DOI: 10.1002/adsc.200700330
PCP- and SCS-Pincer Palladium Complexes Immobilized on
Mesoporous Silica: Application in C
C Bond Formation Reactions
Nilesh C. Mehendale,
a
Jelle R. A. Sietsma,
b
Krijn P. de Jong,
b
Cornelis A. van Walree,
a
Robertus J. M. Klein Gebbink,
a
and Gerard van Koten
a,
*
a
Organic Chemistry and Catalysis, Faculty of Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
Fax: (+31)-30-252-3615; e-mail: G.vankoten@uu.nl
b
Inorganic Chemistry and Catalysis, Faculty of Science, Utrecht University, P.O. Box 80 083, 3508 TB Utrecht, The
Netherlands
Received: July 6, 2007; Published online: November 21, 2007
Dedicated to Prof. Dr. Jan-E. Bckvall on the occasion of his 60
th
birthday.
Abstract: ECE-pincer palladium(II) complexes
{ECE=[C
6
H
3
ACHTUNGTRENNUNG(CH
2
E)
2
-2,6]
,E=PPh
2
and SPh} teth-
ered to a trialkoxysilane moiety through a carbamate
linkage were immobilized on ordered mesoporous
silicas SBA-15 and MCM-41 using a grafting process.
The resulting hybrid materials were characterized by
IR spectroscopy, solid-state CP/MAS NMR (
13
C,
31
P,
and
29
Si), and elemental analyses. These analyses
showed the integrity of the pincer-metal complexes
on the supports, which highlights their stability under
the applied immobilization conditions. An H-bond-
ing interaction between the carbamate carbonyl
group of the complex and free silanol groups on the
silica surface was also established. The hybrid mate-
rials were found to act as Lewis acid catalysts in the
aldol reaction between methyl isocyanoacetate and
benzaldehyde. SBA-15 modified with the PCP-pincer
Pd complex was used in up to five runs without loss
of activity. Control experiments showed the true het-
erogeneous nature of the catalyst in this reaction. Ni-
trogen physisorption data, XRD, and TEM/EDX
analyses of the hybrid materials revealed that the
mesoporous structure of these materials was retained
during the immobilization process as well as during
catalysis.
Keywords: aldol reaction; catalyst recycling; immobi-
lization; MCM-41; pincer complexes; SBA-15
Introduction
Catalyst separation from the product solution is an
important aspect of homogeneous catalytic processes
both from an economical and ecological point of
view. Amongst current methods to separate homoge-
neous catalysts from product streams are distillation
and selective extraction of products and catalysts.
[1]
In
many cases these methods may turn out as unsustain-
able, however, due to the excessive use of energy and
solvents, and could, therefore, also be economically
less attractive. In this respect, heterogeneous catalytic
processes have obvious advantages due to the ease of
separation of the (insoluble) heterogeneous catalysts
from reaction products. Current research is directed
to merge the advantages of both catalytic approaches,
for example, by immobilizing homogeneous catalysts
on an insoluble support.
[2,3]
In this way, properties
such as catalyst selectivity and activity could be com-
bined with ease of catalyst separation and catalyst
reuse. A variety of insoluble supports has been used
for the immobilization of homogeneous catalysts.
[4,5]
ECE-pincer metal complexes {ECE=[C
6
H
3
ACHTUNGTRENNUNG(CH
2
E)
2
-2,6]
,E= NR
2
, SR, PR
2
etc.} are often
highly stable and resistant to metal leaching due to a
strong M
C s-bond that is stabilized through metal
coordination by two heteroatoms (cis to the M
C
bond) forming two five-membered chelate rings.
[6]
ECE-pincer Pd complexes are known to catalyze a
large variety of C
C and C
X bond formation reac-
tions.
[6–10]
The robustness of the ECE-pincer metal
complexes make them attractive candidates for pro-
cesses for which recycling and reuse is a prerequisite.
Various soluble supports have already been used for
the immobilization and recycling of pincer-based
metal catalysts, among which are hyperbranched poly-
mers,
[9,11]
oligo(ethylene glycol),
[12]
dendronized poly-
mers,
[13]
carbosilane dendrimers,
[14]
cartwheel mole-
cules,
[8,15]
polycationic dendrimers,
[16]
and soluble poly-
mers.
[10,17]
Recently, we as well as others have report-
ed on the grafting of ECE-pincer metal complexes on
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silica and on their use in aldol-type
[18,19]
and Heck re-
actions.
[20]
In a recent effort by our group to immobilize NCN-
pincer complexes on insoluble supports, we have used
para-triethoxysilane-functionalized pincer-metal (plat-
inum and palladium) complexes and immobilized
these on conventional silica gel via surface grafting.
[18]
Although the heterogenized pincer Pd complexes
were found to be active in the aldol reaction of
methyl isocyanoacetate (MI) with benzaldehyde [Eq.
(1)], they showed a poor performance in terms of re-
cycling. For a series of conventional silicas differing in
macroscopic particle size (50 mm–1.5 mm) and pore
diameter (15–50 nm) significant activity losses of
more than 50% between the first and second run
were observed for the heterogenized catalysts. No in-
dication could be derived for a relation between the
macroscopic particle size and average pore diameter
of the silicas, and the degree of catalyst leaching from
the support. Presumably during the reaction, attrition
of the catalyst took place due to partial hydrolysis of
the silica surface. As a result, part of the catalyst was
removed from the support in the form of small, diffi-
cult to separate by filtration and thus non-recyclable
silica particles.
Recent investigations on the mechanism of opera-
tion of ECE-pincer palladium(II) complexes as homo-
geneous catalyst have shown that the NCN- and SCS-
pincer Pd complexes undergo isocyanide insertion in
the Pd
C bond and that the resulting cyclometalated
species are the actual catalytically active species.
[21]
Remarkably, this insertion takes place for both the
neutral halide and the cationic aquo complexes. For
the PCP-pincer Pd complexes, no insertion was ob-
served, but replacement of the halide by a h
1
-coordi-
nated isocyanide generated the cationic, catalytically
active species. Finally, this study showed that there is
no need to generate a cationic palladium(II) species
by prior treatment of the neutral species with, for ex-
ample, Ag reagents (note that Ag halide salts are cat-
alysts themselves)
[22]
for the ECE-pincer palladium
species to catalyze the reaction of Eq. (1).
In the present study, we have used our experience
to immobilize NCN-pincer palladium complexes for
the immobilization of the corresponding PCP- and
SCS-pincer palladium complexes on silica.
[18]
To this
end, complexes 1 and 2 (Figure 1) were synthesized
[23]
and subsequently immobilized on silica-based support
materials. The choice of support depended on the re-
action conditions, ease of functionalization of the sur-
face groups required for attaching the catalyst, as well
as on the chemical, mechanical and thermal stability
of the support.
[5]
In the present study, we have used
the ordered mesoporous molecular sieves MCM-41
[24]
and SBA-15,
[25]
which fulfill most of the above-men-
tioned requirements and have been used extensively
by others.
[3,26]
Moreover, their well-defined structure
may provide uniform active sites with well-controlled
steric effects of the support.
[27]
The synthesis and char-
acterization of the pincer palladium-functionalized
MCM-41 and SBA-15 materials as well as their per-
formance in the reaction of Eq. (1) are discussed.
Results and Discussion
Immobilization of PCP- and SCS-Pincer Palladium
Complexes on Silica
The triethoxysilane-functionalized complexes 1 and 2
(Figure 1) were prepared from the corresponding
para-hydroxy ECE-pincer complexes as reported pre-
viously.
[23]
These complexes comprise both a trialkoxy-
silane and an organometallic fragment covalently con-
nected through a carbamate linker. The presence of a
long, non-polar tail increases the solubility of these
complexes considerably. They are soluble in non-
polar solvents such as benzene or toluene, which
allows the use of common grafting protocols of homo-
geneous catalysts on silica in these solvents. This is of
importance to arrive at a uniform distribution of the
catalyst on the support.
Silica surface grafting of 1 and 2 was carried out
using SBA-15 or MCM-41. In a typical process, the
silica support was pre-treated by heating it at 1008C
under vacuum for two hours. The support was then
reacted with complexes 1 or 2 in toluene at 908C for
20 h (Scheme 1). A continuous extraction (Soxhlet) of
the resulting material with boiling dichloromethane
was subsequently performed for 16 h in order to
remove any non-covalently attached material. After
drying under vacuum, the hybrid silicas 3 and 4, re-
spectively, were obtained as white solids. Subsequent-
ly, silica 3 was treated with 1,1,1,3,3,3-hexamethyldisi-
lazane (HMDS) to cap remaining unreacted surface
silanol groups with a trimethylsilyl functionality (silica
5).
Figure 1. Siloxane-functionalized ECE-pincer palladium(II)
chloride complexes.
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These materials were characterized by using IR
spectroscopy (DRIFT), CP/MAS NMR spectroscopy
(
13
C,
29
Si, and
31
P), and elemental analysis. The impact
of the immobilization of 1 and 2 on the structure of
MCM-41 and SBA-15 was studied by powder X-ray
diffraction (XRD), transmission electron microscopy
(TEM), and nitrogen physisorption.
Characterization
IR Studies
Comparison of the IR (DRIFT) spectra of the pris-
tine silicas (SBA-15 in Figure 2) with those of hybrid
materials 3 (see Figure 2) and 4 pointed to a strong
decrease in the number of isolated free silanol groups
in the latter silicas. Comparison of the IR spectra of 1
and 2 with those of the hybrid materials 3 and 4 re-
vealed that signals corresponding to the C
H stretch-
ing, C=O stretching, CNH group, and the N
H bend-
ing vibrations of 1 and 2 are present in the spectra of
3 and 4. The intensity of the signal for isolated free si-
lanol groups in 3 is further decreased in the spectrum
of 5 (see Figure 2), showing that at least part of the
remaining silanol groups in 3 became capped with a
TMS group in 5. By subtracting the spectrum of plain
SBA-15 from that of 5, a difference spectrum was ob-
tained (top, Figure 3). In this difference spectrum, a
sharp decrease in the intensity of the signal at
3745 cm
1
was observed indicating that a large
number of silanol groups had reacted. Other signifi-
cant changes were observed for the carbamate C=O
stretching vibration. In 1 the n
˜
ACHTUNGTRENNUNG(C=O) amounted to
1734 cm
1
(see Figure 3), which changed to 1725 cm
1
in hybrid material 3, and to 1756 cm
1
in TMS-capped
5 (see Figure 2).
We believe that these shifts are indicative for the
presence of H-bonding interactions of the carbamate
carbonyl group with surface silanol groups in 3, which
consequently are much less present in 5. This observa-
tion is consistent with previous observations made on
immobilizing related NCN-pincer complexes on
silica.
[18]
Scheme 1. Synthesis of hybrid materials 3, 4, and 5.
Figure 2. IR (DRIFT) of plain silica (SBA-15) and hybrid
materials 3 and 5.
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PCP- and SCS-Pincer Palladium Complexes Immobilized on Mesoporous Silica
CP/MAS Solid State NMR Studies
13
C and
29
Si solid-state NMR spectroscopy on 3, 4,
and 5 provided further information on the nature of
both the organic (spacer and metal-complex) and in-
organic part of these hybrid materials. All peaks cor-
responding to the
13
C NMR spectra of the parent
complexes 1 and 2 were present in the
13
C NMR spec-
tra of the hybrid materials 3 and 4, respectively. The
31
P NMR spectrum of solid 3 (on SBA-15) showed a
single peak at 36.01 ppm, while for 1 a resonance at
33.4 ppm (solution NMR in C
6
D
6
) was found which
corresponds to a phosphine grouping coordinated to
palladium. Signals at 16 and 58 ppm in the
13
CNMR
spectra of all hybrid materials pointed to the presence
of EtO
Si groups.
[23]
This supports the notion that not
all tethered palladium complexes became immobi-
lized via three SiACHTUNGTRENNUNG(surface)
O
Si bonds, but rather
that grafting occurred through an average of less than
or equal to three bonds. The
29
Si NMR spectrum of 5
indeed showed all three T
n
type signals at 59, 52,
and 45 ppm, corresponding to T
3
,T
2
, and T
1
types
of organosilica species, respectively [Figure 4, inset;
T
n
= RSiACHTUNGTRENNUNG(OSi)
n
ACHTUNGTRENNUNG(OEt)
3n
].
In addition, the latter spectrum also showed a
signal at 13.9 ppm, which can be assigned to Si-
ACHTUNGTRENNUNG(surface)
O
SiMe
3
groups derived from surface sila-
nols capped with a Me
3
Si grouping arising from
HMDS treatment of the hybrid material 3.
[28]
Two sig-
nals were observed for the SiO
2
framework of 3 at
108 and 101 ppm, respectively, corresponding to
Q
4
and Q
3
species [Q
m
= SiACHTUNGTRENNUNG(OSi)
m
(OH)
4m
], whereas
for 5 a signal at 108 ppm (Q
4
species) along with a
shoulder at about 100 ppm (decreased number of
Q
3
species) was observed, which is in accordance with
the partial SiMe
3
capping of silanol groups (Q
3
spe-
cies) in 5 (vide supra).
Elemental Analysis
The palladium, sulfur, phosphorus, and carbon con-
tents of the grafted silicas were determined by ICP
analysis (Table 1). The E/Pd ratio (E = S or P) was
close to two for 4 as expected from its formula. In the
case of 3 and 5, this ratio was found to be higher than
two, which could point to loss of Pd from the PCP-
pincer ligand during the grafting process.
31
PNMR
analysis of these materials did, however, not show the
presence of free phosphine or of phosphine oxide
moieties. In addition, no Pd(0) formation was ob-
served during the immobilization reaction, which jus-
tifies the conclusion that the complex remained
intact. In the case of 5, an increase of the carbon con-
tent was found, which most likely is due to the cap-
ping of part of the free silanol groups of SBA-15 with
SiMe
3
groups.
Figure 3. Difference spectrum between the DRIFT spectra
of 5 and SBA-15 (top) compared with IR spectrum of non-
supported 1 (bottom).
Figure 4. CP/MAS solid-state
29
Si NMR of 5 (inset: expan-
sion of 30 to 70 ppm region).
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Nitrogen Physisorption
In order to gather information on changes in surface
properties at various stages of the synthesis of silicas
3, 4 and 5, the materials were studied using nitrogen
physisorption. The results are summarized in Table 2
and some selected isotherms are shown in Figure 5.
To ascertain the effect of the conditions of the im-
mobilization process on the ordered structure of
SBA-15, we refluxed pristine SBA-15 at 858C in tolu-
ene for 24 h. The nitrogen physisorption isotherm
(Figure 5a) of the obtained product contained the
typical features: a high onset at low relative pressures
originating from the intra-wall microporosity, and hys-
teresis at higher values because of the ordered meso-
porous channels. The small relative pressure region
(p/p
o
= 0.69–0.71) in which the single-step capillary
condensation took place is a consequence of the uni-
form mesopore diameter of 8.7 nm. The BET surface
area and mesopore volume remained unaffected by
the treatment (entry 2, Table 2). Next, the impact of
the silanol capping treatment with HMDS was ad-
dressed by treating blank SBA-15. The isotherm (Fig-
ure 5a) showed that the characteristic SBA-15 fea-
tures were retained.
However, the porosity and average pore size diam-
eter had decreased to 0.49 cm
3
g
1
SiO
2
and 7.8 nm,
respectively (entry 3, Table 2). The observed pore di-
ameter decrease could not account for the total po-
rosity decrease, and most likely some of mesopores of
SBA-15 became blocked. In addition, after the
HMDS treatment no micropore volume was present
anymore. Comparison of the nitrogen physisorption
results of pristine SBA-15 and functionalized material
3 showed that the BET surface area and mesopore
volume decreased from 518 to 442 m
2
g
1
SiO
2
, and
from 0.73 to 0.62 cm
3
g
1
SiO
2
, respectively (entry 4,
Table 2). The mesopore diameter had decreased from
8.7 to 8.2 nm due to immobilization of complex 1
inside the mesopores of SBA-15. The steep capillary
condensation step and hysteresis loop of the isotherm
(Figure 5b) showed that the uniform character of the
cylindrical mesopores had been retained. The ob-
served mesoporosity reasonably matched the expect-
ed value of 0.58 cm
3
g
1
due to the pore diameter de-
crease of 0.5 nm. Comparison of the textural proper-
ties of materials 3 and 5 showed that treatment of 3
with HMDS resulted in a decline of the micropore
volume from 0.05 to 0.00 cm
3
g
1
SiO
2
, and a further
decrease of the mesoporosity to 0.47 cm
3
g
1
SiO
2
Table 1. Elemental analyses of the hybrid materials 3–5.
Sample Wt.%C
[a]
(mmolg)
1[b]
Wt.%Pd (mmolg
1
)Wt.%E
[c]
(mmolg
1
) E/Pd
[d]
C/E
[d]
Calc. Found Calc. Found
3 4.96 (4.13) 0.82 (0.077) 0.85 (0.275) 2.0 3.6 18 15
4 1.71 (1.43) 0.43 (0.040) 0.28 (0.087) 2.0 2.2 15 16
5 7.66 (6.38) 0.57 (0.054) 0.58 (0.187) 2.0 3.4 - -
[a]
% by weight.
[b]
In brackets, mmol per gram silica.
[c]
E= S for SCS and P for PCP.
[d]
Atom ratio.
Table 2. Nitrogen physisorption data of functionalized silica materials.
Material S
BET
[a]
[m
2
·g
1
SiO
2
]d
pore
[b]
[nm] V
micro
[c]
[cm
3
·g
1
SiO
2
]V
meso
[d]
[cm
3
·g
1
SiO
2
]V
tot
[e]
[cm
3
·g
1
SiO
2
]
1 SBA-15 518 8.7 0.09 0.73 0.78
2 SBA-15 (toluene) 504 8.7 0.09 0.71 0.75
3 SBA-15 (HMDS) 460 7.8 0.00 0.49 0.54
4 3 442 8.2 0.05 0.62 0.66
5 5 433 7.9 0.00 0.47 0.51
6 5 after run 1 416 7.9 0.00 0.45 0.49
7 5 after run 5 423 7.9 0.00 0.46 0.50
8 MCM-41 975 4.3 0.01 0.86 1.03
9 4 966 4.1 0.00 0.76 0.95
[a]
S
BET
=BET surface area.
[b]
d
pore
= average pore diameter calculated using NL-DFT.
[c]
V
micro
= micropore volume.
[d]
V
meso
=mesopore volume.
[e]
V
tot
=total pore volume determined at p/p
o
= 0.995.
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PCP- and SCS-Pincer Palladium Complexes Immobilized on Mesoporous Silica
