the bulk solid (AAS), for the grafting of 1-Br onto MK-
Catalytic results
To compare the activity and stability of the supported catalysts
on MK-10 with their homogeneous counterparts, we per-
formed the standard C–C coupling of PhI with styrene in both,
homogeneous and heterogenized conditions (Table 1), using
NaOAc as base. In the first stage of our research, we studied
the catalytic activity of our catalysts at 180 ꢀC for a reaction
time of 2 h. In all cases, as we previously reported,3 the high
stability of the catalysts used allowed us to work under non-
inert atmosphere and without any special precautions. A per-
fect correlation between the conversions in homogeneous
and heterogenized conditions can be observed, therefore the
activity of the catalyst does not decay upon immobilization.
Even slightly higher rates are achieved with the heterogenized
catalytic systems compared with their homogeneous counter-
parts. However, the conversions seem to depend on the cata-
lyst used, being 1-Br the most efficient one (entry 1 and 2,
Table 1).
10 (B.E.T. surface area 221 m2
g
ꢁ1). In order to confirm
whether the structure of the catalyst remains unchanged upon
immobilization, we performed the Raman spectra of both, free
and supported catalyst (Fig. 1). The spectra of both systems
conclusively revealed that, at least, the structure of the preca-
talyst remains the same upon immobilization. Whether the
true catalytic species is the same in both, homogeneous and
heterogenized systems, falls beyond the scope of this work,
and may need a detailed mechanistic study. Remarkably differ-
ence was observed when preheated bentonite (Na+–M), was
used as support. Following the same impregnation procedure,
the amount of metal complex adsorbed was only 0.05% of 1-Br
Fig. 1 Raman spectra of: (a) montmorillonite K10; (b) 1-Br; (c) 1-Br/MK-10.
426
New J. Chem., 2003, 27, 425–431

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Scheme 2
Removal of the supported catalyst by filtration and repeated
C–C coupling reaction recycling is demonstrated, but a signifi-
cant loss of activity is observed. The presence of active species
in solution is not considered because no product was formed
when styrene and phenyl iodide were added to a hot filtrate
of the first run. Leaching of the palladium complex can be dis-
carded because the amount of Pd(II), determined by elemental
(AAS) and XPS analysis, in the solid before and after the cat-
alytic consecutive reactions was mainly unchanged (Table 2).
As far as the nature of the counter anion is concerned, the cat-
alytic activity exhibited by the supported hydrogen bonded
palladium complexes 1-Br/MK-10 and 1-PF6/MK-10 is com-
parable to their homogeneous counterparts 1-Br and 1-PF6 . In
addition, there is not any significant difference in the use of the
ionic palladium complexes with the counterions studied, show-
ing that the counter anion is not directly involved in the cata-
lytic process. The drop of conversion on recycling is also
independent on the nature of X, indicating that the lack of
recyclability of these catalytic systems at that stage is not
related to the stability of the ionic complexes adsorbed onto
the clay.
The drop of recyclability is not an effect of the temperature
either. Thus in order to study the influence of the temperature
on the conversions obtained, we carried out the above men-
tioned C–C coupling at several reaction temperatures (120ꢀ,
140ꢀ and 180 ꢀC) and studied their time dependent-reaction
profiles using the catalytic system that provided the highest
activity, 1-Br/MK-10. Fig. 2 indicates that both homogeneous
and heterogenized systems show the same reaction profile at
120ꢀ, 140ꢀ and 180 ꢀC. We reported that the homogeneous
reaction is consistent with a mechanism in which the slow cat-
alyst reduction step may not be involved,3 so a similar mechan-
ism may be operating in the heterogenized system, although
further studies are being carried out in order to prove this the-
ory. Maximum conversions, even though far from quantitative
at 120ꢀ and 140 ꢀC, were achieved after 3 hours.
Since montmorillonite K-10 readily adsorbs ionic species
from solution, we suspected that NaOAc, used as the base
on each consecutive C–C coupling run, could be simulta-
neously adsorbed onto the solid surface, hence saturating or
blocking the catalytic active sites. Indeed, elemental and XPS
analysis of the solids before and after its recycling, showed that
the amount of Na and C adsorbed considerably increased
upon the consecutive runs, (Table 2). XPS analysis also offers
Table 1 Influence of the catalytic system in the homogeneous and
heterogenized C–C coupling of phenyl iodide and styrenea
Entry
Catalystb
Number of Runs
Stilbene Yieldc /%
1
2
1-Br
1-Br /MK-10
1
1
2
3
1
1
2
3
1
1
2
3
1
1
2
3
1
1
2
3
89
95.2
52.9
23.8
79
3
4
1-PF6
1-PF6/MK-10
83
35
Table 2 Elemental and XPS analysis
14
65
Number
of runs Base
Pd/wt XPS/
% bulk wt%
Washinga
—
5
6
2-Br
2-Br/MK-10
Catalyst
65
35
1-Br/MK-10
0
3
—
0.8
Pd (1.7)
Si(29.3)
C(10.3)
Pd(1.2)
Si(20.6)
C(14.1)
Na(11.6)
Pd(1.9)
Si(30.9)
C(5.8)
17
64
7
8
2-PF6
2-PF6/MK-10
1-Br/MK-10
NaOAc CH2Cl2
0.5
75
44
11
46
9
10
3-PF6
3-PF6/MK-10
H2O–CH3OHb 0.43
46.5
35
12
Na(2.3)
Pd(1.3)
Si(27)
a
Standard conditions: phenyl iodide (4 mmol), styrene (5.6 mmol)
NaOAc (4.4 mmol), solvent (DEA, 5 mL), supported catalyst (0.004
mmol of metal complex, 0.1 mol%, in 0.5 g of MK-10). T: 180 ꢀC.
1-Br/MK-10 10
NEt3
CH2Cl2
0.51
C(18)
b
Time: 2 h; MK-10: commercial montmorillonite K-10, (preheated
c
a
at 100 ꢀC for 24 h); Conversion of the aryl iodide on trans-stilbene
by C.G. using glycol-n-butyl ether as internal standard.
All washings performed on the solid filtrate with 3 ꢂ 2 mL of solvent.
b
Same solid after washed with H2O-CH3OH.
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Table 3 Influence of the solvent in the washings of 1-Br/MK-10
between the consecutive runs in the heterogenized C–C coupling of
phenyl iodide and styrenea
Entry
1
Solventb
CH2Cl2
Number of runs
Stilbene yieldc /%
1
2
3
1
2
3
1
2
3
95.2
52.9
23.8
99
2
3
H2O–MeOH
H2O–CH2Cl2
88
47
Fig. 2 Reaction profiles of the coupling reaction of PhI and styrene
using 1-Br/MK-10 (Het. 0.1% mol) at 120ꢀ (L), 140ꢀ (G) and 180ꢀ
C (S), and 1-Br (Homo, 1% mol) as catalysts at 180 ꢀC (X).
97
65
35
a
Standard conditions: phenyl iodide (4 mmol), styrene (5.6 mmol)
NaOAc (4.4 mmol), solvent (DEA, 5 mL), supported catalyst (0.004
mmol of metal complex, 0.1 mol%, in 0.5 g of MK-10). T: 180 ꢀC.
valuable information about the Pd(II) content on the surface of
the solid. Fig. 3 shows the Pd3d spectrum obtained for 1-Br/
MK-10 before and after reaction. Before reaction, a maximum
at ca. 337.8 eV (Pd3d5/2) was observed, indicating the presence
of Pd+2, as the only Pd species. After reaction at 180 ꢀC in the
presence of NaOAc (Table 1, entry 2) the binding energy cor-
responding to palladium did not experience substantial
changes and only the relative intensity of the peak was slightly
reduced (Fig. 3). These results indicate that the oxidation state
of palladium is not modified after each reaction. The decrease
of the relative intensity of the Pd3d peak could be associated
with the increase of carbon on the surface due to NaOAc
adsorption. Moreover, bulk analysis of the solid before and
after its use in the catalytic reaction, showed that the amount
of C and Na considerably increases upon its use in the presence
of NaOAc (Table 2). Thus, chemical analysis of palladium
before and after reaction revealed that Pd bulk was practically
unchanged. These data confirm our previous considerations
that adsorption of NaOAc on montmorillonite K-10 may be
the cause for the loss of activity. A similar effect is reported
in literature where potassium acetate is used as base in Heck
reaction with palladium-based supported liquid phase cata-
lysts.6 In that situation, the decrease of the reaction rate during
the repeated runs, was explained by the adsorption of potas-
sium acetate onto the catalyst surface.
b
Time: 2 h; Washings performed on the solid filtrate (3 ꢂ 2 mL);
c
Conversion of the aryl iodide on trans-stilbene by C.G. using
glycol-n-butyl ether as internal standard.
between consecutive runs if washing of 1-Br/MK-10 is carried
out with H2O–CH3OH, (entry 2, Table 3). In fact the amount
of Na and C adsorbed on the solid is significantly lower than
those observed when washings were carried out only with
CH2Cl2 or H2O–CH2Cl2 (Table 3). These data confirm that
a more efficient elimination of undesired ionic species is
achieved upon more polar washings.
On the other hand, the use of the non-ionic base NEt3 yields
quantitative conversion of the substrate, even after the ten con-
secutive runs performed (Table 4), and using CH2Cl2 for wash-
ing of the solid. As expected, this base does not show any
significant affinity towards surface adsorption, allowing the
catalytic sites to remain active after being reused. The accumu-
lated TON of this reaction is 13 000. As it can be seen from
Table 4, there is a smooth decrease of catalytic activity after
run 8. We attribute this decrease to the loss of recycled solid
by the consecutive filtrations after each run, which diminishes
the solid catalyst amount from 0.5 g (run 1) to 0.3 g (run 10).
This suggestion is confirmed by the fact that the bulk and XPS
In order to recover the catalytic activity after catalyst recy-
cling, we studied two plausible solutions: i) keeping NaOAc
as base, but washing the solid with H2O–CH2Cl2 or H2O–
CH3OH between the consecutive runs, in order to eliminate
the amount of NaOAc adsorbed; ii) using NEt3 as base,
instead of NaOAc. The first purpose was suggested in basis
to the fact that our catalyst shows great stability in a non-inert
medium. The results obtained are shown in Table 3. As it can
be seen, the catalytic activity can be more efficiently recycled
Table 4 Influence of the base NEt3 on the recyclability of the cata-
lytic system in the heterogenized C–C coupling of phenyl iodide and
styrenea
Number
of Runs
Stilbene
Yieldb /%
Entry
1
Catalyst
1-Br/MK-10
1
2
3
4
5
6
7
8
9
10
1
2
3
4
> 99
95
88
89
88
90
89
85
83
79
2
1-Br/Na+–M
> 99
98
74
67
a
Standard conditions: phenyl iodide (4 mmol), styrene (5.6 mmol)
NEt3 (4.4 mmol), solvent (DEA, 5 mL), supported catalyst (0.004
mmol of metal complex, 0.1 mol%, in 0.5 g of MK-10) T: 180 ꢀC. Time:
b
2 h; Conversion of the aryl iodide on trans-stilbene by C.G. using
glycol-n-butyl ether as internal standard.
Fig. 3 Pd3d photoelectron spectra of 1-Br/MK10, (a) fresh catalyst,
(b) After 3 reaction runs, using NaOAc as base.
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analysis of the solid after being used for ten times, shows very
little decrease in the amount of the Pd(II) content (0.51, bulk;
1.3, XPS; Table 2). Under these optimized conditions for the
reuse of the catalyst, we also explored the influence of the solid
support 1-Br/Na+–M (bentonite) on the recycled C–C cou-
pling reaction. As can be seen, the bentonite clay not only
activated 4-bromobenzaldehyde, the reaction is complete in
only one hour. Recycling of the catalyst, affords 4 consecutive
runs without measurable conversion decay. After the fifth run,
the conversion smoothly decreases, probably due to the rea-
sons described above. For the unactivated bromide (phenyl
bromide), the conversion decreases after the first run. The dif-
ference between these two systems may be due to higher
adsorption of ionic species in the case of the reaction of phenyl
bromide, since high conversions are only achieved for longer
reaction times (10 h vs. 1 h). For the reaction with 4-chloroben-
zaldehyde, the conversion was very low (15%), probably
because the catalyst is deactivated by adsorption of undesired
ionic species before the reaction is finished. In any case, we
would like to point out that, at least, some C–Cl is activated
under the conditions discussed. Further studies are being car-
ried out in order to enhance the recyclability of the catalyst
in the reactions of C–C coupling of chloro- and bromo- aryl
species.
serves as
a good support to immobilize the complex
[PdBr(CNC-Bu2)]Br, 1-Br, but also the catalytic solid result-
ing, (1-Br/Na+–M), provides a comparable catalytic activity
to 1-Br/MK-10. However, the recyclability is not as efficient
from the same consecutive run (Table 4).
It is important to point out that no Pd(0) was found on the
surface of the solids after being used in catalytic reactions.
Once we optimized our results on the Heck coupling of iodo-
benzene with styrene, we decided to extend the study to other
substrates, in order to determine whether our catalysts could
also be operative in C–C coupling of the less reactive C–Br
and C–Cl bonds. The results that we obtained are shown in
Table 5. When the reactions are carried out in the presence
of NEt3 as base, the yields on the desired product were very
low both in homogeneous and in heterogenized conditions.
We have previously reported the homogeneous reactions of
this catalyst, and we saw that the use of NaOAc provided high
yields on the desired products.3 For the more unreactive aryl
halides, the use of tetrabutylammonium bromide (TBABr)
was necessary in order to achieve high conversions. With this
in mind, we decided to carry out new reactions using NaOAc
and TBABr in order to check whether we could reproduce our
results in heterogenized conditions. As can be seen in Table 5,
the conversions obtained when bromoaryl compounds
are used (bromophenyl and 4-bromobenzaldehyde), are
quasi-quantitative in the first run. Furthermore, for the more
Conclusion
Immobilization of a series of CNC biscarbene Pd(II) complexes
on montmorillonite K-10 has afforded highly efficient sup-
ported catalyst for C–C coupling Heck reaction. Reaction pro-
files and chemical analysis of the solids obtained suggest that
in all cases the reaction follows a mechanism similar to the
homogeneous counterparts, where the supported catalyst
remains unchanged after immobilization (Raman and XPS
analysis). Analysis of the supported catalyst before and after
being used has shown that leaching and chemical decomposi-
tion of the catalyst is negligible. The catalyst has been reused
at least ten times, without important activity decay. The cata-
lyst has been used in the activation of more unreactive C–Br
and C–Cl bonds, providing high yields in the C–Br case.
Table 5 Activation of C–Br and C–Cl in C–C coupling using 1-Br as
catalysta
Experimental section
General details
NMR spectra were recorded on a Varian Innova 300 MHz and
500 MHz, using CDCl3 and DMSO-d6. The FT-Raman spec-
tra were recorded on a Bio-Rad spectrometer, Model FT-
Raman II. The 1.064 mm line of a Nd:YAG laser was used
for excitation along with a germanium detector cooled to
liquid nitrogen temperature. The Raman spectra were
recorded at room temperature in the 180ꢀ scattering configura-
tion using high-quality quartz tubes as cells. The laser power at
the samples was 70 mW. The Raman spectra were corrected
for instrumental response using a white light reference spec-
trum. The apparatus was working at ꢃ0.5 cmꢁ1 resolution.
Gas chromatographic analyses were performed on a Hewlett-
Packard 5890 II with a flame ionisation detector equipped with
a column Ultra-2 (5% diphenylmethyl silicone and 95%
dimethyl silicone) of 25 m. Chemical analysis of the catalysts
were performed by atomic absorption spectrophotometry
(AAS) in a SpectrAA-10Plus (Varian) spectrometer, after dis-
solving the solids in concentrated HCl at room temperature for
24 hours. Montmorillonite K-10 (MK-10) was purchased from
Fluka and bentonite Na+–M was purchased as Majorbenton
B, from AEB Iberica S. A. Pre-dried clays were obtained as
follows: 5 g of clay in a melting pot were kept in the oven at
100 ꢀC for 24 h.
N of
Runs
Catal.
Base
R
X
t/h
Yield/%b
1-Br
1-Br/MK-10
NEt3
NEt3
CHO
CHO
CHO
CHO
H
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
2
2
1
1
2
3
1
2
1
2
3
4
5
6
7
8
1
2
3
1
89
12
NEt3
NEt3
2
7
2
2
25
1-Br/MK-10
1-Br/MK-10c
NEt3
NEt3
10
10
1
H
12
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
CHO
CHO
CHO
CHO
CHO
CHO
CHO
CHO
H
> 99
> 99
97
1
1
1
> 99
82
1
1
78
33
1
1
25
1-Br/MK-10c
10
10
10
24
> 99
82
H
H
CHO
40
15
1-Br/MK-10c
Pd oxidation state in the catalysts was determined by X-ray
photoelectron spectroscopy (XPS). XPS spectra were recorded
on a VG-Escalab-210 electron spectrometer equipped with a
multichannel detector. Spectra were excited by using the
Mg Ka (1253.6 eV) radiation of a twin anode in the constant
analyser energy mode with a pass energy of 40 eV. Vacuum
during spectra acquisition was better than 3.10ꢁ9 mbar. For
a
Standard conditions: aryl halide (4 mmol), styrene (5.6 mmol) base
(4.4 mmol), solvent (DEA, 5 mL), supported catalyst (0.004 mmol of
metal complex, 0.1 mol%, in 0.5 g of MK-10) T: 180 ꢀC. b Conversion
of the aryl iodide on trans-stilbene by C.G. using glycol-n-butyl ether
as internal standard. c Reactions performed with NaOAc with 20% of
tetrabutyl bromide relative to the aryl halide.
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calculation of binding energies the peak of the C–(C,H) com-
ponent coming from contamination of carbon 284.6 eV, was
used as an internal standard. The surface composition was esti-
mated from the corresponding XPS peak area ratios by using
the relation:
pyr), 166.3 (C-Pd). Anal Calcd. for C25H21N5Br2Pd: C,
45.66; H, 3.22; N, 10.65. Found: C, 45.15; H, 3.01; N, 11.23%.
For the preparation of the BF4 and PF6 derivatives, we
applied the following general method: 200 mg of the Pd[Br]
compound were dissolved in 5 ml of CH2Cl2 and added to a
column chromatography packed with silica-gel (mesh 60 A)
in hexane. The solution was eluted with gradient CH2Cl2–
MeOH–KPF6 (Pd[PF6] compounds) or CH2Cl2–MeOH–
NaBF4 (Pd[BF4] compounds), affording the corresponding
pure compouds. [PdBr(CNC-Bz2)][PF6] (Yield 69%). Anal
Calcd. for C25H21N5BrPdPF6 : C, 41.54; H, 2.93; N, 9.69.
Found: C, 41.25; H, 3.01; N, 10.23%. [PdBr(CNC-Bz2)][BF4]
(Yield, 52%). Anal Calcd. for C25H21N5BrPdBF4 : C, 45.18;
H, 3.18; N, 10.54. Found: C, 45.19; H, 3.09; N, 11.12%.
[PdBr(CNC-NO2)][PF6] (Yield 63%). Anal Calcd. for
C25H21N7O4BrPdPF6 : C, 36.94; H, 2.36; N, 12.06; O, 7.88.
Found: C, 37.05; H, 2.67; N, 13.02; O, 8.13%. [PdBr(CNC-
NO2)][BF4] (Yield 59%). Anal. calcd. for C25H21N7O4-
BrPdBF4 : C, 39.79; H, 2.54; N, 12.99. O, 8.48. Found: C,
39.85; H, 2.78; N, 13.15, O, 8.92%.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ
ꢁ
X
Y
AX sY lY EK ðXÞ
AY sX lX EK ðYÞ
¼
s
where A, s, l and EK are the integral of each peak after S-
shaped background subtraction, the effective ionization cross
section, the escape depth and the photoelectron kinetic energy
respectively. Cross section values were taken from Scofield20
and the escape depth was calculated from the formulas given
by Vulli and Starke.21
The compounds 2,6-bis(imidazol-1-yl)pyridine,22 [CHNCH-
2
(Bz)2][Br]2 ,22 [CHNCH-(Bu)2][Br]2 and [PdBr(CNC-Bu2)]-
[Br]2 were obtained according to literature methods.
Synthesis of compounds
[CHNCH-(CH2Ph-NO2)2][Br]2. A mixture of 2,6-bis(imida-
zol-1-yl)pyridine (10 mmol) and 4-nitrobenzylbromide (30
mmol) in toluene were refluxed for 4 h. The white solid formed
was filtrated and washed with Et2O yielding pure [CHNCH-
(CH2Ph–NO2)2][Br]2(95%). 1HNMR (d, DMSO-d6): 5.67
(4H, CH2), 7.45 (d, J ¼ 6.3 Hz, 4H, Ph), 7.64 (d, J ¼ 6.3
Hz, 4H, Ph), 8.2 (2H, CH, imid), 8.28 (d, J ¼ 7.8 Hz, 2H,
pyr) 8.62 (t, J ¼ 7.8 Hz, 1H, pyr), 8.90 (2H, Ph), 11.22 (2H,
imid). 13CNMR (d, DMSO-d6): 52.3 (CH2), 115.8 (C, imid),
120.8 (C, imid), 124–130 (m, CH, Ph), 137.3 (C, pyr), 142.1
(C, pyr), 145.9 (NCHN), 149.3 (C, pyr). Anal. calcd. for
C25H21N7O4Br2 : C, 46.67; H, 3.27; N, 15.24; O, 9.96. Found:
C, 46.55; H, 3.18; N, 15.37; O, 10.04%.
Preparation of the supported complexes
The ionic palladium complexes were immobilized in the fol-
lowing manner. Dichloromethane solutions (5 mL) of each
complex (0.2 mmol) were prepared under nitrogen and added
to a suspension of the solid support (MK-10 and Na+–M) in
deoxygenated dichloromethane (10 mL), and then stirred for
24 h under nitrogen at room temperature. The suspension
was filtered off, the solid was washed with dichloromethane
and dried under vacuum. The amount of metal complex immo-
bilized on the clay was determined by AAS and XPS.
Homogeneous catalytic C–C coupling
NaOAc or NEt3 (4.4 mmol) and the catalyst (0.1 mol%) were
placed in a 3-necked flask fitted with a reflux condenser. Aryl
halide (4 mmol), styrene (5.6 mmol) and solvent (5 mL) were
added. The reaction vessel was placed into an oil bath pre-
heated to the desired temperature. Products were characterized
by gas chromatography.
[PdBr(CNC-NO2)]Br. This compound was obtained by a
similar method to that described for [PdBr(CNC-Bu2)][Br].2
A solution of [CHNCH–(CH2Ph–NO2)2][Br]2 (642 mg, 1.0
mmol) and [Pd(OAc)2] (224 mg, 1.0 mmol) was stirred in
DMSO (10 mL) for 12 h at 70 ꢀC. Subsequently, the reaction
mixture was heated at 170 ꢀC for 2 h and then cooled to room
temperature. The solution was poured into CH2Cl2 (20 mL)
and Et2O was added (200 mL). The precipitate was washed
with Et2O, leaving pure [PdBr(CNC-NO2)]Br as a yellowish
solid (73%). 1HNMR (d, DMSO-d6): 5.96 (4H, CH2), 7.61
(d, J ¼ 6.0 Hz, 4H, Ph), 7.73 (2H, CH, imid), 7.84 (d,
J ¼ 6.0 Hz, 4H, Ph), 8.61 (d, J ¼ 7.8 Hz, 2H, pyr), 8.57
(2H, Ph), 8.62 (t, J ¼ 7.8 Hz, 1H, pyr). 13CNMR (d,
DMSO-d6): 52.8 (CH2), 109.9 (C, pyr), 120.1 (C, imid),
124.99 (C, Ph), 124.94 (C, imid), 131.1 (C, Ph), 144.2 (C,
pyr), 150.8 (C, pyr), 167.1 (C-Pd). Anal. calcd. for
C25H19N7O4Br2Pd: C, 40.16; H, 2.56; N, 13.11; O, 8.55.
Found: C, 40.19; H, 3.01; N, 13.28; O, 8.67%.
Heterogenized catalytic C–C coupling
NaOAc or NEt3 (4.4 mmol) and the solid catalyst (0.004 mmol
in 0.5 g of clay; 0.1 mol%) were placed in a 3-necked flask fitted
with a reflux condenser. Aryl halide (4 mmol), styrene (5.6
mmol) and diethylacetamide (5 mL) were added. The reaction
vessel was placed into an oil bath preheated to the desired tem-
perature. The solution was filtered off under vacuum in air and
the filtrates were analysed by gas chromatography. The solid
containing the complex was washed with polar solvents and
dried under vacuum for 10 minutes and introduced to the
3-necked flask for another run.
[PdBr(CNC-Bz2)]Br. This compound was obtained by a
similar method to that described above for [PdBr(CNC-
NO2)][Br]. A solution of [CHNCH-Bz2][Br]2 (553 mg, 1.0
mmol) and [Pd(OAc)2] (224 mg, 1.0 mmol) was stirred in
DMSO (10 mL) for 12 h at 70 ꢀC. Subsequently, the reaction
mixture was heated at 170 ꢀC for 2 h and then cooled to room
temperature. The solution was poured into CH2Cl2(20 mL)
and Et2O was added (200 mL). The precipitate was washed
with Et2O, leaving pure [PdBr(CNC-Bz2)]Br as a yellowish
solid (65%). 1HNMR (d, DMSO-d6): 5.82 (4H, CH2), 7.35
(d, J ¼ 6.2 Hz, 4H, Ph), 7.48 (d, J ¼ 6.0 Hz, 4H, Ph), 7.72
(2H, CH, imid), 8.12 (d, J ¼ 8.1 Hz, 2H, pyr),8.57 (2H, Ph),
8.66 (t, J ¼ 8.1 Hz, 1H, pyr). 13CNMR (d, DMSO-d6): 53.2
(CH2), 108.8 (C, pyr), 119.8 (C, imid), 124.81 (C, imid),
129.2 (m, 129.2), 136.5 (C, Ph), 147.2 (C, pyr), 150.2 (C,
Acknowledgements
We thank the DGESIC (PB98-1044) and BANCAIXA
(P1.1B2001-03) for financial support.
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